IntroductionToday, automotive leather has to meet a multitude of requirements from different automotive brands. In addition to the important traditional aesthetic properties, such as the feel and appearance of the leather, a growing number of measurable specification parameters have been added over time. These analytically measurable specification variables can be subdivided into physical ones, such as rubfastness or lightfastness and

chemical requirements. The chemical restrictions can be further subdivided into substances that can be extracted from the leather with a solvent such as Cr(VI) or azo dyes and substances that are emitted from the leather. This latter group includes the subject area of this article, the Volatile Organic Compounds (VOCs).

Background of VOCsCar interior air quality, and its improvement, is a priority issue. The focus today is increasingly on the toxicology of the substances emitted from the car components including leather.

Decades ago “fogging,” a condensate that forms on the inside of windshields, can be seen as a starting point for this issue. This phenomenon led to the implementation of simple new tests that measured either the weight of the condensate (gravimetric) or how it affected light passing through it (reflectometry). Obviously, these simple test methods provide no information about the composition of the condensate, and there is no correlation between results. VOC (Volatile Organic Compound) analyses with gas chromatography coupled with mass spectrometry

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Minimising VOCs and acetaldehyde

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Dr Volker Rabe, Head of Technical Product Management Crusting, Lanxess.

detection later led to both the quantification of emissions and the identification of many of the components. Today, there are many different test methods and conditions as each OEM has developed its own. Due to these differences each test method tends to highlight different VOCs, therefore, results cannot be compared (Fig. 2).

VOC test methodsThe large number of emission specifications, each with its own limits, represents a major challenge in the development of automotive leather today. Each of these methods has a slightly different substance focus and highlights certain groups of volatile substances in the test result. This often makes a targeted development for a certain leather article with associated test methods necessary. In order to fulfil all the required parameters, it is necessary to have

in-depth knowledge of the required test methods, the associated focus of these methods and the products used for the leather article. The following table provides a rough overview (Fig. 3).

Dr Volker Rabe, Lanxess

Fig. 1: Different specifications and challenges for automotive leather.

Fig. 2: Results of measuring the same leather with different emission methods.

Fig. 3: Table: Overview of common test methods, emission focus and source.

This table can only be used as a first, very rough approximation for article development in the automotive leather sector. Only the combination of measurement experience and evaluation of many different emission test results with the knowledge of the ingredients (and, if applicable, their degradation products) in the products used makes it possible to find one’s way through the jungle of methods/requirements. Such a map was successfully developed in cooperation with the German Research Institute for Leather and Plastic Sheeting (FILK) and Lanxess as one of the leading leather chemicals suppliers, which led to the so-called “VOC Guide”[1]. This guide shows a total of more than 1,500 individual data points in a simple, structured way and enables a quick overview of which substance is measured with which method and in which product group it occurs. This already represents a considerable step forward and allows a deeper understanding and faster progress in the preparation of recipes for modern car leathers.

However, even that information can sometimes not be enough for today’s highest requirements of leading OEMs on automotive leather and more detailed information is necessary. To benefit from the complete know-how behind the database and to work with the best information, ILM readers can contact one of the Lanxess Customer Support Centres directly.

VOC trendsThe increasingly profound measurement of the emissions over time has led to VOCs becoming reduced from grams to micrograms per kilogram of leather. This trend is clearly visible by comparing a VOC spectra from the past with an actual one (Fig. 5). Here the comparison shows a dramatic reduction in the intensity by a factor of around 100. Most probably because of this reduction in VOC levels, the focus of the regulations has switched today from quantity of emissions to the properties of single substances emitted.

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Single substance regulation: AcetaldehydeThe new China Vehicle Interior Air Quality (VIAQ) standard can be regarded as one of the latest examples of the trend towards increased regulation of individual substances emitted from leather. This standard regulates eight substances with limits and related test method for vehicle interiors (Fig. 6). Following the publication of the first proposal a few years ago, OEMs have already taken these regulations as a basis and adapted their individual leather specifications with stricter limits and harsher test conditions. If you now take a closer look at the controlled substances, the limit value of seven of the eight substances for leather can be adhered to. The only exception was and still is acetaldehyde.

