A process for manufacturing a polymer product and an artillery shell casing made using said process

Abstract

The subject of the invention is process of manufacturing a polymer product with increased electrical conductivity and mechanical strength comprising at least one polymer and carbon nanoparticles. The product is an artillery shell casing (1). The process comprises the steps of mixing a masterbatch with a portion of the polymer and carbon nanoparticles, melting the masterbatch with a second portion of the polymer, injecting the polymer composite into the preheated mold followed by a relaxation period, during which a pressure is applied, and a cooling period.

Description

[0001] A process for manufacturing a polymer product and an artillery shell casing made using said process

[0002] Field of the Invention

[0003] The invention relates to manufacturing a polymer product with increased electrical conductivity and mechanical strength used in armaments industry.

[0004] Background of the Invention

[0005] A repeatable cannon artillery shell casing made of plastic material is considered to save material and improve the efficiency of the process, but there are some problems that need to be solved before the actual use.

[0006] Polymer nanocomposites are widely considered to be an alternative substitute for metals in many fields of application, due to competitive price and / or advantage of the functional properties. One such application is a substitution for brass used as a base material for weapon ammunition, in particular - as a material for 122 / 152mm artillery shells casing or other similar. Polymer material and technological process should provide combination of electrical conductivity and mechanical strength. Injection molding is the preferred manufacturing process because of the simplicity and scalability. Polyethylene (PE) is a prime candidate for such substitution due to its thermal shock resistance, impact strength at low temperature, chemical inertness and water impermeability.

[0007] However pure PE is not fully suitable for such application because of poor stability against ultraviolet radiation of the sun and high electrical resistance. Moreover, mechanical strength of pure PE is generally not sufficient for such purpose. High electrical resistivity of PE is critical issue because static electrical charges accumulated on the surface of the casing may potentially cause detonation of the ammunition. Carbon black is commonly used in PEs as ultraviolet absorber and electrically conductive additive. To our knowledge, high density polyethylene (HDPE) filled with 4 wt.% of carbon black is currently used in Russia for production of casings for 152mm artillery shells. However, our tests of this material revealed that this material has high electrical resistance (above 1012Ohm / cm). This disadvantage of the material had been fixed by mechanical treatment of the surface of casings with graphite powder, as described in document RU2096728C1 . However, this additional procedure in production cycle increases production costs / time and, what is most critical, does not provide electrical properties of high-level stability / durability because of pure adhesion of the graphite particles to the polymer surface.

[0008] It is well known that carbon black (CB) can provide electrical conductivity in the polyethylene if the concentration above 4 wt.% is used. On the other hand, high concentration of CB leads to degradation of mechanical properties: increase of brittleness and decrease of impact strength. Moreover, high concentration of carbon black results in significant reduction of a melt flow index (MFI) that restricts the use of the material in the injection molding technological processes where the high MFI is desirable.

[0009] The carbon nanotubes (CNTs) are more effective filler due to high apex ratio and high electrical conductivity. It is well known that percolation threshold in the case of CNTs varies in range of about 0.1-1 .0 wt.%, depending on the type of CNTs, that is significantly lower than in case of CB. However, single wall carbon nanotubes (SWCNTs) commonly provide the smallest percolation threshold, while multiwall carbon nanotubes (MWCNTs) are less effective. The strong shear stresses present in the polymer melt during the injection molding process destroy the electrically conductive percolation network composed by the conductive CNTs (T. Skipa a, D. Lellinger a, W. Bbhma, M. Saphiannikova b, I. Alig, Influence of shear deformation on carbon nanotube networks in polycarbonate melts: Interplay between build-up and destruction of agglomerates, Polymer 51 (2010) 201-210 / doi:10.1016 / j.polymer.2009.1 1 .047). If the wall of the mold is cold, the nonconductive structure of the polymer composite is “frozen” and therefore the surface of the molded product has high electrical resistivity. The electrically conductive percolation network can be recovered in the absence of sheer stresses, i.e. in unmovable melt providing sufficient thermal mobility of the molecules ( I. Alig, T. Skipa, I . M. Engel, D. Lellinger, S. Pegel, and P. Pdtschke, Electrical conductivity recovery in carbon nanotube-polymer composites after transient shear, Phys. Stat. Sol. (b) 244, No.

[0010] I I , 4223-4226 (2007) / doi: 10.1002 / pssb.200776138).

