Vol.2, No.5, 515-518 (2010) Natural Science
http://dx.doi.org/10.4236/ns.2010.25064
Effect of latex conversion on glass transition temperature
Shao-Xiang Li1, Ying-Dong Guan1*, Lu-Mei Liu2
1Department of Micropolymer Materials of Science and Engineering, Qingdao University of Science & Technology, Qingdao, China;
*Corresponding Author:
2Department of Materials Science and Engineering, Qingdao University of Science & Technology, Qingdao, China
Received 29 January 2010; revised 1 March 2010; accepted 25 March 2010.
ABSTRACT
We have synthesized styrene-acrylic latex and
investigated the effect of such reaction condi-
tions as the dosage of initiator, surfactant and
stirring speed on monomer conversion and
glass transition temperature (Tg) of polymer by
means of orthogonal experiment, then we get
the best reaction conditions. Test results prove
that the glass transition temperature of the
polymer is directly related to the monomer con-
version. The improvement of monomer conver-
sion can make the glass transition temperature
close to the theoretical value. In the case of high
final conversion, we can predict the glass tran-
sition temperature of the polymers of different
composition according to the theoretical rela-
tion effectively.
Keywords: Monomer Conversion; Orthogonal
Experiment; Glass Transition Temperature
1. INTRODUCTION
Styrene-acrylic latex is made of styrene and acrylate
monomers, which has many advantages. For example,
it has wide source of raw materials, high function/price
ratio, simple synthetic process and the latex has out-
standing water resistance, alkali resistance, scrub re-
sistance and also the paint film has good outdoor du-
rable, adhesive attraction. So the styrene-acrylic latex
has been widely used in building coating, metal sur-
face coating and so on. Many researchers [1-5] have
studied styrene-acrylic latex. Climates are usually di-
verse across countries, even in one country. Therefore,
a single recipe cannot satisfy different needs in the
different climate. In order to adapt to different envi-
ronment, especially the temperature environment, it
requires the minimum film-forming temperature can
not only has an unchangeable temperature. Scholars in
this area had focused mostly on performance optimi-
zation but ignored the investigation of minimum film-
forming temperature. In fact, there is a big difference
between actual minimum film-forming temperature
and theoretical minimum film-forming temperature,
which brings polymer designers difficulties in pre-
dicting glass transition temperature and designing the
hardness of the polymer, at the same time, brings users
a lot of inconvenience in use. There are many reasons
for the difference between actual minimum film-
forming temperature and theoretical minimum film-
forming temperature, one of the most important is the
monomer conversion. Due to the minimum film-
forming temperature has a good corresponding relation
with the glass-transition temperature [6], so this paper
mainly investigates the glass-transition temperature by
means of optimizing the latex’s polymerization condi-
tions. We obtain latex with high conversion, thus we
can solve the above problems in polymerization tech-
nology aspect and obtain the latex recipe of different
glass transition temperature under the guidance of the
theoretical relation.
2. EXPERIMENTAL
2.1. Materials
Butyl acrylate (BA, 96%), Styrene (St, 97%), Methyl
Methacrylate (MAA, 96%) and Diacetone acryl amide
(DAAM) were purchased from Qingdao Reagent Com-
pany. The anionic surfactant sodium dodecyl sulfate
(SDS), nonionic surfactant nonylphenol polyoxyethyl-
ene ether (OP-10) and ammonium persulfate (APS)
were purchased from Qingdao Chemistry Reagent
Company. All materials were used without further puri-
fication.
2.2. Preparation of Styrene-Acrylic Latex
All emulsifier and deionized water were feeded into
four-necked flask and stirred at high speed first, then
feed monomer mixtures slowly to obtain the before-
hand latex. Take part of beforehand latex for seed latex,
when temperature was wormed up to 75 ± 1℃, feed

