2 PART I. ERASER COMPOSITIONS AND PROPERTIES

2.1 EXPERIMENTAL

Information on the composition of the Magic Rub, Pink Pearl, Kneaded Rubber, and Art Gum erasers and the Opaline Pad was obtained from product literature and from communication with scientists employed by eraser manufacturers (Table I). The infra-red spectra of the Magic Rub, Pink Pearl, Kneaded Rubber, and Art Gum erasers and Opaline Pad crumbs were obtained to confirm the presence of the organic constituents indicated by the manufacturers. A Perkin Elmer 283 B infrared spectrophotometer with infrared data station was used.

The Art Gum and Pink Pearl erasers and Opaline Pad crumbs were dissolved in a warm acetone/hexane mixture. The Kneaded Rubber eraser was dissolved in chloroform, and the Magic Rub eraser in ethyl acetate. The residues after solvent evaporation were either deposited onto AgCl plates or pressed into KBr plates.

All but the Opaline Pad were dry oven aged separately in a Precision Scientific Company Model 17 Oven at 100 � 2�C for 7 days. The organic constituent of the Opaline Pad is identical to that of the Art Gum eraser; this eraser was therefore used for aging experiments since volume changes could be more easily determined.

Differences in color, weight, volume, surface pH, and UV fluorescence were noted, along with any other changes that appeared significant. Colors, recorded using the Munsell system,6 were observed in a Macbeth Spectralight light box set at maximum brightness of the daylight setting. Dimensional changes were determined by measurement before and after aging with Spi 30–415 calipers, Dial type 6932. A Mettler H72 balance was used for weighing. Surface pH measurements were made using a Beckman Expandomatic SS-2 pH meter with an Ingold glass electrode. The UV fluorescence was observed in the Macbeth Spectralight light box at maximum intensity of the UV setting. The results of the observation of aged and unaged erasers and papers by UV fluorescence will be discussed together in Part II.

2.2 RESULTS

Communications revealed that the Art Gum eraser and Opaline Pad crumbs, as well as over 90 brands of gum erasers and two other brands of dry cleaning pads, are manufactured by one manufacturer.7 They are said to consist of vegetable oil vulcanized, or cross-linked, with sulfur bonds. Vulcanized vegetable oil is known industrially as factice. The IR spectra of Art Gum and Opaline Pad crumbs are identical to that of factice. Magnesium silicate is added to dry cleaning pads to facilitate movement of the eraser crumbs through the bag pores.7

Though additional ingredients were also reported, the spectra of the Pink Pearl and Kneaded Rubber erasers showed that factice is the major organic constituent. The presence of soluble material in the erasers corresponding to the known spectrum of factice suggests that substantial organic material in factice is not crosslinked. The Pink Pearl eraser is also said to contain rubber, antioxidants, softeners, pumice, and coloring agents. Less product information was available on the Eberhard Faber Kneaded Rubber eraser than on Faber-Castell's Kneadable Eraser. The composition of the latter is included here, though it is the Eberhard Faber Kneaded Rubber Eraser which was used and tested throughout this report. Like the Pink Pearl eraser, the Faber-Castell Kneadable eraser is said to contain factice, rubber, antioxidants, pumice, and colorants. Unlike the Pink Pearl, it is said to contain mineral oil as a softener to prevent curing, and CaCO3 as a filler.

The Magic Rub eraser proved to be significantly different in composition from the other erasers. The manufacturer reported its composition to be 30% by weight poly (vinyl chloride), 35% CaCO3 as a filler, and 35% dioctyl phthalate as a plasticizer. The spectrum of the Magic Rub is dominated by the dioctyl phthalate, suggesting that the poly (vinyl chloride) is crosslinked and hence insoluble.

The results of the aging behavior of the erasers appear in Table II. After oven aging at 100 � 2�C for 7 days, the Art Gum eraser lost 10% of its weight and 6.3% of its volume. The color change was extreme, from an ochre (Munsell 10 YR 7/8) to a deep brown (10 YR 3/4). The eraser emitted a pungent odor. The Kneaded Rubber eraser after identical aging lost 4% of its weight and 10% of its volume. It emitted a rubber odor and turned warmer in color, from Munsell 7.5 BG 6/0 to 5 GY 6/0. It also became firmer in texture than an unaged Kneaded Rubber eraser. The Pink Pearl eraser also emitted a rubber odor upon aging and lost 2% of its weight, but gained 23% in volume. The large decrease in density of this sample suggests that decomposition products from the oven aging are entrapped in the eraser network. The color grayed slightly, from Munsell 5R 7/8 to 5R 7/6, and the surface of the eraser took on a granular appearance. After heating, the Magic Rub eraser remained dimensionally stable. It did, however, take on a warmer, grayer appearance (Munsell 5Y 9/1 to 7, 5Y 9/2), become soft and sticky, and emit an odor. This eraser remained slightly more flexible and continued to emit an odor even after cooling to room temperature.

Table II AgingaBehavior of Erasers

Surface pH of the erasers was measured before and after oven aging. After oven aging, all erasers remained within approximately 1 pH unit of their original value. The Pink Pearl was the most alkaline. The unaged Magic Rub eraser was neutral and it became slightly acid upon aging. Both the Kneaded Rubber and Art Gum erasers were slightly acid both before and after aging.