The problem was that the sources of this substance were initially completely unknown. Acetaldehyde is neither used directly in the leather production process nor indirectly, since it is not used in industry as a raw material for the production of chemicals for leather production. Consequently, due to the lack of knowledge where this molecule comes from, a solution to meet the limit was not available at that time.

After analysing the results of the emission values of several crusts measured according to VDA 277, Lanxess’ research scientists found that the source described previously cannot be the only source of this substance.

This can be seen by looking at emission values of crust leather, which originate from one skin but show different concentrations of acetaldehyde (Fig. 8). In the graph, four bars represent a skin which was tanned, then separated into four parts and retanned slightly differently by varying only the fatliquor used.

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Fig. 4: Extract from the printed Lanxess/FILK VOC guide.

Fig. 5: VOC spectra from the past compared with today

Fig. 7: Acetaldehyde values from the raw hide to finished leather.

Fig. 6: List of regulated substances for car interior by new China VIAQ standard.

of acetaldehyde results (from 10 l bag test) clearly shows that when applying Tanigan SR values fall below the detection limit of 0.25 μg/sample in this case. However, without application of this auxiliary the result is 5.4 μg/sample. This means that the product Tanigan SR has reduced

the acetaldehyde by at least a factor of 21. However, not only the measured acetaldehyde values were

reduced, but also the values for the sum of the other aldehydes were reduced from 6.6 μg/sample to 1.1 μg/sample. Thus, Tanigan SR removes not only the acetaldehyde but all aldehydes that have already been formed in the raw hide or in the beamhouse/tanning steps.

After having found a solution for the first source, the next question was how to prevent the formation of acetaldehyde due to the fatliquors, the second source identified by the research work. The answer to this question is the newly developed polymer softener Levotan RV.

In order to demonstrate the efficacy of this product, a similar procedure was used here as already described above (Fig 10). A wet-blue was divided after tanning. Both halves were washed with water and further treated according to the same retanning and finishing recipe with only a slight difference. In one half, the fatliquor was partially replaced with Levotan RV. Afterwards both samples were finished including a flesh side binder and measured according to the ISO 12219-2 method (10 l bag test). The results show that this partial replacement of fatliquor with Levotan RV reduce the acetaldehyde concentration from 8 μg/sample to 1.1 μg/sample (Fig. 12). However, not only this emission value was reduced but also the sum of all other aldehydes from 73 μg/sample to 14 μg/sample for this leather. Likewise, the total value for the VOC decreases by almost a factor of 3 to 35 μg/sample from originally almost 95 μg/sample. Hence, Levotan RV not only helps to solve the problem of the emitted acetaldehyde, but is also able to reduce the total emissions of the leather.

With the two products presented for the wet end, it is possible to reduce the acetaldehyde coming from the two identified sources. An additional product in finishing that can further tackle both sources

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So, if the acetaldehyde values are only influenced by the initial concentration of acetaldehyde in the skin used, the four values of the crust leathers (made from one skin) should be approximately on the same level. However, as can be seen in Fig. 9, a considerable fluctuation of the values is measured. This can only be explained if the variation parameter, the fatliquors or their degradation products are another source of acetaldehyde.

VOC - Solutions for acetaldehydeOnce the two sources had been identified, all that remained to be done was to develop appropriate products which, on the one hand, would enable the acetaldehyde to be removed from the raw skin during the process and, on the other, prevent its later formation through the degradation of the fatliquors.

Lanxess has developed Tanigan SR for the first source coming from the raw hide. In order to demonstrate the effectiveness of this product, a wet-blue was separated after tanning in two parts and another half was subjected to a standard automotive retanning formulation after washing with water and then finished with an automotive finishing formulation. To the other half, Tanigan SR was added to the wash water and then finished with the same retanning and finishing formulation as the other leather. The emission values for both finished leathers were then determined using the ISO 12219-2 method (10 l bag test). The graph (Fig. 11) shows the values measured of both tests. The black columns are the values for the leather produced without the use of an auxiliary agent in the washing process. The red columns are the values for the leather in which Tanigan SR was used during the wet-blue washing step. A comparison

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Fig. 12: Reduction of acetaldehyde with Levotan RV.

Fig. 11: Reduction of acetaldehyde with Tanigan SR.

Fig. 8: Acetaldehyde values in crusts originate from one skin.

