Formation of silica/epoxy hybrid network polymers

Abstract

Epoxy-based inorganic–organic hybrid polymers, for use as a matrix in coatings, have been prepared from 3-glycidoxypropyltrimethoxysilane by a sol–gel process. The precursor molecule possesses both epoxy and silicon alkoxide functionality and so interlinked inorganic–organic networks can be formed. Diethylenetriamine was used to open the epoxy rings and form the organic network to an extent determined by the initial ratio of amine to epoxy groups. The materials were cured either at room temperature or with an additional heat treatment at 150 °C. Structural characterisation of the cured hybrid materials was performed using a combination of Raman, and ²⁹Si and ¹³C MAS NMR spectroscopies. These show that the formation of the two networks does not occur independently and the rate or extent of organic cross-linking has a direct effect on the extent of the inorganic network formation, and vice-versa.

Introduction

Inorganic–organic hybrid materials are of increasing interest as constituents of coating materials for a wide variety of applications since they offer the prospect of combining the mechanical toughness and flexibility of the organic component with the hardness and thermal stability of the inorganic component. The molecular structure and the microstructure of the hybrid materials can take several different forms as classified by Novak [1]. Silicones, silicates and silanes are commonly blended with organic resins to prepare coatings with improved weathering and thermal resistance [2]. The hybrid can be formed by cold blending or by synthesis of a dedicated co-polymer. In materials formed by cold blending there is little cross-linking between the organic and inorganic polymers with the result that phase separation often occurs which is usually detrimental to one or more of the properties of the coating (e.g. solvent resistance). In co-polymers the inorganic segments are chemically attached to the organic segments, but the degree of cross-linking is still low. It is now generally accepted that the preferred hybrid structure is one in which there is intimate cross-linking between the organic and inorganic polymer networks (Type II and Type IV of Novak’s classification). One way of increasing the cross-link density, thereby obtaining stronger interaction between the inorganic and organic components, is to use functionalised silanes (coupling agents) as cross-linkers. These can react with functional groups already present on the polymer molecules or they can cross-link by reaction with an added multifunctional molecule (curing agent).

The use of the sol–gel process to prepare highly intermingled inorganic–organic hybrid polymer networks using coupling agents is of current scientific interest since it offers the possibility of tailoring the properties of the materials by variation of the relative composition of the inorganic and organic phases. With such systems, the inorganic and organic networks are formed together (often simultaneously) to achieve homogeneous phase morphologies which are impossible to produce by the other routes. These inter-linked networks are widely reported to offer improved mechanical properties [1]. This, combined with the potential to prepare coatings with an improved scratch resistance over traditional organic coatings and which possess greater flexibility than traditional inorganic systems makes these hybrids an attractive proposition for the coatings industry. For example, Frings et al. [3] describe the preparation of an acrylic-based hybrid sol–gel coating applied as a coil-coat to sheet steel.

Epoxy resins are widely used as the basis for coating materials and epoxy/silica hybrids have the potential to provide good chemical resistance, good thermal resistance and good mechanical properties. For epoxy/silica hybrid systems the coupling agent 3-glycidoxypropyl-trimethoxysilane (GPTMS) is a useful molecule (Fig. 1) to form cross-links between silanol groups on the silica network and be incorporated into the epoxy network. This coupling agent has been used in transparent abrasion-resistant hybrid coatings for polymers [4], [5], [6], [7] and metals [8], [9], [10] and for gas separation membranes [11]. If the coupling agent is present in substantial concentration then it too is capable of polymerising and cross-linking to form interconnected epoxy and silica networks. GPTMS can undergo a variety of reactions during the preparation of a hybrid by a sol-gel route. Hydrolysis of the methoxy groups gives silanol groups which can subsequently condense to form the silica network. The Si atom in GPTMS is tri-functional in terms of reactive methoxy groups and is therefore able to form a three-dimensional branched siloxane silica network with nominal stoichiometry SiO_1.5. The epoxy rings can be opened and polymerised to form a linear poly(ethylene oxide) organic network.

Cross-links between the two networks arise either from the pre-existing link in the GPTMS molecule, by direct reaction of silanols with epoxy rings, or by condensation of silanols with hydroxyl of the opened epoxy rings. The uncatalysed ring opening reaction occurs at a useful rate only at elevated temperature and so thermal curing is required. However, all the condensation reactions are catalysed by other metals (Al, Ti, Zr) introduced into the sol–gel system [12]. Low temperature epoxy ring opening and organic network formation can be achieved by the use of amine curing agents. In principle each active hydrogen in the amine group is capable of opening and linking to one epoxy group. The nitrogen of the amine enters into the organic network and the branching of the network depends on the number of –NH₂ and –NH– groups in the amine molecule.

Since the amines are basic, they also catalyse the condensation of silanol groups to form the silica network. Amine functionalised silane coupling agents have also been used as epoxy ring openers (for example, 3-aminopropyl-triethoxysilane, APTES [13] or 2-aminoethyl-3-aminopropyl-trimethoxysilane [7]). The formation of the hybrid network is therefore a complicated chemical process and, in particular, the network structure is expected to be determined by the relative rates of formation of the organic and inorganic parts and the linkages between them.

In this present study, we have prepared a number of epoxy–silica hybrid materials by a sol–gel/amine cure reaction of GPTMS for use in coatings on steel. The structures of the hybrid systems have been investigated by Raman spectroscopy, Fourier transform infra-red spectroscopy (FT-IR) and nuclear magnetic resonance (NMR) after curing both at room temperature and after thermal curing.