Properties of PTFE

PTFE is ideal for performance due to unique properties. The molecular structure of PTFE is depicted in Fig. . The properties of PTFE widely spread in all the branches and being used for a variety of applications. Various properties of PTFE are represented in Fig. Reports on various properties of PTFE have been discussed in this topic.

Physical properties of PTFE

Barrier properties

PTFE demonstrated superior hydrophobic nature due to the low surface energy. Wrinkled superhydrophobic surfaces, fabricated from two forms of PTFE exhibit the durable and excellent barrier properties as the roll-off angle of the surfaces tend to be very low. The contact angle for single-scale wrinkled PTFE and hierarchical wrinkled PTFE surface was measured at 163° and 172° that has been possessing higher magnitude. Surface modification of PTFE from hydrophobic to hydrophilic property was optimized by the addition of chemical agents amino (-NH₂), carboxyl (-COOH) and sulfonic acid (-SO₃H). On microfiltration analysis, membranes of PTFE adhered with hydrophilic agent’s shows good microfiltration property. The property of plasma modified PTFE is more effective in high-performance direct contact membrane distillation (DCMD). PTFE has been treated with a plasma to obtain pore on the surfaces. Surface morphology study reveals the appearance of parallel pore layers during plasma treatment. Plasma treatment deploys contact angle as a function of treatment time. The bipolar Argon plasma treatment of PTFE also supports the same as with plasma treatment there is an increase in surface free energy. Even though the low surface energy property of virgin PTFE is useful but somehow it is difficult to blend or grind with other polymers. In such a case, the modification of surface is achieved using high-energy irradiation which is in connection with degradation process. Further examination revealed that under irradiation, the PTFE compound has low molecular weight and lower hydrophobicity.

A work on extended PTFE tape demonstrated the increase in water contact angle in a feasible manner. PTFE was stretched using a mechanical device for different ratio of extension. It was portrayed that the increasing extension ratio significantly increases the water contact angle of the surface. The water-repellent property of the surface was mainly due to the decrease in the density of PTFE tape under stretching. More precisely the reason for higher contact angle was the alignment of PTFE microforms on the tape.

The composition of inorganic fullerene-like tungsten disulfide (IF-WS₂) nanoparticles and PTFE improved the hydrophobic property. The drastic change in surface roughness of IF-WS₂/PTFE was revealed by atomic force microscope (AFM) images. Such key factor was the reason for the increase in hydrophobicity. The different contact angles reported for PTFE and PTFE composites are graphically shown in Fig.

Tribological property of PTFE surface

Surface friction of PTFE

Extensive studies have been done on the friction property of PTFE because of interesting low friction coefficient. Friction occurs due to the relative motion of the surface. A virgin PTFE reveals the ultimate friction resistance property therefore optimized for different types of lubrication. As a function of glass fiber, carbon, and graphite loading, there has been a strong influence over friction properties. A wear mechanism was reported for metal precursor-based PTFE composites. Nano-sized PTFE particles were filled in nickel (Ni) and phosphorous (P) coatings. The work revealed the considerable change in friction coefficient (μ) of Ni-P/PTFE coatings when subjected to wear test. Comparatively, Ni-P/PTFE coating exhibited low wear resistance than Ni-P coating because of the presence of PTFE particles.

Surface wear of PTFE

Wear is the most important property of PTFE among surface properties. In general, wear is also associated with mechanical properties, the parameters of wear include weight load, velocity, temperature, contact area, and sliding distance. A familiar method known as Pin-on-Disc setup was used to analyze the wear behavior. This test showed that the friction coefficient for virgin PTFE decreased with the increase in loading of carbon and bronze, increased the wear resistance whereas the friction coefficient was affected slightly. PTFE with filler loadings was effectively have good wear resistance when bearing the weight load over the surface.

Surface lubrication of PTFE

The performance of PTFE as self-lubricant bearings was well known and often examined for its excellent sliding behavior. The mitigation of London dispersive forces in PTFE is due to the highly electronegative fluorine atoms. Furthermore, this property of PTFE was thoroughly examined to improve for high efficiency. In PTFE compound, the fluorine atoms are very close, forming a smooth and cylindrical surface so as the other molecules sliding over easily. In tribological view, PTFE is the topmost material preferred among all.

