Many engineering materials are in truth families in their own right. Take, for example, steel – this ferrous (iron)-based material is in fact a wide-ranging class of materials, with different microstructures, heat treatments and alloy content. Materials range from utility steels to highly specialised variants which are manufactured in small quantities for a narrow range of applications.
And so it is with plastics. Take nylons (or polyamides). There are multiple different types of nylon including:
- Nylon 6
- Nylon 6/6 (aka Nylon 66, or Nylon 6,6)
- Nylon 6/9
- Nylon 6/10
- Nylon 6/12
- Nylon 4/6
- Nylon 11
- Nylon 12/12
The system of identification refers to the number of carbon atoms in the materials from which it is produced. Nylon 6, for example, is produced from caprolactam, which contains six carbon atoms, and nylon 6/6 is formed from hexamethylene diamine, with six carbon atoms, and adipic acid, also having six.
While the variations in properties are not as marked as they are with steels, variations in underlying structure and the fillers and additives used can in some cases lead to a wide variation in properties, with a combination of commodity and specialist grades. One supplier has almost 90 grades of Nylon 11 alone.
Nylon as engineering plastics
As a family, their characteristics of strength, stiffness and toughness have earned them a reputation as engineering plastics. Typical applications include small gears, grilles, door handles, bicycle wheels, bearings, brushes, sprockets, housings for power tools, terminal blocks and slide rollers.
An important design consideration is that nylon absorbs moisture, which can affect its properties and dimensional stability. Reinforcement, usually by glass, reduces this problem and produces an extremely strong, impact resistant material.
Nylon 6/6
Nylon 6 and 6/6 are the cheapest of all the nylon types. Nylon 6/6 is the most widely used group of nylons, although for supplier reasons nylon 6 is historically the most widely used grade in Germany. Unfilled, it exhibits strength across the widest range of temperature and moisture of any nylon. It has good resistance to abrasion and the lowest permeability for gasoline, mineral oils and fluorocarbon refrigerants.
But 6/6 has high moisture absorption and low impact strength and ductility when dry. It is the most sensitive of all the nylons to UV and oxidative degradation. Both 6/6 and 6 have lower resistance to weak acids than 6/10, 6/12, 11 and 12.
In addition to the common applications of nylons, 6/6 is popular for electrical components, stimulated by improved fire retardancy. It is commonly used as a metal replacement for diecast hand tool bodies.
Selecting the best nylon from the family is often a matter of compromising one or more properties against others. This is an area where materials selection systems such as UL Prospector really come into their own, combined with a qualitative understanding of the basic differences, as described below. Here, other engineering polymers, such as acetals, thermoplastic polyesters and others should also be considered.
Nylon 6
One of the differences between Nylon 6 and Nylon 66 is that, under moist conditions, nylon 6 has better impact strength and flex fatigue life than 6/6. Compared to 6/6, it can be processed at lower temperatures and is less crystalline, which means mould shrinkage is low and closer tolerances are possible. Transparent grades are also available.
But it has the highest moisture absorption of any nylon, with attendant dimensional instability and changes in mechanical and electrical properties. Some of these effects can be offset by alloying with low density polyethylene (LDPE).
As a result of these properties, it is used for components requiring higher impact strength than 6/6, but not requiring higher yield strength.
Since the original version of this article was published in 2018, readers have reported that because of longer chains, nylon 6/6 has better chemical resistance than nylon 6 to saturated calcium chloride. Most sources report that 6/6 has better weathering properties. One important advantage of 6/6 compared to 6 is a higher HDT.
All nylons will severely degrade when exposed to fermented 15% ethanol gasoline.
Other nylon grades
Nylon 6/10 and 6/12 are used for electrical insulation where their lower moisture absorption justifies the extra cost. Nylon 6/10 has lower moisture absorption and a very low embrittlement temperature. Nylon 6/12 is progressively replacing 6/10 as it is cheaper (though more expensive than 6 and 6/6) and has better heat resistance. Its properties are generally between 6 and 6/6, with slightly superior creep properties to 6/6 under moist conditions.
Nylons 11 and 12 have still lower moisture absorption, with 12 having the lowest of all unfilled nylons. They have greater flexibility and impact strength than 6, 6/6, 6/10 and 6/12. This comes at the expense of higher cost, lower strength and lower maximum service temperatures. Weathering resistance is the best of nylons, although more recently, grades of so-called super-tough nylons (in effect alloys) have challenged this position. Nylon 12/12 has better properties than 6 and 6/6, but less expensive than 11 or 12.
