Wood is one of the most commonly used natural materials and there is still a relatively abundant supply. However, as more and more articles are being made of wood, the reduction in the supply of large trees and of high quality trees is becoming more serious. Therefore, the forest and lumber industry is trying to make the best use of wood through the use of synthetic adhesives. Examples of articles which use adhesives include particle boards, plywood, oriented strand boards, laminated beams, I-beams, and other engineered woods. An added advantage of making wood products with an adhesive system is that the so-produced woods are sometimes much stronger than natural woods because of the higher physical and chemical stability and the higher strength of glue relative to wood.
One of the most commonly used adhesives in the wood bonding industry is based on phenolic resins. A typical phenolic resin is made from the condensation polymerization of phenol with formaldehyde in the presence of a catalyst such as NaOH. Presently, numerous phenolic resins are tailor-made to suit different types of wood and for different shapes of wood articles. Traditional phenol-formaldehyde resins are chemically not reactive enough, so they have to be cured or used at high temperatures. The use of high temperature not only consumes a lot of energy, but also generates safety issues. The use of high temperatures can cause volatile organic compounds, called VOC's, to evaporate in the ovens. These often-toxic vapors eventually end up being released into the environment, mainly through stack emissions. Another safety concern with traditional phenolic resins is that they always contain some formaldehyde, which is a carcinogen.
If an adhesive can be cured at room temperature, capital and operating costs can be reduced. Therefore, phenolic resin manufacturers and wood manufacturers have concentrated their efforts on developing better adhesives that can cure at room temperature and that are safer to handle and use than traditional phenolic adhesives. The most promising technology that is presently being used is a two-component resorcinol adhesive. One part is a resorcinol-formaldehyde resin that is deficient in formaldehyde; and the other is simply formaldehyde or a formaldehyde donor. This adhesive takes advantage of the high reactivity of resorcinol so as to make possible the curing of the adhesive at room temperature.
Initially, resorcinol-formaldehyde resins were used for these applications. To reduce the impact of the high cost of resorcinol, phenol was later introduced to partially replace some of the resorcinol in the resin. Presently, phenol-resorcinol-formaldehyde resins are widely used as the adhesive for wood bonding purposes. In this two-part adhesive system, one part is usually formaldehyde or paraformaldehyde, called the hardener in the industry. The use of formaldehyde or paraformaldehyde is essential for the adhesive to work. However, formaldehyde and paraformaldehyde bring with them a lot of safety issues, since they are both carcinogens. The transportation, storage, handling, exposure, and stack emissions of these harmful compounds are closely watched by EPA and various other governmental bodies. Paraformaldehyde is presently the most commonly used hardener for resorcinol resins. Paraformaldehyde is a powder, and it is extremely difficult to work with because the dust is very hard to control. It is very toxic, and it readily decomposes and releases formaldehyde, which is very difficult to work with. The handling of paraformaldehyde thus is not very safe and requires special attention. Another disadvantage of paraformaldehyde is that it normally has to be prepared as a dispersion just immediately prior to its use. This is because such dispersions can easily settle and decompose to formaldehyde, which evaporates into the air, resulting in a concentration change of the active ingredient. Therefore, the industry typically has to prepare the dispersion, then use it within a short period of time in order to maintain consistent performance and avoid settling and down time. Therefore, the resorcinol resin-paraformaldehyde adhesive system permits curing at room temperature, but it has not addressed the safety issues of dusting and toxicity.
Recent efforts of wood manufacturers and adhesive manufacturers have been directed to developing hardeners to replace paraformaldehyde, so as to eliminate the dusting, toxicity and safety problems associated with paraformaldehyde and formaldehyde. One of the most promising technologies is based on oxazolidine chemistry. Oxazolidines are made from amino alcohols with formaldehyde. An oxazolidine can be very stable, and no free formaldehyde can be detected, therefore its transportation does not present any exposure problems. An oxazolidine reacts with a resorcinol through chemical transfer, which means that the formaldehyde will only leave the amino alcohol molecule when it is in direct contact with a resorcinol molecule. The whole transfer process does not involve any formation of formaldehyde, thus completely eliminating all the safety issues of paraformaldehyde handling, workplace exposure and emissions. It also does not have the settling problem of paraformaldehyde, thus increasing product consistency and reducing down-time. Resorcinol resin-oxazolidine type adhesives represent a tremendous improvement over resorcinol resin-paraformaldehyde system, and so have gained popularity. However, current oxazolidine hardeners used by the industry have some limitations. A major deficiency of oxazolidine-based hardeners is that the hardener is unable to cure a resorcinol resin at room temperature in less than 24 hours and achieve a strong bond passing the required wet strength (ASTM D 2559). Under similar conditions, paraformaldehyde-based hardeners can cure a resorcinol resin very efficiently at room temperature within 24 hours and provide the needed wet strength. Another limitation is that the oxazolidine-based hardeners typically have a much shorter gel time or working time relative to that of paraformaldehyde-based hardener.
Therefore, it has been the concern of the present inventor to provide a liquid hardener to be used with a resorcinol resin as an adhesive that can cure at room temperature within 24 hours to glue wood articles and which produces strong gluelines passing wet strength specifications. Also, a new liquid hardener should eliminate the safety hazards of formaldehyde and paraformaldehyde and provide a longer gel time for the gluing of wood articles than the present oxazolidine hardeners.