LEATHER FINISHING PROCESSES
1. INTRODUCTION
It is not possible to gain the end-use properties of the skin with wet processes after tanning and tanning. It is aimed to improve some features of the leather, increase its durability, increase the usage area and provide ease of use and support its aesthetic with some applications applied to the leather whose wet processes are finished. All processes in this direction are defined as finishing processes.
In this process, bringing the leather fiber to a certain moisture content and stretching it, increasing the usable area of the skins, gaining a certain form, adjusting the thickness, ensuring the surface smoothness, improving its appearance with printing and pattern applications, ensuring the dye and color smoothness, painting the desired color, developing its attitude, ready for sale by packaging. making and storing applications are carried out.
Definition of Finishing
Finishing covers the finishing of leather production. In the enterprise, this section is the section that improves the use of leather, increases its evaluability and provides appropriate features to the end use area. The staff in this section should be rational and skillful and be able to use their practical intelligence. It is necessary to know that the profitability of the factory depends on successful applications in finishing and to adjust the work flow accordingly.
The aim of the finishing is to give the skin appearance, fastness and durability, and to offer the customer a pleasant and attractive product. In other words, it is to give skins the appearance, attitude and physical features that can meet different demands in the world markets which are constantly changing. Differentiating the demand in the market with the fashion element and the demands that arise in terms of different leather properties day by day increase the importance of finishing day by day. Accordingly, the finishing process; It can be defined as all of the physical and chemical processes carried out in order to give the final properties of the dried leather by making tanning processes, to provide quality appropriate to the place and purposes of use and to support the physical strength values .
The most important factors affecting the appearance and quality of the finishing are the combination of finishing agents with a wide range of chemical structures and properties in leather finishing and performing various mechanical applications. In addition to this, the differences in the properties of the animals from the animal life and the applications and technological possibilities for wet processes on the leather cause the appearance of leathers with various quality features, which causes a variety in the applications and chemicals used. If the changes in properties depending on the difference in usage areas and customer demands are added to these, the importance of the applications to be made in the finishing process and how large the variation possibilities in the materials that can be used will become clear. Also; fondness for innovations and changes in users,
The engineer working in the finishing department will firstly rivet his success by knowing the properties of the leather and the previous applications, knowing the structure and properties of the chemicals to be used, planning the applications to be done, making preliminary experiments on the samples that will represent the party and using the finishing machinery and equipment correctly.
History of Leather Finishing
Coating of materials for protection purposes dates back to very old times. The surface coating for this protection is the basis of the finishing process. The treatment of the surfaces of treated leather with protective materials in this way was an application even in ancient times and these applications are understood from the appearance and forms of various leather goods and clothes exhibited in museums. For this purpose; Various waxes, milk, eggs and blood proteins, various oils and shellac were used in the early applications. Today, some of these substances can still enter finishing formulations.
XX. In parallel with the developments in the chemical industry in the first quarter of the century, studies on today's finishing technology have been started and the application principles of finishing have been established in the first half of the last century. The main need in this regard was dyeing the skin. Until the First World War, natural and synthetic dyestuffs were used mostly in dyeing of leather. The supply of aniline dyes was difficult during the war years, so some finishing companies chose to dye the skin with the help of pigments. In order to ensure the binding of pigments on the skin, they mashed the dyes in this powder form with viscous casein solution in the mills.
By making this kind of pigment dyeing, it was seen that the skins containing surface defects and dyeing irregularities were almost as smooth as the skins without surface defects, and this application increased the merchantability of the leather. In addition, better evaluation of these low quality leather has made leather production more profitable. These approaches are very important in terms of directing the finishing process to the dimensions we understand from its current definition. In today's sense, finishing started in 1931 with BASF producing and selling polyacrylate dispersions for leather finishing. The production of these polymer dispersions was expanded by other chemical industry companies in 1935, and binders with different properties were introduced to the industry, and these developments led to a breakthrough in leather production. In the initial stages, leather skins with high elongation ability were finished with these flexible binders. Later, with the discovery of binder and other finishing chemicals of different properties, finishing applications showed great improvements in the last 50 years.
The Importance of Leather Finishing
After tanning, some features of the leather should be improved and presented to the customer with a more beautiful appearance. Even if the colors of the finished leather after tanning are brought to the desired color tone by dyeing, this color is not sufficient in terms of uniformity, brightness and vitality depending on the characteristic features of the leather. In addition, since the customer orders are given as number, total area and uniform thickness value, such characteristics should be adjusted within the lot.
As can be understood from the previous definition of finishing, the finishing covers the implementation of a series of finishing applications for the surface and structure of the leather. Especially the applications to be made on the leather surface increases the value of the leather and provides its economy in its use with the subsequent processes of the leather. Applications to the surface can be applications aimed at protecting the skin against external factors and increasing its lifetime, as well as improving the surface touch of the skin and providing surface effects for appeal. In this sense, finishing reflects the latest technological developments in polymer films to be applied on the surface. In this process, the surface of the skin is covered by using acrylic and vinyl polymers and copolymers and urethane emulsions,
Since the manufactured leathers will be subjected to ready-to-wear garments after this stage, cutting the combined leather pieces from different parts, with the aim of minimizing the skin losses and clippings, requires keeping the quality of the asort in the foreground. The attitude and appearance homogeneity of all pieces of leather garment will increase the sales value. The fact that a leather jacket is made of an average of 40-45 square feet of leather and consists of many pieces also reveals how difficult it is. Therefore, finishing is a very important process that improves the merchantability of the leather. With its mastery and dexterity, the finisher can produce the leather suitable for demand by applying the most appropriate finishing according to the characteristics of the leather.
Recently, developments have been observed towards the emergence of more intensive new forms of application in the finishing process. There is no doubt that changes in the breeding conditions of animal breeds in the world and deterioration in skin quality are also effective in this. When evaluated from this aspect; it is foreseen that the finishing process will become a much more important step in the coming years.
Features of Finishing
For many years, leather finishing has been evaluated as an area that is very complex, difficult to understand and requires expertise and practical knowledge. Many mistakes made in wet processes can be covered with careful work in finishing, and as a result of mistakes made in the finishing of a leather which is very high quality from wet processes, bad appearance and low fastness properties appear. We can explain some of the modified or modified properties of finished leather in the finishing as follows:
a) The attitude characteristics can be adjusted for the usage area of the skin,
b) To ensure that the appearance of color and brightness or opacity are compatible with the order,
c) Improving the binding of finishing layers to the skin in dry and humid conditions and protecting the skin,
d) Ensuring that the finishing is able to keep pressure and maintain its printing feature,
e) Providing softness and flexibility,
f) Improving the molding properties,
g) Increasing the resistance to wet and dry rubbing,
h) Preventing the cracking of the finishing floor in the cold,
i) Providing impact and wear resistance to the finished leather,
j) Prevention of property changes at any temperature,
k) Sweat fastness,
l) Light fastness,
m) Prevention of heat resistance and thermal yellowing,
n) Providing resistance to chemical substances,
o) Providing resistance to home and body care products,
p) To be able to preserve skin properties for a long time.
In addition to such features, the necessity of working with many objectives makes the applications of the finishing process more complicated and makes it imperative to focus on it.
II. DRYING AND MECHANICAL PROCESSES
A. DRYING
After all the stages of wet processes are completed, the skins must be dried and subjected to some mechanical processes to be prepared for subsequent finishing processes. Since drying is generally done under the influence of temperature, drying conditions should be adjusted according to the tanning process in order to ensure efficiency. The maximum temperature should not exceed 30-35 o C for vegetable / synthetic tanned leathers and 60 o C for chromium tanned leathers to prevent adverse effects on skin properties .
Drying is simply the transfer of water stored in the skin to the outer environment and atmosphere. Drying is briefly an evaporation event. The simplest form of this process is its application in open air or in circulating areas such as porch. The sections used as drying rooms in the factories are arranged to provide air flow. In more technical planning, the necessary equipment is used for heating the air. Since the drying conditions and time must be adjusted depending on the weather, when it is desired to be carried out in a more controlled manner, humidity and heat meter operating the fans that provide the air circulation by controlling the humidity continuously and the heating mechanism providing the controlled temperature increase are also used.
