Transfer sheet and method of manufacturing a transfer sheet

The present invention relates to a transfer sheet and in particular, but not exclusively, to a transfer sheet that may be used for transferring images onto garments or fabrics. The invention also relates to a method of making transfer sheets.

A well-known process for transferring images onto T-shirts and similar garments involves the use of a transfer sheet having a paper backing or substrate and an extruded resin coating. An image can be printed onto the resin coating using an electrostatic imaging device such as a colour laser printer or copier, which forms an image by fusing dry toner powder onto a recording sheet. The toner that forms the image is captured in the resin layer of the transfer sheet and can be transferred together with the resin layer onto a T-shirt by applying heat and pressure. The backing paper can be peeled away while the resin is still hot, leaving the image and the resin layer adhered to the fabric of the T-shirt.

In some known processes, the backing sheet comprises a release paper that is treated on the transfer side thereof with a release agent such as silicone. This ensures complete transfer of the resin coating onto the fabric. However, the use of release papers carries a risk that the resin layer can become detached from the backing sheet during printing, contaminating the printing equipment. Moreover, after transfer to the fabric, the resin coating can sometimes have an undesirable glossy appearance. Production of release papers also requires a separate off-line coating process, which adds to the cost of the product.

US 6395119 describes a transfer sheet having an extruded coating of a hot-melt thermoplastic polymeric film material on a plain paper backing sheet, which is not treated with a release agent. In use, the polymeric film material is designed to split so that approximately 80% of the material is transferred onto the shirt with a residual amount remaining on the backing paper. The transfer sheet provides a high quality image but the

image does not have good washing resistance. The transfer sheet cannot be used with an automatic sheet feeder, as the sheets tend to stick to one another when stacked, causing the sheet feeder to j am. Although the hot peel characteristics of the transfer sheet are generally good, the backing sheet can sometimes be quite difficult to remove, and 100% toner transfer is not always achieved.

US 5614345 describes another transfer sheet in which a thin top coat is provided on top of the resin layer, on a plain paper backing sheet. This top coat contains a low temperature film forming binder and anti-adhesion pigments which reduce the coefficient of friction of the sheet. The top coat provides improved washing performance and it is claimed that sheet feeding is also improved. However, we have found that in practice the sheet does not allow reliable multiple sheet feeding. The sheet also has an uneven surface appearance, which is easily marked during handling. The uneven surface appearance also shows on the transferred image and can be off-putting to some users. Again, although the hot peel characteristics of the transfer sheet are generally good, the backing sheet can sometimes be quite difficult to remove, and 100% toner transfer is not always achieved.

Ideally, the materials used in the transfer sheet should be chosen to have a low peel strength, allowing efficient transfer of the image onto the T-shirt and easy removal of the backing sheet without the use of a release sheet. The transferred image should have strong colours, it should be soft to the touch and it should be durable, having a good washing performance (i.e. good resistance to fading). Ease of printing is also important and when printing several sheets in a batch, it is desirable that the sheet should be compatible with an automatic sheet feeder, allowing multi-sheet feeding. Preferably, the sheet should be suitable for manufacturing using either an on-line (single step) production process or an offline (two-step) process.

It is an object of the present invention to provide a transfer sheet that mitigates at least some of the aforesaid disadvantages. A further object of the invention is to provide a transfer sheet having superb hot peel characteristics. Yet another object of the invention is to provide a method of making such a transfer sheet.

According to the present invention there is provided a transfer sheet for transferring an image onto a textile fabric, including a backing sheet, a lower layer of a thermoplastic film material bonded to the backing sheet, and an upper layer applied to the lower layer, which contains a binder and a polymer wax, wherein the lower layer is constructed and arranged such that when the transfer sheet is applied to a textile fabric and the backing sheet is peeled away, the lower layer splits to leave the upper layer and a first part of the lower layer adhered to the fabric, and a second part of the lower layer adhered to the backing sheet.

