Logo weiterlesen.de
Additives for Water-borne Coatings

Wernfried Heilen

Adalbert Braig | Anne Drewer | Patrick Glöckner
Roman Grabbe | Juergen Kirchner | Frank Kleinsteinberg
Benoît Magny | Thomas Matten | Ingrid Meier
Kirstin Schulz | Heike Semmler | Jean-Marc Suau

Additives for
Water-borne Coatings

2nd Revised Edition

vin_le_hks

Foreword

Since the publication of the 1st edition of this book almost ten years ago, some areas of paint applications have seen further technological developments that have driven significant advances in the coatings industry.

In general, the use of water-borne coatings worldwide has increased by 13 % over the last six years. 92 % are used as architectural coatings, while 8 % find application in industrial coatings. The annual growth rate is expected to be 2.5 to 2.8 % for the next 3 years [1].

Additives help to protect the environment by effectively reducing the use of organic solvents. Many of today’s water-borne coatings could not be formulated without them.

Additives are often used in very small quantities, usually in proportions of less than one percent of the total formulation. When incorporated into water-borne paints, coatings and printing inks, they enhance both the production process and the performance of the applied inks and coating materials.

The present book (2nd edition) is intended to provide an updated, state-of-the-art overview of the chemistry and technology of additives even unique solid products and co-binders for water-borne systems and their use from the industrial viewpoint.

The information it contains will be useful for understanding the chemistry and the action of the newly developed additives in water-borne systems. Experienced coating specialists, too, will benefit from finding out about the latest developments in the industry.

Ideally, this book will make paint chemists’ work easier and provide invaluable suggestions for meeting the challenges of formulating modern and environmentally-friendly coatings that will present themselves in the future.

Many thanks to my colleagues for their assistance in bringing this book to fruition and to Evonik Operations GmbH for supplying literature and illustrative materials.

Bad Grönenbach, Germany, September 2021,

Wernfried Heilen

1Introduction

Wernfried Heilen

Water-borne coatings materials have very different properties from those of conventional solvent-based systems. The reason for this lies in the physical properties of water. The heat of evaporation of water is very high compared to that of many other solvents [1]. Consequently, air-drying systems dry more slowly at lower temperatures and/or higher relative humidity. Significantly more energy must be utilized in heat-curing systems.

Numerous solvents with different heats of evaporation and boiling points can be used to optimize drying and film-forming of solvent-based systems. In contrast, formulators of water-borne coatings have only a limited choice of solvents which can be used as water-soluble co-solvents.

As a strongly polar solvent, water has a comparatively high surface tension. Because of this and the make-up of the binder, which consists of incompletely dissociated polyelectrolytes or colloidal systems or emulsions based on various polymers, characteristic problems can occur during manufacture and application. This necessitates the development of specialist additives.

Essential for the manufacture of water-borne- as well as for solvent-based coatings systems are:

  • wetting and dispersing agents

Especially important in water-borne formulations are

  • defoamers as well as
  • rheology-modifying additives

As is the case with solvent-borne coatings, polymeric wetting and dispersing additives are used nowadays in the production of water-borne automotive and industrial coatings, while polyphosphates and salts of polyacrylic acids are used in the production of emulsion paints.

Aqueous pigment concentrates continue to be produced with the aid of fatty acid and fatty alcohol derivatives, and in addition of alkylphenol ethoxylates – although their ecotoxicity is controversial.

Fortunately, polymeric wetting and dispersing agents also find use instead of alkylphenol ethoxylates.

Foam-forming substances include emulsifiers used in the manufacture of water-based binders but also the wetting and dispersing agents mentioned above. Non-associative thickeners, such as those derived from cellulose, which have many hydrophilic segments in the molecules can also cause foam formation. Modern defoamers comprise a complex mixture of active substances, including mineral oils, polyether siloxanes, waxes, precipitated silicas, etc.

Synthetic as well as inorganic thickeners are used to control viscosity in all shear conditions, as well as properties such as flow, sagging, settling and storage stability. The polyurethane thickeners described in this book belong to the class of associative thickeners. The thickening function of these products is dependent on the system and is strongly influenced by certain constituents in the formulation.

The high surface tension of water can cause surface defects and inadequate adhesion on poorly-cleaned surfaces. Therefore, depending on the surface it is important to use

  • substrate wetting agents and
  • adhesion promoters

as additives or as co-binders in water-borne coatings systems. Deaerators are also indispensible in many formulations and particularly useful during airless application.

Flow and levelling additives based on polyether siloxanes or polyacrylates, which are also utilised in solvent-based coating systems, are only used in water-borne systems such as stoving enamels which contain large amounts of co-solvents.

Such additives are essential in many cases where surface tension gradients occur. Polyether siloxanes and waxes are also used because of positive characteristics such as reduction of friction.

Instead of rheological additives, low-solvent and solvent-free aqueous systems may contain gemini surfactants, which often also improve levelling thanks to effective substrate wetting.

Film-formers have already been discussed extensively in the literature and will therefore not be covered in any detail in this book, although their importance in water-based emulsions is undisputed.

To protect the applied water-borne coating from degradation the use of light absorbers, as well as of film preservatives, is absolutely essential.

Hydrophobing agents, which are mainly used in facade protection as co-binders or additives, are described as well.

The chapters of the book are also organised in the sequence set out above.

Finally, the production and use of a newly developed synthetic solid for preventing burnishing of applied emulsion paints are described.

1.1Literature

[1]

Kittel, “Lehrbuch der Lacke und Beschichtungen”, Volume 3, Hirzel Verlag, Stuttgart-Leipzig 2001

 

Wernfried Heilen et al.: Additives for Water-borne Coatings

© Copyright 2021 by Vincentz Network, Hanover, Germany

2Wetting and dispersing additives

Frank Kleinsteinberg

Dispersing of pigments is indisputably one of the most demanding steps in the manufacture of coatings. Formulators therefore look for easy solutions and additives that fulfil a number of different demands.

Already the first step in pigment dispersing – wetting of the pigment surface, which can have a very low energy – is highly problematic because the high surface tension of water needs to be reduced, without creating too many side effects. Even more problematic is finding the right stabilisation mode to match the water-borne binder. Finally, the pH also plays a key role in pigment wetting, stabilisation and compatibility.

Meeting these complex demands calls for combinations of additives that have different functions. Where applications require outstanding performance by all components, modern, highly sophisticated wetting and dispersing additives are used. The mode of action of wetting and dispersing additives at the various stages of pigment grinding is explained below. Various chemical concepts are elucidated in terms of performance and regulatory constraints and their significance for specific market segments is examined.

2.1Modes of action

The function of wetting and dispersing additives can be considered under three headings:

  • pigment wetting
  • grinding of the pigment particles
  • stabilisation of the pigment particles

2.1.1Pigment wetting

The process of wetting a solid by a liquid is summarised by Young’s equation:

γ s = γ sl + γ l • cosθ or

γ s - γ sl/γ l = cosθ

where γ s: surface tension of the solid, γ sl: interfacial tension solid/liquid, γ l: surface tension of the liquid, θ: contact angle solid/liquid, see Figure 2.1.