Abstract
Properties were evaluated for particleboard made from polymethylene polyphenyl polyisocyanate (pMDI) binder and wheat straw treated with sodium hydroxide (NaOH) and ammonia hydroxide (NH3·H2O) solutions. The crystallinity (Xc) of wheat straw and board properties including internal bond strength (IB), thickness swelling after 24 h (24 hTS), modulus of rupture (MOR), and modulus of elasticity (MOE) were investigated. XRD indicated that the crystallinity of wheat straw was increased with the mass fraction range from 1% to 3%, and the bending properties of board were also improved. MOR and MOE were significantly (P < 0.05) improved, and a positive correlation with solution mass fraction was shown. Bending properties, especially MOE, were superior to the requirements in GB/T 4897-2015. IB of NaOH solution treated board was also significantly higher (P < 0.05), but results for IB of ammonia solution treated and 24 hTS for board treated with both solutions failed to meet the standard. Effects of the two solutions were not the same according to a t-test, and the effects of NaOH were better.
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The Properties of Particleboard Made from Alkaline-treated Wheat Straw and Methylene Diphenyl Diisocyanate Binder
Yang Cao,a,b Wei Song,a,b Zexun Yang,a,b Zhenghao Chen,a,b and Shuangbao Zhang a,b,*
Properties were evaluated for particleboard made from polymethylene polyphenyl polyisocyanate (pMDI) binder and wheat straw treated with sodium hydroxide (NaOH) and ammonia hydroxide (NH3·H2O) solutions. The crystallinity (Xc) of wheat straw and board properties including internal bond strength (IB), thickness swelling after 24 h (24 hTS), modulus of rupture (MOR), and modulus of elasticity (MOE) were investigated. XRD indicated that the crystallinity of wheat straw was increased with the mass fraction range from 1% to 3%, and the bending properties of board were also improved. MOR and MOE were significantly (P < 0.05) improved, and a positive correlation with solution mass fraction was shown. Bending properties, especially MOE, were superior to the requirements in GB/T 4897-2015. IB of NaOH solution treated board was also significantly higher (P < 0.05), but results for IB of ammonia solution treated and 24 hTS for board treated with both solutions failed to meet the standard. Effects of the two solutions were not the same according to a t-test, and the effects of NaOH were better.
Keywords: Particleboard; Wheat straw; Alkaline treatment; Ammonia; Polymethylene polyphenyl polyisocyanate (pMDI)
Contact information: a: Beijing Key Laboratory of Wood Science and Engineering, Beijing Forestry University, Beijing 100083, China; b: MOE Key Laboratory of Wooden Material Science and Application, Beijing Forestry University, Beijing 100083, China; *Corresponding author: shuangbaozhang@163.com
INTRODUCTION
Wheat (Triticum aestivum L.) is the second most widely cultivated crop in the world. A large amount of wheat straw is produced and needs to be reused. In China, about 112 million tons, representing 21.6% of the total production of crop straw, was burned in fields (Li and Wang 2013). The open burning of straw is not only a waste of resources, but also causes air pollution. About 20% of all the agriculture straw is wheat straw. Thus, its industrialized utilization in pulp, bioethanol, pellet fuels, or particleboard manufacturing could be very important.
Particleboard is a very important wood composite product. Unlike wood, the bond strength of straw board is poor when using UF resin or PF resin because of the waxy layer on the surface of straw (Cheng et al. 2004) and a higher silica content (Halvarsson et al. 2010). An alternative high-performance binder, methylene diphenyl diisocyanate (MDI), has been adopted. The bond strength is mostly formed by urethane linkages generated by the reaction between –N=C=O group in MDI and –OH groups in lignocellulosic materials (Johns 1982). This is usually sold and used as PMDI (polymeric MDI) (Papadopoulos 2006) in wood industry. The bond properties and water resistance are excellent, and the resin content is much lower than UF resin or PF resin. Furthermore, the formaldehyde emission of board is almost zero, making it an environmentally friendly binder.
