1.  Field of the Invention
The present invention relates to polyamine-epihalohydrin resin products, particularly polyamine-epihalohydrin resin products which can be stored with at least reduced formation of halogen containing residuals, such as 3-chloropropanediol (CPD).  Moreover, the present invention relates to formation of polyamine-epihalohydrin resins having at least reduced formation of halogen containing residuals, and to various uses of the resins, such as wet strength agents.  More specifically, the present invention relates to polyamine-epihalohydrin resin products which have reduced levels of formation of CPD upon storage, such as paper products.  Moreover, the present invention relates to the production of polyamine-epihalohydrin resins prepared from polyaminoamide prepolymers containing low levels of acid functionalities, and to resins formed thereby. 
2.  Discussion of Background Information
Polyamine-epihalohydrin resins, such as polyaminopolyamide-epihalohydrin resins are cationic thermosetting materials used to increase the wet strength of papers.  Often these materials contain large quantities of epihalohydrin hydrolysis products.  For example, commercial polyaminopolyamide-epichlorohydrin resins typically contain 1-10 wt % (dry basis) of the epichlorohydrin (epi) by-products, 1,3-dichloropropanol (1,3-DCP), 2,3-dichloropropanol (2,3-DCP) and 3-chloropropanediol (CPD).  Production of wet-strength resins with reduced levels of epi by-products has been the subject of much investigation.  Environmental pressures to produce wet-strength resins with lower levels of adsorbable organic halogen (AOX) species have been increasing.  xe2x80x9cAOXxe2x80x9d refers to the adsorbable organic halogen content of the wet strength resin which can be determined by means of adsorption onto carbon.  AOX includes epichlorohydrin (epi) and epi by-products (1,3-dichloropropanol, 2,3-dichloropropanol and 3-chloropropanediol) as well as organic halogen bound to the polymer backbone. 
Commercial papermaking operations typically utilize paper wet strengthening formulations which comprise cationic thermosetting polymers.  In the papermaking process, waste material is frequently disposed of in landfills, etc.  It is desirable to reduce the organohalogen content of such wastes to as low a level as possible.  This waste is a substantially solid mass of material which is exposed to the environment.  The exposure of the waste to the environment results in the selection of microorganisms which feed on the components in the waste.  It is known that there are microorganisms which feed on the organohalogen compounds in the solid waste. 
In the papermaking process the epichlorohydrin hydrolysis products are released into the environment in the water used to make paper, or into the air by evaporation during the paper drying step, or into the paper itself or a combination of these events.  It is desirable to reduce and control these emissions into the environment to as low a level as possible.  Reduced levels of CPD are especially desirable in applications where food is the end use. 
Several ways of reducing the quantities of epihalohydrin hydrolysis products have been devised.  Reduction in the quantity of epihalohydrin used in the synthetic step is an alternative taught in U.S. Pat.  No.  5,171,795.  A longer reaction time results.  Control over the manufacturing process is taught in U.S. Pat.  No.  5,017,642 to yield compositions of reduced concentration of hydrolysis products.  These patents are incorporated by reference in their entireties. 
Post-synthesis treatments are also taught.  U.S. Pat.  No.  5,256,727, which is incorporated by reference in its entirety, teaches that reacting the epihalohydrin and its hydrolysis products with dibasic phosphate salts or alkanolamines in equimolar proportions converts the chlorinated organic compounds into non-chlorinated species.  To do this it is necessary to conduct a second reaction step for at least 3 hours, which adds significantly to costs and generates quantities of unwanted organic materials in the wet strength composition.  In compositions containing large amounts of epihalohydrin and epihalohydrin hydrolysis products (e.g., about 1-6% by weight of the composition), the amount of organic material formed is likewise present in undesirably large amounts. 
U.S. Pat.  No.  5,516,885 and WO 92/22601, which are incorporated by reference in their entireties, disclose that halogenated by-products can be removed from products containing high levels of halogenated by-products as well as low levels of halogenated by-products the use of ion exchange resins.  However, it is clear from the data presented that there are significant yield losses in wet strength composition and a reduction in wet strength effectiveness. 
It is known that nitrogen-free organohalogen-containing compounds can be converted to a relatively harmless substance.  For example, 1,3-dichloro-2-propanol, 3-chloro-1,2-propanediol (also known as 3-chloropropanediol, 3-monochloropropanediol, monochloropropanediol, chloropropanediol, CPD, 3-CPD, MCPD and 3-MCPD) and epichlorohydrin have been treated with alkali to produce glycerol. 
The conversion of nitrogen-free organohalogen compounds with microorganisms containing a dehalogenase is also known.  For example, C. E. Castro, et al. ( xe2x80x9cBiological Cleavage of Carbon-Halogen Bonds Metabolism of 3-Bromopropanol by Pseudomonas sp.xe2x80x9d, Biochimica et Biophysica Acta, 100, 384-392, 1965), which is incorporated by reference in its entirety, describes the use of Pseudomonas sp.  isolated from soil that metabolizes 3-bromopropanol in sequence to 3-bromopropionic acid, 3-hydroxypropionic acid and CO2. 
Various U.S. patents also describe the use of microorganisms for dehalogenating halohydrins, e.g., U.S. Pat.  Nos.  4,452,894; 4,477,570; and 4,493,895.  Each of these patents is hereby incorporated by reference as though set forth in full herein. 
U.S. Pat.  Nos.  5,470,742, 5,843,763and 5,871,616, which are incorporated by reference in their entireties, disclose the use of microorganisms or enzymes derived from microorganisms to remove epihalohydrin and epihalohydrin hydrolysis products from wet strength compositions without reduction in wet strength effectiveness.  Processes of removal are described which remove up to 2.6 weight per cent of halogenated by-product based on the weight of the composition.  The amount of microorganism or enzyme used is in direct proportion to the quantity of halogenated by-product present.  Thus, when present in large quantities (e.g., more than about 1% by weight of the composition) a large proportion of microorganism or enzyme is needed to adequately remove the unwanted product.  Large quantities of halogenated byproduct can be toxic to the microbes employed in such dehalogenation processes.  Each of these documents is hereby incorporated by reference as though set forth in full herein. 
Still further, U.S. application Ser.  No.  08/482,398, now U.S. Pat.  No.  5,972,691 and WO 96/40967, which are incorporated by reference in their entireties, disclose the treatment of wet strength compositions with an inorganic base after the synthesis step (i.e., after the polymerization reaction to form the resin) has been completed and the resin has been stabilized at low pH, to reduce the organo halogen content of wet strength compositions (e.g., chlorinated hydrolysis products) to moderate levels (e.g., about 0.5% based on the weight of the composition).  The composition so formed can then be treated with microorganisms or enzymes to economically produce wet strength compositions with very low levels of epihalohydrins and epihalohydrin hydrolysis products. 
It is also known that epihalohydrin and epihalohydrin hydrolyzates can be reacted with bases to form chloride ion and polyhydric alcohols.  U.S. Pat.  No.  4,975,499 teaches the use of bases during the synthetic step to reduce organo chlorine contents of wet strength composition to moderate levels (e.g., to moderate levels of from about 0.11 to about 0.16%) based on the weight of the composition.  U.S. Pat.  No.  5,019,606 teaches reacting wet strength compositions with an organic or inorganic base.  These patents are incorporated by reference in their entireties. 
Moreover, U.S. application Ser.  No.  09/001,787, filed Dec. 31, 1997, and Ser.  No.  09/224,107, filed Dec. 22, 1998 to Riehle, and WO 99/33901, and which are incorporated by reference in their entireties, disclose amongst other features, a process for reducing the AOX content of a starting water-soluble wet-strength resin comprising azetidinium ions and tertiary aminohalohydrin, which includes treating the resin in aqueous solution with base to form treated resin, wherein at least about 20% of the tertiary aminohalohydrin present in the starting resin is converted into epoxide and the level of azetidinium ion is substantially unchanged, and the effectiveness of the treated resin in imparting wet strength is at least about as great as that of the starting wet-strength resin. 
The use of endcapping agents to prepare polyaminoamide prepolymers of controlled molecular weight is described in U.S. Pat.  Nos.  5,786,429 and 5,902,862, which are incorporated by reference in their entireties.  The endcapping agents described were either monofunctional carboxylic acids, monofunctional carboxylic esters or monofunctional amines.  These polyaminoamides were subsequently reacted with a minimal amount of an intralinker to give highly branched polyamidoamines having either no or very low levels of reactive functionality. 
WO 99/09252 describes thermosetting wet strength resins prepared from end-capped polyaminoamide polymers.  The endcappers used are monocarboxylic acids or monofunctional carboxylic esters, and are used to control the molecular weight of the polyaminamide in order to obtain wet strength resins with a high solids content. 
Each of the foregoing approaches has provided various results, and there has been a continuing need for improvement. 
The present invention is directed to polyamine-epihalohydrin resin products, particularly polyamine-epihalohydrin resin products which can be stored with at least reduced formation of halogen containing residuals, such as 3-chloropropanediol (CPD).  The present invention is also directed to various uses of polyamine-epihalohydrin resins having at least reduced formation of halogen containing residuals, such as wet strength agents. 
The present invention is also directed to polyamine-epihalohydrin resin products which have reduced levels of formation of CPD upon storage, particularly paper products. 
The present invention is also directed to the preparation of polyamine-epihalohydrin resins, especially polyaminopolyamide-epihalohydrin resins and/or the treatment of polyamine-epihalohydrin resins, especially polyaminopolyamide-epihalohydrin resins. 
The present invention is also directed to the preparation of storage stable polyamine-epihalohydrin resins, especially polyaminopolyamide-epihalohydrin resins and/or the treatment of polyamine-epihalohydrin resins, especially polyaminopolyamide-epihalohydrin resins to render such resins storage stable. 
In one aspect of the present invention wherein the polyamine-epihalohydrin resin is treated to obtain a storage stable product, the present invention is directed to a process for rendering a polyamine-epihalohydrin resin storage stable, comprising treating a composition containing polyamine-epihalohydrin resin which includes CPD-forming species with at least one agent under conditions to at least one of inhibit, reduce and remove the CPD-forming species to obtain a gelation storage stable reduced CPD-forming resin so that composition containing the reduced CPD-forming polyamine-epihalohydrin resin when stored for 2 weeks at 50xc2x0 C., and a pH of about 2.5 to 3.5 contains less than about 250 ppm dry basis of CPD, preferably less than about 150 ppm dry basis of CPD after two weeks, more preferably less than about 75 ppm dry basis of CPD after two weeks, even more preferably less than about 40 ppm dry basis of CPD after two weeks, and even more preferably less than about 10 ppm dry basis of CPD after two weeks. 
Moreover, the present invention is also directed to a process for rendering a polyamine-epihalohydrin resin storage stable, comprising treating a composition containing a polyamine-epihalohydrin resin which includes CPD-forming species with at least one agent under conditions to at least one of inhibit, reduce and remove the CPD-forming species to obtain a gelation storage stable reduced CPD-forming resin so that a composition containing the reduced CPD-forming polyamine-epihalohydrin resin, when stored at pH 1 for 24 hours at 50xc2x0 C. and measured at 24 hours, produces less than about 1000 ppm dry basis of CPD, more preferably produces less than about 750 ppm dry basis of CPD, even more preferably produces less than about 500 ppm dry basis of CPD, even more preferably produces less than about 250 ppm dry basis of CPD, even more preferably produces less than about 150 ppm dry basis of CPD, even more preferably produces less than about 100 ppm dry basis of CPD, even more preferably produces less than about 75 ppm dry basis of CPD, even more preferably produces less than about 50 ppm dry basis of CPD, even more preferably-produces less than about 25 ppm dry basis of CPD, even more preferably produces less than about 15 ppm dry basis of CPD, even more preferably produces less than about 5 ppm dry basis of CPD, and even more preferably produces less than about 3 ppm dry basis of CPD, and even more preferably produces less than about 1 ppm dry basis of CPD. 
The present invention is also directed to a storage stable polyaminopolyamide-epihalohydrin resin, the polyaminopolyamide-epihalohydrin resin when stored as an aqueous composition containing the resin, when stored at pH 1 for 24 hours at 50xc2x0 C. and measured at 24 hours, produces less than about 1000 ppm dry basis of CPD, more preferably produces less than about 750 ppm dry basis of CPD, even more preferably produces less than about 500 ppm dry basis of CPD, even more preferably produces less than about 250 ppm dry basis of CPD, even more preferably produces less than about 150 ppm dry basis of CPD, even more preferably produces less than about 100 ppm dry basis of CPD, even more preferably produces less than about 75 ppm dry basis of CPD, even more preferably produces less than about 50 ppm dry basis of CPD, even more preferably produces less than about 25 ppm dry basis of CPD, even more preferably produces less than about 15 ppm dry basis of CPD, even more preferably produces less than about 5 ppm dry basis of CPD, and even more preferably produces less than about 3 ppm dry basis of CPD, and even more preferably produces less than about 1 ppm dry basis of CPD. 
