Hyperbranched polylactide resin compositions

Abstract

Polylactide resins are branched by reaction with a mixture of a polyene compound and a cyclic peroxide. This branching method produces a product that has a very high polydispersity, a high branching number (B.sub.n) and excellent melt strength, without forming large amounts of gelled material. The branched polylactide resins are useful in many melt processing operations, in particular sheet and film extrusion, extrusion foaming, extrusion coating and fiber processing. They are characterized by easy processing and allow for broadened processing windows.

Claims

1. A branched polylactide made by a method comprising the steps of I. forming a molten mixture of: i) a starting linear polylactide resin or mixture of linear polylactide resins; ii) 0.05 to 1.5 weight percent, based on the weight of component i), of at least one polyene compound, the polyene compound having 2 to 6 vinyl groups and an equivalent weight per vinyl group of up to 500; and iii) 0.001 to 0.2 weight percent, based on the weight of component i), of at least one cyclic peroxide; and II. reacting the molten mixture at a temperature sufficient to decompose component iii) and branch at least a portion of the linear polylactide resins, wherein the branched polylactide has an absolute M.sub.w of at least 200,000 g/mol, a polydispersity of at least 3, an absolute Z-average molecular weight of at least 1,250,000 g/mol, an inherent viscosity of 1.1 to 1.35 dL/g, a branching number (B.sub.n) of at least 3, and a gel content of 10% or less by weight based on the weight of the polylactide composition.

2. A branched polylactide made by a method comprising the steps of I. forming a molten mixture of: i) a starting linear polylactide resin or mixture of linear polylactide resins; ii) 0.05 to 1.5 weight percent, based on the weight of component i), of at least one polyene compound, the polyene compound having 2 to 6 vinyl groups and an equivalent weight per vinyl group of up to 500; and iii) 0.001 to 0.2 weight percent, based on the weight of component i), of at least one cyclic peroxide; and II. reacting the molten mixture at a temperature sufficient to decompose component iii) and branch at least a portion of the linear polylactide resins, wherein the branched polylactide has an absolute M.sub.w, of at least 350,000 g/mol, a polydispersity of at least 4, an absolute Z-average molecular weight of 1,500,000 to 3,000,000 g/mol, an inherent viscosity of 1.1 to 1.35 dL/g, a branching number (B.sub.n) of at least 6, and a gel content of 10% or less by weight, based on the weight of the polylactide composition.

3. A branched polylactide having an absolute M.sub.w, of at least 200,000 g/mol, a polydispersity of at least 3, an absolute Z-average molecular weight of at least 1,250,000 g/mol, an inherent viscosity of 1.1 to 1.35 dL/g, a branching number (B.sub.n) of at least 3 branches, and a gel content of 10% or less by weight based on the weight of the polylactide composition.

4. A branched polylactide having an absolute M.sub.w, of at least 350,000 g/mol, a polydispersity of at least 4, an absolute Z-average molecular weight of 1,500,000 to 3,000,000 g/mol, an inherent viscosity of 1.1 to 1.35 dL/g, a branching number (B.sub.n) of at least 6 branches, and a gel content of 10% or less by weight based on the weight of the polylactide composition.

Description

EXAMPLE 1 and COMPARATIVE SAMPLES A and B

(1) Example 1 is prepared by combining the PLA Resin with 0.21% (based on resin) of TAIC AND 0.25% (based on resin) of the Cyclic Peroxide Solution in a Brabender melt mixer operated at 60 rpm, 210° C. for 15 minutes.

(2) Comparative Sample A is neat PLA Resin.

(3) Comparative Sample B is prepared in the same general manner as Examples 1-3 from the PLA resin and the Epoxy Branching Agent.

(4) The complex viscosity and tan delta G″/G′ values are measured on Example 1 and Comparative Samples A and B, at a temperature of 210° C. and shear rates of 1 rad/s and 100 rad/s. Measurements are made using a TA Instruments ARES parallel plate rheometer equipped with TA orchestrator software. Plate diameter is 25 mm and the gap is 2 mm. Tool inertia is 62.5 g-cm.sup.2 and strain is 1.6%. Results are as indicated in Table 1.

(5) TABLE-US-00001 TABLE 1 Sample Designation A* 1 B* 1 sec.sup.−1 shear rate Complex Viscosity, Pa .Math. s 120 5000 60,000 Tan Delta 40 2 1 100 sec.sup.−1 shear rate Complex Viscosity, Pa .Math. s 100 1000 30,000 Tan Delta 2 1.2 0.6

(6) The results for Comparative Sample A represent a baseline. Neat linear PLA resin exhibits a low complex viscosity at both the low and the high shear rates. However, tan delta is quite high at low shear rates. Tan delta is a useful indicator of melt elasticity; high values are indicative of low melt strength. Comparative Sample A is representative of linear PLA resins in having low melt strength and low melt elasticity.

(7) Example 1 shows the effects of this invention. At low shear rates, a viscosity increase is seen compared to Comparative Sample A, but due to the shear thinning behavior of these samples the viscosity at high shear is not significantly different than the baseline case. The small increase in viscosity is more than compensated for by the large increase in melt elasticity, as indicated by tan delta values of approximately 2 at low shear rate, compared to approximately 40 for the baseline case.

(8) Comparative Sample B shows the effect of branching with the Epoxy Branching Agent. A large improvement in melt strength is seen relative to the baseline case, but the viscosity at low shear rate is an order of magnitude greater than Example 1.