The transfer sheet has a low surface friction giving it the capacity to be multi-sheet feedable. This is very helpful for printing batches of transfer sheets. The transfer sheet prints well and is easy to use, having a low peel strength and superb hot peel characteristics, which are superior to those of other currently-available products. It has a uniform appearance and is not easily marked during handling. It is suitable for production in both online and offline production processes. Images transferred onto textile fabric with the transfer sheet have a good appearance and are durable, having a good washing performance.

The image on the recording sheet is preferably electrostatically formed: that is, an image is formed by fusing toner to the transfer sheet using an electrostatic imaging device such as a laser printer or a photocopier. The toner may be either dry toner powder or liquid toner, for example as used in the HP Indigo ^® range of printers.

Advantageously, the polymer wax is a polyethylene wax. The wax may be any suitable natural or synthetic wax material that can be used to improve slip performance. The polymer wax is preferably a micronised modified wax/polymer mixture. Alternatively, it may be a polyethylene wax/paraffin wax mixture. The polymer wax preferably has a melting temperature of approximately 115°C.

Advantageously, the polymer wax content of the upper layer is in the range 2-30%, preferably 5-20%, more preferably approximately 10% by dry weight.

Advantageously, the binder is a polyurethane-based binder. Advantageously, the binder content of the upper layer is in the range 70-98%, preferably 80-95%, more preferably approximately 90% by dry weight.

Advantageously, the upper layer has a weight density in the range 0.3-5. Og/m ² , preferably 0.4-2g/m ² , more preferably approximately 0.4-1. Og/m ² .

The lower layer may contain an ethylene- vinyl acetate (EVA) copolymer, or a mixture of EVA copolymers, or separate layers of different EVA copolymers.

The EVA content of the lower layer is preferably in the range 90-100% by dry weight. The EVA copolymer preferably has a vinyl acetate (VA) content in the range 20-30%, preferably 22-28%, by weight.

The lower layer may contain an anti-block EVA compound. Advantageously, the anti- block EVA compound has a vinyl acetate (VA) content in the range 10-25% by weight. Advantageously, the anti-block EVA compound content of the lower layer is in the range 0-5%, preferably 1-2.5%, by dry weight.

Advantageously, the lower layer has a weight density in the range 20-50g/m ² , preferably 25- 40g/m ² , more preferably approximately 30g/m ² .

Advantageously, the first part of the lower layer comprises a major portion of the lower layer and the second first part of the lower layer comprises a minor portion of the lower layer. For example, first part of the lower layer may comprise from 50% to 95% by weight of the entire thermoplastic film material, and typically comprises approximately 75-80% thereof. Advantageously, the lower layer has a low internal cohesive strength at the pressing temperature so that it splits readily during peeling away of the backing sheet, thus avoiding the problems associated with the use of release sheets.

The backing sheet is preferably made of paper. Preferably, the paper is a bond paper, more preferably a coated bond paper. Preferably, the coated bond paper has a coating on one or both sides of the paper comprising a binder and a pigment, for example starch and calcium carbonate. Advantageously, the coating has a weight in the range 1-12 g/m ² , preferably 3-4 g/m ² . Advantageously, the backing sheet has a weight density in the range 50-200g/m ² , preferably 90-130g/m ² . The backing sheet preferably has a surface roughness on the side facing the lower layer in the range 4-8μm as measured by PPS (Parker Print Surf CP500KPa). This method of measuring roughness is set out in the following standard:

ISO8791 part 4 1992. It is commonly used to determine the roughness of paper and/or board.

We have found that a transfer sheet having a backing sheet made of coated bond paper as specified above has extremely good hot peel characteristics, surpassing the performance of existing available products. The backing sheet is very easy to remove after applying the transfer to a garment, and complete toner transfer is achieved.

Alternatively, the backing sheet may be made of a polymeric sheet material.

Advantageously, the lower layer is applied to the backing sheet as an extruded film. The upper layer is preferably applied to the lower layer as an aqueous coating.

