Extrusion Foam Coating of Coaxial Cables using Butane as Physical Blowing Agent

Transcription

1 Extrusion Foam Coating of Coaxial Cables using Butane as Physical Blowing Agent M. Nazari Marvian, A. H. Behravesh 1 *, M. Mahmoodi and M. Golzar Department of Mechanical Engineering, Faculty of Engineering, Tarbiat Modares University, Tehran, Iran, P.O. Box Received: 30 June 2009, Accepted: 15 October 2009 ABSTRACT This paper presents an experimental study on manufacture and characterization of an insulating foam coating in the coaxial cables where a physical blowing agent (butane) is used. Coaxial cables with foam insulator are widely used in communication industries. The insulating foam plays a key role in the attenuation properties of the coaxial cables. In this research work, an extrusion setup was prepared to produce foam insulated coaxial cable using butane as the physical foaming agent. The effect of die geometry (die gap and die outlet diameter) on foam morphology and loss-attenuation were investigated using a blend of LDPE (90%)-HDPE (10%) as the main material. The results show that an acceptable low loss-attenuation is achievable via using a polyethylene compound and adjusting processing parameters. INTRODUCTION Coaxial cables with polyethylene foam insulation are widely used in mobile telecommunication system, antenna feeders, broadcast transmission system and so on. From the first generation of cellular phones in 1980s, the operating frequency of these telecommunication systems has increased. As the operating frequency increases, the attenuation property increases. The coating material, used for the high frequency transmissions, requires low dielectric loss tangent (tan δ, the relative permittivity of the dielectric) and low dielectric constant (1). In order to attain an appropriate insulation, these factors must be maintained as low as possible. One method to decrease the dielectric constant is to lower the bulk density of the insulation foam to replace more air for the polymeric material (2-4). * The author to whom the correspondence should be addressed: amirhb@modares.ac.ir Smithers Rapra Technology,

2 M. Nazari Marvian, A. H. Behravesh, M. Mahmoodi and M. Golzar Foamed plastics can be produced using both chemical and physical blowing agents. The expansion ratio in chemical foaming method is limited (up to four-times) (5) ; further expansion, leads to an irregular surface appearance and the collapse of cellular structure due to the gas escape phenomenon. A high expansion ratio can be achieved when using a physical blowing agent. Main advantages in using physical blowing agents include high expansion ratio, high purity of the foam insulator and better electrical properties. In addition, when physically foamed insulators are used in signal transmission, they provide better attenuation compared to the chemically foamed ones (4). The authors recent work on production of coaxial cables using a chemical blowing agent indicated that while it is possible to achieve acceptable loss attenuation foam, but the adjustment is critical (6). Hence, further improvement in foam quality requires using a physical blowing agent since it could yield a higher expansion ratio. Examples of commonly used physical blowing agents are butane, pentane, carbon dioxide (CO 2 ) and nitrogen (N 2 ) (1,7-10). Butane and Pentane have much higher solubility compared to those supercritical gasses (CO 2 and N 2 ) and requires less system pressure although they are flammable. The higher solubility at a lower pressure leads to a higher and more stabilized expansion suitable for cable coating purpose. In the present research work, a setup was prepared to produce insulating foam using butane as the physical blowing agent for its very high expansion capability (11). Effects of material, die design on the final coating foam with regards to attenuation properties were also investigated. BACKGROUND ON COAXIAL CABLE MANUFACTURING Coaxial cables are commonly used for transmitting signals (8,9). Radio frequency (RF) coaxial cables are widely used in mobile telephone networks (4). These cables comprise of an inner conductor, a dielectric, and a conductive surrounding sheath, wherein the sheath serves as an outer conductor. The dielectric insulation is positioned between the inner and outer conductor (8,10,12-13). In some coaxial cables, a thin helix polyethylene tape is used which, along with air, constitutes the dielectric material. In some other coaxial cables plastic foams are used, instead (8). Figure 1 shows a typical structure of a foam-dielectric coaxial cable. One of the design criteria to be met for a coaxial cable is having sufficient compressive strength required to withstand bending forces and general severe 304

3 Figure 1. Structure of a typical coaxial cable (8) conditions encountered during regular handling and installation (9). Coaxial cables with foam insulation possess much better bending properties than air dielectric cables, but they are hampered by a somewhat lower velocity of propagation (or high electric loss) than that of the air insulated cables (8). The electric loss is a function of the dissipation factor of polymer, foam density, and signal frequency (10). Recent improvements in polymer foam technology have created opportunities to produce foams with enhanced properties to be effectively used as a more efficient dielectric in transmission lines (9,10,13,14). A descriptive review of the research works conducted on improving the insulating foam in coaxial cables is given in (6). PROBLEM STATEMENT Electrical, physical and mechanical properties of coaxial cables strongly depend on cell size and cell density of the insulating foam. Chemical blowing agents (CBA) strongly affect physical and electrical properties of the foamed insulation via generating moisture which could increase attenuation properties of the coaxial cables. Besides, using butane as the blowing agent is favorable due to its low cost and high solubility. It should be mentioned that using butane blowing agent could lead to the formation of large cells. The main purpose of the present paper is to produce coaxial cable with low attenuation properties utilizing butane gas as the blowing agent and reduction or eliminating of the additives and chemical blowing agent. Efforts were also made to produce smaller cells via modification of the die. 305

4 M. Nazari Marvian, A. H. Behravesh, M. Mahmoodi and M. Golzar EXPERIMENTATION Materials As the physical blowing agent, butane gas produced by Persi Corporation, Iran, was used. As the nucleating agent, azodicarbonamide, grade P1941 LOTNR: BT HA 425 produced by Clarinet Co. was applied. In order to investigate the effect of polymeric material on foam microstructure, two grades of polyethylene were used with the specifications listed in Table 1. HDPE has shorter and fewer side chains in its molecular structure compared with LDPE. Its melt strength is thus lower than LDPE which is not favorable for foaming. However, tan δ, which is defined as dielectric loss tangent of the insulation, decreases via increasing the HDPE amount into the LDPE (3). Besides, adding HDPE into LDPE leads to a lower shrinkage of the final product (21). Hence addition of HDPE into LDPE has some merits which have to be investigated. Table 1. Raw materials used in the experimentation Supplier MFI (gr/10 min) Density, ρ (gr/cm 3 ) Raw Material Material/ Compound Petrochemical Arak/ PE-LD A Tabriz Petrochemical Tabriz PE-LD 0200, 90% PE-HD EX3, 10% B Equipment Figure 2 shows a schematic of the setup prepared for the manufacture of the foamed-insulation coaxial cables, used in this research work. Figure 2. A schematic of the production setup of the coaxial cable with foam insulation 306

5 Figure 3 shows a schematic of the extrusion foaming system as the main unit in producing the cable insulation. A single-screw extruder of 65 mm in diameter with an L-to-D ratio of 37 was utilized. An appropriate screw was designed to ensure creating a homogenous mixture of polymer melt and the blowing agent. A static mixture (dissolution enhancing device) was designed to ensure enhancing diffusion of the injected blowing agent (here butane) into the polymeric melt via heating and shear mixing. Heating increases the diffusivity of blowing agent into the polymeric melt, and shear mixing enhances the convective mixing and reduces the striation thickness by generating a shear field (15-17). The gas charged molten polymer travels through the static mixer, and changes the direction by hitting the conical distributor (Figure 4) toward the radial direction where it is Figure 3. A schematic of the designed extrusion foaming setup Figure 4. Cross section view of the diffusion enhancing static mixer used in this study 307

