Influence of Silane Coupling Agents on the Interlaminar ...cheric.org/PDF/MMR/MR12/MR12-1-0119.pdf · glass fiber surface may form the chemical bonds with reactive amino end groups

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119

Macromolecular Research, Vol. 12, No. 1, pp 119-126 (2004)

Influence of Silane Coupling Agents on the Interlaminar and Thermal Properties of Woven Glass Fabric/Nylon 6 Composites

Donghwan Cho* and Suk Hyang Yun

Department of Polymer Science and Engineering, Kumoh National Institute of Technology, Kumi, Kyungbuk 730-701, Korea

Junkyung Kim, Soonho Lim, Min Park, Sang-Soo Lee, and Geon-Woong Lee

Polymer Hybrid Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea

Received Nov. 6, 2003; Revised Dec. 26, 2003

Abstract: In this study, the influence of silane coupling agents, featuring different organo-functional groups on theinterlaminar and thermal properties of woven glass fabric-reinforced nylon 6 composites, has been by means of short-beam shear tests, dynamic mechanical analysis, scanning electron microscopy, and thermogravimetric analysis. Theresults indicate that the fiber-matrix interfacial characteristics obtained using the different analytical methods agreewell with each other. The interlaminar shear strengths (ILSS) of glass fabric/nylon 6 composites sized with varioussilane coupling agents are significantly improved in comparison with that of the composite sized commercially. ILSSof the composites increases in the order: Z-6076 with chloropropyl groups in the silanes * Z-6030 with methacrylategroups *Z-6020 with diamine groups; this trend is similar to that of results found in an earlier study of interfacialshear strength. The dynamic mechanical properties, the fracture surface observations, and the thermal stability alsosupport the interfacial results. The improvement of the interfacial properties may be ascribed to the differentchemical reactivities of the reactive amino end groups of nylon 6 and the organo-functional groups located at theends of the silane chains, which results from the increased chemical reactivity in order chloropropyl *methacrylate* diamine.

Keywords: interlaminar property, dynamical mechanical property, woven glass fabric/nylon 6 composite, silanecoupling agent.

Introduction

An introduction of interphase by use of coupling or sizingagents to a fiber-reinforced polymer composite material notonly provides the ability to control the interfacial character-istics at the interface between the fiber and the matrix in acomposite, depending on the chemical and physical interac-tion involved, but also contributes to improving variousproperties of a composite material.1-4

The chemical bonding at the fiber-matrix interfaces is crit-ically important to understand the interfacial strength in afiber-reinforced polymer matrix composite. Especially, anapplication of silane-based sizing interphase onto glass fibersurface not only protects the fiber from possible surfacedamages on handling, but also significantly improve theadhesion between the reinforcing fibers and thermosettingor thermoplastic polymer matrix.2,5-9 The chemical bonds can

be formed between the chemical groups at the fiber surfaceand in the polymer matrix resin. For instance, the hydroxylgroups in the silanols [Si(CH3O)] of a silane coupling agentand the hydroxyl groups at the glass fiber surface can formthe chemical bonds. The organo-functional groups locatedat the end of silane chains with the other end anchored at theglass fiber surface may form the chemical bonds with reactiveamino end groups in the nylon 6 molecular chains. Silanecompounds with different organo-functional groups mayimportantly influence the chemical reactivity and the bondingstrength between inorganic glass fiber and organic polymerresin.5

The experimental approaches for examining the interfacialproperties between reinforcing fiber and matrix are largelydivided into two categories. The one is to indirectly use asimplified fiber-matrix specimen for the interfacial mea-surement of a composite material. The typical examples aresingle fiber pull-out test, single fiber microbonding test andembedded single fiber fragmentation test.10-12 These methodsuse a model composite composed of a single filament fiber

