Leather as a high-tech material for car interiors is required to meet exceptionally high quality standards. For the automotive industry and its customers, satisfactory test results for the finished leather before processing are not enough. They expect leather as a durable commodity able to withstand exposure to extreme and varying temperatures, light, moisture and mechanical loading conditions over many years, thereby living up to its reputation as a high quality durable material.
One of the most difficult aspects of materials testing in general, however, is predicting the durability of a material immediately after it has been manufactured. The legal and economic consequences arising from any guarantee declaration mean that manufacturers have to know the ageing properties of their products and the material included in these products.
The studies described below deal with the effect of different tanning methods and of the choice of products used in the wet operations and finishing on the ageing properties of car upholstery leather.
Mechanical influences – changes in the leather due to mechanical influences depend on the individual degree of loading during processing or use and can be controlled to a certain extent by taking appropriate technical measures during the manufacturing process, eg by modifying the finish formulation and increasing the application rate. This improves the abrasion and scuff resistance and the fastness to dry cleaning, but at the same time spoil the look. The appearance is not what the public expects of the highly esteemed natural product leather.
Effect of light and UV radiation – changes in the colour of leather following exposure to light can mainly be attributed to the use of products with poor lightfastness. Leathers finished with a suitable combination of lightfast pigments, binders and auxiliaries seldom give rise to complaints. It tends to be the more natural looking leathers which may cause problems. The finishes applied to these leathers contain little or no pigment and so the lightfastness of the finished product is determined by the crust material, which is coloured by dyestuffs. The lightfastness can be improved marginally by selecting an appropriate tanning process and using dyestuffs and retanning and softening polymers with high lightfastness. However, this is still not sufficient to satisfy the current requirements of the automotive industry.
Effect of temperature and moisture – heat and moisture in car interiors can have a particularly negative effect on leather. The problem of shrinkage is especially acute if leather is used for instrument panels and consoles, where temperatures of well over 100°C are possible. The leather may become brittle and, at worst, the whole component may be destroyed. The interaction of heat and atmospheric humidity is crucial. The ‘hydrolysis properties’ refer to the changes in the physical properties of a material – in our case leather – when it is exposed simultaneously to the effects of heat and moisture.
Depending on the tanning process used, there may be irreversible hardening of the leather on the seats. The colour may also change on exposure to extremely high temperatures due to yellowing of individual finishing components or migration of inadequately fixed dyestuff out of the leather.
Demands on the ageing properties of leather
Laboratory tests carried out on the finished leather before processing can only provide limited information on the possible changes to car upholstery leather caused by everyday influences (see table1). The degree of mechanical loading and other external factors vary too much to permit any accurate prediction of the ageing properties.
Ageing can be defined as irreversible changes in the properties of a material caused partly by its own instability and partly by external influences. Changes in the fibre structure and in the products used for the manufacturing process affect the appearance and feel of the material as well as its physical properties.
If we compare the delivery specifications of various car manufacturers, we see that their demands regarding the ageing properties of leather for car interiors vary widely. This is due to the fact that the required property profile differs considerably, depending on what area of the interior the leather is to be used for.
Different technologies are used during the manufacturing process to ensure that the leather has the ideal properties for the intended application. Both chrome tanned and chrome free leathers are used – and rightly so, as both types have their own particular advantages.
The requirements of the car manufacturers are, therefore, geared to the specific field of application. It is also important to remember that the test methods, in particular those used for determining the hydrolysis stability, differ considerably depending on the tanning process used. It is, therefore, necessary to consider the various ageing methods separately, but nevertheless in connection with the required properties.
Test methods for determining the ageing properties of car upholstery leather
We would now like to present a selection of the most important methods used in our series of tests for evaluating the ageing properties of car upholstery leather.
Heat/light ageing
DIN 75 2022 is a test method devised by the German Standards Committee for motor vehicles and known as the FAKRA test. This standard is used to determine the resistance of the colour and the ageing properties of dyed or printed organic materials on simultaneous exposure to artificial light and temperatures of 100°C for example. DIN 75 202-3A is a variant in which the test specimens are exposed to artificial light twice to four times to simulate extreme conditions. In many cases the specimens are subsequently subjected to physical testings such as Bally flexes and rub fastness.
Hydrolysis resistance according to DIN 53 3443
This standard describes test methods for determining the resistance of leather and its finish coat to moist heat. The specimens are exposed to a temperature of 50°C and a relative humidity of 95% and evaluated after 1,2,3 and 4 weeks.
Saab’s STD 211 delivery specifications regarding the hydrolysis resistance4 are much more stringent. They require the leather to be exposed to a temperature of 70°C and a relative humidity of 95% for 900 hours (approximately 37 days) before it is assessed.
Planning of trials
The aim of the trials was to assess the influence of various products and technologies and at the same time develop practice-orientated formulations for automobile crust leather with good resistance to ageing.
We compared processes for chrome containing and chrome free car upholstery in both beige and black using different retanning materials, softening concepts and finish formulations. The leathers were evaluated at the crust stage and after finishing.
Wet operations
a. Chrome tanned crust leather
In one series of trials, wet-blue side leather with a shaving substance of 1.0-1.2 mm was neutralised to a pH of approx 6.0 with sodium formate and sodium bicarbonate.
The leather was retanned with synthetic replacement tanning materials:
* Vegetable tanning material (tara)
* Polymer retanning materials
* Resin retanning material
* The addition being 5% in each case.
The beige and black dyed leathers were treated in each case with:
* 13% of an acrylic-based polymer softener
* or 11% of a commercial-grade special fatliquoring agent for car upholstery leather
b. Chrome-free crust leather
Wet white leather manufactured on the basis of a modified glutardialdehyde was shaved to 0.9-1.0mm and tanned with 18% of the following types of tanning material or combinations:
* Synthetic tanning materials
* Vegetable tanning materials (tara, mimosa)
* Combination of synthetic and vegetable tanning materials (tara)
* Combination of synthetic, vegetable and polymer tanning materials (tara)
The beige and black dyed leathers were
treated in each case with:
* 22% of an acrylic based polymer softener
* or 18% of a commercial grade special fatliquoring agent for car upholstery
leather
c. Evaluation of the crust leather
Heat/light ageing
As expected, the shade of all the unfinished specimens had changed noticeably after three heat/light ageing cycles. There was no trace of black or beige left, the shade of the leathers was comparable to that of natural look crust leather.
The specimens were then subjected to 10,000 flexes in the Bally flexometer. There was no significant difference between the two colours. Although there were now cracks visible, the surface of all the specimens was scuffed around the area where they were bent, presumably due to the effect of friction during the test.
Chrome leather that had been tanned with synthetic tanning materials showed the least amount of hardening after exposure to heat and light. Similar results were obtained for leathers retanned with tara. In the trials where polymer and resin retanning materials were utilised, the handle of the leather deteriorated noticeably.
Chrome free crust leathers tanned exclusively with synthetic or vegetable tanning materials or a combination of these two types fared well, with no appreciable changes. There was hardly any hardening. The inclusion of polymer tanning materials, on the other hand, did not bring any real advantages with regard to the hardening due to heat/light ageing.
In all the trials, the new acrylic based softening polymers had a clear advantage over conventional fatliquoring agents. Leathers treated with these products showed the least amount of hardening on exposure to high temperatures and UV light and remained soft and supple even when the ageing process was fairly advanced.
Hydrolysis resistance
Specimens of chrome tanned and chrome free leathers were tested in accordance with the Saab delivery specifications at 70°C and 95% relative humidity. While the structure, surface area and softness of the chrome leather remained unchanged, the chrome free leather had shrunk after 900 hours and disintegrated into hard, rigid, charred looking pieces. Looking at how the leather alters as the test progresses, we see that the chrome free leather changes at a very early stage. It is destroyed after only one week, leading to the conclusion that degradation due to hydrolysis starts immediately after the test begins.
Extensive preliminary trials with existing technologies were conducted with products available on the market to establish how these affect the hydrolsis properties of chrome free leather. It was found that none of the process variants using different categories of sytans, vegetable tanning materials, polymers and fatliquoring agents and different wet white processes produced satisfactory results. Like vegetable tanned leather, none of the chrome free car upholsltery leathers, whether crust or finished, were able to withstand the extreme conditions of the hydrolysis test.
We assume that the shrinkage temperature of chrome free leather is reached at 70°C and extremely high atmospheric humidity. The bond between the hide substance and the tanning material is attacked by hydrolysis, stripping the tannage and causing the hide substance to undergo heat transformation. These trials demonstrate clearly how much the test conditions affect the evaluation of the ageing properties1).
Finishing trials
Based on these findings, only the leathers with the best ageing properties at the crust stage were included in the subsequent finishing trials.
Black and beige pigmented chrome containing and chrome free full grain leathers were finished with different aqueous polymer dispersions and auxiliaries and compared. Only products which have already been used successfully in practice for car upholstery leather were used. As in the various wet operations, the aim here was to determine the influence of individual finishing components on the heat/light ageing and hydrolysis properties.
