Adhesion promoter

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Adhesion promoters are substances that create a close physical or mostly chemical bond in the interface of immiscible substances.

Basics

Adhesion promoters are mainly used for highly stressed bonds and coatings such. They are used, for example, in dental technology or in paintwork for unfavorable weather or corrosion conditions as well as difficult substrates such as polyolefin films. The adhesive strength of coatings is defined as a measure of the resistance of a coating to its mechanical separation from the substrate; its testing is standardized in a large number of DIN and ISO standards. By increasing the adhesive strength, an improved resistance to water , chemicals and climatic influences as well as improved corrosion protection is generally achieved. Due to the large number of different composite systems, a wide range of substances is used today for adhesion promotion. Technically particularly important adhesion promoters are organically functionalized silanes ( silane adhesion promoters ) and other organometallic compounds , in particular titanates and zirconates , as well as various polymers such as polyesters and polyethyleneimine .

Mechanisms

The mode of action of adhesion promoters can be based on a large number of properties. It is particularly important to increase the wettability of the substrate surfaces and to be able to form chemical bonds between the substrate surface and the binding agent of the lacquer or adhesive . The adhesion promoters are either used as part of the formulation of paints and adhesives or can be applied separately as part of the surface pretreatment; the latter has the advantage that the adhesion promoters develop their optimal effect directly at the interface and can therefore be used in smaller quantities and achieve better adhesion values.

Pretreatment of the substrate surfaces

In order for adhesion promoters to work optimally, the substrate surfaces must be clean and sufficiently reactive . In most cases, COOH or OH groups are required for the reaction with adhesion promoters . OH groups are found on practically all base metals because they form an oxide layer in the atmosphere , as well as on glass . Since these groups also react with other substances in their environment, they lose their reactivity over time. In order to create a good bond, the surface must therefore be activated before the adhesion promoter is used, e.g. B. by pickling , grinding or blasting . Particularly good adhesion grounds can be achieved by coating with silicon dioxide beforehand. B. by PVD , CVD or flame coating ; the latter can also be used with materials such as plastics or wood. In the case of plastics, the activation can also be carried out by an oxidizing treatment such as plasma processes , flame treatment or gas phase fluorination.

Polyolefinic adhesion promoters

The most commonly used adhesion promoters in the plastics industry are modified polyolefins. Usually a polyolefin is modified so that this previously insoluble polyolefin is soluble in organic solvents. Chlorine and maleic anhydride , the so-called chlorinated polyolefins (CPO) , are mainly used for modification . This technique was developed in the 1960s. In the 1990s, attempts were made to replace chlorine with acrylic , the so-called APOs. Most of the polyolefinic adhesion promoters themselves do not react with the substrate; a reaction can only take place via the maleic anhydride. Approx. 500 tons of these adhesion promoters are processed worldwide every year. The main area of ​​application is the painting of plastic parts in the automotive industry.

Silane coupling agent

Mechanism of a silane coupling agent . In the example, steel is shown as the substrate, which has been activated by the deposition of a fresh silicon dioxide layer. The adhesion promoter is anchored on the substrate surface through condensation reactions; with its amino function it binds covalently to the epoxy resin varnish that is then applied .

Adhesion promoters of this group of substances are used primarily for composites made of an inorganic substrate with an organic material. They were first used on an industrial scale in the manufacture of glass fiber reinforced plastics (GRP) . As a rule, they have the general form R-SiX 3 , where R denotes an organically functionalized radical and X denotes a hydrolyzable group (mostly alkoxy groups , more rarely - Cl ). Through hydrolysis reactions with water they therefore form silanols of the form R-Si (OH) 3 . With inorganic materials that have OH or COOH groups on the surface, silanols can enter into condensation reactions (with elimination of water) and thus form a stable bond through chemical bonds (see figure); ideally, the Si atoms are bound into the substrate surface via all three OH groups. Alternatively, the silane coupling agents used can also react directly with the chemical groups on the surface, the condensation reaction then taking place with elimination of the alcohol or HCl. The organic group of the silane allows binding to the binder of the paint or adhesive; in the ideal case this takes place through a newly formed covalent bond . The organic group R usually consists of a spacer (usually a propyl chain ) and a functional group ; the latter should be specific to the binder of the organic layer. Functions that are widely used are vinyl , methacrylic acid , epoxy , amino , urea or thiol groups.