In another aspect, the present invention is also directed to a storage stable polyaminopolyamide-epihalohydrin resin, the polyaminopolyamide-epihalohydrin resin being capable of forming a paper product, so that a paper product containing said polyaminopolyamide-epihalohydrin resin, when corrected for adding at about a 1 wt % addition level of the polyaminopolyamide-epihalohydrin resin, contains less than about 250 ppb of CPD, more preferably less than about 100 ppb of CPD, more preferably less than about 50 ppb of CPD, more preferably less than about 10 ppb of CPD and even more preferably less than about 1 ppb of CPD. 
A paper product containing the reduced CPD-forming resin, when corrected for adding at about a 1 wt % addition level of the reduced CPD-forming resin, preferably contains less than about 250 ppb of CPD, more preferably less than about 100 ppb of CPD, more preferably less than about 50 ppb of CPD, more preferably less than about 10 ppb of CPD and even more preferably less than about 1 ppb of CPD. 
In another aspect of the present invention involving preparation of the polyamine-epihalohydrin so as to have a reduced acid number, the present invention is directed to the production of polyaminopolyamide-epihalohydrin resins having reduced acid number, compositions and solutions containing such resins, as well as products, such as paper products, containing such resins. 
In another aspect of the present invention, a storage stable polyaminopolyamide-epihalohydrin resin is provided, wherein the polyaminopolyamide-epihalohydrin resin when stored as an aqueous composition containing the resin for 2 weeks at 50xc2x0 C., and a pH of about 2.5 to 3.5 contains less than about 250 ppm dry basis of CPD, preferably less than about 150 ppm dry basis of CPD after two weeks, more preferably less than about 75 ppm dry basis of CPD after two weeks, even more preferably less than about 40 ppm dry basis of CPD after two weeks, and even more preferably less than about 10 ppm dry basis of CPD after two weeks. 
In still another aspect, the present invention is directed to a polyaminopolyamide-epihalohydrin resin formed by reacting polyaminoamide prepolymer with epihalohydrin, the polyaminoamide prepolymer having an acid functionality less than about 0.5 milliequivalents/dry gram of prepolymer, and said polyaminopolyamide-epihalohydrin resin being subjected to a treatment to reduce at least one of epihalohydrins, epihalohydrin hydrolysis by-products and CPD forming species. 
Still further, the polyaminopolyamide-epihalohydrin resin can be a polyaminopolyamide-epihalohydrin resin produced from polyaminoamide prepolymer having an acid functionality less than about 0.5 milliequivalents/dry gram of prepolymer, more preferably less than about 0.25 milliequivalents/dry gram of prepolymer, more preferably less than about 0.1 milliequivalents/dry gram of prepolymer, more preferably less than about 0.075 milliequivalents/dry gram of prepolymer, even more preferably less than about 0.05 milliequivalents/dry gram of prepolymer. 
Still further, the polyaminopolyamide-epihalohydrin resin can be a polyaminopolyamide-epihalohydrin resin produced from polyaminoamide prepolymer having an acid end group concentration of less than about 5%, as measured by 13C NMR analysis, more preferably, an acid end group concentration of less than about 2.5%, as measured by 13C NMR analysis, more preferably an acid end group concentration of less than about 1%, as measured by 13C NMR analysis, more preferably an acid end group concentration less than about 0.7%, as measured by 13C NMR analysis, and even more preferably an acid end group concentration of less than about 0.5%, as measured by 13C NMR analysis. 
In another aspect of the present invention, the prepolymer can have a RSV of about 0.075 to 0.2 dL/g, more preferably about 0.1 to 0.15 dL/g, and is preferably at least about 0.05 dL/g, more preferably at least about 0.075 dL/g, and even more preferably at least about 0.1 dL/g.
As noted above, the composition preferably contains less than about 150 ppm dry basis, more preferably less than about 75 ppm dry basis, more preferably less than about 40 ppm dry basis, more preferably less than about 10 ppm dry basis of CPD after two weeks. 
Moreover, the present invention is also directed to a process for preparing a paper product, comprising treating a compositon containing polyamine-epihalohydrin resin which includes CPD-forming species with at least one agent under conditions to at least one of inhibit, reduce and remove the CPD-forming species to obtain a gelation storage stable reduced CPD-forming resin, and forming a paper product with the reduced CPD-forming polyamine-epihalohydrin resin, so that a paper product, when corrected for adding at about a 1 wt % addition level of the reduced CPD-forming resin, contains less than about 250 ppb of CPD, preferably less than about 100 ppb of CPD, even more preferably less than about 50 ppb of CPD, even more preferably less than about 10 ppb of CPD, and even more preferably less than about 1 ppb of CPD. 
In still another aspect, the present invention is directed to a process of producing a polyaminoamide prepolymer by reacting polyalkyleneamine with dicarboxylic acid and/or dibasic ester in a prepolymer forming reaction, and post-adding at least one amine at a later stage of the prepolymer forming reaction.  The amine can be added in an amount so that a total molar quantity of polyalkylenepolyamine plus post-added amine is greater than a total molar amount of dicarboxylic acid. 
Preferably, the prepolymer forming reaction is at least about 70% complete at time of addition of the post-added amine, more preferably at least about 80% complete, and even more preferably at least about 90% complete. 
The post-added amine can be a monofunctional amine and/or a polyamine, such as a polyalkyleneamine. 
In the various reactions, the dicarboxylic acid can comprise at least one of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid and azelaic acid; the dibasic ester can comprise at least one of dimethyl adipate, diethyl adipate, dimethyl glutarate, diethyl glutarate, dimethyl succinate and diethyl succinate, and the polyalkyleneamine can comprise at least one of diethylenetriamine, triethylenetetramine, tetraethylenepentamine, dipropylenetriamine, methylbisaminopropylamine, bis-hexamethylenetriamine and methylbisaminopropylamine. 
The polyamine-epihalohydrin resin can comprise polyaminopolyamide-epihalohydrin resin, preferably polyaminopolyamide-epichlorohydrin resin, and polyaminoureylene-epihalohydrin resin, preferably polyaminoureylene-epichlorohydrin resin. 
The prepolymer can comprise endcapped prepolymer, amine excess prepolymer and post-added amine prepolymer. 
The at least one agent can comprise at least one acidic agent.  The at least one acidic agent is preferably added to provide an initial pH of less than about 2, preferably less than about 1, or about 1, the temperature is at least about 30xc2x0 C., preferably about 30xc2x0 C. to 140xc2x0 C., more preferably about 40xc2x0 C. to 90xc2x0 C., with preferred temperatures being at least about 50xc2x0 C., and the time is at least about 2 hours.  The at least one acidic agent can be added to provide an initial pH of about 1, the temperature can be about 50xc2x0 C., and the time can be about 24 hours.  The at least one acidic agent can added to provide an initial pH of about 1, the temperature can be about 60xc2x0 C., and the time can be about 12 hours.  The at least one acidic agent can be added to provide an initial pH of about 1, the temperature can be about 70xc2x0 C., and the time can be about 6 hours.  The at least one acidic agent can be added to provide an initial pH of about 1, the temperature can be about 80xc2x0 C., and the time can be about 3 hours. 
The at least one acidic agent can comprise a non-halogen inorganic acid, preferably sulfuric acid. 
Following the treating with the at least one acidic agent, at least one basic agent can be added to raise the pH of the resin solution to at least about 7, preferably to at least about 8, with a preferred range of about 8 to 12.  The resin solution during base treatment preferably has a temperature of about 40xc2x0 C. to 70xc2x0 C. Following the addition of the at least one basic agent, an acidic agent can be added in an amount effective to gel stabilize the resin solution. 
The at least one agent can comprise at least one basic agent.  The resin can comprise a resin formed in a polyamide-epihalohydrin reaction having a molar ratio of epihalohydrin to secondary amine group of less than 1, more preferably the molar ratio of epihalohydrin to secondary amine group is less than about 0.975, with a preferred range of the molar ratio of epihalohydrin to secondary amine group being about 0.5 to 0.975, more preferably the molar ratio of epihalohydrin to secondary amine group being about 0.8 to 0.975.  The at least one basic agent can raise the pH of the composition containing the polyamine-epihalohydrin resin to a pH of at least about 8, more preferably at least about 9, more preferably a pH of at least about 10, and the pH is preferably less than about 12.5, with a preferred pH range pH about 10 to 12.  The composition preferably has a temperature of at least about 20xc2x0 C., more preferably a temperature of at least about 40xc2x0 C., with one temperature range being about 20xc2x0 C. to 80xc2x0 C. The composition can have a temperature of about 50xc2x0 C., a pH of about 11.5, and a treatment time is about 5 minutes.  The composition can have a temperature of about 55xc2x0 C., a pH of about 10.5 to 11.5, and a treatment time is about 5 minutes.  The reduced CPD-forming resin can be acid stabilized, such as to a pH from about 2.5 to 4. 
The at least one agent can comprise at least one enzymatic agent, such as at least one of esterases, lipases and proteases, preferably ALCALASE. 
The at least one agent can comprise at least one pH modifying agent to obtain a pH of about 5.5 to 7.  The composition can have a temperature of about 30xc2x0 C., a pH of about 6 and a treatment time of about 6 days.  The composition can have a temperature of about 50xc2x0 C., a pH of about 6 and a treatment time of about 6 hours. 
Prior and/or subsequent to treating a polyamine-epihalohydrin resin to obtain a reduced CPD-forming resin and/or after production of a low acid number resin, the resin can be contacted with at least one microorganism, or at least one enzyme isolated from the at least one microorganism, in an amount, and at a pH and temperature effective to dehalogenate residual quantities of organically bound halogen.  The at least one microorganism can comprise at least one of Arthrobacter histidinolovorans HK1, Burkholderia cepacia and Aerobacterium radiocacter HK7.  The at least one microorganism can comprise a mixture comprising at least one of Agrobacterium radiobacter HK7 and Burkholderia cepacia, and Arthrobacter histidinolovorans HK1. 
Moreover, prior and/or subsequent to the treating a polyamine-epihalohydrin resin to obtain a reduced CPD-forming resin and/or after production of a low acid number resin, the resin can be treated to reduce at least one of epihalohydrins, epihalohydrin hydrolysis by-products and organic halogen bound to the polymer backbone. 
The present invention is also directed to paper products treated with resins produced according to the present invention, to reduced CPD-forming resin produced according to the present invention, and aqueous compositions comprising the reduced CPD-forming resin according to the present invention, and such aqueous compositions including at least one polyalkylene polyamine-epihalohydrin resin. 
The paper product can comprise a paper product which comes into contact with food products, such as a tea bag or coffee filter, or packaging board, or tissue and towel. 
Unless otherwise stated, all percentages, parts, ratios, etc.,  are by weight. 
Unless otherwise stated, a reference to a compound or component includes the compound or component by itself, as well as in combination with other compounds or components, such as mixtures of compounds. 
Further, when an amount, concentration, or other value or parameter, is given as a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of an upper preferred value and a lower preferred value, regardless whether ranges are separately disclosed. 
Polyamine-epihalohydrin resins according to the present invention include polyaminopolyamide-epihalohydrin resins (which are also known as polyaminoamide-epihalohydrin resins, polyamidepolyamine-epihalohydrin resins, polyaminepolyamide-epihalohydrin resins, aminopolyamide-epihalohydrin resins, polyamide-epihalohydrin resins); polyalkylene polyamine-epihalohydrin; and polyaminourylene-epihalohydrin resins, copolyamide-polyurylene-epichlorohydrin resins, polyamide-polyurylene-epichlorohydrin resins with the epihalohydrin preferably being epichlorohydrin in each instance. 
This invention is also directed towards the preparation, use and treatment of polyamine-epihalohydrin resins, such as polyaminopolyamide-epichlorohydrin resins, made by reacting epihalohydrin, such as epichlorohydrin, with a prepolymer (also interchangeably referred to herein as polymer), such as polyaminoamide prepolymer.  In the case of polyaminopolyamide resins, it is noted that the polyaminoamide prepolymer is also referred to as polyamidoamine, polyaminopolyamide, polyamidopolyamine, polyamidepolyamine, polyamide, basic polyamide, cationic polyamide, aminopolyamide, amidopolyamine or polyaminamide. 
While not wishing to be bound by theory, the present invention is based upon the discovery that CPD that is formed in polyamine-epihalohydrin resins, particularly polyaminopolyamide-epihalohydrin resins, after storage, is due to CPD-forming species that are associated with the oligomeric and/or polymeric component of the resin.  Thus, it has been discovered that polyamine-epihalohydrin resins can be treated during and/or subsequent to production in such a manner so as to prevent the formation of, inhibit and/or remove elements associated with the polyamine-epihalohydrin resin which form CPD upon storage. 
In other words, the resins according to the present invention are capable of being stored without undue formation of CPD.  More specifically, as an example, the solution will contain less than about 10 ppm (parts per million), more preferably less than about 5 ppm, and most preferably less than 1 ppm of CPD, when stored at about 13.5 wt % resin solids content.  In the context of the present invention the phrase xe2x80x9cresin solidsxe2x80x9d means the active polyamine-epihalohydrin of the composition. 
