Omar Khalid
Plastics are widely used across industries due to their lightweight nature, corrosion resistance, and ease of fabrication. However, their thermal conductivity coefficients play a crucial role in determining their suitability for specific applications, particularly in electronics, thermal insulation, and high-temperature environments. Thermal conductivity measures a material's ability to conduct heat, and plastics typically exhibit low values compared to metals, making them excellent thermal insulators. Advancements in filler technologies and polymer chemistry have led to the development of thermally conductive plastic composites, addressing the challenge of increasing thermal conductivity in plastics. These composites utilize fillers such as silver, copper, carbon fibers, aluminium, and ceramic fillers to enhance heat transfer while maintaining electrical insulation properties. The addition of fillers also influences the abrasive wear properties of plastics. Furthermore, the geometry of the plastic part affects its thermal conductivity, as observed in 3D printing applications. Overall, understanding and manipulating the thermal conductivity of plastics are essential for optimizing their performance in various industrial contexts.
| Characteristics | Values |
|---|---|
| Thermal conductivity of plastics | 0.1 to 0.5 W/m·K |
| Thermal conductivity of specialty-engineered polymers | .>0.5 W/m·K |
| Thermal conductivity of amorphous thermoplastics at 0-200°C | 0.125-0.2 W/mK |
| Thermal conductivity of semi-crystalline thermoplastics at 0-200°C | >0.2 W/mK |
| Fillers to increase thermal conductivity | Silver, copper, CNTs, aluminium-hydro-silicate, natural zeolite, carbon fibres, metallic fillers, aluminium oxide, aluminium nitride, boron nitride, boron carbide, silicon carbide, Halloysite nanoclay nanofillers, carbon nanotubes |
| Other methods to increase thermal conductivity | Increasing inter-particle bonding, increasing filler content |
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What You'll Learn
Using fillers like silver, copper, aluminium, CNTs, etc
Plastics are widely used across industries due to their lightweight nature, corrosion resistance, and ease of fabrication. However, they have poor thermal conductivity, which can be a problem in certain applications. The thermal conductivity of plastics can be increased by using fillers such as silver, copper, aluminium, CNTs, etc.
Silver, copper, and aluminium are metallic fillers that can be added to plastics to increase their thermal conductivity. These metals have good thermal conductivity and can improve the heat transfer properties of the plastic. For example, copper is a well-known, highly conductive metal that has been used in engineering applications for its impressive thermal and electrical conductivity. Silver and aluminium are also effective in increasing the thermal conductivity of plastics.
Carbon nanotubes (CNTs) are another type of filler that can be used to increase the thermal conductivity of plastics. CNTs have exceptionally high thermal conductivity due to their rigid carbon-carbon bonds. They can form an interconnecting network within the plastic, providing a pathway for heat conduction. The use of CNTs can result in a sharp increase in the thermal conductivity of the plastic composite.
When using fillers to increase the thermal conductivity of plastics, it is important to consider not only the thermal conductivity of the filler but also the volume fraction and dispersion. There must be enough filler present to create continuous thermal conduction pathways or networks in the plastic composite. Additionally, the existence of a polymer phase can result in high thermal resistance at the interfaces between the fillers and the plastic, affecting the overall thermal conductivity.
By using fillers like silver, copper, aluminium, CNTs, and others, the thermal conductivity of plastics can be significantly improved. This allows plastics to be used in a wider range of applications, especially in thermal management and electronics, where heat dissipation is a critical factor.
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Adding Halloysite nanoclay nanofillers to the resin
Adding Halloysite nanoclay nanofillers to resin is a promising method to increase the thermal conductivity of plastics. This process involves incorporating Halloysite nanoclay particles into a photopolymer resin, which is then cured and characterised for various properties, including thermal conductivity. Initial trials have shown that loading the resin with 3 wt% of Halloysite nanoclay significantly increases its thermal conductivity to 0.721 w/mk, compared to 0.681 w/mk for the neat resin. This increase in thermal conductivity is attributed to the formation of conductive paths created by particle agglomerates in the interfacial surface of the resin and nanofiller composite.
The use of Halloysite nanoclay nanofillers offers a potential solution to enhance the thermal management of plastic moulds used in injection moulding. Injection moulding is a widely employed process where molten polymer is forced into a cooled metal mould, resulting in the polymer solidifying into the desired shape. However, the high costs associated with metal moulds have driven interest in alternative methods, such as 3D printing, for producing the tools utilised in injection moulding.
While 3D printing provides flexibility, the inserts produced through this method have demonstrated susceptibility to wear and breakdown due to the demanding conditions of injection moulding. By incorporating Halloysite nanoclay nanofillers into the resin used in 3D printing, it becomes possible to create reinforced thermally conductive polymer resins. This innovation addresses the challenge of effectively managing the heat generated during the injection moulding process, thereby reducing differential warping in the final product.
It is important to note that while Halloysite nanoclay nanofillers show promising results in enhancing thermal conductivity, further research and development are necessary before these materials can become a viable alternative to metal tooling in injection moulding applications. Additionally, the incorporation of nanofillers may lead to trade-offs in other material properties, such as a reduction in compression strength, which should be carefully considered based on the specific requirements of the application. Nonetheless, the ability to increase the thermal conductivity of plastics through nanofillers like Halloysite nanoclay presents exciting opportunities for improving the performance and versatility of plastics in various industries.
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Increasing the volume fraction and dispersion of fillers
Fillers such as silver, copper, CNTs, aluminium oxide (Al2O3), aluminium nitride (AlN), boron nitride (BN), boron carbide (B4C), and silicon carbide can cause a sharp increase in the thermal conductivity of polymers. The increase in thermal conductivity is influenced not just by the use of a single filler but also by the volume fraction and dispersion of the fillers.
