The influence of sizing conditions on bending properties of continuousglass fiber reinforced polypropylene composites

H. Hamada*, K. Fujihara, A. HaradaDivision of Advanced Fibro-Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan

Received 18 September 1998; received in revised form 22 November 1999; accepted 19 January 2000

Abstract

In this study, the interphase construction by using binding agents was proposed in continuous glass fiber reinforced polypropylenecomposites. Two types of binding agents modified with maleic anhydride, i.e. low- and high-molecular-weight PP binders were employed.Amino silane coupling agent was also used for the sizing of the glass fibers. In the first step, the sizing conditions with non-binding, low- andhigh-molecular-weight PP binders were compared for bending properties. According to the near infra-red (NIR) analysis of the sized glassfibers, the glass fiber finished with the low-molecular-weight PP binders greatly indicated the existence of the amide bond between the PP binderand the silane coupling agent. This result implies that the matrix PP chain tends to entwine with the PP binder chain. As a consequence, low-molecular-weight PP binder specimens gave the highest bending strength. Furthermore, the concentration ratio of the low-molecular-weight PPbinder was varied with three steps. Matrix PP modified with maleic anhydride was also used in order to investigate the effect of matrixmodification on bending properties. The interphase structure formed by a binder agent and a silane coupling agent in the continuous glassfiber reinforced polypropylene composites was discussed in detail. q 2000 Published by Elsevier Science Ltd.

Keywords: Continuous GF/PP composites; E. Surface treatment; B. Interphase; B. Mechanical properties

1. Introduction

Thermoset composite materials had a dominant share inthe market (about 85% in 1995) as compared with thermo-plastic composite materials [1]. Nowadays, the consumptionof thermoplastic composite materials is increasing relativeto thermoset composite materials because of ecologicalissues. This tendency is due to the development of highperformance polymers, such as polyetheretherketone(PEEK), polyphenylenesulphide (PPS) or polycarbonate(PC), the so-called engineering plastics, which offer excel-lent mechanical properties. On the other hand, polypropy-lene, which was firstly polymerized in 1955 by Natta et al.,has also been recognized as a candidate matrix. The reasonlies in the versatile design ability of polypropylene at themolecular level. For instance, the polymerization techni-ques with various catalysts can control the molecular weightwith the narrow distribution. There is another example,which demonstrated that the copolymerization techniquesgenerate copolymers with different comonomer contents

[2,3]. Therefore, polypropylene exhibits many beneficialpropertieshigh thermal stability, easy processing andresistance to corrosionand comes to be used for thecomposites. By using the copolymerization technology, itis well known that Ethylene Propylene Rubber (EPR) elas-tomer is dispersed into matrix PP for improving the impactresistance in GF/PP injection moldings [4].

Grades of PP composites include particulate (calciumcarbonate or talc) filled PP, glass mat reinforced PP andGF/PP injection moldings. Almost all of them have beenused for the automobile industries. This means that PPcomposites should possess not only high strength but alsohigh toughness and energy absorption under impact loading.These problems provoke the interphase control in themechanical properties of PP composites as well as thermo-set composite materials. With regard to the PP composites,except for the particulate filled PP, it is well known that theinterphase control is conducted with a silane coupling agentand a binding agent or an additive agent such as maleic-anhydride into matrix PP [59]. There is an example forimproving the impact properties using a silane couplingagent in the GF/PP stampable sheets [5]. It can be consid-ered that there were no major obstacles using PP as a matrixinstead of engineering plastics. It can be said that such kinds

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* Corresponding author. Tel.: 1 81-75-724-7844; fax: 1 81-75-724-7800.

E-mail address: hhamada@ipc.kit.ac.jp (H. Hamada).

of technology gave a wide selection of material design to thePP composites. Further, there are possibilities for improvingthe mechanical property.

In this study, the use of a binding agent was proposed as anew method for modifying the interphase in the GF/PPcomposites. The reason lies in the production procedure ofthe glass fiber. Continuous filament glass fibers areproduced by melting the raw materials in a reservoir ortank which feeds the molten glass into a series of platinumbushings, each of which has several hundred holes in itsbase [10]. Next, the filament glass fibers are finished witha sizing solution, which consists of both a silane couplingagent and a binding agent, and they become the condition ofglass strand fibers. In the case of injection moldings, thesestrand fibers are chopped and compounded with matrix PP.The selection of the binding agent is also a crucial issue informing a good interphase. In the case of using urethane as abinding agent in the GF/PP injection moldings, it gives theminus effect for the adhesion between fiber and matrix [11].We consider that there are two interfaces on glass fiberreinforced thermoplastic composites, i.e. a silanebinderand a bindermatrix interface. Besides, in the case of ther-moset composites like GF/Vinylester, the interphase struc-ture constructed by a silane coupling agent is very complexand its mechanical properties are greatly affected by theagent concentration [1214]. Even the silane couplingagent concentration influences composite properties.These phenomena indicate that in the case of thermoplasticcomposite materials, the contribution of both types of inter-face to the macro mechanical behavior of the compositematerials should be discussed in detail. Therefore, in thisstudy, three point bending tests of continuous glass fiber

reinforced polypropylene composites were carried out andthe focused issues were clarified into two steps. Firstly, thecomparison of the sizing conditions with and without twotypes of binding agent was discussed. Next, the concentra-tion ratio of the best PP binder was varied and discussed interms of its influence on bending properties. It is well knownthat maleic-anhydride has been used for the improvement ofadhesion between fiber and matrix on the PP matrix compo-sites. This is due to the non-polarity of polypropylene fromthe viewpoint of polymer chemistry. Therefore, the effect ofmatrix modification was also discussed in detail with thebest sizing conditions.

2. Experimental details

2.1. Materials

The material used in this study was E-glass fiber (fiberdiameter: 24 mm, Nippon Glass Fiber Co., Ltd). Matrixes

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Table 1Specimen types I (comparison of the sizing conditions)

Specimenname

Type offiber

Type ofmatrix

Binder agent Coupling agent

HN N Homo-PP Nothing Amino silaneHL1.0 L1.0 Low-molecular-

weight PP bindercoupling agent

HH1.0 H1.0 High-molecular-weight PP binder

Table 2Specimen types II (influence of the contents ratio and matrix modificationof low-molecular-weight PP binder)

Specimenname

Type offiber

Type ofmatrix

Binder agent Coupling agent

HL L1.0 Homo-PP Low-molecular-weight PP binder

Amino silanecoupling agent

HL1.0 L2.0HL3.0 L3.0ML0.5 L1.0 Modified-PPML1.0 L2.0ML3.0 L3.0

Fig. 1. Fabrication method of continuous glass fiber reinforced polypropy-lene composites.

Table 3Molding conditions of specimens

Molding pressure 4.5 MPaMolding temperature 2208CPre-heating time 1 minHolding time at 2208C 20 minCooling condition Gradual cooling (HN and HH type) gradual

cooling with pressure (HL and ML type)

were homo-polypropylene film (thickness: 30 mm, IdemitsuPetrochemical Co., Ltd) and polypropylene film modifiedwith maleic-anhydride (thickness: 27 mm, K1008/XKP707W, Chisso Co., Ltd). They are denoted as Homo-PP and Modified-PP, respectively. Their detailed polymer-ization processes were not informed by the company. Theglass fibers were finished with an amino silane couplingagent. Furthermore, two types of binding agent whichconsist of maleic-anhydride were employed in order todevelop the interphase property by a binding agent. Here,they are called high- (mean molecular weight 20,000 , 40,000) and low- (mean molecular weight 5,000) PP binder, respectively. Nippon Glass Fiber Co.,Ltd also performed the surface treatment procedures, detailsof which are not given for reasons of commercial confiden-tiality. The sizing conditions of the glass fiber are shown inTables 1 and 2. A concentration ratio of 1.0 times is acommercial-based treatment. The concentration of aminosilane coupling agent in an aqueous solution was maintainedas a constant for each condition. The glass fiber that finishedwith an equal weight of high-molecular-weight PP binder isdenoted H1.0. Furthermore, when this fiber is combinedwith Homo-PP, the specimen is denoted HH1.0.

2.2. Viscosity average molecular weight of polypropylenefilm

In the case of thermoplastic polymers, their properties canbe controlled by their molecular weight. Recently, control-ling the molecular weight has led to the development ofvarious types of polypropylene with unique properties.Therefore, when polypropylene is used as a matrix ofcomposite materials, it is a crucial issue to understand itsmolecular weight. The molecular weight of matrix PP usedin this study cannot be disclosed for commercial reasons.Therefore, the viscosity-average molecular weights ofHomo-PP and Modified-PP were measured by the dynamic

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Fig. 2. Bending specimen geometry: (a) 08 specimen; (b) 908 specimen.

