Polymer Composites with Enhanced Thermal Conductivity and Mechanical Properties for Geothermal Heat Pump Pipes

Abstract

In this article, we report a robust strategy to prepare polymer composites with enhanced thermal conductivity and mechanical properties for geothermal heat pump pipes, in which high density polyethylene (HDPE) used as matrix, various inorganic fillers including zero-dimensional (0D) aluminium spherical particles (Al-SPs), one-dimensional (1D) carbon fibres (CF) and two-dimensional (2D) graphite platelets (GPs), used alone or blended as modifier. The as-prepared polymer composites are GPs/HDPE, GPs/CF/HDPE, GPs/Al-SPs/HDPE and GPs/CF/Al-SPs/HDPE. Thermal conductivity and mechanical properties of these polymer composites were characterised. The results indicate that of all these as-prepared polymer composites, GPs/CF/Al-SPs/HDPE composites are the only one which possesses preferable and comprehensive properties both in thermal conductivity and mechanical properties due to the synergistic effect of filler. The thermal conductivity, elongation at break and tensile strength of GPs/ CF/Al-SPs/HDPE composites prepared with GPs (9 wt%), CF (3 wt%) and Al-SPs (3 wt%) were 0.803 W m⁻¹K⁻¹, 70.6 % and 26.69 MPa, respectively, which could meet the commercial requirements of the geothermal heat pump pipes.

1. Introduction

Due to the energy crisis, green energy has been attracting more and more attention^1,2. Geothermal heat pumps system for heating or cooling in residential and commercial buildings is a new, green and energy-saving method, which can store the extracted heat from conditioned buildings for cooling or release heat into conditioned buildings for heating. About 3 or 4 times more energy for heating or cooling will be produced from one unit of electrical energy by the using of geothermal heat pumps system³. A significant component of the geothermal heat pump system is the heat exchange pipes, which decide the efficiency of the heat pump, and has become the subject of intensive engineering evaluations ⁴ . In practical application, thermal conductivity of the pipes plays an important role.

In the geothermal heat pump system, the heat exchange pipes composed of high density polyethylene (HDPE) were the first choice due to their chemical stability, high tensile strength and elongation at break ⁵ . However, compared to the thermal conductivity of commercial heat pump pipes, which is 0.75 W m⁻¹K⁻¹ at least^6,7, the thermal conductivity of HDPE, which just varies from 0.2 to 0.4 W m⁻¹K⁻¹, is too low. In order to achieve enough efficiency of heat pumping pipes, the thermal conductivity of HDPE composites must be enhanced and heat conduction modification of HDPE composites is a good way.^8–11.

Thermally conductive polymer composites typically comprised polymer matrix and conductive fillers^12,13. In the published literature, the combination of different dimensional fillers was reported to achieve a preferable and comprehensive performance in both thermal conductivity and mechanical properties^14–19. Luan et al. ¹⁵ reported that by combining 2D chemically reduced graphene (CRG) with 1D Ag nanowires (NWs), the thermal conductivity and mechanical properties were improved due to chemical and physical crosslinking effects between nano-structures and polymer chains. Kim et al. ¹⁶ combined expanded graphite (EG) and highspeed crusher treated EG (wEG) to prepare a series of polymer composites and observed a synergistic effect on the enhancement of thermal conductivity, which caused by the formation of efficient thermally conductive pathways due to the hybrid of the differently sized. Tian et al. ¹⁸ prepared thermally conductive polymer composites, in which paraffin and EVA were used as matrix, and mixed fillers of two-dimensional expanded graphite (EG) and one-dimensional carbon fibre (CF) used as heat conduction modifiers. The results indicate that EG and CF have synergistic enhancement to thermal conductivity of polymer composites. Ren et al. ¹⁹ studied thermally conductive composite film based on combination of silicon carbide (SiC) and linear low-density polyethylene (LLDPE) by powder mixing and hot press moulding, and the results demonstrate that thermal conductivity of polymer composites depends on the different dimensions of SiC particles strongly. The current research paid much attention to thermosetting resins as materials in electronic products, but few researches on heat pump pipes were reported.

