The oldest engineering plastic is still the largest-volume high-performance polymer.

The first thermoplastic material with physical properties good enough to be considered an engineering material was nylon. This wonderful, versatile material traces its roots to a low-molecular-weight caprolactam polymer discovered in Germany by J. Von Braun in 1907. No commercial use was found for that material.

In 1935, DuPont’s Wallace Carothers successfully polymerized higher-molecular-weight caprolactam polymers, leading to the development of nylon 6/6. In 1938, I.G. Farben introduced nylon 6. Nylons 11 and 12 followed in 1949 and 1966.

DuPont’s original research was aimed at developing a synthetic polymer suitable for textile fiber. The first commercial application was in the bristles of Dr. West’s miracle toothbrush in 1938. This was the beginning of the end of animal hair brushes that started in China with the 1498 description of a toothbrush “with hog bristles perpendicular to a handle of bone.” Nylon fiber proved to be stronger than silk. This led to the 1939 introduction of nylon stockings. Sixty-four million pairs were sold in the first year of production. The Chinese learned to harvest and weave silk around 3000 BC. Nylon would take the large hosiery market away from silk, but not until after it had helped win World War II. Glider tow ropes, parachutes, tire cording, and life rafts were important military applications.

Molding grades of nylon appeared in 1941 and were immediately classified as strategic war effort materials. The first application was for coil bobbins, gears, and bearings. Following the war, some processors made a specialty of molding nylon, which quickly dominated the high-performance part of the thermoplastics market. By 1960, 25 million lb of nylon were sold in the U.S., and this quantity increased dramatically with each decade (see chart). Today molded nylon is the most frequently specified engineering plastic.

Defining characteristics

Nylons are semicrystalline, thermoplastic materials known for their tensile strength, temperature, and chemical resistance. Abrasion resistance coupled with a low coefficient of friction are important characteristics of nylon. Flame-retardant grades are available with UL 94 V-2 ratings.

A few of the less crystalline grades of nylon have a slight yellow haze but good see-through ability. Adding transparency to nylon’s other properties increases its usefulness. None of the other transparent plastic materials can provide nylon’s combination of properties. Light transmission can be 85-90%. These are not the best materials for optical lenses, but their clarity is good enough for many products that require transparency.

No two nylons are the same. The most frequently injection molded nylon is 6/6, which has an as-molded tensile strength of around 13,000 psi, a flexural modulus of 420,000 psi, a notched Izod impact of only .7 ft-lb/in, and a heat deflection temperature of 220°F at a loading of 264 psi. All of these properties can be enhanced by reinforcing the material with different types of fibers and fillers. Nylon is the largest-volume fiber-reinforced thermoplastic material. That important topic will be the subject of a future By Design article.

At the end of 2005, the market selling price for truckload quantities of nylon 6/6 was $1.61/lb and $.066/in3. It is interesting to note that the physical properties of nylon 6 are approaching nylon 6/6 values. The major difference between the two is that nylon 6 is $.14/lb lower in cost. Nylon 6 has a slightly improved impact strength and a heat deflection temperature that is as much as 70 deg F lower than nylon 6/6. As the best-known injection molding grade, nylon 6/6 is frequently specified for low-temperature applications that require the material’s other properties. Nylon 6 can sometimes be substituted in applications that do not require nylon 6/6’s higher heat deflection temperatures. That material change can result in a cost reduction of $.14/lb.

Understanding nylon

The chemical name for this family of materials is polyamides, but all of the suppliers refer to them as nylon. The different materials are identified as nylon followed by a number such as 6 or 12. This number indicates the number of carbon atoms in the original polymerization monomer. A material such as nylon 6/6 or 6/12 would result from polymerizing two monomers with different molecular structures.

All of the different nylons have varying physical properties. In general, as the number of carbons between amide linkages increases, tensile strength and stiffness decrease as impact strength increases.

All nylons absorb varying amounts of moisture from the atmosphere. Moisture absorption decreases with an increase in the number of carbon atoms between amide linkages. A fully saturated nylon 6/6 can absorb 2.8% of water. The absorbed moisture acts as a plasticizer that softens the material. For example, the tensile strength and stiffness of nylon 6/6 are reduced to 9400 and 320,000 psi respectively, while impact strength increases to 1.2 ft-lb/in.

One disadvantage of nylon is that the absorption of moisture also results in an increase in linear dimensions. This is unsettling to the uninitiated. Experienced molders have learned to size cavities to allow for this moisture-related increase in dimensions. Another negative is that nylon must be molded in its dry as-received condition or be dried before molding. A just-molded nylon part is dry and relatively brittle. As the nylon absorbs moisture, it regains its characteristic toughness. The length of time required to reach equilibrium depends on humidity, temperature, and the part’s thickness. Material suppliers provide literature defining these relationships.

Nylon is not an ideal material for just-in-time delivery. Just-molded nylon parts are undersized and may fail due to impact loads applied during assembly, or the part’s inability to flex into position. In the majority of cases, nylon parts must be moisture conditioned before they are used.

The properties of nylon are also affected by the amount of crystallinity in the material. The degree of crystallinity is dependent on part design and processing conditions. A thick-walled part that cools slowly can be 50-60% crystalline. The crystallinity of a rapidly cooled thin-wall part may be only 10%. Generally speaking, as crystallinity increases, tensile strength, stiffness, mold shrinkage, and heat and chemical resistance also increase. Impact strength and moisture absorption decline. It is important to recognize that minor variations in molding conditions can have a major effect on the size and physical properties of a nylon part.

Part two of the nylon story will review its many uses and the guidelines for designing injection molded nylon parts.