Mechanical Properties of Phenolic Composites Reinforced with Flax-g-copolymers Prepared under Different Reaction Conditions-A Comparative Study

Graft copolymers of methyl methacrylate (MMA) and its binary mixtures such as MMA + ethyl acrylate (EA), MMA + acrylonitrile (AN), MMA + acrylic acid (AA), MMA + vinyl acetate (VA), MMA + acrylamide (AAm) and MMA + styrene (Sty) with flax fiber have been prepared in air (IA) and under the influence of microwave radiations (MWR). Synthesized flax-g-copolymers under two different reaction conditions were used as reinforcing material in the preparation of phenol-formaldehyde composites. Mechanical properties such as wear resistance, modulus of rupture (MOR), modulus of elasticity (MOE) and stress at the limit of proportionality (SP) were measured and a comparative studied has been made. It has been found that composites reinforced with flax-g-copolymers-MWR showed better mechanical properties in comparison to composites reinforced with flax-gcopolymers-IA


Introduction
Natural fibers reinforced composites have received much attention due to their distinct advantages such as renewable source, biodegradability, high filling effect and nonabrasiveness.Because of tremendous changes in the quality of natural fibers, they are fast emerging as a reinforcing material in composites.Phenolic resin bonded Flax-particle boards were manufactured for construction industry and structure, chemical composition and mechanical resistance of Flax shives and Flax boards were compared with corresponding properties of wood particleboards 1 .Bentsianova et al 2 have described the manufacturing process for wood particle board from wood alone, wood and hemp scotch or scotch alone.Kolosvary 3 increased the water resistance of wood particle and hemp waste boards by preliminary thermal treatment of the particles.Particle boards made from pretreated materials showed lower water absorption and lower thickness swelling after soaking in water or after exposure to super-saturated water vapour for ten days than particle boards made from untreated materials.Water-proof building materials of increased strength and rigidity were manufactured at low costs from vermiculite, polyester resin binders, natural fiber waste, synthetic fibers, acrylic resins, paper, paperboard and wood fibers 4 .Work on reinforcement of composite soil with barley straw has been reported by Bouhicha et al 5 .Importance of Indigenous natural fiber in their applications in the preparation of composites over synthetic fibers has been reported by Paramasivam and Kalam 6 .Le et al. 7 developed the novel short silk reinforced polybutylene succinate bio-composites by compression molding method and observed that there was enhanced improvement in tensile and flexural properties.Joseph et al. 8 fabricated composites reinforced with banana fibers and glass fibers with varying fiber length and fiber loading.The analysis of tensile, flexural and impact properties of these composites revealed that the optimum length of the type required for banana fiber and glass fiber is different in phenol-formaldehyde resole material.Interlocking between the banana fiber and phenol formaldehyde resins was much higher than that between glass and phenol formaldehyde resin.
Wetting behaviour of Flax fiber and possible replacement of polypropylene fiber with Flax for the preparation of composites as reinforcement was studied by Askargorta et al. 9 .Escamilla et al. 10 observed that grafting of polymethyl methacrylate or polybutylacrylate on the cellulosic fiber results in lower mechanical properties than those of the ungrafted cellulosic fibers for their applications in composites as reinforcing agent.Kaith et al. [11][12][13] and Kalia et al. [14][15] reported the mechanical studies of polymer composites reinforced with flax-g-copolymers.
Since the use of flax-g-copolymers as reinforcement for polymers is meagerly reported, therefore, in this paper reinforcement of phenol-formaldehyde matrix with flax-gcopolymers prepared under different reaction conditions have been reported and a comparative study has been made.

Experimental
Flax fiber (Linum usitatissimum) was obtained from the Department of Agronomy, CSK HP Agriculture University, Palampur (India).Phenol (Sd fine-chem ltd., India) and formaldehyde (CDH, India) were used as received.Monomers were washed with 5% sodium hydroxide followed by water and were dried over anhydrous sodium sulphate.The dried monomers were distilled and the middle fraction was used.Composites were prepared in Compression Molding Machine (SANTEC India Ltd.).Libror AEG-220 (Shimadzu make) electronic balance was used for weighing purpose.
Wear-resistance testing of the composites was carried-out on Wear & Friction Monitor (DUCOM, 20LE).MOR, MOE and SP of samples were tested on Universal Testing Machine (HOUNSFIELD, H25KS).

