Elsevier

European Polymer Journal

Volume 40, Issue 9, September 2004, Pages 2089-2095
European Polymer Journal

Crystallization kinetics of a fluorinated copolymer of tetrafluoroethylene

https://doi.org/10.1016/j.eurpolymj.2004.05.018Get rights and content

Abstract

The so-called “fluoropolymers” gained in recent years a considerable industrial success, and the increasing industrial interest to this class of materials causes a need of a better characterization of the properties of interest for processability. In this work, the crystallisation kinetics of a Fluorinated Copolymer of Tetrafluoroethylene (MFA, produced by Solvay), was studied by both standard calorimetric tests and fast cooling tests performed by an apparatus which allows on line determination of crystallisation phenomena. Material crystallisation kinetics resulted to be so fast that the polymer reached the maximum degree of crystallisation for all solidification conditions, also those obtained at cooling rates of the order of hundreds of degrees per second. Calorimetric tests also gave indications about the dependence of maximum crystallinity degree upon temperature. The crystallisation kinetics was described by the non-isothermal formulation due to Nakamura of the well-known Avrami equation. Results were compared with experimental data.

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

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