TECHNOLOGY | WOOD PLASTIC COMPOSITES
Right: Mixaco’s HM/KM Wood mixing system is designed for the specific challenges of WPCs
capstock formulations, lubricants, compatibilisers, and what is describd as a “next-genera- tion” product that provides both lubrication and compatibilisation. “Historically, process and performance aids have been independent products that sometimes inter- acted with each other in negative ways,” said Jim Keenihans, senior technical service & development scientist, Dow Consumer Solutions. “The most common chemistries of lubricants can actually inhibit wood- polymer compatibilisers such that the full proper- ties cannot be reached. Because of this, formulators have been forced to make compromises on lubrication — which is critical to quality, stability, and rate — and compatibilisation, which is critical for mechanical strength, moisture stability, and melt strength,” Keenihans said. Dow says that its Amplify Si PE 1000 product was developed with the intention that both lubrication and performance work at their peak performance. Keenihans claims that the product can “achieve a whole new echelon in WPC additives.” Dow’s research has shown that the Si PE 1000 additive is more thermally stable than metallic stearates and will not degrade as quickly as process temperatures and rates are increased. “One of our most interesting findings is that mechanical performance is improved so signifi- cantly that strength can be achieved with lower cost materials, for example utilising linear low density polyethylene instead of, or blended with, high density polyethylene,” he said.
Mixing ideas Mixing the high volume of natural fillers required in
WPC compounds is a challenge, according to Nicolas Gehring, global sales manager at equipment manufacturer Mixaco. He explained that in the mixing process in general, it is easier to homogenise materials and to prevent post-mixing separation if the particle sizes and shapes of the components are similar. In WPC compounds, however, the fine low bulk-density wood flour
or fibres are very different from the polymer powders or granules. “Mixing is a matter of moving particles consist- ently in order to create a uniform blend. However, particles with different bulk densities behave very differently. Lighter products need less power to get moved, while heavier products need more energy,” Gehring said.
Dispersing the additives into the blend is also challenging because of the high loading of natural fillers.
Achieving a uniform blend is key for the produc-
tion of the WPC end-product. “Uniformity can affect the intake behavior of an extruder, or the material transport handling,” said Gehring. “The smallest change in the [mixing] process can affect the further processing of the product.” He adds that wear and tear of the mixing system can affect consistency of the blend over time, so compounders should be aware of when maintenance is needed. Moisture of the natural fillers is also a considera-
tion, as these fillers are generally hydrophilic and easily absorb water if stored in a bag without a plastic liner. Before drying, they can contain more than 10% moisture in some cases, says Gehring.
Figure 1: To create a blend of 60% PLA with 40% cellulose fibre, such as that shown above, Mixaco’s HM/KM Wood system regulates parameters including hot mixer rpm (blue), hot mixer temperature (red), and hot mixer torque (green)
18 PIPE & PROFILE EXTRUSION | November/December 2022
www.pipeandprofile.com
SOURCE: MIXACO
IMAGE: MIXACO
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