Calculating Surface Temperature Limits during High Speed Thermal Paperboard Embossing
Maximum surface temperature limits during high-speed paperboard embossing are bounded by latex binder scorching and steam-induced Z-direction blistering.
Optical Brightening Agent Pyrolysis
Thermal Decomposition
High temperatures in the drying and converting sections of a paper mill can degrade chemical additives used to improve board appearance. The occurrence of optical brightening agent pyrolysis causes these fluorescent compounds to decompose, reducing their ability to absorb ultraviolet light and emit blue light. This breakdown leads to a loss of paper brightness and a shift toward a yellowish tint.
Process Limits
Coated paperboard often passes through intense drying hoods and calendering nips where surfaces experience extreme heat. Under these harsh conditions, optical brightening agent pyrolysis is accelerated if the moisture content of the sheet is too low or if the contact time is extended. This degradation limits the maximum operating temperature of the dryer section when producing high-brightness grades.
Formulation Strategy
Chemical suppliers develop thermostable additives that can withstand the high-temperature environments of modern paper machines. Preventing optical brightening agent pyrolysis requires selecting carrier polymers, such as polyvinyl alcohol, that insulate the active molecules from thermal stress. This selection helps maintain the optical properties of the finished paperboard during subsequent converting processes, including high-temperature extrusion coating with polyethylene.
Furthermore, minimizing the dwell time in the drying hoods protects the surface fibers from burning and ensures the OBA remains functional.