Coating processes – Application to opposite sides of sheet – web – or strip – Roller applicator utilized
Reexamination Certificate
2002-08-09
2004-01-06
Bareford, Katherine A. (Department: 1762)
Coating processes
Application to opposite sides of sheet, web, or strip
Roller applicator utilized
C427S428010
Reexamination Certificate
active
06673391
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method of applying coatings to metal substrates, utilizing direct contact with a flat, non-deformable applicator roll; and to substrates coated by such a method. In this invention, the metal substrate is metal sheet, not metal foil.
BACKGROUND OF THE INVENTION
Roll coating has been used for decades to apply liquid coatings to solid substrates for decoration, protection, and functionality. During that time, line speeds have increased, coating uniformity requirements tightened, environmental concerns heightened, and productivity goals raised. Roll coating has evolved according to the industry and coating requirements. Roll configurations vary tremendously; some of the variants are the number of rolls (two to five or more rolls per coating), the position of metering roll(s), coating feed location, coating against a backup roll or sheet tension, roll compositions, and the roll directions relative to each other and the substrate. Wet film thickness varies from 0.05−>100 micrometers and line speeds from 1−>500 m/min. Roll coating is used on paper, metal, plastic, and other substrates.
Liquid coatings are applied to substrates in a variety of methods including spray, dip, roll, knife, electrodeposition, vapor deposition, slot, and curtain coating. For metallic substrates, liquid coating is commonly transported to the applicator roll via direct contact with a bath, by contact with a second roll that has contact with a bath, by direct spraying onto the roll, and the like. The rolls can have a metal, plastic or other type surface depending on the material to be coated. The various rolls in a roll coater are usually given common names associated with their function. In the three roll configuration shown in
FIG. 1
of this invention, the applicator roll
20
applies the coating to the web, and the backup, or impression roll
22
provides support for the web. If the back-up roll is deformable (rubber-covered) it is an impression roll while a non-deformable roll is a backup roll. The pickup/metering/fountain roll
18
lifts coating from the pan and meters coating with applicator roll
20
. Since roll
18
has two functions, it can be called by more than one name. In addition, different industries commonly use different names for the same roll function, hence roll
18
is also called a fountain roll in the printing industry. Depending on the number of rolls in the configuration, additional terms such as transfer roll and spreading roll are commonly used to describe the roll function in a specific industry. The narrow gap between roll
18
and roll
20
is termed the metering nip and the narrow gap between roll
20
and the web is termed the coating nip. The back-up roll supports the web at the point of coating application. If space is maintained between the applicator roll and the web, the back-up roll is usually rigid, and there is no metal-to-metal contact. If the backup roll is forced into near-contact with the applicator roll, one of the two rolls are covered with deformable material to prevent metal to metal contact. Surfaces that move in the same direction at the point of nearest contact (rolls
18
and
20
) are said to be moving in a forward direction, and those in opposite direction (roll
20
and the web) in a reverse direction. For the most uniform surface appearance, the applicator roll moves in a reverse direction for continuous web coating. A description of various roll combinations and descriptions of fluid dynamics can be found in
Liquid Film Coating
, Ed. S. F. Kistler et al. “Knife and Roll Coating” by Dennis J. Coyle, pp 539-542 (1997).
Ceramic surfaced metering rolls, having surface indentations to contain ink and the like, have long been used in the printing industry, as taught in U.S. Pat. Nos. 4,301,730 and 4,009,658 (Heurich et al and Heurich respectively). In Heurich the hard ceramic coating protects the roll from wear and permits a flatter cell profile. Other possible uses for this roll include “glue application rolls, rotogravure coating rolls and the like” but the substrates mentioned are compliant paper products. These inventions were improvements over rolls having a plated metal surface. In offset printing processes, water and ink are alternately fed to a plate surface where the ink selectively adheres to the picture portion. Yokoyama et al., in U.S. Pat. No. 4,991,501, used a ceramic faced dampening water feed roll with a flame sprayed layer of Al
2
O
3
and/or TiO
2
, and also containing SiO
2
as a hydrophilic, inorganic pore-occluding agent within ceramic surface occlusions. These rolls were seen as an improvement in terms of better wettability over flame sprayed Cr
2
O
3
roll surfaces. The dampening water feed roll here is not in direct contact with substrate being coated or the plate cylinder.
In the area of coating metallic substrates for packaging and architectural end uses, metallic substrate are coated in one of two ways: as a continuous web or as approx. one m
2
(10 ft
2
) sheets. A continuous “web” is rewound into a coil for subsequent trimming, slitting, or fabrication. Aluminum and steel are most commonly used as the substrate. Many coatings are applied on metal webs via forward or reverse roll coat with a deformable applicator roll. The applicator roll has a pliable covering about 5-50 mm thick made from polyurethane, EPDM rubber or similar pliable material. The covering Shore A hardness, referred to as durometer, is about 40-80 and the surface roughness R
a
is about 20-80 micro inches. (R
a
is determined by first finding a mean line parallel to the general surface direction, dividing the surface such that the sum of the areas formed above the line is equal to the sum of the areas formed below the line, and computing the surface roughness summing the absolute values of all the areas above and below the mean line and dividing by the sampling length.) This allows the applicator roll to transfer liquid coating to the metallic web and conform to any web surface irregularities or roll irregularities to ensure complete coverage at a uniform film thickness.
Deformable (rubber or polyurethane) applicator rolls are subject to wear and cause mottling, blisters, skips and eyehole defects due to swelling, burnishing, or exhibition of internal defects. When applicator rolls start to produce coating defects, the whole production line must be shut down and the applicator roll replaced. In addition to the cost of resurfacing the roll and finished product scrap, problems associated with stopping and starting a continuous line come to bear. Each line stoppage results in scrap generation and increases the chances for sheet breaks and dents. Another large cost is lost production time.
Two common applicator roll defects are mottling and edge blisters. Mottling occurs from wet coating thickness variations caused by the applicator roll. The applicator roll is thought to pick up oxides from the metal surface or become burnished from repeated contact with the metallic substrate. When a burnished applicator roll receives coating from the metering roll, the burnished surface does not carry as much coating and a low coating weight area results.
Edge blisters are another coating defect that results from deformable rolls wearing at the edge of the web. As the deformable roll covering deforms around the web edge, the web cuts the roll. After switching to a wider web, the cut in the applicator roll leaves an uneven amount of coating on the substrate, and the roll needs to be replaced or resurfaced before production can continue.
Other application problems with deformable applicator rolls, especially in high gloss waterborne colors, are small (1 mm×10 mm) areas of low coating weight and flow lines. The small areas are higher gloss than the surrounding coating and are eliminated only with difficulty by adjusting roll speeds and nip pressures. Flow lines, or ribbing instability from a forward metering roll nip, are unstable when using a deformable applicator roll.
A second mechanism for edge defects i
Bowling Jeffrey W.
Perkett Bruce A.
Schade James A.
Zediak Clinton S.
Alcoa Inc.
Bareford Katherine A.
Cills Daniel
Eckert Seamans Cherime Mellott
Mayald David
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