Several effective techniques to consistently and markedly decrease the melamine content in melamine-urea-formaldehyde (MUF) wood adhesives without any loss of performance are presented, some of which are already in the early stages of industrialization. Among these, first melamine/acid salts such as melamine acetate, melamine formate, and melamine oxalate, function both as efficient hidden hardeners of urea-formaldehyde (UF) resins for plywood as well as upgrading the performance of simple OF resins for plywood through approximately 10 percent by mass melamine grafting to yield comparable strength durability of premanufactured MUF resins of 30 to 40 percent melamine mass content, hence of resins of much higher mass content of melamine. The system works by simple addition of a melamine salt in the OF glue-mix eliminating the need to premanufacture a MUF resin. The effectiveness of melamine grafting in the glue-mix and during hot pressing has been found to depend upon the relative solubility of the melamine salt which depends on both the acid strength of the acid as well as the number of acid functions in the salt. The reasons why traditional, premanufactured MUF resins waste 2/3 or more of the melamine used in them, and why the system presented is so much more effective in not wasting melamine are presented and discussed.
The system of upgrading cured UF adhesives water resistance by addition of small amounts of melamine acid salts directly in the glue mix has also been partly optimized for plywood. The short pot-life of the glue mix initially was found to be. due to an excess of acetic acid present in the melamine salt powder. Proper drying of the salt yielded an almost pure melamine monoacetate which allowed glue-mix pot-lives of over 15 hours and thus needed addition of a standard NH4+-based salt to harden the system. Shorter pot-lives with out ammonium salts hardeners addition can also be achieved depending on the length of the drying period of the salt. Within limited ranges the amount of melamine on OF resin solids can be increased to slightly over 15 percent. This is dependent on the initial molar ratio of the OF resin which determines the amount of free reactive methylol groups on the OF resin onto which the melamine monomer can be grafted. This is the determining limiting factor in the top amount of melamine which can be added. Amounts of melamine greater than 15 to 17 percent are however likely to yield ever-diminishing returns on wet strength performance of the resin and consequently other techniques to further increase the relative proportion of melamine are not needed. Resins containing 10 to 11 percent melamine have been shown to yield performances better than copolymerized MUFs of M:U mass ratio of 30:70. Addition of 16.5 percent on OF solids of melamine by the new system (thus a 20:80 M:U mass ratio) give results which are about half-way between those of a 30:70 and those of a 50:50 M:U MUF, thus comparable to a classical 40:60 MUF A new thermomechanical analysis (TMA) technique was also used to show the likely upper limits of this adhesive system. While such an adhesive system was optimized for plywood, the system appeared to work on particleboard-type panels, with the provision that in the latter case the construction and morphology of the panel has an important influence on this system performance. In OSB panels results almost as good as in plywood can be expected, while in particle-board the results, although excellent, are only comparable-to those obtainable with traditional MUF resins of only double the content of melamine (against triple for plywood).
TMA results indicate that simple post-treatment (such as some postcuring) conditions capable of improving the mechanical performance of aminoplastic resins-bonded particleboard without any degradation do indeed exist. A model to describe the decrease. in. temperature under different conditions of a particleboard after hot pressing already developed for OF resins was shown to be valid for MUF resins too. This model. was shown to correlate well with experimental results of board temperature variation after pressing, both on cooling and during postcuring under different conditions. From this, conditions of temperature and time favorable, to improve panel performance by postcuring treatments can be and were also determined. The validity of the improvements forecasted under such conditions was then confirmed at the molecular level for OF adhesive/wood joints by TMA testing, and had already been confirmed by testing the mechanical performance of laboratory UF-bonded boards prepared under the postcuring treatment conditions identified. Noticeable improvements in MUF board properties, namely on internal bond (IB) strength, can be forecasted by implementing short postcuring periods at temperatures much lower than pressing temperature. The performance improvements observed in the panels were explained on the basis of the shifts in relative importance of already described and well known molecular level rearrangements of the cured adhesive network in modern, lower formaldehyde content aminoplastic (UF and MUF) adhesives. The conclusion was that modern, lower formaldehyde content aminoplastic (UF and MUF) adhesives can considerably benefit from short, hot postcuring periods as regards board performance; a trend in clear contrast with the degradation and loss of performance this practice was known to induce in the older, very much higher formaldehyde content aminoplastic resins of the past.
The coupling of the simple and very rapid TMA technique with the developed model allows the rapid scan of many other post-treatment schedules and thus to forecast other still possible improvement in the performance of MUF-bonded and other-adhesives-bonded particleboard.
Two other systems to decrease melamine content at parity of adhesive performance are reported and discussed in depth: the effect that the resin formulation and its polymerization schedule have on the relative proportion of chemical species formed and as a consequence how the performance of the resin can be and has been improved from the knowledge of its structure-properties relationships..
Furthermore series of additives are presented capable of halving the percentage requirements of a MUF resin in a board while maintaining unaltered its performance. Vice versa, the same additives can double the performance of the panel at parity of MUF resin, leading to much stronger panels. These additives which are exclusively used at the glue-mix stage, improve considerably the performance of OF and PF resins, but not quite as much as MUF resins, due to the number of mechanisms operative.
The same TMA technique allowed the development of a simpler and more rapid method to show fundamental differences in TTT and CHT curing diagrams of MUF resins when within the wood joint.