Polymeric Coating Systems for Artificial Leather

Standard and Latest Technologies

Chris Defonseka

• Publisher: De Gruyter
• Year: 2022
• ISBN:
  9783110716634
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

The manufacture of artifi cial leather using polymeric systems is a vital component as an essential commodity for consumer, industrial and automobile applications. Both practical and exciting possibilities to the standard traditional coatings with PVC and polyurethanes with newer coatings of silicone and graphene induced coatings, and economical biomass materials as non-traditional fi llers, stiffening and softening agents are discussed.

Chapter 1 Introduction

1.1 What is artificial leather?

From ancient times, people used well-cured skins of animals for clothing, rugs, tents, footwear and in other applications. As population grew over the years and these sources diminished, they began to look for alternative materials.

To this day, natural leather is highly valued and considered number one, like preference for natural wood products against synthetic products such as furniture. It is of no doubt that well-cured natural leather from animals has a class of its own but technology has advanced so much that imitation or synthetic artificial leather products are equally good and are much cheaper than natural leather products. In certain applications like upholstery in high-end automobiles, certain manufacturers have their own farms producing these special ‘natural’ leathers from animal hides.

Artificial leather, also called synthetic or imitation leather, is a material made as a substitute for natural leather for upholstery, clothing, footwear, handbags, protective wear and tarpaulins, and the applications are endless. These polymeric coated materials give a very close finish to natural leather and are very cost-effective. Unlike in natural leather, the surface finishes that can be achieved with artificial leather are amazing. In addition to the standard embossed patterns in glossy, matt or fluffy (suede) surfaces and in almost in any colour, there are top-end finishes like imitation of crocodile skin, snake skin, silver and gold colours which are in great demand. Another aspect of artificial leather category is transparent polymeric coatings on synthetic films, paper and printed cloths for clothing. This presentation will also introduce new concepts like the use of bamboo cloth (woven and knitted), handloom cloth and woven cloths from natural fibres with specially formulated polymeric coatings, some of which will be transparent coatings.

Imitation leather, another name for artificial leather, as the name suggests, is a material that looks like real leather. There are two basic methods of manufacturing artificial leather. In direct coating, a fabric is coated with a polymeric mix directly onto the surface of the fabric. This could be either a solid or foamed mix. For high-end foamed artificial leather, the preferred method would be indirect coating, where a solid polymeric mix is first coated onto an embossed release paper and then foam mixes are coated and laminated with a fabric. Since the production of natural leather is expensive due to wastage costs incurred in the cutting process due to the irregular contours and to preserve the inherent grain structures and natural markings, constant attempts have been taking place to develop materials that mimic the surface and feel of natural leather. Advancing technologies have made it possible to produce materials very close to natural leather so much, so that some may find it difficult to distinguish between the two.

Visually, manufacturers have succeeded very well in imitating a natural leather surface. However, artificial leather is unable to match some technical properties of leather like haptics, breathability, water vapour permeability and toughness. Therefore, imitation leather remains as an inexpensive alternative to genuine leather. However, there are also areas that withstand constant contact with sunlight and water, which would damage real leather. Also, in the medical field, in applications like dentist’s chairs, massage beds, examination and treatment beds, artificial leather will be more durable to withstand the use of disinfectants, which are mostly solvent based and would dissolve the finish of leather and finally make it brittle. Artificial leather is also widely used for protective wear such as safety aprons, gloves and industrial boots. These materials will not have an embossed surface and instead have a plain surface and probably with a glossy finish for easy cleaning. In keeping with industrial safety codes, the colour could be yellow with polymeric coatings on both sides with the fabric sandwiched in between.

Other categories with polymeric coatings, although they cannot be strictly called artificial leather but certainly belonging to it as a branched ‘family’, would be paper-coated products, coated or embossed synthetic films and other substrates with polymeric coatings to suit particular end applications.

1.2 Visual differentiating

Differentiating between genuine leather and artificial leather is not always easy, even for experts. Some may recommend even checking them in a laboratory. If these materials are already used, for example, in upholstery, then it is difficult as only the surface is available for scrutiny. On the other hand, if these two materials are available in their normal forms, it is easier to identify each. The author suggests, as a simple test, to examine the back of both materials and the genuine leather will have no ‘backing material’, while artificial leather will have one.

Artificial leather will have a textile backing, generally cotton or synthetic and they will be dyed the same colour as the coating to improve quality, mainly aesthetic value. Genuine leather will have a slightly fibrous back but then some imitation leathers also have this effect. If the material consists of a foam layer that is thick enough, one could squeeze the foam and observe the ‘bounce back’, whereas in genuine leather, it is solid. The grain pattern in genuine leather is irregular, whereas the embossed surface patterns of artificial leather are neat and uniform.

When synthetic leather is cut, the cutting edge is often smooth and clean, whereas when leather is cut it has an almost ‘lint-like’ effect. Natural leather has a fibrous structure that can be seen under a magnifying glass. The grain side has a dense fibre layer which becomes more fibrous towards the centre. In the case of artificial leather, the top layer is very dense and without fibres and will have a fabric backing. In some cases, artificial leather can also take the form of a fabric coated on both sides (protective wear) and also a pure sheet of solid or foam.

