• Density (EN 323)
• Thickness tolerance (EN324-1)
• Tensile strength perpendicular (EN319)
• Screwholding (EN320)
• Bending strength and modulus of elasticity (EN310)
• Swelling in thickness (EN317)
• Wet cyclic test (EN321)
• Boil test (EN112 104-1)
• Dimensional stability (EN 318)
• Grit content (ISO 3340)
• Surface absorption (EN382)
• Formaldehyde measurements

Density (EN 323)   The density of a wood based sheet material is expressed as the weight in kilogrammes of a one cubic meter volume of board (kg/m3). The density of layered boards, for example, furniture grade particleboard, varies widely across the thickness of the board. MDF, with its homogeneous mix of fibres has a more uniform density profile which accounts for its good edge and surface machining characteristics when compared with particleboard.   The density of a 50mm x 50mm test piece is measured by reference to its weight and dimensions and the result calculated using a simple equation.

Although density is often specified in board manufacturer's data sheets, it is not necessarily a reliable guide to performance. For this reason, density levels are not specified in the European Standard EN 622-1. Nevertheless, within any one board type, boards with significantly higher density can reasonably be expected to have higher strength. MDF manufacturers check average density and density profile, often using a highly sophisticated radiation scanning instrument, as part of their quality control procedures. The European Standard EN 622-1 includes a ±7% within board density variation limit as a means of controlling board variability.
 

Thickness tolerance (EN324-1)   The successful operation of highly automated furniture production lines is dependent upon the supply of boards with closely specified properties. As these lines rarely include surface sanding facilities, a close thickness tolerance of supplied boards is desirable. Whilst board manufacturers have complete control of thickness calibration at the mill and can easily achieve close tolerances, any significant changes in moisture content during transportation or storage can adversely affect the thickness tolerance of boards reaching the user. Consequently attention to correct storage conditions at all stages of the supply chain is recommended. A close thickness tolerance is required for MDF used for wood veneering and foil laminating, finishing through roller coaters and membrane pressing. The European Standard EN 622-1 specifies a thickness tolerance of ±0.2mm for boards up to 19mm thickness and ±0.3mm for thicker boards.

Tensile strength perpendicular (EN319)   Tensile strength perpendicular to the plane of the board, often described as internal bond, gives an indication of the resistance of a board to delamination or splitting. This property is measured by bonding two metal or wood holding blocks on either side of a 50mm x 50mm sample cut from the board. The prepared test piece is pulled apart in a tensile testing machine. The tensile strength is the force, in N/mm2 , required to split the test piece, failure normally occuring approximately along the centre line of the test piece. This test which, perhaps more than any other, characterises MDF for interior applications, is widely used for quality control purposes. A high internal bond strength is an important requirement of MDF used for most furniture and fitment applications, but particularly for those applications involving edge fixings and the jointing of panels with mechanical fittings.

Screwholding (EN320)   The ability to hold screws in both face and edges is an important attribute of any wood based sheet material used for the manufacture of furniture or fitments. Many different types and sizes of screws can be used in MDF but for the purpose of determining screwholding strengths according to EN 320, 38 mm length, 4.2 mm diameter, parallel shank, steel screws with two start threads with pitch l.4mm (Gauge No. 8 in ISO 1478/1983) are inserted, where relevant, to a depth of 15 mm into predrilled pilot holes in the face and two edges of the MDF test piece. The resistance to axial withdrawal of these screws is measured using a tensile testing machine.

Whilst the diameter and depth of insertion of a screw will have a significant effect on the measured screwholding strength of MDF, small differences in thread form among commercially available particleboard screws would not be expected to have an effect on screwholding strength although some patented screws have advantages in production, for instance, ease of insertion.

A screwholding capacity requirement of 1000N in the face and 700 to 950N in the edge particular relevance to furniture and fitment manufacturers.
 

Bending strength and modulus of elasticity (EN310)   The deflection of a shelf or other load bearing surface under load is determined by the applied load, the dimensions of the shelf and the modulus of elasticity of the materials used for its construction. Furniture and fitments designers will use the specified modulus of elasticity of MDF as a basis for their calculations on likely deflections of shelves and other horizontal load bearing surfaces in service. Bending strength is more of interest to architects and building materials specifiers concerned with possible structural applications of MDF.

Both these properties are measured by supporting a 50 mm wide test piece on two parallel rollers spaced at a distance 20 times the thickness of the MDF and applying an increasing load through a third roller at the centre of the span. The modulus of elasticity is calculated from the deflection noted when the applied load has reached approximately one third of the failing load.

The bending strength is calculated from the failing load. EN 622-5 includes MDF grades with higher levels of bending strength and modulus elasticity for use in load bearing application in dry and humid conditions.

Swelling in thickness (EN317) Although standard MDF is not intended for prolonged use in extreme damp conditions, the thickness swelling results from a water soak test give an indication of the response of the board to misuse involving intermittent wetting or exposure to extreme damp conditions for short periods. The swelling in thickness of a 50 mm x 50mm test piece during a 24 hours immersion in cold water is measured. Excessive thickness swelling will result in permanent appearance defects and some loss in strength. Moisture resistant boards for use in humid conditions will normally have less swelling in thickness compared with standard grade boards.

Wet cyclic test (EN321) This test consists of a three cycle accelerated ageing treatment followed by strength testing. It is used to assess the likely performance of MDF with improved moisture resistance for applications involving long term exposure to extreme damp conditions with relative humidities consistently up to 80% or the exposure of the boards to intermittent wetting or extreme damp conditions. Each of the three exposure cycles comprises: a) immersion in water at 20°C for three days b) freezing in air at -12°C for one day c) exposure to air at 70°C for three days

The swelling in thickness of the test pieces occurring during the treatment and the tensile strength perpendicular to the plane of the board after the treatment are measured and the results used to characterise board performance in adverse conditions.

