WO2004056156A1

WO2004056156A1 - Plastic film

Info

Publication number: WO2004056156A1
Application number: PCT/EP2003/004125
Authority: WO
Inventors: Sohaib Elgimiabi; Stefan Spiekemann; Elisabeth Cura; Hans-Martin Sauer; Rolf Hempelmann
Original assignee: Sustech Gmbh & Co. Kg
Priority date: 2002-12-16
Filing date: 2003-04-22
Publication date: 2004-07-01
Also published as: AU2003227652A1; DE10258959A1

Abstract

The invention relates to plastic film, particularly adhesive film, which has superparamagnetic, nanoscale particles of a ferrite with a volume-averaged particle diameter ranging from 2 to 100 nm. Said nanoscale ferrite particles are surface-modified, and the ferrite is selected from the group formed from ferrites of general formulas M<a>1-x M<b>xFe2O4 or Li1-xZn2xFe5-xO8, in which: M<a> is selected from among Mg, Ca, Cu, Zn, Y, V, Mn, Fe, Ni and Co; M<b> is selected from among Zn and Cd, and X = 0-1.

Description

"Plastic film"

The present invention relates to a plastic film, in particular an adhesive film, which comprises superparamagnetic, nanoscale particles, a method for producing such a film and a method for heating such a film.

Preparations of particles with particle sizes in the nanometer range (nanoscale particles) have found application in many areas of technology. This applies especially to dispersions that contain particles with magnetic, ferroelectric or piezoelectric properties and that can be heated under the influence of magnetic, electrical or electromagnetic alternating fields. These are used, for example, to produce adhesives and sealants which harden as a result of the heating induced by the application of magnetic, electrical or electromagnetic alternating fields or in which an existing adhesive bond is separated. Such adhesives and sealants are used in many industries, especially in the metalworking industry, such as. B. the aircraft industry, the vehicle industry, in commercial vehicle construction and their supplier industries or in the manufacture of machines and household appliances and in the construction industry increasingly used to bond the same or different metallic and non-metallic substrates adhesive or sealing together. This type of joining of components is increasingly replacing traditional joining methods such as riveting, screwing or welding because they offer a variety of technological advantages, for example with regard to possible recycling of the components used.

EP-A-735 121 describes an adhesive film section for a residue-free and damage-free and releasable bond consisting of a double-sided adhesive film with a handle protruding from the adhesive film, on which the bond can be released by pulling in the direction of the bond plane. However, this method can only be used if the adhesive layer of the adhesive film is a pressure-sensitive adhesive. With such adhesive connections however, only achieve low tensile or shock strengths, so that this method only for fixing small objects such as hooks and the like. Like. Is applicable in the household area.

DE-A 199 23 625 describes a process for producing redispersible metal oxides or metal hydroxides with a volume-weighted average crystallite size in the range from 1 to 20 nm, which are particularly suitable for so-called magnetic liquids (ferrofluids).

DE-A 199 24 138 describes adhesive compositions which contain nanoscale particles with ferromagnetic, ferrimagnetic, superparamagnetic or piezoelectric properties in the binder system and which are suitable for producing detachable adhesive bonds. The adhesive connections can be heated to such an extent under the influence of electromagnetic radiation that an easy solution (detackification) is possible.

WO 01/30932 describes a method for adhesive separation of adhesive bonds. The adhesive composite comprises a thermally softenable thermoplastic or thermosetable thermosetting adhesive layer and a primer layer, the primer layer containing nanoscale particles that can be heated by alternating electromagnetic fields.

WO-A 01/28771 describes a microwave-curable composition which contains particles capable of microwave absorption with a Curie temperature which is higher than the curing temperature of the composition. The particles capable of microwave absorption can be, for example, ferrites.

EP-A-0 498 998 describes a method for microwave heating a polymer material to a predetermined temperature, the polymer material containing dispersed ferromagnetic particles which have a Curie temperature which corresponds to the temperature desired by the heating. The particle diameter of the ferromagnetic material is one Range from 1 to 100 nm and the Curie temperature in a range from 50 to 700 ° C. The desired heating temperature can be, for example, the curing or melting temperature of the polymer material or the temperature required to activate a cleavage reaction.

WO 01/14490 describes a method for gluing substrates with hot melt adhesives, these being used in combination with a primer that can be activated by microwaves.

German patent application DE 100 37 883.8 describes a method for heating a substrate which contains metallic, magnetic, ferrimagnetic, ferromagnetic, antiferromagnetic or superparamagnetic particles with an average particle size between 1 and 5000 nm, the substrate being subjected to microwave radiation with a frequency in the range from 1 to 300 GHz and at the same time exposed to a direct current magnetic field, whose field strength is at least twice as high as the strength of the earth's magnetic field.

A disadvantage of the aforementioned nanoparticulate compositions is the insufficient use of the energy input. Systems of this type generally contain a high content of dispersed particles, of which only a small proportion is capable of absorbing magnetic, electrical or electromagnetic radiation of a specific irradiated frequency and thus contributes to heating. In addition to the high costs for the nanoscale particles used, which lead to an economic disadvantage of such compositions, a high content of dispersed particles generally has further disadvantages, for example with regard to the rheological properties of the systems and the properties of the bond (tendency to embrittlement) ,

The unpublished German patent application DE 101 63 399.8 describes a nanoparticulate preparation containing a coherent phase and at least one particulate phase of superparamagnetic, nanoscale particles with a volume-average particle diameter dispersed therein Range from 2 to 100 nm, which further comprises a mixed metal oxide and a method for heating a nanoparticulate preparation, in which the preparation comprises an alternating electromagnetic field and microwave radiation with a frequency in the range from 0.3 to 300 Ghz and possibly also a direct current Magnetic field with a field strength in the range of 0.001 to 10 Tesla is exposed.

Attempts made so far to incorporate the nanoparticulate compositions described in the prior art into plastic films in order to obtain films which can be selectively heated by irradiation with microwaves and can be glued in this way, for example, have only given unsatisfactory results. When processing such films by extrusion at high temperatures, the nanoparticulate particles only remain well dispersed in the films if surface-modified nanoparticles are used. However, the films obtained in this way do not show a satisfactory property profile. Firstly, the optical quality of the films obtained in this way is limited because the film material is cloudy. On the other hand, in many cases, there are also films whose smooth surface structure is disturbed or roughened by the inclusion of "clumps". These disadvantageous effects are attributed to the fact that the surface modifiers change from high processing temperatures, for example 160 to 180 ° C. dissolve the nanoparticulate particles and also form agglomerates from the nanoparticulate particles, which then impair the optical and mechanical properties of the resulting films, which can also lead to increased brittleness of the films.

The present invention is therefore based on the object of providing plastic films which can be heated by microwave radiation with optimal use of the energy input and which furthermore have good optical and mechanical properties. The knowledge of the invention is that such films can be obtained as microwave absorbers by using surface-modified nanoparticulate zinc or cadmium-containing ferrites.

The invention relates to a plastic film, in particular an adhesive film, which comprises superparamagnetic, nanoscale particles of a ferrite with a volume-average particle diameter in the range from 2 to 100 nm, the nanoscale ferrite particles being surface-modified and the ferrite being selected from the group formed by Ferrites of the general formula M ^a ι- _x M ^b _x Fe ₂ θ4 or Liι _-x Zn _2x Fe ₅ - _x θ8, where M ^a = Mg, Ca, Cu, Zn, Y, V, Mn, Fe, Ni and / or Co, M ^b = Zn and / or Cd, and X = 0-1.

The surface modification of the nanoscale ferrite particles can, for example, be a surface coating of the particles. The surface modification is intended to substantially prevent agglomeration or the growing together of the nanoscale particles and / or to ensure good dispersibility of the nanoparticulate particles in the plastic film according to the invention. Surprisingly, it was found that the surface modifications in the plastic films according to the invention are bonded to the ferrite particles in a very firm and temperature-stable manner. This enables the extrusion of the plastic films or adhesive films at high processing temperatures, for example 130 or 160 to 180 ° C, which survive the surface-modified nanoparticulate ferrite particles largely without damage. This is attributed to the fact that with the zinc and cadmium-containing ferrites used according to the invention the surface modifiers do not detach from the nanoparticulate particles, or only to a minor extent, even at high processing temperatures, and furthermore no or only a subordinate number of agglomerates is formed from the nanoparticulate particles. so that the optical and mechanical properties of the resulting films are not impaired. When using appropriate plastics, the films remain translucent and clear and retain their tough-elastic properties, so that they can still be stretched if necessary. At the same time, the plastic films according to the invention allow Simple processing, since the zinc and / or cadmium-containing ferrites used according to the invention, which in particular as a further metal ion or metal ions of one or more of the metal ions Mg, Ca, Cu, Zn, Y, V, Mn, Fe, Ni, Co or Li contain, can be selectively and quickly heated by microwave radiation by the introduced ferrite particles.

The superparamagnetic, nanoscale particles are preferably dissolved in a coherent phase. M ^a and M ^b represent a divalent metal component, in each case only one of the listed metal ions or more of the listed metal ions can be present, ie ferrites of the general formula M ^a ι_ _x M ^b _x Fe ₂ O ₄ or Liι_ _x Zn _2x Fe ₅ - _x θ ₈ , where M ^{a is} selected from Mg, Ca, Cu, Zn, Y, V, Mn, Fe, Ni and Co, and M ^{b is} selected from Zn and Cd. The stoichiometries of the individual metals are selected so that the ferrites are electrically neutral. Ferrites of the general formula M ^a ι _-x M ^b _x Fe ₂ O ₄ or Liι _-x Zn ₂ χFe _5-x θ _{8 are} preferably used, where M ^a = Mg, Ca, Cu, Zn, Y, V, Mn , Fe, Ni or Co, and M ^b = Zn or Cd, and X = 0-1. Ferrites of the general formula M ^a ι are particularly preferred. _x M ^b _x Fe ₂ O _{4 is} used, where M ^{a is} selected from Mg, Ca, Mn, Co, Fe and Ni, M ^{b is} selected from Zn and Cd, in particular Zn, and X = 0-1.

According to a particularly preferred embodiment, ferrites of the general formula M ^a _1-x M ^b _x Fe ₂ O ₄ or Liι. _x Zn ₂ χFe ₅ . _x O _{8 is} selected, X being at least 0.2, in particular for M ^a _-x M ^b _x Fe ₂ O ₄ . Ferrites of the general formula M ^a ₁ -x Zn _x Fe ₂ O ₄ with X = 0-1, in particular at least 0.2 and preferably 0.2 to 0.8, in particular 0.3 to 0.5, are also particularly preferred.

According to a further particularly preferred embodiment, the plastic film according to the invention contains ferrites of the general formula Mnι. _x M ^b _x Fe ₂ O ₄ , where M ^{b is} selected from Zn and Cd, in particular Zn, and X = 0.2-0.5, in particular 0.3-0.4.

According to a further particularly preferred embodiment, the plastic film according to the invention contains ferrites of the general formula Cθι. _x M ^b _x Fe ₂ O ₄ , where M ^{b is} selected from Zn and Cd, in particular Zn, and X = 0.2-0.8, in particular 0.4-0.6.

According to a further particularly preferred embodiment, the plastic film according to the invention contains ferrites of the general formula Niι. _x M ^b _x Fe ₂ O ₄ , where M ^{b is} selected from Zn and Cd, in particular Zn, and X = 0.3-0.8, in particular 0.5-0.6.

Also particularly preferred are embodiments which contain lithium zinc ferrites of the general formula Liι _-x Zn _2x Fe _5-x O ₈ with X = 0-1, in particular at least 0.1.

According to a further particularly preferred embodiment, the foils contain LiFe ₅ O ₈ .

“Nanoscale particles” in the sense of the present application are particles with a volume-average particle diameter of at most 100 nm. A preferred particle size range is 3 to 50 nm, in particular 4 to 30 nm and particularly preferably 5 to 15 nm. Such particles are characterized by a high degree of uniformity The particle size is preferably determined by the UPA method (Ultrafine Particle Analyzer), for example by the laser light back scattering method, and the nanoscale particles used are also superparamagnetic ,

The plastic films according to the invention are particularly suitable for the adhesive bonding of windshields in the automotive industry, since films are required here which, on the one hand, can be heated selectively and quickly by microwave radiation and, on the other hand, should not show any clouding on the adhesive seams. Because of the excellent optical properties, repair foils for windshields can also be produced with the plastic foils according to the invention. Another area of application for the plastic films according to the invention is the bonding of translucent plastics, for example for laminates.

Magnetic and electromagnetic alternating fields are suitable for introducing energy into the adhesive film according to the invention. When using alternating magnetic fields, the superparamagnetic particles used are distinguished in comparison to paramagnetic particles known from the prior art in that they have no hysteresis and no remanence. This leads to significantly more effective energy input and heating rates of the particles and the coherent phase surrounding them.

Advantageously, the superparamagnetic, nanoscale particles used according to the invention enable simultaneous optimization of the Curie temperature and the magnetic relaxation time of the particles when energy is introduced by means of a high-frequency alternating magnetic field. The Curie temperature is the maximum temperature to which a magnetic substance can be heated by the action of an alternating magnetic or electromagnetic field. It therefore represents intrinsic protection against overheating, since the maximum achievable temperature of the nanoparticles is limited. Thus, by properly selecting the particles according to the Curie temperature, excessive heating of the coherent phase can be effectively avoided. The Curie temperature can be controlled by suitable selection of the type and the respective amount of the different divalent metals. For example, in CoFe ₂ O ₄ , MnFe ₂ O ₄ or NiFe ₂ O ₄ ferrites, the Curie temperature can be reduced with increasing Zn content. The Curie temperatures of the Co-Zn ferrites are above those of the stoichiometrically analogous Mn-Zn ferrites and these in turn are higher than those of the stoichiometric analogous Ni-Zn ferrites. Suitable methods for calculating the Curie temperatures of some ferrites are listed below when dealing with the energy input through alternating electromagnetic fields.

In addition, the nanoscale particles used according to the invention must have a magnetic relaxation time that coincides with that for energy input used magnetic alternating field correlates. In addition, the relaxation times of all the particles used should be essentially identical as far as possible in order to enable optimal absorption of the energy input. As stated at the outset, this is achieved according to the invention by using nanoscale particles which are essentially of the same type with regard to their particle size distribution and morphology. The adaptation of the magnetic relaxation time to the alternating magnetic field used for energy input (τ =! Π f, τ = magnetic relaxation time, f = irradiated frequency) is advantageously achieved like the control of the Curie temperature by suitable selection of the type and the respective amount of the different ones divalent metals of the mixed metal oxide particles used. The magnetic relaxation time can thus be adapted to the frequency of technically available inductors, which enables a particularly effective use of the energy input. The frequency of suitable alternating magnetic fields is generally in a range from approximately 30 Hz to 100 MHz. For example, medium frequencies in a range from about 100 Hz to 100 kHz and high frequencies in a range from 10 kHz to 60 MHz, in particular 50 kHz to 3 MHz, are suitable. The selection of the frequency can depend on the devices available. In a special embodiment, the adhesive films according to the invention contain particles with different relaxation times τi, τ ₂ , etc. A multifunctionality of the films according to the invention can thus be achieved.

According to a second embodiment, the nanoparticulate preparation according to the invention can be exposed to an alternating electromagnetic field to introduce energy. It is preferably the electromagnetic alternating field of microwave radiation with a frequency in the range from approximately 0.3 to 300 GHz. In a preferred embodiment, the adhesive film is simultaneously exposed to a direct current magnetic field, the field strength of which can be approximately in a range from 0.001 to 10 Tesla. Advantageously, the superparamagnetic, nanoscale particles used in accordance with the invention enable an energy input through an alternating electromagnetic field simultaneous optimization with regard to both the Curie temperature and the resonance frequency. With regard to the possibilities for controlling the Curie temperature by selecting the type and amount used of the two different divalent metals in the mixed metal oxide particles, reference is made to the previous explanations on the energy input by alternating magnetic fields. It is also necessary to control the adhesive films for the best possible use of energy, also with regard to their resonance frequency, since the usable microwave frequencies are specified by the authorities.

First of all, the superparamagnetic, nanoscale particles used according to the invention, because of their uniformity with regard to particle size distribution and morphology, enable a sharp resonance frequency to be achieved in the adhesive films and not, as in the case of particulate preparations from the prior art, an ensemble of widely distributed frequencies. In these preparations from the prior art, the microwave absorption frequencies of the individual nanoparticles are never completely the same, so that only that fraction of the dispersed particles absorbs microwave energy whose absorption frequency actually corresponds to the irradiated frequency. All other particles are inactive, which leads to insufficient use of the energy input. In contrast, the nanoparticles in the adhesive films have an almost Lorentz-like relationship between absorption A (B, f) and magnetic field B: A = Ao / [(B-Bo (f)) ^{2 +} ΔB ² ] ^1/2 . The microwave absorption in the compositions is phase coherent to a significantly greater extent than in preparations from the prior art. In the compositions according to the invention, therefore, a significantly larger proportion of dispersed particles is able to absorb microwave radiation of a predetermined frequency. Because of this effect alone, the preparations according to the invention advantageously show significantly improved energy utilization.

In addition, the microwave absorption frequency, such as the Curie temperature, can also be controlled by selecting the type and amount used of the different divalent metals in the mixed metal oxide particles. this will explained below by way of example for Mn-Zn ferrites, Co-Zn ferrites and Ni-Zn ferrites.

1. Mnι_ _x Zn _x Fe ₂ O ₄ :

Calculation of the Curie temperature T _c as a function of the zinc content x (x = 0 to 0.6)

T _C [° C] = 295-221 x -247 x ²

Calculation of the zinc content x for a maximum temperature To, which must not be exceeded during microwave or magnetic field irradiation:

x = (1, 394 - 0.00405 T ₀ ) ^1/2 - 0.447 (with 295> T ₀ > 70)

linearized approximation: x = (295 - T ₀ ) / 375 calculation of the microwave absorption frequency f _ab s as a function of x (0 <x <0.7): fa _bs (x) = γ [B - 16 K (x) / 9 M (x)]

with: γ = 28 GHz / T (gyromagnetic constant);

B external DC magnetic field (in Tesla);

K = -2,800 - 2,350 x (crystal anisotropy energy density in J / m ³ ),

M = 1, 21 10 ^{5 *} (4.18 + 11.74 x - 13.17 x ² ) (spontaneous magnetization in A / m)

2. Co _1-x Zn _x Fe ₂ O ₄ :

Curie temperature T _C [° C] = 520 - 619 x for x = 0 to 0.8

Given the maximum temperature T ₀ , x can be calculated as follows: x = (520 - T ₀ ) / 619; (T ₀ in ° C, 520> T ₀ > 25)

The MW absorption frequency f _ab s is calculated as a function of x

B _A (x) = 0.15 - 0.21 x (in Tesla); (0 <x <0.7)

3. Niι. _x Zn _x Fe ₂ O ₄ :

Curie temperature T _C [° C] = 590 - 547 x - 201 x ² for x = 0 to 0.7

Given the maximum temperature T ₀ , x can be calculated as follows:

x = (4.787 - 0.00498 T ₀ ) ^1/2 - 1.36; (T ₀ in ° C, 590> T ₀ > 105)

or in a linearized approximation x = (590 - To) / 693.

The MW absorption frequency f _{a S is} calculated as a function of x

f _abs (x) = γ [B - 16 K (x) / 9 M (x)]

K = -6,200 - 8,570 x (crystal anisotropy energy density in J / m ³ ),

M = 1, 21 10 ⁵ * (2.24 + 9.15 x - 5.96 x ² ) (spontaneous magnetization in A / m)

The resonance frequency of the superparamagnetic particles used can also be controlled by additionally using a direct current magnetic field. A method and a device for ferromagnetic resonance excitation by the simultaneous action of microwaves and a DC magnetic field are described in German patent application DE 100 37 883.8, to which reference is made in full here. The energy utilization is further increased by resonance optimization with the aid of a direct current magnetic field. The frequency of the microwave radiation used is preferably in a range from 500 MHz to 25 GHz. For example, electromagnetic radiation from the so-called ISM areas (Industrial Scientific and Medical Application) can be used, in which the frequencies are between 100 MHz and 200 GHz. Available frequencies are, for example, 433 MHz, 915 MHz, 2.45 GHz and 24.125 GHz as well as the so-called cell phone bands in the range from 890 to 960 MHz and 1,710 to 1,880 MHz. Further details on alternating electromagnetic fields in the microwave range are described in Kirk Othmer, “Encyclopedia of Chemical Technology”, 3rd edition, volume 15, chapter “Microwave Technology”, to which reference is made here.

According to a special embodiment, the adhesive films according to the invention contain a multifunctional particle system. These contain two or more different types of particles, which have their absorption maximum at different frequencies. Are suitable for. B. particle systems in which one component absorbs at 2.45 GHz and another at 900 MHz. To achieve the most selective absorption possible of only one particle type of a multifunctional particle system, the previously described control of the resonance frequency via the type and amount of use of the different divalent metals and / or a direct current magnetic field can be used.

Processes for producing superparamagnetic nanoscale particles are known in principle. They are based, for example, on precipitation from aqueous solutions of metal salts by raising or lowering the pH with a suitable base or acid. DE-A-196 14 136 describes a process for producing agglomerate-free nanoscale particles using the example of iron oxide particles. DE-A-199 23 625 describes a process for producing nanoscale redispersible metal oxides and hydroxides. The disclosure of these documents is hereby fully referred to. A suitable method for producing superparamagnetic nanoscale particles which can be used according to the invention consists in the precipitation from acidic aqueous metal salt solutions by adding a base. You can do this, for example if appropriate, heated, aqueous hydrochloric acid solutions of metal chlorides for precipitation are mixed with a suitable amount of a heated base. It has proven to be advantageous to maintain a certain temperature range during the precipitation of the particles in order to achieve particles with the desired high magnetic field or microwave absorption capacity. The temperature during the alkaline precipitation is preferably in a range from 20 to 100 ° C., particularly preferably 25 to 95 ° C. and in particular 60 to 90 ° C.

In order to substantially prevent agglomeration or coalescence of the nanoscale particles and / or to ensure good dispersibility of the nanoparticulate particles in the plastic film according to the invention, the particles used are surface-modified or surface-coated. The particles preferably have a single- or multi-layer coating on at least part of their surface which contains at least one compound with ionogenic, ionic and / or nonionic surface-active groups.

These surface modifications are particularly firmly and temperature-stable bound to the ferrite particles, so that even at very high processing temperatures, the surface modifiers do not detach from the nanoparticulate particles, or only to a minor extent, and furthermore, no or only a subordinate number of agglomerates form from the nanoparticulate particles. The resulting films therefore have excellent optical and mechanical properties. This is especially true when fatty acids, esters or silanes are used as dispersing agents. At the same time, the plastic films according to the invention allow simple processing, since the zinc or cadmium-containing ferrites used can be selectively and quickly heated by microwave radiation due to the ferrite particles introduced.

The compounds with surface-active groups are preferably selected from the salts of strong inorganic acids, e.g. B. nitrates and perchlorates, saturated and unsaturated fatty acids, such as palmitic acid, margaric acid, Stearic acid, isostearic acid, nonadecanoic acid, lignoceric acid,

Palmitoleic acid, oleic acid, linoleic acid, linolenic acid and elaosteric acid, quaternary ammonium compounds such as tetraalkylammonium hydroxides, e.g. B. tetramethylammonium hydroxide, silanes such as alkyltrialkoxysilanes and mixtures thereof. DE-A-197 26 282 describes the surface modification of nanoscale particles with at least two shells surrounding the particle. WO 97/38058 describes the production of nanoscale particles modified on the surface with silanes. Full reference is made to the documents mentioned.

If surface-modified nanoparticulate particles are used, the proportion of surface modifier is generally 1 to 50% by weight, preferably 2 to 40% by weight and especially 10 to 30% by weight, based on the weight of the particles used.

Suitable organic dispersants are selected, for example, from oils, fats, waxes, esters of Cβ-Cso-monocarboxylic acids with mono-, di- or trihydric alcohols, saturated acyclic and cyclic hydrocarbons, fatty acids, low molecular weight alcohols, fatty alcohols and mixtures thereof. These include, for example, paraffin and paraffin oils, mineral oils, linear saturated hydrocarbons with generally more than 8 carbon atoms, such as tetradecane, hexadecane, octadecane etc., cyclic hydrocarbons such as cyclohexane and decahydronaphthalene, waxes, esters of fatty acids, silicone oils etc. , B. linear and cyclic hydrocarbons and alcohols.

The theological properties can advantageously be adjusted over a wide range depending on the type and amount of the dispersant.

The proportion of the nanoscale particles is preferably 1 to 70% by weight, in particular 5 to 50% by weight and especially 15 to 25% by weight, based on the total weight of the plastic film. Because of the very good ability of the plastic film according to the invention to absorb energy through absorption Magnetic or electromagnetic alternating fields, the proportion of dispersed particles required to absorb a certain amount of energy can be significantly reduced compared to particulate preparations from the prior art.

In principle, all polymers suitable for plastics or adhesives can be used as the coherent phase (binder matrix) for the plastics film, in particular adhesive film. Examples of the thermoplastic softenable adhesives include the hot melt adhesives based on ethylene-vinyl acetate copolymers, polybutenes, styrene-isoprene-styrene or styrene-butadiene-styrene copolymers, thermoplastic elastomers, amorphous polyolefins, linear, thermoplastic polyurethanes, copolyesters, polyamide resins, polyamide / EVA copolymers, polyaminoamides based on dimer fatty acids, polyesteramides or polyetheramides. In addition, the known reaction adhesives based on one- or two-component polyurethanes, one- or two-component polyepoxides, silicone polymers (one- or two-component), silane-modified polymers, such as those used by G. habenicht, “Gluing: Basics, Technology, Applications ", 3rd edition, 1997 in chapter 2.3.4.4. The (meth) acrylate-functional reactive adhesives based on peroxidic hardeners, anaerobic hardening mechanisms, aerobic hardening mechanisms or UV hardening mechanisms are also suitable as an adhesive matrix. Specific examples for the installation Thermally labile groups in reaction adhesives for the purpose of later cleavage of these bonds are the adhesives according to WO 99/07774, in which at least one constituent component contains di- or polysulfide bonds In a particularly preferred embodiment, these adhesives can also contain solid cleavage reagents in crystalline, encapsulated, chemis ch blocked, topologically or sterically inactivated or kinetically inhibited, finely dispersed form, as disclosed in the as yet unpublished DE-A-199 04 835 on pages 14 to 16. Another possibility is the use of polyurethane adhesives which contain the amine derivatives disclosed in DE-A-198 32 629 as a cleaving agent. The ones disclosed in the two aforementioned publications Cleavage agents are expressly part of the present invention. Polyethylene films, in particular polyethylene films containing polypropylene, ethylene-vinyl acetate copolymers (EVA) and / or polyethylene terpolymer, are particularly preferred.

The coherent phase of thermally activatable chemically reacting adhesives generally contains one or more components which are amenable to a polyreaction. These include, for example, adhesives that contain polyisocyanates with capped thermally activated isocyanate groups and a component with groups that are reactive toward isocyanate groups, such as, for example, B. contain a polyol.

Another object of the invention is a method for producing a plastic film as previously defined, in which plastic granules with nanoparticulate ferrites are dispersed. The extrusion is carried out at temperatures in the range from 120 to 200 ° C., in particular 130 to 190 ° C. and particularly preferably 160 to 180 ° C. It is particularly preferred in this process to use surface-modified nanoparticulate ferrite particles, since in this way the optical and mechanical properties of the resulting plastic films can be further improved.

Another object of the invention is a method for heating a plastic film, as previously defined, in which the film is exposed to an alternating magnetic or electromagnetic field.

As mentioned before, when heating by alternating magnetic fields, both the maximum achievable temperature via the Curie temperature and the magnetic relaxation time can be controlled simultaneously. For heating, the plastic film is preferably exposed to an alternating magnetic field with a frequency in the range from 30 Hz to 100 MHz. The frequencies of conventional inductors are suitable, for example medium frequencies in a range from 100 Hz to 100 kHz or high frequencies in a range from 10 kHz to 60 MHz, in particular 50 kHz to 3 MHz. As also mentioned above, the nanoparticles in the invention enable Plastic films utilize the energy input available electromagnetic radiation in a particularly effective manner. The same applies to heating by alternating electromagnetic fields of microwave radiation. Microwave radiation with a frequency in the range from 0.3 to 300 GHz is preferably used. All previously mentioned officially approved microwave frequencies are suitable, such as the ISM frequencies mentioned, which are in a range from 100 MHz to approximately 200 GHz. It had already been pointed out that when using alternating electromagnetic fields for heating, both the maximum achievable temperature via the Curie temperature and the ferromagnetic resonance frequency can be regulated simultaneously. In order to set the resonance frequency, a direct current magnetic field with a field strength in the range of approximately 0.001 to 10 Tesla is preferably used in addition to the microwave radiation. The field strength is preferably in a range from 0.015 to 0.045 Tesla and in particular 0.02 to 0.06 Tesla. Even when the plastic film is heated by alternating electromagnetic fields, the energy input is used particularly effectively for the reasons mentioned above.

The alternating field-induced heating of adhesive compositions by the method according to the invention is carried out with the aim of modifying the adhesive properties. A distinction can be made between two basic embodiments, each with different variants. On the one hand, an adhesive composition can be used for the heating, the coherent phase of which comprises at least one thermally curable polymer and / or at least one thermally polymerizable monomer, the adhesive composition curing as a result of the heating which is brought about. On the other hand, an adhesive composition can be used for the heating, the coherent phase of which comprises a hardened (set) adhesive, the bonding being able to be released as a result of the heating which is brought about.

The loosening of an adhesive connection by targeted heating of a plastic film (disbond-on-command) by entering energy in the form of magnetic or alternating electromagnetic fields can be based, for example, on a reversible or irreversible softening of the adhesive connection. For example, the adhesive used can be a hot-melt adhesive which reversibly softens as a result of the heating caused by the action of magnetic or electromagnetic radiation. This reversible softening can then be used both for the targeted manufacture and for the targeted loosening of an adhesive connection.

In a further embodiment, the set adhesive film contains thermally labile bonds, which are split as a result of the heating caused. In the case of such adhesives, the adhesive connection can be released without the action of chemicals and under conditions under which the joined substrates are not significantly heated and are therefore thermally damaged.

In a further embodiment, the adhesive composition contains cleavage reagents dispersed in a form which can be activated thermally, for example in encapsulated, crystalline, chemically blocked, topologically or sterically inactivated or kinetically inhibited form. The components which trigger the cleavage are present, for example, in the form of microcapsules which additionally have superparamagnetic, nanoscale particles and are dispersed in the adhesive composition. By irradiation with magnetic or electromagnetic alternating fields of suitable frequency, these microcapsules can be thermally opened and thus the adhesive preparation can be split. Here, too, the heating can be limited to precisely these capsules, so that the energy consumption required can be greatly reduced compared to a homogeneous distribution in the entire composition.

As in the previously described method for producing an adhesive connection, an adhesive composition can also be used in the method for releasing an adhesive connection which has at least one magnetic or electromagnetic alternating fields activatable primer in combination with at least one adhesive.

The plastic films according to the invention can advantageously be modified in a multifunctional manner. Such multifunctional plastic films contain at least two different types of superparamagnetic, nanoscale particles which differ, for example, in their magnetic relaxation times or in their magnetic resonance frequency to such an extent that they can be heated individually. It is also conceivable to excite one type of particle by a magnetic and the other by an electromagnetic alternating field. Multifunctional plastic films are suitable for a variety of individual applications.

The invention is illustrated by the following non-limiting example.

example 1

0.04 mol FeCI ₃ ^* 6 H ₂ O are dissolved in 40 ml H ₂ O and heated to 80 ° C. 0.008 mol NiCI ₂ * 6 H ₂ O and 0.012 mol ZnCl ₂ are dissolved in a mixture of 10 ml H ₂ O and 1 ml HCl and also heated to 80 ° C.

In a separate beaker, 16 g NaOH are dissolved in 400 ml H ₂ O and heated to 80 ° C. The metal salt solutions are combined and added to the NaOH solution with vigorous stirring. The mixture is stirred at 80 ° C for about 30 minutes. The suspension is brought to sedimentation. The precipitate is washed several times with water until a pH of 10 is reached. The powder is then suspended in 300 ml of water and heated to 80.degree. 5 ml of oleic acid are added dropwise with vigorous stirring.

After stirring for 15 minutes, the modified powder is sedimented and washed 3 times with water. The precipitate is dried in a vacuum cabinet at 60 ° C. overnight. The modified powder shows very good dispersion in a non-polar solution with a narrow particle distribution.

Then a hot melt granulate made of EVA (ethylene-vinyl acetate copolymer) is extruded at 160 ° C. with 20% by weight of the surface-modified nanoparticles. After cooling, the resulting material is granulated and extruded in a ratio of 1: 3 with polyethylene at approx. 160 ° C. to form an adhesive film. A clear adhesive film with a proportion of 3.5% by weight of the ferrite used (Nio _{, 4} Zno _{, 6} Fe ₂ θ ₄ , particle diameter approx. 10 nm) is obtained. The film has a smooth, slightly shiny surface (planar surface structure). It has tough elastic properties and, like a pure polyethylene film, can subsequently be stretched.

Comparative Example 2

0.04 mol of FeCl ₃ * 6H ₂ O are dissolved in 40 ml of H ₂ O and heated to 80 ° C. 0.02 mol FeCl ₂ ^* 4H ₂ O are dissolved in a mixture of 10 ml H ₂ O and 1 ml HCl and also heated to 80 ° C.

In a separate beaker, 16 g NaOH are dissolved in 400 ml H ₂ O and heated to 80 ° C. The metal salt solutions are combined and added to the NaOH solution with vigorous stirring. The mixture is stirred at room temperature for about 30 minutes. The suspension is brought to sedimentation. The precipitate is washed several times with water until a pH of 10 is reached. The powder is then suspended in 300 ml of water and heated to 80.degree. 5 ml of oleic acid are added dropwise with vigorous stirring.

