PVDF processing routes mainly include two categories at present, which are emulsion polymerization and suspension polymerization. The two routes are similar, but still exist slight differences. The principles and specific methods will be described in detail below.

Principle of emulsion polymerization^[25]

The main site of emulsion polymerization is sol-increasing beam, monomer droplets play the role of monomer warehouse, with the progress of polymerization reaction, monomer through the dissolved monomer in the water phase to the micelle diffusion, the supply of polymerization required monomer. The particles of the polymerization product are insoluble in water and precipitate from the aqueous phase. Under the action of emulsifier, the stable emulsion is formed. In emulsion polymerization, the stirring rate of the polymerization kettle needs to be strictly controlled.

Emulsion polymerization process^[26]

Reaction vessel: 130L stainless steel autoclave is used, rotational speed 88r/min.

Reaction conditions: polymerization temperature 75~90℃, polymerization time 14h, polymerization pressure 2.0 ~ 3.8 MPa.

Reaction reagent: VDF monomer, emulsifier, initiator, buffer, deionized water.

Reaction steps: First, the sealing property of the autoclave is checked, and then the autoclave is vacuumized and filled with nitrogen. Repeat for many times until the oxygen content in the autoclave reaches the requirement. Then the autoclave is added with VDF monomer and formula reagent and heated to 75~90℃. During the reaction, VDF monomer is continuously added to maintain the pressure in the autoclave. Finally, the polymer emulsion is physically processed to obtain PVDF products, and the unreacted VDF monomers are recovered.

Fig.1. Flow chart of VDF emulsion polymerization process ^{[25] [26]}.

Principle of suspension polymerization^[25]

In VDF suspension polymerization, VDF monomer is suspended in deionized water in dispersion medium in the form of droplet under the action of agitation and dispersant. The oil-soluble initiator is used to make the initiator enter the monomer droplet and trigger the polymerization reaction. The polymerization product PVDF resin is precipitated out in the form of solid particles. VDF suspension polymerization is usually carried out at lower temperatures and high strength initiators are required.

Suspension polymerization process^[26,27]

Reaction vessel: stainless steel autoclave with stirring.

Reaction conditions: polymerization temperature 30 ~ 60℃, polymerization time 15 ~ 22 h, polymerization pressure 2.1 ~ 7.0 MPa.

Reaction reagent: VDF monomer, dispersant, deionized water, initiator.

Reaction steps: First, the airtight of the autoclave is checked, and oxygen is replaced with nitrogen after vacuumization. Secondly, the autoclave is added with a certain amount of dispersant and deionized water, stirred, heated to 50℃, filled with VDF monomer to boost the pressure to 3.5mpa, and added initiator and other additives. During the reaction process, VDF monomer and formula additives are added to the autoclave in a certain proportion and rate to maintain temperature and pressure. Until the polymerization of VDF monomer is finished, the pressure drops to 2.8mpa, and the stirring is stopped. Finally, the polymerizate is physically processed to obtain PVDF products, and the unreacted VDF monomers are recovered.

Fig.2. Flow chart of VDF suspension polymerization process ^{[25] [26]}.

Comparison between VDF emulsion polymerization and suspension polymerization

Process flow comparison:

In the emulsion polymerization, the polymerization temperature was 75 ~ 90℃, the stirring rate was 88 r/min, the appropriate amount of chain transfer agent was 1.0% ~ 3.0% of the monomer mass, and the polymerization pressure was 2.0 ~ 3.8 MPa^[28].

In the suspension polymerization, the polymerization temperature is 30 ~ 60℃, the polymerization temperature is 50℃, the stirring rate is not strict, the appropriate amount of chain transfer agent is 0.05% ~ 0.4% of the monomer mass, the polymerization pressure is 2.1 ~ 7.0 MPa, the polymerization pressure is 3.5 MPa^[28].

Performance comparison chart:

Fig.3. Comparison of performance between emulsion polymerization and suspension polymerization^[29].

[25] specialchem.com. 2022. Polyvinylidene Fluoride (PVDF) Plastic: Material Properties & Other Info. [online] Available at: <https://omnexus.specialchem.com/selection-guide/polyvinylidene-fluoride-pvdf-plastic> [Accessed 11 May 2022].

[26] Habib S. What is PVDF (Polyvinylidene Fluoride) | A Detailed Guide - PlasticRanger [Internet]. PlasticRanger. 2022 [cited 11 May 2022]. Available from: https://plasticranger.com/what-is-pvdf/

[27] Browser Check - Update my Browser [Internet]. Matmatch.com. 2022 [cited 17 May 2022]. Available from: https://matmatch.com/learn/material/polyvinylidene-fluoride-pvdf

[28] Habib S. What is PVDF (Polyvinylidene Fluoride) | A Detailed Guide - PlasticRanger [Internet]. PlasticRanger. 2022 [cited 17 May 2022]. Available from: https://plasticranger.com/what-is-pvdf/

[29] What is PVDF [Internet]. Pvcpipesupplies.com. 2022 [cited 17 May 2022]. Available from: https://pvcpipesupplies.com/what-is-pvdf