Polyvinylidene fluoride, or polyvinylidene difluoride is a highly non-reactive and pure
thermoplastic fluoropolymer produced by the polymerization of vinylidene difluoride.
PVDF is a specialty plastic material in the fluoropolymer family; it is used generally
in applications requiring the highest purity, strength, and resistance to solvents, acids,
bases and heat and low smoke generation during a fire event.
Compared to other fluoropolymers, it has an easier melt process because of its relatively
low melting point of around 177 °C. It has a low density compared to the other
fluoropolymers.
It is available as piping products, sheet, tubing, films, plate and an insulator for
premium wire.
It can be injected, molded or welded and is commonly used in the chemical, semiconductor,
medical and defense industries, as well as in lithium ion batteries.
It is also available as a crosslinked closed cell foam, used increasingly in aviation and
aerospace applications.
A fine powder grade, KYNAR 500 PVDF or HYLAR 5000 PVDF, is also used as the principal ingredient
of high-end paints for metals.
These PVDF paints have extremely good gloss and color retention, and they are in use on
many prominent buildings around the world, e.g. the Petronas Towers in Malaysia and Taipei
101 in Taiwan, as well as on commercial and residential metal roofing.
PVDF membranes are used for western blots for immobilization of proteins, due to its
non-specific affinity for amino acids.
Names PVDF is sold under a variety of brand names
including KF, Hylar, Kynar and Solef.
Properties In 1969, strong piezoelectricity was observed
in PVDF, with the piezoelectric coefficient of poled thin films as large as 6–7 pC/N:
10 times larger than that observed in any other polymer.
PVDF has a glass transition temperature of about −35 °C and is typically 50–60%
crystalline.
To give the material its piezoelectric properties, it is mechanically stretched to orient the
molecular chains and then poled under tension.
PVDF exists in several forms: alpha, beta, and gamma phases, depending on the chain conformations
as trans or gauche linkages.
When poled, PVDF is a ferroelectric polymer, exhibiting efficient piezoelectric and pyroelectric
properties.
These characteristics make it useful in sensor and battery applications.
Thin films of PVDF are used in some newer thermal camera sensors.
Unlike other popular piezoelectric materials, such as PZT, PVDF has a negative d33 value.
Physically, this means that PVDF will compress instead of expand or vice versa when exposed
to the same electric field.
Processing PVDF may be synthesized from the gaseous VDF
monomer via a free radical polymerization process.
This may be followed by processes such as melt casting, or processing from a solution.
Langmuir-Blodgett films have also been made.
In the case of solution-based processing, typical solvents used include dimethylformamide
as well as the more volatile butanone.
In aqueous emulsion polymerization, the fluorosurfactant perfluorononanoic acid is used in anion form
as a processing aid by solubilizing monomers.
For characterization of the molecular weight via gel permeation chromatography, solvents
such as dimethyl sulfoxide or tetrahydrofuran may be used.
Processed materials are typically in the non-piezoelectric alpha phase.
The material must either be stretched or annealed to obtain the piezoelectric beta phase.
The exception to this is for PVDF thin films.
Residual stresses between thin films and the substrates on which they are processed are
great enough to cause the beta phase to form.
In order to obtain a piezoelectric response, the material must first be poled in a large
electric field.
Poling of the material typically requires an external field of above 30 MV/m.
Thick films must be heated during the poling process in order to achieve a large piezoelectric
response.
Thick films are usually heated to 70–100 °C during the poling process.
A quantitative defluorination process was described by mechanochemistry, for safe eco-friendly
PVDF waste processing.
Copolymers Copolymers of PVDF are also used in piezoelectric
and electrostrictive applications.
One of the most commonly used copolymers is P(VDF-trifluoroethylene), usually available
in ratios of about 50:50 wt% and 65:35 wt%.
Another one is P(VDF-tetrafluoroethylene).
They improve the piezoelectric response by improving the crystallinity of the material.
While the copolymers' unit structures are less polar than that of pure PVDF, the copolymers
typically have a much higher crystallinity.
This results in a larger piezoelectric response: d33 values for P(VDF-TFE) have been recorded
to be as high as −38 pC/N versus −33 pC/N in pure PVDF.
Applications PVDF is commonly used as insulation on electrical
wires, because of its combination of flexibility, low weight, low thermal conductivity, high
chemical corrosion resistance, and heat resistance.
Most of the narrow 30-gauge wire used in wire wrap circuit assembly and printed circuit
board rework is PVDF-insulated.
In this use the wire is generally referred to as "Kynar wire", from the trade name.
The piezoelectric properties of PVDF are used to advantage to manufacture tactile sensor
arrays, inexpensive strain gauges and lightweight audio transducers.
Piezoelectric panels made of PVDF are used on the Venetia Burney Student Dust Counter,
a scientific instrument of the New Horizons space probe that measures dust density in
the outer solar system.
PVDF is the standard binder material used in the production of composite electrodes
for lithium ion batteries.
1-2% weight solution of PVDF dissolved in N-methyl-2-pyrrolidone is mixed with an active
lithium storage material such as graphite, silicon, tin, LiCoO2, LiMn2O4, or LiFePO4
and a conductive additive such as carbon black or carbon nanofibers.
This slurry is cast onto a metallic current collector and the NMP is evaporated to form
a composite or paste electrode.
PVDF is used because it is chemically inert over the potential range used and does not
react with the electrolyte or lithium.
In the biomedical sciences PVDF is used in immunoblotting as an artificial membrane,
usually with 0.22 or 0.45 micrometres pore sizes, on which proteins are transferred using
electricity.
PVDF is resistant to solvents and, therefore, these membranes can be easily stripped and
reused to look at other proteins, making it very convenient.
PVDF membranes may be used in other biomedical applications as part of a membrane filtration
device, often in the form of a syringe filter, or wheel filter.
The various properties of this material such as heat resistance, resistance to chemicals,
corrosion and low protein binding properties make this material valuable in the biomedical
sciences for preparation of medications as a sterilizing filter, and as a filter to prepare
samples for High Performance Liquid Chromatography and other advanced analytical techniques in
which small amounts of particulate can damage sensitive and expensive equipment.
PVDF is used for specialty monofilament fishing lines, sold as fluorocarbon replacements for
nylon monofilament.
Optical density is lower than nylon, which makes the line less discernible.
The surface is harder, so it is more resistant to abrasion and sharp fish teeth.
It is also denser than nylon, making it sink faster.
PVDF transducers have the advantage of being dynamically more suitable for modal testing
applications than semi-conductor piezoresistive transducers, and more compliant for structural
integration than piezoceramic transducers.
For those reasons, the use of PVDF active sensors is a keystone for the development
of future structural health monitoring methods, due to their low cost and compliance.
See also Ferroelectric polymers
Klaiber's law Ferroelectricity
Piezoelectricity Pyroelectricity
Plastic Polymer
Lithium ion battery References
External links PVDF Tubing Properties