Effect of impact modifier types on mechanical properties of rubber-toughened glass-fibre-reinforced nylon 66.

Alsewailem, Fares D. ; Gupta, Rakesh K.

The effect of adding rubber on the properties of glass-fibre-filled nylon 66 was investigated in this study. Styrene-Ethylene-Butylene-Styrene and Ethylene-Propylene elastomers grafted with maleic anhydride (SEBS-g-MA and EP-g-MA, respectively) were used to toughen the nylon-matrix composites.

Impact strength and elongation at break were found to increase with increasing rubber content, but flexural strength, tensile strength and stiffness decreased; however, by adding moderate amounts of rubber to glass-fibre-reinforced nylon 66, a desirable balance between stiffness and toughness of the material may be obtained. For example, the addition of 10 wt.% of SEBS-g-MA to nylon 66 with 23.62 wt.% glass fibre loading resulted in 28.3% and 167% increase in tensile strength and impact strength of the composites, respectively, when compared to neat nylon 66. This suggests that combining both glass fibres and rubber with nylon 66 is a useful strategy to optimize and enhance the properties of nylon 66. The procedure may be used to recycle polyamides, in general, and to develop components for under-the-hood automotive applications, in particular.

On a etudie dans ce travail l'effet de l'ajout de caoutchouc sur les proprietes du nylon 66 renforce de fibre de verre. On a utilise des elastomeres de styrene-ethylene-butylene-styrene et d'ethylene-propylene greffe avec de l'anhydride maleique (SEBS-g-MA et EP-g-MA, respectivement) pour renforcer les composites a matrice de nylon.

On a trouve que la force d'impact et l'elongation a la cassure augmentaient avec l'augmentation de la teneur en caoutchouc, mais que la force de flexure, la force extensible et la rigidite diminuaient; toutefois, en ajoutant des quantites moderees de caoutchouc au nylon 66 renforce en fibre de verre, un equilibre souhaitable entre la rigidite et la durete du materiau peut etre obtenu. Par exemple, l'ajout de 10 % en poids de SEBS-g-MA au nylon 66 avec un chargement de 23,62 % de fibre de verre mene a une augmentation de 28,3 % et 167 % de la force extensible et de la force d'impact des composites, respectivement, comparativement au nylon 66 pur. Cela suggere que l'association de fibre de verre et de caoutchouc a du nylon 66 est une strategie utile pour optimiser et accroitre les proprietes du nylon 66. La technique peut etre utilisee pour recycler des polyamides en general, et pour mettre au point des pieces de mecanique automobile en particulier.

Keywords: nylon 66, glass reinforcement, rubber toughening, mechanical properties

INTRODUCTION

Existing neat polymers have known physical, mechanical, and thermal properties that depend both on chemical type and on molecular weight. There is, however, always a need to improve properties of thermoplastics to meet some specific applications such as under-the-hood automotive applications where humidity, high temperature, and repeated impact are encountered. One way to alter properties of thermoplastics is to reinforce them with glass fibres. Indeed, E glass fibres are widely used for this purpose because of their low cost and good mechanical, chemical and electrical properties. Chopped glass fibre strands and polymer powder or pellets may be melt blended in a compounding extruder, and the result is that tensile properties are enhanced by the process of fibre reinforcement. The extent of property enhancement, however, depends on several factors; these include fibre strength and modulus, aspect ratio (fibre length/ diameter), effectiveness of coupling between fibre and matrix, fibre orientation, and concentration. For quantitative work, the rule of mixtures is often used as a first estimate for predicting tensile strength and modulus. It states that the modulus or tensile strength of a composite is the weighted average of the corresponding property of the constituents; the weighting function is the volume fraction. Mathematically, this is given as:

[[sigma].sub.c] = [[sigma].sub.f][[phi].sub.f] + [[sigma].sub.m][[phi].sub.m] (1)

where [sigma] is strength or m odulus and [phi] is the volume fraction. Subscripts c, f, and m refer to composite, fibre, and matrix, respectively. Clearly, the rule of mixtures predicts a linear relationship between strength and volume fraction of fibres. Results sometimes deviate from Equation (1), and, in such a case, a modified rule of mixtures can be used. This takes into account the interactions between the matrix and the fibres and the orientation of fibres in the host polymer (Folkes, 1982). Since the strength and stiffness of glass fibres exceeds the strength and stiffness of the polymer, an increase in these two properties is achieved by incorporating fibres into a polymer. However, the consequence is often a material that is very poor in terms of handling impact loading. This is because conditions that lead to high strength and stiffness usually result in low elongation to break, so that the work of fracture may be very low compared to that of the matrix. Note that the work of fracture of a composite is normally determined as the area under the stress-strain curve up to the failure point. Enhancement of the work of fracture generally depends on the existence of a mechanism for energy dissipation. In particular, the energy required for fibre pull out is considered to be important for composite impact fracture. In this regard, the toughness of a composite is maximized when the fibre is at its critical length (Ramsteiner and Theysohn, 1979). During tensile loading, composites tend to fail in one of two ways: fibre breakage and fibre pull-out (Collyer, 1994). The critical fibre length, which is defined as the length at which energy for fibre breakage equals energy for fibre pull-out, is the determining factor for composite fracture mechanism. The critical fibre length [l.sub.c] is inversely related to the interfacial shear strength (Kelly and Tyson, 1965; Lunt and Shortall, 1980; Mallick, 1993).

