thermoplastic elastomeric (tpe) materials and their

15Thermoplastic elastomeric (TPE) materials and their use in outdoor electrical insulation

© 2011 Adva]ced Study Ce]ter Co. Ltd.

Rev. Adv. Mater. Sci. 29 (2011) 15-30

Corresponding author: Salman Amin, e-mail: [email protected]

THERMOPLASTIC ELASTOMERIC (TPE) MATERIALS ANDTHEIR USE IN OUTDOOR ELECTRICAL INSULATION

Salman Amin1 and Muhammad Amin2

1University of Engineering & Technology, Taxila, Pakistan2CIIT, Wah, Taxila, Pakistan

Received: March 03, 2011

Abstract. The use of polymeric materials has become very popular since last thirty years in manyapplications. The materials in this category can be broadly divided into two categories i.e. thermoset a]d thermo_lastics. With i]creasi]g the world’s co]cer] about recyclability a]d e]viro]me]tprotection issues thermoplastics have gained more attraction over the thermo set. Thermoplas-tic elastomers are novel constructional polymers, which are physically cross linked materialsmade up of a thermoplastic and an elastomer. TPE have been the interest of numerous re-searchers world wide since last three decades. Thermoplastic elastomers have a very wide-spread application domain. This paper reviews the history, physical, chemical, mechanical,electrical characteristics, advantages and uses of these materials along with a particular focuson their use as outdoor electrical insulation. Results of using different kinds of thermoplasticmaterials as outdoor insulation in different environments of world are also reviewed. This reviewexpected to be useful for researchers working on thermoplastic elastomers in general andparticularly for those working in filed of electrical insulation.

1. INTRODUCTION

Polymeric materials can be classified as Thermo-sets and Thermoplastics. Before looking onto ther-moplastic elastomeric materials it is necessary tounderstand the thermosets and thermoplastics first.

1.1. Thermoset

Thermoset is a polymer that is cured by heat orchemical reaction and becomes infusible and in-soluble material. Thermoset polymers have a per-manent irreversible polymerization. Thermoset poly-mers posses a cross linked molecular structure andare formed in two stage polymerization. The firststage is formation of a polymer with linear chains.The second stage of polymerization results in finalcross linked structure. The end product can be maderigid or flexible. The Polymerization is controlled toresult in heavily cross linked short chains for hardproduct and lightly cross linked long chains for softand flexible products [1].

1.2. Thermoplastic

Thermoplastic is a plastic that softens upon heat-ing and hardens when cooled. Thermoplastics aremade up of linear molecular chains and can beshaped by flow into articles by molding or extrusionwithout need of any chemical processing beforemolding. The most useful physical property of a ther-moplastic is its glass transition temperature at whichit begins to soften. Glass transition temperatures ofdifferent thermoplastics can be seen in [1]. Thereare three types of thermoplastics crystalline, amor-phous and semi- crystalline [2,3].

1.2.1. Crystalline thermoplastics

They are usually translucent and their molecularchains have a regular arrangement. They have highand sharp melting temperatures. Compared to amor-phous thermoplastics they have more mechanicalimpact resistance. Their load bearing capacity canbe enhanced considerable with addition of fibers.

16 S. Amin and M. Amin

Upon curing they shrink more as compared to amor-phous thermoplastics. Examples are polypropylene(PP), polyethylene (PE) [1].

1.2.2. Amorphous thermoplastics

They are usually transparent and have diffused melt-ing point. The molecules are arranged randomly andthat’s why do ]ot flow as easily i]to mold com_aredto crystalline polymers. Upon curing they shrink lessas compared to crystalline thermoplastics [3,4].Examples are polycarbonate (PC),polymethylmethacrylate (PMMA), polystyrene (PS),polyphenylene oxide (PPO), acrylonitrile butadienestyrene (ABS) [1].

1.2.3. Semi-crystalline thermoplastics

Semi-crystalline polymers plastics have combinedproperties of crystalline polymers and amorphouspolymers. Examples are polyester Polybutyleneterephtalate (PBT) and Polyamide (Nylon 6, Nylon66).

1.3. Thermoplastic elastomers

Thermoplastic elastomers are one of the most ver-satile _lastics i] today’s market.Thermoplastic elas-tomeric materials are a physical mixture of poly-mers (a plastic and a rubber). They exhibit the prop-erties of both plastics and rubbers. The unique prop-erties of both materials exist because TPE materi-als are created only by physical mixing of a ther-moplastic and elastomer and no chemical or cova-lent bonding exists between the two. This behaviorhas opened a new field of polymer science.Thermopalstic elastomers have become a signifi-cant part of the polymer industry. They are used inmany applications like adhesives, footwear, medi-cal devices, automobile parts house hold goods,etc.

TPE were introduced commercially in 1960.Si]ce the] they have grow] very ra_idly a]d world’sannual consumption of TPE materials is growing ata rate of 11% per year.

The first generation TPE were made using Eth-ylene Propylene Diene Monomer (EPDM) andppolypropylene(PP) [5-7]. Later on Ethylene Pro-pylene Rubber (EPR) replaced EPDM due to its lowcost. Since these materials were mixed only physi-cally and no cross linking existed there so hard ther-moplastic cannot be produced using them. Anotherdraw back is that both EPR and EPDM have lowtensile strength and less resistance to organic sol-vents especially oil or grease.

In order to resolve the problem of low thermaland chemical stability of thermoplastic elastomers,dynamic vulcanization techniques were used in late1970’s to mix the soft elastomer i]to hard thermo-plastic. In dynamic vulcanization the thermoplasticand elastomers are both cross-linked and physi-cally mixed together. This gave rise to a secondgeneration of thermoplastic elastomers which hadbetter tensile strength and oil resistance as com-pared to those formed by physical mixing only [8].I] 90’s third ge]eratio] thermo_lastic elastomers

were introduced which were made by same dynamicvulcanization process but using natural and butylrubbers in place of EPR and EPDM. Natural andbutyl rubbers have the advantages like low cost,very good over molding properties, reusable and veryeasily recyclable scrap [9,10]. Butyl rubber basedthermoplastic elastomers have excellent adhesionwith other thermoplastics hence it is still used todate for many over molding applications [11]. A listof common thermoplastic elastomers used in mod-ern world is given in Table 1.

