The present invention relates to a solution comprising a dye. The invention also relates to a peroxide formulation comprising said solution.

Radically curable resins, such as unsaturated polyester (UP) resins, vinyl ester (VE) resins and (meth)acrylate resins, are usually cured by mixing the resin with an organic peroxide formulation. In addition, a cure accelerator may be added to allow the cure to proceed at lower temperature.

It is generally not visible whether or not the organic peroxide has already been added and if so, whether its dispersion in the resin is homogeneous. For that purpose, peroxide formulations have been developed that contain a dye. The dye indicates the presence of the organic peroxide formulation in the resin, and shows whether or not the formulation is homogeneously dispersed.

If a dye is used that loses its colour during the cure (probably due to radicals attacking the chromophoric group), it is possible to monitor the cure by the fading colour. In addition, the dye will not affect the colour of the cured product.

Suitable dyes for this purpose are solid and need to be dissolved before incorporation in the peroxide formulation. However, these dyes are very difficult to dissolve in many solvents.

Solvents presently used for dissolving such dyes are N-ethyl pyrrolidone and N- methypyrrolidone. These solvents, however, are classified as reprotoxic.

The object of the present invention is therefore the provision of a suitable solvent for such dyes that poses less health risks.

This object has been met by a solvent mixture comprising cyclopentanone and dimethylsulphoxide in a weight ratio 60:40 - 95:5. The present invention therefore relates to a solution comprising:

1 -25 wt% of at least one dye having a structure according to one of the following formulae

wherein

^■ Y is -NO ₂ or -SO ₂ CH ₃ , preferably -NO ₂ ,

^■ R ¹ and R ² are independently selected from -C _X H _2X+I , -C _x H _2x -CºN, -C _X H _2X -OH, -C _X H _2X -0-C(=0)-0-C _X H _2X+I , -C _X H _2X -0-C(=0)-C _X H _2X+I , and

-C _X H _2X -C(=0)-0-C _X H _2X+I , x being an integer in the range 1 -5,

^■ R ³ is selected from H, Cl, Br, -NO2, -OH, and -CºN, preferably H,

^■ R ⁴ and R ⁶ are independently selected from H, Cl, Br, -NO2, -OH, and -CºN, or may together form an aromatic or aliphatic ring, R ⁴ and R ⁶ preferably being H,

^■ n is an integer in the range 0-3, preferably n=0,

^■ each R ⁵ is independently selected from alkyl groups, alkoxy groups, halogens, amide groups, and sulphonamide (-N(H)S0 ₂ CH ₃ ) groups,

75-99 wt% of a solvent mixture, said solvent mixture comprising cyclopentanone and dimethylsulphoxide in a weight ratio 60:40 - 95:5. In the above formulae, R ¹ and R ² are independently selected from -C _X H _2X+I , -

C _X H _2X -CºN C _X H _2x -OH C _X H _2X -0-C(-0)-0-C _X H _2X+I C _X H _2X -0-C(-0)-C _X H _2X+I and -C _X H _2X -C(=0)-0-C _X H _2X+I , X being an integer in the range 1 -5, preferably 1 -2. Specifically preferred R ¹ groups are -C _X H _2X+I and -C _x H _2x -CºN, more preferably -CH ₂ CH ₃ and -CH ₂ -CH ₂ -CºN.

Specifically preferred R ² groups are -C _x H _2x -0-C(=0)-0-C _x H _2x+i and -C _x H _2x -0 - C(=0) -C _X H _2X+I . Specifically preferred examples are -C ₂ H ₄ -0-C(=0)-0-C ₂ H ₅ and -C ₂ H ₄ -0-C(=0)-CH ₃ . Specific examples of suitable dyes with formula

are:

Disperse Blue 79, i.e. 2-[5-acetamido-N-(2-acetyloxyethyl)-4-[(2-bromo-4,6- dinitrophenyl)diazenyl]-2-ethoxyanilino]ethyl acetate, in which Y=-N0 ₂ , R ¹ =R ² =- C ₂ H ₄ -0-C(=0)-CH ₃ , R ³ =Br, R ⁴ =N0 ₂ , n=2, one R ⁵ is -0-C ₂ H ₅ ; the other is -N(H)-

C(=0)-CH ₃ , R ⁶ =H.

