Organic–inorganic hybrid coatings for coil coating application based on polyesters and tetraethoxysilane (2023)

Progress in Organic Coatings

Volume 33, Issue 2,

23 February 1998

, Pages 126-130

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Organic–inorganic hybrid coating systems based on polyesters and tetraethoxysilane (TEOS) are synthesized and evaluated to develop protective coatings with improved performance for prefinish construction steel and aluminium. The coatings have to combine flexibility, necessary for enduring deformation of the metal substrate after coating, and hardness for improved mechanical properties. Two systems have been studied: a polyester–TEOS system and a polyester–epoxide–TEOS system. The polyester–TEOS system contains polyester oligomers which are crosslinked with silica clusters, formed via the sol-gel process. The polyester–epoxide–TEOS system consists of an organic network of polyester and epoxide with silicon compounds, formed by the sol-gel process. Both systems form transparent hybrid organic–inorganic coatings. The influence of the inorganic compound on the König hardness of the coatings is determined. The polyester–TEOS system shows an increase in hardness with increasing silica content. It appeared difficult to incorporate in situ formed silica in the organically crosslinked polyester–epoxide–TEOS system.


Inorganic material can be synthesized via the sol-gel process in organic media at temperatures at which organic compounds are thermally stable [1]. In this way it is possible to combine inorganic and organic components into one material: an organic–inorganic hybrid material. In the sol-gel process hydrolysis and condensation of metal alkoxides takes place. The most frequently used precursors are silicates. Gels are formed in the presence of water and an acid or base catalyst. The reactions of hydrolysis and condensation of alkoxy silane are shown in Fig. 1. Under acidic conditions the rate of hydrolysis is faster than the rate of condensation and a polysiloxane network is formed, while under basic conditions the rate of the condensation reaction is faster and dense silica particles are formed [1].

Schmidt prepared hybrid systems in the sol-gel process by adding organofunctional alkoxy silanes to TEOS (tetraethoxysilane) [2]. Later, Schmidt and colleagues also incorporated organic molecules in these systems. In this way, both bulk [3]and coating materials 4, 5were obtained. The glassy inorganic materials were made more flexible by the addition of organic compounds. The sol-gel technique is also used to improve the properties of organic materials by incorporation of metal alkoxides, for example the heat resistance of polyamides [6]or the mechanical properties of elastomers 7, 8. Wilkes et al. used various polymers and oligomers for this type of hybrid systems to develop abrasion resistant coatings for polymeric substrates 9, 10, 11. In these systems the organic compounds were silane functionalized and combined with metal alkoxides. For the synthesis of harder organic-based coatings on glass substrates, Pascault's group also used the sol-gel process by adding organofunctional alkoxy silanes and metal alkoxides to organic oligomers [12].

Our aim is to prepare organic–inorganic hybrid coatings for coil coating applications, which integrate the flexibility of the organic phase and the hardness of the inorganic phase. In this paper the results obtained with the study of two systems are presented: a polyester–TEOS system and a polyester–epoxide–TEOS system. The polyester–TEOS system, based on polyester oligomers and TEOS, is used as a model system to study the chemical interaction between the organic and inorganic compounds and the influence of the chemical composition on the properties of the coatings [13]. In the polyester–epoxide–TEOS system, an organically crosslinked coating system is combined with TEOS, to study the properties of the coatings for coil coating applications. Emphasis is laid on the influence of in situ formed silica on the hardness of the coatings.

Section snippets


A bifunctional hydroxyl-terminated polyester was synthesized from neopentylglycol, 1,4-cyclohexanedimethylol, esterdiol (HOCH2C(CH3)2CH2OC(O)C(CH3)2CH2OH), isophthalic acid and adipic acid by conventional methods [14]. To obtain an acid-terminated polyester, the hydroxyl-terminated polyester was treated with succinic acid anhydride at 150°C for 1 h. After synthesis, the polyesters were characterized by endgroup titration and GPC. The characteristics of the polyesters are summarized in Table 1.

Interaction between the organic and inorganic phase

Transparent hybrid coatings in the polyester–TEOS system were only obtained from hydroxyl-terminated polyesters and TEOS, when the reaction was catalyzed by an acid (pTSA), in a moist atmosphere. No coatings were obtained from acid-terminated polyesters and TEOS. Neither were coatings were formed with a base catalyst. Under acidic conditions, TEOS hydrolyzes fast [1]and then interaction between the polyester and hydrolyzed TEOS is possible via hydrogen bridging with the carbonyl groups or via


It was shown that transparent organic–inorganic hybrid coatings could be made from hydroxyl-terminated polyester and TEOS. The interaction between the organic and inorganic part occurs via the hydroxyl endgroups of the polyester and the hydroxyl groups of the silanols. Probably, a Si–O–C bond is formed by a condensation reaction. Increasing the TEOS content increases the silica content in the coating and causes an increased König hardness and Tg. It appeared difficult to incorporate silica in

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