Method for detecting a fluid permeating through a layer

Cross reference to a related application

[0001] This application claims priority to European application No. 10185828.0 filed on October 1 ,2010, the whole content of this application being incorporated herein by reference for all purposes.

Technical field

[0002] The invention pertains to a method for detecting the permeation of a fluid through a layer, in particular through a polymer layer, and to the use of this method for determining the map of the permeability coefficient of such layer.

Background art

[0003] The permeation phenomenon is related to the penetration and transport of a permeate, i.e. of a fluid such as a liquid, gas, or vapour through a solid, and is related to a material's intrinsic permeability.

[0004] This intrinsic permeability is strictly related to inherent (micro)structure of the material; in particular, in case of polymer materials, permeability is related to the actual composition of the material (e.g. to the presence of fillers and additives, their amount and distribution), to the nature of polymer chains and their relevant molecular parameters, including crystalline fraction and habits, as well as to the overall layout of the article penetrated by the fluids, i.e. it is sensitive to defects, voids and other ex- processing features.

[0005] Appropriate determination of permeability of fluids through layers of solid materials is thus of interest with many regards, in particular when such determination enables mapping this property throughout the entire layer surface.

[0006] Methods for detecting leakage of fluids through solid surfaces based on the use of coating compositions wherein dyes or pigments undergoing colour changes upon contact with said fluids are known. [0007] Thus, US 5183763 (SOUTHWEST RES INST ) 2/02/1993 discloses a coating composition comprising notably a binder and a dye, which can be notably used, when painted on the external surface of an enclosure, as passive sensor for detecting leakage of a fluid from the same.

[0008] Nevertheless, this method provides no hint or suggestion on how

exploiting these human discernable changes for obtaining quantitative measurements.

[0009] Similarly, US 2003003589 (PHOTONIC BIOSYSTEMS INC ) 2/01/2003 discloses a method for detecting and measuring volatile acid or basic components by means of an indicator film having an indicator dye embedded therein, such that said dye undergoes a colour change inducing modification of spectral properties upon exposure to the component to be detected. This method is notably taught as applicable for detecting holes and defects in objects or materials wherein said coating can be applied on one side and the fluid, ammonia, for instance, on the other side.

[0010] Nevertheless, spectral change determinations, like absorbance or

fluorescence measurements, as provided in this document, are not easy techniques to be used and required the material itself to be submitted to the spectral measurement. This method cannot thus be applied to objects and articles of complex geometry, and or which cannot be easily handled or moved.

[0011] There is thus a current shortfall in the art for an easy, reliable and non destructive method for the determination of permeate through solid layers, which would advantageously enable detection of permeation through surfaces of complex geometry, said method being possibly automated and implemented via involvement of simple tools.

Summary of the invention

[0012] The present invention pertains to a method for detecting the permeation of a fluid through a layer having a first surface (S1) and a second surface (S2), wherein said fluid is contacted with said first surface (S1) and wherein a coating composition comprising a reactive dye is applied to said second surface (S2) to yield a reactive dye coating onto said surface (S2), wherein said reactive dye coating undergoes a colour change when contacted with said fluid, said method comprising:

- obtaining an image of at least a portion of said second surface (S2);

- applying RGB colour model to said image to determine the red (R ), green (G) and blue (B) coordinates of at least one point of said image;

- comparing RGB coordinates of said point to the RGB coordinates of a reference colour; and

- correlating said comparison to the concentration of permeating fluid on the surface (S2).

Brief description of the drawings

[0013] Figure 1 shows a calibration curve useful for applying the method in

accordance with the present invention, wherein a correlation was established between the concentration of a fluid (here, ammonia) in ppm (in abscissa) and the distance in the RGB space from an original point (in ordi nates).

[0014] Figure 2 shows the results of a test on how ammonia permeates through a certain PVDF layer : a plot of the concentration of ammonia as a function of time was determined in accordance with the method according to the present invention, wherein in abscissa the time is expressed in hours, while in ordinates, concentration of ammonia in ppm is provided.

Detailed description of the invention

[0015] The Applicant has found that by means of the method of the invention it can be possible to detect permeation of a fluid through a surface of whichever geometry in an easy and efficient way, with no need of complex apparatuses, nor destruction of the sample, by using easily available devices and components.

