Adhesion improvement with water-based systems

The continuous demand for technical textiles with increased performance combined with sustainability leads to an increased focus on a phenomenon such as "adhesion to textile substrates", writes Theo Breugelmans, elaborating on the parameters which influence the adhesion of coatings and finishes towards the most commonly used textile substrates made of polyamide and polyester.

The demand for technical textiles has increased continuously over the last decade and the expectation is that this will continue for the coming years. But what is considered to be a technical textile? A technical textile is a product based on textile or nonwoven which is produced for non-aesthetic purposes but where functionality is the primary criterion. Functionalities in that respect can be protection, sealing, separation, isolation, repellency, stabilisation, storage, etc. The required functionality need to be translated in the performance of the article. This performance is measurable.

Think about flame-retardency according to many different norms. Think about the resistance to hydrolyses (degradation by water), light, UV and/or weathering. Furthermore properties like (cold) flexibility, the ability to withstand water pressure as well as chemical- and mechanical resistance (scratch and abrasion) are very important.

The main objective of the development of a technical textile is to create new articles with new properties which could be applied in new application areas. The current newly introduced materials in the automotive industry are innovative technical textiles. In many cases to comply with the most important demand; weight reduction. Above mentioned properties are primarily integrated into the textile or nonwoven by the application of one or more coating layers. The principle of coating, still a relatively new technology in textile processing, is applying a layer of film forming products on the textile substrate.

Adhesion

The adhesion of the coating towards the textile is one of the most important parameters and plays a major role in realising the required properties of a technical textile. It also determines how the final product performs in further processing steps, the resistance to ageing during the life cycle of the product and finally the suitability of the product for the specific application. In the early days the industry had a broad range of (aromatic) solvent based binders and crosslinkers available to cope with this problem and thus improve adhesion. Nowadays the usage of these products is prohibited or limited and we have to choose from a range of (sustainable) water based products. This makes it even more challenging.

Before discussing the factors that influence the adhesion it is essential to look in more detail into the definition of adhesion. Adhesion is the force needed to separate two different type of materials, particles or surfaces from each other.

Not to confuse with cohesion which is the force needed to separate two similar type of materials from each other. Or in other words the (internal) adhesion of atoms, molecules inside the material. This has to be considered when conclusions are made after testing. Figure 1 shows the differences between adhesion and cohesion ruptures after a tear test. It is clear to see that both adhesion and cohesion influence the total bonding strength. The total bonding strength is equal to the strength of the weakest component. Useful to mention is that there are different mechanisms of adhesion to bond one material to the other. Each mechanism has its specific influence on the bonding strength.

Adhesion can be realised

• Mechanically. Think about the most obvious examples as sewing and "hook and loop" (Velcro). Coating of rough and irregular surfaces (painting sanded wood).