Functional Modification On The Surface Of Carbon Fiber And Its Effect On The Interfacial Properties Of Rubber Composites

Carbon fiber has been widely used as a reinforcement in rubber composites due to the characteristic of structure-function integration,which corresponds to the development of new generation rubber products.However,there is less active oxygen containing functional group in rubber matrix compared to thermosetting resin matrix.In addition,due to the huge difference in module between carbon fiber and rubber matrix,the deformation of these two phases presents huge difference in stress when mechanical load was applied.As a result,the relative motion between the fiber and rubber was increased,which leads to more significant destruction of interface.Therefore,it is extremely important to solve the weak interfacial adhesion of CFRR composites.According to the characteristics of unsaturated bonds and demanding of vulcanization for rubber matrix,a vinyl polysiloxane double interface layer with good chemical connection,which was composed with carbon fiber-transition layer and transition layer-rubber matrix,was designed and constructed to improve the interfacial performance of CFRR composites.Methacryloxy propyl trimethoxyl silane（MPTS）was utilized as the precursor to obtain the modified solution with rich active functional groups by controlling its hydrolysis conditions.Hydrolysis MPTS（H-MPTS）was successfully used for grafting polymerization on the surface of carbon fibers since the condensation could be occurred simultaneously.The process and mechanism of grafting were also studied.Finally,the effects of MPTS grafting conditions on the surface structure of carbon fiber as well as the interfacial performance of CFRR were investigated by controlling the concentration of Si-OH,treatment temperatures and times.The results indicated that:（1）The hydrolysis and condensation behaviors of MPTS were characterized by in-situ measurements of time dependence of IR and ²⁹Si-NMR.It was found that the hydrolysis of MPTS is a stepwise reaction,meanwhile the progress of hydrolysis and condensation reactions of MPTS were seriously affected by p H value.In acid environment,the methyl in Si-（OCH₃）₃group of MPTS was electrophilic substituted by H⁺,which promoted the hydrolysis reactions to generate active silicone hydroxyl groups（Si-OH）and inhibited the further polycondensation of generated Si-OH.For alkaline environment,the Si-（OCH₃）₃group could be nucleophilic attacked by OH^-,which not only accelerated hydrolysis reactions but also the following polycondensation reaction,resulting in the loss of active silicone hydroxyl groups.The rate of both hydrolysis reactions and polycondensation reactions of MPTS under neutral conditions were slow.It was found that the content of Si-OH reached the highest when hydrolysis time was 80 min by comparing the integrated intensity of characteristic peaks in the in-situ ²⁹Si-NMR spectra.（2）A transition layer based on covalent bonds connection was introduced into the interface between carbon fiber and rubber matrix.The results of Infrared Spectroscopy（IR）and X-ray Photoelectron Spectroscope（XPS）indicated that carbon fiber and H-MPTS were connected by Si-O-C bond formed by dehydration reaction between hydroxyl groups.It was also confirmed that a polysiloxane interface layer was formed due to the further polycondensation of Si-OH on the surface of carbon fiber according to Thermogravimetric Analysis（TGA）results.The connection between the polysiloxane interface layer and rubber was established based on co-vulcanization reaction was proved by the distribution of sulfur after the vulcanization of CFRR observed in the SEM-EDS.（3）Effects of MPTS concentration,treatment temperatures and times on the structure of the surface for carbon fibers were investigated by Scanning Electron Microscope（SEM）,XPS and TGA.It could be indicated that the collision probability of reactive functional groups increased with the increase of MPTS concentration,which promoted the formation of vinyl polysiloxane interface layer.However,the excessive Si-OH could hinder the functionalization modification of MPTS due to agglomeration.As the treatment temperature increases,the grafting rate of vinyl siloxane on the surface of carbon fiber was accelerated as well as the rate of polycondensation reaction.Extending the treatment time could increase the grafting density,while it tended to be stable after 30min due to the need to match with the active sites on the surface of carbon fibers.（4）The effects of different treatment conditions on the wettability of carbon fibers as well as the interfacial adhesion of CFRR composites were evaluated.It was found that interfacial adhesion of CFRR composites increased and then decreased with the increase of MPTS concentration.Raising the treatment temperature and time were also beneficial to improving the interfacial adhesion.When the concentration of MPTS hydrolysate was5wt%,treatment temperature was 75℃and the time was 30 min,the surface energy increased from 18.0 m J·m^-2to 35.9 m J·m^-2compared to oxidized carbon fiber.Meanwhile,the H pull-out force of CFRR composites was increased from 21.4 N to 66.6 N after re-functional modification.An interface layer with good adhesion to both carbon fiber and rubber was introduced,resulted in an effective improvement of interfacial properties for CFRR composites.