ROHSTOFFE UND ANWENDUNGEN
RAW MATERIALS AND APPLICATIONS
EPDM Rubber Mould Fouling
Mould fouling of EPDM rubber compounds has been studied by means of
Fourier IR spectroscopy, X-ray diffractometry, differential scanning calorimetry and other methods. Chemically altered constituents of the plasticizer oil - resinous substances
formed by reaction with the curing
system during heating in contact with
the metal surface of the mould - are
the source of the soluble organic part
of mould fouling. The insoluble, inorganic part is ZnS which forms when
sulphur-containing vulcanizing systems are used. Vulcanization accelerators and other compounding ingredients migrate to the interface between the rubber compound and the
metal of the mould, entrained on the
shoulder of the plasticizer oil. The
metal of the mould has a catalytic effect on the change in the chemical
structure of the oil.
Formverschmutzung bei EPDMKautschukmischungen
EPDM Kautschuk Formverschmutzung
Formverschmutzung bei EPDM-Kautschukmischungen ist mittels Fourier
IR-Spektroskopie, Röntgen-Diffraktometrie, Differentialscanningkalorimetrie und anderen Methoden untersucht
worden. Chemisch veränderte Bestandteile des Plastifizierungsöls –
harzhaltige Substanzen, die sich
durch Reaktion mit dem Vulkanisationssystem während des Erhitzens bei
Berührung mit der Metalloberfläche
der Pressform bilden – liegen dem
löslichen, organischen Teil der Formverschmutzung zugrunde. Der nicht
lösliche, anorganische Teil ist ZnS,
das sich bei der Verwendung von
schwefelhaltigen Vulkanisationssystemen bildet. Vom Plastifizierungsöl getragen und mitgeführt, wandern Vulkanisationsbeschleuniger und andere
Mischungsbestandteile zur Grenzfläche zwischen der Kautschukmischung
und dem Metall der Pressform. Das
Metall der Pressform hat eine katalytische Auswirkung auf die Veränderungen in der chemischen Struktur
des Öls.
172
Mould Fouling of EPDM Rubber
Compounds
M. F. Bukhina, Y. L. Morozov, Moscow (Russia)
P. M. van de Ven, J. W. M. Noordermeer,
Geleen (The Netherlands)
Mould fouling is a deposit which forms on
the surface of metal moulds during the
process of high-temperature moulding
and vulcanization of rubber goods. It is
the result of thermochemical changes
in components of rubber compounds under moulding conditions.
The problems caused by mould fouling
in the rubber industry are well-known. In
order to obtain defect-free rubber articles, mould fouling must be removed
from the mould surfaces at regular intervals. A number of attempts have been
made in the past to understand the
causes of mould fouling, as well as to
study its composition in the case of different rubbers [1 – 13]. Many attempts to
minimize the amount of mould fouling
have also been documented, revealing
results which are often contradictory despite the researchers’ best efforts. This is
also the case for EPDM rubber compounds. Particular reference is made to
the work of Sommer [2] which is among
the earliest attempts to identify the various factors involved in mould fouling occurring with EPDM. The objective of that
study was, however, primarily to investigate the potential of various amine compound ingredients to prevent mould fouling or to clean the mould. The recipes
used in that study now look very unrealistic in the light of current EPDM compounding practice. Valuable pointers to
factors which give rise to mould fouling
in the case of EPDM can nevertheless
be derived from that article.
The objective of the present work is to
revisit the issue of mould fouling by EPDM
on the basis of more current compound
compositions for EPDM rubber. The
aim is to update our understanding of
the causes of mould fouling in the case
of EPDM, as well as to estimate the rela-
tive contributions made by different composition components and processing
variables to the formation of mould fouling in the case of EPDM rubber compounds.
Materials and methods
Rubber compounds
The process of mould fouling formation
was studied for 12 rubber compounds
based on EPDM rubbers: Keltan 312
and Keltan 378 from DSM Elastomers,
as well as SKEPT-50, which is of Russian
origin. The viscosities and typical chemical compositions of the EPDM’s of DSM
Elastomers are as follows [14]:
Keltan
312:
Mooney
viscosity
ML(1 þ 4)125 8C: 33; ethylene content
49 wt.%; ethylidene norbornene (ENB)
termonomer content 4.3 wt.%.
Keltan
378:
Mooney
viscosity
ML(1 þ 4)125 8C: 33; ethylene content
67 wt.%; ENB termonomer content 4.3
wt.%.
For SKEPT-50 the following characteristics apply [15]: Mooney viscosity
ML(1 þ 4)125 8C: 30; ethylene content
60 wt.%; dicyclopentadiene (DCPD) termonomer content 6.3 wt.%.
