Wheat straw-based particleboards with citric acid as adhesive; influence of mat moisture and adhesive content

doi:10.21203/rs.3.rs-3322245/v1

ABSTRAC In this study, the properties of particleboard made from wheat straw using citric acid (CA) as a bio-adhesive were investigated. The effects of mat moisture content (MC) and adhesive on mechanical properties (flexural strength, modulus of elasticity, internal strength) and physical properties (thickness shrinkage and water absorption) of straw particleboard were evaluated. To control the MC of the mat, the resinated straw particles were oven dried at different times (2 to 14 hours) and temperatures (80 and 100 °C) to obtain a mat with MC between 20 and 0.5%. The effects of an adhesive content of 15, 20, 25 and 30% (based on the dry mass of the straw) on the properties of the panels were also studied. The best MC of the mat before hot pressing was about 11% (drying of the particles in 2 hours at 100 °C), and the corresponding board had the highest mechanical and the lowest physical properties. Further decreasing the MC of the mat from about 11% to 0.5% had a negative effect on the mechanical and physical properties of the straw particleboard. Increasing the adhesive content from 15% to 30% improved all mechanical and physical properties of the panels; however, the changes in the range of 20% to 30% of adhesive content on the flexural properties and water absorption were not significant. In general, the results showed that due to the acidic nature of CA, it is possible to produce particleboard from wheat straw without pretreating the straw particles before use.

Wheat straw

Citric acid

mat moisture content

adhesive content

drying time

The increase in global population and changing lifestyles have increased the demand for wood-based panels (WBP), with global production of WBP more than doubling from about 178 million m³ in 2000 to about 396 million m³ in 2021 (FAO, 2023). On the other hand, competition for raw wood supply has intensified, leading to an increase in wood prices, mainly due to increased demand for raw wood from both WBP producers and the bioenergy sector (Guan and Zhang, 2023). In countries like Iran, the wood supply is much worse due to the tripling of production volume in the last decade (more than 3 million m³) and ongoing forest conservation plans (Heidarlou et al., 2023). The use of agricultural waste as a renewable and abundant material in the world and also in Iran is considered one of the most reliable solutions to overcome this challenge. Most agricultural residues are quite similar to wood in terms of the chemical and morphological composition of the fibers (Dukarska et al., 2019). Researchers have showed the potential of various agricultural residues such as bagasse stalks (Legesse et al., 2022), rice straw (Li et al., 2010), bamboo (Bazzetto et al., 2019), almond shells (Guerue et al., 2006), corn husks (Zakaria et al., 2021), palm biomass (Eghtedarnezhad et al., 2020) and rapeseed stalks Hýsek et al. (2018) for particleboard production.

Wheat straw has significant potential to replace wood for the production of WBP as it is much more widespread and abundant than other lignocellulosic sources in the world. Global wheat production is estimated at about 750 million tons (Mancini et al., 2018). It is stated that for every ton of wheat, about 1.3 to 1.6 tons of straw remain in the field (Montane et al. 1998; Neitzel et al. 2022). Thus, It is estimated that at least 975 million tons of wheat straw is available as residue in the fields each year, most of which is burned, negatively impacting global warming potential. Previous research has shown that although wheat straw can be used as a raw material for the production of WBP, the composition and hydrophobic structure of the straw surface leads to incompatibility between the straw and the conventional resins (e.g., urea formaldehyde) used for WBP production (Yao et al. 2003; Boquillon et al. 2004). This leads to a very weak bond and a reduction in the physical and mechanical properties of the resulting straw particleboard (Han et al. 1998; Oh and Lee 2012). To overcome this challenge, some techniques have been proposed. The wheat straw was pretreated with various materials before being used for board production. Pretreatments of wheat straw with enzymes (Zhang et al. 2003), steam, hot water, sulfuric acid (Harlvarsson et al. 2009, 2010), acetic anhydride and soap solution (Bekhta et al. 2013), silane coupling agents (Hafezi et al. 2016), and plasma (Hysek et al. 2018) have been carried out to improve the adhesion of wheat straw with urea formaldehyde adhesives. Others suggested using alternative adhesive systems such as isocyanate (p-MDI) (Boquillon et al. 2004; Cao et al. 2017), epoxidised linseed oil (Luo and Yang et al. 2010), acrylated epoxidised soybean oil (Tasooji et al. 2010) and tannin adhesive (Tabarsa et al. 2011) to make panels from wheat straw. Most of these adhesives consist of harmful or toxic chemical substances derived from fossil raw materials, so their further use is severely restricted. However, there is a lot of emphasis on green or environmentally friendly adhesives in WBP production these days (Sutiawan et al., 2021).

