Sandwich structure with an internal layer made of foam polymeric material
A novel method for producing sandwich structures with a thermoplastic polymer core addresses inefficiencies in existing methods by using vacuum heating and inert gas foaming to create a foam core, enhancing productivity and geometric flexibility.
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- LEONARDO SPA
- Filing Date
- 2025-12-12
- Publication Date
- 2026-06-18
AI Technical Summary
Existing methods for producing sandwich structures with a foam polymer core are inefficient due to long execution times, high costs, operational complexities, and limited applicability to simple geometries, and chemical foaming processes introduce contamination and complexity.
A method involving positioning, vacuum heating, blowing, foaming, and cooling phases to create a sandwich structure with a thermoplastic polymer core, where the central layer has a lower melting point than the external layers, using inert gas at high pressure to form the foam core without contamination, ensuring efficient bonding.
The method significantly improves productivity, reduces costs, and allows for more complex geometries, while maintaining mechanical resistance and soundproofing properties.
Smart Images
Figure IB2025062785_18062026_PF_FP_ABST
Abstract
Description
[0001] SANDWICH STRUCTURE WITH AN INTERNAL LAYER MADE OF FOAM POLYMERIC MATERIAL
[0002] Cross-Reference to Related Applications
[0003] This Patent Appl ication claims priority from Italian Patent Application No . 102024000028449 filed on December 13 , 2024 , the entire disclosure of which is incorporated herein by reference .
[0004] Technical Filed
[0005] The present invention relates to a method for making a sandwich structure with an internal layer made of foam polymeric material .
[0006] Prior Art
[0007] As is known, in the aerospace sector, as well as in the automotive sector, the use of sandwich structures made of foam polymer material covered with reinforcement layers has long been increasingly popular . The sandwich structures mentioned above have the advantage of being extremely light and, at the same time , resistant . Lightness and mechanical resistance are two qualities considered extremely important , and this is why research in the aerospace and automotive sectors is increasingly focused on finding materials with these characteristics .
[0008] Another advantage of foam materials is their soundproofing properties . For this reason, the above- mentioned sandwich structures have long been used in the construction industry as secondary walls (not load-bearing) .
[0009] Today, sandwich structures with the central part made of foam material are made using a method that involves two or more stages . Generally, in the first stage , the external layers and the central layer of foam material are prepared separately and, only in a later stage , are they j oined together to form the sandwich structure .
[0010] When thermoplastic polymers are involved, the coupling phase of the central layer with the external layers usually occurs through fusion or adhesive .
[0011] As it may be immediately apparent to a person skilled in the art , the above method suf fers from a disadvantage in terms of productivity due to long execution times , excessive cost and operational criticalities .
[0012] Furthermore , another disadvantage of the above method lies in the limit of being applicable only to extremely simple flat and curved geometries .
[0013] As an alternative to the above method, a method using chemical foaming agents has been developed .
[0014] This method is a one-step process that uses a chemical foaming agent to create the porous structure while the external layers are placed above and below it . In particular, during the foaming process , the polymer melts due to the heat of reaction and comes into contact with the external layers , to initiate the bond by melting . The entire process is carried out inside a pressuri zed autoclave reactor . This method, which can be called chemical foaming, has the disadvantages of having to use a speci fic foaming chemical for each polymer, the release of C02 and other gases , the chemical contamination of the final product , as wel l as the operational complexity of adding the chemical correctly and ef fectively .
[0015] The need was felt to have a method for the construction of sandwich structures , whose technical features were able to overcome the disadvantages of the prior art methods of above . Description Of The Invention
[0016] The subj ect of the present invention is a method for the production of a sandwich structure composed of a central layer made of foam polymeric material and two external layers comprising polymer material and arranged on opposite sides of the central layer ; said method being characteri zed in that it comprises ( i ) a positioning phase , in which the two external layers are housed inside a mold and a central layer of thermoplastic polymer arranged between the two external layers ; said thermoplastic polymer of the central layer having a melting point lower than that of the polymer of the two external layers by at least 5 ° C, preferably by a value between 5 and 15 ° C, more preferably by a value between 8 and 10 ° C ; ( ii ) a vacuum heating phase , in which the inside of said mold is brought to a pressure lower than or equal to 0 . 01 bar and said layer of thermoplastic polymer is heated to a temperature equal to or greater than the melting temperature of said thermoplastic polymer and lower than the melting temperature of the material of which the two external layers are composed; ( iii ) a blowing phase , in which an inert gas is blown inside the mold at a pressure equal to or greater than 50 bar, preferably equal to or greater than 80 bar ; ( iv) a foaming phase , in which the said central layer of thermoplastic material is brought to a temperature lower than the melting temperature of the thermoplastic polymer by a value between 5 and 20 ° C, preferably by a value between 10 and 15 ° C for the formation of the central layer made of foam polymeric material ; (v) a cooling phase in which the sandwich structure reali zed is brought to room temperature .
