MODIFIED STARCH AND COATING SLIP COMPRISING THE SAME

Abstract: A modified starch and a coating slip containing the same are described. The modified starch is characterised in that it has a degree of substitution in respect of carboxyl groups of 0.005-0.070, a degree of substitution in respect of cationic, hydrophobic groups, which contain a hydrocarbon chain having 7-20 carbon atoms, of 0.008-0.080, and that an aqueous solution of the modified starch has phase separation in the range 0-100 DEG C, preferably about 30-60 DEG C.

Description: MODIFIED STARCH AND COATING SLIP COMPRISING THE SAME The present invention relates to a modified starch and a coating slip comprising the same. In the manufacture of paper and cardboard, the paper web is treated by, inter alia, a coating slip being applied to make the paper surface more even and dense and to achieve a higher quality in the printing process. Such coating slips contain pigments, such as clay, chalk, titanium dioxide etc., in a binder system which contains binding and thickening agents in the form of e.g. starch or a polymer latex, optionally in combination with each other. The binder content is usually about 5-40, prefer¬ ably about 5-20 parts by weight of binder per 100 parts by weight of pigment. For dispersing the pigment, pre¬ ferably also a dispersing agent in an amount of about 0.03-1 parts by weight per 100 parts by weight of pigment is included. In addition, the coating slip may comprise a number of further additives, such as optical whitening agent, wet-strength agent, viscosity-controlling agent, colourants etc. The content of these additives normally amounts to about 5 parts by weight at most per 100 parts by weight of pigment. All coating slips must have such rheological pro¬ perties as to be manageable under the existing conditions of use, which means anything from pumping at low shear- ing rates to application to the paper web at very high shearing rates at a coating station. At the same time the coating slip should be such as to permit optimum results when printing the coated paper. This places high demands in respect of, inter alia, the covering of the surface and the porosity provided by the coating slip when it has been applied to the surface of the paper, and has been immobilised and dried. The above-mentioned criteria of coating slips are to some extent contradictory, and one drawback of coating slips known up to now is that if a coating slip is pro- 4 preferably about 0.010-0.050. The oxidised starch is suitably prepared in the following manner. Potato starch is suspended in water to a dry content of about 45%. The pH of the suspension is adjusted with sodium hydroxide to a pH of 9.5. Sodium hypochlorite having an active sub¬ stance of 150 g/1 available chlorine is added slowly. The pH of the reaction mixture is kept constant at 9.5 by adding sodium hydroxide. When the reaction is completed, i.e. when all chlorine is consumed, the reaction is interrupted by neutralisation to a pH of 5.5 with hydro¬ chloric acid, whereupon the product is dewatered and washed before drying. Through this reaction, carboxyl groups are introduced on the starch molecule. The amount of carboxyl groups, i.e. the degree of substitution (DS) , is controlled by the amount of added hypochlorite and at which pH the reaction is carried out. For further de¬ tails, reference is made to an article by von J. Potze -and P. Hienstra, "ϋber den Einfluss der Reaktionsbeding- ungen auf die Oxydation der Kartoffelstarke mit Hypoklo- rit", Die Starke, No. 6/1963, pp 217-225. In addition to the introduction of carboxyl groups, the starch according to the invention is modified by introduction of cationic hydrophobic groups. The degree of substitution in respect of cationic hydrophobic groups is about 0.008-0.080, preferably about 0.010-0.030. To obtain the desired hydrophobicity, the hydrophobic groups contain a hydrocarbon chain having 7-20 carbon atoms, preferably 10-16 carbon atoms. To make the hydrophobic groups cationic, they contain a positively charged atom or group, such as a quaternary nitrogen atom. According to the invention, it is preferred that the positively charged atom or group is bound to, or constitutes part of the hydrophobic group, but the invention does not exclude that the starch is modified with separate hydrophobic groups and cationic groups. In such a separate modifi¬ cation, the degree of substitution in respect of the which decreases or eliminates the drawbacks of prior-art coating slips. The object of the invention is achieved by a modi¬ fied cationic starch which contains both carboxyl groups and hydrophobic groups and which in aqueous solution has phase separation. More specifically, the invention provides a modified starch, which is characterised in that it has a degree of substitution concerning carboxyl groups of 0.005-0.070, a degree of substitution concerning cationic, hydropho¬ bic groups, which contain a hydrocarbon chain having 7-20 carbon atoms, of 0.008-0.080, and that an aqueous solution of the modified starch has phase separation in the range 0-100°C. According to the