The physical testing of leather

                                                                                          

The physical testing of leather

3.1. General comments

The physical tests, together with the fastnesses tests, serve to evaluate the capacity of the leather

to resist and overcome the strengths and actions it will be subjected to during the process of transformation into a marketable item and during the course of its use by the consumer.

The physical tests deal with properties which depend on the full structure of the cross section of

the leather through its entire thickness, whereas the fastnesses tests study the properties relative to the leather surface.

Results from the measurement of the physical parameters highly depend on such factors as

location, size of the test pieces, characteristics of the instruments employed, environmental conditions and the general particularities of the procedures followed. Bearing all these factors in mind, both the physical and fastnesses tests have to be determined under strict conditions of standardisation, and all their practical variables have to be free of ambiguities. Even then, the results will present a high degree of dispersion.

3.1.1. Precision of test results

Unfortunately, physical test results present a poor precision. This is accounted by the fact that the

physical test, in contrast with the chemical test, is not possible to exactly have two identical test pieces. Given the destructive character of the tests, a test cannot be repeated using the same test piece. Hence, it is required as alternative a test piece that has taken from an adjacent area, but this implies that it is difficult to ensure that it consists of an "identical test material".

The dispersion caused by the heterogeneity of leather is always much greater than that caused by

the operator or the measuring instrument.

3.1.2. Units of measurement

The procedures described in the IUP standards use the International System of Units (SI). The

ASTM standards simultaneously use the British metric system of units and SI units.

The unit of force that has been most commonly used is the kilopond (kp) or the kilogram-force

(Kgf). Many journals use this unit:

1 kp = 1 kg ⋅ 9.81 m/s2 = 9.81 kg⋅m/s2 = 9.81 N

In practical terms, the conversion factor is usually close to ten:         1 kp ∼ 10 N

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3.1.3. Conditioning

The temperature and relative humidity of air in equilibrium with the leather have an effect on most

of its physical properties. Magnitudes such as tensile strength and percentage elongation, water content and leather dimensions such as thickness and surface area vary significantly depending on environmental conditions, especially with humidity.

Generally, the influence of relative humidity of surrounding air can be summarized as follows:

1. Mechanical resistances increase with the degree of external air humidity

2. Surface and thickness increase with the external air humidity

3. The properties of the chrome tanned leather vary more under the effect of humidity

than those of the vegetably tanned leather.

4. The modification of the properties is highly accentuated in extreme environmental

conditions: very low (0 - 25 %) or very high humidities (75 - 100 %).

5. On the contrary, the properties of leather remain almost unchanged in

environmental conditions between 40 and 65 % of relative humidity.

The IUP 3 - EN ISO 2419 is the standard has been subjected to more changes in the last 20

years, both with regard to environmental conditions and time of acclimation. The current draft version (2009) specifies that the test pieces have to remain within one of the normalised atmospheres described in Table 12 or Table 12.b at least 24 hours prior to carrying out the physical tests.

Name                          Temperature (0C)          Relative humidity (%)

Standard atmosphere                   23 ± 2                                50 ± 4

Table 12. Standard atmosphere and tolerance zones

Name                           Temperature (0C)          Relative humidity (%)

Specific standard atmosphere            20 ± 2                                65 ± 4

Tropical standard atmosphere            27 ± 2                                65 ± 4

Table 12b. Alternative standard atmospheres and tolerance zones

The alternative, but not equivalent, atmospheres may be used only if the parties involved agree on

its use. In the cases of dispute the standard atmosphere shall be used.

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The 24 h conditioning is for dry leathers. Leathers with high moisture contents should be dried prior

to conditioning. The report of the results has to specify the conditions that have been applied. Environmental conditions 23/50 are more pleasant and easier to replicate, except for areas such as tropical regions, with high external air humidity.

3.1.4. Classification of physical tests

Section 1.5 presents a thorough account of the IUP physical tests, together with the bibliographic

references necessary to obtain each specific standard. Alternatively, these standards can be

obtained at www.iultcs.org. The physical tests can be classified into five main groups:

1) Measurement of dimensions

2) Assessment of resistance to mechanical actions

3) Assessment of behaviour before water and water vapour

4) Assessment of behaviour before cold and heat

5) Measurement of sensory properties

3.2. Measurement of dimensions

The following characteristics of leather are determined:

a) Thickness

b) Apparent density

c) Thickness of surface coverage

The measurement of the leather surface, which could also be included in this classification, is

widely covered in Machinery in the Tannery by Anna Bacardit and Lluís Ollé.