Fig. 3. Loaddisplacement curves of 908 specimen for each sizingcondition.

Fig. 4. Comparison of the bending strength among three types of sizingcondition.

viscosity method. The viscosity was measured by theCannon-Fanske viscometer. For the Cannon-Fanske visc-ometer, the inclination of the two arms of the U-tube verticalhas the effect of placing the centers of the two liquidsurfaces along an axis, which is vertical, even if there is asmall degree of error in the vertical mounting of the visc-ometer. First, polypropylene was dissolved in tetralinsolvent in a silicone-oil bath at 1358C. The concentrationof this dilute solution is C (g/100 ml). The capillary visc-ometer used for dilute solution measurements was made ofglass. It was operated by filling with a suitable volume of the

dilute solution and drawing the liquid level to a point abovethe upper mark. The flow time of the dilute solution isrelated to the relative viscosity (h rel) of the polymer,which is calculated by the following equation:hrel t=t0 1where t and t0 are outflow times of the solution and tetralinsolvent. From the data of h rel, the specific viscosity (h sp) iscalculated as follows:hsp hrel 2 1 2The intrinsic viscosity [h ] has the relationship with h sp asfollows:hspc h1 Ac 3

where A is a constant, which is dependent on the polymersmolecular weight, the solvent and the measurementtemperature. According to the plot of reduced specific visc-osity (h sp/c) versus concentration (c), it was clear that intrin-sic viscosity values of Homo-PP and Modified-PP were 1.53and 1.63, respectively. Generally, the viscosity-averagemolecular weight Mv of polydisperse linear polymer isgiven by the MarkHouwinkSakurada equation as follows:

h KMva 4where K and a are the material constants for a given polymerat a certain temperature, in a certain solvent. When the poly-propylene solute and the tetralin solvent are used, K and a areknown to have the following values.

K 9:17 105 100 ml=g; a 0:80Finally, the viscosity average molecular weight could beobtained as follows:Homo-PP 192; 000

Modified-PP 205; 000Therefore, the comparison of the matrix modification withmaleic-anhydride could be possible.

2.3. Fabrication method of unidirectional glass fiberreinforced polypropylene composites

Continuous glass fiber reinforced polypropylene compo-sites were fabricated by a film stacking method, as shown inFig. 1. Firstly, a winding of strand glass fibers wasperformed unidirectionally on a metallic frame at 10times. Next, a fiber layer and a polypropylene film layerwere stacked alternately. The total number of layers was25. After performing this procedure, the film stacking mate-rials were placed into a hot metal die heated to the requiredtemperature. The compression moldings were carried out byusing the press machine. The molding conditions are indi-cated in Table 3. The molding temperature and the pressurewere 2208C and 4.5 MPa, respectively. The pre-heating andholding times were 1 and 20 min, respectively, at 2208C.

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Fig. 5. SEM photographs of the fracture surface for each sizing condition.

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Fig. 6. Typical chemical reaction between a matrix PP modified with maleic anhydride and a glass fiber finished with a silane coupling agent.

Fig. 7. Chemical reactions in the time of a sizing treatment of the glass fiber.

Fig. 8. NIR spectra of N, L1.0 and H1.0 glass fibers.

After holding at this temperature, gradual cooling was appliedfor the HN and HH type specimens. However, in the case ofHL type specimens, the holding pressure was also maintainedfor 6 h to avoid the generation of the voids. The importanceof the cooling pressure during molding on thermoplasticcomposite materials was reported by Lin Ye [15]. Thefiber volume fraction of all the specimens was 55 vol%.

2.4. Three-point bending test

The bending specimens of the HN and HH types were cutparallel to the transverse direction (i.e. 908), against the fiberorientation, as illustrated in Fig. 2. Furthermore, in the caseof the HL and ML type specimens, they were also cut paral-lel to the longitudinal direction, i.e. 08. In the case of 908specimens, three-point bending tests were carried out atroom temperature with a cross-head speed of 1.0 mm/minwith a 16 mm span length by using an AUTOGRAPH test-

ing machine, while the 08 specimens were tested at roomtemperature by using an INSTRON universal machineunder the condition with a cross-head speed of 3.0 mm/min with a 32 mm span length. Scanning electron micro-scope (SEM) observations were carried out after the tests.