The technique used in the drying process affects the properties of leather such as attitude, touch, softness, area and thickness and plays an important role in the formation of the skin color. It is also one of the most important processes affecting the finishing process. Therefore; At the beginning of the drying process, the structural properties of the leather and the effectiveness of tanning and lubrication should be checked.
Tanned leather fibers counteract effects such as compression and stretching with a pronounced elastic behavior. A certain shrinkage occurs in the skin fibers when it is brought to 14-25% humidity level uncontrolled while drying. If the skin is kept free in these conditions, it should be expected that greater shrinkage will arise in the area, and less shrinkage occurs in the direction of thickness. Therefore, drying the skins freely gives tighter skin properties. Narrowing of space is limited in nailing, ratcheting and pasting drying or vacuum drying.
AA. Convection Drying
Air can hold a certain amount of water evaporating from the skin. In the process, the ambient air is circulated with the help of a fan and the water holding capacity limit value is increased when relocating the fresh and dry air. Again; In order to increase this limit value, the ambient temperature is also increased. It should be noted that the air at 50 o C will hold 10 times more water than the air at 10 o C. Therefore, the increase in heat will encourage evaporation.
It is possible to utilize all these factors effective in drying in drying tunnels specially produced for this purpose. Effective factors such as air circulation, temperature and humidity are kept under control in this type of drying tunnel. In drying tunnels, working according to reverse current system is applied. Fresh, dry and hot air humidity level is sent on the leather approaching the drying rate. When the age entering from the other end of the tunnel affects the skin, it becomes completely damp.
BB. Hang Drying
In hanging drying, it is usually carried out by hanging the skins on the hooks and nails specially made from the hind legs as they are hanged in the laundry drying. Although the basis of the method is drying in the open air, this process can also be carried out on the sheds and even in the drying rooms with simple drying equipment. Although it is a very cheap method, as mechanization and automation increase, cost increase occurs. Since there is no stretching in drying, shrinkage and loss of space arise areally. More stretchable leathers are obtained after drying with the effect of this areal change in the skin. That's why; The method is used in the production of gloves, garment and leather leather type leather where softness and stretchability are important. Today, the suspended drying process has been mechanized by making use of semi-mobile and mobile systems. The skins hung on the conveyors are dried slowly on the roof of the factory and sometimes passed through a drying cabinet in certain parts of the conveyor in order to speed up the process.
CC. Tension Drying
Depending on the technology changing from the past to the present day, two types of tension drying are applied. Generally, it is a process that is performed to give permanent skin and form to the skins that have been dried, after the burn, and is used to dry the moisture retained in the skin.
AA. Fastening: It is the simplest method to remove the water in the skin pores by stretching and opening well. Leathers stretched in this way are nailed onto wooden plates. Nailing and stretching allows the leather to gain permanent space. The work in the form of a lighter gives flat and flat leather. In addition, the fastening applied to open such a shrunken area makes the leather thin and tight and encourages the appearance of hollow structures in the skirt. It is mostly used in the production of vegetable tanned light leathers and today, especially in the drying of head leather. Drying occurs slowly and is faster than the surface striking the wood on the air exposed surface.
bb. Tensioner Drying: In principle, it is similar to the fastening process. However, instead of the wooden plates, the grills of the tensioner are used. These grids are made of thick wire mesh or metal holes with holes drilled. The skins are attached from various parts with spring pegs and are pulled in various directions and stretched to the holes on the grates. Some leather types such as furniture and upholstery, especially when wet, are applied directly to the tensioner process, resulting in a good area gain and wrinkles are reduced. Working on a tensioner has some advantages over the application of wooden planks. These;
a. Air circulation is better on both sides of the skin, so drying takes place faster.
b. It has a longer life.
c. It has the possibility of clean operation. It is easier to move.
D. The risk of fire is lower.
Tension grids are used in drying tunnels or in rooms heated by stoves. Various techniques have been considered to reduce labor costs in the tensioning process. In one system, the tensioner grid is divided into two. The skins are lightly latched onto it. The skins are fully stretched as a result of the separation of the two parts of the stretch grille with the mechanics. This system is a technique used in the final drying of garment leathers after dry closet and knurling.
With the help of automatic tensioning machines, which are widely used today, the skins are dried more efficiently in the tensioner with less labor and permanent area is gained. In tensioning machines operating on more complex systems, leather is spread with only one operator and the skins are stretched and sent to the drying tunnel with the help of the machine's sensors.
The most important aspect of the tension drying systems is that it allows the skin to be opened by stretching it in order to maximize the efficiency of the area where the leather is sold. Tension drying provides up to 15% space increase in some cases. An important factor here is the loss of area in pruning to remove the pegs that appear in the stretching of the skin.
DDR. Pasting Drying: A glass plate is covered with thin paste and the moist leather is spread on it. The skin part is adhered to the glass. In this way, the leathers that are completely adhered to the plates are dried at 50-60 o C in hot air flow . There is a certain area gain according to the hanging drying. In this way, shrinkage during drying is prevented. Generally, it is applied to sanded and firmly structured upper leathers. The property of the adhesive used is important. When skin is removed from the plate, the adhesive should remain on the glass plate and be easy to remove.
It is necessary to adjust the viscosity of the cake to maintain adhesion during drying. The degree of adhesion; It is taken to the control horse by adjusting the concentration of the starch (usually at 1% starch) or by adding a small amount of special oils or excipients.
EA. Secotherm Drying: This method is lower in use compared to pasting drying. Secotherm drying is carried out in double-skinned glass, stainless steel and enamel panels. The interior of these is heated with the help of hot water, hot oil or electricity. The advantage of this method is that it reduces the drying time. But; It is an intensive workmanship for operators tasked to spread skins on panels that are constantly heated.
FFA. Vacuum Drying: This drying method is based on the easier evaporation of water that boils easily under low air pressure at low temperature. The wet skins are spread on a flat surface metal plate made of stainless steel, which is heated by taking care not to leave any folds with the help of a glass spatula with a stiff end. Then the top plate, which is covered with felt and can form a vacuum, is closed so that air does not leak. While the lower plate is heated to 70-80 o C, a vacuum equivalent to 50-60 mm mercury column is created in the upper plate.
Leather; The felt that is spread on the metal plate and pressed on them is easily dried by conduction under vacuum. It has 5-10% area gain compared to air drying. Thin and fairly flat skin occurs. Since the drying is in a very short time, the problem of tanning agents and oils migrating to the surface does not arise. The skins are dried in such vacuum compartments, depending on the preset vacuuming, the heating temperature of the metal plate, the moisture content and thickness of the skin. Drying does not vary from leather to leather, from batch to batch. Skin properties such as area gain, softness and thickness on the skin are affected by the degree of vacuum and pressure applied and the temperature.
GG. Infrared Drying: In the drying process, heat is provided as infrared heating or radiant heating. In this process, heating is provided by tubes heated by catalytic oxidation of gases such as propane or butane or electric infrared heaters. This heating provides fast heat transfer and can give high temperatures. Extremely high temperatures can burn the skin or the thermoplastic finish may become too soft or sticky. The main use of this method is in the process of drying and removing the water sprayed with the finishing layers. In the process, the skin is wetted on only one face, which increases the usability of this method. Rather, it is used in combination with conveyor drying tunnels and finishing application systems.
HH.High Frequency Drying
The wet leather is passed through 2 metal sheets combined with a generator that produces high frequency alternating current (27 MHz, 12Kw). The advantage of this method is that the final amount of moisture in the skin can be precisely adjusted. It is not too economical to dry wet leather completely. But; It is a very suitable method for re-drying the dried skin after the applications.