To determine storage stability of resin solutions according to the present invention, a resin solution stability test is performed wherein the resin solution is stored for a period of 2 weeks at 50xc2x0 C., and a pH of about 2.5 to 3.5, preferably 2.8, and the CPD content is measured at the end of the 2 week period.  Thus, a solution containing polyamine-epihalohydrin resin according to the present invention will be storage stable if it contains less than about 250 ppm dry basis of CPD when measured at the end of the two week period, more preferably less than about 150 ppm dry basis of CPD when measured at the end of the 2 week period, more preferably less than about 75 ppm dry basis of CPD when measured at the end of the 2 week period, even more preferably less than about 40 ppm dry basis of CPD when measured at the end of the two week period, and even more preferably less than about 10 ppm dry basis of CPD when measured at the end of the 2 week period. 
The resin solution stability test can be performed on solutions containing varying percent resin solids content; however, the CPD produced should be corrected for solids content.  For example, for a 15 wt % resin solids content solution having a measured CPD content of 15 ppm, the corrected CPD, on a dry basis, will be 100 ppm dry basis (15 ppm/0.15 weight resin solids content). 
The resin solution stability test is performed by charging a portion of the polyamine-epihalohydrin resin into a container containing a stirrer.  The container is placed in a 50xc2x0 C. water bath and maintained at 50xc2x0 C. with stirring.  An aliquot is removed from the container and submitted for GC (gas chromatography) analysis according to the GC procedure as set forth in Comparative Example 1.  Typically, a flame ionization detector (FID) is first used to analyze the sample.  An electrolytic conductivity detector (ELCD) or a halogen-specific detector (XSD) is used when increased sensitivity is needed, especially at less than about 20 ppm of the species to be analyzed.  Other sensitive detectors can be used, e.g., electron capture detectors.  This test is an accelerated aging test to model ageing at longer periods of time at about 32xc2x0 C.
Moreover, paper products containing resins according to the present invention are capable of being stored without undue formation of CPD.  Thus, paper products according to the present invention can have initial low levels of CPD, and can maintain low levels of CPD over an extended period storage time.  More specifically, paper products according to the present invention, made with a 1 wt % addition level of resin, will contain less than about 250 parts per billion (ppb) of CPD, more preferably less than about 100 ppb of CPD, even more preferably less than about 50 ppb of CPD and even more preferably less than about 10 ppb of CPD when stored for periods as long as 2 weeks, more preferably as long as at least 6 months, and even more preferably as long as at least one year.  Moreover, paper products according to the present invention, made with about a 1 wt % addition level of resin, will have an increase in CPD content of less than about 250 ppb, more preferably less than about 100 ppb of CPD, even more preferably less than about 50 ppb of CPD, even more preferably less than about 10 ppb of CPD, and even more preferably less than about 1 ppb of CPD when stored for periods as long as 2 weeks, more preferably as long as at least 6 months, and even more preferably as long as at least one year.  In other words, the paper products according to the present invention have storage stability and will not generate excessive CPD content in paper products when the paper products are stored as little as one day and for periods of time greater than one year.  Thus, the resins according to the present invention give minimal formation of CPD in paper products, particularly those exposed to aqueous environments, especially hot aqueous environments, e.g., tea bag, coffee filters, etc.  Further examples of paper products include packaging board grade, and tissue and towel grade. 
Paper can be made by adding the resin at addition levels other than about 1 wt %; however, the CPD content should be corrected for the addition level.  For example, for a paper product made by adding the resin at a 0.5 wt % addition level having a measured CPD content of 50 ppb, the corrected CPD on a 1 wt % addition level basis will be 100 ppb (50 ppb/0.5 percent addition level). 
To measure CPD in paper products, the paper product is extracted with water according to the method described in European standard EN 647, dated October 1993.  Then 5.80 grams of sodium chloride is dissolved into 20 ml of the water extract.  The salted aqueous extract is transferred to a 20 gram capacity Extrelut column and allowed to saturate the column for 15 minutes.  After three washes of 3 ml ethyl acetate and saturation of the column, the Extrelut column is eluted until 300 ml of eluent has been recovered in about 1 hour.  The 300 ml of ethyl acetate extract is concentrated to about 5 ml using a 500-ml Kuderna-Danish concentrating apparatus (if necessary, further concentrating is done by using a micro Kudema-Danish apparatus).  The concentrated extract is analyzed by GC using the instrumentation described in Comparative Example 1.  Typically, a flame ionization detector (FID) is first used to analyze the sample.  An electrolytic conductivity detector (ELCD) or a halogen-specific detector (XSD) is used when increased sensitivity is needed, especially at less than about 20 ppm of the species to be analyzed.  Other sensitive detectors can be used, e.g., electron capture detectors. 
Preferably, the resin according to the present invention contains less than 1 part per million (ppm) each of epihalohydrin, e.g., epichlorohydrin, 1,3-DCP, 2,3-DCP and less than 10 ppm of CPD after storage, at 13.5 wt. %  total solids content, which, when applied to paper at a dosage of up to 1 wt. %  dry basis on fiber, gives a level of less than about 30 ppb of each of epihalohydrin and epihalohydrin byproducts, e.g., epichlorohydrin and 1,3-DCP and 2,3-DCP, and CPD content in paper, and the paper is stable at that level for up to 6 months storage at room temperature, so that after about 6 months, preferably after about 1 year, the level each of these species will be less than about 30 ppb. 
Polyaminopolyamide-epichlorohydrin resins comprise the water-soluble polymeric reaction product of epichlorohydrin and polyamide derived from polyalkylene polyamine and saturated aliphatic dibasic carboxylic acid containing from about 2 to about 10 carbon atoms.  It has been found that resins of this type impart wet-strength to paper whether made under acidic, alkaline or neutral conditions.  Moreover, such resins are substantive to cellulosic fibers so that they may be economically applied thereto while the fibers are in dilute aqueous suspensions of the consistency used in paper mills. 
In the preparation of the cationic resins contemplated for use herein, the dibasic carboxylic acid is first reacted with the polyalkylene polyamine, under conditions such as to produce a water-soluble polyamide containing the recurring groups. 
xe2x80x83xe2x80x94NH(CnH2nNH)xxe2x80x94CORCOxe2x80x94
where n and x are each 2 or more and R is the divalent hydrocarbon radical of the dibasic carboxylic acid.  This water soluble polyamide is then reacted with an epihalohydrin to form the water-soluble cationic thermosetting resins. 
The dicarboxylic acids contemplated for use in preparing the resins of the invention are the saturated aliphatic dibasic carboxylic acids containing from 2 to 10 carbon atoms such as oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, azelaic acid and the like.  The saturated dibasic acids having from 4 to 8 carbon atoms in the molecule, such as adipic and glutaric acids are preferred.  Blends of two or more of the saturated dibasic carboxylic acids may also be used.  Derivatives of dibasic carboxylic acids, such as esters, half-esters and anhydrides can also be used in the present invention, such as dimethyl adipate, diethyl adipate, dimethyl glutarate, diethyl glutarate, dimethyl succinate and diethyl succinate.  Blends of two or more of derivatives of dibasic carboxylic acids may also be used, as well as blends of one or more derivatives of dibasic carboxylic acids with dibasic carboxylic acids. 
A variety of polyalkylene polyamines including polyethylene polyamines, polypropylene polyamines, polybutylene polyamines, polypentylene polyamines, polyhexylene polyamines and so on and their mixtures may be employed of which the polyethylene polyamines represent an economically preferred class.  More specifically, the polyalkylene polyamines contemplated for use may be represented as polyamines in which the nitrogen atoms are linked together by groups of the formula xe2x80x94CnH2nxe2x80x94 where n is a small integer greater than unity and the number of such groups in the molecule ranges from two up to about eight.  The nitrogen atoms may be attached to adjacent carbon atoms in the group xe2x80x94CnH2nxe2x80x94 or to carbon atoms further apart, but not to the same carbon atom.  This invention contemplates not only the use of such polyamines as diethylenetriamine, triethylenetetramine, tetraethylenepentamine and dipropylenetriamine, which can be obtained in reasonably pure form, but also mixtures and various crude polyamine materials.  For example, the mixture of polyethylene polyamines obtained by the reaction of ammonia and ethylene dichloride, refined only to the extent of removal of chlorides, water, excess ammonia, and ethylenediamine, is a satisfactory starting material.  The term xe2x80x9cpolyalkylene polyaminexe2x80x9d employed in the claims, therefore, refers to and includes any of the polyalkylene polyamines referred to above or to a mixture of such polyalkylene polyamines and derivatives thereof.  Additional polyamines that are suitable for the present invention include; bis-hexamethylenetriamine (BHMT), methylbisaminopropylamine (MBAPA), other polyalkylene polyamines (e.g., spermine, spermidine).  Preferably, the polyamines are diethylenetriamine, triethylenetetramine, tetraethylenepentamine and dipropylenetriamine. 
It is desirable, in some cases, to increase the spacing of secondary amino groups on the polyamide molecule in order to change the reactivity of the polyamide-epichlorohydrin complex.  This can be accomplished by substituting a diamine such as ethylenediamine, propylenediamine, hexamethylenediamine and the like for a portion of the polyalkylene polyamine.  For this purpose, up to about 80% of the polyalkylene polyamine may be replaced by molecularly equivalent amount of the diamine.  Usually, a replacement of about 50% or less will serve the purpose. 
Appropriate aminocarboxylic acids containing at least three carbon atoms or lactams thereof are also suitable for use to increase spacing in the present invention.  For example, 6-aminohexanoic acid and caprolactam. 
Polyaminoureylene-epihalohydrin resins, particularly polyaminoureylene-epichlorohydrin resins, are also contemplated in the present invention, such as discussed in U.S. Pat.  Nos.  4,487,884 and 3,311,594, which are incorporated by reference in their entireties, such as Kymene(copyright)450 type of resins (Hercules Incorporated, Wilmington, Del.).  The polyaminoureylene resins contemplated for preparation and use herein are prepared by reacting epichlorohydrin with polyaminoureylenes containing free amine groups.  These polyaminoureylenes are water-soluble materials containing tertiary amine groups and/or mixtures of tertiary amine groups with primary and/or secondary amino groups and/or quaternary ammonium groups.  However, tertiary amino groups should account for at least 70% of the basic nitrogen groups present in the polyaminoureylene.  These polyaminoureylenes may be prepared by reacting urea or thiourea with a polyamine containing at least three amino groups, at least one of which is a tertiary amino group.  The reaction can, if desired, be carried out in a suitable solvent such as xylene. 
The polyamine reactant should preferably have at least three amino groups, at least one of which is a tertiary amino group.  The polyamine reactant may also have secondary amino groups in limited amounts.  Typical polyamines of this type suitable for use as hereinabove described are methyl bis(3-aminopropyl)amine (MBAPA), methyl bis(2-aminoethyl)amine, N-(2-aminoethyl)piperazine, 4,7-dimethyltriethylenetetramine and so on, which can be obtained in reasonably pure form, but also mixtures of various crude polyamine materials. 
To prepare the prepolymer from diacid and polyalkylenepolyarnine, a mixture of the reactants is preferably heated at a temperature of about 125-200xc2x0 C. for preferably about 0.5 to 4 hours, at atmospheric pressure.  Where a reduced pressure is employed, lower temperatures such as 75xc2x0 C. to 150xc2x0 C. may be utilized.  This polycondensation reaction produces water as a byproduct, which is removed by distillation.  At the end of this reaction, the resulting product is dissolved in water at a concentration of about 50% by weight total polymer solids. 
Where diester is used instead of diacid, the prepolymerization can be conducted at a lower temperature, preferably about 100-175xc2x0 C. at atmospheric pressure.  In this case the byproduct will be an alcohol, the type of alcohol depending upon the identity of the diester.  For instance, where a dimethyl ester is employed the alcohol byproduct will be methanol, while ethanol will be the byproduct obtained from a diethyl ester.  Where a reduced pressure is employed, lower temperatures such as 75xc2x0 C. to 150xc2x0 C. may be utilized. 
In converting the polyamide, formed as above described, to a cationic thermosetting resin, it is reacted with epichlorohydrin at a temperature from above about 0xc2x0 C., more preferably about 25xc2x0 C., to about 100xc2x0 C., and preferably between about 35xc2x0 C. to about 70xc2x0 C. until the viscosity of a 20% solids solution at 25xc2x0 C. has reached about C or higher on the Gardner Holdt scale.  This reaction is preferably carried out in aqueous solution to moderate the reaction.  Although not necessary, pH adjustment can be done to increase or decrease the rate of crosslinking. 
When the desired viscosity is reached, sufficient water can be added to adjust the solids content of the resin solution to the desired amount, i.e., about 15 wt % more or less, the product can be cooled to about 25xc2x0 C. and then stabilized to permit storage by improving the gelation stability by adding sufficient acid to reduce the pH to less than about 6, preferably less than about 5, and most preferably less than about 4.  Any suitable inorganic or organic acid such as hydrochloric acid, sulfuric acid, methanesulfonic acid, nitric acid, formic acid, phosphoric acid and acetic acid may be used to stabilize the product.  Non-halogen containing acids, such as sulfuric acid, are preferred. 
