SMART HYBRID LATEXES AS BINDERS FOR PAINTS WITH
IMPROVED CHEMICAL RESISTANCE
S. Piçarra,1,2 J. M. G. Martinho,2 J, P. S. Farinha2
1
ESTSetúbal, Instituto Politécnico de Setúbal, Campus do IPS, Estefanilha,
2910-761 Setúbal, Portugal
2
CQFM-IN, Av. Rovisco Pais, 1040-11001, Lisboa, Portugal
Introduction
Aqueous emulsions of polymer nanoparticles are often used as binders
in paints and coatings. When a polymer emulsion is applied over a surface,
water evaporates and polymer nanoparticles deform as they pack close
together. By annealing at a temperature above the polymer minimum film
formation temperature (MFFT), chains interdiffuse between adjacent
nanoparticles to yield a film with good chemical and mechanical properties.
This is a good alternative to the use of paints and coatings based on organic
solvents, which have been ruled out due to environmental problems.
Controlled crosslinking of the polymer chains can improve films
chemical resistance and mechanical strength, provided that the crosslinking
reactions are slower than the interdiffusion of polymer chains across the
interparticle boundaries. The addition of inorganic fillers can also enhance
these film properties, but slows down chain interdiffusion, which is clearly
disadvantageous.
In this work we followed a different approach and produced smart
hybrid latexes to be used as binders. These especial latexes are formed by
encapsulation of a silica precursor (tetraorthosilacate, TEOS) inside polymeric
emulsified nanoparticles of poly(butyl methacrylate), PBMA. As TEOS is
very hydrophobic, it remains inside PBMA nanoparticles for very long times
at neutral pH, where both TEOS hydrolysis and condensation reactions are
inhibited.
However, during the annealing process of film formation, polymer
chains are allowed to interdiffuse, and so does TEOS. As pH is no longer
controlled, TEOS molecules are expected to react with both other TEOS
molecules, forming silicone oligomers, and with some specific groups of the
chains, forming crosslinks. Both reactions should contribute to the
improvement of films chemical resistance and mechanical strength.
Förster Resonance Energy Transfer, FRET, was used to follow the
kinetics of chains interdiffusion across the particle boundaries in films made
of dye labelled nanoparticles, either constituted only by PBMA or also
containing TEOS. Films resistance to solvent was evaluated in both films and
compared.
The encapsulation of TEOS in polymer nanoparticles was shown to
enhance both chain interdiffusion and film chemical resistance. The films
obtained from dispersions of these particles show excellent properties for
application in high performance coatings and paints.
Experimental
Nanoparticles Synthesis. PBMA nanoparticles either containing
TEOS or not (NP-T and NP, respectively) were prepared by emulsion
polymerization though the seeded semi-batch procedure. Two latexes were
produced for each kind of particles: either labeled with a fluorescence energy
donor – phenantherene (NP-T(P) and NP(P)) - or with an acceptor – antracene
(NP-T(A) and NP(A)). Labeling was performed by introducing 1% of a dyelabeled co-monomer during the emulsion polymerization procedure.1
Characterization. Particle sizes and size distributions were measured
by Dynamic Light Scattering, DLS; molecular weight and molecular weight
distribution of the linear chains forming the latex nanoparticles were
determined by Gel Permeation Chromatography, GPC. The weight fraction of
solids was determined weighing four samples of each emulsion, before and
after drying. Table 1 presents the latexes characterization results:
Table 1. Characterization of the Smart Latexes Particles
latexes
wt%
DLS
GPC
solids
D(nm)
rel.var.
Mw
Mw/Mn
NP(P)
33%
114.3
0.026
126k
1.8
NP(A)
33%
117.1
0.007
111k
1.7
NP-T(P)
29%
104.0
0.014
129k
1.6
NP-T(A)
30%
99.0
0.016
128k
1.8
Sample Preparation. Equimolar mixtures of polymer nanoparticles
either containing or not containing TEOS - NP(P):NP(A) or NP-T(P):(NPT(A) - were prepared, spread over surfaces, dried at 30ºC, and further
annealed between 80ºC and 100ºC.
Results and Discussion
FRET results. Donor fluorescence decay curves were measured by the
time-correlated single-photon counting technique, to determine quantum
efficiencies of energy transfer, ΦET. Apparent fractions of mixing, fm, were
determined from ΦET at different annealing times and for complete mixing.
Figure 1 shows fm evolution with the annealing time for two PBMA films
annealed at 80ºC, formed form homogeneous PBMA particles (NP) and from
smart PBMA particles containing TEOS (NP-T).
NP
NP-T
Figure 1. Apparent fraction of mixing, fm, as a function of the annealing time
for two 1:1 molar mixtures NP(P):NP(A) (□) and NP-T(P):NP-T(A) (♦)
annealed at 80ºC.
Polymer interdiffusion is much faster in the presence of TEOS. Apparent
mean diffusion coefficients, <Dapp>, were also determined by fitting fm data to
the Fickian diffusion model for spherical geometry. Diffusion activation
energies, Ea, were further calculated by a multilinear fitting of <Dapp> with fm.
Results are presented in Table 1.
Table 1. Apparent Mean Diffusion Coefficients and Activation Energies
PN(P):PN(A)
PN-T(P):PN-T(A)
80ºC
6.4 x 10-4
8.2 x 10-2
-3
<Dapp>
90ºC
1.8 x 10
2.8 x 10-2
(nm2s-1)
100ºC
1.6 x 10-2
9.2 x 10-3
-2
110ºC
3.1 x 10
8.8 x 10-1
Ea (kcal/mol)
35 ± 4
26 ± 5
Diffusion coefficients are at least one order of magnitude lower in films
formed from smart hybrid nanoparticles compared to films formed by only
PBMA nanoparticles. Activation energies are also lower for films formed by
smart hybrid nanoparticles. It is then possible to conclude that chains move
faster in the presence of TEOS, which acts as a plasticizer improving chains
interdiffusion. This is an advantage, as more homogenous films are obtained.
Films Resistance to Solvents. Films formed by either homogeneous
PBMA or smart hybrid PBMA nanoparticles, NP or NP-T, were annealed at
80ºC during 90 min. After cooling, both films were washed during 30 minutes
with THF, a very good solvent for the polymer. Films fraction of Remaining
Macroscopic Gel, wt%RMG, were determined by weighting the dry films
before and after washing. While only 37% of the film formed from NP
remained after the chemical attack, 51% of the NP-T films remained under the
same conditions. The hybrid network formed during the annealing, constituted
by silicone oligomers crosslinked to the PBMA chains, does improve the
obtained films chemical resistance.
Conclusions
While TEOS molecules act as plasticizers during chains interdiffusion
at short times, improving films elastic properties, at longer times TEOS react
to form a hybrid network, which enhances film chemical resistance. Such a
material is very promising and has very high commercial perspectives.
(1) Piçarra, S.; Afonso, C.A.M.; Kutreva, V.B.; Fedorov, A.; Martinho,
J.M.G.; Farinha, J.P.S. Journal of Colloid and Interfacial Science 2012,
368, 21.