technical paper | Compounding bioplastics
would require a moulding cycle time of over 5 minutes in a 110°C mould to produce a part with moderate levels of crystallinity[3]
. This lengthy mould cycle time
would typically be cost prohibitive when producing semi-durable parts. To overcome this, RTP Company has developed
nucleating packages that can be melt compounded into the polymer to speed up this rate of crystallization during the injection moulding process. A nucleation package typically consists of a nucleating agent, such as a fine mineral or salt, which provides a site on which crystal growth can initiate. The package can also be enhanced by incorporating a polymer chain mobilizer – usually a low molecular weight polymer, oligomer, or plasticizer – that will allow the PLA molecular chains to migrate more freely to the sites of crystal growth. RTP has developed two such nucleation packages and the performance of each is shown in Table 5. Once the impact modifier and nucleation packages were developed, we then explored what additives, such as minerals, could bring to the table. By compounding PLA with the nucleation package from Trial 5, impact modifier from Trial 3, and incorporating 10% calcium carbonate or talc mineral, Trials 7 and 8 were set up as listed in Table 6 and the results are listed in Table 7. The latter shows that, by choosing calcium carbon-
ate or talc, one can bracket the physical performance of the petroleum based ABS, which is a material of choice in many demanding durable applications because of its engineering level performance.
Upgrading strength performance One of the most efficient methods to impart engineering level perfor- mance into most polymers is to
reinforce the material with a high aspect ratio fibre, such as glass or carbon fibre. Glass fibre rein- forced polypropylene, nylon and PBT polyester have become
RTP has developed a PLA/PC
alloy for consumer electronics applications, such as portable device housings. The 2000
Series grade has 32% renewable resource content, and an HDT of 115˚C. It can be processed in
existing tooling and has the added benefit of being over-mouldable with a
soft-touch elastomer. The grade offers a good balance of stiffness and impact strength
COMPOUNDING WORLD | March 2014
Table 5. Nucleation packages in PLA (c) Nucleation Package Trial 5 Trial 6
Cycle Time (d) 60 seconds 40 seconds
(c) 20 MFR PLA (d) Total closed mould time, 1/8” part thickness, 105°C mould
Table 6. Trial formulations Trial PLA 7 8
70% 70%
10% CaCO3 10% Talc
Mineral Impact mod. Nucleator Trial 3 Trial 3
Trial 5 Trial 5
Table 7. 10% mineral filled PLA Trial 7
Mineral Type CaCO3 Tensile Str.
Flexural Str. Flex Mod. Not. Izod
HDT @ 455 Renewable
ABS —
40 MPa 70 MPa
3,300 MPa 360 J/m 80°C 70%
50 MPa 75 MPa
Trial 8 Talc
50 MPa 80 MPa
2,700 MPa 4,400 MPa 215 J/m 93°C 0%
118 J/m 117°C 70%
materials of choice for a variety of durable engineering applications because they can offer physical properties such as those listed in Table 8. There are bio-based polyamides, such as nylon 11
and nylon 6/10, and bio-based polyesters such a polytrimethylene terephthalate (PTT) that, when reinforced, can offer comparable property performance to the products in Table 8. However, they do so at a significant cost penalty. Can we offer a bio-based compound with equivalent performance as listed in Table 8 but at a reasonable cost? By melt compounding PLA with high aspect glass
fibre reinforcement according to the Trial 9 formulation of 70% PLA with 30% glass fibres and the Trial 5 nucleation package, RTP Company was able to achieve the performance featured in Table 9. One can see that the bio-based PLA has been
upgraded by melt compounding with glass fibre to a performance level that falls predominantly between that of glass fibre reinforced PP and that of glass fibre reinforced PBT or nylon. The raw material cost of the glass fibre PLA also falls between that of the glass fibre reinforced PP and that of the glass fibre reinforced nylon or PBT. From the data in Table 9 it is clearly evident that PLA
can be engineered to compete with the performance of some of the most popular glass fibre reinforced thermoplastics used in the industry.
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