THERMALLY CROSS-LINKABLE THERMOPLASTIC PET-CO-XTA COPOLYESTERS

A series of thermally crosslinkable polyester copolymers were synthesi zed by incorporation of a benzocyclobutene-containing terephthalic aci d derivative (XTA) into polyethylene terephthalate (PET). The cyclobut ene moiety on the XTA monomer allows for reactive crosslinking at temp eratures similar to 350 degrees C requiring no catalyst and causing no change in mass. Copolymers were synthesized containing 1, 5, 10, 20, 50, and 100 mol% XTA. Crosslinking occurred above the melting temperat ure (similar to 250 degrees C) yet below the degradation temperature ( similar to 400 degrees C), providing a window for melt processing of t he copolymer. To demonstrate this point fibres were melt spun. The PET -co-XTA copolymers show systematic variations in the glass transition, recrystallization, melting and degradation temperatures as a function of benzocyclobutene content. The degradation and melting temperature both decrease slightly with increased XTA, while the recrystallization and glass transition temperature were relatively insensitive to XTA c ontent. Thermal gravimetric analysis (TGA) indicated a decrease in the degradation temperature as higher amounts of XTA were incorporated, a lthough an increase in the %char at 800 degrees C was seen. This decre ase in degradation temperature may be due to the generation of free ra dicals. Limiting Oxygen Index (LOI) measurements showed an increase in the oxygen content required to maintain a stable flame in copolymers with increasing amounts of XTA. LOI values ranged from 18 for neat PET to 35 for the copolymer containing 20 mol % XTA. Wide-angle X-ray sca ttering data showed little change in the crystalline structure, but de creasing crystallinity for PET for blends containing up to 20 mol% XTA . The 50 mol% XTA copolymer was amorphous, while the 100% XTA homopoly mer (PEXTA) showed evidence of a new crystalline structure. Crystallin e diffraction peaks showed reduced intensities in data recorded for he at treated samples, and there was evidence for new peaks in the copoly mer containing 20 mol% XTA when heated near 300 degrees C. Transmissio n electron microscopy of cross-sections through burned samples showed a highly crystalline char at the surface of XTA copolyesters. This cry stalline char appeared to protect the underlying copolymer from furthe r flame-induced degradation. Evidence for significantly increased adhe sion of the copolymers to polyimide films was also obtained. (C) 1998 Elsevier Science Ltd. All rights reserved.