1.4. Uses of thermoplastic elastomericmaterials

There are countless applications where thermoplas-tic elastomeric materials are used. Major applica-tions include replacements for artificial and naturalrubber, foam making, soft and hard thermal insula-tion sheets, door and window handles for house-hold and automobiles, car dash boards, car dashboards knobs, bumpers, CV joint boots, suspen-sion bushings, window & door trim, seals, floor Mats,gear Knobs, flexible grip, mirror case, automotivegaskets, colored interior components and manyother body parts, house hold plastic furniture, coathangers, lacquer, varnishes, paints, latex, adhesives,weather stripping applications, shoe soles, sportsequipment, electrical accessories, expansion joints,water sealing rubbers, pipes, belts, ring gaskets,electronics appliance casings, handles for knives,scissors, and other non slip grip equipment, foodpackaging cling films, syringes, nabulizers, cath-eters, components for gas masks, flexible tubingfor various applications, toys, plastic eye wear, as-phalt modification, personnel hygiene equipment likerazors, shavers, safety equipment, casings for leadacid batteries, document lamination films, packingfoam, blister packs of mediices, workshop helmets,lenses for cameras, projectors and copiers, softcontact lenses for eyesight, vials, monitors, medi-cal devices like IV sets, blood bags seal, urine bagsseal, and latest in replacement of metals in many


Amorphous Crystalline

Low cost PVC Poly Vinyl Chloride HDPE Poly Ethylene High DensitySAN Styrene Acrylo Nitrile LDPE Poly Ethylene Low DensityPS Poly Styrene PP Poly PropylenePMMA Poly Methyl Metha

CrylateABS Acrylonitrile Butadiene

StyreneSMA Styrene Maleic

Anhydride

Medium Cost PPO Poly Phenlene Oxide UHMWPE Ultra High Molecular WeightPC Poly Carbonate Poly EthylenePPC Poly Phthalate POM Poly Oxy Methylene

Carbonate PA Poly AmidePTFE Poly Tetra Fluoro PBT Poly Butylene Terephthalate

Ethylene(Teflon) PET Poly Ethylene Terephthalate

High Cost PAR Poly Arylate PA-4,6 Poly Amide-4,6PES Poly Ether Sulfone PPA Poly Phthal AmidePEI Poly Ether Imide PPS Poly Phenylene SulfidePPSU Poly Phenyl Sulfone LCP Liquid Crystal PolymersTPI Thermoplastic PVDF Poly Vinyl Diene Fluoride

Polyimide FP Fluoro PolymersPAI Poly Amide Imide PEEK Poly Ether Ether Ketone

Table 1. Common thermoplastic elastomeric materials.

applications like aircraft, space ships where theyprovide strength like metals but in less weight, in-door/outdoor electrical cable insulation, optical fi-ber sheaths, welding cable insulation, high voltageautomobile cables insulation, coil forms and bob-bins for transformers, parts for motors, aerospaceelectrical parts, flame retardant insulations for cablesand housing for composite high voltage outdoor in-sulators, substation and transformer bushings, overmolded products and encapsulation of coils andmicroelectronic chips etc.[1,2,4-6,12-14].

1.4.1. Over molding [15]

Over molding means outside covering of any mate-rial with any other material to change it appearanceand aesthetic properties. Now a days thermoplas-tic elastomeric materials are very widely used forover molding the rigid products like knives, handheld electronics, house wares and computers, handtools, personnel hygiene tools and a lot of consumermarket products requiring improved soft feel andtouch, good impact absorption, aesthetics anddamping of vibration along with excellent perfor-

mance. Polypropylene and polyethylene are the bestsuitable among the TPE materials for over moldingpurposes.

The major factors responsible for strong bond-ing and adhesion between a TPE and a hard innermaterial or substrate are [6,7]UChemistry of TPEUChemistry of substrateUMatching the surface energy on both

materials(selection of proper TPE in combinationwith substrateUHydrophobicity of TPE on substrate surfaceUOver molding process, machine type and material

preparation,USubstrate design and mold.

1.4.2. Encapsulation of coils andelectronic chips

The encapsulation is covering of an object with somematerial to protect and/ or insulate it. Polymericmaterials are used extensively as encapsulants forcoils, motor windings and micro electronics pack-aging industry. Since late 1980s the thermoset rub-


bers were mainly used for encapsulating coils, elec-tronics chips etc. Use of thermoplastic elastomersspeed manufacturing with low cost compared withany other material and technology [16].

Thermoplastic encapsulation has almost 40%less cost than using thermoset rubbers because ofrapid mold cycles and possibility of injection mold-ing which enables to efficiently integrate separateparts into the encapsulating component. Polyesterand polyamides are best suitable options amongthe TPE materials for encapsulation [17].

Lianhua Fan and C. P. Wong investigated theperformance of thermoplastic materials along withthermosetting materials as encapsulation materi-als for some special microelectronic products. Boththermosetting bisphenol epoxy and thermoplasticphenoxy resins were investigated in terms of theiradhesion capacity durability. The results indicatethat thermoplastic resins having high molecularweight chains show more adhesion capabilities thanthermosetting ones [18]. There results of this workare particularly useful towards the development ofmaterials for future generation of low cost solenoidand microelectronic packaging, etc.

