{"id":19606,"date":"2025-08-13T06:00:51","date_gmt":"2025-08-13T12:00:51","guid":{"rendered":"https:\/\/www.ulprospector.com\/knowledge\/?p=19606"},"modified":"2025-08-15T06:09:06","modified_gmt":"2025-08-15T12:09:06","slug":"pc-adhesion-on-steel-via-covalent-bonding-2","status":"publish","type":"post","link":"https:\/\/ulprospector.ul.com\/19606\/pc-adhesion-on-steel-via-covalent-bonding-2\/","title":{"rendered":"Adhesion on Steel via Covalent Bonding"},"content":{"rendered":"<p><strong>Introduction <\/strong><\/p>\n<p>Excellent adhesion on the substrate is a key requirement for most coating systems. The quality of the substrate-coating interface, and by that the adhesive strength, is governed by the quality of the paint, the condition of the substrate, the application process and film formation.<\/p>\n<figure id=\"attachment_19607\" class=\"thumbnail wp-caption aligncenter\" style=\"width: 1750px\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-19607 size-full\" src=\"https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/1.-Adhesion-is-governed-by-the-quality-of-the-substrate-coating-interface.jpg\" alt=\"\" width=\"1750\" height=\"358\" srcset=\"https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/1.-Adhesion-is-governed-by-the-quality-of-the-substrate-coating-interface.jpg 1750w, https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/1.-Adhesion-is-governed-by-the-quality-of-the-substrate-coating-interface-300x61.jpg 300w, https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/1.-Adhesion-is-governed-by-the-quality-of-the-substrate-coating-interface-1024x209.jpg 1024w, https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/1.-Adhesion-is-governed-by-the-quality-of-the-substrate-coating-interface-768x157.jpg 768w, https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/1.-Adhesion-is-governed-by-the-quality-of-the-substrate-coating-interface-1536x314.jpg 1536w\" sizes=\"(max-width: 1750px) 100vw, 1750px\" \/><figcaption class=\"caption wp-caption-text\">Adhesion is governed by the quality of the substrate-coating interface.<\/figcaption><\/figure>\n<p>Several mechanisms can be used to obtain adhesion<sup>1,2<\/sup>. This article is about adhering coatings on steel via <strong>covalent bonding<\/strong>.<\/p>\n<p><strong>The important start<\/strong><\/p>\n<p>The surface of a steel object that is coated must be pure and clean. This implies that oxidation products of iron, like rust and scale, must be removed before paint is applied. Also, the substrate must be cleaned, to remove dirt, grease et cetera, for example by using alkaline cleaner. After cleaning, the surface must be rinsed and dried. A steel surface, that is filled with hydroxyl (<strong>&#8211;<\/strong><strong>OH<\/strong>) groups, results when the <strong>pre-treatment<\/strong> is done right.<\/p>\n<figure id=\"attachment_19608\" class=\"thumbnail wp-caption aligncenter\" style=\"width: 1750px\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-19608\" src=\"https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/2.-The-surface-of-clean-steel-is-filled-with-hydroxyl-groups.jpg\" alt=\"\" width=\"1750\" height=\"525\" srcset=\"https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/2.-The-surface-of-clean-steel-is-filled-with-hydroxyl-groups.jpg 1750w, https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/2.-The-surface-of-clean-steel-is-filled-with-hydroxyl-groups-300x90.jpg 300w, https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/2.-The-surface-of-clean-steel-is-filled-with-hydroxyl-groups-1024x307.jpg 1024w, https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/2.-The-surface-of-clean-steel-is-filled-with-hydroxyl-groups-768x230.jpg 768w, https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/2.-The-surface-of-clean-steel-is-filled-with-hydroxyl-groups-1536x461.jpg 1536w\" sizes=\"(max-width: 1750px) 100vw, 1750px\" \/><figcaption class=\"caption wp-caption-text\">The surface of clean steel is filled with hydroxyl groups.<\/figcaption><\/figure>\n<p><strong>Key success factors<\/strong><\/p>\n<p>An approach to obtain excellent adhesion of a coating on steel is to assure <strong>interfacial crosslinking<\/strong>. This implies that covalent chemical bonds, that are resistant against hydrolysis, are formed between the steel surface and the coating during film formation. The surface of pure and clean steel is filled with reactive hydroxyl groups, thus enabling interfacial crosslinking. Apart from requirements with respect to the substrate and the paint themselves, the resulting steel-coating interface must be <strong>flexible<\/strong> enough to cope with dimensional changes that are caused by changing temperature or mechanical stress, possibly in combination with the exposure to water.