Conventional methods of making particleboards utilize wood chips. This has resulted in a decrease in the tree cover due to the increase in wood demand. The effect has been climatic change. Wood is bound using phenol formaldehyde resin. Because of the decrease in the forest cover, alternative lignocellulose materials are required. In this study, lignocellulose materials used include sugarcane bagasse, maize stock, and rice husks. The cassava-starch mix with borax was used as a binder in particleboard formulation. The lignin content was determined, and its effect on properties of boards was investigated. The resultant composite material was molded at a pressure of 6.5 N/mm2 and at 30°C. The resultant particleboards had mean densities ranging from 0.604 to 0.611 g/cm3. The modulus of elasticity ranged from 2364.2 N/mm2 to 3329.93 N/mm2, modulus of rupture ranged from 13.55 N/mm2 to 14.83 N/mm2, and internal bonding ranged from 1.613 N/mm2 to 2.370 N/mm2. The performance of the board was dependent on the lignocellulose material used. Fourier transform infrared spectroscopy analysis showed that main chemical bonding in the particleboard resulted from esterification of –COOH from lignocellulose and OH- from starch. The particleboards formulated were found to be of low-density-fibre standard used in a similar manner to the conventional low-density particleboards.
Crop residue is an alternative raw material for use in the construction industry [
Surface properties of the lignocellulose material are of importance in the compatibility of composite materials. Interfacial interactions between components of the composite material are controlled to an extent by chemical composition and morphology. Surface modification is covalently bonded through esterification. Esterification is enhanced through activation of the agents used in modification before treatment of the lignocellulose material. Inactive modifying agents result in insufficient interfacial adhesion that leads to moisture intrusion. Incompatibility results in decreased mechanical properties of particleboards due to poor stress transition from one material to the other. Interfacial adhesion and chemical approaches are enhanced by addition of coupling agents such as alkali treatment, esterification, and use of coupling materials. Coupling agents specifically create significant bonds between the lignocellulose material and other polymer matrices. Coupling agents include a nucleophile-containing molecule.
Lignocellulose materials such as crop residues are composed of lignin, cellulose, and hemicellulose. These components contain hydroxyl functional groups. Hydroxyl groups determine the physical and chemical dimensions of a material and are chemically modified to carboxylate and aldehyde groups. Aldehydes such as formaldehyde are currently used as a component of resins used as a binder. The carboxylate group is the major functional group utilized during the esterification process. The carboxylate group is formed by oxidation of the hydroxyl group using oxidizing agents [
Hydroperoxy anion reacts with undissociated H2O2 to form highly reactive hydroxyl radicals (OH) and superoxide (
Hydrogen peroxide under alkaline media therefore produces peroxide anion that cleaves the side chains of lignin and opens the benzene ring to form oxalic acid, formic acid, and malonic acid [
Cleavage and oxidation of the lignin material in the lignocellulose material using perhydroxyl anion.
Hydroxyl groups in cellulose are converted to carboxylic acid and aldehydes [
Oxidation of cellulose using hydrogen peroxide in alkaline media.
Cleavage of the lignocellulose material with an oxidizing agent leads to synthesis of a mixture of functional groups such as carboxylate and aldehyde. Hydroxyl groups combine through etherification. A hydroxyl group reacts with the carboxylic group through condensation polymerization to form a covalent bond through esterification. Research has shown that chemical treatment of cellulose fibres and lignin enhances the binding efficiency of natural fibres and lignin compounds into the starch composite matrix [
Bio-based adhesives are an alternative to formaldehyde-based adhesives to use as binders in particleboard formulation [
Starch abundance has diverted its use from food to nonfood applications [
Effects of sodium hydroxide on changing the starch morphology by breaking intra- and intermolecular hydrogen bonding.
