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Article
2022, Vol. 51(5S) 8906S–8924S
© The Author(s) 2022
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DOI: 10.1177/15280837221077048
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Thermophysiological
properties of bovine leather in
dependence on the sampling
point, tanning and finishing
agents
Dragana Kopitar
1
, Franka Zuvela Bosnjak
2
, Jadranka Akalovic
2
and Zenun Skenderi
1
Abstract
The influence of differently tanned and finished bovine leather on thermophysiological
properties was investigated. In addition, it was investigated whether sampling has a
significant influence on the thermophysiological properties. The back of the tested
leathers is thicker than the neck because of better microstructure regularity and uni-
formity as well as thicker and denser distributed fibrils than in the neck parts. The neck
parts have a greater proportion of air-filled spaces between the fibrils, resulting in a higher
thermal resistance of the leather neck parts. Considering the thickness of synthetic and
chrome-tanned leathers (dyed and hydrophobized), the thinner chrome-tanned leathers
(0.063 W m
1
°C
1
for the neck part, 0.090 W m
1
°C
1
for the back part part) have
almost the same thermal conductivity as synthetic ones (0.065 W m
1
°C
1
for the neck
part, 0.089 W m
1
°C
1
for the back part). Their thermal and water vapour resistance
show considerable differences. There is no significant difference in water vapour re-
sistance of the neck and back part of chrome tanned, dyed and hydrophobized leather
(25.27 m
2
Pa W
1
for the neck part; 25.15 m
2
Pa W
1
for the back part) in contrast to
equally treated synthetically tanned leather (30.38 m
2
Pa W
1
for the neck part; 26.96 m
2
Pa W
1
for the back part).The presented study could help in choosing the appropriate
1
Department of Textile Design and Management, University of Zagreb Faculty of Textile Technology, Zagreb,
Croatia
2
Study in Varazdin, University of Zagreb Faculty of Textile Technology, Varazdin, Croatia
Corresponding author:
Zenun Skenderi, Department of Textile Design and Management, Faculty of Textile Technology, University of
Zagreb, Prilaz baruna Filipovica 28a, Zagreb 42000, Croatia.
Email: zenun.skenderi@ttf.unizg.hr
point of sampling, tanning as well as finishing agents for obtaining satisfying thermo-
physiological comfort in the wide range of leather application.
Keywords
Bovine leather, sampling, tanning, finishing, thermophysiological properties
Introduction
In order to obtain usable leather for various products, animal hides and skins are processed
in several stages; pretanning (liming, bating and pickling), tanning and post tanning and
finishing. By the action of chemical agents the protein structure bonds are broken and new
bonds are created, respectively, structural, chemical and physical changes of a leather
occurs.
1
At every stage of the process, the optimal state of the micro and macrostructure of
the protein must be achieved in order to produce high quality leather.
The impact of the processes on the porous structure of skin and leather after each
process step has been investigated in several papers.
2,3
The influence of pretanning and
tanning process conditions on goat skins after each stage of the chrome tanning process
(soaking, liming, deliming, pickling and tanning) showed significant variations in pore
size distributions at each stage of leather processing.
3
Chrome-tanned goat leathers
showed an increase in pore size and maintained a high level of water vapour permeability,
which makes the leather very comfortable to wear. Vegetable tanned goat leathers are
characterised by a wider pore size distribution with a certain mesopore volume and
macropores with small pore size.
Nishad Fathima et al. did study changes in the pore structure of the hides (wet salted
and unhaired cowhides), chrome and vegetable tanned leather due to thermal shrinking.
4,5
Before exposure to heat, the porosity of the native hide was 33.17%, that of the chrome-
tanned 16.34% and that of the vegetable-tanned 31.35%, with the tanning process re-
ducing the porosity of the leather. There is a significant difference between the porosity of
differently tanned leathers, which is explained by the nature of the tanning agents and their
interaction with the hides. The lower reduction in porosity after exposure to heat of
chrome-tanned leather compared to native and vegetable-treated leather is explained by
the fact that the chrome treatment does not fill interspaces between fibres, as is the case
with vegetable-treated hides. Tanning with chromium salts and vegetable tannins suggests
that surface groups, inorganic substances and the pore size distribution influence the water
transfer through the hides or leather.
