1.6　Applications of Polyimide Films

Aromatic polyimide films exhibit a unique combination of thermal， mechanical， electrical， dielectric， and properties， making it ideal for a variety of applications in many high-tech industries. Polyimide films can maintain their excellent combined properties over a wide temperature range， and exhibit excellent chemical resistance without any dissolution by most common organic solvents. Additionally， polyimide films do not melt or burn due to its highest UL-94 flammability rating： V-0. Polyimide films can be used both in electrical insulation applications， including wire and cable tapes， formed coil insulation， motor slot liners， magnet wire insulation， transformer and capacitor insulation， magnetic and pressure-sensitive tapes and tubing， and in microelectronic packaging applications， including FPC， TAB， and COF， etc. However， each special use requires the polyimide film to have its desirable combined properties.

1.6.1　Electric Insulating Applications

The traditional applications of polyimide films are the motor and magnet wire industry due to their unique properties. The electrical and mechanical strengths of polyimide films enable thinner insulation designs and conserving space for conductors， yielding more power without increasing motor size. Polyimide films can provide exceptional overload protection and long motor life， even in the most harsh servicing environments and conditions. Polyimide films have superior chemical resistance to most organic solvents， hydrocarbons， lubricants， resins， and varnishes， and UL 94-V0 flammability rating， and will not melt， ignite， or propagate flame.

The typical motor applications include magnet wire， turn-to-turn， strand， coil， slot liner， and ground insulation. Currently， there are several types of polyimide films which can be used in motors， generators， and transformers， including： （1） Kapton HN film which exhibit exceptional and unique balance of physical， chemical， and electrical properties over a wide temperature range， particularly at high temperatures. （2） Kapton CR film which is developed specifically to withstand the damaging effects of partial discharge. The corona produced by the partial discharge can cause the eventual breakdown of an insulation material or system. Kapton CR film has a corona resistance or voltage endurance that is significantly higher than standard Kapton HN film， and also provides great higher thermal conductivity， allowing better dissipation of heat in motors and other electrical equipment. （3） Kapton WR film which can be exposed continuously to hot water， combating the effect of water on insulation systems where hydrolytic stability is important.

The polyimide films used in electrical insulating areas are usually coated with Teflon FEP or Teflon PFA polyfluorocarbon resins， which were melt-extrudable copolymers of poly（tetrafluoro-ethylene）. The heat-sealable Kapton films are always used as primary insulation on magnet wires. The thermally stable coatings confer on polyimide films heat sealability to conductor metals and act as high-temperature adhesives. The film tapes are used to wrap conductors and are then heat-sealed at high temperature. The heat-sealable Kapton films include FN， FCR， FWN， FWR， etc. Tables1.19， 1.20， and 1.21 summarize typical polyimide films for the electrical insulation industry.

1.6.2　Electronic and Optoelectronic Applications

1.6.2.1　Electronic Applications

Polyimide films used for electronic applications such as FPC， TAB， and COF must have low CTE， low coefficients of humility expansion， high elastic modulus， and low moisture uptakes. For instance， TAB tape is a polyimide film substrate on the surface of which is fabricated very fine metal circuits， and the substrate has openings or “windows” for mounting integrated circuit （IC） chips. Sprocket holes for precisely feeding the TAB tape are produced near both edges of the tape. IC chips are embedded in the windows on the TAB tape and bonded to the metal interconnects on the tape surface. TAB tape is used to automate and simplify the process of mounting IC chips， improving manufacturing productivity and enhancing the electrical characteristics of electronic equipment containing mounted IC chips.

There are two constructions for TAB tapes， one is the three-layer construction which was composed of a polyimide substrate film on the surface of which an electrically conductive metal foil has been laminated with an intervening layer of polyester， acrylic， epoxy， or polyimide-based adhesives. Another is the two-layer construction composed of a polyimide substrate film on the surface of which a conductive metal layer has been directly laminated without an intervening layer of adhesive.

