Corrosion-Resistant Polymer Composite Tubes with Enhanced Thermal Conductivity for Heat Exchangers

Abstract

The heat transfer surfaces of heat exchangers are usually made of metals which may suffer from severe corrosion. When corrosive fluids are present, highly corrosion-resistant metals, graphite or ceramics are used, resulting in high costs. This study presents measured data on the thermophysical and mechanical properties of recently developed corrosion-resistant polymer composite tubes for use in heat exchangers. Extruded polymer composite tubes based on polypropylene or polyphenylene sulfide filled with graphite flakes were investigated. The anisotropic thermal conductivities of the polymer composite tubes were measured at various temperatures. The through-wall thermal conductivity of the tubes made of polypropylene filled with 50 vol.% graphite is increased by a factor of 30 compared to pure polypropylene, resulting in a thermal conductivity of 6.5 W/(m K) at 25 °C. The tubes composed of polyphenylene sulfide filled with 50 vol.% graphite have a through-wall thermal conductivity of 4.5 W/(m K) at 25 °C. The mechanical properties of the polymer composites were measured using tensile and flexural tests at different temperatures. The composite materials are more rigid and keep their mechanical properties up to a higher temperature level compared to the unfilled polymers. Surface roughness measurements show the very smooth and sealed surface of the composite tubes. The results contribute to establishing the viability of using polymer composites for heat exchanger applications with corrosive fluids.

1. Introduction

Heat exchangers play a crucial role in many industrial processes, ranging from the chemical, petrochemical, oil and gas, pulp and paper, food, pharmaceutical and power industry; seawater desalination; and concentration of waste waters and brines to heating, ventilation, air conditioning and refrigeration [1]. Metals are the most common materials of construction for heat exchangers due to their favorable thermal and mechanical properties. However, metals may suffer from corrosion and corrosion-related failure, especially in harsh environments when aggressive fluids are used [1]. Therefore, highly corrosion-resistant and very cost-intensive metals such as super duplex and hyper duplex stainless steels, nickel-based alloys and titanium must be used in various heat exchanger applications [2,3], e.g., concentration of waste waters in zero liquid discharge applications or concentration of high-salinity brines to recover valuable minerals and metals as well as energy recovery from exhaust gases, leading to high capital expenditures of the heat exchanger. Apart from their tendency to corrode, metals are susceptible to fouling, which is the accumulation of unwanted deposits on surfaces, particularly those used for heat transfer. Fouling reduces the overall heat transfer coefficient under clean conditions. Thus, it leads to a decrease of the heat duty of an existing heat exchanger or to an additional surface requirement in the design of a new heat exchanger. Over-sizing the heat transfer surface area, fouling mitigation measures and cleaning methods as well as production losses during plant shutdown create considerable capital, operating and maintenance costs [4,5,6]. Furthermore, metals have a high weight, affecting material selection for the superstructure of heat exchangers as well as transportation, installation and maintenance expenses [7]. In very corrosive environments, the capabilities of many corrosion-resistant metals are exceeded. For extremely corrosive applications in chemical processing, graphite, ceramics (e.g., silicon carbide) or highly corrosion-resistant metals such as tantalum are used, but these are very expensive. Since graphite is stable in highly reducing environments, heat exchangers with impregnated graphite plates and tubes are often used in processes involving hydrochloric acid and phosphoric acid [8,9,10,11]. However, graphite tubes are cost-intensive and exhibit a very rough surface, promoting fouling [12]. Therefore, various industries are striving to find alternative materials for heat exchangers.

The high resistance to chemicals and corrosion, the low density, the great freedom in shaping and the low cost of many polymers have led to considerable attention being paid to the development and implementation of polymer heat exchanger technology in recent decades [13,14,15,16]. However, the major disadvantage of polymers in heat transfer applications is their very low thermal conductivity, between 0.1 and 0.5 W/(m K) [17], compared to that of metals such as austenitic stainless steels, super duplex, hyper duplex stainless steels and titanium grade 2, which have a thermal conductivity between 12 and 20 W/(m K) [18,19].

Polymer composite tubes with enhanced thermal conductivity are a promising alternative in heat exchangers in many fields of application. In the following, the state of the art and developments in the field of polymer-based heat exchangers and thermally enhanced polymer matrix composites are briefly summarized.

1.1. Advancements in Polymer-Based Heat Exchangers

Polymers are primarily used in heat exchangers in highly corrosive environments [15,16,20], e.g., for heating and cooling of tanks and vessels in the electroplating industry, heating and cooling of acids, heating and cooling of aggressive exhaust gases, energy recovery from aggressive exhaust gases, condensation of vapors in the chemical and pharmaceutical industries and for the recovery of solvents, evaporative/desiccant cooling, solar water heating systems, seawater applications in marine, aqua zoos, air conditioning and electronic cooling devices. Polymers which are currently used as heat transfer material in heat exchangers are mainly thermoplastics, such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polypropylene (PP), polyethylene (PE), polyphenylene sulfide (PPS) and perfluoroalkoxy (PFA) [15,20]. Commercially available polymer heat exchangers are often shell-and-tube, immersed tube and plate heat exchangers [15]. Few experimental and theoretical studies have been published for other heat exchanger types such as finned tube heat exchangers and hollow-fiber heat exchangers [15].

In general, there are three approaches to overcome the disadvantage of the low thermal conductivity of polymers for heat transfer applications [7,13,21,22,23,24,25,26,27,28,29,30,31,32]. One approach is to use a considerably larger heat transfer surface area to compensate for the lower thermal conductivity [7,24,25,26,27,28]. The overall heat transfer coefficient obtained with unfilled polymers is usually smaller than that obtained with metals. It lies between 100 and 500 W/(m² K) [7]. This disadvantage is offset by a larger surface area obtained, for example, by using many small tubes. The outside diameter of polymer tubes is often between 2 and 8 mm [26]. More recently, hollow-fiber heat exchangers were developed to increase the heat transfer area and maintain the overall volume of the heat exchanger system [27,28]. Zarkadas and Sirkar [28] proposed a polymeric hollow-fiber heat exchanger for lower temperature and pressure applications and measured overall heat transfer coefficients between 647 and 1314 W/(m² K) for water–water systems. However, one major concern in hollow-fiber heat exchangers is the pressure drop caused by the small fiber diameter.

