Epdm Mold Fouling

Mould Fouling of EPDM RubberCompoundsM. F. Bukhina, Y. L. Morozov, Moscow (Russia)P. M. van de Ven, J. W. M. Noordermeer,Geleen (The Netherlands)

EPDM Rubber Mould Fouling

Mould fouling of EPDM rubber com-

pounds has been studied by means of

Fourier IR spectroscopy, X-ray dif-

fractometry, differential scanning ca-

lorimetry and other methods. Chemi-

cally altered constituents of the plas-

ticizer 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, inor-

ganic part is ZnS which forms when

sulphur-containing vulcanizing sys-

tems are used. Vulcanization accel-

erators and other compounding ingre-

dients migrate to the interface be-

tween 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 ef-

fect on the change in the chemical

structure of the oil.

Formverschmutzung bei EPDM-Kautschukmischungen

EPDM Kautschuk Formverschmut-zung

Formverschmutzung bei EPDM-Kau-

tschukmischungen ist mittels Fourier

IR-Spektroskopie, Rontgen-Diffrakto-

metrie, Differentialscanningkalorime-

trie und anderen Methoden untersucht

worden. Chemisch veranderte Be-

standteile des Plastifizierungsols

harzhaltige Substanzen, die sich

durch Reaktion mit dem Vulkanisati-

onssystem wahrend des Erhitzens bei

Beruhrung mit der Metalloberflache

der Pressform bilden liegen dem

loslichen, organischen Teil der Form-

verschmutzung zugrunde. Der nicht

losliche, anorganische Teil ist ZnS,

das sich bei der Verwendung von

schwefelhaltigen Vulkanisationssyste-

men bildet. Vom Plastifizierungsol ge-

tragen und mitgefuhrt, wandern Vul-

kanisationsbeschleuniger und andere

Mischungsbestandteile zur Grenzfla-

che zwischen der Kautschukmischung

und dem Metall der Pressform. Das

Metall der Pressform hat eine kataly-

tische Auswirkung auf die Verande-

rungen in der chemischen Struktur

des Ols.

Mould fouling is a deposit which forms onthe surface of metal moulds during theprocess of high-temperature mouldingand vulcanization of rubber goods. It isthe result of thermochemical changesin components of rubber compounds un-der moulding conditions.

The problems caused by mould foulingin the rubber industry are well-known. Inorder to obtain defect-free rubber arti-cles, mould fouling must be removedfrom the mould surfaces at regular inter-vals. A number of attempts have beenmade in the past to understand thecauses of mould fouling, as well as tostudy its composition in the case of differ-ent rubbers [113]. Many attempts tominimize the amount of mould foulinghave also been documented, revealingresults which are often contradictory de-spite the researchers best efforts. This isalso the case for EPDM rubber com-pounds. Particular reference is made tothe work of Sommer [2] which is amongthe earliest attempts to identify the var-ious factors involved in mould fouling oc-curring with EPDM. The objective of thatstudy was, however, primarily to investi-gate the potential of various amine com-pound ingredients to prevent mould foul-ing or to clean the mould. The recipesused in that study now look very unrealis-tic in the light of current EPDM com-pounding practice. Valuable pointers tofactors which give rise to mould foulingin the case of EPDM can neverthelessbe derived from that article.

The objective of the present work is torevisit the issue of mould fouling by EPDMon the basis of more current compoundcompositions for EPDM rubber. Theaim is to update our understanding ofthe causes of mould fouling in the caseof EPDM, as well as to estimate the rela-

tive contributions made by different com-position components and processingvariables to the formation of mould foul-ing in the case of EPDM rubber com-pounds.

Materials and methods

Rubber compounds

The process of mould fouling formationwas studied for 12 rubber compoundsbased on EPDM rubbers: Keltan 312and Keltan 378 from DSM Elastomers,as well as SKEPT-50, which is of Russianorigin. The viscosities and typical chemi-cal compositions of the EPDMs of DSMElastomers are as follows [14]:Keltan 312: Mooney viscosityML(1 4)125 8C: 33; ethylene content49 wt.%; ethylidene norbornene (ENB)termonomer content 4.3 wt.%.Keltan 378: Mooney viscosityML(1 4)125 8C: 33; ethylene content67 wt.%; ENB termonomer content 4.3wt.%.For SKEPT-50 the following characteris-tics apply [15]: Mooney viscosityML(1 4)125 8C: 30; ethylene content60 wt.%; dicyclopentadiene (DCPD) ter-monomer content 6.3 wt.%.

The compositions of the compoundsare given in Tab. 1: A1A11. Com-pounds A1/A4 and A2/A3, respectively,represent basic starting recipes of a prac-tical nature, which formed the basis forsubsequent variations. They are a blackand a white recipe, sulphur-cured andperoxide-cured. The compounds wereprepared as large masterbatches in a50-litre Banbury internal mixer FarrelBridge 3D containing the rubber,ZnO, stearic acid, TEA, PEG, fillers andoil. The mixing in the internal mixer will

ROHSTOFFE UND ANWENDUNGEN

RAW MATERIALS AND APPLICATIONS

172 KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 4/2003

be indicated as high-rate Banbury mix-ing throughout this paper. Sulphur-basedand peroxide-based curing additiveswere added separately on a two-roll millshortly before the mould fouling experi-ments. The properties of the basic com-pounds and the vulcanisates of the twosulphur-cured recipes A1 black andA2 white are given in Tab. 2.

