Particleboards

Particleboards are defined as panel materials produced under pressure and heat from particles of wood, flax, hemp or other similar lignocellulosic materials. The wood particles may be in the form of flakes, chips, shavings, saw-dust, wafers or strands (BS EN 309: 2005). Boards may be uniform through their thickness or of a muliti-layer structure. Wood parti-cleboard and cement-bonded particleboard are made from wood chips with resin and cement binder respectively. Oriented strand board is manufactured from large wood flakes and is classified in BS EN 300: 1997.

Wood particleboard (chipboard)

Manufacture

Wood particleboard (chipboard) is manufactured from wood waste or forest thinnings, which are converted into wood chips, dried and graded according to size. The chips are coated with adhesive to approximately 8% by weight and then formed into boards

Fig. 4.27 Structural uses of plywood
Fig. 4.28 Core plywoods

(Fig. 4.29). The woods chips are either formed randomly into boards giving a uniform cross-section or distributed with the coarse material in the centre and the finer chips at the surface to produce a smoother product. The boards are then compressed and cured between the plates of a platen press at 200°C. Boards are finally trimmed, sanded and packed. In the Mende process a continuous ribbon of 3-6 mm particleboard is produced by calendering the mix around heated rollers.

The standard sizes are 2440 X 1220 mm, 2750 X 1220 mm, 3050 X 1220 mm and 3660 X 1220 mm, with the most common thicknesses ranging from 12 mm to 38 mm although much larger sheet sizes and thicknesses from 2.5 mm are available.

Extruded particleboard (BS EN 14755: 2005) is manufactured by extruding the mixture of wood chip and resin through a die into a continuous board; however, in this method the wood chips are predominantly orientated at right angles to the board face, thus generating a weaker material. Extruded particleboard is specified within four grades according to its density and whether it is solid or has tube voids.

Fig. 4.29 Manufacture and standard types of wood particleboard (chipboard)

Three layer

Fig. 4.29 Manufacture and standard types of wood particleboard (chipboard)

Types

The durability of particleboards is dependent upon the resin adhesive. Much UK production uses urea-formaldehyde resin although the moisture-resistant grades are manufactured with melamine-urea-formaldehyde or phenol-formaldehyde resins. Wood chipboards are categorised into seven colour-coded types to BS EN 312: 2003 according to the anticipated loading and environmental conditions. The standard specifies requirements for mechanical and swelling properties and also formaldehyde emissions. The first colour code defines the loading and the second colour the moisture conditions.

Grades of wood particleboard:

Purpose/ Loading

Environmental Conditions

Colour codes

Type

General purpose

dry

white, white

blue

P1

Interior fitments

dry

white

blue

P2

Non-load-bearing

humid

white

green

P3

Load-bearing

dry

yellow, yellow

blue

P4

Load-bearing

humid

yellow, yellow

green

P5

Heavy duty,

dry

yellow

blue

P6

load-bearing

Heavy duty,

humid

yellow

green

P7

load-bearing

Grades of extruded particleboard:

ES Extruded Solid board with a minimum density of 550 kg/m3 ET Extruded Tubes board with a minimum solid density of 550 kg/m3 ESL Extruded Solid Light board with a density of less than 550 kg/m3 ETL Extruded Tubes Light board with a minimum solid density of 550 kg/m3 (Grade ET must have at least 5 mm of material over the void spaces.)

Standard particleboards are hygroscopic and respond to changes in humidity. A 10% change in humidity will typically increase the sheet length and breadth by 0.13% and the thickness by 3.5%. Dry grades should not be exposed to moisture even during construction. Humid-tolerant grades are resistant to occasional wetting and relative humidities over 85%. However, no particleboards should be exposed to prolonged wetting, as they are all susceptible to wet rot fungal attack.

All untreated wood particleboards have Class 3 spread of flame (BS 476-7: 1997). However, they can be treated to the requirements of Class 1 by chemical addition in manufacture, by impregnation or the use of intumescent paints. Class 0 may also be achieved. For untreated particleboard with a minimum density of 600 kg/m3 and a minimum thickness of 9 mm, the Euroclass fire performance rating under the conditions specified in BS EN 13986: 2004 is Class D-s2, d0 for non-floor use and Class DFL-s1 for floorings.

A wide range of wood veneer, primed/painted, paper and plastic (PVC, phenolic film or frequently melamine) finishes is available as standard products. Pre-cut sizes are available edged to match. Domestic flooring grade particleboard, usually 18 mm or 22 mm, may be square-edged or tongued and grooved. The industrial flooring grades are typically from 38 mm upwards in thickness.