Citric acid (CA) is an organic polycarboxylic acid with three carboxyl groups that is commercially produced from the fermentation of glucose or materials containing glucose and sucrose (Widyorini et al. 2016). CA can also be extracted naturally from various fruits and vegetables, especially citrus fruits such as oranges, tangerines, lemons, limes, etc. (Tsao et al., 1999). Citric acid has been used for decades as a modifier to improve the dimensional stability of wood and WBP (Umemura et al., 2012). Recently, Menezzi et al. (2018) and Ando and Umemura (2021) studied the chemical bonding between the functional groups of wood carbohydrates and lignin with citric acid using sophisticated analytical techniques. They found that an ester bond forms between the carboxyl groups of citric acid and the hydroxyl groups (aromatic and aliphatic) in lignin and wood carbohydrates. They also suggested the use of citric acid as an environmentally friendly adhesive for the manufacture of wood products. So far, researchers have investigated the potential of citric acid as a bio-based adhesive to produce different types of WBP, especially from agricultural residues. In this context, the use of particles from bamboo (Widyorini et al., 2016), bagasse (Kusumah et al., 2016), maize husks (Prasetiyo et al., 2018), Imperata cylindrica (Syamani et al., 2018) and Nipa frond (Santoso et al., 2019) for the production of particleboard has been reported. The effective processing conditions for CA-bonded particleboard were summarized by Lee et al. (2020) as follows: Pressing temperature of more than 180°C, board density of 800 kg/m³, CA content of 20 wt.% or more and pre-drying of 12 hours at 80°C. The pre-drying of resinated particles with CA was suggested to eliminate moisture from particles before hot pressing (Kusumah et al. 2016; Syamani et al. 2018).

There are no studies on the production of wheat straw boards with citric acid as an adhesive, which was the aim of the current study. It seems that citric acid, due to its acidic nature, can scratch or even dissolve the oily extracts on the straw surface and form chemical bonds with the hydroxyl groups of the straw. In this case, the above-mentioned complex pre-treatment of the wheat straw prior to the production of particleboard is not necessary. The influence of adhesive content and mat moisture on the properties of wheat straw particleboard before hot pressing was investigated. To control the moisture content of the resinated particles, different pre-drying protocols (various drying times and temperatures) were applied.

2.1. Materials preparation

The wheat straw was obtained from the farms in Gonbad (Golestan province, Iran). A hammermill was used to produce the particles. The straw particles were then sieved to obtain particles of uniform size and the particles that passed mesh size 8 and remained at mesh size 50 were used as raw material. The particles were oven dried at 80°C for 12 hours to achieve a moisture content of less than 5% before resination.

Citric acid as an adhesive in commercial grade and powder form was obtained from Hugestone China. CA was used without further purification and dissolved in water at a concentration of 62% wt.%. To reduce the toughness and brittleness of the citric acid (Hakmeh et al., 2021; Zakaria et al., 2021), 2% corn starch powder (per dry weight of straw) was also added to the CA solution. The final concentration of the adhesive was constant at 62% throughout the study. Urea formaldehyde (UF) adhesive supplied from Amol Resin was used as reference adhesive.

