Thermal properties
The transport of heat in paper determines how quickly paper reaches thermal equilibrium with its environment and how much energy is needed for that. Thermal properties are relevant in paper drying, supercalendering, some printing processes and thermal insulation. In all these applications, specific heat and thermal conductivity in the thickness direction of paper are important, but the in-plane thermal conductivity is usually not. In the drying process, the behaviour of the wet web is important. In this article, only the thermal properties of dry paper are discussed. Practical examples on the relevance of thermal properties are introduced in the following.
More discussion on the details related to thermal properties can be found in:
Mechanisms of heat transfer in paper
Specific heat and thermal diffusivity
Effect of furnish and paper structure on thermal properties
In supercalendering, application of heat and moisture softens paper. Smooth surfaces without a loss of bulk result if the action only influences the paper surfaces. In gradient calendering, this is accomplished with a temperature or moisture gradient. The temperature gradient in the thickness direction depends on the thermal diffusivity of paper and the heat transfer from the hot calender rolls to paper 1.
Thermal properties are indirectly relevant in printing processes where heat dries ink. The thermal properties of paper are also important in electrophotographic applications such as laser printing and photocopying. Here, the printed image consists of polymeric toner particles. These fuse to the paper in a hot roller nip (T = 100 °C–200 °C) where the toner melts and adheres to the paper surface. The toner adhesion depends primarily on surface chemistry and roughness. The thermal properties of paper are more important for the print gloss. Gloss improves with the temperature that toner and paper reach during fusing 2,3.
The heating used in ink drying and toner fusion operations causes the paper to lose moisture. The moisture loss leads to curl, cockling and skewed dimensional changes. Sometimes the resulting temperature distribution in paper is asymmetric. This easily leads to curl in paper. In electrophotography, cockling can lead to a “deletion” problem where print is lacking from concave cockles. This happens if the second side of paper is printed after toner has been fused to the first side. Local variations in the thermal properties of paper may also lead to nonuniform print.
Good thermal insulation is sometimes necessary in packaging such as that used for fast food. Corrugated board is an inherently good thermal insulator due to the insulating air gaps between the liners. Recycled paper has some use as an insulating material in buildings after receiving fireproof and moisture repellent treatments.