Copolymerization of natural polymers from crops emerges as a solution to the high cost of biodegradable materials [
Biopolymers derived from renewable resources are an emerging class of advanced materials with a wide range of industrial applications [
Possible interaction mechanism between hydrolyzed starch and oxidized lignocellulose materials [
The starch-lignin matrix showed reduction in enthalpy of combustion [
Maize stalks were collected at random from a farm in Nakuru County (0°18′S, 36°4′E), sugarcane bagasse from Kisumu County (0°5′S, 34°46′E), and rice husks from Mwea (0°37′S, 37°20′E), Kirinyaga County, which are all counties in Kenya. Maize stalk was cut into small sizes less than 20 mm in width using a chuff cutter and ground using a mill crusher to particle size less than 10 mm. Sugarcane bagasse which was obtained from the sugar factory was not ground further. Rice husks were ground using the mill to particle size less than 10 mm. All the three plant residues were dried in an oven at 105°C, to a constant mass. Cassava was obtained at random from Thika, Kiambu County. It was peeled, chopped into smaller pieces, and sun-dried to a constant mass. The dried cassava was ground using a maize mill to 100 mesh size. The ground cassava starch was then placed in an oven at 105°C to a constant mass. Dried cassava was pulverized to 150 mesh size.
Lignin mass was determined through the Klason method [
Formulation of particleboards was adopted from a method used for starch-lignin polymer preparation [
Physical (density, moisture content (MC), water absorption (WA), and thickness swelling (TS)) and mechanical (internal bonding (IB), modulus of rupture (MOR), and modulus of elasticity (MOE)) tests are important specifications for particleboards. They were measured for each formulated particleboard. Data were analyzed using Excel through data analysis (ANOVA: single factor) at the 95% confidence level (
Ten millilitres of 0.0304 M potassium ferricyanide reagent were pipetted into clean 100 ml Erlenmeyer flasks. 5.0 ml of 2.5 M sodium hydroxide was then added and mixed thoroughly while heating. Five drops of the dilute methylene blue indicator were added, and the resulting mixture was titrated with 2.0 ml starch till the indicator changed to colourless solution. The titre readings were noted. Titration was repeated three times for each sample till concurrent titre values were obtained [
Physical tests for density, moisture content, water absorption, and thickness swelling were done. Mechanical tests for internal bonding, modulus of rupture, and modulus of elasticity were done. All tests were done using the standard test methods for evaluating properties of wood-based fibres and particle panel materials (ASTM D 1037-99). Raw materials and formulated boards were subjected to chemical analysis [
Density of the particleboards was determined as a quotient of the difference between air-dried weight and air-dried volume:
Dimensions were taken with a micrometer screw, and weight was measured using a Mettler AJ150 analytical balance.
The moisture content was obtained by determining the mass of the boards before oven-drying and after oven-drying:
Water absorption and thickness swelling tests were done according to the ASTM standard method (ASTMD1037). Test pieces, 300 mm × 50 mm × 2 mm, were cut and immersed in distilled water in a glass vessel and maintained at a temperature of 23°C for every 2 hours and 24-hour interval. The weight and thickness of the samples were measured before and immediately after soaking within the 10-minute interval and used to calculate water absorption and thickness swelling.
Internal bond strength (IB) measurements were performed on 5 cm × 5 cm square probes at a crosshead speed of 1.33 mm/min. IB was calculated using the following equation:
Particleboards formulated were cut into various specimens according to the ASTM standard method (ASTM D1037-99). The rectangular 50 mm wide, 275 mm long, and 20 mm thick pieces were cut from each full particleboard. The pieces were used for thickness swelling and three-point-flex measurement of MOR and MOE, respectively. The 5.1 cm × 5.1 cm square pieces were used for IB measurement. The mechanical properties were determined using an Instron testing machine (model 1122; Instron Corporation, Canton, MA) with the speed of the movable crosshead of 4 mm/min for the TS test and 5 mm/min for three-point-flex and IB tests. The reported value is the average of triplicate measurements.
