RADOME WALL WITH MULTILAYER POLYMER SHEET

Abstract

The invention relates to a radome wall comprising a multilayer sheet, the multilayer sheet comprising: a) at least one sheet layer A comprising at least two monolayers, the monolayers comprising polymeric fibers, and b) at least one first film layer B1 comprising a polyolefin, wherein the multilayer sheet has a thickness of at least 0.05 mm and at most 0.8 mm. The invention also relates to a radome comprising the radome wall, the radome preferably being a non-ballistic radome, preferably suitable for telecommunications.

Claims

1. A radome wall comprising a multilayer sheet, the multilayer sheet comprising: a) at least one sheet layer A comprising at least two monolayers, the monolayers comprising polymeric fibers, and b) at least one first film layer B1 comprising a polyolefin, wherein the multilayer sheet has a thickness of at least 0.05 mm and at most 0.8 mm.

2. The radome wall according to claim 1, wherein said multilayer sheet has a thickness in a range of at least 0.1 mm and at most 0.5 mm and preferably, a thickness in a range of at least 0.1 mm and at most 0.3 mm.

3. The radome wall according to claim 1, wherein the sheet layer A comprises at most 14 monolayers, preferably at most 8 monolayers and most preferably at most 4 monolayers.

4. The radome wall according to claim 1, wherein two monolayers are woven together in one woven fabric.

5. The radome wall according to claim 1, wherein the polymeric fibers comprise a polyolefin, preferably a polyethylene, most preferably ultrahigh molecular weight polyethylene.

6. The radome wall according to claim 1, wherein the polymeric fibers in layer A are polymeric tapes, preferably solid state polymeric tapes.

7. The radome wall according to claim 1, wherein the film layer B1 comprises a polyolefin, preferably a polyethylene, more preferably the film layer B1 is selected from a group comprising low density polyethylene, linear low density polyethylene, very low density polyethylene and/or mixtures thereof.

8. The radome wall according to claim 1, further comprising at least one second film layer B2 comprising a polymer selected from a group comprising a polyolefin and a functional polymer.

9. The radome wall according to claim 1, wherein the film layer B2 comprises a polymer selected from a group comprising polypropylenes, polyethylenes, polyurethanes, polyacrylates, epoxy resins, polyacetates, polycarbonates, polyesters, polyamides, acrylonitrile butadiene styrene; polystyrenes, polyamines, and/or mixtures thereof.

10. A radome comprising the radome wall according to claim 1.

11. The radome according to claim 10, the radome being a non-ballistic radome, preferably a non-ballistic radome suitable for telecommunications.

