Curl is a dimensional stability phenomenon common to all paper products. Paper curls when it deforms out of its plane, thereby acquiring a cylindrical shape. Its cause is a combination of non-symmetric paper structures through the thickness of the sample with asymmetry in the interaction with the outer environment.
Important for converting and for the end user
The presence of curl in a paper product can have negative effects on both quality and productivity. For packaging materials, high-velocity printing and converting lines rely on constant shape and properties of the paper substrate. Curl is not uncommon in graphical papers, both in presence of heat and upon wetting and drying of the surface.
The end user of a paper product can also experience the negative effects of a curled product: for example, the corner of a newspaper can roll up in a rainy day and a re-sealable package can fail to close because of paper curl.
Measuring curl
Innventia has developed several methods to measure curl. For research and detailed characterization, we can measure in variable humidity following the procedure described in Nordic Pulp and Paper Research Journal 1987(2):66-70.
We have recently developed the Curl-O-Meter, a new instrument to characterize curl in paperboard with high levels of automation and large sample throughput. This instrument is designed to meet the needs of quality control laboratories at the paper mill.
Tuning dimensional stability
Paper structure is often the primary cause of the out-of-plane deformation. The scientists at Innventia can characterize the structure of a paper sample with special focus on the features that influence curl. The deformation behaviour can therefore be linked to process parameters and properties of raw materials. This forms the base for suggesting actions to tune the dimensional stability of the product.