POLYMER CHEMISTRY

Polymer Chemistry

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As far back as 1839, Charles Goodyear first improvedthe elastic properties of natural rubber by heating withsulfur (vulcanization). It was not until the 1930s that themacromolecule model of rubber was understood. AfterWorld War II and through the 1950s rapid developmentsin synthetic polymers were made. Most commercial high-performance elastomers trace their origins to the 1960sand 1970s.

Polymers are long chains of repeating chemical units, or monomers. The chemical skeletal structures may belinear, cyclic or branched. When one monomer is poly-merized, the resultant polymer is called a homopolymer.Examples include polyethylene, polstyrene and polytetra-fluoroethylene (PTFE). Copolymers (or dipolymers) arederived from the polymerization of more than one typeof monomer. The distribution of monomers in thesecopolymers can be statistical, random or alternating.Examples include ethylene-propylene and fluorocarbonelastomers (vinylidene fluoride and hexafluoropropylene).

Polymer Chemistry

Polymer chemistry is the backbone of our business. By carefully matching the customer’s needs with Simrit’spolymer technology, the optimum sealing solution for both performance and economy can be chosen.

Terpolymers are three-monomer-unit polymers, such asethylene-propylene-diene (EPDM) and specialty fluoro-carbon grades.

Simrit/Freudenberg–NOK’s technology center, located inPlymouth, Michigan, has capabilities ranging from basicpolymer research to applied material development whichcontinue to meet specific application and market needs.

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There are two general classes of polymers:

1. Thermoplastics(can be melted with the application of heat)

Crystalline—crystallize when cooled

Amorphous—no crystallization when cooled

Semicrystalline—polymers which contain both crystalline and amorphous segments

2. Thermosets/Elastomers (Cross-Linked)(degrade rather than melt with the application of heat)

Types of PolymersPlastics are rigid long-chain polymerswhich are not usually connected or cross-linked. Plastics can either be thermo-plastic—meaning they can be heatedand cooled without changing properties—or thermoset, where an increase in temperaturechanges the chemical structure and properties. As a class,plastics have low elongation and high elongation set.

Elastomers are flexible long-chain polymers which arecapable of cross-linking. Cross-linking chemically bonds

polymer chains, which can preventreversion to a non-cross-linked polymerat elevated temperatures. The cross-linkis the key to the elastic, or rubbery,properties of these materials. The

elasticity provides resiliency in sealing applications.

Thermoplastic elastomers (TPEs) often combine theproperties of elastomers with the ease of processabilityof thermoplastics. They are the chemicalresultant of elastomers and thermal plastics. Thermoplastic elastomers aregenerally classified by their structurerather than their chemical makeup.

Types of Polymerization Reactions

Types of Chain Polymerization Methods

1. Condensation Polymerization: Yields polymers withrepeating units having fewer atoms than the monomersfrom which they are formed. This reaction generallyinvolves the elimination of small molecules such asH2O or HCl.

2. Addition Polymerization: Addition curing of elastomers involves addition of SiH (silicone hydride)across double bonds in the presence of platinumcomplexes (although palladium and rhodium are alsosuitable). The reaction proceeds gradually at roomtemperature or rapidly at elevated temperatures.

There are no volatile by-products produced duringcure, so post-curing is not required.

3. Chain Polymerization/Free Radical Polymerization:

A. Initiation: Formation of free radicals by scission ofa single bond (homolysis), or by the transfer of a single electron to or from an ion or molecule (redox).

B. Propagation: Growth of macromolecular structure.

C. Chain Transfer and Termination: Completing the polymerization step.

1. Bulk Polymerization: Involves only the monomer anda monomer-soluble initiator.

2. Solution Polymerization: A solvent lowers the viscosity,assisting heat transfer and reducing the likelihood ofauto-acceleration.

3. Suspension Polymerization: Reaction mixture is suspended as droplets in an inert medium. Polymer

particles are produced in the form of beads in therange of 0.1 to 2 mm in diameter.

4. Emulsion Polymerization: The initiator is not solublein the monomer but soluble only in the aqueous dispersion medium. Polymer is produced in the form ofa latex with particles in the range of 0.05 to 1 micron.

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Elastomer ProcessingSimrit Elastomer ProductionSimrit/Freudenberg–NOK’s production facilities are typically equipped to provide compounding-to-packagingsteps in elastomer processing. In select elastomer materials,our capabilities begin at the polymerization stage.

PolymerizationThe beginning step for elastomers is the polymerizationof the backbone and cure-site monomers. This is typically

done by large chemical companies suchas DuPont Dow (DDE), Bayer, GE,Ausimont, Daikin and Dyneon. Commontechniques are emulsion, microemulsion,and suspension polymerization.

Polymerization combines two or more process gases(monomers) into an aqueous environment and, under

specific temperature and pressure conditions, connectsthe individual monomers into the desired polymer.Initiating agents, buffers and other chemicals may beadded to the polymer reactor to achieve the desiredchemical properties and polymerization dynamics.

IsolationThe backbone polymers are isolated (brought out of theemulsion), cleaned and dried. Chemical agents may beadded at this step to isolate the polymer“latex” into a more usable form.

Once the polymer is cleaned and dried,the “crumb” polymer is shipped to compounders or molders for mixing.

