Cotton Production

Cotton production practices begin after the previous crop is harvested. The first operations usually include shredding stalks, ripping out roots and disking the soil. Fertilizer and herbicides generally are applied and incorporated into the soil before the land is bedded in preparation for needed irrigation or planting. Since soil characteristics and past fertilization and cropping practices can cause a wide range of fertility levels in cotton soils, fertility programmes should be based on soil test analyses. Control of weeds is essential to obtain high lint yield and quality. Cotton yields and harvesting efficiency can be reduced by as much as 30% by weeds. Herbicides have been widely used in many countries for weed control since the early 1960s. Application methods include pre-planting treatment to foliage of existing weeds, incorporation into pre-plant soil and treatment at pre-emergence and post-emergence stages.

Several factors that play an important role in achieving a good stand of cotton plants include seed-bed preparation, soil moisture, soil temperature, seed quality, seedling disease infestation, fungicides and soil salinity. Planting high-quality seed in a well-prepared seed-bed is a key factor in achieving early, uniform stands of vigorous seedlings. High-quality planting seed should have a germination rate of 50% or higher in a cool test. In a cool/warm test, the seed vigour index should be 140 or higher. Seeding rates of 12 to 18 seeds/metre of row are recommended to obtain a plant population of 14,000 to 20,000 plants/hectare. A suitable planter metering system should be used to ensure uniform spacing of seed regardless of seed size. Seed germination and seedling emergence rates are closely associated with a temperature range of 15 to 38 ºC.

Early-season seedling diseases can hamper uniform stands and result in the need to replant. Important seedling disease pathogens such as Pythium, Rhizoctonia, Fusarium and Thielaviopsis can reduce plant stands and cause long skips between seedlings. Only seed that has been properly treated with one or more fungicides should be planted.

Cotton is similar to other crops with respect to water use during different plant developmental stages. Water use is generally less than 0.25 cm/day from emergence to the first square. During this period, loss of soil moisture by evaporation may exceed the amount of water transpired by the plant. Water use increases sharply as the first blooms appear and reaches a maximum level of 1 cm/day during the peak bloom stage. Water requirement refers to the total amount of water (rainfall and irrigation) needed to produce a crop of cotton.

Insect populations can have an important impact on cotton quality and yield. Early-season population management is important in promoting balanced fruiting/vegetative development of the crop. Protecting early fruit positions is essential to achieving a profitable crop. Over 80% of the yield is set in the first 3 to 4 weeks of fruiting. During the fruiting period, producers should scout their cotton at least twice a week to monitor insect activity and damage.

A well-managed defoliation programme reduces leaf trash that can adversely affect the grade of the harvested cotton. Growth regulators such as PIX are useful defoliators because they control vegetative growth and contribute to earlier fruiting.

Harvesting

Two types of mechanical harvesting equipment are used to harvest cotton: the spindle picker and the cotton stripper. The spindle picker is a selective-type harvester that uses tapered, barbed spindles to remove seed cotton from bolls. This harvester can be used on a field more than once to provide stratified harvests. On the other hand, the cotton stripper is a nonselective or once-over harvester that removes not only the well-opened bolls but also the cracked and unopened bolls along with the burs and other foreign matter.

Agronomic practices that produce a high-quality uniform crop will generally contribute to good harvesting efficiency. The field should be well drained and rows laid out for effective use of machinery. Row ends should be free of weeds and grass, and should have a field border of 7.6 to 9 m for turning and aligning the harvesters with the rows. The border also should be free of weeds and grass. Disking creates adverse conditions in rainy weather, so chemical weed control or mowing should be used instead. Plant height should not exceed about 1.2 m for cotton that is to be picked, and about 0.9 m for cotton that is to be stripped. Plant height can be controlled to some extent by using chemical growth regulators at the proper growth stage. Production practices that set the bottom boll at least 10 cm above the ground should be used. Culturing practices such as fertilization, cultivation and irrigation during the growing season should be carefully managed to produce a uniform crop of well-developed cotton.

Chemical defoliation is a culturing practice that induces abscission (shedding) of foliage. Defoliants may be applied to help minimize green-leaf-trash contamination and promote faster drying of early morning dew on the lint. Defoliants should not be applied until at least 60% of the bolls are open. After a defoliant is applied, the crop should not be harvested for at least 7 to 14 days (the period will vary depending on chemicals used and weather conditions). Chemical desiccants may also be used to prepare plants for harvest. Desiccation is the rapid loss of water from the plant tissue and subsequent death of the tissue. The dead foliage remains attached to the plant.

The current trend in cotton production is toward a shorter season and one-time harvest. Chemicals that accelerate the boll opening process are applied with the defoliant or soon after the leaves drop. These chemicals allow earlier harvests and increase the percentage of bolls that are ready to be harvested during the first harvest. Because these chemicals have the ability to open or partially open immature bolls, the quality of the crop may be severely impacted (i.e., the micronaire may be low) if the chemicals are applied too early.

Storage

The moisture content of cotton before and during storage is critical; excess moisture causes stored cotton to overheat, resulting in lint discolouration, lower seed germination and possibly spontaneous combustion. Seed cotton with a moisture content above 12% should not be stored. Also, the internal temperature of newly built modules should be monitored for the first 5 to 7 days of cotton storage; modules that experience a 11 ºC rise or are above 49 ºC should be ginned immediately to avoid the possibility of major loss.

Several variables affect seed and fibre quality during seed cotton storage. Moisture content is the most important. Other variables include length of storage, amount of high-moisture foreign matter, variation in moisture content throughout the stored mass, initial temperature of the seed cotton, temperature of the seed cotton during storage, weather factors during storage (temperature, relative humidity, rainfall) and protection of the cotton from rain and wet ground. Yellowing is accelerated at high temperatures. Both temperature rise and maximum temperature are important. Temperature rise is directly related to the heat generated by biological activity.