Fig. 9: Acetaldehyde values in crust using different fatliquors.

Fig. 10: Trial concept: to prove each solution individually only one product was varied.

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ConclusionThe sources of VOC and acetaldehyde are diverse and can vary widely from tannery to tannery. This often makes a single solution difficult and complex. This is illustrated by the latest example of individual substance regulation for emitting substances, acetaldehyde. The concentration of this substance found in the finished leather depends on the raw hide used, the type of fatliquors applied as well as on the entire manufacturing process. This illustrates how different the results can be from manufacturer to manufacturer and how difficult it often can be to find a general solution for all possible factors. For this reason, with the modular system solution presented here based on the three products Tanigan SR, Levotan RV and Primal Fleshcoat AR 2 Emulsion, Lanxess has created the preconditions that the concentration of acetaldehyde can be reduced. Whether one, two or all three products are needed, depends on the requirements and the measured concentration of acetaldehyde without the application auxiliaries. This concept gives the tanner flexibility and the opportunity to choose at which stage of the manufacturing process he integrates the necessary auxiliary to achieve the required result.

All three products can not only be used for the reduction of acetaldehyde, but they also enable the reduction of other aldehydes and minimise the measured total VOC-concentration. Therefore, a further improvement can also be achieved for the latter requirement.

There will continue to be a number of challenges for automotive leather in the future, and the next highly complex issue is already emerging: which is odour.

AcknowledgementThe Lanxess’ Leather business unit is grateful for the contribution of C. Tysoe , Dr M. Kleban, Dr Graupner von Wolff, and all the others

from the technical team who made contributions to this essay. Special thanks to the exchange student Y. Koike from the Ochanomizu University for helping to identifying one source of acetaldehyde.

simultaneously in addition to the wet end products would be desirable. For this reason, the flesh side binder Primal Fleshcoat AR 2 Emulsion was developed.

In order to check the effectiveness, a crust leather was divided and one side was treated with Primal Fleshcoat AR 2 Emulsion and the other side with a standard flesh side binder. The emission values were then determined using the ISO 12219-2 method (10 l bag test). And as the measured values show, it is also possible with Primal Fleshcoat AR 2 Emulsion to drastically reduce the measured values for acetaldehyde (Fig 10). In this case from a value of over 14 μg/sample to just over 2 μg/sample. The same applies to the other aldehydes and also the total VOC content could be almost halved for this leather.

Therefore, there are three different ways to reduce the acetaldehyde content. However, the products presented should not only be regarded as individual solutions, but the basic idea behind the three products is a modular concept right from the beginning. This means that these products, depending on the possibilities to adapt the operational procedure in a tannery and the requirements on the final leather article, can be used individually or in combination in order to reduce the total emission values to a minimum.

In order to demonstrate the modular principle, the finished leather, in which the fatliquor in the retanning was partially replaced with Levotan RV, was treated with the flesh side binder

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References: [1] Dr M. Kleban, Dr D. Tegtmeyer Lanxess , Dr H. Schultz, FILK, VOC-free Leathers: A Moving Target, World Leather, June/July 2014

[2] Dr H. Schulz, Dr B. Matthes IGF-Vorhabens-Nr. 18367 BR, „Aufklärung der Acetaldehyd-emissionen aus Leder“, FILK gGmbH, Freiberg

[3] Dr J. Ammen, Dr B. Wegner, Recent Findings in Acetaldehyde Emission from Leather, JALCA, 112, 2017

Fig. 16: Reduction of acetaldehyde with Levotan RV and Primal Fleshcoat AR 2 Emulsion.

Fig. 14: Reduction of acetaldehyde with Primal Fleshcoat AR 2 Emulsion.

Fig. 13: Sources and solutions for acetaldehyde at one glance.

Fig. 15: Trial concept: to proof modular concept two possible parameter were changed.

Primal Fleshcoat AR 2 Emulsion replacing the used standard flesh side binder (Fig 15).

This made it possible to lower the acetaldehyde concentration of 1.1 μg/sample further below the detection limit and also the other aldehydes were further reduced (Fig. 16). The measured volatile components are also minimised by the combination of the two products and the measured value of the total VOC is reduced from 94.6 μg/sample to only 28.1 μg/sample.