6 M. Nazari Marvian, A. H. Behravesh, M. Mahmoodi and M. Golzar forced to pass through many holes machined on the cylindrical piece. The melt will again pass through the similar holes in the other side, but in the opposite direction, and finally exits from the mixer toward the cooling unit. This many separations of the flow causes better mixing of the polymeric melt with the blowing agent via reducing the diffusion distance (18). A thermal static mixer (cooling unit) with nine elements was implemented to uniformly cool the melt, a design which has been found to be effective in suppressing cell coalescence via increasing the melt strength (18-20). A schematic design of the cooling mixer is shown in Figure 5. A set of extruder cross-head was manufactured to produce 7/8 coaxial cable (Figure 6 and 7). An innovative two-stage die was designed. The first stage exerts rapid pressure drop rate on the polymeric melt via a multi-slot passage. Second stage was designed to properly shape the final extrudate product. As the melt passes through the slot (Figure 6a-c and Figure 7), a rapid pressure drop is exerted to promote nucleation (15). The shaping section is used to promote fusion of the separated foamed melt. Due to the limit associated with the cable dimensions (dimensions of the inner and outer copper conductors), the die outlet diameter could be varied only in the range from 13 to 15 mm, as selected for the experimentation. Appropriate calibrating, straightening and take-up units were designed, manufactured and implemented to produce sound foam insulation coaxial cables (Figure 2). In order to measure attenuation characteristics, a network analyzer, manufactured by HP Company, model HP8719A, USA, was used. Fully manufactured cable samples of 100m in length were prepared and two special connectors were attached to the ends of the cables. One connector is special to produce a signal at various frequencies (200, 300, 450, 900, 1500, 1800, 2300, 3000 MHz). The other connector receives the signal and is attached to the analyzer to measure and record the signal attenuation. Figure 5. Schematic view of the melt cooling device 308

7 (a) (b) (c) Figure 6. Cross section view of the designed die: a) tip, b) cross head, c) shaping die assembly 309

8 M. Nazari Marvian, A. H. Behravesh, M. Mahmoodi and M. Golzar Figure 7. Cross section of the designed nucleating die Microstructures of the foams were examined using a scanning electron microscope (SEM, PHLIPS X130). The samples were dipped into liquid nitrogen and then fractured to expose the cellular morphology and then gold coated to be prepared for SEM examination. PROCEDURE Polymeric material was first dry-mixed with 0.5 wt.% of the nucleating agent and then fed into the extruder through the hopper. After melting, butane was injected into the extruder using a positive displacement pump the amount of which was adjusted using a pre-calibrated dosing valve (about 5% wt.). The copper tube was fed through the extruder cross-head, and the foam coats the surface of the inner conductor. The foam-coated inner conductor was then directed through a cooling unit by a caterpillar, and was coiled on the reel. Outer conductor and PE jacket were then added in the following steps. To investigate the effect of the die geometry on the expansion ratio, dies with various diameters were used (13, 14, 15 mm). The change in the gap distance was possible via movement of the conic tip, as shown Figure 6, in forward/ backward directions. Shifting the tip in forward direction decreases the output area, and vice versa. Selected gap distances were from mm with 0.5 mm interval. This leads to a change in the amount of pressure drop. To study the effect of raw material on foam properties, the polymeric material was varied according to Table 1. The attenuation properties, then, measured for 100 m cable, and compared with those coaxial cables produced using 310

9 the chemical blowing agent (6). The target density was gr/cm 3, to simultaneously achieve a good attenuation and maintaining the strength and concentricity of the insulation in bending during installation within an acceptable limit (4,6). Table 2 shows the temperature of extruder zones 1 to 4, the distributor (diffusion enhancing device), the cooling section and the die. Table 2. Set temperatures for the extrusion sections zone Distributor Cooling Die (mixing) section Temperature ( C) variable variable Due to the existing various effective parameters, and to avoid performing too many experiments, the following approach was followed: 1) selecting the optimum temperatures for pure material (preliminary experiments), 2) using the modified material and find optimum gap distance at the selected temperatures, 3) finding the optimum die diameter. It will be shown that this approach was effective in yielding the acceptable final product. PRELIMINARY EXPERIMENTS As an initial step, Material A (LDPE) mixed with the nucleating agent (0.5%wt) was used. The sample collection was started when the melt and die temperature were lowered to 120 C. The foam, produced at this condition was found to have a bulk density of 0.5 g/cm 3. Microstructure of the produced foam is shown in Figure 8. Figure 8. Microstructure of the foam produced using Material A (pure PE 020) extruded at a temperature of 120 C (scale bar is 1 mm) 311