12. Use of the radome wall claim 1 in the construction of a radome.

13. A system comprising the radome according to claim 10 and an antenna and/or a radar.

Description

EXAMPLES

Methods of Measuring

[0080] Flexural strength and modulus of the sheet according to the present invention were measured according to ASTM D790-07. To adapt for various thicknesses of the film layers, measurements were performed according to paragraph 7.3 of ASTM D790-07 by adopting a loading and a support nose radius, which are twice the thickness of the laminate component and a span-to-depth ratio of 32. [0081] Tensile properties of fibers, e.g. tensile strength and tensile modulus, were determined on multifilament yarns as specified in ASTM D885M, using a nominal gauge length of the fibre of 500 mm, a crosshead speed of 50%/min and Instron 2714 clamps, of type Fibre Grip D5618C. For calculation of the strength, the tensile forces measured are divided by the titre, as determined by weighing 10 metres of fibre; values in GPa for are calculated assuming the natural density of the polymer, e.g. for UHMWPE is 0.97 g/cm3. [0082] Tensile properties of tapes, e.g. tensile strength and tensile modulus were defined and determined as specified in ASTM D882 at 25 C., on tapes (if applicable obtained from films by slitting the films with a knife) of a width of 2 mm, using a nominal gauge length of the tape of 440 mm and a crosshead speed of 50 mm/min. If the tapes were obtained from slitting films, the properties of the tapes were considered to be the same as the properties of the films from which the tapes were obtained. [0083] Tensile modulus of film layers was measured according to ASTM D-638(84) at 25 C. and about 50% relative humidity. [0084] Tensile strength of film layers was measured according to ASTM D882-10 at 23 C. and about 50% relative humidity. [0085] Flexibility was measured on a sample of said sheet having a supported end, i.e. the end thereof which was placed on a rigid table; a free end, i.e. the unsupported end; and a length of 500 mm between the rigid support and the free end, will deflect under its own weight with an angle of more than 10 between the sheet and the horizontal axis. A sheet having a total length of about 65 cm was used. A length of 15 cm of the sheet (the first part of the sheet) was positioned on the surface of the table and then pressed against the table with a clamp. The remaining 50 cm in length of the sheet (the second part of the sheet) was positioned between the table and the free end. The angle alpha formed by these two parts of the sheet (deflection angle) was then measured three independent times by applying the same method and calculated with the relation: alpha=cos.sup.1 (P2P1)/L, wherein P1 is the distance (cm) between the floor plane (on which the table stands) and the end of the second part of the sheet (i.e. the end of the 50 cm long sheet, at the side of the floor); P2 is the height of the table from the floor plane to the top of the table (P2=97.4 cm) and L is the length of the second part of the sheet (L=50 cm). The average number of these 3 independent measurements performed on the same sample (for P1 and thickness of the multilayer sheet) is reported in Table 1. [0086] Thickness of any one of the layers in the sheet and of the sheet according to the invention was measured with a micrometer on an original location and on eight peripheral locations, said peripheral locations being within a radius of at most 0.5 cm from the original location, and averaging the values. Melting temperature (T.sub.m), and heat of fusion (H.sub.F) were established by differential scanning calorimetry (DSC) according to ISO-11357-3 by evaluation of the second heating curve at a heating rate of 10 C./min in the interval from room temperature to 200 C. The crystallinity (X.sub.e) was calculated from the equation: X.sub.c=H.sub.F/H.sub.F.sup.0, where H.sub.F.sup.0 is the heat of fusion of perfect crystalline HDPE which is assumed to be equal to 280 J/cm.sup.3. [0087] Intrinsic Viscosity (IV) for UHMWPE was determined according to ASTM D1601/2004 at 135 C. in decalin, while shaking the mixture for 16 hours, with BHT (Butylated Hydroxy Toluene) as anti-oxidant in an amount of 2 g/I solution. IV is obtained by extrapolating the viscosity as measured at different concentrations to zero concentration. [0088] dtex: fibers' titer (dtex) was measured by weighing 100 meters of fiber. The dtex of the fiber was calculated by dividing the weight in milligrams by 10. [0089] Electromagnetic properties, e.g. dielectric constant and dielectric loss, were determined for frequencies of between 1 GHz and 10 GHz with the well-known Split Post Dielectric Resonator (SPDR) technique. For frequencies of above 10 GHz, e.g. of between 10 GHz and 144 GHz, the Open Resonator (OR) technique was used to determine said electromagnetic properties, wherein a classical Fabry-Perot resonator setup having a concave mirror and a plane mirror was utilized. More details on the method of measurement of dielectrics and loss tangent at 100 GHz using a network analyzer can be found in T. M. Hirvonen and P. Vainikainen et al. in IEEE Transactions on instrumentation and measurement, vol. 45, no. 4, August 1996, page 780-786 and on dielectric measurements at 35 Ghz using an open microwave resonator can be found in R. G. Jones, in Proc. IEE, vol. 123, No. 4, April 1976, page 285-290. For both techniques plain samples were used, i.e. samples not having any curvature in the plane defined by their width and length. Since in the case of the SPDR technique, for each frequency at which the dielectric properties are measured a separate setup has to be utilized, the SPDR technique was carried out at the frequencies of 3.9 and 5 GHz. The setups corresponding to these frequencies are commercially available and were acquired from QWED (Poland). The software delivered with these setups was used to compute the electromagnetic properties. For the OR technique, the setup was built in accordance with the instructions given in Chapter 7.1.17 of A Guide to characterization of dielectric materials at RF and Microwave frequencies by Clarke, R N, Gregory, A P, Cannell, D, Patrick, M, Wylie, S, Youngs, I, Hill, G, Institute of Measurement and Control/National Physical Laboratory, 2003, ISBN: 0904457389, and all the references cited in that chapter, i.e. references 1-6, and in particular reference [3] R N Clarke and C B Rosenberg, Fabry-Perot and Open-resonators at Microwave and Millimetre-Wave Frequencies, 2-300 GHz, J. Phys. E: Sci. Instrum., 15, pp 9-24, 1982. The OR technique was carried out at the frequencies of 35, 50 and 72 GHz. The relative permittivity (also known as dielectric constant) is computed form the change in the length of the resonator required to maintain the same resonance mode on inserting the sample. The loss tangent value is computed from the corresponding change in Q-factor. The coefficient of variation of the dielectric constant and the loss tangent in a frequency interval were calculated by measuring 3 values of the dielectric constant and loss tangent in the frequency interval, computing from these values the average dielectric constant and loss tangent and the standard deviation of the dielectric constant and loss tangent, and dividing said standard deviation to said average. [0090] Return loss (RL) measurements were done according to the standard method IEEE Std. 149-1979, Chapter 15, in front of a microwave dish antenna for different frequency bands, as specified herein below: at room temperature, by using an anechoic chamber and carrying out calibration at antenna transmission line port, by using of a parabolic dish and placing the radome perpendicular on the beam. [0091] Wind load test was performed in the following way:

