Use of long-chain citrate esters in aqueous polyurethane dispersions

Abstract

The invention relates to the use of long-chain citrate esters in aqueous polyurethane dispersions. The invention describes the use of long-chain citrate esters as polymer additives in aqueous polymer dispersions for producing porous polymer coatings, preferably for producing porous polyurethane coatings.

Description

Technical Field

[0001] The invention belongs to the field of plastic coating and artificial leather. Background Art

[0002] More particularly, the present invention relates to the use of long chain citrate esters as additives for producing porous polymer coatings, especially porous polyurethane coatings.

[0003] Plastic-coated textiles, such as artificial leather, usually consist of a textile carrier onto which is laminated a porous polymer layer, which in turn is coated with a top layer or top coating.

[0004] In this case, the porous polymer layer preferably has pores in the micrometer range and is air permeable and therefore breathable, that is, permeable to water vapor, but waterproof. The porous polymer layer generally includes porous polyurethane. In order to produce artificial leather based on PU in an environmentally friendly manner, a method based on aqueous polyurethane dispersion (called PUD) has been developed recently. These dispersions are generally composed of polyurethane particles dispersed in water, and the solid content is generally in the range of 30-60% by weight. In order to produce porous polyurethane layers, these PUDs are mechanically foamed, coated on a carrier (layer thickness is generally between 300-2000 μm), and then dried at high temperature. During this drying step, the water present in the PUD system evaporates, which results in the formation of a film of polyurethane particles. In order to further improve the mechanical strength of the film, hydrophilic (poly) isocyanates can also be additionally added to the PUD system during the production process, and these isocyanates can react with the free OH groups present on the surface of the polyurethane particles during the drying step, thereby resulting in additional crosslinking of the polyurethane film.

[0005] The mechanical and tactile properties of the PUD coatings produced in this way are determined to a crucial extent by the cell structure of the porous polyurethane film. In addition, the cell structure of the porous polyurethane film influences the air permeability and breathability of the material. Particularly good properties can be achieved here with very fine, evenly distributed cells. A conventional method for influencing the cell structure during the above-mentioned production process is to add a foam stabilizer to the PUD system before or during the mechanical foaming. The primary role of a suitable stabilizer is to be able to inject a sufficient amount of air into the PUD system during the foaming operation. Secondly, the foam stabilizer has a direct influence on the morphology of the bubbles produced. The stability of the bubbles is also influenced to a crucial extent by the type of stabilizer. This is important, especially during the drying of the foamed PUD coating, because drying defects such as coarse cells or drying cracks can be prevented in this way.

[0006] In the past, various foam stabilizers have been used in the above-mentioned PUD methods. For example, the documents US 2015 / 0284902 A1 or US 2006 0079635 A1 describe the use of foam stabilizers based on ammonium stearate. However, the use of corresponding stabilizers based on ammonium stearate has many disadvantages. A significant disadvantage here is that ammonium stearate has a high migration capacity in the finished artificial leather. The effect of this is that over time, surfactant molecules accumulate at the surface of the artificial leather, which can lead to white discoloration at the leather surface. In addition, this surfactant migration can produce an unpleasant greasy film on the surface of the artificial leather, especially when the corresponding materials come into contact with water.

[0007] Another disadvantage of ammonium stearate is the formation of insoluble lime soaps when in contact with hard water. In the case of artificial leather produced on the basis of ammonium stearate coming into contact with hard water, white blooming may therefore occur on the surface of the artificial leather, which is undesirable, especially in the case of dark leathers.

[0008] Another disadvantage of foam stabilizers based on ammonium stearate is that they do allow efficient foaming of aqueous polyurethane dispersions, but often result in a rather coarse and irregular foam structure. This can have a negative impact on the optical and tactile properties of the finished artificial leather product.

[0009] Another disadvantage of ammonium stearate is that the PUD foams produced often have insufficient stability, which leads to disadvantages in their processing, especially in the drying of PUD foams at high temperatures. As a result, for example, the corresponding foams must be dried relatively gently and slowly, which in turn leads to longer processing times in the production of artificial leather.

[0010] As an alternative to ammonium stearate-based foam stabilizers, polyol esters and polyol ethers have been identified as effective foam additives for aqueous polyurethane dispersions in the past. These structures are described, for example, in documents EP 3487945A1 and WO2019042696A1. Compared with ammonium stearate, the main advantage of polyol esters and polyol ethers is that they migrate only slightly (if at all) in the finished artificial leather, and therefore do not cause undesirable surface discoloration. In addition, polyol esters and polyol ethers are not sensitive to hard water.

[0011] Another advantage of polyol esters and polyol ethers over foam stabilizers based on ammonium stearate is that they generally lead to a significantly finer and more uniform foam structure, which has a favorable effect on the properties of artificial leather materials produced with these substances. Polyol esters and polyol ethers often also produce much more stable PUD foams, which in turn brings process-related advantages in the production of artificial leather.

[0012] Despite these advantages, polyol esters and polyol ethers are not completely without potential disadvantages. One potential disadvantage is that in some cases the foam stabilizing effect of these compounds may be impaired by the presence of other co-surfactants in the PUD system. However, the use of co-surfactants is not uncommon, especially in the production of aqueous polyurethane dispersions. In this case, the co-surfactant is used to improve the dispersion of the polyurethane prepolymer in water and is usually retained in the final product. During the mechanical foaming of an aqueous polyurethane dispersion containing polyol esters or polyol ethers as foam additives, the corresponding co-surfactant may in some cases have an adverse effect on the foaming characteristics of the system. As a result, in some cases, only very little air, if any, may be driven into the system; the resulting foam structure will then be relatively rough and the leather quality will be reduced. Co-surfactants can also have an adverse effect on the stability of the resulting foam, which may lead to foam aging during the processing of the foamed PUD system, and thus to failures and defects in the resulting foam coating.

[0013] Another potential disadvantage is that PUD systems containing polyol esters or polyol ethers as foam additives often require very high shear energies for effective foaming. This in turn can lead to limitations and process-related drawbacks in some cases and restrict the choice of machines for producing foam in the industry.