Ginning process

About 80 million bales of cotton are produced annually worldwide, of which about 20 million are produced by about 1,300 gins in the United States. The principal function of the cotton gin is to separate lint from seed, but the gin must also be equipped to remove a large percentage of the foreign matter from the cotton that would significantly reduce the value of the ginned lint. A ginner must have two objectives: (1) to produce lint of satisfactory quality for the grower’s market and (2) to gin the cotton with minimum reduction in fibre spinning quality, so that the cotton will meet the demands of its ultimate users, the spinner and the consumer. Accordingly, quality preservation during ginning requires the proper selection and operation of each machine in a ginning system. Mechanical handling and drying may modify the natural quality characteristics of cotton. At best, a ginner can only preserve the quality characteristics inherent in the cotton when it enters the gin. The following paragraphs briefly discuss the function of the major mechanical equipment and processes in the gin.

Seed-cotton machinery

Cotton is transported from a trailer or module into a green-boll trap in the gin, where green bolls, rocks and other heavy foreign matter are removed. The automatic feed control provides an even, well-dispersed flow of cotton so that the gin’s cleaning and drying system will operate more efficiently. Cotton that is not well dispersed can travel through the drying system in clumps, and only the surface of that cotton will be dried.

In the first stage of drying, heated air conveys the cotton through the shelves for 10 to 15 seconds. The temperature of the conveying air is regulated to control the amount of drying. To prevent fibre damage, the temperature to which the cotton is exposed during normal operation should never exceed 177 ºC. Temperatures above 150 ºC can cause permanent physical changes in cotton fibres. Dryer-temperature sensors should be located as near as possible to the point where cotton and heated air come together. If the temperature sensor is located near the exit of the tower dryer, the mixpoint temperature could actually be 55 to 110 ºC higher than the temperature at the downstream sensor. The temperature drop downstream results from the cooling effect of evaporation and from heat loss through the walls of machinery and piping. The drying continues as the warm air moves the seed cotton to the cylinder cleaner, which consists of 6 or 7 revolving spiked cylinders that rotate at 400 to 500 rpm. These cylinders scrub the cotton over a series of grid rods or screens, agitate the cotton and allow fine foreign materials, such as leaves, trash and dirt, to pass through the openings for disposal. Cylinder cleaners break up large wads and generally condition the cotton for additional cleaning and drying. Processing rates of about 6 bales per hour per metre of cylinder length are common.

The stick machine removes larger foreign matter, such as burs and sticks, from the cotton. Stick machines use the centrifugal force created by saw cylinders rotating at 300 to 400 rpm to “sling off” foreign material while the fibre is held by the saw. The foreign matter that is slung off the reclaimer feeds into the trash-handling system. Processing rates of 4.9 to 6.6 bales/hr/m of cylinder length are common.

Ginning (lint-seed separation)

After going through another stage of drying and cylinder cleaning, cotton is distributed to each gin stand by the conveyor-distributor. Located above the gin stand, the extractor-feeder meters seed cotton uniformly to the gin stand at controllable rates, and cleans seed cotton as a secondary function. The moisture content of cotton fibre at the extractor-feeder apron is critical. The moisture must be low enough that foreign matter can be easily removed in the gin stand. However, the moisture must not be so low (below 5%) as to result in the breakage of individual fibres as they are separated from the seed. This breakage causes an appreciable reduction both in fibre length and lint turnout. From a quality standpoint, cotton with a higher content of short fibres produces excessive waste at the textile mill and is less desirable. Excessive breakage of fibres can be avoided by maintaining a fibre moisture content of 6 to 7% at the extractor-feeder apron.

Two types of gins are in common use—the saw gin and the roller gin. In 1794, Eli Whitney invented a gin that removed fibre from the seed by means of spikes or saws on a cylinder. In 1796, Henry Ogden Holmes invented a gin having saws and ribs; this gin replaced Whitney’s gin and made ginning a continuous-flow process rather than a batch process. Cotton (usually Gossypium hirsutum) enters the saw gin stand through a huller front. The saws grasp the cotton and draw it through widely spaced ribs known as huller ribs. The locks of cotton are drawn from the huller ribs into the bottom of the roll box. The actual ginning process—separation of lint and seed—takes place in the roll box of the gin stand. The ginning action is caused by a set of saws rotating between ginning ribs. The saw teeth pass between the ribs at the ginning point. Here the leading edge of the teeth is approximately parallel to the rib, and the teeth pull the fibres from the seed, which are too large to pass between the ribs. Ginning at rates above those recommended by the manufacturer can cause fibre quality reduction, seed damage and choke-ups. Gin stand saw speeds are also important. High speeds tend to increase the fibre damage done during ginning.

Roller-type gins provided the first mechanically aided means of separating extra-long staple cotton (Gossypium barbadense) lint from seed. The Churka gin, which has an unknown origin, consisted of two hard rollers that ran together at the same surface speed, pinching the fibre from the seed and producing about 1 kg of lint/day. In 1840, Fones McCarthy invented a more efficient roller gin that consisted of a leather ginning roller, a stationary knife held tightly against the roller and a reciprocating knife that pulled the seed from the lint as the lint was held by the roller and stationary knife. In the late 1950s, a rotary-knife roller gin was developed by the US Department of Agriculture (USDA) Agricultural Research Service’s Southwestern Cotton Ginning Research Laboratory, US gin manufacturers and private ginneries. This gin is currently the only roller-type gin used in the United States.

Lint cleaning

Cotton is conveyed from the gin stand through lint ducts to condensers and formed again into a batt. The batt is removed from the condenser drum and fed into the saw-type lint cleaner. Inside the lint cleaner, cotton passes through the feed rollers and over the feed plate, which applies the fibres to the lint cleaner saw. The saw carries cotton under grid bars, which are aided by centrifugal force and remove immature seeds and foreign matter. It is important that the clearance between the saw tips and grid bars be properly set. The grid bars must be straight with a sharp leading edge to avoid reducing cleaning efficiency and increasing lint loss. Increasing the lint cleaner’s feed rate above the manufacturer’s recommended rate will decrease cleaning efficiency and increase loss of good fibre. Roller-ginned cotton is usually cleaned with non-aggressive, non-saw-type cleaners to minimize fibre damage.

