Yellowing by CR MITCHELL
Following is a lengthy article written by CR Mitchell, (Carrie?) on yellowing.
By C.R. Mitchell
(--Used with permission)
This article clarifies the issue of BHT yellowing of carpet, as the issue has mushroomed
out of proportion in recent
months. Several years ago some carpet manufacturers sent out letters indicating that
claims would not be paid where
BHT originating from the pad caused yellowing of the carpet.
In the early 1990's carpet manufacturers began seeing an increase in the number of
complaints of carpets turning yellow.
While these new complaints occur in a wide variety of circumstances, some common factors
have been identified. The
most common factor is that the carpets are usually installed over rebonded polyurethane
cushion. This type cushion now
comprises about 75 percent of the market. A few instances have occurred where prime
urethane cushions have been
Testing the urethane cushion under the carpet usually shows significant levels of a
chemical called butylated
hydroxytoluene (BHT). This chemical is used as an antioxidant (preservative) in processed
foods, the production of
polyurethane foam, many plastics, etc. It is capable of vaporizing and migrating up
through the carpet and depositing on
the pile. Under some conditions, BHT reacts with atmospheric contaminants to form bright
yellow derivatives which bond
to the fiber. Testing of the yellowed carpets shows the presence of two of the breakdown
products of BHT.
The carpets involved are almost invariably light colors. Obviously, any discoloration is
more visible on light colors.
Lighter colors have predominated in the marketplace during the past decade, hence the
opportunity for occurrence is
greater. All synthetic fibers, polypropylene, polyester and nylon carpets are affected;
Shaw has not seen the phenomenon
on wool carpet.
Several cushion manufacturers have published information suggesting that carpet mills'
position and letters are not
accurate or are misleading. Most claim that the majority of yellowing complaints are due
to causes other than BHT and
cite many other causes of discoloration. While many of the items mentioned do indeed
discolor carpet, few cause
yellowing. None cause the distinctive lemon yellow that is peculiar to BHT yellowing. Most
other materials that cause
yellowing result in dull or brownish yellows, very different from BHT. The characteristics
of these other types of
discoloration are described at the end of this article. BHT and its two breakdown products
can be positively identified in
the laboratory, although few carpet manufacturers have the capability for such chemical
Reviewing the chemistry, BHT is an antioxidant, or preservative, used in a wide variety of
foods, plastics, etc. The
production of slab urethane foam generates high temperatures and BHT is added to prevent
scorching and possible fires.
Even though BHT is a solid material, it slowly vaporizes form the cushion and can migrate
up through the carpet and
deposit on the pile fiber. In rare cases, it reacts with atmospheric contaminants such as
those created by the burning of
fossil fuels to form two breakdown products. Unfortunately these byproducts happen to be
intensely lemon yellow even in
very small amounts - they are visible on white fiber at the single digit parts-per-million
BHT yellowing appears in several scenarios. First, is in the form of bright yellow
splotches which may be more intense
near the backing and less intense near the surface of the pile. Second, is overall
yellowing where large areas are
involved. Third is where yellowing occurs under furniture or area rugs in geometric
shapes. Often, straight lines of
non-yellowed carpet are found along a wall over the tack strip or along seams where
seaming tape prevents migration of
BHT through the carpet backing. While the intensity of the yellow color may vary from
faint to very bright, the color is
always a pure lemon yellow - this is distinctive of BHTs breakdown products.
Most rebonded polyurethane cushion contains some BHT, as well as many prime cushions, and
together these types
comprise about 90 percent of the market. Despite this fact, BHT yellowing is still very
rare. Statistics indicate that less
than one claim in a thousand involved yellowing of any kind.
