Natural rubber coating: A new trend for the rubber tree plantation in Brazil.
As it was pointed out by the United Nations Environment Program (ref. 3), the search for new applications for materials from renewable sources has been one of the most explored means of improving the economy in developing countries.
Natural rubber, as a biomaterial that comes from a tree (hevea brasiliensis), is a traditional material from renewable sources, since it can be extracted without any harm to the tree. The hevea trees can start producing rubber latex commercially within six years from their planting, which means that the rubber producing cycle is short and a permanent natural rubber production is possible.
In Brazil, the National Institute of Technology (INT) has developed, in cooperation with the Espirito Santo State Rubberplanters Association (HEVEACOOP), with funding from the Brazilian Institute of Environment and Renewable Sources (IBAMA), a new alternative to traditional uses and processes of natural rubber latex.
It consists of laminating natural rubber on fabrics, employing simple technology and yet obtaining a product with good mechanical properties, along with nice finishing. In this technology, there is the suppression of the smoke house, the traditional method used.
The smoked rubber sheet on raw cotton fabric is a common practice among the Amazonian natives. They take the rubber latex from isolated trees in the forest, spread it on fabrics and expose them to smoke to vulcanize the rubber. These rubbery fabrics are used in many ways, mostly as bags to carry fruits, fish, etc. However, the lack of temperature control, the use of field latex and the direct contact of the rubber with smoke from wood ashes result in an invariably dark to light brown product that soon starts to lose properties by the action of moisture, fungi and blooming of serum components.
In this regard, the use of a controlled temperature oven for the rubber laminates vulcanization is a great improvement of the technique, considering that this suppresses the hazardous contact of the operator with the phenolic vapors originating from the dense smoke that comes from the wood ashes, plus adding precision of the vulcanization conditions. Also, overheating is avoided, resulting in light colored vulcanized rubber that can be pigmented in any color.
Natural rubber laminates were first prepared at laboratory scale, from 60% centrifuged latex from Espirito Santo state plantations. This latex was diluted to 45%, a concentration which was believed to be ideal for spreading the latex composition onto cotton materials. The composition prepared for testing is shown in table 1. This preparation was a standard one, consisting of vulcanizing agents and accelerators, a thickener to balance the viscosity, antioxidant agent and pigment.
Table 1 - standard composition for 45% centrifuged natural rubber latex Components Standard formulation 45% centrifuged latex 100 phr Thickener 1 phr Antioxidant (50%) 1 phr Vulcanizing dispersion 6 phr
The main objective of this first trial was to set the vulcanization conditions. For this, natural rubber compositions were cast on glass plates and allowed to dry in an oven at 50 [degrees] C for about 30 minutes.
The dry films were then cut, weighed and tested on a rheometer, at a temperature interval between 80 [degrees] and 130 [degrees] C, for 60 minutes each. The optimum conditions were established as being 120 [degrees] C for 30 minutes. Table 2 lists the range of rheometric conditions considered acceptable for the vulcanization of the composition.
Table 2 - rheometric parameters for vulcanization of latex composition Temperature([degrees] C) ML TS2 TC50 TC90 MH Cure 110 24.4 8:10 10:30 17:40 41.9 4.4 120 24.7 6:10 7:20 15:50 41.9 6.8 130 24.9 4:20 5:30 11:40 41.2 8.5
A semi-efficient vulcanization system (ZMBT/ZDEC) was used. Latex films cured with ZMBT have a markedly higher modulus. ZMBT is used as the primary accelerator, along with a fast curing secondary accelerator, ZDEC. This combination is widely used in the natural rubber goods industry (ref. 4).
After setting the process conditions, natural rubber compositions, prepared according to table 1, were laminated on cotton fabrics fitted in wooden 20 centimeter frames, dried and vulcanized within the temperature range set in the rheometer tests for a 30 minute period. Visual inspection had detected oxidation of the laminate at 130 [degrees] C. Mechanical tests had shown that, at 120 [degrees] C, the vulcanization was effective in 30 minutes and these conditions were adopted for all the laminates prepared.
Those conditions were scaled up for fabrics measuring two square meters. A low cost layout building was specially built for centrifugation of latex, composition and manufacture of the laminates. The scheme of the plant is shown in figure 1.
A medium pressure spray gun was used for spreading of the latex composition onto the cotton fabric, tightly fit on an aluminum frame. More than one frame can be laminated at the same time. After drying at 50 [degrees] C for 30 to 45 minutes, they were allowed to vulcanize at 120 [degrees] C for 30 minutes.
Results and discussion
It was observed that when field latex was used in preliminary tests, the onset of vulcanization started sooner than in the case of centrifuged latex, as shown in figure 2. The end of the crosslinking reaction, which is perceived when the torch remains constant, takes also place in shorter times. However, the rate of vulcanization remains, for both types of lattices, practically the same.
