Extrusion coating line

Figure 5.14 displays the principle of a wire extrusion-coating line. 

Fig. 10 Extrusion coating line. 1 Extrusion, 2 unwinding, 3 pretreatment, thickness measurement, temperature control, 4 laminator system, 5 thickness measurement, second pretreatment, 6 winder). (Reproduced with permission from Reifenhauser)...

M. Nolan, S. Greig, N. Jadon, Corona, Ozone and Flame Treaters for Extrusion Coating Lines , p. 4. 

The corona discharge, particularly bare roll and universal roll designs, and flame treatment at atmospheric pressure have both been effective in improving adhesion of various substrates on extrusion coating lines (the image in Figure 5.3 shows a multilayer system). 

Most of the polymeric-based layers are applied using extrusion technology the main equipment is the extruding coating line (see Polymer processing). 

In this process the plastic is directed against the surface of the substrate immediately before its entry into the nip between a pressure roll and a chill roll. The extrusion rate and the line speed of the substrate determine the coating thickness. The chill roll controls the surface finish of the coating. Extrusion coatings are typically applied in thin layers, ranging down to 0.005 mm. 

Computer-controlled automatic profile dies with electrical controlled sensors in closed-loop control systems have developed greater efficiency and accuracy to extrusion coating, cast film, and sheet lines. A scanner measures the web thickness and signals the computer, which then converts the readings to act on thermally actuated die bolts. The individual adjusting bolts expand or contract as ordered by the computer to control the profile. The more sophisticated systems measure adjusting bolt temperature and provide faster response time with less scrap and quicker startups. The scanner is typically an infrared, nuclear, or caliper-type gauge. 

In order to increase the moisture resistance of cardboard (wood product) the extrusion coating of polyethylene (PE) is often applied on the surface of the cardboard. In order to make the surface of PE-coated cardboard printable, the corona discharge was applied on the PE surface. The PE coating as well as the corona discharge treatment is applied in line at the site of cardboard manufacturing. PE-coated cardboards with printable surface manufactured in Canada were shipped to a customer in Japan after printability of the treated surface was confirmed. 

In the first operation, the paper and the foil are extrusion laminated using an acid copolymer as the adhesive. Next, the sealant layer is applied as an extrusion coating. The operation can be accomplished in two passes, or in a single pass in a tandem extrusion line as shown in Fig. 8.2. In such a line, both extrusion heads are usually capable of coextrusion, as well. To understand the process of extrusion coating and laminating, it is necessary to start with the process in its simplest form, which, by the way, is how it is still practiced today by some converters. 

To reduce the need for such high temperatures, primers are used to improve adhesion. These primers are organic compounds that are coated onto the paper in-line with the extrusion coating to provide nonpolar groups to which the PE can adhere, as well as polar groups which can adhere to the paper. Polyethylene imine (PEI) is one such compound. These primers allow the melt temperature to be reduced to 288°C (550°F), so less oxidation occurs and there is less potential for off-odors. 

Uses Component of thermoplastic urethane elastomers for inj. molding, extrusion, thermoset PU, castable prepolymers, spandex fibers, metal and textile coatings/linings, adhesives, elastomeric polyesters, as polyester modifiers soft segment in elastomers food pkg. adhesives, poly-... 

Extrusion coating Poor adhesion a variety of apparently contrary causes related to polymer viscosity, degradation, oxidation, tackiness, temperature melt temperature too low or high/air gap too small/chill roll temperature too cold or hot/line speed too fast/poor match between coating and substrate. 

Extrusion (see Extrusion coating). Adhesive is forced through a die on to the substrate and may be applied as continuous or intermittent lines or bands. 

While the extruder pumps the molten polymer, the die and downstream equipment determine the final form of the plastic. Blown film and flat film extrusion both produce plastic films, but require very different dies and take-off systems. Similarly, different extrusion lines are used for pipes, tubing, profiles, fibers, extrusion coating, and wire coating. 

In extrusion coating (Fig. 5.53 ), a layer of molten polymer is applied to a substrate such as paper, plastic, or metal foil. When the substrate is fed into the extrusion line, it is often preheated and may be pretreated to enhance adhesion. Then an extruder pumps molten plastic onto the substrate. The coated material is then cooled by chill rolls and collected on a wind-up roll. Auxiliary equipment, such as slitters, corona discharge stations, and printing heads, may be placed between the chill and wind-up rolls. Since extrusion coating offers high production rates and ehmina-tion of solvent-based adhesives, it is frequently used for packaging. 

Extrusion coating and laminating are ways to add value to the existing product line of spunbonded PP [165]. 

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