Strength and Chemical Resistance of Composites Based on Epoxy Resins, Filled With Gypsum in the Original and Waterhardened Forms

This paper reports the results of studying epoxy compositions with gypsum taken in the form of dispersed powders in the original and water-hardened form. The exact pattern has been shown in the way the introduction of a gypsum additive in the amount of 50 % by weight affects the strength, chemical stability, and morphology of the composites.

Under conventional heat treatment (60‒110 °C) of the hardened composites, the maximum stress at compression σm and the elasticity module at compression Ес, as well as wear resistance, decrease after the introduction of gypsums (of both types). At the same time, after a hard (destructive) heating at 250‒260 °C, the elasticity module Ес of the hardened composites increases. The maximum stress at compression σm is also increased. The same applies to the wear resistance, which grows especially noticeably after 250 °C.

The micro-hardness after filling is prone to increase but the fragility of epoxy-gypsum composites does not make it possible to measure it when a punch (a steel hemisphere) penetrates it deeper than 20 µm. However, after the heat treatment at 250‒260 °C, the unfilled polymer, on the contrary, is embrittled while the filled ones are plasticized, thus showing a high micro-hardness at significant (30‒50 µm) immersion.

The composites with gypsum, in contrast to the unfilled ones, do not disintegrate in acetone and retain integrity at any aging duration (up to 75 days and beyond). In this case, the original gypsum produces a composite with less swelling in acetone than the hardened gypsum. Based on the data from atomic-strength microscopy (ASM) microscopy, the morphologies of the non-filled composite, the composites with the hardened gypsum and original gypsum are different. The original gypsum forms a composite with a more pronounced (possibly crystalline) filler structure; the morphology for the hardened composite reflects the distribution of inert particles; for the unfilled composite (H-composite), only pores are visible against the background of a relatively smooth relief.