When comparing polymers with metals for application in heat exchangers, it is worth considering the thermal conductivity, the weight per unit of heat transfer surface and the costs of the materials [15]. Zaheed and Jachuck [24] compared these parameters for a reference heat exchanger. They came to the conclusion that a PVDF tube bundle must be six times larger than a tube bundle made of nickel–chromium–molybdenum (Ni-Cr-Mo) alloy. Nevertheless, the weight of the Ni-Cr-Mo tube bundle is around five times larger and the costs are 2.5 times higher than those of the PVDF tube bundle, if all other conditions remain the same. El-Dessouky and Ettouney [7] analyzed the performance of horizontal tube falling film evaporators and plate preheaters constructed of metal or polymer materials for a single-effect evaporator with mechanical vapor compression for seawater desalination. They concluded that the PTFE evaporator and preheater have lower specific costs than those made of copper-nickel 90/10 or titanium. Other factors that favor the use of plastic evaporators and preheaters include ease of construction and machining, lower installation and erection cost.

Heat exchangers with unfilled polymers are applied in niche markets where only highly corrosion-resistant and very expensive metals could be used and/or where the low thermal conductivity of the polymer will not significantly reduce the overall heat transfer coefficient because the heat transfer coefficient on one side of the heat transfer surface is very low like in energy recovery systems from exhaust gases. Their lower thermal performance compared to that of metal heat exchangers has prevented the widespread use and acceptance of polymer heat exchangers. More detailed information about polymer heat exchangers can be found in various reviews [14,15,16,24,33,34].

The second approach to compensate for the poor thermal conductivity of polymers is to lower the relatively high heat conduction resistance of the polymeric wall by reducing the wall thickness, leading to thin polymer films with a thickness lower than 100 µm. Bart et al. [29] developed a heat exchanger similar to a plate-and-frame heat exchanger using thin polymer films supported by a spacer grid. However, using thin polymer films requires a new heat exchanger design.

The third approach to compensate for the poor thermal conductivity of polymers is to add a filler material with a high thermal conductivity to the polymer matrix, which increases the overall thermal conductivity of the composite. Studies on polymer composites with respect to their use in heat exchangers are rare. Bar-Cohen et al. [30] investigated polymer composites for corrosion-resistant compact heat exchangers in seawater-cooled applications. They showed that a polymer composite heat exchanger provides 80% of the heat transfer rate compared to that of a metal heat exchanger. Moreover, Luckow et al. [31] investigated the efficiency of a polymer composite seawater–methane heat exchanger for liquefaction in natural gas applications. In a theoretical study, a polymer composite heat exchanger with a thermal conductivity of 5 or 10 W/(m K) was compared with one made out of metals (aluminum, copper, copper-nickel and titanium). The researchers calculated the total coefficient of performance (COP) which relates the heat transferred during service life to the energy invested in the manufacture of the heat exchanger as well as in the operation (pumping power). Luckow et al. [31] found that a polymer composite heat exchanger with a thermal conductivity of 10 W/(m K) provided a total COP that was two times higher than that of a titanium heat exchanger. They stated that thermally enhanced polymers are a viable heat exchange material due to their low fabrication energy and overall lifetime energy use. Boeck [35] discussed the use of polymer composite tubes in multiple-effect distillation (MED) plants for seawater desalination. A 10-stage MED plant equipped with polymer composite tubes was compared with a 10-stage MED plant using conventional aluminum brass tubes. Boeck [35] showed that the MED plant equipped with polymer composite tubes having a thermal conductivity of 10 W/(m K) would require a 25% greater heat transfer surface area than that of the metal reference plant. Tahir et al. [23] studied the sustainability and economic viability of thermally enhanced polymer tubes for MED plants. They reported that using polymer composite tubes could save 40% of the costs of a titanium evaporator. Kiepfer et al. [36] evaluated polypropylene–graphite composites for use in corrugated heat exchanger plates. These polymer composites, with up to 80 wt.% graphite and thermal conductivities up to 2.74 W/(m K), show suitable mechanical properties and improved thermal performance compared to pure polypropylene. They also exhibit lower crystallization fouling susceptibility and lower fouling resistance to calcium sulfate than stainless steel.

1.2. Developments in Polymer Composites with Enhanced Thermal Conductivity

Research developments in the field of polymer matrix composites with enhanced thermal conductivity are summarized below. A more detailed overview is provided in reviews [15,16,17,33,37,38,39,40].

Polymer composite materials can combine the favorable properties of different material classes. Chemical resistance, low density and good processability of polymers can be combined with the high thermal conductivity of metals, ceramics or carbon-based materials. In general, the polymer composite’s thermal conductivity depends on the thermal conductivity of the polymer, of the filler and the filler volume fraction (filler loading). In addition, the filler type, size and shape, morphology (dispersed individual particles, aggregates, fractal clusters, percolated network, etc.), anisotropy (particularly in the case of non-spherical filler particles), adhesion between the filler and matrix, filler–matrix interfaces and effects of processing history have also a strong influence on the thermal conductivity of polymer composites [17]. Figure 1 shows the relationship between the properties of the polymer composite and the filler content. Unlike electrical conductivity, there is no clear percolation threshold for thermally enhanced composites. In general, the thermal conductivity of composites increases with higher filler volume fraction and is often non-linear [41]. At lower filler volume fractions, the increase in thermal conductivity is relatively small. High thermal conductivities can only be achieved at high filler loadings when thermally conductive pathways are formed due to filler–filler connections [17]. However, high filler volume fractions can cause the composite to be brittle [17], as shown in Figure 1. Thus, a trade-off between mechanical strength and thermal conductivity must be made.

1.2.1. Matrix and Filler Materials

Thermosetting polymers (e.g., epoxy, vinylester, polyester) and thermoplastics (e.g., PP, PA) are used as the matrix material in polymer composites [16]. Thermoplastics are mainly used in heat exchanger applications. They have a wide range of applications because they can be formed and reformed into many shapes and demonstrate a high modification potential. Moreover, thermoplastics can be used in various processing techniques, such as injection-molding and extrusion [44,45].

Thermoplastics exhibit good chemical resistance, fatigue resistance and sliding characteristics, and they are wear-resistant. Generally, thermoplastic materials are divided into commodity plastics, engineering plastics and high-performance plastics. Commodity plastics such as polystyrene (PS) and PP have a relatively low price and show basic mechanical properties. Engineering plastics such as polyester (polybutylene terephthalate (PBT), PET) or polyamides (PA 6, PA 66) show enhanced mechanical properties and higher service temperatures. High-performance plastics such as polyether ether ketone (PEEK) or PPS exhibit the best mechanical values and the highest service temperatures. However, they are more expensive and only available in much smaller volumes than commodity and engineering plastics [13].

For efficient use in heat exchangers, filler material with a high thermal conductivity must be added to the polymer matrix. Metals, ceramics and carbon-based materials can be used in polymer composites to improve thermal conductivity [17]. Depending on the heat transfer application, a suitable filler must be selected. Electrically insulating ceramic fillers can be used for applications requiring both high thermal conductivity and electrically insulating properties, e.g., electronic devices, while metallic and carbon-based particles can be utilized in applications where electrical insulation is not required [17]. The filler material, the size, shape and the filler–matrix interfacial resistance affect the thermal conductivity of the composite [17]. Moreover, other aspects, such as price and availability on the market, are also crucial for selecting an appropriate filler material [13]. Various filler materials are summarized in

Table 1. Thermal conductivities of filler materials based on [17].