Nylon 4/6 has high impact, low creep and higher stiffness at high temperatures. The fatigue behaviour is better than 6/6. The processing window is lower.
Other variants which might be encountered on occasions are 6 and 6/6 flexible copolymers, cast nylon 6, transparent amorphous nylons, sintered grades and more recently forms suitable for 3D printing.
Nylon films exhibit high tensile strength and elongation, good impact strength, impermeable to gases. Low temperature properties (-70C) are good, with good flex-crack resistance and clarity. Increasing moisture content reduced barrier properties but increases elongation and flexibility.
Nylon is also available as fibres, which are notably strong.
Acetals vs nylons
Acetals – sometimes also known as polyoxymethylene (POM) – like nylons, are semi-crystalline thermoplastics, and some of their characteristics overlap, such as their fatigue resistance, chemical resistance, and wear resistance with a sharp melting point. Both are used for small components such as washers, discs, and spacers.
One of the most popular properties of acetals is their ease of machining compared to various other engineering plastics, including nylon, as well as such as HDPE and UHMW. Acetals tend not to deflect away from or grab machining tools and they also chip nicely, making them ideal if an application requires the material to be machined.
There are distinct differences between nylons and acetals:
- nylon offers superior tensile strength and bending stiffness
- nylon can also handle higher loads and higher temperatures
- nylon is susceptible to UV radiation unless special additives are incorporated
- nylon reacts poorly to changes in humidity, which cause it to swell and lose tensile strength
- nylon is a self-lubricating material
- acetal provides higher impact resistance and cold resistance
- acetal is suitable for moderate loads.
- acetal has much better wear resistance and chemical resistance
- acetal has greater dimensional stability and resists moisture and humidity
- acetal is shiny, while nylon appears dull in comparison
Like nylon, acetal comes in different formulations, though not so many, of which the main two are acetal copolymer (acetal-C) and acetal homopolymer (acetal-H).
The differences are slight: H has better mechanical properties than C, including higher strength and stiffness, better creep resistance and higher hardness rating. However, C has the better chemical properties, hydrolysis resistance, higher continuous allowable service temperature in air and less outgassing.
One of the biggest differences between C and H is centreline porosity, which is a characteristic of H but not C. Centreline porosity is caused by gasses trying to escape during the cooling process after extrusion or compression. It can appear as small bubbles in thicker rods or a white line down the middle of each cut edge of a sheet.
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it is very good information on Nylon. generally the understanding is that Nylon 6,6 is much better than Nylon 6. this information clarified the technicality.
we are doing compounding. can you please email more details on Nylon.
Good information . I also observed that because of longer chain Nylon 6/6 has better chemical resistance than Nylon 6.
I was testing for resistance against saturated solution of calcium chloride .
thank you for the input on polyamides. Question for the group- is the below statement accurate? (that 6 is better than 66 for weathering). I have heard it said both ways but I have personally never seen data. Thanks.
But 6/6 has high moisture absorption and low impact strength and ductility when dry. It is the most sensitive of all the nylons to UV and oxidative degradation
Very good technical information, please share more articles to improve my technical knowledge
from PA6GF30 PA66GF30 which one has better oil/grease resistance ?
One important advantage of 66 compared to 6 is a higher HDT
I agree. True statement.
A clamp load relaxation comparison would help us all.
Experience indicates that all nylons will loose about 50% clamp load in 7 days at room temperature, requiring a crush limiter for retained clamp load.
Experience also indicates that all nylons will severely degrade when exposed to Aggressive Water – fermented 15% ethanol gasoline.
I see studies for gasoline exposure but not for aged ethanol laced gasoline i.e.6 month stored lawnmowers, boats, snow blowers etc.
Evidence indicates that PPS does best when ethanol laced gasoline is used, your suggestions?
how it is the resistance Nylon 6/6 with natural gas service and
co2 content and h2s on high pressure for valves components
Andy Pye
Thanks for very good information on Nylon family. Many dubts are clear now.
Pulin Jardosh
This information betwen the diferents Nylon is very important, for choose the better accordin to the application
The comparasion between acetal vs nylon also gives very importan information foy your choise.