The hairspray should dry quickly but be easy to brush and wash out again in the evening. Eco-friendliness is another requirement for hairsprays. With Luviset P.U.R., the new BASF polymer for hair cosmetics, hairsprays can meet these requirements perfectly. The polymers of the polyurethanes class give hair a flexible hold. They are also specially adapted to produce environmentally friendly hairsprays in which water replaces many of the organic solvents.

Conventional polymers for setting lotions are only suitable to a limited extent for use in water-containing sprays. As the water content of the hairspray increases, the viscosity rises dramatically and the sprayability suffers. The polymer films are tackier and leave much more visible residue on the hair on brushing out. Completely new polymers therefore had to be found for use in hairsprays with a high water content.

 
In their search for a new class of polymers for hair setting lotions, BASFs scientists were able to draw upon the broad range of polymer classes already available within the company as well as the associated know-how. One important aspect was recognized from existing knowledge of structure-property relationships: polymers with a similar setting action based on conventional polyacrylates have a markedly higher molecular weight (and thus a higher solution viscosity!) than polymers polymerized by polycondensation (polyesters and polyamides) or polyaddition (polyurethanes).

The special feature of polyurethane: the solution viscosity in water is much lower than for conventional acrylate-based polymers and provides improved sprayability.

 
Which polymers are best suited for use in hairsprays is decided in the application technology department. These investigations involve the use of test hairs on model heads and in the form of single locks, which are washed, blow-dried, treated with the new polymers and combed. The testers need great sensitivity to evaluate the body of the hair and how well the setting lotion can be combed and washed out.

Testing the practicability of new polymers

 
The researchers also wish to describe the combability of polymer-treated hair surfaces with the aid of an objective, physical parameter. This is where the nano-scratching method comes in and involves producing a thin film of the hairspray polymer on a substrate. Using special nanotechnology (nano atomic force microscopy), scratch tests are performed on the polymer films and their mechanical properties such as modulus of elasticity, hardness and coefficient of friction (lateral force divided by normal force, LF/NF) are determined quantitatively. This objective physical parameter allows cosmetic performance to be predicted in the combability test.

 
The modular design concept underlying polyurethane chemistry allows the chemists in polymer research to proceed flexibly and continue offering customers tailor-made products by using different components.