In the polyamide-epichlorohydrin reaction, it is preferred to use sufficient epichlorohydrin to convert most of the secondary amine groups to tertiary amine groups.  For prepolymers that contain tertiary amine groups, it is preferred to use sufficient epichlorohydrin to convert most of the tertiary amine groups to quaternary amine groups.  However, more or less may be added to moderate or increase reaction rates.  In general, satisfactory results may be obtained utilizing from about 0.5 mole to about 1.8 moles of epichlorohydrin for each secondary amine group of the polyamide.  It is preferred to utilize from about 0.6 mole to about 1.5 moles for each secondary amine group of the polyamide. 
Epichlorohydrin is the preferred epihalohydrin for use in the present invention.  The present application refers to epichlorohydrin specifically in certain instances, however, the person skilled in the art will recognize that these teachings apply to epihalohydrin in general. 
As to the CPD-forming species, not to be limited by theory, it is believed that the acid groups in, for example, polyaminopolyamides, react with epichlorohydrin during production of, e.g., polyaminopolyamide-epichlorohydrin resins, to form a small amount of chlorohydroxypropyl ester species (hereinafter also referred to as CPD ester) on the resin backbone.  Hydrolysis of CPD ester upon aging would yield CPD and regenerate the acid group. 
Without wishing to be limited by theory, it is noted that epichlorohydrin is more reactive with secondary amine than with acid groups.  Therefore, by having a lower value of epihalohydrin, the epihalohydrin will preferentially react with the secondary amine than with acid groups.  Also, as the epichlorohydrin to secondary amine ratio increases there are more CPD forming species, and would be more CPD forming species to remove.  Still further, if excess of epichlorohydrin is present, after the secondary amines react with the epichlorohydrin, there would still be epichlorohydrin present to react with the acid groups, which would be capable of forming the CPD-forming species.  Accordingly, it is preferred that the epihalohydrin to secondary amine group molar ratio be less than 1, more preferably less than about 0.975, with a preferred range being about 0.5 to 0.975, a more preferred range being about 0.8 to 0.975. 
Any procedure can be utilized to remove or reduce the amount of already produced CPD-forming species, including CPD-forming species that may already be present in the resin.  For example, the resin can be formed under conditions that prevent and/or reduce the formation of CPD-forming species on the polymer backbone and/or inhibit the CPD-forming ability of already produced species.  Moreover, the resin can be treated, preferably as a last step in its production, or immediately subsequent to its production, to remove, reduce and/or inhibit the CPD-forming species.  Thus, in one aspect, the invention comprises processes for reducing the CPD-forming species, especially in resins that have low amounts of at least one of epihalohydrins, epihalohydrin hydrolysis by-products and organic halogen bound to the polymer backbone.  In particular, the resin can comprise low residual resins such as disclosed in U.S. Pat.  Nos.  5,189,142, 5,239,047 and 5,364,927, U.S. Pat.  No.  5,516,885, WO 92/22601, WO 93/21384, U.S. application Ser.  No.  08/482,398, now U.S. Pat.  No.  5,972,691, WO 96/40967, and U.S. Pat.  Nos.  5,470,742, 5,843,763 and 5,871,616.  The disclosures of each of these documents is incorporated by reference in their entireties.  For example, the concentration of hydrolyzates in the wet strength composition can be preferably less than about 100 ppm (parts per million by weight relative to the total weight of aqueous solution containing wet strength resins), more preferably less than about 50 ppm (parts per million by weight relative to the total weight of aqueous solution containing wet strength resins), more preferably less than about 10 ppm (parts per million by weight relative to the total weight of aqueous solution containing wet strength resins), more preferably less than about 5 ppm (parts per million by weight relative to the total weight of aqueous solution containing wet strength resins), and even more preferably less than about 1 ppm (parts per million by weight relative to the total weight of aqueous solution containing wet strength resins). 
For example, with respect to the removal, reduction and/or inhibition of CPD-forming species in the resin, the following preferred non-limiting procedures are noted.  It is noted that procedures for removing, reducing and/or inhibiting the CPD-forming species in the resin can be used alone or in combination. 
The CPD-forming species in the resin can be reduced and/or removed by treating the resin with an acid to lower the pH of the solution to a pH less than about 2, more preferably less than about 1, and the pH can be as low as 0.5, or even as low as 0.1, for a sufficient period of time and at a sufficient temperature to remove and/or reduce CPD-forming species in the resin to obtain a storage stable product.  In particular, the temperature is preferably at least about 30xc2x0 C., more preferably at least about 40xc2x0 C., and even more preferably at least 50xc2x0 C., with the upper temperature being preferably less than about 140xc2x0 C. Preferably, the temperature ranges from about 30xc2x0 C. to 140xc2x0 C., more preferably about 40xc2x0 C. to 90xc2x0 C., and most preferably from about 50xc2x0 C. to 80xc2x0 C. The time of treatment can be made shorter with increasing temperature and decreasing pH, and is preferably at least about 2 hours, with the time of treatment being preferably about 24 hours at 50xc2x0 C., and preferably about 2 hours at 90xc2x0 C. Preferred combinations of temperature, time and pH, include at 50xc2x0 C., a pH of about 1 and a treatment time of about 24 hours; at 60xc2x0 C., a pH of about 1, and a treatment time of about 12 hours; at 70xc2x0 C., a pH of about 1, and a treatment time of about 6 hours; and at 80xc2x0 C., a pH of about 1, and a treatment time of about 3 hours. 
When referring to the pH, reference is being made to the pH of the solution immediately after addition of the acidic agent.  The pH can vary after addition of the acidic agent, or can be maintained at the initial pH. Preferably, the initial pH is maintained. 
The resin solids for acid treatment can be at least about 1 wt %, preferably at least about 2 wt %, more preferably at least about 6 wt %, more preferably at least about 8 wt %, and most preferably at least about 10 wt %.  The resins solids can be up to about 40 wt %, preferably up to about 25 wt %. 
Both organic and inorganic acids can be used herein in the present invention.  An acid is defined as any proton donor (see Advanced Organic Chemistry, Third Ed.;  Jerry March; John Wiley and Sons: New York, 1985, p 218-236, incorporated herein by reference.)  Suitable acids include hydrochloric acid, sulfuric acid, methanesulfonic acid, nitric acid, formic acid, phosphoric and acetic acid.  Non-halogen containing acids, such as sulfuric acid, are preferred. 
It is noted that the acid treatment reduces the wet strength effectiveness of the resin.  However, the effectiveness can preferably be recovered by a base treatment of the acid-treated resin.  Not to be limited by theory, it is believed that the effectiveness increase is due to an increase in the molecular weight of the polymer during the crosslinking base treatment.  Moreover, it would appear that if the base-treated resin were not long-term stabilized against gelation with an acid treatment, an additional effectiveness boost would also be likely due to the conversion of aminochlorohydrin to the more reactive epoxide.  The base treatment is performed at a pH of at least about 7, more preferably at least about 8, with a preferred range of about 8 to 12.  The base temperature is preferably about 40xc2x0 C., more preferably about 50xc2x0 C., even more preferably about 60xc2x0 C., and can be as high as at least about 70xc2x0 C., and even as high as about 100xc2x0 C.
The base treatment time is determined by the desired level of crosslinking.  The preferred Gardner-Holdt viscosity is dependent upon solids.  At about 12 wt % resin solids, a Gardner-Holt viscosity of about Axe2x88x92M is preferred, with Bxe2x88x92H being more preferred.  Within limits, the higher the crosslinking temperature and pH, the faster the rate of crosslinking.  It is preferred to perform the base treatment for about 0.5 to 6 hours, more preferably about 1 to 4 hours. 
Both organic and inorganic bases can be used as the basic agent in the base treatment.  A base is defined as any proton acceptor (see Advanced Organic Chemistry, Third Ed.;  Jerry March; John Wiley and Sons: New York, 1985, p 218-236, incorporated herein by reference.)  Typical bases include alkali metal hydroxides, carbonates and bicarbonates, alkaline earth metal hydroxides, trialkylamines, tetraalkylammonium hydroxides, ammonia, organic amines, alkali metal sulfides, alkaline earth sulfides, alkali metal alkoxides, alkaline earth alkoxides, and alkali metal phosphates, such as sodium phosphate and potassium phosphate.  Preferably, the base will be alkali metal hydroxides (lithium hydroxide, sodium hydroxide and potassium hydroxide) or alkali metal carbonates (sodium carbonate and potassium carbonate).  Most preferably, the base comprises inorganic bases including sodium hydroxide and potassium hydroxide, which are especially preferred for their low cost and convenience. 
The base treated resin can be used without further treatment, especially when the resin is to be used without storage.  Thus, the resin can be treated directly prior to application, e.g., in papermaking.  However, if the resin is to be stored, it is preferred to add an acid to the base treated resin to lower the pH to less than about 6.0, with a preferred range being about 2.5 to 4.0.  The stabilizing acid can be any suitable inorganic or organic acid such as hydrochloric acid, sulfuric acid, methanesulfonic acid, nitric acid, formic acid, phosphoric and acetic acid.  Non-halogen containing acids, such as sulfuric acid, are preferred. 
The amount of CPD-forming species can be determined using the following test.  A portion of resin to be tested is charged into a container containing a stirrer.  The pH is adjusted to 1.0 with 96 wt % sulfuric acid.  The container is closed and placed in a 50xc2x0 C. water bath and maintained at 50xc2x0 C. with stirring.  An aliquot is removed from the container at 24 hours, and submitted for GC analysis in the manner described in Comparative Example 1 to provide an indication of the CPD-forming species.  The CPD-forming species at 24 hours preferably produces less than about 1000 ppm dry basis of CPD, more preferably less than about 750 ppm dry basis of CPD, even more preferably less than about 500 ppm dry basis of CPD, even more preferably less than about 250 ppm dry basis of CPD, even more preferably less than about 150 ppm dry basis of CPD, even more preferably less than about 100 ppm dry basis of CPD, even more preferably less than about 75 ppm dry basis of CPD, even more preferably less than about 50 ppm dry basis of CPD, even more preferably less than about 25 ppm dry basis of CPD, even more preferably less than about 15 ppm dry basis of CPD, even more preferably less than about 5 ppm dry basis of CPD, even more preferably less than about 3 ppm dry basis of CPD, and even more preferably less than about 1 ppm dry basis of CPD. 
The resin having at least reduced levels of formation of CPD can be a resin as produced in a resin synthesis process without further treatment.  Moreover, the resin can be treated by various processes prior to reduction and/or removal of the CPD-forming species.  Still further, after treatment to reduce and/or remove CPD-forming species, the resin can be treated by various processes.  Yet still further, the resin can be treated by various processes prior to reduction and/or removal of the CPD-forming species, and the resin can also be treated by various processes after treatment to reduce and/or remove CPD-forming species.  For example, the resin can be treated by various processes, such as processes to remove low molecular weight epihalohydrin and epihalohydrin by-products, e.g., epichlorohydrin and epichlorohydrin by-products, for example, CPD in the resin solution.  Without limiting the treatments or resins that can be utilized, it is noted that resins, such as Kymene(copyright) VSLX2, Kymene(copyright)617 and Kymene(copyright)557LX (available from Hercules Incorporated, Wilmington, Del.), could be treated prior to and/or subsequent to reduction or removal of CPD-forming species with a base ion exchange column, such as disclosed in U.S. Pat.  No.  5,516,885 and WO 92/22601; with carbon adsorption, such as disclosed in WO 93/21384; membrane separation, e.g., ultrafiltration; extraction, e.g, ethyl acetate, such as disclosed in U.S. Statutory Invention Registration H1613; or biodehalogenation, such as disclosed in U.S. application Ser.  No.  08/482,398, now U.S. Pat.  No.  5,972,691, WO 96/40967 and U.S. Pat.  Nos.  5,470,742, 5,843,763 and 5,871,616.  The disclosures of each of these documents is incorporated by reference in their entireties. 
For example, with respect to biodehalogenation, such as disclosed in any one of U.S. Pat.  Nos.  5,470,742; 5,843,763 and 5,871,616, or previous base treatment and biodehalogenation as disclosed in U.S. application Ser.  No.  08/482,398, now U.S. Pat.  No.  5,972,691, and WO 96/40967, with or without a previous inorganic base treatment, the wet strength composition may be reacted with a microorganism or enzyme in adequate quantities to process epihalohydrin hydrolyzates to very low levels.  Microorganisms use dehalogenase enzymes to liberate halide ion from the epihalohydrin and haloalcohol and then use further enzymes to break down the reaction products ultimately to carbon dioxide and water. 
While not wishing to be bound by theory, it is noted that when the CPD-forming species is removed or reduced, CPD is released from the oligomeric and/or polymeric component of the resin, and therefore CPD is a component of the resin solution.  With this in mind, the resin is preferably subjected to treatment to remove or reduce the CPD-forming species, and then the resin is biodehalogenated.  In this manner, epihalohydrin and epihalohydrin hydrolyzate (also referred to as hydrolysis by-products), including released CPD, can be removed, such as by the biodehalogenation.  However, the resin can be initially treated, such as by biodehalogenation, and then subjected to treatment to remove, inhibit and/or reduce the CPD-forming species.  In particular, any CPD that will be released by the treatment should be readily soluble, and can therefore be at least partially washed away from the resin.  For example, when the resin with released CPD is included in a paper product, the CPD can be at least partially washed out of the paper product, and, due to the treatment, the resin in the paper product will not produce CPD or will not produce undesirable amounts of CPD. 
