Choosing the right material

What is the best material for a cable sheath – PUR or PVC? Is stainless steel a better material for connector housings than cast zinc or plastic? These questions can only be answered on a case-by-case basis, as each material has its own strengths and weaknesses. Unfortunately, there are no simple rules when it comes to choosing the right materials for connectivity solutions, as the specific application always plays a decisive role. Furthermore, any connection solution is only ever as strong as its weakest link. This is often an unsuitable material or accessory where the manufacturer did not pay enough attention to material properties for seemingly unimportant C-parts.

Requirements can vary greatly from one sector to another. In the food and beverage industry, for example, hygiene is the number-one priority when processing and packaging perishable goods. This calls for materials that can be easily washed down and that do not lose their functionality when they are treated with hot steam and aggressive cleaning agents. The rail industry, on the other hand, requires cables that can be safely routed inside passenger carriages. Here, they must meet strict fire protection standards. As you can see, the demands on the cables are completely different, calling for different materials to be used. These differences are also reflected in connectors, cable glands, seals and other accessories.

To find out where exactly these differences lie in the cables, we need to look at the most important cable components, starting from inside at the conductor and insulator and then working outwards to the sheath. The conductor is often made from bare copper, although tin- or silver-plated conductors can be a more sensible choice depending on the application and environmental influences. Tin plating protects the copper against a type of corrosion called tarnishing, helping to preserve its high conductivity. Silver-plated conductors offer the same advantage, and are often used in high-temperature applications.

The insulation – Not just for protecting against short circuits

Conductors require electrical insulation. In single-core wires, this insulation acts as touch protection. In multi-core wires, it prevents current flowing from one conductor to another and causing a short circuit. You may have already experienced what happens when this insulation fails. When putting away old hair dryers or irons, for example, we often wrap the power cord around the handle. After being wrapped and twisted like this thousands of times, the sheath breaks followed by the insulation, usually at the point where the cable enters the housing. A flash of electricity then shoots out of the cable and the lights in the house go out with a loud bang after the fuse hopefully trips. If unsuitable cables are used in industrial applications, not only can these incidents happen more frequently, but the consequences can be severe, such as hours of production downtime in a factory.

Insulation must do more than just provide protection, however. In data transfer cables like ETHERLINE®, the insulation can determine the transmission quality or, more precisely, the signal losses during transmission. This is because the electrical signal interacts with the plastic and gives off energy. This causes the plastic to heat up slightly and the signal weakens until it causes dropouts. The most important factor here is the dielectric constant of the plastic, which should be as low as possible. Polyethylene and polypropylene are plastics that offer a low dielectric constant. This value can be reduced further by using nitrogen to turn the plastic into a foam when it is extruded onto the conductor. Lapp subsidiary CEAM Cavi Speciali, based in the Italian town of Monselice, produces its high-quality data cables in a complex process. Here, up to three layers are applied simultaneously from three extruders, while the middle layer is foamed. These cables enable high data transfer rates across long distances, and are also slightly thinner than conventional cables. This is because the foam layer offers better insulation properties, and can therefore be thinner.

Fire protection – Beware halogens

The behaviour in case of a fire is another important aspect for cables. Flame-retardant insulation material is required wherever there is a risk of fire, in order to meet the fire protection classes of the European Construction Products Regulation (CPR). The easiest way to achieve good fire protection is by mixing the plastic with substances containing halogens. These are elements of the seventh group in the periodic table, often brominated compounds. This fire protection strategy is highly efficient as it requires few additives, thus maintaining the mechanical properties of the material. These cables are therefore often used by car manufacturers in the engine compartment. In public areas like buses, however, halogen has a major disadvantage. In the event of a fire, halogen creates noxious smoke that mixes with extinguishing water to form corrosive vapours.

HFFR plastics (halogen free flame retardant) are a non-toxic alternative, but they require a filling level of sometimes over 60 percent, which can significantly affect the mechanical properties of the plastic. One new trend is the use of so-called synergistic systems. These combine two substances that together provide better flame protection than either individual material would alone. One possible combination is halogen-free aluminium trihydrate and silane compounds. When it comes into contact with fire, aluminium trihydrate reacts to form aluminium oxide and water. This endothermic reaction draws energy from the fire. A crust of combusted material also forms and acts as a protective barrier.

