TANNING OF SUBSTRATES USING IONIC LIQUIDS

Abstract

The invention provides a process for tanning a substrate using at least one ionic liquid. The at least one ionic liquid can be used in at least one of the following tanning steps: tanning, re-tanning, preservation, liming, pickling, impregnation, lubrication, dyeing, fatliquoring or finishing.

Claims

1. A process for tanning a substrate, comprising applying using at least one ionic liquid to the substrate.

2. The process of claim 1 wherein the substrate is selected from a collagenic biomaterial or a textile material.

3. The process of claim 2 wherein the collagenic biomaterial is selected from animal hides, skins, tendon, ligament and cartilage.

4. The process of claim 1 wherein the at least one ionic liquid is used in at least one of the following steps: (i) tanning; (ii) re-tanning; (iii) preservation; (iv) liming; (v) pickling; (vi) impregnation; (vii) lubrication; (viii) dyeing; (ix) fatliquoring; or (x) finishing.

5. The process of claim 4, wherein at least one of the steps further comprises using reagents which confer desired properties to the substrate and which are incorporated into the ionic liquids as solutes or as components of the ionic liquids themselves.

6. The process according to claim 5 wherein the reagents are selected from graphite, elemental sulphur, metal and semi-metal oxides, inorganic complexes and inorganic complex salts, organic polymers and reactive organic oligomers, Type II Eutectics and Type IV Eutectics.

7. The process of claim 4 wherein at least one of the steps is performed in a substantially non-aqueous system.

8. The process of claim 1 wherein the ionic liquid is in the form of a liquid formulation which is sprayed onto the substrate, preferably wherein the ionic liquid is in the form of a gel.

9. The process according to claim 4 wherein in the dyeing step, the ionic liquids are used to dissolve reactive dyes, preferably wherein the reactive dye is selected from dichlorotriazine or dichloroquinoxaline.

10. The process of claim 1 wherein the ionic liquid is selected from Deep Eutectic solvents, non-reactive ionic liquids with discrete anions and ionic liquids with Brnsted acidic cations.

11. The process of claim 10 wherein the Deep Eutectic solvent is selected from at least one of the following: (i) metal salt+organic salt (ii) metal salt hydrate+organic salt (iii) organic salt+hydrogen bond donor (iv) metal salt hydrate+hydrogen bond donor.

12. The process of claim 10 wherein the Deep Eutectic solvent is a mixture having a freezing point of up to 50 C., formed by reaction between: (A) one molar equivalent of a salt of formula (I)
(M.sup.n+)(X.sup.).sub.n(I) or a hydrate thereof; wherein M represents one or more metallic elements selected from the group consisting of Mg, Ca, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, In, Sn, Tl, Pb, Cd, Hg and Y, X.sup. is one or more monovalent anions selected from the group consisting of halide, nitrate and acetate and n represents 2 or 3; and (B) from one to eight molar equivalents of a complexing agent comprising one or more uncharged organic compounds, each of which compounds has (i) a hydrogen atom that is capable of forming a hydrogen bond with the anion X.sup.; and (ii) a heteroatom selected from the group consisting of O, S, N and P that is capable of forming a coordinative bond with the metal ion M.sup.n+, which reaction is performed in the absence of extraneous solvent.

13. The process of claim 12 wherein the anion X.sup. is an anion selected from the group consisting of chloride, nitrate and acetate.

14. The process of claim 12 wherein the complexing agent (component (B)) consists of one or more organic compounds, each of which compounds has (i) A hydrogen atom that is capable of forming a hydrogen bond with the anion X.sup.; and (ii) An oxygen atom that is capable of forming a co-ordinative bond with the metal ion M.sup.n+.

15. The process of claim 14 wherein the complexing agent consists of one or more compounds of formula II and/or formula III, ##STR00004## wherein R.sup.1 represents H, C.sub.1-4 alkyl (which latter group is optionally substituted by one or more F atoms), or N(R.sup.2)R.sup.3; R.sup.2 and R.sup.3 independently represent H or C.sub.1-4 alkyl (which latter group is optionally substituted by one or more F atoms); A represents C.sub.2-10 alkylene optionally (i) substituted by one or more substituents selected from F, OH, SH and N(R.sup.4)R.sup.5, and/or (ii) interrupted by one or more groups selected from O, S and NR.sup.6; and R.sup.4 to R.sup.6 independently represent H or C.sub.1-4 alkyl (which latter group is optionally substituted by one or more substituents selected from F and OH); provided that the compound of formula (III) does not contain any C-atoms that are bonded to more than one atom selected from the group O, S and N.

16. The process of claim 14 wherein the complexing agent is acetamide, urea, ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol or glycerol.

17. The process of claim 11 wherein the Deep Eutectic solvent is a mixture is selected from: (i) CrCl.sub.3.6(H.sub.2O)+2(urea); and (ii) 1 Kr(SO.sub.4).sub.2.10H.sub.2O: 1 glycerol.

18. A leather or textile which is obtained using the process according to claim 1.

19. (canceled)

20. A method of making a leather, comprising applying at least one ionic liquid to a collagenic biomaterial.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0174] FIG. 1 shows a photograph of the 1010 cm samples of bovine hide pH 3.65 after tanning in 5 ionic liquids. From l to r 1010 cm samples of bovine hide pH 3.65 tanned in (i) mimosa in ethaline, (ii) chestnut in ethaline, (iii) 1ChCl: 2 CrCl.sub.3.6H.sub.2O, (iv) 1CrCl.sub.3.6H.sub.2O: 2 urea, and (v) 1 KCr(SO.sub.4).sub.2.10H.sub.2O: 2 urea for 20 hours.

[0175] FIG. 2 shows samples of tanned leather impregnated with graphite (left from ethaline and right from water).

[0176] FIG. 3 shows: Above from left to right: 1010 cm samples of bovine hide, pH 4, tanned in 1 ChCl: 2 CrCl.sub.36H.sub.2O; 2 urea: 1 CrCl.sub.36H.sub.2O; and 2 urea: 1 KCr(SO.sub.4).sub.2.10H.sub.2O for 18 hours. The samples below show the corresponding cross sections.

[0177] FIG. 4 shows: a) Above: From left to right: 1010 cm samples of bovine hide, pH 4, tanned in mimosa in Ethaline, chestnut in Ethaline. Below: Corresponding cross sections. b) Mimosa tanning powder (middle) in water (left), and in Ethaline (right).

[0178] FIG. 5 shows: a) mass increase in leather soaking in Ethaline as a function of time and temperature, b) appearance of samples soaked in Ethaline at 70 C. for different times (hrs).

[0179] FIG. 6 shows: standard aqueous chromium-tanned bovine leather before (below), after (above) soaking in Ethaline containing 0.15 wt % Sudan Black B at 70 C. for 48 hours, (centre) water sample after washing leather for 15 minutes at 20 C. and (right) cross section of sample before and after dyeing.

[0180] FIG. 7 shows the changes in mechanical properties for the samples shown in FIG. 5 as a function of soaking time.

[0181] Certain embodiments of the invention are illustrated by way of the following examples.

EXAMPLES

Example 1

Tanning/Retanning Using Ionic Liquids

[0182] This example demonstrates the applicability of different types of Deep Eutectic solvents to tanning.

[0183] Two different organic vegetable tanning agents (at a loading of 10 wt %) each in a eutectic mixture of choline chloride and ethylene glycol (1:2 molar ratio) were prepared: [0184] chestnut, a hydrolysable polyphenol; and [0185] mimosa, a condensed polyphenol, largely prorobinetinidin

[0186] Three chromium based Eutectic mixtures: [0187] 1 choline chloride: 2 CrCl.sub.3.6H.sub.2O [0188] 2 urea: 1 CrCl.sub.3.6H.sub.2O [0189] 2 urea: 1 KCr(SO.sub.4).sub.2.10H.sub.2O

[0190] To demonstrate the stabilisation of the hide collagen by chromium, the shrinkage temperature was determined by differential scanning calorimetry (DSC) using a heating rate of 5 C. min.sup.1. Raw hide was mixed with each of the tanning liquids for 5 hours before the samples were washed for 10 min with fresh water. Typical aqueous chromium tanning would normally be carried out over at least 10 hours.