Exemplary microorganisms which contain dehalogenating enzymes capable of dehalogenating haloalcohols and epihalohydrins have been found in the following species:
40271, 40272, 40273 and 40274 were deposited on Apr. 2, 1990.  NCIMB 40383 was deposited on Mar. 11, 1991. 
Such microorganisms are conventional.  Such microorganisms are obtainable by batch or continuous enrichment culture.  Inoculation of enrichment isolation media with soil samples taken from organohalogen-contaminated soil results in mixed microbial communities, which can be sub-cultured, in a plurality of subculturing steps (preferably 2 to 5 subculturing steps), using increasing concentrations of the particular organohalogen-containing compound for which selection is sought. 
The microorganisms containing suitable enzymes are suitably used to dehalogenate the epihalohydrin hydrolyzates contained in the wet strength composition with or without an initial inorganic base treatment.  The enzymes and microorganisms are maintained in a suitable concentration to substantially metabolize the hydrolyzates to chloride ion and ultimately carbon dioxide and water.  Thus the concentration of hydrolyzates in the wet strength composition after treatment is preferably less than about 100 ppm (parts per million by weight relative to the total weight of aqueous solution containing wet strength resins after the bioreaction step), more preferably less than about 50 ppm (parts per million by weight relative to the total weight of aqueous solution containing wet strength resins after the bioreaction step), more preferably less than about 10 ppm (parts per million by weight relative to the total weight of aqueous solution containing wet strength resins after the bioreaction step), more preferably less than about 5 ppm (parts per million by weight relative to the total weight of aqueous solution containing wet strength resins after the bioreaction step), and even more preferably less than about 1 ppm (parts per million by weight relative to the total weight of aqueous solution containing wet strength resins after the bioreaction step). 
To achieve this, the concentration of microorganisms should be at least about 5xc3x97107 cells/ml, preferably at least about 108 cells/ml and most preferably at least about 109 cells/ml.  To maintain optimum active content of cells in the reactor, the reaction is best carried out at about 30xc2x0 C. +/xe2x88x925xc2x0 C. in the presence of oxygen (e.g., from about 5 to about 100% DOT) and nutrients in a stirred tank reactor.  As used herein, the term xe2x80x9cDOTxe2x80x9d refers to xe2x80x9cdissolved oxygen tensionxe2x80x9d and is the amount of oxygen, expressed as a percentage, dissolved in a given volume of water relative to oxygen-saturated water at the same temperature and pressure.  The residence time is controlled by flow rate and monitored to ensure complete reaction.  Thus, at steady state the concentration of epihalohydrin hydrolyzates in the reactor will be from about 1 to about 1000 ppm. 
The present invention also includes the reaction of an enzyme with the organohalogen compound, whereby the organohalogen is dehalogenated.  As used herein, the term xe2x80x9cenzymexe2x80x9d refers to any dehalogenase, i.e., any enzyme capable of dehalogenating a nitrogen-free organohalogen compound.  Preferably, the enzyme is obtained from a living cell, which is thereafter used for the dehalogenation of nitrogen-free organohalogen compounds.  Suitable enzymes include those produced by the microorganisms identified above. 
Although the precise identity of the enzymes of the method has not been determined, the enzymes which effectuate the method belong to the class of enzymes variously termed xe2x80x9chaloalcohol dehalogenasesxe2x80x9d or xe2x80x9chydrogen halide lyase type dehalogenasesxe2x80x9d or xe2x80x9chalohydrin hydrogen-halide lyasesxe2x80x9d. 
Thus, for dehalogenation, the invention contemplates the use of either living cells or an immobilized, unrefined cell-free extract or refined dehalogenase.  The term xe2x80x9cbiodehalogenationxe2x80x9d refers to the dehalogenation of an organohalogen compound using such materials. 
In general, if an enzyme is employed, the enzyme may be added to the composition in an amount of from about 2.5xc3x9710xe2x88x926 to 1xc3x9710xe2x88x924 weight percent, based on the weight of the composition.  However, the enzyme is preferably added to the composition in an amount of from about 2.5xc3x9710xe2x88x925 to 0.75xc3x9710xe2x88x924 weight percent, most preferably in an amount of from about 4xc3x9710xe2x88x925 to 6xc3x9710xe2x88x925 weight percent, based on the weight of the composition. 
Suitable biocatalysts can also be employed.  Such biocatalysts can be readily selected by those of ordinary skill in the art.  Agrobacterium radiobacter HK7 (NCIMB 40272) represents a biocatalyst for use in the method of the present invention. 
The most preferred biocatalyst for use in the method of the present invention is a two-component mixture of one or both of Agrobacterium radiobacter HK7 and Burkholderia cepacia with Arthrobacter histidinolovorans.  To ensure that both bacteria are present in the biodehalogenation process, it is preferred to start the process with one or both of Agrobacterium radiobacter HK7 and Burkholderia cepacia and to subsequently add the Arthrobacter histidinolovorans.  This would especially be the situation wherein the biodehalogenation process is run in a continuous mode. 
As noted above, although the precise identity of the enzymes which make the method operable has not been made, it is believed that the enzymes which effectuate the method belong to the class of enzymes termed xe2x80x9chydrogen halide lyase type dehalogenasexe2x80x9d. 
The method of biodehalogenation in accordance with the present invention is carried out by contacting a microorganism or cell-free enzyme-containing extract with the aqueous composition containing the unwanted organohalogen contaminants.  Such contact is typically achieved by forming a slurry or suspension of the microorganism or cell-free extract in the aqueous composition, with sufficient stirring. 
If desired, the microorganism or enzymes can be removed from the product stream by filtration, sedimentation, centrifugation or other means known to those skilled in the art.  Alternatively the microorganisms or enzymes can remain in the final product and optionally deactivated by thermal sterilization (e.g., by treatment at 140xc2x0 C. for 20 seconds) or by the addition of a suitable concentration of a suitable biocidal agent.  Suitable biocidal agents can be readily selected by those of ordinary skill in the art.  Thus, deactivation of the microorganism can be performed by reducing the pH of the aqueous mixture to 2.8, then adding a proprietary biocidal agent (e.g. Proxell(copyright) BD biocidal agent, which comprises 1,2-benzisothiazolin-3-one) in sufficient quantity, normally 0.02% to 0.1%, based on the weight of the aqueous composition.  The biocidal agent may be added along with potassium sorbate. 
The removal of the microorganism may be performed by one or more of the steps of filtration, centrifugation, sedimentation, or any other known techniques for removing microbes from a mixture.  The microorganisms mineralize the nitrogen-free organohalogen compounds, producing CO2, water, and biomass, with no glycerol left in the resin.  Where the biocatalyst is an immobilized dehalogenase, the product of the reaction is glycidol. 
A problem associated with the removal of the microbes from the mixture is that intensive methods of separation, such as microfiltration, remove not only microbes but also particles of cationic polymer, with the result that the wet strength properties are reduced, which is undesirable.  Therefore, it is preferable to leave the deactivated microorganism in the mixture to avoid the problem of reducing wet strength properties. 
The CPD-forming species in the resin can also be reduced, and/or inhibited and/or removed by base treatment.  In particular, the resin can be treated with at least one basic agent to raise the pH of the solution containing the polyamine-epihalohydrin resin to a pH of at least about 8, more preferably at least about 9, more preferably at least about 10, with a preferred upper limit being about 12.5, and a preferred pH range of about 10 to 12 for a sufficient period of time and at a sufficient temperature to remove and/or inhibit CPD-forming species in the resin to obtain a storage stable product.  The temperature is preferably at least about 20xc2x0 C., more preferably at least about 40xc2x0 C., even more preferably at least about 50xc2x0 C., even more preferably at least about 55xc2x0 C., and even more preferably at least about 60xc2x0 C., with the upper temperature being preferably less than about 80xc2x0 C., and can be as high as 100xc2x0 C.
It is understood that the temperature, time and pH are related so that as the temperature and pH are increased, the time of base treatment can be shortened to remove the CPD-forming species.  Thus, the time of treatment can be made shorter with increasing temperature and pH, and is preferably at least about 1 minute, even more preferably at least about 3 min, and most preferably at least about 5 min.  The treatment time can be as long as about 24 hours, but is preferably up to about 4 hours and most preferably up to about 1 hour.  Preferred combinations of temperature, time and pH include the time of treatment preferably being about 5 minutes at 50xc2x0 C. and pH of 11.5, and 5 minutes at 55xc2x0 C. and a pH of 10.5 to 11.5.  Without being wished to be bound by theory it is noted that for higher pH""s, shorter periods of time should be used, because the molecular weight of the resin may get too high and the solution can gel. 
For base treatment according to the present invention, the polyamide-epihalohydrin reaction, preferably polyamide-epichlorohydrin reaction, has an epihalohydrin, preferably epichlorohydrin, to secondary amine group molar ratio of less than 1, more preferably less than about 0.8, with a preferred range being about 0.5 to 0.8, and a preferred value being about 0.8.  Thus, in other words and being exemplary with respect to epichlorohydrin, less than 1 mole of epichlorohydrin is utilized for each secondary amine group of the polyamide, and more preferably less than about 0.8 mole of epichlorohydrin is utilized for each secondary amine group. 
As noted above and without wishing to be limited by theory, it is noted that epichlorohydrin is more reactive with secondary amine than with acid end groups.  Therefore, by having a lower value of epichlorohydrin, the epichlorohydrin will preferentially react with the secondary amine than with acid end groups.  Also, as the epichlorohydrin to secondary amine ratio increases there are more CPD forming species, and would be more CPD forming species to remove when base treating.  Still further, if excess of epichlorohydrin is present, after the secondary amines react with the epichlorohydrin, there would still be epichlorohydrin present to react with the acid end groups, which would be capable of forming the CPD-forning species. 
It is further noted that there may actually be an increase of CPD during base treatment, such as when Kymene(copyright)ULX is base treated.  However, as discussed above, any CPD that will be released by the treatment should be readily soluble, and can therefore be at least partially washed away from the resin.  For example, when the resin with released CPD is included in a paper product, the CPD can be at least partially washed out of the paper product, and, due to the treatment, the resin in the paper product will not produce CPD or will not produce undesirable amounts of CPD.  Still further, during base treatment, the CPD is reacted to glycidol, which is hydrolyzed to glycerol. 
The resin solids for base treatment, based upon the weight of the composition, can be at least about 1%, preferably at least about 2%, preferably at least about 6%, more preferably at least about 8% and most preferably at least about 10%.  The resin solids for base treatment can be up to about 40 wt %, preferably up to about 25 wt %, and most preferably up to about 15 wt %.  After base treatment, the resin can be diluted, typically, with water. 
When referring to the pH, reference is being made to the pH of the solution immediately after addition of the basic agent.  The pH can vary after addition of the basic agent, or can be maintained at the initial pH.
Both organic and inorganic bases can be used as the basic agent in the present invention.  A base is defined as any proton acceptor (see Advanced Organic Chemistry, Third Ed.;  Jerry March; John Wiley and Sons: New York, 1985, p 218-236, incorporated herein by reference.)  Typical bases include alkali metal hydroxides, carbonates and bicarbonates, alkaline earth metal hydroxides, trialkylamines, tetraalkylanrronium hydroxides, ammonia, organic amines, alkali metal sulfides, alkaline earth sulfides, alkali metal alkoxides, alkaline earth alkoxides, and alkali metal phosphates, such as sodium phosphate and potassium phosphate.  Preferably, the base will be alkali metal hydroxides (lithium hydroxide, sodium hydroxide and potassium hydroxide) or alkali metal carbonates (sodium carbonate and potassium carbonate).  Most preferably, the base comprises inorganic bases including sodium hydroxide and potassium hydroxide, which are especially preferred for their low cost and convenience. 
After the base treatment, the resin is preferably stabilized and stored prior to use.  The resin can be stabilized by the addition of an acid in a manner as discussed above.  Thus, the product can be stabilized to permit storage by improving the gelation stability by adding sufficient acid to reduce the pH to less than about 6, preferably less than about 5, and most preferably less than about 4, with a preferred range being a pH from about 2.5 to 4.  As noted above, any suitable inorganic or organic acid such as hydrochloric acid, sulfuric acid, methanesulfonic acid, nitric acid, formic acid, phosphoric and acetic acid may be used to stabilize the product.  Non-halogen containing acids, such as sulfuric acid, are preferred. 
According to the present invention, the resin is storage stable with respect to CPD and gelation.  With respect to gelation, the resin is storage stable, when stored at 25xc2x0 C., for at least two days, and is more preferably stable for at least one week, more preferably for at least one month, more preferably for at least three months, and is even more preferably for at least six months. 
The acid stabilization is preferably performed about 1 minute to about 24 hours after base treatment, preferably about 1 minute to about 6 hours, most preferably about 1 minute to about 1 hour after the base treatment.  The acid stabilized resin can be stored for extended period of time, such as greater than about 6 months.  Of course, the stabilized resin can be used at any time after stabilization including about 1 minute to 24 weeks after acidification, about 1 minute to 2 weeks after acidification, and about 1 minute to 24 hours after acidification. 
