Studies On The Dispersion Of Expanded Graphite And The Resultant Structure And Properties Of Graphite Filled Polymers

Expanded graphite is a layered material possessing excellentlubrication property and high aspect ratio in an exfoliated state, and it isalso one of the strongest materials per unit weight with unique functionalproperties, e.g. good electrical and thermal conductivities. In this paper,the effect of expanded graphite dispersion on the properties of polymerswas investigated, especially for friction and wear properties. The effect ofcuring process on the dispersion of expanded graphite was also studied.Firstly, in order to prepare expanded graphite (EG) andacrylonitrile-butadiene rubber (NBR) composites, two preparationmethods were adopted, namely directly mechanical blending(micro-composites) and latex compounding technique (nanocomposites),and their resulting mechanical and tribological properties were compared.Meanwhile, the effects of the expanded graphite dispersion state, volumefraction, load and sliding velocity on the friction and wear propertieswere also explored. As expected, nanocomposite showed better tensileproperties, hardness, dynamic storage modulus and lower hysteresis losspeak. The coefficient of friction (COF) for both composites decreased asthe load and sliding velocity increased, while the effect of the expandedgraphite volume fraction was subtle. The wear mechanism could bedescribed as expanded graphite sheets de-bonded from matrix, lubricantfilm formation and breaking, matrix wear, and lubricant film re-formationin cycles. Compared with micro dispersion, the nano-sized dispersion ofexpanded graphite was beneficial to the formation of a continuous,uniform, and stable lubricant film under high sliding velocity and heavy load, contributing to reduce the COF and improve the wear resistance.Secondly,(NBR)/(EG)/carbon black (CB) micro and nanocompositeswere prepared by two different methods, and the resulting mechanicaland tribological properties were compared with those of NBR/CBcomposites. Meanwhile, the effects of expanded graphite dispersion andcontent, as well as the applied load and sliding velocity on thetribological behavior of the above composites under dry friction conditionwere also evaluated. It was found that a small amount of EG couldeffectively decrease COF and specific wear rate (Ws) of NBR/CBcomposites because of the formation of graphite lubricant films.NBR/CB/EG nanocomposite always shows a stable wearing process withrelatively low COF and Ws. The effect of expanded graphite content andcarbon black particle size on the friction and wear properties ofcomposites was little, and the proposed expanded graphite content isbetween3to5g/100g NBR.Third, NBR/EG compounds with initial fine dispersion of nanosizegraphite were prepared by latex compounding method, and then thedispersion evolution of the expanded graphite during subsequent mixingand vulcanization was fully investigated. The effects of expandedgraphite loading amount, shear during mixing, and the pressure andtemperature during vulcanization on the expanded graphite dispersion andthe resultant filler network in the nanocomposites were evaluated byusing rubber process analysis X-ray diffraction and transmission electronmicroscopy. We found that the initial compounds already showed asignificant filler network because of the nanosize dispersion and highwidth/thickness ratio of expanded graphite even at very low contents.During shearing, the variation of expanded graphite dispersion is stronglyrelated to the initial filler network. The initial fine dispersion of expandedgraphite in the rubber/expanded graphite nanocomposites could bemaintained by reducing shear and curing at as high a temperature and lowa pressure as possible.Fourth, in order to evaluate the roles of expanded graphite dispersionon the functional properties of the composites, PVP/expanded graphitenanocomposites were prepared by blending the aqueous suspension of expanded graphite sheets and polyvinylpyrrolidone (PVP) aqueoussolution by ultrasonic treatment, followed by spray drying and directdrying as a comparison individually. The effects of expanded graphiteloading and drying method on the dispersion of expanded graphite andthe resultant properties of the composites such as electrical and thermalconductivity, friction and dynamic mechanical properties were studied.The results showed that the composites prepared by fast spray dryingexhibited a higher degree of exfoliation and a better dispersion ofexpanded graphite sheets in the PVP matrix than the correspondingcomposites prepared by direct drying, leading to a conclusion that fastspray drying can effectively prevent from re-stacking of the exfoliatedgraphite sheets as illustrated. As a result, the spray drying greatlyimproved the electrical and thermal conductivity of the composites. Itwas also found that the electrical and thermal conductivity of thecomposites strongly depended on the expanded graphite dispersion, whilethe friction coefficient unexpectedly does not. Increasing expandedgraphite loading level might enhance the probability of graphite sheetsre-stacking, resulting in poor expanded graphite dispersion.In the final part, EPDM/mica composites were prepared by fillingsynthesized mica and natural mica separately into ethylene-propylenediene terpolymer (EPDM) using melt blending technique. Microstructures,electrical properties, gas resistance, and mechanical properties of twoEPDM/mica composites were investigated systematically. EPDM/synthesized mica composite possessed better mechanical property,electrical insulationproperty, and gas permeability resistance, resultedfrom better dispersion of synthesized mica as well as rubber-fillerinterfacial adhesion.