Thickener, also known as gelling agent, is also called paste or food glue when used in food. Its main function is to increase the viscosity of the material system, keep the material system in a uniform and stable suspension state or emulsified state, or form a gel. Thickeners can quickly increase the viscosity of the product when used. Most of the mechanism of action of thickeners is to use macromolecular chain structure extension to achieve thickening purposes or to form micelles and water to form a three-dimensional network structure to thicken. It has the characteristics of less dosage, fast aging and good stability, and is widely used in food, coatings, adhesives, cosmetics, detergents, printing and dyeing, oil exploration, rubber, medicine and other fields. The earliest thickener was water-soluble natural rubber, but its application was limited due to its high price due to its large dosage and low output. The second-generation thickener is also called emulsification thickener, especially after the emergence of oil-water emulsification thickener, it has been widely used in some industrial fields. However, emulsifying thickeners need to use a large amount of kerosene, which not only pollutes the environment, but also poses safety hazards in production and application. Based on these problems, synthetic thickeners have come out, especially the preparation and application of synthetic thickeners formed by copolymerization of water-soluble monomers such as acrylic acid and an appropriate amount of cross-linking monomers have been rapidly developed.
Types of thickeners and thickening mechanism
There are many types of thickeners, which can be divided into inorganic and organic polymers, and organic polymers can be divided into natural polymers and synthetic polymers.
Most of the natural polymer thickeners are polysaccharides, which have a long history of use and many varieties, mainly including cellulose ether, gum arabic, carob gum, guar gum, xanthan gum, chitosan, alginic acid Sodium and starch and its denatured products, etc. Sodium carboxymethyl cellulose (CMC), ethyl cellulose (EC), hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), methyl hydroxyethyl cellulose (MHEC) in cellulose ether products ) and methyl hydroxypropyl cellulose (MHPC) are known as industrial monosodium glutamate, and have been widely used in oil drilling, construction, coatings, food, medicine and daily chemicals. This kind of thickener is mainly made of natural polymer cellulose through chemical action. Zhu Ganghui believes that sodium carboxymethyl cellulose (CMC) and hydroxyethyl cellulose (HEC) are the most widely used products in cellulose ether products. They are the hydroxyl and etherification groups of the anhydroglucose unit on the cellulose chain. (Chloroacetic acid or ethylene oxide) reaction. Cellulosic thickeners are thickened by hydration and expansion of long chains. The thickening mechanism is as follows: the main chain of cellulose molecules associates with surrounding water molecules through hydrogen bonds, which increases the fluid volume of the polymer itself, thereby increasing the volume of the polymer itself. system viscosity. Its aqueous solution is a non-Newtonian fluid, and its viscosity changes with shear rate and has nothing to do with time. The viscosity of the solution increases rapidly with the increase of concentration, and it is one of the most widely used thickeners and rheological additives.
Cationic guar gum is a natural copolymer extracted from leguminous plants, which has the properties of cationic surfactant and polymer resin. Its appearance is light yellow powder, odorless or slightly scented. It is composed of 80% polysaccharide D2 mannose and D2 galactose with 2∀1 high molecular polymer composition. Its 1% aqueous solution has a viscosity of 4000~5000mPas. Xanthan gum, also known as xanthan gum, is an anionic polymer polysaccharide polymer produced by fermentation of starch. It is soluble in cold water or hot water, but insoluble in general organic solvents. The characteristic of xanthan gum is that it can maintain a uniform viscosity at a temperature of 0~100, and it still has a high viscosity at a low concentration, and has good thermal stability. ), it still has excellent solubility and stability, and can be compatible with high-concentration salts in the solution, and can produce a significant synergistic effect when used with polyacrylic acid thickeners. Chitin is a natural product, a glucosamine polymer, and a cationic thickener.
