PROPERTIES OF BOARDS MANUFACTURED FROM RAPE STRAW DEPENDING ON THE TYPE OF THE BINDING AGENT

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Electronic Journal of Polish Agricultural Universities (EJPAU) founded by all Polish Agriculture Universities presents original papers and review articles relevant to all aspects of agricultural sciences. It is target for persons working both in science and industry,regulatory agencies or teaching in agricultural sector. Covered by IFIS Publishing (Food Science and Technology Abstracts), ELSEVIER Science - Food Science and Technology Program, CAS USA (Chemical Abstracts), CABI Publishing UK and ALPSP (Association of Learned and Professional Society Publisher - full membership). Presented in the Master List of Thomson ISI.

Copyright © Wydawnictwo Uniwersytetu Przyrodniczego we Wroclawiu, ISSN 1505-0297

Dziurka D. , Mirski R. , Łęcka J. 2005. PROPERTIES OF BOARDS MANUFACTURED FROM RAPE STRAW DEPENDING ON THE TYPE OF THE BINDING AGENT, EJPAU 8(3), #05.
Available Online: http://www.ejpau.media.pl/volume8/issue3/art-05.html

 Dziurka, Radosław Mirski, Janina Łęcka

Department of Wood-Based Materials, Poznań University of Life Sciences, Poland

 

ABSTRACT

Properties of boards manufactured from rape straw depending on the type of the binding agent and resination rate were investigated in this study. Tests showed that lignocellulose boards with properties similar to particle boards may be produced with the application of UF, PF, MUPF and PMDI resins as agents binding straw particles, while mechanical properties of boards made from rape straw are affected to a larger extent by resination rate than the type of the binding agent. In contrast, the application of isocyanate resin at lower resination rates (6%) makes it possible to produce boards with higher water resistance and improved mechanical properties in comparison to boards manufactured with the other resins and thus it may be used in industrial practice.

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

INTRODUCTION

The increasing range of applications for wood based board materials, as well as the development of the private house building sector have resulted in the recent years in increasing production of these materials [1]. Further development of the industry of wood based board materials in Poland will require increased supplies of timber used in their production. However, the depleting stocks of these raw materials [21], together with economic reasons, force producers to search for possible extension of the raw material resources. This trend is manifested in a considerable increase in the efficiency of utilization of industrial wood refuse, especially sawdust, medium-sized wastes, recycled wood [24], as well as annual plants. So far such annual plant wastes as flax and hemp shives, sugar cane, rice straw, jute and cotton fiber have been used in the production of lignocellulose boards [10, 14, 11, 25, 6].

In recent years numerous studies have been devoted to the application of cereal straw as an alternative source for the production of lignocellulose boards [23, 2, 22, 7, 12, 20, 4, 5, 13, 17, 1]. It turned out that boards produced according to the developed technologies from straw or wood particle boards with the addition of straw, resinated with isocyanate resins exhibited higher static bending strength and better hydrostatic properties than boards produced solely from wood. They are also characterized by smoother surface. Moreover, a technology was also proposed to manufacture MDF boards from straw, after previous chemico-thermomechanical processing of the raw material, making it possible to apply low pressure and low pressing temperature (from 40 to 100°C). Research projects currently in progress, focusing primarily on the improvement of adhesion to binding agents cheaper than isocyanate resins, suggest that cereal straw, especially wheat, will be used in the production of lignocellulose boards in an increasing range of applications. For instance it is suggested in those studies that the surface of particles may be processed using various chemical methods combined with thermomechanical procedures [8, 9, 26]. Moreover, progress has been observed in research connected with the modification of traditional wood binding agents towards increasing their adhesion to straw [18].

Another raw material which may be applied in the industry of wood based materials is rape straw. The expected increase in rape production in Poland, connected with the intended introduction of biocomponents as additives to conventional motor fuels, results in enhanced interest in the utilization of the expected growing amounts of rape straw. So far the basic method of utilization of rape straw has been its use in agriculture or incineration in order to obtain energy [19]. Attempts have also been undertaken to produce pulp for the industry of wood based boards, although the efficiency of this process, as well as the low quality of the obtained pulp indicate poor prospects for this type of utilization [16].

Up to now the few attempts to use rape straw in the production of particle boards have not been conclusive [3, 15]. For this reason the aim of this study was to investigate the possible application of rape straw as a substitute of particles in the manufacture of particle boards, depending on the type of the binding agent.

MATERIALS AND METHODS

Rape straw particles were obtained as a result of their disintegration in a knife shredder. Before the board manufacturing process the chemical composition of straw was investigated according to the chemical composition determination methodology developed for wood.

Urea-formaldehyde (UF), phenol-formaldehyde (PF), melamine-urea-phenol-formaldehyde (MUPF) and isocyanate (PMDI) resins were applied as binding agents. A characteristic of binding agents used in the experiments is presented in Table 1.

Table 1. Characteristic of binding agents used in the experiments

Kind of determination	Unit	Properties of resin
		UF	MUPF	PF	PMDI
Density at 25°C	g.cm-3	1.29	1.3	1.23	-
No. 4 Ford Cup viscosity	s	123	66	143	-
Dynamic viscosity at 25°C	mPa.s	-	-	-	179
Solids content	%	71	69	51	100
Miscibility with water	-	1.4	0.8	-	-
Gel time at 100°C at 130°C	s	83 -	140 -	- 225	-
pH	-	9.95	9.34	11.2	-
Acidity as HCl	mg.kg-1	-	-	-	96
NCO-content	%	-	-	-	31.9

For the purpose of testing the properties of boards produced from rape particles, single-layer boards with the density of 700 kg . m^-3 and dimensions of 500 × 600 × 12 mm were manufactured under semi-commercial conditions at the Department of Wood-Based Materials, applying the following pressing parameters:

• pressing time 5 min
• unit pressure 2.5 MPa
• temperature 200°C
• resination rate depending on the type of resin was:

8, 10, 12 and 14% for UF and MUPF; 6, 8, 10 and 12% for PF and 4, 6, 8 and 10% for PMDI, respectively.

DISCUSSION OF RESULTS

Properties of rape straw

The chemical composition of wood differs from the chemical composition of straw. In comparison to wood rape straw contains less cellulose and lignin, but more hemicelluloses, mineral compounds and waxes (Table 2).

Table 2. Chemical composition of rape straw in comparison to spruce wood

Component	In relation to dry mass of raw material (%)
	rape straw	spruce wood
Cellulose	37.55	54.09
Hemicelluloses	31.37	23.40
Lignin	21.30	30.15
Extraction substances	3.76	1.47
Mineral compounds	6.02	0.24

Undoubtedly cellulose has an advantageous effect on strength properties of boards; however, a bigger role is played by the contents of extraction substances, as they determine bond quality. Values presented in Table 2 show that amounts of extraction substances in rape straw are slightly higher than the contents of such substances in wood, although - similarly to wood - they are dispersed throughout its mass. For this reason, in contrast to cereal straws, in which olefin substances are mostly gathered on the surface, in this way hindering resination, they should not have a disadvantageous effect on the gluability of rape straw particles, even in case when typical polar wood adhesives are applied.

Mechanical properties of rape boards depending on the type of resin and resination rate

Properties of rape boards resinated with UF, MUPF, PF and PMDI resin in terms of resination rate are presented in Table 3.

Table 3. Mechanical properties of rape boards depending on the type of resin and resination rate

Resination rate	IB				MOR				MOE
%	MPa
	UF	MUPF	PF	PMDI	UF	MUPF	PF	PMDI	UF	MUPF	PF	PMDI
4	-	-	-	0.24 0.01*	-	-	-	6.2 1.0	-	-	-	1090 90
6	-	-	0.27 0.02	0.59 0.01	-	-	9.2 0.9	10.9 1.1	-	-	1380 130	1560 160
8	0.28 0.03	0.31 0.01	0.30 0.03	0.82 0.02	11.0 0.8	11.1 0.9	9.4 1.2	14.7 0.8	1830 90	1750 150	1580 90	1940 135
10	0.32 0.02	0.35 0.02	0.35 0.02	0.83 0.03	12.0 0.9	12.8 1.0	11.2 0.8	15.2 0.9	2010 120	1880 120	1770 120	2160 130
12	0.35 0.02	0.36 0.02	0.54 0.01	-	12.6 0.8	14.0 0.7	12.4 0.7	-	2360 80	2150 110	2020 110	-
14	0.51 0.01	0.37 0.02	-	-	14.2 0.9	16.4 0.9	-	-	2500 160	2410 130	-	-

