Persoz hardness

The hardness of the cured film was evaluated by monitoring the damping of the oscillations of a pendulum (Persoz hardness) which is directly related to the sofmess of the sample. For a 35 jim thick UV-cured film, coated onto a glass plate, Persoz values typically range from 50 s for elastomeric materials to 300 s for harel and glassy polymers. 

For photocured polymer materials, the Persoz hardness was shown to depend mainly on the chemical structure of the prepolymer chain, on the functionality of the reactive diluent, on the photoinitiator system and on the duration of the UV exposure. The hardness of tack-free coatings thus continues to increase with further irradiation, as shown by Figure 6 for polyurethane-acrylates. This slow hardening process is likely to result from some further polymerization of the unreacted acrylic functions since their concentration was found to decrease concomittantly. 

It is well known that adhesives need to be low-modulus elastomers, so that the resin formulation has to be designed to give, after UV-curing, a soft material with a low glass transition temperature (Fg). Fig. 3 shows some typical elastic modulus E) and tan S profiles recorded by dynamic mechanical analysis for a UV-cured polyurethane-acrylate which is suitable for adhesives applications, based on its E and Tg values. It is quite soft, as shown by its Persoz hardness value of 50 s, on a scale that goes up to 400 s for glassy materials. 

The cross-link density of the polymer network, as well as its properties, depend on the functionality, the length and the chemical nature of the polyene (R ) and thiol (R) prepolymer chains, and it can thus be tailored as desired. Low-modulus polymers suitable for adhesive applications were obtained by using aliphatic prepolymer chains, in particular with polybutadiene-based elastomers which were cross-linked very efficiently by UV-irradiation in the presence of a tri- or tetrathiol [45-48]. As only a few cross-links need to be formed between the polymer chains to make the rubber insoluble, low concentrations of thiol (2 wt%) proved to be sufficient to achieve an effective and fast cross-finking. Hardening was found to hardly occur upon UV-curing (increase of the Persoz hardness from 40 to 55 s), which is essential to ensure outstanding adhesion. At the same time, the shear adhesion failure temperature (SAFT) increased from 80 to 160°C, due to the formation of the chemical network (Fig. 4). 

Figure 9.5 shows the effect of adding two different amounts of a glycerolpropylsilyl silica dispersion, 10 wt% and 20 wt% respectively as dry SiO on dry resin, on the Persoz hardness. The system used in Figure 9.5 is a two-pack system (2-k) with a total solids (TS) of 35 wt% [60]. 

In the Persoz hardness test [61], the hardness of a coating is measured by determining the damping time of an oscillating pendulum. The pendulum rests with two stainless steel balls on the coating surface. When the pendulum is set into motion,... 

Figure 93 Persoz hardness as function of coating composition and drying time in days (d) at ambient temperature, silica dosage 10 wt% and 20 wt% SiO, respectively, on dry resin [60].

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