These coatings are meant for Plastic films, paper , Metal Foils etc.
The surface preparation is a very important step while coating on these substrates. The backing / Carrier material is the most important component of coated products . The backing or carrier provided the necessary strength and body to end product. The backing has to be sufficiently strong . the backing needs to have reasonably good tensile strength , elongation , conformability, moderate heat resistance , chemical resistance, abrasion resistance etc. The various backing materials that are common in use are Plastic films Paper Metal Foils The backing materials need a primer coat or chemical or physical , surface preparation / treatment before adhesive coating.
Plastics Films are widely used in Packaging applications such as Self Adhesive Tapes, Laminates, Printing etc. As we know all these films are chemically inert and have very low surface energy. The below mentioned table gives an idea about the surface tension of various polymers. From the tables given below it is very clear that the polymer surfaces need treatment to enhance the wet ability , printability , and adhesion properties. Or in other words the surface tension has to match that of the coating materials.
Measuring Surface Energy The object of surface treating is to improve the wettability of the surface, thereby improving the ability to bond to solvents, adhesives, coatings, and extrusion coating. In order for a surface to be properly wet by a liquid, the surface energy of the plastic must be higher than the surface tension of the liquid. Surface energy is measured in dynes per centimeter. Ideally, the surface energy of the plastic should be 7 to 10 dynes/cm higher than the surface tension of the solvent or liquid. For example, a printing ink having a surface tension of 30 dynes/cm would not 4 adequately wet or bond to a material having a surface energy less than 37 to 40 dynes/cm (see Figure 3).
One method for measuring surface energy known as the Wetting Tension Test (ASTM D-2578) was established a number of years ago (10) . Surface energy testing is frequently done on post-treatment surfaces prior to follow-on converting processes. With this test, a series of mixed liquids with gradually increasing surface tensions are applied to a treated substrate surface until one is found that just wets the surface. The surface energy of the plastic is approximately equal to the surface tension of that particular mixture. Test solutions are available from various manufacturers of corona treating equipment. Table below details the ratio of Formamide and Cellosolve™ for various surface tensions. Cellosolve™ is a registered trademark of Union Carbide for ethylene glycol monoethylether. The wetting tension test method is by far the most prevalent measurement used to determine treatment level of post-treated surfaces.
The common methods of surface treatment for films are
however chemical etching method is outdated.
Flame treatment In this process propane or butane flame is used to treat the polymer surface. Here the electron density is lower and therefore the ionization rate . The higher temp in the flame poses a problem for most polymer surfaces.
Four variables control the optimum treatment conditions of a flame, they are: 1. Air-to-Gas Ratio 2. BTU (kj) Output of the Burner 3. Distance of Surface from Flame Tips 4. Dwell Time of Surface in Oxidizing Zone
Carona treatment Generally, plastics have chemically inert and nonporous surfaces with low surface tensions causing them to be non-receptive to bonding with printing inks. For example, polyethylene and polypropylene are known to have an extremely low surface energy. To activate substrates and make them receptive to ink adhesion printers employ corona treaters. Corona treaters increase the surface energy of substrates by oxidizing the surface and creating positive and negative sites on the surface by adding and deleting electrons.
A corona treating system is designed to increase the surface energy of plastic films, foils and paper in order to allow improved wettability and adhesion of inks, coatings and adhesives. As a result, the materials treated will demonstrate improved printing and coating quality, and stronger lamination strength. The system consists of two major components: 1. the power supply, and 2. the treater station. The power supply accepts standard 50/60 Hz utility electrical power and converts it into single phase, higher frequency (nominally 10 to 30 kHz) power that is supplied to the treater station. The treater station applies this power to the surface of the material, through an air gap, via a pair of electrodes, one at high potential and the other, usually a roll which supports the material, at ground potential. Only the side of the material facing the high potential electrode should show an increase in surface tension
A corona treating system in its simplest form Voltage is applied to the top plate which, in the case of a corona treating system, would be the electrode. The dielectric portion of the capacitor would be made up of some type of roll covering, air, and substrate in the corona treating system. The final component, or bottom plate, would take the form of an electrically grounded roll. In the corona treating system, the voltage buildup ionizes the air in the air gap, creating a corona which will increase the surface tension of the substrate passing over the electrically grounded roll.
