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Accurate bonding with printable adhesives

Thanks to further developments in quality, screenprintable pressure-sensitive adhesives have made their mark in many industrial applications.
Which screenprintable pressure-sensitive adhesives are available on the market and where are their preferred areas of application?

For processing liquid adhesives, the screen printing method has established itself alongside the conventional systems like curtain coating, spraying, anilox and knife. The specific advantages of using the screenprinting method for processing adhesives was already recognised in the early 1970’s. Today pressure-sensitive adhesives, which have been printed on substrates can be found in many industrial areas of application, as for example, the automotive industry, household white goods and the electronic industry. More can be found in the production of optical displays as well as the advertising and sign making industry.
The variety of these areas of application calls for an even greater variety of substrates, which have to be made self-adhesive. This begins with simple cardboard, metallic foils, coated films and the plastics industry. Here film, foam and hard plastic made from the usual materials (PVC, polycarbonate, polyester, polyethylene, polypropylene, PMMA, PUR etc.) are to be given a self-adhesive finish.

Using the screenprinting method, the liquid adhesive is printed through a stencil with a squeegee on to the substrate. The stencil is made from a woven polyester filament, in which the mesh area is only open for the pattern of the adhesive to be printed. The choice of mesh (the number of threads and thread diameter) determines the thickness of the adhesive deposit. Dry coating thicknesses of 50µm are usual for technical applications today; for special applications where more thickness is required, it is possible to lay down 160µm.

Adhesive groups

The screenprintable adhesives, which are liquid at room temperature, are usually solvent based, aqueous dispersions or UV curable 100% systems. All three systems have their advantages and disadvantages.

Solvent based pressure-sensitive adhesives:
The advantages of solvent based systems lie mainly in good printability, very good surface flow and extraordinarily short drying times. After a short time, silicone paper can be applied or other processing steps can be carried out. The disadvantages of the system lie mainly in the flammability of the solvents used, as well as fumes being breathed in. Added to this, solvents have to be used for cleaning and thinning.
The early screenprintable pressure-sensitive adhesives of the 1970’s had low adhesive properties and were mainly used for decorative applications. The products available today have been optimised for industrial applications, with regard to their tear strength, tack value, UV and heat resistance. Rubber and acrylic resin based formulations are available, which differ in price and property profile. Both product groups have very high water and steam resistance, which is especially advantageous for use where condensation occurs.
The economical rubber based pressure-sensitive adhesives have very high initial bond strength, but are yellow to brownish in colour, which intensifies when exposed to light. UV resistance is limited. Acrylic resin based pressure-sensitive adhesives have only light colouration. They are resistant to light and UV and thus are good for outdoor applications. Initial bond strength is not as high as with the rubber based adhesives, but develops within a few hours and then even exceeds the values of the rubber based adhesives.

Dispersion based pressure-sensitive adhesives:
The advantages of the water based systems lie in the environmentally friendly usage. The products are non-flammable and practically odour-free.  The disadvantages are to be found in the printability, which particularly affects the dwell time in the printing stencil. Drying times are longer than solvent based products.
By being able to replace polyvinyl acetate by acrylic resin copolymers as a binding material for screenprintable pressure-sensitive adhesives, it was possible to advance into areas of application which were hitherto the reserve of the solvent based formulations. Good UV resistance, high heat resistance and the ability to prevent plasticiser migration are the advantages of this group. Limited water and steam resistance are the disadvantages.

UV curable pressure-sensitive adhesives:
Only recently, liquid 100% systems have become available, which create pressure-sensitive adhesive when cured by UV rays after printing. The advantages of this range are mainly to be found in the problem free application; long dwell times, good printability and excellent flow, fast “drying” by chemical means and environmentally friendly processing, as the liquid is 100% adhesive. Here additional equipment for drying and more comprehensive process monitoring is required. The range of UV curable pressure-sensitive adhesives does not yet have the application variety of the dispersion based adhesives, but augmentation is being worked on at full speed.

Application Examples

Self-adhesive products made with pressure-sensitive adhesives are in use in a broad range of industrial applications. The particular advantage in using screenprinting for this class of adhesive lies mainly in the adhesive contours being exactly defined. Exact positioning to within millimetres, as well as fine filigree and stand alone adhesive geometry, cannot be done with any other processing technology.

A special area of application for screenable, solvent based pressure-sensitive adhesives is the transfer.
Here we mean a decal, which is used for decorative purposes or in the advertising business. Because of the smooth and even adhesive film, only solvent based rubber and acrylic resin formulations can be used. Where transparency and UV resistance are required, acrylic resin pressure-sensitive adhesives are used. Otherwise, the more economical rubber-based pressure-sensitive adhesives can be applied. Depending on the area of application and size of the motif, a differentiation is made between wet and dry transfers.

The main area of application for screenable adhesives is for the self-adhesive finish of instrument panels.
For optical display panels, polyester and polycarbonate film are mainly used. Display texts, viewing windows and dials are then printed in different coloured and transparent inks. Die cut parts, which have been finished with pressure-sensitive adhesives are then bonded to injection-moulded housings and fibre optics made from PMMA, PS, ABS, PA etc. The main areas of application are instrument panels in the automotive industry and operating components of a wide variety of electrical devices. Screenable acrylate dispersions are predestined for this area of application. Laying down a very high coating thickness is possible due to the solids content. This is necessary when bonding to injection moulded goods with their unavoidable pock holes. Specially formulated pressure-sensitive adhesives have an aggressive tack, which is important for successful bonding of difficult substrates. Finally, the adhesive film is transparent and is resistant to heat and UV light.
Printing is normally done using mesh with a thread count between 21 and 43 per centimetre – in special cases up to 8 threads per cm. After drying in a convector oven or drying tunnel, the product is covered with silicone paper and if required, is subsequently die cut.
In order to achieve the best and reproducible adhesive results for industrial applications, it is recommended that bonding is done with pressure through a heated silicone rubber stamp (e.g. pressure dwell time 4 -5 secs, stamp temperature approx. 40° C with 3 – 4 bar pressure).
A further area of application is membrane switches, the upper film of which is for identifying and operating the circuits underneath. On the bottom side, dispersion or rubber-based (containing solvents) pressure-sensitive adhesive is applied by screenprinting. After drying, the panel film can thus be bonded to the overlay underneath.
This overlay forms the second layer of the entire circuit and this is printed as required with e.g. a silver conductive paste. In order to prevent contact with the underlayer film, a spacer is glued in between, which can be made self-adhesive by screenprinting. Still in evaluation is the development of a spacer, which can be completely printed as pressure-sensitive adhesive.
It is also possible to provide a self-adhesive finish to the underside of the complete membrane switch, in order to facilitate positioning when applying to the final product.

Self-adhesive films and foams.
It is possible to furnish films and foams with a partial finish of pressure-sensitive adhesive where previously adhesive tape was used as a means of fixing. The selection of the appropriate pressure-sensitive adhesive system depends on what it is going to be used for. Restrictions only apply as far as soft PVC is concerned, because there is a danger of plasticizer migration into the adhesive layer, as well as unusual compounds, which contain e.g. PVDF.
Plastic film printed on both sides is used particularly for short term advertising campaigns in the fast food branch. In applications behind glass, the film has to be able to be removed from the glass without leaving any residue – and this can be best achieved by using cohesive and UV resistant acrylic systems.
To finalise from a selection of additional areas of application, we would like to mention foam bonding of mouse pads, screen dot shaped transfer adhesive film, rhomboid and line shaped adhesive systems for photo albums, tacky document security systems and rub-off letters and motifs.

Summary

Advantags of processing with screenprinting:

  • Exactly determinable adhesive contour, even for finest lines, stand alone dots and complicated geometrical patterns
  • No overspray, no die cut waste or loss of adhesive through extensive area coating
  • Definable and reproducible coating thicknesses by selecting different mesh types  (from 5 – 160µm dry coating thickness per cycle)
  • Economical series production through high coating speed and multiple repeat processing

In the foreseeable future, when new raw materials can be combined with the relevant drying and cross-linking technologies in screen printing, we can look forward to even more areas of application for screenable pressure-sensitive adhesives.

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