WO1997025357A1

WO1997025357A1 - Gels with thermotropic properties

Info

Publication number: WO1997025357A1
Application number: PCT/EP1997/000015
Authority: WO
Inventors: Matthias Gerst
Original assignee: Basf Aktiengesellschaft
Priority date: 1996-01-13
Filing date: 1997-01-03
Publication date: 1997-07-17
Also published as: DE19601084A1; AU1309897A

Abstract

The gels are for thermotropic layers are obtained by irradiation with high-energy light of a mixture containing the following: a) a non-cross-linked polymer with a number average molecular weight Mn of between 1,000 and 30,000 g/mol; b) radically polymerisable monomers; and c) water or an organic solvent or a mixture of the two.

Description

Gels with thermotropic properties

description

The invention relates to gels for thermotropic layers, obtainable by irradiating a mixture containing

a) an uncrosslinked polymer with a number average molecular weight M _n of 1000 to 30,000 g / mol b) free-radically polymerizable monomers and c) water or an organic solvent or mixtures thereof

with high-energy light.

The invention further relates to a method for producing the gels and their use for producing thermotropic layers.

The need for rational use of energy and the use of solar energy for heating buildings requires highly effective and intelligent systems that optimally adapt the solar energy systems (windows, collectors, house facades) to the weather conditions and user requirements. A high-quality window in an office facade ensures pleasant temperatures in winter and thus leads to energy savings. At the latest in spring, the same window will lead to overheating and the resulting cooling loads and nullify the energy savings in winter. It is therefore obvious that control mechanisms for regulating the radiation or heat flow are necessary in optimized systems. This control can be done partially passively, such as shading by roofing or balconies or actively mechanically by roller or awning systems. However, many of these systems are not effective enough, are too expensive or are insufficient in their long-term durability, which in turn leads to additional costs.

The idea of using electro- or thermo-optical systems for control comes up again and again, but so far has not led to a decisive breakthrough due to material or system problems.

An effective and inexpensive method for regulating the radiation and heat flow is the use of thermotropic layers. These layers are transparent in the unswitched state and only become cloudy above a certain one Switching temperature on. The clouding is completely reversible, ie when the temperature drops below the switching temperature, the thermotropic layers become completely transparent again.

EP-A-678534 describes gels for thermotropic layers which can be obtained from mixtures of monomers, thermotropic polymers and solvents by irradiation with high-energy light. The molecular weights of the thermotropic polymers are not specified, but are above 30,000 g / mol (M _n ) in the examples given.

The gels obtained still have disadvantages with regard to the stability of the network formed from the monomers by irradiation and all of the transparency below the desired switching temperature. The switching temperature clear / cloudy should naturally be above room temperature, preferably between 20 and 90 ° C.

The object of the present invention was therefore a gel for thermotropic layers or a method for its production, by means of which the disadvantages mentioned above are remedied.

Accordingly, the gel defined at the outset, a process for its preparation and its use for the production of thermotropic layers have been found.

Gels according to the invention can be obtained by irradiating a mixture comprising components a) b) and c) with high-energy light.

Particularly suitable as component a) are uncrosslinked polymers which have thermotropic properties in such a way that a 10 wt. % solution of this polymer in the selected solvent c), ie in water or an organic solvent or mixtures thereof in a switching range comprising less than 20 ° C. a change in the transmission of light at a wavelength of 600 nm and has a layer thickness of 10 mm, so that less than 50% of the incident light is transmitted above the switching range and at least 70% of the incident light below the switching range. (When measuring with stei¬ gender or decreasing temperature for the determination of the transmission is maintained in each case until the polymer or gel the newly set temperature ^¬ adopted.) The switching range or the switching temperature is preferably in the range from 20 to 90, particularly preferably between 20 and 50 ° C. and very particularly preferably in the range from 20 to 40 ° C. in the case of glazing systems.

The switching range should preferably be less than 10 °, particularly preferably less than 5 ° C and very particularly preferably only 1 ° C or less.

Below the switching range, the transmission is preferably at least 90%; above preferably less than 30%.

To determine the transmission, the samples were measured with a light transmission device. The device essentially consists of a point-shaped light source and a photocell that detects the direct beam path of the light source (direct-direct measurement). The photocurrent emitted by the photocell serves as a measure of the intensity of the light passing through. A heatable sample holder is located in the beam path between the light source and the photocell. A cuvette filled with distilled water served as a reference.

The thermotropic properties of the polymer are preferably caused by a limited solubility of the polymer in the chosen solvent or mixture.

The temperature above which the polymers have only a limited solubility is referred to as "lower critical solution temperature" (LCST) or the turbidity temperature. Below the LCST, the polymers are largely soluble in the solvent in the amount selected, above the LCST solutions from this polymer form a multiphase system which consists of at least one polymer-rich phase and one polymer-poor phase. The solvent of the low-polymer phase contains less than 50% of the polymer originally dissolved in the solvent. The polymer-rich phase predominantly contains polymer, but may also contain solvent enclosed in the polymer or solvent attached to the polymer (e.g. water of hydration).

As polymers a) come e.g. radical polymers, polycondensates or polyadducts, e.g. Polyoxymethylene.

The polymer a) is preferably a radical polymer and preferably consists, for example, of the following monomers: 60-100 in particular 90-100% by weight of monomers A

0 - 20 in particular 0 to 10% by weight of crosslinking monomers B

0 - 20% other monomers C

If water is used as solvent c) or main constituent of the solvent (more than 50% by weight of the solvent mixture), suitable monomers A, e.g. N-substituted, ethylenically unsaturated lactams (substituted and unsubstituted N-vinylcaprolactams, N-vinylpyrrolidones, N-vinyloxazolidinones, N-vinylpiperidones, N-vinylsuccinimides, N-vinylmorpholinones, N-vinylbutyrolactam, N-vinylvalerolactam, N -Vinyl- capryllactam, N-vinylhexahydrophtalimide, N-vinylmethyloxalodidone, N-vinylethyloxazolidone, N-vinyloxazidinone). N-Vinyl-2-caprolactam and N-vinyl-2-pyrrolidone are particularly preferred.

N, N '-divinylimidazolidone, N-vinylsuccinimide are also suitable.

Unsubstituted or N-substituted acrylamides or methacrylamides are also suitable.

Hydroxyalkyl esters of unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid are also suitable. The hydroxyalkyl group preferably has 2 to 5 carbon atoms. Examples of suitable monomers are hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate.

Unsubstituted or N-substituted acrylamides or methacrylamides are also suitable. Examples include N- (2-hydroxyethyl) acrylamide, N-metholylacrylamide, acrylamidoglycolic acid, N-butoxymethylacrylamide, N-methoxymethylacrylamide, N-methoxymethyl methacrylamide; N, N-dialkyl acrylamides with alkyl groups from C1-C3 such as N, N-diethylacrylamide, N, N-dimethylacrylamide, N-alkyl acrylamides with alkyl groups from C1-C6, such as N-ethyl acrylamide, N-isopropylacrylamide, N-propylacrylamide.

As monomers A there are also vinyl ethers of the formula I.

H ₂ 0 = CH OR

or hydrophilic vinyl ethers of the formula II H ₂ 0 = CH OR ¹ II

into consideration

R represents a Ci-C ₂₀ alkyl group and R ^{1 represents} an aliphatic or cycloaliphatic radical which may be substituted by hydroxyl groups or may be interrupted by non-adjacent groups -Y-, Y the meaning of an oxygen atom, a sulfur atom, a group NR ² or N ⁺ N ² R ³ X ^~ , wherein R ² and R ³ independently represent a hydrogen atom or a Ci-C ₄ alkyl group and X "stands for a counter anion and the molar ratio of carbon atoms to the sum of groups Y and hydroxyl groups in the aliphatic or cycloaliphatic radical R ^{1 is} 1.01: 1 to 6.5: 1.

Examples of vinyl ethers of the formula I which may be mentioned are in particular Cχ-C ₄ -alkyl vinyl ethers, for example methyl vinyl ether, ethyl vinyl ether, propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether and, for example, dodecyl vinyl ether.

Methyl vinyl ether is particularly preferred.

