GB2287953A - High stability, organic tanning processes - Google Patents

Abstract

A process for tanning leather includes the treatment of animal skins with (a) a cross-linking agent containing aldehydic or active hydroxyl groups, particularly a phosphonium salt or oxazolidine, and (b) a melamine or urea resin prepolymer, such treatment being effected with prepolymer of particle size less than 500 nm. and/or at an effective elevated temperature. The prepolymer may comprise the reaction product of melamine or urea and formaldehyde and may have a molecular weight of 50,000 to 5,000,000 and a degree of polymerisation of 300 to 30,000. The treatment may be a pretannage, followed by vegetable tanning. In a further process, a high-stability vegetable tanned leather is obtained by treating a vegetable tanned animal skin with a cross-linking agent having aldehydic functions or reactive hydroxyl groups as above.

Description

High stability, organic tanning processes In the modern leather industry, the preferred tanning method is to use salts of chromium air).

This is the preferred method because the part processed leather, called wet blue, is very versatile, in so far as the material can be further processed to make a wide variety of leather types for use in a wide variety of applications. A distinct advantage of this tanning method is the high degree of hydrothermal stability conferred to the leather; it is easy to make leather which can resist damage by boiling water for many minutes and shrinkage temperatures, that is the point at which the leather structure is irreversibly damaged, higher than 110 0C are readily achieved. The colour of the part processed leather made in this way, varying from apple green to royal blue, is a disadvantage.

A viable commercial alternative to chrome tanning is of interest to the leather industry, either to avoid the base colour of the leather or to eliminate discharges of chromium (III) in effluent streams or solid waste. Alternative tannages are innumerable, well known in the art, but all suffer from shortcomings and none can be regarded as a substitute for chrome tanning. No other mineral tanning salt can, by itself, match chromium (Ill) in terms of the hydrothermal stability of the leather, although aluminium (III), titanium (IV) or zirconium (IV) salts do make white leather.The onl viable method of producing comparable hydrothermal stability is by semi-metal tannage, that is tanning with polyphenolic plant extracts (vegetable tannins) then retanning with a metal salt, of which the preferred salts are aluminium (III). These processes result in vegetable tannins and metal salts in waste streams, possibly causing an adverse impact on the environment, the leather is typically highly coloured and darkens upon prolonged exposure to light (poor light-fastness).

If high hydrothermal stability is not a requirement of a leather, then there are many options open to the tanner. However, the uses of low stability leather are limited to a few sectors of the industry, such as leather goods (luggage, handbags, wallets, watch straps etc). All the major sectors of the leather industry need leather with high shrinkage temperature. The biggest sector is shoe uppers; these must be able to withstand the effects of steam applied before lasting, the high temperature of heat setting and the application of directly moulded soles. Clothing leathers have to withstand steam pressing, either at the point of manufacture of the garments or following dry-cleaning; shrinkage temperatures below 100"C are not sufficient to prevent incipient shrinking from even mild steam pressing.Upholstery leathers are traditionally believed to be one sector which does not call for high levels of hydrothermal stability. However, this is not generally the case, because steam or dry heat is often used in stretching the panels of the pads. Also, upholstery leathers for automobiles may be subjected to surprisingly high temperatures in direct sunlight; this particularly applies to the tops of rear seats in cars, which may additionally be chemically damaged by the action of perspiration.

The ideal tannage to rival chrome tanning should incorporate the following features: high hydrothermal stability no metal salts white or pale coloured lightfast low environmental impact.

The hydrothermal stability criterion is important, to allow the widest possible application of the leather. Here, the target is to reach shrinkage temperature 1100 C; a tannage which can achieve that target carries a degree of insurance, so that if the processing conditions fall short of ideal, boilfast leather will still be produced. If possible the tannage should be organic rather than inorganic, to allow biochemical effluent treatment. The colour and lightfastness criteria are not critical, since the tanner ordinarily has to contend with both; but a white or pale substrate for dyeing pastel shades or bright colours, which are stable, would be an advantage. Any substitute for chrome tanning must match or exceed the acceptability of chromium (III), both in the workplace and in discharges to the environment.

