Leather is widely used in cars – mainly as upholstery – with smaller quantities for steering wheel covers and as decorative trims. It is often perceived as bringing an otherwise standard vehicle into the ‘luxury’ class.
The leathers used are normally chrome tanned but some car manufacturers are now including ‘chrome free’ (normally vegetable tanned) leathers within their ranges. These new vegetable tanned leathers incorporate modern finishes and consequently have a far higher performance than comparable leathers used 30 years ago.
Performance
Car manufacturers demand a high performance from all components. Extensive testing is normal practice in the automotive industry and leather is not excepted from this rule.
Leather destined for use as car upholstery is subjected to a more rigorous testing regime than any other type of leather. The demands placed on automotive leathers appear to have reached a level where the physical properties have become more important than the reason for including the leather, with its use as a luxury item becoming almost secondary.
To be fair to car manufacturers, the stringent requirements have to some extent been forced on them. Owners expect the upholstery to last the lifetime of the car. After all, no-one wishes to take a car out of service because the seat is showing wear.
Automotive upholstery can be subjected to heavy use over a long period, as well as being exposed to extremes of temperature. It has been reported that the interior of a modern car can reach 100°C if it is parked in strong sunlight. At the other extreme, even in the UK, the temperature can fall below -10°C if left outside overnight.
The combination of customer expectations and severity of end-use means that the majority of automotive upholstery leather is pigment finished to provide a hard wearing surface. Aniline and suede leathers are occasionally seen, but are normally found only in specialist or true luxury uses.
Requirements
Many car manufacturers have their own test methods and specifications and this has resulted in a very large number of tests being available. This lack of standardisation does create difficulties for tanners, who have to both produce the leathers and demonstrate that they meet a particular manufacturer’s specification. Because of these difficulties, a tannery will sometimes only produce leathers for one or two car manufacturers.
While individual manufacturers may specify their own test methods, there will inevitably be similarities in the methods that are tested. The properties tested are listed in the Table 1. Some are similar to the properties required for domestic upholstery, but others are very specific to automotive uses.
* Fogging
Fogging is the condensation of volatiles onto glass surfaces, particularly the windscreen. It is looked on as a hazard because it reduces visibility. Potential sources include interior fittings (particularly plastic components), carpets and upholstery.
The tendency for any material to cause fogging is measured by heating a sample of material in a beaker and collecting the condensate. The fogging is given either by the mass of condensate (gravimetric method) or by the reduction in gloss (reflectance method). This test, although relatively simple in principle, does allow for a large number of permutations of temperature and time and it once seemed that every manufacturer had its own set of test conditions.
* Shrinkage and discolouration
Repeated cycling of leather from warm and humid conditions to hot and dry can leave it with a permanent shrinkage. It can also encourage migration of dyes into the finish.
Both properties can conveniently be tested together by storing alternately for 24 hours at 100°C (dry) and then for 24 hours at 38°C and 98% relative humidity, repeating the procedure three times. The progressive change in dimensions can be measured and the change in colour assessed visually.
* Abrasion resistance
This is not a requirement in all markets. In general terms, it is regarded as more important in North America than in Europe where good rub fastness is often thought more important.
If this property is included, it will most probably be tested using the Taber method (Satra TM 163). Some manufacturers specify the use of highly abrasive wheels which subject the finish to severe test conditions.
* Flammability
Many companies place limits on the rate of burning of upholstery materials and trims. The most common type of test is the horizontal spread of flame, using for example the method in ISO 3795.
* Smell
Leather is normally regarded as having a pleasant smell although this is obviously a subjective judgement. The ‘traditional’ leather smell is a combination of vegetable tans and fish oils and can become overpowering at high temperatures or in a closed space. Some constituents may develop an acrid smell and manufacturers usually specify that the smell is not objectionable.
* Other properties
The other properties listed in Table 1 are those generally associated with physical and fastness testing of leather. Methods and requirements do vary from manufacturer to manufacturer, although in many cases the standard Satra methods provide a perfectly satisfactory method of assessment.
Examples are flex resistance (Satra TM 55), tear strength (Satra TM 162), wet and dry rub fastness (Satra TM 173) and fastness to light (Satra TM 160).
Satra can carry out many tests on automotive leathers and provide informed comment on the results. Steve Ferry is the contact: Tel +44 1536 410000 for more information.
