Self-destructible Honeycomb Laminates

Abstract

This disclosure is directed to on-command, self-destructible, laminated honeycomb structures, e.g., printed circuit assemblies, having a honeycomb core the polygonal cells of which contain a material capable of rapid yet nonexplosive combustion e.g., incendiary or pyrotechnic material, and wherein the cell walls are substantially perpendicular to the longitudinal axis of substantially parallel facing sheets which can be secured directly or indirectly thereto. One of the facing sheets can constitute or contain a printed circuit and a portion or all of the incendiary material can be encapsulated to render it inactive at ambient conditions until the desired time for ignition. Ignition wells and igniters can be provided to communicate with a portion of the incendiary composition and the honeycomb cell walls can have small openings to interconnect some or all of the cells for migration of gases and flame propagation therebetween. An edge sealant, e.g., potting composition, can be used to aid in insulating the incendiary material from exposure to degradative influences and directionally channel the burning to the major desired area(s) to be destroyed, e.g., the facing sheet area(s) containing the printed circuitry.

Description

The present invention is directed to on-command, self-destructible laminated structures comprising a honeycomb core having cells at least some of which contain a material capable of rapid yet nonexplosive combustion wherein the cell walls are secured positionally along at least a portion of their extent, e.g., at their ends, substantially perpendicularly with respect to the longitudinal surface of substantially parallel facing sheets.

Frequently it is desirable, or necessary, esp., in military applications, to destroy or disfigure specific structures and/or equipment so that their identity and function remain anonymous. In such cases the time required for destruction may be of paramount importance. Also it is often necessary that the material or structure which is to be destroyed contain within its structure the materials and method for its destruction. Structures having these capabilities are oftimes referred to as integral, on-command, self-destruct systems.

While it is possible to prepare such systems using explosive compositions wherein an explosive charge or charges are placed on various portions of or proximate to the device to be destroyed; such systems frequently result in the nonselective destruction of the surrounding installation with danger to personnel. Consequently there has developed a need for an integral, on-command self-destruct structure whereby only that desired portion(s) to be destroyed are actually affected by the destruct mechanism. This in turn requires a controlled or limited destruction to take place.

The present invention constitutes what is believed to be a marked advance in that it enables the attainment of a very rapid, on-command, selective self-destruct capability thus reducing risk of personnel injury as destruction takes place. This allows the personnel to supervise destruction with minimal hazard. Hence it has been discovered that by employing the aforesaid laminated structure with a honeycomb core the cells of which contain a rapidly burning yet nonexplosive pyrotechnic or incendiary composition, that upon ignition of said composition; the desired objectives are achieved while largely avoiding the disadvantages attendant to prior structures. Moreover, the present invention enables the self-destruct function to be further controlled by the use of an encapsulated pyrotechnic component(s) to achieve uniform, safe and inert distribution of an otherwise reactive component(s) and hence increase reliability of the self-destruct system. Also the use of encapsulation enhances control over the rate of burning at which the selective destruction takes place. When the cells of the honeycomb laminated sandwich-type structure are filled with a suitable pyrotechnic or incendiary composition; the complete structure can be rapidly reduced to a token quantity of ash without creating a hazard to extraneous structures or personnel in the immediate vicinity. Such self-destruct structures can be made comparatively safe regardless of where they are used.

The sole FIGURE of the drawing shows a partially exposed schematic view of an exemplary on-command self-destructible laminated honeycomb structure in accordance with this invention. In the drawing a honeycomb core section 1 is comprised of a plurality or multitude of individual cells 2 at least some of which are interconnected by holes, openings or perforations 3 to enhance flame propagation by allowing the flame to spread from one cell to another through cell walls 4. At least a portion, and usually most, of these cells contain an incendiary material 5 which is capable of rapid yet nonexplosive combustion when ignited through one or more ignition wells 6 which afford communication of an igniter(s), not shown, with said combustible material. Substantially parallelly arranged lower and upper facing sheets 7 and 7', respectively, are positioned substantially perpendicularly with respect to cell walls 4. The facing sheets can be secured to the core section by reinforced or unreinforced adhesive 8, which can be a "pre-preg" (previously adhesively impregnated) glass cloth type reinforced adhesive component. An edge sealant 9 is usually used to seal an outer ring(s) of cells along the outer periphery of the structure prior to securing upper facing sheet 7'. While the drawing shows use of adhesive securing means, which is preferably employed to secure the core section to the facing sheets; any other suitable securing or fastening media can be used, e.g., nuts and bolts, screws, etc.

A wide variety of materials can be employed to form the honeycomb cell structure which constitutes the core of the article of the present invention. Suitable exemplary honeycomb cell wall materials include, but are not limited to, the following: unperforated and perforated aluminum, copper and fiberglass; steel; paper; plastics, e.g., polypropylene, polyethylene, polystyrene, polycarbonate resins; etc. According to a preferred embodiment of the present invention; the honeycomb material is perforated, e.g., resinbonded fiberglass or aluminum honeycomb wall material is provided with a plurality of minute orifices (holes) in the cell walls to assist in migration of gases among the individual cells. These perforations are so placed as to interconnect the cells, e.g., a vacuum introduced at one location will be communicated to all the cells. In the present invention, these openings allow flame propagation between cells. The honeycomb cell dimensions of height and distance across each cell can be varied widely. Hence the honeycomb cell height and size (distance across each cell between parallel cell faces) will be determined largely by the size and purpose of the self-destruct system. Suitable cell dimensions of height and size (amenable to use in printed circuit boards and other electronic components) are as follows:

The use of a honeycomb core whose cell walls are perpendicular to the facing sheets enables the attainment of structural strength with minimum weight and also serves to assist in channelizing the thermal energy released upon ignition of the incendiary composition in the direction where it is most needed.

A wide variety of incendiary and pyrotechnic compositions, both encapsulated and unencapsulated, can be employed in accordance with this invention. Suitable exemplary encapsulated incendiary and pyrotechnic compositions which can be used include, but are not limited to, the following: encapsulated metallic fuel compounds (aluminum, magnesium, titanium, beryllium, lithium, sodium, phosphorous, silicon, alloys of two or more of these metals, etc); encapsulated oxidizers, e.g., alkali metal perchlorates, such as KClO.sub.4, NaClO.sub.4, LiClO.sub.4, nitrates such as NaNO.sub.3, AgNO.sub.3, etc; encapsulated fluorocarbon chemical liquids such as C.sub.9 F.sub.21 N and C.sub.8 F.sub.16 O. Extremely reactive liquid and/or solid components can be microencapsulated and employed in accordance with this invention to achieve tailored combustion temperatures and burning rates. Without encapsulation many of these materials, either because of their liquid form and/or extremely reactive nature, cannot be formulated into stable incendiary materials. Using encapsulation to contain the reactive component(s) enables both totally solid and liquid-solid hybrid formulations to be used. Either a fuel or oxidizer component (or both) can be encapsulated to enhance system stability. According to a preferred embodiment of this invention, one or more component(s) of the rapidly combustible material are encapsulated because of the greater rapidity and combustion efficiency available through use of such previously unstable fuels and oxidizers. Hence the present invention affords an effective structural environment for employing potentially explosive pyrotechnic chemicals such as sodium, magnesium and aluminum in the presence of oxygen, which pyrotechnics have not been widely employed previously in self-destruct mechanisms due to the frequent necessity of combining them only at the time when destruction is desired. Also in the past the use of such materials was accompanied by definite safety hazards.

The capsules are produced by encapsulation and are in effect spheres having an internal phase, viz, the pyrotechnic component(s), and an external phase, viz, the capsule cell wall material. Moreover, a mixture of two or more components can be encapsulated in the same or different capsules and capsules containing different mixtures can be mixed. Reactive components are placed in separate capsules, or one is encapsulated and the other(s) are not.

A wide variety of both chemical and mechanical encapsulation procedures can be employed to form capsules containing various incendiary components. Suitable chemical encapsulation procedures can be found in U.S. Pat. Nos. 2,800,457 and 2,800,458. Suitable exemplary chemical (en masse) encapsulation procedures for use with highly reactive incendiary components, e.g., metals, oxidizers, fluorochemicals, etc., will be discussed hereinbelow with respect to the use of gelatin-gum arabic plus copolymers of ethylene and maleic anhydride and nitrocellulose external phase materials. Other cell wall materials can be used however depending upon the specific internal phase materials to be encapsulated. Capsules too small to be identified by the naked eye are referred to as microcapsules; and the process of making them is called microencapsulation.