2.2. Adhesive characterization

The properties of CA adhesives before and after the addition of starch were characterized. For this purpose, the adhesive viscosity and gel time were measured according to DIN 53211 and ISO 9396, respectively. The flow time of the adhesive was measured with a DIN flow cup of 4 mm. To measure the gel time, 10 g of adhesive were poured into a glass tube and stirred at 100°C in a water bath. The time that passes until the adhesive can no longer be stirred (becomes a gel) is called the gel time.

The pH value of the adhesive was measured at a temperature of 23 ± 0.5 according to ASTM E70. It should be noted that the instrument was calibrated before the measurement. The pycnometer method according to the ISO 2811 standard was used to measure the density of the adhesive. The following formula was used to calculate the density of the adhesive.

\(\varvec{D}=\frac{{\varvec{M}}_{\varvec{p}}-{\varvec{M}}_{\varvec{a}}}{{\varvec{V}}_{\varvec{p}}}\)

Formula 1

where D is the adhesive density (g/ml), M_p is the pycnometer weight (g), M_a is the adhesive weight (g) and V_p is the pycnometer volume (ml). To obtain each of the above properties of the CA adhesive, five replicates were performed, of which the average was reported.

2.3. Fourier Transform Infrared (FTIR) Spectroscopy

The FTIR spectrum was recorded to analyze and investigate the mechanism of the bonds between citric acid and wheat straw. For this purpose, resinated wheat straw was taken from the edge of the panel. The particles were ground into a powder, and the obtained powder was dried in a vacuum drying oven at 70°C for 10 hours. Infrared (IR) spectral data were obtained with an FTIR spectrophotometer (FT/IR-6300; JASCO Corporation, Japan) using the KBr disk technique and recorded with an average of 32 scans at a resolution of 4 cm^− 1 in the range 4000 − 400 cm^− 1.

2.4. Wheat straw particleboard production

Single-layer particleboards with a thickness of 6 mm were made from resinated wheat straw particles. According to the literature (Syamani et al., 2018; Kusumah et al., 2016), the straw particles were placed in the laboratory oven (at different times and temperatures) to reduce and control the moisture content of the resinated particles. After the oven, the particles were poured into a 400 x 300 mm² box to form the mat. The prepared mat was placed in a hot press for 360 seconds at a temperature of 200°C. In addition, a 6 mm thick steel bar was used to control the panel thickness during hot pressing. The maximum specific pressure was 4 MPa when the top of the press plate reached the steel-thick bar. The density of the panel was kept constant at 800 kg/m³.

In addition, particleboards bonded with 20 wt% UF resin with a similar size and nominal density as the CA-bonded particleboards were to be produced as a reference. However, these reference boards were delaminated after hot pressing and no tests could be performed on these specimens. All panels were taken to the laboratory for conditioning for two days. The process for producing wheat straw particleboard with citric acid as adhesive is shown in Fig. 1.

The moisture content of the resinated particles and the adhesive content are considered as research treatments. To control the moisture content of the mat, the resinated straw particles were stored in a laboratory oven at different time intervals (2, 6, 10 and 14 hours) and temperatures (80 and 100°C). For comparison, panels of resinated straw particles that was not stored in the oven were also prepared. Depending on the duration and temperature of the oven, the mat had a moisture content between 0.5 and 20% before hot pressing. The effect of the adhesive content on the properties of the wheat straw particleboard was also investigated at four levels of 15, 20, 25, and 30% (based on the dry weight of the straw particles). The research treatments are listed in Table 1. Three boards were prepared from each treatment, and at least three test specimens were made from each board. The specimens were conditioned at 20 ± 2°C and 65 ± 5% relative humidity before the physical and mechanical tests.

Table 1

Research treatments for the production of wheat straw particleboard using citric acid as an adhesive.
Samples code	Oven time (h)	Oven temperature (°C)	Mat moisture content (%)^*	Adhesive content (%)
A	0	100	20	20
B	2		11
C	6		2
D	10		1.5
E	14		0.5
F	6	100	2
G	6	80	4.5
H	2	100	11 11 11 11	15
K				20
L				25
M				30
R	Reference sample with UF adhesive			20

* * Moisture content was measured as a function of the corresponding drying time and temperature of the resinated particles in the oven (numbers were rounded for simplicity).

