RAPE STRAW AS A SUBSTITUTE OF CHIPS IN CORE LAYERS OF PARTICLEBOARDS RESINATED WITH PMDI

Dorota Dziurka, Radosław Mirski, Janina Łęcka
Department of Wood-Based Materials, Poznań University of Life Sciences, Poland

ABSTRACT

In this study the potential application of rape straw as a substitute of wood chips in the core was analyzed in boards, which core was resinated with PMDI and outer layers -resinated with PF, MUPF and PMDI. Rape straw particles were added to the outer layers at 10÷100%. As a result of conducted analyses it was found that rape straw may be an alternative material for the production of wood-based materials meeting requirements of Standard EN 312 in terms of strength and water resistance if the proportion of rape straw particles does not exceed 75% or 25%, respectively, for boards resinated in outer layers with PF and PMDI, or MUPF.

Key words: rape straw, particleboard, resins.

INTRODUCTION

Increasing production and consumption of biofuels in the nearest future will be stimulated first of all by such factors as shrinking reserves of crude oil, growing energy consumption and environmental concerns. The future of the biofuel market seems promising, especially since governments of many countries undertake more and more initiatives aiming at the promotion of alternative fuels. In accordance with the EU directive, in the member countries by the year 2010 the share of biocomponents in the market of vehicle fuels should be at least 5.75%, while in 2020 this proportion should be at least 10%. Thus it may be expected that with the development of the biofuel market the area cropped to rape is going to increase since it is primarily rapeseed oil that is used in biofuel production. The Polish Association of Rape Producers reports that yields of rape and turnip-like rape in 2008 were 1.9 million ton, i.e. they were by approx. 64% higher than mean yields for the years 2001–2005.

In case of biodiesel production from rape it is essential to manage by-products of its processing, including straw. It is important as the yield of straw when harvesting rape is at least equal or even slightly higher than the yield of rapeseeds. So far the primary method of rape straw management has been either its utilization in agriculture or combustion. However, an interesting option seems to be its potential adaptation to the needs of wood-based particleboard industry. The first reports on the possible use of rape straw in the production of particlebo.ards were published as early as the turn of the 1950's and 60's [3] At that time in Poland the first attempts were made to produce particleboards at the "Lenwit" Flaxboard Production Plant in Witaszyce. It was found then that particles of rape straw in combination with chips are good quality material for the production of particleboards to be used mainly as insulation material. They are characterized by higher thermal insulating power, lower hygroscopicity and specific gravity. However, high costs of material preparation and a lack of appropriate binding agents (with high adhesion forces) prevented their continuous production. Considerable advances observed since that time in the field of binding agents used in the manufacture of wood-based materials have resulted in a situation when in recent years research has been intensified on the potential use of non-wood lignocellulose materials in their production [1,2,8,9,10,11,12,14,15].

In recent years also at the Department of Wood-based Materials, the Poznań University of Life Sciences attempts have been made to use rape straw as a partial or total substitute of wood in the production of particleboards using different binding agents [4,5,6,7,13]. Experiments conducted by the authors of this study showed that lignocellulose particleboards may be produced from rape straw particles using resins commonly applied in wood-based materials industry, i.e. UF, PF, MUPF and PMDI. It was found that mechanical properties of this type of particleboards to a bigger extent depend on resination rate than the type of applied resin adhesive [5]. Moreover, it was shown that the use of MUPF and PF as binding agents makes it possible to manufacture particleboards with mechanical properties meeting the requirements of Standard EN 312, defined for particleboard type P3, in case of substitution of wood chips with rape straw in an amount up to 50%. However, with an increase in the proportion of rape straw particles in boards resinated with these resins a considerable decrease in water resistance is observed, measured by swelling in thickness and internal bond after the boiling test. Tests showed that particleboard with high water resistance may be produced, even at a 100% substitution of wood chips with rape straw, only when PMDI is used as a binding agent [4,7].

Thus the aim of this study was to investigate the effect of chip substitution in the core of particleboards with rape straw particles on properties of particleboards resinated in that layer with PMDI, and in outer layers resinated with MUPF, PF and PMDI, respectively. The use of solely PMDI to resinate the core on the one hand should facilitate the production of particleboard with high water resistance and on the other hand, thanks to the application of condensation resins in the resination of outer layers, costs of the agent binding lignocellulose material may be reduced and cake sticking to production machines may be prevented.

MATERIALS AND METHODS

In the production of particleboards pine wood chips were used together with rape straw particles, obtained as a result of double shredding at a knife shredder. Characteristics of chips in the core and rape straw particles are presented in Table 1.

