Electrical and Electronics Applications of Polymers and Composites

December 2018

The emergence of the electrical and electronics industry and the emergence of polymers as a new class of materials are both modern phenomena. The electric relay, a remote switch controlled by electricity that was invented in 1835, was the first electronic device. Bakelite, a thermosetting phenol formaldehyde resin that was developed in 1907, was the first completely synthetic polymer. Hence the electrical and electronics industry and the plastics industry have grown contemporaneously for just a little over a century.

Thermoplastic and thermoset polymers and their composites are used increasingly more frequently today, in a wide range of electrical and electronics applications, to perform many very different functions.

The main reason for this trend is the amazing versatility of polymers, which enables the design and manufacture of a vast range of products to meet very different application requirements at acceptable cost.

The abilities to prepare blends of polymer, to incorporate many types of performance-enhancing additives, and to prepare polymer matrix composites by incorporating reinforcing agents (such as fibers, platy fillers, and particulate fillers) all enhance the versatility of polymers far beyond the versatility provided by individual polymers on their own.

At Bicerano & Associates, our expertise in polymers and composites helps our clients to develop polymers and composites for any application they may require.

Material Properties

There are three very broad areas of polymer and composite material use in electrical and electronics applications. Different sets of properties are of the greatest importance in each of these three areas of use. Some properties are considered to be important in all use areas. The properties will, therefore, be discussed under the following four subheadings:

Electrically insulating polymers
Intrinsically conducting polymers
Adhesives, coatings, potting compounds, and sealants
All polymers

Electrically insulating polymers:

Electrical conductivity:

Low electrical conductivity is the first requirement for the use of any material to provide electrical insulation. Most polymers inherently have very low electrical conductivity so that polymers are used commonly as electrical insulation materials. The optimum choice of polymer for a given electrical insulation application depends on the details of that application. For example, flexible polymers are preferred as insulation materials for wires and cables while rigid polymers are preferred as housings, enclosures, and covers providing both mechanical protection and electrical insulation for many other electrical and electronic components.
On the other hand, in some applications, it is preferable to begin with an electrically insulating base polymer possessing good mechanical properties and durability but to then impart some electrical conductivity to this polymer by incorporating small amounts of selected nanofillers or by “doping” it.

Mechanical and thermomechanical properties: The following properties are important in selecting optimum insulation materials, housings, enclosures, and covers:

Flexible insulation and housing materials must have sufficient flexibility, which means that they must have a sufficiently low elastic modulus. They must also be resistant to puncture and tear and able to withstand without damage the maximum temperature that they may be exposed to during use.
Rigid insulation and housing materials must have acceptable tensile properties, impact resistance, and dimensional stability, as well as the ability to manifest acceptable properties over the entire range of temperatures that may be encountered during use.
- Elastic modulus, yield strength (for materials that manifest a yield point), and ultimate strength are generally the most important tensile properties.
- Percent elongation at yield (for materials that manifest a yield point) and ultimate elongation (the strain at which a material ruptures) are also important in some applications.
- Polymers become more brittle, so that their impact resistance decreases, as the temperature is lowered. It is, therefore, very important to verify that the low-temperature impact resistance is acceptable if very low temperatures will be encountered during use.
- Dimensional stability is usually described in terms of the heat distortion temperature which must exceed the maximum temperature that may be encountered during use.
- In practice, a distinction is sometimes made between the maximum acceptable continuous operating temperature and the highest temperature to which a polymer may be exposed briefly without irreversible damage.