invention, a coating slip is also provided, which is characterised in that it contains the above-defined, modified starch. Further features and advantages of the invention are apparent from the following specification and the accom- panying claims. The types of starch that are normally used in coat¬ ing slips are chemically degraded (oxidised) starches or chemically stabilised (hydroxy-propylated)/chemically degraded (oxidised) starches. Also chemically degraded/ cationised starches are used. In contrast thereto, the inventive starch is modified, on the one hand by being chemically degraded (oxidised) so as to contain carboxyl groups and, on the other hand, by containing cationic, hydrophobic groups. The starch used as raw material for the modified starch according to the invention can be of any known origin whatever, such as starch from potato, maize, wheat and tapioca. Carboxyl groups are introduced into the starch according to the invention in conventional manner by oxidation with e.g. sodium hypochlorite or hydrogen per¬ oxide. The degree of substitution in respect of carboxyl groups is according to the invention about 0.005-0.070, 6 separation is reversible. The UCST, i.e. the temperature at which phase separation takes place, lies for the modi¬ fied starch according to the invention in the range 0-100°C, preferably about 30-60°C, most preferred about 50°C. The reversible phase separation of the modified starch according to the invention causes the viscosity of the phase-separated starch solution at temperatures below the UCST to be lower than the viscosity of the starch solution at temperatures above the UCST. This phenomenon is utilised in the invention to provide a coating slip having unique and advantageous properties. If in fact the modified starch according to the invention is added to a coating pigment dispersion, the viscosity of the obtained coating slip at temperatures below the USCT of the starch will be lower than at temperatures above the same. This means that the coating strip after manufacture can be stored, pumped and applied to the paper web without any problems at temperatures below the USCT of the starch, at the same time as drying of the coated paper in the drying section at temperatures above the UCST of the starch results in the starch again being dissolved and its hydrophobic and cationic groups being available to form a structure. As a result, a well-covering coat¬ ing slip having a, relatively seen, high porosity is obtained. The inventive starch is intended to be used in a coating slip alone or together with a dispersion binder of the polymer latex type. When the modified starch is included in a coating slip together with a latex binder, there will be obtain¬ ed, after drying above the UCST, a coating layer having a higher porosity, compared with the condition that the modified starch had been replaced by a conventional, oxidised starch. The tensile stress at break and tensile energy at break for the layers containing the modified starch will not be lower than for the layers containing the conventional starch. This can be explained by the cationic groups should preferably conform with that of the hydrophobic groups. Modification of the starch by introduction of cati¬ onic, hydrophobic groups can take place, for instance by reacting the starch with the chlorohydrin of a quaternary ammonium compound, which contains a hydrophobic hydrocar¬ bon chain as above, for instance, the compound 3-chloro- -2-hydroxypropyl dimethyl dodecylammonium chloride. The introduction of cationic, hydrophobic groups can suitably take place in the following manner. Potato starch is sus¬ pended in water to a dry content of about 45%. The pH of the suspension is adjusted with sodium hydroxide to 11.4, whereupon the cationising reagent is added. During the reaction, the pH is kept constant at 11.4 by adding sodium hydroxide. After 24 h, total nitrogen and bound nitrogen are measured to determine the yield of the reac¬ tion. If the bound nitrogen amounts to the desired value, the reaction is interrupted by adjusting the pH to 5.5 with hydrochloric acid. The product is washed, dewatered and dried. If a starch is modified in this manner by introduc¬ tion of carboxyl groups and cationic, hydrophobic groups and the degree of substitution in respect of these groups as well as the size of the hydrophobic group are selected in a suitable manner within the above-mentioned ranges, a modified starch is obtained, which after dissolution in water, for instance by boiling in a jet boiler, phase- separates (precipitates) as the temperature is decreased. It is the amylose fraction or amylopectin fraction of the starch or both fractions at the same time which phase- separate. This phase separation, which is a distinguish¬ ing feature of the modified starch according to the in¬ vention, takes place at a certain so-called upper criti¬ cal solution temperature (UCST). When the temperature is raised above the UCST, the modified starch is dissolved, whereas, when the temperature is lowered below the UCST, the starch phase-separates (precipitates). This phase- 8 