The basis for organic tannage, the alternative to mineral (metal) tannage, can be syntans or vegetable tannins. These options represent the synthetic and the natural approaches to tanning.

Synthetic tanning agents (syntans) form a generic group of tanning agents, well known in the art; they are characterised as synthetic organic polymers or resins, with chemically active groups capable of interacting with collagen, the protein of skin, typically via multiple hydrogen bonding. They vary in their constituents, often phenolic and polymerised with formaldehyde, but all are incapable of routinely raising the shrinkage temperature of collagen above 90". One class of syntans is the melamine resins, typically formed by reaction between melamine and formaldehyde, both highly toxic compounds, but yielding an innocuous polymer; these resins have all the tanning features set out above, except they produce leather with low hydrothermal stability.

One approach to improving the tanning performance of materials such as melamine resins is to allow them to undergo some polymerisation in situ in the tanning bath, using a crosslinking agent which might in addition bind the polymer to collagen covalently. The approach has a mixed history. Graft polymerisation of acrylates received much attention in USA, but with only limited success. Resorcinol can be polymerised in situ with formaldehyde, to produce a polyphenolic tannage; in this way, using these toxic reagents, a leather with shrinkage temperature > 100"C can be obtained, but it is highly coloured with poor lightfastness.

Melamine itself can be polymerised in situ with formaldehyde, but the result is only moderately hydrothermally stable leather.

A more practical option, avoiding the use of melamine and formaldehyde, is to use prepolymers with defined average particle size, which are incompletely crosslinked and then to complete the polymerisation reaction in situ, using a crosslinking agent. In general, this does not work, yielding only moderate shrinkage temperatures.

Surprisingly, there are conditions which can result in high shrinkage temperatures. The present invention is based on the discovery that effective tanning using melamine resins is dependent on the particle size of the resin, which in turn is dependent on the temperature at which the resin is prepared or reacted with animal skins.

The critical factor is the particle size of the dispersion of melamine resin in water. The reaction conditions of the synthesis of the resins can yield a wide range of particle sizes up to gel formation. The distribution of the particle sizes and the constitution of the particles depend upon the reaction conditions; the constitution of the particles refers to whether they are single component particles or consist of aggregations of smaller particles. In the case of resins which consist primarily of aggregated small particles, the observed distribution of particle sizes is dependent on temperature; there is a critical temperature, above which the resin becomes maximally dispersed. Therefore, the way the resins are made determines the temperature for optimum dispersion and in turn determines the temperature at which tanning must be conducted.

According to one aspect, the present invention provides a process for tanning leather in which animal skins are treated with (a) a crosslinking agent with aldehydic functions or containing active hydroxyl groups, and (b) a melamine or urea resin prepolymer containing an effective amount of particles having a particle size less than 500 nm.

According to another aspect, the present invention provides a process for tanning leather in which animal skins are treated with (a) a crosslinking agent with aldehydic functions or containing active hydroxyl groups, and (b) a melamine or urea resin prepolymer at an effective elevated temperature.

An effective amount and an effective temperature refer to conditions that provide a particle size distribution which will result in a shrinkage temperature of at least 100"C.

The melamine and urea resins are typically reaction products of melamine or urea, preferably melamine, and an aldehyde, preferably formaldehyde. The prepolymers typically have a molecular weight of 50,000 to 5,000,000 and a degree of polymerisation of 300 to 30,000.

The prepolymers can be prepared in the conventional manner, in a reaction vessel equipped with stirrer, thermometer and reflux condenser. Formaldehyde solution is adjusted to the desired pH, heated to the desired temperature and then melamine is added with stirring. The formaldehyde to melamine ratio may be 3-8 : 1, but preferably 4-7 : 1. The mixture reacts for the desired period and then may be diluted and used immediately or, more typically, dried. The pH may be in the range 5-10, but preferably 7-9, the temperature may be in the range 50-100 C, but preferably 70-90"C and the reaction time is controlled to produce the required degree of polymerisation.