2.5. Panel characteristics

The evaluation of physical and mechanical properties of wheat straw particleboards was carried out according to the European standards. For this purpose, the modulus of elasticity and flexural strength (EN 310), internal bond strength (EN 319), moisture content (EN 322), thickness swelling (EN 317) and water absorption were tested.

A displacement speed of 2 and 1 mm/min was used to measure the bending properties and internal bond strength, respectively. In addition, a digital scale with an accuracy of 0.01 gram was used to measure the weight of the specimens and a caliper with an accuracy of 0.01 mm was used to measure the dimensions of the specimens. Nine replicates were tested for each panel variant. Differences between panel characteristics were analyzed with a one-way ANOVA using SPSS software (IBM SPSS Statistics 26). Statistical significance was set at P < 0.05.

3.1. Adhesive characteristics

The specifications of the CA adhesive including density, pH, viscosity, and gel time with and without starch as additive are shown in Table 2. The CA adhesive with and without starch had similar density of about 1.28 g/ml. The results also show that the pH of the CA adhesive with and without starch was 1.5 and 2.5, respectively. In other words, the pH increased with the addition of starch due to its alkalinity (Lee et al., 2019). Other researchers have also reported that the pH of CA adhesive without additives is less than 2, which is consistent with this result (Nitu et al., 2022; Sutiawan et al., 2021). The viscosity of CA adhesive without and with starch was 38 and 45 centipoises, respectively. Due to the low viscosity of both types of adhesives (high water content), the adhesive could be easily sprayed onto the straw particles. Due to the high-water content available in the CA adhesive solution, a drying treatment was required to remove excess moisture from the particles before hot pressing (Syamani et al. 2018).

The gelation time of the CA adhesive without starch was about 50 seconds. The addition of starch to the citric acid shortened the gelation time to 42 seconds, a reduction of about 16%. The reason for this could be the formation of cross-links between citric acid and starch and the acceleration of adhesive curing (Hikmah et al., 2021).

Table 2

Characteristics of citric acid adhesives
Adhesive type	Density (g/ml)	Gel time (s)	Viscosity (cP)	pH
Citric acid solution	1.28	50	38	1.5
Citric acid solution with starch^*	1.27	42	45	2.5

* The starch content was 2%, based on the dry mass of the straw.

3.2. Infrared spectroscopy (FTIR)

FTIR analysis was performed to evaluate the formation of bonds between the wheat straw and the carboxylic acid groups of citric acid, and the results are shown in Fig. 2.

The peak in the range of 1722 and 1733 cm^− 1 in citric acid is due to C = O stretching originating from the carboxyl group or the ester group. Peaks in the range of 1208 cm^− 1 in citric acid have also been associated with C-O stretching and -OH bending (Umemura et al., 2012). The peaks at wavenumbers 1159, 1082, 1014 cm^− 1 in corn starch were also attributed to C-O stretching (Goheen and Wool, 1991). The absorption peaks at wavenumbers 992, 929, 861, 765, 575 cm^− 1 are also attributed to the stretching vibrations of the anhydro glucose ring in starch. A broad peak was observed at 3421 cm^− 1 which is attributed to the hydroxyl groups present in starch (Fang et al., 2004).

