DESIGNING HIGH HARD BLOCK CONTENT THERMOPLASTIC POLYURETHANE (TPU) RESINS FOR COMPOSITE APPLICATIONS

Abstract

The effect of different chain extenders and hard segment (HS) concentrations on the properties of thermoplastic polyurethanes (TPUs) and TPU composites were investigated. The chain extenders used were 2-methyl-1,3-propanediol (2m13pd), 1,2-ethanediol (12ed), 1,4-butanediol (14bd), 1,5-pentanediol (15pd), 1,6-hexanediol (16hd), 1,7-heptanediol (17hpd), 1,8-octanediol (18od) and 1,4-cyclohexanedimethanol (14chdm). The hard segment concentrations of the TPUs investigated were 70%, 80%, 90% and 100%. Only 70% and 100%HS were investigated for the 17hpd and 18od TPUs. DSC results revealed that the cast 2m13pd and 14chdm TPUs had little or no melting transitions. Cast 14chdm TPUs were amorphous which was attributed to the mixture of cis- and trans- geometric isomers present in the 1,4-cyclohexanedimethanol chain extender. The reason for the low crystallinities of 2m13pd TPUs was attributed to the branched structure of the 2m13pd extender as the methyl (-CH3-) pendant group hinders the crystallization of the polymer chains which also imparts flexibility to the polymer chains. 12ed and 14bd TPUs displayed the highest melting transitions (about 220°C) and melting enthalpies (DeltaHTot) due to their high crystallinity levels. Similar trends in thermal properties were observed for the 70%HS cast TPUs chain-extended with 15pd, 16hd, 17hpd and 18od. The microphase mixing transition (TMMT) values of 70%HS cast TPUs chain-extended with 15pd, 16hd, 17hpd and 18od were 180±0.3, 190±0.5, 164±0.5 and 182±0.5°C. The TMMT values show that the even-numbered chain-extended TPUs have higher melting transitions than the odd-numbered chain extended TPUs. This observation could be linked to the odd-even effect of odd-numbered and even-numbered diols (chain extenders). Multiple endothermic transitions were observed for the melt-quenched and slow-cooled TPUs chain-extended with 16hd, 17hpd and 18od. These multiple endothermic transitions were attributed to the existence of polymorphic structures in the polymer chains. The melt-quenching and compression moulding processes decreased the crystallinities of the TPUs whereas the annealing process increased the degrees of crystallinity of all TPU samples. Phase separation was observed for all the cast 70% and 100%HS TPUs as revealed in the SAXS results. The SAXS peaks observed for the 70%HS TPUs come from the HS-Soft Segment (SS) phase whereas the SAXS peaks observed for the 100%HS TPUs come from the crystalline HS and the amorphous HS. Crystalline peaks were seen on the amorphous halos of linear chain-extended cast 70% and 100%HS TPUs as revealed by WAXS results. These peaks correspond to the crystallinity of hard segments. It was observed that annealing the moulded TPU samples at 80°C for 168 hours induced the formation of crystal structures which have d-spacings of about 4.6A (otherwise known as type-II crystals). Compression-moulded TPU composites reinforced with woven glass-fibre mats displayed storage moduli above 2 GPa at 25°C as revealed by the DMTA results. Upon annealing, the storage moduli of the TPU composites increased above 4 GPa. The storage moduli of the unannealed and annealed TPU composites compare well with those of unannealed and annealed composites with polypropylene and commercial TPU matrices. Tensile testing showed that the Young's moduli of the unannealed and annealed 15pd TPU composites were similar to those of polypropylene and commercial TPU composites.

Date of Award	1 Aug 2015
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Alberto Saiani (Supervisor)