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Manufacture of non-resin wheat straw fibreboards
i n d u s t r i a l c r o p s a n d p r o d u c t s 2 9 ( 2 0 0 9 ) 437–445
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Manufacture of non-resin wheat straw fibreboards Sören Halvarsson a,b,∗ , Håkan Edlund a , Magnus Norgren a a
Department of Natural Sciences, Fibre Science and Communication Network (FSCN), Mid Sweden University, SE-851 70 Sundsvall, Sweden b Metso Panelboard AB, Department of Research, Technology and Development (RTD), SE-851 50 Sundsvall, Sweden
a r t i c l e
i n f o
a b s t r a c t
Article history:
Wheat straw was used as raw material in the production of fibreboards. The size-reduced
Received 8 May 2008
straw was pretreated with steam, hot water and sulphuric acid before the defibration pro-
Received in revised form
cess to loosen its physical structure and reduce the pH. No synthetic binder was added.
22 July 2008
Adhesive bonding between fibres was initiated by activation of the fibre surfaces by an
Accepted 25 August 2008
oxidative treatment during the defibration process. Fenton’s reagent (ferrous chloride and hydrogen peroxide) was added. Two different levels of hydrogen peroxide (H2 O2 ), 2.5% or 4.0% were used. The resulting fibres were characterized in terms of fibre length distribution,
Keywords:
shive content, pH and pH-buffering capacity. The properties of finished fibreboards were
Wheat straw
compared with medium-density fibreboard (MDF) with density above 800 kg/m3 produced
Non-resin
from straw and melamine modified UF resin. The modulus of rupture (MOR), modulus of
Non-wood fibres
elasticity (MOE) and internal bond (IB) were lower than those of conventional manufactured
Peroxide
wheat straw fibreboards but close to the requirements of the MDF standard (EN 622-5: 2006).
MDF
The water absorption properties for the H2 O2 activated straw fibreboards were relatively
HDF
high, but were reduced by 25% with the addition of CaCl2 into the defibrator system as a
UF-resin
water-repelling agent. Increased levels of hydrogen peroxide improved the mechanical and
MUF-resin
physical properties of the straw fibreboard. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.
Refining Mechanical properties
1.
Introduction
The manufacture of medium-density fibreboard (MDF) uses wood as a raw material, particularly softwood, hardwood and mixtures of different wood species. However, the shortage of wood, forestry regulations, and the presumed lower cost of non-wood materials have encouraged board makers all over the world to research alternative sources of lignocellulose fibre. This study examines wheat straw, an agricultural waste material. The processing of straw differs from that of wood in the first part of the MDF process. The harvested and baled wheat
straw is size-reduced (chopped), hammer-milled, screened, and pre-wetted before defibration (the fibre-refining process). The subsequent processing steps are similar to those in conventional wood-based systems and involve resination, drying, mat-forming, pre-pressing and, finally, hot pressing. During hot pressing a synthetic resin binder (adhesive) is usually added to glue the fibres together to form a composite material. For several decades the adhesives used for the manufacture of MDF and high-density fibreboard (HDF) have been formaldehyde-based resins such as urea–formaldehyde (UF), urea–melamine–formaldehyde (UMF), and phenol–formaldehyde (PF). The vast quantity of
∗ Corresponding author at: Department of Natural Sciences, Fibre Science and Communication Network (FSCN), Mid Sweden University, SE-851 70 Sundsvall, Sweden. E-mail address: [email protected] (S. Halvarsson). 0926-6690/$ – see front matter. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.indcrop.2008.08.007
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formaldehyde consumed has raised concerns about the need to use more environment friendly chemicals. Removing the synthetic resin from the MDF process without impairing the mechanical properties or quality of the finished panels represents a great challenge, for the adhesive interactions between the fibres will undoubtedly be reduced. However, one possible way to improve the adhesion is to chemically activate the fibre surface by oxidation. Binding between and within the fibres can then be promoted during hot pressing by activated (reactive) components that are part of the lignocellulose material (Back, 1991; Bouajila et al., 2005). Paraffin wax or wax emulsions are added in small quantities (approximately 0.5–1.0%) to improve the poor water resistance of MDF. Fibreboards made using annual plant materials and agricultural waste have even worse water-resistant properties than wood (Sauter, 1996; Markessini et al., 1997; Han, 2001; Mantanis and Berns, 2001; Wasylciw, 2001; Ye et al., 2007). Another way to improve the hydrophobic properties of the fibre is to add small quantities of salts containing di- or trivalent cations (Westin et al., 2001). The most frequently used salt in the papermaking industry is aluminum sulphate. Below pH 9, the cations are primarily adsorbed to the pulp fibres by electrostatic interactions with the carboxyl groups in the lignocellulose material. The electrostatic interactions result in adsorption of small species or colloids on the fibre surface, altering the surface properties of the fibres (Ohman and Wågberg, 1997; Kato et al., 2000). Improved swelling-resistance in wood-based fibreboards has been reported after the addition of CaCl2 (Westin et al., 2001; Widsten, 2002). Wood-based binderless fibreboards without synthetic resin binders have been produced for at least 80 years. One of the early methods of producing these fibreboards was wet processing, generally known as the Masonite process (Mason, 1927). In this wet-process, wood fibres are generated by a steam explosion and activated by chemical reactions. During hot pressing self-bonding of the fibres and softening of the lignin contributes to the formation of high-performance fibreboards. The drawbacks of this type of fibreboard (hardboard) processing include high water consumption, the dark colour of the boards and the relatively long pressing times. Today hardboard (HB) is manufactured in a dry process very similar to the MDF-process and adhesive is normally used. A number of papers deal with binderless or self-bonding pulps produced using steam-exploded or thermo-mechanical processes (Suzuki et al., 1998; Anglés et al., 1999; Velásquez et al., 2003). Spent sulphite liquor (SSL) activated by hydrogen peroxide has also been tested as an adhesive to wood-based particle boards (Pizzi, 1983). However, their results are difficult to correlate with those from the present study because of differences in the processing of the raw materials and in the chemicals added. The binderless manufacturing of straw fibreboards involves the addition of an oxidative reactant. The oxidative-activated fibre surface and the low molecular degradation components are thought to create chemical bonds between activated fibres during hot pressing. The formation of covalent bonds between the lignocellulose polymers results in intermolecular forces that are much stronger than those created by hydrogen bonds (Back, 1991).
In this study, Fenton’s reagent was introduced into the defibration process to decompose the added hydrogen peroxide and induce improved intra- and inter-fibre interactions (Widsten, 2002). The decomposition of hydrogen peroxide by catalytic reactions of ferrous ions generates several types of reactive radicals, as shown in the following formulae: Fe2+ + H2 O2 → Fe3+ + OH− + HO
•
(a)
•
Fe3+ + H2 O2 → HO2 + H+ + Fe2+
(b)
•
HO + Fe2+ → OH− + Fe3+
(c)
•
HO2 + Fe3+ → O2 + H+ + Fe2+ •
H2 O2 + HO → H2 O + HO2
(d)
•
(e) •
The created hydroxyl radicals (HO ) continue to react and attack lignin and carbohydrates (RH), resulting in activated • lignocellulosic components (R ), as shown in formula (f). Cleavage of polymer chains and oxidation of components resulting from low molecular degradation are also possible: •
RH + HO → H2 O + R
•
(f)
Perhaps the most problematic side-reaction is the decomposing of hydrogen peroxide and the formation of oxygen gas, as shown in formula (g). Hydrogen peroxide can auto-decompose in aqueous solutions, and the rate of decomposition can accelerate upon contact with mineral surfaces. 2H2 O2 → H2 O + O2 (g)
(g)
Degraded low-molecular reagents not only oxidize the lignocellulose fibre surface but can also diffuse into the lumen and penetrate the pores in the cell walls, reacting in areas within the fibre cell walls. The bonding capacity between adjacent fibres for this type of reaction inside the fibres (intrafibre bonding) is low. However, the intra-fibre cross-links are thought to improve the resistance of the finished fibreboards to water swelling and increase the dimensional stability of the fibres (Back, 1991). Degradation components of higher molecular mass may have difficulty in penetrating the cell wall due to restrictions in pore size. The macromolecular lignocellulosic degradation compounds are predominately adsorbed on the fibre surfaces and probably contribute to inter-fibre bonding (Back, 1987). The aim of this study was to evaluate the suitability of combining a non-wood resource (wheat straw) and a binderless system to produce fibreboards on a sound environmental and economic basis. Instead of commercial resins, Fenton’s reagent and hydrogen peroxide were used in the defibration process to generate reactive groups in the wheat straw material in order to improve fibre adhesion and cross-linking during hot pressing of the fibreboards.
