Patent Leather

Laminated Leather Soft polymeric (PU) laminates 1.5 ± 0.1 mm of thickness: microporous polyurethane membrane laminated to split leather surface by adhesive layer. Introduction: Physical and mechanical properties of leather can be varied depending on the applications of bags, footwear, etc. The principle and common requirement for leather must be very soft with considerable substance, extensible and moldability to shape for product. Generally after dying various finishing material and technique will done to improve its wear properties and also protect from wetting and soiling.

Abstract: Various multi component polymeric products containing materials of different nature and properties are used in consumer goods. One kind of these composite materials are the soft polymeric laminates used for leather products exploited in aggressive environment. Usually such soft laminates are composed of the layers of leather and microporous or hydrophilic film (membrane). Advanced soft polymeric laminates often combine a set of different properties, for example, water impact, temperature and chemical resistance, and simultaneously a high possible permeability of water vapours. A de-lamination is encommon and critical problem in laminated leather ‘while in process to product the leather has been treated with various chemical and different temperatures. Due to incompatibility between the lamination between the leather and PU will create the cracking and tearing.

On other hand a cyclic load is a very critical loading mode for laminates, because under fatigue failure occurs as the initiation and growth of a crack at loads significant lower than ultimate strength of the material. Multilayer laminates under fatigue loadings experience damage, consisting of a multiplicity of failure modes, such as matrix cracking, delamination and fiber breakage, with the increasing number of loading cycles these results in a continuous decrease not only of the residual strength, but also of the elastic modulus.

Due the low elastic modulus film and proportional elastisity of the leather is lesser than the film will facilitate the carcking

Consequently, the fatigue life of a multi component material can be determined by its loss in elasticity modulus, rather than catastrophic/physical failure. Materials, advanced laminates age, suffer physical or chemical degradation, accumulate micromechanical damage and become weaker during exploitation. Poor fatigue performance can significantly reduce the strength of multilayer composites, decrease the life cycle and compromise safety Microscopic cracks, which emerge at the beginning of exploitation, grow later to macrostructure damage.

The needle and stitch parameters on pigment-coated and polymeric film laminated leather behavior under biaxial loading have been investigated. The Simulation by finite element analyses details .the stress in the middle of a stitch is four to five times higher than stresses at the beginning or at the

end of the stitch. Besides, the increase of the stitch density increases stress values.

The crack formation and propagation depend on the stitch density and shape of the needle.

Patent leather / Coated leather/laminated leather: Laminated leather – Permair leather – used in this study was a commercial grade product. Laminated leather was obtained by hot pressing of microporous polyurethane (PU) membrane to ground-coated split leather surface. Film bonding was performed by water-born polyurethane adhesive/ solvent. Polymeric film provides protection against stuffs, stains and water, influenced on the mechanical properties. It comprises about 1/3 of laminate thickness. The structure of laminated leather and its comparison with pigment finished leather. Leather with polymer coating minimum 0.15mm thick but less than 1/3 of the total thickness on leather substrate conventionally knows as finished leather. There are classification depends upon the finishing, retanning & appearance.

If the coating exceeds 1/3 of the thickness

encommonly named as laminated leather/patent leather. To contrast in

finishing methodology, film thickness & appearance will classified into following type protein finish, pigment finish & upgradiation process. The patent comes under upgradiation, to laminating the leather by coating with polymer material frequently through PU .The thickness ranges from 0.25 mm to 4 mm. This is obvious difference between the leather & coated leather.

SEM view

Roots and causes for the Delamination and cracking: 1. Adhesion failure theory

2. Retanning process of split leather 3. Environmental factors: such has temperature and stress and strain during the stitching (biaxial load will exist during the stitching. 4. Water vapour permeability :

Adhesion failure Theory: Most adhesion failure is directly proportional to the poor preparation, transformation of the material like plasticizer and fat materials. Poor adhesive curing, Poor pretreatment of adherent surfaces were primary reason for the peel off and delaminating.

Classification of patent bonding failures: There are many different forms of “failure” not all of which are as a result of migration and material transfer & poor preparation. However, here are the general categories:

•

Surface film tear –As a result poor film strength

• Non-coalescence – Poor reactivation / fusion of adhesive or as result of aging adhesives

•

Adhesive peel from leather – insufficient preparation of the leather

• Adhesive breakdown – aged adhesive, incorrectly cured adhesives, incompatible adhesive types

Suggestion:

Plating or kiss plating will regenerate the behavior of film

forming material. (Appropriate temperature 100 impact on film forming molecule.

