technical paper | Compounding bioplastics
Table 2. Trial formulations Trial
PLA (a)
Control 1 2 3
100% 95% 90% 85%
Impact mod.
0% 5%
10% 15%
(a) 20 MFR PLA (b) Renewable content as measured by weight % renewable polymer
Table 3. Impact modified PLA Notched Izod 15 J/m 105 J/m 315 J/m 850 J/m 105 J/m 215 J/m 700 J/m
PLA
Trial 1 Trial 2 Trial 3 HIPS ABS
PC/ABS
Tensile 60 MPa 56 MPa 48 MPa 42 MPa 20 MPa 50 MPa 60 MPa
HDT @ 455 kPa 55°C 66°C 63°C 60°C 88°C 93°C
115°C
Table 4. PLA/PC alloy Notched Izod Tensile Strength HDT @ 455 kPa 15 J/m 880 J/m 700 J/m
PLA Trial 4 PC/ABS
60 MPa 50 MPa 60 MPa
55°C
119°C 115°C
Upgrading impact performance With a notched Izod impact of less than 15 J/m, PLA is very brittle and would usually be unsuitable for even the lesser impact demanding semi-durable applications. Fortunately, numerous impact modifiers have been developed in the past to improve the toughness of the traditional petroleum based polyesters including polybutylene terephthalate (PBT) and polyethylene terephthalate (PET). These impact modifiers include such chemistries as butyl acrylates, acrylonitrile-buta- diene-styrene, methacrylate-butadiene-styrene, and a host of others. Through a series of compounding trials, RTP has identified a rubber chemistry that is especially effective with PLA and the trial formulations listed in Table 2 were melt compounded on a twin-screw extruder. These trials were injection moulded and tested per
ASTM standards and compared to published data for the traditional petroleum based polymers known for impact resistance such as HIPS, ABS and PC/ABS alloys. The results are listed in Table 3. They show that by compounding 5% of the impact modifier into PLA, one can produce a bio-based compound that has 95% renewable resource content and can offer physical performance similar to HIPS,
46 COMPOUNDING WORLD | March 2014
Renewable content (b)
100% 95% 90% 85%
which is used in many semi-durable applications. Similarly, by compounding 10% of the impact modifier into the PLA, one can offer a bio-based polymer that has performance comparable to ABS, but with a 90% renewable resource content. Further increasing the impact modifier to 15%, it is
possible to develop a compound with 85% renewable resource content that performs similar to one of the best impact resistant petroleum based materials available, a PC/ABS alloy. One notable exception relating to the performance of this impact modified PLA compound compared to the PC/ABS is with thermal resistance as measured by HDT at 455 kPa. What can be done to improve the thermal performance of biopoly- mers such as PLA?
Upgrading thermal performance With an HDT at 455 kPa of 55°C, one would be con- cerned whether a component manufactured from PLA would be able to survive shipment in a semi-trailer across the southern section of the United States in mid-summer. In order to allow PLA to be confidently used in semi-durable applications, the thermal performance must be improved. PLA is a semi-crystalline resin and the thermal
resistance, as measured by HDT, of semi-crystalline polymers can be improved by the following methods: 1. Alloy polymer with another higher temperature polymer 2. Raise the level of crystallinity within the polymer 3. Compound polymer with a high aspect ratio mineral filler or reinforcing fibre RTP has discovered that PLA can be alloyed via
compounding with several traditional polymers including PC, acetal, ABS, PMMA, and LDPE to produce polymer blends that have unique physical performance. One alloy that has shown potential for significant performance is the PC/PLA blend. For example, the Trial 4 blend of 58% PC, 32% PLA and 10% compatibi- lizer can offer physical performance similar to the versatile PC/ABS alloy in both impact and thermal properties as indicated in Table 4. One of the reasons that PLA has such a low HDT is that the polymer, in its typical neat state, has very low crystallinity and can be considered to be amorphous. Research has shown that taking PLA from its amor- phous morphology to maximum crystallinity can significantly raise the thermal resistance as measured by HDT[2]
. The HDT at 455 kPa is 55°C for amorphous
material and 130°C for crystalline material. Like its polyester cousin PET, PLA has proven to be
very difficult to injection mould into parts that exhibit the crystalline morphology. Studies relating to the rate of crystallization have shown that unmodified PLA
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