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Table 1 – Processing steps in two different straw fibreboard processes: the conventional dry fibreboard process (MDF-process) for straw and the dry fibreboard process without addition of synthetic resin, but with the addition of hydrogen peroxide. Proc. steps
Manufacturing of wheat straw MDF/HDF (SMDF)
1 2 3
Size reduction of straw Wetting/acid pre-treatment Pressurized refining addition of UMF resin addition of wax
4 5 6 7
Drying of fiber (tube flash drying) Forming of resinated fiber Pre-pressing Pressing
2.
Materials and methods
2.1.
Wheat straw substrate
The wheat straw (Triticum aestivium L.) was grown and harvested in Uppsala province, Sweden. The wheat straw (WS) was cut to about 30 cm in length, dried in the field, and finally baled. The moisture content of the baled WS was approximately 15%. The full-length straw material was chopped to 25–50 mm in length, after which it was subjected to a low-energy hammer milling and screening process to remove dust and small straw particles. The size-reduction, hammer milling, and de-dusting were conducted at the Förderanlagen GmbH KG (FMW) facility in Kirchstetten, Austria.
2.2.
Additives and chemicals
The experiments were carried out using hydrogen peroxide of puriss grade (Labservice, Sundsvall, Sweden) supplied at a concentration of 35%. The hydrogen peroxide was diluted and added to the blowline to react with and activate the fibre surface. The hydrogen peroxide reaction was catalyzed by metal ions and ferrous sulphate (FeSO4 ·7H2 O) of pro-analysis grade (Labservice, Sundsvall, Sweden). The size-reduced wheat straw was pretreated to increase its moisture content and temperature and to reduce the pH of the straw material. Diluted sulphuric acid (10%) of analysis grade (Labservice, Sundsvall, Sweden) was added to the hot water, steam, and wheat straw. An aqueous solution of calcium chloride, CaCl2 ·6H2 O, proanalysis grade (Labservice, Sundsvall, Sweden), was added to decrease the hydrophilicity of the fibre surfaces and to improve the water-repellant properties of the finished fibreboards. The reference MDF panels at densities higher than 800 kg/m3 were produced using wheat straw fibres and a commercial melamine-modified formaldehyde resin (UMF) supplied by Dynea (Prefere 11G23). Ammonium chloride was added to the UMF resins as a hardener (1.0%) and hexamethylenetetraamine (0.2%) was added as a retarder. The target content of the added UMF resin was 14% on an oven-dry basis (db). A wax emulsion supplied by Emutech AB, Sweden (Boardwax B100) was added to the fibre as a conventional hydrophobic additive.
2.3.
Manufacturing of peroxide wheat straw fibreboard Size reduction of straw Wetting/acid pre-treatment Pressurized refining addition of ferrous sulphate Addition of hydrogen peroxide Addition of calcium chloride Drying of fiber (tube flash drying) Forming of peroxide treated fiber Pre-pressing Pressing
Experimental
The pilot-plant manufacture of fibreboard based on wheat straw involves seven major processes (Table 1). Except for the use of hydrogen peroxide, these processes are much the same as the Mid Sweden University & Metso method of manufacturing straw MDF and HDF (Halvarsson et al., 2008). Chemicals such as hydrogen peroxide, co-reactants and calcium chloride (CaCl2 ) were added to the DefibratorTM system instead of using a binder such as MUF resin.
2.3.1.
Experimental layout
The actual experimental layout and processing was performed over 2 days. Steam, hot water and sulphuric acid were added in the pretreatment of the size-reduced straw material. This pretreatment was performed the day before the refining and pressing of the non-resin straw fibre material. The refining, forming and pressing of the activated fibres were divided into four major trials denoted RA, RB, and RC (Table 2). Trial RD was only used to produce fibres for evaluation of the fibre properties and was produced without the addition of chemicals during refining. The reference straw MDF/HDF (SMDF) in trial RE had been produced in an earlier investigation (Halvarsson et al., 2008).
2.3.2.
Acid pretreatment of wheat straw
Sulphuric acid was added to reach a target level of 0.75% (db) based on dry wheat straw. Steam and water were used to increase the moisture content and temperature. A short conveyor belt was used for feeding and levelling the wheat straw material flow. A mixer screw was connected to the conveyor and the sulphuric acid solution was sprayed on the straw material at the inlet of the mixer screw. Hot water and steam were injected into the mixer screw to increase the temperature of the straw material to a range of 70–90 ◦ C. The moisture content (MC) was increased and the pH of the straw mixture was decreased to approximately 4–5. The production rate was approximately 120 kg/h and the MC level was set to 100%. A slightly higher MC level of the pretreated straw was obtained (MC = 107%).
2.3.3. Refining in the pilot-plant laboratory defibrator (OHP 20) The acid and water/steam pre-treated wheat straw material mix was fed into a horizontal preheater and refined in a pilot
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Table 2 – Amount of added chemicals, wax, and melamine modified urea–formaldehyde UMF-resin for the different straw fibreboard trials (RA–RE) based on dry wheat straw. Trial
Hydrogen peroxide (%)
RA RB RC RD RE
Ferrous sulphate (%)
2.5 4 4 – –
Calcium chloride (%)
Wax addition (%)
UMF resin addition (%)
2 2 – – –
– – – – 1
– – – – 14.5
1 1 1 – –
Table 3 – The pH and pH-buffering capacity of straw and straw fibres samples after different chemical treatments and the addition of chemicals based on dry wheat straw. Property
Unit
Straw
Straw
Straw pretreatment pH pH-buffering capacity
– – ml
Water 7.8 14.5
Addition of chemical H2 SO4 H2 O2 CaCl2
% % %
– – –
plant pressurized single-disc refiner, type OHP 20” Defibrator (Metso Panelboard, Sundsvall, Sweden) fitted with a horizontal pre-heater. Refining was performed at a rotational speed of 1500 rpm and a pressure of 0.7 MPa. The retention time in the defibrator system was set to 3 min. The refined fibres were vented from the refining house into the blowline. Chemicals were injected into the defibrator system at separate injection points.
2.3.4.
Fibremat forming and pre-pressing
The produced fibre materials were later prepared and formed in a 500 mm × 600 mm forming box PendistorTM (Metso Panelboard AB, Sundsvall, Sweden). The subsequent pre-pressing was performed in a cold daylight press at 1.0 MPa for 60 s.
2.3.5.
Pressing of fibreboards
The pressing cycle was guided by in-house experience of pressing fibre materials from annual plants. The pressing time is one of the critical parameters for production capacity in fibreboard manufacture. The final fibreboard thickness was set to 6 mm and the pressing times were approximately 90 s, or a press factor of 15 s/mm. The temperature of the press plate was set to 200 ◦ C. Thus the resulting maximum temperature in the core of the fibremat during pressing was approximately 110–115 ◦ C. One inconvenience when pressing straw materials is the long de-gassing time necessary to avoid delamination of the pressed panels. To overcome this obstacle, an extra feature was added to the press heating system: Oil at 60–80 ◦ C was circulated through the press plates for cooling. This arrangement reduced the build-up of steam pressure inside the fibreboard at the end of the press cycle. The cooling time was one-third of the total pressing cycle and is included in the total pressing time. The applied pressure at the beginning of pressing was set to 0.5 MPa for 10–15 s to give the panel high surface density. The pressure was then reduced to approximately 0.05 MPa to
RA
RB
RC
RD
Acid 4.7 8.2
Acid 3.3 2.8
Acid 3.1 1.8
Acid 3.1 2.0
Water 5.7 8.5
0.75 – –
0.75 2.5 2
0.75 4 2
0.75 4 0
– – –
adjust the core density. Finally, the pressure was reduced to zero before opening the press.
2.4.
Evaluation of straw fibre and fibreboards
2.4.1.