The compositions of the compounds
are given in Tab. 1: A1 – A11. Compounds A1/A4 and A2/A3, respectively,
represent basic starting recipes of a practical nature, which formed the basis for
subsequent variations. They are a “black”
and a “white” recipe, sulphur-cured and
peroxide-cured. The compounds were
prepared as large masterbatches in a
50-litre Banbury internal mixer – Farrel
Bridge 3D – containing the rubber,
ZnO, stearic acid, TEA, PEG, fillers and
oil. The mixing in the internal mixer will
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 4/2003
Mould Fouling of EPDM Rubber Compounds
Tab. 1. Recipes and mixing characteristics of the practical compounds used throughout this study
Compound number
Ingredient (phr)
A1
A2
A3
A4
A5
A6
A7
A8
A9
A10
A11
A12
EPDM Keltan 312
EPDM Keltan 378
Russian SKEPT-50
ZnO
Stearic acid
TEA
PEG
Carbon black N-550
Carbon black PM-50
SiO2 Ultrasil VN3
Whiting Omya BSH*)
Sillitin Z86**)
Par. oil Sunpar 2280
Par. oil Sunpar 150
Naphth. oil Naphthoplast
MBT-80
TMTD-80
DTDM
S-80
Perkadox 14-40
“High-rate” Banbury
mixing
“Low-rate” mill mixing
“Normal” mill mixing
t v at 200 8C (min)
100
–
–
5
1
–
–
89
–
–
44
–
56
–
–
100
–
–
5
1
1
2
–
–
47
95
–
47
–
–
100
–
–
5
1
1
2
–
–
47
95
–
47
–
–
100
–
–
5
1
–
–
89
–
–
44
–
56
–
–
100
–
–
5
1
2
–
–
47
95
–
47
–
–
100
–
–
5
–
–
89
–
–
44
–
56
–
–
100
–
–
5
1
1
2
–
–
47
95
–
–
47
–
100
–
–
5
1
1
2
–
–
47
–
95
47
–
–
–
100
–
5
1
1
2
–
–
47
95
–
47
–
–
100
–
–
5
1
1
2
–
–
47
95
–
47
–
–
100
–
–
5
1
–
–
45
–
–
–
–
–
–
–
–
–
100
5
1
–
–
–
25
–
–
–
–
–
20
0.94
1.88
–
1.88
–
þ
0.94
1.88
–
1.88
–
þ
–
–
–
–
5.75
þ
–
–
–
–
5.75
þ
–
–
–
–
5.75
–
–
–
–
–
5.75
–
0.94
1.88
–
1.88
–
–
0.94
1.88
–
1.88
–
–
0.94
1.88
–
1.88
–
–
0.94
1.88
–
1.88
–
–
0.94
1.88
–
1.88
–
–
0.5
0.5
1
2
–
þ
–
–
4
–
–
10
–
–
12
–
–
8
þ
–
15
þ
–
8
þ
–
20
þ
–
20
þ
–
18
þ
–
20
–
þ
–
–
8
8
*) Inactive white filler of normal structure **) Inactive white filler of lamellar structure
be indicated as “high-rate” Banbury mixing throughout this paper. Sulphur-based
and peroxide-based curing additives
were added separately on a two-roll mill
shortly before the mould fouling experiments. The properties of the basic compounds and the vulcanisates of the two
sulphur-cured recipes A1 “black” and
A2 “white” are given in Tab. 2.
Compounds A5 – A10, which were designed so as to enable the effects of successive individual changes in compound
ingredients to be studied, were prepared
individually by special low-rate mill mixing: indicated as “low-rate” mill mixing.
Compound A11 was prepared by the
usual “high-rate” milling. The compound
compositions were set up as an experimental design, adjusted so as to obtain
an IRHD hardness of the vulcanisates
of 65 – 70 in all cases:
– two principal recipes, one “black” containing N-550 carbon black and one
“white” containing silica Ultrasil VN3
as a reinforcing filler and additionally
a non-reinforcing, coated whiting
Omya BSH: A1 vs. A2;
– sulphur cure is compared with peroxide cure: e. g. A1 vs. A4, and A2 vs. A3;
– the influence of the high ethylene content of Keltan 378 EPDM is compared
with the low ethylene content of Keltan
312: A9 vs. A10;
Tab. 2. Compound properties and cure properties of basic compounds A1 and A2
Property
Test method
Compound Mooney viscosity at ISO 289
100 8C
Oscillating disk curemeter at
180 8C
scorch time ts2
90 % cure time tc(90)
ML
MH
MH – ML
Hardness
Unit
A1
A2
ML(1þ4)
41
59
min.
min.
N.m
N.m
N.m
IRHD
1.6
4.95
0.59
6.68
6.08
72
1.5
14.4
1.21
3.49
2.28
65
ISO 3417
ISO 48
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 4/2003
– with stearic acid present and without
stearic acid: A3 vs. A5 and A4 vs. A6;
– the effect of a whiting which combines
corpuscular and lamellar primary particle structure: Sillitin Z86 vs. whiting of
spherical corpuscular structure only:
Omya BSH: A8 vs. A10;
– a low-viscosity paraffinic oil Sunpar
150 is compared with its high-viscosity
counterpart Sunpar 2280: A7 vs. A10;
– a recipe without oil and white fillers and
with half the carbon black content:
A11;
– the influence of mixing rate, high shearrate internal mixer vs. normal and lowrate mill mixing: A1 – A4 vs. A5 – A10; at
low-rate mill mixing the mill was run at
an unusually low roll speed;
– recipe A12 represents a compound of
Russian origin, which is industrially
mixed in a batch internal mixer and
used for the moulding of engineering
rubber articles.
All the compounds were finally milled to
1 mm-thick sheets and stored.
The virgin rubbers Keltan 312 and Keltan 378 were extracted in MEK at room
temperature in two steps: first for
1 hour and, after removal of the extract,
for a further day with a new portion of
MEK. The extracted rubbers (i. e. with
173
Mould Fouling of EPDM Rubber Compounds
Tab. 3. Recipes of model mixtures
Mixture number
Ingredient (g)
B1
B2
B3
B4
B5
ZnO
Stearic acid
Sunpar 2280
Sunpar 150
MBT-80
TMTD-80
S-80
5
1
–
–
1
2
2
–
–
4.7
–
1
1.9
2
5
1
5
–
1
1
2
5
1
20
–
1
2
2
5
1
the low molecular weight components removed) were dried; their weights still exceeded 97 % of those of the virgin
rubbers. Both virgin and extracted rubbers, as well as the extracts, were studied.
Another method of studying the effect
of the separate compounding ingredients
on fouling of steel is to create model mixtures containing only a few ingredients:
see Tab. 3. This series of model mixtures
was prepared by swelling small quantities
of sulphur particles and accelerator particles in oil for almost half an hour, and
subsequently mixing with ZnO and stearic acid in a laboratory mortar for 20 min.
to obtain a homogeneous blend.
Metals used for the mould fouling
experiments
To evaluate the effect of steel quality, four
types of steel were used for the fouling
experiments, as indicated in Tab. 4. Steel
types A - C represent materials of different hardness which are commonly used
for the production of rubber moulds.
Steel type R is a common grade of steel
of Russian origin and is characterized by
a high carbon content.
The steels were cut into small 48 9 1 mm plates. These were polished
and hardened as indicated in Tab. 4. Be-
–
20
1
2
2
cause it is easier to analyze small areas of
steel in the various analytical tests than
entire steel moulds, these small steel
plates were used as inlays in the actual
moulds. Large 200 200 5 mm plates
were used as the upper part of the mould:
see paragraph below.
