Discussion
Based on collected data and previous reports, secondary
nucleation is believed to occur under monomer-flooded
conditions only and to follow the inequation below:
Background
2-Hydroxyethyl methacrylate (HEMA) is one of the most
widely used functional monomers in acrylate/styrene
latexes. With hydroxyl groups incorporated by HEMA,
resulting emulsion polymers can cross-link with hardeners
such as melamine-formaldehyde (MF) resins and
isocyanates, thus significantly boosting the coating
performance. However, it has been previously reported
that limited water solubility of HEMA-rich oligomers results
in homogeneous secondary nucleation, leading to an
uncontrollable bimodal particle distribution, which is
undesirable for latex quality control. It is vital to investigate
the conditions where secondary nucleation occurs in
HEMA-rich latexes. In this work, the effects of HEMA
content on secondary nucleation in butyl acrylate/styrene
(BA/St) latexes were investigated.
Secondary nucleation of styrenated hydroxyl-functionalized latexes
Yongan Hu, Martin Ocepek, Mark D. Soucek*
School of Polymer Science and Polymer Engineering, The University of Akron
Objective
Prepare BA/St latexes (BA:St=1:1 by mole) with
different HEMA content (0, 10, 20, 30, 40 mol%).
Charactize resulting latexes and analyze the
phenomenon of secondary nucleation.
Synthesis and Characterization
Latex synthesis:
Seeded semi-batch emulsion polymerization
Characterization:
Polymerization kinetics
Average particle size and particle size distribution (PSD)
Glass transition temperature
Surface tension
Fig. 1. Comparison of (a) normal latex and (b) latex with secondary nucleation.
(a) (b)
Key Result
PSDs are narrowly monodispersed until HEMA is
incorporated.
Fig. 2. Number-based PSDs based on transmission electron microscope (TEM) images.
Control 10 mol% HEMA
20 mol% HEMA
30 mol% HEMA 40 mol% HEMA
With the increment of HEMA content from 10 to 30
mol%, particle number is found to increase, while it
decreases at higher HEMA concentration (40 mol%).
On the other hand, size of secondary particle keeps
increasing from 10 to 40 mol% HEMA.
Fig. 3. Particle number N
p
evolution with increasing HEMA content based on TEM data.
2
8
24
70
11
0
10
20
30
40
50
60
70
80
0 10 20 30 40
Np (×10^15)
HEMA content (mol%)
Fig. 4: Instantaneous and overall conversion evolution during the feed.
HEMA significantly accelerates the increase of
instantaneous conversion as feed process proceeds.
The period before reaching monomer-starved
conditions (monomer-flooded conditions, t
m-f
; x
inst
<80%)
is found to be shortened when more HEMA is used.
Since all of latexes have a final surface tension > 35
dyne/cm, the absence of micelle is proven, suggesting
any secondary nucleation can only be induced by
homogeneous nucleation mechanism.
Fig. 5: Surface tension. Dash line represents the surface tension at critical micelle concentration
(CMC) of sodium dodecyl sulfate (used as surfactant in this work).
Since t
m-f
is shortened with increasing HEMA content, the
time for HEMA-rich oligo-radical formation is longer at
lower HEMA content. However, at low HEMA content (10
mol%), averagely, less monomer units are needed for a
HEMA-rich oligo-radical to become surface-active,
leading to a relatively small ∆. In this case, only a small
number of secondary particles can be produced. In other
words, ∆ is the major factor for the occurrence of
secondary nucleation while t
m-f
is the minor factor. It
exhibits long t
m-f
but relatively small ∆ at 10 mol% HEMA,
medium t
m-f
and medium ∆ at 20 and 30 mol% HEMA,
and short t
m-f
and relatively large ∆ at 40 mol% HEMA. In
this way, the ascending scale of number of secondary
particles produced is 10 mol% HEMA < 40 mol% HEMA <
20, 30 mol% HEMA, which is consistent with the results
obtained.
It is postulated that with increasing HEMA content, the
overall surface area of secondary particles increases,
leading to higher probability of monomer entry, which
results in larger size of final secondary particles.
,
≫
,
+
,
⇔
∆=
,
− (
,
+
,
) ≫
,
= Overall aqueous propagation rate of oligo-radical
,
= Overall radial entry rate
,
= Overall aqueous termination rate
Conclusion
The increasing HEMA content can affect the polymerization
kinetics and thus the occurrence of secondary nucleation.
The number and size of secondary particles are determined
by the HEMA content.
