KONSTRUKTION UND SIMULATION CONSTRUCTION AND SIMULATION 33 KGK · 4 2021 www.kgk-rubberpoint.de EPDM · automotive · light-weighting · Statistical Modeling. The automotive trend of replacing steel doors in cars with light-weight metals like Magnesium/Aluminum (Mg/Al) has a significant impact on the EPDM wea- ther-strip (WS) requirements. To avoid electrochemical degradation of the EP- DM WS and oxidation of metal surface, EPDM WS need to have 3-4 orders high- er volume resistivity (VR) than standard WS. This can be obtained reducing the amount of carbon black (CB) to below its percolation threshold, but this has an adverse effect on other properties. In this paper, the fundamentals behind the electrochemical corrosion of Mg/Al and EPDM profiles are described. It is followed up by a 4-factorial experimen- tal design that can predict the com- pounding formulation to obtain desired final properties or estimate the proper- ties achieved varying EPDM blends, CB, white filler and oil. Statistischer Ansatz für das Design von isolierenden EPDM Kautschuk- Compounds für Leichtgewichtlösungen an ‚Automotive OEMs‘ EPDM · Automotive · Leichtbau · Statis- tische Modellierung. Der Trend im Automobilmarkt, Türen aus Stahlblech durch Leichtmetalle wie Aluminum oder Magnesium (Mg/Al) zu ersetzen, hat einen Einfluss auf die An- forderungen von EPDM-Dichtprofilen. Um die Korrosion der EPDM -Pofile und Oxidation der Metalloberflächen zu verhindern, müssen EPDM- Profile eine höhere Volumenresistenz haben. Diese kann durch eine Reduzierung des Ruß- anteils unterhalb des Perkulationspunk- tes erreicht werden, aber dies beein- flusst andere Eigenschaften negativ. Figures and Tables: By a kind approval of the authors. INTRODUCTION Ethylene Propylene Diene (EPDM) rubber [1-5] is composed of randomly distribut- ed ethylene (C2), propylene (C3), and ethylidene norbornene (ENB) diene along its saturated backbone (Figure 1). EPDM compounds are composed of not only EPDM, but also of CB, white fil- lers (WF), oil and cure additives package. They can be used in many applications but their characteristics like weathering and heat resistance, and ability to accept large amount of fillers and process oil make them ideal for automotive WS, which account for 25% of the global EP- DM rubber consumption [6]. The mole- cular design and microstructure of EP- DMs need to be optimized in order to impart the required properties such as faster curing, superior elasticity and compression set resistance. To satisfy these requirements, Dow has introduced the Advanced Molecular Catalyst (AMC) technology, which can efficiently produ- ce EPDMs with ultra-high molecular weight, higher diene content, broader molecular weight distribution (MWD) and increased levels of long chain bran- ching than conventional single site cata- lyst [7]. In the recent years, there has been a strong automotive trend towards the decrease of the carbon dioxide emissi- ons, which will have to be below 78 g.CO 2/km by 2025 [8]. There are two main strategies that have been emplo- yed by the OEMs to reach this target - light-weighting car parts and electrifica- tion of the vehicles. One of the key light-weighting ap- proaches is the use of lower-density me- tals (Mg/Al) to replace steel doors in the car body. Nevertheless, after substituting steel with Mg & Al doors, electrochemi- cal degradation of the EPDM profile (Fi- gure 2) occur in combination with corro- sion of the metal surface. To hinder the galvanic contact between the Mg/Al doors and steel body structure, which are put into electric contact by the low but still relevantly conductive EPDM profile, the volume resistivity of the EPDM profi- le needs to be increased from the traditi- onal value of 10 6 Ω·cm to at least 10 8 Ω·cm. This increase in the volume resisti- vity can been achieved by reducing the CB content, but it has a negative impact on the overall compound properties. This paper will focus on the under- standing of the fundamentals of electro- chemical corrosion to better explain the process occurring and the requirements imposed to EPDM compounds. Additio- nally, a statistical design of experiments is reported with a predictive model deri- ved from it. It will help to optimally for- mulate the compound recipes in order to counteract the challenges imposed by reduction in CB content. Fundamentals of Electrochemical Corrosion According to International Union of Pure and Applied Chemistry (IUPAC), a corrosi- on is an irreversible interfacial reaction of a