Environmentally-Friendly Acrylates-Based Polymer Latices - Advanced Materials for Agriculture, Food and Environmental Safety - page 146

6.1 Introduction
About the time of the American Revolution, most of the paints used were
based on natural resins, but with the increasing demand for coatings, natu-
ral resins became insui cient. Moreover, the paints based on natural res-
ins do not exhibit satisfactory i lm properties. As a result, today the paint
industry has become dependent mainly on synthetic resins because of
their much superior properties, easy availability to suit the requirements
and also their cost ef ectiveness. Synthetic polymers as binders play a key
role in the process of developing suitable coatings. Since the use of latex
as a coating binder was i rst examined in 1946, synthetic polymers have
become more and more important. Latex dispersions form i lms at ambi-
ent temperatures by coalescence of relatively sot particles containing solid
polymer. h e selection of binder for formulating a coating is based on the
desired i lm properties for the specii ed end use. Industrial coatings dif er
from those used in most household paint applications. h ey are used on
larger-scale industrial buildings and equipment. h ey have a higher glass
transition temperature so that they are tougher than household paints,
and have better barrier properties through copolymerizsation with more
hydrophobic monomers. Industrial coatings and an important class of
binders used in surface coatings are ot en acrylic copolymers, such as poly-
urethane, epoxy, alkyd resin latexes, etc.
Monomers are the building block of the polymer, and their selection is
mainly based on the desired end-product properties (adhesive strength,
water resistance, minimum i lm-forming temperature, etc.). h ese appli-
cations require i lm-forming latex with high clarity, good stability and
high mechanical strength. h e desired properties of the emulsions depend
on the glass transition temperature (Tg), visco-elastic modulus of the
polymer which, in turn, depends upon the type of monomer, sequence
length distribution of the monomer, and the molecular weight of the
polymer. For i lm-forming behavior, Tg must be well below the applica-
tion temperature to allow high molecular mobility for interdif usion and
entanglement.
6.1.1 Alkyds
Alkyd resins are essentially short-branched polyesters derived as the reac-
tion products of vegetable oils triglycerides, polyols and dibasic acids or
their anhydrides. Alkyd resins have been the workhorse for the coating
industry over the last half century. At present, they hold the majority share
of the world market for non-aqueous binders. h ey are used extensively