Influence of Cure Volume Shrinkage of the Matrix Resin on theAdhesion between Carbon Fiber and Vinyl Ester Resin
L.Xu and L.T. Drzal, CompositeMaterials and Structure CenterDepartmentof Chemical Engineering and Materials ScienceMichigan State UniversityEast Lansing, Michigan 48824-1226 USAe-mail:xulanhon@msu.eduTel:517-353-4708 (office)FAX:517-432-1634 Introduction Free radical cured thermosetting vinylester resins have superior toughness and chemical resistance in comparison tounsaturated polyester. The use of vinylester composites reinforced with carbon fibers requires an improvement in thefiber-matrix adhesion levels. Previousdata has shown that the application of a lightly cross-linked amine-cured epoxysizing to the carbon fiber surface creates a beneficial interphase between thecarbon fiber and vinyl ester resin matrix resulting in a substantial increasein fiber-matrix adhesion and the sizing has a optimum thickness[1,2]. However, the exact mechanism by which thiscoating improved adhesion is not known. Vinyl ester resin can undergo as much as a 9% volume shrinkage with curewhile typical epoxy system undergo only 3-4%[3] shrinkage duringcure. This shrinkage could have inducedsignificant stresses in the composite which might be responsible for the loweradhesion to vinyl ester resins. In thisstudy, the influence of the cure volume shrinkage on the adhesion betweencarbon fiber and vinyl ester resin was investigated using the epoxy sizing as acontrolled and designed interphase. Adhesion was evaluated as an interfacial shear strength (IFSS) measuredwith a micro-indentation. The effect of the cure volume shrinkages of vinylester resins to the interphase gradients between them and epoxy sizing havealso been investigated with nano-scratch. Finite element analyses will be used to simulate the stress distributionaround the fiber-sizing-matrix interphases during volume shrinking of thematrix resin and the micro-indentation process as well. Experiments Materials: The fiber used in this study is an AS4 carbonfiber from Hexcel, Inc. The matrix resin is Derakane 411-C50 vinyl ester resinfrom Dow Chemical and Fuchem 891 vinyl ester resin from Shanghai Fuchen Chemicals Co. CHP-5 (diluted cumene hydroperoxide) from Witco Chemical andMEKP (methyl ethylketone peroxide) from Aldrich chemicals were used as theinitiator. Both CoNap and DMA fromAldrich Chemicals are used as promoters and accelerators respectively. Diglycidyl ether of bisphenol A (DGEBA epoxy)and trimethylpropane mine (JEFFAMINE T403) are from Shell and Huntsmanrespectively. Methods: The mixture of DGEBA and JEFAMINE T-403 wasprepared and held for one half hour then added to acetone to form a 5wt% sizingsolution. Fiber sizing was carried outby using a pre-impregnation machine. Thermal Gravimetric Analysis (TGA) was used to measure the thickness ofthe sizing. A digitally controlled, programmableoven was used for specimen curing to make sure all the samples were processedin the same way. Lab-made dilatometerswere used to measure the cure volume shrinkage. United Testing System (UTS), of the composite samples were conducted tomeasure the mechanical properties of the matrix resin formulations. A MTS nano-indentation instrument was used toquantify the gradient of the modulus between the sizing and the DGEBA epoxy andD411-C50 vinyl ester resin matrices. Adhesion was evaluated as an interfacial shear strength (IFSS) measuredwith a micro-indentation system, Interfacial Testing System (ITS). Results and Discussion
1. Cure Volume Shrinkage Measurements Two types of recipe systems were used for cure resins of the samplesfor the investigation of influence of cure shrinkage on the adhesion betweencarbon fiber and vinyl ester resin. Twodifferent systems were of different initiators. One is CHP-5 and the other one is MEKP. The CHP-5 cure system was based on the recipe of D411-C50 recommended bythe manufacturer and the MEKP cure system is based on the recipe of Fuchem 891recommended by the manufacturer, shown in the gray columns of Table 1. For the same initiator, the correspondingrecipes, the white columns of Table 1, were calculated out from the recommendedrecipes, the gray columns, based on the density of the carbon-carbon doublebond, C=C. Table 1 The cure recipe for theinvestigation of the cure volume shrinkage
