Purification of wet strength resins

Certain wet strength resins based on the epichlorohydrin/polyamine reaction which are characterized by extremely long gel times can be purified from epichlorohydrin residuals by azeotropic distillation.

BACKGROUND OF THE INVENTION 
This invention relates to a process for the purification of certain wet 
strength resins and specifically the resins produced by the process 
described in U.S. application Ser. No. 21,414. 
The resins involved are obtained by reacting, in an aqueous medium 
maintained at a temperature of 20.degree. C. or less, an epihalohydrin and 
a polymer comprising a backbone formed of repeating segments at least 10% 
of which comprise an amine group and/or partial salts thereof, 
substantially all of the amine groups being tertiary amine groups pendant 
from the polymer backbone and having a structure selected from the group 
consisting of 
##STR1## 
wherein R' is either methyl or, where the nitrogen atom bears two R' 
groups, these together with the nitrogen can form a heterocyclic ring; R" 
is a divalent radical and R"' is a trivalent radical; with the unattached 
bond in each amine structure being attached directly to a carbon atom of 
the polymer backbone. The ratio of epihalohydrin molecules to tertiary 
amine groups in the reaction mixture is such that the transition ratio is 
equalled or exceeded. 
The transition ratio represents a dividing line between the resins of the 
prior art and those obtained by the above process. The latter are 
characterized by their exceedingly long gel times which may be one or more 
orders of magnitude longer than those characterizing the resins of the 
prior art. The gel time is the time taken for a given resin solution or 
emulsion to set up, (that is to cross-link three dimensionally), once the 
pH is raised to a level at which the resin is activated, generally above 
about 8. 
The resins obtained by the above process are styled "perepiquat" resins and 
that term shall be used in this specification. Perepiquat resins are 
characteristically reaction products of a poly(tertiary amine) with an 
epihalohydrin which have a gel time in excess of 100 minutes for a 10% 
solids solution at room temperature and a pH of from 10 to 13. 
DISCUSSION OF THE PRIOR ART 
Polymers produced by the prior art processes, as indicated above, are 
characterized by very short gel times such that, in the activated form, it 
is not possible to employ conventional purification techniques. In the 
stabilized, non-activated form, i.e. at a pH below about 6, the major 
impurity is usually a dihalopropanol (from the reaction of uncoverted 
epihalohydrin with acid) and this seems to have a strong affinity for the 
resin and water such that it cannot be successfully removed by 
distillation. Above a pH of about 8, raising the temperature greatly 
accelerates gelation such that purification becomes impracticable. Thus in 
the prior art, attempts are not usually made to separate unreacted 
epihalohydrin or dihalopropanol from the resin and the existence of 
otherwise undesirable efficiency-sapping side reactions during use as wet 
strength resins is tolerated. 
Not only does the presence of unreacted monomers reduce efficiency but it 
is wasteful in that the epichlorohydrin is a relatively expensive 
component. 
It is therefore advantageous, from several points of view, to remove any 
unreacted dichloropropanol or epichlorohydrin from the resin before it is 
used and the present invention provides a technique by which this can be 
done for perepiquat resins. 
DESCRIPTION OF THE INVENTION 
The present invention provides a process for removing epihalohydrin and 
dihalopropanol impurities from an aqueous solution of a perepiquat resin 
which comprises adjusting the pH of the solution to a value in the range 8 
to 13 and volatilizing from the solution, at a temperature below 
40.degree. C., an azeotrope of water and epihalohydrin. 
The adjusting of the pH to the above range has the effect of converting the 
dihalopropanol to the corresponding epihalohydrin which readily forms an 
azeotrope with water and is removed as such. The preferred pH range is 10 
to 13 and most conveniently from 10.5 to 11.5 because at such pH levels 
the above conversion is largely complete. At pH levels in this range the 
perepiquat resin is converted to its epoxy or activated form. 
The volatilization is preferably achieved by sparging an inert gas through 
the solution such that the azeotrope is entrained with the gas stream and 
removed. The inert gas can be nitrogen, air, argon or the like. The 
preferred gas is nitrogen. 
The process can be assisted by use of sub-atmospheric pressures if desired 
and in some circumstances the use of such low pressures will make sparging 
unnecessary. The preferred process uses both sparging and subatmospheric 
pressures. 
The temperature at which the purification proceeds has a very significant 
effect on both the gel time of the resin and on the rate at which the 
azeotrope is removed. In practice the purification temperature should be 
between 0.degree. and 40.degree. C. and preferably 15.degree. to 
25.degree. C. 
The process of the invention can be applied to a solution with any desired 
solids content but in practical terms the solids content is most 
conveniently from 5 to 50% by weight and preferably from 10 to 30% by 
weight. 
A preferred process utilizes an aqueous resin solution at a temperature of 
from 15.degree. to 25.degree. C. and a pH of from 10 to 13 through which 
air is bubbled and from which the sparged air along with an entrained 
water/epihalohydrin azeotrope is removed at pressures below 0.5 kg./sq.cm 
and most preferably at laboratory vacuum pressures. Any other process that 
increases the surface/volume ratio of the solution, thus making 
volatilization of the azeotrope easier, can also be used. One such 
approach would be the use of an extraction column in which the solution 
passes down the column against a counter current of gas which entrains the 
azeotrope. Another might be an adaptation of a wiped film devolatilizer. 
All such expedients and others achieving the same affect are considered 
within the purview of this invention. 
As the removal of the epihalohydrin proceeds the pH drops and more alkali 
may need to be added to maintain the pH at the desired level. This 
variation is also within the scope of this invention. 
The preferred perepiquat resin is one having a gel time as a 10% solids 
aqueous solution of at least 100 and preferably from 1000 to 8000 minutes 
or even longer at room temperature and a pH of 10 to 13. Particularly 
preferred resins are based on the reaction of epichlorohydrin with a 
polymer (including a copolymer) of N-methyl diallylamine, vinyl benzyl 
dimethylamine, dimethylaminoethyl methacrylate and the like, which 
polymers are characterized by tertiary amine groups that are pendant from 
a polymer chain and are readily quaternized by reaction with the 
epichlorohydrin. Other forms of perepiquat resins area described in U.S. 
application Ser. No. 21,414 which is incorporated herein by reference. 
The most preferred epihalohydrin is epichlorohydrin (EPI) and the 
effectiveness of the process of the invention using EPI can be gauged from 
a consideration of the EPI/water azeotropic boiling point and the 
composition of the EPI/water azeotrope by weight. 
TABLE 1 
______________________________________ 
Binary Azeotropes Containing Water 
B.P. in .degree.C. at 760 mm 
% by wt. in Azeotrope 
Compound Compound Azeotrope Water Compound 
______________________________________ 
Epichloro- 
117 88.5 26% 76% 
hydrin 
Dichloro- 
182 99.4 88.7% 11.3% 
propanol* 
______________________________________ 
*2,3 dichloropropanol. 
The 1,3-dichloropranol-2 isomer has a boiling point of 174.degree. C. 
From this it will be observed that a second azeotrope, 
dichloropranol/water, is potentially involved. However since, at the 
relevant pH range, the removal of epichlorohydrin will allow further 
dehydrohalogenation of the dichloropropanol to form epichlorohydrin to 
occur and since the epichlorohydrin azeotrope boils at a temperature 
11.degree. C. below that of the dichloropropanol azeotrope, it will be 
appreciated that the dominant mechanism for removal of dichloropropanol is 
via conversion to epichlorohydrin. The dehydrohalogenation reaction occurs 
readily during resin activation of pH 10-13 and though the reaction is 
reversible, at those pH levels the equilibrium is displaced well over 
towards formation of epichlorohydrin.

