CA1267278A - Method for applying a composition to a substrate and composition for use therein - Google Patents

Abstract

ABSTRACT

The present invention provides a process for applying a molten thermoplastic composition as a series of discrete droplets from a non-contact ink jet printing apparatus to form separate drops on a substrate moving relative to the apparatus, characterised in that the molten composition is thermally stable at the temperature of application and is applied at a temperature in excess of 100°C. The invention can be used to apply the molten composition to a variety of substrates using on-demand or continuous non-contact ink jet application techniques. However, the invention is of especial use in applying thermoplastic inks to non-porous substrates using an on-demand ink jet printer.

Description

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The present invention relates to a method for applying a composition to a substrate and to a composition for use therein, notably to a method for applying thermoplastic inks and to a novel thermoplastic ink composition.
It has been proposed to apply inks through an ink jet printing machine where the inks are in the form of wax based compositions which are applied molten through the nozzle of the printer~ In order to reduce problems which would be expected in attempting to operate at elevated temperatures (for example degradation of the composition), the prior proposals have required the use of comparatively low melting point compositions and low temperatures of operation. Thus, for example, in USP No. 3653932 the composition is required to have a melting point which does not exceed 51C and contains a didodecyl sebacate which is a highly viscous material. In order to overcome the problems associated with that formulation, USP No. 4390369 proposes the use of a composition which comprises a natural wax and has a melting point below about 75C. The use of natural waxes i5 also proposed in published European Application No. 097823, where the composition contains mixtures of parafin and natural waxes, eg. carnauba wax, and in published European Patent- Application No. 099682 where the composition comprises a mixture of paraffin wax and ,~, ,.. ...
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72~8 stearic acid. However, such compositions do not adhere satisfactorily to plastics substrates, suffer from smudging, and problems are encountered due to the high viscosity of the components where synthetic materlals are used.
Contrary to the teaching of the prior proposals, we have found that it is advantageous to operate a hot ~elt ink jet printer at a temperature in excess of 10nC. At such elevated temperatures, the waxes of the earlier 10 proposals would often decompose whereas in the pre~ent lnvention they result in an image of improved definition, which resist~ s~udging and has impro~ed adhesion to plastics substrates, and also reduces the problems associated with the use of high viscosity materials.
US Patent ~o 33692S3 discloses a number of compositions for use in a pen type chart recorder in which ink flows con~inuously from a no~zle onto a moving substrate to draw lines ~hereon~ There is direct contact beiween the nozzle and the substrate via the ink 20 compositlon and the composition mus~ ha~e a suficien~1y high surface tension at the temperature of operation to be pulled from ~he nozzle as a continuous stream and not as ~a series of indlvidual droplets. S~ch requirements are totally the reverse of what i8 re~uired in an ink jet 25 printer whese there is no direct contact between th~

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q'IP/126/~ -3-printer and the substrate and the ink issues as a serles o~ discrete droplets. Fur~hermore, in the US Patent the substrate has ~o be of a specified type with a mandatory surface layer in order that the molten composition should be not cause appreciable dimensional changes in the substrate. The need for a special surface on the substrate se~erely limits the possible fields of use of this technique.
Accordingly, the present invention provides a process 10 for applying a thermoplas~ic composition through a non-contact ink jet printing apparatus as a series of discrete droplets onto a substrate moving relati~e to th~
apparatus, characterised in that the molten composition is thermally stable at the temperature of application and 15 is applied at a temperature in excess of 100C~
The invention can be used to apply the molten composition to a variety of substrates using on-de~and or continuous non-contact ink jet application techniques.
However, the inven~ion is of especial use in the 20 application of thermoplastic inks to a .substrate uslng an on-demand ink jet printer.
In an on-demand ink je~ printer, ink is fed under pressure, typically 0.5 to 20 psi gauge, though higher pressures may be used if desiredy from a reservoir to a , -~,~ ,.... .
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series of nozzles via valve mean~ which control the flow of the ink through each nozzleO The valve means is typically an electro-magnetically ac~uated valve, notably a solenoid val~e. The ink is discharqed throuqh the nozzlesas discre~e droplets in the desired sequence to form the required imaga on the substrate. usually, the nozzles are arranged in one or more series transversely to the line of movement of the substrate. ~ypically, such printers have quick acting valves with an operating 10 cycle time of from 1 to 5 mi1liseconds feeding noz~les with orifices ha~ing bore diameters of from 0.01 to 0.45 mms and an internal bore length to diameter ratio of from 3:1 to 1:2, notably ~rom 2:1 to 1:1.
The thermoplastic compositions for present use 15 comprise one or more image forming components, preferably oil miscible or soluble, in a fusible carrier medium.
The image forming materlal can be one which forms a visual image on the subs~rate, eg. it can b~ a dyestuff, or one which is de~ected by other means, eg. it can be a 20 magnetic material to be scanned by a suitable reader, or a fluorescent material, eg. one which is detected by an ultra-violet or other radiation scanner. For convenience, the present invention will be described in terms of a composition contalning a visually detectable 25 dyestuff.

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The composition for present use is preferably a solution or emulsion of the dyestuf in the carrier medium, ie. it i8 subs~antially free from particles which might block the nozzles through which the composition is to be disch~ryed. If neces~ary, the composition can contain a co-solvent or a bridging fluid ~o aid formation of a substan~ially homogeneous composition.
The compositions comprise a fusible carrier medium which is rendered molten at the temperatures encountered 10 in the method of the invention. ~he fusible carrier media for present use are thermally stable at the temperature of application and satisfy the surface tension and viscosity require~ents of the ink jet printer they are to be applied through. In general, the carrier 15 should not be thermally degraded or decomposed at temperatures of up to 160C and should have a viscosity of less than 120 Cps at the temperature of application.
They should have a surface tension which is sufficiently low at the operating tempera~ure for the ink ~o form 20 discrete droplets rather than a continuous jet which forms a bridge bewteen the nozzle and the substrate. In view of the dlfficulties in meas~ring surface tensions at elevated temperatures, the most convenient te~t of suitability for present use is to run the composition 25 throuqh the ink jet printer in which it is to be used to ~ .

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In general, where the composition has a surface tension of 50 dynes per cm or less in the abo~e test, it will be suitable for present use.
The carrier media for present use can be selected from amonst natural waxes having ~he desired porperties, but we prefer to use synthetic materials. We have ~ound that microcxystalline waxes, notably the synthetic forms o~ such waxes, and/or hydrocarhon resins provide 20 particularly advantageous carrier media, in tha~ they can provlde highly mobile molten compositions with reduced risk of degr~dation at high operating temperatures, which will typically be at from 110 to 160~C, notably at from 125 to 150~C~ Moreover, where the composition has a 25 softening point grea~er than 60C, the xapid cooling of ~, ,, .
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the droplets of the composition as they ætrike the substra~e and the partial fusing of the substrate below the hot droplet in the case of plastics substrates gives an image which resis~s smudging.
The in~ention therefore also provides a fusible ink composition comprising one or more oil soluble or miscible indicator materials, notably a dyestuff, in a carrier medium comprising a synthetic microcrystalline wax and/or a hydrocarbon res~n, the composition having a 10 viscosity of less than 120 Cps, a surface tension (as determined by the test method descibed above) of less than 50 dynes per cm at 25~C and a softening point of from 60 to llO~C.
The microcrystalline waxes for present use can be 15 selected from a wide range of such waxes which are available commercially. Typically, the wax will be a synthetic hydrocarbon wax obtained from the processing of petroleum or naphthas, notably the naphthenic, polyethylene or polypropylene waxes which have softening 20 points in the range 60 to 110C, notably 70 to 90~C.
Other s~itable waxes include those obtained by the Fischer Tropsch process, typically those comprisln9 long chain linear parafins with molec~lar weights of from 300 to 1500 and softening points in the range 80 to 110C.
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use include Slack Wax* and the polyethylene waxes obtained from the lighter fractions of the cracking of naphtha and petroleum. If desired, the microcyrstalline wax may be used in the form of a derivative thereof, eg. as an oxidised or maleinised derivative.
The hydrocarbon resins for present use are preferably crystalline resins, notably C5 or Cg chain length aliphatic waxes or polyolefins with softening points greater than 90C, typically 90 to 110C, and melting points greater than 110C, preferably in the range 140 to 160DC.
rrypically, the polyethylene resins will have a mean molecular weight of less than 1500, eg. 500 to 1200 and an acid number of less than 1 (expressad as mg KOH/g.)~
Suitable hydrocarbon resins for present use thus include polyolefins, notably polyethylene, polypropylene or polybutylene; C5 to Cg chain aliphatic resins, eg. those obtained by the steam cracking of naphthas; polyterpenes, notably wood rosins, tall oil or balsam resins, which can be esterified or hydrogenated if desired; and aromatic compounds, eg. styrenes such as methyl styrenes.
Whilst the hydrocarbon resin may often be a microcrystalline material, we have found that the use of a mixture of both an hydrocarbon resin and a synthetic microcrystalline wax as described above is of especial * trade mark .~

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advantage, since the hydrocarbon resin enhances the adhesion of the wax based composition to non-porous substrates, notably to plastics sheet substrates.
Furthermore, by varying the proportions of the resin and wax it is possible to tailor make the properties of the composikion, eg. the viscosity, to suit a wide range of operating conditions.
The compositions of the in~ention may contain other ingredients in addition to the microcrystalline wax, 10 resin and image forming material. Thus, the compositions can contain thermal and/or UV stabilising materials to reduce the degradation of the ingredients of the composition; and minor proportions of one or more solvents or co-solvents for the ingredients to aid 15 formulation of the composition as a substantially homoyeneous mixture.
~ he compositions for present use preferably have a viscosity of from 2 to 120, notably 3 to 20, Cps at the temperature of operation of the printer, typically 120 to 20 160C; and a ~urface tension of less than 40 dynes per cm, notably less than 30 dynes per cm, at 25C using the test metbod described above.
It may be desired to incl~lde one or more viscosity and/or surface tension modifying agents in ~he 25 composition to achieve the desired viscosity and/or 4~ ' '', ~ ~ ''' , '. ' ~` .

surface tension at the actual operating temperature3 for optimum operation of a particular printer. Howe~er, as indicated above, we have found that the viscosity and the surface tension are affected by the relative proportions of the microcrystalline wax and the hydrocarbon resin in the composition and that the viscosity and surface tension can often be adjusted to the desired value4 merely by varying the relative proportions of these two ingredients.
Where the compositions are to be applied using a continuous jet ink printer, it is necessary that the composltion be one which can accep~ an electrical charge. This is conveniently achieved by including one or more ionic or polar materials in the composition, 15 e.g. potassium isothiocyanate. In order to reduce the rlsk of segregation of these materials from the composition, it may be desired to incorporate a bridging compound, e.g. a wetting agent of the alkyl ether sulphate or sulphonate or of the alkyl benzene su~phon~te 20 type, into the composition~ Typically, the composition for applicatior. through a continuous jet printer will have a conductivity of at leas~ 1000, preferably 1500 to 2500, microSiemens.
The compositions of the invention comprise the 25 microcrystalline wax, the hydrocarbon resin and the image . ~ -.: `''':
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TIP~126/11 forming material in any suitable proportion having regard to the nature of the ingredients, the nat~lxe of the substrate it is to be applied to and the operating conditions under which it is applied. Typically, the composi~ion comprises at least one selected from a microcrystalline wax or an hydrocarbon resin, the mlcrocryRtzlline wax being present in up to 99~9~, notably from 40 to 99.5%; the hydrocarbon res~n being presen~ in from 0 to 65%, notably 25 to 55~; and ~he 10 image forming material being present in from 0.05 to 5~, preferably 0.1 to 2~: all percentages being of the active material and by weight on the weight of the total composition.
A particularly preferred composit~on for present use 15 comprises from 40 to 99 parts by weigh~ of a micro-crystalline polye~hylene wax, from 0 to 60 parts by weight of a polyethylene or styrene hydrocarbon resin of molecular wsight Prom 500 to 1200 and 0.1 to 1~5 parts by weight of an oil soluble dyestuff.
The compositions of the invention can be ~ade by any sui~able technique, e.g. by melting the wax and/or rQsin components ther~into.
The compositions can be put up in the form of powders or granules by spraying the mol~en mixtures of the 25 components into a void vessel. Alternatively, they can ~ ., .

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be extruded through a suitable die to form moulded plugs of the compo,~i~ion for insertion into a suitable shaped receptacle in the printer for melting and use.
As stated above, the compositions of -the invention are applied to a substrate by pas~ing them through the nozzle or array of nozzle~ of an ink jet printer appara~us at an elevated temperatur~. The apparatus can be of conventional design, except that those parts of the apparatus thxough which the molten composition is to flow 10 are heated or insulated so as to reduce the risk of the composition solidifying within the appara~us. Such heating can be achieved by any suitable means, e.g. by electrical heating elements around the appropriate ducts or vessels or by infra red or other radiant heaters 15 playing on the apparatus. Typically, the apparatus is operated with the composition flowing therethrough at temperatures of from 125 to 150C, e.g. about 140C.
The composition is fed to the apparatus in solid form, e.g. as chips or granules or the shaped plugs ~0 described abo~e, and is melted in a suitable vessel attached to or ~orming an integral part of the printer apparatus. If desired~ the compositions can be held in a separate heated reservoir and fed ~o one or more individual printer3 through heated or insulated lines.
Surprisingly, the presence of the microcrystalline , ~
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The printlng appara~us can therefore be operated under similar pressures and flow rates as with a converitional ink forrnulation and using nozzle orifices in the range 10 to 450 microns diameter.
The compositions of this invention can be applied to 10 a wide range of porous and non-porous substrates, e.g.
paper, metal, wood, plas~ics or glass without the need to form any special surface layer on the -substrate.
However, the invention is of especial use in forming images on non-porous materials, e.g. plastics, plastics 15 coated materials, glass and ~etalsO The high temperature of the composition as it is desposited on a plastics substrate causes enhanced adhesion of the composition due to partial fusion with the substrate. In the case of porous substrates the compositions penetrate lnto ~he 20 substrate as they cool. In both cases, the compositions of the invention solidify rapidly on the substra~e to give a sharp image resistant to smudging, The invention has been described above in ter~s of the application of an ink-type composition. However r it 25 can also be applied to adhesive composition, for example '~:

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those containing polyacrylamide or similar polymers.
Whilst it is known to apply hot melt adhesives from a gun or other applicator which extrudas the molten adhesive onto a substrate, we believe ~hat it is novel to apply ho~ melt adhesives as droplets emit~ed from an ink jet printer where the droplets are emmitted at a frequency of more than 100 drops per second from a nozzle.
~ he invention therefore also provides a method for applying a molten adhesive composition to a substrate 10 characterised in that the adhesive composition is dispensed at a temperature in excess of 100C through a noz~le to form a serles of droplets which are applied to the substrate, the droplets being formed at the nozzle at a rate of more than 100 per second.
The invention will now be illustrated by the following Examples in which all parts and percentages are glven by weight:
Example 1 An ink formulation ~as prepared by melting the ~0 microcrystalline polyethylene wax commercially available under the trade name Shell Microcrystalline wax MMP (54 part~) in a heated vessel fitted with a stirrer. To the molten Wax were added the oll soluble dyestuff Ceres Blue (1 part) and 45 parts of the hydrocarbon resin sold under 25 the trade name Escorez 5300. The resultant mixture had a .. i.. ~--. - - . ;
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~2~ j ~27~3 softening point of 85C and a melting point of 90C, both as determined by Brookfield thermoset viscometer, and surface tension of less than 40 dynes per cm as determinad by the test method described earlierO
The composition was fed ~o a drop-on-demand ink jet prin~er having electrical heating coils to maintain the ink reservoir, ink lines and printing head at 140C i 5C. The reservoir was pressurised to 2 psi gauge and the molten composition printed khrough the print head 10 using a 0.225 mms bore orifice to produce a series of separate droplets which formed discrete dot i~ages on a paper or polyethylene sheet target placed below the printing head. The dots were sharply defined, vell anchored to the substrate and resistant to smudging.
By way of contrast, when natural car~uba wax was used in place of the microcrystalline wax and hydrocarbon resin, the composition had a melting point of 50C and began to degrade when held ak 120C for 1 hour, as evidenced by charring. When this composition was applied 20 to a paper substrate at 80C, the image was soft and susceptible to smudging. Where a polyethylene sheet was used as the substrate, the image did not adhere to the sheet and was readily wiped off.

25 The process of Example 1 was repeated with a range of . , ,. . ~

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different resins and waxes being applied onto different target substrates including gla~s, metal, polyethylene, polypropylene, PVC, polystyrene, wax and plastics coated paper and card. The compositions are set out below as formulations A, B and C and in all cases the compositions were applied as in Example 1 and gave good dot image~0 When the compositions were replaced by con~entional sovent based formulations or low temperature wax form-ulations and applied ~o a non-porous ~ubstrate, the 10 resultant dots either did not dry rapidly and gave a runny image (in ~he case of a solvent ink) or were soft and readily smudged ~in the case of the low temperature waxes).

15 Formulation A
54.7~ wood rosin hydrocarhon resin 45.0~ low melting point microcrystalline polyethylene wax 0.3% oil soluble dyestuff 20 Formulation B
70.0% medium melting point microcrystalline polyethylene wax 29.5% commercially available C5 - Cg aliphatic hydrocarbon re~in 25 05% oil soluble dyestuf~
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Formulation C
55% commercially availahle water white grade alpha methyl styrene hydrocarbon resin 44.9~ of a commercially available mixture of microcrystalline and paraffin waxes o.1% oil soluble dyestuff .

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Claims (18)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for applying a thermoplastic composition as a series of discrete droplets of the thermoplastic composition in the molten state to a substrate moving relative to a printing apparatus by passing the thermoplastic composition in the molten state through a nozzle of a non-contact ink jet printing apparatus to form the droplets which are applied to the substrate, characterised in that the thermoplastic composition in the molten state is applied at a temperature in excess of 100°C; and in that the thermoplastic composition in the molten state is not significantly thermally degraded at the temperature of application of the thermoplastic composition in the molten state, has a viscosity of less than 120 Cps at the temperature of application, and has a surface tension which is sufficiently low under the operating conditions of the printing apparatus for the thermoplastic composition in the molten state not to form a continuous jet of fluid at the nozzle which bridges between the nozzle and the substrate.

2. A process as claimed in claim 1, characterised in that the thermoplastic composition has a softening point in excess of 60°C.

3. A process as claimed in either of claims 1 or 2, characterised in that the thermoplastic composition has a viscosity of from 2 to 20 Cps at the temperature of application.

4. A process as claimed in claim 1, characterised in that the thermoplastic composition has a surface tension of less than 50 dynes per cm at 25°C.

5. A process as claimed in claim 1, characterised in that the thermoplastic composition comprises one or more oil miscible or soluble image forming components in a fusible carrier medium; and in that the thermoplastic composition in the molten state is substantially free from a solvent and is thermally stable at temperatures of up to 160°C, has a softening point in excess of 60°C and a viscosity of less than 120 Cps at the temperature of application and a surface tension of less than 50 dynes per cm at 25°C.

6. A process as claimed in either of claims 1 or 5, characterised in that the thermoplastic composition in the molten state has a viscosity of from 3 to 20 Cps at, the operating temperature; and a surface tension of less than 40 dynes per cm at 25°C.

7. A process as claimed in claim 1, characterised in that the printing apparatus is operated at a temperature of from 125 to 150°C for the thermoplastic composition.

8. A process as claimed in claim 1, characterised in that the thermoplastic composition comprises a fusible carrier medium selected from a synthetic microcrystalline wax, a synthetic hydrocarbon resin and mixtures thereof.

9. A process as claimed in claim 8, characterised in that the synthetic microcrystalline wax and the synthetic hydrocarbon resin provide in total at least 95% be weight of the total thermoplastic composition.

10. A process as claimed in claim 8, characterised in that the microcrystalline wax is selected from a polyolefin wax or a paraffin wax having a molecular weight of from 300 to 1500 and a softening point in the range 80 to 110°C.

11. A process as claimed in claim 8, characterised in that the hydrocarbon resin is selected from a crystalline C5 to C9 chain length aliphatic resin and polyolefin resin, the resins having a softening point greater than 90°C and a melting point greater than 110°C.

12. A process as claimed in claim 1, characterised in that the thermoplastic composition is fed in the molten state under a pressure of from 0.5 to 20 psi gauge from a reservoir for the composition to at least one nozzle via a valve means which controls the flow of the molten composition through the nozzle.

13. A process as claimed in claim 12, characterised in that the valve means has an operating cycle time of from 1 to 5 milliseconds; and in that the nozzle has an orifice bore diameter of from 0.01 to 0.45 mms and an internal bore length to diameter ratio of from 3:1 to 1:2.

14. A thermoplastic composition suitable for application in the molten state as droplets to a substrate by means of a non-contact printing apparatus, characterised in that the thermoplastic composition comprises at least one oil miscible or soluble image forming component in a fusible carried medium; in that the thermoplastic composition is not significantly thermally degraded at a temperature of 160°C, has a viscosity of less than 120 Cps at 160°C, and has a surface tension of less than 50 dynes per cm at 25°C; and in that the fusible carrier medium comprises at least one component selected from a synthetic microcrystalline wax, a synthetic hydrocarbon resin and mixtures thereof.

15. A composition as claimed in claim 14, characterised in that the microcrystalline wax and the hydrocarbon resin provide in total at least 95% by weight of the total composition.

16. A composition as claimed in claim 14, characterised in that the microcrystalline wax is present in from 40 to 99.5% by weight of the total composition.

17. A composition as claimed in claim 14, characterised in that the hydrocarbon resin is present in from 25 to 65%
by weight of the total composition.

18. A composition as claimed in claim 14, characterised in that the microcrystalline wax is selected from a polyolefin and/or paraffin wax having a molecular weight in the range of 300 to 1500; and in that the hydrocarbon resin is selected from a crystalline C5 to C9 aliphatic resin, a polyolefin resin and/or a styrene resin, said latter two resins having a molecular weight of from 500 to 1200; and in that the said waxes and/or resins have a softening point in the range 80 to 110°C.