COMPOSITE SHEET AND MANUFACTURING METHOD FOR A FOAMED DECORATIVE SHEET FREE OF PVC AND PLASTICIZERS

Abstract

A composite sheet and a method for producing a decorative sheet using the composite sheet are provided. The composite sheet has a base layer and a foamable layer bonded to the base layer. The foamable layer includes 100 parts by weight of a polyolefin material having an elastic modulus of <0.1 GPa, 0.1 - 10 parts by weight of a foaming agent, and 0 - 200 parts by weight of additives. The foamable layer has a thickness of 0.05 to 0.3 mm.

Claims

1. A method for producing a foamed decorative sheet, the method comprising: providing the curling-free composite sheet, the curling-free composite sheet including a base layer and a foamable layer; printing on the curling-free composite sheet; and foaming of the curling-free composite sheet.

2. The method of claim 1, wherein the base layer has a first elastic modulus of > 1 GPa, a first thickness of 0.05 mm to 0.15 mm, and shows a change of dimensions of less than 2% when heated from 0° C. to 200° C. and subsequently cooled to 0° C., wherein the foamable layer includes 100 parts by weight of a polyolefin material having a second elastic modulus of < 0.1 GPa, 0.1 - 10 parts by weight of a foaming agent, and 0 - 200 parts by weight of additives, and wherein the foamable layer has a second thickness of 0.05 to 0.3 mm.

3. The method of claim 2, wherein a ratio of the second elastic modulus of the foamable layer and the first elastic modulus of the base layer is less than 0.05 to prevent curling of the curling-free composite sheet during heating-cooling circles in a wall cover production process.

4. The method of claim 1, wherein providing the curling-free composite sheet comprises: producing the foamable layer by mixing components and forming the components to a sheet; and assembling the foamable layer to the base layer.

5. The method of claim 4, wherein providing the curling-free composite sheet further comprises: performing cross-linking of the foamable layer with at least one of ionizing radiation and employing a cross-linking agent, wherein the ionizing radiation is is conducted by an electron beam with 100 to 300 kV voltage and a dosage of 10 to 200 kGy, and wherein the cross-linking agents decompose to radicals at an elevated temperature.

6. The method of claim 1, wherein printing on the curling-free composite sheet includes at least one of gravure printing, flexo screen printing, inkjet printing, and coronary treatment.

7. The method of claim 2, wherein foaming of the curling-free composite sheet is performed at a temperature between 120° C. and 210° C., and wherein the temperature depends on the foaming agent.

8. The method of claim 1, wherein the curling-free composite sheet is produced in a continuous mode with a width between 0.5 and 3.0 meters.

9. The method of claim 2, wherein the polyolefin material is selected from the group consisting of thermoplastic elastomer polyolefins, ethylene vinyl acetate copolymers, and atactic polypropylene polymers, or mixtures thereof.

10. The method of claim 2, wherein the foaming agent is selected from the group consisting of azodicarbonamide, an azodicarbonamide metal salt, hydrazodicarbonamide, sodium bicarbonate, trihydrazino-sym-triazine, pp′-oxybis-benzenesulfonylhydrazide, dinitroso-pentamethylene-tetramine, azobisisobutyl-odinitrile, p-toluenesulfonylhydrazide, and bisbenzenesulfonylhydrazide.

11. The method of claim 2, wherein the additives are selected from the group consisting of catalysts, pigments, fillers, matting agents, microbial agents, UV stabilizers, fire retardants and release compounds.

12. The method of claim 11, wherein the additives include a catalyst, and wherein the catalyst is selected from the group consisting of zinc oxide, barium ricinoleate, tin methoxy maleate, hydrated calcium silicate, calcium stearate, zinc stearate, lead acetate, zinc laurate, zinc octonoate, and cadmium amyl phosphite.

13. The method of claim 1, wherein the base layer is selected from the group consisting of paper, paper-like material, non-woven material, woven fabrics, nonwoven fabrics and plastic foils.

14. The method of claim 2, wherein the polyolefin material is a polyolefin blend including polyolefins with an elastic modulus > 0.1 GPa and polyolefins with an elastic modulus < 0.1 GPa, such that a total elastic modulus of the polyolefin blend is < 0.1 GPa.