It has been found that the particle size of the resin dispersion should be < 500 nm and preferably < 100 nm; the larger the particle size overall, or the smaller the fraction of the desired particle size, the lower is the final shrinkage temperature. The effect may be related to the chemistry of crosslinking, to the collagen itself and between resin molecules. By analogy with the influence of emulsion particle size on fat-liquoring efficiency, the resin particle size will influence the ability of the reagent to penetrate down the hierarchy of structure and hence affect the uniformity of reaction at the molecular level. The role of particle size of the prepolymer is demonstrated in accompanying Fig. 1, showing that the shrinkage temperature of the leather depends more upon particle size than on the melamine to formaldehyde ratio.

The basis of this invention is the potential for in situ crosslinking of the prepolymer.

Therefore, this technology is not limited to polymers made only from melamine and formaldehyde, but encompasses analogous, related prepolymers, such as prepolymers of melamine with other aldehydes, or urea-formaldehyde resins, or prepolymers made from mixtures of melamine with other amino compounds such as urea of dicyandiamide.

The crosslinking step is critical, because without it the tannage is inadequate. The most efficient reagents for this purpose are those which have an aldehydic function; these include aldehydes themselves, mono- and di-functional, aldehyde derivatives and compounds which have at least partial aldehydic function or reactive hydroxyl function, such as hydroxymethyl phosphonium salts, typically the sulphate or chloride. It is recognised that not all the potential crosslinkers are acceptable in the workplace because of toxicity hazards. In addition, all derivatives of glutaraldehyde produce leathers which are significantly coloured.

Therefore, the preferred crosslinkers are the active-hydroxyl phosphonium salts, which are significantly less toxic than most of the other reagents and produce white leather.

It has been found that the way in which the two components are applied makes a difference to the effectiveness of the process. Applying the crosslinker before the oligomer is more effective than vice versa, because the shrinkage temperature is about 10 C higher.

The pH conditions typically used for fixing the individual components of the tannage are different; aldehydes are less reactive at low pH and more reactive at high pH, but melamine resins are less reactive at high pH and more reactive at low pH. Therefore, there should be a balance stuck between the apparently mutually exclusive pH conditions required by the components. The optimum operating range is pH 3-8 and preferably pH 4-7. It is important that the initial conditions should be set to allow partial reaction of the crosslinking agent, before adding the melamine resin oligomer. Therefore, preparing the substrate at, for example, pH 5, whether depickling pickled pelt starting at pH 2 or acidifying bated pelt starting at pH 9, the pH adjustment should be as close to equilibrium as practically feasible and possible.In solution, melamine formaldehyde polymers typically produce pH 6-7, so the addition of the prepolymer raises the overall pH to about those values; this has the effect of accelerating the reactivity of the crosslinker.

The temperature of the tanning process is important. It has two effects. The first effect, as outlined above, is to disperse the resin in solution and in situ, to create the basis for potent tanning. The second effect is to accelerate the reaction of the crosslinker; it is not possible to control this reaction by pH alone, so having established the optimum pH, the reaction is forced to completion by elevated temperature. It has been found that the minimum effective final temperature is 40"C and ideally should be 2 50"C.

The effect of temperature to disperse a coagulated resin is demonstrated in Fig.2. Two commercial melamine-formaldehyde resins are compared; in both cases they exhibit maximum shrinkage temperature at 50-60"C, at which the mean particle size is about 75nm. The dispersing rate is rapid, contributing to a rapid overall tanning rate. This is demonstrated in Fig. 3, showing that the tanning process is effectively completed in a few hours.

The effectiveness of the tanning reaction depends on the magnitude of the offers, in absolute and in relative terms. The offer of crosslinker should preferably be > 1% on pelt weight and more preferably up to 5 % . The offer of melamine resin should preferably be > 5 % on pelt weight and more preferably up to 10%.

