Low-shear dosing of micro-encapsulated adhesives and liquids
Tests were carried out back in the 1930's using this procedure in the printing industry, in the field of carbonless carbon paper (so-called NCR paper). Since then the process has gained new momentum and found new possible applications due to new technologies and production processes. The point of the microencapsulation is to lend new properties to various forms of media by the addition of microencapsulated fillers.
For example, two-component adhesives can be processed with just one medium because the reactive components are included in the base material, in microcapsules. However, the processing and, above all, the dosing of these media present new challenges to the maker of process machinery.
Microencapsulated products are used in the following branches of industry:
- Printing industry (e.g. carbonless carbon paper)
- Food industry (e.g probiotic bacteria in yoghurt)
- Agricultural chemicals (e.g water-soluble fertilisers with a long-term effect)
- Pharmaceutical industry (e.g. medications that act at a specific location)
- Cosmetics industry (e.g. UV filters in sun screen)
- Adhesives industry (e.g. fillers for model making and aircraft construction) and much more.
In the production process for microencapsulation tiny portions of solid, liquid or gaseous substances are coated with a sheath made of polymer or inorganic wall materials. These capsules (microcapsules) can have a diameter of down to less than 1 µm.
Types of microcapsules
There is a basic distinction between two different types of microcapsules: monolithic (capsules with a solid core) and reservoir microcapsules (capsules with a liquid or gaseous core). The selection of the wall materials depends on the intended purpose and the location and time of release.
The wide variety of applications requires various types of release of the materials. Depending on the type of application, the capsules should be released at the place of use by mechanical work or the dissolving of the wall material, or even not destroyed at all. In the case of microencapsulated adhesives the reactive components are enclosed in the material in a glass or polymer sheath. The chemical basis can include the following, among others: Polyesters, epoxy resins, acrylates and polyurethanes.
Before the material is applied, hence during conveying and dosing, it is essential to prevent the capsules from being destroyed. Engineers from the ViscoTec company have tackled this problem in depth and have developed extraction systems for just about all type of containers that are available on the market (barrel, hobbock, can, cartridge, free feeding, etc.) so as to ensure gentle conveying to the dispensers.
For example, dispensers of this type have been used successfully to coat screw threads with a self-hardening screw locking material. Here the microcapsules are intended to only break open through the shearing forces when the screw is turned in the thread and hence produce an adhesive and sealing effect.
Processing The problems that arise when processing microencapsulated liquids can be seen at once. Extraction from the containers in which they are supplied and the subsequent dosing and application to the target location must be done virtually without producing any shearing forces and friction so that the microscopically small balls are not destroyed.
For that reason piston-type pumps are unsuitable for conveying and dosing because the pressure pulse produced by the piston stroke can lead to the destruction of the fillers. Pressures of more than 10 bar are regarded as dangerous for media that have been microencapsulated. Gear pumps also cannot be used as dosing systems due to the shearing forces that they inherently produce. Which dosing systems can be used then for the handling, transport and application of microencapsulated liquids?
The endless piston principle
The choice must fall on a process that produces little shear, is highly accurate and at the same time handles the product gently. Due to these requirements, the endless piston principle of the ViscoTec company has shown itself to be the ideal technology for the extraction, preparation and dosing of microencapsulated fluids. The dosing pumps that work volumetrically convey regardless of the viscosity or the back-pressure. The special dosing geometry guarantees a pulse-free conveying stream.
The interaction between the rotor and the stator produces enclosed chambers with identical volumes that also do not change during the dosing process. The results is that media that are dosed with as much solids as possible are conveyed with very little shear and with very low friction without the fillers being destroyed. The medium is thus conveyed from the inlet to the discharge side of the pump.
A dosing geometry without any re-entrant angles and optimised (minimum) dead space volumes ensures a constant volume flow on the basis of the "first in first out" principle. The amount to be dosed is regulated by the drive unit proportionally to the speed of rotation and thus can be set continuously and is free of pulses. The pump system forms a sealing line between the rotor and the stator. For that reason the system does not need any additional valves to prevent any dripping or running out of the medium when dosing is stopped. Abrasive and chemically aggressive materials are also no problem for ViscoTec dosing systems.
A number of different elastomers that are chemically very resistant are available for use as the stator material. The dosing pumps can therefore be designed optimally for the most varied kinds of media. This is of special importance with regard to the chemical basis of microencapsulated adhesives. The comparatively difficult handling and the chemical and physical properties of the products require systems that are enclosed as far as possible all the way from material extraction up to to the dosing or filling, without any additional handling steps.
Product feeding, filling and dosing
A compact filling device was developed by ViscoTec to deal with this and other aspects. The dosing system combines all the positive properties of the ViscoTec endless piston technology in a compact unit. It was designed to fill media partially automatically onto all kinds of small containers with great precision and while treating the product gently. Filling can be done with high security and repeat accuracy into everything from syringes to cartridges to customer-specific containers. Filling according to the level of the surface of the material prevents air from getting into the container or product during filling. The extraction of material from the hobbock, feeding freely from a funnel and extraction by means of a suction lance can be done compactly on a frame.
The individual components can be put together specifically for each application. This allows not only the filling but also the dosing to be done on the following components in point and caterpillar form (Fig. 6: Dosing).
The engineers of ViscoTec Pumpen- u. Dosiertechnik GmbH would be glad to provide you with help and consultation on all matters concerning the dosing and preparation of microencapsulated liquids.