Shelf-stable aqueous reducing agent solution

Abstract

A shelf-stable aqueous reducing agent solution comprises a) a sulfinate of formula (Ia), b) a sulfonate of formula (Ib), and c) optionally, a sulfite of formula (Ic) M2SO3, wherein M is an equivalent of an alkali metal; wherein the total concentration of a), b) and c) is more than 10% by weight. The shelf stability of the reducing agent solution is significantly improved. The shelf-stable aqueous reducing agent solution is useful as a shippable form of a sulfinate of formula (Ia), in a method to inhibit decomposition of a sulfinate of formula (Ia) solution during storage and / or shipment, and in a method of performing a reducing step.

Description

[0001] Shelf-Stable Aqueous Reducing Agent Solution

[0002] The invention relates to a shelf-stable aqueous reducing agent solution and its use as a shippable form of sulfinate, and a method to inhibit decomposition of sulfinate during storage and / or shipment.

[0003] BACKGROUND

[0004] Formaldehyde sulfoxylates (hydroxymethane sulfinates), in particular sodium formaldehyde sulfoxylate, have proven to be effective and good value reducing agents, in particular in free-radical-initiated emulsion polymerizations. During the reduction process, however, the formaldehyde sulfoxylates release toxic formaldehyde.

[0005] The disadvantages of formaldehyde sulfoxylates have been overcome by sulfinic acid derivatives which additionally have a carboxylic acid group in the molecule such as 2-hydroxyl-2-sulfinatoacetic acid. These derivatives and their preparation are disclosed in WO 99 / 18067. They have a high reducing power and do not release formaldehyde during or after use they are in wide-spread use as reducing agents, in particular in free-radical-initiated emulsion polymerizations. However, said derivatives are not sufficiently stable in aqueous solution and in particular in acidic media. Consequently, they develop an unpleasant and moreover toxic "sulfur” smell in aqueous or acidic solution, along with a significant loss in reducing power.

[0006] WO 2018 / 108922 relates to mixed salts of hydroxyalkane sulfinic acids and optionally hydroxyalkane sulfonic acids, their preparation and the use of said salts as a reducing agent. While it is claimed that the storage stability of the salts as a solid and as an aqueous solution is significantly higher than that of the corresponding sodium salts, storage periods longer than 21 days have not been investigated.

[0007] The solutions in a ready-to-use concentration of about 2 to 5 % by weight have a shelf life of only several days or less. As a rule, the decomposition continues to accelerate until all of the 2-hydroxyl-2-sulfinatoacetic acid salt has been consumed. Decomposition can proceed both aerobically and anaerobically. While aerobic decomposition would not appear to be a serious factor when the solutions are tightly sealed against ambient oxygen, degradation by anaerobic decomposition is inevitable.

[0008] Due to the relative instability of the 2-hydroxyl-2-sulfinatoacetic acid salt in aqueous solution, solutions are prepared immediately before use from a dry pulverulent form of the 2-hydroxyl-2-sulfinatoacetic acid salt. The users rely heavily on a free-flowing form to be loaded and discharged with conventional handling equipment. Caking and lump formation will render the dissolution more difficult when preparing the ready- to-use solution. Dust generated by the handling of the reducing agent has been an area of concern for a number of years. The dust can be a nuisance and even a health hazard.

[0009] It would be desirable to have a storage-stable aqueous solution that, optionally after appropriate dilution, can be used immediately and dispenses with dry solids material handling equipment. To be commercially attractive, storage-stable aqueous solution should have a sufficient shelf-life of, e.g., at least 6 months, or preferably at least 9 months. The stability of aqueous reducing agent solutions has also been addressed in the patent literature. US 3,798,172 discloses a stabilized reducing agent comprising a mixture of sodium dithionite as reducing agent and a stabilizer for said reducing agent. The stabilizer is an adduct of aldehyde and bisulfite. This reference relates to the inhibition of oxidative decomposition of sodium dithionite upon exposure to air during a dyeing process wherein sodium dithionite is used as a reducing agent. In the examples, vat solutions were kept in open dyepots for a period of two hours. US 3,804,944 discloses a method to stabilize aqueous solutions of sodium dithionite against decomposition during storage thereof comprising the steps of preparing a dilute aqueous solution having a concentration of 5.0% to 30.0% of sodium dithionite, and maintaining the pH of the aqueous solution at 9.0 to 13.0 by adding caustic soda to the aqueous solution of sodium dithionite, while controlling the storage temperature within a defined range. In the examples, aliquots were kept under observation for 84 days at most. The prior art does not teach or suggest stabilization of aqueous solutions of 2-hydroxy l-2-sulfinatoacetic acid salts.

[0010] SUMMARY OF THE INVENTION

[0011] It has been found that the shelf stability of aqueous solutions of 2-hydroxyl-2-sulfinatoacetic acid salts can be greatly improved when the concentration of 2-hydroxy l-2-su Ifin atoacetic acid salts and concomitant salts is maintained within certain limits. Thus, a solution of 2-hydroxyl-2-sulfinatoacetic acid disodium salt at a concentration of 15% by weight, including concomitant salts, has a virtually unchanged residual content of sulfinate active principle after storage for 200 days, whereas a solution at a concentration of 5% by weight rapidly diminishes in sulfinate.

[0012] The invention relates to a shelf-stable aqueous reducing agent solution, comprising a) a sulfinate of formula (la) b) a sulfonate of formula (lb) (lb), and c) optionally, a sulfite of formula (Ic)

[0013] M2SO3 (Ic) wherein

[0014] M is an equivalent of a metal selected from an alkali metal, an alkaline earth metal, aluminum, zinc, and combinations thereof, wherein the total concentration of a), b) and c) is maintained within certain limits. It has been found that the stability of the reducing agent in aqueous solution depends on the nature of the metal M and, hence, the total concentration of a), b) and c) required to achieve a desired stability depends on the nature of the metal M.

[0015] In the shelf-stable aqueous reducing agent solution of the invention, "shelf-stable" means that the concentration of a) does not decrease from an initial concentration upon manufacture by more than 15% after storage for 200 days at ambient temperature under exclusion of ambient oxygen.

[0016] The effects underlying the invention are not fully elucidated, but the available evidence suggests that anaerobic decomposition of 2-hydroxyl-2-sulfinatoacetic acid salts occurs via a sequence of reactions. A first step thereof is the dissociation of 2-hydroxyl-2-sulfinatoacetic acid salts in sulfoxylate HSO2" and glyoxylate. Sulfoxylate then disproportionates in several stages into hydrogensulfite HSO3" and sulfide. Dissociation of 2-hydroxyl-2-sulfinatoacetic acid salts may be facilitated by the fact that glyoxylate forms hydrates in water, which are solvated with other water molecules via hydrogen bonds. In a more concentrated salt solution, a majority of water molecules is bound as a hydrate shell of solvated cations and anions and is not as readily available for hydrate formation with glyoxylate. It is also envisaged that the concomitant 2-hydroxyl-2-sulfonatoacetic acid salt acts as a reservoir for glyoxylate. The 2-hydroxyl-2- sulfonatoacetic acid salt can likewise dissociate under release of glyoxylate, but unlike sulfoxylate HSO2", the resultant sulfite HSO3" is stable under the exclusion of ambient oxygen. The less efficient hydration of glyoxylate and the presence of glyoxylate from 2-hydroxyl-2-sulfinatoacetic acid salts suppresses dissociation of the 2-hydroxyl-2-sulfinatoacetic acid salt. Hence, the first step of anaerobic decomposition is inhibited.

[0017] BRIEF DESCRIPTION OF THE DRAWING

[0018] Fig. 1 shows the stability of the sulfinic component in aqueous solutions of 2-hydroxy l-2-sulfinatoacetic acid disodium salt and concomitant salts at concentrations of 15%, 10% and 5% by weight over time.

[0019] DETAILED DESCRIPTION OF THE INVENTION

[0020] In a first aspect, the invention relates to a shelf-stable aqueous reducing agent solution, comprising a) a sulfinate of formula (la) (la), b) a sulfonate of formula (lb) c) optionally, a sulfite of formula (Io)

[0021] M2SO3 (Io) wherein

[0022] M is an equivalent of an alkali metal, wherein the total concentration of a), b) and c) is more than 10% by weight.

[0023] Preferably, the total concentration of a), b) and c) is more than 10% by weight, preferably at least 12% by weight, more preferably 15% by weight. In one embodiment, the total concentration of a), b) and c) is at most 25% by weight, preferably at most 24% by weight, more preferably at most 22% by weight. In one embodiment, the total concentration of a), b) and c) is in the range of 10 to 25% by weight, preferably 12 to 24% by weight, in particular 15 to 22% by weight.

[0024] The requirement for stability is a stability of at least 200 days, preferably at least 270 days. This will include post-manufacture shipping to the end user and a remaining shelf life for the end user.

[0025] The total concentration of a), b) and c) is sufficiently high such that the concentration of a) does not decrease from an initial concentration upon manufacture by more than 15% after storage for 180 days at ambient temperature under exclusion of ambient oxygen. In an embodiment, the concentration of a) does not decrease from an initial concentration upon manufacture by more than 15% after storage for 270 days at ambient temperature under exclusion of ambient oxygen. In certain embodiments, for example, the concentration of a) does not decrease from an initial concentration upon manufacture by more than 10% after storage for 180 days at ambient temperature under exclusion of ambient oxygen, or does not decrease from an initial concentration upon manufacture by more than 10% after storage for 270 days at ambient temperature under exclusion of ambient oxygen.

[0026] The maximum total concentration of a), b) and c) is limited by the combined solubility of a), b) and c) in water at ambient temperature. The total concentration of a), b) and c) is preferably slightly below the combined solubility in order to avoid precipitation or flocculation when the reducing agent solution is exposed to temperature fluctuations.

[0027] The term "under exclusion of ambient oxygen” means that the solution is stored in a container made of a material with no or only negligible permeability to oxygen. There is no requirement to displace oxygen or to reduce the proportion of oxygen in the headspace of the container, but the headspace volume should be less than 30% of the container volume. The concentration of sulfinate a) may conveniently be determined as disclosed in the experimental section that follows. Herein, the term "ambient temperature” is understood to mean 23° C. In an embodiment, M is sodium and the total concentration of a), b) and c) is more than 10% by weight, preferably from 12% to 24% by weight.

[0028] In a further embodiment, M comprises an equivalent of a metal selected from an alkaline earth metal, aluminum and zinc. In this embodiment, the total concentration of a), b) and c) is preferably more than 20% by weight, preferably more than 20 to 60% by weight, more preferably more than 20 to 45% by weight, in particular more than 20 to 40% by weight. In a particular embodiment, M is a combination of magnesium and zinc. Preferably, the counterions M comprise 40 to 60 mol-% magnesium ions and 40 to 60 mol-% zinc ions, more preferably 45 to 55 mol-% magnesium ions and 45 to 55 mol-% zinc ions.

[0029] Generally, the weight ratio of a) to [b) + c)] is from 3: 1 to 1 :5, such as 2: 1 to 1 :3. The weight ratio of b) to c) is preferably 1 :5 to 5: 1

[0030] The shelf-stable aqueous reducing agent solution may further comprise salts which are essentially non- redox-active, such as sulfates, for example sodium sulfate. Such non-redox-active salt, if present, are preferably comprised in amounts of less than 10% by weight, such as 0.1 to less than 10% by weight.

[0031] The invention also relates to the use of the shelf-stable aqueous reducing agent solution as a shippable form of a sulfinate of formula (la). This may include manufacture of the shelf-stable aqueous reducing agent solution at a location of manufacture and shipment, e.g., via a truck, train, airplane or other motor vehicle, of the shelf-stable aqueous reducing agent solution to a point of application. The shelf-stable aqueous reducing agent solution may be diluted at the point of application prior to use.

[0032] The inherent added volume of water, however, results in higher shipping costs due to the added bulk as opposed to a dry pulverulent composition. Also, the added volume of a liquid requires greater storage space at the location of manufacture. Hence, in particular when the composition is shipped overseas, it is envisaged that a dry pulverulent composition is shipped to a port plant or other plant close to customers and dissolution takes place at the port plant or other plant close to customers. From there, the shelf-stable aqueous solutions can be shipped to customers for on-site use.

[0033] The invention also relates to a method to inhibit decomposition of a sulfinate of formula (la) solution during storage and / or shipment, comprising providing an aqueous reducing agent solution comprising a) a sulfinate of formula (la) b) a sulfonate of formula (lb) c) optionally, a sulfite of formula (Io)

[0034] M2SO3 (Io) wherein

[0035] M is an equivalent of an alkali metal; by providing the aqueous reducing agent solution at a total concentration of a), b) and c) of more than 10% by weight.

[0036] The shelf-stable aqueous solutions can be prepared by dissolution of dry pulverulent compositions containing a), b) and c) defined as above in an appropriate volume of water. Generally, the shelf-stable aqueous solutions do not contain excess amounts of reagents such as glyoxylate or dithionite.

[0037] Dissolution preferably occurs by introducing a dry pulverulent compositions containing a), b) and c) defined as above continuously or in small increments into a volume of water or other aqueous liquid with agitation.

[0038] The dry pulverulent compositions may be prepared in accordance with WO 99 / 18067 or WO 2018 / 108922. Generally, the preparation comprises reacting glyoxylic acid with dithionite in the presence of a base. The slurry obtained may be spray-dried.

[0039] It is known that both aerobic and anaerobic decompositions of sulfur-containing reducing agents are accelerated by an acidic pH value, wherein the stronger the acidic condition, the faster the rate of the decomposition. By the nature of the decomposition reaction products, an acidic environment is created. Thus, the decomposition is likely to promote and accelerate further degradation. Hence, it is preferred to maintain a basic pH value within the shelf-stable aqueous reducing agent solution of the invention. In an embodiment, the shelf-stable aqueous reducing agent solution comprises a buffer agent providing buffering in a pH range of from 8 to 11 , preferably 9 to 10.

[0040] Suitable buffering agents include carbonate I bicarbonate, ammonia I ammonium, phosphate I hydrogen phosphate and boric acid I tetraborate.

[0041] The shelf-stable aqueous reducing agent solution of the invention are used - optionally after dilution - as reducing agents in fields where a high reducing power is required. For example, they can be used as reducing agents in textile printing, in particular in textile discharge printing, in textile bleaching or vat dyeing, or as reducing agents for bleaching minerals, such as kaolin, and fibers, for example cellulose fibers. They are preferably used, however, as an initiator in emulsion polymerization together with peroxidic initiators in order to allow the polymerization to be carried out at a lower temperature. For this purpose, the solutions of the invention may, if desired, be also used together with oxidizable metal ions, such as Fe2+, Mn2+etc. Preferably, the solutions of the invention are used as reducing agents in the main- and / or postpolymerization of an emulsion polymer in order to reduce the residual monomer content to an acceptable level.

[0042] The invention moreover provides a method of performing a reducing step, comprising diluting the shelfstable aqueous reducing agent solution as defined above with an aqueous liquid to obtain a dilute reducing agent solution and bringing the dilute reducing agent solution into contact with a substrate to be reduced.

[0043] The example below illustrates the invention.

[0044] Example

[0045] A dry pulverulent composition containing 48% by weight of 2-hydroxyl-2-sulfinatoacetic acid disodium salt, 9% by weight of 2-hydroxyl-2-sulfonatoacetic acid disodium salt, 38% by weight of disodium sulfite and 5% by weight of sodium sulfate was dissolved in water at concentrations of 15%, 10% and 5% by weight, respectively. The solutions were stored in tightly closed glass bottles and maintained at 23 °C. The concentration of 2-hydroxyl-2-sulfinatoacetic acid disodium salt was determined on days 0, 28, 56, 84, 112, 140, 168 and 200 by iodometric titration.

[0046] The results are reported as percentage of the initial content and shown in Fig. 1. Thus, the solution at a concentration of 15% by weight had a residual content of about 93% of sulfinate after storage for 180 days, whereas the solution at a concentration of 5% by weight rapidly diminished in sulfinate content.

Claims

ClaimsA shelf-stable aqueous reducing agent solution, comprising a) a sulfinate of formula (la)(la), b) a sulfonate of formula (lb)c) optionally, a sulfite of formula (Ic)M2SO3 (Ic) whereinM is an equivalent of an alkali metal; wherein the total concentration of a), b) and c) is more than 10% by weight.

2. The shelf-stable aqueous reducing agent solution of claim 1, wherein M is sodium.

3. The shelf-stable aqueous reducing agent solution of claim 1 or 2, wherein the weight ratio of a) to [b) + c)] is from 3:1 to 1 :5.

4. The shelf-stable aqueous reducing agent solution of claim 3, wherein the weight ratio of b) to c) is 1 :5 to 5:1.

5. The shelf-stable aqueous reducing agent solution of any one of the preceding claims, additionally comprising a buffer agent providing buffering in a pH range of from 8 to 11, preferably 9 to 10.

6. A method to inhibit decomposition of a sulfinate of formula (la) solution during storage and / or shipment, comprising providing an aqueous reducing agent solution comprising a) a sulfinate of formula (la)(la),b) a sulfonate of formula (lb)(lb), and c) optionally, a sulfite of formula (Ic)M2SO3 (Ic) whereinM is an equivalent of an alkali metal; by providing the aqueous reducing agent solution at a total concentration of a), b) and c) of more than 10% by weight.

7. A shelf-stable aqueous reducing agent solution, comprising a) a sulfinate of formula (la)b) a sulfonate of formula (lb)c) optionally, a sulfite of formula (Ic)M2SO3 (Ic) whereinM is an equivalent of a metal selected from an alkali metal, an alkaline earth metal, aluminum, zinc, and combinations thereof, wherein the concentration of a), b) and c) is sufficiently high such that the concentration of a) does not decrease from an initial concentration upon manufacture by more than 15% after storage for 180 days at ambient temperature under exclusion of ambient oxygen.

8. Use of the shelf-stable aqueous reducing agent solution of claim 7 as a shippable form of a sulfinate of formula (la).

9. A method of performing a reducing step, comprising diluting the shelf-stable aqueous reducing agent solution of any one of claims 1 to 5 or 7 with an aqueous liquid to obtain a dilute reducing agent solution and bringing the dilute reducing agent solution into contact with a substrate to be reduced.

Images