Thermal paper is often contacted in our daily life, but have you ever found that as long as there is a slight stroke of your fingernail, a mark will appear on the thermal paper, why is this? Can you write without a pen? Let’s follow Guanwei to find out!
A: In order to solve this problem, two things need to be clarified: one is why thermal paper develops color when exposed to heat, and the other is why scratching paper can generate heat.
First, the principle of color rendering is explained. Ordinary thermal paper consists of a paper base layer as a substrate and a thermal color developing layer that plays the main role. If a protective layer is added on the thermal coating, a three-proof thermal paper with better waterproof, oil-proof and anti-friction properties is obtained. It can be divided into physical type and chemical type according to the color development principle of thermal paper. The difference is whether the color development process contains chemical reactions.
Physical thermal paper is divided into melt transparent type, melt transfer printing and thermal sublimation type, etc., which mainly use the physical state transition of thermal materials after heating. For example, the melting and transparent type is coated with a non-transparent wax layer on the black base paper. After local heating, the wax-like material melts and becomes transparent, showing the black color of the base.
Chemical thermal paper is mainly divided into two types: metal compound chromogenic thermal paper and leuco dye two-component thermal paper. The commonality is that the components used as reactants are fused as a coating. After local heating, the solid substance becomes a molten state and starts a chemical reaction, thereby developing color. For example, the reactants of metal compound color thermal paper are fatty acid-based heavy metal salts and reducing agents, while the reactants of leuco dye two-component thermal paper are leuco dyes and color developers containing internal alicyclic rings.
“Paper can generate heat” utilizes the principle of frictional heat generation. According to the formula.
It’s not hard to get the heat buildup while paddling. Where is the friction coefficient between the fingernail and the thermal paper, is the pressure, and is the relative displacement.
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