Foaming epoxy-amine-carbamate: The effect of different neat amines on rheological and cellular morphology

Abstract

The use of carbamate to foam epoxy depends significantly on the precured modulus to stabilize the cellular structure. The optimum precured modulus is developed from the reaction of epoxy resin and the neat amine. The selection of the neat amine relies on its reaction temperature with epoxy, which is required to be below the decomposition temperature of carbamate. This study investigates the effect of three different neat amines on the rheological behavior of foaming epoxy-carbamate-amine. They are bisphenol-A diglycidyl ether epoxy (DGEBA), isophorone diamine carbamate (IDPA.CO2), N-aminoethylpiperazine (AEP), 2,4-Diamino-1-methyl-cyclohexan (DMC) and isophorone diamine (IDPA). The mixtures of DGEBA-amine-carbamate are filled in 25% and 75% of the volume of a closed mold. Precuring is carried out at 60°C for 2 h. The foaming and complete curing are conducted at 180°C for 1 h. Having H-active at piperazine, AEP reacts with DGEBA faster and develops a higher precured modulus compared to DMC and IDPA. It is important to note that DGEBA-AEP-IDPA.CO2 exhibits viscoelastic behavior beyond 138°C, seen by its rheological storage modulus lower than loss modulus and its tan delta larger than 1. The reaction between DGEBA and the H-active piperazine of AEP leads only to linear linkage and is unable to further crosslink compared to the primary amine (-NH2). This results in a lower glass transition temperature Tg of DGEBA-AEP-IPDA.CO2. The effect of amine on foaming is more obviously at 25% filling level. DGEBA-AEP-IPDA.CO2 has more spherical and homogeneous cellular structure and the density of 285 kg/m3. Having quite similar chemical structure, both DGEBA-DMC-IPDA.CO2 and DGEBA-IPDA-IPDA.CO2 produce the epoxy foams having cell-interconnection and coalescence; their densities are also similar 301 kg/m3 and 305 kg/m3, respectively. All the foams are closed-cell at 75% of filling level. The cell morphologies are well reflecting the foaming modulus and tan delta behavior.