Crystallization kinetics of a fluorinated copolymer of tetrafluoroethylene
Introduction
The so-called “fluoropolymers” have gained in recent years a considerable industrial success in many sectors due to excellence of a range of properties. Basically, fluoropolymers are adopted for special applications, and their use is limited also because of lack of characterisation of main parameters influencing processability, above all those related to crystallisation kinetics during processing.
MFA, a fluoropolymer recently commercialised by Solvay, belongs to the class of PFA (perfluoroalkoxy), having a melting point lower than standard PFA grades. MFA and PFA are semi-crystalline fully-fluorinated melt processable fluoropolymers which offer the highest temperature rating, and broadest chemical resistance of all melt processable fluoropolymers. The unique chemistry of MFA allows for a cost competitive product, whenever PFA type performance is required. In particular, these resins can be easily injection moulded to obtain self-supported items like fittings and valves, where special requirements of thermal and chemical resistance are necessary.
In this work, the crystallisation kinetics of MFA was studied by both standard calorimetric tests and fast cooling tests performed by an apparatus which allows real-time monitoring of crystallisation phenomena.
Section snippets
Material
The resin adopted is a commercial Hyflon® MFA 640, (polytetrafluoroethylene–perfluoromethylvinylether) co-polymer, supplied by Solvay.
Hyflon® MFA shows outstanding thermal resistance, from cryogenic up to 250 °C (the melting point is 285 °C). It also has excellent melt stability––weight loss measured by means of thermogravimetric analysis in air at 380 °C for 60 min is about 0.3% [1], [2].
Calorimetry
Some samples of material were solidified in a DSC apparatus (Mettler DSC30, with liquid nitrogen as cooling
Crystallization kinetics
In the case of random copolymer of TFE, even for low comonomer unit contents, the complex polymorphic behaviour of polytetrafluoroethylene (PTFE) is simplified and only two crystalline forms (I and II) are generally observed. The less ordered crystalline form I is present at room temperature and the low temperature form (form II, ordered structure with a triclinic unit cell) is observed at −40 °C [8], [9].
All tests performed in this work, like in usual processing conditions, start from a melt
Conclusions
Crystallization kinetics of a commercial fluoropolymer was analysed by calorimetric isothermal and cooling tests in the range 0.017–220 °C/s. Cooling tests in the range 2–220 °C/s were performed by a new apparatus based on detection of light intensity transmitted through a film of the polymer.
A simple kinetic model was adopted for the description of the crystallinity evolution. The model keeps into account the dependence of the maximum crystallinity degree on temperature. The parameters of the
References (13)
New method for following rapid rates of crystallisation. I. Poly(hexamethylene adipamide)
Polymer
(1961)A new technique for following rapid rates of crystallisation. II. Isotactic polypropylene
Polymer
(1962)- et al.
Structural variations in random copolymers of tetrafluoroethylene with kind and content of comonomer units
Polymer
(1998) Role of entanglement in crystalline polymers 1. Basic theory
Polymer
(2002)- Hyflon MFA and PFA Design Guide. Solvay Solexis,...
Semi-crystalline fluorinated polymers
Polym. Int.
(2001)