Graft co-polymerization of MMA and its binary monomer mixtures onto flax
Grafting of MMA and its binary monomer mixtures were carried out as per the methods reported earlier [11][12]14 . The ercentage grafting (P g ) was calculated as follows: W 2 -W 1 P g = W 1 x 100 Where W 1 and W 2 are the weights of original flax fibers and grafted flax fibers, respectively.

Preparation of resin
Phenol-formaldehyde resin was prepared as per the method reported earlier 11 .

Preparation of composites
Phenol-formaldehyde resin was thoroughly mixed with fibers (10:0.5 w/w).After thorough mixing of the resin and fibers, the mixture was poured in molds.The surfaces of molds were coated on the inside with oleic acid to avoid adhesion of the mixture and to allow easy removal of the composites.The whole assembly was then placed inside the hot press and cured at 90 o C and a pressure of 10 Kg/cm 2 for 5 hours.Composites thus prepared were subjected for the evaluation of different mechanical properties.The numbers of specimen used for the determination of mechanical properties were three and the tests were conducted at ambient laboratory conditions.

Wear test
Wear resistance of composites was carried-out as per ASTM D 3702 on DUCOM make machine.Wear resistance was conducted against hardened steel disc having hardness of 60 HRC and roughness Ra: 0.5 µm.Counter surface was polished using emery paper and cleaned with acetone before each sliding test.Samples (3 cm) for wear test were held against rotating counter surface at different speeds (100-600 RPM) and normal loads (1-4 Kg).Each test was conducted for 5 minutes of sliding.Loss of weight was used as a measure of wear.Where P = peak load, L = length of sample, b = width of the sample, d = thickness of the sample, P 1 = load at the limit of proportionality and y = rate of bending.

Stress at the limit of proportionality
Stress at the limit of proportionality was calculated by using the following equation: Where P 1 = load at the limit of proportionality, L = length of sample, b = width of the sample and d = thickness of the sample.

Results and Discussion
C 2 , C 3 and C 6 hydroxyls and the C-H groups are the active cites for grafting in cellulosic fibers.The grafting onto flax fibers in presence of FAS-H 2 O 2 takes place as per the mechanism proposed by Misra et al 16 .
It is quite evident from above results that reinforcement of graft copolymers prepared under the influence of MWR resulted in higher MOR.Whereas, the reinforcement with graft copolymers prepared in air resulted in decreased MOR.Maximum values of MOR was found to be 108 N/mm 2 for PF composites reinforced with flax-g-poly(MMA/AA)-MWR.
It has been observed that PF composites reinforced with graft copolymers prepared under MWR showed maximum values for SP followed by reinforcement with graft copolymers prepared in air.Maximum and minimum values for SP were 99.29 and 11.70 N/mm 2 .Phenolic composites reinforced with graft copolymers prepared under the influence of micro-wave radiations showed better mechanical properties due to the reason that optimum reaction time for grafting was very low and thus the fiber underwent a fewer disturbances in its crystal structure.Moreover, fiber faces less surface deformations during grafting process under the influence of microwaves as compared to grafting in air, thereby retaining better mechanical properties with better fiber-matrix interaction.

Conclusion
Microwave enhanced Graft copolymerization is an effective method for modifying the properties of natural fibers in terms of time consumption and cost effectiveness.Flax-gcopolymers-MWR showed better mechanical properties in comparison to flax-gcopolymers-IA and flax fibers reinforcement.
MOR and MOE were determined according to ASTM D 790 in a universal testing machine and were calculated by using the following equations:

Fig. 1
Fig. 1 Effect of Flax Fiber and Graft Co-polymers (IA) Reinforcement onWear Rate of the Phenol-Formaldehyde Composites

Fig. 2
Fig. 2 Effect of Flax Fiber and Graft Co-polymers (MWR) Reinforcement onWear Rate of the Phenol-Formaldehyde Composites

Table 1 .
Effect of concentrations of different binary monomer mixtures on percent grafting under different reaction conditions

Table 2 .
MOR, MOE and SP values of PF and composites reinforced with graft copolymers prepared in air

Table 3 .
MOR, MOE and SP values of PF and composites reinforced with graft copolymers prepared under the influence of microwave radiations.