Thermal Behavior of Insulation Fiberboards Made from MDF and Paper Wastes

• Today, recycling is becoming increasingly important. In recycling process, the product performance should also be considered. In this study, manufacturing insulation ﬁ berboard, as a practical wood product from recycled ﬁ bers, was investigated. For this purpose, two types of waste (MDF wastes and waste paper) were recycled to ﬁ bers and used for producing insulation ﬁ berboards. The target ﬁ berboard density was 0.3 g/cm 3 . The ratio of waste paper to MDF waste recycled ﬁ bers (WP/RF) was considered at two levels of 70/30 and 50/50. Poly vinyl acetate adhesive was used as a variable in the board manufacturing process. The mechanical properties, dimensional stability, thermal conductivity, and ﬁ re resistance of the boards were evaluated. Besides, the thermal stability of ﬁ berboards was studied using thermal analysis including thermogravimetric analysis (TGA) and dif-ferential thermal analysis (DTA). The results showed that the insulation ﬁ berboards had admissible mechanical properties and dimensional stability. The manufactured boards displayed low thermal conductivity, which proved to be well competitive with other insulation materials. The ﬁ berboards manufactured with PVAc adhesive and WP/ RF ratio of 50/50 had higher ﬁ re resistance compared to other treatments. Additionally, results of thermal analysis showed that the use of PVAc adhesive and WP/RF ratio of 50/50 leads to improved thermal stability. Overall, the recycled ﬁ bers from MDF and paper wastes appear to be appropriate raw materials for manufacturing thermal insulation panels, and use of PVAc adhesive can signi ﬁ cantly improve thermal and practical properties of insulation ﬁ berboards.

fi re -resistance of insulation materials is a necessary factor. TGA-DTA and fi re retardant tests can be useful techniques to evaluate the performance of insulation materials. TGA measures the mass losses of materials under the infl uence of elevated temperatures, so it can partially simulate the thermal degradation occurring in fi re events (Ramiah, 1970). In this study, the fi berboards were manufactured by a mixture of recycled fi bers from MDF and paper wastes to achieve better formation of natural bonds. Waste paper, due to its large number of hydroxyl groups, allows natural bonds to be formed, thus contributing to better formation of fi berboard.
The aim of this study is to investigate the fi berboard produced from recycled fi bers without chemicals, along with protecting the natural environment and sustainable use of resources.

MDF wastes recycling process 2.1. Proces recikliranja MDF otpada
For this study, MDF wastes and original fi bers were obtained from Pars Neopan MDF factory in Iran (Nashtarood). This factory uses a mixture of hardwoods as raw material. These wastes always remain after size cutting of produced MDF boards.
Poly vinyl acetate resin was supplied by Shomal Chemical Industry, a manufacturer of resins and adhesives. The properties of PVAc are shown in Table 1. A laboratory chipper crushed the wastes of MDF, and then the chips were treated with hot water at 100 °C for 30 min. After that, the wet chips and waste paper were transferred to the pulper, and the fi bers were mechanically released.
Ključne riječi: izolacijske ploče vlaknatice; recikliranje; toplinska stabilnost; MDF otpad; otpadni papir 1 INTRODUCTION 1. UVOD MDF (medium-density fi berboards) is known as one of the most essential raw materials for the construction and furniture industry (Pan et al., 2018). Due to the growing consumption of MDF, a huge amount of waste has been generated. Despite the well-known problems of toxic gas emissions to human health and environmental problems, in some countries, such as Iran, waste MDF is still burned, instead of being recycled. Due to the decline in the quality of MDF boards made from recycled fi bers, manufacturers are reluctant to recycle this waste. The change in the chemical composition and the urea-formaldehyde resin residues on the surface of the recycled wood fi bers leads to an increase in fi ber pH. (Lykidis and Grigoriou, 2008;Medved and Resnik, 2004). Considering the incompatibility of the recycled fi bers with urea-formaldehyde resin, one of the possible solutions could be to use the recycled fi bers to produce binderless boards. Therefore, insulation fi berboard production is an additional alternative to add value to these materials. Insulation fi berboard is a free-formaldehyde, green material made of natural fi bers with a low density between 0.1 to 0.35 g/cm 3 . One of the most critical applications of insulation fi berboard could be its use as thermal insulator in buildings (Epinoza-Herrera and Cloutier, 2009). Due to climate change trends and the importance of reduced energy consumption, it is necessary to develop materials with high thermal insulation ability to sustain proper temperature in interior environments (Sable et al., 2015;Torres Rivas et al., 2018). The materials used as the building thermal insulation must have high specifi c heat capacity, low density, and low thermal conductivity (Pavelek and Adamova, 2019). Plastic foam and mineral wools are usually used as thermal insulation in building construction. Regarding the petroleum basis of plastic foam and health risks of mineral wools, it is necessary to fi nd a safer and more environmentally friendly alternative for insulation purposes (Kawasaki and Kawai, 2006). Over the past years, raising awareness of environmental issues and harmful effect of chemicals on human health and other organisms have led to the development of green materials (Pavelek and Adamova, 2019 Using the test apparatus, as shown in Figure 2, a load of about 10 mm/min at a mean deforming rate on the surface of test piece was applied. The maximum load (P) shall be measured, and the bending strength shall be obtained from Eq. 2: The test piece shall be adhered to a steel block. The tensile load is applied vertically to the surface test equipment, which let water draining, by gravity and low pressure of hand molding. At the end of this step, the thickness of the board reached 30 to 40 mm. The board was pressed at 3.5 bar pressure to a fi nal thickness of 12 mm in a cold press. Finally, the wet board was dried for 24 h at 100 °C, then cooled, weighed and measured, and its density was calculated (the density of insulation fi berboards must be around 0.3 g/cm 3 ). The manufactured boards were conditioned at 20 °C and 65 % humidity for two weeks before trimming to prepare test samples. The MDF wastes recycling and production process stages are shown in Figure 1.

Morphology of fi bers 2.3. Morfologija vlakana
Franklin method was used for the separation of wood fi bers to study the morphology of fi bers. Specimens of virgin and recycled fi bers were macerated in a mixture (1:1) of 30 % hydrogen peroxide and acetic acid in a 64 °C oven for 24 hours. After maceration, the samples were washed with distilled water. For measuring fi ber dimension, the microscopic images were captured by Motic microscope. After that, the fi ber dimensions were measured in Image j software. The dimension of 50 fi bers was measured for each kind of fi bers.  First the thickness of the center part of the test piece shall be measured to the nearest 0.05 mm by a dial gauge or a micrometer, next the test piece shall be The effect of the recycled fi ber proportion and the adhesive utilization on the properties of insulation fi berboards was assessed by one-way ANOVA statistical analysis. Also, Duncan's Multiple Range Test (DMRT) was used to check for signifi cant differences between treatment groups.

Morfologija vlakana
The appearance of virgin and recycled fi bers is shown in Figure 3. It is obvious that recycled fi bers have been damaged as compared to virgin fi bers. Fiber degradation occurs due to the pressure and high temperature applied to the fi bers during the process of making the initial boards, gluing step, loss of wood polymers, heating and mechanical recycling operations (Lykidis and Grigoriou, 2008).
The measured values of length and diameter of the fi bers are presented in Table 2. The results showed that the length and especially the diameter of recycled fi bers are less than those of virgin fi bers due to the destruction of fi bers in the initial board manufacturing and recycling process. The length of the fi bers plays an important role in the physical and mechanical properties and its reduction leads to a decrease in the quality of the manufactured boards (Dix et al., 2001a).

Physical and mechanical properties of insulation fi berboards 3.2. Fizička i mehanička svojstva izolacijskih ploča vlaknatica
Statistical analysis of physical and mechanical properties is summarized in Table 3. of the test piece, the maximum load (P) at the time of the fracture of the adhesion part shall be measured, and the internal bond shall be calculated by Eq. 3: Where, P -maximum load at time of fracture of adhesion part (N) b -width of sample (mm) L -length of sample (mm) 2.5 Thermal conductivity 2.5. Toplinska vodljivost Thermal conductivity coeffi cient (λ) of the insulation fi berboards was measured across the thickness of the panel using a heat fl ux meter custom-made apparatus.
The full size of the specimens was 500 mm × 500 mm but heat fl ow was measured in the 120 mm × 120 mm midrange of the sample. The temperature difference between the hot and cold plate was set to 10 °C. For each panel type, the thermal conductivity test was carried out on three specimens.