S.-X. Li et al. / Natural Science 2 (2010) 515-518
516
part of initiator solution. After the blue seed latex
formed, the remaining latex was fed gradually, and par-
tially drop initiator, beforehand latex and initiator were
added respectively in 3.5 h and 4 h. Then the tempera-
ture was heated to 85 ± 1 a℃nd kept this temperature
for 1 h, then cooled, adjusted PH = 7-8, Filtered and
Collected latex at last.
2.3. Characterization
The test of solid content:
1
0
100%
m
sm 
where s is the solid content of latex, m1 is the weight of
the latex after dried at 80 in vacuum drying oven, ℃ m0
is the weight of the latex.
The test of monomer conversion:
We calculate the conversion by below relation
%100(%)
0
4321 


W
WWWSW
C
where W1 is the whole output of latex, W2 is the amount
of gel, W3 is the amount of initiator, W4 is the amount of
emulsifier, W
0 is the amount of whole monomers, S is
the solid content of latex.
The theoretical value of copolymer’s glass transition
temperature:
Using the following FOX relation, we can get the
composition of copolymers which have an expectable
Tg.
3 12
123
1...... n
n
WW WW
Tg TgTgTgTg

where Tg is the glass transition temperature of co-
polymers, Tgl, Tg2, Tg3, Tgn are the glass transition
temperature of the respective homopolymers and Wl,
W2, W3, Wn are the weight fraction of the respective
groups.
2.4. Differential Scanning Calorimeter (DSC)
Analysis
Tg was measured by the DSC method in a NEYZSCH
204F1 type differential scanning calorimeter for polymer
samples of ~20 mg. DSC condition measurement: hold for
1.0 min at –100, heat from ℃–80 to 100 at 10 min℃℃
–1
and with nitrogen protection.
3. RESULTS AND DISCUSSION
3.1. Choice of Variables and Level of the
Orthogonal Experiment and its Results
During the experiment we found that there is a big dif-
ference between measured value and theoretical value of
Tg (the theoretical value is calculated by FOX relation).
After analysis, the author believes that the main reason
of this phenomenon is due to a lower conversion of po-
lymerization, the system was not polymerized according
to the expectable proportion, so we do the experiment to
optimize the process parameters of polymerization by
orthogonal experimental firstly in order to obtain the
latex with high monomer conversion.
Based on a large number of references and many re-
peated experiments, we consider that the dosage of ini-
tiator (A), Emulsifier (B) and stirring speed (C) are the
main factors of polymerization, and have designed L9
(33) orthogonal table (three variables, three levels Or-
thogonal design), the results are shown in Table 1.
3.2. The Analysis of Orthogonal Experiment
Results
The weighted average (K) and range (R) are given in
Table 2.
Table 2 shows that the sequence of the effect of vari-
ous factors on conversion is emulsifier > initiator > stir-
ring speed, the best condition is A3B2C2: initiator: 0.8%,
emulsifier: 4%, stirring speed: 180 rpm. But we can see
that the difference between k2 (88.033) and k3 (89.000) is
very small, and as we know the conversion increase with
the increase of the initiator, but the gel will increase ob-
viously and the polymerization will become unstable at
the same time, so we choose A2B2C2: initiator: 0.6%,
emulsifier: 4%, stirring speed: 180 rpm at last.
Table 1. Test results.
Test
NO
A
%
B
%
C
rpm
Conversion
%
1 1(0.4) 1(2) 1(140) 82.6
2 1 2(4) 2(180) 90.0
3 1 3(6) 3(240) 78.3
4 2(0.6) 1 2 87.6
5 2 2 3 92.5
6 2 3 1 84.0
7 3(0.8) 1 3 90.0
8 3 2 1 91.0
9 3 3 2 86.0
Table 2. The analysis of experiment results.
Test Indicators A B C
k1 83.633 86.733 85.867
conversion k2 88.033 91.167 87.867
k3 89.000 82.767 86.933
R 5.367 8.400 2.000
Table 3. Properties of the latex under the condition of A2B2C2.
solid content (%)gel (%)water absorption(%) Conversion(%)
48.6 3.5 7 94

