DESC:FIELD OF THE INVENTION
The present invention relates to storage stable aqueous solutions of optical brightening agents and process for preparing the same.

BACKGROUND OF THE INVENTION
Optical brightening agents have been extensively used to impart higher degree of whiteness to paper, cardboard, woven and non-woven fabrics. The anilino-substituted bistriazinyl derivatives of 4,4’-diaminostilben-2,2’-disulphonic acid are especially important optical brightening agents for the paper and cardboard industry. The anilino-substituent may additionally contain sulphonic acid groups, which impart higher water solubility to these agents owing to its hydrophilicity. The tetrasulpho- and hexasulpho- optical brightening agents derived from 4,4’-diaminostilben-2,2’-disulphonic acid, therefore have higher solubility in water. The disulpho- optical brightening agents derived from 4,4’-diaminostilben-2,2’-disulphonic acid, i.e., with aniline-substituent without any sulphonic acid group, however, have higher affinity for cellulose fibres and are therefore most preferred in the wet-end application in the paper making process.
The paper and cardboard industry demands aqueous formulations of the optical brightening agents in the form of suspensions or clear solutions in water for the obvious advantages during handling, transport, storage and metering. Particularly preferred are the clear concentrated aqueous solutions of the optical brightening agents, which are stable for prolonged period of at least a few months at temperatures ranging from -5 to 40°C. The formulations in the form of a clear liquid also reduce the time required to dissolve the otherwise solid form of the optical brightening agents.
Furthermore, the freezing point of such clear aqueous solutions is of importance when these solutions are used in the regions where the average temperature is below 0°C. It is therefore desirable that the freezing point of the solutions is as low as possible.
Thus, there exists a challenge to develop storage stable aqueous formulations, especially clear solutions of the bistriazinyl derivatives of 4,4’-diaminostilben-2,2’-disulphonic acid optical brightening agents without any sulphonic acid group on the anilino-substituent, which are most preferred in the paper and cardboard industry but have substantially less solubility in water by virtue of their structure.
Still another problem that exists and is unavoidable in the case of all the optical brightening agents based on bistriazinyl derivatives of 4,4’-diaminostilben-2,2’-disulphonic acid is the presence of inorganic salts, sodium chloride being the most common, as a result of the manufacturing process. This problem stems from the fact that these optical brightening agents are manufactured using cyanuric chloride and inevitably generate a minimum of six moles of sodium chloride for every mole of the optical brightening agent produced. Furthermore, the synthesis processes use water as the solvent, resulting in appreciable solubility of salts such as sodium chloride in the reaction medium, which are therefore difficult to remove. These residual salts tend to destabilise the aqueous solutions of the optical brightening agents and call for expensive and tedious processing steps such as membrane filtration.
It has been a practice in the industry to add solubilising additives such as urea, caprolactum, ethylene glycol in amounts as high as ~30% of the formulations, with the content of the optical brightening agent at ~ 20-35%, to achieve the desired solubility and storage stability. These solubilising additives neither contribute to the performance of the optical brightening agent as such nor have any affinity towards cellulose. At the end of the paper manufacturing process therefore, these solubilising additives are released into the effluent.
US 3012971discloses whitening compositions in the form of concentrated aqueous solutions of 4,4’-bis-[2-phenylamino-4-diethanolamino-1,3,5-triazyl(6)]-diaminostilbene-2,2’-disulphonic acid, or an alkali metal salt thereof together with alkanolamines. The patent mentions that, “the proportion by weight of the alkylolamine agents to whitening agent may vary widely, say from 0.5:1 to 3.0:1”. This ratio translates into significantly large excess of the alkanolamines in terms of moles per mole of the whitening agent.
US 4717502 discloses aqueous optical brightener compositions of disulpho-, tetrasulpho- as well as hexasulpho- optical brighteners based on bistriazinyl derivatives of 4,4’-diaminostilben-2,2’-disulphonic acid with the following general formula:

in which,
R1 and R2 are hydrogen or –SO3M;
R3 is hydrogen, C2-3- hydroxyalkyl, C1-4-alkyl, -CH2CH2CN or –CH2CH2CONH2;
R4 is hydrogen, C1-4-alkyl, C2-3-hydroxyalkyl, hydroxyl-ethoxy-ethyl, N,N-Bis-(C1-3-alkyl)-amino-C2-6-alkyl or benzyl; or R3 and R4 together with the neighbouring nitrogen atom signify a morpholine, pyrrolidine, piperidine or N-methylpiperazine ring; and
M is hydrogen or a colourless cation; provided that at most one of R3 and R4 is hydrogen,
with polyethylene glycol with an average molecular weight in the range of 1000 to 3000. The disclosure suggests 10-500 parts by weight of polyethylene glycol per 100 parts of the brightening agent and that at least 20% of the composition is water. The disclosure also suggests the use of mono- and triethanolamine salts of the brightening agents by way of the examples cited. The optical brightener compositions disclosed in both these disclosures are thus unacceptable in current context from the point of view of economics and more particularly the eco-friendliness of the application process.
US 2004/0074021 attempts to solve the problem by providing a mixture of two or more bis(triazinylamino)stilbene derivatives. However, the brightening agents are used in the form of mixed sodium/triethanolammonium salt. Furthermore, it also recommends the use of additive, which is employed at a concentration of 0.2 to 3.0% by weight of the solution. Preferred additives are tertiary alkanolamines, triisopropanolamine being especially preferred.
WO 2005/028749 discloses optical brightener compositions comprising an alkanolamine and a bis(triazinylamino)stilbene derivative of the following general formula:

wherein, X is hydrogen, an alkali metal ion, an ammonium ion or hydroxyalkyl ammonium radical derived from the said alkanolamine and R7, R8, R9 and R10 are independently of the others –OR11, -NR11R12 or a group of formulas
, , or ,
wherein, R11 and R12 are each independently of the other hydrogen, alkylhydroxyalkyl, alkoxyalkyl, carboxyalkyl, dicarboxyalkyl, H2N-CO-alkyl or alkylthio.
Preferred alkanolamines are 2-amino-2-methyl-1-propanol, 1-amino-2-propanol or a mixture of 2-amino-2-methyl-1-propanol and 2-(N-methylamino)-2-methyl-1-propanol. The examples cited in this disclosure indicate that at least 17% molar excess of the alkanolamine per mole of the brightening agent is needed for acceptable stability of the resulting formulation.
US 8221588 discloses storage stable solutions of certain anilino-substituted bistriazinyl derivatives of 4,4’-diaminostilbene-2,2’-disulphonic acid, wherein, at least 25% of the [M+] ions associated with the sulphonate groups have been replaced by (CH3)2NH+CH2CH2OH ions. The general structure of the optical brightener is as follows:

in which, R is hydrogen or a methyl radical, R1 is hydrogen, an alkyl radical with 1 to 4 carbon atoms, a ?-hydroxyalkyl radical containing 2 to 4 carbon atoms, a 13-alkoxyalkyl radical containing 3 or 4 carbon atoms or CH2CH2CONH2; R2 is hydrogen or a methyl radical; M+ is Li+, Na+, or K+ and n is less than or equal to 1.5. It is however evident from the examples 1 and 6 in this disclosure, that only ~50-55% of the dimethylaminoethanol employed during the synthesis is retained in the final formulation.
US 2010/0294447 discloses storage stable solutions of anilino-substituted bistriazinyl derivatives of 4,4’-diaminostilbene-2,2’-disulphonic acid, which are substituted at the triazine rings with diethanolamine, wherein, at least 25% of the [M+] ions associated with the sulphonate groups have been replaced by (CH3)2NH+CH2CH2OH ions. The general structure of the optical brightener is as follows:

in which, R is hydrogen or a methyl radical; M+ is Li+, Na+, or K+, and n is less than or equal to 1.5, and of 0.05 to 5% by weight of an organic acid selected from citric acid, glyoxylic acid, acetic acid or formic acid. Similar to the above mentioned disclosure, only ~38-40% of the dimethylaminoethanol employed during the synthesis is retained in the final formulation. Furthermore, the process of the production of these solutions employs carboxylic acids (e.g., citric acid) in almost equimolar quantities with respect to the dimethylaminoethanol used.
WO 2013/018012 discloses stable aqueous solutions of anilino-substituted bistriazinyl derivatives of 4,4’-diaminostilbene-2,2’-disulphonic acid in the form of a tertialyalkanolammonium salts. 2-(dimethylamino)ethanol is mentioned as the preferred tertiary alkanolamine for this purpose. However, the concentration of the optical brighteners reported in the examples is relatively low (~20%) moreover, ~50% molar excess of the alkanolamine is required to get a stable solution.
WO 2014/009479 discloses stable aqueous solutions of the hydrates of tertiary alkanolamine salts of anilino-substituted bistriazinyl derivatives of 4,4’-diaminostilbene-2,2’-disulphonic acid. Once again, only ~50-65% of the tertiary alkanolamine used during the synthesis of these solutions are retained in the final product.
Unfortunately, although most of the abovementioned methods offer a solution for the problem of effluent generation during the end application, there is, nevertheless, generation of such effluents during the manufacturing of the optical brightening formulations. The effluent generation is thus shifted one step earlier in the supply chain. Also, the freezing point of most of the aqueous solutions described above is between -2 to 0°C, unlike the urea stabilised OBA solutions, in which case, it is around -10°C.
There is therefore a need to provide improved formulations, which gives stable aqueous solutions of anilino-substituted bistriazinyl derivatives of 4,4’-diaminostilbene-2,2’-disulphonic acids, which are easy to manufacture, use easily available and cheaper raw materials, have freezing point at least below -5°C and cause minimum harm to the environment by way of lesser effluent generation during manufacturing as well as application of the optical brighteners.
Surprisingly, it has been found that the use of certain amines for the formation of quaternary ammonium salts with the sulphonic acid groups in anilino-substituted bistriazinyl derivatives of 4,4’-diaminostilbene-2,2’-disulphonic acid leads to the formation of an exceptionally stable solution of optical brightening agents.

