Adhesion method using thin adhesive layers

Abstract

The invention relates to a method for adhering flexible substrates, the connecting adhesive layer being formed in a thin manner. The invention further relates to a composite substrate, wherein the two substrates are connected by a thin flexible adhesive layer.

Claims

1. A method for adhesively bonding two substrates, comprising: providing a curable adhesive that is free of polyurethane; applying the adhesive onto a first substrate at a coating weight of less than 2 g/m.sup.2, providing a second film-shaped substrate made of a thermoplastic at a first temperature, heating the second film-shaped substrate to a second temperature higher than the first temperature to convert the surface of the second substrate into a softened state, bringing the first substrate and applied adhesive together under pressure with the heated second film-shaped substrate made of a thermoplastic, and curing the adhesive to adhesively bond the first substrate to the second substrate immediately after heating the second substrate.

2. The method according to claim 1, wherein the second substrate is a plastic film having a softening point below 200 C.

3. The method according to claim 1, wherein the adhesive is selected from crosslinkable adhesives in solvent-containing, or solvent-free form.

4. The method according to claim 1, wherein the heating is carried out using plasma or laser treatment, ultrasound, NIR radiation, or IR radiation.

5. The method according to claim 1, wherein two film-shaped substrates are adhesively bonded.

6. The method according to claim 1, wherein the first substrate is selected from wood, thermosetting plastic, thermoplastic, organic polymer, paper and paperboard.

7. The method according to claim 1, wherein the adhesive is selected from ethylene-vinyl acetate, polyacrylate and acrylate.

8. The method according to claim 1, wherein the first substrate is a flexible film, optionally coated with metal.

9. The method according to claim 1, wherein the first substrate is not heated when it is brought together with the second substrate.

10. The method according to claim 1, wherein the curable adhesive applied onto the first substrate is selected from EVA, polyacrylate, acrylate, silane-crosslinking adhesive; reactive hot melt adhesive; radiation-crosslinkable adhesive, and combinations thereof.

11. The method according to claim 1, wherein the second temperature is about +/40 C. from the softening point of the second substrate.

12. The method according to claim 11, wherein the first substrate does not have a thermoplastic surface at the heating temperature.

13. A composite object comprising at least two substrates adhesively bonded according to claim 1, wherein the adhesive coating weight is less than 2 g/m.sup.2.

14. The composite object according to claim 13, wherein the adhesive coating weight is less than 1 g/m.sup.2.

15. The composite object according to claim 13, wherein the second substrate is a flexible thermoplastic film, and the first substrate is selected from paper, metal, plastic, or multi-layer substrates.

16. The composite object according to claim 13, wherein the composite object is a flexible packaging film.

17. A method for adhesively bonding two substrates, comprising: providing a curable adhesive; providing a first substrate having a planar first surface and an opposing second surface; applying the adhesive onto the first substrate first surface at a coating weight of less than 2 g/m.sup.2, providing a second film-shaped substrate having opposing first and second planar surfaces made of a thermoplastic at a first temperature, heating the second substrate second surface to a second temperature higher than the first temperature but no higher than 40 C. above a melting point of the second substrate to convert the second surface of the second substrate into a softened state, bringing the first substrate first surface and applied adhesive together under pressure with the heated second substrate second surface, and curing the adhesive to form a adhesive bond between the first substrate first surface and the second substrate second surface, wherein the second substrate first surface remains planar after the adhesive bond is formed.

18. A method for adhesively bonding two substrates, comprising: providing a curable adhesive; providing a first substrate having opposing first and second planar surfaces; applying the adhesive onto the first substrate first surface at a coating weight of less than 2 g/m.sup.2; providing a second film-shaped substrate having opposing first and second planar surfaces made of a thermoplastic at a first temperature; heating only the second substrate second surface to a second temperature higher than the first temperature but no higher than 40 C. above a melting point of the second substrate to convert the second surface of the second substrate into a softened state, wherein the first substrate first and second surfaces and the second substrate first surface are not heated to a softened state; bringing the first substrate first surface and applied adhesive together under pressure with the heated second substrate second surface in a softened state; and curing the adhesive to form a planar adhesive bond between the first substrate first surface and the second substrate second surface.

Description

EXAMPLES

(1) Adhesive 1 (NCO-Terminated Polyester Urethane):

(2) A polyester polyol made of aromatic and aliphatic dicarboxylic acids and polyalkylene diols is reacted with an excess of 4,4-MDI.

(3) The adhesive has an NCO content of 3.4 wt % NCO based on the solids. Solids: 50 wt % in ethyl acetate, based on adhesive 1. Viscosity: 140 mPas (Brookfield LVT at 20 C., spindle 2, shear rate 30 rpm, ISO 2555)

(4) Adhesive 2:

(5) A polyester prepolymer is manufactured using the protocol of Example 1.

