gypsum-bonded particleboard manufactured from agricultural based material

For. Sci. Pract., 2013, 15(4): 325–331DOI 10.1007/s11632-013-0420-6

© 2013 Beijing Forestry University and Springer-Verlag Berlin Heidelberg

RESEARCH ARTICLE

Abstract Gypsum-bonded particleboard (GBPB) panels were made from various mixtures of particles of bagasse (Saccharum officinarum L.) and wheat straw (Triticum aestivum L.), bonded with different ratios of particle/gypsum. This study examined the feasibility of bagasse and wheat straw particles in the production of GBPB. One-layer experimental GBPBs with a density of 1.05 or 1.20 g∙cm−3 were manufactured at different ratios of bagasse/wheat straw, i.e., 100%/0%, 93.75%/6.25%, 87.5%/12.5%, 75%/25%, 50%/50%, 25%/75% and 0%/100% using two particle/gypsum composite ratios, i.e., 1/2.75 and 1/3.25 by weight. Thickness swelling (TS), water absorption (WA), modulus of rupture (MOR), modulus of elasticity (MOE) and internal bond strength (IB) properties of the boards were evaluated and a statistical analysis was performed in order to examine the possible feasibility of these agricultural residues for use in commercial GBPB manufacturing. We determined that WA of panels decreases as the amount of straw increases to 100% and the LR/G (wood/gypsum) ratio decreases to 1/3.25, whereas the TS of panels decreases as the proportion of straw decreases to 0% and the LR/G ratio increases to 1/2.75. The experimental results also show that the MOR and MOE of panels containing 0%, 6.25% and 12.5% wheat straw with a LR/G ratio of 1/2.75 were higher than those of panels made from 25%–100% wheat straw with a LR/G ratio of 1/2.75, as well as those from all other percentages of straw with a LR/G ratio of 1/3.25. On the other hand, the IB of panels containing more than 12.5% straw with LR/G ratios of 1/2.75 and 1/3.25 were lower than those of panels made from 0–12.5% straw also with both LR/G ratios. Panels consisting of 0%, 6.25% and 12.5% wheat straw with LR/G ratios of 1/2.75 and 1/3.25 met the minimum EN standard requirements of mechanical properties for general purposes. All of the panels containing 0–100% wheat straw with a LR/G ratio of 1/2.75 or 1/3.25 met the required level of TS for 24-h immersion.

Keywords wheat straw, bagasse, gypsum-bonded particleboard

Received 22 April 2012; accepted 27 August 2012

Author for correspondence (Morteza NAZERIAN)E-mail: [email protected]

Gypsum-bonded particleboard manufactured from agricultural based material

Morteza NAZERIAN, Meisam KAMYAB

Department of Wood and Paper Science and Technology, University of Zabol, Zabol 9861335856, Iran

Introduction

Developing structural products from industrial and agricultural residual material is necessary for overcoming the waste disposal problem. There is a great amount of agricultural and wood

waste remaining from harvesting of agricultural products, such as wheat straw and sugarcane bagasse and from manufacturing of poplar wood, considered to be one of the most important tree species under management in the entire world. This waste material could be considered the alternative raw material for the production of inorganic- or organic-bonded wood composite material. However, the composition of inorganic binders shows a strong effect on the compatibility between wood or lignocellulosic material and cement and on the resulting mechanical

Forest Science and Practice, Vol.15, No.4, 2013326

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properties. It is well known that mechanical interlocking plays a major role in the bonding mechanism of inorganic-bonded panels (Ahn and Moslemi, 1980). Due to the existence of waxy and silica layers encirculating straw stems, which does not permit sufficient direct contact between binder and straw fibers, the use of crop material as raw fiber material for panel manufacturing has been limited (Sauter, 1996). Moreover, straw stems of this material are hollow and tubular. When the straw is cut into small particles, some of the particles cannot be split and maintain a tubular shape, which prevents the binder from reaching the internal surfaces of the straw (Li et al., 2010).