SUMMARY OF THE INVENTION
The present invention relates to optical brighteners of Formula I :
Formula I
wherein,
R and R’ are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl;
R1 and R’1 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2CH2CONH2, -CH2CH2CN, -CH2CH2COO-, -CH2COO-;

R2 and R’2 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2COO-, CH(COO-)CH2COO-, CH(COO-)CH2CH2COO-, CH2CH2SO3-;
R3 and R4 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2CH2CONH2;
M+ represents H+, alkali metal cation, alkaline earth metal cation, ammonium, or a quaternary ammonium cation derived from mono-, di- or tri- C1-C4 alkylamine; mono- C1-C4 alkyl, di- C1-C4 alkanolamine; di- C1-C4 alkyl, mono- C1-C4 alkanolamine; mono-, di- or tri- C1-C4 alkanolamine, or a mixture thereof, with ‘p’ less than or equal to 0.75 and ‘q’ is the number of functional groups in the optical brightener molecule, that are capable of forming a quaternary ammonium salt;
n = 0, 1 or 2; and
z = 1, 2, 3, 4 or 5.
Another aspect of the present invention relates to a process for preparing compounds of Formula I. The process comprises of reacting compound of Formula II with an amine of Formula III and a suitable alkali metal hydroxide; alkaline metal hydroxide; mono-, di- or tri- C1-C4 alkylamine; mono- C1-C4 alkyl, di- C1-C4 alkanolamine; di- C1-C4 alkyl, mono- C1-C4 alkanolamine; mono-, di- or tri- C1-C4 alkanolamine, or a mixture thereof.

Formula II
wherein,
R and R’ are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl;
R1 and R’1 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2CH2CONH2, -CH2CH2CN, -CH2CH2COO-, -CH2COO-;
R2 and R’2 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2COO-, CH(COO-)CH2COO-, CH(COO-)CH2CH2COO-, CH2CH2SO3-;
n = 0, 1 or 2

Formula III
wherein,
R3 and R4 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2CH2CONH2; and
z = 1, 2, 3, 4 or 5.

DESCRIPTION OF THE INVENTION
An embodiment of the present invention discloses an optical brightener of Formula I:

Formula I
wherein,
R and R’ are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl;
R1 and R’1 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2CH2CONH2, -CH2CH2CN, -CH2CH2COO-, -CH2COO-;
R2 and R’2 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2COO-, CH(COO-)CH2COO-, CH(COO-)CH2CH2COO-, CH2CH2SO3-;

R3 and R4 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2CH2CONH2;
M+ represents H+, alkali metal cation, alkaline earth metal cation, ammonium, or a quaternary ammonium cation derived from mono-, di- or tri- C1-C4 alkylamine; mono- C1-C4 alkyl, di- C1-C4 alkanolamine; di- C1-C4 alkyl, mono- C1-C4 alkanolamine; mono-, di- or tri- C1-C4 alkanolamine, or a mixture thereof, with ‘p’ less than or equal to 0.75 and ‘q’ is the number of functional groups in the optical brightener molecule, that are capable of forming a quaternary ammonium salt;
n = 0, 1 or 2; and
z = 1, 2, 3, 4 or 5.
An objective of the present invention is to provide stable aqueous solutions of optical brighteners, particularly anilino-substituted bistriazinyl derivatives of 4,4’-diaminostilbene-2,2’-disulphonic acids, which can be easily manufactured, safely handled and conveniently used as optical brightening agents in paper, paper board, woven and non-woven fabrics.
Another objective of the present invention is to provide concentrated aqueous solutions of optical brightening agents which cause minimum harm to the environment during manufacturing and application by avoiding the use of large excess of the solubilising agents.
Yet another objective of the present invention is to provide concentrated aqueous solutions of optical brightening agents, which are stable over a wide range of temperature and for a prolonged duration so as to remain in usable condition during storage and transportation, especially, which have freezing point below at least -5°C.
Still another objective of the present invention is to provide a method for the preparation of stable concentrated aqueous solutions of optical brightening agents, particularly anilino-substituted bistriazinyl derivatives of 4,4’-diaminostilbene-2,2’-disulphonic acids using a specific class of amine compounds.
An embodiment of the present invention discloses compounds of Formula I for optically brightening an article, compound of formula I has the following structure:

Formula I
wherein,
R and R’ are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl;
R1 and R’1 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2CH2CONH2, -CH2CH2CN, -CH2CH2COO-, -CH2COO-;
R2 and R’2 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2COO-, CH(COO-)CH2COO-, CH(COO-)CH2CH2COO-, CH2CH2SO3-;
R3 and R4 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2CH2CONH2;
M+ represents H+, alkali metal cation, alkaline earth metal cation, ammonium, or a quaternary ammonium cation derived from mono-, di- or tri- C1-C4 alkylamine; mono- C1-C4 alkyl, di- C1-C4 alkanolamine; di- C1-C4 alkyl, mono- C1-C4 alkanolamine; mono-, di- or tri- C1-C4 alkanolamine, or a mixture thereof, with ‘p’ less than or equal to 0.75 and ‘q’ is the number of functional groups in the optical brightener molecule, that are capable of forming a quaternary ammonium salt;
n = 0, 1 or 2; and
z = 1, 2, 3, 4 or 5
In an embodiment of the present invention, the preferred substituents of compound of Formula I are:
R and R’ each are H;
R1, R2, R’1 and R’2 each are –CH2CH2OH;
M is Na;
R3 and R4 each are H;
p is 0.25;
q is 2;
n is 0; and
z is 1.
In another embodiment of the present invention, the preferred substituents of compound of Formula I are:
R and R’ each are H;
R1, R2, R’1 and R’2 each are –CH2CH2OH;
M is Na;
R3 and R4 each are –CH3;
p is 0.25;
q is 2;
n is 0; and
z is 1.
In an embodiment of the present invention, the preferred substituents of compound of Formula I are:
R and R’ each are H;
R1 and R’1 each are –CH2CH2OH;
R2, and R’2 each are H;
M is Na;
R3 and R4 each are –CH3;
p is 0.2;
q is 2;
n is 0; and
z is 1.
In an embodiment of the present invention, the preferred substituents of compound of Formula I are:
R and R’ each are H;
R1 and R’1 each are –CH2CH2OH;
R2 and R’2 each are –CH3;
M is Na;
R3 and R4 each are –CH3;
p is 0.15;
q is 2;
n is 0; and
z is 1.
In an embodiment of the present invention, the preferred substituents of compound of Formula I are:
R and R’ each are H;
R1, R2, R’1 and R’2 each are –CH2CH2OH;
M is Na;
R3 and R4 each are –CH3;
p is 0.8;
q is 4;
n is 1; and
z is 1.
In an embodiment of the present invention, the preferred substituents of compound of Formula I are:
R and R’ each are H;
R1, R2, R’1 and R’2 each are –CH2CH2OH;
M is Na;
R3 and R4 each are –CH3;
p is 0.75;
q is 6;
n is 2; and
z is 1.
In an embodiment of the present invention, the preferred substituents of compound of Formula I are:
R and R’ each are H;
R1, R2, R’1 and R’2 each are –CH2CH3;
M is Na;
R3 and R4 each are –CH3;
p is 0.7;
q is 6;
n is 2; and
z is 1.
The present invention also discloses a stable aqueous solution for optically brightening an article. The aqueous solution comprises compound of formula I having the structure

Formula I
wherein,
R and R’ are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl;
R1 and R’1 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2CH2CONH2, -CH2CH2CN, -CH2CH2COO-, -CH2COO-;
R2 and R’2 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2COO-, CH(COO-)CH2COO-, CH(COO-)CH2CH2COO-, CH2CH2SO3-;
R3 and R4 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2CH2CONH2;
M+ represents H+, alkali metal cation, alkaline earth metal cation, ammonium, or a quaternary ammonium cation derived from mono-, di- or tri- C1-C4 alkylamine; mono- C1-C4 alkyl, di- C1-C4 alkanolamine; di- C1-C4 alkyl, mono- C1-C4 alkanolamine; mono-, di- or tri- C1-C4 alkanolamine, or a mixture thereof, with ‘p’ less than or equal to 0.75 and ‘q’ is the number of functional groups in the optical brightener molecule, that are capable of forming a quaternary ammonium salt;
n = 0, 1 or 2; and
z = 1, 2, 3, 4 or 5
Another aspect of the present invention relates to a process for preparing compounds of Formula I. The process comprises of reacting compound of Formula II with an amine of Formula III and a suitable alkali metal hydroxide; alkaline metal hydroxide; mono-, di- or tri- C1-C4 alkylamine; mono- C1-C4 alkyl, di- C1-C4 alkanolamine; di- C1-C4 alkyl, mono- C1-C4 alkanolamine; mono-, di- or tri- C1-C4 alkanolamine, or a mixture thereof. The process further comprises of mixing of compound of Formula II with an amine of Formula III in the presence of water and adjusting the pH to 8 to 8.5 with a suitable inorganic or organic base.

Formula II
wherein,
R and R’ are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl;
R1 and R’1 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2CH2CONH2, -CH2CH2CN, -CH2CH2COO-, -CH2COO-;
R2 and R’2 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2COO-, CH(COO-)CH2COO-, CH(COO-)CH2CH2COO-, CH2CH2SO3-;
n = 0, 1 or 2

Formula III
wherein,
R3 and R4 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2CH2CONH2; and
z = 1, 2, 3, 4 or 5.
Another embodiment of the present invention relates to a stable aqueous solution comprising optical brighteners of Formula I prepared from bistriazinyl derivatives of 4,4’-diaminostilben-2,2’-disulphonic acid of Formula II and an amine of Formula III along with a suitable alkali metal hydroxide; alkaline metal hydroxide; mono-, di- or tri- C1-C4 alkylamine; mono- C1-C4 alkyl, di- C1-C4 alkanolamine; di- C1-C4 alkyl, mono- C1-C4 alkanolamine; mono-, di- or tri- C1-C4 alkanolamine, or a mixture thereof.
Preferably, the amines of Formula III used in the present invention are 2-(2-Aminoethoxy)ethanol and 2-[2-(Dimethylamino)ethoxy]ethanol.
In a preferred embodiment, compound of Formula I is obtained by directly adding compound of Formula II, in the form of a dry powder or a wet-cake, to the solution containing the amine of Formula III and finally adjusting the pH of the resulting solution to 8 to 8.5 with the help of a base. The base used for adjusting the pH may be any inorganic or organic base, such as NaOH, KOH, LiOH, mono-, di- or tri-alkylamine or mono-, di- or tri-alkanolamine.
In an embodiment of the present invention, compound of Formula I is prepared by mixing compound of Formula II and Formula III not exceeding the stoichiometric amount. They can also be prepared by a treatment of the alkali metal or alkaline earth metal salt of the optical brightening agent with a general Formula IV

Formula IV
Wherein,
R and R’ are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl;
R1 and R’1 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2CH2CONH2, -CH2CH2CN, -CH2CH2COO-, -CH2COO-;

