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Impact Strength in Thermoplastics: Formulation Strategies Using Specialty Minerals Functional Fillers

There are several ways in which calcium carbonate products can be used to lower the overall cost of a PVC formulation:

  • Maximizing the value from the impact modifier.
  • Extending or replacing the impact modifier.
  • Extending the compound.

Calcium carbonate products make use of the quality and property relationships discussed on the prior two pages of this section, which can be read by clicking on one of these topics:


Ground Calcium Carbonate (GCC) as an Extender for the PVC Compound
This is the traditional use for ground limestone in rigid or unplasticized polyvinyl chloride (PVC). The goal is to add as much filler to a formulation as possible. Since the filler costs less than the compound, the more one adds, the lower the total formulation cost.

An example of this is DWV (drain waste vent) PVC pipe for sewer applications where 30-40 phr of Specialty Minerals Inc.’s (SMI’s) uncoated, 3.5 micron, Vicron® 15-15 GCC can be utilized for this non-pressure-rated application.

The physical requirements of the product govern the amount of GCC to be added. In this situation, the fillers are usually 2 microns or larger and will have a negative effect on some critical property, such as impact strength. This approach is valid as long as no impact modifier is used. If impact modifier is necessary, one should first consider a finer calcium carbonate. It is always more cost-effective to increase impact performance by reducing the filler’s particle size than by adding more impact modifier.


Finer Calcium Carbonates in Formulations with Impact Modifier
Here, impact modifier is added to the PVC formulation to obtain the required impact performance. The filler size is usually finer, around 1 micron, because you don’t want the filler and impact modifier to work against each other. An example of this might be a window profile formulation where Super-Pflex® MS precipitated calcium carbonate (PCC), which has an average particle size of 1.1 micron, is used with an impact modifier to obtain the specified level of impact performance at the lowest formulation cost.

When comparing fillers in this category, one should first determine the lowest level of impact modifier needed for each formulation and then compare formulation costs. It is very likely that the lowest cost filler will not produce the least expensive compound because it may require more impact modifier, which is costlier than the finest fillers.


Fine or Ultrafine Nano Precipitated Calcium Carbonates (PCCs) as Impact Modifier Extenders
Adding a submicron calcium carbonate such as SMI’s 0.35 micron coated Tuffgard® PCC to a rigid PVC formulation can increase impact performance. To obtain this result, the proper loading level must be determined by lab experiment or a series of plant trials; it is usually in the 8-15 phr range, depending on how much titanium dioxide is in the formulation. The total pigment loading is typically kept in the 15 phr range.

This has proved to be a cost-effective approach in formulations containing four or more parts of impact modifier. To determine proper loading levels, an experiment is run testing different filler and impact modifier levels to find a combination that provides the required performance at a lower formulation cost.

Here is an example: A PVC window profile formulation originally made with 4.5 phr of acrylic modifier and a 0.7 micron-coated GCC is replaced by one using only 2.5 parts of acrylic modifier, plus higher loadings of 0.35 micron Tuffgard® PCC. With Tuffgard® PCC, less external lubricant is required, so the loading is reduced to keep fusion time constant.

The first set of profiles tested are called unpigmented, as there is no titanium dioxide or other color pigment present–only calcium carbonate functional fillers. Here, going to a smaller-size calcium carbonate substantially increases the notched Izod impact strength. Meanwhile, the other properties remain essentially equal or better and the total formulation cost is reduced.

In the second set of tests, which are white profiles, the impact modifier level is again reduced, from 4.5 phr to 2.5 phr, the filler loading is again slightly increased and the external lubricant is reduced, giving a lower total cost formulation.

Notched Izod impact strength shows some improvement with other properties remaining essentially equal to or better than the original.

If, instead of using a submicron PCC such as 0.35 micron Tuffgard® PCC, formulators use an ultrafine, true nano PCC such as Ultra-Pflex® or Calofort® S PCC, then even larger reductions in impact modifier level are possible. A formulation that contains a low loading of a one or two micron calcium carbonate and eight parts of impact modifier may be able to do the job with only six parts of impact modifier if the filler is replaced with a nano PCC carbonate at the optimum level. The filler cost in the formulation increases but the impact modifier cost decreases by a much larger amount, resulting in significant savings to the compounder. Tuffgard®, Ultra-Pflex®, and Calofort® S PCCs from SMI are specifically designed to deliver this type of formulation value.

Click on one of the links below to read more about SMI’s calcium carbonates for PVC, to download one of several brochures or papers on our PCCs in PVC, or to download the technical data sheets for Super-Pflex® MS, Super-Pflex® 100, Tuffgard®, Ultra-Pflex® or Calofort® S PCCs for rigid PVC impact strength improvement.

 

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