All the
products used in finishing cure by one of two types – evaporative and reactive.
Each type cures differently. A finish cures by changing from a liquid to a
solid on the wood.
EVAPORATIVE
FINISHES
Evaporative
finishes are made-up of solids that have been dissolved in a solvent. These are
made-up of long, strand-like molecules that pack together and unite when all
the solvent present in the solution evaporates. A thin solution can be achieved
by increasing the solvent dilution which also means the less the solvent added,
the thicker the resulting solution. When all the solvent evaporates only the
solid contents are left. These solids are the same as what they were before
they were dissolved. They then transforms from solid flakes into a film on the
wood.
The strands
are intertwined when soft and they’re interlocked and form a continuous film
when dried; but nothing bonds the strands together, re-introducing of solvents
will soften the strands. Evaporative finishes can change back and forth between
liquid and solid by the introduction or evaporation of the solvent.
Applying a
coat of evaporative finish over an existing cured coat will dissolve the
underlying coat. The solvent puts the strands back into a solution, so the
coats fuse making one thicker layer.
Evaporative
finishes cure from the bottom up. The solvent at the bottom travels through the
film layer which then makes the top layer the last part to cure. When the top
becomes hard, you can be sure the film is cured all the way through.
REACTIVE FINISHES
Reactive
finishes crosslink when they cure. As the thinner evaporates, the resin
molecules move closer together resulting into a chemical reaction that links
the molecules together forming a network. Reintroduction of the thinner doesn't
break the bonds that are formed. This chemical reaction is referred to as
crosslinking or polymerization.
Reactive
finishes don't bond chemically between coats once the previous coat has cured.
The film formed won't fuse to the previous coat. Making fine scratches by
sanding promotes the new coat to lock or adhere mechanically. Also, sanding
dusts must be removed because it won't be dissolved into the new coat.
There are
two categories of reactive finishes those that cure by reacting with oxygen and
those that cure by reacting with a chemical catalyst.
For oxygen
dependent curing, the application of coatings must be kept thin. Oxygen makes
contact with the external part of the coating then through the bottom of the
film. It cures from the top down. Once the top cures and forms a skin, oxygen
is inhibited from penetrating to cure the rest of the coat. Thick coats take
much longer to cure all the way through.
For
catalyst curing, it cures from the bottom to top. The solvent that carries the
material will travel from bottom to top; which then allows the molecules to
crosslink.
COMPARISON
The main
difference between EVAPORATIVE and REACTIVE finish is whether or not the
molecules are cross-linked. Crosslinking makes the produced film more resistant
to heat, scratch, solvents, acids and alkalis and making it less penetrable.
There's little space between cross-linked molecules for liquids or water vapor
gases to pass through.
The strong
bonds of cross-linked molecules produce a durable film but for the same reason
the coat produced is difficult to repair. A new coat of finish won't dissolve
the previous coat; a visible line will separate a repair from the original
finish. There are few solvents and chemicals that can dissolve the film.
Evaporative
finishes are not cross-linked, they are easiest to scratch, dissolve and
penetrate. An advantage with evaporative finishes is that it would be easier to
repair since a number of solvents and chemicals can separate the molecules. A
disadvantage would be that the un-cross-linked molecules have large spaces in-between
that results to water and water vapor penetration.