DRYING ERRORS
Drying in leather production is one of the process steps in which important mistakes are made and unwanted changes in leather properties occur. Because it shows feature changes in different humidity conditions. It should be known that especially the strength and performance properties depend on the moisture and moisture content of the fiber. For this reason, the quality of the finishing depends on the effective drying of the skin. The most common skin faults and defects in drying are as follows.
a) Loss of Skin in the Skin: In drying the skin freely, a shrinkage of 5-15% is observed depending on the type of tanning. This is due to the uncontrolled and rapid evaporation of moisture within the skin. Especially high temperatures towards the end of the drying process have more effect of loss of space than the high temperatures at the beginning of the process. Although the loss of area can be prevented to some extent with the openings and stretching in the mechanical processes of the finishing, it is not possible to correct it completely. It is necessary to choose more moderate drying temperatures and control the drying conditions absolutely.
b) Hardening and Brittleening of the Skin: Using the excessively high drying temperature and drying the leather for a long time under these drying conditions, at the same time, the leather is not able to show the expected soft and elasticity and deterioration of the mechanical behavior as a result of processing the leather by using one of the tanning methods that do not have high heat resistance. To prevent it, extreme conditions should not be tried in order to keep the drying temperature and other conditions at a reasonable level and to speed up the process. In addition, it is important to make a suitable lubrication to prevent tanning and subsequent hardening and to keep the moisture in the skin.
c) Distortion of Surface Color: The use of highly unsaturated lubricants or non-heat resistant vegetable retanning agents that have a tendency to turn yellow in the production processes of the skin causes undesired surface color changes in drying. In order to prevent this, it is absolutely necessary to examine the properties of the retanning and lubricants and their behavior in drying conditions.
d) Dark Color and Yellowing Zones: These are the appearance defects that occur due to the migration of the insoluble substances in the paint float from the skin towards the outer regions, especially during hanging drying. This problem is known as migration and is frequently encountered. It is caused by poorly bound or suspended chemicals. To prevent the problem, it is necessary to guarantee a good intake of the dye bath with sufficient acidification and fixation and, if necessary, a short wash after dyeing.
III. BASIC PRINCIPLES OF FINISH
It is important to explain the basic principles to be considered in this process before explaining the chemicals and applications used in finishing. Finishing process; It is a process that consists of different layers and between these exercises, some mechanical applications are carried out that will ensure good attachment to the skin and improve the attitude, appearance and fastness values of the skin. Applications in finishing are carried out in the form of application of 3 different layers in terms of composition.
a) Primer solid
b) Paint layer
c) Finishing solid
Applications are made from these layers for the purpose of correcting the bottom color of the skin and cutting the absorption, closing the skin faults and forming a base that will allow the subsequent layers to be attached to the skin. While the primer coat was applied in the form of velvet application with the help of hand or machine, today it is made with a pistol.
It is aimed to eliminate the surface and color unevenness of the skin in the paint coat, to increase some fastness values, to create a surface that can keep the skin pressed, to increase the bonding of the dyes and auxiliaries used in the primer coat, to improve the appearance and to create a transition to the applications to be made in the finish.
At the end, efforts are made to improve the fastness values of the leather, to increase the physical strength of the finishing, to gain greasy, slippery, waxy and silky attitudes, as well as to adjust its brightness.
For finishing these purposes, one, two or all three of these layers are applied on the same skin as one or more applications. However, the final finishing layer that gives the finishing properties to the finishing is definitely applied and some properties are modified.
A good finish in leather production is a technological accumulation. It is never a very acceptable behavior to completely cover the skin and eliminate its naturalness. However, it is undesirable that the negativities and skin faults from the sub-processes remain completely clear. Accordingly, it is necessary to pay attention to the following points in order to make a very good finishing.
1. Thin finishing films should be applied to the skin until the desired result is achieved and it should be aimed to ensure that these layers are bonded to the skin by supporting them with mechanical processes.
2. The applied layers should be allowed to penetrate the skin and under the skin and should be worked towards not to spill it with friction.
3. The physical properties of the finishing agents used, such as softness and flexibility, should be parallel to those of the leather.
FINISHING TYPES
Technically, finishing can be classified in several ways. The criterion based on the classification gives information about the application and properties of the finishing. Accordingly, the finishing is classified as follows.
1. By Solvent Type;
a) Water based finishing,
b) Organic solvent based finishing.
2. By Connector Type;
a) Case finishing,
b) Nitro finishing,
c) Polyurethane finishing,
d) Acrylic finishing,
e) Butadiene finishing,
f) Combined finish.
3. According to the Application Method;
a) Velvet finishing,
b) Pistol finishing,
c) Roll-Coat finishing,
d) Curtain-coating,
e) Foam finish.
4. According to its appearance;
a) Antique finishing,
b) Aniline finishing,
c) Semi-Aniline finishing,
d) Opaque finishing (Pigmented finishing-covering finishing),
e) Lacquer finish.
5. According to Technique and Technology;
a) Classic finishing,
b) Splitting finishing,
c) Oil-Wax finishing,
d) Bulk finishing,
e) Crack finishing,
f) Polyurethane finish.
6. According to the Load;
a) Cationic finishing,
b) Anionic finishing,
c) Nonionic finishing,
d) Amphoteric finishing,
e) Combined finish.
7. According to Leather Type and Usage Area;
a) Garment finishing,
b) Shoe finishing,
c) Upholstery finishing,
d) Saddlery finishing,
e) Fur-suede finishing
f) Special purpose leather finishing
LEATHER FINISH CHEMICALS AND CLASSIFICATION
All chemicals and auxiliaries used to improve the attitude, appearance and fastness values of leather finishing processes are called finishing chemicals. These chemicals can be natural products or synthetic products. The most important condition of good finishing applications is to know the compatibility of these chemicals with the skin and their compatibility with each other. Their classification should be considered as a starting point in the recognition of finishing agents. Leather finishing agents can be divided into several groups according to the following basic classification.
LEATHER FINISH CHEMICALS
Colorants Binders Finishing Materials Finishing Aids and Feature Developers
1) Protein Based
1) Pigment Pastes 1) Water soluble colloids a) Casein 1) Filler (Filler) and Waxes
a) Inorganic Pigments b) Egg Albumin
b) Organic Pigments 2) Organic Solvent Soluble colloids 2) Cellulose Derivatives 2) Matting Widows)
a) Nitrocellulose Lacquers
2) Pigmented Lacquers 3) Disperse and Emulsion Systems b) Nitrocellulose Emulsions 3) Attitude Enhancers
3) Effect Paints 3) Vinyl Resins 4) Penetrators
4) Polyurethane Systems 5) Cellulosic Thinners
5) Polyurethane Dispersions 6) Crosslinkers
6) Polyurethane Nitrocellulose Dispersions
7) Waxes
8) Oils
1. Colorants
a) Pigment Pastes
aa) Inorganic Pigments
bb) Organic Pigments
b) Pigmented Lacquers
c) Effect Paints
2. Binders
a) Water Soluble Colloids: Albumin, starch and some synthetic materials, which give film by gelling, are applied in an aqueous environment and form a film by evaporation of water. .
b) Organic Solvent Soluble Colloids: Nitrocellulose and other cellulose ethers and esters, vinyl polymers, polyacrylates, some polyurethanes are applied to surfaces to be used in anhydrous environment. The most common types of these are finishing agents called nitrocellulose lacquers, which are applied in anhydrous environment and have subsequent water resistance. Nitrocellulose lacquers were the first finishing agents used in pigmented finishes in many skin types in the 1920s.
c) Disperse or Emulsion Systems: These are systems where binders are not dissolved in water but are dispersed as a stable suspension of small droplets. They form films with coagulation and coalescence.
3. Finishes
a) Protein Based
aa) Casein: 1-2% alkaline solution is used. It gives a good shine to the leather with its polishing process. In addition, since the transparency and natural appearance of the leather is in the foreground, it is often preferred in finishing applications. It has sufficient skin filling feature.
bb) Egg Albumin: Its solution prepared in 1% concentration in cold water is used in finishing. In addition to having very good glossiness, it has very high polishability but weak skin filling feature. In thick films, stiffness and brittleness also increase proportionally. It becomes insoluble by heating.
b) Cellulose Derivatives
aa) Nitrocellulose Lacquers: It is used by diluting in ester or ether solvents. The film has very good water resistance. Its bonding is often better than water-based emulsions. It has special attitude characteristics.
bb) Nitrocellulose Emulsions: It is used by diluting with 50/50 water. It has the characteristics of water resistance, wet rub fastness, brightness. In addition, it provides non-sticking to the leather in the ironing process.
c) Vinyl Resins
It is solvent based. Although it has very good wet rubbing and fastness values, it also has bending and abrasion resistance.
d) Polyurethane Systems
They are used on a solvent basis. They have high gloss and moist appearance. It has washable and durable properties.
e) Polyurethane Dispersions
They are used by diluting with water. They have the advantage that they have better wear resistance than nitrocellulose lacquers and nitrocellulose emulsions. Its water resistance and flexibility are permanent.
f) Polyurethane and Nitrocellulose Dispersions
They are used by diluting with water. It has similar properties to polyurethane dispersions. But; attitude characteristics are better.
g) Waxes
They are used in some special-purpose finishes to give them an oily and waxy feel. They are also used in ironing processes to prevent sticking.
h) Oils
They are used in some special purpose finishes. By causing the surface to acquire oily and blackened appearance, they provide different attitude and appearance features.
