Photo-graft modification of fluoropolymer surface and its bonding properties with EVA hot melt adhesive

Abstract

<p indent=0mm>As an important part of photovoltaic (PV) modules, backsheet materials play an important role in protecting solar cell from environmental degradation caused by moisture penetration. Fluoropolymers, which have excellent weatherability, UV radiation resistance and chemical stability, are widely used as the outer layer of the backsheet to ensure the long-term reliability and safety of PV modules. However, the low surface energy and poor adhesion to other materials of fluoropolymer markedly limits its practical application in encapsulation of solar cells which require the backsheet to be firmly bonded with PV modules by ethylene-vinyl acetate copolymer (EVA) hot melt adhesive. Although some methods such as corona discharge, plasma treatment, radiation grafting can effectively improve the surface properties of fluoropolymers, UV-induced graft polymerization is still considered as one of the most promising approach due to its advantages in fast reaction rate, low cost of processing, simple equipment, easy industrialization, and no damage to bulk property of materials. In this paper, different copolymers of acrylic acid-butyl acrylate (PAA-BA) or butyl acrylate-trimethylolpropane triacrylate (PBA-TMPTA) were introduced onto fluoropolymer film by photografting polymerization. The effects of UV irradiation time, monomer composition and monomer concentration on the graft efficiency were investigated. The chemical composition, surface morphology and chemical composition of the film were characterized by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and contact angle analysis (CA). The results showed that the UV-induced graft polymerization can be successfully performed on the fluoropolymer film and a dense graft layer could be introduced on the surface. The introduction of PAA-BA changed the surface property of fluoropolymer film and facilitated the spreading of EVA, while the adhesive strength of PAA-BA modified surface bonded with EVA film is only <sc>0.8–4.8 N/cm,</sc> which is almost same as the unmodified fluoropolymer film. Similarly, introducing functional groups such as hydroxyl and sulfate groups by confined photooxidation method did not improve the adhesion properties. When PBA-TMPTA copolymer was grafted on the fluoropolymer film, its laminate with EVA hot melt adhesive had a peeling strength up to <sc>17.6 N/cm</sc> (15 times of unmodified film). The remarkable increase in the adhesive property of modified fluoropolymer was believed to be attributed to the unreacted double bond of grafted PBA-TMPTA. During the melt of EVA, the peroxide additive in it decomposed to release radicals and resulted in the crosslinking of EVA film. Meanwhile, the chain radicals of EVA also reacted with double bond in the PBA-TMPTA layer, which could form covalent bond between the interfaces and improve the adhesive strength. Therefore, the improvement of interfacial adhesion performance is mainly due to chemical bond rather than physical interaction.