[0014] The problem addressed by the present invention is therefore to provide an additive for the production of foam systems and foam coatings based on PUDs which enables efficient foaming of PUD systems and which does not have the disadvantages detailed in the art. It has been surprisingly found that long-chain citrate esters are able to solve said problem. Summary of the invention

[0015] The present invention therefore provides the use of long-chain citrate esters as additives, preferably as foam additives, in aqueous polymer dispersions, preferably aqueous polyurethane dispersions, for producing porous polymer coatings, preferably for producing porous polyurethane coatings.

[0016] The use of the long-chain citrate esters according to the invention surprisingly has various advantages here.

[0017] One advantage is that long-chain citrate esters enable particularly efficient foaming of aqueous PUD systems. It is noteworthy here that the foams produced thereby have a particularly fine cell structure and a particularly uniform cell distribution, which in turn has a very favorable effect on the mechanical and haptic properties of porous polymer coatings produced on the basis of these foams. In addition, the air permeability or breathability of the coating can be improved in this way.

[0018] Another advantage is that long-chain citrate esters enable efficient foaming of PUD systems even at relatively low shear rates, which leads to fewer limitations and broader processability during artificial leather production.

[0019] A further advantage is that long-chain citrate esters enable the production of particularly stable foams. Firstly, this has a favorable effect on their processability, in particular due to the additional time flexibility during the production process. Secondly, the increased foam stability has the advantage that during the drying of the corresponding foam, drying defects such as cell coarsening or drying cracks can be avoided. Furthermore, the improved foam stability enables the foam to be dried more quickly, which offers processing advantages both from an environmental and economic point of view.

[0020] Yet another advantage is that the efficacy of the long-chain citrate ester is hardly compromised by the co-surfactant (if any) present in the PUD system. Thus, even in the case of a PUD system containing a co-surfactant, the surfactant formulation according to the invention enables the system to foam efficiently and to form fine and uniform foam that is also extremely stable.

[0021] A further advantage is that the long-chain citrate esters according to the invention have hardly any migration capacity (if any) in the finished artificial leather product and therefore do not cause undesirable surface discoloration or efflorescence. In addition, the surfactants according to the invention are hardly sensitive to hard water (if any).

[0022] The term "long-chain citrate" throughout the present invention encompasses esters of citric acid (C6H8O7) with long-chain alkanols and enols. By "long chain" in this context is meant an alcohol having at least 12, preferably at least 14, more preferably at least 16 carbon atoms. Both branched and linear hydrocarbon groups are preferred herein.

[0023] The term "co-surfactant" throughout the present invention encompasses further surfactants which may be present in the polymer dispersion together with the long-chain citrate esters according to the invention. These surfactants include in particular the surfactants used during the production of the polymer dispersion. For example, polyurethane dispersions are often produced by synthesizing a PU prepolymer which is dispersed in water in a second step and then reacted with a chain extender. In order to improve the dispersibility of the prepolymer in water, co-surfactants can be used here. In the context of the present invention, co-surfactants are preferably anionic co-surfactants.

[0024] The present invention is further described below by way of example, without intending to limit the present invention to these illustrative embodiments. Where ranges, general formulas or classes of compounds are specified below, these are intended to include not only the corresponding ranges or groups of compounds explicitly mentioned, but also all sub-ranges and sub-groups of compounds that can be obtained by removing individual values ​​(ranges) or compounds. In the case of citing a document in the context of this specification, the entire content of the document (especially the content of the subject matter forming the context in which the document is cited) is intended to form a part of the disclosure of the present invention. Unless otherwise stated, percentages are weight percentages. Unless otherwise stated, when parameters determined by measurement are given below, the measurement is carried out at a temperature of 25° C. and a pressure of 101 325 Pa. In the case of using a chemical (empirical) formula in the present invention, the specified coefficients can be not only absolute values, but also average values. For polymeric compounds, the coefficients preferably represent average values. The structural formulas and empirical formulas given in the present invention are representatives of all isomers, which can be achieved by different arrangements of repeating units.

[0025] In the context of the present invention, particular preference is given to those long-chain citric acid esters which are obtained by reacting long-chain alcohols with citric acid with elimination of water. If desired, these reactions can be carried out using suitable catalysts, for example organic or inorganic acids (such as p-toluenesulfonic acid, sulfuric acid or methanesulfonic acid), acid salts, carbonyl chlorides, metal or amphoteric metal oxides, metal alkoxides or carboxylates, for example tetrabutyl (ortho) titanate or tin(II) 2-ethylhexanoate. The corresponding reactions are known to the person skilled in the art and are described, for example, in –Chemie Lexikon (Thieme-Verlag, 1996).

[0026] In the context of the present invention, it is further preferred that the average degree of esterification of the long-chain citrate esters according to the invention is 1-2.7, preferably 1.3-2.6, more preferably 1.4-2.5, even more preferably 1.5-2.4. The degree of esterification is defined here as the molar ratio of alcohol to citric acid. The average degree of esterification refers to the arithmetic mean.

[0027] The above-mentioned reaction for preparing the long-chain citrate ester according to the invention may not be carried out to completion, so that residues of unconverted alcohol and / or citric acid may also be present in the reaction mixture. Therefore, in the context of the present invention, the term "citrate ester" also encompasses those esters containing free alcohol and / or free citric acid.

[0028] Furthermore, it is possible that, in the above-mentioned reaction for preparing the long-chain citrate esters according to the invention, mixtures of different esters consisting of monoesters, diesters and triesters may be formed during the reaction and these mixtures may further contain free citric acid and / or free fatty alcohols. Therefore, in the context of the present invention, the term "citrate ester" also covers such mixtures.

[0029] Furthermore, it is possible that in the above-mentioned reaction for preparing the long-chain citrate esters according to the invention, linear, branched or cyclic oligomeric or polymeric condensation products of citric acid may be formed during the reaction. Therefore, in the context of the present invention, the term "citrate ester" also encompasses those esters comprising linear, branched or cyclic oligomeric or polymeric condensation products of citric acid as well as esters of these compounds. Throughout the context of this application, the term "oligomeric" means a molecule comprising 2 or more repeating units.

