Wood panel types 2:
|Cement bonded particleboard (CBPB)|
Cement bonded particleboard (CBPB)
Cement bonded particleboard (CBPB) was first commercially manufactured in the early nineteen seventies and has continued to be manufactured in relatively small quantities satisfying the requirements of specialised end-use applications. There are perhaps only about fifty of these mills world-wide (none in the UK anymore) each of which produces on average only about 200 m3/day.
The panel is a mixture of wood particles and Portland cement together with some additives. The first impression of the panel is that it is grey in colour, has a smooth almost polished surface and is heavy. This initial assessment of the panel fails to appreciate its outstanding merits especially in terms of reaction to fire, durability, stability, sound insulation and stiffness.
Composition & manufacturing
Following storage for at least 3 months, the debarked softwood logs of selected species are reduced to flakes some 10 to 30 mm in length and 0.2 to 0.3 mm in thickness using drumknife flaking machines. After passing through a hammermill, the flakes are separated into surface and core material by screening, and are then mixed with Portland cement and water in the ratio by weight of: cement 60%, wood 20%, water 20%. Small quantities of chemicals are added to the wet mix; one of their purposes is to accelerate cement setting. The mat is formed in three layers, the outer layers comprising small chips. Unlike normal particleboard production in a multi-daylight press, the set of cauls in CBPB production must be kept under pressure until the cement has set and this is achieved by fixing a set of clamps to each set of cauls while in the press; these clamps are then released some 6-8 hours later after the set of cauls has passed through a heated chamber at 70 to 80°C. The panels are further dried before shipment. It is the high mass of Portland cement which confers on the product its good reaction to fire behaviour, its very high durability (as a result of the panel having a pH of 11), its high stiffness (E = 4500 N/mm2), it's very good sound insulation and its good dimensional stability relative to other wood-based composites.
Primarily because of its lay up, composition and mass, CBPB is mainly used for specialized applications in construction. Thus, its outstanding merits, especially in terms of reaction to fire, durability, sound insulation and stiffness, render the product most suitable for internal wall construction in public places, lining of lift shafts, construction of cabling ducts, soffits, motorway acoustic fencing and cladding of prefabricated house units.
Flaxboard is an engineered sheet material in which shives from the stalk of the flax plant are bonded together with a synthetic resin adhesive. Flax shives are in fact a by-product of the linen industry.
The flaxboard industry in Europe dates from the late nineteen fifties and until recently, flaxboard was only available in standard panel sizes, because it was produced by daylight presses. Now the technology has developed to produce a continuous board in various lengths, and enable it to be cut into many possible sizes without wastage. Flaxboard can now show excellent surface properties and offers numerous benefits. It is a lightweight board and has natural characteristics which aid fire resistance.
Although flaxboard is similar in some ways to particleboard, it has different properties and applications and should not be used as a chipboard substitute.
Flaxboard is a non-structural product.
Composition & manufacturing process
Flax shives from the stalk of the flax plant comprise the bulk of flaxboard and are prepared in a mechanical chipper and can also contain other raw materials such as particles of wood (wood, flakes, chips, shavings, saw dust and similar materials). These chips are compressed and are generally bound together with synthetic resin systems such as urea formaldehyde (UF) or melamine urea-formaldehyde (MUF), though phenol- formaldehyde (PF) and polymeric methylene di-isocyanate (PMDI) are used by some manufacturers.
The binding system employed depends on the end use intended and the grade of the product. The most common resin employed is urea-formaldehyde, but this is only suitable for use in dry conditions: the other three resin systems confer a measure of moisture resistance to the composite.
Some manufacturers of flaxboard produce a three-layer type, obtained through the separate gluing of the course and fine fractions of flax shives.
Typical constituents of a flaxboard are of the order (by mass) of at least 70% flax shives and which can also contain other raw materials such as particles of wood (wood flakes, chips, shavings, saw dust and similar materials), with the addition of a polymeric adhesive. PMDI, 5-7% water, 2-3 % Nitrated Ammonium and 1-2% paraffin wax solids.
