Physico-mechanical behaviour of thermoplastic co-polyester elastomer/polytetrafluroethylene composite with short fibers and microfillers

Abstract

Short fibers and particulate fillers are known to enhance the mechanical properties of the polymers. The type of fiber and filler morphology, size, loading, and dispersion homogeneity influence extensively the composite’s performance. In the present study, various amounts of short fiber (glass and carbon) and micro-scale particles (silicon carbide, alumina and molybdenum disulphide) were systematically introduced into thermoplastic copolyester elastomer/polytetrafluroethylene (TCE/PTFE) composite for reinforcement purpose. The influence of these fibers and fillers on the tensile, flexural, and impact properties was investigated. All composite samples were fabricated using twin-screw extruder followed by injection molding. The incorporation of short glass fiber (SGF) yielded an effective improvement in mechanical properties of TCE/PTFE composite at a fiber loading of 20 wt.%. Choosing the 20 wt.% SGF-reinforced TCE/PTFE composite, short carbon fiber and microparticles were further added in order to achieve additional improvement in the mechanical properties. In fact, synergistic effects were in the form of a further increase in hardness, tensile modulus, flexural and impact strength. Various reasons to explain these effects in terms of reinforcing mechanisms were discussed. Also, dispersion of the fiber and fillers were studied using scanning electron microscopy.

Keywords

Short glass fiber particulate-filled TCE/PTFE mechanical properties scanning electron microscopy

Introduction

Polymeric materials with synthetic fibers (glass, carbon and aramid) and micro-scale fillers (SiC, Al₂O₃, MoS₂, SiO₂, clay) possess better strength to weight ratio as compared to conventional materials, i.e. wood, concrete, and steel. With the increase of the world's concern about recyclability and environment protection issues, thermoplastics have gained more attraction over the thermoset. Polymer-based composites are very common in situations where combinations of good properties are required. It is often found that such properties are not attainable with a single polymer alone. The methods of polymer modification include copolymerizing, reinforcing, and blending. Polymer reinforcing is fascinating because it has simple processing and unfolds unlimited possibilities of producing materials with improved properties. These advantages have led to its rapid use in applications of polymer composites.

Thermoplastic co-polyester elastomer (TCE) is a unique material combining the strength and processing characteristics of engineering polymers with the flexibility of thermoset elastomers. It offers highly consistent performance over a wide operating temperature range with very low variation in properties between low and high temperature extremes. TCE is a crystalline material and is a widely used engineering thermoplastic material. They are novel constructional polymers, which are physically cross-linked materials made up of a thermoplastic and an elastomer. TCE has been the interest of numerous researchers world wide since last two decades. Applications include flexible couplings, ski boots, gears, high pressure hose lines, outer coverings for wire and optical fiber cables, seals, etc.¹

Polytetraflouroethylene (PTFE) possesses some extraordinary characteristics, such as very low friction coefficient, good high temperature stability, and chemical stability. However, its application is greatly limited by its poor mechanical properties such as low stiffness, low resistance to creep, moderate tensile strength, non-resilient, and difficult to process.² In order to attain better mechanical properties, PTFE is usually blended with other polymers.

Fillers in the form of particulates and fibers are often added to polymeric materials to enhance their processability and mechanical compound properties, as well as reduce material costs. Filler behaviour in the compound is affected by factors such as the particle size distribution, surface area, shape, and surface chemistry.³

For the last two decades, it has been a focus of research for enhancing its mechanical properties by incorporating fibers and fillers and research articles updating the state of art of TCE-based composites for structural applications. Chen et al.⁴ reported the mechanical properties of various proportions of polyamide 66 (PA66)/polyphenolyne sulphide (PPS) blends. They concluded that the blend with 70 wt.% PA66 in 30 wt.% PPS showed the optimum mechanical properties. Bijwe et al.⁵ investigated the role of adding PTFE (7.5 to 30 wt.%) into polyetheretherketone (PEEK) to form blends on mechanical properties. They found that 30 wt.% PTFE into PEEK showed the maximum impact strength and other mechanical properties were decreased. Borggreve et al.⁶ concluded that the impact strength of polyamide6 (PA6) is only moderately improved by blending with Arnitel EL315. Palanivelu et al.⁷ have investigated the tensile and flexural strength of the blends decreased with increasing TPU concentration. However, the impact strength increased with increasing TPU concentration in POM blends. Long and Wang⁸ concluded that the ekonol filled in PEEK was beneficial for enhancing the compressive strength and the hardness, but it was not beneficial for improving the flexure and impact strength. Tham et al.⁹ reported that the flexural modulus of polymethyl methacrylate (PMMA) was increased with the incorporation of hydroxyapatite. Arsad et al.¹⁰ reported that the tensile and flexural properties improved upon the addition of maleic anhydride grafted acrylonitrile-butadiene-styrene (ABS-g-MAH) at its various compositions into the PA6/acrylonitrile-butadiene-styrene (ABS) blends, while toughness decreased with introduction of short glass fiber (SGF) in the composites. Zhang et al.¹¹ reported that in acrylonitrile-styrene-acrylic (ASA)/styrene-acrylonitrile (SAN) blend, increase in SAN content increases tensile strength, flexural strength, flexural modulus, and hardness, but decrease in impact and elongation at break. Rong-guo et al.¹² reported that in PTFE/PA6 and PTFE/PA66 blends, increment of PTFE content reduces tensile strength, flexural strength, and impact strength of the blends. Ghazanfari et al.¹³ reported that in blends of high density polyethylene (HDPE)/date pit particles, the tensile and flexural strength and strain of the composites decreased with increase in the percentage of date pit flour. Alhareb and Ahmad¹⁴ reported that the incorporation of aluminium oxide (Al₂O₃)/zirconium oxide (ZrO₂) into PMMA managed to improve the fracture toughness, tensile modulus, and flexural properties of denture base composite materials. Cao et al.¹⁵ reported the effect of basalt fiber in ultra-high molecular weight polyethylene (UHMWPE). Increase in basalt content led to decrease in toughness and increase in strength, hardness, and creep resistance. Tarawneh et al.¹⁶ reported that the tensile strength, tensile modulus, and impact strength are improved significantly while sacrificing high elongation at break by incorporating multi – walled carbon nanotubes as filler in thermoplastic natural rubber (TPNR) compared to pure TPNR. Kushwaha et al.¹⁷ observed that nickel-coated CF-reinforced polycarbonate composites show no effect on the hardness (Shore-D). Harsha and Tewari¹⁸ reported that the tensile and flexural properties of PEEK are increased by reinforcing glass fiber and carbon fiber which is according to general observation in fiber-reinforced thermoplastics.

PTFE is one of the most important and promising materials to improve fracture toughness of polymer-based composites. In spite of the fact that polymer composites are used in such structural applications, no data are reported on the inﬂuence of PTFE in TCE composite with SGF and other inorganic particulate fillers and lubricants viz. short carbon fiber (SCF), silicon carbide (SiC), alumina (Al₂O₃), and molybdenum disulphide (MoS₂). Keeping this in view, a series of TCE/PTFE composites with short fibers, ceramic fillers and a solid lubricating material were investigated for their physico-mechanical properties. Efforts were made to study the role of fiber, ceramic fillers, and lubricants inclusion on the relevant strength properties.

Experimental details

Materials and Composite preparation

TCE was considered as a matrix material in this study. PTFE was selected as particulate filler to form a polymer composite. A short E-glass fiber was selected as reinforcing fiber. SCF, SiC, Al₂O₃, and MoS₂ were selected as micro-scale fillers. The average diameter of the SGF was approximately 12 µm with an average fiber length of about 4 mm. The average diameter of the carbon fibers was approximately 7 µm with an average fiber length of about 100 µm. The particle size of PTFE was about 5–15 µm and around 5–10 µm for SiC, Al₂O₃, and MoS₂.The source and selected properties of these materials are listed in Table 1.

Table 1.

Characteristics of materials used for the study.

