Environmental Engineering Reference
In-Depth Information
for paints and coatings [55]. h e crosslinking process involves the linking
of polymer chains to obtain a network which results in an increase in i lm
hardness, and resistance to solvents, chemicals and detergents. Polymer
chains are viscoelastic in nature and crosslinking will increase the rigidity
of chains, which retards the segmental motion of chains. Modii cation of
the monomer that leads to a crosslinkable monomer has been receiving
great attention in recent years due to their potential for the development of
products such as thermosetting molding compounds, coatings, superab-
sorbent and ion-exchange resins [102].
h e crosslinking reaction increases the brittleness and adhesion of the
coatings. Present technology on thermosetting latexes has shown the suc-
cessful crosslinking of latexes with dif erent crosslinking agents, either
external or copolymerized within the polymer backbone. However, the
incorporation of crosslinking technology into common thermoplastic
latexes involves additional variables that af ect the processes of latex pro-
duction and property development on i lms. Besides the types of chemistry
involved in the crosslinking reactions, one of the most important variables
is the incorporation of functional monomers and crosslinking agents into
the dispersions. h e type of functional groups and crosslinkers, as well as
the addition and localization, would be expected to inl uence processes
of synthesis, i lm formation and mechanical behavior of latex i lms. It is
anticipated that the presence of increased functionality of monomers dur-
ing synthesis inl uences the physical properties of the particles and the
structure of the i lm. On the other hand, the overall mechanical properties
and solvent resistance of latex i lms may not be enhanced if the polymer is
not sui ciently crosslinked. h us, functionality and crosslinker levels must
be enough to provide solvent resistance and cohesive strength, without dis-
rupting other desired properties mandated by the requirements of specii c
applications such as l exibility or impact resistance.
h e distribution of functional groups within a crosslinked latex i lm
is considered as a controlling factor for structure-property relationships
if it is believed that the enhancement of strength through crosslinking is
achieved primarily when the crosslinker is located in the proximities of
functional groups. Ideally, systems with a specially controlled functionality
location would allow specii c crosslinked structures with similar or even
improved mechanical behavior compared to homogeneous crosslinked
systems. Such control of functionality location could be synergistically
combined with packing optimization through the use of dif erent parti-
cle sizes within a single latex dispersion (e.g., bimodal latex dispersions).
However, as a result of the incorporation of functional groups within the
polymer backbone and the addition of crosslinking agents, packing and
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