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由麦秆和三聚氰胺改性的脲醛树脂(UMF)组成的中密度纤维板(MDF)的性能


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 8 ( 2 0 0 8 ) 37–46

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>Properties of medium-density ?breboard (MDF) based on wheat straw and melamine modi?ed urea formaldehyde (UMF) resin
¨ ? Soren Halvarsson a,b,? , Hakan Edlund a , Magnus Norgren 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

a r t i c l e
Article history:

i n f o

a b s t r a c t
Wheat straw was investigated as a raw material for manufacturing of medium density ?breboard (MDF) in a fully equipped pilot-plant. Commercial urea melamine formaldehyde (UMF) and a mixture of UMF-resin and urea melamine phenol formaldehyde (UMPF) adhesives were used as binders in manufacturing of high performance MDF. The study evaluated the quality of MDF produced of straw (i.e., SMDF). Different qualities of wheat straw and different resin contents (14–17%) were used. Moreover, the SMDF was produced at different thicknesses of 9 and 16 mm and densities of 750–1000 kg/m3 . The properties of the resulting

Received 24 September 2007 Received in revised form 9 January 2008 Accepted 9 January 2008

Keywords: Wheat straw Nonwood ?bres MDF Uf-resin Muf-resin Re?ning Board properties

SMDF were evaluated by analysing mechanical and water absorption (anti-swelling) properties as a function of density. Internal bond (IB), modulus of rupture (MOR), modulus of elasticity (MOE), thickness swelling (TS), and water absorption (WABS) were the properties analysed. SMDF-panels produced with densities above 780 kg/m3 and resin contents above 14% met the requirements for wood-based MDF standard EN 622-5:1997. ? 2008 Elsevier B.V. All rights reserved.

1.

Introduction

Medium-density ?breboard (MDF) is a ?bre composite material comprising of re?ned wood ?bres, adhesive (resin), process additives, and a minor amount of wax. MDF is produced in a dry ?bre process, and by de?nition, wood-based MDFpanels have densities between 450 and 800 kg/m3 (EN 316, 1999). The strength of MDF depends on its ?bres and on the adhesive bonds between them. Thereby, the adhesives are necessary to ensure effective bonding between the ?bres. The most common types of resins used for MDF-products are

based on formaldehyde, for example urea-formaldehyde (UF), melamine-urea-formaldehyde (MUF), and phenol- formaldehyde (PF) resins. Typical MDF-products are cabinet doors, shelves, laminated ?oors, furniture and panels for building construction. Real commercial interest in MDF-products started in the mid 1970s when UF-resin was injected into a tube (blowline) and mixed with re?ned ?bres in a blowline blending process (Haylock, 1977; Hammock, 1982; Gran, 1982). The outcome was successful, and today the MDF production in Europe is at approximately 14.9 million m3 /year as of 2006 (Wadsworth,

Corresponding author. Tel.: +46 60 165741/148858; fax: +46 60 148802/165500. E-mail addresses: soren.halvarsson@miun.se, soren.halvarsson@metso.com (S. Halvarsson). 0926-6690/$ – see front matter ? 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.indcrop.2008.01.005

?