Fourier transform infrared (FTIR) spectroscopy was used to characterize the type of functional groups existing in the raw materials and formulated particleboards. Pellets were prepared by mixing 5 mg of powder of each sample type with 95 mg of finely ground potassium bromide (KBr) and pressing the mixture into pellets that are approximately 1 mm in thickness. Particleboards were maintained at 102°C for 24 hours in an oven. Materials were characterized by the attenuated total reflectance (ATR) mode using the potassium bromide (KBr) disk (model Alpha 1005 4238 from Bruker instrument). Each spectrum was recorded from 4000 to 400 cm−1.
Results of the lignin content in the three lignocellulose materials are shown in Figure
Lignin content in crop residues.
The lignin content in sugarcane bagasse was more than that in rice husks and maize stalk. Maize stalk had the lowest percentage of lignin content among the three lignocellulose materials. This is within the range of 10 to 25 percent of the lignin content found in wood, a conventional lignocellulose material used in formulation of particleboards [
One-dimensional ANOVA using the Tukey method for density, moisture content (MC), thickness swelling (TS), water absorption (WA), internal bonding (IB), modulus of rupture (MOR), and modulus of elasticity (MOE) is presented in Table
Average values of physical and mechanical properties of particleboards formulated.
Boards | Physical properties | Mechanical properties | |||||
---|---|---|---|---|---|---|---|
Density (g/cm3) | MC (%) | WA (%) | TS | IB | MOR | MOE | |
PBR/HS | 0.612a | 9.587a | 76.33a | 19.32a | 1.613a | 13.630a | 2651.61a |
PBR/DS | 0.623a | 9.700b | 74.67a | 20.63b | 1.623a | 13.553a | 2875.67b |
PBR/OS | 0.622a | 9.593c | 64.67b | 18.59c | 1.657b | 13.770b | 2806.67c |
PBR/U/OS | 0.621a | 9.487d | 68.61c | 19.12d | 1.710c | 14.323c | 2891.67d |
|
|||||||
PBM/HS | 0.604a | 9.577a | 81.33a | 22.77a | 2.220a | 14.380a | 2508.34a |
PBM/DS | 0.622b | 9.843b | 83.87a | 23.43b | 2.367b | 13.960b | 2364.20b |
PBM/OS | 0.621c | 9.763c | 72.67b | 19.31c | 2.343c | 14.720c | 2672.27c |
PBM/U/OS | 0.615a | 9.593d | 78.68c | 19.44d | 2.370bd | 14.830d | 2716.57d |
|
|||||||
PBS/HS | 0.608a | 9.543a | 66.67a | 18.94a | 1.720a | 13.880a | 3175.71a |
PBS/DS | 0.627b | 9.770b | 69.27b | 19.88a | 1.677a | 13.630b | 2907.68b |
PBS/OS | 0.619ab | 9.567a | 61.33c | 18.23b | 1.820b | 14.134c | 3229.08c |
PBS/U/OS | 0.611ab | 9.553c | 71.04d | 18.46c | 1.950c | 14.320d | 3329.93d |
Same letters show no significant difference, while different letters show significant difference from each other in the Tukey test at the 5% significance level.
The average densities of particleboards are shown in Figure
Average densities of particleboards and their standard deviation as error bars.
Statistically, no significant difference was observed between the particleboards formulated using rice husks (Table
Particleboards formulated using maize stalk and sugarcane bagasse showed a significant difference, as shown in Table
This minimizes the interaction between lignocellulose materials and the modified starch. These result in formation of voids that increase the volume of the composite material, thus reducing the density of the particleboards.
Figure
Moisture content of particleboards.
Moisture influences the density of particleboard as it serves as a means of heat transfer. Heat transfer from composite mat faces to the composite mat core helps in the curing of the binder [
Aldehydes and carboxylic groups are used during esterification. The hydroxyl groups are therefore reduced in the process of particleboard formulation. Hydroxyl groups transformed into aldehydes and carboxylic groups are used in formation of bondage between the modified starch and the lignocellulose material. Rice husks bonded with oxidized starch and urea showed the lowest moisture content. Sodium hydroxide used reacts with silica to form silicic acid. Silicic acid is an inorganic adhesive that undergoes condensation by cross-linking and dehydration to yield less hydrated silicon dioxide phases. Porosity of the particleboard made from maize stalk is higher than that of the particleboard made from sugarcane bagasse and rice husks [
Water absorption of the particleboards is shown in Figure
Water absorption (WA) of synthesized particleboards.