Filler	Category	Thermal Conductivity k/W/(m K)
Aluminum	Metal	234
Copper	Metal	386
Graphite	Carbon-based	100–400
Carbon fiber	Carbon-based	300–1000
Carbon nanotube (CNT)	Carbon-based	1000–4000
Graphene	Carbon-based	2000–6000
Hexagonal boron nitride (h-BN)	Ceramics	103–200
Aluminum nitride (AlN)	Ceramics	100–300

.

Various metallic fillers, such as copper, aluminum, nickel, zinc and silver, have been investigated in polymer composites to improve thermal conductivity [46]. An increase in thermal conductivity was observed by Boudenne et al. [47], who incorporated copper particles into a PP matrix with a thermal conductivity of 0.24 W/(m K) and achieved a thermal conductivity of 2.45 W/(m K) at a filler content of 45 vol.%. Tekce et al. [48] compared copper spheres, platelets and fibers in a polyamide matrix at different filler volume fractions up to 60 vol.% and measured thermal conductivity with the hot disk method. The polyamide matrix filled with 30 vol.% copper fibers reached a thermal conductivity of 8.71 W/(m K). Platelet-shaped particles resulted in a thermal conductivity of 11.57 W/(m K) at a maximum filler fraction of 60 vol.%, while spheres showed the lowest thermal conductivity of 3.66 W/(m K) at 60 vol.%.

Another option is using carbon-based materials such as carbon black, carbon fibers, graphite, graphene and single-walled and multi-walled carbon nanotubes (CNTs), which increase both thermal and electrical conductivity and improve the mechanical properties of the composites [49,50]. Furthermore, carbon-based structures have low thermal expansion coefficients and are corrosion-resistant. Compared to metallic fillers, most carbon-based fillers show relatively lower densities [17,51]. Graphite is a crystalline allotrope of carbon and comprises a series of stacked layer planes [51]. The peculiar crystal structure of graphite results in considerable anisotropy, especially in terms of electrical and thermal properties. Graphite has an exceptionally high thermal conductivity of up to 398 W/(m K) in the in-plane direction but a relatively low thermal conductivity of about 2.2 W/(m K) in the through-plane direction [51]. This highly anisotropic thermal conductivity of graphite substantially impacts the performance of the polymer composite. Graphite has the advantage of being readily and cheaply available in various shapes and sizes [15]. Moreover, many research studies focus on graphene and CNTs as filler materials for polymer composites with enhanced thermal conductivities [52,53].

Ceramics, such as aluminum nitride, boron nitride, silicon carbide and beryllium oxide, are used when a composite with high thermal conductivity, low electrical conductivity and low thermal expansion is needed [17]. Leung et al. [54] investigated the thermal conductivity of PPS with spherical hexagonal boron nitride (h-BN) particles. They increased the thermal conductivity from 0.22 W/(m K) to 1.8 W/(m K) at a filler content of 33.3 vol.%. Xu et al. [55] investigated polymer matrices of PVDF and epoxy filled with aluminum nitride (AlN) whisker and silicon carbide whisker particles. The highest thermal conductivity, 11.5 W/(m K), was found for a PVDF matrix with 60 vol.% AlN whisker particles. Ishida and Rimdusit [56] and Yu et al. [57] also found increased thermal conductivity for polymer matrices with ceramic fillers. Resin-based composites for the electronics sector were systematically studied by Übler [58], who reported a thermal conductivity of 3 W/(m K) at 30 °C for a filler mixture of 60 vol.% aluminum oxide with 5 vol.% AlN.

1.2.2. Processing and the Importance of Particle Alignment

Most developments have been focused on thermally enhanced polymer composite parts produced by injection-molding for the thermal management of electronic devices, batteries and photovoltaic cells [17,37,42,43,58,59]. However, manufacturing of polymer composite tubes with enhanced thermal conductivity by extrusion has been scarcely investigated. Öztürk [60] found that the maximum filler content in a PA 6-graphite compound for extrusion and injection-molding was about 35 vol.%, indicating the challenges of extruding polymer tubes with high filler loadings. A high filler loading increases the viscosity of the molten composite, which poses challenges for conventional polymer processing techniques. With standard extrusion processes, filler contents of up to 15 to 20 vol.% are common [13]. Sobolčiak et al. [61] prepared composites composed of high-density polyethylene (HDPE) and expanded graphite. The blends were hot-pressed. The researchers found that 50 wt.% of graphite increased the thermal conductivity of HDPE to 2.18 W/(m K). Furthermore, they assumed that filler loadings below 30 wt.% could be processed by the conventional extrusion process, while filler loadings above 30 wt.% required injection-molding due to the complex viscosity profiles involved [61].

Polymer composites generally have an orthotropic thermal conductivity if the thermal conductivity of the filler differs from that of the matrix material, as the filler cannot be arranged uniformly in every direction. In addition, non-spherical filler shapes also lead to anisotropy in the composite materials [17]. However, these fillers can be oriented during processing, which usually results in high thermal conductivity in the direction of orientation but low thermal conductivity in the direction perpendicular to the alignment [17]. This anisotropic behavior is further enhanced if the filler has anisotropic properties, as in the case of graphite and boron nitride [17]. The filler content must be high and the alignment of the filler particles must be optimized to create a network of particles with percolation paths that take advantage of the highly thermally conductive anisotropic fillers. Figure 2 illustrates the importance of filler-filler connections and particle orientation in a tube wall. Random alignment leads to similar thermal conductivities in the axial and radial directions. However, the thermal conductivity is low as there are no percolation paths. The orientation of the particles in the axial direction leads to filler-filler connections and a higher thermal conductivity in the axial direction. High thermal conductivity in the radial direction can be achieved by aligning the particles orthogonally to the wall.

Techniques for aligning filler particles in a polymer matrix have been explored. The alignment of the filler particles can be achieved by flow-assisted alignment and by field-assisted alignment (e.g., electrical, magnetic or acoustic field) [62,63,64]. Controlling the particle orientation leads to changes in functionality and filler content efficiency and influences the mechanical, thermal and electrical characteristics of the composite material.

