Chapter 1: Introduction

Background of the Study

Leather finishing is a sequential process performed in the final phase of
leather production, in which coatings are applied to define the final
appearance and durability of the material[1]. The primary component of a
leather finish is a polymer binder dispersed in water; this binder forms a thin
film on the leather surface that traps pigments and protects the leather.
Polyurethane (PU) and acrylic polymer dispersions are the main types of
binders used in modern finishing systems[1]. In particular, acrylic binders
(polyacrylates) are widely used in leather finishing because they provide a
good balance of properties: they produce soft, flexible films with high block
resistance, excellent adhesion, and good UV and light stability, all at
relatively low cost[2][3]. In fact, acrylic polymers are prized for imparting
softness and stability to finished leather, making them economical and
effective finish components[3]. (Other synthetics such as polybutadiene and
different PU chemistries are also used, but acrylics remain common for
topcoats and pigmented finishes.)
Leather is inherently breathable and moisture-managing, which contributes
to its comfort. It can absorb a large amount of moisture without feeling wet,
and the collagen fiber structure contains many air-filled pores[4]. This means
leather has high water vapor permeability (WVP), often referred to as its
“breathability” – the ability to transport water vapor (gaseous H₂O) through
the material[5]. High WVP is crucial for comfort in applications such as
footwear and garments, because it allows perspiration vapor to escape from
inside the leather to the ambient air[4]. In fact, leather’s ability to exclude
liquid water while still permitting air and vapor transfer (i.e. being water-
resistant yet breathable) underpins its wide use in shoe uppers and
upholstery (no synthetic material has matched this combination)[4].
However, treatments that increase hydrophobicity or seal the pores can
greatly reduce WVP. Classic finishing treatments (often called waterproofing)
can fill or coat the fiber interstices. Such “closed” waterproofing can
eliminate moisture permeability, whereas “open” waterproofing
(impregnating fibers) tries to maintain vapor flow. As noted by industry
sources, any finish that renders leather more water-repellent typically risks
reducing its WVP (and thus comfort) unless carefully controlled[6][7].
In addition to breathability, leather must meet demanding physical
property requirements for its intended use. Mechanical strength tests such
as tensile and tear resistance are standard in the leather industry (ISO and
ASTM protocols exist for these)[8]. High tensile strength and tear resistance
are needed for durability in garments, saddlery, and automotive interiors,
while good abrasion resistance and flex-crack resistance are critical for
footwear and upholstery that undergo repeated bending. For example, ISO
standards exist for measuring tensile strength (ISO 3376) and tear strength
(ISO 3377) of leather[8]. Flexibility (ability to withstand many bending
cycles) and abrasion resistance are similarly key metrics (e.g. ISO 5402 for
flexing resistance). Thus, any finish must not only look and feel good, but
also maintain or improve these mechanical properties. In practice, polymer
coatings (like acrylic binders) often increase surface durability (good rub
fastness and hardness)[7], but they can also stiffen the leather or make it
less breathable if over-applied. Balancing these effects is a core challenge in
finish formulation.

Problem Statement
Although acrylic polymer dispersions are widely used in leather finishes, their
detailed impact on the balance of performance properties is not fully
understood. Many studies have examined individual aspects of finishing (e.g.
improving wet rub fastness or film adhesion), but there is a lack of
comprehensive data on how varying the amount of an acrylic binder in a
finish formulation affects both breathability (WVP) and a range of physical
properties simultaneously. This is a practical concern because leather
products (especially footwear and wearable goods) must remain comfortable
(requiring high WVP) while also being durable (requiring good tensile, tear,
abrasion, and flex resistance). Previous work indicates that applying an
acrylic finish can significantly reduce leather’s water vapor permeability
compared to unfinished leather[6][9]. At the same time, increasing polymer
content typically increases film continuity and hardness, improving
properties like rub resistance and strength[7][9]. However, these trends can
conflict: for example, enhancing water repellency or adding more binder
tends to lower WVP[6][9], while achieving high mechanical fastness may
come at the cost of breathability. Thus, there is a gap in understanding
the trade-offs and interactions in acrylic-based finishes. Addressing this gap
is important for the leather industry: without clear guidance, formulators
may struggle to achieve the optimal finish, leading either to uncomfortable
(poorly breathable) leather or leather that wears out too quickly.

Aim of the Study

The aim of this study is to experimentally investigate how the concentration
of an acrylic polymer dispersion in a leather finish formulation affects the
water vapor permeability and key physical properties (tensile strength, tear
resistance, abrasion resistance, and flex resistance) of the finished leather.

