Open Journal of Polymer Chemistry, 2013, 3, 79-85
Published Online November 2013 (http://www.scirp.org/journal/ojpchem)
http://dx.doi.org/10.4236/ojpchem.2013.34014
Open Access OJPChem
Natural Rubber/Fluoro Elastomer Blends: Effect of Third
Component on Cure Characteristics, Morphology,
Mechanical Properties, and Automotive Fuel Swelling
Manisara Phiriyawirut*, Thalatchanan Limwongwatthananan, Surawut Kaemram,
Sirithada Wiengkaew
Department of Tool and Materials Engineering, Faculty of Engineering, King Mongkut’s University of
Technology Thonburi, Bangkok, Thailand
Email: *manisara.pee@kmutt.ac.th
Received July 11, 2013; revised August 22, 2013; accepted September 5, 2013
Copyright © 2013 Manisara Phiriyawirut et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
The natural rubber (NR) was mixed with fluoro elastomer (FKM), due to the difference of polarity in NR and FKM
made this blend incompatible so the third component was used. NR/FKM blended with the blend ratio of 70/30 was
prepared by using a two-roll mill and vulcanization in a compression mold at 180˚C using peroxide as a curative agent.
Epoxidized natural rubber (ENR) or polyisoprene-graft-maleic acid monomethyl ester (PI-ME) was used as a third
component. The curing characteristics, morphology, mechanical properties, and automotive fuel swelling were investi-
gated. The results indicated that the scorch time and cure time of the blend rubbers were longer as adding ENR or
PI-ME. Both mechanical properties and automotive fuel resistance of the blend rubbers were found to increase with
adding ENR in rubber blend. Conversely for adding PI-ME, automotive fuel resistance of the blend rubbers was found
to decrease progressively with increasing PI-ME content.
Keywords: Natural Rubber; Fluoro Elastomer; Epoxidized Natural Rubber; Blend
1. Introduction
Natural rubber (NR) from Hevea brasilien sis is known to
have highly unsaturated backbone, and it also shows low
oil and organic solvent resistance due to nonpolarity [1].
Owing to the presence of the polarity of the fluorine at-
oms in fluoro elastomer (FKM), NR/FKM blends should
be resistant to ozone, oil, heat, and nonpolar chemicals.
As the matter of fat, based on chemical structure, the NR/
FKM blend becomes incompatible due to the difference
in polarity. The resulting blend exhibits poor mechanical
properties due to the poor adhesion between the phases
[2].
There are a lot publications that have discussed rubber
blends based on the addition of a third homopolymer or
graft or block copolymer that binds with the two phases
and the introduction of bonds between the homopolymer
phases [3-8]. A ccord ing to K arn ik a d e Silv a and Lewwan
[3] the addition of graft copolymer of NR reduced phase
sizes attained in NR/NBR blends over a wide range of
acrylon itrile con ten t of NBR. K. Pr akash an an d cowo rker
[4] studied compatibility problems in polypropylene
(PP)/poly(dimethylsiloxane) (PDMS) elastomer blend.
Maleic anhydride grafted polypropylene (PP-g-MAH)
was reported to act a compatibilizer for these blends.
Epoxidized natural rubber (ENR) contains polyiso-
prene as main chain and epoxidize group as side group
which should be reacted to NR and FKM molecules, re-
spectively (Figure 1). Then, ENR was interested to be
used as a third component for NR/FKM blend. Not only
ENR but also polyisoprene-graft-maleic acid mono-
methyl ester (PI-ME) was interested to be used as a third
component for these blend. PI-ME also contains polyiso-
prene as main chain and maleic acid monomethyl ester as
a grafting group. The grafting group of PI-ME contains
carbonyl group which should react to the FKM molecule.
According to M. Abdul Kader and A. K. Bhowmick [8]
reported that blend of acrylate rubber and FKM was mis-
cibility due to interaction between C=O grou p of acrylate
rubber and –CF3 group of FKM in the blend.
In this study, the cure characteristics, morphology,
*Corresponding a uthor.