The strong broad peak in 3330 cm^− 1 of wheat straw can be associated with the O-H bond (Behrvan et al., 1400). The peak in the wavenumber of 1031 cm-1 is also caused by the stretching vibration of the C-O bond (Zarski et al., 2016). Peaks in the range of 1400 − 1200 cm-1 in wheat straw also correspond to the bending frequencies of C-H, O-H, or CH2 (Sain and Panthapulakkal, 2006). FTIR spectroscopy of resinated straw with CA also revealed a peak at 1732 cm-1 associated with the carbonyl group of the stretching C = O double bond (Žagar and Grdadolnik, 2003). According to the literature, this peak confirms the adhesion between the hydroxyl group of lignocellulosic materials and the carboxylic acid group of CA. Citric acid led to the formation of ester bonds (R-COO-R) by esterification between the carboxyl groups of CA and the hydroxyl groups of wood (Sutiawan et al., 2021). Esterification of wood is one of the most common chemical reactions that occurs at the level of the cell structure of wood to improve the dimensional stability and biological durability of wood and wood products. Esterification is a chemical reaction between an acid (carboxylic acid) and an alcohol (or other -OH) to form an ester and water (Lee et al., 2020). Previous studies have shown that CA is able to form an ester bond with the hydroxyl groups of lignocellulosic materials (Umemura et al., 2012).

3.3. Influence of moisture content (MC)

3.3.1. Moisture content of straw

The effects of time (at a constant temperature of 100°C) and temperature (at a constant time of 6 hours) in the oven on the MC of the mat are shown in Fig. 3. As expected, increasing the temperature and drying time of the resin-containing particles in the oven resulted in a significant decrease in the MC of the particles. The resin-containing particles (20 wt% CA) had an MC of about 20% without drying. The MC of the mat decreased by almost 50% to about 10.6% after 2 hours of drying. With increasing time in the oven, the moisture content decreased further and reached about 0.5% after 14 hours in the oven.

At a fixed time (6 h) in the oven, the oven temperature also had a significant effect of more than 50% on reducing the MC of the resinated particles, so that the MC decreased from about 4% (6 h at 80°C) to about 1.9% (6 h at 100°C). In the literature, a temperature of 80°C is recommended for drying resinated particles with CA (Lee et al., 2020). However, the results of this study show that the drying temperature of 100°C has a greater effect on the internal bond strength, which will be discussed in more detail in Section 3.3.3. This may have a significant impact on the reduction of drying time.

MC of the mat before hot pressing was reported to has an important influence on the properties of WBP (Wolcott et al., 1990). It was seen that when the MC of the mat is increased up to 22%, the mechanical properties of the boards decrease, and their physical properties increase (Böhm, 2009). Halliga and Schniewind (1974) also reported a significant decrease in the flexural properties and internal adhesion of particleboard made with urea-formaldehyde resin when the mat MC was increased by more than 5%. Watkinson and van Gosliga (1990) reported similar results for medium density fiberboard (MDF). Therefore, the precise control of the mat MC before pressing, depending on the type of adhesive used, is an important step to achieve the desired panels properties.

3.3.1. Flexural properties

The Fig. 4 shows the effect of the MC of the straw mats on the flexural properties of the wheat straw particleboard (for simplicity, the MC of the particles is rounded in Figures). The lowest flexural strength and modulus of elasticity were found for the sample with MC of 20% (without drying the particles). By decreasing the MC of the particles, the flexural strength and elastic modulus increased significantly. The highest flexural strength and modulus of elasticity were obtained for specimens with MC of 11% (dried at 2 h and 100°C) at 12 and 3700 MPa, respectively. The mat always needs a certain amount of moisture, especially in the surface layers, to accelerate the heat transfer from the press plates to the core layer of the mat so that the adhesive can cure there. While a higher MC content can accelerate heat transfer to the core, it leads to an undesirable increase in vapor pressure in the core (risk of blister), a negative effect on adhesive curing, and negative effects on panels properties (Thoemen et al., 2010; Shalbafan and Thoemen 2022). Researchers also found that MC plays an important role in the quality of citric acid bonding with lignocellulosic materials (Kusumah et al. 2016; Syamani et al. 2018). Therefore, the highest flexural properties of samples with about 11% MC can be attributed to improved bonding of the carboxylic acid groups of CA with the hydroxyl groups of wheat straw. The optimum MC value for the surface and core layers of the particleboard mat should not exceed 15–18% and 10–13%, respectively, because the accumulation of steam leads to a delay in resin curing (Istek et al., 2019).

