Some newly finished leathers seem to be more sensitive to chemical aggressions (such as perspiration) than ‘older aged’ leathers which can cause worn out areas due to rubbing or mild abrasion. The French footwear and leather research and testing organisation, CTC, have observed such damage on a number of upholstered leather sofas and other leathergoods.

Deterioration originates from contact with organic micro-dirt linked to use.

CTC have analysed the cause of this deterioration by developing a method that simulates the ageing of the finish. Research carried out appears to show that the drying temperature of finishing products, the proportions and the nature of crosslinkers among others seem to influence ageing performance of the final leather product.

Leather sofas that alter around the area that comes into contact with the human head show a loss of colour. This can also be observed where the hand swings around the flap of a handbag, a steering wheel, a gear-stick knob or just in the general deterioration of car seats.

In the context of their technical service activities and expertise, CTC have observed an increase in premature ageing of certain leather sofas. Although finishing properties comply originally with current regulations (EN13336 for example), use linked to a lack of care from the consumer, in many cases, causes a deterioration of the finishing films. This defect materialises in a colour alteration; a sticky feel and a general loss in finish adhesion.

The weakening of the protective film can start after just a few months of use which certainly has an impact on the image of a leather product for the consumer, not to mention the deterioration of the commercial relationship between the seller and the client.

Taking a step back

Analysis carried out at CTC’s laboratories has enabled them to identify the causes of these alterations and thus question the modern use of aqueous finishes, the role of the film thickness in finishing and poor use or incorrect care from the end-user.

The first objective of the analysis is to understand the causes of physical-mechanical durability of the finish when in contact with micro-dirt (such as perspiration, sebum, dust etc) particularly with leather used in home furnishings. CTC looked at methods to enable them to predict such a reaction through suitable ageing tests (for example passing through a climatic enclosure).

The second aim is to define optimum conditions to obtain good quality, lasting properties for use by associating finishing resins and the most recent crosslinkers available on the market with an optimised drying system using an infrared tunnel, which are commonly found in tanneries.

Identifying the cause

Originally, the study had focused on the development of a test reproducing the phenomenon of wear due to use (in the presence of sebum, perspiration etc).

The work was carried out in collaboration with leather seat manufacturers, selecting leathers that had been identified as problematic and others more stable in principle.

They carried out comparative studies:

* With physical tests, artificial rubbing, concentrated perspiration, artificial sebum, body lotion and acetic acid at 20%

* With an ageing test in a climatic chamber with variation of temperature parameters, humidity and duration (duration in weeks)

In these latest trials, the parameters were as follows: heat from 30°C to 60°C, relative humidity from 70 to 90% for a duration of one or four week(s).

Optimum conditions

CTC have developed a test reproducing the ageing/wear phenomenon in a climatic/thermal chamber as this is the only equipment capable of reproducing realistic conditions in a short period of time.

Results showed that the optimum conditions for assessment were: 60°C, 90% relative humidity during two periods (one week and four weeks).

The difference depends on the end use. One week’s test is suitable for leathergoods leathers and four weeks’ test in the case of leather for furniture.

The Veslic test (dry and wet) was carried out before ageing and after passing through the climatic/thermal chamber.

Parameters for obtaining good stability

Analysis was then carried out with respect to the stability of aqueous fixations, from a dozen resins representing the current market.

These have been applied on black, full grain leather which had undergone prior treatment of a pigmented polyurethane/acrylic base coat. The rate of crosslinker in the top coat was calculated to represent the minimum percentage necessary to obtain a good wear to humid rubbing (200 cycles on a Veslic machine) after an ‘optimum drying’.

Different types of crosslinker were used such as isocyanate, carbodiimide and aziridine. Subsequently drying was carried out in an infrared tunnel under two conditions:

* ‘Optimum’ drying, with a conveyor belt speed of 8m/min enabling a temperature on the surface of 90°C to be reached by exit from the tunnel

* ‘Standard’ drying, with a conveyor belt speed of 12m/min enabling a temperature on the surface of 70°C to be reached by exit from the tunnel

Finally the ageing was carried out in a climatic chamber regulated at 60°C, 90% relative humidity for a duration of between one to four weeks (optimum conditions to develop the instability of finishing).

Regarding the analysis of the stabilisation of fixations, performance can be very diverse due to the nature of the resins used (pu or pu mix, as well as polyester or pu- polyester).

It is, therefore, necessary to analyse the rate at which a crosslinker obtains the optimum percentage for application. CTC found that the drying process was also a key factor to obtain good final properties as well as the necessity for certain resins to complete fixation.

The drying temperature is particularly important with a crosslinker made of isocyanate and carbodiimide (high temperature, minimum 90°C); however, using aziridine, a lower moderate temperature is sufficient (70°C).

After the ageing tests the following was observed:

* If the rate of coat application and drying are at their optimum, the properties of all analysed resins are stable after ageing.

* However, if one of the first parameters is not optimised (insufficient percentage of crosslinker or drying temperature not high enough), some fixations lose their properties after ageing regardless of the compound used.

This study focused on leather for furniture. However, the trials and recommendations may be applied to all types of finished leather products.