3.2.1. Measurement of leather thickness

Leather thickness is relevant in commercial terms and necessary for the calculation of properties

such as apparent density or mechanical resistances. The measurement of leather thickness depends on factors such as pressure and dwell time. Spring pressure gauges are employed for common measurements in leather manufacturing as they provide the required accuracy. However, the said gauges are not appropriate to make exact measurements in the laboratory as the operator cannot reproduce the identical pressure in the measurement of all the samples.

Standard IUP 4 employs a micrometric pressure gauge with disc, mounted on a firm base. The

pressure applied is 500 g/cm2. The leather is placed in the gauge with the grain side up. The load is applied softly, and the reading is carried out 5 seconds after the application of the entire load. The measurement of leather thickness in accordance with IUP 4 ensures a very good reproducibility. The results obtained without the use of the load or employing small loads will differ from those obtained using this method.

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3.2.2. Determination of apparent density

Density, defined as the quotient between mass and volume, is a basic physical property of any

material.

When the volume is calculated from the dimensions of a test piece, without taking away the volume

occupied by the air in the pores, the quotient between mass and volume of the test piece is known as apparent density.

The values of apparent density range between 0.52 g/cm3 of very porous leathers such as chamois

and 1.15 g/cm3 of hard sole leather.

Standard IUP 5 determines the apparent density dividing the mass in kilograms of a circular test

piece of 70 mm in diameter by its volume in cubic metres. It is also commonly expressed in grams per cubic centimetre.

The volume is calculated considering that the test piece is a regular cylinder whose diameter and

height correspond to the diameter and thickness measured in the test piece, respectively. The measurement of the diameter is accomplished with the vernier gauge by quadruplicate, with two measurements in perpendicular direction to one another on the grain side, and two more measurements on the flesh side. The thickness is also determined at four points.

The determination of apparent density applies to sole leather as an indirect measure of the

presence of heavy loads. Hence, in the analysis of 20 sole butts manufactured in Spain

13 , the

average value found was 1.05 g/cm . The minimum and maximum densities found were 0.96 and 3

1.13 g/cm3, respectively.

3.2.3. Determination of thickness of surface coverage

Regulations in Spain and in other EU countries which specify the labelling of the materials used in

the main components of footwear do not allow a definition of "leather" of those materials whose surface finish coat exceeds 0.15 mm in thickness. The thickness of the coverage is measured with a microscope in accordance with the instructions specified by the standard EN ISO 17186 (equivalent to IUP 41).

3.3. Resistance to mechanical and abrasive actions

The methods for the measurement of the following physical properties are covered in this section:

a) Tensile strength and resistance to elongation

b) Tear resistance

c) Resistance to grain crack

d) Flex resistance

e) Abrasion resistance

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3.3.1. Measurement of tensile strength and percentage elongation

In order to determine the tensile strength, an extended test piece of leather is fixed between the

pegs of a dynamometer (see Figure 8). Next, the pegs are separated at a constant speed while the force exerted upon the test piece is recorded by means of the load cell of the instrument.

The tension applied results in the deformation of the test piece: it extends continually in the

direction of the exerted force until it breaks.

Tensile strength is usually expressed as the quotient between the break strength and the

transverse section of the test piece. The result is expressed in Newtons by square millimetre.

Tensile strength = F / W ⋅ T

F is the major force expressed in Newtons

W is the average width of the test piece, expressed in millimetres

T is the average thickness of the test piece, expressed in millimetres

In the automotive industry, tensile strength is expressed in absolute Newtons and is measured in

10 mm wide test pieces.

Elongation is calculated as the difference between the final and initial separation of the test piece.

Such a difference is expressed as ratio of the initial separation. Elongation can be determined at a given strength or at break (maximum elongation).

% elongation at break = (L2 - L0 / L0) · 100

L2 is the separation of the clamps at break

L0 is the initial separation of the clamps

Number of replicates

Tensile strength and elongation are properties which vary hugely depending on the testing position

and the direction.

It is therefore important in comparative studies to take the test pieces from the same area and

effect 6 replicates of the test, three in parallel direction and three in perpendicular direction with regard to the backbone.

The determination of tensile strength is one of the most commonly carried out tests in the study of

leather, especially in research and developmental tasks. For the evaluation of the behaviour of

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leather in its practical use, measurement of tear load is generally preferred over that of tensile

strength. However, there an exception is made with splits. In this case, the determination of the absolute value of tensile strength is the most appropriate tool to verify that this type of tanned leather has the sufficient structural force required for its end product, mainly shoe uppers.

As far as automotive upholstery is concerned, the absolute value of tensile strength is one of the

most important parameters, and depending on the type of tannage, it is one of the most difficult ones to achieve.

Practice 2. Determination of tensile strength and percentage elongation according to IUP 6.

(This is an abstract. Refer to the book for the complete text)

Instruments

-Press knife, in accordance with IUP 1 prescriptions; it has to be able to cut a test piece as indicated in

Figure 8, with the dimensions specified in Table 14.

-Vernier callipers

-Thickness gauge, as specified in IUP 4.

-Machine for tension testing (dynamometer), with a uniform speed of separation of clamps of 100

mm/min ± 20 mm/min and a system to determine the extension of the test piece.

-Clamps, with a minimum length of 45 mm in the direction of the applied load, capable of exerting a

constant grip. The texture and design of the internal walls of the clamps have to be in accordance with the maximum load achieved in the test in such a way that the sample does not slip from the said grip more than 1% in relation to the initial position.

Figure 8. Shape of the test piece.

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Procedure

- Measurement of the thickness of each test piece in accordance with IUP 4.

Effect the measurements in three positions: in the middle point and in the roughly equidistant positions

between the middle point and lines AB and CD (Figure 8). The thickness of the test piece is the arithmetic mean of the three measurements.

- Determination of tensile strength:

Place the clamps of the tensile strength test device at a distance of 50 mm from one another (if the

normal test piece is used), or at 100 mm if the larger test piece is employed. Clamp the test piece in such a way that the furthest ends coincide with lines AB and CD. When the test piece is firmly fixed, make sure tat the grain side is laid out flat.

Start the machine until the test piece breaks, and then record the greatest force applied as break force

F.

- Determination of percentage elongation at break.

Effect this measurement together with the measurement of tensile strength.

3.3.2. Measurement of tear load

Tear load testing is used to assess the capacity of the leather to resist the multidirectional tensions

it is subjected to in its practical use. Tear resistance is particularly necessary around the sewn areas, stitched slits and around all the pieces with orifices or carvings that are subjected to tension. Standards and quality guidelines of most leather goods specify the compliance with the values of tear resistance.

The essential characteristic of the tear load test is that, in contrast with the tensile strength test, the

force applied to the test piece is distributed across the fibrous lattice of the leather to adjacent sections, and in practice the test piece behaves as if it was simultaneously subjected to tensions in all directions. For this reason, the tear load test is more representative of the normal conditions of use of the leather.

Different procedures

There exist several procedures to measure the tear strength of leather. The method IUP 8 is called

double edge tear.

A groove is cut off from the test piece with the shape indicated in Figure 9. The curved ends of two

L-shaped pieces are introduced in the groove that has been put on the test piece. These pieces are fixed by their other end between the clamps of a dynamometer like the one used in tensile testing. When the device is activated, the L-shaped pieces introduced in the test piece separate at a constant speed in perpendicular direction to the longest side of the hole, causing the tear of leather until its complete break.

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Tear resistance can be expressed in relative terms as the quotient between the maximum force

and the thickness of the test piece (in Newton/millimetre), although in practice it is more useful to express such a force in absolute terms.

At present, there are two drafts of the new methods IUP 40 and IUP 44 and three further ASTM

methods for the measurement of tear resistance under different conditions.

Practice 3. Determination of tear load according to IUP 8.

(This is an abstract. Refer to the book Análisis y ensayos en la industria del curtido for the complete

text).

Apparatus

-Thickness cutting gauge, as specified in IUP 4.

-Machine for tension testing (dynamometer), with a uniform speed of separation of clamps of 100

mm/min ± 20 mm/min.

-Clamps and accessories for the determination of tear strength by the double edge method.

Figure 9. Shape of the test piece for IUP 8.

Procedure

i)           Measurement of the thickness of each test piece in accordance with IUP 4:

Carry out two measurements. Take the arithmetic mean average of the two measurements for the

thickness of the test piece.

ii)         Determination of tear resistance:

Adjust the dynamometer so that the folded ends of sample holders are slightly in contact with one

another. Place the test piece over the turned up ends, so that they protrude through the slot with the widths of the turned up ends parallel to the straight edges of the slot. Press the test piece down firmly on to the holders.

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Run the machine until the test piece is break, and record the maximum force reached as the tear

strength.

3.3.3. Measurement of grain crack resistance

Grain crack resistance of light leathers

In shoes manufacturing, the leather for the toecap experiences a sudden deformation that takes it

from its flat shape to a three-dimensional shape. Such a transformation produces a hard tension on the grain layer since the surface has to stretch out more than the rest of the leather in order to be able to adapt to the spatial shape. If the grain is not sufficiently elastic to accommodate to the new situation, it cracks and breaks. The sharper the toecap is, the higher the demand on grain resistance of the leather.

Method IUP 9 is used to test the aptitude of shoe lasting and it is based on the lastometer. This

device contains a clamp that grips the rounded leather test piece firmly, grain side out, plus a drive that propels the clamp towards a steel ball at constant speed. The ball remains still at the middle of the grain side of the test piece (see Figure 10). The descending action of the clamps deforms the leather progressively until it resembles a cone, with the grain subjected to growing tension until the first crack occurs. At this point, the force applied by the steel ball has to be written down, as well as the distance in millimetres between the initial position of the camp and the position when the first crack in the grain occurs.

Such a distance is known as distension.

The action is not stopped until the complete break of the leather. The value of distension is written

down again, as well as the load, although these data are only referential in nature.

Distension during the first grain crack is the most significant parameter to judge leather aptitude for

shoe lasting. The quality guidelines for shoe uppers (see Chapter 6) specify a minimum of 7 mm, although the distension should be at least 8 mm to be more confident about its behaviour in the footwear toecap.

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Figure 10. Cross section of steady head with test piece in testing position and already

subjected to a mild distension. Reference: IUP 9.

Grain crack resistance of heavy leathers

Please, consult the book Análisis y ensayos en la industria del curtido.

3.3.4. Measurement of flex resistance by flexometer method

All leathers that flex repeatedly during their practical use are exposed to the deterioration of their

finish. A typical example is that of shoe uppers and the deterioration occurring in the flexion area. The most common defect is the cracking of the finish, with the formation of cracks to a certain extent, although a change of colour may also occur owing to the loss of adhesion between finish coats or between finish coat and leather. The leather may also be damaged, e.g. formation of thick pleats or the crack of the grain.

The behaviour of a finish to the flexion depends on its elasticity, thickness, adhesion to the leather and adhesion between coats. The most commonly used testing process is IUP 20, "Measurement of flex resistance by flexometer method". This method mainly applies to uppers, although it is currently employed for almost all types of tanned leathers. In this test, the test piece is folded as indicated in Figure 12 and fixed between two pincers.

The lower pincer is fixed, whereas the upper one moves back and forth, within an angle of 22.5

degrees, similar to the foot's average degree of flexion on walking.

A counter records the number of flexions carried out. The device can be programmed to stop after

a certain number of flexions, and the state of the test piece can be visually analysed using a magnifying glass.

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As for leathers intended for cold climates, it is advisable that the flexometry test will be carried out at low temperatures. Some finishes resist 100,000 flexions at 20 ºC, but at -10 ºC they start cracking before reaching 10,000 flexions.

Fig.11. Loading of test piece between clamps: A, test piece in upper clamp. B, test piece folded back. C, test piece fixed between upper and lower clamp. Reference: IUP 20.

3.3.5. Measurement of abrasion resistance

(Refer to the book Análisis y ensayos en la industria del curtido for information about this subject).

3.4. Behaviour to water and water vapour

Water resistant leathers have been in demand for a long time. The ideal behaviour of leather to

humidity consists of a material that has the attribute of being permeable to water vapour while being impermeable to water.

