Formaldehyde emission

The primary adhesive used ia hardwood plywood is urea—formaldehyde (UF) mixed with wheat flour as an extender to improve spreadabiUty, reduce penetration, and provide dry-out resistance. A catalyst may also be added to UF resias to speed the cure or to cause the UF to cure. Scavengers also may be added to reduce formaldehyde emissions from finished panels. If more water-resistance is requited using a UF bond, small amounts of melamine maybe added, producing a melamine—urea—formaldehyde (MUF) adhesive. 

Testing—includes test specimen preparation, bond durabiHty tests, and stmctural performance tests. It should be noted that formaldehyde emission tests of phenoHc bonded products such as stmctural plywood are not required because emissions are normally about 0.02—.03 pl/L (ppm), weU below the previously noted safe level of 0.10 p.L/L (ppm). 

The exterior form of MDF is used in special appHcations requiring durabiHty and resistance to water or weather exposure. Highway signs would be an example of this use of exterior MDF. It is an extremely expensive product and thus is used only for special appHcations requiring its special properties. Another example of use would be where a customer would be willing to pay the additional cost to use a composite which has the exceptional quaHties of MDF, but also has virtually no formaldehyde emissions. 

Methanol substitution strategies do not appear to cause an increase in exposure to ambient formaldehyde even though the direct emissions of formaldehyde have been somewhat higher than those of comparable gasoline cars. Most ambient formaldehyde is in fact secondary formaldehyde formed by photochemical reactions of hydrocarbons emitted from gasoline vehicles and other sources. The effects of slightly higher direct formaldehyde emissions from methanol cars are offset by reduced hydrocarbon emissions (68). 

Wood Bonding. This appHcation requires large volumes of phenoHc resins (5—25% by weight) for plywood, particle board, waferboard, and fiberboard. Initially, phenoHc resins were used mainly for exterior appHcations, whereas urea—formaldehyde (UF) was used for interiors. However, the concern over formaldehyde emission has caused the replacement of UF by phenol-formaldehyde adhesives. 

Even though UF adducts are known to be present in OSB, formaldehyde emissions are not elevated over those expected of an unmodified PF. There are three reasons for this. First, the molar ratio of formaldehyde-to-urea in these situations is very low. It is at least an order of magnitude lower than practical molar ratios for curable UF resin binders. Second, UF adducts are quite stable under the alkaline conditions that prevail in PF-bonded OSB. Finally, the urea only reacts with the formaldehyde that was left behind during polymerization and would have been largely emitted in pressing and cool-down. Urea additions have been shown to reduce PF formaldehyde emissions from hot pressing [121 ]. 

Formaldehyde emission from finished panels is due to the residual formaldehyde present as a gas in the wood and recesses of UF-bonded and MUF-bonded boards as well as dissolved in the boards residual moisture content. On the other... 

Actual regulations cone, the subsequent formaldehyde emission from wood-based panels (Germany) according to the German Regulation of Prohibition of Chemicals (former Regulation of Hazardous Substances)... 

The so-called El-emission class describes a wood panel presenting formaldehyde emission which is low enough to prevent any danger, irritation or inflammation of the eyes, nose and mouth mucous membranes. However, it is important that not only the boards themselves, but also the veneering and carpenter s adhesive resins, laquers, varnishes and other sources of formaldehyde are under control, since they also might contribute to the mixture steady state formaldehyde concentration [9]. Table 3 gives an overview on some European regulations. However, it is necessary here to introduce the principal types of composite wood products, especially panels, that are produced in this industry ... 

This problem, namely the formaldehyde emission of the wood panel in service, after its manufacture, can be fortunately regarded to day as more or less solved, due to unequivocal and stringent regulations in many European countries, and successful long-term joint R D in the adhesives and wood-working industries. 

Despite the fact that the two main components of UF-resins are urea and formaldehyde, a broad variety of possible reactions and structures in the resins can be obtained. At the molecular level, the basic characteristics of UF-resins can be explained as follows (1) high reactivity (2) water solubility and dispersibility, which renders the resins ideal for use in the woodworking industry and (3) the reversibility of the aminomethylene link, which also explains the low resistance of the UF-resins against the influence of water and moisture, especially at higher temperatures. This is also one of the reasons for the subsequent formaldehyde emission. 

Forced by the necessity to limit the subsequent formaldehyde emission, the UF-resin molar ratio, F/U, has been progressively decreased to very low values. The main differences between UF-resins with high and with low content of formaldehyde, are (1) the reactivity of the resin due to the different content of free formaldehyde, and (2) the degree of crosslinking in the cured network. 

However, it has to be considered that it is neither the content of free formaldehyde itself nor the molar ratio which eventually should be taken as the decisive and the only criterion for the classification of a resin concerning the subsequent formaldehyde emission from the finished board. In reality, the composition of the glue mix as well as the various process parameters during the board production also determine both performance and formaldehyde emission. Depending on the type of board and the manufacturing process, it is sometimes recommended to use a UF-resin with a low molar ratio F/U (e.g. F/U = 1.03), hence low content of free formaldehyde, while sometimes the use of a resin with a higher molar ratio (e.g. F/U = 1.10) and the addition of a formaldehyde catcher/depressant will give better results [17]. Which of these two, or other possible approaches, is the better one in practice can only be decided in each case by trial and error. 

Below 1. 00 Special glue resins for boards with a very low formaldehyde emission in most cases modified or fortified with melamine. 

Special resins for boaids with a very low subsequent formaldehyde emission. 

Only a small amount of work has been done up to now concerning the prediction of bond strengths and other properties based on the results of the analysis of the resin. Ferg et al. [59] worked out correlation equations evaluating the chemical structures in various UF-resins with different F/U molar ratios and different types of preparation on the one hand and the achievable internal bond as well as the subsequent formaldehyde emission on the other hand. These equations are valid only for well defined series of resins. The basic aim of such experiments is the prediction of the properties of the wood-based panels based on the composition and the properties of the resins used. For this purpose various structural components are determined by means of - C NMR and their ratios related to board results. Various papers in the chemical literature describe examples of such correlations, in particular for UF, MF, MUF and PF resins [59-62]. For example one type of equation correlating the dry internal bond (IB) strength (tensile strength perpendicular to the plane of the panel) of a particleboard bonded with PF adhesive resins is as follows [17]... 

Tannins can also be hardened by addition of hexamethylenetetramine (hex-amine) [146,147], whereby these boards show very low formaldehyde emission [16-18,148-151]. 

European Norm EN 717-2, Wood-based panels. Determination of formaldehyde emission. 

MUF resin is widely used as an adhesive in wood industries, coating technology, paper industries and a main material in kitchenware production. In various applications, different resin properties are needed to suit its application. Important resin properties are for example higher resin solubility, low curing period with lower temperature and catalyst amount, good stability for longer shelf life, and lower free formaldehyde emission, as formaldehyde is very toxic, and can cause cancer [1]. One of the factors that affecting the MUF resin properties is the mole composition. The mole composition is a ratio of formaldehyde to amino compoimd... 

Formaldehyde emission regulations, 15 776 Formaldehyde gas, 12 110 Formaldehyde-hydroquinone developers, 19 210... 

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