Adhesive Applications Adhesive application specifics By Tom Kerchiss of RK Print Coat Instruments. R
egardless of whether the product being processed is a flexible bag for frozen peas, a peel back and apply wound care patch, or a PS- tape for the fixing, fastening and bundling together of electronic and electrical cables for an automotive or aerospace applications:– every segment of industry has different adhesive requirements. In flexible packaging adhesives must provide good clarity and bond strength. For frozen or chilled items, paperboard cartons and other packaging materials must be able to withstand lengthy periods of low temperatures and rapid thaw conditions. Even though adhesives are often applied in small quantities, without adhesives and the coating systems and devices that apply them many products simply couldn’t function. For instance, it would be more than inconvenient if content package end-flaps couldn’t be sealed. Fortunately, various speciality adhesives are available to safely and securely seal packs including certain dispersion adhesives and hot melts. For many applications the use of a single material may not provide the degree of functionality that is required. To meet specific
barrier and other performance requirements, multi-layered composite structures often only a few microns thick are created in order that product performance is fully optimised. A common method of producing a composite material is to laminate various polymeric materials to other films or to foils, papers, etc., coating them with a polymeric adhesive. This is one way to ensure that performance requirements such as tensile strength, moisture and high gas permeability can be met.
When laminating substrates together thought must be given to the adhesive that is to be used in order to avoid problems such as tunnelling or de-lamination. This can occur when one ply of film proves to be more extensible or has a different rate of relaxation to the substrate that it is being bonded. As far as adhesives are concerned there is a wider choice now than ever before. Solventless laminating options include low isocyanine-monomer chemistries. Water-based polyurethane and emulsion based laminating adhesives. Of course, there are alternatives to adhesive laminating. Heat seal coatings is
some instances may be appropriate. These can be applied to a substrate via RK’s VCM pilot/ production coater or VCML-Lab/Pilot Coater. When needed the roll of coated material is unwound and the heat seal adheres to another substrate using heat and pressure. Heat seal coating is used for many electronic and medical applications. An advantage of this type of adhesive is that it does not have to bond immediately, unlike laminating adhesives. With heat seal, bonding is distinctly separate and controllable. RK Print Coat supplied a print/coat machine for a heat- sealing application in which the converter needed to provide printed and coated lacquered layers of discrete patterns. Hot melt adhesives made their debut as long ago as the 1960s. Their use at that time was generally restricted to low-speed case sealing, corrugated box end flaps, etc. Adhesive technology progressed with pressure sensitive hot melts used for labelling applications and for many high-speed production purposes. Hot melts play a key role in boat building, automotive and other assembly applications. Hot melts form a strong bond quickly and are compatible with most materials.
They are less sensitive to water, moisture and humidity than many other adhesives. Hot melt coating can be undertaken on RKs VCM and VCML-Lab/Pilot Coater via extrusion coating or alternatively via gravure coating. Extrusion coating is suitable for high viscosity products while gravure meets the requirements of lower viscosity products. With hot melt adhesives the bond is formed by solidification of the molten adhesive through the loss of heat. Resins, waxes and thermoplastics that melt in the temperature range of 100 to 250 C are used.
RK Print Coat Instruments hot melt coating head when configured for extrusion is able to regulate coating weight down as low as 10 gsm. For those adhesives with a viscosity of below 5,000 centipoise and with a maximum temperature of below 150C products can be coated with gravure. In this case gravure cylinders are employed using a reverse angle doctor blade setting.
Xwww.rkprint.com
12
July/August 2022
www.convertermag.com
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