A poluição da emissão de formaldeído no ambiente

ON THE PERFORMANCE OF UF RESINS AS ADHESIVES FOR PLYWOOD

Wei Gao1, Jianzhang Li2

ABSTRACT

Uron resin, a kind of urea-formaldehyde (UF) resin containing much more uron structure, were prepared. Several modifi ed UF resins were obtained from mixing uron resins with normal UF resins as well as synthesizing UF resin with uron resin as raw material. Th is work demonstrated that the introducing of the uron structure reduced free formaldehyde content of UF resin and formaldehyde emission levels in bonded plywood panels signifi cantly, the former were reduced by 76% and 84% in the latter. Th e synthesis time of uron resin can be shortened to 8 hours. Th e test result of free formaldehyde content, formaldehyde emission levels and bond strength indicated that specimen of 20 parts uron resin synthesized in 8 hours mixed with 100 parts UF resins (F/U molar ratio of 1.3) had a potential advantage to industrial application. Th e benefi cial eff ect of uron resin on the performance of UF resin can be attributed to the opening of uron cycle structure and the following reaction with free formaldehyde and its oligomeric glycol forms. Keywords: Uron structure; UF resin; plywood; formaldehyde emission; 13C NMR.

INTRODUCTION

Th e contamination from formaldehyde emission on the environment limits the wide application of urea-formaldehyde (UF) resin. Th us, considerable eff orts have been paid on reducing formaldehyde emission level from UF bonded wood composites, such as mending the prescription and procedure of UF resin synthesis, optimizing the application craft of adhesives, as well as the use of formaldehyde scavengers with adhesives (Abdullah and Park 2010) . Uron initially aroused interest due to its lower formaldehyde emission eff ect on UF resin which was synthesized at very acid pH (Gu et al. 1995). Th e following research confi rmed that the introduction at lower reaction temperatures of the additional strongly acid condensation step was the formation of sometime considerable quantities of Uron (Gu et al. 1995). Th us, the critical parameters for maximizing the portion of Uron present at the end of the UF resin synthesis was proposed, and the particle board bonded with uron-rich resins showed the considerable bond strength (Soulard et al. 1999). However, the preparation of UF resins containing high proportion of urons is not feasible in practice simply because the quantity of free formaldehyde left at the end of the reaction is too high and would lead to a very high formaldehyde emission from panels. Futher, little is known about the mechanisms of the reduction eff ect of uron structure on reducing free formaldehyde content and formaldehyde emission levels.

Th e approach of this work is attempted to simplify the preparation process of uron resin and develop it as a type of additive to UF resin. Furthermore, the special objective is to reduce formaldehyde content of UF resin and formaldehyde emission levels of it bonded plywood via introducing uron resins, as well as to interpret the reaction mechanism of the former result.

1Faculty of Material Engineering, Southwest Forestry University, Kunming, Yunnan Province, P. R. China, 650224. 2Faculty of Material Science and Technology, Beijing Forestry University, Haidian, Beijing, P. R. China, 100083. Correspondence Author: [e-mail] Received: 22.06.2011 Accepted: 18.09.2011

maderas.indd 3 06-06-12 11:38

Universidad del Bío -Bío

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EXPERIMENT

Resin preparation and testing

Urea formaldehyde resins Th ree diff erent UF resins were prepared according to the following typical resin procedure: 955.9g (11.79mol) of a 37% formaldehyde solution (AR) was charged under continuous mechanical stirring in a glass reactor equipped with a refl ux condenser, thermometer and pH electrode and temperature raised to 45ºC. Th e pH of the formalin was adjusted to 7.5-8.0 with 30% NaOH water solution. Th e fi rst urea of 353.7g (5.90mol) was added to the reactor and the temperature was slowly raised to 90±2ºC within 40∼60min and maintained at 90±2ºC for 60min. Aft er that, the pH was adjusted to 4.4 with 20% formic