Boil test (EN1087-1)

The boil test can be used to characterise the likely performance of some types of moisture resistant MDF and exterior MDF in conditions of high relative humidity or exposure to intermittent wetting. For this test, 50 mm x 50 mm test pieces are boiled in water for two hours and then checked for tensile strength perpendicular to the plane of the board.

Dimensional stability (EN 318) MDF is a relatively stable material compared with solid wood which has large movement across the grain as a result of changes in moisture content. Although the movement of MDF is small, furniture and fitment manufacturers require information on the likely movement in length, width and thickness resulting from exposure to extremes of relative humidity for use as a basis for setting machining tolerances to ensure the optimum fit of parts. Dimensional stability is measured by noting the length, width and thickness of the test pieces after conditioning to constant mass at 35% rh, 25°C and after conditioning at 85% rh, 25°C. The dimensional stability is expressed as the sum of the percentage changes in each dimension between these limits.

The lower limit of 35% rh, equivalent to an equilibrium moisture content of about 6% for MDF, represents the dry conditions associated with use in hot climates or in buildings with central heating. The upper limit of 85% rh, equivalent to about 13% moisture content, represents storage in a damp warehouse or the fitting out of components in a new building before the moisture has been dissipated from the building materials.

Grit content (ISO 3340)

The working lives of saw blades, moulding cutters and routers used on any wood based sheet materials will be reduced by the presence of abrasive particles within the board. As intensive machining operations are often carried out on MDF to take advantage of its good face and edge machining characteristics, the selection of a board with a low average grit content is desirable.

The amount of grit in MDF is measured by reducing a weighed sample to ash by burning and then washing the ash in acid to dissolve any chemical salts. The incombustible, acid insoluble residue consists of small grit particles which can abrade cutting edges. MDF manufacturers using selected timber generally bark free, and appropriate screening or washing treatments can easily ensure that the grit content of their boards stays well below 0.05%.

Surface absorption (EN382) For direct painting of any wood based sheet material to be successful, the surfaces have to be smooth, stable and relatively non absorbent. Unlike smoothness, the surface absorption characteristics cannot be assessed visually. Instead, absorption is measured by allowing a measured amount of paint solvent, toluene is specified for use in the standard method, to fall on the surface of a test panel supported at an angle of 60° to the horizontal and noting the length of board surface wetted before the toluene is absorbed. Boards with high surface absorption as indicated by a short length of wetting, will require higher paint coating weights to achieve a satisfactory paint finish.

Formaldehyde measurements

Many different methods are used to measure the amount of formaldehyde contained in or emitted from wood based sheet materials manufactured with urea formaldehyde binder. These methods can be divided into three groups as follows.

A. Large chamber tests (EN 717-1).

Clean, formaldehyde free air at controlled temperature (typically 23°C) and relative humidity (typically 45% rh) is passed through a chamber containing a specified amount of board with its surface area related to the volume of the chamber which may vary from 12 m3 to about 40 m3. The formaldehyde concentration of the air surrounding the board is measured at intervals until an equilibrium concentration is reached when the amount of formaldehyde emitted from the board surfaces is equal to the amount of formaldehyde removed from the chamber in the controlled flow air stream. This equilibrium concentration, measured in parts per million (ppm), is used as a means of defining the formaldehyde emission characteristics of the board being tested.

The large size of the chamber and the long time to reach an equilibrium concentration makes this type of test unsuitable for quality control purposes and of doubtful value for surfaced or coated boards.

B. Small scale emission testing (EN 717-1, EN 717-2 and EN 717-3).

Several small scale tests are being used in different countries for the measurement of the amount of formaldehyde emitted from the surfaces and edges of test pieces contained in small chambers. In some, the equilibrium concentration, in parts per million (ppm) of formaldehyde, is measured, following the same principle adopted for the large chambers. In others, the total amount of formaldehyde emitted into a controlled air stream in a specified time is measured with the result expressed as a true emission value per unit area of test piece (mg/m2/hour). In a third group of test methods within this general classification of small scale emission tests, the test piece is suspended over water in static air in an enclosed space. The amount of formaldehyde emitted from the test piece and absorbed by the water in a specified time is measured. Having regard for the significantly increased rate of emission of formaldehyde from boards in damp conditions, this method is considered to have some limitations.

Some of these small scale emission methods are being used for certification and for quality control purposes but at present, most MDF manufacturers are using the European standard method, EN120, to measure the total amount of extractable formaldehyde within a board as an alternative to measuring the emission from a board under controlled conditions.

C. Formaldehyde content (EN 120)

The total extractable formaldehyde content of wood based sheet materials including MDF is measured using the perforator method specified in EN 120 and now widely used throughout Europe. For this test, approximately 500 g of board, cut into small pieces, is boiled in toluene in a special set of laboratory glassware. The toluene absorbs the formaldehyde contained in the board pieces. It, in turn, is washed with water and the amount of formaldehyde

originally contained in the board sample but now in solution in the water, is determined by chemical analysis using a colourimetric method. The results are expressed as milligrams of formaldehyde in 100 grams of board (mg/100g).

The perforator method is widely used for quality control purposes by board manufacturers and by independent test houses for certification of compliance with published specifications.

The European Standard EN 622-1 refers to Class E1 boards with a formaldehyde content not exceeding 8mg/100g and Class E2 boards with an upper limit of 30mg/100g.