[0191] From FIG. 1, it can be seen that the tanning processes produce deeply coloured leather showing that the dyes bind to the hide in all cases. A mass balance of the liquid and hide showed that less than 5 wt % of liquid was lost during the process.

Example 2

Stabilising Graphite Dispersions in Ionic Liquids

[0192] This example demonstrates how graphite dispersions can be stabilised in a range of ionic liquids.

[0193] The graphite particles were firstly stirred into Eutectic mixtures of ethylene glycol and choline chloride. These Eutectic mixtures were then passed through a piece of blue crust leather which had previously been fat liquored. The particles were taken into the leather structure and even when the sample was washed with water most remained within the leather (FIG. 2). When the same experiment was repeated using water the graphite was totally washed out. A graphite impregnated leather could be useful due to its stabilising properties, its colour or it ability to conduct electricity.

Example 3

Chromium and Vegetable Tanning

[0194] The synthesis of the DESs was carried out using the method described in the literature (A. P. Abbott et al, Chem Commun, 70 (2003)). Samples of bovine hide were pre-treated and pickled according to a conventional leather manufacturing process. The final pH of the pickled hide was approximately 4. The bulk water was removed from the hide using a sammying machine and the final water content of the hide prior to tanning with Deep Eutectic Solvent (DES) was determined gravimetrically to be 62 wt %. Bovine hide samples (100 cm.sup.2) weighing 502 g were contacted with 232 g of DES for 18 hours at room temperature.

[0195] Following tanning, the excess DES was skimmed from the surface using a metal blade and the hide reweighed. The sample was then washed in cold water or 1 mol dm.sup.3 sodium sulphate solution and allowed to air dry before being analysed by the techniques listed below. Negligible leaching of the DES from the hide into the water was observed during the washing stage. The Cr(III) reference sample was tanned according to a conventional aqueous recipe. Following the tanning trials the chromium content was determined, following the total digestion of the tanned leather samples, using an inductive coupled plasma-optical emission spectrometer (ICP-OES) according to the standard method BS EN ISO 5398-4:2007. The vegetable tanning was carried out in the same way as the chromium tanning using a eutectic mixture of choline chloride and ethylene glycol (1:2 molar ratio) at a loading of 10 wt % of either mimosa bark or chestnut wood.

[0196] For the dyeing experiments: 0.4 g of Sudan Black B was mixed with 200 ml of Ethaline 200 to make a 4.3810.sup.3 mol dm.sup.3 dye solution. Samples of dried wet-blue leather (5 cm.sup.2) were fully submerged in the Ethaline 200 and Sudan Black B solution. The samples were then left in the solution at 70 C. for 24 hours. Once completed, the samples were washed for 15 minutes in deionised water, placed on absorbent paper and air-dried for a minimum of 24 hours at room temperature. The shrinkage temperature (Ts) was determined using differential scanning calorimetry (DSC) with 40 L gold-lined high pressure pans with a heating rate of 5 C. min.sup.1 from 20 to 140 C. The measurement of the shrinkage temperature is important to determine the efficacy of a tanning agent to stabilise the collagen fibres and usually measured in a wet state. Samples were conditioned to 20 C. and 65% relative humidity according to BS EN ISO 3376:2011 and the tensile strength and elongation at break were determined using an lnstron tensiometer according to BS EN ISO 3376:2011.

RESULTS

[0197] Chromium Tanning:

[0198] The applicability of three different types of DES solvents to tan hides was tested: [0199] 1 Choline chloride: 2 CrCl.sub.3.6H.sub.2O [0200] 2 Urea: 1 CrCl.sub.3.6H.sub.2O [0201] 2 Urea: 1 KCr(SO.sub.4).sub.2.10H.sub.2O

[0202] By creating highly concentrated Cr(III)-based liquids, it is possible to formulate a liquid-active ingredient. The aim is to liquefy the tanning agent through complexation to avoid the use of a solvent and thus minimise the total amount of material used. Any excess chromium salt can be mechanically removed and reused as it does not change the overall chemical composition since the leather absorbs both the anionic and cationic component of the liquid.

[0203] The majority of the costs with the tanning process is associated with the chromium salt and the recovery from dilute aqueous solutions after processing. Using DESs removes the solvent from the process and increases the efficiency of chromium salt uptake into the hide while minimising the treatment of aqueous effluent.

[0204] To demonstrate the uptake of chromium into the hide, the shrinkage temperature and chrome content was determined for samples of bovine hide treated with the three DESs listed above. Table 1 below shows the chromium content and shrinkage temperatures of the hide obtained from conventional Cr(III) tanning process using aqueous chromium (III) sulfate (33% basified, 25 wt % Cr.sub.2O.sub.3) with the three chromium-based DESs.

TABLE-US-00001 TABLE 1 Chromium content, shrinkage temperature and mechanical properties of a bovine hide tanned with a conventional process as well as DESs. Leather Cr Thick- Tensile Elonga- content/ T.sub.s/ ness/ strength/ tion/ Tanning agent % C. mm MPa % Conventional aq. 33% 3.04 109 3.02 32.6 50.8 basic Cr(III) tanning salts 1 ChCl: 2.27 71 2.55 37.7 39.3 2 CrCl.sub.36H.sub.2O 2 Urea: 3.43 80 2.84 27.4 34.9 1 CrCl.sub.36H.sub.2O 2 Urea: 3.52 83 3.10 30.3 42.5 1 KCr(SO.sub.4).sub.210H.sub.2O Mimosa extract 83 2.92 56.6 50.5 Chestnut extract 78 2.62 43.2 65.7

[0205] It can be seen that all three DESs yield samples with a chromium content that is comparable with aqueous chromium tanning. The urea eutectics produce higher chromium content than the choline chloride based eutectics which is thought to be due to the charge on the chromium species. Both urea eutectics produce cationic chromium species whereas the choline chloride species is predominantly anionic. The hide samples increased in mass between 9 and 19%. Given that the chromium content of the 3 liquids is between 9 and 18% this corresponds approximately to the Cr content listed in Table 1. The data shows that both components of the DESs are absorbed into the collagen structure.

[0206] Despite the differences in speciation, the shrinkage temperatures for DES-treated samples are similar. The indicative shrinkage temperatures for hide tanned using DESs are however lower than conventional chrome tanning, although it should be noted that sulfate also plays a role in increasing the shrinkage temperature. The various chromium species diffuse rapidly into the hide samples but chemical binding may be slower than in aqueous solutions potentially due to the higher ionic strength of DESs. It should however be noted that no attempt has been made to optimise the process and fix the chromium in the DES-treated samples, whereas this is a requirement of the aqueous process. Fixing is generally achieved by raising the pH, increasing the temperature and/or with the addition of complexing agents.

[0207] Washing the sample treated with 1 ChCl: 2 CrCl.sub.3.6H.sub.2O with 1 mol dm.sup.3 sodium sulphate solution increased the shrinkage temperature from 71 to 86 C. at pH 4 and this increased to 96 C. when the pH was increased to 8. This shows that the chromium can be fixed to the collagen structure irrespective of the anion in the DES solvent. In addition the shrinkage temperatures are comparable to those obtained using conventional chromium tanning solutions.

[0208] Table 1 also shows the strength and ductility of the tanned samples. The samples exhibited similar mechanical strength and elongation at break as the conventional aqueous Cr(III)-tanned leather, showing that the DES solvents have not exhibited deleterious effects on the mechanical properties of the hide.

[0209] FIG. 3 shows the optical photographs and cross-sections of the Cr-DES samples listed in Table 1. It can be seen that all samples are intensely coloured by the tanning process. The cross-sectional images show that the tanning agent has penetrated through the material. Cross-sections taken during the tanning process showed that the urea-based liquids had permeated the hide rapidly suggesting that the treatment period utilised could be considerably optimised with the potential to be more rapid than the aqueous tanning methods. It should also be noted that the three chromium DES-tanned samples all exhibited various colours originating from different speciation.

