Abstract:
Cross-linked cationic polymers are prepared by alkylating a polyaminoalide with 5 to 30 mole percent, relative to the amount of nitrogen available for alkylation, of a quaternary ammonium alkylating agent and cross-linking the resulting alkylated polyaninoamide with a molar amount of epichlorohydrin corresponding to the non-quaternized nitrogen content. The cross-linked cationic polymers are suitable for use as wet-strength resins in the treatment of paper.

Description:
FIELD OF THE INVENTION 
     This invention relates to a new process for the production of cross-linked cationic polymers for the treatment of paper and to the use of the crosslinked cationic polymers as wet strength resins. 
     DISCUSSION OF THE RELATED ART 
     The generic term &#34;paper&#34; encompasses about 3,000 different types and articles which can differ, in some cases considerably, in their fields of application and their properties. For their production, aqueous 1 to 5% by weight suspensions of cellulose fibers are applied in thin layers to screens on which they are dewatered at high speeds to a solids content of around 40% by weight by application of heat and mechanical pressure. While the still moist raw paper is dried in further steps, the water squeezed off returns to the circuit. 
     In modern paper mills, it is regarded as important for economic and ecological reasons to keep the water circuit substantially closed. To this end, the process water must of course be free from waste or should only contain waste which is easy to reuse. Particular importance is attributed to the so-called AOX content of the process water, i.e. the content of unwanted halogen compounds. 
     Unfortunately, the papermaking process unavoidably involves the addition of various additives, including for example fillers, defoamers, dyes, retention agents and auxiliaries which improve the mechanical strength of the paper and which are known as dry or wet strength resins, depending on the field of application. Since the additives accumulate in the process water together with cellulose fibers over a period of time, there is a considerable need for ecologically safe products. 
     The additives which are regarded as critical in this respect include, in particular, wet strength resins which are crosslinked polyaminoamides. These substances, which are also known commercially as Fibrabones®, are produced from polyaminoamides of which the chains are crosslinked via nitrogen atoms using epichlorohydrin. U.S. Pat. No. 5,116,887, U.S. Pat. No. 5,120,773, U.S. Pat. No. 5,239,047, U.S. Pat. No. 5,350,796 (Henkel Corp.) and European patent application EP-A1 0 553 575 (Hercules) are cited as representative of the extensive prior art available on this subject. 
     It has been found that there is a direct connection between the degree of crosslinking of the wet strength resins and the improvement in the strength of the paper treated with them. From this point of view, it is advisable to crosslink all the nitrogen groups in the polyaminoamide available for crosslinking through epichlorohydrin units. On the other hand, the reaction of each nitrogen group with epichlorohydrin leads to the elimination of hydrogen chloride (which remains bound as adduct) and, hence, to the presence of AOX in the product which is extremely undesirable for the reasons explained above. Accordingly, attempts have hitherto been made to maintain a balance between the two conflicting parameters (degree of crosslinking and AOX content). In actual fact, however, the results are unsatisfactory because, in many cases, the performance properties are not entirely acceptable, nor is the ecological compatibility of the products by any means adequate. 
     Accordingly, the problem addressed by the present invention was to provide a process for the production of wet strength resins which would be free from the disadvantages mentioned. 
     DESCRIPTION OF THE INVENTION 
     The present invention relates to a process for the production of crosslinked cationic polymers, in which polyaminoamides 
     (a) are first reacted with a quantity of 5 to 30 mole-%, based on the nitrogen available for quaternization, of a quaternizing agent and 
     (b) the resulting quaternized polyaminoamides are then crosslinked with a molar quantity of epichlorohydrin corresponding to the content of non-quaternized nitrogen. 
     It has surprisingly been found that the performance properties of the crosslinked polyamides can be significantly improved by quaternizing a particular quantity of nitrogen groups in the polymer before crosslinking without the wastewater being additionally polluted with AOX. 
     Dicarboxylic acids 
     The polyaminoamides suitable as starting materials for the purposes of the invention are commercial polycondensation products derived from dicarboxylic acids corresponding to formula (I): 
     
