MAY 6 7 years The technical service magazine for the rubber industry Volume 5, No. EPDM sponge profile formulations: New approach with metallocene elastomers by Eric Jourdain, Brian Burkhart and Mark Welker, ExxonMobil Chemical www.rubberworld.com
New approach to EPDM sponge profile formulations with metallocene elastomers by Eric Jourdain, Brian Burkhart and Mark Welker, Exxon Mobil Chemical EPDM is one of the most widely used rubbers in automotive exterior sealing systems. It is the material of choice for automotive sponge weatherstrips around doors, trunks and hoods. Its unique properties of air and ozone resistance, coupled with its compounding versatility, produce high performing profiles at relatively low cost while sealing out water, dirt and noise over the lifetime of the car. Advances in metallocene polymerization technology over the past decade have provided precise control over the molecular architecture and comonomer introduction, enabling the ability to design high performance elastomers with tailored properties. In this article, we present different polymer approaches to Specific gravity Figure density reduction of sponge profiles made from Vistalon 7 EPDM rubber using Exxon 5 5 Sponge profile density 7 7 5 phr 7 5 phr formulate sponge compounds, such as an amorphous, high diene, medium molecular weight, metallocene EPDM rubber grade; two amorphous, bimodal, high molecular weight, conventional EPDM rubber grades; blends of EPDM grades with a metallocene propyleneethylene elastomer; and blends of metallocene and bimodal EPDM rubber grades. These polymer combinations offer the potential for reduced sponge profile density; optimized sealing attributes, such as load deflection and compression set; and adaptability to meet stringent automotive OEM weatherseal specifications. Elastomer properties We selected elastomers with different molecular weight structures and properties (table ). Vistalon 7 EPDM rubber is a metallocene EPDM with medium molecular weight and high diene. Vistalon 8 EPDM rubber is a conventional EPDM with higher molecular weight, bimodal molecular weight distribution and high diene. Vistalon 8 EPDM rubber is a conventional EPDM with high molecular weight, bimodal molecular weight distribution, and very high diene. Exxon is a propyleneethylene elastomer that does not vulcanize with sulfur (no unsaturation). Exxon interaction in the EPDM foaming process Exxon is a propylene based elastomer. It has a predominantly amorphous structure of ethylenepropylene laced with a network of isotactic polypropylene crystallites. A reduction in crystallinity is obtained through the introduction of amorphous regions of ethylenepropylene in the polypropylene sequences. has a lower molecular weight than Vistalon 7 EPDM rubber. During the mixing stage, it is easily dispersed in the EPDM compound matrix along with the fillers and plasticizers. Since propylene units dominate the backbone structure of Exxon, it is not miscible with most EPDM polymers. It is a fully saturated copolymer and does not react with the curing agent due to the absence of double bonds. is used to modify the EPDM compound rheology by viscosity reduction and the kinetic balance blow over cure (ref. ). Table Vistalon EPDM rubber and Exxon properties (ref. ) Mooney viscosity ML +, 5 C C content, weight ENB content, weight MWD Softening temperature, C Vistalon 7 65 55 7.5 Medium NA Vistalon 8 8 (ML +8) 57.5 8.9 Bimodal NA Vistalon 8 73 (5 phr oil) 53.5 Bimodal NA 6 Narrow 5 Sponge profile formulation and rheology Exxon was substituted for 5 phr and 5 phr of Vistalon 7 in a standard sponge formulation (table ). acts as a polymeric processing aid, reducing the compound viscosity. Since has no unsaturation, the scorch time (Ts5) is slightly increased, while the cure rate and cure state are decreased (table 3). Sponge profile properties Sponge profiles were produced using an Omega shaped die in a semiindustrial extrusion line at 8 RUBBERWORLD.COM MAY 6