[0011] An effective method of increasing the surface electrical conductivity of the polymer has been suggested in the patent EP2151830. In this patent, the use thermal heating of the surface of a polymer molded body manufactured using CNT-filled polymer is suggested. Indeed, by providing proper concentration and dispersion of CNTs in the thermoplastic, it is possible to form electrically conductive surface on the polymer body by short-term heating of the surface to the melting point of the polymer. However, the method has some disadvantages. Firstly, this method provides a formation of a very thin conductive layer on the surface. Our examinations showed that thickness of this layer is about few microns and can be easily removed by mechanical wear. The wear of the thin surface layer is accelerated by UV radiation of the sun. Formation of the thicker conductive layer requires a longer melting process that leads to corrugation of the body surface. Secondly, thermal heating of the selected surface could lead to the generation of strains and mechanical stresses due to the inhomogeneous temperature distribution over molded body and the corresponding inhomogeneous thermal expansion. This effect critically reduces mechanical strength of the body and may cause uncontrollable change of geometrical parameters of the molded body. Both of these effects are very critical for the polymer casings manufactured by injection molding.

[0012] Therefore, it would be advantageous to suggest a material and technological process that provide a combination of mechanical strength and an electrical conductivity for polymer products manufactured by injection molding.

[0013] Summary of the Invention

[0014] The shortcomings of the solutions known in the prior art are to some extent eliminated by a process of manufacturing a polymer product with increased electrical conductivity and mechanical strength comprising at least one polymer and carbon nanoparticles, where the process comprises the steps of

[0015] (i) manufacturing a masterbatch by mechanical premixing of o first portion of at least one polymer, carbon nanoparticles and a dispersion agent,

[0016] (ii) melt mixing of the masterbatch obtained in step (i) with a second portion of at least one polymer,

[0017] (iii) injection of the melted polymer composite obtained in step (ii) into a mold preheated to a temperature not lower than a melting point of the polymer used, followed by a relaxation period at a temperature not lower than melting point of the polymer used,

[0018] (iv) mold cooling, wherein a holding pressure is applied during the relaxation period and at least a part of the cooling period. The relaxation period can last at least 5 seconds. The temperature at the relaxation period can be different from the temperature of the preheated mold. When multiple polymers are used, the mold temperature, as well as the relaxation temperature, can be higher than or equal to the melting point of each used polymer. Step (ii) can in some cases occur multiple times, i.e., the polymer can be added more gradually, e.g., in three or more portions.

[0019] To overcome disadvantages of the state of art, the use of carbon nanoparticles, especially carbon nanotubes (CNTs), as a nanofiller instead of carbon black is suggested. Due to high aspect ratio, CNTs provide a development of electrical and mechanical properties at much lower concentration. The lower concentration of the filler allows to keep the melt viscosity low enough, thus enabling injection molding processing.

[0020] The content of the carbon nanoparticles can be 0.2-4.0 weight %, e.g., 0.2-1 .0 weight %. Polyethylene can be used as the at least one polymer.

[0021] Our finding is that some of the problems of the product described in EP2151830 can be avoided by injection of the polymer melt into the mold heated to the melting point (or higher) of the polymer, followed by relaxation of immovable melt at the melting point for short relaxation period and then cooling down. Such procedure provides formation of the electrically conductive percolation network of CNTs not only on the surface but in the bulk of the polymer body as well. Moreover, the homogeneous temperature distribution over the body and the relaxation period reduces mechanical stresses. Electrical conductivity of thermoplastic product, manufactured by the injection molding process, is not an intrinsic property of the raw material used for manufacturing process but strongly depends on the injection molding condition. Even with proper concentration and ideal dispersion of the conductive nanofiller, the electrical conductivity of the surface of the product would be determined by parameters of the injection molding process, most importantly - by the mold temperature. Therefore, the optimal functional properties of the material can be obtained by combining the proper selection of the polymer matrix, proper dispersion of the carbon nanoparticles and proper parameters of injection molding process at the final stage of the casing production.

