Mastering electrical conductivity of plastics

Plastics are recognized for their insulating properties. However, plastics have undergone a transformative evolution and challenged preconceptions. They have emerged as promising candidates for applications demanding electrical conductivity.

Plastic conductivity unravels the advancements that have propelled plastics in many areas. From insulators to conductors, opening new possibilities in electronics, sensing technologies, and beyond. The electrically conducting polymers are classified into two main categories:

 

Intrinsic conductivity

Intrinsic conductivity refers to the inherent ability of certain polymers to conduct electricity without the need for external modifications. This remarkable property arises from the unique electronic structure of conjugated polymers.

Conjugated systems within the polymer chain are characterized by alternating single and double bonds. They create a pathway for the efficient movement of electrons, facilitating electrical conduction.

Examples of intrinsically conductive polymers include:
 

• polyaniline,
• polypyrrole, and
• polythiophene

Extrinsic conductivity

Extrinsic conductivity involves enhancing the electrical properties of polymers through intentional modifications. This is achieved by introducing external elements into the polymer matrix. Such extrinsically conductive materials include:
 

• conductive additives,
• fillers, or
• dopants

The goal is to create pathways for charge carriers to move more freely. This further improves the overall electrical conductivity of the polymer.
 

Several electrically conductive fillers are used in plastics to enhance their electrical properties. The choice of electrically conductive filler depends on factors such as:
 

• the desired level of conductivity,
• mechanical properties,
• cost considerations, and
• the specific requirements of the application

Each filler type has its unique advantages. They may be selected based on the intended use of the conductive plastic composite. Table 1 shows a list of some widely employed electrically conductive fillers along with their pros and cons.

 

Discover the commercially available conductive fillers and fibers available in our database:
 

The concentration of electrically conductive additives has a significant impact on the electrical conductivity of polymers. As the concentration of conductive fillers increases, the electrical conductivity of the composite material generally improves.

However, the relationship is not always linear as shown in Figure 1, and there are several factors to consider.

 

Figure 1. Electrical Conductivity vs. Nanofiller Content¹

Here are some key effects of electrically conductive additive concentration on the electrical conductivity of polymers:

 

Linear increase in conductivity

At lower concentrations of conductive additives, the increase in electrical conductivity may exhibit a relatively linear relationship. This is particularly true in the early stages of filler incorporation, where the additives start forming a conductive network within the polymer matrix.

 

Percolation threshold

The percolation threshold is a critical concentration point at which the conductive fillers form a connected network. This leads to a sharp increase in electrical conductivity. Below this threshold, the conductivity may be very low. And above it, a significant improvement is observed.

The percolation threshold is influenced by several factors of the conductive fillers like:
 

• shape,
• size, and
• aspect ratio

Saturation effect

At higher concentrations, there may be a saturation effect where the increase in electrical conductivity starts to plateau. This saturation is due to factors like:
 

• filler-filler interactions,
• agglomeration, and
• limitations in the formation of an efficient conductive network

The electrical conductivity of some polymers with conductive additives may be temperature-dependent. Concentration levels may need to be adjusted based on the intended operating temperature range of the application.

 

Challenges you may face while increasing the additive concentration

Increasing the concentration of conductive fillers can sometimes lead to:
 

• processing challenges, affecting the ease of manufacturing and the choice of processing methods
• compromise the mechanical properties of the polymer
• significantly increase material costs

Balancing electrical conductivity with mechanical strength is crucial. It ensures the overall performance of the composite material. Thus, the ideal concentration of conductive additives depends on the specific requirements of the application.

For some applications, a low to moderate concentration may be sufficient. While others may demand higher concentrations to meet stringent conductivity standards.

 

Electrically conductive fillers are available in many different forms. Each form can have different electrical conductivity. Several forms include:
 

• powders,
• flakes,
• spheres, etc.

Major factors that affect the conductivity of composite are properties of filler such as form, particle size, morphology, and aspect ratio. Let's discuss some of these parameters in detail.

 

Aspect ratio

Aspect ratio refers to the ratio of the length of a conductive filler to its diameter or width. It determines the effectiveness of the filler in improving electrical conductivity. The aspect ratio affects the pathways available for electron transport within the polymer matrix. Key considerations include:

 

High aspect ratio

Fillers with high aspect ratios provide elongated conductive pathways. Key features of high aspect ratio fillers include:
 

• promote efficient electron transport and enhance electrical conductivity.
• employed in applications where maximizing electrical conductivity is a primary goal. For example, in sensors, electronic devices, and conductive coatings.