Moulding
Repeated compression mouldings of
compounds A1 – A11 were performed in
order to study the quantity and composition of mould fouling. A conventional curing press was used. A 150 150 1.05 mm rectangular mould was contained between two flat steel plates, the
upper plate of the mould being the large
metal plate, as described in the preceding paragraph. The small steel plates
were used as inserts and placed at the
bottom of the mould. Compound was
placed on top of the small plates in the
form of milled sheets; the upper sides
of these sheets contacted the large steel
plates. Vulcanization was carried out at
temperatures Tv of 180 or 200 8C, under
a load of 200 bar for the duration tv, obtained from rheometer cure curve as representing the optimum vulcanization
time tc(90) of the particular rubber compound. Tab. 1 shows the vulcanization
times tv as employed at 200 8C. A variable
number, N, of mouldings were made in
order to record the evolution of the fouling.
Besides normal moulding, what we
shall term “model moulding” was also applied: a piece of rubber compound was
placed between two small metal plates
under load and heated in an oven at a
temperature of generally 180 8C. The
pressure obtainable under these conditions was obviously far lower than in actual moulding. This sort of “model moulding” was used to study the effects of the
separate compounding ingredients. In
the case of the model mixtures of
Tab. 3, small aliquots were also heated
between two small metal plates in the
oven.
The metal plates on which the deposits
had formed were used for analytical purposes. The weight of the deposits on the
metal plates was determined at intervals.
The weight changes were commonly extremely low. The metal plates were
furthermore washed off with solvents
such as chloroform or methyl ethyl ketone
and the “wash-off’s” subjected to analytical investigation. The purity of the
chloroform and MEK had been checked
in advance by means of IR, so as to ensure that no peaks occurred in those
spectral areas which were of interest. Similarly, the surfaces of the cured rubber
facing the small metal plates and the
large plate on the top of the mould
were also washed off and the “washoff’s” analyzed in order to compare the
material which been deposited on the
steel with that which adhered to the rubber surface.
Analytical techniques
The mould fouling obtained, either in the
pure form or in the form of “wash-off’s”,
Tab. 4. Steel types used for the fouling experiments
Steel
code
Material
code
A
1.2311
B
1.2767
DIN classification
C
Si
Mn
Cr
(%)
Mo
Ni
V
W
Polishing
<Ra> (lm)
40CrMnMo7
0.42
0.30
1.50
2.0
0.2
–
–
–
N2-N3
X45NiCrMo4
0.45
0.25
0.40
1.35
0.25
4.0
–
–
N2-N3
0.05 – 0.1
0.05 – 0.1
C
1.2510
100MnCrW4
1.00
0.30
1.10
0.6
–
–
0.1
0.6
N5
0.4
R
Steel 3*)
**)
0.22
0.06
0.8
0.3
–
0.3
–
–
Different
*) Steel of common use, GOST 380-71 **) Similar to FeE235BFU
174
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 4/2003
Mould Fouling of EPDM Rubber Compounds
was studied by means of various methods:
– Fourier transform IR spectroscopy (FTIR),
– thin layer chromatography (TLC), with a
mixture of petroleum ether, diethyl
ether and acetic acid in a ratio
70:30:5 as eluent and a 1% solution
of 2,6-dichloroquinone-4-chloramide
in ethanol as developer,
– Auger spectrometry,
– X-ray diffraction,
– differential scanning calorimetry (DSC),
– mass spectroscopy,
– light microscopy,
– visual observation,
– additional chemical analytical methods.
IR spectroscopy – in transform mode
(FT-IR) and in the attenuated total reflection (ATR) of surfaces mode - was found
to be the most powerful investigative
technique.
Results and discussion
Carbon black-filled compounds,
sulphur-vulcanized
Repeated compression moulding at
Tv ¼ 200 8C of the Keltan 312-based,
carbon black-containing rubber compound with the sulphur vulcanizing system A1, at all times using the same steel
plates of metal B, shows that in general
the first visible traces of mould fouling appear after 30 – 40 moulding cycles. A persistent deposit forms after about 100
moulding cycles. The quantity of mould
deposit increases up to about 225 cycles.
Further moulding results only in a visible
colour change in both the small and the
large metal top plates. The colour change
in the large metal top plates is particularly
intense in the free edges of their surface
remote from the mould cavity, which
made no direct contact with the rubber
compound.
The ATR spectrum of the small plates
of metal B after N ¼ 222 mouldings is
shown in Fig. 1a. This spectrum exhibits
seven characteristic spectral regions,
which are designated I to VII. The transmission spectrum of wash-off from the
surface of the vulcanized rubber sheet
after almost the same number of mouldings N ¼ 227 essentially shows the same
characteristic bands, as shown in Fig. 1b.
This result opens up the possibility of
using the transmission spectra of washoff’s from the surface of vulcanized rubber
sheets instead of the reflection spectra of
the deposits on the metal plates. The
spectra of the wash-off’s are of higher
quality. It also means that some of the
mould fouling is “dragged” from the metal
surface by the rubber sheets during
mould release.
Fig. 1. 1a – Fourier IR spectrum in ATR mode of the small plates of metal B after N ¼ 222 mouldings at Tv ¼ 200 8C of compound A1; 1b –
transmission spectrum of MEK wash-off from surface of rubber sheet of A1 after N ¼ 227; 1c – after N ¼ 292; 1d – transmission spectrum of
MEK wash-off from the large metal top plate of metal B after N ¼ 300; 1e – transmission spectrum of wash-off from rubber compound A2 at
Tv ¼ 200 8C after N ¼ 4
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 4/2003
175
Mould Fouling of EPDM Rubber Compounds
Of the characteristic spectral regions in
these spectra, the absorption bands at
2853 and 2923 cmÿ1 (region II in Fig. 1a)
are specific to hydrocarbons. The other
bands are characteristic of the composition of the mould fouling. Two of these regions, region I comprising the wide absorption band at 3100 – 3500 cmÿ1, and
region IV comprising the absorption bands
between 1385 and 1460 cmÿ1, are particularly important. In the initial stages of
mould fouling region IV is a doublet of relatively narrow bands. In the course of
further mould fouling, the shape of this region changes: a new intermediate absorption band appears at 1401 – 1407 cmÿ1
between the two original bands in region
IV – initially as a shoulder – and the doublet
becomes a triplet: see Fig. 1b and 1c.