It is well known that the serum components of natural rubber latex are composed mainly by low molecular substances such as fatty acids, proteins, sugars, carbohydrates and also lutoids, which are carotene rich particles that impart the yellowish color to the dry natural robber (refs. 5 and 6). These substances are almost all extracted from latex through the centrifugation process, raising the latex concentration from 30-35% to about 60%.
The accelerating role of serum components is an already known phenomenon and some studies have been conducted with the aim of preparing vulcanizing agents from latex serum components industrially (ref. 7).
The first laminates prepared presented a slightly tacky surface that was suppressed with the use of industrial talc spread onto the dried laminates, prior to vulcanization. Yet, the need of a surface agent that would be able to provide an additional UV protection, along with a smoother touch, was placed.
Polyolefins and paraffin emulsions were then added to the compositions, in amounts according to the total solids content of each emulsion. A 10% solution of polyethylene glycol (PEG) was also employed, with good results.
The use of paraffin waxes on rubber is commonly employed in the rubber industry, for the blooming of the paraffin on rubber parts surfaces protects against oxidation by ultraviolet radiation (ref. 8).
Infrared surface analysis (FTIR-ATR) had shown that the additives, markedly, low density polyethylene (LDPE) emulsions and PEG solution, migrate towards the laminate surface on film drying, improving greatly the washability of the laminates, i.e., the removing of dirt and stains is much more effective than for PVC laminates.
Mechanical tests such as tensile strength, elongation, tear strength, abrasion resistance, blocking resistance and washability were performed for laminates prepared using different surface additives. Table 3 lists these results for each composition.
Table 3 - mechanical properties for laminates with different surface additives Laminate Surface Tensile Elongation Tear number additive used strength (MPa) (%) strength (KN/m) 2 PEG 39.2 650 33 5 Paraffin 33.6 800 47 emulsion 3 PEBD 33.8 600 42 emulsion 4 PEAD 34.1 670 48 emulsion Laminate Abrasion Blocking Washability number resistance resistance (% weight loss) 2 0.097 NB 7-10 (No blocking) 5 0.066 NB 7-10 3 0.078 NB 7-10 4 0.069 NB 7-10
As can be seen, the mechanical properties that are not directly related with surface phenomena do not vary significantly for any of the compositions, except for the one that used knitted cotton/polyester fabric.
Properties such as tension stress and elongation and tear strength are also related to the type of fabric used. For regular threaded cotton fabrics, tension strength is maximum at the fabric rupture, with maximum elongation reached for the rubber sheet alone. In the case when knitted cotton is mixed with synthetic yarn, such as polyester fibers, synergistic behavior occurs and then the maximum stress and elongation are taken for the laminate as a whole. For 100% cotton fabrics, only surface related properties like washability and blocking present more notable differences according to the additive used.
The natural rubber laminates prepared according to controlled conditions developed in this work were found to be very stable and repeatable, regarding mechanical properties. This enables the preparation of light colors, smooth finish rubber accessories, like bags, shoes and coats, creating finished products with higher value, that will bring up the development of the rubber tree producing areas, giving to the communities involved the opportunity of diversifying the market for their products and boosting local economies.
(1.) A.S. Siqueira Filho, D.M.R. Costa, L.M.K. Nakamura and R.C.C.A. Cid, "Desenvolvimento do processo de producao do couro vegetal em escala de campo," 7th Brazilian Congress on Rubber Technology, Sao Paulo, 1997, p. 51.
(2.) A. Vieira, A.S. Siqueira Filho, M.C. Bo and D.M.R Costa, "Desenvolvimento de uma formulacao para confeccao de couro vegetal a partir do latex de borracha natural," 3 [degrees] Congresso Brasileiro de Polimeros, Rio de Janeiro, 1995, p. 78.
(3.) "Way Beyond", issue 1, volume 1, communications from the United Nations Environmental Program's Working Group on Sustainable Product Development, Amsterdam, 1997.
(4.) Coran, A.Y., "Vulcanization" in Mark, H.F., BikaIes, N.M., Overberger, C.G. and Mendes, G., Encyclopedia of Polymer Science and Engineering, John Wiley & Sons, New York, 17, 1988, p. 666.
(5.) A. Subramaniam, "The Vanderbilt Rubber Handbook," R.T. Vanderbilt Company, Inc., 1978, p. 23.
(6.) Cyr, D.R.St., "Natural rubber" in Mark, H.F., Bikales, N.M., Overberger, C.G. and Mendes, G, Encyclopedia of Polymer Science and Engineering, John Wiley & Sons, New York, 14, 1988, p. 687.
(7.) Tanaka, Y., Hioki, Y. and Ichikawa, N., Eur. Pat. Appl. EP 584,597, 02 March 1994.
(8.) Bevilacqua, E.M., "Oxidation and antioxidants in rubber," cap. 18, em W.O. Lundberg, Autoxidation and Antioxidants, Interscience Publishers, New York, 2, 1966, p. 857.
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|Article Type:||Statistical Data Included|
|Date:||Nov 1, 2000|
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