Environmental Engineering Reference
In-Depth Information
According to Shah [53], the key aspects in developing this kind of materials
include: fibre type (nature, extraction process, surface modification); fibre volume
fraction; fibre/filler geometry and interfacial properties; reinforcement packing
arrangement and orientation; and matrix type and composite manufacturing tech-
nique. From his literature review, the author concluded that it was better to use
bast fibres (mainly from flax, hemp or jute), prepared especially for composite
applications (as opposed to textile processes) with high fibre content (up to 60%
in volume), using pre-impregnation techniques followed by compression-mould-
ing or infusion. Concerning the matrix, thermoset matrices (especially epoxy-
based) are more suitable due to better compatibility, lower viscosity and processing
temperature, and better mechanical properties.
6.4.4
Wood-Based Boards
When wood sheets or particles of wood are bound together to create a stiff board,
many applications are possible. These applications depend on the mechanical prop-
erties and the granulometry of the wood employed and on the density of the board.
In a first classification, we can distinguish between boards made with (1) an
external synthetic binder, such as urea-formaldehyde or phenol-formaldehyde res-
ins for thermosets, and (2) internal natural binders. The products described in the
following sections do not constitute an exhaustive list.
6.4.4.1
Wood Boards with Synthetic Binder
Different commercial products are classified according to the size or the form of
the raw material (wood). All of them require the binder (around 10%) to be
sprayed on the wood material before undergoing hot pressing at temperatures and
pressures of 150-200°C and 20-25 bar.
Wood sheets, obtained by 'unrolling' steamed wood rods with a blade, can be
bound together to form thicker boards. The main characteristic of these 'sand-
wiches' is the alternation of the fibre orientation by 90° in every layer. The anisot-
ropy of the whole board is therefore limited in the length and in the width. Three,
five or seven layers are usually used in these panels, referred to as
plywood
.
The inner layers of plywood are often replaced by oriented strand board (OSB),
obtained from wood chips, to reduce costs; the two outer layers of wood sheets
ensure the esthetical appearance of the panel. The orientation is desired to simu-
late the characteristics of a wood panel, with limited swelling and higher resist-
ance in the fibre direction. Chips are orientated by air with a blowing machine.
These panels are commonly used in building sites and often comprise part of the
walls in particular houses, especially in the United States. If no orientation is
given, the panel is called
waferboard
.
When sawdust is used, particleboards are obtained. The bigger granulometries
are used for
fibreboard
, which is the material mostly used in the fabrication of
