UNCORRECTED PROOF 1 2 Springback in acetylated wood based composites 3 Behbood Mohebby a, * , Maryam Gorbani-Kokandeh b , Mojtaba Soltani c 4 a Department of Wood and Paper Sciences, Faculty of Natural Resources, Tarbiat Modares University, P.O. Box 46414-356, Noor, Iran 5 b Department of Wood and Paper Sciences, Faculty of Natural Resources, Mazandaran University, Sari, Iran 6 c Department of Wood and Paper Sciences, Azad University, Chaloos, Iran Q1 7 9 article info 10 Article history: 11 Received 12 November 2008 12 Received in revised form 7 February 2009 13 Accepted 8 February 2009 14 Available online xxxx 15 Keywords: 16 Acetylation 17 Spring back 18 Particleboards 19 Medium density fibreboard 20 21 abstract 22 Reasons of the strength loss of the acetylated wood based composites were still under question. This 23 research was considered to study springback in the acetylated particleboard and the medium density 24 fibreboard. Chips and fibres were acetylated by using the acetic anhydride to gain different percentages 25 of the weights (WPG). The boards were made based on the target thicknesses. Thickness of the boards as 26 well as the modulus of elasticity (MOE) and the modulus of rupture (MOR) were determined after con- 27 ditioning based on three point static bending test. 28 Results showed that the moisture content and the thickness swelling of the boards were reduced as the 29 WPG was increased. However, increase of the springback and reduction of the MOE and the MOR were 30 revealed as the WPG increased in the test boards after condition. It was also revealed that the springback 31 was correlated with the weight gain and increased as the WPG was raised in the test boards. Reduction of 32 the MOE and the MOR was also correlated with the springback in the test boards. 33 Ó 2009 Published by Elsevier Ltd. 34 35 36 1. Introduction 37 During the last decades, demands in the market obliged wood 38 technologists to introduce new composites with enhanced proper- 39 ties, such as particleboard, oriented strand board, plywood, and 40 hardboards. Those products are currently known as interested 41 engineering materials for building and construction. Even though, 42 they were fascinating and reached higher technological properties 43 in comparison with solid wood, however such products had similar 44 problems as wood. For example, those products are potentially at- 45 tacked by the biological agents, they become dimensionally insta- 46 ble when they are exposed to the changing humidities, influenced 47 by the UV-light and burnt when exposed to the fire. Scientists are 48 in challenge to find solutions for the mentioned problems. Acetyla- 49 tion is one of the interesting processes which have been recently 50 industrialized to modify the wood and the wood based composite 51 to enhance properties of the products for application in the severe 52 conditions and also remove the noticed problems. The acetylation 53 is a reaction between the acetic anhydride with the wood poly- 54 mers, lignin, cellulose and also hemicelluloses. 55 Advantages of the acetylation have been reported by many 56 authors. The wood and the wood based composites become mois- 57 ture repellent due to the acetylation reaction [1,2]. The reaction is 58 accompanied by substitution of the hydrophilic hydroxyl groups 59 with the hydrophobic acetyl groups in the wood cell wall polymers 60 and causes increase in the wood bioresistance [3–8]; resistance 61 against the weathering [9] and decrease in the board roughness 62 [9], reduction in the moisture absorption [5,10–13]. 63 In spite of those advantages, there are different reports indicat- 64 ing that the mechanical strengths are diminished in the acetylated 65 wood based composites [10,11,2,14,15]. There are several opinions 66 to explain reasons of the strength loss; e.g. type of the adhesive 67 [16], less press pressure, etc. It has been proposed to apply higher 68 press pressure to increase the strengths [10,11,2]. However, no re- 69 port has paid detailed attention to springback in the acetylated 70 composites and its correlation with the strength losses. 71 It is known that the springback in the wood based composites 72 occurs usually after manufacture of the boards. The springback is 73 an irreversible thickness swelling which occurs after wetting of 74 the composites and is attributed to the release of applied stresses 75 accompanied by some loss of the glue bonds [17,18]. The spring- 76 back indicates debonding of the adhesion between the wood ele- 77 ments and the adhesives. Result of the debonding in the boards 78 is loss of the strengths [19], which is occurred in the specimens 79 subjected to different exposures because of a lowering of the board 80 density [19]. The mechanical properties that were presumably 81 most directly affected by the springback are the shear strength, 82 the modulus of elasticity, and the modulus of rupture, because 83 these properties depend somewhat on the strength that is devel- 84 oped by the mechanical interlocking of the compressed particles 85 in the composites [20]. 0950-0618/$ - see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.conbuildmat.2009.02.007 * Corresponding author. Tel.: +98 122 6253101/9111255972; fax: +98 122 6253499. E-mail addresses: mohebbyb@modares.ac.ir (B. Mohebby), ghorbani_mary@ yahoo.com (M. Gorbani-Kokandeh), mo_slty@yahoo.com (M. Soltani). Construction and Building Materials xxx (2009) xxx–xxx Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat JCBM 1686 No. of Pages 4, Model 5G 25 February 2009 Disk Used ARTICLE IN PRESS Please cite this article in press as: Mohebby B et al. Springback in acetylated wood based composites. Constr Build Mater (2009), doi:10.1016/j.conbuildmat.2009.02.007