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 (ɛr) 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 ɛr and tanδ varied and demonstrated in many
reports. The improved ɛr and tanδ for various PTFE-based composites are
shown in Fig. . It shows the PTFE composites tested under different
frequency ranges and their respective ɛr 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 ɛr 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 ɛr and tanδ. PTFE/AlN (aluminum nitrate) showing
improved ɛr 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 ɛr 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 ɛrhere 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 ɛr and tanδ which
were demonstrated. Over different frequency ranges, PTFE is stable and possess
low dielectric constant ɛr ∼ 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 ɛr ∼ 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.
