MONTMORILLONITE CLAY FILLED SKIM NATURAL RUBBER NANOCOMPOSITE AS A VALUE ADDED RAW RUBBER

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MONTMORILLONITE CLAY FILLED SKIM NATURAL RUBBER NANOCOMPOSITE AS A VALUE ADDED RAW RUBBER U.N Ratnayake1 L.D.A.N Kumari2, A K W Prasad1 W D R Wijesinghe2 ( Rubber Research Institute, Telawala Road, Ratmalana, Sri Lanka, 2Department Chemistry, University Sri Jayewardenepura, Nugegoda, Sri Lanka. Correspondence: [email protected] 1

ABSTRACT: Skim natural rubber (SNR), a by-product of centrifuged latex manufacturing process, contains a higher percentage of non-rubber substances, especially nitrogenous substances. SNR, known as a low quality grade of raw rubber, limits the industrial applications because of the inherited unfavourable curing characteristics and inconsistent raw rubber properties especially green strength and lower resistance to thermo-oxidative degradation. Montmorillonite (MMT) clay filled SNR was prepared by incorporating aqueous dispersion of MMT clay into skim natural rubber latex followed by a coagulation. The nanostructures of MMT clay in the skim rubber was evaluated with X-ray diffraction technique while process based characterization and un-cured tensile measurements were used to study the effect of the nanostructured of MMT clay on viscoelasticity and green strength respectively. Plasticity Retention Index (PRI), which measures the resistance to thermo-oxidative degradation, data have shown that montmorillonite clay loading has increased the thermo-oxidative stability of the skim rubber. Improved green strength of MMT clay filled SNR nanocomposite has revealed the presence of MMT nanostructures and its effect on strain-induced crystallization. However, processability, as measured with low shear strain rate viscoelastic behaviour, is affected by the added MMT clay. With this latex compounding methodology, a novel MMT clay filled SNR with improved physical properties, especially green strength and resistance to thermo-oxidative degradation has been developed as a value added raw rubber for different applications. Keywords: Skim natural rubber, montmorillonite clay, green strength, thermo-oxidative degradation, nanocomposite, Mooney Viscosity

INTRODUCTION Skim natural rubber latex, a protein rich by-product of centrifugation process of natural rubber (NR) latex containing about 4-7 % dry rubber content and large quantities of nonrubber substances, converts into skim natural rubber (SNR), known as a lower grade of NR, with acid coagulation (Tillekeratne at al., 2003). As a result of less percentage of dry rubber and higher percentage of nitrogen rich non-rubber substances, characteristic features of natural rubber such as higher strength and resilient, resistance to thermo-oxidation and viscoelasticity/processability are not favourable for most of the applications in comparison to good quality raw rubbers like NR latex crepe and Ribbed Smoked Sheet (RSS). However, continuous attempts have been made to improve the quality of SNR and thereby make suitable for wide range of applications (Ismail and Veerasamy, 2011, Alex and Nah, 2006, Rattanaphan at el.,2011). In general most of the applications, NR requires reinforcement with particulate materials and recently, it was realized the potential of nano-scale materials (i.e. nanofillers) for reinforcement of NR as an alternative for conventional reinforcing material especially carbon black and silica (Arroyo at el., 2003, Aarasiri at el., 2013). Recent past, montmorillonite (MMT) clay, which belongs to 2:1 layered silicate has been identified as a potential reinforcing nanomaterial for preparing rubber nanocomposites because of a higher surface area to volume ratio of MMT clay platelets and its ability to surface modification through different methodologies (Galimberti, , 2012)

The objectives of this study were to prepare novel MMT clay filled SNR nanocomposite, with improved material properties, as a value added raw rubber compared to that of conventional skim rubber. The effect of clay nanostructures on viscoelasticity, strength and stiffness of the SNR is also discussed here MATERIALS AND METHODS Skim natural rubber latex was provided by Bulathsingala Rubber Factory, Lalan Rubbers (Pvt) Ltd. Montmorillonite clay supplied by Southern Clay Products, USA, with a cation exchange capacity of 80 meq/100 g and an interlayer distance of 1.24 nm was used as the clay mineral. Preparation of MMT clay filled SNR nanocomposite Aqueous dispersion of MMT clay was added into the skim latex while stirring. MMT cay added skim latex was acid coagulated and followed by a standard milling process to prepare the SNR nanocomposite. The exact MMT clay loading in each SNR samples are presented in Table 1. Table 1 MMT clay filled skim natural rubber (SNR) nanocomposite SNR nanocomposite code SNR SNR-3 SNR-6 SNR-9 SNR-12 SNR-15

