Mid Sweden University

Rice straw, a waste agriculture material grown and harvested in Willows, CA, was used,is a raw material in the production of thin medium- and high-density fiberboards (MDFs and HDFs). The rice straw was cleaned, size-reduced, and soaked in water before being refined. Defibration was performed in it pressurized pilot-plant single-disk refiner, OHP 20". The fiber production capacity reached a level of 63 kg/h. and the proper fiber quality for MDF/HDF production was established. Analysis of the produced fiber showed an average fiber length of approximately 0.9 mm, in average fiber width of 31 mu m, a shive weight of below 24%, and a dust content of less than 30%. Production of fiberboards was performed by addition of 3%, 4%, and 5% methylene diphenyl diisocyanate (MDI). The flexural properties, internal bond strength, and thickness swelling of the produced fiberboards were evaluated according to ASTM methods. The finished fiberboards based on rice straw and MDI resin showed excellent properties. The internal bond (IB) reached levels of 2.6 MPa, and the modulus of rupture (MOR) and modulus of elasticity (MOE) showed levels comparable to those of wood-based fiberboards and were acceptable according to the requirements or medium-density fiberboard (MDF) for interior applications (American National Standards Institute, ANSI A208.2-2002). The water-repelling properties of the 3-min rice-straw fiberboards were encouraging; the thickness swelling, (TS) was in the range of 15-30%. Two different methods to avoid adhesion between the press plates and the resinated fiber material during hot pressing were investigated: protective paper sheets were placed between the fiber mat and press plates, or a press-release agent was sprayed oil steel plates that were then placed ill the press before pressing Satisfactory results were obtained with both methods, and no adhesion was observed between the fiberboard and the steel plates. The method of using press-release agent during pressing had no notable negative effects oil the fiberboard properties.

The purpose of this thesis was to develop an economical, sustainable, and environmentally friendly straw Medium Density Fibreboard (MDF) process, capable of full-scale manufacturing and to produce MDF of requested quality. The investigated straw was based on wheat (Triticum aestivum L.) and rice (Oryzae sativa L.). In this thesis three different methods were taken for manufacture of straw MDF; (A) wheat-straw fibre was blowline blended with melamine-modified urea-formaldehyde (MUF), (B) rice-straw fibre was mixed with methylene diphenyl diisocyanate (MDI) in a resin drum-blender, and (C) wheat-straw fibre was activated in the blowline by the addition of Fenton’s reagent (H₂O₂/Fe²⁺) for production of non-resin MDF panels.  The MUF/wheat straw MDF panels were approved according to the requirements of the EN standard for MDF (EN 622-5, 2006). The MDI/rice-straw MDF panels were approved according to requirements of the standard for MDF of the American National Standard Institute (ANSI A208.2-2002). The non-resin wheat-straw panels showed mediocre MDF panel properties and were not approved according to the requirements in the MDF standard. The dry process for wood-based MDF was modified for production of straw MDF. The straw MDF process was divided into seven main process steps.

1. 1.       Size-reduction (hammer-milling) and screening of straw
2. 2.       Wetting and heating of straw
3. 3.       Defibration
4. 4.       Resination of straw fibre
5. 5.       Mat forming
6. 6.       Pre-pressing
7. 7.       Hot-pressing

The primary results were that the straw MDF process was capable of providing satisfactory straw MDF panels based on different types of straw species and adhesives. Moreover, the straw MDF process was performed in pilot-plant scale and demonstrated as a suitable method for producing straw MDF from straw bales to finished straw MDF panels. In the environmental perspective the agricultural straw-waste is a suitable source for producing MDF to avoid open field burning and to capture carbon dioxide (CO₂), the biological sink for extended time into MDF panels, instead of converting straw directly into bio energy or applying straw fibre a few times as recycled paper. Additionally, the straw MDF panels can be recycled or converted to energy after utilization.

A relationship between water retention value (WRV) of resinated straw fibres, the thickness swelling of corresponding straw MDF panels, and the amount of applied adhesive was determined. WRV of the straw fibre increased and the TS of straw MDF declined as a function of the resin content. The empirical models developed were of acceptable significance and the R² values were 0.69 (WRV) and 0.75 (TS), respectively. Reduced thickness swelling of MDF as the resin content is increased is well-known. The increase of WRV as a function of added polymers is not completely established within the science of fibre swelling. Fortunately, more fundamental research can be initiated and likely a simple method for prediction of thickness swelling of MDF by analysis of the dried and resinated MDF fibres is possible.