[l.sub.c] = ([[sigma].sub.f] / 2[[tau].sub.i]) [d.sub.f] (2)

in which [[sigma].sub.f] is the ultimate fibre strength, [[tau].sub.i] is the shear strength at the fibre/matrix interface or the shear strength of the matrix whichever is less, and d is the fibre diameter. Equation (2) implies that for a given fibre diameter and length, the critical fibre length can be changed relative to composite fibre length by increasing or decreasing the interfacial shear strength of the composite. This is important because a composite having fibres whose length is greater than the critical length will be strong and stiff and failure will occur due to fibre breakage, while a composite having a fibre length less than the critical value will be tough but not as strong and stiff. In the latter case, fibre debonding and fibre pull-out occur if poor adhesion is encountered. However, at high interfacial shear strength, i.e., good fibre-matrix adhesion, failure occurs in the matrix material.

As a practical matter, the toughness of brittle or semi ductile polymers is increased by incorporating a rubber phase into the polymer matrix since rubber particles act as stress concentrators, forcing material to yield at a lower stress. The matrix material must deform, and, as a consequence, one observes shear yielding and/or crazing (Bucknall, 1977). Crazing consists of an array of voids and fibrils whose diameters are in the range of 10-20 nm and 10-40 nm, respectively. This network of fibrils and voids can break down to form cracks when tensile stress is applied on the polymeric material (Bucknall, 1977; Collyer, 1994). The drawback of the process of rubber toughening is the drastic decrease in some important properties such as modulus and tensile strength as the rubber concentration increases in the blend. A number of factors can contribute to the failure of toughened polymer when impact occurs. These factors include rubber phase size and distribution, notch and surface flaws, temperature, rubber type, and its reactivity with matrix material. Miscibility between polymer matrix and rubber phase has to be very good in order to have a system that is thermodynamically stable. For both brittle and pseudo-ductile polymers, the maximum toughness may be achieved at an optimum rubber phase size (Wu, 1985; Oshinski et al., 1992). The inter-particle distance is also an important factor in rubber-toughening of thermoplastics. It has been reported that a transition from brittle to tough mode as measured by notched Izod impact strength is observed at a critical particle-particle distance regardless of the rubber volume fraction (Wu, 1985; Margolina and Wu, 1988; Wu, 1988). Several theories have been suggested in the past to explain this phenomenon. According to the damping theory, the rubber phase absorbs impact energy by mechanical damping (Bucknall, 1977). Despite its early appeal, the damping theory has failed to fully explain some other rubber toughening aspects such as large strain deformation. Some other theories that have been suggested as improvements over the damping theory are: "microcrack theory" by Merz et al. (1956); "multiple crazing theory" by Bucknall and Smith (1965); and "shear yielding theory" by Newman and Strella (1965). The theory of multiple crazing takes into account the role of matrix material that has been ignored by the microcrack theory. Despite all its success in describing the behaviour of some rubber-toughened materials, the multiple crazing theory has failed to predict the behaviour of some toughened polymers that exhibit notable necking under tensile yielding without detectable stress whitening (Bucknall, 1977). Also, since the shear yielding theory cannot explain many aspects of rubber toughening such as stress-whitening, density change, and elongation without necking, it has been suggested that crazing is the principal mechanism of toughening, and shear yielding may contribute to the toughening process mainly in ductile polymers where interaction between crazes and shear bands is expected to take place (Bucknall, 1977).

The yield strength of a polymer is another important property. For rubber-toughened polymers, this may be predicted by the effective area model (Ishai and Cohen, 1968):

[[sigma].sub.b] = [[sigma].sub.m] (1 - .21 [[phi].sub.r.sup.2/3]) (3)

where [[sigma].sub.b] is yield strength of the blend, [[sigma].sub.m] is the yield strength of the matrix, and [[phi].sub.r] is rubber volume fraction. Since rubber particles are considered as voids in the model, an over or underestimation of yield stress may be obtained depending on the type of yield stress test (compression or tensile) considered (Bucknall et al., 1986).

Here we have worked with nylon 66. This is a high melting-point engineering-thermoplastic with excellent mechanical properties and good chemical properties. The toughness of nylon 66 may be increased by blending the nylon with thermoplastic elastomers such as EPDM. The subject of rubber toughening of neat nylon 66 has been extensively investigated in the past, and the reader is referred to the literature for details (Wu, 1983, 1985, 1987, 1988, 1990; Margolina and Wu, 1988; Wu and Margolina, 1990). We merely mention that styrene/ethylene/ butylene/styrene copolymer and ethylene propylene elastomer grafted with maleic anhydride (SEBS-g-MA and EP-g-MA) have been recommended as good impact modifiers for nylon (Kohan, 1995). Indeed, SEBS-g-MA blended with nylon 66 has been employed to formulate a super tough nylon that has a very high value of Izod impact strength (Gelles et al., 1988; Modic et al., 1989; Takeda et al., 1992; Oshinski et al., 1992).