MDI is the preferred binder in manufacturing boards with agriculture residues (Sellers 2001). However, MDI is expensive and mostly used as a structural binder. The properties of wood-MDI particleboard are very stable after being exposed to outdoor conditions for 5 years, as the IB still exceeds standard requirements (Korai et al. 2015). Compared with the boards made of MDI and wood particles, the properties of straw board are poor, and the straw should be pretreated if a higher quality straw board is required.
There are several methods of pretreatment for wheat or rice straw before particleboard manufacturing. These methods include enzymes (Zhang et al. 2003), oxalic acid and steam (Li et al. 2011), acid, and alkaline treatments (Liu et al. 2004; Zhang and Hu 2014). Alkaline treatment, or NaOH treatment, is a very effective method for the pretreatment of wheat straw for bio-ethanol manufacturing because it dissolves lignin and hemicellulose (Wan et al. 2011). Additionally, the NaOH treatment breaks ester linkages between lignocellulose and the wax layer on the exterior surface of straw (Binod et al. 2010). Liu finds that the surface wax layer becomes rough as a consequence of NaOH solution treatment and calls this phenomenon “chemically etched”; the wettability of binders is also improved by such treatment (Liu et al. 2004). The removal of the wax layer increases the bonding and does not decrease the water resistance (Kurokochi and Sato 2015); thus, it is very important in improving the properties of straw particleboard. NH3·H2O is easily recycled and also breaks the ester linkages of wax on the surface of straw (Yang et al. 2012), so this method also would be effective.
The NaOH treatment of particles has been shown to improve the properties of wheat straw particleboard. Research involving NH3·H2O pretreatment in board manufacturing has been uncommon, though in theory such treatment would be feasible. This research evaluated the properties of boards made from wheat straw after two different alkaline treatments and also compared the difference between the two alkaline solutions.
EXPERIMENTAL
Material Preparation
The wheat straw (Triticum aestivum L.) was collected in Henan Province, Central China, stored for more than 3 months, and then processed to particles using a hammer mill. The particle length was mostly between 5 mm and 10 mm. Particles were screened by standard sieves to a size between 0.70 mm (24 mesh) and 1.40 mm (12 mesh) to manufacture particleboard.
PMDI (WANHUA WANNTE® PM-200, Wanhua Chemical Group Co., China) was used as a binder; the solid content was considered 100%. NaOH powder (analytical reagent (AR), Beijing Chemical Works, Beijing, China) and 25 wt.% ammonia (AR, Beijing Chemical Works) were dissolved into 1 wt.%, 2 wt.%, and 3 wt.% solutions in distilled water separately.
The straw was sprayed with an alkali aqueous solution at a ratio of 1 mL to 1 g (oven dried weight), for each group 800 g wheat straw (original moisture content = 9%) was selected, and the consumption was 720 mL. The treated straw was stored at room temperature (25 °C) for 12 h and then dried in an air-dry box (DF82-8, Beijing Tongxian Experiment Instrument Works, China) at 90 °C to reduce the moisture content to 5% prior to processing.
Particleboard Manufacturing
MDI binder was dissolved in acetone (1:1 by weight) and mixed with straw particles in a blender (FTR 250, Draiswerke GmbH, Mannheim, Germany) using a spray gun at a pressure of 0.40 MPa. The resin content was 4%, as calculated by the ratio of resin consumption and the oven-dried weight of the particles.
The mat was formed in a paving mold on steel plates coated with Teflon film, and then it was pressed at 3.5 MPa for 7 min in a hot-presser (BY302 × 2/15 150T, Xinxieli group, Suzhou, China). The temperature was 180 °C, and the thickness of the board was conditioned to 10 mm iron bars. To reduce the error, two boards were made for each formulation. After hot-pressing, the board was cut into 300 by 300 mm pieces and conditioned at room temperature for more than 24 h until a constant weight was reached.
