The present invention relates to polyester copolymer compositions suitable for synthetic filaments and to fibers and fabrics that can be made from such compositions. In particular, the invention relates to compositions that will produce fibers that can be blended and dyed with cotton under conditions that are more typically favorable for cotton than for polyester.
The use of synthetic compositions to produce filaments, fibers, and then fabrics is well established. Accordingly, improvements in such entrenched compositions can be particularly advantageous. Such improvements are, of course, more valuable when they enhance desired characteristics of filaments, fibers, fabrics, and items—very often clothing—made from such compositions.
Working backwards, a garment is typically formed of a fabric that is either woven or knitted from yarns. In turn, yarns are formed from individual fibers joined together, most commonly using well known and well established spinning processes.
Natural fibers—the most common are cotton and wool—have characteristics that produce desired properties in yarns, fabrics, and garments. For example, wool has (among other advantages) excellent thermal properties, and remains insulating when wet. Unless treated properly, however, wool can be abrasive and thus uncomfortable when in contact with skin for extended intervals. Cotton produces fabrics that are comfortable and breathable, but can lose its thermal insulation properties when wet. Further advantages of cotton, wool, and other natural fibers are generally well understood in the art.
In the same manner, synthetic fibers have some properties that are subjectively better then natural fibers, some of which can include (particularly in the case of polyester) strength, durability, and “memory.”
Accordingly, one of the goals in producing or designing or developing synthetic compositions for eventual use as fibers, yarns and fabrics is to take advantage of some of the favorable properties of synthetics, while matching as closely as possible—or in some cases improving upon—the desired properties of natural fibers (e.g., the thermal insulation of wool, but less abrasive; the comfort of cotton, but with better thermal properties when wet).
In the clothing industry, the ability to produce garments with desired colors is a fundamental goal. The nature of both natural and synthetic fibers and their underlying chemical compositions requires, however, that color be obtained by some type of dyeing process. Depending upon circumstances, fibers can be dyed as fiber, filament, yarn, fabric, or even as a garment. Furthermore, because in many cases consumers expect to be able to wash and dry garments in machines many times, an associated goal is to obtain garments that can withstand such repeated machine washing and drying while still maintaining most or all of the desired color. Related goals include light fastness (typically with respect to exposure to sunlight) and (using active wear as another example) color stability when exposed to perspiration.
Fundamentally, the relationship between the color of a garment and its lifetime will be based upon the chemical composition of the underlying fibers and the chemical composition of an appropriate dye composition. As is well understood in the art, a dye is technically defined as “a colorant that becomes molecularly dispersed at some point during application to a substrate and also exhibits some degree of permanence.” Tortora, FAIRCHILD'S DICTIONARY OF TEXTILES, Seventh Edition, 2009 Fairchild Publications.
Dye is typically categorized as either natural (e.g., from plants) or synthetic (e.g., typically developed from other compositions using principles and techniques of organic chemistry).
The dyeing characteristics of a fiber are based upon the composition from which the fiber is formed. The desired property is referred to as “dyeability,” which is defined as the “capacity of fibers to accept dyes.” (Tortora, supra).
In the manufacture or garments, it is also common to blend synthetic fibers with natural fibers in proportions that produce a finished garment with desired properties. For a number of reasons, blends of cotton and polyester have long been popular. Based on that, compositions and methods for producing dyed color in cotton-polyester blends has been and remains a desired outcome. The natures of the two different fibers, however, present practical problems. For example, cotton can be conveniently dyed with “reactive dyes” that can be successfully added to a cotton substrate at temperatures of about 150° F.
On the other hand, the properties of polyester (i.e., the polymer formed from the condensation esterification and then polymerization of terephthalic acid and ethylene glycol) required that polyester be dyed with “disperse dyes;” i.e., small particles of colorant suspended in water.
Coloring polyester with disperse dyes tends to require significantly higher temperatures; typically above 250° F. and frequently on the order of 270° F. or higher. In many cases, high pressure (i.e., above atmospheric pressure) is also required to successfully dye polyester, or to reach the temperatures required to dye the polyester.
As further comparative factors, cotton dyeing tends to be driven by the pH of the dye solution or composition (typically in a basic environment); while polyester dyeing tends to be driven by the temperature, and conventionally requires the addition and performance of supplementary chemicals commonly referred to as “carriers” or “leveling agents.” From the standpoint of economics, disperse dyes (sometimes referred to as “high energy” dyes because of the conditions required) are more expensive than reactive dyes, and sometimes by as much as a factor of 5-10 times on a comparative basis.
