WO2017095223A1

WO2017095223A1 - A stitch-reinforced sandwich panel

Info

Publication number: WO2017095223A1
Application number: PCT/NL2016/050840
Authority: WO
Inventors: Keiko ENOMOTO; Michiel Harjon Wassink; Roland Frederik SMITH; Thijs VAN DER AA
Original assignee: Vdl Fibertech Industries B.V.
Priority date: 2015-12-01
Filing date: 2016-11-30
Publication date: 2017-06-08
Also published as: NL2015889B1; EP3383639A1

Abstract

The present invention relates to a stitch-reinforced sandwich panel and to a method for manufacturing such a stitch-reinforced sandwich panel. In addition the present invention also relates to the use of such a stitch-reinforced sandwich panel. The stitch-reinforced sandwich panel comprises a foam core having a pair of opposite first and second faces; first and second skins of fiber reinforcing material respectively covering the first and second faces of the foam core; and a reinforcing thread stitched through each of the skins and through the foam core to form stitches arranged in a pattern over substantially the entire faces of the foam core, the stitching joining the skins and the core together to form a stitch-reinforced sandwich panel.

Description

Title: A stitch-reinforced sandwich panel.

Description

The present invention relates to a stitch-reinforced sandwich panel and to a method for manufacturing such a stitch-reinforced sandwich panel. In addition the present invention also relates to the use of such a stitch-reinforced sandwich panel.

Fibre reinforced plastic composites are used in a variety of technical products such as cars, aeroplanes, wind turbine blades, storage tanks etc. In many cases the fiber parts are placed in a mould. The mould is closed by a second mould part or a plastic foil is placed over the mould, and vacuum is applied to the fiber filled hollow structure. Hereafter the cavity is infused with a liquid resin, which later is cured to form a matrix material. The fiber parts are commonly supplied as fabrics of roving yarns, stitched together with thermoplastic sewing threads. The stitching process mechanically stabilizes the roving yarns in a geometrically desirably pattern. The reinforced composite materials are highly effective in their strength to weight ratio and thereby afford significant advantages over the conventionally used structural materials of wood, metals and the like.

The weight of an aircraft is a factor in the operating expense and performance of the aircraft. For example, an aircraft with a lighter weight may consume less fuel and may be able to travel for longer distances, as compared to an aircraft with a greater weight. For example, performance, such as stall speed, maximum range air speed, maximum endurance air speed, rate of climb, maneuvering speed, and other performance factors are related to the weight of an aircraft.

In an effort to reduce weight and improve performance, the commercial aircraft industry has increased the use of composite materials. In this effort, aircraft are being designed and manufactured with greater and greater percentages of composite materials. Some aircraft may have more than 50 percent of their primary structure made from composite materials. Composite materials are used in aircraft to decrease the weight of the aircraft resulting in improved payload capacities and fuel efficiencies. Those composite materials may be tough, light-weight materials, created by combining two or more dissimilar components, such as fibers and resins wherein the fibers and resins are combined to form a cured composite material.

Honeycomb sandwich panels are known, for example from US patent application No 2015/239226, and these panels are made by laminating a honeycomb core made of aramid paper with prepreg laminates on both sides and curing the prepreg laminates while bonding the prepreg laminates to the honeycomb core as so-called co- curing.

US patent application number US2009/ 155403 recites that panel-shaped, flat or spherically curved sandwich components, for example for interior lining components for passenger cabins of aircraft, are, for example, formed with a honeycomb-shaped core structure from aramide paper and phenolic resin, which core structure comprises on both sides cover layers made from a so-called prepreg material made of phenolic resin. This prepreg material comprises reinforcement fibres, reinforcement mats or reinforcement scrim pre-impregnated with a phenolic resin. The sandwich component is cured in an autoclave at overpressure and at an elevated temperature. In this process an intensive connection of the cover layers to the core structure takes place so that very considerable mechanical strength values of the finished sandwich components result, because the cover layers are primarily exposed to tensile forces and pressure forces, while the core structure essentially fulfils the task of a spacer between the cover layers.

A stitch-reinforced sandwich panel is known from US Patent No. 6,187,41 1. According to this US patent the stitch-reinforced sandwich panel comprises a foam core, skins of fiber reinforcing material covering the foam core, and a reinforcing thread stitched through the skins and through the foam core, wherein the foam core is formed of a plurality of discrete foam pieces that are spaced apart to form intervening spaces there between.