material (metal, ceramic, polymer) with its environment which results in consumption of the material or in disso- lution into the material of a component of the environment [9]. On the other hand, galvanic corrosion (also called ‚dis- similar metal corrosion‘) refers to corrosi- on damage induced when two dissimilar metals are brought into electrical con- tact [10]. Statistical Approach of Designing Isolating EPDM Rubber Com- pounds to Enable Light-Weighting Solutions at Automotive OEMs Authors Veronica Colombo, Tarragona, Spain, Sharon Wu, Texas, United States, Varun Thakur, Horgen, Switzerland Corresponding Author: Varun Thakur Dow Europe GmbH Bachtobelstraße 3 8810 Horgen, Switzerland E-Mail: [email protected]
Transcript
KONSTRUKTION UND SIMULATION CONSTRUCTION AND SIMULATION
The automotive trend of replacing steel doors in cars with light-weight metals like Magnesium/Aluminum (Mg/Al) has a significant impact on the EPDM wea-ther-strip (WS) requirements. To avoid electrochemical degradation of the EP-DM WS and oxidation of metal surface, EPDM WS need to have 3-4 orders high-er volume resistivity (VR) than standard WS. This can be obtained reducing the amount of carbon black (CB) to below its percolation threshold, but this has an adverse effect on other properties. In this paper, the fundamentals behind the electrochemical corrosion of Mg/Al and EPDM profiles are described. It is followed up by a 4-factorial experimen-tal design that can predict the com-pounding formulation to obtain desired final properties or estimate the proper-ties achieved varying EPDM blends, CB, white filler and oil.
Statistischer Ansatz für das Design von isolierenden EPDM Kautschuk- Compounds für Leichtgewichtlösungen an ‚Automotive OEMs‘EPDM · Automotive · Leichtbau · Statis-tische Modellierung.
Der Trend im Automobilmarkt, Türen aus Stahlblech durch Leichtmetalle wie Aluminum oder Magnesium (Mg/Al) zu ersetzen, hat einen Einfluss auf die An-forderungen von EPDM-Dichtprofilen. Um die Korrosion der EPDM -Pofile und Oxidation der Metalloberflächen zu verhindern, müssen EPDM- Profile eine höhere Volumenresistenz haben. Diese kann durch eine Reduzierung des Ruß-anteils unterhalb des Perkulationspunk-tes erreicht werden, aber dies beein-flusst andere Eigenschaften negativ.
Figures and Tables:By a kind approval of the authors.
INTRODUCTION
Ethylene Propylene Diene (EPDM) rubber [1-5] is composed of randomly distribut-ed ethylene (C2), propylene (C3), and ethylidene norbornene (ENB) diene along its saturated backbone (Figure 1).
EPDM compounds are composed of not only EPDM, but also of CB, white fil-lers (WF), oil and cure additives package. They can be used in many applications but their characteristics like weathering and heat resistance, and ability to accept large amount of fillers and process oil make them ideal for automotive WS, which account for 25% of the global EP-DM rubber consumption [6]. The mole-cular design and microstructure of EP-DMs need to be optimized in order to impart the required properties such as faster curing, superior elasticity and compression set resistance. To satisfy these requirements, Dow has introduced the Advanced Molecular Catalyst (AMC) technology, which can efficiently produ-ce EPDMs with ultra-high molecular weight, higher diene content, broader molecular weight distribution (MWD) and increased levels of long chain bran-ching than conventional single site cata-lyst [7].
In the recent years, there has been a strong automotive trend towards the decrease of the carbon dioxide emissi-ons, which will have to be below 78 g.CO2/km by 2025 [8]. There are two main strategies that have been emplo-yed by the OEMs to reach this target - light-weighting car parts and electrifica-tion of the vehicles.
One of the key light-weighting ap-proaches is the use of lower-density me-tals (Mg/Al) to replace steel doors in the car body. Nevertheless, after substituting steel with Mg & Al doors, electrochemi-cal degradation of the EPDM profile (Fi-gure 2) occur in combination with corro-sion of the metal surface. To hinder the galvanic contact between the Mg/Al doors and steel body structure, which are put into electric contact by the low but
still relevantly conductive EPDM profile, the volume resistivity of the EPDM profi-le needs to be increased from the traditi-onal value of 106 Ω·cm to at least 108
Ω·cm. This increase in the volume resisti-vity can been achieved by reducing the CB content, but it has a negative impact on the overall compound properties.