MMT clay loadig phr 0 3 6 9 12 15

Properties of MMT clay filled SNR nanocomposite Plasticity (Po) and Plasticity Retention Index (PRI) were analysed as per the ISO standard test method, ISO 2007: 1991(E) and ISO 2930:1995(E)) respectively using rapid Wallace Plastimeter to study the effect of clay nanostructures on stiffness and resistance to thermosoxidative degradation. Processability of the MMT clay filled SNR, as measured with low shear stain rate viscoelasticity, was evaluated based on Mooney Visocosity and Mooney stress relaxation, which characterizes the elasticity of a rubber, using Mooney Viscometer, model EKT-2001M . Stress relaxation of rubber can usually be explained with by power law model, M= k(t)a where M is torque value in Mooney units and a is the rate of stress relaxation (Malac 2011). Effect of MMT clay nanostructures on green strength was investigated with stress-strain behaviour of the SNR nanocomposite at a strain rate of 300 mmmin-1 using INSTRON, model 3365 Universal Testing Machine. RESULTS AND DISCUSSION The effect of MMT clay structure on Po and PRI was investigated (Figure 1) since one of the characteristic draw backs of SNR is lower resistance to thermal oxidation. As shown in Figure 1, PRI increases with the increase of clay loading up to 12 and beyond that PRI shows a reduction. However, all SNR nanocomposites containing MMT clay have shown a better resistance to thermal oxidation than conventional skim natural rubber, indicating that

nanostructures of MMT clay improves the thermo-oxidative stability of the SNR nanocomposites. Figure 2 illustrates the effect of clay on Mooney viscosity and Mooney stress-relaxation of the SNR nanocomposites while Table 2 presents the Mooney viscosity data and elastic energy retention exponent (a+1), which directly relates to the elastic response of an un-cured rubber, for each MMT clay filled SNR nanocomposites. Low shear strain rate viscoelastic data show (Table 2) that resistance to flow has gradually increased while elastic response has marginal increased with MMT clay loading. 70 60

Po/PRI

50 40 30 20

Po

PRI

10 0

0

2 4 6 8 10 12 14 Montmorillonite clay loading, phr

16

Figure 1 Wallace Plasticity No. (Po) and Plasticity Retention Index (PRI) of MMT clay filled SNR nanocomposites

Mooney viscosities of the nanocomposites suggest that exfoliated MMT clay platelets/stacks interacts with latex particles and as a result hinder the flow behaviour at the low shear strain rate measured in the Mooney viscosity analysis. In addition, elasticity of the MMT clay filled SNR’s has slightly increased in comparison to conventional SNR but, with the increase of clay loading, elasticity has shown a slight reduction trend, probably due to the reduction degree of exfoliation of MMT clay with in the SNR. 450 Mooney Viscosity, M.U'

400

SNR SNR-3 SNR-6 SNR-9 SNR-12 SNR-15

350 300 250 200 150 100 50 0 0

2

4 6 8 Time, min Figure 2 Mooney viscosity and Mooney stress-relaxation curves of MMT clay filled SNR nanocomposites

These viscoelastic responses of the nanocomposites have clearly shown that processability is negatively affected by the MMT clay.

