Basic of Shoes- Bottom Parts

Last part of the Basic of shoes series. Here is the first three parts
1. Basic of Shoes
2. Basic of Shoes- Leather Upper
3. Basic of shoes- Synthetic and Others

Rubber: Rubber is used for the upper when the sole is made of the same material.
Bottoms: The types of materials commonly used in shoe bottoms are:

Leather
Rubber
Plastic

Leather: The larger and thicker leather skins are used for bottoms. The insoles are cut from the bellies and the outsoles from the skin cut along the backbone where the leather has the greatest abrasion resistance. The properties of leather of bottoms are:

Great abrasion resistance
Expensive: Leather is generally more expensive then most rubbers or plastic. That is why it is mainly used in high quality footwear.
Less durable than most other materials: Leather is chosen for its attractive appearance rather than durability, which is lower compared with most other materials.

Rubber: Rubber can be:

Natural
Synthetic(SBR)

The properties of rubber for bottoms are:

Hard wearing
Flexible
Waterproof
Easier and less expensive to produce than leather

Today, most types of rubber are likely to be based on the general-purpose synthetic rubber denoted S.B.R., rather than the traditional natural rubber obtained from rubber trees. Soles made from these two types of rubber are vulcanized (except crepe) and have similar properties. Vulcanized soles are sometimes referred to as "Composition rubbers" because they are made up of a number of ingredients added to the basic rubber to control the properties. Some black compounds may have a tendency to mark floors depending on the quantity of carbon black in the mix. The soles are used in a wide variety of footwear from heavy-duty to general purpose.

Rubber bottoms vary in type, including:

Translucent rubber-vulcanized: Translucent rubber is a high-quality vulcanized rubber, either natural or synthetic. It contain silica and it is usually available in "natural" colors ranging from off-white to light brown.
Resin rubber-vulcanized: Resin rubber was developed in the early 1950s to simulate leather. It is a firm type of vulcanized rubber based on a synthetic SNR-rubber, reinforced with a high styrene resin.
Microcellular rubber-vulcanized: Microcellular rubber is basically a resin rubber with a cellular structure. During vulcanization the rubber is expanded by a blowing agent which forms numerous small, non-interconnection cells.
Sponger rubber-vulcanized: Sponge rubber is used for sole for felt or cloth slippers. It has a spongy, cellular structure and it is covered with a thin, solid skin. it is vulcanized directly to the upper.
Crepe-not vulcanized: Crepe is obtained directly from the rubber latex, the milky liquid from rubber trees, by coagulation the rubber particles and milling them into sheets. It is lightweight, flexible, hard-wearing and comfortable to wear. The downside of crepe is that it will deteriorate if contaminated by oil, petrol or chemicals. If subjected to heat, sole will discolor and become soft and tacky. It is frequently used for soles of stitchdown shoes and sandals.

Plastic: Polyvinyl chloride (PVC) is cement lasted as a molded unit or directly injected to the upper. PVC is the main synthetic used for shoe soling. The PVC polymer is hard, but when used for soling it is softened by the addition of plasticisers. It can be either used as "direct injection" to the upper or as a stuck-on unit, "Cement lasted." The Properties of PVC bottoms are:

Hard wearing
Flexible
Soft
Can slip when cold

Thermoplastic Rubber (TPR) is directly injected to the upper. TPR can be injection molded. It was originally developed for soling to simulate crepe and soft vulcanized rubbers. The properties of TPR of bottoms are:

Looks and feels rubbery
Lighter than vulcanized rubber or PVC
Good traction
Excellent cold track resistance
Low cost

Ethylene Vinyl Acetate (EVA) is used in an expanded form. Normally used in expanded form either on its own or blended with S.B.R. It is used fro outsole in lightweight footwear and midsoles in trainers. The properties of EVA for bottoms are:

Much lighter than conventional SBRs microcellular rubbers but with the same durability.
Flexible and resilient
Good ground insulation
Used as outsoles in lightweight footwear
Used as midsoles in trainers.

Polyurethane(PU) can be blended to be solid, cellular, flexible or rigid. Normally for shoe the blend is a flexible cellular structure. For top-pieces the solid, rigid from is used. PU is a very hard wearing polymer resistant to most chemicals. Used frequently as a two-part sole material with a thin solid PU-polymer and a blown PU-material for most of the sole making. It is frequently used for safety footwear. The properties of PU for bottoms are: