14 Rubber & Plastics News • March 22, 2021 www.rubbernews.com www.rubbernews.com Technical Technical volatiles can vaporize. Once this filler surface moisture evapo- rates it allows more filler inter- How to control porosity in action with other ingredients such as PEG and other process aids. Desiccants like CaO are added late in the mixing cycle to extruded EPDM dense pro le prevent absorption of moisture. A two-roll mill below the mixer By Greg Li, Santosh Bawiskar The authors further can be used to complete the and Sharon Wu Executive summary mixing, optimize dispersion and let Dow Packaging & Specialty Plastics Greg Li is a research scien- volatiles escape by creating and re- Introduction The fully saturated hydrocarbon backbone of EPDM rubber results in outstanding weatherability, tist in North newing the surface. The roll speed EPDM dense profile and manu- ultraviolet stability and low/high temperature resistance. Therefore, EPDM rubber has been estab- America Pack- is adjusted so air is not trapped in facturing process lished as the primary polymer choice for automotive sealing applications (both dense and sponge aging & Spe- the compound, as this can result in The fully saturated hydrocar- weatherstrip). cialty Plastics porosity as well. Finishing ingredi- bon nature of EPDM rubbers re- A typical manufacturing process of automotive dense weatherstrip involves profile extrusion fol- Technical Ser- ents like slab dips must be appro- sults in outstanding weather- lowed by continuous vulcanization. Excessive porosity in extruded and fully cured dense profiles re- vice & Develop- priately selected as these can be ability, ultraviolet stability and mains a big challenge to many automotive weatherstrip tier suppliers. ment at Dow sources of volatiles. low/high temperature resistance. In this study, we systematically studied the mechanism of porosity formation in EPDM dense pro- Chemical Co. Therefore, EPDM rubber has file. The major factors for porosity in EPDM dense profile were summarized as compound formulation; His responsibil- Extrusion been established as the primary extrusion conditions; and vulcanization conditions. ities include the Li The extrusion step also can be polymer choice for automotive A porosity-free technical solution for EPDM dense profile also was presented. development used to control porosity. As the sealing applications. of Nordel-brand EPDM prod- compound is being conveyed ucts, formulations and applica- through the extruder, it imparts TECHNICAL NOTEBOOK • It reduces mechanical prop- ation and bubble growth has to result in porosity. tion and processing develop- additional mixing due to the mo- Edited by John Dick erties such as tensile, tear, ad- be mitigated by controlling the Calcium oxide can help bind ments. tion of the screw and the type of hesion, etc.; volatiles/air entrapment, or by the moisture by functioning as a Santosh Bawiskar is a fellow screw design. With a two-stage Automotive body sealing (both • It changes performance char- applying external pressure on desiccant. Further oils get ab- at Dow. He has more than 20 screw, with a devolatilization sec- dense and sponge weatherstrip) acteristics (e.g. sealing) as the the extrudate. sorbed on porous filler surfaces, years of experi- tion, it is possible to pull vacuum, represents one of the largest ap- stiffness and density are reduced which may or may not incorpo- ence in materi- which allows extraction of the vola- plications, accounting for about (in cases of high porosity); and Variables affecting porosity rate into the EPDM matrix and al development, tiles. The back pressure due to the 20 percent of the total worldwide • Customer specifications don’t The compound design (formu- can act as a source of volatiles. compounding die allows volatiles to escape from 1,2 consumption of EPDM. allow porosity even if perfor- lation) and the subsequent man- Low molecular weight oils also and processing the vent and proper screens/die A typical manufacturing pro- mance is unaffected. ufacturing steps can impact po- are more volatile and a source of of elastomers. design can be used to adjust this. cess of automotive dense weather- The mechanism of porosity or rosity of the cured profile parts. porosity. Many low molecular B a w i s k a r Generally, higher extrusion strip includes the following steps: bubble formation involves bubble The manufacture of an extruded weight ingredients like process graduated with temperatures and screw speeds • Compound formulating and nucleation followed by bubble dense profile broadly involves three aids and curatives at low concen- a doctorate in help devolatilization, although mixing; growth. During extrusion, nucle- steps: compound mixing; extrusion trations can have high volatility, polymer engi- Bawiskar there is an upper limit for the • Profile extrusion; and ation occurs when the rubber and shaping; and curing. which can cause porosity. neering from extrudate temperature consider- • Curing. material is supersaturated with Each of these steps can con- Purity of these ingredients also