Syftet med denna avhandling var att lägga grunden för en ekonomisk, hållbar och miljövänlig MDF process för halmråvara, kapabel för fullskalig produktion av MDF och goda skivegenskaper. Framställningen av MDF skivor utgick från halm av vete (Triticum aestivum L.) och ris (Oryzae sativa L.). Tre olika metoder användes för att producera MDF av halm; (A) fibrer av vetehalm belimmades i blåsledning med ett melaminmodifierat urea-formaldehydlim (MUF), (B) fibrer av rishalm belimmades i en limblandare med metylen difenyl diisocyanate (MDI), (C) Limfria MDF skivor av vetehalm framställdes med aktivering av fibrer genom tillsats av Fenton´s reagens (H₂O₂/Fe²⁺) i blåsledning utan någon tillsats av syntetiskt lim. Sammanfattningsvis kan det understrykas att framställda MDF-skivor av MUF/vetehalm var godkända enligt standard för MDF (EN 622-5, 2006). Dessutom var framställda MDF skivor av MDI/rishalm också godkända enligt krav i standard för MDF ”American National Standard Institute” (ANSI A2008.2-2002). Limfria vetehalmskivor visade på måttliga skivegenskaper och klarade inte kraven i MDF standard.

Fiberframställningsprocessen för MDF modifierades till en produktion utgående från halm. MDF processen för halm delades upp i sju primära processoperationer.

(1)            Storleksreducering och sållning av halm

(2)            Vätning och uppvärmning av halm

(3)            Defibrering

(4)            Belimning av halmfiber

(5)            Mattformning

(6)            Förpressning

(7)            Pressning

De viktigaste resultaten från denna studie är att MDF av halm kunde produceras utgående från olika typer av halmsorter och lim. Dessutom utfördes MDF-processen i pilotskala och visade på en lämplig metod för framställning av MDF-skivor från halmbalar till färdiga halmfiberskivor. Det miljömässiga perspektivet på att använda jordbruksavfall till framställning av halmskivor är att undvika förbränning av halm ute på fältet, men det är även möjligt att binda koldioxid (CO₂) i halmskivor under längre tid än att omsätta halmråvaran omedelbart som bioenergi eller använda halmfiber som returpapper några få gånger. Dessutom kan MDF återanvändas eller bli omsatt till energi efter användning.

Ett förhållande mellan ”water retention value” (WRV), av belimmade halmfiber, tjocklekssvällning för motsvarande MDF av halmskivor och mängden av tillsatt lim vid olika nivåer har undersökts. Med ökande limhalt tilltog WRV fibersvällning, vidare minskade tjocklekssvällning för motsvarande MDF skivor. De framtagna empiriska modellerna var godtagbara och beräknade R² värden var 0.69 (WRV) och 0.75 (TS). Minskad tjocklekssvällning med ökad limhalt är dokumenterad sen tidigare. Ökad fibersvällning WRV vid tillsats av polymerer (limmer) är inte fullständigt etablerad inom vetenskapen för fibersvällning. Lyckligtvis kan grundläggande forskning initieras och sannolikt föreligger en enkel metod för att prediktera tjocklekssvällning av MDF genom analyser av torkade och belimmad MDF fiber.

Wheat straw was used to produce medium-density fiberboard (MDF). The chemical and physical characteristics of fractionated size-reduced wheat straw were investigated. The pH, pH-buffering capacity, ash, and silicon content increased as wheat straw particle size decreased. Ash of the finest straw, <0.2 mm, had high ash (15%) and silicon (18%) contents. The outer and inner parts of size-reduced straw were analyzed using scanning electron microscopy (SEM). The SEM micrographs revealed a complex ultrastructure containing a notable portion of thin-walled cells approximately 1 mu m thick. Pressurized defibration of size-reduced wheat straw produced lignocellulosic fibers nearly 1.0 mm long combined with approximately 24% of small particles and dust. The high water uptake of straw-based MDF was significantly reduced using melamine-modified urea-formaldehyde (UF) resin and removing wheat straw particles and dust by screening. UF resin was added at levels of 12.5%, 13.1%, and 14%. In terms of water resistance, 12-mm-thick straw MDF displayed thickness swelling below 10%, acceptable according to the EN 622-5 MDF standards. It was concluded that manufacturing wheat straw MDF entails straw size reduction (hammer-milling), removing small particles and dust, and adding melamine-modified UF resin to attain necessary MDF quality standards.