Examples of high aspect ratio fillers are carbon nanotubes or nanofibers.

 

Low aspect ratio

Fillers with lower aspect ratios include spherical nanoparticles or short fibers.
 

• They may offer advantages in terms of ease of dispersion and processing.
• They contribute to electrical conductivity while maintaining other desirable properties.

Polymer composites with low aspect ratio fillers are used in applications where a balance between electrical performance and processing considerations is crucial.

 

Type of alignment

Random alignment

Randomly dispersed conductive fillers contribute to overall electrical conductivity. However, they may not provide preferential conductivity in specific directions. For example:
 

• in applications where isotropic conductivity is acceptable, or
• in situations where achieving a specific alignment is challenging

Oriented alignment

Controlled alignment of conductive fillers along specific directions enhances anisotropic conductivity. This is particularly helpful in applications requiring directional electrical properties. For example, electronic components, conductive films, and applications where conductivity in specific directions is critical.

 

3D network formation

Creating a 3D network of conductive fillers throughout the polymer matrix is essential. It maximizes the number of conductive pathways. Examples of applications where efficient 3D electron transport is essential include:
 

• batteries,
• supercapacitors, and
• other energy storage devices

Understanding the above factors allows tailoring electrical properties to meet specific application requirements.

 

Processing methods

Several processing methods are employed for the preparation of conductive polymer composites. Each method offers unique advantages and characteristics. The choice of method depends on factors such as:
 

• the type of polymer,
• the conductive filler,
• the desired properties of the composite, and
• the intended application

Figure 2 shows some of the common processing methods for the preparation of conductive polymer composites.

 

Figure 2. Common Conducting Polymer Composites Preparation³

Melt compounding

Conductive fillers are directly mixed with the molten polymer. This is done using equipment like twin-screw extruders or internal mixers. Key factors to take into consideration in this method are:
 

• filler distribution,
• potential for filler agglomeration, and
• compatibility of the filler with the polymer melt

Solution compounding

The polymer is dissolved in a suitable solvent, and conductive fillers are dispersed in the polymer solution. The solution is then cast into a mold, and the solvent is evaporated to form the composite. It is important to consider the following:
 

• solvent selection,
• potential for residual solvent, and
• control over film thickness

In-situ polymerization
 

Polymerization occurs in the presence of conductive fillers or nanoparticles, resulting in the formation of the composite. Factors that need to be taken into consideration include:
 

• control over polymerization conditions
• potential for side reactions, and
• optimization of filler loading

This method is often used with conductive polymers like polyaniline or polypyrrole.


Find out other processing methods used in manufacturing conductive polymer composites:
 

Processing conditions

The processing conditions of electrically conductive polymers include the following factors:

 

Processing temperature

Higher temperatures generally lead to reduced viscosity. This makes it easier to disperse and orient conductive fillers within the polymer.
 

• This is particularly important for achieving a homogeneous distribution throughout the polymer matrix.
• Some conductive fillers may have temperature limitations due to degradation or phase changes.

Optimal processing temperatures must balance the need for good dispersion with the thermal stability of both the polymer and the fillers.

 

Shear forces

Shear forces help break down filler agglomerates. This promotes a more even distribution within the polymer matrix. This is crucial for preventing localized regions of high filler concentration, which could negatively impact the mechanical and electrical properties.

Depending on the processing method, the shear forces can be controlled to achieve the desired orientation of conductive fillers. This is particularly relevant in applications where anisotropic electrical conductivity is advantageous.

 

Processing time

Longer mixing times generally contribute to improved dispersion of conductive fillers. Adequate mixing time ensures that fillers are evenly distributed throughout the polymer matrix. This thereby reduces the likelihood of agglomeration. It is essential to avoid excessive heat generation, potential filler degradation, or undesired changes in polymer properties.

Optimizing processing conditions involves a delicate balance of the above-mentioned factors. Tailoring them ensures a uniform distribution and controlled orientation of conductive fillers. This ultimately influences the electrical conductivity and performance of the resulting composite material. Experiments, modeling, and careful process control are essential to achieving the desired properties in conductive polymer applications.

