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Background knowledge for the production of PAVATEX wood fiber boards

PAVATEX wood fiber boards are manufactured using the so-called eco-friendly wet process. The wet process production uses the woods own cohesiveness (predominantly lignin resin) for the bonding of the finished material. This is done by decomposing the wood into its fibers in a thermomechanical process. The resulting pressed fiber will then be brought to set with the application of heat. This process does not require any additional chemical bonding agents.

History of the development of the wet process
The production of wood fiber boards using a wet manufacturing process started with the invention of a method to industrially and efficiently defibrate wood in 1926. This lead wood fiber boards to be among the first derived timber products which initially were used to make furniture and, later on, used increasingly in the building industry. Due to its advantages such as dimensional stability or large formats, the new product quickly experienced rapid further development.

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Wood fibers/raw sheets
Fiber boards are predominantly made from coniferous woods. The preferred wood types are spruce and fir, with the occasional use of pine and larch. Among the essential advantages of coniferous woods are their ample availability and a very good fiber quality that render the finished boards a high stability compared to their gross density. Among the raw materials is mostly residual wood fresh from the saws of sawmills in the shape of slabs and splinters as well as wood chips. Slabs and splinters are further processed to chips inside the plant. In thisphase the wood moisture is above 35 %.

Description of wet process wood fiber board production
The wood chips are exposed to water vapor and soaked at a pressure level of 3 to 8 bar and prepared for the defibration process to follow.

These days this is mostly done by applying the defibration process between molded metal grinding blocks Depending on the specific requirements of the various products, the fibers may undergo a second milling process on a raffinator.

By way of these decomposition processes the surface of the fibers will be activated to the extent that the wood’s own cohesiveness, during the subsequent drying process of the wood fiber insulating material, will create the bonding with the addition of water. Any addition of adhesives to promote the bonding is therefore not necessary. If, for example, binders containing bitumen (approx. 2 mass percent) are still added for individual products, this is done for the purpose of quality improvements which cannot be attained with the raw material alone.

The fibers, which are up to 98% slurried in water, will at first be temporarily stored in tubs and subsequently formed to pressed fiber using a shaping machine. After mechanically squeezing out the majority of the water, the pressed fiber will be cut lengthwise and be transported to the drying tunnel. Inside the drying tunnel, the wood fiber insulating sheets are dried at temperatures ranging from 160 to 220°C.

Pavataex achieved significant reductions in the use of fresh water as a result of a closed water circuit with connected purification, as well as reductions in their energy consumption for the defibration, drying, treatment and other steps in the production process. These efforts were honored with the European Environmental Award by an international jury.

Forming line/ stock
The thickness producible for single-layer for wood fiber insulation sheets is approx. 8 to 30 mm. The raw sheets (approx. 2.50 x 5.25 m) are glued with PVAC glue in order to manufacture multilayer blocs. Afterwards they will be cut into the desired shape and, depending on the product, equipped with tongue and groove/crown using a double-end tenoner.