Compounds A5A10, which were de-signed so as to enable the effects of suc-cessive individual changes in compoundingredients to be studied, were preparedindividually by special low-rate mill mix-ing: indicated as low-rate mill mixing.Compound A11 was prepared by theusual high-rate milling. The compound

compositions were set up as an experi-mental design, adjusted so as to obtainan IRHD hardness of the vulcanisatesof 6570 in all cases: two principal recipes, one black con-

taining N-550 carbon black and onewhite containing silica Ultrasil VN3as a reinforcing filler and additionallya non-reinforcing, coated whitingOmya BSH: A1 vs. A2;

sulphur cure is compared with perox-ide cure: e. g. A1 vs. A4, and A2 vs. A3;

the influence of the high ethylene con-tent of Keltan 378 EPDM is comparedwith the low ethylene content of Keltan312: A9 vs. A10;

with stearic acid present and withoutstearic acid: A3 vs. A5 and A4 vs. A6;

the effect of a whiting which combinescorpuscular and lamellar primary parti-cle structure: Sillitin Z86 vs. whiting ofspherical corpuscular structure only:Omya BSH: A8 vs. A10;

a low-viscosity paraffinic oil Sunpar150 is compared with its high-viscositycounterpart Sunpar 2280: A7 vs. A10;

a recipe without oil and white fillers andwith half the carbon black content:A11;

the influence of mixing rate, high shear-rate internal mixer vs. normal and low-rate mill mixing: A1A4 vs. A5A10; atlow-rate mill mixing the mill was run atan unusually low roll speed;

recipe A12 represents a compound ofRussian origin, which is industriallymixed in a batch internal mixer andused for the moulding of engineeringrubber articles.All the compounds were finally milled to

1 mm-thick sheets and stored.The virgin rubbers Keltan 312 and Kel-

tan 378 were extracted in MEK at roomtemperature in two steps: first for1 hour and, after removal of the extract,for a further day with a new portion ofMEK. The extracted rubbers (i. e. with

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 312 100 100 100 100 100 100 100 100 100 100 EPDM Keltan 378 100 Russian SKEPT-50 100ZnO 5 5 5 5 5 5 5 5 5 5 5 5Stearic acid 1 1 1 1 - - 1 1 1 1 1 1TEA 1 1 1 1 1 1 1 PEG 2 2 2 2 2 2 2 Carbon black N-550 89 89 89 45 Carbon black PM-50 25SiO2 Ultrasil VN3 47 47 47 47 47 47 47 Whiting Omya BSH*) 44 95 95 44 95 44 95 95 95 Sillitin Z86**) 95 Par. oil Sunpar 2280 56 47 47 56 47 56 47 47 47 Par. oil Sunpar 150 47 Naphth. oil Naphtho-plast

20

MBT-80 0.94 0.94 0.94 0.94 0.94 0.94 0.94 0.5TMTD-80 1.88 1.88 1.88 1.88 1.88 1.88 1.88 0.5DTDM 1S-80 1.88 1.88 1.88 1.88 1.88 1.88 1.88 2Perkadox 14-40 5.75 5.75 5.75 5.75 High-rate Banburymixing

Low-rate mill mixing Normal mill mixing tv at 200 8C (min) 4 10 12 8 15 8 20 20 18 20 8 8

*) Inactive white filler of normal structure **) Inactive white filler of lamellar structure

Tab. 2. Compound properties and cure properties of basic compounds A1 and A2

Property Test method Unit A1 A2

Compound Mooney viscosity at100 8C

ISO 289 ML(14) 41 59

Oscillating disk curemeter at180 8C

ISO 3417

scorch time ts2 min. 1.6 1.590 % cure time tc(90) min. 4.95 14.4ML N.m 0.59 1.21MH N.m 6.68 3.49MH ML N.m 6.08 2.28

Hardness ISO 48 IRHD 72 65

Mould Fouling of EPDM Rubber Compounds

KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 4/2003 173

the low molecular weight components re-moved) were dried; their weights still ex-ceeded 97 % of those of the virginrubbers. Both virgin and extracted rub-bers, as well as the extracts, were stu-died.

Another method of studying the effectof the separate compounding ingredientson fouling of steel is to create model mix-tures containing only a few ingredients:see Tab. 3. This series of model mixtureswas prepared by swelling small quantitiesof sulphur particles and accelerator par-ticles in oil for almost half an hour, andsubsequently mixing with ZnO and stea-ric acid in a laboratory mortar for 20 min.to obtain a homogeneous blend.

Metals used for the mould foulingexperiments

To evaluate the effect of steel quality, fourtypes of steel were used for the foulingexperiments, as indicated in Tab. 4. Steeltypes A - C represent materials of differ-ent hardness which are commonly usedfor the production of rubber moulds.Steel type R is a common grade of steelof Russian origin and is characterized bya high carbon content.

The steels were cut into small 48 9 1 mm plates. These were polishedand hardened as indicated in Tab. 4. Be-

cause it is easier to analyze small areas ofsteel in the various analytical tests thanentire steel moulds, these small steelplates were used as inlays in the actualmoulds. Large 200 200 5 mm plateswere used as the upper part of the mould:see paragraph below.

Moulding

Repeated compression mouldings ofcompounds A1A11 were performed inorder to study the quantity and composi-tion of mould fouling. A conventional cur-ing press was used. A 150 150 1.05 mm rectangular mould was con-tained between two flat steel plates, theupper plate of the mould being the largemetal plate, as described in the preced-ing paragraph. The small steel plateswere used as inserts and placed at thebottom of the mould. Compound wasplaced on top of the small plates in theform of milled sheets; the upper sidesof these sheets contacted the large steelplates. Vulcanization was carried out attemperatures Tv of 180 or 200 8C, undera load of 200 bar for the duration tv, ob-tained from rheometer cure curve as re-presenting the optimum vulcanizationtime tc(90) of the particular rubber com-pound. Tab. 1 shows the vulcanizationtimes tv as employed at 200 8C. A variable

number, N, of mouldings were made inorder to record the evolution of the foul-ing.