A very special combination of atmospheric contaminants, moisture, light and other
conditions must come together for
this phenomenon to occur. Most complaints occur where the heating seasons are long and
from fossil fuels are present. Carpet cleaners are often blamed for causing yellowing. In
BHT yellowing cases the
cleaning process only triggers or accelerates the color change. The cleaning provides the
moisture and raises the pH of
Determining conclusively whether BHT is involved requires sophisticated lab equipment and
analysis has usually been by a technique called gas chromatography/mass spectroscopy
(GC/MS). While normally
considered reliable, in the case of analysis of BHT-related yellowing its results are
often misinterpreted. BHT is
semi-volatile, meaning it vaporizes very, very slowly at room temperature. The GC/MS
technique involves heating the
carpet or pad sample in a small closed chamber, collecting whatever vaporized material
comes off, and passing it
through a separatory column. The separated chemicals are then identified by the mass
spectrometer. GC/MS can identify
the colorless BHT easily and reliably.
Not generally recognized is the fact that the yellow material is not BHT, but the
derivatives left when BHT oxidizes. Two
compounds, diphenquinone and bis-phenozyquinone are the most common. Both compounds are
intensely yellow at
very low concentrations. They cannot be analyzed by GS/MS techniques because they are not
readily volatile. They
must be extracted off the fiber using a suitable solvent and the solvent containing the
derivatives analyzed. Analysis by
liquid chromatography, HPLC, which, as the name implies does the separation in the liquid
state, is the preferred
When yellowed fibers are analyzed by CG/MS, a negative result for BHT is often
misinterpreted to mean that BHT was not
involved. The fact is that there is no BHT present on the carpet fiber because it has all
reacted into the quinone
derivatives, which cannot be found by this method. Since most labs do the analysis by
GC/MS because it is fast and
usually accurate, much erroneous data has been developed. Unlike the GC/MS, the HPLC
detects both BHT and the
Even more confusion has been caused by the belief that treatment with citric acid always
reverses the yellowing, and is
therefore a conclusive test for BHT. This is not true.. While acid treatment does reverse
the yellowing in many cases, it is
not universal and is absolutely unreliable as a test for BHT, as it may also reverse other
types of yellowing. Further, it may
not reverse BHT yellowing that has existed for some time. Some say that the chemical
reaction can only proceed at high
pH levels (highly alkaline). The fact is that it can occur at the neutral point, although
more slowly. While treatment with a
mild acid often removes the yellowing by converting the yellow byproducts to colorless
compounds - this may not be
permanent and the yellow color may return, especially if a volatile acid such as acetic
(vinegar) is used.
Some of the information being circulated indicates that BHT yellowing can only be
reproduced in the lab at
unrealistically high concentrations. The polyurethane foam industry tried without success
for several years to cause
yellowing in the lab at concentrations commonly found in complaints. Shaw Industries has
succeeded in reproducing it
at concentrations similar to those commonly found in complaints. That lab technique was
shared with the Carpet Cushion
Council and the Polyurethane Foam Association at their Technical Conference in 1997.
The whole issue of BHT yellowing has mushroomed somewhat out of proportion for several
First, some carpet manufacturers issued disclaimers for this phenomenon several years ago.
Others have taken a
case-by-case approach without issue of a disclaimer. Most mills have decided that BHT
yellowing is not the fault of the
carpet and the focus has shifted to the cushion manufacturers, who are now faced with
claims for carpet which are for
more costly per square yard than cushion. Despite claims that most yellowing complaints do
not involve cushion, a recent
Floor Covering Weekly article quoted the Director of the Carpet Cushion Council
"....combine the increased use of
bonded or prime urethane foam with the increasing consumer preference for light colored
carpets and the result is more
complaints." Several cushion manufacturers have recently announced that they now
offer BHT-free prime urethane
Second, much confusion exists in this situation. There have been allegations that carpet
raw materials, particularly
polypropylene fibers and backings, contain BHT for various reasons, including to prevent
yellowing! The truth is that BHT
is not used in the production of these materials. The use of acid treatment as a
"test" for BHT has added to the confusion;
this is not a reliable test. It gives a "maybe" at best.
Third, some carpet manufacturers have tried to capitalize on the situation. One has
offered to pay BHT yellowing claims -
in the gulf coast area. This is a very safe offer since very, very few complaints occur
here; most occur in the snow belt
where heating seasons are long.
