The soft cell – softness and directionality in automotive upholstery20 June 2016
Dr Patricia Casey, Nicolás Delgado, Silvia Metz and Leonardo Pileggi deliver a report on the importance of the relationship between softness and directionality of physical properties on leather for automotive upholstery.
The interiors of high-quality cars are characterised by having a sophisticated design, sometimes minimalist, but very demanding. The seats of an automobile are important to secure comfort and safety to the driver and other passengers. Not only are colour, feel and the overall aesthetic of the seat important, but equally so is the comfort. If the seats fail, the travel experience can be unpleasant.
An improper seat leads to greater fatigue and back pain. Leather is the material preferred for upholstering the interior of high-quality cars and, it being a natural product, faces demanding challenges to adapt it to the range that each automotive terminal applies. Often, companies engaged in cutting, sewing and assembling car-seat covers raise claims against leather suppliers because of defects such as:
- wrinkles that, when the cover is manufactured for the seat or the headrest, appear on some identical parts but not on others
- parts that, when sewn, stretch and deform, not returning to their original size or aspect
- covers of which the pieces are cut from the same lot of leather and when placed on the seat, show unwanted wrinkles or look distorted.
Each automotive terminal issues technical specifications of which the purpose is to ensure the consistent quality of the leather to be processed, while considering the influence of physical, chemical and organoleptic properties during the stages of manufacture. This article describes the relationship between the softness of the leather, measured by different methods and directionality of physical properties, and the influence they have to obtain a defect-free product.
Leather as the material of choice
Leather continues to be the preferred material to upholster premium-class automotive seats, because of the high value it adds to the vehicle. This is, in part, because leather characteristics are quite different from those of other materials; its natural and, at the same time, luxurious aspect offers a pleasant and warm touch; it is also noble, wear-resistant and comfortable. The present competition level among automotive companies to offer high-performance and quality vehicles, with daring or simple designs, present new and demanding challenges to tanners.
Tanneries do not always work directly with the automotive terminals. Usually, there is an intermediary that cuts, sews and assembles the seat covers.
This situation puts tanneries in a doubly demanding position; on one hand, they have to interpret and reflect on the leather they produce, what the auto terminal designer imagined as the proper material to upholster the seat and, on the other hand, satisfy the demands of the company making the cover; to obtain the maximum cutting yield with the minimum variation in the physical properties of the leather on its whole surface, facilitating their staff work and preventing ergonomic problems.
This report studied how to resolve a customer’s request by analysing the interaction of the physical softness properties, and static and permanent elongation of finished leather for automotive upholstery, in order to prevent the appearance of wrinkles on the seat, particularly in the areas were long and narrow pieces are sewn together, and also to minimise rejections because of dimensional variation.
Topographical distribution of leather physical properties
Leather is a material with anisotropy: the value of some physical parameters varies depending on the direction under consideration. It is a unique case among non-uniform materials because its heterogeneity is systematic and predictable, since it is the same from one hide to the other.
During the 1950s, the National Bureau of Standards (now the National Institute of Standards and Technology), carried out a very extensive study on the physical and chemical properties of leather. From the results obtained from the different tests done by the NBS, some of the conclusions were: a hide can be considered as two adjacent symmetrical sheets, with properties substantially the same on the points symmetrically located with respect to the backbone; and the topographic distribution of physical properties is substantially the same from one hide to the other.
Description of the test methods
Bending length or ‘flexibility’ of leather when bent under its own weight (cantilever) is used to determinate flexibility, based on the longitudinal bending and its stiffness by measuring the distance covered by the test tube up to the extreme without support bends by its own weight and contacts the tilted plate of the device. The test is carried out with the leather specimen grain up and also down. The results are expressed as:
Bending length: is the average of the lengths of the tested material, grain side and flesh side, up to the tilted plate. A standard deviation is calculated.
Resistance to bending: the average mass of the specimen (m) is in grams, averaging the weight of each specimen
and mass per area (ma) is calculated.
For leather softness, specific specimen size is not required. Pieces or whole hides can be used (sampling must be representative). A method is used to determine softness based on the depression caused by the action of a rod on the leather surface, held by a round hollow part. The result is expressed in millimetres.
Stretch and set: elongation
To carry out this test, tubes must be marked in the middle, equidistant from each end, at 10.0cm with a 0.1mm accuracy, and then clamped on the narrower end. Another clamp is to be placed at the other end with a weight on it during the stipulated time. After that, distance between the marks at the beginning is remeasured then the weight and the clamps are taken out, and the specimens are left to rest on a flat surface for ten minutes, measuring the distance between marks once again.
Determination of the most favourable leather zone and direction
The static and permanent elongation tests were carried out on five hides, as per the customer’s specifications, cutting five specimens from bellies and central zone of the leather, aside the backbone, parallel and perpendicular to it.
All the results obtained comply with specifications; static and permanent elongation on the central zone of the leather near the backbone are lower than on the bellies and the best direction is parallel to the backbone.
The purpose of these comparative tests was to find a correlation and establish a specification for softness (non-destructive test) that ensures compliance with the bending length (destructive test), which was one of the methods used by the customer.
In the light of these results, we found a softness range, using a non-destructive method, can be applied to measure softness on cut pieces before sewing them so that a customer can confirm all the leather is within specs. It is recommended that the pieces, because of their shape, tend to wrinkle when sewn together, are cut from A and B zones (head and mid of the hide; see Figure 1, page 29), parallel to the backbone, and to avoid cutting longer pieces perpendicular to the backbone and particularly that their narrower ends are not cut from bellies, where permanent stretching is higher. By working in this way, rejection of cut pieces was reduced from 20 to 5%.
Finally, die-cuts on the hide should be placed in such a way that the narrower part of them is the part from which it goes into the press, to reduce stretching, which will result in a lower reject percentage of cut parts due to dimensional problems.
All references, methodology and equipment lists available upon request.