The present invention is directed to a process for making starch, or polysaccharide particles for the paper industry in a micro fluidic reactor. In particular the present invention relates to a process of preparing cross-linked functionalized starch or polysaccharide particles using a micro fluidic device. These particles are generally suitable for use in the paper/board industries to enhance mechanical strength. Moreover the present invention is directed to the use of starch or polysaccaharide particles and to the use of a device for forming starch or polysaccharide particles for the production of paper or cardboard products .

In the last years it was shown that paper shows especially good mechanical properties when particle size distribution in the paper is as homogenous as possible. For obtaining improved mechanical properties of the paper the use of an emulsion made of a polysaccharide solution in an oil phase was proposed. Herewith the conventional water-in-oil (w/o) emulsion method is the most commonly used method for preparing spherical polysaccharide particles. Generally, a polysaccharide solution is emulsified in an oil phase, such as heptane, cyclohexane, or even vegetable oil with the assistance of a lipophilic surfactant, such as sorbitan monooleate .

The starch particles are usually cross-linked by reactions of the hydroxyl group with polyfunctional reagents or by radical polymerization of unsaturated groups created on the starch. In US-A 2007/0122487 a water-in-oil emulsion method is used to prepare hydroxyethyl starch gel particles by radical polymerization of acryloyl groups on hydroxyethyl starch.

In CN-A 1502648 a starch solution is emulsified together with some monomers in vegetable oil and consecutively cross-linked by radical polymerization.

In US-A 6,755,915 a special water-in-oil emulsion method was investigated, where the water-in-oil emulsion was converted from an oil-in-water emulsion by temperature control, then starch particles were cross-linked and separated by conventional separation techniques .

In Journal of Colloid and Interface Science 246, 48-59

(2002) and in Journal of Material Science: Material

Medicine 17, 371-377(2006), starch particles were prepared by water-in-oil emulsification and the particles cross- linked using epichlorohydrine and trisodium trimetaphos- phate, respectively. It is easy to control the size of the particles using this method, but solvent and surfactants have to be removed from the final particles. This process uses a totally different route to the one that will be described in this patent.

The solvent exchange method is also often used. In CN-A 1951979 a starch solution, e.g. in dimethyl sulfoxide, is added dropwise to a mixture of water/ethanol/surfactant, e.g. sodium dodecyl benzene sulphate, to form starch particles . According to the article of El-Tahlawy, K., et al .

[Carbohydrate Polymers 67(3), 319-331 (2007)], starch microcellular foam particles were prepared by precipitation of a cooked starch solution cross-linked with glutaralde- hyde under shear with ethanol . Effects, such as the viscosity of the starch solution, stirring speed during the precipitation and cross-linking content on the particle morphology and size were studied. The method was also applied in US-A 6,703,048.

Furthermore, micro fluidic systems have gained great interest over recent years in fields such as biomedicine and biochemistry to produce controlled drug delivery systems . Micro fluidic systems are also known as micro mixers, micro reactors, micro fabricated systems and lab- on-a-chip. Micro fluidic refers to a device that contains a network of chambers connected by channels, in which the channels have mesoscale dimensions, e.g., having at least one cross-sectional dimension in the range from about 0,1 μm to about 500 μm.

It is also known to use micro fluidic systems or micro fluidic reactors for preparing emulsions used for different purposes .

Micro fluidic reactor systems have been used for the fabrication of 10 μm sized monodisperse micro gels by emulsifying an aqueous dextran hydroxyethyl methacrylate phase within an oil phase at the junction of micro fluidic channels (De Geest, B. G., et al . , Langmuir 21(23), 10275- 10279 (2005)). This is the first use of micro fluidics in grafting onto starch particles and using an oil phase. These particles are predominantly suited for use in the biomedical and drug delivery industries and do not include functional groups in the gel particles that can be applied as retention aid for the paper/board industry.

The present invention therefore aims at producing extremely homogeneous emulsions and especially producing starch micro gel particles which can be used in the paper industry which starch particles show a homogeneous size distribution and which particles can therefore enhance the properties of the final product, namely the paper or the card board.