10 M. Nazari Marvian, A. H. Behravesh, M. Mahmoodi and M. Golzar Then, the die and cooling unit temperatures were gradually reduced down to 102 ºC, and the samples were collected at the intermediate points at stabilized condition. Figure 9 shows the relative density of the specimens produced at different temperatures. Relative density of the extruded specimens shows low values at a temperature range between C. At higher temperatures bulk density increased due to the gas escape phenomenon (15). The produced foam at 102 C showed a bulk density of 0.23 gr/cm 3. Although the foam density was suitable, but the cell density was found to be too low about cells/cm 3 indicating a structure of having too large cells, and therefore was not acceptable for the application. Considering above results, in the following experiments, two temperatures were selected as 105 and 112 C. To enhance the cell structure, following procedure was also premised: using a blend of LDPE (90%)-HDPE (10%) and changing the die geometry. The die geometry was varied via variation of the die gap. It should be noted that blending LDPE with high density polyethylene, HDPE, improves some physical and mechanical properties of the material (1). Figure 9. Relative density of the extruded foams from Material A at various temperatures RESULTS AND DISCUSSION To produce fine cells, two main parameters were varied: i) melt temperatures at two levels of 105 C and 112 C, and ii) shaping die. 312

11 Effect of Raw Material on Foam Morphology Experiments were carried out to investigate the effect of Compound B (Table 1) on foam morphology. The previous study (6) revealed that a Compound of LDPE-HDPE leads to an acceptable structure with chemical foaming process. Adding HDPE to LDPE decreases foam shrinkage after expansion. It should be mentioned that using butane as the blowing agent results in a large shrinkage in foaming as it tends to be quickly replaced by the air (22). In this experiment, die geometry and nucleating agent amount were maintained unchanged. Compound B was tested at the melt temperatures of 105 and 112 ºC and at various gap distances. Figure 10 shows the microstructure of the foamed specimens produces with Compound B at temperature of 105 ºC. The foam bulk density was found to (a) (b) (c) (d) Figure 10. SEM micrographs of the foamed specimens for Compound B processed at 105 ºC and various gap distances (scale bar is 500 μm). a) gap 1 mm; b) gap 2 mm; c) gap 3 mm; d) gap 4 mm; e) gap 5 mm (e) 313

12 M. Nazari Marvian, A. H. Behravesh, M. Mahmoodi and M. Golzar Figure 11. Bulk density vs. gap distance for Compound B processed at two temperatures: 105 and 112 C be larger than 0.5 gr/cm 3 for all specimens (Figure 11). The calculated cell densities of the foamed specimens shown in Figure 12, revealed a relatively high variation. Increasing the gap distance from 1 to 3 mm, noticeably increased cell density. At a gap distance of 3 mm (found to be the optimum condition for this compound and at 105 C) cell density was measured to be about cells/cm 3. It is an interesting observation that when increasing the die gap, the cell density increases, as opposed to the conventional notion that decreasing the gap could enhance foam density due to a higher developed pressure. However, this is in fact the same scenario occurring at different condition. In the current die, the design was planned to initiate nucleation in the slotted section before the die tip (Figure 6a and b). The governing parameter for nucleation is the pressure difference between P 1 and P 2 ( P 12 ). Thus, when the gap distance increases, pressure P 2 decreases more significantly than pressure P 1. The consequence is that a higher pressure drop rate is created at the nucleation site (slotted section) causing a higher nucleation. Further increasing above an optimum limit could adversely affect the nucleation via decreasing both pressures at the same order. The results could then indicate the efficient function of the nucleation stage of the designed die as fairly decoupled from the shaping stage. Here, it should be noted that while the material was extruded at a temperature of 105 ºC, the product surface finish was poor indicating promotion of melt fracture (Figure 13). 314