Lint cleaners can improve the grade of cotton by removing foreign matter. In some cases, lint cleaners may improve the colour of a lightly spotted cotton by blending to produce a white grade. They may also improve the colour grade of a spotted cotton to light spotted or perhaps white colour grade.

Packaging

The cleaned cotton is compressed into bales, which must then be covered to protect them from contamination during transportation and storage. Three types of bales are produced: modified flat, compress universal density and gin universal density. These bales are packaged at densities of 224 and 449 kg/m³ for the modified flat and universal density bales, respectively. In most gins cotton is packaged in a “double-box” press wherein the lint is initially compacted in one press box by a mechanical or hydraulic tramper; then the press box is rotated, and the lint is further compressed to about 320 or 641 kg/m³ by modified flat or gin universal density presses, respectively. Modified flat bales are recompressed to become compress universal density bales in a later operation to achieve optimum freight rates. In 1995, about 98% of the bales in the United States were gin universal density bales.

Fibre quality

Cotton quality is affected by every production step, including selecting the variety, harvesting and ginning. Certain quality characteristics are highly influenced by genetics, while others are determined mainly by environmental conditions or by harvesting and ginning practices. Problems during any step of production or processing can cause irreversible damage to fibre quality and reduce profits for the producer as well as the textile manufacturer.

Fibre quality is highest the day a cotton boll opens. Weathering, mechanical harvesting, handling, ginning and manufacturing can diminish the natural quality. There are many factors that indicate the overall quality of cotton fibre. The most important ones include strength, fibre length, short fibre content (fibres shorter than 1.27 cm), length uniformity, maturity, fineness, trash content, colour, seedcoat fragment and nep content, and stickiness. The market generally recognizes these factors even though not all are measured on each bale.

The ginning process can significantly affect fibre length, uniformity and the content of seedcoat fragments, trash, short fibres and neps. The two ginning practices that have the most impact on quality are the regulation of fibre moisture during ginning and cleaning and the degree of saw-type lint cleaning used.

The recommended lint moisture range for ginning is 6 to 7%. Gin cleaners remove more trash at low moisture but not without more fibre damage. Higher fibre moisture preserves fibre length but results in ginning problems and poor cleaning, as illustrated in figure 1. If drying is increased to improve trash removal, yarn quality is reduced. Although yarn appearance improves with drying up to a point, because of increased foreign-matter removal, the effect of increased short-fibre content outweighs the benefits of foreign-matter removal.

Figure 1. Moisture-ginning cleaning compromise for cotton

Cleaning does little to change the true colour of the fibre, but combing the fibres and removing trash changes the perceived colour. Lint cleaning can sometimes blend fibre so that fewer bales are classified as spotted or light spotted. Ginning does not affect fineness and maturity. Each mechanical or pneumatic device used during cleaning and ginning increases the nep content, but lint cleaners have the most pronounced influence. The number of seedcoat fragments in ginned lint is affected by the seed condition and ginning action. Lint cleaners decrease the size but not the number of fragments. Yarn strength, yarn appearance and spinning-end breakage are three important spinning quality elements. All are affected by length uniformity and, therefore, by the proportion of short or broken fibres. These                                                                                                                     three elements are usually preserved best when cotton is                                                                                                                                 ginned with minimum drying and cleaning machinery.

Recommendations for the sequence and amount of gin machinery to dry and clean spindle-harvested cotton were designed to achieve satisfactory bale value and to preserve the inherent quality of cotton. They have generally been followed and thus confirmed in the US cotton industry for several decades. The recommendations consider marketing-system premiums and discounts as well as the cleaning efficiency and fibre damage resulting from various gin machines. Some variation from these recommendations is necessary for special harvesting conditions.

When gin machinery is used in the recommended sequence, 75 to 85% of the foreign matter is usually removed from cotton. Unfortunately, this machinery also removes small quantities of good-quality cotton in the process of removing foreign matter, so the quantity of marketable cotton is reduced during cleaning. Cleaning cotton is therefore a compromise between foreign matter level and fibre loss and damage.

Safety and Health Concerns

The cotton ginning industry, like other processing industries, has many hazards. Information from workers’ compensation claims indicates that the number of injuries is highest for the hand/fingers, followed by back/spine, eye, foot/toes, arm/shoulder, leg, trunk and head injuries. While the industry has been active in hazard reduction and safety education, gin safety remains a major concern. The reasons for the concern include the high frequency of accidents and workers’ compensation claims, the large number of lost work days and the severity of the accidents. Total economic costs for gin injuries and health disorders include direct costs (medical and other compensation) and indirect costs (time lost from work, downtime, loss in earning power, higher insurance costs for workers’ compensation, loss of productivity and many other loss factors). Direct costs are easier to determine and much less expensive than indirect costs.

Many international safety and health regulations affecting cotton ginning are derived from US legislation administered by the Occupational Safety and Health Administration (OSHA) and the Environmental Protection Agency (EPA), which promulgates pesticides regulations.

Other agricultural regulations may also apply to a gin, including requirements for slow-moving vehicle emblems on trailers/tractors operating on public roadways, provisions for rollover protective structures on tractors operated by employees and provisions for proper living facilities for temporary labour. While gins are considered agricultural enterprises and are not specifically covered by many regulations, ginners will likely want to conform to other regulations, such as OSHA’s “Standards for General Industry, Part 1910”. There are three specific OSHA standards that ginners should consider: those for fire and other emergency plans (29 CFR 1910.38a), exits (29 CFR 1910.35-40) and occupational noise exposure (29 CFR 1910.95). Major exit requirements are given in 29 CFR 1910.36 and 29 CFR 1910.37. In other countries, where agricultural workers are included in mandatory coverage, such compliance will be compulsory. Compliance with noise and other safety and health standards is discussed elsewhere in this Encyclopaedia.

