Title

Molecular toughening of epoxy resins through siloxane modification

Date of Award

2005

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical and Chemical Engineering

Advisor(s)

George C. Martin

Keywords

Molecular toughening, Epoxy resins, Siloxane

Subject Categories

Chemical Engineering | Engineering | Materials Science and Engineering

Abstract

The fracture resistance of epoxy resins is significantly improved through a new molecular toughening mechanism without sacrificing the desired thermal and mechanical properties. A liquid diglycidyl ether of bisphenol-A type epoxy resin (DGEBA) was modified using a methylphenyl siloxane (MPS) oligomer under the catalysis of tetraisopropyl titanate (TPT). A variety of characterization techniques confirmed that the methoxyl end group of the MPS modifier was reacted with the hydroxyl group of the DGEBA resin forming a grafted molecular structure.

The curing kinetics between DGEBA and an aromatic amine, metaphenylenediamine (mPDA), was extensively investigated using isothermal and dynamic differential scanning calorimetry (DSC) techniques. The MPS modified DGEBA/mPDA systems follow the same autocatalytic curing mechanism as the unmodified one. Based on a time-temperature-transformation (TTT) diagram, the optimum curing conditions were determined as 130°C for 2 hours curing and 170°C for 3 hours postcuring. The DiBenedetto and Wisanrakkit/Gillham models provide a satisfactory prediction of the T g -α relationship over the whole conversion range.

Both the critical stress intensity factor (K Ic ) and the critical strain energy release rate (G Ic ) of the modified DGEBA/mPDA samples increase with an increasing MPS content. At 15 wt% MPS content, K Ic shows a 2.5-fold increase, and G Ic shows a 8.8-fold increase, compared with the unmodified DGEBA/mPDA system. For the MPS modified DGEBA/mPDA systems, the high glass transition temperature and the thermal stability were well maintained. The tensile and the flexural strengths and strains were improved while the Young's modulus and the flexural modulus were slightly decreased. The moisture resistance was improved.

The morphologies of the unmodified and the MPS modified DGEBA/mPDA systems were studied using both optical and scanning electron microscopy techniques. With the incorporation of the MPS oligomer, a two-phase microstructure was observed on the fracture surface. The second phase particle may be caused by the MPS oligomer coalescing during curing. The toughening mechanisms identified include localized shear deformation of the epoxy matrix, particle tearing, fracture and cativation, and crack deflection.

The factors including modification method, MPS modifier type, and curing agent type affect the structure-property relationship of modified epoxy resin. The chemical modification method is more efficient in enhancing the fracture toughness of DGEBA/mPDA system than the physical blending method. The differences in the fracture toughness of various types of siloxane-modifier DGEBA systems can be attributed to their different morphological structures. The MPS modifier is more efficient in improving the fracture toughness of aromatic amine, mPDA, cured DGEBA system than that of aliphatic amine, polyoxypropylene diamine (POPDA), cured DGEBA system.

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