Toughness enhancement of poly(ethylene terephthalate) with chemically modified ultra high molecular weight polyethylene
The present work was concentrated on exploring and developing a toughening material system to be used in engineering products made of high crystalline PET [poly(ethylene terephthalate)]. The work addressed some fundamental objectives in the technology. of crystallised PET (CPET) related to toughness and thermoforming. PET is widely used in fibres and oriented films but is also an attractive material for moulded products, owing to its high melting point and solvent resistance. Crystalline PET, however, is brittle and susceptible to hydrolytic degradation through water absorption. The polymeric toughening system was based on suitably modified UHMWPE (ultra high molecular weight polyethylene) a crystalline polymer with very high fracture toughness under plane strain conditions, low water absorption, but with a relatively low melting point. Being a thermoplastic it has a modulus about 1,000 times higher than the equivalent hydrocarbon elastomer usually used to toughen PET and polyamides. Hence, deterioration of strength and stiffness is minimized. Blending PET with UHMWPE produces coarse microstructures and poor mechanical properties due to lack of compatibility of the two polymers. Compatibility is expected to be improved by reducing the interfacial tension and increasing the adhesion between the two phases; the first will reduce the size of the dispersed phase, while the latter would improve the mechanical properties. Compatibilisation of a pair of immiscible polymers, like in blend of PET with UHMWPE, can be achieved by incorporation of reactive units along the main chains of the UHMWPE, which are capable of strong interactions with the matrix component (PET). The methods used in this investigation involved smface grafting and irradiation treatment to produce chemically active groups on the UHMWPE polymer chain to achieve the required level of interfacial adhesion assisting the agglomerate particle breakdown into small size and to develop strong bonding to the PET when the two polymers are blended in a high shear mixing. The results obtained with several modified UHMWPE powders incorporated into PET by melt mixing showed a clear formation of small primary particles of UHMWPE strongly bonded to the matrix, thereby achieving a vast improvement in toughness while preserving other important fundamental properties related to both the end product and the material processing.