Abstract:
A microcapsule system is used to store active agents and release them as a function of specific environmental parameters. The active agents are contained in capsules having a capsule wall comprising an environment-induced material. To allow sequential, time-controlled release of the active agents, provision is made for a plurality of capsules to be accommodated in at least one outer capsule, thereby forming an inner encapsulation layer, wherein the outer capsules form an outer encapsulation layer. Like the capsule wall of the inner capsules, the capsule wall of the outer capsule is made of an environment-induced material.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a microcapsule system for environment-induced release of active agents. 
     BACKGROUND ART 
     “Environment-induced” is understood to mean, here and below, that the walls of the capsules of the microcapsule system comprise a material which becomes unstable under the influence of one or more specific environmental parameters, such that the corresponding capsules become permeable or burst open and release the active agent optionally contained therein. Examples of such environmental parameters are pressure, temperature, deformation, light and other electromagnetic radiation, particulate radiation, chemical substances etc. Time should also be viewed as an environmental parameter for the purposes of the invention. 
     Such a microcapsule system is known from DE 40 00 920 A1, for example. In said patent, microcapsules filled with skin care oil are applied to the surface of a cloth. In this way, the skin care oil is protected against evaporation during storage of the cloth. If the cloth is rubbed against the skin, for example, the capsule walls burst as a result of deformation, such that the skin care oil contained in the capsules is released into the surrounding area and may perform its action on the skin of the user. 
     However, in many instances of use, it may be desirable for not all the active agents to be released at once, but for release of the active agents from the respective microcapsules to proceed in a specific sequence over time. 
     BRIEF SUMMARY OF THE INVENTION 
     This object is achieved by a microcapsule system for environment-induced release of active agents, comprising a plurality of capsules having a capsule wall comprising an environment-induced material, an active agent being contained in at least some of the capsules, wherein the plurality of capsules are accommodated in at least one outer capsule and thus form an inner encapsulation layer and the capsule wall of the outer capsule likewise comprises an environment-induced material and the outer capsule forms an outer encapsulation layer. 
     Through the “encapsulation” provided according to the invention of inner capsules in at least one outer capsule, it is possible to achieve sequential release of the active agents, i.e. release occurring in stages over time. The active agents stored in the microcapsule system are thus not necessarily released at once from the respective microcapsules, but rather in a freely selectable sequence over time. This opens up numerous new possibilities, precisely adaptable to the respective requirements, for use of the microcapsule system according to the invention. 
     Advantageous modifications of the invention are indicated in subclaims. 
     A modification of the invention increases the number of possible stages over time. The number of possible encapsulation stages is limited only by the maximum possible size of the capsules of the outermost encapsulation layer, which may vary from one application to another. 
     It is possible to release relatively large amounts of active agent from an encapsulation layer at the same time when said encapsulation layer is broken open. 
     Complex release sequences for the active agents contained in the capsules of an encapsulation layer may be achieved by two modifications of the invention. 
     In many instances, it is desirable to release not the same but different active agents in sequence. 
     The encapsulation layers can be broken open from the outside inwards. The active agents are thus not merely released from the corresponding capsules in sequence, but also discharged into the surrounding area in sequence. 
     In a modification of the invention, breaking open proceeds in the opposite direction, i.e. from the inside outwards. This is particularly advantageous if, for example, different active agents are firstly to be mixed together before the outermost encapsulation layer is broken open and the active agent mixture thus produced is released to the surrounding area. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described in more detail below with the aid of two exemplary embodiments and with reference to the drawings, in which 
     FIG.  1 : shows a greatly enlarged section through afirst exemplary embodiment of a microcapsule system; 
     FIG.  2 : is a view similar to FIG. 1 after bursting open of the microcapsules of an outer encapsulation layer of the first exemplary embodiment; 
     FIG.  3 : is a view similar to FIG. 2 after bursting open of the microcapsules of an inner encapsulation layer of the first exemplary embodiment; 
     FIG.  4 : shows a greatly enlarged section through asecond exemplary embodiment of a microcapsule system; 
     FIG.  5 : is a view similar to FIG. 4 after burstingopen of the microcapsules of an inner encapsulation layer of the second exemplary embodiment; 
     FIG.  6 : is a view similar to FIG. 5 after burstingopen of the microcapsules of a middle encapsulation layer of the second exemplary embodiment; 
     FIG.  7 : is a view similar to FIG. 6 after burstingopen of the microcapsules of all the middle encapsulation layers of the second exemplary embodiment; 
     FIG.  8 : is a view similar to FIG. 7 after burstingopen of the microcapsules of a further inner encapsulation layer of the second exemplary embodiment; 
     FIG.  9 : is a view similar to FIG. 8 after burstingopen of the microcapsules of the outermost encapsulation layer of the second exemplary embodiment; and 
     FIG.  10 : shows a section through an example ofapplication of the microcapsule system of FIGS. 4 to  9 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, a microcapsule system bears the overall reference numeral  10 . It comprises a plurality of outer capsules  12  of an outer encapsulation layer, only one of which is illustrated in FIGS. 1 to  3 , however. This is designated as outer capsule  12 , for the sake of simplicity. 
     The outer capsule  12  contains an active agent W 1 , and a plurality of capsules  14  of an inner encapsulation layer. These contain an active agent W 2  and are known hereinafter as inner capsules  14 . 
     The capsules  12  and  14  are each surrounded by a very thin capsule wall  16  and  18  respectively, which comprises an environment-induced material. In the present exemplary embodiment, this means that the outer capsule wall  16  bursts open under the influence, lasting for a certain period, of a temperature at the level of human body temperature, whereas the material of the capsule wall  18  of the inner capsules  14  comprises a material, which breaks down under the action, of longer duration, of human sweat and thus allows the capsules  14  to burst open. 
     In the present exemplary embodiment, the active agent W 1  is a relatively mild chemical substance which reacts with human sweat to neutralize it. Active agent W 2  is a similar substance which is more highly concentrated, however, and is thus more effective. 
     The microcapsule system  10  may be applied to a cloth with a binder in the manner described in DE 40 00 920 A1. Alternatively, the microcapsule system  10  may also be suspended in an emulsion, which is applied directly to the appropriate points on human skin. 
     As is clear from FIG. 2, the capsule wall  16  of the outer capsule  12  bursts first of all under the influence of body temperature, whereby the active agent W 1  and the inner capsules  14  are released. The active agent thus reaches the skin of the user directly and may prevent the breakdown of sweat present there and stop further sweat formation. However, if, for example, after a relatively long period of heavy physical activity, a large amount of sweat is exuded, which can no longer be adequately combated by active agent W 1 , the capsule wall  18  of the inner capsules  14  is broken down under the action of this additional amount of sweat, thereby releasing the more effective active agent W 2 . This allows a need-oriented sequential release of active agents W 1  and W 2 , which display different levels of efficacy. 
     Reference will now be made to the second exemplary embodiment of a microcapsule system  10  illustrated in FIGS. 4 to  9 . Functionally equivalent elements are provided therein with the same reference numerals as in the exemplary embodiment illustrated in FIGS. 1 to  3 . The following discussion will, moreover, concentrate in particular on the differences with regard to the first exemplary embodiment. 
     The microcapsule system  10  illustrated in FIGS. 4 to  9  has a three-layer structure and serves to produce a layer comprising a pressure-activatable multi-component adhesive. 
     The microcapsule system  10  again comprises a plurality of capsules  12  of an outer encapsulation layer, which comprise a thin and relatively porous, gas-permeable capsule wall  16 . FIGS. 4 to  8  show only one of the capsules  12  of the outer encapsulation layer. For the sake of simplicity, this is known hereinafter as outer capsule  12 . 
     This outer capsule  12  encloses a plurality of capsules  20  and  22  of a middle encapsulation layer, which are each surrounded by a thin capsule wall  24  and  26  respectively and are designated below as middle capsules  20  and  22 . The middle capsules  20  in turn contain an active agent B, in this case hollow microspheres, which, in the final adhesive, serve as a filler, which is useful for example in filling unevennesses in the surfaces to be adhered, together with inner capsules  14  having a capsule capsule wall  18 , containing an active agent, namely epoxy resin A. The capsules  14  of the inner encapsulation layer are designated hereinafter as inner capsules  14 . The middle capsules  22  contain as active agent D a curing accelerator together with capsules  28  of a further inner encapsulation layer, which contain a curing agent as active agent C and comprise a thin capsule wall  30 . 
     The materials of the capsule walls  16 ,  18 ,  24 ,  26  and  30  are environment-induced in the following manner: the capsule wall  16  of the outer capsule  12  breaks down under heat, more precisely at a temperature above 40° C. The capsule walls  24 ,  26  and  30  of the middle capsules  20  and  22  and the inner capsules  28  respectively break down under the influence of the active agent mixture AB, i.e. the epoxy resin/hollow microsphere mixture. The capsule walls  18  of the inner capsules  14  burst under mechanical load, i.e. under pressure or deformation for instance. 
     The mode of operation of the microcapsule system  10  illustrated in FIGS. 4 to  9  will now be described with reference to the example of application illustrated in FIG.  10 . 
     First of all, the outer microcapsules  12  of the microcapsule system  10  are applied with a binder layer  32  to an appropriate surface  34  of a workpiece  36 . The microcapsules  12  are then covered by a soft, impact-repellent film  38 . In this state, the workpiece may be stored with the microcapsule system  10  for a relatively long period, without the adhesive layer becoming active and without breakdown of the individual components from which the adhesive layer is made. 
     To activate the adhesive layer, i.e. to trigger the release sequence of the active agents A, B, C and D contained in the microcapsules  12 ,  16 ,  20 ,  22  and  28 , the film  38  is peeled off, as indicated in FIG. 10 by the arrow  40 . The workpiece  36  with the layer consisting of the microcapsule system  10  is then pressed against the wall to which it is to be adhered. In this way, the capsule walls  18  of the inner capsules  14  containing the epoxy resin A are deformed, thereby bursting and releasing the epoxy resin A. In this way, the hollow microspheres B and the epoxy resin A mix together inside the middle capsules  20 . This is illustrated in FIG.  5 . 
     Since the capsule walls  24  of the middle capsules  20  are unstable with regard the epoxy resin A, these also burst, such that the mixture of hollow microspheres B and epoxy resin A reach the interior of the outer capsule  12  (c.f. FIG.  6 ). 
     The capsule wall  26  of the middle capsule  22  then also comes into contact with the epoxy resin A, thereby disintegrating and, in addition to the hollow microsphere/epoxy resin mixture AB, the curing accelerator D and the capsules  28  containing the curing agent C also spread out inside the-outer capsules  12  (c.f FIG.  7 ). 
     Since, however, the capsule wall  30  thereof is likewise unstable with regard to the hollow microsphere/epoxy resin mixture AB, said wall disintegrates, such that finally the four substances hollow microspheres B, epoxy resin A, curing agent C and curing accelerator D are present inside the outer capsule  12  in a mixed state (c.f FIG.  8 ). 
     In the case of the selection of components under consideration, this causes an exothermic reaction, which leads to an increase in temperature, which opens the capsule wall  16  of the outer capsules  12 , such that the now ready-mixed multi-component epoxy resin adhesive is present in the form of a reacting and setting continuous adhesive layer (c.f FIG.  9 ). 
     A microcapsule system  10  constituted in this way may thus produce a high-strength multi-component adhesive, the individual components of which are initially separate from one another and may therefore be stored for a relatively long period. Through the simple application of pressure, the release sequence of the microcapsule system  10  is triggered, whereby the individual components of the multi-component adhesive are mixed together and the adhesive is activated. 
     The present multi-layer microencapsulation may naturally also be used in other sectors, for example for the sequential release of aromatic substances, drugs, lubricants or even fertilizers or pesticides. 
     Mention is made above of capsule walls bursting in response to environmental factors. It goes without saying that more gentle, incomplete opening of the capsules is feasible. All that is important is that the permeability of a capsule wall with regard to the capsule contents changes through the action of an environmental parameter.