We studied the development of adhesive bond strength for natural lignin adhesives. We tested several different adhesive mixtures made of lignin (L), hexamine (H) and hexamethylenediamine (HMDA). Based on preliminary testing, we selected the best two adhesive mixtures, L-H-HMDA and L-HMDA. We used L-H as the reference adhesive. The adhesive curing process was studied using the ABES method, in which we used different pressing parameters. The test specimens were pressed at four different temperatures, namely 125 °C, 150 °C, 175 °C and 200 °C. At each temperature, measurements were performed at ten different times from 15 s to 600 s. When testing with the ABES method, we found that the best results were achieved by the L-HMDA adhesive mixture at a temperature of 200 °C and a time of 10 minutes. We then performed a standard test for shear strength according to the SIST EN 205:2016 standard. In the standard test, we used the L-HMDA adhesive mixture that achieved the best results according to the ABES method. The pressing parameters were 200 °C and 15 minutes. The standard test was used to test whether the adhesive could be classified into the C1 application class (test specimens exposed to a climate of 20 °C and 65% humidity) and C2 (test specimens subjected to an additional 24 hours in cold water). The adhesive did not fall into either of these two classes, as it achieved shear strengths of 8.91 N/mm2 and 5.51 N/mm2, whereas it should have achieved at least 10 N/mm2 and 7 N/mm2.
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