[0030] The alcohols used for preparing the long-chain citric acid esters are preferably lauryl alcohol (1-dodecanol), myristyl alcohol (1-tetradecanol), cetyl alcohol (1-hexadecanol), margaryl alcohol (1-heptadecanol), stearyl alcohol (1-octadecyl alcohol), arachidyl alcohol (1-eicosanol), behenyl alcohol (1-docosanol), xylanyl alcohol (1-tetracosanol), ceryl alcohol (1-hexacosanol), montanyl alcohol (1-octacosanol), myristyl alcohol (1-triacontanol), palmitoleyl alcohol (cis-9-hexadecen-1-ol), oleyl alcohol (cis-9-octadecen-1-ol) and / or elaidoyl alcohol (trans-9-octadecen-1-ol) and structural isomers having the same overall structural formula in each case and mixtures of these substances, particular preference being given to cetyl alcohol and stearyl alcohol and mixtures of these two substances.

[0031] The source of the above-mentioned long-chain alcohols can be vegetable or animal fats, oils or waxes. For example, lard, beef tallow, goose fat, duck fat, chicken fat, horse fat, whale oil, fish oil, palm oil, olive oil, avocado oil, seed kernel oil, coconut oil, palm kernel oil, cocoa butter, cottonseed oil, pumpkin seed oil, corn kernel oil, sunflower oil, wheat germ oil, grape seed oil, sesame oil, linseed oil, soybean oil, peanut oil, lupine oil, rapeseed oil, mustard oil, castor oil, jatropha oil, walnut oil, jojoba oil, lecithin (e.g. based on soybean, rapeseed or sunflower), bone oil, neatsfoot oil (neatsfoot oil) can be used. oil), borage oil, lanolin, emu oil, deer tallow, marmot oil, mink oil, safflower oil, pumpkin oil, evening primrose oil, tall oil, and also carnauba wax, beeswax, candelilla wax, carnauba wax, sugar cane wax, retamo wax, palm wax, raffia wax, esparto wax, clover wax, bamboo wax, douglas fir wax, cork wax, sisal wax, flax wax, cotton wax, dammar wax, tea wax, coffee wax, rice wax, oleander wax, beeswax and / or wool wax.

[0032] It is further preferred to use branched long-chain primary or secondary alcohols for the preparation of the citrate esters according to the invention. Guerbet alcohols, ie branched alcohols formed by Guerbet condensation, and branched secondary alcohols formed by paraffin oxidation by the Bashkirov process are particularly preferred.

[0033] It is further preferred in the context of the present invention to use alkoxylated, preferably ethoxylated / propoxylated derivatives of the alcohols detailed above for the preparation of the long-chain citrate esters according to the invention. In this case, it is preferred that each alcohol group carries not more than 10, preferably not more than 7, more preferably not more than 5, even more preferably not more than 3 alkoxy units.

[0034] In the context of the present invention, it is particularly preferred that the citrate esters according to the invention are monopalmityl citrate, dipalmityl citrate, monostearyl citrate, distearyl citrate and mixtures of these, particularly preferably dipalmityl citrate and / or distearyl citrate and / or monostearyl palmityl citrate and mixtures of these.

[0035] In the context of the present invention, particular preference is given to those citrates which conform to the general formula (I):

[0036]

[0037] Where R 1 The radicals are independently identical or different monovalent aliphatic or aromatic, saturated or unsaturated hydrocarbon radicals having 12 to 40 carbon atoms, preferably 14 to 30, more preferably 16 to 24 carbon atoms, or H, provided that all R 1The statistical numerical average of at least 0.3, preferably 0.4, more preferably 0.5, even more preferably 0.6 of the radicals is H. Thus, formula (I) also describes mixtures of various monoesters, diesters and triesters of citric acid with long-chain alcohols. Such mixtures are likewise preferred according to the invention. The statistical numerical average is the arithmetic mean.

[0038] As already described, during the above-mentioned reaction for preparing the long-chain citrate esters according to the invention, linear, branched or cyclic oligomeric or polymeric condensation products of citric acid may also be formed. Thus, the long-chain citrate esters as well as those conforming to the general formula (I) may additionally contain those conforming to the general formula (II). In the context of the present invention, this is likewise preferred.

[0039] M w D x T y Q z Formula (II)

[0040] in

[0041]

[0042] W=0 to 11, preferably 0 to 5, particularly preferably 0 to 2,

[0043] x=0 to 10, preferably 0 to 5, particularly preferably 0 to 2,

[0044] y=0 to 5, preferably 0 to 5, particularly preferably 0 to 2,

[0045] z=0 to 3, preferably 0 to 2, particularly preferably 0 to 2,

[0046] And R 1 As defined above, the condition is that all R 1 The statistical average of at least 10%, preferably at least 15%, more preferably at least 20% of the groups is H.

[0047] With regard to the general formula (II), it should be noted that the structural units M, D, T and Q are in each case connected via an oxygen bridge. 1 / 2 The groups are connected to form an oxygen bridge (-O-), where any O 1 / 2 The group can only be combined with another O 1 / 2 Group connection.

[0048] In terms of structure, the citrate esters according to the invention can be characterized by wet chemical coefficients, for example their hydroxyl number, their acid number and their hydrolysis number. Suitable determination methods for determining the hydroxyl number are in particular those according to DGF CV 17a (53), Ph.Eur. 2.5.3 Method A and DIN 53240. Suitable methods for determining the acid number are in particular those according to DGFC-V 2, DIN EN ISO 2114, Ph.Eur. 2.5.1, ISO 3682 and ASTM D 974. Suitable determination methods for determining the hydrolysis number are in particular those according to DGF CV 3, DIN EN ISO 3681 and Ph.Eur. 2.5.6. In the context of the present invention, preference is given to those citrate esters having an acid number of 35 to 150 mgKOH / g, preferably 40 to 140 mgKOH / g, more preferably 45 to 125 mgKOH / g.