D. Cho et al.

120 Macromol. Res., Vol. 12, No. 1, 2004

as representative reinforcing fiber and of a thermoplastic orthermosetting resin as representative matrix to mimic thepractical composite material with corresponding fibers andmatrix. The test results may provide useful information onpredicting the interfacial property of a composite. The otheris to directly use a real composite specimen in the interfacialmeasurement. The typical examples are short-beam sheartest,13,14 microindentation test13,15 fracture toughness test16

and dynamic mechanical analysis.17-19 These methods are alsowidely used because they may provide practical informationon the interfacial properties of a composite material. All themethods above-mentioned may give quantitative information.Also, qualitative interpretation on the interfacial phenomenonmay be obtained from microscopic observations of the fiber-matrix fracture surfaces of a composite.

Consequently, the objective of the present study is toinvestigate the effect of various silane coupling agents withdifferent organo-functional groups on the interlaminar andthermal properties of glass fabric/nylon 6 composites throughshort-beam shear test, dynamic mechanical analysis, fracturesurface observations, and thermal stability test.

Experimental

Materials. E-glass woven fabrics with plain texture (KN1800-HS, Kangnam Chemical Co., Korea) were used asreinforcing fibers in this work. The ‘as-received’ fiber sur-face is commercially sized but the information has not beenreleased from the manufacturer. The average diameter of acontinuous single fiber filament in the fabric is about 16µm. The thickness per ply of the ‘as-received’ fabric is 0.18· 0.02 mm. Pellet type nylon 6 resin (Grade MC100L),supplied in the pellet form from Kanebo Gohsen, Ltd.,Japan, was used as matrix resin. Glass fabrics and nylon 6were sufficiently dried in an oven prior to fabrication ofeach composite specimen.

Four silane coupling agents with different organo-functionalgroups, supplied from Dow Corning Corp. U.S.A., were usedto explore their effects on the interlaminar and dynamicmechanical characteristics of woven glass fabric/nylon 6composites. The commercial sizes on the glass fiber surface

were completely desized at 400oC for 45 min prior to sizingtreatment in the present study. The sizing content of 1.0 wt%has been used throughout this work. Table I summarizes thechemical structure and name of silane-based couplingagents with different organo-functional groups.

Fabrication of Woven Glass Fabric/Nylon 6 Composites.Woven glass fabric/nylon 6 composites were fabricated in acompressive molding machine using a film stacking method.A number of nylon 6 films were prepared repetitively bymelting at 228οC for 30 seconds and then naturally coolingin between two clean and smooth stainless steel plates usinga hot press. A number of glass fabrics and nylon 6 filmswere stacked alternately in a steel mold. The nylon 6 filmsinterleaved between the glass fabrics were remelted, suffi-ciently impregnated and consolidated under given meltingand cooling cycles. The composites were fabricated bymelting at 228oC for 5 min, pressing at 1,000 psi for 1.5 min,and then naturally cooling down to ambient temperature.The applied pressure was 1,000 psi during processing. Thedimensions of the composite fabricated were 50Ý 50 mm.The thickness of each composite specimen was preparedaccording to individual test requirements. The averagethickness per ply in a composite is about 0.32 mm. The ratioof the resin to the fiber in the composite is 6 : 4.

Analysis. A thermogravimetric analyzer (TGA 951, TAInstrument) was used to examine the content of the com-mercial size of ‘as-received’ glass fibers and the thermalstability of the composites. A heating rate of 10oC/min wasused purging a N2 gas of 50 cc/min.

The short-beam shear tests of the composites were con-ducted according to ASTM D-2344 using a UTM (Instron4467). The crosshead speed was 1.3 mm/min and the span-to-depth ratio was 5. Ten specimens from each compositewere used to have the average value of interlaminar shearstrength.

The storage modulus, loss modulus and tan δ of the com-posites were measured in the vertical mode using a dynamicmechanical analyzer (DMA 983, TA Instrument). The heat-ing rate of 2oC/min was used with purging a N2 gas of 50cc/min. Use of the slow rate is to minimize the possiblethermal lag between the program temperature and the com-

Table I. Four Silane-based Coupling Agents Used in the Present Study

No Trade Name Chemical Name Chemical Structure Organo-functional Group

1 Z-6020 N-2-Aminoethyl-3-aminopropyl-trimethoxysilane NH2CH2CH2NH(CH2)3Si(OCH3)3

Diamine

2 Z-6030 3-MethacryloxypropyltrimethoxysilaneCH2=CCH3COO(CH2)3Si(OCH3)3

Methacrylate

3 Z-6040 3-GlycidoxylpropyltrimethoxysilaneOCH2CHCH2O(CH2)3Si(OCH3)3

Epoxide

4 Z-6076 3-ChloropropyltrimethoxysilaneCl(CH2)3Si(OCH3)3

Chloropropyl


Macromol. Res., Vol. 12, No. 1, 2004 121

posite specimen temperature. A fixed frequency of 1 Hz andthe oscillation amplitude of 0.25 mm were used.

The delamination pattern after the short-beam shear testand the fracture pattern of each composite specimen wereobserved using a scanning electron microscopy (SEM, JEOLJSM-5900).

Results and Discussion

In the present work, TGA thermograms for commerciallysized and desized glass fabrics were studied to examine thesizing content in the ‘as-received’ fabric, as shown in Figure1. The result informed that the commercial size starts to loseits weight around 150oC and is completely removed below400oC due to decomposition of the sizing ingredients. Thecontent of the commercial size is estimated to be approxi-mately 1~2% of the total fiber weight used. It is known thatcommercial applications of silane coupling agents includesilanes as an ingredient with additional reactive or nonreac-tive chemicals such as lubricants, antistatic agents, etc.5

Even though the silane is partial constituent of the formula-tions in the sizing system, this TGA result provides usefulinformation to determine the concentration of silane couplingagents to be 1 wt% in the present study. It has been foundfrom microscopic observations that sizing of 1 wt% does notalter significantly the appearance of the surface, as reportedpreviously.12 The thickness has been expected to be lessthan 0.1µm. It is noted that the surface chemistry of glassfiber may be changed due to different interactions betweenthe hydroxyl groups in the fiber surface and the hydroxylgroups of silanols in the silane coupling agents having differ-ent organo-functional groups onto the fiber surface, althoughthe appearance of the surface does not change significantly.

Interlaminar Shear Strength. Composite laminate testsare often used to directly measure fiber-matrix adhesion in afiber-reinforced polymer matrix composite. Short-beam sheartest is one of the frequently used test methods because thefiber-matrix interface dominates the test results and the testmethod is relatively simple with minimum complication ofspecimen preparation.2 Figure 2 compares the interlaminarshear strengths (ILSS) for glass fabric/nylon 6 compositessized with four silane coupling agents. The short-beamshear testing was done with removing the applied load assoon as the initial delamination between the fiber and thematrix takes place through the thickness in a compositeright after the yield point in the stress-strain behavior. Theaveraged ILSS values were obtained from the maximumload of the stress-strain curve for each specimen using thefollowing equation.

τmax = 3Pmax/4b⋅t

where τmax is the ILSS, Pmax is the maximum load, b is thewidth of the specimen, and t is the thickness of the specimen.

The ILSS value is the greatest with the Z-6076 that haschloropropyl organo-functional groups at the molecular ends.Comparing with the commercially sized composite, theILSS is remarkably improved by 196%. The ILSS value inthe case of the Z-6030 is significantly increased by 82%. Itis also slightly increased by 6% in the case of the Z-6020.Such the improvement has been found similarly in theinterfacial shear strength (IFSS) results studied for a singleglass fiber system embedded in nylon 6 microdroplet usinga microbonding test method in an earlier report.12 Eventhough the percent improvement in the ILSS is relativelylower than that in the IFSS, the tendency of the improvementquite agrees with each other, with the exception of the Z-6040. The ILSS value of the Z-6040 is even lower than thatof the commercially sized one. In the single fiber micro-bonding test, the composite sized with the Z-6040 exhibits alarge increase of the IFSS by about 115%. The reason for

Figure 1. TGA result showing the weight change of commer-cially sized material in the glass fabric before and after desizing.