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2007), and has an estimated increase (2006–2010) by at least 4% annually (Dunky and Lukkaroinen, 2005). Consequently, demand for nonwood lignocellulosic ?bre resources has also increased, due to lack of wood raw materials and for economic and environmental considerations. Most wood raw materials are primarily used for sawn lumber products and in the pulp and paper industry. The availability of alternative inexpensive ?bre raw materials is critical if manufacturing is to increase in several regions of the world. The most promising annual plant waste materials for manufacturing MDF are wheat and rice straw. Globally, wheat and rice are the most important food grains ranking second (wheat) and third (rice) in terms of the total cereal production after corn (maize). The production of wheat grain was estimated at 620 million metric tons in 2005–2006 (WAOB, 2007, p.18). According to an earlier investigation the same amount of waste straw materials as grains can be generated (Russel, 1996). Part of the agricultural by-products such as straw are likely to be used as industrial raw materials at a signi?cant scale in the near future, which is an alternative of being burned or left in the ?elds. The lignocellulosic ?bres used in manufacturing conventional MDF entirely comprise wood materials, the ?bres of which vary in kind and quantity depending on the source woods. Wood ?bres differ in length, which is approximately 1–1.5 mm for hardwoods and 2–4.5 mm for softwoods. Reported ?ber length of wheat straw are almost the same length as the hardwood at approximately 0.7–1.4 mm (Markessini et al., 1997; Subrahmanyam et al., 1999; Rowell et al., 2000). The main chemical components of wood and straw materials are cellulose, hemicellulose, and lignin. However, the proportions and chemical structures of hemicellulose and lignin differ between straw and wood (Markessini et al., 1997; Sun et al., 1997; Rowell et al., 2000; Donaldson et al., 2001), the lignin content being lower and the hemicellulose content higher in straw materials. Morphological analysis of straw materials reveals a higher amount of non?brous thin-walled cells, parenchyma and epidermal cells than in wood materials (Zhai and Lee, 1989; Subrahmanyam et al., 1999; Liu et al., 2005). The manufactured straw ?bres are shorter and the ?nished straw particleboards and MDF (SMDF) panels display greater water swelling than do wood-based boards (Sauter, 1996; Han, 2001; Eroglu and Istek, 2000; Mantanis and Berns, 2001; Wasylciw, 2001). Furthermore, the straw contains more silica and ash (Markessini et al., 1997; Ghaly and Al-Taweel, 1990; Rowell et al., 2000). The outer part, or epidermis, of the straw contains micro-sized silica particles called phytoliths (microfossils), which are unique to each species (Ball et al., 1999; Sangster et al., 2001). A previous study found that fractions of size-reduced and screened wheat straw materials were enriched by silica as the fractions length was reduced (Halvarsson et al., 2005). Removing the dust and short-?bre fractions is undoubtedly a necessary process operation to improve SMDF quality. This report describes a method for manufacturing of MDF based on wheat straw and melamine modi?ed UF-resins. The parameters discussed include processing steps involved in the SMDF manufacturing and the in?uence of the average density of MDF panels at different thicknesses and resin content on various panel properties.

2.
2.1.

Materials and methods
Raw materials

Straw of wheat (Triticum aestivum) was harvested and baled in north-eastern China. The wheat straw (WS) was harvested in different seasons, and some of the WS was stored in dry conditions for 1 year. The dry content of the delivered straw was approximately 90%. Two types of commercial melamine-modi?ed Urea Formaldehyde (UMF) resins were used. The UMF-resins were supplied by Dynea NV (Prefere 11G321) and Akzo Nobel-Casco Adhesives AB (UMPF 1074-0837). The latter UMF-resin contained a high amount of melamine (30%) and a minor amount of phenol. Some of the UF-resins were mixed to reduce the melamine content to an estimated 20%. An aqueous solution of ammonium chloride was added to the UMF resins as a hardener (1.0%) and hexamethylenetetraamine (0.2%) was added as a retarder. Both the commercial UMF resins and the mixed resin mixture were ?nally diluted to 50% before application. The target content of the UMF-resin ranged from 14 to 17% in relation to the dry wheat straw (dry basis, d.b.). A small amount of wax is generally added to the ?bre ?ow to improve the water resistance of the ?nished woodbased MDF panels. In this study, the wax was injected at the infeed of the re?ning process. The wax emulsion was supplied by Emutech AB, Sweden, Boardwax B100 and was diluted to a solid content of 30% before use. The target level of wax emulsion was 1.0% (d.b.) in relation to the wheat straw.

