Aqueous Coating Defects

Transcription

1 Aqueous Coating Defects Barrier Coating Frustration Presented by: Tony Lyons Director of Research

2 Most Aqueous Coating Defects Can Be Traced to the Following Root Causes: Raw material quality Entrained air Poor coating rheology and improper water retention control Base paper defects and quality Inadequate operator training Poorly maintained equipment Mismatch of equipment for the application Coating make down errors

3 Barrier Coating Frustration Customer has adopted a new technology for moisture barrier coating that gave excellent results in the laboratory 5 g/m 2 /day under tropical conditions with rod draw downs On their 500 M/min rod coater they were able to achieve the target coat weight and surface coverage, but got disappointing results when the moisture barrier property was tested 35 g/m 2 /day under tropical conditions The coat weights and the formulation were double checked and everything was at it should be What might be wrong?

4 Roll and Web Defect Terminology Classification: Web Defects - Coating Ref. Number WD-62 COATING PITS DEFECT DESCRIPTION Coating Pits - Defined as microscopic holes in the coating. Appearance can be enhanced by a Croda Red stain. SYNONYMS Also known as: Pinholes, Pits- Coating, Holes (Microscopic)-Coating ILLUSTRATION SEM image of coating pits at low magnification. Courtesy of Imerys. Optical micrograph of a coating pit. Note how rounded the edges of the pit are.

5 Roll and Web Defect Terminology Classification: Web Defects - Coating Ref. Number WD-62 COATING PITS CAUSES OF COATING PITS 1. Micro air bubbles in the coating. 2. Fiber crossings that didn't wet out. REMEDIES OF COATING PITS 1. Install air separation system (deaerator). 2. Use a defoamer in coating formulation.

6 Roll and Web Defect Terminology Classification: Web Defects - Coating Ref. Number WD-68 Optical micrograph of foam streak. Courtesy of Imerys. FOAM STREAKS DEFECT DESCRIPTION Foam Streaks - Defined as relatively short 6 mm (0.25") [Brown says that they can be as short as 2-3 mm] long, extremely faint marks in the coated paper surface. About 3 mm (1/8") in width, they are usually not in a straight line but tend to wander to one side or the other. Foam Streaks on puddle type blade coaters are caused by air bubbles on the color surface in the pan riding against the incoming web. Bubbles may ride back and forth and/or follow the web down into color. Foam streaks on an applicator roll/trailing blade coater will line up with the roll split pattern. Foam Streaks are best seen when inspected at a very low angle and with low angle lighting. They may or may not calendar out, and they may or may not show up in the printed surface. SYNONYMS Also known as: ILLUS TRATION

7 Roll and Web Defect Terminology Classification: Web Defects - Coating Ref. Number WD-48 FOAM STREAKS CAUSES OF FOAM STREAKS 1. Excessive turbulence in head causing foam. 2. Inadequate defoaming. Excessive oil-based defoamer causes the defect. 3. Excessive free fall in recirculating line. 4. Air being sucked into color at pumps. 5. Pond level too low. 6. High jet flow impingement angle (jet coater). 7. Contamination such as lubricating oil leaking from coating processing equipment. REMEDIES OF FOAM STREAKS 1. Minimize turbulence in head with appropriate baffling. 2. Check type and amount of defoamer. 3. Eliminate free falls. 4. Repair leaky pump seals on suction side. 5. Raise pond level. 6. Decrease jet impingement angle. 7. Opening the jet to backing roll application gap. 8. Reducing coater speed. REFERENCES 1. Brown, A., and Cruthoff, B., A Pilot Coater Trial to Investigate the Occurrence of Coating Surface Foam Streak, TAPPI Coating Conference Proceedings, 2002.

8 Resolution Customer discovered an open fall of coating from the make down tank to the coating supply tank Extend the piping to reduce the chance for air entrainment It was also noticed that excessive foam was developing in the coating make down tank Contacted their coating additive supplier for a defoamer recommendation that would not interfere with the barrier property they were after Were able to achieve a marked improvement to 15 g/m 2 /day under tropical conditions, but still not what they achieved in the lab Customer invested in deaeration equipment to further reduce the pin holes and now achieve about 8+\- 2 g/m 2 /day under tropical conditions

9 Thank you PRESENTED BY Tony Lyons Director of Research IMERYS Please remember to turn in your evaluation sheet...