Oils for Paints & Coatings

OIL is defined as an unctuous, combustible substance, liquid or easily liquefiable on warming, not miscible with water (but soluble in ether)

For example, castor oil, mustard oil, linseed oil, coconut oil, petroleum oil, fish oil, essential oils etc. Are unctuous (buttery, soapy) and are combustible and some of these are liquid at ambient temperature and coconut oil is solid at little lower temperature which can be liquefied.

Significance of oil in paints and coatings:-

  • Oils were major binders or film formers in paints/varnishes/lacquers in earlier days with or without modification.
  • Even today they are in use though in less quantity as binder.
  • They are used as raw material for some resin as, alkyd resin, epoxy ester, urethane oil.
  • Oils are being replaced by synthetic resins as resins have better properties than oils for examples the paints for house and buildings are now in emulsion form based upon acrylic resins.
  • Alkyd resin is still back bone for enamel paints where alkyd resin is used in bulk quantity and oil is the major ingredient in alkyd resin.
  • Oils are also used as plasticizer & additives (Dispersing agent-TRO from castor oil)
Pigments
Resins
Extenders
Solvents
Additives

Introduction & synthesis of Oil

Vegetable oils are common raw materials in the coating industry, especially for alkyd resins. These are in common use, but in many industrial coating systems they bacame second choice because of substitutes based on petrochemical raw materials and which provide an improved technological performance.

In architectural coating systems alkyd resins still play an important role because of their benefits such as the economical balance, an uncomplicated application and an outstanding appearance of the films.

Water-borne and high solids systems as well as alkyd/acrylic-hybrids will increase their market share for ecological reasons.

Apart from alkyd resins, oils are used as reactive diluents e.g. in printing inks, and fatty acids are used as educts for a series of specialty products in the coating industry, e.g., electric insulation systems.

please watch the given lecture to know more.

Classification of oils

OILS used in paint and coatings are classified as non drying, semi drying and drying oils based upon the unsaturation in the molecule. For example coconut, castor oil are non drying, soybean, safflower oils are semidrying and linseed oil, Tung oils are drying

Depending upon origin-

  • Animal oil-from animal like fish oil
  • Mineral oil-from mineral source like petroleum oil
  • Vegetable oil-from seeds, nuts, flowers etc. like linseed oil, coconut oil

Depending upon behavior of heating-

  • Volatile oils (essential oils: essence of odor and flavor) e.g., rose oil, peppermint oil
  • Fixed oils: non-volatile

Two major sources oils that are being in surface coatings.

  1. Vegetable -Seeds, nuts, or fruits e.g., linseed oil, castor oil, coconut oil for paints and coatings
  2. Marine – from species of fish but are not much in use due to smell, poor drying and other inferior film properties.
Vegetable Oils-

“Vegetable oils are triglycerides of long chain mixed fatty acids”.

The composition of crude vegetable oils is as-

  • Glyceride
  • Non-glyceride

GLYCERIDE: consist of triglyceride, Chain length of fatty acids may vary from C8 to C22 but C18 is most common in vegetable oils. The physical and chemical properties of oils depend upon fatty acid composition.

Fatty acids may have.

  • Saturation and unsaturation with one double bond or poly double bond
  • Polar groups in the long chain like =CO, -OH etc.

More the unsaturation better and faster will be the drying of oils.

For air drying paint there should be good percentage of three unsaturation. The two unsaturation in the fatty acid of an oil is normally suitable for stoving or baking paints.

Fatty Acids vary in terms of:

  • Number of double bonds and their position
  • Number of double bonds- saturated, mono-unsaturated, and polyunsaturated
  • Position – conjugated and non-conjugated

Characteristics of Fatty Acids:

  • Saturated FA-no drying,
  • One double bond –no drying
  • Two double bond –baking type of coating
  • Three double bond- air drying and baking/stoving.

Position of unsaturation: with same degree of unsaturation, oils with conjugated FA dry faster than non-conjugated oils

NON-GLYCERIDE– Gums, waxes, phosphatides, sterols, tocopherol, fatty alcohol, colouring matter, free fatty acids, hydrocarbon etc.

  • Non glycerides are in small quantity but affect gloss, drying rate, colour, clarity of film.
  • So, should be removed by refining mostly by acid or alkali refining.
  • Alkali refined oils are used in resin/binder/film former like alkyd resin, epoxy ester, urethane oil etc.
  • Edible oil is also normally alkali refined.
  • Acid refined has high acid value so rarely used in binder as it can make the film water sensitive.
Drying mechanism of OILS​

Drying of paint and coating means conversion of wet film into dry film. This is possible by two general ways:

  1. Small molecules of resins/oils/polymers (liquid state) react and form big molecules which are in dried form e.g., enamel or paints used in houses on doors or iron surfaces.
  2. Resins/polymers are big enough and are in solid form which form film after evaporation of solvent/thinner e.g., water-based emulsion paints for houses.

Oil can form film in two ways-

  1. By reacting with oxygen
  2. By reacting among its molecules due to heat.

please watch the given lecture to know more.

Non-conjugated structure of oil converts into conjugated structure in both the above case and then the big molecule of oil is formed which provide a dried film.

  • Presence of drier accelerate the drying as driers are the oxygen carrier.
  • The film or coating contains C-O-O-C, C-O-C and C-C link.
  • Colour of film changes with time
  • Coating becomes brittle with time.
Modification of oils/ TREATED OILS

OILS in their raw form are rarely used in paints and coating as they cannot provide desired properties to the paint such as levelling, gloss, drying, etc. rather they can furnish some defects in the coatings e.g., sagging etc. Therefore the modification or treatment of oils are done to fetch some suitable properties.

In this video we will discuss some of the treatments– 

  • BOILED OIL
  • STAND OIL/HEAT BODIED OILS
  • BLOWN OIL
  • LIMED OIL
  • Effect of heat, air, time, temperature, driers have been discussed in this video.

In this video we will discuss some of the treatments– 

  • ISOMERISED OIL- The isomerised oil explains how the non-conjugation changes to conjugation and what benefit we get from this.
  • DEHYDRATED CASTOR OIL- How castor oil (non-drying oil) is converted to dehydrated castor oil (drying oil), has also been explained with chemical reaction.
  • MALEIC TREATED OIL- Maleic treated oils have wide application in coating and allied industries.
  • COPOLYMERISED OIL- Copolymerised oil improves the properties of oil e.g., hardness, drying, impact resistance etc.
  • SOLVENT SEGREGATION- Solvent segregation is the process of segregating desired range of molecular weight of heat bodied oil from less important part of molecular weight of heat bodied oil.
  • OIL SPLITTING AND RE-ESTERIFICATION WITH ALCOHOLS OTHER THAN GLYCEROL- Oil splitting helps us to take out the high unsaturation portion of oil from less unsaturation portion thus increasing the drying rate of oils.
  • CATALYSED OIL