Engineering the Properties of Eco -Friendly Medium Density Fibreboards Bonded with Lignosulfonate

• Free formaldehyde emissions from wood-based panels, especially in indoor applications, pose serious risks to human health at certain concentrations. Prolonged exposure to formaldehyde can cause adverse health effects including eye, nose and throat irritation, other respiratory symptoms and cancer. As a consequence, new formaldehyde emission limits for composite wood products were established in Europe, USA and Japan. This, together with the stricter environmental legislation are the main driving factors for shifting the scienti ﬁ c and industrial interest from the traditional formaldehyde-based synthetic resins to the new bio-based adhesives for production of eco-friendly wood-based panels. The lignin-based products are one of the most prospective ecological alternatives to the traditional formaldehyde resins. The main interest in lignin is due to its phenolic structure with several favourable properties for the formulation of wood adhesives such as high hydrophobicity and low polydispersity.The present article is aimed at studying the possibilities for using lignosulfonate as an adhesive for the production of eco-friendly MDF. Regression models describing the impact of lignosulfonate concentration and hot pressing temperature on the exploitation properties of MDF panels were developed. The individual and combined impact of both factors was analysed in order to determine the optimal exploitation properties of the panels.

Wood fi bres were mixed with the lignosulfonate and paraffi n for 50 s in a high-speed glue blender (850 min -1 ). The hot pressing was performed on a laboratory press type РМС SТ 100, Italy. The applied pressing regime of MDF was as follows: in the fi rst stage, the pressure was increased for 20 s to 3.0 MPa and maintained for 20 % of the whole pressing cycle, then the pressure was evenly decreased for 10 s to 1.2 MPa and maintained for a duration of 30 % of the whole pressing cycle; the third (last) pressing period was carried out at the pressure of 0.6 MPa for 50 % of the whole cycle. The pressing factor was 90 s/mm 1 . The physical and mechanical properties of the obtained MDF panels were determined in accordance with the requirements of the respective EN standards (EN 310; EN 317; EN 319; EN 322 and EN 323). The mechanical properties of MDF panels were determined on a universal testing machine Zwick/Roell Z010.

UVOD
The production of medium density fi breboards (MDF) is one of wood-based industries with the fastest growth rate. Production in the European Panel Federation member countries grew by 2 % in 2016 to 12 million m 3  Analytical study on engineering the properties of eco-friendly MDF panels produced in laboratory conditions using lignosulfonates as adhesives is presented in this article. Two main factors were studied -the concentration of adding lignosulfonate in the wood-fi bre mass and hot pressing temperature.

MATERIJALI I METODE
In order to design the properties of eco-friendly MDF, panels were produced at three different lignosulfonate concentrations -20 %, 30 % and 40 %, and three hot pressing temperatures -200 ºC, 210 ºC and .... Savov, Antov: Engineering the Properties of Eco-Friendly Medium Density Fibreboards... On this basis the following regression equation in a coded form was derived: (1) where is the predicted value of the respective MDF output parameter, X 1 is the lignosulfonate concentration in a coded form and X 2 is the hot pressing temperature in a coded form.
A standard methodology was used to determine the adequacy of the model by calculating the Fisher criterion and comparing it with the table values. Variance relation was calculated, where is the residual variance and is the variance of random disturbance determined on the basis of the variance under the null experiment (at 30 % lignosulfonate concentration and 210 ºC hot pressing temperature). The testing was carried out on 8 test samples for each parameter and variational and statistical processing of the results was made to test the adequacy of the model, encoded by the following formula: (2) where is the natural meaning of the factor, is the variation interval and is the variation interval median.
The accuracy of the applied model was determined by the multiple correlation coeffi cient.

