Water-Based Adhesives

Objects and advantages of this invention are illustrated in the following examples.

Polymers useful in the adhesives of this invention were prepared by the following procedure.

De-ionized water (D.I. water) was charged to a 1-liter resin kettle equipped with a mechanical stirrer, thermometer, an IR.sup.2 Therm-O-Watch, a nitrogen inlet, and a reflux condenser. Water, acidic monomers, and HCl if used, were added and the solution heated to 55.degree. C. At 55.degree. C. the basic monomer(s) was added. An acid/base neutralization reaction occurred and the reaction temperature rose to about 65.degree. C. Then the polymerization mixture was heated to the initiation temperature. About 20 minutes after the addition of the basic monomer(s) and at the initiation temperature, the catalyst, pre-dissolved in water, was added. A slow nitrogen purge was started at this time in preparing most of the polymers and started earlier in the preparation of some of the other polymers and to reduce coloring of the polymer solution during the ensuing exothermic polymerization. The reaction temperature peaked between 88.degree. and 98.degree. C. within 5-15 minutes and would slowly cool and be maintained at the initiation temperature for a total polymerization time of 2 hours. The polymer was removed from the reactor and the amount of non-volatile polymer solids and Brookfield viscosities determined. The polymers were colorless. Specific preparation for each polymer type is described below and Table I shows formulations and reaction parameters for other examples of three polymer types, where "Z" denotes a zwitterionic polymer (illustrated by formula II, supra), "A" denotes an amphoteric polymer (illustrated by formula III, supra), and "Z/A" denotes a zwitterionic-amphoteric polymer (illustrated by formula IV, supra).

Illustrating the preparation of a preferred zwitterionic polymer, Ex. 4/Z, a 53:47:4 mole ratio DMAEMA:AA:HCl zwitterionic polymer, D.I. water, 198.7 g, was charged to a 1 liter resin kettle equipped as above. A fast nitrogen purge was used to clear the reactor of air, then a slow purge was maintained throughout the polymerization. Acrylic acid, 25.7 g, and 2.9 g of conc. HCl were added next. The reaction mixture was heated to 55.degree. C. and then 63.2 g of DMAEMA added. The reaction mixture exothermed to 66.5.degree. C. and then it was heated to 75.degree. C. About 20 minutes after the addition of the DMAEMA, 0.5400 g of ammonium persulfate, pre-dissolved in 9.0 g of D.I. water, was added. The reaction exothermed rapidly to 89.degree. C. The polymerization mixture was allowed to cool back down to 75.degree. C. and maintained there for a total polymerization time of 2 hours. The polymer was removed from the reactor and was water-clear, colorless, and had a Brookfield viscosity of 54,100 cps.

The preparation of a preferred amphoteric polymer, a 51:49:2 mole ratio DMAEMA:MAA:HCl, Ex. 14/A, was as follows. D.I. water, 263.8 g, was charged to a 1-liter resin kettle equipped as above. Methacrylic acid, 41.1 g, and 1.9 g of conc. HCl were added next. A slow nitrogen purge was started to clear the reactor of air and then maintained throughout the polymerization. The reaction mixture was heated to 55.degree. C. and 78.2 g of DMAEMA was added. The reaction mixture exothermed to about 65.degree. C. and then was heated to 80.degree. C. Twenty minutes after the addition of the DMAEMA, 0.7200 g of potassium persulfate, pre-dissolved in 15.0 g. distilled water, was added. The reaction exothermed rapidly to 89.7.degree. C., cooled back down to 80.degree. C. and was maintained at 80.degree. C. for a total polymerization time of 2 hours. The polymer was removed from the reactor. It was water-clear and colorless and had a 32,000 cps Brookfield viscosity of 32,000 cps.

The preparation of a preferred zwitterionic/amphoteric polymer, Ex. 24Z/A, a 51:24.5:24.5:2 mole ratio DMAEMA:MAA:AA:HCl, was as follows. D.I. water, 228.6 g, was charged to a 1 liter resin kettle equipped as above. Methacrylic acid, 24.7 g, 20.7 g of acrylic acid and 2.3 g of conc. HCl were added next. The reaction mixture was heated to 55.degree. C. and 93.9 g of DMAEMA was added. The reaction mixture exothermed to 68.degree. C. and then was heated to 80.degree. C. About 20 minutes after the addition of the DMAEMA, 0.700 g of potassium persulfate, pre-dissolved in 30.0 g of water, was added and a slow nitrogen purge was started. The reaction exothermed rapidly to 91.degree. C., cooled back down to 80.degree. C. and was maintained at 80.degree. C. for a total polymerization time of 2 hours. The polymer was removed from the reactor. It was water-clear and colorless and had a Brookfield viscosity of 37,800 cps.