4. Finishing Aids and Feature Developers
These substances are used both in the application of the finishing and in improving the appearance and attitude of the finishing in the skin.
a) Elephants (Fillers) and Waxes
They are added to finishing formulations in small amounts. Different attitudes and features are gained depending on the type of skin, the amount used, the application and the residence or penetration of the skin. They give flat and flat surface appearance.
b) Dulling agents (Widows)
They are added very little to the finish. Therefore, the main component of the finish is not accepted. These chemicals are fine dispersions of silica. They give a full and flat surface appearance next to the desired opacity.
c) Attitude Developers
They give the desired surface attitude to the skin. These are auxiliary materials used in obtaining smooth, coarse, flexible, dry, wax and oily finishes.
d) Penetrators
It is used to provide deeper penetration of finishing formulations to the skin and under the skin. These products are water-miscible organic solvents and / or capillar-active substances.
e) Cellulosic Thinners
It is used to dissolve and thin nitrocellulose based finishing agents. The effect of the finishing film on the technical properties is great.
f) Crosslinkers
When it is added to the finishing formula in very small amounts, it is used to improve some fastness values of the finishing by providing cross-linking.
I. COLORING
A) Pigment Dispersions: Aniline production in leather production and consequently maintaining the natural skin appearance adds a different value to the finished leather, which will turn into profitability by increasing the sales value. But; working in this way depends on the high quality of raw leather and using a very special technological unit. However; Leathers that can be evaluated as aniline skin are 10-20% of all leathers produced if a generalization is made. Leathers out of these ratios require the application of dense finishing layers both in order to improve skin and surface properties and to increase their merchantability. In this process, the leather surface is coated with a homogeneous and certain thickness finishing layer and painted to improve the surface properties. From this point of view, pigment dispersions are the most preferred materials in the dyeing of the leathers in order to eliminate the surface defects of the skins, to eliminate the color unevenness and inter-party color differences, to provide color changes depending on the customer demands, to improve their appearance and fastness.
Pigments are, in definition, colored substances in powder form, obtained from natural or synthetic materials used for coloring and dyeing of all kinds of goods. Pigment in industry; It is the general name given to colored powders with very small particle structure, which are utilized in solid form, insoluble in water and organic materials. They are usually suspended in a liquid medium and thus applied to surfaces. Since the pigments used for coloring paints, ink, plastics, fabrics and other materials are a dry colorant, they must be prepared by wetting them before use.
Pigments selectively adsorb some of the visible spectrum and reflect others while providing color. There is a significant difference between pigment and dye terms. Pigments are insoluble in a carrier, while dyes are dissolved.
Unlike dyestuffs, pigments used for dyeing in leather finishing are virtually insoluble in systems consisting of water or organic solvent, depending on the general characteristics described above. Therefore, they must be properly dispersed before being used in aqueous or solvent systems. It should be noted here that some lacquer pigments and organic synthetic pigments are slightly soluble in plasticizers and organic solvents. Therefore; They can migrate to the surface when nitrocellulose lacquers and nitrocellulose emulsions and other solvent based binders are used in the subsequent layers.
Pigments that are extracted in water-insoluble form can be dispersed in order to be used in the industry for various purposes and these dispersion systems must be stabilized in order to prevent precipitation problems during their storage. While preparing the pigments, it is first brought into powder form by crushing and grinding. Then, it is mixed with water with a wetting agent and a binder such as casein solution. This mixture is then ground together so that the surface of each small pigment particle is wetted and coated with a binder or wetting agent solution.
In pigment preparations, steric stabilization is provided with nonionic dispersers, and its properties such as film appearance, color strength and brightness are also increased. The most obvious indicator of flocculation during storage is increased viscosity. Disperse agents prevent this flocculation and provide a stable emulsion.
The most important point of preparing a very good pigment cake is the great mastery of the color and other properties of the pigment, the choice of binder or wetting agent and the grinding technique. Pigment pastes are commercially available either in 20-30% concentration or as 45-50% concentrated preparations.
Persons dealing with leather finishing usually choose colorants, ie pigments, only by color, to find the closest color tone to the assorted and matched sample and increase ascordability. Indeed, it is one of the most important aspects to consider in choosing these pigments. But; There are other pigment characteristics that are at least as important and effective on some properties such as fastness properties, chemical resistance and long-term durability of finished leather. At the same time, when evaluated in terms of quality, the pigment should show as narrow a particle size distribution as possible, and also should not contain coarse particles, ultra-fine particles and harmful impurities. Chart to make a general assessment 1.
Table 1: Pigment Preparations and Technical Properties
Pigment Color | Chemical structure | Technical specifications | forms |
White | Titanium dioxide | High covering, opacity 7, light fastness 7. | Anatas: Creamy white Rutile: Bluish white |
White | Zinc oxide and zinc sulfur mixture | Low covering, no dusting. | Zinc white is less concealing than titanium dioxide. |
Yellow | Lead chromate | Opacity 7, light fastness 7. | Chrome yellow, not ultramarine |
Yellow | Cadmium sulfide | Glossy, high opacity and coloring | It gives lemon yellow, bright golden yellow color. |
Yellow | Organic yellow soil paint | Opacity 2, light fastness 2. | |
Yellow | Iron oxide | Opacity 7, light fastness 8. | Pale yellow, red, black |
Red | Cadmium sulfide | Bright, Opacity 6, light fastness 7. | Shades of red |
Red | Molybdenum Salts | is cheap | Cadmium is not as bright as red |
Red | Red organic soil paint | Opacity 2, light fastness 5. | |
Oranje | Basic lead chromate | Opacity 7, light fastness 7. | Not ultramarinated |
Oranje | Molybdenum salts | is cheap | Orange shades |
Green | Chrome oxide | Opacity 4, light fastness 7. | |
Green | Organic green | Opacity 2, light fastness 5. | |
blue | Organic phthalocyanine | Opacity 2, light fastness 5. | |
blue | Ultramarine | Opacity 6, light fastness 7. | Moderately blue, No lead pigments. |
Lacquer pigments | Opacity 3, light fastness 2. | Various colors |
Important factors to be considered in the selection and use of pigment pastes; The pigment has a crowing power, color intensity and stability. The hiding power and opacity is the reflection of the light coming to the pigmented surface not passing this surface and reaching the real skin surface. Thus, finishing; It must pass as little light as possible with absorption, refraction and internal reflection. Refractive index is a measure of the degree of refraction of light to the material. The hiding power is partly to the particle size, the color intensity is partly to the refractive index of the pigment and the environment, and the stability; depends on the chemical properties of each pigment.
Pigments; It is divided into two groups as organic and inorganic pigments. Inorganic pigments are mainly metal oxides. In general, the distinction between both pigment groups is evident by the fact that inorganic pigments show very good hiding power, while organic pigments form transparent film due to the smaller particles. While inorganic pigments mostly resist chemical, thermal and light effects, organic pigments give very high gloss value. The particle size of organic pigments is between 0.01-1.0 microns and inorganic pigments are between 0.1-2.5 microns.
Pigments are also divided into two main groups: casein and non-casein pigments. Again; It can be anionic and cationic according to its charge. Anionic pigments are used in anionic finishes, and cationic pigments are used in cationic finishes. Nonionic pigments can be used in all finishing formulations.
When the solid is taken as the base, the pigment content in one finish should be less than 40% if almost all of the other part is binder. There are some points to be considered when working with pigment preparations. These;
1. Pigment preparations with high concentrations should be brought to the appropriate consistency and density by using other finishing aids that are required for dyeing skins.
