MITRA Forum Question Details

There are resins in both formulations. Resin is the material is left to create the coating in either case. It is true that resins dispersed in an aqueous system have other additives to allow for the dispersion (surfactants, defoamers, etc).

The synthetic resin Paraloid B-67 can be dissolved in  mineral spirits to become a solvent-borne varnish or be dispersed in an aqueous system that would allow for initial application diluted in water but dries insoluble in water after drying. Both forms would contain a resin, the same resin in this case. Mowilith 20 and white glue both contain PVA resins.

I hope that makes sense.

I am only including MWs in the below for illustraion, not for you to force on non-science people.

Paraloid B-72 is very HMW 105,000 units in general.

Paraloid B-67 MSA is still HMW but not as much as above.  60,000 units in general. This is often mixed with F-10 (which helps diminish brittleness) is a HMW. 142,000 units in general. What is called MSA is often a mixture of these.

Laropal A-81 is LMW but I cannot find an exact number. I do see that it has a relatively broad molecular weight distribution. That is probably why no specific MW is listed.

Regalrez is super LMW under 900 units. Again, I am not sure if it is lower or higher than the above.

In general, LMW resins make varnishes that are more saturating since their LMW allows them to level out while drying. They do not  almost immediately gel up, preventing leveling. This leveling is a primary component in color saturation even beyond refractive index (think about how fully saturating a surface is when covered by a film or drop of water even though water has a very low refractive index compared to many pigments).

The high saturation of LMW may only be the case when used as solvent-borne varnishes as the other additives in dispersions might diminish the leveling. How much or how little I do not know. I will shoot the Golden people an email to see if they want to comment.

Laropal® A81 is a urea aldehyde resin with a refractive index of 1.50, an average Molecular Weight (Mw) of 3,566, and a polydispersity of 2.76.

Regalrez® 1094 is a hydrogenated hydrocarbon resin with an RI of 1.52, and 815 Mw.

Dammar is triterpenoid resin with a refractive index of 1.54, 1,463 Mw, and a polydispersity of 3.50.

Paraloid® B-67 is a poly(isobutylmethacrylate) resin with RI of 1.48, and 24,876 Mw.

Paraloid® B-72 is an ethylmethacrylate/methylacrylate copolymer with an RI of 1.49, and 75,455 Mw.

Source: E. René de la Rie, John K. Delaney, Kathryn M. Morales, Christopher A. Maines & Li-Piin Sung (2010) "Modification of Surface Roughness by Various Varnishes and Effect on Light Reflection", Studies in Conservation, 55:2, 134-143, DOI: 10.1179/sic.2010.55.2.134

​I do recomemnd using the number designations of these resins since there are several different, Paraoloid, Laropal and Reglarez resins used in commerce.

George is right about the number codes. There was another Laropal called K80 which was a poylcyclohexanone resin that had quite different qualities from A81. It was also used in the past as a picture varnish but is no longer being manufactured. It did become less soluble over time.

I neglected to add the 1094 to the Regalrez since it is the only one that is generally used in painting conservation, but there are a number of other resins that also use the Regalrez name but with a different number code. Precision is always a good idea.

​MSA is made of HMW resins. I also think it would be important to give the Laropal and Regalrez numbers, since there are alternative resins available from conservation suppliers (like Regalrez 1126) and artists might get confused. 

​Koo, since both polymer dispersion and polymer solutions are made of resins, I think using the term 'resin' exclusively for solutions might be confusion. Typically polymer dispersions are higher in molecular weight than solution polymers. Therefore solution varnishes can sink into paint layers more than water borne varnishes. Solvent borne varnishes with slower evaporation solvents tend to have better flow and leveling, which can also increase saturation and clarity. Polymer dispersions require lots of other stuff (particularly coalescing agents, but also surfactants, defoamers, anti-freeze agents etc.) that can affect the clarity and refractive index.

​Thanks, everyone, for your replies.  It's all helpful and informative.  Koo