[0017] The temperature of phase ( ii ) should prevent the external layers from foaming and ensure melting bonding at the external layer / central layer interface during the foaming process . Additionally, the layer of thermoplastic polymer must be completely melted to achieve a better di f fusion of the foaming agent . The heating is performed under a vacuum to avoid the evaporation of the thermoplastic polymers during heating .
[0018] Preferably, the two external layers are made of a composite material consisting of resistant fibers embedded in a matrix of a thermoplastic polymer .
[0019] Preferably, the thermoplastic polymer of the composite material is the same as the thermoplastic polymer of the central layer .
[0020] When the polymer of the composite material of the external layers i s the same as the thermoplastic polymer, a better coupling between the central layer and the two external layers is obtained .
[0021] Preferably, the foaming phase involves a cooling rate between 0 . 37 and 0 . 87 ° C / min .
[0022] Preferably, said inert gas is included in the group composed of N2 , CO2 and He .
[0023] Description Of The Drawings
[0024] Below are some examples of implementation for purely illustrative and non-limiting purposes with the aid of the attached figure , which illustrates in a simpli fied and schematic manner the phases of the method which is the obj ect of the present invention .
[0025] Embodiments Of The Invention
[0026] In the figure , 1 indicates a mold, within which two external layers 2 and a central layer 3 of thermoplastic polymeric material placed between the two external layers 2 are housed. In phase C, 4 represents the foaming agent, i.e. the inert gas that is blown in, while in phase D, 5 indicates the central layer made of foam polymeric material of the sandwich structure, which is indicated as a whole with 6.
[0027] EXAMPLES
[0028] Four examples of invention have been created which differ from each other in terms of the process conditions as will be described below.
[0029] - example 1
[0030] Phase A (positioning phase) : A layer 3 of polypropylene (PP) with a melting point of 153°C determined by DSC ("Differential scanning calorimetry") analysis and two external layers 2 each of which is made of a PP polymer c- mingled with reinforcing glass fibers (marketed under the trade name TWINTEX® R PP 60) and hot compressed to get a uniform skin were inserted into mold 1. The PP polymer of the two external layers has a melting point of 180°C.
[0031] Layer 3 has smaller dimensions than external layers 2 as it must be subject to foaming.
[0032] Phase B (vacuum heating phase) : Mold 1 was inserted into an autoclave. A pressure of 0.01 bar was created inside mold 1. Once the pressure of 0.01 bar was reached, the autoclave heating was turned on to reach a temperature of 160°C. With thermocouples, the temperature of layer 3 was also measured, verifying a temperature equal to 153-154°C and, therefore, equal to or higher than the melting temperature of the PP that constitutes layer 3.
[0033] Phase C (blowing phase) : Once the temperature of layer 3 stabilized at 153°C, 002 was injected into mold 1 at a pressure of 100 bar as blowing agent 4. After 5-10 minutes of saturation time, the temperature decreased at a rate of 0.53°C / min until reaching the temperature of 140°C (foaming temperature) of layer 3.
[0034] Phase D (foaming phase) : After reaching the foaming temperature, blowing agent 4 was released and the temperature of the autoclave is reduced to 100°C without using a specific cooling rate.
[0035] Phase E (cooling phase) : Mold 1 is removed from the autoclave and left to cool. Then, the mold 1 is opened and the sandwich structure 6 is extracted.
[0036] In phase E, water cooling can also be used to reduce the process time.