Fire resistance 2.6. Vatrootpornost
Fire resistance was assessed by the ignitability of products subjected to direct impingement of fl ame. The test procedure was performed according to STN EN ISO 11925-2:2010.
The test sample with a size of 170 mm × 150 mm was exposed to a small fl ame with a height of 30 mm for a specifi ed time. The burner must be inclined by an angle of 45° to the vertical axis.
The fl ame is applied to the sample for 60 s. The measured properties consisted of mass reduction, infl ammation time, fi re endurance and burnt area.
10 mg of each sample was placed on a balance located in the furnace, and heat was applied over the temperature range of room temperature to 600 °C. The analysis was performed under a nitrogen atmosphere fl owing at 20 mL/min and a scanning rate of 20 °C/min.  The analysis showed that the effect of different ratios of fi bers and adhesive utilization on both physical and mechanical properties of insulation fi berboards was signifi cant at a 99 % confi dence interval (Sig <0.01). Duncan's multiple range classifi cation of the effect of different treatments is shown in Table 4.
As shown in Table 4, instead of thickness swelling of the insulation fi berboards, which is better than values defi ned in the standard, the other measured properties are lower than those specifi ed in the standard, but they are relatively acceptable regarding their low density and manufacturing condition (without synthetic resin and hot pressing). The application of the manufactured fi berboards is restricted; they are intended to be used as insulator, and they cannot be used in applications where high strength is required. The use of PVAc adhesive resulted in the improvement of the physical and mechanical properties of insulation fi berboards. PVAc can improve the adhesion of fi bers (Sable et al., 2015). PVAc, as an insulator, provides not only mechanical strength but also prevents the exposure of fi bers to the atmosphere and leads to high stability in the environment (Hosseini and Entezami, 2005). In terms of thickness swelling and internal bonding, WP/RF ratio of 50/50 is suggested. However, considering MOR and MOE, WP/RF ratio of 70/30 leads to better results due to more fl exibility of waste paper as compared to fi bers recycled from MDF wastes, which is the result of a lower amount of lignin and decreased stiffness of waste paper. As the results show, an increased amount of waste paper leads to a decrease in the internal bonding of boards because of heterogeneous forming of boards, which results in a lack of proper bonding between the different components of two types of fi ber (Rassam, 2008). Paper fi bers also have a higher specifi c surface area than that of wood fi bers because of their small size and high slenderness ratio, which results in better absorption of the adhesive and prevents uniform spreading between the fi bers. Because of this phenomenon, the internal adhesion of boards with a higher proportion of waste paper is reduced (Ghahri, 2017). Each type of fi ber has its own unique characteristics due to their different preparation methods. Considering MDF fi bers, hemicelluloses and lignin of the intercellular material have been partially degraded, also due to the mechanical method of fi ber separation, fractures occurred in the fi bers and the dissolved lignin resulted in fusion and reconnection of the fi bers in hot press (Ghahri, 2017). However, in the case of paper fi bers, chemical and semi-chemical pulping have a profound effect on cellulose fi bers and cell walls. These include the removal of large amounts of hemicelluloses and lignin from the reticular cell walls and their conversion into larger pores reticular system that creates space and locality for water absorption. The partial removal of lignin and hemicellulose polymers results in the availability of large numbers of hydroxyl groups in the cell wall of the fi bers, which leads to excellent hydrogen bonding of the fi bers and subsequently dimensional stability of the produced fi berboards. Waste paper fi bers and recycled fi bers from MDF wastes have different morphology. The waste paper fi bers lose much lignin, while the recovered fi bers from MDF wastes have more lignin, which makes them stiffer as compared to paper fi bers (Hwang et al., 2005). When the combination of paper fi bers and recycled fi bers from MDF wastes are used for fi berboard manufacturing, inter-fi ber bonds are formed better.In addition, the paper fi bers have a bearded state due to refi ning operations, which makes them more likely to have hydrogen bonds (Rassam, 2008).

