Title:
Hardening with a heterocyclic carbamoyl ammonium compound of a photographic material containing a silver halide layer
United States Patent 3880665
Abstract:
As rapidly acting hardeners for protein-containing photographic layers carbamoyl ammonium compounds are used the quaternised nitrogen atom of which being present in a 5- or 6-membered heterocyclic aromatic group.
US Patent References:
Hardening of gelatin
Allen et al. - August 1960 - 2950197
/3676143.html
Himmelmann et al. - July 1972 - 3676143
Hardening of gelatin
Allen et al. - August 1960 - 2950197
/3676143.html
Himmelmann et al. - July 1972 - 3676143
Application Number:
05/362484
Publication Date:
04/29/1975
Filing Date:
05/21/1973
View Patent Images:
Export Citation:
Assignee:
Agfa-Gevaert Aktiengesellschaft (Leverkusen, DT)
Primary Class:
Other Classes:
527/201, 530/354, 106/150.100, 530/373, 530/360, 524/23, 527/203, 427/338
International Classes:
C07D213/20; C07D213/30; C07D213/75; C07D213/81; C07D213/82; C07D217/10; C07D277/22; C07D295/205; G03C1/30; C07D213/00; C07D217/00; C07D277/00; C07D295/00; G03C1/30
Field of Search:
260/117 96/111 106/125 117/34,62.2
Primary Examiner:
Torchin, Norman G.
Assistant Examiner:
Louie Jr., Won H.
Attorney, Agent or Firm:
Connolly, And Hutz
Claims:
I claim
1. A process for providing a protein-containing binder selected from the group consisting of gelatin, casein and zein in a photographic silver halide emulsion layer coated on a support wherein the improvement comprises incorporating in a binder for the layer a hardening amount of a hardener of the formula ##SPC9##
2. A process according to claim 1, characterised in that the hardener is applied from aqueous solution.
3. A process according to claim 1, characterised in that the hardener is applied from alcoholic solution.
4. A process according to claim 1, characterised in that the hardener is applied from aqueous-alcoholic solution.
5. A process according to claim 1 in which the hardener is added to the casting solution of the layer which is to be hardened and the hardener is used in quantities of 0.2 to 6% by weight based on the weight of the protein-containing binder in the casting solution.
6. A process as claimed in claim 1 in which the hardener is applied to the layer before processing, said hardener being in a 0.2 to 10% solution.
7. A process as claimed in claim 5 in which the hardener is coated on the layer in a 0.2 to 5% solution and the layer is then dried.
8. A process according to claim 1 characterized in that the hardener is used for hardening photographic multilayer color materials.
1. A process for providing a protein-containing binder selected from the group consisting of gelatin, casein and zein in a photographic silver halide emulsion layer coated on a support wherein the improvement comprises incorporating in a binder for the layer a hardening amount of a hardener of the formula ##SPC9##
2. A process according to claim 1, characterised in that the hardener is applied from aqueous solution.
3. A process according to claim 1, characterised in that the hardener is applied from alcoholic solution.
4. A process according to claim 1, characterised in that the hardener is applied from aqueous-alcoholic solution.
5. A process according to claim 1 in which the hardener is added to the casting solution of the layer which is to be hardened and the hardener is used in quantities of 0.2 to 6% by weight based on the weight of the protein-containing binder in the casting solution.
6. A process as claimed in claim 1 in which the hardener is applied to the layer before processing, said hardener being in a 0.2 to 10% solution.
7. A process as claimed in claim 5 in which the hardener is coated on the layer in a 0.2 to 5% solution and the layer is then dried.
8. A process according to claim 1 characterized in that the hardener is used for hardening photographic multilayer color materials.
Description:
The invention relates to a process for hardening photographic layers which contain protein and in particular contain gelatin.
Numerous substances have already been described as hardeners for portein and in particular for gelatin, for example metal salts such as chromium, aluminium or zirconium salts, aldlehydes and halogenated aldehyde compounds, in particular formaldehyde, dialdehydes and mucochloric acid, 1,2- and 1,4-diketones such as cyclohexane-1,2-dione and quinones as well as chlorides of dibasic organic acids, the anhydrides of tetracarboxylic acids, compounds which contain several reactive vinyl groups such as vinyl sulfones, acrylamides, compounds containing at least two heterocyclic three-membered rings which can easily be split off, such as ethylene oxide and ethylene imine, polyfunctional methane sulfonic acid esters and bis-α-chloracylamido compounds.
High molecular weight hardeners which are used particularly as hardeners of which the effect is to be confined to the layer having recently become known, for example polyacrolein and its derivatives or copolymers and alginic acid derivatives.
Many of the above mentioned compounds, however, are unsuitable, in particular for photographic purposes. Some of them are photographically active and therefore unsuitable for hardening photographic materials whereas others cannot be used because they have such a deleterious effect on the physical properties, for example the fragility of the gelatin layers. Others again cause discoloration or alter the pH in the course of the hardening reaction. Furthermore, for hardening photographic layers it is particularly important that the hardening effect should attain its maximum as soon as possible after drying so that the material which is being hardened will not constantly change its permeability to the developer solution as, for example in the case of mucochloric acid or formaldehyde.
Some cross-linking agents for gelatin, for example the ethylene imine compounds, also have a harmful effect on the skin and are therefore in any case unsuitable on physiological grounds.
It has long been known to use trichlorotriazine and dichloroaminotriazines as hardeners. The disadvantage of these compounds is their relatively high vapour pressure and their physiological effect. Water-soluble derivatives which contain carboxylic and sulphonic acid groups and are obtained by reacting cyanuric acid chloride with 1 mol of a diaminoalkyl or diaminoarylsulfonic acid or carboxylic acid to not have these disadvantages and have therefore recently been proposed as hardeners. Their usefulness in practice is, however, limited because owing to their high solubility they decompose when left to stand in aqueous solutions and therefore rapidly lose their effectiveness. HYdroxydichlorotriazine has also been proposed as hardener. Lastly, in the case of a hardener used for photographic layers which contain gelatin, it is of major importance both for manufacturing and for processing reasons that the onset of the cross-linking reaction should be controllable within certain limits, for example by choice of the correct dyring temperature of pH.
Compounds which contain two or more acrylic acid amide groups or vinylsulphone groups in the molecule are also known as hardeners for photographic gelatin layers, e.g. divinylsulfone, arylene-bis-vinylsulfones, N,N', N"-tris-acryloyl-hydrotriazine or methylene-bis-vinyl-sulfonamide.
