polymers Review Techniques and Processes for the Realization of Electrically Conducting Textile Materials from Intrinsically Conducting Polymers and Their Application Potential Toty Onggar *, Iris Kruppke and Chokri Cherif Institute of Textile Machinery and High Performance Material Technology, Technical University Dresden, 01062 Dresden, Germany; [email protected] (I.K.); [email protected] (C.C.) * Correspondence: [email protected]; Tel.: +49-351-463-44033 Received: 28 September 2020; Accepted: 24 November 2020; Published: 30 November 2020 Abstract: This review will give an overview on functional conducting polymers, while focusing on the integration of intrinsically conducting, i.e., self-conducting, polymers for creating electrically conducting textile materials. Thus, different conduction mechanisms as well as achievable electrical properties will be introduced. First, essential polymers will be described individually, and secondly, techniques and processes for the realization of electrically conducting textile products in addition to their application potential will be presented. Keywords: electrical conducting; intrinsically polymers; textile; spinning; coating 1. Introduction The textile industry has a strong interest in electrically conducting polymers. This is due to their unique combination of properties, offering amongst others, electrical conductivity, electromagnetic shielding, bendability, and stretch ability in addition to the high mechanical flexibility provided by substrates. Within the last years, there have been different printing technologies developed, such as screen printing, inkjet printing, gravure printing, transfer printing, flexographic printing, and nanoimprinting to apply intrinsically conductive polymers (ICP) onto textile surfaces. The used substrates are natural fibers like cotton (CO), wool (WO), and silk (SE), as well as man-made fibers like polyethylene terephthalate (PET), polypropylene (PP), polyamide (PA), polyacrylonitrile (PAN), and polyurethane (PU). In contrast to this, ICP are achieved using electro spinning, wet spinning, or dry spinning, whereas the spun yarns exhibit a good electrical conductivity but reduced textile mechanical properties. In consequence, the manufacturing of intelligent materials, which consist of electro conductive yarns, is a challenging target. In order to get good mechanical properties with coincident electrical conductivity, coating technologies for textiles and fabrics are a promising alternative. For the application of ICP onto textile materials, two main coating technologies are prevailingly used: dipping-and-drying technique and chemical solution/vapor polymerization. Intrinsically conducting polymers, so-called conducting polymers, are plastics that are characterized by electrical conductivity and properties that are comparable with metals [1]. The conductivity of polymers is based on conjugated double bonds between atoms enabling the free mobility of doped charge carriers [1]. Electrical conductivity requires freely moving charge carriers; hence, electrically self-conducting polymers have an extensive π-electron system in the form of conjugated double bonds with holes serving as charge carriers. Aromatic or heteroaromatic rings as well as triple bonds also belong to the group of polyconjugated bond systems. The term Polymers 2020, 12, 2867; doi:10.3390/polym12122867 www.mdpi.com/journal/polymers Polymers 2020, 12, 2867 2 of 46 Polymers 2020, 12, x FOR PEER REVIEW 2 of 48 ICPto, for refers example, to, for polyacetylene example, polyacetylene (PAc), polypy (PAc),rrole polypyrrole (PPy), polythiophene (PPy), polythiophene (PT), poly(3,4- (PT), poly(3,4-ethylenedioxythiophene)ethylenedioxythiophene) (PEDOT), (PEDOT), polyaniline polyaniline (PAni), (PAni),polyselenophene polyselenophene (PSe), polyfuran (PSe), polyfuran (PFu), (PFu),poly(para-phenylene) poly(para-phenylene) (PPP), (PPP),and andpoly(p-phenylene poly(p-phenylene vinylene) vinylene) (PPV). (PPV). Figure Figure 11 illustrates thethe conductivity of ICPs in comparison to other solid materialsmaterials [[1–5].1–5]. The The electrical electrical conductivities conductivities of these 13 8 polymer classesclasses areare inin thethe rangerange ofof 1010−−13–10–108 S/cmS/cm in in a a doped doped and and undoped undoped state state (Table1 1))[ [1–5].1–5]. These polymers—depending on theirtheir state—canstate—can actact asas insulator,insulator, semiconductor,semiconductor,or orconductor. conductor. Figure 1.1. ElectricalElectrical conductivity conductivity of of intrinsically intrinsically co conductingnducting polymers polymers compared to other solid materials [[1–5].1–5]. Table 1. Electrical conductivity of ICP in a doped an undoped state [1–5]. Table 1. Electrical conductivity