[0022] The main issue of the good quality of PE:CNTs nanocomposite is a quality of the dispersion of the carbon filler in polymer matrix, i.e. proper deagglomeration of CNTs from micrometer to nanometer scale. Improved dispersion quality can be provided by a multi- step pretreatment of the CNTs filler, which comprises multiple additions of the polymer:

[0023] - preparation of the masterbatch by mechanical mixing of CNTs (or other carbon nanoparticles), the dispersion agent and a first portion of the polymer, especially in form of a polymer powder, with a rotation speed preferably in range of 500-1 ,500 rpm,

[0024] - melt homogenization of the masterbatch in twin-screw extruder,

[0025] - melt compounding and homogenization with a second portion of the polymer (preferably polymer powder) in twin-screw extruder.

[0026] The input mass of the polymer can thus be divided into at least two portions and every portion is added to masterbatch melt and processed in twin-screw extruder. The weight of the polymer portions can be approximately the same. The permissible weight deviation of individual portions is e.g., 30 %.

[0027] The content of the dispersion agent can be in range of 1.0-3.0 weight %. Advantageously, the content of 2.0 weight % is used which gives the best results, which was experimentally determined. Polyethylene glycol or polyethylene glycol(N)cetyl ether (N=20-23) can be used as the dispersion agent. Polyethylene glycol(N)cetyl ether (N > 15), polyethylene glycol ethers with another substituent (decyl, undecyl, dodecyl etc.) or another polyethylene glycols with N > 15 can also be used.

[0028] The dispersion quality and mechanical / electrical properties are also dependent on the concentration of CNTs which should be proper to provide required functional properties and avoid degradation. The proper concentration and dispersion of the filler provide improved mechanical properties of the composite, i.e., high elastic modulus, tensile strength and impact strength that is critical to withstand explosion of the ammunition. The optimal concentration of CNTs is in range of 0.3-1 .0 weight %. At the lower concentration of CNTs, problems with formation of electrically conductive percolation throughout the whole volume of the material could occur. At concentration higher than 2.0 weight %, problems with mechanical strength could occur.

[0029] The main principle of the manufacture of the electrically conductive polymer mold body is based on injection of the composite material into the mold preheated to the temperature not lower than the melting point of this material, or in case of polymer mixture, the mold is preheated to the temperature not lower than the melting point of a polymer which have the lowest melting point. Injection must be followed by the relaxation period of the immovable melt inside the hot mold at the temperature not lower than the melting point. This step is necessary for the formation of the electrically conductive percolation network through thermally activated movement of the carbon nanoparticles. Cold water or other cooling agent is then used to cool down the mold and the part to the temperature around the material ejection temperature. The ejection temperature means a temperature for a removal from the mold. The holding pressure has to be applied during the relaxation period and for a certain period of the cooling time to ensure that the product is free of sinkmarks and inner irregularities - voids, inner stress, etc.

[0030] The value of holding pressure applied to the mold depends on used injection molding machine, on temperature of material, mixing speed. In general, the holding pressure is typically 30-120 MPa. The pressure can be applied e.g., for 10-60 seconds.

[0031] Above mentioned advantages can be achieved by the following heating and cooling stages using well known methods. It is important to maintain temperature homogeneity on the mold surface to keep temperature above the melting point during the relaxation period.

[0032] The nanoparticles used as a filler are typically branched multiwall carbon nanotubes (BMWCNTs). Other kinds of multiwall carbon nanotubes (MWCNTs) and also single wall carbon nanotubes (SWCNTs) can be used, provided the dispersion is good. High density polyethylene (HDPE), linear medium density polyethylene (LMDPE) or linear low-density polyethylene (LLDPE) with sufficiently low melting flow index (MFI) is suggested to be used as a basic polyethylene matrix.

[0033] The disadvantages of the prior art are to some extent also eliminated by a polymer product which is made via the process of the invention. The mold can be shaped such that the resulting product can be used as an artillery shell casing. The shell casing can have a conical or cylindrical hollow body, e.g., closed at one end and provided with a flange at its other end, which is open.

[0034] An artillery shell casing made of such material ensures the reusability, up to 5 times. Additionally, this material is UV stable and resistant to temperature and moisture fluctuations.

[0035] Abbreviations

[0036] PE - polyethylene

[0037] HDPE - high density polyethylene

[0038] LMDPE -linear medium density polyethylene

[0039] LLDPE - linear low-density polyethylene

[0040] MFI - melting flow index

[0041] CNTs - carbon nanotubes

[0042] MWCNTs - multiwall carbon nanotubes

[0043] SWCNTs - single wall carbon nanotubes

[0044] BMWCNTs - branched multiwall carbon nanotubes rpm - revolutions per minute

[0045] UV - ultraviolet light of Drawings

[0046] A summary of the technical solution is further clarified using exemplary embodiments thereof, which are described with reference to the accompanying drawings, in which: fig. 1 schematically shows an artillery shell casing. Embodiments of the Invention

[0047] The invention will be further described by means of exemplary embodiments with reference to the respective drawings.

