technical paper | Compounding bioplastics
Table 8. Glass fi bre engineered resins 30% Glass fi bre
PP
Tensile Str. Flexural Str. Flex Mod. Not. Izod
HDT @ 455
75 MPa 110 MPa
4,800 MPa 105 J/m 160°C
PBT
125 MPa 185 MPa
8,300 MPa 75 J/m 215°C
Table 9. Glass fi bre PLA comparison 30% glass fi bre
PP
Tensile Str Flexural Str. Flex Mod. Not. Izod
HDT @ 455 Renewable
PLA PBT Nylon 6
75 MPa 110 MPa 125 MPa 160 MPa 110 MPa 145 MPa 185 MPa 230 MPa 4,800 MPa 11,000 MPa 8,300 MPa 8,300 MPa 105 J/m 60 J/m 160°C 0%
165°C 70%
75 J/m 215°C 0%
105 J/m 215°C 0%
Next generation PLA As PLA increases in popularity, manufactures are continuing to refi ne their processes and develop new grades with improved properties and processability. The crystallization speed and melt temperature of PLA is dependent upon the purity of the monomers. The monomer is chiral and exists in both D and L versions. Impurities in the D monomer must continue to be reduced to allow more pure PLA to be polymerized. PLA with improved performance can be obtained through better monomer synthesis, monomer separation, and/ or polymerization catalysts. Currently, higher performing, next generation PLA grades have increased the melt point for crystalline PLA from the 160-170°C range up to 180°C, and there is potential for further increases in the future. Other improvements include reductions in crystallization time. Today’s high performance grades crystallize at three to four times the speed of prior generation PLA resins. For example, glass fi bre rein- forced PLA can now achieve short 20-30 second cycle times and provide an HDT greater than 165°C at 455kPa.
Environmental stability? But what about the environmental stability of PLA? Hasn’t PLA been widely promoted as being biodegrad-
able? Would one want to go through all the work and effort to develop an engineered PLA compound for a semi-durable application and have it
50 COMPOUNDING WORLD | March 2014
Nylon 6 160 MPa 230 MPa
8,300 MPa 105 J/m 215°C
revert to organic matter after being exposed to the environment? First one must understand that PLA does not
undergo biodegradation unless certain environmental conditions exist relating to high heat and high moisture. The PLA used for trials in this paper is high molecular weight polyester and, in this state, the molecules are too large for the microbes to eat and digest. In order for the molecular weight of polyester, such as PLA, to be reduced, it must typically fi rst undergo hydrolysis. Much research has been carried out on the hydrolysis of the traditional thermoplastic polyesters of PET and PBT. Through these studies, it has been learned that the hydrolysis reaction in polyesters will greatly accelerate when the polyester is exposed to high moisture conditions at temperatures above the polymer’s glass transition temperature (Tg)[4]
. The Tg of PLA is 54°C (130°F), compared to 68°C
(155°F) for PET and 57°C (135°F) for PBT. Based on the relationship of hydrolysis to Tg, it has become common knowledge that PET should not be subject to prolonged exposure to high moisture environments above 68°C and that PBT should not be subject to prolonged exposure to high moisture environments above 57°C. In order to ensure that PLA will provide reliable perfor- mance for semi-durable programs, the same polyester rule must followed and it is advised that PLA com- pounds not be used in high moisture environments at temperatures above 54°C (130°F). Another result from the many studies relating to the
hydrolysis resistance of the traditional thermoplastic polyesters is that stabilization packages have been developed that will slow down this degradation reaction and allow that polyesters to survive short to moderate exposures to high moisture above their Tg[5]
. Recent
work has also shown that these stabilizers are also effective in PLA[6]
. Thus, by ensuring that the PLA is not
subject to prolonged exposure to high moisture above 54°C and by using a proper stabilization package, PLA compounds can offer reliable performance in semi- durable applications.
An impact modifi ed PLA grade was developed by RTP for this promotional divot tool. The RTP 2000 Series grade has 84% renewable content and it delivers an excellent balance of strength and toughness. Other benefi ts include good fl ow for thin walls, plus high durability, gloss and colour
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