Formaldehyde concentration particleboard

Figure 7. Effect of air change rate and loading on chamber formaldehyde concentration - particleboard.

If we consider as an example a relative air humidity of 50% and a temperature of 25 l, the wood moisture content would be 9.2 wt% (3. If we further consider that the product manufacturing process leaves about 1 wt% of the formaldehyde content of the UF resin as unreacted formaldehyde, we obtain for particleboard or medium density fiberboard (MDF), where UF-resin makes up 6-10 wt%, an approximate formaldehyde concentration of 0.2 M in the S-2 cell of the wood. 

Guideline for the Use of Particleboards with Respect to the Avoidance of Unacceptable Formaldehyde Concentration in Room Air Commission for Uniform Technical Building Specifications, Beuth Publishers Berlin, April 1980. 

When we place a piece of particleboard with a surface area of A in an enclosed space with a volume of V m, in which at time zero no formaldehyde gas is present in the air (Figure 1), it is known that the particle board will release formaldehyde into the air and that, viewed over a period of time, the rate of release will not be constant but decreases as the formaldehyde concentration Cg in the environment increases, until a certain maximum concentration... 

From the results it can be concluded that the formaldehyde concentrations in the exit air in situation 2 differ from those in situations 1 and 3, which are almost the same. The reason is that in situation 2 the exchange of formaldehyde between the particleboard and the air concerned, was not complete. So the measurements in situation 2 do not fit in with the equilibrium determined. 

The effect on particleboard of an ammonia treatment can also be shown using this testing method. In figure 8 again the ideal mixing model is applied. Notice that the line with the lowest emission is the one on the top. The reason is that the reciprocal values and not the steady state formaldehyde concentrations as such, are plotted. Here the slope is different as well. 

Figure 9 illustrates the effect of veneering on formaldehyde emission of particleboard. For the veneering the same type of resin was used as in the production of the particleboard. Pressing conditions are not comparable. Veneering has increased the equilibrium value a little, from 0.48 to 0.56 mg/m. The mass transfer coefficient however, decreased very much. The mass transfer resistance shows an increase from 2,400 sec/m to 11,000 sec/m. In the case at issue, the formaldehyde concentration, at a loading factor of 1 m /m of the veneered particleboard, is below that of the bare particleboard, only at a ventilation rate in excess of 0.2 per hour. 

After a few hours of circulating, different steady state concentrations are in fact found in the two burettes. In other words, one particleboard continually absorbs formaldehyde from the other. In this case particleboard 1 absorbs formaldehyde from particleboard 2. Table VIII shows the formaldehyde emission parameters of the two boards. Especially the equilibrium values are different, the mass transfer coefficients do not differ much. 

Table IX presents chamber data obtained in only one large test chamber identified as A on medium density fiberboard made at one plant. A medium density fiberboard "set" is a specific production run. The columns are labeled the same as the particleboard Table VIII described above. The "Normalized Chamber Concentration" is based on a 0.6 ppm formaldehyde concentration at an N/L ratio of 0.96. The choice of 0.6 ppm concentration is purely arbitrary. Figure 8 graphically represents the normalized formaldehyde chamber concentrations to loadings at air changes of 0.5, 1.0 and 1.5. The points which define the curves are averages of the normalized concentrations.

If the mass transfer coefficient is sufficiently low, the emission will be so slow that the ventilation can manage to remove the formaldehyde at almost the same rate as it is liberated, resulting in a very low formaldehyde concentration in the air. This presentation deals with what can be achieved in terms of reduced mass transfer coefficient and emission rate by applying some sort of diffusion barrier to the surface of the particleboard. The diffusion barriers studied comprise overlays or surface finishes commonly applied when particleboard is used as a building material, such as wall paper, painting and floor covering, but even overlays that are used by the furniture and joinery industries, such as veneers, melamine facing and resin saturated paper foils (finish foils). 

It is important to distinguish between those emission tests that measure the emission in a closed, or unventilated, system and those that measure in a ventilated system. If a particleboard is kept in an unventilated system, the formaldehyde concentration will increase until it levels off at an equilibrium concentration which will depend on the formaldehyde content of the board under test, the temperature and the relative humidity. The particleboard loading, on the other hand, will not influence the equilibrium concentration, just the time it takes to reach it. The time to reach the equilibrium concentration is also influenced by the mass... 

Finishing or overlaying particleboard can be an efficient way to reduce the formaldehyde concentration of the air in rooms where particleboards are used e.g. as building panels or in furniture. 

The need for control of formaldehyde emission from UF-bonded wood products has been recognized since Wittmann (4) reported in 1962 that extensive use of particleboard in furniture and building envelopes can cause indoor formaldehyde concentrations exceeding occupational threshold levels. However, it proved to be difficult to define the problem because formaldehyde emission from finished products was not regularly measured, and the correlation between emission rate and the environmental factors were not yet well established. 