The polyimide substrate film in TAB tapes are thus required to be excellent in thermal resistance， ensuring that the substrate film is able to withstand the high-temperature operations in the soldering of IC chips bonded to the metal interconnections on TAB tape and when the TAB tape carried with IC chips is bonded to a printed circuit for wiring electronic equipment. However， the heat was incurred in the process of laminating polyimide film with metal foil or a metal layer. Then， the chemical etching of the metal foil or metal layer to form metal interconnections may elicit different degrees of dimensional change in the polyimide film and metal， sometimes causing considerable deformation of the TAB tape. Such deformation can greatly hinder or even render impossible subsequent operations. Hence， making the thermal expansion coefficient of polyimide film close to that of the metal so as to reduce deformation of the TAB tape becomes a key issue in the production of polyimide films. Moreover， reducing dimensional change attributed to the tensile and compressive forces in TAB tape is important for achieving finer-pitch metal interconnects， reducing strain on the metal interconnects and reducing strain on the mounted IC chips. Hence， polyimide film used as the substrate must have a higher elastic modulus.

In order to satisfy the special requirements in microelectronic packaging and electrical insulating applications， polyimide films with different backbone structures have been developed. The most successful commercial polyimide films are the PMDA-based Kapton H films by Dupont and the BPDA-based Upilex R and S films by Ube. Upilex R， which was prepared using the same diamine as Kapton H， has a significantly lower T_g （285 °C versus about 385 °C）， but is comparable to Kapton H in mechanical and electrical properties both at ambient and elevated temperatures. Moreover， Upilex R shows lower H₂O uptake， lower 250 °C shrinkage and excellent hydrolytic resistance， particularly to aqueous NaOH solution， and has found extensive application in traction motors， which require mechanical and thermal durability above 300 °C.

Upilex S film is significantly different from Kapton H. It is much stiffer， has substantially higher tensile strength， significantly lower high-temperature shrinkage， and lower thermal and hygroscopic coefficients of expansion. It has much lower permeability to gas and to water vapor. Upilex S film shows excellent hydrolytic durability， which is superior to Kapton H. It is particularly in circuit applications that Upilex S appears to offer significant potential competition to Kapton H.

Kapton H film has played a major role in aerospace wire and cable， traction motors， magnet wire， transformers， capacitors， and many other areas. Upilex S has lower elongation at breakage than Kapton H； this brittleness limits its utility as electrical insulating tapes for wire and cable as well as motor and generator applications. Table 1.21 compares the mechanical properties of PMDA-based and BPDA-based polyimide films.

Ube and Kaneka have developed several commercialized polyimide films for electronic applications （Table 1.22）. The Apical HP films by Kenaka exhibit a great combination of mechanical and thermal properties with heat shrinkage of 0.06% at 200 °C/2 h， CTE of 11×10-6 °C−1， elastic modulus of 5.6GPa-6.0 GPa， elongation at breakage of 42%-47%， and moisture uptake of 1.2%.

1.6.2.2　Optoelectronic Applications

CTPI films have been intensively investigated for potential applications in flexible optoelectronic devices， such as flexible light-emitting diodes （F-LED）， flexible solar cells or photovoltaic cells （F-PV）， flexible thin-film transistors （F-TFT）， flexible printing circuit boards （FPCs）， and so on. The optoelectronic applications require that the CTPI films have excellent combined properties which are not available from the commodity polyimide films. To satisfy these requirements， engineers have screened and tested many types of high-performance polymer films， such as PET， PEN， PEI， etc. Up to now， CTPI films have been considered as one of the most competitive candidates as the flexible plastic substrates for flexible optoelectronic devices， including FPC， flexible display （TFT-LCDs or AMOLEDs， etc.）， touch panel， electronic paper， and thin photovoltaic cells. The flexible plastic substrates with both optical transparency and high temperature resistance have great potential applications in these areas due to their superior flexibility， lightness， cost-effectiveness， and processability compared with the fragile and expensive glass analogs.