Combining rubber and glass fibres with a neat polymeric material should result in a trade-off relationship between two important properties: toughness and stiffness. Thus, while tensile strength and modulus may be drastically reduced in rubber toughened thermoplastics, by proper addition of glass fibres to rubber toughened thermoplastics, one should be able to restore tensile strength and modulus to a large extent. Consequently, blending of both glass fibres and rubber with thermoplastics seems to be a logical way to optimize important properties of thermoplastics. Surprisingly, though, the number of such studies conducted, mainly in the last decade, is small (see, for example, Kinloch et al., 1985; Kelnar, 1991; Nair and Shiao, 1992; Pecorini and Hertzberg, 1994; Shiao et al., 1994; Wong et al., 1995; Azari and Boss, 1996; Din and Hashemi, 1997; Nair et al., 1997a; Nair et al., 1997b, 1998; Laura et al., 2000; Cho and Paul, 2001; Sui et al., 2001). Among these studies, nylon 66 has received less attention in terms of selecting the appropriate impact modifiers for it (Pecorini and Hertzberg, 1994; Wong et al., 1995; Nair et al., 1997a; Nair et al., 1997b, 1998; Sui et al., 2001). In fact, toughening of glass-fibre-reinforced nylon 66 with SEBS-g-MA and EP-g-MA, appears not to have been investigated at all. In this study, we report the effect of separately adding SEBS-g-MA and EP-g-MA to a glass-fibre-reinforced nylon 66 on the mechanical properties (i.e., tensile, impact, and flexural) of this reinforced polymer. Our initial motivation for conducting this research was to develop a method of recycling post-industrial nylon 66 waste that was generated during the compounding of this polymer with glass fibres.

Materials Used

Glass-reinforced-nylon 66 (GRZ 70G) with two different glass contents, i.e., 13 and 33 wt.% was obtained courtesy of the DuPont Company. According to the manufacturer, the expected tensile and notched Izod impact strengths of this material are 120.67 MPa (17.5 kpsi) and 48.13 J/m (0.9 ft-lb/in) at 13 wt.% glass fibre content and 186.16 MPa (27 kpsi) and 117.65 J/m (2.2 ft-lb/in) at 33 wt.% glass fibre content, respectively.

The elastomers used in this study were EP-g-MA (Exxelor VA 1801) and SEBS-g-MA (KRATON FG1901X), and these were supplied by ExxonMobil and KRATON polymers, respectively. These two rubbers are semicrystalline and are typically produced by a maleic anhydride grafting process. The maleic anhydride group is expected to react with the amine group in nylon 66, and this should promote the miscibility of the blend during melt extrusion (Kohan, 1995). Table 1 gives some of the properties of these two rubbers.

EXPERIMENTAL DETAILS

Glass Fibre Content

The glass fibre content in the samples was verified by means of the ash test (ASTM D2584). This was done by burning a pre-weighed sample at 650[degrees]C and measuring the ash weight.

Sample Composition

The research was not carried out using the as-received, glass-reinforced nylons. Instead, the two nylons were mixed together in order to match the glass percent of a recycled nylon 66 that was investigated in our prior study (Alsewailem, 2002). The resulting glass percentages in the nylon, and these were used in this study, were 14.79 wt.% and 23.62 wt.%. These two percentages were checked and confirmed after each extrusion run by the use of the ash test. In addition, 5, 10, 15, and 20 wt.% of rubber (based on total sample wt.), i.e., EP-g-MA and SEBS-g-MA, were melt blended with the nylon 66 at the two glass fibre loadings (i.e., 14.79 and 23.62 wt.%).

Sample Preparation

Although one may dry mix materials and directly injection mold them without pre-blending them in an extruder, this can result in mouldings having composition variations. This is because an injection moulding machine screw is not intended to perform mixing, but instead it is used as a metering device. For this reason, it is important to have good blended samples that represent all constituents involved prior to the injection-moulding step. Blending rubbers and nylon using a twin-screw extruder should give a homogeneous blend. Consequently, a C. W. Brabender intermeshing counter-rotating twin-screw extruder with 42 mm diameter screws and ~ 4 kg/h (8 lb/h) maximum flow rate was used to prepare samples of glass-fibre-reinforced nylon 66 containing rubber. It should be recognized, though, that this procedure can result in reduction in glass fibre lengths that exist in the nylon; however, this factor was ignored since all samples were prepared using the same conditions of temperature and screw speed (rpm). In order to minimize fibre attrition in the extruder, a moderate screw speed of 40 rpm was used. The extrusion temperature used was 275[degrees]C. Before each extrusion run, samples were dried overnight at 82[degrees]C, and, when performing extrusion, the hopper was purged by argon gas to prevent polymer degradation. Samples of the two rubbers and the two glass-fibre-reinforced nylon 66 were dry mixed and fed into the extruder. The extrudates were then drawn into long strands in a water bath and pelletized using a Brabender strand pelletizer. In order to mold test samples, i.e., Izod bars and dog-bone shapes, by injection moulding, at least 2 kg of material was produced during each extrusion run. Pellets of glass-fibre-reinforced nylon 66 blended with SEBS-g-MA or EP-g-MA rubbers (prepared by the process of extrusion just described) were injection moulded using a Unilog B4 injection moulding machine manufactured by Battenfeld. In each case, the sample thickness was ~ 3.18 mm (0.125 in). After injection moulding, the samples were immediately put into double-sealed plastic bags and stored in a sealed container containing silica gel desiccant in order to prevent moisture pickup by the nylon. The samples were later taken out of the container only at the time of the test, and all tests were conducted at "dry as moulded" condition.

