Thermal paper is used broadly throughout the world for receipts, tickets, tags, and labels. The printed image is formed by heating the paper, which contains a reactive combination of leuco dye and acidic developer, such as BPA (Bisphenol-A) or BPS (Bisphenol-S). In the presence of heat, the developer protonates the dye to facilitate a color change from white to black. Testing shows that human exposure to BPA is widespread and that thermal paper is a source of exposure. Paper manufacturers and regulatory agencies, such as the U.S. Environmental Protection Agency, expended significant effort to find replacement chemical developers and no suitable replacement was identified. A new thermal paper technology that eliminates chemical developers and dyes present in current thermal paper is presented herein. Air-filled, reflecting spheres are chemically produced from otherwise clear polymers by precise control of particle size and void diameter. A layer of these polymer spheres effectively mask or hide an underlying colored layer. Exposure to heat collapses the voided particles making the colored layer visible. The resulting paper is compatible with existing thermal printers and, unlike conventional thermal paper, creates an irreversible image. The synthetic white pigment has an outer shell comprised of hard, high Tg polymer, and a hollow core. It is an engineered structure that, because of the particle design, transforms a clear polymer into a light reflecting white pigment. It is analogous to snow hiding a black road. Each snowflake is clear ice but due to the size and interaction of the collection of particles, a layer of snow can completely obscure even an asphalt road, making it appear white. The degree of light scattering in opaque polymers results from the refractive index differential between the air void and the shell. The void size is carefully controlled to facilitate optimal scattering ability and consistency. Thermal printing is the most efficient printing mechanism from the perspective of material consumption, printer size, electrical consumption and speed. Due to the irreversible imaging, this new thermal paper could substitute inefficient and less environmentally friendly printing solutions such as (heavy metal containing) laser printing and thermal film printing.

Voided polymer imaging technology for thermal printing

255th National Meeting (2018)

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