Polyvinylidene fluoride (PVDF) is a semi-crystalline thermoplastic fluoropolymer. It is readily melt-processible. It can be fabricated by injection and compression molding. It combines high mechanical strength with good processability.
In recent years, polyvinylidene fluoride (PVDF) has shown considerable interest in the market. This is due to its strongest piezoelectric properties. The polymer is widely used in high-tech applications. These include chemical process equipment, electrical and electronics, specialty, and energy-related applications. But what makes PVDF a high-performance plastic in several sectors? Read on to know more.
What is PVDF?
Polyvinylidene fluoride (PVDF) or polyvinylidene difluoride (PVF2) is a semi-crystalline thermoplastic fluoropolymer. It has a high purity with service temperatures up to 150°C.
The polymer chain has alternating CH2 and CF2 groups. The polarity of these groups leads to:
- Insolubility
- Electrical properties
Molecular Structure of Polyvinylidene Fluoride (PVDF)
PVDF has become the largest volume of fluoropolymers after PTFE. Thanks to its excellent combination of properties and processability. It is commercially available in a wide range of melt flow rates and with various additives. This enhances the processing or end-use properties.
How to produce PVDF?
PVDF is generally synthesized by the free radical polymerization of 1,1-difluoroethylene (CH2=CF2). The polymerization takes place in the suspension or emulsion at:
- Temperature: 10-150°C
- Pressure of 10-300 atm
The material obtained is then processed into film or sheets.
Chlorotrifluoroethylene (CTFE) or hexafluoropropene (HFP) are most commonly employed to prepare PVDF copolymers.
- PVDF copolymers with HFP contain increased flexibility over PVDF homopolymer grades.
- Copolymers with CTFE are among the most flexible PVDF products. The show excellent low-temperature performance and low shrinkage.
Copolymers are ideal for wire & cable and tubing applications, which require enhanced flexibility.
What are the properties of PVDF?
Crystal structure
It is typically a semi-crystalline polymer that is approximately 50% amorphous. It has a highly regular structure with most of the VDF units joined head-to-tail. A very less percentage of these monomer units joined head-to-head.
This fluoroplastic exists in four possible conformations named α, β, γ, and δ phases.
- The C–F bonds are polar. The highest dipole moment is obtained with the alignment of all dipoles of the polymer in the same direction. This corresponds to the β-phase of the PVDF. β-phase is the desired phase for the piezoelectric characteristics of the polymer.
- The dipole moments of α crystallites are oriented in opposite directions. This results in zero net polarization.
Alpha- and Beta-phase Structure of PVDF
(Source: Royal Society of Chemistry)
Physical properties
It has one of the lowest melting points of commercial fluoropolymers. But has the highest heat deflection temperature under load.
| Polymer | Deflection Temperature, °C | Melting Point, °C | |
| 0.5 MPa | 1.8 MPa | ||
| PVDF | 148 | 113 | 178 |
| PCTFE | 126 | 75 | 218 |
| PTFE | 121 | 56 | 327 |
| ECTFE | 116 | 77 | 240 |
| ETFE | 104 | 74 | 270 |
| PFA | 73 | 48 | 310 |
| FEP | 70 | 51 | 270 |
PVDF provides very low permeation values compared to other fluoropolymers. This is due to high crystallinity and surface tension properties. PVDF shows low permeabilities to gases and liquids. Though, the permeability of PVDF is influenced by:
- the crystalline degree and
- the modification of crystalline parts.
Mechanical properties
PVDF shows the following characteristics when compared to ETFE and ECTFE.
- PVDF has similar tensile modulus but lower impact strength.
- Modification with HFP or CTFE lowers the modulus but increases elongation and impact strength.
- When exposed to flame, PVDF is non-flammable and non-dripped. It is self-extinguishing – It is TL V0 compliant. The LOI is 44%.
- It also exhibits good resistance to UV light.
