Influence of Cure Volume Shrinkage of the Matrix Resin on the Adhesion between Carbon Fiber and Vinyl Ester Resin

L. Xu and L.T. Drzal,

Composite Materials andStructureCenter

Department of Chemical Engineering and Materials Science

Michigan StateUniversity

East Lansing,Michigan48824-1226

USA

e-mail: xulanhon@msu.edu

Tel: 517-353-4708 (office)

FAX: 517-432-1634

Introduction

Free radical cured thermosetting vinyl ester resins have superior toughness and chemical resistance in comparison to unsaturated polyester.  The use of vinyl ester composites reinforced with carbon fibers requires an improvement in the fiber-matrix adhesion levels.  Previous data has shown that the application of a lightly cross-linked amine-cured epoxy sizing to the carbon fiber surface creates a beneficial interphase between the carbon fiber and vinyl ester resin matrix resulting in a substantial increase in fiber-matrix adhesion and the sizing has a optimum thickness^[1,2].  However, the exact mechanism by which this coating improved adhesion is not known.  Vinyl ester resin can undergo as much as a 9% volume shrinkage with cure while typical epoxy system undergo only 3-4%^[3] shrinkage during cure.  This shrinkage could have induced significant stresses in the composite which might be responsible for the lower adhesion to vinyl ester resins.  In this study, the influence of the cure volume shrinkage on the adhesion between carbon fiber and vinyl ester resin was investigated using the epoxy sizing as a controlled and designed interphase.  Adhesion was evaluated as an interfacial shear strength (IFSS) measured with a micro-indentation. The effect of the cure volume shrinkages of vinyl ester resins to the interphase gradients between them and epoxy sizing have also been investigated with nano-scratch.  Finite element analyses will be used to simulate the stress distribution around the fiber-sizing-matrix interphases during volume shrinking of the matrix resin and the micro-indentation process as well.

Experiments

Materials:  The fiber used in this study is an AS4 carbon fiber from Hexcel, Inc. The matrix resin is Derakane 411-C50 vinyl ester resin from Dow Chemical and Fuchem 891 vinyl ester resin from Shanghai Fuchen Chemicals Co. CHP-5 (diluted cumene hydroperoxide) from Witco Chemical and MEKP (methyl ethylketone peroxide) from Aldrich chemicals were used as the initiator.  Both CoNap and DMA from Aldrich Chemicals are used as promoters and accelerators respectively.  Diglycidyl ether of bisphenol A (DGEBA epoxy) and trimethylpropane mine (JEFFAMINE T403) are from Shell and Huntsman respectively. 

Methods:  The mixture of DGEBA and JEFAMINE T-403 was prepared and held for one half hour then added to acetone to form a 5wt% sizing solution.  Fiber sizing was carried out by using a pre-impregnation machine.  Thermal Gravimetric Analysis (TGA) was used to measure the thickness of the sizing.  A digitally controlled, programmable oven was used for specimen curing to make sure all the samples were processed in the same way.  Lab-made dilatometers were used to measure the cure volume shrinkage.  United Testing System (UTS), of the composite samples were conducted to measure the mechanical properties of the matrix resin formulations.  A MTS nano-indentation instrument was used to quantify the gradient of the modulus between the sizing and the DGEBA epoxy and D411-C50 vinyl ester resin matrices.   Adhesion was evaluated as an interfacial shear strength (IFSS) measured with 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 samples for the investigation of influence of cure shrinkage on the adhesion between carbon fiber and vinyl ester resin.  Two different 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 by the manufacturer and the MEKP cure system is based on the recipe of Fuchem 891 recommended by the manufacturer, shown in the gray columns of Table 1.  For the same initiator, the corresponding recipes, the white columns of Table 1, were calculated out from the recommended recipes, the gray columns, based on the density of the carbon-carbon double bond, C=C.    

Table 1 The cure recipe for the investigation of the cure volume shrinkage

The results of the cure volume 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 resin cured with the recommended recipe, 2% of MEKP and 0.1% of CoNap, the cure volume shrinkage was very small, only 1.73% comparing with 8.10% of D411-C50.  It is interesting that when Fuchem 891 vinyl ester resins were cured with CHP-5 as initiator using amount of 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 of Matrix Materiasl

The mechanical properties of matrix, which are very important for ITS interfacial shear strength measurements, were tested by UTS and Nano-indentation as well.  During cure process, it was found that styrene vaporization was very fast.  The vaporization would be different for those UTS samples made with open sample molds from those ITS sample which were cut in the middle of the sample bar made with less open molds especially different from the samples measuring cure volume shrinkage with totally closed lab-made dilatometer.  To avoid the influence of styrene vaporization, nano-indentation was used to measure the bulk mechanical properties of the material.  It was also found that the bulk elastic moduli measured by nano-indentation were compatible to the tensile moduli measured by UTS.  The influence of the cure volume shrinkage of the vinyl ester matrix materials on their bulk moduli were shown in Fig. 2.  The bulk moduli of the vinyl ester materials were increased with the increase of the cure volume shrinkage of the materials.  It might indicate that the more shrinkage of the cure the tighter the polymer chains held together resulting in an increase of the material mechanical property.

3.     Influence of the Cure Volume Shrinkage of the Matrix Material to the Adhesion between Carbon Fiber and Vinyl Ester

Two sets of ITS composite sample were tested by the ITS to find out the influence of the cure volume shrinkage of the matrix material on the adhesion between fiber and matrix.  One set of composite samples was the vinyl esters having different cure volume shrinkage reinforced by AS4 carbon fiber as received and the other set was the vinyl esters having different cure volume shrinkage reinforced by AS4 carbon fiber with 5% of DGEBA-Jeffamine T-403/Acetone sizing solution.

The test results were shown in Fig. 5.3.   The ITS test results were very interesting, for the samples made with AS4 carbon fiber as received, the interfacial shear strength showed a progressive decrease with the cure volume shrinkage of the matrix material, the bigger the cure volume shrinkage the more significant the decrease of the interfacial shear strength.  In contrast, for the samples made with carbon fiber with DGEBA-T403 sizing, the value of the interfacial shear strength had little change even for big cure volume shrinkage of 8.2%.  This suggested that the cure volume shrinkage would bring thermal residual stress which brought negative effect on the adhesion between carbon fiber and vinyl ester resin.  And it was also suggested that the sizing material might play a very important role in elimination of the residual stresses caused by the cure volume shrinkage. 

Conclusions

The bulk moduli of the vinyl ester resin measured by nano-indentation would increase as the cure volume shrinkages increase. 

The cure volume shrinkage would introduce residual stress which could decrease the interfacial shear strength (IFSS) values suggesting a negative effects on the adhesion between carbon fiber and vinyl ester resin. 

The epoxy sizing on the fiber surface is very important for the relaxation of thermal residual stress caused by cure volume shrinkage.

References

1.     Xu, L., Al-Ostas, A., and Schalek, R., Drzal, L.,  “Improvement of adhesion between vinyl ester resin and carbon fibers by a controlled and designed interphase”, the 2002 Adhesion Society/WCARP-II Meeting, February 2002, Orlando, FL.

2.     S. M. Corbin, M.S. Thesis, Department of Chemical Engineering,MichiganStateUniversity, 1998

3.     L. S. Penn and T. T. Chiao, “Epoxy Resins”, Hand Book of Composite, pp.57~88