Polymer/Filler	Designation	Trade name	Melting point (℃)	Density (g/cm³)	Source and Supplier
Thermoplastic Copolyester Elastomer	TCE	Arnitel EM740	221	1.29	DSM and M/s Gargi enterprise, Bangalore
Polytetrafluroethylene	PTFE	MP1000	327	2.16	Du Pont Co. Ltd.
Short glass fibre	SGF (φ12 µm × 4 mm)		1056	2.50	Fine organics Mumbai
Short carbon fibre	SCF (φ7 µm × 100 µm)		>1000	1.74	Fine organics Mumbai
Silicon carbide	SiC (5–10 µm)		2700	3.21	Carborundum India Ltd.
Alumina	Al₂O₃ (5–10 µm)		2030	3.95	Triveni groups
Molybdenum Disulfide	MoS₂ (5–10 µm)		1185	4.70	Omkar speciality chemicals ltd, Thane

Compounding

Before compounding, the polymer granules and fillers were dried at 75℃ for 10 h in an oven. Selected compositions were mixed and extruded in Barbender co-rotating twin-screw extruder ( Figure 1, Make: CMEI, Model: 16CME, SPL, chamber size 70 cm³). The mixing speed of 100 rpm was maintained for all the compositions. The melt blending temperature profile of the extrusion is as follows: zone 1 (200℃), zone 2 (210℃), zone 3 (220℃), zone 4 (240℃), and zone 5 (260℃). The extrudates of the compositions were pelletized in pelletizing machine.

Injection molding

The pellets of the extrudates were predried at 100℃ in vacuum oven for 24 h and injection molded in a reciprocating screw injection molding machine ( Figure 2, DGP Windsor, 50 T) to produce tensile, flexure, and impact test samples. Photographs of the specimen are shown in Figure 3. The processing temperature for zone 1 (220℃) and zone 2 (250℃). The mold temperature was maintained at 35℃. The compositional details of each material are presented in Table 2.

Table 2.

Constituents of the polymer material composite system for present study.

Composites	Matrix (wt.%)	Fiber (% wt)	Filler (wt.%)	Measured Density (g/cm³)	Theoretical Density (g/cm³)	Void Fractions (%)
TCE + PTFE	TCE (85)	–	PTFE (15)	1.38	1.420	2.81
TCE + PTFE + SGF	TCE (85)	SGF (20)	PTFE (15)	1.60	1.637	2.26
TCE + PTFE + SGF + Ceramic fillers	TCE (85)	SGF (20) + SCF (2.5)	PTFE (15) + SiC (6.25) + Al₂O₃ (6.25)	1.81	1.850	2.16
TCE + PTFE + SGF + Ceramic fillers + MoS₂	TCE (85)	SGF (20) + SCF (2.5)	PTFE (15) + SiC (5) + Al₂O₃ (5) + MoS₂ (2.5)	1.82	1.875	2.93

Density test

Density test was conducted as per ASTM D792 using METTLER AE 200 densometer. The specimen was weighed in air, and then weighed when immersed in distilled water at 23℃ using a sinker and wire to hold the specimen which was completely submerged and the density was calculated.

Microhardness test

The Vickers hardness of the polymer and their composite materials were tested using Mitutoyo HM 113 model, series 810-Micro Vickers hardness testing machine. Surfaces of the specimens were polished by fine sand papers in order to produce a flat surface for the indentation test. The unit and magnitude of the hardness are defined by Vickers hardness; HV. Specimens were indented at a constant load of 50 g for a dwell time of 5 s. The diagonals of the indentation were measured and the Vickers hardness was automatically computed and read on a digital display. Average values of five readings were reported as the microhardness of the samples.

Tensile test

The tensile properties were performed according to ASTM D638- Type 1 using Universal testing machine (Lloyds, capacity 1–20 kN). Dog-bone shaped tensile specimens of 3.3 mm thickness were fabricated from the twin-screw extruder followed by injection molding and the test specimen is shown in Figure 3(a). In all of the experiments, the stretching direction is along the length of the specimen.¹⁹ Tensile modulus and strength were evaluated from the stress–strain diagram.

Flexure test

The flexural properties were determined by three-point bending test and were performed in accordance with ASTM D790-Type B using Universal testing machine (Lloyds, capacity 1-20 kN). The span length was set at about 50 mm. Testing speed was set at 2 mm/min and carried out at room temperature, and specimen dimensions are 125 mm × 12.5 mm × 3.3 mm ( Figure 3b). Flexural modulus and strength were evaluated from the stress–strain diagram.

