Size press application, starches

Before discussing specialty starches for size press applications, it should be noted that the largest volume of starch applied at the size press is either enzyme converted or ammonium persulfate hydrolyzed native corn starch. These are economic considerations and the properties resulting from these treatments do not compare to the properties provided by specialty starches. It is also fair to state that all grades of paper do not require a specialty starch and it is only through the need for improved paper performance that specialty starches are required. 

The dominant specialty starch used in size press applications is the hydroxyalkyl derivative commonly made wjth ethylene oxide to yield a hydroxyethyl starch. Low substituted hydroxypropy1 starch has similar properties. These starches were introduced to the industry in 1950 and they have grown in importance each year. Recent expansions in production by major producers has indicated their continuing dominance of the specialty starch surface sizing and coating field. 

Uses Optical brightener for broad range of paper applic., paper stock even at low pH, size-press with starch and other prods., and paper coatings based on latex, starch, CMC, and PVOH Eeatures Cone. 

Application of surface size to paper carries with it the transfer of a substantial quantity of water. As an example, surface sizing of a 75g/m2 (501b/3300fit2) sheet (with 1% residual water content) by a 5% starch solution for a coat weight of 1.5 g/m2 (1 lb/3300ft2/side) will raise the water content of the sheet to 43%. This large quantity of water will weaken the paper. Web breaks at the size press can occur, particularly when the sheet is also weakened as a result of edge cracks or holes. 

Many kinds of native and modified starches are being used on the size press.207 The governing factors are dispersion viscosity, film formation and resistance to retrogra-dation. For low-cost applications, native corn starch is depolymerized in the paper mill by enzymic or thermal-chemical conversion. In-plant converted starches are... 

Paper specimens were obtained from Mead Paper, Printing and Writing Paper Division (Chillicothe, Ohio 45601). Samples were identified as being free from other materials, i.e., no preservatives, slimicides, defoamers, etc. Internally-sized samples were sized by conventional alum/rosin sizing surface-sized materials were prepared by an application of starch at the size press and all the other samples were unsized. Whatman filter paper was extracted with a 68/32 mixture of chloroform/acetone to remove any organic binder from the fiber surface. The fiber composition of the Whatman filter paper was identified by optical microscopy as cotton. The sample identified as "No Sizing" was... 

To improve productivity and reduce production costs, effective and well controlled starch application at the wet end is required (see Fig. 3.6). New practical experience shows that a starch-polyvinylamine complex can be an alternative to a surface treatment by size-press for strength improvement. A hoHstic picture of the application of starch based on analytical investigations (in produced paper, exhaust air, clarified waste water and soil) shows that starch is an extraordinary, environmentally friendly additive with a diversified functionality. 

For surface application, first the compatibility with the other chemicals used in the preparation and also the shear stability have to be checked (e. g. with starch, optical brightener, dyes). It is advisable to meter the undiluted polymer by means of a piston pump to the circuit of the size-press, e. g. before the feed pump. To achieve a good effect, the PSA must penetrate sufficiently into the paper, e. g. an adequate quantity of sizing solution must be taken up. Quantities of 2-4 g solid polymer in combination with 40-80 g 1 starch are normally sufficient to achieve a hard sized paper sheet 1-2 g 1 solid polymer is sufficient for final sizing of a paper that has been presized internally. 

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