Guideline on the Use of Particleboard with Respect to Avoiding Intolerable Formaldehyde Concentrations in Room Air," Committee for Uniform Technical Construction, Institute for Construction Technology, (ETB), Berlin, translated by U.S. HUD, 1980. 

The curing conditions are equally important for reducing formaldehyde emission. The curing process is not yet fully understood. In fact, there is even still some question about the nature of the reactive resin. The latter subject is described in a later chapter by Johns. Appropriate resin cure conditions must take into account the wood moisture content and wood acidity, as well as resin concentration, temperature gradients, and press duration. In excessively cured UF bonded wood products, and in products that are stacked while still hot from the press, UFR can hydrolyse so strongly that particleboard loses internal bond strength. 

Three situations are shown. In each of them the concentration of formaldehyde in the exit air has been measured for four rates of airflow. The equilibrium value of the examined particleboard sample has been determined as well (1.06 mg/m ). 

Out of the results of the intersection should follow an equilibrium concentration of 0.35 mg/m, which is not in accordance with the determined equilibrium value. So this experimental set up is a case of a situation which is not well defined and therefore not suitable for measurement of the relevant formaldehyde release parameters of the particleboard. 

Table VIII presents chamber data on underlayment particleboard, mobile decking particleboard, and industrial particleboard obtained from four different chambers identified A, B, C and D. A particleboard "set" is a specific production run of a particleboard type. The observed concentration is the formaldehyde level actually determined in the chamber for a specific loading and air change rate. "N" represents the air change rate (number per hour). The column labeled "L" is the loading (m2/m3) that the test was conducted. The column "N/L" ( m/hr) is the ratio of air change rate to the loading. Finally, the column labeled "Normalized Chamber Concentration" is the actual chamber concentration (first column) normalized to 0.3 ppm at N/L = 1.16. The 0.3 ppm chamber...

The H.U.D. formaldehyde standards of 0.2 ppm and 0.3 ppm for hardwood plywood paneling and particleboard, respectively, were chosen because the combination of these products at their specific loadings and air change rate would result in a chamber concentration of less than 0.4 ppm. This assumption was based on four studies. 

The purpose of this study was to evaluate laboratory formaldehyde release test methods for predicting real-life formaldehyde air concentrations human exposure levels, and health risk. Three test methods were investigated the European perforator test, the gas analysis method at 60 C and 3% RH, and the gas analysis method at 23 C and 55% RH. Different types of particleboard bonded with urea-formaldehyde and urea-melamine-formaldehyde resins were tested. The results were used to rank boards as a function of test method, conditioning, short-term humidity, and temperature variations during storage. Additional experiments were conducted in small experimental houses at a Dutch research institute. Our conclusions are that relative ranking of products is influenced by the test method and by change in relative humidity. The relationship between test method and release in real-life situations is not clear. In fact, it seems impossible to use laboratory measurements to predict real-life product performance of board if the board is not fully in equilibrium with the atmosphere. 

During the manufacture (hot pressing) of the particleboard the formaldehyde is concentrated in the core of the board. Tests run on laboratory made particleboard with the same binder level throughout the board, have shown about 75% higher content of extract-able formaldehyde in the core than in the face ( ). Emission tests indicate an even greater difference between the two layers of the board. 

The concentration gradient that exists between the core and the face, leads to a migration of formaldehyde to the surface of the particleboard. From the surface layer it is released to the surrounding air. 

The concentration of formaldehyde in the air of a room containing particleboards, will depend on the content of formaldehyde in the boards and on the rate of its release. The formaldehyde content of a particleboard is determined by the binder used to manufacture the board and a number of production parameters. The release rate is affected by the temperature and the relative humidity of the surrounding air, but also by some of the physical properties of the board. The most important one probably is the diffusion resistance of the surface layer, which may be expressed by means of a mass transfer coefficient. 

In a ventilated system the exhaust air will remove some of the emitted formaldehyde, and a steady state concentration will be established. The steady state concentration will be lower than the equilibrium concentration. How much lower, will depend on the ventilation rate, the particleboard loading and the mass transfer coefficient. 

Formaldehyde is directly emitted into the air from vehicles. It is released in trace amounts from pressed wood products such as particleboard and plywood paneling, from old sick bnildings, and from cotton and cotton-polyester fabrics with selected crosslink finishes. Formation of formaldehyde has been observed in some frozen gadoid fish due to enzymic decomposition of the additive trimethylamine oxide (Rehbein 1985). Its concentration can build up during frozen storage of fish (Leblanc and Leblanc 1988 Reece 1985). It occurs in the upper atmosphere, cloud, and fog it also forms in photochemical smog processes. 

Tea may have yet another health benefit. Japanese researchers examined the use of teabags to cure sick-building syndrome. They scattered them around buildings to absorb gases that were making people sick. The concentration of formaldehyde, which can outgas from particleboard, was decreased by over 60 percent. It seems that tea may be healthy in more ways than one. 

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