SUPERABSORBIENT POLYMERS THAT UNDERSTAND COVALENT LINKS BETWEEN POLYMERIC CHAIN SEGMENTS. AND THE METHODS FOR THE ELABORATION OF THEMSELVESFIELD OF THE INVENTIONThe present invention relates to superabsorbent polymers comprising polymeric chain segments, which are directly linked together by covalent bonds. Moreover, the invention relates to processes for making these superabsorbent polymer particles and also relates to absorbent articles containing these superabsorbent polymers.
BACKGROUND OF THE INVENTIONThe superabsorbent polymers (SAPs) are well known in the industry. These are usually applied in absorbent articles, as in the case of diapers, training pants, adult incontinence products and feminine care products to increase the absorption capacity of these products, while reducing their total volume. . The SAPs are capable, in general terms, of absorbing and retaining quantity of aqueous fluids equivalent many times to their own weight. Commercial production of superabsorbent polymers began in Japan in 1978. The first superabsorbent was a cross-linked starch polyacrylate. The partially crosslinked polyacrylic acid replaced the first superabsorbents in the commercial production of the SAPs, and is the main polymer used for the current SAP. These generally consist of a network of slightly cross-linked and partially neutralized polymers, which are hydrophilic and allow swelling of the network once it has been immersed in water or in an aqueous solution, such as, for example, physiological saline. The crosslinks between the polymer chains ensure that the superabsorbent polymer does not dissolve in water. SAPs are often applied in the form of small particles such as fibers or granules. After absorption of an aqueous solution, the swollen SAP particles become very soft and deform easily. When deformed, the voids between the superabsorbent polymer particles can be blocked, which drastically increases the resistance to flow of the liquids. This is usually referred to as"gel block". In gel blocking situations, the liquid can travel through the swollen SAP particles only by diffusion, which is much slower than the flow in the gap between the SAP particles. One form that is frequently applied to reduce gel blockage is to make the particles more rigid, which allows the superabsorbent polymer particles to retain their original shape, thus creating or maintaining the empty spaces between the particles. A well-known method for increasing stiffness is to cross-link the carboxyl groups exposed on the surface of the SAP particles. This method is commonly known as surface crosslinking. European patent EP 0 509 708 B1 refers to particles of absorbent resin coated with a surfactant and cross-linked on its surface and to a method for its preparation. The surface crosslinking agent in the European patent EP 0 509708 B1 is a polyhydroxyl compound consisting of at least two hydroxyl groups, which react with the carboxyl groups on the surface of the superabsorbent polymer particles. In European patent EP 0 509 708 B1, surface crosslinking is carried out at temperatures of 150 ° C or higher. These particles are preferably exposed to elevated temperatures for at least 5 minutes, but for less than 60 minutes. U.S. Pat. No. 5,164,459 describes another method for surface cross-linking absorbent resins, wherein the carboxyl groups of the polymer that are surface-compounded by the resin react with the polyhydric alcohol. The reaction is carried out at temperatures in the range of 90 ° C to 250 ° C. In WO 01/89591 A2, the hydroxyalkylurea is used as the crosslinking agent. WO 01/89592 applies to the hydroxyalkylamide as the crosslinking agent. In both applications, the surface crosslinking reaction is carried out at temperatures of about 90 ° C to 170 ° C for 60 to 180 minutes. A water soluble peroxide radical initiator, such as the crosslinking agent, is known from European patent application EP 0 248 437 A2. An aqueous solution containing the surface crosslinking agent is applied on the surface of the polymer. The surface crosslinking reaction is achieved by heating to a certain temperature, so that the peroxide radical initiator decomposes as long as the polymer does not. European patent application EP 1 199 327 A2 describes the use of an oxetane compound and / or an imidazolidinone compound to be used as a surface crosslinking agent. The surface crosslinking reaction is carried out with heat, wherein the temperature is preferably in the range of 60 ° C to 250 ° C. Alternatively, in the European patent EP 1 199 327 A2 the surface crosslinking reaction is achieved by a photo-irradiation treatment, preferably using ultraviolet rays. All processes known from the prior art result in crosslinked surface particles, wherein the reaction products of the surface crosslinking molecules are incorporated in the superabsorbent polymer particles. Therefore, the cross-linked surface superabsorbent polymer particles contain the reaction product of the crosslinking molecules. In general, the surface crosslinking agent is applied on the surface of the SAP particles. Therefore, the reaction occurs on the surface of the superabsorbent polymer particles, which results in improved crosslinking on the surface of the particles while essentially not affecting the core of the particles. Accordingly, the SAP particles become rigid and the gel block is reduced. A disadvantage of the commercial surface crosslinking process described above is that it takes a relatively long time, often at least 30 minutes. However, the more time is required for the surface crosslinking process, the more surface crosslinking agent will penetrate the superabsorbent polymer particles, producing greater crosslinking within the particles, which has a negative impact on the ability of the particles of superabsorbent polymer. For this reason, it is convenient to have short process times for surface crosslinking. In addition, short processing times are also convenient with respect to a global process of economic processing of the SAP particle. Another disadvantage of common surface crosslinking processes is that they occur only at relatively high temperatures, often around150 ° C or higher. At these temperatures, not only the surface crosslinking agent reacts with the carboxyl groups of the polymer, but other reactions are also activated, for example the formation of anhydride of neighboring carboxyl groups within or between the polymer chains, and the dimeric cleavage of the d acrylic acid incorporated into the particles of superabsorbent polymer. These secondary reactions affect the nucleus, decreasing the capacity of the SAP particles. In addition, exposure to elevated temperatures can lead to color degradation of the superabsorbent polymer particles. Therefore, in general terms, these secondary reactions are undesirable. The superabsorbent polymers known in the industry are typically partially neutralized, for example with sodium hydroxide. However, in processes known in the industry, a careful balance must be made between the neutralization and the need for surface crosslinking: The surface crosslinking agents known in the industry react only with the free carboxyl groups comprised by the polymer chains, but they are not able to react with neutralized carboxyl groups. Thus, carboxyl groups can be applied for surface crosslinking or for neutralization, but the same carboxyl group can not be applied to fulfill both tasks. The surface crosslinking agents known in the industry do not react with chemical groups other than carboxyl groups, for example they do not react with aliphatic groups. In the process of making the SAP particles, the neutralization of the free carboxyl groups is generally carried out before the surface crosslinking takes place. In fact, often the neutralization step is carried out at the very beginning of the process, before the monomers undergo polymerization and cross-linking to form the SAP. Such a process is called "prior neutralization process". Alternatively, the SAP can be neutralized at half polymerization or after polymerization ("post-neutralization"). Moreover, a combination of these alternatives is also possible. Since the total number of carboxyl groups on the outer surface of the SAP particles is limited by said neutralization, it is very difficult to obtain particles with a high degree of surface crosslinking and, consequently, a high stiffness to reduce gel block. . Even more, it is very difficult to obtain particles ofSAP with uniformly distributed surface crosslinking, since the remaining free carboxyl groups are not only few in number, but are also generally distributed randomly, which sometimes results in SAP particles with more surface crosslinking regions well dense and regions of poor surface crosslinking. Therefore, it is an object of the present invention to provide SAP particles that have a high degree of surface crosslinking and at the same time allow a high degree of neutralization. It is another object of the present invention to provide SAP particles with uniformly distributed homogeneous surface crosslinking. Moreover, the surface comprising the surface crosslinking should be as thin as possible. In addition, another additional object of the present invention is to provide superabsorbent polymers and superabsorbent polymer particles; wherein the polymer chain segments contained in these polymers as well as in their particles are crosslinked with each other without the need for a crosslinking molecule to be incorporated in the superabsorbent polymers. This objective is the most desired with respect to surface crosslinking, ie, it is desired to provide cross-linked surface superabsorbent polymer particles, which do not contain the reaction product of the crosslinking molecules. furtherIt is an object of the present invention to provide a process for producing superabsorbent polymers and superabsorbent polymer particles with the aforementioned advantages. It is still another object of the present invention to provide a process for producing SAP particles, wherein the step of cross-linking the process surface can be carried out in a rapid manner to increase the efficiency thereof. In addition, another objective of the present invention is to provide a process for producing superabsorbent polymer particles, which can be carried out at moderate temperatures for the purpose of reducing unwanted side reactions, initiated by elevated temperatures, such as anhydride formation and cleavage. dimerBRIEF DESCRIPTION OF THE INVENTIONThe present invention relates to superabsorbent polymers containing polymer chain segments. At the same time, the polymer chain elements are crosslinked with each other through covalent bonds, where covalent bonds are formed directly between the polymer chain segments. The present invention also relates to a method for crosslinking superabsorbent polymers; the method comprises the steps of: a) Providing a superabsorbent polymer comprising polymeric chain segments, and b) providing a monofunctional radiation activatable radical former, and c) exposing the superabsorbent polymer and the radical former operable by monofunctional radiation to electromagnetic radiation. , thus forming direct covalent bonds between the polymer chain segments. Moreover, the present invention further relates to another method for making superabsorbent polymers and comprising the steps of: a) Providing a superabsorbent polymer comprising polymeric chain segments, and b) exposing the superabsorbent polymer to electromagnetic irradiation, preferably with beam of electrons, thus forming direct covalent bonds between the polymer chain segments. In addition, the present invention relates to absorbent articles containing superabsorbent polymer particles comprising direct covalent bonds between polymeric chain segments.
