Size press

Starches used to increase the internal strength of paper as measured by tensile, edge cmsh resistance, Mullen burst, and Scott bond strength are added at the wet end, whereas starch used to increase surface strength as measured by pick and resistance to Hnting are added at the size press. 

Though functionally and chemically similar, fillers and pigments ate distinguished from one another in that fillers are added at the wet end of the paper machine, and serve to fill the sheet pigments are added at the size press and serve to alter the surface of the sheet. The most common fillers are mineral pigments, eg, clay, titanium dioxide [13463-67-7] calcium carbonate, siUca [7631-86-9], hydrated alumina [21645-51 -2], and talc [14807-96-6]. 

Processes. The most common method for the appHcation of chemicals to the surface of a paper web is by a size press. In the size press, dry paper, which usually is sized to prevent excess water and chemical penetration, is passed through a flooded nip or pond, and a solution or dispersion of the functional chemical contacts both sides of the paper. Excess Hquid is squeezed out in a press and the paper is redried. 

The gate-roU size press is used for the appHcation of high soHds, high viscosity compositions to the surface of the sheet (92). The material to be appHed is transferred over a series of roUs, and a thin film from the appHcator roU is appHed to the sheet. 

At high speeds, the pond of the flooded nip size press becomes turbulent. RoU maintenance is a problem with the gate-roU size press. To avoid these problems, the blade/rod metering size press was developed. Short-dweU coater heads are used to apply a precisely controUed quantity of chemicals to the size press roUs. This quantity is controUed with either a metering blade or a metering rod. Blade or rod metering eliminates the pond, and does not increase the number of roUs required for surface chemical appHcation. 

Spray appHcations to the surface of the sheet have been useful, especially for appHcation of creping aids and release agents in towel and tissue miUs. The spray appHcation of functional chemicals has not been used widely. Generally, the uniformity of appHcation is mote difficult to control when functional chemicals are sprayed than when they are appHed by a size press. Eunctional chemicals also can be appHed to heavier grades of paper or paperboard at the calender stack. 

Curl Control. Many grades of paper tend to cud, especially as humidity varies, because of the stresses produced duting the dryiag process. This is especially troublesome when only one side of the paper receives a surface treatment. Judicious appHcation of water to the opposite side of the dry sheet followed by redryiag may correct the curling. Water may be appHed by surface appHcation at the size press, water box, or calender stack or it may be sprayed on. Small amounts of water can be appHed to the paper surface as a foam with exceUent results. 

The resistance of the polymer to oils and organic solvents can be direcdy attributed to the hydroxyl fiinctionahty and the fihn-forrning properties of the polymer. Treated paper substrates display a significant amount of oil resistance, which make them valuable for packing papers and food-grade paperboard containers. This performance improvement is achieved despite the fact that the 1—3 wt % add-on level, typical of size press appHcations, is too low to provide a continuous poly(vinyl alcohol) film. 

In most cases, some internal size is stiU required to control the depth or penetration of the surface size, the mimabiHty through a size press, or other properties. Surface sizing reviews and leading references are available (66,67). 

Paper may also be colored by surface appHcation of dyestuff solutions after the paper has been formed and dried or partially dried by utilizing size-press addition, calendar staining, or coating operations on the paper machine. In addition, paper may be colored in off-machine processes by dip dyeings or absorption of dyestuff solution and subsequent drying, such as for decorative crepe papers. 

Apparel and other finished products Cutting, sewing, sizing, pressing Cloth, fibers, metals, plastics, rubber... 

Service No. Req d. Size Press Class Facing Work No. ... 

For use from the size press it is necessary for the FBA to be compatible with the chosen size, such as starch, casein or urea-formaldehyde resin. Since sizes tend to be yellowish and to absorb ultraviolet radiation, brighteners are generally less effective in sized paper. 

Pulp and size press Pulp and size press... 

The on-machine size press is the most common method of applying surface sizes to a base paper. The base paper is normally passed... 

A further problem of the size press operation is that of films splitting. At the exit of the nip, the film sometimes splits into two layers with one part following the paper and the other remaining on the roll surface. The problem manifests itself as small scale nonuniformities on the paper surface caused by threads of the size solution being drawn out and deposited unevenly on the surface to give a kind of orange peel effect. 

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. 

Significant volumes of low oxidized starch are used at the size press. These starches are made by treatment in alkaline suspension with sodium hypochlorite so that from 1 to 2% active chlorine acts on the starch. The reaction is simple to perform. However, the reaction products are complex. Chain scission occurs at the same time that carboxyl and carbonyl groups are formed in the starch. It is most desirable to prepare the highest ratio of carboxyl to carbonyl as possible and this reaction is a function of the pH in the slurry. 

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