UREA RESINS:
HISTORICAL DEVELOPMENT
- Among the various amino plastics, Urea and Melamine formaldehyde resins are the most important commercially
- The interest in amino plastics dates from the publication of a patent by John in 1918
- In 1926, as a result of work by E.C. Rossiter, moulding powders based on urea-thiourea-formaldehyde were marketed.
- In 1935 Henkel patented, the production of resins based on melamine. Today these resins are important in the manufacture of decorative laminates and in tableware.
- The bulk of the amino resins are used in the woodworking industry
UREA – FORMALDEHYDE RESINS
Preparation of Urea
- Urea is prepared by the reaction of liquid carbon dioxide and ammonia in silver-lined autoclaves, at temperature in the range 135-195ºC and pressure of 70-230 atm.
- Urea is a white crystalline compound with a melting point of 132.6ºC and is highly soluble in water.
Preparation of Formaldehyde
- As in the case of phenolic resin, formalin with both high and low methanol content is used to prepare formaldehyde.
MANUFACTURE OF UREA FORMALDEHYDE RESINS:
- Urea-formaldehyde ratios normally employed are in the range 1: 1.3 to 1: 1.5
- Urea-formaldehyde resins are manufactured by a two-stage reaction.
- First stage of resin preparation is to dissolve urea into the 36% w/w formalin which has been adjusted to a pH of 8 with caustic soda.
- Solution at the end of the first stage process contains urea, formaldehyde, and mono- and dimethylol urea
Manufacture of Urea and Melamine Formaldehyde Resins
CURING OF UREA-FORMALDEHYDE RESINS
- Product of the first stage, contains unreacted urea and formaldehyde, is then subjected to acid conditions at elevated temperatures during the second stage.
- In the first part of the second-stage methylol ureas condense with each other by reaction of an –CH2OH group of one molecule with an –NH2 group.
- More soluble resins produced on continuation of the reaction contain pendant methylol groups formed by reactions of the groups with free formaldehyde - I
- Furthermore the ether linkages on heating may break down to methylene linkages with the evolution of formaldehyde
- These methylol groups and the methylol groups on the chain ends of the initial reaction product can then react with other methylol groups to give either linkages or with amine groups to give methylene linkages.
- When reactions II and III occur on average more than twice per molecule the resin gels, and cross-linking may be considered to have occurred.
UREA FORMALDEHYDE mouldING MATERIAL
Compounding Ingredients
Fillers
- Bleached wood pulp is employed for the widest range of bright colours and in slightly translucent mouldings,
- Woodflour, which is significantly cheaper, also be used.
Pigments
- A wide variety of pigments is now used in U-F moulding compositions
Accelerator
- In order to obtain a sufficient rate of cure at moulding temperatures it is usual to add about 0.2-2.0% of a 'hardener' (accelerator).
- This functions by decomposing at moulding temperatures to give an acidic body that will accelerate the cure rate.
Example :- Ammonium sulphamate, ammonium phenoxyacetate, ethylene sulphite and trimethyl phosphate.
Stabilizer
- Urea-formaldehyde powders have a limited shelf-life but some improvement is made by incorporating a stabiliser such as hexamine into the moulding power
Plasticizers
- Their main virtue is that they enable more highly condensed resins to be used and thus reduce curing shrinkage whilst maintaining good flow properties.
- Glyceryl a - tolyl ether (monocresyl glycidyl ether) is often used for this purpose.
Lubricants
- Metal stearates such as zinc, magnesium or aluminium stearates are commonly used as lubricants at about I % concentration
Compounding of moulding Compositions
- Urea-formaldehyde is mixed with the filler (usually with a dry weight resin-filler ratio of about 2:1) and other ingredients except pigment in a tough mixer.
- Resulting wet base is then fed to a drier which may be either of the turbine or rotary type.
- On emerging from the drier the base is hammer-milled and then ball-milled.
- For densification the powder is heated as it passes along a belt and to drop the heated powder into the nip of a two-roll mill,
In this process the material passes directly through the rolls to form a strip which is then hammer milled to give powder.
- More recent processes involve the use of continuous compounders, such as the Buss Ko-Kneader.
STRUCTURE AND GENERAL PROPERTIES
- The structure of the pre-condensate is a short chain molecule combined with CH2 bridges.
- Low cost. The cheaper grades are sometimes lower in weight cost than the general purposes phenolics. (It is to be noted that urea formaldehyde have a somewhat higher density).
- Wide colour range.
- They do not impart taste and odour to foodstuffs and beverages with which they come in contact.
- Good electrical insulation properties with particularly good resistance to tracking.
Resistance to continuous heat up to a temperature of 70℃.
PHYSICAL PROPERTIES
Mechanical Properties
- UF and MF are the hardest of all plastic materials.
- UF has a hardness of M-110 to 120 and MF of M-115 to 125 Rockwell Scale.
- Although they are brittle at normal room temperatures, UF and MF compounds exhibit very high modulus of elasticity
Thermal Properties
- UF the continuous service temperature is 77ºC and for MF is 99ºC.
- Heat deformation temperature (HDT)under load (1.8 MPa) is 130ºC for UF whereas HDT of MF is 183ºC.
Optical Properties
- UF and MF resins are transparent
- Colour possibilities of both the materials are unlimited.
CHEMICAL PROPERTIES
Resistance to chemicals
- UF resins are resistant to solvents, oils, fats, weak acids and alkalies.
- They are not resistant to strong acids and alkaline solutions, boiling water, oxidizing and reducing agents.
Weather Resistance
- On immersion in cold water, UF mouldings absorb more water than MF.
- Wood flour filled UF compounds have less weathering effect than cellulose filled compounds.
Resistance to High Energy Radiation
- Radiation resistance of UF resin is high. In the case of moulding compounds the nature of the reinforcing materials is the deciding factor
Flammability
- UF moulded materials burn with a yellow flame and are self-extinguishing
- The combustion products have a choking smell of amines (fishy) and formaldehyde.
Toxicological Assessment
- Only special grades of UF are permitted for contact with food.
AVAILABILITY
- Although available as fine powders, UF moulding compounds are mainly available as granules.
TYPICAL APPLICATIONS
Adhesives
- UF resins are used as adhesives for the particle board, plywood and furniture industries
- Interior decoration panelling is the major end use of the UF particle board and plywood.
- As gap filling resins by incorporating plasticizers.
Moulding compounds
- The first big application of UF resin is the moulding area.
- The bulks of these applications are for plugs, sockets and switches switch box, plugs, sockets, lamp holders, screw cap for cosmetics, hair drier housings, table mats, telephone sets and cookware handles etc.
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