COMPOUNDING | BIOPLASTICS
Natureworks has been working with Farrel Pomini to develop low-shear compounding options for its PLA
bioplastics.Peter Mapleston takes a look at the findings
Low shear mixing of PLA
Many compostable bioplastics are polyesters and are shear-sensitive by nature so low-shear process- ing is required to help maintain polymer integrity and achieve optimal physical properties. Major PLA producer NatureWorks has been working with Farrel Pomini, which makes the Farrel Continuous Mixer (FCM), in a joint research program to compare the processing of its Ingeo compounds on a twin-screw extruder (TSE) and the FCM. The objective of the study was to evaluate the effects on molecular weight loss in PLA while adding a talc mineral filler. “PLA is often compounded with mineral fillers
and/or additives to improve physical properties. The benefits of the FCM would allow for lower process- ing costs, higher mineral fill levels and improved physical properties relative to twin screw extruders,” says Farrel Pomini. It has published the results of the research in a White Paper written by Patrick
Lahmann from Farrel Pomini and Nicholas Knowl- ton from NatureWorks. This article reviews some of the key findings. The FCM uses a non-intermeshing, counter
rotating twin rotor design which imparts controlled levels of shear on the polymer blend, making it a good technology for handing shear and tempera- ture sensitive materials such as PLA. For the purposes of the study, Farrel Pomini used a Compact Processor (CP Series II), which is a combination of an FCM that hot-feeds an extruder used to create the pressure necessary to produce pellets for analysis. Variables studied included molecular weight (Mw) retention, melt temperature, dispersion of the talc (measured by the Filter Pressure Value or FPV), and specific energy (SE). Figure 1, showing molecular weight retention vs talc filler levels, indicates that with the CP mixing
Main image: A study by Natureworks and Farrel Pomini finds continuous mixers a good choice for processing shear sensitive PLA resins
Summary of missing trials showing molecular weight retention of different PLA compounds processed on twin screw and continuous compounding equipment
Sample ID Process Method Mw Retention 926-29-01 926-29-02 926-29-03 926-29-04 926-29-05
TSE TSE TSE TSE TSE
CP CP CP CP
93% 82% 63% 60% 37%
Processed at NatureWorks on 40mm TSE 16-A 17-A 18-A 24-A
99% 97% 95% 88%
Processed at Farrel on CPeX Compact Processor Source: Natureworks/Farrel
www.compoundingworld.com September 2019 | COMPOUNDING WORLD 35
% Filler 0%
20% 40% 50% 60%
20% 40% 50% 60%
Melt Temp C 222 223 226 228 235
178 188 202 212
FPV
0.39 2.93 2.50 0.84
0.38 0.54 0.54 0.81
SE kW-hr/kg
0.346 0.308 0.308 0.231
0.127 0.144 0.166 0.181
PHOTO: FARREL POMINI
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