Impact of nucleating agents of PVDF on the crystallization of PVDF/PMMA blends
Introduction
PVDF is widely used for its mechanical properties, chemical and weathering resistance and processing properties [1]. The major application of PVDF is in external architectural coatings (paints) [2]. In order to improve performance and reduce costs, PVDF is often blended with miscible acrylic polymers, which act as binders and help in the dispersion of pigments [3]. For PVDF/PMMA blends, DSC curves show a single glass transition and a depression of the melting point for PVDF compositions above 50% [4], [5], [6], [7], [8], [9], [10]. However, the thermal history of the sample has important bearings on the compatibility characteristics of the PVDF/PMMA system: for example, a single glass transition is observed only for completely amorphous blends.
For coatings formulations, earlier investigations have shown that a PVDF crystalline phase must be present, which implies that PVDF contents must be higher than 50%. A composition containing 70% of PVDF and 30% of PMMA (70/30) was found to have optimal physical and optical properties: gloss, hardness and solvent resistance. For PVDF contents higher than 80%, the hardness and flexural modulus decrease. Hence, the properties of PVDF/PMMA blends are optimal only in a quite narrow window of PVDF contents and are, in ultimate analysis, highly dependent on the PVDF crystallinity.
Design of optimal crystalline-amorphous blend compositions is therefore a major challenge, especially when considering that amorphous polymers may have a major detrimental impact on the crystallization rates of their crystalline partners. As an illustration, addition of only a small amount (0.5–5%, i.e. one to two orders of magnitude less than in the present PVDF-based systems) of poly(vinylbutyral) to linear polyesters (e.g. poly(ϵ-caprolactone)) reduces drastically the growth rates and reduces by several orders of magnitude the concentration of active, heterogeneous nuclei [12]. Similar observations have been reported for the PVDF–PMMA blends under investigation: sepiolite (a hydrated magnesium silicate) is a nucleating agent for PVDF, but this nucleating effect vanishes in PVDF/PMMA blends [13].
Building on the experience gained while investigating nucleating agents (NA) of PVDF reported in the companion paper [14], we present here the impact of NA (in particular PTFE) on the crystallization behavior of PVDF when blended with PMMA. We show that the kinetics of PVDF crystallization and the ultimate crystallinity of PVDF/PMMA samples are significantly enhanced by the use of NA. As a consequence, PVDF/PMMA blends with higher PMMA contents can be used as coatings without loss of physical and optical properties.
Section snippets
Materials
The experiments are performed with the three different grades of commercial latexes of PVDF produced by ELF-Atochem and used in the companion paper: Kynar 500, 740 and 1000.
One PMMA homopolymer and two copolymers of methyl–methacrylate (MMA) and ethyl-acrylate (EA) (copolyMMA–EA) were produced by batch emulsion polymerization at the ELF-Atochem Research Center in Lacq (GRL). The two copolymers have identical compositions (90/10), slightly different molecular weights (5.65 and 10.7×105), and
Glass transitions and melting temperature
Variation of glass transition temperatures and melting points with composition is a convenient indicator in the analysis of the blend compatibility of semi-crystalline and amorphous polymers — in the present case PVDF/PMMA. The resulting ‘phase diagram’ (Fig. 1) displays many features known from earlier studies [4], [5], [6], [7], [8], [9], [10], [11], namely:
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for low PVDF concentrations (<≈40–50%), and even though only few data points are available (owing to the experimental difficulties
Discussion
The above results indicate that PTFE is a very potent nucleation additive for blends of PVDF and PMMA, or PMMA-co-EA polymers. If anything, PTFE would appear as more efficient for the blends than for pure PVDF, on account of the larger increase of Tc — which however translates in comparable efficiencies (i.e. taking into account the larger ΔTc gap for the blends). Furthermore, the nucleating additive makes it possible to reach sizeable PVDF crystallinities for blends which would not ‘normally’
Conclusion
The nucleating impact of PTFE on PVDF, which has been shown to induce significant variations of the crystallization kinetics of the homopolymer, has an even more profound impact on the crystallization behavior of blends of PVDF and acrylic polymers. This impact is best attested by the unusually high overall efficiency, which amounts to 90% of the ‘ideal’ standard set by self-nucleated samples. It is also attested by the significant increase in Tc (over 25°C) achieved on simple cooling from the
Acknowledgements
The generous financial support of ELF Atochem to S.S. and the ICS is gratefully acknowledged.
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