Mechanical Tests

Izod impact strength

Notched Izod impact strength was measured according to ASTM D 256. The test was done by employing an impact testing machine (Instron model BLI) with a pendulum capacity of 2.7 J (2 ft-lb) at room temperature. A manual Notchvis device manufactured by Ceast was used to make notched samples. The energy required to fracture the sample was measured from the reading dial. The correction due to wind friction was made, and the actual energy was then divided by the thickness of the sample at the notch. The measurements were conducted over five specimens for each test and the average was reported. Also the pendulum machine was put in a heated chamber, and the test was conducted at several temperatures to examine the behaviour of impact resistance at elevated temperatures.

Tensile strength

Tensile properties were measured according to ASTM D 638 using an Instron machine model 8501 at an extension rate of 5.08 mm/min (0.2 in/min). Elongation at break was measured by the help of an extensometer. Five samples were tested for each composition, and the average was reported.

Flexural strength

Flexural strength was measured according to ASTM D 790.

Glass fibre length

In order to assess any reduction in glass fibre length due to the processes of extrusion and injection moulding, the glass fibre diameter and length in samples as received, after extrusion, and after injection moulding were measured by an optical microscopy technique. The procedure involved burning the sample and spreading the remaining fibres on a glass slide gently with a drop of silicone oil. The fibres then were viewed under a microscope with a digital camera attached to a computer. Fibre lengths were measured by an image analysis program. The expected value of the fibre diameter was confirmed manually from the pictures (D = 13 [micro]m). The fibre length was computed from the area calculated by the program. At least 200 fibre lengths were measured, and the average was reported.

Morphology of the fractured surface

The fracture surface of the samples, mainly the Izod and tensile samples, was sputter coated with gold by an SPI sputtering machine. The coated samples were then tested for the morphology of the fracture surface using AMR model 1000 scanning electron microscope at a voltage of 10 kV.

RESULTS AND DISCUSSION

Tensile Properties of the Composites

Unless otherwise specified, in all the results presented in this study, glass fibre weight percentage is based on nylon 66 plus glass, while the rubber weight percent is based on total sample weight. The tensile strength of the various composites is plotted against weight % of rubber at the two glass fibre loadings in Figure 1. As demonstrated by Figure 1, the addition of rubber to glass-fibre-reinforced nylon 66 tends to lower strength. This is because the rubber phase acts as a stress concentrator forcing material to yield at lower values of stress. Figure 1 also shows that strength varies fairly linearly with rubber content, in accord with the rule of mixtures. This behaviour contrasts with the reported tensile strength versus rubber content behaviour of a glass-fibre-reinforced nylon 66 toughened with ABS (containing 14 wt.% butadiene) wherein the tensile strength of the reinforced nylon 66 increased upon increasing ABS content until around 50 wt.% based on total weight of nylon 66 and ABS (Nair et al., 1997a); this increase indicates a lack of toughness that is supposed to be the major role of the rubber phase, perhaps due to incompatibility of the nylon 66/ABS blend even though a compatibilizer was used (Nair et al., 1997a). For the present study, both rubbers contain the maleic anhydride group that can react with the amine group in nylon 66, helping make a miscible blend. Here, all composites showed a decrease in tensile strength upon increasing both SEBS-g-MA and EP-g-MA rubber content. It is also seen from Figure 1 that, as glass fibre content increases, composites with SEBS-g-MA give better tensile strength than composites with EP-g-MA rubber. It is also worthwhile to note that unprocessed (virgin) glass-fibre-reinforced nylon 66 has a higher tensile strength but smaller elongation at break (estimated based on DuPont data) than that of similar composites prepared and tested in the current study (see Table 2). A reason for this difference may be the reduction in glass fibre length during processing by extrusion and injection moulding. Note also that the nylon 66, by itself, is more ductile (but not as stiff) as the glass-reinforced nylon, but it is stiffer (but not as ductile) than the toughened glass-reinforced polymer.

[FIGURE 1 OMITTED]

In Figure 2, the elongation at break is plotted against rubber content. The measured elongation of the glass-fibre-reinforced nylon 66 alone is slightly higher than the value expected based on interpolating the data reported by the manufacturer. This is probably due to the reduction in fibre length upon processing by extrusion and injection moulding. Increasing the amount of rubber in the composites is seen to increase the elongation at break. Also, at the highest rubber content (i.e., 20 wt.%) no significant change in elongation was observed when the glass content was almost doubled. Further, composites with SEBS-g-MA have a higher elongation at break than those with EP-g-MA rubber, especially at the high rubber content (i.e., 20 wt.%). This may imply that SEBS-g-MA is more ductile than EP-g-MA. However, both rubbers, i.e., SEBS-g-MA and EP-g-MA, are seen to increase elongation of the composites as rubber content is increased. Despite the increase in elongation upon increasing rubber content, the presence of glass fibres does limit the ductility of the composites even at high rubber loadings.

[FIGURE 2 OMITTED]

The modulus of elasticity of some of the composites is given in Figure 3. The values of modulus were calculated from the slope of the initial (linear) portion of stress-strain data. From Figure 3, it can be clearly seen that addition of rubber to glass-fibre-reinforced nylon 66 reduces modulus. This is not unusual since rubber has a low value of the modulus. Note that composites containing SEBS-g-MA rubber show better modulus retention than composites with EP-g-MA rubber.

