Cellulose chains, cross-linking

The most important cotton etherification treatments are those that produce wrinkle resistance in fabrics [331,333,334]. The aldehydes, formaldehydes, and glyoxals, react with the OH groups of two cellulose chains as well as those of one chain. Reaction in which a bond is established between the two cellulose molecules is called cross-linking and is the basis for profound changes in the cotton fiber. Cross-linking produces resiliency in the fiber to give the needed dimensional stabilization, wrinkle resistance, and crease retention for modern durable-press cellulosic fabrics. Cross-links based on etherification reactions traditionally have been used because of their durability to repeated laundering and wear. 

Dimethyloldihydroxylethyleneurea (DMDHEU) molecules penetrate into the cellulose stmcture, cross-link with the hydroxyl groups in the amorphous region and thus reduce the slipping of cellulose chains. Macroscopically, fabric appearance, crease retention and wrinkle recovery performance can be increased by DMDHEU treatment, while the fabric strength can be significantly reduced. 

Secondary bonds are considerably weaker than the primary covalent bonds. When a linear or branched polymer is heated, the dissociation energies of the secondary bonds are exceeded long before the primary covalent bonds are broken, freeing up the individual chains to flow under stress. When the material is cooled, the secondary bonds reform. Thus, linear and branched polymers are generally thermoplastic. On the other hand, cross-links contain primary covalent bonds like those that bond the atoms in the main chains. When a cross-linked polymer is heated sufficiently, these primary covalent bonds fail randomly, and the material degrades. Therefore, cross-linked polymers are thermosets. There are a few exceptions such as cellulose and polyacrylonitrile. Though linear, these polymers are not thermoplastic because the extensive secondary bonds make up for in quantity what they lack in quahty. 

In terms of tonnage the bulk of plastics produced are thermoplastics, a group which includes polyethylene, polyvinyl chloride (p.v.c.), the nylons, polycarbonates and cellulose acetate. There is however a second class of materials, the thermosetting plastics. They are supplied by the manufacturer either as long-chain molecules, similar to a typical thermoplastic molecule or as rather small branched molecules. They are shaped and then subjected to either heat or chemical reaction, or both, in such a way that the molecules link one with another to form a cross-linked network (Fig. 18.6). As the molecules are now interconnected they can no longer slide extensively one past the other and the material has set, cured or cross linked. Plastics materials behaving in this way are spoken of as thermosetting plastics, a term which is now used to include those materials which can in fact cross link with suitable catalysts at room temperature. 

By using labelled, specific cross-linking agents in a polymer it is possible to count the number of cross-links introduced. Bishop (63) has used labelled di-acid chlorides (oxalyl, pimelyl and dodecandioyl) to cross-link cellulose acetate monofilaments (Reaction 6). He also used a tracer technique to correct for side chain formation (Reaction 7). 

Cellulose, an important constituent of wood, has long chains of glucose molecules linked by glycoside bonds. These chains are cross-linked by hydrogen bonds. Many biological polymers have unusual mechanical properties, not at present matched by the properties of artificial macromolecules. For instance, arteries are... 

In this context, some experimental results relevant to these open questions of enzymatic degradation will be presented and will be discussed from the viewpoint of cellulose chemistry, together with a summary of our recent work on thermohydrolysis and acid hydrolysis of cellulose, performed in connection with research on cellulose powder manufacture (7). After a short survey of the experimental techniques applied, this contribution will be centered on three problems (1) the interaction of chain degradation and cross-linking in thermal and thermo-hydrolytic treatments of cellulose, (2) the influence of mechanical strain... 

Interaction of Chain Destruction and Cross-linking in Thermal and Thermohydrolytic Degradation of Cellulose. Starting points of these investigations were the statement of a thermal level-off-DP by Golova and Krylova (10) and observations of our own (8) of an increase... 

Series III Cross-Linking Resins. The third series of experiments attempted to create cross-links within the cellulose chains using resins traditionally used to impart crease-resistant properties to cotton and rayon fabrics (4,8). Fabrics treated with the materials in Table IV were prepared. 

Natural or artificially accelerated aging of papermaking pulps is characterized by two important reactions, scission of the polymeric cellulose chains and some cross-linking reaction (7,8,9), the exact nature of which remains unknown. Since the mechanical properties of aged paper are modified by these two simultaneous reactions, it was of interest to determine whether these chemical effects influence the thermograms of artificially aged papers. 

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