In a typical embodiment,this invention provides the following new process steps for tanning leather: 1. Adjust pelt to pH 3-8 and preferably pH 4-7 2. Agitate the pelt with up to 5% of an aldehydic crosslinker, preferably a phosphonium salt, at ambient temperature for up to 2 hours.

3. Add up to 10% of a dispersible melamine resin, preferably melamine formaldehyde and agitate at ambient temperature for up to 3 hours.

4. Raise the temperature of the bath to preferably > 50 C and agitate for several hours.

The products of the tannages are white or pale coloured leathers, with shrinkage temperatures up to 118"C, but typically 105-112"C. They possess the interesting property that they dry soft; air drying, without previously incorporating a lubricant, results in a relatively soft product that is readily softened by mechanical action, without significantly disrupting the fibre structure and thereby making the leather loose. This property also means that dried, part processed leather could be marketed as a commodity, at minimum transport cost.

The tannages are general, that is they are not limited in the rawstocks to which they are applicable; this new technology can be used to tan bovine hide, fellmongered ovine skin and even woolskins. The tannages are stable, resisting dyeing processes at 60"C at pH 3.5 or 6.

Lightfastness tests of undyed leathers to Blue Wool Scale 6 demonstrated that they bleached rather than darkened.

For some circumstances, it is not an option for a tanner to change tannages to organic processes as described above. If the basic material available is vegetable (plant polyphenol) tanned, then high hydrothermal stability is only achievable by retanning with metal salts, preferably aluminium (III). It has been discovered that a similar effect can be achieved with organic crosslinking agents capable of reacting with polyphenol hydroxy groups,including those described above for use in the synthetic organic tanning process of this invention.

Surprisingly, it was found that not only are the effects of the crosslinkers different, but also the effect can be very temperature dependent.

Accordingly, in a further aspect the present invention provides a process for producing highstability vegetable tanned leather, which comprises treating a vegetable tanned animal skin with a crosslinking agent capable of reacting with polyphenol hydroxy groups, such as agents having aldehydic functions or reactive hydroxyl groups, such as phosphonium salts and especially oxazolidines. This treatment can be carried out as a continuation of the tanning process, or on already tanned skins, preferably after wetting back.

This is illustrated in the following table, in which 10% of crosslinker was applied to EI buffalo calf on received weight; the leather was wetted back, the crosslinker added to cold float, agitation continued at ambient temperature, then the system was warmed to the required temperature and running continued for several hours.

Shrinkage temperature (OC) Reaction temperature (OC) 20 40 50 Control, no crosslinker 84 Phosphonium salt* 94 93 93 Glutaraldehyde derivative 90 91 92 Oxazolidine 100 105 110 * THPS (tetra-kis hydroxymethyl phosphonium sulphate) The effectiveness of the crosslinkers is related to the individual reactivities and the results in the table do not reflect any difference in pH at which the reactions were conducted. Whilst there is some benefit to be gained from each crosslinker, the preferred reagent is oxazolidine.

The effectiveness of the preferred crosslinker and its dependence upon fixation temperature is demonstrated in Fig. 4.

The effect of crosslinking by oxazolidine is not universal for all vegetable tannins; the achievable shrinkage temperature is dependent upon the nature of the vegetable tannin, as shown in the following table. The results relate to sheepskin, pretreated with 5% syntan (Tanigan RFS, to aid vegetable tannin penetration), tanned with 20% vegetable tannin on pickled weight, then retannage with 5 % oxazolidine for two hours at ambient temperature, followed by running for several hours at 60"C.

Vegetable tannin Shrinkage temperatures (OC) Veg. Tanned After only oxazolidine Condensed: mimosa 84 111 quebracho 84 105 Hydrolysable: chestnut 73 87 tara 75 86 An alternative approach to high stability vegetable tanning is to use organic tannage in accordance with the present invention prior to vegetable tannage; the shrinkage temperature can be controlled by the pretannage, as illustrated in the following table.

