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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
Testing Leather - Unit 3
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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.
Testing Leather - Unit 3
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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.
Testing Leather - Unit 3
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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
Testing Leather - Unit 3
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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
Testing Leather - Unit 3
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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.
Testing Leather - Unit 3
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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.
Testing Leather - Unit 3
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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.
Testing Leather - Unit 3
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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.
Testing Leather - Unit 3
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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.
Testing Leather - Unit 3
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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.
Testing Leather - Unit 3
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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
Testing Leather - Unit 3
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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
Testing Leather - Unit 3
14
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).
Testing Leather - Unit 3
15
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
Testing Leather - Unit 3
16
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.
Testing Leather - Unit 3
18
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.
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