Preparation of acrylic resins modified with epoxy resins and their behaviors as binders of waterborne printing ink on plastic film

Abstract

Polymers have low-energy surfaces and are difficult to be adhered by waterborne inks due to the higher surface tension of the waterborne inks. An approach to prepare acrylic resin used as a binder of waterborne printing ink on plastic film with excellent adhesion and water resistance is described. The epoxy resin is incorporated into the acrylic resin to enhance the molecular interactions between binder molecules in ink and surface molecules of plastic film. The binder resin is obtained by esterifying epoxy resin with methyl acrylic acid and then co-polymerizing the esterification product with several acrylic monomers via a free radical polymerization. The acrylic resin modified by bisphenol A epoxy resins (EP-MA-AR) shows significant enhancement over the acrylic resin without modification (AR) in adhesion between ink and polyethylene terephthalate (PET) film surface and as well water resistance performance. N-Hydroxymethyl acrylamide (NHMA) is introduced into the synthesis recipe to further enhance the interactions between the polymer chains in ink and the polar groups on the surface of non-polar polypropylene(PP) film caused by the corona discharge treatment and improves the adhesion between ink on and surface of PP film dramatically, whatever using AR or EP-MA-AR as binder. The adhesion between ink and PET film surface is also further improved by this method. The water resistance of AR and EP-MA-AR is also greatly improved under the appropriate content of NHMA. Two Tgs appear on the DSC curve of EP-MA-AR, suggesting the separated status of the epoxy modified acrylic chains and the acrylic chains without modification in EP-MA-AR. The experimental data of waterborne ink on PET and PP indicate the EP-MA-AR resin prepared in this way is an applicable binder of waterborne ink on plastic film.

Introduction

Polymers usually have low-energy surfaces (surface free energy typically 18–46 mJ/m² [1], [2], [3], [4], [5], which can cause wetting and adhesion problems due to the higher surface tension of the waterborne inks. However, The demand for environmentally friendly materials has tremendously accelerated the use of waterborne printing inks on the polymer materials. To achieve good wetting and print adhesion on the surface of polymers, it is necessary to carry out a pretreatment, e.g. corona discharge treatment, which can increase the surface energy of plastic [6], [7], [8]. Corona treatment can increase the polar characteristics of the polymer surface by introducing new oxidized functionalities onto the surface (ketones, aldehydes, carboxylic acids, esters, ethers, peroxides, etc.) [6]. The polar resin in the ink will form ionic and hydrogen bonds with the treated surface [9], [10].

When formulating the water-based ink, it is important to choose the right binder resin to get required adhesion between substrate and ink. Acrylic resin is the most widely used binder resin for water-based coating, paints and printing ink, because of its good stability, high gloss and good surface adsorption [11], [12], [13], [14], [15], [16], [17]. Acrylic waterborne coatings, paints and printing inks usually include their formulation pigments, fillers, water, multi-purpose additives, and binders. The binders are aqueous polymer or latexes that are responsible for binding the pigments and fillers, and forming a homogeneous film with a good adhesion to the substrate [18], [19]. While acrylic resin is employed as a binder of waterborne ink for plastic film, the attractive force between ink and substrate surface is weak because of the absence of strong molecular interaction at the interface. The water resistance of the ink is also unsatisfactory to the practical application as the waterborne acrylic resins contain many hydrophilic groups. In our work, the epoxy resin was incorporated into the acrylic resin during synthesizing binder resin in order to enhance the molecular interactions between binder molecules and surface molecules of plastic film. Meanwhile, the water resistance of the ink also was improved due to the increased molecular interactions across the interface. Epoxy resins are characterized by low shrinkage, good toughness, adhesion and chemical resistance [20], [21], [22]. There are several commercially available methods [23], [24], [25] to combine epoxy resin with carboxyl acrylic polymer together. For example, epoxy-acrylate composition is formed from the epoxy resin by the grafting of addition polymer onto aliphatic backbone carbons of the epoxy resin. In another preferred method, self-emulsifiable mixtures are prepared by esterifying epoxy resin with carboxyl acrylic polymer in an organic solvent medium in the presence of tertiary amine [25], [26], [27]. Then the adduct react with a base and render it self-dispersible into water in neutralized form. Min Liu et al. suggested a two-step esterification process to prepare epoxy-acrylic-graft-copolymer waterborne resins used in anticorrosion coating on metal substrate [13].

The present work is aiming at the synthesis of waterborne acrylic binders used in waterborne printing ink for plastic film with good adhesion between ink and plastic film surface and good water resistance. The acrylic resin was obtained by co-polymerizing the esterification product of epoxy resin with methyl acrylic acid and several acrylic monomers. We use polyethylene terephthalate (PET) and polypropylene (PP) as examples of plastic films to analyze the effects of epoxy resin modification, monomer ratio and hydroxymethyl acrylamide content on the adhesion property and water resistance. It is founded that the adhesion between waterborne ink and PET film surface increases greatly owing to the epoxy resin modification. The water resistance of acrylic resin also is improved greatly. But the adhesion between ink and non-polar PP surface was not improved effectively by this way. Therefore, N-hydroxymethyl acrylamide (NHMA) was introduced into the synthesis recipes aim to further enhance the molecular actions between polymer chains in ink and polar groups on the surface of PP film caused by the corona discharge treatment.The results indicated the adhesion between ink and non-polar PP film surface is increased dramatically by the incorporation of N-hydroxymethyl acrylamide. The experimental data of waterborne ink on PET and PP indicate the EP-MA-AR resin synthesized in this way is an applicable binder for waterborne ink on plastic film.