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Study on the Milling Behavior of Chloroprene RubberBlends With Ethylene–Propylene–Diene MonomerRubber, Polybutadiene Rubber, and Natural Rubber
Chen Fulin, Cen Lan, Lei CaihongFaculty of Material and Energy, Guangdong University of Technology, Guangzhou, Guangdong 510006, China
The viscoelastic properties of the blends of chloroprenerubber (CR) with ethylene–propylene–diene monomer rub-ber (EPDM), polybutadiene rubber (BR), and natural rubber(NR) at different temperature were studied using rubberprocessing analyzer (RPA). Mooney viscosities of com-pounds were measured and tight milling and sheetingappearance were observed on a two-roll mill. The resultsshowed that Mooney viscosities and the elastic modulusof the blends decreased with the increase of the temper-ature from 60 to 1008C. And the decreasing trends ofpure CR, pure NR, and CR/NR blend compounds weremore prominent than that of pure EPDM, pure BR, CR/EPDM, and CR/BR blend compounds. For CR/EPDMblend compounds, the decreasing trend became slowerwith the increase of EPDM ratio in the blend. Comparedwith pure CR, pure NR and CR/NR blend compounds,pure EPDM, pure BR compounds, and the blend com-pounds of CR/EPDM and CR/BR showed less sensibilityto temperature and they were less sticky to the metalsurface of rolls and could be kept in elastic state athigher temperature, easy to be milled up and sheeted. Atthe same blend ratio and temperature, the property of tightmilling of the blends decreased in the sequence of CR/EPDM, CR/BR, and CR/NR. With the increase of EPDM,BR, or NR ratio in CR blends, its property of tight millingwas improved. POLYM. COMPOS., 28:667–673, 2007. ª 2007Society of Plastics Engineers
INTRODUCTION
The mixing is one of the most important procedures in
the rubber processing, and is also one kind of high energy
consumption procedures. Therefore, it is important to study
the milling behavior of rubber or its compounds since it is
significant to reduce batch-difference of compounds or vul-
canizates and nonessential energy consumption.
Generally, the mixing of rubber compound is carried out
in its elastic state, because the elastic shear forces can make
the compounding ingredients disperse well [1–3]. Chloro-
prene rubber (CR) has outstanding antiaging and self-rein-
forcement properties, and it is not easy to stick to rolls at or
close to room temperature. But, the viscous flow transition
temperature for amorphous CR is within the range of 70–
908C, close to the mixing temperature in industrial produc-
tion [4]. For the CR products with low hardness values for
vulcanizate, the green strength of its compounds is often
low and it is easy to stick to rolls and hard to tight milling
so that the compounding ingredients are difficult to disperse
evenly and that the vulcanizate is easy to frost and its prop-
erties are easy to undulate [4].
Rubber blends are being used extensively in numerousapplications. A blend can offer a set of properties that cangive it the potential of entering application areas not possi-ble with either of the rubbers comprising the blend. It hasbeen reported that the blending of CR with natural rubber(NR), polybutadiene rubber (BR), or ethylene–propylene–diene monomer rubber (EPDM) can improve the process-ability of CR and physico–mechanical property of its vul-canizate [5–18]. Previous researches have proved that CRblend with NR, BR, or EPDM is incompatible [7–9]. Theproperty of a blend depends on components and blend ratio,type and dosage of compatibilizer, processing condition ofmixing and curing [6–23]. Also, using methods of master-batch-way [5, 6] and two-step blend [19] can improve theproperty of blends. In addition, to improve the processabil-ity of CR, the ratio of CR and NR, BR or EPDM often usedis 90/10–70/30 [5, 6].
Obviously, owing to the structure difference of NR, BR,
and EPDM molecular chains, there is difference in the vis-
coelasticity at a given temperature [4], so mixing processing
properties of CR blends with EPDM, BR, or NR are affected
greatly. For the systems of CR blended with NR, BR, or
EPDM, the previous researches about these blends mainly
focused on the influences of filler, compatibilizer, blend ra-
tio, co-curing of blend on their properties [8–18]. The mill-
ing behavior of NR, BR, EPDM, bromobutyl rubber, and
blends of NR/BR and EPDM/BIIR had been studied over a
range of temperatures, nip gaps, and speed ratios [20], and
relative studies for rheological properties, adhesive tack, and
green strength of other blends had been reported [21–23],
Correspondence to: Chen Fulin; e-mail: [email protected]
DOI 10.1002/pc.20328
Published online in Wiley InterScience (www.interscience.wiley.com).
VVC 2007 Society of Plastics Engineers
POLYMER COMPOSITES—-2007
but the study of the milling processing behavior of the CR
blends with NR, BR, or EPDM has not been systemically
and deeply reported.
Many factors such as viscosity, elasticity, green strength,
and recipe of compound influence processability of com-
pound [4, 24]. There is a big difference in the elastic modu-
lus and elastic viscosity as to the different viscoelastic state
of the compound [4, 24, 25]. At given temperature, strain
and frequency, elastic modulus, and elastic viscosity of
compound can be obtained by holding a time by RPA [24,
25]. The time is variable, and it depends on the time when
the elastic modulus and elastic viscosity are invariability on
the whole. The curves of variation of elastic modulus of
compounds with the temperature can indicate directly their
difference in sensitivity to temperature and changing trend
of viscoelastic property.
In this article, the milling behavior on an open mill and
viscoelastic property of CR blends with three commercial
rubbers, NR, BR, and EPDM are studied under different
temperatures. It is known that CR and NR are self-reinforc-
ing rubbers, but BR and EPDM need to be reinforced by
high reinforcing carbon black because of their low strength
[4]. Hence, for easy comparison with one another, we
chosed semireinforcing furnace black (N774) and clay as
the filler of CR, high abrasion furnace black (N330) as the
filler of NR, BR or EPDM, processing aid Struktol WB212
(a mix of fatty acid ester with high molecular weight and
medium activated filler) to prevent CR compound sticking
to rolls in early stage mixing, homogenizing agent Struktol
60NSF (a mix of aromatic, cyclone, and aliphatic hydrocar-
bons) to disperse the rubber components [27], the different
softening agents that are well compatible with CR, NR, BR,
and EPDM [28], respectively to decrease the viscosities of
the compounds and the hardness of vulcanizates, so as to
obtain the four recipes that the Shore A hardness of vulcani-
zates was about 35–45 with similar weight percents of raw
rubber components in the compounds. Meanwhile, the com-
pounds were mixed by using the methods of masterbatch-
way [5, 6] and two-step blend [19]. The experimental results
would be of practical importance to improve mixing proc-
essing properties of CR compounds.
EXPERIMENTAL
Materials
CR (Baypren 126) and EPDM (Buna EPG 3850) were sup-
plied by the Guangzhou Sanpu Trade. NR (RSS No. 1, Mn,
267 thousand, dispersion index, 7) was provided by Zhanjiang
Bureau of Farm, Guangdong Province, China. BR (9,000)
was obtained from Beijing Yanshan Petrochemical Industry.
Processing agent Struktol WB212 and homogenizing agent
Struktol 60NSF were provided by Shanghai Rachem Chemi-
cal Industry. Other ingredients used for compounding are
commercial materials usually used in the rubber industry.
Basic Recipes
CR masterbatch (phr): CR, 100; stearic acid, 1.5; anti-
oxidant ODPA (octylated diphenylamine), 1.5; antioxidant
IPPD (N-isopropyl-N0-phenyl-p-phenylene–diamine), 1.0;
processing agent Struktol WB212, 2.0; semireinforcing
furnace black (N774), 20; clay, 15; aromatic oil, 25.
EPDM, BR, or NR masterbatch (phr): raw rubber, 100; ste-
aric acid, 1.5; antioxidant ODPA, 1.5; antioxidant IPPD, 1.0;
coumarone resin, 5.0; homogenizing agent Struktol 60NSF,
4.0; high abrasion furnace (N330), 30; softening agent (par-
affin oil in EPDM and alkyl oil in NR and BR), 30.
Sample Preparation
The mixing procedure for every masterbatch was car-
ried out on a laboratory size two-roll open mill (XJ-160,
supplied by Shanghai Rubber Machinery Works No. 1,
China) with the size of 6 � 13 in., a friction ratio of 1:1.12
and a speed of 16 r/min for slow roll, and the temperature
of the rolls was maintained at 20–408C by the circulation
of running water through the rolls. Raw rubber (500 g) for
each one was weighted, and the compounding ingredients
were prepared according to the basic recipes.
