Polypropylene (PP) has the characteristics of good mechanical properties, non-toxicity, good chemical stability, good heat resistance, and easy processing and molding. However, due to the influence of PP crystal size, its light transmittance and glossiness limit its application in daily necessities and transparent packaging.

After transparent modification, the obtained transparent PP not only has the performance of traditional PP, but also makes its transparency and gloss comparable to polystyrene resin and polycarbonate. Therefore, in order to improve the market value of PP, it needs to be transparently modified.

Transparent PP development process

At present, there are three main transparent PP development routes that meet the conditions for industrial scale-up production and have commercial production value: (1) using catalysts to produce transparent PP;

(2) adding transparent nucleating agents to modify PP;

(3) blending with other resins to produce transparent PP.

01 Using catalysts to produce transparent PP

Using Z-N catalysts to produce random copolymer PP

The process of industrial production of ethylene-propylene random copolymer PP using Z-N catalysts is as follows:

(1) fully mixing propylene and ethylene gas;

(2) using catalysts to prepare comonomers and various monomer polymer segments;

(3) forming PP molecular chains through chain growth and chain transfer, and finally obtaining random copolymer PP.

The transmittance of random copolymer PP can exceed 94%, which is basically close to the transparency of transparent polyethylene.

Synthesis of high transparent PP using metallocene catalysts

The transmittance enhancement effect of metallocene catalysts is better than that of Z-N catalysts. In the synthesis of transparent PP, it can play a role in controlling crystallinity, precise molecular weight control, and comonomer embedding mode control. Produce a mixture of syndiotactic and isotactic PP with high transparency and high strength.

Companies with this technology include: UnivatExxpol/Unipol technology developed by ExxonMobil, Insite/Spheripol technology developed by Dow Chemical, JPC technology developed by Mitsubishi Chemical, Mitsui technology, Spheripol and Metocene technology developed by Mitsui Chemicals, Borecene technology developed by Borealis, Univation/Unipol technology developed by Basel, Atofina technology developed by Total, and BP technology developed by British Petroleum.

02 Adding transparent nucleating agent to modify PP

The method of adding transparent nucleating agent to PP can effectively improve the transparency of the product, not only that, but also improve its rigidity and melt flow rate.

Transparent nucleating agent modification mechanism

The process of compound generation of crystal structure includes two stages: nucleation and crystal growth. For PP, its nucleus growth process includes two modes: heterogeneous nucleation and homogeneous nucleation. The homogeneous nucleation method has a small number of nuclei, large crystal size and low crystallinity. In the process of heterogeneous nucleation, the molecular chain can be adsorbed on the surface of the "impurities" of the melt to form a crystal nucleus. Under this condition, the crystallization rate is fast, the crystal size is small, and the crystallinity is high. The transparent modifier can be considered as an "impurity". As the core of the crystal nucleus of heterogeneous nucleation, it improves the heterogeneous nucleation effect in the crystallization process of PP, inhibits the homogeneous nucleation of PP, reduces the crystal size, improves the crystallinity, and reduces the scattering and refractive index of light on the crystal interface.

Types and applications of transparent nucleating agents

① Inorganic transparent nucleating agents

Inorganic transparent nucleating agents are mainly non-metallic compounds such as talcum powder and mica. The characteristics of this transparent nucleating agent are wide sources of raw materials and low cost. Only a small amount of addition is required to play a role in enhancing transparency. However, its disadvantages are also obvious. As inorganic compounds, they are difficult to disperse in organic melts and have the effect of light shielding.

② Organic transparent nucleating agents

Among organic transparent nucleating agents, sorbitol compounds are the most widely used additives. According to the type of substituent functional groups on the benzene ring in sorbitol compounds, the development of sorbitol nucleating agents has gone through three series.

In the first series, there is no functional group on the benzene ring in sorbitol compounds, and its preparation cost is low, but the nucleating effect is poor, and aldehyde compounds will be released during use;

In the second series, chlorine, methyl and other functional groups are used to replace the hydrogen atoms on the para position of the benzene ring in sorbitol compounds. This series of sorbitol compounds increases the nucleation efficiency and product transparency, but still cannot solve the problem of aldehyde compound release;

The third series, first developed by Milliken, effectively solves the problem of aldehyde compound release.

Adding 0.3% of the mass fraction of sorbitol nucleating agents produced by Milliken Chemical Company to PP can reduce the haze of PP to less than 15%. At present, there are many explanations for the nucleation mechanism of DBS in academia, but no unified view has been formed.

In addition to sorbitol nucleating agents, researchers have also developed a series of organic transparent nucleating agents such as aromatic phosphate transparent nucleating agents, carboxylic acid metal salt transparent nucleating agents, dehydropolar acid and its salt transparent nucleating agents, branched acid amine transparent nucleating agents, and polymer transparent nucleating agents.

Methods of adding transparent nucleating agents

When producing transparent PP, there are two ways to add transparent nucleating agents. One is to use mechanical stirring and other methods to evenly disperse the transparent nucleating agent in a container containing the reaction raw materials to ensure the uniformity of the performance of the PP product. This method is limited by the stirring intensity, which will interfere with the polymerization activity and is not used frequently. The other is the addition method outside the reactor, adding the nucleating agent during the PP granulation process, which is convenient for industrial application.

03 Blending to increase transmittance PP

The method of blending to increase transmittance is to use one or more polymers with a refractive index similar to that of PP and a dispersed phase particle size smaller than the wavelength of visible light to blend with PP, and reduce the crystal size of PP by exerting the effect of heterogeneous nucleation, thereby increasing the transmittance of the product.

Studies have shown that low-density polyethylene and ethylene-propylene-diene copolymer are suitable blending agents. Adding 10% blending agent can reduce the size of PP crystals, increase crystallinity, and increase the light transmittance of the product. In addition, the impact performance of the product can also be improved through the effect of the blending agent. However, the method of using blending agents to increase light transmittance has obvious limitations. Since another one or more polymers are used as blending agents, they must ensure similar refractive index and interfacial compatibility with PP, which makes development very difficult.

Application areas of transparent PP

Transparent PP can be produced into products suitable for various fields of daily life through various processing technologies such as injection, thermoforming, and blow molding. The performance and field of use of PP depend not only on the transparency and mechanical properties of PP itself, but also on its processing technology. Table 1 lists the application areas of transparent PP products under different processing technologies.

At present, transparent PP products prepared by blow molding, injection molding, thermoforming and other processing technologies have occupied a certain market share and have a trend of replacing other competing materials. For example, currently, polyethylene terephthalate (PET) is mainly used in beer and beverage bottles in the market. The highly transparent PP produced by AMCO through the "injection stretch blow molding" process has the potential to replace PET.

Development trend of transparent PP

With the development of transparent PP resin products, the market pays more attention to the quality, environmental protection and high performance of products. The Metocene series products developed by Basel based on the metallocene catalytic process have the characteristics of high transparency, high fluidity, heat resistance, high rigidity and easy molding and processing, and are widely used in the fields of fiber and food packaging. The EILexPK176 transparent PP product developed by Solvay Group based on the two processes of blending anti-reflection and transparent nucleating agent has good chemical stability, light stability and impact resistance, and is widely used in single-layer transparent bottles and extruded sheets.