Abrasion property of PTFE

Abrasion property of PTFE interlinked with wear rate and friction coefficient. Pure PTFE compound are good in abrasion resistance but fixing it on the surface is the challenging task. Glass fiber (GF) and carbon fiber (CF) filled PTFE were tested for the abrasion resistance capacity. The abrasiveness and surface morphology of the worn surfaces of GF/PTFE and CF/PTFE was studied using scanning electron microscope (SEM). The wear volume was certainly lost in GF/PTFE than CF/PTFE. Under various weight loads, CF/PTFE poses better abrasion resistance because of the adhesion of carbon fibers with the PTFE matrix. Although PTFE possesses lower friction property than any other polymer, the addition of filler makes it suitable for interfacing with good friction resistance.

Mechanical properties of PTFE

Tensile, hardness, stress, and strain tests on PTFE

The mechanical property of PTFE deals with the study of tensile strength, stress and strain, ductility, hardness, and molding ability. PTFE is ductile in nature and obviously remains low in mechanical phase when compared to other polymers but PTFE has a good advantage in constructing mechanical device parts by loading filler components. Compression test on two grades of PTFE exhibited good mechanistic performance. Significantly the mechanical properties are affected by temperature hence the samples of PTFE were also tested with the load of 50% at a temperature varying from − 198 to 200 °C. During deformation, PTFE undergoes a structural change of approximately 30% in comparison with metals which are less than 10%. The rearrangement of molecules due to strain is temporary because of the viscoelastic nature of polymers and permanent damage when it reaches the physical aging.

Generally, the unfilled PTFE exhibits very poor flexural properties. An improvement over mechanical property has been studied in detail for the composite material Polyamide6 (PA6)/PTFE. Flexural and tensile properties test were conducted for different PA6 content. The samples were analyzed by keeping constant load for five specimens of different magnitudes and the morphology was observed using SEM. Under stress, the deformation of PTFE occurs and improves the flexural toughness due to the absorption of energy. Results showed that the 30% PA6-reinforced PTFE composites have a significant improvement in mechanical performance. The improved tensile strength of PTFE composites is depicted in Fig.

Improvement of mechanical property as a function of temperature

PTFE filled with expanded graphite nanoparticles (nano-EG) with reinforcement of nano-aluminum oxide_, nano-copper, nano-silicon dioxide were studied to explore its mechanical properties. Dynamic mechanical thermal analysis (DMTA) method was used to analyze the mechanical property. It is noteworthy that the composites reinforced with nano-materials have a remarkable improvement in strength and hardness in comparison with the pure PTFE. DMTA provided good results under testing of the composites and different types of reinforcement showed different distinct mechanical properties. Notably, the composite added with nano-Al₂O₃ showed higher tensile strength and the composite added with nano-SiO₂ showed high elastic modulus. Dynamic mechanical testing proved that the increase of hardness in PTFE/Nano-EG composites with an increase in stress relaxation time and limit.

Creep resistance properties of PTFE

Creep test is important for engineering polymers. Lower creep rate increases the ability of the material to withstand under harsh physical conditions. PTFE exhibits high creep and causes hindrance to utilize in applications. The improvement in creep properties of PTFE with the addition of micro and nanoscale fillers are an important case of study. Directional PTFE/nano-SiO₂ thin films were tested for the improved creep property. Epoxy based nano-SiO₂ mixed with powder PTFE before it is executed for sintering process. The addition of SiO₂nanoparticles increases the crystalline form of PTFE. Thermal mechanical analyzer (TMA) was used to analyze the mechanical properties of the composite. The tensile properties of PTFE and PTFE composites (nano-SiO₂) were measured and it shows the difference in modulus, tensile strength, and elongation at break at a different weight percent (wt.%) of PTFE/Nano-SiO₂. The results clearly indicate that the addition of nano-SiO₂ considerably improves the tensile strength and hardness and in particular, it reduces the creep strain and creeps rate. The reinforcement of short carbon fibers and short glass fibers significantly improved the tensile strength of 18 wt.% and 20 wt.% of the filler ratio to the PTFE which was reported by the authors.

