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Blog Post By Inky Fingers

Adhesives On Trial

Tom Kerchiss from RK Print Coat Instruments Ltd talks about adhesives in another instalment of Inky Fingers

The development of aqueous flexible packaging laminating adhesive mirrors closely the history of solvent and other adhesives that are used in paper, board, film and foil packaging and converting. Developmental history involved a substantive period of correcting inconsistencies, compensating for known variables; finding workarounds when new or unexpected problems arose, and at all times working as swiftly as possible to bring products to market. Monitoring, quality control and product development tools have had an important part to play in all of this and no doubt this will continue to be the case.

Laminating adhesives were initially cellulose-related structures dissolved in solvents. Second generation thermoplastic rubber soon followed; then two component polyurethane systems and isocyanate cured polyester materials came on the scene; the latter engineered for demanding applications such as boil-in-the-bag products.

Profit is not the only driver of product development; the decline in the popularity of solvents in the light of environmental awareness was a prime motivator for more efficient and environmentally acceptable laminating adhesive systems to be developed. The first aqueous system to make its debut was the relatively simple pressure sensitive acrylic co-polymers, followed by the then more complex polyurethane dispersion systems cured with isocyanate.

One should not forget those polymer chemists, the formulators and the companies that they worked for had their work cut out when developing aqueous adhesives for flexible packaging. Unlike aromatic hydrocarbons, esters and other substances, which readily dissolve polymers and resins – water dissolves few of these substances.  Furthermore, additives added initially to the dispersion or emulsion interfered with final adhesion.  Success was not over night but in time laboratory research and shop floor trials paid off and products eventually earned end user trust and became accepted.

Packaging is thought to account for around 40 per cent of adhesive usage, and although used in relatively low quantities adhesives such as pressure sensitive, energy curable, hot melts and many others including natural rubber and latex adhesives are essential for packaging related applications and for many industrial processes. For instance, natural rubber lattices formulated with acrylic resins are used in the preparation of cold seal adhesives, which are then employed in package wrapping of heat sensitive foods such as chocolate.

Do it yourself, or let someone else do it for you? For many converters and product developers this has been a common, yet sometimes difficult decision to make

The work outlined earlier with regard to aqueous laminating adhesives could in many respects stand for product development work in other adhesive product areas, for instance UV curable PSAs: substantial laboratory work and then a lot of real world product testing.

Improved curing technology, higher coating productivity, lower overall production costs and reduced VOC emissions have made UV curable PSAs a more attractive proposition with UV curable styrenic block co-polymers, cationic UV curable epoxy functional liquid rubbers and acrylic polymers being developed over time and in order to meet the varying PSA requirements of customers and their applications.

UV laminating adhesives are used in some segments of the narrow web market in place of PSA. In applications such as flexible package printing they protect a printed graphics from mechanical and chemical abrasion and from scuffing and tearing. Products laminated with these adhesives can be processed immediately due to instant cure and bond setting, and are useful in just-in-time production scenarios.  Issues that both formulators and users need to be aware of include the risk of chemical migration/product contamination, temperature stability, reactivity and clarity.  

Innovative adhesives continue to be developed, adapted or refined. For example, natural rosin obtained from conifers was the first hot melt. Although still in use as a single component, if formulated with ethylene-vinyl acetate (EVA) co-polymer the cohesion of the melt is enhanced making it suitable for glass jar and can labeling.

Other types of hot melt include the pressure sensitive hot melts based on thermoplastic rubbers, polyamide and polyester hot melts.                           

It is important to take into account factors such as the adhesive coating thickness prior too and after application. A hot melt advantage is that coating thickness is not affected by curing. Whereas solvent-based adhesives may lose as much as 70 per cent of the applied thickness while being dried/cured; meaning that it may be necessary to tinker with the amount of adhesive being applied in order that a desired ‘final’ coating adhesive thickness can be assured.

The successful application of new adhesive technology requires the input of multiple decision makers including, product development engineers, and adhesive suppliers and of course converters and other supply chain providers such as substrate providers and equipment vendors.  The end customer is the ultimate governor; they are the ones that have the final say with regard to whether a product meets the requirements or not.

Do it yourself, or let someone else do it for you? For many converters and product developers this has been a common, yet sometimes difficult decision to make. If work is farmed out to outside contractors with pilot/lab coating lines, some of which may also have test rigs, there can be a trade off between cost and control. Moreover, and for reasons of commercial security many organizations increasingly prefer to keep every aspect of their business in-house and under their control.

In addition, the need to control and document procedures for reasons such as meeting accreditation requirements has led to a greater demand for customized and bespoke quality control and product development systems that enable users to trial new formulations, undertake coat/laminate pilot runs and evaluate how substrates/inks/coatings and adhesives, etc., perform over time and under realistic real world production conditions.

Pilot print/coat and laminating systems such as RK’s Rotary Koater or the customer bespoke VCM (Versatile Converting Machine) speed product development and assist in bringing products to market quicker. Many of the devices, for instance the Control Coater are compact, multi-functional devices that are easily sited on any convenient bench top – enabling users to monitor quality at any time, eliminating a dependence on third parties.

The Rotary Koater offers a high degree of flexibility, allowing users to select from more than 15 different print head and coating technologies. The versatility of this system does not stop there in that a choice of web paths are available and a choice of drying/curing and laminating technologies. For this reason manufactures, including many of the adhesive producers have opted for the Rotary Koater or the high tech VCM, which may be considered top of the range.

In conclusion, technological advancements in adhesives, coatings, varnishes and UV inks have been made easier by the advances in quality control and monitoring equipment. Additional benefits have included reductions in material and energy waste.  Moreover, the breadth of range and low system cost has put test, monitoring and product development tools within everyone’s reach.  For instance, RK Print Coat Instrument portfolio of products ranges from the K Hand Coater on up to the VCM. The former being a simple but effective means of applying coatings onto many substrates including, glass plates, wood, plastic films, foils, paper and board. Two or more coatings can be applied side-by-side in a single operation. Other devices available include hand held trouble shooting devices such as the Esiproof for flexo printing and the Printing Proofer, as well as the FlexiProof 100,  FlexiProof UV and FlexiProof UV/LED.

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