THE EFFECT OF AMOUNTS OF RAPE STRAW ADDED TO CHIPS ON PROPERTIES OF PARTICLE BOARDS DEPENDING ON THE TYPE OF BONDING AGENT

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

ABSTRACT

Properties of particle boards with varying contents of rape straw were investigated, depending on the type of the applied bonding agent. Rape straw as a substitute of wood chips was added in the manufactured boards in the amounts of 0, 10, 20, 30, 50, 75 and 100%. Tests showed that it is possible to partially substitute wood chips with rape straw in the manufacturing process of particle boards resinated with MUPF, PF and PMDI resins. An up to 30% replacement of wood chips with rape straw in case of MUPF resin and up to 50% for PF resin makes it possible to produce particle boards, the mechanical properties of which meet the requirements of standard PN-EN 312-3. However, the addition of rape straw to boards resinated with MUPF and PF resins resulted in a considerable deterioration of their water resistance, proportional to the amounts of straw particles added to chips. In turn, the application of PMDI resin as a bonding agent makes it possible to manufacture boards with high water resistance and mechanical properties even at a 100% substitution of wood chips with rape straw.

Key words: rape straw, binding agents, lignocellulose board.

INTRODUCTION

The rapid development of the wood based material industry, especially in case of particle boards, and the extending range of their applications have resulted in recent years in an increasing demand for wood material [2]. One of the problems in the developing wood based material industry is to provide appropriate amounts of this raw material. Wood is becoming a deficit material, which encourages board producers to use it more effectively and search for possible substitutes. A source of considerable amounts of lignocellulose material, potentially available for the wood based material industry as a substitute of wood, is agriculture, especially wastes of annual plants, such as flax and hemp shives, bagasse, jute, cotton stems, grasses, oat glumes, straw of the four staple cereals [4, 15, 14, 6 16, 7, 8, 3, 10, 13, 11, 17, 18 9, 1]. This has resulted in a situation, in which in recent years studies on their utilization in the production of board materials have been dynamically developing, especially since in this respect new possibilities have been created by the considerable progress in the synthesis of bonding agents used in their production. An increasing production of biofuel, predicted in the nearest future in numerous countries, including Poland, justifies the assumption that the cropping area of rape, one of the main raw materials for its production, will increase considerably. A by-product in this case would be large amounts of rape straw, up to now used almost exclusively as a fertilizer. It results from investigations conducted by the authors of this study on the possible manufacture of boards solely from rape particles using such bonding agents as UF, PF, MUPF and PMDI resins [5] that it is possible to produce boards with properties similar to those of particle boards, where mechanical properties of these boards were affected to a larger degree by the resination rate that the applied type of resin. However, boards resinated with MUPF and PF resins, used in industrial practice to manufacture boards with enhanced water resistance, exhibited low water resistance. In contrast, the application of PMDI as a bonding agent made it possible to manufacture boards not only exhibiting high water resistance, but also better mechanical properties in comparison to boards manufactured using condensation resins. Studies conducted so far on the possible partial substitution of wood chips with rape straw using UF resin as a bonding agent also showed that the presence of straw particles in the board resulted in the deterioration of its physical and mechanical properties proportional to their share [12]. The aim of this study was to investigate the effect of the amounts of rape straw particles added to wood chips on the properties of particle boards in case when such bonding agents were used in the manufacture as MUPF, PF and PMDI resins, commonly applied in the wood based material industry in the production of materials with enhanced water resistance.

MATERIALS AND METHODS

In the production of experimental boards commercially available pine chips were used along with rape straw particles obtained as a result of double shredding in a knife shredder. Figure 1 presents the fraction composition of wood chips and rape particles on the basis of screen analysis.

Fig. 1. Fraction composition of wood chips and rape particles

The following resins were used as bonding agents: melamine-urea-phenol-formaldehyde (MUPF), phenol-formaldehyde (PF) and isocyanate resin (PMDI). Characteristics of resins used in the study are presented in Table 1.

Table 1. A characteristic of resins applied in the tests

In order to test properties of particle boards with varying contents of rape particles in the board furnish, depending on the type of the bonding agent, single-layer boards with the density of 700 kgˇm^-3 and dimensions of 500 × 600 × 12 mm were produced under semi-commercial production conditions, using the following pressing parameters:

• pressing time 25 s/mm board thickness,
• unit pressure 2.5 N/mm²,
• temperature 200°C,
• resination for MUPF and PF - 10%, PMDI - 8%,
• percentage of rape particles in board furnish: 0, 10, 20, 30, 50, 75 and 100%.

In the manufactured boards the following properties were determined:

• modulus of rupture (MOR) and modulus of elasticity (MOE) according to EN 310
• internal bond (IB) according to EN 319,
• internal bond after the cooking test (V100) EN 1087-1,
• swelling in thickness after 24 h according to EN 317 and additionally absorbability after 24 h soaking in water.

RESULTS AND DISCUSSION

The effect of the amount of rape straw added to wood chips on mechanical properties, depending on the type of resin

Mechanical properties of manufactured boards are presented in Figures 2 – 4. It results from Figure 2 that irrespective of the applied bonding agent internal bond of these boards decreases along with an increase in the content of rape particles in the board, reaching the lowest values for boards manufactured solely from rape particles.

Fig. 2. The effect of the amount of rape straw added to wood chips on internal bond depending on the type of bonding agent

The decrease in IB is highest in case of the application of condensation resins MUPF and PF – by 57% and 64%, respectively, while in case of the application of PMDI this value decreases by only 20%. However, boards produced with a 30% content of rape straw particles and resinated with MUPF and PF meet the requirements of standard PN-EN-312-3 in terms of internal bond (0.40 N/mm²). It also needs to be stated that in spite of the observed trend for the IB value to decrease in boards resinated with PMDI along with the increase in its rape content, even boards manufactured only from rape straw using this resin showed a considerably higher IB value than boards made from only wood chips resinated with MUPF or PF. A slight decrease of IB along with an increase in the share of straw particles may thus indicate a comparable adhesion of this resin both to wood and rape straw. Results of testing for modulus of rupture and modulus of elasticity (Figs. 3 and 4) show that an increase in the content of rape straw particles in relation to wood chips in the board to 30%, using MUPF and PF resins as bonding agents, does not cause as considerable changes in these properties as it was with IB.

Fig. 3. The effect of the amount of rape straw added to wood chips on modulus of rupture of particle boards depending on the type of bonding agent

Fig. 4. The effect of the amount of rape straw added to wood chips on modulus of elasticity of particle boards depending on the type of bonding agent

Increasing their contents in the board to 50 – 75% in both cases results in a slight decrease in bending strength (10%) and modulus of elasticity (14%). A distinct drop in MOR by 23 and 27% and in MOE by 27% and 16% is caused only by the total substitution of wood chips with rape straw particles. In turn, these properties exhibit completely different trends in case of PMDI applied as a bonding agent. The use of this resin results in a situation when along with an increase in the share of rape straw in the board its strength and moduli increase as well, reaching maximum values for boards manufactured solely from straw particles. Such a behaviour of PMDI in relation to rape straw particles on the one hand indicates better availability on their surface of functional groups, capable of reacting with isocyanate, while on the other hand it shows that the reactivity between straw and resin increases along with the increase in temperature, i.e. the advantageous action of this bonding agent is manifested especially in the surface layers of boards, responsible for both MOR and MOE.

The effect of the amount of rape straw added to wood chips on water resistance of particle boards, depending on the type of applied resin

Water resistance of manufactured boards was determined by measuring their swelling in thickness and absorbability after 24 h soaking in water and internal bond after the cooking test (V-100). Mean values of the measurements are given in Table 2.

Table 2. The effect of the amount of rape straw added to wood chips on water resistance of particle boards depending on the type of bonding agent

* standard deviation

The presented numerical data show that in boards in which MUPF and PF resins were applied as bonding agents an increase in the share of rape straw had a negative effect of their water resistance measured with these three parameters. It needs to be stated that values of swelling in thickness for MUPF resin differ considerably from the requirements of the respective standard. It results from the fact that the boards were single-layer boards devoid of the layer of microchips with higher resination and no water proofing substances were added in their manufacture under laboratory conditions. However, while the decrease in swelling in thickness and absorbability along with an increase in the share of rape particles in the boards may be considered comparable in case of the application of both MUPF and PF resin, these differences in internal bond after the cooking test were considerable. The use of PF resin makes it possible to produce boards with distinctly better properties in this respect and even a 30% substitution of chips with rape straw makes it possible to manufacture boards meeting the requirements of standard PN-EN 312-5. Similarly as in case of mechanical properties, also for parameters connected with water resistance boards with a share of rape particles resinated using PMDI resin behave differently. The application of this resin as a bonding agent results in a decrease both in absorbability and swelling in thickness along with the increasing share of rape straw. This phenomenon may be related to a different structure of rape straw particles in comparison to that of wood chips, their considerably lower bulk density, and first of all the capacity to form stable bonds between resin and particles of rape or wood. This is confirmed by the results of measurements of internal bond after the cooking test for boards manufactured using different contents of rape straw. Although the value of this property indicates a downward trend also with an increase in the amount of rape straw in the board, even a 100% substitution of wood chips with rape makes it possible to manufacture boards meeting the requirements of standard PN-EN 312-5 in this respect, in spite of the fact that no water proofing agents were applied in the process of their manufacture.

CONCLUSIONS

1. Partial substitution of wood chips with rape straw particles in the process of manufacturing particle boards is possible when PF, MUPF and PMDI resins are used as bonding agents.

2. The application of MUPF resin as a bonding agent makes it possible to manufacture boards with mechanical properties meeting the requirements of standard PN-EN 312-3 in case of an up to 30% substitution of wood chips with rape straw. The application of PF resin makes it possible to increase the share of rape straw in the board furnish to 50%.

3. Water resistance of the manufactured boards decreases along with the increase in the share of rape straw particles in boards resinated with both PF and MUPF resins.

4. The application of PMDI resin as a bonding agent makes it possible to manufacture boards with high water resistance and very good mechanical properties even at a 100% substitution of wood chips with rape straw.

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Accepted for print: 06.09.2006

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Dorota Dukarska
Department of Wood-Based Materials,
Poznań University of Life Sciences, Poland
Wojska Polskiego 38/42, 60-637 Poznan, Poland
Phone: +48 61 8487558
email: ddukar@au.poznan.pl

Dorota Dziurka
Department of Wood-Based Materials,
Poznań University of Life Sciences, Poland
Wojska Polskiego 38/42, 60-637 Poznan, Poland
email: ddziurka@up.poznan.pl

Janina Łęcka
Department of Wood-Based Materials,
Poznań University of Life Sciences, Poland
Wojska Polskiego 38/42, 60-637 Poznań, Poland
Phone: +48 61 8487419
email: janinal@up.poznan.pl

Radosław Mirski
Department of Wood-Based Materials,
Poznań University of Life Sciences, Poland
Wojska Polskiego 38/42, 60-627 Poznań, Poland

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