 | |
| Names | |
|---|---|
| IUPAC name poly(1-fluoroethylene)[1] | |
| Other names poly(vinyl fluoride) | |
| Identifiers | |
| Abbreviations | PVF |
| ChEBI | |
| ChemSpider |
|
| MeSH | polyvinyl+fluoride |
| Properties | |
| (C2H3F)n | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Polyvinyl fluoride (PVF) or –(CH2CHF)n– is apolymer material mainly used in theflammability-lowering coatings of airplane interiors andphotovoltaic module backsheets.[2] It is also used inraincoats and metal sheeting. Polyvinyl fluoride is athermoplasticfluoropolymer with a repeatingvinyl fluoride unit, and it is structurally very similar topolyvinyl chloride.
The PVF-basedfilm was first commercialised in 1961 byDuPont under the name Tedlar.[3][4]
The most widely usedpolymerizations of VF are inaqueous suspensions oremulsions. Highpressures are required because of theVFvolatility. The highelectronegativity offluorine makes thepolymerization more difficult when compared to othervinyl halides.[4] Thepolymerizationtemperatures range from 50 °C to 150 °C and can affect thecrystallinity,melting point andbranching of the product.Initiation is done byperoxides orazo compounds.[3]
Theresonance stabilization of thepropagatingintermediate (VFradical) is poor, which often leads tomonomer reversals,branching andchain-transfer reactions. The presence ofimpurities greatly affects themolecular weight andthermal stability of the product, as theVFradical is highly reactive. This also limits the choice ofpolymerization mediums,surfactants,initiators or other additives.[4]
TheliquidVF issuspended inwater and stabilized either bywater-solublepolymers based oncellulose orpolyvinyl alcohol.Inorganicsalts can also act as stabilizers. Thesuspension polymerization is usuallyinitiated byorganic peroxides (eg diisopropyl peroxydicarbonate), butUV light orionizing radiation can also be used. However, when there are noradicals present, theUV radiationdecomposes theVF intoacetylene andHF.[4]
Emulsion polymerization can be done at highly reducedpressures and lowertemperatures compared tosuspension polymerization. The improved process control andreaction heat removal lead to increase inmolecular weight,rate of reaction andyield.Fluorinatedsurfactants such asperfluorinatedcarboxylic acids maintain a highrate of reaction even after 40%conversion, they arethermally andchemically stable and their incorporation does not impair PVF properties. Otheremulsifiers (fatty alcoholsulfates, alkane sulfonates etc) are not as effective.[4]
PVF is usually converted intothin films andcoatings. However, due to its hydrogen bonds andcrystallinity, atemperature above 100 °C is necessary to dissolve PVF in latentsolvents. The processing bymelt extrusion depends on the latentsolvation of PVF in highlypolarsolvents and its subsequentcoalescence. The incorporation of additives (plasticizers,pigments,stabilizers etc.) is done bydispersion with PVF in the latentsolvent. Thesolvent isevaporated afterextrusion.[4]
To create biaxially orientedfilms, the PVFdispersed insolvent must be trailed by bothtransverse directions and biaxial orientations, which results in highertensile strength. The unoriented films are also slightly stretched aftercasting. They are more compliant andformable and exhibit higherelongation at break than the oriented films.[5]
The majority of linkages in PVF are head-to-tail, and only 12-18 % of linkages are head-to-head. These irregularities are probably the cause of the variations inmelting point, which ranges from 185 °C to 210 °C. Thecrystallinity of PVF ranges from 20 to 60%, depending on thepolymerization method and thermal history of thepolymer. It has been found that lowerpolymerizationtemperature leads to a decrease in head-to-head linkages and subsequently increase inmelting point since the highly regular structures display highercrystallinity. As forstereoregularity, PVF is mostlyatacic, but this does not significantly affect themelting point. The commercialatactic PVFfilm shows amelting point peak at 190 °C.[3][4][5]
Several transition phases occur below themelting point, mainly at lowerTg from -15 to ‑20 °C, and at upperTg with the temperature range of 40 to 50 °C.[6]
PVF isinsoluble in commonsolvents below 100 °C. When thetemperature is raised, it becomessoluble inpolarsolvents (amides,ketones etc.). At roomtemperature, the PVFfilms are resistant to bothacids andbases as well asaliphatic,aromatic andalcoholliquids.[3]
Thethermal stability of PVF is better than that of othervinyl halidepolymers, reportingbackbonecleavage andHF loss in aninert atmosphere at 450 °C, while inair theHF loss occurs at 350 °C.[4]
Since PVF has exceptionalthermal stability, it is far safer thanPVC, which degrades more easily. If PVF degradation happens, the highly reactiveHF acid is generated but is quickly absorbed into the surrounding materials and dissipates.[7]
Themonomer,VF isflammable and highly reactive, forms anexplosive mixture withair and is classified as "probablycarcinogenic tohumans".[7]
PVF has not caused anyskin reaction ortoxic effects, although after excessive exposure thefluoride content inurine increased. The overheating of PVF products may result in interaction with the additives such aspigments orfillers, which may pose as an additional risk. Some formulations of the Tedlarfilms may containheavy metal compounds, which can be present indust created by secondary operations (egsanding).[7]
Exterior and interior PVF finishes do not create an additional danger regardingfire in residential and industrial buildings, because thecarbon monoxide created by thecombustion of otherconstruction materials is far more dangerous.[7]
The main applications of PVF are protective and decorativecoatings, thanks to itsthermal stability, and generalinertness towardschemicals,corrosives andstaining agents. There are two ways to apply PVF, either as a preformed film (laminating) or fromdispersion (coating). Thesecoatings can betransparent orpigmented.[4]
In theautomotive industry, PVFprimer is used to improvepaintadhesion, while in theaerospace engineering industry, the PVF film is applied toinsulating bags containingglass fibre, which are used on exteriorairplane walls, incargo space andair condition ducts.[4]
On the top ofphotovoltaic cells, thetransparent PVFfilm protects againstmoisture, while thewhite-pigmentedfilm is used on their bottom surface.[4]
PVFfilms are non-adhering tophenolic,acrylic andepoxy resins and can be therefore used as releasefilms, usually in high-temperature processing of theseresins.[4]
