The present invention relates to a material for casting release paper and a process for the preparation thereof.

State of the art

Different types of casting paper and release paper are known, usually consisting of a layer of paper, or other suitable material, onto which a release layer is applied by extrusion, in the case of thermoplastic polyolefins, by "Roll Coating", in the case of acrylic resins, or by coating and sizing, in the case of silicone resins. Embossing is a separate and subsequent step. A further process uses an acrylic resin cross-linked with Electron Beam.

EP2386684 discloses a material comprising a mixture of polyesters saturated with urea-formaldehyde or melamine-formaldehyde. The presence of solvents, formaldehyde and VOCs (Volatile Organic Compounds) affects the industrial applicability of the material described in EP2386684.

US 4 288 479 discloses an adhesive or transfer design laminate comprising a release layer obtained by a liquid composition comprising a radiation polymerizable material and an effective amount of a release material having low adhesion to said adhesive. The radiation polymerizable material are selected from monomers and prepolymers having ethylenic unsaturation, such as acrylate monomers and polyurethane/acrylate prepolymers, polyamides with a plurality of reactive amine groups reacted with ethylenically unsaturated polybasic carboxylic acids, acrylated epoxides, and epoxide resins. The preferred radiation polymerizable material is an unsaturated polyether-polyurethane. Unsaturated polyesters are also disclosed. The release material is selected from waxes, silanes, siloxanes, silicones, fluorocarbons, forming a thin layer which migrates to the surface to provide the desired release properties and to exclude oxygen from the film to permit radiant curing in air.

According to said document, the waxy release material is an essential and critical feature of the disclosed material. US 2004 0220340 discloses unsaturated polyesters resins useful in the field of gel coats, quite different from the technical field pertaining to the present invention.

Unsaturated polyester resins having allyl substituents are also known from US 3 326 710 as coatings specifically for wood or metal substrates.

Description of the invention

The object of the present invention is a release paper (CR) or casting paper which, compared to currently known papers, is characterized by better performance in terms of strength and stability, formability, aesthetic appearance and cost-effectiveness together with compliance with environmental legislation.

It has been found that these purposes can be achieved by a material obtainable by impregnation of a paper substrate with an unsaturated polyester resin with allyl functionalities cross-linked with acrylic monomers in the presence of radical initiators.

Contrary to the cited prior art, particularly US 4288 479, the present invention does not require the exclusion of oxygen from the film, providing a material which is simpler and more advantageous than the previously known materials.

Thus, the invention relates in a first aspect to a material obtainable by impregnation of a paper support with an unsaturated polyester resin with allyl functionalities cross-linked with acrylic monomers in the presence of radical initiators: the resin penetrates the paper support where cross-linking between the allyl-modified polymer and the acrylic monomers takes place with the formation of an original composite material.

The invention enables improved control of the kinetics of resin cross-linking, higher CR paper strength and lower production costs compared to conventional methods.

The material and process of the invention also provide the following further advantages:

Possibility of increasing the number of CR paper reuses;

Reduction in the cost of articles prepared with it, whether technical coated with PU or PVC or synthetic leather;

Reduction of CR paper disposal costs; Greater flexibility in the implementation process;

In-line process with embossing or printing step;

Improved aesthetic characteristics of the texture on the imitation leather;

Realization of surfaces with pattern, lettering or design in contrasting glossy/matt finish.

The material of the invention can be used in the production of release liners for films and labels with pressure-sensitive adhesive, polyurethane or PVC coatings for technical uses or synthetic leather, or decorative furniture panels made of melamine resin on HPL and CPL equipment.

Detailed description of the invention

Unsaturated polyester resins with allyl functionalities are known and commercially available. Preferably, the invention uses an unsaturated polyester resin with allyl functionalities characterized by: a. Viscosity at 23°C between 200 and 800 mPa.s; b. Gardner scale color less than 1; c. Distribution of molecular weights with poly dispersity around 3.