[0048] Example 0

[0049] A basic exemplary embodiment of the process of the invention is as follows: The process for manufacturing a polymer product, especially an artillery shell casing, with increased electrical conductivity and mechanical strength uses at least one polymer and carbon nanoparticles. The process has the following steps:

[0050] (i) manufacturing a masterbatch by mechanical premixing of a first portion of the at least one polymer, the carbon nanoparticles and a dispersion agent;

[0051] (ii) melt mixing of the masterbatch obtained in step (i) with a second portion of the at least one polymer; A third portion, fourth portion etc. can also be added in some embodiments, i.e., this step can be repeated I divided into multiple substeps such that the polymer is added even more gradually.

[0052] (iii) injection of the melted polymer composite obtained in step (ii) into a mold preheated to a temperature not lower than a melting point of the polymer used, followed by a relaxation period at a temperature not lower than melting point of the polymer used; The mold is preferably shaped such that the molded product is an artillery shell casing T

[0053] (iv) mold cooling; A pressure is applied during the relaxation period and at least a part of the cooling period.

[0054] The subsequent examples can expand or elaborate on the basic embodiment and can provide further improvement of the material properties of the resulting polymercarbon nanoparticle composite product.

[0055] Example 1

[0056] The first example of embodiments of the invention is a manufacturing process for a polymer structure with electrically conductive surface.

[0057] High density polyethylene premix is melted and mixed with 0.5 weight % of branched multiwall carbon nanotubes (BMWCNTs). The dispersion is prepared by mechanical mixing at 500-1 ,500 rpm with 2.0 weight % of polyethylene glycol as the dispersion agent and melt compounding.

[0058] The homogenized melted mixture is injected into the mold preheated to a temperature equal to or higher than the melting point of this material which in this case means a temperature of approximately 135°C. The injection must be followed by the relaxation period at the temperature not lower than the melting point. The relaxation period lasts 60 seconds.

[0059] Cold water or other cooling method is then used to cool down the mold and the part to temperature around the material ejection temperature. The holding pressure has to be applied during the relaxation period and for a certain period of the cooling time. The exact holding pressure value is dependent on the type of an injection molding machine but in general the pressure value is in range 30-120 MPa and exerts for 10-60 seconds. In laboratory conditions a lower pressure is sufficient, in industrial conditions a higher pressure can be needed

[0060] This cycle can be achieved by one of the following heating and cooling principles or their combinations.

[0061] Hot-water and cold-water cycles - in each cycle, water at temperature of 130— 200°C is used to heat the mold elements that are in contact with plastic, such as a core, cavity, sliders, etc. This stage is followed by the relaxation period which is important for the development of the electrical conductivity. This stage is followed by cooling where the mold with plastic material has to be cooled down to a temperature close to the ejection temperature of the plastic material.

[0062] Steam and cold-water cycles - principle is the same as in the previous point but instead of hot water, hot steam is used for heating.

[0063] Inductive heating can be used for external or internal heating of core / cavity inserts of other parts of the mold. The heating has to be sufficient to keep the core / cavity temperature over 130°C to maintain a constant temperature.

[0064] Use of the electrical heater and cold water, where the cooling principle is the same as in the first point, but the electric heater is used as a heat source.

[0065] Use of the hot oil or other fluid and cold water, where the cooling principle is the same as in the first point, but the hot oil or other fluid is used as a heat source.

[0066] Use of any form of radiation heating, where the cooling principle is the same as in the first point, but the radiation is used as a heat source.

[0067] Example 2

[0068] The masterbatch is prepared by mechanical mixing at 1 ,500 rpm of LLDPE, CNTs and polyethylene glycol as the dispersion agent. The CNTs used are any types of SWCNTs or MWCNTs. The content of CNTs is 1 .0 weight %, the content of polyethylene glycol is 3.0 weight %.

