Technical Papers

Effects of carbon black on compression set
with peroxide cross-linking elastomer

analysis, we can contribute to controlling trade-offs

1. Introduction

between mechanical properties and CS, and to establish

One of the important parameters showing heatguidelines for selection of Carbon in accordance withresistance of elastomer sealing materials is compression

demanded characteristics.

set （CS）. CS is an indicator forthe permanentdistortion in case of loading at a certain temperature fora certain amount of time to the elastomer material incompression, and it is calculated as Figure1. For
example, CS100% indicates that it is completely distortion
Figure1

Calculation of CS

Winter 2021

and highly likely to end up in leakage. It is an effective
measure to check durability of sealing materials.
Usually, carbon black（Carbon）is contained in elastomer
materials as a reinforcing filler, and we can adjust their

2. Experimentmechanical properties including tensile strength and2-1）Preparation of samples

stretch by changing the primary particle-size or theComposition of Carbon mixture in each sample is shown

amount of Carbon added. However, it has become anin Table1. We chose FKM（VDF-HFP-TFE copolymer）

issue in the product development scenes that there wereand EPDM（ethylene-propylene-diene copolymer）fromoften trade-offs between the mechanical properties andamong peroxide cross-linked systems, which are

CS gained from blending Carbon.

especially suitable to be used in high temperature

It is well known that CS is correlated with effective

environments, as the elastomer base materials. We

cross-link density of elastomer in non - Carbon filled

prepared samples that were each blended with 3system. Therefore, it is possible that this trade-off is also

different particle-sizes of Carbon（HAF: primary particle

related with effective cross-link density. However, wesize is about 28 nm, MAF : primary particle size is about

cannot properly evaluate properly the cross-link density

38 nm, MT : primary particle size about 450 nm）forin Carbon filled system with calculations of a generalboth polymers, and also made non-Carbon filled system

swelling test due to an effect of reinforcement of Carbon,

as the control system. As for blended materials otherand it was an issue that we could not analyze thethan Carbon, we used 2, 5-Dimethyl - 2, 5 - di（tert-butyl

correlation with CS.

peroxy）hexane as peroxide for both FKM and EPDM

Based on this, in this report, we analyzed thesystems, and used triallyl isocyanurate as a cross-linking

relationship between effective cross-link density ofcoagent. The same blending amount was set for each

Carbon-filled systems and CS, using mainly dynamic

polymer system. In addition, with EPDM system, as

viscoelasticity measurements. We also examined theother blended materials, the same blending amount ofcauses of changes in CS of elastomer material with

anti-aging substance, zinc oxide and processing aid weredifferent types of Carbon fillers. We think that with thisadded to each sample.
14No.40Effects of carbon black on compression set with peroxide cross-linking elastomer

Table1Sample

Carbon blend compositions and physical properties

F-Control

F-A20

Polymer species

F-B20

F-C20

E-Control

E-A50

FKM

E-B50

E-C50
HAFEPDM

Carbon black species

─MTMAFHAF─MTMAF

Primary particle size（nm）

─4503828

─4503828

N2 absorption specific surface area（m2/g）

─749

79

─749

79
50

Amount of carbon black（phr）

020

20

20

050

50Hardness（JIS A）

57

72

84

86

48

65

70

70

Tensil strength（MPa）

5.2

14.9

21.6

22.5

1.7

9.7

16.6

16.8

Elongation at break（％）

330

245

220

230

185

190

215

230

100% Modulus（MPa）

1.2

2.6

5.9

5.4

1.2

2.6

5.3

3.4

CS（％）at 200℃×72hr

N.D

14.8

17.6

21.2

9.1

13.4

21.4

31.6

Polymers and each blended material were kneaded by

While with EPDM system, the smaller the particle size8 - inch rolls. Afterwards, 2 mm-wide sheets and CS discof Carbon used, the higher the increase in breaking

pieces（Φ29×12.5t）were created by compression

elongation, FKM system showed a different tendencypressure molding.

and F - Control, which is non - Carbon filled substance,
showed the largest value.