The Maxwell equation considers the volume fraction and thermal conductivity of the filler and polymer, assuming that the filler particles are dispersed in a continuous matrix phase and are not interacting with each other. This model is valid only for low filler volume, under 25%. The Lewis-Nielsen model, on the other hand, takes into account the shape and orientation of the filler particles. The Halpin-Tsai model also describes the relationship between filler loading and thermal conductivity.
The thermal conductivity of composites increases with the increase in filler fraction. When the filler content is less than 50% by weight, the thermal conductivity of all fillers increases slowly. At 50% by weight, the percentage by volume of fillers varies. For example, in one experiment, the filler volume content of all PA6 composites was about 50% except for Al2O3, which was 43%. With a large amount of the second phase, a thermal conduction pathway is assumed to form. However, a gap between the filler and the matrix impairs thermal conductivity.
The dispersion state of the fillers also plays a crucial role in enhancing thermal conductivity. Fillers such as zeolite, Al2O3, Fritt1, and Fritt2 exhibited relatively good dispersion into the PA6 phase. The Lewis-Nielsen model and the additive model have been used to predict the thermal conductive enhancement factor for different fillers.
In summary, increasing the volume fraction of fillers and improving their dispersion can effectively enhance the thermal conductivity of plastics. Various models, such as the Maxwell equation, Lewis-Nielsen model, and Halpin-Tsai model, can be used to understand and predict the relationship between filler loading and thermal conductivity.
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Coating Polyamide 12 with CNT (carbon nanotubes)
Polyamide 12 (PA12) is a thermoplastic engineering polymer known for its toughness, tensile strength, impact strength, and ability to flex without easily fracturing. The thermal conductivity of plastics can be increased by adding fillers such as silver, copper, and CNTs. CNTs, or carbon nanotubes, are nano-allotropes of carbon with graphitic characteristics.
Coating Polyamide 12 with CNTs can be achieved through a process called laser sintering. This involves coating the CNTs onto the surface of PA12 powder particles, resulting in a PA12-CNT nanocomposite powder. The CNTs are typically multiwalled carbon nanotubes (MWCNTs) with an average diameter of 10 nm and a length of 1.5 µm. The loading of CNTs can vary, with some studies using 0.1 wt %, while others use higher loadings of 2.5, 5.0, and 10.0 wt %.
The PA12-CNT nanocomposite powder exhibits near-spherical morphology, with the CNTs uniformly coated on the surface. This powder is then laser sintered to create a solid object. The laser sintering process involves heat absorption, heat transfer, and phase change. Physical experiments and three-dimensional simulation models have shown that the addition of CNTs into the PA12 matrix increases thermal conductivity.
The increased thermal conductivity of the PA12-CNT composite is due to the greater heat conduction of the CNTs, which allows the laser heat to be conducted wider and deeper into the material. This reduces the interspace between two successive layers, increasing the parts' density and enhancing their mechanical properties. The flexural, impact, and tensile properties of the PA12-CNT composite are improved compared to neat PA12. Additionally, the electrical conductivity of the composite is anisotropic in the through-layer and cross-layer printing directions.
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Utilising ceramic fillers, aluminium-hydro-silicate, and their mixtures
Plastics are widely used across industries due to their lightweight nature, corrosion resistance, and ease of fabrication. However, they have poor thermal conductivity, which is a crucial factor in determining their suitability for specific applications, especially in electronics and high-temperature environments.
To address this issue, researchers have explored the use of ceramic fillers, aluminium-hydro-silicate, and their mixtures to enhance the thermal conductivity of plastics. One example of a ceramic filler is natural zeolite, a crystalline aluminium-hydro-silicate powder that contains smectites, which are clay minerals. Natural zeolite is commercially available and has been utilised in experiments to improve the thermal conductivity of polymers.
In one study, natural zeolite powder was used as a filler in a polymer matrix at proportions of 50 and 70% by weight. The results showed that the thermal conductive enhancement factor was highest when the polymer composite contained 70% natural zeolite. Additionally, a hybrid filler-containing composite was created by mixing PA6/70 wt% MgO and PA6/80 wt% zeolite, which also exhibited a high thermal conductive enhancement factor.
Another experiment used epoxy molding compounds (EMCs) with aluminosilicate (AlS) and aluminum oxide (AlO) as fillers to improve the thermal conductivity of composites. The fillers were combined with a polymer matrix using a twin-screw extruder and injection molding. The results indicated that the thermal conductivity of the composites increased significantly when the filler content exceeded 50% by volume.
By utilising ceramic fillers, aluminium-hydro-silicate, and their mixtures, the thermal conductivity of plastics can be enhanced. These fillers provide new opportunities for developing thermally conductive composites that are more economical and versatile for various applications. However, it is important to consider the volume fraction and dispersion of fillers to ensure effective thermal conduction pathways and maintain the desired physical characteristics of the plastic.
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Frequently asked questions
Thermal conductivity is the transfer of heat from one body to another body in contact with the first. It is measured as W/mK. Plastics typically have poor thermal conductivity, which makes them excellent thermal insulators.
Thermally conductive fillers, carbon fibres, or metallic fillers such as copper, silver, and aluminium can increase the thermal conductivity of plastics. Fillers like CNTs (carbon nanotubes) and natural zeolite can also be used.
The C-Therm Trident Thermal Conductivity Instrument is a fast and easy way to measure the thermal conductivity of plastics. The Transient Plane Source (TPS) and Modified Transient Plane Source (MTPS) can also measure the thermal conductivity of polymers under a variety of environmental conditions.
