Adhesive Application Methods
A variety of application methods are used in the converting industry to apply adhesives and coatings to the primary web. Each method has its advantages and disadvantages. Some methods are more common in the wide web, long-run format, while others are more prevalent in the narrow web, shorter-run format. In addition, some methods will be better suited to the adhesive or coating chemistry being used. Often the type of equipment dictates which methods are available and it’s up to the converter to work with their adhesive and coating supplier to find the best product solution for the coating methods available.
The predominant coating methods used in the industry are known as pre-metered. These include methods such as direct, reverse, and offset gravure (flexographic coating falls in this category) as well as smooth roll transfer coatings. Less prominent, is known as the flood coat method with Mayer rod being the most common. The pre-metered method accurately measures and applies a pre-determined amount of coating or adhesive to the web. The flood coat method applies an excess of coating which is then metered or removed to leave the desired amount on the coated web.
The most common coating method in wide web converting is direct gravure. In this type of application, the coating or adhesive is applied directly to the substrate using an engraved cylinder. These cylinders are etched/engraved either mechanically, chemically or with a laser. The etching/engraving yields a line of cells with a line count in number of lines/inch and a cell volume in BCM(Billion Cubic Microns). The most common cell pattern is quadrangular with tri-helical and pyramidal also seen. Most commonly the cylinder is partly immersed into an open pan of coating/adhesive, the cylinder picks up the coating to be applied. Excess is removed with a doctor blade prior to the cylinder coming into contact with the web, transferring the coating to the web. The coating weight applied is determined by the cell pattern, cell volume, and the solids/viscosity of the material being applied. Coating weight is generally not affected by line speed. Some disadvantages of this method are that coating weight is limited by cell volume and only minor adjustments can be made without changing cylinders. Due to the cell pattern appearance on lay down, appearance may appear mottled until adhesive/coating can flow and level. A smoothing bar can help improve this appearance if needed. A version of the above coating method is called reverse gravure. In this method the above criteria all apply; however, the cylinder moves in the opposite direction of the web creating a “wiping effect”. Cell pattern is removed leaving a smoother coating. This method is most used for barrier coatings in which flow and leveling is critical to achieve proper barrier properties.
In narrow web applications the predominant application is flexographic transfer or offset gravure. In these methods an etched/engraved cylinder applies the adhesive/coating to a rubber transfer roller, which then applies the coating or adhesive to the web. This method is ideally suited to short and medium runs. The rubber plates are very economical and can allow for adhesive/coating to be applied in patterns or full face easily. Care must be taken to the chemical make-up of the coating/adhesive being applied to insure compatibility with the rubber roll. Water-based chemistries are most often used with this application method.
The last application method for discussion is smooth roll transfer coating. While transfer coating can be used with other chemistries, it is most common with solvent-less adhesives. In this method, a series of rotating rolls in-close proximity to each other split and meter the adhesive onto the web to be coated. Coating weight is determined by roll gaps and roll rotation as a percentage to line speed. These rolls can vary from 3-6, but most common are 3 and 4 roll transfer coaters. The first two rolls are known as gate rolls and hold the mixed adhesive in what’s known as the “puddle”. The next roll is the transfer roll which applies the adhesive to the applicator roll which then applies the adhesive to the web to be coated. Due to the relatively short pot-life (work-life) of the mixed adhesives, the “puddle” is the only area in which mixed adhesive is held. It is supplied (fed) with a mix, meter dispense pump (MMD).
As one can see there are a variety of application methods used in the converting industry to apply both adhesives and coatings. Each has its advantages and disadvantages but all work successfully in the industry. It is advisable to work closely with your supplier of the adhesives/coatings to match the best product/chemistry to the available application equipment and of course end-use needs.