Winter 2021
No.402-2）Evaluation methods

With both FKM and EPDM systems, the smaller theWe have measured hardness, tensile strength, tensile

particle size of Carbon used, the more CS increased andbreaking extension, 100% modulus and CS of eachheat resistance was lowered. In addition, from thesample at 25℃ according to JIS K6253, K6251 andresults of mechanical properties mentioned earlier, it wasK6262. We conducted swelling test by soaking EPDM

confirmed that there is a trade-off relationship between

sample for 72 hours, using toluene as solvent andchanges in CS dependent on the blended Carbon type,calculated effective cross-linking density（vs）using aand hardness, tensile strength and 100% modulus . It1）

modified Flory - Rehner equation .

should also be mentioned that it was not possible toWe carried out dynamic viscoelasticity measurements

measure CS of F- Control because the sample wasusing DMS6100（Seiko Instruments）under tension

broken by compression at measurement.

mode, rate of rising temperature at 2 ℃/ min, frequency
at 10 Hz, applied distortion at 0.05 % and temperature
range between -50℃ and 150℃ .

3-2）Correlation between effective crosslinking density and CS
Although it is well known that CS is correlated with

3. Results and discussion

effective cross - linking density measured by swelling3-1）Relationship between mechanicaldifferent with Carbon-filled system. Figure2 shows theproperties and CS

test in non - Carbon filled system, the situation isrelationship between effective cross-linking density

Table1 shows mechanical properties and CS of eachand CS measured by swelling test of EPDM system

sample. These are the results from blending 20phr ofsamples produced in this experiment.

each Carbon in FKM system and 50phr in EPDM

It cannot be said that there is a correlation betweensystem.

effective cross - linking density and CS from theseWith both FKM and EPDM systems, the smaller theresults. We consider this is due to the reinforcement

particle- size of Carbon used, which is said to be better in

effect of Carbon.

reinforcement in general, the higher the increase in

Cross - linking density is calculated in swelling tests

hardness and tensile strength. Although 100% modulus

according to the degree of swelling of the sample

also showed similar tendencies, it decreased with HAF

soaked in good solvent. Therefore, we consider that it

filled substance with the smallest particle size.

would be difficult to calculate accurate effective cross-15

Technical Papers

Figure2 R elationship between effective cross-lining density
and CS from swelling test

linking density if natural swelling is disturbed byreinforcement property of Carbon.

Figure3

Relationship between tanδ and temperature in
EPDM systems

Winter 20213-3）Relationship between dynamic
viscoelasticity properties and CS
Since the past, there is loss tangent,（tanδ）, as anotherindicator to show cross-linking density in non - Carbon
filled system, which can be acquired by dynamicviscoelastic measurements. For example, Imoto andcoworkers 2） reported the higher the cross-linking

No.40

density, the lower tanδ on the higher temperature side
becomes. They said that the lower the cross-linking
density is, the more terminal molecular chains whicheasily flow increase, and the larger tanδ becomes, whichreflects energy loss due to forced vibration.
Iwabuki and coworkers3） report a good correlation
between tanδ within the high temperature range andthe cross-linked component amount（or uncross-linked

Figure4

Relationship between tanδ at 150℃ and CScomponent amount）obtained by pulsed NMR despite
any type of filler. The tanδ within a high temperaturetanδ becomes, which we consider is exhibiting therange reflects relaxation due to the movement ofreinforcement property of Carbon. On the contrary toterminal molecular chains even in Carbon - filled systems,

the peak value, the smaller the particle size of Carbon

and this can be an effective measure when analyzing theadded, the higher the value of tanδ on the highcorrelation between effective cross-linking density andtemperature side, which is regarded as reflectingCS.

relaxation due to the movement of terminal molecular

Figure3 shows the relationship between tanδ andchains.

temperature in EPDM system samples. The smaller theFigure4 shows the relationship between tanδ and CS atparticle size of Carbon used , the lower the peak value of150℃ in FKM and EPDM systems respectively. Unlike
16Effects of carbon black on compression set with peroxide cross-linking elastomer

the results of Figure2, tanδ increases according to thefor free radicals4）. This means that increase of tanδ inincrease of CS both in FKM and EPDM systems, whichaccordance with increase of the total surface area oftells us there is a correlation. If we assume tanδ at 150℃

