Polymeric materials for cables insulation and sheath

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There are many different polymeric materials used today to manufacture the vast array of electric cables available in the marketplace. It is, therefore, important that users have a basic understanding of the various polymeric materials that are used, to assist them in the selection of the correct cable to suit their particular application.

Almost all the materials used to make cables are compounds, not pure polymers, and they are unique to each manufacturer. As such, their characteristics and performance will not be exactly the same or give exactly the same results in tests. 

Important: This guide is simply intended to provide the generic characteristics of the most popular materials; it is not intended to be used for writing detailed contracts or specifications

Types of Polymeric Materials

The most common types of insulation and sheath used are referred to as Thermoplastic; there are, however, a significant number of cables with insulations and sheaths referred to as Thermosetting.

List of Polymeric materials for cables used:

Thermoplastic

A thermoplastic polymer is one that becomes pliable or mouldable at a certain elevated temperature and solidifies upon cooling. Most thermoplastics have a relatively high molecular weight. The polymer chains associate by intermolecular forces, which weaken rapidly with increased temperature, yielding a viscous liquid. In this state, thermoplastics may be reshaped and are typically used to produce parts by various polymer processing techniques such as injection moulding, compression moulding and extrusion. Different thermoplastics require different temperatures to soften to the point where they will flow and become capable of being extruded or moulded. Most thermoplastics require extrusion and moulding temperatures in the region of 110oC to 180oC. Once cooled, the material reverts to its normal tough character hence the term thermoplastic.

Typical thermoplastic polymers used in cable compounds include PVC, Polyethylene, Nylon and Polyurethane.

Thermoplastics differ from thermosetting polymers, which form irreversible chemical bonds during the curing process. Thermosets do not melt when heated but can decompose and do not reform upon cooling.

Thermosetting:

Thermosetting compounds are characterised by their retention of physical properties over a wide operating temperature range. The polymers used include EPR, Silicone, PCP and CSP. They are all subject to further processing after extrusion, to promote the cross-linking required for them to attain the enhanced properties exhibited by the finished product.

These materials are also known as elastomeric compounds, or more simply known as rubbers. The polymers used do not have the same strong bond between adjacent polymer chains and thus tend to be softer at room temperature and will tend to flow when subjected to prolonged pressure. To provide resistance to deformation and to enhance chemical and physical properties, these polymers are cross-linked. 

Because their properties are permanently fixed following the cross-linking process, these materials are termed thermosets.

Cross-linking, also known as vulcanisation or curing, is a chemical reaction in which adjacent polymer chains are chemically linked together, usually by the application of heat, although irradiation is sometimes used for cables for specialised applications.

The combination of being softer and the fact that after cross-linking, the polymer chains are locked together allows these materials to show elastic properties. When stretched, the polymer chains try to slide over one another, but the cross-links prevent this.

When the tension is released, the polymer returns to its original shape, which gives it elastic or elastomeric characteristics.

In general, the term Rubber and Elastomer can be considered to have the same meaning.

Compound or Polymer

Regardless of being thermoplastic or thermosetting, the most common materials used in the manufacture of cables are compounds, not pure polymers. A typical compound will contain

• Polymer
• Fillers &#; added to improve electrical or physical properties
• Plasticiser &#; added to enhance flexibility, particularly at cold temperatures
• Antioxidants &#; added to prevent premature heat ageing
• Antiozonants &#; added to prevent attack by ozone
• UV stabilisers &#; added to prevent degradation by UV
• Colour &#; pigments added to obtain the desired colour

And in the case of elastomeric and thermosetting compounds 

Curing agents &#; added to promote cross-linking

Accelerators &#; added to speed up the cross-linking process

The level of additives incorporated into a compound can vary greatly depending on the polymer and the application. For some elastomeric sheathing compounds, the polymer content may be as low as 30%, whereas, for MV and HV XLPE insulation, the polymer content could be as high as 97%.