Straw and fibre properties
The pH and the pH-buffering capacity of the size-reduced straw material were used to characterize the fibreboard after pretreatment. These properties were measured using a modified method suggested by Johns and Niazi (1980). The straw and fibre samples were diluted in water at room temperature. The pH-buffering capacity data in Table 3 is expressed as the added amount of 0.1 M H2 SO4 titration solution required to reach a pH of 3.0. The fibre length and the size of the refined and dried fibres were measured by image analysis using a laser-based PQMTM 1000 pulp quality monitoring system (Metso Paper, Sundsvall, Sweden). The average fibre length, the fibre length distribution, and other properties related to fibre quality were determined (Table 4). The amount of short fibres and dust particles were evaluated for each trial. The oversized fibre bundles
Table 4 – Properties of samples of straw fibre from trials RA to RD, measured in PQMTM 1000 fibre classifier and Pulmac Instruments, 0.15 mm slot. Property
Unit
RA
RB
RC
RD
Average length Average width Curl index Coarseness Shive weight Short fibres <1.45 mm Dust <0.45 mm Pulmac 0.15 mm
mm m % mg/m % % % %
1.1 25 7 0.34 15 68 21 17
1.1 26 7 0.36 18 68 21 19
1.1 25 7 0.40 17 69 23 20
1.1 26 7 0.35 19 65 21 24
Average values calculated for two separate fibre samples.
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are defined as shives and the shive content was determined by the PQM 1000 instrument and the Pulmac shive analyzer with 0.15 mm slots (Pulmac Instruments International, Vermont, Canada).
2.4.2.
Mechanical properties of fibreboards
Straw fibreboards were cut into 50 mm × 50 mm pieces to determine the internal bond (IB) properties and density profiles. Bending strength, modulus of rupture (MOR) and modulus of elasticity (MOE) were measured on 4 cm × 32 cm samples. The mechanical properties (IB, MOR and MOE) were determined according to EN standard methods (EN 310: 1993; EN 319: 1993) in an Alvetron Model Tc10 testing instrument (Lorentzen and Wettre, Sweden). The pressed panels were stored for one week at room temperature after pressing. Before testing, the specimens were conditioned in a room for 48 h at 65% relative humidity and a temperature of 20 ◦ C.
2.4.3. Thickness swelling and water adsorption of fibreboards The thickness swelling (TS) and water adsorption (WABS) properties were determined on 50 mm × 50 mm test pieces according to EN standard EN-317: 1993. The fibreboard specimens were immersed vertically in water for 24 h to determine their thickness and weight.
2.4.4.
Vertical density profile (VDP)
The density as a function of thickness, or the vertical density profile (VDP), was measured in a Grecon instrument (Laboratory Density Analyzer DA-X, Alfed, Germany).
3.
Results and discussion
3.1.
Wheat straw fibre properties
The pH and a modified pH-buffering capacity analysis performed after the pretreatment of the straw and after defibration of fibres from each trial (RA, RB, RC, and RD) are presented in (Table 3). Different loadings of hydrogen peroxide during defibration influenced the pH and the buffering capacity. The pH buffering capacity is presented as the amount of titration solution (ml of 0.1N H2 SO4 ) required to reduce the pH of straw and fibre samples to a level of pH 3.0. The pH of the wetted straw reference sample was 7.8. Acid treatment of the straw reduced the pH to 4.7. The wetted straw reference sample required 14.5 ml of titration solution, while the acid pre-treated straw required 8.3 ml of titration solution. The acid treatment was found to be effective in reducing the pH and the buffering capacity of the straw specimens. The hydrogen peroxide treatment of fibre during defibration produced an additional reduction in the pH. The analysis showed a pH level very close to pH 3 and consequently a minor amount of titration solution was required to reach the required value. The lowest pH value (pH 3.2) and the least amount of titration solution (1.8 ml) were observed after acid pre-treatment of the straw in combination with the addition of 4.0% hydrogen peroxide to the processed fibres (trials RB and RC).
441
An interesting observation was that after defibration and drying the pH value of the untreated straw fibre (trial RD) decreased from pH 7.8 to 5.7. This drop is probably due to the generation of carboxyl groups of the lignocellulose material. The type of raw material, defibration pressure, and retention time influence the final pH and buffering capacity of the produced fibres (Johns and Niazi, 1980; Widsten et al., 2002; Holmbom et al., 2005). Samples of refined wheat straw fibres were analyzed in the PQM-1000 fibre classifier system. The fibre properties from all the separate trials (RA to RD) are presented in Table 4. The fibre length distribution was homogenous in all the trials. Varying the amounts of hydrogen peroxide and calcium chloride had no major effect on fibre length, width or curl. The homogenous fibre quality reflects constant defibration processing conditions throughout the process. These conditions were maintained by an even input and control of the raw material feed. The average length of the straw fibre was approximately 1.1 mm and the average width was around 25–26 m. The fibre curl index was constant (7%) and the mass per length property (coarseness) increased with higher doses of H2 O2 . The amount of oversized straw fibre bundles or shives varied slightly between the different trials. The shive weight was in the range of 15–19%. Similar results were obtained by using the Pulmac instrument (0.15 mm slot, Table 4). Defibration of straw materials will always result in high levels of dust and oversized fibre bundles compared to wood-based materials. The geometry of the shives in wheat straw differs from that in wood-based species. Straw shives have a more compact and flat structure that is related to the thin outer layer of the straw. It is assumed that the flat shives consist of a relatively high amount of fibre bundles that could yield high tensile strength. However, it is unknown whether the wheat straw shives have any dramatic effect on other mechanical properties. In Table 4 short fibre and dust are defined as the amount of fibres shorter than 1.45 and 0.45 mm respectively. The amount of dust in straw fibres is generally higher than in refined wood fibres. The obvious explanation is the additional types of plant cells in the straw structure—the parenchyma, epidermis, and vessel cells. The thin cell walls in these types of cell generate more dust due to the thermo-mechanical work during the defibration process.
3.2.
The straw fibreboard properties
After the pressing, the fibreboards manufactured seemed to be of sufficiently high quality and close to the EN standard. The best results were found with a high loading of hydrogen peroxide (4%) and densities above 1000 kg/m3 . The high-density range of produced straw fibreboards is above the typical density range of MDF and should be named high-density fibreboard (HDF). Hydrogen peroxide-activated fibreboards showed moderate mechanical properties compared to those of glued (UF resin) SMDF. The internal bond and bending properties, modulus of rupture and modulus of elasticity were acceptable. The water resistance or thickness swelling (TS) was the major problem. Even though the addition of calcium chloride improved the fibreboards’ properties, the water-swelling properties were adversely affected. None of the manufactured
442
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Fig. 1 – The internal bond of wheat straw fibreboards as a function of average density. The line represents requirements in the MDF standard EN-622-5: 2006. See Table 2 for further explanation of the figure legend.
straw fibreboards met the European wood-based MDF standard (EN 622-5: 2006). TS and water absorption (WABS) were several times higher than the requirements in the MDF standard.
3.2.1.
Internal bond
Fig. 1 shows the internal bond of the non-resin straw fibreboards as a function of the average density. The IB was below or close to the requirements in the MDF standard (EN 622-5: 2006). However, a much higher IB strength is displayed for the reference SMDF glued with a commercial UMF-resin. The reference SMDF showed excellent IB values above 1.0 MPa. The different levels of hydrogen peroxide and calcium chloride additions are shown in Table 2. The enhanced adhesive effect obtained by activation of the wheat straw fibres by hydrogen peroxide is obvious. The most probable contribution to the increased strength is the formation of covalent bonds between chemical groups on adjacent fibres surfaces. High water absorption of the finished fibreboards is observed and
could be regarded as contradictive to the suggested covalent bonding. However, it should be remembered that hydrogen peroxide oxidises the fibre material and increases the overall number of charged groups, and thus, the prerequisites for water sorption. Other adhesive effects such as electrostatic interactions between adjacent fibres might also be possible when divalent metal ion salts are added. Increasing the H2 O2 from a level of 2.5–4.0% improved the strength of the fibreboards. The internal bond increased from approximately 0.45 to around 0.65 MPa at a density of 1000 kg/m3 or higher. Finally, the addition of CaCl2 seemed to increase the IB strength (compare trials RB and RC). The IB specimens were used to measure the vertical density profiles (VDP). The density profiles of typical straw fibreboards are displayed from each trial (RA, RB and RC) and are plotted as a function of thickness in Fig. 2. Generally, the VDP of wood-based fibreboards is designed to have a high surface density and lower core density. The benefits are hard surfaces and acceptable bending properties. The core density is normally correlated to the internal bond (Xu and Winistorfer, 1995; Schulte and Früwald, 1996; Wong et al., 2000). The fibreboards related to trial RA displayed VDP variations due to inhomogeneous fibre mats and/or difficulties in the pressing. The subsequent pressing of fibreboards in trials RB and RC was improved. Their vertical density profiles displayed greater homogeneity and had roughly the same shape. A plausible explanation for this effect is the improved adhesive effect between activated straw fibres at higher loadings of hydrogen peroxide. The surface density was around 1150–1200 kg/m3 and the core density was approximately 1000 kg/m3 . In addition, the thicknesses of the fibreboards in the RB and RC trials varied between 6 and 7 mm.