3. Experimental results and discussion

3.1. Comparison among three types of sizing condition

Fig. 3 gives the loaddisplacement curves of HN, HL1.0

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Fig. 9. Stressdisplacement curves of 08 specimen: (a) homo-PP matrix; (b)modified-PP matrix.

Fig. 10. Stressdisplacement curves of the comparison of the matrix modi-fication for each concentration: (a) L0.5; (b) L1.0; and (c) L3.0 glass fibers.

and HH1.0 specimens. Here, the influence of a binder typeon bending properties has been discussed. Because of ourequipment problem, load data was used in the curve insteadof stress. In the case of the HN specimen without a bindingagent, the bending load remained approximately a constantvalue after the maximum load. On the other hand, the othertypes specimen, i.e. HL1.0 and HH1.0, showed a steady fallafter reaching maximum load; especially, a large load dropwas seen in HL1.0. The bending strength of each specimenis shown in Fig. 4. The bending strength of the HL1.0 speci-men exhibits the highest value. The HH1.0 specimenfinished with a high-molecular-weight PP binder, and alsogave a slightly higher value than the HN specimen. It wasfound that the PP binder agent is effective for improving thebending strength of the continuous GF/PP composites. Afterthe bending tests, SEM observations were performed as seen

in Fig. 5. There is a remarkable difference on the fracturesurface between the specimens finished with the PP binderand the non-binder specimen. The matrix PP was notobserved on the fracture surface of the HN specimen. Onthe other hand, PP adhering to the fiber surface wasobserved on the HL1.0 and HH1.0 specimens.

When the glass fibers are finished, a solution that consistsof a silane coupling agent and a binding agent is employed.Generally, maleic-anhydride is used for matrix PP modifi-cation, and it is considered that the chemical reactionbetween glass fibers treated with a silane coupling agentand matrix PP is as illustrated in Fig. 6 [6,9]. Amide groupsare generated due to the reaction between the maleic groupin matrix PP and the amino group in the silane couplingagent. However, in the case of this study, maleic-anhydridewas used for the PP binders. Therefore, the chemicalreactions on the surface of the glass fibers may be different.

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Fig. 11. Relationship between bending strength and concentration ratio oflow-molecular-weight PP binder: (a) homo-PP matrix; (b) modified-PPmatrix.

Fig. 12. Loaddisplacement curves of 908 specimen: (a) homo-PP matrix;(b) modified-PP matrix.

It is considered that chemical reactions between thecoupling agent and the glass fibers, and between the PPbinder and the coupling agent, occur simultaneously, asseen in Fig. 7.

Therefore, near infra-red (NIR) spectroscopy analysis inthe 10002500 nm region was carried out with a Bran 1Luebbe Infraprover II FT-NIR spectrometer to evaluate thesurface condition of glass fibers [16]. Fig. 8 gives the NIRspectra of the N, L1.0 and H1.0 glass fiber strands. Thespectrum of the L1.0 sized glass strongly indicated a bandin the 19002000 nm region, where the combination modeof the amide group is expected. The appearance of the amideband in the spectrum proved that the amide band wasformed between the amino silane coupling agent and thelow-molecular-weight polypropylene binder. Therefore, itis considered that the PP binder chain reacted with the silanecoupling agent and might entwine with the matrix PPchains. On the other hand, in the case of the other glass

fibers, the amide band was not observed. It was expectedthat H1.0 would also give a band due to the amide group inthe region of 19002000 nm. The molecular weight of bothbinders allows us to consider that the high-molecular-weight PP binder is an isotactic polypropylene, while thelow-molecular-weight PP binder is an atactic polypropylenewith low-molecular-weight. Therefore, the number ofmaleic-anhydride groups per unit mass may be smaller inthe former, and seems to lead to the small number of amidegroups in the H1.0 type glass fiber. As a result, the bendingstrength of the HL1.0 specimen gave the highest value.

3.2. Low-molecular-weight PP binder

3.2.1. 08 specimenIn the previous section, the low-molecular-weight PP

binder seems to possess large numbers of amide groupsand to generate an excellent bending strength. Therefore,the concentration ratio of low-molecular-weight PP binderwas changed in three steps in order to determine the effect ofconcentration ratio in detail. Furthermore, the influence ofmatrix modification was also investigated. Stressdisplace-ment curves of 08 specimens are given in Fig. 9. Firstly,specimens with Homo-PP matrix are discussed. It can beunderstood that the stress increased linearly for each speci-men, and reached a maximum stress at 1.1 , 1.3 mm displa-cement. The feature that the stress remains approximatelystable after yield was recognized, and HL0.5 gave thelargest displacement after yield. On the other hand, in themodified-PP specimen, the behavior of stressstrain curveswas dramatically changed and demonstrated the suddenstress drop in the ML0.5 and ML1.0 specimens afteryield. However, the ML3.0 specimen, which possesses thehighest concentration ratio, showed approximately constantstress after yield. Fig. 10 compares stressdisplacementcurves for the PP homopolymer and copolymer for eachconcentration of the binder. The interesting point is thatevery specimen with a modified-PP showed a large stressincrease above about 1.2 mm displacement. Furthermore, itis clear that the linear stress increase region in stressstraincurves is due to matrix modification. Fig. 11 shows therelationship between the bending strength and the concen-tration ratio of the low-molecular-weight PP binder in the 08specimens. Bending strength was 440 MPa in the HL0.5specimen. However, the increase of the concentration ratioled to an improvement in the bending stress (21% higher). Inthe case of the specimen with the modified-PP matrix,amazingly, the opposite tendency was obtained, whichmeans that the ML0.5 specimen gave the highest bendingstrength (1 GPa) and its strength decreased with the increas-ing concentration ratio.

3.2.2. 908 specimenFig. 12 gives the loaddisplacement curves of the 908

specimens. The curves of the Homo-PP specimens showeda small load drop (2 , 3 N) after the maximum load. On the

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Fig. 13. Relationship between bending strength and concentration of low-molecular-weight PP binder: (a) homo-PP matrix; (b) modified-PP matrix.

other hand, the modified-PP specimens gave a large loaddrop after the maximum load, which suggests a brittle fail-ure manner. Fig. 13 indicates the relation between the bend-ing strength and the concentration ratio of low-molecular-weight PP binders in the 908 specimens. With regard to themodified-PP specimens, the bending strength indicatedabout 36 MPa at 0.5 times, and decreased about 30% withthe increasing binder content. On the other hand, the Homo-PP specimens had a peak at 1.0 times and the higheststrength was 16 MPa. The SEM observations of the 908specimens were carried out after the tests, as shown inFig. 14. The observations were focused on the tensile side.On the surface of the glass fibers of the Homo-PP speci-mens, the amounts of matrix PP increased with the concen-tration ratio. The modified-PP specimens formed werestriking fracture surfaces, which means substantial amountsof resin adhere to the fibers for each concentration ratio. It

was impossible to distinguish between the SEM photo-graphs. Besides, many resin cracks, which imply a brittlefracture manner, were also observed (see in Fig. 15).

3.3. Interphase structure in continuous glass fiberreinforced polypropylene composites

With respect to the Homo-PP specimens with the 908 fiberorientation, it can be noticed here that there was a mismatchbetween the strength tendency and the appearance of thefracture surfaces. This means that the HL3.0 specimenwas expected to show the highest strength because muchPP adhered to the surface of the glass fiber in the SEMphotographs. Therefore, glass fibers after just sizing treat-ment procedure were observed in order to confirm thesurface conditions. Fig. 16 shows SEM photographs ofraw glass fibers. It can be understood that the amounts of

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Fig. 14. SEM photographs of 908 specimen for each concentration ratio.

the PP binder increase with the increasing concentrationratio. It was also clear that the PP binder did not adhere tothe fiber surface uniformly. The NIR spectroscopy analysiswas also a great candidate for investigating the surfaceconditions such as Fig. 8. However, the difference seen inthe SEM photographs of the raw glass fibers was notdetected by NIR spectroscopy.

The interphase structure model in the GF/Thermosettingtype composite material, which was reported by Ikuta et al.[17,18], can explain these phenomena. According to thereports, the interphase structure created by a silane couplingagent varies with the concentration of coupling agent in thesolvent. At extremely low concentrations, few silane mole-cules exist on the surface of the glass fibers. On the otherhand, molecular chain networks generated by the silanemolecules were recognized at medium concentrations,which means that the chemical bond between the silanecoupling agent and the glass fibers increases and tends toreact with the matrix resin. Here, the silane coupling agentforming a chemical bond is called chemi-sorbed silane.Furthermore, when a solution with a high concentrationagent was used, the number of chemical bonds betweensilane molecules and glass fibers would be saturated. As aconsequence, the silane molecules physically adhere to theglass fibers or exist between chemi-sorbed silane molecules(called physi-sorbed silane). The physi-sorbed silaneprevents the reaction between the chemi-sorbed silane andthe matrix resin.

Therefore, we consider that the fixation phenomena of thePP binder agent to the glass fiber is the same as that of thesilane coupling agent in the GF/Thermosetting type compo-sites, as shown in Fig. 17. There were few PP binder mole-cules which reacted with silane molecules on the surface ofthe glass fiber at 0.5 times concentration ratio. The optimumconcentration was obtained at 1.0 times, which means thenumber of maleic groups in the PP binder is appropriate forgenerating the amide bond with the silane coupling agent.Finally, at 3.0 times concentration, many PP binders whichdid not react with the silane molecules (physi-sorbed binder)exist on the surface of the glass fibers. According to this

hypothesis, in the case of the 908 specimens with the Homo-PP matrix, there are not many amide bonds due to a shortageof the PP binding agent at 0.5 times concentration. This isthe reason why HL0.5 indicated the lowest bendingstrength. On the other hand, the bending strength increaseddue to the increase in the chemical bonds at 1.0 times.Furthermore, matrix PP around a glass fiber seems to beembrittled by the excessive maleic groups of the PP binderat 3.0 times concentration. In this case, the crack tends topropagate into the region of the modified matrix PP, i.e.around the bindermatrix interface. Therefore, it can beunderstood that the bending strength of the 908 specimensdecreased at 3.0 times concentration, regardless of theobservations of matrix PP adhering to the glass fiber surface.

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Fig. 16. SEM photographs of the raw glass fibers.

Fig. 15. SEM photograph of resin cracks.

In the case of the ML type specimens with the modifiedmatrix PP, the opposite tendency in the bending strengthof the 908 specimens against the HL type specimens isdiscussed as follows. At 0.5 times concentration, the maleicgroups of the modified PP react with the silane moleculeseasily because of the scarcity of the PP binders. Conse-quently, many amide groups are generated between matrixPP and the silane coupling agent and lead to the higheststrength. However, the bending strength decreased withthe increasing concentration ratio. It is considered thatthere might be chemical interruption between the maleicgroup of matrix PP and that of the physi-sorbed binderbecause of the high polarity of the maleic-group. Therefore,the ML3.0 specimens showed the lowest strength. Withregard to the 08 specimens, the tendency of the bendingstrength also can be explained with the above discussion.Therefore, it can be noted here that even in the glass fiberreinforced polypropylene composite material, the theory,which Ikuta et al. has proposed, i.e. the molecular structurein the interphase, should be considered.

4. Conclusion

In the glass fiber reinforced thermoplastic compositeswith the polyolefin polymer matrixes, the surface treatmentof the glass fibers with only a silane coupling agent is notenough to obtain high mechanical properties. This is due tothe non-polarity of the matrix polymers. In the case of theGF/PP composites, maleic-anhydride has been used gener-ally for matrix modification. However, in this study, atten-tion is focused on the binding agent that is needed for theperformance of the textile configurations, and the two typesof the PP binder agents with different molecular weightswere employed. They were modified with maleic-anhy-dride. In the first step, the influence of the sizing conditionswith and without the binder agents on the bending properties

were investigated. As a result, a low-molecular-weight PPbinder was found to be appropriate for generating the high-est bending strength. It was clear that the NIR analysis was agreat candidate for investigating the chemical condition ofthe sized glass fiber. Furthermore, it can be noted that thebending properties of continuous glass fiber reinforced poly-propylene composites are dependent on the molecularweight of the PP binder. In the next step, the concentrationratio of the low-molecular-weight PP binder was varied inthree steps in order to determine the influence of the PPbinder agent on the bending properties in detail. The bend-ing strengths of the 0 and 908 specimens with the modified-PP matrix (i.e. ML type specimens) decreased with theincreasing concentration of the PP binder agent. On theother hand, the bending strength of the HL type specimenswas saturated with the increasing concentration ratio in boththe 0 and 908 specimens. It can be concluded that there wasan optimum concentration of the PP binder in order toobtain high bending properties. The optimum concentrationalso depends on the chemical condition of matrix PP.

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Fig. 17. Interphase structure in continuous glass fiber reinforced polypropylene composites: (a) 0.5 times concentration; (b) 3.0 times concentration.

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