M. PHIRIYAWIRUT ET AL.
80
CH2
C
H3C
C
H
CH2n
Epoxide natural rubber,
ENR
Natural rubber,
NR
n
CH2CF2CF
CF3
CF2
Fluoro elastomer,
FKM
CH2
C
H3C
C
H
CH2
O
CH3
O
O
R
O
O
R
CH3
xy
R = H or CH3
Polyisoprene-graft-maleic acid
monomethyl ester, PI-ME
Figure 1. Chemical structure of investigated polymers.
mechanical properties, as well as automotive fuel resis-
tances of 70/30 NR/FKM blen ds with and without a th ird
component, ENR or PI-ME, were investigated. The se-
lected automotive fuels were B5-biodiesel, diesel, gaso-
line and gasohol.
2. Experimental
2.1. Materials
Fluoro elastomer (FKM, Viton GF-S600) and natural
vegetable wax (VPA No. 2) was supplied from Dupont
Dow Elastomer Co., Ltd., USA. The natural rubber (NR)
was STR 5L. Epoxidized natural rubber (ENR) was 25
mol% of epoxide groups. Polyisoprene-graft-maleic acid
monomethyl ester (PI-ME) with MW = 25,000 was sup-
plied from Aldrich. Organic peroxide, 2,5-dimethyl-2,4-
bis (t-butyl peroxy) hexane 45% active ingredient (Lu-
perox® 101-XL 45) was supplied from Atofina, Singa-
pore. A coagent, triallyl isocyanurate (SR533) was pro-
vided from Sartomer, USA. Other chemicals were pro-
cured from indigenous sources and were used as such.
The selected automotive fuels in this experiment were
gasoline, gasohol, diesel and B5-biodiesel which were
supplied from a petroleum station of PTT Public Co.,
Ltd., Thailand. The gasohol is a 10% ethanol blend with
conventional gasoline and B5-biodiesel is a 5% biodiesel
blend with conventional diesel.
2.2. Mixing and Vulcanization Procedure
The compound ingredients as shown in Table 1 were
mixed on a two-roll mill at room temperature. The NR
was first masticated to soften it. Subsequently, given
Table 1. Compound formulation used in the preparation of
the NR/FKM blends.
Content (phr)
Ingredients 1 2 3 4 5
NR (STR 5L) 70 70 70 70 70
FKM 30 30 30 30 30
ENR or PI-ME 0 2.5 5 7.5 10
Carbon black (N 660)10 10 10 10 10
6PPDa 2 2 2 2 2
TMQb 2 2 2 2 2
Peroxidec 3 3 3 3 3
SR533 3 3 3 3 3
VPA No. 2 0.5 0.5 0.5 0.5 0.5
a N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine; b Polymeriz ed 2,2,4-
trimethyl-1,2-dihydroquinoline; c 2,5-dimethyl-2,4-bis(t-butyl peroxy) hex-
ane 45% active ingredient.
amounts of all chemicals except Luperox® 101-XL 45
and SR533 were added to the masticated rubber. The
mixture was then blended with FKM and further mixed.
Lastly SR533 and Luperox® 101-XL 45 were added. To
vulcanize the blends, they were compression molded
using a hydraulic hot press at 180˚C [2], under a pressure
of 15 MPa according to their respective cure times, as
determined by oscillating disk rheometer (ODR GT-
707052). ODR gives digitals outputs of curing character-
istics such as scorch time, cure times and torque value.
2.3. Testing
Morphological tests: Morphological study was carried
out using a Philips, XL30CP scanning electron micro-
scope with a 20 kV accelerating voltage. Phase contrast
was improved with a complementary surface preparation
techniques via OsO4 staining. Each sample was coated
with a thin layer of gold prior to observation under SEM.
Mechanical tests: Tensile and tear specimens were
punched out from the molded slab using an ASTM stan-
dard die. The mechanical tests were carried out per
ASTM D412 and ASTM D1004 for tensile test and tear
test, respectively. An Instron 2532 tensile tester and a
LLOYD tear resistance tester were both used at a cross-
head speed of 500 mm/min, with a 500 N load cell.