A stitch-reinforced sandwich panel is also known from KR 2000-0018198. Such a panel is manufactured by a step for adhering first surface materials on a structure being formed by stitching a glass fiber thread on second surface materials attached to a phenolic foam and a step for infiltrating a phenolic resin in a hardening process to the glass fiber thread in the phenol prepreg state of the first surface materials. Examples of first and second surface materials are glass fiber reinforcing materials and glass cloth.

US2010196654 relates to a process of producing a resin-impregnated laminate composite structure comprising a core between resin-impregnated fabric layers, the process comprising stitching a roving in a preliminary structure comprising the core so as to structurally reinforce the core, the core being a closed-cell material and the roving passing through through-holes in the core and traversing opposite outer surfaces of the preliminary structure, wherein the roving and a resin close the through-holes in the core. This US patent application further comprises a step of placing the preliminary structure on a mold such that a first of the at least two fabric layers is disposed between a surface of the mold and a first of the outer surfaces of the core, a second of the at least two fabric layers is disposed at a second of the outer surfaces of the core, and the preliminary structure conforms to the surface of the mold, infusing the resin into the preliminary structure to impregnate the at least two fabric layers; and then curing the resin to cause the at least two fabric layers to bond to the core and thereby form the resin-impregnated laminate composite structure.

The above mentioned honeycomb structures have some disadvantages, such as weight adding surface finish and potting for inserts needed. In addition, the above mentioned use of phenolic resins used may result in a toxic working environment. Furthermore, there is also a risk of delamination of the individual layers and the manufacturing process can be qualified as intensive.

For the use of materials in a cabin interior of an airplane the fire-smoke- toxicity and heat release of those materials are the most critical properties. In addition, mechanical properties of those materials are also important.

An aspect of the present invention is to provide a light weight sandwich panel that meets fire-smoke-toxicity tests.

Another aspect of the present invention is to provide a light weight sandwich panel that meets heat release tests.

Another aspect of the present invention is to provide a light weight sandwich panel that meets both the fire-smoke-toxicity tests and the heat release tests.

These tests are standard tests in the field of the aircraft industries, for example FAR 25.853.

The present invention thus relates to a stitch-reinforced sandwich panel, comprising:

a foam core having a pair of opposite first and second faces; first and second skins of fiber reinforcing material respectively covering the first and second faces of the foam core; and

a reinforcing thread stitched through each of the skins and through the foam core to form stitches arranged in a pattern over substantially the entire faces of the foam core, the stitching joining the skins and the core together to form a stitch-reinforced sandwich panel; characterized in that said stitch-reinforced sandwich panel further comprises first and second skins of carbon fiber reinforcing material respectively covering the first and second skins of fiber reinforcing material, wherein a resin is present between said first and second skins of carbon fiber reinforcing material and said first and second skins of fiber reinforcing material, said resin being impregnated into said foam core.

On basis of such a stitch-reinforced sandwich panel one or more aspects are covered. The terms "stitch", "stitching" and "stitched" as used here also cover "fastening", "attaching" or "tying" and its derivatives.

The present inventors found that the application of additional skin layers on top of the already applied stitch-reinforced sandwich panel provides unexpected results regarding weight and stiffness.

The first and second skins of fiber reinforcing material are preferably composed of fibers chosen from the group of glass fibers, carbon fibers and aramid fibers. These types of fibers provide an acceptable balance between strength, weight, fire - and smoke properties. These first and second skins of fiber reinforcing material are located close to the first and second faces of the foam core. In other words, the top surface and the bottom surface of the core layer are both covered by a layer of fiber reinforcing material, i.e. first and second skins of fiber reinforcing material respectively. By stitching both skins and the core together to form a stitch-reinforced sandwich panel. In an embodiment the type of material used for the first and second skins of fiber reinforcing material is glass fibers, especially glass weave, more especially glass weave in the configuration of 0 90° and in a range of 30 - 80 g/m². In another embodiment the type of material used for the first and second skins of fiber reinforcing material is carbon fibers, especially carbon weave in the configuration of 0 90° and in a range of 30 - 80 g/m².

From the viewpoint of strength it is preferred when the first and second skins of carbon fiber reinforcing material are of the type plain weave.

In the stitch-reinforced sandwich panel according to the present invention the foam is preferably chosen from the group of poly(ether sulfone), polymethacrylimide, polyphenylsulfone and polyethylene terephthalate. These types of foam provide the desired strength and weight properties, besides the stringent requirements of fire-smoke- toxicity tests and the heat release tests. A preferred type of foam is a poly(ether sulfone) foam having a density of 40-60 kg/m³ (according to ASTM D 1622) or having a density in a range of 40-50 kg/m³ (according to ASTM D 1622).

Another preferred type of foam is a polyphenylsulfone foam having a density of 40 - 50 kg/m³ (according to ASTM D 1622).

Another preferred type of foam is a polyethacrylimid foam having a density of 50 - 70 kg/m³ (according to ASTM D 1622).

Another preferred type of foam is a polyethylene terephthalate foam having a density of 65 - 100 kg/m³ (according to ASTM D 1622).

The resin applied in the present stitch-reinforced sandwich panel is preferably chosen from the group of hybrid epoxy/polyurethane, epoxy, polyurethane, polyester and acrylate, or a mixture thereof. Such a resin provides a good bonding between the individual components of the present stitch-reinforced sandwich panel. In addition, these types of resin provide a fast curing rate which is beneficial in a continuous process for manufacturing the present stitch-reinforced sandwich panel. The acrylate type resin is preferably used for thermoformable purposes.

A preferred example of the resin in the present stitch-reinforced sandwich panel is a resin is of the epoxy type, especially of the hybrid epoxy/polyurethane type, for example epoxy/isocyanate.

The reinforcing thread is preferably chosen from the group of carbon, glass and aramid. An embodiment of a reinforcing thread used in the present stitch-reinforced sandwich panel is a carbon fiber made from polyacrylonitrile (PAN).

In an embodiment the first and second skins of carbon fiber reinforcing material for covering the first and second skins of fiber reinforcing material respectively are of the type carbon spread tow (30 - 200 g/m²). In another embodiment the first and second skins of carbon fiber reinforcing material for covering the first and second skins of fiber reinforcing material respectively are of the type carbon weave (0 90°, 30 - 300 g/m²). In a special embodiment the carbon fiber reinforcing material in the first and second skins of carbon fiber reinforcing material for covering the first and second skins of fiber reinforcing material is replaced by a glass weave (30 - 350 g/m²).

According to a preferred embodiment of the present stitch-reinforced sandwich panel the combination of technical features comprises polyphenylsulfone (40 - 50 kg/m³) as foam core material, carbon as reinforcing thread, glass weave (0 90°, 30 - 80 g/m²) as inner skin layers, epoxy/isocyanate as resin and carbon spread tow (30 - 200 g/m²) as outermost skin layers. More specific, polyphenylsulfone, 50 kg/m³ (+/- 10% mass tolerance), carbon reinforcing thread , glass weave, 0 90°, 49 g/m² as first and second skins of fiber reinforcing material, epoxy/isocyanate hybrid resin, and carbon spread tow, 0 90°, 160 g/m² as first and second skins of carbon fiber reinforcing material, i.e. outermost layer.

According to another preferred embodiment of the present stitch-reinforced sandwich panel the combination of technical features comprises polyethylene terephthalate, 65 kg/m³ (+/- 10% mass tolerance) as foam core material, carbon reinforcing thread, glass weave, 0790°, 49 g/m² as first and second skins of fiber reinforcing material, epoxy/isocyanate hybrid resin and carbon spread tow, 0790°, 160 g/m² as first and second skins of carbon fiber reinforcing material, i.e. outermost layer. This embodiment is especially preferred for VIP jets.

According to another preferred embodiment of the present stitch-reinforced sandwich panel the combination of technical features comprises polyethylene terephthalate, 65 kg/m³ (+/- 10% mass tolerance) as foam core material, glass reinforcing thread, glass weave, 0790°, 49 g/m² as first and second skins of fiber reinforcing material, acrylate resin and glass weave, 0790°, 350 g/m² as first and second skins of glass fiber reinforcing material, i.e. outermost layer.

