Allylic monomers as reactive plasticizers of polyphenylene oxide. Part II: Cure kinetics

Abstract

As part of an investigation of the processing of polyphenylene oxide (PPO) with polymerisable plasticizers, the curing kinetics of various diallylic monomers – diallyl ortho phthalate (DAOP), diallyl terephthalate, triethylene glycol diallyl ether and diethylene glycol diallyl carbonate) – and triallylic – triallyl cyanurate and triallyl isocyanurate – with radical initiators of differing activity were studied. All the monomers exhibited similar cure kinetics with each initiator and the exotherm peak temperatures correlated well with the reactivity of the initiators as measured by the 1 h half-life temperatures. Multiple scan-rate dynamic DSC studies gave similar activation energies to those obtained from isothermal rheology studies of gelation. The effects of the presence of PPO on the curing of diallyl ortho phthalate (DAOP) were studied using dicumyl peroxide (DCP) and tert-butyl hydroperoxide (TBHP) as initiators and differing behaviour was observed. In the PPO:DAOP/DCP system, the reaction rate was reduced with increasing PPO due to a dilution effect but the heat of reaction was generally unaffected. However, in the PPO:DAOP/TBHP system, a significant increase of cure rate was observed in the presence of 20 wt.% PPO due to the catalysis of TBHP by Cu²⁺ and Co³⁺ impurities in the PPO, but the cure rate was slightly reduced with increasing PPO content. In addition, the heat of polymerisation for the PPO:DAOP/TBHP blends were usually less than that for the pure monomer. This information can be used in the processing of PPO with DAOP as a polymerisable plasticizer.

Introduction

Some thermoplastics such as polyphenylene oxide (PPO), polyethersulfone and polyetherimide possess a range of desirable properties such as high dimensional stability, stiffness, strength and toughness. However, their applications are often limited due to difficulty in their processing resulting from their high melt viscosity. In addition, they may undergo degradation under the high processing temperature or long processing time (such as with rotational moulding) that is required, causing loss of their desirable properties.

It has been reported [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11] that when a large quantity of thermoplastic is blended with a crosslinkable monomer, the monomer can act like a plasticizer thus enhancing processability and avoiding potential degradation that would results from the use of high temperature processing. Unlike other plasticizers, the monomer can be subsequently polymerised into a crosslinked phase and depending upon a several parameters (e.g. blend composition [1], cure conditions [4], thermoplastic molecular weight [1] etc.), the resulting morphology is either a co-continuous structure or a dispersion of one phase in the other continuous phase. The potential advantages of this system are that the desirable properties of the thermoplastic in the blend can be retained because the thermoplastic forms the continuous phase and the crosslinked phase can either reinforce or toughen the blend, depending whether they are rigid or rubbery.

To be an effective reactive plasticizer, the monomer should be readily miscible with the thermoplastic and not form a network at an early stage of cure or react too quickly at the processing temperature of the blend. Due to their high gelation conversion [12] and the ability to alter their cure kinetics by selection from a wide range of curatives, several researchers have reported the use of epoxy monomers as reactive plasticizers for various thermoplastics such as PPO [1], polystyrene [5] and polyetherimide [6], [7]. The use of these monomers successfully improved the processing of the thermoplastics [3], [5] without sacrificing the properties of the thermoplastics [1]. All of these studies used monomers that undergo step growth polymerisation [1], [2], [3], [4], [5], [7], but few researchers [8], [9], [10] have worked with monomers that undergo chain growth polymerisation, probably because this polymerisation mode usually results in early gelation [12] and thus limited processing time.

Compared with the free radical chain growth polymerisation of common multivinyl monomers, such as acrylates, allylic monomers undergo gelation at relatively high conversion [13] and have been found useful as reactive plasticizers to improve the processing of thermoplastics [8], [9], [10], [14]. In addition, the occurrence of chain transfer to monomer and the presence of easily abstractable allylic hydrogen cause these monomers to have lower reactivity, thus requiring higher temperatures and longer times to polymerise, and these factors also increase the usefulness of allylics as reactive plasticizers.

In this paper we study the blends of PPO and allylic monomers. Fujiwara et al. [15] have studied the phase diagram of blends up to 60 wt.% PPO in triallylisocyanurate/PPO blends by cloud point studies and have measured the isothermal DSC cure kinetics, chemorheology, DMTA, flexural strength and TEM of these blends. In a follow-up study, Yang et al. [16] have studied the phase diagram of blends up to 40 wt.% PPO in diallyl phthalate monomer by cloud point studies, and have used Fourier Transform Infrared spectroscopy, light scattering and transmission electron microscopy of the curing and cured blends to study the morphology of blends with 20 wt.% PPO. In the present study, we investigate the curing kinetics of a wider range of diallylic monomers with radical initiators of differing activity and investigate the effect of blending with up to 80 wt.% PPO on these kinetics to provide information required for the application of such monomers as processing aids of PPO.