[0092] A conical shaped sample holder clamping device having a 13 cm diameter was used in a tensile tester to analyze the maximum force that can be applied on a radome made with the multilayer sheet according to the present invention. A conical shaped sample holder was used in order to be able to generate more clamping force on the sheet to reduce the chance of slipping. A round stamp with a diameter of 11 cm was used to apply the maximum force on the radomes containing the sheet of the invention. The maximum force is defined as the force at the moment when there is no slipping of the multilayer sheet according to the invention in the conical holder and the sheet is not damaged.

[0093] A wind load was simulated by applying sand bags on the radome. A wind speed of 250 km/h was simulated by applying 450 kg of sand bags on a surface of a 4 ft radome (1.2 m) in diameter multilayer sheet according to the invention and then applying a pressure of 3982 N/m.sup.2 (0.0039 MPa) on the radome. The force applied by wind on the flat radome surface was calculated according to the well-known Drag equation: F=Cd**p*V.sup.2*A, wherein Cd=the Drag coefficient and is 1.17 for a flat surface; V.sup.2=wind speed (m/s)=250 km/h=69.4 m/s; A=radome surface (m.sup.2)=4 ft diameter=(4*0.3 m).sup.2*/4=1.13 m.sup.2; air=1.293 kg/m.sup.3. Thus, F=1.17**1.293*(69.4).sup.2*1.13=4117 N. Thus, a very high force of 4117 N can be applied on the radome, without damaging or braking the surface of the radome.

[0094] When applying 51 N with a stamp on the 13 cm diameter sheet, the same average wind load force of 250 km/h was applied/simulated as with the 450 kg sand bags on a 1.2 m diameter sheet radome.

[0095] The wind load force was simulated by pressing the stamp on the multilayer sheet with a speed of 10 and 100 mm/min and started at 10 N to evaluate the maximum obtainable wind load force on the multilayer sheet in the conical shaped sample holder. It was observed that a 2500 N force can be applied on the 13 cm diameter on the sheet clamped in the conical shaped sample holder (0.25 MPa) without slip in the conical shaped sample holder and the sheet stays intact, i.e. does not damage or break. The fixation in the conical shaped holder of the high strength multilayer sheet was very good in order to be able to obtain these high forces.

[0096] A constant force test with 500 N for 60 min was performed to analyze constant high wind loads on the multilayer sheet used in a radome in the same way as the maximum wind load test as described herein above. The measurement started at 10 N force. An additional displacement in the tensile tester occurred of only 0.5 mm in order to maintain the force of 500 N for 60 min on the multilayer sheet.

[0097] Production of a Solid State Tape Comprising UHMWPE

[0098] A powder bed of UHMWPE powder was compacted in a double belt press at a pressure of 40 bar and a temperature of 130 C. The aerial density of the powder bed was 1 kg per square meter. The resulting product was compressed between two calendar rolls at a temperature of 135 C., down to a thickness of 270 microns and subsequently was drawn with a factor 10 in an oven at 147 C. and then it was drawn again in another oven with a factor 2.5 at a temperature of 150 C. The resulting oriented tape had a thickness of 42 microns, a tensile strength of 1.7 GPa, a tensile modulus of 115 GPa and a width of 35 cm. The ratio of tensile strength of the tape in longitudinal direction to tensile strength of the tape in transversal direction was 150.

[0099] Production of Woven Sheets Comprising UHMWPE

[0100] 4 monolayers of the tape as obtained as described under Production of a solid state tape comprising UHMWPE, each monolayer containing a 10 cm width and 0.042 mm thick solid-state UHMWPE tape produced as described herein above, were woven into two plain woven structures, each plain woven structure containing two 0.90 cross-plied monolayers consisting of said tapes. The two plain weave structures were stacked on top of each other under ambient condition (room temperature of about 23 C.) to produce one fabric sheet layer having 4 monolayers, each monolayer consisting of said tape. The obtained sheet was cut in a sheet sample having a width of 40 cm, a length of 40 cm and a thickness of 0.17 mm, a tenacity of 8.5 cN/dTex in the 0.90 direction of the tape and an area density of 168 g/m.sup.2.