2.2. Manufacture of straw medium-density ?breboard (SMDF)
Several processing steps were necessary to convert the agricultural waste materials to a high-performance MDF. The wheat straw material was ?rst size reduced before re?ning, to obtain a proper bulk density for conveying and handling. Moreover, undersized particles and dust were removed via screening. The dry straw was wetted in a pre-treatment process preceding the re?ning. Re?ning was performed in a pressurised 508 mm (20 in.) single-disc De?bratorTM re?ner (Metso Panelboard, Sundsvall, Sweden) provided with a horizontal pre-heater. The re?ned ?bres were vented from the De?bratorTM via a blow valve into a tube (blowline). The resin was injected into the ?ow of steam and re?ned ?bres inside the blowline. The resulting resinated, wet ?bres were dried in a connected ?ash tube dryer, whereafter the dried resinated ?bre mass reaches a dry content (DC) of approximately 90%. A schematic of the ?bre preparation system in the SMDF process is presented in Fig. 1. The dried and resinated ?bre materials were later formed into 500 mm × 600 mm ?bre mats in a mat-forming unit (PendistorTM , Metso Panelboard AB, Sundsvall, Sweden), this was followed by pre-pressing and then ?nal pressing in a daylight press at 190 ? C and at suf?cient pressure to achieve the required panel density and thickness.

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Fig. 1 – Schematic of a straw ?bre preparation system in an MDF pilot plant for handling wheat straw: (1) hammermill, (2) dry screen, (3) pre-treatment screw, (4) conveyer, (5) infeed screw, (6) pre-heater (digester), (7) De?bratorTM (re?ner), (8) blowline, (9) dryer, and (10) ?bre outlet (cyclone).

2.2.1.

Size reduction of the straw material

Wheat straw was size-reduced using two connected hammermills. The ?rst hammermill was a Jeffery Swinghammer Shredder, 15 in. × 8 in. and installed with 25 mm screens. The second hammermill, model H-30.C (Kamas Industri AB, Karlstad, Sweden), was equipped with oval screening holes with 10 mm aperture. The rotation speed of the ?rst hammermill was 1000 rpm and of the second was 1500 rpm. A production rate of 60 kg/h was selected. The resulting straw length of the size-reduced straw feedstock was approximately 10–15 mm. The straw material was sieved in typical screening equipment using a 0.7 mm mesh.

(10%) was added to the straw material in some of the trials to decrease the pH below 6 to ensure effective UF-resin curing. The sulphuric acid solution was added to a level of 0.75% (d.b). A short conveyor belt was used for feed the WS material ?ow into the de?brator system and to level it.

2.2.3.

Re?ning the straw material

2.2.2.

Water pre-treatment of the straw material

The size-reduced straw materials were pre-treated with steam and hot water (60 ? C), added to a mixer screw to increase the temperature of the size-reduced straw to approximately 80–90 ? C. The dry content of the wheat straw after pretreatment was approximately 50%. Sulphuric acid solution

The pre-treated wheat straw material was fed into a horizontal pre-heater (digester) and re?ned in a pressurized single-diskre?ner, type OHP 20 laboratory De?brator (Metso Panelboard, Sundsvall, Sweden) with a plate diameter of 508 mm (20 in.). Re?ning was done at a rotational speed of 1500 rpm and a pressure of 0.6 MPa. The retention time in the De?bratorTM system was set to 2 min. The re?ned ?bres were vented from the re?ning house into a blowline and mixed with UMF-resin. The resinated ?bres in the blowline were dried in a connected continuous ?ash dryer. The average dry content of dried resinated ?bres was approximately 90%.

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Table 1 – Material- and process parameters in the SMDF-process Trial
A B C D E F

Target RC (%)
14 17 16 15 16 14

Target density (kg/m3 )
750 950 950 950 750 950

Target thickness (mm)
16 9 9 9 16 9

Storage time (year)
0 0 0 1 1 1

Straw pre-treatment
Water Water Acid Acid Acid Acid

Target variables; resin content (RC), SMDF density, and thickness.

2.2.4.