         HOOC--(A)--COOH                                            (I) 
    
     in which A is a linear or branched, aliphatic, cycloaliphatic and/or aromatic, optionally hydroxysubstituted alkylene group containing 1 to 52 carbon atoms and preferably 1 to 10 carbon atoms. 
     Typical exarmples of dicarboxylic acids from which the polyaminoamides may be derived are, for example, succinic acid, azelaic acid, dodecanedioic acid, phthalic acid and technical dimer or trimer fatty acids. The polyaminoamides are preferably based on adipic acid. 
     Polyamines 
     The polyaminoamides suitable as starting materials for the purposes of the invention are commercial polycondensation products additionally derived from polyamines corresponding to formula (II): 
     
         H.sub.2 N--(B)--NH--((C)NH).sub.z H                        (II) 
    
     in which B and D independently of one another represent linear or branched, optionally hydroxysubstituted alkylene groups containing 1 to 4 carbon atoms and z=0 or a number of 1 to 10. 
     Typical examples are polyamines corresponding to formula (II) in which B and C represent ethylene groups and z=2 or 3. Accordingly, the preferred polyamines are diethylenetriamine and triethylenetetramine. Overall, polyaminoamides based on adipic acid, and diethylenetriamine are preferred. 
     Quaternization 
     Quaternization may be carried out by the method known from the production of conventional cationic surfactants. Suitable quaternizing agents are, for example, alkyl halides corresponding to formula (III): 
     
         R.sup.1 --Hal                                              (III) 
    
     in which R 1  is an alkyl group containing 1 to 4 carbon atoms and Hal represents chlorine or bromine. Typical examples are methyl bromide, butyl chloride and, in particular, methyl chloride. 
     Other particularly preferred quaternizing agents are dialkyl compounds corresponding to formula (IV): 
     
         R.sup.2 O--XO.sub.2 --OR.sup.2                             (IV) 
    
     in which R 2  is an alkyl group containing 1 to 4 carbon atoms and X represents sulfur or phosphorus. Typical examples are diethyl sulfate, dibutyl sulfate, dimethyl phosphate, diethyl phosphate, dibutyl phosphate and, in particular, dimethyl sulfate. 
     Finally, 2,3-epoxypropyl trimethyl ammonium chloride, 3-chloro or 3-bromo-2-hydroxypropyl trimethyl ammonium chlorides marketed by Degussa AG of Hanau, FRG under the name of QUAB® may also be used as quaternizing agents. 
     To carry out the quaternization reaction, the polyamidoamines are dissolved in water or alcohol and the quaternizing agent is added in portions to the resulting solution at temperatures of 40 to 70° C. The reaction time is normally 0.5 to 3 h. It has been found to be of advantage to base the quantity of quatemizing agent used (5 to 30 mole-% and preferably 15 to 25 mole-%, based on the nitrogen available for quaternization) on the amine value. The quatemization is preferably carried out with dimethyl sulfate or with a QUAB reagent. In addition, it has proved to be of advantage to adjust the pH value of the intermediate product to 9-11 by addition of an aqueous alkali base after the quatemization reaction. 
     Crosslinking 
     The crosslinking of the quaternized polyaminoamides may also be carried out in aqueous or alcoholic solution. It is advisable to add the epichlorohydrin in portions and to carry out the reaction at low temperatures of 10 to 40° C. on account of the considerable heat effect. The quantity of epichlorohydrin to be used normally corresponds to the molar quantity of nitrogen groups available for crosslinking. Accordingly, it is advisable for practical reasons to determine the amine value of the quatemized product once more before the crosslinking reaction. After the epichlorohydrin has been added, the product is stirred for 12 to 48 h and preferably from 24 to 36 h. It is advisable to increase the temperature to around 40° C. towards the end of the reaction and to terminate crosslinking by addition of mineral acid and adjustment of a pH value of 2 to 4 when the viscosity reaches a critical value of around 500 mPa-s (as measured by the Brookfield method, 40° C., spindle 1). 
     Reaction scheme 
     The chemical reactions involved are illustrated by the following scheme which relates by way of example to quaternization/crosslinking with dimethyl sulfate (DMS) and epichlorohydrin (EPI): ##STR1## 
     Quaternization may be similarly carried out using alkyl halides, 2,3-epoxypropyl trimethyl ammonium chloride, 3-chloro- and 3-bromo-2-hydroxypropyl trimethyl ammonium chlorides. 
     Commercial Applications 
     The crosslinked cationic polymers obtainable by the process according to the invention provide paper with improved strength for low AOX levels. By contrast, known products either show a comparable improvement in strength for higher AOX levels or, conversely, poorer strength for comparable AOX levels. 
     Accordingly, the present invention also relates to the use of the crosslinked cationic polymers obtainable by the process according to the invention as wet strength resins in papermaking. 
     The following Examples are intended to illustrate the invention without limiting it in any way. 
    
