SELF-HEALING SANDWICH COMPOSITE STRUCTURE

Field of the Invention

The present invention relates to a macro/meso capsule production method that contains a sandwich structure that is capable of self-healing macro-scale damage and repair agents.

The present invention particularly relates to a self-healing sandwich composite structure that comprises, at least one thermoset polymer resin filled capsule, at least one thermoset polymer resin activator filled capsule, at least one sandwich core whose cells are filled with capsules containing thermoset polymer resin and activator, at least one upper and lower surface plate holding the structure together and a combining layer that joins the surface plates to the sandwich core and a production method thereof.

State of the Art

Today, sandwich materials are used as structural materials due to their being lightweight and having very high bending rigidity, in the fields such as military vehicles, airplanes, space crafts, wind turbines, satellites, ship and yacht sector, high speed trains, electric vehicles and the like.

Critical damages happen in some structures produced from sandwich materials, as exemplified herein below:

- Vehicle tanks, similar fuel tanks such as liquid and gas, tankers and similar material containers that are dangerous when suddenly damaged for any reason,

- Perforation of yachts, boats and ship hulls caused by rock collisions in shallow seas,

- Sudden cracks in airplanes caused by bird strikes and fatigue,

- Damages that occur by effects such as wind, bird strikes on turbine blades, lightning strikes and storms as well as striking hard parts flying in the air.

The subject of the present invention is to develop the sandwich structures as a novel material that is self- healing after the damages in said examples. When the self-improving (self-healing) structures developed in the literature are considered, many differences attract the attention according to the subject of the invention. Some of the studies have insufficiencies in terms of appropriateness for mass production. Although the structures developed by some researchers are suitable for serial production, they have limited fields of use due to their high costs. It draws the attention that the structures with superior improvement performances have high costs in terms of production, quality control and characterization after production. It is seen that the ease of production is neglected in most of the studies.

The Chinese patent application numbered CN103467791 A relates to the production of a double-wall rubber-based microsphere containing a repair agent and activators.

Another related patent is the Chinese application numbered CN108947319A; it is a patent comprising repair agents in the form of microcapsules embedded in the foam structure.

This patent relates to a sandwich material filled with macro/meso capsules containing repair agents contained in nuclear cells. This kind of a sandwich material is not disclosed in the two patents mentioned above.

Consequently, developed structure has a different production system and method from the self-healing materials and structures used in the literature. Due to the abovementioned disadvantages and the insufficiency of the current solutions regarding the subject matter, a development is required to be made in the relevant technical field.

Brief Description of the Invention

The present invention is related to a self-healing sandwich composite structure which fulfills the abovementioned requirements, eliminates all disadvantages and brings some additional advantages.

The main aim of the invention is to perform self-healing in the damaged areas by means of the interaction of the repair agents that are transferred to the fracture or damage area after the damage occurred within the structure. An aim of the invention is to provide a production method with high production speed, low cost and simple inspection processes, different from the self-healing materials in the state of the art.

In order to fulfill the abovementioned aims, the present invention is a self- healing sandwich composite structure, comprising the following; at least one thermoset polymer resin filled capsule, at least one thermoset polymer resin activator filled capsule, at least one sandwich core whose cells are filled with capsules containing thermoset polymer resin and activator, at least one upper and lower surface plate holding the structure together and a combining layer that joins the surface plates to the sandwich core.

The production method of the inventive self-healing sandwich composite structure comprises the following process steps:

- Forming a polymer shell on the polystyrene grains,

- Forming pores on the polymer shell,

- Obtaining the capsule structure by dissolving and removing the polystyrene foam sphere from the porous shell with chemical solvent,

- Strengthening the capsules by coating them with polymer for the second time,

- Injecting the recovery agents into capsules formed with automatic dosing system,

- Filling the sandwich core pores with capsules that contain repair agents consisting of thermoset resin and thermoset resin activator.

