Converting Case Study
pressure sensitive adhesives. Because of the risk of ridging, this wire wound rod coating method is limited to low viscosity formulations that offer good flow characteristics. As the range of substrates and product requirements expand to meet the complex needs of today, so too does the range of adhesives and sealants, many of which can be applied in the converting and/or product development environments using pilot/ production coating and laminating systems.
PLAYING A VITAL ROLE
Adhesives punch above their weight. Despite the fact that adhesives are generally only used in discrete low volume amounts many packaged food items, labels, tapes and many industrial products and commercial goods simply wouldn’t be able to function effectively without some form of adhesive input. Synthetic adhesives only made their debut in the 1940s, so in many respects the development of modern adhesives is recent and very much on-going. One of the first beneficiaries of adhesive system advancement was the corrugated sector when mineral-based sodium silicate glue was replaced with resin-modified starches. Synthetic polymers formed the cornerstone of modern adhesive development, invariably with many of those utilising thermo-plastic or cross-linking polymers.
Laminate films provide flexible packaging solutions for many product sectors, including frozen foods and for many luxury items. The strength and flexibility, excellent printability and presentation and customisable nature of laminate structures makes for an attractive proposition. In addition to holding the various plies together for the lifetime of the laminate; adhesives, along with the completed structure itself, may be engineered to meet specific performance requirements. The requirements may include, gas permeability and/or optical clarity, heat resistance, moisture and light resistance, conductivity and perhaps thermo forming capability or even flameproofing – all of which must be factored for.
The laminate adhesive must be resistant to the effects of tunnelling: a localised separation or del-lamination of the substrate that sometimes arises as a result of using substrates with differing degrees of extensibility. Also, if the adhesive is not applied correctly for instance, the coat weight is too light or too heavy the process of laminating will fail as it will bubble, blister or again it might delaminate from the base material.
HOT MELT ADHESIVES
Hot melt adhesives are highly versatile and can be optimised for many different applications including carton and case closing; chocolate/confectionary wrap- around boxes, as well as for fruit and vegetable case/tray making and for labelling. Many converters will be most familiar with the pressure sensitive and hot melt adhesives. The former belongs to a class of drying adhesives, whereby most of the drying takes place before the surfaces or adherends are married together. It is pressure that optimises contact bonding. Pressure sensitives are based on soft polymers or elastomers. Because they stay tacky for long periods, which is why they are ideal for many labelling products and for tape either from the roll or produced in die cut shapes. With hot melts the bond is formed by solidification of molten adhesive and then by loss of heat. They are based on resins, waxes and thermoplastics that melt in the temperature range of between 100 to 250 degrees centigrade.
In recent times formulae have become more complex, resulting in hot melts that provide instant adhesion on almost any substrate. Hot melt is favoured for many reasons: no large drying ovens are needed, there is no use of solvents and consequently no emission control equipment is needed. Hot melts must flow, wet substrates thoroughly, then cool rapidly. The final bond strength is attained when the temperature falls to ambient levels.
Viscosity must be low at the application temperature, but not so low that excessive
spreading and penetration occurs, otherwise blocking could be problematic. Hot melt coating can be accomplished on equipment and systems via extrusion coating or gravure coating. When it comes to extrusion, the hot melt coating head is comprised of the extrusion head, heater hose and heated tank with pump. Adjustments can be made to the temperature of the system and the pump speed in order that coat weights down to as low as 10 gsm can be obtained. For adhesives with a viscosity of below 500 centipoises and with temperatures of up to 150 degrees centigrade can be coated on the VCM or VCML-Lab/pilot coater with gravure cylinders using a reverse angle doctor blade setting.
To the uninitiated, coating and laminating may, at first glance, seem straightforward but that is far from the case. Trialling and developing adhesive and other material solutions takes up time; especially when using a production coating machine. One reason being that a production machine lacks the flexibility and does not allow the degree of experimentation that a pilot, print/coat and laminating system allows for. In addition to being able to print, coat and wet or dry bond laminate on a wide range of flexible substrates the system also enables users to undertake small-scale production. Hot air, infrared and UV curing is available while print and coating options include screen, slot die, reverse roll, gravure, gravure offset, flexography, etc., are selectable.
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convertermag.com
July/August 2021
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