Durability: Electrical and electronic devices are ubiquitous. They are used in locations ranging from the terrestrial environment of a mobile phone to subsea and outer space environments. They are used in very hot and very cold locations, in deserts and rainforests, and in the most polluted locations on our planet. Depending on the environment of use, the assessment of durability may, therefore, include the determination of whether the product will manifest sufficient weathering and aging resistance under the influence of factors such as chemical exposure (to cleaning solutions, acid rain, other pollutants, etc.), continuously high levels of relative humidity, temperature extremes, thermal shock, UV exposure, and exposure to high-energy radiation. The following are some examples:

Housings, enclosures, and covers of electrical systems must be able to withstand any environmental exposure they may encounter during use.
Elastomers possessing excellent environmental resistance can be used to provide an environmental seal for the housings, enclosures, and covers of electrical systems.
In addition to the housing materials and any elastomeric environmental seals being resistant to environmental factors themselves, these materials must also be sufficiently impermeable to any molecules present in the environment that could damage the electrical systems being housed if they are able to permeate through.
The housing materials must not themselves create a hostile environment to the electrical systems they are housing, for example, by releasing volatile molecules such as sulfur or amine compounds that can cause corrosion.
Polymer matrix composites are used in many demanding rigid insulation and housing applications to obtain adequate mechanical and thermomechanical properties along with outstanding durability. Glass fibers are the most common reinforcing agents in such composites. Polymer matrix nanocomposites are also used as the materials of construction of many housings and covers; such as power tool housings, lawn mower hoods, and covers for portable electronic equipment such as mobile phones and pagers.

Thermal conductivity: The inherently low thermal conductivity of polymers, and their resulting ability to provide thermal insulation, enhances the safety of many electrical appliances (such as toasters) where polymers are used to provide cool surfaces (such as plastic or rubber handles) for the user to touch.

Fire resistance: Fire resistance is important in the electrical and electronics applications of polymers. A familiar example of the use of fire resistance as a material selection criterion is the extensive use of PVC in wire and cable insulation, where it is often selected over other inexpensive flexible polymers because of its inherently higher fire resistance resulting from the chlorine atom in its repeat unit. Flame retardant additives are incorporated into many polymers used in devices such as computers, mobile phones, pagers, TVs, electrical connectors, printed circuit boards, and cables. Continuing research and development on non-halogenated fire retardants is driven by considerations of environmental sustainability.

Antistatic performance: Antistatic performance is also important in many electrical and electronics applications of polymers. An obvious example is that a consumer does not want to receive an electric shock from a mobile phone or other handheld device due to the buildup of static electricity on its housing. Antistatic additives are often incorporated into polymers to improve their antistatic performance.

Intrinsically conducting polymers:

The electrical conductivity range of intrinsically conductive polymers (ICPs) overlaps with the vast range spanned by inorganic semiconductors and metallic conductors. ICPs are used as alternatives to such conventional materials in a growing number of applications.

The repeat unit structure is the main factor determining the electrical conductivity of an ICP. ICP chains possess delocalized electrons, in aromatic rings, in double bonds, or both in aromatic rings and in double bonds, conjugated along their entire lengths.

"Doping" an ICP by means of reactions such as oxidation or reduction can increase its conductivity by up to several orders of magnitude and make it suitable for an application for which the precursor undoped ICP would not have sufficient conductivity. Hence a precursor ICP is doped rather than being used in its unmodified form in many applications.

Poor processability and high manufacturing costs are the main factors that have limited the use of ICPs in many large-scale applications in the past. ICPs cannot be processed directly in the melt state. Poor solubility makes solvent processing difficult as well. Nanostructured versions of ICPs which form dispersions that are then used in the fabrication of devices are helping overcome such processing challenges. Fabrication via dispersion (rather than via melt or solution) should lead to a major expansion in the large-scale applications of ICPs.

Toxicity is also a limiting factor in some applications of ICPs.

Adhesives, coatings, potting compounds, and sealants:

Adhesives: Adhesives are used extensively in electrical and electronics equipment to bond components to each other.

The most important requirement for any adhesive is the ability to bond components to each other sufficiently well that they will remain bonded to each other. This requirement implies that the adhesive strength must be sufficiently high for the needs of the application, and that the adhesive joint must be able to withstand the complete operating temperature range of the device as well as any environmental factors that it may be exposed to during use for as long as the device will be used.
Polymeric adhesives tend to have low electrical and thermal conductivity. These attributes are advantageous for most electrical and electronics applications.
There are, however, some electronics applications requiring the use of an electrically and/or thermally conductive adhesive. Such adhesives are obtained by incorporating electrically and/or thermally conductive additives into a base adhesive.

