Effect of Process Variables on the Tensile Properties of Fibreglass Nonwoven Composites

Transcription

1 Effect of Process Variables on the Tensile Properties of Fibreglass Nonwoven Composites Effect of Process Variables on the Tensile Properties of Fibreglass Nonwoven Composites Sheraz Hussain Siddique Yousfani 1, R.H. Gong 2 and I. Porat 2 1 Textile Engineering Department NED University of Engineering & Technology, Karachi, Pakistan 2 School of Materials, The University of Manchester, Manchester M13 9PL, United Kingdom Received: 14 August 2014, Accepted: 2 February 2015 SUMMARY Fibreglass nonwoven composites are a new class of fibre-reinforced composites, and in this work composite were manufactured by using nonwoven webs as reinforcement and thermo-setting resin as matrix. The were prepared by using a resin infusion technique, and the nonwoven webs were manufactured by a wet-laying method. In this research work, the main purpose was to investigate the effects of different process variables, i.e. dispersion, fibre length and multiple layering, on the tensile properties of these composites. It was found that the tensile strength was improved by better dispersion of fibre strands. Tensile strength and stiffness of the composite were also improved by an increase in fibre length and by multiple layering. Keywords: Fibreglass, Nonwoven, Tensile properties, Composites, Wet-laid, Nonwoven webs, Dispersion, Stiffness, Fibre length, Multiple layering 1. Introduction Chopped fibreglass strands are used as reinforcements for making random fibre composites such as sheet moulding compounds, bulk moulding compounds, chopped strand mats, spray lay-up and hand lay-up products 1-3. The products manufactured by these techniques are generally used for lowimpact applications as compared to those manufactured by using fibreglass woven fabrics as reinforcement. This is due to the fact that chopped strands are distributed in the matrix in a noncontinuous manner and also they are not dispersed uniformly in the matrix as the fibres are not opened to an individual state. The main purpose of this research work was to investigate a new technique of 1 Corresponding author. siddique@neduet.edu.pk Smithers Information Ltd., 2016 composite manufacturing. It had the following objectives: a) To manufacture fibreglass nonwoven webs by dispersion of fibreglass strands in water and then making the webs by draining the water from the slurry using a lab scale paper hand sheet machine, as explained in 4 b) Fibreglass nonwoven webs were used as reinforcement, the composite were manufactured by using resin infusion technique, as explained in 5 c) Three variables were to be considered, i.e. fibre length, dispersion and multi-layering of nonwoven webs d) The tensile properties i.e. strength, strain and Young s modulus were compared for different types of composite It is believed that if the chopped strands are converted to nonwoven webs, the fibres have more chance of even distribution in the composite structure. It was found during previous research conducted by the authors that the fibres were randomly distributed in the nonwoven webs 4. From the literature review it was found that fibreglass nonwoven composites were manufactured by a group of researchers using fibreglass strands as reinforcement and polyester fibre as matrix. Both these fibres were mixed; nonwoven webs were manufactured by a wet-laying method and bonded with hydro-entanglement. The webs were then compression moulded to form composites in which the polyester was melted to form the matrix and glass fibre used as reinforcement 6,7. In this research work, the composite were manufactured by using thermo-set resin as matrix and fibreglass nonwoven webs were used as reinforcement. These composite were tested for their tensile Polymers & Polymer Composites, Vol. 24, No. 1,

2 Sheraz Hussain Siddique Yousfani, R.H. Gong and I. Porat properties, and the effects of different variables were investigated. 2. Materials and methods Chopped fibreglass fibres were supplied by PPG Fibreglass Company 9. Two fibre lengths of 6 and 9 mm were used for manufacturing of composites. The specifications of these fibres are shown in Table 1. In order to manufacture fibreglass webs from these fibres, a wet-laying method was used 4. Composites were manufactured by using these webs as reinforcement by using the method of resin infusion. For this purpose thermo-set resin Araldite LY 5052 and Hardener HY 5052 were used to produce the matrix. The process of composite manufacturing is explained in detail in 5. Fibreglass nonwoven composites were manufactured by using these nonwoven webs as reinforcement. The following conditions were used for making composites as shown in Table 2. In order to determine the tensile properties, five were chosen from each category of the flat-circular composite. 2.1 Tensile Tests Tensile properties i.e. tensile stress and strain at break and Young s modulus, were determined by using the standard test method as explained in 10. Rectangular specimens of 140 by 25 mm were cut prepared. The gauge length was fixed at 100 mm, lower than the recommended length of 150 mm because it was not possible to obtain longer due to the limitation of the web former. The tests were done at a constant extension rate of 2mm/min. The following equations were used to determine the stress and strain at break. (1) σ t is the tensile stress at break in MPa F is the force in N at break and A cs is the cross sectional area in mm 2. (2) ε(%) is the strain at break expressed in % Lo is the gauge length of the specimen expressed in mm Lo is the increase in specimen length between the gauge marks expressed in mm. The stress values at 0.05% strain and 0.25% strain were taken in order to determine the tensile modulus by using the following equation 8 : (3) E t is the Young s modulus of elasticity expressed in MPa σ 1 is the stress in MPa measured at the strain value ε 1 = 0.05% σ 2 is the stress in MPa measured at the strain value ε 2 = 0.25%. 3. Results Tensile stress and strain at break and Young s modulus of different types of composite was determined by Equations (1), (2) and (3) respectively. For both the single and multi-web it was observed that these composites were brittle in nature. Stress-strain curves for some of the are shown as examples in Figures 1 and 2. Table 1. Specification of glass fibres 9 Sample No. Fibre length mm Fibre diameter µm Moisture content % Silane Sizing content % The results for the different are shown in Table 3. These results are graphically represented in Figures 3 to 8. Table 2. Conditions for making fibreglass nonwoven composites Sample No. Condition Description for making nonwoven webs Coding for nonwoven webs Description for making composites 1 6 mm-ndnd No dispersion no de-flocculation W6 ndnd Single and multi-web 2 9 mm-ndnd No dispersion no de-flocculation W9 ndnd Single and multi-web 3 6 nn-10-0 Single step 10 minutes dispersion process 4 9 mm Two step dispersion process, each step being 10 minutes W W Single and multi-web Single and multi-web Coding for composite Cs 6 ndnd and Cm 6 ndnd Cs 9 ndnd and Cm 9 ndnd Cs and Cm Cs and Cm Polymers & Polymer Composites, Vol. 24, No. 1, 2016

