# Mechanical Properties and Formaldehyde Release of Particleboard Made with Lignin-Based Adhesives
## 1 Introduction  
Formaldehyde emission represents a key issue for the wood-based composite industry. Formaldehyde issues are related to urea-formaldehyde resin (UF) as a dominating bonding adhesive used in the production of wood-based panels [1, 2]. Melamine-formaldehyde (MF), melamine-urea-formaldehyde (MUF) and phenol-formaldehyde (PF) adhesives are less dangerous in terms of their subsequent formaldehyde emission, but they have not convinced industrial producers, due to their higher price or lower reactivity [3, 4]. Researchers use various methods to modify the traditional urea-formaldehyde (UF), formulations [5, 6] in order to meet regulations concerning formaldehyde release. Exposure to formaldehyde is possible during the adhesive-mixing, mat-forming and hot-pressing operations. Formaldehyde exposure can be harmful to human health. At low levels, formaldehyde can cause eye, nose and throat irritation, and skin rashes, shortness of breath, wheezing and changes in lung function at higher levels of exposure [7]. The International Agency for Research on Cancer (IARC) has reclassified formaldehyde into Group 1-\"carcinogenic to humans\" [8]. In Europe, the European Chemicals Agency (ECHA) reclassified formaldehyde in category 1B \"presumed human carcinogen\" and \"germ cell mutagen category 2 (acute toxicity)\" [9]. With growing interest in indoor air quality, efforts have been made to reduceexposure limits to formaldehyde both in workplaces and in living spaces. There is legislation in the European Union, USA, China, and Japan limiting the levels of formaldehyde emission (FE) from wood and wood-based products. The limits of formaldehyde emissions set by CARB (California Air Resources Board) are the following: 0.05 ppm for hardwood plywood, 0.09 ppm for particleboard, 0.11 ppm for MDF (medium-density fibreboard) and 0.13 ppm for thin MDF (up to 8 mm thick) [10]. Such regulations established the allowable limits of formaldehyde emissions at a level 10 to 20 times lower than that that existed 30 years ago [2], and, in a few years, the limits could be lowered, forcing producers to adopt alternative methods. Among these, several methods can be mentioned: binderless particleboard manufacturing [11], the use of formaldehyde scavengers [4; 12; 13; 14; 15], the surface treatment of wood composites [16; 17] and the use of bio-adhesives prepared from natural raw materials, such as lignin [18; 19; 20], starch [21; 22], soy or tannin [23]. The studies were especially focused on three biopolymers: lignin, starch and plant proteins, but lignin seems to be the most used one in the experimental research and industrial trials to manufacture particleboards. L Lignin-based adhesives can be classified into two groups: formaldehyde-free lignin-based adhesives and lignin phenol formaldehyde adhesives, when lignin is used as a partial replacement of phenol [24; 25]. For the first group of adhesives, mixtures between kraft lignin and polyethyleneimine [26], lignin with added glyoxal [27], or oxygen-plasma-treated enzymatic hydrolyzed lignin [28] were investigated. A study [29] referred to lignin-based copolymer adhesives for the production of wood based composites and found that up to 50%wt. of the phenol can be replaced by kraft lignin. Other study [30] found that up to 50% of phenol in phenol formaldehyde adhesive could be substituted by bark lignin with improved adhesive properties. Lots of research has been done to improve the reactivity of lignin as the substitute of phenol in phenol-formaldehyde (PF) adhesive synthesis by modifying the chemical structure of lignin to increase its reactivity and reduce the long pressing time and high pressing temperature [31]. The most-used methods to fulfill this objective are [32; 33] methylation (hydroxymethyl), phenolation, demethylation, reduction, hydrolysis and oxidation [34; 35; 36; 31]. The oxidation of lignin has been described as a way to weaken the lignin structure, making it more susceptible to depolymerization [36] and a good procedure to improve the properties of lignin [31].