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Polymeric foams, used in the production of aircraft, are consideredin the article. Thermogravimetric analysis and differential scanning calorimetry were used to conduct thermal studies of rigid polyimide foam materials. It was shown, that the process of thermal destruction of these foams hadthree stages of different intensity: the destruction of rigid segment with the formation of initial components and the emission of volatile elements; the activation of thermo-oxidative processes; the decomposition, accompanied by considerable heatrelease. Infrared spectroscopy was used to estimate the influence of the content of volatile components on the glass transition temperature of rigid polyimide foams. It was found, that the presence of volatile substances led to a decrease in the glass transition temperature of the foam materials.
At present, various brands of elastic and rigid gas-filled materials are used in all branches of industry and construction [1-5]. The issue of development of new compositions of polymer foams, whichcan meet the requirements of lowering their prime costs, reducing toxicity in production, and also the possibility of their operation under different temperature conditions, becomes topical. The use of a large number of initialcomponents during production makes it possible to vary the properties of the resulting polymer foams [2, 4]. For example, the introduction of fillers is widely used for the production of materials with specified technological and operational properties, and for the reduction of their cost. In addition, filling leads to a certain decrease in the combustibility of polymers .
As the fillers of polymeric foam materials, traditional disperse and reinforcing fibrous fillers (RFF) can be used [7-8].
The mechanical properties of fibrous polymer composites are determined by reinforcing fibers and filaments (by their types and properties, size and location, that is, the structure of the reinforcing fibrous filler). Fibers, filaments, strings, rovings, slubbings, cables, fabrics, woven fabrics, knitted fabrics, nonwoven fabrics, paper and othermaterials are used as reinforcing fibrous fillers.
Using different types of RFF and technological methods of their location in the fiber composite, it is possible to optimize the positioning of reinforcing fibers and filaments in a finished composite or a product on its base, to achieve the most rational variant, wherethe majority of reinforcing fibers are located in the direction of the main acting mechanical stresses under operating conditions.
Glass-filled polymeric foams make it possible to manufacture large-sized elements with a multi-layer structure, composed of various combinations of solid and porous foam material and short glass fibers, formeeting various functional and aesthetic requirements in different fields of application. Glass-filled polymeric foams are widely used for the production of carcasses, heat-insulating screens in the automotive industry [3, 9-11].
Polymeric foams also find wide application in the production of aircraft. Polymethacrylimide foam materials are the most widely used in the production of laminated materials, which along with high thermal stability, low density and increased mechanical properties, are well combined with resins, that makes them suitable materials for aircraft construction.
It is known, that the main method of production the polymer foam materials is the foamingof copolymer half-finished product in a mold. The copolymer is synthesized by block copolymerization of acrylonitrile and acrylic acids with simultaneous introduction into the monomer mixture of all components for foaming and adjusting the properties of the future foam material [12, 13].
The world leader in the supply of a wide range of rigid sheet polyimide foams for structural and radio-technical purposes is Evonik (Germany), which has developed various brands of this material under the general name Rohacell.
At present, the structural profiles with filler, made of foam material Rohacell, are successfully used in the designs of aircraft.
An example is the stringer-profiles, filled with foam polyimide material, used for the construction of rear pressure bulkhead of Airbus A340 and A 340-360. The key to the successful introduction of apolyimide foam-filled trapezoidal stringer construction was the ease of mechanical machining and the final thermoforming of Rohacell. Filled with polyimide foam, trapezoidal profile contributes to the increase in the stability of thin-walled structures, made ofcarbon fiber reinforced plastic and their resistance to longitudinal bending and axial load. The appliance of Rohacell as a stiffener allows to use one or two layers of CFRP on the walls of the structure and thus to obtain weight gain .
In the helicopter industry, high-tech blades with a Rohacell foam core were developed. A high level of mechanical properties of the foam improves the resistance to longitudinal bending of the thinnest U-shaped spars, used for the construction of blades. Due to passive behavior, which is not typical for any other foam, polyimide foam materials withstand the high dynamic loads, to which the main rotor blade is subjected during its operation .
Conducted studies showed, that the process of thermal destruction of rigid polyimide foams had undergone three stages. At the first stage, there were the destruction of rigid segment, with formation of initial components and the emission of volatile elements. The sample can contain them up to 7% of mass. This leads to a decrease in the onset temperature of decomposition and the temperature of the maximum decomposition rate. At the second and the third stages, there werethe activation of thermo-oxidative processes and decomposition, accompanied by a significant heat release. This fact is explained by the acceleration of thermooxidative reactions by the products, formed at the first stage of thermal decomposition.
The work was carried out jointly with the specialists of the “Central Laboratory of Non-Metallic Materials”, Public Joint Stock Company “KAMAZ”.
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