Although in common with almost all cable makers, Tratos purchase the specialised insulations that they use to produce their HV and EHV cables, Tratos have developed and compound in-house all of the other insulation and sheathing compounds that they use in the manufacture of their cables, this allows them to optimise the properties required for the particular application or market.

Insulation or sheath

Regardless of the type of polymer, when applied to cables, materials fall into two broad categories of application &#; insulations and sheaths:

The purpose of insulation

The purpose of insulation is simply to:

• separate the conductors from each other and from earth (ground in the USA)
• be completely effective in preventing short circuits from developing in the cable.

In considering the function of insulation, you must remember that the conductors in the cable are at different voltages; the voltages are different from one conductor to the next, and different from the general mass of earth.

Insulation of a cable

Definition: Insulating materials incorporated in a cable with the specific function of withstanding voltage. (IEV 461-02-01)

The purpose of a sheath

Definition: The purpose of a sheath is simply to protect the components within the cable from harmful external influences. 

Examples of external influences include:

• Temperature
• Presence of rain, steam or accumulation of water
• Presence of corrosive, chemical or polluting substances
• Mechanical stresses (such as through holes or sharp edges in metal work)
• Fauna (such as rodents)
• Flora (such as mould)
• Radiation (such as sunlight)

Sheath (Jacket of a cable)

Definition: A non-metallic uniform and continuous layer is applied over an assembly of cores or components to ensure the protection of the cable from external influences. (IEV 461-05-03 Modified)

Information pages

Tratos have produced a series of information pages containing the basic characteristics of the most popular materials available, including: 

• (Cross-linked elastomeric insulating compounds &#; Ethylene polyethylene rubber (EPR), High Modulus Ethylene polyethylene rubber HEPR), Silicone rubber (SiR), Cross-linked Polyethylene (XLPE).
• (Cross-linked elastomeric sheathing compounds &#; Chloroprene rubber (CR or PCP), Chloroprene rubber (CM or CSP), Chlorinated polyethylene (CPE), Ethyl Vinyl Acetate (EVA). Polyurethane Rubber (PUR)
• Thermoplastic insulation compounds &#; Polyethylene (Pe), Polyvinyl Chloride (PVC),
• Thermoplastic sheathing compounds &#; Polyvinyl Chloride (PVC), Medium density polyethylene (MDPE), High-density polyethylene (HDPE), Nylon, and Polykynar.

Finally, remember, if in doubt, ask!

Tratos employ engineers and material technologists with years of experience and industry knowledge, who will be only too pleased to answer your questions regarding our cables and the materials that we use to make them

In the world of cable manufacturing, cable sheathing (also known as cable extrusion and cable jacketing) stands as a crucial process &#; ensuring the durability, safety, and functionality of all cables. 

From retractable cables for data transfer to braided cables for protection against electromagnetic interference, cable sheathing is a constant, yet often unrecognisable, key component. 

Whether it&#;s large-scale production or bespoke custom cables, the application of a cable sheath is a fundamental step. But what is cable sheathing, and how is it done?

Regardless of whether you&#;re a manufacturer, an installer, or a consumer, understanding the intricacies of cable sheathing can help you make informed decisions and appreciate the complexities behind the cables that power our modern world.

Keep reading as we delve into the materials used for cable sheathing, the step-by-step process of sheathing cables, and the myriad benefits this process brings to cable construction, along with its challenges. 

What is cable sheathing?

Cable sheathing is an essential step in the cable manufacturing process, where an outer sheath or covering is extruded onto the cable. 

Cable sheathing isn&#;t just for large-scale, mass-produced cables. All cables, including custom cables, require cable sheathing to pass rigorous testing and quality control during the cable manufacturing process. 

Materials used in cable sheathing

There are typically 3 materials used when cable sheathing; PVC, PE, and LSZH sheaths. The type of sheath chosen will depend on the cable&#;s specifications and intended use. 

1. PVC (Polyvinyl Chloride)

PVC, or Polyvinyl Chloride, cable sheaths are most commonly used in household wiring, construction, and telecommunications. This is because PVC is known for its durability, flexibility, and chemical resistance. 

