Low-shear dosing of micro-encapsulated adhesives and liquids0 pages

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Low-shear dosing of micro-encapsulated adhesives and liquids

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Micro-encapsulated materials are

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taking an increasingly important

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place in almost all branches of

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modern industry. Tests were carried

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out back in the 1930's using this

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procedure in the printing industry,

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in the field of carbonless carbon

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paper (so-called NCR paper). Since

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then the process has gained new

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momentum and found new possible

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applicationsduetonewtechnologies

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and production processes. The point

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of the microencapsulation is to lend

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new properties to various forms of

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media by the addition of micro-

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encapsulated fillers. For example,

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two-component adhesives can be

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processed with just one medium

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because the reactive components

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are included in the base materi-

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al, in microcapsules. However, the

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processing and, above all, the do-

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sing of these media present new

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challenges to the maker of process

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machinery.

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Microencapsulated products are

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used in the following branches of

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industry:

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- Printing industry (e.g. carbonless

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carbon paper)

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- Food industry (e.g probiotic

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bacteria in yoghurt)

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- Agricultural chemicals (e.g water-

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soluble fertilisers with a long-term

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effect)

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- Pharmaceutical industry (e.g.

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medications that act at a specific

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location)

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- Cosmetics industry (e.g. UV filters

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in sun screen)

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- Adhesives industry (e.g. fillers

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for model making and aircraft

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construction) and much more.

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In the production process for micro-

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encapsulation tiny portions of so-

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lid, liquid or gaseous substances

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are coated with a sheath made of

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polymer or inorganic wall materials.

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These capsules (microcapsules) can

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have a diameter of down to less

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than 1 urn.

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Types of microcapsules

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There is a basic distinction between

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two different types of microcap-

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sules: monolithic (capsules with a

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solid core) and reservoir microcap-

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sules (capsules with a liquid or gase-

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ous core). The selection of the wall

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materials depends on the intended

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purpose and the location and time

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of release.

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The wide variety of applications

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requires various types of release of

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the materials. Depending on the

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type of application, the capsules

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should be released at the place

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of use by mechanical work or the

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dissolving of the wall material, or

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even not destroyed at all. In the

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case of microencapsulated adhe-

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sives the reactive components are

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enclosed in the material in a glass or

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polymer sheath. The chemical basis

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can include the following, among

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others: Polyesters, epoxy resins,

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acrylates and polyurethanes. Before

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the material is applied, hence during

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conveying and dosing, it is essential

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to prevent the capsules from being

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destroyed. Engineers from the

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ViscoTec company have tackled this

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problem in depth and have deve-

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loped extraction systems for just

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about all type of containers that

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are available on the market (bar-

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rel, hobbock, can, cartridge, free

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feeding, etc.) so as to ensure gentle

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conveying to the dispensers.

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(Figs 2a+b: Coating of screw

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threads with self-hardening screw

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locking material)

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For example, dispensers of this type

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have been used successfully to coat

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screw threads with a self-hardening

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screw locking material. Here the

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microcapsules are intended to only

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break open through the shearing

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forces when the screw is turned in

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the thread and hence produce an

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adhesive and sealing effect.

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Mikroverkapselter

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Klebstoff

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Zerstorung der

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Kapselwand

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durch Scherung

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Kiebschicht

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mit Kapselwandeinschlussen

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Processing

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The problems that arise when

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processing micro-ecapsulated liquids

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can be seen at once. Extraction

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from the containers in which they

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are supplied and the subsequent

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dosing and application to the target

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location must be done virtually

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without producing any shearing

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forces and friction so that the

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microscopically small balls are not

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destroyed. Forthatreasonpiston-type

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pumps are unsuitable for conveying

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and dosing because the pressure

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pulse produced by the piston stroke

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can lead to the destruction of the

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ViscoTec

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