What are the secondary properties of textile fibers?
1. Morphology/Physical Shape:
Morphology also referred to as the physical structure textile fiber, is a broad term which includes characteristics of fiber such as physical shape i.e. longitudinal and cross-section of fiber along with color, luster, fineness, crimp, etc. fiber morphology helps to establish the properties of various fibers and influences the end-use and performance of the fiber.
Let’s discuss each one of them:
• Physical Shape: The shape of fiber covers longitudinal and cross-section, surface contour, irregularities, and the average length of textile fibers.
The longitudinal section of cotton, wool, and silk are quite distinct and can be used to identify these fibers easily. The longitudinal section of cotton is flat and twisted while wool has the presence of overlapping scales and silk possesses a smooth rod-like structure.
And thus, each natural fiber has some characteristic features. However, the identification of certain synthetic fibers such as nylon, polyester, and acrylic can not be done since their longitudinal section does not show any distinction from one another.
• Colour: There is a wide range of colors in the natural fibers. The fibers obtained from natural sources vary in color such as wool obtained from sheep has different colors based on the breed and area of the body from which it is sheared. Similarly, plant-based fiber also differs in color due to the variety in crop and processing techniques.
The color of the synthetic fibers is generally off-white or yellowish and can be further whitened by the process of bleaching.
• Luster: Lustre is the sheen or gloss that a fiber possesses. It is determined by the amount of light that is reflected by the fiber. Thus, the fiber exhibits high luster when it reflects the incident light evenly while a scattered incident light is observed in fiber which is dull.
In addition, the surface irregularity and cross-section of fiber are some of the factors that influence whether the incident light will reflect evenly or scattered. For example, wool has the presence of overlapping scales and cotton has convolutions at irregular intervals as a result of which the incident light gets scattered on reflection.
Let us understand the effect of cross-section on luster: round, oval and trilobal cross-section impart a high degree of luster while irregular, kidney-shaped, and octagonal cross-section has a low degree of luster.
Among natural fibers such as silk has a high luster while unmercerised cotton has a low luster. The luster of man-made cotton can be regulated by controlling the amount of delusterant (a substance added before the spinning process) added.
• Translucency: It is the property of the fiber to transmit light. Natural fibers such as cotton and wool staple fibers have some surface irregularity and so tend to be opaque while synthetic fibers and silk exhibit high luster and are translucent.
2. Specific Gravity:
The specific gravity of a fiber is the density relative to that of water (at 4). Also, the density of water at that temperature is 1. The performance and laundering are affected by fiber density. The fiber will float, if the specific gravity of fiber is less than 1, and so the procedures such as washing and dyeing are difficult to carry out. For example, olefins.
The specific gravity also determines density or mass per unit volume, it can be said the higher is the specific gravity, the heavier is the fabric. Thus, specific gravity and density are two related properties.
3. Elongation and Elastic Recovery:
The amount of extension or stretch a fiber accepts is referred to as elongation or extension. It can be considered that elongation occurs in two situations either when the fiber is subjected to some force or elongation at break.
Elastic recovery is defined as the ability of fibers to return to their original length after being stretched. A fiber with 100% elastic recovery returns to its original position after being stretched to a specific extent or degree for a specific time period. Thus, time duration, amount and type of force applied, number of times stretching takes place and the degree to which a fiber is stretched are some of the factors that determine the elastic recovery of the fiber.
Both elongation and elastic recovery must be thought of together in fabric examination since they guide factors such as comfort and appearance through shape retention. One such example is the lycra blends.
The property of the fabric to return back to its original position after being creased or folded is called resiliency. Resiliency indicates the crease recovery and elastic recovery of the fiber.
This property is assessed qualitatively and ranges from excellent to poor.
Excellent resiliency is shown by fibers such as polyester, wool, and nylon while fibers such as flax, rayon, and cotton have low resilience.
5. Moisture Regain:
The ability of a bone-dry fiber to absorb moisture is called moisture regain. Moisture regain is expressed in percentage. In other words, moisture regain is the ratio of the weight of water in material to oven-dry weight. Moisture may be absorbed by some fibers, while others may absorb it. Absorption is defined as the movement of moisture along the surface of the fiber. The factors that promote the absorption of water in fiber are the chemical and physical structure and properties of the fiber, as well as the temperature and humidity of the surroundings.
6. Flammability and Thermal Reactions:
The flammability of a fiber deals with the burning characteristics. Each fiber group behaves differently when exposed to some amount of heat and thus is also a method of identification.
When we study the reaction of a particular fiber group to flame the following stages are considered: 1. Behavior when approaching flame 2. When in the flame 3. After being removed from flame 4. Odor 5. Residue. The thermal properties of a fiber are also studied along with the flammability. Thermal characteristics are important in their use and care. For instance, in every stage of washing, drying and ironing certain temperatures are mentioned which indicate the ability of fiber to withstand heat.
Lastly, synthetic fibers are less flame-resistant as compared to natural fibers because they are composed of thermoplastic materials which are sensitive to heat and so reach the Glass Transition Temperature (Tg) easily at which they soften and melt. Although this property helps in the heat setting of synthetic fibers. Melting temperature (Tm) is that at which the material initiates to liquefy and is permanently damaged.
7. Abrasion resistance:
When a fiber is worn out or damaged when being rubbed against another surface, then it is referred to as abrasion. There are three types of abrasion: 1. Flex Abrasion (when fabric bends or folds and is rubbed against another surface) 2. Flat or plane Abrasion (when a flat surface is rubbed against another surface) 3. Edge Abrasion (when the curved edges such as collars and cuffs are rubbed).
However, abrasion resistance is the ability of the fiber to resist the force applied that may lead to abrasion. This force or stress is resisted by fiber when a mobile surface comes in contact with the fiber. Thus, the fiber exhibits good abrasion resistance when it absorbs and disseminates the force without being damaged.
Abrasion resistance is often measured in terms of toughness and hardness of fiber which is bought by the physical, chemical and morphological properties of fibers. Abrasion resistance of synthetics such as nylon and polyester is excellent, acrylics have good, viscose has fair, while silk has poor resistance.
Pilling is another property related to abrasion. Due to the abrasion, the surface of fiber breaks and forms small bundles of fiber that cling to the surface or may fall off.
This property of abrasion resistance is important since to understand the end-use and wear characteristics of a fiber. These fibers are employed effectively in various areas of technical textiles such as medical textiles, safety and protective gear, marine industry etc.
8. Chemical reactivity:
It is vital to understand the chemical composition and reactivity of fibers since the manufacturing and processing of fibers involve the use of the chemicals. This property is effectively studied as it plays an important role in manufacturing, application of finishes, and care of fabrics.
The fibers with the same chemical composition react in a similar manner when reacted with acids, alkalies and organic solvents. For example, cellulosic fibers are fairly resistant to alkalies but get harmed by acids.