Coatings: Coatings are used to protect electrical and electronics equipment from damage through exposure to environmental factors; such as moisture, dust, chemicals, temperature extremes, UV radiation, and high-energy radiation.

Protective coatings must be highly resistant to the environmental factor(s) they are protecting against and sufficiently impermeable to protect the substrate from exposure.
Conformal coatings, defined as thin polymeric films that conform to the contours of the substrate, are often preferred for protecting sensitive components such as printed circuit boards. A conformal coating adds only a negligible amount of weight to a coated part because it is very thin. Some of the chemistries used to obtain conformal coatings are similar to the chemistries used to obtain adhesives.
Nanotechnology is leading to the development of new coatings with especially attractive performance attributes. The applications of polymer matrix nanocomposites include coatings that provide both protection and enhanced aesthetic appeal. For example, the incorporation of optimum quantities of nanoclays enhances the transparency (optical clarity) and reduces the haze of many polymer films, while also improving the strength, toughness, hardness, and abrasion resistance. It has also been shown that polymer matrix nanocomposites can be used in thin film capacitors for computer chips.

Potting compounds: Potting compounds are also used to protect electrical and electronics equipment from damage. The performance criteria summarized above for protective coatings also apply to potting compounds. The main difference is that, while conformal coatings are thin polymeric films that conform to the contours of the substrate, when using a potting compound a complete electronic assembly is filled with a solid or gelatinous compound to provide resistance to shock and vibration and to also exclude moisture and corrosive agents. Hence potting results in a deeper protective layer but adds more weight because it uses more material and also the depth of the protective layer can vary over the part rather than being the same everywhere. Some of the chemistries used in adhesives and conformal coatings are also favored for use in potting compounds.

Sealants: Sealants are also used to protect electrical and electronics equipment from damage. Sealants are used both in industrial settings and in do-it-yourself applications. Some of the chemistries used in adhesives, conformal coatings, and potting compounds are also favored for use in sealants.

All polymers:

Environmental sustainability is becoming an increasingly important consideration in material selection. It is, therefore, important to compare the expected environmental impacts of different choices of materials that meet the requirements of an application.

Cost and profit margins are usually also among the important factors in selecting a material from among candidate materials which all meet the performance requirements.

Examples of Applications

The following highlights provide a sampling of the vast range of uses of polymers and composites in electrical and electronics applications. There are three very broad areas of polymer and composite material use in electrical and electronics applications. Materials and applications will, therefore, be tabulated under the following three subheadings:

Electrically insulating polymers
Intrinsically conducting polymers
Adhesives, coatings, potting compounds, and sealants

Electrically insulating polymers:

Materials	Applications
Acetal homopolymer	See polyoxymethylene
Acrylonitrile-butadiene-styrene (ABS) copolymers, their PC/ABS blends with polycarbonate, and their glass fiber reinforced grades	Housings, enclosures, and covers for electrical and electronic devices, lids for electrical accumulators, electrical switches, distribution boards, power sets, electronic boards, telephones, pagers, connectors, power tool boxes, air conditioner fans, instrument panel parts
Cellulose	Carbon nanotubes can be incorporated into paper, which is not electrically conducting, to obtain conductive paper which can be soaked in an electrolyte to obtain flexible batteries. Since cellulose is the main component of paper, the conductive paper is a polymer matrix nanocomposite. While not being commercial yet as of 2018, this technology is highlighted here because it is a significant step forward in the emerging field of flexible (bendable) electronics.
Fluoropolymers	See perfluoroelastomers (FFKM) and polytetrafluoroethylene (PTFE)
Organic polymers of undisclosed proprietary compositions	The patented ACCUFLO T-27 Series of sacrificial fill materials from Honeywell constitute a family of organic polymers formulated in an environmentally friendly solvent system designed to fill and planarize a wide spectrum of aggressive topographies in manufacturing integrated circuits.
Perfluoroelastomers (FFKM)	FFKM is a thermoset (cross-linked) elastomer. FFKM parts and seals resist over 1800 different chemicals while offering temperature stability of up to 327 °C. These exceptional properties help maintain seal integrity, reduce maintenance and operating costs, and improve safety. The main electrical and electronics application of FFKM is in seals for contamination (particulates, outgassing, and extractables) control during semiconductor chip fabrication processes.
Phenolic resins	Electrical switch and receptacle covers
Poly(butylene terephthalate) (PBT), including its glass fiber reinforced grades	PBT is easily melt-processed. It offers good stiffness, toughness, heat resistance, dimensional stability, moisture resistance, and electrical insulation. These properties make PBT an excellent choice for use in lighting bezels, connectors, electrical components, circuit breakers, distribution boxes, and fiber optic cable jackets. Non-halogenated product grades containing alternative flame retardants are also available to facilitate compliance with recycling programs for discarded electronic products.
Poly(ethylene terephthalate) (PET), including its glass fiber reinforced grades	PET is a thermoplastic polymer. Its high melt flow rate enables the fabrication of thin-walled and/or miniaturized parts. Glass fiber reinforced PET is used in applications requiring durable electrical and electronic performance; such as coil forms, electrical encapsulation, electrical devices, solenoids, smart meters, photovoltaic panels, solar junction boxes, electrical switches, and other critical energy components. It is lightweight, stiff, UV-resistant, and dimensionally stable, so that it is an excellent candidate for the replacement for both die-cast metals and thermoset polymers. Non-halogenated product grades containing alternative flame retardants are also available to facilitate compliance with recycling programs for discarded electronic products.
Poly(polyphosphonate-co-carbonate) copolymers	Used as flame retardants in polycarbonate and in its blends (such as PC/ABS blends). Have high impact resistance and glass transition temperature while maintaining a high melt flow rate and a high limiting oxygen index.
Poly(polyphosphonate-co-carbonate) reactive oligomers	Used as flame retardants in unsaturated polyesters, epoxies, polyurethanes, and polyureas. Phenolic end groups enable the oligomers to react with the other reactive precursors and become incorporated into the thermoset polymer network. Transparent and highly soluble in the typical solvents used in thermoset resin processing.
Poly(p-phenylene sulfide)	Electrical insulation materials that are highly resistant to heat and chemical agents; ICP precursor which can be converted into a semiconducting ICP by doping
Poly(vinyl chloride) (PVC)	Flexible (plasticized) PVC is used in wire and cable insulation. It is often selected over other inexpensive flexible polymers for such uses because of its inherently higher fire resistance resulting from the chlorine atom in its repeat unit. On the other hand, the phthalates used as plasticizers in many flexible PVC formulations and the release of hydrochloric acid (HCl) during a fire are environmental concerns.
Poly(vinylidene fluoride) (PVDF)	PDVF has many electrical and electronics applications: Insulation on electrical wires, due to its flexibility, low weight, low thermal conductivity, high corrosion resistance, and heat resistance. Tactile sensor arrays, inexpensive strain gauges, and lightweight audio transducers, all take advantage of the piezoelectric and/or pyroelectric properties of PVDF. Binder material in composite electrodes for lithium-ion batteries because PVDF does not react either with the electrolyte or with lithium over the operating condition range of such batteries.