    
     EXAMPLES 
     I. Production Examples 
     Example H1: 
     Quatemization. In a 1 liter stirred reactor, 450 g of a polyaminoamide based on adipic acid and diethylenetriamine (amine value 101 mg KOH/g corresponding to 0.81 equivalent of nitrogen, a product of Henkel Corp., Gulf Mills/USA) are suspended in 100 ml of water and 44 g (0.2 mole) of 2,3-epoxypropyl trimethyl ammonium chloride (70% by weight in water, QUAB® 151, Degussa AG, Hanau, FRG) were added in portions to the resulting suspension. After the addition, the reaction was heated for 2 h to 60° C. The mixture was then cooled to 17° C. and the pH value was adjusted to 9.5-10 by addition of 50% by weight sodium hydroxide solution. The resulting intermediate had an amine value of 81 mg KOH/g. 
     Crosslinking. After the addition of another 115 ml of water, 80 g (0.96 mole) of epichlorohydrin were added in portions to the reaction mixture so that the temperature did not exceed 20° C. The reaction mixture was then stirred for 36 h at 30° C. After heating to 40° C., the mixture was stirred until an adequate viscosity of about 500 mPa-s was reached. The reaction was then terminated by addition of aqueous hydrochloric acid and adjustment to pH 2-3. The resulting crosslinked cationic polymer had a dry residue of 13% by weight. 
     Example H2 
     Quatemization. As in Example 1, 450 g of the polymer of Example 1 were suspended in 150 ml of water at 40° C. Instead of the QUAB, however, 25.6 g (0.2 mole) of dimethyl sulfate were used for quaternization. After the addition, the reaction mixture was again heated for 2 h to 60° C., cooled to 17° C. and adjusted to a pH of about 9.5 with sodium hydroxide. The amine value was 66.2 mg KOH/g. 
     Crosslinking. The reaction mixture was diluted with 42 ml of water and 70.7 g (0.76 mole) of epichlorohydrin were added at a temperature of 20 to 35° C. The reaction time was 25 h. The reaction mixture was then heated as in Example 1 up to a viscosity of about 500 mPa-s and adjusted to a pH value of about 2.5 by addition of aqueous hydrochloric acid. The dry residue was 13% by weight. 
     II. Application Examples 
     The products according to the invention of Production Examples H1 and H2 and a commercial non-quatemized market product from Hercules Inc. were used in the following in the form of 13% by weight aqueous solutions. 
     For performance evaluation, an aqueous suspension of 1200 g of cellulose fibers (ratio by weight of long to short fibers =70:30) in 6 liters of water was first prepared. Quantities of 5 and 10% by weight of the test solutions (based on dry matter) were added to quantities of 250 ml of these pastes which were then stirred for 2 minutes and coated onto a substrate. Finally, the still moist raw paper was dried for 30 minutes at 105° C. 15 mm wide strips of paper (weight of the samples about 2 g, paper weight about 55 g/m 2 ) were tested for tensile strength. The results are expressed in Table 1 below as the force required to tear the paper (average of five measurements). 
     
                       TABLE 1______________________________________Determination of Tensile Strength              Quantity  Tensile strengthEx.     WSR        % by weight                        N______________________________________1       H1         5         5.582       H1         10        8.713       H2         5         5.224       H2         10        7.52C1      None       --        0.21C2      Standard   5         4.78C3      Standard   10        6.36______________________________________ Legend: WSR = Wet strength resin