Equipment Description:Sizing
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Sizing refers to the quality of the board which allows it to repel a liquid, so that the absorbance of the liquid is reduced. The paper thus becomes more suitable for after-processing such as printing or the packaging of liquids such as milk cartons. Two main types of sizing are internal and surface. Internal sizing refers to the process of adding size at the wet end of a paper/board machine. Surface sizing uses the size press as the addition point typically. |
sizing |
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The degree of water repellency for a particular paper/board is determined by the Cobb test. This measures the weight of water absorbed, usually in a one minute period, by an area of 1m2. Other liquids apart from water can be used. The following groups are indicative of the main types of internal sizing agents. AKD: alkyl ketene dimer: a type of sizing agent in the synthetic sizing class. Adopted for commercial use in the late 1970s. Off-machine sizing is not always available with this type of sizing system. In this case, a test to predict the cured Cobb result is necessary. An addition point from the machine chest onwards is typical for this type of size. Over-addition of AKD size can lead to slip problems, which can cause problems on subsequent stacking of the paper/board and in converting machinery. Rosin: a type of size, the use of which dates back to the 1880s. It is composed mainly of resin acids. Available either as a bi-product of the kraft pulping process (tall oil rosin) or by distillation from tree stumps. Always applied with some kind of fixative, such as papermakers alum (aluminium sulphate) or PAC (polyaluminium chloride). This serves to precipitate the rosin onto the anionic fibre surface. Since alum carries a cationic charge in the pH range from 4-7, rosin sizing is most effective in this pH range, hence the commonly used term acid sizing. The use of PAC has extended the pH spectrum of this type of size, so that a pseudo-neutral application, up to app pH 7.5 can be used, which allows the use of calcium carbonate filler. Off-machine curing is usually a feature of this type of size. Modern practice tends toward the addition of the size and the alum species close together, sometimes pre-mixed, to an addition point after the machine chest. Fan pump is typical. Cationic rosin: This is a variant of anionic rosin and allows this type of size to be used either without alum/PAC or with substantially reduced amounts. Either cationic starch or PAC is incorporated into the size. ASA (alkenyl succinic anhydride): This is another type of synthetic size and is usually prepared on the papermill site, by means of an emulsification unit. It is sometimes referred to as a reactive size, in view of its high reactivity with cellulose and water. Due to the need to avoid hydrolysis of the ASA, an addition point such as the fan pump or further on in the stock approach flow system is usually chosen. A typical as received dosage for ASA is 1-2kg/tonne of paper. The pH spectrum is typically from 6.5 to 8.5, hence this sytem is referred to as neutral sizing. Sometimes, size is added to the size press and in this case is referred to as surface sizing as distinct from internal sizing. Typical chemical groups in this category are polyurethane dispersions (PUD), styrene maleic anhydride (SMA), styrene acrylate (SA) |
Sizing was also mentioned in the following Fact Sheets (6): |
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