Mechanical Properties of the Composite Material Based on Modified Scrap Tires and Polymer Binder R. Plesuma*, A. Megne, I. Mateusa-Krukle and L. Malers Institute of Polymer Materials, Riga Technical University, Latvia Received: 10 February 2012, Accepted: 25 October 2012 Summary The present work focuses on the investigation of the mechanical properties of the composite material composed of preliminary modified rubber crumb and a polymer binder. The modification of rubber crumb was realized by means of treatment with water or sulphuric acid, in order to change the degree of polymer crosslinking or to intensify adhesion interaction between rubber particles and polymer binder. The Shore C hardness, compressive stress at 10% deformation and compression modulus of elasticity, ultimate tensile strength and elongation at break of composite material, in correlation with its composition and adhesion between rubber and polymer binder were investigated. Obtained results show possibility of attaining significant changes in the mechanical properties of composite material by selected rubber crumb treatment methods, in correlation with the composite material composition. This gives additional opportunity for meaningful improvement of composite material production technology. INTRODUCTION Recycling of used automobile tyres must be considered as an important activity from the point of view of not burdening the environment from the non-degradable waste. Various ways of utilization and recycling methods * Corresponding author: Tel. +37129570655; fax: +37167615765. E-mail address: renate.plesuma@gmail.com Smithers Rapra Technology, 2013 177
R. Plesuma, A. Megne, I. Mateusa-Krukle and L. Malers exist [1];one of the most common approaches for recycling tyres is mixing of rubber crumb with a polymer binder to produce composite materials [2, 3]. In our previous investigations, optimization of the composition and technology of composite material production from mechanically ground scrap tyres and a polyurethane type binder, were realized [4, 5, 6]. It was elucidated that the properties of the studied composite material are highly dependent, not only on the material composition and the type of polyurethane binder used, but also on several technological parameters and production conditions such as molding pressure and formation temperature [6]. The present work must be considered as a continuation of previous investigations on composite material and focuses on clarifying the influence of preliminary rubber crumb modification on selected mechanical properties of the composite material, in what must be considered as insufficiently investigated question. Two different methods of modification were selected based on experience and published composite material research [7, 9, 11-13]. Hence,the methods of rubber crumb modification detailed in this study could be useful in improving the mechanical properties of composite material and production technology. EXPERIMENTAL Description and Preparation of Materials and Samples Mechanically ground at ambient temperature, modified and non-modified rubber crumb obtained from used tyres, with a particle size ranging from 0.2-7.0 mm (Figure 1), and polyurethane type binder with a different reactivity (isocyanate group content - 2.42%, 5.4% and 7.4%) were used in order to produce composite material samples. Different compositions of rubber crumb and polymer binder (from 8-23 wt%) were used in the composite material. Uniform samples of composite material were prepared under constant and defined conditions: formation temperature (18-22 C), pressure (0.004 MPa), molding time (24 h) and relative air humidity (23-30%). It is well known [7] that environmental moisture content directly affects the hardening degree of polyurethane polymers, containing isocyanate groups, due to the degree of change of polymer crosslinking. Direct preliminary rubber crumb modification using water was implemented in order to clarify the possibility of intensifying the degree of crosslinking of the polymer binder used directly in the composite material. The conditions employed for the 178
Figure 1. Particle size distribution of scrap tyres before and after surface modification with sulphuric acid rubber treatment with water in the present work were selected taking into account our previous investigations [8]. The modification of rubber crumb was realized by the direct treatment of previously dried rubber crumb with a defined amount of water. Chemical modification of rubber crumb with 96% sulphuric acid at different reaction times was used in order to explore the possibility of intensifying the adhesion between the polymer binder and rubber particles, thereby affecting the mechanical properties of the investigated composite material [9, 11, 12]. The particle size distribution of scrap tyres before and after surface modification with sulphuric acid is shown in Figure 1. Testing In order to examine constituent materials used in the production of the composite material and determine the essential characteristics which could affect the final properties of the material, the following test methods were used: Determination of moisture content in rubber crumb was ascertained by simple calculations with preliminary dried (different drying times were selected) and modified rubber particles. In order to determine some wetting properties, the contact wetting angle q of a water droplet on a rubber surface of the non-modified and 179
