UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

416 IMMC 2016 | 18th International Metallurgy & Materials Congress

Surface Characterization Study on EPDM Rubber Seal Profi les

Ekrem Altuncu¹, Uğursoy Olgun¹, Ali Erkin Kutlu², Yusuf Güner², Müfi t Çağlayan²

¹Sakarya University, ²Standard Profi l Automotive - Türkiye

Abstract Followed by an increased quality perception of the end customers, automotive equipment manufacturers focus on visual aspects (color and gloss) of diversified components used on vehicles. Consequently, all suppliers that produce visual parts are required to deliver parts according to internal technical specifications of the OEM’s. In this paper, a surface characterization research on a roof top profile of an automotive weatherstrip system is presented. The extruded part of concern is produced through a compounding and consequent extrusion process. Aim is fundamentally to reveal process effect on the color change of a profile. 1. Introduction

Subsequent to rapid growth in rubber based processes brought improved compounding techniques. Increased use of accelerators, antioxidants, fillers and carbon blacks added complexity to chemical processing. As the number of applications increased, demand for the raw material grew and added diversity and geographically different sources of compound materials. Technology to process raw materials has also changed and became complex. Based on the application and market requirements, even the synthetic rubber production surpassed the production of natural rubber. Among the wide usage of elastomers, current study focuses on weatherstrip compounding and extrusion process. Weatherstrip used in automotive industry is assembled along the edges of automobile vehicle body (door, window, trunk, hood etc.). A weatherstrip system prevents water leakage, dust, air, exterior noises, and body and window vibrations and provides shock-absorbing capacity. Ethylene propylene diene monomer (M-class) - EPDM is a basic elastomer mainly used in automotive sealing applications. EPDM is selected in the weatherstrip industry due to properties of high resistance against severe temperatures, solar ageing, and high elasticity under compression, harsh mechanical environments,

high insulating, wide hardness range and low density [1, 2]. The extrusion and curing a compound depends on the flow and thermal properties of the chemical design. Totally, the process converts the raw material in the form of powder or pallets into a saleable finished or semi-finished product. Whole process series is composed of shaping by an extrusion die and then cross-linking (curing) to achieve above-mentioned material properties. The detail of the curing (post extrusion) is highly specific to the material and product [3] Therefore much of it is confidential to producer. 1.1. Description of the case In the current case, a Gerlach SHF 147 type curing process is investigated for an identified surface color change abnormality. Colour is visible to the eye as a brown colour (Figure 1). Clearly, color change is an objective concern of the customer where the color change is quantified through a colour measurement.

Figure 1. Photograph of the colour change on the profile surfaces Extrusion speed is 12 m/min, tunnel length is 12 m and temperature measured in the curing tunnel is 247°C.Segmented line trials throughout the extrusion line indicate that the colour change occurs specifically in the curing tunnel. Brown profile surface has a gradient colour layout as shown in Figure 2.

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41718. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2016

Figure 2. Failure map of browning on the profile surface The colour that is visually observed is a brownish colour defined on a colour scale with a > 0, 4, b > 0, 82. Non-brown profiles were available for comperative analysis after an experimental similar system trial. 1.2. Literature Survey The browning effect is known for more than 10 years in the sealing market. Tacit knowledge within the industry is not explicit for scientific research for sealing extrusion process and related mechanisms. The different scientific studies tried to explain the effect, till now there is no general statement and solution given. For that reason, current case is one of the first steps to literally explain the phenomenon. The compound composition, used polymers, aging agents, UV agents and the related compound mixing process cause primarily the browning effect. However, homogeneous mixing can take influence to surface appearance as well. The browning effect is known to be happening immediately after curing or after UV lab aging of some days after process. As validated in the current case, browning occurs after the curing tunnel. Generally, such a colour change is addressed as a degradation of the rubber compound. Available literature focuses on artificial weathering environment and associated as aging due to cross-linking performance [4,5]. Additionally, available literature indicates that discolouration on the surface of a product develops over a longer period of time while the product is in service and is exposed to environmental weathering conditions. The cause can sometimes be attributed to amount of sulphur present in the compound but mainly two types of degradation exist; photo degradation or thermo degradation [6]. Latter can be associated with the irregular temperature distribution inside the curing tunnel, thus can affect the browning however, available literature does not advice specific addressin for process adjustment or characteristics change of the compound.

2. Experimental Procedure A priori to any analysis of the browning effect, several industrial trials are performed; Heat map measurements with the IR thermal camera revealed the fact of extreme premature curing of the lip of the profile. Where the lip temperature and body temperature of the profile are evidently different. Besides, lip thickness was mainly inferior than that of the main body (Figure 2) The result is inline with the failure map shown in Figure 3.

Figure 3. Heat map of profile surface after curing

Effect of the directionality of the high temperature air blow is observed by reversing the extrusion orientation of the profile (Figure 4). The outcome of the trial was again a brown profile. Temperature profile was same as shown as Figure 3. Which has confirmed an irrevelant effect and assured that curing system is responsible for the undesired effect of bronzing.

