The effects of fillers on the surface morphology, mechanical properties and protein content in natural rubber latices / Ruhida Ab Rahim

Ruhida , Ab Rahim (2021) The effects of fillers on the surface morphology, mechanical properties and protein content in natural rubber latices / Ruhida Ab Rahim. PhD thesis, Universiti Malaya.

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Abstract

The purpose of adding fillers into rubber products is not only as a cheapening aid but also to improve the properties of the end products. In this study, the untreated and stearic acid (SA)-treated calcium carbonate (CaCO3) were used as a filler into the mixed clones natural rubber latex concentrate (MCLC). The MCLC was prepared by processing the monoclones field latex, namely RRIM2023, RRIM2024 and RRIM2025, at a similar weight ratio. The untreated filler loadings mixed into MCLC varied from 10 to 40 parts per hundred rubber (pphr). The CaCO3 treated with SA at various concentrations of 0.5%, 1.0%, 3.0%, 4.0%, 5.0% and 7.0% by % (w/v) in chloroform was mixed into MCLC at 30 pphr filler loading. The MCLC films at different untreated filler loadings were evaluated for their properties chemically and physically. Results obtained were evaluated and compared to observe the occurrence of chemical interferences, if any, on the determination of extractable proteins (EP) content. The addition of the SA-treated CaCO3 at various concentrations of SA was to improve the filler-rubber interaction and to reduce the amount of EP content extracted from the MCLC films. The SA-treated CaCO3 was characterised by using Fourier-Transform Infra-Red (FTIR) and Thermogravimetric analysis (TGA) to identify the presence of stearic acid on the surfaces of CaCO3 particles. The percentage of nitrogen (N) and EP contents were measured using the Automatic Analyser and UV Microplate Reader, respectively. The EP content of MCLC films was also measured before and after exposure to ozone at 10 parts per hundred million (pphm) concentration, and the surface morphology of MCLC films was monitored before and after exposure to ozone. Their tensile and tear strengths were determined as well as their total, combined and free sulphur contents. The results of latex properties such as dry rubber content (DRC), total solids content (TSC), alkalinity (NH3) and volatile fatty acid number (VFA No.) showed that the MCLC prepared from MCFL is clean, stable and free from foreign matters and non-polymer impurities. The cross-link concentration of samples was also being measured by the Flory-Rehner equation to identify that the presence of CaCO3 in samples has the ability to influence the cross-link concentration. It was found that the addition of the filler below 20 pphr affected the cross-link concentration. The percentage of combined sulphur of unaged samples decreased from 22% until 37% and aged samples from 24% until 43%. The tensile strength of the unaged sample decreased from 10% to 20% and aged samples from 10% to 29%. Therefore, decreasing the percentage of combined sulphur led to a reduction of the tensile strength of MCLC films. The total protein contents on the MCLC films at different filler loadings showed that the values were higher in the unaged samples as compared to aged samples (70 ºC for 7 days). From TGA analysis, the presence of stearic acid can only be detected at filler treated at 3.0% concentration of stearic acid. FTIR analysis showed that the absorption peak at 875 cm-1 indicated that the stearic acid might be attached to the CaCO3. The nitrogen (N) content of the unaged and aged samples at 3.0% stearic acid treated CaCO3 was similar. However, all corrected and non-corrected EP contents for unaged samples were lower compared to the Control sample (untreated CaCO3). The highest value of corrected EP content was 417.4 μg/g obtained from latex mixed with 3.0% treated filler. The results of the EP contents were reported as non-corrected and corrected values. In the corrected method, the copper reagent was replaced by water in an alkaline tartrate solution in the colour development stage during the protein assay. The results of corrected and non-corrected EP contents for the aged, treated samples were found to be inconsistent and was attributed to the heating and ageing processes. Contrary to expectation, the non-corrected EP content was lower compared to the corrected EP content. The tensile strength of unaged SA-treated CaCO3 results shows an increase from Control 1 to 0.5% SA-treated before being reduced from 1.0% to 7.0%. Nevertheless, the MCLC films with the SA-treated CaCO3 showed no significant difference in tensile strength after ageing but higher than the Control 1 (untreated CaCO3). The tear strength of unaged samples was higher as compared to aged samples.

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