METHOD OF MANUFACTURING SANDWICH PANEL

Abstract

The invention relates to a method of manufacturing a sandwich panel comprises the steps of: a) providing a plate-shaped assembly of a first cover part and a second cover part and between these cover parts a core part of a thermoplastic material containing a physical blowing agent, b) heating the assembly resulting from step a) under pressure between press tools in a press to a foaming temperature below the glass transition temperature of the thermoplastic material in the core part, thereby effecting adhesion of the foamed core part to the first and second cover parts c) foaming the thermoplastic material in the core part under pressure and at the foaming temperature wherein the spacing between the press tools is increased; d) a cooling step of cooling the foamed sandwich panel resulting from step c), while the sandwich panel is maintained under pressure between the press tools; e) removing the thus cooled sandwich panel from the press; and f) drying the sandwich panel thus obtained;

wherein the cooling step d) comprises.a first substep d1) of cooling the foamed assembly from the foaming temperature to an intermediate temperature in the range of 70-100 C. at a first cooling rate and a second substep d2) of cooling the foamed assembly from the intermediate temperature to ambient temperature at a second cooling rate, the second cooling rate is less than the first cooling rate.

Claims

1. Method of manufacturing a sandwich panel comprising the steps of: a) an assembling step of providing a plate-shaped assembly of a first cover part and a second cover part and between these cover parts a core part of a thermoplastic material containing a physical blowing agent; b) a heating step of heating the assembly resulting from step a) under pressure between heated press tools in a press to a foaming temperature below the glass transition temperature of the thermoplastic material in the core part, thereby effecting adhesion of the foamed core part to the first and second cover parts; c) a foaming step of foaming the thermoplastic material in the core part under pressure and at the foaming temperature wherein the spacing between the press tools is increased; d) a cooling step of cooling the foamed sandwich panel resulting from step c), while the sandwich panel is maintained under pressure between the press tools; e) a discharging step of removing the thus cooled sandwich panel from the press; and f) a drying step of drying the sandwich panel thus obtained; wherein the cooling step d) is carried out in two substeps, comprising a first substep d1) of cooling the foamed assembly from the foaming temperature to an intermediate temperature in the range of 70-100 C. at a first cooling rate and a second substep d2) of cooling the foamed assembly from the intermediate temperature to ambient temperature at a second cooling rate, wherein the first cooling rate is at least 140 C./min and wherein the second cooling rate is less than the first cooling rate.

2. Method according to claim 1, wherein the first cooling rate is more than 200 C./min.

3. Method according to claim 1, wherein the second cooling rate is less than half of the first cooling rate.

4. Method according to claim 1, wherein the second cooling rate is 25 C./min or less.

5. Method according to claim 1, wherein during substeps d1) and d2) the temperature difference between the press tools is less than 2 C.

6. Method according to claim 1, wherein the surface of the press tools is provided with a layer of a material having a heat conductivity coefficient, which is higher than that of the construction material from which the press tools are made.

7. Method according to claim 6, wherein the layer of material having the higher heat conductivity coefficient is a layer made of copper or aluminium.

8. Method according to claim 1, wherein at least one of the first and second cover parts comprises a layer of a thermoplastic.

9. Method according to claim 8, wherein the thermoplastic of a cover part is selected from the group consisting of polyetherimide (PEI), polyethersulfone (PES), polyphenylsulfone (PPSU) and polysulfone (PSU).

10. Method according to claim 1, wherein the thermoplastic of the core part is polyetherimide (PEI).

11. Method according to claim 1, wherein the physical blowing agent is acetone.

12. Method according to claim 8, wherein at least one of the first and second cover parts comprises a layer of a fibre-reinforced thermoplastic.

13. Method according to claim 2, wherein the second cooling rate is less than half of the first cooling rate.

14. Method according to claim 2, wherein the second cooling rate is 25 C./min or less.

15. Method according to claim 2, wherein during substeps d1) and d2) the temperature difference between the press tools is less than 2 C.

16. Method according to claim 13, wherein during substeps d1) and d2) the temperature difference between the press tools is less than 2 C.

17. Method according to claim 2, wherein the surface of the press tools is provided with a layer of a material having a heat conductivity coefficient, which is higher than that of the construction material from which the press tools are made.

18. Method according to claim 13, wherein at least one of the first and second cover parts comprises a layer of a thermoplastic.

19. Method according to claim 13, wherein the thermoplastic of the core part is polyetherimide (PEI).

20. Method according to claim 18, wherein the thermoplastic of the core part is polyetherimide (PEI).

Description

EXAMPLES

[0032] The invention is further illustrated by means of the following Examples.

Example 1

[0033] First and second cover parts: each one layer US-style 7781 glass fabric PEI (polyetherimide) impregnated and consolidated with 33+2% PEI, layer thickness=0.23 mm;

[0034] Thermoplastic core part: two films of PEI, (Polyetherimide) Ultem 1000, impregnated with 12.1-12.9 wt. % acetone, film thickness in the range of 250-300 micrometres.

[0035] The percentage of acetone in the film is determined as ((weight of film+acetone in g) minus (weight of the neat film in g)) divided by (weight of the neat film in g).

