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At LANXESS the sector involved in developing products that soften leather we call “softening technology” not fatliquoring. This is because our latest developments in this sector, we believe, have little to do with oily or fatty materials which no doubt led to the term “fat-liquoring”.

 

Read any paper on fatliquoring and it will usually begin with water insoluble oils and fats originally used in hand stuffing, then oil in water emulsions, which were first based on soap added to raw oils, then sulphated fatliquors (sulphuric acid added to raw oils) then sulphited oils (sodium bisulphite etc added to raw oils) cationic and non-ionic fatliquors (made by adding ammonium compounds and surfactants to raw oils respectively). All these natural based fatliquors are chemically related. The exception are the fatliquors based on mineral oil e.g., the paraffins. These are usually chlorinated, the short chain versions of which are already banned in Europe (bio-accumulative) and sulphochlorinated paraffins which are chlorinated paraffins that have been sulphated. Some of these give good performance, at least in terms of consistency, but potential problems too, for example, with biodegradability.

 

As you can see, virtually all these products have one main theme in common, adding something to existing raw oils so that it can be made water soluble, at least temporarily, and then in most cases penetrate into the leather, to varying degrees.  

 

Polymer softening agents based on, for example, acrylics have been around for some years now and are well established in the market, their distinct advantages and performance means that customers are prepared to pay for this chemistry where such properties are necessary especially in automotive applications but now increasingly in the quality end footwear and leather goods etc., markets.

 

Tanners ask us for a multitude of properties for our products which soften leather. Examples include consistent product and performance season to season, no migration, no potential for spew or fatty stain formation, oxidation resistant, high light and heat fastness, neutral odour, no dyestuff bleaching, no uneven dyeing, no loose grain, no dark flanks and belly areas, no area loss, last but not least makes the leather soft! There is also increasing concern about environmental issues, including biodegradability.

 

We believe such demands can only be ultimately satisfied by special polymer chemistry, namely modified polyamide carboxylic acid polymers. I will refer to these as PCA (modified polyamide carboxylic acid) polymers.  

 

This entirely “new” category of polymer for softening has been largely overlooked, and it is this unique sector that LANXESS have achieved success. We have successfully synthetically constructed our interpretation of a polymer class that actually occurs in nature which then gives us the performances of synthetic polymer softeners, but with certain advantages of a natural product, including biodegradability which we can demonstrate is superior to, for example, certain other classes of softening polymers. Further general comparisons between conventional natural based fatliquors and PCA polymers are given in table (1) and a general formula for PCA polymers is given in fig. (1) below:-

 

 

 

 

	Convention Natural fatlikqour	PCA Polymer
Spew formation:	possible	impossible, no free fatty acids
Exhaustion:	variable	high and constant
Fixation:	no significant chemical bonding	chmical fixation to the fibres
Smell:	variable	neutral an constant
Mould resistance:	vulnerable to attack	resistant
Washability:	easily washed out, hardens	highly resistant, stays soft
Consistency:	seasonal variations possible	constant
Finish adhesion:	can cary	excellent, constant
Fat stains:	possible	impossible, contains no fat
Oxidation:	vulnerable	highly resistant
Bleaching of dye:	variable	low
Heat & light fast:	low - good	excellent
Uneven dyeing:	possible	not usually possible
Leather density:	heavier, denser	lightweight  and fluffy
Mill pattern:	variable	increased uniformity

PCA polymer can be applied during or after neutralisation, even together with most fatliquors and acrylics. However, it is important to make sure, as usual, that the float is exhausted before acidifying.

 

Colour intensity and economy – PCA polymers are specifically designed not to bleach dyestuffs, bleaching components are not added. In the case of common fatliquors which, for example, have had gaseous oxides of sulphur bubbled through them, or chlorine added, these are actual chemical bleaches and means that with such “cheap” products can be huge hidden costs as more dyestuff has to be applied to reach a similar depth of shade as that attained with PCA polymers. Fig. (2) gives an example illustrating this bleaching point. Bleaches can have other unwanted side effects, on physical properties for example, such as reduced tear strength. On the contrary, PCA polymers can actually contribute to increased tear strength.