The preparation of another preferred zwitterionic/amphoteric polymer, Ex. 34Z/a, a 50.3:25.1:24.6:2.5 mole ratio DMAEMA:MAA:AA:HCl polymer, was as follows. D.I. water, 308.0 g, was charged to a 1 liter resin kettle equipped as above; 27.2 grams of methacrylic acid, 22.3 g of acrylic acid and 3.1 g of conc. HCl were added next. The reaction mixture was heated to 65.0.degree. C. and 99.4 g of DMAEMA was added. The reaction mixture exothermed to 76.5.degree. C. and was maintained at 77.0.degree. C. About 20 minutes after the addition of the DMAEMA, 0.3380 g of potassium persulfate, pre-dissolved in 40.0 g of D.I. water, was added and a slow nitrogen purge started. The reaction exothermed rapidly to 90.0.degree. C. and then cooled back down to 77.0.degree. C. The polymerization mixture was maintained at 77.0.degree. C. for a total time of 2 hours. The polymer was removed from the reactor. It was water-clear and colorless and had a 106,200 cps Brookfield viscosity at 30% solids.

The preparation of a representative zwitterionic polymer (a 53:47:4 mole ratio DMAPMAm:AA:HCl, Ex 35/Z) utilizing N,N-dimethylaminopropyl methacrylamide, DMAPMAm, is as follows. D.I. water, 159.6, was charged to a 1 liter resin kettle equipped as above. Acrylic acid, 34.5 g, and 3.9 g of conc. HCl were added next. The reaction mixture was heated to 55.0.degree. C. and then 92.0 g of DMAPMAm added. The reaction mixture exothermed to about 67.degree. C. and then heated to 75.0.degree. C. About 20 minutes after the addition of the DMAPMAm, 0.7680 g of potassium persulfate, pre-dissolved in 30.0 g of D.I. water, was added. A slow nitrogen purge was started. The reaction exothermed to 86.3.degree. C. in approximately 6 minutes and was allowed to cool back down and maintained at 75.0.degree. C. Two hours after the first initiator charge, 0.0768 g of potassium persulfate was added and the polymerization continued for another hour. The polymer was cooled and removed from the reactor. It was water-clear with a very slightly yellow color and had a 17,440 cps Brookfield viscosity.

The preparation of a representative amphoteric polymer (a 51:49:2 mole ratio DMAPMAm:MAA:HCl, Ex 36A) utilizing an acrylamide monomer, is as follows. D.I. water, 191.4 g, was charged to a 1 liter resin kettle equipped as above. Methacrylic acid, 39.0 g, and 1.8 g of conc. HCl were added next. The reaction mixture was heated to 55.0.degree. C. and 80.3 g of DMAPMAm was added. The reaction mixture exothermed to 67.5.degree. C. and then was heated to 80.0.degree. C. About 20 minutes after the addition of the DMAPMAm, 0.7200 g of potassium persulfate, pre-dissolved in 30.0 g of D.I. water, was added. A slow nitrogen purge was started at this time and maintained throughout the polymerization. The reaction exothermed to 84.8.degree. C. in 4 minutes, cooled back down and maintained at 80.0.degree. C. Two hours after the first initiator addition, 0.0720 g of ammonium persulfate was added and the reaction stirred for another hour. The polymer was removed from the reactor and labeled. It was water-clear with a slightly yellow color and had a 2970 cps Brookfield viscosity.

The preparation of a representative zwitterionic/amphoteric polymer (a 50.3:25.1:24.6:2.5 mole ratio DMAPMAm:MAA:AA:HCl, Ex 37Z/A) utilizing an acrylamide monomer was as follows: D.I. water, 191.4 g, was charged to a 1 liter resin kettle equipped as above. Methacrylic acid, 20.6 g, 16.9 g of acrylic acid and 2.3 g of conc. HCl were added next. The reaction mixture was heated to 55.0.degree. C. and 81.6 g of DMAPMAm added. The reaction mixture exothermed to 70.5.degree. C. and then was heated to 77.0.degree. C. About 20 minutes after the addition of the DMAEMA, 0.7200 g of potassium persulfate, pre-dissolved in 30.0 g of D.I. water, was added and exothermed to purge was started. The reaction exothermed to 82.6.degree. C., cooled back down and was maintained at 77.0.degree. C. for a total polymerization time of two hours. The polymer was removed from the reactor and labeled. It was water-clear with a slightly yellow color and had a 17,680 cps Brookfield viscosity.