 

Tuning the electrical conductivity of plastics involves exploring synergies between conductive additives and hybrid systems. The combination of different filler types:
 

• reduces cost and
• balances the conductivity mechanism of polymer composites

Such an approach allows for synergistic effects. It leverages the unique properties of each filler to achieve a desired level of conductivity.

The factors that optimize the electrical and mechanical properties of the resulting hybrid system are:
 

• choice of filler combination,
• polymer matrix, and
• processing conditions

Some examples of hybrid systems for tuning conductivity in plastics are as follows:

 

Carbon-based hybrid systems
 

Combining different forms of carbon-based fillers can create hybrid systems. Carbon-based fillers include:
 

• carbon black,
• carbon nanotubes,
• graphene, and
• carbon fibers

Each carbon form contributes to unique electrical and mechanical properties. This tailors the conductivity and reinforcement in the polymer matrix as shown in Figure 3.

 

Figure 3. Schematics of the Microstructure of the Composites and the Forming of Conductive Paths in the Hybrid Composites⁴

Carbon nanotubes (CNTs) and graphene hybrid

CNTs provide a high aspect ratio and effective electron transport pathways. Graphene contributes to enhanced surface area and additional conductive paths.

 

Silver nanoparticles and carbon black hybrid

Silver nanoparticles offer high conductivity. Carbon black enhances the mechanical properties and provides additional conductive pathways.

 

Copper nanowires and polyaniline (PANI) hybrid

Copper nanowires offer a unique one-dimensional conductivity. PANI contributes to additional conductive paths and compatibility with the polymer matrix.

 

Carbon fibers and silver nanoparticles hybrid

Integrating carbon fibers and silver nanoparticles in a polymer matrix creates a hybrid system. The system offers enhanced electrical conductivity and mechanical strength.
 

• Carbon fibers provide reinforcement
• Silver nanoparticles contribute to high conductivity

Graphene oxide (GO) and polypyrrole (PPy) hybrid

The combination of GO and PPy results in a hybrid system with improved electrical conductivity and flexibility.
 

• Graphene oxide enhances the surface area and stability.
• Polypyrrole provides additional conductivity and compatibility with the polymer.

MWCNTs and nickel nanoparticles hybrid

MWCNTs offer excellent conductivity. Nickel nanoparticles contribute to additional conductive paths and magnetic properties.

 

Zinc oxide (ZnO) nanoparticles and carbon nanofibers hybrid
 

The hybrid system of ZnO nanoparticles and carbon nanofibers combines the benefits of:
 

• piezoelectricity from ZnO
• electrical conductivity of carbon nanofibers.

This is advantageous for applications requiring both electrical conductivity and sensing capabilities.

 

When incorporating electrically conductive filler into plastics, making trade-offs is a common practice. Making trade-offs in plastics requires:
 

• a comprehensive understanding of the specific requirements of the application
• an optimal balance of electrical conductivity, cost, processability, and mechanical properties

Achieving the desired performance often involves finding a compromise among these conflicting requirements. Here are some strategies for making trade-offs in plastics with electrically conductive composites:

 

Conductivity vs. cost
 

Consider the cost implications of different conductive fillers. While materials like silver nanoparticles offer high conductivity, they can be expensive. Cost-effective alternatives such as carbon-based fillers or hybrid systems balance performance and cost.

 

Conductivity vs. processability
 

Evaluate the impact of conductive fillers on the processability of the polymer. Some highly conductive fillers, like metallic nanoparticles, may pose challenges during processing. This is due to issues such as agglomeration. Use appropriate fillers that maintain:
 

• good dispersion and
• do not compromise the processing characteristics of the polymer

Conductivity vs. mechanical properties

Achieving high electrical conductivity often involves incorporating a significant amount of conductive fillers. This may impact the mechanical properties of the polymer. Explore hybrid systems that combine conductive fillers with reinforcing materials (e.g., carbon fibers). This combination maintains a balance between electrical conductivity and mechanical strength.

 

Cost vs. mechanical properties
 

Consider the overall cost implications of the conductive composite, including the cost of fillers and any additional processing steps. Evaluate the following:
 

• whether the enhancement in mechanical properties justifies the increased cost, or
• if a more cost-effective solution with acceptable mechanical performance is sufficient

Processability vs. electrical performance

Adjust processing conditions to achieve a balance between ease of processing and the desired electrical performance. For example, optimizing extrusion or injection molding parameters can:
 

• enhance the dispersion of conductive fillers and
• improve electrical conductivity without compromising processability

Conductivity vs. weight/density

Consider the weight implications of incorporating conductive fillers. This is especially done in applications where weight is a critical factor. Evaluate the balance between achieving the required electrical conductivity and minimizing any adverse effects on the overall weight of the plastic component.