 

 

 

As fatliquor manufacturers increasingly compete for scarce raw materials with e.g., bio-fuel producers, this means fatliquor prices are likely to only continue to increase as feedstocks decrease, and worse still, in the case of marine oils certain species are becoming increasingly endangered or even extinct.

 

High constant performance – unlike many fatliquors, all components of our PCA polymers are aliphatic and not aromatic. This chemistry results in its general high- light and heat etc fastness properties, as shown in fig.(3) below. They also have a high affinity for the fibre and so fixes extremely well and cannot be washed out. Most natural fatliquors use -S03 solubilising groups so they can be emulsified in water, and persistence of this group after fixation causes many problems including making the leather more hydrophilic, on repeated wash cycles the leather becomes harder and harder as the fatliquor is removed. Our PCA polymer has extraordinary self emulsifying powers and does not have any -S03 groups. Being synthetically engineered PCA polymers offer constant performance batch to batch, season to season.

 

 

 

Maintenance of crust softness during finishing - made tight and soft crust leather, but after finishing it is harder and looser than required? Then you have to increase the fatliquor offer, increasing your costs further and changing the leather character? This is an all too common problem that has been resolved over and over again with the application of PCA polymers. This product moderates water / finish absorption keeping the leather soft, and most importantly uniformly and constantly, without adversely affecting adhesion, and thus is superior in these respects to a variable natural oil based product.

 

Waterproof – PCA polymers are particularly recommended for softy articles. The polymer can be incorporated into leathers that use waterproofing chemicals without problem. Being –S03 emulsifier free, softening is easily achieved without increasing the hydrophilic character of the leather. 

 

Flame resistance - is remarkably improved with PCA polymers. Of all the products added to leather in wet end processing the fatliquor is probably the single biggest contributor to the burning problem. Removing fatliquors and replacing with a PCA polymer means that flame resistance is maximised. We have tested virtually all major flame retardant products and found none so far that perform, unless they contain AOX (Halogens) i.e., Bromine in some form or another. Certain bromine containing compounds act as flame retardants, but on burning threaten the life of air, sea and train etc., passengers with the lethal gases they can release. For best results remove the fatliquor and replace with an AOX (Halogen) free PCA polymer. This way you may not need to add a flame retardant to make a flameproof crust and this makes the real cost of a PCA polymer insignificant!

 

Faster, easier and cheaper processing - With fatliquors you can get greasy leather on removal from the drum, obvious from the water pearling usually visible on the grain. In this condition the leather cannot be further processed, it has to be horsed up and stored for many hours, usually over night, a big problem if you want to compete on the world stage with a shift system without pause. You have to wait until the water in the leather has been displaced and the fat has crept along the fibres and away from the surface, but then don’t wait too long as the fat can, thanks to gravity, start heading towards the necks and butts at the bottom of the horse if handling sides, or worse still the bellies if handling hides. This can result in uneven softness and looser bellies, common tanner’s complaints. This is not the case with PCA polymers, they have a high affinity for leather fibres and remain evenly distributed, thus maintaining leather quality and saving time and money.

 

PCA polymers are quickly and easily diluted. You can add them to water or vice versa no problem. With most fatliquors they usually have to be added to water, not the other way round or the emulsion can break resulting in a greasy surface, hard leather and uneven dyeing.

 

For fatliquors the temperature of the water is usually very important. Most fatliquors should be added to hot water, at least 50°C, and at least four to five times their volume. (This volume of water is not easy to handle when adding large volumes of product for softy leather) this costs time and money. Drum heating systems, if used, are also expensive to run. If the water is too hot again the fatliquor emulsion can break and cause the same problems mentioned previously. PCA polymers can be readily diluted in much lower temperature water thus saving energy, time and money.

