LETTERS TO THE EDITOR
1.1 To the Editor:
When I wrote my brief note regarding the confusion among members of the AIC in the usage of three terms in adhesive technology (“On Hot-Melt, Heat-Seal and Hot-Set Adhesives,” JAIC 18 (1978), 44–45) I did not expect it to develop into a debate on the merits of the adhesives themselves. Since it did, I welcome the development, because it shows an acute need to further familiarize the profession with certain facts which, if overlooked, may add disaster to confusion. Perhaps a special column should be introduced for questions and answers on adhesives.
I am referring to Bernard Rabin's letter in reply to my above mentioned note (JAIC 18 (1979), 129). He ascribes to Beva, as well as to the PVA adhesive, qualities which are contrary to those published by the respective formulators and manufacturers. Mr. Rabin states that “When heat is applied, AYAA and AYAC become viscous with enough flow to reach into the recesses of the linen and grab hold of the outer layer of threads without penetrating the fibers. Because of this flow quality, it is misleading to couple PVA-AYAA, AYAC with Beva.”
This paragraph contradicts the published data of the manufacturer. Union Carbide states that “Bakelite AYAC flows at 200°F” (Product Standards Bakelite Vinyl Acetate Resin—AYAC, Union Carbide, F-45670, 1975). According to the National Paint & Coatings Association the softening point of AYAC is 90°C (194°F) and that of AYAA—150°C (302°F).
When two PVA's of different melting points are mixed, the resultant melting point is proportional to the percentages of the materials in the mixture. Since Mr. Rabin's mixture contains AYAA and AYAC in equal parts, the softening point is exactly in the middle, that is 120°C (248°F).
A Hot Melt adhesive is by definition one which is applied to the surfaces to be bonded in molten form. The PVA mixture of AYAA and AYAC is not applied to paintings in molten form, and attempts to do so would destroy any painting. Among conservators it is considered bad practice to heat an oil painting above 65–70°C (150–160°F). Therefore, at the accepted, or safe range of lining temperatures for oil paintings, Rabin's PVA mixture is still solid, not a Hot Melt, and therefore the flow referred to in his letter is negligible. On the other hand, the nap-bond behavior which he describes in typical of all the heat-seal adhesives; otherwise they could not attach themselves to irregular surfaces.
Although the flow of Rabin's PVA adhesive is minimal at the acceptable lining temperatures, the adhesive might be tacky and give under pressure. This brings us to another, important point: Union Carbide recommends a pressure of 60 lbs/square inch for the PVA adhesives to be effective. This is four times the maximum pressure available on the vacuum hot table and about ten times what a painting can safely take. Although PVA has been used by conservators for many years, it is unfortunate that Mr. Rabin never published any tests he may have performed on the mixture which he formulated for lining. The only printed information is the results of the research conducted by Rustin Levinson of the Metropolitan Museum of Art in New York which concluded: “For the 50:50 PVA mixture—toluene adhesive the peel strength was good but the tensile strength was too low” (That means it is brittle—G.B.), in ‘A New Method for Strip Lining Easel Paintings’ (ICOM, 5th Triennial Meeting, Zagreb, 1978, 78/2/8).
Beva is an adhesive which has been extensively tested. The tests and their results have been published in the professional literature and its usage treated in more than thirty-five articles by various authors in the past eleven years. In my reports on tests of Beva, I repeatedly stated that at the temperature of about 85°C (185°F) Beva adhesives melt and are absorbed by the materials to which they are applied: “Beva 371 was formulated especially for the impregnation of paintings” (“Formulating Adhesives for the Consolidation of Paintings” in ‘Conservation of Paintings and the Graphic Arts,’ 5th IIC Congress, Lisbon, 1972, p. 616, Xerox copy enclosed). Application at this temperature might result in the same type of staining as is caused by the application of wax hot-melt adhesives. On the other hand, it is easy to lower the melting point of Beva adhesives: “Its (that of Beva 371) activation temperature of 60°C can be temporarily lowered to 40–50°C with small amounts of solvents” or waxes of low-melting point (Ibid, p. 616, also “Consolidation of Delaminating Paintings,” ICOM, 5th Triennial Meeting, Zagreb, 1978, 78/2/1).
To sum up: Beva has a lower melting point, is softer and more elastic, and has lower viscosity than the PVA adhesive, AYAA, AYAC.Gustav A.Berger