Resin acids and Rosin acids, esters with diethylene glycol

Endpoint summary

Additional information

The category of Rosin Esters consists of rosin which has been esterified with alcohols, typically methanol, ethylene glycol, diethylene glycol, triethylene glycol, glycerol and pentaerythritol. Resin acids are the predominant components of rosin (>85%). Resin acids are composed of three skeletal classes of tricyclic carboxylic acids which share similar structure, but vary in the position of the double bonds and methyl groups. Hydrogenated rosin is implicitly included in the definition of rosin as disproportionated rosin which is a combination of hydrogenated and dehydrogenated rosin naturally produced when rosin is heated. The category therefore also includes the hydrogenated rosin forms of these substances. Due to the reactivity of resin acids, dimers can be formed. These rosin dimers are also known as oligomers or by the trivial name of polymerised rosin.

As each member of this category is a UVCB, the resin acid components will vary in type and proportion.

The number of ester bonds that can be formed is driven by the alcohol. Methanol can form mono-esters, ethylene glycol, diethylene glycol and triethylene glycol can form mono- and di- esters, glycerol can form mono-, di- and tri-esters and pentaerythritol can form mono-, di-, tri- and tetra-esters. In each case, the esterification reaction results in a UVCB containing esters with varying numbers of ester bonds formed by the reaction of the various resin acids with the alcohol.

A number of aquatic toxicity studies have been performed on Rosin Esters with varying results. Many of the historic studies utilized techniques that attempted to increase the solubility of the substances for testing, such as the use of solvents, ultra-sonication, etc., or were conducted by dilution of Water Accommodated Fraction (WAF) instead of preparing WAF from several loading rates. These studies are not believed to be reliable indicators of inherent toxicity, even though they show EC50 values > 166 mg/L. These studies have been given Klimisch 3 scores (not reliable).

More recent studies have utilized WAF techniques according to current guideline recommendations, which are believed to be more reliable. GLP-compliant, guideline studies are available for fish, Daphnia and algae (Harlan Laboratories 2010 and 2014, Inveresk 2001a-e).

Based on the available ecotoxicity data, substances in the Rosin esters category with lower molecular weights are more toxic to aquatic organisms than those with higher molecular weights. This is likely to be due to the lower molecular weight substances being more soluble, whereas higher molecular weight substances are poorly soluble and less bioavailable. 

The substances with the lowest molecular weights in the category are Resin acids and rosin acids, Me esters and Resin acids and rosin acids, hydrogenated, Me esters. These two substances are considered to be the low molecular weight members of the category and read across between these two substances is used to complete REACH ecotoxicity endpoints, and to determine the classification of these substances. The lowest EC50 available for these substances is 27 mg/L, from a Daphnia study with the test material Resin acids and rosin acids, hydrogenated, Me esters (Inveresk 2002). This result is read across to Resin acids and rosin acids, Me esters. Studies with fish and algae determined EL50 values above the highest concentration tested. 

For higher molecular weight esters (with molecular weights higher than for the Methyl esters), no effects were seen at the limit of solubility in the available acute ecotoxicity studies. Acute ecotoxicity studies are available that ‘bracket’ higher molecular weight category members. Studies are available for the substance resin acids and rosin acids, esters with ethylene glycol, which has the lowest molecular weight of any of the 'high molecular weight' substances, and for resin acids and rosin acids, esters with pentaerythritol and resin acids and rosin acids, esters with glycerol, the highest molecular weight members of the category. All these studies determine EL50 values above the highest concentration tested.

Due to the differences in ecotoxicity observed between low and high molecular weight esters, one set of PNECs has been derived to cover low molecular weight substances in this category and one set to cover high molecular weight substances.