Chip-rape particleboards were manufactured in the following system:

• mass ratio of outer layers to the core of 1:2

• the proportion of rape particles in the chip-rape mixture to be used as the core: 0, 10, 20, 30, 50, 75 and 100% (percentage by weight), with PMDI used in the resination of particleboard at 6%

• resination rate for outer layers of 12% for PF and MUPF, and 8% for PMDI.

In order to test properties of particleboards with different proportions of rape particles in the core, depending on the type of the binding agent used to resinate outer layers, three-layer particleboards with density of 700 kg/m³ and thickness of 19 mm were manufactured under laboratory conditions, using the following pressing parameters:

• pressing time 22 s/mm particleboard thickness

• unit pressure 2.5 N/mm²

• temperature 200ºC

Properties of manufactured particleboards were tested according to the respective European Standards (EN):

• modulus of rupture (MOR) and modulus of elasticity (MOE) [16]

• internal bond (IB) [17]

• internal bond after the boiling test (V100) [18]

• swelling in thickness after 24 h soaking in water and additionally absorbability after 24 h soaking in water [19]

• formaldehyde content [20]

RESULTS AND DISCUSSION

Mechanical properties and water resistance of particleboards depending on the degree of wood chip substitution in the core with rape straw particles are presented on Figures 1–3 and in Table 2. As it results from the presented data, chip substitution with rape straw generally does not affect bending strength and modulus of elasticity of manufactured particleboards (Fig. 1 and 2). Tests showed that irrespective of the type of resin used to resinate outer layers (MUPF, PF or PMDI), strength of particleboards is comparable. In turn, total substitution of chips with rape straw results in a decrease in their strength by only 10%. A similar trend was also observed for modulus of elasticity. Such a trend in strength results from the fact that bending strength is modified first of all by outer layers, characterized by a much higher density than the core, since they are manufactured from microchips with a higher resination rate. Moreover, it also results from conducted investigations that with an increase in the proportion of rape particles in the core, irrespective of the type of resin used to resinate outer layers, internal bond decreases, with the lowest values recorded for particleboards, which core was manufactured solely from rape particles (Fig. 3). A decrease in IB with an increase in the proportion of straw particles indicates poorer chemical adhesion of PMDI to rape straw in comparison to wood. This is because rape straw contains less cellulose and lignin than wood does [5] and it is commonly known that the presence of these compounds has an advantageous effect on strength properties of particleboards. However, even a 100% substitution of wood chips with rape straw facilitates the manufacture of particleboards meeting the requirements of the standard for particleboards carrying loads and used under humid conditions (0.45 N/mm² according to EN 312).

In turn, water resistance tests showed that with an increase in the proportion of rape shreds in the particleboard core hydrophobicity of manufactured boards increases, measured by swelling and water absorption after 24 h soaking (Table 2). The observed decrease in values of these parameters is caused by the less porous structure of rape straw, as a result of which it exhibits poorer absorbability towards water. Moreover, in relation to wood chips it is characterized by bigger elasticity, thanks to which the volume of free spaces at the cross-section is reduced, thus limiting the penetration of water inside the particleboard. Moreover, it needs to be stressed that in case of particleboards resinated in outer layers with PF and PMDI, even a 75% substitution of chips in the core with rape straw makes it possible to manufacture particleboards meeting the requirements of the respective standard in terms of internal bond after the boiling test for particleboards type P5, i.e. particleboards carrying loads, used under humid conditions (0.14 N/mm² according to EN 312).

Due to the type of applied resins manufactured particleboards were characterized by very high hygienic quality and the determined formaldehyde content was comparable to the amount of this compound in wood material alone (Table 2). As could have been expected, wood substitution with rape straw resulted in a further reduction of its content in particleboards.

CONCLUDING REMARKS

Summing up these tests it may be stated that the substitution of chips with rape straw particles in the core of tested particleboards does not have a significant effect on their mechanical properties. Moreover, it was shown that even a 100% substitution of wood chips with rape straw makes it possible to manufacture particleboards meeting the requirements for bending strength, modulus of elasticity and internal bond of the standard for particleboards carrying loads and used under humid conditions (type P5). This was true irrespective of the type of the binding agent applied in outer layers of particleboards. In turn, in terms of water resistance measured by internal bond after the boiling test, the requirements of this standard were met by particleboards, in which the proportion of rape straw in the core was 75% for particleboards resinated in outer layers using PF and PMDI, and 25% for those resinated with MUPF.

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Accepted for print: 9.09.2010

Responses to this article, comments are invited and should be submitted within three months of the publication of the article. If accepted for publication, they will be published in the chapter headed 'Discussions' and hyperlinked to the article.