Effect of Epoxy Resin Reinforcement on Screw Withdrawal Strength of Fiberboard and Particleboard Used in Furniture Industry

• The study aimed to increase the screw withdrawal strength of medium density fiberboard and par-ticleboard used in furniture strength by using epoxy resin in the screw pilot hole. Therefore, the effects of pilot hole diameters, screw diameter, and amount of epoxy resin on screw withdrawal strength of medium density fiberboard and particleboard from face and edge were investigated. According to TS EN 13446, 50 mm × 50 mm specimens were cut from commercial medium density fiberboard and particleboard boards. A static load was applied parallel to the screw direction. The screw withdrawal strength of medium density fiberboard was higher than the screw withdrawal strength of particleboard because of its density. Besides, the screw withdrawal strength of medium density fiberboard and particleboard samples with a 3.5 mm screw diameter was higher compared to those with a 4.5 mm screw diameter. A decrease in pilot hole diameter and an increase in the amount of epoxy resin provided higher screw withdrawal strength of materials. Using 20 % epoxy resin of the volume of the pilot hole resulted in two times better screw withdrawal strength values. The study showed that a higher amount of epoxy resin, smaller pilot hole diameter, and smaller screw diameter contribute to better screw withdrawal strength of both medium density fiberboard and particleboard from the face and edge