Fibre forming and prepressing

Resinated dry ?bres were formed batch wise in a (500 mm × 600) mm forming box PendistorTM (Metso Panelboard, Sundsvall, Sweden). Depending on the target densities and the thickness of SMDF-panels, different ?bre weights were selected. Finally, the ?bre mats were pre-pressed in a cold daylight press at 1.0 MPa for 60 s.

length and the ?bre length distribution were determined. The amounts of short ?bres and dust particles were evaluated for each trial.

2.3.2.

Mechanical properties of SMDF

2.2.5.

Pressing of MDF

The pressing cycle was guided by in-house experience of pressing annual plant ?bre materials. The overall goal was to press panels for a time of 12–13 s per millimetre of the panel thickness. The total pressing times were 130 or 200 s, depending on the ?nal panel thickness of 9 or 16 mm. The press-plates temperature were set to 190 ? C and the resulting maximum temperature in the core of the ?bre mat during pressing was approximately 105–110 ? C. One inconvenience when pressing straw materials is the long de-gassing time necessary to release gas and to avoid delamination of pressed panels. To overcome this obstacle an extra feature was added to the press heating system. Oil at a temperature of 60–80 ? C was circulated into the press plates for cooling. This was arranged in the end of the press cycle to reduce the build-up of steam pressure inside the panel. The time for cooling was one third of the total pressing cycle. The applied pressure in the beginning of pressing was set to 0.5 MPa during 10–15 s to create a high surface density of the panel. The pressure was then regulated to approximately 0.05 MPa for adjusting of the core density. Finally, the pressure was set to zero for opening of the press.

Straw MDF panels were cut into 50 mm × 50 mm pieces for determining of internal bond (IB) and for measuring density pro?les. Bending strength, modulus of rupture (MOR) and modulus of elasticity (MOE) were measured on 4 cm × 32 cm. The mechanical properties, i.e., IB, MOR, and MOE, were determined according to the EN standard methods (EN 310, 1993; EN 319, 1993) in an Alvetron TC 10 testing instrument. The pressed panels were stored for 1 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.

2.3.3.

Resin content of SMDF

The resin content was analysed using the Kjeldhal method and the nitrogen content of the ?nished SMDF panels was measured using the Kjeltec System 1026 Distilling Unit.

2.3.4.

Thickness swelling and water adsorption of SMDF

Thickness swelling (TS) and water adsorption (WABS) properties were determined according to the EN standard (EN 317, 1993) from 50 mm × 50 mm pieces of SMDF. The SMDF specimens were immersed vertically in water for 24 h to determine thickness and weight.

2.4.

Experimental set-up

2.3. Evaluation of straw ?bre and medium-density ?breboard 2.3.1. Straw ?bre properties

Resinated and dried straw ?bres were sampled after the dryer cyclone. The size and shape of ?bres were measured by image analysis using a laser-based PQM 1000 pulp quality monitoring system (Metso Paper, Sundsvall, Sweden). The average ?bre

The experimental set-up was designed to achieve the speci?cations presented in Table 1. The different trials (Trials A to F) also aimed to distinguish the effects of different straw harvesting seasons (storage time). Some of the wheat straw material was stored for 1 year in dry conditions before testing. The size-reduced straw was subjected to different pretreatments (water or acid) to reduce the pH for Trials A and B. Moreover, different amounts of resin (14–17%) were added in combination with different SMDF panel thicknesses and den-

Table 2 – Properties of re?ned wheat straw ?bres measured using the PQM 1000 system Fibre Property
Average length Average width Fibres <1.45 mm Dust <0.45 mm

Unit
mm m % %

Trial A
1.02 25.4 64 21

Trial B
0.90 25.5 72 26

Trial C
0.99 25.5 71 25

Trial D
1.02 26.4 66 23

Trial E
0.97 26.3 70 25

Trial F
0.95 25.6 71 24

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Fig. 2 – Internal bond (IB) as a function of average density. SMDF was produced with resin contents (RCs) of 14% (Trials A and F), 15% (Trial D), 16% (Trials C and E), and 17% (Trial B) at thicknesses of 9 and 16 mm. MDF-S1 represents the MDF standard at thicknesses >9 mm and SMDF-S2 at thicknesses >12 mm in dry conditions. .

sities. Finally, the effect of a highly water-resistant adhesive, a mixture of a commercial UMF-resin and an UMPF-resin was evaluated in Trial F.

