Natural fiber composites durability

W. Woods. Efficacy of a unique fungicide in wood filled plastic. In Conference Proceedings of Durability in Wood Plastic Natural Fiber Composites 2006, Intertech, Portland, ME, San Antonio, TX, December 4-5, 2006. 

Synthetic polymers have become extremely important as materials over the past 50 years and have replaced other materials because they possess high strength-to-weight ratios, easy processabiUty, and other desirable features. Used in appHcations previously dominated by metals, ceramics, and natural fibers, polymers make up much of the sales in the automotive, durables, and clothing markets. In these appHcations, polymers possess desired attributes, often at a much lower cost than the materials they replace. The emphasis in research has shifted from developing new synthetic macromolecules toward preparation of cost-effective multicomponent systems (ie, copolymers, polymer blends, and composites) rather than preparation of new and frequendy more expensive homopolymers. These multicomponent systems can be "tuned" to achieve the desired properties (within limits, of course) much easier than through the total synthesis of new macromolecules. 

D.J. Gardner and N.M. Stark. Understanding the durability of wood-plastic composites. In The Global Outlook for Natural Fiber and Wood Composites, Intertech, Portland, ME, New Orleans, LA, December 8-10, 2004. 

Kumar et al. [83] studied the weathering properties of ethylene-propylene copolymer (EPC) matrix composites with three different reinforcement materials, namely, 3% NaOH-treated jute fibers, 17.5% NaOH-treated jute fibers and commercial microcrystalline cellulose powder, using maleated EPC as a compatibilizer. The samples were subjected to UV radiation at 60°C in air for 150 hours. Again, the neat polymer samples were more resistant to weathering than the composites. The samples reinforced with commercial microcrystalline cellulose were the most stable of the composites and those made with fibers treated with the lower concentration of NaOH were the most susceptible to photo-oxidation. It was concluded that optimizing the durability and mechanical properties of the natural fiber-reinforced composites was closely dependent on selecting the appropriate treatment for the fibers. 

A.P. Kumar, D. Depan and R.J. Singh, Durability of natural fiber-reinforced composites of ethylene-propylene copolymer under accelerated weathering and composting conditions. 7. Jhermoplast. Compos. Mater. 18, 489-508 (2005). 

Gutierrez et al. [50] investigated the effect of replacing part of Portland cement by pozzolans as silica fume, fly ash and metakaolin on the mechanical properties of cement mortar reinforced with synthetic and natural fibers. The mechanical behavior and durability of composite materials was improved by replacing 15 wt% of cement with metakaolin or silica fume however, because of its low pozzolanic power, fly ash had no effect on the durability of these materials. Khorami and Ganjian [62] increased flexural strength in 20% of cementitious materials reinforced with straw fibers and eucalyptus, by replacing 5 wt% of Portland cement with silica fume. 

The alkaline pH of cement matrix can also be reduced by reaction between carbon dioxide (CO ) and Ca(OH. This reaction is known as carbonation of cementitious matrix and occurs in three stages (i) Initially CO diffuses in matrix through pores and is dissolved (ii) subsequently CO reacts with sodium hydroxide (NaOH) of cement matrix, decreasing the pH which favors Ca(OH formation and (iii) finally a reaction occurs between COj and Ca(OH that forms Ca(HCQ ) and CaCO. Several authors have used this reaction to decrease pH of cement matrix and increase durability of cement composites reinforced with natural fibers [31, 37, 69]. To allow this reaction occurs, composite is cured in an environment rich in COj, which favors the formation of CaCQ and therefore decreases the pH in the matrix [32, 70-73]. However, cement... 

Toledo Filho et al. [32] conducted a comprehensive study to determine specific modifications of cement matrix and natural fibers that were effective increasing durability. Results indicated that carbonation of the specimens for 109 days at conditions of 26.5 C is a promising alternative for increasing the durabihty of cement composites reinforced with cellulosic fibers. 

Modifications of Natural Fibers to Increase Durability of Cement Composites... 

Table 19.2 Effect of natural fiber treatments on properties and durability of Portland cement composites. (Tmeans that property increased, >lmeans that property decreased). 

Toledo Filho et al. [32] studied the effect of a series of treatments on natural fiber and cement matrix in order to increase durability of cement mortars reinforced with sisal fibers. Based on results from their investigation, it was concluded that incorporation of sisal fibers significantly reduced mechanical properties of composite material. However, the mechanical properties of composite were maintained over time if the sisal fibers were immersed in a suspension of silica fume. This is because the silica fume particles contact the fiber and their size allows them to be located in smaller defects in the fiber, preventing that compounds such as calcium hydroxide, come into contact with the fiber and destroy it by growth of crystals. 

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