Felt is a fibrous material made by interlocking fibres of fur, hair or wool through the application of heat, moisture, friction and other processes into an unwoven, densely matted fabric. There are also needleloom felts, in which the felt is attached to a loosely woven backing fabric, usually made of wool or jute.
Fur Felt Processing
Fur felt, used most frequently in hats, is usually made from the fur of rodents (e.g., rabbits, hares, muskrats, coypus and beavers), with other animals used less frequently. After sorting, the skins are carroted using hydrogen peroxide and sulphuric acid, and then the following processes are performed: cutting of hair, hardening and dyeing. For dyeing, synthetic dyestuffs are usually used (e.g., acid dyes or dyes containing complex metal compounds). The dyed felt is weighted using a shellac or vinyl polyacetate.
Wool Felt Processing
Wool used for felt manufacture may be unused or reclaimed. Jute, generally obtained from old sacks, is used for certain needlefelts, and other fibres such as cotton, silk and synthetic fibres may be added.
The wool is sorted and selected. To separate the fibres, it is ragged in a rag-grinding machine, a spiked cylinder that rotates and tears up the fabric, and then garnetted in a machine that has rollers and cylinders covered with fine saw-toothed wires. The fibres are carbonized in an 18% sulphuric acid solution and, after drying at a temperature of 100 ºC, they are blended and, when necessary, oiled with mineral oil with emulsifier. After teasing and carding, which further blends the fibres and arranges them more or less parallel to one another, the material is deposited on a moving belt as layers of a fine web that are wound up on poles to form batts. The loose batts are taken to the hardening room, where they are sprinkled with water and pressed between two heavy plates, the top one of which vibrates, causing the fibres to curl and cling together.
To complete the felting, the material is placed in bowls of dilute sulphuric acid and pounded by heavy wooden hammers. It is washed (with the addition of tetrachloroethylene), dewatered and dyed, usually with synthetic dyestuffs. Chemicals may be added to make the felt rot-resistant. The final steps include drying (at 65 °C for soft felts, 112 °C for hard felts), shearing, sanding, brushing, pressing and trimming.
Safety and Health Hazards
The machines used in felt manufacturing have driving belts, chain and sprocket drives, rotating shafts, spiked drums and rollers used in garnetting and teasing, heavy presses, rollers and hammers, and so on, all of which must be properly guarded and have lockout/tagout systems to prevent injuries when they are being serviced or cleaned. Good housekeeping is also necessary to avoid slips and falls.
Many of the operations are noisy; when safe noise levels cannot be maintained by enclosures, baffles and proper lubrication, personal hearing protection must be made available. A hearing conservation programme featuring periodic audiograms is required in many countries.
Felt workplaces are dusty and are not recommended for persons with chronic respiratory diseases. While, fortunately, the dust is not associated with any specific disease, adequate exhaust ventilation is necessary. Animal hair can evoke allergic reactions in sensitive individuals, but bronchial asthma appears to be infrequent. Dust also can be a fire hazard.
The sulphuric acid solution used in felt making is usually dilute, but care is needed when diluting the supply of concentrated acid to the desired level. The danger of splashes and spills requires that eyewash facilities be nearby and that workers be fitted with protective clothing (e.g., goggles, aprons, gloves and shoes).
Tanning of certain papermakers’ felts may involve the use of quinone, which can cause severe damage to skin and mucous membranes. The dust or vapour of this compound can cause staining of the conjunctivae and cornea of the eye and, with prolonged or repeated exposures, may affect vision. Quinone powder should be dampened to prevent dusting, and it should be handled in enclosed hoods or chambers fitted with LEV, by workers fitted with hand, arm, face and eye protection.
Heat and fire
The high temperature of the material (60 °C) involved in the manual hat-shaping process dictates the use of hand skin protection by the workers.
Fire is a common hazard during the early, dusty stages of felt manufacture. It may be caused by matches or sparks from metallic objects left in the waste wool, hot-running bearings or faulty electrical connections. It may also occur in finishing operations, when vapours of flammable solvents may collect in the drying ovens. Because it damages the material and corrodes the equipment, water is less popular for fire extinguishing than dry-powder extinguishers. Modern equipment is fitted with vents through which the extinguishing material can be sprayed, or with an automatic carbon dioxide releasing device.
Although rare, cases of anthrax have occurred as a result of exposure to contaminated wool imported from areas where this bacillus is endemic.