1.5. Advantages.

TPE materials have performance properties like thatof thermoset rubbers but with the process and de-sign flexibility like plastics, thereby providing widedesign options and enhanced cost reduction oppor-tunities [1]

They can be processed speedily, efficiently andeconomically. Thermoplastics and can be insertmolded with other olefin based materials, such aspolypropylene, without the use of adhesives. Theother advantages include lightweight, elastic recov-ery properties with in a specific temperature range,metal replacement, noise reduction by self lubrica-tion, very good electrical insulation properties, heatresistance with in a specific temperature range, oilresistance, improved adhesion, tear resistant sur-face, low permeability and colorable and can bemade in a variety of hardness grades[2,6,12,13].Worldwide consumption of TPE rubbers of all typeswas estimated about 3.2 million metric tons for theyear 2009 and the % share of different types amongthem is shown on a pie chart in Fig. 1 [19,20].

1.6. Disadvantages

In comparison to thermoset rubbers TPE materialshave decreased chemical & environmental resis-tance and thermal resistances as they melt or be-

Fig. 1. Percentage share of different TPE materialsin global consumer market.

come soft at high temperatures and lose their rub-bery behavior.

The cost of TPE materials are higher than thethermoset materials, in addition to this they can-not be used with fillers in order to make productsrequiring large amounts of material at low cost. TPEmaterials also lose their surface hydrophobicity inshort time when use din outdoor environment espe-cially in high voltage insulation applications.

2. CLASSIFICATION AND CHEMISTRYOF THERMOPLASTICELASTOMERIC MATERIALS

Thermoplastic elastomers are classified by theirchemistry into following six categories [2,5,7,21].1. Poly Styrenes2. Poly Olefins3. Poly Ether Imides4. Poly Urethanes5. Poly Esters6. Poly Amides

Each category has a slight difference in its chem-istry and therefore offers different properties. Thermoset rubbers have chemical cross linking which isachieved by the process of vulcanization. Vulcani-zation is an irreversible and slow process whichtakes place upon heating. The thermoplastic elas-tomers are formed by a physical mixing and/or crosslinking of two components, which is a reversible pro-cess and occurs upon cooling the reactants. TPEmaterials can be polar (copolyesters, copolyamides,polyurethanes) or non-polar (PP, PE EPM, EPDM,NR, BD).

2.1. Chemical structure

In simplest form TPE materials are like an X-Y-Xblock copolymers. Where X is a hard thermoplastic


at room temperature but soften at elevated tempera-tures, and Y is a soft elastomer. Examples for X arepolystyrene, polyethylene, or polypropylene,whereas examples for Y are polydimethylsiloxane,polybutadiene, polyisoprene or poly (ethylene-pro-pylene) [2]. Most polymer pairs are incompatiblethermodynamically due to different surface energiesand so in hot state their mixture separates into twophases [5,22]. This is the reason thermoplastic elas-tomers are not heat resistant beyond a certain tem-perature range.

In order to understand the chemistry of TPEmaterials consider polystyrene which is an exampleof styrenic block copolymers. Polystyrene has hardspherical styrene domains connected with soft elas-tomer segments as shown in Fig. 2. The ends ofelastomer segments are dispersed in hard styrenedomains [23]. Polystyrene is hard at normal tem-perature and melts on high temperatures.

This structure resembles the one present in vul-canizing rubbers having sulphur cross linking. Themain difference is that thermoplastic domains loseelastomer segments in TPE materils upon heatingor dissolving in solvents. This causes the moltenmaterial to flow. Again upon cooling the material oron the evaporation of solvent the physical links be-tween thermoplastic domains and elastomers areformed and material hardens itself [24]. The aboveexplanation applies to all kinds of thermoplasticelastomers which have structure like X–Y–X–Y-X....

The block copolymers having structures like X–Y or Y–X–Y where X is a]y hard thermo_lastic a]dY is any soft elastomer) are not called thermoplas-tic elastomers. The reason is that for a thermoplas-tic elastomer to be stable at any given temperature,both end points of elastomer segment should betrapped in hard thermoplastic domains. The materi-als formed by structure Y-X-Y are much weaker like

Fig. 2. Structure of Poly Styrene.

un-vulcanized synthetic rubbers [25]. The chemis-try described until now applies particularly to styrenicblock copolymers but in general to all six catego-ries of thermoplastic elastomers. Each category hasits own specific properties which will be discussednow.

2.1.1. Poly Styrenes

Poly Styrenes are the larget group among TPEmaterials and probably the most versatile, they canbe produced over a variety of hardness values.Styrenic block copolymers are amorphous andopaque polymers. Styrenic block copolymers haveelastomer segments with relatively short lengths soeach elastomer may pass through a few hard do-mains before it ends. These are most cheap andwidely used category of thermoplastic elastomersfor making different kinds of foam and packagingmaterials. The typical value of glass transition tem-perature for these polymers can be exceeding 95°C. They are resitant to polar solvents where as theydisssolve in non-polar solvents [5,25,26]. A styreneblock is shown in Fig. 3a. This block acting as hardthermoplastic can be attahed with many differentelastomers to make different styrenic blockthermplastic ealsotmers. The most commonly useelastomers with styrene are butdiene, isoprene andethylene-butylene. The structure fromed using theseelastomers are shown in Figs. 3b - 3d [26, 27,28].

2.1.2. Poly olefins

This is a new class among thermoplastic elasto-meric materials as compared to styrenic block co-polymers and they are also called Poly Alkenes.Polyolefins are simple long chain hydrocarbons,polyolefins are produced by reaction of an alkeneas a monmer with general formula CnH2n. Low andhigh density polyethylene copolymer, polypropylenecopolymer and poly methyl pentene are most promi-nent polyolefins in use [29]. Polyethylene andpolypropylene are produced by long chian polymer-ization of olefins ethylene and propylene respec-tively as shown in Fig. 4.

