Polyamide

The synthesis of the first polyamides derived from the polymerization of dibasic acids and diamines occurred in the 1930s in the DuPont laboratories, coining the name currently used in common language as “Nylon”. Initially used as a fiber for a cotton substitute, it was later introduced in all fields in which a heightened mechanical performance was required, due to the possibility of it being strengthened with added filler. According to legend, “Nylon” derives from the acronym Now You Lose Old Nippon in response to the trade bloc on cotton between the United States and Japan, during the Second World War, for the production of fabrics for parachutes. With the arrival of polymerization from amino carboxylic monomers like Caprolactam (production of Polyamide 6) in the 1940s, a new nomenclature was introduced, which used the number of repeating units of carbon, for example, Nylon 6.6, Nylon 6, etc. In the same period, experimentation led to the discovery of all the other polyamides with the most varied combinations of alternations between -CONH- and -CH2- leading to the creation of Polyamide 3, 4.6, 11, 6.10, 6.12, etc. An exception to that is Polyamide 12 which was synthesized in the 1970s. Another innovation happened in the 1990s when the Polyphthalamide PPA was presented to the market; PPA is a polyamide that exhibits, within the base monomer, an aromatic ring shape that lends rigidity to the structure.

All polyamides are opaque semi-crystalline polymers or semi-transparent polymers with the exception of polyamide 12, which can also be found in the transparent amorphous version that was introduced on the market in the 1970s.

For all of the polyamides, the fundamental structural units are repeated identically along the whole length of the chain. That factor that makes them fall within the polyamide family is the presence of alternating amide and aliphatic groups.

The success of the polyamides is due to a combination of characteristics that make them ideal for many applications. Polyamides combine optimal mechanical properties with excellent chemical resistance; they can be used as metal substitutes or even as elastomers; they can be used in both protected and aggressive environments (UV rays, bad weather conditions, etc.); this is possible due to the numerous types of polyamides that manifest varied and specific characteristics.

Polyamide 6.6 shows the best performance regarding mechanical rigidity, higher resistance to temperature, higher resistance to abrasion, less water absorption, and a lower speed of absorption than polyamide 6. On the other hand, polyamide 6 exhibits better impact resistance, better resistance to low temperatures, and better workability and surface finish. The chemical properties of these two polyamides are very similar.

Polyamide 11 compared to polyamide 12, (with the same additives and reinforcements) shows double impact resistance at low temperatures; the deflection temperatures under load are similar, but the Vicat temperature (penetration of a tip under load) is higher and therefore, albeit with minimal differences, the performance of polyamide 11 at high temperatures is better. As a general guideline it can be stated that polyamide 11 offers chemical resistance to hydrocarbons and, in general, to other substances that attack polyamides, better than polyamide 12. It also guarantees a barrier effect to carbides and hydrocarbons up to twice as effective. On the other hand, polyamide 12 is the lightest polyamide of all, to the advantage of all those applications for which the mass is a discriminating factor and offers the lowest percentage of water absorption ever.

Polyamide 11 and 12 are more resistant to hydrolysis than polyamide 6 and 6.6 as they absorb less water. They also have slightly lower friction coefficients. The mechanical properties at room temperature and at high temperatures of polyamide 6 and 6.6 are higher than that of polyamide 11 and 12 but their propensity to water absorption makes performance difficult to predict because they are inconsistent. The last two lines are dedicated to polyphthalamide (PPA) which embodies the advantages of aliphatic polyamides, such as the excellent mechanical properties of polyamide 6.6, 6, exceptional chemical resistance and reduced water absorption of polyamide 11 and 12, to which it is added an exceptional improvement in performance, even in the long term, at high temperatures.