Disperse Brown 1 , i.e. 2-[3-chloro-4-[(2,6-dichioro-4-nitrophenyl)diazenyl]-N- (2-hydroxyethyl)anilino]ethanoi, in which Y=-N0 ₂ , R ¹ =R ² = -C ₂ H ₄ -OH, R ³ =R ⁴ =CI, n=1 , R ⁵ = Cl, R ⁶ =H.

Disperse Violet 24, i.e. 2-[[4-[2-(2-Bromo-4,6-dinitrophenyl)diazenyl]-3-methyl phenyl]butylamino]ethanol, in which Y=-N0 ₂ , R ¹ =C ₄ H ₉ , R ² = -C ₂ H ₄ -OH, R ³ =Br, R ⁴ =N0 ₂ , n=1 , R ⁵ =CH ₃ , R ⁶ =H.

Disperse Red 73, i.e. 2-[[4-[2-cyanoethyl(ethyl)amino]phenyl]diazenyl]-5- nitrobenzonitrile, in which Y=-N0 ₂ , R ¹ =C ₂ H ₅ , R ² = -C ₂ H ₄ -CºN, R ³ =H, R ⁴ = -CºN, n=0, R ⁶ =H. Disperse Orange 25, i.e. 3-[A/-Ethyl-4-(4-nitrophenylazo)phenylamino] propionitrile, in which Y=-N0 ₂ , R ¹ =C ₂ H ₅ , R ² = -C ₂ H ₄ -CºN, R ³ =R ⁴ =H, n=0, R ⁶ =H. Disperse Red 1 , i.e. A/-Ethyl-/V-(2-hydroxyethyl)-4-(4-nitrophenylazo)aniline, in which

Specific examples of suitable dyes with formula

are:

Disperse Blue 148, i.e. methyl N-ethyi-N-(4-((5-nitro-2,1-benzisothiazol-3- yl)azo)phenyl)-beta-alaninate, in which Y=-N0 ₂ , R ¹ =C ₂ H ₅ , R ² = -C ₂ H ₄ -C(=0)-0- CHs, n=0

Disperse Red 177, i.e. 2-((2-cyanoethyl)(4-((6-nitrobenzothiazol-2-yl)azo) phenyl)amino)ethyl acetate, in which Y=-N0 ₂ , R ¹ =-C ₂ H ₄ -CºN, R ² = -C ₂ H ₄ -0- C(=0)-CH ₃ , n=0

Disperse Red 179, i.e. 3-[N-ethyl-[3-methyl-4-(6-nitrobenzothiazol-2- yl)diazenyl-phenyl]amino]propanenitrile, in which Y=-N0 ₂ , R ¹ =C ₂ H ₅ , R ² = -C ₂ H ₄ - CºN, n=1 , and R ⁵ = CH ₃

Disperse Red 145, i.e. 3-[N-ethyl-4-[(6-nitro-1 ,3-benzothiazol-2-yl)diazenyl] anilino]propanenitrile, in which Y=-N0 ₂ , R ¹ =C ₂ H ₅ , R ² = -C ₂ H ₄ -CºN, n=0

Disperse Red 154, i.e. 2-[2-cyanoethyl][4-[6-nitrobenzothiazol-2-yl)azo]phenyl] amino]ethyl ethyl carbonate, in which Y=-N0 ₂ , R ¹ = -C ₂ H ₄ -CºN, R ² =-CH ₂ CH ₂ -0- C(=0)-0-C ₂ H ₅ , n=0

Disperse Red 177, i.e. 3-[[2-(acetyloxy)ethyl][4-[2-(6-nitro-2-benzothiazolyl) diazenyl]phenyl]amino]propanenitrile, in which Y=-N0 ₂ , R ¹ = -C ₂ H ₄ -CºN, R ² =- CH ₂ CH ₂ -0-C(=0)-CH ₃ , n=0 The latter two dyes, i.e. Disperse Red 154 and Disperse Red 177, are the preferred dyes for use in the present invention. These dyes are the most compatible with organic peroxides and have the best discoloration properties.

The solution according to the present invention (“the dye-containing solution”) contains 1 -25 wt%, more preferably 2-20 wt%, and most preferably 5-15 wt% of the dye, dissolved in a mixture cyclopentanone and dimethylsulphoxide (DMSO).