This paper will focus on the under-standing of the fundamentals of electro-chemical corrosion to better explain the process occurring and the requirements imposed to EPDM compounds. Additio-nally, a statistical design of experiments is reported with a predictive model deri-ved from it. It will help to optimally for-mulate the compound recipes in order to counteract the challenges imposed by reduction in CB content.
Fundamentals of Electrochemical CorrosionAccording to International Union of Pure and Applied Chemistry (IUPAC), a corrosi-on is an irreversible interfacial reaction of a material (metal, ceramic, polymer) with its environment which results in consumption of the material or in disso-lution into the material of a component of the environment [9]. On the other hand, galvanic corrosion (also called ‚dis-similar metal corrosion‘) refers to corrosi-on damage induced when two dissimilar metals are brought into electrical con-tact [10].
Statistical Approach of Designing Isolating EPDM Rubber Com-pounds to Enable Light-Weighting Solutions at Automotive OEMs
Where ∆E is the potential difference bet-ween the two metals, and Ri are the re-sistances: Relectrolyte is the resistance of the electrolyte, RP,A and RP,C are the polarizati-on resistances respectively at the anode and at the cathode, while Rconn is the re-sistance of the conductive connection (reinforced EPDM) [10]. Therefore, the effect of these four resistivity values will sum up and determine the velocity of corrosion.
Moreover, the difference in potential between steel and Mg/Al exposed to the atmosphere are:
In fact, the substitution of steel with Mg/Al in car doors has enabled to reduce the weight of the car body, thus making it a promising solution to decrease CO2 emis-sions by improving car’s mileage and fuel economy. Nevertheless, the presence of dissimilar metals put into electric con-tact via a conductive EPDM profile con-nection leads to the creation of a galva-nic couple. Under these circumstances, the less noble material becomes the ano-de and corrodes faster than it would all by itself, while the other metal becomes the cathode and corrodes slower than it would alone. Therefore, the cathode is protected as the less noble metal (anode) “sacrificially” corrodes. The driving force for corrosion is an electric potential diffe-rence between the different materials. In Table 1, a list of metals from least to most noble ones is reported. As such, each metal will corrode when put in con-tact with one below it. Hence, Mg/Al corrodes when put in contact with steel in new car bodies.
Nevertheless, the risk of galvanic cor-rosion does not depend only on the diffe-rence in potential but also on other fac-tors (Figure 3a), namely the presence of a conductive connection between the two metals and an electrically conductive humidity film (such as water from the atmosphere or rainwater).
In EPDM sealing profiles, the conduc-tive connection is the EPDM rubber filled with reinforcing fillers such as carbon black above percolation threshold, while the electrolyte is air and rain or humidity from the atmosphere or salt spread on the road in winters. Therefore, the con-tact of two metals with different poten-tials in a conductive environment leads to the flow of electrons from the anode to the cathode. An EPDM profile can con-tribute to corrosion via two principal mechanisms (Figure 3b):
● Connecting two different metals, hence, creating the required condi-tions to have galvanic contact;
● Acting as a cathode, hence, favoring corrosion of Al when put in contact with it.
Both these two processes can be hinde-red by increasing the volume resistivity of the EPDM WS.
Figure 3b shows the process occurring when Al and Steel are put in contact via a non-isolating EPDM compound in an environment containing oxygen and wa-ter (such as humid air), or when Al is put in contact with such a conductive EPDM. In fact, electrons flow from Al through the rubber and this enables galvanic cor-rosion to occur, which is accompanied by the transformation of Al to Al-hydroxide.The corrosion extent will be proportional to the current of electrons flowing, which is in turn given by a constant term plus a
Figure 1: General mole-cular structure of an EP-DM rubber.
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Figure 2: Electrochemi-cal corrosion on the EP-DM rubber sealing
2
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1 Table 1. List of metals from least to most nobleANODIC (Least Noble) MagnesiumZinc Aluminium Carbon steel or cast iron Copper alloys (brass, bronze ) LeadStainless Steel Nickel alloys (Incoloy 825,Hastelloy B) TitaniumGraphiteCATHODIC (Most Noble)
Figure 3: (a) Galvanic corrosion occurs in the presence of two dissi-milar metals, an elec-trically conductive se-paration and an elect-rolyte connection. The process does not occur when the two metals are isolated or when there is no electrolytic connection (9); (b) Cor-rosion mechanism oc-curring between alu-minum and steel with a not-completely-isola-ting EPDM [11].
