Introduction
Excellent adhesion on the substrate is a key requirement for most coating systems. The quality of the substrate-coating interface, and by that the adhesion strength, is governed by the combination of the quality of the paint, the condition of the substrate, the application process and film formation.
Several mechanisms can be used to obtain adhesion1,2. This article is about adhering coatings on steel via covalent bonding.
Looking for adhesion promoters for your coating formulations?
Prospector has listings for more than 500 adhesion promoters from global suppliers. Find technical data, order samples and more. Register today for access!
Steel surface pre-treatment: The important start
The surface of steel that is to be coated must be pure and clean. This implies that oxidation products of iron, like rust and scale, must be removed from the steel surface. Following that, the substrate must be cleaned, for example by using alkaline cleaner, rinsed and dried. When the pre-treatment is done right, you get a steel surface that is filled with hydroxyl (-OH) groups.
Key coating adhesion success factors
An approach to obtain excellent adhesion of a coating on steel is to assure interfacial crosslinking. This implies that covalent chemical bonds, which are resistant towards hydrolysis, are formed between the steel surface and the coating during film formation. The surface of the steel must be filled with suitable reactive groups. Apart from requirements with respect to the substrate and the paint themselves, the resulting steel-coating interface must be flexible enough to cope with dimensional changes under the influence of changing temperature or mechanical stress, possibly in combination with exposure to water.
Adhesion promoters
Adhesion promoters, also known as coupling agents, are bi-functional reactive additives designed such that one part of the molecules forms covalent bonds with the substrate and another part participates in the crosslinking of the binder system of the paint during film formation3,4. In general, bi-functional silanes, based on silicon (Si) chemistry, are used as adhesion promoters for steel.
A chemical reaction of one part of the silanes with the hydroxyl groups on the steel surface takes place when the adhesion promoter comes in contact with the steel under the right circumstances. During this so-called chemisorption, alcohol (ROH) is released and covalent bonds (Fe-O-Si) are formed.
The chain between the reactive groups, the spacer, must be such that the resulting substrate-coating interface is flexible, to prevent possible stress building-up at the interface. It is said that the spacer gives interfacial flexibility.
An example
3-Aminopropyltriethoxysilane (APTES) is a molecule that can be used as adhesion promoter to couple a 2-component (2K) primer with a steel surface.
The metal reactive block, consisting of three ethoxysilane groups, can react with hydroxyl groups on the steel surface under the release of ethanol (H3C-CH2-OH). The primary amine group is reactive towards both epoxy groups and isocyanate groups. This implies that APTES can be used as an adhesion promoter for both 2K epoxy-amine systems and 2K polyurethane systems.
Let’s take a closer look at APTES. The three metal reactive groups are close to one another. This is critical with respect to both reactivity and flexibility. The second critical aspect is that the molecules contain only one binder system reactive group. Finally, the spacer has a critical length of only three methylene (-CH2-) units, providing limited interfacial flexibility.
Oligomers
System developers recognise the critical aspects of conventional adhesion promoters like APTES. Monomeric silanes are more and more substituted by oligomeric adhesion promoters.
CoatOSil MP 200 is an example of an innovative oligomer, based on silane chemistry, that can be used as adhesion promoter for 2K metal primers that are based on epoxy-amine binder systems.
The metal reactive block contains several methoxysilane groups that can react with hydroxyl groups on the steel surface, under the release of methanol (H3C-OH). Also, each molecule has multiple epoxy groups thus ensuring multiple co-crosslinking with the binder system of the primer. Finally, MP 200 has flexible polymeric spacers, giving increased reactivity as well as enhanced interfacial flexibility.
Application
Basically, there are two options with respect to the use of adhesion promoters. First, a dilution of adhesion promoter can be applied directly on the clean steel surface. In this way, a thin reactive layer is formed on the steel surface, a so-called conversion layer: the surface is converted from hydroxyl-functional to epoxy- or amine-functional. The next step is the application of the paint on the modified surface in such a way that the binder system of the paint forms covalent bonds with the conversion layer.
The second option is to use the adhesion promoter as additive in the paint that is applied on the steel. This can, for example, be a primer based on a 2K epoxy-amine or on a 2K polyurethane binder system. A critical factor of the second option is that undesired pre-reaction of the adhesion promoter with paint ingredients can take place before the paint is applied on the steel. Also, the adhesion promoter molecules must be able to migrate from the bulk of the paint to the steel-paint interface during film formation. Both critical factors imply that the best option is to first apply the adhesion promoter as conversion layer before paint is applied. The disadvantage of this approach is that an extra application step is needed.
References
- Metal Surface Treatment – The Key to Successful Performance, Ron Lewarchik, 4 November 2016
- Achieving Superior Coatings Adhesion, Jochum Beetsma, 13 June 2014
- Reactive Silanes for Enhancement of Coating Performance, Ron Lewarchik, 6 March 2015
- Adhesion Promoters 101, Marc Hirsch, 19 May 2016
The views, opinions and technical analyses presented here are those of the author or advertiser, and are not necessarily those of ULProspector.com or UL Solutions. The appearance of this content in the UL Prospector Knowledge Center does not constitute an endorsement by UL Solutions or its affiliates.
All content is subject to copyright and may not be reproduced without prior authorization from UL Solutions or the content author.
The content has been made available for informational and educational purposes only. While the editors of this site may verify the accuracy of its content from time to time, we assume no responsibility for errors made by the author, editorial staff or any other contributor.
UL Solutions does not make any representations or warranties with respect to the accuracy, applicability, fitness or completeness of the content. UL Solutions does not warrant the performance, effectiveness or applicability of sites listed or linked to in any content.
What will happen if the paint containing the oligomeric adhesion promoters also contains metallic particles (aluminium, zinc or iron). Will the oligomeric adhesion promoters react with the metalllic particle of the paint and not be anymore available to react with the substrate?
Indeed, this is a risk. Therefore, it is important to consider all the raw materials that are in your paint and find out if any pre-reactions can be expected. An option is to use a conversion layer (see article) instead of adding an adhesion promoter to your paint.
The post shared informative content and I have obtained useful insights for making changes in the planning of my own business house. Information shared on the product with relation to keeping them ductile and durable the use of the coated product is very important and most extreme in those scenarios where the product is exposed to water. thanks mate your post was helpful and informational.