These changes in the spectra show the
formation of new substances in the course
of moulding which appear to be characteristic of the mould fouling. The chemical
nature of this adsorption band at 1401 –
1407 cmÿ1 has remained unclear
throughout this study, as it corresponds
to no commonly documented IR spectrum. It is believed to result from the formation of a new substance, e. g. from cyclization of some of the oil molecules as
well as of low molecular weight components of the rubber due to protracted
heating and the catalytic effect of the metal surfaces.
The density of this new absorption
band between 1401 and 1407 cmÿ1 enables the formation of mould fouling on
the number of mouldings N to be evaluated semi-quantitatively by measuring
the relative density (D/Do) of this new
band in the triplet. Do is the optical density
of the absorption band at 2923 cmÿ1 in
region II, which is used as an internal
standard. There is a slight increase in
the density of the new middle band in
the triplet up to about N ¼ 200. At around
N ¼ 300 there is a sharp increase in D/Do,
and after about 300 mouldings a decrease can be observed. At the same
time, the visible amount of mould fouling
also decreases, suggesting that some
sort of self-cleaning of the steel takes
place.
The visual appearance of the mould
fouling on the surface of both the small
and the top metal plates changes in the
course of storage of the metal plates after
moulding. Storage of vulcanized rubber
sheets also results in some changes in
composition of the fouling on their surface, as is revealed by IR spectroscopy:
the middle band in the characteristic triplet disappears again. In order to obtain
data with high reproducibility, therefore, a
precise, short storage time has to be observed prior to the IR measurement. Increasing the storage time of the raw, as
yet unvulcanized, rubber compound prior
Fig. 2. Auger spectrum of the thin layer of mould fouling on the surface of plates of metal A after N ¼ 283 mouldings of compound A1 at
Tv ¼ 200 8C; elements corresponding to the peaks are indicated in
the spectrogram
176
to moulding also results in the disappearance of the essential IR features of the
mould fouling. Re-milling of the raw compound results in a reappearance of these
features. We believe that a small amount
of excess oil migrates to the surface of the
raw rubber sheets, thus preventing the
formation of new substances essential
for mould fouling. Re-milling of uncured
rubber sheets results in the reabsorption
of this excess oil into the compound.
Fourier IR spectroscopy detects the organic components of mould fouling. The
use of Auger spectrometry and X-ray diffraction shows the presence of inorganic
components. Fig. 2 shows the Auger
spectrogram of the thin layer of mould
fouling of compound A1 on the surface
of metal A after N ¼ 283 mouldings. In
addition to peaks corresponding to the
steel itself – reflecting its composition, indicated as Fe, Cr, Mn – peaks are visible
which correspond to the mould fouling
and are indicated as Zn and S (at E ¼ 1
and 8.7 KeV for Zn and E ¼ 3.3 KeV for
S). Fig. 3 shows the X-ray diffraction pattern of the same thin layer of mould fouling on metal A. In addition to peaks corresponding to steel itself again (at angles
2H ¼ 458, 2H ¼ 488 and 2H ¼ 658)
peaks are shown which correspond to
mould fouling (at 2H ¼ 298 and
2H ¼ 578). These peaks are characteristic of crystals of ZnS (of cubic crystal
Fig. 3. X-ray diffraction patterns of the thin layer of mould fouling on
the surface of the small plates of metal A after N ¼ 283 mouldings of
compound A1 at Tv ¼ 200 8C; numbers indicated at the peaks are
lattice spacings in Ȧ
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 4/2003
Mould Fouling of EPDM Rubber Compounds
Fig. 4. DSC thermograms: powder of
mould fouling of
A12: 4.1 – 1st and 4.2
– 2nd scan; dry residue of extract of
mould fouling of
A12: 4.3 – 1st and 4.4
– 2nd scan
structure). The data obtained by both Auger spectroscopy and X-ray diffraction
therefore confirm the presence of ZnS
in the mould deposit of the carbon
black-filled rubber compound A1 cured
with the sulphur vulcanizing system. It
is commonly assumed that ZnS is formed
during sulphur vulcanization of rubbers
[16].
As an additional check, mould fouling
of compound A12 based on Russian
EPDM SKEPT-50 with a sulphur vulcanizing system of a different composition, and
filled with carbon black was investigated.
The Fourier IR transmission spectrum of
this mould fouling, applied as powder to a
KBr crystal, was also similar to that of the
spectra of the mould fouling discussed
above. Despite the differences in compound recipe, therefore, the composition
of mould fouling as seen in IR spectra is
almost independent of the EPDM type
and the quantity and composition of
the sulphur vulcanizing system.
A DSC thermogram of this mould fouling recorded in the heating mode is
shown in Fig. 4, curve 1. There are multiple endothermic peaks within the temperature range between 200 and
230 8C: a wide peak and a narrow one.
The temperatures of these far exceed
the melting temperature of Zn stearate
and fall far below that of ZnS, as well
as below the decomposition temperature
of the zinc salt of MBT, which may form
during the vulcanization process [16]. It
is possible that the source of these peaks
is the presence of some other organic Zn
salt in the mould fouling.
The exothermic peak at about 240 8C
is evidence of a change in the mould fouling composition upon heating. The ab-
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 4/2003
sence essentially of any exothermic and
endothermic peaks from the thermogram
obtained in a second scan – curve 2 in
Fig. 4 – confirms this conclusion. The decomposition temperature of EPDM rubber, particularly in the presence of other
compounding ingredients, is known to
be far higher than 240 8C. It is therefore
unlikely that, for example, the high molecular weight fraction of the rubber itself
gives rise to this exothermic peak.
A thin layer chromatogram of the soluble fraction – extract – of the mould fouling shows the presence of sulphur, plasticizer oil and some of the stearic acid in
the extract, on top of two unidentified
substances. In a DSC thermogram of
the dry residue of this extract almost
the same exothermic and wide endothermic peaks are present as in the initial
mould fouling: curves 3 and 4, Fig. 4.
Only the narrow endothermic peak at
about 240 8C has disappeared. This
means that an essential part of the mould
fouling remains in the insoluble part of the
fouling.
Qualitative chemical analysis reveals
the presence of Zn ions and sulphide
ions in the insoluble residue of the mould
fouling. This confirms the importance of
ZnS as the main inorganic component
of mould fouling of the rubber compounds containing the sulphur vulcanizing system.