(6) The adhesive has an NCO content of 4.0 wt % NCO based on the solids. Solids: 60 wt % in ethyl acetate, based on adhesive 2. Viscosity: 300 mPas (Brookfield LVT at 20 C., spindle 2, shear rate 30 rpm, ISO 2555)

(7) Substrate 1: PET film, 12 m.

(8) Substrate 2: LLDPE film, 60 m (melting point: 114 C., DSC per DIN EN ISO 11357-3:2011; heating rate: 10 K/min).

(9) Adhesive Bonding Methods:

(10) Method 1: Apply the adhesives with a blade onto substrate 1. Heat substrate 2 with an IR radiator (1.5 to 1.8 m wavelength; 10 cm spacing). Immediately thereafter, adhesively bond the substrates manually.

(11) Method 2: Mechanically apply a dilute adhesive (approx. 10 wt % solids content) onto substrate 1 using a screen roller, at a web speed of 10 m/min. Remove the solvents in a three-zone dryer. Adhesively bond using a laminating unit at 60 C. Ultrasound treatment (20 kHz) of composite at 5 mm spacing from substrate 1.

(12) Method 3: Mechanically apply a dilute adhesive (approx. 10 wt % solids content) onto substrate 1 using a screen roller, at a web speed of 60 m/min. Remove the solvents in a three-zone dryer. Heat substrate 2 with an IR radiator (see above), 40 kW, over a length of 1 m along the travel direction. Adhesively bond using a laminating unit at 60 C.

(13) Determine the application weight by weighing adhesively bonded films and films cleaned with solvent.

(14) Bonding adhesion (peel test on 15-mm wide strips, 100 mm/min) measured per DIN 53278, 290.

(15) Seal adhesion (peel test on 15-mm wide strips, 100 mm/min) measured per DIN 53278, 290).

(16) Boiling test: after four days of storage, adhesively bonded samples were subjected to a boiling test in boiling water. For this the samples were firstly cut up (30 cm16 cm) and then folded so that the LLDPE film lies against itself and the long side of the laminate is halved (15 cm16 cm). The two short sides (15 cm) are sealed with a laboratory bench sealing unit having two heating sealing jaws, over a width of 1 cm, at 150 C. for one second and under a pressure of 50 N/cm.sup.2. The pouch thus produced, open on one side, is filled with 100 ml water, and the open 16-cm long side is again sealed as above over a width of 1 cm. The sealed pouch is then heated in boiling water for 30 minutes.

Experiments

(17) TABLE-US-00001 Bonding Layer Bonding adhesion, 7 Seal wt. adhesion, 12 days (N/15 adhesion Experiment (g/m.sup.2) Heating hr (N/15 mm) mm) (N/15 mm) 1 Adhesive 1, 0.5 IR 2.5 (material method 1 break) 2 Adhesive 1, 0.5 ultrasound 2.7 (material method 2 break) V 1 Comparison: 0 IR no bonding no bonding method 1, no adhesion adhesion adhesive V 2 Comparison: 0 ultrasound no bonding no bonding method 2, no adhesion adhesion adhesive 3 Adhesive 2, 0.5 IR 2.5-3.0 2.7 (material 52 method 3 (material break) break) .sup.[b] 4 Adhesive 2, 0.2 IR 2.5-3.0 2.8 (material 53 method 3 (material break) break) V 3 Comparison: 0 IR no bonding no bonding method 3, no adhesion adhesion adhesive 5 Adhesive 2, 0.6 IR 4.7 (material 3.4 (material method 3, LLDPE break) .sup.[b] break .sup.[b] (white pigmented) V 5 Comparison: 0.6 no 1.3 (peel) 1.8 (peel) adhesive 2, heating method 3, no heating, LLDPE (white pigmented) 6 Adhesive 2, 0.2 IR 1.2-1.5 (peel) method 3, PET (imprinted) .sup.[a] V 6 Comparison: 0.2 no no adhesion no adhesion adhesive 2, heating method 3, no heating, PET (imprinted) .sup.[a]
[a] PET imprinted with an approx. 1-m thick layer of a blue printing ink.
[b] PET film is torn.

(18) The laminates in which substrate 2 had been subjected to a heat treatment and in which the two films had been adhesively bonded to one another were all visually unobjectionable (transparent, no bubbles).

(19) These adhesively bonded films were furthermore still firmly bonded after boiling, with no delamination. Bonding adhesion remained unchanged (peel test produced material break).

(20) Laminates for which substrate 2 was heated, but on which adhesive was omitted, exhibited no bonding adhesion.

(21) Laminates for which the films were adhesively bonded, but substrate 2 was not heated, exhibited little or no bonding adhesion.

(22) It was furthermore observed that in laminates that had been manufactured with no heating of substrate 2, visual shortcomings were able to occur. The smaller the quantity of adhesive applied, the more likely those shortcomings were to occur.