Curing of gypsum is caused by chemical reactions between water and calcium sulphate hemihydrate (CaSO4∙0.5H2O) which is readily available in the formulation of the gypsum. Straw and bagasse are characterized by lower amounts of lignin and higher amounts of extractives. In the presence of extractives containing sugars, morphology and size of the hydrate crystals are affected and impermeable hydrates are formed around unhydrated mineral grains, which inhibit or delay curing times. As a consequence, they can affect the adhesion of inorganic binders to wood (Ahn and Moslemi, 1980; Simatupang et al., 1988; Simatupang and Geimer, 1990). On the other hand, the initial curing of gypsum usually takes about ten minutes, which is too short to meet production requirements (Feng et al., 2007). In fact, the mixing, spreading and pressing processes may be not completed before hydration temperature of this mixture of gypsum-wood-water reaches its highest point. Moreover, the pH of aqueous extracts from non-wood lignocellulosic material is significantly higher than that of wood. Species with a pH higher than 4.9 are considered incompatible. On the other hand, those with a pH lower than 3.9 are considered compatible (Hachmi and Moslemi, 1989). Therefore, extractives with a high pH can act as natural retarders and delay the initial setting and curing of the mixture.

However, there have been few investigations using different lignocellulosic resources on properties of cement-bonded particleboard and the effect of these materials on the characteristics of gypsum-particleboard have not yet been

studied. The aim of this study therefore was to determine properties of gypsum-bonded particleboard (GBPB) manufactured from mixtures of wheat straw (Triticum aestivum L.) and bagasse (Saccharum officinarum L.) particles at various bagasse/wheat straw ratios using two particle/gypsum compositions.

Materials and methods

One-layer boards were made using bagasse (S. officinarum L.) and wheat straw (T. aestivum L.) particles. Bagasse was collected from a local pulp and paper mill (Pars Paper Co. Haft Tapeh, Iran). After harvesting, sugarcane stalks were directly separated by a cane separator to obtain their outer layers. These layers were then stored for three months after being treated by the cane separator. Wheat straw was approximately 1-m in length at harvest and cut above the water line leaving the lower third as stubble in the field. Then, the wheat straw and outer layers of the bagasse were broken down by using a hammer mill and screened. Particles remaining on the 2.4-mm sieves were used in panel production. Fourteen series of gypsum-bonded particleboard (GBPB) with dimensions of 350 mm × 350 mm × 12 mm,

Table 1 Experimental designBoard type Bagasse (%) Wheat straw (%) LR/G ratioA 100 0 1:2.75B 93.75 6.25 1:2.75C 87.5 12.5 1:2.75D 75 25 1:2.75E 50 50 1:2.75F 25 75 1:2.75G 0 100 1:2.75H 100 0 1:3.25I 93.75 6.25 1:3.25J 87.5 12.5 1:3.25K 75 25 1:3.25L 50 50 1:3.25M 25 75 1:3.25N 0 100 1:3.25

Note: LR, lignocellulosic resources; G, gypsum.

Morteza NAZERIAN and Meisam KAMYAB: Gypsum-bonded particleboard manufactured from... 327

were produced at two wood:gypsum ratios (1:2.75 and 1:3.25). Target board densities for the specimens were 1.05 and 1.20 kg∙m−3. The amount of water added was maintained at 40% of gypsum weight and 0.05% citric acid as retarder was added. In order to manufacture GBPB, wood particles were wetted with water + retarder and then gypsum was added. After 15 min of manual mixing, a single layered mat was formed into a caul. Cold pressing took place under an initial pressure of 3 MPa, to a 14-mm thickness, after which the board was retained under compression for 4 h. Three pieces of boards were produced from each series. The panels were loaded into an oven-dryer and thermally treated at (40 ± 2)°C for 4 h. After manufacturing, the boards were conditioned at (25 ± 2)°C at (60 ± 10)% RH for 7 days, trimmed and tested for WA (water absorption), TS (thickness swelling, after 24-h immersion in water), MOR (modulus of rupture), MOE (modulus of elasticity) and IB (internal bond strength), according to the EN 310 (1993), EN 319 (1993) and EN 317 (1993). Mechanical tests were carried out by a HONSFILD material testing device (H25KS, Redhill, England). Table 1 shows the experimental design of the study. For the study of the physical and mechanical properties of the GBPB, an analysis of variance (ANOVA) was used

considering the factors: bagasse/straw particle compositions and lignocellulosic resources (LR)/gypsum (G) ratios. Duncan’s tests were used to make comparisons between experimental panels.