<\/p>\n<hr \/>\n<h3>Work smarter and win more, with powerful software to manage regulatory, supply chain and sustainability challenges, learn more about ULTRUS\u00a0<a href=\"https:\/\/www.ul.com\/software\/ultrus?utm_source=KnowledgeCenter&amp;utm_medium=article&amp;utm_campaign=covalent_bonding&amp;utm_term=2025PC&amp;utm_content=Beetsma\" target=\"_blank\" rel=\"noopener\">here<\/a>!<\/h3>\n<hr \/>\n<p><strong>Adhesion promoters<\/strong><\/p>\n<p><a href=\"https:\/\/www.ulprospector.com\/en\/na\/Coatings\/Product\/search?k=Adhesion+Promoters&amp;sug=1&amp;st=31&amp;utm_source=KnowledgeCenter&amp;utm_medium=article&amp;utm_campaign=covalent_bonding&amp;utm_term=2025PC&amp;utm_content=Beetsma\" target=\"_blank\" rel=\"noopener\">Adhesion promoters<\/a>, also known as <a href=\"https:\/\/www.ulprospector.com\/en\/na\/Coatings\/Product\/search?k=Coupling+Agents&amp;sug=1&amp;st=31&amp;utm_source=KnowledgeCenter&amp;utm_medium=article&amp;utm_campaign=covalent_bonding&amp;utm_term=2025PC&amp;utm_content=Beetsma\" target=\"_blank\" rel=\"noopener\"><strong>coupling agents<\/strong><\/a>, are bi-functional reactive additives that are designed as such that one part of the molecules forms covalent bonds with the substrate and another part of the molecules participates in the crosslinking of the binder system of the paint during film formation<sup>3,4<\/sup>. Bi-functional <strong>silanes<\/strong>, based on silicon (<strong>Si<\/strong>) chemistry, are often used as adhesion promoters for steel.<\/p>\n<figure id=\"attachment_19609\" class=\"thumbnail wp-caption aligncenter\" style=\"width: 1750px\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-19609\" src=\"https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/3.-Chemical-reaction-of-a-silane-adhesion-promoter-with-the-surface-of-steel.jpg\" alt=\"\" width=\"1750\" height=\"732\" srcset=\"https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/3.-Chemical-reaction-of-a-silane-adhesion-promoter-with-the-surface-of-steel.jpg 1750w, https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/3.-Chemical-reaction-of-a-silane-adhesion-promoter-with-the-surface-of-steel-300x125.jpg 300w, https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/3.-Chemical-reaction-of-a-silane-adhesion-promoter-with-the-surface-of-steel-1024x428.jpg 1024w, https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/3.-Chemical-reaction-of-a-silane-adhesion-promoter-with-the-surface-of-steel-768x321.jpg 768w, https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/3.-Chemical-reaction-of-a-silane-adhesion-promoter-with-the-surface-of-steel-1536x642.jpg 1536w\" sizes=\"(max-width: 1750px) 100vw, 1750px\" \/><figcaption class=\"caption wp-caption-text\">Chemical reaction of a silane adhesion promoter with the surface of steel.<\/figcaption><\/figure>\n<p>A chemical reaction of one part of the silane adhesion promoter with a hydroxyl group on the steel surface takes place when the adhesion promoter molecule comes in contact with the steel under the right conditions. During this so-called <strong>chemisorption<\/strong>, an alcohol molecule (R-OH) is released and a covalent bond (Fe-O-Si) is formed.<\/p>\n<p>The chain between the two types of reactive groups, the <strong>spacer<\/strong>, is designed as such that the resulting substrate-coating interface is flexible enough, thus preventing possible <strong>stress<\/strong> building-up at the interface. It is said that the spacer gives <strong>interfacial flexibility<\/strong>.<\/p>\n<p><strong>An example<\/strong><\/p>\n<p>3-Aminopropyltriethoxysilane (<strong>APTES<\/strong>) is a molecule that can be used as adhesion promoter to couple a 2-component (<strong>2C<\/strong>) primer with a steel surface.<\/p>\n<figure id=\"attachment_19610\" class=\"thumbnail wp-caption aligncenter\" style=\"width: 1750px\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-19610\" src=\"https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/4.-Chemical-structure-of-APTES.jpg\" alt=\"\" width=\"1750\" height=\"558\" srcset=\"https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/4.