Crust Formation On Natural Rubber

1. Fluid Structure Interactions Research Group Crust Formation On Natural Rubber Shamsul B.Kamaruddin1, Grant E. Hearn1, Alan H. Muhr2, P-Y.Le Gac3, Y.Marcor4, 1University of Southampton; 2Tun Abdul Razak Research Centre; 3IFREMER, 4ENSTA Bretagna sbk1m09@soton.ac.uk • Motivation • Accelerated ageing tests suggest no crust would be formed at ambient temperature (Lindley & Teo, 1977). This conflicts with some observations on naturally aged natural rubber (NR). Objectives • Characterise the crust that has formed on rubber aged naturally over a period of 80 years. • Probe the chemical and physical character of the crust and interpret the results to assess the significance of hypothetical mechanisms • Seek to resolve the conflict with extrapolation from accelerated ageing tests by identifying the primary mechanism and modelling it. Distribution of particle diameters and total area contribution of each tranche of particles Reduced modulus from micro-indentation according to the analysis of Oliver & Pharr (1992) Discussion Bramfield tyre: aged ~80 years in woodland UV light catalyses oxidation, but doesn’t penetrate deeply: where where I is intensity,  is volume fraction of black and L is effective length scale of black agglomerates I falls to 1% after only ~12mm. Ozone is too low in concentration and too reactive to penetrate more than ~0.5m. Only oxidation could penetrate as deep as ~1mm to form the crust; with ozone attack, photo-oxidation and loss of scission products by leaching and evaporation contributing to microcracks at the surface. General model for diffusion-limited oxidation: Analysis Results Table: TGA weight loss and Tmax Needs to be solved numerically, but analytical solutions exist if D and r are at least piecewise constant, that is : (i) On moving front, and (ii) In steady state, and (iii) In steady state, Section showing the hard layer and cracks into the transition layer (~4mm deep) and Conclusions • A crust ~1mm thick has formed on NR aged naturally over 80 years in conflict with the extrapolation of Lindley & Teo (1977). • Ozone attack and photo-oxidation could only degrade a layer a few microns thick. • The primary mechanism determining thickness is oxidation. • A model based on diffusion limited oxidation is being developed. FTIRspectra from different depths (labelled in mm) into rubber Table : IR Peak ratios /CH2 backbone (1460-1450cm-1) References: Lindley ,P.B and Teo,S.C., (1977), “High temperature ageing of rubber blocks “, Plastics & Rubber:Materials & Applications, 2, 82-88 Li G.Y., Keong J.L., (2005), “A review of rubber oxidation“, Rubber Chemistry Technology, 78, 355-389 Normalized C=O intensity, at different distances from the surface (left) to bulk (right) Acronym: TGA= Thermogravimetric Analysis, FTIR=Fourier Transform Infrared Spectroscopy (FTIR) , IR = Infrared FSI Away Day 2012 Acknowledgement: This project is supported by funds from the Malaysian Rubber Board