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AIR3-CT94-2485
New Paper Industry Uses of Advanced Chemically Modified and Biologically Improved Starch Grades |
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Proposal No: | AIR3-CT94-2485 |
| Date Prepared: | September 1999, April 1998 | |
| Source: | Final technical report 1998
Progress Report, Year 2 |
Introduction
This project investigated the modification and selection of starches to be used as chemical additives in paper-making. Such paper chemicals are in general divided into process chemicals and functional chemicals. The first group includes those added in order to flocculate small particles, to increase the drainage or to combat deleterious substances in the process water. The second group of paper chemicals are added to produce a desired function in the paper, e.g. to increase the mechanical strength properties in dry or humid climate, to increase the hydrophobicity of the paper or to increase the brightness.
Starch is the most used additive in paper fabrication. Modified starches have been used for some time as paper chemicals, both as flocculents to retain small particles in the paper web and as dry strengthening agent. Up until recently, its consumption in the paper industry increased more than paper production. This use has now swung back, starch consumption now increases less than paper production. Most of this starch is layered on the paper surface as "raw starch" to affect mechanical characteristics, but for more technical applications, more efficient synthetic polymers are now often used in place of starch derivatives. To reverse this trend, new types of modifications have been made in this project and the use of such derivatives, as paper chemicals, has been investigated, since new opportunities could arise in the paper industry associated with an increase in the technical level of starch derivatives.
Objectives
The project has focused on three tasks:
Activities
A range of modifications were made both on conventional potato starch and a new, biologically improved, starch which contains 100 % amylopectin instead of the usual mixture of 20 % amylose and 80 % amylopectin in conventional potato starch. Three main fields have been investigated:
For wet-end chemistry and coating, the new starch derivatives could have two main origins. New chemical derivatives produced by starch chemistry could bring new properties to paper, favour the machinability or improve paper characteristics. A common starch is a mixture of two kinds of molecules: amylose, a linear polymer of low molecular weight, and amylopectin, a branched polymer with a high molecular weight. Recent genetically modified plants can produce a starch made of pure amylopectin. The properties of chemical derivatives of pure amylopectin will be different and possibly improved compared to those of the starch chemical derivatives marketed today for the paper industry.
Starch as a retention aid Cationic starch has been much used as a retention aid and is still much used in wet end, because it is a multi-purpose product bringing cohesion to the wet and dry paper sheet, and because it helps neutral sizing. But it seems that flocculating forces developed by this polymer are too weak for use on modern very fast paper machines. New 100% amylopectin starches, highly cationic starches and other starch derivatives could develop stronger forces and had to be tested for retention at lab scale and on a fast pilot paper machine.
Starch treatment of white water Paper makers have to reduce the effluent of the paper mills. Deleterious substances, especially anionic colloids, accumulate in the white water circuit. This problem, important today, will increase over the next few years. Hence, the project aimed to answer the question - could starch derivatives and especially the new highly cationic starches act as scavengers of such substances and maintain their dry strength effect?
Modified starch as dry and wet strength agent The mechanism of action of dry and wet strength agents is not well known. A better understanding of the mechanisms should help to the implementation of the products. Some derivatives of synthetic polymers with specific groups can develop wet strength. Hence, the project aimed to answer the question - could the grafting of similar groups on starch turn this natural polymer into a wet strength agent?
Modified starch for improving the print quality of coated paper and board A pigment aggregation into the coating should increase the porosity of the coating layer and thus improve the printing qualities of paper. The necessary flocculation to obtain this porosity must be carefully controlled. An increase of the viscosity would reduce the runnability. Too weak flocs would break during the drying phase and would have no effect on the optical and mechanical characteristics of the coating layer. This can be done at lab scale by measurement of the rheological properties of the coating and has to be controlled at pilot scale and through industrial printing tests.
Starch based fillers To act as a filler particle, starch granules, intact or ground, must remain insoluble in water, even after one pass in the drying section of a paper machine. This could be obtained through chemical modifications, especially three-dimensional cross-linking. The fixing of chemical group on the surface of the starch granule could favour retention and produce a scavenger of anionic colloids in white water. This could lead to a 'biological paper' of very low ash content, made only of renewable bio-products, and producing no residues after incineration. The surface of starch granules can be a support for inorganic precipitates. Precipitated starch based copolymers could act as pigment. In that case, their effect on the surface properties of paper will be more interesting than their specific optical properties.
Results
Starch for the wet-end chemistry and for coating applications A range of modifications have been made both on conventional potato starch and a new biologically improved potato starch containing pure amylopectin instead of 20% amylose and 80% amylopectin as in a conventional potato starch. These starch derivatives have been tested for various applications at laboratory and pilot scale.