3.2.2.
Straw fibreboard bending properties
Fig. 3 shows the modulus of rupture while Fig. 4 shows the modulus of elasticity as a function of average density. The
Fig. 2 – Vertical density profile of typical wheat straw fibreboards in trial RA, RB, and RC. See Table 2 for further explanation of the figure legend. (For interpretation of the references to colour in the artwork, the reader is referred to the web version of the article.)
i n d u s t r i a l c r o p s a n d p r o d u c t s 2 9 ( 2 0 0 9 ) 437–445
Fig. 3 – The modulus of rupture (MOR) of wheat straw fibreboards as a function of average density. The line represents requirements in the MDF standard EN-622-5: 2006. See Table 2 for further explanation of the figure legend.
Fig. 5 – The thickness swelling of wheat straw fibreboards as a function of average density. The line represents requirements in the MDF standard, EN-622-5: 2006. See Table 2 for further explanation of the figure legend.
3.2.3.
Fig. 4 – The modulus of elasticity (MOE) of wheat straw fibreboards as a function of average density. The line represents requirements in the MDF standard, EN-622-5: 2006. See Table 2 for further explanation of the figure legend.
different hydrogen peroxide loadings and calcium chloride additions had only minor effects on the MOR and MOE. Pressing was initially problematic and some of the fibreboards in the RA trials delaminated. In trial RA the hydrogen peroxide loading was set to 2.5%. The average density of the fibreboards was low and the numerical values of MOR and MOE were below 20 and 3000 MPa, respectively. The bending properties of conventional wood-based panels are strongly dependent on the average density (Wong et al., 2000; Shi et al., 2005). However, proper pressing conditions and modified VDP improved the bending strength of the subsequent samples. The MOR for fibreboards in both RB and RC trials was approximately 15–25 MPa, but still within the same density range, namely 900–1000 kg/m3 . The higher loading of hydrogen peroxide (4%) in trials RB and RC yielded increased densities and bending strengths. Almost all fibreboards above densities of 975 kg/m3 met the requirements in the MDF standard (EN 6225: 2006). The reference SMDF panels showed numerical MOR values above 40 MPa.
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Thickness swelling of fibreboards
Thickness swelling (TS) and water absorption (WABS) of the non-resin, non-wood fibreboards showed high water swelling levels (Figs. 5 and 6). Apart from the differing chemical composition of wood and non-wood fibres, the proportions of cellulose, hemicellulose and lignin components are different. In most cases the amounts of cellulose and lignin are higher in wood-based materials while the hemicellulose component is lower. Hemicellulose is a branched, non-crystalline polymer which has the ability to absorb water and swell (Berthold et al., 1996). Of the different wood polymers in the cell wall, the hemicelluloses swell the most and lignin the least (Lindström and Westman, 1980). Consequently the natural mixture of cellulose, lignin and hemicelluloses in wood-based fibreboards possesses a better resistance to water and water absorption than what is possible for fibreboards based on straw and other annual plant materials. The ability of attracting water to the oxidised ligno-cellulosic material will also increase and contribute to a higher water sensitivity of the non-resin straw fibreboards. The wood-based MDF-standard (EN 622-5: 2006) requires a TS level below 30% for fibreboard thicknesses between 4 and 6 mm and a TS below 17% for thicknesses between 6 and
Fig. 6 – The water absorption of wheat straw fibreboards as a function of average density. See Table 2 for further explanation of the figure legend.
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9 mm. Fibreboards treated with CaCl2 and 4% hydrogen peroxide (trial RB) showed approximately 80–100% swelling in the density range 950–1000 kg/m3 . The WABS of produced straw fibreboards was estimated at 110–130%. The water-repelling properties improved at higher densities. Increased loadings of hydrogen peroxide and the addition of calcium chloride reduced the thickness swelling by approximately 25%. The lowest thickness swelling and water absorption properties were achieved with the reference SMDF composed of wheat straw, UMF resin and wax.
4.
Conclusion
Wheat straw fibreboards were produced in the dry board process from agricultural waste without the addition of synthetic resin. To achieve the required fibreboard properties, oxidative activation of wheat straw fibres was performed by adding hydrogen peroxide (Fenton’s reagent) during defibration. The addition of hydrogen peroxide into the defibrator system reduced the pH of the pre-treated straw material by several units. From approximately pH 4.5 of straw to pH values close to 3 of the refined straw fibres. The pH-buffering capacity displayed the same type of behaviour. Produced wheat straw fibre showed an average length and width of approximately 1.1 mm, 26 m, respectively. The fibre properties or fibre length distribution was not affected by the different processing conditions in this investigation. The fibreboard properties were improved by increased levels of added hydrogen peroxide (2.5–4.0)%. Addition of a fibreboard water-repelling agent, calcium chloride (CaCl2 ), improved the water resistance and reduced the thickness swelling by approximately 25%. As could be expected, the oxidative activated fibreboards are very sensitive to water and moisture. The mechanical and physical properties of the non-resin straw fibreboards are not acceptable according to requirements in the EN standards of MDF. However, use of these fibreboards in applications designed for dry conditions is possible. Moreover, the non-resin straw fibreboards do not emit formaldehyde, and can be produced at a considerable lower raw material cost than conventional wood-based MDF. Further research should concentrate on methods for improved surface activation of refined ligno-cellulosic material for reduction of water absorption and improved mechanical strength of fibreboards. Non-resin, non-wood fibreboards may have potential for future board applications when wood-based fibre resources decline.
Acknowledgements The authors are grateful to the Swedish Knowledge and Competence Foundation who supported this research financially. Thanks for support and advice is also extended to all those involved at Metso Panelboard AB, FMW, Akzo Nobel, and Dynea.