This developing trend provides great opportunities for the CTPI optical films. With the structural support and optical signal transmission pathway and medium， flexible substrates will play very important roles in advanced optoelectronic display devices. The characteristics and functionalities of flexible substrates are the important factors that affect the quality of flexible devices. Currently， there are mainly three types of substrates for flexible displays： thin glass， transparent plastic （polymer）， and metal foil. The transparent polyimide films not only have good optical transmittance comparable to the thin glass， but also possess good flexibility and toughness similar to metal foils. Thus， CTPI films are ideal candidates for flexible display. Use of flexible plastic substrate is considered to be one of the promising technologic breakthroughs in optical displays due to its attractive features， such as thinness， light weight， and good flexibility. The development of flexible substrates is experiencing a roadmap from plane （current） to bended （2015） then to rollable （2018） finally to foldable （2020） in the coming years ［101］. The radius of curvature for highly transparent flexible substrates might reach <3 mm at 2020. At that time， transparent polymer film substrates might be the best candidates to meet the demands in device flexibility.

In order to achieve a practical application for transparent polymer film substrates in flexible display， several issues have to be addressed. First， the thermal stability of the transparent substrate should meet the application demands. For instance， thin film transistors （TFTs） are currently fabricated on the common optical polymer films or sheets， which are produced at low temperature due to the low thermal stability of used plastic substrates （< 250 °C）. To date， there are four types of producing technologies for TFT fabrications in AMOLED， including amorphous silicon （a-Si） TFTs， low-temperature polysilicon （LTPS） TFTs， oxide TFTs， and organic TFTs （OTFTs） ［66］. The LTPS TFTs exhibit the highest field-effect mobility and stable electrical performance. However， the procedure requires a high process temperature of about 500 °C during silicon crystallization. a-Si TFTs process has been widely used to produce AMOLED devices， which show uniform electrical characteristics over large areas， reasonable field-effect mobility， low-temperature process （<300 °C）， and low cost compared to the other techniques. Hence， colorless and transparent polymer substrates with good thermal resistance above 300 °C are highly desired in advanced flexible display engineering.

Second， the CTPI film substrate should have low water vapor transmission rate （WVTR） and oxygen transmission rate （OTR） features. When a polymer film substrate is used for the flexible OLED application， the WVTR and OTR feature of the film matrix become critical because most high- performance semiconductor organic compounds built on the substrate show degraded performance when exposed to environmental moisture ［48］. WVTR and OTR of the currently flexible substrates are severely limited to be below 10−4 cm3·m−2 per day and 10−6 g·m−2 per day， respectively， for AMOLED and organic solar cells ［102］. Hence， polymer film substrates cannot effectively protect the water and oxygen permeants. In general， CTPI films have WVTR values of 100-102 g·m−2 per day depending on the aggregation structures of their molecular chains.

Third， the polymer film substrate should have comparable CTE values to the inorganic or metal components in display devices. The CTE value of polymer film substrate is quite important for its application as flexible substrate， especially when it is used with other heterogeneous materials， such as metal， glass， or ceramic. The CTPI film substrates usually have CTE values of higher than 30×10−6 °C−1； However， the inorganic components， such as SiNx gas barrier layer， have CTE values of lower than 20×10-6 °C−1. The unmatched CTE values between the polymer films with other materials are thought to be one of the most important reasons for delamination， cracking， and other failures in the devices ［73］. Hybrid with inorganic additives， such as silica， titania via sol-gel route has been attempted to reduce the CTE values of CTPI films ［103，68］.

Recently， ITRI （Industrial Technology Research Institute， Taiwan） have developed a unique flexible-universal-plane （FlexUP） solution for flexible display applications ［104］. This new technique relies on two key innovations： flexible substrate and a debonding layer （DBL）. As for the flexible substrate， ITRI developed a CTPI substrate， which contains a high content （>60%（w）） of inorganic silica particles in the PI matrix. The CTPI substrate exhibits good optical transmittance （90%）， high T_g （>300 °C）， low CTE （28×10-6 °C−1）， and good chemical resistance. In addition， the CTPI substrate with additional barrier treatment shows a WVTR value less than 4×10−5 g·m−2 per day. Moreover， this barrier property suffered only to a minor drop， to 8×10−5 g·m−2 per day， after the flexible panel had been bent 1000 times at a radius of 5 cm. A 6-inch flexible color AMOLED display device was successfully fabricated using this substrate. Using this CTPI substrate， the flexible touch panel was successfully prepared.