Impact test

Izod impact strength was performed according to ASTM D256-Type A using INSTRON (impact hammer with a mass of 1.3 kg) impact testing machine at the striking rate of 3.2 m/s. A notch of 2.5 mm width with an included angle of 45° was generated on the sample with a thickness of 3.3 mm and width and length of the sample was 12.5 mm and 63.5 mm, respectively ( Figure 3c).

Scanning electron microscopy

Fractured surface morphology of the composites was investigated using E-SEM, Quanta 200 model, FEI – Netherland make, scanning electron microscope (SEM) instrument with a voltage of 20 kV. The micrographs were used to study the fiber dispersion and voids distribution in the composites.

Results and discussion

Density

Physical properties of TCE/PTFE composites with varying content of SGF, SCF, SiC, Al₂O₃, and MoS₂ in wt.% are summarized in Table 2. TCE/PTFE composite exhibits a density of 1.38 g/cm³ slightly greater than that of pure TCE. This is due to the presence of PTFE in TCE/PTFE composite. Bijwe et al.⁵ reported that inclusion of PTFE in PEEK increases the density. All other samples, i.e. TCE/PTFE and its composites, exhibit higher density value. The presence of fibers, microfillers, and MoS₂ makes TCE/PTFE hybrid composite a denser material in this study.

Microhardness

The microhardness of a material has close relationship with the mechanical strengths of the material. It has been shown that a material with a higher hardness value will be accompanied with lower elongation/deflection and load-bearing capacity. Measured microhardness of TCE/PTFE and their microcomposites are listed in Table 3 and plotted in Figure 4. The hardness of TCE/PTFE composite is 10.1 HV, and it increases with 20 wt.% of SGF to 11.4 HV for the TCE/PTFE, to 13.6 HV for SGF-reinforced particulate-filled TCE/PTFE microcomposites and 12.5 HV for SGF-reinforced particulate and MoS₂-filled TCE/PTFE hybrid composites. The increase in microhardness might be attributed to higher microhardness of short fibers (glass and carbon) and ceramic fillers (Al₂O₃ and SiC) compared to TCE/PTFE. Moreover, relatively uniform distribution of short fibres and decrease in inter-fiber/particle distance of PTFE in the matrix result in increase of resistance to indentation of TCE matrix. For a given wt.%, microparticles (12.5 wt.%, Al₂O₃ + SiC) are much closer to each other compared to short fibers in the matrix, and hence microparticles will resist more strongly the penetration of the indentation in the matrix. This results in higher microhardness for SGF reinforced with particulate-filled TCE/PTFE microcomposite than that of SGF-reinforced TCE/PTFE and its hybrid composites.

Table 3.

Physico-mechanical properties of TCE/PTFE composites.

Property	Composites
Property	TCE + PTFE	TCE + PTFE + SGF	TCE + PTFE + SGF + Ceramic fillers	TCE + PTFE + SGF + Ceramic fillers + MoS₂
Micro Vickers hardness (HV)	10.1	11.4	13.6	12.5
Tensile strength (MPa)	30.58	57.84	30.16	29.72
Tensile modulus (MPa)	624.4	797.8	693.8	555.0
Elongation at break (%)	18.9	16.36	9.87	10.22
Flexural strength (MPa)	41.0	80.7	51.49	53.6
Flexural modulus (GPa)	1.068	2.528	3.038	6.333
Deflection (mm)	10.54	8.27	4.54	5.19
Izod impact strength (J/m)	60.0	80.0	40.31	75.0