[FIGURE 3 OMITTED]

Flexural Properties of the Composites

Flexural strength of the composites is given in Figure 4. It is seen that adding EP-g-MA and SEBS-g-MA rubbers to glass-fibre-reinforced nylon 66 tends to decrease flexural strength, and this mirrors the same trend seen in tensile strength data. However, composites with SEBS-g-MA rubber show relatively higher values of flexural strength. Indeed, the composites did not break within the strain on the outer surface of the fibres, i.e., 5%, as specified in ASTM D 790. This is not an unusual observation since composites become more ductile upon incorporating the rubber phase. In case of composites with 0 wt.% rubber, breakage did happen. The breaking of sample within the 5% strain may be attributed to glass fibres length or aspect ratio. As in the case of tensile strength, the variation of flexural strength of the composites with rubber content seems to conform to the rule of mixtures.

[FIGURE 4 OMITTED]

Yield Strength of the Composites

The dependence of the yield stress on rubber volume fraction in rubber-toughened polymers may be predicted theoretically by use of the effective area model (Equation (3)) developed by Ishai and Cohen (1968). However, for the current research, one expects that this model will underestimate the yield stress data since both rubber and glass fibres are present in the nylon. Glass fibres, which act as reinforcement agents, tend to increase yield stress of the composite material. Ishai and Cohen have also proposed a relation for calculating yield stress for reinforced polymers in the absence of rubber as:

[[sigma].sub.c] A + B log [epsilon] + C [[phi].sub.f] (4)

where [[sigma].sub.c] is composite yield stress, [epsilon] is strain rate that is defined as extensional rate applied to the specimen divided by the original length of the specimen, [[phi].sub.f] is volume fraction of the reinforcement, and A, B, and C are constants. Equation (4) is based on the fact that strain rate and reinforcement content influence yield stress independently. At a fixed strain rate, yield stress of the reinforced polymer is found to increase linearly with increasing reinforcement content. Similarly, at a fixed content of the reinforcement, yield stress increases linearly with increasing strain rate (Ishai and Cohen, 1968). The constants A, B, and C depend on the matrix material used, and Ishai and Cohen mention that the equation is valid for up to 50 vol.% reinforcement. Also, the range of strain rate that they used varied from 0.0027 [min.sup.-1] to 1.35 [min.sup.-1]. Conceptually, one may argue that since the current study deals with incorporation of rubber to a glass-fibre-reinforced matrix, combining both equations, i.e., Equations (3) and (4) would account for the presence of both the rubber and the glass reinforcement. The result is:

[[sigma].sub.c] = C[[phi].sub.f] + [[sigma].sub.m](1 - 1.21 [[phi].sup.2/3]) (5)

where [[phi].sub.f] and [[phi].sub.r] are volume fraction of glass fibre and rubber, respectively, based on total weight of sample. Note that the first two terms in Equation (4), i.e., A + B log [epsilon], are not included in Equation (5) due to the fact that they represent the yield strength of the matrix at a fixed strain rate that is already included in Equation (3) as [[sigma].sub.m]. By examining Equation (5), it is easy to notice that when no rubber is present, i.e., when [[phi].sub.r] = 0, Equation (5) reduces to Equation (4), which is the yield stress relation for the reinforced material. On the other hand, in the absence of reinforcement (glass fibres), i.e., .f = 0, Equation (5) reduces to Equation (3). Figure 5 shows a comparison between the yield stress predictions and data for rubber-toughened nylon 66 at the higher glass fibre content (23.62 wt.%). It is clear that while the Ishai and Cohen model given by Equation (3) underestimates the actual experimental data since it does not account for the effect of the reinforcement, Equation (5) does a good job of predicting the experimental data.

[FIGURE 5 OMITTED]

Impact Properties of the Composites

Figure 6 gives Izod impact strength data for glass-fibre-reinforced nylon 66 toughened with EP-g-MA and SEBS-g-MA. Figure 6 clearly shows that the addition of 5-20 wt.% of EP-g-MA or SEBS-g-MA to glass-fibre-reinforced nylon 66 increases toughness significantly. The two rubbers seem to be equally effective at toughening the reinforced nylon at the lower fibre content, but at the higher fibre content, EP-g-MA appears to be superior. The reported Izod impact strength for un-reinforced nylon 66 toughened by SEBS-g-MA at a weight ratio of (20/80) (SEBS-g-MA/nylon 66) (Oshinski et al., 1992) is about 1070 J/m (20 ft-lb/ in). Needless to say, the cause of the lower Izod impact strength in the present study is the presence of glass fibre in the matrix. In order to examine the effectiveness of these two rubbers for toughening nylon 66, blends containing 15 wt.% of both rubbers, i.e., SEBS-g-MA and EP-g-MA, were formulated with nylon 66 (Zytel 101 L), but with no glass fibres. These blends were prepared by extrusion and injection moulding using the same conditions as used with the reinforced composites. The measured Izod impact strength for (15/85) wt.% of (EP-g-MA/ nylon 66) and (SEBS-g-MA/nylon 66) were 166.5 and 289.1 J/m (3.11 and 5.40 ft-lb/in), respectively. This indicates that both rubbers are effective in toughening nylon 66. While the reinforced blends having 15 wt.% of SEBS-g-MA and 23.62 wt.% glass fibre suffers ~ 40% reduction in toughness, the blend consisting of 23.62 wt.% glass fibres and 15 wt.% EP-g-MA has a slightly increased toughness when compared to the un-reinforced blend (Alsewailem, 2002). This may imply that composites with EP-g-MA have some brittleness that would lead to some increase in toughness upon reinforcing with glass fibres. Note here that incorporating 33 wt.% glass fibre into nylon 66, which is semiductile at room temperature, increases its toughness by a factor of 2.2. The increase in impact strength when a material is reinforced may be related to the elongation. The elongation at break data given in Figure 2 clearly indicate that reinforced nylon containing EP-g-MA has less elongation than in the case of SEBS-g-MA. It seems that the extent of reaction between EP-g-MA and nylon 66 up to the weight percent of rubber specified in this study made the rubber phase not sufficient enough for super toughness. A similar observation has been reported by others (Oshinski et al., 1992).

