Tutorial on Rubber Technology and Vulcanized Rubber Footwear

Introduction to Rubber

Rubber is a "polymer", a material made of repeating molecular units ("monomers"), which are strung into long chains. Generally, when a polymer shows some “springy” characteristics, we call it “rubber”, and when springiness is less evident, we call it "plastic". About 40% of the world’s consumption of rubber is derived from plant-based, renewable sources, normally the rubber tree (Hevea brasiliensis), and this is referred to as natural rubber or "NR". NR is produced as a watery sap, or "latex", from the rubber tree, similar to the sap of many other plants. However, this sap contains a high concentration of microscopic particles of polyisoprene, the polymeric resin of natural rubber. Some rubber goods (like gloves and condoms) are made directly from latex; but, by far, the largest volume of natural rubber latex is solidified and dried, so that it can be processed as “gum” rubber. NR is valued for its excellent physical strength properties, resistance to certain chemicals, and for its ease of processing into finished products. For example, up to 70% of the rubber used in automobile tires can be natural rubber, NR. For some interesting ways to visualize Natural Rubber at the molecular level, look at the collection of schematics at the bottom of this page.

Rubber can also be a synthetic material, made from chemicals derived from natural gas, or other hydrocarbons, which are reacted together into polymers similar to natural rubber. Synthetic rubber can often be prepared as a liquid latex, but it is usually produced in a dry “gum” form. Synthetic rubber is referred to as SR, but there are many types of SR, including:

  • SBR – styrene butadiene rubber
  • BR – butadiene rubber
  • EPDM – ethylene propylene diamine monomer rubber
  • CR – polychloroprene (neoprene) rubber
  • NBR – acrylonitrile (nitrile) rubber
  • IIR – isobutylene-isoprene (butyl) rubber

Different raw rubber types are used to achieve different physical properties for the finished products. Frequently, raw rubber of different types are blended, to achieve a balance of properties in a finished product.

Currently, world consumption of rubber breaks down into approximately these percentages: 40% NR (solid and latex), 20% SBR (solid), 12% SBR (latex), 12% BR, 5% EPDM, 2% CR, 2% NBR, and 7% other synthetic types.

Since this website focuses on footwear, the more detailed discussion of rubber technology below will also focus on footwear.

Rubber Footwear

Rubber boots share many of the characteristics of leather boots, in their purpose and basic design. But differences in materials, and differences in how they are manufactured, have a big impact on their performance, and the markets that each product reaches.

Vulcanized rubber boots have a lot in common with other rubber products, in terms of materials and technology of production. However, the amount of hand labor required for building rubber boots is really unique within the rubber industry. Vulcanized rubber boots are almost always built by hand, layer by layer -- not molded (although a sole or other part may be pre-molded). The method of producing rubber boots can be briefly described:

Materials (typical):

  • Natural rubber -- Referred to as NR, various grades are given names like RSS, SIR, SMR, etc., and these grades have various sub-grades that distinguish them by purity, color, or physical properties.
  • Synthetic rubbers -- including SBR (styrene butadiene rubber), EPDM (ethylene propylene diene monomer), Neoprene (polychloroprene), etc. There are different types of synthetic rubber, and different grades of each type. The composition and structure of synthetic rubbers (and natural rubber) determine what physical and chemical properties they have.
  • Vulcanization chemicals -- Vulcanization is the chemical reaction process in which rubber is turned from a sticky, soft material, into a material that has good strength and resilience. This process is often called "curing". Vulcanization chemicals are materials that change independent rubber molecules into a continuous network. These include sulfur (which builds cross-links between rubber molecules), vulcanization promoters (like zinc oxide and fatty acids), and accelerators (which speed up vulcanization, and improve final properties).
  • Antidegradants – These chemicals prevent or slow down the deterioration of rubber by heat, oxygen, ozone, acid attack, fire or other causes of rubber deterioration.
  • Fillers – These materials are used to improve processing, reduce cost, or increase strength or other desired properties in the final product. Commonly used fillers are ground limestone, clay, silica powder, carbon black, and many others. Plasticizer oils, colorants/pigments, resins and other special purpose materials can also be considered as fillers.

