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RUBBER

RUBBER

An extraordinary Material!

Let’s think about the life we lead every day, we may have never noticed it but if there weren’t tires it wouldn’t be the same: riding a bicycle, moving around by car, playing ball, watching television…

Since the appearance of Caucciù (natural rubber), the rubber industry has really made great strides!

The rubbers we know today, that is, with the elastic and tenacious characteristics that distinguish them, were born only after the discovery of vulcanization, a discovery that probably occurred by chance by Charles Goodyear. Legend tells us that the mixture of sulfur and latex that Goodyear used to make the rubber more consistent accidentally fell on the stove in his laboratory, producing a rigid, elastic and unalterable layer. In honor of the god Vulcan this process was called vulcanization.

Formation of sulfur bridges (S) between natural rubber chains

In fact, rubber is a thermosetting material because the macromolecules of the chains that make up the material are linked by bridges or bonds (cross-links) giving rise to a chemical-physical transformation, during which the rubber passes from the plastic state to the elastic one.

Not all rubbers are cross-linked with sulfur primarily because this process can only take place if the polymer chains contain double bonds, but today the term vulcanization means any rubber cross-linking process even if it does not involve sulphur.

As the cross-linking density increases (links between the chains), for example, the elastic forms and hardness increase, while the values of permanent deformation after compression (Compression-set) decrease.

Rubber – Elastomer and Compound

The macromolecular chain unit that is repeated inside the rubber is defined as an elastomer.

There are many types of elastomers, each with different chemical, physical and mechanical properties. Here is a quick list:

NAME  ISO SYMBOL
Natural Rubber NR
Styrene Butadiene SBR
Polychloroprene CR
Nitrile NBR
Ethylene/Propylene/Diene EPDM
Hydrogenated Nitrile HNBR
Chlorinated Polyene CM
Epichlorohydrin ECO
Ethylene/Acrylate AEM
Polyacrylic ACM
Silicone VMQ
Silicone fluorine FVMQ
Fluorinated FKM

We are talking about rubber compounds because in addition to the elastomer inside the material we have a “recipe” which also contains 20 different ingredients in order to obtain peculiar and specific properties. This is where the term formulation is used. Broadly speaking, we can divide the products that make up a “recipe” as follows:

  • Elastomer: which is the main ingredient;
  • Vulcanizers (cross-linking, accelerating, activating and retarding agents): to create the three-dimensional lattice and modify the cross-linking speed;
  • Protective (anti-ozone and anti-aging): which protect the rubber from oxidative phenomena;
  • Plasticizers and processing aids: to reduce the viscosity of the compound, incorporate the fillers and increase resistance at low temperatures;
  • Carbon Black definitely the most important reinforcing filler of the compound, produced from petroleum to impart not only reinforcement, resistance to UV rays, electrical and thermal conductivity;
  • White fillers (kaolin, silica, calcium carbonate, talc…): fillers used to reduce costs, semi-reinforcing fillers or reinforcing fillers used to achieve high performance in light-colored articles;
  • Tackifiers (vegetable and mineral resins): They allow the improvement of the attack on substrates such as metal, textiles;
  • Various ingredients (hardeners, dyes, flame retardants, abrasives, blowers and etchants).

Surely the profession of the formulator requires careful knowledge of each ingredient, each with its own variability, in order to obtain a compound that achieves the set objectives of unique characteristics.

Laboratory Test on Vulcanized Rubbers

Carrying out a rubber characterization is necessary to define its properties; the first step is to obtain the specimens for the tests which can be die-cut directly from the component as it is or, in most cases, after thinning to make the thickness uniform by means of a splitting machine. Instead, in the case of characterization of the laboratory compound, the samples can be printed directly as sheets (plates) and cylinders (tombolini).

However, it is essential to start from the assumption that the characteristics of the materials must be taken directly from the finished products to take into account the transformation and vulcanization process.

         

 

Here are some characterization tests on rubber compounds:

  • Hardness: Shore A, IRHD and micro IRHD;
  • Tensile: Breaking load [Mpa] and Elongation at break [%];
  • Tearing [N/mm]: evaluate the tear resistance index of the elastomers on different types of specimens, notched and not;
  • Residual deformation [%] (Compression-set with constant deformation): it is the permanent deformation that remains when the previously applied force is removed through an imposed deformation.
  • Required for components that work with an imposed deformation, for example O-Rings;
  • Resistance to low temperatures: Embrittlement [°C] and Elastic Return Temperature;
  • Resistance to fluids: the samples are aged in the liquids used, such as oils, fats, petrol, etc., at a specific temperature and time;
  • Resistance to atmospheric agents, light and ozone [index of merit]: these are both aging tests to verify the resistance of materials to attack from the external environment;
  • Differential Scanning Calorimetry (DSC): verifies the glass transition temperature of the material. It is also used on raw compounds to analyze the vulcanization process as it is an exothermic reaction. In some cases it could also be useful for evaluating the degree of vulcanisation
  • Thermogravimetry (TGA): measures the weight variation of a material as a function of temperature, as a result of any decompositions (breaking of bonds due to pyrolysis and thermo-oxidation). It is a quantitative analysis from which we can identify the % of volatile substances, elastomer, carbon black and inorganic residue. A sort of identity card for the compound.
  • Infrared spectroscopy (FTIR): the absorption bands of the functional groups of the material allow the identification of the nature of the elastomer.

In our laboratories we can perform the tests indicated and many others, visit our. site on LABORATORY TESTS !

A special thank you goes to my colleague Dr. Angelica Pasquariello, part of the Plastlab Team as a laboratory technician, who raises awareness on this issue so as to provide us with new content for these #laboratory pills!

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