MELAMINE - FORMALDEHYDE
Preparation of Melamine
- A number of methods of producing melamine have been described.
1. Heating dicyanodiamide, either with ammonia or on its own under pressure.
2. Fusion of dicyanodiamide with a guanidine salt.
3. From urea.
- Melamine, a non-hygroscopic, white crystalline solid, melts with decomposition above 347°C and sublimes at temperatures below the melting point.
MANUFACTURE MELAMINE FORMALDEHYDE
- In a typical process a jacketed still fitted with a stirrer and reflux ‘condenser is charged with 240 parts 37% w/w formalin and the pH adjusted to 8.0-8.5 using sodium carbonate solution with the aid of a pH meter.
- 126 parts of melamine (to give a melamine formaldehyde ratio of 1:3) are charged into the reactor and the temperature raised to 85°C.
- The melamine goes into solution and forms methylol derivatives
Manufacture of Urea and Melamine Formaldehyde Resins
CURING OF MELAMINE FORMALDEHYDE RESIN
- Reaction of melamine with neutralised formaldehyde at about 80-100°C leads to the production of mixture of water-soluble methylolmelamines.
- The principal resinification reaction involves methylol-methylol condensations.
- Methylene links may also be formed by the following reactions.
Cross linked structure of melamine formaldehyde
MELAMINE FORMALDEHYDE MOULDING POWDERS
- MF moulding powders are prepared by methods similar to those used with UF material.
- In typical process aqueous syrup, containing resin with a MF molar ratio of 1:2 is compounded with fillers, pigments, lubricants, stabilizers and in some cases accelerators in a dough-type mixer.
- The product is then dried and ball-milled.
- Magnesium carbonate is employed to act as a pH stabilizer during storage
- Decorative moulding powders a - cellulose is used as filler.
- Industrial grade materials employ fillers such as asbestos, silica and glass fibre
Mouldings from MF powders are superior to the UF plastics in a number of respects
- Lower water absorption, especially with mineral-filled resins.
- Better resistant to staining by aqueous solutions such as fruit juices and beverages.
- Electrical properties, which are initially similar to those of UF resins, are maintained better in damp conditions and at elevated temperatures.
- Better heat resistance.
- Greater hardness.
- MF moulding materials are more expensive than general purpose UF and PF resins.
- MF is transparent
GENERAL PROPERTIES
- High surface hardness and scratch resistance,
- High surface gloss,
- High resistance to creep,
- Resistance to heat (up to 250 °C for special grades),
- Resistance to moisture,
- Not suitable for continuous contact with boiling water,
- High shrinkage as for UF moulding compounds, tendency to crack formation,
- Curing properties better than UF moulding compounds,
- Grades, which are free of smell, are permitted for the manufacture of utensils for food contact applications.
CHEMICAL PROPERTIES
Resistances to Chemicals
- Fundamentally the Melamine formaldehyde mouldings are more resistants to solvents, oils, fats, weak acids and alkaline solutions than UF mouldings.
Weathering Resistance
- Under high humidity conditions MF is having less water absorption than UF. However the water absorption depends on the nature of filler used in the composition. The weathered surfaces bleach and turn grey with micro cracks
Resistance to High Energy Radiation
- The radiation resistance of cellulose reinforced MF moulding compounds is the same as that of PF resins and special UF grade
Flammability
- MF mouldings burn with a yellow flame and are self extinguishing on removal of source of ignition. The combustion products have a choking smell of amines (fishy) and formaldehyde
Toxicological Assessment
Special grades are permitted for food contact. The special conditions are summarized below:
Behaviour in boiling test: The boiling water must not be coloured or turbid.
Freedom from taste or smell: The taste and smell of the water used for boiling is not allowed to differ from that of the control.
Formaldehyde Release: More than 3 ppm of formaldehyde release is not allowed.
PROCESSING OF MELAMINE
FORMALDEHYDE COMPOUNDS
Compression and Transfer moulding
- MF based compositions are easily moulded in conventional compression and transfer moulding equipment.
- A 3.2 mm thick moulding required about 190 seconds cure time at 150ºC but shorter times are possible with preheated powder.
- An interesting use of melamine resins in compression moulding involves decorative foils. A printed or decorated grade of paper is impregnated with resin and dried.
- During compression moulding of MF, shortly before the cure is completed the mould is opened and the foil is placed in position and the resin in the foil cured in the position so that the foil actually bonds on the moulding.
Injection moulding
- Injection moulding of MF moulding powder is carried out on a small scale. The temperatures are somewhat higher than UF. The barrel temperatures 100-115ºC; the mould temperatures 163-177ºC.
- Cure time employed depends on the properties required for the finished product
AVAILABILITY
- Melamine moulding compounds are available as power, fibers, chips or granules.
TYPICAL APPLICATION
Principal application of melamine - formaldehyde moulding compositions is for the manufacture of tableware, largely because of their wide colouration range, surface hardness and stain resistance.
- Cellulose-filled compositions also find a small outlet for trays, clock cases and radio cabinets and other purposes.
- Mineral filled powders are used in electrical applications and knobs and handles for kitchen utensils.
Laminates Containing Melamine-Formaldehyde Resin
- Use of laminates prepared using only melamine resins as the bonding agent is, however, limited to some electrical applications because of the comparatively high cost of the resin compared with that of P-F resins
- Very large quantity of decorative laminates are produced in which the surface layers are impregnated with melamine resins and the base layers with phenolic resins
- Their availability in a wide range of colours has led to their well-known applications in table tops and as interior wall-cladding in public buildings and public transport vehicles.
- Glass-reinforced melamine-formaldehyde laminates are valuable because of their good heat resistance coupled with good electrical insulation properties; including resistance to tracking.
Miscellaneous Applications
- MF is now widely used in conjunction with urea (and formaldehyde) to produce adhesives of good strength, reactivity and water resistance
- MF condensates are also useful in textile finishing
Example : Useful agents for permanent glazing, rot proofing, wool shrinkage control and, in conjunction with phosphorus compounds, flame proofing water repellence.
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