Figure 2. Interlaminar shear strengths for various silane-sizedwoven glass fabric/nylon 6 composites obtained by short-beamshear tests.

D. Cho et al.


this discrepancy has been found from some possible causesoccurred during the composite fabrication in the presentwork. There has been no wetting problem with the formationof the nylon 6 microdroplet on a single glass fiber filamentprepared for microbonding tests. However, in the glassfabric/nylon 6 laminate prepared using the Z-6040 couplingagent only, it has been found that the glass fiber surface isnot sufficiently wetted or impregnated with the nylon 6 resinunder the same processing condition as used for fabricatingother composites. Such the poor wetting is because the Z-6040 of relatively high viscosity puts the fiber filamentstogether. As a result, the resin cannot be properly impreg-nated into the individual filaments bound together, causingthe incomplete fabrication of the composite with pores.Such the wetting problem obviously weakens the interfacialadhesion between the fiber and the matrix and results in thelowest value of ILSS. Therefore, the short-beam shear test

result suggests that the Z-6040 is not appropriate for applyingon the fiber surface to make a glass fabric/nylon 6 compositealthough it increases the IFSS value, as found previously.12

Figure 3 shows the scanning electron microphotographsobserved in the delaminated region after the short-beamshear test for various glass fabric/nylon 6 composites sizedwith different coupling agents. The photos were taken at thesame magnification of Ý 1000. Interlaminar failure wasmicroscopically verified by the delamination pattern ofvariously sized composite specimens. The result indicatesthat the composite sized with the Z-6076 exhibits thestrongest adhesion between the fiber and the matrix, resultingin the highest ILSS with a larger amount of the resinadhered to the glass fibers than in the other compositespecimens, as seen in Figure 3(E). The composite sizedwith the Z-6040 in Figure 3(D) exhibits the weakest inter-laminar failure leaving many debonded fibers behind. It is

Figure 3. Scanning electron microphotographs (Ý 1000) in the delaminated region after the short-beam shear test for woven glass fabric/nylon 6 composites sized with different coupling agents. (A) commercially sized, (B) Z-6020, (C) Z-6030, (D) Z-6040, and (E) Z-6076.



likely that the composite in Figure 3(C) (Z-6030) has thebetter adhesion than the composites in Figure 3(A) (com-mercial size) and 3(B) (Z-6020). These microscopic viewsalso support qualitatively the ILSS measurements.

Fracture Surface. Qualitative information on the effectof silane coupling agents on the interfacial adhesionbetween the fiber and the matrix may be more apparentlyfound from the fracture surface examination of each com-posite. Figure 4 represents the scanning electron micropho-tographs showing the fracture surfaces of nylon 6composites reinforced with glass fabrics desized and sizedwith various coupling agents. The result indicates that theglass fabric/nylon 6 composite (E) sized with the Z-6076exhibits the highest adhesion between the fiber and thematrix resulting in the greatest extent of the resin adheringon the pulled-out fiber surface upon fracture, reflecting a

ductile fracture pattern. As expected, Figure 4(D) micro-scopically demonstrates that use of the Z-6040 decreases thefiber-matrix adhesion in the glass fabric/nylon 6 composite,leaving a number of traces of clearly pulled-out fibersbehind. It is also observed that there is a greater extent of theresin remaining on the fiber surface after fracture in Figure4(B) (Z-6020) and 4(C) (Z-6030), compared with the com-mercially sized counterpart in Figure 4(A) (commercial size).