[0037] The central layer 5 made of foam polymer material obtained in example 1 has a density of 0.11 g / cm3.
[0038] - example 2
[0039] The conditions of this example differ from those of example 1 in that in phase C the foaming temperature was 141°C.
[0040] The central layer 5 made of foam polymeric material obtained in example 2 has a density of 0.13 g / cm3.
[0041] - example 3
[0042] The conditions of this example differ from those of example 1 in that in phase C the expansion temperature was 138°C.
[0043] The central layer 5 made of expanded polymeric material obtained in example 3 has a density of 0.40 g / cm3.
[0044] - example 4
[0045] The conditions of this example differ from those of example 1 in that in phase C the expansion temperature was 139°C.
[0046] The central layer 5 made of foam polymeric material obtained in example 4 has a density equal to 0.15 g / cm3. The sandwich structures 6 obtained from examples 1 - 4 were subj ected to mechanical peeling measures according to the linear elastic fracture mechanics through modi f ied beam theory . In the modified beam theory, a load vs displacement curve is assumed to be linear during the delamination of a thin skin from its core .
[0047] Table I shows the results of the mechanical peeling measurements .
[0048] Table I
[0049] From the above it appears that the method of the present invention involves an improvement in terms of production times , costs and energy inputs . Therefore , with the present invention the realization of sandwich structures is greatly improved in terms of productivity .
[0050] Furthermore , the method obj ect of the present invention is compliant with the obj ectives of sustainable development .
[0051] The sandwich structures reali zed with the method o f the present invention can be used in the aerospace sector, in the automotive sector and in the construction sector .
Claims
CLAIMS1. A method for the production of a sandwich structure (6) composed of a central layer (3) made of foam polymeric material and two external layers (2) comprising polymer material and arranged on opposite sides of the central layer (3) ; said method being characterized in that it comprises(1) a positioning phase, in which the two external layers(2) are housed inside a mold (1) and a central layer (3) of thermoplastic polymer arranged between the two external layers (2) ; said thermoplastic polymer of the central layer(3) having a melting point lower than that of the polymer of the two external layers (2) by at least 5°C; (ii) a vacuum heating phase, in which the inside of said mold (1) is brought to a pressure lower than or equal to 0.01 bar and said central layer (3) of thermoplastic polymer is heated to a temperature equal to or greater than the melting temperature of said thermoplastic polymer and lower than the melting temperature of the polymer of the two external layers (2) ; (iii) a blowing phase, in which an inert gas (4) is blown inside the mold (1) at a pressure equal to or greater than 50 bar; (iv) a foaming phase, in which said central layer (3) of thermoplastic material is brought to a temperature lower than the melting temperature of the thermoplastic polymer by a value between 5 and 20°C for the formation of the central layer made of foam polymeric material (5) ; (v) a cooling phase in which the sandwich structure (6) realized is brought to room temperature.
2. The method according to claim 1, characterized in that said thermoplastic polymer of the central layer (3) has a melting point lower than that of the polymer of the two external layers (2) by a value between 5 and 15°C.
3. The method according to claim 1, characterized in that said thermoplastic polymer of the central layer (3) has a melting point lower than that of the polymer of the two external layers (2) by a value between 8 and 10°C.
4. The method according to any of the previous claims, characterized in that in the blowing phase, the inert gas (4) is blown inside the mold at a pressure equal to or greater than 80 bar.
5. The method according to any of the previous claims, characterized in that in the foaming phase, the said central layer (3) of thermoplastic material is brought to a temperature lower than the melting temperature of the thermoplastic polymer by a value between 10 and 15 °C for the formation of the central layer (6) made of foam polymeric material .
6. The method according to any of the previous claims, characterized in that the two external layers (2) are made of a composite material consisting of resistant fibers embedded in a matrix of a thermoplastic polymer.
7. The method according to claim 6, characterized in that the thermoplastic polymer of the composite material is the same as the thermoplastic polymer of the central layer (3) .
8. The method according to any of the previous claims, characterized in that the foaming phase involves a cooling rate between 0.37 and 0.87°C / min.
9. The method according to any of the previous claims, characterized in that said inert gas (4) is comprised in the group composed of N2, CO2 and He.