ENCAPSULATION OF METALS AND FLUOROCARBONS

The encapsulation procedure is conducted in three basic steps: sol(colloidal solution) preparation of the external phase, dispersion of the internal phase and coacervation. Subsequent to coacervation optional capsule cell wall hardening and air drying can be performed in conventional manner. For gelatin-gum arabic type cell walls, hardening can be effected using glutaraldehyde. When encapsulating metals and fluorocarbons with a combined gelatin-gum arabic, ethylene-maleic anhydride copolymer external phase system; the following overall parameters are observed: ##SPC1##

The gelatin sol is prepared by adding 25 grams of gelatin into 202 grams of deionized water. The gelatin is allowed to swell thoroughly then heated to 55.degree. C. with continuous stirring until a clear solution is obtained. The pH is then adjusted to 6.5 with a 10 percent solution of aqueous sodium hydroxide.

The gum arabic sol is prepared by adding 25 grams of gum arabic into 202 grams of deionized water with stirring until a clear solution is secured. This solution is usually filtered to remove foreign material. Then the pH thereof is adjusted to 6.5 with a 20 percent aqueous sodium hydroxide solution.

Two separate ethylene-maleic anhydride (EMA) copolymer aqueous solutions are prepared with each having a 2 percent by weight concentration of the respective EMA material and the pH of each solution being adjusted to 9 using 10 percent aqueous sodium hydroxide. One EMA solution is formed using an ethylene-maleic anhydride copolymer having a molecular weight of approximately 6,000, e.g., a commercially available Monsanto Company product designated "EMA-21." The other EMA solution is formed using a copolymer of ethylene and maleic anhydride having a molecular weight ranging from 60,000 to 70,000, e.g., a commercially available Monsanto Company product designated "EMA-31."

Then 200 grams of the internal phase material are dispersed in the external phase sols by charging 180 grams of the gelatin solution and 180 grams of the gum arabic solution (pH adjusted to 9) to a coacervation vessel, e.g., a 3-liter glass beaker, followed by the addition of 1,500 cubic centimeters of deionized water and 40 grams of the aqueous solution of the 6,000 molecular weight copolymer of ethylene and maleic anhydride. The resulting solution is heated to 45.degree. C. while stirring, e.g., using a flat-bladed impeller. The internal phase material is then added to the heated solution and stirring is continued to achieve proper dispersion.

After dispersion the solution pH is lowered to 5 with 10 percent by weight acetic acid aqueous solution. Thin walls appear and coacervate spheres appear in the external phase. The pH is then raised to 6 with 20 weight percent sodium hydroxide aqueous solution to decoacervate the spheres. Then 40 grams of the aqueous solution of the 60,000 to 70,000 molecular weight ethylene-maleic anhydride copolymer is added and the pH is lowered to 5.3 with 10 percent by weight aqueous acetic acid. As the pH is lowered, capsule wall formation becomes thicker. Maximum thickness is obtained by cooling to approximately 25.degree. C. at pH of 5.3.

ENCAPSULATION OF INORGANIC OXIDIZERS

When encapsulating inorganic oxidizers with a nitrocellulose external phase material, a phase separation inducing agent is usually employed, e.g., a commercially available, carboxy-terminated, low molecular weight, liquid polybutadiene homopolymer (molecular weight of approximately 2,500) marketed under the trade designation "Butarez." ##SPC2##

The nitrocellulose solution is formed by adding 4 grams of nitrocellulose into 196 grams of methyl ethyl ketone followed by stirring at 25.degree. C. until a clear solution is obtained.

Then the internal phase material, 16 grams, having the desired particle size is added to the nitrocellulose solution with stirring to obtain a uniform dispersion thereof. Phase separate and encapsulation are secured by slowly adding 60 grams of "Butarez" (preheated to 50.degree. C.) while stirring at 25.degree. C. At this point the capsules are formed. Usually the encapsulation medium is stirred for about 15 minutes or more to insure that equilibrium has been reached. The capsules can then be decanted from the solution, washed with a nonsolvent (for the cell wall material) and air-dried. Other suitable procedures for encapsulation will be apparent to those skilled in the art.

While the use of incendiary materials, containing one or more encapsulated component(s) is preferable; less preferable unencapsulated highly reactive incendiary materials, such as those metals fuels and inorganic and organic oxidizers mentioned hereinabove, can be used provided that they are contained in a binder or matrix capable of rendering the reactive components inert until the desired time for ignition. Thus organic binder or matrix materials such as vaseline, low-melting paraffin waxes, etc., can be used to inertly isolate oxidizer and metallic fuel from one another, oxygen and moisture. Also relatively nonreactive, wholly inorganic, nonencapsulated incendiary materials can be contained in the honeycomb cells, e.g., 50 to 90 weight percent black gunpowder (charcoal, KNO.sub.3 and sulfur) mixed with 10 to 50 weight percent of a burning rate retardant such as ground glass, quartz, magnesium carbonate, etc., or mixtures thereof.

When using encapsulated incendiary compositions, it is possible to control burning rates and combustion temperatures by varying the concentration of encapsulated fuel, e.g., metals, and encapsulated oxidizer, e.g., perchlorates or nitrates. This is possible due to the superior uniformity of distribution and inert protection afforded by encapsulation. For example, to raise the burning temperature, more encapsulated aluminum is used. To increase the rate of burning, the concentration of encapsulated perchlorate or nitrate is increased. It is also possible to elevate both combustion temperatures and burning rate, e.g., by using a hybrid incendiary system, viz, unencapsulated fuel, e.g., aluminum granules or powder, with encapsulated oxidizer; thus the unencapsulated fuel reacts with atmospheric oxygen substantially immediately upon ignition. Usually the combustion temperature level desired will be dictated by the specific type of laminated structure, its associated hardware to be destroyed and the extent of destruction desired; whereas the rate of burning desired will be determined by the environment in which burning is to take place. Usually it will be desirable to burn at a combination of combustion temperatures and burning rates sufficiently low to avoid significant sputtering and wise distribution of residual ash, yet sufficiently high to reduce substantially all metalic components to their respective oxides.

As noted above, the incendiary materials present in the honeycomb cells are protected with facing layers on both sides of the honeycomb core. These layers are secured to the honeycomb core section by conventional systems, e.g., adhesives, nails, etc. Suitable facing layer materials which can be employed with this invention include, but are not limited to, the following: various metal foils, such as tin, copper, aluminum, palladium-clad aluminum pyrochemical foil, viz, foil comprised of thin core of aluminum having on both its upper and lower surfaces even thinner films of palladium, steel foil, including stainless steel foil; etc.; wood facing layers; fiber and fabric reinforced adhesive facings; molded plastics; woven and nonwoven glass fabric facing layers; resin-impregnated paper and other fibrous facing sheets; polyurethane-coated nylon cloth etc. Combinations of any two or more of the above facing materials or alloys containing any one or more of them as a predominant component can be used. According to one of the preferred embodiments of this invention, the pyrochemical metal foil is used due to its self-destruction capability plus the adaptability of the metal cladding material, e.g., palladium, to development of a printed circuit, e.g., by etching away a portion of the palladium or deposition of a printed circuit thereon or a combination of both procedures.

According to another preferred embodiment of this invention, each facing can be a plurality of layers or combination of different or similar materials, e.g., a combination of metal foil, e.g., aluminum foil, and a woven glass fabric impregnated with a suitable polyester, epoxy or other adhesive. The adhesive usually constitutes from about 10 to 50 weight percent of such facings, based on the total of adhesive and facing material. In the latter case the glass fibers not only add structural strength to the self-destruct system, but also assist in protecting the incendiary from degradative exposure.

The resin impregnated or coated glass fiber material is laid upon the honeycomb core (in the cells of which the incendiary pyrotechnic composition has previously been placed) followed by the placement of the aluminum or other metal foil facing sheets thereon. Then when the composite structure is heated to set the adhesive; the epoxy or other adhesive employed with the fiberglass can adhere the aluminum foil and fiberglass to the honeycomb core, the glass fabric being incorporated into the facing layer.

In addition to epoxy resins, various other adhesive compositions can be employed depending largely upon the selection of core and facing materials, electrical properties and temperature stability desired, etc. Moreover, the adhesive layer need not be reinforced with glass fiber or other reinforcing media. Suitable adhesive compositions include, but are not limited to, the following: epoxy resins, alkyd resins; silicone resins, polycarbonate resins, various phenolic condensates, e.g., phenol-formaldehyde resins; various aminoplast condensates, e.g., urea-formaldehyde and melamine-formaldehyde resins; polyester resins; polyurethane resins, etc.

While glass fibers have been mentioned hereinabove as a reinforcing media for the adhesive layer, other reinforcing media (both fibrous and nonfibrous in nature) can be used. Such reinforcing media include, but are not limited to, the following; fibers, strands and woven and nonwoven fabrics containing nylon, cotton, glass fibers, scrim cloth and metal, e.g., aluminum and magnesium, wire cloth.

The facing layers usually represent about 0.05 to 50 percent of the total thickness of the self-destructible laminated honeycomb articles. The thickness of the individual facing layers can range from 0.025 to 1 centimeters. Usually, however, the thickness of the facing layers range from about 0.025 to 0.5 centimeters.

According to a preferred embodiment of this invention, one or both of the facing layers contain or are provided with a printed or other circuitry so that the composite, self-destruct article in effect constitutes a readily destructible electronic circuit, electronic component, or other electronic equipment. In such cases, the method whereby the circuitry is imposed on the facing layer will vary according to the specific facing layer material used, the electrically conductive circuit material, and other factors. When using aluminum foil as a facing layer material, the printed circuitry will usually be copper or aluminum material deposited by conventional adhesive bonding or vacuum coating techniques. Another suitable structure results from depositing a copper circuit facing on a fiberglass, stainless steel or aluminum honeycomb core, e.g., using 1 to 2-ounce-per-square-foot copper foil as external surfaces (facings) of the honeycomb during the initial fabrication whereby the copper foil is bonded to a reinforced adhesive layer at the same time the adhesive is bonded to honeycomb cells.

The self-destruct mechanism is provided with an igniter(s) whereby ignition of the incendiary or pyrotechnic composition is secured. Conventional igniters can be employed. To achieve most rapid on-command self-destruction, it is preferable to employ igniters which operate under electric impulse. Such igniters usually are comprised of pyrofuse wire which ignites by heat from an electrically triggered alloying reaction between palladium and aluminum metals. This type of igniter can be fabricated in different sizes to fit particular ignition requirements and laminated structure sizes. Of course, it is also within the purview of this invention to use nonelectrical igniters triggered by heat, e.g., which can be activated by a match or lit cigarette. Usually the igniter(s) are located so that they are in direct communication with at least one of the honeycomb cell cores which contains the incendiary composition.

The edge(s) of the self-destruct structure can be protected from moisture and other degradative influences by sealing them along at least a portion of their peripheral edges, for example, using conventional sealant and potting compositions, such as polysulfide rubber sealants, butyl rubber sealants, epoxy resin potting compositions, etc. Preferably the outer periphery of the honeycomb core is sealed throughout substantially all of its edge extent.

The self-destruct laminated honeycomb structures of this invention can be readily prepared with the following illustrative procedure: adhesive bond one face to the honeycomb core. Fill the cells with an incendiary destructive system (composition) and install the igniter(s). After this is done, the second face can be secured to the honeycomb core section. The adhesive system can be a nongassing epoxy type. Heat and pressure are usually required in the bonding process.

The invention will be illustrated in greater detail by the examples which follow. It should be understood, however, that the present invention in its broadest aspects is not necessarily limited to the specific incendiary or pyrotechnic compositions, specific adhesives and reinforcements therefore, specific facing layer materials and dimensions, specific igniters and igniter mechanisms, et., set forth hereinbelow in the examples. ##SPC3##