Polyamides (nylons), including both aliphatic and semi-aromatic molecular structures and glass fiber reinforced grades of all molecular structures	Polyamides are easy to process, tough, lightweight, and durable in hot, chemically aggressive, and humid environments. They are used in applications such as enclosures and electrical switch and receptacle covers. High-performance versions provide excellent flow and dimensional stability, and are able to withstand high-temperature circuit assembly methods (including those using lead-free solder), enabling the cost-effective fabrication of thinner and lighter components (such as electronic connectors, relays, light-emitting diode components, micro-speakers, micro-receivers, micro-switches, and camera modules) in advanced electrical and electronic devices including handheld devices. They are suitable for replacing metals in many applications. Non-halogenated product grades containing alternative flame retardants are also available to facilitate compliance with recycling programs for discarded electronic products.
Polycarbonate (PC) and related copolymers and blends	PC, related copolymers, and blends of PC with polymers such as ABS and ASA (acrylonitrile styrene acrylate copolymer), provide excellent melt processability. The fabricated products offer low electrical conductivity and thermal conductivity, high heat resistance, excellent mechanical properties, high dimensional stability, and high durability. Their electrical and electronics applications include circuit breakers, connectors, cable ducts, housings for electrical tools and light fixtures, enclosures, power sets, electrical switches, sockets, and distribution boards. PC is also used as the substrate in optical data storage media such as CDs, DVDs, and Blu-Ray discs, where is usually protected by a moisture-barrier layer and a lacquer coating.
Polycarbonate/ABS blends	Electrical switches, distribution boards, power sets
Polyesters	See poly(butylene terephthalate) (PBT), poly(ethylene terephthalate) (PET), and thermoplastic polyester elastomers
Polyethylene (PE)	Electrical conduits and other applications (low-cost option for many electrical and electronics applications similar to those of PTFE, but only if high performance is not needed)
Polyimides	High-performance polymers that combine heat resistance, lubricity, dimensional stability, chemical resistance, and creep resistance, and that can be used in hostile and extreme environmental conditions. Electrical and electronics applications include insulators requiring dielectric properties as well as high heat resistance and chemical compatibility.
Polyoxymethylene (POM)	POM is a highly crystalline thermoplastic homopolymer. It has excellent load bearing ability. It combines low friction and high wear resistance with the stiffness and strength needed to replace metals. It is competitive with polyamides for many electrical and electronics applications; including housings, enclosures, covers, sockets, and switches.
Polystyrene (PS)	PS foam packaging materials are used widely as extremely lightweight cushioning materials in the shipment of electrical and electronic components and devices.
Polytetrafluoroethylene (PTFE)	PTFE is a thermoplastic perfluoropolymer. It is used in wiring in aerospace and computer applications such as hookup wire and coaxial cables. It is an excellent insulator in connector assemblies and cables, and in printed circuit boards used at microwave frequencies. Its high temperature resistance and other outstanding properties make PTFE the more expensive high-performance substitute for PE which is usually preferred in low-cost applications.
Silicates and boron nitrogen polymers	Silicates possess an –Si-O- backbone. Several types of silicates are used in manufacturing integrated circuits: Phosphosilicate polymers doped with 2.0% to 4.0% by weight of phosphorus (P) are used for metal and polysilicon planarization, overcoat passivation, and non-etchback interlayer dielectric in the manufacture of integrated circuits. They also provide Na⁺ gettering ability similar to P-doped chemical vapor deposited (CVD) oxides, where “gettering” is defined as the process of removing device-degrading impurities from the active circuit regions of the wafer. Methylsiloxane polymers are used for passivation, planarization, and gap filling in the manufacture of integrated circuits. Organosiloxane (R_xCH_3ySiO_z) polymers (R = organic chromophore) are used for advanced patterning applications. Borosilicate and boron nitrogen polymers are used for p-type diffusion; while arsenic-doped, antimony-doped, and phosphorus-doped silicones are used for n-type diffusion. Note that, while the undoped silicate polymers are electrically insulating, their doped versions (allowing p-type or n-type diffusion) are not electrically insulating.
Thermoplastic polyester elastomers	These materials combine the flexibility of high performance elastomers with the strength and processability of thermoplastics. They provide resilience, heat resistance, chemical resistance, and durability. They outperform typical flexible PVC compounds in tensile strength, elongation, and low temperature toughness, while also offering the benefit of having a halogen-free molecular structure. The use of non-halogenated flame retardants leads to completely halogen-free products. Applications include laptop B-cover frame sealing, hard disk drive brackets, and computer feet.

Intrinsically conducting polymers:

Materials

Applications

Examples include the polyacetylenes, poly(p-phenylene vinylene), polypyrroles, polyindoles, polyanilines (PANI), polythiophenes, poly(3,4-ethylenedioxythiophene) (PEDOT), and poly(p-phenylene sulfide)

Current and potential ICP applications as of the end of 2018:

PEDOT and PANI are the only ICPs used on a large scale.
- PEDOT is used when antistatic protection or a transparent electrically conductive material is needed. Its applications include antistatic and electrostatic discharge coatings, capacitors, touch panels, organic light emitting diodes, organic solar cells, and printed electronics.
- PANI is used in printed circuit board manufacturing, in antistatic and electrostatic discharge coatings, and to provide corrosion protection.
Polyindoles may become the next family of ICPs to be used in large-scale applications in the future.
ICPs may also find uses in actuators, electrochromic devices, optical devices for data storage and processing, supercapacitors, chemical sensors, biosensors, flexible transparent displays, electromagnetic shielding, microwave-absorbent coatings, and replacement for the transparent electrical conductor alloy indium tin oxide.

Adhesives, coatings, potting compounds, and sealants:

Materials	Applications
Adhesives: Epoxy, acrylate, epoxy acrylate, urethane, urethane acrylate, cyanoacrylate, silicone, UV-curable formulations of various chemistries	The most important use of adhesives is in the assembly of the printed circuit board (PCB) which is the basic building block of the electronics industry. A typical PCB consists of a multi-laminate reinforced polymer board (most often a glass fiber reinforced epoxy thermoset matrix) with a protective plastic coating (permanent solder mask). PCB assembly also makes use of adhesive materials in bonding surface-mount components (SMCs), wire tacking, conformal coatings, and potting and encapsulating components. Epoxy is the acceptable material of lowest cost that can be used for the solder mask. Acrylate, epoxy, and urethane acrylate adhesives are used for bonding SMCs. Cyanoacrylate, epoxy, and UV-curable acrylate adhesives are used for wire tacking. Epoxy, urethane, silicone, and acrylate adhesives are used for encapsulation and potting. See below for conformal coating materials.
Adhesives: Polyimide	Polyimide adhesives are used for high-technology electronics applications requiring the adhesive to be able tolerate temperatures of up to 300 ^oC. Examples include wire coatings and flexible circuits used in aerospace applications.
Adhesives: Electrically conductive	The adhesives listed above are not electrically conductive. Some applications, such as integrated circuits and surface mount devices, require electrically conductive adhesives. An electrically conductive adhesive is usually obtained by adding a conductive filler (such as silver, nickel, or carbon) to a base material (most often an epoxy resin).
Adhesives: Thermally conductive	Miniaturized electronic circuitry may build up heat and fail if its maximum operating temperature is exceeded. Thermally conductive adhesives are used to prevent such heat buildup. A thermally conductive adhesive is obtained by blending a metallic (electrically conductive) or non-metallic (electrically insulating) powder into a base material (most often an epoxy, silicone, or acrylate).
Conformal coatings: Acrylate, epoxy, urethane, silicone, fluorinated or non- fluorinated poly(p-xylylene) (Parylene), amorphous fluoropolymers	The most important application is in the protection of a PCB and its components from all possible environmental hazards by the placement of a thin (typically with thickness in the range of 25 to 250 microns) and lightweight polymeric film which conforms to the contours of the PCB. The electrically insulating nature of a conformal coating also helps in the miniaturization of electronics by allowing the conducting components of the PCB to be placed closer to each other.
Potting compounds: Epoxy, silicone, urethane	A complete electronic assembly is filled with a solid or gelatinous compound to provide resistance to shock and vibration and to also exclude moisture and corrosive agents. Potting is performed by (1) placing an electronic assembly inside a mold, (2) filling the mold with an insulating liquid compound, and (3) allowing the insulating liquid compound to harden. In many potting applications, the mold becomes a part of the finished article and provides additional protection to the article. In some other potting applications, a removable mold is used, and this mold is removed after the insulating liquid has hardened.
Sealants: Silicones, epoxies, urethane-modified epoxies, polysulfide-modified epoxies, UV-curable formulations	Sealants are also used to protect electrical and electronics equipment and/or components of such equipment from damage. Sealants are used both in industrial settings and in do-it-yourself applications.

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