In general, the addition of micro- or nanoparticles to polymer matrices significantly increases the mechanical properties, particularly modulus and hardness, of the composites if the particles are strongly bonded to the polymer matrix. Recently, the effect of morphology on the scratch hardness of polymer/clay nanocomposites was investigated by Misra and co-workers.^20–22 They reported that scratch hardness of polypropylene (PP)-filled clay nanocomposite increases with increasing clay nanoparticles due to an increase in crystallinity and lamellar thickness, and decrease in spherulite size. An increase in lamellae thickness plays an important role for controlling hardness property of the particle-filled polymer composites.²³ They investigated that the clay nanoparticles influence strongly the micromechanism of scratch deformation and reduce the extent of plastic deformation, whereas the mineral microparticles did not influence significantly. The surface nanomechanical properties of the carbon films were studied by Zhang and Komvopoulos using surface force microscopy (SFM).²⁴ They found that the effective hardness was found to depend on the fractions of tetrahedral (sp³) and silicon-carbon composition profile. The formation of different carbon atom bonds, film growth mechanisms, and optimum process conditions for synthesizing ultrathin carbon films were interpreted. Bijwe et al.⁵ reported that addition of PTFE into PEEK composite reduces the hardness value. The results for the TCE/PTFE composite had revealed that the hardness increases with addition of short fibers and inorganic fillers weight fraction. This increase in hardness is attributed to increasing wettability or bonding (interaction) between the matrix, fibers, and the filler particles. Bijwe et al.²⁵ also reported that the addition of 20 wt.% glass fiber to neat polyetherimide (PEI) increases the hardness by about 5%. Furthermore, inclusion of 25 wt.% of glass fiber, 15 wt.% PTFE, and 15 wt.% (MoS₂ + graphite) deteriorated the hardness of the composite which is less than that of neat PEI. In the present study, inclusion of fibers and particulate fillers increased the microhardness of TCE/PTFE composite and these results are in good agreement with the previous findings.

Tensile properties

Tensile properties provide information about the behaviour of the material when it is subjected to stretching or pulling force before it fails. The characterization of the composites reveals that inclusion of short fibers and particulate fillers has very strong influence on the mechanical properties of composites. By incorporating SGF into the TCE/PTFE composites, synergistic effects, as expected, were achieved in the form of modified tensile and flexural properties. A comparative study of modified behaviour of the composites against the fiber and particulate fillers is presented.

Figure 5 shows the tensile stress–strain curves for all the composites subjected to tensile test. Tensile properties of TCE/PTFE composites are shown in Figure 6 for tensile strength and Figure 7 for tensile modulus (Table 3). The TCE/PTFE composite exhibits tensile strength and tensile modulus of 30.58 MPa and 624.4 MPa, respectively. From the tensile-fractured sample as shown in Figure 10(a), it is evident that PTFE particles (indicated by P) are not aggregated and evenly dispersed in matrix material (indicated by M). Because of their wide melting temperature, the compounds had weak bond strength resulting in particulate pull-out (indicated by F), and crack (indicated by C) resulting in poor tensile properties. In order to have proper aggregation, a suitable compatibilizer has to be used. Jian and Tao²⁶ reported that the incorporation of PA6 as a compatibilizer contributes to significant increase in the tensile strength and wear resistance of CF/PPS composites, which is closely related to the cross-linking reinforcing action of the compatibilizer. TCE/PTFE reinforced with SGF possesses tensile strength of 57.84 MPa and tensile modulus of 797.8 MPa with an increase of 89% and 27.77%, respectively, than that of the TCE/PTFE composite. It is also seen that in all the samples, irrespective of the filler material, the tensile strength decreases with increase in filler content. Likewise, there is a decrease in tensile strength of TCE/PTFE composite with fibers and particulate fillers and TCE/PTFE composite with fibers, particulate and lubricating fillers with a corresponding decrease of 1.39% and 2.81%, respectively, than that of the TCE/PTFE composite. The modulus of fiber plus particulate filled TCE/PTFE composites with and without lubricating filler are 693.8 MPa and 555 MPa, respectively. It is evident from the above result that, with increase in fibre and filler content, the stiffness of the composites increased and toughness decreased. It is well known that the properties of short fiber composites are determined by the nature of the fiber, the fiber volume fraction, fiber orientation factor, aspect ratio of the reinforcement and by the quality of the fiber/matrix interface. TCE/PTFE composite with SGF exhibited the best tensile properties among all the composites selected in this study. The result might be attributed to the relatively strong interfacial adhesion between the polymer matrix and the SGF (indicated by I in Figure 10(b)). The interface could transfer the stress from polymer matrix to SGF which has good tensile strength (3100–3800 MPa), and could improve the tensile properties of the TCE/PTFE composite with SGF. The addition of SiC and Al₂O₃ in TCE/PTFE and SiC, Al₂O₃ and MoS₂ in TCE/PTFE has deteriorated their tensile properties. Microfillers taken in this study cause maximum reduction in the composite’s tensile strength and elongation at break. It may occur due to the poor interface bonding between the TCE/PTFE, microfillers, and fibers resulting in poor adhesion of particles and fiber pull-out (indicated as PA and F, respectively) as shown in Figure 10(c) and (d), which are not good to transfer the tensile stress. Another reason is that the corner points of the irregular-shaped particulates result in stress concentration in the polymer composites.

