New Plastic Packaging Material PEN

[Abstract] This article reviews the manufacturing methods and properties of polyethylene naphthalate (PEN), and introduces the application fields of PEN, the blending or copolymerization with PET, and the development prospects.

Keywords: PEN; plastic beer bottle; packaging container; film; packaging material

With the continuous development of the national economy, the demand for plastic packaging materials is getting higher and higher, which has led to the rapid development of research and development and production of packaging materials. There are many new types of plastic packaging materials. PEN is one of them. Because its widely used polyethylene terephthalate (PET) has excellent mechanical properties, chemical properties and unique thermal stability, it has attracted people's attention in the world packaging industry and entered the packaging field.

1 PEN manufacturing method

PEN is oxidized from naphthalene compound 2.6-dimethylnaphthalene (2.6-PMN) or 2.6-diisopropylnaphthalene to produce 2.6-naphthalenedicarboxylic acid (2.6-NDCA), 2.6-ND-CA reacts with methanol to produce 2.6-NDC 2.6-naphthalenedicarboxylic acid. Finally, 2.6-NDC is condensed with ethylene glycol to form PEN.

The key to the production and commercialization of PEN is the source and cost of the raw material 2.6-NDCA or 2.6-NDC. PEN first appeared in the 1960s, but it has not entered the practical stage because of the high cost of PEN's raw material 2.6-dimethylnaphthalene (2.6-DMN) or 2.6-NDCA and complicated production process. . After entering the 1990s, a wave of development and application of PEN has been set off worldwide. Many foreign resin companies have invested a lot of manpower, material resources, and financial resources to conduct PEN synthesis and application research and have obtained a number of patents. With the advancement of PEN's raw material technology production, industrialization has been realized, which has made PEN commercialized.

At present, there are two companies in the world that formally produce 2.6-NDC. The United States Amoco Chemical Company, first in July 1995, realized the industrial production of 2.6-NDC, and the production capacity has reached 45,000 t/year. The method adopted by the company is: using o-xylene and butadiene as starting materials, 2.6-DMN is obtained through alkenylation reaction, cyclization reaction, dehydrogenation reaction and isomerization reaction, and then undergoes oxidation reaction. 2.6-ND-CA was obtained and esterified and distilled to obtain 2.6-NDC. Mitsubishi Gas Corporation of Japan used 2-methylnaphtalidation to obtain 2-methyl-6-isobutyl naphthalene, and oxidized to obtain 2.6-NDC. After 1995, the company changed the synthetic route and adopted the same method as Amoco Chemical Co., Ltd. The production capacity has reached 40,000 t/year.

PEN and PET synthesis have similarities, can be divided into direct esterification and transesterification, because the current raw material used in the actual production of PEN is 2.6-NDC, the current industry uses esterification method, that is, 2.6 - NDC and ethylene glycol under the action of the catalyst to produce ethylene naphthalate, and then polycondensation to PEN, which transesterification method at 170 ~ 250 °C, 2.6-NDC and ethylene glycol feed The ratio is 1:1.5~3.5(m/m). Catalysts generally use acetates of metal ions such as palladium, zinc, cobalt, and magnesium. The polycondensation reaction temperature is controlled within the range of 285~293°C. The catalyst is titanium, lanthanide oxide or acetate. The addition amount of catalyst is 0.01%~0.04% (molar to 2.6-NDC. percentage). Because pure PEN will degrade at 300~315°C, it must be modified. Commonly used modified components are CA-C36 dibasic acid and C3-C10 glycol. The modified PEN has a viscosity of 0.4 to 1.0 dL·g-1, a glass transition temperature of not less than 100°C, and can improve the processability while maintaining heat resistance, hot-filling properties, and barrier properties.

2 PEN features

The resin structure of PEN is very similar to that of PET. The benzene ring of PET is replaced by a naphthalene ring, which is PEN. The molecular structure of the two is shown in Figure 1.

