Coating process
A thermoplastic polymer advanceable by solid state polymerization is blended with at least one dissimilar thermoplastic polymer, or an organic or inorganic particulate filler. The blend is solid state polymerized to provide a modified polymer alloy or filled polymer blend having at least one physical or chemical property different from that of the blend before solid state polymerization. A substrate is coated with an adherent layer of the modified polymer alloy or filled polymer blend. The modified polymer alloy or filled polymer blend may also be coextruded with a layer of thermoplastic extrusion polymer having a melt viscosity similar to that of the modified polymer alloy or filled polymer blend and applied to a substrate to form an adherent coating.
1. A process for applying an adherent multilayer coating to a substrate comprising:
a) providing a first thermoplastic polymer whose melt viscosity has a given value at a chosen temperature and extrusion rate;
b) providing a second thermoplastic polymer advanceable by solid state polymerization and having a melt viscosity at the chosen temperature and extrusion rate sufficiently unlike the given value so that the first and second thermoplastic polymers can not be coextruded;
c) blending the second thermoplastic polymer with at least one dissimilar thermoplastic polymer;
d) solid state polymerizing the resulting blend to provide a modified polymer alloy whose melt viscosity at the chosen temperature and extrusion rate is sufficiently close to the given value so that the modified polymer alloy and the first thermoplastic polymer may be coextruded, wherein the modified polymer alloy has a melt viscosity within about ±15% of the first thermoplastic polymer at temperatures of about 200-300° C. and shear rates of about 50-500 l/s; and
e) coextruding a layer of the modified polymer alloy and a layer of the first thermoplastic polymer onto a metal substrate to form a coated substrate having an adherent multilayer coating on the substrate.
2. A process according to claim 1 wherein the second thermoplastic polymer comprises a condensation polymer.
3. A process according to claim 1 wherein the second thermoplastic polymer comprises an alkyd.
4. A process according to claim 1 wherein the dissimilar thermoplastic polymer is advanceable by solid state polymerization.
5. A process according to claim 1 wherein the dissimilar thermoplastic polymer is not advanceable by solid state polymerization.
6. A process according to claim 1 wherein the second thermoplastic polymer comprises a polyester and the dissimilar thermoplastic polymer comprises a copolyester.
7. A process according to claim 1 wherein the second thermoplastic polymer comprises polyethylene terephthalate, the dissimilar thermoplastic polymer comprises a copolyester and the modified polymer alloy has greater melt viscosity than that of the blend before solid state polymerization.
8. A process according to claim 1 wherein the modified polymer alloy has a melt viscosity within about ±10% of the first thermoplastic polymer at temperatures of about 200-300° C. and shear rates of about a 50-500 l/s.
9. A process according to claim 1 wherein the modified polymer alloy has a melt viscosity within about ±5% of the first thermoplastic polymer at temperatures of about 200-300° C. and shear rates of about 50-500 l/s.
10. A process according to claim 1 comprising coextruding onto the substrate a first layer containing polyethylene terephthalate and a copolyester, a second layer containing polyethylene terephthalate, and a third layer containing polyethylene terephthalate and a copolyester.
11. A process according to claim 1 further comprising fabricating the coated substrate into a container.
12. A process according to claim 11 wherein the adherent layer is an interior coating on the container.
13. A process according to claim 12 wherein the interior coating is suitable for storage in contact with food.
14. A process according to claim 1 wherein the substrate comprises steel.