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ments Mg degradation is experienced (Figure 4) [13].
Similarly, in the presence of oxygen and water the following reactions occur with Al:
4𝐴𝐴𝑒𝑒 → 4𝐴𝐴𝑒𝑒3+ + 12𝑒𝑒−
3 𝑂𝑂2 + 12𝑒𝑒− + 6𝐻𝐻2𝑂𝑂 → 12𝑂𝑂𝐻𝐻−
4𝐴𝐴𝑒𝑒 + 6𝐻𝐻2𝑂𝑂 + 3 𝑂𝑂2 → 4𝐴𝐴𝑒𝑒(𝑂𝑂𝐻𝐻)3
The right part of Figure 4 shows the Pour-baix diagram of Al [10]. Al(OH)3 is an amphoteric electrolyte, which means that in an acidic environment is behaves like a base givingAl3+ and H2O, while in a basic environment give AlO2- and H2O, not protecting the metal against corrosi-on. On the contrary, at neutral pH, a compact layer of alumina (Al2 O3· H2O) is formed, which partially protects the me-tal against corrosion by blocking the pas-sage of oxygen towards the surface (pas-sivation region).
Therefore, Mg will not form the pro-tective layer seen in Al when exposed to the atmosphere. This, combined with the higher difference in electric potential between Mg and steel than in Al and steel explains why higher volume resisti-vity is required in car designs where the EPDM profile is in contact with Mg rather than with Al.
In the following section, a DOE model is reported which allows to understand how the compounds’ properties are de-termined by the synergic effect of the different ingredients (CB, WF, EPDM, oil).
2 Table 2: Names and values of the con-tinuous variables considered in the DOE
Continuous Variables Values
Oil -1, 0 , 1
CB -1, 0 , 1
WF -1, 0 , 1
Ratio between EPDM NORDEL™6565XFC/
NORDEL™4725P-1, 0 , 1
3 Table 3: List of necessary factors and interactions to be estimatedMain Effects Quadric Effects 2-way interactions 2-way interactions
Oil Oil*oil Oil*WF WF* Ratio EPDM N6565/N4725
WF WF*WF Oil*CB CB* Ratio EPDM N6565/N4725
CB CB*CB WF*CB 3-way interactions
Ratio EPDM N6565/N4725
Ratio EPDM N6565/N4725* Ratio EPDM N6565/N4725 Oil* Ratio EPDM N6565/N4725 WF*CB* Ratio EPDM N6565/N4725
Figure 4: Pourbaix dia-gram of Magnesium and Aluminum.
4
∆𝐸𝐸𝑀𝑀𝑀𝑀/𝑆𝑆𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 = 0.7 − 1.1 𝑉𝑉
∆𝐸𝐸𝐴𝐴𝑒𝑒/𝑆𝑆𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 = 0.1 − 0.3 𝑉𝑉
The higher potential difference between Mg/steel than between Al/steel results in a higher volume resistivity require-ment for Mg(1010 Ω·cm) than Al (108 Ω·cm).
In addition, the two metals Mg and Al have a different behavior towards corro-sion when exposed to the atmosphere, as shown by the Pourbaix diagram [12]. The three regions can be distinguished as:
● “immunity”, occurs when the metallic phase is stable and corrosion does not occur;
● “corrosion”, when corrosion can occur, and
● “passivation”, where a 10-nanometer thick layer forms on the metal sur-face, shielding it from the environ-ment and hindering corrosion
When unalloyed magnesium is exposed to the air at room temperature, an oxide forms on its surface and this is converted by moisture in magnesium hydroxide at basic pH (Equation 4-6):
2𝑀𝑀𝑀𝑀 → 2𝑀𝑀𝑀𝑀2+ + 4𝑒𝑒−
𝑂𝑂2 + 4𝑒𝑒− + 2𝐻𝐻2𝑂𝑂 → 4𝑂𝑂𝐻𝐻−
2𝑀𝑀𝑀𝑀 + 𝑂𝑂2 + 2𝐻𝐻2𝑂𝑂 → 2𝑀𝑀𝑀𝑀(𝑂𝑂𝐻𝐻)2
The Mg(OH)2 formed is stable only at basic pH values (“passivation” region); as a result, in neutral and low pH environ-
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Predicting Compound Properties Using Statistical ModelingA design of experiment (DOE) is a cont-rolled set of tests designed to model and explore the relationship between factors and one or more responses in order to construct process knowledge, making process improvements and create higher quality/ cost-effective products, thus, providing a competitive advantage.