references
Anglés, M.N., Reguant, J., Montané, D., Ferrando, F., Farriol, X., Salvadó, J., 1999. Binderless composites from pretreated residual softwood. J. Appl. Polym. Sci. 73, 2485–2491. Back, E.L., 1987. The bonding mechanism in hardboard manufacture. Holzforschung 41 (4), 247–258. Back, E.L., 1991. Oxidative activation of wood surfaces for glue bonding. Forest Prod. J. 41 (2), 30–36. Berthold, J., Rinaudo, M., Salmén, L., 1996. Association of water to polar groups; estimation by an adsorption model for ligno-cellulosic materials. Colloids Surf. Part A 112, 119–131. Bouajila, J., Limare, A., Joly, C., Dole, P., 2005. Lignin plastification to improve binderless fiberboard mechanical properties. Polym. Eng. Sci. 45 (6), 809–816. EN 310, 1993. Wood based panels: determination of modulus of elasticity in bending strength. CEN (European Committee for Standardization). EN 317, 1993. Particleboards and fibreboards: determination of swelling in thickness after immersing in water. CEN (European Committee for Standardization). EN 319, 1993. Particleboards and fibreboards: determination of tensile strength perpendicular to the plane of the board. CEN (European Committee for Standardization). EN 622-5, 2006. Fibreboards – Specifications. Part 5. Requirements for dry process boards (MDF). CEN (European Committee for Standardization). Halvarsson, S., Edlund, H., Norgren, M., 2008. Properties of medium-density fibreboard (MDF) based on wheat straw and melamine modified urea formaldehyde (UMF) resin. Ind. Crops Products 28 (1), 37–46. Han, G., 2001. Development of high-performance UF-bonded reed and wheat straw composite panels. Wood Res. 88, 19–39. Holmbom, B., Konn, J., Pranovich, A., 2005. What is the true yield of TMP and CTMP? What is lost in refining and bleching. In: International Mechanical Pulping Conference, IMPC 2005, June 7–9, 2005, Oslo, Norway, pp. 98–101. Johns, E.W., Niazi, A.K., 1980. Effect of pH and buffering capacity of wood on the gelation time of urea–formaldehyde resin. Wood Fiber 12 (4), 255–263. Kato, M., Isogai, A., Onabe, F., 2000. Studies on interactions between aluminum compounds and cellulosic fibres in water by means of 27 Al NMR. J. Wood Sci. 46, 310–316. Lindström, T., Westman, L., 1980. The colloidal behaviour of kraft lignin. III. Swelling behaviour and mechanical properties of kraft lignin gels. Colloid Polym. Sci. 258, 390–397. Mantanis, G., Berns, J., 2001. Strawboards bonded with urea–formaldehyde resins. In: Proceedings of the 35th Washington State University International Symposium on Particleboard/Composite Materials, April 3–5, 2001, Pullman, WA, USA, pp. 137–144. Markessini, E., Roffael, E., Rigal, L., 1997. Panels from annual plant fibres bonded with urea–formaldehyde resins. In: Proceedings of the 31st Washington State University International Symposium on Particleboard/Composite Materials, April 8–10, 1997, Pullman, WA, USA, pp. 147–160. Mason, W.H., 1927. Pulp and paper from steam exploded wood. Paper Trade J. 84 (8), 131–136. Ohman, L.-O., Wågberg, L., 1997. Freshly formed alumium (III) hydroxide colloids: Influence of aging, surface complexation and silicate substitution. J. Pulp Paper Sci. 23 (10), 475–480. Pizzi, A., 1983. Wood Adhesives Chemistry and Technology. Marcel Dekker, Inc., New York, USA, pp. 247–288. Sauter, S.L., 1996. In: Wolcot, M.P. (Ed.). Proceedings of the 30th Washington State University International Symposium on Particleboard/Composite Materials, April 16–18, 1996, Pullman, WA, USA, pp. 197–214.
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available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/indcrop
Manufacture of non-resin wheat straw fibreboards Sören Halvarsson a,b,∗ , Håkan Edlund a , Magnus Norgren a a
Department of Natural Sciences, Fibre Science and Communication Network (FSCN), Mid Sweden University, SE-851 70 Sundsvall, Sweden b Metso Panelboard AB, Department of Research, Technology and Development (RTD), SE-851 50 Sundsvall, Sweden
a r t i c l e
i n f o
a b s t r a c t
Article history:
Wheat straw was used as raw material in the production of fibreboards. The size-reduced
Received 8 May 2008
straw was pretreated with steam, hot water and sulphuric acid before the defibration pro-
Received in revised form
cess to loosen its physical structure and reduce the pH. No synthetic binder was added.
22 July 2008
Adhesive bonding between fibres was initiated by activation of the fibre surfaces by an
Accepted 25 August 2008
oxidative treatment during the defibration process. Fenton’s reagent (ferrous chloride and hydrogen peroxide) was added. Two different levels of hydrogen peroxide (H2 O2 ), 2.5% or 4.0% were used. The resulting fibres were characterized in terms of fibre length distribution,
Keywords:
shive content, pH and pH-buffering capacity. The properties of finished fibreboards were
Wheat straw
compared with medium-density fibreboard (MDF) with density above 800 kg/m3 produced
Non-resin
from straw and melamine modified UF resin. The modulus of rupture (MOR), modulus of
Non-wood fibres
elasticity (MOE) and internal bond (IB) were lower than those of conventional manufactured
Peroxide
wheat straw fibreboards but close to the requirements of the MDF standard (EN 622-5: 2006).
MDF
The water absorption properties for the H2 O2 activated straw fibreboards were relatively
HDF
high, but were reduced by 25% with the addition of CaCl2 into the defibrator system as a
UF-resin
water-repelling agent. Increased levels of hydrogen peroxide improved the mechanical and
MUF-resin
physical properties of the straw fibreboard. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.
Refining Mechanical properties
1.
Introduction
The manufacture of medium-density fibreboard (MDF) uses wood as a raw material, particularly softwood, hardwood and mixtures of different wood species. However, the shortage of wood, forestry regulations, and the presumed lower cost of non-wood materials have encouraged board makers all over the world to research alternative sources of lignocellulose fibre. This study examines wheat straw, an agricultural waste material. The processing of straw differs from that of wood in the first part of the MDF process. The harvested and baled wheat
straw is size-reduced (chopped), hammer-milled, screened, and pre-wetted before defibration (the fibre-refining process). The subsequent processing steps are similar to those in conventional wood-based systems and involve resination, drying, mat-forming, pre-pressing and, finally, hot pressing. During hot pressing a synthetic resin binder (adhesive) is usually added to glue the fibres together to form a composite material. For several decades the adhesives used for the manufacture of MDF and high-density fibreboard (HDF) have been formaldehyde-based resins such as urea–formaldehyde (UF), urea–melamine–formaldehyde (UMF), and phenol–formaldehyde (PF). The vast quantity of
∗ Corresponding author at: Department of Natural Sciences, Fibre Science and Communication Network (FSCN), Mid Sweden University, SE-851 70 Sundsvall, Sweden. E-mail address: [email protected] (S. Halvarsson). 0926-6690/$ – see front matter. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.indcrop.2008.08.007
438
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formaldehyde consumed has raised concerns about the need to use more environment friendly chemicals. Removing the synthetic resin from the MDF process without impairing the mechanical properties or quality of the finished panels represents a great challenge, for the adhesive interactions between the fibres will undoubtedly be reduced. However, one possible way to improve the adhesion is to chemically activate the fibre surface by oxidation. Binding between and within the fibres can then be promoted during hot pressing by activated (reactive) components that are part of the lignocellulose material (Back, 1991; Bouajila et al., 2005). Paraffin wax or wax emulsions are added in small quantities (approximately 0.5–1.0%) to improve the poor water resistance of MDF. Fibreboards made using annual plant materials and agricultural waste have even worse water-resistant properties than wood (Sauter, 1996; Markessini et al., 1997; Han, 2001; Mantanis and Berns, 2001; Wasylciw, 2001; Ye et al., 2007). Another way to improve the hydrophobic properties of the fibre is to add small quantities of salts containing di- or trivalent cations (Westin et al., 2001). The most frequently used salt in the papermaking industry is aluminum sulphate. Below pH 9, the cations are primarily adsorbed to the pulp fibres by electrostatic interactions with the carboxyl groups in the lignocellulose material. The electrostatic interactions result in adsorption of small species or colloids on the fibre surface, altering the surface properties of the fibres (Ohman and Wågberg, 1997; Kato et al., 2000). Improved swelling-resistance in wood-based fibreboards has been reported after the addition of CaCl2 (Westin et al., 2001; Widsten, 2002). Wood-based binderless fibreboards without synthetic resin binders have been produced for at least 80 years. One of the early methods of producing these fibreboards was wet processing, generally known as the Masonite process (Mason, 1927). In this wet-process, wood fibres are generated by a steam explosion and activated by chemical reactions. During hot pressing self-bonding of the fibres and softening of the lignin contributes to the formation of high-performance fibreboards. The drawbacks of this type of fibreboard (hardboard) processing include high water consumption, the dark colour of the boards and the relatively long pressing times. Today hardboard (HB) is manufactured in a dry process very similar to the MDF-process and adhesive is normally used. A number of papers deal with binderless or self-bonding pulps produced using steam-exploded or thermo-mechanical processes (Suzuki et al., 1998; Anglés et al., 1999; Velásquez et al., 2003). Spent sulphite liquor (SSL) activated by hydrogen peroxide has also been tested as an adhesive to wood-based particle boards (Pizzi, 1983). However, their results are difficult to correlate with those from the present study because of differences in the processing of the raw materials and in the chemicals added. The binderless manufacturing of straw fibreboards involves the addition of an oxidative reactant. The oxidative-activated fibre surface and the low molecular degradation components are thought to create chemical bonds between activated fibres during hot pressing. The formation of covalent bonds between the lignocellulose polymers results in intermolecular forces that are much stronger than those created by hydrogen bonds (Back, 1991).