Said mixture comprises cyclopentanone and dimethylsulphoxide in a cyclopentanone : dimethylsulphoxide weight ratio of 60:40 to 95:5, preferably 70:30 - 90:10, most preferably 70:30-80:20.

Said solvent mixture preferably consist solely of cyclopentanone and dimethylsulphoxide.

The present invention also relates to a peroxide formulation comprising:

^■ one or more organic peroxides and

^■ 0.05-2.0 wt%, preferably 0.1 -1.5 wt%, most preferably 0.2-1.0 wt% of the dye-containing solution described above,

wherein the active oxygen content of the formulation is the range 2-15 wt%, preferably 4-12 wt%, and most preferably 6-10 wt%.

The active oxygen content of the formulation can be determined by iodometric titration, according to ASTM D2180-89(2008).

Examples of suitable organic peroxides are ketone peroxides, organic hydroperoxides, peroxyesters, and combinations thereof.

Examples of ketone peroxides are acetylacetone peroxide (AAP), methyl ethyl ketone peroxide (MEKP), methyl isopropyl ketone peroxide (MiPKP), cyclohexanone peroxide, cyclopentanone peroxide, methyl isobutyl ketone peroxide (MiBKP), and combinations thereof. Ketone peroxides can have the formula

These formulae represent methyl isopropyl ketone peroxide when R ¹ and R ⁴ are methyl and R ² and R ³ are isopropyl. They represent methyl ethyl ketone peroxide when R ¹ and R ⁴ are methyl and R ² and R ³ are ethyl. They represent methyl isobutyl ketone peroxide when R ¹ and R ⁴ are methyl and R ² and R ³ are isobutyl. They represent cyclohexanone peroxide when R ¹ and R ² are linked to form a cyclohexane ring and R ³ and R ⁴ are linked to form a cyclohexane ring. They represent cyclopentanone peroxide when R ¹ and R ² are linked to form a cyclopentane ring and R ³ and R ⁴ are linked to form a cyclopentane ring.

The first formula reflects a so-called type-4 (T4) ketone peroxide; the second formula a so-called type-3 (T3) ketone peroxide. Both types are generally present in ketone peroxide formulations, in addition to hydrogen peroxide.

Examples organic hydroperoxides are cumyl hydroperoxide (CHP), isopropyl cumyl hydroperoxide, 1 ,1 ,3,3-tetramethylbutyl hydroperoxide, tert-butyl hydroperoxide (TBHP), tert-amyl hydroperoxide, pinane hydroperoxide, para- menthane hydroperoxide, and combinations thereof.

Examples of peroxyesters are tert-butyl peroxybenzoate (TBPB), tert-butyl peroxy-2-ethylhexanoate, 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane, tert-butylperoxy-3,5,5-trimethylhexanoate, and combinations thereof.

In addition to the peroxide and the dye-containing solution, other compounds may be present in the peroxide formulation, such as an organic solvent other than cyclopentanone and dimethylsulphoxide. This solvent may serve as a diluent/phlegmatizer for the organic peroxide. Suitable solvents are compatible with cyclopentanone and dimethylsulphoxide and at the same time are able to stabilize organic peroxides. Examples of suitable solvents are phthalates, such as dimethyl phthalate or diisononyl phthalate, 2,2,4-trimethyl pentanediol diisobutyrate (TXIB), mineral spirits, diacetone alcohol, and methylbenzoate.

Furthermore, other materials such as diethyl acetoacetamide, 2,4- pentanedione, water, residual ketone, H2O2, and fillers (e.g. silica or clay) may be present in the peroxide formulation.

The invention further relates to a process for curing a radically curable resin using the peroxide formulation.

Suitable radically curable resins to be cured according to the invention include alkyd resins, unsaturated polyester (UP) resins, vinyl ester resins, and (meth)acrylate resins. Preferred resins are (meth)acrylate resins, UP resins, and vinyl ester resins. The radically curable resin may be combined with other types of resins, such as epoxy resins.

In the context of the present application, the terms“unsaturated polyester resin” and “UP resin” refer to the combination of unsaturated polyester resin and ethylenically unsaturated monomeric compound. The term“vinyl ester resin” refers to a resin produced by the esterification of an epoxy resin with an unsaturated monocarboxyl ic acid, and dissolved in an ethylenically unsaturated monomeric compound (e.g. styrene). UP resins and vinyl ester resins as defined above are well known and commercially available.

Suitable UP resins to be cured by the process of the present invention are so- called ortho-resins, iso-resins, iso-npg resins, and dicyclopentadiene (DCPD) resins. Examples of such resins are maleic, fumaric, allylic, vinylic, and epoxy- type resins, bisphenol A resins, terephthalic resins, and hybrid resins.

Acrylate and methacrylate resins without an additional ethylenically unsaturated monomeric compound like styrene are referred to in this application as (meth)acrylate resins. Examples of suitable ethylenically unsaturated monomers to be present in UP and vinyl ester resins include styrene and styrene derivatives like a-methyl styrene, vinyl toluene, indene, divinyl benzene, vinyl pyrrolidone, vinyl siloxane, vinyl caprolactam, stilbene, but also diallyl phthalate, dibenzylidene acetone, allyl benzene, methyl methacrylate, methylacrylate, (meth)acrylic acid, diacrylates, dimethacrylates, acrylamides; vinyl acetate, triallyl cyanurate, triallyl isocyanurate, allyl compounds which are used for optical application (such as (di)ethylene glycol diallyl carbonate), chlorostyrene, tert-butyl styrene, tert- butylacrylate, butanediol dimethacrylate, and mixtures thereof. Suitable examples of (meth)acrylate reactive diluents are PEG200 di(meth)acrylate, 1 ,4- butanediol di(meth)acrylate, 1 ,3-butanediol di(meth)acrylate, 2,3-butanediol di(meth)acrylate, 1 ,6-hexanediol di(meth)acrylate and its isomers, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, glycerol di(meth)acrylate, trimethylol propane di(meth)acrylate, neopentyl glycol di(meth)acrylate, dipropylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, PPG250 di(meth)acrylate, tricyclodecane dimethylol di(meth)acrylate, 1 ,10-decanediol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, trimethylol propane tri(meth)acrylate, glycidyl (meth)acrylate, bismaleimides, biscitraconimides, bisitaconimides, monoitaconimides, monocitraconimides, monocitraconimides containing a functional group - e.g. an additional unsaturated functionality, for example n-allylcitraconimides - and mixtures thereof.

The amount of ethylenically unsaturated monomer to be used is preferably at least 0.1 wt%, based on the weight of the resin, more preferably at least 1 wt%, and most preferably at least 5 wt%. The amount of ethylenically unsaturated monomer is preferably not more than 50 wt%, more preferably not more than 40 wt%, and most preferably not more than 35 wt%.

The amount of peroxide formulation to be used in the curing process is preferably at least 0.1 wt%, more preferably at least 0.5 wt%, and most preferably at least 1 wt%, based on the weight of resin. The amount of peroxide formulation is preferably not more than 8 wt%, more preferably not more than 5 wt%, most preferably not more than 2 wt%, based on the weight of resin.

Other optional additives may be employed in the curing process according to the invention, such as cure accelerators, fillers, glass fibres, pigments, inhibitors, and promoters.

In the curing process of the present invention, the resin is first mixed with the monomer. The peroxide formulation can be added directly to the resulting mixture. However, it can also be first mixed with the monomer or resin.

The curing process can be carried out at any temperature from -5°C up to 250°C. Preferably, it is carried out at ambient temperatures commonly used in applications such as hand lay-up, spray-up, filament winding, resin transfer moulding, coating (e.g. gel-coat and standard coatings), button production, centrifugal casting, corrugated sheets or flat panels, relining systems, kitchen sinks via pouring compounds, etc. However, it can also be used in SMC, BMC, pultrusion techniques, and the like, for which temperatures up to 180°C, more preferably up to 150°C, most preferably up to 100°C are used. EXAMPLES

Example 1 - solubility in various solvents

1 .0 gram of Disperse Red 154 powder was stirred for 30 minutes in 9.0 g solvent or solvent mixture. Samples of the resulting mixture were subsequently stored for at least 12 hours at different temperatures: -15°C, 5°C, and 20°C.

After storage, the mixture was filtered. The residue was dried and its weight was determined. Table 1 - solubility in different solvents

Solubility (wt%)

-15°C 5°C 20°C

DMSO froze froze >10

Cyclopentanone (CP) 7.34 8.40 9.35

Cylcohexanone (CH) 3.79 4.19 4.68

Propylene carbonate (PC) 2.24 2.22 2.41

Dimethylphthalate (DMP) 2.13 2.15 2.26

Poly(methyl acrylate) (PMA) 1.58 1.56 1.66

Diacetone alcohol (DAA) 0.87 0.88 1 .1 1

Trimethyl Pentanyl Diisobutyrate (TXIB) 0.67 0.69 0.91

PC/DAA (90/10) 2.69

PC/DAA (80/20) - 2.79

PC/DAA (50/50) T89 T77 2.54

PC/PMA (50/50) T50 2L Ϊ 2.25

DMP/TXIB (50/50) - 1.50

CP/DMSO (90/10) 09 ^ 11.58

CP/DMSO (80/20) 10.11 12.60

CP/DMSO (70/30) 12.57 ^ 13.98

CP/DMSO (50/50) froze 07 9.66

CP/DEG ² - 7.78

CP/TEP" (50/50 - 7.68

CH/DAA (80/20) - 4.78

CH/DAA (50/50) 3.28

¹ - = not analysed

² DEG = diethylene glycol

³ TEP = triethyl phosphate Example 2

2.5 grams of Disperse Red 177 powder was stirred for 24 hours at room temperature in 22.5 grams of a CP/DMSO 80/20 mixture. Disperse Red 177 completely dissolved.

Example 3

2.5 gram of Disperse Red 1 powder was stirred for 24 hours at room temperature, followed by 4 hours at 40°C, in 22.5 grams of a CP/DMSO 80/20 mixture. Disperse Red 1 completely dissolved.

Example 4 - chemical stability of peroxide formulations

Peroxide formulations were prepared by adding 0.2 g of a freshly made solution of 10 wt% Disperse Red 154 in 80/20 CP/DMSO to 100 g of different peroxide formulations.

The formulations were stored in 30 ml glass bottles for 4 or 8 weeks at 30°C or 40°C.

After storage, the decrease in total active oxygen content was determined by way of iodometric titration according to ASTM D2180-89(2008). Samples having a decrease in total active oxygen content of >10% after eight weeks storage at 40°C are considered unstable.

Table 2 - Relative active oxygen loss (%) after t=weeks/temperature

Peroxide Dye sol. 4 wks 4 wks 8 wks 8 wks

_ (wt%) 30°C 40°C 30°C 40°C

Butanox® M-50 0.2 0.7 1 .6 0.5 2.3

50% MEKP in

dimethyl phthalate _

Trigonox® 75 0.2 0.0 0.4 0.1 1 .3

VRN

MEKP and TBHP in

dimethyl phthlate _

Trigonox® 44B 0.2 0.7 3.7 0.7 6.9

33% AAP in DEG,

water, and diactone

alcohol _

Trigonox® 239 0.4 0.6 4.7 1 .3 7.3

45%CHP in solvent

mixture _

Butanox® P-50 0.2 0.8 2.6 1 .1 6.1

50% MiPKP in

dimethyl phthalate _

Cadox® D-50 0.5 0.2 0.7 0.3 4.8

50% MEKP in TXIB _

Cadox® D-50 1 .1 0.3 0.9 0.3 8.4

50% MEKP in TXIB _

Cadox® L-50A 0.5 0.1 0.6 0.2 7.5

50% MEKP in TXIB _

Cadox® M-50A 0.5 0.1 0.4 0.2 1 .9

50% MEKP in TXIB _

Butanox® M-60 0.1 - - 0.6 1 .8

60% MEKP in

dimethyl phthalate _

Butanox® LPT-IN 0.2 - - 0.35 1 .4

MEKP in diisononyl

phthalate _

Trigonox® 249 0.2 - - 0.4 1 .5

MEKP and CHP in

dimethyl phthalate _

Trigonox® 289 0.2 - - 0 0.6

MEKP and CHP in

dimethyl phthalate

and methyl benzoate _

Trigonox® V388 0.2 - - 1 .1 4.7

MEKP in solvent

mixture Example 5

A gelcoat was cured with Butanox M-50, with and without 0.2 wt% of the 10 wt% Disperse Red 154 in 80/20 CP/DMSO solution. The colour of the mold- and back side of the resulting gel coat was following in time using an X-rite Color I-5. It showed that red colour of the mould side disappeared completely during the curing reaction; the back side did retain some red colour after cure, which faded completely after one month of storage.