The encapsulated components were mixed mechanically to provide a uniform mixture. A 4-inch by 4-inch piece of epoxy adhesive coated, electrically conductive, 1-mil-thick copper foil facing was placed on a platen press with the adhesive side facing upwards. A comparably sized piece of aluminum (perforated) foil honeycomb core section having 1/8-inch cells having a height of approximately 1/4-inch, was placed on the copper foil. Heat was applied to the platen and the press was closed. At temperatures of 250.degree. to 350.degree. F. and pressures of 10 to 50 p.s.i.g., the copper facing is bonded to the aluminum honeycomb core section. The epoxy adhesive used was a commercially available epoxy resin formed by reaction of "bis-phenol A" and 2-epichlorohydrin and marketed under the read designation "DER-331" from Dow Chemical Company. The catalyst was a commercially available tri(dimethyl amino) phenol available under the trade designation "DMP-30" from Dow Chemical Company. Twelve weight parts of catalyst were employed per 100 weight parts of epoxy resin. After bonding one face to the honeycomb core, the core was provided with insulated electrical terminals and was edge sealed by filling the outer ring of cells with the same epoxy resin and catalyst used for bonding the face to the core, the insulated terminals being inserted while the sealing composition is still liquid or workable. After curing, one or two cells joining the ends of the electric terminals were removed and a small piece of pyrofuse wire was soldered to the terminal ends, care being taken in installing the pyrofuse wire to avoid contact of the wire with the honeycomb core in order to avoid short circuiting the igniter. The remaining cells were then filled (loaded) with the above-tabulated premixed encapsulated incendiary material. The loaded structure was placed back in the press and preheated at 250.degree. to 350.degree. F. While the assembly was still hot, the second epoxy adhesive-coated copper foil facing is placed on the core and bonded thereto.

The laminated self-destruct structure is in an environmentally sealed condition with the incendiary under a slight negative pressure. It will usually remain at this pressure at normal ambient storage conditions and even at elevated temperatures provided such temperatures are below those used for curing. However, if the environment of storage or intended use (prior to ignition) reach or exceed assembly adhesive curing temperatures, e.g., 250.degree. F. and higher, the self-destruct structure should be insulated against this heat or it may lose its internal negative pressure, which loss can be undesirable for certain applications.

The laminated self-destruct structure is now ready for use until its thermal destruction is desired. The latter is illustrated in example II below. While the laminated self-destruct structure of example I is a flat panel, curved laminated panels can likewise be produced, e.g., using preformed, curved honeycomb core sections or curved dies and autoclaves.

EXAMPLE II

Two laminated self-destruct structures, assembled as per example I, are thermally destroyed "on command" by applying an electromotive force across the pyrofuse, the electromotive force necessary for ignition depending upon the size (diameter) of the pyrofuse wire. One laminated structure employed 0.001-inch-diameter pyrofuse wire and was ignited using approximately 1.5 volts. The other structure used 0.008-inch-diameter pyrofuse wire and was ignited with approximately 12 volts. In both structures essentially complete thermal destruction was achieved in approximately 3 minutes. The burning proceeded uniformly but rapidly with no noticeable sputtering.

While this example illustrates ignition with one type of electrically triggered system, other ignition systems, electrical and nonelectrical, can be used. For example, conventional resistance heating elements, dynamite-type fuses, etc., can be utilized. The dynamite-type fuses can be triggered by a match or burning cigarette whereas the resistance heating element fuses are ignited by electrical energy with a somewhat greater level of energy being required than with the pyrofuse ignition. These fuses can be installed in essentially the same manner as the pyrofuse igniter.

Claims

What is claimed is:

1. A self-destructible laminated structure comprising a honeycomb core having cells at least some of which contain incendiary material capable of rapid yet nonexplosive combustion wherein the cell walls are secured positionally along at least a portion of their extent substantially perpendicularly with respect to a longitudinal surface of substantially parallel facing sheets.

2. A structure as in claim 1 wherein at least one of said facing sheets has an electroconductive portion.

3. A structure as in claim 2 wherein said electroconductive portion comprises a printed circuit.

4. A structure as in claim 1 wherein at least a portion of said incendiary material is contained in capsules and wherein said capsules are located within cells of the honeycomb core.

5. A structure as in claim 1 wherein at least some of the honeycomb cells are interconnected by openings.

6. A structure as in claim 1 wherein the outer periphery of said core is sealed over at least a portion of its extent.

7. A structure as in claim 1 which includes an igniter communicating with said incendiary material to cause ignition thereof on command.

8. A structure as in claim 1 wherein said honeycomb core comprises perforated aluminum and said facing sheets are comprised of metal foil.

9. A structure as in claim 8 wherein said metal foil comprises copper.

10. A structure as in claim 8 wherein said facing sheets are secured to said core by epoxy adhesive.