1 AN ACRYLIC PRECIPITATION METHOD

IN VERY PRELIMINARY TRIALS, I have observed that an acrylic precipitation consolidant can work well for strengthening some leathers. The acrylic preciptation method was described in some detail in 1982, in respect to test applications on wood, stone, and brick.10 Essentially, an acrylic monomer is mixed with a solvent and intiating materials and soaked into a porous substrate. Polymer is formed after impregnation; at present a thermally activated initiator is used which requires the impregnated object to be heated to about 40�C or higher. Evaporation of the consolidant during polymerization is prevented by wrapping the treated object in foil. The object is unwrapped after polymerization to allow the solvent to evaporate. A solvent is chosen in which the polymer to be formed will be insoluble: thus, the newly formed polymer is not drawn out toward the surface with the evaporating solvent.

The precipitation method may sometimes circumvent limitations inherent in other methods of using acrylics as consolidants: the higher viscosities of polymer solutions and their tendency to consolidate the surface of an object more than the core; and stresses that may occur when undiluted monomer is polymerized within an object. In addition, the precipitation method often seems to darken an object less than other polymeric treatments that impart similar strength, perhaps because the resin is deposited as a light-scattering gel or as particulates. The amount of monomer used, in relation to the solvent, can be varied over a wide range.

In earlier work on wood and masonry materials I used methyl methacrylate as monomer: the current trials on leather have been done with methyl acrylate, which forms a flexible polymer. The brand of monomer used contained 15 p.p.m. of p-methoxy phenol inhibitor. A “starting” formulation that seems promising is, by weight,

methyl acrylate

100

50

2

PercadoxR16N is bis [4-t-butyl cyclohexyl] peroxydicarbonate.11 Exxon IsoparRG is an industrial grade of mixed isomers of mostly C-10 and C-11 aliphatic hydrocarbons. Laboratory grades of decane or perhaps dodecane should work similarly. In previous work on masonry materials and wood, heptane was used, but there may be some advantage in using solvents more viscous than heptane, as the polymerization rate and the molecular weight of the polymer formed might be enhanced by higher viscosity.12