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• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61K—PREPARATIONS FOR MEDICAL, DENTAL OR TOILETRY PURPOSES
  • A61K31/00—Medicinal preparations containing organic active ingredients
  • A61K31/74—Synthetic polymeric materials
  • A61K31/765—Polymers containing oxygen
• • A—HUMAN NECESSITIES
  • A23—FOODS OR FOODSTUFFS; TREATMENT THEREOF, NOT COVERED BY OTHER CLASSES
  • A23L—FOODS, FOODSTUFFS OR NON-ALCOHOLIC BEVERAGES, NOT OTHERWISE PROVIDED FOR; PREPARATION OR TREATMENT THEREOF
  • A23L29/00—Foods or foodstuffs containing additives; Preparation or treatment thereof
  • A23L29/20—Foods or foodstuffs containing additives; Preparation or treatment thereof containing gelling or thickening agents
  • A23L29/206—Foods or foodstuffs containing additives; Preparation or treatment thereof containing gelling or thickening agents of vegetable origin
  • A23L29/262—Cellulose; Derivatives thereof, e.g. ethers
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61L—METHODS OR APPARATUS FOR STERILISING MATERIALS OR OBJECTS IN GENERAL; DISINFECTION, STERILISATION OR DEODORISATION OF AIR; CHEMICAL ASPECTS OF BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES; MATERIALS FOR BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES
  • A61L15/00—Chemical aspects of, or use of materials for, bandages, dressings or absorbent pads
  • A61L15/16—Bandages, dressings or absorbent pads for physiological fluids such as urine or blood, e.g. sanitary towels, tampons
  • A61L15/22—Bandages, dressings or absorbent pads for physiological fluids such as urine or blood, e.g. sanitary towels, tampons containing macromolecular materials
  • A61L15/225—Mixtures of macromolecular compounds
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61L—METHODS OR APPARATUS FOR STERILISING MATERIALS OR OBJECTS IN GENERAL; DISINFECTION, STERILISATION OR DEODORISATION OF AIR; CHEMICAL ASPECTS OF BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES; MATERIALS FOR BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES
  • A61L15/00—Chemical aspects of, or use of materials for, bandages, dressings or absorbent pads
  • A61L15/16—Bandages, dressings or absorbent pads for physiological fluids such as urine or blood, e.g. sanitary towels, tampons
  • A61L15/22—Bandages, dressings or absorbent pads for physiological fluids such as urine or blood, e.g. sanitary towels, tampons containing macromolecular materials
  • A61L15/28—Polysaccharides or their derivatives
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61L—METHODS OR APPARATUS FOR STERILISING MATERIALS OR OBJECTS IN GENERAL; DISINFECTION, STERILISATION OR DEODORISATION OF AIR; CHEMICAL ASPECTS OF BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES; MATERIALS FOR BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES
  • A61L15/00—Chemical aspects of, or use of materials for, bandages, dressings or absorbent pads
  • A61L15/16—Bandages, dressings or absorbent pads for physiological fluids such as urine or blood, e.g. sanitary towels, tampons
  • A61L15/42—Use of materials characterised by their function or physical properties
  • A61L15/60—Liquid-swellable gel-forming materials, e.g. super-absorbents
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61P—SPECIFIC THERAPEUTIC ACTIVITY OF CHEMICAL COMPOUNDS OR MEDICINAL PREPARATIONS
  • A61P3/00—Drugs for disorders of the metabolism
  • A61P3/04—Anorexiants; Antiobesity agents
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08B—POLYSACCHARIDES; DERIVATIVES THEREOF
  • C08B11/00—Preparation of cellulose ethers
  • C08B11/20—Post-etherification treatments of chemical or physical type, e.g. mixed etherification in two steps, including purification
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J3/00—Processes of treating or compounding macromolecular substances
  • C08J3/24—Crosslinking, e.g. vulcanising, of macromolecules
  • C08J3/246—Intercrosslinking of at least two polymers
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61K—PREPARATIONS FOR MEDICAL, DENTAL OR TOILETRY PURPOSES
  • A61K31/00—Medicinal preparations containing organic active ingredients
  • A61K31/70—Carbohydrates; Sugars; Derivatives thereof
  • A61K31/715—Polysaccharides, i.e. having more than five saccharide radicals attached to each other by glycosidic linkages; Derivatives thereof, e.g. ethers, esters
  • A61K31/716—Glucans
  • A61K31/717—Celluloses
