FAIR-CT96-1604 Non-Toxic Aminoplastic Adhesive for Medium Density Fibreboard and New Application Panel Products of Improved Weather and Biological Resistance

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Biocomposites/Boards : FAIR Area 1.3 - Forestry-Wood Chain : Wood (Lignocellulose)

	Contract No:	FAIR-CT96-1604
	Date Prepared:	January 2000
	Source:	Final Report Executive Summary

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Final Report Executive Summary

Introduction

Recent concerns over the effect of formaldehyde on health are posing limitations to conventional formaldehyde-based adhesive systems that emit formaldehyde during use. Alternative adhesive systems containing isocyanates, phenolics, or tannins all have inherent problems of perceived health risks, or higher production cost. The main goal of this project has been the development of a new generation of urea-formaldehyde (UF) based adhesives with near-zero emission coupled with improved weather and biological resistance.

Two main pathways were followed in order to achieve this goal. The first was the incorporation, into UF based resins of low F/U molar ratio, of additives, that even in small amounts improved significantly the wet properties or imparted biological resistance. The second was the optimisation of the hardening of the resins so that the final wood-based panel had superior properties. This was achieved by using a resin production process that optimised the resin's reactivity as well as with the use of special hardeners/crosslinkers during the final setting of the resin. The above-mentioned additives and special hardener were combined in a single formulation that was further improved with the addition of a formaldehyde catcher. Optimisation of the final formulation and board pressing parameters was achieved in the lab as well as in pilot scale for both particleboard and NMF. It was proved that the two types of final product require very different formulations as a result of the different process conditions that apply.

Results

Direct Results

• Improved water resistance and consequently lower formaldehyde emission by incorporation into the resins of long chain aliphatic primary diamines and triamines.
• Development of effective hardeners for the hardening of resins of very low formaldehyde content.
• Optimisation of the reaction parameters in order to influence the reaction pathway and minimise the necessary formaldehyde.
• Improved biological resistance by the fixation in a non-reversible manner of an easily diffusible non-toxic preservative such as boron, to the resin.

Indirect Results:

• Better understanding of how various reaction parameters affect the final properties of formaldehyde-based resins that will result in better control and increase in the reproducibility of such preparations.
• Correlation of the microscopic structure with the macroscopic properties of the resins that could be useful for the development of adhesives for specialised applications.

Discussion

It was shown that the use of a special hardener and additives in glue mix for particleboard production can result in boards of increased weather resistance (higher V100 values) while the level of formaldehyde emission is kept low with the use of a catcher. Furthermore, results from biological tests have proved that boric acid added to the glue mix is imparting to the boards biological resistance. Therefore, we have developed a glue mix formulation for particleboards that provides improved weather and biological resistance with low formaldehyde emission levels. It was also shown that the presence of a fixating agent is not necessary in order to avoid leaching of boric acid, presumably because the MUF resin itself acts as a fixating agent. This is significant from the financial point of view since the cost of the preservative system is in this way greatly reduced.

The ability to use such a preservative in OSB production was also examined. Typical resin systems for OSB are phenolic type resins and their development was beyond the scope of this project. However, the compatibility of these resins with boric acid and various other boron containing compounds was examined. Even though we did not find a system to satisfy completely the pot life requirements, we managed to press boards using phenolic resins and boric acid added directly to the chips. Leaching tests indicated that in this case also albumin was not necessary for the fixation of boric acid.

In the case of MDF production, results form pilot scale and industrial tests performed have indicated that the use of the special hardener is not possible due to pre-curing in the dryers. The most effective way of increasing the wet strength in this case was found to be the addition of melamine to the resins. An extensive study has been carried out in order to find out what is the best way of melamine addition. We have concluded so far that co-condensation of methylol-ureas and methylol-melamines leads to better performing resins. However, the extent of co-condensation seems not to be crucial and therefore more practical reasons can dictate the method of resin synthesis.

Finally, boric acid can be added in the glue mix formulations used for NOF production only when a reactivity modifier such as hexamethylenetetramine is also added to the system. The mechanical properties of the boards thus produced are not affected by the addition of the boric acid. The presence of albumin as a fixating agent was not found necessary from the preliminary leaching experiments.

Conclusions

A glue mix formulation was achieved, where the use of a special hardener combined with a crosslinker, results in increased weather resistance (up to 40%) while the formaldehyde emission level is kept low by using special additives such as formaldehyde catchers. Furthermore, by coupling a wood preservative with this novel formulation, it is now possible to improve the durability in terms of biological resistance as well. This allows the use of panels for new exterior applications, in particular those where ground contact is required. The technology was evaluated with pilot scale trials and feasibility studies and will be protected by a patent application. The resins developed will be primarily marketed as adhesives for MDF for exterior uses. Other applications that need low health risk, weather and biologically resistant adhesives will also be considered.