Dynamic Mechanical Behavior. Since dynamic mechan-ical analysis has been widely used as a powerful tool forstudying the viscoelastic behavior of macromolecularstructure and motion as well as polymer morphology, it hasbeen increasingly utilized for characterizing the interfacialadhesion in fiber-reinforced polymer composite systems interms of storage modulus, loss modulus, and tan δ.20,21 Figures5 and 6 represent the dynamic mechanical behavior of glass/

Figure 4. Scanning electron microphotographs showing the fracture surfaces of nylon 6 composites reinforced with various woven glassfabrics (A) commercially sized, (B) Z-6020, (C) Z-6030, (D) Z-6040, and (E) Z-6076. The scale bar indicates 10µm at magnification ofÝ 1000.

D. Cho et al.


nylon 6 composites fabricated using glass fabrics sized withfour different silane coupling agents and also sized commer-cially for comparison. Figure 5 depicts the variation of thestorage modulus as a function of temperature. The storagemodulus of the composites sized with Z-6030 and Z-6076 isgreater than that of the commercially sized counterpart. Thecomposite sized with Z-6020 exhibits the comparable log E'values with the commercially sized one. However, the mod-ulus of the composite sized with Z-6040 is the lowest. Thisdynamic mechanical result is quite consistent with the ILSSresult. The lowest storage modulus for Z-6040 is attributedto the poor interfacial adhesion between the fiber and thematrix, as described earlier.

It is known that nylon 6 is a semicrystalline polymer havinga glass transition temperature (Tg) of about 55oC and melt-ing point of about 225oC. The Tg of nylon 6 is significantlyenhanced by an incorporation of reinforcing glass fibers.The Tgs of the composites are observed to be in the range of

60~70oC depending on the silane coupling agents used. It is5~15oC greater than the Tg of nylon 6 without reinforcement.The Tg enhancement in the composite is because themolecular motion in the polyamide 6 chain is significantlyrestricted by the reinforcing fibers. Therefore, couplingagents with different organo-functional groups influence theTg due to their different contribution to the fiber-matrixadhesion. The Tg in a fiber reinforced polymer composite, ingeneral, increases with increasing the interfacial strengthbetween the fiber and the matrix. The Tg can be normallydetermined from the maximum peak temperature of a tan δcurve. Figure 6 shows the variation of the tan δ as a functionof temperature for the composites. The dynamic mechanicalresults show that the composite sized with the Z-6076exhibits the greatest Tg while the one with the Z-6040 exhibitsthe lowest. The Tg of the glass/nylon 6 composites sizedwith the silane coupling agents used in this work is increasedby 5~8oC in comparison with the commercially sized one,except for the case of Z-6040. The reason for the lowestvalue of Tg in the case of the Z-6040 is found from theexplanation described earlier.

Thermal Stability. The silane compounds with differentchemical structures may have different thermal stabilities,influencing some possible integrity at the interfaces betweenthe fiber and the matrix once the silane coupling agents areappropriately applied for the adhesion improvement. Figure7 compares the thermal stability of the glass fabric/nylon 6composites studied. All the composite specimens degradebeyond 350oC, showing a dramatic weight loss, which isresponsible for the nylon 6 resin. The composite sized withthe Z-6076 exhibits the highest thermal stability. The resultalso indicates that the composites sized with the silanecoupling agents used here have slightly improved thermalstability in the temperature range prior to degradation. Thisis due to the increased interfacial adhesion between the fiberand the matrix in the composites, which may somewhat

Figure 5. Variations of the storage modulus for woven glass fab-ric/nylon 6 composites sized with different silane couplingagents as a function of temperature.

Figure 6. Variations of the tan δ for woven glass fabric/nylon 6composites sized with different silane coupling agents as a func-tion of temperature.

Figure 7. Thermal stability of woven glass fabric/nylon 6 com-posites sized with different silane coupling agents.



contribute to slightly deactivating and retarding the thermaldegradation. However, one cannot expect exactly the effectof the sizing interphase at elevated temperature above thedegradation. The thermal stability result indicates that theinfluence of the silane coupling agent on the thermal stabil-ity of glass fabric/nylon 6 composites is not distinguishableas much as found in the other examinations in this work.