Vegetable tannin Shrinkage temperatures (OC) 2.5% phosphonium salt 5% phosphonium salt 5% melamine resin 10% melamine resin 10% vegetable tannin 15% vegetable tannin Condensed: mimosa 111 129 quebracho 111 126 Hydrolysable: chestnut 103 117 tara 101 113 Additional stability may be obtained by retanning the leather with an organic (or mineral) crosslinker, as described above.

A marked advantage of this process is the rapidity with which the vegetable tannin penetrates through the cross section of the pretanned substrate. As described for the synthetic organic tannage, either approach to high stability vegetable tannage can be applied to any rawstock.

This invention is further described in the following examples.

Example 1. Synthesis of Resin A 203g of formaldehyde solution, 37 % w/w, adjusted to pH 8.3 with 1.0 M sodium bicarbonate solution was heated to 80"C in a reaction vessel. 64g of melamine was added with stirring and the mixture was allowed to react at 85-87"C for 3.5 hours. The mixture was diluted with 200g of water, then used for tanning within 24 hours.

Example 2. Synthesis of Resin B 248g of formaldehyde solution, 37% w/w, adjusted to pH 8.3, was heated in a reaction vessel to 80"C, then 55.6g melamine was added with stirring. The mixture was allowed to react at 85-90"C for 3 hours. The product was frozen then freeze dried.

Example 3. Properties of resins used in the Examples

Resin Formaldehyde to Average particle sizes (errors are melamine ratio standard deviations) 20"C 60 C A 5 77% 150+15 nm 23% 1100+150nm 0% gel B 7 74% 70+35 nm 26% 833+260no 0% gel C 4.5 10% 355nm 44% 103# 19 nm 55% 1132nm 46% 600#170 nm 35% gel 10% gel D 5.5 27% 182nm 80% 54+ 16 mn 63% 580nm 20% 500+185 nm 10% gel 0% gel Resin C is a melamine-formaldehyde resin sold by Hoechst AG under the trade name GRANOFIN MHN. Resin D is a melamine-formaldehyde resin sold by Hoechst AG under the trade name GRANOFIN MH.

Example 4. Tannage results The following table contains some results of tannages using Resins A-D with different crosslinkers. In each case, the processes were conducted on pieces of depickled sheepskin, basing offers on pickled weight and processing in bottles rotating in a temperature controlled environment. In each case the pelt was agitated with the crosslinker at ambient temperature (typically 15"C) for 1 hour, then the resin was added as a dispersion in cold water, agitation continued at ambient temperature for 2 hours, after which the temperature was elevated over about one hour and was maintained overnight, typically for 15 hours. Shrinkage temperatures were determined on undried leathers by differential scanning calorimetry using sealed pans.

Pelt Crosslinker offer Resin offer Overnight Shrinkage pH 1 (10%) temperature ("C) temperature (0C) 3 5% oxazolidine A** 40 104 7 2% glyoxal D 50 106 6 5% oxazolidine D 50 106 5 4% phosphonium salt* B 50 110 5 4% phosphonium salt* C 50 ~ 111 5 4% phosphonium salt* D 50 112 * THPS ** 6% offer Example S.

A melamine-formaldehyde resin, reactant ratio 5.5:1.0, was prepared according to the outline procedure set out in Examples 1 and 2. The reaction was conducted to yield a resin with mean particle size 95nm.

The resin was used for tanning in conjunction with THPS or an oxazolidine (trade name Neosyn TX), the reaction being completed at 50 C. The results are set out in the following table

THPS THPS Neosyn TX Resin Ts ( C) offer (%) offer (%) offer (%) 5 10 103 3 6 101 5 10 104 3 6 97 Example 6 The shrinkage temperature achieved using a crosslinker and a prepolymer depends upon the amounts of the reagents used. The following table shows how the shrinkage temperature of sheepskin leather depends on the reagent offers (on pickled weight of pelt) applied as given in Example 5.