BRIEF DESCRIPTION OF THE FIGURESAlthough the specification concludes with the claims that particularly state and clearly claim the invention, it is believed that the present invention will be better understood from the following drawings when considered together with the accompanying description, in which like components are designated with the same reference number. Figure 1 shows the 300 MHz 1 H-NMR spectrum of dibenzoyl peroxide (BP) and benzoic acid (BA). Figure 2 shows the 1H-NMR spectrum of the ether extract before photolysis, after the photolysis for 1 min. and after the 10 min photolysis.
DETAILED DESCRIPTION OF THE INVENTIONThe superabsorbent polymers are available in a variety of chemical forms, including substituted and unsubstituted natural and synthetic polymers, such as carboxymethylstarch, carboxymethylcellulose, and hydroxypropylcellulose; nonionic types such as polyvinyl alcohol, and polyvinyl ethers; cationic types such as polyvinyl pyridine, polyvinyl morpholinone, and N, N-dimethylaminoethyl or N, N-diethylaminopropyl acrylates and methacrylates, and their respective quaternary salts. In general, the SAPs useful for the present invention have a multiplicity of anionic functional groups, such as, for example, sulfonic acid and more specifically carboxyl groups. Examples of polymers suitable for use in the present invention include those that are prepared from monomers containing polymerizable and unsaturated acid. Thus, these monomers include the anhydrides and olefinically unsaturated acids containing at least one carbon-to-carbon olefinic double bond. More specifically, these monomers can be selected from olefinically unsaturated carboxylic acids and acid anhydrides, olefinically unsaturated sulfonic acids and mixtures thereof. In the preparation of the SAPs, some non-acidic monomers, usually in minor amounts, may be included. These non-acidic monomers can include, for example, water-soluble or water-dispersible esters of the acid-containing monomers, as well as monomers that definitely do not contain sulfonic or carboxylic acid groups. The optional non-acidic monomers / may thus include monomers containing the following types of functional groups: esters of the carboxylic acid or sulfonic acid, hydroxyl groups, amide groups, amino groups, nitrile groups, saline groups of quaternary ammonium, aryl groups (for example, phenyl groups, such as derivatives of styrene monomers). These n-acid monomers are well known materials and are described in more detail, for example, in U.S. Pat. 4,076,663 and 4,062,817. The anhydride monomers of the carboxylic acid and the olefinically unsaturated carboxylic acid include the acrylic acids typified by the same acrylic acid, methacrylic acid, ethacrylic acid, α-chloroacrylic acid, α-cyanoacrylic acid, β-methacrylic acid (crotonic acid), a- phenylacrylic acid, β-acryloxypropionic acid, sorbic acid, chlorosorbic acid, angelic acid, cinnamic acid, p-chlorocinamic acid, β-sterilacrylic acid, itaconic acid, citroconic acid, mesaconic acid, glutaconic acid, aconitic acid, maleic acid, fumaric acid, maleic anhydride and tricarboxyethylene The olefinically unsaturated sulfonic acid monomers include aliphatic or aromatic vinyl sulphonic acids, such as vinylsulfonic acid, allylsulfonic acid, vinyltoluenesulfonic acid and styrenesulfonic acid; acrylic and methacrylic sulfonic acid, such as sulfoethyl acrylate, sulphoethyl methacrylate, sulfopropyl acrylate, sulfopropyl methacrylate, 2-hydroxy-3-methacryloxypropyl sulfonic acid and 2-acrylamide-2-methylpropane sulfonic acid. The superabsorbent polymers useful for the present invention contain carboxyl groups. These polymers consist of hydrolyzed starch-acrylonitrile graft copolymers, partially neutralized hydrolyzed starch-acrylonitrile grafted copolymers, grafted copolymers of starch-acrylic acid, partially neutralized grafted starch-acrylic acid copolymers, saponified acrylic ester vinyl acetate-copolymers , hydrolyzed acrylonitrile or acrylonitrile copolymers, lightly cross-linked polymers of any of the above copolymers, partially neutralized polyacrylic acid and slightly network crosslinked polymers of partially neutralized polyacrylic acid, partially neutralized polymethacrylic acid, and slightly network cross-linked polymethacrylic acid polymers partially neutralized. These polymers can be used alone or in the form of a mixture of two or more different polymers, which when used as mixtures, individually do not have to be partially neutralized, while the resulting copolymer does. Examples of these polymeric materials are described in U.S. Pat. num. 3,661, 875; 4,076,663; 4,093,776;4,666,983; and 4,734,478. The superabsorbent polymers useful for the present invention preferably comprise a partially neutralized α, β-unsaturated carboxylic acid homopolymer or a partially neutralized β, unsaturated carboxylic acid copolymer copolymerized with a monomer which can be copolymerized therewith. One suitable method for polymerizing the monomers is aqueous solution polymerization, which is well known in the art. An aqueous solution comprising monomers and a polymerization initiator is subjected to a polymerization reaction. The aqueous solution may contain, for example, monomers of α, β-unsaturated carboxylic acid, or may, as another alternative, contain monomers of α, β-unsaturated carboxylic acid and additional monomers, which are copolymerizable with the carboxylic acid monomers a , ß-unsaturated. At least the α, β-unsaturated carboxylic acid must be partially neutralized, either before, during or after the polymerization of the monomers. If the α, β-unsaturated carboxylic acid has been partially neutralized prior to polymerization, the monomers (including the α, β-unsaturated carboxylic acid monomers and the possible comonomers) are neutralized, more or less preferably, less 50%, at least 70%, at least 75% and between 75% and 95%. The monomers in aqueous solution are polymerized by standard techniques of free radicals, which usually use a photoinitiator for activation, such as ultraviolet (UV) light. Alternatively, an oxide-reduction initiator can be used. However, in this case increased temperatures are necessary. The polymer chains will preferably be slightly crosslinked so that they become insoluble in water. The desired cross-linked structure can be obtained by copolymerization of the selected water-soluble monomer and a cross-linking agent having at least two polymerizable double bonds in the molecular unit. The crosslinking agent is present in an amount effective to crosslink the water soluble polymer. The desirable amount of the crosslinking agent is determined by the desired degree of absorption capacity and the desired strength to retain the absorbed fluid, i.e., the desired absorption under pressure. As a rule, the crosslinking agent is used in amounts ranging from 0.0005 to 5 parts by weight per 100 parts by weight of the monomers (including possible monomers and comonomers of α, β-unsaturated carboxylic acid) used. If an amount greater than 5 parts by weight of the crosslinking agent is used per 100 parts, the resulting polymer has a fairly high crosslink density and exhibits a reduced absorption capacity and an increased force to retain the absorbed fluid. If the crosslinking agent is used in an amount less than 0.0005 parts by weight per 100 parts, the polymer has a fairly low crosslink density and when it makes contact with the fluid to be absorbed they become rather sticky, soluble in water. water and exhibits low absorption performance, particularly under pressure. The crosslinking agent will generally be soluble in the aqueous solution. Alternatively to the copolymerization of the crosslinking agent with the monomers, it is also possible to crosslink the polymer chains in a separate process step after the polymerization.