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2. PE (Polyethylene)

PE, or Polyethylene, cable sheaths are more often found in underground cables and automotive wiring. This is due to their toughness, moisture resistance, and suitability for outdoor environments. 

3. LSZH (Low Smoke Zero Halogen)

LSZH, or Low Smoke Zero Halogen, cable sheaths are much preferred in environments where stringent safety regulations are in place; such as confined spaces and public buildings. This is because it has extremely low toxicity and minimal smoke emission properties during combustion. 

Cable sheathing process

But how are cables sheathed? The cable sheathing process has 7 different steps to it, two being optional &#; depending on the application of the cable being manufactured. These steps are:

Step 1: Material selection

The first step is to choose your cable sheathing material: PVC, PE, or LSZH. Additives can also be added in here, such as plasticisers, stabilisers, flame retardants, and colourants &#; depending on the desired properties of the cable and regulatory requirements. 

Step 2: Preparation of raw materials

Then you need to mix the cable sheathing material and additives. These are combined in precise proportions, using mixers, to achieve uniform dispersion and desired properties, like flexibility and flame resistance.  

Step 3: Extrusion of sheathing

The third step is to apply the cable sheath to the cable. 

Here, the sheath is fed into an extruder, where it is heated and pressured, before being shaped through a specialised die to form the desired cross-sectional shape and dimensions.  Then, the sheath is rapidly cooled, using water or an air cooling system, to solidify the polymer onto the cable. 

Step 4: Cross-linking (if applicable)

One way to enhance the mechanical and thermal properties of your cable is to use the cross-linking method. However, this only applies if using cross-linked polyethylene (XLPE). 

Cross-linking is achieved through chemical methods, using peroxide, or physical methods, using radiation. 

Step 5: Surface treatment

The next necessary step in the cable sheathing process is surface treatment. Here, a discharge treatment is applied to the surface of the sheath. This treatment improves adhesion for the subsequent steps in the process, such as printing and marking.  

Step 6: Printing and marking (optional)

This step is optional, though is usually a part of international and industry standards,l or individual customer requirements (if producing custom cables). 

Using inkjet or laser printing techniques, product information, specifications, and safety markings are printed onto the sheath surface. Colour coding may also be applied here, as per industry standards or customer requirements. 

Step 7: Testing and quality control

Before the cables move to the next stage of the overall cable manufacturing process, they will need to go through testing and quality control. This includes:

• Dimensional inspection: Measuring the outer diameter, thickness, and concentricity of the sheath, to ensure compliance with specifications.
• Visual inspection: Inspecting for surface for defects, uniformity of colour, and adherence to print or marking requirements.
• Physical testing: Conducting tests such as tensile strength, elongation at break, impact resistance, and heat ageing, to verify mechanical and thermal properties.

Benefits of cable sheathing 

Cable sheathing hosts a wide range of benefits to the overall cable construction. These benefits include:

1. Mechanical protection

Cable sheathing provides a cable with mechanical protection. 

This protects the cable from physical damage, caused by abrasion, frictions, and external impact during installation, handling, and use &#; alongside protection from mechanical impact and crushing forces. This helps maintain their structural integrity and longevity. 

2. Environmental resistance

Cable sheaths also give a cable environmental resistance. 

This means that the cable will become resistant to various environmental conditions, depending on the type of sheathing used. Environmental resistance often includes: 

• Moisture barrier: Sheathing prevents water and moisture from getting into the cables, safeguarding them from corrosion and electrical insulation degradation.
• Chemical resistance: Resistant sheathing materials protect the cables from exposure to chemicals, oils, solvents, and other harsh substances present in industrial and outdoor environments &#; which could damage the internal wiring or cause them to erode.
• UV resistance: Certain types of sheathing materials offer UV stabilisation, protecting the cables from degradation and loss of mechanical properties due to sunlight exposure.