Table model compound formulations for automotive sponge Formulation and mixing Masterbatch: Exxon Vistalon 7 EPDM rubber Spheron 5 Omya BSH (whiting) Flexon 85 Stearic acid Active ZnO PEG 335 Rhenocure ZAT (7) Rhenogran TP/G (5) Final: Rhenogran CaO Sulfur MBTS () MBT () DOTG ZBEC (7) Vulkalent E Celogen OT Total phr Vistalon 7...8..5 9. 7/5 phr 5 85...8..5 9. 7/5 phr 5...8..5 9. ExxonMobil. Profiles were extruded at approximately meters/ minute and then cured using a UHF oven followed by a hot air oven. The increased thermoplasticity of Exxon reduces the pressure at the extruder head. The final density of the profile was also reduced by about (figure ). It appears that allows the blowing agent to expand more easily, while the reduced diene content lowers the cure rate and cure state. These effects may also be influenced by soft thermoplastic domains from the dispersed into the compound. deflection (CLD) is a measure of stiffness and serves as a proxy for the seal s influence on the effort to Table 3 rheology of Vistalon 7 EPDM rubber/ Exxon model compounds Rheology Mooney ML (+) C on MVE TS ML (+) (MU) Mooney scorch 5 C TS ML t (min. min.) ML t5 (min. min.) ML t (min. min.) Minimum (MU) MDR arc ±, C TS3 ML MH MHML Ts T (dnm) (dnm) (dnm) (min.) (min.) 7 3.5. 8. 9.6.3.8 7/5 phr 3.5.5 6 8.5 7.5.37 3. deflection N/ cm close a door. The profile s CLD is virtually unchanged after adding 5 phr of Exxon, despite the lower profile density. The CLD increases at 5 phr, which reflects the natural hardness and increased abundance of domains in the compound (figure ). Compression set reflects the sealing performance over time of a sponge profile held at constant deflection. A compression set improvement is observed using 5 phr of Exxon, but the data suggest the optimum morphology of the foamed structure requires much less than 5 phr (figure 3). At the higher loading of, where the thermoplasticity of becomes more predominant and the cure state is reduced, the compression set gets worse. Possible mechanism of Exxon interaction in the EPDM foaming process In compounds containing Exxon, the chemical blowing 7/5 phr Figure compression load deflection of sponge profiles made with Vistalon 7 EPDM rubber and Exxon 39 3.7.3.8.9 7. 6..5 Sponge profile CLD 7 7 5 phr 7 5 phr agents have more latitude to expand. decreases green strength, reduces the cure rate, creates thermoplastic domains and lowers the crosslink density. As a result, a lower profile density can be achieved when using the same amount of blowing agent. Alternatively, the same density can be obtained with less blowing agent. Since has higher crystallinity than the EPDM rubber, the sponge profiles with also have a higher stiffness at similar densities, resulting in a higher compression load deflection. Alternatively, a similar compression load deflection can be obtained at a lower density. A surprising effect is seen in the compression set at 7 C, which maintains similar performance despite lower crosslink density and lower profile density. It appears that the Exxon improves the cell structure, creating more open cells of smaller size, which improves the compression set in a sponge profile. FOLLOW US ON TWITTER @rubberworld 9
Figure 3 compression set of sponge profiles made with Vistalon 7 EPDM rubber and Exxon Compression set,7 days at 7 C, deflection, 3 Best elastic recovery Sponge profile compression set 7 7 5 phr 7 5 phr Compression set on profile, deflection, 35 3 5 5 Compression set and temperature hours/ C, 7 hours/7 C, hours/ C, 7 7 5 phr Relative concentration Figure typical GPC trace of a bimodal EPDM grade ( C trichlorobenzene refractive index detector).e 8.E5 6.E5.E5.E5 Bimodal grade Lower MW mode Higher MW mode.e+.e+.e+5.e+6.e+7.e+8 Mw (g/mol) Sponge formulations with bimodal Vistalon EPDM rubber ExxonMobil s bimodal EPDM elastomers are composed of a major polymer fraction with narrow MWD with a controlled level of very high molecular weight component (figure ). Since their introduction in the 9s, bimodal EPDM grades have been very successful in extrusion applications, as they offer the elastic performance of a high molecular weight EPDM with the processing capability of a medium molecular weight polymer (ref. 3). Sponge profile formulation and