Method of producing a particle or group of particles having a coating of polymers interacting with each other

Method of producing a particle or group of particles having a coating of at least two, preferably at least three, outside each other located thin layers of interacting polymers, at which the particle or group of particles is treated in consecutive steps with solutions of the interacting polymers. Excess of the previous polymer is removed between each treatment step alternatively the respective polymer is added only in such an amount in each step that substantially all polymer is absorbed to the particle surface. It is also referred to a paper- or nonwoven product containing fibers and/or fillers containing particles or groups of particles of the mentioned kind.

DESCRIPTION OF THE INVENTION According to the present invention particles or groups of particles, e g fibers or filler particles, are treated with interacting polymers in order to build up thin multilayers of the interacting polymers on the particle surface. In principle the technique described in for example above mentioned articles from thin Solid Films is used in case alternating cationic and anionic polyelectrolytes are used as interacting polymers, with the difference that according to the invention the substrate is fibers or other particles or groups of particles. The particles are treated in consecutive steps with solutions of the interacting polymers, at which the treatment time for each step is sufficient for forming a layer of the desired molecular thickness, preferably of the magnitude 5-100 &angst;. The interaction between the particles can be in the form of electrostatic forces, at which the polymers consist of alternating cationic and anionic polyelectrolytes, or by interaction between nonionic polymers by means of e g dispersion forces or hydrogen bonds. Examples of this type of interaction between nonionic polymers are adsorption of polyethylene oxide on unbleached cellulosic fibers and complex formation between polyethylene oxide and polyacrylic acid. In case the interacting polymers are alternating cationic and anionic polyelectrolytes the first layer should be a cationic polymer for particles or groups of particles having an anionic surface, which for example is the case for cellulosic fibers, and vice versa. Possible excess of the previous polyelectrolyte can be removed between every treatment step, e g by rinsing with water. Alternatively the addition is controlled in such a way that no excess amount of the respective polymer is added in each step, so that substantially all polymer is adsorbed to the particle surface. The method is based on electrostatic attraction between oppositely charged polyelectrolytes for building the desired multilayers. By treating the fibers in consecutive steps with a solution containing polyions of opposite charge and permit these spontaneously to adsorb to the particle surface, multilayers of the stated kind are built up. In principle all types of polyelectrolytes may be used. Through such a treatment of particles or groups of particles, such as fibers, it is possible to make new types of surface modifications to them. By for example treating fibers with consecutive layers of hydrophobic, charged polyelectrolytes it would be possible for example to develop new types of hydrophobizing chemicals for the hydrophobization of paper. It would also be possible to build up “intelligent” surface layers on fibers, which alter the properties with temperature, pH, salt content etc. These changes could for example be based on fundamental knowledge about modern theories on interaction between polymers and surfactants. Further applications are ion-exchanging fibers where “membranes” with ion-exchanging properties are provided on the fiber surface, wet strength agents where the added polymers are reactive with the fibers and with each other, in order to provide permanent bonds between the fibers and for the production of highly swelling surface layers, where the added chemicals form swollen gel structures on the fiber surface for use in absorbent hygiene products. Another possible application are new types of fibers for printing paper, where the adsorbed polymers change colour when they are exerted to an electric, magnetic or electromagnetic field. Such polymers are available today. The fibers that are treated with the method according to the invention can be of optional kind, natural as well as synthetic fibers. Mainly cellulosic fiber are concerned. However it would be possible to treat synthetic fibers, for example for giving them a more hydrophilic surface. Also groups of fibers or particles can be treated according to the method. Examples of suitable anionic and cationic polyelectrolytes which may be used in the method according to the invention are given below. Anionic polyelectrolytes: Anionic starch with different degrees of substitution, polystyrene sulphonate, carboxy methyl cellulose with different degrees of substitution, anionic galactoglucomannan, polyphosphoric acid, polymethacrylic acid, polyvinyl sulphate, alginate, copolymers of acryl amide and acrylic acid or 2-acrylic amide-2-alkylpropane sulphonic acid. Cationic polyelectrolyte: Cationic galactoglucomannan, polyvinyl amine, polyvinyl pyridine and its N-alkyl derivatives, polyvinyl pyrrolidone, chitosan, alginate, modified polyacryl amides, polydiallyl dialkyl, cationic amide amines, condensation products between dicyane diamides, formaldehyde and an ammonium salt, reaction products between epichlorhydrine, polyepichlorhydrine and ammonia, primary and secondary amines, polymers formed by reaction between ditertiary amines or secondary amines or dihaloalkanes, polyethylene imines and polymers formed by polymerisation of N-(dialkylaminoalkyl)acrylic amide monomers. 
 EXAMPLE 1 The example below shows the increase of tensile strength of sheets made in a dynamic sheet former. The pulp that is used was bleached SWK (softwood sulphate pulp) beaten in accordance with SCAN-C 18:65, diluted to 3 g/l and pH adjusted to 8. PVAM (polyvinyl amine), a cationic polymer, was added in excess and was given time to react after which the excess of polymer was washed away from the fiber suspension by means of water. After that CMC (carboxy methyl cellulose), an anionic polymer, was added in excess, and after 10 minutes non-adsorbed polymer was removed through washing. Admixture of PVAm and CMC was repeated in several steps. After each addition of CMC the so called dynamic sheet former was used for making sheets having a basis weight of 80 g/m 2 . The sheets were tested with respect to tensile strength according to SCAN-P 67:93. The results are shown in Table 1 below and proves clearly an improvement of tensile strength index with the number of applied polymer layers. 1 TABLE 1 Polymer layer (P &equals; PV Am, C &equals; CMC) Tensile strength index (kNm/kg) No polymer 39.0 P 53.6 PC 56.6 PCPC 69.4 PCPCPC 75.8 The results are also shown in FIG. 1 in the drawings. 
 EXAMPLE 2 In this example fillers for papermaking have been used which have been treated through multilayer adsorption with the same polymers as in Example 1 above, i e polyvinyl amine and carboxy methyl cellulose. Paper sheets of 80 g/m 2 were made in a dynamic sheet former. The sheets were tested with respect to ash content, tensile strength index and opacity. The results are shown in Table 2 below. Ash content here means the content of filler treated as above and which has been added to the paper. 2 TABLE 2 Tensile Polymer layer strength index Opacity (P &equals; PV Am, C &equals; CMC) Ash content (%) (kNm/kg) (%) No polymer 0.0 47.2 78.8 No polymer 20.6 19.7 87.1 P 7.5 38.3 82.9 P 14.3 30.5 86.7 P 21.7 23.8 89.5 PCP 7.5 37.9 83.5 PCP 13.9 31.7 87.8 PCP 24.3 26.6 90.7 PCPCP 9.0 40.0 84.7 PCPCP 18.0 35.9 87.0 PCPCP 28.0 30.0 191.0 The results are also shown in FIG. 2 and 3 . FIG. 2 shows the effect on strength in the paper containing different amounts of filler particles treated according to the invention. FIG. 3 shows the effect on the opacity of paper when using different amounts of fillers treated according to Example 2 above. 
 EXAMPLE 3 Two types of polymers were used: an anionic polyacryl amide (A-PAM), Percol 155 from Ciba, and a cationic polydimethyl diallyl ammonium chloride (DMDAAC) also from Ciba. The pulp that was used was unrefined, fully bleached longfiber pulp, pH 8 in all tests. The dosing of the polymers were done in two different ways: A) all at the same time and in this case 3.9 kg/ton A-PAM was added first and then 6.6 kg/ton polyDMDAAC was added. B) Six layers of 1.1 kg/ton polyDMDAAC and 0.65 kg/ton A-PAM in the respective layers were added. Excess of polymer was removed between the dosings. Handsheets were then made and the strength (tensile strength index) was measured. In FIG. 4 the strength increase in % is shown for the two different cases. As can clearly be seen it is much more effective to add the polymers in layers in a controlled manner.