Although these claims have indeed increased in recent years with the increasing market
share of rebonded urethane
cushions, the total number of complaints remains very small, but the issue has become
highly visible. Carpet mills should
continue to address yellowing claims on a case by case basis, and those found to be the
result of causes other than BHT
should be handled appropriately. Where BHT and/or its breakdown products are present, the
complaints should be
referred to the cushion manufacturer.
Various types of discoloration
Information has been circulated suggesting that a number of conditions listed below cause
yellowing similar to BHT.
Some clarification is in order. In considering yellow discoloration on site, the first
step is to decide whether the yellowing
represents a gain in color or a loss of color.
Ozone fade - ozone fade is rare in moist areas of the country today, occurring typically
where humidity is high for most of
the year; i.e., coastal areas and Hawaii. Ozone preferentially destroys blue dyes first.
With most carpet colors, this leaves
the yellow and red components intact and the carpet turns a dull orange, not yellow.
Further, this is a loss rather than a
gain of color.
Oxides of nitrogen yellowing - This rare phenomenon occurs in the manufacturing plant,
when undyed nylon is stored
for long periods in areas where air circulation is poor. If oxides of nitrogen are present
from gas-fired process equipment or
gas-powered lift trucks, yellowing may occur. Dye sites are blocked and the affected areas
of the carpet cannot be dyed.
This yellowing occurs in the mill, not in the consumer's home, and carpet manufacturers
took steps to eliminate stale air
Formaldehyde - Some discoloration under furniture was traced to formaldehyde emissions
from particle board subfloors
and furniture during the 1970s. This caused the yellow to turn dark green, not yellow.
Since formaldehyde emissions were
brought under control by environmental regulations many years ago, this has not been seen
in more than two decades.
Asphalt sealers - These contaminants are very dull brownish yellow in color and are found
in areas where foot traffic
enters directly from newly paved or sealed asphalt parking lots. The discoloration is most
visible in the traffic lanes near
the door and is less noticeable farther into the building. This occurs primarily in
commercial situations while BHT
yellowing typically occurs in residential uses.
Calcium chloride - this material is used to melt snow and ice and is tracked onto carpet
in the winter. It is a dirty white,
not yellow in color, and is removed with water. Over time this area will attract soil and
the color can turn from white to
dingy yellow to brown to black.
Excessive stain blocker application - The phenolic stain blockers used in the early days
of stain resist treatments (ca.
1987-89) did turn yellow under some circumstances. This was a very dull, brownish yellow,
very different from the lemon
yellow caused by BHT. Today's stain blockers are chemically different and do not turn
"Optical brightner" carpet dyes - Optical brighteners are not carpet dyes as
alleged, but are sometimes used in cleaning
agents. They sometimes turn yellow. Shaw Industries' maintenance literature has
specifically recommended against their
use for years, not because of yellowing but because of their tendency to mask the carpet
color resulting in a chalky
Oxidation of fiber spin finish - While some spin finishes used in years past have been
demonstrated to turn a very faint,
dull or dingy yellow, carpet manufacturers routinely screen their finishes to insure that
these are not used. This has
become even more important over the past decade due to the move toward light colors. This
particular yellow color did
not resemble BHT yellowing at all and is easily distinguished; it does not respond to acid
Pesticides - In the early 1980s, two new dyestuffs were introduced to the carpet industry.
It was soon learned that these
were susceptible to discoloration by organophosphate pesticides. In this case, the carpet
turned green, not yellow. These
dyes were withdrawn from the market within a year of the discovery.
Silicone-based carpet protectors - some of these products were used in a base of crude
mineral spirits in the early
1980s. The mineral spirits residues sometimes turned dull yellow. After a number of fires
and instances of carpet
delaminating, the use of mineral spirits was discontinued. No carpet manufacturer
recommends these products and most
warranties are voided if silicones are used.
Soil abrasion of fiber - In heavy commercial traffic areas with sandy soil, the surface of
the fiber gets scratches, which
caused the fiber's luster to become dull, but no change in color occurs.
Urine - obviously, urine can result in yellow spots. Again, distinguishing from BHT
yellowing is easy. Urine fluoresces a
yellow-green under UV light (black light) and the odor is a dead giveaway. The yellow is
very dull, not lemon yellow as in