Employee participation in safety programmes

The most effective loss control programmes are those in which management motivates employees to be safety conscious. This motivation can be accomplished by establishing a safety policy that gets the employees involved in each element of the programme, by participating in safety training, by setting a good example and by providing employees with appropriate incentives.

Occupational health disorders are lessened by requiring that PPE be used in designated areas and that employees observe acceptable work practices. Hearing (plugs or muffs) and respiratory (dust mask) PPE should be used whenever working in areas having high noise or dust levels. Some people are more susceptible to noise and respiratory problems than others, and even with PPE should be reassigned to work areas with lower noise or dust levels. Health hazards associated with heavy lifting and excessive heat can be handled by training, use of materials-handling equipment, proper dress, ventilation and breaks from the heat.

All persons throughout the gin operation must be involved in gin safety. A safe work atmosphere can be established when everyone is motivated to participate fully in the loss control programme.

Back

Cotton accounts for almost 50% of the worldwide consumption of textile fibre. China, the United States, the Russian Federation, India and Japan are the major cotton-consuming countries. Consumption is measured by the amount of raw cotton fibre purchased and used to manufacture textile materials. Worldwide cotton production is annually about 80 to 90 million bales (17.4 to 19.6 billion kg). China, the United States, India, Pakistan and Uzbekistan are the major cotton-producing countries, accounting for over 70% of world cotton production. The rest is produced by about 75 other countries. Raw cotton is exported from about 57 countries and cotton textiles from about 65 countries. Many countries emphasize domestic production to reduce their reliance on imports.

Yarn manufacturing is a sequence of processes that convert raw cotton fibres into yarn suitable for use in various end-products. A number of processes are required to obtain the clean, strong, uniform yarns required in modern textile markets. Beginning with a dense package of tangled fibres (cotton bale) containing varying amounts of non-lint materials and unusable fibre (foreign matter, plant trash, motes and so on), continuous operations of opening, blending, mixing, cleaning, carding, drawing, roving and spinning are performed to transform the cotton fibres into yarn.

Even though the current manufacturing processes are highly developed, competitive pressure continues to spur industry groups and individuals to seek new, more efficient methods and machines for processing cotton which, one day, may supplant today’s systems. However, for the foreseeable future, the current conventional systems of blending, carding, drawing, roving and spinning will continue to be used. Only the cotton picking process seems clearly destined for elimination in the near future.

Yarn manufacturing produces yarns for various woven or knitted end-products (e.g., apparel or industrial fabrics) and for sewing thread and cordage. Yarns are produced with different diameters and different weights per unit length. While the basic yarn manufacturing process has remained unchanged for a number of years, processing speeds, control technology and package sizes have increased. Yarn properties and processing efficiency are related to the properties of the cotton fibres processed. End-use properties of the yarn are also a function of processing conditions.

Yarn Manufacturing Processes

Opening, blending, mixing and cleaning

Typically, mills select bale mixes with the properties needed to produce yarn for a specific end-use. The number of bales used by different mills in each mix ranges from 6 or 12 to over 50. Processing begins when the bales to be mixed are brought to the opening room, where bagging and ties are removed. Layers of cotton are removed from the bales by hand and placed in feeders equipped with conveyors studded with spiked teeth, or entire bales are placed on platforms which move them back and forth under or over a plucking mechanism. The aim is to begin the sequential production process by converting the compacted layers of baled cotton into small, light, fluffy tufts that will facilitate the removal of foreign matter. This initial process is referred to as “opening”. Since bales arrive at the mill in various degrees of density, it is common for bale ties to be cut approximately 24 hours before the bales are to be processed, in order to allow them to “bloom”. This enhances opening and helps regulate the feeding rate. The cleaning machines in mills perform the functions of opening and first-level cleaning.

Carding and combing

The card is the most important machine in the yarn manufacturing process. It performs second- and final-level cleaning functions in an overwhelming majority of cotton textile mills. The card is composed of a system of three wire-covered cylinders and a series of flat, wire-covered bars that successively work small clumps and tufts of fibres into a high degree of separation or openness, remove a very high percentage of trash and other foreign matter, collect the fibres into a rope-like form called a “sliver” and deliver this sliver in a container for use in the subsequent process (see figure 1).

Figure 1. Carding

Wilawan Juengprasert, Ministry of Public Health, Thailand

Historically, cotton has been fed to the card in the form of a “picker lap”, which is formed on a “picker”, a combination of feed rolls and beaters with a mechanism made up of cylindrical screens on which opened tufts of cotton are collected and rolled into a batt (see figure 2). The batt is removed from the screens in an even, flat sheet and then is rolled into a lap. However, labour requirements and the availability of automated handling systems with the potential for improved quality are contributing to the obsolescence of the picker.

Figure 2. A modern picker

Wilawan Juengprasert, Ministry of Public Health, Thailand

The elimination of the picking process has been made possible by the installation of more efficient opening and cleaning equipment and chute-feed systems on the cards. The latter distribute opened and cleaned tufts of fibres to cards pneumatically through ducts. This action contributes to processing consistency and improved quality and reduces the number of workers required.

A small number of mills produce combed yarn, the cleanest and most uniform cotton yarn. Combing provides more extensive cleaning than is provided by the card. The purpose of combing is to remove short fibres, neps and trash so that the resulting sliver is very clean and lustrous. The comber is a complicated machine composed of grooved feed rolls and a cylinder that is partially covered with needles to comb out short fibres (see figure 3).

Figure 3. Combing

Wilawan Juengprasert, Ministry of Public Health, Thailand

Drawing and roving

Drawing is the first process in yarn manufacturing that employs roller drafting. In drawing, practically all draft results from the action of rollers. Containers of sliver from the carding process are staked in the creel of the drawing frame. Drafting occurs when a sliver is fed into a system of paired rollers moving at different speeds. Drawing straightens the fibres in the sliver by drafting to make more of the fibres parallel to the axis of the sliver. Parallelization is necessary to obtain the properties desired when the fibres are subsequently twisted into yarn. Drawing also produces a sliver that is more uniform in weight per unit of length and helps to achieve greater blending capabilities. The fibres that are produced by the final drawing process, called finisher drawing, are nearly straight and parallel to the axis of the sliver. Weight per unit length of a finisher-drawing sliver is too high to permit drafting into yarn on conventional ring-spinning systems.