The special properties of flaxboard have several advantages in a wide range of non-load bearing applications. Its lightweight properties and natural characteristics which aid fire resistance make it a natural choice for fire resistant door cores and partitions. Different grades of the product are available for different environmental conditions, ranging from general purpose board for use in dry conditions (for filling purposes and veneering) to non load bearing board for use in humid conditions. The higher grades also find use for interior fitment (including furniture and worktops).
Flaxboard can also find uses for doors, partitioning walls and packaging (protection sheets), table tennis tables, and warehouse shelves.
Plywood is a versatile product with a comparatively high strength to weight ratios that can combine attractive surface appearance with superior performance in a range of uses from dry interior, through high humidity to exterior. It is available in a range of wood species hardwoods, softwoods and mixtures thereof, with the natural durability of the species and the glue type used determining the end use conditions for which the Plywood is suitable. It was developed to provide panels with dimensional stability and good strength both along and across the panel. The term “plywood” includes both the true “veneer plywood” and also the core plywoods of which “blockboard” and “laminboard” are examples.
Composition & manufacture
Standard plywood veneer is still produced using a lathe, which peels a log in a similar manner to a blade pencil sharpener. Most decorative veneer is sliced from flitches after the log is cut into quarters. Small strips of veneer may be jointed into full-size sheets by edge gluing, stitching or using perforated paper adhesive tape. Open defects, such as knot holes, may be repaired using plugs or filler to upgrade the panel in accordance with grading rules. The dried, clipped or reconstituted veneers are sorted into grades, usually by visual inspection.
Synthetic resin adhesive is applied to the veneers which are then assembled with the grain of each normally at 90° to the adjacent veneer. (Plywood with special characteristics is produced when this rule of bonding at right angles is not followed.) The resultant assembly is known as a lay-up which are then subjected to pressure and heat in batches, most commonly in a multiopening (multi-daylight) press. This results in a compressed and cured panel which, after cooling, is trimmed to size and, if necessary, sanded.
Core plywood (Blockboard/Laminboard)
The technique of manufacturing blockboard and laminboard developed alongside the plywood industry from the turn of the century. Blockboard uses strips of wood about 25mm wide for its core, whilst laminboard cores are composed of strips of veneer on edge (or occasionally strips cut from plywood). Introduction of block or laminboard manufacturing facilities can help a mill to utilize residues by producing types of panel suited to interior purposes such as joinery, door blanks, furniture and shopfitting. The method of production is similar to that for plywood and the 'wet' stages of veneer manufacture are identical.
Main types of resin used for plywood manufacture
The relevant European standard describing Plywood bond performance is EN 314 which sets out three bond performances classes:
EN 314 Class 1 for Dry Interior uses
EN 314 Class 2 for High Humidity environments such as covered exterior uses
EN 314 Class 3 for Exterior uses out of ground contact
Plywoood manufactuers should describe their product using one of the three bond classes above, but there is still common useage of the withdrawn term WBP which in performance terms should be equivalent to EN 314 Class 3. This standard makes no restriction on the type of resin to be used but the most common types are:
Urea-formaldehyde (UF) Panels made with this type of resin normally achieve EN 314 Class 1 Bonds and are therefore only suitable for interior use.
Melamine Urea-formaldehyde (MUF) Panels made with this type of resin normally achieve an EN 314 Class 2 bond as they are more resistant than UF to moisture/weather. However, use of additional Melamine allows some manufacturers to achieve exterior EN 314 Class 3 bonds or even marine plywoods using melamine based adhesive.
Phenol-formaldehyde (PF) Panels made with this type of resin will achieve EN 314 Class 3 bonds and are therefore suitable for use in humid or exterior situations.
The relevant European standard for specification is EN 636 which allows manufacturers to declare their Plywood as suitable for use in particular end use circumstances.