Besides normal moulding, what weshall term model moulding was also ap-plied: a piece of rubber compound wasplaced between two small metal platesunder load and heated in an oven at atemperature of generally 180 8C. Thepressure obtainable under these condi-tions was obviously far lower than in ac-tual moulding. This sort of model mould-ing was used to study the effects of theseparate compounding ingredients. Inthe case of the model mixtures ofTab. 3, small aliquots were also heatedbetween two small metal plates in theoven.

The metal plates on which the depositshad formed were used for analytical pur-poses. The weight of the deposits on themetal plates was determined at intervals.The weight changes were commonly ex-tremely low. The metal plates werefurthermore washed off with solventssuch as chloroform or methyl ethyl ketoneand the wash-offs subjected to analy-tical investigation. The purity of thechloroform and MEK had been checkedin advance by means of IR, so as to en-sure that no peaks occurred in thosespectral areas which were of interest. Si-milarly, the surfaces of the cured rubberfacing the small metal plates and thelarge plate on the top of the mouldwere also washed off and the wash-offs analyzed in order to compare thematerial which been deposited on thesteel with that which adhered to the rub-ber surface.

Analytical techniques

The mould fouling obtained, either in thepure form or in the form of wash-offs,

Tab. 3. Recipes of model mixtures

Mixture number

Ingredient (g) B1 B2 B3 B4 B5

ZnO 5 5 5 5Stearic acid 1 1 1 1Sunpar 2280 4.7 5 20 Sunpar 150 20MBT-80 1 1 1 1 1TMTD-80 2 1.9 1 2 2S-80 2 2 2 2 2

Tab. 4. Steel types used for the fouling experiments

Steelcode

Materialcode

DIN classifi-cation

C Si Mn Cr(%)

Mo Ni V W Polishing (lm)

A 1.2311 40CrMnMo7 0.42 0.30 1.50 2.0 0.2 N2-N30.050.1

B 1.2767 X45NiCrMo4 0.45 0.25 0.40 1.35 0.25 4.0 N2-N30.050.1

C 1.2510 100MnCrW4 1.00 0.30 1.10 0.6 0.1 0.6 N50.4

R Steel 3*) **) 0.22 0.06 0.8 0.3 0.3 Different

*) Steel of common use, GOST 380-71 **) Similar to FeE235BFU



was studied by means of various meth-ods: Fourier transform IR spectroscopy (FT-

IR), thin layer chromatography (TLC), with a

mixture of petroleum ether, diethylether and acetic acid in a ratio70:30:5 as eluent and a 1% solutionof 2,6-dichloroquinone-4-chloramidein 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 reflec-tion (ATR) of surfaces mode - was foundto be the most powerful investigativetechnique.

Results and discussion

Carbon black-filled compounds,sulphur-vulcanized

Repeated compression moulding atTv 200 8C of the Keltan 312-based,carbon black-containing rubber com-pound with the sulphur vulcanizing sys-tem A1, at all times using the same steelplates of metal B, shows that in generalthe first visible traces of mould fouling ap-pear after 3040 moulding cycles. A per-sistent deposit forms after about 100moulding cycles. The quantity of moulddeposit increases up to about 225 cycles.Further moulding results only in a visiblecolour change in both the small and thelarge metal top plates. The colour changein the large metal top plates is particularlyintense in the free edges of their surfaceremote from the mould cavity, which

made no direct contact with the rubbercompound.

The ATR spectrum of the small platesof metal B after N 222 mouldings isshown in Fig. 1a. This spectrum exhibitsseven characteristic spectral regions,which are designated I to VII. The trans-mission spectrum of wash-off from thesurface of the vulcanized rubber sheetafter almost the same number of mould-ings N 227 essentially shows the samecharacteristic bands, as shown in Fig. 1b.This result opens up the possibility ofusing the transmission spectra of wash-offs from the surface of vulcanized rubbersheets instead of the reflection spectra ofthe deposits on the metal plates. Thespectra of the wash-offs are of higherquality. It also means that some of themould fouling is dragged from the metalsurface by the rubber sheets duringmould release.

Fig. 1. 1a Fourier IR spectrum in ATR mode of the small plates of metal B after N 222 mouldings at Tv 200 8C of compound A1; 1b transmission spectrum of MEK wash-off from surface of rubber sheet of A1 after N 227; 1c after N 292; 1d transmission spectrum ofMEK wash-off from the large metal top plate of metal B after N 300; 1e transmission spectrum of wash-off from rubber compound A2 atTv 200 8C after N 4



Of the characteristic spectral regions inthese spectra, the absorption bands at2853 and 2923 cm1 (region II in Fig. 1a)are specific to hydrocarbons. The otherbands are characteristic of the composi-tion of the mould fouling. Two of these re-gions, region I comprising the wide ab-sorption band at 31003500 cm1, andregion IV comprising the absorption bandsbetween 1385 and 1460 cm1, are parti-cularly important. In the initial stages ofmould fouling region IV is a doublet of re-latively narrow bands. In the course offurther mould fouling, the shape of this re-gion changes: a new intermediate absorp-tion band appears at 14011407 cm1

between the two original bands in regionIV initially as a shoulder and the doubletbecomes a triplet: see Fig. 1b and 1c.These changes in the spectra show theformation of new substances in the courseof moulding which appear to be character-istic of the mould fouling. The chemicalnature of this adsorption band at 14011407 cm1 has remained unclearthroughout this study, as it correspondsto no commonly documented IR spec-trum. It is believed to result from the forma-tion of a new substance, e. g. from cycli-zation of some of the oil molecules aswell as of low molecular weight compo-nents of the rubber due to protractedheating and the catalytic effect of the me-tal surfaces.

The density of this new absorptionband between 1401 and 1407 cm1 en-ables the formation of mould fouling onthe number of mouldings N to be evalu-ated semi-quantitatively by measuringthe relative density (D/Do) of this newband in the triplet. Do is the optical densityof the absorption band at 2923 cm1 inregion II, which is used as an internalstandard. There is a slight increase inthe density of the new middle band inthe triplet up to about N 200. At aroundN 300 there is a sharp increase in D/Do,and after about 300 mouldings a de-crease can be observed. At the sametime, the visible amount of mould foulingalso decreases, suggesting that somesort of self-cleaning of the steel takesplace.

