Use of a mixture of starch and polyvinylalcohol

Abstract: 50 to 95 parts by weight starch and 5 to 50 parts by weight polyvinyl alcohol with a degree of saponification between 87 and 100% are thoroughly mixed and furthermore the moisture content of the mixture is regulated between 10 and 25 weight %. Subsequently the mixture obtained is subjected to a thermo-mechanical treatment at a temperature situated between 110 and 180°C. During the extrusion the mixture is subjected to shearing forces so that the product obtained when it is dispersed in cold water and subsequently is centrifuged, divides into a watery phase and a coacervate phase without a sediment of undissolved polyvinyl alcohol being formed.

Description: The invention relates to a use of a mixture of starch and polyvinyl alcohol in coating compositions for the coating of paper. Said mixtures are applied in paper coating compositions as they exert, in the presence of optical whiteners, an advantageous influence on the whitness of the paper. However the suitable polyvinyl alcohols are badly soluble in cold water and in the known methods. A polyvinyl alcohol dispersion is first prepared which is then heated in order to dissolve the polyvinyl alcohol. The solution of polyvinyl alcohol is then added to the slurry of water and pigment and optical whiteners. The previous solution of the polyvinyl alcohol by cooking means adding additional water so that the dry substance content of the coating composition is lowered. In the above mentioned use, it would be very advantageous that the polyvinyl alcohol would be very quickly soluble in cold water whereby then the polyvinyl alcohol could also be added directly in dry form to the pigment slurry. On the one hand therefore the coating installation would be simplified because no solution and heating installation for the starch and the polyvinyl alcohol are necessary. On the other hand a coating agent with a higher dry material content would be obtained through which during the drying of the coated paper less energy needs to be employed in order to evaporate the superfluous moisture and/or through which the speed of the coater can be increased. The purpose of the invention is to provide a use of a mixture of starch and polyvinyl alcohol as a binder in a coating composition for the coating of paper, whereby a mixture is obtained that is soluble in cold water. For this purpose a mixture of starch and polyvinyl alcohol, manufactured by: mixing of 50 to 95 parts by weight starch and 5 to 50 parts by weight polyvinyl alcohol having a degree of saponification between 87 and 100%, regulating the moisture content of the mixture between 10 and 25 wt % with demineralized water and optionally adding acid so that the pH of a 10% suspension of the mixture is situated between 2 and 2.5, subsequently extruding the mixture at-a temperature situated between 110 and 180°C, and finally grinding the extrudate; whereby the ground extrudate is added to an aqueous pigment slurry or coating premix, is Used as a binder in coating compositions for the coating of paper. The addition of a modified starch in dry form to a pigment slurry, with the intension of raising the dry material content of the coating composition is already described in US-A-4.425.452. This method offers a advantage that during the drying of the coated paper less moisture needs to be evaporated, which results in a more economical procedure. According to this oatent no use is made however of polyvinyl alcohol. The extrusion of mixtures of starches and polyvinyl alcohol for manufacturing shaped products is known as such from BE-A-653.335 and GB-B-2.190.093. According to BE-A-653.335, the amount of starch is however much smaller than the amount of polyvinyl alcohol and large quantities of water are added before extrusion. According to GB-B-2.190.093 texturing agent and a lubricant/release agent and/or melt-flow accelerator are added to the mixture. In both cases a shaped product is made and no cold water dispersable powder. No reference is made to coating compositions. DE-A-1.494.148 discloses the manufacturing of a cold water soluble foil comprising cold water soluble starch and a special cold water soluble polyvinyl alcohol. No extrusion is performed. GB-A-881.073 discloses a method for producing an aqueous solution of polyvinyl alcohol by extruding a mixture of water and polyvinyl alcohol. No starch is used. US-A-3 767 604 discloses a dry mixture of polyvinyl alcohol and starch for use in a sizing composition for paper, which is different from a coating composition. Moreover, in order to have sufficient polyvinyl alcohol in the mixture a special modified starch and a polyvinyl alcohol with a degree of polymerization ranging from about 300 to 1400 and a degree of hydrolyses greater than 95% are used. Heating is necessary to dissolve the polyvinyl alcohol. No extrusion is performed. In a notable embodiment of the invention the moisture content is regulated between 12 and 20 weight % preferably between 14 and 18 wt % during the mixing or the starch and the polyvinyl alcohol. The starch employed can be a natural or modified starch. The modified starch can be etherified or esterified and/or oxidatively, by acid or enzymatically broken down. A granular etherified starch is preferably employed. Other details and advantages on the invention will appear from the following description. For the use according to the invention a mixture of starch and polyvinyl alcohol is prepared by thoroughly mixing, 50 to 95 and preferably 85 to 90 parts by weight starch and 5 to 50 and preferably 10 to 15 parts by weight polyvinyl alcohol. The polyvinyl alcohol has a minimum degree of saponification of 87%. This degree of saponification can go to 100% and particularly for use in paper coating strongly saponified polyvinyl alcohol is preferably employed. The starch can be natural or modified. The modified starch can be etherified or esterified and/or be oxidatively, by acid or enzymatically broken down. A granular etherified starch is preferably employed. The moisture content of the mixture is regulated between 10 and 20 weight %, preferably between 12 and 20 weight % and best between 14 and 18 weight %. The mixture obtained is subjected to a thermo-mechanical treatment at a temperature situated between 110 and 180°C. During this treatment the mixture is subjected to a shearing force so that the product obtained, when it is dispersed in cold water and subsequently centrifuged, divides into a watery phase and a coacervate phase without a third phase consisting of a sediment of undissolved polyvinyl alcohol being formed. A similar phase would be formed without or without sufficient thermo-mechanical treatment. A suitable thermo-mechanical treatment is a continuous extrusion. This extrusion can be performed in an extrusion apparatus that contains a cylinder with one or more rotating screws which give a buildup of pressure in the mixture, whereby the mixture present melts through the internal heat which is released and/or through externally supplied heat. The melted mixture is then transported by means of the screw or the screws through the pressure cylinder toward the extremity of the extrusion apparatus, whereby the mixture is subjected to the shearing forces. At the extremity of the pressure chamber there are one or more openings through which the mixture is pressed outward. A suitable extrusion apparatus is for example an apparatus found on the market of the type BUSS-PR 46. Such an extruder has a capacity of 18 kg of product per hour and has a length diameter ratio of 7. The extruder contains two screws with a diameter of 46 mm, namely a dosing screw which rotates at 13 revolutions per minute and an extrusion screw which rotates at 97 revolutions per minute. The time the mixture stays there amounts to 10 to 20 seconds at a temperature of between 130 and 150°C and a pressure of between 5 and 10 kg per cm². Other extruders, for example of the type CREUSOT-LOIRE can likewise be employed subject to the adjustment of the reaction conditions to the characteristics of the apparatus in order to obtain the desired shearing forces. To a mixture an acid can possibly be added in order further to break down the starch. The added amount of acid is preferably selected so that the pH of a 10% suspension of the mixture is stirred between 2 and 2.5. It is surprisingly determined that with use of the extruded mixtures the polyvinyl alcohol present dissolves very well and very quickly in cold water too. In this way there is a clear difference to be observed in the dissolving speed between polyvinyl alcohol with a low degree of saponification, which is also considered as soluble in cold water, and extruded mixtures of starch and such a polyvinyl alcohol. The difference in dissolving speed is still much more clearly to be observed when strongly or completely saponified or hydrolized polyvinyl alcohol is compared with an extruded polyvinyl alcohol on the basis of starch and such a polyvinyl alcohol. The extruded mixtures obtained are used as a binder in coating compositions for the coating of paper. These mixtures can be added in dry form to the pigment slurry, whereby the moisture content of the mixture decreases, which of course is advantageous. The dry material content of the thus prepared coating composition can moreover be increased with 2 to 4 weight %, which results in a decrease of 7 to 14 weight % of the moisture that needs to be evaporated afterward. Furthermore it was surprisingly also determined that the application of the extruded mixtures according to the invention as binder in coating compositions gave a significantly greater whiteness to the treated paper than the corresponding non-extruded mixtures. These non-extruded mixtures nave to be boiled up beforehand. It is clear that there is question here of a synergic effect. The invention will be further explained hereafter with the help of some examples. Examples 1 through 5 25 kg of etherified starch, brought onto the market by the depositor under the name of AMYLUM A2, are placed in a mixer and stirred for 5 minutes. After that the moisture content and pH are determined. To the starch that has a moisture content of 13.2%, is then added an amount of completely hydrolized (98%) polyvinyl alcohol with low viscosity, obtainable on the market under the name of AIRVOL 107 (r) of Air Products in the ratio according to the following table 1. The whole is further homogenized and is subsequently mixed with the necessary amount of demineralized water in order to obtain the desired moisture content according to table 1. Finally the homogenized mixture is measured into doses by means of a dosing screw in an extrusion apparatus of the type CREUSOT-LOIRE EXTRUDER. The mixture is extruded under the extrusion conditions according to the following table 1. In this H.S. indicates a screw with kneading elements and L.S. a standard screw. After extrusion the mixture obtained is finely ground. example 1 2 3 4 5 Composition mixture starch 90 90 50 50 90 PVA 10 10 50 50 10 moisture 20 12 20 12 20 extrusion conditions T°C 110 180 110 180 110 rotation speed screw (rpm) 100 150 150 100 100 configuration H.S. L.S. H.S. L.S. L.S. Examples 6 through 8 The procedure according to examples 1 through 5 is repeated but with the composition of the mixtures and the extrusion conditions as in the following table 2, whereby moreover together with the demineralized water for the adjustment of the moisture content also 4 weight % concentrated sulfuric acid, calculated on the amount of starch, is added. After the extrusion sodium bicarbonate is added to the extruded mixture in order to neutralize the acid present. Example 8 is only taken for comparison since there is no PVA present and therefore falls outside of the invention. Example 6 7 8 Composition mixture starch 90 90 100 PVA 10 10 - moisture 12 15 15 extrusion conditions T°C 110 130 130 rotation speed screw (rpm) 150 100 100 screw configuration H.S. L.S. L.S. (H.S. is a screw with kneading elements and L.S. a standard screw). Examples 9 and 10 The procedure according to examples 6 through 8 is repeated but with the composition of the mixtures and the extrusion conditions as in the following table 3 and furthermore with another type of polyvinyl alcohol, namely an only partially hydrolized (88-90%) polyvinyl alcohol with low viscosity that is obtainable on the market under the name of AIRVOL 203® of Air Products. Example 9 10 Composition mixture starch 50 90 PVA 50 10 moisture 16 16 extrusion conditions T°C 130 130 rotation speed screw (rpm) 100 100 screw configuration L.S. L.S. (standard screw) Results The extruded mixtures obtained according to the examples described above were dispersed in cold water, in a concentration of 5 weight % and stirred for 3 minutes. The dispersed mixtures were subsequently centrifuged for 10 minutes at 3000 revolution per minute. In table 4 shown hereafter the viscosities of the dispersions and the centrifuge results are shown. The distribution of the various phases are shown in percentages. Example Viscosity in mP Watery phase % Gel % Sediment % 1 96 52 48 - 2 92 59 41 - 3 38 79 21 - 4 42 88 12 - 5 82 50 50 - 6 30 95 5 - 7 20 99 1 - 8 20 99 1 - 9 32 98 2 - 10 26 99 1 - An additional test was performed on the layer of viscous mixtures in order to verify to what extent the polyvinyl alcohol present was dissolved. During this the sample material was dispersed in cold water under violent stirring. The concentration of the dispersions amounted to 28% dry material. The dispersions obtained were then filtered through a 53 micron sieve. The residue on the sieve was then dried and weighed. The residue was expressed as a percentage of the amount of dry material used. The residual polyvinyl alcohol content was then determined from the difference with a reference sample, which contained no polyvinyl alcohol. The results are shown in the following table 5. residue residual polyvinyl alcohol 6 0.7% 2.7% 7 0.8% 3.9% 10 0.56% 1.3% reference 0.43% - The residual polyvinyl alcohol was expressed as a percentage of the polyvinyl alcohol present, which is not dissolved. From table 5 it clearly appears that the polyvinyl alcohol present with the examples according to the invention is practically completely dissolved. In the case that partially hydrolized polyvinyl alcohol was employed it appears that here also the solubility is greatly improved. Applications. "Coating" compositions were produced by means of products according to the invention and compared with known compositions. Moreover the dry material and the viscosity of the "coating" mixture, as well as the dry material after dilution of the pigment slurry were determined at 1000 mP. After treatment of the paper with this composition the whiteness was measured with the assistance of a Hunterlab apparatus. The products of the invention were added to the "coating-premix" in dry form. In the other cases the binder or co-binder had to be dissolved beforehand with exception of the extruded starch from example 8. In the following examples the following precoating composition was employed: pigment : 80 parts calcium carbonate (Hydrocarb 90)® 20 parts China clay (Amazon 88)® binder : 8 parts styrene butadiene latex (DL 685)® additive : 1 part Urecoll® 1 part calcium stearate 1.5 parts optical whitener (Blancophor P) This slurry has a dry material content of 72.0%. The added materials and the results obtained are shown in the following table 6: Composition Binder Parts %d.m. viscosity %d.m. whiteness. I latex 2 69.0 2450 65.0 90.9 CMC 1 II latex 2 67.4 920 67.4 90.8 AIRVOL 107® 1 III from ex.8 4 72.5 3000 66.3 89.7 IV from ex.7 4 72.5 1840 69.3 92.2 V from ex.10 4 72.5 2560 68.2 91.4 From the above table it therefore clearly appears that in the case of the compositions IV and V "coating" mixtures are obtained with a distinctly higher dry material, when compared with the compositions I and II. In addition to this the greater whiteness for paper "coated" with this composition is striking; a gain of 0.5 to 1% in whiteness is considered as a great advantage in the paper industry.

Claims: Use as a binder in coating compositions for the coating of paper, of a mixture of starch and polyvinyl alcohol, manufactured by: mixing of 50 to 95 parts by weight starch and 5 to 50 parts by weight polyvinyl alcohol having a degree of saponification between 87 and 100%, regulating the moisture content of the mixture between 10 and 25 wt % with demineralized water and optionally adding acid so that the pH of a 10% suspension of the mixture is situated between 2 and 2.5, subsequently extruding the mixture at a temperature situated between 110 and 180°C, and finally grinding the extrudate; whereby the ground extrudate is added to an aqueous coating premix comprising cold water and at least pigment. Use according to claim 1, characterized in that the moisture content is regulated between 12 and 20 wt% during the mixing of the starch and the polyvinyl alcohol. Use according to claim 2, characterized in that the moisture content is regulated between 14 and 18 wt% during the mixing of the starch and the polyvinyl alcohol. Use according to either one of the claims 1 to 3, characterized in that a mixture is manufactured of 85 to 90 parts by weight starch and 10 to 15 parts by weight polyvinyl alcohol. Use according to either one of the claims 1 to 4, characterized in that as starch a granular etherified starch is employed.

Inventors: NACHTERGAELE WILLY A M, VAN NUFFEL JAN H, NACHTERGAELE, WILLY A.M., VAN NUFFEL, JAN H.
Applicants: AMYLUM NV, AMYLUM, N.V.
Application year: 1989
Main classification: C08L3
IPC codes: C08J3, C08L29, C08L3, D21H19, D21H17