Green strength, which directly affects the processability and mechanical properties, of the conventional SNR is comparatively lower than raw rubbers such as RSS and crepe rubber. Figure 3 illustrates the effect of MMT clay structures on stress-strain behaviour of the uncured skim natural rubber as measured under tensile deformation. Table 1 Mooney viscosity and elastic

Composite code

MMT clay loading phr 0 3 6 9 12 15

SNR SNR-3 SNR-6 SNR-9 SNR-12 SNR-15

energy retention exponent Mooney Viscosity MU 93.76 95.36 96.38 98.4 104.8 109.26

Elastic energy retention exponent (a+1) 0.793 0.818 0.817 0.814 0.812 0.807

As shown in stress-strain behaviour (Figure 3), MMT clay has improved the tensile properties such as elastic modulus, stress at yield and stress at break. However, strength characteristics of the un-cured MMT clay filled SNR samples as measured with stress at yield and stress at break has reached to a maximum value when the clay loading is around 9-12 phr and then shown a reduction. Latex particle in the skim rubber latex adsorbed or interacted effectively with the exfoliated MMT clay platelets probably due to a higher percentage of non-rubber substances associated with latex particle and hence increase both stiffness and strength characteristics of the nanocomposites. However, reduction in stress at yield and stress at break at higher clay loading, especially above 9 phr of MMT clay would suggest a drop in degree of exfoliation 0.9

1.00

0 Phr 3 Phr 6 Phr 9 Phr 12 Phr 15 Phr

Tensile stress /MPa

0.7 0.6

0.80 Tensile stress, M Pa

0.8

0.60

0.5 0.4

0.40

0.3 0.2

Stress at yield

0.20

Stress at break

0.1 0

0.00 0

200

400

600

800

Tensile strain %

1000

1200

0

3

6

9

12

15

18

Montmorillonite clay loading, phr

Figure 3 Tensile stress-strain behaviour of un-cured MMT clay filled SNR nanocomposites

CONCLUSION The effect of MMT clay structure on processability and green strength of skim n natural rubber (SNR) was investigated in view of improving the quality of the skim rubber.

Thermo-oxidative stability, as measured with PRI, of the clay filled SNR has shown that nanostructured MMT clay has improved the ability to process at higher temperatures without a significant degradation. Improved green strength of the nanocomposites indicates the higher degree of MMT clay exfoliation and its interaction/adsorption with the latex particles. Although nanostructured MMT clay likely to improve reinforcement, MMT clay is negatively affected the processability in terms of both viscous and elastic behaviour at a lower shear strain rate. Acknowledgement Authors wish to acknowledge Lalan Rubber (Pvt) Ltd for providing materials and financial support to carry out this research project. REFERENCES Alex, R. & Nah, C. (2006). Preparation and characterization of organoclay-rubber nanocomposites via a new route with skim natural rubber latex. Journal of Applied Polymer Science. 102(4), 3277–3285. Amarasiri, A., Ratnayake, U.N., De Silva, U. K., Walpokage, S., Siriwardena, S. (2013). Natural rubber latex-clay nanocomposite: use of montmorillonite clay as an alternative for CaCO 3. Journal of National Science Foundation of Sri Lanka. 41(4), 293-302. Arroyo, M., Lo´pez-Manchado, M.A. & Herrero, B. (2003). Organo-montmorillonite as substitute of carbon black in natural rubber compounds. Polymer. 44(8), 2447–2453. Galimberti, M. (2012). Rubber Clay Nanocomposites. In A. Boczkowska (Ed.), Advanced Elastomers - Technology, Properties and Applications (pp. 91-120). Intech online publishers. Ismail, N. I. N. & Veerasamy, D. (2011) Value-added Natural Rubber Skim Latex Concentrate/Montmorillonite as Environmentally-friendly Nanocomposite Materials. Journal of Rubber Research. 14(4) (2003): 216–229. Malac J., 2011. Mooney Viscosity, Mooney Elasticity and Processability of Raw Natural Rubber .Journal of Materials Science and Engineering with Advanced Technology 3:67-87 Rattanaphan, O., Danwanichakul, D. & Danwanichakul, P. (2011). Reduction of protein content in skim rubber via both Extractions in skim latex and from rubber films. Proceedings of the 1st Mae Fah Luang University 2011 International Conference. (pp. 1-7). Chiang Rai, Thailand. Tillekeratne, L.M.K., Nugewela, A., Seneviratne, W.M.G. (2003). Hand Book of Rubber, Volume 2: Processing Technology. Rubber Research Institute of Sri Lanka.

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