DESCRIPTION OF A PREFERRED EMBODIMENT 
The invention is further described by reference to the following Example 
which is for the purposes of illustration only and is intended to imply no 
limitations on the essential scope of the invention. 
EXAMPLE 
A 25 ml pyrex test-tube was charged with approximately 5 ml of a 20% 
solution of a perepiquat polymer obtained by reacting poly(N-methyl 
diallylamine) with epichlorohydrin in a EPI/amine group ratio of 1.75 at a 
temperature of 5.degree. C. rising to room temperature during a 24 hour 
reaction period. A pipette was introduced into the tube such that the end 
touched the bottom of the tube and a moderate flow of sparging nitrogen 
was initiated. Aqueous sodium hydroxide solution was added dropwise and 
the steadily rising pH was monitored. All operations were performed at 
ambient temperatures. 
When the pH reached 11.52 and the temperature was 24.degree. C., the sodium 
hydroxide addition was stopped. The nitrogen flow caused azeotropic 
removal/volatilization of epichlorohydrin for about 4 to 5 minutes, the 
odor being very readily detectable. Gradually thereafter the intensity of 
the epichlorohydrin odor in the sparging gas flow and in the immediate 
vicinity of the solution diminished and finally disappeared. 
The nitrogen sparging was continued for a total time of 10 minutes after 
which the solution temperature was 19.degree. C. and the pH was 11.48. 
Because water was lost along with the epichlorohydrin the solution was 
slightly more viscous. Addition of 1 ml of water gave a clear, colorless, 
odorless resin solution with a pH of about 11.3. 
Filter paper circles, when wetted with a 1.0% aqueous solution of the 
purified perepiquat resin solution and allowed to dry/air-heat cure in a 
120.degree. C. circulating air oven for 7 minutes exhibited excellent wet 
tensile strength showing that the perepiquat resin had lost none of its 
effectiveness as a result of the removal of the unreacted epichlorohydrin. 
A perepiquat resin, stripped of epichlorohydrin monomer by the above 
process and stabilized by a reduction of the pH to 4.5 maintained its 
original wet strength efficiency. This demonstrates the impressive 
stability to degradation possessed by the perepiquat resins even after the 
demonomerization process.