For solving the above-mentioned problem according to the present invention a process for making starch particles in a micro fluidic reactor is provided comprising at least two immiscible liquid inlet streams, in which process the two liquid streams are forced through a plurality of adjacent micro channels of the micro fluidic reactor and joined together to form an outlet stream of a water-in-oil (w/o) emulsion system, where a discontinuous phase will be a mixture based on aqueous starch or polysaccharide and a continuous phase will be an oil/solvent based system and wherein a polyvinyl alcohol with a molecular weight of between 5.000 and 140.000, preferably between 5.000 and 30.000 is added to the mixture based on starch or poly- saccharide.

With such an inventive process stable water-in-oil polysaccharide emulsions can be prepared using micro fluidic reactors. Furthermore it is possible with such a process to produce starch particles with various particle sizes and with extremely narrow size distributions. Therefore a great uniformity of the final micro gel starch particles can be obtained with such a process and in the following the whole paper show excellent properties, especially mechanical properties . By adding a polyvinyl alcohol with a molecular weight of between 5.000 and 140.000, preferably between 5.000 and 30.000, to the mixture based on starch or poly- saccharide, it is preferably possible to prepare particles with stronger adhesive ability. Especially, if the ratio of polysaccharide to polyvinyl alcohol is between 100:0 and 40:60, preferably between 100:0 and 70:30, the adhesive ability of the prepared starch micro gel particles can be further enhanced. In such a mixture the solid content can be as high as 30 wt%, but preferably 3 to 15 wt%, for obtaining the best results.

Especially micro fluidic systems offer various benefits as a process of emulsification, such as a continuous process for preparing particles, wherein starch particles can be emulsified and cross-linked directly in a pipe-flow reactor system and introduced directly to the process . A further benefit is that the particle size and distribution can be manipulated easily by adjusting the pressure and/or flow rates of the feed streams through the micro fluidic reactor. Generally it is possible to produce very mono- disperse particles using this process. Furthermore, since there are no mechanically moving parts, energy input is much less than conventional batch reactor systems. The process can easily be scaled-up using several micro fluidic devices in parallel.

Because of the fact that the two liquid streams are forced through a plurality of adjacent micro channels of the micro fluidic reactor and joined together to form an outlet stream of a water-in-oil emulsion, one can obtain an extremely homogeneous and fine emulsion, in which emulsion the starch particles are uniformly distributed.

If the continuous phase and especially the solvent in the continuous phase is a hydrophobic solvent, such as for example cyclohexane or heptane, it is possible to obtain an extremely homogeneous and stable emulsion.

Especially for stabilizing the polysaccharide emulsion which is formed after passing through the micro channels of the micro fluidic reactor a lipophilic surfactant such as sorbitan monooleate or a w/o emulsifier such as poly- hydroxystearic acid-polyethyleneglycol-polyhydroxystearic acid is introduced into the joined streams. The stability of the emulsion is of great importance for the final product which can be obtained and especially for the homogeneity of the starch micro gel particles which will be produced according to the present invention.

Especially for obtaining a defined particle size distribution of the starch micro gel particles in the process according to the present invention the starch or polysaccharide particles in the emulsion, which was formed in the micro channels of the micro fluidic reactor, are cross linked with a cross linking agent, by radical polymerisation, UV, thermo or gamma ray irradiation.

The best results during the cross-linking can be obtained, if the cross linking agent is added to the emulsion in the presence of a base such as sodium hydroxide. With such a process the obtained starch or polysaccharide micro gel particles show a uniform or narrow particle size distribution and especially a great homogeneity concerning the dimensions of the particles.

For obtaining a good and homogeneous product and for avoiding any undesired by-products in the process according to the present invention the cross linking agent is added in an amount of 1 to 10 wt%, preferably 1 to 5 wt% based on the weight of the polysaccharide or starch contained in the emulsion. By adding amounts of 1 to 10 wt%, preferably 1 to 5 wt%, of the cross-linking agent, the cross-linking of the starch or polysaccharide particles can be performed in a sufficient manner and moreover it will be shown that the products obtained have a uniform and homogeneous particle size distribution.

Paper or cardboard products produced with starch or polysaccharide micro gel particles which are produced according to the process of the present invention show improved properties, especially an enhanced bending stiffness, tear length, tear resistance and folding endurance, and especially such properties can be obtained in the process according to the present invention, wherein the starch or polysaccharide particles are ionized by addition of an ionic substance such as carboxymethyl chloro ether or quaternary ammonium chloro ether, in the presence of a base .

For obtaining a faster cross-linking and ionization of the particles the process according to the present invention is performed such that after passing the micro channels of the micro fluidic reactor and after addition of the cross linking agent and the ionic substance, the produced mixture of the reaction partners is heated to a temperature between 50 to 100 ⁰ C in a heating section.

For obtaining an extremely narrow particle size distribution of the starch or polysaccharide micro gel particles the outlet stream from the micro fluidic reactor is heated in at least two series of heating sections. With such a process it is possible to precisely control the process and therefore also the particle size distribution of the final product, which shows excellent mechanical properties .

In the process according to the present invention it is possible that either one or all components of the reaction system, namely the inlet streams of the liquids, the micro fluidic reactor and/or the outlet stream of the micro fluidic reactor are located in at least one heat exchanger. With such a process it is especially possible to obtain a very good energy balance of the process and also a final product showing excellent mechanical properties.

Furthermore, the present invention aims at the use of starch or polysaccharide particles and at the use of a micro fluidic reactor for producing homogeneous and uniform starch polysaccharide micro gel particles for obtaining paper cardboard with enhanced or better properties .

The use of polysaccharide particles prepared according to the process of the present invention allows the production of paper or card board products with enhanced or better properties especially of paper or cardboard products showing excellent mechanical properties. By using of a micro fluidic reactor comprising micro channels, at least two separate liquid inlets and a liquid outlet, in which reactor immiscible liquids are unified and then introduced into the liquid outlet for forming starch or polysaccharide particles for the production of paper or cardboard products it is possible to provide paper or cardboard having a uniform quality and homogenous and defined properties.

By using a device in which at the liquid outlet at least one further inlet or duct is attached for introducing a cross-linking agent and/or ionic derivative to the unified immiscible liquids one can obtain a basic mixture for obtaining starch or polysaccharide particles which show a defined and homogeneous emulsion and especially a stable emulsion for reaction and therefore also the final product which can be produced with a device according to the present invention shows homogeneous and defined properties .

Especially for obtaining an extremely small particle size distribution the use according to the present invention preferably comprises the micro fluidic reactor which contains furthermore at least one of a mixing section, a heating section and a cooling section. Therefore, one or all of mixing sections, heating sections or cooling sections can be contained in the micro fluidic reactor and even if not all of these additional sections are contained the size distribution of the final product can be adjusted according to the use of the present invention.

The invention is in the following further described on the basis of examples and drawings, wherein: Fig. 1 is a diagram showing the device layout of a micro fluidic system for preparing polysaccharide/starch micro gel particles according to the process of the present invention; Fig. 2 is an image of a multi-lamination micro mixer according to the present invention which is used for emulsification of two immiscible fluids;

Fig. 3 shows a particle size distribution of starch and polysaccharide micro gel particles which are prepared using different polysaccharide/polyvinyl alcohol to solvent flow ratios ;

Fig. 4 shows polysaccharide gel particles which are prepared in a micro fluidic device according to the present invention at various flow rates.

Fig. 1 shows a diagram explaining the process layout of a micro fluidic system according to the present invention for preparing polysaccharide or starch particles in solution.

In this diagram a micro fluidic reactor 1 is schematically shown, two inlet ducts 2 are connected therewith and both inlet ducts 2 are connected on their other ends with a special solution reservoir or tank 3. On the outlet duct 4 of the micro fluidic reactor 1 there are provided two further inlet ducts 5 and 6 for introducing a cross-linking agent and an ionic derivative which are to be mixed with the fluid which leaves the micro fluidic reactor 1. After having introduced all components to the liquid or fluid, the fluidic system will be introduced into a mixing section 7 and after leaving the mixing section 7 into a heating section 8 and thereafter into a cooling section 9, where after the final starch particle solution leaves the micro fluidic reactor system.

In Fig. 2 the micro fluidic device 1 is shown in greater detail. Especially it is shown how an emulsion of two immiscible fluids or liquids can be prepared in such a micro fluidic device 1.

On the basis 10 of the micro fluidic reactor 1 there are attached two different inlet ducts 3 which inlet ducts 3 lead a charge of two immiscible liquids into micro channels 11 of the micro fluidic reactor 1. At an outlet slit droplets of fluid 13 penetrate. Because of the fact that the droplets are of a very small dimension there can be prepared an emulsion of two immiscible fluids which emulsion is characterized in that the droplets of two immiscible fluids are very small in dimension and therefore one can obtain a highly intense or homogeneous and stable emulsion. This emulsion penetrates the outlet duct 4 of the micro fluidic reactor 1 and will be further introduced in the mixing section 7, heating section 8 and/or cooling section 9 for producing starch and polysaccharide particle solutions .

Furthermore, the invention is described by an example in which also Fig. 3 and 4 are described.

Example

Preparation of starch particles using a micro fluidic reactor according to the invention An anionic starch solution with DS = 0.3 was prepared by- adding maize starch (20 g) and acetone (20 g) to a reaction flask. The starch mixture was slurried and sodium hydroxide solution (5 wt%, 100 ml) added. The mixture was heated to 60 ⁰ C and sodium monochloroacetate (4.3 g) added. The reaction mixture was stirred continuously for 2 - 3 h. The carboxymethyl starch solution was then allowed to cool and neutralized with hydrochloric acid (32 wt%, 14.3 g) . The gel was precipitated in acetone (500 ml) . The starch gel was subsequently redissolved in water (200 ml) and once again precipitated in acetone. The redissolving and precipitating steps were repeated until there was no trace of chloride present in the modified starch and could be dried to remove all traces of acetone and water.

An anionic starch/polyvinyl alcohol (90:10) mixture was dissolved in NaOH solution (1 %) at about 95 ⁰ C. The total solids content of the solution was 2,5 wt%. This was loaded into the first of two vessels and heptane, premixed with Span 80 surfactant (sorbitan monooleate) (4 %) , was loaded into the second vessel. The flow rates of the two feed streams through the pumps were adjusted to ensure that the emulsification in the micro fluidic reactor occurred at a feed ratio of 40:60 heptane to starch. Epichlorohydrine (rate adjusted to 2 wt% based on total solids) was fed inline to the emulsion just after passing through the micro fluidic reactor. The emulsion was passed through a mixing section to allow sufficient mixing of the cross-linking agent with the emulsion. Subsequently, the mixture was passed through a heating section (90 ⁰ C) to allow time for sufficient cross-linking to occur. Upon completion of the reaction the particle solution passed through a cooling section (25 ⁰ C) before being collected at the outlet. The final dispersion was neutralized with hydrochloric acid (32 wt %) to a pH of between 7 and 8. The dispersion was centrifuged, and the particles washed and diluted with water to a total solids content of 10 wt%.

The range of particle sizes obtained varied between 10 and 1000 microns.

Fig. 3 presents typical particle size and distributions of polysaccharide particles prepared using various ratios of polysaccharide/polyvinyl alcohol : solvent flow rate.

The ratio of polysaccharide/polyvinyl alcohol : solvent flow rate in graph A is 50:650 ml/h which results in a mean particle size of 11., 6 μm.

The ratio of polysaccharide/polyvinyl alcohol : solvent flow rate in graph B is 100:600 ml/h which results in a mean particle size of 11.6 μm.

The ratio of polysaccharide/polyvinyl alcohol : solvent flow rate in graph C is 200:500 ml/h which results in a mean particle size of 16.4 μm.

The ratio of polysaccharide/polyvinyl alcohol : solvent flow rate in graph D is 300:400 ml/h which results in a mean particle size of 16.8 μm.

The ratio of polysaccharide/polyvinyl alcohol: solvent flow rate in graph E is 350:350 ml/h which results in a mean particle size of 26.4 μm. The ratio of polysaccharide/polyvinyl alcohol : solvent flow rate in graph F is 400:300 ml/h which results in a mean particle size of 208.8 μm.

As the polysaccharide/polyvinyl alcohol flow rate increased, compared to the solvent flow rate, the mean particle size gradually increased and ultimately became unstable after 350:350 ml/h flow ratio.

Fig. 4 presents optical microscope images of poly- saccharide/polyvinylalcohol particles prepared using the micro fluidic reactor according to the present invention in which the flow rate of the polysaccharide and solvent is increased, producing particles that gradually decrease in average size.