Text|Mate Polymer Winlong
Plastic products made of PP polypropylene, PE polyethylene, HIPS, ABS and other polymer alloys and filled polymers are often subjected to external stresses such as stretching, bending and impact during use. Whitening may occur. If heat treatment (such as heating above glass transition temperature, etc.) or high-pressure stretching is performed on a portion where stress whitening already exists, portion of whitening may partially or completely disappear.
There are approximately two definitions of stress bleaching: first definition refers to local bleaching of polymer materials under stress. Irregularities, etc. The latter definition assumes that polymer material forms a large number of crazes under stress, and refractive index in region of crazes decreases and becomes silvery white. The latter definition equates phenomenon of stress whitening with phenomenon of crazing, or that cracking causes stress whitening. To best of current research, first definition is more general. For this, we can refer to first definition to improve stress bleaching of polymers.
Why are TPVs with high hardness susceptible to stress bleaching? The main reason is that content of polypropylene in high hardness TPV is relatively high. For example, in TPV 90A, content of PP reaches about 35% of total. As content of polypropylene increases further, phenomenon of stress whitening becomes more evident.
So, how to improve high hardness TPV stress whitening? First, it is unrealistic to reduce hardness and reduce amount of PP, because such a high hardness is really required. According to research of some scientists and precursors of modified plastics, stress bleaching of high hardness TPV can usually be improved by following methods:
1. The use of homopolymer polypropylene is better than bleaching under influence of copolymerization and high-impact polypropylene, whitening of high-impact polypropylene under stress is especially obvious;
2. Add certain polyolefin elastomers such as POE, POP, etc.;
3. If it is a black product, consider increasing use of black masterbatch (carbon black);
4. The addition of some nucleating agents may improve stress whitening.
5. Add low crystallinity polypropylene homopolymer.
The use of Asahi Kasei low crystalline polypropylene can significantly improve stress whitening (SW)
However, methods 1, 3, and 4 cannot significantly improve stress whitening; Methods 2 and 5 can greatly improve, but result in a significant reduction in TPV, which must also be carefully considered.
Meteor Polymer has specially released TPV 1060DB-UWE folding tape in white, which is mainly used for sealing edges of car shelves and does not leave white pinhole marks when sewing on edge seals. If you are interested, please leave a message or private message!
Recommended Reading: Key Features and Influencing Factors of TPV(1)