Working with polypropylene sheet for the first time often presents new challenges for thermoformers including shrinkage, low rigidity, and low stiffness. The addition talc or calcium carbonate mineral fillers can help remedy these problems while contributing added benefits compared to resin-only formulations. But while talc and calcium carbonate both offer benefits, we often find our customers asking, “what’s the difference?”
When considering the use of a mineral filler, increased stiffness, increased rigidity, heat deflection, and cost are typically of concern to our customers. But what minerals or combination of minerals should you use to achieve desired part function? This answer of course varies from case to case depending on the end use application and desired properties – but to make things a bit easier, we have outlined the tradeoffs and benefits that come as a result of using these mineral fillers in PP sheet.
Mineral Structure
To start, the structure of each mineral has a drastic impact on how it behaves when added to plastic, impacting the properties of the sheet and as a result, the formed part. Talc has a plate-like structure, which means that when extruded, the plates will orient in the direction of extrusion. This mineral structure results in increased rigidity and resistance to heat. Calcium carbonate has more of a spherical or cube shape. The addition of calcium carbonate will increase stiffness, but not as much as in talc filled polypropylene sheet.
Impact Strength
While the plate-like structure of talc contributes to increased rigidity and heat resistance, it results in a significant decrease in impact strength. Calcium carbonate, however, does not have an adverse affect on impact strength. In fact, in studies conducted by Heritage Plastics, calcium carbonate was found to improve this property at higher loading levels.
This increase in impact strength ultimately allows for a more durable part: “The significant finding here is that CaCO3 can be used at levels up to 35% by weight in polypropylenes without adversely affecting impact strength. This finding will be key in maintaining performance in parts with reduced wall thickness when switching from resin only systems to formulations that contain calcium carbonate.” (Holly Hansen, Heritage Plastics)
Heat Deflection
Despite the sacrifice of impact resistance when using talc, heat deflection, or thermal conductivity, of the talc allows it to cool faster reducing shrinkage in formed parts. Reduced shrinkage can significantly help with the predictability and appearance of the formed part. Calcium carbonate is also able to combat temperature changes in PP sheet, with higher loading being more efficient than lower loading, although to a lesser extent than talc.
Since this property is increased with both minerals, it comes as no surprise that a blend of the two produced the best results: “Coefficient of Linear Thermal Expansion or CLTE monitors the expansion and contraction of the part as it undergoes thermal changes. Talc with its platy structure is much more efficient in minimizing expansion and contraction of the final part due to temperature changes. However, calcium carbonate is also able to suppress temperature related changes in polypropylene. Higher mineral loads were more efficient than lower loadings. And blends of the two minerals were extremely effective in reducing thermal changes.” (Holly Hansen, Heritage Plastics)
Final Thoughts
As with every application, it all depends on what you’re trying to accomplish when deciding what mineral to use. Both talc and calcium carbonate will reduce shrinkage during thermoforming; both minerals will increase rigidity; both minerals will increase heat deflection temperature, with talc being the most effective. If stiffness is your main concern, then talc is the way to go, but if you’re worried about impact resistance, calcium carbonate filled polypropylene sheet is the clear choice.