[0016] The method also advantageously enables detecting the permeation of said fluid through selected areas of the layer under investigation.

[0017] Actually, depending upon resolution of the device used for obtaining the image of the surface (S2), said surface can be divided in 'pixels' of well defined dimensions, each of said pixel being thus possibly associated to a value of concentration of permeating fluid, and hence to a value of permeability coefficient. [0018] Thus, by means of the method of the invention, a mapping of the permeation of the fluid through the layer can be notably advantageously obtained, which can enable, e.g., identification of inhomogeneities in thickness of such layer or other structural local defects.

[0019] As a consequence, variations of the R,G,B coordinates can be recorded for one point of each of those pixels, thus providing via appropriate computations methods the concentration of the permeating fluid in each of the selected areas of said layer.

[0020] Thus, the method can be advantageously used for establishing a map of permeation of the fluid through said layer.

[0021] The nature of the layer is not particularly limited. Generally polymer layer will be submitted to the method of the invention. Among polymers whose layers might be submitted to the method of the invention mention can be notably made of elastomeric and thermoplastic polymers, in particular of polyolefins, aromatic and aliphatic polyamides, polyetherketones, polysulphones, halopolymers, including chlorinated polymers (PVC, PVDC) and fluoropolymers (PVDF, PTFE, ECTFE, ETFE, PFA, MFA).

[0022] The layer may have whichever geometry.

[0023] The layer might be comprised in articles intended to confine fluids, like, notably, enclosures, bottles, recipients, pipes, tubings and the like, or can be a (part of) a substantially bidimensional article, i.e. a film or a sheet.

[0024] The method of the invention can be used to determine permeation of

various fluids, including but not limited to hazardous and non hazardous gases such as, notably, ammonia, carbon monoxide, chlorine, arsine, diborane, dimethylamine, fluorine, formaldehyde, hydrogen chloride, hydrogen cyanide, hydrogen fluoride, hydrazine, methylamine, methyl hydrazine, nitric acid, nitrogen dioxide, phosgene, phosphine, sulfur dioxide, trimethylamine hydrogen sulfide (F S) and hydrogen (Kb), and carbon dioxide, water vapour, nitrogen; and hydrocarbon vapours such as benzene, toluene, alkanes, alcohols like methanol, ethanol. Further, in addition, the fluid can be a liquid, preferably a low boiling liquid, such as notably water, hydrogen peroxide, organic solvents, and the like. [0025] The reactive dye coating is designed and dimensioned to be gas and liquid permeable so as to allow the fluids to contact the reactive dye held within the coating matrix.

[0026] The reactive dye comprised in the reactive dye coating is advantageously selected from a group of compounds that change colour perceptibly in response to the presence, interaction with, or binding to the fluid; as a consequence, RGB coordinates of reactive dye in the presence or in the absence of the fluid are modified. It is also understood that the reactive dye might be coloured when kept in the absence of said fluid or can be colourless, in said conditions. Similarly, the reactive dye might be coloured or colourless when in the presence, interaction with or binding to the fluid. As a consequence, the colour change, as above mentioned can be a change from colourless to coloured, from coloured to colourless or may be, more often, a change between two different colours.

[0027] The reactive dye used can be tailored to the specific environment, the fluid to be detected and other factors, well known to the skilled in the art. Any reactive dye that is known in the industry can be used so long as a colour change occurs in response to the presence, interaction with, or binding to the fluid which permeation should be detected.

[0028] The group of useful reactive dyes generally comprises: transition metal oxides, permanganate salts such as silver permanganate and potassium permanganate, chromate salts such as sodium dichromate, metal hydroxides, including notably Copper hydroxide, metal carbonates, metal sulfates such as tribasic lead sulfate and copper sulfate, metal chlorides such as cobalt chloride and mercury chloride, cobalt bromide, silver nitrate, tetrephenylporphyrin, a metal complex of tetrephenylporphyrin, lead acetate, uranine, gold complexes, indophenol sodium salt, sodium salt of the leuco sulfuric derivative of Vat Green 1 , phosphomolybdic acid, and organic indicator compounds.

[0029] The transition metal oxides used as reactive dyes in the reactive dye

coating are generally selected from the group comprising: scandium oxide, titanium dioxide, chromium(ll) oxide, chromium(IV) oxide, manganese oxide, manganese(IV) oxide, iron(lll) oxide, cobalt(ll) oxide, nickel(lll) oxide, zirconium dioxide, niobium(V) oxide, technetium oxide, ruthenium(IV) oxide, palladium(ll) oxide, silver(ll) oxide, lutetium(lll) oxide, hafnium(IV) oxide, tantalum(V) oxide, rhenium trioxide, rhenium(VII) oxide, osmium tetroxide, vanadium(V) oxide, tungstic oxide, molybdenum oxide, molybdenum dioxide, yttrium oxide, and combinations thereof. The transition metal oxide pigments may also be combined with a catalyst to facilitate the reaction of the metal oxide with hydrogen or hydrogen sulfide. The catalysts may be selected from the group comprising platinum, palladium, rhodium, nickel, combinations of these metals, or alloys of these metals with other metals such as copper, cobalt, iridium,

magnesium, calcium, barium, strontium, and combinations thereof.

[0030] Among organic indicators compounds used as reactive dyes in the

reactive dye coating, mention can be notably made of anthraquinone dye, anthraquinone dye with alkylammonium salts, methyl orange, bromocresol green, methyl red, bromocresol purple, bromothymol blue, phenol red, bromothymol purple, thymol blue, bromophenol blue, methylene red, bromophenol red, bromocresol green, tetraiodophenolphthalein,

tetraiodophenoltetrachlorophthalein, 3,3',5,5'-tetraiodophenol

sulfonephthalein, 3',3"-dibromo-5',5"-dichlorophenolsulfonephthalein, 3', 3"- dibromophenolsulfonephthalein, 3,3-dichlorophenolsulfonephthalein, naphthoic acid, quinaldine red, martius yellow, 2-nitroaniline, 4-nitroaniline, cresol red, crystal violet, dimethyl yellow, thymolphthalein, a- naphtholbenzein, and combinations thereof.

[0031] It is generally preferred for the reactive dye coating to possess a

homogenous thickness throughout the entire surface of the surface (S2) of the layer. This preferred embodiment would enable avoiding possible spurious contributions from different thicknesses and thus colour intensities of the reactive dye coating when mapping permeability through the entire surface of the layer.

[0032] The coating composition may further comprise a polymer binder

component; the inclusion of this component might impart tixotropic properties to those reactive dyes, which, in the absence of the polymer binder component, are unable to appropriately provide for a suitable reactive dye coating.

[0033] The polymer binder can be notably selected from the group consisting of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF),

ethylene/chlorotrifluoroethylene (ECTFE) and ethylene/tetrafluoroethylene (ETFE) copolymers, polyamides, polyacrylamides, polyacrylate, polyalkylacrylates, polystyrenes, polynitriles, polyvinyls, polyvinylchlorides, polyvinyl alcohols, polydienes, polyesters, polycarbonates, polysiloxanes, polyurethanes, polyolefins, polyimides, cellulosic polymers, including ester and ether derivatives, like notably Cellulose triacetate, Cellulose

propionate, Cellulose acetate propionate, Cellulose acetate butyrate, methylcellulose, ethylcellulose, ethyl methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose (HPC), hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose (HPMC), ethyl hydroxyethyl cellulose, carboxymethyl cellulose (CMC), and heteropolymeric combinations thereof. It is also understood that embodiments wherein above referred polymers are further functionalized, e.g. by grafting, so as to possess reactive groups, including, notably, epoxy groups, maleic anhydride groups, ammine groups, carboxylic acid groups, are still encompassed by the present invention.

[0034] According to certain embodiments, the polymer binder can be selected from thermosetting resins; these resins might be lead to coating

compositions which do not required the use of additional solvents and might provide increased adhesion to the surface (S2) of the layer; further, in addition, thermosetting resins available from renewable sources can be used when limiting environmental impact is required.

[0035] In general, the selection of the polymer binder will be made also having regards to the chemical nature of the layer to be submitted to the method of the invention. In case of polymer layer, it is generally understood that the selection of the polymer binder will be made so as to generally ensure that the polymer binder and the polymer of the layer are compatible with each other; this requirement is considered to advantageously ensure increased adhesion and interaction between the reactive dye coating and the layer.

[0036] The coating composition may further include other ingredients such as solvents and fillers.

[0037] Solvents dissolve or disperse constituents of the coating composition, resulting in a liquid mixture that may be applied as a liquid to the surface (S2) as above detailed by standard coating techniques, like casting, spin coating, curtain coating, roll coating, air knife coating, metering rod coating.

[0038] Most appropriate technique will be selected as a function of the nature of the coating composition and of the geometry of the layer; typically, techniques ensuring achievement of an homogenous thickness of reactive dye coating overall the entire surface (S2) of the layer will be preferred.

[0039] The solvents are generally selected to have a low boiling point and

evaporate easily, or to be easily removable by distillation, to form the reactive dye coating onto said surface (S2).

[0040] Among the variety of solvents suitable to be used in the coating

composition mention can be made of water; aromatic solvents, like notably benzene, toluene, xylenes; alcohols, in particular methanol, ethanol, propanols; ketones, like notably acetone, methylethylketone, methyl isobutyl ketone, cyclohexanone; esters, like ethylacetate, butylacetate; polar aprotic solvents, like dimethylformamide, triethyl phosphate, propylene carbonate, triacetin (also known as 1 ,3-diacetyloxypropan-2-yl acetate), dimethyl phthalate, butyrolactone, isophorone and carbitol acetate; mineral spirits, turpentine, mineral turpentine, naptha, and combinations thereof.

[0041] Fillers may also be added, in addition to the other constituents of the

coating composition, to thicken the coating, to lend additional support to the structure of the coating and/or to even enhance reactive dye's response to the fluid. Fillers may be selected from the group comprising: glass flake, glass flake coated with silver, hollow glass spheres, solid glass spheres, talc, fibrous talc, lime, calcium carbonate, barite, clay, gypsum, chalk dust, marble dust, fumed silica, amorphous silica, glass fibres, zeolite, and mica.

[0042] Various other thickeners, stabilizers, emulsifiers, texturizers, adhesion promoters, de-glossing agents and combinations thereof can also be used. Any of a variety of constituents may be used to form the reactive coating composition so long as an adequate coating is formed that can be placed on the surface (S2) with adhesion.

[0043] According to certain embodiments, it may well be that the surface (S2) of the layer is preliminary treated for ensuring increased adhesion to the reactive dye coating. Among techniques which can be used, mention can be notably lade of corona treatment, sulfonation, oxyfluorination, plasma, and the like.

[0044] The reactive coating composition is deposited onto surface (S2) as a film, with the reactive dye incorporated therein.

[0045] The method of the invention comprises obtaining an image of at least a portion of said second surface (S2), as above detailed.

[0046] Said image can be obtained by conventional techniques; either traditional camera or digital camera can be used, being understood that in former case further treatment with suitable software will be required for applying RGB colour model to said image to determine the red (R ), green (G) and blue (B) coordinates of at least one point of said image.

[0047] In case of digital camera, RGB colour model may be integrated in data acquisition of the camera itself, so that output of the same can already be a set of RGB coordinates for each section (pixel) of the image, according to the chosen resolution.

[0048] The RGB coordinates of said at least one point are then compared with the RGB coordinates of a reference colour. This can be achieved by various means for obtaining a quantitative comparison. It may be useful, for instance, to determine the spectral distance between said at least one point having spectral coordinates (R _p , G _p and B _p ) and said reference colour (Rref, Gref, B _re f) according to formula:

D = - ^R " ) ² + ( ^Q P - Grcf f + (B _p - B _ref † [0049] According to this embodiment of the method of the invention, this latter thus comprises:

- determining spectral distance D in the RGB colour space of said point from a reference colour; and

- correlating said spectral distance to the concentration of permeating fluid on the surface (S2).

[0050] The method of the invention can be notably applied for determining fluids transport coefficients through layers, and more particularly through polymer layers.

[0051] Actually, appropriate elaboration of the concentration of fluid onto the

surface (S2) can provide for the amount Q of fluid actually permeating through the layer from surface (S1) to surface (S2). This amount can be monitored in time (t) as a function of temperature, concentration and/or pressure (p) of the fluid contacting surface (S1).

[0052] The permeability or permeability coefficient Pe is generally defined as follows:

Pe =

Αφ

wherein Q is the flux of fluid through the layer, I is the thickness of said layer and A<p is the so-called potential difference between both sides of the layer. This potential ^ can be a pressure, a mass concentration or a molar concentration. As a consequence, dimensions of flux and permeability will be consequently expressed.

[0053] When fluid is a gas, potential φ is generally expressed as a pressure; in this case, by plotting gas flow rate (Q/t) versus the applied pressure in steady state conditions, a straight line is obtained, the slope of which being directly proportional to the permeability coefficient Pe. Thus, Pe of the layer can be determined according to the equation herein below:

wherein Pe is the permeability coefficient, Q is the volume of gas permeated through the layer at time t, I is the thickness of the layer, A is the diffusion area exposed to said gas, through which the same

permeates, t is the time and p is the pressure of the gas. [0054] Diffusion coefficients (D) and solubility coefficient (S) can be further determined notably applying the "time lag" method; in particular, 6 · ® wherein D is the diffusion coefficient, I is the thickness of the layer and Θ is the 'time lag', obtained by extrapolating the linear part of the integrated flow of gas (Q) versus time and determining the interception of said straight line with the abscissa; and

wherein Pe and D have the meanings as above detailed.

[0055] The method also enables determining such coefficients (Pe, D, S) for each of defined area of the layer under investigation, as above detailed. A permeability mapping can thus be established by appropriate

computations methods so as to advantageously determine values

Si for each of area i of the layer.

[0056] Exploitation of such information can be notably useful for quality control.

[0057] The invention will be now described in more details with reference to the following examples, whose purpose is merely illustrative and not intended to limit the scope of the invention.

[0058] Manufacture of the coating composition

A coating composition was prepared by mixing:

- 2.97 g of ethyl cellulose having a viscosity of 4 cPs;

- 7.34 g of butyl acetate;

- 7.34 g of toluene;

- 0.53 g of acetone;

- 0.40 g of glycerol;

- 0.04 g of 3,3\5,5'-tetraiodophenol sulfonephthalein;

-1.32 g of Talc commercially available from LUZENAC 10MO;

- 0.06 g of nitric acid.

Said composition was used for coating a film made from SOLEF(R) 1008 VDF homopolymer, commercially available from Solvay Solexis, having a thickness of 16 pm.

To the purpose of calibration, this coating was used alone, by casting onto a substrate. [0059] Establishment of Calibration curves

The establishment of these curves has enabled settling quantitative correlation among the concentration of the fluid and the value of the spectral distance in the RGB space from initial state of the dye to the final state of the dye, when contacted/in the presence of the fluid.

As fluid, ammonia was selected, fed through a bubbling system LENZ of inner volume of 100 ml though a metering device Bronkhorst to a measuring cell exposed to a scanner EPSON PERFECTION V750 PRO. Concentration of ammonia was made to vary between 2 ppm and 10 ppm as a function of time; after each pulse of ammonia, enough time was left to the system for desorbing ammonia and recovering initial status of the reactive dye.

An automated acquisition of images via the scanner devices was obtained by means of a suitable software specifically designed to this aim. Figure 1 shows the correlation established between the concentration of ammonia in ppm (in abscissa) and the distance in the RGB space from original point.

[0060] Determination of permeation of ammonia through a PVDF layer

[0061] This test was performed at 23°C for determining the permeation of

ammonia through a film of PVDF as above detailed, coated with the reactive dye composition as above mentioned.

A vial having an inner volume of 12 ml was filled with 5 ml of a saturated ammonia aqueous solution and was capped by means of the coated PVDF layer as above mentioned, retained by means of aluminium caps, letting a surface of about 0.5 cm ² exposed.

A digital camera NIKON D700 equipped with a AF-S Micro NIKKOR 68 mm 1 :2.8 G ED objective was operated through a suitable NIKON software for taking pictures as a function of time of the exposed layer to ammonia vapours.

RGB coordinated as a function of time were thus determined and their distance from the reference point (recorded on the coated film before exposure to ammonia) determined. Using calibration curve as obtained above, a plot of the concentration of ammonia as a function of time was determined; such plot is reproduced in Figure 2, wherein in abscissa the time is expressed in hours, while in ordinates, concentration of ammonia in ppm is provided. From these data, permeability coefficient can be notably determined by simple correlation, on the basis of calibration data.