Gas chromatography combined with
mass spectroscopy provided no further
evidence of the chemical composition
of the mould fouling because its constituents were non-volatile.
White filler-loaded compounds vs.
carbon black loading
The first traces of mould fouling formation
at Tv ¼ 200 8C for the Keltan 312-based
rubber compound A2 containing only
white fillers are already observed at the
very first moulding, even with a sulphur
vulcanizing system. “Dragging” of mould
deposit from the metal plates onto the
rubber surface starts at N ¼ 9. A much
more intense colour change in the free
edge surfaces of the large metal plates,
which are not in contact with rubber compounds, is also seen. The rate of mould
fouling formation is almost two orders
of magnitude faster for the white fillerloaded than for the carbon black-loaded
compounds. This difference in rate was
177
Mould Fouling of EPDM Rubber Compounds
confirmed consistently throughout this
study with the other compound recipes
as well, in actual moulding experiments
as well as in model mouldings.
The Fourier IR spectra of the wash-off
from rubber sheets of this compound A2
are practically identical to that of A1:
Fig. 1e. The only difference is that all
the features characteristic of the spectra
of mould fouling after many moulding cycles now appear as early as N ¼ 4. The Xray diffraction pattern of the layer of
mould fouling of this rubber compound
on the surface of the metal plate at
N ¼ 10 is also similar to that of A1 at
N ¼ 283. Again it indicates the presence
of ZnS.
All these data show that the composition of the mould fouling as seen in the IR
spectra and X-ray data is independent of
the filler type, whether carbon black or
white. The more rapid mould fouling of
white filler-loaded compounds irrespective of the curing system can therefore
only be due to a faster rate of migration
of ingredients to the surface of the rubber
sample during curing. The lower rate of
diffusion through the medium containing
carbon black may in turn be considered
to result from adsorption of the low molecular weight components – such as vulcanization ingredients – onto the carbon
black. Similarly, the stronger interaction
between the rubber and the carbon black
results in a higher level of physical crosslinking, also widely known to slow down
migration.
Sulphur vulcanization vs. peroxide
vulcanization
Exchanging the sulphur vulcanizing system for peroxide does not, furthermore,
influence the composition of the organic
part of mould fouling. The shapes of the
IR spectra are identical for both sulphur
vulcanisates and peroxide vulcanisates.
Only the inorganic components as seen
by X-ray diffraction differ between the
two systems: there are no signs of ZnS
in the case of peroxide-vulcanized compounds. However, other, unknown (most
probably organic) Zn salts still appear to
be present.
Influence of the EPDM type
In compounds A10 and A9 the low ethylene-containing amorphous EPDM Kel-
178
tan 312 is replaced with the high-ethylene
crystalline EPDM Keltan 378, in the white
filler loaded, sulphur-cured recipe. Model
moulding, i. e. heating these compounds
between two metal plates in a thermostatic oven under low pressure, shows
an insignificant increase in the rate of
mould fouling formation when changing
from a low to a high ethylene content.
The composition of the organic part of
the mould fouling is again the same.
The Fourier IR transmission spectra of
films of the pure, non-compounded
EPDM rubbers Keltan 312 and Keltan
378 – dissolved in chloroform and applied
to a KBr crystal – are very similar. The
spectrum of Keltan 312 is shown in
Fig. 5a. In addition to the bands around
2937 and 2852 cmÿ1 common to all hydrocarbons, bands are present at 1377
and 1462 – 1469 cmÿ1, the doublet characteristic of EPDM (as well as of plasticizer oil; see below). These bands correspond to similar bands of the mould fouling in the initial stages of moulding. An
absorption band is also visible within
the range 720 – 730 cmÿ1, which also occurs in the spectra of mould fouling. Exposing films of pure Keltan 312 and 378
between metal plates in the oven (model
moulding at 180 8C) results in the appearance of a band at 1407 cmÿ1. Other
changes also occur, as seen in Fig. 5b.
This indicates that mould fouling formation can even result from moulding of
the pure EPDM rubber itself. Only the
rate of formation is slightly higher for Keltan 312 than for Keltan 378.
Extraction of the EPDM rubbers, which
results in the removal of low molecular
weight portions of rubber as well as impurities, prevents the formation of mould
fouling, especially after a great many
moulding cycles. Spectra of the “washoff’s” from the surfaces of metal plates
after exposure (model moulding) to these
extracted EPDM’s for a contact time
tv ¼ 3 h at 180 8C show only traces of
fouling. These are by no means comparable to those of fully compounded compositions. It is an indication that only low
molecular weight components are responsible for the formation of mould fouling of pure rubber. Because they are contained at low levels in both Keltan 378
and Keltan 312, the formation of mould
fouling due to the pure rubber is either
low or negligible.
Effect of the choice of extender oil
Extender oils are included in rubber compounds as plasticizing components. In
the case of EPDM rubbers paraffinic
oils are most commonly used owing to
their good compatibility with the saturated olefinic nature of EPDM. Two types
of paraffinic oil were investigated from
among the range available: Sunpar
2280 and Sunpar 150, which have approximately the same composition – aromatic, naphthenic and paraffinic groups –
but differ in viscosity. Sunpar 2280 is the
high-viscosity and Sunpar 150 the lowviscosity variant.
The Fourier IR transmission spectra of
the pure oils Sunpar 2280 and 150 are
identical and are similar to those of
EPDM rubbers (Fig. 6a). This is obviously
due to the similarity of the chemical structure of EPDM rubber and these paraffinic
oils. No change in the shape of their
spectra, nor of their viscosity resulted
from heating of the pure oils; only their
colour darkened. However, if these oils
are heated in contact with metal plates
some changes which are typical of mould
fouling observed after a great many
moulding cycles appear in the shape of
the spectra. The earliest observed occurrence is with Sunpar 150 (Fig. 6b). This
indicates that the composition changes
in the plasticizer oils which are typical
of mould fouling arise during heating in
contact with metal surfaces. An accelerating, catalytic effect by these metal surfaces is evident.
The difference in mould fouling tendency between Sunpar 2280 and Sunpar
150 is demonstrated by comparing compounds A7 and A10, which were obtained by the same mixing procedure
and then exposed between metal plates
in the thermostatic oven (model moulding) for the same time at the same temperature. Replacing Sunpar 2280 with
Sunpar 150 results in a fivefold increase
in the fouling rate: Fig. 7.