JMPTM statistical software was used to create the experimental design and to analyze the data. After identifying res-ponses and variables of interest, a design was created to minimize the average va-riance of prediction and model interac-tions. The results coming from the expe-rimental part were fitted using the JMPTM modeling platforms in order to create a predictive model.
In order to predict all the final compound’s characteristics, a broad mo-del of 35 runs was created. Four factors were chosen (Table 2) and for each factor, 3 values were considered and coded as -1 (lowest value for each factor), 0 (medi-um) and 1 (highest). The EPDMs used were NORDELTM 6565XFC (EPDM N6565) and EPDM NORDELTM 4725P (EPDM N4725), which show remarkable proper-ties in WS applications and are widely used in the industry. The NORDELTM EP-DM ratio was coded -1 for the lowest amount of N6565 (and highest of N4725), 0 for the medium, and +1 for the highest (lowest amount of N4725).
The effects to be estimated are repor-ted in Table 3. The factors are used to identify linear and quadratic dependen-
KONSTRUKTION UND SIMULATION CONSTRUCTION AND SIMULATION
4 Table 4: C2, ENB, and Mooney viscosity of EPDMs used in the DOE study.
NORDEL™ 6565 XFC NORDEL™ 4725P
C2 55 % 70 %
ENB 8.5 % 4.9 %
Mooney Viscosity ML1+4 @125°C 65 MU 25 MU
Figure 5: Actual vs predicted value for VR and hardness.
5ces of the properties on the parameters chosen, and two-way and three-way in-teractions.
Experimental
Compound CompositionsThe raw materials used are NORDELTM EPDMs, carbon black, white filler, paraffi-nic oil and additives, while curing packa-ge stays same for all the compounds.
The most important characteristics of EPDM N6565 XFC and EPDM N4725 are reported in Table 4.
NORDELTM 6565XFC is an amorphous fast curing dense EPDM grade with a po-lymer Mooney viscosity of 65Mooney units, diene content of 8.5wt-%, and an ethylene content of 55wt-%. It is widely used in the production of dense and mic-ro-dense compounds for the automotive weather-strips. The high molecular weight and broader MWD of this grade results in improved in filler uptake, while still providing superior physical and pro-cessing properties.
NORDELTM 4725P is a semi-crystalline pellet EPDM grade with a Mooney visco-sity of 25 Mooney units, diene content of 4.9wt-%, and an ethylene content of 70wt-%. It is widely used in the produc-tion of hard compounds as well as pro-cessing modifiers in EPDM compound for the automotive weather-strips. The pel-let form of this grade provides enhanced manufacturing efficiency.
CompoundingThe compounds were mixed in a Harburg Freudenberger internal mixer equipped with intermeshing rotors using a stan-dard mixing procedure. The 1.5 L net chamber was filled to a filling level of 75 %. The rotor speed was kept constant at 45 rpm during the mixing cycle. The feed temperature was 50°C and the com-pounds were mixed for 200 seconds or to a drop temperature of 110°C whichever came first. The compounds were then homogenized and sheeted out with a two-roll mill (50°C, friction ratio 1:1.27) for three minutes.
Characterization
Cure dynamicsCure characteristics were measured using an Alpha Technologies Moving Die Rheometer (MDR) 2000 E according to ASTM D5289 at 180°C with 0.5deg arc, and for 30 minutes.
Mooney viscosity, stress relaxation, and scorch measurementsMooney viscosity & stress relaxation (for compounds: 100 °C, using a large rotor) measurements were recorded with an Alpha technologies MV2000E viscometer (ML1+4+3min) according to ASTM D1646. The preheating time was 1 minute.
