GB2055393A - Heat and moisture sensitive ink compositions - Google Patents

Abstract

An ink composition suitable for use in contact or ink jet printing operations, comprises a solution of (a) a soluble resole resin; (b) a combination of dyes of different colour wherein one dye is water soluble and the other is relatively water insoluble, and (c) a solvent for the resin and the dye combination selected from the lower aliphatic monohydric alcohols or a mixture thereof, the composition being formulated to undergo a colour change upon exposure to water at a temperature of 120 DEG F or greater. The inks are suitable for use in printing coated and uncoated metal and plastics surfaces, and are useful as sterilization or pasteurization indicators.

Description

SPECIFICATION Heat and moisture sensitive ink compositions The present invention relates to printing ink compositions suitable, inter alia, in ink jet printing. The compositions according to the invention are particularly useful as sterilization and/or thermal exposure indicators, since they change colour when exposed to water vapour and elevated temperatures.

Ink jet printing techniques, although of comparatively recent development in the art of applying decorative and/or identifying indicia to a substrate, are of increasing importance. In general, such techniques impose rigid requirements on ink compositions. To be suitable for use in jet printing, ink compositions must meet rigid requirements of viscosity, resistivity, solubility, compatibility of components and wettability of substrate. The ink compositions must be quick-drying and smear resistant, but must not clog an ink jet nozzle, and they must permit rapid clean-up of the jet printing machine components with minimum effort. At the same time the compositions must also be adapted for satisfactory performance in the particular end use for which they are specifically intended.For example, where the ink is to be employed in the printing of plastic or metal substrates, for example, coated and uncoated tinplate or aluminium, the ink must properly wet the substrate and, where the surface is coated, (as ti may be in food or beverage containers) it is highly desirable that some penetration of the coating be effected. In addition to proper wetting and/or penetration of the surface to be printed, the ink must also adhere strongly and be resistant to abrasion or moisture, It has been particularly difficult to obtain and maintain satisfactory adhesion especially where the printed surfaces are subjected to sterilization processes involving the combination of moisture and high temperature.

Apart from the jet ink printing applications, another area of interest in the art is the provision of printable pasteurization or sterilization indicators. Usually, such compositions have utilized pigment combinations and have been applied primarily in determining whether proper sterilization heat has been applied to objects used in medical and surgical procedures.In the food packaging industry, for example, where metal cans, plastic pouches or coated metal cans are printed, filled with product, sealed and the package subjected to conditions of high temperature and moisture during pasteurization or sterilization techniques, there is a need for printable compositions that exhibit sufficient adhesion to the substrate to withstand the moisture and high temperature conditions of sterilization and which, at the same time, undergo a visible and permanent colour change indicative of proper sterilization having been attained. The indicator ink can be applied howsoever convenient, for example by ink jet printing.

The provision of such an ink composition which shows a distinct difference in colour among unsterilized and completely sterilized packages provides for ready, visible inspection and permits tracing a particular package or packages after they have been processed. The invention thus provides a positive and visible indication that sterilization or pasteurization has in fact been carried out on the product to which the ink is applied.

It is therefore an object of this invention to provide ink compositions, suitable inter alia for use in ink jet printing or for contact printing, that exhibit excellent adhesion when exposed to wet immersion or other moisture at elevated temperature conditions and which undergo a visible colour change when so exposed.

Desirably, such compositions are capable of exhibiting excellent adhesion to both plastics substrates, coated or uncoated metal substrates.

According to one aspect of the invention, there is provided an ink composition suitable for use in contact or ink jet printing operations, comprising a solution of (a) a soluble resole resin; (b) a combination of dyes of different colour wherein one dye is water soluble and the other is relatively water insoluble, and (c) a solvent for the resin and the dye combination selected from the lower aliphatic monohydric alcohols or a mixture thereof, the composition being formulated to undergo a colour change upon exposure to water at a temperature of 1 20"F or greater.

According to another aspect of the invention, useful in the testing or control of manufacture of commestible products, for instance, there is provided a method of indicating low-temperature sterilization of articles, which comprises (i) applying markings to a surface of the articles using a thermotropic ink composition comprising a solution of (a) a soluble resole resin (b) a combination of extractible and non-extractible dyes of different colour; and (c) a solvent for the resin and dyes; and (ii) exposing the marked articles to water at a temperature of at least 1 20"F for a time sufficient to effect a visible colour change in said markings.

