Formaldehyde-urea ratio particleboard

Figure 10. Formaldehyde loss ratios at 20 percent relative humidity for various materials. (Formaldehyde removed from a material divided by that removed from urea-formaldehyde particleboard. Board elution by nitrogen. Resin liberation by weighing bottle test. PF = phenol-formaldehyde) (ML85 5437)...

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.

High temperatures and high relative humidity can result in odor problems in a room containing particleboard manufactured with UF resins [25]. The release of formaldehyde from UF particleboard is caused by two factors. It can be due to free formaldehyde present in the board that has not reacted, and it can be due to formaldehyde formed by hydrolysis of the aminoplastic bond as a result of temperature and relative humidity [2,25]. While the first type of release lasts only a short time after manufacture of the particleboard, the second type of release can continue throughout the entire working life of the board. A considerable number of variables influence the emission of formaldehyde from a UF-bonded particleboard. The main ones are the molar ratio of urea to formaldehyde (which influences both types of release), the press temperature, and in service, the ambient temperature and relative humidity. 

UF resins for particleboard with urea/formaldehyde molar ratios of 1 1.45, 1 1.32, and 1 1.25 have free formaldehyde contents of 0.8%, 0.3%, and less than 0.2%, respectively [15]. While the current tendency internationally is to use UF resins that have a urea/formaldehyde molar ratio lower than or much lower than 1 1.2, which release much less formaldehyde, these resins perform less well in the production of UF-bonded particleboard [15,17]. In particular, they do not allow as much flexibility in particleboard production as do resins with higher formaldehyde/urea molar ratios. This fact stresses the need for greater control and supervision of the production at particleboard plants where UF resins of low molar ratio are used. An example of the variation in properties between particleboard manufactured with different molar ratio resins is given in Table 1. 

Reduction in the emission characteristics of unfinished hardwood plywood is currently being achieved primarily by the use of low formaldehyde to urea molar ratio formulations. For the manufacture of hardwood plywood and particleboard, formaldehyde to urea molar ratios have been reduced to a range of 1.15/1 to 1.3/1. An important caveat low F/U ratios perhaps should be considered a proxy for the potential to reduce emissions through improved urea-formaldehyde adhesive technology rather than the exclusive means for improvement. Reducing the F/U ratio is not always the most effective way of reducing emissions in consideration of the variety of hardwood plywood constructions, products, and thicknesses. ... 

---