Varnishes and Surface Coatings: Additives

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Authors: James Bourdeau, Richard Wolbers
Date: Submitted January, 1996
Compiler: Wendy Samet

Phenolic Antioxidants, Stabilizers, and UV Absorbers

A variety of antioxidants is currently available. The additives are added in low concentrations (typically 0.5–3% to the weight of the resin) to inhibit degradation processes (de la Rie 1988a). What follows here is a brief discussion of stabilizers that conservators may add to varnishes with reference to some that are no longer used.

Hindered Amine Light Stabilizers

Stabilizers of the hindered secondary or tertiary amine type are known by the acronym HALS (Hindered Amine Light Stabilizers). Because of their superior effectiveness as radical scavengers in the presence of “light,” HALS have largely supplanted phenolic antioxidants as stabilizers for damar and mastic picture varnishes used in the conservation of paintings. Their mechanism of stabilization is complex and not fully understood. Hindered amines form nitroxyl radicals which regenerate themselves in a cyclical process. HALS are not consumed by their stabilizing reaction. This makes them more suitable in picture varnishes where the additive must function indefinitely in order to increase the lifespan of a coating. Tertiary amines are, however, transformed through an oxidative process into a different species, the nitroxyl radical (N - R + O₂ → NO ·), which itself actually does the work of polymer stabilization. It does this in two ways, first, by reacting with free radicals in binder impurities of the binder/polymer itself to form an amine-ether nonradical species and, second, by this latter compound's terminating of peroxy radicals (R'OO · → 6 R'OOR) found in the binder. This last reaction regenerates the nitroxyl radical to allow it to continue its radical scavenging.

(de la Rie 1988a)

To summarize the steps described above:

1. nitroxyl radical formation

2. free radical scavenging

3. termination of peroxy radicals

Product Example: Tinuvin® 292, Ciba-Geigy

a) Shelf Life and Longevity

Although Ciba Ltd. (Canadian subsidiary of Ciba-Geigy) recommends that the shelf life of Tinuvin® 292 is 36 months, this suggestion is aimed at the retail industrial user. Tinuvin® 292 should be stored in a nonreactive plastic or glass container, in a cool dark environment. I have achieved acceptable stabilization of damar films using Tinuvin® 292 which was over six years old. The effectiveness of old batches of Tinuvin® 292 cannot be quantified without a certain amount of analytical expense or experimental difficulty. Thus, it is probably worthwhile to purchase this additive in very small quantities and to discard batches which are between three and ten years old depending upon storage history (Tennant, A. (Ciba Limited, Toronto), 1995, personal communication).

Resins and stabilizers degrade more rapidly in solution, so varnishes with Tinuvin® 292 should be discarded after one month.

b) HALS Treatment Example

As recommended in the literature, Tinuvin® 292 should be used to stabilize coatings like Regalrez® 1094, Arkon® P-90, and MS2A®. It can be used to stabilize damar and, to a less effective degree, mastic varnishes in an environment free of ultraviolet radiation. HALS can be used to stabilize numerous varnish polymers and resins with less than optimum aging characteristics; however, each new application should be thoroughly tested first. It is difficult to add stabilizers to proprietary varnishes because the resin concentration must be known to calculate the additive. As reported by de la Rie, several observations based upon experimental evidence can guide our use of HALS.

(1) Damar can be stabilized only in an ultraviolet-free environment (de la Rie and McGlinchey 1989).

(2) Laropal® K80 is not effectively stabilized by HALS (de la Rie and McGlinchey 1990b).

(3) MS2A® was stabilized only for a limited time with 0.5% Tinuvin® 292 (de la Rie and McGlinchey 1990b).

(4) Regalrez® 1094 and the experimental aldehyde resin tested required as little as 0.5% Tinuvin® 292 for good stabilization although concentrations up to 2% are recommended (de la Rie 1993).

(5) Solubility changes in Arkon® P-90 are much less pronounced with the addition of 2% Tinuvin® 292 than in the unstabilized polymer. Arkon® P-90 stabilized with 2% Tinuvin® 292 remained unchanged for up to 6,000 hours of aging in a Weather-ometer, whereafter some polarity changes occurred (de la Rie 1993).

