Polysaccharides, crosslinking

Many crosslinked gel treatments are currently available for injection profile modification in secondary oil recovery waterflood operations. The gels usually consist of a water-soluble polymer, such as polyacrylamide or polysaccharide, crosslinked with a metal ion (A13+, Cr3+, Zr4+) or organic compounds. Due to the large number of systems available and possible conditions which must be considered, a rapid testing procedure must be used to screen these systems on a timely basis. 

Mocanu, Q, Souguir, Z., Picton, L. and Le Cerf, D. (2012). Multi-responsive car-boxymethyl polysaccharide crosslinked hydrogels containing Jeffamine side-chains. Carbohydrate Polymers, 89,578-585. 

An example is the preparation of a starch xanthate sludge polymer with herbicides, such as thiocarbamates. The starch xanthate was formed by using oxidatively crosslinked aqueous starch xanthate in the presence of active agents to produce a granular matrix that entraps the agent. The sludge is formed by a heterogeneous mix of polysaccharides, crosslinked... 

As mentioned before, lignin is covalently bonded to hemicellulose and crosslinks different plant polysaccharides, thereby reinforcing the cell wall and the plant as a whole. The polysaccharide crosslinked... 

Different types of gel materials, such as polysaccharides, proteins and synthetic polymers, are now used to entrap biocatalysts. Among them, photo-crosslinkable resin prepolymer ENTP-4000 as shown in Eig. 7 is more useful compared to others. Entrapment of biocatalysts should be carried out under the illumination of near ultraviolet hght within 3-5 min, by which high temperatures, shifts of pH to extremely alkahne or acidic sides are avoided. ENTP-4000, hydrophobic photo-crosslinkable resin prepolymer, is one of the most suitable prepolymers for entrapment of p-glucosidase. Molecular weight of its main chain is about 4000. 

Partially hydrolyzed polyacrylamides, carboxymethylcellulose, polysaccharides, and acrylamido methylpropane sulfonate have been screened to investigate the performance of aluminum citrate as a chelate-type crosslinker. An overview of the performance of 18 different polymers has been presented in the literature [1646]. The performance of the colloidal dispersion gels depends strongly on the type and the quality of the polymer used. The gels were mixed with the polymers at two polymer concentrations, at three polymer-to-aluminum ratios, and in different concentrations of potassium chloride. The gels were quantitatively tested 1,7, 14, and 28 days after preparation. 

Drying the borate-crosslinked polysaccharide formed in step 2... 

Delayed Crosslinking Additives. Glyoxal [458,460,461] is effective as a delay additive within a certain pH range. It bonds chemically with both boric acid and the borate ions to limit the number of borate ions initially available in solution for subsequent crosslinking of a hydratable polysaccharide (e.g., galactomannan). The subsequent rate of crosslinking of the polysaccharide can be controlled by adjusting the pH of the solution. 

Various zirconium compounds are used as delayed crosslinkers, (see Table 17-12). The initially formed complexes with low-molecular-weight compounds are exchanged with intermolecular polysaccharide complexes, which cause delayed crosslinking. 

A breaker an enzyme (at T<140°F), strong oxidizing agent, or an acid, is used to depolymerize polysaccharides and break crosslinks such that viscosity declines at a controlled rate so that the proppant may be deposited in the fracture. Too rapid proppant dropout would cause a premature "sand-out" which prevents future extension of the fracture. Peroxydisulfates are the most frequently used breakers. Less reactive organic peroxides may be preferred for high temperature formations (85). 

The use of other crosslinking metals developed simultaneously with the use of antimony, chromium, and boron(borate). Tiner, et al.(242) introduced titanium (IV) crosslinkers in 1975 as ammonium tetralactonate or bis(triethanolamine)bis(isopropyl)titanium(IV). Upon contact with water soluble titanium (IV) derivatives ordinarily form orthotitanic acid, Ti(0H)4, which rapidly forms oligimeric metatitanic acid, [Ti(0H)2] and titanium dioxide. Electron donors such as the hydroxyl groupsxof polysaccharides, if properly oriented, can participate in the sequence of titania reactions and a crosslinked gel network results. Various titanium metal crosslinkers remain in common use today. More will be said about titanium crosslinked gels later. 

Carbohydrate molecules containing amine groups, such as D-glucosamine, easily may be conjugated to other macromolecules using a number of amine reactive chemical reactions and crosslinkers (Chapter 2, Section 1 and Chapter 3). Some polysaccharides containing acetylated... 

The use of other functionalities (either indigenous or created) on polysaccharide molecules to effect a crosslinking reaction can be done in similar 2- or 3-step strategies. 

The carbonyl-reactive group on these crosslinkers is a hydrazide that can form hydrazone bonds with aldehyde residues. To utilize this functional group with carbohydrate-containing molecules, the sugars first must be mildly oxidized to contain aldehyde groups by treatment with sodium periodate. Oxidation with this compound will cleave adjacent carbon-carbon bonds which possess hydroxyl groups, as are abundant in polysaccharide molecules (Chapter 1, Sections 2 and 4.4). 

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