Flow-assisted alignment has been observed during shear-dominant processes such as extrusion, injection-molding and post-elongation processes (e.g., fiber spinning). In shear-dominant processes, fillers change their orientation due to flow-induced shear forces, a phenomenon which has been investigated theoretically and experimentally [64]. Sulong and Park [65] studied the influence of shear rate on CNT alignment during extrusion. They concluded that higher shear rates lead to higher alignment toward extrusion direction. Similarly, Eken et al. [66,67] performed fiber-level simulations which confirm that high shear rates orientate particles along the flow direction. However, low-shear forces cause agglomerates and a low degree of alignment. In addition, Fan and Advani [68] investigated the Brownian motion effect theoretically and found that only a little or no filler alignment takes place after the shear force is interrupted. In general, during extrusion or injection-molding, shear forces run parallel to the walls. Thus, the filler particles tend to align in parallel with the extrusion direction, especially close to the tube wall. Therefore, the alignment of the filler perpendicular to the extrusion direction can be better achieved in the center of the tube wall, where the shear forces are lower.

Polymer matrix composites are a promising alternative to metals in heat exchangers that operate under harsh conditions. Extrusion is the most cost-effective process for the mass production of continuous polymer profiles and is therefore best suited for the production of polymer composite tubes. In order to achieve high thermal conductivity of the composite material, high filler loadings are required. However, the mechanical strength can decrease with high filler contents. Furthermore, measuring the direction-dependent thermal conductivity of curved, thin-walled and anisotropic composite tubes is a difficult task.

Recently developed extruded polymer composite tubes with a high filler content have been investigated for use in heat exchangers. The present study explores the thermophysical and mechanical characteristics of the polymer composite tubes, which are necessary to assess the suitability of the material for a specific application and for the design of the heat exchanger.

2. Materials and Methods

In the following, the material selection of the polymer matrices and the filler, the extrusion process and the analysis methods used for material characterization are briefly described.

2.1. Material Selection

For the production of the polymer composite tubes, the plastic extrusion process, in which thermoplastics are melted and formed into continuous profiles, was chosen. After evaluating a wide range of thermoplastic materials, polypropylene (PP) and polyphenylene sulfide (PPS) were selected as suitable matrix materials for heat exchanger applications [13]. The thermal and mechanical properties of unfilled PP and PPS are listed in

Table 2. Material characteristics of PP and PPS [45,69].

	Unit	PP	PPS
Density	g/cm3	0.90–0.907	1.34
Tensile modulus	MPa	1100–1300	3400
Tensile strength	MPa	21–37	75
Elongation at break	%	10–140	3–8
Flexural modulus	MPa	1240–1600	3750–4200
Flexural strength	MPa	41	125–145
Thermal conductivity	W/(m K)	0.17–0.22	0.25
Linear coefficient of thermal expansion	1/K	110–170·10−6	55·10−6
Specific heat capacity	kJ/(kg K)	2	not spec.
Heat deflection temperature HDT/A	°C	55–70	135
Max. service temperature (short term)	°C	140	260
Max. service temperature (long term)	°C	100	200–240
Min. service temperature (long-term)	°C	0 to −30	not spec.
Melting temperature	°C	160–170	285
Glass transition temperature	°C	0 to −10	85
Water absorption, equilibrium in water (23 °C)	%	0.02–0.04	0.01–0.03

.

Polypropylene is a semi-crystalline thermoplastic and belongs to the group of olefins. It exhibits higher strength, stiffness and crystalline melting temperature at lower densities than polyethylene. Stiffness and hardness lie between those of polyethylene and engineering plastics. The maximum service temperature is in the range of many low-temperature heat exchanger applications (see Table 2). Polypropylene exhibits only low water absorption and permeability. It is highly chemically resistant to aqueous solutions of salt, strong acids, alkalis and brines up to 120 °C, due to its non-polar structure [45].

Polyphenylene sulfide is a suitable polymer matrix material for composite tubes due to its superior chemical, thermal and mechanical properties. It is a semi-crystalline polymer in which aromatic monomer units are connected by sulfur atoms. It exhibits exceptionally high heat stability, high chemical resistance and strength. Moreover, PPS shows an enhanced long-term service temperature range (Table 2), and it is famous for its excellent resistance to acid attack. It is highly resistant to organic solvents and does not dissolve in any known solvent below 200 °C [45]. Results of tests in 85% sulfuric acid at 120 °C for up to 5000 h suggested that PPS performs better than PTFE and PVDF in acidic conditions [16].

Metallic, ceramic and carbon-based filler materials were evaluated, and graphite flakes at µm-scale were selected as the most promising filler for polymer composites with enhanced thermal conductivity in heat exchanger applications. Graphite offers the best overall performance regarding high thermal conductivity, low price, low density, good availability, low wear on processing equipment and chemical inertness. Graphene and CNTs have sparked interest in research and development as filler materials for thermally enhanced polymer composites [52,53]. However, they are expensive and currently unavailable in high quantities.

Polymer composite tubes based on polypropylene or polyphenylene sulfide filled with graphite flakes with a filler content of 50 vol.%, in the following referred to as PP-GR and PPS-GR respectively, were investigated. The polymer composite tubes were developed and supplied by Technoform Heat Transfer Solutions (Fuldabrück, Germany). A filler content of 50 vol.% was found to be favorable for the PP- and PPS-based composite tubes to reach a relatively high thermal conductivity on the one hand and to ensure sufficient mechanical strength on the other hand. The PP-GR tubes can be used at temperatures up to 100 °C, while the PPS-GR tubes can be applied up to 240 °C depending on the mechanical load.

2.2. Tube Extrusion

Extrusion is a continuous process in which a screw transports granules of a polymer composite material through a heated cylinder. Heat and friction melt the granules, and the screw homogenizes and pressurizes the melt and transports it to a die. The die forms the tube and guides the melt to a cooling section (calibration) wherein it solidifies. Technoform has developed an advanced extrusion process that makes it possible to reach high filler contents of up to 60 vol.% [13], which are considerably higher than filler contents reported in literature [60,61], and to calibrate the inner and outer surface of the tube for high surface quality and precision.

2.3. Analysis Methods

In the following, the methods used for characterizing the polymer composite tubes are described.

2.3.1. Particle Alignment

Particle orientation considerably affects the thermal conductivity of a composite when anisotropic filler materials such as graphite are used. Filler particles in tubes produced by an extrusion process show the natural alignment behavior in the melt flow direction, leading to poor thermal conductivities in the radial (through-wall) direction [21]. Hence, flow-assisted particle alignment must be employed. For imaging the particle orientation, the cross-section of the extruded composite tubes was prepared and scanning electron microscopy (SEM) was used (Supra 40, Carl Zeiss AG, Oberkochen, Germany).

2.3.2. Thermophysical Properties

One of the benefits of using polymers in heat exchanger applications is their low density compared to metal tubes. The low density can reduce transportation and installation costs and minimize downtime due to faster tube exchange. The densities of the composite materials were experimentally determined at ambient temperature (20 °C). First, the mass of a sample was measured with a balance (Sartorius LC4200, Sartorius AG, Göttingen, Germany) with an accuracy of ±0.01 g. Afterwards, a measuring cylinder was filled with deionized (DI) water and weighed. The sample was put into the cylinder, and the increase in water head was noted. Subsequently, the sample was removed from the cylinder and dried. The cylinder was then filled with DI water to the noted water head and weighed. The sample volume was determined with the density of DI water, and the composite material’s density was calculated. The measurement was repeated at least five times.