R. Plesuma, A. Megne, I. Mateusa-Krukle and L. Malers modified solid rubber samples were examined. The possible change of hydrophilic properties, depending on the rubber surface energy, was estimated (Figure 2). Special samples were prepared in order to examine the strength of adhesive bonding between the polymer binder and modified rubber surface. For this purpose Peel test (90, speed 50 mm/min) was used according to LVS EN 28510-1. The apparent density AD (kg/m 3 ) of the composite material was determined according to LVS EN 1602. Shore C hardness was investigated by using a Shore tester (Type C; ISO 7619, ISO 868). Mechanical properties of the composite material (compressive stress at 10% deformation and compressive modulus E) were determined by using the testing apparatus, Zwick/Roell7020, according to EN 826. In the tensile mode of loading, the ultimate tensile strength and elongation at break, for specially prepared samples with defined dimensions, were determined according to LVS EN ISO 527-3. Figure 2. Contact angle q of solid rubber samples before and after modification with sulphuric acid 180
RESULTS AND DISCUSSION In previous investigations it was clarified that the composite material composition and its hardness is highly interdependent [5]. Close correlation between mechanical properties and Shore C hardness was demonstrated, hence proving that Shore hardness can be used as a tool to determine other mechanical properties (for example, compressive stress at 10% deformation) without direct material testing [5, 6]. It was also elucidated that elevated air humidity, during formation of the composite material, reduces the composite material hardening time of due to intensification of polymer crosslinking reactions: thus making it possible to achieve significantly higher values of Shore C hardness after 24 h hardening, compared with the testing results of composite material samples hardened at ambient air humidity [10]. Therefore, the effect of moisture content on the properties of the composite material must be considered as potentially significant, and for that reason additional investigations were carried out using a rubber crumb special treatment with water spraying before production of the composite material samples. The results show that values of apparent density increase upon increasing the sprayed water and binder content (Figure 3a). The correlation between characteristics and properties of the composite material with the binder content, used in the production of the samples, has also been investigated and demonstrated in several previous investigations [4-6]. Similar tendencies were observed in the determination of Shore C hardness for composite material samples containing modified rubber particles (Figure 4a). This can possibly be explained by the specific nature of polymer binder in improving the hardening process, as the rubber particle-polymer binder interface is highly dependent on the environmental moisture [8]. Chemical modification of rubber crumb was carried out by treatment with sulphuric acid under defined conditions prior to the production of the composite material samples. The results show a significant decrease of apparent density compared with the composite material samples prepared under the same conditions using non-modified rubber crumb (Figure 3b). This can possibly be explained by the changes in the particle size distribution of rubber crumb after chemical modification (Figure 1). At the same time, experiment showed that Shore C hardness increased by up to 15% when modified rubber crumb was used; probably a result of the destruction of double bonds on the rubber surface and consequent loss of elasticity (Figure 4b). In addition, a more similar shape of rubber particles was observed after chemical modification. 181
R. Plesuma, A. Megne, I. Mateusa-Krukle and L. Malers Figure 3. Variation in composite material apparent density AD (kg/m 3 ) with composition of material and modification of rubber crumb: a) treated with water, b) treated with sulphuric acid The chemical modification of the rubber surface with sulphuric acid was carried out mainly in order to investigate subsequent changes of adhesive interaction (bonding) between the rubber and polymer binder. Surface oxidant treatment, in order to improve interfacial compatibility in composite materials, has also been demonstrated in systems containing polyolefins [12, 13]. Peel test results show that the chemical modification of the rubber surface promotes adhesive bonding for selected samples (Figure 5). The results of the Peel test correlate with rubber surface wettability measurements shown 182
Figure 4. Variation in composite material Shore C hardness with composition of material and modification of rubber crumb: a) treated with water, b) treated with sulphuric acid in Figure 2. A visual evaluation of the failure character of adhesive joints after the Peel test demonstrated mainly adhesive type of destruction. It was shown that with the increase of concentration of reactive isocyanate groups in the polymer binder, Peel strength increased due to a more active interaction between components of the adhesive joint (Figure 5).This fact establishes the importance of the individual properties of constituent materials used in the production of composite material. 183