Results and discussion

Water absorption and thickness swelling

Water absorption (WA) and thickness swelling (TS) test results are shown in Table 2. As can be seen from this table and Fig. 1, WA varied between 13.37% and 41.04%. This means that the boards produced under the conditions studied can be partitioned into eight groups (a-h). The binder content and type of LR as variables had a significant impact on WA. Duncan’s multiple range tests indicated that the panels made from 100 per cent straw mixed with the 1/3.25 ratio of LR/G have a significantly lower WA capacity than the panels made from a lower proportion of straw and gypsum. The board types L, M, G and N were found to be resistant to water penetration and hence, these also showed the lowest water absorption. This is due to the fact that straw itself is known to be resistant to water, even at decreased levels of gypsum used.

Table 2 Various properties of gypsum-bonded particleboard manufactured from agricultural based material and Duncan’s test results Board type Density (g∙cm−3) WA (%) TS (%) MOR (N∙mm−2) MOE (N∙mm−2) IB (N∙mm−2)A 1.05 41.04 ± 0.34a 0.66 ± 0.11g 13.26 ± 0.34a 4505.00 ± 54.56b 0.55bB 1.05 35.35 ± 0.23b 0.77 ± 0.12f 13.00 ± 0.54ab 4756.00 ± 45.34ab 0.51bC 1.05 29.28 ± 0.45c 0.89 ± 0.21e 12.56 ± 0.28b 4992.00 ± 35.67a 0.42cD 1.05 28.69 ± 0.19c 1.02 ± 0.11cd 9.50 ± 0.12c 4168.00 ± 23.67c 0.31dE 1.05 27.39 ± 0.46c 1.11 ± 0.13bc 7.57 ± 0.27d 3754.00 ± 26.78d 0.27dF 1.05 19.84 ± 0.65ef 1.18 ± 0.15b 5.90 ± 0.46f 3197.00 ± 34.12f 0.17eG 1.05 16.78 ± 0.31fgh 1.28 ± 0.11a 6.57 ± 0.28e 3474.00 ± 35.67e 0.17eH 1.20 30.70 ± 0.45c 0.51 ± 0.10h 5.87 ± 0.21f 3763.00 ± 31.67d 0.63aI 1.20 28.30 ± 0.24c 0.68 ± 0.18g 6.47 ± 0.34e 4203.00 ± 53.12c 0.53bJ 1.20 23.76 ± 0.38d 0.80 ± 0.19f 6.50 ± 0.45e 4143.00 ± 45.67c 0.42cK 1.20 20.46 ± 0.29e 0.89 ± 0.17e 5.20 ± 0.54g 4000.00 ± 35.67cd 0.30dL 1.20 17.71 ± 0.31fg 0.98 ± 0.21de 5.10 ± 0.67g 2603.00 ± 31.34g 0.30dM 1.20 15.00 ± 0.49gh 1.09 ± 0.16bc 5.23 ± 0.37g 2641.00 ± 35.67h 0.21eN 1.20 13.37 ± 0.28h 1.10 ± 0.19bc 5.07 ± 0.16g 2481.00 ± 32.67i 0.18e



The panels made from 100%, 93.75%, 87.5%, 75%, 50%, 25% and 0% bagasse presented significantly different WA levels. It was found that board types A and B showed less resistance against water penetration and showed the highest water absorption values. This was assumed to be also due to the lower density of these boards compared with other ones. Besides, as other non-wood plant material, bagasse particles have higher amounts of extractives, polysaccharides and hemicellulose, resulting in a higher water absorption rate.

Generally, the water absorption of GBPB seems to be improved by increasing the board density and the amount of gypsum (Table 2 and Fig. 1). At the minimum density (1.05 g∙cm−3) and LR/G ratio (1/2.75), the GBPB produced provided the maximum WA (from 16.78% to 41.04%). The WA was further decreased when the density was increased to 1.20 g∙cm−3 and LR/G ratio decreased to 1:3.25, reaching a minimum value of 13.37%.

As shown in Table 2 and Fig. 2, an increase in the LR/G ratio and bagasse particles resulted in a decrease in thickness swelling (TS) of the boards. The samples A and H are identical in respect to density, straw/bagasse particle ratio and gypsum content. The observed results indicated that all of the panels met the minimum EN standard requirements of TS for general purposes.