Sodium alginate (C6H7O8Na)n is mainly composed of the sodium salt of alginic acid, which is composed of aL mannuronic acid (M unit) and bD guluronic acid (G unit) connected by 1,4 glycosidic bonds and composed of different GGGMMM fragments of copolymers. Sodium alginate is the most commonly used thickener for textile reactive dye printing. The printed textiles have bright patterns, clear lines, high color yield, uniform color yield, good permeability and plasticity. It has been widely used in the printing of cotton, wool, silk, nylon and other fabrics.
synthetic polymer thickener
1. Chemical cross-linking synthetic polymer thickener
Synthetic thickeners are currently the most sold and widest range of products on the market. Most of these thickeners are microchemical cross-linked polymers, insoluble in water, and can only absorb water to swell to thicken. Polyacrylic acid thickener is a widely used synthetic thickener, and its synthesis methods include emulsion polymerization, inverse emulsion polymerization and precipitation polymerization. This type of thickener has been developed rapidly due to its rapid thickening effect, low cost and less dosage. At present, this type of thickener is polymerized by three or more monomers, and the main monomer is generally a water-soluble monomer, such as acrylic acid, maleic acid or maleic anhydride, methacrylic acid, acrylamide and 2 acrylamide. 2-methyl propane sulfonate, etc.; the second monomer is generally acrylate or styrene; the third monomer is a monomer with cross-linking effect, such as N, N methylenebisacrylamide, butylene diacrylate ester or dipropylene phthalate, etc.
The thickening mechanism of polyacrylic acid thickener has two kinds: neutralization thickening and hydrogen bonding thickening. Neutralization and thickening is to neutralize the acidic polyacrylic acid thickener with alkali to ionize its molecules and generate negative charges along the main chain of the polymer, relying on the repulsion between the same-sex charges to promote the molecular chain stretching Open to form a network structure to achieve thickening effect. Hydrogen bonding thickening is that polyacrylic acid molecules combine with water to form hydration molecules, and then combine with hydroxyl donors such as non-ionic surfactants with 5 or more ethoxy groups. Through the same-sex electrostatic repulsion of carboxylate ions, the molecular chain is formed. The helical extension becomes rod-like, so that the curled molecular chains are untied in the aqueous system to form a network structure to achieve a thickening effect. Different polymerization pH value, neutralizing agent and molecular weight have great influence on the thickening effect of the thickening system. In addition, inorganic electrolytes can significantly affect the thickening efficiency of this type of thickener, monovalent ions can only reduce the thickening efficiency of the system, divalent or trivalent ions can not only thin the system, but also produce insoluble precipitate. Therefore, the electrolyte resistance of polycarboxylate thickeners is very poor, which makes it impossible to apply in fields such as oil exploitation.
In the industries where thickeners are most widely used, such as textiles, petroleum exploration and cosmetics, the performance requirements of thickeners such as electrolyte resistance and thickening efficiency are very high. The thickener prepared by solution polymerization usually has a relatively low molecular weight, which makes the thickening efficiency low and cannot meet the requirements of some industrial processes. High molecular weight thickeners can be obtained by emulsion polymerization, inverse emulsion polymerization and other polymerization methods. Due to the poor electrolyte resistance of the sodium salt of the carboxyl group, adding non-ionic or cationic monomers and monomers with strong electrolyte resistance (such as monomers containing sulfonic acid groups) to the polymer component can greatly improve the viscosity of the thickener. Electrolyte resistance makes it meet the requirements in industrial fields such as tertiary oil recovery. Since inverse emulsion polymerization started in 1962, the polymerization of high molecular weight polyacrylic acid and polyacrylamide has been dominated by inverse emulsion polymerization. Invented the method of emulsion copolymerization of nitrogen-containing and polyoxyethylene or its alternating copolymerization with polyoxypropylene polymerized surfactant, cross-linking agent and acrylic acid monomer to prepare polyacrylic acid emulsion as a thickener, and achieved Good thickening effect, and has good anti-electrolyte performance. Arianna Benetti et al. used the method of inverse emulsion polymerization to copolymerize acrylic acid, monomers containing sulfonic acid groups and cationic monomers to invent a thickener for cosmetics. Due to the introduction of sulfonic acid groups and quaternary ammonium salts with strong anti-electrolyte ability into the thickener structure, the prepared polymer has excellent thickening and anti-electrolyte properties. Martial Pabon et al. used inverse emulsion polymerization to copolymerize sodium acrylate, acrylamide and isooctylphenol polyoxyethylene methacrylate macromonomers to prepare a hydrophobic association water-soluble thickener. Charles A. etc. used acrylic acid and acrylamide as comonomers to obtain a high molecular weight thickener by inverse emulsion polymerization. Zhao Junzi and others used solution polymerization and inverse emulsion polymerization to synthesize hydrophobic association polyacrylate thickeners, and compared the polymerization process and product performance. The results show that, compared with the solution polymerization and inverse emulsion polymerization of acrylic acid and stearyl acrylate, the hydrophobic association monomer synthesized from acrylic acid and fatty alcohol polyoxyethylene ether can be effectively improved by inverse emulsion polymerization and acrylic acid copolymerization. Electrolyte resistance of thickeners. He Ping discussed several issues related to the preparation of polyacrylic acid thickener by inverse emulsion polymerization. In this paper, the amphoteric copolymer was used as a stabilizer and methylenebisacrylamide was used as a crosslinking agent to initiate ammonium acrylate for inverse emulsion polymerization to prepare a high-performance thickener for pigment printing. The effects of different stabilizers, initiators, comonomers and chain transfer agents on the polymerization were studied. It is pointed out that the copolymer of lauryl methacrylate and acrylic acid can be used as a stabilizer, and the two redox initiators, benzoyldimethylaniline peroxide and sodium tert-butyl hydroperoxide metabisulfite, can both initiate polymerization and obtain a certain viscosity. white pulp. And it is believed that the salt resistance of ammonium acrylate copolymerized with less than 15% acrylamide increases.
2. Hydrophobic association synthetic polymer thickener
Although chemically cross-linked polyacrylic acid thickeners have been widely used, although the addition of monomers containing sulfonic acid groups to the thickener composition can improve its anti-electrolyte performance, there are still many thickeners of this type. Defects, such as poor thixotropy of the thickening system, etc. The improved method is to introduce a small amount of hydrophobic groups into its hydrophilic main chain to synthesize hydrophobic associative thickeners. Hydrophobic associative thickeners are newly developed thickeners in recent years. There are hydrophilic parts and lipophilic groups in the molecular structure, showing a certain surface activity. Associative thickeners have better salt resistance than non-associative thickeners. This is because the association of hydrophobic groups partly counteracts the curling tendency caused by the ion-shielding effect, or the steric barrier caused by the longer side chain partly weakens the ion-shielding effect. The association effect helps to improve the rheology of the thickener, which plays a huge role in the actual application process. In addition to the hydrophobic associative thickeners with some structures reported in the literature, Tian Dating et al. also reported that hexadecyl methacrylate, a hydrophobic monomer containing long chains, was copolymerized with acrylic acid to prepare associative thickeners composed of binary copolymers. Synthetic thickener. Studies have shown that a certain amount of cross-linking monomers and hydrophobic long-chain monomers can significantly increase the viscosity. The effect of hexadecyl methacrylate (HM) in the hydrophobic monomer is greater than that of lauryl methacrylate (LM). The performance of associative crosslinked thickeners containing hydrophobic long-chain monomers is better than that of non-associative crosslinked thickeners. On this basis, the research group also synthesized an associative thickener containing acrylic acid/acrylamide/hexadecyl methacrylate terpolymer by inverse emulsion polymerization. The results proved that both the hydrophobic association of cetyl methacrylate and the non-ionic effect of propionamide can improve the thickening performance of the thickener.
Hydrophobic association polyurethane thickener (HEUR) has also been greatly developed in recent years. Its advantages are not easy to hydrolyze, stable viscosity and excellent construction performance in a wide range of applications such as pH value and temperature. The thickening mechanism of polyurethane thickeners is mainly due to its special three-block polymer structure in the form of lipophilic-hydrophilic-lipophilic, so that the chain ends are lipophilic groups (usually aliphatic hydrocarbon groups), and the middle is Water-soluble hydrophilic segment (usually higher molecular weight polyethylene glycol). The effect of hydrophobic end group size on the thickening effect of HEUR was studied. Using different test methods, polyethylene glycol with a molecular weight of 4000 was capped with octanol, dodecyl alcohol and octadecyl alcohol, and compared with each hydrophobic group. Micelle size formed by HEUR in aqueous solution. The results showed that the short hydrophobic chains were not enough for HEUR to form hydrophobic micelles and the thickening effect was not good. At the same time, comparing stearyl alcohol and lauryl alcohol-terminated polyethylene glycol, the size of micelles of the former is significantly larger than that of the latter, and it is concluded that the long hydrophobic chain segment has a better thickening effect.
Main application areas
Printing and Dyeing Textile
The good printing effect and quality of textile and pigment printing depend largely on the performance of printing paste, and the addition of thickener plays a vital role in its performance. Adding a thickener can make the printed product have high color yield, clear printing outline, bright and full color, and improve the permeability and thixotropy of the product. In the past, natural starch or sodium alginate was mostly used as a thickener for printing pastes. Due to the difficulty in making paste from natural starch and the high price of sodium alginate, it is gradually replaced by acrylic printing and dyeing thickeners. Anionic polyacrylic acid has the best thickening effect and is currently the most widely used thickener, but this kind of thickener still has defects, such as electrolyte resistance, color paste thixotropy, and color yield during printing. The average is not ideal. The improved method is to introduce a small amount of hydrophobic groups into its hydrophilic main chain to synthesize associative thickeners. At present, printing thickeners in the domestic market can be divided into natural thickeners, emulsification thickeners and synthetic thickeners according to different raw materials and preparation methods. Most, because its solid content can be higher than 50%, the thickening effect is very good.
water-based paint
Appropriately adding thickeners to the paint can effectively change the fluid characteristics of the paint system and make it thixotropic, thus endowing the paint with good storage stability and workability. A thickener with excellent performance can increase the viscosity of the coating during storage, inhibit the separation of the coating, and reduce the viscosity during high-speed coating, increase the viscosity of the coating film after coating, and prevent the occurrence of sagging. Traditional paint thickeners often use water-soluble polymers, such as high-molecular hydroxyethyl cellulose. In addition, polymeric thickeners can also be used to control moisture retention during the coating process of paper products. The presence of thickeners can make the surface of coated paper smoother and more uniform. Especially the swellable emulsion (HASE) thickener has anti-splash performance and can be used in combination with other types of thickeners to greatly reduce the surface roughness of the coated paper. For example, latex paint often encounters the problem of water separation during production, transportation, storage, and construction. Although water separation can be delayed by increasing the viscosity and dispersibility of latex paint, such adjustments are often limited, and the more important Or through the choice of thickener and its matching to solve this problem.
oil extraction
In oil extraction, in order to obtain high yield, the conductivity of a certain liquid (such as hydraulic power, etc.) is used to fracture the fluid layer. The liquid is called fracturing fluid or fracturing fluid. The purpose of fracturing is to form fractures with a certain size and conductivity in the formation, and its success is closely related to the performance of the fracturing fluid used. Fracturing fluids include water-based fracturing fluids, oil-based fracturing fluids, alcohol-based fracturing fluids, emulsified fracturing fluids, and foam fracturing fluids. Among them, water-based fracturing fluid has the advantages of low cost and high safety, and is currently the most widely used. Thickener is the main additive in water-based fracturing fluid, and its development has gone through nearly half a century, but obtaining a fracturing fluid thickener with better performance has always been the research direction of scholars at home and abroad. There are many kinds of water-based fracturing fluid polymer thickeners currently used, which can be divided into two categories: natural polysaccharides and their derivatives and synthetic polymers. With the continuous development of oil extraction technology and the increase of mining difficulty, people put forward newer and higher requirements for fracturing fluid. Because they are more adaptable to complex formation environments than natural polysaccharides, synthetic polymer thickeners will play a greater role in high-temperature deep well fracturing.
Daily Chemicals and Food
At present, there are more than 200 kinds of thickeners used in the daily chemical industry, mainly including inorganic salts, surfactants, water-soluble polymers and fatty alcohols/fatty acids. They are mostly used in detergents, cosmetics, toothpaste and other products. In addition, thickeners are also widely used in the food industry. They are mainly used to improve and stabilize the physical properties or forms of food, increase the viscosity of food, give food a sticky and delicious taste, and play a role in thickening, stabilizing and homogenizing. , emulsifying gel, masking, flavoring and sweetening. Thickeners used in the food industry include natural thickeners obtained from animals and plants, as well as synthetic thickeners such as CMCNa and propylene glycol alginate. In addition, thickeners have also been widely used in medicine, papermaking, ceramics, leather processing, electroplating, etc.
2.Inorganic thickener
Inorganic thickeners include two classes of low molecular weight and high molecular weight, and low molecular weight thickeners are mainly aqueous solutions of inorganic salts and surfactants. The inorganic salts currently used mainly include sodium chloride, potassium chloride, ammonium chloride, sodium sulfate, sodium phosphate and pentasodium triphosphate, among which sodium chloride and ammonium chloride have better thickening effects. The basic principle is that surfactants form micelles in aqueous solution, and the presence of electrolytes increases the number of micelles associations, resulting in the transformation of spherical micelles into rod-shaped micelles, increasing the movement resistance, and thus increasing the viscosity of the system. However, when the electrolyte is excessive, it will affect the micellar structure, reduce the movement resistance, and thus reduce the viscosity of the system, which is the so-called salting-out effect.
Inorganic high molecular weight thickeners include bentonite, attapulgite, aluminum silicate, sepiolite, hectorite, etc. Among them, bentonite has the most commercial value. The main thickening mechanism is composed of thixotropic gel minerals that swell by absorbing water. These minerals generally have a layered structure or an expanded lattice structure. When dispersed in water, the metal ions in it diffuse from the lamellar crystals, swell with the progress of hydration, and finally separate completely from the lamellar crystals to form a colloidal suspension. liquid. At this time, the surface of the lamellar crystal has a negative charge, and its corners have a small amount of positive charge due to the appearance of lattice fracture surfaces. In a dilute solution, the negative charges on the surface are larger than the positive charges on the corners, and the particles repel each other without thickening. However, with the increase of the electrolyte concentration, the charge on the surface of the lamellae decreases, and the interaction between particles changes from the repulsive force between the lamellae to the attractive force between the negative charges on the surface of the lamellae and the positive charges at the edge corners. Vertically cross-linked together to form a house of cards structure, causing swelling to produce a gel to achieve a thickening effect. At this time, the inorganic gel dissolves in water to form a highly thixotropic gel. In addition, bentonite can form hydrogen bonds in solution, which is beneficial to the formation of a three-dimensional network structure. The process of inorganic gel hydration thickening and card house formation is shown in schematic diagram 1. Intercalation of polymerized monomers to montmorillonite to increase the interlayer spacing, and then in-situ intercalation polymerization between the layers can produce a polymer/montmorillonite organic- Inorganic hybrid thickener. Polymer chains can pass through montmorillonite sheets to form a polymer network. For the first time, Kazutoshi et al. used sodium-based montmorillonite as a cross-linking agent to introduce a polymer system, and prepared a montmorillonite cross-linked temperature-sensitive hydrogel. Liu Hongyu et al. used sodium-based montmorillonite as a cross-linking agent to synthesize a new type of thickener with high anti-electrolyte performance, and tested the thickening performance and anti-NaCl and other electrolyte performance of the composite thickener. The results show that the Na-montmorillonite-crosslinked thickener has excellent anti-electrolyte properties. In addition, there are also inorganic and other organic compound thickeners, such as the synthetic thickener prepared by M.Chtourou and other organic derivatives of ammonium salts and Tunisian clay belonging to montmorillonite, which has a good thickening effect.
Post time: Jan-11-2023