Adhesion promoters (AP) are bi-functional materials that increase adhesive strength between the coating and the substrate and sometimes are referred to as coupling agents. Unlike priming systems, adhesion promoters are generally applied at thinner film thicknesses. An adhesion promoter’s effectiveness depends on both the substrate and the adhesive being used. Surface pretreatments, such as solvent cleaning or mechanical etching and corona treatment, can be used with adhesion promoters as part of a pretreatment method. Within a class of materials, the functionality on the backbone of the molecule will vary based on the resin system employed as well as the substrate to which it is attached.
Adhesion is one of the most important properties of both decorative and protective coatings. Many of the additives used in coating formulations can have a negative impact on the overall adhesion behavior, even for surfaces that would not be considered difficult for adhesion. Avoidance of deleterious additives is preferred, but at times they are needed to impart other properties to the coating.
Titanates and zirconates are coupling agents and adhesion promoters that increase impact strength and chemical resistance and enhance physical, compounding and application properties. Common applications include polymers in industrial coatings, silicone and rubber sealants, adhesives, surface primers, oil and gas, electronics, printing, ceramics, glass, catalysts in esterifications and transesterification. Zircoaluminates are manufactured by several suppliers.
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Monomeric phosphate chemistries and phosphate esters
Monomeric phosphates are materials used in free-radical emulsion polymerization and provide wet adhesion bonding to the substrate. The phosphorus-containing functional groups can be incorporated into the polymers during the polymerization, usually through delayed addition from the start of the polymerization. When these resins are used in coatings or adhesives, the adhesion and anti-corrosion performance of the final products are significantly improved.
Several companies produce phosphate ester surfactants and phosphate acids, precursors to the esters. These have been found to benefit adhesion as well as improve corrosion inhibition of coatings over ferrous surfaces.
Silanes are effective APs for glass- and silica-containing materials. Hence, they are used widely for ultrahydrophobic treatments that rely on a system package of a silanol and silica, with silane adhesion used in epoxy, urethane and acrylic systems. One of the concepts used to obtain excellent adhesion is to assure that covalent bonds are formed between the substrate and the coating. This concept is called coupling or chemisorption. Silanes can be used as additives to couple substrates that contain hydroxyl (-OH) groups, such as metals, and coatings. Apart from an alkoxy (-OR) group, which forms a covalent bond with the metal surface, the silane additive contains a chemical group that can crosslink with the binder system of the coating during film formation.
The effectiveness of silanes depends on the substrate being used. Smooth, high-surface-energy substrates are better than low-surface-energy or discontinuous substrates.
The first-generation coupling agents that are still used to improve adhesion are mono-functional. These mono-functional silane adhesion promoters are more and more substituted by oligomers. The oligomer molecules have a moderately high molecular weight and multiple functionality in both types of reactive groups:
- Methoxy (-O-CH3) groups, directly linked to silicon (Si) atoms, which form a covalent bond with the hydroxyl (-OH) groups of the metal substrate
- Epoxy groups that co-crosslink with the binder system during film formation
The core of the additive is formed by a strong siloxane network that has excellent resistance against, for example, hydrolysis, chemicals and UV radiation. Despite the increased molecular weight, compared to mono-functional silanes, the oligomer has a low viscosity and a broad compatibility.
Chlorinated and Non-Chlorinated Polyolefin Chemistries
For plastic adhesion, some chlorinated polyolefin (CPO) chemistries are effective in the improvement in adhesion of coatings to thermoplastic olefin (TPO). This is particularly important in automotive coatings, as an increasing amount of the external portion of the vehicle is composed of plastics.
It is possible to apply an adhesion-promoting primer from a waterborne system or to promote adhesion of a waterborne coating through use as an additive. CPO dispersions are useful as the base resin in adhesion-promoting primers and as an additive by being added directly to waterborne coating formulations to improve adhesion to polypropylene-based substrates.
There are also non-chlorinated adhesion promoters that provide excellent adhesion to TPO and polypropylene substrates.
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