Toughening of isothermally polymerized cyclic butylene terephthalate for use in composites

Interest in thermoplastic composites is growing because of their advantages over thermosets in recyclability and in toughness. The melt viscosity of thermoplastic polymers is very high, which makes fibre impregnation difficult. This can be solved by using in-situ polymerization with cyclic butylene terephthalate (CBT). However this leads to a brittle PBT. To solve this problem physical and chemical modification of the polymerized CBT (pCBT) was performed, to disturb the crystallization. Chemical modification with PC and with PTHF has an embrittling effect because of a bad chemical interaction. When polycaprolactone is added to the CBT a copolymer is formed which leads to a lower crystallinity, resulting in a higher toughness of the pCBT. This tougher matrix material was used in composites and a two times tougher composite is produced when only 7 wt% PCL is added to the CBT. The physical modification evaluated was the addition of carbon nanotubes (CNT). Although an increase in stiffness and strength of the pCBT is seen when CNTs are added up to 0.1 wt%, the failure strain decreases.