-Chemical-structure-of-APTES.jpg 1750w, https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/4.-Chemical-structure-of-APTES-300x96.jpg 300w, https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/4.-Chemical-structure-of-APTES-1024x327.jpg 1024w, https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/4.-Chemical-structure-of-APTES-768x245.jpg 768w, https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/4.-Chemical-structure-of-APTES-1536x490.jpg 1536w\" sizes=\"(max-width: 1750px) 100vw, 1750px\" \/><figcaption class=\"caption wp-caption-text\">Chemical structure of APTES.<\/figcaption><\/figure>\n<p>The metal reactive block, consisting of three ethoxysilane groups, can react with hydroxyl groups on the steel surface under the release of ethanol (H<sub>3<\/sub>C-CH<sub>2<\/sub>-OH). The primary amine group (<strong>&#8211;<\/strong><strong>NH<sub>2<\/sub><\/strong>) is reactive towards both epoxy groups and isocyanate groups. This implies that APTES can be used as adhesion promoter for both 2C epoxy-amine systems and 2C polyurethane systems.<\/p>\n<p>Let\u2019s take a closer look at the APTES molecule. The three metal reactive groups are close to each other. 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 3 methylene (-CH<sub>2<\/sub>-) units, providing limited interfacial flexibility.<\/p>\n<p><strong>Oligomers<\/strong><\/p>\n<p>System developers recognize the critical aspects of conventional adhesion promoters like APTES. Because of that, monomeric silanes are more and more substituted by oligomeric adhesion promoters. An <a href=\"https:\/\/www.ulprospector.com\/en\/na\/Coatings\/Product\/search?k=Oligomers&amp;sug=1&amp;st=31&amp;utm_source=KnowledgeCenter&amp;utm_medium=article&amp;utm_campaign=covalent_bonding&amp;utm_term=2025PC&amp;utm_content=Beetsma\" target=\"_blank\" rel=\"noopener\"><strong>oligomer<\/strong><\/a> is defined as a polymer with a relatively low molecular weight<sup>5<\/sup>.<\/p>\n<figure id=\"attachment_19611\" class=\"thumbnail wp-caption aligncenter\" style=\"width: 1765px\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-19611\" src=\"https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/5.-Indicative-structure-of-CoatOSil-MP-200.jpg\" alt=\"\" width=\"1765\" height=\"603\" srcset=\"https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/5.-Indicative-structure-of-CoatOSil-MP-200.jpg 1765w, https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/5.-Indicative-structure-of-CoatOSil-MP-200-300x102.jpg 300w, https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/5.-Indicative-structure-of-CoatOSil-MP-200-1024x350.jpg 1024w, https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/5.-Indicative-structure-of-CoatOSil-MP-200-768x262.jpg 768w, https:\/\/ulprospector.ul.com\/wp-content\/uploads\/2025\/07\/5.-Indicative-structure-of-CoatOSil-MP-200-1536x525.jpg 1536w\" sizes=\"(max-width: 1765px) 100vw, 1765px\" \/><figcaption class=\"caption wp-caption-text\">Indicative structure of CoatOSil MP 200 (courtesy of Momentive).<\/figcaption><\/figure>\n<p><a href=\"https:\/\/www.ulprospector.com\/en\/na\/Coatings\/search?k=%22CoatOSil+MP+200%22&amp;st=31&amp;utm_source=KnowledgeCenter&amp;utm_medium=article&amp;utm_campaign=covalent_bonding&amp;utm_term=2025PC&amp;utm_content=Beetsma\" target=\"_blank\" rel=\"noopener\"><strong>CoatOSil MP 200<\/strong><\/a> is an example of an oligomer, based on silane chemistry, that can be used as adhesion promoter for 2C metal primers that are based on epoxy-amine binder systems.<\/p>\n<p>The metal reactive block contains several methoxysilane groups (Si-O-CH<sub>3<\/sub>) that can react with hydroxyl groups on the steel surface, under the release of methanol (HO-CH<sub>3<\/sub>) molecules. Each molecule has multiple epoxy groups, thus enabling multiple co-crosslinking with the binder system of the primer. Finally, the spacers in MP 200 are more flexible than the spacer in APTES, giving both increased reactivity and more interfacial flexibility.<\/p>\n<p><strong>Application<\/strong><\/p>\n<p>Basically, there are two options with respect to the use of <a href=\"https:\/\/www.ulprospector.com\/en\/na\/Coatings\/Product\/search?k=Adhesion+Promoters&amp;sug=1&amp;st=31&amp;utm_source=KnowledgeCenter&amp;utm_medium=article&amp;utm_campaign=covalent_bonding&amp;utm_term=2025PC&amp;utm_content=Beetsma\" target=\"_blank\" rel=\"noopener\">adhesion promoters<\/a>.