[0049] The present invention encompasses the use of neutralized and unneutralized long-chain citrates as additives in aqueous polymer dispersions, with particular preference given to the use of neutralized long-chain citrates. Throughout the present invention, the term "neutralization" also encompasses partial neutralization. For neutralization (including partial neutralization), conventional bases can be used. These conventional bases include water-soluble metal hydroxides, such as barium hydroxide, strontium hydroxide, calcium hydroxide, and preferably alkali metal hydroxides that dissociate into free metal ions and hydroxide ions in aqueous solution, especially NaOH and KOH. These also include anhydrous bases that react with water to form hydroxide ions, such as barium oxide, strontium oxide, calcium oxide, lithium oxide, silver oxide, and ammonia. In addition to these abovementioned alkali metals, solid substances which can be used as bases are those which do not have HO (in the pure compound) but which also react alkalinely when dissolved in water; examples of these substances include amines such as monoalkylamines, dialkylamines and trialkylamines, for example, in the case of amidoamines, monoalkanolamines, dialkanolamines and trialkanolamines or monoaminoalkylamines, diaminoalkylamines and triaminoalkylamines, the alkyl group can also be a functionalized alkyl group, and for example salts of weak acids such as potassium carbonate, sodium carbonate, trisodium phosphate, etc. In addition, it is also possible to use higher-functionality amines such as ethylenediamine, diethylenetriamine or triethylenetetramine for neutralization.

[0050] As already mentioned, the present invention contemplates the use of the long-chain citrate esters as described in detail above as additives in aqueous polymer dispersions, preferably in aqueous polyurethane dispersions. The polymer dispersions are preferably selected from aqueous polystyrene dispersions, polybutadiene dispersions, poly(meth)acrylate dispersions, polyvinyl ester dispersions and / or polyurethane dispersions. The solids content of these dispersions is preferably in the range of 20-70% by weight, more preferably in the range of 25-65% by weight. According to the invention, the use of long-chain citrate esters as additives in aqueous polyurethane dispersions is particularly preferred. Polyurethane dispersions based on polyester polyols, polyesteramide polyols, polycarbonate polyols, polyacetal polyols and / or polyether polyols are particularly preferred.

[0051] In the context of the present invention, it is preferred that the concentration of the long-chain citrate is in the range of 0.1-20% by weight, more preferably in the range of 0.2-15% by weight, particularly preferably in the range of 0.5-10% by weight, based on the total weight of the aqueous polymer dispersion.

[0052] Preferably, long-chain citrate esters are used in aqueous polymer dispersions as foaming aids or foam stabilizers for foaming the dispersion, i.e. as foaming additives. In addition, however, they can also be used as drying aids, leveling additives, wetting agents and rheology additives, which likewise corresponds to a preferred embodiment of the invention.

[0053] Besides the long-chain citrate esters according to the invention, the aqueous polymer dispersions may also comprise further additives / formulation components, for example color pigments, fillers, matting agents, stabilizers such as hydrolysis or UV stabilizers, bactericides, antioxidants, absorbers, crosslinkers, leveling additives, thickeners and further cosurfactants.

[0054] The long-chain citrate esters can be added to the aqueous dispersion in a suitable solvent in pure or blended form. In this regard, preferred solvents are selected from the group consisting of water, propylene glycol, dipropylene glycol, polypropylene glycol, diethylene glycol butyl ether, triethylene glycol butyl ether, ethylene glycol, diethylene glycol, polyethylene glycol, polyalkylene glycols based on EO, PO, BO and / or SO, alcohol alkoxylates based on EO, PO, BO and / or SO and mixtures of these substances, very particularly preferably aqueous diluents or blends. The blend or dilution of the long-chain citrate ester preferably contains at least 5% by weight, more preferably 10% by weight, even more preferably 15% by weight of the long-chain citrate ester according to the invention.

[0055] In the case of aqueous dilutions or blends of the long-chain citrate esters according to the invention, it may be advantageous to add hydrotropic compounds to the blend in order to improve the formulation properties (viscosity, homogeneity, etc.). Hydrotropic compounds here are water-soluble organic compounds which consist of a hydrophilic part and a hydrophobic part but whose molecular weight is too low to have surfactant properties. They lead to an improvement in the solubility or solubility properties of organic substances, in particular hydrophobic organic substances, in aqueous formulations. The term "hydrotropic compound" is known to the person skilled in the art. In the context of the present invention, preferred "hydrotropic compounds" are alkali metal and ammonium toluenesulfonates, alkali metal and ammonium xylenesulfonates, alkali metal and ammonium naphthalenesulfonates, alkali metal and ammonium cumenesulfonates and phenol alkoxylates, in particular phenol ethoxylates with up to 6 alkoxylate units.

[0056] It may also be advantageous to use the long-chain citrate esters not in pure form but in combination with other cosurfactants as additives in aqueous polymer dispersions, preferably in aqueous polyurethane dispersions. These can be used, for example, for improved compatibility of the system or, in the case of preformulated surfactant mixtures, for improved formulation properties. In this case, preferred cosurfactants according to the invention are, for example, free fatty alcohols, fatty acid amides, ethylene oxide-propylene oxide block copolymers, betaines, for example amidopropyl betaines, amine oxides, quaternary ammonium surfactants, amphoacetates, ammonium and / or alkali metal salts of fatty acids, alkyl sulfates, alkyl ether sulfates, alkyl sulfonates, alkyl benzene sulfonates, alkyl phosphates, alkyl sulfosuccinates, alkyl sulfosuccinamates, alkyl sarcosinates and mixtures of these substances, very particularly preferably free fatty alcohols having 12 to 40, more preferably 14 to 30, even more preferably 16 to 24 carbon atoms, and alkyl sulfates having 12 to 40, more preferably 14 to 30, even more preferably 16 to 24 carbon atoms, and mixtures of these substances. In addition, the cosurfactant may comprise a surfactant based on silicones, for example a trisiloxane surfactant or a polyether siloxane. In the case of ammonium and / or alkali metal salts of fatty acids, it is preferred that they contain less than 25% by weight of stearate and in particular are free of stearate.

[0057] As mentioned above, in the case of combinations of the long-chain citrate esters according to the invention with other co-surfactants, it is particularly preferred that these combinations contain between 1 wt. % and 60 wt. %, preferably between 2 wt. % and 50 wt. %, more preferably between 3 wt. % and 40 wt. %, even more preferably between 5 wt. % and 30 wt. % of co-surfactant, based on the combination of the long-chain citrate ester according to the invention and the co-surfactant.

[0058] Since, as described above, the use of the inventive long-chain citrate esters leads to significant improvements in porous polymer coatings produced from aqueous polymer dispersions, the present invention likewise provides aqueous polymer dispersions comprising at least one inventive long-chain citrate ester as described in detail above.

[0059] The invention also provides a porous polymer layer produced from an aqueous polymer dispersion obtained by using a long-chain citric acid ester according to the invention as detailed above.