1.2.1 Behavioural patterns

Imitation leather melts at high temperatures and burns well, while natural leather will only glow and solidify without catching fire. Burnt artificial leather will smell like burnt plastic, whereas leather smells like burnt hair.

1.2.2 Stretchability

Synthetic leather is often thinner and stretches more when heated. If the materials are glued on hard surfaces, for example, steering wheels and table tops, they cannot be tested and an inexperienced person will not be able to recognize the difference. Some may argue that artificial leather is colder than leather but there are exceptions. Good imitations with smooth or rough surfaces can feel warmer than some leathers. With modern-day technology, it is possible to copy the natural haptic of a thick aniline leather.

1.2.3 Breathability

A distinctive feature between the two is the breathability aspect. Even a heavily coated leather has more breathability than unperforated artificial leather. In rubber boots, one would sweat more than in waterproof boots made of leather. When one sits on a synthetic leather surface, after sometime, one would sweat more quickly than on real leather. This is due to the porous factor of leather. However, artificial leather can be micro-perforated to make it more vapour-permeable.

1.2.4 Other characteristics

Some may argue that rubbing artificial leather would produce more creaking noises than leather. This may well depend on the quality of the materials. High-end materials of both kinds of patent leather and top-end artificial leather like glossy plain surfaces will produce ‘creaking’ noises, while matt finishes like suede will not.

If artificial leather is damaged due to age and long use, it is usually clear immediately. Synthetic leather often breaks and the fabric backing underneath becomes visible. On the other hand, natural leather suffers damage slowly and will have a different appearance.

Sometimes, both types of leathers are used in one application. A good example is the upholstery in automobiles, where identical looking natural leather and artificial leather are used. Usually, the contact surfaces such as seats, back rest and armrests are upholstered in natural leather, while the rest is done in artificial leather. For an untrained eye, it will be almost impossible to tell the difference.

It is often said that genuine leather can be recognized by its smell. Good well-cured natural leather has a characteristic ‘leather’ smell which is very hard to achieve in artificial leather. Also, different types of genuine leather may have different smells. Most manufacturers of artificial leather use ‘leather bouquets’ (perfumes compatible with surface coating) to match natural leather smells and also to give off a nice fragrance. If odour-giving stabilizers like amines or others are used in polyvinyl chloride or polyurethane coatings, then the use of a ‘leather bouquet’ to mask the odour becomes essential. These fragrances can be single or a combination of more than one to get the maximum effect. A lingering soothing ‘smell’ over a long period of time is most. Here, some caution may be required to ensure that these fragrances used to have a neutral effect on people to counter possible allergies but in general terms this has not been an issue.

1.3 Different surface finishes

In keeping with the needs for different types of artificial leather for market like consumer, automobile, furniture, industrial, footwear, fashionwear, decorative, entertainment, travel goods, safety clothing or other, each application will need different qualities with different surface finishes like matt, glossy, suede, psychedelic, embossed, plain, wood grain and animal skin imitations.

Bibliography

[1]The Leather Dictionary. www.leather-dictionary.com. 

[2]Defonseka, Chris. Defonseka Technology Consultants. 

Chapter 2 Types of artificial leather

2.1 Brief history

Over the years, many different types of artificial leathers have been developed, with probably the USA, the UK and Germany being the pioneers. Today, several countries are into the manufacture of these synthetic or imitation leathers for natural leather, which are expensive and in short supply. Since there is an expanding market demand for these products, constant research is being carried out to find more and more versatile polymeric coatings, not necessarily dependent on traditional ones, to meet newer challenges in diverse applications.

Probably, one of the earliest examples was Presstoff invented in the nineteenth century in Germany. It was made of specially layered and treated paper pulp. It was in great demand during the Second World War in place of natural leather which was in short supply and rationed. Presstoff could be used in most applications where natural leather was used, except in applications where flexing and moisture, for example in footwear, would delaminate the layers due to loss of cohesion.

Another early example was Rexine, a leather cloth produced in the UK by Rexine Ltd. in Manchester. It was made of cloth coated with a mixture of nitrocellulose, camphor oil, alcohol and pigment and embossed to look like leather. It was used as a bookbinding material, upholstery in motor vehicles and railway carriages. A big advantage was its c...

Table of contents

• Title Page
• Copyright
• Contents
• Chapter 1 Introduction
• Chapter 2 Types of artificial leather
• Chapter 3 Understanding dyes, pigments and colouring
• Chapter 4 Selection of fabrics/base materials for coating
• Chapter 5 Raw materials for polymeric coatings – PVC, polyurethanes and silicones
• Chapter 6 Concepts, theory of formulating and formulations
• Chapter 7 Machinery and equipment
• Chapter 8 Coating systems
• Chapter 9 A small manufacturing plant for artificial leather for an entrepreneur
• Chapter 10 Safety factors and production efficiency
• Some terms in artificial leather industry