Full spectrum fluorescent dye composition for the optimization of leak detection processes

A fluorescent dye composition for inclusion in a fluid for leak detection applications is provided which is responsive to a broad range of UVA, blue, and UVA-blue light sources. The composition comprises a mixture of a perylene dye, a naphthalimide dye, and a hydrocarbon-based fluid, wherein the combination of the perylene dye, the naphthalimide dye and the hydrocarbon-based fluid are miscible in the fluid. A fluorescent dye composition is also provided that is effective for all automotive working fluids, including heavy lubricants and transmission fluids, synthetic lubricants and internal combustion oils, refrigerants, liquid and gaseous fuels, and hydraulic fluids. A fluorescent dye composition is also provided containing only a combination of perylene dye and naphthalimide dye.

FIELD OF THE INVENTION 
This invention relates, generally, to the field of leak detection by 
illumination of fluorescent material, and more particularly to fluorescent 
dye compositions utilized in leak detection. 
BACKGROUND OF THE INVENTION 
Fluorescent additives provide an excellent leak detection technique for 
determining the site of leakage of an operating fluid from a working 
system. Operating fluids such as lubricants, hydraulic fluids, heat 
transfer fluids, and refrigerants are treated with a dye additive which 
fluoresces when illuminated by suitable ultraviolet or visible blue light. 
Fluorescence is generally understood to be a property that enables certain 
materials to absorb light energy and radiate visible light at a longer 
wavelength than the absorbed light. According to generally accepted 
theory, electrons in fluorescent materials are excited upon being 
illuminated by light energy of a specific wavelength, and light energy of 
a longer wavelength is radiated from these materials as the electrons 
return to the unexcited or ground state. The specific excitation and 
radiation wavelengths are characteristics of the particular fluorescent 
materials. The apparent brightness of a fluorescent material's 
luminescence is dependent on the wavelength emitted by the material and 
the intensity of the incident radiation that excites the material. For 
example, a fluorescent dye which has its excitation peak at a specific 
wavelength may quickly emit a much reduced luminescence as the wavelength 
of incident light deviates from the excitation peak, and will lose the 
ability to fluoresce when the incident light does not have enough energy 
within the specific excitation range. 
The visibility of the fluorescent response is much increased when the 
intensity of other visible light is reduced, so that the fluorescent 
response is not masked or washed-out by other light. Thus, 
ultraviolet/blue leak detection lamps directed in otherwise dark 
conditions at an operating system containing a UV/blue responsive 
fluorescent dye will reveal leak sites which glow against the dark 
background. 
The most common UV/blue fluorescent leak detection dyes used today are 
either perylene-based fluorescent compounds or naphthalimide-based 
fluorescent compounds. Perylene dyes produce an intense yellow fluorescent 
response when exposed to incident radiation in a band of the 
electromagnetic spectrum which includes the long wave ultraviolet (UV-A) 
wavelength range of about 315 nm to about 400 nm, with a strong peak 
between about 340 to 375 nm. Long-wave ultraviolet is also referred to as 
"black light", as it includes a small segment of the visual violet range. 
Naphthalimide dyes fluoresce a brilliant green when exposed to incident 
radiation of visible violet/blue light. The visible violet/blue range 
extends from about 400 nm to about 480 nm within the electromagnetic 
spectrum. Both perylene and naphthalimide dyes are useful for leak 
detection in oil-based working fluids or fluids in which oil is miscible. 
The various inspection lamps used to illuminate the exterior of the 
fluid-containing portions of the operating systems can be grouped by 
emission wavelength into three general types: (a) UV filter (see FIG. 1), 
(b) UV/BLUE glass filter (FIG. 2) and (3) broad spectrum UV to blue thin 
film filters (FIG. 3). 
The UV filter lamps emit long wave ultraviolet light which provides optimal 
energy for use with the perylene dyes. These high intensity/ narrow band 
UV lamps will also emit enough energy in the naphthalimide dyes' broader 
excitation band to produce a fluorescent response. However, these high 
intensity lamps tend to be larger than those used to provide visible light 
and can be cumbersome in situations where working space is very limited. 
The lower intensity lamps with UV/BLUE glass filters provide visible 
violet/blue illumination are optimally suited for use with the 
naphthalimide dyes, whose excitation peak lies within this wavelength 
band. These lamps may cause the perylene dyes to fluoresce, though only 
poorly. These lamps are also typically less expensive and more compact 
than high intensity lamps. 
The third type is the relatively new broad spectrum lamps which use thin 
film filters to pass a substantial amount of visible violet to blue light 
as well as long wave ultraviolet light. These lamps are the most 
versatile, as they can be used effectively with both perylene and 
naphthalimide fluorescent dyes, and can provide intense illumination even 
in compact form. Being relatively new, however, there are fewer of these 
lamps in use than the other types. 
The sensitivity of the human eye to low Intensity light is greatest for 
wavelengths between about 540-570 nm (yellow green). Perylene dyes emit an 
intense, but narrow band, fluorescent response, which has a main intensity 
peak between about 520 and 550, and a smaller peak between about 560 to 
580. Thus, the color of the perylene fluorescent response is close to that 
wavelength range which the human eye is most sensitive, and the perylene 
dyes exhibit a more intense fluorescence than that of the naphthalimide 
dyes. Under ideal conditions in clean, clear fluids, perylene dyes provide 
a superior fluorescence to that of naphthalimides. 
However, what the leak inspector sees is the combination of the 
fluorescence of the dye and any natural fluorescence of the fluid. This 
combination may "wash out" the apparent brightness. Many of the lubricants 
to which fluorescent leak detection dyes are applied have some natural 
fluorescent response to UV/visible blue light energy. These fluids may 
also have a color or may be tainted with contaminants which can mask the 
fluorescent response of the leak detection dye. The combination of a 
natural fluorescence from host fluids, and the presence of contaminants 
(such as dirt or combustion residue) may mask the fluorescent response of 
a perylene dye. 
Naphthalimide dyes emit a broad fluorescent response with an intensity peak 
between about 480-520 nm. The naphthalimide dyes do not fluoresce as 
brilliantly as the perylenes, but the green fluorescence of a 
naphthalimide dye will not be as noticeably diminished by the natural 
fluorescence of a host fluid. 
The majority of fluorescent leak detection dyes presently being marketed 
for use in oil-based working fluids, or in fluids in which oil is soluble, 
employ perylene-based dye materials. These dyes are applied in lubricating 
systems, fuel systems, transmission fluids, and hydraulic fluids. 
Naphthalimide dyes are primarily used in applications within some air 
conditioning and refrigeration systems where polyolester or polyalkylene 
glycol-based lubricants are used with R-134a or similar refrigerant. 
Typical automotive fluids encompass a wide range of physical properties and 
include air conditioning refrigerant, engine lubricating oil, transmission 
fluid, brake fluid, power steering fluid, radiator coolant, diesel oil, 
and gasoline. In an auto repair facility, where diagnostic leak detection 
using fluorescent dyes is commonly performed on both air conditioning and 
other fluid systems, there is generally only one inspection lamp available 
for leak detection purposes. This light source may not be optimally, or 
even altogether, effective on the dye in the target fluid. 
It would be desirable to have a fluorescent dye for the various types of 
working fluids which is effective when used with any of the available 
inspection lamps. It would also be desirable to have a dye which is 
effective in all automotive working fluids. 
Thus, one objective of this invention is to create blended dye compositions 
of perylene and naphthalimide dye in various proportions to create 
effective dyes for particular fluids with a substantial fluorescent 
response to illumination by the entire range of inspection lamps. Another 
objective is to create a single all-purpose blend that is satisfactory for 
all automotive fluids and all inspection lamps. 
SUMMARY OF THE INVENTION 
In its general aspect, the invention is a fluorescent dye composition for 
leak detection of working fluids, the composition comprising a mixture of 
a perylene dye, a naphthalimide dye, and a hydrocarbon, wherein the 
combination of the perylene dye, the naphthalimide dye and the hydrocarbon 
are miscible in the working fluids. 
In its more restricted aspects, the invention is a fluorescent dye 
composition tailored to the physical and chemical properties of a working 
fluid. 
In another aspect, the invention is a fluorescent dye composition effective 
for all automotive working fluids, including heavy lubricants and 
transmission fluids, synthetic lubricants and internal combustion oils, 
refrigerants, liquid and gaseous fuels, and hydraulic fluids.

DETAILED DESCRIPTION OF THE INVENTION 
FIGS. 1 through 3 show the irradiance output of the three common types of 
UV/BLUE inspection lamps commonly used in leak detection. FIG. 1 shows the 
transmission spectra characteristic of a high intensity UV light source 
having a UVA filter. This lamp produces an intense output essentially 
restricted to a narrow band between 330 nm and 390 nm. FIG. 2 shows the 
characteristic transmission curve from a lamp having a broad UV/blue glass 
filter. Here, a lower intensity light is spread over a broader wavelength 
range of between about 350 nm and about 510 nm, with a peak intensity at 
about 420 nm. FIG. 3 shows a transmission curve characteristic of another 
type of UV/blue lamp which uses thin-film coating technology to construct 
a filter which passes a broader band of relatively uniform intensity 
between about 320 nm and 148 nm. 
FIG. 4 shows the excitation spectra of a fluorescent perylene dye in the UV 
and visible spectrum. As used herein, the term "perylene dye" refers to 
the class of water-insoluble organic dyes that includes perylene and 
substituted perylene. The perylene dye shows good excitation in the long 
wave ultraviolet (UV-A) wavelength range, with a strong peak between about 
340 to 375 nm. The dye also shows moderate excitation in the visible blue 
region, and another excitation peak at around 530 nm. 
Perylene-based fluorescent additives exhibit optimal fluorescent properties 
when illuminated by a high-intensity narrow band UVA lamp (as exemplified 
by the lamp of FIG. 1). The high intensity of radiation at around 365 nm 
which is characteristic of these lamps falls right at the primary 
excitation peak of the perylene dye. Perylene-based fluorescent additives 
may also be excited (and hence fluoresce) by broader spectrum light 
sources, though to a lesser degree. 
FIG. 5 shows the excitation spectra of a fluorescent naphthalimide dye in 
the UV and visible spectrum. As used herein, the term "naphthalimide dye" 
refers to the class of water-insoluble organic dyes that includes 
naphthalimide and substituted naphthalimide. The naphthalimide dye 
exhibits a broad excitation band in the violet-blue region, with a peak at 
about 410 nm. The characteristic excitation spectra of the naphthalimides 
explains why the high intensity UV lamps are not as effective in 
illuminating these dyes. The high intensity UV lamps indicate little or no 
light above 400 nm at the peak excitation band of the naphthalimides. 
Combinations of both perylene and naphthalimides dyes, blended in 
proportions as set forth in this description, are effective to the full 
range of excitation available from filtered long wave ultraviolet light or 
visible blue light sources. The resulting mixtures are a multipurpose, 
full spectrum dye having the benefits of each of its constituent 
components for different types of working fluids. 
FIG. 6 shows an excitation spectra profile for a combined perylene 
dye/naphthalimide dye composition. As can be seen in the Figure, such a 
dye combination has multiple excitation peaks, including peaks at about 
345 nm, 360 nm, 410 nm, 490 nm, and 535 nm. 
Solvent effects with the carrier fluid may account for shifts in the 
excitation spectra. FIG. 6 illustrates the versatility and responsiveness 
of a combination dye to all of the available light sources--UVA, Blue, and 
UVA-Blue. It is shown in FIG. 6 that the two dyes complement each other in 
that the absence of a fluorescent response in one is supplemented by the 
response of the other. 
The perylene and naphthalimides dyes are blended with a carrier fluid, 
which is a liquid in which oil is miscible, to make the dye compositions 
of the present invention. The carrier fluid is a hydrocarbon-based fluid. 
"Hydrocarbon-based fluid" is understood to refer to fluids substantially 
containing hydrocarbon constituents, but may further contain substituted 
hydrocarbons or non-hydrocarbon additives. Typical carrier fluids include, 
for example, a paraffinic or naphthenic mineral oil, a polyalkylene 
glycol, a polyolester, or an alkyl benzene, depending upon the particular 
working fluid. 
The relative concentrations of perylene and naphthalimide dye, as well as 
the carrier liquid, are generally dependent on the target fluid to which 
the dye mixture will be introduced. In fluids where there is little 
natural fluorescence of the target fluid and a low likelihood of 
contamination, such as liquid or gaseous fuels or hydraulic fluids, the 
perylene dye is used in significantly larger proportion than that of the 
naphthalimide dye. Liquid fuels include, for example, kerosene and the 
variety of grades of gasoline and diesel fuels. Gaseous fuels include, for 
example, propane (which is initially delivered as a liquid under pressure) 
or natural gas. Hydraulic fluids are defined as fluids based on paraffinic 
and cycloparaffinic petroleum fractions, with additives that improve wear, 
viscosity or flame resistance. 
An example of an appropriate leak detection dye composition for working 
fluids with little or no natural fluorescence has a volumetric ratio of 
perylene to naphthalimide in the range from about 20:1 to about 2.3:1, 
most preferably about 4:1. The resulting greater contribution of the 
perylene to the naphthalimide in the fluorescent response will result in 
an increase in brilliance and viability for superior leak detection. The 
naphthalimide dye, though added in a smaller proportion, provides 
sufficient fluorescent intensity if the target is illuminated with a 
UV/blue lamp instead of a high intensity UV lamp. 
In fluids that exhibit some interfering natural fluorescence and where some 
contamination is typical, such as in some heavier weight lubricants, 
synthetic lubricants and internal combustion engine oils, the perylene dye 
would still be used in larger proportion than the naphthalimide dye, but 
to a lesser degree. Synthetic lubricants, for example, include silicone 
oils, polyglycol (hydraulic and brake fluids), polyphenyl ethers (high 
temperature resistance), silicates (aircraft hydraulics), phosphate esters 
(such as tricresyl phosphate, for fire resistance), neopentyl polyolesters 
(turbine engines), and synthetic hydrocarbons. Internal combustion engine 
oils include commercially available motor oils (e.g., 10W-30 grade) and 
gear lubricating oils. In such a case, a leak detection dye may be 
formulated having a volumetric ratio of perylene to naphthalimide in the 
range from about 4:1 to about 1.5:1, most preferably 2.3:1. The green 
fluorescence of the naphthalimide component overcomes the blue hue of the 
naturally fluorescent working fluid which would otherwise interfere with 
the perylene dye. 
In very dark and heavy fluids where there is intense natural fluorescence, 
or where contaminants are typical, such as in some heavy weight lubricants 
and transmission fluids, the perylene dye is used in about equal 
proportions to that of the naphthalimide dye. Heavy weight lubricants are 
petroleum-derived lubricating oils having greater specific gravities than 
lighter weight lubricants, as well as greater viscosities for turbine, 
aircraft, or gear applications. Transmission fluids are petroleum-derived 
lubricating oils and generally contain a variety of additives to inhibit 
oxidation and rusting, improve viscosity, and promote wear. In this case, 
an example of a dye mixture consists of a leak detection dye having a 
volumetric ratio of perylene to naphthalimide from about 1.5:1 to about 
0.5:1, most preferably 1:1. The green fluorescence of the naphthalimide 
component would overcome the natural blue fluorescence of the target fluid 
or the adverse affects on the fluorescence of the perylene attributed to 
contaminants. 
A dye mixture may also be formulated to be universally applicable for all 
of the above described applications. This universal dye has a volumetric 
ratio of perylene to naphthalimide in the range from about 2.33:1 to about 
1.5:1, most preferably 1.86:1. In such case, a single fluorescent dye 
mixture would be effective in all working fluids and under all inspections 
lamps. 
Refrigeration systems present a unique set of design parameters for a leak 
detection dye mixture. The carrier fluid that is used to deliver the 
fluorescent dye mixture is generally chosen to be the same material the 
refrigeration system employs as the system lubricant. Refrigeration 
systems utilizing CFC or HCFC refrigerants use mineral oil, polyolester, 
or alkyl benzene lubricants. HFC refrigerants generally require a 
polyalkylene glycol or polyolester lubricants. Blended refrigerants can 
use any of these oils, depending on the characteristics of the blend's 
constituents. 
The present invention contemplates that a leak detection dye mixture for a 
refrigeration system utilizing HFC fluids (such as R-134a) containing 
polyalkylene glycol as a system lubricant contains naphthalimide and 
perylene dyes having a volumetric ratio of naphthalimide to perylene in 
the range from about 19:1 to about 5.7:1, most preferably about 9:1. 
For refrigerant fluids using a polyolester as the system lubricant, the 
fluorescent dye mixture contains naphthalimide to perylene in the range 
from about 5.7:1 to about 1.5:1, most preferably about 2.3:1 (volumetric 
ratios). 
In preparing the dye mixtures of the present invention, the perylene, 
naphthalimide, and carrier fluid components are blended in a low-shear 
mixing fashion. The amount of carrier fluid to which the dye components is 
added is determined by the desired volume of the final composition. A 
desirable volume of dye composition to be added to a working fluid may be 
one-quarter ounce. This quantity is sufficient to carry the dye product 
without adding an excessive amount of extra fluid to a working system. 
The dye mixtures of the present invention may be prepared for inclusion in 
a working fluid in powdered form without a carrier fluid. The volumetric 
ratio of the dye components is unchanged in such cases. 
It is further understood that the present invention is not limited to the 
particular embodiments shown and described herein, but that various 
changes and modifications may be made without departing from the scope and 
spirit of the invention.