[FIGURE 6 OMITTED]

Since one of the important uses of glass fibre reinforced nylon 66 is under-the-hood applications in automobiles where the temperature may be high, it is important to know the impact strength behaviour of the reinforced nylon 66 when toughened with rubber at high temperatures. In case of nylon 6 toughened with EP, it has been shown that increasing glass fibre content in the composite tends to drastically reduce the transition in impact strength versus temperature (Dijkstra et al., 1991). For the present study, Figure 7 shows Izod impact strength for composites with EP-g-MA as a function of temperature. The impact strength increases as temperature increases at all rubber contents except for those composites that contain 20 wt.% of EP-g-MA rubber where the impact strength at temperatures greater than 50[degrees]C remains almost unchanged. The transition from brittle to tough upon increasing temperature is not seen to be large. The presence of glass fibres seems to suppress the transition from brittle to tough in impact strength versus temperature relationship for reinforced nylon 66 toughened by EP-g-MA rubber. At a temperature below the [T.sub.g], nylon 66 is considered semi ductile material because the amorphous part is below the [T.sub.g] where chains are frozen. Therefore, the nylon phase in the composite will probably not contribute to enhancement in elongation of the blend so that the presence of glass fibres in the composite will not affect elongation significantly and impact strength increases. In this case, increasing glass fibre content is seen to increase impact strength. Beyond the [T.sub.g], the chains that occupy the amorphous part start to move and become rubbery and when impact occurs they act as stress concentrators, which leads to absorption of energy before failure. However, the presence of glass fibres will drastically reduce the elongation and, as a result of that, impact strength does not change, especially at high rubber contents (>5 wt.%). Here, increasing glass fibre content does not change impact strength regardless of the content of rubber phase in the composite. Figure 7 also shows that for untoughened composites (i.e., composites having 0 wt.% rubber) the transition in impact strength occurs at temperature above 70[degrees]C while, when rubber is introduced, the transition occurs at a temperature below 70[degrees]C. Note here that a typical [T.sub.g] for nylon is between 70 and 80[degrees]C. It seems that addition of reacted rubber to glass-fibre-reinforced nylon 66 may have resulted in a reduction in [T.sub.g].

[FIGURE 7 OMITTED]

Toughness-Stiffness Trade-Off Relation

Generally, toughness of thermoplastics tends to drastically reduce or remain unchanged upon glass fibre incorporation. At the same time, important properties such as strength, stiffness and dimensional stability are improved. On the other hand, the addition of rubber can improve toughness, but there is a reduction in the strength and stiffness. It is interesting to note that by adding both glass fibres and rubber to thermoplastics, one may optimize the mechanical properties of the polymer. To show this trade-off relationship between toughness and strength, the Izod impact strength has been plotted against tensile strength for the composites with SEBS-g-MA at different glass fibre and rubber contents as given in Figure 8. Figure 8 clearly shows that increasing rubber content leads to increase in impact strength, but, at the same time, tensile strength decreases. This clearly shows the possibility of balancing strength and toughness by adding appropriate amounts of rubber and glass fibres to the polymer. For example, tensile and impact strengths of nylon 66 may increase by 28.3% and 167%, respectively, upon incorporating 23.62 wt.% and 10 wt.% of glass fibre and SEBS-g-MA rubber, respectively. It is interesting to note that the tensile strength-impact strength relationship, given by Figure 8, for the current research is fairly linear. The linear equations that govern the experimental data are:

(TS) = 21.959 - 2.355 (IS) 23.62 wt.% glass fibre (6)

(TS) = 15.577 - 1.284 (IS) 14.79 wt.% glass fibre (7)

where TS refers to tensile strength, while IS to impact strength. This says that for nylon 66 toughened with the rubbers employed in this study, i.e., EP-g-MA and SEBS-g-MA, and reinforced with short glass fibres, at given glass fibre and rubber contents, it is possible to predict the tensile strength when knowing the value of the impact strength and visa versa. Similar analysis may be done with the stiffness-toughness relationship where modulus may be plotted against impact strength.