INTRODUCTION 1. UVOD
Joints are the weakest component of furniture construction (Smardzewski, 2009).In the case of any joint failure, all construction fails.Therefore, the reusability of the furniture decreases over time due to the fact that furniture joints loosen or cannot provide its serviceability.Especially, in case furniture, screw joints are a typical example where a furniture member or screw is not damaged, but the screw hole enlarge.Besides, screws are used for demountable furniture to be easily assembled and disassembled.However, screw holes enlarge after screwing off, so the screw pilot hole diameter increases and the joint does not have the initial strength after re-assembly.Larger screws could be preferred for re-assembly efforts, which increases the strength of the screw joints because the pilot hole diameter was increased in that screw withdrawn from the material (Uysal et al., 2015).However, the use of larger joints may not be suitable for wood-composite materials, especially edgewise, because of their thickness and internal bond strength.On the contrary, the use of adhesive in screw pilot holes is another method to increase the screw withdrawal strength (SWS) of materials.Epoxy resin (EP) is a better selection to reinforce screw withdrawal strength compared to other adhesives because it provides higher mechanical properties for adhesives containing EP as reinforcement.
An increase in joint strength decreases the environmental impact of furniture because its serviceability and life span are increased since the rate of discarded furniture in landfills decreases while the rate of reusability increases.Besides, the repairability of the furniture construction with larger screws or the use of adhesive could increase the life span of the furniture.Remanufacturing furniture is another way to increase furniture life span and decrease its environmental impact before being discarded in landfills.However, the remanufacturing market of furniture is responsible for 0.4 % of the furniture industry, and the number of employees is less compared to employees in the reuse network (Parker et al., 2015).In addition, furniture remanufacturing requires more energy than furniture izvlačna sila vijka bila je veća za srednje gustu ploču vlaknaticu nego za ploču ivericu.Osim toga, izvlačna sila vijka promjera 3,5 mm u srednje gustoj ploči vlaknatici i ploči iverici bila je veća od izvlačne sile vijka promjera 4,5 mm.Smanjenje promjera pilot-rupe i povećanje količine epoksidne smole omogućilo je veću otpornost materijala na izvlačenje vijaka.Upotrebom 20 % epoksidne smole u odnosu prema volumenu pilot-rupe rezultiralo je dvostruko boljim vrijednostima izvlačne sile vijka.Istraživanje je pokazalo da veća količina epoksidne smole, manji promjer pilot-rupe i manji promjer vijka pridonose boljoj izvlačnoj sili vijka na plohi i rubu srednje guste ploče vlaknatice i ploče iverice.KLJUČNE RIJEČI: epoksidna smola; ojačanje; izvlačna sila vijka; srednje gusta ploča vlaknatica; ploča iverica reuse, according to the product recovery hierarchy in the end-of-life options (Östlin et al., 2009).Hence, increasing the service life of furniture with screw joints could be provided by increasing the SWS of materials used in construction, so the second users can reuse furniture after it fulfills its service life for the first users.
Screw-type joints are mainly used in demountable furniture for easy assembly and disassembly.Therefore, it should be considered how joint strength would be increased after the first use or dissembling.Fasteners like screws, minifixes, confirmats, etc. are effective joinery systems to join elements made of wood-based composites and are used in case furniture.The strength of joints depends on the material, joinery system, screw pilot hole, number of fasteners and distance between fasteners.However, it is significant to have a self-locking system for each fastener to hold material.In addition, freezing and heating significantly affect material withdrawal capacity (Gašparík et al., 2023).Thus, increasing the SWS of materials comes into prominence to improve the overall strength of furniture joints.It depends on their density and internal bonding strength, diameter of pilot hole, length of screw penetration, diameter of screw shank and thread, and filling material injected in a pilot hole (Smardzewski et al., 2016).Besides, thread geometry such as thread height, flank distance, teeth angle, and thread angle significantly affect the SWS of the material (Hoelz et al., 2022).
Fiberboard (MDF) and particleboard (PB) are widely used as raw materials for Ready-to-Assembly (RtA) furniture.Screw-type fasteners are not only used in joints but also in handles, hinges and locks (Örs et al., 1998).Studies related to joints with screws have been conducted with the technological development in wood composites and engineered wood since the strength design of furniture was defined in the 1940s (Eckelman, 1973;Smardzewski, 2009;Uysal and Haviarova, 2019).Such studies will continue to be conducted to enhance the strength of screw joints as long as the furniture is the greatest need of people's living spaces and as long as new technological developments emerge in the field.The SWS of material increased by 9-45 % and 7-39 % with injected polyvinyl acetate ad-hesive in pilot holes for MDF and PB compared to those without adhesive.In addition, an increase in the thread depth of the screw provided higher SWS (Örs et al., 1998).The correlation between SWS and internal bonding strength is better than that of density and was studied to predict the internal bond strength of PB from its SWS (Semple and Smith, 2005).The larger-diameter screws had a greater SWS than smaller-diameter screws, but thread depth is more critical than screw diameter for the SWS of materials (Abu and Ahmad, 2015).Polyvinyl acetate and polyurethane adhesive were used in screw pilot holes to increase joint strength in RtA furniture (Imirzi et al., 2016).The type of material and type of screw affected the SWS of the material (Wolpiuk and Sydor, 2016).The SWS of a material increases with higher material density (Bal et al., 2017;Jivkov et al., 2017).The SWS of wood-based materials was inversely proportional to the modulus of elasticity because of high stiffness (Yunus et al., 2019).The effects of screw type, screw orientation, and adhesive on the SWS of MDF and PB were studied.Results showed that the use of adhesive in pilot holes increased the SWS of both MDF and PB (Yorur et al., 2020).The pilot hole diameter and density effects on the SWS of MDF were studied.Results revealed that pilot diameter had more impact on SWS than material density.Besides, coarse thread screws have lower SWS than fine thread screws; namely, the distance between screw threads is also a significant factor.SWS from the face was greater than SWS from the edge because the face density of MDF is higher than the core density.SWS is also affected by the adhesive composition used in particleboard (Farajollah Pour et al., 2022).The reinforcement type in a material increases its SWS due to higher density (Perçin and Uzun, 2022).Different size and amount of waste melamine impregnated paper (WMIP) as adhesive in particle boards were studied (Başboğa et al., 2023).SWS of particleboard increased with narrower size and higher amount of WMIP.
EP was recommended for frame construction with biscuit joints (Yildirim et al., 2017).EP provides a more durable adhesion with its mechanical adhesion force.It achieves more and faster penetration to the adhesion surface pores on the material than other adhesives due to its higher elasticity (Karaman et al., 2017).Also, an increase in the mechanical interlocking mech-anism between wood material and the surface of metal fasteners with EP enhances the SWS of wood-based composite materials.In light of this information, the present study was carried out with the idea of adding EP as reinforcement to the screw joint to increase the SWS of materials.In doing so, the effects of pilot hole diameters, screw diameter, and amount of EP on SWS of MDF and PB from face and edge were investigated.

