Mineral Technologies Mineral TechnologiesMinteqSpecialty Minerals
CareersContact UsProductsMSDSAnalytical ServicesSitemapSearch
About SMIOur MineralsPaperSpecialty ApplicationsExhibitionsPublicationsUseful Links
 
Impact Strength in Thermoplastics: Functional Fillers and Their Properties

The Role of Fillers in Thermoplastics
Fillers are used for a wide variety of reasons. They can extend resin, increase stiffness and strength, improve impact performance, and shorten cycle times. They prevent hang-up in dies and neutralize the products of degradation. Fillers can also be used to add color, opacity, and conductivity to a compound. Unique property combinations can be achieved through the use of fillers.

Traditionally a filler was a low-cost material of relatively large particle size that lowered a formulation’s cost simply because it was less expensive than the other ingredients in the formulation. Today a “filler” can be a true performance additive. Advances in compounding technology allow the use of much finer fillers that could not be used in the past. Today’s filler products are tailored for specific applications and designed to deliver value in new and interesting ways.

Types of Fillers Used in PVC and Other Thermoplastic Polymers
Approximately 80 percent of the filler used in PVC in the U.S. is calcium carbonate. Titanium dioxide is second at around 12 percent, followed by calcined clay at about 5 percent. The remaining few percent is taken up by other materials including glass and talc.

Calcium carbonate products are available in a wide range of sizes. They are produced by grinding limestone and by precipitation. The precipitation process can produce true nanoparticles of calcium carbonate (defined as less than 100 nanometers or 0.1 micron) whereas the grinding process is typically limited to an average particle size of around 1 micron. Titanium dioxide is used as a white pigment and UV stabilizer. Calcined clay goes into wire and cable formulations where it improves electrical properties. The remaining fillers find their role in a variety of specialty applications.


Primary Filler Properties: Particle Size and Shape
Mineral fillers tend to be described by their properties which influence the filler’s performance in a resin system. Normally the first two considered are pigment particle size and shape.

  • Particle Size - Most fillers are grouped and ranked by their particle size. There are a number of different ways to measure and report particle size. When comparing two fillers, one must make sure that the comparisons are made using equivalent measurement techniques. Even small differences can be significant, especially where fine fillers are involved.

The term “particle size” is itself misleading. Even a small sample of a filler will contain many particles of different sizes. What we are actually dealing with is a particle size distribution. Most data sheets give the average size or the midpoint (median) value in their product’s size distribution.

“Top size” is another term used to describe a filler’s particle size. It is a carry-over from the manufacture of coarser, screened stone, where the “top size” of the product was the finest screen that all the material would pass through (that is, the one on the top of the stack of screens). Its definition and applicability become less clear when dealing with fine fillers. Classifiers do not produce perfect top cuts and fine fillers tend to stick together causing agglomerates that act like coarser particles. It is technically more appropriate to speak of a “95 percent finer than” or “99 percent finer than” size.

  • Particle Shape - Filler particles come in a variety of shapes as well as sizes: spheres, rods, platelets, and irregular shapes of varying proportions. Here are scanning electron micrographs of three of Specialty Minerals Inc.’s (SMI’s) functional filler products—ground calcium carbonate (GCC), two precipitated calcium carbonates (PCCs), and talc.

One sub-feature of shape that has a significant influence on a composite’s physical properties is aspect ratio. The aspect ratio of a filler particle is the ratio between the particle’s largest and smallest dimensions. 

In the case of a rod, it would be length divided by diameter. In the case of a talc platelet, it would be length divided by thickness. A sphere would have an aspect ratio of 1:1, while a platelet or fiber can be 20:1. Shape plays an important role in defining a filler’s reinforcing characteristics.


Some Additional Filler Properties
Other filler or extender pigment properties that influence performance include:

  • Hardness – Abrasion is a function of hardness and particle size. This relationship to particle size is not linear. Coarser particles are much more abrasive to plastics processing equipment than finer ones. The contribution of impurities must also be considered.
  • Color – A filler can contribute to the color of a compound. If color is important, the pigment package must be adjusted to compensate for the effect of the filler. The dry color of the filler can be misleading. A bright white filler may produce a gray color in the polymer. The only way to know how a filler will affect color is to test it in the compounded formulation.
  • Specific Gravity – The specific gravity of the filler must be taken into account when calculating the cost of a compound because the parts produced are sold on the basis of volume, not weight. Most mineral fillers have a relatively high specific gravity and will therefore raise the specific gravity of the compounds they are used in. As a result, it will take more pounds of highly filled compound to make the finished part.
  • Surface Treatment – Most of the fillers sold to PVC applications are surface treated. This treatment is usually a fatty acid, such as stearic acid. A coating can improve the dispersion of the filler particles during melt compounding, reduce the adsorption of other formulation ingredients onto the filler’s surface, improve the filler’s dry flow properties, and change its processing characteristics.

 

More on Impact Strength Fundamentals
This section of the SMI web site contains a number of pages that describe the basics of impact strength in polymers. Click on one of these topics to read more:

 

Learn more: