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polypropylene composites
Composites: Part A 30 (1999) 815–822
The effect of fibre content on the mechanical properties of glass fibre mat/polypropylene composites Nam-Jeong Lee, Jyongsik Jang* Department of Chemical Technology, Seoul National University Shinlimdong San 56-1, Kwanakku, Seoul, South Korea Received 20 January 1998; received in revised form 30 July 1998; accepted 15 October 1998
Abstract Glass fibre mat was prepared by the fibre mat-manufacturing machine developed in our laboratory. Glass fibre mat reinforced polypropylene (PP) composites were fabricated with the variation of glass fibre content. Tensile, flexural and high rate impact test was conducted to investigate the effect of glass fibre content on the mechanical properties of the glass fibre mat/PP composite. Deformation and fracture behaviour of the glass fibre mat/PP composites was investigated to study the relationship with the mechanical property data. The tensile and flexural modulus increased with the increment of glass fibre content. However, the tensile and flexural strengths exhibited maximum values and showed a decrease at the higher glass fibre content than this point. The impact absorption energy also exhibited a similar result with the tensile and flexural property data. 䉷 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Glass fibres; A. Thermoplastic resin; Polypropylene; B. Mechanical properties; B. Impact behaviour
1. Introduction Glass fibre mat reinforced thermoplastic (GMT) is a sheet material that can be compression-moulded to manufacture final products. GMT has many advantages such as short processing time, high impact strength, low cost, and weight saving and its product can be manufactured by one forming operation. Therefore, GMT has been competing with sheet moulding compound (SMC), injection moulded thermoplastics, and sheet metal. GMT is commonly used as automotive components, including bumper beam, front ends, and noise shields[1]. In addition, GMT has its potential applications in other fields such as pallets, shipping container, blower housings, and door skins. GMTs are classfied into swirled mat and short fibre mat according to the structure of glass fibre mat. Swirled mat is composed of undispersed glass fibre roving and it has continuously looped structure. However, short fibre mat comprises well-dispersed short fibres. The word ‘‘short fibre’’ in GMT does not mean short fibre in the case of injection moulded composite. In reality, short glass fibre mat has about 5 ⬃ 20 mm long glass fibres, which are much longer than those of injection moulding product. There are various manufacturing methods of GMT. They include melt-impregnation method, slurry deposition * Corresponding author. ⫹82 2 880 7069; fax: ⫹82 2 888 1604. E-mail address: [email protected] (J. Jang)
method, and preforming method [2–5]. Melt impregnation and slurry deposition methods were commercialized to produce semi-finished sheets. In the melt-impregnation method, GMT is fabricated by belt pressing of glass fibre mat and molten polypropylene (PP). The slurry deposition method is based on paper-making method. Glass fibre and PP powder are mixed in water and the mixture is taken out of water. After being dried, glass fibre/PP powder mixture is moulded. In the preforming method, glass fibre/PP powder preform is produced with the aid of blown air with high pressure and the friction through the small diameter pipe. Glass fibre/PP preform is directly heated in hot air and moulded to final product. The main difference between the preforming and the slurry deposition method is mixing medium. Preforming method has no need to manufacture semi-finished sheet, and more complicated structure and controlled microstructure can be made [2]. Additionally, it is dry process, so it does not need to dry water like the slurry deposition method. Fabrication with oriented fibres was also possible by other authors [4]. Thomason et al. reported the effect of the length and the concentration of glass fibre on general mechanical properties of glass fibre reinforced polypropylene [6–8]. Researches about scattering of the mechanical properties [9,10] and fracture behaviours [11–13] were performed. R. Schledjewski et al. performed the dynamic mechanical analysis of glass fibre reinforced PP and have tried to describe the stiffness of GMT-PP as a function of glass
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mat reinforced polypropylene composites were estimated as a part of researches concerning glass fibre mat/PP composite. The tensile and flexural property was examined with the variation of glass fibre content. High-rate impact behaviour of glass fibre mat/PP composite was also observed. Moreover, the deformation behaviour of the glass fibre mat/ PP composite was investigated using optical microscopy and scanning electron microscopy to examine the correlation with the mechanical property data.
2. Experimental
Fig. 1. Schematic presentation of the fibre mat-manufacturing machine.
fibre content and temperature [14]. Notch sensitivity and damage mechanism of glass mat reinforced PP were studied by other workers [15]. We have developed the glass fibre mat-manufacturing machine, which can control the length of glass fibre from millimeters to centimeters. The present manufacturing method is similar to the preforming method aforementioned. However, it differs in the manner of inclusion of PP matrix. By using this machine, glass fibre mat has been able to be prepared with the various content and length of glass fibre. In this study, the mechanical properties of the glass fibre
Fig. 2. The shapes and sizes of specimens for various mechanical test (A) tensile; (B) flexural; (C) impact test.
Glass fibre roving (ER 2310) used as a reinforcing fibre was purchased from Hankook Fibreglass Co. (Korea). The density of ER 2310 is 2.56 g/cm 3. Isotactic polypropylene (HF411X, Samsung Chemical, Korea) was used as a matrix resin. HF411X has its melt index of 6.0 g/10min and its density is 0.91 g/cm 3. Glass fibre mat was prepared using the fibre mat-manufacturing machine, which was designed in our laboratory. The schematic presentation of the fibre-mat manufacturing machine is shown in Fig. 1. It is composed of feeding, cutting, and spreading parts. Glass fibre roving was fed to the cutting part and cut into short fibres of the same length. The length of fibre can be controlled by various speed of cutter with the feeding-roller speed being fixed. In this study, the length of glass fibre was fixed at 10 mm. The glass fibres were dispersed onto the preforated glassfabric/ PTFE sheet with the aid of vacuum operation. The prepared glass fibre mat was heat treated for 3 h at 500⬚C in a furnace to remove presizing and organic impurities on glass fibre surface. PP was weighed according to the fibre volume fraction, and it was dissolved in boiling xylene. The solution was poured into glass fibre mat and the mixture was dried on 60⬚C hot plate in hood for more than 2 days. Afterwards, the glass fibre mat/PP mixture was dried in 80⬚C vacuum oven for 24 h or more. The mixture of glass fibre mat/PP was heated in mould up to 210⬚C. Moulding pressure was 4 MPa and this pressure was maintained during the cooling ( ⫺ 5⬚C/min) to ambient temperature. The volume fraction of glass fibre was varied from 10% to 30% by changing the amount of glass fibre and the calculated PP weight. Void content of the glass fibre mat/PP composite was measured using the Eq. (1) (ASTM D2734). ÿ 1 Vd 100 Td ⫺ Md =Td ; where, Vd is void content (%) Td, theoretical density (g/cm 3), Md, measured density (g/cm 3). Md was measured from the nominal volume and the mass of rectangular specimen. Td was obtained by matrix burning-off method. The specimen was burnt off for 5 h at 500 ⬚C in a furnace. The weight of specimen before and after burning-off was measured. The shape and size of the specimens for mechanical tests are represented in Fig. 2. Tensile test was performed by a
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adhesive onto slideglass. Then, the specimen was sliced with low speed saw and carefully polished to be very thin and clear section. Transmission optical microscope (Labolux 12Pol S) was used to observe the thin section sample. The fractured and/or deformed surfaces of the specimen resulting from mechanical test were observed by scanning electron microscopy (SEM). Scanning electron microscope was JSM T-200 (Jeol) and the specimen was coated with thin gold to prevent charging effect. 3. Results and discussion 3.1. Formation of glass fibre mat
Fig. 3. Photographs of glass fibre mat prepared with (A) as-received glass fibre; (B) ethanol-treated glass fibre.
universal testing machine (Lloyd LR10K). External extensometer was used to measure the elongated strain. The gauge length was 27 mm and the crosshead was moved up with the speed of 1.0 mm/min. Three point bending flexural test was performed according to ASTM D790M. The span length was fixed at 48 mm and crosshead speed was 2.5 mm/min. The impact absorption energy of the glass fibre/PP composite was evaluated using an instrumented impact tester (Radmana ITR2000). The specimen thickness was ca. 2 mm. Driving velocity was ca. 3.26 m/s and the diameter of a striking nose was 15.6 mm. Crack initiation, crack propagation, and total impact absorption energy were calculated from the load-displacement curves. In order to observe the structure and failure behaviour of the glass fibre mat/PP composite during the mechanical test, the sample with thin section was prepared. The specimen was embedded using an epoxy resin and cut by low speed saw. After polishing the surface, it was fixed with epoxy
In this experiment, the modified preforming method was used to manufacture glass fibre mat. Other preforming methods used small diameter pipe and high-pressure air blowing to facilitate friction between glass fibres and pipe walls [2–5]. However, this modified method did not use air blowing with high pressure and long pipe. Therefore, in order to get a piece of glass fibre mat with good dispersion, glass fibre filaments of the roving need to be spread during the cutting and spreading process. When glass fibre roving was used as received, the glass fibre filaments could not be dispersed (see Fig. 3(A)). It was because of good adhesion between glass fibre filaments originating from sizing on the as-received glass fibre surface. In order to remove the sizing, glass fibre roving was immersed in ethanol for 24 h and dried in hood for 2 days or more. This treatment removed some sizing from the glass fibre surface to weaken the adhesion force between glass filaments. Fig. 3(B) shows the shape of the glass fibre mat prepared with desized glass fibre roving. It is clearly shown that the glass fibre filaments are well dispersed. When the glass fibre mat was impregnated with PP/xylene solution and dried, the well-wetted mixture of glass fibre mat and PP could be obtained. The mixture could be hotpressed to produce the final specimen. In Fig. 4, the thin polished section of the glass fibre mat/ PP composite shows the glass fibre distribution in PP matrix. Even if some glass fibres are in agglomerated state, most of glass fibres are well dispersed. Compared with the case of 10 vol% glass fibre content (Fig. 4(A)), the composite with 30 vol% glass fibre (Fig. 4(B)) has more agglomerated glass fibres. In the case of the composite with 30 vol% glass fibre (Fig. 4(C)), the stream-lined void agglomerate is found, which is thought to influence the mechanical property of the composite. 3.2. Tensile properties Fig. 5 represents the tensile modulus of the glass fibre mat/PP composite as a function of the glass fibre volume fraction. It was observed in this experiment that there was large scatter in the tensile property values, so that the error bar was not included in the figure. This phenomenon is
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Fig. 5. The tensile modulus of the glass fibre mat/PP composite as a function of the glass fibre volume fraction.
Fig. 4. Optical micrographs of the thin section of the glass fibre mat/PP composite (A) 10 vol%; (B) 30 vol%; (C) 30 vol% (void with stream-line shape).
because of the non-uniform and anisotropic nature of glass fibre mat in the composite. Similar observations were reported from other researches [9,10]. Ericson et al. described the local inhomogeneities as low fibre content, very high fibre content with poor melt impregnation or unfavorable fibre orientation [2]. The tensile moduli of the glass fibre/PP composite showed 3–7 GPa according to the nominal glass fibre content. These values are in agreement with tensile modulus data from other studies [13]. The tensile modulus of the glass fibre mat/PP composite
increases almost linearly with the increment of glass fibre volume fraction. The tensile modulus seems to obey the rule of mixture well. However, the composite with 30% glass fibre has lower tensile modulus than expected by the rule of mixture. This result can be explained by its void content. The void content of the glass fibre mat/PP composite is shown in Table 1. Although the data show large scatter, the composite with higher glass fibre content has higher void content. Large content of glass fibre is thought to prevent PP melt from wetting glass fibre mat at the moulding process. Moreover, it is well known that the melt viscosity of PP is relatively high. It was reported in the glass/PBT system[3] that the reinforcing efficiency of the fibres began to fall when the fibre concentration exceeded 20 vol%. This was because of the inability to produce fully wet fibres at concentrations above this level. It was reported that the tensile strength of GMT increases with glass fibre volume fraction up to 20 vol% [2]. It was said in another report [7] that the tensile strength increases approximately with increasing fibre content up to 35%. However, in this study, the tensile strength of the glass fibre/PP composite does not increase continuously as the glass fibre content increases, but rather shows maximum value (Fig. 6). In spite of the highest tensile modulus, the composite involving 30 vol% glass fibre has the lowest tensile strength. It is because of insufficient wetting of PP into glass fibre and poor fibre-matrix adhesion from heat treatment of glass fibre. In the case of higher fibre volume fraction, there are many fibre ends in unit volume. This higher stress concentration results in crack propagation at finite localized region and the region can not resist given tensile stress any more. Whereas, in the case of low content of glass fibre, the number of fibre ends in unit volume is relatively low. This prevents the localized stress concentration, and consequently the deformation takes place in the entire gauge length of the tensile specimen.
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Table 1 Void content of the glass fibre/PP composite as a function of glass fibre content Glass fibre content (vol%) Void content (vol%)
10 2.5 ^ 0.7
15 3.1 ^ 0.9
In conclusion, the maximum tensile strength occurred at 15% or 20% glass fibre volume fraction. It results from the combination of the higher modulus and relatively non-localized deformation at the gauge length region. The tensile strength values of the composite are relatively lower than the other reported values. In this experiment, glass fibre was treated with hot air to remove the presizing, so that the fibrematrix adhesion is very poor. This treatment also seems to lessen the strength of glass fibre itself. 3.3. Flexural properties Fig. 7 represents the flexural modulus of the glass fibre mat/PP composite with the variation of glass fibre content. As the content of glass fibre is raised, the flexural modulus of the composite increases. Up to 20% by volume, the flexural modulus shows linear increment with the content of glass fibre, showing good correlation with rule of mixture. At the content of 25% or more, however, the flexural modulus deviates from the linearity, as observed in the tensile modulus data. The flexural strength of the glass fibre mat/PP composite is shown in Fig. 8. There is the maximum flexural strength at 15 vol% glass fibre content, exhibiting decreased values over that content. The typical stress–strain curves for the flexural specimens with the variation of glass fibre content are shown in Fig. 9. It can be seen that in the case of lower glass fibre content (10%–20%), the slope (viz. stress/strain) varies just before maximum stress. Whereas, in the case of higher glass fibre content (25%–30%), the slope does not
20 3.3 ^ 1.2
25 4.2 ^ 1.1
30 6.4 ^ 2.0
deviate from the linearity nearly up to the maximum stress. In addition, the stress falls drastically after the maximum point. However, the composite with lower fibre content can endure the stress after the maximum to a considerable extent. In order to investigate the stressed state during the flexural test the thin section of the specimen, which had been given tensile stress, was prepared. Fig. 10 shows the optical micrographs of the bent composite containing 10 vol% and 30 vol% of glass fibre. In the case of 10 vol% glass fibre content (Fig. 10(A)), there exist many small cracks (especially around the glass fibre) at the opposite side of loaded surface of the flexural test specimen. These cracks are formed in the same direction of flexural loading. Many small cracks means that the stress given by flexural loading can be released over large area. Additionally, the resulted shape of bent specimen exhibits large curvature. However, the composite with glass fibre content of 30 vol% induces a large crack in one spot of the specimen (Fig. 10(B)). Although there exist several small cracks around the large crack, the number and size of small cracks are very small. The curvature of the bent specimen is very small and the straight line of the specimen remains nearby the large crack. Many glass fibre ends at the specimen surface result in easy crack initiation because of the stress concentration. Moreover, the initiated crack propagates more easily, which means that the cracked specimen cannot persist the given load any more. The above result agrees well with the stress–strain behaviour of the composite (Fig. 9). 3.4. Impact properties
Fig. 6. The tensile strength of the glass fibre mat/PP composite as a function of the glass fibre volume fraction.
The crack initiation, crack propagation and total impact absorption energies of glass fibre mat/PP composites according to glass fibre content are given in Table 2. The data show very large scatter because of the non-homogeneous structure of the glass fibre mat/PP composite. The crack propagation energy is higher than the crack initiation energy through the whole range of glass fibre content. This means that more impact energy is dissipated at the crack propagation stage than at the stage before crack is initiated. When the composite contains glass fibre of 20% by volume, the total impact absorption energy has the maximum value. This result indicates that as glass fibre is included in the composite, impact energy dissipation originated from fibre inclusion occurs more largely. Main mechanisms are regarded as the debonding between glass fibre and PP matrix and fibre pull-out [12,13,16]. The plastic deformation of the PP matrix also contributes to the impact energy absorption. Gupta et al. suggested that in glass/PP
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Fig. 7. The flexural modulus of the glass fibre mat/PP composite as a function of the glass fibre volume fraction.
Fig. 9. Typical stress-strain curves for the flexural specimen with the variation of glass fibre content.
system, the fracture energy absorption was dominated by contribution from matrix plasticity [17]. The matrix plastic deformation occurs either through homogeneous deformation of the matrix or from localized deformation around fibre ends. When more amount of glass fibre content than 20% by volume is included in the composite, the total impact absorption energy exhibits a decreased value. From other result[8], it could be observed that impact strength obtained by charpy test increased almost linearly with fibre concentration. However, that result was obtained from the composite having up to about 20% by volume of glass fibre. In the photograph of the specimen after impact test (Fig. 11), it is shown that the composite containing 10 vol% glass
fibre shows large damage area, but in reality, the fracture was dominated by brittle failure of matrix resin itself. As glass fibre content increases, the composite is deformed through large damaged area in bent state. However, the
Fig. 8. The flexural strength of the glass fibre mat/PP composite as a function of the glass fibre volume fraction.
Fig. 10. Optical micrographs of thin section of the specimen after flexural test; (A) 10 vol%; (B) 30 vol%.
N.-J. Lee, J. Jang / Composites: Part A 30 (1999) 815–822 Table 2 Impact absorption energy of the glass fibre mat/PP composite as a function of glass fibre content Glass fibre content (vol%)
Ei (J/mm)
Ep (J/mm)
Et (J/mm)
10 15 20 25 30
1.10 1.26 1.28 1.29 0.68
2.20 2.26 2.52 1.31 1.18
3.30 3.52 3.80 2.60 1.86
composite containing more than 20 vol% glass fibre has smaller damaged area at the impact loading spot, and does not give rise to any deformation around the spot. Fig. 12 depicts the scanning electron micrographs of the fracture surfaces of the glass fibre mat/PP composite after impact test. In the case of the composite containing 10 vol% of glass fibre (Fig. 12(A)), the number of fibres are small and the glass fibres from the fibre pull-out process are short in comparison with that of 20 vol% case (Fig. 12(B)). The fracture surface of matrix region shows the brittle failure of PP, showing little plastic deformation resulting from fibre ends. When glass fibre content is 20% by volume, the glass fibres protruded from the fracture surface plane are longer. Additionally, the plastic deformation of PP matrix is easily observed near the fibre pulled-out region. When 30 vol% of glass fibre is included in the composite (Fig. 12(C)), the length of fibres is shorter than that of 20% case. When the composite has more amount of glass fibre than optimum content, there is insufficient PP matrix to be plastically deformed. In addition, the fibre breakage takes place more frequently than the case of 20 vol% fibre content. However, fibre breakage is not known as main impact energy absorption mechanism. When fibre content is low, fibre-originated mechanisms do not play a role of impact energy absorption and the amount of localized deformation from fibre ends is small. When fibre content is higher, the volume of matrix resin is too small to deform plastically. In addition, strength
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reduction of the composite is thought to be one of the reason of impact absorption energy reduction. Strength reduction of the composite is revealed by the lowest crack initiation energy of the composite with 30% glass fibre among any other ones. When the impact load is given to the composite with lower strength, the composite cannot transfer the energy around the loading spot. Hence, the impact energy absorption mechanisms such as fibre debonding, fibre pullout does not occur. Conclusively, the optimum fibre content exists from the point of view of impact absorption energy and it is determined by the relative amount of each impact energy dissipation mechanism; fibre–matrix debonding, fibre fracture, fibre pull-out, matrix plastic deformation. 4. Conclusions From the investigation of the effect of glass fibre content on the mechanical properties of the glass fibre mat reinforced polypropylene composite, the following results were obtained. 1. As glass fibre content increased, the tensile and flexural modulus of the glass fibre mat/PP composite showed a linear increment. However, the higher volume fraction of glass fibre than optimum content induced a drop of the reinforcing efficiency of fibres because of higher void content. 2. The tensile and flexural strength of the composite increased up to glass fibre content of 15–20 vol% and then decreased with more glass fibre content. This behaviour is explained by the localized crack formation induced by the stress concentration from glass fibre ends and poor fibre-matrix adhesion strength. 3. The total impact absorption energy of the composite showed the maximum value at 20 vol% glass fibre content, and then decreased at more fibre content. This phenomenon resulted from the variation of each impact absorbing mechanism according to glass fibre content.
Fig. 11. Photographs of the glass fibre mat/PP composite after instrumented impact test (A) 10 vol%; (B) 15 vol%; (C) 20 vol%; (D) 25 vol%; (E) 30 vol%.
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was limited and the reduced strength of the composite prevented other fibre-originated fracture mechanisms.
References
Fig. 12. SEM micrographs of fracture surfaces of the glass fibre mat/PP composite from impact test (A) 10 vol%; (B) 20 vol%; (C) 30 vol%.
As glass fibre content increased, failure processes such as debonding, fibre pull-out and locally plastic deformation were increased. However, at the fibre content above optimum point, the plastic deformation
[1] Berglund LA, Ericson ML. Glass mat reinforced polypropylene. In: Karger-Kocsis J, editor. Polypropylene: structure, blends and composites, vol. 3: Composites. London: Chapman and Hall, Cambridge, 1995, p. 202–227. [2] Ericson ML, Berglund LA. The effect of microstructure on the elastic modulus and strength of preformed and commercial GMTs. Polymer Compos 1993;14(1):35–41. [3] Hiscock DF, Bigg DM. Long-fibre-reinforced thermoplastic matrix composites by slurry deposition. Polymer Compos 1989;10(3):145– 149. [4] Ericson ML, Berglund LA. Processing and mechanical properties of orientated preformed glass-mat-reinforced thermoplastics. Compos Sci Technol 1993;49:121–130. [5] Ericson M, Berglund L. Deformation and fracture of glass-mat-reinforced polypropylene Compos Sci Technol 1992;43:269–281. [6] Thomason JL, Vlug MA. The influence of fibre length and concentration on the properties of glass fibre-reinforced polypropylene Part 1. Tensile and flexural modulus. Composites Part A 1996;27A:477–484. [7] Thomason JL, Vlug MA, Schipper G, Krikor HGLT. The influence of fibre length and concentration on the properties of glass fibre-reinforced polypropylene Part 3. Strength and strain at failure. Composites Part A 1996;27A:1075–1084. [8] Thomason JL, Vlug MA. The influence of fibre length and concentration on the properties of glass fibre-reinforced polypropylene Part 4 Impact properties. Composites Part A 1997;28A:277–288. [9] Stokes VK. Random glass mat reinforced thermoplastic composites. Part I Phenomenology of tensile modulus variations. Polymer Compos 1990;11(1):32–44. [10] Stokes VK. Random glass mat reinforced thermoplastic composites. Part II Characterization of the tensile strength. Polymer Compos 1990;11(6):354–367. [11] Kander RG, Siegmann A. Mechanism of damage and failure in an orthotropic glass/polypropylene composite. Polymer Compos 1992;13(3):154–168. [12] Gupta VB, Mittal RK, Sharma PK, Mening G, Wolters J. Some studies on glass fibre-reinforced polypropylene. Part II Mechanical properties and their dependence on fibre length, interfacial adhesion and fibre dispersion. Polymer Compos 1989;10(1):16–27. [13] Karger-Kocsis J. Microstructural aspects of fracture in polypropylene and in its filled, chopped fibre and fibre mat reinforced composites. In: Karger-Kocsis J, editor. Polypropylene: structure, blends and composites, vol. 3, Composites. London: Chapman and Hall, 1995, p. 142– 201. [14] Schledjewski R, Karger-Kocsis J. Dynamic mechanical analysis of glass mat reinforced polypropylene (GMT-PP). J Thermoplastic Compos Mater 1994;7:270–277. [15] Lindhagen J, Berglund L. Notch sensitivity and damage mechanisms of glass mat reinforced polypropylene. Polymer Compos 1997;18:40–47. [16] Wells JK, Beaumont PWR. Debonding and pull-out processes in fibrous composites. J Mater Sci 1985;20:1275–1284. [17] Gupta VB, Mittal RK, Goel M. Energy absorbing mechanisms in short-glass fibre-reinforced polypropylene. Compos Sci Technol 1990;37:353–369.
The effect of fibre content on the mechanical properties of glass fibre mat/polypropylene composites Nam-Jeong Lee, Jyongsik Jang* Department of Chemical Technology, Seoul National University Shinlimdong San 56-1, Kwanakku, Seoul, South Korea Received 20 January 1998; received in revised form 30 July 1998; accepted 15 October 1998
Abstract Glass fibre mat was prepared by the fibre mat-manufacturing machine developed in our laboratory. Glass fibre mat reinforced polypropylene (PP) composites were fabricated with the variation of glass fibre content. Tensile, flexural and high rate impact test was conducted to investigate the effect of glass fibre content on the mechanical properties of the glass fibre mat/PP composite. Deformation and fracture behaviour of the glass fibre mat/PP composites was investigated to study the relationship with the mechanical property data. The tensile and flexural modulus increased with the increment of glass fibre content. However, the tensile and flexural strengths exhibited maximum values and showed a decrease at the higher glass fibre content than this point. The impact absorption energy also exhibited a similar result with the tensile and flexural property data. 䉷 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Glass fibres; A. Thermoplastic resin; Polypropylene; B. Mechanical properties; B. Impact behaviour
1. Introduction Glass fibre mat reinforced thermoplastic (GMT) is a sheet material that can be compression-moulded to manufacture final products. GMT has many advantages such as short processing time, high impact strength, low cost, and weight saving and its product can be manufactured by one forming operation. Therefore, GMT has been competing with sheet moulding compound (SMC), injection moulded thermoplastics, and sheet metal. GMT is commonly used as automotive components, including bumper beam, front ends, and noise shields[1]. In addition, GMT has its potential applications in other fields such as pallets, shipping container, blower housings, and door skins. GMTs are classfied into swirled mat and short fibre mat according to the structure of glass fibre mat. Swirled mat is composed of undispersed glass fibre roving and it has continuously looped structure. However, short fibre mat comprises well-dispersed short fibres. The word ‘‘short fibre’’ in GMT does not mean short fibre in the case of injection moulded composite. In reality, short glass fibre mat has about 5 ⬃ 20 mm long glass fibres, which are much longer than those of injection moulding product. There are various manufacturing methods of GMT. They include melt-impregnation method, slurry deposition * Corresponding author. ⫹82 2 880 7069; fax: ⫹82 2 888 1604. E-mail address: [email protected] (J. Jang)
method, and preforming method [2–5]. Melt impregnation and slurry deposition methods were commercialized to produce semi-finished sheets. In the melt-impregnation method, GMT is fabricated by belt pressing of glass fibre mat and molten polypropylene (PP). The slurry deposition method is based on paper-making method. Glass fibre and PP powder are mixed in water and the mixture is taken out of water. After being dried, glass fibre/PP powder mixture is moulded. In the preforming method, glass fibre/PP powder preform is produced with the aid of blown air with high pressure and the friction through the small diameter pipe. Glass fibre/PP preform is directly heated in hot air and moulded to final product. The main difference between the preforming and the slurry deposition method is mixing medium. Preforming method has no need to manufacture semi-finished sheet, and more complicated structure and controlled microstructure can be made [2]. Additionally, it is dry process, so it does not need to dry water like the slurry deposition method. Fabrication with oriented fibres was also possible by other authors [4]. Thomason et al. reported the effect of the length and the concentration of glass fibre on general mechanical properties of glass fibre reinforced polypropylene [6–8]. Researches about scattering of the mechanical properties [9,10] and fracture behaviours [11–13] were performed. R. Schledjewski et al. performed the dynamic mechanical analysis of glass fibre reinforced PP and have tried to describe the stiffness of GMT-PP as a function of glass
1359-835X/99/$ - see front matter 䉷 1999 Elsevier Science Ltd. All rights reserved. PII: S1359-835 X( 98)00 185-7
816
N.-J. Lee, J. Jang / Composites: Part A 30 (1999) 815–822
mat reinforced polypropylene composites were estimated as a part of researches concerning glass fibre mat/PP composite. The tensile and flexural property was examined with the variation of glass fibre content. High-rate impact behaviour of glass fibre mat/PP composite was also observed. Moreover, the deformation behaviour of the glass fibre mat/ PP composite was investigated using optical microscopy and scanning electron microscopy to examine the correlation with the mechanical property data.
2. Experimental
Fig. 1. Schematic presentation of the fibre mat-manufacturing machine.
fibre content and temperature [14]. Notch sensitivity and damage mechanism of glass mat reinforced PP were studied by other workers [15]. We have developed the glass fibre mat-manufacturing machine, which can control the length of glass fibre from millimeters to centimeters. The present manufacturing method is similar to the preforming method aforementioned. However, it differs in the manner of inclusion of PP matrix. By using this machine, glass fibre mat has been able to be prepared with the various content and length of glass fibre. In this study, the mechanical properties of the glass fibre
Fig. 2. The shapes and sizes of specimens for various mechanical test (A) tensile; (B) flexural; (C) impact test.
Glass fibre roving (ER 2310) used as a reinforcing fibre was purchased from Hankook Fibreglass Co. (Korea). The density of ER 2310 is 2.56 g/cm 3. Isotactic polypropylene (HF411X, Samsung Chemical, Korea) was used as a matrix resin. HF411X has its melt index of 6.0 g/10min and its density is 0.91 g/cm 3. Glass fibre mat was prepared using the fibre mat-manufacturing machine, which was designed in our laboratory. The schematic presentation of the fibre-mat manufacturing machine is shown in Fig. 1. It is composed of feeding, cutting, and spreading parts. Glass fibre roving was fed to the cutting part and cut into short fibres of the same length. The length of fibre can be controlled by various speed of cutter with the feeding-roller speed being fixed. In this study, the length of glass fibre was fixed at 10 mm. The glass fibres were dispersed onto the preforated glassfabric/ PTFE sheet with the aid of vacuum operation. The prepared glass fibre mat was heat treated for 3 h at 500⬚C in a furnace to remove presizing and organic impurities on glass fibre surface. PP was weighed according to the fibre volume fraction, and it was dissolved in boiling xylene. The solution was poured into glass fibre mat and the mixture was dried on 60⬚C hot plate in hood for more than 2 days. Afterwards, the glass fibre mat/PP mixture was dried in 80⬚C vacuum oven for 24 h or more. The mixture of glass fibre mat/PP was heated in mould up to 210⬚C. Moulding pressure was 4 MPa and this pressure was maintained during the cooling ( ⫺ 5⬚C/min) to ambient temperature. The volume fraction of glass fibre was varied from 10% to 30% by changing the amount of glass fibre and the calculated PP weight. Void content of the glass fibre mat/PP composite was measured using the Eq. (1) (ASTM D2734). ÿ 1 Vd 100 Td ⫺ Md =Td ; where, Vd is void content (%) Td, theoretical density (g/cm 3), Md, measured density (g/cm 3). Md was measured from the nominal volume and the mass of rectangular specimen. Td was obtained by matrix burning-off method. The specimen was burnt off for 5 h at 500 ⬚C in a furnace. The weight of specimen before and after burning-off was measured. The shape and size of the specimens for mechanical tests are represented in Fig. 2. Tensile test was performed by a
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adhesive onto slideglass. Then, the specimen was sliced with low speed saw and carefully polished to be very thin and clear section. Transmission optical microscope (Labolux 12Pol S) was used to observe the thin section sample. The fractured and/or deformed surfaces of the specimen resulting from mechanical test were observed by scanning electron microscopy (SEM). Scanning electron microscope was JSM T-200 (Jeol) and the specimen was coated with thin gold to prevent charging effect. 3. Results and discussion 3.1. Formation of glass fibre mat
Fig. 3. Photographs of glass fibre mat prepared with (A) as-received glass fibre; (B) ethanol-treated glass fibre.
universal testing machine (Lloyd LR10K). External extensometer was used to measure the elongated strain. The gauge length was 27 mm and the crosshead was moved up with the speed of 1.0 mm/min. Three point bending flexural test was performed according to ASTM D790M. The span length was fixed at 48 mm and crosshead speed was 2.5 mm/min. The impact absorption energy of the glass fibre/PP composite was evaluated using an instrumented impact tester (Radmana ITR2000). The specimen thickness was ca. 2 mm. Driving velocity was ca. 3.26 m/s and the diameter of a striking nose was 15.6 mm. Crack initiation, crack propagation, and total impact absorption energy were calculated from the load-displacement curves. In order to observe the structure and failure behaviour of the glass fibre mat/PP composite during the mechanical test, the sample with thin section was prepared. The specimen was embedded using an epoxy resin and cut by low speed saw. After polishing the surface, it was fixed with epoxy
In this experiment, the modified preforming method was used to manufacture glass fibre mat. Other preforming methods used small diameter pipe and high-pressure air blowing to facilitate friction between glass fibres and pipe walls [2–5]. However, this modified method did not use air blowing with high pressure and long pipe. Therefore, in order to get a piece of glass fibre mat with good dispersion, glass fibre filaments of the roving need to be spread during the cutting and spreading process. When glass fibre roving was used as received, the glass fibre filaments could not be dispersed (see Fig. 3(A)). It was because of good adhesion between glass fibre filaments originating from sizing on the as-received glass fibre surface. In order to remove the sizing, glass fibre roving was immersed in ethanol for 24 h and dried in hood for 2 days or more. This treatment removed some sizing from the glass fibre surface to weaken the adhesion force between glass filaments. Fig. 3(B) shows the shape of the glass fibre mat prepared with desized glass fibre roving. It is clearly shown that the glass fibre filaments are well dispersed. When the glass fibre mat was impregnated with PP/xylene solution and dried, the well-wetted mixture of glass fibre mat and PP could be obtained. The mixture could be hotpressed to produce the final specimen. In Fig. 4, the thin polished section of the glass fibre mat/ PP composite shows the glass fibre distribution in PP matrix. Even if some glass fibres are in agglomerated state, most of glass fibres are well dispersed. Compared with the case of 10 vol% glass fibre content (Fig. 4(A)), the composite with 30 vol% glass fibre (Fig. 4(B)) has more agglomerated glass fibres. In the case of the composite with 30 vol% glass fibre (Fig. 4(C)), the stream-lined void agglomerate is found, which is thought to influence the mechanical property of the composite. 3.2. Tensile properties Fig. 5 represents the tensile modulus of the glass fibre mat/PP composite as a function of the glass fibre volume fraction. It was observed in this experiment that there was large scatter in the tensile property values, so that the error bar was not included in the figure. This phenomenon is
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Fig. 5. The tensile modulus of the glass fibre mat/PP composite as a function of the glass fibre volume fraction.
Fig. 4. Optical micrographs of the thin section of the glass fibre mat/PP composite (A) 10 vol%; (B) 30 vol%; (C) 30 vol% (void with stream-line shape).
because of the non-uniform and anisotropic nature of glass fibre mat in the composite. Similar observations were reported from other researches [9,10]. Ericson et al. described the local inhomogeneities as low fibre content, very high fibre content with poor melt impregnation or unfavorable fibre orientation [2]. The tensile moduli of the glass fibre/PP composite showed 3–7 GPa according to the nominal glass fibre content. These values are in agreement with tensile modulus data from other studies [13]. The tensile modulus of the glass fibre mat/PP composite
increases almost linearly with the increment of glass fibre volume fraction. The tensile modulus seems to obey the rule of mixture well. However, the composite with 30% glass fibre has lower tensile modulus than expected by the rule of mixture. This result can be explained by its void content. The void content of the glass fibre mat/PP composite is shown in Table 1. Although the data show large scatter, the composite with higher glass fibre content has higher void content. Large content of glass fibre is thought to prevent PP melt from wetting glass fibre mat at the moulding process. Moreover, it is well known that the melt viscosity of PP is relatively high. It was reported in the glass/PBT system[3] that the reinforcing efficiency of the fibres began to fall when the fibre concentration exceeded 20 vol%. This was because of the inability to produce fully wet fibres at concentrations above this level. It was reported that the tensile strength of GMT increases with glass fibre volume fraction up to 20 vol% [2]. It was said in another report [7] that the tensile strength increases approximately with increasing fibre content up to 35%. However, in this study, the tensile strength of the glass fibre/PP composite does not increase continuously as the glass fibre content increases, but rather shows maximum value (Fig. 6). In spite of the highest tensile modulus, the composite involving 30 vol% glass fibre has the lowest tensile strength. It is because of insufficient wetting of PP into glass fibre and poor fibre-matrix adhesion from heat treatment of glass fibre. In the case of higher fibre volume fraction, there are many fibre ends in unit volume. This higher stress concentration results in crack propagation at finite localized region and the region can not resist given tensile stress any more. Whereas, in the case of low content of glass fibre, the number of fibre ends in unit volume is relatively low. This prevents the localized stress concentration, and consequently the deformation takes place in the entire gauge length of the tensile specimen.
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Table 1 Void content of the glass fibre/PP composite as a function of glass fibre content Glass fibre content (vol%) Void content (vol%)
10 2.5 ^ 0.7
15 3.1 ^ 0.9
In conclusion, the maximum tensile strength occurred at 15% or 20% glass fibre volume fraction. It results from the combination of the higher modulus and relatively non-localized deformation at the gauge length region. The tensile strength values of the composite are relatively lower than the other reported values. In this experiment, glass fibre was treated with hot air to remove the presizing, so that the fibrematrix adhesion is very poor. This treatment also seems to lessen the strength of glass fibre itself. 3.3. Flexural properties Fig. 7 represents the flexural modulus of the glass fibre mat/PP composite with the variation of glass fibre content. As the content of glass fibre is raised, the flexural modulus of the composite increases. Up to 20% by volume, the flexural modulus shows linear increment with the content of glass fibre, showing good correlation with rule of mixture. At the content of 25% or more, however, the flexural modulus deviates from the linearity, as observed in the tensile modulus data. The flexural strength of the glass fibre mat/PP composite is shown in Fig. 8. There is the maximum flexural strength at 15 vol% glass fibre content, exhibiting decreased values over that content. The typical stress–strain curves for the flexural specimens with the variation of glass fibre content are shown in Fig. 9. It can be seen that in the case of lower glass fibre content (10%–20%), the slope (viz. stress/strain) varies just before maximum stress. Whereas, in the case of higher glass fibre content (25%–30%), the slope does not
20 3.3 ^ 1.2
25 4.2 ^ 1.1
30 6.4 ^ 2.0
deviate from the linearity nearly up to the maximum stress. In addition, the stress falls drastically after the maximum point. However, the composite with lower fibre content can endure the stress after the maximum to a considerable extent. In order to investigate the stressed state during the flexural test the thin section of the specimen, which had been given tensile stress, was prepared. Fig. 10 shows the optical micrographs of the bent composite containing 10 vol% and 30 vol% of glass fibre. In the case of 10 vol% glass fibre content (Fig. 10(A)), there exist many small cracks (especially around the glass fibre) at the opposite side of loaded surface of the flexural test specimen. These cracks are formed in the same direction of flexural loading. Many small cracks means that the stress given by flexural loading can be released over large area. Additionally, the resulted shape of bent specimen exhibits large curvature. However, the composite with glass fibre content of 30 vol% induces a large crack in one spot of the specimen (Fig. 10(B)). Although there exist several small cracks around the large crack, the number and size of small cracks are very small. The curvature of the bent specimen is very small and the straight line of the specimen remains nearby the large crack. Many glass fibre ends at the specimen surface result in easy crack initiation because of the stress concentration. Moreover, the initiated crack propagates more easily, which means that the cracked specimen cannot persist the given load any more. The above result agrees well with the stress–strain behaviour of the composite (Fig. 9). 3.4. Impact properties
Fig. 6. The tensile strength of the glass fibre mat/PP composite as a function of the glass fibre volume fraction.
The crack initiation, crack propagation and total impact absorption energies of glass fibre mat/PP composites according to glass fibre content are given in Table 2. The data show very large scatter because of the non-homogeneous structure of the glass fibre mat/PP composite. The crack propagation energy is higher than the crack initiation energy through the whole range of glass fibre content. This means that more impact energy is dissipated at the crack propagation stage than at the stage before crack is initiated. When the composite contains glass fibre of 20% by volume, the total impact absorption energy has the maximum value. This result indicates that as glass fibre is included in the composite, impact energy dissipation originated from fibre inclusion occurs more largely. Main mechanisms are regarded as the debonding between glass fibre and PP matrix and fibre pull-out [12,13,16]. The plastic deformation of the PP matrix also contributes to the impact energy absorption. Gupta et al. suggested that in glass/PP
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Fig. 7. The flexural modulus of the glass fibre mat/PP composite as a function of the glass fibre volume fraction.
Fig. 9. Typical stress-strain curves for the flexural specimen with the variation of glass fibre content.
system, the fracture energy absorption was dominated by contribution from matrix plasticity [17]. The matrix plastic deformation occurs either through homogeneous deformation of the matrix or from localized deformation around fibre ends. When more amount of glass fibre content than 20% by volume is included in the composite, the total impact absorption energy exhibits a decreased value. From other result[8], it could be observed that impact strength obtained by charpy test increased almost linearly with fibre concentration. However, that result was obtained from the composite having up to about 20% by volume of glass fibre. In the photograph of the specimen after impact test (Fig. 11), it is shown that the composite containing 10 vol% glass
fibre shows large damage area, but in reality, the fracture was dominated by brittle failure of matrix resin itself. As glass fibre content increases, the composite is deformed through large damaged area in bent state. However, the
Fig. 8. The flexural strength of the glass fibre mat/PP composite as a function of the glass fibre volume fraction.
Fig. 10. Optical micrographs of thin section of the specimen after flexural test; (A) 10 vol%; (B) 30 vol%.
N.-J. Lee, J. Jang / Composites: Part A 30 (1999) 815–822 Table 2 Impact absorption energy of the glass fibre mat/PP composite as a function of glass fibre content Glass fibre content (vol%)
Ei (J/mm)
Ep (J/mm)
Et (J/mm)
10 15 20 25 30
1.10 1.26 1.28 1.29 0.68
2.20 2.26 2.52 1.31 1.18
3.30 3.52 3.80 2.60 1.86
composite containing more than 20 vol% glass fibre has smaller damaged area at the impact loading spot, and does not give rise to any deformation around the spot. Fig. 12 depicts the scanning electron micrographs of the fracture surfaces of the glass fibre mat/PP composite after impact test. In the case of the composite containing 10 vol% of glass fibre (Fig. 12(A)), the number of fibres are small and the glass fibres from the fibre pull-out process are short in comparison with that of 20 vol% case (Fig. 12(B)). The fracture surface of matrix region shows the brittle failure of PP, showing little plastic deformation resulting from fibre ends. When glass fibre content is 20% by volume, the glass fibres protruded from the fracture surface plane are longer. Additionally, the plastic deformation of PP matrix is easily observed near the fibre pulled-out region. When 30 vol% of glass fibre is included in the composite (Fig. 12(C)), the length of fibres is shorter than that of 20% case. When the composite has more amount of glass fibre than optimum content, there is insufficient PP matrix to be plastically deformed. In addition, the fibre breakage takes place more frequently than the case of 20 vol% fibre content. However, fibre breakage is not known as main impact energy absorption mechanism. When fibre content is low, fibre-originated mechanisms do not play a role of impact energy absorption and the amount of localized deformation from fibre ends is small. When fibre content is higher, the volume of matrix resin is too small to deform plastically. In addition, strength
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reduction of the composite is thought to be one of the reason of impact absorption energy reduction. Strength reduction of the composite is revealed by the lowest crack initiation energy of the composite with 30% glass fibre among any other ones. When the impact load is given to the composite with lower strength, the composite cannot transfer the energy around the loading spot. Hence, the impact energy absorption mechanisms such as fibre debonding, fibre pullout does not occur. Conclusively, the optimum fibre content exists from the point of view of impact absorption energy and it is determined by the relative amount of each impact energy dissipation mechanism; fibre–matrix debonding, fibre fracture, fibre pull-out, matrix plastic deformation. 4. Conclusions From the investigation of the effect of glass fibre content on the mechanical properties of the glass fibre mat reinforced polypropylene composite, the following results were obtained. 1. As glass fibre content increased, the tensile and flexural modulus of the glass fibre mat/PP composite showed a linear increment. However, the higher volume fraction of glass fibre than optimum content induced a drop of the reinforcing efficiency of fibres because of higher void content. 2. The tensile and flexural strength of the composite increased up to glass fibre content of 15–20 vol% and then decreased with more glass fibre content. This behaviour is explained by the localized crack formation induced by the stress concentration from glass fibre ends and poor fibre-matrix adhesion strength. 3. The total impact absorption energy of the composite showed the maximum value at 20 vol% glass fibre content, and then decreased at more fibre content. This phenomenon resulted from the variation of each impact absorbing mechanism according to glass fibre content.
Fig. 11. Photographs of the glass fibre mat/PP composite after instrumented impact test (A) 10 vol%; (B) 15 vol%; (C) 20 vol%; (D) 25 vol%; (E) 30 vol%.
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was limited and the reduced strength of the composite prevented other fibre-originated fracture mechanisms.
References
Fig. 12. SEM micrographs of fracture surfaces of the glass fibre mat/PP composite from impact test (A) 10 vol%; (B) 20 vol%; (C) 30 vol%.
As glass fibre content increased, failure processes such as debonding, fibre pull-out and locally plastic deformation were increased. However, at the fibre content above optimum point, the plastic deformation
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