In this study, Fenton’s reagent was introduced into the defibration process to decompose the added hydrogen peroxide and induce improved intra- and inter-fibre interactions (Widsten, 2002). The decomposition of hydrogen peroxide by catalytic reactions of ferrous ions generates several types of reactive radicals, as shown in the following formulae: Fe2+ + H2 O2 → Fe3+ + OH− + HO
•
(a)
•
Fe3+ + H2 O2 → HO2 + H+ + Fe2+
(b)
•
HO + Fe2+ → OH− + Fe3+
(c)
•
HO2 + Fe3+ → O2 + H+ + Fe2+ •
H2 O2 + HO → H2 O + HO2
(d)
•
(e) •
The created hydroxyl radicals (HO ) continue to react and attack lignin and carbohydrates (RH), resulting in activated • lignocellulosic components (R ), as shown in formula (f). Cleavage of polymer chains and oxidation of components resulting from low molecular degradation are also possible: •
RH + HO → H2 O + R
•
(f)
Perhaps the most problematic side-reaction is the decomposing of hydrogen peroxide and the formation of oxygen gas, as shown in formula (g). Hydrogen peroxide can auto-decompose in aqueous solutions, and the rate of decomposition can accelerate upon contact with mineral surfaces. 2H2 O2 → H2 O + O2 (g)
(g)
Degraded low-molecular reagents not only oxidize the lignocellulose fibre surface but can also diffuse into the lumen and penetrate the pores in the cell walls, reacting in areas within the fibre cell walls. The bonding capacity between adjacent fibres for this type of reaction inside the fibres (intrafibre bonding) is low. However, the intra-fibre cross-links are thought to improve the resistance of the finished fibreboards to water swelling and increase the dimensional stability of the fibres (Back, 1991). Degradation components of higher molecular mass may have difficulty in penetrating the cell wall due to restrictions in pore size. The macromolecular lignocellulosic degradation compounds are predominately adsorbed on the fibre surfaces and probably contribute to inter-fibre bonding (Back, 1987). The aim of this study was to evaluate the suitability of combining a non-wood resource (wheat straw) and a binderless system to produce fibreboards on a sound environmental and economic basis. Instead of commercial resins, Fenton’s reagent and hydrogen peroxide were used in the defibration process to generate reactive groups in the wheat straw material in order to improve fibre adhesion and cross-linking during hot pressing of the fibreboards.
i n d u s t r i a l c r o p s a n d p r o d u c t s 2 9 ( 2 0 0 9 ) 437–445
439
Table 1 – Processing steps in two different straw fibreboard processes: the conventional dry fibreboard process (MDF-process) for straw and the dry fibreboard process without addition of synthetic resin, but with the addition of hydrogen peroxide. Proc. steps
Manufacturing of wheat straw MDF/HDF (SMDF)
1 2 3
Size reduction of straw Wetting/acid pre-treatment Pressurized refining addition of UMF resin addition of wax
4 5 6 7
Drying of fiber (tube flash drying) Forming of resinated fiber Pre-pressing Pressing
2.
Materials and methods
2.1.
Wheat straw substrate
The wheat straw (Triticum aestivium L.) was grown and harvested in Uppsala province, Sweden. The wheat straw (WS) was cut to about 30 cm in length, dried in the field, and finally baled. The moisture content of the baled WS was approximately 15%. The full-length straw material was chopped to 25–50 mm in length, after which it was subjected to a low-energy hammer milling and screening process to remove dust and small straw particles. The size-reduction, hammer milling, and de-dusting were conducted at the Förderanlagen GmbH KG (FMW) facility in Kirchstetten, Austria.
2.2.
Additives and chemicals
The experiments were carried out using hydrogen peroxide of puriss grade (Labservice, Sundsvall, Sweden) supplied at a concentration of 35%. The hydrogen peroxide was diluted and added to the blowline to react with and activate the fibre surface. The hydrogen peroxide reaction was catalyzed by metal ions and ferrous sulphate (FeSO4 ·7H2 O) of pro-analysis grade (Labservice, Sundsvall, Sweden). The size-reduced wheat straw was pretreated to increase its moisture content and temperature and to reduce the pH of the straw material. Diluted sulphuric acid (10%) of analysis grade (Labservice, Sundsvall, Sweden) was added to the hot water, steam, and wheat straw. An aqueous solution of calcium chloride, CaCl2 ·6H2 O, proanalysis grade (Labservice, Sundsvall, Sweden), was added to decrease the hydrophilicity of the fibre surfaces and to improve the water-repellant properties of the finished fibreboards. The reference MDF panels at densities higher than 800 kg/m3 were produced using wheat straw fibres and a commercial melamine-modified formaldehyde resin (UMF) supplied by Dynea (Prefere 11G23). Ammonium chloride was added to the UMF resins as a hardener (1.0%) and hexamethylenetetraamine (0.2%) was added as a retarder. The target content of the added UMF resin was 14% on an oven-dry basis (db). A wax emulsion supplied by Emutech AB, Sweden (Boardwax B100) was added to the fibre as a conventional hydrophobic additive.
2.3.
Manufacturing of peroxide wheat straw fibreboard Size reduction of straw Wetting/acid pre-treatment Pressurized refining addition of ferrous sulphate Addition of hydrogen peroxide Addition of calcium chloride Drying of fiber (tube flash drying) Forming of peroxide treated fiber Pre-pressing Pressing
Experimental
The pilot-plant manufacture of fibreboard based on wheat straw involves seven major processes (Table 1). Except for the use of hydrogen peroxide, these processes are much the same as the Mid Sweden University & Metso method of manufacturing straw MDF and HDF (Halvarsson et al., 2008). Chemicals such as hydrogen peroxide, co-reactants and calcium chloride (CaCl2 ) were added to the DefibratorTM system instead of using a binder such as MUF resin.
2.3.1.
Experimental layout
The actual experimental layout and processing was performed over 2 days. Steam, hot water and sulphuric acid were added in the pretreatment of the size-reduced straw material. This pretreatment was performed the day before the refining and pressing of the non-resin straw fibre material. The refining, forming and pressing of the activated fibres were divided into four major trials denoted RA, RB, and RC (Table 2). Trial RD was only used to produce fibres for evaluation of the fibre properties and was produced without the addition of chemicals during refining. The reference straw MDF/HDF (SMDF) in trial RE had been produced in an earlier investigation (Halvarsson et al., 2008).
2.3.2.
Acid pretreatment of wheat straw
Sulphuric acid was added to reach a target level of 0.75% (db) based on dry wheat straw. Steam and water were used to increase the moisture content and temperature. A short conveyor belt was used for feeding and levelling the wheat straw material flow. A mixer screw was connected to the conveyor and the sulphuric acid solution was sprayed on the straw material at the inlet of the mixer screw. Hot water and steam were injected into the mixer screw to increase the temperature of the straw material to a range of 70–90 ◦ C. The moisture content (MC) was increased and the pH of the straw mixture was decreased to approximately 4–5. The production rate was approximately 120 kg/h and the MC level was set to 100%. A slightly higher MC level of the pretreated straw was obtained (MC = 107%).
2.3.3. Refining in the pilot-plant laboratory defibrator (OHP 20) The acid and water/steam pre-treated wheat straw material mix was fed into a horizontal preheater and refined in a pilot
440
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Table 2 – Amount of added chemicals, wax, and melamine modified urea–formaldehyde UMF-resin for the different straw fibreboard trials (RA–RE) based on dry wheat straw. Trial
Hydrogen peroxide (%)
RA RB RC RD RE
Ferrous sulphate (%)
2.5 4 4 – –
Calcium chloride (%)
Wax addition (%)
UMF resin addition (%)
2 2 – – –
– – – – 1
– – – – 14.5
1 1 1 – –
Table 3 – The pH and pH-buffering capacity of straw and straw fibres samples after different chemical treatments and the addition of chemicals based on dry wheat straw. Property
Unit
Straw
Straw
Straw pretreatment pH pH-buffering capacity
– – ml
Water 7.8 14.5
Addition of chemical H2 SO4 H2 O2 CaCl2
% % %
– – –
plant pressurized single-disc refiner, type OHP 20” Defibrator (Metso Panelboard, Sundsvall, Sweden) fitted with a horizontal pre-heater. Refining was performed at a rotational speed of 1500 rpm and a pressure of 0.7 MPa. The retention time in the defibrator system was set to 3 min. The refined fibres were vented from the refining house into the blowline. Chemicals were injected into the defibrator system at separate injection points.