According to another preferred embodiment of the present stitch-reinforced sandwich panel the combination of technical features comprises polyethersulfone (40 - 60 kg/m³) as foam core material, glass as reinforcing thread, carbon weave (0790°, 30 - 80 g/m²) as first and second skins of fiber reinforcing material, epoxy as resin and carbon weave (0790°, 30 - 300 g/m²) as first and second skins of carbon fiber reinforcing material, i.e. outermost layer.

The present invention furthermore relates to a method for manufacturing a stitch-reinforced sandwich panel as discussed above, wherein the present method comprises the steps of applying to a foam core on opposite surfaces thereof, first and second skins of fiber reinforcing material, stitching said skins through said foam core with a reinforcing thread over substantially the entire surface area of the core, impregnating with a resin and placing and pressing in a mould under heat and pressure, wherein said method further comprises, before said step of impregnating with a resin, a step of applying to said already stitched first and second skins of fiber reinforcing material, first and second skins of carbon fiber reinforcing material.

The present method has been shown in the attached Figure.

In the Figure step 1 comprises the preparation of the raw material, i.e. the foam core. In step 2 both sides of the foam core are provided with first and second skins of fiber reinforcing material respectively covering the first and second faces of the foam core. In step 3 a reinforcing thread is stitched through each of the skins and through the foam core to form stitches arranged in a pattern over substantially the entire faces of the foam core, the stitching joining the skins and the core together to form a stitch-reinforced sandwich panel. On the thus prepared stitch-reinforced sandwich panel first and second skins of carbon fiber reinforcing material are applied thereby respectively covering the first and second skins of fiber reinforcing material (step 4). The thus obtained intermediate composite material, i.e. a sandwich structure, is fed into a mold comprising one or more an injection chambers (step 5). In the injection chamber a resin is injected (step 6) into the composite material, i.e. the sandwich structure, in such a way that the resin is present between the first and second skins of carbon fiber reinforcing material and the first and second skins of fiber reinforcing material, the resin being impregnated into the foam core. After the step of impregnating there is a step of curing the resin in a heating zone of the mold (step 7). The transport of the composite material through the mold takes place via pultrusion (step 8). In an embodiment of the application of a thermoplastic type resin, i.e. an acrylate, the cured panel is heated (step 9) and thermoformed (step 10).

The present inventors surprisingly found that the amount of resin used in the manufacturing process can be dramatically reduced without deteriorating the mechanical properties of the final panel, especially the stiffness and the risk of delamination of the individual layers. Therefore, it is preferred to apply a pre-pressing step on the foam core and after such a pre-pressing step the first and second skins of fiber reinforcing material are applied on opposite surfaces of the thus pre-pressed foam core.

Such a step of pre-treating, i.e. pre-pressing the foam core, is partly done with pressure, but mostly by temperature. The foam has to be heated up to a temperature close to its glass transition temperature TG, for example ~150°C, to make it soft. And after reaching the required temperature the surface of the foam can be smoothened when applying pressure. The present invention furthermore relates to the use of a stitch-reinforced sandwich panel, as discussed above, or a stitch-reinforced sandwich panel manufactured according to the process, as discussed above, in aircraft interiors. Examples of aircraft interior parts are for example galleys, lavatories, stowage's, partition walls, armrest and armrest segments, doors, foldable tables etc. Besides these aircraft interior parts other applications of the present panel are interior for trains, boats and automotive.

The features, functions, and advantages can be achieved independently in various embodiments of the present disclosure or may be combined in yet other embodiments in which further details can be seen with reference to the following description and drawings.

In the next tables the results from the experiments carried out by the applicant have been shown. The first table shows the material combinations and its name (see the last column), the second table the experimental results. In this second table the first column refers to the name as shown in the last column of the first table. Each example of a panel consists of an outer layer, i.e. identified as skin in the Table below, a glass weave, a foam core, a glass weave and a skin as well. Both layers of glass weave are stitched to the foam core on both sides and the resin penetrates the whole sandwich of skin-glass-foam-glass-skin.

The material identified as TeXtreme 100 g/m² originates from Oxeon (supplier, 1000 15K, Spread tow weave 0 90°, TR50S, 15K) and TeXtreme 160 g/m² originates from Oxeon (supplier, 1000 15K, Spread tow weave, 0 90°, TR50S, 15K).