Chemical properties of PTFE

The peculiar property of PTFE is chemical inertness. Naturally, PTFE is non-reactive and insoluble due to the strongly bonded carbon-fluorine atom. The high molecular weight is responsible for chemical inert behavior. PTFE is not affected by common reagents such as hydrofluoric, hydrochloric, and chlorosulfonic acids. Even above the transition temperature (327 °C), PTFE is insoluble in organic solvents like hydrocarbons, chlorinated hydrocarbons, or ester and phenol. This is due to the very fewer interaction forces between fluorocarbon and other molecules.

Solubility of PTFE

A detailed and comparative examination has been made on the solubility of PTFE under thermodynamic observations. The solvents chosen are oligomers, non-oligomeric perfluorocarbons, aromatic perfluorocarbons, and non-perfluorocarbons. The report was consolidated the different types of thermodynamic solubility influence on PTFE. The solubility of PTFE involves various factors such as temperature, pressure, solvent polarity and swelling in solvents.

There are many practical issues of PTFE in terms of solubility. Several methods were employed to understand the solubility of PTFE with commercial solvents such as perfluorocarbon and other halogenated fluids. Autogenous and superautogenous methods were involved in the solubility of PTFE under applied pressure. The report suggested that the entropy effects cause insolubility due to the less intermolecular forces. The molecular weight of the solvent can influence the solubility with the increase of lower critical solution temperature.

2.4 Thermal properties of PTFE

The performance in terms of thermal conductivity of PTFE over a wide range of temperature is excellent than other polymers. The thermal stability is due to the linear high crystalline arrangement of carbon-fluorine atoms that shows a high melting point of about 342 °C. For the measurement of crystallinity, different techniques can be preferred such as X-ray diffraction, density and dynamic mechanical analysis (DMA). The differential scanning calorimetry (DSC) technique was used to prepare the material from the melt with different crystallinity as a function of temperature. The sample was further tested with reference to one another. The thermal conductivity was measured using Lee’s disk apparatus clearly indicates the improvement in heat transport of aluminum flakes included PTFE. The increase in thermal conductivity at 232 °C was noted for different levels of crystallinity. A detailed study on the thermal behavior was carried out by incorporating ceramics (Sr₂ZnSi₂O₇) as a filler with PTFE. This work explains that how the filler fraction is responsible for the increase in thermal conductivity of the composites. It was measured that the thermal conductivity of Sr₂ZnSi₂O₇ is 16.5 W/mK which is large when compared with PTFE (0.283 W/mK). The increase in thermal conductivity depends upon the filler material’s shape, size, and thermal properties. The fillers generally provide the heat transfer path which was the reason for the increase in thermal conductivity.

2.4.1 Thermal transport property of PTFE composites

Thermal transport property of Al/PTFE nanocomposite with graphene and CNT were reported. Graphene and CNT are widely involving in numerous applications and significantly influence the material behavior which is added along with them. By introducing graphene into Al/PTFE, increasing thermal conductivity was observed. Al acts as a mediator for heat transportation throughout the composite. Thermal diffusivity analysis of Al/PTFE portrayed about how quickly the material responds to the heated environment. The addition of graphene in Al/PTFE increases thermal diffusivity in contrast to the addition of nano carbon (C) allotrope and CNT. The amorphous nature of nano C and CNT is due to the random arrangement of sp² and sp³carbons which results in low thermo-physical property.

