3056 Chapter 24 



Marra et al. found that when blended in a ratio of 50 percent southern red oak, 

 25 percent sweetgum, and 25 percent hickory, mats of particles were somewhat 

 more difficult to diffuse with foaming resin than mats produced from northern 

 red oak. Limb wood produced more particles of desired shape than stemwood. 

 Panels made with the southern woods and urethane resin in a ratio of 2:1 had 

 strength properties proportional to density over a range from 10 to 42 Ib/cu ft; 

 modulus of rupture was about 500 psi at a panel density of 10 Ib/cu ft, and about 

 3,800 psi at 42 Ib/cu ft. 



Marra et al. found that wood particles of low moisture content yielded prod- 

 ucts of higher strength and greater stability than moist wood, as follows (wood at 

 70°F): 



Wood moisture content 



Property 10 percent 60 percent 



Mechanical 



Modulus of elasticity, psi 175,000 100,000 



Modulus of rupture, psi 1 ,800 800 



Internal bond strength, psi 200 70 



Stability during 24-hour water soak 



Thickness swell, percent 2 7 



Linear expansion, percent 3 5 



Weight increase, percent 17 62 



Because of the high proportion of resin needed to insure formability, material 

 costs are closely tied to the cost of resin. At costs prevailing in 1975, a board foot 

 of panel at a density of 24 Ib/cu ft required materials (wood and resin) costing 

 about $0.23. For this reason, Marra et al. (1975) concluded that only molded 

 products such as speaker cabinets or possibly caskets could absorb process costs. 

 A further economic analysis of the potential was provided by Marra (1981); his 

 conclusions are summarized in sect. 28-14. 



24-25 CEMENT-BONDED BOARDS 



Dinwoodie (1978) reviewed the numerous attempts to produce a particleboard 

 bonded with portland cement. The German firm Bison- Werke Bahre & Greten 

 GmbH, in partnership with the Swiss Durisol AG, began large-scale industrial 

 DURIPANEL production in 1974 at Dietkon, Switzerland. This cement parti- 

 cleboard has superior dimensional stability, stiffness, fire resistance, durability, 

 insect and fungus resistance, and resistance to freezing/thawing. Its density is 

 about twice that of more conventional particleboard and its cost is between that 

 of particleboard and asbestos-cement board. 



In 1976 Fulgurit-Vertriebsgesellschaft mbH began large-scale production of 

 an air classified, fine-surfaced, wood-cement particleboard in Wunstorf, Feder- 

 al Republic of Germany. The wood used is all spruce {Picea sp.) pulpwood 

 debarked in the forest, chipped and ring flaked, but not dried (Fraser 1977). 

 Water, cement, and other chemicals are added and blended in a horizontal drum 

 with agitators. Air classifying heads form a graduated mat with a superfine 



