An Overview of Transparent Modified PP

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As the production volume of polypropylene continues to grow rapidly, its properties are also constantly being improved, which in turn changes the scope and extent of its applications. In recent years, through improvements during the polymerization process or by taking measures during granulation after polymerization, new varieties of polypropylene with unique properties have been developed, such as transparent polypropylene and polypropylene with high melt strength. The crystallization of transparent modified PP is the main cause of opacity. By using rapid cooling to suppress the crystallization tendency of PP, transparent films can be obtained. However, in products with a certain wall thickness, heat conduction takes time, so the core layer cannot be cooled and frozen quickly; therefore, rapid cooling cannot be relied on to improve the transparency of products of this thickness. It is necessary to focus on the crystallization patterns of PP and the factors that influence them. Modified PP obtained through certain technical methods can possess excellent transparency and surface gloss, and can even rival typical transparent plastics such as PET, PVC, PS, etc. What makes transparent PP more advantageous is its high heat distortion temperature, which is generally above 110°C; in some cases it can even reach 135°C, whereas the heat distortion temperatures of the three aforementioned transparent plastics are all below 90°C. Due to the obvious performance advantages of transparent PP, it has seen rapid development worldwide in recent years. It is widely used in a wide range of applications, from household items to medical devices, and from packaging materials to heat-resistant containers (for use in microwave ovens). The transparency of PP can be improved through the following three methods: (1) Polymerizing PP with transparency using metallocene catalysts ； (2) Obtaining transparent PP through random copolymerization ； (3) Transparent modifiers (mainly nucleators) are added to ordinary polypropylene to improve its transparency. Domestic and international development trends: According to Nippon Rika Co., Ltd., 7% of PP in Japan is transparent PP, with the production volume of this type of PP exceeding 400 kt/year. In Japan’s transparent PP market, the largest consumption is seen in microwave oven appliances and furniture. Nippon Degussa Chemical Company has developed transparent PP that possesses the same transparency and luster as PVC. It can now be used extensively as a substitute for conventional transparent PVC in the production of packaging materials for stationery and notebooks, at a cost of only 20%-30% of that of PVC. In 1999, 1,200 tons of this transparent PP were sold. South Korea’s LG Caitex has introduced transparent PP as a substitute for PET, using it in water bottles, detergent bottles, and packaging for personal care products. Fina Company’s marketing department claims that their new transparent PP product will compete with PS food packaging, which has a market capacity of 300 kt/a. BASF’s PP random copolymer Novolen3248 TC features high fluidity (melt flow rate of 48 g/10 min), low warping, a transparency of 90%, and a haze level of 10%; it is suitable for use in thin-walled packaging and daily household items. The PP random copolymer EltexPKLl76 developed by Solvay contains ethylene and a clarity agent; it is primarily used for manufacturing single-layer transparent bottles and extruded sheets, which can be thermally pressed into various containers and decorations. Its products feature a glass-like luster, excellent chemical stability, resistance to environmental stress cracking, and impact strength. German companies Schneioler and Klein use transparent polypropylene instead of PVC for transparent rigid packaging. Water bottles made from polypropylene resin produced by the American company Amoco using transparent modifiers, and processed through injection, stretching, and blowing techniques, can serve as alternatives to polyester water bottles. Montell Polyolefins has recently introduced α-olefin-modified PP resins, with grades 273RCXP and 276RCXP, primarily for injection molding. Neither of the two grades of resin contains nucleators or transparency enhancers; the melt flow rate of the 273RCXP resin is 14 g/10 min, and it exhibits low odor and good resistance to stress whitening. The light-transmitting properties of this resin are on par with those of the best PP random copolymers; it possesses a high level of gloss, and can be formulated into masterbatch form for use in producing solid materials or semi-transparent masterbatches similar to those made from nylon. The melt flow rate of 276RCXP resin is 16 g/10 min; its light transmittance and gloss are slightly lower, but it exhibits excellent low-temperature impact resistance. After storage at low temperatures, it can withstand repeated heating and impacts, making it suitable for use in containers for microwave ovens. The resin with grade 721RCW has a melt index of 10 g/10 min; it is primarily used for extrusion blow molding or casting. This resin features excellent transparency and gloss, low haze, a wide heat-adhesion range, and a sealing temperature of 118–120°C. This resin is used in single-layer films or as an adhesive layer in co-extruded structures. In 1985, Japan’s Idemitsu Petrochemical Co., Ltd. developed transparent PP sheets using processing technology. This technology improves the transparency of PP sheets by extruding PP resin in its molten state and then enabling rapid cooling for crystallization, as well as by improving heat treatment techniques and utilizing Idemitsu Corporation’s crystallization control and high-temperature surface treatment technologies. This technology has been granted a patent. The development and application of transparent PP in China are relatively lagging behind, but its progress is very rapid. According to preliminary research, transparent PP is currently widely used in films, sheets, plastic cups, microwave ovens, and other injection-molded products in China. Manufacturers that use transparent PP are mainly concentrated in cities along the southeast coast. In 1996, China’s demand for transparent PP was 5 kt, with all of it being imported. By 2000, the market demand was around 100 kt per year; as new application areas were explored, domestic demand rose to 200–300 kt per year by 2005. The continuous increase in demand for transparent PP has stimulated the enthusiasm of domestic PP manufacturers for innovation. The Research Institute of Yangzi Petrochemical Co., Ltd. has carried out