The material identified as CarboNXT (supplier) is a Recycling veil, 30 g/m², Non woven recycled fiber. The term F40 refers to Divinycell F40, supplier, DIAB, weight 40kg/m³. The term S51 refers to Rohacell S 051 , supplier Evonik, weight 52 kg/m³. The term IGF refers to Rohacell IG-F 031 , supplier Evonik, weight 32 kg/m³.

For the resins, the term B refers to Baydur FR RP. CS01 PL0061 mod. SR01924, supplier Bayer, polyurethane type, the term A refers to FST Araldite 40002/40003, supplier Huntsman, modified epoxy type, the term H refers to Loctite BZ 9120 AERO, supplier Henkel, modified benzoaxine, the term F refers to Firestop 8175-W- 1 , supplier DSM/BLJFA, polyester type.

In the table of the test results the second column FST refers to Flammability A (60 sec. vertical burn test, AITM 2.0002 A, length: 300 ± 1 , width: 75 ± 1 , thickness: 19,05, number of specimens: 4), Flammability B (12 sec. vertical burn test, AITM 2.0002 B, length: 300 ± 1 , width: 75 ± 1 , thickness: 19,05, number of specimens: 4), Smoke Density (Optical smoke, AITM 2.0007 A, length: 73 ± 2, width: 73 ± 2, thickness: 19,05 a number of specimens: 10), and Toxicity (AITM 3.0005, length: 73 ± 2, width: 73 ± 2, thickness: 19,05 a number of specimens: 10), all taken from the FAA "Aircraft Materials Fire Test Handbook", April 2000 (chapter 1-1).

For the heat release test method AITM 2.0006, length: 150 ± 2, width: 150 ± 2 and thickness: 19,05, number of specimens: 5).

For the 4PB (4-Point Bending) a sandwich panel is placed in a 4 point bending test machine, lying on two support roles. Two load point apply force on the panel, with a constant rotation velocity until the material fails. Maximum load, deflection at maximum load and failure mode are monitored (test method: AITM 1.0018, length: 400, width: 50, thickness: 19,05, number of specimens: 5).

The peeling test, i.e. climbing drum peel test, is defined in EN 2243-3. The skin of the sandwich is attached on a drum with a certain diameter. While rolling the drum over the sandwich, the skin peels off. The force which is needed for that is monitored.

As shown in the first table seven different material combinations have been tested. Example 8 did pass all the tests.

For the Fire Smoke Toxity Test example numbers 1-5 passed the test, whereas example 6 failed due to the type of foam.

The present inventors further found that due to the high resin uptake and inferior mechanical properties the Recycling Veil (CarboNXT) was not considered option anymore. In addition, due to production and manufacturing advantages Baydur resin has been identified as a favorable resin.

Tests were only performed with example 1 as the influences on the mechanical properties are dominated by the fibers, not the resin. Focus of this test was to investigate the effect of stitching compared to a non-stitched foam-sandwich-panel. However, the present method is in no way restricted to a specific type of stitching as shown here. Therefore, other ways of stitching, such as stitching in non-regular patterns, i.e. higher densed stitching and higher loaded areas, are also covered by the present invention.

Claims

1. A stitch-reinforced sandwich panel, comprising:

a reinforcing thread stitched through each of the skins and through the foam core to form stitches arranged in a pattern over substantially the entire faces of the foam core, the stitching joining the skins and the core together to form a stitch-reinforced sandwich panel;

characterized in that said stitch-reinforced sandwich panel further comprises first and second skins of carbon fiber reinforcing material respectively covering the first and second skins of fiber reinforcing material, wherein a resin is present between said first and second skins of carbon fiber reinforcing material and said first and second skins of fiber reinforcing material, said resin being impregnated into said foam core.

2. The stitch-reinforced sandwich panel of claim 1 , wherein said first and second skins of fiber reinforcing material are composed of fibers chosen from the group of glass fibers, carbon fibers and aramid fibers.

3. The stitch-reinforced sandwich panel of claim 2, wherein said glass fibers are of the type glass weave in a configuration of 0 90°, preferably in a range of

30 - 80 g/m².

4. The stitch-reinforced sandwich panel of claim 2, wherein said carbon fibers are of the type carbon weave in a configuration of 0 90°, preferably in a range of

30 - 80 g/m².

