Examining structure property relationships in coatings based on substituted linear aromatic polycyanurates

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Abstract

Three series of halogenated and non-halogenated polycyanurates are prepared in good yield and purity, and fully characterised. Many of the resulting polymers, formed at room temperature using phase transfer catalysis, can be cast into films with good resilience and high thermal stability (some examples suffer no mass loss when held isothermally at 190 °C and only display appreciable losses when held continuously at 300 °C). Char yields of 35–65% are achieved in nitrogen depending on backbone structure. Some problems were encountered with solubility, particularly with heavily halogenated dichlorotriazines, which limited molecular weights (Mn = 2000–4000 g mol−1 depending on backbone structure) although when the phase volume ratio was altered to 0.25 higher molecular weights (Mn = 10,000–30,000 g mol−1) were possible. Best solubility was achieved by using aromatic diols with at least two equivalent phenylene units per dichlorotriazine unit. DSC reveals polymerisation exotherms with maxima at 190–260 °C (ΔHp = 35–57 kJ/mol) followed by embrittlement and shrinkage (when heated to 300 °C). These phenomena may be due to the formation of poorly formed crystallites (activation energies span 155–380 kJ/mol) combined with thermal isomerisation.

Introduction

The reactivity of chlorotriazines towards alcohols in the presence of base was first used to obtain polymeric products by Picklesmeyer in 1965 [1], albeit insoluble and intractable crosslinked films. However, from these beginnings efforts were directed towards a more controlled approach involving capped dichlorotriazines to prevent crosslinking. Historically, the formation of linear polycyanurates was undertaken from single solvent systems and high temperatures to condense aromatic bisphenols with a number of 2-substituted-4,6-dichloro-s-triazines in the presence of base [2]. Nakamura et al. showed [3] that similar results could be obtained at much lower temperatures using a water-immiscible organic solvent in the presence of a cationic phase transfer catalyst (PTC), typically quaternary salts of organic amines. However, despite the wide scope of the work (some 40 polycyanurates were produced to achieve high thermal stability and good mechanical properties), little attention was paid to the processibility of the resulting polymers or the refinement of the synthesis. It was reported that all alkoxy-cyanurates either decomposed or crosslinked on heating at 200 °C for 60 min and some limited data examining the reduced solubility of the polymers was presented in support. The present work concentrates on examining polycyanurates for their potential use as thermally stable coating materials or pigment binders. Consequently, our study presents data to show the effects of the structure of the polycyanurate backbone and the degree of halogenation on the resulting thermal and mechanical properties and the kinetics of crystallisation.

Section snippets

Equipment

Fourier transform infrared (FTIR) spectra were obtained using a Perkin Elmer 1750 FTIR spectrometer interfaced with a Perkin Elmer 7300 computer; a minimum of 24 scans was collected at a resolution of 2 cm−1. Materials that could be cast from solution were analysed directly as films, otherwise diffuse reflectance infrared spectroscopy (DRIFTS) was used. The spectral data are presented in absorbance units to allow easy comparison of the normal transmission spectra with DRIFTS data in Kubelka–Munk

Results and discussion

All of the interfacial polymerisations were performed under phase transfer (PT) conditions using Cetrimide as the phase transfer catalyst (PTC). Initially, the polymerisation was performed using a reported method [10] in which a phase-volume ratio (PVR) of 1 was used and the PTC (6 mol%) was added, but later studies suggested that a PVR of 0.25 [11] yielded a high molecular weight product. All reactions were carried out under nitrogen to prevent termination by oxidation of phenoxide (to a

Conclusions

Some materials formed from m-dichlorotriazine and a variety of bisphenols can be cast into films with good resilience and these films exhibit good thermo-oxidative stability. Problems were encountered with solubility, particularly aromatic substituted dichlorotriazines and those containing high halogen content, which led to the formation of polymers with limited molecular weights. In order to ensure adequate solubility throughout the polymerisation range, it was found necessary to use aromatic

Acknowledgement

We thank the Ministry of Defence (formerly DRA, Malvern) for providing sponsorship in the form of a studentship (DAT).

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