EN 636-1 for Dry Interior uses
EN 636-2 for High Humidity environments such as covered exterior uses
EN 636-3 for Exterior uses out of ground contact
Clearly the primary criteria for determining Plywood durability is selection of the appropriate bond performance class hence the close link between the classes in EN 314 and EN 636. However, for exterior end uses the durability of the veneer species should also be taken into account when selecting plywood because of the potential for expose to prolonged high moisture content. In order to achieve long term performance in exterior (EN.636-3) uses all veneers used should ideally have a durability rating of Class 3 (moderately durable) or better in accordance with EN 350-2. See DD ENV 1099 for additional guidance on plywood durability.
The classification of Plywood by surface appearance is covered by many different standards around the world but in Europe this is dealt with by EN 635 “Plywood - Classification by surface appearance”, as follows:
• EN 635 Part 1. General.
• EN 635 Part 2. Hardwood Plywood
• EN 635 Part 3. Softwood Plywood
Limits are set for 5 different grades of veneer with the best grade “E” being virtually clear of all defects. Grades I, II , III permit increasing levels of defects with the lowest class IV having almost no limit on type and size of defect. The limits vary between softwood (EN 635-3) and hardwood (EN 635-2), so care has to be taken to select the correct definition of the grades. Manufacturers, declare the face grade followed by the back grade to achieve panel descriptions such as I/II or II/III.
Although the quality of veneer used in the manufacture of plywood will affect its mechanical properties, the veneer classes given in EN 635 are intended only for use in determining the visual appearance of the panel. They are not intended to be used as a basis for defining a structural grade of plywood.
The final colour of the panel will be affected by the species of veneer used, the finish applied and the effects of weathering and ageing. . Part 4 of EN 635 provides additional guidance for finishing/coating of plywood.
The range of species and bond qualities means that plywood can be engineered to have specific properties making it suitable for a wide range of applications. It is the only wood-based panel having established design values that can be used in structural applications under external conditions in accordance with EN 1955-1-1 Eurocode 5 (EC5) or BS 5268-2.
As necessary with all structural wood based panels, plywood like any other has to be fit for the intended end use in terms of the testing that has been carried out and / or claims made for that end use. This applies to both the specification and the purchasing of plywood. A structural plywood is denoted by an 'S' after the Technical Classification e.g.EN 636-3 S on the product marking e.g. CE mark. However, this simple designation gives no indication of the level of performance. Therefore buyers should always obtain performance data to ensure the specific product they are purchasing is suitable for the end use to which it is destined, and the structural role it will be expected to perform.
Common uses for structural plywoods are in:
• floor decking
• wall sheathing
• flat roofing
• concrete formwork
• external cladding
Marine plywood (BS 1088)
Marine plywood to BS 1088 2003 is manufactured using timbers having a durability rating of Class 3 (moderately durable) or better in accordance with EN 350-2. Exceptionally, low density species such as gaboon, with a durability rating of Class 4 (slightly durable) or better may also be used. In both cases, high quality veneers are used and must be bonded using either a phenolic resin or a melamine-formaldehyde (amino) resin to meet bonding Class 3 (exterior)of EN 314. Marine plywood is available from Israel, France, Malaysia Singapore, and other sources.
Marine plywood was developed for ship/boat building and can have a very high performance under severe exposure conditions. It is also commonly specified for use in construction applications where durability performance is required or where the cost of replacement or consequences of failure warrant the additional cost. In such circumstances in addition to meeting the requirements of BS 1088 2003 the Marine Plywood Manufacturer must also demonstrate compliance with the Construction Products Directive , e.g. by complying with EN 13986. Where such Construction applications are Structural the Manufacturer must also provide Structural performance data for the Plywood.
Other Types of Plywood
There are a wide range of Plywood products available ranging from Utility grades through to those destined for specific applications. Such as those overlaid with veneers, phenolic films or other finishes to give a decorative or hard- wearing finish. Other examples are flexible plywoods, able to be bent into complex curves, to highly compressed, “bullet proof” plywoods. Lightweight panels and panels with an aggregate or non-slip finish are also available. These products are usually advertised by a brand name and should always be used in accordance with the manufacturer's specification.
Wood panel products on GreenSpec
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