[0101] Production of a Radome Wall

[0102] Press pads of a silicone rubber with a hardness of 60 shore A (commercially available from Hofland Deltaflex) were applied during the press process on one side (i.e. on the upper surface) of the sheet, for the homogenous distribution of the pressure. One two-layered finishing film was then applied on both sides of the sheet sample according to Examples and Comparative Examples herein and then the assembly so formed was pressed in a one step process in a hydraulic press at a temperature of 145 C., under a pressure of 50 bars and a dwell time of 10 min. Afterwards, the assembly was cooled to room temperature (about 23 C.), the silicone rubber pads were removed and a multilayer sheet was thus obtained.

[0103] The multilayer sheet obtained by Production of woven sheets comprising UHMWPE and according to Examples 1 and Comparative Examples herein was then mounted on a plastic ring frame by clamping it to the frame by using bold and nut system to form a radome wall.

Example 1

[0104] A two-layer film with a thickness of 0.04 mm was applied on the lower and the upper sides of the multilayer sheet obtained as described above in Production of woven sheets comprising UHMWPE. The film comprises one 0.02 mm thick layer film comprising LDPE commercially available from SABIC under the name HP2023N and LLDPE commercially available from SABIC under the name 6135NE, 0.8 wt % UV-stabilizer commercially available as UVS225 made by A. Schulman Company, 0.2 wt % Chimassorb 944 commercially available from BASF and 0.4 wt % TiO.sub.2 commercially available from BASF and one 0.02 mm thick layer film containing HDPE commercially available from SABIC under the trade name F10750, 0.2 wt % Chimassorb 944 UV stabilizer, commercially available from BASF and 0.4 wt % TiO.sub.2 commercially available from BASF The sheet sample was facing the LDPE/LLDPE film layer at the lower side and the HDPE film layer at the upper side. The multilayer sheet obtained had a thickness of 0.21 mm. The tenacity of the sample was 8.5 cN/dTex.

Example 2

[0105] Example 1 was repeated with the only difference that the film layer containing HDPE had a thickness of 0.04 mm. The thickness of the multilayer sheet was 0.29 mm and the areal density was 288 g/m.sup.2.

Example 3a

[0106] Example 1 was repeated with the only difference that one multilayer sheet was produced with a film having an areal density of 65 g/m.sup.2 and made of one 0.05 mm thick LDPE film and one 0.015 mm thick polyamide-6 film commercially available from DSM under the trade name Akulon F130. The multilayer sheet had a thickness of 0.29 mm and an areal density of 298 g/m.sup.2. The results on flexibility measurements of this sample are shown in Table 1.

Example 3b

[0107] Example 3a was repeated with the only difference that the layer sheet A had 8 monolayers formed in 4 plain woven structures, each woven structure containing two 0.90 cross-plied tape monolayers, the woven structures being stacked on top of each other. The results on flexibility measurements of this sample are shown in Table 1.

Example 3c

[0108] Example 3a was repeated with the only difference that the layer sheet A had 12 monolayers formed in 6 plain woven structures, each woven structure containing two 0.90 cross-plied tape monolayers, the woven structures being stacked on top of each other. The results on flexibility measurements of this sample are shown in Table 1.

Comparative Example 3a

[0109] Example 3a was repeated with the only difference that the layer sheet A had 16 monolayers formed in 8 plain woven structures, each woven structure containing two 0.90 cross-plied tape monolayers, the woven structures being stacked on top of each other. The results on flexibility measurements of this sample are shown in Table 1.

Comparative Example 3b

[0110] Example 3a was repeated with the only difference that the layer sheet A had 24 monolayers formed in 12 plain woven structures, each woven structure containing two 0.90 cross-plied tape monolayers, the woven structures being stacked on top of each other. The results on flexibility measurements of this sample are shown in Table 1.

Example 4

[0111] Example 1 was repeated with the only difference that the film had an areal density of 20 g/m.sup.2 and was consisting of one 0.020 mm thick LDPE film located on both sides of sheet layer A. The multilayer sheet had a thickness of 0.21 mm and an areal density of 208 g/m.sup.2.

