Equipment Description:Dry strength additives
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Dry strength additives are added to the paper/board machine to impart an increase in strength properties to the final sheet. The mechanism for this is usually an increase in bonding sites on the fibres, although some dry strength aids function by providing stronger bonds. Typical additives for this purpose are starch (usually cationic, but sometimes amphoteric), resin type polymers and cationic polymers. Carboxy methyl cellulose (CMC) and various types of naturally occuring gums can also be used, either as single additions or in conjunction with other dry strength aids. |
dry strength strength starch CMC polymer |
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A brief description of the more commonly used dry strength system follows:- 1. Cationic starch - this gives better natural retention than unmodified starch due to its positive charge, which makes it adhere to negatively charged fibres under normal papermaking conditions. The prevalent method for starch preparation is cooking, which renders the starch soluble and therefore able to react with furnish components. The jet cooker, which is a piece of equipment that reacts a slurry of starch with superheated steam is mainly used for this purpose. Potato and maize based cationic starch are typical types used in the UK, but other types such as waxy maize and tapioca are more prevalent in non-European countries. A typical addition point for wet-end cationic starch is the machine chest, but later additions are sometimes found. Typical addition levels are 0.5-1.5%. Further information on starch, relating type and charge level to effects in the papermaking system will be presented in the Level 3 factsheets. 2. Amphoteric starches - of which potato starch because of its higher amount of naturally occuring phosphate groups is a natural type. Others have negatively charged phosphate groups added by chemical synthesis. This type of starch is very useful in complicated multicomponent furnishes and some contaminated systems and allows for operational effectiveness over a higher pH range. Starch, it should be noted does not function solely as a dry strength aid, but will also, at moderate addition levels improve drainage and retention. It is also used as the basis for some retention systems, such as the combination with anionic silica of which more detail will be found in the retention/drainage fact sheet. 2. Polymeric type dry strength aids These are usually low molecular weight polyacrylamides, which function by increasing the strength of the fibre-fibre bonds and have the minimum detrimental effect on formation due to their limited flocculation of the furnish components. Typical dosage level is 0.2-0.5% dry basis on fibre. Ther are several variants on this type of system, inluding the use of cationic and anionic polymeric resins. 3. CMC - carboxy methyl cellulose These can be added either as a standalone addition or in conjunction with another dry strength aid. They function by increasing the number of negative sites on the fibre which, later in the process, allow increased interaction with cationic chemicals such as cationic starch. CMC produces its optimum strength effect when added early in the stock preparation system, in the pulper ideally. Typical addition levels are 0.3-0.5% dry basis on fibre. |
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