A 7-inch flexible VGA transmissive-type active matrix TFT-LCD display with a-Si TFT was successfully fabricated on CTPI film substrate by ITRI ［105］. The CTPI film substrate has the features of high T_g （>350 °C） and high light transmittance （>90%）， which ensure the successful fabrication of 200 °C a-Si：H TFT in the flexible device. The flexible panel showed resolution of 640×RGB×480， pixel pitch of 75×225 mm， and brightness of 100 nit. This technique is fully a-Si TFT backplane compatible， making it very attractive for applications in high-performance flexible displays. Similarly， a-Si TFTs depo sited on clear plastic substrates （from DuPont） at 250 °C-280 °C was reported ［106］. The free-standing clear plastic substrate has a T_g value of higher than 315 °C and a CTE value of lower than 10×10-6 °C−1. The maximum process temperature of 280 °C is close to the temperature used in industrial a-Si TFT production on glass substrates （300 °C-350 °C）.

Toshiba Corp. Japan have successfully fabricated a flexible 10.2 in. WUXGA （1920×1200） bottom-emission AMOLED display device driven by amorphous indium gallium zinc oxide （IGZO） TFTs on a CTPI film substrate ［107］. First， a transparent PI film was formed on a glass substrate and then a barrier layer was deposited to prevent the permeation of water. Then， the gate insulator， IGZO thin film， source-drain metal， and passivation layer were successively deposited to afford the IGZO TFT. Second， the flexible AMOLED panel was fabricated using the IGZO TFT， color filter， white OLED， and encapsulation layer. Finally， the OLED panel was debonded from the glass substrate to afford the final AMOLED panel. The threshold voltage shifts of amorphous IGZO TFTs on the PI substrates under bias-temperature stress have been successfully reduced to less than 0.03 V， which is equivalent to those on glass substrates. ITRI also reported high-performance flexible amorphous IGZO TFTs on CTPI-based nanocomposites substrates ［79］. Similarly， a high-heat-resistant PC film with the T_g of 240 °C and optical transmittance higher than 90% in the visible light region has been reported by General Electric ［108］. A transparent， high barrier， and high heat substrate for organic electronics was successfully prepared based on the CTPI film.

Over the years， the flexible printing circuit boards （FPC） have been the largest market for high-temperature polymer films， such as PI， polyamideimide， and polyetherimide films. The flexible nature of FPCs allows their convenient use in compact electronic equipment such as portable computers， digital cameras， watches， and panel boards. Generally， the traditional FPC is mainly prepared from FCCLs. FCCLs consist of a layer of PI film bonded to copper foil. Depending on the intended use of the laminate， copper may be applied to one （single-sided） or both sides （double-sided） of the PI film. PI film almost dominates the portion of the FCCL market in which heat resistance is needed to withstand the soldering temperatures. Recently， with the development of flexible displays， necessity for a transparent film substrate in place of glass substrate is increasing. Correspondingly， a transparent film substrate for FCCLs is increasingly desired. However， most of the all-aromatic PI films currently used in FCCLs show colors from yellow to deep brown， and thus cannot be used in transparent FCCLs.

Very recently， there has been vigorous activity in developing and commercializing transparent FPC products. This is mainly driven by the urgent needs for such products for mobile communication optoelectronics. Various polymeric optical films， including PEN， PAI， and PI films have been used as the substrates together with the transparent conductive films （usually indium-tin oxide （In₂O₃ + SnO₂） （ITO） film） in these new products. Toyobo Corp.， Japan recently patented a colorless and transparent FCCL and the derived FPCB based on a PAI film. The PAI film was synthesized from 1，2，4-cyclohexanetricarboxylic anhydride （HTA） and aromatic diisocyanate monomers and the curing procedure was 200 °C/1 h， 250 °C/1 h， and 300 °C/30 min under nitrogen. The film exhibited good thermal stability with T_g of 300 °C， light transmittance of 89%， good tensile properties with tensile strength of 140 MPa， elongation at break of 30%， tensile modulus of 3.9 GPa， and low CTE of 33×10-6 °C−1. The single-side FCCL from the PAI film and copper coil showed good soldering resistance， high bonding strength （10.6 N·cm−1）， and good dimensional stability under the condition of 150 °C for 30 min. In addition， the FCCL showed good optical transparency with a transmittance of 75% at the wavelength of 500 nm. It can be anticipated that polyimide optical films will play an increasingly important role in the future development of transparent FPCs.

Thin film solar cells or flexible photovoltaics （PV） have been intensively investigated in energy industries due to their potential ability to reduce the cost per watt of solar energy and improve lifetime performance of solar modules ［109］. Conventional thin-film solar cells are usually manufactured on transparent conducting oxide-coated 3 to 5mm-thick soda-lime glass substrates and offer no weight advantage or shape adaptability for curved surfaces. Fabricating thin film solar cells on flexible polyimide substrates seems to offer several advantages in practical applications， such as weight saving， cost saving and easy fabrication. The polyimide substrates for thin film solar cell fabrications should be optically transparent and withstand the high processing temperatures. For instance， the current cadmium telluride （CdTe） cell fabrication techniques， the processing temperatures are in the range of 450 -500 °C. Most of the transparent polymers will degrade at such high temperatures. Undoubtedly， the lack of a transparent polymer film which is stable at the high processing temperature of solar cells is one of the biggest obstacles for the application of polymer substrates in flexible solar cells. Wholly aromatic polyimide films， such as Kapton and Upilex films can withstand high temperatures of around 450 °C. However， they exhibit deep colors and strongly absorb visible light. CdTe solar cells on such polyimide substrates will yield only low current due to large optical absorption ［83］. Hence， the development of CTPI films with good high-temperature stability makes it possible to produce highly efficient solar cells. One of the most promising reports on the successful applications of CTPI films in flexible solar cells fabrication might be the work carried out in Swiss Federal Laboratories for Materials Science and Technology （Empa） ［84］. As part of the Empa’s continuous work on developing high-efficiency thin-film solar cells aiming at enhancing their performance and simplifying the fabrication processes， they utilized colorless PI film （developed by DuPont） as the flexible substrate for CdTe thin-film PV modulus in 2011. A conversion efficiency of 13.8% using the new substrates was achieved， which was the new record among this type of solar cells at that time.

Overall， CTPIs represent a class of new materials with both high technological content and high additional value. Excellent comprehensive properties make them good candidates for advanced optoelectronic devices. It can be anticipated that， with the ever-increasing demands of optoelectronic fabrication， CTPI films will attract more attention from both academia and industry. The demand will continue to grow for displays of smart phones， tablet PCs， and other types of mobile electronic devices. Furthermore， these displays will be continuously improved in terms of visibility， flexibility， durability， and light weight. Currently， CTPI optical films are facing great development opportunities. However， several obstacles still exist at present， that should be overcome for the wide applications of CTPI films in advanced high-tech fields. First， very limited commercially available CTPI film products greatly increase their cost， which leads to a very limited application only in high-end optoelectronic products. Low-cost CPIs are highly desired for their wide applications. Second， the combined properties of current CTPI films should be further enhanced， such as their optical transmittances at elevated temperatures， and gas barrier properties. Third， the manufacturing technology for CTPI films should be optimized in order to increase their uniformity， colorlessness， and dimensional stability at high temperatures.