Because the naphthalene ring has a larger conjugated structure than the benzene ring, the molecule has high rigidity and a more planar structure. Although the two have similar characteristics, PEN performs better than PET in all aspects. It has the following characteristics: 1) PEN's heat distortion temperature is 30°C higher than PET and can be used for hot filling. 2) The glass transition temperature of PEN is about 40°C higher than that of PET, and the tensile strength, flexural modulus, and bending strength are also high. Therefore, PEN has good dimensional stability, low thermal shrinkage and good long-term heat resistance. 3) PEN is superior to PET in terms of acid and alkali resistance, hydrolysis resistance and general chemical resistance. 4) PEN is a kind of good gas barrier property in all kinds of plastics. The barrier property to oxygen, carbon dioxide and water is 4 times, 5 times, 3.5 times higher than that of PET. 5) Compared with PET, PEN has less adsorption to organic solvents, and has a low freeness and precipitation. 6) PEN crystallinity is lower than PET, easy to make thick-wall transparent bottles. 7) Has good UV resistance. 8) PEN has good hygienic properties, and the US Food and Drug Administration (FDA) has allowed PEN to be used for food packaging.

3 PEN/PET copolymerization or blending

As mentioned above, PEN has excellent characteristics and is an ideal packaging material. However, PEN is expensive, which limits its wide use as a packaging material. Since both PEN and PET are thermoplastic polyesters and their chemical structures are similar, a certain ratio of PEN and PET are melt-blended to form a polymer. Alloys, which can take into account the economics of PET, heat resistance and gas barrier properties of PEN, are currently the focus of PEN application research and development, and are also one of the main ways to bring PEN to market (especially packaging). When 10% to 20% of PEN is blended with PET, the permeability to oxygen and carbon dioxide can be increased by 30% to 50% and 23% to 37%, respectively, and the shielding wavelength of ultraviolet rays can be increased to 380 nm. PEN/PET blends or copolymers have been used for the packaging of edible oils, alcoholic beverages, carbonated soft drinks, and beer.

4 PEN application in the packaging industry

PEN first appeared on the market as a film product, but at present, bottle packaging containers have become PEN's main application market.

4.1 Film

At present, Teijin, ICI, and Shell Chemical Co., Ltd. formally sold PEN films. Teijin is a pioneer in the production of biaxially oriented PEN films in the world. As early as 1989, it successfully developed highly functional PEN films. In 1993, it built a 4,000-ton/year film production line. In addition, world film giants such as Toray and Du Pont have also entered the PEN film production market. PEN can be manufactured using the same equipment as PET. The process is the same as PET film production. PEN film (BOPEN) is produced by melt extrusion-biaxial stretching. BOPEN has excellent heat resistance, gas barrier property, hydrolysis resistance, dimensional stability, etc., and it is easy to obtain an extremely thin film with a thickness of 0.8 μm. By using these characteristics of the film, packaging materials for different purposes can be obtained. Which occupy a larger application market in food packaging, pharmaceutical packaging, fragrant packaging and precision instruments resistant to impulse packaging.

4.2 PEN packaging container

At present bottle containers have become PEN's main market. Due to the excellent characteristics of PEN, PEN bottles blown with PEN perform better than PET bottles. PEN bottle transparent and hot filling temperature up to about 100 °C, good barrier to ultraviolet, oxygen, carbon dioxide, and chemical resistance, used in beverages, beer, cosmetics, baby food packaging, has great practicality and market . Although PEN bottles can be processed by various blow molding methods of PET, the most commonly used is stretch blow molding.

5 Domestic Status and Development Prospects

PEN, a functional polyester with naphthalene as its skeleton, has become a hot spot for many foreign companies to develop with great features. The annual production volume has reached more than 100,000 tons, and it has been widely used in product packaging, construction, electronics, electrical appliances, and aviation. Aerospace, textiles and other fields. In contrast, domestic research and development of PEN is in its infancy. From the synthesis of single 2.6-NDC, PEN performance characterization and application, much work needs to be done. Yizheng Chemical Fiber Co., Ltd., Angang University, Guilin Electronics Research Institute and Beijing Chemical Research Institute have done some research work and made some progress. With the rapid development of other industries such as the packaging industry and the electronic communications industry, the demand for PEN will increase significantly, and China will also become a PEN consumer. Only beer packaging, currently the total domestic consumption of beer in mainland China ranks second in the world, beer packaging uses 35 billion beer bottles (including recycling bottles) and 7 billion cans annually, if there is 10% glass bottled beer by plastic Replacement of bottle packaging, the demand for 4 billion, the market is huge. Therefore, it is very urgent to develop China's own PEN industry. Domestic related enterprises and research institutes should increase human and financial investment to accelerate the development of PEN technology and industrialization, and to welcome the arrival of PEN consumption boom. (Liang Yanshu)