1. Base coat products compared:
Acrylic dispersions
Polyurethane dispersions
Filler
Anti-tack agent
Black pigment
Pigment mixture (white, caramel,
brown)
2. Top lacquer products compared:
Acrylic dispersion
Acrylic/polyurethane dispersion
Polyurethane dispersions
Matting agent with polyurethane as a
binder
Matting agent without binder
We compared acrylic and polyurethane based binders, these binders in combination with a filling and anti-tack auxiliary in the base coat and different matting agents in the top lacquer. The proportion of covering pigments in the base coat was 6% in the case of the black leathers and 10% for the beige leathers. The top lacquers were crosslinked with an isocyanate based product.
The finish consisted in each case of two spray base coats with a coating weight of around 17 g/ft2 and intermediate drying, followed by an extremely dull top lacquer with a coating weight of approx. 4g/ft2, also applied by spraying.
Evaluation of the finished leather
Heat/light ageing after three cycles according to DIN 75 202 (method A 65°C test room/100°C black standard/80 hours):
Because of the covering pigments included, there was no change in the shade of any of the base coated leathers,whether made from chrome tanned or chrome free crust material and irrespective of the shade and the finishing products used.
It was only on the finished leather, after the application of the colourless, unpigmented top lacquers, that differences could be seen, and even then they were only very slight and within the specifications. Generally, the leather turned darker, without changing the shade. This applies to both chrome and chrome free leathers in black and beige.
The differences in the results of the subsequent flexometer tests were more obvious and confirmed our previous findings with regard to the influence of the product selection. Polyurethane dispersions have considerable advantages over pure acrylates or combinations of acrylates and polyurethane dispersions.
A particularly striking feature was the pronounced chalking out when using binder free matting agents in formulations with polyurethane dispersions. This did not happen with acrylic dispersions or combinations of acrylic and polyurethane dispersions.
The flex resistance of the black leathers was better than that of the beige specimens, due to the type and amount of pigments used.
Hydrolysis resistance (70°C/95% relative humidity / 900 hours)
As we saw already in the tests on crust leather, this more stringent method is not suitable for chrome free leather, as the fibre structure begins to disintegrate after only a few days and continues until the leather is completely destroyed. The findings described below, therefore, refer only to chrome tanned leather.
Despite the high pigment content, there were noticeable differences in the extent to which the colour of the base coated leathers changed, depending on the polymer dispersions used. Only selected acrylic and polyurethane based products with good hydrolysis resistance gave satisfactory results.
The beige leathers became darker, and in the case of the black leathers there was pronounced greying, although in most cases the shade was still within the accepted tolerance range. When it came to assessing the completely finished leather, however, there were considerable deviations in the shade of the beige specimens, while in the case of the black leathers there were no significant changes.
The change in shade of the beige leathers could only be reduced to an acceptable level by increasing the pigment addition, eg up to 5% in the top lacquer.
All the specimens passed the subsequent flexometer test without damage.
Summary
Extensive series of trials in the wet operations and finishing sector have shown that it is possible to fulfil the requirements regarding the ageing properties, at least in the case of highly pigmented car upholstery leathers, by selecting suitable processes and products.
While the resistance to mechanical influences, and to heat and light, eg according to DIN 75 202, is primarily determined by the properties of the finish, the hydrolysis properties on simultaneous exposure to heat and humidity depend on the type of tannage and the tanning products used.
Colour changes can only be avoided by the use of selected finishing products in combination with elevated amounts of pigments in the top coat.
The best results with regard to heat/light ageing and hydrolysis resistance were obtained with wet-blues retanned with selected synthetic and vegetable tanning materials in combination with mainly softening, acrylic based polymers. Chrome free leathers treated with similar products also achieved good heat/light ageing properties.
A continuing problem is the highly unsatisfactory lightfastness of the resultant chrome containing and chrome free crust leathers, which prevents the use of low pigment and, therefore, more attractive finishes for car upholstery leathers.
Thus research scientists will have to continue their search for ways of producing lightfast crust leather with improved thermostability.
Finishes formulated with special heat and light stable pigments, acrylic and polyurethane dispersions and suitable auxiliaries make it possible to meet most of the demands concerning the ageing properties of automotive upholstery leathers.
With automotive upholstery leather, as with other types of leather, it is generally essential to adapt the test methods and specifications to suit the particular field of application and type of tannage.