Thermal conductivity of insulation fi berboards 3.3. Toplinska vodljivost izolacijskih ploča vlaknatica
Thermal conductivity is an indicator of the value of a material as a heat insulator (Xu et al., 2004). Wood based panels manufacturing methods, wood based materials and wood particle sizes were indicated as having a large effect on thermal conductivity (Tsalagakas et al., 2019). As it can be observed in Figure 4, the insulation fi berboards made from PVAc adhesive and the WP/RF ratio of 50/50 is a better insulator compared to other treatments. The use of PVAc decreased thermal conductivity.
Utilization of waste paper and fi bers recycled from MDF wastes as a component appears to be an appropriate choice for manufacturing insulation fi berboards. Paper has been employed as insulation material due to its low thermal conductivity value. The thermal conductivity of paper varies upon the paper sheet density, fi ller content, nature of fi bers, etc. (Tsalagakas et   al., 2019). The measured thermal conductivity ( of the obtained boards was within the interval of 0.025-0.037 W/m·K, which is in the range of building bio-based insulation materials (Theasy et al., 2017). Due to the different structure of the two types of fi bers (obtained from waste paper and MDF wastes) and process of manufacturing, the structure of the insulation fi berboards is very porous. Low thermal conductivity of the insulation fi berboards is due to the low conductivity of the air trapped in the pores. The large number of spaces and voids inside the insulators impeded heat transfer, resulting in considerably lower thermal conductivity (Sihabut and Laemsak, 2010). The thermal conductivity of air is lower than that of solid materials (Nguyen  et al., 2018). For better comparison, the measured values of for different kinds of insulators, according to the results reported by other researchers, are presented in Table 5.

Fire resistance of insulation fi berboards 3.4. Vatrootpornost izolacijskih ploča vlaknatica
According to Table 6, samples lose their weight about 3 % to 15 % of the weight of the initial value after 60 s of fl ame exposure.
The results of fl ame resistance values of insulation fi berboards showed that the lowest mass reduction was related to fi berboards manufactured with PVAc adhesive and WP/RF ratio of 50/50 (lower amount of paper), which can confi rm the positive effect of the use of adhesive on the thermal stability of insulation fi berboards. It is obvious that the use of PVAc can increase the fi re resistance of insulation boards. PVAc is degraded by elimination of acetic acid, yielding a char that provides a transitory phase as the fi ller particles fuse into a ceramic mass (Al-hassany et al., 2010). Insulation fi berboards manufactured without adhesive and WP/RF ratio of 50/50 (N-50-50) had the lowest ther- mal stability as compared to other treatments due to low internal bonding and weakness in bonding formation. Reducing internal bonding between fi bers and the fuzzy surface of boards caused more surface burns and lead to an increased mass reduction of the boards, which is an important criterion for assessing fi re resistance of boars (Hojati et al., 2018). Figures 5 and 6 show the TGA-DTG and DTA curves of insulation fi berboards, respectively.

