What we Should Know About PVDF (Polyvinylidene Fluoride) Materials

The name PVDF may sound a little strange, but it has a wide range of applications in our lives.

Its name is Polyvinylidene fluoride, and it also has aliases like polyvinylidene fluoride, polyvinylidene fluoride, polyvinylidene fluoride resin, polyvinylidene fluoride resin, and so on. In terms of appearance, it is usually a white powder or granule.

Poly(vinylidene fluoride), abbreviated as PVDF, mainly refers to homopolymers of vinylidene fluoride or copolymers of vinylidene fluoride and a small amount of other fluorine-containing vinyl monomers, which combines the characteristics of fluorine resins and general-purpose resins, and also has piezoelectricity, dielectricity and other special properties, such as thermoelectricity, etc. It is the second largest product in the production of fluorine-containing plastics with a global production capacity of 10,000 tons, In addition to good chemical resistance, high temperature resistance, oxidation resistance, weather resistance and radiation resistance, it also has piezoelectricity, dielectricity, thermoelectricity and other special properties.

The chemical structure of fluorocarbon is combined with a fluorine-carbon bond, and this structure with short bonding properties forms the most stable and strongest bond with hydrogen ions. Therefore, fluorocarbon coatings have special physical and chemical properties, not only has a strong wear resistance and impact resistance, but also in the extremely harsh and harsh environment has a high resistance to fading and ultraviolet performance.

Physical Properties

PVDF is mainly used where extreme purity is required, as well as resistance to solvents and acids and bases. PVDF has a lower density (1.78 g/cm) than other fluoropolymers such as PTFE.

PVDF can be used to produce tubing, sheet, film, substrate, and insulating jackets for cables. It can also be injection molded or welded, and is widely used in the chemical, semiconductor, pharmaceutical, and defense industries, where it can be used, for example, to make lithium-ion batteries. In addition, it can be made into cross-linked closed-cell foams, which are increasingly used in aerospace applications.

Fine powder grades of PVDF, such as KYNAR 500 PVDF and HYLAR 5000 PVDF, can be used for high-end metal coatings. These coatings have an extremely high gloss and color stability. These coatings can be found in many famous buildings such as the Twin Peaks Tower and Taipei 101. It is also used in commercial buildings and residential paved metal roofs.

PVDF films can be used to immobilize proteins in Western blotting assays due to their non-specific affinity for amino acids.

In 1969, researchers discovered that PVDF has a strong piezoelectric effect: the piezoelectric coefficient of the polarized (i.e., put into a strong electric field to produce a net dipole moment) film was 6-7 PC/N, which was more than 10 times larger than the corresponding value for polymers found at that time.

PVDF has a glass transition temperature (Tg) of about -35°C and a crystallinity of typically 50-60%. In order to impart piezoelectric properties to the material, the material is usually mechanically stretched in the direction of the molecular chains and then polarized under tension.PVDF is available in a variety of solid phases: α phase (TGTG'), β phase (TTTT), and γ phase (TTTGTTTG'). The difference between these phases is whether the molecular chain is cis (T) or trans (G.) PVDF becomes a ferroelectric polymer when polarized and has good piezoelectric and pyroelectric properties. These properties allow it to be used in the production of sensors and batteries, such as some of the new thermographic camera sensors that use PVDF films.

Unlike some other piezoelectric materials, such as PZT, PVDF has a negative d33 value. In physical terms, this means that while other materials expand in an electric field, PVDF contracts and vice versa.

Main Properties

PVDF is generally produced by suspension polymerization or emulsion polymerization of vinylidene fluoride, the reaction equation is: CH₂=CF₂--(CH₂CF₂)n .

In terms of physical properties, PVDF has a relative density of 1.76 to 1.79, a melting point of 160°C to 170°C, can be used in a wide range of temperatures from -60°C to +150°C, and is soluble in strong polar solvents such as dimethylacetamide.

PVDF has many outstanding properties. First, it has good heat resistance and maintains stability even at high temperatures. Secondly, it has excellent chemical stability and is resistant to a wide range of chemicals. Weather resistance, UV resistance and resistance to high-energy radiation are also excellent. In addition, PVDF has a high polarity, a high dielectric constant, and a high dielectric loss angle tangent.

In terms of processing, PVDF is relatively simple, and common molding, extrusion, and injection molding methods can be applied.

In terms of molding conditions, drying is not required for the original package; the injection temperature is usually controlled at 180-230℃; the mold temperature is 60-90℃; the extrusion temperature is 180-265℃; and the die head temperature is 66-140℃.

For storage, PVDF should be stored at 5-30℃ in a clean and dry environment to prevent impurities such as dust and moisture from mixing. Transportation should be treated as non-hazardous goods, and heat, moisture or violent vibration should be avoided during the process.

PVDF plays a vital role in many fields

In the petrochemical field, because of its good chemical resistance, excellent processability and excellent fatigue and creep resistance, it has become one of the ideal choices for the whole or lined pumps, valves, pipelines, pipeline fittings, tanks and heat exchangers in the fluid handling system of petrochemical equipment.