According to another aspect of the invention there is provided a method of making a transfer sheet, comprising coating a backing sheet with a lower layer of a thermoplastic film material and applying an upper layer over the lower layer, the upper layer containing a binder and a polymer wax.

The upper layer is preferably applied as an aqueous coating and the lower layer is preferably applied as an extruded coating. The lower layer is preferably subjected to a Corona® electric discharge treatment before the upper layer is applied. The backing sheet is preferably a paper substrate, for example a coated bond paper, that does not have a release treatment on the transfer side thereof.

According to another aspect of the invention there is provided a method of printing a textile fabric, comprising providing a transfer sheet as defined by any one of the preceding statements of invention, printing the transfer sheet to form an image, transferring the image to the textile fabric by placing the transfer sheet against the fabric and applying heat and pressure, and removing the backing sheet to leave the image, the upper layer and a major part of the lower layer adhered to the fabric.

According to a preferred aspect of the present invention there is provided a transfer sheet for transferring an image onto a textile fabric by a process that involves printing the transfer sheet digitally for example via a colour printing device to form an image. Usually, the

image is printed in reversed (or "mirror") format to provide a non-reversed image on the final garment. The image is subsequently transferred to the textile fabric by placing the transfer sheet against the fabric and applying heat and pressure. The materials of the backing sheet and the upper and lower layers are selected such that after applying heat and 5 pressure, the backing sheet can be peeled away to leave the upper layer and a major part of the lower layer adhered to the fabric, while a minor part of the lower layer remains adhered to the backing sheet.

Various embodiments of the invention will now be described by way of the following examples and with reference to the accompanying drawing, which provides in Fig. 1 a 10 schematic representation showing in cross-section a transfer sheet according to an embodiment of the invention.

The transfer sheet includes a backing sheet 2 with a polymeric coating 4 comprising an extrusion coated lower layer 6 and an upper layer 8 that is applied as an aqueous coating over the lower layer.

15 In this example, the backing sheet is made of paper and typically has a weight density of 90, 110 or 130g/m ² . The paper is preferably white and has good surface smoothness, for example with a roughness in the range 4-8μm as measured by PPS (Parker Print Surf CP500KPa). Papers rougher than this exhibit a speckle phenomenon after printing, which is wholly undesirable.

20 Suitable papers for the backing sheet 2 are:

1) Ultra smooth commercial papers having a roughness value close to 3μm as measured by PPS (Parker Print Surf CP500KPa). These however are generally expensive.

2) Bond type papers. These are generally uncoated papers that are machine finished and surface treated. These bond papers can be mineral filled with an ash content up to

25 35%.

3) Coated bond papers. These have a surface coating on the fibre surface applied in situ on the paper machine via a size press, film press or speed sizer. The coating normally

contains at least a binder such as starch and a pigment, for example calcium carbonate. The coating weight of the coated bond paper is typically 12 g/m ² per side or less.

Of these, coated bond papers are particularly suitable as they have superb hot peel properties, being better in this respect than current marketplace products. Suitable coated bond papers may have a coat weight as low as 3.5 g/m ² per side.

Alternatively, a polymeric sheet material, for example polyethylene terephthalate (PET), may be used for the backing sheet.

The lower layer 6 preferably comprises an extruded film of an ethylene copolymer, and preferably an ethylene-vinyl acetate (EVA) copolymer with a weight density of approximately 3 Og/m ² . This layer may consist of a single EVA copolymer or a mixture of EVA copolymers. For example, we have used Exxon εscorene Ultra UL04028, Evatane 28-40 grade EVA from Arkema and Elvax 3200 grade EVA from DuPont. The Evatane 28-40 grade and the Exxon εscorene Ultra UL04028 have a relatively high vinyl acetate (VA) content of 28% by weight, which makes them flexible and soft to the touch. These materials provide a good "feel" when transferred onto fabric. However, they are very sticky and a backing sheet coated only with these materials would therefore be very difficult to handle or use in an offline (two-stage) production process. The Elvax 3200 grade has a vinyl acetate (VA) content of 22.5% and is stiffer than the Evatane 28-40 but less sticky and therefore easier to handle. These materials may be mixed in various proportions to produce the desired handling and touch characteristics, and a low internal cohesive strength at the pressing temperature to ensure that the lower layer 6 splits easily when peeling away the backing sheet.