 

Conductivity vs. environmental impact

Assess the environmental impact of different conductive fillers and their manufacturing processes. Conductive filler must align with sustainability goals and environmental regulations. They must balance electrical performance with eco-friendly considerations.

 

Conductivity vs. thermal properties
 

Evaluate the impact of conductive fillers on the thermal properties of the polymer. Some fillers may enhance or hinder thermal conductivity, depending on the application. Balance the need for electrical conductivity with thermal requirements in specific use cases.

 

Compatibility of additives
 

Be mindful of potential interactions between conductive fillers and other additives. Some additives may enhance or hinder the performance of conductive fillers. Optimize the formulation to achieve synergies and minimize negative interactions.

 

Regulatory compliance
 

Consider regulatory requirements and standards related to the use of certain conductive materials. This is important, especially in applications such as electronics or medical devices. Balance the need for compliance with the desired electrical performance.
 

Conductive polymer composites find diverse applications across various industries. This is due to their unique electrical and mechanical properties. Here are some notable applications of conductive polymer composites:

 

EMI shielding

Conductive polymer composites are used in electromagnetic interference (EMI) shielding applications. They protect electronic devices and systems from unwanted electromagnetic radiation. The composites provide a lightweight and flexible alternative to traditional metal-based shielding materials. They also help in maintaining effective EMI protection.

 

Antistatic materials
 

Conductive polymer composites are employed as antistatic materials. The benefits include:
 

• prevent the buildup of static electricity on surfaces
• reduce the risk of electrostatic discharge (ESD) damage to electronic components
• offer controlled electrical conductivity
• ensure the safe handling of sensitive electronic devices

Electronics packaging

Conductive polymer composites are used in electronic packaging. They provide both mechanical support and electrical conductivity. They are utilized as housings, enclosures, or coatings for electronic components. The properties that make these composites suitable for electronic packaging include:
 

• enhanced electrical conductivity,
• lightweight, and
• ability to tailor mechanical properties

Sensors and actuators

Conductive polymer composites serve as sensing materials in the development of various sensors such as:
 

• strain sensors,
• pressure sensors, and
• gas sensors

They can also be employed in actuators for responsive applications. The electrical conductivity of these composites can be modulated in response to external stimuli. This enables their use in diverse sensing and actuation applications.

 

Heating elements

Conductive polymer composites with resistive properties are utilized as flexible and lightweight heating elements in applications such as:
 

• heated clothing,
• automotive seats, and
• de-icing systems

The composites offer a convenient way to generate heat with improved flexibility and formability. This makes them suitable for applications requiring controllable and uniform heating.

 

Smart textiles

Conductive polymer composites are integrated into fabrics to create smart textiles with electronic functionalities. These textiles may include:
 

• sensors,
• wearable electronics, and
• energy storage components

The flexibility and lightweight nature of conductive polymer composites make them well-suited for incorporating electronic functionalities into textiles, paving the way for wearable technology.

 

Flexible printed circuit boards (FPCBs)

Conductive polymer composites are used in the fabrication of flexible printed circuit boards. They offer an alternative to traditional rigid counterparts. These flexible circuits find applications in various electronic devices.

The flexibility and lightweight nature of these composites enable the production of bendable and foldable electronic circuits, expanding the design possibilities for electronic devices.

 

Lightning strike protection
 

Conductive polymer composites are employed in aircraft and aerospace applications to:
 

• help dissipate electrical charges and protect critical components
• offer a lightweight and effective solution for lightning strike protection

They also reduce the potential for damage to aircraft structures.

 

Biosensors
 

Conductive polymer composites can be functionalized for use in biosensors.
 

• They detect biological molecules and facilitate applications in healthcare and diagnostics
• Their electrical conductivity and biocompatibility make them suitable for interfacing with biological systems in biosensing applications

Energy storage devices

Conductive polymer composites are utilized in energy storage devices including:
 

• supercapacitors
• batteries

They contribute to electrical conductivity and structural integrity. Enhanced electrical conductivity and mechanical properties improve the performance and reliability of energy storage devices. This makes these composites valuable in the field of energy storage.