Other organometallic adhesion promoters

Zirconates and titanates as well as zirconium- aluminates are often used, especially for printing inks . The most important compounds here are titanium tetraisopropoxide and zirconium tetrabutoxide; chelates such as titanium acetylacetonate are also used less frequently . Similar to the organically functionalized silanes, these compounds can hydrolyze and react with OH and COOH groups on the substrate surfaces. For applications on plastics or metals, the same pretreatments of the substrate surfaces are necessary as for the use of organically functionalized silanes. In contrast to the silanes, functional groups specially adapted to the organic layer are unusual. Instead, titanates and zirconates bind directly to the OH and COOH groups of the binder; Their higher reactivity compared to the silanes also plays a role here.

Areas of application

Automobile manufacturing

In the automotive industry , windows are glued in using adhesives based on polyurethane ( PUR for short ). The panes are automatically cleaned in advance and prepared for the application of adhesive using a primer or adhesion promoter. The adhesive can then be applied using an industrial robot in combination with a corresponding application system.

Metal construction or lightweight construction

Before gluing bare metal, oxidized or phosphated metal surfaces, a primer is applied, possibly also as corrosion protection.

Dentistry

Adhesion promoters are also used in dentistry : a plastic layer connects the tooth substance with the filling material.

Metal-plastic composite

Adhesive systems in the form of adhesive lacquers or adhesive foils are used to connect metallic foils or sheets to plastics (for example in the case of metal foil back-molding ).

literature

  • Martina Ortelt: primer In: Johan Bieleman (ed.): Paint additives , Wiley-VCH, Weinheim 1998, ISBN 3-527-28819-8 , pp 114-130.
  • Bodo Müller, Ulrich Poth In: Ulrich Zorll (Ed.): Paint formulation and paint recipe , Vincentz Verlag, Hanover 2003, ISBN 3-87870-746-0 , pp. 37-40.

Individual evidence

  1. ↑ Christian Bonten: Plastics Technology Introduction and Basics, Hanser Verlag, 2014
  2. ↑ DIN55945: Coating materials and coatings - Supplementary terms to DIN EN ISO 4618 " . Beuth Verlag.
  3. ↑ A. Miszczyk, H. Szalinska: Laboratory evaluation of epoxy coatings with adhesion promoter to by impedance . In: Progress in Organic Coatings . tape 25 , 1995, pp. 357-363 .
  4. ↑ L. Cecchetto, A. Denoyelle, D. Delabouglise, J.-P. Petit: A silane pre-treatment for improving corrosion resistance performances of emeraldine base-coated aluminum samples in neutral environment . In: Applied Surface Science . tape 254 , 2008, p. 1736-1743 .
  5. ↑ A. Heft, T. Hädrich, T. Struppert, A. Pfuch, M. Homuth, B. Grünler: Deposition of thin functional layers at atmospheric pressure . In: Yearbook Surface Technology . tape 64 . Leuze Verlag, 2008, ISBN 978-3-87480-245-1 , p. 137-149 .
  6. ↑ Questions and answers on lightweight construction from Professor Schürmann , Question 219, 2006
  7. ↑ dental lexicon, adhesion promoters , accessed on August 18, 2016
  8. ↑ IMC Wiki, Dentinadhäsive (Dentin Bonding Agent) , accessed on August 18, 2016
  9. ↑ Dirk Schreckenbach. Dentistry - a question of filling , accessed August 18, 2016