Panels produced using only wheat straw resulted in higher TS values (1.28% and 1.10%) compared to the boards made from bagasse particles (0.66% and 0.51%). This could be due to the following: the lower the bonding strength between straw particles and gypsum, the greater the springback after 24 h of water immersion due to lower internal bonds. In a cross section, the epidermic cells are the outermost surface cells and covered by a thin layer of wax (Mantanis and Berns, 2001). This layer decreases the wettability of straw with water and hence prohibits gypsum paste to penetrate into the cell walls and creates good mechanical interlocking between the mineral matrix and particles, which results in extra losses of IB. This then affects the TS of the panels and could be considered an important cause of reduced dimensional stability of GBPB made from straw particles.

The TS of GBPB made from straw or bagasse

particles decreases as the density increases. Not only did the WA increase significantly from 30.70% to 41.04% and from 13.37% to 16.78%, respectively, but as well the TS increased from 0.51% to 0.66% and from 1.10% to 1.28%, respectively, when the density increased from 1.05 to 1.20 g∙cm−3. The density of the particleboard was controlled using a fixed mold volume, meaning that the dimension of the particleboard was constant. To produce a higher density panel, the mass of the binder (gypsum) needs to be higher, causing a greater pressure within the composite and consequently, stronger contact between LR particles, resulting in a lower TS.

As observed earlier, straw particles are thinner than bagasse particles. At a fixed mass ratio of binder and LR, the total surface area of straw could be larger than that of bagasse particles. In

Fig. 1 Comparison of the WA values of manufactured panels

Fig. 2 Comparison of the TS values of manufactured panels


this case, the binder was not enough to cover the straw surface, resulting in a higher TS.

Mechanical properties

The MOR ranged from 5.07 to 13.26 N∙mm−2 and the MOE from 2481 to 4992 N∙mm−2 (Table 2, Figs. 3–4). The MOR requirement is 12.5 N∙mm−2 for general-purpose boards by EN 312 (2003) and for MOE 1800 N∙mm−2 according to EN 312. Mean values of MOE for all panels were higher than the requirement for general purposes. With the LR/G ratio of 1/2.75 and the amount of straw varying from 25% to 100%, the boards did not meet the required level of MOR, while the boards containing 0%, 6.25% and 12.5% straw particles

had higher values of MOR than the requirement level for general purposes. When the LR/G ratio was 1/3.25, none of the panels could meet the requirement level for MOR.

The range of IB was from 0.17 to 0.63 N∙mm−2 (Table 2; Fig. 5). The IB requirement is 0.28 N∙mm−2 for general-purpose boards according to EN 312 (2003). It is seen that the IB of panels was affected by the production variables used in the study. The lowest IB values were obtained with a panel density of 1.05 g∙cm−3, the LR/G ratio of 1/2.75 and 100% straw. The IB of panels increased as panel density, the amount of bagasse and the binder ratio increased. The highest IB values were obtained with the panels H, A, B and I. The H-type board achieved the highest IB values, while F-, G-, M- and N-type boards gave the lowest IB values as expected. Considering only IB values, approximately half of the board types complied with the minimum requirements for general grade particleboards. Test results indicated that A-, B-, C-, and D-type boards which were manufactured using the LR/G ratio of 1/2.75 with 0–25% straw and H-, I-, J-, K- and L-type boards, manufactured using the LR/G ratio of 1/3.25 with 0–50% straw, conform to the minimum requirements for general grade particleboards as indicated by EN 312 (2003). A- and H-type boards also meet the minimum requirements for furniture grade particleboards used under dry conditions as indicated by EN 312 (2003).