EXAMPLES 2 and 3 and COMPARATIVE SAMPLE C

(9) Examples 2 and 3 and Comparative Sample C are prepared by processing the starting materials listed below through a 30 mm twin screw extruder at a rate of 40 pounds (18.2 kg) per hour. Barrel temperatures are 215° C. The raw materials are dry blended and fed together through the throat. The extrudate is extruded into a water bath at 30-40° C. and chopped underwater to form pellets.

(10) Example 2 is made using the PLA Resin, 0.2% (based on resin) of TAIC and 0.25% of the Cyclic Peroxide Solution (based on resin).

(11) Example 3 is made using the PLA Resin, 0.2 wt.-% of TAIC and 0.1% of the Cyclic Peroxide Solution.

(12) Comparative Sample C is neat PLA Resin.

(13) Each of Examples 2 and 3 and Comparative Sample C are evaluated by gel permeation chromatography using a Viscotek GPCmax VE2001 GPC/SEC system (Malvern) equipped with a Viscotek TDA 302 triple detector array module (light scattering, viscometer, refractive index detectors). The mobile phase is THF (refractive index 1.405) at a rate of 1.0 mL/min and a temperature of 30° C. RI dn/dc is 0.185 for polystyrene standards and 0.046 for polylactide samples. Absolute M.sub.n, absolute M.sub.w, polydispersity, absolute M.sub.z, inherent viscosity, branching number, and Mark-Houwink slope are all determined using OmniSEC version 4.7 software. For branching calculations, branching calculations are made using the “star” option. “MH exponent” is inputted as 0.68, “MH intercept” is inputted as −3.39, “structure factor” is inputted as 0.75 and “repeat factor” is inputted as 72,000. Results are as indicated in Table 2.

(14) TABLE-US-00002 TABLE 2 M.sub.n, M.sub.w, M.sub.z, Mark-Houwink Designation g/mol g/mol PDI g/mol IV (dL/g) B.sub.n slope (a) C* 59,000 109,000 1.8 154,000 1.136 2.00 0.7 2 97,000 409,000 4.2 2.5 MM 1.190 7.48 0.42 3 75,500 205,000 2.8 1.6 MM 1.129 3.78 0.50

(15) The B.sub.n and Mark-Houwink slope values for Comparative Sample C are indicative of a linear polymer. The polydispersity (PDI) for that sample is typical for commercial grades of linear polylactide resins.

(16) The large increases in polydispersity, M.sub.z and branching number and the large decrease in Mark-Houwink slope for Examples 2 and 3 demonstrate the effectiveness of the combination of TAIC and the cyclic peroxide as branching agents. A significant amount of branching is seen even at the lower level of cyclic peroxide (Ex. 3). The branching number of over 7 and Mark-Houwink slope of 0.42 for Example 2 are indicative of a hyperbranched structure. Gel contents are less than 6% (as indicated by % recovery of polymer introduced into the test apparatus) for each of Examples 2 and 3, despite the high degree of branching.

(17) The haul-off force for each of Examples 2 and 3 and Comparative Sample C are measured at 225° C. using a Rosand capillary rheometer equipped with a haul-off system (Malvern Instruments). The capillary is 20 mm long and 2 mm in diameter. The piston is fixed as 10 mm/minute. Haul-off speed is varied from 5 meters/minute to 60 meters/minute. The average haul-off force over the haul-off speed range to 5 to 60 meters/minute is taken as the haul-off force for the sample. The haul-off force for Comparative Sample C is only about 0.5 centinewtons (cN) under those conditions.

(18) By contrast, Example 3 exhibits a haul-off force of about 3.6 cN, representing a very large increase in melt strength. Example 2 exhibits a haul-off force of well over 20 cN, which is larger than that which can be achieved by branching with the Epoxy Branching Agent, despite the higher viscosity of the latter. In addition, Example 2 exhibits zero extensibility, which is consistent with very high elasticity.

(19) The melt strengths of Example 3 and Example 2 in particular are higher than are needed for some applications, which allows for the possibility of diluting these samples with additional polylactide resin. A blend of PLA resin with as little as 5% (based on PLA resin) of Example 2 exhibits a significant increase in haul-off force, compared to Comparative Sample C. When the amount of Example 2 is increased to 20-40%, haul-off forces in the range of 0.75-3 cN are obtained. These represent substantial improvements that are adequate for many melt-processing operations. In addition, these blends have excellent melt extensibility, as indicated by their ability to be drawn at haul-off speeds of 100 to 250 meters/minute or more on this test.

EXAMPLE 4

(20) A branched polylactide resin (Example 4) is produced in the same general manner as Examples 2 and 3, from the PLA resin, 0.23% TAIC and 0.25% of the Cyclic Peroxide Solution.

(21) Let-down compositions containing 80% of the PLA resin and 20% of Example 4, and 90% of the PLA resin and 10% of Example 4 are prepared by melting blending the materials in a 30 mm twin-screw extruder.

(22) Extruded sheet is produced from neat PLA Resin on a Leistritz 3 Roll Sheet line at a processing temperature of 225° C., a screw speed of 200 rpm and a throughput of 22.7 kg/hr. This temperature is somewhat higher than the optimal processing temperature for this grade, in order to reduce the melt viscosity. When the sheet is examined under polarized light, prominent stress bands are visible. These stress bands are indicative of poor processing due to uneven extrusion dimensions across the melt curtain and calendaring.

(23) When the experiment is repeated by using the blend of 90% of the PLA Resin and 10% of Example 4, the stress bands are barely visible, indicating improved processing and better melt strength. When the experiment is again repeated using the blend of the 80% of the PLA Resin and 20% of Example 4, the stress bands virtually disappear.