Design of experiment 2.1. Postavke eksperimenta
In this study, MDF and PB materials, widely used in panel-type furniture and commercially produced, were investigated.A complete four-factor factorial experiment with five replications in each sample group was conducted to determine the SWS of the MDF and PB.For this purpose, two materials (MDF and PB), two screw diameters (3.5 mm and 4.5 mm), three pilothole diameters (80 %, 90 % and 100 % of screw diameter) and four levels of the EP amount (no-resin, 20 %, 40 % and 60 % of volume of the pilot-hole) factors were selected for edge and face withdrawal from specimens.In doing so, 480 specimens in 96 sample groups were tested to examine the effect of the EP on the SWS of the materials.

Preparation of specimens 2.2. Priprema uzoraka
According to TS EN 13446, all specimens were cut into 50 mm × 50 mm nominal dimensions from 18 mm × 2100 mm × 2800 mm of MDF and PB boards obtained from a local store in the Inegol furniture cluster in Turkey.Table 1 shows the average density and moisture content of the MDF and PB used in the study.The dimensions of the specimens and orientations for the pilot holes are given in Figure 1.a and b.All pilot holes were drilled according to screw diameter and depth of screw penetration in MDF and PB.The depth of the screw penetration is equal to 8d (d: screw diameter) or at most 30 mm for SWS from the edge and 16 mm for SWS from the face of the materials.Besides, the depth of the pilot hole was 70 % of the depth of the screw penetration.Table 2 shows pilot hole dimensions and the amount of EP for each sample group.Epoxy and hardener, obtained from a local store, were mixed to a 5/2 ratio for 3 minutes.Epoxy and hardener mix were exposed to air for at most 15 minutes before the screws were driven.After each specimen was drilled for a pilot hole and an appropriate amount of EP was injected, 3.5 and 4.5 mm diameter screws were driven into pilot holes to the above length.Table 3 shows the average length, penetration depth, diameter, and root diameter of the screws used in the study.

