Cellulose strength

Plant cell walls are made of bundles of cellulose chains laid down in a cross-hatched pattern that gives cellulose strength in all directions. Hydrogen bonding between the chains gives cellulose a sheetlike structure. 

The strengths of the resulting dried sheets were tested by applying mechanical compression forces to determine the relative effects of the bacterial strains. As shown (Table 2), there were no significant differences between the textures of all bacterial cellulose strength levels derived from the three strains. Both coconut and pineapple juices yielded the same strength rating. The mechanical properties of bacterial cellulose, both air-dried and hot-pressed... 

Strong acids completely hydrolyse cellulose to glucose very mild hydrolysis gives hydrocelluloses with shorter chains and lower viscosity and tensile strength. Under special conditions a large yield of cellobiose is obtained. 

As Figure 25 8 shows the glucose units of cellulose are turned with respect to each other The overall shape of the chain however is close to linear Consequently neigh boring chains can pack together m bundles where networks of hydrogen bonds stabilize the structure and impart strength to cellulose fibers... 

Pentaerythritol in rosin ester form is used in hot-melt adhesive formulations, especially ethylene—vinyl acetate (EVA) copolymers, as a tackifier. Polyethers of pentaerythritol or trim ethyl ol eth an e are also used in EVA and polyurethane adhesives, which exhibit excellent bond strength and water resistance. The adhesives maybe available as EVA melts or dispersions (90,91) or as thixotropic, one-package, curable polyurethanes (92). Pentaerythritol spko ortho esters have been used in epoxy resin adhesives (93). The EVA adhesives are especially suitable for cellulose (paper, etc) bonding. 

The next significant strength improvement followed the 1950 Du Pont (19) discovery of monoamine and quaternary ammonium modifiers, which, when added to the viscose, prolonged the life of the ziac cellulose xanthate gel, and enabled even higher stretch levels to be used. Modifiers have proliferated siace they were first patented and the Hst now iacludes many poly(alkylene oxide) derivatives (20), polyhydroxypolyamines (21—23), and dithiocarbamates (24). 

Fully modified yams had smooth, all-skin cross sections, a stmcture made up of numerous small crystaUites of cellulose, and filament strengths around 0.4 N/tex (4.5 gf/den). They were generally known as the Super tire yams. Improved Super yams (0.44—0.53 N/tex (5—6 gf/den)) were made by mixing modifiers, and one of the best combiaations was found to be dimethylamine with poly-(oxyethylene) glycol of about 1500 mol wt (25). Ethoxjlated fatty acid amines have now largely replaced dimethylamine because they are easier to handle and cost less. 

Other patents (81,82) coveted the preparation of cellulose solutions using NMMO and speculated about their use as dialysis membranes, food casings (sausage skins), fibers, films, paper coatings, and nonwoven binders. NMMO emerged as the best of the amine oxides, and its commercial potential was demonstrated by American Enka (83,84). Others (85) have studied the cellulose-NMMO system in depth one paper indicates that further strength increases can be obtained by adding ammonium chloride or calcium chloride to the dope (86). 

It has been claimed that solutions containing 10—15% cellulose in 55—80% aqueous zinc chloride can be spun into alcohol or acetone baths to give fibers with strengths of 0.13—0.18 N /tex (1.5 to 2 gf/den). However, if these fibers were strain-dried (ie, stretched) and rewetted while under strain, strengths of 0.46 N/tex (5.2 gf/den) were achieved (92). 

A common surface cartridge is the pleated paper constmction type, which allows larger filtration areas to be packed iato a small space. Oil filters ia the automobile iadustry are of this type. The paper is impregnated, for strength, with epoxy or polyurethane resia. Any other medium ia sheet form, similar to cellulose paper, such as wool, polypropylene, or glass may be used. 

Ammonia—Gas-Cured Flame Retardants. The first flame-retardant process based on curing with ammonia gas, ie, THPC—amide—NH, consisted of padding cotton with a solution containing THPC, TMM, and urea. The fabric was dried and then cured with either gaseous ammonia or ammonium hydroxide (96). There was Httle or no reaction with cellulose. A very stable polymer was deposited in situ in the cellulose matrix. Because the fire-retardant finish did not actually react with the cellulose matrix, there was generally Httle loss in fabric strength. However, the finish was very effective and quite durable to laundering. 

The most commonly used reinforcement for high pressure decorative and industrial laminates is paper (qv). The strong substrate layers, or filler, are kraft paper. Kraft is a brown paper made from a sulfate pulp process (8). It consists of both short cellulose fibers from hardwoods and long fibers from conifers. The long fibers impart most of the wet strength required for resin saturation processes. 

Air and Oil Filters. Liquid resole resins are used to coat and penetrate the cellulose fibers of filters and separators in order to increase strength and stiffness and protect against attack by the environment. The type of phenoHc to be used depends on both the final property requirements and the papermaking process. 

The recovery of fiber from broke (off-specification paper or trim produced in the paper mill) is compHcated by high levels of urea—formaldehyde and melamine—formaldehyde wet-strength resin. The urea resins present a lesser problem than the melamine resins because they cure slower and are not as resistant to hydrolysis. Broke from either resin treatment may be reclaimed by hot acidic repulping. Even the melamine resin is hydrolyzed rapidly under acidic conditions at high temperature. The cellulose is far more resistant and is not harmed if the acid is neutralized as soon as repulping is complete. 

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