A surprising effect related to the choice
of oil – mentioned earlier – is that storage
of the 1 mm-thick sheets of compound at
room temperature prior to moulding affects mould fouling. The essential features of the IR spectra of fouling after
heating between metal plates no longer
appear if the compound is stored for
longer than 2 weeks at room temperature
in the case of compound A7 – containing
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 4/2003
Mould Fouling of EPDM Rubber Compounds
Fig. 5. Fourier IR transmission spectra of: 5a – film of virgin Keltan
312 on the surface of a KBr crystal; 5b – chloroform “wash-off” from
surfaces of plates of metal C, after low pressure contact with Keltan
312 at Tv ¼ 180 8C for 2.5 h
the low-viscosity Sunpar 150 – or 3
months in the case of rubber compounds
containing Sunpar 2280. This lends support to the theory that an excess of oil,
which migrates to the surface of the
raw rubber sheets, prevents the formation of substances responsible for mould
fouling.
The composition of compound A11,
Tab. 1 is for comparison with that of the
actual rubber compound A1. These compounds differ in that A11 contains no extender oil, no white filler and only half the
amount of carbon black. A comparison of
Fig. 8, which shows a chloroform washoff from the metal plates after moulding
of this rubber compound, with
Figs. 1b – d for the wash-off’s from compound A1 (as well as with IR spectra of
other oil-containing compounds) shows
no similarity between essential parts of
the spectra. This result also supports
the conclusion that the plasticizing oil
Fig. 6. Fourier IR transmission spectra of: 6a – virgin plasticizer oil
Sunpar 150 on the surface of a KBr crystal; 6b – chloroform “washoff’s” from surfaces of metal plates after heating of Sunpar 150 at
180 8C for 2.5 h on a metal C
has a major influence on the formation
of mould fouling.
Effect of other compounding ingredients
A comparison of the effects of the other
compounding ingredients – for example
a change in the structure, from a corpuscular to a lamellar structure of the inactive
white filler – shows them to be negligible,
at least in the case of the compounds
prepared by low-rate milling. This corresponds to similar observations made by
Oggermüller and Risch [17], who also
found relatively small differences in the
mould fouling tendency between corpuscular whiting and a Sillitin Z86 consisting
of a combination of corpuscular and lamellar primary particles. This contrasts
with a fully plate-like lamellar filler, such
as kaolinite, which showed a greatly aggravated fouling effect.
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 4/2003
Influence of the processing
conditions
The rate of formation of mould fouling is
influenced by the processing method.
The rate is lower for rubber compounds
prepared by low-rate mill-mixing than
for those prepared by Banbury mixing.
A surprising side-effect is the optimum
vulcanizing time observed for rubber
compounds having the same composition but mixed differently – low-rate mill
mixing vs. high-rate Banbury mixing –,
which is twice as long and is reflected
in tv.
Despite the difference in the fouling
rate and the behaviour of these compounds, the mould fouling composition
remains the same. The vulcanizing temperature Tv also has no effect on the
mould fouling composition, only on the
rate of its formation. Comparison of the
data for Tv ¼ 180 8C and Tv ¼ 200 8C
179
Mould Fouling of EPDM Rubber Compounds
Fig. 7. Relative
density D/Do of absorption band
1401 – 1407 cmÿ1 in
Fourier IR transmission spectra of
“wash-off’s” from
surfaces of small
plates of metal A,
after heating in an
oven at Tv ¼ 180 8C
for 2.5 h in contact
with rubber compounds A7 and A10
of mould fouling of rubber compound
A10.
The differences in composition between the various steels are indicated
in Tab. 4, as is their surface roughness.
The data in Fig. 9 support the concept
that it is primarily the chemical composition of the steel, which has the catalytic
effect on mould fouling. The next step
is to relate this to the elemental composition of the steel alloys used, which is also
given in Tab. 4. The presence of Ni and a
low level of Mn in Steel B are highly conspicuous, being associated with the most
fouling except for Steel R, while the presence of V and W, respectively, in Steel C
is associated with the least fouling of all.
The major differences in mould fouling
as a function of the steels used are very
interesting. However, insufficient data
was obtained to enable firm conclusions
to be drawn. This is a more difficult area of
investigation because the possibilities for
varying the composition of the steels are
restricted quite simply by the need to use
commercially available types. Nevertheless, we believe the data obtained are
of sufficient interest to warrant further research.
Model mixtures for further investigation of chemical interactions between compounding ingredients
Fig. 8. Fourier IR
transmission spectrum of chloroform
“wash-off” from
surfaces of metal
plates after model
moulding of rubber
compound A11 at
Tv ¼ 180 8C for
tv ¼ 2 h
shows that the increase of 20 8C accelerates mould fouling by a factor of more
than 2.5.
The earlier conclusion about the lower
rate of mould fouling in the presence of
carbon black relative to white fillers, in
the case of rubber compounds with
both sulphur vulcanization and peroxide
vulcanization, remains valid irrespective
of variations in the processing method.
180
Metal types
The metal type also exerts no influence
on the mould fouling composition. However, it does exert a powerful influence on
the rate of formation. The amount of
mould fouling deposited decreases in
the following sequence: R, B, A, C, as
was seen in a comparison (Fig. 9) of
the relative densities of the IR-absorption
bands at 1401 – 1407 cmÿ1 in the spectra
In order to study the effect of the separate
compounding ingredients in even more
detail, in particular the effect of the extender oil and the curatives, model mixtures
B1 – B5 containing no rubber and fillers
were prepared: see Tab. 3. Model mixture
B1 is completely oil-free. The IR spectrum
of its mould fouling after heating on metal
plates was recorded directly from powder
applied to a KBr pellet: see Fig. 10a. It differs significantly from that of actual mould
fouling, as recorded earlier. This is yet
more evidence of the importance of the
extender oil to the formation of mould
fouling.
