Leather Testing

From LeatherLink Knowledge Base

Contents 1 Introduction 2 Strength 3 Wear 4 The Grade of Leather 5 Health and Safety tests 6 Comfort tests 7 Others

Introduction

If you are one of the chosen few, who work with leather to make products of different types, this page is for you. Even if you not, you may come across a piece of leather in your life and really want to know what the heck it is! Not interested! Still read on! No knowledge goes waste!!

If you have come here from the page dealing with judging the quality of leather on a product you already have a good idea about how to judge the surface of the leather. If you are coming from somewhere else, it does not hurt to go there and have a peek first.

If you want to know just how to test the leather surface for various properties click here to go direct to the section dealing with that.

Properties of Leather to be tested are basically of three categories.

• The strength and Wear properties required for the specific use as footwear, upholstery, garment or leather good material
• The properties which indicate the quality of raw material used and the standard of processing employed in making.
• Leather's chemical nature should not be harmful to the consumer. In addition, governments agencies require that the leather does not affect the environment by controlling the presence of certain chemicals in the leather. These chemicals are not to be used in leather processing and these tests are just to confirm that they are not present.

In each category we will list the tests that can be carried out and briefly describe the crude on-the spot test method. The more elaborate tests which requires special equipments are also mentioned for reference.

Strength

To test the strength of a piece of leather, fold the leather as a single fold perpendicular to one of the edges and cut the folded edge with a knife. When you open the fold you have created a loop parallel to the edge. Put your index finger in the hole and pull the loop and try to tear the piece off. You can feel the resistance of the leather fibres to tearing. If it tears off like a paper, the strength is low. If you are not able to pull it off, it is quite strong.
Once you try this test on a few samples you will get a feel of the typical pressures required by different types of leathers. Perfect it by practicing it often! (so long as the leather belongs to someone else, you do not want to tear away all your leathers!!!)
There are special equipment to measure the tensile strength and tear strength. The specifications are different for leathers meant for different end uses.

Wear

1. Leather fibers are subjected to flexing during use. Leather may be exposed to light, heat, water, sweat and acidic/alkaline media during use. The leather surface may be abraded and rubbed. Wear tests measure the ability of leather to resist changes due to these factors
2. The fastness of the surface color can be tested by rubbing with a white cloth (cotton hand kerchief?) held around the index finger. By trial and error you can choose the pressure and number of strokes of rubbing. After rubbing by looking at the color change of the rubbed leather surface AND the cloth surface, we can judege the rub fastness of the leather surface.
   By wetting the cloth with water, sweat or any other solution we want to test, we can determine the rub fastness of against that solution.
   The official tests work on the same principles using more accurate equipment and standard scales for comparison of color change. The tests are dry rub fastness, wet rub fastness and fastness to perspiration.
3. Light fastness is determined by measuring the change in color of the surface on intensive exposure to a specified light for fixed time duration.
   The other way to determine this property is by cutting a sample of leather into two pieces and leaving one piece exposed to sun light and keeping the other piece in the dark for a number of days. We can compare the color of each and judge the change.
4. The strength of the finish coating's adhesion to the leather surface can be checked by folding the test sample into four (creating a pointed edge) and abrading the edge against a rough surface and determine whether the finish coating is removed or not. By subjecting the leather piece into rapid flexing by hand (holding the edges by each hand and moving them to and fro) and looking at the change in the finished surface, we can judge resistance of the leather to flexing.
   The official test methods measure the force required to tear away the grain or finish from the leather surface for finish adhesion. The samples are subjected to a specified number of flexing cycles and the surface is examined under magnification to determine the flex performance of the leather
5. By placing spots of water, oil or any other solvent on the leather surface and observing how they are absorbed, we can judge the leather's resistance to wetting by these solvents. By making the leather surface into a cup form and holding water to see the time it takes to penetrate to the other side, we can judge the water resistance property of the sample.
   The official methods measure the amount of water abosorbed by leather in a given time (water absorption test), change on the surface on water spotting etc and also can test the resistance to water during flexing (water resistance) by measuring the number of cycles of flexing after which water can penetrate from one side of the leather to the other side.