0

c with 5 sec) can do

Plating of leather during the course of finishing is an operation of fundamental importance as a guarantee of adhesion, good leveling of the film, and the final appearance and feel of the leather. General fastness also tends to increase considerably after heavy finishes are plated, because of the closed, continuous and compact layer that the thermoplastic film creates on the leather. 1. The adhesion property of the film can be regenerate by heating process. (Leather is obtained by hot pressing of the microporous polyurethane membrane to split the leather surface)

Retanning: In general patent leather is upgradiation process. Conversion of lower range to acceptable range frequently from chrome split leather. In retanning tanning material ratio to fat liquor is about 2:5. The fatliquor will fix chemically 80% and remain 20% unfixed or free to reason will mobile to some extent and above up to the finish layers. The mobility of fatliquor is lower than the plasticizer. The plasticizer apparently added during the finishing process to increases the flexibility of the film.

Mobility of the fat liquor molecule & causes in patent leather problem: Plasticizer: During the finishing this has been using to rectify the brittleness of the film .These material are organic additives having high mobility than the fatliquor molecule. This is primary compound for film flextiator in

polymer sheet and adhesive film. If the base sets to a glassy and brittle compound then it is likely to crack as the base shrinks due to the solvent evaporating. Plasticizers’ are often added to the adhesive

1,This is primary reason to peel off finishing coat from the leather .however the leather had rough surface difficult to maintain the cohesive strength between adhesive and leather • Film peel from upper – insufficient preparation of leather.

2. In ageing the plasticizer will cleave the physically dyed molecule and create the stain/ dye migration in between leather surface & finish film of the patent leather. (The same pink color had staining problem in previous order plan). Chrome tanned split is common substrate with PU coat will enhance the scuff resistance. Coating can be done by straight or with release paper. PU coat are either almost solid even though it appear like micropores or micro cellular structure. This microporous remains & elevated on deep embossing to an extent considering the depth before the embossing of patent leather is vital important

Environment factor: It was well known that at fatigue loading the environment temperature has a great influence on mechanical properties of elastomeric PU coating film. Tensile strength of PU film reduces twice when the environment temperature decreases from 20°C to 0°C , some may (about 40%) fractured when flexing at - 10°C and all the samples – at -20°C.

The load-elongation and recovery behavior of leather will show considerable Differences for the reason of differences structure. Especially, differences in the long-term properties show up in the case of laminated leather due to the rearrangement and reorientation at the interface between layers.

Needle influences in product quality:

The force of leather

perforation, the disturbance of material or threads and heat of the needle are affected by the shape of the needle spike. These factors significantly influence wear properties and the esthetical view of footwear or other products made from leather.

The needle and stitch parameters have a significant influence on pigment Coated and polymeric film laminated leather and polyurethane film behavior under biaxial loading. Needle shape, stitch density and the nature of the stitched material significantly influence the principal stress distribution in leathers and PU film during membrane deformation. Stresses reach their maximal values in the middle of the stitch. Stress concentration increases as

stitch density increases. Investigations of the influence of the needle and stitch parameters on mechanical properties of leathers and polyurethane films during punching show that crack formation and propagation during punching depend upon the nature of the investigated materials and on the shape of pricks. The strength of materials is higher when prick is oval shaped. The punching strength and elongation of the leather at break slightly increases when prick density is low. Punching strength of PU film decreases As prick density increases.

Recap of the patent leather issue: 1. Delaminating base film & breakage of top film: Especially in the pressure existing area, prominent with 10 tkt thread.

2. The peel of problem shows obviously the film adherent issues to leather between the base & top film 3. Considering the base coat has no transformation between the leather & PU base film 4. Top film tearing easily without extending & add on pressure.

Recommendation for patent leather to avoid the issue’s later on: 1. Like resting panel. 2. Have to avoid the #10 thread stitch. 3. The welting/piping having no effect in complete bag (good way to use it but minimizing the closing construction stitch will increase the durability) 4. Needle point is vital concern: Neither narrow wedge point nor ball point will not solve rather it will increase the chances to peel off. Acute round point will help.