Polymer Chemistry POLYMER CHEMISTRY

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1 Polymer Chemistry POLYMER CHEMISTRY

Polymer Chemistry As far back as 1839, Charles Goodyear first improved the elastic properties of natural rubber by heating with sulfur (vulcanization).

2 Polymer Chemistry As far back as 1839, Charles Goodyear first improved the elastic properties of natural rubber by heating with sulfur (vulcanization). It was not until the 1930s that the macromolecule model of rubber was understood. After World War II and through the 1950s rapid developments in synthetic polymers were made. Most commercial highperformance elastomers trace their origins to the 1960s and 1970s. Polymers are long chains of repeating chemical units, or monomers. The chemical skeletal structures may be linear, cyclic or branched. When one monomer is polymerized, the resultant polymer is called a homopolymer. Examples include polyethylene, polstyrene and polytetrafluoroethylene (PTFE). Copolymers (or dipolymers) are derived from the polymerization of more than one type of monomer. The distribution of monomers in these copolymers can be statistical, random or alternating. Examples include ethylene-propylene and fluorocarbon elastomers (vinylidene fluoride and hexafluoropropylene). Terpolymers are three-monomer-unit polymers, such as ethylene-propylene-diene (EPDM) and specialty fluorocarbon grades. Simrit/Freudenberg NOK s technology center, located in Plymouth, Michigan, has capabilities ranging from basic polymer research to applied material development which continue to meet specific application and market needs. Polymer chemistry is the backbone of our business. By carefully matching the customer s needs with Simrit s polymer technology, the optimum sealing solution for both performance and economy can be chosen. 2

3 Types of Polymers 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) Plastics are rigid long-chain polymers which are not usually connected or crosslinked. Plastics can either be thermoplastic meaning they can be heated and cooled without changing properties or thermoset, where an increase in temperature changes the chemical structure and properties. As a class, plastics have low elongation and high elongation set. Elastomers are flexible long-chain polymers which are capable of cross-linking. Cross-linking chemically bonds polymer chains, which can prevent reversion to a non-cross-linked polymer at elevated temperatures. The cross-link is the key to the elastic, or rubbery, properties of these materials. The elasticity provides resiliency in sealing applications. Thermoplastic elastomers (TPEs) often combine the properties of elastomers with the ease of processability of thermoplastics. They are the chemical resultant of elastomers and thermal plastics. Thermoplastic elastomers are generally classified by their structure rather than their chemical makeup. Types of Polymerization Reactions 1. Condensation Polymerization: Yields polymers with repeating units having fewer atoms than the monomers from which they are formed. This reaction generally involves the elimination of small molecules such as H 2 O or HCl. 2. Addition Polymerization: Addition curing of elastomers involves addition of SiH (silicone hydride) across double bonds in the presence of platinum complexes (although palladium and rhodium are also suitable). The reaction proceeds gradually at room temperature or rapidly at elevated temperatures. There are no volatile by-products produced during cure, so post-curing is not required. 3. Chain Polymerization/Free Radical Polymerization: A. Initiation: Formation of free radicals by scission of a 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. Types of Chain Polymerization Methods 1. Bulk Polymerization: Involves only the monomer and a monomer-soluble initiator. 2. Solution Polymerization: A solvent lowers the viscosity, assisting heat transfer and reducing the likelihood of auto-acceleration. 3. Suspension Polymerization: Reaction mixture is suspended as droplets in an inert medium. Polymer particles are produced in the form of beads in the range of 0.1 to 2 mm in diameter. 4. Emulsion Polymerization: The initiator is not soluble in the monomer but soluble only in the aqueous dispersion medium. Polymer is produced in the form of a latex with particles in the range of 0.05 to 1 micron. 3

4 Elastomer Process Diagram The production of elastomeric components involves several steps. This diagram is a basic schematic of the process flow from monomer to finished part. UPDI Extrusion Molding & Curing Finishing & Inspection Cleaning Polymerization N 2 Compounding Packaging Isolation Flash Removal Post-Curing Elastomer Processing Simrit Elastomer Production Simrit/Freudenberg NOK s production facilities are typically equipped to provide compounding-to-packaging steps in elastomer processing. In select elastomer materials, our capabilities begin at the polymerization stage. Polymerization The beginning step for elastomers is the polymerization of the backbone and cure-site monomers. This is typically done by large chemical companies such as DuPont Dow (DDE), Bayer, GE, Ausimont, Daikin and Dyneon. Common techniques 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, connects the individual monomers into the desired polymer. Initiating agents, buffers and other chemicals may be added to the polymer reactor to achieve the desired chemical properties and polymerization dynamics. Isolation The backbone polymers are isolated (brought out of the emulsion), cleaned and dried. Chemical agents may be added 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. 4

Elastomer Processing continued Compounding (Mixing) The crumb polymer is mixed with a cross-linking agent and other

The cross-linking agent allows chemical bonds to form between the polymer backbones, thus providing resiliency to the

These ingredients are typically mixed together on a 2-roll mill or other custom mixing machinery.

A preshaped form is inserted into a multi-section mold and transferred to a heated press.

to take place, depending on the specific elastomer compound).