3.
3.1.

Results and discussion
The wheat straw ?ber properties

The produced resinated wheat straw ?bres were sampled after the drying step. Fibre length was analysed in a PQM 1000 laboratory analyser by means of image analysis. The average ?bre length and width average were approximately 1.0 mm and 26 m, respectively, for re?ned ?bres in all trials (see Table 2). The measured length was in line with the length of wheat straw reported in the literature (Markessini et al., 1997; Subrahmanyam et al., 1999). Re?ning wheat straw generates

a higher amount of short ?bres and dust than does re?ning wood-based ?bres, short ?bres and dust are de?ned as particles shorter than 0.45 mm. One general explanation for the dust generation in the re?ning process of wheat straw is the presence of non?brous plant cell materials, i.e., parenchyma and epidermal tissues which have thinner walls compared with the ?brous cell types found in wood (Subrahmanyam et al., 1999). The different storage times and pre-treatment conditions of the wheat straw resulted in only minor differences in the ?bre properties as measured in the PQM 1000.

3.2. 3.2.1.

The straw MDF properties Internal bond (IB)

SMDF panels produced at an average density above 780 kg/m3 and at a resin content higher than 14% met the requirements of the MDF standard (EN 622-5, 1997). The ?nal thicknesses of

Fig. 3 – The average IB (Avg) and standard deviation (Sdev) of manufactured SMDF panels in Trials A to F.

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Fig. 4 – Modulus of rupture (MOR) versus SMDF average density. SMDF was produced with resin contents (RCs) of 14% (Trials A and F), 15% (Trial D), 16% (Trials C and E), and 17% (Trial B) at thicknesses of 9 and 16 mm. MDF-S1 represents the MDF standard at thicknesses >9 mm and SMDF-S2 at thicknesses >12 mm in dry conditions.

the ?nished SMDF panels were approximately 9 and 16 mm. The average densities ranged from 750 to 1025 kg/m3 and the resin content varied between 14 and 17%. Fig. 2 presents the internal bond (IB) of manufactured SMDF plotted as a function of average density, IB increasing with average density. The numerical values of the resin content and the thickness of the SMDF can be seen in Table 1. In general, the mechanical properties of wood-based particle board (PB) and MDF are strongly dependant on the average density and to some extent on the amount of UF-resin added (Suzuki and Kato, 1989; Hague et al., 1999; Wong et al., 2000; Shi et al., 2005). Straw-based MDF has been observed to behave in similar fashion (Halvarsson et al., 2004; Ye et al., 2007). Increased numbers of ?bre–?bre contact points create crosslinks (inter-bonds) between resinated ?bres, which increases

the density and improve the IB strength of the ?nished MDF panels. Investigated material and process variables see Table 1, had only minor effects on IB compared with the effect of average density. Thus, the dominating parameter for variation in IB is average density. SMDF produced with a thickness of 16 mm had a density range of 750–850 kg/m3 . In contrast the 9 mm thick SMDF panels were set to a higher density, of 850–1075 kg/m3 . In Fig. 3 the average values of IB are summarized for Trials A–F. SMDF panels in the 750–850 kg/m3 density region (Trials A and E) displayed the lowest IB values. SMDF panels in Trials B, C, D, and F displayed enhanced average IB-strength. SMDF panels produced in Trial F showed excellent IB strength even if the measured resin content was at the lower

Fig. 5 – Modulus of elasticity (MOE) versus SMDF average density. SMDF was produced with resin contents (RCs) of 14% (Trials A and F), 15% (Trial D), 16% (Trials C and E), and 17% (Trial B) at thicknesses of 9 and 16 mm. MDF-S1 represents the MDF standard at thicknesses >9 mm and SMDF-S2 at thicknesses >12 mm in dry conditions.

