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POLYMER HISTORY
 
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 high-performance 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, polystyrene 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.

  
 

TYPES OF POLYMERS

 
There are three 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 (degrade rather than melt with the application of    heat) 

3. Elastomers (cross-linked)

Plastics are rigid long-chain polymers which are not usually connected or cross-linked. 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 flexiblelong - 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 result of a physical  combination of soft, elastic polymer segments and hard, crystalline segments which are capable of cross-linking. Thermoplastic elastomers are generally classified by their structure rather than their chemical makeup.

 

RUBBER PROCESSING

 
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 Du Pont, Dow, 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 O-ring molders) for mixing.
 
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.

 

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 H2O or HCl. 

2. Addition Polymerization 3. Chain Polymerization/Free Radical Polymerization� 

  1. 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). 
  2. Propagation: growth of macromolecular structure. 
  3. Chain Transfer and Termination: completing the polymerization step. 

Types of Chain Polymerization Methods 

  • Bulk Polymerization� involves only the monomer and a monomer-soluble initiator. 
  • Solution Polymerization� a solvent lowers the viscosity, assisting heat transfer and reducing the likelihood of auto-acceleration. 
  • 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. 
  • 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.
 
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.

Molding    Most of the elastomeric O-rings used in the semiconductor industry are compres-sion molded. A preshaped form is inserted into a multi-section mold and trans-ferred 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). After a period of time ranging from several seconds to several minutes, the 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.

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.

  
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.

Finishing and Inspection  After the parts are removed from the curing ovens, the parts are again cleaned and inspected to ensure the parts meet the material and dimensional specifications.

Cleaning  After the parts are inspected, acceptable parts are delivered to the Class 100 clean room for cleaning and packaging. An ultrapure deionized water (UPDI) rinsing cycle removes surface contamination from the parts.

Packaging  Acceptable parts are then counted and packaged, either individually or in bulk, in a heat-sealed clean inner bag. The parts are then packaged in an outer bag, with a complete description of the parts, lot number, the batch and cure date, as well as any specific information relative to the customer�s part number.
 
 

COMMON POLYMERS
 
 
ASTM Polymer Trade Names Monomers

PLASTICS
  Polyamideimide (PAI) TORLON�  
  Polybenzimidazole (PBI) CELAZOLE� -(C7H6N2)-
  Polycarbonate (PC)   -COOC6H5C(CH3)2C6H5O-
  Polyethylene (PE)   -CH2CH2-
  Polyetheretherketone (PEEK)

KETRON�

 -C6H5-CO-C6H5-O-C6H5-
  Polyetrherimide (PEI)

ULTEM�

  
  Polyimide (PI)

DURATRON�

 N(C2O2)C6H5(C2O2)N-R-
  Polypropylene (PP)   CH2CH(CH3)-
  Polyphenylenesulfide (PPS)

TECHTRON�

  
  Polyvinylidine Fluoride (PVDF)   -CH2CF2-
  Fluorinated Ethylene-Propylene (FEP)

TEFLON� FEP

 -CF2CF2-CF2CF(CF3)-
  Perfluoroalkoxy (PFA)

TEFLON� PFA

 -CF2CF2-CF2CF(OCF3)-
  Polytetrafluoroethylene (PTFE)

TEFLON� PTFE

 -CF2CF2-

ELASTOMERS
NBR Nitrile (Buna-N) PARACRIL�, CHEMIGUM� -CH2CH=CH(CH2)2CHCH(CN)-
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 VITON�, ETP� -CF2CF2-CF2CF(OCF3)-CH2CH2-
  Fluoroelastomer TFE/P AFLAS� -CF2CF2-CH2CH(CH3)-

FFKM

 Perfluoroelastomer AEGIS�, CHEMRAZ�, KALREZ� -CF2CF2-CF2CF(OCFnCF3)-

OTHER
  VF2/CTFE

KEL-F�

 -CH2CF2-CF2CFCI-
 
 

SEALING ELASTOMERS
 
Sealing elastomers may best be initially understood and compared by examining their chemical structure. It is this chemical structure that forms the foundation for a seal�s ability to withstand certain chemical, thermal or physical environments. 

The seal industry uses many tests to determine an elastomer�s chemical and thermal compatibility as well as physical properties which can have a great influence on the performance in high-pressure or vacuum environments. These properties can provide an insight into the mode of degradation or the retention of sealing properties�all useful information in predicting seal life or comparing economic alternatives. 

Another difference in elastomer compounds is the compounding (or mixing) of ingredients. These factors can provide unique pigmentation, improved specific chemical or thermal properties, improved dynamic performance, reduced cost, improved electrical properties, reduced friction or sticking, and many other aspects of seal performance.

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