The visual appearance of the mouldfouling on the surface of both the smalland the top metal plates changes in thecourse of storage of the metal plates aftermoulding. Storage of vulcanized rubbersheets also results in some changes incomposition of the fouling on their sur-face, as is revealed by IR spectroscopy:the middle band in the characteristic tri-plet disappears again. In order to obtaindata with high reproducibility, therefore, aprecise, short storage time has to be ob-served prior to the IR measurement. In-creasing the storage time of the raw, asyet unvulcanized, rubber compound prior

to moulding also results in the disappear-ance of the essential IR features of themould fouling. Re-milling of the raw com-pound results in a reappearance of thesefeatures. We believe that a small amountof excess oil migrates to the surface of theraw rubber sheets, thus preventing theformation of new substances essentialfor mould fouling. Re-milling of uncuredrubber sheets results in the reabsorptionof this excess oil into the compound.

Fourier IR spectroscopy detects the or-ganic components of mould fouling. Theuse of Auger spectrometry and X-ray dif-fraction shows the presence of inorganiccomponents. Fig. 2 shows the Augerspectrogram of the thin layer of mouldfouling of compound A1 on the surfaceof metal A after N 283 mouldings. Inaddition to peaks corresponding to thesteel itself reflecting its composition, in-dicated as Fe, Cr, Mn peaks are visiblewhich correspond to the mould foulingand are indicated as Zn and S (at E 1and 8.7 KeV for Zn and E 3.3 KeV forS). Fig. 3 shows the X-ray diffraction pat-tern of the same thin layer of mould foul-ing on metal A. In addition to peaks cor-responding to steel itself again (at angles2H 458, 2H 488 and 2H 658)peaks are shown which correspond tomould fouling (at 2H 298 and2H 578). These peaks are characteris-tic of crystals of ZnS (of cubic crystal

Fig. 2. Auger spectrum of the thin layer of mould fouling on the sur-face of plates of metal A after N 283 mouldings of compound A1 atTv 200 8C; elements corresponding to the peaks are indicated inthe spectrogram

Fig. 3. X-ray diffraction patterns of the thin layer of mould fouling onthe surface of the small plates of metal A after N 283 mouldings ofcompound A1 at Tv 200 8C; numbers indicated at the peaks arelattice spacings in A



structure). The data obtained by both Au-ger spectroscopy and X-ray diffractiontherefore confirm the presence of ZnSin the mould deposit of the carbonblack-filled rubber compound A1 curedwith the sulphur vulcanizing system. Itis commonly assumed that ZnS is formedduring sulphur vulcanization of rubbers[16].

As an additional check, mould foulingof compound A12 based on RussianEPDM SKEPT-50 with a sulphur vulcaniz-ing system of a different composition, andfilled with carbon black was investigated.The Fourier IR transmission spectrum ofthis mould fouling, applied as powder to aKBr crystal, was also similar to that of thespectra of the mould fouling discussedabove. Despite the differences in com-pound recipe, therefore, the compositionof mould fouling as seen in IR spectra is

almost independent of the EPDM typeand the quantity and composition ofthe sulphur vulcanizing system.

A DSC thermogram of this mould foul-ing recorded in the heating mode isshown in Fig. 4, curve 1. There are multi-ple endothermic peaks within the tem-perature range between 200 and230 8C: a wide peak and a narrow one.The temperatures of these far exceedthe melting temperature of Zn stearateand fall far below that of ZnS, as wellas below the decomposition temperatureof the zinc salt of MBT, which may formduring the vulcanization process [16]. Itis possible that the source of these peaksis the presence of some other organic Znsalt in the mould fouling.

The exothermic peak at about 240 8Cis evidence of a change in the mould foul-ing composition upon heating. The ab-

sence essentially of any exothermic andendothermic peaks from the thermogramobtained in a second scan curve 2 inFig. 4 confirms this conclusion. The de-composition temperature of EPDM rub-ber, particularly in the presence of othercompounding ingredients, is known tobe far higher than 240 8C. It is thereforeunlikely that, for example, the high mole-cular weight fraction of the rubber itselfgives rise to this exothermic peak.

A thin layer chromatogram of the solu-ble fraction extract of the mould foul-ing shows the presence of sulphur, plas-ticizer oil and some of the stearic acid inthe extract, on top of two unidentifiedsubstances. In a DSC thermogram ofthe dry residue of this extract almostthe same exothermic and wide endother-mic peaks are present as in the initialmould fouling: curves 3 and 4, Fig. 4.Only the narrow endothermic peak atabout 240 8C has disappeared. Thismeans that an essential part of the mouldfouling remains in the insoluble part of thefouling.

Qualitative chemical analysis revealsthe presence of Zn ions and sulphideions in the insoluble residue of the mouldfouling. This confirms the importance ofZnS as the main inorganic componentof mould fouling of the rubber com-pounds containing the sulphur vulcaniz-ing system.

Gas chromatography combined withmass spectroscopy provided no furtherevidence of the chemical compositionof the mould fouling because its constitu-ents were non-volatile.

White filler-loaded compounds vs.carbon black loading

The first traces of mould fouling formationat Tv 200 8C for the Keltan 312-basedrubber compound A2 containing onlywhite fillers are already observed at thevery first moulding, even with a sulphurvulcanizing system. Dragging of moulddeposit from the metal plates onto therubber surface starts at N 9. A muchmore intense colour change in the freeedge surfaces of the large metal plates,which are not in contact with rubber com-pounds, is also seen. The rate of mouldfouling formation is almost two ordersof magnitude faster for the white filler-loaded than for the carbon black-loadedcompounds. This difference in rate was

Fig. 4. DSC thermo-grams: powder ofmould fouling ofA12: 4.1 1st and 4.2 2nd scan; dry resi-due of extract ofmould fouling ofA12: 4.3 1st and 4.4 2nd scan



confirmed consistently throughout thisstudy with the other compound recipesas well, in actual moulding experimentsas well as in model mouldings.

The Fourier IR spectra of the wash-offfrom rubber sheets of this compound A2are practically identical to that of A1:Fig. 1e. The only difference is that allthe features characteristic of the spectraof mould fouling after many moulding cy-cles now appear as early as N 4. The X-ray diffraction pattern of the layer ofmould fouling of this rubber compoundon the surface of the metal plate atN 10 is also similar to that of A1 atN 283. Again it indicates the presenceof ZnS.

All these data show that the composi-tion of the mould fouling as seen in the IRspectra and X-ray data is independent ofthe filler type, whether carbon black orwhite. The more rapid mould fouling ofwhite filler-loaded compounds irrespec-tive of the curing system can thereforeonly be due to a faster rate of migrationof ingredients to the surface of the rubbersample during curing. The lower rate ofdiffusion through the medium containingcarbon black may in turn be consideredto result from adsorption of the low mo-lecular weight components such as vul-canization ingredients onto the carbonblack. Similarly, the stronger interactionbetween the rubber and the carbon blackresults in a higher level of physical cross-linking, also widely known to slow downmigration.

Sulphur vulcanization vs. peroxidevulcanization

Exchanging the sulphur vulcanizing sys-tem for peroxide does not, furthermore,influence the composition of the organicpart of mould fouling. The shapes of theIR spectra are identical for both sulphurvulcanisates and peroxide vulcanisates.Only the inorganic components as seenby X-ray diffraction differ between thetwo systems: there are no signs of ZnSin the case of peroxide-vulcanized com-pounds. However, other, unknown (mostprobably organic) Zn salts still appear tobe present.

Influence of the EPDM type

In compounds A10 and A9 the low ethy-lene-containing amorphous EPDM Kel-

tan 312 is replaced with the high-ethylenecrystalline EPDM Keltan 378, in the whitefiller loaded, sulphur-cured recipe. Modelmoulding, i. e. heating these compoundsbetween two metal plates in a thermo-static oven under low pressure, showsan insignificant increase in the rate ofmould fouling formation when changingfrom a low to a high ethylene content.The composition of the organic part ofthe mould fouling is again the same.

The Fourier IR transmission spectra offilms of the pure, non-compoundedEPDM rubbers Keltan 312 and Keltan378 dissolved in chloroform and appliedto a KBr crystal are very similar. Thespectrum of Keltan 312 is shown inFig. 5a. In addition to the bands around2937 and 2852 cm1 common to all hy-drocarbons, bands are present at 1377and 14621469 cm1, the doublet char-acteristic of EPDM (as well as of plastici-zer oil; see below). These bands corre-spond to similar bands of the mould foul-ing in the initial stages of moulding. Anabsorption band is also visible withinthe range 720730 cm1, which also oc-curs in the spectra of mould fouling. Ex-posing films of pure Keltan 312 and 378between metal plates in the oven (modelmoulding at 180 8C) results in the appear-ance of a band at 1407 cm1. Otherchanges also occur, as seen in Fig. 5b.This indicates that mould fouling forma-tion can even result from moulding ofthe pure EPDM rubber itself. Only therate of formation is slightly higher for Kel-tan 312 than for Keltan 378.

Extraction of the EPDM rubbers, whichresults in the removal of low molecularweight portions of rubber as well as impu-rities, prevents the formation of mouldfouling, especially after a great manymoulding cycles. Spectra of the wash-offs from the surfaces of metal platesafter exposure (model moulding) to theseextracted EPDMs for a contact timetv 3 h at 180 8C show only traces offouling. These are by no means compar-able to those of fully compounded com-positions. It is an indication that only lowmolecular weight components are re-sponsible for the formation of mould foul-ing of pure rubber. Because they are con-tained at low levels in both Keltan 378and Keltan 312, the formation of mouldfouling due to the pure rubber is eitherlow or negligible.

Effect of the choice of extender oil

Extender oils are included in rubber com-pounds as plasticizing components. Inthe case of EPDM rubbers paraffinicoils are most commonly used owing totheir good compatibility with the satu-rated olefinic nature of EPDM. Two typesof paraffinic oil were investigated fromamong the range available: Sunpar2280 and Sunpar 150, which have ap-proximately the same composition aro-matic, naphthenic and paraffinic groups but differ in viscosity. Sunpar 2280 is thehigh-viscosity and Sunpar 150 the low-viscosity variant.

The Fourier IR transmission spectra ofthe pure oils Sunpar 2280 and 150 areidentical and are similar to those ofEPDM rubbers (Fig. 6a). This is obviouslydue to the similarity of the chemical struc-ture of EPDM rubber and these paraffinicoils. No change in the shape of theirspectra, nor of their viscosity resultedfrom heating of the pure oils; only theircolour darkened. However, if these oilsare heated in contact with metal platessome changes which are typical of mouldfouling observed after a great manymoulding cycles appear in the shape ofthe spectra. The earliest observed occur-rence is with Sunpar 150 (Fig. 6b). Thisindicates that the composition changesin the plasticizer oils which are typicalof mould fouling arise during heating incontact with metal surfaces. An acceler-ating, catalytic effect by these metal sur-faces is evident.

The difference in mould fouling ten-dency between Sunpar 2280 and Sunpar150 is demonstrated by comparing com-pounds A7 and A10, which were ob-tained by the same mixing procedureand then exposed between metal platesin the thermostatic oven (model mould-ing) for the same time at the same tem-perature. Replacing Sunpar 2280 withSunpar 150 results in a fivefold increasein the fouling rate: Fig. 7.