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Fig. 6 – Thickness swelling (TS) versus SMDF average density. SMDF was produced with resin contents (RCs) of 14% (Trials A and F), 15% (Trial D), 16% (Trials C and E), and 17% (Trial B) at thicknesses of 9 and 16 mm. MDF-S1 represents the MDF standard at thicknesses >9 mm and SMDF-S2 at thicknesses >12 mm in dry conditions.

level (14%). The SMDF specimens analysed in Trial F were in the 850–950 kg/m3 density range, see Fig. 2. Furthermore, more melamine (20%) was incorporated into the resin mixture than into the conventional UF-resin (10% melamine). Possibly, the difference in IB strength is more related to the adhesive used and the resultant quality of bonding between ?bres than to the ?bre strength itself. However, the expected improvement of IB strength with increased resin content was not clearly demonstrated in all trials.

3.2.2.

SMDF bending properties

Figs. 4 and 5 present the bending properties modulus of rupture (MOR) and modulus of elasticity (MOE) as a function of average density, revealing the same tendency as can be observed in the relationship between IB strength and den-

sity. The bending properties of the SMDF panels were strongly dependent on the average density, and two regions of MOR and MOE are shown in Figs. 4 and 5. Increased average density resulted in signi?cant improvement of the bending properties. The higher density of the 9 mm thick SMDF panels generates a higher number of contact points (inter-bonding between ?bres), which consequently improve the bending strength. For SMDF produced at the lower density and with a thickness of 16 mm had MOR values in the range of 25–32 MPa, the corresponding MOE values reached 3.5 GPa. Higher density in the produced SMDF panels resulted in improved bending properties, causing MOR to increase to a 39–44 MPa and the corresponding MOE values to reach approximately 4.6 GPa. The wheat straw-based MDF thus met the requirements in the MDF-standard (EN 622-5, 1997), see Fig. 4. The amount of

Fig. 7 – Water absorption (WABS) versus SMDF average density. SMDF was produced with resin contents (RCs) of 14% (Trials A and F), 15% (Trial D), 16% (Trials C and E), and 17% (Trial B) at thicknesses of 9 and 16 mm.

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Table 3 – Calculated average thickness swelling (TS) and water absorption (WABS) of wheat straw MDF at different resin contents (RC) and average densities Trial
A B C D E F

RC (%)
14 17 16 15 16 14

Thickness (mm)
16 9 9 9 16 9

Panel density (kg/m3 )
800–850 900–1000 900–1000 900–1000 750–800 850–1100

TS 24 h (%)
9 8 11 20 15 6

WABS 24 h (%)
43 22 33 43 56 20

added resin, type of resin, storage time, and type of straw pretreatment (water or acid) had no major effect on the bending properties. The observed MOE values are slightly in?uenced by the resin content of the SMDF.

3.2.3.