Tensile strengthTensile properties were measured accor-ding to ASTM D412 using a Zwick Roell Z010 device. Dumbbells (type 5A) were cut from cured plates (t90+3 min, 180°C).
Die-T Tear strengthTear strength was measured according to ASTM D624 type-T on a Zwick Roell Z010 device. Test specimens were cut from cured plates (t90+3 min, 180°C).
Compression setCompression set was measured at 23°C, 70°C and 100°C for 22h as described in ASTM D395 (25% deflection method B) and at 90°C for 22h (50% deflection, wai-ting 2h before removing the compres-sing plates after taking them out of the
oven and having placed them at room temperature). Test specimens have been cured at 180°C for t90+8 minutes under standard pressure.
HardnessShore A type hardness was measured according to ASTM D2240 using a 3 layer ply of cured plates (t90+3min, 180°C). Hardness of aged compounds was mea-sured after heat-treating the cured com-pounds in an oven at 100°C for 168h.
Volume resistivity Compression molded sheets were used for volume resistivity measurements ac-cording to DIN IEC 93.
Differential Scanning Calorimetry (DSC)Two heating runs were carried out and the second one was used for evaluation. The temperature range was from -100°C to 200°C, with a ramp rate of 10°C and cooling rate of -10°C.
Results and DiscussionIn order to analyze the correlations bet-ween the responses and the factors consi-dered in the study, JMPTM Pro 14 platform was used to construct a model able to predict the property’s value once the com-position is known and, on the other hand, to give indication on the formulati-on to be used to obtain the required performances. The electrical (VR), physi-cal, compression set, processing, curing and thermal properties were analyzed.
Moreover, to create a reliable and sta-ble model, only the parameter estimates with a strong statistical probability were considered significant. This was done by selecting only those factors with a p-va-lue < 0.1, which are those giving a low probability of making a mistake by rejec-ting the “null hypothesis”. Therefore, by considering p<0.1, only those values with a low probability of giving a wrong
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Figure 6: (a) Prediction profile for VR; (b) Prediction profile for hardness.
6
Figure 7: Example of contour plot showing a quite broad region of operability to obtain the desired properties.
7
model (or high probability to give a right one) are taken into account.
Figure 5 represents the predicted ver-sus actual volume resistivity (VR) and hardness values. Most of the value points are well interpolated and there is a good fit between the actual values measured experimentally and those obtained using the predicting model. Only one com-pound showed a measured VR lower than predicted by the model and has be-en considered as an outlier. In the graphs, the blue line is the average value of all tested compounds.
Additionally, using JMPTM it was possible to obtain the prediction profilers show-ing the dependence of the properties from the four factors considered. The VR and hardness prediction profilers are re-ported in Figure 6.
As expected, VR is highly dependent on CB due to the high conductivity of these particles in the percolation region, and is affected up to 9 orders of magni-tude (Figure 6a) going from the mini-mum (-1) to the maximum (+1) CB con-tent. The effect of the other components (WF, oil & EPDM ratio) is less significant.
In the prediction profiler the target value of VR (red number on the y-axis) has be-en set to approximately 8, being 108 ohm·cm the new VR requirement impo-sed by the OEMs for Al metal car doors. A similar analysis is possible for Mg, for a VR target of 1010 ohm.cm.
A different trend is shown by the hardness, which increases with increa-sing the amount of fillers (CB and WF) inside the compound (Figure 6b). On the other hand, an increase of EPDM N6565, due to its lower C2 content with respect to EPDM N4725, decreases it. Increasing the oil content decreases hardness, ma-king the compound softer. The target value has been set to 75 Shore A, and by looking at the prediction profiles it is clear how a wide range of values can be obtained by changing the amount of oil and fillers while using the NORDELTM EP-DMs considered in this study.
Once the regression analysis is carried out, a simulating model can give guidance on the optimal formulation to obtain the desired set of final properties. This enables to find optimal combination of CB-WF-oil- NORDELTM EPDM ratio to give several properties simultaneously.
It is shown by the contour plots impo-sing different required property values of the final compounds. The CB & NOR-DELTM EPDM ratio was selected as direct variables, but the same can be done by using oil and/or WF. Moreover, the two variables not shown in the x-y axis (oil & WF in the current case), can be changed to increase or decrease the white area of operability. On Figure 7, a contour plot is reported where the operability region (white area) is quite large, meaning that several different formulations can give the properties required.