2.3.4.
Fibremat forming and pre-pressing
The produced fibre materials were later prepared and formed in a 500 mm × 600 mm forming box PendistorTM (Metso Panelboard AB, Sundsvall, Sweden). The subsequent pre-pressing was performed in a cold daylight press at 1.0 MPa for 60 s.
2.3.5.
Pressing of fibreboards
The pressing cycle was guided by in-house experience of pressing fibre materials from annual plants. The pressing time is one of the critical parameters for production capacity in fibreboard manufacture. The final fibreboard thickness was set to 6 mm and the pressing times were approximately 90 s, or a press factor of 15 s/mm. The temperature of the press plate was set to 200 ◦ C. Thus the resulting maximum temperature in the core of the fibremat during pressing was approximately 110–115 ◦ C. One inconvenience when pressing straw materials is the long de-gassing time necessary to avoid delamination of the pressed panels. To overcome this obstacle, an extra feature was added to the press heating system: Oil at 60–80 ◦ C was circulated through the press plates for cooling. This arrangement reduced the build-up of steam pressure inside the fibreboard at the end of the press cycle. The cooling time was one-third of the total pressing cycle and is included in the total pressing time. The applied pressure at the beginning of pressing was set to 0.5 MPa for 10–15 s to give the panel high surface density. The pressure was then reduced to approximately 0.05 MPa to
RA
RB
RC
RD
Acid 4.7 8.2
Acid 3.3 2.8
Acid 3.1 1.8
Acid 3.1 2.0
Water 5.7 8.5
0.75 – –
0.75 2.5 2
0.75 4 2
0.75 4 0
– – –
adjust the core density. Finally, the pressure was reduced to zero before opening the press.
2.4.
Evaluation of straw fibre and fibreboards
2.4.1.
Straw and fibre properties
The pH and the pH-buffering capacity of the size-reduced straw material were used to characterize the fibreboard after pretreatment. These properties were measured using a modified method suggested by Johns and Niazi (1980). The straw and fibre samples were diluted in water at room temperature. The pH-buffering capacity data in Table 3 is expressed as the added amount of 0.1 M H2 SO4 titration solution required to reach a pH of 3.0. The fibre length and the size of the refined and dried fibres were measured by image analysis using a laser-based PQMTM 1000 pulp quality monitoring system (Metso Paper, Sundsvall, Sweden). The average fibre length, the fibre length distribution, and other properties related to fibre quality were determined (Table 4). The amount of short fibres and dust particles were evaluated for each trial. The oversized fibre bundles
Table 4 – Properties of samples of straw fibre from trials RA to RD, measured in PQMTM 1000 fibre classifier and Pulmac Instruments, 0.15 mm slot. Property
Unit
RA
RB
RC
RD
Average length Average width Curl index Coarseness Shive weight Short fibres <1.45 mm Dust <0.45 mm Pulmac 0.15 mm
mm m % mg/m % % % %
1.1 25 7 0.34 15 68 21 17
1.1 26 7 0.36 18 68 21 19
1.1 25 7 0.40 17 69 23 20
1.1 26 7 0.35 19 65 21 24
Average values calculated for two separate fibre samples.
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are defined as shives and the shive content was determined by the PQM 1000 instrument and the Pulmac shive analyzer with 0.15 mm slots (Pulmac Instruments International, Vermont, Canada).
2.4.2.
Mechanical properties of fibreboards
Straw fibreboards were cut into 50 mm × 50 mm pieces to determine the internal bond (IB) properties and density profiles. Bending strength, modulus of rupture (MOR) and modulus of elasticity (MOE) were measured on 4 cm × 32 cm samples. The mechanical properties (IB, MOR and MOE) were determined according to EN standard methods (EN 310: 1993; EN 319: 1993) in an Alvetron Model Tc10 testing instrument (Lorentzen and Wettre, Sweden). The pressed panels were stored for one week at room temperature after pressing. Before testing, the specimens were conditioned in a room for 48 h at 65% relative humidity and a temperature of 20 ◦ C.
2.4.3. Thickness swelling and water adsorption of fibreboards The thickness swelling (TS) and water adsorption (WABS) properties were determined on 50 mm × 50 mm test pieces according to EN standard EN-317: 1993. The fibreboard specimens were immersed vertically in water for 24 h to determine their thickness and weight.
2.4.4.
Vertical density profile (VDP)
The density as a function of thickness, or the vertical density profile (VDP), was measured in a Grecon instrument (Laboratory Density Analyzer DA-X, Alfed, Germany).
3.
Results and discussion
3.1.
Wheat straw fibre properties
The pH and a modified pH-buffering capacity analysis performed after the pretreatment of the straw and after defibration of fibres from each trial (RA, RB, RC, and RD) are presented in (Table 3). Different loadings of hydrogen peroxide during defibration influenced the pH and the buffering capacity. The pH buffering capacity is presented as the amount of titration solution (ml of 0.1N H2 SO4 ) required to reduce the pH of straw and fibre samples to a level of pH 3.0. The pH of the wetted straw reference sample was 7.8. Acid treatment of the straw reduced the pH to 4.7. The wetted straw reference sample required 14.5 ml of titration solution, while the acid pre-treated straw required 8.3 ml of titration solution. The acid treatment was found to be effective in reducing the pH and the buffering capacity of the straw specimens. The hydrogen peroxide treatment of fibre during defibration produced an additional reduction in the pH. The analysis showed a pH level very close to pH 3 and consequently a minor amount of titration solution was required to reach the required value. The lowest pH value (pH 3.2) and the least amount of titration solution (1.8 ml) were observed after acid pre-treatment of the straw in combination with the addition of 4.0% hydrogen peroxide to the processed fibres (trials RB and RC).
441
An interesting observation was that after defibration and drying the pH value of the untreated straw fibre (trial RD) decreased from pH 7.8 to 5.7. This drop is probably due to the generation of carboxyl groups of the lignocellulose material. The type of raw material, defibration pressure, and retention time influence the final pH and buffering capacity of the produced fibres (Johns and Niazi, 1980; Widsten et al., 2002; Holmbom et al., 2005). Samples of refined wheat straw fibres were analyzed in the PQM-1000 fibre classifier system. The fibre properties from all the separate trials (RA to RD) are presented in Table 4. The fibre length distribution was homogenous in all the trials. Varying the amounts of hydrogen peroxide and calcium chloride had no major effect on fibre length, width or curl. The homogenous fibre quality reflects constant defibration processing conditions throughout the process. These conditions were maintained by an even input and control of the raw material feed. The average length of the straw fibre was approximately 1.1 mm and the average width was around 25–26 m. The fibre curl index was constant (7%) and the mass per length property (coarseness) increased with higher doses of H2 O2 . The amount of oversized straw fibre bundles or shives varied slightly between the different trials. The shive weight was in the range of 15–19%. Similar results were obtained by using the Pulmac instrument (0.15 mm slot, Table 4). Defibration of straw materials will always result in high levels of dust and oversized fibre bundles compared to wood-based materials. The geometry of the shives in wheat straw differs from that in wood-based species. Straw shives have a more compact and flat structure that is related to the thin outer layer of the straw. It is assumed that the flat shives consist of a relatively high amount of fibre bundles that could yield high tensile strength. However, it is unknown whether the wheat straw shives have any dramatic effect on other mechanical properties. In Table 4 short fibre and dust are defined as the amount of fibres shorter than 1.45 and 0.45 mm respectively. The amount of dust in straw fibres is generally higher than in refined wood fibres. The obvious explanation is the additional types of plant cells in the straw structure—the parenchyma, epidermis, and vessel cells. The thin cell walls in these types of cell generate more dust due to the thermo-mechanical work during the defibration process.
3.2.
The straw fibreboard properties
After the pressing, the fibreboards manufactured seemed to be of sufficiently high quality and close to the EN standard. The best results were found with a high loading of hydrogen peroxide (4%) and densities above 1000 kg/m3 . The high-density range of produced straw fibreboards is above the typical density range of MDF and should be named high-density fibreboard (HDF). Hydrogen peroxide-activated fibreboards showed moderate mechanical properties compared to those of glued (UF resin) SMDF. The internal bond and bending properties, modulus of rupture and modulus of elasticity were acceptable. The water resistance or thickness swelling (TS) was the major problem. Even though the addition of calcium chloride improved the fibreboards’ properties, the water-swelling properties were adversely affected. None of the manufactured
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Fig. 1 – The internal bond of wheat straw fibreboards as a function of average density. The line represents requirements in the MDF standard EN-622-5: 2006. See Table 2 for further explanation of the figure legend.
straw fibreboards met the European wood-based MDF standard (EN 622-5: 2006). TS and water absorption (WABS) were several times higher than the requirements in the MDF standard.