6
Sundar and Muralidharan succeeded in improving water vapour permeability of goat
skins using enzymatic and chemical methods during pretanning and after chrome tanning
without affecting other mechanical properties of leather. The reason could be a more open
fibre structure if proteases are used, respectively, integrated treatment with acetic acid and
urea provides a better leather porosity than conventionally processed leathers.
7
The influence of different tanning agents (chromium, vegetable and chromium-
vegetable combinations) on the physical and fastness properties of sheep skins of
Kopitar et al. 8907S
English and Persian origin shows that the type of tanning agent has an important influence
on the physical properties of leather, even if the origin is the same.
8
A study of thermal comfort properties and water vapour permeability on sheep skins
tanned with chrome, zirconium, vegetable, phosphonium and glutardialdehyde tanning
agents showed a significant influence of tanning agents on the thermal and vapour
permeability of sheep skins.
9
The leather with the highest thermal resistance was the
leather treated with glutardialdehyde and the leather treated with chrome active sub-
stances had the lowest thermal resistance. The conclusion of the research is that the use of
specific tanning agents and leather tanning processes may result in different thermo-
physiological properties of leather or that the use of leather products should serve as a
guide for the selection of suitable tanning agents.
Besides the tanning process, finishing and fatliquoring significantly influences the
parameters of moisture absorption and desorption.
6
Finishing reduces the size of the
monolayer and increases the binding energy constants, while fatliquoring reduces the
maximum absorption capacity and hysteresis, which partially affects the reduction of the
monolayer size.
Research of structural, chemical and physical properties of Nile Perch fish leather
showed that there is a significant difference of physical properties depending on the part of
sapling, respectively, the part towards the head and towards the tail.
10,11
Recent literature related to the leather field mainly deals with the cleaner approach to
chrome tanning in terms of cleaning up waste and residues after leather processing as well
chrome-free leather tanning using eco-friendly and functional polymers.
12
In addition, a lot of research is being done in the area of tannin extraction from natural
and renewable resources such as plant tannins and tannins mediated nanoparticles for use
in tanning process.
13,14
Considering cruelty free and vegan leather, some new approaches
to obtain leather from bagasse, mushroom, banana and pineapple are done.
15–18
In the recent and relevant literature, a whole series of studies on tanning agents and the
influence of processes on the different properties of various mammalian skins were found.
The available literature lacks a systematic study of thermophysiological properties
differently tanned and finished bovine leather. Among the leathers of different animal
species, the bovine leather is the most commonly produced type of animal leather as it is
suitable for a wide range of products. Depending on pretanning, tanning and post tanning
processes and their parameters, different thermophysical properties of leather will be
achieved. A systematic study of the influence of sampling, tanning and post tanning
processes on thermophysiological properties could help to choose the appropriate point of
sampling, tanning as well as finishing agents to obtain satisfying thermophysical comfort
regarding the application of leather.
Experimental
Materials
The designation and description of six samples, which were differently tanned, processed
and sampled from the back and neck of the leather are given in Table 1.
8908S Journal of Industrial Textiles 51(5S)
Legend:
Figure 1.a) - Back of the leather: B = tail root, R = shoulder point, BR = longitudinally
along the spine, D’= on the edge of the abdomen, AD’= vertical on BR, AB/BR/2, AB =
AR, AE = (50±5 mm), AF = FD’, JK = EF, GE = EH, HL = ML = HK/2. Lines GH, JK,
HN and LM are parallel on BR.
When sampling the back part of the leather (Figure 2a), point A is half value of length
BR (BR/2 = A). A parallel is drawn in regard to the length BR. The length of the AF point
is half the length of AD’(AD’/2 = AF). From half the length EF, the length JF is obtained.
A parallel line is drawn in regard to length AD
0
through the point J, the left point G is
obtained. The length EF is equal to the length GJ and the length HK. The length HL is half
the length HK and we repeat the drawing of the square.
Figure 1.b) - C = top of the neck, R = shoulder point, CR = line along the spine, S =
shoulder edge, KP = 20±2mm, HQ = 50±5mm, RP = PS, GH = JK = HL = LM = MN =
GK/2, lines RS, HJ, GK and LM are vertical on CR. Lines GL, KJ and NM are parallel
on CR.
