May 2000 Volume 22 Number 2

The Development of MS2A Reduced Ketone Resin in Painting Conservation

Historical Development¹

Ketone Resins based on condensation reactions involving methylcyclohexanone and/or cyclohexanone were first patented in the 1920s² and were produced commercially as AW2 (BASF, Germany) and MS2 (Howards, UK) from the 1930s. By the late 1930s several hundred tons per year of these resins were sold into the textiles and coatings industries. They were used particularly in a range of specialty paints and varnishes as light-fast additives which improved gloss and hardness.

The resins have a high solubility in low polarity solvents coupled with a high refractive index. This enables the formulation of high-solids, low viscosity varnishes which are fairly easy to apply by brush or spray and provide attractive optical properties. During the 1950s ketone resins were evaluated as a more stable alternative to dammar in conservation varnishes for works of art. Whilst fast drying rate was an advantage in many cases (though not all) the varnish films were found to break down even in museum conditions. A major problem associated with these resins is the tendency of the ketone groups and double bonds to degrade in light, resulting in yellowing and a loss of solubility over time.

In 1959 Howards, with the support of Garry Thompson, Scientific Advisor at the London National Gallery, developed a reduced form of MS2 in which the double bonds and ketone groups were chemically removed by reduction with borohydride. The reduced product, designated MS2A, was evaluated extensively as a conservation varnish and was enthusiastically accepted. Handling characteristics and stability were superior to MS2 and to the natural resin varnishes, and the optical appearance of the applied varnish was described a being beautifully soft and silky. From 1962 MS2A was produced by Howards specifically for the conservation of works of art.

As general demand for the unmodified ketone MS2 increased, the plant process was changed to increase production. The new MS2 product was not suitable for reduction to MS2A. In 1963 the BASF AW2 product was examined as an alternative source of ketone resin for reduction, and was found to be entirely acceptable. Because of its different chemical nature (being produced from methyl cyclohexanone admixed with cyclohexanone rather from methyl cyclohexanone as in the case of MS2) the reduced product was subtly different to MS2A in terms of its ease of application and appearance, and was designated MS2B. MS2B was produced commercially for paintings conservators until 1967. In 1967 the BASF AW2 ketone resin was replaced by a resin derived wholly from cyclohexanone, and known a Ketone N. The reduction product of Ketone N was useless as a conservation varnish. (The batch process for Ketone N was succeeded by a continuous process to produce Larapol K80.)

In view of the continuing demand from painting conservators for MS2A, Howards agreed in 1967 to install dedicated small scale plant equipment which utilized the traditional process and materials to provide small amounts of MS2A exclusively for art conservation. MS2A production could not be justified commercially. Despite attempts by Howards and their successors Laporte to phase it out in subsequent years, persuasion from a number of important sources ensured its continuance. When Howards' Ilford site was closed down in 1973, stockpiled MS2A was not promoted but was used to satisfy ongoing demand. This continued to 1983 when MS2A stocks were exhausted, and required Laporte to resume production from dedicated equipment at their Widnes pilot plant facility. Again, MS2A was not promoted, but was produced intermittently as required and sold at the cost price of production. This amounted to �22/Kg in 1984, rising to �375/Kg by 1992. In 1993 the Widnes pilot plant facility was closed down, and production equipment and know-how transferred to Linden Chemicals Ltd. (now Linden Nazareth). Linden Nazareth have promoted MS2A, and increasing demand has stabilized the price at Laporte's 1992 levels (approx. $600/kilo with shipping and customs as of 3/00). Work-in-progress will enable MS2B as well to be commercially available in the near future.

MS2A Structure and Properties

MS2A is the chemical reduction product of a complex mixture of methylolation and condensation products formed by reactions involving methyl cyclohexanone, methanol and their derivatives, e.g.,

"Typical" MS2A resin has a weight average molecular weight of 1800 and a number average molecular weight of 800, and will contain around 8 ring units per molecule. Low boiling, lower molecular weight impurities are removed during processing by steam and vacuum distillation, whilst ketone groups and double bonds are removed by chemical reduction. The purified product has the following properties. MS2A is produced as yellow granules having a high solubility (50%w/w) in mineral spirit (white spirit). The high solids, low viscosity varnish, which appears colourless once applied as a thin film, rapidly reaches gel (no flow) point. As a chemically reduced ketone resin, the main colour forming bodies have, in effect, been removed.

Accelerated aging trials have shown that MS2A can be additionally stabilized by the use of HALS stabilizers (Tinuvin 292, Ciba Geigy)⁵. Even under abnormal aging conditions and in cases where oxidative crosslinking is favored, the low molecular resin is unlikely to be come insoluble⁶. It has been reported that unstabilized MS2A varnishes applied to paintings almost 40 years ago have shown no signs of aging, and remain fully soluble in white spirit. Synthetic microcrystalline wax (Cosmolloid 80H) or bleached beeswax have been incorporated into MS2A varnishes to enable gloss to be modified across the extremes, from total saturation to almost unvarnished appearance. The addition of wax also provides a plasticising effect to MS2A varnishes, which have a brittle nature attributable to hydrogen bonding between the many hydroxyl groups present in the product. These aspects of MS2A varnishes are described more fully in the following section.