Automotive fuel resistance tests: the automotive fuel
resistance tests were carried out per ASTM D471 . Sq u are
test specimens of 2 × 2 × 2 cm3 were weighed accurately
before immersion into the fuel at 25˚C. After a specific
time of immersion, a specimen was removed from the
fuel and weighed again after removing surface fluids by
blotting with filter paper. Th e p ercentage of swelling was
calculated according to the following equation:
21
1
WW
% swelling100
W

(1)
Open Access OJPChem
M. PHIRIYAWIRUT ET AL. 81
where W1 and W2 represent the weight of the specimens
after and prior to immersion into automotive fuel.
3. Results and Discussion
3.1. Cure Characteristic
The effect of third component content of the NR/FKM
blend on the cure characteristics was investigated by
ODR. The cure characteristics of the NR/FKM blends;
scorch time, cure time and different torque values of NR/
FKM rubber blends between before and after curing
(MH-ML) are shown in Figures 2 and 3.
Scorch time is the time taken for the minimum torque
value to increase by two units. It is a measure of prema-
ture vulcanization of the material. The scorch times of
the blends were found to increase by adding ENR but not
significantly chang e with increasing ENR content. As for
scorch time, the cure time value also increase by adding
ENR and also not significantly change with increasing
ENR content. The longer cure characteristic time of the
NR/FKM blend with ENR was due to the presence of
ENR mean increase the NR phase which susceptible to
bonding with peroxide slower than FKM molecule [2].
(a)
(b)
Figure 2. Cure characteristic time of 70/30 NR/FKM blends
with and without third component at 180˚C (a) scorch time
and (b) cured time.
Figure 3. MH-ML of 70/30 NR/FKM blends with and with-
out third component at 180˚C.
However, increasing ENR content was not effect to the
cure characteristic time due to insufficient of ENR con-
tent in the blend that can enhance peroxide cure reaction.
While adding PI-ME to the blend, the scorch times of
the blends were found to increase by increasing PI-ME
content. As for scorch time, the cure time value also in-
creased by increasing PI-ME content in the blend. Per-
oxide curative agent possessed lower efficiency in the
presence of acid because the peroxide molecule can react
with the acid functional group of PI-ME [9]. Then low-
ering content of peroxide, rubber blend with PI-PE, slow
the cure rate compared to rubber blend without PI-ME.
From Figure 3, ENR content at 2.5 to 5 phr in NR/
FKM blend had no effect on the different torque values
(MH-ML), but it was gradually increased by increasing
ENR content from 5 to 10 phr. NR/FKM blending with-
out ENR has low MH-ML value due to less interaction
between NR and FKM molecules [2], however, addition
2.5 to 5 phr of ENR do not increase the MH-ML value
due to insufficient of ENR content in the blend that can
enhance binding between two phases. The higher ENR
content between 5 to 10 phr, the higher torque of ODR
was achieved. The increasing of ODR torque is the effect
on increasing of interaction between two phases of rub-
ber blend. In addition, the higher MH-ML value is rep-
resented crosslink density in rubber phases. Addition 10
phr of ENR into NR/FKM blend gives the highest cross-
link in the rubber blend.
While adding PI-ME to the blend, PI-ME content at
2.5 to 7.5 phr in NR/FKM blend had no effect on the
torque difference (MH-ML), but it was gradually de-
creased by increasing PI-ME content from 7.5 to 10 phr.
The decreasing of ODR torque is the effect on decreasing
of the number of crosslink created from the lower mole-
cule of peroxide in the presence of PI-ME. Then adding
of PI-ME in NR/FKM has least effect on peroxide cures.
In addition, PI-ME was lower molecular weight (ca.
2500) and presented in the liquid form. Higher content of
PI-ME, particularly 10 phr, PI-ME would act as a lubri-
Copyright © 2013 SciRes. OJPChem
M. PHIRIYAWIRUT ET AL.
82
cant in the system instead of penetrate into the rubber
phase for link between NR and FKM molecules. Thus
lower ODR torque was achieved.
3.2. Morphological Properties
The SEM micrographs of the cryofracture surfaces of the
70/30 NR/FKM blend without third component were
showed in Figure 4. The holes were present in the matrix
because phase separation occurred during the fracture.