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61K—PREPARATIONS FOR MEDICAL, DENTAL OR TOILETRY PURPOSES
  • A61K31/00—Medicinal preparations containing organic active ingredients
  • A61K31/70—Carbohydrates; Sugars; Derivatives thereof
  • A61K31/715—Polysaccharides, i.e. having more than five saccharide radicals attached to each other by glycosidic linkages; Derivatives thereof, e.g. ethers, esters
  • A61K31/738—Cross-linked polysaccharides
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61K—PREPARATIONS FOR MEDICAL, DENTAL OR TOILETRY PURPOSES
  • A61K47/00—Medicinal preparations characterised by the non-active ingredients used, e.g. carriers or inert additives; Targeting or modifying agents chemically bound to the active ingredient
  • A61K47/30—Macromolecular organic or inorganic compounds, e.g. inorganic polyphosphates
  • A61K47/36—Polysaccharides; Derivatives thereof, e.g. gums, starch, alginate, dextrin, hyaluronic acid, chitosan, inulin, agar or pectin
  • A61K47/38—Cellulose; Derivatives thereof
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61K—PREPARATIONS FOR MEDICAL, DENTAL OR TOILETRY PURPOSES
  • A61K9/00—Medicinal preparations characterised by special physical form
  • A61K9/06—Ointments; Bases therefor; Other semi-solid forms, e.g. creams, sticks, gels
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08B—POLYSACCHARIDES; DERIVATIVES THEREOF
  • C08B15/00—Preparation of other cellulose derivatives or modified cellulose, e.g. complexes
  • C08B15/005—Crosslinking of cellulose derivatives
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08B—POLYSACCHARIDES; DERIVATIVES THEREOF
  • C08B15/00—Preparation of other cellulose derivatives or modified cellulose, e.g. complexes
  • C08B15/02—Oxycellulose; Hydrocellulose; Cellulosehydrate, e.g. microcrystalline cellulose
  • C08B15/04—Carboxycellulose, e.g. prepared by oxidation with nitrogen dioxide
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J2301/00—Characterised by the use of cellulose, modified cellulose or cellulose derivatives
  • C08J2301/08—Cellulose derivatives
  • C08J2301/26—Cellulose ethers
  • C08J2301/28—Alkyl ethers
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J3/00—Processes of treating or compounding macromolecular substances
  • C08J3/24—Crosslinking, e.g. vulcanising, of macromolecules
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E50/00—Technologies for the production of fuel of non-fossil origin
  • Y02E50/30—Fuel from waste, e.g. synthetic alcohol or diesel
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02W—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
  • Y02W30/00—Technologies for solid waste management
  • Y02W30/40—Bio-organic fraction processing; Production of fertilisers from the organic fraction of waste or refuse

Definitions

• the present invention relates to superabsorbent polymer hydrogels and a method of preparing thereof.
• Polymer hydrogels are cross-linked hydrophilic polymers which are capable of absorbing high amounts of water.
• hydrogels capable of absorbing an amount of water in excess of 95% of their overall weight are defined as "superabsorbent". Some of these materials are even capable of absorbing over 1 litre of water per gram of dry polymer.
• crosslinks or cross-linking knots i.e. the physical or chemical bonds between the macromolecular chains forming the hydrogel network, guarantee the structural integrity of the polymer-liquid system, on the one hand preventing the complete solubilisation of the polymer, and on the other hand allowing the retention of the aqueous phase within the molecular mesh.
• the superabsorbent hydrogels which are currently available 4 on the market are characterised not only by their marked absorbent properties, but also by their biocompatibility, which is probably due to the high water content, and above all by the possibility of adjusting their absorption properties according to the external stimuli. Consequently, such hydrogels may be used as intelligent materials, for example for the manufacture of sensors or actuators for a number of industrial applications.
• absorbent cores in the field of personal hygiene absorbent products
• there are more recent and innovative applications such as for example in the biomedical field, for the development of controlled release drugs, artificial muscles, sensors, etc., and in agriculture and horticulture, for example in devices for the controlled release of water and nutrients in arid soils.
• biodegradable polymers used to obtain superabsorbent hydrogels are the starch and cellulose derivatives. *
• Anbergen and Oppermann proposed a method for the synthesis of a superabsorbent material made entirely from cellulose derivatives. Particularly, they used hydroxyethylcellulose (HEC) and a carboxymethylcellulose sodium salt (CMCNa), chemically crosslinked in a basic solution with divinylsulphone. However, the absorption properties of such materials are not high if compared to those of the acrylic-based superabsorbent materials.
• HEC hydroxyethylcellulose
• CMCNa carboxymethylcellulose sodium salt
• CMCNa may be chemically crosslinked with any reagent which is bifunctional with respect to cellulose.
• any reagent which is bifunctional with respect to cellulose.
• divinylsulphone used in the synthetic process according to Anbergen and Opperman, the use of epichlorhydrin, formaldehyde and various diepoxides is also known. However, such compounds are highly toxic in their unreacted states [3].
• Some carbodiimides are known amongst the unconventional crosslinking agents. Particularly, the use of carbodiimides in order to crosslink salified or non-salified carboxymethylcellulose (CMC) was described in [4].
• Carbodiimide induces the formation of ester bonds between cellulose macromolecules, without participating in the bonds themselves, but simply giving rise to an urea derivative having a very low toxicity [5].
• a ⁇ uperabsorbent hydrogel obtained cross-linking carboxymethylcellulose sodium salt and hydroxyethylcellulose with carbodiimide as the cross-linking agent is disclosed in the international patent application WO 2006/070337 [6].
• the object of the present invention is to provide superabsorbent polymer hydrogels which overcome the above-mentioned drawbacks which result from the use of carbodiimide as a cross-linking agent.
• the hydrogel of the invention is based on the replacement of carbodiimide with citric acid as the cross-linking agent, as well as on the use of a molecular spacer.
• Citric acid (3-carboxy-3-hydroxy-l,5-pentanedioic acid), hereinafter designated as CA, is a naturally occurring substance which is non-toxic and available on the market at low costs.
• CA is used in the prior art as a cross-linking agent for polymers such as cellulose, hydroxypropylmethylcellulose and starch, in textile and food applications [7-11].
• the possibility of using CA in order to cross-link carboxymethylcellulose for preparing superabsorbent hydrogels was never shown before.
• the method of preparing a superabsorbent hydrogel according to the present invention comprises the step of cross-linking an aqueous solution of a precursor comprising a carboxymethylcellulose salt, optionally in combination with a further and different cellulose derivative polymer, with citric acid as the cross-linking agent and in the presence of a molecular spacer.
• the carboxymethylcellulose salt is preferably carboxymethylcellulose (CMCNa) sodium salt.
• the further and different cellulose derivative polymer is preferably hydroxyethylcellulose (HEC).
• the total precursor concentration in the aqueous solution is of at least 2% by weight referred to the weight of the water of the starting aqueous solution, and the concentration of the cross-linking agent is comprised between 1% and 5% by weight referred to the weight of the precursor.
• the term "precursor” indicates the whole amount of the cellulose derivative polymers used as the precursors for the formation of the hydrogel polymer network (i.e., preferably, the sum of CMCNa and HEC).
• the swelling ratio is a measure of the hydrogel ability to absorb water. SR is obtained through swelling measurements at the equilibrium (using, for example, a Sartorius micro scale with a sensibility of 10 "5 ) and it is calculated with the following formula:
• the molecular spacer used to synthesise the superabsorbent hydrogel is selected from the group consisting of sorbitol, sucrose and plant glycerol.
• a molecular spacer is a substance which, although not taking part into the reaction resulting in the formation of the hydrogel polymer network, exerts the function of increasing the length of the cross-linking bridge, thereby enhancing the possibility of the polymer network to expand so as to greatly increase the hydrogel absorption properties. »
• sorbitol is used as the molecular spacer, at a concentration within the range of 0,5 to 10% by weight referred to the weight of water, preferably within the range of 2 to 8% by weight referred to the weight of water, still more preferably at a concentration of 4% by weight referred to the weight of water.
• the cross-linking reaction is preferably carried out at a temperature comprised between about 60°C and 120 0 C. Variating the temperature during this stage of the process will enable to increase or decrease the cross- linking degree of the polymer network. A cross-linking temperature of about 80 0 C is preferred.
• the precursor used to form the polymer network is a carboxymethylcellulose salt, preferably carboxymethylcellulose sodium salt (CMCNa), optionally in combination with a further and different cellulose derivative polymer, preferably hydroxyethylcellulose (HEC).