Isolation

UPDI

Finishing & Inspection

Packaging

Polymerization

Extrusion Molding & Curing

Compounding

N2

Flash Removal

Post-Curing

Cleaning

Elastomer Process Diagram

The production of elastomeric components involves several steps. This diagram is abasic schematic of the process flow from monomer to finished part.

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Elastomer Processing—continued

Compounding (Mixing)The “crumb” polymer is mixed with a cross-linking agentand other functional fillers. The cross-linking agent allows

chemical bonds to form between the polymer backbones, thus providingresiliency to the material. Functional fillers include reinforcing fillers, pigments,anti-degradants, acid scavengers and

process aids. These ingredients are typically mixedtogether on a 2-roll mill or other custom mixing machinery.

Once the material is compounded, it is shaped intosheets and then shipped to o-ring molders.

ExtrusionThe sheet compound is extruded into a configuration

similar to the desired finished part.

Molding and CuringMost of the elastomeric o-rings used are compressionmolded. A preshaped form is inserted into a multi-sectionmold and transferred to a heated press.Under heat and pressure, the elastomerflows into the mold cavities and chemicalcross-linking takes place (or begins totake place, depending on the specificelastomer compound). This cross-linking is commonlyreferred to as curing or vulcanization. After a period oftime ranging from several seconds to several minutes, the

parts are removed from the hot molds. Depending on thecompound, mold releases are often used. These dilutedspray coatings are often a derivative of fluoropolymers,or silicone-based polymers.

Clean Extrusion

Compression Molding

Mill Mixing

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Elastomer Processing—continued

Finishing and InspectionAfter the parts are removed from the curing ovens, they

are inspected to ensure that the partsmeet the material and dimensionalspecifications.

Post-Cure Ovens

Automatic Inspection Machine

Flash RemovalAfter the parts are removed from the molds, they contain

thin “flash” as a result of the elastomer flowing in the multi-section mold. This“flash” is typically removed by exposingthe parts to a cryogenic tumbling process.The elastomer is cooled and tumbled,

causing the thinner “flash” section to become brittle andbreak away from the main part. Additional tumbling orhand-deflashing may be required on some part designsor compounds.

Post-CuringSome high-performance elastomers are subjected to apost-curing operation. Elastomer parts areexposed to high temperatures in carefullycontrolled environments for several hoursto complete the curing process. Additionally,this post-curing step removes excess watervapor and volatile process additives, thereby improvingvacuum and contamination performance.

Cryo-Deflash Unit

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Elastomer Processing—continued

Common Elastomers

CleaningAfter the parts are inspected, they are thoroughly cleaned.In the case of perfluoroelastomers, an ultrapure deionized

water (UPDI) rinsing cycle is used toremove surface contamination from the parts.

Packaging Parts are then counted and packaged, either individuallyor in bulk. Shipping labels include a complete descriptionof the parts, lot number, the batch and curedate, as well as any specific information,including the customer’s part number, when applicable.

ASTM POLYMER TRADE NAMES MONOMERS

NBR Nitrile (Buna-N) PARACRIL®, CHEMIGUM® -CH2CH=CH(CH2)2CHCH(CN)-

HNBR-HSN Hydrogenated Nitrile/ THERBAN®, ZETPOL® -CH2-CH(CN)-CH2-(CH2)2-CH2- Highly Saturated Nitrile

EPDM Ethylene-Propylene Diene VISTALON®, NORDEL® -CH2CH2-CH2CH(CH3)-

VMQ Silicone SILASTIC®, SILPLUS® -OSi(CH3)2-OSi(CH3)(CH=CH2)-

FVMQ Fluorosilicone SILASTIC® LS, FSE® -OSi(CH3)(CH=CH2)-OSi(CH3)(CH2CH2CF3)-

FKM Fluoroelastomer A VITON®, FLUOREL® -CH2CF2-CF2CF(CF3)-

Fluoroelastomer B -CH2CF2-CF2CF(CF3)-CF2CF2-

Fluoroelastomer GF -CH2CF2-CF2CF(OCF3)-CF2CF2-

Fluoroelastomer ETP VITON®, VITON ETP® -CF2CF2-CF2CF(OCF3)-CH2CH2-

Fluoroelastomer TFE/P AFLAS® -CF2CF2-CH2CH(CH3)-

FFKM Perfluoroelastomer SIMRIZ®, CHEMRAZ®, -CF2CF2-CF2CF(OCFnCF3)-KALREZ®

AFLAS® is a registered trademark of Asahi Glass Co., Ltd.CHEMIGUM® is a registered trademark of GoodyearCHEMRAZ® is a registered trademark of Greene, Tweed & Co.FLUOREL® is a registered trademark of DyneonFSE® is a registered trademark of General ElectricKALREZ® is a registered trademark of Du Pont Dow Elastomers, LLCNORDEL® is a registered trademark of Du Pont Dow Elastomers, LLCPARACRIL® is a registered trademark of Uniroyal

SILASTIC® and SILASTIC® LS are registered trademarks of Dow Corning STISILPLUS® is a registered trademark of General ElectricSIMRIZ® is a registered trademark of Simrit Division of Freudenberg-NOKTHERBAN® is a registered trademark of BayerVISTALON® is a registered trademark of Exxon Chemical Co.VITON® and VITON ETP® are registered trademarks of Du Pont Dow Elastomers, LLCZETPOL® is a registered trademark of Zeon Chemical

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0603PC5M Publication No. 2103

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