What is PET?

Polyethylene terephthalate Chemical formula -OCH2-CH2OCOC6H4CO-

English name: Polyethylene terepthalate/Poly(ethylene terephthalate)

PET is a milky white or light yellow, highly crystalline polymer with a smooth, shiny surface. Excellent physical and mechanical properties in a wide temperature range, long-term use temperature up to 120 ° C, excellent electrical insulation, even at high temperature and high frequency, its electrical performance is still good, but poor corona resistance, anti-corrosion Creep resistance, fatigue resistance, abrasion resistance, and dimensional stability are all good.

Study on the heat and humidity resistance of PET

1.PET wet heat aging mechanism

The wet heat aging process of PET is mainly the process of hydrolysis and corresponding aggregation state of PET molecular chain under hot and humid conditions. The ester bond on the PET backbone is a gene for the thermal and aging of polyester. When water is present, the active carboxyl group at the 2nd end of the PET chain will induce and accelerate the hydrolysis of the ester bond, and the temperature rise will further accelerate. This process.

At the initial stage of damp heat aging, the water adsorbed on the PET surface and slowly penetrated. Due to the etching and solvation of PET over time, the crystallinity of PET gradually increased, and the surface micropores and cracks increased. At the same time, PET In the molecular chain, the ester bond is broken by water molecules, and the PET with shorter molecular chain is formed. The crystallinity is further increased under the action of water growth rate, the brittleness of PET is increased, and the penetration of moisture into the PET is accelerated. The above process is repeated repeatedly. The PET loses its original mechanical properties and moisture barrier properties. The increase of temperature accelerates the crystallization of PET on the one hand, thereby increasing the degree of cracking; in addition, it increases the activity of the terminal carboxyl group and accelerates the occurrence of hydrolysis reaction.

2. Method for improving PET moisture and heat resistance

It is known from the mechanism of polyester heat and aging. The ester bond and terminal carboxyl group in the PET molecular chain are the key factors in the hydrolysis. Therefore, there are two main ways to improve the heat and humidity resistance of PET. One is to reduce the content of ester bond; the other is to reduce The content of terminal carboxyl groups.

The first route is mainly to add a third component through the polyester polymerization process, reduce the content of ester bonds in the molecular chain, and introduce chemical bonds or substances with higher hydrolysis resistance. For example, patent CN102898632A discloses the introduction of a certain amount of polyethylene glycol in the PET polymerization process, so that its block is distributed in the polyester molecular chain, since the polyethylene glycol ether bond structure is more resistant to hydrolysis than the ester bond structure, The hydrolysis resistance of the obtained copolyester can be improved, and in addition, in order to further improve the hydrolysis resistance, a benzoic acid compound having three substituents on the benzene ring can be introduced, and a three-chain structure can be formed in the copolyester. Toray has published a flame retardant polyester with excellent hydrolysis resistance in its patent CN101186688A. Since the phosphorus-containing flame retardant component is usually added, the hydrolysis resistance of the polyester is greatly reduced. The patent adds naphthalene in the polymerization process. The acid component introduces a naphthyl group having higher hydrolysis resistance, which effectively improves the hydrolysis resistance of the polyester.

The disadvantage of this method is that the addition of the third component has an effect on the physical and mechanical properties and processing properties of the polyester. Therefore, the current mainstream method for improving the hydrolysis resistance of the polyester is through the introduction of reactive groups and polyester. The terminal carboxyl group reacts to form a stable group, thereby improving the hydrolysis resistance of the polyester. The substances which can usually be added are an epoxy compound or a carbodiimide compound. Patent CN1312237A discloses a hydrolysis resistant polyester prepared by using a polymer type blocking agent such as an epoxy group and an amino group. The benchmarking mass fraction of the blocking agent is (0.05-10%). Because the molecular weight of the blocking agent is relatively high, it is not easy to decompose during the melt-kneading process, and the sealing efficiency is higher. Therefore, the prepared polyester has higher hydrolysis resistance. performance.

Dongli Patent CN101955579A discloses a hydrolysis-resistant polyester prepared by adding any one of a silane coupling agent compound, a carbodiimide compound and an oxazoline compound containing a reactive functional group, wherein the quality of the blocking agent The score is (0.01 to 10)%. There are also many patents reported abroad that epoxy compounds or carbodiimide compounds are used as blocking agents to prepare hydrolysis-resistant polyesters. For example, U.S. Patent Nos. 5,763,568 and 6,974,846, and the like.

In the paper, Wei Wenliang chose Taiwanese F-11 and British P-1002 end-capping agent in the development of anti-aging PET film for photovoltaic battery backsheet. The test results show that P-100 can prepare hydrolysis resistance aging. Good polyester, but easy to decompose and release irritating odor during processing, and F-11 can not only prepare polyester with excellent hydrolysis and aging resistance, but also has good processing performance.

From the patents reviewed and related researches, it is known that the blocking agents such as epoxy compounds or carbodiimide compounds are low molecular type, have poor heat resistance, and are easily decomposed and volatilized during the mixing process, so it is necessary to meet the corresponding application requirements. It is necessary to add an excessive amount of blocking agent, and the price of the blocking agent is relatively high, so that the application of the type of blocking agent is limited to some extent.

3. Photothermal performance of photovoltaic PET backsheet

At present, the heat and humidity resistance test of PV film manufacturers is usually carried out by observing the appearance change of PET film after aging in a damp heat aging box for a certain period of time, such as delamination, bubble and change of color value b value. The damp heat condition was 85 ° C, 85% HR, and the aging time was 1000 h. In order to further grasp the changes in the degree of aging and related performance after damp heat aging, other tests will be carried out in academic research after aging at different times (500h, 1000h, 1500h, 2000h, etc.).

Such as mechanical properties test after wet heat aging, crystallinity test, infrared test and surface tension test. For example, Tang Jing and Wang Li respectively tested the mechanical properties, DSC test and infrared performance of the PET film at 85 ° C, 85% HR under heat and humidity for 500 h, 1000 h, 1500 h, 2000 h. The results show that the mechanical properties, molecular weight and crystallinity of the PET film are affected by each other. When the crystallinity is below 38%, the back film still has good protection and support properties.

Since the PET film in the multilayer composite film is connected with other films through the EVA film, the surface tension of the film determines the bonding ability between the films, so Li Wei also tested the PET film after aging in the study. The change of surface tension showed that the surface tension decreased from 48 dyn/cm to 42 dyn/cm after 500 h of moist heat aging, and decreased to below 36 dyn/cm at 1500 h. The change of surface tension was closely related to corona treatment and PET hydrolysis. Mainly due to the failure of the corona effect, the degree of hydrolysis increases in the later stage, the PET molecular chain becomes shorter, the molecular structure changes, and the surface tension decreases.

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