Water particleboard, formaldehyde

El-particleboard- and El-MDF-resin for water-resistant boards (EN 312-5 and 312-7 [2]). For boards according to option 1 (V313-cycle test) MUF-resins can be used for boards according to option 2 (VI00-boiling test) MUPF with approval are necessary. In case, espeeially for the MDF-production, formaldehyde catchers are added. 

Adhesive. Urea-formaldehyde water-based dispersions are the most widely used particleboard binders. The low-cost, rapid curing, and colorless properties of urea-formaldehyde adhesives make them the adhesive of choice for most interior particleboard. These adhesives have been continuously improved by the resin manufacturers, resulting in reduced press times without detrimental effects on their storage life or handling characteristics. 

Dost in 1971 reported on a study where redwood bark fiber was used in three-layer particleboard (44). Amount of bark in the furnish, by weight, was 0, 10, 20, and 30% hammemilled disk flakes or Pallmann flakes of redwood wood made up the remainder of the furnish. Urea formaldehyde resin was applied at three percentages. Test results showed surface smoothness and strength properties (MOR, MOE, and IB) decreased with increasing bark content in the boards. Water absorption decreased, but thickness swelling and linear expansion increased as the amount of bark increased. 

Roffael (15) measured formaldehyde emissions from a phenolic particleboard using the WKI-Method which involves suspending small samples over 50 cm of distilled water in tightly closed polyethylene bottles and measuring formaldehyde levels in the water after varying times. Temperatures were maintained at 42 C. This work indicated that formaldehyde release from the phenolic particleboards ceased after a relatively short reaction period (approximately 96 hours). This finding is consistent with the resin stability considerations discussed previously under theoretical considerations. 

Figure 9. Formaldehyde liberation in water at 25 C and pH 3 from particleboard and CH20-sorbed wood all materials -80 mesh. (Sodium azide in water at 100 mg/L as preservative symbols and abbreviations as in Figure 7.) (ML85 5436)...

Figure 11. Formaldehyde loss ratios at 80 percent relative humidity (RH) and in water. (Loss ratio = CH2O liberated relative to that from urea-formaldehyde particleboard in same test.

The UF adhesives for plywood generally contain less than 2 mole of formaldehyde per mole of urea, and most of them are condensed to a slightly viscous, hydrophilic stage and are quite soluble in water. The degree of polymerization, and hence the viscosity under comparable conditions of UF resins for plywood is generally higher than those of UF resins for particleboard. 

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