ZINC SULPHIDE (ZnS)

MINAR DHONDE (11SUR1007)

RONAK VAGHANI (11POL1006)

About ZnS

• Inorganic Pigment.•Pigment White 7C.I. No. 77975Chemical formula ZnS( MW= 97.44)Chemical Composition (wt%)Zn= 67.15% ; S =32.85%.•Costlier than TiO2 pigments, hence not preferred

over them.

Physical Properties

Quantity ValueAppearance Fine White Powder

Particle Shape Spherical

Particle Size (Mean) (0.25-4.0)x 10^-6m

Density( g/ml) 4.0

Refractive Index 2.37

Oil absorption( g/100g) 12-20

Tinting Strength 800

Hardness( Mohs Scale) 3

Fume Proof Yes

Specific Surface Area (Sq.m/gm) 6-8

Chemical Properties

Quantity ValueResistance to acids/bases Soluble in Strong acids

Resistance to Organic Solvents InsolubleStability in aqueous Solutions Stable between pH 4-10

pH value 6-7Heat Stability and Light Fastness Very good

Water Solubility(18 C), wt% 6.5x 10^-5

Lithopone

•Was an earlier version of ZnS based pigments, but is now replaced by Sachtolith.•Contains 30-60% ZnS and the rest, Barium

Sulphate.•ZnS gives good hiding characteristics while Barium

Sulphate acts as an extender.•Has lesser Lightening power than Sachtolith( almost

1/4th).• Lithopone imparts good light fastness and clear

bluish-white shade plastics.

Manufacturing of ZnS(SACHTOLITH)

• A sodium sulphide (Na2S) solution is mixed with zinc salt solution under precisely controlled conditions.• The resulting zinc sulphide precipitate is calcined and processed

to give the finished product.

• Na2S + ZnSO4 → ZnS + Na2SO4

• This is mainly used in plastics. It provides excellent hiding power. • During the dispersion process it does not cause abrasion of

metallic production machinery or adversely affect the polymer, even at high operating temperature or during multistage processing.

Manufacturing of Lithopone

The reaction of equimolar quantities of zinc sulphate(ZnSO4) and barium sulphate (BaS) produces a white, water insoluble co-precipitate.

ZnSO4 + BaS → ZnS +BaSO4

HYDROTHERMAL PROCESS :

• The raw lithopone is precipitated with a slight excess of sulphide at pH 8.5 . The pH is then adjusted to 12-13 with sodium hydroxide(NaOH) solution and 0.5% sodium carbonate. The suspension is then autoclaved for 15-20 min at 250-300 °C. The hydrothermal product has a sphalerite structure with 10% greater scattering power.• Limitation of hydrothermal process is that it is less economic due to high

cost of materials required for lining the autoclave.

Grades

Sachtolith L HD HD-S

ZnS (%) 98 98 98

pH 6 7 7

Hardness (Mohs) 3 3 3

Relative Scattering Power

>50 >50 >52

Dispersibiltiy Satisfactory Good Very Good

Oil Absorption Value (g/100g)

11-13 11-13 11-13

Pure ZnS is marketed as Sachtolith

Applications

• Imparting Color to Thermoplastics due to Low abrasion. Even at high Temperatures or High M.W. thermoplastics .•Used as White Pigment in thermosetting molded

compounds due to low hardness and abrasion.•Glass Fiber Reinforced Plastics: Unlike TiO2, Soft ZnS

gives protection to glass fibers, also acts as a dry lubricant while processing.•Confers good light fastness to Elastomers and Synthetic

Fibers.•ZnS is stable in aqueous media between pH 4-10 and is

inert to organic solvents. When Zinc Sulphide crystals are doped during their production with cobalt they impart better UV protection.

Applications

•The curing of white UV-coatings pigmented with SACHTOLITH is easier, faster and complete – even at higher film thickness due to short wavelength absortion.• It has excellent semiconductor properties.•Paints: Special Advantages are Good Brightness, Pure

color shade, low oil absorption, advantageous rheological properties, soft texture, and low abrasivity•Printing Inks: due to low oil absorption and low abrasion.•Decorative Papers and laminates due to bright coloring.•They have fungicidal and algicidal effects, hence help

prevent growth of such microbes

References

•Pigment Handbook- Properties and Economies by PETER A. LEWIS•Pigment Handbook- Applications and Markets by

TEMPLE C. PATTON• Industrial Inorganic Pigments by GUNTER BUXBAUM

and GERHARD PFAFF.•Application Properties of Pigments by Dr. ASHOK B.

KARNIK