Semi-crystalline ethylene vinyl acetate emulsion polymers for heat seal applications
This invention is directed to aqueous-based ethylene-vinyl acetate (EVA) polymer emulsions suited for use in heat seal applications. The EVA polymers of this invention are produced by a relatively low-pressure process, i.e., less than 2000 psig, preferably from about 1000 to about 2000 psig. The aqueous-based EVA based polymer emulsions contain crystalline segments resulting from ethylene linkages and are prepared by emulsion polymerizing ethylene and vinyl acetate, preferably with a carboxylic monomer, in the presence of a stabilizing system containing surfactant or a cellulosic protective colloid in combination with surfactant.
1. An ethylene-vinyl acetate polymer emulsion suited for use in heat seal applications said ethylene-vinyl acetate polymer comprised of crystalline ethylene segments prepared by emulsion polymerizing ethylene and vinyl acetate in the presence of a stabilizing system consisting essentially of a surfactant or a cellulosic protective colloid in combination with a surfactant, said ethylene-vinyl acetate polymer having:
(a) a crystalline melting point ranging from 35 to 110° C. as measured at a heat rate of 20° per minute; and,
(b) a tensile storage modulus of at least 1×10 5 dynes/cm 2 at a temperature of 115° and measured at 6.28 rad/sec.
2. The polymer emulsion of claim 1 wherein the polymer is comprised of from 15 to 90% by weight of polymerized units of vinyl acetate and from about 10 to 85% by weight of polymerized units of ethylene based upon the total weight of the polymer.
3. The polymer emulsion of claim 1 wherein the polymer is comprised of from 25 to 80% by weight of polymerized units of vinyl acetate and from about 20 to 75% by weight of polymerized units of ethylene based upon the total weight of the polymer.
4. The polymer emulsion of claim 1 wherein the polymer is comprised of from 35 to 75% by weight of polymerized units of vinyl acetate and from about 25 to 65% by weight of polymerized units of ethylene based upon the total weight of the polymer.
5. The polymer emulsion of claim 1 wherein the polymer is comprised of from 30 to 50% by weight of polymerized units of vinyl acetate and from about 50 to 70% by weight of polymerized units of ethylene based upon the total weight of the polymer.
6. The polymer emulsion of claim 2 wherein polymerized carboxylic acid units are present in said polymer in an amount from about 0.2 to about 10% by weight of said polymer.
7. The polymer emulsion of claim 6 wherein said polymer has a tensile storage modulus of at least 2×10 5 dynes/cm 2 at 115° and measured at 6.28 rad/sec.
8. The polymer emulsion of claim 7 wherein the polymer is comprised of polymerized units of ethylene, vinyl acetate, and acrylic acid.
9. The polymer emulsion of claim 7 wherein the crystalline heat of fusion of said polymer is from about 5 to 100 joules per gram as measured at a heat rate of 20° C. per minute.
10. The polymer emulsion of claim 7 wherein the glass transition temperature is from +25° C. to about −35° C. as measured at a heat rate of 20° C. per minute.
11. The polymer emulsion of claim 8 wherein crystalline thermal melting point ranges from 50 to 90° C. as measured at a heat rate of 20° C. per minute.
12. The polymer emulsion of claim 8 wherein a portion of the emulsion polymerization is carried out at a pressure of from 1000 to 2000 psig (6,996 to 13,891 kPa).
13. The polymer emulsion of claim 12 wherein the stabilizing system consists essentially of hydroxyethyl cellulose in combination with a surfactant.
14. The polymer emulsion of claim 13 wherein the vinyl acetate is present in an amount from 15 to 90% by weight, the ethylene is present in an amount from 10 to 85% by weight, and the acrylic acid is present in an amount from 0.5 to 5% by weight of the polymer.
15. The polymer emulsion of claim 14 wherein the crystalline heat of fusion ranges from preferably 15 to 70 joules per gram as measured at a heat rate of 20° C. per minute.
16. The polymer emulsion of claim 1 , wherein polymerized units of acrylamide or Veova-10 ester are present in said ethylene-vinyl acetate polymer.
17. The polymer emulsion of claim 1 , wherein a chain transfer agent is used during emulsion polymerization.
18. A process for making an aqueous semi-crystalline ethylene vinyl acetate polymer emulsion which comprises reacting vinyl acetate and ethylene with optionally one or more other ethylenically unsaturated monomer, under emulsion polymerization conditions in the presence of a stabilizing system consisting essentially of a surfactant or a cellulosic protective colloid in combination with a surfactant, said ethylene-vinyl acetate polymer having:
(a) a crystalline melting point ranging from 35 to 110° C. as measured at a heat rate of 20° C. per minute; and,
(b) a tensile storage modulus of at least 1×10 5 dynes/cm 2 at a temperature of 115° C. and measured at 6.28 rad/sec.
19. The process of claim 18 wherein the addition of monomers, except ethylene, to the emulsion polymerization are completed within the first 75% of the total reaction time.
20. The process of claim 18 wherein a portion of the process is carried out at pressures of from 1000 to 2000 psig (6,996 to 13,891 kPa).
21. The process of claim 18 , wherein the one or more other ethylenically unsaturated monomer is selected from the group comprising acrylamide and Veova-10 ester.
22. The process of claim 18 , wherein a chain transfer agent is used during emulsion polymerization.
23. A process for forming a paper stock for heat seal application which comprises:
coating a paper substrate with a polymer emulsion comprising emulsion polymerized units of vinyl acetate and ethylene with one or more other ethylenically unsaturated monomer, wherein said polymer emulsion is stabilized with a stabilizing system consisting essentially of a surfactant or a cellulosic protective colloid in combination with a surfactant, said polymer containing crystalline ethylene segments and having (a) a crystalline melting point ranging from 35 to 110° C. as measured at a heat rate of 20° C. per minute; and, (b) a tensile storage modulus of at least 1×10 5 dynes/cm 2 at a temperature of 115° C. and measured at 6.28 rad/sec; and then,
drying the coating, said dried coating being non-blocking at ambient temperature.
24. A multi-layer heat sealable material comprising
(a) at least one substrate; and
(b) at least one coating of a polymer emulsion comprising emulsion polymerized units of vinyl acetate and ethylene with one or more other ethylenically unsaturated monomer, wherein said polymer emulsion is stabilized with a stabilizing system consisting essentially of surfactant or a cellulosic protective colloid in combination with surfactant, said polymer containing crystalline ethylene segments and having (a) a crystalline melting point ranging from 35 to 110° C. measured at a heat rate of 20° C. per minute; and, (b) a tensile storage modulus of at least 1×10 5 dynes/cm 2 at a temperature of 115° C. and measured at 6.28 rad/sec.