Determination of screw withdrawal strength 2.3. Određivanje izvlačne sile vijka
All tests for the SWS were conducted on the SHI-MADZU universal test machine according to TS EN 13446.Withdrawal loads were obtained by applying a withdrawal load parallel to the screw axis from the edge and face of specimens with a rate of 0.8 mm/min and 2 mm/min, respectively, and continued until the ultimate load was reached (Figure 2).SWS, f (MPa), was calculated by using Eq. 1.
(1)    According to test results, the average SWS from the edge of MDF was 9.26 MPa with a standard deviation of 0.56 MPa provided that the pilot hole was 80 % of the 3.5 mm screw diameter.An increase in the pilot hole diameter from 80 % to 90 % and 100 % decreased SWS of the MDF by 7.89 MPa and 3.42 MPa with standard deviations of 1.32 MPa and 0.47 MPa, respec-tively.If 4.5 mm diameter screws were used, the average SWS from the edge of MDF was 8.05 MPa with a standard deviation of 0.50 MPa, 6.80 MPa with a standard deviation of 0.28 MPa and 2.88 MPa with a standard deviation of 0.13 MPa for 80 %, 90 % and 100 % pilot hole of 4.5 mm-screw-diameter, respectively.If PB was used, the average SWS from the edge was 6.91 MPa with a standard deviation of 0.63 MPa, 4.92 MPa with a standard deviation of 0.44 MPa, and 3.50 MPa with a standard deviation of 0.22 MPa for 80 %, 90 % and 100 % pilot hole of 3.5 mm-screw-diameter, respectively.Moreover, those of 4.5 mm-screw diameter had 6.27 MPa with a standard deviation of 0.69 MPa, 5.17 MPa with a standard deviation of 0.78 MPa, and 2.80 MPa with a standard deviation of 0.19 MPa, respectively.
It could be observed that the average SWSs from the of MDF are higher than those of PB not only because of internal bond strength but also because the density of the material directly affected the SWS (  shown in Table 4 and Table 5 documented that the type of material significantly affected SWS from the edge (pvalue = 0.000 < 0.05), and that the mean values of SWS from the edge of MDF and PB were significantly different from each other (α = 0.05), respectively.Furthermore, larger screw diameter levels yielded decreasing SWS values from the edge of both MDF and PB.Although larger screws provide higher SWS for materials, screw diameter affected SWS when drilling a pilot hole but not the screw insertion technique (Abu and Ahmad, 2015).Therefore, 4.5 mm screw sizes might have an adverse impact on SWS because of using EP.The average SWS from the edge for a 3.5 mm-screw diameter was 11.3 % higher than those of 4.5 mm.Also, Table 4 and Table 5 showed that screw diameter had a significant impact on the SWS from the edge (p-value = 0.000 < 0.05) and that the mean values of SWS from the edge for 3.5 mm and 4.5 mm screw diameter were significantly different (α = 0.05), respectively.Pilot diameter is vital to the strength of joints with screws because the difference between its shank diameter and screw teeth length affects SWS depending on the material.Therefore, it should be close to the shank diameter so that the screw is easily driven into the material without damaging it (Eckelman, 2003).On the other hand, joinery systems with screws allow easily assembled and disassembled furniture construction.However, every effort to disassemble furniture construction enlarges the pilot hole, and re-assembling the furniture results in lower strength of joints.The test results showed that larger pilot hole diameters decreased the SWS of materials, while the increased levels of EP injected in the pilot hole enhanced SWS.The amount of EP was increased by 20 %, 40 % and 60 % of the pilot hole diameter.In the case of MDF samples with a 3.5 mm screw diameter and 60 % EP, SWS from the edge was increased by 119.67 %, 156.56 %, and 425.59 % for 80 %, 90 % and 100 % pilot hole diameters, respectively.Those with 40 % and 20 % EP, increased SWS as much as 69.49%, 67.33 %, 292.66 % and 79.32 %, 111.55 % and 322.94 %, respectively.
Furthermore, SWS was increased by 94.24 % when the pilot hole diameter increased from 80 % to 100 %, and the amount of EP increased from 0 to 60 %.SWS increase in those with 40 % and 20 % EP was 45.08 % and 56.27 %, respectively.Not only did both the pilot hole (p-value =0.000) diameter and amount of EP (p-value = 0.000) have a significant effect on SWS from the edge, but the interaction of pilot hole diameter and amount of EP (p-value = 0.000) also significantly affected SWS from the edge (Table 4).Furthermore, as shown in Table 5, based on Tukey pairwise comparisons, the mean values of SWS from the edge for pilot hole diameter and amount of EP significantly differed at the 95 % confidence level, respectively.An SWS increase should be achieved with a screw pilot hole close to the root diameter (Farajollah Pour et al., 2022).The pilot hole diameter should be at least 70 % of the screw diameter (Eckelman, 2003).The holding mechanism of the screw on materials depends on the screw thread rather than the whole screw diameter.In this study, specimens with 70 % pilot hole diameter and EP failed in the case of drilling screws, so these sample groups were omitted from the experiment.
As can be observed in Table 4, interactions of (i) screw diameter and pilot hole diameter, (ii) screw diameter, amount of EP, and material, (iii) screw diameter, amount of EP, and pilot hole diameter, (iv) screw diameter, material, and pilot hole diameter, (v) amount of EP, material, and pilot hole diameter, and (vi) screw diameter, amount of EP, material, and pilot hole diameter had no significant effects on SWS from the edge.

Screw withdrawal strength from the face 3.2. Izvlačna sila vijka na plohi
Test results for SWS from the face are given in Figure 4.According to test results, the average SWS from the face of MDF was 11.80 MPa with a standard     6 showed that all independent variables, namely, screw diameter (p-value = 0.000), material type (p-value = 0.000), amount of EP (p-value = 0.000), and pilot hole diameter, significantly affected the mean values of SWS from the face of specimens.In terms of the impact of material type on SWS from the face, a similar pattern was obtained with those from the edge; namely, the mean SWS from the face of MDF (11.80 MPa) was higher than that of PB (8.97 MPa) because MDF is a denser material with a higher internal bonding strength than PB.Tukey pairwise comparisons identified and documented the presence of SWS from the face differences across various levels of each independent factor.Table 7 demonstrated that the mean SWS from the face of MDF was significantly different from that of PB (α = 0.05).Table 5 also showed that changing screw diameter levels resulted in significantly different mean SWS from the face values.Besides, the mean SWS from the face for 3.5 mm and 4.5 mm screw diameters were significantly different from each other (α = 0.05).As given in Table 6, the interaction of screw diameter and material also significantly affected the average SWS from the face (p-value = 0.000).
A smaller pilot hole diameter and higher of EP resulted in higher mean SWS from the face of both MDF and PB.According to Table 6, the interaction of pilot hole diameter and amount of EP (p-value = 0.001) impacted SWS from the face.The average SWS from the face of MDF increased by 69.94 %, 73.09 %, and 101.28 % for 80 %, 90 % and 100 % pilot hole diameter when using a 3.5 mm screw diameter, and the amount of EP was 60 % of pilot hole diameter, respectively.For the same pilot hole diameters, the increase in SWS from the face for the samples with 4.5 mm screw diameter and the amount of EP of 60 % of pilot hole diameter was 68.74 %, 46.85 % and 117.86 %, respectively.Similar increases in the mean SWS from the face of MDF and PB were observed for the samples with the amount of EP equal to 20 % and 40 % of the pilot hole diameter.According to Tukey pairwise comparison results, the mean SWS from the face for the samples with the amount of EP equal to 60 % of the pilot hole diameter and without resin was significantly different.However, there was not enough statistical evidence to claim that the mean SWS from the face values for the groups with an amount of EP equal to 20 % and 40 % of the pilot hole diameter (Table 7) were different.Furthermore, Table 7 also demonstrated that the mean SWS from the face values for 80 %, 90 % and 100 % pilot hole diameters were significantly different from each other.Table 6 also showed that interactions of (i) screw diameter, amount of EP, and pilot hole diameter and (ii) all parameters had no significant effects on the SWS from the face.

