Nowadays, we always seem to be sticking something somewhere: "Post-It" notes on the fridge door, adhesive plasters on an injured finger, carpets on the floor, price labels on goods. Sometimes we want them to stick like the devil, other times they should be easy to remove. BASF has now developed a new generation of pressure sensitive adhesives under the name acResin, in which the adhesive effect can be varied in such a way that different adhesive products can be manufactured using the same adhesive raw material.

Since acResin pressure sensitive adhesives are also free of water and solvents, they have clear advantages over conventional adhesives both in processing and ecological terms. The special trick: irradiation with UV light. The UV dose with which the film is radiated controls the crosslinking of the macromolecules in the adhesive film, the factor responsible for the adhesive properties.

 
When two surfaces have to be joined together, the thin film of adhesive creates an intimate contact between the surfaces. On one side, the long-chain polymers in this layer cling firmly to certain groups on the contact surface, on another side, they are crosslinked with each other. The overall adhesive effect depends on how strong are the following forces: adhesion to the surface of the object or crosslinking of the polymers in the adhesive layer.

If the polymers are highly crosslinked among each other, the adhesive film is relatively firm within itself and holds together when subjected to tensile loading. The contact between an object and the adhesive gives way first which is why, for example, a price label can be removed easily without leaving residues. If the polymer chains are relatively weakly bound together, however, then they are lost before the adhesive detaches from the surface. Depending on the pealing strength, residues of the adhesive or even the price label itself will remain on the surface.

 
Bridges between the long polymer chains in the adhesive layer are created by small molecules which react rapidly on exposure to UV light and are thus called photoreactive groups. Adhesive properties are easy to control by varying the period of light exposure: when acResin is exposed to a great UV-C dose, a large number of the photoreactive groups go into action. A lot of crosslinks are formed between the polymers. A lower UV-C dose means fewer links between the polymer chains.

 
At the very beginning, polymer chemists set out to answer the following basic questions: what would such a polymer have to be like? Which monomers, i.e. building blocks can be regarded as likely candidates? What would the chain length distribution of the polymers have to be like? What initiators would be suitable for inducing the photocrosslinking? What kind of manufacturing process would be needed? What would production cost?
All these are typical questions that present themselves at the start of product development.

The synthesis chemists then developed a special polymerizable photoinitiator which had to satisfy a number of requirements: it should be easy to integrate in the polymer and allow very efficient photocrosslinking. The synthesis chemists finally found a way of producing this photoinitiator on a large scale within the BASF integrated production system (Verbund).

To develop a simple process for manufacturing high viscosity acrylate resins (to which the acResin brands belong) on a large scale, the know-how of the engineers was indispensable. Reducing the amount of the residual volatile organic components, for example, represented a challenge.

The polymer physicists were then enlisted to answer the following important questions: What is the optimal radiation dose for the new UV crosslinking adhesives? How do the mechanical properties of film change during radiation?

To optimize the application properties of acResin, the BASF experts conducted coating experiments in their own coating pilot plant, and also tested a variety of UV radiation sources.

Experienced formulation developers devised suitable guide formulations for adhesives.

Now that acResin has gained itself a place on the market, the time has come to extend the range of products and add new applications.