Composition and manufacturing process
The timbers used in OSB manufacture include both softwoods (spruce, pine) and hardwood (aspen). Wood strands are cut tangentially from debarked logs which are held longitudinally against rotating knives. The ribbon of flakes produced is usually about 75mm wide and this breaks up on handling to produce individual flakes which are 75mm along the grain and from 5 to 50mm across the grain.
After drying, these flakes are generally sprayed with a synthetic resin, though one or two mills employ powdered resins. One of the important points in OSB manufacture is the removal of fines prior to resin application: this results in the use of lower amounts of resin in OSB (2-3%) compared with other resin bonded panel products.
The three main adhesives used in the production of OSB are phenol formaldehyde (PF), Isocyanates (MDI or PMDI) and melamine urea formaldehyde (MUF). These are either used on their own or the core and the surface layers may use two different types of adhesive.
All these resins confer a measure of moisture resistance to the composite.
In the formation of the mat the strands are aligned either in each of the three layers of the panel, or, more frequently, in only the outer two layers. The degree of orientation varies widely within any one panel, and also in panels from different manufacturers; thus, in panels from different manufacturers it is possible to obtain ratios of property levels in the machine- to cross-direction of 1.25:1 to 2.5:1, thereby emulating the ratios found in plywood.
The boards are subsequently pressed with heat to cure the adhesive and compress the strands to form a compact and dense product to the desired thickness.
There may be a sanding process following pressing and therefore it is possible to obtain sanded boards as well as un-sanded versions.
Applications
Because of its lay-up and composition, OSB is primarily a panel for construction and is widely used for flooring, flat roof decking and wall sheathing but is used in furniture where it is not seen. Different grades of the product are available for different levels of loading and different environmental conditions. Guidance on the use of OSB in these load-bearing applications is given in DD CEN/TS 12872. Generally, for the same loading conditions, a thinner board of OSB can be used than a load-bearing particleboard.
There are currently “deemed to satisfy” tables for domestic floor and roof applications in BS 8103-3 (Code of practice for timber floors and roofs for housing), for plywood and particleboard. However there is no deemed to satisfy route for non-domestic floor applications at present.
Large quantities of OSB are also used for sarking and industrial packaging and in the construction of site hoardings and pallet tops.
