FR2692605A1 - Liq. aq. dispersions contg. mineral filler and anionic co-polymer of acrylic acid - and acrylamide with low mol.wt., and use in paper industry - Google Patents

Abstract

A liq. aq. dispersion contains (a) a mineral filler and (b) an anionic dispersant comprising a copolymer of acrylic acid and acrylamide in ratio by wt. of 95:5-25:75, with a distribution of mol. wt., determined by analytical exclusion chromatography, such that at least 60 wt.% of the mol. wt. is below 500. The ratio by wt. of acrylic acid:acrylamide is 90:10-50:50. The dispersion contains 25-85 (55-65) wt.% of filler, pref. CaCO3, kaolin, gypsum or talc, and has pH 4-9. USE/ADVANTAGE - The dispersions are used in the paper industry. The dispersant has a dispersing power at least equal to that of the usual polyacrylate dispersions, with a much lower electrical charge, giving better affinity between filler and binder, redn. in migration of fillers or pigments during drying, better coverage and better appearance of the paper, and fewer problems in recycling waste from the paper machine. In an example, (1) 600 g of CaCO3 was added gradually to 400 g of water contg. 1 g of a copolymer of Na acrylate and acrylamide (75:25) (FR2604712, Example 1), followed by addn. of acetic acid to pH 7.5, giving a dispersion with Brookfield viscosity 96 mPa, and density of electrical charge 4.8 mmols (C+)/g/l. (2) A similar dispersion, prepd. with Na polyacrylate of mol. wt. 5000 had viscosity 150 mPa, and density of electrical charge 9.6 mmols (C+)/g/l. Paper sheets were made from a dispersion contg. 1000 g of water, 0.5 g of a cationic epoxidised polyaminoamide to improve the wet resistance of the paper, and 0.1 g of the copolymer from (1), or (2) 0.1 g of Na polyacrylate with mol. wt. 5000, or (3) no additive, followed by drying at 105 deg.C for 20 mins. The wet bursting strength of the paper, kg/cm2, was: (1) 2.35, (2) 1.9, (3) 2.5.

Description

 The present invention relates to the use of a particular dispersing agent for producing concentrated dispersions of mineral fillers, intended for the paper industry.

 The paper industry uses significant amounts of mineral fillers. Their use is essential to improve certain properties or characteristics of the papers produced, such as whiteness, opacity, smoothness, etc.

 Two types of application of mineral fillers can be involved, either the use of mineral fillers in formulations intended for coating paper, or the use of these fillers in the wet process used to produce the sheet.

 Depending on the application conditions and the results sought, the nature of the mineral fillers can be varied. However, fillers such as calcium carbonate, kaolin, talc or gypsum are used more frequently.

 Concentrated dispersions of fillers are generally prepared before using them in one or other of the abovementioned paper operations. It is therefore desirable to have fluid dispersions, as concentrated as possible in solid materials, in order to be transportable economically to the various factories.

 Frequently used to produce these concentrated dispersions of mineral fillers, dispersing agents of highly anionic polarity and having a very high dispersing power. The most frequent of these agents are in particular polyacrylates and / or phosphonates.

 Effective in terms of dispersing power, the use of strongly anionic products has however a certain number of drawbacks, drawbacks precisely linked to their charge. One of the main drawbacks is, for example, a lower affinity of the charge vis-à-vis the binding agent used to fix the charge on the cellulosic part, and above all the incidence of strongly anionic products in the recovery. and the recycling on the paper machine of waste or non-conforming articles designated more frequently in terms of trade by the name of "broken broken".

 The introduction of this waste into the wet part of the paper machine, which often contains cationic agents, can indeed cause parasitic effects due to the anionicity of the recycled products, such effects having as a consequence a modification of the retention of fillers, or even flocculation of the products or a reduction in the binding of the fibrous material.

 It has been observed unexpectedly that the use of acrylic acid derivatives having a relatively lower polarity than that of the products currently used, makes it possible to produce concentrated dispersions of mineral charges while at the same time making it possible to avoid the abovementioned drawbacks due to too strong anionicity of the agents.

 It has been found, according to the invention, that the use, as a charge dispersing agent, of the copolymers described in French patent 2 604 712, the teachings of which are incorporated here by reference, makes it possible to obtain a very high dispersing power. on the various mineral fillers without presenting, or to a lesser extent, the various known drawbacks inherent in the use of strongly anionic products.

 The present invention therefore relates to an aqueous and fluid dispersion, of at least one mineral filler containing an anionic dispersing agent, characterized in that the dispersing agent is an anionic copolymer comprising units derived from acrylic acid and l acrylamide in a weight ratio ranging from approximately 95: 5 to 25:75, preferably from 90:10 to 50:50, the distribution of the molecular weights of the copolymer, as determined by analytical exclusion chromatography, being such that At least 60% of the molecular weights by weight are less than 500.

The use of the abovementioned copolymers has been found to have a number of important advantages
- a very high dispersing power, equal to or greater than that of the polyacrylates currently used to carry out the dispersion of mineral fillers;
- an electric charge density very much lower than that of the polyacrylates currently used for the same dispersing efficiency, this property having the consequence of providing
an improvement in the affinity between filler and bonding agent leading to being able to increase the charge rate fixed by the bonding agent,
a reduction in the migration of fillers or pigments during drying,
an improvement in the appearance of the papers produced due to better coverage of the load; and
a lower incidence or parasitic effect in the case of recycling of the broken ones lying on the paper machine

 The proportion of the mineral filler in the dispersion of the invention can vary, depending on the intended application and the nature of the filler, from 25 to 85% by weight, preferably from 55 to 65% by weight.

 The filler can be any filler used in the paper industry, the most common being calcium carbonate, kaolin, talc, and gypsum.

 The proportion of anionic copolymer incorporated in the dispersion of the invention can range from 0.01 to 1%, preferably from 0.08 to 0.15%.

 Preferably, according to the invention, the dispersion of the invention is carried out by adding the mineral filler to an aqueous medium added with the dispersing agent, the pH of which is between 4 and 9.

 The following nonlimiting examples are given for the purpose of illustrating the present invention.

Example 1
A concentrated dispersion is prepared containing 60% by weight of calcium carbonate by gradually introducing with stirring 600g of calcium carbonate into 400g of water supplemented with 1g of the copolymer of Example 1 of French patent 2 604 712, derived from 75 parts sodium acrylate and 25 parts of acrylamide. The pH is adjusted to 7.5 using acetic acid.

 After adding all of the calcium carbonate, a fluid dispersion is obtained, the viscosity of which, determined using a Brookfield viscometer (Mobile N "2; Speed 100 rpm), is 96 mPa.

 Comparatively, a dispersion produced under the same conditions with a sodium polyacrylate having an average molecular weight of 5000 gives a viscosity of 150 mPa.

Example 2
A concentrated dispersion is prepared containing 65% by weight of kaolin by gradually introducing 650g of kaolin into 350g of water maintained with stirring and added with 1 gram of the copolymer of Example 2 of French patent 2 604 712, derived from 50 parts of sodium acrylate and 50 parts of acrylamide. The pH is adjusted to 3 using acetic acid.

 A fluid dispersion of kaolin is obtained, the viscosity of which, determined under the conditions of Example 1, is 20,000 mPa.

 Comparatively, a dispersion produced under the same conditions with a sodium polyacrylate with an average molecular weight of 5000 gives a viscosity of 16,000 mPa.

Example 3
The following dispersions are prepared in order to produce a sheet of paper.

A / 1000g of water
3g of cellulosic fibers used for
paper making
0.5 g of a cationic agent consisting of an epoxidized polyaminoamide used to improve the resistance to the wet state of the paper.

 B / A part of the dispersion A is taken up into which 0.1 g per liter of the copolymer used in Example 1 is introduced.

 C / A part of the dispersion A is taken up into which 0.1 g / l of a sodium polyacrylate of an average molecular weight of 5000 is introduced.

 A sheet of paper is then produced on a form with the previous dispersions.

 The sheets obtained are dried for 20 minutes at 105 ° C. and then the wet strength of the papers produced is determined using an eclatometer.

The following results are obtained
A. 2,5 Kg / cm2
B. 2.35 Kg / cm2
C. 1.9 Kg / cm2
This example very clearly illustrates the disruptive effect of a strongly anionic product on the cationic products frequently used in the wet process.

 The observation of the three dispersions above, before the sheet is produced, also reveals a rapid destabilization or flocculation of dispersion C.

A determination of the electric charge density carried out on solutions (a) containing lg / l of the copolymer used in Example 1 and (b) lg / l of sodium polyacrylate with an average molecular weight of 5000, respectively gives the following values
at. 4.8 mmol (C +) / g / l
b. 9.6 mmol (C +) / g / l
It goes without saying that the embodiments described are only examples and they could be modified, in particular by substitution of technical equivalents, without departing from the scope of the invention.

Claims (6)

 1. An aqueous and fluid dispersion of at least one mineral filler containing an anionic dispersing agent, characterized in that the dispersing agent is an anionic copolymer comprising units derived from acrylic acid and acrylamide in a ratio weight ranging from about 95: 5 to 25:75, the distribution of the molecular weights of the copolymer, as determined by analytical exclusion chromatography, being such that at least 60% of the molecular weights by weight are less than 500.  2. A dispersion according to claim 1, characterized in that said weight ratio ranges from 90:10 to 50:50.  3. A dispersion according to claim 1 or 2, characterized in that the mineral filler comprises at least one of the following fillers: calcium carbonate, kaolin, gypsum, and talc.  4. A dispersion according to any one of claims 1 to 3, characterized in that the dispersion contains 25 to 85% by weight of filler.  5. A dispersion according to claim 4, characterized in that the dispersion contains 55 to 65% by weight of filler.  6. A dispersion according to any one of claims 1 to 5, characterized in that it has a pH of 4 to 9.