Wheat straw was used as raw material in the production of fibreboards. The size-reduced straw was pretreated with steam, hot water and sulphuric acid before the defibration process to loosen its physical structure and reduce the pH. No synthetic binder was added. Adhesive bonding between fibres was initiated by activation of the fibre surfaces by an oxidative treatment during the defibration process. Fenton’s reagent (ferrous chloride and hydrogen peroxide) was added. Two different levels of hydrogen peroxide (H₂O₂), 2.5% or 4.0% were used. The resulting fibres were characterized in terms of fibre length distribution, shive content, pH and pH-buffering capacity. The properties of finished fibreboards were compared with medium-density fibreboard (MDF) with density above 800 kg/m³ produced from straw and melamine modified UF resin. The modulus of rupture (MOR), modulus of elasticity (MOE) and internal bond (IB) were lower than those of conventional manufactured wheat straw fibreboards but close to the requirements of the MDF standard (EN 622-5: 2006). The water absorption properties for the H₂O₂ activated straw fibreboards were relatively high, but were reduced by 25% with the addition of CaCl₂ into the defibrator system as a water-repelling agent. Increased levels of hydrogen peroxide improved the mechanical and physical properties of the straw fibreboard.

Wheat straw was used as raw material in production of medium density fibreboard (MDF) and high-density fibreboard (HDF). The straw fibreboard process was performed without addition of synthetic binders. Adhesive bonding between fibers was initiated by activation of the fibre surfaces by an oxidative pre-treatment in the defibration process. Hydrogen peroxide was added into the blowline to get a fast and effective process. Adhesive bonding between the activated fibres were later formed when pressing. The fiber quality and pH and pH buffering capacity was analysed at different hydrogen peroxide loadings. The effects of mechanical and physical properties of the non-resin wheat straw MDF/HDF were evaluated. Mechanical bending properties, Modulus of Rupture (MOR), Modulus of Elasticity (MOE), and Internal Bond (IB) were generally lower than conventional wood-based MDF/HDF but close to the requirements of the MDF-standard. Modulus of Elongation (MOE) was surprisingly high and exceeded the levels in the MDF-standard. Moreover, the thickness swelling of the non-resin wheat straw MDF was high but was reduced by the addition of a hydrophobic agent (metallic ion). Increased level of hydrogen peroxide improved the panel properties and the usage of the hydrophobic agent reduced the thickness swelling.

Wheat straw was investigated as a raw material for manufacturing of medium density fibreboard (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 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.

Wheat straw waste materials were processed in a fully equipped pilot-scale MDF-process. Hammermilled wheat straw was used as a raw material in combination with a commercial melamine modified Urea Formaldehyde (UF) resin. Approximately 15 wt % of the wheat straw particles smaller than 0.7 mm was removed in a sifting operation. Medium density fiberboard was produced in the range of 790 to 860 kg/m3 average densities and at a resin content of 12.5, 13.1 and 14 wt %. The panels produced were approved according to the European Standard for MDF (EN 622-5:1997). Four different wheat straw fractions were investigated and sifted at 1.0 mm, 0.6 mm and 0.2 mm screen hole diameter, including the fine particles less than 0.2 mm. The ash content of the four wheat straw fractions varied between 7 wt % and 15 wt %. The maximum level of ash (15 wt %) was observed for the finest particle fraction based on materials less than 0.2 mm. The silicone (Si) content in corresponding ash samples of the sifted wheat straw was analysed by Energy Dispersive X-ray analysis, EDX. The silicone content increased from 18 % to 24 % at a reduced particle size. Moreover, the pH-buffering capacity of the four wheat straw fractions was reduced as the particle size of the specific fractions was increased.

Production of fiber composite materials such as Medium Density Fiberboard (MDF) and particleboard (PB) is in general based on wood as a raw material. However, cereal straws and other annual agriculture waste materials have regained an interest as a potential raw material for production of MDF. The cereal straws are among the most common lignocellulosic materials that are easily accessible, non-expensive and renewable. The aim of this investigation was to produce high performance MDF based on wheat straw and urea formaldehyde (UF) resin. The usage of UF-resin for wheat straw MDF-panels has so far resulted in acceptable strength properties but poor moisture resistance and thickness swelling (TS). Application of melamine modified UF-resin for wood based MDF has improved the moisture resistance of produced MDF panels. In this investigation two commercial melamine modified UF-resins were used as binders (adhesives) in the production of wheat straw MDF. Hammer milled wheat straw was treated with water and sulfuric acid (0.6 %) before refining. The reason was to improve the curing, conditions of the UF-resins by a reduction of the pH and the pH-buffering capacity of refined wheat straw fiber. Refining of wheat straw was performed at slightly lower pressure and retention time compared with refining of wood material. However, a lot of fines and dust (wheat straw fibers < 0.5 mm) were generated during refining. A hi-h resin content of the melamine modified UF-resin was necessary (15 %) to compensate for the high ratio of wheat straw fines and dust. Final panel properties of wheat straw MDF could meet the requirements of the MDF standard (EN 622-5:1997), including the TS. Strength properties as internal bond (IB) and modulus of rupture (MOR) were increased as a function of density. Thickness swelling was reduced as a function of density. The usage of wheat straw as a raw material in combination with a melamine modified UF-resin, as an adhesive, is a possible route for manufacturing of high performance Medium Density Fiberboard.