[0210] Chromium salts are used for approximately 80-85% of tanned leather. From a Green perspective, the concern with chromium tanning of leather relates to the emission of large volumes of dilute aqueous chromium which has to be treated. The tanned leather will retain a variable amount of moisture which is integral to the stability of the collagen structure, making the calculation of Green-metrics quite complex since an exact mass balance is difficult to quantify. The conventional aqueous tanning process starts with an equal mass of aqueous Cr(III) salt solution so nominally the Sheldon E factor is >1 since the hides are subsequently treated with an aqueous base to fix the chromium to the collagen. The water, which is the major component by mass, is recycled and the remaining chromium content is usually recovered through a series of precipitation, adsorption or ion exchange processes. Notwithstanding, the wastewater may contain between 500 and 3000 ppm. Recovery of the chromium could bring the E factor down in the region of 0.002 to 0.005 for this stage.

[0211] DESs have the potential to decrease the total volume of chemical applied during the tanning process. The DESs are viscous and may be applied as a cream to both sides of the hide similar to the roller-coating process observed during the application of surface coatings to leather. Since both components of the DES are absorbed by the hide any liquid remaining can be physically squeezed from the hide and directly reused.

[0212] Vegetable Tanning

[0213] Chromium tanning is the technique used for the majority of leathers due to its relatively short tanning time, and high shrinkage temperature allowing the tanned leathers to be processed at higher temperature. Vegetable tanning agents form a smaller part of the market due primarily to the slow reaction kinetics and lower shrinkage temperatures. These tanning agents are potentially greener since the polyphenolic active ingredients are biodegradable and do not persist in the environment.

[0214] In addition to the three chromium-based tanning DESs shown above, two organic vegetable tanning agents chestnut wood (Castanea sativa) and mimosa bark (Acacia meamsii) were used, each in a eutectic mixture of choline chloride and ethylene glycol (1:2 molar ratio) at a loading of 10 wt %. The hide samples were treated as described previously. Data are shown in FIG. 4.

[0215] Vegetable tanning agents have poor solubility in water and are slow to solubilise. FIG. 4b shows the mimosa extract tanning solution in Ethaline, and for comparison sake, the comparable aqueous system. It can clearly be seen that the extract is considerably more soluble in Ethaline producing a more transparent solution. Vegetable tanning agents are polyphenolic compounds which tend to be poorly dispersed in aqueous solutions. They are used extensively in the retanning process prior to dyeing and fatliquoring. In the DESs, vegetable tannins form intensely coloured homogeneous solutions and this evidently aids their dispersal into the collagen structure. It is unsurprising that the vegetable tanning agents dissolve readily in DESs, as these solvent systems are good hydrogen bond donators as well as organic and relatively hydrophobic.

[0216] It is also evident from the cross sectional images in FIG. 4a that the fibrous structure of the leather is retained during the tanning process in ionic liquids.

[0217] Despite the differences in the chemical composition, the tanned leathers have similar properties. The mimosa-tanned sample exhibited the highest tensile strength, while chestnut showed a large strain at break (Table 1). Although the samples shown in FIG. 4 were undertaken for 18 hours for comparison sake with the samples shown in FIG. 3, considerably shorter tanning periods may possibly be used.

[0218] A partial volume of the ethylene glycol: choline chloride-DES in the vegetable tanning liquid, is retained in the collagen structure following washing with water and drying. Both vegetable tanned samples increased their mass by 19% showing that the DES becomes trapped within the collagen structure. This may act as a lubricating phase imparting greater flexibility to the dried leather. Since the DES liquid exhibits very low volatility, it will be retained within the tanned leather. The lubricating behaviour may be seen from the greater strain at break values for the vegetable-tanned samples. As discussed below, the trapped DES liquid may act as a lubricant that would normally be added during the post-tanning process. This is traditionally achieved using natural and synthetic fats and oils in a process known as fatliquoring.

