Drainage aids

Papermaking additives can be categorized either as process additives or as functional additives. Process additives are materials that improve the operation of the paper machine, such as retention and drainage aids, biocides, dispersants, and defoamers they are primarily added at the wet end of the paper machine. Functional additives are materials that enhance or alter specific properties of the paper product, such as fillers (qv), sizing agents, dyes, optical brighteners, and wet- and dry-strength additives they may be added internally or to the surface of the sheet. 

Recycled paper contains more fines, short fibers, and anionic trash. This will increase demand for process chemicals such as drainage aids and both wet- and dry-strength additives (43). 

Retention and drainage aids are chemicals which are added to the fibre and filler suspension to assist the efficiency of the filtration process. Growth in recent years in the use of retention aids has been greater than that of almost any other paper chemical additive. It has been caused by a combination of factors increased machine speeds, the increased use of filler in alkaline systems, the increased use of recycled paper and the growing tendency to use fillers in newsprint. Retention aids are water-soluble polymers which may be cationic,... 

Linear water-soluble anionic poly(acrylamide-co-ammonium acrylate) has been prepared by a water-in-oil polymerization. These materials are characterized by a Huggins constant in brine greater than 0.75 and a storage modulus for a 1.5 wt% actives polymer solution at 4.6 Hz greater than 175 Pa. These agents are particularly useful as drainage aids and contamination control aids in papermaking. 

Anionic copolymers consisting of acrylamide and styrene sulfonic acid sodium were prepared by Harrington [1] and used as drainage aids for cellulosic fiber compositions. Doherty [2] anionicaUy prepared high molecular weight poly (acrylamide-co-styrene sulfonic acid sodium), which was also effective as a drainage aid. 

Cationic copolymers consisting of acrylamide and quaternary ammonium salts such as [2-(acryloyloxy)ethyl]trimethyl ammonium chloride, (1), prepared by HoUomon [3] were effective as drainage aids. In the absence of crosslinking these cationic materials were characterized by a Huggins constant greater than 0.3 with a storage modulus at 6.3 Hz of over 50 Pa. 

Agesbt [Q Ptdymers retention aid, pigment dispersant drainage aid, stabilizer, raw and waste water churifier for pj r industry. 

Defoamer/Dralnage Aid. [Crompton Knowles] Fat acids blends defoamer and drainage aid for paper industry. 

Drainaid. [Hart Chem. Ltd.] Drainage aid for pulp and paper industry. 

Unidri. [Hart Chem. Ltd.] Dewatering agent low-foaming surfactant for metallic oxides, coat vacuum filoudon of iiunerals drainage aid for pu and paper processing. 

Increased water system closure and iuCTcased water temperatures are also altering the environment within which the retention and drainage aid has to function, requiring an alternative approach in many cases. 

The paper machine and system chemistry factors which can affect the retention and drainage aid performance are discussed along with some troubleshooting guidelines and examples. 

What does a papermaker expect from a retention and drainage aid ... 

For the supplier, having a wide range of retention and drainage aid products and systems is essential as everything starts with the needs of the papermaker, the products made and the papermaking machinery used. 

The following products form part of the considerations for the retention and drainage systan which will eventually be used and should therefore be looked upon as components of the retention and drainage system - as any change in the product used or the dosage, application point, etc. can have a significant effect on the retention and drainage aid efficiency on the machine ... 

The products normally considered as the effective retention and drainage aid additives are ... 

Polyethyleneimine low molecular weight but structured cationic products available as liquids, generally these are more effective as drainage aids... 

As machine speeds have increased and wet-end system chemistry has become more complex, the demands on the retention and drainage aid have increased considerably. This has led to the development of an increasing number of retention and drainage aid systems which incorporate two, three or even four components selected from the above list. A very general representation of the time frame for this development is shown in Fig. 3.3. 

Occasionally, two products can be mixed prior to addition or more than one product can be added at the same addition point - out of necessity or because laboratory testing has shown this to be the optimum addition strategy. The main addition points used for the retention and drainage aid components are illustrated in Fig. 3.4 - points 1-6. 

All of the above retention and drainage aid products and systems are in use currently. The system used depends on machine type, machine speed, paper/ board type, production rate, requirements, etc. 

The basic criteria used for selection of the most suitable retention and drainage aid product or system, include the following ... 

Stock approach flow This can be a limiting factor in application of the most efficient products. The papermaking systems need to be able to cope with the changes necessary for implementation of an efficient retention and drainage aid system. Restrictions on pulper capacity, fan pump capacity etc. can reduce the suitability of the most effective system, requiring a less powerful system to be used, as can limited machine speed, water removal capacity and drying capacity. 

Paper/board quality All retention and drainage aids products and systems affect the sheet formation. The need to maintain acceptable formation can limit the type of system that can be applied and also the level of retention and drainage performance which can be sustained. As many trials fail due to an over-performing retention aid as fail due to an under-performing retention aid. 

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