The structural and characteristic features of the present invention will be understood clearly by the following drawings and the detailed description made with reference to these drawings and therefore the evaluation shall be made by taking these figures and the detailed description into consideration.

Figures Clarifying the Invention

Figure 1 , is an illustrative view of the self-healing sandwich composite structure

Figure 2, is an illustrative view of the filled sandwich core structure with honeycomb form.

Figure 3, is an illustrative view of the polystyrene foam and polymer shell structures.

Figure 4, is an illustrative view of the automatic dosing system.

Figure 5, is an illustrative view of the capsule production process. Figure 6, is an illustrative view of the sandwich composite production process.

Figure 7, is a graph showing the press test results.

The figures are not required to be scaled and the details which are not necessary for understanding the present invention may be neglected. Moreover, the elements that are at least substantially identical or have at least substantially identical functions are shown by the same number.

Description of the References

1 - Thermoset polymer resin filled capsule

2- Thermoset polymer resin activator filled capsule

3- Sandwich core

4- Surface plate

5- Combining layer

6- Polystyrene foam

7- NaHCO ₃ filled polymer shell

8- Porous polymer shell

9- Non-porous polymer shell formed on the porous shell

10- Thermoset polymer resin

11- Thermoset polymer resin activator

12- Automatic dosing system

13- Acetone solution

14- Capsule consisting of polymer shell outside and polystyrene foam inside

15- Capsule filled with acetone solution after the dissolution of the polystyrene foam Detailed Description of the Invention

In this detailed description, the inventive self-healing sandwich composite structure is described only for clarifying the subject matter in a manner such that no limiting effect is created.

The present invention is a self-healing sandwich composite structure. The inventive structure; comprises the following, at least one thermoset polymer resin filled capsule (1 ), at least one thermoset polymer resin activator filled capsule (2), at least one sandwich core (3) whose cells are filled with capsules containing thermoset polymer resin and activator, at least one lower and upper surface plate (4) holding the structure together and a combining layer (5) that joins the surface plates to the sandwich core (Figure-1).

In a preferred embodiment of the invention; capsules (1 and 2) are in meso and macro sizes.

In a preferred embodiment of the invention; the sandwich core (3) is honeycomb, square, rectangular, cylindrical, corrugated and suitable auxetic forms. It is preferably in honeycomb form (Figure-2).

In a preferred embodiment of the invention; the thermoset polymer resin is selected from the group consisting of epoxy, polyester, vinyl ester, polymethyl methacrylate, polyurethane and/or silicone. Preferably epoxy is used.

This sandwich material can repair itself after damage. Repair agents are encapsulated in macro/meso capsules. Since macro/meso capsules are filled into the pores of the sandwich core (3) and the macrospheres will also break after the damage, the liquid recovery agents inside them will leak and fill the pores of the sandwich core (3). It reacts with the flowing liquid polymer (10) and its activator (11 ) and solidifies in the sandwich structure and enables the damaged structure to carry the load when subjected to reloading. Therefore, it provides self-healing in the sandwich structure.

The operating principle of the inventive structure is explained in detail herein the following:

Capsules (1 , 2) that contain thermoset resin and thermoset resin activator produced in different locations are placed in the pores of a core (honeycomb, etc.) structure (3). This structure may be used in larger sized structures and at different points apart from the ones mentioned above. The example shown here is a representative configuration and the operation of this structure against the loads that may occur during the use of this structure in different situations will be as follows. On one hand, the core sandwich (3) in the form of honeycomb will be crushed on the other hand during this crushing, the capsules (1 , 2) containing thermoset resin and thermoset resin activator will also be broken and repair chemicals therein will leak out. Consequently, repair chemicals will advance through the cracks and they gather and cause the curing process to initiate. The damaged area in the system will be repaired and the strength and rigidity and geometric healing of the structure will be provided, after completing this curing process. Thus, the structure will obtain its ability to fulfill its function, to a large extend compared to its original state, and will continue to fulfill its task. Furthermore, larger and inevitable damages that may arise in the future will be prevented by healing possible micro-scale structural damages.