In this paper, we report research on HDPE polymer composites with enhanced thermal conductivity and mechanical properties for geothermal heat pump pipes. The effects of different hybrid fillers, such as Al-SPs, CF and GPs, which added alone or in combination to generate a synergistic effect on the enhancement of thermal conductivity and mechanical properties of polymer composites, were studied in detail. Compared with the previous studies^10,20,21, this is a robust strategy to prepare polymer composites with enhanced thermal conductivity and mechanical properties for geothermal heat pump pipes. Due to the synergistic effect between differently dimensioned fillers, the as-prepared GPs/ Al-SPs/CF/HDPE composites have excellent thermal conductivity and mechanical properties, which can meet the commercial requirements of the geothermal heat pump pipes. As such, it stands out as a promising method to prepare polymer composites with excellent comprehensive properties including thermal conductivity, especially used in the fields of geothermal heat pump pipes.

2. Experimental

2.1 Materials

HDPE with trade mark 2200J was produced by Yangzi Petrochemical Company. Al spherical particles were supplied by Tianjin lung Ying Da Trade Co., Ltd. and its average diameter was 30 μm. Colloidal graphite platelets were provided by Shanghai Yifan graphite Co., Ltd. and its average diameter was 10 μm. Carbon fibre was produced by TORAY Industries Inc, Japan, and its average diameter and length were 10 μm and 2 mm, respectively. Aluminate coupling agents were provided by Nanjing Shuguang Chemical Group Co. Ltd. All the materials were commercial grade products and were used without further purification.

2.2 Surface Modification of Fillers

Surface modification of fillers was carried out with aluminate coupling agents. After drying for 2 h at 100°C, the fillers were added into a high-speed mixer (SHR-10A, Zhangjiagang Baer Machinery Co., Ltd.) and heated to 80°C, then the aluminate coupling agents were directly added into the high-speed mixer within 3 min. The mixture continued to mix for 2 min. The final fillers with surface modification were obtained after the fillers had been put in the oven at 80°C for 2 h.

2.3 Composites Preparation

HDPE and modified fillers were mixed in a high-speed mixer at room temperature for 10 min according to recipe of Table 1 , and the mixtures were extruded through melt-extrusion. Melt-extrusion was carried on a twin-screw extruder (SJSH-30, Nanjing Rubber Machinery Factory) and the screws were set in co-rotation mode. The extrusion temperature was 190–220°C and the screw speed was set at 100 rpm. After passing through the extruder, the composites strands with the diameter of 4 mm went through a water bath and then a pelletiser to produce pellets with the diameter of about 3 mm. After the pelletised HDPE composites were dried at 80°C, they were injected into specimens via an injection moulding machine at 210°C.

Table 1

Composition of composites

Samples	HDPE (wt %)	GPs (wt %)	Al-SPs (wt%)	CF (wt %)
A1	90	10	0	0
A2	85	15	0	0
A3	80	20	0	0
A4	75	25	0	0
B1	85	12	3	0
B2	85	9	6	0
B3	85	6	9	0
C1	85	12	0	3
C2	85	9	0	6
C3	85	6	0	9
D1	85	9	1	5
D2	85	9	2	4
D3	85	9	3	3
D4	85	9	4	2
D5	85	9	5	1

2.4 Characterisation

The mechanical properties, including tensile strength and elongation at break of polymer composites, were measured by universal testing instrument (CMT4304, Shenzhen Xinsansi material detection Co., Ltd.) according to ISO527/1–1993(E). The test speed was 5 mm/min and the specimen length between benchmarks was 50 mm. The fracture surface of polymer composites was examined with a scanning electron microscopy (SEM) (JEOLJSM-5900LV, JEOL Ltd., Japan). The thermal conductivity was measured by a Hot Disk instrument (TPS 2500S, Hot Disk Ltd., Sweden) according to ASTM E 1461–01.

3. Results and Discussion

3.1 GPs/HDPE Composites

Figure 1 shows the mechanical properties of GPs/ HDPE composites as a function of the content of GPs. With increasing content of GPs, the tensile strength of GPs/HDPE composites increased but elongation at break decreased. Figure 2 shows the thermal conductivity of GPs/HDPE composites as a function of the content of GPs. The thermal conductivity of GPs/ HDPE composites improved with increasing content of GPs. When the content of GPs varied from 15 wt% to 20 wt%, the sharp increase of thermal conductivity dependent on content of GPs was observed. When the content of GPs increased to 15 wt%, the thermal conductivity of GPs/HDPE composites was just 0.6 W m⁻¹K⁻¹, which was far away from the required thermal conductivity of 0.75 W m⁻¹K⁻¹. But when the content of GPs increased to 25 wt%, the thermal conductivity of GPs/HDPE composites reaches 0.82 W m⁻¹K⁻¹, while elongation at break decreased significantly to 28 %.