[0060] Preferably, the porous polymer coating according to the present invention can be produced by a method comprising the following steps:

[0061] a) providing a mixture comprising at least one aqueous polymer dispersion, at least one long-chain citrate ester according to the invention and optionally further formulation components,

[0062] b) frothing the mixture to produce foam,

[0063] c) optionally adding at least one thickener to adjust the viscosity of the wet foam,

[0064] d) applying the coating of the foamed polymer dispersion to a suitable carrier,

[0065] e) Drying / curing the coating.

[0066] The porous polymer coating has pores preferably in the micrometer range, preferably with an average cell size of less than 350 μm, more preferably less than 200 μm, particularly preferably less than 150 μm, very particularly preferably less than 100 μm. The average cell size can preferably be determined microscopically, preferably by electron microscopy. For this purpose, a cross section of the porous polymer coating is observed using a microscope with sufficient magnification and the size of at least 25 cells is determined. The observed cells or the arithmetic mean of the cell sizes are then taken as the average cell size.

[0067] For preferred configurations, in particular for the long-chain citrate esters and polymer dispersions preferably used in the process, reference is made to the preceding description and to the preferred embodiments described above, in particular as described in detail in the claims.

[0068] Obviously, the processing steps of the method according to the present invention as described above are not restricted to any fixed time sequence. For example, processing step c) can be performed at an early stage and simultaneously with processing step a).

[0069] A preferred embodiment of the present invention is to foam the aqueous polymer dispersion in process step b) by applying high shear forces. Foaming can be carried out here with the aid of shearing devices familiar to the person skilled in the art, such as Dispermats, dissolvers, Hansa mixers or Oakes mixers.

[0070] Furthermore, it is preferred that the viscosity of the wet foam produced at the end of treatment step c) is at least 5 Pa·s, preferably at least 10 Pa·s, more preferably at least 15 Pa·s and even more preferably at least 20 Pa·s, but not more than 500 Pa·s, preferably not more than 300 Pa·s, more preferably not more than 200 Pa·s and even more preferably not more than 100 Pa·s. The foam viscosity can be determined here, for example, with the aid of a Brookfield LVTD viscometer equipped with a LV-4 spindle. Corresponding test methods for determining the viscosity of wet foams are known to the person skilled in the art.

[0071] In a preferred embodiment of the invention, the foam has maximum homogeneity and cell fineness in process step b). If necessary, the person skilled in the art can verify this in a conventional manner based on their typical experience by simple direct visual inspection with the naked eye or with optical aids, such as a magnifying glass or a microscope. "Cell fineness" is related to the cell size. The smaller the average cell size, the finer the foam cells. If necessary, the fine cell content can be determined, for example, using an optical microscope or using a scanning electron microscope. "Homogeneous" refers to the cell size distribution. A homogeneous foam has a very narrow cell size distribution, so that all cells have approximately the same size. This can in turn be quantified using an optical microscope or using a scanning electron microscope.

[0072] As already mentioned above, further thickeners can be added to the system in order to adjust the wet foam viscosity. The thickeners here are preferably selected from associative thickeners. Associative thickeners here are substances which cause a thickening effect by association at the surface of the particles present in the polymer dispersion. This term is known to the person skilled in the art. Preferred associative thickeners are selected from polyurethane thickeners, hydrophobically modified polyacrylate thickeners, hydrophobically modified polyether thickeners and hydrophobically modified cellulose ethers. Very particular preference is given to polyurethane thickeners. In addition, in the context of the present invention, it is preferred that the concentration of the thickener is in the range of 0.01 to 10% by weight, based on the total dispersion composition. More preferably in the range of 0.05 to 5% by weight, most preferably in the range of 0.1 to 3% by weight.

[0073] In the context of the present invention, it is further preferred that the coating of the foamed polymer dispersion produced in process step d) has a layer thickness of 10 to 10000 μm, preferably 50 to 5000 μm, more preferably 75 to 3000 μm, even more preferably 100 to 2500 μm. The coating of the foamed polymer dispersion can be produced by methods familiar to the person skilled in the art, for example by knife coating. Direct or indirect coating methods (known as transfer coating) can be used here.

[0074] In the context of the present invention, it is also preferred that the drying of the foamed and coated polymer dispersion in process step e) is carried out at elevated temperature. It is preferred here that the minimum drying temperature according to the invention is 50° C., preferably 60° C., more preferably at least 70° C. Furthermore, the foamed and coated polymer dispersion can be dried in a plurality of stages at different temperatures in order to avoid drying defects. The corresponding drying techniques which take into account the temperature, ventilation and relative humidity of the atmosphere are widely used in industry and are known to the person skilled in the art.

[0075] As already described, process steps c) to e) can be realized by means of widely practiced methods known to the person skilled in the art, which are reviewed, for example, in “Coated and laminated Textiles” (Walter Fung, CR-Press, 2002).

[0076] In the context of the present invention, those porous polymer coatings which contain long-chain citrate esters and have an average cell size of less than 350 μm, preferably less than 200 μm, particularly preferably less than 150 μm, most preferably less than 100 μm are particularly preferred. The average cell size can preferably be determined microscopically, preferably by electron microscopy. For this purpose, a cross section of the porous polymer coating is observed through a microscope with sufficient magnification and the size of at least 25 cells is determined. In order to obtain sufficient statistical data for the evaluation method, the magnification of the microscope selected should preferably be such that at least 10 x 10 cells are present in the observation field. The average cell size is then calculated as the arithmetic mean of the observed cells or cell sizes. The determination of the cell size by microscopy is familiar to the person skilled in the art.

[0077] The porous polymer layer (or polymer coating) according to the invention, comprising at least one long-chain citrate ester according to the invention and optionally further additives, can be used, for example, in the textile industry, for example for artificial leather materials, in the construction industry, in the electronics industry, in the sports industry or in the automotive industry. For example, based on the porous polymer coating according to the invention, everyday items such as shoes, insoles, bags, suitcases, small boxes, clothing; automotive parts; preferably seat covers, coverings for door parts, dashboard parts, steering wheels and / or handles and shift covers; assembly items such as table mats, cushions or seat furniture; gap fillers in electronic devices; cushioning and shock-absorbing materials in medical applications; or adhesive tapes can be produced. DETAILED DESCRIPTION

[0078] Example

[0079] Material:

[0080] YS:3000: polyurethane dispersion based on MDI (methyldiphenyl diisocyanate) from DOW. As a result of its preparation process, the product contains 1-3% by weight of the anionic co-surfactant sodium dodecylbenzenesulfonate (CAS: 25155-30-0);

[0081] DLU: aliphatic polycarbonate ester-polyether-polyurethane dispersion from Covestro;

[0082] WX 151: Aqueous polyurethane dispersion from Cromogenia;

[0083] PC 287PRG: Aqueous polyurethane dispersion from Cromogenia;

[0084] STA: ammonium stearate from Bozetto (about 30% in H2O);

[0085] SR: sodium sulfosuccinate based on beef tallow from Bozetto (about 35% in H2O);

[0086] Sodium dodecylbenzene sulfonate (LAS; CAS: 25155-30-0) was from Sigma Aldrich. This is a standard co-surfactant used in the production of aqueous polyurethane dispersions;

[0087] ECO Pigment Black: Aqueous pigment dispersion from Cromogenia (black);

[0088] 250: polyethersiloxane-based leveling additive from Evonik;

[0089] PV 301: polyurethane-based associative thickener from Evonik Industries AG.;

[0090] TH 27: Isocyanate-based leveling additive from Cromogenia.

[0091] Viscosity measurement:

[0092] All viscosity measurements were performed using a Brookfield viscometer LVTD equipped with a LV-4 spindle at a constant speed of 12 rpm. For viscosity measurements, the sample was transferred to a 100 ml jar in which the measuring spindle was immersed to a specified depth. A constant viscometer measurement was always waited for to be displayed.

[0093] Example 1: Synthesis of stearyl citrate with a degree of esterification of 2.1:

[0094] While stirring and introducing N2, stearyl alcohol (≥95%, 275.2 g, 1.02 mol, 2.1 eq.) was heated to 70°C, and then citric acid (anhydrous, 93.10 g, 0.485 mol, 1.0 eq.) was added. The reaction mixture was heated to 140°C and stirred at a pressure of 150 mbar for 3 hours while introducing N2 until the acid value reached 62 mg KOH / g. The mixture was filtered at 2 bar at a temperature of 120°C using a filter press (1 μm). After cooling, a colorless solid with an acid value of 57 mg KOH / g was obtained.

[0095] Example 2: Synthesis of stearyl citrate with a degree of esterification of 2.4:

[0096] While stirring and introducing N2, stearyl alcohol (≥95%, 244.0 g, 0.902 mol, 2.4 eq.) was heated to 70°C, and then citric acid (anhydrous, 72.23 g, 0.376 mol, 1.0 eq.) was added. The reaction mixture was heated to 140°C and stirred at a pressure of 150 mbar for 4 hours while introducing N2 until the acid value reached 50 mg KOH / g. The mixture was filtered at 2 bar using a filter press (1 μm) at a temperature of 130°C. After cooling, a colorless solid with an acid value of 48 mg KOH / g was obtained.

[0097] Example 3: Synthesis of Behenyl Citrate with a Degree of Esterification of 2.1:

[0098] Behenyl alcohol (70-80%, 245.7 g, 0.763 mol, 2.1 eq.) was heated to 80°C while stirring and introducing N2, and then citric acid (anhydrous, 69.78 g, 0.363 mol, 1.0 eq.) was added. The reaction mixture was heated to 140°C while introducing N2 and stirred at a pressure of 150 mbar for 2 hours until an acid value of 63 mg KOH / g was reached. The mixture was filtered using a filter press (1 μm) at a temperature of 120°C at 2 bar. After cooling, a colorless solid with an acid value of 61 mg KOH / g was obtained.

[0099] Embodiment 4: Comparative example:

[0100] In addition to the citrate according to the invention, a comparative surfactant based on polyglyceryl-3 stearate was used, which was prepared by reacting 103.3 g of polyglycerol (OHN=1124 mg KOH / g, Mw=240 g / mol) with 155.0 g of technical grade stearic acid.

[0101] Embodiment 5:

[0102] The citrate esters and the comparative surfactants based on polyglycerol esters from Examples 1-3 were blended according to the compositions detailed in Table 1 and then homogenized at 80°C. The inventive surfactant formulations 1-3 were then neutralized with KOH to pH = 7. The pH of comparative surfactant 4 was already 7 after blending and was not neutralized.

[0103] Table 1: Composition of the surfactant blend used below:

[0104]

[0105] Example 6: Foaming test:

[0106] In order to test the efficacy of the additive combination according to the invention, a series of foaming experiments were carried out. For this purpose, in a first step, a foaming agent from Covestro was used. DLU polyurethane dispersion. The foam stabilizers used were the surfactant formulations 1 to 3 according to the invention (see Table 1) and a combination of the two surfactants Stokal STA (ammonium stearate) and Stokal SR (sodium sulfosuccinate) as comparators. Table 2 summarizes the compositions of the corresponding experiments.

[0107] All foaming experiments were performed manually. For this purpose, the polyurethane dispersion and surfactant were first placed in a 500 ml plastic cup and homogenized for 3 minutes at 1000 rpm using a dissolver equipped with a dispersing disk (diameter = 6 cm). In order to foam the mixture, the shear rate was then increased to 2000 rpm, ensuring that the dissolving disk was always immersed in the dispersion to a degree sufficient to form a suitable vortex. The mixture was foamed to a volume of about 425 ml at this speed. The mixture was then sheared for another 15 minutes at 1000 rpm. In this step, the dissolver disk was immersed deeply enough in the mixture so that no further air was introduced into the system, but the entire volume was still in motion.

[0108] Table 2: Overview of foam formulations:

[0109]

[0110] In all cases, fine homogeneous foams were obtained at the end of the foaming operation. It is noteworthy that the foams produced with the surfactants 1 to 3 according to the invention have a higher viscosity (see Table 2). The foams were applied to a siliconized polyester film by means of a film applicator (AB3220 from TQC) equipped with an application frame (coating thickness = 800 μm) and then dried at 60° C. for 5 minutes and then at 120° C. for a further 5 minutes.

[0111] Compared to sample # 4, the dried inventive samples # 1-# 3 are characterized by a more uniform macroscopic appearance and a softer feel. In addition, a finer cell structure can be determined in electron microscopy studies.

[0112] Example 7: Migration test:

[0113] In order to evaluate the surface migration of the surfactant according to the present invention, an artificial leather material is produced by the following method. First, a top coat is applied to a siliconized polyester film (layer thickness 100 μm). It is then dried at 100° C. for 3 minutes. Subsequently, a foam layer (layer thickness of 800 μm) is applied to the dried top coat and dried at 60° C. for 5 minutes and at 120° C. for 5 minutes. In the last step, an aqueous adhesive layer (layer thickness of 100 μm) is applied to the dried foam layer, and then a textile carrier is laminated to the still wet adhesive layer. The completed laminate is dried again at 120° C. for 5 minutes and then peeled off from the polyester film.

[0114] All coating and drying operations were carried out here using a Labcoater LTE-S from Mathis AG. The topcoat and adhesive layers were formulated according to the compositions listed in Table 3; the foam layer used was the foam formulation listed in Table 2, which was foamed by the method described in Example 6.

[0115] Table 3: Top coating and adhesive formulations used to produce artificial leather materials:

[0116]

[0117] To evaluate surfactant migration, artificial leather samples were placed in water at 100° C. for 30 minutes after production and then dried overnight at room temperature. After this treatment, the comparative sample prepared with Stokal STA / SR surfactant (foam formulation #4, Table 2) had clearly visible white spots on the artificial leather surface, while no such surface discoloration was observed in the case of the samples prepared with the surfactants according to the invention (foam formulations #1-#3, Table 2).

[0118] Example 8: Co-surfactant compatibility:

[0119] To evaluate the co-surfactant compatibility, use The YS:3000PUD system was subjected to further foaming tests. It contained 1-3 wt% of anionic co-surfactant sodium dodecylbenzene sulfonate (CAS: 25155-30-0). The surfactants used in these experiments were surfactant formulations 1-4 listed in Table 1. Table 4 gives an overview of the composition of the foam formulations.

[0120] Table 4: Overview of foam formulations:

[0121]

[0122] Based on these formulations, foam coatings were prepared by the method described in Example 6. It is noteworthy here that sample #8 prepared with comparative surfactant 4 has a much coarser and less uniform foam structure. After the foam coating has dried, clear cracks can also be observed in the foam structure, which indicates that the stability of the foam is insufficient. In contrast, samples #5-#7 prepared with the surfactants of the present invention again show very fine cells and a uniform foam structure. They also have no drying cracks.

[0123] In addition, a series of foaming experiments were performed in which the foam was deliberately Sodium dodecylbenzene sulfonate (a common co-surfactant described for PUD stabilization) was added to the DLU system. In these experiments, the surfactants used were also the surfactant formulations 1 to 4 listed in Table 1. Table 5 gives an overview of the composition of the foam formulations.

[0124] Table 5: Foam formulation overview:

[0125]

[0126] Here again, a foam coating was prepared by the method described in Example 6. It is noteworthy here again that sample #12 prepared with comparative surfactant 4 has drying cracks and a much coarser cell structure, while samples #9-#11 according to the invention again show a fine and uniform cell structure without defects. Almost no difference was observed here from the similar samples #1-#3 without co-surfactant (see Example 6). Therefore, these experiments demonstrate a significant improvement in the co-surfactant compatibility of the surfactants according to the invention.

Claims

1. Use of long-chain citrate esters as foam additives in aqueous polymer dispersions, wherein the aqueous polymer dispersion is an aqueous polyurethane dispersion, wherein the solids content of the dispersion is in the range of 20 to 70% by weight, based on the total dispersion, wherein the long-chain citrate esters are esters of citric acid with long-chain alkanols and enols, long chain meaning that the alcohol has at least 12 carbon atoms.

2. The use according to claim 1, characterized in that The long-chain citrate ester can be obtained by condensing a long-chain alcohol with citric acid.

3. The use according to claim 1 or 2, characterized in that The average degree of esterification of the long-chain citrate esters is 1-2.7, wherein the degree of esterification is defined as the molar ratio of alcohol to citric acid used.

4. The use according to claim 3, characterized in that The average esterification degree of the long-chain citrate ester is 1.3-2.

6.

5. The use according to claim 3, characterized in that The average esterification degree of the long-chain citric acid ester is 1.4-2.

5.

6. The use according to claim 3, characterized in that The average esterification degree of the long-chain citric acid ester is 1.5-2.

4.

7. The use according to any one of claims 1-2, 4-6, characterized in that The alcohol used is lauryl alcohol (1-dodecanol), myristyl alcohol (1-tetradecanol), cetyl alcohol (1-hexadecanol), margaric alcohol (1-heptadecanol), stearyl alcohol (1-octadecyl alcohol), arachidyl alcohol (1-eicosanol), behenyl alcohol (1-docosanol), lignocerol (1-tetracosanol), wax alcohol (1-hexacosanol), montanol (1-octacosanol), myristyl alcohol (1-triacontanol), palmitoleyl alcohol (cis-9-hexadecene-1-ol), oleyl alcohol (cis-9-octadecene-1-ol), anti-oleyl alcohol (trans-9-octadecene-1-ol) or the corresponding structural isomers with the same empirical formula, as well as mixtures of these substances, and / or branched long-chain primary alcohols and / or secondary alcohols.

8. The use according to claim 7, characterized in that Alcohols used are cetyl alcohol and / or stearyl alcohol, Guerbet alcohols, ie branched alcohols formed by Guerbet condensation, and branched secondary alcohols formed by paraffin oxidation by the Bashkirov process.

9. The use according to any one of claims 1-2, 4-6, characterized in that Long chain citrate esters conform to the general formula (I): Where R 1 The radicals are independently identical or different monovalent aliphatic or aromatic, saturated or unsaturated hydrocarbon radicals having 12 to 40 carbon atoms, or H, provided that all R 1 The statistical numerical mean of at least 0.3 of the group is H.

10. The use according to claim 9, wherein R 1 The radicals are independently the same or different monovalent aliphatic or aromatic, saturated or unsaturated hydrocarbon radicals having 14 to 30 carbon atoms.

11. The use according to claim 9, wherein R 1 The radicals are independently the same or different monovalent aliphatic or aromatic, saturated or unsaturated hydrocarbon radicals having 16 to 24 carbon atoms.

