Oil-repellent microvoid-imaging material

Claim 1

A self-supporting sheet material which is adapted to being provided with markings by the application of a colorless liquid of predetermined volatility, comprising in combination: a. a self-supporting dark-colored base sheet having a face side and a back side and b. bonded over each side of said base sheet, a diffusely reflective opaque white to pastel microvoid-containing layer comprising an organic polymer having a refractive index in the range of about 1.3 to 2.2, c. present at least at the exposed surface of the layer on only the face side of the base sheet, an oleophobic fluorochemical which prevents the microvoids at the face side from being readily penetrated by oil and grease, so that the microvoids at the face side of sheet material can be penetrated and transparentized only by polar liquids or liquids having a low surface tension, such penetration effecting local transparentization of the face side layer to expose the dark-colored base, whereas the microvoids at the back side of the sheet material are readily penetrated by both polar and non-polar liquids having either low surface tension or high surface tension, whereby potentially transparentizing contamination of the layer on the face side of said sheet material, caused by contact with oil and the like, can be removed by promptly placing the face side layer in contact with the back side layer of another portion of the sheet material, the contaminating liquid being absorbed into the back side layer of the sheet material.