13 Figure 12. Cell density vs. gap distance for Compound B processed at two temperatures: 105 and 112 C Figure 13. Quality of the surface finish of foamed Compound B extruded at 105 C showing melt fracture To investigate the effect of temperature on the foamed microstructures of Compound B, the above experiments were repeated only that the cooling unit and die temperatures were changed to 112 C. Bulk and cell density of the specimens extruded at this temperature are shown in Figure 11 and 12. Also the microstructure of Compound B extruded at 112 C is shown in Figure 14. Increasing the temperature could lead to a better surface finish of the extrudate thus enable the cable production. The surface quality for this test is shown in Figure

14 M. Nazari Marvian, A. H. Behravesh, M. Mahmoodi and M. Golzar (a) (b) (c) (d) (e) Figure 14. SEM micrographs of the foamed specimens for Compound B processed at 112 ºC and various gap distances (scale bar is 500 μm). a) gap 1 mm; b) gap 2 mm; c) gap 3mm; d) gap 4 mm; e) gap 5 mm Figure 15. Surface finish of the foamed produced with Compound B at a temperature of 112 C 316

15 The amount of bulk density at the melt temperature of 112 ºC was measured to be larger than 0.6 gr/cm 3 at all gap distances, with very little difference with those produced at temperature of 105 ºC. However, cell density was found to be lower at the temperature of 112º with relatively higher variation. The highest number of cell density, about , occurred at a gap distance of 3 mm. The surface finish of the final product improved highly to yield an acceptable surface for further downstream processing. The experiment revealed that Compound B, processed at a melt temperature of 112 ºC, yields a better structure compared with that processed at 105 ºC. The cell density was relatively high and the surface finish improved. However, bulk density of this product was too high (about 0.6 g/cm 3 ) to be utilized as coaxial foam insulation. The element which could have a major effect on expansion phenomenon is the shaping element. Hence, in order to enhance the bulk foam density, experiments were extended to investigate the effect of shaping die diameter. Effect of Die Diameter on Expansion and Cell Density In order to study the effect of shaping die geometry on the expansion ratio and cell density, three shaping dies with outlet diameters of 13, 14, and 15 mm were designed and implemented (Figure 6c). Other processing parameters such as physical blowing agent percent (5%), Compound B and melt temperature (112 C) were maintained unchanged. For each shaping die, the gap distance was varied. Figures 16 and 17 show the effect of shaping die diameter on bulk and cell densities at various gap distances. At the die diameter of 13 mm, the foam density was obtained to be between gr/cm 3, still higher than the acceptable value, with a high variation of cell density between and cells/cm 3. Increasing the die diameter to 14 mm caused a dramatic decrease in bulk density. At a gap distance of 0.5 mm, the foam density was found to be about 0.25 gr/cm 3. Via increasing the gap distance, the bulk density gradually decreased so that at a gap distance of 2 mm the obtained bulk density was 0.15 gr/cm 3. Increasing the gap distance to 4 mm caused an increase in bulk density to 0.5 gr/cm 3. However, at the die diameter of 14 mm, cell density noticeably decreased. Cell density decreased as the gap distance increased. The highest number of cells, cells/cm 3, was obtained at a condition where the bulk density was 0.23 gr/cm 3 (gap distance of 1 mm). Here, it should be mentioned that an inappropriate condition of die geometry may lead to an increase in the cell size, although the bulk density improved noticeably. 317