Particleboards formulated with maize stalk and dextrinized starch as a binder had the highest water absorption of 83.87%. Maize stalk contains high content of cellulose and hemicellulose materials. Cellulose and hemicellulose contain high hydroxyl groups. Hydroxyl groups influence the physical properties of the composite materials formulated. Hydroxyl groups form hydrogen bonding with water, thus absorbing a lot of water. Particleboard with a high lignin content of 21.5% showed reduction in water absorption. Lignin is a hydrophobic material and reduces the WA of particleboards [
Moisture movement was restricted within the matrix. Water moves easily into large cavities of maize stalk than into small cavities of sugarcane bagasse [
Particleboards formulated showed thickness swelling as in Figure
Thickness swelling (TS) of the particleboards.
Thickness swelling showed a direct relationship to the results obtained on water absorption. Particleboards made from maize stalks bound with dextrinized starch showed the highest TS of 23.43%. This is attributed to the presence of the cellulose material in maize stalk. PBM/DS showed the highest thickness swelling of 27.08%, and PBS/OS showed the least at 18.23 and PBR/HS at 19.32% and 25.59%, respectively.
Particleboards based on the ASTM D 1037-99(1999) standard should have a maximum thickness swelling value of 25% for general uses [
Chemical analysis for lignocellulose materials and cassava starch.
Na | Zn | Ca | Mg | |
---|---|---|---|---|
Rice husk | 0.001 ± 0.001 | 0.005 ± 0.001 | 1.029 ± 0.012 | 0.001 ± 1.841 |
Maize stalk | 0.002 ± 0.001 | 0.055 ± 0.001 | 8.021 ± 0.540 | 17.085 ± 2.842 |
Bagasse | 0.002 ± 0.003 | 0.012 ± 0.001 | 3.893 ± 0.350 | 12.862 ± 2.877 |
Cassava starch | 0.003 ± 0.002 | 0.038 ± 0.021 | 10.087 ± 0.321 | 0.001 ± 0.217 |
Sodium ions (Na+) were low in both lignocellulose materials and starch. Sodium enables the curing process of the cassava adhesive. Treatment of the starch with sodium hydroxide increased the concentration of the sodium ions. Zinc is involved in the synthesis and breakdown of carbohydrates. Chemical additives containing zinc and magnesium influence the bonding strength of the particleboards [
FTIR spectra for cassava starch, rice husks, sugarcane bagasse, and PBM/OS board.
A major peak was found at 3414.06 cm−1 which was attributed to the presence of intramolecular hydrogen-bonded hydroxyl groups in single bridge compounds. Another peak was observed at 1626.98 cm−1 which was for –OH bending. The peaks at 2484.36 cm−1 and 1362.73 cm−1 corresponded to C-H bending. A peak was observed at wavenumber 1229.64 cm−1 that corresponds to C=O vibration and at 1102.34 cm−1 that corresponds to C-O stretching. There were major functional group transformations from rice husks and gelatinized cassava starch, leading to reduction in the peak size at wavenumber 3414.06 cm−1, -OH stretching, and increase in 2484.36 cm−1 for C-O. Reduction in the OH peak was attributed to formation of esters and hence the formation of C-O in C-O-C. The –OH spectra that remained can be attributed to the presence of sodium hydroxide used in the pretreatment of the lignocellulose matrix, water formed through condensation polymerization, and also boric acid formed during hydrolysis of borax in sodium hydroxide. The C-H peak increased because of the addition of C-H functional groups from both cassava and lignocellulose matrices. Peaks observed at 1150 cm−1 and 950 cm−1 were attributed to B-O-H because of hydrolysis of borax.
IB of the particleboards formulated using rice husks, maize stalk, and sugarcane bagasse.