When the MC content of the mat was further reduced from about 11% to about 0.5% (dried at 100 C from 2 to 14 hours), the flexural strength and elastic modulus decreased by 36% and 28%, respectively, reaching about 7.5 and 2700 MPa. To further reduce MC, the straw particles were left in the oven at a temperature of about 100 C for a longer period of time. During this time, pre-curing of the adhesive or even excessive penetration of the adhesive into the straw particles may have occurred, resulting in the formation of weak spots between the straw particles. Of course, it should also be noted that excessive reduction of MC of the mat weakens the process of heat transfer to the core during pressing, which may also negatively affect the intensity of resin curing (Shalbafan and Thoemen 2022).

Figure 4 also shows that although the flexural properties of panels made with straw dried at 80 C (MC of about 4 percent) were slightly higher than those of samples dried at 100 C (MC of about 1.9 percent), this difference was not statistically significant. In other words, a change in MC in the range of 2 to 4 percent had no significant effect on the flexural properties of the panels.

3.3.3. Internal bond strength (IB)

The effect of mat MC on IB was also studied and the results are shown in Fig. 5. The samples prepared with undried straw particles (MC of 20%) had the lowest IB. Reducing the MC of the particles from about 20–11% (dried at 2 h and 100°C) increased the IB by more than 280%. The carboxyl groups of CA were found to react with the hydroxyl groups of lignocellulosic materials to form ester bonds (Liao et al., 2016; Umemura et al., 2013; Kusumah et al., 2016). The results observed in this study show that the moisture content of the mat definitely affects the intensity and quality of CA adhesion. Excessive MC content led to an excessive increase in vapor mass in the core layer and negatively affected the adhesive bonds formed (Maloney, 1977; Halligan and Schniewind, 1974; Ndazi et al., 2006). Further reduction of MC from about 11–0.5% (dried at 14 h and 100°C) reduced the IB values by about 23%, which could be due to the negative effects on the heat transfer mechanism in the pressing cycle. (THoemen et al. 2010). Also, the prevention or excessive penetration of the adhesive into the straw particles during the drying period seems to affect the IB.

Figure 5 also shows that reducing MC by varying temperatures (80 and 100°C) leads to an increase in IB of about 15%. It is possible that the CA had better flowability on the surfaces of the particles during the drying process at 100°C and therefore could form better and more bonds.

3.3.4. Physical properties

The effects of mat MC on thickness swelling (TS) after 2 and 24 hours of water immersion are shown in Fig. 6. According to the results, the TS of the panels did not change significantly with the change in MC of the mat (by varying the drying temperature and time of the particles). Kusumah et al. (2016) observed in their study that pre-drying the resinated particles with CA reduced the TS of the samples, which was not consistent with the results of this study. Of course, in this study, the TS for all samples after 2 and 24 h of immersion in water was less than 6% and 8%, respectively, which is much lower than the values for conventional particleboard or for boards made with CA, which were 9.02% (Thoemen et al., 2010; Kusumah et al. 2016).

Figure 7 shows that the WA of the panels increased with decreasing MC of the mat. The lowest WA was observed for the panels made of undried particles having an MC of about 20%. In this sample, the WA after 2 and 24 h of immersion was about 17 and 43%, respectively. Reducing the MC of the mat to about 0.5% (drying for 14 hours at 100°C) increased the WA to about 39% and 56% after 2 and 24 hours of immersion, respectively. When the MC of the mat was reduced by changing the oven temperature from 80 to 100°C, the WA increased by about 20% after 24 hours of immersion. Previous studies have also shown that deviation from the optimal MC in the mat has a negative effect on the WA of particleboard (Istek et al., 2019). The accumulation of vapor in the core layer (high MC of the mat) and the weakening of the heat transfer process in the core (low MC of the mat) were the main factors affecting the intensity of adhesive curing, which also affects the WA of the specimens.