Assuming the absence of voids, the specific heat capacity of a composite material depends on the specific heat capacity of the filler and matrix material and the mass fraction of the filler. According to Batch and Macosko [70], the specific heat capacity of a composite material $c_{p,\mathrm{C}}$ can be expressed as

$$c_{p,\mathrm{C}}={\varphi }_{\mathrm{w}\mathrm{t}}{c}_{p,\mathrm{F}}+\left(1-{\varphi }_{\mathrm{w}\mathrm{t}}\right)c_{p,\mathrm{M}}$$

(1)

with the mass fraction ${\varphi }_{\mathrm{w}\mathrm{t}}$ of the filler and the specific heat capacity of the filler $c_{p,\mathrm{F}}$ and the matrix $c_{p,\mathrm{M}}$.

The specific heat capacity of the PPS-GR composite was determined using differential scanning calorimetry (Q1000, TA Instruments, New Castle, DE, USA), referred to as DSC. Four samples were taken from different tube positions and measured at 20 °C, 110 °C and 200 °C.

Among the thermophysical properties, thermal conductivity is considered the most important property for heat transfer applications. The measurement of the anisotropic thermal conductivity of the thin-walled and curved composite samples is a difficult task. Methods for thermal conductivity measurement can be divided into steady-state and transient methods. For determining the thermal conductivity of the polymer composites, transient methods were used, namely the laser flash method and the transient hot bridge (THB) method. When applied to materials with anisotropic thermal conductivity, it is important to understand the principles of the measuring methods, in order to be able to correctly interpret the measured values.

The transient hot bridge and the laser flash method are illustrated in Figure 3. The transient hot bridge method is based on a foil sensor which contains a printed circuit of nickel which is embedded between two polyimide sheets. The sensor consists of four identical strips. These are arranged in parallel and connected to form a Wheatstone bridge circuit. At uniform temperature, the bridge is initially balanced. The nickel wire acts as a resistance heater to induce a local change in temperature that turns the bridge into an unbalanced condition. At the same time, the nickel wire can be used as a resistance thermometer. The foil sensor must be clamped between two equivalent samples with homogeneous temperature. During a measurement, the constant electric current flows through the sensor and establishes an inhomogeneous temperature profile within the samples due to the sensor layout. This time-dependent temperature profile leads to a change in the bridge voltage. The voltage rise in time is a measure of the thermal conductivity. The transient hot bridge method is often used to measure the direction-independent thermal conductivity of isotropic materials [71,72].

To measure the thermal conductivity of the thin-walled tubes, a small (3 × 3 mm) THB sensor (Linseis GmbH, Selb, Germany) was used. The THB sensor allows for rapid thermal conductivity measurements of solid and fluid specimens from 0.02 to 30 W/(m K). The measurement uncertainty for the thermal conductivity is better than 2% [71,73]. The thermal conductivities of PP-GR tubes with a wall thickness of 1.5 mm and 1.25 mm and of PPS-GR tubes with a wall thickness of 1.5 mm were measured using the THB method at a temperature of 25 °C.

Figure 3. Schematic illustration of the (a) laser flash method and (b) transient hot bridge method [74].

Figure 3. Schematic illustration of the (a) laser flash method and (b) transient hot bridge method [74].

The laser flash method allows for the experimental determination of the direction-dependent thermal diffusivity $a$ of a material [75]. The sample, which is located in a furnace, is heated by a short laser pulse on the front side and the resulting temperature increase at its rear side is monitored by an infrared (IR) detector, as shown in Figure 3. The temperature signal is plotted over time. If the density $\rho$ and the specific heat capacity $c_p$ are known, the thermal conductivity $k$ can be determined as

$$k=a·c_p·\rho .$$

(2)

The laser flash measurement is referred to as a through-plane measurement with respect to the directional dependence of the measured thermal diffusivity because the heat flow penetrates the sample perpendicularly to the sample surface [75].

The thermal diffusivity was measured with a laser flash analysis system (LFA 457 Micro-Flash, Netzsch-Gerätebau GmbH, Selb, Germany). Since the laser flash method is designed for flat samples, the effect of the curvature of the sample on the measurement was first determined by comparing the thermal diffusivity of a flat metal sample with the measured thermal diffusivities of various curved samples of the same material. Titanium alloy grade 5 was selected for comparison because the thermal conductivities of the polymer composite tubes were expected to be similar to the thermal conductivity of titanium alloy grade 5. A flat titanium alloy grade 5 sample and curved samples representing tube sections with outside diameters of 10, 12, 19, 24 and 25.4 mm were prepared. All samples had the same wall thickness of 1.5 mm. The laser flash measurements were conducted at 25 °C, 40 °C, 60 °C, 80 °C and 100 °C. Then the thermal diffusivities of PP-GR, PPS-GR and unfilled PP were measured at 25 °C, 40 °C, 60 °C, 80 °C and 100 °C and compared with titanium alloy grade 5. The PP-GR, PPS-GR and unfilled PP tubes had an outside diameter of 24 mm and a wall thickness of 1.5 mm. Additionally, the thermal diffusivity of PP-GR tubes with a reduced wall thickness of 1.25 mm was determined.

2.3.3. Mechanical Properties

The mechanical strength highly depends on the polymer matrix, the filler material and shape, the filler loading and the temperature. High filler loadings increase the thermal conductivity but tend to decrease the mechanical performance [42,43].

Tensile tests and three-point flexural tests were performed using a material testing machine (Z010, ZwickRoell GmbH & Co. KG, Ulm, Germany) according to DIN EN ISO 527-1 [76], DIN EN ISO 527-2 [77] and DIN EN ISO 178 [78], respectively. The international standards specify methods for determining the tensile and flexural properties of rigid and semi-rigid plastics and plastics filled with short fibers. The standards define the dimensions of the multi-purpose test specimens type A and B. Thus, injection-molded polymer composite specimens (type 1A) were used, as shown in Figure 4. The injection-molded PP-GR and PPS-GR specimens had the same filler content (50 vol.%) as the extruded composite tubes. A low test speed of 0.5 mm/min was chosen for the tensile tests. The tensile tests were performed at ambient temperature as well as at elevated temperatures. The PP-GR specimens were tested at 23 °C, 60 °C and 80 °C. In addition, the PPS-GR specimens were tested at 23 °C, 80 °C and 150 °C to show the broader application possibilities of PPS-GR. For PP-GR and PPS-GR, three tensile tests were performed at each temperature.

Flexural tests were performed with a 1 mm/min test speed, as stated in DIN EN ISO 178 [78], and at the same temperatures as the tensile tests. For PP-GR and PPS-GR, three flexural tests were performed at each temperature.