Polyolefin block copolymers are amorphous andtransparent polymers. Polyolefins are chemicallyinert, extremely flexible, nontoxic, very light weightand sterile. They are resitant to all kinds of solvents.The typical value of glass transition temperature forthese _olymers ca] be exceedi]g 140 °C. Polyolefi]block copolymers are mostly injection moulded andcan be produced in any shape like cast, wire orfilm. The films are most commonly used in food andblister packaging of medicines. They are also used


a) Structure of a styrene block

b) Styrene-Butadiene-Styrene

c) Styrene-Isoprene-Styrene

d) Styrene-Ethylen-Butylene-Styrene

Fig. 3. Chemical structure of different kinds of poly styrenes.

Fig. 4. Chemical structure of different polyolefins.

to make lenses for cameras, projectors and copi-ers, LCD monitors, contacnt lenses for eye and re-inforced tubes or pipes for medical use [30]. Wiresmade up of Polyolefin block copolymers are usedas fiber optical media because of there excellentglass like transparency, low dielectric loss over timeand high resistance to dielectric breakdown. Longterm exposure to sunlight and strong oxidizingagents causes brittle ness in them which may leadto shattering [31].

2.1.3. Poly ether imides

Polyetherimides are also called thermo plasticvulcanizates. Thermoplastic vulcanizates are com-pounds with fully cross linked rubber. They are madeup of long elastomers bonded with thermoplastics.Structure of a typical polyethermide is shown in Fig.5. They are crystalline, highly elastic and have ex-cellent impact resistance due to long elastomersegments present in them. Their natural color isamber or either black but can be dyed in any color

[29]. They are light weight and have very goodweather and chemical resistance as compared toother TPE materials. They are highly resistant toflame and fire. They have high resistance to surfacechalking.


Fig. 5. Chemical structure of a typical polyetherimide.

The typical value of glass transition temperaturefor these _olymers ca] be exceedi]g 225 °C. Theyare resitant to majority of solvents. Their uses in-clude electrical & electronics cable insulations, flameretaredant insulations, industrial tubing, hoses, pipesand seals for industry and medical applications [30].

2.1.4. Poly urethanes

Polyurethanes are amorphous polymers which areclass of polyurethane plastics. These are elasticand transparent polymers with outstanding high andlow teamprture performance [29]. The typical valueof glass transition temperature for these polymersca] be exceedi]g 190 °C. Thermo_lastic _olyure-thanes are composed of both long chain elastomers(Polyols) and short chain elastomers (Diyols) withshort and hard segments of urethane thermoplasticin alternating positions, the segments are linkedtogether by covalent bonds [32]. Chemical struc-ture of a polyurethane elastomer is shown in Fig. 6.

Therefore we can make unlimited combinationsof tthermoplastic polyurethanes with characteristicsas desired, by varying the structure and/or molecu-lar weight of the polyols, diols and thermpalstic [33].That’s why a large variety of thermo_lastic _olyure-thanes is now available in market. Thermoplasticpolyurethanes have very good resistance to mechani-cal impact, oil, grease, and abrasion.

2.1.5. Poly esters

Polyesters get their name due to presence of estergroup in them which has hydro carbon chains at-tached with them. Copolyestors are are crystallineand transparent polymers. The typical value of glasstransition temperature for these polymers can beexceedi]g 185-220 °C. they have very high mecha]i-cal and tensile strength. They are resistant to al-most all kinds of solvents and chemicals. The twovery common copolysetors are Poly EthyleneTerephthalate (PET) and Poly Ethylene

Naphthalate(PEN). The structure of PET is shownin Fig. 7. PET is most commonly used for makingshatter free bottles for beverages.

The ester is polar group, with the negatively ion-ized oxygen atom and positively ionized carbonatom. The positive and negative poles of differentester groups are attracted together. This causes alinear alignment of ester groups to result in a crys-talline form. Due to this crystalline structurecopolyestors can form strong fibers [6].

Poly Ethylene Naphthalate (PEN) is a new kindof polyester as compared to PET. PEN has a higherglass transition temperature than PET. The chemi-cal structure of PEN is just like PET except in placeof one there are two benzene rings attached together[27]. Presence of one extra benzene ring makesthe PEN more resistant to heat than PET. To makea bottle heat resistant a small amount of PEN ismixed with PET which gives a bottle that can takethe heat a lot better than only PET. Copolyestorsare cheap and widely used in making high volume,low cost plastic items, packaging of many kinds ofindustrial, commercial, medical and domestic flu-ids, drinks, etc. [29].

2.1.6. Poly amides

Polyamides contain amide as their repeated unitwhich is linked by different elastomers. An amidegroup has the formula CONH2 and the structure asshown below.

Polyamides are amorphous and opaque poly-mers which very high resistances to temperatureand electrical stress, mechanical stress, flame andchemicals [29]. The typical value of glass transitiontemperature for these polymers can be exceeding


Fig. 6. Chemical structure of a typical polyurethane.

Fig. 7. Chemical structure of PET (a very commonly used polyester).

220-275 °C. Polyamides ca] be made i] a largevariety, each type is useful for specific applications,and the most widely used polyamides are PA6, PA6.6 (also known as Nylon), kevlar and PA 4.6. Thekevlar and PA 4.6 rein forced with glass fibers areused for metal replacement [34]. Polyamides showlow viscosity and high fluidity at elevated tempera-tures, therefore they are very to mold in thin partsusing injection molding.

Nylon and nylon 6.6 are formed by combing theamide groups with chains of five and six carbon at-oms which is shown in Figs. 8a and 8b respec-tively. The structure of kevlar is shown in Fig. 8c.

Kevlar

Kevlar is a very strong material, about five times asstrong as steel in equivalent weight. Kevlar is madewhen amide groups are linked by a benzene ringinstead of long carbon chains. The presence of ben-zene ring imparts extreme strength to kevlar andhence it is used for metal replacement. Besides allthe advantages a major issue with polyamides istheir affinity to absorb moisture. Moisture uptakecauses variation in the dimensions. Amides are alsoeasily attacked by acids or alkalis even in diluteconcentrations in case of acids. The reaction iscatalyzed by high temperatures. The reason is thatamide linkage is readily broken down by dilute acid,and the elastomer fibers in the material are detachedand destroyed. The net result is a hole in the mate-rial. Kevlar is more resistant to acid or alkali attackas compared to nylon [36].

