Meaning
Organic structural polymers that remain in paper fibers after mechanical refining cause the paperboard to turn yellow when exposed to light. The presence of mechanical pulp lignin is characteristic of lower-cost paperboards used for bulk packaging. This compound provides high yield and stiffness but compromises the long-term optical quality of the board.
Fibre Quality
Mechanical pulping retains almost all of the wood’s natural components, resulting in a high yield from the raw logs. This retention means that mechanical pulp lignin is present in the final paperboard sheet, which increases the material’s bulk and compressive strength. This stiffness makes the board ideal for protective transit packaging where optical properties are less important.
Optical Stability
Exposed wood chemicals react with oxygen in the atmosphere, causing a distinct shift in the appearance of the printed sheet. Under UV light, mechanical pulp lignin degrades and turns the board from white to a dull yellow color. This degradation ruins the appearance of high-end graphics, meaning that boards with high lignin content are rarely used for luxury brand packaging.
Contractual Hazard
Procurement contracts for retail packaging often restrict the use of mechanical pulps to ensure that the carton’s appearance does not degrade while sitting on the shelf. If a supplier uses board with excessive mechanical pulp lignin without disclosure, the resulting yellowing can lead to a breach of contract claim by the retailer. The manufacturer must then replace the faded packaging at their own expense or face significant financial chargebacks, making pulp composition a central clause in packaging supply agreements.
