Release coating for drying cylinders improves quality, case study

Overcoming challenges in the drying section

Poor dryer cylinder release causes runnability problems, sheet picking and dusting. At their worst, these problems may place limits on cylinder temperatures and operating speeds. Dusting and fiber picking are common problems for fine paper machines.

Dirty dryer cylinder surfaces reduce drying capacity and cause sheet moisture profile deviations. Typically, cylinders and rolls can be contaminated by stickies (OCC & DIP furnish) or coating paste and starch (after coating/sizing).

Cylinder cleanliness and release properties can be improved with a functional release coating. However, onsite coating processes need to be fast and flexible.

Valmet Dryer Roll Cover DOH overcomes these challenges with a combination of carefully layered carbide and fluoropolymer cover, fast curing time and low curing temperature. The result is excellent surface properties including release, non-stick, coating lifetime, corrosion and wear resistance.

Case study - Sun Paper improved product quality with onsite dryer cylinder service

Sun Paper was suffering from sticking and corrosion problems on their dryer cylinder surfaces due to poor surface release properties. Fluff and break times had increased and sheet moisture had deviated. Their target was to improve end product quality.

Valmet provided our DOH anti-sticking coating service. Our mill improvement experts assigned two recoating teams with two sets of equipment to work simultaneously. Twenty-five drying cylinders on four different machines were coated with Valmet Dryer Roll Cover DOH coating. Three suction roll shells were also serviced.

The head office was very satisfied with the results. Before, the dryer cylinder needed to be cleaned three or four times per shift, but after the application of DOH it cleaning was only required once a shift. "Since minimizing downtime is very precious to us, it was crucial that the break times were reduced and the release properties significantly improved. Valmet saved lots of valuable time for us with their efficient and professional service," says Mr. Li, Mill Manager of PM17 at Sun Paper.

Valmet Dryer Roll Cover DOH

The hard carbide base layer is wear and corrosion resistant. A special polymeric release layer is layered on top of the carbide base. The polymeric release layer is made of a unique fluoropolymer, exclusive to Valmet, with optimal release and doctoring properties. The top polymer coating wears after some time to a level where carbide particles come to the surface. These carbide particles prevent further wear of the coating. The release polymer will remain in the valleys and cavities of the carbide coating for years, providing a clean surface. The polymer’s fast curing time makes it particularly well-suited for onsite operation. Also, the process temperature remains below 100 °C. Other release materials require higher temperatures which can lead to cylinder deformation.

Valmet Dryer Roll Cover DOH is based on the latest polymer chemistry, modern thermal spraying technology, and long experience. The DOH fluoropolymer has been optimized specifically for excellent release and doctoring properties in combination with high wear resistance and durability. On a nano-scale, the polymer has a large surface area due to small partical size, providing a greater benefit to properties such as release and stiffness-hardness. Its low friction properties and low surface energy are created by customized fluoropolymer composition, where surface chemistry, strength and wear features are in optimal balance. The high wear resistance is based on a hard carbide base layer. New generation carbide technology ensures maximal durability and lifetime.

Designed for superior performance

• Excellent cleanliness properties: Thanks to the latest polymer technology, the surface energy of Valmet Dryer Roll Cover DOH release coating is very low.
• High doctoring wear resistance: The wear resistance and stiffness of the new Valmet Dryer Roll Cover DOH polymer have been further improved with the latest polymer technology. This leads to longer coating lifetime and stable cleanliness properties.
• Savings due to low doctoring friction: Thanks to special additives used in the DOH polymer matrix, the friction coefficient is 80% lower. This provides more stable doctoring, with less heat buildup on the blade tip and a better cleaning result, as well as lower drive power consumption.
• High hardness at elevated temperature: The DOH polymer also holds its high hardness at higher dryer cylinder operating temperatures, resulting in less wear and longer coating lifetime. Higher temperatures make plastics softer, but with the DOH polymer the change is minimal. This means the polymer will withstand more wear, as wear resistance is related to hardness.

Onsite coating procedure

Valmet will bring the required equipment to the mill site for onsite coating. We can coat up to 5 cylinders at the same time. The required shutdown is usually around 3 days (3 x 24 hours). The specific circumstances need to be checked prior to committing to a shutdown schedule. The onsite activies include:

1. Removal of runnability boxes, etc. and installation of onsite coating equipment
2. Cylinder surface cleaning or profile grinding (depending on cylinder condition)
3. Grit blasting
4. Carbide coating
5. Intermediate (light) grinding
6. DOH polymer spraying
7. Polymer curing (UV)
8. Removal of onsite equipment and reinstallation of runnability boxes, etc.
9. Finishing of polymer surface with startup doctoring

Fast onsite coating process

The fast DOH fluoropolymer curing process uses UV light; no additional cylinder heating is needed. Other onsite fluoropolymers are cured using IR radiators. Due to the heating and curing process, the total coating time is longer.

Unlike Valmet's DOH polymer, PTFE/FEP (thermoplastic type fluoropolymer) requires a high sintering temperature (oven treatment over 300°C ). This is a risk for cylinder deformation and cylinder end seals need to be changed. Taking the cylinder out of the machine and transportation take a lot of time and work and the mill also needs a spare cylinder.

For other onsite fluoropolymers (Teflon particle containing polymers), temperatures ranging from 150°C to 200°C are typically needed for curing the binder material. Gas IR radiators are needed, corresponding to several otherwise unnecessary gas cylinders onsite and attendant safety concerns.

With the DOH polymer, coating can be done over a shorter shutdown, or more cylinders can be coated during a shutdown. And there is no risk to cylinder condition due to deformations, balancing changes or head seal damage.

Applications

Typical applications for Valmet Dryer Roll Cover DOH include:

• First cylinders of pre-dryer sections
• Cylinders after sizing or coating stations
• Last cylinder in a dryer group

For more information on how to improve your drying process with onsite application of Valmet Dryer Roll Cover DOH, contact your Valmet representative.