EoW May 2008

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Due to the chlorination process, CPE is always in the powder form independent of the crystallinity. Crystallinity has a strong influence on the physical properties. For example, with increasing crystallinity, modulus and tensile strength along with hardness and tear strength increase, but low temperatures set, elastic recovery, and flex resistance decreases. With increasing crystallinity, cold green strength also improves. Higher molecular weight polymers can accept more filler and have better physical properties including increased tensile and tear strength, but need to be optimised for processability. 2.2 Molecular weight distribution MWD of ethylene containing elastomers affects processing characteristics. A broad MWD shows improved mill mixing, higher hot green strength, extrusion characteristics, and generally better surface appearance on extrusion. However, cure rate, and cure state can be lower for broad MWD elastomers. In general, broader MWD polymers have better processing but poorer cure characteristics than their narrow MWD counterparts. 2.3 Diene Similar to crosslinked polyethylene, polyolefin elastomers, EPDM and CM can be crosslinked by peroxide. For EPDM, the incorporation of the diene provides the capability for curing with sulphur.

However, peroxide cure is preferred for power cable insulation because better electrical properties can typically be achieved through peroxide cure than sulphur cure. The most widely used diene termonomers includes ethylidene norbornene (ENB) and dicyclopentadiene (DCPD). For these dienes, ENB has the fastest curing rate, and DCPD has the slowest curing rate. The presence of diene enhances the probability for EPDM to branch and results in repeating CH2 units from a side chain from the main polymer backbone. Branching lowers polymer viscosity at higher shear rates to provide improved extrusion characteristics, and it increases polymer viscosity at lower shear rates to improve sag resistance. In general, DCPD has the most probability to create more long chain branching vs ENB. 2.4 Chlorine content Similar to the addition of halogen containing additives to polymer for the improvement of ignition resistance, the presence of chlorine on the backbone of CM improves the ignition and combustion resistance characteristics of the polymer. As demonstrated in Figure 7 , resistance to ignition (as indicated by higher limiting oxygen index (LOI) values, increases as the chlorine content of the CPE increases. The presence of chlorine enhances the resistance to hydrocarbon oils and fuels for CPE based compounds, therefore, higher

chlorine containing CPE is typically selected when resistance to oil and fuels is a critical performance attribute of cable applications.

3. Compounding Extrusion of high molecular weight ethylene containing elastomer alone usually results in a rough extrudate surface due to melt fracture. Filler addition improves the surface smoothness of the extrudate as well as the mechanical strength of the compound. Other ingredients such as plasticisers, lubricants, antioxidants, crosslinking agents, UV absor- bers, and flame retardants are frequently used to enhance the functionality of the compound. The composition of the elastomer compound determines the end use performance of the product, but the properties of the elastomer matrix determine the type and quantity of the various ingredients that can be or should be used in the compound. Continuous screw mixers offer excellent rates and reasonable dispersion for effective plastic compound production. However, compound ingredients must be in the form of pellets or powder for proper feeding of continuous mixers. Internal batch mixers such as Banbury® (Farrel Corporation) or two-roll mills are typically used to prepare elastomer compounds. Amorphous low viscosity polymers with broad MWD are best for mill mixing. Compounds based on high viscosity and highly crystalline elastomers are very difficult to mix on two-roll mills and are best suited for Banbury mixer for high mixing efficiency and reasonable cycle time. Elastomers in pellet or friable bale form can be easily mixed in the batch mixer, because friable bales are loosely compacted and can break apart easily in the Banbury under the shearing action. Depending on the application, elastomer compounds produced from roll mill or batch mixer could either go through strainer-extruders or directly feed into the extruder as shown in Figure 8 . 4. Wire extrusion For wire and cable production, compounds based on amorphous elastomers are best extruded with a rubber-type screw, while semi-crystalline elastomers run best with a polyethylene-type screw. Amorphous elastomer compounds are normally strip fed while semi-crystalline elastomers are typically fed as pellets or in diced form. Ethylene containing elastomer compounds can be extruded using either cold or warm feed equipment. Cold feed, long barrel extruders take compounded strips at ambient temperatures and usually require a compound with high green strength at room temperature. Elastomers with sufficient crystallinity should be used in cold feed compound or the extruder screw will bite off the strip causing irregular feeding. Warm feed, short barrel extruders are fed strips of stock previously warmed on a mill. These stocks should contain broad molecular weight elastomer to give green strength at elevated

Figure 4 ▼ ▼ : Heat and oil resistance of various elastomers

Figure 5 ▼ ▼ : Flexibility range of olefinic ethylene elastomers

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EuroWire – May 2008