15. The method of claim 1, wherein the foamed decorative sheet is a wall cover.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0049] The invention will now be described with reference to the drawings wherein:

[0050] FIG. 1 shows the curling of a laminate;

[0051] FIG. 2 is a photograph showing the curling of a composite sheet with a foamable layer made of a polyolefin material having an elastic modulus of <0.1 GPa (left) and two composite sheets with a foamable layer made of a polyolefin material having an elastic modulus of >0.1 GPa;

[0052] FIG. 3A is a microscope view of foam cross-section of a foamable layer with an elastic modulus <0.1 GPa;

[0053] FIG. 3B is a microscope view of foam cross-section of a plasticized PVC composite sheet;

[0054] FIG. 4A is a photograph of embossed foam of a foamable layer with an elastic modulus <0.1 GPa; and,

[0055] FIG. 4B is a photograph of embossed foam of a plasticized PVC composite sheet.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0056] Exemplary embodiments of the invention will now be described.

[0057] A major problem of producing wall covers using composite sheets including a base layer and a foamable layer directly bonded to the base layer is curling. In particular, producing wall covers with a width of 0.5 to 3 m is nearly impossible when significant curling occurs. In principle, the effect is based on different thermal expansion properties of the base layer and foamable layer. Plastics are well known for their high thermal expansion rate, moreover crystallization that takes place upon cooling further increases shrinkage. However, heating is an inevitable process of the wall cover production, for example during coating of the foamable layer on the base layer, after printing of top ink layers or foaming in a heating channel. During the phase of subsequent cooling, the top polyolefin layer may contract significantly more than the base layer. In that case, curling occurs as illustrated in FIG. 1.

[0058] Referring to FIG. 1, the reference number 10 represents the foamable layer with a high thermal expansion/contraction property and reference number 12 represents the base layer with a low thermal expansion/contraction property. X represents the curling that occurs during the phase of cooling.

[0059] The curling depends on each layer’s mechanical and thermal properties as well as on the thicknesses from the well-known theories of laminate thermal stresses. In the well-known Timoshenko’s analytical solution of a beam laminate consisting of two layers (FIG. 1 with top layer 10 and bottom layer 12) the beam had no curling at the temperature T.sub.0. When the temperature changed to T, the beam’s curvature k.sub.b became:

[00001]$k_b=\frac{6\text{Δ}T\text{Δ}\alpha \left(1+m^2\right)}{\left(h\left(3{\left(1+m\right)}^2+\left(1+mn\right)\left(m^2+\frac{1}{mn}\right)\right)\right)}$

wherein m=t.sub.1/t.sub.2, the elastic modulus ratio n=E.sub.1/E.sub.2, the total thickness h=t.sub.1+t.sub.2, the temperature change ΔT=T-T.sub.0, and the thermal coefficient mismatch Δα=α.sub.2-α.sub.1.

[0060] The equation can be used to study the dependence on the curvature radius and hence curling, on film thickness, on elastic modulus of each layer, on the thermal expansion coefficient, et cetera. When the elastic modulus ratio E.sub.1/E.sub.2 is less than 0.05, the curvature radius and hence curling rapidly decreases with a decreasing E.sub.1/E.sub.2 ratio. Therefore, soft top layers with an elastic modulus <0.1 GPa are needed. Standard polyolefins like LDPE and HDPE (high-density polyethylene) or PP (polypropylene) have an even higher modulus (>1 GPa) and are hence worse in curling. Therefore, with standard polyolefins, it is practically impossible to get the needed coating thickness without curling.

[0061] However, by using specific groups of soft plastics, it is possible to minimize the curling effect and to produce curling-free laminate. This group of soft plastics includes thermoplastic elastomer polyolefins, ethylene vinyl acetate copolymers, atactic polypropylene polymers or mixtures thereof with modulus <0.1 GPa.

[0062] Different composite sheets have been manufactured by a melt processing of a foamable layer (directly) on a base layer as follows:

[0063] The base sheet was made of paper or non-woven material, had a thickness of 0.1 mm, an elastic modulus of >1 GPa, and a shrinkage/expansion of less than 2% up to 220° C.

