Paints Pigments and Varnishes Questions and Answers

Answer: A

Correct answer: the binder

The binder, or vehicle, is what actually becomes the paint film: it wets the surface, holds the pigment in place and adheres to the substrate. Only the solvent leaves. Pigment gives colour and opacity but cannot form a film by itself, and extenders are cheap solids that modify the properties of the film the binder makes.

Answer: B

A paint formulation is a balance among these four, and changing any one alters the behaviour of the others. The solvent is the only constituent intended to leave the film entirely. Changing one of the four affects all the others.

Answer: C

It evaporates during drying, and its whole purpose is to make the paint applicable and then leave. Environmental regulation of volatile organic compounds is aimed precisely at that loss. Water based paints avoid the problem altogether.

Answer: D

Correct answer: titanium dioxide

Titanium dioxide accounts for the overwhelming majority of white pigment used, and it is the single most valuable ingredient in most white paints. It displaced white lead on toxicity grounds and zinc oxide on performance, for the reason the next item gives.

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the optics. Opacity comes from scattering, and scattering depends on the difference in refractive index between the particle and the medium around it. Rutile titanium dioxide has an index of about 2.7 against a binder’s 1.5, the largest such contrast available in a practical white pigment, so it hides in the thinnest film.

Answer: B

China clay, calcium carbonate and talc have refractive indices close to that of the binder, so they scatter little light. They are used to control gloss, flow and cost rather than opacity. Their refractive index is close to the binder.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is the same principle applied in reverse. Calcium carbonate has a refractive index near 1.6 and the binder about 1.5, so there is almost no contrast, almost no scattering and effectively no hiding power. It is used as an extender for bulk, rheology and cost, not for opacity - and in a dry film above the critical pigment volume concentration it does contribute, because the surrounding medium is then air.

Answer: D

Correct answer: pigment volume to total solids volume

\[ \mathrm{PVC} = \frac{V_{pigment}}{V_{pigment}+V_{binder}}, \] expressed on the dry film, so solvent does not enter it. It is a volume ratio rather than a mass ratio because what matters is whether there is enough binder to fill the space between the particles - which is the question the critical value answers.

Answer: A

Correct answer: 1 and 2 only

Statements 1 and 2 describe the critical pigment volume concentration correctly: above it the binder can no longer fill the voids between pigment particles, air is left in the film, and gloss collapses while porosity and permeability rise sharply. Statement 3 is inverted - gloss paints are formulated well *below* the critical value so that the surface is a continuous resin film. Matt paints are formulated above it, deliberately.

Answer: B

Thirty of the hundred cubic centimetres of solids is pigment, so the concentration is thirty per cent. That is a typical figure for a gloss paint, well below the critical value. Matt paints run well above the critical figure.