Hardening of the compounds is quite satisfactory after a certain time but the compounds are only sparingly soluble in water so that hardening is liable to be irregular within the layer.
The consequences of the undesirable properties of known hardness indicated above are of great importance for photographic purposes because important photographic properties such as the gradation and sensitivity and in many cases also the silver covering power depend on the degree of cross-linking of the layer-forming colloid and undergo changes in the course of storage. Although this disadvantage can be attenuated by a brief after-treatment of the solidified layer with ammonia or an amine, it cannot be completely eliminated. To this is added the fact that aliphatic divinylsulfones tend to damage the skin.
It is an object of this invention to develop rapidly acting hardeners for layers which contain proteins, and in particular for gelatin layers, which are used for photographic purposes.
A process for hardening photographic layers which contain proteins and preferably gelatin has now been found which is characterised by the use of a hardener which contains in the molecule at least one carbamoyl radical with a heterocyclic, aromatic 5-membered or 6-membered ring which in addition to a quaternary nitrogen atom may also contain other hetero atoms.
The hardeners of this invention correspond to the following general formula ##SPC1##
in which
R 1 represents a substituted or unsubstituted alkyl group preferably containing 1 to 3 carbon atoms, an aryl group optionally substituted with a lower alkyl radical or with halogen, e.g., phenyl, if desired substituted with methyl, ethyl or propyl, Cl or Br, or an aralkyl group, e.g., benzyl, which may be substituted in the same manner as the aryl group;
R 2 has the same meaning as R 1 or it may represent a divalent optionally substituted alkylene, arylene, aralkylene or alkyl-aryl-alkyl radical, e.g., an ethylene, propylene, phenylene or xylylene radical which is attached by its second bond to a further carbamoyl ammonium group of the formula ##SPC2##
or
R 1 and R 2 may together represent a group required to complete a substituted or unsubstituted heterocyclic ring, such as piperidine, piperazine, or morpholine ring which may be substituted e.g., with an alkyl group containing 1 to 3 carbon atoms or with halogen such as Cl or Br;
R 3 represents hydrogen or an alkyl group containing 1 to 3 carbon atoms or the group ##SPC3##
in which
A represents a vinyl group of a polymerised vinyl compound or of a copolymer with other copolymerisable monomers and
αrepresents a number such that the molecular weight of the resulting homopolymer of copolymer with other copolymerisable monomers is greater than 1,000,
R 4 represents hydrogen, an alkyl group containing 1 to 3 carbon atoms or if Z represents the atomic grouping required to complete a pyridinium ring and R 3 is absent R 4 represents one of the groups ##SPC4##
R 5 represents an alkyl, aryl or aralkyl group but R 5 is absent if the nitrogen to which R 5 is attached is involved in a double bond in the heterocyclic aromatic ring formed by Z,
Z represents the atomic grouping required to complete a 5-membered or 6-membered aromatic ring optionally substituted on the nitrogen atom, e.g., with an alkyl group containing 1 to 3 carbon atoms or it may represent the atomic grouping required to complete a condensed system such as isoquinoline, which atomic grouping may contain other hetero atoms in addition to the nitrogen atom, e.g., O or S, and
X represents an acid anion such as halogen - , BF 4 - , NO 3 - , So 4 - , ClO 4 - , CH 3 OSO 3 - .
The following compounds have proved to be particularly useful. The list is given only by way of example and is not intended to limit the scope of the invention. ##SPC5##
The compounds may be prepared by simple methods well known from the literature. Carbamic acid chlorides may be prepared, for example, by reacting secondary amines with phosgene, and these carbamic acid chlorides are then reacted (with the exclusion of light) with aromatic, heterocyclic compounds which contain nitrogen. The preparation of compound 3 has already been described in Chemische Berichte 40 (1907) page 1831. The method of preparation of the other compounds will be explained with reference to N,N-phenyl-methyl-carbamoyl-pyridinium chloride (compound 6) which is used as an example:
A. phenyl-methyl-carbamoyl chloride
49.5 g of phosgene are carefully introduced into 400 cc of absolute toluene with cooling (22° to 25°C). A solution of 107 g of distilled N-methylaniline in 450 cc of absolute toluene is added dropwise to this solution with vigorous stirring. The mixture is then heated for 1/2 hour at 80° to 90°C and cooled and the precipitated hydrochloride is suction filtered. Lastly, the solution in toluene is evaporated to dryness under vacuum with exclusion of moisture. The residue crystallises on cooling.
Yield: 81 g, melting point 85°C.
B. 33.9 g of Compound A are stirred into 400 cc of absolute pyridine the reaction being conducted in a dark room with exclusion of moisture. The carbamoyl-pyridinium chloride formed precipitates after 3 hours. After the addition of an equal quantity of ether, the crystals are suction filtered, taken up with a small quantity of absolute ethanol and again precipitated with ether. The product is again suction filtered, washed thoroughly with ether and dried in a desiccator.
Yield: 41 g, melting point 108°-110°C.
The poly-4-vinyl-N,N-dimethyl-carbamoyl-pyridinium chloride corresponding to Compound 15 is prepared as follows:
10.5 g of poly-4-vinyl-pyridine (molecular weight) 10,000) are dissolved in 150 ml of absolute ethanol, and 10.8 g of dimethylcarbamic acid chloride are added with vigorous stirring.
The mixture is stirred at room temperature for 5 hours and concentrated under vacuum. It is then run into ether with stirring and the precipitated product is suction filtered. It is washed thoroughly with ether. The compound is dried in a desiccator.
Yield 18 g.
The folliwng example may characterize the method of preparation of the pryidinium compounds 19 to 40 which are substituted on the pyridinium ring. The example refers to compound 30:
A. preparation of morpholino-chlorocarbonyl ##SPC6##
49.5 g phosgene are slowly introduced into 400 ml of absolute toluene. A solution of 87 g of destilled morpholine in 450 ml of absolute toluene is added dropwise to this solution with vigorous stirring. The mixture is then heated for one-half hour at 80° to 90°C and coated, and the precipitated hydrochloride is suction filtered and washed with absolute toluene. Lastly the filtrate is evaporated to dryness under vacuum with exclusion of moisture. It remains on oil which is destilled.
Yield: 50 g, boiling point 72°C.
B. preparation of compound 30 ##SPC7##
1/20 mol of compound A were stirred into a solution of 1/20 mol of 3-acetaminopyridine in 60 ml of chloroforme. The mixture is stirred for 2 hours at 50°C with exclusion of moisture. The chloroforme is then evaporated under vacuum and the highly viscous residue is mixed with a small amount of absolute ethyl ether. Then the crystalline product is suction filtered, washed with absolute ethyl ether and dried in a desiccator.
Yield 10 g, melting point 118°-123°C.
The preparation of the other compounds is carried out in a corresponding manner. The salts can be easily prepared by addition of the suitable acids.
The compounds used according to the invention are preferably added to the protein layers which are to be hardened immediately before they are cast, preferably in the form of aqueous or alcoholic solutions. It is necessary to add them only shortly before the layers are cast because they react very rapidly with gelatin or other proteins normally used in photography. Once the compounds have been added, the casting solutions should be cast within a few minutes. The rate at which the hardening reaction takes place depends primarily on the concentration of proteins in the casting solution.
Another possiblity consists in casting solutions which have not been hardened and then coating the resulting layers with a solution of the hardening compounds. Alternatively, photographic layers which have not been hardened or only slightly hardened may be bathed in aqueous solutions of the compounds containing sodium sulfate at some stage of processing the photographic material, for example before development.
The term photographic layers is used in this context as a general term to denote any layers used in photographic materials, for example light sensitive silver halide emulsion layers, protective layers, filter layers, anti-halation layers, back coating layers or, in general, any photographic auxiliary layers.
The light sensitive emulsion layers for which the hardening process according to the invention is particularly suitable are, for example, those layers which are based on non-sensitized emulsions, orthochromatic, panchromatic or infra-red emulsions, X-ray emulsions and other emulsions which are spectrally sensitized. The hardening process according to the invention has also proved suitable for hardening gelatin layers used for various black-and-white photographic processes and colour photographic processes.
The process according to the invention has proved particularly advantageous for hardening composite photographic layers used for colour photographic processes, e.g., those which contain emulsion layers with colour couplers or emulsion layers which are designed to be treated with solutions which contain colour couplers.
The effect of the compounds used according to the invention is not impaired by the usual photographic additives and the hardeners are also uneffected by photographically active substances such as water-soluble or emulsified water insoluble dry components, stabilizers, sensitizers and the like. Moreover, they have no influence on the light sensitive silver halide emulsions. They may be combined with any compounds from the known classes of hardeners, for example formalin, mucochloric acid, triacrylic formal, bis-vinylsulfones, bis-vinylsuflonamides, dialdehydes or bis-chloroacetamides.
Apart from gelatins, the layers may also contain water-soluble high polymer compounds, in particular polyvinyl alcohol, polyacrylic acid sodium and other copolymers which contain carboxyl groups, polyvinyl pyrrolidone, polyacrylamide or high molecular weight natural substances such as dextrans, dextrins, starch ether, alginic acid or alginic acid derivatives.
The concentrations at which the hardeners according to the invention are used may vary within wide limits and depend mainly on the hardening compound used.
Satisfactory results are obtained with quantities of 0.1 to 10 % by weight and particularly 0.2 to 6 % by weight, based on the dry weight of binder.
As already mentioned above, the hardening reaction between the compounds according to the invention and gelatin or proteins sets in immediately so that the optimum degree of hardening is reached more or less simultaneously with drying of the layers after they are cast or processed.
The activity of the hardening compounds is determined by means of the melting point of the layers, which can be found as follows:
Half of a layer cast on a support is dipped in water which is continuously heated up to 100°C. The temperature at which the layer runs from the support (formation of streaks) is taken as the melting point or melting off point. When measured by this method, no increase in melting point is found in any pure protein layers or gelatin layers which contain no hardener, on the contrary the layers run off at 30°-35°C.
The compounds according to the invention react surprisingly quickly with proteins and therefore enable materials which contain proteins to be hardened to their optimum degree of hardness within a very short time. This unexpected effect of the compounds is particularly important for the hardening of photographic materials which contain proteins as binders. Hardening can be adjusted to the required degree in a readily controlled nammer at the stage of preparation of the materials and does not require prolonged storage times with the concomitant unreliability of uncontrollable after hardening.
An additional advantage is brought by the carbamolypyridinium compounds the pyridine ring of which is substituted with hydrocarbon residues having more than 3 carbon atoms. Examples for compounds of this group are compounds 19 - '.
Compounds of this shape are practically unodorous, so that the application as hardeners can be carried out without odorous annoyance. The advantage of the inodorousness of the pyridinium compounds becomes particularly clear considering the fact that the limit of perceptibility of pyridine through the sens of smell already begins at 0.0004 mg/m 3 in air. Since the above pyridinium compounds turn out to be odorless they give sufficient security that exceeding the "Threshold limit value" of pyridine which is of the order of 15 mg/m 3 is virtually impossible.
The invention will now be explained with the aid of the following examples.
EXAMPLE 1
1 % by weight and 2 % by weight, respectively, of compounds 2, 19 and 20 based on the dry weight of gelatin, are added in the form of an aqueous solution at pH 6.2 to 100 ml portions of a photographic silver bromide gelatin emulsion containing 10 % by weight of gelatin which is ready for casting. The mixture is viigorously stirred and immediately cast on a prepared cellulose triacetate support by means of a conventional casting apparatus and dried. The usual additives are not altered. After a storage time of 24 hours at room temperature, the melting points of the layers are measured.
______________________________________ compound quantity layer melting points added in °C ______________________________________ -- 0 % (without 34 hardening) 2 1 % >100 2 % >100 19 1 % >100 2 % >100 20 1 % >100 2 % >100 ______________________________________
The cross-linking reaction is so intensive that the layers do not dissolve even after 5 minutes in boiling water.
EXAMPLE 2
Sodium sulphate is added to a 5 % aqueous solution each of Compound 1 and Compound 30 until the solution is almost saturated. Unhardened photographic silver halide gelatin layers are dipped in this solution for various lengths of time at 22°C. The layers are then briefly washed, dried and then stored at room temperature for 12 hours. The effectiveness of these preliminary baths is determined by measuring the melting points.
______________________________________ immersion time layer melting points remarks in minutes in °C compound 1 compound 30 ______________________________________ 0.5 50°C 60°C 1 100°C >100°C soft layer 2 >100°C >100°C increased strength of layer 3 >100°C >100°C very high strength of layer untreated layer 34°C ______________________________________
At a bath temperature of 40°C, the required immersion times are much shorter and layer melting points above 100°C are obtained after only 45 seconds.
The stability of the hardeners according to the invention in aqueous solutions depends on the additives used. In distilled water, the hardeners are still active after 3 days.
EXAMPLE 3
A 10 % casein solution in water is prepared by the addition of sodium hydroxide solution. The solution is divided in 9 parts of 100 ml and 0.1 g of tartrazine is added as filter dye to each solution. 3 % by weight of compounds 2, 19, 20, 21, 24, 25, 30, 31 and 37 dissolved in water are added at pH 7 before casting. The mixtures are cast on glass plates and hardened filter foils which are no longer soluble in aqueous alkali are obtained after drying.
EXAMPLE 4
A 20 % by weight of solution of zein is prepared in a mixture of ethanol and water (8:2 ) and cast on the back of a cellulose acetate film. After drying, a layer which is readily soluble in a mixture of ethanol and water is obtained.
If strips of the film are bathed in solutions of
2 g of compounds 2, 19, 20, 21, 26, 31, 32, 35, 37, 38 or 39 15 g of sodium sulfate and 80 ml of water
for 3 minutes and then briefly washed and dried in a heating cupboard at 50° to 60°C, the layers are insoluble in all solvents and effectively cross-linked.
EXAMPLE 5
An unhardened silver halide emulsion containing 10 % by weight of gelatin as binder is cast on a triacetyl cellulose support without the addition of a hardener. The layer contains all the other usual additives. Samples of the dried layer are coated with 0.5, 1, 2 and 3 % aqueous solutions of compounds 1, 2, 5, 24, 29 and 33 and then dried. The melting points, degree of swelling and resistance to scratching when wet are then determined on the layers. The results are summarized in the following Table.
The degrees of swelling are determined gravimetrically after 10 minutes treatment of the layers in distilled water at 22°C and are indicated in percent.
To determine the resistance to scratching when wet, a metal point of a specified size is passed over the wet layer and loaded with an increasingly heavy weight. The wet scratch resistance is indicated by the weight at which the point leaves a visible scratch trace on the layer. The larger the weight the higher the scratch resistance.
______________________________________ compound layer swelling wet scratch melting point in % resistance in p ______________________________________ compound 1: 0.5 % 355 750 1 % 310 850 2 % 10'100°* 280 950 3 % 250 950 ______________________________________ compound 2: 0.5 % 325 950 1 % 295 950 2 % 10'100° 260 1150 3 % 235 1150 ______________________________________ compound 5: 0.5 % 420 650 1 % 370 750 2 % 10'100° 280 950 3 % 260 1100 ______________________________________ compound 24: 0.5 % 400 650 1 % 300 750 2 % 10'100°* 250 850 3 % 200 900 ______________________________________ compound 29: 0.5 % 320 900 1 % 300 950 2 % 10'100° 250 1100 3 % 250 1200 ______________________________________ compound 33: 0.5 % 400 650 1 % 350 750 2 % 10'100° 300 950 3 % 260 1100 not after- treated 36° 600 - 800 300 ______________________________________ *The layer is not dissolved off after 10 minutes in boiling water.
EXAMPLE 6
25 % by weight, based on the weight of the gelatin, of a blue green coupler of the following formula ##SPC8##
are added to an unhardened silver halide emulsion which contains 10 % by weight of gelatin. The usual casting additives with the exception of a hardener are then added to the emulsion. The mixture is cast on a prepared polyethylene terephthalate support and dried.
Samples of this layer are then coated with 0.5 to 3 % aqueous solutions of the compounds according to the invention. After drying and 10 hours' storage, layers with an exceptionally high degree of cross-linking are obtained. The results are summarized in the following Tables.
______________________________________ top coating melting swelling wet scratch point in % resistance in p ______________________________________ compound 1: 0.5 % 10'100°C 355 750 1 % 310 850 2 % 280 950 3 % 250 950 ______________________________________ compound 2: 0.5 % 10'100°C 325 950 1 % 295 950 2 % 260 1150 3 % 235 1150 ______________________________________ compound 5: 0.5 % 10'100°C 350 850 1 % 320 850 2 % 270 1150 3 % 250 1150 ______________________________________ compound 19: 0.5 % 10'100°C 410 650 1 % 350 750 2 % 300 800 3 % 280 900 ______________________________________ compound 20: 0.5 % 10'100°C 380 650 1 % 310 800 2 % 280 850 3 % 250 1000 ______________________________________ compound 25: 0.5 % 10'100°C 410 600 1 % 340 650 2 % 300 700 3 % 270 800 ______________________________________ only treated 42°C 800 with water ______________________________________ The photographic properties were not affected.
EXAMPLE 7
The usual additives with the exception of a hardener are added to 100 ml of a photographic silver bromide gelatin emulsion containing 10 % by weight of gelatin. The mixture is cast
a. on baryta paper and
b. on paper backed with polyethylene on both sides. After drying, samples of the two materials are bathed for 2 minutes in aqueous solutions containing 3 g of compounds 2, 5, 10, 11, 12, 13, 14, 19, 20, 22, 25, 28, 32, 34, 36 and 37 in 100 ml of water. The layers obtained after drying and 12 hours' storage are in all cases resistant to boiling (layer melting points above 100°C). The hardening effect obtained is independent of the support used. Layers which have not been treated melt at 37°C.
EXAMPLE 8
An unhardened multilayered colour film consisting of
1. A bottom red sensitive layer 4μ in thickness which contains 35 g of silver bromide, 80 g of gelatin and 24 g of 1-hydroxy-4-sulfo-2-naphthoic acid heptodecylamide per kg of emulsion,
2. a 2μ interlayer of gelatin,
3. a 4μ green sensitive middle layer which contains 35 g of silver bromide, 80 g of gelatin and 16 g of 1-(4'-phenoxy-3'-sulfo)-3-heptadecyl-pyrazolone-5 per kg of emulsion,
4. a 2μ yellow filter layer of colloidal silver in gelatin,
5. a. 482 blue sensitive top layer which contains 35 g of silver bromide, 80 g of gelatin and 20 g of 3-stearylaminobenzoyl acetyl-5', 3'-dicarboxyanilide per kg of emulsion and
6. a 2μ protective layer of gelatin
is cast in known manner on a layer support of cellulose triacetate 120μin thickness and dried. The film is coated with a 1 % aqueous solution and a 2 % aqueous solution of compounds 2, 5, 19 and 30.
The layer melting points and the temperatures at which the layers dissolve off are determined after drying and 12 hours' storage at room temperature.
______________________________________ temperature with top coating at which layer melting point of dissolves off layer ______________________________________ compound 2 1 % 75°C 10'100°C 2 % 100°C 10'100°C compound 5 1 % 100°C 10'100°C 2 % 100°C 10'100°C compound 19 1 % > 100°C 10'100°C 2 % >100°C 10'100°C compound 30 1 % > 100°C 10'100°C 2 % > 100°C 10'100°C Material without top coating used for comparison 40°C 40°C ______________________________________
The results show that the multi-layered colour films are effectively cross-linked by the top coating right down to the lowermost layers.
EXAMPLE 9
40 ml of a 5 % aqueous solution of polyacrylic acid sodium salt and 10 ml of a 40 % SiO 2 suspension are added to 100 ml of a 10 % gelatin solution. The solution is vigorously mixed. 0.2 g of compounds 6 and 20 are added to each 100 ml of the solution. Then the pH is adjusted to 6.2 and the mixtures are cast on cellulose triacetate supports. Layers with a melting point above 100°C and exellent wet strength are obtained after drying and 12 hours' storage. A layer without the addition of the compounds according to the invention melts in water at 40°C.
EXAMPLE 10
0.2 g of compounds 6 and 23 are added to 100 ml of 10 % aqueous solutions of acetyl gelatin which have been obtained by reacting gelatin with 20 % acetic anhydride and the mixtures are cast on cellulose acetate films. A layer containing 0.2 g of formalin instead of compounds 6 and 23 is cast for comparison. After drying, the layers containing compounds 6 and 23 are resistant to boiling whereas the layer which has been hardened with formalin melts at temperatures below 100°C.
Numerous substances have already been described as hardeners for portein and in particular for gelatin, for example metal salts such as chromium, aluminium or zirconium salts, aldlehydes and halogenated aldehyde compounds, in particular formaldehyde, dialdehydes and mucochloric acid, 1,2- and 1,4-diketones such as cyclohexane-1,2-dione and quinones as well as chlorides of dibasic organic acids, the anhydrides of tetracarboxylic acids, compounds which contain several reactive vinyl groups such as vinyl sulfones, acrylamides, compounds containing at least two heterocyclic three-membered rings which can easily be split off, such as ethylene oxide and ethylene imine, polyfunctional methane sulfonic acid esters and bis-α-chloracylamido compounds.
High molecular weight hardeners which are used particularly as hardeners of which the effect is to be confined to the layer having recently become known, for example polyacrolein and its derivatives or copolymers and alginic acid derivatives.
Many of the above mentioned compounds, however, are unsuitable, in particular for photographic purposes. Some of them are photographically active and therefore unsuitable for hardening photographic materials whereas others cannot be used because they have such a deleterious effect on the physical properties, for example the fragility of the gelatin layers. Others again cause discoloration or alter the pH in the course of the hardening reaction. Furthermore, for hardening photographic layers it is particularly important that the hardening effect should attain its maximum as soon as possible after drying so that the material which is being hardened will not constantly change its permeability to the developer solution as, for example in the case of mucochloric acid or formaldehyde.
Some cross-linking agents for gelatin, for example the ethylene imine compounds, also have a harmful effect on the skin and are therefore in any case unsuitable on physiological grounds.
It has long been known to use trichlorotriazine and dichloroaminotriazines as hardeners. The disadvantage of these compounds is their relatively high vapour pressure and their physiological effect. Water-soluble derivatives which contain carboxylic and sulphonic acid groups and are obtained by reacting cyanuric acid chloride with 1 mol of a diaminoalkyl or diaminoarylsulfonic acid or carboxylic acid to not have these disadvantages and have therefore recently been proposed as hardeners. Their usefulness in practice is, however, limited because owing to their high solubility they decompose when left to stand in aqueous solutions and therefore rapidly lose their effectiveness. HYdroxydichlorotriazine has also been proposed as hardener. Lastly, in the case of a hardener used for photographic layers which contain gelatin, it is of major importance both for manufacturing and for processing reasons that the onset of the cross-linking reaction should be controllable within certain limits, for example by choice of the correct dyring temperature of pH.
Compounds which contain two or more acrylic acid amide groups or vinylsulphone groups in the molecule are also known as hardeners for photographic gelatin layers, e.g. divinylsulfone, arylene-bis-vinylsulfones, N,N', N"-tris-acryloyl-hydrotriazine or methylene-bis-vinyl-sulfonamide.
Hardening of the compounds is quite satisfactory after a certain time but the compounds are only sparingly soluble in water so that hardening is liable to be irregular within the layer.
The consequences of the undesirable properties of known hardness indicated above are of great importance for photographic purposes because important photographic properties such as the gradation and sensitivity and in many cases also the silver covering power depend on the degree of cross-linking of the layer-forming colloid and undergo changes in the course of storage. Although this disadvantage can be attenuated by a brief after-treatment of the solidified layer with ammonia or an amine, it cannot be completely eliminated. To this is added the fact that aliphatic divinylsulfones tend to damage the skin.
It is an object of this invention to develop rapidly acting hardeners for layers which contain proteins, and in particular for gelatin layers, which are used for photographic purposes.
A process for hardening photographic layers which contain proteins and preferably gelatin has now been found which is characterised by the use of a hardener which contains in the molecule at least one carbamoyl radical with a heterocyclic, aromatic 5-membered or 6-membered ring which in addition to a quaternary nitrogen atom may also contain other hetero atoms.
The hardeners of this invention correspond to the following general formula ##SPC1##
in which
R 1 represents a substituted or unsubstituted alkyl group preferably containing 1 to 3 carbon atoms, an aryl group optionally substituted with a lower alkyl radical or with halogen, e.g., phenyl, if desired substituted with methyl, ethyl or propyl, Cl or Br, or an aralkyl group, e.g., benzyl, which may be substituted in the same manner as the aryl group;
R 2 has the same meaning as R 1 or it may represent a divalent optionally substituted alkylene, arylene, aralkylene or alkyl-aryl-alkyl radical, e.g., an ethylene, propylene, phenylene or xylylene radical which is attached by its second bond to a further carbamoyl ammonium group of the formula ##SPC2##
or
R 1 and R 2 may together represent a group required to complete a substituted or unsubstituted heterocyclic ring, such as piperidine, piperazine, or morpholine ring which may be substituted e.g., with an alkyl group containing 1 to 3 carbon atoms or with halogen such as Cl or Br;
R 3 represents hydrogen or an alkyl group containing 1 to 3 carbon atoms or the group ##SPC3##
in which
A represents a vinyl group of a polymerised vinyl compound or of a copolymer with other copolymerisable monomers and
αrepresents a number such that the molecular weight of the resulting homopolymer of copolymer with other copolymerisable monomers is greater than 1,000,
R 4 represents hydrogen, an alkyl group containing 1 to 3 carbon atoms or if Z represents the atomic grouping required to complete a pyridinium ring and R 3 is absent R 4 represents one of the groups ##SPC4##
R 5 represents an alkyl, aryl or aralkyl group but R 5 is absent if the nitrogen to which R 5 is attached is involved in a double bond in the heterocyclic aromatic ring formed by Z,
Z represents the atomic grouping required to complete a 5-membered or 6-membered aromatic ring optionally substituted on the nitrogen atom, e.g., with an alkyl group containing 1 to 3 carbon atoms or it may represent the atomic grouping required to complete a condensed system such as isoquinoline, which atomic grouping may contain other hetero atoms in addition to the nitrogen atom, e.g., O or S, and
X represents an acid anion such as halogen - , BF 4 - , NO 3 - , So 4 - , ClO 4 - , CH 3 OSO 3 - .
The following compounds have proved to be particularly useful. The list is given only by way of example and is not intended to limit the scope of the invention. ##SPC5##
The compounds may be prepared by simple methods well known from the literature. Carbamic acid chlorides may be prepared, for example, by reacting secondary amines with phosgene, and these carbamic acid chlorides are then reacted (with the exclusion of light) with aromatic, heterocyclic compounds which contain nitrogen. The preparation of compound 3 has already been described in Chemische Berichte 40 (1907) page 1831. The method of preparation of the other compounds will be explained with reference to N,N-phenyl-methyl-carbamoyl-pyridinium chloride (compound 6) which is used as an example:
A. phenyl-methyl-carbamoyl chloride
49.5 g of phosgene are carefully introduced into 400 cc of absolute toluene with cooling (22° to 25°C). A solution of 107 g of distilled N-methylaniline in 450 cc of absolute toluene is added dropwise to this solution with vigorous stirring. The mixture is then heated for 1/2 hour at 80° to 90°C and cooled and the precipitated hydrochloride is suction filtered. Lastly, the solution in toluene is evaporated to dryness under vacuum with exclusion of moisture. The residue crystallises on cooling.
Yield: 81 g, melting point 85°C.
B. 33.9 g of Compound A are stirred into 400 cc of absolute pyridine the reaction being conducted in a dark room with exclusion of moisture. The carbamoyl-pyridinium chloride formed precipitates after 3 hours. After the addition of an equal quantity of ether, the crystals are suction filtered, taken up with a small quantity of absolute ethanol and again precipitated with ether. The product is again suction filtered, washed thoroughly with ether and dried in a desiccator.
Yield: 41 g, melting point 108°-110°C.
The poly-4-vinyl-N,N-dimethyl-carbamoyl-pyridinium chloride corresponding to Compound 15 is prepared as follows:
10.5 g of poly-4-vinyl-pyridine (molecular weight) 10,000) are dissolved in 150 ml of absolute ethanol, and 10.8 g of dimethylcarbamic acid chloride are added with vigorous stirring.
The mixture is stirred at room temperature for 5 hours and concentrated under vacuum. It is then run into ether with stirring and the precipitated product is suction filtered. It is washed thoroughly with ether. The compound is dried in a desiccator.
Yield 18 g.
The folliwng example may characterize the method of preparation of the pryidinium compounds 19 to 40 which are substituted on the pyridinium ring. The example refers to compound 30:
A. preparation of morpholino-chlorocarbonyl ##SPC6##
49.5 g phosgene are slowly introduced into 400 ml of absolute toluene. A solution of 87 g of destilled morpholine in 450 ml of absolute toluene is added dropwise to this solution with vigorous stirring. The mixture is then heated for one-half hour at 80° to 90°C and coated, and the precipitated hydrochloride is suction filtered and washed with absolute toluene. Lastly the filtrate is evaporated to dryness under vacuum with exclusion of moisture. It remains on oil which is destilled.
Yield: 50 g, boiling point 72°C.
B. preparation of compound 30 ##SPC7##
1/20 mol of compound A were stirred into a solution of 1/20 mol of 3-acetaminopyridine in 60 ml of chloroforme. The mixture is stirred for 2 hours at 50°C with exclusion of moisture. The chloroforme is then evaporated under vacuum and the highly viscous residue is mixed with a small amount of absolute ethyl ether. Then the crystalline product is suction filtered, washed with absolute ethyl ether and dried in a desiccator.
Yield 10 g, melting point 118°-123°C.
The preparation of the other compounds is carried out in a corresponding manner. The salts can be easily prepared by addition of the suitable acids.
The compounds used according to the invention are preferably added to the protein layers which are to be hardened immediately before they are cast, preferably in the form of aqueous or alcoholic solutions. It is necessary to add them only shortly before the layers are cast because they react very rapidly with gelatin or other proteins normally used in photography. Once the compounds have been added, the casting solutions should be cast within a few minutes. The rate at which the hardening reaction takes place depends primarily on the concentration of proteins in the casting solution.
Another possiblity consists in casting solutions which have not been hardened and then coating the resulting layers with a solution of the hardening compounds. Alternatively, photographic layers which have not been hardened or only slightly hardened may be bathed in aqueous solutions of the compounds containing sodium sulfate at some stage of processing the photographic material, for example before development.
The term photographic layers is used in this context as a general term to denote any layers used in photographic materials, for example light sensitive silver halide emulsion layers, protective layers, filter layers, anti-halation layers, back coating layers or, in general, any photographic auxiliary layers.
The light sensitive emulsion layers for which the hardening process according to the invention is particularly suitable are, for example, those layers which are based on non-sensitized emulsions, orthochromatic, panchromatic or infra-red emulsions, X-ray emulsions and other emulsions which are spectrally sensitized. The hardening process according to the invention has also proved suitable for hardening gelatin layers used for various black-and-white photographic processes and colour photographic processes.
The process according to the invention has proved particularly advantageous for hardening composite photographic layers used for colour photographic processes, e.g., those which contain emulsion layers with colour couplers or emulsion layers which are designed to be treated with solutions which contain colour couplers.
The effect of the compounds used according to the invention is not impaired by the usual photographic additives and the hardeners are also uneffected by photographically active substances such as water-soluble or emulsified water insoluble dry components, stabilizers, sensitizers and the like. Moreover, they have no influence on the light sensitive silver halide emulsions. They may be combined with any compounds from the known classes of hardeners, for example formalin, mucochloric acid, triacrylic formal, bis-vinylsulfones, bis-vinylsuflonamides, dialdehydes or bis-chloroacetamides.
Apart from gelatins, the layers may also contain water-soluble high polymer compounds, in particular polyvinyl alcohol, polyacrylic acid sodium and other copolymers which contain carboxyl groups, polyvinyl pyrrolidone, polyacrylamide or high molecular weight natural substances such as dextrans, dextrins, starch ether, alginic acid or alginic acid derivatives.
The concentrations at which the hardeners according to the invention are used may vary within wide limits and depend mainly on the hardening compound used.
Satisfactory results are obtained with quantities of 0.1 to 10 % by weight and particularly 0.2 to 6 % by weight, based on the dry weight of binder.
As already mentioned above, the hardening reaction between the compounds according to the invention and gelatin or proteins sets in immediately so that the optimum degree of hardening is reached more or less simultaneously with drying of the layers after they are cast or processed.
The activity of the hardening compounds is determined by means of the melting point of the layers, which can be found as follows:
Half of a layer cast on a support is dipped in water which is continuously heated up to 100°C. The temperature at which the layer runs from the support (formation of streaks) is taken as the melting point or melting off point. When measured by this method, no increase in melting point is found in any pure protein layers or gelatin layers which contain no hardener, on the contrary the layers run off at 30°-35°C.
The compounds according to the invention react surprisingly quickly with proteins and therefore enable materials which contain proteins to be hardened to their optimum degree of hardness within a very short time. This unexpected effect of the compounds is particularly important for the hardening of photographic materials which contain proteins as binders. Hardening can be adjusted to the required degree in a readily controlled nammer at the stage of preparation of the materials and does not require prolonged storage times with the concomitant unreliability of uncontrollable after hardening.
An additional advantage is brought by the carbamolypyridinium compounds the pyridine ring of which is substituted with hydrocarbon residues having more than 3 carbon atoms. Examples for compounds of this group are compounds 19 - '.
Compounds of this shape are practically unodorous, so that the application as hardeners can be carried out without odorous annoyance. The advantage of the inodorousness of the pyridinium compounds becomes particularly clear considering the fact that the limit of perceptibility of pyridine through the sens of smell already begins at 0.0004 mg/m 3 in air. Since the above pyridinium compounds turn out to be odorless they give sufficient security that exceeding the "Threshold limit value" of pyridine which is of the order of 15 mg/m 3 is virtually impossible.
The invention will now be explained with the aid of the following examples.
EXAMPLE 1
1 % by weight and 2 % by weight, respectively, of compounds 2, 19 and 20 based on the dry weight of gelatin, are added in the form of an aqueous solution at pH 6.2 to 100 ml portions of a photographic silver bromide gelatin emulsion containing 10 % by weight of gelatin which is ready for casting. The mixture is viigorously stirred and immediately cast on a prepared cellulose triacetate support by means of a conventional casting apparatus and dried. The usual additives are not altered. After a storage time of 24 hours at room temperature, the melting points of the layers are measured.
______________________________________ compound quantity layer melting points added in °C ______________________________________ -- 0 % (without 34 hardening) 2 1 % >100 2 % >100 19 1 % >100 2 % >100 20 1 % >100 2 % >100 ______________________________________
The cross-linking reaction is so intensive that the layers do not dissolve even after 5 minutes in boiling water.
EXAMPLE 2
Sodium sulphate is added to a 5 % aqueous solution each of Compound 1 and Compound 30 until the solution is almost saturated. Unhardened photographic silver halide gelatin layers are dipped in this solution for various lengths of time at 22°C. The layers are then briefly washed, dried and then stored at room temperature for 12 hours. The effectiveness of these preliminary baths is determined by measuring the melting points.
______________________________________ immersion time layer melting points remarks in minutes in °C compound 1 compound 30 ______________________________________ 0.5 50°C 60°C 1 100°C >100°C soft layer 2 >100°C >100°C increased strength of layer 3 >100°C >100°C very high strength of layer untreated layer 34°C ______________________________________
At a bath temperature of 40°C, the required immersion times are much shorter and layer melting points above 100°C are obtained after only 45 seconds.
The stability of the hardeners according to the invention in aqueous solutions depends on the additives used. In distilled water, the hardeners are still active after 3 days.
EXAMPLE 3
A 10 % casein solution in water is prepared by the addition of sodium hydroxide solution. The solution is divided in 9 parts of 100 ml and 0.1 g of tartrazine is added as filter dye to each solution. 3 % by weight of compounds 2, 19, 20, 21, 24, 25, 30, 31 and 37 dissolved in water are added at pH 7 before casting. The mixtures are cast on glass plates and hardened filter foils which are no longer soluble in aqueous alkali are obtained after drying.
EXAMPLE 4
A 20 % by weight of solution of zein is prepared in a mixture of ethanol and water (8:2 ) and cast on the back of a cellulose acetate film. After drying, a layer which is readily soluble in a mixture of ethanol and water is obtained.
If strips of the film are bathed in solutions of
2 g of compounds 2, 19, 20, 21, 26, 31, 32, 35, 37, 38 or 39 15 g of sodium sulfate and 80 ml of water
for 3 minutes and then briefly washed and dried in a heating cupboard at 50° to 60°C, the layers are insoluble in all solvents and effectively cross-linked.
EXAMPLE 5
An unhardened silver halide emulsion containing 10 % by weight of gelatin as binder is cast on a triacetyl cellulose support without the addition of a hardener. The layer contains all the other usual additives. Samples of the dried layer are coated with 0.5, 1, 2 and 3 % aqueous solutions of compounds 1, 2, 5, 24, 29 and 33 and then dried. The melting points, degree of swelling and resistance to scratching when wet are then determined on the layers. The results are summarized in the following Table.
The degrees of swelling are determined gravimetrically after 10 minutes treatment of the layers in distilled water at 22°C and are indicated in percent.
To determine the resistance to scratching when wet, a metal point of a specified size is passed over the wet layer and loaded with an increasingly heavy weight. The wet scratch resistance is indicated by the weight at which the point leaves a visible scratch trace on the layer. The larger the weight the higher the scratch resistance.
______________________________________ compound layer swelling wet scratch melting point in % resistance in p ______________________________________ compound 1: 0.5 % 355 750 1 % 310 850 2 % 10'100°* 280 950 3 % 250 950 ______________________________________ compound 2: 0.5 % 325 950 1 % 295 950 2 % 10'100° 260 1150 3 % 235 1150 ______________________________________ compound 5: 0.5 % 420 650 1 % 370 750 2 % 10'100° 280 950 3 % 260 1100 ______________________________________ compound 24: 0.5 % 400 650 1 % 300 750 2 % 10'100°* 250 850 3 % 200 900 ______________________________________ compound 29: 0.5 % 320 900 1 % 300 950 2 % 10'100° 250 1100 3 % 250 1200 ______________________________________ compound 33: 0.5 % 400 650 1 % 350 750 2 % 10'100° 300 950 3 % 260 1100 not after- treated 36° 600 - 800 300 ______________________________________ *The layer is not dissolved off after 10 minutes in boiling water.
EXAMPLE 6
25 % by weight, based on the weight of the gelatin, of a blue green coupler of the following formula ##SPC8##
are added to an unhardened silver halide emulsion which contains 10 % by weight of gelatin. The usual casting additives with the exception of a hardener are then added to the emulsion. The mixture is cast on a prepared polyethylene terephthalate support and dried.
Samples of this layer are then coated with 0.5 to 3 % aqueous solutions of the compounds according to the invention. After drying and 10 hours' storage, layers with an exceptionally high degree of cross-linking are obtained. The results are summarized in the following Tables.
______________________________________ top coating melting swelling wet scratch point in % resistance in p ______________________________________ compound 1: 0.5 % 10'100°C 355 750 1 % 310 850 2 % 280 950 3 % 250 950 ______________________________________ compound 2: 0.5 % 10'100°C 325 950 1 % 295 950 2 % 260 1150 3 % 235 1150 ______________________________________ compound 5: 0.5 % 10'100°C 350 850 1 % 320 850 2 % 270 1150 3 % 250 1150 ______________________________________ compound 19: 0.5 % 10'100°C 410 650 1 % 350 750 2 % 300 800 3 % 280 900 ______________________________________ compound 20: 0.5 % 10'100°C 380 650 1 % 310 800 2 % 280 850 3 % 250 1000 ______________________________________ compound 25: 0.5 % 10'100°C 410 600 1 % 340 650 2 % 300 700 3 % 270 800 ______________________________________ only treated 42°C 800 with water ______________________________________ The photographic properties were not affected.
EXAMPLE 7
The usual additives with the exception of a hardener are added to 100 ml of a photographic silver bromide gelatin emulsion containing 10 % by weight of gelatin. The mixture is cast
a. on baryta paper and
b. on paper backed with polyethylene on both sides. After drying, samples of the two materials are bathed for 2 minutes in aqueous solutions containing 3 g of compounds 2, 5, 10, 11, 12, 13, 14, 19, 20, 22, 25, 28, 32, 34, 36 and 37 in 100 ml of water. The layers obtained after drying and 12 hours' storage are in all cases resistant to boiling (layer melting points above 100°C). The hardening effect obtained is independent of the support used. Layers which have not been treated melt at 37°C.
EXAMPLE 8
An unhardened multilayered colour film consisting of
1. A bottom red sensitive layer 4μ in thickness which contains 35 g of silver bromide, 80 g of gelatin and 24 g of 1-hydroxy-4-sulfo-2-naphthoic acid heptodecylamide per kg of emulsion,
2. a 2μ interlayer of gelatin,
3. a 4μ green sensitive middle layer which contains 35 g of silver bromide, 80 g of gelatin and 16 g of 1-(4'-phenoxy-3'-sulfo)-3-heptadecyl-pyrazolone-5 per kg of emulsion,
4. a 2μ yellow filter layer of colloidal silver in gelatin,
5. a. 482 blue sensitive top layer which contains 35 g of silver bromide, 80 g of gelatin and 20 g of 3-stearylaminobenzoyl acetyl-5', 3'-dicarboxyanilide per kg of emulsion and
6. a 2μ protective layer of gelatin
is cast in known manner on a layer support of cellulose triacetate 120μin thickness and dried. The film is coated with a 1 % aqueous solution and a 2 % aqueous solution of compounds 2, 5, 19 and 30.
The layer melting points and the temperatures at which the layers dissolve off are determined after drying and 12 hours' storage at room temperature.
______________________________________ temperature with top coating at which layer melting point of dissolves off layer ______________________________________ compound 2 1 % 75°C 10'100°C 2 % 100°C 10'100°C compound 5 1 % 100°C 10'100°C 2 % 100°C 10'100°C compound 19 1 % > 100°C 10'100°C 2 % >100°C 10'100°C compound 30 1 % > 100°C 10'100°C 2 % > 100°C 10'100°C Material without top coating used for comparison 40°C 40°C ______________________________________
The results show that the multi-layered colour films are effectively cross-linked by the top coating right down to the lowermost layers.
EXAMPLE 9
40 ml of a 5 % aqueous solution of polyacrylic acid sodium salt and 10 ml of a 40 % SiO 2 suspension are added to 100 ml of a 10 % gelatin solution. The solution is vigorously mixed. 0.2 g of compounds 6 and 20 are added to each 100 ml of the solution. Then the pH is adjusted to 6.2 and the mixtures are cast on cellulose triacetate supports. Layers with a melting point above 100°C and exellent wet strength are obtained after drying and 12 hours' storage. A layer without the addition of the compounds according to the invention melts in water at 40°C.
EXAMPLE 10
0.2 g of compounds 6 and 23 are added to 100 ml of 10 % aqueous solutions of acetyl gelatin which have been obtained by reacting gelatin with 20 % acetic anhydride and the mixtures are cast on cellulose acetate films. A layer containing 0.2 g of formalin instead of compounds 6 and 23 is cast for comparison. After drying, the layers containing compounds 6 and 23 are resistant to boiling whereas the layer which has been hardened with formalin melts at temperatures below 100°C.