SILK

or more precisely, natural silk, is produced by certain worms, caterpillars, and moths. The most important product is the high-quality silk that comes from the mulberry silk moth Bombyx mori Linne) the larvae use it to enclose themselves in a coccoon, which consists of 78% silk fibroin and 22 % silk glue (sericin). 

In each separate thread there are 10-nm-wide microfibrils bound into fibrillar bands of up to 2000 nm in width. The protein chains are present in the microfibrils as folded chains. These so-called silk fibroins can be separated with chymotrypsin into an X-ray crystalline (60% by wt) and an amorphous part. The crystalline portion consists of uniform hexapeptide units (-ser-gly-ala-gly-ala-gly-). In fibroins from Bombyx mori L, there are ten of these hexapeptides. The 60 a-amino acid residues of the crystalline part are joined together with 33 amino acid units of the amorphous component in one peptide chain. The amino acids of the amorphous part vary widely in composition and sequence length. 

The dense packing is responsible for the high strength and the amorphous part is responsible for the stretchability. The properties are also considerably influenced by the number of amino acids with short side chains present, as is shown in Table 30-7 for fibroins of different silk worms. 

To recover the silk, the pupae are killed with steam or hot air. By immersing the coccoons in hot water, the silk glue is softened. Rotating brushes catch the end of the silk fiber and 4-10 of the threads are wound together onto a reel and dried. Of the 3000-4000 m of thread per coccoon, only about 90 m can be unwound. The outer and inner layers are too impure and are used along with damaged coccoons in schappe spinning. 

Percent amino acids with short side chains 

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It is used in the dispersed form as a dye for acetate silk, though it has no affinity for other fibres. It is also used as a starting point for alkyl- or acyl-aminoanlhraquinones which are used either as vat dyes or, after sulphona-tion, as acid wool dyes. 

M.G.Silk, The use of diffraction based time-of-flight measurements to locate and size defects , Br.J.Non-Destr. Test., 1984, 26(4), 208-213. 

One procedure makes use of a box on whose silk screen bottom powdered desiccant has been placed, usually lithium chloride. The box is positioned 1-2 mm above the surface, and the rate of gain in weight is measured for the film-free and the film-covered surface. The rate of water uptake is reported as u = m/fA, or in g/sec cm. This is taken to be proportional to - Cd)/R, where Ch, and Cd are the concentrations of water vapor in equilibrium with water and with the desiccant, respectively, and R is the diffusional resistance across the gap between the surface and the screen. Qualitatively, R can be regarded as actually being the sum of a series of resistances corresponding to the various diffusion gradients present ... 

Carbon disulphide is an excellent solvent for fats, oils, rubber, sulphur, bromine and iodine, and is used industrially as a solvent for extraction. It is also used in the production of viscose silk, when added to wood cellulose impregnated with sodium hydroxide solution, a viscous solution of cellulose xanthate is formed, and this can be extruded through a fine nozzle into acid, which decomposes the xanthate to give a glossy thread of cellulose. 

The leader of DuPont s effort was Wallace H Carothers who reasoned that he could reproduce the properties of silk by constructing a polymer chain held together as is silk by amide bonds The neces sary amide bonds were formed by heating a dicar boxylic acid with a diamine Hexanedioic acid adipic acid) and 1 6 hexanediamme hexamethylenedi-amine) react to give a salt that when heated gives a polyamide called nylon 66 The amide bonds form by a condensation reaction and nylon 66 is an example of a condensation polymer... 

The production of polyester fibers leads that of all other types Annual United States production of poly ester fibers is 1 6 million tons versus 1 4 million tons for cotton and 1 0 million tons for nylon Wool and silk trail far behind at 0 04 and 0 01 million tons re spectively... 

Schweitzer s reagent (dissolves cotton, linen, and silk, but not wool) add NH4CI and NaOH to a solution of copper sulfate. The blue precipitate is filtered off, washed, pressed, and dissolved in ammonia (sp. gr. 0.92). 

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