Porous crystalline cellulose

Due to their large surface area for adsorption, porous materials are useful excipients for solid dispersions. For example, 2-naphthoic acid (2-NPA) solid dispersion with porous crystalline cellulose (PCC) has been successfully prepared by heat treatment of 2-NPA and PCC mixture. " PCC is derived from MCC, but with a larger surface area. Different from 2-NPA mixed with PCC, 2-NPA mixed with MCC still maintained a crystalline form under the same mixing and heating conditions. Various experimental data such as X-ray powder diffraction, Fourier transform infrared (FT-IR) spectroscopy, and solid-state fluorescence measurements suggest that 2-NPA is adsorbed onto the surface of PCC and becomes molecularly dispersed into the system. 

Tozuka, Y. Yonemochi, E. Oguchi, T. Yamamoto, K. Molecular states of 2-naphthoic acid in solid dispersions with porous crystalline cellulose, as investigated by fluorescence spectroscopy. Bull. Chem. Soc. Jpn. 2000, 75, 1567-1572. 

The powder X-ray diffraction patterns of porous crystalline cellulose (PCC) -10% ethenzamide (EZ) mixtures before and after storage of the mixtures for 1 month at 40°C and 0, 40.0, and 97.0% relative humidity are shown in Fig. 3 [7]. In the freshly prepared mixture (A), X-ray diffraction peaks were observed at 20 = 14.5, 19.3, and 25.3° that were attributable to EZ crystals. Following storage at 0 and 40.0% RH (represented by patterns B and C in Fig. 3), the X-ray diffraction peaks of EZ crystals disappeared. It was found that the mixing of EZ with PCC under dry conditions led to the transformation of crystalline EZ into the amorphous state. EZ molecules would be adsorbed physically onto the pore surface of PCC. In the case of 97.0% RH (Fig. 3D), X-ray diffraction peaks of EZ crystals were still observed EZ remained in the crystalline state under this condition. Matsumura et ai. [8] reported that coexisting water vapor caused a decrease in the adsorption of methanol onto porous materials. At 97.0% RH, the maximum pore diameter for water condensation was calculated as 42 nm. All capillaries of PCC were filled with water at 97.0% RH, and molecules of EZ had little chance to adsorb onto the surface of PCC. These results indicated that the indispensable condition for amorphization of EZ by mixing with PCC was storage under dry conditions. 

All of the eommereial alkyl eyanoaerylate monomers are low-viseosity liquids, and for some applications this can be an advantage. However, there are instances where a viseous liquid or a gel adhesive would be preferred, sueh as for application to a vertical surface or on porous substrates. A variety of viscosity control agents, depending upon the desired properties, have been added to increase the viscosity of instant adhesives [21]. The materials, which have been utilized, include polymethyl methacrylate, hydrophobic silica, hydrophobic alumina, treated quartz, polyethyl cyanoacrylate, cellulose esters, polycarbonates, and carbon black. For example, the addition of 5-10% of amorphous, non-crystalline, fumed silica to ethyl cyanoacrylate changes the monomer viscosity from a 2-cps liquid to a gelled material [22]. Because of the sensitivity of cyanoacrylate esters to basic materials, some additives require treatment with an acid to prevent premature gelation of the product. 

Microcrystalline Cellulose. Microcrystalline cellulose is a purified, partially depolymerized cellulose that occurs as a white, odorless, tasteless, crystalline powder composed of porous particles. It is widely used in pharmaceutical dosage forms, primarily as a filler-binder in oral tablets and capsules with both wet granulation and direct compression processes. Microcrystalline cellulose was marketed first in 1964 by the FMC Corporation under name Avicel PH in four different particle size grades, each with different properties.37 Addition of Avicel into a spray-dried lactose-based formulation overcame compressibility problems. At the same time, the lactose enhanced the flowability of the Avicel products available at that time. The direct compression tableting process became a reality, rather than a concept, partially because of the availability of Avicel. As of 2007, Avicel PH is commercially available in 10 types with different particle size, density, and moisture grades that have different properties and applications (Table 7.6).38 Other brands of microcrystalline cellulose are also available on the pharmaceutical market, including Pharmacel 101 and 102 from DMV International and Emcocel 50 M and 90 M from JRS Pharma. 

Microcrystalline cellulose is a purified, partially depolymerized cellulose that occurs as a white, odorless, tasteless, crystalline powder composed of porous particles. It is commercially available in different particle sizes and moisture grades that have different properties and applications. 

Schultz and Asunmaa (40) have suggested that there exists a salt-free 22 A thick liquid-crystalline hydration sheath lining the pores of both cellulose acetate and porous glass membranes, while Ohya, 1 have recently shown that salt rejection improves significantly with a decrease in membrane pore radius below 20-22... 

Solvent bonding is suitable for all amorphous plastics. It is used primarily on ABS, acrylics, cellulosics, polycarbonates, polystyrene, polyphenylene oxide, and vinyls. Solvent welding is not suitable for crystalline thermoplastics. It is not effective on polyolefins, fluorocarbons, or other solvent-resistant polymers. Solvent welding is moderately effective on nylon and acetal polymers. Solvent welding cannot be used to bond thermosets. It can be used to bond soluble plastics to unlike porous surfaces, including wood and paper, through impregnation and encapsulation of the fibrous surface. 

Many studies have shown that the accessibility and reactivity depend on the structural parameters of cellulose-specific surface, degrees of amorphicity and crystallinity, as well as paracrystallinity parameters. So, the improvement of acetylation process of cellulose under effect of acetic acid is explained by the increase of specific surface of the porous system (Papkovetal., 1976]. As was established, non-crystalline domains of cellulose are accessible for water, while crystallites and their paracrystalline layers are inaccessible for this polar liquid (loelovich et al., 2009 loelovich et al., 2010]. 

BNC demonstrates some unique properties such as high degree of crystallinity followed hy high water retention value and porous structure enabling the infusion of solids and liquids. BNC is safe in terms of the health risks caused by distributed and mobile nanoparticles, because the nanofibers of bacterial cellulose are immobilized in a stable network. The in situ production of BNC also enables obtaining products of designed, final shape (Fig. 21.13) [23,24,28]. 

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