Melamine-formaldehyde resins moulding powders

Phenol-formaldehyde was reported as the first commercially synthetic polymer (1899) which was introduced as BakeliteT by Baekeland in 1909. This was the period which marked the dawn for the production of commercial synthetic thermosetting polymers. Other advances in the field included the discovery of urea-formaldehyde resins in 1884 and the beginning of their commercialization as Beetle moldable resin in 1928, followed by thiourea-formaldehyde (1920), aniline-formaldehyde (Cibatine by Ciba, 1935) and melamine-formaldehyde (1937) moulding powders. The year 1909 marked the discovery of epoxy compounds by Prileschaiev, which were not used until World War 2. The first thermoset polyesters, invented by Ellis, date back to 1934 and in 1938 was reported their first use in the forms of glass-reinforced materials [1]. 

The term aminoplastics has been coined to cover a range of resinous polymers produced by interaction of amines or amides with aldehydes. Of the various polymers of this type that have been produced there are two of current commercial importance in the field of plastics, the urea-formaldehyde and the melamine-formaldehyde resins. There has in the past also been some commercial interest in aniline-formaldehyde resins and in systems containing thiourea but today these are of little or no importance. Melamine-phenol-formaldehyde resins have also been introduced for use in moulding powders, and benzoguanamine-based resins are used for surface coating applications. 

As with the U-F moulding powders the relative importance of M-F moulding powders for other plastics materials and also in other uses for melamine-formaldehyde resins has declined. 

In addition to their use in moulding powders and laminates, melamine-formaldehyde resins are widely used in many forms. 

At one time thiourea-urea-formaldehyde resins were of importance for moulding powders and laminating resins because of their improved water resistance. They have now been almost completely superseded by melamine-formaldehyde resins with their superior water resistance. It is, however, understood that a small amount of thiourea-containing resin is still used in the manufacture of decorative laminates. 

Melamine was first isolated by Liebig in 1834 from the mixture obtained by heating ammonium thiocyanate. A technically feasible route to melamine was developed in 1935 by Ciba AG (Switzerland) and at the same time Henkel patented the production of resins from melamine and formaldehyde. In general, melamine-formaldehyde polymers resemble urea-formaldehyde pol)miers but they have improved resistance to heat and water. The two materials have therefore found application in similar areas, melamine-formaldehyde resins now being widely used in the production of moulding powders, laminates, adhesives, surface coatings and textile finishes. 

As with resins, moulding powders based on melamine are used for specialised applications. The two main areas of application are tableware, where their improved water and scratch resistance compared to urea based materials makes them ideally suitable for this purpose, and for high performance electrical goods. Their electrical properties are superior to urea formaldehyde based materials under conditions of high temperature and humidity. They can be made in more attractive colours than phenolic moulding powders and have superior track resistance. 

Moulding powders based on melamine-phenol-formaldehyde resins were introduced by Bakelite Ltd, in the early 1960s. Some of the principal physical properties of mouldings from these materials are given in Table 24.1. 

Resins prepared from melamine, phenol and formaldehyde have found some use in the preparation of moulding powders. These materials have properties which are intermediate between those of melamine-formaldehyde and phenol-formaldehyde moulding powders. In particular, mouldings have better dry-heat dimensional stability than those based on straight melamine-formaldehyde and they may be produced in a much wider range of colours than phenol-formaldehyde mouldings. Typical applications of melamine-phenol-formaldehyde materials include domestic mouldings such as iron handles where a combination of heat resistance and decorative appeal is required. 

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