Coating techniques Coating is a very Important process involved in Manufacture of Coated Products Today we shall review some of these as listed below.
Gravure Coating Comma Coating Reverse Roll Coating Hot Melt Coating Metering Rod / Myer Bar Coating Knife over roll coating Slot Orifice coating Immersion / Dip coating Curtain Coating
The Coating methods Their Capabilities and Limitations Coating Method Viscosity in CPS Max Speed per Min Coating Wt . gm per sq. mtr Gravure 100-2000 700 Mtrs. 3.-20. Comma 1000-6000 300 10-200 Reverse Roll Nip 1000-6000 300 10-200 Reverse Roll Pan 200-6000 100 10-200 Hot Melt 500-20000 300 10-300 Myer Bar 10-200 200 3.-25. Knife over Roll 100-20000 1500 3.-15. Slot Orifice 500-20000 200 20-100 Immersion / Dip Coating 1000-10000 50 Heavy Curtain 500-20000 200 20-100
We will study Comma coating Reverse roller coating and Metering Rod / Myer Bar Coating Today
Reverse Roll Coating In this procedure, the coating material is measured onto the applicator roller by precision setting of the gap between the upper metering roller and the application roller below it. The coating is 'wiped' off the application roller by the substrate as it passes around the support roller at the bottom. The diagram illustrates a 3-roll reverse roll coating process, although 4-roll versions are common.
Reverse Roll Coater The three roll reverse coater is used for many specialty applications. This method is more expensive and generally slower in line speed than the Mayer rod coater. However, the versatility in terms of coat weight range and coating width, and the good coat quality of the three roll reverse has made it popular with many converters. In this process, the middle roll is coated with a precise thickness of adhesive, determined by the size of the gap between the top and middle rolls. The adhesive is then transferred as the rubber backing roll brings the web into contact. The adhesive coat weight can also be varied by increasing or decreasing the speed of the applicator roll relative to the web speed. The metering roll speed can be varied to adjust the smoothness of the coating. Figure 2 : Reverse Roll Coater There are several variations of the reverse roll coating method, such as four-roll, nip fed, and pan fed systems. A nip-fed, three roll reverse coater is shown here. Medium to high coat weights are possible with reverse roll.
Myer Bar Coating In this coating process, an excess of the coating is deposited onto the substrate as it passes over the bath roller. The wire-wound metering rod, sometimes known as a Meyer Rod, allows the desired quantity of the coating to remain on the substrate. The quantity is determined by the diameter of the wire used on the rod. This process is remarkably tolerant of non-precision engineering of the other components of the coating machine .
The market for wire-wound rods has grown rapidly during the past few decades, because they provide predictable, accurate coatings time after time, at a minimal cost. Although the technology goes back almost a century, today's high quality materials, multi-wire designs and special wire surfaces have made this system more popular than ever before. Rods give users the ability to fine-tune coating thickness quickly and easily, without altering the chemistry of their coating material, and without time-consuming and expensive changeovers. Wire-wound rods were first used in coating machines built by Charles Mayer in the 1900's to manufacture waxed paper and carbon paper. They are still called "Mayer Bars“ by many coaters. Wet coating thickness can be accurately predicted within one tenth of a mil (.0001"). Rod selection tables allow coaters to consider coat weight and percent of solids in choosing the proper rod size for each production run. Bottom-line profits have encouraged many coaters to modify existing machines, in order to take advantage of the flexibility, the ease of use and the dollar savings associated with rod coating.
Coating Rods are accurate easy to use inexpensive
One of the more common methods of coating is the Mayer rod coater, sometimes called a metering rod coater. This equipment has advantages such as low capital cost, ease of coat weight adjustment, ease of operation, and a broad range of coat weights are possible.
Figure 1 : Mayer Road Coater In this coating method, an applicator roll delivers adhesive to the substrate being coated. The applicator roll may turn either with or against the primary web, delivering an excess of adhesive. Wire wound rods are then used to remove the excess. The rods may also turn in either direction. The amount of adhesive removed depends upon the diameter of the wire which is wound around the steel rod. Coat weight is increased by simply switching to a rod wrapped with larger diameter wire. Two rods in series often give better coating results than a single rod. The first rod has larger diameter wire and removes most of the excess adhesive. The second rod, with smaller diameter wire, smoothes the coating and produces the final desired coat weight. Mayer rod coating can deliver a broad range of coat weights.
TWO-WIRE DRAWDOWN RODS Where heavier coatings are required, manufactures provide special drawdown rods with two wires. A standard single-wire rod is over wound with a smaller wire which follows the spiral of the base wire. The result is a modified groove between the wires that will produce wet coatings up to 19 mils (.019") thick.
TRI-WIRE DRAWDOWN RODS Three Rod technology developed by for Coatings up to 56 mils (.056") thick can be produced, using three standard wires wound in a unique configuration. Two wires are wound side by side on a core rod, then a third wire follows one spiral of the base wires. The resulting groove will maximize the area between the wires, producing a coating more than six times the thickness produced by a single-wire rod!
Normally paper is coated with some specialty coatings.
Why paper is coated ?
The four main reasons for coating paper are increased
Printability (the faithfulness of an image’s reproduction) is clearly better when the ink does not penetrate the paper’s fibers and spread out. By printing on a coated sheet, the ink is applied to the coating instead of the fibers; and the coating has just enough porosity to improve drying speed, but not enough to spread the ink. However, the consistency of the ink holdout is dependent on the method used in applying the coating. Smoothness is achieved when the coating fills in the valleys of the base sheet, and a smoother surface is easier to print on. However, the degree of smoothness is greatly determined by the method used in applying the coating.
The microphotograph on the left reveals the rough surface of an uncoated offset sheet. The photo on the right shows the same type of sheet after coating and supercalendering.
A sheet’s smoothness determines its gloss . Coated papers’ three levels of gloss—matte, dull, and enamel—are usually the result of the amount of polishing that the coating receives. Matte coated papers are not polished; dull coated papers receive moderate polishing; and enamel papers receive a high degree of polishing. An exception to this rule will be explained later. Coated papers can achieve higher brightness levels than uncoated sheets because the coating pigments can be bleached more heavily than can cellulose fibers (excessive bleaching weakens the fibers).
Coating application methods There are several coating methods, but three are the most commonly used. The blade coater rolls the liquid coating onto the bottom of the paper, then uses a flexible blade to scrape away the excess coating. This method produces a sheet with high gloss, but the resultant variation in coating thickness can cause inconsistent ink holdout and produce uneven printed solids (mottle). The trailing blade coating method scrapes away excessive coating, but leaves a thick-and-thin effect. The trailing blade coating method scrapes away excessive coating, but leaves a thick-and-thin effect.
A second coating method solves this problem of an inconsistent coating thickness. The air knife coating method uses a strong blast of air to remove excess coating and leaves a layer of uniform thickness, but a sheet with poor smoothness and reduced gloss. Of course, the sheet will become smoother after being polished during the supercalendering stage, but it will still not attain a high gloss level. The air knife coating method replaces the blade with a strong blast of air. The result is a sheet with consistent coating thickness, but low smoothness.
Anti fungus is a highly versatile Isothiazolone based biocide and antimicrobial used in all water containing formulations to prevent bacteria growth and odours. The dose levels are extremely low for this highly effective product. Anti Curl Sealable coatings – both Hot and cold We have discussed this in detail Release coating – Silicone , or non silicone Release coating are based on Silicone Polymers There is a wide choice now a days They are available with Water base , solvent base or solvent less technology. The silicone polymer meant for release coating is mixed with a cross linking catalyst , generally in the proportions of n10: 1 , diluted with , water or solvent , and the solution is coated , using Gravure Coating technology , on Paper , Films , Metal Foil etc. The coated web is passed through drying and curing Hot Air Tunnel.
Varnishes / Lacquers for overlay, scratch off , peeloff Overprint varnishes are to enhance and protect the printed substrate. They are applied with special coating units that are predominantly coupled in-line to the printing press; that is to say, after single or multicolor prints, the varnish application takes place immediately. .Various varnishes are used,depending on the requirements for the coating, and thetransfer technology used. Oil-based Varnishes Oil-based varnishes can be compared to colorless offset printing inks. The main components are resins, drying oils, mineral oils, and drying agents. The oil-based varnishes are transferred onto the paper with offset inking units. They dry on the paper by absorption and oxidation. Oil-based varnishes are used to: • increase printing ink brilliance (glossy varnishes), • achieve specific matte effects (matte varnishes and satin finish varnishes), • improve abrasion resistance (protective varnishes). Oilprint varnishes tend to turn yellow. For web offset printing, special heat-set varnishes are used.
Water-based/Dispersion Varnishes Today, dispersion varnishes are widely used and offer diverse technical possibilities in offset printing. The principal components of dispersion varnishes are: • polymer dispersions, • hydrosols (resins dissolved in water), • wax dispersions for grinding and abrasion resistance, • film-forming aids, • cross-linking agents and antifoaming agents. Dispersion varnishes dry extremely quickly. They are therefore not used in standard offset inking units but in special coating units that only have few rollers or work with a chambered doctor blade system. The drying takes place purely physically by evaporation of the water, often supported by hot air application. As soon as a part of the water contained in the varnish is removed, the surface becomes non-tacky.
The most significant advantages of these modern varnishes are: • odor free, • rapid film formation (drying), • no yellowing, • zero, or only slight, powder spraying in the delivery on sheet-fed printing presses necessary, • high processing speed, • smooth surface, • dilutable and washable with water. The following qualities of the printed product are achieved by coating with a dispersion varnish: • abrasion protection, scuff resistance in a wet condition (labels), • high gloss, silk finish, or matte effects, • hot-seal resistance, • gliding quality, • deep freeze resistance, • fixing of metallic inks.
Other special functional varnishes that fulfill an additional task alongside surface finishing are: • blister varnishes to enable adhesion for blister packaging • primer (bonding agent), for instance, for further UV varnish finishing, • varnishes that can be ultrasonically welded for special finishing methods, • scented varnishes (varnishes with micro-encapsulated aromatics), • label varnishes, • two-component varnishes (varnish and hardener). Dispersion varnishes are also used for surface finishing in gravure printing . They are essentially identical to the dispersion varnishes used for offset printing.The principal difference lies in the typically much lower viscosity of those varnishes used in gravure printing.The following categories of aqueous dispersion varnishes are offered for gravure printing: • preprint and intermediate print varnishes, • high gloss and matte finishing varnishes, • special varnishes, for instance, calendar varnishes (high gloss after calendering), and varnishes with gliding properties for special finishing techniques, as well as varnishes with barrier properties, for instance, for drink packaging.
Solvent-based Varnishes Varnishes containing solvents are varnishes in which resin components are dissolved in organic solvents. The hardening takes place through evaporation of the solvent. The varnishes are called nitro varnishes, or nitro combination-varnishes, and are applied in a separate process with coating machines. Solvent-containing gravure printing varnishes are of similar composition to solvent-containing gravure printing inks. The drying of these one-component varnishes takes place purely physically. Two-component varnishes for gravure printing consist of the actual varnish and a hardening component.The cross-linking reaction begins the moment the hardening agent is added to the varnish.When running through the hot air dryer of the press, the two-component varnish receives an energy feed,which induces the further cross-linking reaction in the film. In the course of five to seven days the varnish layer hardens fully. It is then heat-resistant and resistant to many chemicals.
UV Varnishes UV varnishes are varnishes cured under UV radiation. They have a similar composition to UV inks – and are therefore completely different to oil-based and dispersion varnishes – and thus contain no volatile substances.