The properties here imposed are lis-ted under “response”. VR was set to be at least 108 Ω·cm and hardness greater than 70 Shore A. Some of the values ob-tained using this combination of fillers and NORDELTM EPDM were clearly better than the required ones (for instance, VR was 1010 Ω·cm).
Conversely, Figure 8 shows an examp-le in which the operability region is subs-tantially smaller, because the hardness value was increased from 70 Shore A (previous case) to 80 Shore A, while maintaining the same VR requirement. This combination is challenging to achie-ve due to the opposite dependence of VR and hardness from the variables conside-red in the study (especially from the CB content). Nevertheless, several solutions
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Figure 8: Example of contour plot showing a narrow operability region to obtain the desi-red properties.
8were found using a blend of EPDM NOR-DELTM 6565XFC and EPDM NORDELTM 4725P.
Therefore, using this statistical ap-proach, a large variety of contour plots can be obtained imposing different re-quired property values of the final com-pounds, leading to a larger or smaller operability region. Hence, the model de-veloped can give guidance on the possib-le recipes to use to get the targeted pro-perties by using Dow’s NORDELTM EPDMs.
ConclusionsEPDM compounds are widely used in automotive applications, which constitu-te more than 50% of the global EPDM consumption. The automotive trend of replacing steel doors with less noble me-tals like Mg/Al for weight reduction, to comply with the new regulations regar-ding CO2 emission, has had a significant impact on the car sealing. In fact, an un-desired electrochemical degradation of the EPDM WS has been observed. In or-der to avoid galvanic corrosion between the two dissimilar metals, the OEMs ha-ve asked to provide rubber profiles with 3-4 orders of magnitude higher VR than that of standard WS.
In this paper, we highlighted the pro-cess by looking at the fundamentals of electrochemical corrosion. In fact, the presence of two dissimilar metals, an external electrolyte and a conductive EPDM connection enables galvanic corro-sion to occur between Al/Mg and steel. The extent of this corrosion process is inversely proportional to the resistivity, and therefore it becomes fundamental to increase the VR of the compounded EPDM.
Moreover, by looking at the different tendencies of Mg and Al to undergo cor-rosion when exposed to the environ-ment with and without coupling with steel, it is possible to justify why a higher resistivity is required with Mg (1010 Ω·cm) than with Al (108 Ω·cm).
First, when in contact with the atmos-phere, the two metals have different tendencies to undergo corrosion. This is well shown by their Pourbaix diagram: at neutral pH, Mg tends to corrode more than Al, which is partially shielded be-cause of the formation of a thin layer of Al2O3 on its surface which hinders the direct contact with the humidity in the air.
Most importantly, it was shown that the corrosion extent is directly proportio-nal to the difference in potential bet-
ween the two metals. Therefore, as the couple Mg and steel has a difference al-most ten times higher than the couple Al and steel, a higher VR is required with Mg than with Al.
Understood the corrosion process oc-curring, a DOE-based model analyzing the effect of CB, WF, oil and the ratio between the two NORDELTM EPDMs through their linear and quadratic ef-fects as well as synergies was construc-ted.
In order to do so, tests were carried out on the electrical, physical, processing and curing properties, followed by a JMPTM statistical analysis to determine the correlations among the 4 factors used and the final properties.
Once parameters estimates were ob-tained, the information was used to de-velop contour plots able to give indica-tions on the best compound recipe to use in order to achieve a set of determined properties. Using CB and the ratio bet-ween the two NORDEL™ EPDMs as varia-bles, the graph shows a variety of proper-ties can be obtained by changing these two variables.
This tool can be used to provide the model recipes to follow in order to achie-ve a certain set of properties using Dow’s EPDMs, namely, NORDELTM 4725P and NORDELTM 6565XFC, and to guide the future WS development to better satisfy the OEM’s requirements. � n
Dieser Beitrag beschreibt die Grundlagen der elektrochemi-
schen Korrosion zwischen Mg/Al und EPDM-Profilen. Ein vier
Faktoren-„Design of Experiment“ beschreibt die Formulie-
rung, um die gewünschten Endeigenschaften zu erreichen
oder wie die Endeigenschaften als Funktion von EPDM-
Blend, Ruß, Füllstoff und Öl vorherzusagen sind.
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