Calcium carbonate (CaCO3) and talc are two widely used mineral fillers in the plastics industry, and their use in rigid packaging has increased significantly over the years. Once thought of simply as resin extenders, the addition of these mineral fillers to materials such as polypropylene, has been shown to contribute to increased performance, improved processing, and improved sustainability of the finished part - properties which can be further influence by loading level.
But if you're new to the world of mineral fillers, or considering using them in an application for the first time, you might be asking yourself, how do I determine the proper loading level for my application? To shed some light on the subject our friends at Heritage Plastics have contributed the following guest post discussing methods to help determine the optimum loading level for thermoforming applications:
The predominant minerals used in thermoformed parts are talc and calcium carbonate. Decisions regarding why to use mineral fillers (5 Reasons to Use Calcium Carbonate Filled Polypropylene Sheet), which minerals to use (Deciding Between Calcium Carbonate and Talc Filled Polypropylene Sheet), and at what loading level can be somewhat confusing but needn’t be. The following blog hopes to offer some guidance in thinking through the approach.
Mineral levels can vary from 3% to 50% depending on the application, part design, targeted part properties, and desired part costs. Identifying the correct loading level can be roughly determined prior to running any trials by the part geometry and targeted properties. For example, a thin polystyrene cold drink lids would use lower levels of calcium carbonate - typically somewhere between 3-10%. Polystyrene, even HIPS, is somewhat brittle. For that reason, adding talc, which has been shown to lower impact strength, would not be a good fit. And too much calcium carbonate could weaken the structure potentially causing splits along the rim-edge or the straw slot.
So how to determine that optimum level? Trials are pretty straightforward, where stepping through a range of levels can be accomplished relatively easily since no changes in processing conditions are required. Just ensure your part weights remain constant for a fair comparison, which can be accomplished by down-gauging the sheet. For example, trials targeting 3%, 5%, and 10% could be run, with samples collected for performance testing. The results would identify the optimum mineral level to use in the part.
Thicker parts such as deli trays or continers for hot food items can typically use the highest levels of mineral filler content. In the case of roasted-chicken trays, for example, up to 50% calcium carbonate can be used. This higher loading level adds to the heat deflection strength. These parts are usually well designed with a lot of ribbing that further adds to the part strength under heated load. Talc is also well suited for this application, and can accomplish the higher heat deflection strengths at lower percentages, approximately 25-30% talc, making it possible to additionally down-gauge or lightweight the part. In both cases, the part weight must be maintained by lowering the sheet thickness. And again, very straightforward trials can be run to determine the best level for a give part.
In some cases, a blend of talc and calcium carbonate should be considered. To cite a real-world example - a customer was seeking cost savings and environmental advantages for a part that had already been approved. In this case, trials with calcium carbonate alone did not generate the needed heat deflection strengths. Since cost was also a major concern, using talc alone was not an option because talc can be roughly 2-3 times the cost of calcium carbonate. But a blend of talc and calcium carbonate proved just right.
In this case, 22% calcium carbonate and 10% talc for a total mineral load of 32% generated the cost savings and the heat strengths needed, as well as delivered environmental advantages due to the displacement of resin. For this particular application identifying the optimum loading levels was a little more complicated than in the previous examples. Trials were run using two concentrates, an 80% calcium carbonate filled in hPP (homo-polymer polypropylene) and a 60% talc filled in the same hPP. A simple ratio study was set up to look at several combinations of the talc and calcium carbonate and each concentrate added accordingly. Once the ideal ratio of talc and calcium carbonate are identified, a single blended mineral concentrate can be produced; in this case 55% calcium and 25% talc in hPP, where the customer uses at 40% to achieve the targets identified from the trials. For this example, two rounds of trials were needed but it eventually identified a solution that maximized cost saving while delivering the needed performance.
Author Bio: Holly Hansen is VP Technical Services. She and her team focus on customer technical support and new product development.
Heritage Plastics produces a wide variety of mineral compounding and is very focused on working with customers to find cost effective solutions for their applications, both existing and new opportunities. For more information visit www.heritage-plastics.com
For more information on calcium carbonate and talc filled plastic sheet, contact our Impact Plastics team!