• CMCNa carboxymethylcellulose sodium salt
• HEC hydroxyethylcellulose
• the preferred weight ratio of CMCNa to HEC will be 3/1.
• Hydroxyethylcellulose is used in order to stabilise the polymer network and to improve its mechanical properties.
• Carboxymethylcellulose alone tends to create intramolecular rather than intermolecular cross-links, resulting in the creation of a gel with poor mechanical properties.
• the material obtained with the cross-linking reaction is a hydrogel. This material is then purified by washing in a polar organic solvent. The hydrogel immersed into the solvent swells up releasing everything that was not incorporated into the polymer network. Water is preferred as the organic solvent, distilled water is still more preferred. The volume of water required during this step in order to reach the maximum swelling degree of the gel, is approximately 10- to 20-folds the initial volume of the gel itself. Taking into account the huge amounts of water which would be involved during this step on an industrial scale, as well as their disposal and/or recycling, the importance of avoiding the presence of any toxic by-products in the synthetic process becomes evident. *
• the hydrogel washing step may be repeated more than once, optionally changing the organic solvent employed.
• the organic solvent employed for example, methanol followed by distilled water may be used as the organic solvent in the washing step.
• the subsequent step in the synthesis process is the hydrogel drying.
• the drying step is carried out by phase inversion in a cellulose nonsolvent.
• a suitable cellulose nonsolvent is for example acetone or ethyl alcohol. Drying by phase inversion results in a final micro porous structure which improves the absorption properties of the gel by capillarity. Moreover, if the porosity is interconnected or open, i.e. the micropores communicate with one another, the absorption/desorption kinetics of the gel will be improved as well.
• the gel undergoes phase inversion with the expulsion of water, until the gel precipitates in the form of a vitreous solid as white coloured granules.
• Various rinses in the nonsolvent may be necessary in order to obtain the dried gel in a short period of time.
• further air drying or oven drying may be useful in order to eliminate any residual traces of nonsolvent.
• the latter drying step may be carried out at a temperature of e.g. approximately 30-45°C until the residual nonsolvent is completely removed.
• hydrogels obtainable by the method of the invention possess a number of advantages over the polyacrylic hydrogels which have bee employed up to now in consumer products, such as for example in the filed of the absorbent products for personal care (i.e., babies' napkins, sanitary towels, etc.) and in the field of the products for agriculture (e.g., devices for the controlled release of water and nutrients).
• Such advantages are essentially associated with the biodegradability and the complete absence of any toxic by-products during the manufacturing process.
• Such features enable a real employment of the superabsorbent hydrogels of the invention in the biomedical and pharmaceutical fields as well.
• the absorption properties of these materials which depend on the amount of carboxymethylcellulose employed and which may be improved by the induction of a microporosity in the gel structure, are comparable to those of polyacrylic gels.
• the hydrogels obtainable by the method of the present invention therefore possess mechanical properties which make them suitable for use in all of the above-mentioned fields.
• the scope of the present invention also includes the use of the polymer hydrogels obtainable by the method of the invention as an absorbent material in products which are capable of absorbing water and/or aqueous solutions and/or which are capable of swelling r when brought into contact with water and/or an aqueous solution.
• the superabsorbent hydrogels of the present invention may be used as the absorbent materials in the following fields, which are provided by way of non-limiting example:
• - dietary supplements for example, as the bulking agents in dietary supplements for hypocaloric diets capable of conferring a sensation of lasting satiety being retained into the stomach for a limited period of time, or as water and low molecular weight compounds supplements, such as mineral salts or vitamins, to be included into drinks in a dry or swollen form;
• agricultural products for example, in devices for the controlled release of water and/or nutrients and/or phytochemicals, particularly for cultivation in arid, deserted areas and in all cases where it is not possible to carry out frequent irrigation; such products, mixed in a dry form with the soil in the areas surrounding the plant roots, absorb water during irrigation and are capable of retaining it, releasing it slowly together with the nutrients and phytochemicals useful for cultivation);
• toys and gadgets such as for example in products which are capable of significantly changing their size once brought into contact with water or an aqueous solution
• biomedical field for example, in biomedical and/or medical devices such as absorbent dressings for the treatment of highly exudative wounds, such as ulcers and/or burns, or in slow-release polymeric films suitable to slowly release liquids adapted for use in ophthalmology;
• the body fluid management field i.e., for controlling the amount of liquids into the organism, for example in products capable of promoting the elimination of fluids from the body e.g. in the case of oedema, CHF (chronic heart failure), dialysis,etc.
• the superabsorbent hydrogels were prepared cross-linking an aqueous solution of carboxymethylcellulose sodium salt (CMCNa) with hydroxyethylcellulose (HEC), using citric acid (CA) as the cross-linking agent and sorbitol as the molecular spacer.
• CMCNa carboxymethylcellulose sodium salt
• HEC hydroxyethylcellulose
• CA citric acid
• sorbitol sorbitol
• the precursor weight concentration (%) the total mass of polymers in the solution (e.g. CMCNa + HEC) (g) x 100/mass of water (g);
• CMCNa to HEC weight ratio mass of CMCNa (g) in the solution/mass of HEC in the solution (g);
• the cross-linking agent (CA) weight concentration (%) mass of CA in the solution (g) x 100/mass of the precursors in the solution (g);
• CMCNa is the polyelectrolyte species, it is possible to achieve the desired absorption properties adjusting the weight ratio of carboxymethylcellulose sodium salt (CMCNa) to hydroxyethylcellulose (HEC).
• CMCNa/HEC weight ratio of between 0/1 and 5/1, preferably between 1/1 and 3/1, was observed to enable in any event the synthesis of a hydrogel having optimum absorption properties.
• Preparation of gel A in a beaker containing distilled water, sorbitol at a concentration of 4% by weight referred to the weight of distilled water is added and mixed until complete solubilisation, which occurs within a few minutes.
• the CMCNa and HEC polymers are added at a total concentration of 2% by weight referred to the weight of distilled water, with a CMCNa/HEC weight ratio of 3/1. Mixing proceeds until sohibilisation of the whole quantity of polymer is achieved and the solution becomes clear.
• citric acid at a concentration of 1% by weight referred to the weight of the precursor is added to the solution, whose viscosity has greatly increased.
• the solution thereby obtained is poured into a vessel and dried at 48 0 C for 48 hours.
• the macromolecules are stabilised into a polymeric network which is the backbone of the hydrogel.
• the hydrogel is washed with distilled water for 24 hours at room temperature.
• the hydrogel swells up thereby eliminating the impurities, hi order to obtain the maximum swelling degree and elimination of all of the impurities, at least 3 rinses with distilled water are performed during the 24 hours washing step.
• the hydrogel is dried by phase inversion in acetone as the nonsolvent, until a glassy white precipitate is obtained. The precipitate is then placed into an oven at 45°C for about 3 hours, to remove any residual trace of acetone.
• Gel B was prepared as gel A, with the only exception that the polymer is made only of CMCNa, and that the CMCNa concentration is 2% by weight referred to the weight of distilled water.
• Gel C was prepared as gel B, with the only exception that the citric acid concentration is 2% by weight referred to the weight of CMCNa.
• Preparation of gel D Gel D was prepared as gel B, with the only exception that the citric acid concentration is 0.5% by weight referred to the weight of CMCNa.
• the absorption measurements essentially consist of placing the dry sample, obtained from the drying step, in distilled water, so that it swells up until an equilibrium condition is reached.
• the absorption properties of the gel are assessed based on its swelling ratio (SR), defined according to the formula illustrated above, hi order to minimise the influence of experimental errors, each test was performed on three samples from each' gel, and then the mean value of the results of the three measurements was taken as the effective value.
• SR swelling ratio
• Table 1 reports some of the results obtained, in terms of the swelling ratio, varying the concentrations of the reagents and the cross-linking times (6 hours, 13 hours, 18 hours, 24 hours).
• nr not cross-linked It is pointed out that the increase in the polymer concentration exerts a negative effect on the swelling properties of the final product and it is also pointed out that the cross-linking time exerts a significant effect of the absorbing properties.
• Table 2 shows that the sample having the best swelling ratio is the sample designated as g22, which is characterised by a citric acid (CA) concentration of 1%.
• CA citric acid
• nr not cross-linked
• the highest swelling ratio is associated with a cross-linking time of 13 hours and a citric acid concentration of 1%. It is also to be noticed that higher citric acid concentrations together with shorter cross-linking times lead to equally satisfactory swelling ratios, although the reaction is very fast and less easy to control.

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