rheology To aid comparing the performance of different Vistalon EPDM rubber grades, it is useful to target a similar compound viscosity (ML, +, C). Therefore, we used the same formulation for Vistalon 7 and Vistalon 8. Because Vistalon 8 has higher molecular weight, we formulate it with a higher filler loading (3 EPDM content vs. 33) and a lower carbon black to oil ratio (ref. ). The cure package has been modified for each EPDM grade s properties in order to have compounds with a similar scorch time (table ). Compound rheology and profile properties Figure 5 shows a high cure state for Vistalon 7 EPDM rubber compound, as measured by MDR at C. This is related to the uniform molecular structure of metallocene EPDM, which improves ENB efficiency during vulcanization. Figure 6 shows the effects of the high molecular weight and bimodal EPDM structure on profile properties, including smooth surface aspect, low profile density, low CLD and low compression set. The Vistalon 8 EPDM rubber compound has a higher filler content and a lower carbon black to oil ratio. Its profile has similar performance to a Vistalon 8 EPDM rubberbased profile. This illustrates the advantage of selecting a very high molecular weight EPDM to formulate sponge compounds. Sponge profile compressibility Bimodal Vistalon EPDM rubbers offer lower profile load deflection. Vistalon 8 profiles are slightly softer than the Cure state, MHML, MDR at C (dnm) Figure 5 cure state (MHML) of Vistalon EPDM rubber compounds 9 8 7 6 5 Sponge formulation comparison 7 8 8.5 5. Scorch time, Ts5 at 5 C 3 RUBBERWORLD.COM MAY 6
Figure 6 sponge profile comparisons of Vistalon EPDM rubber compounds Profile surface roughness, Ri, um Sponge profile comparison 8 7 7 6 8 5 8. 5.7 deflection N / cm Sponge profile comparison 5 7 3 8 8 5 5 3 Profile density (g/cm 3 ) Compression set, 7 days/7 C () deflection, N/ cm Figure 7 CLD of Vistalon EPDM rubber compounds at room temperature Sponge profile CLD at RT Vistalon 7 Vistalon 8 Vistalon 8 3 5 Profile compression in of initial height Vistalon 8 profiles, and both are softer than Vistalon 7. The following improvements were noted with the bimodal EPDMs: Low CLD with the modified formulations (figure 7) Reduced low temperature stiffening due to low ethylene content (figure 8) Lower CLD loss upon heat aging under compression (figure 8) due to high crosslink density Sponge formulations with blends of Vistalon EPDM rubber Blends of Vistalon 7 EPDM rubber and Vistalon 8 EPDM rubber can be developed to optimize cost/performance balance. In the comparative example below, formulations have Table sponge formulations comparing Vistalon EPDM rubbers Ingredients, phr EPDM rubber Carbon black FEF N55 CaCO 3 Paraffinic oil high viscosity ZnO Stearic acid PEG 335 CaO Sulfur MBT MBTS ZDEDC TMTDS DPTTS ZDBDC DPG Vulkalent E OBSH Total phr V8.3. 99.7 V8 5.3..5 35.7 V7 6.7. 3 Figure 8 CLD of Vistalon EPDM rubber compounds at C and after heat aging deflection, N/ cm Sponge profile CLD at C Vistalon 8 Vistalon 8 Vistalon 7 3 5 Profile compression in of initial height Load deflection loss, Sponge profile CLD loss after aging days at 7 C 3 5 Vistalon 7 Vistalon 8 Vistalon 8 Profile compression in of initial height FOLLOW US ON TWITTER @rubberworld 3
Figure 9 profile properties of Vistalon EPDM rubber compounds Surface roughness, um Sponge compound with EPDM blends 5.5 5.3 5..9 8 7/8.7.5. 3 5 Profile density deflection, N/ cm Sponge compound with EPDM blends 7/8 8 7 5 7 9 3 5 Compression set, (7 days/7 C, deflection) Figure Vistalon EPDM rubber selection and sponge profile performance Profile properties, index based on density, CLD and compression set Sponge grade positioning 7/ Blend 7/ 8 7 8 8 Compound processibility, index based on MLRA/ML and compound viscosity been adjusted to obtain similar compound viscosity and cure parameters (table 5). When blending Vistalon 7 EPDM rubber, having a medium molecular weight distribution, with Vistalon 8, which has a bimodal molecular weight distribution, profile properties such as density and compression set are impacted. However, the Vistalon 7 content can