[0036] Several FITS panels (planar dimensions 5030 cm) were manufactured with the following configuration:

[0037] A symmetrical stack was assembled with the two acetone impregnated PEI films as core part between the identical first and second cover parts, either each consisting of one or two glass fabric layer(s) as indicated above. This assembly was placed between the heated press plates of the press. After closing the press the assembly was heated in seconds to the required foaming temperature of 178-180 C. The centre of the temperature measuring device (Pt element type K) is located 4 mm below the surface of the press plates. Pressure is 4 Mpa. Upon reaching this foaming temperature the presswhile maintaining pressure at essentially the same valuewas opened according to a certain foaming curve to a predetermined thickness (as specified below) of the final sandwich panel, after which the press plates and consequently the thermoplastic sandwich panel were cooled from the foaming temperature to 90 C. in 25 seconds, and further down to ambient temperature at an average cooling rate of 20 C./min. Finally the sandwich panels thus obtained were subjected to a drying step according to WO2006080833 A1 by taping the edges to reduce peripheral outflow of acetone and direct it through the cover parts using temperature increases of 10 C. between intervals of 2-4 hours at a given temperature.

[0038] In this way sandwich panels with thicknesses of 9.5 and 7.5 mm were manufactured. The sandwich panels were tested for the adhesion between the fibre-reinforced thermoplastic PEI cover parts and the in-situ foamed PEI core part using an in plane tensile strength test procedure according to ASTM C297.

[0039] The 9.5 mm thick in-situ foamed thermoplastic sandwich panel having a foam density of 85 kg/m.sup.3 (core part made from 2 PEI films acetone impregnated of 300 micrometers) showed an average flatwise tensile strength of 3.4 MPa. The 7.5 mm thick in-situ foamed thermoplastic sandwich panel having a foam density of 90 kg/m.sup.3 (core part made from 2 PEI films acetone impregnated of 250 micrometers) showed an average flatwise tensile strength of 3.9 MPa.

[0040] Typically, failure of the test samples occurred in the thermoplastic core part, indicating that the adhesion between the core part and cover parts is adequate. The cover parts could not be peeled manually from the foam core.

Example 2

[0041] First and second cover parts: each one layer US-style 7781 glass fabric PEI (polyetherimide) impregnated and consolidated with 33+2% PEI, cover part thickness=0.23 mm; or each two layer US-style 7781 glass fabric PEI (polyetherimide) impregnated and consolidated with 33+2% PEI, cover part thickness=0.46 mm

[0042] Thermoplastic core part: three films of PEI, (Polyetherimide) Ultem 1000, impregnated with 12.1-12.9 wt. % acetone, film thickness of 250 micrometres.

[0043] An assembly was prepared from the thermoplastic core part in between the first and second cover parts. This assembly was subjected to in situ foaming as outlined in EXAMPLE 1 using the same conditions.

[0044] Sandwich panels (2525 cm) having a thickness of 11.3 mm were obtained. The thermoplastic sandwich panel having cover parts comprising one layer glass fabric impregnated with PEI had a foam density of 87 kg/m.sup.3 and showed an average flatwise tensile strength of 3.5 MPa. The thermoplastic sandwich panel having cover parts comprising two consolidated layers glass fabric impregnated with PEI had a foam density of 91 kg/m.sup.3 and showed an average flatwise tensile strength of 3.9 MPa.

[0045] Typically, failure of the test samples occurred in the thermoplastic core part, indicating that the adhesion between the core part and cover parts is adequate. The cover parts could not be peeled manually from the foam core.

[0046] The same test results were obtained with sandwich panels of 5030 cm.

Example 3 (Comparative)

[0047] First and second cover parts: each one layer US-style 7781 glass fabric PEI (polyetherimide) impregnated and consolidated with 33+2% PEI, cover part thickness=0.23 mm; or each two layer US-style 7781 glass fabric PEI (polyetherimide) impregnated and consolidated with 33+2% PEI, cover part thickness=0.46 mm

[0048] Thermoplastic core part: three films of PEI, (Polyetherimide) Ultem 1000, impregnated with 12.1-12.9 wt. % acetone, film thickness of 250 micrometres.

[0049] An assembly was prepared from the thermoplastic core part in between the first and second cover parts. This assembly was subjected to in situ foaming as outlined in EXAMPLE 1 except that the obtained sandwiches were cooled from the foaming temperature to 90 C. in 40 seconds. Sandwich panels (2525 cm) having a thickness of 11.3 mm were obtained. The thermoplastic sandwich panel having cover parts comprising one layer glass fabric impregnated with PEI had a foam density of 87 kg/m.sup.3 and showed an average flatwise tensile strength of 1.8 MPa. The thermoplastic sandwich panel having cover parts comprising two consolidated layers glass fabric impregnated with PEI had a foam density of 91 kg/m.sup.3 and showed an average flatwise tensile strength of 2.3 MPa.

[0050] Failure of the test samples occurred at the interface between the fibre-reinforced thermoplastic cover part and the in situ foamed core part, indicating that the adhesion at the interfaces was less than the strength of the foam. Also the cover parts could be peeled manually of the foam core part rather easily

[0051] The same results were obtained with panels having dimensions of 5030 cm.