2.4. Rheological Analysis

24 h after the specimens were cleaned using a water jet washer, the authors conducted rheological tests in the form of relaxation tests involving stresses determined during the uniaxial compression test. Specimens were fixed between universal testing machine discs, and then compressed at a rate of 1 mm/s, to a strain value of 10% of the specimen height (as per standard ISO 3384-1:2019 Rubber, vulcanized or thermoplastic—Determination of stress relaxation in compression—Part 1: Testing at constant temperature). Next, for a period of 60 min, the specimens were compressed while maintaining a constant strain value (10%—0.63 mm). After taking the measurements, the authors determined the stress relaxation value as per Equation 1 and calculated standard deviation—SD (three samples for each type).

where:

σ_o—initial stress; σ_t—stress after predetermined relaxation time t.

In the course of further description of relaxation measurements, the relative decrease in the compressive stress (1) will be called the stress relaxation index.