PVDF is also used in the electrical and electronic fields. With good chemical stability and electrical insulation properties, it is widely used in the semiconductor industry for the storage and transportation of high-purity chemicals in equipment that meets TOCS and flame retardant requirements.

PVDF is also one of the main raw materials for fluorocarbon coatings, which are now in their sixth generation. These coatings are used in power stations, airports, highways, high-rise buildings and other places due to their superior weather resistance, which allows them to be used outdoors for a long time without maintenance.

In addition, PVDF resin can also be blended with other resin modification. For example, the composite materials obtained by blending with ABS resin have been widely used in construction, automobile decoration, appliance shells and other fields.

PVDF also has some special properties and uses

For example, it has piezoelectricity, thermoelectricity and other functions, can be made into a variety of complex shapes of sensors, medical materials. In protein sequencing, PVDF membrane is an ideal alternative to nitrocellulose membrane, which can bind proteins and separate small fragments of proteins.

They need to be pretreated with anhydrous methanol before use to activate the positively charged groups on their surface, which makes them easier to bind to negatively charged proteins. Methanol-free transfer buffer can be used to transfer pretreated PVDF membranes.

Application Areas

PVDF is often used as an insulating jacket for electric wires due to its multiple excellent properties such as elasticity, low weight, low thermal conductivity, high chemical resistance, and heat resistance. Thin 30-gauge wire, which is often used in wire-wound circuits, and printed circuit boards are often insulated with PVDF. Cables with PVDF insulation are often called [Kynar wire" under the trademark name of PVDF.

Because of its piezoelectric properties, PVDF is often used in the production of tactile sensor arrays, inexpensive strain gauges, and lightweight audio transducers.

PVDF is also the standard binder for composite electrodes for lithium batteries: PVDF dissolved in N-methyl-2-pyrrolidone (NMP) with a specific gravity of 1-2% is mixed with graphite, silicon, tin, active lithium storage materials such as LiCoO2, LiMn2O4 or LiFePO4, and conductive additives such as carbon black or carbon nanofibers. The slurry is then poured onto the metal collector and the NMP is evaporated to form a composite electrode or paste electrode.PVDF can be used in this application because it is chemically inert in the potential range of battery charging and discharging and does not react with the electrolyte or lithium.

In the biomedical field, PVDF films are often used for immunoblotting, where proteins are electrophoresed. Because PVDF is resistant to solvent corrosion, the film used in the assay can be easily peeled off and reused to detect other proteins.PVDF films can also be used to make syringe- or wheel-type membrane filtration devices. The material's heat and chemical resistance and low protein binding properties allow it to be used as a sterilizing filter in the preparation of pharmaceuticals, and as a filter in the preparation of samples for analyses such as HPLC, preventing expensive equipment from being damaged by small amounts of particulate matter in these samples.

PVDF is also used to make specialty monofilament fishing line as an alternative to traditional nylon monofilament. Its hard surface makes it more resistant to abrasion from sharp fish teeth, and its optical density is lower than nylon, making the line less visible to the keen eye of the fish. It is also denser than nylon, allowing the line to sink faster toward the fish.

PVDF transducers are better suited for dynamic modal testing than semiconductor piezoresistive transducers and have advantages in structural integration over piezoceramic transducers. Due to lower cost and greater compatibility, active transducers utilizing PVDF are important for future developments in structural health monitoring.

New energy field is now one of the most important application directions

In the field of new energy vehicles, PVDF (polyvinylidene fluoride) plays a significant role.

It mainly serves as positive binder and diaphragm coating material. With the rapid growth of electric vehicle sales, the demand for lithium batteries is growing explosively.

PVDF has become one of the indispensable key materials in the lithium battery industry chain due to its excellent chemical resistance, electrical insulation and other characteristics.

As anode binder, PVDF is difficult to be replaced in the short term.

Lithium battery has high requirements on the performance of binder, which should be able to be infiltrated by organic electrolyte and resist its erosion, not dissolved, less dissolved, and maintain good bonding performance under the environment of organic electrolyte, and at the same time, its electrochemical stability should be excellent, and the decomposition voltage needs to be above 4.5V.

With excellent chemical stability and corrosion resistance, PVDF can effectively resist the erosion of polar organic solvent electrolyte, and has good bonding performance, so it is widely used in lithium battery cathode binder, which accounts for up to 90% at present.

In order to effectively improve energy density, lithium battery binder needs to be used in small amount and good bonding effect. In order to ensure excellent bonding performance, battery-grade PVDF generally needs to have a high molecular weight, usually more than 1.1 million, and the binder is required to have a low water content.

In addition, in order to ensure the consistency of lithium-ion batteries, battery manufacturers require a high degree of consistency in the performance of different batches of purchased PVDF.

Summarize

PVDF is a material with excellent performance and wide application. With the continuous progress and development of science and technology, its application areas may be further expanded and deepened. However, in the process of use, it is important to fully understand its characteristics and possible risks to ensure its safe and effective function.