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Impact Strength Fundamentals: Particle Size Effect and Impact / Stiffness Balance

Particle Size and Impact Strength
The size of the filler used can have a dramatic effect on the impact strength of a PVC compound. The graph below shows the relationship between impact strength and filler particle size for calcium carbonate in rigid PVC.

The relationship is not linear. When the carbonate size is larger than 1 micron, the impact strength is relatively low. However, as the carbonate size is reduced into the submicron range, the impact strength of the compound increases dramatically.

The low impact strength imparted by particles larger than 1 micron is a function of points of weakness in the polymer matrix. Likewise, the submicron particles impart enhanced impact strength because they are points where stresses can be relieved in the polymer matrix.

Particle Size Distribution and Impact Strength: Precipitated vs. Ground Calcium Carbonates
Considering the median or average particle size alone is not enough to ensure high impact strength. The impact strength of PVC or other polymer is limited by the size of the largest particles present. Two fillers with equal median particle sizes, but different particle size distributions, will confer different impact strengths.

An advantage of precipitated calcium carbonates (PCCs) over ground calcium carbonates (GCCs) is the narrower particle size distribution of the PCCs. The particle size distributions of a PCC and a GCC of the same median particle size are compared in this plot:


The precipitated calcium carbonate has fewer large particles than the ground, and the size of the largest particles is smaller for the PCC than for the GCC. This can be seen in scanning electron micrographs. Here are a PCC and a GCC, each with a median particle size of 0.7 microns:



Specialty Minerals Inc.’s (SMI’s) submicron PCC fillers such as Tuffgard®, Calofort® S, and Ultra-Pflex® PCCs are particularly effective in building impact strength in rigid PVC because of their steep particle distribution curves. The PCCs do not have the large particles found in the GCCs, reducing stress concentrator points in the polymer matrix.


Impact/Stiffness Balance
In theory, one can get the best of both worlds with a high-aspect ratio filler of very small particle size because impact strength is a function of particle size and stiffness is a function of aspect ratio. Unfortunately, it is very difficult to produce a filler with both properties because the critical size is the largest dimension of the filler particle and a very small particle, in order to have a high aspect ratio, would also have to be extremely thin, thus becoming quite fragile. Still, some interesting property balances can be achieved by using the finest platy fillers available—such as SMI’s ultrafine talcs, notably UltraTalc® 609 or FlexTalc® 610—with increased levels of impact modifier. Click here to download a SMI publication on the use of these ultrafine talcs in formulating a strong and stiff rigid PVC.

With this approach, it is possible to produce a rigid PVC with high performance, enabling it to replace acrylonitrile-butadiene-styrene (ABS) in many applications. Click on the link below to read more.


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:

 

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