L-30: SOLVENTS & THINNERS FOR PAINTS & COATINGS (PART-II)

Описание: “Solvent is a material whose molecules are capable of forming an intimate mixture with molecules of other material(s) and is present in higher percentage in the mixture”, and this mixture is known as solution, the other material is known as solute.
Such an intimate mixture depends upon
-chemical character
-size
-shape
of molecules

We can have solution of gases, liquids and solids
Gases : oxygen+ nitrogen + carbon dioxide = air
Liquids : sulfuric acid + water = acid solution
Solid : copper + tin = bronze

There is a saying “like dissolves like”

Water + alcohol=solution but water + kerosene = separate so on why

Role of Chemical character, size and shape of molecules in solubility
• mutual attraction among molecules of polar and non polar are large enough to prevent mixing of one in other
• In Water, Glucose, Carboxy methyl cellulose (CMC) and Cellulose
-glucose is small molecule and polar so soluble in water
-cellulose is large and is made up of glucose anhydride is insoluble
cellulose is made more polar as in CMC become soluble though large

• Let us see the effect of shape, polarity and size
solubility of alcohol in water (gm/100ml water) Methyl and ethyl alcohols are highly soluble in water but butyl and amyl alcohols are les soluble. Solubility decreases with increase in non polar chain but increases with change in shape
Similarly a non polar system as
mineral spirits---mineral oil-----paraffin wax--------polyethylene
Here, small molecules of mineral oil are soluble in mineral spirit and paraffin wax is poorly soluble (large size of molecule) and polyethylene is insoluble ( very large molecule)
But replace hydrogen (alternate) in polyethylene with OH it becomes polyvinyl alcohol which is highly polar and is soluble even in water.
In coating industry the resins/binder are large to very large, non polar to polar but the solubility in solvent is basic requirement. Accordingly solvents are chosen. Solvents are generally small molecules compare to resin/binder


FUNCTION OF SOLVENTS AND THINNERS
• Reduce viscosity of resin –ease processing of resin and good wetting of pigments
• Ease of application
• Improve flow, level, gloss and substrate wetting
SOLVENT CHARACTERISTICS

• SOLVENT POWER
• BOILING POINT
• RATE OF EVAPORATION
• FLASH POINT
• FLAMMABILITY
• CHEMICAL STABILITY
• COLOUR
• ODOUR
• TOXICITY
• CORROSION RESISTANCE
• WT./LITER
COST PER Kg

TYPES OF VOLATILE SOLVENT
SOLVENTS FOR COATING INDUSTRIES MAY BE
CLASSIFIED AS :
• Terpene solvents
• Hydrocarbon solvents : (i) aliphatic (ii) naphthenic (iii) aromatic
• Oxygenated solvents : (i) alcohols (ii) esters (iii) ketones
(iv) ether-alcohols
• Chlorinated compounds
• Nitroparaffins
• Furans
COMPONENTS OF SOLVENTS AND THINNERS
TRUE SOLVENT
LATENT SOLVENT
DILUENT

SOLVENT RELATED PAINT FILM DEFECTS
• Very fast drying – blistering, trap moisture in humid environment, stresses in film resulting weakness in coating, precipitation of polymer if remaining solvent is not the solvent for resin
• Orange peel surface –fast drying solvent –consistency at point – orange peel