2. If the diluted pigment preparations consist of inorganic and organic pigments, precipitation may occur in solution. This situation occurs when heavy pigments settle in the lower parts of the container and cause discolored shade, making the finishing irregularities clear. To prevent this, the solution must be mixed frequently.
3. If the finishing solutions are kept in containers with open mouth for a long time, solid particles form on the edges of the containers. By mixing such solid particles into the finishing solutions, the film formation is disturbed and the appearance of the smooth finishing surface is eliminated. To prevent this, solutions should not be kept open for a long time. Also, the solutions must be filtered before use to separate the solid particle content.
Pigment dispersions used in finishing processes are classified as chemical and content as follows;
1. INORGANIC PIGMENTS
IRON OXIDES: One of the most widely used inorganic pigments is iron oxides. They color oxide yellow (caramel), oxide red (tile), brown and black.
CHROMATE AND MOLIBBATES: These are inorganic pigments that are increasingly used. Chromate gives yellow to orange, molybdate gives orange and red colors. They are present as chromates, zinc chromate, lead chromate and mixtures thereof.
Cadmium salts: These inorganic pigments give the same color as chromates. They are not very preferred because they are very expensive. Light and heat fastness is very high. They do not vomit and are mostly used for upholstery leathers.
TITAN DIOXIDE: This inorganic pigment is used in white colors or bleaching of the mixture. It is the best white pigment with all kinds of fastness. It has two different structures as rutile and anatas. Both are used in leather finishing. Anatas is less covering, whiter and cheaper. Rutile is mainly preferred white. Hiding power is higher.
Pigments are covering and coloring. In color making, the main color consists of a single pigment. For example, the oxide consists of yellow, yellow iron oxide. It is one color and the main color.
Various colors are mixed in color productions. In general, a mixture of more than three primary colors is undesirable due to problems arising from incompatibility of pigments at a later stage. In the production of a brown mixture pigment, it is necessary to use oxide yellow, oxide red and black. There is no need for an expensive black pigment in the production of this mixture pigment. Iron oxide black is preferred.
The density of all pigments is different. Inorganic pigments have higher densities. Pigments begin to decompose due to their differences in density, and this is the most important known problem of pigment blends.
Pigment mixtures are known as casein and non-casein pigments for use as disperse products with or without casein. Since casein-free products are more concentrated, these pigments are dispersed very finely to eliminate the risk of excessive pigment delivery. If pigment dispersions are prepared in water, they are called pigment pastes. In preparing a water-based formulation, it is necessary to mix the pigment well with water and wet it. In the wetting of the pigment, it is necessary to add some heaters as well as dispersing facilitators. Also, some carrier polymeric materials are added after the very small particles are dispersed in water. Drying, Such auxiliary substances used to prevent caking should also be a water-soluble substance. These are substances often called carriers.
The most used casein is used as a carrier. Natural polymer, casein is obtained from milk. It is preferred more because it is cheap, and at the same time; it is also used in other branches of industry. The solution of casein in water prepared by dissolving in basic medium is used. After the casein taken as powder is dissolved by heating, its pH is adjusted. The prepared cake of casein solution has a carrier property in such a formulation. In addition to casein, water-soluble acrylic polymers are used as carriers as a carrier. Pigments prepared with these are also called casein-free pigments. In the preparation of pigment pastes, the pigment is first grinded and dispersed at high temperature, then it is wetted by the addition of wetting agent, with the addition of carrier and thinner.
Anionic pigments are used in anionic finishes. In addition, cationic pigments are prepared for use in cationic finishes. Nonionic pigments can be applied in both anionic and cationic formulations.
ORGANIC PIGMENTS
These are pigments that give brighter shades, and their coverage is less than inorganic pigments. These pigments present some difficulties in obtaining full color shades. The most commonly used organic pigment in the leather industry is carbon black and phthalocyanine.
CARBON BLACK: Black is the most commonly used pigment in leather finishing, and in many brown tones, black pigment is used. Most of the black pigments used in the modern paint industry are composed of elemental carbon, which is known as carbon black. Carbon is amorphous in nature and is not crystalline. Carbon black is obtained in petrochemical processes and is used mostly in the tire industry. An increase in friction and rupture strength is observed by using polymers in the carbon black tire industry for painting. There are different types when used as pigment. With different technologies, carbon black pigment is produced in different particle sizes. Smaller particle sizes are used in the leather industry. The smaller the size is produced, the deeper the colors known as “gel black” are obtained.
Carbon black pigments are quite pure in the form of carbon black, soot black and synthetic graphite, or a mixture of this with silica in natural graphite. In addition, iron oxide based black pigments are also available and used.
Carbon blacks are included in the CI Pigment Black 7 group in the international grouping.
FITALOCIAN: Available in clean blue or black blue. Phylo green is available for green color. Phthalocyanine has very little vomiting properties.
LAC PIGMENTS: These are compounds made with metallic salts of dyes. It is cheap, they have vomiting tendencies. They are solvent based products. Pigments are used in finishing after they are prepared by dispersing the pigments with a solvent and a suitable polymer in a polymer solution. The binders here can be of very different structures. If dispersed in nitrocellulose solution, cellulosic based pigmented lacquer is called and used as transparent finishing materials. In addition to these, pigmented patent varnishes are also available due to some advantages.
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POWDER PIGMENTS: These are special products prepared for finishing . They must be dispersed and dispersed in a certain environment. The medium is either water-based or solvent-based.
DIFFERENCES BETWEEN ORGANIC AND INORGANIC PIGMENTS
1- The particles of inorganic pigments are larger.
2- Inorganic pigments have higher light and heat fastnesses.
3- Economically inorganic pigments are cheaper.
4- Inorganic pigments do not give clean color. Of course they are colors. It is not alive. Organic pigments are more vivid and cleaner pigments .
5- Inorganic pigments are covering. The grain size is larger. But their coloring power is lower. Organic pigments are less covering because they are smaller grained, transparent and have high coloring power.
LIQUID (LIQUID) DYES
Paint solutions are generally products of metal complex type prepared in solvents that can be mixed with water. They are used for spray painting, base coat mixes and double tone effects. When used on the upper floors, they also add brightness to the finishing.
Light fastnesses are lower than pigments. Therefore, these products should not be used on furniture and auto upholstery leathers. The most important issue to be considered or evaluated in the selection of liquid paints; It is the tendency to leave a ring-shaped water drop stain when water is dropped on some spray-dyed leathers. Another important point is the tendency of these dyes to migrate to plastic PVC. This causes slight coloration on plastic soles when used in shoe production.
BINDERS (BINDERS)
It covers the leather surface of the purpose of using binders in leather finishing with a smooth and continuous layer or film. Such covering the skin surface, but also the pigments used in dyeing leather and connecting themselves deep transparent film generally giving the polymeric material to binder with Turkish name binding is called. With the use of binders, the color, brightness, opacity, attitude, softness, pressure-holding feature, water resistance and some different physical properties of the leather can be adjusted.
Binders are the most widely used finishing materials in leather production. They are the main components that reveal the properties of the finishing and, accordingly, the final leather properties. The most important binders are polymerasates that have been in use of the leather industry since the 1930s. The starting products or monomers in the production of these polymer dispersions are derivatives of ethylene. This group mainly consists of acrylic acid esters. Depending on the properties, reaction conditions and copolymerization of different monomers, polymer molecules of different structure and properties are formed, which are dispersed in water in spherical droplets with emulsifiers. In this way, many binder dispersions are available. Polymerization conditions,
a) Smooth film formation,
b) Elasticity,
c) Stability against acetone,
d) Swelling resistance,
e) Bond strength,
f) Influence,
g) Filling effect,
h) Skin thinness,
i) Resistance to hot iron (iron fastness)
j) Wet and dry rubbing fastness
In addition to the most commonly used acrylic acid esters in finishing, it began to be used in leather finishing after 1965, and the shortened aqueous polyurethane dispersions, expressed as PUR, have gained great importance since then. By using these binder types, very high fastness finishes can be obtained. The film is very flexible and fixed by crosslinking.
It is possible to explain the binder types most used in finishing and their film properties as follows.