Mixing for CR Masterbatch. Raw CR was dropped ver-
tically on the bank of the roll of the mill and rolled up for
three times (the nip gap is 0.6–0.8 mm, similarly below),
then the nip distance was reset a little wider to have ad-
equate rubber on the mill, meanwhile stearic acid, Struktol
WB212, antioxidant, carbon black, clay, and aromatic oil
were added in the sequence. The samples were sheeted af-
ter rolling up for five times.
Mixing for EPDM, BR, and NR Masterbatch. Raw
rubber was dropped vertically on the bank of the roll of the
mill and rolled up for five times, then the nip distance was
reset a little wider to have adequate rubber on the mill,
meanwhile stearic acid, antioxidant, coumarone resin,
Struktol 60NSF, carbon black, softening agent were added
in the sequence. The samples were sheeted after rolling up
for five times.
Mixing for the Blends. In terms of the blend ratios and
method of two-step blend, masterbatch of EPDM, BR, or
NR and about 1/3 (wt/wt) CR masterbatch were rolled up
for five times on the open mill firstly, then the surplus CR
masterbatch was added. The samples were sheeted after roll-
ing up for five times. The sheeted samples was conditioned
at a temperature of (23 6 2)8C for 24 h before testing.
Measurements
Mooney Viscosities Measurements. Mooney viscosities
of the compounds were measured using a Mooney Viscom-
eter GT-7080S2 (Gotech Testing Machines, Taiwan) as
described by GB/T1232 (equated to ISO 289) at three dif-
ferent temperatures, 60, 75, and 908C.
668 POLYMER COMPOSITES—-2007 DOI 10.1002/pc
Milling up and Sheeting Behaviors Test. Using the
two-roll mixing mill (XS-160, supplied by Shanghai Rub-
ber Machinery Works No. 1, China) with the size of 6 �13 in., a friction ratio of 1:1.12 and a speed of 16 r/min for
the slow roll, and heating the rolls and maintaining the
temperature at (60 6 3)8C, (75 6 3)8C, or (90 6 3)8C by
adjusting the voltage of the transformer for heating up the
rolls, respectively, and the rolls working distance was 8–
10 cm and the nip gap was 1.9–2.1 mm, about 120 g com-
pound sample was added on the mill and banded on the
front roll for 2 min, then peeled from the rolls, meanwhile
the behavior of the adherence of compounds to roll was
closely observed. If the compound was not obviously
sticky to rolls, then the nip gap was reset to 0.6–0.8 mm,
meanwhile the compound was milled up and sheeted for
five times. It was observed whether the compound was
sticky to the rolls.
Viscoelastic Property Measurements. About 5–6 g com-
pound was tested by 2,000 rubbers processing analyzer
(RPA) (American ALPHA Science and Technology) and the
viscoelastic properties of the compound were obtained. The
strain of the experiment was set at 0.5 deg and the frequency
was at 100 cpm. Firstly, the temperature was set at 608C and
was held for 3 min. Then the temperature was increased to
708C in 2 min and was also held for 3 min. Similar proce-
dure was used for the temperature increasing from 70 to
1008C at a step of 108C. The elastic modulus and elastic vis-
cosity were obtained when time and temperature changed.
Then the curves of the variation of elastic modulus of com-
pound with the temperature can be drawn.
RESULTS AND DISCUSSION
Mooney Viscosities of the Compounds
Table 1 gives Mooney viscosity of the compounds at
different temperatures. From Table 1, Mooney viscosity of
all compounds decrease on increasing the temperature, and
the decreasing trends of pure CR, pure NR, and the CR/
NR blend are more prominent than that of pure EPDM,
pure BR and the blends of CR/BR, and CR/EPDM. With
the increase of EPDM ratio in CR/EPDM blends, the fall-
ing trends of Mooney viscosity of the blends become
slower while the temperature rises.
