Present article demonstrates a viable method of modification of mango wood (MW) through reactive reinforcement of polyacrylonitrile (PAN). Reactive reinforcement of PAN was conducted through sewlling of MW planks (moisture content: 12.5%) into methanolic solution of acrylonitrile (AN, 20-60%,v/v) supplemented with 2,2-azobisisobutyronitrile (1.0% w/v) at 30 ± 10C over 48h followed by curing of planks at 80 ± 10C over subsequent 3h.This has afforded a series of wood polymer composites (WPCs) with PAN loading (%) in the range of 5.5 to 15.5. Formation of WPCs was ascertained through scanning electron microscopy. With loading of PAN, WPCs has shown enhanced mechanical durability with improved resistance against organic , hydrolytic media and a decay fungus Coriolous Versicolor. The proposed method of reactive reinforcement of PAN offers a viable way of modification of MW making this suitable for development of durable furniture and building components. Modification of wood through reinforcing polymer materials has received growing attention since decades. This has been in attempt to develop the viable substitute for plastics and steel components in construction applications. The developed products were popularized as wood plastic composites (WPCs) by North Americans. Leo H. Arthur Baekeland was supposed to pioneer in development of WPCs through blending wood flour with a phenolic resin around early 20th century. In subsequent years, American Wood Stocks from North America had accelerated the production and commercialization of WPCs panels for automotive applications. This has made a tremendous industrial growth in wood plastic composites from North America to Europe and Asia as a low-maintenance, high-durability product. Such panels were derived through implication of Italian technology of extrusion wherein wood floor was extruded with equal weight fraction of polypropylene. Around 1960s, Mayer, in interest to enhance the compatibility of wood flour with polymer component , has introduced coupling agents during processing of moulds of WPCs [1-2].
Presently WPCs are developed through either of a non reactive or reactive process. A non reactive process is based on extrusion of wood flour with thermoplastic or thermosetting polymers. Non reactive process delivers highly finished panels of WPCs for immediate applications [3-4]. A reactive process involves infusion of monomers along with co monomers, coupling agents, initiators into properly shaped wood panels swollen in organic media followed by thermal or radiation polymerization [5]. Reactive method delivers WPCs with improved mechanical, thermal dimensional stability and microbial resistance. Many of low grade wood varieties viz; Akamatsu pinus and Cryptomeria japonica, Birch, Poplar-alder, Eucalyptus, Cedar, spruce and beech has been modified using vinyl monomers with acrylate, methacrylate functionalities and styrene [5-8].In this context, a few efforts on development of WPCs using AN has been documented [9-12].Development of wide variety of WPCs from different wood varieties has made difficult to discuss their relative performance and stability. The properties and applications of WPCs are best defined on the basis of their method of processing, product design, service environment, physical and biological durability of wood substrates, reinforced polymer and their mutual compatibilities [13-14].
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Socio-economic benefits associated with cultivation of MW for production of nutritious fruit and use of hardwood as construction materials [15-18] and pharmaceutical applications [19] has been realized by many ancient civilizations. Vegetating mango trees suffer from invasion of microorganisms [20], that on harvesting delivers deteriorated wood. Harvested MW exposed under humid environments suffers from dimensional instability, breaking or cracking [21], wherein the wood texture is protected through application of coating finish, that often discolors the texture and native color of MW. [22-23]. For such reasons, MW has been the subject of modification through reinforcing polymer materials. A combined documentation of existing reports interestingly reveals that AN as a monomer has been used for modification of limited number of wood varieties [9-12], no reports are available on modification of MW 2,2-azobisisobutyronitrile (AIBN) assisted through reactive reinforcement of PAN. In the present work, a simplified and straight forward method of synthesis of WPCs has been developed through reactive reinforcement of PAN into MW (Mangifera Indica, family Anacardiaceae). Wood and respective WPCs were collectively defined as specimens and were investigated for their mechanical, thermo-oxidative stability, fungal degradation and solubility behavior. AN and AIBN were procured from Ms Sigma Aldrich. AN and purified through multiple number of extractions with aqueous sodium hydroxide solution (10%), followed by washing with distilled water. Fraction of AN collected at 780C was used for development of WPCs. Rest of the chemicals and solvents involved in the study were locally purchased with purity > 98% and were used without further purifications. Planks of MW were fabricated into dimensions as per guidelines of S 1708 66, WPCs were prepared through modifications in the early protocol of wood treatment. [9, 11-12]. A representative protocol demonstrates steps of soxlet extraction of properly finished MW planks in required dimensions (moisture content:12.5%) with toluene: ethanol mixture (2:1,v/v) over 10h , followed by thermal aging at 95±10C and subsequent leaching with water over additional 10 h. The conditioned MW planks were then swollen in methanolic solution of acrylonitrile (20-60%,v/v) supplemented with 2,2-azobisisobutyronitrile (1.0% w/v) at 30 ± 10C 48h. The treated MW planks were subjected to heat curing at 80 ± 10C over subsequent 3h to derive the WPCs. PAN loading (%) into MW was calculated on the basis of dry weights of unextracted MW and corresponding treated specimens [9]. Electron micrographs of gold coated specimen were scanned at 150x magnifications over LEO-435. Compression and static bending strength of specimen were evaluated over ENKAY-UT-40 Universal testing machine with capacity of 40 tons and least count of 80 kg. Impact testing was carried out over indigenous swinging pendulum machine. Moisture content (ASTM D1037 72a 79), solubility of WPCs in hot water, NaOH (%), organic media (ASTM D 1109 56 72) lignin content (ASTM D 1106-56) were investigated.