coating slip was smaller than if no phase separation had taken place. Coating slips prepared as described above at 23°C and 55-60°C, respectively, were applied to a web and dried at 23°C and 105°C, whereupon the properties of the coating layer were measured. The obtained results are stated in Table 1. Table 1 Temp. in Temp, in Dry content Density Porosity prep, of slip drying °C °C (% by weight) (kg/dm 3 ) (%) 23 23 52.8 1.66 32.3 23 105 52.8 1.46 40.7 55-60 23 54.2 1.47 40.3 55-60 105 54.2 1.26 51.3 Porosity values of 40% and above are to be consider¬ ed high. As is evident from Table 1, the highest porosity is obtained if the coating slip is prepared at a tempe¬ rature above the UCST of the starch and if the applied coating layer is dried at 105°C. This phenomenon can by explained by bridging via adsorption. EXAMPLE 2 Two coating slips consisting of 100 parts of kaolin clay dispersed with 0.3 parts of a polyacrylate dispers¬ ing agent, 6 parts of carboxylated styrene-butadiene latex and 6 parts of starch were prepared. The latex had the trade name DL 935 and was a coating quality supplied by Dow Sverige AB, Norrkόping, Sweden. The kaolin clay and the dispersing agent were identical with the quali¬ ties described in Example 1. For the one coating slip, use was made of a merely oxidised potato starch which had been oxidised with sodium hypochlorite to a degree of substitution (DS) of 0.034. This starch was used as an example of a conventional, oxidised starch and it was boiled in a jet boiler at 120°C. The slip containing this starch was designated "Slip A". For the other coating interaction between starch and latex/pigment caused by the cationic, hydrophobic groups. In order to further improve the understanding of the invention, it will be elucidated by means of a non- restricting Example. All parts in the Example concern weight, unless otherwise stated. EXAMPLE 1 A coating slip consisting of 100 parts of kaolin clay dispersed with 0.3 parts of a polyacrylate dispers¬ ing agent and 12 parts of modified starch according to the invention was prepared. The kaolin clay was of coat¬ ing quality and supplied by Speswhite, English China Clay International, Ltd., Cornwall, England. The dispersing agent was a low-molecular sodium polyacrylate under the trade name Dispex N40 supplied by Allied Colloids, Ltd. , Bradford, England. The modified starch according to the invention was a potato starch which had been oxidised with sodium hypochlorite to a degree of substitution (DS) of 0.020. Besides, the starch was modified by introduc¬ tion of cationic, hydrophobic groups. This modifica¬ tion was effected by reacting the potato starch with 3-chloro-2-hydroxypropyl dimethyl dodecyl ammonium chlo¬ ride. The degree of substitution (DS) of the modified starch in respect of cationic, hydrophobic groups was 0.021. The modified starch was boiled in a jet boiler at 140°C. The boiled starch had a UCST of about 50°C. The starch was added to the dispersed kaolin clay for prepar¬ ing the coating slip of the above composition. Since the modified starch in the coating slip had a UCST of about 50°C, the plastic viscosity of the coating slip (measured at 25°C) was higher if the coating slip was prepared and stored at 55-60°C (312 mPas) than if the coating slip was prepared and stored at room temperature (223 mPas) . At room temperature, the starch phase-sepa¬ rated, and therefore its effect on the viscosity of the 10 was 10 mm/ min. and the clamping length 100 mm. The obtained results are shown in Table 2. The tensile energy at break of the layers is given as tensile energy absorption index (TEA* ), defined as follows 1 ε ' TEA * = — / Edε ω 0 where ω is the surface weight, F is the tensile force per width, ε is the elongation and ε t is elongation at break. Table 2 Type Slip Density Poro¬ Tensile Tensile of dry sity strength energy slip content at break absorption (% by index weight) (kg/dm 3 ) (%) (MPa) (J/kg) Slip A 57.1 1.78 24.5 11.5+2.2 3.2+_l .2 Slip B 57.1 1.73 26.7 13.4+1.9 4.5+1.9 Table 2 shows that a higher porosity was obtained by using the modified starch according to the invention, compared with that obtained by using a conventional, oxidised starch. Table 2 also shows that the mechanical properties of the coating layer were improved by using the modified starch according to the invention. EXAMPLE 3 Aqueous solutions of three different modified pota¬ to starches, all oxidised with sodium hypochlorite, were prepared by boiling in a jet boiler. Of the starches, only one was oxidised (designated S-I ) , while the two other starches were both oxidised and modified with cationic, hydrophobic groups according to the invention (designated S-II and S-III ) . The degree of substitution (DS) for the three starches is stated in Table 3. Starch S-III was reacted with 3-chloro-2-hydroxypropyl dimethyl slip, use was made of starch that had been oxidised to a degree of substitution of 0.020 and besides was modified by introduction of cationic hydrophobic groups to a DS of 0.021. This starch is identical with the starch described under Example 1. This starch was boiled in a jet boiler at 140°C. The slip containing this starch was designated "Slip B" . The coating slips were prepared by adding first the starch and then the latex to the dispersed kaolin clay. The temperature of the starch solution was 70°C when it was mixed with the kaolin suspension. It was established that the starch included in Slip B had a UCST of about 50°C. This phenomenon was utilised during the preparation of the coating slip. The cationic groups will, at temperatures above the UCST, produce a structure in the wet coating slip which increases the porosity of the dry coating layer. The dry content of the coating layers was 57.1%. Since the starch used for the preparation of Slip A had no cationic groups, Slip A had a lower plastic viscosity at 25°C (50 mPas) than Slip B (119 mPas). Thanks to the existence of the UCST, the plastic viscosity of Slip B is considerably lower than if a quaternary ammonium starch ether without UCST should have been used instead. If thus the starch in Slip A is replaced with a starch that is oxidised to a DS = 0.020 and modified with 3-chloro-2-hydroxypropyl trimethylammo¬ nium chloride to a DS = 0.007, a coating slip having a plastic viscosity at 25°C of 200 mPas will be obtained. This result is remarkable, especially in view of the fact that the plastic viscosity normally increases as the DS increases. The coating Slips A and B, which were prepared as described above, were applied to a paper web at 23°C and dried at 23°C, whereupon the properties of the coating layer were measured. The mechanical properties were measured at 23°C and 50% relative humidity in tensile testing equipment (Alwetron THI, Lorentzen & Wettre, Stockholm, Sweden). In these tests, the rate of tension 12 CLAIMS 1. A modified starch, c h a r a c t e r i s e d in that it has a degree of substitution concerning carboxyl groups of 0.005-0.070, a degree of substitution concern¬ ing cationic, hydrophobic groups, which contain a hydro¬ carbon chain having 7-20 carbon atoms, of 0.008-0.080, and that an aqueous solution of the modified starch has phase separation in the range 0-100°C. 2. The modified starch as claimed in claim 1, c h a r a c t e r i s e d in that it has a degree of sub¬ stitution concerning carboxyl groups of 0.010-0.050. 3. The modified starch as claimed in claim 1 or 2, c h a r a c t e r i s e d in that it has a degree of substitution concerning cationic, hydrophobic groups of 0.010-0.030. 4. The modified starch as claimed in any one of claims 1-3, c h a r a c t e r i s e d in that the hydrophobic groups contain a hydrocarbon chain having 10-16 carbon atoms. 5. The modified starch as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that, in aqueous solution, it has phase separation at a tempera- ture of about 30-60°C. 6. A coating slip, c h a r a c t e r i s e d in that it comprises a modified starch according to claim 1. dodecyl ammonium chloride to a DS within the range stipu¬ lated according to the invention. Starch S-II was reacted with the same reagent, but to a lower DS than stipulated according to the invention. Starch S-I was boiled at 120°C, while Starches S-II and S-III were boiled at 140°C. Table 3 Desig¬ Starch quality Carboxyl Cationic nation groups, DS hydrophobic groups, DS S-I Conventional oxidised 0.03 0 S-II Modified with the same reagent as the invention 0.02 0.004 S-III Modified according to the invention 0.02 0.21 The jet-boiled starches were examined immediately after boiling without dilution as well as diluted to a 5% by weight solution. Without dilution, the starch con¬ centration was about 20%. Only starch S-III had a sudden phase separation as the temperature decreased below a certain so-called upper critical solution temperature (UCST) in the range 0-100°C. The phase separation observ¬ ed for starch S-III was reversible and took place at 50°C. This Example demonstrates that a UCST in the tempe¬ rature range 0-100°C does not appear in solutions of a conventional oxidised starch. This example also demon¬ strates that the existence of a UCST in the temperature range 0-100°C is not the general case for a starch that was allowed to react with a 3-chloro-2-hydroxypropyl dimethyl dodecyl ammonium chloride. The starch must have a DS within the limits stipulated according to the inven¬ tion in order to have a UCST in the temperature range 0-100°C.

Claims: INTERNATIONAL SEARCH REPORT Inte: ..dϋonal application No. PCT/SE 96/01278 C (Continuation). DOCUM ENTS CONSIDERED TO BE RELEVANT Category* Citation of document, with indication, where appropriate, of the relevant passages Relevant lo claim No. US 4373099 A (E. DANIEL HUBBARD ET AL), 1-6 8 February 1983 (08.02.83), column 1, line 10 - line 17; column 2, line 35 - line 54, claims 1,12,14,22 Form PCT/1SA/210 (continuation of second sheet) (July 1992)

Inventors: JAERNSTROEM LARS, WIKSTROEM OLLE, JAERNSTROEM, LARS, WIKSTROEM, OLLE
Applicants: JAERNSTROEM LARS, SVERIGES STAERKELSEPRODUCENTER, WIKSTROEM OLLE, JAERNSTROEM, LARS, SVERIGES STAERKELSEPRODUCENTER, FOERENING UPA, WIKSTROEM, OLLE
Application year: 1996
Main classification: C08B31
IPC codes: C08B31, D21H19