The neck portion is sampled with a parallel line from the centre line R at a distance of
50mm. Draw a vertical line RS to the centre line R. Divide the length RS in half and obtain
a point P (RS/2 = P). On the parallel 20 mm away from RS, a point K is obtained and on
the left, at a distance of 50 mm from the midline of the point G is drawn (GK/2 = GH). The
obtained rectangle and square are drawn according to the points marked in Figure 2b.
Methods
The preparatory operations of soaking, liming, deliming and pickling were carried out
under industrial conditions according to the standard procedure for all samples. The first
process of raw hide processing is soaking where hide regains the necessary moisture (hide
rehydration), impurities are removed from the hide surface and salt is applied during the
canning phase. The structure is filled with water which prevents mechanical damage of the
fibres and allows the transport of chemicals. When the hide is completely filled with water,
Table 1. Bovine leather designation and description.
Sample mark
Sample description
Tanning agent Finishing Sampling position
Syn-D Synthetic Dyed Neck
Syn-D-CF Synthetic Dyed, Completed leather face Neck, back
Syn-D-H Synthetic Dyed, Hydrophobic Neck, back
Cr Chrome Split leather Neck
Cr-H Chrome Hydrophobic Neck, back
Cr-D-H Chrome Dyed, Hydrophobic Neck, back
Letters before dash denotes type of tanning (Syn for synthetic tanning, Cr for chrome tanning), while letters after
dash denotes finishing process (D for dyed leather, CF for completed leather face and H for hydrophobic
leather).
Kopitar et al. 8909S
soaps and/or enzymes penetrate between collagen fibres, otherwise each subsequent
chemical treatment will be uneven and the result will be unevenly treated skin.
19
This is
followed by a technological operation to remove the flesh, which consists of mechanical
processing of the hide to remove subcutaneous tissue with remnants of muscle and fat
tissue. The liming process removes hair, wool and epidermis, loosens hide tissue, that is,
removes structured proteins and saponifies natural fats. Proteins that are not chemically
stable are removed, which increases the number of collagen reaction groups and leaves
space for tannins that bind more easily to collagen. A solution of lime and sulphide is most
commonly used for liming process. The lime remaining in the hide after leaching leads to
insoluble calcium tannins in the further course of tanning with tanning agents, which
cause a rough grain side of the hide in addition to losses during tanning. Besides removing
lime and sulphides, the deliming process flattens the hide and keeps the pH value at about
eight in order to facilitate the bating process.
7
Bating completely flattens the collagen
fibres, so it is easier to remove all the unnecessary remaining substances from the hide
(remnants of hair roots, elastin fibres, inter-fibre proteins, fats, pigments and lime).
Prior to the tanning process, the hides were sorted into two groups of samples. First
group of samples is tanned with chrome tanning agents. Second group is tanned with
synthetic tanning agents. The samples were subjected to processing procedures shown in
Figure 2.
Figure 1. Schematic presentation of leather sampling according to the standard ISO 2419: 2017.
(a) Back of the leather. (b) Neck of the leather. The bovine leather was sampled according to the
standard ISO 2419:2012.
9
8910S Journal of Industrial Textiles 51(5S)
Acidification of the hide (pickling) achieves dehydration of collagen fibres, which
allows better tanning of the hide. The pH value of the hide, which is also the pH value of
chrome tanning solution (2.5–3.5), is achieved which enables the gradual penetration of
the tanning agent, that is, the astringency of chromium salts is reduced.
Tanning process
During the tanning process, a significant change in the structure of collagen occurs due to
irreversible crosslinking within and between the collagen fibres and the tanning agent
(Figure 3). Different tanning agents have different bond strength and the binding
mechanism itself, which results in differences in the structure and properties of tanned
leather.
The chrome-tanned leather first underwent the pickling process where 1.6%–1.8% acid
(formic, sulphuric) and 5%–7% sodium chloride were applied, following by the tanning
process with 3.2%–3.6% basic chromium sulphate (commercially agent 25%–27%
Cr
2
O
3
; 330Sch). In the process of basification 0.25%–0.32% of basification agents
(sodium bi carbonate; commercial agents composed of low alkaline reactivity salts
mixture for obtaining pH 10.0–12.0) are used. All chrome-tanned samples went through
this tanning process (Cr, Cr-D and Cr-D-H).
For the synthetic tanning process 10% a synthetic organic tanning triazine-based
product was applied (Figure 4). This tanning process is applied on all synthetic tanned
Figure 2. Production and finishing scheme of the leather samples.
Kopitar et al. 8911S
samples (Syn-D, Syn-D-H, and Syn-D-CF). Sulfonate ions on the triazine compound
contribute to tanning effect and increase solubility of triazyne compound in water.
After the process of tanning, ageing, scraping and sorting the post tanning and finishing
processes follow. The main purpose of post tanning processes is to fill the hide structure
and give more uniform thickness and appearance, desired properties, appearance and feel
as well as to make a grain side suitable for further mechanical processing and finishing.
Figure 3. Crosslinking of chromium and collagen.
20
Figure 4. Schematic presentation of side chain crosslinking bridge of collagen with triazine
compound.
23
Post tanning and finishing processes.
8912S Journal of Industrial Textiles 51(5S)
Hydrophobing reduces the water permeability of the leather (reduced permeability of
the leather), whereby hydrophobic leather is permeable to air and water vapour. Finishing
is the final process in the leather production with the task to protect and improve the
quality as well as the appearance and touch of the leather. Leather achieves the final
desired appearance depending on the purpose and the desired properties of the final
product.
Samples Syn-D, Syn-D-H and Syn-D-CF went through the post tanning process
according to Table 2. After post tanning, the finishing agents such as aqueous dispersion
of aliphatic polyurethanes and polyacrylates, penetrators, pigments, crosslinking agents,
inorganic fillers, water-based resin and aqueous varnish were applied on the grain side of
the sample Syn-D-CF. The finishing agents were applied to the grain side (front) of the
leather using a hand spraying pistol (WALCOM; Slim Kombat, Italy, pressure 5 bar).
Samples Cr-H and Cr-D-H went through the post tanning process according to Table 3.
Test methods
Leather thickness was tested according to ISO 2589:2016, while thermal and water
vapour resistance were tested under steady-state conditions using sweating guarded-
Table 2. Agents for post tanning process of synthetic tanned leather.
Sample Syn-D Syn-D-H Syn-D-CF
Post tanning
Cr-free agents for post tanning (condensed phenol sulfonic
acid and and aromatic oxysulfonates; a combination of
synthan and phenolic compounds, natural fats and
vegetables tanning agents)
20–25% 20–25% 20–25%
Despersant agent up to 1% up to 1% up to 1%
Grease (combination of sulphited natural grease, synthetic
grease and grease based on acrylic polymers)
3–5% - 3–5%
Grease (a combination of sulphited lanolin and synthesized
grease)
-1–3% -
Polymer for hydrophobing - 2–4% -
Dye in portions - 1.2% -
Formic acid 0.5% 1.8% 0.5%
Greasing
Tanning and filling agent (based on fish oil, condensed phenol
sulfonic acid and aromatic oxysulfonates and acrylic
copolymers)
8–10% - 8–10%
Polymer for hydrophobing - 3% -
Polymer for post tanning based on acrylic copolymer - 1,5% -
Formic acid 3.5% - 3.5%
Metal salt - 6% -
Where “-”means the agent is not added in post tanning process.
Kopitar et al. 8913S
hotplate test according to the standard ISO 11,092:2014. The leather is placed on the heated
plate with the conditioned air ducted to flow along and parallel to its upper surface with a
speed of 1 m s
1
. For the determination of thermal resistance (R
ct
) the air temperature is set
at 20 °C, a relative humidity of 65% and air speed at 1 m s
1
. After reaching the test
conditions and establishing steady–state condition the recording of values is started. During
the determination of water vapour resistance on the surface of the porous plate a water
vapour permeable, but liquid-water impermeable cellophane membrane is fitted and kept
constantly moist by a water-dosing device. Test conditions for the determination of water
vapour resistance (R
et
) are air temperature set at 35 °C, a relative humidity of 40% and air
speed at 1 m s
1
. Thermal (R
ct
) and water vapour resistance (R
et
) of the leather is de-
termined as the arithmetic mean of the values of three individual specimens.
The leather morphology of the grain surface and cross section was observed with JEOL
JSM-35 CF scan electron microscope set up to an accelerating voltage of 20 kV with a
working distance of 15 mm. The samples were mounted on stubs with carbon tape and
coated with gold in a sputter coater Balzers 7120-A.