2.5 Electrical properties of PTFE

2.5.1 Dielectric property of PTFE

PTFE would play a role of a dielectric medium or insulating medium in an electronic component was consumed potentially because of distinct electric properties. The dielectric constant (ɛ_r) and the dissipation factor (tanδ) are very important for a material operating as a dielectric medium in the charge storing devices. Recently, many works were explored the dielectric properties of PTFE-based composites. Depending upon the filler property, the ɛ_r and tanδ varied and demonstrated in many reports. The improved ɛ_r and tanδ for various PTFE-based composites are shown in Fig. . It shows the PTFE composites tested under different frequency ranges and their respective ɛ_r and tanδ values. It is obvious that depending upon the frequency, the polarization mechanism varies for different types of composites. For PTFE filled with SiO₂ (silicon dioxide), the values of ɛ_r and tanδ increased at 5GHZ of frequency when compared to Virgin-PTFE. The large surface area of the SiO₂ and their moisture absorbance and contaminants were taken into account for explaining the function of ɛ_r and tanδ. PTFE/AlN (aluminum nitrate) showing improved ɛ_r and tanδ as a function of filler loading. The values were obtained in the low frequency range from 100 Hz to 1 MHz which was suggested for electronic packaging. PTFE/TeO₂ showed excellent ɛ_r and tanδ stability tested under 1 MHz and 7 GHz of the frequency range. The increase in tanδ was observed due to the interfacial polarization of the ceramic TeO₂ particles at higher volume fraction in the PTFE matrix. The experimental results showed the improved dielectric constant of MgTiO₃ceramic filled PTFE. The results were good in agreement with the Maxwell-Garnett theoretical model which considers the occupation of ceramic particles in the host polymer system. The calcium copper titanate incorporated PTFE and its dielectric property was studied. The ɛ_rhere reported at low frequency (100 Hz) and attributed to interfacial polarization mechanism. The size of the particle present in the composites obviously changing the value of ɛ_r and tanδ which were demonstrated. Over different frequency ranges, PTFE is stable and possess low dielectric constant ɛ_r ∼ 2.1 and low loss tangent because of the neutralization of dipole moment exhibited by C-F bonds. A work was reported on the moisture absorbance of PTFE/Micron-rutile and PTFE/Nano-rutile composites. The moisture absorbing phenomena is important here because the water molecules are polar in nature having high ɛ_r ∼ 70 which can significantly affect the dielectric nature of the PTFE composition. It is to understand from the above notes that the filler compositions, the size of the particles, frequency, and property of the host polymer system are the important parameters for the dielectric properties.

Optical and spectral properties of PTFE

The inherent optical and spectral properties of PTFE greatly help in the instrumentation of efficient optical devices. The light reflectance and diffusion parameters of PTFE are extremely high; hence, the material has been inevitable in optical instrumentation. Reflectance factor is the measurement of the surface’s ability to reflect light which is equal to the ratio of reflected flux to the incident flux. PTFE exhibits good optical characteristics from a broad ultra-violet to near infra-red spectrum and good in performance when exposed to light or any other electromagnetic radiation. The reflectance angle measurements were studied using reflectometer which was used to measure the bidirectional reflectance of the PTFE pallet. The applications of PTFE as a light diffuser in radiometry were very attractive. The Lambertian surfaces (an ideal surface having high diffusive reflectance) are constructed with PTFE. Previous works considering that low density PTFE functions as a Lambertian diffuser. Measurement of bidirectional reflectance distribution function (BRDF), directional hemispherical reflectance (DHR) and directional hemispherical reflectance (DHT) were taken for two samples namely high density PTFE (HD PTFE) and low density PTFE (LD PTFE). To cover the entire wavelength of the spectrum, the aforesaid measurements were carefully done with the help of Fourier transform infrared Raman spectroscopy (FTIR) and LAMBDA 950 spectrophotometer. The results shown were in favor of LD PTFE because of the order of magnitude for DHR is less than HD PTFE.

The reflectance factor of PTFE is extreme to sustain at high intense electromagnetic radiation. For all optics-based instrumentation works, PTFE was suggested as a white light diffuser. A work was conducted to study the reflectance factor of pressed PTFE powder with a standard reflectance factor scale ratio (45°/0°). The sample was pressed and examined with 45°/0° reflectometer for wavelength varying from 380 to 770 nm. Analysis of samples was done by taking two variabilities: one is an operator (samples collected from 10 different laboratories) and another one is the material (various composition of PTFE). Final result evolved with the expanded uncertainty of 45°/0° reflectance factor due to material and operator variability.

Amorphous PTFE commercially known as Teflon®AF is having a glass-like transparency and possess good optical properties and highly preferred in optical devices. Teflon® AF is a copolymer of PDD and TFE. A detailed study was conducted to calculate the refractive index, extinction coefficient (k), the absorption coefficient (α) and optical absorbance (A) of three different grades of Teflon®AF. The purpose of this work was to compare all the three grades for their respective optical properties. The samples were analyzed using spectroscopic ellipsometer. Further results revealed that the optical characteristics varied for three different grades of Teflon®AF with respect to the TFE content.