Property Testing
The board properties, including internal bond strength (IB), thickness swelling after 24 hours (24hTS), modulus of rupture (MOR), and modulus of elasticity (MOE) were tested according to GB/T 17657-2013 (2013). 24hTS was measured by using a 0 to 25 mm micrometer; IB, MOR, and MOE were tested by a mechanical testing machine (MWW-50, Tianhua Test Device Co., Jinan, China). The standards for boards are described in GB/T 4897-2015, this standard is equivalent to ISO 16893-2: 2010.
Internal Bond Strength
To test IB, the specimens were cut into 50 × 50 mm pieces, and 5 specimens were selected from each board (Fig. 1A). The load speed was 2 mm/min and IB was calculated by Eq. 1,
(1)
where IB represents the internal bond strength (MPa), Fmax represents the maximum load of tensile failure (N), l is the length of specimen (mm), and b is the width of specimen (mm).
Thickness Swelling
To test 24hTS, the specimens were cut into 50 × 50 mm pieces; 5 specimens were selected from each board and soaked in distilled water (pH 7 ± 1, temperature 20 ± 1 °C) for 24 h. The thickness was measured by a micrometer and was finished within 10 min. The 24hTS was calculated by Eq. 2,
(2)
where t1 represents the initial thickness of specimen (mm) and t2 represents the thickness of the specimen soaked after 24 h (mm).
Static Bending Properties
The MOR and MOE were tested in a 3-point bending test. The specimens were cut to 250 by 50 mm pieces, and for each board, 3 specimens were chosen. The test is displayed in Fig. 1B . MOR and MOE was tested with a load speed of 10 mm/min and calculated by Eqs. 3 and 4 respectively,
(3)
(4)
where σb represents MOR (MPa), Eb represents MOE (MPa), Fmax represents the maximum load of failure (N), b represents the width of the specimen (mm), l1 is the length of the span (mm), t is the thickness of the specimen (mm), and ΔF (N) and Δx (mm) represent the variation of load and deformation of the straight-line portion of the load-deflection curve, respectively.
Fig. 1. Test of internal bond strength (L) and static bending properties (R)
Crystallinity of Wheat Straw
Wheat straw particles are ground to 0.250 mm (60 mesh) and scanned by X-Ray Diffractomer (D8 Advance, Bruker AXS GmbH, Karlsruhe, Germany). The crystallinity of cellulose (Xc) was calculated through software DiffracPlus EVA (Bruker AXS GmbH, Karlsruhe, Germany) by Eq.5,
(5)
where Ac represents the area of crystalline phrase and Aam represents the area of amorphous phrase.
Statistical Analysis
One-way analysis of variance (ANOVA) was adopted to compare the relationship between each property and the mass fraction of the alkaline solution. SPSS Statistics 23 software (IBM, Armonk, United States) was used for the analysis with a confidence level of 95% (α = 0.05). p < 0.05 was considered a significant difference in the mass fraction of alkaline solution.
Requirements in GB/T 4897-2015
The general requirements of particleboard used as load-bearing board in dry or humid conditions are shown in Table 1. Besides, optional tests inducing IB were carried out after the boiling test or MOR after 70 °C the dipping test if the board could be used in humid conditions.
Table 1. Requirements of Particleboard (thickness =10 mm) Used as Load-Bearing Boards in Dry Conditions in GB/T 4897-2015
RESULTS AND DISCUSSION
Static Bending Properties: MOR and MOE
The results of MOR and MOE are shown in Table 2 and illustrated in Figs. 2 and 3. The MOR of NaOH-treated straw particleboard increased from 18.29 MPa to 26.62 MPa and 22.65 MPa, respectively, when the alkaline mass fraction of NaOH and NH3·H2O increased from 0% to 3%.