Model mixture B2, a mixture of extender oil Sunpar 2280 and vulcanizing
agents, shows an increase in viscosity
when heated at 180 8C. In the Fourier
IR transmission spectrum of this mixture
after heating for 10 min. at 180 8C some
additional bands of low intensity appear
alongside the bands typical of oil. Of particular significance is the slight trace of a
band at 1401 cmÿ1, see Fig. 10b. This is
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 4/2003
Mould Fouling of EPDM Rubber Compounds
Fig. 9. Relative
density D/Do of absorption band
1401 – 1407 cmÿ1 in
Fourier IR transmission spectra of
“wash-off’s” from
surfaces of small
metal plates after
heating rubber
compound A1 in an
oven at Tv ¼ 180 8C
for 2.5 h in contact
with metals
Fig. 10. Fourier IR
transmission spectra of: 10a – powder
of fouling of model
mixture B1 after
heating at 180 8C for
0.5 h on metal B,
applied to the surface of a KBr crystal;
10b – model mixture
B2 after heating at
180 8C for 10 min;
10c – chloroform
“wash-off” from
surfaces of metal
plates after model
moulding at
Tv ¼ 180 8C for
21.5 h of model
mixture B4
further evidence of a chemical reaction
between the oil and the vulcanization ingredients and suggests that the origin of
the band at 1401 – 1407 cmÿ1 may be
sought in these reaction products.
Model mixture B3 and its counterpart
B4 – somewhat lower in oil content for
reasons of reproducibility – resemble
most closely the composition of the original compounds A1 and A2. There is a si-
milarity between the spectra of chloroform wash-off’s from these compounds
after heating on metal plates for
tv ¼ 21.5 h and that of actual mould fouling, as is shown in Fig. 10c. This result
shows the importance of catalytic effects
of the steel surface for the interaction between oil and curatives.
A thin layer chromatogram of the solvent extract of the fouling deposit of mix-
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 4/2003
ture B3 on the metal plates after
tv ¼ 21.5 h at 180 8C was prepared. For
comparative purposes, the chromatograms of the oils, and those of sulphur
and stearic acid, were prepared as well.
Oil gives a characteristic spot at
Rf ¼ 0.73 and sulphur at Rf ¼ 0.64.
This allows an unequivocal assignment
of the spots in the chromatogram of B3
to the original substances. After curing,
the spot of pure sulphur has disappeared,
and the oil spot and the spot which relates to stearic acid are still there, along
with a certain amount of immobile material which remains at the baseline.
Thin layer chromatograms of solvent
extracts of the deposits of mixtures B4
and B5 on the metal plates after exposure
at 180 8C for different curing times tv
show the development of the spots
over time. The initial TLC pictures of the
model mixtures with both plasticizers
are the same, as are the IR transmission
spectra. The change in density d of specific spots for oil, MBT and TMTD as well
as the total mass is shown in Figs. 11a –
b. The lower viscosity oil Sunpar 150 in
mixture B5 in Fig. 11b shows a much
higher rate of change of total mass of soluble material after extraction than Sunpar
2280 in Fig. 11a. The change in density of
the spots which relate to the oils, i. e. the
rate of decrease of their (soluble) content
in the course of heating, is higher in the
case of mixture B5 with Sunpar 150
than for mixture B4 with Sunpar 2280.
By contrast, the rate of change of the
MBT moiety is, somewhat unexpectedly,
much lower with Sunpar 150 than with
Sunpar 2280. The reason for this is at
present unclear. It would tend to indicate
that the rate of fouling formation is determined by the rate of oil diffusion, rather
than by the rate of their chemical reactions with the vulcanization accelerators.
Chemical analysis of the mixtures B4
before any heating takes place shows
that 53 % of the original virgin mixture reappears as soluble matter in the extract,
as opposed to 84 % if only ZnO were considered as insoluble. This means that interaction between the components in the
model mixtures is already taking place at
room temperature. It is interesting that almost the same amount of soluble material is found in the fouling itself which is obtained after heating these mixtures in
contact with metal plates for 21.5 h:
55 – 45 %. During heating the combined
181
Mould Fouling of EPDM Rubber Compounds
Fig. 11. Relative changes with heating time tv at Tv ¼ 180 8C of the total mass and the relative densities of the spots on thin layer chromatograms of solvent extracts of model mixtures: 11a – B4, containing Sunpar 2280; and 11b – B5, containing low-viscosity Sunpar 150
mass of all soluble extractable components of mixture B4 decreases further
to 80 % of its initial value before heating:
see Fig. 11a. Of the insoluble non-extractable matter only 15 % by mass remains
after heating in an oven at 600 8C. H2S is
released after treatment of this insoluble
residue with HCl, much as in the case
of actual mould fouling. This supports
the conclusion that ZnS is formed not
only in the actual moulding compounds,
but also in these model mixtures. An Auger spectrogram of the fouling on the surfaces of the metal plates after heating of
model mixture B3 at Tv ¼ 180 8C for
tv ¼ 21.5 h confirms this conclusion: it
is similar to that of actual mould fouling.
Conclusions regarding the mechanism of mould fouling
All data show the key role played by the
plasticizer oil in the formation of mould
fouling. Some low molecular weight fractions of the pure EPDM rubbers – which
have a chemical structure similar to the
oils – do contribute, but only slightly, to
the formation of mould fouling. However,
because of the very low content of such
low molecular weight material in EPDM
rubbers such as Keltan 312 and Keltan
378, this effect can be disregarded in
the case of fully filled, technical rubber
compounds.
Chemically altered constituents of the
plasticizer oil – resinous substances
182
formed by reaction with the curing system during heating in contact with the
metal surface of the mould – are the
source of the soluble/extractable organic
part of mould fouling. The insoluble, inorganic part is ZnS which forms when sulphur-containing vulcanizing systems are
used [16]. According to recent data presented by Fraser et al. [13], ZnS is also
found in mould fouling of sulphur-containing rubber compounds of rubbers
other than EPDM.
Vulcanization accelerators and other
compounding ingredients migrate to the
interface between the rubber compound
and the metal of the mould, entrained on
the shoulder of the plasticizer oil. The difference in viscosity between the two oils
investigated results in different migration
rates. This is the major reason for the
higher rate of formation of mould fouling
by rubber compounds containing Sunpar
150, compared with those containing
Sunpar 2280. A higher rate of oil migration in rubber compounds containing
only white fillers than in rubber compounds containing both white and black
fillers is also the most probable cause of
the higher rate of mould fouling formation
by the white filler compounds. The effect
of the processing method and of the
duration of storage of the compounds
before moulding can also be related to
differences in the migration rate of the
plasticizer oils.