Crosslinking was not observed in the materials above.

Tinuvin® 292 should be added to the Regalrez® 1094 solution to a concentration of no more than 2% weight/weight resin as outlined below. At higher concentrations, the additive could separate from the film as a distinct phase. The Tinuvin® 292, which is a rather viscous liquid, is added drop-wise, into the varnish mixture while the container is on a balance which measures to at least two decimal places.

Regalrez® 1094, 15% wt to wt solution (National Gallery of Art 1993a)

25% solution:

Regalrez® 1094	50.0 g (25% resin/solvent)
Tinuvin® 292	1.0 g (2% HALS/resin)
solvent	150.0 g
	201.0 g

Regalrez® 1094 and Kraton G rubber, 25% wt to wt solution (National Gallery of Art 1993a)

25% resin/polymer solution:

Regalrez® 1094	46.0 g (23% resin/solvent)
Kraton® G1650	4.0 g (2% rubber/solvent)
Tinuvin® 292	1.0 g (2% HALS/resin)
solvent	150.0 g
	201.0 g

Significant stabilization of damar films was achieved with a concentration of 3% Tinuvin® 292 behind an ultraviolet filter in a xenon arc Weather-ometer, with up to 6,000 hours of aging. The data projected, assuming reciprocity, virtually complete stabilization for up to 68 years at 1,000 lux with minor oxidative changes occurring thereafter (de la Rie 1990, 160–4).

Tinuvin® 292 should be added to a damar solution in a concentration of no more than 3% weight/weight resin as outlined below. At higher concentrations, the additive could separate from the film as a distinct phase.

Tinuvin® 292 stock solution, damar 25% wt to wt solution (National Gallery of Art 1993b)

Stabilized damar 25% solution

damar resin	50.0 g
Tinuvin® 292	1.5 g (3%)
solvent	150.0 g
	201.5 g

At smaller quantities, and without the use of an analytical balance, it is difficult to measure the exact amount of Tinuvin® 292. Instead, it can be added to the varnish solvent in a known concentration. The resin can then, in turn, be added to the Tinuvin® 292 solution to achieve the desired concentration. (See Measuring Tinuvin® 292 in Section IX. General Application Techniques.)

10% Tinuvin® 292 solution

Tinuvin® 292	10.0 g
solvent	90.0 g
	100.0 g

25% damar solution

damar resin	50.0 g
10% Tinuvin® 292 solution	15.0 g (3%)
solvent	90.0 g
	100.0 g

UV Absorbers

These additives are also called ultraviolet inhibitors. However, this term too easily confuses their function with that of antioxidants or other additives that modify the behavior of the coating in which they are dispersed. When added to a stable acrylic like Paraloid® B-72, ultraviolet absorbers normally interact only with ultraviolet and not with their polymer matrix; however, the ultraviolet absorbers discussed below have exhibited synergistic behavior when combined with HALS. The ultraviolet absorber makes the HALS a more effective stabilizer and the HALS makes the absorber less volatile under high temperature test conditions and less vulnerable to interaction with carbonyl groups. Their activity as excited-state quenchers has also been reported (Vink 1980; Tudos, Balint, and Kelen 1983, in de la Rie, 1988a). The ultraviolet absorbers, discussed below, can interact with polymer impurities or oxidative species present in their film matrix leading to their chemical consumption. They should only be used in a polymer which has few of these reactive sites, thus, their use is recommended in stable top coats of Paraloid® B-72. Their long-term effectiveness may be diminished by using other materials. Please note that UV absorbers will not protect a film from degradation. They can only screen a layer below (de la Rie 1988a).

a) Substituted 2-hydroxybenzophenones

The benzophenones are one of the most widely used ultraviolet absorber/stabilizers for not only are they strong absorbers in the 290–400nm range, with some variations in their cut-off limits, but they also appear to act as radical scavengers at ambient temperatures (Tamblyn and Newland, 1965). In a stable matrix with few oxidative sites their primary activity is ultraviolet absorption. Their absorption mechanism is a self-regenerating one involving the molecule “jumping” to a higher energy state when ultraviolet radiation is absorbed, then rapidly dissipating this energy as harmless heat as it returns to its un-excited state. Technically this process involves a keto-enol tautomerism.