After polymerization, cross-linking and partial neutralization, the viscous superabsorbent polymers are dehydrated (i.e., dried) to obtain dried superabsorbent polymers. The dehydration step can be performed by heating the viscous SAPs to a temperature of about 120 ° C for about 1 or 2 hours in a forced air oven or by heating the viscous SAPs overnight at a temperature of about 60 ° C. The residual water content in the dehydrated superabsorbent polymer after drying depends predominantly on the time and the drying temperature and may vary between 0.5% and 50% by weight of dry superabsorbent polymer. Preferably, the residual water content in the superabsorbent polymer dehydrated after drying is 0.5%- 45% by weight of the dry superabsorbent polymer, more preferably 0.5% -30%, even more preferably 0.5% -15% and more preferably 0.5% -5%. The superabsorbent polymers can be transferred to particles in many ways. The term "particles" refers to granules, fibers, flakes, spheres, powders, platelets and other shapes and forms known to persons with experience in the SAP industry. For example, the particles may be in the form of granules or beads, having a particle size of about 10 to 1000 μm, preferably about 100 to 1000 μm. In another embodiment, the superabsorbent polymers may be in the form of fibers, ie acrylated elongated superabsorbent polymer particles. In these embodiments, the superabsorbent polymer fibers have a smaller dimension (ie, fiber diameter) of less than about 1 mm, usually less than about 500 μm, and preferably less than 250 μm to 50 μm. The length of the fibers is preferably from 3 mm to 100 mm, approximately. The fibers can also be in the form of a long filament that can be woven.
The present invention relates to superabsorbent polymers containing polymeric chain segments, wherein at least a portion of the polymer chain segments are crosslinked together through covalent bonds formed directly between the polymer chain segments. A "direct covalent bond", according to the present invention, is a covalent bond characterized in that the polymer chains are linked together only by a covalent bond without intermediate atoms, such as the atoms included in a crosslinking molecule. In contrast, the known reactions of crosslinking between polymer chains always produce covalent bonds between these polymer chains, wherein the reaction product of the crosslinking molecule is incorporated between the polymer chains. In this way, the known crosslinking reactions do not produce a direct covalent bond, but an indirect covalent bond containing the reaction product of the crosslinking molecule. The direct covalent bond is formed between a carbon atom of the main chain of a first polymer chain and a carbon atom of the main chain of a second polymer chain. The bonds are formed intraparticulate within the superabsorbent polymer, more specifically, they are formed on the surface of the superabsorbent polymer particles, while the core of the superabsorbent polymer particles is substantially free of said direct covalent bonds. The method for making superabsorbent polymers of this type can be applied to polymer chains, which have not cross-linked with each other. Therefore, the polymer chains are provided as a plurality of polymer chains, wherein the polymer chains can be, at least partially, branched. As another alternative, the method can be applied to polymer chains, which have already been crosslinked by a crosslinker known in the art, comprising at least two polymerizable double bonds in the molecule unit. For example, the method can be applied to polymer chains contained in superabsorbent polymer particles, such as surface crosslinking. However, the direct covalent bonds between polymer chain segments according to the present invention are not intended to bind together different superabsorbent polymer particles. Thus, the method of the present invention, when applied to superabsorbent polymer particles, does not lead to any appreciable interparticulate direct covalent bond between the different superabsorbent polymer particles, but only produces direct intraparticulate covalent bonds within an absorbent polymer particle. When they are present, direct interparticulate covalent bonds would therefore require additional interparticulate crosslinking materials, such as crosslinking molecules. For applications where the polymer chains have already been crosslinked and are thus provided in the form of a network, the term "polymer chain segment" refers to the part of the polymer chains between two neighborhoods, to existing crosslinks or to the of the polymer chains between sites, where the polymer chain branches. However, if the polymer chains have not been crosslinked before subjecting them to the crosslinking process of the present invention, the term "polymer chain segments" refers to a complete individual polymer chain. In a preferred embodiment of the present invention, the polymer chain segment contains polycarboxylic acid units. In accordance with the present invention, the term "polycarboxylic acid unit" refers to a unit consisting of at least two monomeric carboxylic acid units, which have polymerized with each other and which are part of a larger polymer. The term "monomeric carboxylic acid units" refers to the reaction product of the carboxylic acid monomer after the polymerization reaction and refers, therefore, to the carboxylic acid monomer incorporated in the polymer. In a preferred embodiment of the present invention, the polycarboxylic acid units consist of polyacrylic acid units or polymethacrylic acid units. A polyacrylic acid unit consists of at least two monomer units of acrylic acid, which have been polymerized with each other. A polymethacrylic acid unit consists of at least two monomeric units of methacrylic acid, which have been polymerized with each other. As another alternative, the carboxylic acid unit can also consist of monomeric acrylic acid units and methacrylic monomeric units which have been copolymerized. In accordance with the present invention, the polycarboxylic acid units are, at least partially, neutralized, that is, at least a part of the carboxylic acid units is neutralized. In addition to the polycarboxylic acid units, the polymer chain segments may also consist of other units, such as polystyrene units. In accordance with the present invention, the term "polystyrene unit" refers to a unit consisting of at least two monomeric units of styrene, which have polymerized with each other and which are part of a larger polymer. The term "monomeric units of styrene" refers to the reaction product of the styrene monomer after the polymerization reaction and therefore refers to the styrene monomer incorporated in the polymer. The polymer chain segment containing, for example, polycarboxylic acid units in combination with other polymeric units, such as polystyrene units, is referred to as the "block polymer chain segment".