Results and discussion
The results of bovine leather thickness (T, mm), thermal resistance (R
ct
,m
2
°C W
1
) and
water vapour resistance (R
et
,m
2
Pa W
1
) with the corresponding indicators of the results
dispersion (standard deviation and coefficient of variation) are presented in Table 4 and
Table 5. The samples were taken from the back and neck parts of bovine leather, whereby
Table 3. Agents for post tanning process of chrome-tanned leather.
Sample Cr-H Cr-D-H
Post tanning
Agents for post tanning (hydrated sulphate salt of the chromium complex;
polycarboxylate, a synthetic tanning agent)
4–6% -
Sulphate natural oil for hydrophobing 1–2% -
Synthetic tanning agent (based on polycarboxylate, sulfone, resin) - 10–14%
Dye in portions - 4%
Greasing
Emulsion of natural and synthetic grease 2–4% -
Synthetic copolymer for hydrophobing 2,5–3,5% -
Sulphate salt of the chromium complex, in portions 2–3,5% -
Natrium salt of formic acid 0.5% -
Synthetic polymers (copolymer for hydrophobing, acrylic, silicone and
synthetic greases)
-2–5%
Formic acid in portions - 3%
Hydrated sulphate salt of the chromium complex - 2–4%
Sodium formate - 0.5%
Where “-”means the agent is not added in post tanning process.
8914S Journal of Industrial Textiles 51(5S)
B is the designation for the back, and N is the designation for the neck. The two-sample
t-test, to determine is there a statistical difference between thermophysiological properties
of bovine leather sampled from back and neck, is performed.
Chrome and synthetically tanned bovine leather of the back is thicker than that of the
neck, regardless of the finishing (Table 4.). This is a consequence of the quality, regularity
and uniformity of the bovine leather back microstructure, which depends on the density
and thickness of the fibrils. On the back of bovine leather, the fibrils are thicker and more
densely distributed as well as arranged more regularly (vertically) in relation to the neck,
giving the back a more uniform structure. A greater deviation in the thickness of bovine
leather was observed mainly in the neck area, which was expected regarding the pre-
viously given explanation of the neck and back microstructure. The synthetically tanned,
dyed and hydrophobic leather (Syn-D-H) deviates from this, where the deviation is
greater in the back area which can be explained by the unique structure of each leather.
Bovine leather thermal resistance and water vapour resistance
Synthetically (chrome-free) tanned leathers have higher thermal resistance compared to
chrome-tanned bovine leathers regardless of the finishing process but considering the
sampling point (Tabl e 5 ).
Comparing neck and back parts of equally finished leathers it can be seen that re-
gardless of the tanning and finishing process as well as thickness, the leathers from neck
parts have higher thermal resistance. The t-test showed that the significant difference of
thermal resistance between neck and back obtained only with synthetic tanned leather
(Table 6). The higher thermal resistance of the neck part can be explained by the different
leather structure of the back and neck parts. The neck parts have a greater proportion of
air-filled spaces between the fibrils which offer greater thermal resistance than the back
parts with a greater thickness of fibrils that are denser and more properly distributed. The
Table 4. Thickness of bovine leather back and neck.
Sample mark Thickness
X, mm SD, mm CV, %
Syn-D N 1.57 0.13 8.52
Syn-D-H N 2.01 0.06 2.76
B 2.22 0.17 7.71
Syn-D-CF N 1.51 0.11 7.37
B 1.57 0.04 2.50
Cr N 1.02 0.05 4.97
Cr-H N 1.40 0.14 10.08
B 1.44 0.11 7.88
Cr-D-H N 1.57 0.11 6.86
B 1.71 0.05 2.71
where: T is thickness, mm; X is average value, mm; SD is standard deviation, mm; CV is coefficient of variation, %;
N is leather taken from neck part of bovine leather, B is leather taken from back part of bovine leather.
Kopitar et al. 8915S
deviation from the above values is only visible for the chrome-tanned split leather which
has a significantly lower thickness.
Within the group of synthetically tanned leathers, the dyed leathers from the neck part
(Syn-D) have the highest thermal resistance since they contains more air in the interfiber
spaces, resulting in higher thermal resistance (Table 5,Figure 5).