After a period of time ranging from several seconds to several minutes, the Mill Mixing Once the material is

5 Elastomer Processing continued Compounding (Mixing) The crumb polymer is mixed with a cross-linking agent and other functional fillers. The cross-linking agent allows chemical bonds to form between the polymer backbones, thus providing resiliency to the material. Functional fillers include reinforcing fillers, pigments, anti-degradants, acid scavengers and process aids. These ingredients are typically mixed together on a 2-roll mill or other custom mixing machinery. Molding and Curing Most of the elastomeric o-rings used are compression molded. A preshaped form is inserted into a multi-section mold and transferred to a heated press. Under heat and pressure, the elastomer flows into the mold cavities and chemical cross-linking takes place (or begins to take place, depending on the specific elastomer compound). This cross-linking is commonly referred to as curing or vulcanization. After a period of time ranging from several seconds to several minutes, the Mill Mixing Once the material is compounded, it is shaped into sheets and then shipped to o-ring molders. Extrusion The sheet compound is extruded into a configuration similar to the desired finished part. Clean Extrusion Compression Molding parts are removed from the hot molds. Depending on the compound, mold releases are often used. These diluted spray coatings are often a derivative of fluoropolymers, or silicone-based polymers. 5

Elastomer Processing continued Flash Removal After the parts are removed from the molds,

This flash is typically removed by exposing the parts to a cryogenic tumbling process.

Finishing and Inspection After the parts are removed from the curing ovens, they are

Automatic Inspection Machine Cryo-Deflash Unit Post-Curing Some high-performance

Elastomer parts are exposed to high temperatures in carefully controlled environments for

6 Elastomer Processing continued Flash Removal After 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 exposing the parts to a cryogenic tumbling process. The elastomer is cooled and tumbled, causing the thinner flash section to become brittle and break away from the main part. Additional tumbling or hand-deflashing may be required on some part designs or compounds. Finishing and Inspection After the parts are removed from the curing ovens, they are inspected to ensure that the parts meet the material and dimensional specifications. Automatic Inspection Machine Cryo-Deflash Unit Post-Curing Some high-performance elastomers are subjected to a post-curing operation. Elastomer parts are exposed to high temperatures in carefully controlled environments for several hours to complete the curing process. Additionally, this post-curing step removes excess water vapor and volatile process additives, thereby improving vacuum and contamination performance. Post-Cure Ovens 6

7 Elastomer Processing continued Cleaning After the parts are inspected, they are thoroughly cleaned. In the case of perfluoroelastomers, an ultrapure deionized water (UPDI) rinsing cycle is used to remove surface contamination from the parts. Packaging Parts are then counted and packaged, either individually or in bulk. Shipping labels include a complete description of the parts, lot number, the batch and cure date, as well as any specific information, including the customer s part number, when applicable. Common Elastomers ASTM POLYMER TRADE NAMES MONOMERS NBR Nitrile (Buna-N) PARACRIL, CHEMIGUM -CH 2 CH=CH(CH 2 ) 2 CHCH(CN)- HNBR-HSN Hydrogenated Nitrile/ THERBAN, ZETPOL -CH 2 -CH(CN)-CH 2 -(CH 2 ) 2 -CH 2 - Highly Saturated Nitrile EPDM Ethylene-Propylene Diene VISTALON, NORDEL -CH 2 CH 2 -CH 2 CH(CH 3 )- VMQ Silicone SILASTIC, SILPLUS -OSi(CH 3 ) 2 -OSi(CH 3 )(CH=CH 2 )- FVMQ Fluorosilicone SILASTIC LS, FSE -OSi(CH 3 )(CH=CH 2 )- OSi(CH 3 )(CH 2 CH 2 CF 3 )- FKM Fluoroelastomer A VITON, FLUOREL -CH 2 CF 2 -CF 2 CF(CF 3 )- Fluoroelastomer B -CH 2 CF 2 -CF 2 CF(CF 3 )-CF 2 CF 2 - Fluoroelastomer GF -CH 2 CF 2 -CF 2 CF(OCF 3 )-CF 2 CF 2 - Fluoroelastomer ETP VITON, VITON ETP -CF 2 CF 2 -CF 2 CF(OCF 3 )-CH 2 CH 2 - Fluoroelastomer TFE/P AFLAS -CF 2 CF 2 -CH 2 CH(CH 3 )- FFKM Perfluoroelastomer SIMRIZ, CHEMRAZ, -CF 2 CF 2 -CF 2 CF(OCFnCF 3 )- KALREZ AFLAS is a registered trademark of Asahi Glass Co., Ltd. CHEMIGUM is a registered trademark of Goodyear CHEMRAZ is a registered trademark of Greene, Tweed & Co. FLUOREL is a registered trademark of Dyneon FSE is a registered trademark of General Electric KALREZ is a registered trademark of Du Pont Dow Elastomers, LLC NORDEL is a registered trademark of Du Pont Dow Elastomers, LLC PARACRIL is a registered trademark of Uniroyal SILASTIC and SILASTIC LS are registered trademarks of Dow Corning STI SILPLUS is a registered trademark of General Electric SIMRIZ is a registered trademark of Simrit Division of Freudenberg-NOK THERBAN is a registered trademark of Bayer VISTALON is a registered trademark of Exxon Chemical Co. VITON and VITON ETP are registered trademarks of Du Pont Dow Elastomers, LLC ZETPOL is a registered trademark of Zeon Chemical 7

8 Aerospace & O-Ring Business Unit Arctic Ocean Drive Lake Forest, CA Phone: Fax: North American Headquarters East Anchor Court Plymouth, M I Phone: SIMRIT Fax: Fluid Power Business Unit 3600 West Milwaukee Spencer, IA Phone: Fax: Industrial & Consumer Business Unit 2250 Point Blvd., Suite 230 Elgin, I L Phone: Fax: SIMRIT 0603PC5M Publication No. 2103

MATERIALS SCIENCE POLYMERS

MATERIALS SCIENCE POLYMERS POLYMERS 1) Types of Polymer (a) Plastic Possibly the largest number of different polymeric materials come under the plastic classification. Polyethylene, polypropylene, polyvinyl chloride, polystyrene,