Trace the history of nylon and polyester

In many ways, nylon and polyester fibers seem to be interchangeable, but the two material families have some interesting differences in performance due to their different chemical structures.

Nylon and polyester have some overlapping properties, each with its own advantages and disadvantages that require careful comparison to make the best choice for any particular application (Image from Domo Chemicals)

The history of polyester fiber and nylon is intertwined. DuPont was the inventor who first created polyester fiber, but abandoned it in the early stages, preferring nylon in favor. However, while other researchers used aromatic acids to make polymers to improve the chemistry of polyester fibers, DuPont eventually bought ownership of the product in the 1940s and commercialized it in 1950, becoming one of the pillars of the company's polymer products.

Since polyester fibers and nylon were developed simultaneously using similar chemical compositions, their applications often overlapped, and in many ways the two materials seemed interchangeable, for example, as were quickly used as synthetic fibers to replace natural fibers such as silk, wool, and cotton.

However, there are some interesting differences in the properties of the two material families, which are caused by their chemical structure. It is well known that the functional groups characterizing these materials are amides and ester groups. In most commercial nylons, short carbon segments extend from the amide group, the length of which depends on the type of nylon, with short segments associated with nylon 46 and nylon 66, etc., while long segments appear in materials such as nylon 612 and nylon 1212.

These segments are called fat chains. Aliphatic structures have greater molecular fluidity than aromatic ring structures, so they are often associated with less performing materials, such as polyethylene and polypropylene. However, these fatty segments in nylon allow the material to have relatively high properties because the amide groups provide strength.

By looking at how their spacing affects the performance of various nylons, the importance of amide groups can be understood. The melting point of nylon 46 is 285 °C, while the melting point of nylon 612 is only 217 °C, and the glass transition temperature of most aliphatic nylons is as low as 60 to 75 °C. When the same aliphatic structure is used for polyester fibers, the results are less impressive, with a glass transition temperature of about room temperature and a melting point below 200 °C.

This difference illustrates the role of hydrogen bonds. When hydrogen is bonded to a highly electronegated atom such as nitrogen or oxygen, the only electron around the hydrogen nucleus is pulled far enough away to produce an unshielded positive charge, which forms a very powerful attraction for the negatively charged neighboring molecule (such as the carbon-oxygen bond on the nylon chain) and requires a lot of energy to break, which ultimately translates into strength, stiffness, and heat resistance.

This hydrogen bond is characterized by the same as the high boiling point in water. The ester-based bureaucracy does not provide such an opportunity for hydrogen bonds, so aromatic rings must be used on the polyester backbone for competitive performance. Even with these aromatic rings, the glass transition temperature and melting point of the most common polyester fibers are only close to or match the glass transition temperature and melting point of mainstream nylon.

However, the presence of hydrogen bonds comes at a cost. Since the hydrogen bonds in nylon are the same as those in water, nylon has a strong hygroscopicity, and its moisture absorption is 10 times that of polyester fibers. In the process from dry molding to humidity conditioning, nylon produces well-known changes in mechanical and electrical properties. In general, the strength and stiffness of unfilled nylon at room temperature can be reduced by 50% to 60% due to humidity, while the volume resistivity can be reduced by 4 to 5 orders of magnitude.

In addition, parts made of nylon will also change in size after moisture absorption, while polyester fibers exposed to this environment will not change so drastically. When injectable polyester fibers were introduced to the market in the 1970s, the inventors expected its advantages to lead to a significant reduction in nylon's market share. Polyester fibers were originally used in the electrical industry, where they are more popular than nylon due to their higher consistency in material properties and molding sizes.

However, nylon is still a mainstream material for several reasons. First, the fatty structure in nylon enhances the fluidity of the molecules, resulting in higher crystallinity, allowing the material to maintain its superior properties in environments above the glass transition temperature. Above the glass transition temperature, unfilled nylon 46 and nylon 66 retain 25% of the modulus they have when dried and molded at room temperature, while PBT can only maintain 15%, a difference that is more pronounced with the addition of glass fibers. Among all the engineering resins that are usually reinforced with glass fibers, glass fibers have the greatest degree of improvement in the mechanical properties of nylon, for example, adding 30% glass fibers to PBT can double the tensile yield strength of the material, while adding the same percentage of glass fibers to nylon 6 and nylon 66 will increase the tensile yield strength to 2.25 to 2.4 times of the unfilled polymer.

The high level of crystallinity also ensures excellent long-term mechanical properties. Nylon 6 and nylon 66 are better resistant to creep and fatigue than those of PBT and PET. Although moisture absorption significantly reduces these properties of nylon, it is still better than PET, and not a single nylon part has been returned due to fatigue failure in the past 20 years.

Not only that, but in the 1980s, when commercial nylon combining aromatic rings and polymer backbones was introduced, the performance gap between nylon and polyester fibers became increasingly large. Today, the glass transition temperature of some aromatic nylons can reach 140 ° C, and the melting point is higher than 300 ° C, while mitigating the effects of moisture absorption.

Both nylon and polyester fibers are hydrolyzed easily, and reacting with water breaks the covalent bonds in the polymer chain. This can happen if the material is not properly dried during processing, especially for PET. This can also be long-standing if the molded part is exposed to high temperatures and humidity. Anti-hydrolysis additives for nylon have been commercialized for more than 40 years, and advances in this technology have even allowed nylon 66 to be used in radiator parts of automobiles. The hydrolysis resistant technology for polyester fibers appeared long after that and is mainly used for PBT.

Although the nylon family has these inherent advantages, polyester fibers also have their strengths. In general, amide groups are easily oxidized, and when exposed to high temperatures, nylon exhibits a well-known color change, a mechanism that also explains why nylon is embrittled during a strong drying process and its relative heat index (RTI) is low, especially when considering impact resistance. Even with the addition of heat stabilizers, the RTI value of nylon is difficult to be higher than 130 ° C, while the RTI value of PBT can easily reach 140 ° C, and the RTI value of some levels of PET even reaches 155 ° C. While some of the advantages of this are offset by some of the new heat stabilizers recently developed for nylon, most commercial nylon materials are less resistant to oxidation than polyester fibers, and polyester fibers are also better resistant to UV.

A good example of the overlapping properties of these two material families can be seen in the tire cord industry. Nylon was the first material used to make tire cords and was the first material used in the industry. But over time, polyester fibers have made significant progress and captured a large market share in this business area. Therefore, objectively speaking, although the tire cord industry still recognizes nylon because performance is important to it, polyester fiber also occupies a considerable share in this market.