3.2.1.
Internal bond
Fig. 1 shows the internal bond of the non-resin straw fibreboards as a function of the average density. The IB was below or close to the requirements in the MDF standard (EN 622-5: 2006). However, a much higher IB strength is displayed for the reference SMDF glued with a commercial UMF-resin. The reference SMDF showed excellent IB values above 1.0 MPa. The different levels of hydrogen peroxide and calcium chloride additions are shown in Table 2. The enhanced adhesive effect obtained by activation of the wheat straw fibres by hydrogen peroxide is obvious. The most probable contribution to the increased strength is the formation of covalent bonds between chemical groups on adjacent fibres surfaces. High water absorption of the finished fibreboards is observed and
could be regarded as contradictive to the suggested covalent bonding. However, it should be remembered that hydrogen peroxide oxidises the fibre material and increases the overall number of charged groups, and thus, the prerequisites for water sorption. Other adhesive effects such as electrostatic interactions between adjacent fibres might also be possible when divalent metal ion salts are added. Increasing the H2 O2 from a level of 2.5–4.0% improved the strength of the fibreboards. The internal bond increased from approximately 0.45 to around 0.65 MPa at a density of 1000 kg/m3 or higher. Finally, the addition of CaCl2 seemed to increase the IB strength (compare trials RB and RC). The IB specimens were used to measure the vertical density profiles (VDP). The density profiles of typical straw fibreboards are displayed from each trial (RA, RB and RC) and are plotted as a function of thickness in Fig. 2. Generally, the VDP of wood-based fibreboards is designed to have a high surface density and lower core density. The benefits are hard surfaces and acceptable bending properties. The core density is normally correlated to the internal bond (Xu and Winistorfer, 1995; Schulte and Früwald, 1996; Wong et al., 2000). The fibreboards related to trial RA displayed VDP variations due to inhomogeneous fibre mats and/or difficulties in the pressing. The subsequent pressing of fibreboards in trials RB and RC was improved. Their vertical density profiles displayed greater homogeneity and had roughly the same shape. A plausible explanation for this effect is the improved adhesive effect between activated straw fibres at higher loadings of hydrogen peroxide. The surface density was around 1150–1200 kg/m3 and the core density was approximately 1000 kg/m3 . In addition, the thicknesses of the fibreboards in the RB and RC trials varied between 6 and 7 mm.
3.2.2.
Straw fibreboard bending properties
Fig. 3 shows the modulus of rupture while Fig. 4 shows the modulus of elasticity as a function of average density. The
Fig. 2 – Vertical density profile of typical wheat straw fibreboards in trial RA, RB, and RC. See Table 2 for further explanation of the figure legend. (For interpretation of the references to colour in the artwork, the reader is referred to the web version of the article.)
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Fig. 3 – The modulus of rupture (MOR) of wheat straw fibreboards as a function of average density. The line represents requirements in the MDF standard EN-622-5: 2006. See Table 2 for further explanation of the figure legend.
Fig. 5 – The thickness swelling of wheat straw fibreboards as a function of average density. The line represents requirements in the MDF standard, EN-622-5: 2006. See Table 2 for further explanation of the figure legend.
3.2.3.
Fig. 4 – The modulus of elasticity (MOE) of wheat straw fibreboards as a function of average density. The line represents requirements in the MDF standard, EN-622-5: 2006. See Table 2 for further explanation of the figure legend.
different hydrogen peroxide loadings and calcium chloride additions had only minor effects on the MOR and MOE. Pressing was initially problematic and some of the fibreboards in the RA trials delaminated. In trial RA the hydrogen peroxide loading was set to 2.5%. The average density of the fibreboards was low and the numerical values of MOR and MOE were below 20 and 3000 MPa, respectively. The bending properties of conventional wood-based panels are strongly dependent on the average density (Wong et al., 2000; Shi et al., 2005). However, proper pressing conditions and modified VDP improved the bending strength of the subsequent samples. The MOR for fibreboards in both RB and RC trials was approximately 15–25 MPa, but still within the same density range, namely 900–1000 kg/m3 . The higher loading of hydrogen peroxide (4%) in trials RB and RC yielded increased densities and bending strengths. Almost all fibreboards above densities of 975 kg/m3 met the requirements in the MDF standard (EN 6225: 2006). The reference SMDF panels showed numerical MOR values above 40 MPa.
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Thickness swelling of fibreboards
Thickness swelling (TS) and water absorption (WABS) of the non-resin, non-wood fibreboards showed high water swelling levels (Figs. 5 and 6). Apart from the differing chemical composition of wood and non-wood fibres, the proportions of cellulose, hemicellulose and lignin components are different. In most cases the amounts of cellulose and lignin are higher in wood-based materials while the hemicellulose component is lower. Hemicellulose is a branched, non-crystalline polymer which has the ability to absorb water and swell (Berthold et al., 1996). Of the different wood polymers in the cell wall, the hemicelluloses swell the most and lignin the least (Lindström and Westman, 1980). Consequently the natural mixture of cellulose, lignin and hemicelluloses in wood-based fibreboards possesses a better resistance to water and water absorption than what is possible for fibreboards based on straw and other annual plant materials. The ability of attracting water to the oxidised ligno-cellulosic material will also increase and contribute to a higher water sensitivity of the non-resin straw fibreboards. The wood-based MDF-standard (EN 622-5: 2006) requires a TS level below 30% for fibreboard thicknesses between 4 and 6 mm and a TS below 17% for thicknesses between 6 and
Fig. 6 – The water absorption of wheat straw fibreboards as a function of average density. See Table 2 for further explanation of the figure legend.
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9 mm. Fibreboards treated with CaCl2 and 4% hydrogen peroxide (trial RB) showed approximately 80–100% swelling in the density range 950–1000 kg/m3 . The WABS of produced straw fibreboards was estimated at 110–130%. The water-repelling properties improved at higher densities. Increased loadings of hydrogen peroxide and the addition of calcium chloride reduced the thickness swelling by approximately 25%. The lowest thickness swelling and water absorption properties were achieved with the reference SMDF composed of wheat straw, UMF resin and wax.
4.
Conclusion
Wheat straw fibreboards were produced in the dry board process from agricultural waste without the addition of synthetic resin. To achieve the required fibreboard properties, oxidative activation of wheat straw fibres was performed by adding hydrogen peroxide (Fenton’s reagent) during defibration. The addition of hydrogen peroxide into the defibrator system reduced the pH of the pre-treated straw material by several units. From approximately pH 4.5 of straw to pH values close to 3 of the refined straw fibres. The pH-buffering capacity displayed the same type of behaviour. Produced wheat straw fibre showed an average length and width of approximately 1.1 mm, 26 m, respectively. The fibre properties or fibre length distribution was not affected by the different processing conditions in this investigation. The fibreboard properties were improved by increased levels of added hydrogen peroxide (2.5–4.0)%. Addition of a fibreboard water-repelling agent, calcium chloride (CaCl2 ), improved the water resistance and reduced the thickness swelling by approximately 25%. As could be expected, the oxidative activated fibreboards are very sensitive to water and moisture. The mechanical and physical properties of the non-resin straw fibreboards are not acceptable according to requirements in the EN standards of MDF. However, use of these fibreboards in applications designed for dry conditions is possible. Moreover, the non-resin straw fibreboards do not emit formaldehyde, and can be produced at a considerable lower raw material cost than conventional wood-based MDF. Further research should concentrate on methods for improved surface activation of refined ligno-cellulosic material for reduction of water absorption and improved mechanical strength of fibreboards. Non-resin, non-wood fibreboards may have potential for future board applications when wood-based fibre resources decline.
Acknowledgements The authors are grateful to the Swedish Knowledge and Competence Foundation who supported this research financially. Thanks for support and advice is also extended to all those involved at Metso Panelboard AB, FMW, Akzo Nobel, and Dynea.
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