Terms such as repellence, resistance and water impermeability/waterproofness are commonly believed to have the same meaning, but they do not mean the exact same.

Impermeability means that the leather does not allow the passage of water by any means: absorptions in the test IUP 10 are practically zero after 24 hours. As for leather, this is only possible by means of finish foils, which prevents also the transpiration by water vapour permeability. This is suitable for leather goods, but not for shoe or clothing leather. At present, there is no waterproofing treatment that can provide systematic impermeability to the leather.

Waterproofness is obtained when in the dynamic test IUP 10 the leather is in contact with water and the latter does not penetrate through its cross section before two hours of testing. It is also needed that within that period of time the amount of water absorbed is lower than 25% of its weight. However, some particular specifications require 3, 4 or even 12 hours without causing the penetration through the cross section. Waterproofness is achieved by means of an exhaustive waterproofing.

Water repellence is a term that applies to those leathers with a limited degree of waterproofing

treatment, which allows for the leather to successfully pass the water spotting test (IUF 420). This said it does not reach the high levels mentioned above. It is a characteristic demand for leather goods and upholstery.

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To summarize:

Impermeability > Resistance > Repellence

This chapter is a study of the procedures involved in the testing of the following properties:

a) Water absorption is static conditions.

b) Water resistance.

c) Water vapour absorption. d) Water vapour desorption.

d) Water vapour permeability.

3.4.1. Determination of water absorption in static conditions

Water absorption is determined by immersing a round leather test piece in a glass of water and

leaving it to stand for a specific period of time. After that, the test piece is weighed after drying the surplus water on its surface. Water absorption is expressed as a percentage which relates to the initial weight of the test piece.

This is the method IUP 7.

Water absorption percentage over time t = 100 · w / m

w = water weight absorbed by test piece (in grams).

m = weight of test piece (in grams).

The commonly employed periods of time are 1, 2, 8 or 24 hours.

In contrast with dynamic methods (see 3.4.2), method IUP 7 is referred to as the static method. At

present, dynamic test methods are widely used as they reflect the real situation of use more accurately. However, the static method is still being used because it is cheap and simple.

3.4.2. Determination of water resistance

The penetration of water into the leather would be very slow if they simply came into contact with

the water in standing conditions. The movement of the foot when walking and the continuous succession of flexions produce a pumping effect that favours the penetration of water through the interfibril spaces of both sole and upper. Hence, the determination of waterproofness has to be carried out in laboratory conditions that can reproduce the type of movement which occurs in the

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actual use of the leather good. In order to accomplish this aim, dynamic tests of waterproofness

have been devised. Trough these tests, three parameters are determined:

1) Percentage of water absorption. This is the amount of water absorbed by the test piece within a specified period of time, expressed in percentage of the initial mass of the leather.

2) Penetration time. It is the interval of time that has passed from the beginning of the test

to the precise moment the water has penetrated through the leather. It is also know as the time for the first pass of water.

3) Amount of water transferred. It is the volume of water that flows through the leather

within a specific period of time, expressed in grams.

The value of penetration time is the most important commercial reference as far as waterproofed leathers are concerned. However, leather waterproofness does not have to be assessed exclusively based on the parameter of penetration time. Instead, water resistance should be assessed considering the three parameters measured in the test.

Flexible leathers and sole leathers have to be tested using different dynamic procedures, given that there are such large differences in behaviour and function.

Determination of water resistance of flexible leathers

The need to be able to adequately control the waterproofness of footwear caused the development of several dynamic test methods both in the USA and in Europe. The "Bally" flexometer method was chosen for method IUP 10, "Water resistance of flexible leather". On the other hand, the USA adopted the methods based on the Maeser water penetration testers (ASTM D 2099) and Dow Corning (ASTM D 2098).

Nowadays, Maeser is the most influential method in the USA. Its procedure is more aggressive and demanding than that of the Bally method. Method IUP 10 corresponds to standards DIN 53338 T1 and UNE 59028.

According to method IUP 10, a rectangular leather test piece (60 mm x 75 mm) is fixed, by means of clamps, to two coaxial metallic cylinders whose axle is parallel to the horizontal. One of the cylinders is static, whereas the other moves along its axle and the amplitude of the movement can be regulated by the operator. The test piece is formed into the shape of a trough and placed over a bucket containing de-ionised water (see Figure 12). When the test begins, the grain side of the test piece comes into contact with water while it is continuously flexed as a result of the oscillating movement of the moveable cylinder.

By increasing the flexing action, penetration speed increases in most leathers. Hence, the amplitude of the movement is an essential factor in the reproducibility of the test. The machine allows for the pressing of four possible amplitudes: 5, 7.5, 10, and 15 %. Thick and rigid leathers flex to a lesser extent than soft ones in its practical use. In order to reproduce these differences in

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the laboratory, the thick leather has to be tested with a lesser amplitude than that of the thinner leather.

In order to determine the amount of water absorbed once the testing time has passed, the test piece is put aside, slightly dried to remove the humidity which has adhered to the surface, and finally weighed. Water absorption is expressed in percentages which refer back to the initial mass of the test piece.

Figure 12. Loading of test

piece in IUP 10.

As for the testing of footwear materials, absorption is determined generally after a period of 120

minutes. In research and developmental studies, absorption is measured after 240 minutes,

although on certain occasions the period of time is as large as six or twenty-four hours.

In practice, penetration time is reported in minutes only if the value is lower than two hours.

Otherwise, it is commonly expressed as "over two hours". The waterproofed materials tested may be classified depending whether they resist more than two, four, six or twenty-four hours.

The amount of water transferred is determined by the increase in weight of an absorbent fabric

which is wrapped up and placed in the interior of the channel that the test piece forms. In many practical studies, this is only determined in leathers whose penetration time is lower than two hours.

Determination of water resistance for sole leather

(Consult the book Análisis y ensayos en la industria del curtido for information about this subject).

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3.4.3. Determination of water vapour absorption

Certain types of leather have the capacity to absorb the humidity caused by the user's transpiration.

That is mainly the case of insole leather, although this also applies to lining leather, uppers and gloving.

Generally, static absorption of water (see 3.4.1) for insole leather is determined as a measurement of

its capacity to absorb the humidity. As for lining leather, uppers and gloving, it is preferable to determine their capacity to absorb water vapour since it is a property that resembles more what occurs in its actual use. The most commonly employed method, which is summarised in the book, is EN ISO 20344.

3.4.4. Determination of permeability to water vapour

One of the most important attributes of leather is its capacity to transmit water vapour. This quality

defines leather as the best suited material for manufacturing footwear, garments and gloving.

Leather permeability is intrinsically very high. The addition of high amounts of fat and impregnation to

achieve the waterproofing of leather decrease the vapour permeability, although not to such a degree that the comfort of leather would be seriously affected.

However, very thick finishes, and specially transfer finishes and foil finishes reduce drastically the

permeability to water vapour. Table 16 is a summary of the permeability values of several types of leather.

Type of leather

Suede, velour and nubuck

Full grain box-calf, casein finish

Corrected grain with a finish

Patent (polyurethane finish)

Polyurethane covered split (transfer

process)

GERIC recommendation for uppers

Permeability to water vapour

(IUP 15)

11 - 14 mg/cm2⋅hour

5 - 8 mg/cm2⋅hour

0.3 - 3 mg/cm2⋅hour

0.4 - 0.9 mg/cm2⋅hour

0.0 - 0.2 mg/cm2⋅hour

minimum 1.0 mg/cm2⋅hour

Table 16. Characteristic values of permeability to water vapour

The most commonly employed procedure for the measurement of permeability to water vapour is

method IUP 15. In this method, the leather test piece is fixed by means of a screwed lid with a circular opening around the mouth of a glass bottle that contains dried silica gel. The only possible air inlet is through the leather (Figure 13). The bottle is kept in motion, and a fan ensures the circulation of fresh air.

The temperature and relative humidity of the air outside the bottle is set by the laboratory conditions

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specified in IUP 3.

The leather test piece is placed with the side that will be exposed in its practical use to greater humidity

(usually the flesh side) facing the exterior of the bottle.

After conditioning for 20 ± 4 hours, the test piece and the screwed lid are transferred to a second bottle

which contains silica that has just be regenerated. Next, the set is weighed, the instrument is switched

on, and at 11.5 ± 4 hours it is weighed again in order to determine the vapour mass that has come

through the leather and absorbed by the silica gel.