Figure 8 illustrates proposed reaction schemes betweenthe different organo-functional groups in the silane couplingagents and the amino groups in the nylon 6 resin occurringduring the composite fabrication. It may be assumed that thetrimethoxysilane groups located at the end of each silanecoupling agent result in the same concentration of silanolgroups upon hydrolysis, generating the correspondingnumber of -Si-O- linkages between the glass and silane cou-pling agent at the glass fiber surface. The organo-functionalgroups located at the other end can then react with the nylonmatrix once a reactive functionality is present. In the presentsystem, the chemical reactivity between the organo-func-tional groups located at the end of silane coupling agentswith the other end anchored at the glass fiber surface and thereactive amino end groups of nylon 6 resin differs. Consider-ing nucleophilicity and the unshared electron pair in theamino group, it is believed that the chemical bonds betweenthe silane coupling agent and the nylon 6 can be formedwith increasing frequency based upon the increased chemicalreactivity in the order of chloropropyl*methacrylate*

diamine organo-functional groups. It is expected that theamino group in the resin may not react preferably with thediamine group in the Z-6020 coupling agent. Even thoughthe glass fabric/nylon 6 composite has some difficulties inmaking an acceptable composite for the short-beam sheartest in the case of Z-6040 due to insufficient wet-out betweenthe reinforcing fibers and the resin and accordingly the ILSSof the composite sized with the Z-6040 is observed to belower than that of other counterparts, it may be expectedthat the amino-epoxide reaction substantially takes place withthe comparable reactivity with the Z-6030 having methacry-late groups, as found from the earlier result on the increasedinterfacial adhesion between the single glass fiber and thenylon 6 microdroplet in a microbonding testing system.

Conclusions

The interlaminar shear strengths of woven glass fabric/nylon 6 composites sized with silane coupling agents usedin the work are significantly improved in comparison withthe composite sized commercially. A silane coupling agent(Z-6076) with chloropropyl organo-functional groups showsthe highest interfacial adhesion, followed by the Z-6030with methacrylate group and by the Z-6020 with diaminegroup. The dynamic mechanical properties, the SEM obser-vations of the fracture surfaces, and the thermal stabilityalso support the interfacial result. Combining all the analyti-

Figure 8. Proposed reaction scheme between the different organo-functional groups in the silane coupling agents and the amino groupsin the nylon 6.

D. Cho et al.


cal results, the fiber-matrix interfacial characteristics agreewell with each other.

The improvement of the interlaminar shear strength maybe interpreted in terms of different chemical reactivitybetween organo-functional groups located at the end ofsilane coupling agents with the other end anchored at theglass fiber surface and the reactive amino end groups at thenylon 6 resin. The possibility of the chemical bond formationbetween the silane coupling agent and the nylon 6 increaseswith increasing the chemical reactivity in the order of chlo-ropropyl*methacrylate*diamine organo-functional groups.

In a fiber-matrix composite system, the solubility betweenthe matrix and the sizing interphase apparently influence theinteraction and chemical composition at their interfaces.Also, the interphase materials near the fiber surface maydepend on the concentration of silane coupling agents, givinga gradient of properties with the distance from the fibersurface. The concentration effect will be discussed in afuture paper.

Acknowledgements. This research was financially sup-ported from the Center for Advanced Materials Processing(CAMP) of the 21st Century Frontier R & D Programfunded by the Ministry of Science and Technology, Korea.The authors also thank Mr. Y. Choi for his technical assis-tance of SEM observations.

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Influence of Silane Coupling Agents on the Interlaminar ...cheric.org/PDF/MMR/MR12/MR12-1-0119.pdf · glass fiber surface may form the chemical bonds with reactive amino end groups