The various aqueous polymer solutions were evaluated as water-based adhesives by the following test method to determine the "180.degree. peel adhesion" of the adhesive.

The 180.degree. peel adhesion test was run by coating a 0.63.times.5.08.times.17.78 cm (1/4.times.2.times.7 inch) piece of wood (pine) with 250 microns of the test adhesive. A 2.54.times.22.86 cm (1.times.9 inch) piece of No. 10 "cotton duck" cloth was centered over the adhesive-coated, wood substrate and a 1 Kg roller was pulled back and forth over the cotton duck for 5 cycles. The test panels were allowed to dry for 24 hours in a 22.2.degree. C. (72.degree. F.)/50% relative humidity room. The 180.degree. peel adhesions were determined on a Sintech 1/S using a 5.06 cm (2-inch)/minute jaw speed. All tests were run in triplicate. Scotch NetO adhesive was run each time as a control or reference and, on average, gave 10.0N/cm adhesion.

For the zwitterionic adhesives, Ex 1Z through 8Z and 35Z, the higher 180.degree. peel adhesions were obtained with those systems containing 50 to 55% DMAEMA at 35% solids, see Ex. 3/Z, 7/Z and 8/Z. As the basic monomers increased above 55%, the neutralizing agent (HCl), had to be increased to maintain adhesion, see Ex 1Z, 2Z and 3Z. Higher adhesion for these adhesives with higher levels of basic monomers can be obtained by increasing the percent solids of the adhesive as Ex 1Z shows.

Representative amphoteric adhesives are depicted by Ex 9A through 21A and 36A. These polymers provide adhesions similar to their zwitterionic analogs at similar percent solids, as Ex. 12/A and 3Z and 15A and 8Z demonstrate. When maleic acid and itaconic acid were used, high percent solids of the adhesives were required to obtain higher levels of adhesion, see Ex 18A and 19/A and 20A and 21/A.

The combination of the amphoteric functionality and the zwitterionic functionality into the same adhesive polymer to give the zwitterionic/amphoteric polymers (Ex. 22Z/A through 34Z/A and 37Z/A) was most surprising in that better adhesives than either the zwitterionic polymers or the amphoteric by themselves were obtained. This can be seen in Table I where the best amphoteric polymers Ex. 3Z or 8/Z, and the best zwitterionic polymer Ex. 15/A, appear to be inferior to these new zwitterionic/amphoteric polymers Ex. 32Z/A to 34Z/A. The significance of this is that the zwitterionic/amphoteric adhesives "at lower percent solids (polymer)" provide excellent adhesion which is better than the other two. In general, the preferred zwitterionic/amphoteric adhesives, at the same percent solids, are stronger than the best amphoteric polymer and the best zwitterionic polymer. The preferred zwitterionic/amphoteric adhesives have higher bond strengths at much lower viscosities than the zwitterionic polymers, compare Ex 23Z/A with 4Z, and at lower percent solids than the zwitterionic and amphoteric adhesives.

The effect of the percent solids of these adhesive polymers can be seen by comparing polymer Ex 6/Z with Ex 8/Z, Ex 11/A and 12/A with Ex 13/A, Ex 14/A with Ex 15/A, Ex 20/A with Ex 21/A and Ex 23Z/A with Ex 24Z/A and Ex 25Z/A. In general, higher adhesion values are obtained with the higher solids adhesives. The adhesive polymer viscosity also affects adhesion and in general, a higher viscosity yields higher adhesion values. For these zwitterionic, amphoteric and zwitterionic/amphoteric polymers, a preferred adhesive viscosity range would be from about 2000 to 300,000 cps. A more preferred range would be from 5000 to 150,000 cps and a most preferred range would be from 20,000 to 100,000 cps. Again, this would depend on the specific polymer composition and the percent solids of the adhesive.

Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention.