Polypropylene (PP) has the characteristics of good mechanical properties, non-toxic and harmless, good chemical stability, good heat resistance, easy processing and forming. However, due to the influence of the crystal size of PP, its light transmittance and gloss limit its application in the fields of daily necessities and transparent packaging.
After transparent modification, the transparent PP obtained not only has the performance of traditional PP, but also makes its transparency and luster comparable to polystyrene resin and polycarbonate. Therefore, in order to improve the market value of PP, it is necessary to carry out transparent modification.
Transparent PP development process
At present, the transparent PP development routes that meet the conditions of industrial scale-up production and have commercial production value mainly include three kinds:
(1) Use catalyst to produce transparent PP;
(2) Adding transparent nucleating agent to modify PP;
(3) Blending with other resins to produce transparent PP.
01
Use catalyst to produce transparent PP
Random copolymerization of PP was produced by Z-N catalyst
The process of industrial production of ethylene-propylene random copolymerization PP using Z-N catalyst is as follows:
(1) the propylene and ethylene gas is fully mixed;
(2) Copolymerization monomer and various monomer polymerization chain segments are prepared by catalyst;
(3) PP molecular chain is formed through chain growth and chain transfer, and finally random copolymerization PP is obtained.
The light transmittance of random copolymer PP can exceed 94%, which is basically close to the transparency of transparent polyethylene.
High transparent PP was synthesized by metallocene catalyst
Metallocene catalyst has better anti-reflection effect than Z-N catalyst, and can play a role in controlling crystallinity, molecular weight and copolymer embedding mode in transparent PP synthesis. The PP mixture with high transparency and high strength is produced.
Companies with this technology are: UnivatExxpol/Unipol technology developed by ExxonMobil Corporation, Insite/Spheripol technology developed by Dow Chemical Company, JPC technology developed by Mitsubishi Chemical Company, Mitsui Technology developed by Mitsui Chemical Company, Spheripol and Metocene Technology, Borecene technology developed by Nordic Chemical Company, Univation/ Unipol technology developed by Basel Company, Atofina technology developed by Total Company, BP technology developed by BP company.
02
PP was modified by adding transparent nucleating agent
By adding transparent nucleating agent to PP, the transparency of the product can be effectively improved, not only that, but also improve its rigidity and melt flow rate.
Mechanism of transparent nucleating agent modification
The process of compound formation of crystal structure includes two stages of nucleation and crystal growth. For PP, the growth of crystal nuclei includes two ways: heterophase nucleation and homogeneous nucleation. The homogeneous nucleation method has fewer crystal nuclei, larger crystal size and lower crystallinity. In the process of heterogeneous nucleation, the molecular chain can be adsorbed on the surface of the melt "impurity" to form a crystal nucleus, which has a fast crystallization rate, small crystal size and high crystallinity. The transparent modifier can be considered as a kind of "impurity", as the nucleus core of heterogeneous nucleation, improve the heterogeneous nucleation during the crystallization process of PP, inhibit the homogeneous nucleation of PP, reduce the crystal size, improve the crystallinity, and reduce the scattering and refraction of light on the crystal interface.
Types and applications of transparent nucleating agents
① Inorganic transparent nucleating agent
Inorganic transparent nucleating agent is mainly talc powder, mica and other non-metallic compounds, this transparent nucleating agent is characterized by a wide source of raw materials, low cost, only a small amount of addition can play a role in anti-reflection. However, the disadvantages are also obvious, as inorganic compounds, they are very difficult to disperse in organic melt, and there is a role of light shielding.
② Organic transparent nucleating agent
Sorbitol compounds are the most widely used additives in organic transparent nucleating agents. According to the types of substituent functional groups in the benzene ring of sorbitan compounds, the development of sorbitan nucleating agents has gone through three series.
The first series, sorbitol compounds have no functional group on the benzene ring, its preparation cost is low, but the nucleation effect is poor, and aldehyde compounds will be released during use;
In the second series, chlorine, methyl and other functional groups were used to replace the hydrogen atoms on the benzene ring para-position in the pyritol compounds. This series of sorbitol compounds increased the nucleation efficiency and the transparency of the product, but still could not solve the problem of aldehyde compound release.
The third series, pioneered by Milliken, effectively addresses the issue of aldehydes release.
When the sorbitol nucleating agent produced by Milliken Chemical Company with 0.3% mass fraction is added to PP, the fog degree of PP can be reduced to less than 15%. At present, there are many explanations about the nucleation mechanism of DBS, but no unified view has been formed.
In addition to sorbitan nucleating agents, researchers have also developed a series of organic transparent nucleating agents such as aryl phosphate transparent nucleating agents, carboxylic acid metal salts transparent nucleating agents, dehydropolar acids and their salts transparent nucleating agents, branched acid amines transparent nucleating agents, and polymer transparent nucleating agents.
Method of adding transparent nucleating agent
In the production of transparent PP, there are two ways to add transparent nucleating agents. One is to use mechanical stirring and other ways to evenly disperse the transparent nucleating agent in a container equipped with reaction raw materials to ensure the uniformity of PP product performance. This method is limited by the stirring intensity, which will interfere with the polymerization activity, and the frequency of use is not high. The other is to add the nucleating agent outside the reactor in the process of PP granulation, which is convenient for industrial application.
03
Blend anti-reflection PP
The method of blending to increase the transmittance is to use one or more polymers with similar refractive index to PP and dispersed phase particle size smaller than the wavelength of visible light to blend with PP. By playing the role of heterogeneous nucleation, the crystallization size of PP is reduced and the transmittance of the product is increased.
The results show that low density polyethylene and ethylene propylene diene copolymer are suitable blends, adding 10% blend can reduce the crystal size of PP, improve the crystallinity and increase the transmittances of
PP Products. In addition, the impact performance of the product can also be improved through the action of the blending agent. However, the use of blends to increase light transmittance has obvious limitations. Due to the use of another polymer or more as a mixture, they and PP must ensure similar refractive index and phase interface compatibility, development is very difficult.