9
Leather thermal re-
sistance was reduced by hydrophobing (Syn-D-H).
20
It should be noted that although the
thickness of the dyed and hydrophobically treated leather (Syn-D-H) is greater than that of
the only dyed leather (Syn-D), their thermal resistance is lower. The process of
Table 5. Thermal and water vapour resistance of bovine leather back and neck.
Sample mark R
ct
,m
2
°C W
1
λR
et
,m
2
Pa W
1
X SD CV, % W m
1
°C
1
X SD CV, %
Syn-D N 0.034 0.003 9.1 0.046 15.83 0.49 3.1
Syn-D-H N 0.031 0.003 10.3 0.065 30.38 3.55 11.7
B 0.025 0.001 5.5 0.089 26.96 0.43 1.6
Syn-D-CF N 0.027 0.001 4.7 0.056 21.77 2.68 12.3
B 0.022 0.002 7.5 0.071 22.26 2.46 11.1
Cr N 0.015 0.002 12.7 0.068 11.27 0.99 8.9
Cr-H N 0.023 0.004 17.5 0.061 20.41 2.03 9.9
B 0.019 0.003 15.7 0.076 18.93 1.94 10.2
Cr-D-H N 0.025 0.005 21.2 0.063 25.27 1.84 7.3
B 0.019 0.001 1.8 0.090 25.15 1.08 4.3
where: R
ct
is thermal resistance, m
2
°C W
1
;R
et
is water vapour resistance, m
2
Pa W
1
;λis thermal con-
ductivity, W m
1
°C
1
; X is average value, SD is standard deviation,; CV is coefficient of variation, N is leather
taken from neck part of bovine leather, B is leather taken from back part of bovine leather.
Table 6. F and t test for thermal resistance of bovine leather back and neck.
Sample
mark F-test t-test
VF
ca
l
F
0
(0.05) F
cal
to F
0
sd t
cal
t
0
(0.05) p
Syn-D-H N 9 × 10
6
9 19.0 < 0.0025 3.3 2.78 <0.05 (difference
B1×10
6
Syn-D-CF N 1 × 10
6
4 19.0 < 0.0013 3.89 2.78 <0.05 (difference)
B4×10
6
Cr-H N 1.6 × 10
6
1.8 19.0 < 0.0029 1.39 2.78 >0,05 (no
difference)>B9×10
6
Cr-D-H N 25 × 10
6
25 19.0 > 0.0029 2.05 2.78 >0,05 (no
difference)B1×10
6
Where V is variance, F
cal
is calculated F value, F
0
(0.05) is value for probability P(F>F
0
) = 0.05, SD standard
deviation of t-test, t
cal
is calculated t value, t
0
(0.05) value of probability (P(|t|> t
0
), p-probability value.
8916S Journal of Industrial Textiles 51(5S)
hydrophobing binds water repellent agents into the leather structure, which reduces the
binding of moisture. By reducing the moisture content in the structure, thermal resistance
increases, that is, thermal conductivity decreases.
21,22
The penetration of the dye and the
water repellent into the structure of the leather and the binding to the collagen fibres
pushes the water molecules out, reducing the moisture in the structure and thus the
conductivity of the leather (Table 5). In addition to reducing the water content, the
hydrophobing process binds a greater amount of fats into the leather structure, where a
greater fat amount increased thermal conductivity of the hydrophobic and dyed leather
(0.065 W m
1
°C
1
) compared to only dyed leather (0.046 W m
1
°C
1
).
21,23
The lowest thermal resistance within a group of synthetically tanned leathers has
leather with a face finish (Syn-D-CF). Comparing to dyed leather (Syn-D) with almost
equal thickness, their thermal resistances differ significantly. Finish applied on grain side
partially closes the face of the leather, which prevents the penetration of moisture and
water into the leather structure. It is assumed that agents applied by spraying on the grain
side of the Syn-D-CF sample reduced the thermal resistance and increased the thermal
conductivity to a greater extent, thus cancelling the effect of moisture in the sample. On
the contrary, although the thicknesses of dyed and hydrophobic (Syn-D-H) is significantly
greater then dyed and finished (Syn-D-CF), their thermal resistances are approximately
equal. It seems that the process of hydrophobing reduces thermal resistance to a greater
extent than the finishing of the leather face due to larger amount of fat in the structure.