[0069] The homogenized melted mixture is injected into the mold preheated to 1 15°C which is a temperature higher than the melting point of this material. Injection has to be followed by relaxation period at the temperature not lower than melting point. Cold water or other cooling method is then used to cool down the mold and the part to a temperature around the material ejection temperature. The holding pressure has to be applied during the relaxation period and for certain period of cooling time.

[0070] Example 3

[0071] The masterbatch is prepared by mechanical mixing at 800 rpm of MDPE, 0.2 weight % of CNTs and 1 .0 % of polyethylene glycol cetyl ether as the dispersion agent. The homogenized melted mixture is injected into the mold preheated to 135°C which is a temperature equal to or higher than the melting point of this material. Injection must be followed by the relaxation period at the temperature not lower than melting point. Cold water or other cooling method is then used to cool down the mold and the part to a temperature around the material ejection temperature. The holding pressure has to be applied during the relaxation period and for a certain period of the cooling time.

[0072] Example 4

[0073] A standard casing for an ammunition comprising at least two parts - a plastic artillery shell casing 1 (shown in Fig. 1 ) with a flange and a cap, preferably, not exclusively, metal. This document discusses the production of the plastic part of the casing for an ammunition 1. only.

[0074] It is possible to use the polymers or their mixture, CNT and the dispersion agent which are mentioned in text of this application, but it is experimentally found that the best results are given by the process described in Example 1. The homogenized melted mixture is injected into the preheated shell-shaped mold. The relaxation and cooling period are applied.

[0075] The temperature not lower than melting point of the polymer used and the holding pressure are applied during the relaxation period. The exact values of temperature, pressure or time are dependent on the injection molding machine type, the type of material and so one. After the relaxation period, it can be started the cooling period and during at least a part of the cooling period is applied the holding pressure.

[0076] The shell casing 1. can have a conical or cylindrical hollow body, e.g., closed at one end and provided with a flange at its other end, which is open.

[0077] The casing 1 manufactured by described process is conductive throughout its whole volume which is important for the intended advantages which is the possibility of reusability. No less important is the increased mechanical strength which is also the result of the used process.

[0078] A different product that the casing can also be mode in further embodiments, wherein these products are made with the present process for manufacturing. In further embodiments of the process, the mold can thus have a shape and size for creating such a polymer product. List of reference numbers

[0079] 1 - casing

Claims

CLAIMS1 . A process for manufacturing a polymer product with increased electrical conductivity and mechanical strength comprising at least one polymer and carbon nanoparticles, characterized in that the process comprises the steps of(i) manufacturing a masterbatch by mechanical premixing of a first portion of the at least one polymer, the carbon nanoparticles and a dispersion agent,(ii) melt mixing of the masterbatch obtained in step (i) with a second portion of the at least one polymer,(iii) injection of the melted polymer composite obtained in step (ii) into a mold preheated to a temperature not lower than a melting point of the polymer used, followed by a relaxation period at a temperature not lower than melting point of the polymer used,(iv) mold cooling, wherein a pressure is applied during the relaxation period and at least a part of the cooling period.2 The process for manufacturing a polymer product according to claim 1 , characterized in that the carbon nanoparticles are carbon nanotubes.3 The process for manufacturing a polymer product according to any one of the previous claims, characterized in that the content of carbon nanoparticles is 0.2-4.0 weight %.4 The process for manufacturing a polymer product according to claim 3, characterized in that the content of carbon nanoparticles is 0.2-1 .0 weight %.5 The process for manufacturing a polymer product according to any one of the previous claims, characterized in that the dispersion agent used in step (i) is polyethylene glycol and / or polyethylene glycol(N)cetyl ether, wherein N = 20-23.6 The process for manufacturing a polymer product according to any one of the previous claims, characterized in that the polymer is polyethylene.7 The process for manufacturing a polymer product according to claim 6, characterized in that the polymer is chosen from HDPE, LMDPE or LLDPE.8 The process for manufacturing a polymer product according to any one of the previous claims, characterized in that the pressure applied during the relaxation period and at least a part of the cooling period is in range of 30-120 MPa.

9. The process for manufacturing a polymer product according to any one of the previous claims, characterized in that the pressure applied during the relaxation period and at least a part of the cooling period is exerted for 10-60 seconds.

10. A polymer product made by the process according to any one of the previous claims, characterized in that the polymer product is an artillery shell casing (1 ).1 1. The polymer product according to claim 10 characterized in that the artillery shell casing (1 ) has a conical or cylindrical hollow body.

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