Because of the differences in the dyeing compositions and the dyeing conditions, it is conventional practice to dye cotton and polyester separately.
In some conventional methods, blended cotton-polyester fabric is dyed in two separate steps. In a first step, the fabric is dyed in a slightly acidic bath at a temperature of about 270° F. or higher (e.g., using a disperse dye) in order to get the polyester to accept the dye. The partially dyed fabric is then scoured or rinsed, and thereafter dyed in a cotton-appropriate dye (e.g., a direct or reactive dye) at a basic pH and at a temperature of about 150° F. Because many cotton dyestuffs will degrade at the polyester dying temperatures, the two steps cannot be combined.
As another factor that must be addressed, high dyeing temperatures tend to degrade the elasticity of stretch fibers such as spandex that are often included in cotton-polyester fabrics and garments. Some versions of spandex can withstand high dyeing temperatures (e.g. 270° F.), but are proportionately more expensive than versions that have essentially the same end-use properties, but that tend to degrade when dyed at such higher temperatures.
As yet another factor, perceived color (e.g., of a garment) is a combination of the interaction of light, the material the light illustrates, and the resulting perception of the human eye. In terms of textile dyeing, the color of the dye is based upon the functional groups in the dye molecules. Stated differently, different colors in textiles are a function of dye molecules with different compositions. Not all dye colors (i.e., the underlying molecules) perform, however, in the same manner with either natural or synthetic fibers, yarns, and garments. Thus a fiber, yarn, blend, or fabric may accept certain dye colors relatively straightforwardly while rejecting (to some greater or lesser extent) other dye colors under the same conditions.
Furthermore, additives are often used to control or adjust the properties of a polymer melt, and the features of such additives are likely to change either the dyeing characteristics or the spinning characteristics or both.
As another factor, synthetic fibers—and certainly including polyester—are typically manufactured by polymerizing the starting materials and thereafter extruding a melt of the polymer through small openings in a device referred to as a spinneret; a process referred to as “spinning.” Those experienced in synthetic and natural fibers will immediately recognize that the term “spinning” is used to refer to two entirely different processes. In one meaning (and since antiquity) spinning refers to the step of twisting individual fibers together and pulling them into a yarn. In the manufacture of synthetic fibers, the extrusion of filaments from a melt into solidified polymer filaments is also referred to as “spinning.” The difference is normally clear in context. Typically, the solidification of the extruded filaments is encouraged or advanced using a quenching step, in which a carefully controlled airflow is directed against the extruded filaments.
The properties required of a composition that can be melt and spun in this fashion, however, may be unrelated to, or disadvantageous in combination with, the properties that produce good dyeing characteristics. Composition characteristics that produce the proper viscosity for spinning may be entirely unrelated, and in some cases directly opposite to, those properties that produced good dyeing characteristics. Thus, designing or adjusting the composition of a polymer, copolymer or copolymer blend to improve the spinning properties may result in less desired or even unacceptable dyeing properties.
For example, in order to “spin” properly, a melted polymer must have a certain fluidity (viscosity) that permits the extrusion to produce coherent liquid filaments (i.e. that won't separate) at the spinneret while avoiding a viscosity that too low (“watery”) to control the spinning process for its intended purpose. Because the viscosity of a polymer melt is proportional to temperature, the degree of polymerization, and to other polymer properties, the spinning temperature must be appropriate as well. Stated differently, the melted polymer must be able to perform at the indicated temperature.
In the context of synthetic fibers and their manufacture, the term “melt viscosity” refers to the specific resistance of the melted polymer to deformation or flow under any given conditions. The term “intrinsic viscosity” is used to describe a characteristic that is directly proportional to the average molecular weight of a polymer. Intrinsic viscosity is calculated on the basis of the viscosity of a polymer solution (in a solvent) extrapolated to a zero concentration. Thus, the intrinsic viscosity is a characteristic that will affect the melt viscosity, but the melt viscosity is also related to other factors, particularly including the temperature of a melt.
As yet another factor, because synthetic fibers originate as a filament, they must be cut and textured (not necessarily in that order) to gain other properties that are desirable in a finished yarn, fabric, or garment. In most cases, the texturing step requires that the synthetic filament or fiber be mechanically or thermally formed into a shape other than a straight extruded filament. Accordingly, the need to texturize polyester adds another set of properties that must be accounted for and that may compete against the properties that enhance polymerization, spinning, or dyeing.
Thus, a need exists for polymer compositions that can produce a fiber that can be dyed with cotton in a single step.