Polylactide fibers

Abstract

Polylactide fibers are made from a blend of polylactides. One of the polylactides has a ratio of R-lactic and S-lactic units from 8:92 to 92:8. The second polylactide has a ratio of the R-lactic and S-lactic units of 97:3 or 3:97. The ratio of the R-lactic units to S-lactic units in the blend is from 7:93 to 25:75 or from 75:25 to 93:7. The polylactide fiber contains at least 5 Joules of polylactide crystallites per gram of polylactide resin in the fiber.

Claims

1. A polylactide fiber that contains at least 75% by weight polylactide resin, wherein (a) the polylactide resin is a blend formed by melt blending, solution blending or both melt blending and solution blending (1) 20 to 90% by weight of a first polylactide in which the ratio of the R-lactic and S-lactic units is from 50:50 to 10:90 and (2) from 80 to 10% by weight of a second polylactide in which the ratio of the R-lactic and S-lactic units is 3:97, and wherein the R-lactic units and S-lactic units combined constitute at least 90% of the weight of the second polylactide (b) the ratio of the R-lactic units to S-lactic units in the blend is from 8:92 to 20:80; and (c) the polylactide fiber contains at least 5 up to 30 Joules of polylactide crystallites having a melting temperature of 140 C. to 190 C. per gram of polylactide resin in the fiber and no more than 5 J/g of other crystallites that melt in the temperature range of 20 C. to 250 C.

2. The fiber of claim 1, wherein the blend of polylactide resins contains about 15 to 50 milliequivalents of carboxyl end groups per kilogram of polylactide resins.

3. The fiber of claim 2, wherein the first polylactide resin has a weight average molecular weight from 40,000 to 125,000.

4. The fiber of claim 3, which contains at least 10 Joules of polylactide crystallites per gram of polylactide resin in the fiber.

5. The fiber of claim 4, which contains 12 to 22 Joules of polylactide crystallites per gram of polylactide resin in the fiber.

6. The fiber of claim 1, which loses 7 to 20% of its mass upon immersing 0.48 g of the fiber in 100 mL of a 0.1 M phosphate buffer solution for 6 days at 65 C.

7. The fiber of claim 6, which exhibits 5 to 25% shrinkage when heated in air at 80 C. for 10 minutes.

8. The fiber of claim 1, which contains an agent that increases the hydrophilicity of the polylactide resin, or a catalyst for the hydrolysis of the polylactide resin, or both.

9. A plant covering comprising the fiber of claim 1.

10. A blend of polylactide resins formed by melt blending, solution blending or both melt blending and solution blending (1) 20 to 90% by weight of a first polylactide in which the ratio of the R-lactic and S-lactic units is from 50:50 to 10:90 and (2) from 80 to 10% by weight of a second polylactide in which the ratio of the R-lactic and S-lactic units is 3:97, and wherein the R-lactic units and S-lactic units combined constitute at least 90% of the weight of the second polylactide, wherein the ratio of the R-lactic units to S-lactic units in the blend is from 8:92 to 20:80.

Description

EXAMPLES 1-3

(1) A polylactide resin containing 88% of S-lactic units and 12% of R-lactic units and having a weight average molecular weight of about 170,000 is passed multiple times through an extruder. This reduces the molecular weight to about 130,000 g/mol. The so-treated polymer is formed into pellets. 70 parts by weight of these pellets are mixed with 30 parts by weight of pellets of a second polylactide resin that contains 98.6% of S-lactic units and 1.4% of R-lactic units and has an M.sub.w of about 160,000. The mixture of polylactide resins contains about 8.8% R-lactic units and 91.2% S-lactic units. This mixture is melt-spun, heat-set and drawn to produce multifilament staple fiber having a denier of 15-20/filament. The resulting fiber designated as Example 1. It contains 15 J/g of polylactide crystallites by DSC, which is very significantly in excess of the value expected to be obtained with a PLA resin that contains a ratio of 8.8:91.2 R- to S-lactic units. The resin processes easily at high spinning speeds.

(2) Fiber Example 2 is made in the same way, except the ratio of the first polylactide resin to the second polylactide resin is 65:35. The mixture of polylactide resins contains about 8.3% R-lactic units and 91.7% S-lactic units. This fiber contains 9 J/g of polylactide crystallites, which is very significantly in excess of that expected given the ratio of R- to S-lactic units in the resin blend.

(3) Fiber Example 3 is again made in the same way, except the ratio of the first polylactide resin to the second polylactide resin is 60:40. The mixture of polylactide resins contains about 7.8% R-lactic units and 92.2% S-lactic units. This fiber contains 22 J/g of polylactide crystallites, which again is unexpectedly high given the ratio of R- to S-lactic units in the resin blend.

(4) Comparative Fiber A is prepared in the same manner, using only the first polylactide resin. It contains no measurable crystallinity. These fibers block when baled and stored at ambient temperatures.

(5) The fiber samples are heated at 57 C. for to assess shrinkage. Shrinkage on this test is a good proxy for the tendency of the fibers to block (i.e., become stuck together) upon storage at slightly elevated temperatures as might be encountered during storage and/or transportation. Greater shrinkage indicates a greater tendency to block.

(6) Comparative Sample A exhibits 11.5% shrinkage on this test. Examples 1-3 exhibit only 8.4, 6.25 and 0% shrinkage, respectively, demonstrating that the blend of polylactides is much more resistant to blocking at moderately elevated temperatures than the single resin.