R. Plesuma, A. Megne, I. Mateusa-Krukle and L. Malers Figure 5. Variation in Peel strength with polymer binder activity before and after chemical modification of rubber surface In order to investigate subsequent changes of mechanical properties - ultimate tensile strength (σ t ) and elongation at break (ε t ) in the tensile mode of loading of the composite material (polymer binder with 2.42% isocyanate group content) were determined. The samples of composite material were prepared using modified and non-modified rubber crumb. Results are given in Table 1. The results presented in Table 1 show that the rubber crumb treatment with water demonstrated mainly higher ultimate elongation values and lower values of tensile strength for the tested material samples. This can be possibly explained due to some of the water not being involved in the crosslinking reactions of the polyurethane binder and therefore acting as a plasticizer of the composite material. Chemical modification of the rubber crumb resulted in the ultimate tensile strength exhibiting lower values compared with the non-modified samples. However, the elongation at break in the tensile mode of deformation demonstrated an inverse situation. The explanation for this must take into account the fact that rather different rubber particle shape and size distributions (Figure 1) were observed for modified and non-modified rubber crumb, and therefore the compactness and also properties of the composite material samples differ [11]. Similar samples of composite material, produced with the methods mentioned above, were also tested in a compressive mode of loading; compressive modulus of elasticity and compressive stress at 10% deformation were also determined (Table 1). 184
Table 1. Ultimate tensile strength (σ t ),elongation at break (ε t ), compressive stress at 10% deformation (σ 10% ) and compressive modulus of elasticity (E) of the composite material Nr. Description of rubber crumb Polymer binder content (wt.%) σ t (MPa) ε t (%) σ 10% (MPa) E (MPa) 1. Non-modified 13 0,068 66 0.21 0,25 18 0,152 57 0,28 0,36 23 0,175 51 0,39 0,42 2. Modified with water 2.1. Preliminary dried ( 0.5 h; 80 C) 13 0,086 58 0,42 1,09 18 0,088 - - 1,10 23 0.090 63 0,51 1,40 2.2. Treated with water -1 ( 4 wt% ) 13 0,070 73 0,59 1,38 18 0.118-0,60 1,60 23 0.116 75 0,69 1,63 2.3. Treated with water -2 (6 wt% ) 13 0,074 67 0,61 1,30 18 0.088 71-1,36 23 0,142 75 0,79 1,70 3. Modified with sulphuric acid 13 0,064 67 0,80 1,50 18 0,104-1,00 6,80 23 0,106 62 1,38 7,00 In the case of using water spraying as a modification method, test results show that the mechanical properties of the composite material are highly dependent on moisture content; explained by the improvement of the particular polymer binder hardening. The significance of water treatment prior to the production of composite material samples was established and therefore the role of moisture presence is emphasized once again. Similar tendencies were observed when chemical modification of the rubber crumb was used (Table 1). In this case, mechanical properties such as modulus of elasticity and compressive stress at 10% of deformation were enhanced due to the improvement of adhesion between the rubber and polymer binder. This is highly connected with the improvement of wetting capability (hydrophilicity) of the rubber crumb due to the processes arising from the influence of sulphuric acid on the chemical structure of the rubber surface [9]. This was also demonstrated by the experiment investigating the 185
R. Plesuma, A. Megne, I. Mateusa-Krukle and L. Malers adhesion between the preliminary treated rubber substrate and the polymer binder (Figure 5). CONCLUSIONS The results show that the selected properties of the composite material are highly dependent on both the rubber crumb treatment with water spraying and surface modification with sulphuric acid. The remarkable influence of rubber crumb treatment with sulphuric acid on selected properties (Shore C hardness, compressive stress and modulus of elasticity, peel strength) of the composite materials was observed; attributed to the increase of rubber surface hydrophilicity and thereby improved adhesion with the polymer binder. It is postulated that the rubber treatment with water leads to the increase of Shore C hardness and ultimate tensile strength of composite material, due to the intensification of crosslinking and thus hardening process of the polyurethane binder. Therefore, the methods of the modification of rubber crumb, detailed in this study, could lead to a meaningful improvement of mechanical properties and production technology of composite material. REFERENCES 1. Mark J.E., Erman B., and Erich R., The Science and Technology of Rubber,3 d. Ed., Elsvier Inc., USA, (2006) 763. 2. Lund H.F., McGraw Hill recycling handbook, R.R.Donelly & Sons Company, USA, (1993) 18.2-18.35. 3. Adhikari B. and Maiti S., Progress in Polymer Science, 25, (2000) 909-948. 4. Malers L., Plesuma R., and Locmele. L., Mech. Comp. Mat., 45, (2009) 1-6. 5. Malers L., Plesuma R., and Locmele L., Scientific Journal of Riga Technical University: Material Science and Applied Chemistry, 21, (2010) 35-38. 6. Malers L., Plesuma R., Locmele. L., Megne A., and Mateusa-Krukle I., Polymer composites: Book of abstracts, Zapodočeska Univerzita v Pilzen, the Czech Republic, ( 2011) 135-136. 7. Wicks Z.W., Jones F.N., and Pappas S.P., Organic Coatings: Science and Technology, Volume I, John Wiley & Sons Inc., USA, (1992) 189-191. 186
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