Table 2. Properties of Boards Manufactured in this Research
Fig. 2. Modulus of rupture and the comparison of two pretreatment methods
A statistical difference was found for NaOH (p = 0.000 < 0.05) and NH3·H2O (p = 0.019 < 0.05). Thus, the mass fraction of alkaline had significant impact on MOR, which is also reflected in the bar chart. The MOE increased from 2070 MPa to 2880 MPa and 2070 MPa to 3070 MPa for NaOH and NH3·H2O, respectively, when the alkaline mass fraction increased from 0% to 3%. The mass fraction of NaOH (p = 0.000 < 0.05) and NH3·H2O (p = 0.000 < 0.05) had significant impact on MOE. MOE showed a positive correlation with the mass fraction and it was obviously that the effect of NH3·H2O was better than NaOH.
The strength of particleboard is formed mainly between MDI and cellulose hydroxyls. The improvement of MOR and MOE and of alkaline-treated boards may be because of increased crystallinity. Generally speaking, the MOE of materials such as plastic, timber, and wood-based composites are improved with increased crystallinity. Increased cellulose crystallinity could increase the MOR and MOE of boards (Wu et al. 2011). NaOH solution treatment destroys the amorphous region of the cellulose first (Zheng et al. 2012). Kim also found the ammonia treatment would remove the amorphous part of biomass (Kim et al. 2003). Thus, the crystallinity of cellulose in the straw may increase and the bending properties may be improved. This phenomenon could also certified by XRD spectrum in Fig. 4. The crystallinity of untreated wheat straw was 32.80% and the crystallinity (Xc) raised to 35.90% and 36.90% by NaOH and ammonia solutions treated.
Fig. 3. Modulus of elasticity and the comparison of two pretreatment methods
The MOR of NaOH-treated board was significantly improved, and MOE of NH3·H2O treated board was better at the same alkaline mass fraction. Alkaline solution treatment could improve the bending properties of particleboard, and the properties were superior to the requirements for use as a load-bearing board in dry (MOR > 15 MPa, MOE > 2200 MPa) or humid conditions (MOR > 17 MPa, MOE > 2450 MPa) as specified in GB/T 4897-2015.
Fig. 4. XRD of untreated and alkaline solution treated wheat straw particle
Internal Bond Strength
The results are shown in Table 2 and illustrated in Fig. 5. For the NaOH treatment, the average IB increased from 0.30 MPa to 0.55 MPa when the mass fraction increased from 0% to 3%. For NH3·H2O treated boards, the value was 0.30 MPa, 0.25 MPa, 0.26 MPa, and 0.31 MPa when the mass fraction increased from 0% to 3%. A statistically significant difference (p =0.000 < 0.05) was found between mass fraction of NaOH and the IB of 3 wt.% treated board is 83% higher than the untreated. The results are also reflected in Fig. 2. Alkaline treatment by NaOH could increase the specific surface of particle by reducing the content of silica and waxy layer (Han et al. 1998) and resulting in more chemical linkages formed with binder and increased bonding quality. Additionally, the alkaline solution treatment could also improve the wettability on the surface of straw, which could improve the penetration ability of binder (Shen et al. 2011) and achieve higher bond strength.
Fig. 5. Internal bond strength and the comparison of two pretreatment methods
Unlike NaOH, NH3·H2O could not improve the IB of board. Besides, ANOVA indicated that no statistically significant difference (p = 0.615 > 0.05) was found between IB and mass fraction of NH3·H2O.The reason could be the reaction between NH3 and MDI binder and the impact of hydrogen bond.
MDI binder could react with the residual NH3 or -NH2 groups and form urea and biuret keys (Johns 1982), and the related reaction could partly substitute the reaction between binder and lignocellulosic materials and so the bonding efficiency would be decreased. Besides, NH3 could partly form OH···NH3 keys instead of OH···OH keys between cellulose molecules (Yang et al. 2012). Therefore, the existence of H···N hydrogen bond would impact the bond between cellulose, lignin, and hemicellulose and result in a weaker bond strength.
