Impact protection by means of a non-Newtonian fluid, and

Object with such impact protection

TECHNICAL FIELD OF THE INVENTION

The invention relates to a method for producing a non-Newtonian fluid in particular for impact protection, a method for producing an impact protection by means of a non-Newtonian fluid, an impact protection by means of a non-Newtonian fluid and an object with such impact protection.

BACKGROUND OF THE INVENTION

Known from US 5599290 A and GB 2349798 A are polymer shells for protecting objects from impacts, in which a non-Newtonian fluid is used in each case. US 2541851 A, to which reference is made in its full scope for the disclosure of non-

Newtonian materials and their properties, teaches non-Newtonian fluid

compositions. Corresponding products based on polyborosiloxanes are known from Dow Corning Corp., Midland, USA under the trade name "3179" and the Crayola LLC, Easton, Pennsylvania, USA, under the trade name "Silly Putty". The latter is a plasticine also designated as "intelligent plasticine" or "bouncing putty". US

2002/0187859 A1 , JP-H07-216138 and WO 2005/000966 A1 disclose mixtures of such materials and elastomers in applications for absorbing impacts.

Non-Newtonian fluids such as bouncing putty are difficult to process on account of their viscosity properties and are therefore usually processed into shells or extruded or kneaded plastic mixtures to form products.

WO 2005/000966 A1 discloses moulding material comprising mixtures of bouncing putty and elastomers which are produced by mixing, in particular kneading of bouncing putty and molten elastomer at high temperature and subsequent extrusion. US 6548177 B2 discloses a combination of two elastomer layers having different shear moduli, which are provided between a shatter protection layer and an adhesive layer and which can be applied to a glass display surface by means of the adhesive layer. In order to produce this combination, for example, a film serving as a shatter protection layer is coated with a pressure-sensitive film on which a fracture- protective film is laminated which in turn is coated with a pressure-sensitive adhesive.

DISCLOSURE OF THE INVENTION

It is the object of the invention to provide a method that significantly improves the handling ability of the non-Newtonian fluid so that various advantageous

applications of the non-Newtonian fluid are possible. For example, the method according to the invention should enable impact protection to be formed, which is improved at least under the following aspects: high impact resistance,

small layer thickness,

low manufacturing costs,

possible optical transparency,

homogeneity and

touch sensitivity.

Here the term "impact protection" is understood to be a material layer, e.g. in the form of a layer or a layer introduced into a shell, which serves to protect any objects, layers, body parts etc. located thereunder from damage or injuries by impacts. The invention also relates to applications of the method e.g. for the manufacture of impact protection as well as impact protection and an object provided with impact protection.

The said object is solved by a method in which at least one starting material that can be cross-linked to form a non-Newtonian fluid and a cross-linking agent dissolved in a polymer are provided, wherein the solvent has an inhibiting effect on the cross- linking of the starting material, these components are mixed and the solvent is removed from the mixture so that the cross-linking agent is released and the starting material is cross-linked by means of the cross-linking agent. Advantageously the viscosity of the mixture can be adjusted by means of the solvent for easy processing, in particular easy and homogeneous application of the mixture to, for example, a plastic film. To this end, the mixture can comprise suitable viscosity modifiers. Suitable methods can be used for application such as, for example, doctor coating, roll coating or casting.

According to the invention, the starting material is a non-cross-linked or partially cross-linked polymer. The term polymer comprises homopolymers, copolymers and mixtures thereof. The crosslinking of such polymer to form the non-Newtonian fluid is only carried out after application of the mixture to, for example, a plastic film and the crosslinking is started by removing (e.g. evaporating) of at least a part of the solvent and thus releasing the cross-linking agent in the mixture. This can take place at room temperature, but heat and/or negative pressure can also be used.

The invention enables a defined synthesis of an impact-energy-absorbing polymeric material with a non-Newtonian fluid in which impact-energy-absorbing properties adapted to the particular application can be adjusted. For adjusting these properties, in particular the degree of cross-linking of the partially cross-linked polymer to form a non-Newtonian fluid and/or its chain length,

the chain length of the non-cross-linked polymer to form a non-Newtonian fluid,

the concentration of the non-cross-linked or partially cross-linked polymer to form a non-Newtonian fluid in the solvent,

the composition of the non-cross-linked or partially cross-linked polymer from various non-cross-linked or partially cross-linked polymers, the concentration of the cross-linking agent,

an addition of fillers and/or

the creation of interpenetrating network structures can be suitably selected. Also a plurality of layers of the material can be combined with respectively different impact-energy-absorbing properties. The invention makes it possible to produce impact protection which can be adjusted to individual requirements. It is thus possible to adapt the absorption maximum of the impact energy according to the particular requirement and to optimize the impact protection for different properties such as, for example, impact resistance, small layer thickness, optical transparency, low manufacturing costs, homogeneity and touch sensitivity. Thus, for example, it is possible to produce impact protection with a minimal thickness and optimal impact energy absorption properties for a mobile telephone according to the specific weight of the device and a typical drop height of, for example, about 80-100 cm, or impact protection which is optimized for larger weights and drop heights, e.g. for electronic measurement equipment used by craftsmen. It is also possible to produce impact protection with several layers/plies each of different or the same impact protection properties in order to thus create impact protection optimised to different cases of application, e.g. a plurality of different drop heights.

In particular non-cross-linked and/or partially cross-linked polyorganosiloxanes are suitable for optimal adjustment of the impact energy absorption properties. In the method according to the invention, polyorganosiloxanes can also be adjusted to a corresponding molecular weight before and/or after a (partial) cross-linking by forming a specific chain length in order to ensure an impact energy dependent absorption maximum required in a product. The synthesis of the non-cross-linked polyorganosiloxanes can, for example, be accomplished by means of methods known from the literature such as, for example condensation reaction of linear oligomers, for example with acid catalysis, or anionic or cationic initiated ring opening polymerisation of cyclic siloxanes.

The properties and/or impact absorption regions for impact reactions of the impact protection can be adjusted in a required and defined manner by mixing

polyorganosiloxanes of different molecular weight and/or with different organic side groups.

The non-Newtonian material properties can additionally be optimised in different ways, for example by adding additives such as stearic compounds, for example stearic acid, or aluminium stearate, and/or mechanically, for example by repeatedly pressurising the cross-linked polymer in a manner known per se. The pressurisation can be accomplished by rolling, milling, pressing, kneading and also, in a non- contact manner, by ultrasound.

The invention advantageously uses non-Newtonian fluids which can be obtained by cross-linking polyorganosiloxanes having molecular weights between 92 and

150,000 g/mol, for example, between 700 and 55,000 g/mol, where a boron compound such as, for example, boric acid, is used as a cross-linking agent for the cross-linking. By means of a suitable choice of molecular weight or chain length in this range, impact reactions of the impact protectors can be adjusted in the manner required by the user.

The invention enables a very much thinner impact protection layer compared with the prior art to be formed using a non-Newtonian fluid on a substrate. In general, any solid product surface can be used as a substrate. The substrate can advantageously comprise a plastic film which can adhere to most diverse surfaces so that an improved impact protection can easily be provided for different products. It is then preferable to proceed such that the impact protection is inserted in a film shell or after applying the impact protection to a film, a second film is laid on the impact protection layer and is connected, e.g. welded to the first film in the edge zone to form a film shell. If a film shell is provided, impact protection having a low viscosity can be used.

The film shell formed by welding two films or in another suitable manner can be very thin in the finished state. Such a shell can be coated on one side with an adhesive layer so that the resulting impact protective film can easily be applied by a user onto an object to be protected, e.g. to a contact-sensitive screen of a smartphone.

The layer of non-Newtonian fluid can advantageously be configured to be very thin and typically has a thickness between 20 μηη and 500 μηη, in particular between 5 μηη and 200 μηη. If one film is provided, this preferably has a thickness between

50 μηη and 1 mm, preferably between 100 μηη and 500 μηη, further preferably between 150 μηη and 300 μηη.

The impact protection and one or more optionally provided films can be configured to be transparent so that a surface provided therewith can be translucent. This is suitable in particular for applications in touch-sensitive screens. The impact protection of the present invention can also be configured to be non- transparent and/or coloured by adding fillers such as, for example, titanium oxide or silicon oxide, to the non-Newtonian fluid. By means of such fillers, the impact resistance properties can be further optimised and the absorption maximum of the impact protection can be increased. Such coloured and/or non-transparent impact protections are suitable for surfaces which do not need to be translucent such as, for example, the rear side of a mobile telephone. It is also possible to produce a combined impact protection which has a transparent region for example for the touch-sensitive screen and a non-transparent and/or coloured region for example for the frame of the screen.

Applications of the impact protection for protection of humans and animals in sport, leisure and professionally are also possible, e.g. in protective vests, in protectors or in protective glazings.

The impact protection according to the invention can also be formed as an adhesive formulation. Adhesives which harden by pressure, temperature, light and/or drying can be used to produce the adhesive formulation. In particular, it is possible to use such adhesives which harden by drying which also harden when solvent is extracted according to the invention. In this way, an impact protection can be applied rapidly and cost-effectively to a substrate.

If the impact protection is to be present as adhesive formulation, the non-cross- linked or partially cross-linked polymer which can be cross-linked to form a non- Newtonian fluid can at least partially have a functionalisation, in particular a vinyl group, which enables it to form a covalent bond with an organic unsaturated adhesive component. It is thereby possible to influence the adhesive properties in addition to the impact absorption properties by appropriate modifications.

Introduction of vinyl substituents into the non-cross-linked or partially cross-linked polymer into the polymer chain thereof can be achieved, for example, by means of an equilibration reaction, or a ring opening polymerisation from the following monomers:

Octamethyl cyclotetrasiloxane Tetramethyl tetravinyl cyclotetrasiloxane

Preferably a linear alpha, omega-hydroxy-terminated polydimethyl siloxane which can be obtained by the ring opening polymerisation shown above, having a molecular weight between at least 222 and 150,000 g/mol, for example, 20,000- 30,000 g/mol, is used. More preferably the molar ratio of the monomers octamethyl cyclotetrasiloxane to tetramethyl tetravinyl cyclotetrasiloxane lies in the range between 5 and 50, in particular between 10 and 20.

In order to produce the impact-energy-absorbing adhesive formulation the non- cross-linked and/or partially cross-linked polymer can, for example, have terminal dimethylsiloxy, hydroxy and/or vinyl groups, where the functional groups form the adhesive domains of the fluid adhesive formulation.

The mixture according to the invention for producing the impact protection comprising a non-Newtonian polymer can also be formed by addition of an adhesive composition disclosed in EP 0 688 847 B1 having vinyl groups and organometal catalysts as adhesive having an unsaturated adhesive component.

The fluid used can be stabilised by forming co-polymers by means of reaction of double bonds or other suitable active chemical groups between the unsaturated organic adhesive component and the functional group of the non-cross-linked or partially cross-linked polymer such that the resulting material retains its impact- energy absorbing properties but no longer flows under the action of gravity. The resulting adhesive formulation can then be dimensionally stabilised depending on the composition, e.g. by pressure, temperature, light and/or drying. A copolymer which cures in these ways can be used such that it is also dimensionally stabilised in the process step of evaporation of the solvent according to the invention in which the cross-linking agent is dissolved. The non-Newtonian fluid produced according to the invention, formulated as adhesive, having impact energy absorption potential can thus be applied very rapidly and cost-effectively to a substrate and serve to adhesively bond various layers of an impact-energy-absorbing material.

The invention is particularly suitable for protecting touch-sensitive screens, for example, of smartphones and tablet PCs since the impact protection according to the invention can be manufactured to be transparent, have small thickness and chemical surface inertness. The transmittance of the impact protection or an impact protective film formed thereby can be adjusted so that for visible light it lies in the range of 70% or more.

A vacuum lamination can advantageously be used for extracting the solvent and laminating films to form impact protection in the method according to the invention. However, volatile solvents or very small solvent fractions can be used which allow operation at atmospheric pressure without vacuum.

According to the invention, the solvent is the carrier of the cross-linking agent and is used to adjust the viscosity and to inhibit cross-linking reactions before extracting the solvent. By this means it is possible to provide a system or a composition with which cross-linking can be started at a defined time (specifically during extraction of the solvent) and under defined conditions. The invention therefore provides a "chemical switch" in a method for applying impact protective layers. To this end, in particular a boron compound, in particular boric acid can be used as a cross-linking agent, a volatile solvent such as, for example, ethanol or methanol and

polyorganosiloxane can be used to form a non-Newtonian fluid. Since the boron compound has a higher affinity to the solvent, that is for example to ethanol or methanol, than to polyorganosiloxanes, an instantaneous cross-linking of the polyorganosiloxanes by the boron compounds only occurs after extracting the solvent. Furthermore, a solvent can be used in order to positively influence the surface roughness in the coating process, e.g. for a very smooth and/or very homogeneous coating.

Further, as ethanol and water form an azeotropic mixture, in the moment ethanol is removed also water will be removed from the reaction system. Thus, the equilibrium of the following condensation reaction (exemplary a condensation reaction of polydimethylsiloxane and boric acid is shown) is shifted into the direction of the desired product, i.e. non-Newtonian fluid consisting of boron cross-linked polyorganosiloxane:

^• H O

Known techniques such as roll to roll coating, spray coating etc. can be used to apply the fluid mixture to a substrate such as a film, for example, before extracting the solvent.

Further details and advantages of the invention are obtained from the following, purely exemplary and non-restrictive description of exemplary embodiment of the invention in conjunction with the drawings.

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 shows highly schematically an impact protective layer according to the invention in a film shell and

Fig. 2 shows highly schematically an impact protective layer applied to an object to be protected.

DESCRIPTION OF PREFERRED EMBODIMENTS

Figure 1 shows highly schematically and not to scale an impact protection designated in its entirety by 10, in which a non-Newtonian transparent fluid 14 is enclosed in a transparent film shell 12. The layer of non-Newtonian fluid 14 here typically has a thickness between 5 μηη and 500 μηη and the impact protection overall has a thickness between 50 μηη and 1 mm. In one example the impact protection was formed from alpha-omega

hydroxyterminated polydimethyl siloxane having molecular weights between 20,000- 30,000 g/mol and an average molecular weight of 25,000 g/mol by cross-linking with boric acid in a region on a first optical, 100 μηη thick, transparent PET film as substrate. A second identical film was applied to the layer of non-Newtonian fluid and welded in the edge zone with the first film. Then a corresponding shape was stamped out from the film shell thus formed with the included non-Newtonian fluid and welded in the edge zone. The film shell thus formed can be provided with an adhesive layer on one side in order to produce a self-adhesive impact protective film which can be glued, for example, onto screen glass substrates such as contact- sensitive screens. Another film or another layer can be applied to the film shell, which forms a scratch-resistant layer and which forms the outermost side facing a user of the object when the film is mounted as intended on a surface to be protected.

Figure 2 shows highly schematically and not to scale a cross-section through an impact protective layer 18 applied to an object 16 to be protected, where the layer 18 consists of adhesive (cured in the state shown) and the material of the impact protection described in connection with Fig. 1. The layer 18 is applied to a scratch- resistant self-healing film 20 whilst extracting solvent. The adhesive force and the surface energy of the impact-absorbing dimensionally stable adhesive formulation (layer 18) were adjusted so that this exhibits a so-called "one-touch effect" so that after placing on a surface to be protected and gently pressing, the impact protective film is pulled over the surface to be protected. It should also be noted that instead of one impact protective layer 18, several of such layers each of different or the same impact protection properties may be foreseen and layered on top of each in order to to create an impact protection optimised to different cases of application, e.g. a plurality of different drop heights.

Example

The following polydimethylsiloxane (silanol) was obtained from ABCR Dr. Braunagel GmbH & Co. KG, D-76187 Karlsruhe:

Silanol Molecular weight (g/mol) Viscosity (cSt) DMS-S31 20000-30000 800-1200

Boric acid (99.5% pure from Merck) was dissolved in ethanol and mixed with silanol. The formulation was produced with the following quantities:

Silanol Boric acid

g mmol g mmol

DMS-S31 5 0.200 0.020 0.326

The formulation is heated to 85°C whilst agitating. After 5 minutes, the stirring process is interrupted and the formulation is applied thinly to a glass object slide.

The object slide is left to stand for 30 min at 85°C and then for two hours at 150°C in order to obtain transparent films on the object carriers.

Within the scope of the inventive idea, numerous modifications and further developments are possible which relate, for example, to the specific physically objective embodiment of the impact protection and the number and shape of the impact protective layers and impact protective elements used, e.g. using a shell filled with a non-Newtonian fluid produced according to the invention.