Particleboard formulated with maize stalk and dextrinized starch showed the highest internal bonding of 2.367% N/mm2. Maize stalk contains low percentage of lignin and high content of cellulose and hemicellulose. The hemicellulose content increases the fibre joint strength. The bonding of fibres is described by the bonded area and the bonding strength between fibres. The high density of particleboards made from maize stalk and dextrinized starch increases fibre-to-fibre bonding strength. Density results in the improvement of fibre-to-fibre bonding [
Particleboards formulated from rice husks showed the lowest internal bonding of 1.613 N/mm2. This is attributed to low hemicellulose content in the rice husks that reinforce the bondage in composite materials. The lignin content in rice husks was higher than that in maize stalk. This implies that the cellulose content is lower in rice husks. Reduction in the cellulose and hemicellulose content reduced the amount of hydroxyl groups that influences cohesion between components of the particleboard.
Particleboards met minimum requirements of IB between 0.24 and 0.4 N/mm2 for both general use and furniture manufacturing based on the European standard EN 312:2010 [
MOR of the three particleboards formulated in this study.
Particleboard is a random blend of lignocellulose materials that require no orientation during board formulation. In this study, composite materials were put in a mold with no bias towards the forming direction expected. Tests for the MOR and MOE properties were taken parallel and perpendicular to the forming directions. Analysis of the test data revealed a significant difference between samples from the parallel and perpendicular directions. Particleboards from maize stalk showed the highest MOR of more than 13.96 N/mm2. MOR is influenced by the cross-linking of the lignocellulose materials with the binder and the particle geometry. Maize stalk contains cellulose and hemicellulose that provide hydroxyl groups that underwent esterification with carboxylic groups from oxidized starch. Esterification leads to cross-linking of starch and lignocellulose materials, hence increasing the MOR. A covalent bond is formed during the esterification process which is a strong bond. Reddy and Yang [
Low-density particleboards formulated exceeded the minimum requirements of 2.8 N/mm2 for interior applications based on ASNI 208.2-2009. Rice husks contain very high percentages of silica. Silica reacts with sodium hydroxide to form sodium silicate which is an adhesive [
MOE of the three formulated particleboards.
Particleboards made from sugarcane bagasse had the highest MOE of 3175.71 N/mm2, those from rice husks had 2647.23 N/mm2, and those from maize stalk had 2508.34 N/mm2. All low-density boards attained the MOE of 1241 N/mm2 based on ASNI 208.2-2009 for interior applications. Trends in MOE are similar to those of MOR where there is no significant difference between particleboards formulated from rice husks. In addition, there was no significant difference among particleboards formulated with maize stalk and sugarcane bagasse. Low-density particleboards formulated exceeded the minimum MOR of 500 N/mm2.
Chemical treatment of the lignocellulose material and starch led to the formation of essential functional groups used in esterification. This resulted in compatibility of lignocellulose materials and starch materials used in formulation of composite materials. Composite materials were molded to form the particleboard. The particleboard formulated showed high water absorption and thickness swelling. The results showed that mechanical properties of the formulated low-density particleboards exceeded the requirements of American Society for Testing and Materials (ANSI 208.1 1999). Furthermore, all physical properties, except water absorption and thickness swelling, met the ANSI 208.1 1999 standards. Water absorption and thickness swelling were due to a high number of hydroxyl groups. Hydroxyl groups formed hydrogen bonding with water, making the composite material interact with water. Reduction of hydroxyl groups through chemical modification resulted in introduction of carboxylate groups that formed a covalent bond between the lignocellulose material and the starch binder. Particleboards formulated showed characteristics of low density. From this study, it was apparent that rice husks, sugarcane bagasse, and maize stalk formulate low-density particleboards for interior housing applications.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The authors acknowledge the immense support from the University of Embu for availing their chemistry laboratory for research; the Ministry of Infrastructure and Roads, where most physical and mechanical properties were investigated; the Ministry of Geology and Mines for XRF analysis; and Kenyatta University, where FTIR spectroscopy analysis was performed.