20,22
In contrast to thermal resistance, the lowest water vapour resistance have synthetically
tanned and dyed neck part of leather (Syn-D) which can be explained by the untreated
natural face of the sample providing lower water vapour resistance.
20,22
The finishing
after leather dyeing increased the resistance to water vapour passage (Syn-D-CF) (Table 5
and Figure 6).
22
The significantly highest water vapour resistance of synthetically tanned,
Figure 5. Thermal resistance of synthetic tanned leathers sampled from the neck and back.
Kopitar et al. 8917S
dyed and hydrophobic leather (Syn-D H) can be explained by the hydrophobing process.
Waterproofing agents bind to the collagen fibres of the leather with functional groups
forming a hydrophobic film. Water vapour penetrates into the interfiber spaces where
vapour, due to surface tension, cannot wet the inner surface of the leather. This reduces the
water vapour permeability, that is, increases the water vapour resistance.
20
Same as synthetically tanned leather, chrome-tanned leather have higher thermal
resistance on neck part regardless of thickness and finish (Table 5 and Figure 7).
Neck part of chrome-tanned split leather has the lowest thermal resistance, and thus the
highest thermal conductivity. Split leather is characterized by higher permeability that can
be associated with the structure of leather split through the cross section.
Although chrome-tanned, dyed and hydrophobic leather (Cr-D-H) has a greater
thermal resistance regarding to only hydrophobic leather (Cr-H), if we consider the leather
thickness through the thermal conductivity it is obvious that Cr-D-H have greater
conductivity then only hydrophobic leather (Cr-H). Therefore, thermal resistance of
chrome-tanned, dyed and hydrophobic leather would be less with respect to chrome-
tanned hydrophobic leather of equal thickness.
Although thermal conductivity is closely related to the material moisture content, the
hydrophobing and dyeing process binds more fat in the leather structure (compared to the
pure hydrophobing process), which increases the thermal conductivity. A higher content
of fat added to the leather fills the inter-fibre spaces and improves the thermal conductivity
of leather. This effect is more visible on the back part of the leather. The back structure is
more uniform and compact, and a larger number of structural elements of the leather are
available for the binding of dye molecules and hydrophobing agents that also fill the free
Figure 6. Water vapour resistance of synthetic and vegetable tanned leathers sampled from the
neck and back.
8918S Journal of Industrial Textiles 51(5S)
spaces between the fibrils.
1
Due to the reduction of the pore size of the leather structure,
and thus the reduction of the air content in the pores, the thermal conductivity of the back
part is greater than that of the neck part.
Chrome-tanned neck parts of leather have higher water vapour resistance compared to
the back part, regardless of leather finishing. The reason is the same as for synthetically
Figure 7. Thermal resistance of chrome-tanned leathers sampled from the neck and back.
Figure 8. Water vapour resistance of chrome-tanned leathers sampled from the neck and back.
Kopitar et al. 8919S
Table 7. F and t test for water vapour resistance of bovine leather back and neck.
Sample mark F-test t-test
Var F
cal
F
0
(0.05) F
cal
to F
0
sd t
cal
t
0
(0.05) p
Syn-D-H N 12.60 68.16 19.0 > 2.054 1.66 2.78 >0.05 (no
difference)B 0.19
Syn-D-CF N 7.18 1.19 19.0 < 2.089 0.23 2.78 >0.05 (no
difference)B 6.05
Cr-H N 4.12 1.09 19.0 < 1.613 0.92 2.78 >0.05 (no
differenceB 3.76
Cr-D-H N 3.39 2.90 19.0 < 1.226 0.09 2.78 >0.05 (no
differenceB1×10
6
Where V is variance, F
cal
is calculated F value, F
0
(0.05) is value for probability P(F>F
0
) = .05, SD standard
deviation of t-test, t
cal
is calculated t value, t
0
(0.05) value of probability (P(|t|> t
0
), p-probability value.
Figure 9. Scanning electron micrographs showing grain view of the leather at ×130 magnification.
(a) Leather neck part, Cr-D-H. (b) Leather back part, Cr-D-H. (c) Leather neck part, Syn-D-H.
(d) Leather back part, syn-D-H.
