Specification II: Fibers

In the introductory article to this series, I defined performance as “the ability to withstand use and maintenance while retaining physical integrity and minimizing changes in appearance.”

I’ll tackle the “minimizing changes in appearance” part first. However, before I do that, it’s important to do a brief refresher on fiber characteristics, so I’ll use this post to review the properties that are most relevant to the life of a carpet fiber.

Before we get started, I should qualify the sweeping generalizations I’m going to make. Every time I cover this subject, someone gets fixated on some tangential exception and misses the point of the overview, starting with the fact that it’s an overview. So if you find yourself thinking that’s not always true, you’re probably right. Few things are always true. This article describes things that are generally true most of the time, and in fact these generalizations hold up in all but the most exceptional cases.

So, lest anyone split hairs, get their undies in a bunch, or start speaking in clichés, here are the qualifiers:

1. The comparisons below assume all other factors are equal, so when I compare the resilience of polyester to that of nylon, I’m comparing them in equivalent constructions. None of this “well, if you make polyester carpet extra dense…” stuff here. Strictly apples to apples.

2. This article is only about carpet fibers — nylon, wool, polyester, acrylic and polypropylene. (For now I’ll skip specialty fibers like sisal, cotton, coir and silk.)

When you assess a performance property of a carpet fiber, the only comparisons that matter are those against other carpet fibers, so those are the only ones we’re concerned with here. When I say wool has poor stain resistance (and I’m going to), don’t respond with something like, “well, not compared to linen.” That’s relevant for clothes, but not for carpet.

3. Finally, yes, there are variations in how each fiber type exhibits each characteristic. For example, there are noteworthy differences between nylon 6, nylon 6,6 and cationic-dyeable nylon; there are variations between different polyesters and huge differences between various wools. Even so, those variations usually aren’t enough to change the way one fiber type ranks relative to other types.

The Big Six

Of all the fiber properties we could talk about, six are of particular interest to carpet specifiers:

Resilience — Resilience is the ability of fibers to return to their original upright position after being bent over or compressed under dynamic (momentary) or static (prolonged) loads. It’s the characteristic that enables pile to spring back after being stepped on and prevents it from being prematurely compacted under traffic.

Nylon offers the best resilience. Wool comes in second, followed by acrylic. Both nylon and wool recover well with cleaning. Polyester has only moderate resilience and tends to flatten rather quickly in trafficked areas. Polypropylene has the poorest resilience of the bunch and becomes crushed even more quickly and severely than polyester.

Twist retention — Twist retention is the ability of tufts in cut-pile constructions to resist bursting, flaring and untwisting under traffic. (Twist retention is not a factor in loop-pile constructions.) In most cases, twist retention is roughly inversely correlated with resilience; the better the twist retention, the worse the resilience, and vice-versa.

Based on that, you might conclude correctly that polypropylene is the king of twist retention, followed by polyester and acrylic, which also do quite well. Relatively speaking, nylon and wool rank last; however, it’s worth noting that their twist retention is highly variable and can range from very bad to quite good according to the size of the yarn (smaller is better) and the quality of yarn processing.

Abrasion resistance — Abrasion resistance is the ability of a fiber to resist becoming scratched and dulled by abrasive soil and traffic. When fibers become abraded, they appear dull and cloudy in much the same way a piece of clear plastic would be scratched and dulled by sandpaper. Regardless of how effectively it’s cleaned, the surface would be permanently dulled. (Figures 1 and 2.)

Abrasion can be a visible problem when a high-luster fiber becomes dulled by traffic and soil. The original luster of the pile cannot be restored regardless of the effectiveness of cleaning. Cleaners call this condition “traffic lane gray.”

The visual effects of abrasion can be disguised by delustering the fiber with a pigment such as titanium dioxide. Nylon and polyester exhibit very good abrasion resistance, whereas polypropylene tends to become abraded rather easily.

Wool exhibits the poorest abrasion resistance, and it is not terribly unusual to see wool carpet in heavy traffic areas or on stairs that has worn completely through to the backing materials. On the other hand, aside from physically wearing away the pile, abrasion on wool doesn’t degrade its appearance in the same way it does on, for instance, polypropylene. Wool often tends to “age gracefully” with a kind of classic-looking patina, rather than just looking “old” like synthetics tend to.

Colorfastness — Colorfastness is the ability to resist becoming discolored by light (particularly ultraviolet light in sunlight), cleaning, chemicals, atmospheric agents (i.e., oxidizing gases such as ozone and oxides of sulfur and nitrogen), and the friction of traffic (crocking).

Polypropylene has outstanding colorfastness. Polyester and acrylic also are generally excellent. Nylon and wool exhibit comparatively poor colorfastness; however, nylon can be pigmented (solution dyed) to give it colorfastness equal to or better than polyester and acrylic.

I said that I wasn’t going to dig into the variances within fiber types, but as it relates to colorfastness it’s worth two short detours. The first is that nylon 6,6 provides substantially greater colorfastness than nylon 6, particularly to gases like ozone, nitrogen oxides and sulfur oxides. The vast majority of gas-fading cases are on type 6 nylon.

The second point is that nylon can be solution-dyed, like polypropylene, in which case its colorfastness is nearly as good as that of polypropylene.

Cleanability — Cleanability is the ability of a fiber to readily release soil — water-soluble, oily and insoluble particulates — during vacuuming and cleaning.

Generally, more absorbent fibers like nylon and wool have better cleanability; however, absorbency also makes a fiber more susceptible to staining. Conversely, more hydrophobic fibers like polypropylene and polyester, which also have an affinity for oily soils, are more difficult to clean, but have excellent stain resistance. Though hydrophobic fibers often require more aggressive cleaning, their chemical makeup allows them to withstand it.