3. Electrical insulation

Cable sheathing also enhances the electrical insulation of the cable. 

The materials used in cable sheathing help maintain the cable&#;s electrical integrity and reduce the risks of electrical faults, leakage currents, and short circuits from occurring. This helps maintain consistent electrical performance and prevent interference or signal loss in data transmission cables.

4. Fire safety

Fire safety is another benefit of cable sheathing. 

Fire-retardant cable sheathing materials can inhibit the spread of flames and reduce smoke emissions during a fire, contributing to enhanced safety in buildings, vehicles, and industrial settings. Some sheathing materials, such as LSZH (Low Smoke Zero Halogen), also minimise toxic emissions, making them more suited for confined spaces and public areas with more stringent safety standards.

5. Durability and longevity

Cable sheathing makes cables more durable, and therefore increases their longevity. 

By protecting the cables from external elements and mechanical stress, cable sheathing extends the operational life of cables &#; and therefore reduces maintenance costs and downtime. Cable sheathing also enhances the reliability of electrical and data transmission systems, ensuring consistent performance over time.

6. Flexibility and ease of installation

A further benefit of cable sheathing is that it provides flexibility and ease of installation. 

The flexible materials in cable sheaths makes them easier to handle, bend, and install within various applications &#; including in complex routing and confined spaces. This allows the cables to accommodate installation practices and enables efficient cable management, reducing labour and installation time. 

7. Regulatory compliance

Sheathing cables helps with regulatory compliance. 

The materials used in cable sheathing often comply with industry standards and regulations. This supports all standards concerning electrical safety, environmental impact, and fire resistance &#; ensuring the cables are regulatory compliant and reliable in diverse applications and uses.

8. Customisation and identification

Customisation and identification is another benefit of cable sheathing. 

Cable sheathing, especially within custom cable construction, allows for customisation through colour coding and labelling, aiding in cable identification, maintenance, and troubleshooting. This means that you can mark for product information, specifications, and safety warnings &#; enhancing the overall operational efficiency of the cables and follow stringent safety protocols.

Challenges of cable sheathing

Cable sheathing does, however, come with challenges &#; as with any manufacturing process. These challenges include:

1. Material selection and compatibility

Choosing the correct material and ensuring it is compatible with the cable&#;s intended use can be a challenge with cable sheathing. 

Choosing the right cable sheathing material involves considering factors such as environmental conditions, chemical exposure, mechanical stress, and regulatory requirements. If you select an inappropriate material for the cable&#;s intended use, it can lead to premature degradation, reduced performance, or safety risks.

2. Flexibility vs protection balance

It can be a challenge to find the right balance between flexibility and protection when cable sheathing. 

Whilst flexible sheathing is easier to install and route, it may offer less protection against physical damage compared to more rigid materials. If this is the case, the imbalance could lead to damage to the cable, or safety risks. 

3. Cost considerations

Cost is often a challenge both cable manufacturers and their customers face &#; especially in regard to custom cable construction. 

High-quality cable sheathing materials can be expensive, and therefore impact the overall cost of cable production. Balancing cost-effectiveness with performance and regulatory compliance requirements is crucial for manufacturers and users, but also a challenge. If costs are too high, this can lead to lower customer purchases and reduced profitability. 

4. Environmental impact

Cable sheathing can sometimes have a negative environmental impact. 

Some cable sheathing materials contain chemicals or additives that pose environmental concerns during production, use, and disposal. Addressing environmental impact through sustainable material choices and recycling programs is increasingly important, but can be inaccessible (especially to smaller manufacturing companies), due to the cost and innovation needed.

5. Installation challenges

Installing cables with sheathing can be a challenge in some settings. 

When installing cables with sheathing in complex or tight spaces, can often be difficult and require specialised tools and techniques. Flexibility and ease of handling are critical factors that affect installation efficiency and labour costs &#; and therefore could put users off.

Looking to explore cable sheathing for your custom cables? Contact our expert team today for more information, or find more resources on our blog. 

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