Melamine-based and other resins

By T.K. UNNIKRISHNAN

Melamine formaldehyde resin is the most important and a very highly sophisticated resin among the amino group because melamine, due to its inherent properties, contributes a lot of characteristics such as surface resistance to scratches, wear and tear, water repellence, stain resistance, limited resistance to heat, flame retardant and weather resistance.

This is a water-based resin that is is soluble in water as well as methyl alcohol; but it has a short shelf life – at room temperature it lasts about a week. However, this resin in spray-dried powder form has long shelf life under normal room temperature, and an even longer life under lower temperature and humidity.

This is polycondensation and the reaction is a heating-cooling cycle with its  endpoint decided on the values of water tolerance, viscosity, specific gravity, gel time, solid content, water content, etc.

Melamine – C3N3(NH2)3 – has a melting point of 3540C and is obtained by heating dicyandiamide at 209 degrees C. Di-cyanamide is obtained by heating cynamide at 800C. The recently preferred and universally accepted method is by heating urea in the presence of a catalyst.

Thermo-setting compounds

Melamine and formaldehyde (37w\w) are drawn into a stainless steel reaction kettle in a molecular ratio of 1:2.50. The reaction kettle is fitted with a stirrer, two reflux condensers, one vertical and a horizontal type.

The kettle may be either a jacketed type or of limpet coil construction. The heating medium used is generally steam (thermic fluid systems are also in use) and cooling is done by cold water.

After charging the reaction kettle with melamine and formaldehyde, caustic soda solution (sodium hydroxide) is used to adjust the pH of the reaction mixture to 9.0 under constant stirring, before heating.

Heating is conducted at a rate of 20C per minute and the temperature is raised to 900C. Necessary care should be taken to control the heating as this is an exothermic reaction.

Melamine, when it reacts with formaldehyde, forms resinous thermo-setting compounds similar to the methylol ureas. Ureas, as we know, forms mono- and di-methylol compounds by loss of water between an amino group and formaldehyde reacting as methylene glycol.

This complexity results in better properties compared to urea formaldehyde resins; and that is the reason behind the acceptance of melamine formaldehyde resins, polymers and plastics by the industry.

The use of urea formaldehyde resins in the laminate industry, especially for the manufacture of decorative laminates, has now become an accepted technology due to its cheaper cost when compared to melamine formaldehyde resins.

But the substitution of urea formaldehyde resins will automatically lead to the deterioration of most of the surface properties: resistance to abrasions, scratches and stains. Loss of weather resistance and increased moisture absorption of the laminates can lead to poor bonding.

However, there is one silver lining – an improvement in the fire-retardant properties of the laminates. Urea (NH2-CO-NH2) has two neutral gases in its structure: nitrogen and carbon dioxide, which drastically reduce the combustion properties of the product.

Urea formaldehyde resins

The term amino-plastics (amino resins) has been used as a general name to describe step reaction polycondensation products of aldehydes such as formaldehyde, and nitrogen compounds such as urea, melamine, guanidine and thio-urea.

Each urea molecule contains four reactive hydrogens but only two are methylolated by formaldehyde under alkaline conditions. Cross-linking of the dimethylol urea occurs under acidic conditions to form infusible urea resins.

The addition of monohydric alcohols such as methanol, n-butanol, etc., yields a more soluble resinous product, with a bulky pendant group that can be used for coatings, and as bonding resins for the manufacture of laminates, plywood, particle board, light, medium and high-density boards, and many such allied products.

The industry follows a double-stage reaction in order to obtain a water-clear resin used for impregnation. After charging the urea and formaldehyde as per the molecular ratios described above, sodium hydroxide solution is added to the mixture under constant stirring to a pH of about 9.0.

This mixture is then heated to about 900C and maintained at that temperature for nearly 30 minutes. It is then cooled down to a temperature of 650C and acetic acid is added to achieve a pH of 6.0.

The temperature is raised in a gradual manner to 900C until a water tolerance of 1-3 is achieved. This mixture is then cooled down to room temperature and brought to a pH of 8.0.

Re-adjusting the pH is of great importance as at a lower pH of around 5-6 the resin will continue to polymerise and thus affect the shelf life of the resin.

Epoxy resins

The most popular and accepted epoxy resin intermediates are prepared from the reaction of bis-phenol-A and Epichlorhydrin. Liquids are produced when high ratios of epichlorhydrin to bisphenol-A are used. Solids with N values of 10-12 are produced by using a higher proportion of polyphenols.

These oxyranes may undergo ring-opening polymerisation in the presence of acidic or alkaline catalysts. Polyamides and diethylene triamine are widely used for the production of network polymers at room temperature.

Amides with amine end groups (versamides) obtained from the reaction of diamines and dicarboxilic acids are also used as curing agents for epoxy resins at room temperatures.

The ring-opening reactions take place in the presence of traces of proton donors like water or methyl alcohol. Epoxy resins may also be cured by heating with cyclic anhydrides such as phthalic anhydride, maleic anhydride, etc.

A semi-ester is formed by this reaction of the hydroxyl group in the epoxy resin. The carboxyl group formed can then react with either a hydroxyl group or an oxyrane ring.

Phenoxy resins

This type of resin is not a popular or widely-used resin in the high-pressure laminate industry, but is used in the moulded plastics industry because of its qualities like exceptionally good dimensional stability and resistance to creep.

Under the circumstances, it was decided not to go into the details of this grade of resin. However, developmental work was carried out by some companies in the industry and they have established the manufacturing feasibility of laminates, though having a very limited use in the industry.

The only advantage of phenoxies over epoxies is the cost factor; but at the cost and sacrifice of certain properties.

(To be continued)

 

–The writer is an expert in chemistry in the high-pressure laminates industry, with 55 years’ experience. This is an abridged extract from his new book, ‘Chemistry & Technology of High Pressure Laminates’. To buy the book, write to ravs6@yahoo.co.in. This is the second part of an article that first appeared in the Sept-Oct 2022 edition of WoodNews.

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