Uses

Significant quantities of wood particleboard (chipboard) are used in the furniture industry. Much flat-pack DIY furniture is manufactured from painted or veneered particleboard. Particleboard can be effectively jointed by use of double-threaded particle-board wood screws and various specialist fittings. Where high humidity is anticipated the moisture-resistant grades should be used. The domestic housing market uses large quantities of flooring-grade parti-cleboard as it is competitively priced compared to traditional tongued and grooved softwood. Joist centres should be at 450 mm and 610 mm centres maximum for 18/19 mm and 22 mm particleboard, respectively. Edges should be tongued and grooved or fully supported and the standard panel size is 2400 X 600 mm. For heavy-duty flooring, flat-roof decking and structural work, the moisture-resistant structural grade must be used. Phenolic film-coated parti-cleboard offers a suitable alternative to plywood as formwork to concrete.

Cement-bonded particleboard

Manufacture

Cement-bonded particleboard is manufactured from a mixture of wood particles or filaments (usually softwood) and cement. The boards which are light grey in colour have a uniform cementitious surface. The material has up to 75% cement by weight, with the cement filling all the void spaces, producing a material with a density of 1000-1250 kg/m3, (c.f. 650-690 kg/m3 for standard grade particleboard).

Types and uses

The material based on Portland cement has good resistance to fire, water, fungal attack and frost. The standard (BS EN 634-2: 1997) specifies only one grade, which is suitable for use both internally and externally. It should be colour coded white, white (non-load-bearing) and brown (suitable for dry, humid and exterior conditions). The standard BS EN 633: 1994 refers to both Portland and magnesium-based cements. Magnesite-bonded par-ticleboard is used as a lining board but it is not frost-resistant and is unsuitable for use in humid conditions.

Boards frequently have a core of coarse wood chips, sandwiched between finer material, producing a good finish, which may be further treated by sanding and priming. Because of its density, cement-bonded particleboard has good sound-insulation properties. Typically, 18 mm board will give sound reduction of 31-3 dB. The material is frequently used for soffits, external sheathing and roofing on both modular and timber-frame buildings, particularly where racking resistance is required. The heavier grades, generally tongued and grooved, are suitable for flooring, due their resistance to moisture, fire, impact and airborne sound.

The material has a Class 0 Surface Spread of Flame to Building Regulations (Class 1 to BS 476:

Part 7: 1997). The Euroclass fire performance rating under the conditions specified in BS EN 13986:2004 for 10 mm cement-bonded particleboard is Class B-s1, d0 for non-floor use and Class BFL-s1 for floorings.

Board sizes are typically 1200 X 2440,2600 or 3050 mm, with standard thicknesses of 12 and 18 mm although sheets up to 40 mm in thickness are made. However, because of the density of the material, a 1200 X 2440 X 12 mm board weighs approximately 45 kg and should not be lifted by one operative alone.

Gypsum-bonded particleboard

Gypsum-bonded particleboard, available in sheets of 6 mm thickness upwards, is an alternative multipurpose building board. It is not included in the scope of BS EN 633: 1994.

Oriented strand board

Manufacture

Oriented strand board (OSB) is manufactured from 0.5-mm- thick timber flakes tangentially cut and measuring approximately 75 X 35 mm. These are dried and coated with wax and 2.5% of either phenol formaldehyde or melamine-urea-formaldehyde resin. The mix is laid up in three (or occasionally five) layers with the strands running parallel to the sheet on the outer faces and across or randomly within the middle layer. The boards are then cured under heat and pressure, sanded and packaged (Fig. 4.30). Standard panel sizes are 2440 or 3660 X 1220 mm with densities usually in the range 600 to 680 kg/m3.

Grades and uses

Orientated strand board is graded according to the anticipated loading and environmental conditions (BS EN 300: 1997). Large quantities are used as sheathing in timber-frame housing. The moisture-resistant grade is suitable for roof sarking, whilst the higher specification grade with enhanced strength properties is suitable for flat-roof decking. Thicker panels are used for heavy-duty flooring, and OSB is often used as the web material in timber I-beams. Oriented strand board is manufactured to a thickness range of 6-38 mm, although 9-18 mm sheets predominate. In Europe it is manufactured from Scots pine and spruce, but in North America from aspen and Southern pine.

Grades of oriented strand board:

Grade Purpose/Loading Environmental Colour codes conditions

OSB 1

General purpose,

dry conditions

white

blue

Interior fitments

(hazard class 1)

OSB 2

Load-bearing

dry conditions

yellow, yellow

blue

(hazard class 1)

OSB 3

Load-bearing

humid conditions

yellow, yellow

green

(azard class 2)

OSB 4

Heavy duty,

humid conditions

yellow

green

Load-bearing

(hazard class 2)

The Euroclass fire performance rating under the conditions specified in BS EN 13986:2004 for 9 mm untreated oriented strand board with a minimum density of 600 kg/m3 is Class D-s2, d0 for non-floor use and Class DFL-s1 for floorings.

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