Another factor to bear in mind is that flame retardants are hydrophilic, meaning they have the unwanted characteristic of attracting water. In a worst-case scenario, this can cause an electric breakdown. This is not a problem for cables that do not come into contact with water. But in some sectors like the food industry, where hot water is used for cleaning, different materials are needed. Flame retardants coated with silane compounds are more suitable here. These make the plastic hydrophobic, i.e. water repellent.

Cable sheath – Armed against the elements

The sheath is subject to even tougher demands than the insulation, as it is directly exposed to environmental influences. It must withstand abrasion, chemicals, cleaning agents, UV light, temperature and much more. Unfortunately, there is no one material that meets all requirements. The material must be tailored precisely to the respective purpose. Depending on the application, cables must withstand lubrication oil, greases and cleaning agents to name just a few. The mechanical engineering sector uses tried-and-tested cables with sheaths made from polyvinyl chloride or polyurethane (PUR). PUR is the workhorse of sheath materials. It offers some of the strongest chemical bonds available. It is difficult to process, however, when producing both the cable and assemblies, as the sheath does not cut easily. PUR is also flammable and expensive. The cable types ÖLFLEX® 408P and ÖLFLEX® 409P find a compromise that combines the toughness of PUR with the easy processing of PVC. These cables feature a PUR outer sheath and a interstice filler functional layer made from PVC.

Cables used outdoors are exposed to the sun, and require a different combination of materials. In this case, the sheath must contain UV stabilisers. In solar cables (such as ÖLFLEX® SOLAR), soot is added to the mix to block sunlight, hence the fact that these cables are usually black. The ideal solution for outdoor cables is radiation crosslinking. Here, the cable is bombarded with electron beams. The plastic molecules absorb the energy from the electrons and become interlaced, making the material much more resistant. This allows the cables to withstand extreme temperature swings from minus 40 to 120 degrees Celsius, as well as high mechanical loads. This mechanical resistance is also the reason why crosslinking is also used for cables in the rail industry, such as ÖLFLEX® TRAIN. Several materials are suitable for crosslinking, including polyethylene (PE), polyolefin elastomers (POE), ethylene vinyl acetate (EVA) or ethylene ethyl acrylate (EEA). Additives are generally also added in crosslinking, usually around one percent, in order to improve the bonds between the molecule chains. They also reduce the amount of energy required in the crosslinking process.

Unlike unlinked material, which eventually softens, crosslinked material has no melting point. When heated, it oxidises and becomes brittle, which makes it necessary to add antioxidants and stabilisers. Crosslinking offers no advantage at very low temperatures, as the material will inevitably become brittle at some point. This makes it all the more important to select a base polymer that is suitable for low temperatures. Possible materials include polyolefin elastomers (POE), linear low-density polyethylene (LLDPE), certain kinds of ethylene vinyl acetate copolymers (EVA) or thermoplastic elastomers (TPE). If the application calls for tougher mechanical properties, suitable materials include high-density polyethylene (HDPE) or polypropylene (PP, for greater strength), as well as polyolefin elastomers (for greater elasticity).

Cables for the foodstuffs industry – The battle against bacteria

In the foodstuffs industry, the top priority is resistance against biological influences like microbes and fungi. In cheese factories, the bacteria that help the cheese ripen can corrode a conventional cable in a matter of months, leading to short circuits. Sheath materials made from special TPE, such as those used on Lapp’s ROBUST cables, repel bacteria and are easy to clean. The secret of Lapp’s special thermoplastic elastomer is the smooth surface. This is achieved with a smart mixture of additives that fill microscopic gaps in the material and that stay bonded in the plastic matrix even after heavy cleaning with a steam jet. The combination of strong substances with flexible polymer chains in between gives the mixture rubber-like properties, while being as easy to process as thermoplastics. Some suppliers offer PUR cables for use in the foodstuffs industry that offer extremely high mechanical strength. But PUR is hydrophilic, meaning that it attracts water. Special TPE, by contrast, is hydrophobic.

All of these mixtures, however, are powerless against larger lifeforms like rodents. That is why underground cables are protected with flavourings like vanillin or are specially reinforced. Alongside steel reinforcement, other materials are available that splinter when bitten and spoil the hungry animals’ appetite. When it comes to the devastation that martens can cause, certain materials that do not take on odours may provide a solution. Scientists believe that the reason why martens attack cables has to do with battles over territory. Martens mark their territory with their droppings, urine and sweat from glands in their paws. Their rivals try to destroy these markings. A cable that does not take on odours cannot be marked and is therefore not attacked.

Rust-free connections – Stainless steel the material of choice

From the cable to the gland to the connector, from plastic to metal. Here, the answer seems clear: stainless steel is the material of choice when connector housings or cable and hose glands need to withstand chemicals or cleaning agents. In the foodstuffs industry, stainless steel is often essential. It does not rust and there is no coating that could eventually flake off. But the situation is not as easy as it appears, as there are different types of stainless steel. Conventional V2A stainless steel is relatively low-cost but is often not sufficiently robust when it comes to chemical resistance. Stains can appear on the metal when it is immersed in substances containing chlorides. The foodstuffs industry often uses hypochlorous acid that disintegrates into hydrochloric acid and kills organic substances. V2A stainless steel is not suitable here. V4A stainless steel offers a tougher alloy and is also used on expensive Swiss watches. It is extremely hard and withstands impacts and cleaning with coarse brushes.

As ever in life, however, for every advantage there is a disadvantage. Stainless steel is harder than brass or standard steel, and is therefore more difficult to process. This is especially true of V4A due to the alloy elements chromium, nickel and molybdenum. If its surface is left untreated, V4A is rougher, leading to higher abrasion. Screws that have to withstand high forces across their thread would therefore be stuck. This is why Lapp gives its products made from V4A stainless steel (such as the EHEDG-certified SKINTOP® HYGIENIC cable gland) a special surface treatment that reduces abrasion and makes it easy to tighten and release the cable gland.

Stainless steel cannot be used everywhere. One example of this is in rectangular connectors. Stainless steel is unsuitable here because the metal is too hard to be processed. The connector would have to be milled from a complete block, which would be far too expensive for customers. Lapp therefore found a different solution for its EPIC® ULTRA. The housing on this rectangular connector is made from nickel-plated cast zinc. This material resists corrosion, such as from salt spray on oil platforms or in the foodstuffs industry. For bolts and brackets, however, Lapp recommends using stainless steel. This is because any coating would be quickly worn off by the frequent opening and closing when separating the modules, such as during cleaning. Cast-on bolts are not suitable here, as the clamping forces and therefore the tightness reduces over time due to the low stability of the material, whether plastic or cast aluminium.

But bolts and brackets pose a dilemma for the foodstuffs industry. These components contradict the principles of hygienic design, as the nooks and corners can trap splashes from food processing. Other locking methods are either not secure enough or are not compatible with the market standard. Lapp therefore recommends not using such connectors in the product zone, but rather only in areas that do not come into direct contact with food.

Some companies use plastic housings that offer some resistance against acids and alkaline solutions. But plastic housings present the risk of low dimensional stability under mechanical or environmental influences. This can also lead to leaks, presenting a safety risk. There is a risk of accidents and high follow-up costs for maintenance and service. Plastic is also unsuitable for applications in which electromagnetic compatibility is important. At the very least, they must be coated with metal to protect against interference. In practice, the results in terms of screening are often disappointing.

Sealing – Rubber vs. silicon

Wherever metal meets metal, such as a connector and a control cabinet, there is usually a seal sitting in between. It must have similar temperature and media resistance characteristics to the materials that make up the other components in the connector or gland, otherwise it will become the weak link in the chain. Fluorocarbon elastomers (FKM) are the best solution here, but are expensive. These resist the effects of weather, ageing, ozone and chemicals, and withstand temperatures of up to 200 degrees Celsius. In moderate ambient conditions, ethylene propylene diene monome rubber (EPDM) is a good alternative. FKM does have one small disadvantage. It is not suitable for very cold environments, and should not be used at temperatures under -20 degrees Celsius. Silicon is a better choice for refrigerated warehouses or other very cold or hot environments. On the other hand, it is not suitable for use at very high temperatures such as in furnaces. Just as important as the seal’s material is its design. Seals used in food processing should not have any gaps or crevices where residues could settle. The seals of the SKINTOP® HYGIENIC range, for example, adjust to the shape of the sealed surfaces and join flush without creating any gaps.

For users, finding the right products is not always easy. Many order stainless steel components just to be on the safe side, without knowing the media with which the connectors will come into contact. Alternatively, users dial back their requirements and use improved standard products for their cables and connectors, in the knowledge that these will often need replacing. There is no one right answer here, the best way to proceed is to weigh up the various advantages and disadvantages. The experts at Lapp know their products extremely well. Users should always consult them before potentially making the wrong choice.