Synthesis, Characterization and Mechanical Evaluation of the Phenol-Formaldehyde Composites

Phenol: formaldehyde ratio was varied in the synthesis of phenolformaldehyde resin and used to prepare the composites. These composites were then evaluated for their mechanical strength on the basis of tensile strength, compressive strength and wear resistance. Composite with better strength was characterized by IR, SEM, XRD, TGA/DTA and further studies were carried out for its physico-chemical and mechanical properties like viscosity, modulus of rupture (MOR), modulus of elasticity (MOE) and stress at the limit of proportionality (SP) etc.


Introduction
Phenoplasts or the phenolic (PF) resins are produced by the condensation of phenol and formaldehyde. These were the first synthetic thermosets. They have a wide range of applications as lacquers and varnishes, molding compounds and laminates for wall panels and table tops etc. About 41% of the PF resins are used in plywood manufacture and about 14% in insulation 1 Composites are the wonder materials, which after meeting the challenges of aerospace have cascaded down for catering to the domestic and industrial avenues. In the recent times many successful efforts have been made to prepare phenol-formaldehyde composites and to increase their mechanical potential by using different reinforcements like wood particle, hemp scotch, 2 banana fiber 3 , cellulose fiber 4 etc. Kaith et al [5][6][7] reported the mechanical studies of the polymer composites reinforced with flax-g-copolymers.
The versatility of the phenol: formaldehyde resin based composites inspired to prepare its composites by altering the phenol: formaldehyde ratio and screening them on the basis of tensile strength, compressive strength and wear resistance. The best results thus obtained would then be further evaluated for their physico-chemical, mechnical & thermal behavior.
Wear resistance testing of the composites was carried out on the Wear & Friction Monitor (DUCOM-TR-2OL), Compressive/Tensile strength, MOR, MOE and SP of the samples were tested on the Universal Testing Machine (HOUNSFIELD, H25KS). Curing and degasification were done on Compression Molding Machine (Santech India Ltd.).

Preparation of the composites
The resin was degassed and poured into molds. The molds were coated inside with oleic acid to avoid adhesion of the mixture and for the ease in removal of the composites. The whole assembly was then placed inside the hot press and cured at 100 0 C and a pressure of 100 Kg/cm 2 for 6-7 hours was applied. Mean of three specimen samples were used as a result to determine the mechanical properties. The mechanical tests were conducted in ambient conditions.
Wear test, modulus of rupture, modulus of elasticity and stress at limit of proportionality wear resistance was calculated as per ASTM D 3702 and MOR, MOE and SP were tested on the basis of ASTM D 790 as per the method reported earlier 8 .

Reaction mechanism
In phenolic resin the major reactions involved are the addition to give methylol phenols, condensation of a methylol phenol and a phenol to give a methylene bridge, condensation of two methylol phenol and a phenol to give a methylene bridge, condensation to two methylol groups to give an ether bridges and decomposition of the ether bridges to methylene bridge and formaldehyde 9 . The mechanism used for the formation of the resole type PF resin involves the protonation of the carbonyl group of the formaldehyde followed by the electrophilic aromatic substitution at the ortho and para positions of the phenol. Phenol reacts with excess formaldehyde under basic conditions to form mono, di and tri methylolphenols. Phenol is present as resonance stabilized anion and substitution is exclusively at ortho, para. On heating the methylolphenols condense to give resole, which on further heating under basic or neutral or slightly acidic condition gives resite 9,1 .

Mechanical screening of the P-F composite Wear resistance
As is evident from the Figure 1, that the phenol-formaldehyde resin formed in the ratio of 1: 0.75 of formaldehyde: phenol, had better wear resistance of 0.09, 0.15 and 0.2 x 10 -4 g/m at all loads of 1, 2 and 3 Kg, respectively. On the other hand, formaldehyde: phenol in 1: 0.5 ratio showed 0.21, 0.22 and 0.25x 10 -4 g/m; while 1:1 ratio showed 0.3, 0.32, 0.35 x 10 -4 g/m and 1: 1.25 ratio had 0.42, 0.44 and 0.45 x 10 -4 g/m wear resistance at 1, 2 and 3 Kg, respectively. The weight loss in these composites mainly takes place due to abrasion and frictional heat generated due to sliding.

Infra red analysis
The IR spectrophotometer shows significant peaks at 3422 cm -1 due to -OH, 2920 cm -1 of CH 2 stretch, C=C stretch at 1609.9 & 1508.6 cm -1 ,1475.5 cm -1 due to CH 2 bend, C=C bend at 1374.9 cm -1 and 1235 cm -1 due to C-O stretch.

X-ray powder analysis
The X-ray powder diffractometer studies depict the d-values, continuous scan were taken between 3 to 40 at 2θ scale at 3 sec scan speed and increment rate of 0.05. Copper was the source of X-ray, Ni filter and scintillation counter mechanism. PMMA holder with zero background was used. Corundum was used to calibrate the system. The small crystal form of sample was subjected to analysis

Scanning electron microscopy
Since the composite had non conducting behavior so, they it was gold plated to have an impact. The Figure 4 clearly shows the well packed smooth surface of the composite.

Physico-thermal, mechanical evaluation
The formaldehyde: phenol in 1: 0.75 after being characterized were subjected to study their physical, thermal and mechanical behavior.

Viscosity and the pH
The viscosity of the resin was tested at different temperatures as it varies with the change in temperature as shown in Table 1. The pH of the resin was found to be 8.5.

Thermal study
The each sample of weight 6.0 mg was taken into the silica crucible for TGA / DTA analysis in air with temperature ranging from 50 to 1000 o C. The heating rate of the sample was 10 o C per min and scanned for 60mins. Air was taken as the reference for zero and the results, Delta-M (mg) vs. temperature ( o C) in case of TGA and DTA signal (µV) vs. temperature ( o C) in case of DTA, were obtained. The degradation occurs in various forms as deacetylation, dehydration, decarboxylation and chain scissions etc. The TGA /DTA results as shown in Table 2 depicts the high initial and final decomposition temperature i.e. IDT and FDT which signifies the higher thermal stability. The major weight loss is between 500 to 551°C. TGA results have been supported by the DTA results. In DTA, there are exothermic peaks, the first transition hump at 527°C revealed the dehydration, adsorption and oxidation from the semi-crystalline host and the sharp major peak at 551°C signifies the irreversible dissociation of the crystallites whereas the small peak at 560°C may be due to fusion.

Mechanical studies
The MOR, MOE and SP have been shown in the Table 3 which clearly show the increase in the mechanical strength 8.40. When the P-F composite was tested for its hardness it was found to be brittle.

Conclusion
The phenol-formaldehyde resin has immense applications in the development of latest technologies and one of the best uses is to prepare the composites. These composites not only have better physico-chemical, mechanical and thermal properties for domestic and industrial application avenues but can further be improved by reinforcing them with natural fibers to incorporate high strength and avail better use of waste biomass