A surprising effect related to the choiceof oil mentioned earlier is that storageof the 1 mm-thick sheets of compound atroom temperature prior to moulding af-fects mould fouling. The essential fea-tures of the IR spectra of fouling afterheating between metal plates no longerappear if the compound is stored forlonger than 2 weeks at room temperaturein the case of compound A7 containing



the low-viscosity Sunpar 150 or 3months in the case of rubber compoundscontaining Sunpar 2280. This lends sup-port to the theory that an excess of oil,which migrates to the surface of theraw rubber sheets, prevents the forma-tion of substances responsible for mouldfouling.

The composition of compound A11,Tab. 1 is for comparison with that of theactual rubber compound A1. These com-pounds differ in that A11 contains no ex-tender oil, no white filler and only half theamount of carbon black. A comparison ofFig. 8, which shows a chloroform wash-off from the metal plates after mouldingof this rubber compound, withFigs. 1bd for the wash-offs from com-pound A1 (as well as with IR spectra ofother oil-containing compounds) showsno similarity between essential parts ofthe spectra. This result also supportsthe conclusion that the plasticizing oil

has a major influence on the formationof mould fouling.

Effect of other compounding ingre-dients

A comparison of the effects of the othercompounding ingredients for examplea change in the structure, from a corpus-cular to a lamellar structure of the inactivewhite filler shows them to be negligible,at least in the case of the compoundsprepared by low-rate milling. This corre-sponds to similar observations made byOggermuller and Risch [17], who alsofound relatively small differences in themould fouling tendency between corpus-cular whiting and a Sillitin Z86 consistingof a combination of corpuscular and la-mellar primary particles. This contrastswith a fully plate-like lamellar filler, suchas kaolinite, which showed a greatly ag-gravated fouling effect.

Influence of the processingconditions

The rate of formation of mould fouling isinfluenced by the processing method.The rate is lower for rubber compoundsprepared by low-rate mill-mixing thanfor those prepared by Banbury mixing.A surprising side-effect is the optimumvulcanizing time observed for rubbercompounds having the same composi-tion but mixed differently low-rate millmixing vs. high-rate Banbury mixing ,which is twice as long and is reflectedin tv.

Despite the difference in the foulingrate and the behaviour of these com-pounds, the mould fouling compositionremains the same. The vulcanizing tem-perature Tv also has no effect on themould fouling composition, only on therate of its formation. Comparison of thedata for Tv 180 8C and Tv 200 8C

Fig. 5. Fourier IR transmission spectra of: 5a film of virgin Keltan312 on the surface of a KBr crystal; 5b chloroform wash-off fromsurfaces of plates of metal C, after low pressure contact with Keltan312 at Tv 180 8C for 2.5 h

Fig. 6. Fourier IR transmission spectra of: 6a virgin plasticizer oilSunpar 150 on the surface of a KBr crystal; 6b chloroform wash-offs from surfaces of metal plates after heating of Sunpar 150 at180 8C for 2.5 h on a metal C



shows that the increase of 20 8C acceler-ates mould fouling by a factor of morethan 2.5.

The earlier conclusion about the lowerrate of mould fouling in the presence ofcarbon black relative to white fillers, inthe case of rubber compounds withboth sulphur vulcanization and peroxidevulcanization, remains valid irrespectiveof variations in the processing method.

Metal types

The metal type also exerts no influenceon the mould fouling composition. How-ever, it does exert a powerful influence onthe rate of formation. The amount ofmould fouling deposited decreases inthe following sequence: R, B, A, C, aswas seen in a comparison (Fig. 9) ofthe relative densities of the IR-absorptionbands at 14011407 cm1 in the spectra

of mould fouling of rubber compoundA10.

The differences in composition be-tween the various steels are indicatedin Tab. 4, as is their surface roughness.The data in Fig. 9 support the conceptthat it is primarily the chemical composi-tion of the steel, which has the catalyticeffect on mould fouling. The next stepis to relate this to the elemental composi-tion of the steel alloys used, which is alsogiven in Tab. 4. The presence of Ni and alow level of Mn in Steel B are highly con-spicuous, being associated with the mostfouling except for Steel R, while the pre-sence of V and W, respectively, in Steel Cis associated with the least fouling of all.

The major differences in mould foulingas a function of the steels used are veryinteresting. However, insufficient datawas obtained to enable firm conclusionsto be drawn. This is a more difficult area ofinvestigation because the possibilities forvarying the composition of the steels arerestricted quite simply by the need to usecommercially available types. Neverthe-less, we believe the data obtained areof sufficient interest to warrant further re-search.

Model mixtures for further investi-gation of chemical interactions be-tween compounding ingredients

In order to study the effect of the separatecompounding ingredients in even moredetail, in particular the effect of the exten-der oil and the curatives, model mixturesB1 B5 containing no rubber and fillerswere prepared: see Tab. 3. Model mixtureB1 is completely oil-free. The IR spectrumof its mould fouling after heating on metalplates was recorded directly from powderapplied to a KBr pellet: see Fig. 10a. It dif-fers significantly from that of actual mouldfouling, as recorded earlier. This is yetmore evidence of the importance of theextender oil to the formation of mouldfouling.

Model mixture B2, a mixture of exten-der oil Sunpar 2280 and vulcanizingagents, shows an increase in viscositywhen heated at 180 8C. In the FourierIR transmission spectrum of this mixtureafter heating for 10 min. at 180 8C someadditional bands of low intensity appearalongside the bands typical of oil. Of par-ticular significance is the slight trace of aband at 1401 cm1, see Fig. 10b. This is

Fig. 7. Relativedensity D/Do of ab-sorption band14011407 cm1 inFourier IR trans-mission spectra ofwash-offs fromsurfaces of smallplates of metal A,after heating in anoven at Tv 180 8Cfor 2.5 h in contactwith rubber com-pounds A7 and A10

Fig. 8. Fourier IRtransmission spec-trum of chloroformwash-off fromsurfaces of metalplates after modelmoulding of rubbercompound A11 atTv 180 8C fortv 2 h



further evidence of a chemical reactionbetween the oil and the vulcanization in-gredients and suggests that the origin ofthe band at 14011407 cm1 may besought in these reaction products.