The WA values of the specimens in this study were less than 60% after 24 h immersion in water, which is much lower than conventional particleboard or board made of wheat straw (Behrwan et al., 1400; Azizi et al., 2011; Tabarsa et al., 2011; Bekhta et al., 2013).

3.4. Influence of adhesive content

3.4.1. Flexural properties

The effects of adhesive content on the flexural properties of wheat straw particleboard are shown in Fig. 8. Increasing the adhesive content from 15 to 30% increased the flexural strength and modulus of elasticity by 30% and 40%, respectively, from 9 and 2385 MPa to 13 and 3934 MPa. The increase in adhesive content resulted in more bonding sites between the carboxylic acid groups of citric acid and the hydroxyl groups of wheat straw, which had a positive effect on the mechanical properties, especially the flexural properties (Kusumah et al., 2016). Further increasing the adhesive content from 20 to 30 percent showed no significant effect on the flexural properties of the panels. Azeredo et al. (2015) also found that flexural properties increased with the increase of CA content up to 20 percent, and the further increase had no significant effect on flexural properties. At a fixed board density, a further increase in the adhesive content is associated with a drastic reduction in the consumption of lignocellulosic material in the board. This leads to a reduction in the compressibility of the mat in the board structure, especially at the board surfaces, which drastically affects the flexural properties of the board (Shalbafan and Thoemen, 2023).

3.4.2. Internal bond strength

The effect of the different adhesive content on the IB is shown in Fig. 9. The lowest IB value was about 0.12 MPa for the sample with 15% citric acid, and the highest value was about 0.26 MPa for the sample with 30% citric acid. In other words, the IB increased by 217% when the adhesive content was increased from 15–30%. The reason for this could be the increased bonding points between the hydroxyl groups of the straw and the carboxylic acid groups of the citric acid. In their study, Kusumah et al. (2016) also reported the highest IB for panels made of bagasse particles and CA at an adhesive content of 30%. Increasing the CA content increased the ester bonds formed with bagasse. The formation of ester bonds improved the adhesion of particleboard made with CA (Syamani et al., 2018).

3.4.3. Physical properties

The TS decreased significantly when the adhesive content was increased from 15–20% (Fig. 10). The TS after 24 h immersion decreased from 17% to less than 6.5% when the adhesive content was increased from 15–20%. It has long been known that CA improves the dimensional stability of wood and wood products by reacting with the hydroxyl groups of lignocellulosic materials to form ester bonds (Umemura et al., 2012; Sutiawana et al., 2020).

However, further increasing the adhesive content from 20–30% had no significant effect on the TS, which is important from an economic point of view to reduce adhesive consumption. The reason was probably the lower percentage of wheat straw on the board when a higher content of adhesive was used. Reducing the consumption of lignocellulosic material creates more voids in the board structure, which facilitates the access of water molecules to free hydroxyl groups of the wheat straw (Shalbafan et al., 2012). In other words, it seems that the positive effect of increasing the adhesive content to improve the reaction with the hydroxyl groups of wheat straw was compensated by the accessibility of more hydroxyl groups to water. Another possible reason is that almost all hydroxyl groups present in wheat straw can be bound with up to 20% citric acid and increasing the adhesive by more than 20% has no effect on the formation of new bonds. Researchers have also found that the binding capacity of the cell wall becomes saturated with increasing amounts of citric acid and the cell wall is no longer able to form compounds with ester groups (Liao et al., 2016).

A similar trend was observed for the WA as for the TS (Fig. 11). An increase of the adhesive content from 15–20% led to a significant decrease of the WA. For the samples with 15% and 20% adhesive content, the WA after 24 hours of water soaking was 65% and 47%, respectively. With increasing adhesive content, more ester bonds appear to have formed between the hydroxyl groups of the straw and the CA, leading to a decrease in WA. Kusumah et al. (2016) also reported similar results by increasing CA content to decrease WA. However, the change in WA was not statistically significant when the adhesive content was further increased from 20–30%. The reason for this, as mentioned earlier, could be the additional reduction in the amount of wheat straw in the panel or even the saturation of all hydroxyl groups in the straw by CA, although more detailed analyses are needed to support this assumption.