 

 

 

Furthermore, as the PCA polymers are engineered synthetically the design of the molecule is such that we can be sure that exhaustion is optimised and complete. In the case of natural fatliquors with their variable composition this is more difficult, a certain fraction of the product can remain in the float even after acidification and give that characteristic, and all too common, milky, opaque appearance to the waste water, as shown in fig. (5) below.

 

 

 

Tanners have applied PCA polymers by replacing at least half of the fatliquors in their recipes and achieved the required technical benefits, with the same softness as full fatliquored leathers and without increasing costs. However, to achieve this tanners don’t normally replace the fatliquor 1:1, for example, leathers have been produced with 6% PCA polymer and 4% natural fatliquor (i.e., 10% total softening materials) instead of 16% natural fatliquored leather. In such cases, this is a true win-win situation.

 

Print retention and definition – the embossing properties of PCA polymers are very good. When comparing the difference between leathers processed with fatliquors against those with PCA polymer the improvement in the bellies and other auxiliary areas with the polymer on embossing can be clearly seen, as shown in the below fig. (6).

 

 

 

 

In common cases of grain crack on embossing, a traditional “quick-fixes” are to decrease pressure, resulting in reduced print definition or add more fatliquor.  Increasing the fat content of leather increases costs, changes the character of the leather which becomes more heavy and greasy, and may adversely affect finish adhesion, and does not usually completely cure the underlying grain crack problem neither.

 

PCA polymers have been successfully deployed to solve such issues, allowing maximum embossing pressure and print definition without grain crack. The leather maintains its softness and there is no increase in density.

 

Faster drying - One important advantage of polymers in general in processing is that they tend to give up the moisture during drying faster than conventional fatliquors.  With natural fats normally great care has to be taken in their selection with reference to the type of drying. With the low migration rate of PCA polymers you don’t have the risk you do with natural fats migrating towards the hot plate and / or blocking the mesh. PCA polymers help reduce energy expenditure, machine down time and cleaning costs.

 

Light weight, low density and fluffy character – leather containing PCA polymer is less dense than that made with traditional fatliquors. This property can be further developed by milling, however, one important point to note is that the conditioning is important. Before staking and/or milling conditioning must be optimised. For transport applications, especially aircraft leather, or hi-tech sports shoe leather low density is critical, here PCA polymers give you a competitive edge.

 

 

Competition – We have encountered a product claiming to have certain similar properties to ours. However, this you cannot really compare as it contains a significant quantity of volatile components which when removed results in an actual concentration of only 23%. (i.e., ca., 50% less compared to our product, without such volatiles). Comparative testing, even adjusting the weight of product offered for this low concentration, still gave poorer results, namely less softness, drier feel, courser flesh fibres and more bleaching of the dyestuff with duller dyeing.

 

We are not suggesting the end of fatliquors, several hundreds of thousands of tons of which are sold each year, they still have a role to play, at least today. They are currently virtually indispensable, for example, as touch modifiers, here their sheer variety means that they are difficult to substitute. However, they are facing grave problems, for example, with the important fish oil based products, here dangerously decreasing fish stocks (a similar scenario which ended sperm whale oil production in the 1960’s) threatens their future. Other serious environmental pressures and roller-coaster price fluctuations (more up than down these days) add to this predicament. These problems coupled with increasing demands for performance means that such customer needs, we believe, can only be satisfied by increasingly replacing traditional fatliquors with at least a certain proportion of PCA polymers. Tanners report that they are increasingly replacing between 55% and 75% of traditional fatliquors with the PCA polymer for optimum results and, with the right recipe, can also reduce the total offer of softening products.

 

Tanners often comment on the price per kilo of a product, however, in the case of PCA polymers if you really calculate the cost per square foot they are usually cost neutral. If you consider the savings, including reduced processing time and energy usage plus the benefit from all the performance advantages offered by this unique class of product then they are actually both economical and environmentally friendly!

 

Acknowledgements: Dr. René Graupner-von Wolff, Christopher Henzel and Alexander Hippel for their contributions.