R.A. Laidlaw
Dr. R.A. Laidlaw is the Principal Scientific Officer at the Laboratory of the Building Research Establishment, Princes Risborough Laboratory, United Kingdom.
Durability of aminoplastic and phenolic glues for plywood is discussed on the basis of studies carried out at the Princes Risborough Laboratory in the United Kingdom.
In plywood manufacture, the choice of adhesive and timber species will determine not only the physical properties such as strength, but also the product's ability to withstand the degrading effects of any environment to which it might be subjected in service. It is clear that plywood made from unprotected, non-durable timbers will be unsuitable for applications involving exposure to conditions of persistent dampness, whatever the glue employed. However, if durable or preservative-treated wood species are used, the performance of the plywood will be governed by the properties of the adhesive. This article offers guidance on the types of plywood glues found by experience and experiment to be suitable for different end-use applications.
Plywoods may meet a wide variety of exposure conditions in service, ranging from sheltered situations indoors to full outdoor exposure, and may even be subject to aggressive chemical environments. Adhesive formulations are known to exhibit a wide variation in their resistance to degradation by such agents as heat and moisture, and in order to gain information on glue performance a series of exposure trials of laboratory made plywood has been carried out at the Princes Risborough Laboratory in the United Kingdom.
In the most recent studies, lasting over 18 years and involving the testing of 65 glues in some 18000 specimens, the behaviour of phenolic and aminoplastic glues in a wide variety of environments was examined in detail. In these experiments, the progress of glue breakdown was followed at intervals by means of the knife test of British Standard 1455:1972 (Plywood manufactured from tropical hardwoods, British Standards Institution). The main conclusions are presented here. It should be stressed that these figures represent the results obtained with one set of glues only; considerable variations in behaviour were encountered even among glues of similar types.
In BRE Digest 175 Choice of glues for wood (April 1975) five main categories of exposure condition were enumerated. These were: (a) high-hazard exterior (full exposure to weather); (b) low-hazard exterior (protected from direct sun and rain); (c) high-hazard interior (warm and damp); (d) low-hazard interior (dry conditions); and (e) special (chemically polluted atmospheres). The findings of the Princes Risborough Laboratory studies point to the suitability of the various plywood glues for use in this range of conditions.
Aminoplastic glues
The exposure trials established the lower durability of conventional urea-formaldehyde (UF) resins compared with phenol-formaldehyde (PF) or PF/resorcinol-formaldehyde (PF/RF) glues. It was concluded that where a long plywood life was required UF glues are completely suitable only for low-hazard interior applications, although they may give satisfactory service for shorter periods under more severe conditions.
Better results, in both outdoor and "severe indoor" exposures, were obtained with mixes of melamine and urea (MF/UF) resins and with formulations fortified with, for example, phenol or resorcinol (F-UF and F-MF). In outdoor trials it was shown that the particular F-UF glues tested could maintain an adequate bond for as long as the melamine resins (four to six years), and that the single fortified melamine resin tested gave a bond lasting for at least ten years. The increased durability of these glues made the derived plywoods suitable for use under more severe conditions - for example, inside the roofs of open sheds and porches, in laundries and in roof spaces in buildings.
Phenolic glues
It was found that PF or PF/RF glues which had been designed for use without fillers or extenders, or with only small percentages of filler included, maintained in all instances an efficient bond over prolonged periods. Plywoods made from these glues are recommended for use in the most severe environments, where other glues would have only limited application.
Fillers such as mineral powders or fibres are incorporated in glue formulations to modify properties such as shrinkage and viscosity, and usually lower the cost of the system. The effect of these materials will, of course, vary with the nature and amount of filler used. Generally, a rapid loss of bonding efficiency was found to result from high filler content, and when this exceeded 100% (dry resin basis) the performance of the glue was severely affected.
Effect of surface protection on plywood life
In experiments with boards made with UF, MF and PF glues it was shown that their life under exterior conditions could be extended threefold by surface protection treatments, such as painting, provided that these are efficiently and regularly maintained. However, when such films become damaged deterioration of the substrate may be accelerated by moisture trapped under the film. In general, therefore, it would be unwise to rely on the protective qualities of the paint alone, and the glue specified for any application should be itself fit for the job.
Glue classification by British Standard tests
In British Standard 1203:1963 Specification for synthetic resin adhesives (phenolic and aminoplastic) for plywood (British Standards Institution) and British Standard 1455:1972 Plywood manufactured from tropical hardwoods (British Standards Institution), methods of test for classifying glues for different applications are described, and four groups are distinguished. These are: WBP (weather and boil-proof), BR (boil-resistant), MR (moisture-resistant and moderately weather-resistant) and INT (interior). The results of such tests indicate the probable performance of the plywoods in use. In the trials it was found that in general the glue assessments for both tests agreed, and these findings were reflected in the performance of the corresponding plywoods on exposure. Some inconsistencies were encountered, however, where the results of the two tests were at variance with each other and/or with the behaviour of the panels on weathering. Unexpected panel failure after short exposure could frequently be explained by the fact that at excessively high filler content the bond quality can diminish rapidly on exposure. Such results emphasize the importance of long-term exposure trials.
Care must be taken in the interpretation of BS 1455 knife-test results. When plywood made from high-density wood species is tested, the veneers may be more difficult to part but leave less fibre on the glue line, indicating lower ratings on the Bond Quality scale. With rotted wood, however, the force necessary to separate the veneers is low but excessive amounts of fibre may be left on the glue line.
The studies indicated that only properly formulated phenolic resins (WBP type) can be recommended without reservation for use in high-hazard exterior conditions; excess filler in the glueline may cause poor performance. Adhesives based on melamine or fortified ureas (BR type) can usually only give satisfactory service for shorter periods under these conditions, but may perform adequately in less demanding (low-hazard exterior or perhaps high-hazard interior) situations. The unmodified urea glues (MR type) examined were, in general, only suitable for long-term exposure in dry interior situations.
Charging the press in a Philippines plywood mill: the glue may outlast the wood
Different brands of the same glue type may exhibit considerable variations in performance. It is recognized that many of the adhesives examined may now have been superseded by new formulations with improved properties, but the general conclusions drawn from these studies remain valid. Wherever possible, an assurance should be sought from the manufacturer that a particular glue formulation employed is suitable for the service conditions and for the length of life envisaged.