In vinyl ethers of the formula II, -Y- is preferably an oxygen atom.

Vinyl ethers of formula I or II can e.g. used alone or in a mixture with one another, preferably from 40 to 100% by weight of the vinyl ethers I and 0 to 60% by weight of the vinyl ethers II as monomers A.

If polymer a) should also be crosslinkable, monomers B can also be copolymerized, which can bring about a covalent crosslinking of the polymer chains during or after the polymerization.

Such monomers B can be two or more ethylenically unsaturated compounds, such as Methylene bisacrylamide, poly (ethylene oxide) diacrylates and dimethacrylates; Poly (propylene oxide) diacrylates, etc.

Suitable monomers B can also be ethylenically unsaturated compounds which, through the action of radiation such as UV radiation, can undergo addition reactions with themselves or other groups present. Such monomers are, for example, cinnamic acid glycidyl methacrylate, furylacrylic acid glycidyl methacrylate; Cinnamic acid (4-acryloxybutyl) ester; Cinnamic acid (2-acryloxyethyl) ester; Cinnamic acid (2-acryloxy-1-hydroxyethyl) ester, furylacrylic acid (4-acryloxybutyl) ester; Furylacrylic acid (2-acryloxyethyl) ester; Fury1acrylic acid (2-acryloxy-1-hydroxyethyl) ester etc.

Coumarin derivatives such as those e.g. in Macromolecules, 23; Pp. 2693-2697 (1990) or p-formylstyrene derivatives as described in J. Polym. Be. Polym. chem. Ed. 20; Pp. 1419-1432 (1982).

The presence of monomers C is generally not necessary and is therefore not preferred.

Polymer a) particularly preferably consists essentially of only monomers A and also contains no crosslinking monomers B.

The polymer a) is therefore preferably not crosslinked before and after irradiation with high-energy light.

Very particularly preferred polymers a) are poly-N-vinylcaprolactam, copolymers of N-vinylcaprolactam with at least 20% by weight, N-vinylcaprolactam and polyvinyl ether homo- or copolymers.

The number average molecular weight M _{n of} the polymer a) is 1000 to 30,000, preferably 5000 to 28,000 and particularly preferably 10,000 to 25,000. The weight average molecular weight M _w is preferably 5000 to 350,000 and particularly preferably 10,000 to 200,000 .

The molecular weight M _n is determined, like that of M _w, by gel permeation chromatography using a polystyrene standard and dimethylformamide as the eluent.

The manufacture of low molecular weight polymers is well known. The molecular weight can e.g. can be reduced by using regulators in the radical polymerization.

In addition to the polymers a), the mixture contains free-radically polymerizable monomers b).

After irradiation with high-energy light, the monomers b) form a three-dimensional network, ie a gel which is insoluble or hardly soluble in the selected solvent or solvent mixture. The b) network formed has preferential ^¬ as no thermotropic properties from monomers or comprises at most a switching range that is at least 20 ° C above the switching range of the polymer a). An optionally available switching range is preferably above the boiling point of the selected solvent or solvent mixture at normal pressure (1 bar).

The monomers b) are preferably free-radically polymerizable monomers. For example, a mixture of monomers is suitable

70 to 99.9% by weight, preferably 85 to 96 free-radically polymerizable, non-crosslinkable monomers D and

0.1 to 30% by weight, preferably 4 to 15% by weight, of crosslinkable

Monomers E

Suitable monomers D are, for example, vinyl esters of carboxylic acids containing 1 to 20 carbon atoms, some or preferably all of which can be saponified to vinyl alcohols, (meth) acrylamide, (meth) acrylic acid, N-substituted (meth) acrylamides, hydrophilic vinyl ethers of the formula II. Furthermore, hydroxyl-containing (meth) acrylates, such as Ci-C ₂ o-hydroxyalkyl (meth) acrylates, or N-methylol methacrylamide are suitable.

Suitable as crosslinking monomers E are those which have at least 2 copolymerizable, ethylenically unsaturated groups or at least one ethylenically unsaturated group and at least one further reactive, e.g. contain a group capable of condensation reactions.

Examples include Monomers with 2 or more acrylic groups such as polyethylene oxide diacrylate, methylene bisacrylamide.

Suitable solvents c) as part of the mixture are e.g. Glycols such as ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, glycol ethers such as glycol methyl ether, diethylene glycol mono- and diether and water.

Water is preferred as the solvent or a solvent mixture which contains at least 60% by weight, preferably 90% by weight of water, based on the solvent mixture.

The combination of water with one of the above solvents has the advantage that the frost stability of the gels is increased. The thermotropic layers should remain in the application even in winter. They should have the same function as in Summer. It is therefore necessary to stabilize the gels against freezing. This is necessary because the freezing water can cause permanent damage to the gel or to the entire system (ie gel and support structure). The freezing point of the gel is reduced by adding glycols, glycol ethers or alcohols with low alkyl chains. The same effect can also be obtained if the solvent contains soluble salts. In the case of the salts, however, care must be taken that they are compatible with the polymer system and on the other hand that the switching temperature is not adversely affected. The amount of agent required to lower the freezing point depends on the ambient temperature in which the thermotropic layer is used.

In addition to components a), b) and c), the mixtures defined at the outset can also contain further constituents.

In order to maintain the long-term usability of the thermotropic layers of gels, they can be stabilized against bacterial attack. Therefore it makes sense to add biocides to the mixtures. This can either be done by organic compounds, e.g. by formaldehyde or 1,2-benzisothiazolone or inorganic compounds such as sodium fluoride. The amount of biocide depends on the application (accessibility through microorganisms) and the activity of the biocide. The basics of biocide stabilization are e.g. in color and varnish; JL2_; P. 108 ff (1976); Paint and varnish; 9_9_; Pp. 105 ff (1993) or in the "Textbook of Pharmaceutical Chemistry" (W. Schmack, K. Mayer, M. Haake; Verlag Vieweg and Sohn, Braunschweig 1983; e.g. p. 537 ff).

Furthermore, photoinitiators for radical polymerization are preferably added to the mixtures. Suitable amounts are between 0.01 to 5% by weight, particularly preferably 0.03 to 3% by weight, based on the free-radically polymerizable monomers b).

A general overview of suitable photoinitiators can be found e.g. through the article by H.F. Gruber in Prog. Polym. Sei., Vol. 11, pp. 953 ff. (1992).

Compounds such as water-soluble benzophenone derivatives (J. Appl. Polym. Sei; 32, pp. 6209-26 (1986); Polym. Paint Color J.; 179, p. 684) are particularly suitable for the preparation of hydrogels with water as solvent -687 (1989); Polym. Paint. Resin; 175; pp. 246-251 (1985)), sulfonated aromatic ketones (Eur. Polym. J., Vol. 27, pp. 69-75 (1991)), water-soluble Thioxanthone derivatives (J. Appl. Polym. Sei, 34; pp. 477-488 (1987)) such as 4-benzoyl-N, N-dimethyl-N- (2-oxo-2-propenyloxy) ethylbenzene-methanaminium bromide, 4-benzoyl, N, N, N, trimethylbenzene-methanaminium chloride, 4- (3-dimethylamino-2-hydroxypropoxy) -benzophenone methyl chloride, 2- (3-dimethylamino-2-hydroxypropoxy-oxy) -3, 4-dimethyl-9H- thioxanthene-9-one methochloride.

It is preferably

R ³ 0

R6

where R ¹ and R ^{2 are} independently a

Ci-Cχo-alkyl group and R ³ to R ⁷ independently of one another for a hydrogen atom, a Ci-Cio-alkyl group or

C ₅ -C 10 aryl group can be provided that at least one of the radicals R ³ to R ^{7 represents} a hydrophilic radical, which is a carboxylic acid or carboxylate group, a sulfonic acid or sulfonate group, a hydroxyl group , a polyalkylene oxide group or an organic radical having up to 30 carbon atoms, which contains at least one of the above hydrophilic groups.

In the case of the polyalkylene oxide group as the hydrophilic group, it is preferably a polyethylene oxide, polypropylene oxide group or mixed polyethylene propylene oxide groups with preferably 2 to 20 alkylene oxide units.

In formula I, R ¹ and R ^{2 are} preferably a C ₁ -C ₄ alkyl group, particularly preferably they are a methyl group. R ³ to R ⁷ are preferably a C ₁ -C ₄ alkyl group or particularly preferably an H atom. At least one, preferably one to three, particularly preferably one of the radicals R ³ to R ⁷ represents one of the specified hydrophilic radicals.

The hydrophilic radical is preferably an aliphatic radical, for example an alkyl radical or alkoxy radical, having 1 to 10 carbon atoms, which may optionally contain ether groups (-O-) and is substituted by at least one hydroxyl group. It is particularly preferably a photoinitiator of the formula

which is available as Darocure ^® 2959 from Ciba Geigy.

The mixture defined at the outset preferably contains, based on the sum of a) + b) + c),

0.1 to 40% by weight, particularly preferably 0.5 to 10% by weight and very particularly preferably 0.5 to below 5% by weight (in particular up to 4.5% by weight) of the uncrosslinked polymer a),

1 to 30% by weight, particularly preferably 3 to 27% by weight, of radically polymerizable monomers b) and

30 to 98.9% by weight, particularly preferably 70 to 96.5% by weight

Water, organic solvents or mixtures thereof.

The mixture can be easily prepared by combining the components and homogenizing, e.g. by stirring.

The gel can then preferably be produced directly in the device provided for the thermotropic layer.

For this, the mixture is placed in this device, e.g. a glazing system, filled in and irradiated with high-energy light.

However, the radiation can also be carried out in advance and the gel obtained can then be filled into the corresponding device.

During the irradiation, the monomers b) polymerize and crosslink so that they form a three-dimensional network or a gel which contains the preferably uncrosslinked polymer a), the solvent c) and, if appropriate, further additives. The irradiation is preferably carried out with UV light or electron beams.

The duration of the UV exposure for complete gelling is sufficient if, in a dynamic mechanical analysis of the hydrogels at 25 ° C., the storage module G '[Pa] is greater than the loss module G''[Pa] at a measuring frequency of ω = 1 [rad / s]. The data were obtained using a heavy-speed controlled rotary viscometer (RFS) Rheometrics, with a cone-plate geometry (diameter = 25 mm, cone = 0.04 rad) at 25 ° C with an angular frequency of 0.1 s " ¹ <ω <100 s" ¹ measured.

Dynamic mechanical measurements (DMA) subject the hydrogel samples to a temporally sinusoidal deformation with a certain amplitude and frequency and measure the resulting mechanical stress. The complex dynamic module of the sample material can be determined from deformation and tension. It consists of two components, the storage module as a measure of the reversibly stored deformation energy and the loss module, which characterizes the energy irreversibly converted into heat.

The method is well known and e.g. in A. Zosel, Farbe und Lack, 94 (1988) 809.

The irradiation is preferably carried out at a temperature of at least 5 ° C., particularly preferably at least 10 ° C., very particularly preferably at least 20 ° C. below the switching range or below the cloud point as the lower temperature limit of the switching range.

The gels according to the invention are suitable for the production of thermotropic layers which meet the conditions described above for polymer a) with regard to switching range and change in transparency.

At the same time, the gels show high transparency in the homogeneous state, i.e. below the switching range. Even after many repetitions of the switching process, the transparency difference remains large and the switching range is narrow. The short switching time is also advantageous.

The gels according to the invention are therefore particularly suitable for the production of glazing systems with thermotropic properties and of components for thermal insulation.

Examples Production of poly-N-vinylcaprolactam with low molecular weight (thermotropic polymer).

Production Example 1

To a mixture of 333.5 g demineralized water, 180.0 g N-vinylcaprolactam, 45.0 g Mowiol 30-92 (10%) as a protective colloid and 9.0 g mercaptoethanol as a regulator (5 wt. %, based on monomers), a mixture of 59.7 g of demineralized water and 0.3 g of 2,2'-azobis (2-amidinopropane) dihydrochloride as initiator was left at 75 ° C. over a period of 30 minutes at 200 revolutions drop per minute. The internal temperature rose to 85 ° C. After the polymerization, the mixture was stirred for about an hour. The polymer precipitated during the polymerization, but dissolved on cooling to room temperature. An approximately 30% aqueous, clear solution was isolated as the product.

Production Examples 2 to 5

In the further preparation examples, the procedure was as in preparation example 1, but the amount of regulator (mercaptoethanol) according to Table 1 was changed (based on monomers).

^xx for comparison.

* The molecular weights M _n and M "were determined by gel permeation chromatography with polystyrene as the standard and dimethylformamide as the eluent.

Preparation of the gels

A mixture of 2.67 g of a 30 wt. % solution of poly-N-vinyl-caprolactam in water (thermotropic polymer, 2% by weight, based on all components of the gel), 32.49 g of water, 2.72 g of aqueous acrylamide solution (50%), 2, 10 g aqueous methylenebisacrylamide solution (2%), (weight ratio acrylamide: methylenebisacrylamide 93: 7), 0.02 g photoinitiator Darocure® 2959 (solid, ent speaks 2% by weight, based on all components of the gel), was degassed and filled between two float glass panes (thickness of the panes 4 mm, spacing of the panes 1 mm). The filled glazing was then irradiated for 15 min at 10 ° C. under a UV lamp (λ ^max = 367 nm). The sample crosslinked to a gel.

The polymers from preparation examples 1 to 5 were used as thermotropic polymers.

The transparency of the gels obtained at T = 22 ° C, i.e. below the switching temperature, was assessed visually (Table 2).

Claims

claims

1. Gels for thermotropic layers, obtainable by irradiating a mixture containing

a) an uncrosslinked polymer with a number average molecular weight M _n of 1000 to 30,000 g / mol

b) radically polymerizable monomers and

c) water or an organic solvent or mixtures thereof

with high-energy light.

2. Gels according to claim 1, which is a mixture containing

a) 0.1 to 40% by weight of an uncrosslinked polymer,

b) 1 to 30% by weight of radically polymerizable monomers and

c) 30 to 98.9% by weight of water or an organic

Solvents or mixtures thereof

is.

3. Gels according to claim 1 or 2, wherein the proportion by weight of polymer a) is 0.1 to less than 5% by weight, based on the sum of components a), b) and c).

4. Gels according to one of claims 1 to 3, wherein component c) is water.

5. Gels according to one of claims 1 to 4, wherein the uncrosslinked polymer a) has thermotropic properties in such a way that a 10 wt. -% solution of this polymer in water or an organic solvent or mixtures thereof in a switching range comprising less than 20 ° C causes a change in the transmission of light at a wavelength of 600 nm and a layer thickness of 10 mm, so that at temperatures above the Switching range less than 50% of the incident light and below the switching range at least 70% of the incident light.

6. Gels according to one of claims 1 to 5, wherein the polymer a) is poly-N-vinylcaprolactam, copolymers of N-vinylcaprolactam with at least 20% by weight of N-vinylcaprolactam or polyvinyl ether.

7. Gels according to one of claims 1 to 6, wherein the monomers or the monomer mixture b) after irradiation with high-energy light forms a three-dimensional network which has no thermotropic property, as defined in claim 4, or which has a switching range of at least 20 ° C above the switching range of the polymers a).

8. Gels according to one of claims 1 to 7, wherein the mixture additionally contains a photoinitiator.

9. A process for the preparation of gels with thermotropic properties according to one of claims 1 to 8, characterized in that a mixture containing

a) an uncrosslinked polymer with a number average molecular weight M _n of 1000 to 30,000 g / mol, b) free-radically polymerizable monomers and c) water or an organic solvent or mixtures thereof

is irradiated with high-energy light.

10. Use of gels according to one of claims 1 to 8 for the production of thermotropic layers which change their transparency in a switching range comprising less than 20 ° C in such a way that above the switching range less than 50% of the incident light at a wavelength of 600 nm are transmitted at a layer thickness of 10 mm and at least 70% of the incident light is transmitted below the switching range.

11. Glazing system with thermotropic properties, obtainable using a gel according to one of the claims

1 to 8.

12. Components for thermal insulation, obtainable using a gel according to one of claims 1 to 8.