Which one among the following is commonly used as an 'anti-skinning agent' in paints?
- (a)Gelatin
- (b)N-methyl pyrrolidone
- (c)Pyridine
- (d)Polyhydroxy phenol
Correct — D, Polyhydroxy phenol. 'Skinning' is the tough film that forms on oil/alkyd paint in a partly used can because the drying oil oxidises at the air surface. Polyhydroxy phenols are antioxidants: they interrupt that surface oxidation while the can is stored, and being volatile they escape when the paint is spread, so the coating still dries normally. That makes a phenolic antioxidant the standard anti-skinning agent.
- (a)Gelatin — Gelatin is a protein used as a protective colloid, thickener or emulsion stabiliser — it does not act as an antioxidant, so it cannot stop the oxidative skinning of paint.
- (b)N-methyl pyrrolidone — N-methyl pyrrolidone (NMP) is a strong polar solvent and paint stripper, not an antioxidant; it dissolves coatings rather than preventing a skin from forming.
- (c)Pyridine — Pyridine is a basic, foul-smelling organic solvent and reagent. It is used as a solvent/base, not as an anti-skinning antioxidant in paint.
Oil-based (alkyd) paints dry by oxidative polymerisation of the drying oil in the binder. During storage, that oxidation starts at the paint-air interface and forms an unwanted 'skin'. Anti-skinning agents are volatile antioxidants — phenolic compounds (such as polyhydroxy phenols) and ketoximes — that block this oxidation inside the can.
The trap is that NMP and pyridine are genuine paint-industry solvents, so they 'sound' right. But the job of an anti-skinning agent is antioxidant, not solvent — you need something that stops oxidation and then leaves. Phenolic antioxidants do exactly that, and they are consumed or evaporate on application so the film can still cure.
- Anti-skinning agents are antioxidants that stop oil/alkyd paint from forming a skin during storage.
- Polyhydroxy phenols (e.g., hydroquinone, catechol) and oximes (e.g., methyl ethyl ketoxime) are the usual anti-skinning agents.
- They are volatile — they leave as the paint is applied, so the film still dries.
- N-methyl pyrrolidone and pyridine are solvents; gelatin is a colloid/thickener.
A phenolic antioxidant stops skinning in the can, then evaporates so the applied coat can still cure.
- Picking NMP or pyridine because they are well-known paint solvents — the anti-skinning agent must be an antioxidant.
- Confusing an anti-skinning agent (delays oxidation in the can) with a drier/siccative (speeds oxidative curing of the film).
Asked as additive-function matching — 'which acts as an anti-skinning agent / antioxidant / drier'.
Regular intake of fresh fruits and vegetables is recommended in the diet since they are a good source of antioxidants. How do antioxidants help a person maintain health and promote longevity?
- (a) They activate the enzymes necessary for vitamin synthesis in the body and help prevent vitamin deficiency
- (b) They prevent excessive oxidation of carbohydrates, fats and proteins in the body and help avoid unnecessary wastage of energy
- (c) They neutralize the free radicals produced in the body during metabolism
- (d) They activate certain genes in the cells of the body and help delay the ageing process
Answer(c) They neutralize the free radicals produced in the body during metabolism
Same core idea — how an antioxidant works. UPSC tested it in a diet/health context (antioxidants neutralise free radicals), and that very property is why a polyhydroxy phenol acts as an anti-skinning agent, inhibiting the oxidation that would otherwise skin the paint.
- practice — not a real PYQ
In oil-based paints, driers (siccatives) are added mainly to:
- (a)Slow down drying
- (b)Accelerate the oxidative drying of the film
- (c)Dissolve the pigment
- (d)Prevent skinning in the can
Answer(b) Accelerate the oxidative drying of the film — driers do the opposite job to an anti-skinning agent.
- practice — not a real PYQ
An antioxidant works by:
- (a)Supplying oxygen
- (b)Neutralising free radicals / inhibiting oxidation
- (c)Increasing acidity
- (d)Absorbing water
Answer(b) Neutralising free radicals / inhibiting oxidation — the same property that lets a phenolic antioxidant prevent skinning.