1. Crust Formation On Natural Rubber Shamsul B.Kamaruddin 1, Grant E. Hearn 1, Alan H. Muhr 2, P-Y.Le Gac 3, Y.Marcor 4, 1 University of Southampton; 2 Tun Abdul Razak Research Centre; 3 IFREMER, 4 ENSTA Bretagna sbk1m09@soton.ac.uk Fluid Structure Interactions Research Group Acknowledgement: This project is supported by funds from the Malaysian Rubber Board FSI Away Day 2012 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. References: Conclusions Bramfield tyre: aged ~80 years in woodland Analysis Results Section showing the hard layer and cracks into the transition layer (~4mm deep) Rubber (wt%) Black (wt%) Ash (wt%) Tmax Surface47.247.75.0445 1-3mm55.038.35.4446 5-7mm59.531.58.6418 Bulk59.231.99.0413 Table: TGA weight loss and T max FTIR spectra from different depths (labelled in mm) into rubber 3360 cm -1 -OH 1708 cm -1 C=O ketone 1014 cm -1 C-O 831 cm -1 C=CH Surface1.670.67 8.3 with silica 0.33 1mm1.000.33 8.67 with silica 0.67 2mm0.350.291.650.59 4mm0.220.040.890.96 Internal bulk 0.07<0.01<0.300.63 Table : IR Peak ratios /CH 2 backbone (1460-1450cm -1 ) Normalized C=O intensity, at different distances from the surface (left) to bulk (right) 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 UV light catalyses oxidation, but doesn’t penetrate deeply: 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: Needs to be solved numerically, but analytical solutions exist if D and r are at least piecewise constant, that is : (i) On moving front, (ii) In steady state, (iii) In steady state, where and  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. 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 Acronym: TGA= Thermogravimetric Analysis, FTIR=Fourier Transform Infrared Spectroscopy (FTIR), IR = Infrared