8920S Journal of Industrial Textiles 51(5S)
tanned leather, namely, because of the differences in the structure of these parts of the
leather. Although, the values of water vapour resistance differ, the t-test showed that there
is no significant difference between neck and back parts of the leathers (Table 7).
The higher water vapour resistance of dyed and hydrophobic chromed leather
compared to only hydrophobing is explained by the additional influence of oiling in the
dyeing process of leather.
Synthetic and chrome-tanned leathers treated in the same way by dyeing and hy-
drophobing show considerable differences in thermal and water vapour resistance.
Considering their thickness, thinner chrome-tanned leathers have the same thermal
conductivity as synthetic ones. The effect of oiling when dyeing chrome leather is visible.
It was found that there is no significant difference in water vapour resistance of the
neck and back part of chrome tanned, dyed and hydrophobing leather in contrast to
synthetically tanned, dyed and hydrophobing leather in which the difference is
significant.
Figure 10. Scanning electron micrographs showing cross section view of leather at ×660
+magnification. (a) Leather neck part, Cr-D-H. (b) Leather back part, Cr-D-H. (c) Leather neck
part, syn-D-H. (d) Leather back part, syn-D-H.
Kopitar et al. 8921S
Bovine leather scanning electron micrographs
Scanning electron micrographs of surface and cross sections of chrome and synthetically
tanned leathers (dyed, hydrophobized) sampled from the back and neck at magnifications
of ×130 and 660x are presented in Figures 9 and 10.
The surface micrographs of the chrome and synthetically tanned leathers sampled from
the back and neck show similar characteristics, that is, the pore sizes have a similar shape
and size. This was to be expected, since the samples were equally finished, with the finish
applied to the grain side partially closing the face of the leather.
Scanning electron micrographs of the cross sections of the chrome and synthetically tanned
leathers show thicker and more densely distributed as well as more uniform arranged fibrils on
the back part. The neck parts have more empty inter-fibre spaces between the fibrils and a less
uniform structure that is more visible in the synthetically tanned leather cross section.
The micrographs are consistent with the obtained results, that is, the neck parts have a
greater proportion of air-filled spaces between the fibrils providing higher thermal resistance
than the back parts. Likewise, the more uniform and better arranged structure of chrome-
tanned leather provides less thermal resistance compared to synthetically tanned leather.
Conclusion
The back parts of bovine leathers are thicker than the neck parts regardless of the type of
tanning and finishing because the fibrils are thicker and more densely distributed and more
regularly arranged vertically. The sampling point (neck and back part of the bovine
leather) affects the leather thermal resistance in all examined leathers, regardless of the
type of tanning and leather finishing. The t-test showed that the significant difference of
thermal resistance between neck and back obtained only with synthetic tanned leather.
Synthetically (chrome-free) tanned leathers have a higher thermal resistance compared to
chrome-tanned bovine leathers, regardless of finishing, but taking into consideration the
sampling point.
Synthetically tanned leathers from the neck area have the highest thermal resistance, as
they contain more air in the fibre interstices and have a higher thermal resistance. The
thermal resistance of the leather was greatly reduced by the hydrophobic treatment.
In contrast to thermal resistance, the untreated face of dyed leather provides the lowest
resistance to water vapour. Water vapour resistance increases with the finishing of the
leather face. Hydrophobic leathers have the highest water vapour resistance.
Chrome tanned, dyed and hydrophobic leather has a higher conductivity than hy-
drophobic leather only due to the higher fat content in the leather structure. In addition,
oiling chrome leather during the dyeing process increases water vapour resistance
compared to only hydrophobic treated leather. Equally treated synthetic and chrome-
tanned leathers have the same thermal conductivity due to oiling during chrome leather
dyeing process.
The research results reveal an influence of the type of tanning and post tanning
processes on the thermophysiological properties of bovine leathers.
8922S Journal of Industrial Textiles 51(5S)
The presented study could help in choosing the appropriate sampling point, tanning as
well as finishing agents to achieve satisfactory thermophysical comfort in the application
of leather.
Acknowledgements
This is work has been fully supported by the Croatian Science Foundation under project no. IP-
2016-06-5278.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship and/
or publication of this article.
Funding
The author(s) received no financial support for the research, authorship and/or publication of this
article.
ORCID iD
Zenun Skenderi https://orcid.org/0000-0003-0758-1111
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