Study on incorporating wattle tannin in polyvinyl acetate emulsion and its effect on properties for wood bonding applica
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Study on incorporating wattle tannin in polyvinyl acetate emulsion and its effect on properties for wood bonding application Rajesh Prabhu1 · Ramanand Jagtap1 · Mohanlal Digar2 Received: 7 April 2020 / Accepted: 15 September 2020 © Springer Nature Switzerland AG 2020
Abstract Polyvinyl acetate emulsion (PVAc) is an essential class of adhesives for woodworking applications. However, it suffers from low water resistance. In the present research, wattle tannin was incorporated in synthesised PVAc emulsion at ratios ranging from 0 to 6 wt.% in PVAc emulsion. The study was carried out in two parts. The first part focuses on understanding the effect of the presence of tannin in PVAc emulsion on its adhesive performance. The second part deals with the study of the variation in adhesive properties of tannin PVAc emulsion adhesive by introducing isocyanate crosslinker in the system. DMA was employed to understand the glass transition and rheology behaviour. FTIR was used to assess the reaction of tannin with isocyanate crosslinker. Contact angle measurement was measured using goniometry to understand the surface wettability achieved by incorporating tannin in the PVAc emulsion system. Detail study on adhesive strength was carried out by bonding wood substrates followed by measuring bonding strength in wet conditions as per BS EN 204 and heat resistance as per DIN EN 14257 (WATT 91). The presence of tannin in the system enhances water and heat resistance property which gets further improved in the presence of isocyanate crosslinkers. The observed effect is due to better wetting of substrate by adhesive, hydrogen bonding between tannin and components in adhesive, and dense crosslinking formed by the reaction of isocyanate crosslinker. Keywords Tannin · Polyvinyl acetate · Adhesive · Water resistance · Crosslinking · Emulsion · Wood adhesive · Heat resistance
1 Introduction Tannin is a natural water-soluble polyphenolic compound ranging from the molecular mass of 300–3000 Da [1] found abundant in plants [2, 3]. Tannin is present in superior plants such as mimosa, chestnut and oak to protect lingocellulosic material against biological and radiative degradation due to their chemistry and antioxidant property [4–6]. Tannin is commercially essential due to its usefulness in pharmaceutical, food and nutrients-based applications. [7]. Mimosa, quebracho and pine tannins find use in commercial purposes due to their availability. The industrial grade of tannin available contains a significant portion as
tannin, while sugar and hydrocolloid gum form the nontannin part [2, 8]. The hydroxyl group of tannin reacts with the isocyanate group [9] and has different reactivity. The viscosity change of the two-component adhesive system with time shows the reaction rate and molecular growth of the polymeric chain. Wattle tannin (resorcinol), when reacted with paraformaldehyde, has a gel time of 950 (s) as compared to pine tannin extract (phloroglucinol) which has a gel time of 65 (s) [10] which makes wattle tannin a preferred choice for adh
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