This phenomenon indicates that the interfacial adhesion
between NR and FKM is not sufficient and could say this
blend was incompatible. To improv e blend compatibility,
the third component with segments, that was chemically
identical or similar to NR and FKM phases, was added.
In this study, ENR or PI-ME was used as a third compo-
nent. It should promote good compatibility between NR
and FKM phases in blends.
The influent of ENR content on cryofracture surfaces
of the 70/30 NR/FKM blend was showed in Figure 5 . A
roughness surfaces of the blend can be observed even
thought adding EN R. This suggests that adding ENR did
not have a remarkable effect on the morphology of NR/
FKM blen d.
Figure 6 showed the SEM micrographs of the cry-
ofracture surfaces of the 70/30 NR/FKM blend with 2.5
to 10 phr PI-ME content. Unexpectedly, the addition of
PI-ME appeared to increase roughness of the blend.
However, PI-ME content did not have a remarkable ef-
fect on the surface of the blend. In addition, holes were
still presented in the matrix even through added ENR or
PI-ME, but more appeared in PI-ME added blend than
did in ENR added blend.
3.3. Mechanical Properties
The mechanical properties, in terms of tensile strength,
elongation at break and tear strength were determined for
NR/FKM rubber blends with and without third compo-
nent, ENR or PI-ME, and the results are reported in Fig-
ure 7.
For the ENR added NR/FKM blends, the tensile
strength, elongation at break and tear strength w e re found
to not increase significantly with the ad dition of up to 7.5
a b
Figure 4. Scanning electron micrograph of cryofracture
surface of 70/30 NR/FKM blends without the third compo-
nent (a) magnificent ×200 and (b) magnificent ×1000.
a b
c
e
g
f
d
h
Figure 5. Scanning electron micrograph of cryofracture
surface of 70/30 NR/FKM blends with different ENR con-
tent ((a), (b)) 2.5 phr ((c), (d)) 5.0 phr ((e), (f)) 7.5 phr and
((g), (h)) 10 phr; ((a), (c), (e), (g)) magnificent ×200 and ((b),
(d), (f), (h)) magnificent ×1000.
phr ENR, but subsequently increase progressively at 10
phr ENR at the expense of the tear strength, which was
found to decrease with increasing ENR content. It is evi-
dent that the mechanical properties of the blend exhibited
some extent of improvement with the addition of up to
10 phr ENR. This is due to the achievement of interac-
tion between NR and FKM by ENR molecule at enough
ENR content.
For PI-ME added NR/FKM blends, the ten sile strength,
and tear strength were found to decrease progressively
with the addition of up to 10 phr PI-ME. However, the
elongation at break was found to not remarkable different
with or without PI-ME. It is evident that the mechanical
properties of the blend were deteriorated by the addition
of up to 10 phr PI-ME. This was due to PI-ME would act
as a plasticizer in the system instead of link between NR
and FKM molecules. Physically, PI-ME added NR/FKM
blends had trend to soft and sticky with increasing PI-ME
content.
Open Access OJPChem
M. PHIRIYAWIRUT ET AL. 83
a b
c
e
g
d
f
h
Figure 6. Scanning electron micrograph of cryofracture
surface of 70/30 NR/FKM blends wi th different PI-ME con-
tent ((a), (b)) 2.5 phr ((c), (d)) 5.0 phr ((e), (f)) 7.5 phr and
((g), (h)) 10 phr; ((a), (c), (e), (g)) magnificent ×200 and ((b),
(d), (f), (h)) magnificent ×1000.
Furthermore, the strength of the blend was depended
on degree of crosslinking which confirmed by ODR re-
sults. The high strength of the ENR-added blend was due
to high degree of crosslinking, while the low strength of
the PI-ME-added blend was due to low degree of cross-
linking. The results are clearly in good agreement with
the cure characteristic results.
3.4. Automotive Fuel Resistance
The effect of third component content on automotive fuel
resistance of NR/FKM blend was investigated by deter-
mined the percentages of swelling as shown in Figure 8.
For a given blend compositio n, the percentages of swell-
ing increased with increasing immersion time. The rate
of swelling increased exponentially with time.
