Methods for reducing the flammability of cellulosic substrates

Details Methods for reducing the flammability of cellulosic substrates Methods for reducing the flammability of cellulosic substrates

1999-06-09

2002-12-10

Delcotto, Gregory (Department: 1751)

Bleaching and dyeing; fluid treatment and chemical modification

Chemical modification of textiles or fibers or products thereof

Cellulose fibers

C008S116100, C008S116400, C008S120000, C008S129000, C008S184000, C008S185000, C008S127100

Reexamination Certificate

active

06491727

ABSTRACT:

FIELD OF THE INVENTION
The present application relates to methods for reducing the flammability of cellulosic substrates, including cotton fiber carpets and raised surface apparel.
BACKGROUND OF THE INVENTION
Cotton, like most textile fibers, is combustible. Whenever cotton is in the presence of oxygen and the temperature is high enough to initiate combustion (360-420° C.), untreated cotton will either burn (flaming combustion) or smolder (smolder combustion). The degree of flammability depends on the fabric construction. Fabrics have different flammability requirements depending on the particular end use. Cotton fabrics, without the use of special flame-retardant finishes, meet practically all of these requirements for most existing end-uses. However, some new cotton product developments require special constructions or finishes to reduce their flammability. This is especially true in certain countries, such as the United States, which have strict regulations governing the flammability of these products.
Resistance to burning is one of the most useful properties that can be imparted to cotton fibers and textiles. Some end uses for cotton in textiles for apparel, home furnishings, and industry, can depend on its ability to be treated with chemical agents (flame-retardants) that confer flame resistance (FR). End uses requiring flame-retardant finishes include protective clothing (e.g., foundry workers apparel and fire fighters uniforms), children's sleepwear, furnishing/upholstery, bedding, carpets, curtains/drapes, and tents.
Chemical agents for reducing the flammability of products containing cotton fiber and other cellulosic fibers are well known and generally grouped into two categories: durable and non-durable. The durable type tend not to be removed in conventional washes and the non-durable type are typically removed in conventional washes.
The variable manufacturing cost of a typical durable flame-retardant treatment is about $1-2 per yard, depending on fabric weight and other factors. This can be a major limitation. The flammability and flame resistance of cotton has been studied extensively and several comprehensive reviews of the subject are available.
Cotton is not currently the raw material of choice in the carpet industry. The carpet fiber business in the U.S. is roughly a 7,000,000 bale/year market, and cotton is less than one percent of this overall market. One reason that cotton has been almost excluded from this large market for fibers is the difficulty in complying with the Flammable Fabrics Act. This regulation requires that all carpets which are six feet by four feet or larger and are sold for residential use pass a flammability test. This test is commonly referred to as the “Pill Test”. It calls for igniting a methenamine pill, which is placed in the center of a nine-inch by nine-inch carpet specimen. The specimen fails if the flame spreads to within one inch of a metal template containing an eight-inch diameter hole, which is placed on top of the carpet specimen prior to igniting the pill. The specimen passes if the flame does not spread to within one inch of the metal template.
For a residential carpet to be saleable, at least seven out of eight specimens must pass the test. Furthermore, if the carpet has been treated with a flame-retardant (with the exception of alumina trihydrate added to the back coating), then the carpet must be washed ten times as described in AATCC 124-1967 prior to testing.
There are numerous man-made fiber carpets which are currently available, many of which do not require any special treatments to pass federal flammability requirements because of the nature of the test. Many synthetic carpet fibers will melt away from the burning pill during the pill test, such that the pill eventually self extinguishes. The fuel load provided by these carpets in a fire, which is already burning, is not considered by the test method.
Other synthetic fiber carpets, such as polypropylene, require a flame-retardant such as alumina trihydrate. Alumina trihydrate is often added to a backcoating (or backing), as opposed to application directly to the carpet fibers. Synthetic thermoplastic fibers such as polypropylene melt quickly when exposed to a flame, for example, during the pill test. The burning pill then quickly falls, due to gravity, onto the backing. The backing typically includes three layers: a thermoplastic (usually polypropylene) primary backing layer, a latex adhesive layer (which may contain the flame-retardant) and a secondary thermoplastic (usually polypropylene) backing layer. Since the primary backing is also a low melting point thermoplastic, it quickly melts and allows the burning pill to come into direct contact with the latex. Since the latex often includes a flame-retardant, it can then suppress the spread of flames.
Certain other fibers, such as wool and modacrylic, are inherently flame resistant. These can be made into carpets which require no special treatments to pass the required pill test.
Cotton carpets can also be made which require no special treatments to pass the pill test. For example, a cut pile carpet can be made from a 3/2 Ne yarn composed of 90 percent cotton and 10 percent low melt thermoplastic fiber. The low melt fiber is allowed to melt, typically prior to tufting of the carpet. A carpet which includes 12 stitches per inch, {fraction (1/11)}-inch gauge, and ¼ inch pile height can be constructed from this yarn. Such a carpet is generally dense enough, with a sufficiently low pile height, that it will pass the pill test without any additional treatment.
A disadvantage of relying on such low pile height constructions when manufacturing cotton carpets is that it is very limiting from a design and marketing standpoint. The consumer in the U.S. today has become accustomed to a wide variety of choices when selecting a carpet. Substantially limiting the choices of carpet construction is not a practical option for a successful marketing program.
Another disadvantage of attempting to reduce the flammability of a cotton (or cellulosic) carpet by construction alone is that achieving reduced flammability often means increasing the area density (oz./square yard) of the carpet. As the area density of the carpet increases, the cost also generally increases. This approach is therefore very restrictive and would limit the market to the small, upper price end.
Alumina trihydrate, which is effective on certain thermoplastic fiber carpets, is not typically effective on cotton-containing carpets. On cotton-containing carpets, the cotton yarn which is under and in the vicinity of the burning pill will tend to char but maintain sufficient integrity to support, insulate and separate the burning pill from the carpet backing. There is not a sufficient heat flux reaching the alumina trihydrate contained in the latex backing for the alumina trihydrate to be effective at suppressing the flame.
The use of flame-retardant low melt fibers in place of the typical non-flame-retardant low melt fiber used in the yarn has been attempted. The low melt fiber, in general, offers the advantages of improved resilience and tuft definition and minimizes shedding of loose fibers from the tufts. Testing has shown that flame retardant low melt fiber used in the yarn is not effective. Although various explanations have been offered, the mechanism is not understood.
Since federal law in the U.S. requires that any carpet which has a flame-retardant treatment (other than alumina trihydrate) be laundered ten times prior to flammability testing, any such flame-retardant which is applied for that purpose must remain effective after the ten home launderings. Because home launderings are rather effective at removing materials which are not chemically bonded to the fibers, durable flame retardants are generally the most effective.
There have been many techniques for imparting durable flame resistance properties to cellulosic substrates described in the literature. However, there are relatively few that are practiced today, due to commercial availability

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