Figure 1

Mechanical properties of GPs/HDPE composites

Figure 2

Thermal conductivity of GPs/HDPE composites

SEM images of GPs/HDPE composites are shown in Figure 3 . GPs dispersed in parallel direction. When the content of GPs exceeded to 20 wt%, GPs did not disperse in the matrix uniformly and more agglomerates and isolated islands were observed. Thus, more interface defects and stress concentration points, which had a negative effect on the mechanical properties and thermal conductivity^20,21, were involved in the composites. When the content of GPs was 15 wt%, a considerable performance of dispersion was observed, so 15 wt% was selected as the optimal content of total fillers.

Figure 3

SEM images of GPs/HDPE composites as a function of the GPs contents: (a) 10 wt%; (b) 15 wt%; (c) 20 wt%; (d) 25 wt%

3.2 GPs/CF/HDPE Composites

In the GPs/CF/HDPE composites, the total loading of hybrid fillers was 15 wt%. Figure 4 and Figure 5 show the mechanical properties and thermal conductivity of GPs/CF/HDPE composites as a function of the fraction of CF, respectively. As the content of CF increased, a significant enhancement of tensile strength of the composites was observed, which was attributed to the large aspect ratio. However, the elongation at break of the composites decreased. In Figure 5 , the thermal conductivity of GPs/CF/HDPE composites improved with the increasing content of CF. When the content of CF arrived at 6 wt%, the thermal conductivity and tensile strength of GPs/CF/HDPE composites increased to 0.64 W m⁻¹K⁻¹ and 30.14 MPa, respectively, but the elongation at break decreased to 44.3 %.

Figure 4

Mechanical properties of GPs/CF/HDPE composites

Figure 5

Thermal conductivity of GPs/CF/HDPE composites

Figure 6 shows the SEM image of a GPs/CF/HDPE composite. The orientation of CF was observed and CF with large aspect ratio crossed in the resin. It is well established that the factor enhancing the thermal conductivity of GPs/HDPE composites was to construct thermally conductive network ¹⁶ . CF could cross the resin to bridge the thermally conductive areas of GPs to form thermally conductive network and enhance thermal conductivity of GPs/ CF/HDPE composites. The most profitable contact between GPs and CF was the end fragments of CF, which were vertical to the flat surfaces of GPs and bridged them²⁰. However, due to the rigidity of GPs and CF, the probability of such bridge was very low. Apart from such a contact type, other contact types were point-type with a small contact area ²⁰ , so the enhancement of thermal conductivity was not obvious. Furthermore, due to the strong rigidity of CF, the sharp decrease of elongation at break of composites was observed.

Figure 6

SEM of GPs/CF/HDPE (9 wt%/6 wt%/ 85 wt%) composites

3.3 GPs/Al-SPs/HDPE Composites

In the GPs/Al-SPs/HDPE/composites, the total loading of hybrid fillers was 15 wt%. In Figure 7 , with the increasing content of Al-SPs, the elongation at break of GPs/Al-SPs/HDPE composites was enhanced while the tensile strength decreased. Figure 8 show the thermal conductivity as a function of the fraction of Al-SPs. The thermal conductivity of composites improved with increasing content of Al-SPs. When the content of Al-SPs arrived at 6 wt%, the thermal conductivity of GPs/Al-SPs/HDPE composites reached 0.7 W m⁻¹K⁻¹, which indicated that the combination of GPs and Al-SPs could have a more positive influence on thermal conductivity. Compared to GPs/HDPE composites, the elongation at break of the GPs/Al-SPs/HDPE composites increased to 76.85 %, but tensile strength decreased to 23 MPa. When the content of Al-SPs increased to 9 wt%, the thermal conductivity of GPs/Al-SPs/HDPE composites increased to 0.75 W m⁻¹K⁻¹, but the tensile strength was reduced too much to meet the requirements of geothermal heat pump pipes.