The metal of the mould, which acts as
a substrate during the formation of mould
fouling, has a catalytic effect on the alteration of the chemical structure of the
oil, i.e. on the formation of the organic
part of mould fouling. The catalytic activity of the metal is related to its composition. Further study is required in order to
relate the level of catalytic activity to the
particular composition of the steel. Surface roughness of the steel has been
shown to have no conclusive effect other
than to increase the contact area and
consequently the potential quantity of
fouling collected.
Processing variables such as vulcanizing temperature Tv, vulcanizing time tv,
pressure and the metal type of the mould,
as well as the composition of the rubber
compound itself – oil type and type of filler
– do not influence the composition of the
mould fouling itself. Differences are only
seen in the rate of formation of mould
fouling.
Fig. 12 shows a semi-quantitative
summary picture of the relative importance of the various factors investigated
in this study to the rate of formation of
mould fouling.
Acknowledgements
The authors wish to acknowledge the financial support provided by DSM Elastomers for this work.
They are indebted to L. K. Berents for the Fourier IR
measurements and the preparation of the model mixtures; N. M. Zorina for the DSC experiments, light microscopy and repeated transfer mouldings; B.I. Revyakin for compounding; L. N. Gribanova for the TLC
experiments and chemical analyses; R. I. Kabetova
for untiring assistance; and all of the above, as well
as Yu. G. Chekishev, O. A. Govorova, A. A. Lapshova
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 4/2003
Mould Fouling of EPDM Rubber Compounds
Fig. 12. Relative influence of the various factors investigated on the acceleration of mould
fouling formation:
12.1 – Tv: 200 8C vs.
180 8C; 12.2 – tv increased by a factor
of 2.5; 12.3 – metal B
vs. metal C; 12.4 –
Sunpar 150 vs. Sunpar 2280; 12.5 – Keltan 378 vs. Keltan
312; 12.6 – white filler
Sillitin Z86 vs. Omya
BSH; 12.7 – sulphur
vulcanization vs.
peroxide vulcanization; 12.8 – white fillers vs. carbon black
and Z. N. Nudelman for fruitful discussions. They are
also grateful to E. N. Vlasova and N. B. Dyakonova
(NTC NIIChermet) for X-ray diffraction measurements, and A. E. Chalich and A. A. Abbasov (IFCh
RAN) for the Auger spectrometry.
Finally, they acknowledge the cooperation of their
colleagues at DSM Elastomers Europe RATD for
the preparation of the compounds; and of Mr. Vadim
Gaevoi of DSM Moscow for liaising between the two
parties involved in this research.
Glossary
DCPD
DTDM
ENB
MBT-80
Dicyclopentadiene
4,4’-Dithiodimorpholine
Ethylidene norbornene
2-Mercaptobenzothiazole,
80% masterbatch
N
Number of mouldings
PEG
Polyethylene glycol
Perkadox 14-40 2,5-Bis(tert.-butylperoxy)-isopropyl benzene,
40 % masterbatch,
trademark of Akzo Company
S-80
Sulphur, 80 % masterbatch
TEA
Triethanolamine
TMTD-80 Tetramethylthiuram disulphide, 80 % masterbatch
vulcanization time (min)
tv
Vulcanization temperature
Tv
(8C)
Literature
[1] Maclean, A., Rapra Members Journal 2 (1974)
296.
[2] Sommer, J. G. et al., Rubber Chem. Techn. 49
(1976) 1129.
[3] van Pul, J. P., Rubber Sales Office, Dutch State
Mines, the Netherlands (1981).
[4] Ludwig, H. J., Gummi Asbest Kunststoffe 35
(1982) 72.
[5] Larsen, L. C. et al., Rubber and Plastics News
11 (1982) 12.
[6] Menges, G. and Benfer, W., Gummi Asbest
Kunststoffe 36 (1983) 161.
[7] Reed, D. and School, R., Eur. Rubber J. 166
(1984) 26.
[8] Grossman, R. F. and McKane, F. W., 131st ACS
Rubber Div. Meeting Montreal, 26 – 29 May
1987, paper # 13.
[9] Reeves, L. A. et al., Kautschuk Gummi Kunststoffe 45 (1992) 369.
[10] Meirtoberens, U. et al., Int. Pol. Sci. Techn. 21
(1994) T/1-9.
[11] Yamaguchi, K. and Yukawa, A., Int. Pol. Sci.
Techn. 21 (1994) T/38-49.
[12] Van Baarle, B., Kunstst. Rubber 51 (1998) 4.
[13] Fraser, C. and Hoover, J., 156th ACS Rubber
Div. Meeting Orlando, 21 – 23 September
1999, paper # 39.
[14] DSM Elastomers Survey of Keltan EP(D)M
Grades.
[15] Synthetic rubbers, Garmanov, I.V. ed., Leningrad, Chimia, 1983.
[16] Dogadkin B. A. et al., “Chemistry of Elastomers”, Moscow, Chimia, 1981.
[17] Die-Plating, Oggermüller, H. and Risch, A., Hoffmann Mineral Technical Information 2000.
Autors
Prof. Dr. Maya Bukhina is Leading Researcher-Consultant of the Joint Stock Company “Scientific Institute of Elastomeric Materials and Articles (NIIEMI)” in
Moscow, and Deputy Editor of the journal “Kauchuk I
Rezina”.
Prof. Dr. Yuri L. Morozov is Deputy General Director of
the Joint Stock Company “Scientific Institute of Elastomeric Materials and Articles (NIIEMI)” in Moscow,
General Director of the Association “Elastomers”
and Deputy Editor of the journal “Kauchuk I Rezina”.
Peter M. van de Ven was formerly responsible for Keltan EPDM Application Development at DSM Elastomers R&D.
Prof. Dr. Jacques W.M. Noordermeer is presently employed at the University of Twente, Dept. of Rubber
Technology and as a Consultant to DSM Elastomers
R&D.
Corresponding adress:
Prof. Dr. J. W. M. Noordermeer
DSM Elastomers BV
Research þ Development
P.O. Box 1130
6160 BC Geleen, Netherlands
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 4/2003
183