Product Example - Univil 400, 497, 490, BASF

(may form colored degradation products)

b) Substituted 2-hydroxybenzotriazoles

This compound also functions via an excited state/ground state mechanism involving the internal hydrogen bonding of the phenolic hydrogen to a nitrogen of the triazole ring. The usual theoretical model involves internal hydrogen bonding of the 2-hydroxyl group with the adjacent nitrogen in the excited-state, followed by the a mild exothermic conversion back to an unexcited state. The molecule regenerates and, in theory, goes on continually absorbing and dissipating ultraviolet energy which explains their long-lived effectiveness.

This class of compounds will function in limited applications as radical scavengers by forming phenoxy radicals; however, they apparently are consumed rapidly in polyolefins like damar due to their reaction with the ketonic groups present in films of damar, mastic, or polycyclohexanone resin (Vink, 1980 in de la Rie 1988a, 9–22). They are most effectively used in a two-varnish application where the ultraviolet-sensitive varnish layer is screened by the benzotriazole in a stable top coat. And certainly substituting another class of additives which are not ultraviolet absorbers, such as an antioxidant or a HALS, in the top coat, will protect neither a damar nor mastic varnish, nor any ultraviolet-sensitive material beneath, from autoxidation when exposed to ultraviolet.

Of all of the ultraviolet absorbers tested to date, Tinuvin® 327 (Ciba Ltd. AG, Basel, Switzerland) appears to have had the best performance, i.e., longevity, ultraviolet absorption cut-off at 400nm, and the least ultraviolet-induced yellowing of top-coated damar test samples (Bourdeau, 1988). This compound is a substituted 2-hydroxybenzotriazole and it is recommended in the application below.

Product Example - Tinuvin® 327, 328, Ciba-Geigy

c) Shelf Life and Longevity

Because Tinuvin® 327 is supplied in a powder form, it appears to have a long shelf life when stored in an opaque, tightly-sealed container. Acrylic films cast with 10-year-old Tinuvin® 327 in a 3% concentration showed identical spectral curves, i.e., ultraviolet absorption, as did films cast with fresh batches of the additive. Five-year-old Tinuvin® 1130, an ultraviolet absorber which is supplied in a solution of ethylene glycol, also had the same spectral curves as those of fresh batches of Tinuvin® 1130 when cast in films of Paraloid® B-72. The shelf life of these products appears to be fairly long.

Films cast from two-year-old solutions of Paraloid® B-72 and 3% Tinuvin® 327 had almost identical spectral curves to those of fresh solutions. The solutions were stored in ultraviolet-opaque, brown glass containers. Ultraviolet absorption by the old solutions did not decrease, thus this ultraviolet absorber additive appears to have a fairly long shelf life when in a solution of Paraloid® B-72 and xylenes. However, films cast from this old solution showed decreased ultraviolet-visible transmittance from 395 to 430nm, suggesting that it had yellowed slightly in the bottle. It is not known whether the solvent, polymer, or additive, or their interaction, causes this slight color change which is not distinguishable to the naked eye when this old solution was applied in a thin film. A good rule of thumb is to avoid using old polymer solutions and to mix fresh batches when required.

d) Treatment Example using a UV Absorber

UV Barrier Acrylic Top Coat Solution

10% Paraloid® B-72 solution

Paraloid® B-72	40.0 g
solvent (xylenes)	360.0 g
add-Tinuvin® 327	1.2 g

This varnish recipe may be used over a damar varnish layer in an environment with high ultraviolet light levels (the UV absorber is not necessary in a UV filtered environment). If too much of the additive is used, the picture will appear black in an ultraviolet light examination. For an indepth description of this technique, see Section V.B.I. Paraloid® B-72, in “Special Application Techniques and Tricks of the Trade,” page 144.