The Grade of Leather

1. The ability of leather to transfer water vapour is measured by water vapour permeability test. This is an importance test for footwear leathers as a measure of foot comfort.
   The type finish has the greatest influence on this property. If the leather is finished by coating heavily, the water vapour permeability will be relatively poor.
2. By cutting a piece of leather and looking at the cross section, we can determine the difference between the surface color (finishing color) and cross section color (dyeing color). If the cross section color is closer to the surface color, it indicates that the leather is dyed accurately and thoroughly. The official methods also follow a similar procedure and use standard scales for comparison.
3. Softness of leather can be judged subjectively. Softness Gauge can help in providing a value to softness. Even though thickness is measured by thickness gauge, some people are able to tell the thickness of leather just by passing their hands over a folded surface.
4. The quality of tannage can be judged by comparing the shape of a leather before and after placing in boiling water for five minutes. The shape can be compared by drawing out the outline before testing.
   This test is widely used for testing the quality of wet blue in tanneries without test facilities. The official methods measure the Shrinkage temperature which is the temperature at which the leather fibres start shrinking. .It is also possible to measure the effect of sweat on leather shrinking.

Health and Safety tests

1. pH (acid/alkali balance) of the leather is measured by determining the pH of the extract of leather sample. The crude method is to soak the leather in water for one or two days and measure the pH of the solution.
2. Detecting and measuring Formaldehyde, PCP, Chrom VI and aromatic amines involve sophiscated equipment and may not be possible to be carried out outside specialized laboratories.

Comfort tests

Personal comfort is a key factor now every user is desiring out of the products they buy. Atleast everyone curses the products which reduces their comfort, nowadays no one likes to wear just for look and appeal alone, comfort of wear is a key factor of like for a brand

Comfort shall be measured as

1. Sweat permeability
2. Perspiration impact
3. Light Weight (Water proof?)

• Sweat permeability:

Sweat is the way a human breathes through his skins, a man sweats around 6 mg/min per palm to regulate temperature of skin. This sweat should be let out as vapor by the wear (garment/shoe) for better comfort avoiding moist slippery smelly legs / body. For this the wear should possess permeability (breathing) property. Permeability is measured in g/SqM/day, its technical term is Moisture Vapor Transmission Rate (MVTR) Leather is highly permeable by nature (thanks its three dimensional weave of fibers). However the cheaper processing for upgradation and water proofing reduces Moisture vapor permeability. The standard minimum Moisture vapor permeability of Leather for shoes is 10mg/Sqcm/day.

• Perspiration:

As said above Sweat is associated with human, this Sweat contains undoubtedly Water and the chemicals such as chlorides and the odorants 2-methylphenol (o-cresol) and 4-methylphenol (p-cresol), as well as a small amount of urean. These chemicals (salty water) is capable of stripping some of the Leather chemicals used during its processing, chiefly coloring chemicals namely dyes and dye solutions. This on stripping soils the inner wear such as socks and causes skin blemishes for those who are allergic to these kinds of chemicals. Perspiration is tested by soaking the leather in Sweat solution(pseudo Sweat) with a perspiration strip (a strip of fabric made with different layers made of seven different fabric, namely Cotton, Nylon, Silk, PVC, etc). After 15 hours of standard humid and temperature control, the strip and leather are taken of the sweat solution. The sweat solution, leather and fabric are observed for color change and graded in grey scale, normally Grey scale of above 3 to 4 is appreciated in the Grey scale of range 0 - 5

• Light Weight(water proofing?):

Leather being three dimensional and porous takes up water easily to increase the weight of shoe and needs ample of time to get dried. Imagine some one walking with Leather shoes in rainy street or some soldier with long leather boots walking in snow filled terrain, they should have additional energy to carry their protecting shoes. Water proofing helps a lot to address these issues. Water proofing coats leather fibers and reduces water absorbency (observe the word 'coats', so it does not impair breath-ability. Tests on water proofing are

1. Static Water uptake
2. Water proofing time

Others

In addition to the above tests, special requirements such as oil content, chrome content, fogging tests, flammability tests etc. can be tested in laboratories.