Nylon is used in textiles for clothing and car-pets, tire cords, ropes. Kevlar is used in bulletproofvests, lightweight skis, in replacement of metals in

many applications like space and aircraft bodies,light weight automobiles etc.

2.2. Environmental Issues

Thermoplastic elastomers are stable materials. Theydecompose slowly under normal circumstances.When decomposed they usually convert into theproducts which are not hazardous and do not poseany problems [2,3]. However the products contain-ing polyolefins and PVC should be handled with careso as to avoid their overheating. Also they shouldbe disposed off properly to avoid their burning be-cause they emit isocyanates and caprolactam,which are potential pollutants and cancer produc-ing carcinogenic agents [6,25].

3. MECHANICAL PROPERTIES OFTHERMOPLASTIC ELASTOMERICMATERIALS

With a huge variety of TPE materials, the futureexpectations from TPE materials are very wide.Most thermoplastic elastomers are phase separatedso they show many characteristics of the individualpolymers constituents [2]. For example, each poly-mer has its own glass-transition temperature (Tg orcrystal melting point Tm if it is crystalline). This inturn determines the temperatures at which a par-ticular thermoplastic elastomer will melt. So broadlyspeaking, elasticity modulus curve of any certainthermoplastic elastomer shows three has three dis-tinct regions as shown in Fig. 9.

At very low temperatures, both the thermoplas-tic and elastomer are in hard and brittle state. At Tg

or Tm they start melting. At temperature mentionedin useful state both of them are in rubber state. In-


Fig. 8. Chemical structure of different poly amides, replotted from [29,36].

creasing the temperature finally results in moltenstate where both thermoplastic and elastomer arein a weak liquid and moldable state. The thermo-plastic elastomer has the properties which are in-termediate between its two ingredients and shownby a grey line in Fig. 9. So a thermoplastic elas-tomer will have two service temperatures. The lowerservice temperature which is dependant on the Tg

of the elastomer and the upper service temperatureis dependant on the Tg or Tm of the hard thermo-plastic phase [5].

There are some other factors which govern themechanical properties of thermoplastic elastomers,which include effect of individual molecular weightof soft elastomers and hard thermoplastic used andthe proportion of hard and soft phases.

Thermoplastic elastomers have high meltingpoints and tensile strength when compared withhomo polymers of same molecular weight [6,11,23].The reason for this is the two phase separated struc-ture as discussed earlier. One phase is hard ther-moplastic other is soft elastomer, whereas both ofthem are retaining their properties in the mixtureand hence called two phase structure. If the ther-moplastic content is held constant while varyingcontent of elastomer, no change in tensile strengthof thermoplastic occurs. This shows that tensilestrength has no relation with the amount of elas-tomer used. In fact the tensile strength is relatedwith composition of elastomer used and the type ofbonding that exists within elastomer or betweenelastomer and thermoplastic. In addition to that ten-sile strength of a thermoplastic elastomer depends Fig. 9. Temperature Vs elasticity of TPE materials.

a lot on the amount of hard phase thermoplastic.When the amount of hard thermoplastic is increasedas compared to soft elastomer, the product firstbecomes strong rubber then leather like and finallyhard flexible plastic [25,33].

The amount of hard thermoplastic also influencesthe melting point and thermal stability of resultingthermoplastic elastomer. The very popular low costand yet having good solvent/oil resistance thermo-plastic is propylene. It is crystalline and results in athermoplastic elastomer having high upper servicetemperatures [29].

The amount and type of elastomer used in ther-moplastic elastomeric materials controls the chemi-cal resistance of resulting material. The elastomerswith double or triple carbon bond show more sol-vent and chemical resistance. The use of polar elas-


tomers like ethylene also increases oil resistancemany fold [36].

3.1. Ultra sonic welding

A very useful and advantageous property of thermo-plastic elastomers is the possibility of welding to-gether using ultra sound waves. Ultrasonic wavescause local melting of thermoplastic elastomericmaterials due resonant energy absorption [37]. Ul-trasonic welding is a very fast method of joining theplastics together than using any solvent or adhe-sive. The process can be automated and weldedjoint has excellently clean look. The application ofultrasonic welding for thermoplastic elastomers wasfirst introduced in 1965 by Soloff and SeymourLinsley [38,39]

Crystalline and semi crystalline thermoplasticelastomers are most suitable candidate for ultra-sonic welding, however amorphous thermoplasticelastomers can also be welded together by thismethod but with less tensile strength. Details of ul-trasonic welding systems can be seen in [40,41]

Ultrasonic welding is most widely used in pack-aging, sealing containers of explosive materialsbutane cigarette lighters, and lead acid batterieswhich cannot be sealed using heat or flame, seal-ing tubes and blister packs of medicines are amongother important to mention [42].

3.2. Recycling

Easy recycling and repairing is the factor that em-phasis the use of thermoplastic elastomers overconventional thermoset rubbers. For recycling thescrap, the first step is to grind it. To grind it easilymaterial should be absolutely dry. Grinding involvescutting material with sharp and narrow gap bladeshredders and/or grinders. Often some amount ofnew and virgin hard thermoplastic is mixed withground material to increase its tensile strength andreduce brittleness. This mixture is then melted andremolded [25].

There are many requirements where a part madeup of rubber or plastic needs repair for example incase of automobile exterior parts, and in applica-tion where replacement is difficult due to financialor availability constraints. The broken or crackedportion of any part made up of thermoplastic elas-tomer can be easily repaired by just melting andjoining it to settle again. However the molten jointafter settlement some times may not have sametensile strength as before, to counter this metalstrips or fibers can be easily be incorporated in the

joint while maintaining the aesthesis of part sameas before.

