Evaluation of Two-Roll Mill Method for Preparing Short Glass Fibre Reinforced NBR-Phenolic Composites

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1 Evaluation of Two-Roll Mill Method for Preparing Short Glass Fibre Reinforced NBR-Phenolic Composites Evaluation of Two-Roll Mill Method for Preparing Short Glass Fibre Reinforced NBR-Phenolic Composites A.M. Rezadoust and M. Esfandeh Iran Polymer Institute P.O. Box 14965/115, Tehran, I.R. Iran Received: 29th January 2001; Accepted: 27th June 2001 SUMMARY Mechanical properties of short fibre composites depend strongly on the fibre length. Therefore, in preparing short fibre composites, processing conditions are of great importance as they affect the final fibre length. In this study 50 mm glass fibres were used to prepare acrylonitrile butadiene rubber (NBR) phenolic-glass fibre composites by tworoll mill method. Fibre breakage during processing was studied and the fibre critical length was calculated. On the basis of the critical length, the usefulness of the two-roll mill method for preparing glass-nbr-phenolic composites with the specified formulation was assessed. 1. INTRODUCTION Elastomeric composites including glass-nbrphenolic composites, used as ablative thermal insulators, have been of interest for a long time because of their improved thermal properties. Many attempts have been made to prepare ablative thermal insulator compositions with improved char and erosion resistance 1,2. To improve the mechanical properties of these compositions, fibres such as glass and silica are used as reinforcement. The requirements for a reinforcing fibre in an elastomeric matrix are as follows 3,4. It should have aspect ratio (length divided by effective diameter) of around 100 to 200, develop good adhesion with the matrix, and be flexible enough to be processible with minimum breakage. Both long and short fibres may be added. Short fibre reinforced composites show a great advantage over long fibre composites in their relatively easy manufacture and processing 5. A review of various types of short fibres, highlighting their properties and shortcomings as reinforcements for polymers,has been given by Milewski 6. High initial aspect ratio can be obtained with glass fibres, but brittleness causes breakage of the fibres during processing. The aspect ratio can thus be drastically reduced, and reinforcement becomes less effective 7. For short fibre composites, good dispersion of the fibres is necessary to achieve the full advantage of the fibre reinforcement. Because of the high viscosity, rubber compounds are usually compounded in high intensity mixers. High shear stresses are produced during normal mixing. Brittle fibres such as glass and carbon are very hard to incorporate without drastic reduction of the fibre length. O Connor 8 compares the mixing of nylon, glass, carbon and Kevlar fibres into nitrile and natural rubber compounds using a mill for a sufficient time to obtain complete dispersion. By comparing the fibre length and the aspect ratio of the fibres before and after mixing, it can be seen that the glass and carbon fibres have been reduced to such a low aspect ratio as to give a relatively poor performance as a reinforcement for elastomers. From the above introduction it is obvious that the processing conditions and the process itself affect the final fibre length and thus the mechanical properties of the product. Considering service conditions of a product, it is important to find the optimum processing conditions in order to obtain the required properties. In the manufacture of glass reinforced rubber composites by the two-roll mill method, there is a fibre length distribution. Indeed, a significant decrease in fibre length occurs as a result of fibre breakage during processing. The objectives of this work are: (i) to investigate the variations of the fibre length distribution during processing of NBR-phenolic compounds at various processing stages, and (ii) to Polymers & Polymer Composites, Vol. 9, No. 6,

