Improving BOPP Special Materials for 5 Major Applications


By Zhang Liyang, Chen Shangtao, Research Institute of Petroleum Processing, Zhao Zhiqing,
China National Petroleum Overseas Audit Center


BOPP films are important products made of PP. Making BOPP films consumes around 20% of all the PP used in China. Today, almost all continuous-process PP units are used to make BOPP special materials. But the products are mostly low-end varieties, with few high quality, high grade varieties. The performance of special materials is a key factor influencing the quality of BOPP films.
BOPP films have great tensile strength, good air blocking behavior, good printability and good tear resistance, and are therefore extensively used in food packaging, medical packaging, industrial packaging, tobacco packaging, electrical films and lithium battery separator material.
Here we discuss the development prospects for five high-end application areas.

1. High-speed film

An important index in the production of BOPP films is the rate of extension. Rapid production speed and wide breadth can effectively reduce the production cost. In the years around 2 000 special materials produced in China were mostly used in medium and low-speed films. In July 2002 Shanghai Petrochemical Co., Ltd. developed a new product called F280S. Quite a few polyolefin producers in Sinopec and PetroChina followed suit and also developed high-speed BOPP special materials. Examples included L5D98 in Guangxi Petrochemical Co., Ltd. and Fushun Petrochemical Co., Ltd., YF3003 in Yangzi Petrochemical Co., Ltd. and PPH-F03D in Zhongyuan Petrochemical Co., Ltd. In 2005 Shanghai Petrochemical Co., Ltd. made major adjustments to the F280S process, both improving product performance and achieving super high-speed production.
Major imported brands of high-speed BOPP special materials include S28C from Basell, HP425J from Daelim of Korea, FS3011 from TPC of Singapore and KS409 from Solvay of Belgium.

2. Cigarette film  

All cigarette films today must satisfy high requirements for blocking, gloss, absence of haze, antistatic performance and heat sealing performance. As BOPP films alone can hardly be given such numerous properties, complex multi-layer structures are used.
BOPP special resins for cigarette films are classified into homopolymers and copolymers. Homopolymers are used mainly in the core layers of complex three-layer cigarette films. The core layer accounts for 80% of the total cigarette film mass. According to the specific requirements of different cigarette makers, upper surface layers and lower surface layers of three-layer complex cigarette films can be anti-counterfeiting information layers, printing layers or heat sealing layers. Special materials used in these layers are usually biopolymers, terpolymers, olefin polymers and other polymers of PP.
Among biopolymers, compared with propylene-ethylene atactic copolymer the outstanding feature of propylene-butene atactic copolymer is that its transparency, rigidity and surface non-adhesion are more satisfactorily balanced.
Propylene-ethylene-butene terpolymer is a sound resin for heat-sealing layers. Some companies also use propylene-higher olefin (1-hexene or 1-octylene) atactic copolymer to achieve satisfactory heat sealing performance. A new approach is to use PVDC-coated cigarette film to replace BOPP cigarette film. In such film 2-3g/m2 PVDC latex (the solid content being 50%) is coated on both the front side and the back side of conventional BOPP film. In this way the heat sealing performance of the original BOPP film is greatly improved. The new film has excellent low-temperature heat sealing performance.

3. Lithium battery separators

The structure of lithium battery separators and their physiochemical properties play an important role in overall cell performance. The main separator materials in domestic and overseas markets are wet-process PE diaphragms, dry-process PP diaphragms and coated diaphragms with diaphragms or nonwoven fabrics as matrixes. Production processes for dry-process lithium cell diaphragm materials can be classified by production process as uniaxially-oriented extension and biaxially-oriented extension. Production of dry-process lithium battery separators in China usually uses the single-layer biaxially-oriented extension method. BOPP special battery separator materials are the main materials used.
Petrochemical enterprises in China have researched and produced special materials for lithium battery separators for nearly a decade. Nevertheless, further improvements are needed the micro-pore uniformity, physical strength and closed-pore characteristics of separators produced in China today with domestic materials.

4. Capacitor film  

BOPP film is the core medium of capacitors, enabling excellent electric behavior, good reliability, high-temperature resistance and a wide range of capacitance to choose from. It is therefore the most important raw material for film capacitors. China started making capacitor films in the 1980s. A competition pattern of 12 enterprises with over 30 production lines has already been formed today. Both product quality and R&D capabilities have been improved drastically. Products can satisfy the high-end technical and quality requirements of domestic markets, and great quantities are exported.
Special capacitor film materials are usually imported. After success with special materials for lithium battery separators, Shanghai Petrochemical Co., Ltd. and Zhongyuan Petrochemical Co., Ltd. have accelerated R&D in recent years for new polyolefin products such as special BOPP capacitor film materials and have made some breakthroughs.

5. Aluminum-plated film

Aluminum-plated film is a complex flexible packaging material whose plastic film surface is plated with an extremely thin metal aluminum by unique process. It has special features of both plastic film and metal, so it is a very practical packaging material with low price, nice appearance and excellent performance.
Aluminum-plated films on the market today typically have a five-layer complex structure including surface layer A, sandwich layer 1, the core layer, sandwich layer 2 and surface layer B. Surface layers are usually made of PP biopolymer, PP terpolymer or another PP polymer. Core and sandwich layers are all made of PP homopolymer. One side of each surface layer is a heat sealing layer and the other side is an aluminum-plated layer.
Due to the poor fastness of aluminum plating on BOPP films, aluminum-plated BOPP films have a low market share in the aluminum-plated film market compared with other materials. Aluminum-plated BOPP films remain irreplaceable however, owing to their low cost and excellent blocking behavior, the extensive application of BOPP films in general and progress in printing technology.

Conclusion

Today, BOPP enterprises in China emphasize R&D and production of high-end functional films such as electrical films with a high dielectric constant, cigarette films with high gloss, aluminum-plated films and lithium cell diaphragms, aiming to optimize industrial structure and enhance competitiveness.