Cleanability can be enhanced considerably by soil-repellent fluorochemical finishes, usually Scotchgard or Teflon. These finishes enhance fibers’ willingness to release particulate and oily soils from their surface and increase the effectiveness of vacuuming and cleaning.

Wool is the only carpet fiber with inherently good cleanability without the addition of a fluorochemical. The cleanability of flourochemical–treated nylon is also quite good. Polyester and acrylic don’t rank quite as well; they have only fair cleanability and can be relatively difficult to clean even if treated with a fluorochemical. It is important to note that fluorochemical finishes alone add little resistance to staining by dyes.

Polypropylene is quite difficult to clean, but its outstanding chemical resistance allows the fiber to withstand unusually aggressive cleaning.

Stain resistance — Stain Resistance is the ability of a fiber to resist becoming permanently stained (having color added) by foreign substances like soil and dyes. Stains differ from soil in that stains are not removed by cleaning alone.

Stain resistance is determined primarily by a fiber’s inherent chemical resistance, lack of absorbency, and affinities for foreign substances.

Polypropylene is the undisputed winner in the stain resistance category. Polyester and acrylic also offer impressive stain resistance, though prolonged exposure to oily soils may leave them stained.

Nylon has the poorest inherent stain resistance of any of the synthetics. On acid-dyeable nylon, stain resistance may be enhanced by modifying the fiber with a finish that inhibits its receptiveness to anionic acid dyes; e.g., some food colorings like FD&C red #40. (Note that this “stain blocking” finish imparts no real resistance to any type of stain other than acid dyes.) Even the best stain-resistant nylon ranks a distant last among the synthetics; however, solution-dyed nylon can withstand some aggressive stain-removal chemicals, including careful, professional use of dilute hypochlorite bleach. (Hypochlorites are extremely destructive oxidizers and cause physical damage to nylon, even when its color remains intact.)

Wool’s stain resistance ranks dead last among carpet fibers, and the fiber can neither be modified enough to change its ranking nor cleaned with aggressive chemistry.

Some Notes About Blends

There are at least two reasons a manufacturer would design a carpet using two or more pile fibers:

Cost — One reason fibers are blended is to reduce cost. A wool/nylon blend costs less than pure wool, at least in the North American market. A nylon/polyester blend costs less than 100% nylon.

Styling — There are some interesting things you can do with carpet containing different yarn types, particularly with coloration. For example, you can cross-dye it or print a pattern that only colors some of the yarns, or dye two different fibers two different colors in a single dying process.

Despite those reasons, the specifier should think carefully before choosing a product with mixed pile fiber types.

Blending different fiber types introduces issues that don’t come into play when the carpet has only one pile fiber. When you mix two fibers, you don’t get the best of both; you get the combined weaknesses and limitations of both.

For example, a 50/50 nylon/polyester blend performs very much like 100% polyester, but costs more. And while you could use more aggressive chemicals to remove difficult stains on polyester, you would be limited by the presence of nylon. In other words, it’s more like an overpriced polyester than a bargain-priced nylon, and it lacks polyester’s colorfastness and chemical toughness.

A more extreme example I saw once was a berber loop style with 50/50 wool/polypropylene pile, which flattened and fuzzed just like you would expect. Cleaning-wise, the two fibers make a terrible mix: Polypropylene often requires aggressive cleaning, but wool can’t tolerate it.

Another problem with blends is that different fibers respond differently to things like light, traffic and cleaning, which can result in some pretty strange-looking performance characteristics. For example, a nylon/polypropylene level loop will develop a rough or patterned texture (depending on the construction) in traffic paths, where the polypropylene yarns become permanently flattened and the nylon remains resilient. This behavior can lead to the traffic paths appearing to change color, and sometimes this phenomenon appears suddenly when the carpet is cleaned because the nylon springs back to life and the polypropylene stubbornly remains flattened.

The Optimum Balance

If the overview above made fiber selection seem too complicated, rest assured that it’s not that bad. It boils down to selecting the fiber with the best balance of the “Big Six” properties. Which fiber is that?

The long, textbook answer: It depends. The fiber that is best for the job is determined by how its mix of properties matches the traffic, soiling and cleaning requirements, with special consideration for job-specific things like sunlight exposure, indoor swimming pools, and the like.

The short answer: Solution-dyed nylon.

More on this in the next article in this series.

One Response to “Specification II: Fibers”

1. cousin Says:
   November 30th, 2007 at 6:49 pm

   All things being equal makes for an excellent control, but equal is the last thing carpet manufacturers want. Australian carpet manufacturer’s early foray into solution dyed nylon was predominantly nylon 6.6, and squarely aimed at the commercial market.
   I would have to agree, it makes one of the best carpet fibres I’ve come across (all things being equal!).
   Unfortunately the carpet industry in Australia seems to have re discovered sdn, but for the most are targeting nylon 6.
   To make matters worse, the (overseas) supplier is sourcing the sdn from different countries which is leading to significant differences in batches.
   The two big problems we are facing is differences in amine end groups, leading to rapid take up of dye stuffs (spillages) and in many cases production listening too much to marketing.
   This leads to a reduction in dye blocker application, and a change from ‘wet’ to dry application of dye blocker.
   So not only do we have a fibre with a higher propensity to absorb stains, the problem is exacerbated by a reduced application of dye blocker.
   And just to compound this issue, one carpet manufacturer, that has been frustrated with the problems this yarn has put forward, has now decided to bring in nylon 6.6 to promote to the residential market, but the old nylon 6 they were struggling with, is being shunted off to the commercial marked to create havoc there.
   Unfortunately it’s resulting in sdn getting a black eye here in Australia.

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