Viscosities of the compounds can reflect their flowing
character directly and they are correlated with states of the
compounds. With the increase of temperature, the rubber
undergoes the glassy, elastic, and viscous flow state in the
sequence [4, 25]. At a given temperature, the state of rub-
ber compounds largely depends on the molecular structure,
which also influences the temperature range of compounds
in elastic state. The effect of temperature on the visco-
elastic property of rubber compound can be indicated by
flow activation energy of the rubber [4]. With respect to
the molecular structure of rubber, the flow activation
energy decreases in the sequence of CR, NR, BR, and
EPDM because of the difference of side groups and unsat-
urated degrees of rubber molecules. And the similar trend
of the sensitivity of viscoelastic states to temperatures can
be obtained.
Because of the polarity of CR molecules, the effect of
temperature on CR morphology is prominent. At high tem-
peratures, the temperature to keep CR in elastic state is
lower than that of NR, BR, and EPDM, and CR transforms
to be in viscous flow state at 70–908C [1]. When the tem-
perature is higher, part of CR molecules change to be in
plastic state, hence the compound is in the particulate state
being situated between elastic state and plastic state [1]. At
this time, the compound is easy to viscous flow at strain to
show comparatively low Mooney viscosity. EPDM is a
low unsaturated and nonpolar rubber and it can keep in
elastic state at a wide range of temperature [4, 22]. The po-
larity and activity of BR moleculars are relatively low
because there is no side group like that of NR. However,
because of the side groups and unsaturated degree of NR
molecules, the sensitivity to temperature of NR in visco-
elastic state is stronger than that of BR and EPDM whereas
weaker than that of CR.
In fact, for the rubbers customarily used in rubber
industry, the temperature for NR to keep it in elastic state
is slightly higher than that for CR about 208C, but lowerthan that for BR and EPDM [4]. Therefore the tempera-
ture-sensitivity of the rubber state decreases in the order of
CR, NR, BR, and EPDM. When CR is blended with NR,
BR, or EPDM, the temperature-sensitivity of blends falls.
That is to say, at higher temperature, the compounds of
EPDM and BR are easier to keep rubber in elastic state
than that of NR and CR, therefore the trend of decreasing
for their Mooney viscosities with increasing temperature
becomes slow, and the trend being influenced decreases,
EPDM, BR, NR in order. For CR/EPDM blend, the falling
trend of Mooney viscosity on increasing temperature is
less prominent with the increase of EPDM ratio in the
blend, as shown in Table 1.
TABLE 1. Mooney viscosity of the compounds at different temperature.
The constituents
of blends CR 100 EPDM 100 BR 100 NR 100
CR/EPDM
90/10
CR/EPDM
80/20
CR/EPDM
70/30
CR/BR
70/30
CR/NR
70/30
The test temperatures (8C)60 44.6 47.8 48.4 47.6 42.4 43.5 45.6 43.4 42.9
75 31.5 40.7 42.9 38.2 32.8 34.6 37.1 32.5 28.4
90 19.6 34.3 35.8 29.5 24.9 28.5 32.2 27.8 23.6
DOI 10.1002/pc POLYMER COMPOSITES—-2007 669
The Milling Behaviors of the Compounds
The milling behavior of rubber compounds can be clas-
sified into four states in terms of mill band formation char-
acteristics, and the most favorable state to mixing of rubber
compounds and dispersing of compounding ingredients in
rubber is that rubber can enter into nip gap of two rolls
automatically, at this time rubber is plastic flow and forms
a tight elastic band adhering to roll (named for Region 2)
[1, 2]. Another state is that the rubber compound forms a
transparent fluid film band and it is very soft to be lack of
elasticity, meanwhile green strength of rubber is low so
that the rubber compound adheres to the roll (named for
Region 4) [1, 2]. At this time, compounding ingredients
can be added into rubber easily, but they cannot be dis-
persed well in rubber. When the compound is mixed on an
open mill, the state of the compound is not only related
with the recipe of rubber, but also dependent on conditions
of mixing. Certainly, the temperature of rolls is one of the
most important factors that affect milling behavior of rub-
ber compounds [1, 2].