5. The stitch-reinforced sandwich panel of claims 1-4, wherein said first and second skins of carbon fiber reinforcing material are of the type plain weave, especially of the type carbon weave (0790°, 30 - 300 g/m²).

6. The stitch-reinforced sandwich panel of claims 1-4, wherein said first and second skins of carbon fiber reinforcing material is of the type carbon spread tow (30 - 200 g/m²).

7. The stitch-reinforced sandwich panel of claims 1-6, wherein said foam is a foam chosen from the group of poly(ether sulfone), polymethacrylimide, polyphenylsulfone and polyethylene terephthalate.

8. The stitch-reinforced sandwich panel of claim 7, wherein said foam is a poly(ether sulfone) foam having a density of 40-60 kg/m³ (according to ASTM D 1622).

9. The stitch-reinforced sandwich panel of claim 7, wherein said foam is a polyphenylsulfone foam having a density of 40 - 50 kg/m³ (according to ASTM D 1622).

10. The stitch-reinforced sandwich panel of claim 7, wherein said foam is a polyethacrylimid foam having a density of 50 - 70 kg/m³ (according to ASTM D 1622).

1 1. The stitch-reinforced sandwich panel of claim 7, wherein said foam is a polyethylene terephthalate foam having a density of 65 - 100 kg/m³ (according to ASTM D 1622).

12. The stitch-reinforced sandwich panel of claims 1-1 1 , wherein said resin is chosen from the group of hybrid epoxy/polyurethane, epoxy, polyurethane, acrylate and polyester, or a mixture thereof.

13. The stitch-reinforced sandwich panel of claim 12, wherein said resin is of the epoxy type.

14. The stitch-reinforced sandwich panel of claim 12, wherein said resin is of the epoxy/isocyanate type.

15. The stitch-reinforced sandwich panel of claims 1-14, wherein said reinforcing thread is chosen from the group of carbon, glass and aramid.

16. The stitch-reinforced sandwich panel of claim 15, wherein said reinforcing thread is a carbon fiber made from polyacrylonitrile (PAN).

17. The stitch-reinforced sandwich panel of claims 1-16, wherein said stitches are arranged in a non-regular pattern.

18. The stitch-reinforced sandwich panel according to any one or more of claims 1-17, wherein said panel comprises polyphenylsulfone (40 - 50 kg/m³) as foam core material, carbon as reinforcing thread, glass weave (0 90°, 30 - 80 g/m²) as first and second skins of fiber reinforcing material, epoxy/isocyanate as resin and carbon spread tow (30 - 200 g/m²) as first and second skins of carbon fiber reinforcing material.

19. The stitch-reinforced sandwich panel according to any one or more of claims 1-17, wherein said panel comprises polyethersulfone (40 - 60 kg/m³) as foam core material, glass as reinforcing thread, carbon weave (0 90°, 30 - 80 g/m²) as first and second skins of fiber reinforcing material, epoxy as resin and carbon weave (0790°, 30 - 300 g/m²) as first and second skins of carbon fiber reinforcing material.

20. A method for manufacturing a stitch-reinforced sandwich panel according to any one or more of the preceding claims, said method comprises the steps of applying to a foam core on opposite surfaces thereof, first and second skins of fiber reinforcing material, stitching said skins through said foam core with a reinforcing thread over substantially the entire surface area of the core, impregnating with a resin and placing and pressing in a mould under heat and pressure, wherein said method further comprises, before said step of impregnating with a resin, a step of applying to said already stitched first and second skins of fiber reinforcing material, first and second skins of carbon fiber reinforcing material.

21. The method for manufacturing a stitch-reinforced sandwich panel according to claim 20, wherein, before applying to said foam core on opposite surfaces thereof, first and second skins of fiber reinforcing material, said foam core is pre-pressed.

22. The method for manufacturing a stitch-reinforced sandwich panel according to claim 21 , wherein said step of pre-pressing said foam core comprises preheating the foam close to its glass transition temperature and then applying pressure to the foam for smoothening of the foam.

23. The use of a stitch-reinforced sandwich panel according to any one or more of the claims 1-19 or a stitch-reinforced sandwich panel manufactured according to any one or more of the claims 20-22 in interiors for aircrafts, trains, boats and/or automotive.