Thermal stability of insulation fi berboards 3.5. Toplinska stabilnost izolacijskih ploča vlaknatica
Thermal degradation mechanism of wood occurred in three regions, including 60-100 °C (evaporation of water and extractives), 130-350 °C (decomposition of major wood chemical constituents) and after 350 °C (complete decomposition of wood) (Feng et al., 2012;Bodirlau et al., 2009;Yunchu et al., 2000). Above 200 °C, the pyrolysis speed increases, hemicelluloses and cellulose decompose into gases (Yunchu et al., 2000). In a temperature range between (250 to 300 °C, lignin and cellulose are degraded, and tar, gas and char are produced during the pyrolysis (Hakiki-Uner et al., 2016). After heating to 350 °C, since the amount of fl ammable gas is very small, the fl ame burn transforms into the fl ameless charcoal burn. In this step, weight loss slows down (Yunchu et al., 2000). After 430 °C, an exothermal peak of wood burn appears on the DTA curve ( Figure 6).
Instead of the N-70-30 curve, which is related to insulation fi berboards made by WP/RF ratio of 70/30 and without adhesive, in other curves, the sharp exothermal peak is smoothed and the temperature of the peak is increased. This means that the utilization of a higher amount of wood fi bers and adhesive increased the thermal stability of fi berboards, and the rates of heat released slowly down. The broadest peak is related to W-50-50 insulation fi berboards, which are manufactured by the use of PVAc adhesive and a lower amount of waste paper. Concerning the N-70-30 curve, the main component is waste paper. Waste paper is mainly composed of cellulose. The shape of this curve and the intensity of the decomposition peak are very similar to those of the cellulose curve (Sobol et al., 2020). In fact, there is a larger share of cellulose in this sample and therefore its shape is different from other   curves. Summary data of TGA-DTA for fi bers can be observed in Table 7.
The results show that the degradation began faster with insulation fi berboard in which PVAc adhesive was used. In addition, when the lower percentage of waste paper was used, the degradation reaction occurred faster. T m as a crucial factor, was higher with insulation fi berboards made from WP/RF ratio of 70/30 with PVAc and lower with insulation fi berboards manufactured without adhesive and WP/RF ratio of 50/50. The maximum weight loss was observed in insulation fi berboard manufactured with PVAc adhesive and WP/ RF ratio of 70/30. The lowest weight loss with a signifi cant difference occurred in insulation fi berboards manufactured with PVAc adhesive and WP/RF ratio of 50/50 (W-50-50). The higher weight loss means lower thermal stability (Aydemir et al., 2011). It can be said that the use of adhesive and a lower percentage of waste paper results in a higher thermal stability. There is not a signifi cant difference in terminal mass residues of different treatments. The lowest mass residue (9.7 %) was observed with insulation fi berboard manufactured without adhesive and with 50 % waste paper, the reason probably being the weak bonding of this kind of board. Overall, it can be concluded that the use of adhesive results in increased thermal stability of insulation fi berboard. Also, the higher amount of waste paper decreased the thermal stability of insulation fi berboards. DTA peak temperature in N-70-30 fi berboards was signifi cantly lower than in other treatments, which confi rmed previously obtained results and showed that this treatment caused lower thermal stability of fi berboards. The thermal resistance of a compound is related to the breakdown of the weakest bond at a specifi c temperature as the temperature was raised (Bachari, 2015).

ZAKLJUČAK
This study showed that it is possible to manufacture insulation fi berboard from wastes of MDF and paper by the wet process.
According to the measured values, which were similar to mineral wool, it can be said that the manu-factured insulation fi berboards can be properly used as a structural insulator.
The use of PVAc adhesive and a lower percentage of waste paper (WP/RF ratio of 50/50) in manufacturing fi berboard result in a higher fi re resistance and thermal stability, in better functional properties and lower thermal conductivity values of insulation fi berboards.
Better insulators have better thermal stability.
Overall, it can be concluded that waste MDF and paper have good potential to be used as raw material for producing green insulation fi berboards. with better results obtained from treatment in which PVAc adhesive and the WP/RF ratio of 50-50 (waste paper to recycled MDF fi bers) were used. Table 7 TGA -DTA results from insulation fi berboards manufactured by waste MDF and paper fi bers: N-70-30 (without adhesive, WP/RF ratio of 70/30), N-50-50 (without adhesive, WP/RF ratio of 50/50), W-70-30(with PVAc adhesive, WP/RF ratio of 70/30) and W-50-50 (with PVAc adhesive, WP/RF ratio of 50/50) Tablica 7. TGA -DTA rezultati za izolacijske ploče vlaknatice proizvedene od vlakana MDF otpada i otpadnog papira: N-70- 30  T i -T f -temperature corresponding to the beginning and end of decomposition, respectively / temperatura koja odgovara početku i kraju razgradnje; T m -Temperature corresponding to the maximum rate of mass loss / temperatura koja odgovara najvećem gubitku mase; Wweight loss / gubitak mase