Microbial degradations were experimented through incubation of specimens under exposure of decay fungus Coriolous Versicolor with reference to streptomycin at 25 0C over 10 days [6-7]. AN, PAN, AIBN, flours of MW and WPC (60 mesh size) and solvents as control. Mineral salt medium (MSM) was prepared through mixing (g/L) of Na2HPO4.2H2O(7.8),KH2PO4, (6.8), MgSO4 (0.2), NH4 Fe (CH3COO) 3, (0.01), Ca (NO 3) 2. 4H 2O (0.05) at PH 7± 0.1. Flours were separately mixed with MSM thereafter inoculated in presence of fungus at 300 C. Culture filtrate were removed at selected intervals over 24, hours, centrifuged @7500 rpm for 50 min. The supernatant of culture was examined for protein reducing sugars content and enzyme activities. A mixture of Na2CO3 (2.0%), NaOH (0.1N, 50 mL), CuSO4 (0.5g), Sodium potassium tartarate (1.0%, 5 mL) and Folin Ciaclateau reagent (0.50 mL) was incubated at 250C for 30 min. (λ max 660 nm). Standard curve of Bovine Serum albumin (0.5 mL, 20-200 mg/L) was prepared. Standard solutions were also treated as the sample and calibration curve was then plotted. Productions in protein and reducing sugar contents were estimated (mg/mL) with reference to incubation time. CMCase was assayed using CMC (2%) in sodium citrate buffer (PH 4.8, 0.05M). Pre-incubated enzyme (0.5mL) was added to substrate (0.5mL), and incubated at 500 C at 30 min. Xylanase assay was determined by monitoring the release of reducing sugar from oat spelt Xylem. The reaction mixture (0.9 mL enzyme +0.1mL birch Xylem) was incubated at 500C for 10 min. The reducing sugars liberated were spectrally characterized (λ max 540 nm) and estimated .A reference standard of xylose, which gave linear curve at 10 min, was used as positive control. FPase activity was determined through mixing of enzyme (0.5mL) with 1.0 mL citrate buffer (PH 4.8, 0.05M) and Whatman filter paper (N0.1, 1x5 cm =50±0.025 mg) followed by incubation at 500C for 60 min [6,10]. A method of reactive reinforcement of PAN into MW was developed through free radical polymerization of various concentrations of AN in methanol medium. This has afforded a series of WPCs with quantitative loading (wt%) of PAN in the range of 05.50 to 15.50 (Table 1).In order to have further insight into dispersion and compatibility of PAN. MW and a representative WPC with 15.5 % loading were imaged through SEM (Fig 1). SEM image reveals characteristic granular morphology with knots and voids into MW (Fig 1a). Such knots and voids were filled with PAN into MW in a non uniform manner (Fig 1b). PAN reinforcement has reduced the moisture content of MW from 12.50 to 04.45 [Fig.2].Such reduction in moisture content may protect the MW from dimensional instability, breaking or cracking under humid environments and microbial decay of MW [20].This may also circumvent the additional efforts on protection of MW against decay by humid environment through implication of coating finish [22-23]. The complexity and compositional non uniformity makes difficult to dissolve the MW in solvent media. The overall solubility of MW is combination of the ability of individual components of wood that comes into contact with the specified solvent or their mixtures with other solvents. Wood dissolves at the cost of solubility of its any one of the component into the solvent. Organic solvents and water dissolves some of the wood components, however the complex cellular structure makes wood to be highly resistant against many of solvents at ambient temperatures [19].
