Synthesis and thermal cure of high molecular weight polybenzoxazine precursors and the properties of the thermosets

Abstract

High molecular weight polybenzoxazine precursors have been synthesized from aromatic or aliphatic diamine and bisphenol-A with paraformaldehyde. The precursors were obtained as soluble white powder. Molecular weight was estimated from the size exclusion chromatography to be several thousands. The structure of the precursors was confirmed by IR, ¹H NMR and elemental analysis, indicating the presence of cyclic benzoxazine structure. The ratio of the ring-closed benzoxazine structure and the ring-opened structure in the high molecular weight precursor was estimated from ¹H NMR spectrum and also from the exotherm of DSC, showing that the ratio of the ring-closed benzoxazine structure was 77–98%. The precursor solution was cast on glass plate, giving transparent and self-standing precursor films, which was thermally cured up to 240 °C to give brown transparent polybenzoxazine films. The toughness of the crosslinked polybenzoxazine films from the high molecular weight precursors was greatly enhanced compared with the cured film from the typical low molecular weight monomer. Tensile measurement of the polybenzoxazine films revealed that polybenzoxazine from aromatic diamine exhibited the highest strength and modulus. While, polybenzoxazine from longer aliphatic diamine had higher elongation at break. The viscoelastic analyses showed that the glass transition temperature of the polybenzoxazines derived from the high molecular weight precursors were as high as 238–260 °C. Additionally, these novel polybenzoxazine thermosets showed excellent thermal stability.

Introduction

The traditional phenolic resins possess excellent characteristics such as high heat resistance and flame retardance, good electric and chemical resistance, low water absorption, and low cost. Therefore, they are widely applied in various fields such as matrix for fiber-reinforced plastics (FRP), structural materials, adhesives, paints and others. The disadvantages of the traditional phenolic resins are their brittleness, the need of catalyst for polymerization, formation of voids because of the volatiles formed during the cure, and large volumetric shrinkage upon cure. Also the volatalization of phenol and formaldehyde into the air during the cure process causes some health concern.

A series of polybenzoxazines obtained by the ring-opening polymerization of cyclic monomers, benzoxazines, has been developing as a novel type of phenolic resin [1]. The monomers are easily prepared from phenols, primary amines and formaldehyde. The wide variations of raw materials, phenols and amines, allow considerable molecular-design flexibility for the cyclic monomers. Polymerization proceeds through the ring-opening of the cyclic monomers only by heat treatment without the need of catalyst and without generating byproducts or volatiles, and thus excellent dimensional stability is obtained. The structure of a typical benzoxazine monomer (B-a) prepared from bisphenol-A, aniline and formaldehyde along with the structure of its polybenzoxazine (PB-a) are shown in Scheme 1.

Polybenzoxazines provide characteristics found in the traditional phenolic resins such as high heat resistance and flame retardance. They also provide characteristics that are not found in the traditional phenolic resins such as excellent dimensional stability, low water absorption and good dielectric properties. Various applications as electronic materials, matrix resin for FRP, and adhesives are expected. However, there are some shortcomings for polybenzoxazines. The cured materials are brittle and a relatively high temperature is needed for the ring-opening polymerization. Also, processing into thin film from the typical monomers is difficult because most monomers are powder and the polymers are brittle due to the short molecular weight of the network structure.

Recently, aiming for performance enhancement and lowing the polymerization temperature, various approaches have been examined. One approach is the modification of monomer. Introduction of another crosslinkable functional units is very effective to enhance the thermal properties [2]. As another approach, polymer alloys of polybenzoxazine with high performance polymers or with elastomers resulted in high performance and tough films [3]. Third approach of hybridization with inorganic materials such as layered clay [4] and metal oxide nanoparticles [5] was also successful in obtaining polybenzoxazine with improved properties.

Until now, however, only the low molecular weight cyclic monomers have been studied in detail as precursors of polybenzoxazines. Although Ishida reported in his patent the modification of polyphenol with monofunctional amine to get benzoxazine-functional polymer [6], he did not mention details about the preparation and characterization as well as the properties of the polymer. So far, a high molecular weight polymer containing cyclic benzoxazine structure as a repeating unit in the main chain prepared from the reaction of bisphenols with diamines has not yet been prepared. This is probably due to the belief that the formation of cyclic benzoxazine monomer is usually accompanied with a ring-opening polymerization of the once-formed cyclic monomer, which eventually cause a crosslinking and hence insolubility of the obtained polybenzoxazine precursor. If it is possible to obtain soluble high molecular weight polybenzoxazine precursors, processing into thin films should become very easy, and application into the fields for which the low molecular weight cyclic monomer has not used would realize.

In this study, we report on the preparation of high molecular weight polybenzoxazine precursor from bisphenol-A and aromatic or aliphatic diamine in the presence of formaldehyde. The structure of the obtained precursor was characterized, and the properties of the cured polybenzoxazine films were compared with the typical polybenzoxazine film from typical low molecular weight cyclic monomer.