Optimization of polyester wet paint to improve surface quality in coil coating
Ritala, Akseli (2017)
2017
Materiaalitekniikka
Teknisten tieteiden tiedekunta - Faculty of Engineering Sciences
This publication is copyrighted. You may download, display and print it for Your own personal use. Commercial use is prohibited.
Hyväksymispäivämäärä
2017-06-07
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tty-201705191437
https://urn.fi/URN:NBN:fi:tty-201705191437
Tiivistelmä
Coil coating is a widely used coating method where a coating is continuously applied onto a steel strip. At SSAB the topcoat is applied using reverse three-roll coating. There are various coating errors that are typical for this coating method that are difficult to eliminate. The aim of this thesis was to reduce the formation of striping errors in polyester coatings at SSAB's continuous color coating lines by modifying the properties of the paint.
The focus of this thesis is on two specific common coating errors: the longitudinal ribbing stripe and the transverse cascade stripe. Cascade occurs when the meniscus between the rolls travels too far inside the nip due to a high roll speed ratio. This causes air to get caught in the meniscus, which leads to a transverse stripe. Ribbing occurs when pressure gradients in the meniscus cause small disturbances to widen instead of leveling.
The conditions where striping errors form are complicated and often difficult to control. In practice, the surface quality in reverse roll coating is controlled by modifying the roll speeds. Suitable roll speed ranges constitute a coating window, in which the produced surface quality is considered to be acceptable. Based on literature, it was identified that the coating window could be widened by increasing the surface tension of the paint.
To confirm the effect of surface tension on error formation, a series of trial runs with modified polyester paints with increased and lowered surface tension was performed. The line trials were designed to go through a trial cycle, where the striping of the paints was comprehensively tested at various roll speed conditions. Viscosity and surface tension of the paints were measured. In addition to the polyester paints, a particle structured paint was also modified and a line trial was performed.
The results of the trial runs were positive with the high surface tension polyester paints exhibiting superior surface quality compared to production paints in all trial runs. The high surface tension particle structured paint also had significant improvement in striping performance, which means that the improvements gained by increasing the surface tension could potentially be extended to other coatings used at SSAB.
The focus of this thesis is on two specific common coating errors: the longitudinal ribbing stripe and the transverse cascade stripe. Cascade occurs when the meniscus between the rolls travels too far inside the nip due to a high roll speed ratio. This causes air to get caught in the meniscus, which leads to a transverse stripe. Ribbing occurs when pressure gradients in the meniscus cause small disturbances to widen instead of leveling.
The conditions where striping errors form are complicated and often difficult to control. In practice, the surface quality in reverse roll coating is controlled by modifying the roll speeds. Suitable roll speed ranges constitute a coating window, in which the produced surface quality is considered to be acceptable. Based on literature, it was identified that the coating window could be widened by increasing the surface tension of the paint.
To confirm the effect of surface tension on error formation, a series of trial runs with modified polyester paints with increased and lowered surface tension was performed. The line trials were designed to go through a trial cycle, where the striping of the paints was comprehensively tested at various roll speed conditions. Viscosity and surface tension of the paints were measured. In addition to the polyester paints, a particle structured paint was also modified and a line trial was performed.
The results of the trial runs were positive with the high surface tension polyester paints exhibiting superior surface quality compared to production paints in all trial runs. The high surface tension particle structured paint also had significant improvement in striping performance, which means that the improvements gained by increasing the surface tension could potentially be extended to other coatings used at SSAB.