the University of Akron and an ing the possibility of scorch. A volatile materials and the vapor tribute to the formation of poros- is critical. Volatiles also can result MBA from Clarion University. good screw design will avoid Porosity in dense EPDM profile pressure of the volatiles exceeds ity in an extruded part. Con- from byproducts of the curing re- He serves on the Board of the dead spots, ensure mixing and Porosity is the unintentional the external pressure on the ex- versely, optimizing these steps action. For example, peroxide de- TPE Special Interest Group of surface renewal, allow a depres- formation of bubbles or voids in trudate. This is followed by diffu- can help prevent porosity. composition can result in volatiles the Society of Plastics Engineers. surized zone in a two-stage screw an extruded or molded dense sion of the volatile materials to like acetone being formed. Sharon Wu is a product develop- and result in adequate tempera- part. In the case of sponge or the nuclei, which results in bub- Compound design Besides the compound formula- ment leader who oversees the ture buildup. foamed parts, the bubbles or cells ble growth. An EPDM formulation consists tion, the choice of EPDM rubber Nordel EPDM product and In the case of hot feed extru- are intentionally created by us- Although porosity formation is of fillers, oils, process aids, cure also is critical. Typically, EPDM technology innovation pipeline, sion, the additional warming ing physical or chemical blowing mostly due to volatiles as trapped package and other ingredients. with low volatile organic compound and leads a global team to de- step on a two-roll mill also can agents. In the case of dense ex- air during mixing, milling or ex- Many of these ingredients are (VOC) and high Mooney viscosity is velop next-generation EPDM help reduce volatiles and mois- truded parts (for example weath- trusion also can cause this. polar and not easily wetted by preferred for compound design to with new, valu- ture, which may have been ab- erstripping, hose, profiles, etc.) Therefore, in order to control the non-polar EPDM. If not fully incorporate all the ingredients and able properties. sorbed during storage. the porosity is undesired because: porosity formation during dense wetted, the filler surface can act enable the compound to resist any Her career • It is difficult to control di- profile manufacturing process, as heterogeneous nucleation bubble formation and growth. Due with Dow be- Curing mensions of a porous part; the mechanism of bubble nucle- sites. This is especially true with to the inherent feature of Nor- gan in 2008, As the material exits the die, non-black fillers, which tend to del-brand EPDM manufacturing and she has led the external pressure drops to Table 1: List of dense compound formulations. be more polar. process, low VOC Nordel EPDM is multifunction- atmospheric levels and bubble Use of process aids like poly- a favorable polymer choice for an al global teams nucleation and growth may oc- ethylene glycol (PEG) helps filler EPDM-dense compound. toward the cur. This is especially true with wetting. Another issue with commercial- Wu lower viscosity or lower melt many non-black fillers (clay, etc.) Mixing ization of a se- strength feedstocks, which do is that they inherently have a Mixing is a critical step to en- ries of polyolefin elastomers for not resist bubble nucleation. high moisture content, which can sure good dispersion of all ingre- the transportation, infrastruc- Lowering extrudate tempera- ture and packaging markets. Table 2: One-pass mixing procedures. ture can help if bubble formation She received her doctorate from occurs instantly after exiting the the Georgia Institute of Technolo- die. Curing also helps build com- gy, and her research has covered pound viscosity and prevent po- many aspects of polyolefin mate- rosity. Thus, the kinetics of bub- rials science, including structure/ ble formation must be slower property relationship, prod- than the kinetics of curing in or- uct-process development and der to prevent porosity. polyolefin characterization. The cure rate and cure set-up Wu’s research has resulted (hot air, microwave, salt bath, in 16 peer-reviewed journal autoclave, etc.) play a significant articles, 15 granted U.S. pat- role in controlling porosity. The ents and more than 26 patent autoclave process is most forgiv- applications. ing for porosity as external steam pressure can be applied to dients and can be used to elim- prevent porosity development. inate volatiles like moisture In the hot air cure and salt bath coming from fillers and other process, curing starts from the ingredients. Good dispersion surface and progresses inward, ensures that the filler surface allowing time for porosity develop- is not a source for heteroge- ment. In microwave curing, cure neous nucleation. Good mixing rate is more uniform throughout also ensures that the oil is well the cross-section, and using a incorporated into the EPDM shock oven can help form a skin phase rather than remaining layer quickly.