2 A.M. Rezadoust and M. Esfandeh find the critical fibre length (l c ) of the fibre. As for the proper distribution of the fibres in the matrix, a certain level of mixing is necessary, so that the information on the critical fibre length can be used to assess suitability of the process for the manufacture of glass-nbr-phenolic composites. 2. THEORY In a composite material, a load is applied to the matrix material and transferred to the fibre through the fibre ends and also through the cylindrical surface of the fibre near the ends. When the length of the fibre is much greater than the length over which the stress transfer takes place, the end effects can be neglected and the fibre may be considered continuous. In the case of short fibre composites, the end effects cannot be neglected and the composite properties are dependent on the fibre length. The minimum fibre length, in which the maximum fibre stress, σ fmax, can be achieved, can be defined as a load-transfer length, l t. It is over this length of the fibre that the load is transferred from the matrix to the fibre. It is given by lt d = ( σf ) max 2τy where d (=2r) is the fibre diameter, (σ f ) max is the maximum fibre stress and τ y is the interfacial shear stress 9. Since σ fmax is a function of the applied load, the load transfer length (l t ) is also a function of the applied stress. The critical fibre length, l c, independent of the applied stress, is defined as the minimum fibre length in which the fibres ultimate strength, σ fu, can be achieved 9, thus lc d = σ fu 2τ y The critical fibre length is an important system property and it affects the ultimate composite properties. In order to establish l c, the shear stress along the fibre-matrix interface, τ y, needs to be calculated. In the case of a strong interfacial bond, τ y is limited by the shear strength of the matrix, τ m. Assuming isotropy of the matrix 10, this results in τ y σ = τ m m = 3 where σ m is the tensile strength of the matrix. (1) (2) (3) 3. EXPERIMENTAL 3.1 Materials The NBR used in this study was Hycar 1051 (BFGoodrich), which contains 40% by weight of acrylonitrile. Phenolic novolac resin (containing 10% HMTA by weight) was supplied by Moheb Co. of Iran. Chopped glass fibre (50mm) was obtained from Fibretech Co. Stearic acid, paraffin wax, ZnO, SiO 2 and TMTD (Tetra Methyl Thiuram Disulphide) were of commercial grade and obtained from local suppliers. 3.2 Preparation of NBR-Phenolic Sheet NBR was masticated in a two-roll mill (POLYMIX 200L) at room temperature. The gap between the rolls was set at 2 mm and the roll speed was 13 rpm (revolutions/min). Paraffin wax, stearic acid and ZnO were added to the rubber in a pre-determined weight ratio. A mixture of phenolic resin and SiO 2 was then added to the blend. To reduce fibre breakage, fibres were added to the sheet of material step by step. Each time a portion of fibres was put in the middle of the sheets and then it was folded over and passed through the rollers. Before the fibres were fully dispersed, the curing agents (TMTD and sulphur) were added and then mixing was continued. The formulation used is given in Table 1. Table 1 Formulation used for NBR-phenolic sheets (in parts by weights) Material NBR Stearic acid ZnO Paraffin wax Antioxidant Phenolic TMTD Sulphur SiO2 Glass Fibre Analysis of Curing Conditions In order to establish curing conditions (temperature and time) of the compound, rheometric measurement was carried out using a Zwick 4308 rheometer. For this purpose a sample (7 g) was placed between the cone and plate of the rheometer and the temperature was set at 150 C. The torque versus time curve was plotted (Figure 1) and the optimum curing time was determined, defined as a point at which the curve reaches a plateau. Using the information obtained from the torque-time curve, samples were cured at 150 C for 30 minutes in a hydraulic press. 404 Polymers & Polymer Composites, Vol. 9, No. 6, 2001

3 Evaluation of Two-Roll Mill Method for Preparing Short Glass Fibre Reinforced NBR-Phenolic Composites Figure 1 Rheometer curve of NBR-phenolic compound at 150 C Time (min) 3.4 Samples Burn-Off To follow the extent of fibre breakage during mixing, fibres must be separated from the matrix by heating the sample in a furnace. For this purpose, after mixing, part of the sheet (prepared as in Section 3.2) was cut, weighed in a crucible and heated at 600 C in air. The sampling intervals were 30, 78, 95, 120 and 146 seconds after finishing fibre feeding. For comparison, a sample was also taken as the fibre feeding was completed (zero time). The remaining fibres were inspected for breakage using an optical microscope. The corresponding micrographs are shown in Figure 2 (a-f). Figure 2 Optical micrographs of burn-off samples showing a reduction in fibre length as a function of mixing time. a) Onset of mixing, b) 30 s, c) 78 s, d) 95 s, e) 120 s and d) 146 s. (Magnification x 32) (a) (b) (c) (d) (e) (f) Polymers & Polymer Composites, Vol. 9, No. 6,

4 A.M. Rezadoust and M. Esfandeh 3.5 Mechanical Properties In order to obtain the shear strength of the matrix, τ m (Equation 3), the tensile strength of the cured matrix (σ m ) must be measured. For this purpose an Instron testing machine (Model 6025) was employed and the test was performed according to ASTM D638. Three dumbbell-shaped specimens with standard dimensions were used for testing. The result was shown as a load-displacement curve and σ m was calculated automatically by the software provided with the instrument. The cross-head speed was maintained at 25 mm/min. Figure 3 SEM micrographs of the broken surface of glass-nbr-phenolic composites, showing a strong interfacial fibre-matrix bond 4. RESULTS AND DISCUSSION 4.1 Fibre Breakage In the mixing process which involved the two-roll mill method, fibre length is reduced as a result of shear forces. The extent of breakage depends on factors such as the mixing time and the roller nip. In this work the roller nip was kept constant and the effect of the mixing time on the fibre breakage was considered. The results (Figure 2 (a-f)) showed a reduction in the fibre length, which became more evident as the mixing time increased. Visual inspection of the mixing process shows that the optimum mixing level was achieved after 78 seconds. At this stage the bulk material appeared to be uniform and there was no aggregation of fibres. Measurement of fibre lengths from burn-off samples showed that the maximum length after 78 seconds (Figure 2c) was only 1.47 mm. The maximum length was 3.43 mm after 30 seconds of mixing (Figure 2b). Considering that the fibre breakage occurs during the mixing process, it is reasonable to say that the reduction in length may reach a point in which fibres lose their maximum reinforcing effect. Obviously, when selecting a processing method the extent of fibre breakage must be considered. The fibre length should not fall below the critical fibre length, which has a definite value for a particular matrix. In order to evaluate the usefulness of the two-roll mill process for preparing glass-nbr-phenolic composites under the stated conditions, the critical fibre length must be calculated. 4.2 Calculation of the Critical Fibre Length The critical fibre length can be calculated from equation 2. As d and σ fu can be obtained from the supplier, only τ y needs to be calculated. Scanning Electron Microscopy (SEM) studies of the broken specimen surfaces (Figure 3) revealed that the breakage occurred mainly through the matrix. This suggests that the matrix is the determinant factor for the breakage. The evidence shows that the interfacial fibre-matrix bond was stronger than the shear strength of the matrix, τ m. Thus, equation 3 can be used for calculating τ y. Combining equation 2 and 3 gives equation 4 for calculation of l c. lc = 3 2 d σfu σm (4) From the tensile test, σ m is 3.04 MPa. With d = 13 µm and σ fu = 3400 MPa, the critical fibre length, l c, was obtained (12.3 mm). Considering the extent of the fibre breakage (Section 4.1), it is clear that the fibre length is well below the critical length before achieving optimum mixing (3.43 mm). 406 Polymers & Polymer Composites, Vol. 9, No. 6, 2001

5 Evaluation of Two-Roll Mill Method for Preparing Short Glass Fibre Reinforced NBR-Phenolic Composites 5. CONCLUSIONS In manufacturing glass-nbr-phenolic composites using the two-roll mill process, the fibre length is significantly reduced to about 2mm. To achieve the maximum reinforcing effect of the fibres, the critical fibre length must be above the maximum fibre length that can be obtained in the process. For this particular formulation and under the specified processing conditions, the two-roll mill method is not suitable as the fibre length is reduced to about 2 mm, which is below the critical length (12.3 mm). If the two-roll mill method is to be used as a processing method, a stronger matrix must be used so that the critical length is reduced below the minimum achievable fibre length. REFERENCES 1. Danowski R.J., US Patent 4,183,841, January 15, (1980) 2. Herring L.G., US Patent 4,507,165, March 26, (1985) 3. Boustany K. and Coran A.Y., US Patent 3,697,364, October 10, (1972) 4. Moghe S.R., Amer. Chem. Soc. Rubber Div. Meeting, Chicago, (1982) 5. Rezadoust A.M. et al, Fifth Iranian Seminar on Polymer Science and Technology, (2000) 6. Milewski J.V., Plastics Compounding, 53, (1982) 7. Goettler L.A. and Shen K.S., Rubber Chem. Technol., 56, (1983), O Connor J.E., Rubber Chem. Technol., 50, (1977), Matthews F.L. and Rawlings R.D., Composite Materials: Engineering and Science, Chapman and Hall, Oxford, England (1995) 10. Van Hattum F.W.J. and Bernardo C.A., Polymer Composites, 20, (1999), Polymers & Polymer Composites, Vol. 9, No. 6,

6 A.M. Rezadoust and M. Esfandeh 408 Polymers & Polymer Composites, Vol. 9, No. 6, 2001