4. ELECTRICAL PROPERTIES OFTHERMOPLASTIC ELASTOMERICMATERIALS

With growing transmission voltages high reliabilityinsulating materials are required. The performanceof a material as electrical insulation is determinedby its dielectric breakdown strength [43]. Thisbreakdown strength depends on the factors like fre-quency and waveform of voltage applied, moisturelevel, temperature, partial discharges, impurities inthat material, thickness and layers of insulation,dielectric constant and volume resistivity [44,45].

For a material to qualify for the use in electricalinsulation there are some particular requirementson its properties. These properties include low spe-cific weight, good mechanical, chemical and ther-mal strength, good surface leakage resistance, abil-ity to maintain good surface hydrophobicity, easyprocessing and production [46]. A good combina-tion of these properties is found in modern thermo-plastic elastomers (TPE). Before the introduction ofTPE materials ble]ds of ethyle]e–_ro_yle]e–die]emonomer (EPDM) were introduced for electrical in-sulatio] for the first time i] 1960’s. But they _rovedto be a failure shortly [44]. Ethyle]e–_ro_yle]e co-polymer (EPR) and silicon rubber (SiR) were theni]troduced till 1980’s. I] late 90’s TPE materialswere first time introduced in the field of electricalinsulation. In recent years the importance of ther-moplastic elastomeric materials has increased ex-ponentially for indoor, outdoor, machine and cableinsulations.

Olefinic block copolymers are well known in theelectro technical field. Among them polyethylene isused for insulation in high and medium voltage un-derground cables.

Polyesters and polyamides are particularly use-ful in encapsulation of coils and electronic devices.For manufacturing motors glass reinforced polya-mides and polyesters are very suitable. Polyestershave very good dimensional stability and high resis-tance to thermal deformation. Among polyestersPET has been most widely used today to encapsu-late many electronics devices in order to protectthem from environmental effect in addition to elec-trical insulation. When compared with the metals,PET polyester and polyamides reinforced with 30to 45% glass fibers, show sufficient strength to beused for making all parts of a motor provided thatthe designed enhances the beam strength [47].


Encapsulation of coils in polyester or polyamidesreplaces the need of tape wrapping.

In modern electrical manufacturing the coil bob-bins for transformers are made with 6,6 nylon. Sta-tor, end bracket sand housings for motors are madewith PET polyester. PET does not corrode and henceparts made up of it require no painting or machin-ing. The bearings are generally made with polya-mides which have natural lubrication and can beused without any other lubricant. Since loss of lu-bricant cannot occur so chance of failure is com-pletely eliminated, and startup becomes fast in coldweather conditions [47]. A more detailed study onelectrical properties of polyamides is presented in[48] where the authors rate polyamide among thevery successful for making indoor insulation, how-ever authors report that polyamides due to theirextreme affinity for moisture, may not be suitablefor making outdoor insulation. Polyimides such asKapton is used largely for making the electrical in-sulation in protected environments, but it is onlyavailable as a film [49].

PET is also used in gas insulated substation.Yannick Kieffel and his co workers [49] performedan investigation to see suitability of PET for makinggas insulated substation (GIS) post insulators. Allthe results obtained on virgin material as well as onaged material fully indicate that the selected PETis suitable for thick insulation in high voltage appli-cations. The thermal as well as electrical and me-chanical characteristics satisfy the specification ofthe GIS. In conclusion, the recyclability and reli-ability of GIS can be improved by using TPE mate-rials [49]. However for application as outdoor insu-lator, it does not have sufficient chemical resistance[50].

Sam J. Ferrito and his coworkers in their study[51] investigated the change in electrical propertiesof the three polyester materials, i.e., PBT, PET, andPCT, along with two polyphthalamide materials PPA) at 60 °C a]d a relative humidity level of 100%.The results show that the PBT and PET materialsare the most prone to hydrolytic degradation whencompared to PCT and PPA. The obvious reasonbehind it seems to be lack of symmetry in the mo-lecular weight distribution among PBT and PET.S. Grzybowski and his co workers in [52] performeda detailed study on break down voltage of samplesof polyester cups of different thickness used forencapsulating high voltage coils and electronicscomponents. Results of this study show that in drystate DC breakdown voltage of samples was muchhigher than that of the AC breakdown voltage. BothAC and DC breakdown voltages reduce upon wet-

ting but DC breakdown voltage degraded more sig-nificantly as compared to AC breakdown voltage.DC breakdown voltages are widely scattered ascompared to AC breakdown voltages.

As already discussed in Section 2.1.5 polyes-ter and polyamides are very prone to undergo hy-drolysis. Even they can hydrolyze processing soextreme care should be taken in drying them priorto molding. Another source of hydrolytic degrada-tion is environmental aging. Hydrolytic degradationof the polymer causes random chain scissions tooccur, normally at the ester and amide linkages,which causes a reduction in molecular weight andhence a reduction in mechanical integrity [53-55].Also, ester hydrolysis produces hydrophilic acid(carboxyl) end groups in the polymer which increasethe ability for the polymer to absorb water decreaseits surface resistance and hydrophobicity [54,56,57].This degradation process also occurs in case ofpolyetherimides and polyurethanes particularly athigh temperature and humidity [58]. So each ther-moplastic polymer category demands a comprehen-sive evaluation before its use in any environmentand/or application [59].

Polyetherimides (PEI) are used for high voltageswitchgear applications. For semi-crystalline ther-moplastic, mechanical and electrical propertiesdepend on the morphology of the material especiallythe ratio between crystalline and amorphous parts[49]. A change of components share in copolymersis an effective way to obtain materials with requiredparameters. This is the reason many researchersare focusing their work to see the effect of nanofillersaddition to polymeric materials in order to enhancetheir mechanical and electrical properties.