research and development, as well as market promotion, on the technology for producing transparent PP using PPF401 and PP grades similar to it as base materials, along with DBS series nucleators; good progress has been made, and the related products have already entered the market. In addition, based on the ordinary PP produced by our company, by adding an appropriate amount of transparentizing agents and other related additives, optimizing the formula design, and adjusting the processing techniques, a specialized transparent PP material named PPJ301G has been developed for use in industrial facilities. This material retains the advantages of ordinary PP, such as light weight, high temperature resistance, and ease of molding; its transparency and surface gloss can compare with those of other transparent polymer materials. Moreover, its mechanical properties, such as heat deformation temperature and flexural elastic modulus, have also been significantly improved. In 2001, Yangzi Petrochemical developed two types of transparent specialty materials: PIYF680 and PFF700. The Research Institute of Luoyang Petrochemical Complex used homopolypropylene PPF401 as the base resin, and by adding transparentizers serta self-made masterbatches A and B, it developed a material specifically designed for transparent PP sheets. Experiments show that the material produced using twin-screw extrusion followed by the addition of a transparentizer exhibits good properties; the transparentizer significantly improves the transparency and gloss of this special material, while the addition of special masterbatches enhances its impact resistance and aging resistance. The polypropylene with a bimodal molecular weight distribution produced using the new catalyst system developed by Sinopec features high rigidity, high transparency, and good thermal stability; its flexural modulus can increase by 45%, rising from 1500 MPa to 2200 MPa. This transparent PP can be used to produce food containers for use at high temperatures, as well as various products that require good gloss and stiffness, such as detergent bottles and mineral water bottles. In addition, random copolymer products with high melt flow rates have been developed; they feature high fluidity, high transparency, high gloss, and antistatic properties, making them particularly suitable for injection-molded containers and thin-walled packaging where high transparency is required. The principle of improving transparency through the addition of nucleators involves incorporating nucleators into the already polymerized polypropylene to alter its crystallization behavior, thereby enhancing its transparency – this is currently the most commonly used method. As PP gradually cools from its molten state, its crystallization behavior can be divided into two types: homogeneous nucleation and heterogeneous nucleation. Homogeneous nucleation: Relying solely on the autonomous movement of PP macromolecular chains, at temperatures within a certain range, a certain region first forms a crystalline core, which then gradually expands to become an orderly arranged crystalline region. Heterophase nucleation: The polymer chains of PP arrange themselves in an orderly manner by attaching to substances other than PP. These substances can be catalysts remaining in the polymer or impurities already present in the propylene monomer, or they can be organic or inorganic substances added intentionally. A transparent modifier – a nucleating agent – is added to PP to act as nuclei; this reduces homogeneous nucleation and enhances heterogeneous nucleation during the cooling process of PP polymers. As the number of nuclei increases, more microcrystals are formed in PP, and the size of these crystals becomes smaller, which helps to improve its light transmittance. The added nucleators can be divided into two main categories: non-melting transparent modifiers and melting transparent modifiers. The former includes substances such as talcum powder, sodium benzoate, and organophosphates, while the latter includes compounds like sorbitol condensates. These can be melted in advance and form a homogeneous mixture with the molten PP; as the system cools, the transparent modifiers crystallize first to form a fibrous network, with fiber diameters of around 10 nm, which is smaller than the wavelength of visible light. The macromolecules of PP gradually crystallize around these network fibers, thereby yielding fine crystals. The properties of transparent PP: compared to the raw PP, after being modified for transparency, its haze decreases while its gloss increases; its stiffness and Vicat softening point also increase, but its tensile strength decreases, with the other properties remaining unchanged. See table for performance details:
Material type: Ordinary polypropylene, Transparent polypropylene
Humidity/%: 56.6, 10.1
Gloss/%: 99.6, 129.2
Melt flow rate/(g/10min): 2.8, 3.0
Crystallization temperature/°C: 118.9, 212.86
Vicat softening temperature/°C: 140.0, 156.9
Rockwell hardness/R: 95, 107
Tensile strength/MPa: 35.0, 8.9
Flexural modulus/GPa: 1.49, 1.50
Flexural strength/MPa: 38.8, 39.8
Notch impact strength/(J/m): 24.0, 28.0
According to other reports, PP made by adding 0.25% of a certain nucleating agent and 0.4% of an organic amine can achieve a light transmittance of 71.9%; its tensile strength, flexural modulus, and impact strength all increase by more than 20%, the elongation at break can reach 300%, and the Vicat softening temperature also increases. The properties of the specialized transparent PP material developed by units such as the Resin Research Institute of Daqing Petrochemical Research Institute, using homopolymer polypropylene as the base resin and with nucleators added, are shown in the table. Performance of transparent PP TX30C: According to other reports, PP made by adding 0.25% of a certain nucleating agent and 0.4% of an organic amine can achieve a light transmittance of 71.9%. Its tensile strength, tensile modulus, and impact strength all increase by more than 20%, the elongation at break can reach 300%, and the Vicat softening temperature also increases. The properties of the specialized transparent PP material developed by units such as the Resin Research Institute of Daqing Petrochemical Research Institute, using homopolymer polypropylene as the base resin and with nucleators added, are shown in the table. Performance of transparent PP TX30C: Parameter, Design value, Measured value. MFR/g·(10min) -1, Tensile yield strength/MPa, Fog density/%, Light transmittance/%, Isotacticity/%, Cleanliness/units·kg-1: 8~11, ≥28.0, ≤15.0, ≥85.0, ≥94.0, ≤20, 10.3, 3, 6.09, 9.39, 0.49, 7.82, 15