Below are some notes on calendering related to process variables and their effect on paper properties, response to compressive load, mechanisms of smoothness and gloss and web plasticity.
- Paper: moisture, temperature, furnish and formation
- Coating: coat weight, binder, pigment and coating method
- Nip mechanics: number of nips, linear load (maximum pressure, nip width), dimensions and soft cover (modulus, surface roughness)
- Thermo roll: surface temperature and surface roughness
- Machine speed
Changes of paper properties during calendering
- Favorable changes: smoothness +++, gloss +++, air permeability ---, oil absorption --- and two-sidedness -
- Unfavorable changes: density +++, caliper ---, stiffness ---, compressibility ---, opacity --, brightness -, breaking length -- and tensile -
Calendering ability of paper is created on the paper machine or coating station.
When paper is subjected to compressive load, it responds in three ways:
- It is elastic and compresses like a spring until its elastic limit is exceeded.
- It behaves like a plastic material that flows under stress. Paper is non-uniform and flocs are calendered differently, causing mottle.
- It has a memory of what happened to it previously. Paper partially returns to its original shape when exiting a nip.
Under compression, if sufficient moisture is present, flocs tend to bond together and become more transparent (more blackening).
More gloss is developed against the harder roll. The steel roll surface has to be very smooth.
The bulk coming out of the calender is proportional to the bulk going into the calender, the largest changes occurring in the first nips. Run the rougher sheet side on early nip steel rolls.
The maximum compression increases as the linear load and modulus of elasticity increase and roll diameters decrease - nip width decreases.
The nip dwell time is affected by linear load, roll diameters, modulus of elasticity, speed and number of nips.
The significance of dwell time in a single nip decreases as the number of nips increases - normally after nine nips. The speed impact also decreases.
Mechanisms of smoothness and gloss in calendering
- Compression: Hills are compressed more than valleys, reducing surface pores. This can be more or less permanent.
- Transferring and polishing: Material is transferred from hills to valleys or removed from the hills altogether.
- Orientation: Oblong and flat particles settle in the direction of the surface.
- Reproduction: The surface pattern of the rolls is reproduced in the paper. Chilled iron rolls are smoother than polymer rolls and result in better smoothness.
Web plasticity - different components of the paper are plastic in different ways
- Lignin: Softening temperature of 115 °C no matter what the paper moisture content is.
- Hemicellulose: The softening temperature depends on sheet moisture and decreases linearly when moisture is increased.
- Cellulose: Depends on its crystallinity. The softening temperature decreases as sheet moisture increases.
- Heat: Heat is required to plasticize lignin, and moisture is required to soften hemicellulose and cellulose.
The most significant effect that calendering has on coating is to replicate the surface against which it is pressed.
For more information on optimizing your calender, contact your Valmet representative.