PAINTS and PIGMENTS

Paints are liquid substances, which hold solid colouring material suspension, known as pigments. These stable mechanical mixtures can be applied evenly to a surface for protective purpose, decorative purpose or both.

Essential ingredients of paints are---

Pigments

Various inorganic or organic insoluble substances which are widely used in surface coatings are known as pigments. Pigments are naturally occurring or manufactured and the colour of the paint depends upon the colour of the pigments.

The important properties of pigments are---

(1)         Good covering power.

(2)         Good mixing ability with oil.

(3)         Opacity

(4)         Chemical inertness of pigments.

(5)         Toxicity level of pigments are very low or nil.

(6)         High hiding power.

(7)         High limiting strength.

(8)         Pigments reflect the destructive UV light and protect the film.  

Extenders or Fillers

Extenders or fillers are inert solid substances. Some examples of extenders or fillers are talc, BaSO₄, china clay, asbestos etc. In order to decrease the cost of the paints and to supplement the pigments in increasing the covering and weathering power of the film, the extenders or fillers are added to the paint. Extenders or fillers improve the consistency, levelling, durability and setting of the paints.

Driers

In order to accelerate the drying of the film through oxidation and polymerization, certain driers also been used in the paint. The polymerization of the unsaturated drying oils occurs by a reaction mechanism which involves peroxide intermediate. Due to catalytic activity of the drier they act as a catalyst and promoters in the oxidation polymerization process. Driers are generally mixed with hot boiled linseed oil. Driers dissolve in the hot oil and they reduced the drying time. Some examples of driers are Co, Mn, Pb, cobalt linoleates, cobalt tungstates, MnO₂, litharge, lead acetate, manganese acetate etc.

Thinners or Diluents

In order to dissolve film forming material and to thin concentrated paints for better handling, thinners or diluents is also added to the paints. Some examples of thinners or diluents are petrol, benzene, naphtha, rosin spirit etc.

Film Forming Materials

The film forming materials acts as carriers for the pigments.

Antiskinning Agents

Certain antiskinning agents such as polyhydroxy phenols are added to the paint in order to prevent gelting and skinning of the finished product.

Plasticizer

Plasticizers are added to the paint, in order to reduce certain aspects of cracking in paints.

Resins

A variety of resins are also added to the paints.

Classification of pigments

According to the colour of the pigments, they are classified in the following types---

(A) White Pigments------

(i) White lead [Pb(OH)₂.2PbCO₃]

(ii) Zinc oxide [ZnO]

(iii) Titanium dioxide [TiO₂]

(iv) Lithopone [BaSO₄ + ZnS]

(B) Blue Pigments----

(i) Ultramarine

(ii) Cobalt blue

(iii) Iron blue

(C) Red Pigments----

(i) Red lead [Pb₃O₄]

(ii) Cadmipone [CdS + BaSO₄]

(iii) Iron oxide [Fe₂O₃]

(iv) Chrome red [Pb(OH)₂.PbCrO₄]

(D) Black Pigments-----

(i) Carbon black

(ii) Furnace black

(iii) Lamp black

(E) Green Pigments----

(i) Chrome green [Cr₂O₃]

(ii) Malachit green [Cu(OH)₂.CuCO₃]

(iii) Rinman’s green [CoO.ZnO]

(F) Orange Pigments----

(i) Cadmium orange

(ii) Basic lead chromate

(G) Yellow Pigments----

(i) Lead chromates

(ii) Zinc chromates

(iii) Litharge

(iv) Ocher [A naturally occurring pigment]

(H) Brown Pigments----

(i) Burnt umber

(ii) Burnt sienna

(I) Metallics Pigments----

(i) Zinc dust

(ii) Copper powder

(J) Metal Protective Pigments----

(i) Zinc and basic lead

(ii) Red lead

Due to durability and colouring power of toners (insoluble organic dye), they are also used directly as pigments.

Lakes are organic dyes on an inorganic absorbent, have also been used as pigments.

Requisites of a good paint

(i) Paint should be resistant to heat and light.

(ii) Due to small change of temperature, the colour of paint should not fade.

(iii) Due to exposing to light, the colour of paint should not fade.

(iv) Paint should be resistant to the corrosive action of acids and bases.