5. PERFORMANCE OF TPEMATERIALS AS OUTDOORELECTRICAL INSULATION

Since biological effects are different in each regionof earth, so it requires aging of polymeric insulatorsin each region of world prior to use its use. Aging isthe application of stresses to an insulator, that it isexpected to encounter in real life service environ-ment, in order to see what would be its status and/or performance after a certain time.

5.1. Natural aging

Installation of an insulator in actual outdoor environ-ment for the period of time it performance is to bemeasured/estimated. The Natural aging has the drawback it takes a long time to age a sample. For ex-


ample if we want to see the effect of aging on aninsulator after 30 years, no one can wait 30 years.To resolve this problem, H.M. Schneider and hisassociates first ever put the concept of acceleratedmulti stress lab aging in 1991.

5.2. Accelerated multi stress lab aging

In the accelerated multi stress lab aging, all thestresses are applied on insulators like they appearin real life, but in a calculated accelerated manner,e.g. day and night cycles are simulated faster thanactual. The other factors such as temperature, UVradiation simulating sunlight, rain, etc. are also in-creased. The increase is not straight. Some stressescancel each other, whereas others enhance eachother, e.g. effect of UV is nullified by rain, but theheat multiplies the effect of UV. So while applyingacceleration one has to take into account these fac-tors [59].

Accelerated multi stress aging of polymeric in-sulators simulates real field conditions and producesresult just like natural aging .It also has advantageof causing these changes to occur in less time.Hence effect of long term natural aging can beachieved in short time. The accelerated multi stresslab aging has now become very popular to test anymaterial and especially polymeric insulators.

5.3. Accelerated multi stress aging ofTPE insulators in differentenvironments

S.M. Oliveria and C.H. Tourreil [60] studied the com-posite EPR (Ethylene Propylene Rubber) underIEEE/IEC 1000 h test procedures. The authors statethat EPR formulations perform better than the firstgeneration of epoxy resin insulators. The authorsclaim that aging test methods and the acceptancecriteria proposed by IEC and the IEE are not suit-able to describe the performance of several poly-meric insulator materials under different environ-ments of the world. So test procedures for everyenvironment should be developed individually.

H. M. Schneider et al.[61] presented the behav-ior of 138 kV non-ceramic (polymeric) line post in-sulators investigated by means of clean fog testsconducted before and after aging in a specially de-signed accelerated aging chamber simulating theenvironment of coastal regions of Florida. The re-sults of this study show that clean fog flashover volt-age of polymeric insulators exceeded that of thehigh strength porcelain line post insulators. Authorsstate that ESDD is not an adequate index of con-

tamination severity for polymeric insulators whichhave high hydrophobicity. Polymeric insulators pro-duce increased audible noise but decreased radiointerference as a result of aging.

J. L Fierro Chavez, et al. [62] developed a sys-tem for measurement of leakage current and sur-face resistance of TPE insulators in Monterrey,Mexico. This system was installed on several tow-ers along the 400 kV line as a diagnostic tool tomonitor the surface condition of polluted insulators.The results showed values from 50-150 mA in firsttwo months of installation. But such high values neverappear again when some of faulty insulators werereplaced once. The authors suggest that cleaningor replacement of insulators is necessary whenpeaks of 250 mA or more are observed in rainy sea-son.

R. Matsuoka et al. [63] presented an overview ofdifferent techniques use to study the aging of TPEinsulator. Authors suggest a most suitable set offollowing five techniques namely Visual inspection,FTIR, SEM, hydrophobicity, and leakage currentmeasurements.

R. Sundararajan et al. [64] presented a one yearfield and lab aging study of 28 kV distribution deadend type insulators in two different US environments.Weather cycles simulating summer and winter ofPhoenix and Boston were developed. Good correla-tion between field and lab aging was observed. Au-thors conclude that meaning full studies can becarried out to observe the long term performance ofa number of insulators at a given environment usingaccelerated multi stress lab aging. Based on re-sults authors have guaranteed a 15 year life of theseinsulators. However the actual life may be morelonger than this.

P. Cygan and J.R. Laghari [65] presented anoverview of multi stress aging techniques. The au-thors have presented the several methods used byresearchers all over the world to accelerate the ag-ing process, but suggest that most popular are theexperiments performed at voltages and tempera-tures. The authors classify all methods in two cat-egories. In first category stresses are kept constanttime to failure or satisfactory period is noted. In thesecond category stresses are increased in stepsuntil the failure occurs. The results from both of themare then correlated to get an empirical formula ormodel of aging. The authors have also discussedfeasibility of setting many mathematical models, toget equation of aging from experimental data. Atthe end authors predict that multi stress aging willbe the most suitable option in a few years to dem-onstrate the performance of insulating materials.


Ali Naderian, Majid Sanaye, Hosein Mohseni [66]presented a review of artificial contamination withstand test methods used for polymeric materials.The authors give details of equivalent fog test, pollu-tion rain test, variable voltage test, rapid flashovertest, dust Cycle test, salt fog test, clean fog, accel-erated multi stress tests, etc. Authors state thatalthough several standards and recommendationshave been published by many recognized organiza-tions in order to carry the aging of composite insu-lators, but still there remains some uncertainty aboutthe contamination behavior of these insulators. Be-cause of this there are several non-standard testrecommendations formulated to evaluate the behav-ior of insulators under different conditions which areproving to be better than standard methods. At theend authors stress that it is necessary to deter-mine age polymeric insulators for field applicationof different environments so that users and custom-ers can select proper composite insulators accord-ing to the each environment.

C. Olave et al. [67] conducted 5000 h multi stressaging of 28 kV TPE insulators under slightly modi-fied IEC conditions. There was no major degrada-tion, such as flashovers, heavy arcing, erosion,cracking or tracking. In general the insulators per-formed well showing only superficial changes likecorrosion of the HV end fittings, light arcing, andshallow cracking. Authors conclude that TPE insu-lators can perform well in many service environmentsin excess of 20 years at least. Further more accel-erated multi stress aging is very beneficial to seeperformance of any insulator in nay service environ-ment.

