http://www.revmaterialeplastice.ro MATERIALE PLASTICE ♦55♦No. 3 ♦2018 351 Aspects Regarding Ageing of Compounds Based on Natural Rubber and Plasticized Starch MARIA DANIELA STELESCU 1 , ELENA MANAILA 2 *, GABRIELA CRACIUN 2 , MIHAI GEORGESCU 1 1 National Research and Development Institute for Textiles and Leather, Division Leather and Footwear Research Institute, 93 Ion Minulescu Str., 031215, Bucharest, Romania 2 National Institute for Laser, Plasma and Radiation Physics, Electron Accelerators Laboratory, 409 Atomistilor Str., 077125, Magurele, Romania This paper presents a study on the behaviour of composites based on natural rubber and plasticized starch after aging under temperate climate conditions. Natural rubber blends containing different amounts of glycerine-plasticized starch as biodegradable filler were made. Analysis of samples after being kept for 1 year in temperate weather conditions shows a change in the surface of vulcanizates due to the influence of atmospheric factors. Surface oxidation and degradation was highlighted by: FTIR analysis (appearance of -OH, -COOH groups, etc.), SEM microscopy (appearance of cracks on the surface), immersion in water. The degradation of the materials is also highlighted by the decrease of the physical-mechanical properties, the gel fraction and crosslinking degree. The best aging behaviour was observed in the mixture containing a low amount of biodegradable starch and the worst behaviour was that of the control starch-free mixture. Keywords: polymer composites, natural rubber, starch, degradation, properties Products made from cured rubber undergo various physical and chemical influences over time, causing significant changes in rubber properties. These changes have led to the introduction of the notion of ageing. Vulcanizate ageing in the atmosphere is a destructive form of simultaneous action of aggressive factors such as oxygen, ozone, light, heat and mechanical stress causing degradation over time of vulcanizates if they are not provided with effective protection [1]. In order to evaluate the relative resistance of rubber to deterioration with the passing of time, accelerated ageing and heat resistance tests can be carried out. For this purpose, rubber is subjected to controlled deterioration conditions, over determined periods of time, and then characteristics are measured and compared with the properties of unaged rubber. Accelerated ageing does not reliably and under all circumstances reproduce changes caused by natural ageing [2]. This paper makes a comparison between accelerated ageing and ageing under temperate climate conditions of mixtures based on natural rubber and plasticized starch. It is known that natural rubber (NR) aging in air affects its properties. Due to the high level of unsaturation of NR, it can be easily attacked by oxygen. It was found that the oxidation of NR is further accelerated by heat, light, impurities, and mechanical strainstress [3]. In this study, various amounts of biodegradable filler, namely starch plasticized with glycerine, were added to natural rubber mixtures. Ageing of starch-based materials leads to increased hardness, depending on their physical condition and environmental conditions. This increase in hardness may be linked to two processes. If materials are stored in a glassy state (Tg < storage temperature) increased rigidity is generally associated with a structural relaxation phenomenon or physical ageing that occurs in the material. If they are stored in an amorphous state (Tg > storage temperature), increased hardness is usually linked to a molecular reorganization process known as retrogradation. In 1968, Collison [4] defined retrogradation * email:elenam_inflpr@yahoo.com as a crystallization process occurring due to the very strong tendency to form hydrogen bonds between hydroxyl groups on adjacent starch molecules. Retrogradation can eliminate water from the polymer network. This process is known as syneresis. Retrogradation is directly linked to the ageing of bread [5-6]. Starch introduced in the rubber mixtures was plasticized with a polyol - glycerine. Although the main functionality of these compounds is their plasticizing effect, papers were published suggesting that polyols may interact with the polymer chains of starch, reducing the retrogradation rate of these materials [7]. Several research studies have highlighted the influence of biodegradable fillers such as starch on improving rheological and physical-mechanical characteristics of some mixtures based on natural rubber [8-9]. Studies on ageing behaviour in temperate climate conditions or accelerated ageing of these types of mixtures have not been carried out yet. Experimental part The following materials were used in this study: Natural rubber (NR) Crep from Sangtvon Rubber Ltd, in the form of white rubber sheets, Mooney viscosity 67.64 ML (1 ‘+ 4’) 100°C, volatile matter content of 0.5%, nitrogen content 0.45%, ash content of 0.25%, impurity content of 0.026%; Starch - produced by Lach-Ner - soluble potato starch (water insoluble substances 0.28%; loss on drying 16.9%, easily biodegradable: BOD 5 - 0.6 g/g - and COD -1.2 m g/g); Glycerine produced by SC Chimreactiv SRL (free acidity 0.02%, density 1.26 g/cm 3 , purity 99.5%); Richon IPPD antioxidant (4010 NA) N-isopropyl - N- phenyl - phenylene diamine, 98% purity, molecular mass 493.6374; For crosslinking blends, the following were used: di(tert- butilperoxi-izopropil)benzen Perkadox 14-40B-GB (density 160 g/cm 3 , 3.8% active oxygen content, 40% peroxide content, p H 7) and polyfunctional monomer