<\/p>\n<p>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 <strong>conversion layer<\/strong>: the steel surface is converted from hydroxyl-functional to epoxy-functional 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.<\/p>\n<p>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 2C epoxy-amine binder system or a primer based on a 2C polyurethane binder system. A critical factor of the second approach is that an undesired pre-reaction of the adhesion promoter molecules with <a href=\"https:\/\/www.ulprospector.com\/en\/na\/Coatings?utm_source=KnowledgeCenter&amp;utm_medium=article&amp;utm_campaign=covalent_bonding&amp;utm_term=2025PC&amp;utm_content=Beetsma\">paint ingredients<\/a> 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.<\/p>\n<p>&nbsp;<\/p>\n<p><strong>\u00a0\u00a0\u00a0 References<\/strong><\/p>\n<ol>\n<li><a href=\"https:\/\/ulprospector.ul.com\/search\/A+Guide+to+Providing+Initial+and+Long-Lasting+Coating+Adhesion%2C+Ron+Lewarchik?utm_source=KnowledgeCenter&amp;utm_medium=article&amp;utm_campaign=covalent_bonding&amp;utm_term=2025PC&amp;utm_content=Beetsma\" target=\"_blank\" rel=\"noopener\"><em>A Guide to Providing Initial and Long-Lasting Coating Adhesion<\/em>, Ron Lewarchik<\/a>, 12 January 2022<\/li>\n<li><a href=\"https:\/\/ulprospector.ul.com\/11212\/pc-achieving-superior-adhesion-of-coatings\/?utm_source=KnowledgeCenter&amp;utm_medium=article&amp;utm_campaign=covalent_bonding&amp;utm_term=2025PC&amp;utm_content=Beetsma\" target=\"_blank\" rel=\"noopener\"><em>Achieving Superior Adhesion of Coatings<\/em>, Jochum Beetsma<\/a>, 13 November 2020<\/li>\n<li><a href=\"https:\/\/ulprospector.ul.com\/8630\/pc-superior-coatings-performance-with-organosilane-components\/?utm_source=KnowledgeCenter&amp;utm_medium=article&amp;utm_campaign=covalent_bonding&amp;utm_term=2025PC&amp;utm_content=Beetsma\" target=\"_blank\" rel=\"noopener\"><em>Superior Coatings Performance with Organosilane Components<\/em>, Ron Lewarchik,<\/a> 10 August 2018<\/li>\n<li><a href=\"https:\/\/ulprospector.ul.com\/4524\/adhesion-promoters-101\/?utm_source=KnowledgeCenter&amp;utm_medium=article&amp;utm_campaign=covalent_bonding&amp;utm_term=2025PC&amp;utm_content=Beetsma\" target=\"_blank\" rel=\"noopener\"><em>Adhesion Promoters 101<\/em>, Marc Hirsch<\/a>, 9 November 2022<\/li>\n<li><a href=\"https:\/\/ulprospector.ul.com\/15576\/pc-oligomers-small-group-big-impact\/?utm_source=KnowledgeCenter&amp;utm_medium=article&amp;utm_campaign=covalent_bonding&amp;utm_term=2025PC&amp;utm_content=Beetsma\" target=\"_blank\" rel=\"noopener\"><em>Oligomers \u2013 Small Group, Big Impact<\/em>, Wally Kesler<\/a>, 27 September 2023<\/li>\n<\/ol>\n<p>&nbsp;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>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 adhesive strength, is governed by the quality of the paint, the condition of the substrate, the &hellip; <a href=\"https:\/\/ulprospector.ul.com\/19606\/pc-adhesion-on-steel-via-covalent-bonding-2\/\">Continued<\/a><\/p>\n","protected":false},"author":11,"featured_media":18573,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"episode_type":"","audio_file":"","podmotor_file_id":"","podmotor_episode_id":"","cover_image":"","cover_image_id":"","duration":"","filesize":"","filesize_raw":"","date_recorded":"","explicit":"","block":"","itunes_episode_number":"","itunes_title":"","itunes_season_number":"","itunes_episode_type":"","footnotes":""},"categories":[16],"tags":[1059,1060,1062,1063,1073,1074,1075,1076,1077,1078,1079,1080,1081,1082],"ppma_author":[1235],"class_list":{"0":"post-19606","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-paint-coatings","8":"tag-adhesion","9":"tag-interface","10":"tag-pre-treatment","11":"tag-substrate","12":"tag-adhesion-promoter","13":"tag-chemisorption","14":"tag-conversion-layer","15":"tag-coupling-agent","16":"tag-covalent-bonding","17":"tag-flexibility","18":"tag-interfacial-crosslinking","19":"tag-oligomer","20":"tag-spacer","21":"tag-stress","22":"entry"},"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.4 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Adhesion on Steel via Covalent Bonding<\/title>\n<meta name=\"description\" content=\"Excellent adhesion on the substrate is a key requirement for most coating systems. 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