Figure 4. Extrusion directionality profile surface

A reproduction of the curing process is sought for production of a non-brown extrudate. For that purpose a similar extrusion tunnel with an IR pre-shock unit is used. The outcome of the trial was a non-brown extrudate. Which served as a comparison basis for analysis and case solution (Figure 5)

Figure 5. Extrusion configuration difference

UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

418 IMMC 2016 | 18th International Metallurgy & Materials Congress

3. Measurements and Equipment Used Measurements realized served as a means for identifying the current issue and well known methods are employed [4,5,6]. Surface structure and mechanical forms are analysed by an optical microscope.Scanning Electron Microscope (SEM) - EDX provided elemental analysis and chemical characterization. The surface chemical changes, chemical composition relations were detected by Specular Reflection Fourier Transform Infrared (SR-FTIR) spectroscopy.The colour measurements are realized by LABX-3500. Heat mapping is performed by an Fluke IR, non contact thermal camera. 3.1. Measurement Results Optical Microscope comparative analysis results for surfaces are presented in Figure 6. The browning surface are visualized to be detoriated, deformed and colored, non-browning sample surfaces are homogenous and dark colored. Assuring the fact of the presence of a degradation [6]

Figure 6. Optical microscope (X100) results a) brown b) nonbrown During SEM analysis, image capturing was not possible, because surfaces are deteriorated due to vacuum and irradiation by electron beam. However, EDS spectrum comparative results are presented in Table 1. Essentially, C percentage in brown samples are found to be relatively high, whereas S and O percentages were relatively low. The notable difference in Si percentage can only be attributed to detorition on the surface since Si cannot be thermally degraded and migrate from the surface in brown profiles.

Table1. EDS Spectrum Results

Elements Intensity

(c/s) Brown

Intensity (c/s) Non-

Brown

Conc (wt%) Brown

Conc (wt%) Non-

Brown C 171.37 81.76 65.88 51.92 O 13.45 18.23 22.05 32.71 Si 2.58 9.54 0.33 1.65 S 13.39 12.99 1.67 2.17

Ca 65.38 57.08 10.07 11.56 Comparative FT-IR surface analysis (see Figure 7) primarily depict a low signal strength of brown profiles compared with non-brown ones. Probably, based on

Table 1 results, presence of C has lowered the signal strength. In details Signal strength is lower at 1540 for C-C, C=C), 1850 for C-H, 1900 for C-H and 3200 nm for O-H.

Figure 7. FT-IR Spectrums of surfaces (EPDM bronze means brown, whereas EPDM black means non-brown) 4. Results and Discussion The phenomenon under investigation was explainable by the differences of pre-shock unit processes. Basically, browning disappeared immediately when the gas operated shock unit is interchanged by an electrically operated IR type. The constructional difference of two exrusion line pre shock units lead the researchers to following conclusions, provided that compound type, process parameters are kept constant. Mainly gas preshock unit forms surface structure impurities. Since IR shock unit did not produce the brown surface. In gas pre-shock unit, specifically, the specific compound surface is degraded by direct hot gas exposure to produce free radicals. The free radicals attack the polymer backbone in the presence of oxygen and cause bond breakup, introducing further absorbing impurities from burned natural gas inside the shock unit and curing tunnel. The carbonyl groups and peroxide observed under FT-IR can be related to this fact. Additionally, high temperature processing inside the tunnel can prematurely initiate cross-linking and polymerization on the impurities. Because of this, surface texture changes and materials depict a color change, which lead to a brown looking color to the huma. The microscope results shown in figure 6 assures that in by IR pre-shocking, pre-micro skin cross-linkage is easily formed. It is known that the cross-linking of polymers by ionizing radiation provides increased stability and improved mechanical properties. Under the action of high-energy radiation, ethylene propylene elastomers are able to form intermolecular links, especially in the presence of polyfunctional compounds, to attain a proper crosslinking level with a minimum degree of oxidation [6]. In some compounding applications antioxidants are utlisied which interrupt the oxidative process that occur during processing.

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5. Conclusion This paper aims to explicitly reanalyse tacit knowledge within the industry. Contrary to academical studies dominant knowledge inside the industry can be misleading. The current case of browning includes chemistry, mechanics, metallurgy and thermodynamics in extrusion curing. Even the surface analysis lead the authors to resolve the issue, it is clear that interpretation of the standard chemical reaction kinetics of EPDM curing is scarce. Obviously rheometer and alike system studies does not explain whole process. For that reason, further investigation is planned are as follows;

• Relation of the tunnel curing temperature profile with the associated compound rheological properties

• Temperature measurements of the specific tunnel components and segmented parts. Use them for CFD modelling of a curing tunnel. Match temperature profile with rheological curve of each compound formulae.

6. Acknowledgement The authors would like to thank to Mr. Reimund Plottnik from Gerlach GMBH, Philip Hough from Lanxess, Kübra K lnaz from Standard Profil for their contribution for the current case identification. 7. References [1] F. Wang, J. Xu, H. Luo, J. Wang, and Q. Wang, Molecules, 14(2009) 4087-4097. [2] N. Saleema, D. Gallant, Applied Surface Science, 282(2013) 98-104. [3] Stevens, M.J., Covas, Jose, Extruder Principles and Operation, Chapman and Hall, 1995 [4] Quanlin Zhaoa, Xiaogang Lia, Jin Gaoa, Surface degradation of ethylene–propylene–diene monomer (EPDM) containing 5-ethylidene-2-norbornene (ENB) as diene in artificial weathering environment, Polymer Degradation and Stability, Vol.93, Issue 3, pp 692-699 [5] Quanlin Zhaoa, Xiaogang Lia, Jin Gaoa, Aging of ethyleneepropyleneediene monomer (EPDM) in artificial weathering environment, Polymer Degradation and Stability, Vol.92, Issue 10, pp 1841–1846 [6] M.M. Abdel-Aziz, A.A. Basfar, Aging of ethylene-propylene diene rubber (EPDM) vulcanized by gamma-radiation, Polymer Testing 19 (2000) 591–602 [7] M. van Duin, Geleen, Chemisrty of EPDM Cross-Linking, Elastomers and Plastics, Issue 4, 2002