3 Effect of Process Variables on the Tensile Properties of Fibreglass Nonwoven Composites 4. Discussion 4.1 Tensile Strength From Table 3 and Figures 3 and 6, it was observed that with the dispersion process, the tensile strength increased slightly. This was perhaps because, in the dispersion process, the fibre strands opened up to individual fibres, which distributed evenly in the composite structure and increased the number of fibre crossing points. For the fibres to contribute to the composite strength, it is necessary that the fibre must be longer than its critical length because if it is shorter than the critical length, the strength of the fibre is not utilized properly as illustrated in Figure The critical length of the fibres is determined by using the following equation. Figure 1. Stress-strain curves from tensile testing of single-web (4) The second reason for the increase in the tensile strength was perhaps the decreasing void content 5. It was reported in a study that for the glass vinyl ester resin laminate manufactured by resin transfer moulding, the tensile strength decreased with the increase in the void content 11. From Table 3 and Figures 3 and 6, it was observed that with the increase in fibre length the tensile strength increased slightly. This was perhaps because longer fibres offered more contact surface to the matrix resulting in better load transfer between the matrix and the reinforcement. For short fibre composites the tensile load applied on a single fibre is not uniform throughout its length. For a tensile load applied on a single fibre aligned in the direction of the load, the load is transferred from the matrix to the fibre with the help of shear stress between the fibre and the matrix. The shear stress is at a maximum at the fibre ends and it reduces to a minimum at the centre of the fibre. Normal stress also acts on the fibre and it is at a minimum at the fibre end and reaches a maximum at the centre of the fibre. Figure 2. Stress-strain curves for tensile test of multi-web Table 3. Tensile properties for different types of composite Tensile properties Single web Multi-web Cs 6 ndnd Cs 9 ndnd Cs Cs Cm 6 ndnd Cm 9 ndnd Cm Cm Tensile stress at break (MPa) Tensile strain at break (%) Young s modulus (MPa) Polymers & Polymer Composites, Vol. 24, No. 1,

4 Sheraz Hussain Siddique Yousfani, R.H. Gong and I. Porat l c is the critical length of the fibres σ fu is the ultimate tensile strength of the fibre τ i is the interfacial shear strength between the fibres and the matrix d f is the diameter of the fibre Figure 3. Tensile stresses at break of the single-web composite From Equation (4) it was found that the critical length of the fibres is dependent on the interfacial shear strength between the matrix and the fibres and if the shear strength increases then the critical length of the fibres decreases. The adhesion of the fibres and the matrix is dependent on the surface area of the fibres, i.e. if the surface area offered by the fibres is higher, the chances of adhesion are also higher resulting in a higher interfacial shear strength between the matrix and the fibre. The surface area of the fibre is dependent on its length and diameter. Figure 4. Tensile strains at break of the single-web composite It was reported in a study that for random in-plane glass fibre-reinforced polypropylene laminates, the laminate strength increased with the increase in the fibre length but at fibre lengths greater than 3 to 6 mm, the strength reached a plateau and was dependent on the fibre volume fraction 13. It was also reported that for sheet moulding compounds, the tensile strength increased with the fibre length (ranging from 12.5 to 37.5 mm), and then remained constant for a fibre length of 50 mm 12. Figure 5. Young s modulus of the single-web composite The tensile strength of the composite is dependent on the interfacial shear strength between the matrix and the fibres and as the length of the fibre increases its surface area also increases. This causes the interfacial strength to increase as well. However, when the interfacial shear strength reaches a maximum value, it becomes constant and does not increase with the increase in the fibre length, so the composite strength reaches a plateau level. 68 Polymers & Polymer Composites, Vol. 24, No. 1, 2016