Additionally, the EVA mixture may include an EVA compound with anti-block/slip additives to aid release of the extruded film from the chill roll and prevent production blocking between the extrusion coating and aqueous coating steps when the transfer sheet is manufactured in roll form. For example, we have used Elvax anti-block grade CE9619-1 from DuPont. We have found that this greatly improves production speed and efficiency.

Some suitable recipes for the extrusion coating are as follows, wherein the composition is expressed as a percentage by weight:

Data on the EVA materials used is as follows:

MFI = melt flow index

Alternatively, the different EVA copolymer materials may be co-extruded to provide, for example, a thick layer of the softer Evatane 28-40 copolymer covered by a thin layer of the less sticky Elvax 3200 copolymer. This maximises the amount of the softer material to provide good touch and feel characteristics, while also providing good handling properties. The thin covering layer of the stiffer Elvax 3200 copolymer may also if required include a proportion of the Elvax anti-block grade CE9619-1.

We have found that EVA copolymers with a VA content of less than 20% give poor washing, poor touch and feel characteristics when transferred onto the fabric. They are not soft or flexible. The use of an EVA layer on its own (without an upper layer providing good anti-friction and washing properties) is also undesirable, as this results in poor sheet feeding and fading of the image during washing. These characteristics of the EVA copolymers, and the target characteristics for the transfer sheet, are illustrated below:

In the above table, a tick {/) indicates that sheet has the required characteristic, whereas a cross QC) indicates that it does not have the required characteristic.

The upper layer 8 preferably comprises a layer of non-tack coating, applied over the lower layer as an aqueous coating. Production methods for applying the top layer include for example spray coating, flexo printing, the gravure process or using a rod coater. It can be applied immediately after the lower layer 6 has been extruded onto the backing sheet (that is, in a single on-line production process) or subsequently (off-line) in a separate process. The lower layer 6 is subjected to Corona® electric discharge treatment before the upper layer 8 is applied. The upper layer preferably has a weight density of approximately 1.0g/m ² or less.

The upper layer 8 contains a mixture of a polyurethane-based binder and a polyethylene (PE) wax, typically with a PE wax content of approximately 10% and a binder content of approximately 90% by dry weight. It may also contain other additives, for example a wetting agent, a matting agent and/or a cross-linking material.

The PE wax may for example be a micronised modified wax/polymer mixture such as Aquaflour 400 PE wax from BYK-Chemie, which has a wax melt temperature of 115°C.

The polyurethane (PU) can be the sole binder or a mixture of hard and soft polyurethanes can be used. This is used as a water-borne dispersion or emulsion. Examples of suitable water-borne PU dispersions are Sancure 815, 835 and Permax 200 grades from Noveon,

the former being a carboxylated urethane polymer whilst the latter is an aliphatic polyether urethane. Other PU materials such as Lamberti grades LB2 and AK4 have been shown to produce acceptable results. A polyurethane can be described as a polymer formed by repeating urethane links.

The PU binder ensures strong toner adhesion and thus provides good washing performance, with little fading of the transferred image. The PE wax removes tack and provides a low coefficient of friction, which allows multiple sheet feeding. The upper layer has a uniform appearance and is resistant to finger marking.

Crosslinkers can be added to improve the crosslink density of the coating formulation to improve frictional and water resistance. Suitable materials could be selected from polyfunctional aziridines and zirconium salts.

Matting agents may be added to the top coat recipe to control the finished gloss of the transfer. Suitable materials could be selected from organic/inorganic materials known to give a matting effect.

The upper layer may for example be made according to the following formulation:

The recording sheet may be manufactured in an offline (two-stage) process by first applying the lower EVA layer 6 to the backing sheet 2 in an extrusion coater, rolling the coated sheet, and subsequently applying the upper layer 8 as an aqueous layer on top of the lower layer 6, for example using a rotogravure process. Alternatively, the recording sheet may be manufactured in an online (single stage) process by extruding the lower EVA layer 6

onto the backing sheet 2 and immediately applying the upper layer 8 as an aqueous layer on top of the lower layer 6.

To create and transfer an image onto a garment such as a T-shirt, the recording sheet is first printed with the required image using a digital printing device, for example a laser printer or a photocopier. This deposits toner on the resin coating and then fuses the toner to the coating to form an image by the application of heat. The image is subsequently transferred to the T-shirt by placing the transfer sheet on top of the garment and pressing the transfer sheet with the T-shirt in a heated press, typically for 10-20 seconds at a temperature of about 180°C. The shirt is then removed from the press and the backing sheet is peeled away whilst still hot. This causes the resin coating to split, leaving the toner image and a major portion of the resin coating adhering to the fabric of the T-shirt, while a minor portion of the resin coating remains adhering to the backing sheet.

Examples

Various recording sheets according to the invention were made and tested, along with comparative examples with different formulations. The results of these tests are summarised in Table 1.

Comparative example 1 (Comp 1) is a recording sheetofthe type describedin US 5614345, having a backing sheet of very smooth base paper with a weight of 110g/m ² and a roughness of approximately 3-4μm as measured by Parker Print Surf. The backing sheet carries a lower extruded layer of EVA copolymer and an upper layer comprising pigments (rice starch and synthetic particles) in a styrene acrylate binder. The pigments and the binder constitute respectively approximately 90% and 10% of the upper layer by dry weight. The recording sheet has very poor surface sensitivity, being easily marked by finger prints, and works only partially in an automatic sheet feeder (that is, it does not work reliably). Garments printed using the transfer sheet have good washing performance, with little fading in a 4O ⁰ C wash cycle.

Comparative example 2 (Comp 2) is a recording sheet of the type described in US 6359119, having a backing sheet of a plain commercial base paper with a weight of 110g/m ² and a

roughness of 8-9μm. The backing sheet supports an extruded layer of EVA copolymer, but no additional layer on top of the extruded layer. The recording sheet has good surface sensitivity but is not capable of multi-sheet feeding. Garments printed using the transfer sheet have a poor washing performance, with significant fading in a 40°C wash cycle.

Examples 1 to 6 (Ex 1 to Ex 6) are all recording sheets having a paper backing sheet made with a bond type paper (Conqueror® CX22 Ultra Smooth from Arjo Wiggins) having a weight of 90-100g/m ² and a roughness of 4.0-4.5μm. The backing sheet supports an extruded layer of EVA copolymer. Example 1 (Ex 1) does not have an additional layer on top of the extruded layer. Example 2 (Ex 2) does have an upper layer on top of the lower extruded layer, which is applied as an aqueous coating and includes a mixture of two PU binders but no PE wax. Examples 1 and 2 do not fall within the scope of the claims and are included for comparative purposes only.

Examples 3 to 6 (Ex 3 to Ex 6) each include an aqueous coating on top of the extruded layer, which includes a mixture of two PU binders and a PE wax material.

Example 7 (Ex 7) uses a coated bond paper backing sheet (HS 90 from Arj o Wiggins) having a weight of approximately 90g/m ² and a roughness of 6.0-6.7μm. The backing sheet supports an extruded layer of EVA copolymer with an aqueous top coat comprising a mixture of PU binders and a PE wax.

Example 8 (Ex 8) is a recording sheet comprising an ultra smooth paper backing sheet as per Example 1 with a roughness of 3-4μm as measured by Parker Print Surf. This example is EVA coated and top coated with a coating as per example 7, having a coat weight of 0.43g/m ² .

Example 9 (Ex 9) uses a coated bond type paper for the backing sheet, as per Example 7, with a co-extruded polymer structure where the lower polymer layer is a soft EVA with a VA content 28% and the upper layer is a less soft EVA with a VA content of 22.5%. The coat weight of each layer is 15g/m ² . On the image side a top coat comprising of the same recipe and chemistry as per examples 3-8 was applied at a coat weight of 1.0g/m ² .

Examples 10 and 11 (Ex 10 and Ex 11) are similar to example 7, except that the upper layer comprises only a single PU binder material.

Examples 3-11 all fall within the scope of the invention and, of these, examples 7 and 9-11 are particularly preferred, owing to their superb hot peel characteristics.

The coated bond paper used in preferred examples 7 and 9-11 has a roughness of less than 8μm PPS. It has the following composition:

A. ~85g/m ² cellulose fibre from Peace River and Stora. Mineral filler is added to give an ash content of approximately 14%.

B. ~ 3.5g/m ² dry coating applied to the both paper surfaces via a metered or flooded nip size press.

C. The coating has the following formula:

Filler: calcium carbonate (Carbital 60SP) = 100 parts

Binder: starch (wheat or potato) = 333,3 parts

Water softener: sodium hexametaphosphate = 0,695 parts Total solid content : 34 %

Target characteristics for the coated bond paper as measured after the paper machine process are:

Water hold out: Cobb = 29 g/m ² (ISO 535, 1991) Internal strength: Scott Bond - 320 J/m ² (Tappi 5699)

Smoothness: Bendtsen = 300 ml/min (ISO 8791-2, 1990)

Smoothness: Bekk = 50 s (felt) (ISO 5627, 1995) = 35 s (wire)

Absolute Humidity = 2.5 %

Example 1 prints well and has good surface sensitivity, owing to the presence of the extruded EVA layer. However, it has poor washing performance and cannot be used in a automatic sheet feeder. This is because it has no upper layer to provide good toner retention or to reduce the high friction of the EVA coating.

Example 2 again prints well and has good surface sensitivity, and it has good washing performance owing to the PU binders in the aqueous coating. However, it cannot be used

to make multiple copies as a batch in a copier machine owing to the high friction of the binders in the upper layer.

Examples 3 to 11 all print well and have good surface sensitivity. They can all be stack fed in a photocopying machine, owing to inclusion of PE wax in the upper layer. This significantly reduces the static and dynamic coefficients of friction, allowing a stack of recording sheets to be drawn one at a time into the printer by the sheet feeder without jamming. The PU binder of the upper layer provides good adhesion of the toner, ensuring that the washing performance for each of examples 3 to 11 is good. The hot peel characteristics of examples 7 and 9-11 are superb owing to the base paper characteristics of the coated bond paper. We have found that after peeling away the backing sheet, typically approximately 77% by weight of the polymeric coating (comprising all of the upper layer and a major portion of the lower layer) adheres to the fabric, while approximately 23% by weight of the polymeric coating remains adhered to the backing sheet.

Various modifications of the invention are of course possible. For example, the ethylene copolymer forming the lower layer may be an EVA copolymer or an ethylene-(meth-) acrylic acid alkyl ester copolymer, or a mixture of said copolymers.

Table 1

Comp 1 Very smooth base = US 5614345.

Comp 2 Very rough base = US 6395119.

Ex 1 - 6 Base type Conqueror CX22 Ultra Smooth from ArjoWiggins.

Ex 7, 9-11 HS90 base from ArjoWiggins.

Ex 8 Very smooth base commercially available - as Comp 1.

Ex 9 EVA applied by coextrudiπg two layer of EVA simultaneously.

EVA resin Grade used or % Vinyl Acetate analysed

Aq. Coat = Aqueous coating

Binder type St/Acr = Styrene acrylate, PU - AK4 Polyurethane ex Lamberti, PU - LB2 Polyurethane ex Lamberti, PU - Sancure 815 ex Noveon, PU - Permax 200 ex Noveon

Additive type PE wax = micronised polyethylene wax - modified wax mixture.

Additive grade Aq 400 = BYK - Chemie Aquaflour 400

Printing Konica Minolta colour copier model 5430DL