1.6.3　Aerospace Applications

1.6.3.1　Multilayer Thermal Blanket

Aromatic polyimide films， especially the Kapton H film derived from PMDA and ODA were extensively used in aerospace until the 1970s. However， there is no standard definition for high-performance polymers because the requirements and environments for different applications vary significantly. Multilayer insulation， the so-called multilayer thermal blanket， is a typical example of aromatic polyimide films for space applications. Spacecraft themselves and each of the instruments within are generally covered with various metalized polymer films for controlling the temperature of the instruments in the spacecraft. Surface temperature of a metalized polymer film is a function of the ratio of solar absorptivity （α） to low-temperature emissivity （ε） of the film. At equilibrium， a low α/ε ratio provides a low surface temperature （high values provide a high surface temperature）. Therefore， by choosing the proper film and the appropriate metal， it is possible to specify a thermal control surface within a wide range of α and ε values. A passive （nonactive） thermal control system such as MLI significantly helps to maintain spacecraft systems and components at specified temperature limits. Kapton polyimide films and Teflon fluoropolymer films have long been accepted as space-stable insulating materials. The outer layer uses 50 µm-thick aluminized Kapton film with 11 layers of double aluminized thin Kapton films. The ASCA spacecraft was a powerful X-ray observatory satellite of ISAS in Japan launched in 1993. Because the maximum allowable fluctuation of the focal point is 0.5 mm， the surface of 3.4 m-long， high-precision Extendible Optical Bench （made of carbon fiber reinforced plastics， CFRP） was covered with a Upilex-R polyimide film MLI. Upilex R film， derived from BPDA and ODA， possesses a higher optical transparency of the film compared with Kapton H derived from PMDA and ODA. This is an advantage for the system due to the low absorptivity. A new MLI covering over 90% of the outer layer covering of the spacecraft ASCA is composed of two outer layers of aluminized 25 µm Upilex R and five layers of double-aluminized 12 µm polyester films with a separator net. Until 1987， Kapton MLI was the only MLI for the high-temperature areas of spacecraft and ASCA is the first satellite completely covered by an another polyimide MLI. Colorless polyimide films were first reported by NASA for varying the molecular structure and reducing the electronic interaction between chromophoric centers for space applications. Because of the requirement for radiation resistivity in space， the highly transparent polyimide films were prepared derived from aromatic dianhydride and special diamine monomers ［89］.

1.6.3.2　Flexible and/or Rigid Extendable Structures

A flexible solar array is an attractive example of combining an extendable advanced composite with aromatic polyimide films. Japan’s first spacecraft SFU deployed the two large flexible solar arrays in low earth orbit （LEO）. Due to the high T_g and outstanding mechanical properties， even in very-low-temperature environments， aromatic polyimide films are the most successful， widely used polymeric material in space. Until 10 years ago， a spacecraft was usually equipped with a rigid type power generator （solar paddle）. However， as a spacecraft becomes larger， it requires much more electric power. It is thought that a flexible solar array is the most attractive way for power generation in space. In the SFU spacecraft and the array configuration with the extendable mast， each deployed solar array is 2.4 m wide and 9.7 m long. The array is composed of two assembly boards and the mast canister. The extendable/retractable mast is a continual coilable mast consisting of three GFRP spring rods （longerons） and radial spacers. The main source of its spring force is generated by the bending strain energy of the GFRP longerons. The radial spacers were made of molded Upilex R， because of its excellent space environmental stability without creep behavior originating from the high intermolecular interaction of benzoimide rings. Therefore， no mechanical backlash exists （since there are no pin-joint hinges）， resulting in high dimensional stability. Each array blanket consists of 48 hinged low CTE Upilex S film， whose dimensions are 202 mm wide and 2400 mm long. About 27，000 solar cells are mounted on two array blankets and generate 3.0 kW of power. One-hundred-micrometer-thick silicon cells with 100 µm cover glass are adhered by S-691-RTV silicon type adhesive on the polyimide panels.

1.6.3.3　Large Deployable Antenna

Development of a large deployable antenna on Muses-B is another example of advanced technology in space. The Muses-B antenna launched in 1997 was used aboard the satellite for Space-VLBI （very long baseline interferometry）. A 10 m-diameter parabolic antenna with a mesh surface was deployed with steps extending the six extendable masts in LEO. This incredibly complex system consists of 6000 fine cables of high modulus Kevlar 149 aramid covered by a Conex aramid net. In order to achieve high surface accuracy， a cable must precisely keep its length without creep under a tension field in space. The high-strength Kevlar 149 cable exhibits very little elongation when stressed and has negative CTE over a wide range of temperatures. Because the lengths and tension force of the cable were critical， the tension force of each extendable mast was strictly controlled by the tensioners on the top of the masts. It was the first application of high-performance organic fibers for a large deployable parabolic antenna surface in space.