The results of the curevolume shrinkage measured by the lab-made dilatometer were shown in Fig.5.37. It has shown that D411-C50 gave a volume shrinkage of 7~8% upon curing. Whereas when Fuchem 891 vinyl ester resincured with the recommended recipe, 2% of MEKP and 0.1% of CoNap, the curevolume shrinkage was very small, only 1.73% comparing with 8.10% ofD411-C50. It is interesting that whenFuchem 891 vinyl ester resins were cured with CHP-5 as initiator using amountof concentration the same as that of D411-C50 recommended by the manufacturer, the cure volume shrinkage is 5.85%. 2. Influence of Cure Volume Shrinkages to the Mechanical Properties ofMatrix Materiasl Themechanical properties of matrix, which are very important for ITS interfacialshear strength measurements, were tested by UTS and Nano-indentation aswell. During cure process, it was foundthat styrene vaporization was very fast. The vaporization would be different for those UTS samples made with opensample molds from those ITS sample which were cut in the middle of the samplebar made with less open molds especially different from the samples measuringcure volume shrinkage with totally closed lab-made dilatometer. To avoid the influence of styrenevaporization, nano-indentation was used to measure the bulk mechanicalproperties of the material. It was alsofound that the bulk elastic moduli measured by nano-indentation were compatibleto the tensile moduli measured by UTS. The influence of the cure volume shrinkage of the vinyl ester matrixmaterials on their bulk moduli were shown in Fig. 2. The bulk moduli of the vinyl ester materialswere increased with the increase of the cure volume shrinkage of the materials. It might indicate that the more shrinkage ofthe cure the tighter the polymer chains held together resulting in an increaseof the material mechanical property.
3. Influence of the CureVolume Shrinkage of the Matrix Material to the Adhesion between Carbon Fiberand Vinyl Ester Two sets of ITS composite samplewere tested by the ITS to find out the influence of the cure volume shrinkageof the matrix material on the adhesion between fiber and matrix. One set of composite samples was the vinylesters having different cure volume shrinkage reinforced by AS4 carbon fiber asreceived and the other set was the vinyl esters having different cure volumeshrinkage reinforced by AS4 carbon fiber with 5% of DGEBA-JeffamineT-403/Acetone sizing solution.
The test results were shown inFig. 5.3. The ITS test results were veryinteresting, for the samples made with AS4 carbon fiber as received, theinterfacial shear strength showed a progressive decrease with the cure volumeshrinkage of the matrix material, the bigger the cure volume shrinkage the moresignificant the decrease of the interfacial shear strength. In contrast, for the samples made with carbonfiber with DGEBA-T403 sizing, the value of the interfacial shear strength hadlittle change even for big cure volume shrinkage of 8.2%. This suggested that the cure volume shrinkagewould bring thermal residual stress which brought negative effect on theadhesion between carbon fiber and vinyl ester resin. And it was also suggested that the sizingmaterial might play a very important role in elimination of the residual stressescaused by the cure volume shrinkage. Conclusions The bulk moduli of the vinyl ester resinmeasured by nano-indentation would increase as the cure volume shrinkagesincrease. The cure volume shrinkage would introduceresidual stress which could decrease the interfacial shear strength (IFSS)values suggesting a negative effects on the adhesion between carbon fiber andvinyl ester resin. The epoxy sizing on the fiber surface isvery important for the relaxation of thermal residual stress caused by curevolume shrinkage.
References 1. Xu, L., Al-Ostas, A., andSchalek, R., Drzal, L., “Improvement ofadhesion between vinyl ester resin and carbon fibers by a controlled anddesigned interphase”, the 2002 Adhesion Society/WCARP-II Meeting, February 2002,Orlando, FL.2. S. M. Corbin, M.S. Thesis, Department of Chemical Engineering, Michigan State University, 19983. L. S. Penn and T. T. Chiao,“Epoxy Resins”, Hand Book of Composite,pp.57~88