Flexural properties

Flexural properties provide information about the behaviour of material in bending. The tested result of flexural strength and flexural modulus values of TCE/PTFE composites with varying content of SGF, SCF, SiC, Al₂O₃, and MoS₂ in wt.% is shown in Figures 6 and 7 and Table 3. The TCE/PTFE composite showed poor flexure mode properties. This is due to the addition of PTFE as second matrix material, which reduces the flexural properties because of poor compatibility resulting in early yielding (indicated as Y Figure 11(a)) to deformation with low load-carrying capacity. Many of researchers have showed that the incorporation of fibers increased both flexural strength and modulus.^25,27 The result of the investigation reveals that SGF-reinforced TCE/PTFE exhibits better flexural properties. Figure 11(b) shows a SEM micrograph of fracture surface of the composite, which reveals the uniform distribution of SGF and strong adhesion of polymer matrix material to the SGF (indicated by I). The applied load penetrates the matrix material and gets transferred around the fiber instead of penetrating it. These result in high load-bearing capacity for the deferred fiber rupture. TCE/PTFE composite with fibers, particulate, and lubricating fillers consist 10% ceramic microfillers and has better flexural properties than TCE/PTFE composite with fibers and particulate fillers which has 12.5% ceramic microfillers. This composition could act as very good reinforcing filler in the TCE/PTFE composite. Also, the stresses are received by these fillers, which leads to a moderate deflection of 5.19 mm. Bijwe et al.²⁵ concluded that addition of PTFE, MoS₂, and graphite to glass fiber-reinforced PEI composites further improves the flexural modulus, but with decrease in flexural strength compared to glass fiber-reinforced PEI composite. Microfillers commonly increase the stiffness on the one hand, but on the other hand they may have a detrimental effect on the flexural strain to break.²⁸ The flexural strength of micro-particle-filled composites is known to be reduced with rising filler content.^28,29 This can be better understood by the quality of the interface in the composites, i.e. the static adhesion strength as well as the interfacial stiffness usually plays a very important role in the materials capability to transfer stresses and elastic deformation from the matrix to the fillers.³⁰ If filler matrix interaction is poor, the particles are unable to carry any part of the external load. In that case, the strength of the composite cannot be higher than that of the neat polymer matrix. If the bonding between fillers and matrix is instead strong enough, the yield strength of a particulate composite can be higher than that of the matrix polymer.³¹ Figure 8 shows bending stress–strain curves for all the composites. As mentioned above, the strain at break usually declines with rising filler content. Due to the rigid nature of the fillers, most of the deformation comes from the polymer. The actual deformation is experienced only by the polymer matrix which is much larger than the measured deformation of the sample, with the result that the polymer reaches the failure strain limit at a lower total deformation. Hence, the strain at break decreases. TCE/PTFE composite with fibers and particulate fillers has lower elongation in the study group followed by TCE/PTFE composite with fibers, particulate, and lubricating fillers, SGF-reinforced TCE/PTFE composite, and TCE/PTFE composites. Flexure test failed sample of SGF-reinforced TCE/PTFE composites and an even dispersion of fibers in the matrix is achieved. Good interfacial bonding between fibers and matrix material (indicated with I) can be seen in Figure 11(b). In addition, some holes generated by the pulling out of fiber from the matrix (indicated with F) are observed in Figure 11(c). This proves that fibers and matrix combine well and SGF is effective in increasing the strength and modulus of the composites. When the material is affected by external force, the matrix can transfer it effectively to the interface between fibers and matrix and lead to the increase of the flexural strength and modulus of the composites. With the addition of more fiber and filler, the non-uniform distribution of fibers in the matrix is found as shown in Figure 11(d). Meanwhile, high filler and fiber mass fraction will lead to decrease in flexural properties in TCE/PTFE composite with fibers and particulate fillers, but increase in flexural modulus in TCE/PTFE composite with fibers, particulate, and lubricating fillers, which is because of the presence of MoS₂.

Impact strength

It is the ability of the material to resist the fracture under stress applied at high speed. The specimens are deformed within a short time and therefore exposed to high strain rates. The impact strength of TCE/PTFE and their composites is shown in Figure 9 and also in Table 3. SGF-reinforced TCE/PTFE composites had the highest impact strength among all the composites. This increase is due to fiber-related energy dissipation mechanisms, such as fiber debonding, pull-out, bridging, and fracture, which induce plastic deformation of the polymeric matrix before failure. Bridging and fiber fracture are likely to occur as a consequence as a set of glass fibers with length longer than the critical value for effective reinforcement, while debonding and fiber pull-out are expected to occur as the result of a set of glass fibers with length shorter than the critical value. Mandadori et al.²⁷ showed that the increase in SGF content reinforced with recycled PET leads to increase in impact strength. TCE/PTFE composite with fibers and particulate fillers possess low impact strength compared to others in the group. This is due to the high wt% (12.5%) content of micro fillers, which is hard and brittle in nature, and absorbs less amount of impact energy for failure. Riley et al.³² confirms that the impact properties of polymers are mainly enhanced by small particles with low aspect ratio (nano sized), since large aspect ratio particles are able to induce large stress concentrations near their edges.

Figure 12(a) to (d) shows the SEM micrographs of Izod impact fracture surface of TCE/Teflon composites, respectively. Some voids (indicated with V) may come from bubbles/air traps generated during fabrication as shown in Figure 12(a). Figure 12(b) depicts the fractured surface of SGF-reinforced TCE/PTFE composite micrograph, which clearly highlights the presence of considerable fiber pull-out (indicated with F) as well as regions of micro-cracking (indicated with C) within the matrix and it also suggests that residual matrix material is apparent on the surfaces of the SGF suggesting that the fiber-matrix bond strength was good (indicated with I). The examination of the fractured specimens indicated that many fibers had been pulled out during the failure process. Work by Beaumont³³ has shown that the fiber pull-out is one of the principal energy-absorbing mechanisms in fiber-reinforced composites. The distribution of SiC and Al₂O₃ particles with some agglomeration can be seen on the fracture surfaces (indicated with G) at a magnification of 1000× as shown in Figure 12(c) and (d). Angularities and edges of the fillers would facilitate failure under impact conditions due to localized stress concentrations. Weizhou Jiao³⁴ states the ability of filler on the basis of Al₂O₃, SiC, and SiO₂ to influence the impact strength. In the present study, the results obtained are in line with the literature.^33,34 In all cases except TCE/PTFE composite with fibers and particulate fillers, adding fibers and fillers into the TCE/PTFE composite, the values of the impact strength increased substantially.

Conclusions

An experimental study has been carried out for physico-mechanical properties of TCE/PTFE composite with SGF and particulate fillers such as carbon, SiC, Al₂O_3, and MoS₂ micro-particles. The following conclusions can be drawn from the present study:

SGF (20 wt.%) as reinforcement increased the ultimate tensile strength (57.84 MPa) and tensile modulus (797.8 MPa) of TCE/PTFE composite. However, addition of fine SCF and ceramic and lubricating fillers into TCE/PTFE composite showed improved modulus and unaltered tensile strength, while the hardness of SGF-reinforced TCE/PTFE increased with inclusion of micro fillers.

Composites filled with short glass/carbon fiber, SiC, Al₂O₃, and MoS₂ into TCE/PTFE exhibited improved flexure strength and modulus. However, for TCE/PTFE filled with fibers, particulate and lubricating fillers showed significant improvement in the flexural modulus.

TCE/PTFE composite with short fibers and fillers showed increased impact strength in all cases, except TCE/PTFE composite with fibers and particulate fillers. Adding fibers and fillers into the TCE/PTFE composites, the values of the impact strength increased substantially.

Mechanical properties of TCE/PTFE composites by injection molding suggested that short fiber and micro-particles as fillers are able to improve the tensile and flexural properties while promoting the dissipation of impact energy.

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