| Property (Standard) | PVDF values | vs. ETFE values | vs. ECTFE values |
| Melting point, °C, ASTM D 3418 | 154-184 | 250-275 | 236-246 |
| Specific gravity, g/cm3, ASTM D 792 (Solid) | 1.75-1.80 | 1.72 | 1.7 |
| Tensile Strength @ 23°C, MPa, ASTM D 638 | 36-56 | 38-48 | 45-60 |
| Elongation @ 23°C, %, ASTM D638 | 25-500 | 100-350 | 150-250 |
| Tensile Modulus @ 23°C, MPa, ASTM D 638 | 1340-2000 | 830 | – |
| Izod impact strength @ 23°C, J/m, ASTM D 256 | 160-530 | no break | no break |
| Coefficient of thermal expansion, ASTM D 696 | ~10-4 | 9 x 10-5 | 5 x 10-5 |
| Processing temp. range, °C | 200-300 | 300-345 | 260-300 |
| Dielectric constant, (1 kHz), ASTM D 150 | 7.5-13.2 | 2.6 | 2.6 |
| Dielectric strength, kV/mm, ASTM D 149 | 260-950 | 59 | 80-90 |
| Dissipation factor, (1 kHz), ASTM D 150 | 0.0163-0.019 | 0.0008 | 0.0024 |
| LOI, %, ASTM D 2863 | 44 | 30 | 64 |
Chemical resistance
The chemical inertness varies between the different fluoropolymers.
- PTFE, FEP, PFA, and MFA exhibit chemical inertness to a wider range of chemicals.
- Partially fluorinated polymers show chemical inertness to a narrower range of chemicals. Examples of partial fluoropolymers include CTFE, PCTFE, and ECTFE.
At elevated temperatures, PVDF can be dissolved in organic solvents such as esters and amines. This allows PVDF to be applied as corrosion-resistance coatings on chemical process equipment and architectural panels.
Parts made of PVDF display great resistance to:
- mineral and organic acids,
- aliphatic and aromatic hydrocarbons,
- alcohols and halogenated solvents.
View the complete range of PVDF grades that show good resistance to chemicals.
Electrical properties
PVDF is primarily used in wire and cable isolation. Thanks to its high dielectric constant and dissipation factor. However, their poor electrical properties allow the production of PVDF films with:
- piezoelectric behavior and
- pyroelectric behavior
These films are prepared from extruded films in B-phase conformation. Both surfaces of the film are metalized and then subjected to high voltage. This leaves it permanently polarized. Such films generate a voltage when stretched or compressed (piezoelectricity) or heated (pyroelectricity) at a temperature close to the melting point. The polymer films also show some ferroelectricity.
How to process PVDF?
PVDF grades are available in a wide range of melt viscosities as powders and pellets. It can be processed using techniques applicable to standard thermoplastics:
- Extrusion
- Injection, compression and transfer molding
- Machining
Drying of resin before processing is usually not necessary Care must be taken to eliminate “hang-up” areas where molten resin can collect and thermally decompose if residence time is excessive
Processing temperature: 190 to 280°C
Injection Molding
- Melt temperature should be between 200-270°C
- A mold temperature of 50-95°C is recommended
- The relatively high melt viscosity of PVDF makes proper mold design an important factor in injection molding
- The shrinkage (3-4 %) of injected molded pieces, has to be take into consideration during conception
Extrusion
- In extrusion, attention should be paid to the elimination of dead spots where degradation of polymer can start
- Extrusion temperature : from 230 to 290°C
- No extrusion aids, lubricants, or heat stabilizers are needed to extrude PVDF
- L/D ratio of at least 20 is recommended
What are the uses of PVDF membranes?
PVDF is one of the most used membrane materials. It has received great attention due to its outstanding properties. PVDF is ideally used in biomedical membrane applications and waste-water treatment. This is due to properties such as thermal stability, chemical resistance, and processability.
Thank to these features, applications of PVDF membranes are currently found in:
- Pressure-driven water- and waste-treatment treatment (e.g., microfiltration, ultrafiltration, and membrane bioreactor)
- Membrane contactors operations (e.g., membrane distillation, acid gas absorption and stripping, volatile organic compounds removal)
Today, most commercial membranes are produced via the phase inversion method. This is mainly because of its simplicity and flexible production scales. Hence, there is a low cost of production.
But there are certain drawbacks associated with PVDF membranes such as wetting or fouling which can be easily solved by:
- enhancing surface roughness or
- increasing the hydrophilic nature of the membrane.
Though it cannot be absolutely prevented even with modification.
Is PVDF recyclable?
Yes, PVDF (Polyvinylidene fluoride) is recyclable. It can be melted and reprocessed multiple times. Thus, it doesn’t lead to any significant degradation of its properties. However, it’s important to note that the recycling of PVDF might not be as readily available as common materials (e.g., PET or HDPE).
Are PVDF resins toxic?
PVDF is generally considered safe for many applications. It is a thermoplastic polymer that has a high resistance to chemicals, UV radiation, and extreme temperatures. This makes it suitable for various industries. In terms of human health, PVDF is generally regarded as non-toxic and biocompatible. Hence, it has been used in medical applications such as implants, prosthetics, and drug delivery systems.