The production method of the inventive self-healing sandwich composite structure comprises the following process steps:

- Forming a polymer shell (7) on the polystyrene grains,

- Forming pores on the polymer shell,

- Obtaining the capsule structure by dissolving and removing the polystyrene foam sphere from the porous shell (8) with chemical solvent,

- Strengthening the capsules by coating them with polymer for the second time,

- Injecting the recovery agents into capsules formed with automatic dosing system (12),

- Filling the sandwich core (3) pores with capsules (1 and 2) that contain repair agents consisting of thermoset resin and thermoset resin activator.

The inventive production method and exemplary experimental steps (Figures 5 and 6) of the invention are explained in detail herein the following: a) Forming an epoxy shell (7) on the polystyrene grains:

Polystyrene foams are one of the most commonly used foam structures in industry. It was used as the core material of the self-healing structure developed due to its advantages such as low cost and easy accessibility. As a shell material NaHCO ₃ (sodium bicarbonate) filled polymer (epoxy etc.) is used. As the polymer shell material, a polymer resistant to chemicals like acetone (CaHeO), which can dissolve polystyrene foam can be selected. Epoxy is selected in the described application. The polystyrene spheres (6) coated with epoxy, resin activator and NaHCO, powder mixture were kept at room temperature for 48 hours to complete the shell formation and hardened and a hard shell (7) structure, which has polystyrene foam spheres (6) inside and is coated with a mixture of polymer and NaHCO ₃ outside, was obtained (Figure-3). b) Forming pore on the polymer shell:

NaHCO ₃ shall be removed from the solidified polymer shell (7) so as to remove the coated polystyrene material and obtain the hollow shell structure. Therefore, the capsule particles (14) consisting of polymer solid shell outside and polystyrene spheres inside were mixed for 5 minutes in water at 500. Therefore, a porous polymer shell (8) structure was obtained by dissolving the NaHCO ₃ particles in water that are found in dispersed manner in the polymer solid shell (7) (Figure-3). c) Removal of polystyrene foam sphere having porous polymer solid shell outside with the chemical solvent:

Acetone (CsHeO), one of the chemical solvents, was used so as to remove the polystyrene foam sphere (6) from the porous shell (8). 25% acetone + 75% distilled water solution (13) is applied and is penetrated into porous samples. In Figure-5, the formation of a capsule filled with acetone solution after the dissolution of the polystyrene foam (15) is shown. The spheres (6) of the polystyrene foam are dissolved within 15 minutes, they are mixed with the liquid solution and removed and macro/meso capsules with polymer shell that is porous on the outside and is empty inside (Figure 5). d) Strengthening the porous shell structure:

After removal of NaHCO _:s from the polymer shell structure, the rigidity of the shell with very thin walls has decreased. Furthermore, it has become possible for the repair agents to be filled in to leak out. Therefore, it is provided to strengthen the weakened porous polymer shell by coating the same with polymer for the second time. In Figure-3 the non-porous polymer shell formed on the porous shell (9) structure is shown. e) Injecting the repair agents with the automatic dosing system (12),

Thermoset resin (10) and thermoset resin activator (11 ) were injected into the shell as repair agents (Figure-4). f) Filling the sandwich core pores (3) with capsules (1 and 2) that contain repair agents consisting of thermoset resin and activator: After these processes, the process step of filling thermoset resin (10) based repair chemicals into the capsules, whose polymer shell structures are completely solidified, are filled. Thermoset resins generally consist of two components, polymer resin and activator. The polymer resin is the main repair chemical (component A) and the activator (component B) is the curing of the polymer resin (10) (required for the formation of crosslinks), which leads to the formation of chemical bonds and solidification of these two components, which were initially liquid. Each component in two-component recovery chemicals are filled into the capsules separately and it can be put from both capsules in the core cells in accordance with the mixing ratio of these components. In case the repair chemicals consisting of one-component polymers are used, cells are filled with capsules containing only that component. In case it is required, the process step of foaming occurs with the mixture of polymer resin (A component) and B components mixed with foaming agents therein, and the filling process of repair chemical volume by increasing by 2-20 folds and solidifying (with a porous structure) after it fills the opened gap. Polymer repair agents using natural agents such as moisture can also be used as activators.

