Chemistry Reference
In-Depth Information
applications, numerous efforts have been made to enhance or increase the
ultimate T
g
of the final film without affecting the latex's film-formation
properties. Self-cross-linking latexes enable molecular weight advancement
to occur subsequent to film formation and enhance the separation between
MFFT and T
g
. The auto-oxidative cross-linking capabilities of vegetable oils
are an attractive option in this regard. While blending functionalized vege-
table oil derivatives such as maleinized drying oil with latexes has shown
promise,
40
they also exhibited issues such as phase separation and in-
homogeneous cross-linking. Hybrid waterborne alkyd-acrylic dispersions
(solid content 40%), free from surfactant and solvent, were synthesized by a
melt co-condensation reaction between an acrylic pre-polymer bearing
carboxylic groups and a long-oil alkyd resin.
41
Insertion of anhydride moi-
eties within the acrylic pre-polymer ensured ecient coupling between the
acrylic and alkyd resin and prevented phase separation. The authors re-
ported that their coatings were stable for two months. In general, long-term
stability is usually a challenge with alkyd emulsions due to the profusion of
hydrolysis-sensitive ester groups.
Vegetable-oil-based macromonomers (VOMMs) constitute a series of
vegetable oil acrylate and (meth)acrylate derivatives functionalized for
ecient incorporation into emulsions. The synergistic combination of
vegetable oil derivatives and an acrylic backbone provides storage-stable,
auto-oxidatively cross-linking systems for architectural and/or industrial
coatings with reduced or zero VOC emissions. Generically, VOMMs can be
synthesized from any vegetable oil, independent of its composition. VOMMs
have three distinct characteristics that are advantageous to environmentally
responsible emulsions: (1) by virtue of their molecular length and large
monomer size, they are excellent plasticizing monomers that facilitate co-
alescence without the necessity for solvent-based coalescing agents; (2)
VOMMs readily co-polymerize with vinyl monomers through their acrylate
functionality and are therefore retained during film formation reducing the
T
g
and MFFT; and (3) the allylic functionalities within the VOMM tail react
auto-oxidatively during coalescence at ambient temperature, creating films
with highly cross-linked networks that attain mechanical strength through
room-temperature cross-linking of already high-molecular-weight entangled
polymer chains.
42
VOMM-based latexes are amenable for formulating low-
VOC coatings.
43
Kinetic investigation of VOMMs in bulk as well as emulsion
polymerization has also been reported.
44-46
The most well-studied VOMM, SoyAA-1, is based on soybean oil and offers
a free radically co-polymerizable derivative from a renewable resource.
47
SoyAA-1 is synthesized by reacting soybean oil with N-methyl ethanolamine
followed by (meth)acrylation with (meth)acrylic acid (Figure 12.1). The syn-
thesis is not energy intensive and is accomplished at high yields (495%).
SoyAA-1 is characterized by high vegetable oil content (466% by weight),
favorable hydrophobic-hydrophilic balance for facile emulsion synthesis
under standard conditions, and is an excellent flexibilizing monomer (esti-
mated T
g
67.5 1C). SoyAA-1 is co-polymerizable with a variety of common