ZAKLJUČAK
This study investigated the effect of EP in the pilot hole on the SWS of MDF and PB.For this purpose, three pilot hole diameters, four amounts of EP and two screw diameters were investigated.
Internal bond strength and density of material have a significant effect on SWS.No matter what pilot hole diameter, amount of EP and screw diameter were used, the SWS of MDF was higher than the SWS of PB.An increase in screw diameter from 3.5 to 4.5 mm significantly affects the SWS of MDF and PB.In the edge and face withdrawal of screws from materials, smaller screw diameter has higher SWS.Increase in pilot hole diameter decreases the SWS of materials.80 % pilot hole diameter had greater SWS than that of 90 % and 100 % because it was the closest to the root diameter.
In addition, an increase in the amount of EP in the pilot hole increases the SWS of the material.EP dispenses more and faster in material pores, so it works better after drilling screws in pilot holes.If a piece of furniture with screw-type fasteners were disassembled, its pilot hole would be close to 100 % of or larger than the screw diameter.Using EP in the pilot hole for the following assembly would be beneficial.SWS of materials with 80 % pilot hole diameter and no-EP increased by 40 % compared to that of 100 % pilot hole with 60 % EP of the volume of the pilot hole.
The study showed that the existence of EP in the pilot hole increases the SWS of both MDF and PB.In this way, it could be used to repair furniture with screwtype fasteners.Moreover, it has a longer life span before being discarded in landfills and increases the reusability and repairability of case furniture to decrease its environmental impacts.

Figure 2 1 .
Test configuration for a) edge withdrawal and b) face withdrawal Slika 2. Ispitna konfiguracija: a) za izvlačenje iz plohe, b) za izvlačenje iz ruba 2.4 Statistical analysis 2.4.Statistička analiza Data collected for all specimens were checked for determining statistical significance through Fourway ANOVA analysis, and Tukey pairwise comparisons were carried out in Minitab Statistical Analysis Software (Minitab Inc. 2013).Izvlačna sila vijka na rubu Results for SWS from the edge are given in Figure 3.

Table 4
Four-way ANOVA for SWS from the edge Tablica 4. Četverosmjerna ANOVA za izvlačnu silu vijka na rubu MPa if the pilot hole was 80 % of the 3.5 mm screw diameter.Increasing pilot hole diameter from 80 % to 90 % and 100 % decreased SWS of the MDF by 9.45 MPa and 7.34 MPa with standard deviations of 0.85 MPa and 0.31 MPa, respectively.If 4.5 mm diameter screws were used, the average SWS from the face of MDF was 9.71 MPa with a standard deviation of 0.47 MPa, 7.96 MPa with a standard deviation of 0.85 MPa and 5.26 MPa with a standard deviation of 0.31 MPa for 80 %, 90% and 100 % pilot hole of 4.5 mm-screw-diameter, respectively.If PB was used, the average SWSs from the face were 8.97 MPa with a standard deviation of 0.57 MPa, 6.49 MPa with a standard deviation of 0.72 MPa, and 5.93 MPa with a standard deviation of 0.60 MPa for 80 %, 90 % and 100 % pilot hole of 3.5 mm-screw-diameter, respec-