be easily adjusted to meet or exceed the aspect and sealing requirements of many sponge specifications (figure 9). Conclusions The rubber formulator can conveniently select the most appropriate Vistalon EPDM rubber grade or blend to produce finely tailored automotive sponge profiles according to the particular technical specification desired (figure ). Replacing 5 phr of Vistalon 7 EPDM rubber with Exxon is a novel approach to formulating EPDM sponge compounds. Since the blowing agent becomes more efficient, Exxon blends provide a convenient route to cost and performance improvements through lower profile densities and/or reduced use of blowing agents. Blends of Vistalon 7 EPDM rubber with bimodal Vistalon 8 further improve sponge profile properties such as density and compression set. Vistalon 7/8 blends offer a convenient route to optimize the cost/performance balance of a profile. The highest performing automotive sponge profiles are achieved with the high molecular weight bimodal Vistalon EPDM rubber grades. Vistalon 8 and Vistalon 8, which are the grades of choice to produce the best performing sponge profile, have unique molecular architectures to consistently produce the highest quality low density profiles of complex geometry and best compression load deflection and compression set over a wide range of temperatures. This article is based on a paper presented at the 88th Technical Meeting of the Rubber Division, ACS, October 5. Table 5 formulation and rheology of Vistalon EPDM rubber compounds Ingredients Vistalon 8 Vistalon 7 Carbon black FEF N55 CaCO 3 Paraffinic oil high viscosity ZnO Stearic acid PEG 335 CaO Sulfur MBT MBTS ZDEDC TMTDS DPTTS ZDBDC DPG Vulkalent E OBSH Total (phr) Mooney viscosity ML (+) C (MU) Mooney scorch (+9 ) 5 C EEB Ms t (min. min.) Ms t5 (min. min.) Ms t (min. min.) Minimum (MU) MDR arc ±, C ML (dnm) MH (dnm) MHML (dnm) Ts (min.) T (min.) 8 5.3..5 35.8 9.6 5..7 6.5 5..3.3 8/7 57.5 5 65.3..5 3.8 9 8.7 5.7.8 7.3 6..33. 3 RUBBERWORLD.COM MAY 6
Appendices Profiles detail properties and test methods used. Appendix Exxon modification Eclipse Number Profile properties Density, g/cm 3 Profile dimensions Height, Width, Wall thickness, Surface roughness Ra, m Rt, m Ri, m Physicals Modulus 5, Modulus, TS, EB, Compression set 7 days/7 C/ deflection, Retest Load deflection RT/ deflection, N/cm 8 7 6 6. 6. 3...5.. 37 37 3 7 7 w/ 5 phr 6.7 6..8.9.3.7. 35 8 7 7 7 w/ 5 phr 5.8 6. 3.8 8 6.5.3. 35 Appendix Vistalon EPDM rubber grade comparison Sponge profile properties Dimensions Height Width Thickness Density Surface roughness Ra Rt Ri Compression set, compression 7 days at 7 C deflection deflection Physical properties on sponge profile Modulus 5 Modulus 5 Modulus Tensile strength Elongation at break g/cm 3 N/ cm 7 5.7. 7 3.9 3 7. 9 38.3.9 3. 35 8 5.8 5. 3 3 5.6 8. 7 37 38 8 5.8.8 3. 5. 8.3. 3. 7 Appendix 3 EPDM rubber Sponge profile properties Dimensions Height Width Thickness Density Surface roughness Ra Rt Ri Compression set, compression 7 days at 7 C deflection deflection Physical properties on sponge profile Modulus 5 Modulus 5 Modulus Tensile strength Elongation at break Test methods g/cm 3 References. Vistalon EPDM rubber technical data sheet.. Eric Jourdain and Caroline Szala, "A new approach in EPDM sponge profiles with metallocene elastomers," IRC 5. 3. Eric Jourdain, Guy Wouters and Mark F. Welker, "New EPDM grade with improved processing characteristics," DKT Conference.. Brian Burkhart, Milind Joshi and Eric Jourdain, "A metallocene EPDM grade for use in automotive and industrial sponge applications," 86th Technical Meeting of the Rubber Division, ACS, October. N/ cm 8 6. 5. 3.8.9 9 89. Appendix test methods Compound Mooney viscosity Compound Mooney scorch Compound MDR RPA Density Profile surface roughness Compression set sponge profile deflection sponge Physical properties Test methods technical data sheet EPDM Mooney viscosity EPDM Mooney stress relaxation Ethylene content ENB content 8/7 6.3.8.9 9.6 5...9 5 ExxonMobil reference TS TS TS 3 TS TS 35 TS 57 TS 3 TS 33 TS 35 ASTM D 66 D 66 D 3 D 7 FOLLOW US ON TWITTER @rubberworld 33