1. Thermoplastic and Crosslinked Binders:
Some polymers become thinner, lighter and in some cases melt when heated. When they become molten, these polymers can be taken into any mold and turned into articles of use. Also; It can be forced to transform into fiber form by passing through the fine hole in the spinneret. After these processes, the polymer will be cooled and solidified again as a mold or as a fiber. Polymers that melt under the effect of heat in this way are called thermoplastic polymers.
The term thermoplastic is a frequently used term in the definition of polymers and gives information about the heat effect on the polymer. In finishing, these properties of binders have to be considered in terms of their use. Thermoplasticity is a definition that expresses the ability to change the way with heat exchange, and is the name given to substances that tend to soften and harden-solidify with repeated heating-cooling. As a polymer, plastics can soften, become fluid and become deformed or reshaped as desired by applying sufficient heat and pressure. This is due to physical properties rather than chemical properties.
The term thermoplastic is a frequently used term in the definition of polymers and gives information about the heat effect on the polymer. In finishing, these properties of binders have to be considered in terms of their use. Thermoplasticity is a definition that expresses the ability to change the way with heat exchange, and is the name given to substances that tend to soften and harden-solidify with repeated heating-cooling. As a polymer, plastics can soften, become fluid and become deformed or reshaped as desired by applying sufficient heat and pressure. This is due to physical properties rather than chemical properties.
A thermoplastic polymer shows plastic properties and this substance becomes liquid when heated, becomes brittle, crunchy when frozen and has a rather glassy appearance by cooling the film. This is the most important feature of binder films suitable for spreading and coating. Many thermoplastic polymers have high molecular weight. Chains of these polymeric materials; As in polyethylene, weak Van says Waals forces can be combined with stronger dipole-dipole interactions and hydrogen bonds, as in nylon, or even packaging aromatic chains such as in polystyrene. Unlike thermoset polymers, thermoplastic polymers differ in that they can be remelted and remolded. Many thermoplastic materials are substances produced according to addition polymerization. In other words, they are polymers grown on the chain such as polyethylene and polypropylene.
Thermoplastic polymer fibers can be heat-set. This phenomenon causes the deformation of the fibers and a permanent deformation occurs in the fibers connected to heating at a certain temperature for a certain period of time. An example of the use of a heat barrier is pleats on the pants and skirt.
Many mechanical properties and behaviors of polymers depend on these properties. The formation of polymer films based on their thermoplastic properties is the basis of finishing processes, covering the skin with them. The emergence of these properties of binders in the formation of the film and the fact that the film is suitable for the purpose in terms of strength and rheology is possible with the abandonment of the solvents, which takes place in 3 stages.
1. Rapid Evaporation: this is the stage where continuous evaporation occurs since the finishing film has been discarded and is also described as the fixed evaporation phase. if
2. Viscous Phase: At this stage, the polymer particles come into irreversible contact with each other and an increase in viscosity occurs in the finishing layer. Evaporation rate decreases to 5-10% of the initial value.
3. Diffusion: In this stage, which is the final stage, the stage in which the last water molecules remaining in the film are slowly discharged from the film by superficial effects or diffusion. The diffusion rate is correct with the thermal energy supplied to the system and inversely with the increase in the viscosity of the film with drying. On the other hand, surface-related effects play a very important role in this phase and they control the removal of volatile organic components on the surface of the coating, especially in the case of low auxiliary chemicals.
Figure: Film Formation in Water Based Finishing Systems.
a) Polyacrylates:
The starting products of polyacrylates, known as acrylic binders in finishing processes, are acrylic acid esters. Due to the fact that acrylates have good resistance to UV rays, sufficient resistance to chemicals, their tensile properties are good and superior resistance to the effects of acid and alkali salts, they have been widely used as a binder in the dye industry and also in the leather industry. It is used in all stages of leather finishing, from impregnation processes to finishing layers. Due to its flexibility, it gives the finishing film good flexibility and bonding as well as high light fastness. These are also self-crosslinking polymers or binders with free reactive groups.
High elasticity values against light and oxygen, as well as elastic films, increase its usability for leather finishing. With the increased chain length of the alcohols used in esterification, the softness and stickiness of the film increases, however, their resistance to cold and moisture also increases. It is possible to obtain many products with very different film properties depending on the degree and conditions of the polymerization and also depending on the structure and properties of the different substances included in the copolymerization. These products and all other polymer dispersions are sold in concentrations of 30-60%.
b) Polymethacrylate: The starting products are esters of methacrylic acid. Methacrylates also have good fastness properties like acrylates. But these give harder films.
c) Polyacrylonitrile: The starting products are nitriles of acrylic acid. They give hard films with high gloss. They have very high water resistance and have light fastness like acrylates.
d) Polyurethane (PUR): Today, the use of aqueous polyurethane dispersions in leather finishing processes is increasing and constitutes the second polymer group used in finishing processes. Undoubtedly, it is thought that their share in finishing will be much more in the future than today. The advantages of these products are their good penetrability, excellent bonding of finishing layers, the ability to preserve the skin's properties and naturalness, and the good fastness values of the films obtained. They are used in many combinations with polyacrylates, polyesters and other thermoplastic binders to obtain very different leather properties. This binder class can be divided into two basic groups. These ;
a) Aliphatic Polyurethanes: These products are widely used in finishing in response to almost all needs. Aliphatic polyurethanes are so wide to use as they are offered to the market with a wide range of products ranging from very low particle size binders to high coverage products. It can be said that polyurethane binders have the best physical and mechanical properties that can be obtained from finishing polymers.
b) Aromatic Polyurethanes: This group of polyurethanes is more economical in terms of usage. Although they have excellent binding properties, light fastnesses are weak. Since they tend to turn yellow over time, they are not preferred for finishing leather types such as upholstery.
d) Polybutadiene: These binders are especially high coverage. For this reason, their use is especially important in the finishing of sanded and split leather. Due to its low light fastness and aging tendencies, its use is limited in long-term products such as furniture and car upholstery leather, especially white leather.
Their films are rubbery and soft. It has good filling properties and has a high resistance to cold. Also; films are not sticky and do not swell.
e) Polystyrene: The films given by these binders are hard and fragile. Also; they do not show sticky properties. It is resistant to swelling. Although it shows a moderate film bond strength, it has good cold resistance. These products are used only as copolymers.
f) Polyvinyl acetate: They give transparent film with harder and higher strength. They are used only as copolymers, since they do not possess proper usage properties for leather finishing alone.
g) Polyvinylidene chloride: The films of this binder have low elasticity. Also; it is slightly sensitive to light. For this reason, it is used as a copolymer with acrylic acid esters.
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2. Non-Thermoplastic Binders:
Polymers defined as thermosets do not melt under the influence of heat and their viscosity does not decrease and thin. It is only possible to add a certain form to these polymers by injecting the building blocks into the mold and then polymerizing them by heating. This polymer is defined as thermoset polymers. These cannot be used to obtain fibers.
Thermoset plastics are polymers with dense crosslinking (mesh) between their chains that do not harden with heat and show no softening. They are hard because of their cross-linked structure. They retain their shape, do not dissolve in solvents, but decompose at sufficiently high temperatures. When thermoset plastics are heated, they do not soften and melt like thermoplastics, and even harden on the contrary.
Cross-linked polymers are connected to each other by the same primary covalent bonds as in the main chain. If the thermal energy exceeds the energy of the primary covalent bonds, the main chain and cross links break off randomly and the polymer becomes gradient (decomposes, breaks down). This is a characteristic feature for thermoset polymers. In linear and branched thermoplastic polymers , the forces that hold the chains together with the temperature increase become insignificant next to the thermal energy and the chains slide freely on each other. They flow with the application of stress (tension, pressure, etc.).
a) Water Soluble Products:
aa) Protein Binders: It is widely used in the leather industry. They are based on casein, albumin and synthetic polyamide. These belong to the group of non-thermoplastic binders. These binders cannot soften by increasing the temperature in the finishing process. For this reason, the skins can be ironed and pressed at high temperatures before the finishing layer becomes sticky and the finishing feature does not deteriorate in these processes. In addition, these binders can be used to give a high gloss and gloss effect.
When used with acrylics, butadiene binders and polyurethane binders, the structural stickiness provided by these products is reduced. Protein binders can withstand the hot iron test. However, it is necessary to crosslink with formaldehyde or polyaziridine to improve wet rub fastness values.