[FIGURE 8 OMITTED]

Effect of Processing Type on Fibre Length

It is known that during plastic fabrication by injection moulding, fibre breakage (attrition) is likely to occur. This may lead to a large population of fibres in the moulded article that have lengths that are very small to be effective in ensuring good mechanical properties such as strength and stiffness. For the current study, the average glass fibre length for the composites was determined for the following cases:

I. As received;

II. After extrusion;

III. After injection moulding;

IV. Extrusion followed by injection moulding;

V. After extrusion with 20 wt.% rubber;

VI. Extrusion followed by injection moulding with 20 wt.% rubber.

The situations listed above arise in practice, and it is necessary to assess the change in fibre length when the material is subjected to different processes such as injection moulding and extrusion. The results of fibre length analysis are presented in Table 3. As can be seen from this Table, a drastic reduction in fibre length occurs when material is processed by extrusion followed by injection moulding. Material that has been processed by direct injection moulding (method III) has a smaller fibre length than material that has only been extruded. This is probably due to the mild shear conditions (low screw speed) chosen for extrusion.

In the injection moulding machine, a high shear rate is expected to be applied to the material that would cause significant fibre breakage. Incorporating rubber into glass-fibre-reinforced nylon 66 leads to further fibre length reduction. During blending in the extruder, the rubber phase tends to disperse in the nylon. This interaction between rubber and nylon and glass fibres may result in fibre breakage. Taking a typical value for fibre strength as 2470 MPa (Din and Hashemi, 1997) and assuming good matrix-fibre adhesion so that shear strength of the material may be taken as shear strength of nylon 66 (typically 66.2 MPa), the critical fibre length in the present case may be calculated using Equation (2). After introducing the numbers, the critical fibre length is found to be ~ 234 [micro]m. The typical critical fibres length for glass fibre-nylon 66 system is about 230 [micro]m (Folkes, 1982). The fibre lengths of the specimens tested morphologically are less than the critical length (see method IV and VI in Table 3). This implies that the fracture mechanism of the composites will be dominated by fibre pull-out, and this is indeed what the morphology of the fracture surfaces revealed as shown in the following section. Also, since fibre length is less than the critical length, the failure is expected to be due to matrix fracture or fibre-matrix debonding if the adhesion is poor.

Morphology of the Fractured Surfaces of the Composites

Studying the fracture surface of the samples is a useful way to assess different aspects of the toughening process. Electron microscopy allows one to actually see fibres upon fracture. Whether fibres are pulled out from the matrix or are broken the degree of adhesion with the matrix may be easily visualized. Also, one can see the degree of alignment of fibres in the sample. In principle, fibres tend to align themselves in the direction of flow during injection moulding. For the current research, we examined the fracture surface of rubber-toughened glass-fibre-reinforced nylon 66 at two extremes of strain rate: Izod samples that represent a high strain rate (impact speed ~ 3.05 m/s (10 ft/sec)), and tensile samples, which represent a low strain rate. Figures 9 and 10 show the morphology of the fracture surface of some Izod samples of the composites. The test was done at room temperature, which implies that the matrix, i.e., nylon 66, was semibrittle since its [T.sub.g] is above room temperature. Therefore, in the absence of rubber phase, nylon 66 is not expected to absorb much energy before fracture. As is clearly evident from the fracture surfaces, fibre pull-out is dominant with the blends with 0 wt.% rubber. When rubber is introduced to the glass-fibre reinforced nylon 66, the extent of fibre pull-out is reduced considerably (see Figures 9a and 9c). A maximum toughness is expected to be achieved at the fibre critical length. Here the morphology of the fracture surface of the Izod samples shows that the addition of rubber to glass-fibre-reinforced nylon 66 reduces fibre pull-out. Indeed, fibre breakage was observed with some of the blends (see Figure 10d). This morphology correlates with mechanical properties, i.e., an increase in impact strength of the composites. The rubber phase increases ductility of the composites resulting in large deformations that increase the energy absorption before fracture. While composites with no rubber have less deformation and clean surface of fibres being pulled out, those composites with high rubber content have a large degree of plastic deformation and fibres that are surrounded by much matrix material. In other words, there is good adhesion between matrix and fibres (see Figures 9, 10, and 11). The morphology of the fracture surface of the Izod samples also shows evidence of shear yielding and cavitation. Shear yielding and cavitation are believed to be the main mechanisms for rubber toughening in nylon 66. Figure 9d clearly shows that shear bands were formed around a fibre in circular pattern. Also, cavitation around the fibre is seen in Figure 10d. Since the properties of the glass-fibre-reinforced composite are greatly dependent of the orientation of fibres in the moulded samples, one needs to examine this important parameter. As mentioned previously, fibres are expected to align in the flow direction in processes such as injection moulding. For the current research, Izod bars were cut in a direction parallel to the flow direction and examined by SEM. Figure 12 shows that, in general, fibres were aligned in the flow direction as expected. The morphology of fractured Izod samples tested at a temperature of 103.5[degrees]C was examined and is shown in Figure 13. At this temperature, the matrix material, i.e., nylon 66, is at a temperature above its [T.sub.g], which will make nylon 66 act in a ductile fashion. Consequently, shear deformation is very likely to take place as a mechanism of absorbing the energy of impact. Figure 13, in fact, demonstrates that deformation has been increased in comparison with Izod samples tested at room temperature. Unlike the fracture surface of the Izod sample that has 23.62 wt.% glass fibre with no rubber (tested at room temperature) as given in Figure 9(a), here nylon 66 looks more deformed and the glass fibres that are pulled out from the matrix have some matrix material sticking on them (see Figure 13a). This observation becomes clearer when rubber content increases as demonstrated by Figures 13b-13e. However, when rubber content is increased, the extent of fibre pull-out is diminished. The morphology of the fracture surface of the tensile samples reveals similar behaviour as seen with Izod fracture surface. Increasing rubber content is seen to enhance the adhesion between matrix and fibre and cause nylon 66 to deform more.