R2 and R’2 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2COO-, CH(COO-)CH2COO-, CH(COO-)CH2CH2COO-, CH2CH2SO3-;
M+ represents alkali metal cation or alkaline earth metal cation;
n = 0, 1 or 2
with a mineral acid such as hydrochloric acid or sulphuric acid or any mono-, di- or tri-carboxylic acid or by a sequential treatment of compound of formula II with a cation-exchange resin followed by amine of the present invention, not exceeding the stoichiometric amount. The pH of the resulting solution is adjusted to 8 to 8.5 with the help of any inorganic or organic base.
The aqueous solutions of these compounds are stable on prolonged storage over a wide range of temperature with minimal or no extra solubilising agent and do not freeze even at temperature as low as -5°C
The aqueous solutions of the present invention may optionally contain one or more further whitening agents, one or more carriers, antifreezes, defoamers, solubilising aids, preservatives, complexing agents, inorganic salts, etc., as well as organic by-products formed during the synthesis of the optical brightening agent.
Surprisingly, it has been found that the use of amines of the present invention in the process leads to the formation of an exceptionally stable solution of optical brightening agents of Formula I.
The present invention also discloses a method of optically brightening an article. The method comprises of adding compound of Formula I to the article. Preferably, aqueous solution of compound of Formula I is added to the article to make it optically brightened. The suitable articles for optical brightening are paper, paperboards, woven and non-woven textiles, etc.

Any conventionally known process can be incorporated for optical brightening of an article with compound of Formula I.
The optical brighteners of compound of Formula I are used for optically brightening or whitening articles such as woven and non-woven fabrics, paper, paperboards and the like.
The optical brighteners of the present invention can be easily manufactured, safely handled and conveniently used. The stable aqueous solution of the optical brighteners obtained as per the present invention, causes minimum harm to the environment during manufacturing as large excess of solubilizing agents are not incorporated in the process. Still, more preferably, extra solubilizing agents are not used in the solution of optical brighteners.
The aqueous solution of optical brighteners of the present invention is highly stable over a wide range of temperature for a prolonged duration and therefore remains in usable condition during storage and transportation.
EXAMPLES
The following examples illustrate the present invention but are not limiting thereof.
EXAMPLE 1
108g wet-cake containing 42g of the compound of Formula V and 0.1% salt content was homogeneously mixed with 50g demineralized water. 7.2g 2-(2-aminoethoxy)ethanol was added and the resulting slurry was mixed with vigorous stirring. 30% aqueous sodium hydroxide solution was added drop wise to the slurry till a clear solution was obtained. The pH was finally adjusted between 8 - 8.5 with the help of sodium hydroxide solution. The resulting liquid was clarified through a hyflo bed. The hyflo bed was rinsed with little quantity of demineralized water. Weight of the clear filtrate was adjusted to 200g with demineralized water.

Formula V
Extinction value of the resulting clear solution of Compound A was found to be 137.3. The solution was stored at 25°C and -5°C to check the storage stability. The solution was found to be stable for a period of at least 180 days at these temperatures. No crystal formation or turbidity was observed in the solution during and after the storage.

Compound A
EXAMPLE 2
107.4g wet-cake containing 42g of the compound of Formula V and 0.1% salt content was homogeneously mixed with 50g demineralized water. 9.1g 2-(2-(dimethylamino)ethoxy)ethanol was added and the resulting slurry was mixed with vigorous stirring. 30% aqueous sodium hydroxide solution was added drop wise to the slurry till a clear solution was obtained. The pH was finally adjusted between 8 - 8.5 with the help of sodium hydroxide solution. The resulting liquid was clarified through a hyflo bed. The hyflo bed was rinsed with little quantity of demineralized water. Weight of the clear filtrate was adjusted to 200g with demineralized water.
Extinction value of the resulting clear solution of Compound B was found to be 137. The solution was stored at 25°C and -5°C to check the storage stability. The solution was found to be stable for a period of at least 180 days at these temperatures. No crystal formation or turbidity was observed in the solution during and after the storage.

Compound B
EXAMPLE 3
141g wet-cake containing 61g of the compound of Formula V and 0.12% salt content was homogeneously mixed with 10.5g 2-(2-aminoethoxy)ethanol and the resulting slurry was mixed with vigorous stirring. 30% aqueous sodium hydroxide solution was added drop wise to the slurry till a clear solution was obtained. The pH was finally adjusted between 8 - 8.5 with the help of sodium hydroxide solution. The resulting liquid was clarified through a hyflo bed. The hyflo bed was rinsed with little quantity of demineralized water. Weight of the clear filtrate was adjusted to 200g with demineralized water.
Extinction value of the resulting clear solution of Compound A was found to be 198.8. The solution was stored at 25°C and -5°C to check the storage stability. The solution was found to be stable for a period of at least 180 days at these temperatures. No crystal formation or turbidity was observed in the solution during and after the storage.
EXAMPLE 4
141g wet-cake containing 61g of the compound of Formula V and 0.12% salt content was homogeneously mixed with 13.3g 2-(2-(dimethylamino)ethoxy)ethanol and the resulting slurry was mixed with vigorous stirring. 30% aqueous sodium hydroxide solution was added drop wise to the slurry till a clear solution was obtained. The pH was finally adjusted between 8 - 8.5 with the help of sodium hydroxide solution. The resulting liquid was clarified through a hyflo bed. The hyflo bed was rinsed with little quantity of demineralized water. Weight of the clear filtrate was adjusted to 200g with demineralized water.
Extinction value of the resulting clear solution of Compound B was found to be 200. The solution was stored at 25°C and -5°C to check the storage stability. The solution was found to be stable for a period of at least 180 days at these temperatures. No crystal formation or turbidity was observed in the solution during and after the storage.
EXAMPLE 5
120g wet-cake containing 40g of the compound of Formula VI and 0.12% salt content was homogeneously mixed with 50g demineralized water. 10.3g 2-(2-(dimetylamino)ethoxy)ethanol was added to the mixture and the resulting slurry was mixed with vigorous agitation. 30% aqueous sodium hydroxide solution was added drop wise to the slurry till a clear solution was obtained. The pH was finally adjusted between 8 - 8.5 with the help of sodium hydroxide solution. The resulting liquid was clarified through a hyflo bed. The hyflo bed was rinsed with little quantity of demineralized water. Weight of the clear filtrate was adjusted to 200g with demineralized water.

Formula VI
Extinction value of the resulting clear solution of Compound C was found to be 136.2. The solution was stored at 25°C and -5°C to check the storage stability. The solution was found to be stable for a period of at least 180 days at these temperatures. No crystal formation or turbidity was observed in the solution during and after the storage.

Compound C

EXAMPLE 6
120g wet-cake containing 40g of the compound of Formula VII and 0.12% salt content was homogeneously mixed with 50g demineralized water. 10.3g 2-(2-(dimetylamino)ethoxy)ethanol was added to the mixture and and the resulting slurry was mixed with vigorous agitation. 30% aqueous sodium hydroxide solution was added drop wise to the slurry till a clear solution was obtained. The pH was finally adjusted between 8 - 8.5 with the help of sodium hydroxide solution. The resulting liquid was clarified through a hyflo bed. The hyflo bed was rinsed with little quantity of demineralized water. Weight of the clear filtrate was adjusted to 200g with demineralized water.

Formula VII
Extinction value of the resulting clear solution of Compound D was found to be 136.2. The solution was stored at 25°C and -5°C to check the storage stability. The solution was found to be stable for a period of at least 180 days at these temperatures. No crystal formation or turbidity was observed in the solution during and after the storage.

Compound D
EXAMPLE 7
53.7g dry powder containing 47.28g of the compound of Formula VIII and 2.3% salt content was dispersed in 100g demineralized water. 6.4g 25% aqueous sulphuric acid was added and the mixture was mixed with vigorous agitation. 4.3g 2-(2-(dimetylamino)ethoxy)ethanol was added to the mixture and the resulting slurry was mixed with vigorous agitation. The pH was finally adjusted between 8 - 8.5 with the help of 30% aqueous sodium hydroxide solution. The resulting liquid was clarified through a hyflo bed. The hyflo bed was rinsed with little quantity of demineralized water. Weight of the clear filtrate was adjusted to 200g with demineralized water.

Formula VIII
Extinction value of the resulting clear solution of Compound E was found to be 130. The solution was stored at 25°C and -5°C to check the storage stability. The solution was found to be stable for a period of at least 180 days at these temperatures. No crystal formation or turbidity was observed in the solution during and after the storage.

Compound E
EXAMPLE 8
41.8g dry powder containing 33.4g of the compound of Formula IX and 2.5% salt content was dispersed in 100g demineralized water. 9g 25% aqueous sulphuric acid was added and the mixture was mixed with vigorous agitation. 6.1g 2-(2-(dimetylamino)ethoxy)ethanol was added to the mixture and the resulting slurry was mixed with vigorous agitation. The pH was finally adjusted between 8 - 8.5 with the help of 30% aqueous sodium hydroxide solution. The resulting liquid was clarified through a hyflo bed. The hyflo bed was rinsed with little quantity of demineralized water. Weight of the clear filtrate was adjusted to 200g with demineralized water.

Formula IX
Extinction value of the resulting clear solution of Compound F was found to be 81. The solution was stored at 25°C and -5°C to check the storage stability. The solution was found to be stable for a period of at least 180 days at these temperatures. No crystal formation or turbidity was observed in the solution during and after the storage.

Compound F
EXAMPLE 9
62.3g wet cake containing 27.72g of the compound of Formula X and 2.5% salt content was dispersed in 50g demineralized water. 16.9g 25% aqueous sulphuric acid was added and the mixture was mixed with vigorous agitation. 11.4g 2-(2-(dimetylamino)ethoxy)ethanol was added to the mixture and the resulting slurry was mixed with vigorous agitation. The pH was finally adjusted between 8 - 8.5 with the help of 30% aqueous sodium hydroxide solution. The resulting liquid was clarified through a hyflo bed. The hyflo bed was rinsed with little quantity of demineralized water. Weight of the clear filtrate was adjusted to 200g with demineralized water.

Formula X
Extinction value of the resulting clear solution of Compound G was found to be 69.8. The solution was stored at 25°C and -5°C to check the storage stability. The solution was found to be stable for a period of at least 180 days at these temperatures. No crystal formation or turbidity was observed in the solution during and after the storage.

Compound G

INVESTIGATIONS
1. Application of the product of Example 1 in paper pulp.
Pulp suspension having 1% by weight of dry fibre with 80% short fibre and 20% long fibre was bought from a vendor and utilized for determining the whitening efficiency of the product from Example 1. The pulp was beaten to a Schopper Riegler wetness of 35°SR, 20% by weight of precipitated calcium carbonate (PCC) was mixed well into the pulp and the slurry was divided into seven parts. The optical brightener solution from Example 1 was added to six of the pulp suspensions obtained above in varying dosages and stirred for 10 min. One part was left without the addition of the optical brightening agent for reference. Paper sheets were made by drawing each of the pulp slurry through a wire mesh. Paper sheets thus obtained were pressed and dried in an oven at 108°C for 2 hours. Whiteness of the dried paper sheets were measured using Data Colour Elerpho Brightness tester. The results are summarised in the following table 1:
Table 1
Sr. No. Dosage of the Optical brightener solution from Example 1 CIE Whiteness Delta
1 0% 75.92 -
2 0.10% 97.23 + 21.31
3 0.20% 108.69 + 32.77
4 0.40% 120.48 + 44.56
5 0.60% 124.96 + 49.04
6 0.80% 129.09 + 53.17
7 1.00% 132.26 + 56.34

The results from table 1 clearly demonstrate the effectiveness of the optical brightener solution. The degree of whiteness in the absence of optical brightener was 75.92 and addition of optical brightener led to a high increase in the degree of whiteness.
2. Application of the product of Example 7 in paper pulp.
Pulp suspension having 1% by weight of dry fibre with 80% short fibre and 20% long fibre was used for determining the whitening efficiency of the product from Example 7. The pulp was beaten to a Schopper Riegler wetness of 35°SR, 20% by weight of precipitated calcium carbonate (PCC) was mixed well into the pulp and the slurry was divided into seven parts. The optical brightener solution from Example 7 was added to six of the pulp suspensions obtained above in varying dosages and stirred for 10 min. One part was left without the addition of the optical brightening agent for reference. Paper sheets were made by drawing each of the pulp slurry through a wire mesh. Paper sheets thus obtained were pressed and dried in an oven at 108°C for 2 hours. Whiteness of the dried paper sheets were measured using Data Colour Elerpho Brightness tester. The results are summarised in the following table 2:
Table 2
Sr. No. Dosage of the Optical brightener solution from Example 7 CIE Whiteness Delta
1 0% 73.84 -
2 0.10% 88.12 + 14.28
3 0.20% 96.45 + 22.61
4 0.40% 103.38 + 29.54
5 0.60% 107.92 + 34.08
6 0.80% 110.35 + 36.51
7 1.00% 112.64 + 38.8

The results from table 2 clearly demonstrate the effectiveness of the optical brightener solution.
3. Application of the product of Example 7 in a size press.
Oxidised Starch solution of 5% solids on dry basis was prepared in distilled water. The prepared solution was divided in to 5 parts. The product from Example 7 was added to six of starch solutions obtained above in varying dosages and one part was left as is for reference. These starch solutions were stirred continuously for 10 min while maintaining the temperature at 75°C. Each of the above 5 starch solutions was applied on base paper strips of known CIE whiteness using zero number laboratory bar coater. After the application, the strips were air dried for 1 hour and the whiteness was measured using Data Colour Elerpho Brightness tester. The results are summarised in the following table 3:

Table 3
Sr. No. Dosage of the product from Example 7 (gpL) CIE Whiteness Delta
1 0 68.56 -
2 1.5 80.84 + 12.28
3 3.5 89.87 + 21.31
4 5.5 94.45 + 25.89
5 7.5 95.28 + 26.72

The results from table 3 clearly demonstrate the effectiveness of the optical brightener solution.
4. Application of the product of Example 8 in paper coating.
The coating composition was prepared by mixing 70 parts of Ground Calcium Carbonate, 30 parts of Ultra Gloss 90, 11.5 parts of Binder (Sternol Latex), 0.75 parts of Lubricant (LB – 50), 0.18 parts CMC (FIN FIX) and 0.12 parts of NaOH in demineralised water so as to get a solid content of 70%. The composition was divided in to 5 parts. The product from Example 8 was added to six of the compositions obtained above in varying dosages and one part was left as is for reference. The composition was stirred for 10 min before applying on base paper strips of known CIE whiteness using zero number laboratory bar coater (Coat weight of 10gsm). After the application, the strips were air dried for 1 hour and the whiteness was measured using Data Colour Elerpho Brightness Tester. The results are summarised in the following table 4:
Table 4
Sr. No. Dosage of the product from Example 8 (gpL) CIE Whiteness Delta
1 0 68.23 -
2 1.5 85.08 + 16.85
3 3.5 95.82 + 27.59
4 5.5 102.84 + 34.61
5 7.5 109.03 + 40.8

The results from table 4 clearly demonstrate the effectiveness of the optical brightener solution.
5. Application of the product of Example 1 in textile (Exhaust Method):
4 pieces of a white 100% cotton bleached fabric weighing 5 ± 0.05g each were used for the exhaust application. Three pieces were immersed in solutions obtained by dissolving varying quantities of the product obtained in Example 1 in 100 mL distilled water at 85-90°C for 45 min. One piece was immersed in distilled water without the optical brightening agent for reference. After 45 min, the pieces were taken out and washed with distilled water and air dried at 25-30°C. Whiteness of the fabric was measured using Data Colour Elerpho Brightness Tester. The results are summarised in the following table:
Table 5
Sr. No. Loading of the product from Example 1 (%) CIE Whiteness
1 0 77.57 --
2 0.2 140.28 + 62.71
3 0.4 145.67 + 68.1
4 0.6 149.13 + 71.56

The results from table 5 clearly demonstrate the effectiveness of the optical brightener solution.
,CLAIMS:1. An optical brightener of Formula I:

Formula I
wherein,
R and R’ independent of each other are hydrogen, C1-C4 straight chain or branched alkyl;
R1 and R’1 independent of each other are hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2CH2CONH2, -CH2CH2CN, -CH2CH2COO-, -CH2COO-;
R2 and R’2 independent of each other are hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2COO-, CH(COO-)CH2COO, CH(COO-)CH2CH2COO-, CH2CH2SO3-;
R3 and R4 independent of each other are hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2CH2CONH2;
M+ represents H+, alkali metal cation, alkaline earth metal cation, ammonium or a quaternary ammonium cation derived from mono-, di- or tri- C1-C4 alkylamine; mono- C1-C4 alkyl, di- C1-C4 alkanolamine; di- C1-C4 alkyl, mono- C1-C4 alkanolamine; mono-, di- or tri- C1-C4 alkanolamine, or a mixture thereof, with ‘p’ less than or equal to 0.8 and ‘q’ is the number of functional groups in the optical brightener molecule, that are capable of forming a quaternary ammonium salt;
n is 0, 1 or 2; and
z = 1, 2, 3, 4 or 5
2. The optical brightener as claimed in claim 1, wherein
R and R’ each are H;
R1, R2, R’1 and R’2 each are –CH2CH2OH;
M is Na;
R3 and R4 each are H;
p is 0.25;
q is 2;
n is 0; and
z is 1.
3. The optical brightener as claimed in claim 1, wherein
R and R’ each are H;
R1, R2, R’1 and R’2 each are –CH2CH2OH;
M is Na;
R3 and R4 each are –CH3;
p is 0.25;
q is 2;
n is 0; and
z is 1.
4. The optical brightener as claimed in claim 1, wherein
R and R’ each are H;
R1 and R’1 each are –CH2CH2OH;
R2, and R’2 each are H;
M is Na;
R3 and R4 each are –CH3;
p is 0.2;
q is 2;
n is 0; and
z is 1.
5. The optical brightener as claimed in claim 1, wherein
R and R’ each are H;
R1 and R’1 each are –CH2CH2OH;
R2 and R’2 each are –CH3;
M is Na;
R3 and R4 each are –CH3;
p is 0.15;
q is 2;
n is 0; and
z is 1.
6. The optical brightener as claimed in claim 1, wherein
R and R’ each are H;
R1, R2, R’1 and R’2 each are –CH2CH2OH;
M is Na;
R3 and R4 each are –CH3;
p is 0.8;
q is 4;
n is 1; and
z is 1.
7. The optical brightener as claimed in claim 1, wherein
R and R’ each are H;
R1, R2, R’1 and R’2 each are –CH2CH3;
M is Na;
R3 and R4 each are –CH3;
p is 0.75;
q is 6;
n is 2; and
z is 1.
8. The optical brightener as claimed in claim 1, wherein
R and R’ each are H;
R1, R2, R’1 and R’2 each are –CH2CH3;
M is Na;
R3 and R4 each are –CH3;
p is 0.7;
q is 6;
n is 2; and
z is 1.

9. A process for preparing an optical brightener of Formula I, the process comprising mixing compound of Formula II and compound of Formula III to obtain optical brightener of Formula I:

Formula II

Formula III
wherein,
R and R’ are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl;
R1 and R’1 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2CH2CONH2, -CH2CH2CN, -CH2CH2COO-, -CH2COO-;
R2 and R’2 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2COO-, CH(COO-)CH2COO, CH(COO-)CH2CH2COO-, CH2CH2SO3-;
R3 and R4 are, independent of each other, hydrogen, C1-C4 straight chain or branched alkyl, C1-C4 straight chain or branched hydroxyalkyl, -CH2CH2CONH2;
z = 1, 2, 3, 4 or 5; and
n = 0, 1 or 2.
10. The process as claimed in claim 9 comprising adjusting pH of the mixture with a base.
11. The process as claimed in claim 10, wherein the base is selected from NaOH, KOH, LiOH, mono- di- or tri-alkyl amine or mono-, di- or tri-alkanolamine.
12. The process as claimed in claim 9 comprising mixing compound of Formula II and compound of Formula III in the presence of water.
13. A stable aqueous solution comprising the optical brightener claimed in claim 1.
14. The stable aqueous solution as claimed in claim 13 optionally comprising additives such as carriers, antifreeze, defoamers, solubilizing agents, preservatives, complexing agents, inorganic or organic salts.
15. A method for optically brightening woven fabrics, non-woven fabrics, paper and paperboards by adding the optical brightener of Formula I as claimed in claim 1.
16. The method as claimed in claim 15 comprising adding an aqueous solution of the optical brightener of Formula I to woven fabrics, non-woven fabrics, paper and paperboards.