2.3.4. Surface Roughness

The surface properties of the tubes are important for the design and operation of heat exchangers, as they strongly affect fouling and cleaning and thus heat transfer. The surface properties of the novel PP-GR and PPS-GR composite tubes were characterized in terms of surface roughness and compared to stainless steel 316L (EN 1.4404) and aluminum brass (CW702R), hereafter referred to as 316L and Al brass, which are established materials for evaporator tubes. The tube outside diameter of the PP-GR, PPS-GR and 316L tubes was 24 mm, that of the Al brass tubes 25 mm.

The surface topography was examined using a stylus device (MarSurf GD25, Mahr GmbH, Göttingen, Germany). The stylus device measures surface roughness parameters based on DIN EN ISO 21920 [79]. For the characterization of the topography of the outer tube surface, the roughness depth Rz and the mean roughness Ra were examined. The roughness depth Rz describes the greatest single depth of roughness in the observed reference distance given by

$$Rz={\displaystyle \frac{1}{5}}\sum _{i=1}^5{Rz}_i.$$

(3)

The mean roughness Ra is the mean value of all deviations $x_i$ of the roughness profile from the mean line within the reference distance expressed as

$$Ra={\displaystyle \frac{1}{n}}\sum _{i=1}^n{x}_i.$$

(4)

The surface roughness of the tubes was measured at the circumferential angles 0°, 90°, 180° and 270°. At each circumferential angle, the surface roughness was determined at three positions in the axial direction (125 mm, 225 mm and 325 mm) of the 550 mm long tubes.

3. Results

In the following, the results are shown.

3.1. Extrusion and Particle Alignment

Figure 5a shows SEM images of the cross-section of the tube wall of a PP-GR tube with a wall thickness of 1.5 mm and a high graphite content of 50 vol.%. The SEM images display the agglomerated grey graphite flakes and the black PP matrix. The image in Figure 5b shows the outer edge of the cross-section, and the image in Figure 5d shows the inner edge of the cross-section. The images indicate that the agglomerated graphite flakes have a more horizontal direction (marked with blue arrows). In contrast, Figure 5c shows that the graphite flakes in the middle zone of the cross-section of the tube wall have a more radial (through-wall) direction (marked with red arrows). The particles near the tube wall are aligned horizontally because of higher shear forces close to the tube wall during tube extrusion. Particles in the middle zone face less shear forces during extrusion and thus are more aligned in the radial direction. Increasing the portion of radially aligned graphite particles could lead to a higher thermal conductivity of the composite in radial (through-wall) direction.

3.2. Thermophysical Properties

Table 3. Densities of PP-GR and PPS-GR with a filler content of 50 vol.% measured at 20 °C and densities of unfilled PP and PPS (marked with an asterisk *) taken from literature [45,69].

Material	Unit	Density ρ
PP	kg/m3	907 *
PPS	kg/m3	1340 *
PP-GR	kg/m3	1556 ± 10.6
PPS-GR	kg/m3	1782 ± 8.3

shows the measured densities of the novel polymer composites PP-GR and PPS-GR and the densities of the unfilled polymers taken from literature [45,69]. The densities of PP-GR and PPS-GR are higher than those of the unfilled polymers due to the density of the graphite particles, which ranges between 1550 kg/m³ [80] and 2260 kg/m³ [51]. Moreover, the PPS-GR tube has a slightly higher density than PP-GR due to the higher density of unfilled PPS (1340 kg/m³). Both composite materials show considerably lower densities than common metals.

The specific heat capacity of a composite material depends on the filler and matrix materials and the filler content. It was calculated for the PP-GR composite according to Equation (1). The calculated specific heat capacities of the polymer composite PP-GR at different temperatures are based on values for PP and graphite provided by Neubronner et al. [80], as summarized in

Table 4. Calculated specific heat capacities of PP-GR with a filler content of 50 vol.% at different temperatures according to Equation (1) based on the specific heat capacities of unfilled PP and graphite (GR) (marked with an asterisk *) taken from literature [80].

Material	Unit	Specific Heat Capacity ${\mathit{c}}_{\mathit{p}}$
		25 °C	40 °C	60 °C	80 °C	100 °C
PP	J/(kg K)	1745 *	1850 *	1920 *	2178 *	2350 *
GR	J/(kg K)	894 *	903 *	909 *	915 *	927 *
PP-GR	J/(kg K)	1132	1168	1196	1277	1334

.

The specific heat capacities of PPS-GR determined using DSC are shown in

Table 5. Specific heat capacity of PPS-GR with a filler content of 50 vol.% determined using DSC at different temperatures.

Material	Unit	Specific Heat Capacity ${\mathit{c}}_{\mathit{p}}$
		20 °C	110 °C	200 °C
PPS-GR	J/(g K)	0.844 ± 0.0391	1.1 ± 0.0304	1.34 ± 0.0390

. The specific heat capacity of PP-GR was higher than that of PPS-GR, and the specific heat capacities slightly increased with rising temperature.

Measuring thermal conductivity with the THB method resulted in a very high thermal conductivity of 14.5 W/(m K) for the PP-GR tubes with a wall thickness of 1.25 mm and 13.1 W/(m K) for the PP-GR tubes with a wall thickness of 1.5 mm. For PPS-GR with the same wall thickness of 1.5 mm, a slightly higher thermal conductivity of 14.1 W/(m K) was measured, as shown in

Table 6. Thermal conductivity determined using the transient hot bridge (THB) method.

Tube Material	Wall Thickness s/mm	Thermal Conductivity k/W/(m K) at 25 °C
PP-GR	1.25	14.5
PP-GR	1.5	13.1
PPS-GR	1.5	14.1

.

Measuring the anisotropic thermal conductivity of thin-walled and curved composite samples is a challenging task. The thermal diffusivity was measured with the laser flash method, as described in Section 2.3.2. The effect of the curvature of the sample on the measurement was determined by comparing the measured thermal diffusivity of a flat titanium alloy grade 5 sample with the measured thermal diffusivities of various curved titanium alloy grade 5 samples at different temperatures, as presented in Figure 6a. Figure 6b shows the ratio of the thermal diffusivity of the curved sample and the flat sample at different temperatures. The difference between the measured thermal diffusivities of the curved and the flat samples relative to that of the flat sample is less than or equal to ±2%. Thus, a measurement error of ±2% is assumed for the curved polymer composite samples.

The measured thermal diffusivities of PP-GR, PPS-GR, unfilled PP and titanium alloy grade 5 are presented in Figure 7a. The thermal conductivities, calculated according to Equation (2), are shown in Figure 7b.