EXAMPLES 4-8 AND COMPARATIVE SAMPLES B-E

(7) The following PLA resins are used to make fiber Examples 4-8 and Comparative Samples B-E:

(8) TABLE-US-00001 Designation M.sub.n M.sub.w % R enantiomer A 57,000 111,000 11.7 B 60,000-65,000 125,000 50 C 60,500 100,000 1.6 D 66,000 127,000 0.6 E 53,500 99,500 4.3

(9) Fibers are spun from PLA resins A-E or blends thereof as indicated in Table 1 below by melt-spinning through a 0.3 mm spinneret, drawing and heat-setting to form circular cross-section solid filaments having a diameter of 12 microns.

(10) The molecular weight of the resins is determined by gel permeation chromatography. The glass transition temperature, melting temperature and enthalpy of melting are determined on a sample of the resin or resin blend by differential scanning calorimetry. Melting temperature and enthalpy of melting are measured by heating from 25 C. to 225 C. at the rate of 50 C./minute. Glass transition temperature is measured by heating from 0 C. to 210 C. at 20 C./minute.

(11) Acid end group content is determined by titration.

(12) The fibers are evaluated for blocking by chopping 2.5 g of fiber into 2.5-5 cm lengths. The chopped fibers are placed in a preheated cup and a preheated 1 kg weight is applied to the fibers. The assembly is then placed in a preheated oven at 80 C. for 10 minutes. The sample is then removed and visually inspected to evaluate whether the fibers have stuck together to form a mass.

(13) To evaluate hot air shrinkage, the fibers are cut into approximately 25 cm lengths, measured, and placed on a Teflon sheet. The fibers and sheet are then placed in a preheated 80 C. oven for five minutes. The fibers are then removed and their lengths re-measured.

(14) Hot water degradation is measured as follows: 0.48 grams of 2.5-10 cm fibers are fully immersed in 100 mL of a 0.1M phosphate buffer solution. The container is then heated in a water bath at 65 C. for 6 days. The flask contents are then filtered through a glass filter and rinsed twice with 30 mL aliquots of deionized water. The filtered and rinsed fibers are then dried to constant mass in a vacuum oven and weighed to determine % mass loss.

(15) Results are as indicated in the Table 1.

(16) TABLE-US-00002 TABLE 1 Property B* Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 C* D* E* Resin(s) A B/C B/C B/C A/C A/D E C D (50/50) (30/70) (20/80) (70/30) (60/40) % R 11.7 24.1 15.0 11.0 8.1 7.0 4.3 1.6 0.6 enantiomer M.sub.n 1000 g/mol 57 60 66 61 63 64 53 60 66 M.sub.w 1000 111 112 109 106 109 117 99 100 127 g/mol T.sub.m, C. N/A 172 172 171 168 177 155 174 182 Enthalpy N/A 25.5 32.8 36.0 19.9 24.0 30.3 45.6 53.1 melting (J/g) T.sub.g, C. 51.7 48.6 52.6 53.4 55.0 55.5 54.3 56.3 56.0 80 C. Hot Air 70 21.8 13.5 10.3 19.0 9.8 6.2 6.2 1.5 Shrinkage, % Hot water 25.5 27.8 17.7 12.9 10.8 5.4 5.6 0 4.2 degradation, % mass loss 80 C. Fail Fail Pass Pass Pass Pass Pass Pass Pass Blocking

(17) Comparative Sample B shows the effect of using a single, amorphous grade PLA resin that has about 12% of the R-enantiomer. The material cannot be crystallized, and thus has a very high hot air shrinkage value and blocks badly at 80 C. (and lower temperatures).

(18) Comparative Samples C-E show the effect of using a single, semi-crystalline grade of PLA resin that has 0.6 to 4.3% of the R-enantiomer. Shrinkage is very low, but so is degradation, and these single resins are unsuitable for use in applications in which somewhat rapid degradation is necessary.

(19) Examples 4-8 show the effect of using a blend of an amorphous grade of PLA resin and a semi-crystallizable grade. Example 6 has an overall R-enantiomer content very close to that of Comparative Sample B. It degrades more slowly than Comparative Sample B, but does not block and exhibits much less shrinkage on the 80 hot air shrinkage test. Examples 4 and 5 show that very high levels of R-enantiomer can be tolerated if a blend of PLA resins is used instead of a single resin (as in Comparative Sample B), and also show how degradation rates can be tailored by adjusting the overall level of the less-predominant enantiomer (the R-enantiomer in this case). As Example 4 shows, degradation rates as high as the pure amorphous grade polymer (Comp. Sample B) can be obtained with the blend, while avoiding the very large shrinkage problem exhibited by Comp. Sample B. Example 4 resides at the limits of the invention, as some tendency to block is seen with this example.

(20) The results in Examples 4-6 are particularly surprising because the amorphous grade of polylactide resin is a polymer of meso-lactide, which contains the R- and S-enantiomers in nearly equal amounts and which cannot be crystallized by itself at all. The use of the poly(meso-lactide) results in a very high overall R-enantiomer content in the blend, yet the blend is capable of being crystallized enough to prevent blocking while at the same time providing useful degradation rates.

(21) Example 8 resides at the low limit of overall R-enantiomer content. The degradation rate is low for this sample. In this sample, the semi-crystalline resin has a very low R-enantiomer content. That semi-crystalline resin is believed to crystallize very efficiently (as evidenced by the high crystalline melting temperature for that sample). That efficient crystallization, together with the low overall R-enantiomer level, is believed to account for the low degradation rate. As indicated by the other experiments, a slightly higher overall R-enantiomer content (as in Example 6) is expected to lead to an increase in degradation rate for that sample.