As with the acid treatment to remove, inhibit and/or reduce CPD-forming species, for the base treatment, the resin having at least reduced levels of formation of CPD can be a resin as produced in a resin synthesis process without further treatment.  Moreover, the resin can be treated by various processes prior to reduction and/or removal of the CPD-forming species.  Still further, after treatment to reduce and/or remove CPD-forming species, the resin can be treated by various processes.  Yet still further, the resin can be treated by various processes prior to reduction and/or removal of the CPD-forming species, and the resin can also be treated by various processes after treatment to reduce and/or remove CPD-forming species.  For example, the resin can be treated by various processes, such as processes to remove low molecular weight epihalohydrin and epihalohydrin by-products, e.g., epichlorohydrin and epichlorohydrin by-products, for example, CPD in the resin solution.  Without limiting the treatments or resins that can be utilized, it is noted that resins, such as Kymene(copyright)SLX2, Kymene(copyright)617 and Kymene(copyright)557LX (available from Hercules Incorporated, Wilmington, Del.), could be treated prior to and/or subsequent to reduction or removal of CPD-forming species with a base ion exchange column, such as disclosed in U.S. Pat.  No.  5,516,885 and WO 92/22601; with carbon adsorption, such as disclosed in WO 93/21384; membrane separation, e.g., ultrafiltration; extraction, e.g, ethyl acetate, such as disclosed in U.S. Statutory Invention Registration H1613; or biodehalogenation, such as disclosed in U.S. application Ser.  No.  08/482,398, now U.S. Pat.  No.  5,972,691, WO 96/40967 and U.S. Pat.  Nos.  5,470,742, 5,843,763 and 5,871,616.  The disclosures of each of these documents is incorporated by reference in their entireties.  In particular, one preferred manner of applying base treatment to remove or reduce CPD-forming species includes base treatment after biodehalogenation of the resin. 
As another method to produce polyamine-epihalohydrin resin products which have reduced levels of formation of CPD upon storage and minimized levels of CPD in paper products is by treating the resin utilizing either organic or inorganic bases, or organic or inorganic acids, such as described above, to raise or lower the pH of the resin solution to a pH less than 7, with a preferred pH range being about 5.5 to less than 7, with one preferred pH being about 6 for a sufficient period of time and at a sufficient temperature to remove and/or inhibit CPD-forming species in the resin.  This method of treatment is referred to herein as the pH modified treatment.  Preferably, the base comprises alkali metal hydroxides (lithium hydroxide, sodium hydroxide and potassium hydroxide) or alkali metal carbonates (sodium carbonate and potassium carbonate) or alkali metal bicarbonates (sodium bicarbonate and potassium bicarbonate).  Preferred acids include hydrochloric acid, sulfuric acid, methanesulfonic acid, nitric acid, formic acid, phosphoric acid and acetic acid.  Non-halogen containing acids, such as sulfuric acid, are preferred. 
The time of treatment can be made shorter with increasing temperature and increasing pH. The temperature is preferably within the range of about 30xc2x0 C. to 80xc2x0 C., the pH is preferably about 6, and the time of treatment is preferably about 3 hours to 14 days, with a preferred time of treatment being up to about 24 hours, more preferably up to about 6 hours.  Preferred combinations of temperature, time and pH, include at 30xc2x0 C., a pH of about 6 and a treatment time of about 6 days; and at 50xc2x0 C., a pH of about 6, and a treatment time of about 6 hours. 
As with the other treatments to remove, inhibit and/or reduce CPD-forming species, for the pH modified treatment, the resin having at least reduced levels of formation of CPD can be a resin as produced in a resin synthesis process without further treatment.  Moreover, the resin can be treated by various processes prior to reduction and/or removal of the CPD-forming species.  Still further, after treatment to reduce and/or remove CPD-forming species, the resin can be treated by various processes.  Yet still further, the resin can be treated by various processes prior to reduction and/or removal of the CPD-forming species, and the resin can also be treated by various processes after treatment to reduce and/or remove CPD-forming species.  For the sake of brevity, a complete description of these processes is not being repeated. 
As still another method of producing polyamine-epihalohydrin resin products which have reduced levels of formation of CPD upon storage and minimized levels of CPD in paper products, the resin can be treated with other catalysts that will remove and/or reduce the CPD forming species.  For example, the resin can be treated with enzymes.  Thus, for example, the CPD-forming species in the resin can also be reduced and/or removed by treating the resin with an enzymatic agent that is capable of releasing CPD-forming species from the resin.  The enzymatic agent can comprise one or more enzymes that are capable of releasing the CPD-forming species from the resin, such as at least one of esterases, lipases and proteases.  A particularly preferred enzymatic agent according to the present invention is ALCALASE, which is obtainable from Novo Nordisk Biochem, North America, Inc. Franklinton, N.C.
It is noted that following the guidelines set forth in the instant application one having ordinary skill in the art would be capable of determining enzymatic agents that are useful to remove CPD-forming species. 
The use of enzymatic agents to release the CPD-forming species is beneficial in that base treatment to rebuild molecular weight, such as that which is utilized with an acid treatment is utilized to remove CPD-forming species, as described in the acid treatment embodiment of the present invention, is not needed.  However, base treatment to ensure a desired molecular weight can be utilized with the enzymatic aspect of the present invention in a similar manner to the base treatment utilized with the acid treatment.  Also, enzyme-treated resin provides greater wet strength effectiveness relative to the acid treatment that utilizes base treatment to rebuild molecular weight. 
The at least one enzymatic agent is preferably added to the resin under conditions to provide a concentration of enzyme and suitable conditions to achieve sufficient hydrolysis of CPD forming species in the resin.  For example, depending upon the enzymatic agent, the temperature can be at least about 0xc2x0 C., preferably about 10xc2x0 C. to 80xc2x0 C., and more preferably about 20xc2x0 C. to 60xc2x0 C. The reaction time can be about 3 minutes to 350 hours, more preferably about 30 minutes to 48 hours, more preferably about 1 hour to 24 hours, and even more preferably about 2 hours to 6 hours.  The pH of the enzymatic reactions will depend on the pH dependence of the specific enzyme.  The pH can be from about 1 to 11, more preferably about 2 to 10, even more preferably about 2.5 to 9, and even more preferably about 7 to 9.  The concentration of the enzyme will depend upon its activity.  For example, in the case of ALCALASE, the enzyme can be present in an amount of about 0.0025 g of ALCALASE (as received) to 30 g (as received) polyaminopolyamide-epichlorohydrin resin to 2.5 g of ALCALASE (as received) to 30 g (as received) polyaminopolyamide-epichlorohydrin resin, also the enzyme can be present in an amount of about 0.025 g of ALCALASE (as received) to 30 g (as received) polyaminopolyamide-epichlorohydrin resin to 0.25 g of ALCALASE (as received) to 30 g (as received) polyaminopolyamide-epichlorohydrin resin. 
The preferred reaction conditions can be varied by using the appropriate types and amounts of enzymes.  For example, if the enzymatic agent has protease activity with a polyaminopolyamide-epichlorohydrin resin, reaction conditions above about pH 8 and 40xc2x0 C. are practical.  Practical being defined as obtaining a reduced CPD-forming resin while having a resin with the desired viscosity. 
As with the above-discussed procedures for removing and/or reducing CPD-forming species, the enzyme treatment can be applied on resins as produced in a resin synthesis process without further treatment.  Moreover, the resins can be treated by various processes prior to reduction and/or removal of the CPD-forming species.  Still further, after treatment to reduce and/or remove CPD-forming species, the resin can be treated by various processes.  Yet still further, the resin can be treated by various processes prior to reduction and/or removal of the CPD-forming species, and the resin can also be treated by various processes after treatment to reduce and/or remove CPD-forming species.  For the sake of brevity, a complete description of these processes is not being repeated. 
While the above-mentioned processes are directed to removal of the CPD forming species from the polymer backbone in a late stage of the resin synthesis, as noted above, there are other approaches directed to the inhibition, reduction and/or elimination of the amount of CPD forming species, such as CPD ester, that can be formed during the epichlorohydrin reaction.  Without wishing to be bound by theory, CPD ester is formed by the reaction of epichlorohydrin with residual carboxylic acid groups present in the prepolymer, such as polyaminoamide prepolymer.  Usually the carboxylic acid groups are end groups.  Reducing the amount of residual carboxylic acid groups present in the prepolymer will result in a reduction of the amount of CPD ester formed in the resin.  This may be achieved by reducing, minimizing or completely eliminating carboxylic acid groups (also referred to as acid groups or carboxylic acids) or residual carboxylic acid functionality (also referred to as acid functionality and carboxylic functionality) in the polyaminoamide prepolymer, to thereby obtain, as discussed below, a low acid number prepolymer. 
Preferably, the polyaminopolyamide-epihalohydrin resin is produced from a polyaminoamide prepolymer having an acid functionality less than about 0.5 milliequivalents/dry gram of prepolymer, more preferably less than about 0.25 milliequivalents/dry gram of prepolymer, even more preferably less than about 0.1 milliequivalents/dry gram of prepolymer, even more preferably less than about 0.07 milliequivalents/dry gram of prepolymer, and even more preferably less than about 0.05 milliequivalents/dry gram of prepolymer, and most preferably would be undetectable, i.e., it is preferred that the acid functionality be zero or as close to zero as possible. 
Expressed in another manner, the polyaminopolyamide-epihalohydrin resin is produced from a polyaminoamide prepolymer having an acid end group concentration of less than about 5% as measured by 13C NMR analysis, more preferably less than about 2.5%, even more preferably less than about 1%, even more preferably less than about 0.7%, and even more preferably less than about 0.5%, and most preferably would be undetectable, i.e., it is preferred that the acid end group concentration be zero or as close to zero as possible. 
The amount of carboxylic acid groups present in a polyaminoamide prepolymer can be determined by spectroscopy (NMR, IR) or by titration.  Preferably, the carboxylic acid groups are determined utilizing NMR, because this technique is more sensitive, especially when measuring low amounts of acid groups in the resin, such as when the acid groups are equal to 0.25 milliequivalents/dry gram of prepolymer or less.  A typical procedure for determining the acid number of prepolymer by 13C NMR analysis is described in Example 60 with respect to adipic acid and diethylenetriamine (DETA). 
Moreover, as noted above, titration can be utilized to determine the number of acid groups, especially when the number of acid groups is greater than 0.25 milliequivalents/per dry gram.  The procedure for determining the amount of acid groups utilizing titration is set forth in Example 12. 
The procedure for determining RSV is also set forth in Example 12. 
Preferably, the prepolymer has an RSV of at least about 0.05 dL/g (deciliter per gram), more preferably at least about 0.075 dL/g, even more preferably at least about 0.1 dL/g. The RSV is preferably less than about 0.5 dL/g, more preferably less than about 0.25 dL/g, even more preferably less than about 0.2 dL/g, and even more preferably less than about 0.15 dL/g. The RSV is preferably about 0.075 to 0.2 dL/g, more preferably about 0.1 to 0.15 dL/g.
Preferred combinations of acid functionality of the prepolymer from which the polyamidopolyamine resin is produced and the RSV of the prepolymer are wherein the prepolymer has an acid functionality less than about 0.5 milliequivalents/dry gram of prepolymer and a RSV of about 0.05 to about 0.25 dL/g, more preferably about 0.075 to 0.2 dL/g, and even more preferably about 0.1 to 0.15 dL/g; the prepolymer has an acid functionality of less than about 0.25 milliequivalents/dry gram of prepolymer and a RSV of about 0.05 to about 0.25 dL/g, more preferably about 0.075 to 0.2 dL/g, and even more preferably 0.1 to 0.15 dL/g; the prepolymer has an acid functionality of less than about 0.1 milliequivalents/dry gram of prepolymer and a RSV of about 0.05 to about 0.25 dL/g, more preferably about 0.075 to 0.2 dL/g, and even more preferably about 0.1 to 0.15 dL/g; the prepolymer has an acid functionality of less than about 0.07 milliequivalents/dry gram of prepolymer and a RSV of about 0.05 to about 0.25 dL/g, more preferably about 0.075 to 0.2 dL/g, and even more preferably about 0.1 to 0.15 dL/g; and the prepolymer has an acid functionality of less than about 0.05 milliequivalents/dry gram of prepolymer and a RSV of about 0.05 to about 0.25 dL/g, more preferably about 0.075 to 0.2 dL/g, and even more preferably about 0.1 to 0.15 dL/g.
Preferred combinations of acid end group concentration, as measured by 13C NMR analysis, of the prepolymer from which the polyamidopolyamine resin is produced and the RSV of the prepolymer are wherein the prepolymer has an acid end group concentration of less than about 5% and a RSV of about 0.05 to about 0.25 dL/g, more preferably about 0.075 to 0.2 dL/g, and even more preferably about 0.1 to 0.15 dL/g; the prepolymer has an acid end group concentration of less than about 2.5% and a RSV of about 0.05 to about 0.25 dL/g, more preferably about 0.075 to 0.2 dL/g, and even more preferably 0.1 to 0.15 dL/g; the prepolymer has an acid end group concentration of less than about 1% and a RSV of about 0.05 to about 0.25 dL/g, more preferably about 0.075 to 0.2 dL/g, and even more preferably about 0.1 to 0.15 dL/g; the prepolymer has an acid end group concentration of less than about 0.7% and a RSV of about 0.05 to about 0.25 dL/g, more preferably about 0.075 to 0.2 dL/g, and even more preferably about 0.1 to 0.15 dL/g; and the prepolymer has an acid end group concentration of less than about 0.5% and a RSV of about 0.05 to about 0.25 dL/g, more preferably about 0.075 to 0.2 dL/g, and even more preferably about 0.1 to 0.15 dL/g.