Considering that internal bonds are strongly correlated with binder bond strengths, it is clear that very limited mechanical interlocking was achieved between gypsum and straw particles. Apparently, the wax coating of straw obstructs the development of a strong mechanical bond with mineral binders. It seems that straw contains higher amounts of hydrophobic substances (i.e., wax) than bagasse and hence, panels containing straw in amounts of 50% or more showed a higher TS and lower IB than pure bagasse panels, probably as a result of the inferior bond quality of straw with gypsum. When the amount of bagasse particles increases in a mat, the proportion of outer waxy surface relative to the overall particle surface is reduced and hence, stronger bonding between particles may occur. As a consequence,

Fig. 3 Comparison of the MOR values of manufactured panels

Fig. 4 Comparison of the MOE values of manufactured panels



TS can also be decreased.Given our results, it appears that board density,

the binder ratio and the type of lignocellulosic resources significantly affect the MOR, MOE and IB of agricultural based GBPB. Therefore, it is possible to manufacture stronger and stiffer boards by varying the production variables involved, as seen in this study. It is interesting to discover that not only the amount of straw but also the level of binder adversely and significantly affect the MOR and MOE. The results showed a reduction in IB of the samples as the LR/G ratio increased from 1/3.25 to 1/2.75. The relatively lower degree of bending strength deterioration of bagasse/straw panels can be attributed to the higher length to thickness ratio of bagasse compared to straw particles.

The higher gypsum usage had an adverse effect on MOR and MOE of the GBPB, possibly due to the presence of high brittleness and low MOE values of the gypsum. It should be noted that every gypsum-based material is brittle and exhibits very limited deformation before fracture (Duke et al., 2000) and hence, larger loads are restricted by the brittleness of the gypsum. However, incorporation of organic fibrous material (such as wood) into the gypsum matrix can improve the brittle-fracture resistance and strength properties of gypsum boards, thus providing more versatility to the finished

product (Sattler and Lempfer, 1989). In this way, the addition of particles increases the possibility of load transfer and acts as a load spreader, resulting in a more even load transmission between different layers of the panels. Therefore, high amounts of particles as filler can be added in order to improve mechanical properties. Besides, pores are known to affect strongly the mechanical properties of brittle material. With an increase in the amount of particles in panels, pores could be increased significantly. Callister (1994) determined that increase in strength can occur due to the presence of no more than 10% porosity.

Gram (1986) reported that due to a high alkaline environment of the mineral matrix, the sisal fiber cement composites become brittle. This may be due to the reaction of alkaline pore water with the lignin and hemicellulose present in the middle lamellae of fiber cells (weakening the link between individual fiber cells). Another reason may be densification of the matrix in the fibers due to the gradual filling of the fiber cell cores with hydration products (brittle breakage of fiber under stress) (Aggarwal et al., 2008). Consequently, with increasing amounts of gypsum in the system, disintegration of lignin and hemicelluloses increases, resulting in a higher brittleness of composites. Moreover, it appears that the wood-mineral binder composites need enough binder to encapsulate fully the wood material to obtain a cohesive mix with acceptable properties (Simatupang and Geimer, 1990). A lower wood-gypsum ratio also results in weak bonds. However, if the amount of gypsum is too high, the compaction ratio will be reduced, leading to a brittle material.

Our results show that the sugarcane bagasse contained fibers with a mean length of 1590 μm. These fibers are longer than those of wheat straw (1140 μm) (Hemmasi et al., 2011). The shorter average length of wheat straw fibers will adversely affect the bending strength of panels. Moreover, the cell wall thickness of sugarcane bagasse fibers is thicker (5.64 μm) (Samariha and Khakifirooz, 2011) than those of wheat straw (1.00 μm) (Halvarsson et al., 2010). The higher cell length and cell wall thickness of fibers provide for more flexibility of fibers in panel manufacturing process.

Fig. 5 Comparison of the IB values of manufactured panels


Conclusions

The results of our investigation indicate that the manufacture of gypsum-bonded particleboards from bagasse or a mixture of bagasse and wheat straw using gypsum as binder is technically feasible.

The wheat straw decreases the mechanical properties of particleboard and increases thickness swelling due to low internal bonding of panels. However, increasing its use decreases the water absorption of the panels. This is due to the presence of greater amounts of wax and extractives in the straw and anatomical structural differences between the straw and bagasse.

I t w a s f o u n d t h a t a d e c r e a s e o f t h e lignocellulosic material/gypsum ratio resulted in a decrease in MOR and MOE, due to the presence of high brittleness and low values of MOE of the gypsum. However, panels bonded with the lower amount of gypsum had higher water absorption and thickness swelling and lower internal bond strength than the particleboards with lower gypsum because of insufficient encapsulation of the particles at low gypsum/particle ratios.

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gypsum-bonded particleboard manufactured from agricultural based material

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