Granofin THPS offer (%) offer (%) 2 3 4 5 4 89 100 97 89 6 93 101 102 95 8 101 105 106 101 10 : 103 112 ; 103 109 Example 7 The shrinkage temperatures obtained in Examples 4, 5, 6 are influenced by the scale of the process in each case; the tannages were conducted in bottles rotating in a temperature controlled environment, applied to loads of up to 100g of pelt. Because the chemistry of these new redactions depends upon the penetration of the reagents, the low level of mechanical action can adversely effect the result.This is demonstrated in the following table, in which the substrates were either 2mm bovine grain split or 1mm ovine flesh split.

Substrate THPS offer (%) Granofin MHN Ts ("C) offer (%) Hide 3 6 89 Hide 5 10 93 Ovine flesh split 1 2 91 Ovine flesh split 2 4 101 Ovine flesh split 3 6 110 The more open structure of the split sheepskin is less of a carrier to the reagents, and high shrinkage temperatures can be achieved on the laboratory scale. This aspect of the precess will be less important when conducting organic tanning on a larger scale, which necessarily incorporates higher levels of mechanical action.

Example 8 A side of EI vegetable tanned buffalo calf leather was wetted back with water, then tumbled for two hours with 10% oxazolidine on received weight in 100% float at room temperature.

Thereafter the temperature was raised to 60"C and running continued for a further 16 hours (overnight). The shrinkage temperature was 112"C; the matched side received exactly the same treatment, with the exception that the oxazolidine was omitted and at the end of the processing the shrinkage temperature was 84"C.

After being dyed and receiving treatments to make the leather water and soil resistant, the high stability leather was used to make shoes. Classic designs, incorporating foot lasting, heat setting and direct moulding of soles, were used and the leather was found to be entirely suitable for the purpose.

The chemical name of the oxazolidine used is l-aza-3 ,7-dioxabicyclo-5-ethyl-(3 ,3 ,0)octane.

Claims (15)

CLAIMS:

1. A process for tanning leather in which animal skins are treated with (a) a crosslinking agent containing aldehydic or active hydroxyl groups, and (b) a melamine or urea resin prepolymer containing an effective amount of particles having a particle size less than 500 nm.

2. A process for tanning leather in which animal skins are treated with (a) a crosslinking agent containing aldehydic or active hydroxyl groups, and (b) a melamine or urea resin prepolymer at an effective elevated temperature.

3. A process according to claim 1 or 2, in which the melamine or urea resin is a melamine or urea-aldehyde resign.

4. A process according to claim 1 or 2, in which the melamine or urea resin is a melamine or urea-formaldehyde resin.

5. A process according to any one of claims 1 to 4, in which the crosslinking agent is a phosphonium salt.

6. A process according to any one of claims 1 to 4, in which the crosslinking agent is an aldehyde or aldehyde derivative.

7. A process according to any preceding claim, in which at least 40% of resin particles have a particle size less than 500 nm.

8. A process according to any preceding claim, in which the animal skin is treated with an aqueous dispersion of the melamine or urea resin prepolymer at a temperature of at least 40"C.

9. A process according to any preceding claim, in which animal skins are treated as follows:

1. Adjust pelt to pH 3-8,

2. Agitate the pelt with up to 5% of an aldehydic crosslinker at ambient temperature for up to 2 hours,

3. Add up to 10% of a dispersible melamine resin and agitate at ambient temperature for up to 3 hours,

4. Raise the temperature of the bath to > 50 C and agitate for several hours.

10. A process according to any preceding claim, in which steps (a) and (b) are carried out as a pretannage, followed by treatment with a vegetable tannin.

11. A process according to claim 10, in which the vegetable tanning is followed by treatment with a crosslinking agent.

12. A process according to claim 11, in which the crosslinking agent has aldehydic functions or reactive hydroxyl groups.

13. A process for producing high-stability vegetable tanned leather, which comprises treating a vegetable tanned animal skin with a crosslinking agent having aldehydic functions or reactive hydroxyl groups.

14. A process according to claim 12 or 13, in which the crosslinking agent is an oxazolidine.

15. A process according to claim 1, 2 and 3, substantially as described in any one of the Examples.