3.4.4. Comparison of the properties with the literature

One of the major challenges in using wheat straw to produce WBP is the inappropriate bonding with urea-formaldehyde adhesive due to the composition and hydrophobic surface structure of the straw surface. This resulted in an increase in the TS and WA content of the manufactured boards with UF adhesion (Tabarsa et al. 2011; Bekhta et al. 2013). It should be noted that the reference board manufactured with 20% UF adhesive was not able to perform physical and mechanical tests in the present study due to its inappropriate bonding quality. The comparison of the physical and mechanical properties of the panels produced in this study with the corresponding values from the literature (Table 3) shows that the CA successfully overcame the challenge of the surface structure of the wheat straw. CA forms a good bond with the hydroxyl groups in the inner layers of the straw. Compared to other studies on wheat straw particleboard, the lowest percentage of TS and WA after 24 hours was obtained in this study. However, optimization of the parameters of the production process still seems to be necessary.

Table 3

Comparison of the properties of wheat straw particleboard with citric acid as an adhesive with the properties of the corresponding boards reported in the literature.
Source	Adhesive type	Straw treatment or additive used	Flexural strength (MPa)	Modulus of elasticity (MPa)	Internal bond (MPa)	Thickness swelling (24 h)	Water absorption (24 h)
Han et al., 1998	UF	Amino silanes	22		0.11	40
Zhang et al., 2003	UF	Enzyme treatments	16		0.42	8.6
Luo and Yang, 2010	PF		31		0.79	18.8
Tasooji et al., 2010	Oxidized soybean oil	Ammonium chloride	18	2447	0.55	16
Azizi et al., 2011	UF	Ammonium chloride	27	1463	0.49	24	70
Tabarsa et al., 2011	PF	tannin and zinc acetate	17	3022	0.70	11	55
Hematabadi and Behrooz, 2012	UF	Urea treatment	13	4000	0.18	34	93
Bekhta et al., 2013	UF	Acetic anhydride, steam and boil in soapy water	13		0.30	27	66
Cao et al., 2017	PMDI	Ammonia	26	2880	0.55	21
Current research	CA	-	13	3900	0.26	6.5	46

In this study, wheat straw, an abundant agricultural residue, was used to produce particleboard with citric acid as a bio adhesive. Analysis of the results shows that CA can be used as an adhesive for the production of particleboard from wheat straw. When CA is used, no pretreatments of wheat straw such as thermal heating, acid, alkali, plasma, enzymes, etc. are required. Due to its acidic nature, CA is able to overcome the composition and hydrophobic properties of the wheat straw surface and form ester bonds with the hydroxyl groups of the straw. The FTIR results indicated the formation of an ester bond between the carboxyl groups of CA and the hydroxyl groups of wheat straw (peak at wave number of 1732 cm^− 1 in resinated straw). The results also showed that a high mat MC negatively affected the bonding quality of CA with straw. In addition, a lower mat MC was not preferred because it weakened the heat transfer to the core layer of the mat. The results also showed that a drying temperature of 100 C was more favorable than 80 C for improving the bonding quality, which was probably due to the better flowability of CA on wheat straw surfaces. The best flexural properties and internal bonding were found for the panels with an MC value of about 11%. Increasing the adhesive content increased the bonding ability of the CA and improved the physical and mechanical properties of the panels. Increasing the CA content from 20 to 30%, which is due to the reduction of the compressibility of the mat and possibly to the saturation of the bonding ability of the hydroxyl groups present in the wheat straw, does not seem to improve the properties of the wheat straw particleboards.

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No competing interests reported.

Wheat straw-based particleboards with citric acid as adhesive; influence of mat moisture and adhesive content

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Abstract

Figures

1. Introduction

2. Materials and Methods