Finishing Properties of Poly Urethane Coating on Bleached and Ammonia Fumigated Mango Wood Surface

Aims: The impact of bleaching chemicals on the gloss and film thicknesses of poly urethane (PU) coated surface of mango wood (Mangifera indica) with ammonia fumigation and an exposure towards sunlight was investigated

Study Design: Thirty six wood samples of size 10.6 cm (length) C 7.62 cm (width) were prepared from 2.5 cm thick kiln dried mango wood (Mangifera indica). Specimens were divided into six groups, each group having 6 samples for the study.

Place and Duration of Study: The experiment was carried out in year 2016 at the Laboratory of Forest Product Division, Forest Research Institute, Dehradun, Uttarakhand, India.

Methodology: All the samples were sanded with 80 grit size sand paper and coated with poly urethane. Two chemicals were used for bleaching purpose like an aqueous solution of oxalic acid (H2C2O4) was prepared with 30% concentration (by mixing 42.9 g of the solute in 100 ml of hot water) and a commercially available Hydrogen peroxide (H2O2) was used as such (30%). For the purpose of bleaching, these chemicals were applied on to the sanded surfaces using brush at room temperature. The bleached sample surfaces were allowed to dry before proceeding to the next step. The PU (Poly Urethane) coated samples were treated with ammonia fumigation before and after bleached with the two chemicals. Ammonia fumigation was carried out in a fumigation chamber for 24 hours for all the samples (except T1).

Results: On exposure to natural sunlight for 60 hours, all the samples showed reduction in film thickness in PU coating but in a wide range of 2.8 % to 11.4%. The highest loss of coating thickness was shown by samples which were neither fumigated nor bleached. Among the fumigated samples, those bleached with oxalic acid resulted in high film thickness loss. Samples bleached with hydrogen peroxide after ammonia fumigation resulted in least film thickness reduction. Both ammonia fumigation and bleaching in general brought down the original gloss of poly urethane coating. The gloss value reduced as expected on exposure to sunlight. Bleaching by hydrogen peroxide prior to ammonia fumigation resulted in highest loss in gloss

Conclusion: The un-fumigated and unbleached surface resulted in minimum gloss reduction due to long exposure to sunlight.