High density compounds | materials
Not every plastics application is driven by weight reduction. Mark Holmes explains that high density polymer compounds are fi nding an increasing number of specialist applications
Making weight matter
The benefi t of using lightweight thermoplastic com- pounds to substitute much heavier traditional materi- als, such as metals, in automotive and aerospace applications is widely appreciated. However, not all potential plastics applications require a reduction in weight. There is a whole range of applications - extend- ing from sound and vibration damping to radiation shielding – that require plastic compounds with high, and sometimes very high, density. Sound dampening is one of the largest application
areas for high density polymer compounds, in particular for reduction of engine noise and vibration in automo- biles. Other functional applications include parts such as washing machine counterweights. Some industries, however, demand high density plastics for less specifi c purposes – in the premium perfume sector, for example, the heft of a heavy closure helps to create the perception of high quality. Increasing the fi ller loading of a polymer compound
is a pretty straightforward route to increasing its density. However, fi llers such as calcium carbonate, talc or mica can only go so far in meeting this objective. “The role of a high density fi ller has traditionally been taken by barium sulphate,” says plastics industry consultant and President of Phantom Plastics Dr Chris DeArmitt. He says the material offers a density of 4.0-4.5 g/cm3
, is relatively cheap and pure, electrically
insulating, and is white in powder form (allowing compounds to be coloured). Other benefi ts of barium sulphate are that it is highly inert, provides good
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resistance to chemicals and is x-ray opaque so it allows detectability. However, in terms of adding density even barium sulphate has its limits. “Then you need something else. Other high density fi llers include zinc oxide, titanium dioxide and micaceous iron oxide – MIOX. Tungsten is used when very high densities are demanded and high cost can be accepted. Although magnetite is now increasingly being looked at as a more effective high density fi ller,” he says.
Non-linear performance DeArmitt emphasises that density versus weight percentage of a fi ller is not a linear function and as such certain fi llers can prove more effective at higher weight percentages. In other words, a 20 wt% loading can be the equivalent of 10 vol%. Figure 1 shows the density that can be obtained using different fi llers and wt% loadings in polypropylene. The black dot on each curve indicates the limit of processibility and in the case of magnetite shows 85 wt% is the maximum loading in PP, giving a density of 3g/cm3
. This corresponds to 49.5
vol% magnetite. Above this level the polymer is too viscous to process. The maximum loading of 49.5 vol% fi ller for calcium carbonate and barium sulphate is also marked. The chart clearly shows that fi llers such as magnetite can provide a higher density for the same vol% loading, so less compound is required for any given application. Higher loading levels can also mean lower wall thicknesses and faster processing cycles. Aside from its high density, magnetite (an oxide of
March 2016 | COMPOUNDING WORLD 57
Main image: Premium perfume
closures are just one of a growing number of applications that call for high density plastics
PHOTO:
ISTOCKPHOTO.COM/POLYONE
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