Starch for Paper : Products and Market Figures
Starch plays a dominant role amongst chemical additives that are used for the manufacturing and upgrading of paper and board (see Fig. 3.1 and 3.2). It was used in papermaking even before the invention of handmade papers, being de¬tected in sheets of papyrus. Starch derivatives are mainly used for dry strength improvement of paper and board and as a binder for pigment coating. As a bio¬polymer, starch can be provided in sufficient quantities.
The total world production of all kinds of starch at present is 48.5 V 106 tonne p. a., 52 % of which is produced in the USA, 17 % in Europe and the remaining 31 % in the rest of the world. The dominant raw material for starch production is corn (maize) with a share of 75 %, followed by 10 % tapioca, 8 % wheat and 7 % potato (Fig. 3.4). In the paper industry about 5.5 V 106 tons p. a. of starch were used worldwide in 2004, with nearly equal amounts in North America, Asia and Europe. The total spectrum of starch applica¬tions in the paper industry now includes wet-end addition for dry strength im¬provement as well as improvement of filler and fines retention, and application in the surface sizing, pigment coating, and converting adhesive functions (Fig. 3.5)
Chemistry, Modification and Conversion Technologies, Properties
Starch is chemically similar to cellulose. It is a polymer composed of glucopyr¬anose units linked through carbons. The vast majority of starches occur as a mix¬ture of linear and branched molecules. The linear fraction is called amylose and the branched one amylopectin. For efficient usage of starch in papermaking the native starch has to be modified. Unmodified starches are available that differ in properties such as amylopectin to amylose ratio, granule dimensions, gelatinisa¬tion temperature, and the molecular weights of the component fractions. In addi¬tion to these variables, modified starches greatly extend the number of potential wet-end starch additives available for a given application. Much research has been carried out that has resulted in the development of new applications for starch in papermaking. Physical, chemical, and genetic modifications and combinations have further improved starch application potential. The following modification methods are practised:
. • Physical modification
. • Fractionation into amylose and amylopectin components
. • Thermomechanical conversion
. • Acid hydrolysis
. • Chemical modification
. • Oxidation
. • Derivatization
. • Enzyme conversion
Thermomechanical conversion, acid hydrolysis, and enzyme conversion all lead to a decrease in the molecular weight of the starch and a corresponding decrease in the solution viscosity. This makes the starch more manageable, for instance for its application onto the paper surface in a size-press or film-press. For that a certain low viscosity of the starch solution is necessary. On the other hand a reduction in the molecular weight of the starch means a loss in its potential for improvement of paper strength. This means that maximum strength improvement by surface ap¬plication requires the optimal ratio between the viscosity and the pick-up of the starch solution in sizing. Improvement of up to 30–60 % in paper strength proper¬ties is reached by surface treatment of paper. This explains why today 62 % of all starches used in the paper industry are for surface sizing (Fig. 3.5).
Native starch as such is not suitable for wet-end usage because of its very low retention in the paper sheet () 40 %), which is undesirable from both the economic and environmental standpoints. To obtain good starch retention on papermaking stock, it is necessary to attach cationic substituents to the starch molecule. Starches are cationized with either tertiary (diethylamine ethyl chloride hypochloride) or quaternary (epoxy propyl trimethyl ammonium chloride) amine groups. The cati¬onic charge of the tertiary group depends on pH, with decreasing charge at higher pH values above 6.
Quaternary cationic groups retain their complete charge at all pH levels. Typical commercial wet-end cationic starches have a degree of substitution (d. s.) of cationic groups ranging from 0.01 to 0.05. In other words, 1–5 of 100 anhydroglucose units contain a cationic group. For example, a starch of molecular weight 4 V 106 and a d. s. of 0.01 will have approximately 250 cationic charges per molecule. These cationic starches are produced by the starch manufacturers. Due to the ionic reaction with the paper stock, addition rates above 1.5 % lead to an undesirable change in the charge in the stock suspension, causing problems dur¬ing paper production and leading to a high COD load in the circuit water system and in the untreated waste water.
Alternatives to these covalently bonded cationic charges are cationic polymers, which react with native or slightly anionic starches by fixation and/or precipitation. Such polymers can be used in the paper mill during or just after the cooking process of starch. A very effective polymer is polyvinylamine . It forms, e. g. with native potato starch or mildly anionic corn starches and the paper fibers, a very stable complex, which results in very good retention up to an addition level of 5 % starch. Figure 3.6 shows a model of such an on-site process. This wet-end application allows much greater improvements in paper strength than with cati¬onic starches and gives results very close to those obtained with surface application of starch .
The mechanism of dry strength improvement by wet-end starch is based on interfiber bonding. Here its free glucose hydroxy groups participate in hydrogen bonding with fiber surface cellulose molecules, which means a “chem¬ical hydration” of the fiber compound system. Additionally wet-end starch im¬proves the sheet formation by improved retention of fines and fillers and also provides more uniformly distributed fiber-fiber bonds by hydrogen bonding.
As can be seen from Fig. 3.5, 80 % of total starch consumption in the paper in¬dustry is used for surface application, either for surface sizing (62 %), or for spray¬ing (3 %) on the wet paper web or between two layers in board manufacturing, or as a coating binder (15 %). The main role of surface size is to promote surface properties, e. g. strengthen the paper surface and to bind particles such as fibers and pigments more strongly to the surface. Additionally, the starch is expected to add internal strength to the sheet through a liquid penetration in the z-direction. Opposite to wet-end application, where starch retention is crucial, starch applied to the surface is generally 100 % retained.
For production of woodfree uncoated and coated fine papers up to 40 kg starch per ton of paper are applied. 3 to 10 kg starch is added at the wet end, with the aim of internal strength improvement and retention increase. The major share of the starch is added to the sheet in surface treatment. A mass balance on a typical fine paper machine has shown, that more than 90 % of the added starch is retained in the final paper product. Losses occur mainly during the sheet forming process in the wire section due to insufficient retention. Starch which is not held back in the paper is discharged with the process effluents to the waste water treatment plant, where a complete biodegradation process follows.
Packaging paper, e. g. testliner and corrugating medium, made from 100 % re¬covered paper, can only be produced economically and in the required quality by adding cost effective dry-strength agents, i. e. biosynthetic starch products. There¬fore these papers are produced with an average starch consumption of 40 kg t–1, mainly by surface application. A further 25 kg (t corrugated box board)–1 is applied as an adhesive in the converting plant. This means that a high amount of starch is returned to the production process via recovered papers, where it is nearly not retained in the paper sheet. Therefore this uncontrolled starch quantity leads to a considerable load in the white water circuit (usual COD levels from 5.000 to 30.000 (mg O2) l–1) and finally also in the waste water. By applying starch with a size-press to the paper surface, the paper strength will be increased by 30 to 60 %. Additional energy is necessary for extra drying of the paper web and the paper machine runnability may decrease.
To improve productivity and reduce production costs, effective and well con¬trolled 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 holistic 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, envir¬onmentally friendly additive with a diversified functionality.