The unfilled PP tubes have a very low thermal conductivity, as shown in Figure 7b. The PP-GR tubes show a significantly enhanced thermal conductivity of 6.5 W/(m K) at 25 °C in the radial direction, which is already similar to titanium alloy grade 5 with 6.3 W/(m K). The thermal conductivity of the PPS-GR tubes has been notably improved to 4.5 W/(m K) at 25 °C, but it is lower than that of the PP-GR tubes, as shown in Figure 7b. The thermal conductivity of titanium alloy grade 5 increases with increasing temperature, while that of the PP-GR and PPS-GR tubes decreases, as shown in Figure 7b.

3.3. Mechanical Properties

The mechanical properties of the polymer composite tubes are massively influenced by their high filler content (see Figure 1). The fillers are the reason for the brittle behavior of the composites, which results in a low tensile and flexural elongation at break, as shown in Figure 8. The elongation at break of both composite materials is lower than that of unfilled PP and PPS (PP: 10–140%; PPS: 3–8%; see Table 2). The elongation at break increases with rising temperature for PP-GR and PPS-GR because of the softening of the polymer matrix.

The tensile modulus E_t and the tensile strength σ_t of PP-GR and PPS-GR are presented in Figure 9a. The tensile moduli E_t are significantly higher than those of unfilled PP and PPS (PP: 1100–1300 MPa; PPS: 3400 MPa; see Table 2). The tensile strength σ_t of PP-GR lies in the range of unfilled PP (21–37 MPa). However, the tensile strength σ_t of PPS-GR is lower than that of unfilled PPS (75 MPa). Both polymer composites show a decreasing tensile modulus E_t and tensile strength σ_t at higher temperatures. The PPS-GR composite exhibits a higher stiffness and a higher strength compared with that of the PP-GR composite.

The results of the flexural tests are shown in Figure 9b. The flexural moduli E_b of PP-GR and PPS-GR are significantly higher than those of unfilled PP and PPS (PP: 1240–1600 MPa; PPS: 3750–4200 MPa; see Table 2). The flexural strength σ_b of PP-GR lies in the range of unfilled PP (41 MPa). However, the flexural strength σ_b of PPS-GR is lower than that of unfilled PPS (125-145 MPa). Furthermore, the flexural modulus E_b and the flexural strength σ_b of PP-GR and PPS-GR decrease with increasing temperature, as shown in Figure 9b. Compared to PP-GR, the PPS-GR composite material shows higher strength and modulus in flexural tests even at high temperatures due to the higher long-term service temperature of PPS (see Table 2).

3.4. Surface Roughness

The surface roughness parameters of the tube materials are presented in

Table 7. Measured surface roughness parameters of the polymer composite tubes PP-GR and PPS-GR as well as of metal tubes for comparison.

Material	Mean Roughness Ra/μm	Roughness Depth Rz/μm
PP-GR	0.37 ± 0.06	2.37 ± 0.52
PPS-GR	0.45 ± 0.07	2.52 ± 0.31
316L	0.42 ± 0.03	3.66 ± 0.31
Al brass	0.62 ± 0.22	4.40 ± 1.21

.

The PP-GR tubes exhibit a very smooth surface with mean surface roughness Ra and roughness depth Rz values lower than those of the 316L and aluminum brass tubes. The mean roughness Ra of PPS-GR is similar to that of 316L, but the roughness depth Rz is lower.

4. Discussion

The thermal conductivities of the polymer composite tubes were determined using the laser flash method and the transient hot bridge method. The results show the significant enhancement of the thermal conductivity of the polymer composite tubes PP-GR and PPS-GR compared to that of the unfilled polymers. The application of polymer composite tubes in heat exchangers requires a high thermal conductivity in the radial direction (through the tube wall). The laser flash method allowed for the measurement of the thermal diffusivity in the through-wall direction. As shown in Figure 6, the curvature of the samples had only a minor effect on the measurement results. The polymer composite tubes had a remarkably high thermal conductivity in the radial direction, as presented in Figure 7. The through-wall thermal conductivity of the tubes made of polypropylene filled with 50 vol.% graphite was increased by a factor of 30 compared to pure polypropylene, resulting in a thermal conductivity of 6.5 W/(m K) at 25 °C, which is already similar to the thermal conductivity of titanium alloy grade 5 (6.3 W/(m K)). The tubes composed of polyphenylene sulfide filled with 50 vol.% graphite had a through-wall thermal conductivity of 4.5 W/(m K) at 25 °C. Compared to polymer composites investigated by Sobolčiak et al. [61] and Öztürk [60], the thermal conductivities of the polymer composite tubes in this work were significantly higher due to the achieved high filler content and enhanced filler alignment in radial direction.

The thermal conductivity of titanium alloy grade 5 slightly increases with increasing temperature, while that of PP-GR and PPS-GR tubes slightly decreases, as shown in Figure 7b. In solids, heat may be transported by charge carriers (such as electrons and holes) or by phonons (energy quanta of atomic lattice vibrations). In metals, the electronic contribution greatly outweighs the phonon contribution [17]. According to the classical theory of thermal conductivity of metals, the relationship between thermal conductivity, electrical conductivity and temperature obeys the Wiedemann–Franz law. Thus, the thermal conductivity of titanium alloy grade 5 increases with temperature.

The heat conduction mechanism in polymers is mainly governed by phonon transport (lattice vibration) [81]. According to the Debye equation, the thermal diffusivity $a$ of polymers can be described by the phonon velocity and the phonon mean free path. The low thermal diffusivity of most polymers can be explained by the small phonon mean free path as a result of phonon–phonon scattering [17]. As temperature rises, the amplitude of vibrations increases, leading to more frequent scattering. This increased scattering reduces the mean free path of phonons, thereby decreasing the thermal diffusivity. Moreover, higher temperatures can lead to a decrease in the degree of crystallinity in polymers. Crystalline regions conduct heat more effectively than amorphous regions [17]. As temperature increases, these crystalline regions can break down into more amorphous structures, reducing the overall thermal conductivity. Additionally, elevated temperatures increase the mobility of polymer chains due to thermal expansion, leading to more disordered molecular arrangements [82]. Zhang and Luo [83] reported that thermal expansion at higher temperatures frees up inter-chain spacing, enabling segment rotation, destroying along-chain order and thus leading to lower thermal conductivity. Furthermore, the thermal expansion of the polymer matrix at higher temperatures leads to an enhanced boundary layer between filler and matrix material, which increases the interfacial thermal resistance, leading to lower thermal diffusivity at higher temperatures.

The thermal conductivities based on the measurements using the laser flash method and the transient hot bridge method showed significantly different values. The thermal conductivities measured with the THB method (see Table 6) were considerably higher than the thermal conductivities determined using the laser flash method (see Figure 7).