The invention will now be explained in more detail, by way of non-limiting example, in the following description.

The thermotropic ink compositions described hereinafter are adapted for use as jet ink compositions. The constituents comprise a resin binder or combination of such binders, a combination of dyes, a solvent blend and a surfactant. Solution-compatible resins include resins of the phenol-formaldehyde or resorcinolformaldehyde class. The dyes chosen are such that leaching out or extraction of the more soluble dye occurs at elevated temperature in the presence of steam or water to result in a visible colour change. The solvent blend consists essentially of a lower alcohol or mixture of lower alcohols with methyl or ethyl cellosolve, dimethyl formamide or dimethyl sulfoxide. a surfactant and, optionally, an electrolyte.

As described above, the ink compositions according to this invention contain an extractible/nonextractible dye combination, a solvent blend, resinous binder, a surfactant and other optional components, each of which must be in carefully balanced proportion to achieve successful operation of the ink in a jet printing apparatus and suitable properties as colour-change indicators for use on coated and uncoated metal surfaces.

In general, the inks exhibit the following characteristics for use in ink jet printing systems: 1) a viscosity of about 1-5 cps at ambient temperature; 2) surface tensions for use on metal surfaces of between about 20 and 27 dyne cm, and 3) specific resistivity within the range of about 500 ohm cms to about 1500 ohm cms.

The inks upon application should be water resistant within 30 seconds, touch dry within 60 seconds and should exhibit a visible colour change when exposed to temperatures of at least 1 200F (48 C) to about 1 60"F (71 C) or higher.

The Resin Binder.

Resins preferred for use herein are best classified as phenolic heat-sensitive resins of the resole type. Such resins include those derived e.g. from phenol-formaldehyde, and resorcinol-formaldehyde. Suitable resole resins are solution-compatible, alcohol-soluble cross-linked polymers or prepolymers having molecular weights within the range of 1,300 to 10,000. Such resins include known alkaline catalyzed phenolformaldehyde condensation products in which the ratio of formaldehyde to phenol is greater than one and are usually identified as "B-stage" resins which are curable at elevated temperature by further condensation and/or cross-linking through hydroxymethyl groups to insoluble, chemically resistant, adherent polymers.

Preferred commercially available resins are sold under the trade names BLS-2700 and BKS-2600 by Union Carbide Corp.; Varcum 29108 from Reichold Chemicals; and Methylon 75108 from General Electric. Mixtures of these resins are especially preferred. Such alcohol-soluble binders may be employed on their own or they may be modified by admixture with other resins including polyvinyl butyral, polyvinyl acetate, polyacrylics, ethylene-acrylic copolymers, polyamides, etc. In general, the binder will be present in the composition in amounts ranging from 5 to 10% by weight, preferably from 5% to 8% by weight of the ink composition, and e.g. 6% by weight.

The use of resole phenolic resins is believed to be critical to the successful operation of the inks as colour-change indicators, for it seems essential to achieve the intended result that the resins be susceptible to further curing, after application to the substrate, underthe same conditions by which the dye components function to undergo a colour change.Resole phenolic resins as described herein are soluble in the ink solvent, solution compatible with the other components of the ink composition and, when applied to the substrate, they undergo further condensation upon subjection to an elevated temperature of at least 120 F, to produce an adherent, insoluble binderforthe non-extractible dye component, The mechanism by which the resin-dye combination functions to convey the desired characteristics to the ink composition is not fully understood, but it is believed tha while the resin cures under the process conditions with an accompanying release of water, the more soluble or extractible dye is leached out causing a change in colour.

If desired, various acidic compounds may be incorporated in the resin component in catalytic amounts to accelerate the resin cure and/or the colour transition of the composition. Suitable compounds for this purpose include inorganic acids, for example phosphoric acid; esters of such acids, for example dibutyl amino pyrophosphate; organic acids, for example p-toluene sulfonic acid and oxalic acid. Also suitable for this purpose are metal salts, for example stannous chloride and ferric chloride.

The Dye Combination.

Suitable dyes are those which function in combination to undergo a visible and permanent colour change when exposed to temperatures equal to or greater than 120"F in the presence of moisture. In addition, the dyes must also be soluble in the liquid ink base and be compatible with the other ink components.

As contemplated herein, pairs of dyes having differing solubilities or extractibility in water are employed.

For example, a specific combination may contain a water-soluble blue dye and a relatively water-insoluble red dye in a solvent blend also comprising a resole binder resin. The weight ratio of water-soluble dye to water insoluble dye of about 2:1 may be employed. With the said blue and red dyes, markings formed from the composition when applied and dried will be blue. Upon exposure to water at 120"F or higher, the substrate colour changes from blue to a permanent and visible red colour. The leaching step, which is critical to the successful operation of the composition, is not observed when water at temperature below 1 20"F is employed. Substantially no change in colour or leaching takes place when the resin binder is fully cured or cross-linked.It is believed that the use of resole resins which cure over a period of time with the release of water is critical to permit leaching at a satisfactory rate as the products to which the markings are applied are processed.

Particularly good results have been obtained when using Safrinine 0 as the non-extractible red dye and either of Aniline Blue or Light Green SF as the soluble, leachable blue dyes. Other suitable extractiblel non-extractible combinations may be illustrated by permanent dyes selected from the phenylsafranines, Indamine blue or Rhoduline Violet dyes; extractible dyes from sodium su Ifonate salts of triphenyl methane type dyes such as Food Green No, 1, Acid Blue No.7 or from sodium sulfonate salts of Indulines such as Acid Blue No. 20, Indulene ZB, etc. The foregoing examples are not exhaustive: other extractible/non-extractible dye pairs of the same or different colour combinations not specifically enumerated hereinabove, but of sufficient solubility and compatibility with the remaining components of the ink composition may be employed.

In general, the dyes will preferably be present in amounts varying from 2% to 5% by weight of the composition with the extractible dye to inextractible dye weight ratio being at least 2:1. Alternatively, the dyes may be combined in such proportions that it is unnecessary that one predominate the other in amount.

For example, they may blend to form a colour the hue of which changes upon leaching out of the extractible dye.

Ink compositions according to the invention have been found to exhibit, for example, a dark blue colour upon application to the substrate and to undergo a colour change ranging from pink to red after being subjected to a temperature of at least 1 20"F in the presence of water or steam for periods ranging from about 50 to 90 minutes. In general, a readily visible colour change will be evidenced in as little as 30 minutes depending on the particular temperature.

The Solvent Blend.

Although minor amounts of other solvents may be included in the overall ink composition, the primary solvent is selected from lower aliphatic alcohols having 1 to 3 carbon atoms. The solvent can be one or more of these alcohols blended together. Methanol, ethanol and mixtures thereof are preferred. Auxiliary solvents in which the dyes have a high solubility or which aid in penetrating organic coatings on the substrate may also be included. Certain of the relatively low molecular weight glycol ethers such as ethylene glycol monomethyl ether (methyl "Cellosolve") and ethylene glycol monoethyl ether (ethyl "Cellosolve"), as well as other more polar solvents such as dimethyl-formamide or dimethyl sulfoxide may also be included, either alone or in combination. The solvent blend will generally comprise from about 65 to about 97% of the ink composition.The solvent itself may vary from 100% lower alcohol to as little as about 30% alcohol with the remainder of the blend being one or more of the auxiliary solvents mentioned above. "Cellosolve" is a Registered Trade Mark.

Surfactants and electrolytes are optional components that may be added to compositions according to the invention, if desired. Suitable examples of each are anionic and cationic surfactants including sodium lauryl sulfate, alpha methyl sodium lauryl sulfate, fluorinated alkyl esters (commercially available underthetrade designation FC-430 from Minnesota Mining and Manufacturing Co.) FC-430 is the preferred surfactant and may be present in amounts ranging from 0.01% to 0.1% by weight Salts such as lithium chloride or dimethyl amine hydrochloride may be used to improve conductivity. The preferred electrolyte is dimethyl amine hydrochloride which may be present in amounts from 0% up to about 2% by weight of the composition.

The following examples are illustrative of ink compositions according to this invention which are effective thermotropic jet inks.

Example 1.

The following compositions were formulated: Composition A 4% Varcum 29108 phenol-formaldehyde resin 2% BLS-2700 phenol-formaldehyde resin 1% Safranine O 2% Aniline Blue 0.40% Phosphoric acid 49% Methanol 41% Dimethyl formamide 0.04% FC-430 surfactant Composition B 7% BLS-2700 phenol formaldehyde resin 0.80% Safranine 0 2% Light Green SF 45% Methanol 45% Dimethyl formamide 0.04% FC-430 surfactant The ink resulting from composition A had a viscosity of 2 cps, a resistivity of 700 ohm cm, and a surface tension of 25 dynes while that of composition B had a viscosity of 3 cps, a resistivity of 1200 ohm cm, and a surface tension of 20 dynes.

Each ink was used in the ink jet printing of bare tinplate and aluminium cans, as well as coated aluminium and tinplate cans and plastics containers. Printed indicia dried very quickly to form blue images displaying excellent adhesion to the substrates. Subjecting the cans to sterilization or pasteurization in the presence of water at 1 20"F for 60 minutes or 1 60"F for 30 minutes, resulted in a visible colour change of the printed indicia from blue to red. Longer time intervals of water immersion up to 8 hours at 160OF, which are typical conditions in meat processing, also resulted in a permanent colour change.

Example 2.

Example 1 was repeated except the cans were subjected to water at a temperature of about 1 OO"F for a period oftime ranging up to 120 minutes. No change or transition in colour was observed. This example illustrates that the compositions of the invention undergo a colour change at an elevated temperature of at least about 120"F, i.e. they function as low-temperature sterilization indicators and do not undergo a colour change in the absence of these conditions. Merely heating the composition in the absence of water does not cause a colour change.

It will be seen from the above that the compositions according to the invention are valuable low-temperature sterilization or pasteurization indicators which can provide multiple functions in the packaging industry. For example, use of the compositions to imprint indicia on metal cans or on plastics ham cans, permits the packager to determine, by visual inspection of any given batch of cans, that they have been processed at the proper temperature. Additionally, the presence of such visible indicia permits the ready rejection of individual containers that have not been processed and traceability of origin of the container in the event of defects either in the container or its contents. Additionally, provision of an indicator ink usable in jet printing provides for obtaining the above mentioned character changes and eliminates damage to the containers caused by many of the prior contact-printing methods.

Finally, because the present ink compositions are applicable to both coated and uncoated metal substrates, and plastics substrates as well, they are unusually versatile affording a wide spectrum of suitable substrates on which they may be used.

Claims (24)

1. An ink composition suitable for use in contact or inkjet printing operations, comprising a solution of (a) a soluble resole resin; (b) a combination of dyes of different colour wherein one dye is water soluble and the other is relatively water insoluble, and (c) a solvent for the resin and the dye combination selected from the lower aliphatic monohydric alcohols or a mixture thereof, the composition being formulated to undergo a colour change upon exposure to water at a temperature of 120"F or greater.

2. An ink composition as claimed in claim 1, wherein the weight ratio of the soluble dye to the insoluble dye is at least 2:1.

3. An ink composition as claimed in claim 1 or claim 2, wherein the resole resin has a molecular weight within the range of 1300 to 10,000.

4. An ink composition as claimed in claim 3, wherein the resole resin is a phenol-formaldehyde or resorcinol-formaldehyde condensation product.

5. An ink composition as claimed in claim 1,2,3 or 4, wherein the solvent comprises a blend of a lower aliphatic monohydric alcohol or mixture thereof with one or more solvents selected from ethylene glycol monomethyl ether, dimethyl formamide and dimethyl sulfoxide.

6. An ink composition as claimed in any of claims 1 to 5, which contains 5 to 10% by weight of a soluble resole resin which has a molecular weight of 1300 to 10,000 and 2%to 5% by weight of the dye combination.

7. An ink composition as claimed in claim 6, wherein the resole resin is a combination of phenol-formaldehyde resins, the dye combination is Safranine O/Aniline blue or Safranine O/Light Green SF, and the solvent is methanol.

8. An ink composition as claimed in any preceding claim, additionally comprisirg a surfactant.

9. An ink composition as claimed in claim 8, wherein the surfactant is a fluorinated alkyl ester.

10. An ink composition as claimed in claim 1, suitable for use in printing coated and uncoated metal and plastics surfaces, comprising a solution of 6% phenol formaldehyde resin or mixtures thereof derived from the alkaline catalyzed condensation of phenol and formaldehyde, wherein the proportion of formaldehyde to phenol is greater than 1, and the resin has a molecular weight within the range of 100 to 10,000; two dyes comprising 1% Safranine 0 and 2% Aniline Blue; and 0.40% phosphoric acid, 49% methanol, 41% dimethyl formamide and 0.04% fluorinated alkyl ester surfactant, the stated percentages being by weight of the composition.

11. An ink composition as claimed in claim 1, suitable for use in printing coated and uncoated metal and plastics surfaces, comprising a solution of 7% phenol formaldehyde resin derived from the alkaline catalyzed condensation of phenol and formaldehyde, wherein the proportion of formaldehyde to phenol is greater than 1, and the resin has a molecular weight within the range of 1300 to 10,000; two dyes comprising 0.80% Safranine 0 and 2% Light Green SF; and 45% methanol, 45% dimethyl formamide and 0.04% fluorinated alkyl ester surfactant, the stated percentages being by weight of the composition.

12. A method of indicating low-temperature sterilization of articles, which comprises (i) applying markings to a surface of the articles using a thermotropic ink composition comprising a solution of (a) a soluble resole resin (b) a combination of extractible and non-extractible dyes of different colour; and (c) a solvent for the resin and dyes; and (ii) exposing the marked articles to water at a temperature of at least 1200Ffor a time sufficient to effect a visible colour change in said markings.

13. A method as claimed in claim 12, wherein the articles are coated or uncoated tinplate and aluminium cans.

14. A method as claimed in claim 12 or 13, in which said resole resin is a phenol-formaldehyde resin having a molecular weight of 1300 to 10,000.

15. Amethod as claimed in anyofclaims 12to 14, wherein the solvent comprises a loweraliphatic monohydric alcohol.

16. A method as claimed in any of claims 12 to 15, wherein the markings are applied by jet ink printing.

17. A method as claimed in any of claims 12 to 16, wherein the markings are applied by contact printing.

18. A method as claimed in any of claims 12 to 17, wherein the articles are coated with an ink composition comprising a solution of (a) from 5 to 10% by weight of a soluble phenolformaldehyde resole resin; (b) from 2 to 5% by weight of the dye combination in which the extractible dye is present in proportion to the non-extractible dye in a ratio of at least 2:1; (c) from 0.01 to 0.1% by weight of a fluorinated alkyl ester surfactant and (d) sufficient solvent to constitute a complete solution for said resin and dyes, the solvent consisting of a lower aliphatic monohydric alcohol or mixture of such alcohols.

19. A method as claimed in any of claims 12 to 18, wherein the non-extractible dye is Safranine 0.

20. A method as claimed in claim 19, wherein the dye combination is Safranine O/Aniline Blue or Safranine O/Light Green SF.

21. A method according to claim 12, wherein the markings are applied using an ink composition comprising a solution of about 6% phenol formaldehyde resin or mixtures thereof derived from the alkaline catalyzed condensation of phenol and formaldehyde, wherein the proportion of formaldehyde to phenol is greater than 1, and the resin has a molecular weight within the range of 1300 to 10,000; an extractible/non-extractible dye pair comprising 1% Safranine 0 and 2% Aniline blue and 0.40% phosphoric acid, 49% methanol, 41% dimethyl formamide and 0.04% fluorinated alkyl ester surfactant; the stated percentages being by weight of the composition.

22. A method as claimed in claim 12, wherein the markings are applied using an ink composition comprising a solution of 7% phenol formaldehyde resin derived from the alkaline catalyzed condensation of phenol and formaldehyde, wherein the proportion of formaldehyde to phenol is greater than 1, and the resin has a molecular weight within the range of 1300 to 10,000; an extractible/non-extractible dye pair comprising about 0.80% Safranine 0 and 2% Light Green SF, and 45% methanol, 45% dimethyl formamide and 0.04% fluorinated alkyl ester surfactant, the stated percentages being by weight of the composition.

23. Ink compositions capable of changing colour when subjected to moisture and elevated temperature, substantially as herein described.

24. A method of indicating low-temperature sterilization useful in testing or control of manufacture, substantially as herein described.