Example 5

[0112] Example 1 was repeated with the only difference that the finishing film having an areal density of 60 g/m.sup.2 and was consisting of one 0.045 mm thick LDPE film and one 0.015 mm thick PET film commercially available from SABIC under the name BC111. The multilayer sheet had a thickness of 0.29 mm and an areal density of 288 g/m.sup.2.

[0113] It was observed that the radome wall obtained with Examples 1-5 remained intact (e.g. the sheet was not damaged or broken) when high forces were applied. It was specifically observed that when a 4500 N force was applied on a 11 cm diameter radome in the wind load test as described herein above, the radome wall was not damaged or broke, resulting thus a radome with high safety factor. Furthermore, due to its high flexibility, the multilayer sheet of Examples 1-5 could be easier rolled, bent and folded and transported without any damages (e.g. brakes or cracks), whereas the multilayer sheet obtained according to Comparative Examples 3 was rigid, panel-type sheet also when used for large size radomes. In addition, the multilayer sheet was a simple flat product form, suitable also for construction of flat shape radomes and also for large size radomes. These flat product form allowed a more simple and more cost-efficient attachment method to a radome frame or reflector or alike. The multilayer sheet had high strength, was environmentally friendly and translucent for light, this property being relevant for a good quality control of the radome construction. Moreover, the radome wall obtained according to Examples 1-5 had high scratch resistance and showed no delamination and no loose filaments during use compared with the radome wall obtained by Comparative Examples 3 that showed low scratch resistance and showed high delamination and loose filaments during use.

[0114] Table 1 shows that the multilayer sheets according to the present invention (Examples 3 a-c) had much higher flexibility compared with the multilayer sheets having a thickness of higher than 0.8 mm (Comparative Examples 3a-b). It was also observed that the multilayer sheets according to the present invention did not show any (visual or in their internal structure) damage nor delamination upon rolling, bending and folding it, whereas the multilayer sheets obtained according to the Comparative Examples 3a-b showed that visual and internal damages (present as voids or cracks) and delamination of the sheet occurred upon rolling, bending and folding said sheet.

[0115] Furthermore, RF return loss measurements were performed at frequencies between 7.1 and 40 GHz on: i) a sheet as produced in Example 1, with the difference that 2 monolayers of tape were used instead of 4 monolayers of tape; the thickness of the sheet was 0.09 mm; ii) a sheet as produced in Example 1; iii) a 0.5 mm thick sheet consisting of 3 layers of sheets as produced in Example 1, each layer of sheet having 4 monolayers of tape; the 3 layers of sheets were consolidated using the above described pressing procedure under Production of woven sheets comprising UHMWPE. It was observed that at increasing thickness of the sheet, the RL of the electromagnetic radiation is increasing. The 0.09 and 0.17 mm thick sheet is obtaining a RL of 19 dB and 18 dB, respectively. The 0.5 mm thick sheet obtained a RL of 15 dB. Furthermore, multilayer sheets with thicknesses of higher than 0.8 mm (panels shape) were not flexible enough to be able to fold, bent or roll without damaging it (visible cracks and delamination occurred in the sheet while bending, folding or rolling) and showed lower transparency for electromagnetic waves.

[0116] RF return loss measurements were also performed between 7.1 and 40 GHz on the multilayer sheet of Example 3. Suitable return loss values were obtained by applying different frequency bands: 7.1 to 8.5 GHz=19 dB; 10 to 11.7 GHz=17 dB; 12.7 to 13.25 GHz=22 dB; 14.2 to 15.35 GHz=21 dB; 17.7 to 19.7 GHz=18 dB; 21.2 to 23.6 GHz=19 dB; 24.25 to 26.5 GHz=19 dB; 27.5 to 29.5 GHz=17 dB; 31 to 33.4 GHz=17 dB; 37 to 40 GHz=17 dB. It can be seen that the radome wall according to the present invention can be used over a broad frequency range, the low RF values obtained showing that the radome wall according to the invention results in little or no detuning of the antenna and that the transmission efficiency of the radome wall is very high.

TABLE-US-00001 TABLE 1 Thickness Number of multilayer monolayers P1 Angle sheet Sample in sheet A [cm] [] [mm] Ex. 3a 4 64.1 48.24 0.29 Ex. 3b 8 88.7 79.98 0.47 Ex. 3c 12 93.6 85.64 0.64 Comp. Ex. 3a 16 94.5 86.67 0.83 Comp. Ex. 3b 24 95.8 88.17 1.15