Phenolic Antioxidants

Historic Note: Phenolic antioxidants are thermal stabilizers and protect against thermally induced autoxidation at room or elevated temperatures. They generally perform poorly when light is present due to the poor light stability of the additives themselves. They are therefore of limited use when photochemical degradation reactions play a role (de la Rie 1988a). This is a class of compounds used in the work of Raymond Lafontaine, who tested 19 types of antioxidants and settled on Irganox® 565 as a stabilizer for damar (Lafontaine 1979, 14–22). In his research, the samples were heat-aged only, and it was eventually determined (after later aging in a Weather-ometer which included ultraviolet light) that the addition of Irganox® 565 led to increased insolubility and yellowing (Lafontaine 1979b, 114–71).

Product example - Irganox® 565, Ciba, Ltd.

Conclusion

Varnish additives that are either radical scavengers or ultraviolet absorbers are finding common use not only in conservators' varnish mixtures but also in commercial coatings which are available to the conservator and nonconservator alike. Other sections of this catalog discuss the use of such additives, antioxidants, stabilizers, and ultraviolet absorbers, and knowing the limits of an additive's performance should help conservators decide when one or another of these compounds could be of use in their own conservation treatments. (For more information, see de la Rie 1988a.)

Matting Agents

Generally, a flatting (matting) agent or agents used in the present context will be defined as an additive (or additives) which reduce the gloss or angular sheen from a varnish or clear coating film. In a commercial sense, these additives have often been comprised of metal soaps, specifically, aluminum stearate (although calcium, magnesium, and zinc stearates have also found some limited use in this regard). Optically, the inclusion of a flatting agent will reduce the specular gloss of a transparent coating by either of two strategies: creating surface irregularities that scatter light from the surface of the dried varnish film, and/or by making the overall dried film less transparent and therefore less prone to second order internal reflections.

Flatting agents such as aluminum stearate must necessarily be extremely and finely dispersed in, and substantially wetted by, the solvent system that the varnish is ultimately dissolved in. In a commercial sense, grinding mills are used to create flatting agent/solvent pastes that are then milled to produce the requisite dispersion, and then the milled paste is added as a premix to the finished commercially flatted varnish. In the conservation lab, it is difficult to produce the degree of dispersion required for a stable, flatted preparation. A second problem often encountered in the handling of flatted commercial varnishes, is the “solvent shock” that may occur if commercial flatted varnishes are haphazardly blended in the lab with solvents that disrupt the wetting of the fatty acid (stearate) moiety by the original commercial solvent blend.

Metallic soaps are sometimes used with silicas or other inorganic materials to increase the degree of flatting achieved by the metallic soap alone. Metallic soap dispersions are generally constructed so that narrow particle size range (on the range of 5–20μ/particle) is produced in milling. Again, it is difficult to produce these kinds of uniform and small size dispersions in the normal conservation lab setting. Milling times of 3–5 hours are typical of commercial preparations using 5–10% weight/volume aluminum stearate-to-milling solvent ratios in the preparation of premix flatting pastes. Final concentrations of commercial flatting agents in dried films typically range from about 2–6% weight/weight. When measured as a standard 60° gloss value (the amount of light reflected off of the dried flatted film surface at a 60° to normal angle), reflection efficiencies of between 50–80% (based on a 96% efficiency in the unflatted film) are typically achieved. Some “stir in” types of flatting pastes that have been premilled are commercially available, but have yet to find application in conservation. It should be noted that heating tends to increase the solubilization of metal stearates in carrier solvents (and, for that matter, varnish films), and can disrupt their ability to efficiently flatten films, as particle sizes drop below the optimal or initial size range for light scattering.

The refractive indices of flatting agents are critical to their ability to render varnish coatings relatively less transparent. Aluminum stearates typically range between values of 1.35 and 1.55; at the lower ends of this range is a concomitant increase in transparency in the films produced.

Parenthetically, the inclusion of metal soaps in varnish films can render them less water-permeable and more durable to washing.

References

Bourdeau, J. 1989. An Examination of the barrier properties of selected ultraviolet absorbers within acrylic surface coatings. In Papers presented at the fourteenth Conservation Training Programs Annual Conference, 1988. Buffalo, N.Y.: Buffalo State College:41–61.

Bourdeau, J. 1995a. Practical considerations in the use of acrylic UV barrier top-coats for the protection of dammar picture varnishes. In Arbeitsgemeinschaft der Restauratoren, Annual Conference. Bremen:AdR.

Bourdeau, J. 1995b. The Use of UV absorbers in acrylic top coats as a remedial treatment for dammar varnishes containing Irganox 565. In Varnishes: Authenticity and permanence. Proceedings on audio cassette Ottawa: Department of Canadian Heritage - Canadian Conservation Institute: Cassettes 6–7.

Ciba Limited, Toronto, Ontario. 1995. Conversation with Mr. Andrew Tennant, Chemist, Ciba Limited, Toronto, Ontario.

de la Rie, E.R. 1988a. Polymer stabilizers:A survey with reference to possible applications in the conservation field. Studies in conservation 33(3):9–22.

de la Rie, E.R. 1988b. Photochemical and thermal degradation of films of dammar resin. Studies in conservation 33 (2):53–70.

de la Rie, E.R. 1988c. An Evaluation of Irganox 565 as a stabilizer for dammar picture varnishes. Studies in conservation 33 (3):109–14.

de la Rie, E.R. 1993. Polymer additives for synthetic low-molecular-weight varnishes. In ICOM Committee for Conservation, 10th triennial meeting, Washington, DC, USA, 22–27 August 1993, Preprints, Vol. 2:566–73.

de la Rie, E.R. and C.W. McGlinchey. 1989. Stabilized dammar picture varnish. Studies in conservation 34(3): 137–46.

de la Rie, E.R. and C.W. McGlinchey. 1990a. The effect of a hindered amine light stabilizer on the aging of dammar and mastic varnish in an environment free of UV light. In Cleaning retouching and coatings: Technology and practice for easel paintings and polychrome sculpture. Preprints of the contributions to the Brussels Congress, 3–7 September 1990. J.S. Mills and P. Smith, eds. London: International Institute for Conservation of Historic and Artistic Works: 160–4.

de la Rie, E.R. and C.W. McGlinchey. 1990b. New synthetic resins for picture varnishes. In Cleaning retouching and coatings: Technology and practice for easel paintings and polychrome sculpture. Preprints of the contributions to the Brussels Congress, 3–7 September 1990. J.S. Mills and P. Smith, eds. London: International Institute for Conservation of Historic and Artistic Works: 168–73.

Feller, R.L. and M. Curran. 1970. Solubility and crosslinking characteristics of ethylene/vinyl acetate copolymers. Bulletin of the IIC-American Group 11(1):42–5.

Lafontaine, R.H. 1979a. Decreasing the yellowing rate of dammar varnish using antioxidants. Studies in conservation 24(1):14–22.

Lafontaine, R.H. 1979b. Effect of Irganox 565 on the removability of dammar films, Studies in conservation 24(4):179–81.

Michalski, S. 1995. Yellowness and removability: How much change? How fast? How important? In Varnishes:Authenticity and Permanence. Proceedings on audio cassette. Ottawa: Department of Canadian Heritage - Canadian Conservation Institute: Cassette 6.

Newland, G.C. and J.W. Tamblyn. 1965. Metal-organic stabilizers and antistabilizers for polyolefin plastics. In Journal of Applied Polymer Science 9: 1947–1953.

Tudos, F., G. Balint, and T. Kelen. 1983. Effect of various photostabilizers on the photo-oxidation of polypropylene. In Developments in Polymer Stabilization - 6, G. Scott, ed. London: Applied Science Publishers: 121–72

Vink, P. 1980. Loss of UV stabilizers from polyolefins during photo-oxidation. In Developments in Polymer Stabilization - 3, G. Scott, ed. London: Applied Science Publishers: 117–38.

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