The choice of dicarboxylic acid or dicarboxylic acid derivative used in the synthesis of the polyaminoamide can have a significant effect on the acid end group concentration of the polyaminoamide and the polyamidopolyamine resin prepared from it.  In particular and without wishing to be bound by theory, it is hypothesized that 6 and 7 carbon aliphatic dicarboxylic acids and their derivatives, such as adipic and pimelic acids, and to a lesser extent an 8 carbon aliphatic dicarboxylic acid and its derivatives, such as suberic acid, can undergo side reactions during the course of the polyaminoamide synthesis that result in increased levels of acid end groups.  These side reactions are believed to originate with a deprotonation of the carbon alpha to the carbonyl group in the dicarboxylic acid, its derivatives or in the polyaminoamide backbone.  The conditions of the polyaminoamide synthesis are conducive to such a deprotonation reaction because of the basic conditions under which the reaction is carried out.  The deprotonation reaction is then followed by an intramolecular reaction of the resulting anion with the other carbonyl group of the diacid moiety to form a 5-membered ring in the case of adipic acid, a 6-membered ring in the case of pimelic acid and a 7-membered ring in the case of suberic acid.  These cyclic byproducts can generate carboxylic acid end groups either under the conditions of the polyaminoamide synthesis or when the polyaminoamide is dissolved in water.  Dicarboxylic acids that have the potential to form 5, 6, and 7-membered rings as a result of this type of intramolecular reaction are less favored than dicarboxylic acids that will not form these structures.  The use of glutaric acid or its derivatives significantly reduce the formation of such a cyclic byproduct since the intramolecular reaction would result in the formation of a 4-membered ring which is much less favored energetically than formation of 5, 6, and 7-membered rings.  Similarly, succinic acid, malonic acid, oxalic acid, azeleic acids and their derivatives would be expected to have a much lower tendency to undergo this type of side reaction.  Moreover, esters are preferred over acids.  For example, with respect to the above, it is noted that glutaric acid provides a lower concentration of acid end groups than adipic acid, dimethyl glutarate provides a lower concentration of acid end groups than glutaric acid, dimethyl adipate is preferred over adipic acid, and preferred esters include dimethyl glutarate and dimethyl succinate.  Exemplary preferred materials include DBE 4, DBE 5 and DBE 9, which are respectively, dimethyl succinate, dimethyl glutarate, and a 2/1 mixture of dimethyl glutarate/dimethyl succinate, obtainable from Dupont. 
One method to minimize carboxylic acid groups is to use endcapping agents in the preparation of the prepolymer (generally referred to herein as xe2x80x9cendcappingxe2x80x9d or xe2x80x9cendcapped prepolymerxe2x80x9d).  For example, when preparing an endcapped polyaminoamide prepolymer one may replace a portion of the diacid with a monofunctional acid and/or may replace a portion of the polyalkylenepolyamine with a monofunctional amine.  Various procedures, conditions and materials can be utilized when preparing the prepolymer, including conventional procedures, conditions and materials, and include those described herein.  Starting with an equimolar mixture of diacid and polyalkylenepolyamine, for every 1 mole of diacid or polyalkylenepolyamine removed a quantity of preferably about 2 moles of monofunctional acid or monofunctional amine endcapper is used.  In this regard, as the replaced moles of monofunctional acid is lowered below 2, the prepolymer ends up with increased amine end groups, whereas the molecular weight of the prepolymer is lowered as the replaced moles of monofunctional acid is raised above 2 moles.  In contrast, as the replaced moles of monofunctional amine is lowered below 2, the prepolymer ends up with acid groups, whereas the molecular weight of the prepolymer is lowered as the replaced moles of monofunctional amine is raised above 2 moles. 
One can control the molecular weight of a condensation polymer by adjusting the relative amounts of bifunctional and monofunctional reactants (endcappers) in the system.  The theory of molecular weight control and the effect of monofunctional additives for condensation polymer is well known, as see, for example, P. J. Flory, xe2x80x9cPrinciples of Polymer Chemistryxe2x80x9d, pp.  91-95, Cornell University Press, Ithaca N.Y. (1953), which is incorporated by reference in its entirety.  DPn is defined as the number-average degree of polymerization or the average number of monomer units in a polymer chain.  Equation 1 defines the DPn in terms of the molar ratios of the components, assuming complete reaction of all functional groups. 
DPn=(1+r)/(1xe2x88x92r)xe2x80x83xe2x80x83[1.] 
where r is defined as the ratio of the monomer units and is calculated as follows:
r=A/(B+2C)xe2x80x83xe2x80x83[2.] 
A and B are the difunctional monomer components and C is the monofunctional component (end-capper).  The quantity r will always be less than 1. 
In this invention, a controlled molecular weight prepolymer is prepared by using specific amounts of a monofunctional reactant.  The prepolymer composition may be defined in terms of a polyaminoamide prepared from A parts dicarboxylic acid, B parts polyalkylenepolyamine and C parts monofunctional endcapping moiety, all parts given as molar quantities. 
When A greater than B the endcapping moiety will be a monofunctional amine and C will equal about 2(Axe2x88x92B).  When B greater than A the endcapper will be a monofunctional acid and C will be equal to about 2(Bxe2x88x92A).  For this case Equation [2.]  is rewritten as:
r=B/(A+2C)xe2x80x83xe2x80x83[3.] 
The prepolymers should have a molecular weight that is sufficiently high so that the resulting resin is capable of providing sufficient wet strength to a substrate, such as paper.  Moreover, the molecular weight of the prepolymers should not be so high so that the resulting resin gels.  Preferably, the prepolymers have a range of DPn of from about 5 to 50, more preferably a range of from about 10 to 50, and most preferably a range of DPn is from about 15 to 50. 
Various temperatures and reaction times can be utilized in the reaction, including conventional temperatures and time forming prepolymers.  Temperatures of between about 125xc2x0 C. and 260xc2x0 C. are preferred, more preferably between about 165xc2x0 C. and 200xc2x0 C., and the reaction mixtures are maintained at these temperatures for preferably between about 3 to 12 hours, more preferably between about 6 to 10 hours. 
Suitable monofunctional amines include, but are not limited to, monofunctional primary amines, including monoalkyl amines and monoalkanol amines, and monofunctional secondary amines, including dialkyl amines and dialkanol amines. 
Monofunctional primary amines include, but are not limited to butylamine, ethanolamine (i.e., monoethanolamine, or MEA), cyclohexylamine, 2-methylcyclohexylamine,3-methylcyclohexylamine,4-methylcyclohexylamine, benzylamine, isopropanolamine (i.e., monoisopropanolamine), mono-sec-butanolamine, 2-amino-2-methyl-1-propanol, tris(hydroxymethyl)aminomethane, tetrahydrofurfurylamine, furfurylamine, 3-amino-1,2-propanediol, 1-amino-1-deoxy-D-sorbitol, and 2-amino-2-ethyl-1,3-propanediol.  Monofunctional secondary amines include, but are not limited to, diethylamine, dibutylamine, diethanolamine (i.e., DEA), di-n-propylamine, diisopropanolamine, di-sec-butanolamine, and N-methylbenzylamine. 
Monofunctional carboxylic acids suitable for the endcapped polyaminoamide prepolymer include, but are not limited to, benzoic acid, 2-hydroxybenzoic acid (i.e., salicylic acid), 3-hydroxybenzoic acid, acetic acid, phenylacetic acid, propionic acid, butyric acid, valeric acid, caproic acid, caprylic acid, 2-ethylhexanoic acid, oleic acid, ortho-toluic acid, meta-toluic acid, and para-toluic acid, ortho-methoxybenzoic acid, meta-methoxybenzoic acid, and para-methoxybenzoic acid. 
Monofunctional carboxylic acid esters suitable for the endcapped polyaminoamide prepolymer include, but are not limited to, methyl acetate, ethyl acetate, methyl benzoate, ethyl benzoate, methyl propionate, ethyl propionate, methyl butyrate, ethyl butyrate, methyl phenyl acetate, and ethyl phenyl acetate. 
The volatility of the endcapping agent should be low enough so that this agent remains in the prepolymerization reaction at the temperature at which the reaction is being conducted.  Particularly, when the prepolymer is prepared by thermally driven polycondensation, volatility is a significant feature of the endcapping agent; in this instance, an endcapping agent of lesser volatility is preferred.  The boiling point of the endcapping agent should be high enough so that, at the temperature being employed to drive off the condensation productxe2x80x94i.e.,  water where a diacid reactant is used, and alcohol in the case of diesterxe2x80x94the agent is not also removed. 
These endcapped polyaminoamide prepolymers can then be converted to polyaminoamide-epihalohydrin resins, preferably polyaminoamide-epichlorohydrin resins, according to the practices and procedures described earlier.  The resins produced from these polyaminoamide prepolymers can also be subjected to biodehalogenation to remove epihalohydrin, e.g., epichlorohydrin, based residual by-products, and these resins form CPD in the wet strength resin solution or in the paper product at a much reduced rate.  In addition to biodehalogenation, the polyaminoamide-epichlorohydrin resins may be treated to reduce or remove CPD forming species by any desired treatment, such as by utilizing the above-described acid treatment, and/or treated with any procedure for removing halogen-containing residuals. 
Expanding upon the above, it is once again noted that any combination of treatments may be employed in order to bring about desired low levels of CPD forming species and/or low levels of halogen-containing residuals in the resin.  Thus, the reduced acid group resin can be treated to reduce or remove CPD forming species and/or halogen-containing residuals, and therefore obtain even lower levels of formation of CPD upon storage or reduce the level of halogen-containing residuals therein.  For example, the resin can be treated by various processes, such as processes to remove low molecular weight epihalohydrin and epihalohydrin by-products, e.g., epichlorohydrin and epichlorohydrin by-products, for example, CPD in the resin solution, and/or to remove CPD forming species that may still be present in the resin.  Without limiting the treatments that can be utilized, it is noted that produced low acid resins could by various techniques, such as the acid treatments disclosed herein, and as in U.S. application Ser.  No.  09/330,200 to obtain an even further reduction of CPD-forming species.  Still further, the resins could with treated with a base ion exchange column, such as disclosed in U.S. Pat.  No.  5,516,885 and WO 92/22601; with carbon adsorption, such as disclosed in WO 93/21384; membrane separation, e.g., ultrafiltration; extraction, e.g, ethyl acetate, such as disclosed in U.S. Statutory Invention Registration H1613; or biodehalogenation, such as disclosed in U.S. application Ser.  No.  08/482,398, now U.S. Pat.  No.  5,972,691, WO 96/40967 and U.S. Pat.  Nos.  5,470,742, 5,843,763 and 5,871,616.  The disclosures of each of these documents is incorporated by reference in their entireties. 
Moreover, the acid groups can be reduced by variation of the dicarboxylic acid/polyalkylenepolyamine molar ratio and the cook profile in the prepolymer synthesis.  This route to obtaining polyaminoamide prepolymers with low levels of acid groups employs an excess of polyalkylenepolyamine in the synthesis.  This variation is generally referred to herein as xe2x80x9camine excess reactionxe2x80x9d or xe2x80x9camine excess prepolymerxe2x80x9d.  This involves using a polyalkylenepolyamine to diacid molar ratio of greater than 1 which results in a polyaminoamide with a preponderance of amine endgroups.  Moreover, various procedures, conditions and materials can be utilized when preparing the prepolymer, including conventional procedures, conditions and materials, and include those described herein. 
Expanding upon the above, it is noted that altering the stoichiometry of polyalkylenepolyamine to dibasic acid, i.e., diethylenetriamine to adipic acid, to have an excess of the polyalkylene polyamine results in more carboxyl groups being converted to amide groups, thereby reducing the acid group concentration in the prepolymer.  The stoichiometry of polyalkylenepolyamine to dibasic acid, e.g., diethylenetriamine to adipic acid, can range from greater than about 1.0:1.0, on a molar basis, to 1.7:1.0, more preferably, greater than about 1.01:1.0 to 1.4:1.0. 
While changing of the stoichiometry of the reagents in favor of excess polyalkylenepolyamine results in polyaminoamides with lower acid group concentrations for a given time/temperature cook profile, it also results in lower molecular weights for the polymer.  This lower molecular weight has a detrimental effect on the ability of the resulting resin to impart significant wet strength properties into paper.  In order to maintain the desired molecular weight characteristics of the polymer, extended cook times and/or higher temperatures are employed to build prepolymers with low acid group concentrations.  Therefore, temperatures between about 125xc2x0 C. and 260xc2x0 C. are used to cook the prepolymer reaction mixture, preferably between about 165xc2x0 C. and 200xc2x0 C., and the reaction mixtures are maintained at these temperatures for between about 3 to 12 hours, preferably between about 6 to 10 hours.  These conditions result in polyaminoamides with reduced acid groups.  As with the above-discussed end-capping, the prepolymers should have a molecular weight that is sufficiently high so that the resulting resin is capable of providing sufficient wet strength to a substrate, such as paper.  Moreover, the molecular weight of the prepolymers should not be so high so that the resulting resin gels.  Thus, as discussed above with respect to end-capping, the prepolymers preferably have a range of DPn of from about 5 to 50, more preferably a range of from about 10 to 50, and most preferably a range of DPn is from about 15 to 50. 
