Fundamentals of Flexural Modulus or Stiffness in Reinforced Plastics
In many applications for reinforced plastics, especially reinforced polyolefins such as polypropylene and thermoplastic olefins, the product must not only be strong, but also stiff or rigid to perform as designed. This stiffness is known as the flexural modulus of the plastic and is expressed in Pascals (Pa) or as pounds per square inch (psi), or as kg/cm2.
Plastic stiffness begins with plain resin without additives, or the nest resin. The neat resin stiffness is a function of the polymer type and molecular weight, as well as the thickness and shape of the plastic part. Most polypropylenes and thermoplastic olefins (TPOs) used for components in automobiles and appliances are not stiff enough as a neat resin, to meet the requirements of high-performance parts.
Filler Loading and Flexural Modulus or Stiffness
Adding a fine mineral will increase the stiffness or flexural modulus of a polymer system. Generally, the more mineral used, or the higher the filler loading, the greater the increase in flexural modulus. The figure below demonstrates this with a 1 micron average particle size talc product in a typical polypropylene copolymer resin.
Aspect Ratio and Flexural Modulus or Stiffness
In choosing a mineral to increase flexural modulus, there are two important factors to consider: a mineral's aspect ratio and its particle size. The aspect ratio of a particle is defined as the greatest length of the particle divided by its thickness.
Particles that are essentially spherical, such as those of ground calcium carbonate (GCC), have equal lengths and thicknesses and have aspect ratios of 1:1. Talc is a long, thin, platy mineral, as can be seen in the scanning electron micrograph below. For talc, the aspect ratio is high, typically about 20:1.
Minerals with aspect ratios of 1:1 will increase stiffness, but those with higher aspect ratios increase it even more. With its high aspect ratio, talc is one of the most efficient minerals for improving flexural modulus.
In addition to increasing flexural modulus, high-aspect ratio fillers such as talc will also increase the tensile strength, flexural strength, and heat deflection temperature of the polymer and improve the dimensional stability of the part, while decreasing creep, mold shrinkage, coefficient of thermal expansion, and part warpage.
Particle Size and Flexural Modulus
The particle size of the product is the second factor to consider when choosing the mineral filler to improve polymer stiffness. Normally, the aspect ratio of a mineral does not change as it is more finely ground. But with special talc milling technology, such as that employed by SMI to produce its fine and ultrafine talc grades, the aspect ratio will actually increase because the particle's thickness is reduced faster than its length. Accordingly, the smaller particle SMI talc products will give greater increases in stiffness than expected based on particle size change alone. This graph shows the effect of reducing the talc particle size from 10 microns to 1 micron, in the polypropylene copolymer, with a 30 percent talc loading.
Once the talc particle size is reduced to about 3 microns, the aspect ratio then stays fairly constant with further particle size reduction, so there is only a relatively small increase in flexural modulus from using a smaller talc as can be seen in this second graph above.
But there is another excellent reason to choose a very small, ultrafine particle-sized talc: impact strength. As the loading of talc is increased, the impact strength of the plastic is reduced, being lower than that of the original neat or unfilled resin. This example uses a 3 micron talc product, in the polypropylene copolymer, with loadings up to 30 percent. The significant drop in impact strength is readily apparent at higher loading as seen in the following graph.
However, when the 3 micron particle talc product is replaced by a 1 micron particle talc product, several things happen. There is a small increase in the flexural modulus and the impact performance is improved (see following graph).
Adding 30 percent talc did not have a great effect on the Dynatup impact strength numbers at room temperature, but with the 3 micron talc product all the failures were brittle, while with the smaller talc they remained mostly ductile. Also, with cold testing, at -30° C, the 1 micron talc product retained more impact strength. When impact was measured with the notched Izod test, the smaller talc again kept more impact strength.
The results of replacing a 1.8 micron talc product with a 0.9 micron talc product are presented in the following table. Going from a 1.8 micron talc product (MicroTuff® AG191) to a 0.9 micron talc product (MicroTuff AG609) also had a positive effect. Similar patterns are seen in a thermoplastic olefin (TPO) resin, typical of those used in automotive and appliance applications. There is an increase in flexural modulus when the talc loading is increased from 10 to 20 percent, but relatively little change in the modulus when reducing particle size. However, there are positive effects on the Notched Izod impact strengths both in increasing the loading and reducing the talc particle size.