Albumin: Usually blood albumin is used, but albumin from eggs can also be used. Only dark red blood is suitable in blood albumin. In order to prevent coagulation of blood cells, it must be shaken vigorously as soon as it is extracted. Preservatives should be added to ensure deterioration resistance and should be kept in cool places. When added in black glazed finishes, it improves color depth and brightness.
In the production of blood albumin, the blood serum is separated from the red blood cells and fibrin and then evaporated and dried. Dissolution temperature should not exceed 40-45 0 C. Otherwise, coagulation may occur.
Egg albumin is obtained from the chicken egg white and is prepared in the form of dried powder. It is suitable for cold glazed finishes and adds a clear and transparent gloss to the finishing film.
Casein:They are commonly called caseins because they are obtained from milk casein. It is widely used in leather industry together with albumin and polyamides. It belongs to the group of non-thermoplastic binders and its physical and chemical structures do not change at a certain temperature as in thermoplastic binders. In this way, the skins can be treated and printed without finishing at higher temperatures. Also; It has very suitable properties for polishing. When added to acrylic, butadiene and polyurethane binders, they reduce their natural adhesiveness. To increase wet rubbing fastness values, crosslinking agents such as formaldehyde and polyaziridine should be flicked and crosslinked. It is obtained by precipitation with skim milk from hydrochloric acid and the product should not have a high content of fat. Properly dried casein should have a light, whitish yellow tint. The ones dried in high temparatures have a brown color tone and are difficult to dissolve. Water solubility of casein can be achieved by dissociation with the help of ammonia, borax or sodium bicarbonate. Depending on the application and composition, commercial products are available with different viscosities. The basic working style in finishing is to apply soft casein products on the first layers and harder formulations on the finish layers. The addition of plasticizers only leads to improved flexural strength, and does not affect flexibility. In casein finishes, these should be treated with fixing agents to ensure sufficient water resistance. Despite this, the drop of water causes staining to occur due to the swelling of the casein film and this cannot be prevented completely. Such stains can be removed by polishing with a dry cloth.
Modified Casein: Commercially available products are in a modified form with polyamide. This process improves fastness value and flexibility against moisture. Also; The fixing agents can be added directly to these solutions without causing precipitation. The benefit of this is that intermediate fixation is not necessary.
Gelatin: It is an additive of Glase finishes. It gives the films translucent and high gloss effect like glass. Due to its poor fixation properties, it should not be added in excessive amounts. It is especially used in the finishing of snake, lizard and crocodile skins to pale colors.
Shellac: Derived from resin material, which is the secretion of the Indian lacquer beetle. It enhances the gloss effect and the attitude of casein coat coats. Due to its insufficient fixation properties and fragility, it should only be used in very small quantities.
b) Soluble Soluble Products:
They are used as additives to soluble pigment preparations in solvent and as finishing coats. It gives matte, semi-matte and high gloss effect. Depending on the formulation and type of the product used, it has attitude properties and certain fastnesses.
aa) Nitrocellulose Lacquers: The main product of these lacquers is nitrocellulose. This substance is obtained from cellulose or cotton by nitration with a mixture of nitric acid and sulfuric acid. Cellulose nitrate contains 12-13% nitrogen. The degree of nitration and subsequent processes; determines the properties of the product such as viscosity, solubility in organic solvents, filling effect, flexural strength and elasticity. High viscosity products show high elasticity and cracking stability, while low filling effect and low gloss properties. For low viscosity products, the opposite properties apply. Commercially available NC lacquers contain plasticizers. Sometimes, it may contain some resins besides solvents and thinners to provide intended use properties. Silicates, calcium, magnesium or zinc soaps can be added to provide opacity. The films of nitrocellulose lacquers have high fastness to alcohols. In these products; With prolonged exposure to light or heat, ultraviolet radiation and amine vapors, various negativities such as yellowing, fading, and easy breakage of films can occur. This is another disadvantage for naturally produced leather in white and pastel tones. Also; these products are 100Ironing should not be done at temperatures exceeding 0 C. To prevent poor binding of the films, nitrocellulose lacquer systems should be applied only to completely dry finishes.
bb) CAB Lacquers: The product forming the film is cellulose aceto butyrate. Instead of nitromas, cellulose is broken down with acetic acid and butyric acid. The application and film-forming properties are very similar to those of nitrocellulose. However; The advantages of these products are that they are extremely resistant to light, heat and amine vapor. Therefore, it is very suitable for finishing white leathers and furnished leathers. Moreover, the remnants of these products are not flammable and flammable. There are desired brightness and opaque products such as NC lacquers.
cc) Polyurethane Lacquers: These are high polymerized polyation compounds of polyether or polyester polyols and aromatic or aliphatic polyisocyanates. These products; There are many possible combinations, particularly due to the cross-linking reactivity of isocyanates. Products with many different film features are obtained. There are some differences between reactive and non-reactive polyurethane systems. Reactive systems can be divided into two subgroups as two-component systems, respectively. In two-component systems, polyester and polyether polyols are mixed with finely weighed isocyanate before use. The final reaction that will form the polyurethane occurs on the skin. If a thick coat is applied, a mirror-gloss lacquer film is obtained. If these products are sprayed in thin layers and in various compositions, films called so-called easy-to-clean finishes that are particularly resistant to peeling and staining are obtained. During the production of non-reactive, partially cross-linked one-component systems are also used for this purpose. After they are sprayed on the skin, they react with the moisture contained in the skin or moisture in the air, even with the amino groups of the skin substance.
FINISHING AIDS
Such chemicals are not the main components of the finishing, but they are added to the finishing formulations in very small amounts, improving their application characteristics with some finishing appearance and attitude features. The auxiliaries used in finishing are the last alternative in modifying some features that tanning and subsequent wet-end processes should bring to the skin. They can be added to the finishing formulations or applied alone on the skin. Although it is generally used in small amounts, it can be used at higher rates in order to gain certain physical and appearance features, as in the rustic finishing, which is one of the new fashion appearance effects recently.
1. Influence Aids (Penetrators)
It is the chemicals and mixtures that help the finishing solution to penetrate the skin and under the skin by breaking the surface tension of this layer on the skin of the skin, which has insufficient penetration under normal conditions and has a level of surface tension that may prevent absorption of the film thrown on the skin. Since the manufactured skins have a certain level of water resistance due to the wet processes in the cabinet, especially with the effect of the lubrication process, it can also prevent the penetration of water-based finishing chemicals to be applied in the finishing process. Likewise, this may cause different unexpected formations to occur in film formation and the film to be physically inadequate in later stages. In order to prevent these disruptions and unexpected effects, the use of some auxiliary substances called penetrators is an opportunity for smooth film formation. Penetrators are added to finishing formulations in very small amounts.
In addition to the pigments and binders used in finishing, substantial auxiliary substances are also used in formulations. Although these materials are necessary for the finishing process, they also play a role in determining certain surface properties. Generally, they are used for viscosity adjusters, adhesives, fillers, softeners, matters, touchers, etc. The most commonly used aids for these purposes are;
1. Viscosity adjusters
Viscosity is a parameter that determines flow properties in aqueous finishing systems. The required viscosity varies depending on the method applied. The air spray system requires a different viscosity compared to the airless spray or roll-coat system. There are generally 2 types of thickeners for aqueous systems. The most commonly used product group is neutralized high molecular weight linear (straight chain) polyacrylic acid. Higher molecular weight results in higher viscosity in the aqueous solution. Low molecular weight polyacrylic acids do not work as thickeners, but can be used for leveling and leveling.
High molecular weight nonlinear high hydrophilic polyacrylic acids can bind up to 1000 times their own weight. The viscosity of these neutralized products is a gel consistency. These are also called “super-absorbents”. Another group is “associative thickeners”. Their general chemical structure is in the form of a mixture of main chain containing hydrophilic groups and long hydrophobic side chains. In this way, they form a network based on intermolecular and intermolecular interactions with other finishing agents (binder, pigment, filler, etc.) present in the aqueous medium. This network effectively enhances viscosity by creating micellar cloud and rosette structures. This type of associative thickeners work better with smaller particle binders.