As described in Section 1.2.2, polymer composites usually have orthotropic thermal conductivity if the thermal conductivity of the filler differs from that of the matrix material. In addition, non-spherical filler shapes also lead to anisotropy in the composite material. This anisotropic behavior is further pronounced if the filler has anisotropic properties as in the case of graphite. The laser flash method allows for the measurement of the direction-dependent thermal diffusivity of anisotropic materials, while the transient hot bridge method is usually applied to isotropic materials.

Gaiser et al. [84] refined the evaluation of transient hot bridge measurements and explored an enhanced measuring procedure to determine the direction-dependent thermal diffusivity of anisotropic materials using the THB technique. They conducted sensitivity studies to estimate the influence of the samples’ anisotropic thermal diffusivity on the measurement signal. They characterized the influence of the samples’ thermal diffusivities in the three spatial directions of the THB sensor. The x-direction pointed in the thickness direction of the flat sample, while the y-direction was perpendicular to the heating strips, and the z-direction was parallel to the heating strips. The thermal diffusivity was gradually increased in one spatial direction, while the thermal diffusivities in the other spatial directions were kept constant. It was found that the measurement depends mainly on $a_y$, when $a_y\gg a_z$. Furthermore, the researchers concluded that the equation, which was derived to determine the thermal diffusivity of isotropic materials with the THB method, depended directly on $a_y$ as long as $a_y\gg a_z$.

Based on the findings of Gaiser et al. [84], it can be assumed that the thermal conductivity measured with the THB sensor is predominantly the thermal conductivity in the y-direction, perpendicular to the heating strips of the THB sensor, and might be strongly influenced by the thermal diffusivity in z-direction parallel to the heating strips.

Using the THB method, the thermal conductivity of the PP-GR tubes with a wall thickness of 1.25 mm was 14.5 W/(m K), while the thermal conductivity of the PP-GR tubes with a wall thickness of 1.5 mm was 13.1 W/(m K). Furthermore, the thermal conductivity of PPS-GR with a wall thickness of 1.5 mm measured with THB was higher than that of PP-GR, although the thermal conductivity of PPS-GR determined using the laser flash method was lower than that of PP-GR. This can be also explained by the anisotropy of the thermal conductivity of the composites and the measurement methods.

The SEM images in Figure 5 reveal that the graphite flakes close to the outer and inner tube walls were mainly aligned in the extrusion direction, while the graphite flakes in the middle zone faced less shear force during extrusion and thus were more aligned in the radial direction. Overall, the polymer composites exhibited high thermal conductivity in the axial and circumferential directions and a lower, but quite high thermal conductivity through the tube wall. As the middle zone of the cross-section with particle alignment in radial direction was smaller in tubes with a lower wall thickness and the outer and inner zones with filler alignment in extrusion direction were more dominant, the thermal conductivity, which was determined using the THB method, was higher in the PP-GR tubes with a wall thickness of 1.25 mm and the thermal conductivity, which was determined with the laser flash method, was lower compared to the PP-GR tubes with a wall thickness of 1.5 mm. Compared to PP-GR, the higher viscosity of the PPS-GR compound posed a greater challenge during tube extrusion. Thus, fewer graphite particles were aligned in the radial direction in the PPS matrix and the thermal conductivity in the radial direction, measured with the laser flash method, was slightly smaller, but the thermal conductivities in the other two directions, which were mainly measured with the THB sensor, were greater.

Tensile tests and flexural tests were performed at ambient and elevated temperatures to determine the mechanical properties of the polymer composite tubes. As shown in Figure 9, the PP-GR composite had a tensile modulus of about 8295 MPa and a tensile strength of about 29.5 MPa at 23 °C. The PPS-GR composite exhibited a higher tensile modulus and thus a higher stiffness. Furthermore, it had a higher tensile strength than the PP-GR composite. The tensile and flexural tests showed that the composite materials were more rigid than unfilled PP and PPS. The tensile and flexural moduli as well as the tensile and flexural strengths decreased at elevated temperatures. However, the composites had a major advantage over the unfilled polymers because their mechanical properties decreased less with temperature, and they kept their mechanical properties up to far higher temperatures and were not prone to creep effects as unfilled polymers.

The surface roughness measurements show the very smooth surface of the extruded composite tubes even at the high filler content of 50 vol.% (see Table 7). The results confirm that only the polymer was present on the surface of the composites. As shown in the SEM images in Figure 5, a thin polymer layer was present on the outer and inner surfaces, leading to a smooth and sealed surface. Very smooth surfaces are advantageous in heat exchangers with regard to fouling and cleaning.

In ongoing work, finite element analysis of the thermal conductivity of composite materials is being performed in order to supplement the experimental studies and gain a more comprehensive understanding. Furthermore, surface treatments are being tested to increase the wettability of the polymer composite surfaces for applications with falling films, e.g., in falling film evaporators. Moreover, the fouling propensity of the polymer composite tubes is being studied and compared to the fouling propensity of common metal tubes. The low weight of the polymer composite tubes allows for the construction of tube plates, tube support plates and the shell from polymer-based materials, leading to reduced corrosion problems in corrosive environments and to further weight and cost savings. A fully polymer-based heat exchanger comprising polymer composite tubes and glass-fiber reinforced resins for the tube plates and the shell has been developed and will be further explored in future work.

As the polymer extrusion process offers a high degree of freedom as opposed to the manufacturing process of metal tubes, shapes other than plain circular tubes can be easily produced, as illustrated in Figure 10. The great freedom in shaping polymer composite tubes offers promising future prospects.

5. Conclusions

The thermophysical and mechanical characteristics of recently developed corrosion-resistant polymer composite heat exchanger tubes based on polypropylene or polyphenylene sulfide filled with graphite flakes have been explored. The through-wall thermal conductivity of the tubes made of polypropylene filled with 50 vol.% graphite is increased by a factor of 30 compared to pure polypropylene, resulting in a thermal conductivity of 6.5 W/(m K) at 25 °C which is already similar to the thermal conductivity of titanium alloy grade 5 (6.3 W/(m K)) and record thermal conductivity for extruded polymer composite tubes. The tubes composed of polyphenylene sulfide filled with 50 vol.% graphite have a through-wall thermal conductivity of 4.5 W/(m K) at 25 °C.

The PP-GR tubes can be used up to a maximum temperature of 100 °C and the PPS-GR tubes can be applied up to 240 °C depending on the mechanical load. The composite tubes made of PPS filled with graphite may open up cost-effective solutions in many applications, e.g., heat exchangers in flue gas applications or evaporators for acids. The PP-based composite exhibits a slightly higher thermal conductivity and it is a more cost-effective solution for low-temperature heat exchangers.

The thermophysical and mechanical properties are necessary to assess the suitability of the polymer composite tubes for a specific application and for the thermal and mechanical design of the heat exchanger. The results of this work contribute to establishing the viability of using polymer composite tubes for many heat exchanger applications with corrosive environments.