16 M. Nazari Marvian, A. H. Behravesh, M. Mahmoodi and M. Golzar Figure 16. Foam density vs. gap space for foamed Compound B extruded with different die diameter and at various gap distances Figure 17. Cell density vs. gap space for foamed Compound B extruded with different die diameter and at various gap distances Figure 18 shows the microstructure of Compound B processed at die diameter of 14 mm. 318

17 (a) (b) (c) (d) Figure 18. Microstructure of foamed Compound B extruded at 112 C and die outlet diameter of 14 mm (scale bar is 500 μm). a) gap 1 mm; b) gap 2.5 mm; c) gap 3.5 mm; d) gap 5 mm At a shaping die diameter of 15 mm, the foam density was found to decrease significantly to 0.12 gr/cm 3. Increasing the gap distances caused a gradual increase in foam density. However, cell density was too low to be suitable for coaxial cable insulation, compared with the cell densities obtained with the die diameters of 13 and 14 mm. The highest cell population was found to be about cells/cm 3 at the gap distance of 1 mm. The microstructures of Compound B foamed with a die diameter of 15 mm and different gaps are shown in Figure 19. By evaluating the results, it can be proposed that the desirable condition (appropriate expansion and high population density) when using compound B is obtained with, shaping die diameter of 14 mm and at a gap distance of 1 and 2.5 mm. It should be noted that at the gap distance of 1 mm, thickness of foam insulation was not acceptable (less than 6mm in thickness), and thus, in terms of manufacturability, was not suitable. Although the smaller cell size (at a higher cell density) yields a better mechanical properties of the insulation, but the foam bulk density is the main factor to directly affect the attenuation property of the cable (1,2). 319

18 M. Nazari Marvian, A. H. Behravesh, M. Mahmoodi and M. Golzar (a) (b) (c) (d) Figure 19. microstructure of foamed Compound B extruded at 112 C and die outlet diameter of 15 mm (scale bar is 1 mm). a) gap 1 mm; b) gap 2 mm; c) gap 3 mm; d) gap 4 mm Figure 20. Comparison between attenuation properties of the produced coaxial cables at different frequencies with both chemical and physical blowing agents 320

19 At this selected (optimum) condition, a complete production was carried to produce at least 100 meters of the coaxial cable. The attenuation characteristics was measured given in Figure 20. It is seen that the attenuation properties is well improved compared to that of the cable produced using a chemical blowing agent (6), especially when compared at the higher frequencies. This is mainly due to the higher expansion, higher purity of the physical blowing agent compared with chemical ones and also eliminating moisture created from chemical reactions when using a physical blowing agent. CONCLUSIONS An experimental study was carried out to produce a foam-insulated coaxial cable using physical blown agent, butane gas, to achieve a desirable attenuation. The purpose was to simultaneously achieve low bulk density foam critical for attenuation properties and high cell density foam (thus smaller cell size) for an improved mechanical strength. PE compound, melt and die temperatures, and die geometries (die outlet diameter and gap distances) were the processing variables, to optimize the process. The results showed that an optimum result can be obtained when adding HDPE into LDPE processed at a melt-die temperature of 112 C, and appropriately adjusted outlet die diameter and die gap distance. At this condition, bulk and cell densities were 0.25 gr/cm 3 and cells/cm 3, respectively, with a good surface finish. The attenuation property was examined and showed an improvement compared to the cable produced with a chemical blowing agent. ACKNOWLEDGMENT The authors would like to thank Saba Engineering Cooperative for their technical support to this project. REFERENCES 1. Masahiro A., Takanori Y., Kimihiro Y., Minoru K., and Yasuyuki U., Development of low-loss highly foamed polyethylene coaxial cable (LHPX) for base station for cellular phone Hitachi Cable Review, No.22, (August 2003). 2. Wilkenloh F.N., Wilson P.A., and Fox S.A. Coaxial cable with improved properties and process of making same U.S. Patent 4,104,481, (1978). 3. Boysen R.L. and Lebanon N.J., Coaxial cables with improved properties, U.S. patent, 3,968,463, (1976). 321

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