Polypropylene, due to its relatively rigid molecular chain, has a glass transition temperature of only about 0°C. Therefore, measures to improve low-temperature toughness are usually required in winter. In this article, we will introduce some effective methods to improve the low temperature toughness of transparent PP products.
1.How to choose polypropylene base material?
Homopolymer polypropylene and random copolymer polypropylene are the main raw materials used in the preparation of transparent polypropylene products (T30S, L5E89, V30G, Z30S, etc.), of which random copolymer polypropylene is generated by the co-polymerization of propylene and a small amount of ethylene, the random copolymerization of the two monomers will destroy the polypropylene crystallization and reduce the crystallinity.
At the same time, because ethylene has better low-temperature toughness, these two factors make the random copolymer polypropylene has better low-temperature toughness than the former. Therefore, the blending of these two polypropylene materials can improve the low-temperature toughness to a certain extent and improve the drop resistance of the products.
However, since the low-temperature toughness of random copolymerization itself is only slightly higher than that of homopolymerized polypropylene, the improvement in low-temperature toughness of the blends is limited. At relatively low temperatures, special tougheners are still needed to improve low temperature toughness. At the same time, the molecular structure of the homopolymer itself also affects its toughness. The main influencing factors include isotropic index and melt index. Generally, PP with higher isogauge index and melt index is less tough.
Therefore, the same T30S, produced by different manufacturers there are differences in toughness, mainly because of the difference in the isotonicity index of T30S from different manufacturers. High melt index homopolymer polypropylene (such as Z30S) is less tough than low melt index (T30S).
2.Use of suitable toughening agents
There are many kinds of toughening agents that can be used for polypropylene, but most of them ( PE, POE, EPDM and SBS rubber, etc.) will reduce the transparency and gloss of polypropylene products, and only a relatively small number of toughening agents have less impact.
The reason for all this is that the toughening agent is mainly dispersed in the form of particles in the part, and the refractive index of both is different from that of the transparent PP, which causes light refraction and light scattering at the interface of the incident light at the interface between the toughening agent and the PP substrate, thus reducing the light transmission.
Due to the differences in the chemical structure of different types of tougheners, including their own glass transition temperature, their compatibility with PP and the ease of their dispersion in the PP system are different, so the toughening effect of different tougheners also differs to some extent.
Like LLDPE toughening effect will be poor, usually need a relatively high addition amount to achieve satisfactory toughening effect, and at the same time has a great impact on the transparency of the product.
And POE toughening agent itself and PP has a certain compatibility, while its own glass transition temperature is as low as -60 ℃, so it has a very good toughening effect, but POE usually affects the transparency.
3.Optimization of product design
Important factors affecting polypropylene products also include product design, uneven product design thickness variations, edges, rolled edges, reinforcement, etc. All of these factors can produce relatively large internal stresses during the production and deformation of the product, resulting in lower toughness and reduced drop resistance.
Plastic products internal stress distribution chart
4.Optimization of the molding process
Even if the material formulation is the same, differences in theproduct molding processwill also affect the residual internal stress in the PP product, thus affecting the drop resistance of the product.
The higher the internal stress, the worse the drop resistance of the product. The generation of internal stress is directly related to the product and the molding process.