Preparation, Application, and Properties of MS2A varnishes

MS2A varnish recipes are commonly based on ones devised by Mr. Herbert Lank, reported in published articles and in personal communications ⁷,⁸. A comprehensive review of MS2A varnish formulations has been provided by Ms. Sarah Fisher in the Paintings Conservation Catalog⁹. A frequently used formulation is as follows¹⁰:

(A) Basic varnish for brushing or spraying: 50g MS2A resin broken down to small granules and suspended in a muslin bag (if desired) is dissolved in 110 mls Stoddards Solvent (white spirits to BS 245 having less than 20% aromatics content). The resin dissolves overnight, or more quickly with frequent stirring and a little heating. (The final conc. is 31% weight/final volume; 50g of MS2A plus 110 ml Stoddard equals a total volume of 160 ml. The Editor.)

This varnish may be used from a spray gun as prepared or diluted with white spirit or odourless kerosene (4 parts to 1 part diluent) to provide a more slowly drying varnish solution suitable as a brushed isolating layer. (25%)

(B) Basic matt varnish solution: 7 g Cosmolloid 80H microcrystalline wax (or bleached beeswax) is dissolved, with the aid of heating, in 150 mls white spirit, followed by the addition of 18 mls basic varnish solution type (A). The wax reduces brittleness in the dried varnish and enables the gloss to be varied. (4% wax and 3% MS2A)

Diluted matt varnish as finishing layer: 3 parts basic soln. (A) is mixed with 1 part basic matt varnish (B). (24% MS2A and 1% wax) This formulation diminishes the brittleness of the applied varnish without affecting the gloss. By increasing the proportion of (B) to (A) the matt effect is introduced and heightened. This is further controlled by adjusting the spray aperature and distance from the painting, and can be modified again by gently polishing the dried varnish with a soft cloth.

Open/working time. MS2A may be easily worked with a brush following application. Dried varnish may be reworked with additional varnish application by brush to build in uneven areas. The silky soft optical appearance is optimal in the thinnest coats.

Cleaning. Dust which may accumulate as surface film, often over a number of years and particularly on paintings frequently transported for exhibitions, is easily removed by gently washing with distilled water, followed by gentle polishing with a soft cloth. Varnishes most affected by dust accumulation have been reported to be those containing higher levels of wax.

Ageing. Accelerating ageing trials have shown that oxidation/ crosslinking can occur to induce slight yellowing and reduce solubility in white spirit¹¹. Such effects should occur much more slowly than in the case of natural or ketone resins, and should be futher reduced by the addition of 2% Tinuvin 292 (commonly used, particularly by conservators in the USA).

In practice, MS2A varnishes examined to date, almost 40 years after first application, and in the absence of Tinuvin 292, have remained unchanged to the naked eye in terms of gloss and clarity, and have remained fully soluble in white spirit with no evidence of discoloration in removed varnish.

References

1. Information taken from the archives of Howards of Ilford and Laporte Industries Ltd.

2. I. G. Farbenindustrie 1929. See A.W. Hill, Modern Plastics, 119, (1947).

3. MS2A Batch 15021. GPC determination carried out by R.A. Evans Solvay Intertox against polysterene standards using a MAXIMAC 820 GPC Analysis report.

4. "Typical" data as reported by R.de la Rie in Studies in Conservation 32, (1987), pp. 1-13.

5. de la Rie, E.R., McGlinchey, C.W. Cleaning, Retouching, and Coatings eds. J.S. Mills and P. Smith, Int. for Cons. of Historic Works, London, (1990), p. 168.

6. de la Rie, E.R. Stability and Function of Coatings used in Conservation, from Polymers in Conservation, eds. N.S. Allen, M. Edge, C.V. Horie, RSC Publication, (1992), p.75.

7. Herbert Lank, Personel comm. to Linden Chemicals Ltd. and to Sarah Fisher (see ref. 9). Lank, H. Chemistry and Industry, (July 1992), pp. 484-7.

8. Lank, H. Information presented in the 1972 Lisbon Conference, published 1976 (see ref. 9).

9. Fisher, S.L. from Paintings Conservation Catalog, (1997), pp. 81-91.

10. Based on information supplied by M. Twist, C. Booth-Jones, M. Roberts, R. Woudhuysen-Keller.

11. de la Rie, E.R., Shedrinsky, A.M., Studies in Conservation, 34, (1989), pp. 9-19.

(percentages in italics courtesy of C. Stavroudis)