Polylactic acid shrink films and methods of manufacturing same
A heat-shrinkable polylactic acid (PLA) film and a method of its manufacture are provided. In an exemplary embodiment, the PLA films exhibit heat-induced growth in the cross direction with concomitant shrinkage in the machine direction. The films may comprise any grade of PLA polymer, optionally including additives, such as antiblock, slip, viscosity enhancers and combinations thereof. A method of manufacture is disclosed which includes a post-extrusion temperature conditioning step.
1. A method of making a blown PLA shrink film comprising 97.5 to 100 percent, by weight, polylactic acid polymer blend, which film, upon exposure to 95° C. heat for 10 seconds, exhibits shrinkage in the length direction and negative shrinkage (a negative value indicates growth) in the cross direction relative to the original dimensions of the shrink film, the method comprising (a) providing dry pellets of PLA, (b) melting the pellets to form a molten mass at a first desired viscosity value ranging from about 1,000 P to about 5,000 P at about 480° F. at an apparent shear rate of about 55 seconds −1 , (c) increasing the viscosity of the molten mass to a second desired viscosity value or range of values, (d) forming a bubble from the resulting molten mass at a blown up ratio ranging from 1:0.75 to 1.2.0, and (e) collapsing the bubble to form a film.
2. The method of claim 1 , in which the melting step is carried out at a temperature range of about 325° F. to about 485° F.
3. The method of claim 1 , in which the melting step is carried out at a temperature range of about 375° F. to about 425° F.
4. The method of claim 1 , in which the first viscosity value ranges from about 2,000 P to about 4,000 P at about 480° F. at an apparent shear rate of about 55 seconds −1 .
5. The method of claim 1 , wherein the bubble is formed at a draw down ratio ranging from 1:1 to 1:300.
6. A method of making a blown polylactic acid (PLA) shrink film comprising 97.5 to 100 percent, by weight, polylactic acid polymer blend, which film, upon exposure to 95° C. heat for 10 seconds, exhibits shrinkage in the length direction and negative shrinkage (a negative value indicates growth) in the cross direction relative to the original dimensions of the shrink film, the method comprising (a) obtaining dry pellets of PLA, (b) melting the pellets to form a molten mass at a first desired viscosity value or range of values, (c) increasing the viscosity of the molten mass to a second desired viscosity value ranging from about 14,000 P to about 16,000 P at about 375° F. at an apparent shear rate of about 55 seconds −1 , (d) forming a bubble from the resulting molten mass at a blown up ratio ranging from 1:0.75 to 1:2.0, and (e) collapsing the bubble to form a film.
7. The method of claim 6 , wherein the bubble is formed at a draw down ratio ranging from 1:1 to 1:300.
8. The method of claim 6 , in which the second viscosity value ranges from about 15,500 P to about 16,000 P at about 375° F. at an apparent shear rate of about 55 seconds −1 .
9. The method of claim 6 , in which the viscosity increasing step is carried out in a polymer cooling unit.
10. A method of making a blown polylactic acid (PLA) shrink film comprising 97.5 to 100 percent, by weight, polylactic acid polymer blend, which film, upon exposure to 95° C. heat for 10 seconds, exhibits shrinkage in the length direction and negative shrinkage (a negative value indicates growth) in the cross direction relative to the original dimensions of the shrink film, the method comprising (a) obtaining pellets of a PLA polymer blend, (b) melting the pellets to form a molten mass at a first desired viscosity value ranging from about 1,000 P to about 5,000 P at about 480° F. at an apparent shear rate of about 55 seconds −1 , (c) increasing the viscosity of the molten mass to a second desired viscosity value ranging from about 14,000 P to about 16,000 P at about 375° F. at an apparent shear rate of about 55 seconds −1 , (d) forming a bubble from the resulting molten mass at a blown up ratio ranging from 1:0.75 to 1:2.0, and (e) collapsing the bubble to form a film.
11. The method of claim 10 , wherein the bubble is formed at a draw down ratio ranging from 1:1 to 1:300.
12. The method of claim 10 , in which the step of forming a bubble includes a stretching step, which orients the film.
13. The method of claim 12 , in which the stretching step is carried out using a blown film orientation process.
14. The method of claim 10 , further comprising annealing the film.
15. The method claim 14 , in which the annealing step is carried out at a temperature ranging from about 120° F. to about 285° F.
16. The method of claim 15 , in which the annealing step is carried out at a temperature ranging from about 140° F. to about 250° F.
17. The method of claim 10 , in which the viscosity increasing step is carried out in a polymer cooling unit.