Preferably, the temperature of the reaction for forming the prepolymer is varied from one or more lower temperatures during one or more initial stages of the reaction and raised to one or more higher temperatures during one or more later stages of the reaction.  In this manner, the molecular weight of the prepolymer can be built up during the lower temperature stage, while avoiding volatization of low molecular species, e.g., monomers.  The temperature can then be raised to reduce or remove the acid groups while raising the molecular weight.  For example, the prepolymer reaction can be initially performed at temperatures of about 125 to 200xc2x0 C., preferably about 140 to 180xc2x0 C., for about 0.5 to 5 hours, more preferably about 1 to 4 hours.  The reaction temperature can then be raised to about 150 to 260xc2x0 C., more preferably about 180 to 225xc2x0 C., in one or more stages, and maintained at this one or more higher temperatures for about 0.25 to 10 hours, more preferably about 0.5 to 5 hours. 
Alternatively, instead of raising the temperature, longer cooking times can be utilized to build-up the molecular weight of the prepolymer.  Additionally, the temperature can be raised to a lower extent, with an increase in cook time. 
The amine excess prepolymer can then be converted to polyaminoamide-epihalohydrin resins, such as polyaminoamide-epichlorohydrin resins, according to the practices and procedures described earlier.  These resins can also be subjected to any treatment and/or any combination of treatments, such as discussed herein with respect to end capping.  For example, the resin can be subjected to any treatment and/or any combination of treatments to reduce or remove CPD forming species and/or reduce and/or remove halogen-containing residuals. 
Another method of making polyaminoamide prepolymers with low levels of residual acid functionality is to add a reactive amine at later stages of the polycondensation reaction in forming the prepolymer with continued heating in order to amidate any residual acid groups.  This method is referred to herein as xe2x80x9cpost-added amine reactionxe2x80x9d or xe2x80x9cpost-added amine prepolymerxe2x80x9d.  Preferably, the polycondensation reaction is at least about 70% complete, more preferably at least about 80% complete, and even more preferably at least about 90% complete when the reactive amine is added.  The degree of conversion, and hence the degree of completion of the polycondensation reaction can be determined by monitoring the amount of distillate, i.e., the amount of water or alcohol, that is formed during the reaction and comparing this to the theoretical value. 
In order to facilitate the reaction with the reactive amine, a vacuum, e.g., a slight vacuum to a high vacuum, may be applied to the reactor to aid in removal of the byproducts formed in the condensation reaction of the reactive amine with carboxylic acid groups.  Also, a gas sparge, e.g., an inert gas sparge, such as with nitrogen, argon and/or helium, may be introduced to the reactor to aid in the removal of condensation byproducts.  This procedure can be performed while applying vacuum or under conditions of normal atmospheric pressure. 
While not wishing to be bound by any particular theory, monofunctional amines can be utilized in which instance it would appear that amide, alkyl and/or hydrocarbon end groups would form.  Moreover, polyfunctional amines can be utilized in which instance it would appear that amides would be formed. 
The initial stages of the prepolymer reaction can be initially performed at temperatures of about 125xc2x0 C. to 200xc2x0 C., preferably about 140xc2x0 C. to 180xc2x0 C., for about 0.5 to 5 hours, more preferably about 1 to 4 hours.  After the post-addition of the amine compound, the reaction temperature can then be maintained or can be raised to about 150xc2x0 C. to 225xc2x0 C., more preferably about 170xc2x0 C. to 225xc2x0 C., in one or more stages, and maintained at this one or more higher temperatures for about 0.25 to 10 hours, more preferably about 0.5 to 5 hours. 
The post-added amine should be added in an amount such that the total molar quantity of polyalkylenepolyamine plus post-added amine is greater than the total molar amount of dicarboxylic acid.  The initial molar ratio of polyalkylenepolyamine to dicarboxylic acid can range from about 0.6:1.0 to 1.4:1.0, preferably about 0.8:1.0 to 1.2:1.0, more preferably about 0.9:1.0 to 1.1:1.0 and most preferably about 0.95:1.0 to 1.05:1.0.  The post-added amine is preferably added in a quantity such that the ratio of polyalkylenepolyamine to dicarboxylic acid to post-added amine is in the range of about 0.6:1.0:0.7 to 1.4:1.0:0.3, preferably about 0.8:1.0:0.4 to 1.2:1.0:0.2, more preferably about 0.9:1.0:0.2 to 1.1:1.0:0.1 and most preferably about 0.95:1.0:0.1 to 1.05:1.0:0.05. 
A polyaminoamide prepared from an equimolar mixture of polyalkylenepolyamine and diacid will theoretically have an equal number of amine and carboxylic acid groups.  By adding a reactive amine in the later stages of the reaction, the acid groups present in the polyaminoamide can be amidated.  The reactive amine may be any substance that contains at least one primary or secondary amine functionality and may contain a mixture of primary and secondary amine functionalities.  This may be a monofunctional amine, difunctional amine or polyfunctional amine.  This reactive amine is referred to as a xe2x80x9cpost-added aminexe2x80x9d.  Preferred post-added amines are aliphatic amines. 
Suitable monofunctional primary amines include, but are not limited to, butylamine, amylamine, hexylamine, heptylamine octylamine, nonylamine, decylamine, ethanolamine (i.e., monoethanolamine, or MEA), cyclohexylamine, allylamine, 2-methylcyclohexylamine, 3-methylcyclohexylamine, 4-methylcyclohexylamine, benzylamine, isopropanolamine (i.e., monoisopropanolamine), mono-sec-butanolamine, 2-(2-aminoethoxy)ethanol, 2-amino-2-methyl-1-propanol, tris(hydroxymethyl)aminomethane, tetrahydrofurfurylamine, furfurylamine, 3-amino-1,2-propanediol, 1-amino-1-deoxy-D-sorbitol, morpholine aminoethylmorpholine and 2-amino-2-ethyl-1,3-propanediol.  Among the monofunctional secondary amines which are suitable are diethylamine, dibutylamine, diethanolamine (i.e., DEA), di-n-propylamine, diisopropanolamine, di-sec-butanolamine, pyrrolidine, piperidine, diallylamine, and N-methylbenzylamine. 
Examples of appropriate diamines include, but are not limited to, ethylenediamine, propylenediamine, hexamethylenediamine, 1,10-diaminodecane, 1,3-diamino-3-hydroxypropane, 2-(2-aminoethylamino)ethanol, 1,2-diaminocyclohexane, 1,10-diaminodecane, and piperazine. 
Polyfunctional amines that may be used as the post-added amine include, but are not limited to, aminoethyl piperazine, the polyalkylene polyamines, including polyethylene polyamines, polypropylene polyamines, polybutylene polyamines, polypentylene polyamines, polyhexylene polyamines and so on and their mixtures may be employed of which the polyethylene polyamines represent an economically preferred class.  More specifically, the polyalkylene polyamines contemplated for use may be represented as polyamines in which the nitrogen atoms are linked together by groups of the formula xe2x80x94CnH2nxe2x80x94 where n is a small integer greater than unity and the number of such groups in the molecule ranges from two up to about eight.  The nitrogen atoms may be attached to adjacent carbon atoms in the group xe2x80x94CnH2nxe2x80x94 or to carbon atoms further apart, but not to the same carbon atom.  This invention contemplates not only the use of such polyamines as diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and dipropylenetriamine, which can be obtained in reasonably pure form, but also mixtures and various crude polyamine materials.  For example, the mixture of polyethylene polyamines obtained by the reaction of ammonia and ethylene dichloride, refined only to the extent of removal of chlorides, water, excess ammonia, and ethylenediamine, is a satisfactory starting material.  The term xe2x80x9cpolyalkylene polyaminexe2x80x9d employed in the claims, therefore, refers to and includes any of the polyalkylene polyamines referred to above or to a mixture of such polyalkylene polyamines and derivatives thereof. 
The post-capped polyaminoamides can then be converted to polyaminoamide-epihalohydrin resins, such as polyaminoamide-epichlorohydrin resins, according to the practices and procedures described earlier.  These resins can also be subjected to any treatment and/or any combination of treatments, such as discussed herein with respect to end capping and amine excess treatment during the reaction.  For example, the resin can be subjected to any treatment and/or any combination of treatments to reduce or remove CPD forming species and/or reduce and/or remove halogen-containing residuals. 
Moreover, any manner of providing polyamine-epihalohydrin resin having reduced and/or removed CPD-forming species can be utilized alone or in combination according to the instant invention.  When utilized in combination, the techniques can be utilized simultaneously, sequentially or in an overlapping manner.  For example, and without limiting the combinations according to the present invention, treatment with an enzymatic agent can be followed by acid or base treatment. 
Moreover, it noted that a mixture of wet strength agents can be utilized according to the present invention.  For example, it is noted that cationic water-soluble resins, derived from the reaction of epihalohydrins, such as epichlorohydrin, and polyalkylene polyamines, such as ethylenediamine (EDA), bis-hexamethylenetriamine (BHMT) and hexamethylenediamine (HMDA) have long been known.  These polyalkylene polyamine-epihalohydrin resins are described in patents such as U.S. Pat.  No.  3,655,506 to J. M. Baggett, et al.  and others such as U.S. Pat.  Nos.  3,248,353 and 2,595,935 to Daniel et al.  from which their generic description as xe2x80x9cDaniel""s Resinsxe2x80x9d arises.  The disclosures of these patents are incorporated by reference herein in their entireties. 
While not wishing to be bound by theory, these polyamine-epihalohydrin resins do not have acid end groups, and therefore appear to not include CPD-forming species, e.g., CPD esters.  Thus, while their wet strength abilities are less than those of polyaminopolyamide-epihalohydrin resins, it is beneficial to include the polyalkylene amine-epihalohydrin resins in admixture with the polyaminopolyamide-epihalohydrin resins in view of their lower cost and their lack of formation of CPD upon storage. 
The polyalkylene polyamine employed in the present invention can preferably be selected from the group consisting of polyalkylene polyamines of the formula:
H2Nxe2x80x94[CHZxe2x80x94(CH2)nxe2x80x94NRxe2x80x94]xxe2x80x94H
where:
n=1-7,
x=1-6
R=H or CH2Y,
Z=H or CH3, and
Y=CH2Z, H, NH2, or CH3,
polyalkylene polyamines of the formula:
H2Nxe2x80x94[CH2xe2x80x94(CHZ)mxe2x80x94(CH2)nxe2x80x94NRxe2x80x94]xxe2x80x94H
where:
m=1-6, n=1-6, and m+n=2-7,
R=H or CH2Y,
Z=H or CH3, and
Y=CH2Z, H, NH2, or CH3,
and mixtures thereof. 
Polyalkylene polyamine-epihalohydrin resins comprise the water-soluble polymeric reaction product of epihalohydrin and polyalkylene polyamine.  In making Daniel""s Resins the polyalkylene polyamine is added to an aqueous mixture of the epihalohydrin so that during the addition the temperature of the mixture does not exceed 60xc2x0 C. Lower temperatures lead to further improvements, though too low a temperature may build dangerously latent reactivity into the system.  The preferred temperatures fall within the range of about 25xc2x0 C. to about 60xc2x0 C. More preferred is a range of from about 30xc2x0 C. to about 45xc2x0 C.
Alkylation of the polyamine occurs rapidly proceeding to form secondary and tertiary amines depending on the relative amounts of epihalohydrin and polyamine.  The levels of epihalohydrin and polyamine are such that between about 50% and 100% of the available amine nitrogen sites are alkylated to tertiary amines.  Preferred levels are between about 50% and about 80% alkylation of the amine nitrogen sites.  Excess epihalohydrin beyond that required to fully alkylate all the amine sites to the tertiary amine is less preferred because this results in increased production of epihalohydrin byproducts. 
Following complete addition of the polyamine, the temperature of the mixture is allowed to rise and /or the mixture is heated to effect crosslinking and azetidinium formation.  The crosslinking rate is a function of concentration, temperature, agitation, and the addition conditions of the polyamine, all of which can be readily determined by those skilled in the art.  The crosslinking rate can be accelerated by the addition of small shots of the polyamine or other polyamines of the present invention or addition of various alkalies at or near the crosslinking temperature. 
The resin can be stabilized against further crosslinking to gelation by addition of acid, dilution by water, or a combination of both.  Acidification to pH 5.0 or less is generally adequate. 
The preferred polyamines are bishexamethylenetriamine, hexamethylenediamine, and their mixtures. 
In order to more clearly describe the present invention, the following non-limiting examples are provides for the purpose of representation, and are not to be construed as limiting the scope of the invention.  All parts and percentages in the examples are by weight unless indicated otherwise.  Moreover, ND in the Examples indicates xe2x80x9cNot Detectedxe2x80x9d.