Examples of acrylic monomers that can be used for radical crosslinking of the resin include ethylene glycol diacrylate, diethylene glycol diacrylate, dipropylene glycol diacrylate, 1,6-hexanediol diacrylate, tripropylene glycol diacrylate, glycerin triacrylate, trimethylolpropane triacrylate, pentaerythritol tetraacrylate, phenoxyethyl acrylate, hydroxy ethyl methacrylate, hydroxypropyl methacrylate. The use of monomers classifiable as non-volatile organic compounds (i.e., having a vapor pressure of less than 0.01 KPa at 293.15 K (20°C)) is preferred, such as dipropylene glycol diacrylate, tripropylene glycol triacrylate, hydroxy ethyl methacrylate, trimethylolpropanotriacrylate, 1,6-hexanediol di acrylate.

The resin typically comprises between 30 and 80 parts by weight of unsaturated polyester and 70 to 20 parts by weight of acrylic monomers.

Examples of radical initiators include substituted pinacoles, benzopinacol and its alkylated, hydroxylated and/or silanised derivatives.

After cross-linking the unsaturated polyester with the acrylic monomers with a treatment cycle of 30 min at 140°C and 24 h at room temperature, followed by post-treatment of 3 h at 100°C and a further 24 h at room temperature, the cross-linked resin has the following characteristics: a. SHORE hardness D: 40 - 60 (ASTM 2240) b. Elongation at break greater than 3 (DIN EN ISO 7500-1) c. Breaking load: 20 - 30 MPa (DIN EN ISO 7500-1).

Typically, 5 to 25 parts by weight of cross-linked resin per 75 to 95 parts by weight of support material, e.g. cellulose fibers, are used.

The material of the invention may further comprise release agents, levelling agents, fillers.

The process of preparing the material of the invention comprises: a) mixing of polyester resin, acrylic monomers, initiators and any releasing agents, levelling agents, fillers; b) deposition of one or more layers of the mixture obtained in step a) on the support material; c) radical cross-linking by heat transfer and/or UV irradiation.

Step b) can be carried out by immersion, blade coating, rollers or printing cylinders.

The process may also include finishing steps of one of the surfaces by embossing with an engraved cylinder and/or by gravure or flexographic printing.

A schematic diagram of an embodiment of a plant for preparing the material of the invention is illustrated in Figure 1; indicated by 1 is the reel onto which the support 20, typically a continuous paper tape or the like, is wound. The support 20 is conveyed to the coating station, where the feeder 3 pours the mixture of polymeric materials 10 onto the support, which is spread by the doctor blade 4 in the form of the first coating layer 21. Subsequently, the support 2, with the first layer 21 are passed through the drying oven 5, and then sent to a subsequent deposition of the mixture of polymeric materials 10 by the roller 6, which collects the mixture from the container 106 and applies it in the form of the second layer 22 on the support 20, which is then sent to a further drying oven 7. At this point, the release paper thus obtained can be sent towards a further heat treatment in the oven 8, and then collected on the winding reel 101. Alternatively, it may instead be treated with the embossing roller 9 provided with the heating means 109.

For a base paper of 130 -200 gsm, typically 5 to 40 gsm of cross-linked polyester resin is applied.

In the case of thermal cross-linking, the polymerization temperature is generally between 120 and 180°C. The total weight of the final product varies between 135 and 240 g/sqm.

The invention is illustrated in more detail in the following examples.

Example 1 - glossy

A reel of long cellulose fiber paper material weighing 150 g/sq.m. and 190 micron thick is unwound on a metal doctor blade coating machine with oven in succession at the appropriate speed; 20 g/sq.m. of polyester resin mixed with appropriate spreading and viscous depressant additives is applied to one side of this paper material; this resin consists of 50 parts by weight of a polyester obtained by polycondensation reaction at 180°C of a mixture consisting of 30% monoethylene glycol, 42% maleic anhydride and 28% trimethylolpropane diallyl ether, carried out by distillation in the presence of 200 ppm toluenehydroquinone, 9 parts hydroxy ethyl methacrylate, 38 parts dipropylene glycol diacrylate, 3 parts benzopinacol.

The suitable spreading additive for the formulation is a polyethoxylated alcohol- based surfactant, such as Tergitol TMN-100X, at a rate of 0.5-3.0%, with an associated viscodepressant effect.

The material is then subjected to cross-linking in an oven with an increasing temperature gradient from 140° to 180°C. At the end of the cross-linking heat treatment, a glossy casting release material weighing 180 g/sq.m. and 193 microns thick, called M01, is obtained and rewound onto a reel. Example 2 - matt

A roll of long cellulose fiber paper material weighing 150 g/sq.m. and 190 microns thick is unwound on a metal doctor blade coating system with oven in succession at the appropriate speed; 20 g/sq.m. of polyester resin is applied to one side of this paper material, as described in Example 1, mixed with spreading, release, viscous-depressant and matting additives; in particular, the spreading additives are surfactants, poly ethoxylated alcohols from the Tergitol range, to be used in percentages by weight of between 0.5 and 2.0%, the release agents are silicone-free polymers such as BYK 3560, to be used in percentages of less than 1%, and the opacifiers are siliceous polymers, such as Acematt TS100, in percentages of between 2.0 and 5.0%, while the dispersants with a viscodepressant effect are of the Dipserbyk 2159 type, in percentages of between 1 and 2.0%.

The material is then subjected to cross-linking in an oven with an increasing temperature gradient from 140° to 180°C. At the end of the cross-linking heat treatment, an opaque casting release material with a weight of 180 g/sqm and a thickness of 193 microns, called M02, is obtained and rewound onto a reel.

Example 3 - textured

A roll of long cellulose fiber paper material weighing 150 g/sq.m. and having a thickness of 190 microns is unwound on a metal doctor blade coating system with oven in succession at the appropriate speed; 20 g/sq.m. of polyester resin is applied on one side of this paper material as per Example 1, mixed with appropriate dispersing additives and fillers such as glass microspheres or microflakes of plastic material: In this case, polymeric powder such as Deuteron MKF6 and silicone-free dispersing agent as in Example 2 are used.

The material is then subjected to cross-linking in an oven with an increasing temperature gradient from 140° to 180°C. At the end of the cross-linking heat treatment, the textured casting release material weighing 180 g/sq.m. and with a thickness of 195 microns, called M03, is obtained and rewound on a reel. Example 4 - embossed

A reel of paper material in long cellulose fiber weighing 150 g/sq.m. and having a thickness of 190 microns is unwound on a metal doctor blade coating plant with oven in succession at the appropriate speed; 20 g/sq.m. of polyester resin, described in example 1, mixed with appropriate additives is applied to one side of this paper material; the material is then cross-linked in an oven with an increasing temperature gradient from 140° to 180°C and embossed with an engraved cylinder. At the end of the treatment, the embossed casting release material weighing 180 g/sq.m. and of variable thickness, called M04, is obtained and rewound on a reel.

Example 5 - printed

On a rotogravure press with UV dryer, a reel of long cellulose fiber paper material weighing 150 g/sqm and 190 microns thick is unwound in succession at the appropriate speed; 10 g/sq.m. of polyester resin mixed with a photoinitiator and suitable spreading additives is applied to one side of this paper material; the suitable photoinitiator used in this case is Omnirad TPO, in a weight percentage of less than 1%, and the spreading additive is similar to that described in the previous examples. The material is then subjected to crosslinking in a drying tunnel under UV lamps with a power of 120 W/cm. At the end of the treatment, the printed casting release material with a weight of 170 g/sqm and a thickness of 193 microns, named M05, is obtained and rewound on a reel.

Figure 2 shows the characterization of the CR material of example 2 (M02) during its use in the in-line manufacture of polyurethane coatings in comparison with a conventional material referred to as LM. The data obtained, relating to the degree of opacity at 60°C after 10 uses, show that the reference LM material showed a noticeable decay in the degree of opacity, becoming more glossy; it is assumed that the decay continues even after the tenth use. In contrast, the degree of opacity of the material of example 2 M02 remained stable throughout the ten uses, without any sign of decay.

The data shown in Table 1 below demonstrate that even typical mechanical characterization and evaluation indices for casting release papers show better performance of the product of the invention compared to LM both in its original state and after the tenth use. The decay percentage is also equal or lower.

Table 1 : Mechanical properties: