The stabilization effect of two flavonoid type natural antioxidants, silymarin (Si) and quercetin (Q), was compared in polyethylene (PE). Additive concentrations changed between 0 and 500 ppm in several steps and 1000 ppm Sandostab PEPQ phosphorus containing secondary stabilizer was also added to each compound. Stabilization efficiency was determined by changes in functional group (vinyl, residual PEPQ) content, melt flow rate (MFR), oxygen induction time (OIT), color and the consumption of the secondary antioxidant during multiple extrusions. The results showed that silymarin is a much less efficient stabilizer in polyethylene than quercetin. The consumption of vinyl groups is faster, melt flow rate and residual stability is smaller in its presence. Silymarin contains less active phenolic hydroxyls than quercetin, but comparison on equal molar basis also shows the inferiority of the compound. The difference can be partially explained by the larger bond dissociation enthalpies of the hydrogens in silymarin, but this antioxidant also accelerates the consumption of the phosphorous secondary stabilizer that must contribute to its smaller efficiency as well. DSC measurements indicate the interaction of the two compounds probably leading to the faster consumption of the phosphorous antioxidant and poor stabilization. Unlike quercetin and dihydromyricetin, the flavonoid type natural antioxidants studied earlier, silymarin is not a good candidate as stabilizer for practical applications.

The efficiency of the natural extract, silymarin, was compared to its main component, silybin in the processing stabilization of polyethylene. The two compounds were applied as primary antioxidants in the concentration range of 0–500 ppm in combination with 1000 ppm Sandostab PEPQ phosphorous secondary stabilizer. The efficiency of the stabilizers was determined by the measurement of the concentration of unsaturated groups, the amount of residual PEPQ, melt flow rate (MFR), oxidation induction time and color of polymer samples taken after consecutive extrusion steps. The comparison showed that at large concentrations, the extract is more efficient than the pure compound; the vinyl group content and MFR of the polymer are preserved more in the presence on the extract than with silybin. The residual stability of the polymer containing silymarin is also slightly better at the same additive content than that prepared with silybin. Larger efficiency is explained by the smaller bond dissociation enthalpies of the most active phenolic hydroxyl groups of some of the components of the extract. The larger solubility of silymarin resulting from its amorphous character and the presence of the accompanying components of the extract also contributes to its better efficiency. At small concentrations, silymarin proved to be inferior to silybin, which was explained by the interaction of the components. The use of the extract seems to be more advantageous because it is more efficient and significantly cheaper than its pure main component. On the other hand, the stabilizing efficiency of silymarin and the related compounds is inferior to other flavonoids like quercetin, dihydromyricetin or rutin.