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Film Formation (Drying)


Crystal Formation — Whitewash

material: calcium oxide: hydrated, pre-hydrated lime
drying rate
pigments (lime-proof)
added organic binders: milk, tallow (proteins)
cement paints
index of refraction

Example: whitewash
Excercise: slaking quicklime

Solvent Evaporation — Distempers

pigments + small amount of water-soluble organic binder (protein glue) + inert calcium carbonate (in lighter colors)
solvent evaporation -> intertwined, entangled long (organic) molecular chains
non-volatile binder coats pigment/filler without silling pores
weak paint with little abrasion resistance, reversible
use: ceilings, protected areas

Examples: distemper, calcimine, casein, poster colors
Excercise: calcimine (calsomine, kalsomine) mixing, with pigments

Solvent Evaporation and Coalescence — Acrylic-resin solution paints

The Acrylic Drying Process, Golden Artists Colors

[no laboratory mixing of acrylic resin paints]

Oxidation and Cross Linking — Oil Paints

Oil Paint, Wikipedia

  • When exposed to air, oils do not undergo the same evaporative process that water does. Instead, they oxidize into a dry solid. Depending upon the source, this process can be very slow, resulting in paints with an extended drying time.
  • The earliest and still most commonly used vehicle is linseed oil, pressed from the seed of the flax plant.
  • Once the oil is extracted additives are sometimes used to modify its chemical properties. In this way the paint can be made to dry more quickly if that is desired, or to have varying levels of gloss.

Linseed oil, WIkipedia

  • Linseed oil is a "drying oil" as it can polymerize into a solid form. Due to its polymer-forming properties, linseed oil is used on its own or blended with other oils, resins, and solvents as an impregnator and varnish in wood finishing, as a pigment binder in oil paints, as a plasticizer and hardener in putty and in the manufacture of linoleum. The use of linseed oil has declined over the past several decades with the increased use of synthetic alkyd resins, which are functionally similar but resist yellowing.
  • Most applications of linseed oil exploit its drying properties, i.e. the initial material is liquid or at least pliable and the aged material is rigid but not brittle. The water-repelling (hydrophobic) nature of the resulting hydrocarbon-based material is advantageous.
  • Boiled linseed oil is used as a paint binder or as a wood finish on its own. Heating the oil causes it to polymerize and oxidize, making it thicker and shortening its drying time. Today most products labeled as "boiled linseed oil" are a combination of raw linseed oil, petroleum-based solvent, and metallic dryers (catalysts to accelerate drying). The use of metallic dryers makes boiled linseed oil inedible.

linseed oil: short molecular-chained polymer, a liquid without a solvent (turpentine): it cannot dry by solvent evaporation but by oxidative cross-linking.
non-reversible, or hard to re-dissolve using solvents
pores filled with polymer
gloss characteristic
raw and boiled linseed oil
driers: oil-soluble metal ion to promote oxidation
lead pigment as drier
embrittlement, oxidation, UV

linseed oil + pigment (pre-1820) gloss
linseed oil + pigment + turpentine (post-1820) semi-gloss
prime, intermediate, finish

Drying by Cross Linking — Alkyd Resin Paints

Alkyd, Wikipedia

oil/resin blends (flexibility/hardness):
linseed oil/natural resin (copal) copolymers
alkyd blend:
oil/glycerine/phthalic acid components->
obtained by eliminating water from polyhydric alcohols
(glycol/glycerol) with dibasic acids (phthalic, etc.)
dry fast, are strong and hard
high molecular weight requires solvent thinner to dry by evaporation of thinner and then oxidative cross-linking
short/long oil = quantity of oil in alkyd
less reactive sites to cause emprittlement

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