5 Effect of Process Variables on the Tensile Properties of Fibreglass Nonwoven Composites Figure 6. Tensile stresses at break of the multi-web composite The maximum value of the interfacial shear strength between the fibre and the matrix is considered as either the shear strength of the fibre-matrix interfacial bond or the shear strength of the matrix, whichever is lower 12. It was also reported in a study of polypropylene fibreglass composites manufactured by injection moulding, that the tensile strength was higher for long-fibre (up to 4.5 mm residual length) as compared to shortfibre (1 to 2 mm residual length) 14. Figure 7. Tensile strains at break of the multi-web composite The effect of fibre length on the strength of the composites is dependent on the adhesion between the resin and the fibres, so when the method of manufacturing or the resin system changes, the critical length of the fibre also changes, affecting the shear strength of the composites. The second reason for the increasing trend in the tensile strength due to the increase in the fibre length was perhaps a decreasing void content 5. Figure 8. Young s moduli of the multi-web composite From Table 3 and Figures 3 and 6, it could be concluded that with the multiple layering process, the tensile strength slightly increased because the fibre volume fraction showed an increasing trend and the void content decreased 5. This agrees with other published findings Tensile Strain From Table 3 and Figures 4 and 7 it can be observed that with the dispersion process, the tensile strain increased slightly, but the changes in the tensile strain are so small that this phenomenon is not significant. From Table 3 and Figures 4 and 7, it appears that the fibre length and layering process had no obvious effect on tensile strain. Polymers & Polymer Composites, Vol. 24, No. 1,

6 Sheraz Hussain Siddique Yousfani, R.H. Gong and I. Porat Figure 9. Effect of fibre length on the normal stress distribution along the length of a single fibre at the point of a composite failure Young s Modulus From Table 3 and Figures 5 and 8, it can be seen that fibre length and dispersion had no significant effect on the Young s modulus. However, it was observed that due to the layering process, the Young s modulus increased slightly. This was mainly the result of increasing tensile strength as discussed above. This also agrees with previous findings 11, Conclusions The effect of different process variables (i.e. dispersion, fibre length and multiple layering) on the tensile properties of fibreglass nonwoven composites were studied in detail. The conclusions are summarized as follows: Wet laying is a feasible method for manufacture of fibre glass composites. The tensile properties are comparable with those of products from existing techniques such as sheet moulding and bulk moulding. Due to the dispersion process, the tensile strength increases. With an increase in fibre length, the tensile strength and modulus increase slightly. The multiple layering process increases the tensile strength and stiffness of the composites. References 1. Verpoest I. Composites preforming techniques; in: Comprehensive Composite Materials, Edited by Kelly A and Zweben C. Elsevier Ltd., Australia (2000). 2. Mallick P.K. Particulate and short fibre reinforced polymer composites; in: Comprehensive Composite Materials, Edited by Kelly A. and Zweben C. Elsevier Ltd., Australia (2000). 3. Cripps D. Open mould techniques for thermoset composites. Comprehensive composite materials. Edited by Kelly, A. and Zweben, C. Elsevier Ltd., Australia (2000). 4. Yousfani S.H.S., Gong R.H. and Porat I. Manufacturing of Fibreglass Nonwoven Webs Using a Paper Making Method and Study of Fibre Orientation in These Webs. Fibres & Textiles in Eastern Europe, 91 (2), (2012) Yousfani S.H.S. Wet-laid Fibreglass Composites. Ph.D. Thesis, Textiles. The University of Manchester, UK (2010). 6. Vaidya N. The Manufacturing of wet-laid hydro-entangled glass fibre composites for industrial applications. Masters Dissertation, Textiles. North Carolina State University (2002). 7. Vaidya N., Pourdeyhimi B and Shiffler D. The Manufacturing of wetlaid hydro-entangled glass fibre composites: Preliminary Results. International Nonwoven Journal, (Winter 2003), Baker A., Dutton S. and Kelly D. Composite materials for aircraft structure, American Institute of Aeronautics and Astronautics, U.S.A. (2004). 9. PPG Fibreglass Europe, (2001). Data sheet of chopped strands 3075, 8031 and from ppg.com/glass/fiberglass/products/ Pages/wet_chop.aspx. 10. ISO Plastics determination of tensile properties - Part 4: Test conditions for isotropic and orthotropic fibre reinforced plastic composites. International Standard Organization (1997). 11. Varna J., Joffe R. and Berglund L.A. Effect of void content on failure mechanism in RTM laminates. Composites Science and Technology, (53) (1995), Mallick P.K. Particulate and short fibre reinforced polymer composites. Comprehensive Composite Materials. Edited by Kelly, A. and Zweben, C. Elsevier Ltd., Australia (2000). 13. Thomason J.L., Vlug M.A., Schipper G. and KriKort H.G.L.T. Influence of fibre length and concentration on the properties of glass fibre reinforced polypropylene - 1: Tensile and flexural modulus. Composites Part A: Applied Science and Manufacturing, 27 (6), (1996), Thomason J.L. The influence of fibre length and concentration on the properties of glass fibre reinforced polypropylene - 5: Injection moulded long and short fibre PP. Composites Part A: Applied Science and Manufacturing, 33 (12), (2002), Polymers & Polymer Composites, Vol. 24, No. 1, 2016