The most preferred polymers for use in the present invention are lightly crosslinked polymers of partially neutralized polyacrylic acids, lightly crosslinked polymers of partially neutralized polymethacrylic acids, their copolymers and starch derivatives thereof. Most preferably, the superabsorbent polymers are composed of slightly networked and partially neutralized crosslinked polyacrylic acid (ie, poly (sodium acrylate / acrylic acid)). Preferably, the SAPs are in order of least to highest preference saturated at least 50%, at least 70%, at least 75% and at least 75% to 95% neutralized. The crosslinking of the network makes the polymer essentially insoluble and determines, in part, the absorbent capacity of the absorbent polymers forming hydrogels.
The processes for network networking these polymers and common network crosslinking agents are described in detail in U.S. Pat. no. 4,076,663. In the most preferred embodiment of the present invention, the method for bonding directly to each other polymeric chain segments by means of a covalent bond is applied to superabsorbent polymer particles of surface crosslinking instead of conventional surface crosslinking or in addition thereto. In accordance with the present invention, direct covalent bonds of this type can be introduced by two different methods: One method applies electromagnetic irradiation together with radical forming molecules and the second method applies electromagnetic radiation only without the need for any additional chemical, such as radical-forming molecules, to initiate cross-linking. If only electromagnetic radiation is used to generate direct covalent bonds, such as electromagnetic irradiation, it has to be much more powerful than in the case where more radical-forming molecules are used. This, in turn, can lead to the degradation of the main chain polymer chain in the superabsorbent polymer (chain cleavage). Therefore, if only electromagnetic irradiation is used, new covalent bonds of superabsorbent polymer are generated and existing covalent bonds are broken. Therefore, electron beam dosing (depending for example on electron beam intensity and exposure time) has to be chosen in such a way that this ratio is in favor of creating more links than breaking those existing during the irradiation.a) Use of radical-forming molecules (hereinafter referred to as radical formers) together with electromagnetic irradiation: It has been found that a radical former that contains a group that can be activated by radiation, when activated by electromagnetic radiation, can distill out a hydrogen radical from a polymer chain segment. Two of these radicals induced in the polymer chain segments can be combined to form a direct covalent bond between polymer chain segments. In accordance with the present invention, a radical former that contains only one radiation-activatable group is monofunctional. Preferred monofunctional radical formers according to the present invention include: dialkyl peroxydicarbonates, benzyl ketals, di-tert-butyl peroxide, dibenzoyl peroxide, bis- (aroyloxy) peroxides, such as bis- (4-methoxy) dibenzoyl peroxide or bis- (4-methyl) dibenzoyl peroxide or bis- (4-chloro) dibenzoyl peroxide, 2,4,6-trimethyl dibenzoyl peroxide, 3-benzoyl benzoic acid, 1,3-diibenzoyl propane, trifluoromethyl phenylketone, acetophenone, benzophenone, terephthalophenone, fluoronone, xanthone, thio-xanthone, anthraquinone, benzyl, a-ketocoumarins, camphorquinone, a-alkoxy-peroxybenzoins, a, dialkyloxysoxybenzoins, a, a-dialcoxyacetophenones, a, a-hydroxyalkylphenones, O-acyl a-oximinoketones, dibenzoyl disulfide, S-phenyl thiobenzoates, acylphosphine oxides, benzoylphosphinoxides, aryl-aryl sulfides, dibenzoylmethanes, phenylazo-diphenyl sulfone, substituted dialkyl peroxydicarbonates, substituted benzyl ketals, di-tert-butyl substituted peroxides, substituted dibenzoyl peroxides, substituted bis- (aroyloxy) peroxides, such as bis- (4-methoxy) d ibenzoyl peroxide, substituted or substituted oesophoxide, is- (4-methyl) dibenzoyl or bis- (4-chloro) peroxide substituted dibenzoyl, substituted 2,4,6-trimethyl dibenzoyl peroxide, substituted 3-benzoyl benzoic acid, 1,3-substituted di-benzoyl propane, substituted trifluoromethyl-phenylketone, substituted acetophenones, substituted benzophenones, substituted terephthalophenones, substituted fluoronones, substituted xanthones, thio -substituted xanthones, substituted anthraquinones, substituted benzyl, substituted a-ketocoumarins, substituted camphorquinones, substituted a-alkoxysoxybenzoins, a, substituted a-dialkyloxysoxybenzoins, substituted a-dialkoxyacetophenones, substituted a-hydroxyalkylphenones, substituted O-acyl a-oximinoketones, substituted dibenzoyl disulfide, substituted S-phenyl thiobenzoates, substituted acylphosphine oxides, substituted benzoylphosphinoxides, aryl- substituted aryl sulphides, substituted dibenzoylmethanes, substituted phenylazo-diphenyl sulfone, In a preferred embodiment of the invention, the derivatization is carried out either to allow solubility in water or to further improve it. The reactions that occur potentially when applying a monofunctional radical former to polymeric chain segments for dibenzoyl peroxide as a radical former and for polymer chain segments containing polyacrylic acid units (polyacrylic acid or PAA) are illustrated below with examples. ): 1) After the initiation of UV radiation, the dibenzoyl peroxide forms benzoyl benzoic acid radicals according to Formula 1a.
Formula 1a:The benzoic acid radical can, theoretically, be decarboxylated to form benzene radicals as illustrated in Formula 1b.
Formula 1b:After this initial reaction, all of the following reactions 2) to 9) can, in theory, take place: 2) The benzoyl benzoic acid radicals can recombine to form dibenzoyl peroxide again, as illustrated in Formula 2.
Formula 2:3) As another alternative to the reaction of Formula 2, the benzoyl radical of benzoic acid can react with the benzene radical to form the corresponding ester, according to Formula 3.
Formula 3:4) It is also possible, in theory, a recombination of two benzene radicals to form biphenyl, as in Formula 4.
Formula 4:2 - - cor o) As a further alternative, the benzoyl benzoic acid radical can react with the polyacrylic acid to form benzoic acid and a polyacrylic acid radical, according to Formula 5.
Formula 5:The benzoic acid can be subjected to an oxidative reaction to regenerate the dibenzoyl peroxide of the radical former.6) In addition, the benzene radicals, if formed, could also react with the polyacrylic acid according to Formula 6 to form benzene and a polyacrylic acid radical.
Formula 6:If the polyacrylic acid radicals have been formed in accordance with Formulas 5 and 6, the following reactions comprising polyacrylic acid radicals can mainly take place: 7) The polyacrylic acid radical can, at the same time, react again with the benzoyl benzoic acid radical , whereby the benzoyl benzoic acid radical is attached to the polyacrylic acid polymer chain segment, as illustrated in Formula 7.
Formula 7:8) As another alternative, the polyacrylic acid radical can react with the benzene radical to bind the benzene radical to the polymer chain, as illustrated in Formula 8.
Formula 8:9) '+ PAA © - PAA9) As a second alternative, two polyacrylic acid radicals can be recombined, according to Formula 9.