The roving process reduces the weight of the sliver to a suitable size for spinning into yarn and inserting twist, which maintains the integrity of the draft strands. Cans of slivers from finisher drawing or combing are placed in the creel, and individual slivers are fed through two sets of rollers, the second of which rotates faster, thus reducing the size of the sliver from about 2.5 cm in diameter to that of the diameter of a standard pencil. Twist is imparted to the fibres by passing the bundle of fibres through a roving “flyer”. The product is now called “roving”, which is packaged on a bobbin about 37.5 cm long with a diameter of about 14 cm.

Spinning

Spinning is the single most costly step in converting cotton fibres to yarn. Currently, over 85% of the world’s yarn is produced on ring-spinning frames, which are designed to draft the roving into the desired yarn size, or count, and to impart the desired amount of twist. The amount of twist is proportional to the strength of the yarn. The ratio of the length to the length fed can vary on the order of 10 to 50. Bobbins of roving are placed onto holders that allow the roving to feed freely into the drafting roller of the ring-spinning frame. Following the drafting zone, the yarn passes through a “traveller” onto a spinning bobbin. The spindle holding this bobbin rotates at high speed, causing the yarn to balloon as twist is imparted. The lengths of yarn on the bobbins are too short for use in subsequent processes and are doffed into “spinning boxes” and delivered to the next process, which may be spooling or winding.

In the modern production of heavier or coarse yarns, open-end spinning is replacing ring spinning. A sliver of fibres is fed into a high-speed rotor. Here the centrifugal force converts the fibres into yarns. There is no need for the bobbin, and the yarn is taken up on the package required by the next step in the process.

Considerable research and development efforts are being devoted to radical new methods of yarn production. A number of new spinning systems currently under development may revolutionize yarn manufacturing and could cause changes in the relative importance of fibre properties as they are now perceived. In general, four of the different approaches used in the new systems appear practical for use on cotton. Core-spun systems are currently in use to produce a variety of specialty yarns and sewing threads. Twistless yarns have been produced commercially on a limited basis by a system that bonds the fibres together with a polyvinyl alcohol or some other bonding agent. The twistless yarn system offers potentially high production rates and very uniform yarns. Knit and other apparel fabrics from twistless yarn have excellent appearance. In air-vortex spinning, currently under study by several machinery manufacturers, drawing sliver is presented to an opening roller, similar to rotor spinning. Air-vortex spinning is capable of very high production speeds, but prototype models are particularly sensitive to fibre length variations and foreign matter content such as trash particles.

Winding and spooling

Once the yarn is spun, the manufacturers must prepare a correct package. The type of package depends on whether the yarn will be used for weaving or knitting. Winding, spooling, twisting and quilling are considered preparatory steps for weaving and knitting yarn. In general, the product of spooling will be used as warp yarns (the yarns that run lengthwise in woven fabric) and the product of winding will be used as filling yarns, or weft yarns (the yarns that run across the fabric). The products from open-end spinning by-pass these steps and are packaged for either the filling or warp. Twisting produces ply yarns, where two or more yarns are twisted together before further processing. In the quilling process yarn is wound onto small bobbins, small enough to fit inside the shuttle of a box loom. Sometimes the quilling process takes place at the loom. (See also the article “Weaving and knitting” in this chapter.)

Waste handling

In modern textile mills where control of dust is important, the handling of waste is given greater emphasis. In classical textile operations, waste was collected manually and delivered to a “wastehouse” if it could not be recycled into the system. Here it was accumulated until there was enough of one type to make a bale. In the present state of the art, central vacuum systems automatically return waste from opening, picking, carding, drawing and roving. The central vacuum system is used for cleaning of machinery, automatically collecting waste from under machinery such as fly and motes from carding, and for returning unusable floor sweeps and wastes from filter condensers. The classical baler is a vertical upstroke press which still forms a typical 227-kg bale. In modern wastehouse technology, wastes are accumulated from the central vacuum system in a receiving tank which feeds a horizontal bale press. The various waste products of the yarn manufacturing industry can be recycled or reused by other industries. For example, spinning can be used in the waste spinning industry to make mop yarns, garnetting can be used in the cotton batting industry to make batting for mattresses or upholstered furniture.

Safety and Health Concerns

Machinery

Accidents may occur on all types of cotton textile machinery, though the frequency rate is not high. Effective guarding of the multiplicity of moving parts presents many problems and needs constant attention. Training of operators in safe practices is also essential, in particular to avoid attempting repairs while the machinery is in motion, the cause of many of the accidents.

Each piece of machinery may have sources of energy (electrical, mechanical, pneumatic, hydraulic, inertial and so on) that need to be controlled before any repair or maintenance work is attempted. The facility should identify energy sources, provide necessary equipment and train personnel to ensure that all hazardous energy sources are turned off while working on equipment. An inspection should be performed on a regular basis to ensure that all lockout/tagout procedures are being followed and correctly applied.

Cotton dust inhalation (byssinosis)

Inhalation of the dust generated where cotton fibre is converted into yarn and fabric has been shown to cause an occupational lung disease, byssinosis, in a small number of textile workers. It usually takes 15 to 20 years of exposure to higher levels of dust (above 0.5 to 1.0 mg/m³) for workers to become reactors. OSHA and the American Conference of Governmental Industrial Hygienists (ACGIH) standards set 0.2 mg/m³ respirable cotton dust as measured by the vertical elutriator as the limit for occupational exposure to cotton dust in textile yarn manufacturing. The dust, an airborne particulate released into the atmosphere as cotton is handled or processed, is a heterogeneous, complex mixture of botanical trash, soil and microbiological material (i.e., bacteria and fungi), which varies in composition and biological activity. The aetiological agent and pathogenesis of byssinosis are not known. Cotton plant trash associated with the fibre and the endotoxin from gram-negative bacteria on the fibre and plant trash are thought to be the cause or to contain the causative agent. The cotton fibre itself, which is mainly cellulose, is not the cause, since cellulose is an inert dust that does not cause respiratory disease. Appropriate engineering controls in cotton textile processing areas (see figure 4) along with work practices, medical surveillance and PPE can, for the most part, eliminate the byssinosis. A mild water washing of cotton by batch kier washing systems and continuous batt systems reduces the residual level of endotoxin in both lint and airborne dust to levels below those associated with the acute reduction in pulmonary function as measured by the 1-second forced expiratory volume.

Figure 4. Dust extraction system for a carding machine

Noise

Noise can be a problem in some processes in yarn manufacturing, but in a few modern textile mills the levels are below 90 dBA, which is the US standard but which exceeds noise exposure standards in many countries. Thanks to the abatement efforts of machinery manufacturers and industrial noise engineers, noise levels are continuing to decrease as machinery speeds increase. The solution for high noise levels is the introduction of more modern, quieter equipment. In the United States, a hearing conservation programme is required when noise levels exceed 85 dBA; this would include noise-level monitoring, audiometric testing and making hearing protection available to all employees when noise levels cannot be engineered below 90 dBA.

Heat stress

Since spinning sometimes requires high temperatures and artificial humidificaton of the air, careful monitoring attention is always necessary to ensure that permissible limits are not exceeded. Well designed and maintained air-conditioning plants are increasingly used in place of more primitive methods of temperature and humidity regulation.

Occupational safety and health management systems

Many of the more modern textile yarn manufacturing mills find it useful to have some type of occupational safety and health management system in place to control the workplace hazards that workers may encounter. This can be a voluntary programme like the “Quest for the Best in Health and Safety” developed by the American Textile Manufacturers Institute, or one that is mandated by regulations such as the US State of California Occupational Injury and Illness Prevention Program (Title 8, California Code of Regulations, Section 3203). When a safety and health management system is used, it should be flexible and adaptable enough to allow the mill to tailor it to its own needs.

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Adapted from 3rd edition, Encyclopaedia of Occupational Health and Safety.

The origins of the wool industry are lost in antiquity. Sheep were easily domesticated by our remote ancestors and were important in satisfying their basic needs for food and clothing. Early human societies rubbed together the fibres collected from the sheep to form a yarn, and from this basic principle the processes of manipulating the fibre have increased in complexity. The wool textile industry has been in the forefront in developing and adapting mechanical methods and was therefore one of the early industries in the development of the factory system of production.

Raw Materials

The length of fibre when taken from the animal is the dominant, but not the only, factor determining how it is processed. The type of wool available may be broadly classified into (a) merino or botany, (b) crossbreds—fine, medium or coarse and (c) carpet wools. Within each group, however, there are various grades. Merino usually has the finest diameter and a short length, while the carpet wools are long-fibred, with a coarser diameter. Today, increasing quantities of synthetic fibres simulating wool are blended with the natural fibre and are processed in the same manner. Hair from other animals—for example, mohair (goat), alpaca (llama), cashmere (goat, camel), angora (goat) and vicuña (wild llama)—also plays an important, although subsidiary, role in the industry; it is relatively expensive and is usually processed by specialized firms.

Production

The industry has two distinctive processing systems—woollen and the worsted. The machinery is in many ways similar, but the purposes are distinct. In essence, the worsted system uses the longer stapled wools and in the carding, preparing, gilling and combing processes the fibres are kept parallel and the shorter fibres are rejected. Spinning produces a strong yarn of fine diameter, which then is woven to yield a light fabric with the familiar smooth and firm appearance of men’s suits. In the woollen system, the aim is to intermingle and intertwine the fibres to form a soft and fluffy yarn, which is woven to give a cloth of full and bulky character with a “woolly” surface—for example, tweeds, blankets and heavy overcoatings. Since uniformity of fibre is not necessary in the woollen system, the manufacturer can blend together new wool, shorter fibres rejected by the worsted process, wools recovered from tearing up old wool garments and so on; “shoddy” is obtained from soft, and “mungo” from hard waste material.

It should be borne in mind, however, that the industry is particularly complex and that the condition and type of the raw material used and the specification for the finished cloth will influence the method of processing at each stage and the sequence of those stages. For example, wool may be dyed before processing, at the yarn stage or towards the end of the process when in the woven piece. Moreover, some of the processes may be carried on in separate establishments.

Hazards and Their Prevention

As in every section of the textile industry, large machines with rapidly moving parts pose both noise and mechanical injury hazards. Dust can also be a problem. The highest practicable form of guarding or enclosure should be provided for such generic parts of the equipment as spur gear wheels, chains and sprockets, revolving shafting, belts and pulleys, and for the following parts of machinery used specifically in the wool textile trade:

• feed rollers and swifts of various types of preparatory opening machines (e.g., teasers, willeys, garnetts, rag-grinding machines and so on)

• licker-in or taker-in and adjacent rollers of scribbling and carding machines

• intake between swift and doffer cylinders of scribbling, carding and garnetting machines

• rollers and fallers of gill-boxes

• back shafts of drawing and roving frames

• traps between the carriage and headstock of mules

• projecting pins, bolts and other securing devices used on the beaming-off motion of warping machines

• squeeze rollers of scouring, milling and cloth-wringing machines

• intake between cloth and wrapper and roller of blowing machines

• revolving-knife cylinder of cropping machines

• blades of fans in pneumatic conveying systems (any inspection panel in the ducting of such a system should be at a safe distance from the fan, and the worker should have indelibly impressed on his or her memory the length of time it takes for the machine to slow and come to a stop after the power has been cut off; this is particularly important since the worker clearing a blockage in the system usually cannot see the moving blades)

• the flying shuttle, which presents a special problem (looms should be provided with well-designed guards to prevent the shuttle from flying out of the shed and to limit the distance it might travel should it fly).

The guarding of such dangerous parts presents practical problems. The design of the guard should take into account the working practices connected with the particular process and particularly should preclude possible removal of the guard when the operator is at the greatest risk (e.g., lockout arrangements). Special training and close supervision are required to prevent waste removal and cleaning while machinery is in motion. Much of the responsibility devolves on machinery manufacturers, who should ensure that such safety features are incorporated into new machines at the design stage, and on supervisory personnel, who should ensure that workers are adequately trained in safe handling of equipment.

Spacing of machinery

The risk of accidents is increased if insufficient space is allowed between the machines. Many older premises squeezed the maximum number of machines into the available floor area, thereby reducing the space available for aisles and passageways and for the temporary storage of raw and finished materials within the workroom. In some old mills, the gangways between the carding machines are so narrow that enclosure of the driving belts within a guard is impracticable and recourse has to be made to “wedge” guarding between the belt and the pulley at the in-running point; a well-made and smooth belt fastener is particularly important in these circumstances. Minimum spacing standards, as recommended by a British Government committee for certain wool textile machinery, are required.

Materials handling

When modern mechanical load-handling methods are not employed, there remains the risk of injury from the lifting of heavy loads. Materials handling should be mechanized to the fullest extent possible. Where this is not available, the precautions discussed elsewhere in this Encyclopaedia should be employed. Proper lifting technique is particularly important for workers who manipulate heavy beams into and out of looms or who handle heavy and cumbersome bales of wool in the early preparatory processes. Wherever possible, hand-trucks and movable carts or skids should be used to move such bulky and heavy loads.

Fire

Fire is a serious hazard, especially in old multistorey mills. The mill structure and layout should conform to local regulations governing unobstructed gangways and exits, fire-alarm systems, fire extinguishers and hoses, emergency lights and so on. Cleanliness and good housekeeping will prevent accumulations of dust and fluff, which encourage the spread of fire. No repairs involving the use of flame cutting or flame-burning equipment should be carried on during working hours. Training of all staff in procedures in case of fire are necessary; fire drills, conducted if possible in concert with local fire, police and emergency medical services, should be practised at appropriate intervals.

General safety

Emphasis has been placed on those accident situations which are especially to be found in the wool textile industry. However, it should be noted that the majority of accidents in mills occur in circumstances that are common to all factories—for example, falls of persons and objects, handling of goods, use of hand tools and so on—and that the relevant fundamental safety principles to be followed apply no less in the wool industry than in most other industries.

Health Problems

Anthrax

The industrial disease usually associated with wool textiles is anthrax. It was at one time a great danger, particularly to wool sorters, but has been almost completely controlled in the wool textile industry as a result of:

• improvements in production methods in exporting countries where anthrax is endemic

• disinfection of materials liable to be carrying anthrax spores

• improvements in handling the possibly infected material under exhaust ventilation in the preparatory processes

• microwaving the wool bale sufficiently long to a temperature that will kill any fungi. This treatment also assists in the recovery of lanolin associated with the wool.

• significant advances in medical treatment, including immunization of workers in high-risk situations

• education and training of workers and the provision of washing facilities and, when necessary, personal protective equipment.

Besides anthrax fungal spores, it is known that spores of the fungus Coccidiodes immitis can be found in wool, especially from the southwestern United States. This fungus can cause the disease known as coccidioidomycosis, which, along with the respiratory disease from anthrax, usually has a poor prognosis. Anthrax has the added hazard of causing a malignant ulcer or carbuncle with a black centre when entering the body through a break in the skin barrier.

Chemical substances

Various chemicals are used—for example, for degreasing (diethylene dioxide, synthetic detergents, trichloroethylene and, in the past, carbon tetrachloride), disinfection (formaldehyde), bleaching (sulphur dioxide, chlorine) and dyeing (potassium chlorate, anilines). The risks include gassing, poisoning, irritation of the eyes, mucous membranes and lungs, and skin conditions. In general, prevention relies on:

• substitution of a less dangerous chemical

• local exhaust ventilation

• care in labelling, storage and transport of corrosive or noxious liquids

• personal protective equipment

• good washing facilities (including shower baths where practicable)

• strict personal hygiene.

Other hazards

Noise, inadequate lighting, and the high temperatures and humidity levels required for wool processing may have a deleterious effect on general health unless they are strictly controlled. In many countries, standards are prescribed. Steam and condensation may be difficult to control effectively in dyeing sheds, and expert engineering advice is often needed. In weaving sheds, noise control presents a serious problem on which much work remains to be done. A high standard of lighting is necessary everywhere, particularly where dark fabrics are being manufactured.

Dust

As well as the specific risk of anthrax spores in the dust produced in the earlier processes, dust in high quantities sufficient to induce irritation of the respiratory tract mucosae is produced at many machines, especially those with a tearing or carding action, and should be removed by effective LEV.

Noise

With all the moving parts in the machinery, particularly the looms, woollen mills are often very noisy places. While attenuation can be achieved by proper lubrication, the introduction of sound baffles and other engineering approaches should be considered as well. By and large, prevention of occupational hearing loss depends on the workers’ use of ear plugs or muffs. It is essential that workers be trained in the proper use of such protective equipment and supervised to verify that they are using it. A hearing conservation programme with periodic audiograms is required in many countries. As equipment is replaced or repaired, appropriate noise-reduction steps should be taken.

Work stress

Work stress, with its attendant effects on workers’ health and well-being, is a common problem in this industry. Since many of the mills operate around the clock, shift work is frequently required. To meet the production quotas, the machines operate continuously, with each worker being “tied” to one or more pieces of equipment and unable to leave it for bathroom or rest breaks until a “floater” has taken his or her place. Coupled with the ambient noise and use of noise protectors, their heavily routinized, repetitive activity makes for de facto isolation of the workers and a lack of social interaction that many find stressful. The quality of supervision and the availability of workplace amenities have a great influence on workers’ job stress levels.

Conclusion

While larger enterprises are able to invest in new technological developments, many smaller and older mills continue to operate in old plants with out-dated but still functioning equipment. Economic imperatives dictate less rather than greater attention to workers’ safety and health. Indeed, in many developed areas, mills are being abandoned in favour of new plants in developing countries and areas where cheaper labour is readily available and where health and safety regulations are either non-existent or are generally ignored. Worldwide, this is an important labour-intensive industry in which reasonable investments to workers’ health and well-being can bring significant dividends to both the enterprise and its workforce.

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Adapted from 3rd edition, Encyclopaedia of Occupational Health and Safety.

Silk is a lustrous, tough, elastic fibre produced by the larvae of silkworms; the term also covers the thread or cloth made from this fibre. The silk industry originated in China, as early as 2640 BC according to tradition. Towards the 3rd century AD, knowledge of the silkworm and its product reached Japan through Korea; it probably spread to India a little later. From there silk production was slowly carried westward through Europe to the New World.

The production process involves a sequence of steps not necessarily carried out in a single enterprise or plant. They include:

• Sericulture. The production of cocoons for their raw silk filament is known as sericulture, a term which covers feeding, cocoon formation and so on. The first essential is a stock of mulberry trees adequate to feed the worms in their larval state. The trays on which the worms are reared have to be kept in a room with a constant temperature of 25 °C; this involves artificial heating in colder countries and seasons. The cocoons are spun after about 42 days of feeding.

• Spinning or filature. The distinctive process in silk spinning is called reeling, in which the filaments from the cocoon are formed into a continuous, uniform and regular strand. First, the natural gum (sericin) is softened in scalding water. Then, in a bath or basin of hot water, the ends of the filaments from several cocoons are caught together, drawn up, attached to a reeling wheel and wound to form raw silk.

• Throwing. In this process, the threads are twisted and doubled into more substantial yarns.

• Degumming. In this phase, the raw silk is boiled in a solution of soap and water at approximately 95 °C.

• Bleaching. The raw or boiled silk is then bleached in hydrogen peroxide or sodium peroxide.

• Weaving. The silk thread is next woven into fabric; this usually takes place in separate factories.

• Dyeing. Silk may be dyed while in the filament or thread form, or it may be dyed as a fabric.

Health and Safety Hazards

Carbon monoxide

Symptoms of carbon monoxide toxicity consisting of headache, vertigo and sometimes nausea and vomiting, usually not severe, have been reported in Japan, where sericulture is a common home industry, as a result of the use of charcoal fires in poorly ventilated rearing rooms.

Dermatitis

Mal des bassines, a dermatitis of the hands of female workers reeling raw silk, was quite common, particularly in Japan, where, in the 1920s, a morbidity rate of 30 to 50% among reeling workers was reported. Fourteen per cent of the affected workers lost an average of three working days each year. The skin lesions, localized mainly on fingers, wrists and forearms, were characterized by erythema covered with small vesicles which became chronic, pustular or eczematous and extremely painful. The cause of this condition was usually attributed to the decomposition products of the dead chrysalis and to a parasite in the cocoon.

More recently, however, Japanese observations have showed that it is probably related to the temperature of the reeling bath: until 1960 almost all reeling baths were kept at 65 °C, but, since the introduction of new installations with a bath temperature of 30 to 45 °C, there have been no reports of the typical skin lesions among reel workers.

The handling of raw silk may produce allergic skin reactions in some reel workers. Facial swelling and ocular inflammation have been observed where there was no direct local contact with the reeling bath. Similarly, dermatitis has been found among silk throwers.

Respiratory tract problems

In the former Soviet Union, an unusual outbreak of tonsillitis among silk spinners was traced to bacteria in the water of reeling basins and in the ambient air of the cocoon department. Disinfection and frequent replacement of reel bath water, combined with exhaust ventilation at the cocoon reels, brought about a swift improvement.

Extensive long-term epidemiological observations also carried out in the former USSR have shown that workers in the natural silk industry may develop respiratory allergy featuring bronchial asthma, asthmatiform bronchitis and/or allergic rhinitis. It appears that natural silk can cause sensitization during all stages of production.

A situation causing respiratory distress among spinning-frame workers when packaging or repackaging silk on a spinning or winding frame has also been reported. Depending upon the speed of the machinery, it is possible to aerosolize the proteinaceous substance surrounding the silk filament. This aerosol, when respirable in size, will cause a lung reaction very similar to that of the byssinotic reaction to cotton dust.

Noise

Noise exposure can reach harmful levels for workers at machines spinning and winding the silk threads, and at looms where fabric is woven. Adequate lubrication of the equipment and the interposition of sound baffles may reduce the noise level somewhat, but the continuing exposure throughout the working day can have a cumulative effect. If effective abatement is not obtained, resort will have to be made to personal protective devices. As with all workers exposed to noise, a hearing protection programme featuring periodic audiograms is desirable.

Safety and Health Measures

Control of temperature, humidity and ventilation are important at all stages of the silk industry. Home workers should not escape supervision. Adequate ventilation of rearing rooms should be ensured, and charcoal or kerosene stoves should be replaced by electric heaters or other warming devices.

Lowering the temperature of reeling baths may be effective in preventing dermatitis. The water should be replaced frequently, and exhaust ventilation is desirable. Direct skin contact with raw silk immersed in reeling baths should be avoided as far as possible.

The provision of good sanitary facilities and attention to personal hygiene are essential. Hand washing with a 3% acetic acid solution has been found effective in Japan.

The medical examination of new entrants and medical supervision thereafter are desirable.

The hazards from machinery in silk manufacture are similar to those in the textile industry in general. Accident prevention is best achieved by good housekeeping, adequate guarding of moving parts, continuing worker training and effective supervision. Power looms should be provided with guards to prevent accidents from flying shuttles. Very good lighting is required for the yarn preparation and weaving processes.

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