The building industry faces great challenges, such as the reduction of energy consumption from construction to the demolition of buildings [1
]. Globally, buildings account for about 40% of the total primary energy requirement and contribute to more than 30% of the CO2
]. Thus, construction companies are continually searching for ways to improve the insulation performance of the building envelope [3
]. Insulation materials are defined as construction products and are meant to fulfil the building-physical features of thermal or sound insulation. Building insulation materials are commonly manufactured using materials obtained from petrochemicals or from a natural source processed with high-energy consumption [1
]. Materials such as fiberglass, mineral wool, or polyurethane foams have good thermo-mechanical properties (e.g., low thermal conductivity, good moisture protection, and fire resistance), but can be hazardous to human health and to the environment [4
]. These materials emit high levels of carbon during production and cannot be reused or recycled [3
]. Different types of insulation materials are available, the ones made with polymeric foams and the ones made with fibers. Those made with fibers can be classified into two subgroups: Inorganic (e.g., glass fiber, amorphous fiber (rock wool), ceramic fiber) and organic (e.g., hemp fiber, cotton fiber, wood fiber) fibers [5
]. Recently, the use of natural, sustainable, renewable, and environmentally friendly materials has gained interest because of increasing environmental awareness. The research in the field of insulation materials in the building industry has been focusing increasingly more on the use of natural, local materials that are nontoxic, are recyclable, and can assure good thermal insulation [1
]. Insulation products incorporating natural fibers are available on the market. These products have promising properties in building applications [8
]. Asdrubali et al. [1
] report some building insulation products made of natural unconventional material, which show good thermo-acoustic properties such as reeds, bagasse, cattail, corn cob, cotton, durian, rice, etc.
The raw material for wood fiber insulation boards’ manufacturing is typically the chips and shavings recovered as waste from other wood manufacturing processes. The two processes currently used in the insulation boards manufacturing are a wet process and dry process. On the one hand, the wet process, which is widely used, involves the use of water for mixing the wood fiber pulp, paraffin, with latex as a binder. The mix forms a fiber mat, while the water is removed through pressing and vacuum pumping. The insulation boards are dried in a dryer and sawed. On the other hand, in dry process, the dry fiber is sprayed with paraffin and polyurethane resin as a binder. The fiber mats are placed in a press where the resin is cured through exposure to a mixture of air and water vapor. Finally, the insulation boards are sawed and milled.
In the last few years, researchers analyzed different insulation boards with bio-based adhesive as a binder or binderless [4
]. These researchers investigated the physical and mechanical properties of some insulation boards with low thermal conduction. Crude glycerol is a major by-product of the biodiesel refining process derived from vegetable oils [13
]. World production of biofuels and, hence, of crude glycerol has been rapidly increasing in recent decades, making imperative the development of sustainable processes for the use of this by-product [15
]. Heating a blend of glycerol and citric acid can form a three-dimensional polymeric structure, where only water is produced and could be easily removed by evaporation during the reaction [17
]. Previous studies have shown that esterification of the citric acid and glycerol mixture yield polymer networks of varying complexity, depending upon the reactant ratio and esterification parameters used [17
]. The temperature and polymerization time are the most important parameters in the curing process [14
]. In previous works, different temperatures and curing times, such as 100 °C, 140 °C, 160 °C, and 180 °C during 60 or 120 min, were used to polymerize crude glycerol with citric acid. The results showed the presence of ester bonds and cross-linking levels, according to the catalyst and citric acid content used and related to the curing time and temperature [14
The aim of the present study was to develop a wood fiber insulation board from a dry process using a crude glycerol and citric acid mixture as bio-based adhesive. The study evaluated the effect of reactant ratio and adhesive content on thermo-mechanical properties of insulation board such as thermal conductivity, traverse strength, tensile strength parallel and perpendicular to surface, and compressive strength. Finally, the study carried out a characterization of the bio-based adhesive and wood fiber insulation boards.