Model mixture B3 and its counterpartB4 somewhat lower in oil content forreasons of reproducibility resemblemost closely the composition of the origi-nal compounds A1 and A2. There is a si-

milarity between the spectra of chloro-form wash-offs from these compoundsafter heating on metal plates fortv 21.5 h and that of actual mould foul-ing, as is shown in Fig. 10c. This resultshows the importance of catalytic effectsof the steel surface for the interaction be-tween oil and curatives.

A thin layer chromatogram of the sol-vent extract of the fouling deposit of mix-

ture B3 on the metal plates aftertv 21.5 h at 180 8C was prepared. Forcomparative purposes, the chromato-grams of the oils, and those of sulphurand stearic acid, were prepared as well.Oil gives a characteristic spot atRf 0.73 and sulphur at Rf 0.64.This allows an unequivocal assignmentof the spots in the chromatogram of B3to the original substances. After curing,the spot of pure sulphur has disappeared,and the oil spot and the spot which re-lates to stearic acid are still there, alongwith a certain amount of immobile mate-rial which remains at the baseline.

Thin layer chromatograms of solventextracts of the deposits of mixtures B4and B5 on the metal plates after exposureat 180 8C for different curing times tvshow the development of the spotsover time. The initial TLC pictures of themodel mixtures with both plasticizersare the same, as are the IR transmissionspectra. The change in density d of spe-cific spots for oil, MBT and TMTD as wellas the total mass is shown in Figs. 11ab. The lower viscosity oil Sunpar 150 inmixture B5 in Fig. 11b shows a muchhigher rate of change of total mass of so-luble material after extraction than Sunpar2280 in Fig. 11a. The change in density ofthe spots which relate to the oils, i. e. therate of decrease of their (soluble) contentin the course of heating, is higher in thecase of mixture B5 with Sunpar 150than for mixture B4 with Sunpar 2280.By contrast, the rate of change of theMBT moiety is, somewhat unexpectedly,much lower with Sunpar 150 than withSunpar 2280. The reason for this is atpresent unclear. It would tend to indicatethat the rate of fouling formation is deter-mined by the rate of oil diffusion, ratherthan by the rate of their chemical reac-tions with the vulcanization accelerators.

Chemical analysis of the mixtures B4before any heating takes place showsthat 53 % of the original virgin mixture re-appears as soluble matter in the extract,as opposed to 84 % if only ZnO were con-sidered as insoluble. This means that in-teraction between the components in themodel mixtures is already taking place atroom temperature. It is interesting that al-most the same amount of soluble materi-al is found in the fouling itself which is ob-tained after heating these mixtures incontact with metal plates for 21.5 h:5545 %. During heating the combined

Fig. 9. Relativedensity D/Do of ab-sorption band14011407 cm1 inFourier IR trans-mission spectra ofwash-offs fromsurfaces of smallmetal plates afterheating rubbercompound A1 in anoven at Tv 180 8Cfor 2.5 h in contactwith metals

Fig. 10. Fourier IRtransmission spec-tra of: 10a powderof fouling of modelmixture B1 afterheating at 180 8C for0.5 h on metal B,applied to the sur-face of a KBr crystal;10b model mixtureB2 after heating at180 8C for 10 min;10c chloroformwash-off fromsurfaces of metalplates after modelmoulding atTv 180 8C for21.5 h of modelmixture B4



mass of all soluble extractable compo-nents of mixture B4 decreases furtherto 80 % of its initial value before heating:see Fig. 11a. Of the insoluble non-extrac-table matter only 15 % by mass remainsafter heating in an oven at 600 8C. H2S isreleased after treatment of this insolubleresidue with HCl, much as in the caseof actual mould fouling. This supportsthe conclusion that ZnS is formed notonly in the actual moulding compounds,but also in these model mixtures. An Au-ger spectrogram of the fouling on the sur-faces of the metal plates after heating ofmodel mixture B3 at Tv 180 8C fortv 21.5 h confirms this conclusion: itis similar to that of actual mould fouling.

Conclusions regarding the me-chanism of mould fouling

All data show the key role played by theplasticizer oil in the formation of mouldfouling. Some low molecular weight frac-tions of the pure EPDM rubbers whichhave a chemical structure similar to theoils do contribute, but only slightly, tothe formation of mould fouling. However,because of the very low content of suchlow molecular weight material in EPDMrubbers such as Keltan 312 and Keltan378, this effect can be disregarded inthe case of fully filled, technical rubbercompounds.

Chemically altered constituents of theplasticizer oil resinous substances

formed by reaction with the curing sys-tem during heating in contact with themetal surface of the mould are thesource of the soluble/extractable organicpart of mould fouling. The insoluble, inor-ganic part is ZnS which forms when sul-phur-containing vulcanizing systems areused [16]. According to recent data pre-sented by Fraser et al. [13], ZnS is alsofound in mould fouling of sulphur-con-taining rubber compounds of rubbersother than EPDM.

Vulcanization accelerators and othercompounding ingredients migrate to theinterface between the rubber compoundand the metal of the mould, entrained onthe shoulder of the plasticizer oil. The dif-ference in viscosity between the two oilsinvestigated results in different migrationrates. This is the major reason for thehigher rate of formation of mould foulingby rubber compounds containing Sunpar150, compared with those containingSunpar 2280. A higher rate of oil migra-tion in rubber compounds containingonly white fillers than in rubber com-pounds containing both white and blackfillers is also the most probable cause ofthe higher rate of mould fouling formationby the white filler compounds. The effectof the processing method and of theduration of storage of the compoundsbefore moulding can also be related todifferences in the migration rate of theplasticizer oils.

The metal of the mould, which acts asa substrate during the formation of mould

fouling, has a catalytic effect on the al-teration of the chemical structure of theoil, i.e. on the formation of the organicpart of mould fouling. The catalytic activ-ity of the metal is related to its composi-tion. Further study is required in order torelate the level of catalytic activity to theparticular composition of the steel. Sur-face roughness of the steel has beenshown to have no conclusive effect otherthan to increase the contact area andconsequently the potential quantity offouling collected.

Processing variables such as vulcaniz-ing temperature Tv, vulcanizing time tv,pressure and the metal type of the mould,as well as the composition of the rubbercompound itself oil type and type of filler do not influence the composition of themould fouling itself. Differences are onlyseen in the rate of formation of mouldfouling.

Fig. 12 shows a semi-quantitativesummary picture of the relative impor-tance of the various factors investigatedin this study to the rate of formation ofmould fouling.

Acknowledgements

The authors wish to acknowledge the financial sup-port provided by DSM Elastomers for this work.They are indebted to L. K. Berents for the Fourier IRmeasurements and the preparation of the model mix-tures; N. M. Zorina for the DSC experiments, light mi-croscopy and repeated transfer mouldings; B.I. Re-vyakin for compounding; L. N. Gribanova for the TLCexperiments and chemical analyses; R. I. Kabetovafor untiring assistance; and all of the above, as wellas Yu. G. Chekishev, O. A. Govorova, A. A. Lapshova

Fig. 11. Relative changes with heating time tv at Tv 180 8C of the total mass and the relative densities of the spots on thin layer chromato-grams of solvent extracts of model mixtures: 11a B4, containing Sunpar 2280; and 11b B5, containing low-viscosity Sunpar 150



and Z. N. Nudelman for fruitful discussions. They arealso grateful to E. N. Vlasova and N. B. Dyakonova(NTC NIIChermet) for X-ray diffraction measure-ments, and A. E. Chalich and A. A. Abbasov (IFChRAN) for the Auger spectrometry.Finally, they acknowledge the cooperation of theircolleagues at DSM Elastomers Europe RATD forthe preparation of the compounds; and of Mr. VadimGaevoi of DSM Moscow for liaising between the twoparties involved in this research.

Glossary

DCPD DicyclopentadieneDTDM 4,4-DithiodimorpholineENB Ethylidene norborneneMBT-80 2-Mercaptobenzothiazole,

80% masterbatchN Number of mouldingsPEG Polyethylene glycolPerkadox 14-40 2,5-Bis(tert.-butylper-

oxy)-isopropyl benzene,40 % masterbatch,trademark of Akzo Company

S-80 Sulphur, 80 % masterbatchTEA TriethanolamineTMTD-80 Tetramethylthiuram disul-

phide, 80 % masterbatchtv vulcanization time (min)Tv Vulcanization temperature

(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 StateMines, the Netherlands (1981).

[4] Ludwig, H. J., Gummi Asbest Kunststoffe 35(1982) 72.

[5] Larsen, L. C. et al., Rubber and Plastics News11 (1982) 12.

[6] Menges, G. and Benfer, W., Gummi AsbestKunststoffe 36 (1983) 161.

[7] Reed, D. and School, R., Eur. Rubber J. 166(1984) 26.

[8] Grossman, R. F. and McKane, F. W., 131st ACSRubber Div. Meeting Montreal, 2629 May1987, paper # 13.

[9] Reeves, L. A. et al., Kautschuk Gummi Kunst-stoffe 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, 2123 September1999, paper # 39.

[14] DSM Elastomers Survey of Keltan EP(D)MGrades.

[15] Synthetic rubbers, Garmanov, I.V. ed., Lenin-grad, Chimia, 1983.

[16] Dogadkin B. A. et al., Chemistry of Elasto-mers, Moscow, Chimia, 1981.

[17] Die-Plating, Oggermuller, H. and Risch, A., Hoff-mann Mineral Technical Information 2000.

Autors

Prof. Dr. Maya Bukhina is Leading Researcher-Con-sultant of the Joint Stock Company Scientific Insti-tute of Elastomeric Materials and Articles (NIIEMI) inMoscow, and Deputy Editor of the journal Kauchuk IRezina.Prof. Dr. Yuri L. Morozov is Deputy General Director ofthe Joint Stock Company Scientific Institute of Elas-tomeric Materials and Articles (NIIEMI) in Moscow,General Director of the Association Elastomersand Deputy Editor of the journal Kauchuk I Rezina.Peter M. van de Ven was formerly responsible for Kel-tan EPDM Application Development at DSM Elasto-mers R&D.Prof. Dr. Jacques W.M. Noordermeer is presently em-ployed at the University of Twente, Dept. of RubberTechnology and as a Consultant to DSM ElastomersR&D.

Corresponding adress:Prof. Dr. J. W. M. NoordermeerDSM Elastomers BVResearch DevelopmentP.O. Box 11306160 BC Geleen, Netherlands

Fig. 12. Relative in-fluence of the var-ious factors investi-gated on the accel-eration of mouldfouling formation:12.1 Tv: 200 8C vs.180 8C; 12.2 tv in-creased by a factorof 2.5; 12.3 metal Bvs. metal C; 12.4 Sunpar 150 vs. Sun-par 2280; 12.5 Kel-tan 378 vs. Keltan312; 12.6 white fillerSillitin Z86 vs. OmyaBSH; 12.7 sulphurvulcanization vs.peroxide vulcaniza-tion; 12.8 white fil-lers vs. carbon black



Date post:	09-Oct-2015
Category:	Documents
Upload:	cronorom
View:	137 times
Download:	9 times

Epdm Mold Fouling

Documents