The first article in this series defined performance as “the ability to withstand use and maintenance while retaining physical integrity and minimizing changes in appearance.” In the second article, I discussed the properties that influence appearance retention at the fiber level.

In this installment, I’ll cover the aspects of carpet construction and coloration that have the greatest impact on appearance retention.

Appearance Retention in Two Easy Steps

It is helpful to think of performance in terms of characteristics that, at a minimum, don’t exaggerate appearance changes and, where possible, actively contribute to appearance retention. In other words, the specifier should try to:

• Accentuate the positive — Maximize visual features that change least under traffic.

• Eliminate the negative — Minimize visual features that change most under traffic.

Minimizing Appearance Changes

Regardless how carpet looks when it is new, traffic affects its appearance in some pretty predictable ways. If you’re thinking that the carpet that would show the least change in appearance is one whose appearance has already been changed by traffic, then you’re thinking in the right direction.

Now you might be wondering, “You want me to buy carpet that already looks old? Are you insane?” No, that’s not what I’m suggesting. What I mean is that since traffic is going to cause certain inevitable appearance changes, new carpet that already shares certain visual characteristics with trafficked carpet will show the least change in appearance. Makes sense?

Think about it this way: Compared to unused areas, trafficked pile typically has:

• less tip-definition
• a flatter, less textured surface
• less luster

So which would show the greatest change in appearance: A highly-textured cut-pile style with highly lustrous fibers, or a short, dense, untextured style with finer yarns, less tip definition and delustered fibers? The answer is the latter, because its characteristics differ less from those of trafficked pile.

Continuing with that premise, here are the most important factors to consider to minimize traffic-induced appearance changes:

Pile type — Loop pile carpet outperforms cut pile carpet of similar density and fiber type. Loop-pile constructions also eliminate the problem of twist loss. Additionally, performance characteristics like shading and pile reversal are greatly reduced — virtually eliminated, in fact — in loop-pile styles.

Pile weight — I’m listing pile weight here primarily to call out the fact that its importance is greatly exaggerated in relation to appearance retention. Remember that “total pile yarn weight” is just that — all of the pile yarn in the carpet. In many commercial products, as much as half (and sometimes more) of the yarn is buried in the carpet’s back, where it contributes nothing to the carpet’s performance. Some residential styles like shag might have an awful lot of yarn, but the density is so low that it performs terribly. What’s more interesting than weight is density, described below.

Yarn type — All other things being equal, carpet carpet of continuous-filament (BCF) yarns tends to retain its appearance better than carpet with staple pile yarns. Appearance problems like fuzzing are greatly reduced or completely eliminated with BCF yarns.

Yarn Size — In cut-pile constructions, smaller yarns tend to retain their twist considerably better that larger yarns. Large three– and four–ply yarns, once used in popular residential “cable” styles, are especially susceptible to twist loss. The largest yarns virtually explode under foot traffic and it’s fair to say that those styles are only suitable as decorative floor coverings in untrafficked areas.

Pile fiber — In my previous post in this series, I made light of the “ideal fiber” question by answering without explanation that solution-dyed nylon had the best balance of the Big Six performance properties a carpet fiber needs.

It is true that the suitability of a fiber for a particular application depends upon how well its characteristics match the use conditions, and since use conditions vary from job to job — and even within large installations — it is reasonable to conclude that no single fiber is perfect for all applications. However, nylon has the best balance of properties for the vast majority of uses, so much so that the cases where it is not the best choice are the exceptions rather than the rule.

Solution-dyed nylon is rare in residential styles (the range of colors is too limited), its availability in the contract arena arguably makes it the ideal fiber for commercial applications where the budget allows.

Pile thickness — Pile thickness should balance the desired aesthetics with the performance needed for the application. Generally, the shorter the pile, the better the performance.

Pile density — Density is the mass per volume unit of the carpet’s pile. Generally, the higher the density, the better the carpet performs. However, in cut–pile styles, excessively high density may result in concentration of the effects of traffic on the tips of the tufts, resulting in accelerated flaring and flattening. This rarely is a concern in residential styles, but is sometimes observed in extremely dense commercial styles, particularly with fibers of higher luster.

Tuft (or loop) density — The number of tufts or loops per area, usually expressed in tufts or loops per square inch. Higher tuft density decreases the definition of individual ends; the less tip definition the pile has to begin with, the less definition there is to lose under traffic.

Twist (applies only to cut-pile styles) — Generally, the tighter the twist the better, unless the twist is to the point of imparting texture, in which case it may be more of a detriment than a benefit.

Disguising Appearance Changes

While the characteristics above can be controlled to minimize appearance changes, this second group of characteristics are those that actively disguise the effects of traffic. They do this by confusing the eye — giving it a lot more visual information to process so that the traffic-related changes are only a small part of the carpet’s overall appearance.

Pile texture — Texture can either help or hurt appearance retention, depending on how the texture is created. Textures that depend upon features that will be changed fairly quickly by traffic exaggerate texture change. As a general rule, the less texture there is to begin with, the less there is too lose, and the less texture change will occur between trafficked and untrafficked areas. On the other hand, texture helps disguise the normal shading characteristics of cut-pile styles as well as helping to hide footprints and lines from vacuuming.