For the ENR added NR/FKM blends, it is interesting
to note that the p ercentage of swelling of NR/FKM blend
without ENR was greater than that of NR/FKM blend
(a)
(b)
(c)
Figure 7. Mechanical properties of NR/FKM blends with
and without the third component (a) tensile strength (b)
elongation at break and (c) tear strength.
with ENR for all automotive fuel studies. With increas-
ing ENR content, the percentage of swelling after 100
hours of immersion time of NR/FKM blends was found
to decrease when ENR content increased from 2.5 to 10
phr. The NR/FKM blend with high ENR content showed
lower degree of swelling than those of the lower ENR
content. Because of the improvement of interaction be-
tween NR and FKM by ENR molecule at enough ENR
Copyright © 2013 SciRes. OJPChem
M. PHIRIYAWIRUT ET AL.
84
(a)
(b)
Figure 8. Percentage of swelling in selected automotive fuels
of NR/FKM blends with and without the third component
(a) ENR and (b) PI-ME for various immersion time at 25˚C.
content, a result in compatib lized phase in microstructure
was occurred. However, the presence of ENR which po-
lar rubber was also increased the oil resistance of the
blend.
For the PI-ME added NR/FKM blends, the reverse re-
sults were observed. The percentage of swelling of NR/
FKM blend without PI-ME was lower than that of NR/
FKM blend with PI-ME. With increasing PI-ME content,
the percentage of swelling of NR/FKM blends was found
to increase especially at 10 phr. Physically, PI-ME added
NR/FKM blends had trend to soft and sticky with in-
creasing PI-ME content. PI-ME would be plasticizer or
lubricant in this NR/FKM blend and then allowed auto-
motive fuels penetrate in to the rubber phase. Further-
more, the lower degree of crosslinking in rubber blend in
the presence of PI-ME and soft characteristic, a result in
higher degree of automotive fuel penetration into rubber
phase was occurred. The another reason of high automo-
tive fuel swelling of PI-ME added NR/FKM blend is
non-automotive fuel resistance of PI-ME, then increasing
PI-ME content result in high percentage of swelling of
NR/FKM blend in automotive fuel.
It was found that the percentage of swelling of NR/
FKM blend with ENR in gasoline-based fuel was quite
greater than that in diesel-based fuel (Figure 8(a)).
Gasoline is predominantly a mixture of paraffins, naph-
thanes, and olefins, while diesel is composed of about
75% saturated hydrocarbons, and 25% aromatic hydro-
carbons [10]. The gasoline-based fuels have a smaller
average molecular size than that of diesel-based fuel.
Thus gasoline-based fuels were penetrated in ENR added
rubber sample easily than diesel-based fuels. This result
was as same as previous report [11]. However, for com-
paring in percentage of swelling of these rubber blends in
gasoline-based fuel, gasoline and gasohol, it was found
no more different. For NR/FKM blends with PI-ME, the
percentage of swelling in all automotive fuel stu dies was
not considerable different (Figure 8(b)).
4. Conclusion
In this contribution, NR/FKM blended with blend ratio of
70/30 with and without the third component, ENR or
PI-ME was prepared. The curing characteristics, mor-
phology, mechanical properties, and automotive fuel re-
sistance of the rubber blends were investigated. Form the
curing characteristics investigation, the cure time and
scorch time of the blends with the third component were
found to be longer than those of the blends without the
third component. Adding ENR did not have a remarkable
effect on the morphology of NR/FKM blend but the ad-
dition of PI-ME appeared to increase roughness of the
blend. The mechanical properties and automotive fuel re-
sistance of the blend rubbers were found to increase with
adding 10 phr of ENR due to sufficient ENR content for
enhanced interaction between NR and FKM. In contrary
results, adding PI-ME into the NR/FKM decreased the
mechanical properties and automotive fuel resistance of
the blend rubbers especially at 10 phr. Fro m the results, it
has been concluded that ENR could function as a third
component that link NR and FKM phases in the blend
system study. The optimum content of ENR was 10 phr.
5. Acknowledgements
The authors would like to thank for the “Small Projects
on Rubber, SPR 50” under supervision of the Thailand
Research Fund (TRF) as well as Department of Tool and
Materials Engineering, King Mongkut’s University of
Technology Th on b uri for financial sup po rt.
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