Coating binder, starch

The other important function of the binder is its effect upon the rheological properties of the coating mix. Starch, which is widely used, is unsuitable for use in its unmodified form because its solution viscosity is generally too high and also because of the problem of retrogradation. It is usually modified by reducing its molecular weight by either oxidative or hydrolytic (sometimes enzymatic) procedures. 

The performance and quality of starch can be improved through chemical modification (see Chapter 17). Chemical modifications provide processed foods, such as frozen, instant, dehydrated, encapsulated and heat-and-serve products, the appropriate texture, quality and shelf life (see Chapter 21), and improved processing condition tolerance, such as improved heat, shear and acid stability. Modification also allows starches to be used in the paper industry (see Chapter 19) as wet-end additives, sizing agents, coating binders, and adhesives and as textile sizes. 

Dextrins are produced by dry heating (roasting) starch in the presence of an acid catalyst. They are produced in a range of viscosity and color specifications. Dextrins are primarily used as adhesives in paper conversion, such as laminating and envelope production. A low-viscosity dextrin is used in Europe as a total chlorine free (TCF) coating binder for application on TCF paper. 

A previous trend in the paper industry of limiting starch purchases to unmodified grades and effecting modification on-site in the paper mill has changed. The variance in products thus obtained was frequently wider than in products supplied by the starch manufacturer. As a result, there is now more preference to utilize modified starches with specific application properties. Growth in paper recycling should lead to an increased use of starch as a coating binder in place of synthetic materials. 

The converter will operate most efficiently when it supplies coating binder at the same rate as it is consumed. Problems may occur when the converter has to be shut off and flushed with water. Channel flow and vortices during the purge with water affect cleaning time and may make it necessary to sewer starch product. In another converter, a single pump is used to transport the starch through the retention vessel. Jet cookers of the venturi type are placed at the entrance to the vessel and at its exit.114 In an alternative design, a retention coil is used instead of a retention vessel.115... 

Starch can be enzymically converted in the presence of pigment. The conversion follows a similar time-temperature cycle as in neat starch conversion. The pigment will adsorb a portion of the enzyme adsorption can be minimized by the addition of sodium silicate to the mixture prior to the addition of the enzyme (Vanderbilt process). Even with silicate treatment, a higher quantity of enzyme will be required to reach a specific viscosity target. Other coating components, such as latex and lubricants, have to be added after the conversion. The Vanderbilt process is now rarely used for the preparation of coating binder. 

Pregelatinized starch is sometimes used as a coating binder,258 but these products are difficult to wet out uniformly and may cause problems due to clumping. Small granules of starch are used in specialty coatings.259... 

Special starch products have been developed for use as thickeners in coatings that are based on synthetic latex or resin binders. Starch-based thickeners can have associative properties when substituted with a hydrophobic reagent. They provide an alternative for cellulose-based thickeners, but their use is limited. 

About 600 million pounds of starch were used in 1986 as paper-coating binders, and again, most were unmodified by the starch supplier (3). The paper mills convert the starch to the necessary viscosity by either enzyme or the thermal-chemical converting process. Hydroxyethyl starch is also used quite successfully as is some oxidized starch. 

Starch and various lattices are used as coating binders. Upon recycling (as coated broke in the mill itself or with the waste paper) the starch will not deposit while certain lattices have been found prone to deposition. Suitable lattices are SBR and polyvinyl acetate. 

Uses Coating insolubilizer for pigmented paper coatings and size press applies. renders coating binders more hydrophobic, resulting in improved paper surf, str. allows reduction of starch solids resulting in improved bulk, opacity, smoothness component of paper/paperboard in contact with dry foods... 

Regulatory FDA 21 CFR 176.170,176.180 Properties Dens. 8.8 Ib/gal vise. < 500 cps pH 6.5 50% solids Storage Protect from freezing Resyn 25-1151 [Natl Starch Chem.] them. Descrip. Vinyl acetate CAS 108-05-4 EINECS/ELINCS 203-545-4 Uses Coating binder for offset and rotogravure coatings. delivers exc. holdout for special inks and varnishes adds controlled adhesion in special coated paper grades component of paper/paperboard in contact with aq./fatty/dry foods... 

Uses Starch insolubilization thixotropic agent crosslinking agent for coating binders Trade Names Zirmel 1000 ... 

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