Shade — Colors that are too dark tend to show texture change and abrasion more quickly than lighter colors; however, colors that are too light have poor soil hiding characteristics and require a higher level of maintenance. The optimum midrange colors provide a proper balance between being dark enough to minimize soil visibility but not so dark that they emphasize texture changes and shading.

Color — The closer to the color of dirt, the better. Browns, including taupe, darker tans and the like are closest to the color of most soil.

Pattern and Coloration — Generally, the more boldly and busily patterned a carpet is, the better it will disguise changes in appearance, provided that the pattern itself is not dependent on factors that are quickly altered by traffic, like texture. (Patterns created by texture variations may “walk out” quickly under heavy use.) More durable patterns of construction and/or color are better for long-term appearance retention.

Patterns of pre-dyed yarn generally remain more sharp and vibrant than printed patterns. Yarns with heathered (multiple shades of a single color) and tweed (multiple colors) colorations help break up the uniformity of solid-colored styles and are very effective at enhancing appearance–retention. Graphic and random (e.g., floral) patterns are even more effective at hiding changes in appearance.

Shedding is a term used to describe the release of loose fiber — usually unsecured staple or shear lint — from carpet. This condition is most evident when carpet is new, and it usually diminishes within a few weeks or months with frequent, effective vacuuming.

Consumers sometimes notice that dust settled on furniture consists largely of small fibers, and fibers can sometimes be seen floating in the air in beams of sunlight.

These observations lead some consumers, especially those with allergy problems, to question the appropriateness of carpet as a floor covering; they wonder whether such fibers may be inhaled and cause respiratory irritation. Even some physicians have, without really thinking about it, recommended that patients with allergies remove carpet.

What are those floating fibers?

A concerned homeowner in Northern California contacted me with this exact question. She had just moved into a new home with new carpet, and noticed fibers floating in the air and accumulating on table tops and other horizontal furniture surfaces.

I asked her to sent me dust samples collected from furniture, the contents of her vacuum cleaner’s bag, and a sample of the installed carpet. Microscopic examination of the dust samples confirmed that the dust was in fact comprised almost entirely of fibers; however, the fibers in the dust were found to be primarily cotton and polyester, evidently from clothing and other textiles in the home.

The carpet sample she sent confirmed what she had told me: Its pile was 100% continuous filament nylon. Shedding is normally observable only in carpet of staple yarns; continuous-filament yarns release very little fiber, if any, and usually they release virtually none.

I sifted through the dust samples and the contents of her vacuum cleaner bag and, as you might expect, not a single carpet fiber was found in any of the samples.

Can airborne fibers be inhaled?

An attorney once asked me, “How large a particle can a person inhale?” My first answer was, “It depends how determined they are to inhale it.” More or less accurate, but more levity than he was looking for.

Here’s the serious answer:

This chart shows the size ranges in microns (millionths of a meter) of various particles and fibers. The human respiratory system is remarkably effective at filtering out airborne particles larger than 10 microns, which is about the threshold of visibility. So as a general rule, if it’s visible to the unaided eye, it’s too big to be inhaled into the lungs. Particles larger than 10 microns are arrested in the nose and sinus, where they are trapped and either disposed of or expelled.

Even small apparel fiber fragments, which are around 3-10 microns across, are at least thousands of microns long. (One quarter inch is over 6,000 microns.) That’s small enough to remain suspended in the air for quite some time, but far too large to make it into the lungs.

Can carpet fibers remain airborne?

No, carpet fibers cannot stay aloft for more than a few seconds at most. As textile fibers go, carpet fibers are enormous. Their smallest dimension — their cross section — is around 20-50 microns across, hundreds of times the cross-sectional area of fibers used in applications like clothing, upholstery, towels and bedding materials. (See figures 1 and 2.)

If they do somehow become airborne, kicked up by foot traffic for example, their sheer size and density cause them to drop out of the airspace very quickly. (You may demonstrate this for yourself by holding a handful of loose carpet fiber and gently releasing individual filaments; they fall like stones.)

What about allergies?

The word allergy is frequently used incorrectly to refer to just about any irritation. For example, a person who experiences lactose intolerance may say that they are allergic to milk, when in fact an allergy to milk proteins is an altogether different ailment.

A true allergic reaction is a response of the body’s immune system. In the indoor environment, the most common allergens are animal dander and saliva, dust mite excreta, cockroaches and other insects, food products, and pollen.

As discussed above, there is no scenario where carpet fibers (or even apparel fibers) can present an inhalation problem, and I am unaware of any case of an actual allergic reaction being cause by any component of carpet itself.

Tests have demonstrated that well-maintained carpet makes a strong positive contribution to the quality of the indoor environment, including the cleanliness of the air. Available evidence indicates that carpet tends to collect and hold soil, dust, and other materials until they can be removed by cleaning, rather than allowing them to float freely in the air. This is true, of course, as long as this “soil trap” is kept clean.

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.