Consequently, as an example of thermoset polymers that can be filled in macro/meso capsules as a repair chemical in this method developed for general purpose, the following can be listed: Epoxy resin and hardener; polyester resin and hardener; vinyl ester resin and hardener; PMMA resin and hardener, polyurethane resin and hardener, silicone resin and hardener. These two-component repair chemicals are filled into individual macro/meso spheres by an automatic dosage system (12) and obtained macro/meso capsules filled with the repair chemical are filled into the sandwich cores having macro cell divisions (3) with required ratios for solidification and lower and upper plates (4) are combined thereon and closed. Therefore, the self-healing sandwich structures are obtained.

Different curing agent filled macrosphere and sandwiches can be provided for different systems. Thermoset polymer and hardeners or single component thermoset polymer components can be selected according to the repair task inside the macro/meso capsules, depending on the damage types, sizes and environmental conditions that occur where the sandwich structure will be used or it can be made to carry out the desired function. For example, polyurethane resin among two-component thermoset polymer resins is provided in both rigid and foamable (porous) structure and can be used for self-healing agent. Therefore, sandwiches with suitable filling can be provided according to the type of possible damages and the size of the breaks, holes and tears in the area where the sandwich material will be used.

Macro/meso capsules in core cells in the damage area are also damaged, in case the sandwich material is damaged due to environmental and load conditions in its environment, and the repair chemicals therein flow towards the damaged area and mix in the space in the damaged area.

In the following, the self-healing application results for an agent pair consisting of epoxy thermoset polymer resin among repair agents that form a rigid filling by solidifying after mixing and hardening chemical are described:

In this application, the macro/meso capsules are filled with epoxy polymer (Epoxy A) resin and a hardening chemical (Epoxy B) separately and after the filling hole on the sphere is closed, 2 spheres from Epoxy A and 1 capsule from Epoxy B were placed in the cell spaces (2: 1 ratio) such that they are on the core of the sandwich plate. In order to protect the desired rigidity after the capsules were filled, the spaces between the macro/meso capsules placed in the core cells and the cell walls were filled with epoxy thermoset resin mixed with a solidifier and kept at room temperature for 48 hours to harden. g) Improving the structure as a result of the agents’ in the spheres filling into the damaged space after the damage and solidifying as a result of the reaction.

The samples obtained in the graphic in Figure-7 have been tested and damaged. Then, it was kept for 4 hours. In the meanwhile, the self-heal of the sample, which was found in the deformed sandwich honeycomb cells during the first loading, was simultaneously damaged and the repair agents flowed and filled the gap in the broken areas, were realized. The self-healing sample is subjected to test again. The sample which was made unable to carry load in the first test became able to carry load again after self-healing and its mechanical original state was regained.

The fields of use of the inventive sandwich panels, which can show recovery after the damage is explained herein the following; - It can be used for cutting or preventing leakage after cracks that may occur for any reason in fuel (liquid or gas) tanks and similar structures of air, land and sea vehicles, military vehicles.

- It serves for healing so as to prevent water intake in ships, assault boats, amphibious vehicles and submarines as a consequence of holes and cracks in the hull for any reason.

- It is possible to find a field of application area so as to prevent leakages that may occur after puncture and cracking in the hulls of tankers used in the transport of flammable, burning materials or hazardous chemicals, in the tanks of chemical plants, fuel tanks of vehicles and the like.

It has many advantages for eliminating these damages, due to the self-healing property of the sandwich material.

It seems possible to gain the required time for taking full maintenance and precaution by ensuring increased economy and safety with the ease of repair provided by the self- healing ability, which is the property of the sandwich panel given above or to be developed in similar application areas and preventing the danger from growing.