After stirring for 30 min, the modified powder is sedimented and washed 3 times with water. The precipitate is dried in a vacuum cabinet at 60 ° C. overnight. A hot melt granulate made of EVA is then extruded at 160 ° C. with 20% by weight of the surface-modified nanoparticles. The resulting material is already cloudy and almost opaque after this process step, which is attributed to the agglomeration of the nanoparticles. After cooling, the material is granulated and extruded in a ratio of 1: 3 with polyethylene at approx. 160 ° C to form an adhesive film. An almost opaque and relatively brittle adhesive film with a matt and rough surface is obtained (proportion Fe ₃ θ ₄ : 3.5% by weight, particle diameter approx. 10 nm). Subsequent stretching of the film is difficult because of the brittleness of the film.

The comparison of the plastic film produced according to the invention according to Example 1 with a “conventionally” produced film according to Comparative Example 2 shows the advantageous material properties of the plastic film according to the invention. While the nanoparticles in the “conventionally” produced film apparently agglomerate strongly, the ferrite particles do not or do not agglomerate in the plastic film according to the invention only to a very small extent, so that the film remains translucent and tough-elastic and has a largely planar surface.

Claims

claims

1. Plastic film, in particular adhesive film, which comprises superparamagnetic, nanoscale particles of a ferrite with a volume-average particle diameter in the range from 2 to 100 nm, the nanoscale ferrite particles being surface-modified and the ferrite being selected from the group formed by ferrites of the general formula M ^a ι_ _x M ^b _x Fe ₂ O ₄ or Liι- _x Zn2 _X Fe5 _-x O ₈ , where

M ^a = Mg, Ca, Cu, Zn, Y, V, Mn, Fe, Ni and / or Co, M ^b = Zn and / or Cd, and X = 0-1.

2. Plastic film according to claim 1, characterized in that M ^a = Mg, Ca, Cu, Zn, Y, V, Mn, Fe, Ni or Co, and

M ^b = Zn or Cd.

3. Plastic film according to claim 1 or 2, characterized in that the film contains ferrites of the general formula M ^a ι _-x M ^b _x Fe ₂ O ₄ , wherein M ^{a is} selected from Mg, Ca, Mn, Co, Fe and Ni .

M ^{b is} selected from Zn and Cd, in particular Zn, and X = 0-1.

4. Plastic film according to one of the preceding claims, characterized in that X> 0.2.

5. Plastic film according to one of the preceding claims, characterized in that the film contains ferrites of the general formula M ^a ι _-x Zn _x Fe ₂ O ₄ , where X = 0.2-0.8.

6. Plastic film according to claim 5, characterized in that M ^a = Mn and X = 0.2-0.5.

7. Plastic film according to claim 5, characterized in that M ^a = Co and X = 0.2-0.8.

8. Plastic film according to claim 5, characterized in that M ^a = Ni and X = 0.3-0.8.

9. Plastic film according to claim 1, characterized in that the ferrite is a lithium zinc ferrite of the general formula Liι _-x Zn _2x Fe _5-x θ ₈ with X = 0-1, in particular X> 0.1.

10. Plastic film according to claim 1, characterized in that the ferrite is UFe ₅ O ₈ .

11. Plastic film according to one of the preceding claims, characterized in that the nanoscale particles have a volume-average particle diameter in the range from 3 to 50 nm, in particular 4 to 30 nm, in particular 5 to 15 nm.

12. Plastic film according to one of the preceding claims, characterized in that the material of the film contains polyethylene, polypropylene, ethylene-vinyl acetate copolymer (EVA) and / or polyethylene terpolymer, in particular polyethylene and EVA.

13. Plastic film according to one of the preceding claims, characterized in that the nanoscale ferrite particles have a single- or multi-layer coating on at least part of their surface, comprising at least one compound with ionogenic, ionic and / or nonionic surface-active groups.

14. Plastic film according to claim 13, wherein the compound with surface-active groups is selected from the salts of strong inorganic acids, saturated and unsaturated fatty acids, quaternary ammonium compounds, silanes and mixtures thereof.

15. A method for producing a plastic film according to one of the preceding claims, characterized in that plastic granules are dispersed with nanoparticulate ferrites.

16. The method according to claim 15, characterized in that the extrusion is carried out at temperatures in the range from 130 to 200 ° C, in particular 160 to 180 ° C.

17. A method for heating a plastic film according to one of claims 1 to 14, characterized in that the plastic film is exposed to an alternating magnetic or electromagnetic field.

18. The method according to claim 17, characterized in that the plastic film is exposed to an alternating magnetic field with a frequency in the range of 30 Hz to 100 MHz.

19. The method according to claim 17, wherein the plastic film is exposed to the alternating electromagnetic field of microwave radiation with a frequency in the range from 0.3 to 300 GHz and optionally additionally to a direct current magnetic field with a field strength in the range from 0.001 to 10 Tesla.

20. The method according to any one of claims 17 to 19, wherein the plastic film can only be heated up to a maximum temperature regardless of the energy introduced by the alternating field, nanoscale particles with divalent metals selected in such a way that their Curie temperature is at the maximum temperature corresponds.