of ICP in a doped an undoped state [1–5]. Electrical Conductivity (S/cm) Electrical Conductivity (S/cm) PolymerPolymer PAc PAcPPy PPyPT PTPEDOT PEDOT PAni PAni PPV PPV PPPPPP 6 8 2 4 2 2 2 2 3 2 DopedDoped 106–0108 –0 102–10104 –10 10210 102–30010 –300 10 102 10102–10–10−3 1010–5002–500 −2 −82 8 −8 8 −8 8 −8 8 −8 8 −10 10 −88 −1313 8−8 UndopedUndoped 10 –1010− –10−10 10− 10 10 − 10 10 − 1010−–10–10− 1010− –10–10− 1010− In spite of of the the high high interest interest according according to to conductive conductive polymers, polymers, there there are are still still many many challenges challenges in intheir their handling handling and and processing processing because because of ofthe the properties properties of ofthe the melt, melt, the the susceptibility susceptibility to moisture to moisture as aswell well as asthe the weak weak stability stability at athigh high temperatures, temperatures, the the unsolublity unsolublity in in the the most most solvents, and the instability ofof oxygenoxygen containing containing gases. gases. Within Within this this review, review, conductive conductive polymers polymers will will be described be described with theirwith their principles principles of electric of electric conductivity conductivity and theand achievable the achievable electric electric properties. properties. The The processes processes for thefor realizationthe realization of electrical of electrical conductive conductive textile textile products products within within the the assets assets and and drawbacks drawbacks ofof thethe several printing, coating, and spinning techniques will be introduced for the didifferentfferent conductive polymers in accordanceaccordance to thethe scopescope ofof application.application. It It is described in consideration of the used doping and oxidation materials as well as solvents and other additives, which are are used used to to prepare prepare printing printing pastes, pastes, coatings, or dopes for spinning. 2. Electrical Conductivity of ICP To achieveachieve polymerspolymers with a desirable amount of electrical conductivity and stability, they must be fully doped. doped. The The el electricalectrical conductivity conductivity of of PAc PAc in in a doped a doped stat statee is higher is higher than than that that of other of other ICP. ICP. In Inan anundoped undoped state, state, all all ICP ICP possess possess similar similar levels levels of of electrical electrical conductivity conductivity (Table (Table 11).). Doping reactionsreactions can be subdividedsubdivided into p-typep-type dopingdoping ((pp == positive; oxidation), i.e., a polymer reaction with an oxidizer (acceptor carrying electrons),electrons), and n-type doping (n == negative; reduction), i.e., a polymer reaction with a reducer (donor carrying electrons) [11–5].–5]. Due to their large number of double bonds, undoped conductingconducting polymers polymers possess possess high high reactivity. reactivity. Therefore, Therefore, the cross-linkingthe cross-linking reactions reactions as a result as a result of oxidation and addition are possible at room temperature. PPP being resistant to oxidation Polymers 2020, 12, 2867 3 of 46 Polymers 2020, 12, x FOR PEER REVIEW 3 of 48 ofand oxidation temperature and additionis an exception are possible to this atgeneral room temperature.rule. However, PPP reactivity being resistant plays a minor to oxidation role in andthe temperaturecase of doped is polymers an exception due toto thistheir general oxidized rule. state However, [1,2]. reactivity plays a minor role in the case of dopedAlthough polymers researchers due to their have oxidized investigated state [1 ,this2]. topic for at least 40 years, it is still not clear which structuralAlthough characteristics researchers of have(intrinsically) investigated conducting this topic polymers for at least are 40beneficial years, it or is essential still not clear in terms which of structuralhigh conductivity. characteristics It is generally of (intrinsically) assumed conducting that high crystallinity polymers are and beneficial a linear or array essential of chains in terms allow of high conductivity.conductivity, which It is generally has been assumed confirmed that for high technically crystallinity and andcommercially a linear array relevant of chains polymers, allow highsuch as conductivity, PT and PEDOT. which In hascontrast, been this confirmed assumption for technically could not be and confirmed commercially for PAni, relevant which polymers, is mostly suchamorphous as PT and and PEDOT. has little In to contrast, no differences this assumption in crystallinity could not [6]. be Other confirmed polymers, for PAni, e.g., which PAc and is mostly PPP, amorphousexhibit a negatively and has char littleged