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Synthetic enamels - III

This is a common range is given there is no hard and fast rule - if oil % is 52 then some times it also called as medium or medium short alkyd.

Alkyds are classified as:

Air drying alkyds - where drying oils are used.

Air cum stoving Drying Alkyds - where semi drying oils are used.

Stoving or non drying alkyds - where non drying oils are used.

So we have different combinations:

  • ® Air drying alkyd - Used in high quality paints
  • ® Air drying alkyd – Used in second quality paints
  • ® Fast drying alkyd – Used in furniture/ hammer tone/ wrinkle paints
  • ® Air drying alkyd – Used in high quality non yellowing paints
  • ® Air cum stoving alkyds – Used in forced drying paints
  • ® High% oil required high % cross linking agent
  • ® Generally not prepared because high % oil required high % cross linking agent
  • ® Generally not made because high% oil required high % cross linking agent
  • ® Stoving alkyd – used in baking finishes

Long oil alkyds based on the drying oil – glossy, good gloss retention property, soft, good in drying, brushability and exterior durability.

Medium oil alkyds based on drying oils – air drying property better than long oil alkyds, high viscosity, better for spray.

Short non drying alkyds – baking / stove at 120°C to 150°C temperature for 15 to 30 minutes. Spraying only applicable.

The accidental discovery besides these above alkyds is rosinated short oil alkyds. In one of the resin plant chemist saved jellying batch by addition of rosin as peptising agent and saved the batch. Next day to his surprise results were excellent in many aspects. Therefore information of this product is not available in standard old resin technology books. Rosin modify alkyds have HMW polymer chain. Low solid contain have sufficient high viscosity. Shows good gloss, hardness, fast drying (fast solvent released), but have not gloss retention more than 6 to 8 months, no exterior durability, drop in viscosity after ageing due to rosin contents.

Linseed based long oil alkyds have good flexibility, drying properties, exterior durability than DCO long oil alkyds, they show residual tack after drying, soft film, while soya long oil alkyds, show non yellowing comparable with linseed in flexibility, but have some poorer drying characteristics. Tung oil phenolic resin have very fast drying properties resulted into wrinkle finish or jelling. Selection of resin is depend on the end use, application method & cost.

Pigments:

Powder which is not soluble in the vehicle is called pigment.

Pigments are classified in two main categories.

Colour pigments and extenders.

Colour pigments are further classified as (a) inorganic & (b) organic pigments.

Important properties of pigments – particle size & its distribution, shape, oil absorption, hiding, density, bulk volume, dispersibility, strength, masstone, undertone, reduced tone / tinting strength, chemical resistance and reactivity.

But for us important required properties are – oil absorption, tinting strength, dispensability, density, tone and undertone, moisture content, contamination or adulteration, hiding.

Colour pigments are opaque while extenders do not have opacity. Extenders are cheaper in cost and used in paints to introduce certain properties like – permeability, porosity, sanding, gloss reduction (mattness), bulky appearance, thixotropy, impactness, roughness.

Extenders generally used are talc, china clay, whiting, barites, calcite, dolomite, aluminium silicate, silica, etc.

Colour pigments generally used are white & colour. White – titanium dioxide, antimony oxide, zinc oxide( white seal), lithophone, etc.

Inorganic pigments are bright, dull depend on the source (natural or synthetic or treated), but have good light-fastness, good hiding power. Good solvent resistance.

Organic pigments are bright, clean, but poor hiding, poor in light-fastness, prone to bleeding with different solvents, not have heat resistance in general.

Depending on the end use, shade, availability of material, inventory & cost select the pigments combination.

Author Details

Mukund Hulyakar

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