Example 1

[0064] The foamable layer has been made of polyolefin material having an elastic modulus of <0.1 GPa. The components of the foamable layer were mixed at 140° C., 60 RPM. Then the foamable layer has been laminated on paper at 140° C.

[0065] More specifically the foamable layer consisted of:

TABLE-US-00001 Ethylene-octene copolymer, melt flow index 5, modulus 0.011 GPa 100 parts by weight Azodicarbonamide 5 parts by weight ZnO 1 parts by weight

[0066] The layer had a thickness of 0.15 mm and had good adhesion to the substrate. No curling occurred after cooling to room temperature.

Comparative Example 1

[0067] The foamable layer consisted of:

TABLE-US-00002 LDPE, melt flow index 10, modulus 0.32 GPa 100 parts by weight Azodicarbonamide 5 parts by weight ZnO 1 parts by weight

[0068] The layer had a thickness of 0.15 mm and had good adhesion to the substrate. Some curling occurred after cooling to room temperature.

Comparative Example 2

[0069] The foamable layer consisted of:

TABLE-US-00003 HDPE, melt flow index 3, modulus 1.5 GPa 100 parts by weight Azodicarbonamide 5 parts by weight ZnO 1 parts by weight

[0070] The layer had a thickness of 0.15 mm and had good adhesion to the substrate.

[0071] Significant curling occurred after cooling to room temperature.

[0072] FIG. 2 shows three different composite sheets that have been manufactured as described above. The composite sheet of Example 1 with a foamable layer made of a polyolefin material having an elastic modulus of <0.1 GPa showed no curling (left). For comparative purposes, two composite sheets with a foamable layer made of a polyolefin material having an elastic modulus of >0.1 GPa were manufactured (comparative Example 1 and comparative Example 2). In contrast to the curling-free composite sheet on the left side, both of the comparative sheets showed a characteristic curling (medium curling of comparative Example 1 in the center and extreme curling of comparative Example 2 on the right-hand side of FIG. 2).

[0073] The Examples show that for obtaining a composite sheet without curling, the top layer has to be made from a soft plastic with an elastic modulus <0.1 GPa with a thermal expansion coefficient that is as low as possible.

[0074] The samples were cross-linked by electron beam radiation at 160 kV,10 mA with variable dosage 20-100 kGy and foamed at 225oC. All samples showed foaming.

Example 2

[0075] The foamable layer consisted of:

TABLE-US-00004 Ethylene-octene copolymer, melt flow index 5, modulus 0.011 GPa 100 parts by weight Calcium carbonate 60 parts by weight Activated azodicarbonamide 5 parts by weight Titanium dioxide 10 parts by weight Dicumyl peroxide 1 parts by weight

[0076] The layer thickness was around 0.15 mm. In the next step, the layer was laminated to the base layer and cross-linked at 170° C. for 1 min. The layer was printed on a laboratory gravure printing machine with water-based ink suitable for polyolefins surface and foamed in a convection oven at 1 min 205oC. FIGS. 3A and 3B show that the composite sheet including the foamable layer with <0.1 GPa (FIG. 3A) exhibited good foaming that is comparable to PVC based foams used for wall cover (FIG. 3B).

Example 3

[0077] The foamable layer consisted of:

TABLE-US-00005 Ethylene-octene copolymer, melt flow index 17, modulus 0.06 GPa 100 parts by weight

TABLE-US-00006 LDPE, melt flow index 10, modulus 0.32 GPa 10 parts by weight Calcium carbonate 60 parts by weight Azodicarbonamide 5 parts by weight Zinc oxide 1 parts by weight

[0078] The layer with 0.2 mm thickness was laminated to 0.1 mm non-woven material and foamed in a convection oven at 1 min 215oC. Afterwards, it was hot-embossed at about 140oC. FIGS. 4A and 4B show that the embossing structure of the composite sheet including the foamable layer with <0.1 GPa (FIG. 4A) was comparable to PVC-composite sheets (FIG. 4B).

[0079] It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.