Vinyl Resins

22
Formulation of Clear Coatings
Clear vinyl coatings can be modified with plasticizers,
heat and light stabilizers, and other materials for specific
performance properties. Before incorporating any modifier
in the formulation, understand clearly how the modifier
meets the demands of the application. Do not use clear
vinyl coatings for applications that involve long-term
exposure to ultraviolet light.
Plasticizers
The addition of a plasticizer in the coating formulation
will enhance flexibility and help to minimize solvent
retention in the film. The typical phthalate, adipate,
citrate, epoxy, and phosphate plasticizers are compatible
with UCAR® Solution Vinyl Resins. In general, compatibility
decreases as the hydrocarbon nature of the plasticizer
increases. Polymeric plasticizers are less efficient than
monomeric plasticizers.
Other factors to consider in selecting plasticizers
include solubility, volatility, the effect on outdoor durability,
the need for low-temperature flexibility, and suitability
for contact with food. Certain citrates, epoxies, and
phthalates are permitted under FDA regulations.
Monomeric plasticizers are most commonly used,
although the polymeric plasticizers are used to provide
special film characteristics, such as low extractability
or migration. Phosphate plasticizers are generally not
recommended for outdoor exposure because of poor
light stability.
When a bake cycle is required, the volatility of the
plasticizer is particularly important. The plasticizer may
volatilize sufficiently to lower the concentration below
what was originally intended for the dried or cured
formulation.
The optimum level of plasticizer for a formulation
will depend upon the specific resin used and the performance
property required by the application. To obtain
equivalent degrees of flexibility, higher molecular weight
resins require more plasticizer than lower molecular
weight resins. Proportions of 10 to 25 parts plasticizer
per 100 parts of resin are typically used.
Table 6 provides a list of plasticizers having good
compatibility with UCAR Solution Vinyl Resins.
Heat Stabilizers
As with all vinyl resins, UCAR Solution Vinyl Resins
are degraded upon prolonged exposure to heat. The
degradation products include hydrogen chloride, which
accelerates further resin degradation and leads to the
development of unsaturated polymer structures that can
be easily oxidized. The result is embrittlement, loss of
flexibility, and discoloration of the vinyl film. To minimize
the degradation of vinyl films, add suitable heat stabilizers.
Baking at temperatures above 248ºF (120ºC) for more
than five minutes will usually require a thermal
stabilizer to avoid degradation of the film. The use of
a tin mercaptide stabilizer (1 percent*) in combination
with a liquid epoxy resin, such as ERL-4221, or diglycidyl
ether of bisphenol A resin (3 to 5 percent*) gives the
best results.
Do not use barium, cadmium, or zinc stabilizers with
the carboxyl-modified vinyl resins; they tend to react with
the carboxyl groups. Zinc stabilizers also tend to develop
color quickly, especially in low plasticizer systems. Iron
and zinc surfaces can accelerate decomposition and
discoloration.
*on weight of vinyl resin