Fig.3 demonstrates the characteristic solubility behavior of WPCs in various solvent media. Solubility of WPCs was found in reducing order from alcohol benzene media, to hot water and sodium hydroxide (1%) [Fig.3]. With PAN loading, solubility of WPCs in MeOH/benzene mixture was reduced ranging 84.80 to 81.25.However, their solubility reduction in ether was found marginal by 1% in the range of 93.80 to 92.80. Decrease in solubility of WPCs in organic media attributes to protection in leaching of waxes, fats, resins and oil contents from MW. Low solubility of WPCs in NaOH ranging 21.11 to 20.35 attributes to reductions in the proportion of alkali soluble component of wood that impart decrease in the pulp yield [7]. With PAN loading, water solubility of MW was decreased from 37.50 to 33.50. Such reduction in water solubility with simultaneous reduction in moisture content attributes to importance of PAN reinforcement towards improving the performance of MW under out door conditions [21-23]. Mechanical properties, specially the dimensional stabilities of different wood varieties are enhanced with ratio of lignin to cellulose. Mechanical properties, especially dimensional stabilities of various grades woods are best improved through bulking their voids via method of polymer reinforcement [24]. Table 2 demonstrates the effect of quantitative loading of PAN on modification in mechanical properties of WPCs. In general, PAN reinforcement has enhanced the impact strength (Nm) of MW ranging 8.85 to 19.96.This was followed by increase in static bending and compressive strengths (X107, N/m2) ranging 08.08 to 10.67 and 3.23 to 5.00.Mechanical data reveals that reinforcement of PAN was much productive towards gross (%) enhancement in compressive strength of MW (54.79), followed by overall impact strength (34.46) and static bending strength (32.05).Enhancements in mechanical stability of MW attributes to bulking of the voides of MW through reinforcement of PAN [Fig 4]. Fungal degradation was investigated through exposing a representative WPC (15.50 wt% PAN loading) under Coriolous Versicolor. Growth of fungus was noticed over WPC within 10 days of incubation, whereas no fungal growth was observed on WPC under identical experimental conditions. Screening of components involved in the synthesis of WPC against the fungus at 250 ppm reveals 67% inhibition in the growth of fungus by AN. Under identical conditions, benzene and AIBN has shown inhibition in the growth of fungus by 38% and 20% inhibitions respectively. PAN had shown excellent growth in media at £ 15 ppm. Production of protein content was started at 12hr of incubation and reached to a maximum level after 96 hr in the MW and representative WPC. Total amount of protein was increased from 0.14mg/mL at 12 hr to 0.31 mg/mL at 96 hr in the MW. Loading of PAN into MW has decreased protein content (mg/mL) to 0.09 within 12hr. Reducing sugar content (mg/mL) was increased from 0.17 at 12hr to 0.49 at 168 hr. WPC showed decrease in reducing sugar content by 57.12% up to 24 hr. Such increase in the production of protein and reducing sugar by MW and WPCs indicate the availability and accessibility of substrate for the fungus Coriolous Versicolor. Decrease in production of protein after 96 hr of incubation attributes to utilization of most of the substrate by Coriolous Versicolor. Such increase in microbial biomass would add to the total protein availability and degradation of substrate by Coriolous Versicolor in MSM [Fig.5a]. Production of cellulolytic and xylanolytic enzymes was maximized after 168 hr in presence of MW and WPC . Production of Xylanase was found higher over ellulytic enzyme in MW than WPC. In presence of MW, the Xylanase activity was initiated after 12 hr of incubation (0.22 IU/mL) and reached to maximum at 168hr of incubation (0.63 IU/mL). Maximum reduction in the Xylanase activity to 56.5% was assayed for WPC. In presence of MW, FPase activity was started after 12hr of incubation (0.14 IU/mL) and reached to a maximum at 168 hr of incubation (0.45 IU/mL). Maximum reduction in the FPase activity was assayed as 32% for CMCase activity started after 12 hr of incubation (0.15 IU/mL), and it reached to a maximum at 168 hr (0.46 IU/mL) in MW [Fig.5b].
Control over production of protein and reducing sugar contents along with enzyme activities attributes to reduction in moisture content of WPC due to loading of PAN into the MW (Fig 2).Such reduction in moisture content has blocked the blocked transport of moisture which impart resistance to WPCs against degradation by Coriolous Versicolor. [6-8]. Reactive reinforcement of polyacrylonitrile (PAN) into mango wood (MW) has developed wood polymer composites (WPCs) with PAN loading (wt %) of 05.50 to 15.50. Scanning electron microscopy in combination with moisture and lignin content data reveals the the presence of PAN into the matrix of MW. With PAN loading, the mechanical properties of WPCs were increased with simultaneous reduction in their solubility behavior in organic solvents, hot water and aqueous NaOH were WPCs. Effect of PAN was more pronounced towards modification in compressive strength of MW. A representative WPC bearing 15.5 wt % loading of PAN has rendered enhanced resistance against Coriolous versicolor .The presence of PAN has controlled over the release of Protein , reducing sugar contents and enzyme activities in WPCs due to block in the transport of moisture across the cell lumens of MW ,resisting the decay of WPCs in presence of decay fungus. Research grant of Govt. of India. Defense Research Development Organization Letter No EPIR/ER/0003266/M/01/13-9-2001is hereby acknowledged .There is no conflict of interest among authors from different academic Institutions.
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