REZULTATI I RASPRAVA
The summarised results of the exploitation properties of the laboratory produced MDF panels at different lignosulfonate concentrations and hot pressing temperature are presented in Table 2.
Data about the tests in the null experiment are presented in Table 3.
All derived equations correspond to the critical value F (0.005; 3; 7) = 4.35, and the values for the respective models are: for the water absorption -2.37; for the swelling in thickness -3.17; for the bending strength -3.85; for the modulus of elasticity in bending -3.02; and for the internal bond strength -4.21.
The coded form of the regression equation for the water absorption of MDF panels is: where X 1 is the lignosulfonate concentration and X 2 is the hot pressing temperature in coded form. The equation is characterised by the coeffi cient of determination R 2 = 0.96. From Eq. 3, it can be concluded that the effect of lignosulfonate concentration was signifi cantly stronger in the studied range. The impact of the factor at the variation range from 20 to 40 % was about 2.6 greater than the effect of hot pressing temperature within the  Figure 1. It can be concluded that an improvement in water resistance (decreased water absorption) was determined when lignosulfonate concentration was increased from 20 % to 40 % and hot pressing temperature was increased from 200 ºC to 220 ºC, respectively. The water absorption values were signifi cantly improved when the lignosulfonate concentration was increased from 20 % to 35 % and the hot pressing temperature -from 200 ºC to 210 ºC.
The coded form of the regression equation for the swelling in thickness of the laboratory produced MDF panels is: , % where X 1 is the lignosulfonate concentration and X 2 is the hot pressing temperature in coded form. The equation is characterised by a coeffi cient of determination R 2 = 0.95. Decreased swelling in thickness of MDF panels was determined with increasing the concentration of lignosulfonate solution and hot pressing temperature. Again, lignosulfonate concentration was the more signifi cant factor in the studied variation range. However, the effects of both studied factors were comparable. Strong quadratic dependence was determined between the swelling in thickness of MDF panels and lignosulfonate concentration.
The variation of swelling in thickness of MDF panels depending on lignosulfonate concentration and hot pressing temperature is presented in Figure 2.
The analysis of the obtained results of this property is similar to the water absorption of the panels. Signifi cantly greater relative improvement was determined with increasing the lignosulfonate concentration up to 35 % and less with increasing the concentration from 35 % to 40 %. Regarding the hot pressing temperature, the relatively greater improvement was determined with increasing the temperature up to 210 ºC.
The coded form of the regression equation for the bending strength is as follows: , N·mm -2 (4) where X 1 is the lignosulfonate concentration and X 2 is the hot pressing temperature in coded form. The coeffi cient of determination of Eq. 4 is R 2 = 0.90.
The effect of lignosulfonate concentration on this property was signifi cantly greater than the effect of hot pressing temperature. The quadratic dependence between the lignosulfonate concentration and bending strength of the panels was strongly pronounced. A strong impact of the interaction of both studied factors on bending strength values was determined in comparison with the water absorption and swelling in thickness of the laboratory produced MDF panels. In other words, processes of destruction of wood components occur at the relatively higher hot pressing temperature and lignosulfonate concentration and low moisture content of the pressed material, which results in increased panel brittleness.
The graphical representation of the dependence of bending strength on lignosulfonate concentration and hot pressing temperature after encoding Eq. 4 is given in Figure 3. Improvement of bending strength values was determined with increasing the lignosulfonate concentration from 20 % to 30 % and increasing the hot pressing temperature from 200 ºC to 220 ºC. Further increase of hot pressing temperature and lignosulfonate concentration above 30 % resulted in deterioration of the studied property. This deterioration was even more strongly pronounced at concentrations above 35 %.
To achieve the EN 622-5 standard requirement of 27 N/mm 2 for MDF panels with general application in humid environment, the lignosulfonate concentration should be at least 25 %.
where X 1 is the lignosulfonate concentration and X 2 is the hot pressing temperature in coded form. The coeffi cient of determination of equation (5) is R 2 = 0.97. Similar to the bending strength, the modulus of elasticity in bending was more dependent on lignosulfonate concentration compared to the hot pressing temperature, i.e. the concentration factor is more appropriate for engineering the respective MDF property. The interaction of the studied factors resulted in decreased values of the modulus of elasticity in bending. The graphic representation of the dependence of the modulus of elasticity in bending on the lignosulfonate concentration and hot pressing temperature, after encoding Eq. 5, is presented in Figure 4. Improvement of the modulus of elasticity in bending was determined with increasing the hot pressing temperature and lignosulfonate concentration up to 35 %. Further increase of temperature and concentration resulted in decreased values of the studied prop-erty. This can also be explained by the occurring destruction of some wood components under the impact of the increased lignosulfonate concentration, hot pressing temperature and pressing duration.
The regression equation of the internal bond strength of the panels is as follows: , N·mm -2 (6) where X 1 is the lignosulfonate concentration and X 2 is the hot pressing temperature in coded form. The coeffi cient of determination of Eq. 5 is R 2 = 0.98. Separately, the studied factors had positive impact on the internal bond strength of MDF panels. Once again, the negative effect of their interaction was reported. The graphic representation of the dependence of the internal bond strength on the lignosulfonate concentration and hot pressing temperature, after encoding Eq. 6, is presented in Figure 5.
Improvement of internal bond strength values was determined at lignosulfonate concentration of up to 35 % and increase of the hot pressing temperature up to 220 ºC. Further increase of hot pressing temperature resulted in decreased values of the studied property.

ZAKLJUČAK
The use of lignosulfonate as an adhesive is a perspective approach for producing eco-friendly MDF panels without harmful free-formaldehyde emissions. The exploitation properties of MDF panels can be successfully engineered by varying the lignosulfonate concentration from 20 % to 40 % and hot pressing temperature from 200 ºC to 220 ºC. The concentration of adding lignosulfonate solution to wood-fi bre mass has a stronger impact on the panel properties within the determined variation range in comparison with the hot pressing temperature. This can be explained by the relatively high (more than 200 ºC) hot pressing temperature and increased press time. For this reason, a deterioration of the strength properties of the panels is  T e m p e r a t u r e T , °C t e m p e r a t u r a T , °C observed with increasing the hot pressing temperature at lignosulfonate concentrations above 35 %. Decreasing the concentration below 30 % is not justifi ed since this does not lead to additional utilisation of the lignosulfonate hydroxyl groups but requires increasing the hot pressing temperatures.
Despite the signifi cant positive results achieved in the production of lignosulfonate-based MDF panels, there are some considerable drawbacks that should be noted, e.g. the diffi culties in activating lignosulfonates and their binding to wood fi bres, as well as the required substantial increase of pressing temperature and extended pressing time. The latter outlines the future trends of extensive research in the fi eld of MDF production on the basis of lignosulfonate adhesives.