Water vapour permeability (P) is defined as the mass of water vapour that is transferred by the leather

per surface and time unit, expressed in milligrams per square centimetre and hour:

P = vapour mass / test piece area · time = 1.273 m / d2 ⋅ t

m = increase in weight of the bottle after weighing it twice (mg)

d = actual diameter of the test piece (cm)

t = time interval between the two weighings (hours)

PROBETA DE CUERO          Figure 13. Loading of test piece in the bottle

containing silica gel (Standard IUP 15).

EXTERIOR: 65 % H.r.

0 % H.r.

GEL DE SILICE

Testing Leather - Unit 3

3.5. Evaluation of behaviour to heat and cold

This chapter describes the methods used to measure the following physical properties:

a) Cold resistance of the leather finish

b) Shrinkage temperature

c) Heat resistance of the leather

d) Propensity to form a veil (causing fogging)

3.5.1. Resistance to cold of the leather finish

17

(Consult the book Análisis y ensayos en la industria del curtido for information about this topic).

3.5.2. Determination of shrinkage temperature

The most noticeable effect suffered by a strip of leather subjected to a gradual heating is the sudden

contraction that it suffers at a specific temperature. Shrinkage temperature is a very important source of information as a means of measuring the degree of stabilisation of the collagen, and it is used both in control routines of factory processes and in research. It is determined by immersing a strip of leather in a fluid subjected to a slow increase in temperature (2 ºC per minute). Shrinkage temperature is produced when there is a clearly defined contraction of the leather test piece. Standard IUP 16 specifies in full detail the methods to employ to determine the shrinking up to 100 ºC. As for higher temperatures, the de-ionised water is substituted by mixtures of water and glycerine, although this option is not part of standard IUP 16.

3.5.3. Dry heat resistance of leather

(Consult the book Análisis y ensayos en la industria del curtido for information about this topic).

.

3.5.4. Testing the tendency of leather to cause fogging

The high temperatures inside the cars parked in the sun may cause the evaporation and sublimation of

unfixed volatiles of upholstery leather. On cold days, while the outer temperature may be as low as zero degrees, the temperature of upholstery leather inside a car can be higher than 90 ºC, especially when its colour is dark. The glass pans, mainly the windshield, which are refrigerated by the external environment, remain cold and the substances emitted from the leather condense forming a halo known as the fogging, which makes sight difficult for the driver.

The chemical analysis by head space - gas chromatography reveals that the deposits of fogging mainly

consist of plasticisers such as phtalates, fatty acid esters, paraffins, alkylbencenes and other mineral oils.

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Consequently, both degreasing and fatliquoring of leather are essential in the tendency towards

fogging, although the use of volatile components with poor fixation to the leather has to be the minimum possible throughout all the different manufacturing stages.

Leather is not the only material likely to cause the appearance of the fogging. Hence, all car interior

materials are presently tested. The fogging test was first applied in Volvo factories by late 1970s and despite different versions, they all agree on the essential points. Test temperature is a relevant factor, and despite efforts to make this value uniform (at 90 ºC), some manufacturers use different temperatures (100 ºC).

Another important factor is the humidity content of the test piece, which due to an effect similar to steam

distillation increases the degree of fogging. It is thus necessary to standardise the conditioning strictly, which has to be effected smoothly and for a lengthy period of time.

There are two versions of the fogging test; each differs in the method employed to measure the amount

of veil formed:

a)          Measurement of specular reflectance

b)         Gravimetric measurement

Reflectometric fogging method (DIN 75201-A):

(Read the book for more details)

Gravimetric procedure (DIN 75201-B):

This version of the test lasts 16 hours. A light aluminium foil is used, and it is weighed before and after

the test in order to determine the increase in weight owing to the condensation of volatiles emitted from the leather. The result is expressed in milligrams of fogging per square centimetre of leather. The requirements from automotive manufacturers vary from the different brands, but a referential value of the maximum limit is 5 mg (for a 50 cm2 foil).

The results from the two versions of the fogging test do not show a strong correlation. The gravimetric

method is currently preferred over the reflectometric procedure because of its better reproducibility.