Table 2 shows the tight milling behavior of the com-
pounds at different roll temperatures. From Table 2, at low
temperatures, every compound didnot stick to the rolls and
could be milled up well and peeled from the rolls that was
similar to Region 2, as described in related document [1,
2]. When the temperature increased, CR compound and
70/30 CR/NR blend became sticky to the rolls and the bulk
compound entered into the clearance of two rolls hardly,
meanwhile it was hard to be milled up and peeled from the
rolls that was similar to Region 4, as described in related
document [1, 2]. Pure EPDM compound and pure BR com-
pound as well as the CR/EPDM blend with high EPDM
blend ratio could be milled up well and peeled from the
rolls easily even at higher temperatures, and it was not
sticky to the rolls. Besides, at the same blend ratios and
temperatures, the milling up and sheeting behaviors of CR/
BR blend were between that of CR/EPDM blend and that
of CR/NR blend.
Usually, the mixing of rubber compound uses shear force
in elastic state to make the compounding ingredients dis-
perse well [1–3]. Under some mixing conditions, the milling
up and sheeting behaviors of compounds are related with
the state of rubber compound, and also the character and
green strength of rubber compound [1–3, 22]. When CR
compound is mixed, it is usually prone to scorch because of
the heat released during mixing, and even if some process-
ing agents are added, its green strength in particulate state is
too weak at higher temperatures [4]. CR molecules are po-
lar, which makes itself easy to stick to surface of polar rolls.
Meanwhile we know that the compound can easily be
peeled from the rolls and sheeted only that the green
strength of compound is higher than the adhesive strength
between compound and rolls. The adding of EPDM, BR, or
NR slowed the trend of CR blends to become particulate
state, and the blends could keep high green strength in elas-
tic state to improve the milling properties of blends.
Generally, the distribution of relative molecular weight
of NR is wider than that of synthetic rubber. NR has excel-
lent processing property, mainly because of the high green
strength of its chains with high molecular weight and the
plastication of its chains with low molecular weight [4].
TABLE 2. The tight milling appearance of the blends.
Temperatures
of the rolls/8C
The constituents of blends
CR 100
EPDM
100 BR 100 NR 100
CR/EPDM
90/10
CR/EPDM
80/20
CR/EPDM
70/30
CR/BR
70/30
CR/NR
70/30
60 6 3 Normala Normal Normal Normal Normal Normal Normal Normal Normal
75 6 3 Hard processb Normal Normal Normal Hard process Normal Normal Normal Hard process
90 6 3 Stickyc Normal Normal Hard process Sticky Hard process Normal Hard process Sticky
a The compound is not sticky to the rolls and the bulk compound can enter into the clearance of the two rolls, meanwhile it could be milled up well
and peeled from the rolls.b The compound is not sticky to the rolls, but it is hard to be milled up and peeled from the rolls.c The compound is sticky to the rolls and the bulk compound can not enter into the clearance of the two rolls.
FIG. 1. Variation of elastic modulus with temperature of pure EPDM,
BR, CR, and NR.
670 POLYMER COMPOSITES—-2007 DOI 10.1002/pc
But obviously, the NR phase in lower 70/30 CR/NR blend
is a dispersion phase, which is no use of reinforcing in the
compound, and the self-adhesion and adhesion to others
are better than that of EPDM and BR. The carbon black-
reinforcing EPDM or BR masterbatch has lower self-adhe-
sion and adhesion to others, meanwhile reduces the adhe-
sion between the compounds and surface of metal rolls [4,
22]. Therefore, at the same blend ratios, the milling prop-
erty of CR compounds can be greatly improved when CR
is blended with EPDM or BR, respectively, compared with
CR/NR blend. From Table 2, the same milling property of
80/20 CR/EPDM blend and 70/30 CR/BR blend at the
same roll temperature can be noticed. Therein, the improv-
ing effect of CR/EPDM blend on the milling property is
better than that of CR/BR blend.
Viscoelastic Properties of the Compounds
Under certain strain, frequency, and temperature, the
curves of elastic modulus versus temperature of the com-
pounds tested by RPA were shown in Figs. 1–4. Figures 1–4
show that the elastic modulus of all compounds decreases on
increasing the temperature. From Figs. 1 and 2, the falling
trend of CR, NR compound is faster than that of BR, EPDM
compound, meanwhile the similar falling trend of 70/30 CR/
NR, CR/BR, and CR/EPDM can be noticed. From Fig. 2, we
can see that at the same temperature and blend ratio, the
elastic modulus of the compounds decrease in the order of
CR/EPDM, CR/BR, and CR/NR, and the higher the test tem-
perature is, the more obvious the difference is. We also con-
clude from Figs. 3 and 4 that the more the BR or NR ratio in
blend is, the bigger the value of elastic modulus is.
FIG. 2. Variation of elastic modulus with temperature of CR/EPDM,
CR/BR, and CR/NR.
FIG. 3. Variation of elastic modulus with temperature of CR/BR com-
pounds at different blend ratios.
FIG. 4. Variation of elastic modulus with temperature of CR/NR com-
pounds at different blend ratios.
FIG. 5. Elastic modulus of the blends versus time.
DOI 10.1002/pc POLYMER COMPOSITES—-2007 671
The curves of elastic modulus and elastic viscosity of
CR/EPDM blends versus time were shown in Figs. 5 and 6.
The elastic modulus and elastic viscosity of CR/EPDM
blends decrease when temperature rises, but at the same
temperature they increase on increasing the EPDM ratio in
the blend.
The changing trend of the elastic modulus and elastic
viscosities with temperature directly reflects the difference
of temperature-sensitivity of compound and changing trend
of the rubber states [4, 24, 25]. When the compound is in
elastic state, the molecular chains of rubber entangle to
have elastic deformation and viscous flow at stress. When
temperature is lower, the molecular chains entangle tightly,
and chains are hard to have viscous flow, so the elastic
modulus and elastic viscosity are higher. As temperature
rises, the thermal movement of rubber molecules is more
intense to make chains get rid of entanglement, and chains
are easy to have viscous flow, so the elastic modulus and
elastic viscosity decrease [4, 24, 25].
With respect to the analysis above, the difference of
sensitivity to temperature of CR, EPDM, BR, and NR is
embodied in the difference of decreasing extent in the elas-
tic modulus and elastic viscosity with temperature. The
changing trends of elastic modulus and elastic viscosity of
all blends when temperature rises (shown in Figs. 1–6) has
good corresponding connection with the changing trends
of the Mooney viscosity (Table 1) and the milling up and
sheeting behaviors of the compounds on an open mill
when temperature changes (Table 2).
It can be concluded from the results above that the extent
that the rubber state is affected by the sensitivity to tempera-
ture decreases in order of CR, NR, BR, and EPDM. The
poor self-adhesion and adhesion to others of EPDM and BR
reduce the adhesion between the compounds and surface of
metal rolls. Compared with CR and NR compounds and
CR/NR blend, BR, and EPDM compounds and CR/BR and
CR/EPDM blends can keep in elastic state and high green
strength at comparative high temperatures, so that the mill-
ing up and sheeting property of the compounds on an open
mill can be greatly improved. At the same roll temperature
and blend ratio, the milling up and sheeting property of CR
compounds can be improved by blending with EPDM, BR,
or NR, and the improved effect is best when CR is blended
with EPDM, and BR, NR in order.
CONCLUSIONS
From this study, at the temperature range of 60–1008C,the following conclusion can be obtained:
(1) The Mooney viscosity and elastic modulus of the com-
pounds decrease on increasing temperature. And the
decreasing trends of pure CR, pure NR, and CR/NR
blend are more prominent than that of pure EPDM, pure
BR, CR/EPDM, and CR/BR blends. With the increase
of EPDM ratio in CR/EPDM blends, the decreasing
trend of Mooney viscosity, elastic modulus, and elastic
viscosity of the CR/EPDM blend becomes slower as
temperature rises.
(2) Compared with pure CR, pure NR, and CR/NR blend,
the sensitivity to temperature of pure EPDM, pure BR,
CR/EPDM, and CR/BR blends of above 80/20 de-
creases on increasing temperature, their adhesion to the
surface of metal rolls is smaller, and the compounds can
keep their elastic states and have good milling up and
sheeting property on an open mill even at higher roll
temperature.
(3) At the same roll temperature and blend ratio, the milling
up and sheeting properties of the blends on an open mill
decrease in the sequence of CR/EPDM, CR/BR, and
CR/NR. For CR/EPDM, the milling up and sheeting
properties increase with the increase of EPDM ratio in
the blend.
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