P Jaskólowski 
  
Borysiuk 
  
M Ł Mamiński 
  
M E Król 
  
W Jaskółowski 
  
P Borysiuk 
  
EJWWP470_source Wood-mineral wool hybrid particleboards

   

Wood-mineral wool hybrid particleboards

Introduction

There are several types of insulating materials commonly used in engineering. The three main groups of the boards used in house construction are: (1) natural wood-based materialslike low density fiberboard, insulation board from kenaff, hemp or wood fiber; (2) synthetic foams (polystyrene or polyurethane) and (3) boards based on natural inorganic materials (glass wool or mineral wool). On the contrary to wood-based materials or synthetic foams, the main advantage of mineral wool, apart from excellent insulation properties, is its inflammability [START_REF] Karamanos | The impact of temperature and moisture on the thermal performance of stone wool[END_REF].

There are some reports on compounding wood material with mineral components -like vermiculite (Kozłowski et al. 1999), cement [START_REF] Okino | Physico-mechanical properties and decay resistance of Cupressus spp. cement-bonded particleboards[END_REF][START_REF] Qi | Effect of carbon dioxide injection on production of wood cement composites from waste medium density fiberboard (MDF)[END_REF] or potassium aluminosilicate [START_REF] Giancaspro | Fire resistance of inorganic sawdust biocomposite[END_REF].

In some applications low flammability of the materials accompanied by retention of mechanical properties is required. In such cases, the use of particleboards with reduced flammability seems to be reasonable. Thus, an attempt of hybrid wood -mineral wool boards manufacturing by compounding of mineral wool with wood particles was undertaken. In this paper some thermal, sorptive and mechanical properties of the manufactured boards were described.

Materials and methods

Three series of one-layer particleboards (330 x 330 x 12 mm 3 , density 600 kg/m 3 ) with 10, 20 or 30 wt% on dry wood basis of mineral wool were prepared. The boards were made of industrial grade chips (6.0% moisture content) and mineral wool (2.2% moisture content). A commercial urea-formaldehyde (UF) resin was used as adhesive. Glue formulation: UF 50 parts by weight, 10% ammonium chloride 1.2 parts by weight, water 12 parts by weight. Glue rate 10 wt%. Same boards without mineral wool were used as controls.

Compounding of the materials involved three steps: (1) shredding of the wool into small pieces (ca. 5-10 mm), (2) mixing of the wool with wood particles, (3) blending of the mixture with glue. Then, a mat was formed and pressed under the following conditions: platens temperature 180ºC, maximum unit pressure 2.5 MPa, time 291 s. Prior to testing, the boards were conditioned at 20±2 ºC and 65±5% RH for 7 days.

The ignitability was tested according to EN ISO 11925-2, modulus of rupture (MOR) according to EN 310 and internal bond (IB) according to EN 319. Thickness swelling was measured according to EN 317. Thermal conductivity and thermal capacity were analyzed on an ISOMET 2140 heat transfer analyzer (Applied Precision) instrument. Density profiles were measured on an X-ray density analyzer DA-X (GreCon).

Results and discussion

During ignitability tests neither 15 nor 30 s flame application caused self-sustainable fire. At last, 120 s flame treatment occurred to be necessary for ignition and path smouldering onto samples. As indicated in Table 1, there is no difference between the length of the smouldering path of the control and the 10% series. Only 20% addition of wool affected and shortened smouldering path by 19%, while 30% addition of wool reduced that parameter by 30% when compared to the controls. The reduction in ignitability is proportional to the content of the mineral component.

The values of thermal conductivity and thermal capacity measured for the studied boards are shown in Table 1. The data indicate that as small as 10% content of wool did not alter significantly those parameters, however, 20% and 30% additions resulted in a stepwise decrease in thermal conductivity by 7%, whilst changes in thermal capacity were monotonic and achieved 11% and 19%, respectively, when compared to the controls. Having in mind that thermal conductivity for uncompressed mineral wool is 0.033 W/mK and that wool within the board is somehow compressed, the effect of the resultant efficacy of insulating is reasonable.

Unlike thermal properties, the highest modulus of rupture (MOR) and internal bonding (IB) values were observed for the controls. Thus, it is obvious that addition of wool resulted in a decrease in mechanical properties -MOR by 27% -65% and IB by 32% and 71% for 20% and 30% of wool, respectively. However, 10% addition of wool did not affect IB. The deterioration of mechanical performance is ascribed to the low cohesion of the wool domains within the board. It is worth noting that the density profiles of the hybrid boards and that of the control shown in Fig. 1 are very much alike which proves satisfying dispersion of wool EJWWP470_source among wood particles, and that required density of a board can be achieved. Therefore, the proposed method of compounding was found efficient.

Table. 1

Fig. 1 Surprisingly, the obtained values of thickness swelling exhibited by 10%-series were slightly reduced which can be explained by compensation of dimensional changes within the wool domains, so that overall swelling of the board was avoided. For the higher content of wool, the effect was overwhelmed by an intense overall swelling.

Conclusion

In summary it can be concluded that hybrid particleboards with 20% mineral wool exhibit improved insulating properties and reduced ignitability, which -unfortunately -are accompanied by a deterioration of the mechanical properties. The described approach based on typical particleboard preparation procedure allows for compounding of mineral wool with wood particles. However, for the improved performance of the resultant hybrid particleboards, degree of wool shredding and wool pieces dimensions as well as glue load and pressing parameters should be considered for future work. 

Fig. 1

 1 Fig. 1 Density profiles of the studied boards

  

Table . 1

 . Selected properties of the hybrid boards studiedTabelle 1 Ausgewählte Eigenschaften der untersuchten Hybridplatten Values in parentheses are standard deviations. **λ for mineral wool is 0.033 W/mK.

	EJWWP470_source						
	Wool	Density Smouldering	Thermal	Thermal	MOR	IB	Thickness
	content		path length	conductivity	capacity			swelling
								after 24 h
								soaking
	[wt%]	[kg/m 3 ]	[cm]	λ [W/mK]	Cp [J/m 3 K]	[N/mm 2 ]	[N/mm 2 ]	[%]
	0	597 (18)	13.5	0.144	0.765	10.9 (0.9) 0.31 (0.03)	36.1 (5.0)
	10	595 (31)	13.5	0.143	0.755	7.9 (1.4)	0.31 (0.04)	31.6 (2.3)
	20	596 (32)	11.0	0.134	0.680	6.4 (0.8)	0.21 (0.03)	39.6 (6.0)
	30	570 (42)	9.5	0.133	0.619	3.8 (0.7)	0.09 (0.01)	49.1 (9.1)
				Fig. 1				

*