12. The use according to claim 9, wherein the condition is that all R 1 The statistical numerical mean of at least 0.4 of the group is H.

13. The use according to claim 9, wherein the condition is that all R 1 A statistical numerical average of at least 0.5 of the group is H.

14. The use according to claim 9, wherein the condition is that all R 1 The statistical numerical mean of at least 0.6 of the group is H.

15. The use according to any one of claims 1-2, 4-6, 8, 10-14, characterized in that The long-chain citrate esters additionally contain free unreacted alcohol and free citric acid.

16. The use according to any one of claims 1-2, 4-6, 8, 10-14, characterized in that The long-chain citrate esters additionally comprise linear, branched or cyclic oligomeric or polymeric condensation products of citric acid and esterified derivatives thereof.

17. The use according to any one of claims 1-2, 4-6, 8, 10-14, characterized in that The long chain citrate ester is neutralized, and the neutralization also includes partial neutralization.

18. The use according to claim 17, characterized in that Neutralization is accomplished using: water soluble metal hydroxides, and / or anhydrous bases that react with water to form hydroxide ions, and / or basic amines, and / or salts of weak acids.

19. The method according to claim 18, wherein the water-soluble metal hydroxide is barium hydroxide, strontium hydroxide, or calcium hydroxide, the anhydrous base that reacts with water to form hydroxide ions is barium oxide, strontium oxide, calcium oxide, lithium oxide, silver oxide, and / or ammonia, the basic amine is a monoalkylamine, a dialkylamine, and / or a trialkylamine, the alkyl group is optionally a functionalized alkyl group, or an amine with higher functionality, and the salt of the weak acid is potassium carbonate, sodium carbonate, and / or trisodium phosphate.

20. The use according to claim 18, wherein the water-soluble metal hydroxide is an alkali metal hydroxide that dissociates into free metal ions and hydroxide ions in aqueous solution.

21. The use according to claim 20, wherein the alkali metal hydroxide is NaOH and KOH.

22. The use according to claim 18, wherein the basic amine is an amidoamine, a monoalkanolamine, a dialkanolamine, a trialkanolamine, a monoaminoalkylamine, a diaminoalkylamine and / or a triaminoalkylamine.

23. The use according to claim 19, wherein the higher functionality amine is ethylenediamine, diethylenetriamine or triethylenetetramine.

24. The use according to any one of claims 1-2, 4-6, 8, 10-14, 18-23, characterized in that The long-chain citrate esters are combined with at least one other co-surfactant as an additive in aqueous polymer dispersions, wherein the co-surfactant includes fatty alcohols, fatty acid amides, ethylene oxide-propylene oxide block copolymers, betaines, amine oxides, quaternary ammonium surfactants, amphoacetates, ammonium and / or alkali metal salts of fatty acids, alkyl sulfates, alkyl ether sulfates, alkyl sulfonates, alkyl benzene sulfonates, alkyl phosphates, alkyl sulfosuccinates, alkyl sulfosuccinamates, alkyl sarcosinates or silicone-based co-surfactants and mixtures of these substances.

25. The use according to claim 24, wherein the betaine is amidopropyl betaine.

26. Use according to claim 24, wherein the co-surfactant comprises free fatty alcohol and / or alkyl sulfate having 12 to 40 carbon atoms.

27. Use according to claim 26, wherein the free fatty alcohol has from 12 to 40 carbon atoms.

28. The use according to claim 26, wherein the free fatty alcohol has 14 to 30 carbon atoms and the alkyl sulfate has 14 to 30 carbon atoms.

29. The use according to claim 26, wherein the free fatty alcohol has 16 to 24 carbon atoms and the alkyl sulfate has 16 to 24 carbon atoms.

30. The use according to any one of claims 1-2, 4-6, 8, 10-14, 18-23, 25-29, wherein the solids content of the dispersion is in the range of 25-65% by weight, based on the total dispersion.

31. The use according to any one of claims 1-2, 4-6, 8, 10-14, 18-23, 25-29, characterized in that The concentration of the long-chain citrate ester is in the range of 0.1 to 20% by weight, based on the total weight of the aqueous polymer dispersion.

32. The use according to claim 31, characterized in that The concentration of the long chain citrate ester is in the range of 0.2-15 wt%.

33. The use according to claim 31, characterized in that The concentration of the long chain citrate ester is in the range of 0.5-10 wt%.

34. An aqueous polyurethane dispersion comprising a long chain citrate ester for use as claimed in any one of claims 1 to 33, wherein the long chain citrate ester is neutralized, wherein the neutralization further comprises partial neutralization.

35. A method for producing a porous polymer coating using a long-chain citrate ester as an additive in an aqueous polymer dispersion as claimed in any one of claims 1 to 33, comprising the following steps: a) providing a mixture comprising at least one aqueous polymer dispersion, at least one long-chain citrate ester and optionally further additives, wherein the aqueous polymer dispersion is an aqueous polyurethane dispersion, b) frothing the mixture to produce foam, c) optionally adding at least one thickener to adjust the viscosity of the wet foam, d) applying the coating of the foamed polymer dispersion to a suitable carrier, e) Drying the coating.

36. The method of claim 35, wherein the porous polymer coating is a porous polyurethane coating and the foamed polymer dispersion is a foamed polyurethane dispersion.

37. Porous polymer coatings obtainable by using a long-chain citrate ester as an additive in an aqueous polymer dispersion in the production of such polymer coatings for use as claimed in any one of claims 1 to 33.

38. The porous polymer coating of claim 37 which is a porous polyurethane coating and the aqueous polymer dispersion is an aqueous polyurethane dispersion.

39. The porous polymer coating according to claim 37, obtainable by the method according to claim 35.

40. An everyday item comprising the porous polymer coating according to any one of claims 37 to 39.

41. The everyday item of claim 40, which is a shoe, an insole, a bag, a suitcase, a small box, a garment; an automotive component; an assembly item; a gap filler in electronic equipment; a cushioning and shock absorbing material in medical applications; or an adhesive tape.

42. The daily necessities according to claim 41, wherein the automobile parts are door part covers, dashboard parts, steering wheels, handles and shift covers; and the assembled products are cushions or seat furniture.

43. The daily necessities according to claim 42, wherein the assembled product is a table mat.

Images