Effect of polyhedral oligomeric silsesquioxane on thiol-ene UV curing kinetics of waterborne polyester

Highlights

• A curing kinetics is reported.

• Modification of waterborne polyester by a new Mercapto-based POSS.

• The hydrophobicity and adhesion of polyester coatings are obviously enhanced.

• It provides a theoretical basis for the modification of waterborne polyester.

Abstract

The research on UV curing kinetics of waterborne polyester is the basis for regulation of film formation rate. In this paper, a kind of reinforcing agent, namely polyhedral oligomeric silsesquioxanes (POSS), is adopted as modifiers to enhance hydrophobicity and adhesion of polyester coatings, and the effect of POSS on the thiol-ene UV curing kinetics including with the aspects of activation energy, reaction order and control steps are investigated by differential scanning calorimetry (DSC) and Fourier transform infrared (FT-IR). The results show that POSS is not found to significantly affect the reaction order, but change the activation energy obviously. A small amount of POSS can affect the click reaction rate of thiol-ene. The UV curing reaction rate of thiol-ene modified by POSS is mainly controlled by propagation growth rate. There is a resistance parameter in the reaction process, which can effectively control the reaction rate of thiol-ene UV curing of waterborne polyester.

Introduction

The advantages of waterborne polyester coatings appear with the increasing concern for human health and environmental friendliness, and solvent-based coatings are limited due to the release of volatile organic solvents (VOC) [[1], [2], [3]]. Waterborne polyester has the advantages of excellent mechanical properties, chemical properties, excellent impact resistance, safety, environmental protection and pollution-free [[4], [5], [6], [7]]. It is considered to be a good alternative to traditional organic solvents [8]. The traditional film formation was through unsaturated bond crosslinking under the action of thermodynamics, this lead to the high energy consumption and long curing time, the coatings also have other limitations such as oxygen inhibition, non-uniform crosslinking, wide mechanical transition ranges and high internal stress levels [9]. The application of thiol-ene UV curing reaction in waterborne polyesters can increase the reaction rate, increase productivity, reduce energy consumption and reduce the emission of volatile organic compounds (VOC) [10,11].

A great deal of interest has recently focused on UV curing reaction [12,13]. The UV curing system is mainly composed of photoinitiator, photoactive resin, diluent and auxiliary agent. Compared with the traditional thermal curing reaction, UV-cured reaction of thiol-ene has exhibited several distinct advantages such as rapidly curing with a little photoinitiator and fast curing rate, insignificantly oxygen inhibition, delay gel point, little consumption of energy, and simply curing equipment. In order to provide a stable technology for rapid preparation of highly cross-linked networks through the UV curing reaction of thiol-ene, it is absolutely necessary to study the UV curing kinetics of thiol-ene. Polymerization reactivity of thiol-ene UV curing system has been hot topic in recent years [14,15]. For example, allyl sulfide functional groups were incorporated into methacrylate polymerizations to determine their effects on stress relaxation via addition–fragmentation chain transfer (AFCT) [16]. Photopolymerization of tert-butyl acrylate was carried out with a small amount of thiol chain transfer agent. Meanwhile, grafted acrylic acid film with a certain thickness was formed to control the chain length and promote the free radical transfer to the surface. However, excessive reaction speed will lead to the retardation of polymer diffusion and the uneven mixing of groups, which will affect the performance of the paint film.

Polyhedral oligomeric silsesquioxane (POSS) is a kind of core-shell structure of organic-inorganic hybrid. The special structure brings very well performance [17,18]. The properties of polymer, such as glass transition temperature, decomposition temperature, mechanical properties, flame retardance and thermal conductivity [19], would be substantial increased when POSS was added to [20,21]. In addition, adding POSS to the thiol-ene system can significantly decrease water absorption and increase contact angle and gel content, at the same time, the cured film has more close network structure and higher crosslink density. The Si-O-Si which is inorganic cage structure has very well thermal stability [22]. Although POSS has a great influence on the properties of polymers, little research has been done on the effect of POSS on the curing system of mercapto-olefins. During the curing process of water-borne polyester with mercapto-olefin, does POSS influence the curing kinetics and the mechanism while exerting its reinforcing and toughening properties? These questions have important theoretical and practical significance for the study of the curing properties of water-borne polyester. The kinetics research of adding POSS to thiol-ene is an important direction in the study of thiol-ene dynamics [23].

The UV-curing reaction of thiol-ene was very easy to happen at room temperature, and it was convenient to determine the kinetics by FT-IR. Because the thickness of the sample was constant, the concentration was directly proportional to height or area of absorption peak [24]. PUSH, a water-soluble monomer with four thiol groups, was used as a cross-linking agent for curing of water-based polyester. We could intuitively analyze the measured concentration changes. Therefore, FT-IR was chosen to determine the kinetics of thiol-ene reaction in this article.

In UV-curing of water-borne polyester, there is a competition between POSS and PUSH, and the octahedral cubic structure of POSS increases the steric hindrance and reduces the reaction rate. Therefore, the thiol-ene UV curing kinetics of waterborne polyesters could be significantly different before and after the addition of POSS. Herein, we describe the effect of POSS on thiol-ene UV curing kinetics of waterborne polyester by investigating the changes of activation energy, reaction order and control steps changes caused by POSS to explain the thiol-ene UV curing polymerization kinetics of waterborne polyester. At the same time, the influence mechanism of POSS on the curing kinetics of thiol-ene was speculated.