Objectives of the Study

 To prepare a series of leather finishing formulations containing
increasing proportions of a selected acrylic polymer dispersion.
  To apply these finish formulations to identical leather substrates (with
one set left uncoated as control).
 To measure and compare the water vapor permeability (WVP) of the
coated and uncoated leather samples.
 To conduct standard mechanical tests (tensile strength, tear strength,
abrasion resistance, and flex resistance) on the finished leathers and
compare them to the control.
 To analyze the relationships between acrylic binder content and each
measured property, thereby identifying any trade-offs or optimal
formulations.

Research Questions
 How does varying the acrylic binder concentration in the finish affect
the water vapor permeability (breathability) of the coated leather?
 What is the effect of acrylic binder content on the leather’s tensile
strength, tear resistance, abrasion resistance, and flexural endurance?
 Is there a trade-off between improving mechanical durability (e.g. rub
fastness, strength) and maintaining high WVP as binder content
increases?
 Which formulation (binder concentration) provides the best balance of
water vapor permeability and physical performance?
 How can the findings inform practical recommendations for formulating
leather finishes for comfort-sensitive applications?

Justification of the Study

Understanding the effects of acrylic finishing on leather performance is
practically significant for leather technologists, manufacturers, and material
scientists. Leather is prized in consumer products for its combination of
comfort and durability[4]. For applications like footwear and clothing,
maintaining breathability (and thus wearer comfort) is critical, but finishing is
also needed for appearance and durability. This study will provide
quantitative data on how finish formulation choices influence both comfort-
related properties (WVP) and mechanical strength. Such information can help
manufacturers optimize finish recipes to meet product specifications. For
example, shoe leather producers need to know how much polymer can be
applied without compromising breathability, and upholstery makers need
guidelines to ensure abrasion resistance without making the leather overly
stiff. By bridging the knowledge gap on these trade-offs, the research will
enable more informed finish design and quality control. In turn, material
scientists gain insight into the interaction between polymer films and the
collagen structure, which may guide the development of new binder
chemistries. Supporting literature emphasizes that WVP and fastness are
often opposing goals in finishing[7], so this study directly addresses an
industry-relevant problem by seeking an optimal balance.
Scope of the Study
This research is experimental and focuses on finished leather performance
under controlled laboratory conditions. The study uses one type of leather
substrate (e.g. a crust leather from a chosen animal source) and a
commercial acrylic polymer dispersion as the binder; results are specific to
these materials. Only the binder concentration in the finish is varied (with all
other additives held constant) to isolate its effects. The properties
investigated include water vapor permeability (as per relevant ISO/DIN
standards) and a broad range of mechanical attributes: tensile strength, tear
strength, abrasion resistance, and flex resistance (each measured by
standard test methods). Other finish characteristics (such as color, gloss, or
specific chemical composition of additives) are not considered. Similarly,
long-term aging or environmental exposure effects are beyond the scope.
The aim is to evaluate the initial performance of the different acrylic-based
finishes. These bounds ensure that conclusions are directly tied to the
experimental variables, recognizing that different leather types or binder
chemistries might yield different outcomes.

References
Keskin, S., Cheaburu-Yilmaz, C. N., Altinisik Tagac, A., Darie-Nita, R. N., &
Yilmaz, O. (2025). Synthesis of acrylic–urethane hybrid polymer dispersions
and investigations on their properties as binders in leather finishing.
Polymers, 17(3), 308. https://doi.org/10.3390/polym17030308[1][2]
Leather International. (2001, September 18). Manufacture and care of water
vapour permeable leather. Retrieved from
https://www.leathermag.com/analysis/manufacture-and-care-of-water-
vapour-permeable-leather/[4][6]
Ugbaja, M. I., Ejila, A., Mamza, P. A. P., Uzochukwu, M. I., & Opara, H. (2016).
Evaluation and application of acrylic based binder for leather finishing.
International Journal of Innovative Research in Science, Engineering and
Technology, 5(4), 4635–4644.
https://doi.org/10.15680/IJIRSET.2016.0504005[3][10]
Leather Naturally. (2023). Current international standards for leather testing
[PDF]. Leather Naturally. Retrieved from https://www.leathernaturally.org[8]

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[8] Modern Cow Leather Processing
https://www.leathernaturally.org/wp-content/uploads/2023/02/LN-
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