additives feature | Nucleation and reinforcement
FIGURE 2. CROSSHATCHED PP QUADRITE SHOWING THE SWITCHING OF THE A AND C AXES
Minimal and Isotropic Shrinkage
High Shrinkage
FIGURE 3. ISOTROPIC SHRINKAGE PROPERTIES OF HYPERFORM HPN-20E
high-performance nucleating agents for PP: Hyperform HPN-68L, Hyperform HPN-20E and Hyperform 600ei, respectively. While effectively increasing the crystalliza- tion temperature of PP and the accompanying physical properties that result from nucleation, these three nucleating agents were the first commercial nucleating agents for PP that directed balanced crystalline orientation in the MD/TD plane of processed PP parts. This balanced orientation has proven to give excellent dimensional stability and reduced warpage/shrinkage issues in a number of applications.
The importance of orientation While a full description of PP nucleation and crystalline orientation is beyond the scope of this paper, further information can be found in the 10 references listed at the end of the article. In general, PP does not crystallize into cubes, rectangles, or other single crystal objects like small molecules. In the absence of nucleators, the crystalline objects found in PP articles are spherical in shape, and consist of strands of crystalline lamellae growing from a central core (Figure 1). While each crystalline strand has a very definite crystallography and specific orientation, the net
FIGURE 5. OPTICAL PERFORMANCE OF 600EI VS PHOSPHATE ESTER SALT IN 2MFR HPP
FIGURE 4. SPIDER CHART OF HPN-20E IN INJECTION MOULDED HPP (25 MFR)
orientation is zero due to the spherical nature of the crystallite. If one lamellar strand from a spherulite could be magnified, it would look something like Figure 2. It is known that the B-axis of the PP crystal is the
highest shrinkage direction, while A and C are minimal. Therefore, the direction of the crystallography of PP by nucleation can drastically affect the shrinkage and physical property performance of PP parts after fabrication.
Orientation effect of HPN-20E Hyperform HPN-20E particles have a high aspect ratio, and orient very strongly in polymer melts during processing. This particle orientation, followed by directional nucleation, induces a strong crystalline orientation of PP lamellae through the thickness direction (ND) of PP parts such that the shrinkage in MD/TD is minimised (Figure 3). The net effect is a remarkable balance of shrinkage and physical properties. Figure 4 shows a spider chart in an injection moulded 25 MFR homopolymer. This improvement in properties is a direct result of not
34 COMPOUNDING WORLD | April 2012
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