Preparation of a light color cardanol-based curing agent and epoxy resin composite: Cure-induced phase separation and its effect on properties

Abstract

A light color cardanol-based epoxy curing agent (MBCBE) was synthesized from cardanol butyl ether, formaldehyde and diethylenetriamine. In comparison, a phenalkamine with a similar structure was also prepared. The chemical structures were confirmed by GC–MS and FTIR. The cure behaviors of diglycidyl ether of bisphenol A (DGEBA) with these two curing agents was studied by differential scanning calorimetry (DSC). The morphology, mechanical properties, thermal properties of the cured epoxies were also investigated. The DSC results indicated that MBCBE is less reactive than the phenalkamine. The morphology of the cured MBCBE/DGEBA consisted of cavities dispersed within a continuous epoxy matrix. The cavities markedly improved the lap shear strength and impact strength of the cured resin. Both the two cured resins indicated a two-stage decomposition mechanism. Compared with PKA/DGEBA, the weight loss of MBCBE/DGEBA at the first stage was mainly resulted from the dispersed phase in the epoxy matrix.

Introduction

Cashew nut shell liquid (CNSL), an agricultural byproduct abundantly available in the world, is one of the few major and economic sources of naturally occurring phenols and can be regarded as a versatile and valuable raw material for polymer production. Cardanol is obtained by distillation of CNSL, a sustainable, low cost and largely available natural resource. Cardanol and its derivatives can be considered nowadays very attractive precursors in order to develop new materials from renewable bio-sources to be used in eco-friendly processes. The cardanol has been the attraction for many researchers for the production of phenolic resins [1], epoxy resins [2], vinyl ester [3] resins and other bio-based polymers [4] in the past.

Phenalkamines as commercial epoxy curing agents are a series of Mannich bases obtained by reacting cardanols, aldehyde compounds, and polyamines [5]. Phenalkamines are good epoxy resin hardeners for room temperature or low temperature curing applications. The phenalkamine obtained from cardanol has a rapid cure rate at a low temperature and allows application under wet or humid conditions even on a wet or damp surface [6], [7]. However, phenalkamines suffer from the disadvantage that the cured epoxy products are very dark in color (Gardner color greater than 14). Furthermore, the phenol hydroxyl in phenalkamines will be oxidized by air, and the formation of quinonoid chromophoric groups causes bad color stability. Phenol's hydroxyl group also has the potential for producing contact dermatitis. Due to these, epoxy resin compositions containing phenalkamines have been limited to be used by workers of skin allergy and generally cannot be used as top coat material. The classic phenalkamines have a short pot life and a high viscosity, which induced non-uniform curing process, more defects such as bubbles and incomplete crosslinking in the cured resin. These could decrease the toughness or other mechanical properties of the materials.

In this study, a novel epoxy curing agent derived from cardanol butyl ether with light color, good color stability and low viscosity was successfully prepared. The curing agent was synthesized from cardanol butyl ethers, formaldehyde and diethylenetriamine by Mannich reaction. It was interested that this curing agent was initially compatible with the epoxy resin but later seemed to precipitate from the matrix during the cure. After cure, the cured resin displays fairly good toughness. A lot of literatures reported that liquid rubbers or some thermoplastic polymers were used to toughen the epoxy resins, the cure-induced phase separation occurred. But very few study referred to the phase separation caused by a curing agent of such low molecular weight.

In order to get more information of the properties, curing of diglycidyl ether of bisphenol A (DGEBA) with the curing agent, the cure-induced phase separation and its impact on properties of the cured resins was investigated by relevant tests. For comparison, a commercial phenalkamine with similar structure was also synthesized and studied in company with it. The difference between the new curing agent and the commercial phenalkamine was also discussed in this work.