J.L. Bessede and his coworkers [68] analyzedthe suitability of thermoplastic polymer used assupport insulator in high voltage gas insulated sub-station using a semi crystalline thermoplastic poly-ethylene-terephthalate (PET) using accelerated multistress aging. A plate shape insulator for 72.5 kVgas insulated substation was made and tested.Results show that main electrical properties of thematerial were not affected by aging under used SF6.Authors have estimated a 24 year satisfactory lifeof these insulators.

Raji Sundararajan et al. [69] performed multistress accelerated aging of polymer housed ethyl-ene propylene silicone blend (EPSB) and SiliconRubber (SiR) surge arrestors under coastal Floridaconditions. The authors simulated 15 lab years. SiRshowed less physical changes as compared toEPSB. But leakage current was higher for SiR ascompared to EPSB. However on the overall boththe materials preserved excellent surface resistance

at the end of 15 years of lab aging Authors statethat is very beneficial to use multi stress aging toevaluate the long term performance of polymericarresters and insulators in the field.

R. Sundararajan [70] presented the results of a2544 hours multi stress aging of TPE and SiR poly-meric insulators under simulated conditions ofMexico City. The results indicate that the siliconerubber insulators perform better than TPE and thereis discoloration, light arcing loss of hydrophobicityand bond breaking in both type of insulators, butstill they have preserved good electrical and me-chanical x-tics and are estimated to serve severalyears in the field.

Salman Amin et al. performed a 10000 hourscomparative multi stress accelerated aging of TPEand SiR insulators in the environment of HATTAR,Pakistan under the effect of UVA, heat, salt fog,acid rain and electric stress. The analyses weremade using electrical (leakage current), physical(discoloration, hydrophobicity classification) vari-ables, and sophisticated material analysis tech-niques, such as Fourier Transform Infrared Spec-troscopy (FTIR) and Scanning Electron Microscopy(SEM). It was seen that all insulators showed ex-cellent performance till 13th year. But in 14th and 15th

year all of them showed a drastic change, howeverstill preserving good electrical properties. There wasa drastic change in results of all techniques of analy-sis at the 9000 hours point which is equivalent to 15lab years. So an aging exceeding this point mustbe carried. To investigate this more a 30 years simu-lated aging is under way by same authors.

6. CONCLUSIONS

The use of thermoplastic elastomeric materials iscontinuously increasing for huge applications in dailylife. Over molding and encapsulation are most widelyused applications of TPE materials. About 85% ofplastic products in the modern world are made upof TPE materials, because they provide competitiveprice performance as compared to other natural rub-bers. The selection of a TPE material for any par-ticular application requires a special focus on itsphysical, chemical, mechanical and electrical prop-erties. The glass transition temperature Tg of a TPEmaterial is the temperature at which many of itsproperties start changing. Above the Tg the polymeris most susceptible to degradation.

All TPE materials are based on two major com-ponents a hard thermoplastic and a soft elastomer.The physical properties are showing the advantagesof both the rubber and plastics. TPE materials are


environment friendly and very easy to recycle. Inaddition to that their major disadvantages are lowthermal and chemical resistance and a bit high cost.But the advantages offered by them dominate thedisadva]tages that’s why their use is i]creasi]g dayby day.

Thermoplastic elastomers are classified into fol-lowing six categories, namely polystyrenes,polyolefins, polyetherimides, polyurethanes, poly-esters, polyamides. Each category has a slight dif-ference in its chemistry and therefore offers differ-ent properties. Styrenic block copolymers are cheap-est and most widely used. Next popular arepolyolefins and then polyetherimides. Polyamideshave highest thermal resistance and cost. Polyes-ters shave highest tensile strength.

For electrical insulation applications the surfaceof material is most important which is character-ized in terms of it hydrophobicity, resistance andreactance. The insulation properties of a materialare dependant on the physical displacement ofcharge in the bulk of that material upon applicationof a voltage. Any impurities present or contamina-tion introduced into the system can greatly affectthe dielectric characteristics of the material. TPEmaterials when used in electrical applications canbe exposed to many kinds of stresses which in-clude heat, UV radiation, humidity, biological deg-radation, mechanical stress etc. Multi-stress agingmust be designed into the testing and qualificationprocess in order to achieve a representative modelof the interactions encountered. An example of amulti-stress test could include a setup which ex-poses a thermoplastic elastomer to temperature,humidity, UV, mechanical, and electrical stressessimultaneously. For indoor electrical insulation poly-esters and polyamides are most commonly used.For outdoor insulation the most popular are blendsof polyolefins. Polyethermides are also used in highvoltage switch gear application but on limited scale.

Accelerated multi stress aging has become verypopular since last 15 years to investigate the longterm aging and performance of any kind of insula-tions all over the world. Many authors have investi-gated the aging and electrical performance of TPEinsulators in various outdoor environments to vary-ing extents. To estimate the sate of aging visualobservation, hydrophobicity classification, leakagecurrent, SEM, and FTIR are the most commonlyused techniques by many authors all over the world.The results are quite satisfactory except that TPEmaterials cannot maintain their surface hydropho-bicity. IEEE/ IEC standards have been reported by

authors to be inappropriate as they reflect only spe-cific regional impact, and authors suggest using theindividual environmental stress values instead ofthese standards. Maximum Multi stress aging doneby any author is of 10000 hours. Maximum naturalaging done is of 7 years; however various insulatorstested after removing from service environments haveages of up to 20 years. The life estimation is doneby some authors and ranges between 15 to 30 yearswith most typical value of 24 years. A very long termmulti stress accelerated aging of thermoplastic in-sulators under varying environments is needed.

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