Wet end chemistry, laboratory and pilot trials Starch derivatives have been characterised through their effect on:
The cooking conditions and their effect on mechanical properties of paper have been carefully studied. The floc strength indicates that highly cationic starches as well as cationic starches are acting in flocculation in a bridging mechanism.
Cationic amylopectin was found to give a higher efficiency as a retention aid, even if the adsorbed amount of starch on the cellulosic fibres used in the retention measurement is almost the same. This is probably due to the much higher molecular weight in the amylopectin fraction than in the amylose fraction. Cationic amylopectin also gave better dry strength performances to paper. A prerequisite for the better performances seems to be that the amylopectin starches are prepared at lower cooking temperature than for the conventional potato starch. This result suggests that the starch cooking conditions should be carefully optimised in the paper mills. Cationic hydrophobic starch can increase both dry strength and hydrophobicity of paper. Combinations of synthetic sizing agents, such as AKD and cationic hydrophobic starch can lower the amount of synthetic sizing agent needed. The cationic starch provides dry strength and partially the hydrophobicity effect.
The development of a new starch based wet-strength additive was not successful, probably because the reactive groups were able to react into the starch molecule itself, bringing no strength to paper.
Starch as a binder in coating colours Laboratory and pilot trials were carried out on base paper. Several starch products were investigated at laboratory scale, with an oxidised starch used as a reference. These included:
Cationic hydrophobically modified starch and amphoteric starch caused a more pronounced aggregation in the coating colour than the reference starch, resulting in a more porous coating layer. Unfortunately the aggregate was weak and broke down when sheared. Most stables were the aggregates with the amphoteric starch but these were more viscous. The amylopectin was the only product that met the demands of a high optical and topographical coverage combined with a favourable rheology in terms of a low viscosity at a given solids concentration. In addition was found that amylopectin was easy to dissolve and that the solution was stable.
The drying strategy to apply starch-containing coating on a wood free base paper, to counter mottling, has been optimised on a fast pilot coater. A precise three stage drying induces less mottling than a one stage drying. The following method had to be used to obtain the best quality.
Pure amylopectin starch gave less viscosity to coating colours than common starch. This would allow an increase of the solid content and decrease the energetic cost of the drying section without any viscosity trouble. On uncalendered papers, amylopectin containing colour led to higher permeability and gloss than those containing common starch. On calendered papers, permeability and gloss reach a higher level with low gloss heterogeneity and a greater smoothness. These results were obtained in the precoat and in the topcoat when amylopectin was used as a substitute to oxidised starch.
Starch based filler Starch chemical derivatives, including inorganically covered starch, have been prepared and tested in paper applications. The chemical insolubilisation of a suitable selection of starches has been obtained by cross-linking. Since it was been found impractical to reduce the particle size of coarser starches in a grinding process, oat starch and the fine fraction of wheat starch were selected for their particle size. The effect of these starch pigments on paper properties has been studied at lab scale. A moderate cationization of the surface was found to improve their retention.
In fibre sheets, insoluble starch pigments have poorer optical properties than mineral fillers. When starch pigments are introduced into wood-free fine paper sheets in substitution of 30% chalk, brightness decreases by 1-2% and light scattering by 2-10 m2/kg. Pre-cooking of starch changed the optical properties very little. Starch pigment can not compete with soluble starch in creating sheet strength. The efficiency of starch pigment per added amount was approximately only ten percent of that of soluble cationic starch. Pre-cooking of starch improved some strength properties but not as much as soluble starch.
Inorganically modified granular starch produced superior results. When introduced into the paper stock, the products were found to have enhanced brightness and opacity without any drop in strength.
Starch based pigments were also introduced into coating colours, substituting for mineral pigments. Both inorganically modified starch and precipitated styrene-starch copolymers improved the printability of the paper surface. Quality improvement increased with the pigment dosage and, for the inorganically modified starch, also with the amount of calcium carbonate precipitated on the starch granules. An interesting feature of the coating layers containing starch pigments was that their polarity was much higher than in layers containing mineral or mineral plus plastic pigments. This property might be exploited in water based printing.
Conclusion
No new chemical modification of conventional starch produced a soluble starch able to compete with marketed products, except perhaps the cationic hydrophobic starch for which marginal uses could be found after additional testing.
In contrast, chemical modifications of conventional starches, applied to biologically improved starch (pure amylopectin) gave derivatives with better properties than existing marketed products. As wet-end additives they could improve slightly the retention and mechanical characteristics of paper. In coating, where amylopectin can be a substitute to common potato starch in blade coating colours or metering size press colours, there were advantages in terms of runability of the coating colours and the printability of the coated papers.
Commercial development of these derivatives will depend on the price of amylopectin extracted from genetically modified potatoes. Insolubilised starch will never compete with mineral fillers in terms of price and optical characteristics. The idea of a low ash content paper made only of renewable bio-products has been raised but not followed up.
Inorganically covered fine grained starch granules and precipitated styrene-starch copolymer introduced in coating colours improve the printability of coated papers. The same precipitated styrene-starch copolymer modified strongly the surface polarity of the coating layer. However, use of these results would require further development.
Introduction
Starch is the most widely used additive in paper
fabrication. Over the last few years the rate of increase in starch use was
faster than the growth rate of the paper industry. More recently this situation
has changed, and now the rate of increase in starch consumption lags behind the
rate of increases in paper production. Most of the starch used at present is use
to coat the paper surface as raw starch, to enhance the mechanical properties.
For more technical applications, more efficient synthetic polymers are often
used in place of starch derivatives. If the performance of starch derivatives
can be improved, then new possibilities should appear in the paper industry. The
objective of this project is to investigate this possibility. This was done
through a series of tasks as follows:
These can be grouped into three fields of activity as:
Approach
Task 1:
Starch as retention aid. Cationic starch as been much
used as a retention aid and is still much used in the wet-end of pulping. Here,
it is has a multi-purpose role resulting in cohesion of both wet and dry paper
sheet. It also helps neutral sizing. However, it would appear that the
flocculating forces developed by this polymer are to weak for modern very fast
paper machines. New 100 % amylopectin starches, highly cationic starches and
other starch derivatives could produce stronger forces and have to be tested for
retention at the laboratory scale and on fast pilot paper machines, alone or as
part of multi-component systems. Several starch derivatives have been tested at
laboratory scale, for performance as retention aids in one and two component
systems. New methods of starch cationisation produced retention aids that were
effective at a moderate degree of substitution.
Task 2:
Starch compounds for control of pollutants in white
water. Papermakers have to reduce the effluent of paper mills. Substances,
especially anionic colloids, accumulate in the white water circuit. This
problem, already important, are expected to increase in the near future. The
project aimed to answer the question: Could starch derivatives and especially
the new cationic starches, act as a scavenger of these substances and serve as
a good retention aid in a highly polluted white water circuit. Precipitation of
anionic colloids was studied at laboratory scale using various starch
derivatives. Their efficiency appeared to be correlated with the degree of
cationisation.
Task 3:
Modified starch as dry and wet strength agent. The
mechanism of action of dry and wet strength agents is not well understood. A
better understanding of the mechanisms enable better products to be designed.
Some derivatives of synthetic polymers with specific groups are known to enhance
wet strength. This raises the question: could the grafting of similar groups on
starch turn this natural polymer into a wet strength agent ? The starch
derivatives tested to date did not give results good enough to be classified as
wet strength additives. Further information will be obtained by studying the
effect of dry and wet strength agents at the level of the individual fibres
using scanning electron microscopy.
Task 4:
Modified starch for improving the print quality of
coated paper and board. Pigment aggregation into the coating depends on the
porosity of the coating layer, increased porosity would thus improve the
printing qualities of paper. The flocculation required to obtain this porosity
must be carefully controlled to preserve the optical and mechanical
characteristics of the coating layer and to avoid runnability problems. This can
be done at lab scale by the measurement of the rheological properties of the
coating colours. But these results indicate only a tendency and need to be
confirmed by printing tests using industrial printing machines on paper coated
at pilot scale. Several starch derivatives have been investigate. After an
initial screening they have been introduced in coating colours for trials at
pilot scale. The results were quite satisfactory with oxidised amylopectin. This
starch fraction is easier to handle than common starches that also contain
amylose and could replace common mixed starches in future.
Task 5:
Starch based fillers. To act as filler particles,
starch granules, intact or ground, must remain insoluble in water, even after
one pass in the drying section of a paper machine. This could be obtained
through chemical modifications, especially three-dimensional cross linking. The
fixation of chemical group on the surface of the starch granule could favour
retention and produce a scavenger of anionic colloids in white water. However,
these properties alone are not enough and the insoluble particles obtained in
this way must also result in good optical properties to paper and opacity in
particular. The effects of three insolubilised starches were investigated in
order to determine:
However, to date the performance has not been very good. Further work includes the investigation of starch based pigments and the ability of inorganically-modified starch granules to enhance brightness and opacity as a filler in sheet coating.
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by CPL Press:
03/07/2007
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