Cellulose, 1—»4-/3-links

On the other hand, native cellulose is an abundant and inexpensive macromolecular compound that reinforces most plant cell walls. During recent years, attention has been devoted to the use of cellulose (nanocellulose or nanofibrillated cellulose), and important studies have been published [13-22]. The outstanding mechanical properties of nanocellulose or nanofibrillated cellulose, linked to its wide availability, biodegradability, and extensive number of alternatives for chemical modification, have been the driving force for its utilization as reinforcement in polymers. These cellulosic materials are composed of nanosized cellulose fibrils with a high aspect ratio (length-to-width... 

Our time has seen The synthesis of polyisoprene And many cross-linked helixes unknown To Robert Hooke but each primoridal bean Knew cellulose by heart. . . ... 

Figure 3 shows the production of acetaldehyde in the years 1969 through 1987 as well as an estimate of 1989—1995 production. The year 1969 was a peak year for acetaldehyde with a reported production of 748,000 t. Acetaldehyde production is linked with the demand for acetic acid, acetic anhydride, cellulose acetate, vinyl acetate resins, acetate esters, pentaerythritol, synthetic pyridine derivatives, terephthaHc acid, and peracetic acid. In 1976 acetic acid production represented 60% of the acetaldehyde demand. That demand has diminished as a result of the rising cost of ethylene as feedstock and methanol carbonylation as the preferred route to acetic acid (qv). 

THPC—Amide Process. The THPC—amide process is the first practical process based on THPC. It consists of a combination of THPC, TMM, and urea. In this process, there is the potential of polymer formation by THPC, melamine, and urea. There may also be some limited cross-linking between cellulose and the TMM system. The formulation also includes triethanolamine [102-71-6J, an acid scavenger, which slows polymerization at room temperature. Urea and triethanolamine react with the hydrochloric acid produced in the polymerization reaction, thus preventing acid damage to the fabric. This finish with suitable add-on passes the standard vertical flame test after repeated laundering (80). 

Other Polymers. Besides polycarbonates, poly(methyl methacrylate)s, cycfic polyolefins, and uv-curable cross-linked polymers, a host of other polymers have been examined for their suitabiUty as substrate materials for optical data storage, preferably compact disks, in the last years. These polymers have not gained commercial importance polystyrene (PS), poly(vinyl chloride) (PVC), cellulose acetobutyrate (CAB), bis(diallylpolycarbonate) (BDPC), poly(ethylene terephthalate) (PET), styrene—acrylonitrile copolymers (SAN), poly(vinyl acetate) (PVAC), and for substrates with high resistance to heat softening, polysulfones (PSU) and polyimides (PI). 

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. 

Phenylenediamines are used in a variety of other appHcations, such as corrosion inhibitors, cross-linking agents for epoxy resins, toners for electrostatic image development (35), and to improve wrinkle resistance of cellulose acetate fibers (36). 

Sihcone products dominate the pressure-sensitive adhesive release paper market, but other materials such as Quilon (E.I. du Pont de Nemours Co., Inc.), a Werner-type chromium complex, stearato chromic chloride [12768-56-8] are also used. Various base papers are used, including polyethylene-coated kraft as well as polymer substrates such as polyethylene or polyester film. Sihcone coatings that cross-link to form a film and also bond to the cellulose are used in various forms, such as solvent and solventless dispersions and emulsions. Technical requirements for the coated papers include good release, no contamination of the adhesive being protected, no blocking in roUs, good solvent holdout with respect to adhesives appHed from solvent, and good thermal and dimensional stabiUty (see Silicon COMPOUNDS, silicones). 

Oxidation has also been cited as occurring in the cure of polymethyUiydrosiloxane [9004-73-3] (PMHS) on cellulose acetate fibers. Investigation of the cured, cross-linked siUcone shows no evidence of the Si—H bond. The same compound under an atmosphere of nitrogen does not cure and retains the Si—H bonds (99). 

Mechanisms for Formation and Hydrolysis of Finishes. The general mechanism for acid-cataly2ed formation and hydrolysis of /V-methy1o1 cellulose cross-links has been shown to pass through a catbonium ion intermediate as in equations 4 and 5 (41) ... 

DMEU represented the first cross-linking agent that was a pure chemical rather than a mixture of components. As such, it provided research workers a tool to investigate the changes that take place in cellulose with cross-linking in a more exact manner (70—73). 

For ink vehicles based on hydroxyl group containing binders such as nitrocellulose and cellulose acetate, the tetraalkyl titanates cross-link the binder prematurely, limiting the storage stabiUty of the printing ink. Chelated organic titanates such as TYZOR AA and TYZOR TE are preferred for use in these cases because they only initiate cross-linking when the ink is heated to temperatures above 80°C (503). 

Cross-Linking of Polyols. Polyols such as natural polysaccharides, eg, cellulose, starch, guar gum and their derivatives, and polyvinyl alcohol and its derivatives can be cross-linked by organic titanates. 

Cellulose. Cellulose or starch xanthate cross-linked by titanates can adsorb uranium from seawater (536). CarboxymethylceUulose cross-linked with TYZOR ISTT is the bonding agent for clay, talc, wax, and pigments to make colored pencil leads of unusual strength (537). 

Some commercial durable antistatic finishes have been Hsted in Table 3 (98). Early patents suggest that amino resins (qv) can impart both antisHp and antistatic properties to nylon, acryUc, and polyester fabrics. CycHc polyurethanes, water-soluble amine salts cross-linked with styrene, and water-soluble amine salts of sulfonated polystyrene have been claimed to confer durable antistatic protection. Later patents included dibydroxyethyl sulfone [2580-77-0] hydroxyalkylated cellulose or starch, poly(vinyl alcohol) [9002-86-2] cross-linked with dimethylolethylene urea, chlorotria2ine derivatives, and epoxy-based products. Other patents claim the use of various acryUc polymers and copolymers. Essentially, durable antistats are polyelectrolytes, and the majority of usehil products involve variations of cross-linked polyamines containing polyethoxy segments (92,99—101). 

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