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517
As shown in Tables 1 an d 2, the final conversion can
reach 94% under the condition of A2B2C2 and it is higher
than the others, in addition, some other properties are
ideal too.
3.3. DSC Test Analysis
In this paper, the initial composition of monomer is BA:
MMA:St = 33:19:18, the theoretical value of Tg which is
calculated by FOX relation is 8. But when we adopt the ℃
flowing factor: initiator: 0.6%, emulsifier: 4%, speed:
180 rpm, the final conversion reaches 94% and the meas-
ured value of Tg achieved by DSC test is 9.8℃ (Figure 1),
the difference between them is small. That is to say at the
condition of high conversion, the measured value of Tg is
very close to its theoretical value and so we can design the
hardness of copolymers according FOX relation.
3.4. The Relationship of Monomer
Conversion and Glass Transition
Temperature
Figure 2 shows us the relation between Tg and mono-
mer conversion (BA:MMA:St = 33:19:18) and we can
-80-60-40-200 20406080100120
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Glass Transition:
Mid: 9.8℃
DSC /(mW/mg)
Temperature /℃
Figure 1. DSC curve of Styrene-acrylic latex (BA:MMA:St =
33:19:18).
82 84 86 88 90 92 94
8
10
12
14
16
18
20
Tg /℃
Monomer conversion /%
Figure 2. The relation between Tg and monomer conversion.
see from it that with the increase of the monomer con-
version, the glass transition temperature of the latex de-
crease gradually, when the final conversion is over 90%,
Tg reaches a plateau and the value is about 10.0. ℃ The
mainly reason is that during the radical copolymerization,
when the final conversion is low, the monomer with a
strong conjugacy is easier to polymerize than the others,
styrene is such a hard monomer and the glass transition
temperature of its homopolymer is 105, so the Tg ℃of
copolymer will be a little higher than usual; on the con-
trary, when the final conversion is high, the system is
able to polymerize according to the expectable propor-
tion, the measured value and theoretical value of Tg
match very well. However, the conversion of polymeri-
zation can not reach 100%. In addition, Tg will increase
because of the hydrogen bonds formed between the
Components [7]. Some references [8-11] introduce that
some additives and functional monomers will have a
certain impact on glass transition temperature, therefore,
there will be a difference between measured and ex-
pected value of Tg inevitably.
3.5. The Latex Recipe of Different Tg
In this paper, the total mass of the monomer is fixed at
70 g and the ratio of two hard monomer (St and MMA)
will not change at about 1:1, we changes the proportion
of soft and hard monomer only. Table 4 shows the latex
recipe with different Tg which are obtained at the condi-
tion of A2B2C2.
Table 4 shows that when the final conversion is at a
high level, the measured value and theoretical value of
Tg matches very well, thus researcher can be able to
forecast the Tg of polymer according to the proportion
monomers.
4. CONCLUSIONS
1) The results of the orthogonal experimental shows that
emulsifier > initiator > stirring speed in terms of their
effects on conversion. And we get the best condition:
emulsifier 4%, initiator 0.6%, stirring speed 180 rpm.
Table 4. Latex recipe with different Tg.
BA(g) MMA(g)St(g)
theoretical
value of Tg
measured
value of Tg
1# 48 11 11 –20.0℃ –18.3℃
2# 43 14 13 –10.0℃ –9.3℃
3# 38 16 16 –3.0℃ –2.2℃
4# 33 19 18 7.8℃ 9.8℃
5# 28 21 21 20.0℃ 20.4℃
6# 26 22 22 24.0℃ 25.2℃
7# 24 23 23 30.0℃ 32.7℃
8# 20 25 25 40.0℃ 42.7℃
9# 17 27 26 45.0℃ 46.9℃

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2) There is a direct relationship between conversion
and glass transition temperature, the improvement of the
final conversion has made the measured value close to
the theoretical value.
3) At the condition of high conversion, the measured
value and theoretical value of Tg matches very well.
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