Formula 9:2 PAA * or PAA - PAAThe reaction, in accordance with Formula 9, produces polymer chain segments that link together covalently and directly. To determine which of the above reactions have actually taken place, the reaction samples are extracted with ether after the initiation of ultraviolet radiation and the extracts are analyzed by 1 H-NMR and 19 F-NMR (for example, if use tri-fluoromethyl phenyl ketone as a radical former). Figure 1 illustrates the 300 MHz 1 H-NMR spectrum of dibenzoyl peroxide (BP) and benzoic acid (BA). In the above-mentioned example, where the radical former is dibenzoyl peroxide, the 1 H-NMR spectrum of the ether extract reveals the presence of benzoic acid, in accordance with Figure 2. Benzoic acid is formed only in the reaction , in accordance with Formula 5. Moreover, the reaction according to Formula 5 can only take place if the benzoyl benzoic acid radicals are generated according to Formula 1a. Therefore, the ether extract showed that the reactions actually occur, as illustrated in Formulas 1a and 5. Thus, the polyacrylic acid radicals according to Formula 5 must have been actually formed. Unexpectedly, the ester according to Formula 3 could not be detected in the ether extract. Moreover, in the ether extract neither the biphenyl was found, as a product of the reaction according to Formula 4, nor the reaction products of Formula 7 or Formula 8. In this form, no none of the reactions that include benzene radicals. As a consequence, no benzene radicals are formed, in accordance with Formula 1 b. Consequently, while the 1 H-NMR spectrum of Figure 2, derived from the ether extracts, shows that outside of the reactions illustrated in Formulas 1 to 8, only the reactions of Formulas 1a, 5 and possibly Formula 2 (recombination of the radical former) actually took place, the reaction of Formula 9 also occurred: The polyacrylic acid radicals, which are formed as illustrated in Formula 5 do not react with the benzoyl benzoic acid radicals according to Formula 7 or the benzene radical in accordance with Formula 8. However, since free radicals are not stable and no other reaction has been available, the polyacrylic acid radicals must have recombined in accordance with the reaction illustrated in Formula 9. Thus, from the analysis of the ether extract it can be concluded that, in effect, a covalent bond has been incorporated. Direct between different segments of polymer chain. This direct link can not include the reaction product of the radical former because the radical former only contains a group activatable by radiation and, therefore, can not form a crosslinking after ultraviolet irradiation since this would require at least two activatable groups by radiation. Consequently, when ibenzoyl peroxide is applied as the radical former to segments of the polymer chain containing polyacrylic acid (PAA) and subsequently ultraviolet radiation is initiated, the following reaction occurs:Formula 10:The crosslinking agents known in the art had to be at least bifunctional in order to be able to covalently crosslink the different polymer chain segments with each other: Examples of these are thermally activated surface crosslinking agents, such as di or polyhydric alcohols, or their derivatives, as stated in the above. Contrary to this, the radical former of the present invention are monofunctional. Radicals can form radicals with exposure to electromagnetic radiation. Electron beams, as well as ultraviolet light, can produce adequate electromagnetic irradiation. Preferably, in accordance with the present invention ultraviolet light with a wavelength of 220-380 nm is used, depending on the selected radical formers. UV light can be used in combination with an electron beam, and together with an infrared (IR) light. In case of combining irradiation with ultraviolet light with other electromagnetic radiation, it is not critical if the application of ultraviolet light occurs simultaneously with the other electromagnetic radiation (ie electron beam or infrared light) or if the irradiation is performed in a series of different irradiation steps. For radical formers that require a relatively high amount of activation energy, activation with electron beams may be necessary.
The irradiation with the UV light can preferably be carried out in a conventional manner with ultraviolet light lamps having an energy of between 50 W and 2 kW, more preferably between 200 W and 700 W, and even more preferably between 400 W. and 600 W. The irradiation time is preferably about 0.1 second, 30 minutes, more preferably 0.1 seconds to 15 minutes, even more preferably 0.1 seconds to 5 minutes and most preferably 0.1 seconds to 2 minutes. Conventional mercury pressure ultraviolet light lamps can be used. The selection of the lamp depends on the absorption spectrum of the radical formers used. Lamps that have higher energy generally allow faster crosslinking. The distance between the ultraviolet light lamps and the SAP to be crosslinked varies, preferably, from 5 cm to 15 cm. With electromagnetic irradiation, such as ultraviolet irradiation, radical formers form free radicals. The highly reactive free radicals formed in this way can react with polymer chain segments contained in the superabsorbent polymer. When a free radical formed from the radical former reacts with a polymer chain segment, the polymer chain segment forms a "polymer chain segment radical". It is believed that the reaction occurring within the segment of the polymer chain has to take place in an aliphatic group (group C-H) contained in the polymer chain segment. Alternatively, the reaction can also occur in those carboxylic groups contained in the polymeric chain segment, which have not been neutralized. An additional alternative is that the reaction occurs in another functional group contained in the polymer chain segment if the functional group contains a hydrogen radical that can be extracted. Examples of functional groups of this type are sulphonic acid, carboxylic acid or sulfonic acid esters, hydroxyl groups, amide groups, amino groups, nitrile groups, quaternary ammonium salt groups, aryl groups (for example, phenyl groups, such as derivatives). of styrene monomer). When two polymer chain segment radicals of this type react with each other, a direct covalent bond is formed between the polymer chain segments. It is believed that the reaction that produces the direct covalent bonds between polymer chain segments occurs, preferably in the molecules contained in the main polymer chain. Preferred radical formers in accordance with the present invention have a molecular weight, which is lower than the reference, of the M w =g / mol, of at least Mw = 60 g / mol, of at least Mw = 120 g / mol, of at least Mw = 180 g / mol and of at least Mw = 240 g / mol The radical formers which have Frequently, a more stable molecular weight tends to form more stable radicals, as the charge of the radical can be distributed better within the radical. Therefore, it is more likely that the radical reaches a polymer chain segment within the reaction solution and is likely to react with the polymer chain segment to form a "polymer chain segment radical". If the radical were very stable, it would be more likely to react to recombine with the radical former, as illustrated for example in Formula 2. In addition, the preferred radical formers according to the present invention will comprise aromatic groups, such as let us go. This also leads to more stable radicals as the charge can be distributed through the entire aromatic group. b) Use of electron beam only (the use of additional chemical products is not required to initiate crosslinking) With electromagnetic irradiation, the radicals are formed in the main polymer chain of the superabsorbent polymer. The highly reactive free radicals formed in this way can react with polymer chain segments contained in other chains of the same superabsorbent polymer. The electron beam induces a free radical in the polymer chain, therefore, a "polymer chain segment radical" is formed. If two of the "polymer chain segment radicals" react with each other (in accordance with the reaction illustrated in Formula 9 above), a direct covalent bond is introduced between two polymer chain segments. It is well established that atoms are composed of neutrons and hydrogen radicals that are located in the nucleus, as well as negatively charged electrons, which are much lighter and their orbit is around the nucleus. Because they are light and are only slightly attracted to the nucleus, electrons can be separated from the atom relatively easily, accelerated using electric and magnetic fields and can focus on a beam of energy. This resulting beam can then be scanned by means of an electromagnet to produce a "curtain" of accelerated electrons. The strength of the beam and the amount of energy it can deposit on a target are determined by the difference in voltage between the cathode, where the electrons are released, and the positively charged anode that accelerates the electrons, as well as the current , which is defined as the number of electrons in the beam that pass through a certain area per second. For example, the tube inside a television set accelerates the electrons up to,000 V, while modern industrial accelerators can increase the energy of electrons up to 10,000,000 V. In this embodiment of the present invention, electron beam irradiation is used to have a high-energy electron impact on the bonds of electrons. carbon-hydrogen, or near these, in the main chain polymer of the superabsorbent polymer, and to supply enough energy to the molecules to break some of the bonds, and release the corresponding low molecular weight degradation products and leave the polymer superabsorbent with excited carbon atoms (free radicals). When this process occurs in two adjacent segments of the polymer chain, the excited carbon atoms can release excitation energy that forms between them a chemical bond, known as crosslinking. The amount of electron beam irradiation absorbed by the superabsorbent polymer is called the dose, which is generally defined in terms of kiloGrays (where 1 kGy = 1000 J / kg.) Or MegaRads (where 1 MRad = 1, 000,000 erg / g). The electrons will lose a part of their energy due to the interaction with the air, which is the reason why most electron beams operate in a vacuum environment. Unlike gamma irradiation, which includes the use of a source of radioactive cesium or cobalt, electron beam technology does not produce or store any irradiation in the target materials once those materials are left outside the beam. The surface crosslinking of the superabsorbent polymers by means of electron beam processing can be effected using accelerators of which are commercially available, which are equipped with a variety of material handling systems, and can have important performances. A typical direct current accelerator consists of the voltage generator, the electron gun, the accelerator tube, the scanning tube and the control system. This accelerator creates an electron beam of approximately 2.5 cm. in diameter and energizes it up to near the speed of light. The beam of light passes through a scanning tube, where the magnet explores it back and forth at approximately 200 Hz, creating a curtain of electrons 1-2 m wide. The target materials are passed under the examination tube by the use of conveyors, carts, reel equipment or other specialized management means. Around the world there are currently about 700-800 electron beam accelerators in use in the industry. Accelerators are typically described in terms of their power and power. Low-energy accelerators vary between 150 keV and 2.0 MeV. The medium energy accelerators have power of2. 5 and 8.0 MeV. High-energy accelerators have beam energy above 9.0 MeV. The beam energy depends directly on the application for which it is to be used. Accelerators with energies between 150 keV and 5.0 MeV can be used for the crosslinking of superabsorbent polymers. The power of an accelerator is a product of beam current and electron energy. The available beam powers vary between 5 and approximately 300 kW. For example, an accelerator of 5.0 MeV at 30 mA will have the power of 150 kW. Accelerators can be classified, in general, in accordance with the exact way in which they generate accelerated electrons. The five main types of accelerators are: electrostatic direct current (DC), DC electrodynamics, linear accelerators (LINACS) or radiofrequency (RF) accelerators,LINACS magnetic induction and continuous wave machines (CW, for its acronym in English). In general, DC accelerators are characterized by their high power supply and high efficiency, while LINAC systems are typically much more compact and can generate higher beam energies. However, they are also considerably less efficient. Similarly, continuous wave machines can be very compact and achieve high beam energies. Regardless of the exact nature of the accelerator, in all high beam energy installations (EBPs) the target materials are passed under the acceleration scanning tube through the use of conveyors, trolleys, rail equipment or other specialized management means. Around the world there are currently about 700-800 electron beam accelerators in use in the industry. With respect to processing economics, electron beam processing generally requires a lower energy expenditure than that required by conventional thermochemical processes to produce the same network effects. According to the present invention, the dehydrated superabsorbent polymer particles undergo a process step of surface crosslinking. The term "surface or surface" describes the boundaries of the particle that are oriented outward. For porous SAP particles, exposed internal surfaces may also be on the surface. The term "superabsorbent polymer particle crosslinked on the surface" refers to a particle having its polymeric chain segments present in the vicinity of the superabsorbent polymer particle surface crosslinked with each other. It is known in the art that the surface of the polymer chain segments present in the vicinity of the particle surface is reticulated by a compound known as a surface crosslinking agent. The surface crosslinker is applied to the surface of the particle. In an SAP particle cross-linked on its surface, the level of cross-links near the surface of the SAP particle is, generally, higher than the level of cross-links inside the SAP particle. The surface crosslinking agents that are generally available are thermally activated surface crosslinkers. The term "thermally ctivated surface crosslinking agents" refers to surface crosslinking agents that only react upon exposure to elevated temperatures, generally around 150 ° C. The thermally activated crosslinking agents known in the prior art are, for example, di- or polyfunctional agents which can build additional crosslinks between the polymer chains of the bsorbent polymers. Other thermally activatable surface crosslinking agents include, for example, di- or polyhydric alcohols, or their derivatives, capable of forming di- or polyhydric alcohols. Illustrative examples of this type of agents are: alkylene carbonates, ketals and di- or polyglycidyl ethers. Moreover, the (poly) glycidyl ethers, haloepoxy compounds, polyaldehydes, polyols and polyamines are also well known thermally activatable surface crosslinking agents. The crosslinking is based on a reaction between the functional groups constituted by the polymer, for example an esterification reaction between a carboxyl group (constituted by the polymer) and a hydroxyl group (constituted by the surface crosslinker). Since typically a relatively large portion of the carboxyl groups of the polymeric chain segments is neutralized prior to the polymerization step, often only few carboxyl groups are available for this surface crosslinking process known in the art. For example, in a 70% neutralized polymer, only 3 to 10 carboxyl groups are available for covalent surface crosslinking. According to the present invention, surface crosslinking does not have to comprise a surface crosslinking agent, which reaction product will be incorporated into the superabsorbent polymer particle after surface crosslinking. On the contrary, according to the present invention, it is possible to crosslink the polymer chain segments at the surface by directly linking the polymer chain segments to each other through a covalent bond. The radical former, which initiates the reaction, fails to be incorporated into the superabsorbent polymer particle. As another option, the final reaction product of the radical former can be regenerated after the surface crosslinking and, therefore, after the regeneration can be used again for surface crosslinking. No additional monomers, such as styrenes or carboxylic acids, are required if the radical former is used for the surface crosslinking of superabsorbent polymer particles. Direct covalent bonds introduced between the polymer chain segments on the surface of the superabsorbent polymer particles according to the present invention are formed inside the particulate. They are not intended to form bonds between particles. In addition, if the radical former is used for the surface crosslinking of superabsorbent polymer particles, the radical former can be sprayed onto the superabsorbent polymer particles by means of a fluidized bed spray chamber. At the same time, IR irradiation can be applied to perform the drying and simultaneously UV light can be applied to achieve crosslinking in the fluidized bed. However, in certain cases the drying and crosslinking may take place in a series of two stages, which could be carried out in any order. Instead of that or in combination with I uz I R, any conventional drying equipment can be used in the drying step. However, in certain embodiments of the present invention little or no drying is required, for example, in cases where only small amounts of surface crosslinking agents are applied in small amounts of solution. The surface crosslinking of the prior art has been restricted to the carboxylic groups contained in the polymer chain segments exposed on the surface of the superabsorbent polymer particle. Advantageously, the crosslinking process of the present invention is not limited to the carboxyl groups, but also comprises various functional groups and aliphatic groups within the polymer chains of the superabsorbent polymer. Accordingly, in accordance with the present invention the number of reaction sites available for the surface cross-linking process of the SAP particles is considerably increased. For this reason, it is possible to achieve a uniform surface cross-linking and much more homogeneous compared to the surface crosslinking known in the industry. Even more, it is possible to surface crosslink the SAP to a greater degree than those known in the industry. This makes it possible to produce much more rigid superabsorbent polymer particles, and thus more effectively inhibit the blocking effect of the gel at a certain degree of neutralization. The surface crosslinking of superabsorbent polymer particles occurs mainly on the surface of the superabsorbent polymer particles. This means that the mainly polymer chain segments, which are exposed in the vicinity of the surface of the superabsorbent polymer particles, undergo a crosslinking process, which produces superabsorbent polymer particles with a high degree of crosslinking on their surfaces while not they substantially affect the inner core of the superabsorbent polymer particles. Therefore, the covalent bonds formed directly between the polymer chain segments are formed mainly on the surface of the superabsorbent particles, while the core is virtually free of the covalent bonds. The irradiation with ultraviolet light for surface crosslinking can preferably be carried out in conventional manner with ultraviolet light lamps having an energy of between 50 W and 2 kW, more preferably between 200 W and 700 W, and even more preferably between 400 W and 600 W. The irradiation time is preferably between 0.1 second, 30 minutes, more preferably 0.1 seconds to 15 minutes, even more preferably 0.1 seconds to 5 minutes and most preferably 0.1 seconds to 2 minutes. Conventional mercury pressure ultraviolet light lamps can be used. The selection of the lamp depends on the absorption spectrum of the radical formers used. Lamps that have higher energy generally allow faster crosslinking. The distance between the ultraviolet light lamps and the SAP to be crosslinked varies, preferably, from 5 cm to 15 cm. In comparison with the known surface crosslinking of the prior art, the surface crosslinking according to the present invention is much faster. The surface crosslinking reactions of the prior art are carried out at elevated temperatures, usually taking up to 45 minutes to perform them. This stage of the process is laborious, which makes the process of SAP particle preparation less economical and desirable. On the contrary, the crosslinking process according to the present invention can be carried out very quickly and therefore, it is strongly added to a total manufacturing process much more efficient and economical. Furthermore, since the surface crosslinking reaction is carried out rapidly, the surface crosslinking molecules applied on the surface of the SAP particles have less time to penetrate the interior of the particles of the particles.
SAP. As a result, the surface crosslinking process is restricted primarily on the surface of the SAP particles and additional undesirable crosslinking reactions within the SAP particles are avoided. Another advantage of the present invention relates to the neutralization step. The α, β-unsaturated carboxylic acid monomers are often neutralized before the polymerization step (preneutralization). The compounds, which are useful in the eutralization of the acid groups of the monomers, are usually those which will sufficiently neutralize the acid groups without producing a detrimental effect on the polymerization process. These compounds include alkali metal hydroxides, carbonates and bicarbonates of alkali metals. Preferably, the material used for the neutralization of the monomers is sodium or potassium hydroxide, or carbonate. This neutralization compound is preferably added to an aqueous solution comprising the α, β-unsaturated carboxylic acid monomers (pre-neutralization). As a consequence, the carboxyl groups including the α, β-unsaturated carboxylic acid monomers are at least partially neutralized. Accordingly, after the polymerization step, also the carboxyl groups included by the α, β-unsaturated carboxylic acid of the polymer are at least partially neutralized. In case of using sodium hydroxide, the neutralization produces sodium acrylate, which dissociates in water in acrylate monomers with negative charge and positively charged sodium ions. If the final superabsorbent polymer particles are in the swollen state, after having absorbed the aqueous solution, the sodium ions can move freely in the superabsorbent polymer particles. In absorbent articles, as in the case of diapers or training pants, SAP particles usually absorb urine. Compared with distilled water, urine contains a relatively high amount of salts that are present, at least partially, in dissociated form. As the liquid is absorbed against an osmotic pressure caused by ions of dissociated salts, the salts dissociated in the urine make the absorption of liquids in SAP particles more difficult. Sodium ions that can move freely within the SAP particles greatly facilitate the absorption of liquids in the SAP particles because they reduce the osmotic pressure. Therefore, a high degree of neutralization can formidably potentiate the capacity of SAP particles and the speed of liquid absorption. The surface crosslinkers known in the industry react with the carboxyl groups of the polymer. Accordingly, the high degree of neutralization has to balance with the need for surface crosslinking, since both process steps make use of carboxyl groups. In accordance with the present invention, the surface crosslinking reaction using radical formers that form direct covalent bonds between the polymer chain segments is not limited to carboxylic groups, but also comprises other groups within the polymer chain segment, such as aliphatic groups. Therefore, it is possible to neutralize the monomers to a greater degree without significantly diminishing the possibility of a subsequent surface crosslinking. According to the present invention, the carboxyl groups included by the α, β-unsaturated carboxylic acid monomers are preferably at least 50%, more preferably at least 70%, even more preferably at least 75% and even more preferably 75 % to 95% neutralized. Accordingly, also the carboxy groups including the α, β-unsaturated carboxylic acid of the polymer are at least 50%, more preferably at least 70%, even more preferably at least 75% and even more preferably 75% at 95% neutralized. Still another advantage of the present invention is the reduction of undesired side reactions during the surface crosslinking process. The known surface crosslinking of the previous industry requires high temperatures, frequently around or above 150 ° C. At these temperatures, not only the surface crosslinking reaction is achieved, but many other reactions also occur, for example, formation of anhydride within the polymer or dimeric cleavage of the dimers previously formed by the acrylic acid monomers. These side reactions are quite undesirable, since they diminish the capacity of the SAP particles. Since the surface crosslinking process according to the present invention does not necessarily require high temperatures but can also be carried out at moderate temperatures using electromagnetic irradiation, such as irradiation with ultraviolet light, these actions are considerably reduced. According to the present invention, the surface crosslinking reaction using radical formers and electromagnetic irradiation can preferably be carried out at temperatures, lower or higher preferably lower than 100 ° C, lower than 80 ° C, lower at 50 ° C, below 40 ° C and between 20 ° C and 40 ° C. At the elevated temperatures of about 150 ° C or higher generally applied in the known surface crosslinking process of the prior industry, the superabsorbent polymer particles sometimes change color from white to yellowish. Since, according to the surface crosslinking process of the present invention, it is possible to perform the process of surface crosslinking at moderate temperatures, the problem of color degradation of the superabsorbent polymer particles is considerably reduced. In the embodiments of the present invention, which use radical formers, a radical former can be selected or, alternatively, two or more different radical former can be applied. As a further alternative, no or more radical formers can be applied together with one or more thermally activatable surface crosslinking agents, for example, 1,4-butanediol. In this embodiment, the superabsorbent polymer particles must comprise carboxyl groups wherein at least some of the carboxyl groups are at least partially exposed to the outer surface of the superabsorbent polymer particles and wherein the thermally activated surface crosslinking agent is covalently bound to at least a portion of the carboxyl groups exposed at least partially on the surface of said superabsorbent polymer particles.
In the case where a radical former is used together with a thermally activated surface crosslinking agent, both irradiation with ultraviolet light and high temperatures (above 140 ° C) are necessary for the surface crosslinking process. The radical former is preferably used in a liquid solution, and more preferably in an aqueous solution. In order to obtain particles of superabsorbent polymer with uniformly distributed surface crosslinking, the radical former must be evenly distributed over the superabsorbent polymer particle before or during irradiation with ultraviolet light. Therefore, the radical former is preferably applied by spraying on the superabsorbent polymer particles. However, although preferred, the present invention is not limited to the surface crosslinking of superabsorbent polymer particles. It is also possible to directly cross-link covalently polymeric chain segments well before the superabsorbent polymer particles have formed. For example, the radical former can be applied to polymer chains formed from the polymerization reaction of the respective monomers before the polymer chains have been crosslinked together to form a network. In this embodiment, crosslinking with the radical former can substitute the crosslinking processes known in the art. As another alternative, the crosslinking according to the present invention can be carried out in addition to the known crosslinking processes, either before the known processes, simultaneously or subsequently to them. In these embodiments, the radical former does not apply to superabsorbent polymers that have formed into particles. Consequently, if the polymer is transformed into superabsorbent polymer particles, the direct covalent crosslinks between the polymer chain segments are not restricted primarily to the surface of the superabsorbent polymer particles, but the direct covalent bonds between polymer chain segments will be present through all of the superabsorbent polymer particles, possibly the direct covalent bonds will be homogeneously distributed through all the superabsorbent polymer particles. As another alternative, the direct covalent bonds between polymer chain segments will be homogeneously distributed throughout the superabsorbent polymer particle: For example, it is possible to mix different polymers containing different polymer chain segments. In this case, the different polymer chains can be crosslinked (directly or indirectly by a process known in the art) to a different degree or the polymer chains in certain regions of the superabsorbent polymer particles may not be absolutely crosslinked. It is also possible to mix different polymers to form the superabsorbent polymer particles containing different polymer chain segments. In this case, the different polymers can include mixtures of different homopolymers, copolymers and / or block polymers. However, all superabsorbent polymer particles of this type that contain direct covalent bonds through all of the superabsorbent polymer particles may undergo surface crosslinking. In this case, surface crosslinking can be achieved by subjecting the superabsorbent polymer particles to the radical former of the present invention, subjecting them to a surface crosslinking process known in the art, or by a combination of both.
Absorbent articles The superabsorbent polymer particles of the present invention are preferably applied to absorbent cores of absorbent articles. As used herein, the term "absorbent article" refers to devices that absorb and contain liquids, and being more specific, refers to devices that are placed against or in proximity to the body of the user to absorb and contain different exudates excreted from the body. Absorbent articles include, but are not limited to: adult garments with incontinence, fasteners and liners for diapers, sanitary napkins, and the like. Diapers are the preferred absorbent articles of the present invention. As used herein, the term "diaper" refers to an absorbent article that is generally used by infants and people with incontinence around the lower torso. Absorbent articles especially suitable for the present invention generally comprise an outer cover that includes a fluid-permeable top sheet, a lower fluid-impermeable sheet and an absorbent core generally placed between the upper canvas and the lower canvas. The absorbent core may comprise any absorbent material which is generally compressible, conformable, non-irritating to the wearer's skin, and capable of absorbing and retaining liquids, such as, for example, urine and other certain body exudates. In addition to the superabsorbent polymer particles of the present invention, the absorbent core can include a wide variety of absorbent materials commonly used in disposable diapers and other absorbent articles, such as crushed wood pulp, which is generally known as air felt. Illustrative absorbent structures for use as absorbent units are described in U.S. Pat. no. 5,137,537 entitled "Absorbent Structure Containing Individualized, Polycarboxylic Acid Crosslinked Wood Pulp Cellulose Fibers (Absorbent structures containing individualized cellulose fiber from wood pulp crosslinked with polycarboxylic acid), issued to Herron et al., August 11, 1992, no. 5,147,345 titled "High Efficiency Absorbent Articles ForIncontinence Management ", issued to Young et al., September 15, 1992, No. 5,342,338 entitled" Disposable Absorbent Article for Low-Viscosity Fecal Material "(Disposable Absorbent Item for Low Viscosity Stools) ) issued to Roe on August 30, 1994, No. 5,260,345 entitled "Absorbent Foam Materials For Aqueous Body Fluids and AbsorbentArticles Containing Such Materials "(Absorbent sponge materials for containing aqueous body fluids and absorbent articles containing these materials) issued to DesMarais et al., November 9, 1993, No. 5,387,207 entitled" Thin-Until-Wet Absorbent Foam Materials For Aqueous Body Fluids And Process For Making Same "(Absorbent spongy materials that are kept thin until wet, which are used to contain aqueous body fluids and their processing process) granted to Dyer et al on February 7, 1995; 5,397,316 entitled "Slitted Absorbent Members For Aqueous Body Fluids Formed Of Expandable Absorbent Materials" (Absorbent elements with slits for aqueous body fluids made with expandable bsorbent materials) awarded to LaVon et al on March 14, 1995; No. 5,625,222 entitled "Absorbent Foam Materials For Aqueous Fluids Made From High In." on July 22, 1997. Further, the SAP particles of the present invention can be applied as absorbent materials. The SAP particles of the present invention are preferably present in amounts of at least 50% by weight of the total absorbent core, more preferably at least 60%, even more preferably at least 75% and still more preferably at least 90% by weight of the total absorbent core.
MethodsEther extraction: After photolysis, the reaction products are subjected to ether extraction. 10 mL of dry diethyl ether (0.005% water), commercially available, are added to the reaction product after photolysis, these samples are magnetically stirred for 10 min, and are subjected to centrifugation for 5 min at room temperature. The ether phase is removed with an Eppendorf pipette for further analysis, for example for NMR analysis.
Nuclear Magnetic Resonance Spectroscopy: For nuclear magnetic resonance (NMR) spectroscopy, either deuterated trichloromethane, CDCI3, or deuterated methylene chloride, CD2CI2, is used as a solvent for ether extracts. For example, a Bruker Avance 300 MHz NMR spectrometer with Oxford 7.05 T Oxford superconducting magnet (54 mm perforation) and gradient compensation, controlled by a computer can be used750 MHz PC / NT Pentium III. For example, a 5-mm four-core probe can be used, observable for 1H, 13C, 19F and 31P with z gradient, variable temperature capability (-150 ° C to +200 ° C ) and boron-free glass. The respective 1H, 13C, 19F spectra are acquired by high resolution standard nuclear magnetic resolution (solution) techniques well known to people with industry experience.
Photolysis: 200 mg of PAA is mixed with 20 mg of the respective radical former, either in the dry state or dissolved / suspended in 1.5 mL of water. The photolysis is carried out either for 10 or 60 min with a medium Hg pressure lamp of 450 W as a source of ultraviolet rays and using a pyrex filter to decrease the light below a wavelength of 300 nm. A lamp of this type generates mainly light at a wavelength of 365 nm. All samples are degassed before photolysis either by pumping at 10"5 torr or by three cycles of freezing-pumping-thawing All documents cited in the Detailed Description of the Invention are incorporated in their relevant part, as a reference in the present, the citation of any document should not be construed as an admission that it is a prior industry with respect to the present invention, While particular embodiments of the present invention have been illustrated and described.It will be evident to those skilled in the industry that various changes and modifications can be made without departing from the spirit and scope of the invention. It has been intended, therefore, to cover in the appended claims all changes and modifications that are within the scope of the invention.