[0219] A mass balance on the tanning process showed that the tanning liquid could be quantitatively recovered at the end of the process resulting in negligible waste. This is probably due to the low viscosity of these liquids compared with the chromium-based eutectics which tend to be a 100 times more viscous. Given that the liquid is not significantly depleted of tanning agent and so reused, this step of the tanning process has a Sheldon E factor of approximately zero.

[0220] Plasticising Leather

[0221] Once the tanning step is complete, the leather is usually plasticised with an oil in a process known as fatliquoring. The oils used are mostly from plant or fish origins, and are poorly miscible with water and lead to turbid waste solutions which are difficult to treat. Previous experiments from vegetable tanning showed that that the leather was flexible and soft and this is thought to originate from the DES trapped in the collagen structure. This could mean that the DES acts as an in-built fatliquor. A sample of aqueous chromium-tanned bovine leather was soaked in Ethaline at various temperatures and periods of time. The amount of Ethaline absorbed is shown in FIG. 5 as a function of time and temperature.

[0222] Following the soaking process, samples were washed in water and air dried. Interestingly the water content of the leather (determined by TGA) was lower than the untreated sample. A significant amount of DES could be absorbed into the leather e.g. at 70 C. for 24 hours the leather sample absorbed 73% by weight DES. It is evident from the appearance (FIG. 5b) that the structure of the leather can be changed by absorbing the DES.

[0223] FIG. 6 shows the cross-sections of the leather before and after soaking in Ethaline. It is immediately apparent that the sample has swelled (56%). The swelling also appears to be homogeneous across the cross-section, with no change in the grain structure of the material. Furthermore, the DES does not leach from the sample and will not bleed when pressed with filter paper. It is therefore evident that the DES is bound to the collagen structure. This expansion of the collagen matrix appears to enable flexibility in the quaternary structure.

[0224] FIG. 7 shows the mechanical properties of the chrome-tanned leather that has been soaked in Ethaline for different periods of time. It is clear that the tensile strength is approximately constant and the tensile strain doubles when soaked. One of the largest changes is in the flexibility of the material which may be seen by the change in chordal modulus which decreases by approximately an order of magnitude when soaked for as little as 2 hours. While the results in FIGS. 3 and 4 show an extreme change in the properties of the leather they do show the potential to tune the properties of collagen using ionic fluids.

[0225] The amount of DES absorbed increased with time and temperature as would be expected, however the water content of the hide remained at about 12%, irrespective of the DES content compared to 18% in the untreated chromium tanned leather. This shows that the DES does not act as a hydrophilic additive as might be expected, instead the basicity of the anion is neutralised by interacting with hydrogen bond donators in the collagen structure.

[0226] Dyeing

[0227] Acid dyes are currently the most prevalent dye type in the leather industry due to the miscibility with water, and they can be fixed to collagen under acidic conditions. The colours available are wide ranging and exhibit good colour fastness. The molecules tend to be small and hydrophilic and generally anionic, binding electrostatically to protonated amino groups. The dyes also exhibit hydrogen bonding through auxochrome groups. Basic dyes are cationally charged and often more hydrophobic than the acid dyes with an affinity for anionic leather however, interaction also occurs via hydrogen bonding. Whilst they can produce vivid, bright colours they have poor colour fastness in comparison to acid dyes. A variety of dyes were solubilised in Ethaline including cationic dyes (Janus Black) and anionic dyes (Fast Black, Nuclear Fast Red and Polar Brilliant Red), however these dyes showed poor penetration into the leather and readily leached out when washed with water.

[0228] A non-ionic, lysochromic dye Sudan Black B (FIG. 8) was found to be soluble in DES and absorbed evenly throughout the leather. The dye produced an intense black shade which showed no evidence of leaching when the sample was washed in water. The dye penetrated throughout the cross-section of the leather (FIG. 8) showing that the DES has transformed the collagen into a more hydrophobic environment. In principle absorbing the dye and DES into the tanned leather as a gel should remove all waste water treatment from the post tanning process.