Ozdemir, Nazli Gulsine (2015) Toughening of epoxy matrix composites with nanorubber for advanced industrial applications. (PhD thesis), Kingston University, .
Abstract
Current work is a thorough investigation on the processing and mechanical properties and the toughening mechanism of nanorubber-modified epoxy and the carbon and glass fibre reinforced composites produced using these matrices. The study is aiming to assess the applicability of these novel nanorubber-modified matrices to advanced industrial applications where fracture toughness is the main concern. In the first section of this research, process optimisation of the nanorubber-modified epoxy was studied. Dispersion techniques of triple milling, high shear mixing and ultrasonic bath were used in order to enable an even and nano-size dispersion of the acrylonitrile-based nanorubbers in the epoxy matrix. Triple milling was chosen as the dispersion technique which enabled an even and nano-scale dispersion of the nanorubber particles in the epoxy resin matrices. Scanning electron microscopy (SEM) images revealed that CNBR-NP could be dispersed evenly in the epoxy matrix using industrial mixing process whereas partial agglomeration was observed in NBR-NP blends. The difference in the dispersion ability of these two nanorubbers in epoxy was related to the difference in van der Waals forces between single nanoparticles, the chemical formula and the polarity of the systems. In the second stage of this study, the effects of nanorubber on the rheological properties, cure characteristics and mechanical properties of epoxy resin were investigated. The dynamic mechanical behaviour of the carbon fibre reinforced polymer composites (CFRP) with the neat and the nanorubber-modified matrices was also studied. Rheological study showed that the NBR-NP blends attained lower viscosity in comparison to CNBR-NP blends and both systems exhibited shear-thinning behaviour. The dynamic mechanical analysis (DMA) data showed that the addition of nanorubber has negligible effect on the glass transition temperature of the epoxy. The effects of the nanorubber on the interlaminar shear strength, fracture toughness and tensile fatigue behaviour of carbon and glass fibre reinforced polymer composites (CFRP, GFRP) was studied. Mechanical properties of tensile strength, apparent shear strength, Mode I and Mode II delamination fracture toughness, impact toughness and tension-tension fatigue were analysed. The results showed that the nano-size dispersion of rubber significantly improved the Mode I delamination fracture toughness (G[sub]IC) of the CFRP by 251% and its Mode II delamination fracture toughness (G[sub]IIC) by 81% with the addition of 20 phr of CNBR¬NP to the resin matrix. For the NBR-NP system, the G[sub]IC and G[sub]IIC delamination fracture toughness of the CFRP were improved by 203% and 83% respectively with the addition of 20 phr of NBR-NP to the matrix;. Scanning electron microscopy (SEM) images of the CFRP fracture surfaces revealed that the toughening was mainly achieved by the de-bonding of nanorubber, crack path deflection and fibre bridging. The Mode I delamination fracture toughnesses (G[sub]IC) of the GFRP with the addition of 20 phr of CNBR-NP and NBR-NP were improved by 190% and by 150% respectively. Similarly, the Mode II delamination fracture toughness (G[sub]IIC) of the GFRP panels was improved by 73% and 67% with the addition of 20 phr of CNBR-NP and NBR-NP respectively. SEM images of the Mode I delamination fracture surface of the GFRP panels proved that the interfacial adhesion between the individual glass bundles and the matrix was improved with the addition of nanorubber. The tensile-fatigue behaviour of the CFRP panels with X CNBR-NP/ R matrices was studied. The normalised test data revealed that the high cycle fatigue life was enhanced by twice with the addition of 15 phr of CNBR-NP in the neat epoxy matrix. SEM images of the fracture surfaces proved that the better. adhesion of nanorubber-modified matrix to the carbon fibres supressed fibre pull-out and contributed towards the enhanced fatigue life of the panels.
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