Figure 7

Mechanical properties of GPs/Al-SPs/HDPE composites

Figure 8

Thermal conductivity of GPs/Al-SPs/HDPE composites

As the content of GPs was 15 wt% in GPs/HDPE composites, GPs were separated in the polymer network, so the large distances among GPs introduced a large thermal resistance ²³ . The diameter of Al-SPs was larger than that of GPs. It can be noticed in the SEM images ( Figure 9 ) that Al-SPs with large diameters surrounded by GPs shortened the distances among thermally conductive particles and formed effective thermally conductive network Al-SPs/GPs “thermally conductive islands”, in which the thermal resistance of composites decreased and thermal conductivity was improved ²² . Due to the lack of bridges among Al-SPs/ GPs “thermally conductive islands”, the thermal resistance among Al-SPs/GPs was larger than that among GPs/Al-SPs. The thermal conductive network was not integrating, and more attention must be paid to that point.

Figure 9

SEM images of GPs/Al-SPs/HDPE (9wt%/ 6wt%/85wt%) composites

3.4 GPs/CF/Al-SPs/HDPE Composites

In GPs/CF/Al-SPs/HDPE composites, the total loading of hybrid fillers was 15 wt% and the loading of GPs was 9 wt%. In Figure 10 , with decreasing content of Al-SPs content, the tensile strength of GPs/CF/Al-SPs/HDPE composites increased, but the elongation at break decreased. Figure 11 shows the thermal conductivity of composites as a function of the ratio of CF and Al-SPs. When the ratio of the three contents of Gps, Al-Sps and CF arrived at 9:3:3, the thermal conductivity of composites reached the maximum value of 0.803 W m⁻¹K⁻¹, which could meet the required thermal conductivity of geothermal heat pump pipes. The thermal conductivity of the composites could be increased by a synergistic effect, which caused by the formation of efficient thermally conductive pathways due to the hybrid of fillers with different sizes and morphologies ¹⁶ . At the same time, when the thermal conductivity reaches the maximum, the tensile strength and the elongation at break of GPs/CF/Al-SPs/HDPE composites were 26.69 MPa and 70.6 %, respectively. Compared to GPs/HDPE composites with thermal conductivity of 0.82 W m⁻¹K⁻¹, GPs/CF/Al-SPs/HDPE composites had more excellent comprehensive performance.

Figure 10

Mechanical properties of GPs/Al-SPs/CF/ HDPE composites. X-axis is the ratio of three contents of Gps, Al-Sps and CF in the GPs/Al-SPs/CF composites. (a) Y-axis is tensile properties; (b) Y-axis is elongation at break

Figure 11

Thermal conductivity of GPs/Al-SPs/ CF/HDPE composites. X-axis is the ratio of the three contents of Gps, Al-Sps and CF in the GPs/Al-SPs/CF composites

The SEM images ( Figure 12 ) indicated that Al-SPs were surrounded by GPs to form Al-SPs/GPs “thermally conductive islands”. Due to the large diameter of Al-SPs, the contact probability between Al-SPs and CF increased. CF with large aspect ratio could cross the polymer matrix and linked the Al-SPs/GPs “thermally conductive islands” to construct effective three-dimensional thermally conductive network. Thus, the thermal interface resistance of GPs/CF/Al-SPs/HDPE composites decreased and the thermal conductivity was enhanced ²⁴ . Moreover, Al-SPs and reinforcement of GPs and CF had a positive effect on the elongation at break of composites, so both the tensile strength and the elongation at break of GPs/Al-SPs/CF/HDPE composites could be improved.

4. Conclusions

In this paper, polymer composites based on HDPE and different hybrid fillers and possessing enhanced thermal conductivity and mechanical properties for geothermal heat pump pipes, were prepared and investigated. With increasing content of GPs in GPs/HDPE composites, the tensile strength and the thermal conductivity were enhanced, but the elongation at break decreased a lot. The introduction of CF into GPs/HDPE composites could enhance the thermal conductivity and the tensile strength of GPs/CF/HDPE composites, but the elongation at break had a sharp decrease. Al-SPs were introduced into GPs/HDPE composites to form effective Al-SPs/GPs “thermally conductive islands” to improve the thermal conductivity of the GPs/ Al-SPs/HDPE composites, but the tensile strength decreased. When GPs, CF and Al-SPs were combined in GPs/HDPE composites, due to the synergistic enhancement, the thermal conductivity, elongation at break and tensile strength of GPs/CF/Al-SPs/HDPE composites prepared with 9 wt% of GPs, 3 wt% of CF and 3 wt% of Al-SPs have the optimal comprehensive performance and meet the commercial requirements of the geothermal heat pump pipes.

Figure 12

SEM images of GPs/Al-SPs/CF/HDPE (9 wt%/3 wt%/3 wt%/85 wt%) composites