[FIGURES 9-13 OMITTED]

CONCLUSIONS

The current research has demonstrated the effect of incorporating a ductile rubber phase, i.e., SEBS-g-MA or EP-g-MA, into a semi brittle material, i.e., nylon 66, reinforced with glass fibres on the properties of the composite. The approach of combining both reinforcement and tougheners with a thermoplastic material is the appropriate way to balance strength, stiffness and toughness of the material. The results of the current research have shown that both rubbers, i.e., SEBS-g-MA and EP-g-MA, are effective in toughening glass-fibre-reinforced nylon 66. Tensile test results have shown that as rubber content increases, tensile strength decreases. This is not an unusual finding since the rubber phase acts as a stress concentrator forcing material to yield at lower stress. Elongation at break was found to increase with increasing rubber content. All elongation data were less than 11% even at the highest rubber content (i.e., 20 wt.%); this is perhaps due to the dominant role of glass fibres in the blends. This finding is consistent with previous work done by others (Laura et al., 2000; Cho and Paul, 2001). Note that glass fibre typically has a value of elongation at break ~ 5%. The variation of both tensile and flexural strengths with rubber content was found to obey the behaviour given by the rule of mixtures. Although the effective area model developed by Ishai and Cohen was found to underestimate the yield data obtained in the present research due to the presence of the glass fibres, a combined equation based on the work of these same authors (Ishai and Cohen, 1968), which accounts for both rubber and glass reinforcement, showed good agreement with the data. The experimental data of the yield stress versus rubber volume fraction were in good agreement with the results predicted theoretically. As expected, impact strength of the composites was found to increase with increasing rubber content. The plot of strength or stiffness vs. toughness shows that it is possible to optimize strength or stiffness and toughness of nylon 66 by incorporating both glass fibres and rubber. For example, a composite having 23.62 wt.% glass fibre and 10 wt.% SEBS-g-MA resulted in 28.3% and 167% increase in tensile and impact strengths, respectively, of a neat nylon 66. Thus, adding rubber and perhaps additional glass reinforcement to glass-fibre-reinforced nylon 66 recovered from obsolete cars may be considered a good strategy for polymer recycling since this provides flexibility in tailoring polymer properties to potential applications. The morphology of the fractured surfaces was successfully related to the mechanical properties of the composites. When rubber content was increased, composites exhibited a great degree of plastic deformation in the form of cavitation and shear bands as revealed by SEM micrographs, and fibre pull-out was greatly diminished. This allowed the material to absorb much energy before fracture so that impact strength was raised.

ACKNOWLEDGEMENTS

The polymers and elastomers used in this study were obtained courtesy of Mr. J. F. Lathrop of DuPont, Mr. K. Ankney of Kraton and Mr. R. Liotta of ExxonMobil.

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Manuscript received December 23, 2005; revised manuscript received June 29, 2006; accepted for publication July 25, 2006.

Fares D. Alsewailem (1) and Rakesh K. Gupta * (2)

* Author to whom correspondence may be addressed. E-mail address: rakesh.gupta@mail.wvu.edu

(1.) King Abdulaziz City for Science and Technology, Petroleum and Petrochemicals Research Institute, P.O. Box 6086, Riyadh 11442, Saudi Arabia

(2.) Department of Chemical Engineering, West Virginia University, P.O. Box 6102, Morgantown, WV 26506, U.S.

Table 1. Rubber properties *

Property EP-g-MA SEBS-g-MA

Maleic Anhydride content 0.45-0.75 1.4-2.0
(wt.%)

Polystyrene content (wt.%) -- 30

Specific gravity 0.87 <1

Melt flow index (g/10min) 9 (10 21.2 (5
 kg/230[degrees]C) kg/230[degrees]C)

Tg ([degrees]C) -42

* Provided by the suppliers

Table 2. Comparison of the properties of glass-fibre-reinforced nylon
66 used in this study with the corresponding virgin material

 Glass fibre content (wt.%)

Virgin (unprocessed)
 14.79
Material used in this study

Virgin (unprocessed)
 23.62
Material used in this study

 Tensile strength (MPa)

Virgin (unprocessed) 126.52

Material used in this study 98.28

Virgin (unprocessed) 155.41

Material used in this study 134.16

 Elongation (%)

Virgin (unprocessed) 3.00

Material used in this study 3.77

Virgin (unprocessed) 3.00

Material used in this study 5.74

Table 3. Effect of processing method on fibre length
for glass-fibre-reinforced nylon 66

 Fibre length ([micro]m) Fibre length ([micro]m)
Method 14.79 wt.% glass fibres 23.62 wt.% glass fibres

I 305.08 305.08
II 285.86 277.99
III 243.04 230.39
IV 222.47 201.64
V 228.14 259.06
VI 223.27 195.81