Thickness swelling and water absorption of SMDF

In the 24-h water soaking test, most of the analysed samples displayed low thickness swelling (TS) and water absorption (WABS). The TS and WABS of the SMDF specimens are plotted as a function of average density in Figs. 6 and 7. The variation in the TS of SMDF due to different average densities is not evident. In general, the test results indicated a reduction of TS and WABS with increased SMDF density. The low TS of SMDF panels achieved in Trial F was unique and probably an effect of the increased melamine content of the UF-resin, which acted as a hydrophobic component of the ?nished SMDF. It is generally accepted that the increased melamine content in conventional UF -resins improves the water resistance of ?nished wood-based panels. Modifying wood ?bres to make them more water resistance, by adding hydrophobic components (e.g., melamine-modi?ed UF-resin) or by adding wax to coat the wood ?bres, will probably lower the surface energy and improve the water resistance. Furthermore, modifying the cell wall polymers in lignocellulosic ?bres by adding chemicals and substituting hydroxyl groups with more bulky and hydrophobic chemicals would reduce the tendency of wood to swell and shrink (Rowell, 1982). Boards produced of chemically modi?ed wood, straw, and other lignocellulosic materials showed reduced thickness swelling compared to untreated boards (Rowell et al., 1986; Rowell et al., 1995; Han et al., 2000; Gomez-Bueso et al., 2000). Likewise, the TS of SMDF was reduced by changing the chemical composition of the added UMF resin. The SMDF in Trial F had the lowest TS values, approximately 6% over a wide density range of 850–1100 kg/m3 . SMDF panels with a thickness of 16 mm (Trials A and E) displayed a strong density dependence, the TS declining from 21 to 6% as the density of the SMDF increased. No major difference in TS was observed between SMDF containing resin contents of 14 or 16% as a function of density, see Fig. 6. In contrast, the thinner SMDF panels displayed a signi?cant reduction in average TS and WABS with increased resin contents 14%, 15%, and 17% in Trials D, C, and B, respectively. The TS was reduced from 20% to 8% compared within the same density range i.e., 850–1000 kg/m3 , see Table 3. The reduction in TS and WABS indicates improved bonding strength between ?bres in the SMDF panels as the amount of UF-resin was increased, a behavior also observed for wood-based MDF panels (Hague et al., 1999). Presumably, increased numbers of

sustainable links between ?bres are created as the resin content and density increase. Fibres link together so that moisture cannot separate them. Furthermore, WABS of the SMDF will be restricted due to the increased forces holding the ?bres together as the resin content increases. Water absorption (WABS) is de?ned as the per cent increase in weight of a sample immersed in water for 24 h. In Fig. 7, the WABS of the wheat straw MDF specimens is plotted as function of average density. A reduction of WABS can be observed as the density increases. Moreover, the WABS of 9 mm thick SMDF improved as the resin content increased (Trials B, C, and D), consistent with the measurements of thickness swelling. Trial F is excluded from this discussion due to the hydrophobic contribution of melamine in the mixed UF-resin, which significantly reduced the water absorption. Finally, the 16 mm thick SMDF panels displayed no major difference in WABS with different resin loading at densities between 750 and 850 kg/m3 .

4.

Conclusion

The produced wheat straw MDF met the MDF industry standards for densities above 780 kg/m3 at a resin content above 14%. The fundamental properties of the manufactured SMDF panels were evaluated.

? The mechanical strength properties, i.e., IB, MOR, and MOE were strongly depending on average density, and increased as a function of density. ? The amount of added resin 14–17%, storage time of wheat straw after harvesting, and the pre-treatment of straw with water or acid before re?ning had only restricted effects on the SMDF panels mechanical strength. ? The thickness swelling (TS) and water absorption (WABS) declined with increased SMDF density. ? Modifying the UMF-resin by increasing the melamine content strengthened the internal bond (IB) and improved the water resistance, TS, and WABS. No improvement of the bending properties was observed with increased melamine content. ? Increasing the resin content of 9 mm thick SMDF improved its water resistance and reduced its TS and WABS. The water resistance of 16 mm thick SMDF was not affected by the variation of the resin content. ? Modifying the UMF resin by increasing melamine content resulted in the lowest TS and WABS values. The SMDF was made more water-resistant by adding a resin containing a higher content of melamine than that of conventional UMFresins.

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Acknowledgements
The authors are grateful to the Swedish Knowledge and Competence Foundation for ?nancially supporting this research. Thanks are also extended to all those involved at Metso Panelboard AB and Akzo Nobel—Casco Adhesives AB and Dynea, Gent, Belgium for supplying the formaldehyde resins.

references

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