Carbon which was confirmed in Figure5 could havereflects the amount of relaxation due to the movement

occurred because peroxide radicals, which was anof terminal molecular chains, we consider that increase

initiator for cross-linking, was trapped due to theof tanδ means decrease of effective cross-linking density

increase of defect areas on the surface of Carbon in

led to increase of CS. In addition, it suggests a possible

elastomer and effective cross-linking density has

effectiveness of tanδ within the high temperature range

decreased.

as an indicator of cross-linking density in Carbon - filled

Together with the results of Figure4, this leads us tosystem.

conclude that increase of the total surface area ofCarbon in elastomer inhibits cross - linking reaction with3-4）Relationship between the total surface
area of Carbon and tanδcross - linking density.

Figure5 shows a relation between the total surface area

Thus, when we design the recipe, since adding too muchof Carbon filler in each sample and tanδ at 150℃ . The

Carbon or small particle-sized Carbon increases CS, it istotal surface area of Carbon was calculated bynecessary to consider and determine appropriate Carbon

multiplying the nitrogen absorption specific surface area

type and blending amount while considering the balance2（m /g）for each Carbon shown in Table1, by theWinter 2021
No.40

peroxide, and increases CS by decreasing the effectivewith mechanical properties.

compound amount（g）.
As a result, we found that tanδ increased according to
the increase of the total surface area of Carbon filler for

4. Conclusions

both FKM and EPDM systems and had a good

We found the following through analysis of thecorrelation.

relationship between CS of peroxide cross-linked FKM

The basic structure of Carbon is a graphite structure,and EPDM in Carbon-filled systems and effective cross-however on the surface, it possesses defective areas

linking density using dynamic viscoelasticity.

including oxygen, hydroxyl groups and structures in（1） The results of dynamic viscoelasticity

which graphite has undergone ring cleavage bymeasurements suggest the smaller the particle sizehydrogenation, which are reported to become receptorsof Carbon filler in both FKM and EPDM systems,
the higher tanδ（150℃）increased and the larger
the relaxation due to the movement of terminal
molecular chains.
（2）
 tanδ
（150℃）has a good correlation with CS in
both FKM and EPDM systems, and CS increased
as tanδ increased, suggesting the possibility of anincrease of CS in not only in non - Carbon filled but
also in Carbon filled system with decrease ofeffective cross-linking density.
（3）
 With increase of the total surface area of Carbon
filler, tanδ（150℃）increased. This brings us to theconclusion that increase of the total surface area ofCarbon in elastomer inhibits cross-linking reaction

Figure5 R elationship between the total surface area of filled
Carbon and tanδ at 150℃
17of peroxide and that decrease of effective crosslinking density would bring increase of CS.

Technical Papers

5. Acknowledgment

6. References

This report analyzed a limited number of Carbon types.

1）‌Edited by the Society of Rubber Science andFor planning better guidelines of Carbon selection, weTechnology, Japan:“Rubber Test Methods”New

think it is necessary to conduct a more multifaceted

Edition, 211, The Society of Rubber Science andanalysis that analyzes more varieties of Carbon and uses

Technology, Japan（1980）

other analysis methods, which we plan to carry out inthe future.

2）‌Minoru Imoto et al.: Nippon Gomu Kyokaishi, 41,1103（1968）

Finally, to proceed in this study, we were given a lot ofsupport and advice from Junji Mizukado, Group Leaderof the Chemical Materials Evaluation Group, Research3）‌Hitoshi Iwabuki et al.: Nippon Gomu Kyokaishi, 75,409（2002）
4）‌V. A. Garten, D. E. Weiss: Aust. J. Chem., 8, 68（1955）

Institute for Sustainable Chemistry, National Institute ofAdvanced Industrial Science and Technology. We thankhim very much.

Winter 2021

Ryosuke Nishi
Elastomer Team,
Product Development Department,
Research and Development Division

No.4018