Formulation (also known as “compounding”):

Rubber formulas are written for specific purposes. Outsole rubber has different requirements from upper rubber, interior rubber layers have their own special requirements, and some rubber formulas are intended to provide resistance to certain chemicals, or meet the challenges of environmental conditions (like slippery surfaces, cold temperature, extreme heat, and so on). A typical rubber boot can use four, five, or more, very different rubber formulas, for different parts of the boot. A formula may use one type of rubber, or blends of different rubbers, and a selection of different materials to match its required function in the boot.

Mixing:

Rubber and other materials are mixed together in a very precise way, to achieve good dispersion of the materials, and uniformity from batch to batch. Mixing is usually done on an open mill, or in an enclosed “Banbury” mixer. In either method, two steel rollers or screws, rotating inward toward each other, work the materials into a cohesive and uniform mass. Cooling or heating can usually be applied to the mixing machine to control temperature, so that the rubber compound can be mixed in a "plastic" state, when it is soft and “workable”.

Processing:

  • Calendering – A calender is a machine that has 3 or more heated steel rollers, that is used to produce rubber sheeting. The word “calender” derives from the word “cylinder”, and refers to the steel rollers. The rubber sheeting is set to a specific thickness, as determined by the gap between the rollers. Often, a calender will have a textured or patterned surface on one of the rollers. This is how a pattern on leather-grain rubber upper, or a calendered outsole, is created. Also, frequently, rubber is laminated onto fabric under pressure, by running fabric through the gap between rollers at the same time that rubber is fed into the gap.
  • Cutting – Rubber sheeting (or rubberized fabric) has to be cut into parts that will be built into a rubber boot. This cutting can be done (by hand or machine) by a knife (often heated) which runs around the edges of a metal pattern, or it can be performed by die cutting the parts using a press and cutting dies.
  • Molding – Some parts, most commonly molded outsoles, are created by pressing or injecting a rubber compound into a mold under the influences of high heat and pressure. The heated mold is kept closed until the chemical reaction called “vulcanization” is complete enough to remove the part from the mold, without the part’s shape becoming distorted.
  • Cementing – Although rubber is naturally sticky, it is often useful to increase the stickiness of parts to make the boot assembly easier, and insure against leaks. This is achieved by applying an adhesive to the surfaces of rubber parts. Often, this adhesive is simply rubber dissolved in organic solvent. ("Organic", in this case, refers to a carbon-based chemical.) Sometimes, a special adhesive or primer is necessary when bonding together parts made of materials that are difficult to stick together.

Assembly and vulcanizing:

Like leather footwear, rubber footwear is assembled from pieces wrapped around the outside of a "last", or foot-shaped form. While leather boots rely heavily on sewing to bond the parts together, rubber boots rely almost totally on adhesive bonding of the rubber parts. The strength of this bond can only be achieved by placing the boot (on its metal last) in a “vulcanizer” for some period of time. A vulcanizer is a large, heated, pressurized autoclave, that creates the conditions necessary to cure the rubber to its fully vulcanized state. These conditions can be described by the parameters of temperature/pressure/time:

  • The temperature inside a vulcanizer is typically in the range of 280 to 305 degrees Fahrenheit (approximately 138 to 152 degrees Celsius).
  • The pressure inside is typically set at about 10-20 psi above the ambient air pressure, and this is done by injecting extra air into the vulcanizer at the beginning of a cycle.
  • A vulcanizer cycle would normally last about an hour, after proper air pressure and temperature is reached.

Rubber compared to leather:

When the manufacture of leather and rubber footwear is completed, each type of boot is able to offer its own advantages (and disadvantages) to the end user. Here's a short summary, comparing leather and rubber boots of about the same general style:

Characteristic: Leather Rubber
Cost usually more expensive usually less expensive
Weight usually lighter usually heavier
Permeability to liquids not naturally resistant naturally resistant, and can be made highly resistant to liquid chemicals
Breathability usually breathable not breathable
Fit and comfort good, usually adjustable usually good, sometimes limited by the design or materials
Optional components many choices heat of vulcanization precludes some plastic components
Natural rubber graphic