* - standard deviation

It results from the data presented in the table that along with the increase in the resination rate of the boards, irrespective of the type of binding agent used, static bending strength increases as well, while in terms of the specific resination rate of the boards with condensation resins the best strength properties were found for boards resinated with a quaternary MUPF resin. Similarly, along with the increasing resination rate of the boards an increase in the modulus of elasticity is observed. However, in that case at identical resination rates the values of moduli of elasticity for boards resinated with PF resin were slightly lower. This phenomenon could have been caused by the difference in the resin polycondensation rate, as well as the reactivity of the resins measured by gelation time at the temperature of 100°C and 130°C (Table 1). It is probably the cause for which PF resin cured in the board remains more elastic. The measured values of internal bond also remain in direct proportion to the resination rate of boards with these resins. No distinct differences were observed in the results of determinations for resination rate from 8 to 12% for UF and MUPF resins and from 6 to 10% for PF resin. In case of the two former resins an increase in resination rate to 14% results in internal bond in boards resinated with UF resin increasing by 38% than the value for the board resinated with MUPF resin and it is similar to the level in the board resinated with PF resin at 12%. Also in that case the reactivity of the applied liquid resins seems to be responsible for that fact. It may generally be stated that the requirements of standards for particle boards for furniture production and interior fittings in terms of their mechanical properties (PN-EN 312-3) were met only by rape boards resinated with UF resin at 14%. Lowering resination rate in case of that resin resulted in a decrease in static bending strength and internal bond below values admissible by the standard (14 MPa and 0.40 MPa respectively). Rape boards resinated with MUPF resin met the requirements of the standard in terms of bending strength and modulus of elasticity starting from 12% resination, although even at 14% resination rate they did not reach internal bond required by the standard. As could have been expected, at identical resination rates boards resinated with PMDI resin exhibited considerably better mechanical properties than boards resinated with condensation resins. It needs to be stated that low values of static bending strength and modulus of elasticity for 4% resination with PMDI resin make it inadmissible to apply such a low resination rate under commercial conditions. On the other hand, high values of internal bond starting from 6% resination of boards with this resin are worth mentioning. They are definitely better than those obtained for the other types of resins. It may have been caused by the chemical adhesion of isocyanate adhesive to the components of rape straw.

Water resistance of rape boards depending on the type of resin and resination rate

Results of water resistance tests depending on the type of resin and resination rate, measured by their swelling in thickness after 24 h and internal bond after a boiling test are presented in Figures. 1 and 2. It results from the data presented in the figures that water resistance of boards is significantly dependent both on the resination rate and the type of the adhesive. Definitely the lowest swelling (Fig. 1) was found for boards manufactured from rape particles resinated with isocyanate resin, while an increase in resination rate from 4 to 6% increased their hydrophobicity in the highest degree, to as many as 62%. While comparing the calculated values of swelling in thickness for boards produced with the application of all the resin types it may be stated that after 24 h soaking the swelling of boards resinated with PF and UF resins, only at their maximum addition amounting to 12% and 14%, respectively, makes it possible to obtain comparable values with those for boards resinated with 4% share of PMDI resin. Slightly better hydrophobicity was observed for rape boards resinated with a quaternary resin, as for maximum resination (14%) their swelling generally is at the level of swelling of boards resinated with a 6% addition of PMDI resin. However, the obtained values of swelling in thickness of boards resinated with condensation resins considerably diverge from the values required by the appropriate standard specification. The cause for such a situation may be considered the fact that hydrophobic substances were not added to boards manufactured under experimental conditions, as well as the fact that these were single-layer boards, devoid of the external layer of fine particles with a higher resination rate. This layer in commercial boards constitutes an effective barrier delaying the penetration of water inside the board. In this way the slight internal bond of boards resinated with MUPF after a boiling test may be explained (Fig. 2). In turn, both in case of phenol and isocyanate resins an increase in this value in direct proportion to resination rate of these boards is observed, while boards resinated with isocyanate resin reach values provided in the standard PN-EN 312-5 (0.15 MPa) already at 6% resination rate, while boards resinated with PF resin reach this value only at resination rate of 12%.

Fig. 1. Swelling in thickness of rape boards depending on the type of resin and resination rate

Fig. 2. Moisture resistance after a boiling test of rape boards depending on the type of resin and resination rate

CONCLUSIONS

1. The investigations showed that it is possible to produce lignocellulose boards from rape straw particles applying UF, MUPF, PF and PMDI adhesives, commonly used in particle board production, as binding agents.
2. Mechanical properties of boards made from rape straw, such as static bending strength and Young´s modulus are dependent to a higher degree on resination rate of straw particles than the type of the binding agent used. 
3. Boards resinated with MUPF and PF resins, applied under industrial conditions to manufacture particle boards with increased water resistance, showed very low water resistance measured by their swelling in thickness after 24 h and internal bond after a boiling test. On the other hand, the application of PMDI resin as a binding agent made it possible to produce boards with considerable water resistance already at 6% resination rate.
4. Although the high price of isocyanate resin the application of this resin as an agent binding rape straw particles makes it possible - at lower resination rates - to manufacture boards not only with higher water resistance, but also better mechanical properties in comparison to those produced with cheaper condensation resins.

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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

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

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

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