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: flchen@gdut.edu.cn

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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