GB2359556A

GB2359556A - An ink for decoration of substrates such as polycarbonate

Info

Publication number: GB2359556A
Application number: GB9927357A
Authority: GB
Inventors: Brian Smith; Andrew Lane Phillips
Original assignee: Sericol Ltd
Current assignee: Sericol Ltd
Priority date: 1999-11-19
Filing date: 1999-11-19
Publication date: 2001-08-29
Anticipated expiration: 2019-11-19
Also published as: GB2359556B; GB9927357D0

Abstract

An ink for decoration of substrates such as polycarbonate which are to be used in a moulding process. The ink comprises a polyurethane resin as a binder. The ink may also include a solvent, a pigment and a matting agent. The ink is useful for printing on polycarbonate, especially for screen process printing on polycarbonate sheets used for in-mould decoration. The ink has good flexibility, elasticity, adhesion and resistance to heat.

Description

The present invention concerns an ink that is suitable for decorating substrates such as, for example, polycarbonate or coated polycarbonate. In particular, this invention concerns an ink that is suitable for printing black, white or coloured images on polycarbonate substrates to be converted into polycarbonate arcticles. One method of producing of black, white or coloured images on shaped polycarbonate articles is a process known as in-mould decoration. In this process, an image is first printed on a thin sheet or film of polycarbonate resin. The printed sheet is then shaped on a press to produce the intended contours and design of the surface of the article. This shaping is often carried out on a thermal vacuum-forming press, although other shaping processes such as, for example, hydro-forming, air pressure forming and match metal forming can be used. After shaping, the resulting sheet is inserted into a mould of the article which is to be produced. Polycarbonate resin, from which the bulk of the article is to be made, is forced into the mould under heat and pressure at the rear of the printed shaped sheet. Typically, the resin is injected into the mould at around, for example, 300 C. Under these conditions, the polycarbonate resin bonds strongly to the sheet so that when the article is taken out of the mould it is decorated. Depending on whether the printed image was on the outer or inner side of the shaped sheet as it was inserted into the mould, the decoration is either on the surface of the printed article or below its surface at a depth equal to the thickness of the sheet. Both methods are used, although in the latter case, the shaped sheet must be of a transparent material so that the decoration is visible. In the latter case, the decoration is thereafter protected from wear. The decorating of polycarbonate articles in this way is advantageous when compared to two known alternative methods. In the first method, the article is decorated after it has been moulded. This may be a difficult method if the article has a complicated three-dimensional shape, in particular with marked projections or depressions or features of small radius of curvature relative to the size of the article. In the second method, as described in EP 0 311 079, a flat elastic sheet is printed with the desired decoration but it is not formed or shaped before it is inserted into the mould. Pressure and heat of the moulding press are used to deform simultaneously the resin forming the bulk of the article and the flat elastic printed sheet. Although this method enables more complex shapes to be made with a decorated surface, it suffers from the problem that a fault in the surface is only discovered on final inspection of the article. Inks for in-mould decorating methods using a pre-formed printed sheet must possess many properties. In particular, they must be capable of being printed by an appropriate process such as, for example, screen process printing and pad printing. They must also show good wetting and adhesion on sheet substrates such as sheet polycarbonate. After printing on the sheet substrate used in in-mould decoration, the printed layer must be elastic and flexible in order to undergo deformation of the sheet substrate on a thermal vacuum forming press or on another shaping press without cracking, peeling or loss of adhesion. After insertion of the formed sheet into the mould, the printed image must survive the heat of the moulding process without decomposition or discolouration. Decomposition, particularly resulting in the formation of gas or vapour, weakens the shaped article and makes it useless. Discolouration destroys the intended decorative effect. If, as in the case explained above, the printed image is on the inner side of the sheet when it is put into the mould, the layer of ink forming the image must not prevent the bonding or union between the sheet and the bulk of polycarbonate resin being moulded. In particular it must be completely wetted by the liquid polycarbonate resin in the mould and remain as a bonding layer between the two elements when the article is released from the mould. EP 0 688 839 describes a flexible ink resistant to high temperatures useful for in-mould decoration of polycarbonate. This ink uses as binder a novel thermoplastic aromatic polycarbonate resin. EP 0 814 115 describes a composition for in-mould decoration of polycarbonate which is cured or hardened by irradiation with ultraviolet light. An aim of the present invention is to provide an ink for decoration of substrates such as polycarbonate.

A further aim of the present invention is to provide an ink for decorating substrates such as polycarbonate, which is capable of being printed by a variety of processes such as, for example, screen process printing and pad printing. s A further aim of the present invention is to provide an ink for decorating substrates such as polycarbonate, which exhibits good wetting and adhesion to the substrate. A further aim of the present invention is to provide an ink for decorating substrates such as polycarbonate, which is elastic and flexible in order to undergo deformation of the substrate without cracking, peeling or loss of adhesion.

A further aim of the present invention is to provide an ink for decorating substrates such as polycarbonate, which can survive the heat of a moulding process without decomposition or discolouration.

In accordance with the present invention there is provided an ink for decoration of substrates such as polycarbonate, the ink comprising a polyurethane resin as a binder.

The polyurethane resin is preferably a solvent-borne, fully reacted polyurethane that forms films solely by loss of the solvent. Such polyurethanes are described in, for example, the Encyclopaedia of Polymer Science and Technology, second edition, Volume 13, page 292. Coating compositions based on aromatic polyurethane resins are known for other purposes such as, for example, applying to textiles to form synthetic leather. However, it is surprising to find that these resins can give the combination of flexibility, elasticity, adhesion and thermal resistance required in an ink for in-mould decoration processes for decoration of substrates such as polycarbonate. By `fully reacted' is meant a polyurethane which is made from an isocyanate and a_ stoicheiometric or greater than stoicheiometric proportion of polyol or other reagent, so that the resulting resin contains no free isocyanate groups.

The inks of this invention have the advantage that the polyurethanes are available commercially.

When printed, the inks of this invention do not require a separate stage of hardening under ultraviolet light as is required for the inks of EP 0 814 115. In the process of EP 0 311 079, where the inks require properties similar to those in the present invention, the binder is said to be a vinylite resin rather than a polyurethane as in the present invention.

The ink comprises a thermoplastic polyurethane resin which is preferably a fully reacted thermoplastic aromatic polyurethane resin. Such materials are commercially available, for example, under the tradenames Desmolac (from Bayer), Estane (from B.F. Goodrich Chemical Inc.) and Uraflex (from DSM Resins bv). A mixture of these materials may also be used. The thermoplastic polyurethane resin preferably makes up from 10 to 40%, preferably from 15 to 25%, of the ink.

The ink also preferably comprises a solvent or solvent blend in which the polyurethane is soluble. Preferred solvents are esters including cyclic esters, for example, methoxypropyl acetate, isopropoxyethyl acetate, ethoxyethyl propionate, ethyl lactate, isopropoxyethyl acetate, methoxypropyl acetate, ethyl ethoxypropionate, ethoxypropyl acetate, dimethyl succinate, dimethyl glutarate, dimethyl adipate, butyldiglycol acetate, butoxyethyl acetate, butyrolactone; ketones, for example, cyclohexanone, isophorone, diacetone alcohol, acetophenone, cyclohexanone, di-isobutyl ketone; and amides, for example, N-methylpyrrolidone. A mixture of these solvents may also be used. Part of the solvent may be replaced by one or more diluent solvents, for example, aromatic hydrocarbons having boiling points between 160 and 280 C, and glycol ethers such as ethoxypropanol, propoxyethanol, butoxypropanol, isopropoxyethanol and dipropyleneglycol methyl ether. The solvent is preferably present in the ink in an amount from 40 to 90%, preferably from 50 to 70%. The relative proportions of solvent and polyurethane are such that the latter is completely dissolved in the solvent.

The ink also preferably comprises a pigment. The pigment should be selected to be stable to heat and to be compatible with the other components of the ink. Suitable pigments are Pigment Red 144, Pigment Orange 43, Pigment Green 7, Pigment Blue 15:3 and Pigment Yellow 151. Such pigments are commercially available such as, for example, under the tradenames Sandorin (from Clariant plc), Hostaperm (from Hoechst UK) and Chromophthal (from Ciba Pigments). A white ink may be made using as the pigment titanium dioxide available, for example, under the tradename Finntitan from Kemira OY. A black ink may be made using as the pigment carbon black, available, for example, under the tradename Printex from Degussa AG. A mixture of these pigments may also be used. The pigment is preferably present in the ink in an amount from 5 to 35%, preferably from 8 to 24%, of the ink. The ink also preferably comprises a matting agent. We have found that a matting agent can be used to improve adhesion of the printed sheet to the resin from which the bulk of the article is made, so that on removal from the mould the article has a decorated surface to which it is well bonded. A preferred matting agent is silica, for example, those available commercially under the trade names Aerosil (from Degussa AG), Cab-o-Sil (from Cabot GmbH). A mixture of matting agents may also be used. The matting agent is preferably present in the ink in an amount from 0.2 to 25%, preferably from 1 to 10%, and more preferably from 1 to 5%. The following components may also be added to the ink: extenders, surfactants, stabilisers, reodorants, biocides, identifying tracers, defoamers, flow aids or other film forming resins such as, for example, polyesters or acrylics.

Components having a deleterious effect on the desirable properties of the ink should not be incorporated into the ink. One such component is a silicone flow aid which can reduce adhesion. If an exceptionally resistant print is required, the ink may be prepared as a two-pack system in which the ink is in the first pack and an isocyanate is in the second pack, which is added to the ink before use.

The inks are preferably applied to the substrate by screen process printing, but are capable of being printed by other methods such as, for example, by pad printing. This ink may be used for in-mould decoration, they may also be used to decorate articles made from injection moulding.

By way of example, the invention will now be described with reference to the following examples, in which all parts are by weight: EXAMPLE 1 The following components were mixed together using a Grieves stirrer in a water-cooled pot to give a homogeneous solution:

Isopropoxyethyl acetate 30 parts Cyclohexanone 5.6 parts Estane 5715 19.7 parts When the solid had dissolved the following component was added with slower stirring for 30 minutes:

Printex Fluffy 30 8 parts The mixture was transferred to a triple-roll mill and milled until no nibs were visible on a Hegman gauge. The product was then stirred using a Torrance stirrer and the following components were added:

Isopropoxyethyl acetate 2.7 parts Aerosil 200 2.5 parts EFKA 2020 (defoamer from Efka NV) 1 part Isopropoxyethyl acetate 30.5 parts The product was a black ink which was printed through a 120 threads per centimetre screen stencil on to a 380 micron Hiform polycarbonate sheet (from Autotype Ltd). The sheet was thermally formed into a three-dimensional shape using a hydro forming process. The formed three-dimensional shape was then inserted into a 100 ton clamp pressure injection moulding machine and fused to polycarbonate resin (NF 110 from General Electric) as part of the injection moulding process. The resin temperature at injection was between 280 and 340 C. EXAMPLE 2 s The following components were stirred in a water-cooled pot using a Torrance stirrer to give a homogeneous solution:

Isopropoxyethyl acetate 29.6 parts Cyclohexanone 5.5 parts Estane 5715 19.1 parts When the solid had dissolved the following were added:

Finntitan RDIS 22.0 parts The mixture was stirred until no nibs were visible on a Hegman gauge. Then with slower stirring the following components were added in the order given:

Aerosil 200 3.2 parts Isopropoxyethyl acetate 10 parts EFKA 2020 1 part Isopropoxyethyl acetate 11 parts The mixture was stirred until it became homogeneous. The product was a white ink which was printed as for Example 1 and in combination with the black ink used in Example 1. The ink showed good adhesion, flexibility and heat resistance on a polycarbonate substrate. EXAMPLE 3 The following components were stirred in a water-cooled pot using a Grieves stirrer to give a homogeneous solution:

Isopropoxyethyl acetate 31.8 parts Cyclohexanone 5.5 parts Estane 5715 19.2 parts The speed of the stirrer was reduced and the following components were added:

Isopropoxyethyl acetate 14.3 parts

Hostaperm Orange G-R 9.7 parts When the addition was complete the mixture was stirred for a further 30 minutes, then transferred to a Turbomill and milled for 8 hours until the mixture showed no nibs on a Hegman gauge. The product was then stirred slowly on a Grieves stirrer and the following were added and stirred until the mixture became homogeneous:

Cab-o-sil M5 2.5 parts EFKA 2020 1.0 part Isopropoxyethyl acetate 16.0 parts The product was an orange ink which was printed as for Example 1 and in combination with the black ink used in Example 1. The ink showed good adhesion, flexibility and heat resistance on polycarbonate substrate.

Claims

Claims 1. An ink for decoration of substrates such as polycarbonate which are to be used in a moulding process, the ink comprising a polyurethane resin as a binder.
2. The ink claimed in claim 1, wherein the polyurethane resin is a solvent-irorne, fully reacted thermoplastic polyurethane that forms films solely by loss of the solvent.
3. The ink claimed in claim 2, wherein the thermoplastic polyurethane resin is a fully reacted thermoplastic aromatic polyurethane resin.
4. The ink claimed in claims 2 or 3, wherein the thermoplastic polyurethane resin is present in an amount from 10 to 40%, preferably from 15 to 25%.
5. The ink claimed in any one of the preceding claims, wherein it further comprises a solvent or solvent blend in which the polyurethane is soluble.
6. The ink claimed in claim 5, wherein the solvent is selected from esters, ketones, or amides, or mixtures thereof.
7. The ink claimed in claim 6, wherein the solvent is selected from methoxypropyl acetate, isopropoxyethyl acetate, ethoxyethyl propionate, ethyl lactate, cyclohexanone, isophorone, diacetone alcohol and N-methylpyrrolidone, and mixtures thereof.
8. The ink claimed in claim 5, wherein the solvent is a diluent solvent is an aromatic hydrocarbon preferably having a boiling point between 160 and 280 C or a glycol ether preferably selected from ethoxypropanol, propoxyethanol, butoxypropanol, isopropoxyethanol or dipropyleneglycol methyl ether.
9. The ink claimed in claims 6, 7 or 8, wherein the solvent is present in an amount from 40 to 90%, preferably from 50 to 70%, of the ink.
10. The ink claimed in any one of the preceding claims, wherein it further comprises a pigment.
11. The ink claimed in claim 10, wherein the pigment is present in an amount from 5 to 35%, preferably from 8 to 24%, of the ink. s
12. The ink claimed in any one of the preceding claims, wherein it further comprises a matting agent.
13. The ink claimed in claim 12, wherein the matting agent is silica.
14. The ink claimed in claims 12 or 13, wherein the matting agent is present in the ink in an amount from 0.2 to 25%, preferably from 1 to 10%, and more preferably 1 to 5%.
15. The ink claimed in any one of the preceding claims, wherein the ink is prepared as a two-pack system in which the ink is in the first pack and an isocyanate is in the second pack.
16. Use of the inks claimed in any one of claims 1-15 for printing on polycarbonate substrates which are to be used in a moulding process.
17. Use of the ink claimed in any one of claims 1-15 as an ink in an in-mould decoration process.
18. A method for printing a substrate using an in-mould decorating process, the method comprising the step of applying the ink claimed in any one of claims 1-15 to the substrate.
19. The method claimed in claim 18, wherein the substrate is polycarbonate or coated polycarbonate.
20. The method claimed in claims 17 or 18, wherein the ink is applied to the substrate by a screen process printing or pad printing.