IB of NaOH solution treated board was significantly improved and superior to the requirement in the related standard, but the NH3·H2O solution treatment could not improve the IB.
Thickness Swelling
The thickness of swelling after 24 h is shown in Table 2 and illustrated in Fig. 6. The 24 hTS for the NaOH-treated board decreased from 22.75% to 20.81% when the mass fraction increased from 0% to 3%. There was a slightly negative correlation with mass fraction, but no significant difference was found (p = 0.579 > 0.05). Thus, the influence of mass fraction in 24 hTS was not significant.
Fig. 6. 24-h thickness swelling and the comparison of two pretreatment methods
NH3·H2O solution treatment slightly increased the 24 hTS of board, but the correlation between mass fraction and 24 hTS was the same as NaOH-treated board and the impact of mass fraction on 24 hTS was also not significant (p = 0.388 > 0.05).
Alkaline treatment could remove hydrophobic lignin and hydrophilic hemicellulose partly and the change of crystallinity could also affect the dimensional stability of board. It was difficult to explain the change of 24 hTS with the above factors. But it was known that thickness swelling was affected by bond quality and adhesive properties (Mo et al. 2003), which could inhibit the ‘spring-back’ of board (Okino et al. 2004). IB and 24 hTS also showed a negative correlation in this experiment, as illustrated in Fig. 7. Thus, the change of 24 hTS could be related to the change in IB. Besides, the change in crystallinity could also impact the dimensional stability of board, but the impact seemed to be not significant in this experiment.
The thickness swelling of particleboard could not meet the requirement of board used as load-bearing board in dry conditions (24 hTS < 19%)
Fig. 7. Correlation between 24 hTS and IB in this experiment
Difference between NaOH and NH3·H2O solution on properties
T-test of samples was given in Table 2 to evaluate the difference between the two alkaline solutions. The differences of IB, 24 hTS and MOR were significant (P <0.05), which meant that the results of NaOH and NH3·H2O were different. The difference of MOE was not significant (P > 0.05), so the impact of NaOH and NH3·H2O solution on MOE was the same. According to the t-test, there was a significant difference between the NaOH and NH3·H2O solution. Based on the results, the effect of NaOH solution treatment on improving the properties of board was better than NH3·H2O solution.
Table 2. Independent Samples T-test of Properties
CONCLUSIONS
- XRD indicated that the crystallinity was increased by alkaline treatment and showed a positive correlation with mass fraction in this experiment. Statistical differences were found between bending properties (MOR and MOE) and mass fraction. MOR and MOE were both significantly improved, and the highest values were all found at a mass fraction of 3%.
- The internal bond (IB) strength of NaOH-treated board met the requirements of load-bearing board in dry or condition. Both solutions could increase MOR and MOE and the values were superior to the requirements but IB of NH3·H2O solution treated and the 24 hTS of all the specimens in this experiment were fail to meet the requirement.
- NaOH or NH3·H2O solution treatment could be used to improve some properties of wheat straw particleboard. Compared with NH3·H2O, the effect of NaOH was better at the same mass fraction. A mass fraction of 3% showed the best results in this experiment.
- Statistical difference was found between the effect of NaOH and NH3·H2O solution treatment on the properties of board. The effect of NaOH treatment was better.
ACKNOWLEDGMENTS
This research is supported by the Co-built Foundation with Beijing Municipal Education Commission, “R & D on Key Technology in Scientific Utilization of Non-wood Plant Raw Materials”, National Natural Science Foundation of China (31670571), Beijing Natural Science Foundation (6162019) , and the Co-Built Foundation with Zhejiang Province (CZXC201410), “R & D on Bamboo Composites and Environmentally Friendly Adhesives”.
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Article submitted: November 21, 2016; Peer review completed: December 30, 2016; Revised version received and accepted: March 3, 2017; Published: March 15, 2017.
DOI: 10.15376/biores.12.2.3265-3276