2. Adhesives
The part responsible for the stickiness of a finishing formulation is binders. Soft binders in the basement tend to stick more than hard binders in the finishing layer. In addition to the hardness, the type and amount of hydrophilic groups contained in the binder also have a significant effect on the stickiness.
- Low molecular weight and high hydrophilic groups in a polymer are necessary for dispersion stability.
- The effect of the hydrophilic group type on stickiness; It depends on the counter load, the electrodegativity or polarity of the group and the tendency to retain water during drying.
-CO 2 - <-SO 3 - <non - ionic <cationic
stickiness increases These properties depend not only on binders added to finishing solutions, but also binders and other auxiliaries used in the production of pigments.
Besides the binder characteristics, there are other process parameters;
- with ambient temperature and humidity
- the quality of drying and cooling also affects stickiness
The methods for removing these stickiness problems are as follows;
- all the basic thermoplastic polymers used in finishing formulations as long as the temperature is the main problem of adhesion. Lowering the temperature during drying requires that the conveyor belt speed is automatically reduced so that sufficient energy can be applied to the skin. These alternatives are not preferred by companies due to production speed or cost. There is not even a good cooling equipment or tunnel connected to drying tunnels in tanneries.
- Harder surfaces are less sticky. Therefore, crosslinkers can be used to reduce adhesiveness. However, in this case, the necessary flexibility will decrease.
- One of the most common methods of reducing stickiness is the addition of inorganic particles, such as pigment or matting agent. However, this is limited depending on the brightness level and pigment carrying capacity. All particles weaken the film properties of the polymer and reduce the physical properties of the finish.
- The most common method used in reducing stickiness is the use of waxed or oily substances. The excessive use of this type of products may also cause problems in the binding of future finishing floors. At the same time, they also affect the surface behavior when used on the last floor.
Adhesives are used as a mixture of the last two groups. In addition to this chemical solution, tanneries have started to give more importance to cooling units in solving stickiness problems.
3. Matting agents
If the angle of the beam coming to the surface is the same as the angle of the reflected beam, this surface appears bright (mirror example). If these two angles are different from each other and an irregular beam reflection beam is formed, then the surface will look matte.
Matting agents are added to the finish formulations to obtain a matte finish. The most commonly used matting agents are silicates. The matting effect of these products is caused by irregular reflections of the incoming light, creating roughness on the surface. Apart from silicates, other organic and inorganic coarse particles also give the same effect.
Another method used to obtain a matt surface is transfer coatings applied without using any matte. In this application, the matte surface is provided with a rough paper.
Each method used to flatten the surface brightens the surface, such as smoothing the surface in the test area in wet and dry rub testing. This can be briefly called polishing. A tight and hard finishing binder with a high melting point results in less polish effect.
4. Attitude developers (touch items)
The “touch”, which is defined as the surface attitude of the skin, is actually related to the contact of the human with its environment thanks to the sense of touch. In addition, the optical effects received through the eyes significantly affect the image created about the material. The feeling of touch is taken by the receptors in the skin and transmitted to the brain. This; is the way in which temperature, pressure, vibration, the structure of a surface and pain are transmitted. Parameters that determine the surface attitude of the skin; characteristic features such as thickness, softness and stiffness, the warmth of the skin and the smoothness of the skin surface or the surface retention of a binder or an adjuvant. Waxes that give a characteristic attitude to the skin surface,
Friction and adhesive forces play an important role when two skin surfaces come into contact with each other. The wider contact surface causes the emergence of more adhesive forces. For example, a smoother skin surface causes more adhesiv force than a rough skin surface. The smooth surface shows a lower friction resistance. If there is a certain friction between the two surfaces, the lubricating additives form a film between the two surfaces to increase slip and eliminate the blocking effect of the binders. The substances used for this purpose are waxes and silicones.
Basic principles in the preparation of finishing mixes
A finishing solution to be applied on the skin is in its simplest form; The pigment that will be used to color the skin is prepared so as to bind it to the skin and provide wax to reduce the stickiness of the binder and binder to provide film formation on the skin. Pigments used to color the skin surface do not have the ability to bind on the skin and form a film. For this reason, it is imperative to use polymer binders with solids content ranging from 10-45%. In practical applications, the binder ratio to be used when preparing the finishing mixes is calculated based on the amount of pigment. So; In a mixture containing 100 units of pigment, the binder rate to bind this pigment is adjusted to be at least 40 units based on the solid substance it contains. In other words;at least 100 units are used. In practice, binder solid content is used between 40-60% on pigment to ensure effective bonding.
When the amount of binder falls below this rate (40), the problems called paint removal after finishing can be observed since there are not enough binder particles to bind all the available pigment particles; this is also undesirable. However, when excessive binder is used; Since the amount of binder used after each application to fit the color will increase abnormally, a thick finishing layer is formed, which will result in a plastic-like attitude and divergence from the natural appearance of the skin. In binders with a smaller particle size, usage rates can be reduced slightly. The particle size of the binders on the market is usually between 50 and 600 nm. As the surface areas of the low particle size binders (<100nm) increase significantly, their pigment binding capabilities also increase,
Wax rates to be used in finishing mixes are calculated over the amount of binder. The glass transition temperatures of the polymers used as binders in the leather finishing must be below the room temperature in order to show sufficient flexibility under the conditions of use; usually this value is below 0 ° C. As these Tg temperatures of polymers decrease, their adhesiveness increases in conditions with high ambient temperatures (such as drying and ironing-press). For this reason, undesirable situations such as the sticking of the skins together at the spa or sticking to the mirror during ironing may be observed after the finishing application. In such cases, waxes or oily substances are also included in the formulations to reduce the stickiness of the binders. Waxes that are natural or synthetic or can be a combination of these, Because they have an easily meltable structure, they go to the surface and reduce the stickiness of the binders. Besides, they can be used for purposes such as filling the skin, softening it, increasing the water resistance. The point to be considered in their use is that if they are used excessively, they may cause reduction in peeling and peeling between the finishing layers. In practice, they are used between 20-40% over the total weight of the binder.
Usage rates of other materials can be summarized as follows;
- Water-based lacquers (hydrolac) are used by diluting 1: 1 with water, solvent-based lacquers with nitrocellulose solutions, 1: 2 with solvent (thinner).
- The keys are used between 5-15% of the lacquer used. Using more than 5% of some silicone keys can cause a blurred appearance on the surface.
- Casein binders are usually added as half or the amount of wax.
The amount of water used in finishing is also important; because it determines the final solids amount of the total finishing solution. The amount of water to be added in practice is calculated to be equal to the total amount of all other components (pigment, binder, wax, etc.) in the finishing solution. This ratio is mostly valid for situations requiring low viscosity in terms of ease of application, such as spray applications. However, in applications such as roll-coat, higher viscosity is required and the amount of water used is much lower. Sometimes viscosity adjustment can be made with various thickeners. Theoretically, the amount of water to be used can be calculated based on the application method (spray or roal coat) based on the amount of solid substance to be applied per square foot and how many layers this solid substance will be applied to the skin. For example; If it is 2 and you want to discard it lightly, you can adjust the total solids content of your finishing solution to 15% and calculate your total finishing solution requirement. In light-colored leather finishes such as white, you will need to increase your solids and the amount of finishing solution.
Mixing order is also important when preparing finishing mixes. There should be no flocking and a homogeneous mixture should be prepared. For this purpose, some amount of water to be used is put into the container first. Then, pigment, wax, casein, the rest of the water and the last binder are added respectively. Although the order of these may change from time to time, it is useful to add binder last. As a matter of fact, when binder is added directly on the pigment, particle and flocculation can be observed. However, the loads of the materials in the mixture are also important. When the cationic and anionic materials are mixed, precipitation will occur. Therefore, it is necessary to mix the same charged materials with each other in the mixtures. Also, after the solution is prepared, it should be mixed very well and a homogeneous solution should be obtained. In some cases, solutions can increase homogeneity by filtering, thus preventing possible blockages in the system.
Basic composition and usage rates of finishing products;
Component | Amount of usage |
Paint Solid | |
That | 420 units |
Pigment | 100 units |
wax | 60 units |
Protein binder | 60 units |
Binder (20%) | 200 units (total 40 units of solid matter) |
Finishing solid | |
That | 100 units |
Hydrolacs | 100 units |
Touch item | 15 units |
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