Adhesion-strong polymeric layers on polyethylene terephthalate films modified by irradiation with accelerated electrons

Abstract

Regular trends in variation of the functional group composition, surface energy, and amorphouscrystalline structure of oriented polyethylene terephthalate fi lms modifi ed by irradiation with accelerated electrons to the absorbed dose in the range 25-300 kGy were studied by the methods of surface tension, indicator dye adsorption, and X-ray diffraction. The possibility of preparing gelatin emulsion layers exhibiting high adhesion strength was examined. on the surface of biaxially oriented polyethylene terephthalate (PET) fi lms is of particular interest [1]. It is known [1] that the hydrophobic properties of PET depend on the specimen density and extension ratio. As the fi lm crystallinity increases, the molecular volume of PET decreases. Therefore, because of high dispersion energy of wetting of such PET specimens [2], introduction of hydrophilic polymeric adhesives into the surface layer of amorphous-crystalline fi lms is hindered [3].Prior to application of polymeric layers onto the PET fi lm surface, it is made hydrophilic, e.g., by exposure to corona discharge [1,4]. The required degree of the fi lm hydrophilization, ensuring their wetting in aqueousorganic media, is attained by application of several layers in the order of increasing hydrophilicity [1]. Copolymers of styrene, butyl acrylate, and acrylic acid are used as materials for the fi rst layer. To attain high strength, heat resistance, and low swelling in aqueous medium, the gelatin and its hardeners are frequently included in formulations for second and further polymeric layers [1,3].At the same time, the known methods for hydrophilization of the surface of PET fi lms [1], including treatment with corona [4] and glowing discharge [5], have certain drawbacks. In particular, devitrifi cation of the fi rst polymeric layer on fi lms preliminarily exposed to corona discharge leads to an appreciable decrease in the strength of the multilayer system [1].Studies of PET fi lms irradiated with accelerated electrons showed [6,7] that the concentration of hydroxy and carboxy groups formed by hydrolysis of ester groups of the polymer increased at the absorbed dose D close to 100 kGy. The molecular weight of the samples in the range D = 25-300 kGy decreased insignifi cantly [6].A study of the adsorption centers on the PET surface showed [6] that the maximal degree of the fi lm hydrophilization was attained at D close to 100 kGy owing to a decrease in the amount of all the adsorption centers except Brønsted acid centers with рK а 5 [8]. An increase