Electrochemical migration

From gluing

Gluing is a very old joining technique. For a long time, nobody really knew why adhesives stick at all. It wasn't until around 130 years ago that chemists and physicists set out to track down this phenomenon.

The reason for this was the start of industrial production, which led to the mass production of goods. And these required increasingly cost-effective and simple packaging options. Stronger adhesives that could be processed more quickly were needed.

The most important task for construction adhesives is to join two parts - called substrates.

The so-called "van der Waals forces" are one of the main reasons why surfaces stick together. In principle, they describe the attractive forces that molecules exert on each other. However, these forces only act over extremely short distances (approx. 0.5 nm). 

Therefore, even polished surfaces do not adhere to each other, because under the microscope they have a rough, jagged surface. They only touch at a few points. This is where the adhesive comes in. It fills the cavities between the two bonding partners, bringing them closer together, so to speak. This allows the van der Waals forces to take effect. This ability to adhere to surfaces is called adhesion. 

With this knowledge, it is now easier to understand why pressure improves adhesion during bonding: the pressure reduces the distances between the molecules of the adhesive and the surface. The bonding forces required for bonding are increased. In addition, the adhesive has the opportunity to "flow" into any unevenness in the bonded surface.

However, adhesion alone (e.g. in the case of honey) does not provide a stable adhesive bond. The internal strength of the adhesive must be added so that the substrates can be permanently bonded together. This property is known as cohesion (e.g. in the case of superglue, which becomes glass-hard).

An optimum combination of adhesion and cohesion are adhesives that are initially liquid (adhesion, wetting of the surface) and then become solid through drying or chemical reaction (cohesion, internal strength).

In general, adhesives basically consist of chain molecules that interlock with the surface of the substrates in a similar way to a zipper. Other properties such as elasticity and strength are also derived from this structure. The interconnected chain molecules form an elastic and strong network.

At the very beginning of the production of most adhesives are the so-called monomers (mono=one; meros = particles) or prepolymers. These are combined through chemical reactions to form polymers (poly=many). This significantly increases the cohesion (internal strength) of the adhesive.

Adhesives on adhesive tapes differ from curing adhesives (construction adhesives, reaction adhesives) in that they adhere permanently. Even after years, adhesive tapes should adhere reliably to the desired surface when removed from the roll and applied. (The fact that this is best achieved with pressure is described by the term PSA = pressure sensitive adhesive).

For adhesive tapes to continue to work even after years, they must have the ability to flow into the surface roughness of substrates over the long term. Adhesion (remember: e.g. honey) must therefore be guaranteed. For this reason, the adhesive must not have too much internal strength (cohesion), as it would then resemble a solid material. 

Adhesive for adhesive tapes is therefore always a compromise, as it has to fulfill two opposing conditions - and therefore does not achieve the final strength of construction adhesives such as two-component adhesives.

Bonding as a joining technique offers the following advantages, among others:

The adhesive creates a flush connection. Unlike with screws or rivets, there are no mechanical peaks. The load on the joint is distributed over a larger area. Due to its elastic properties (e.g. double-sided adhesive tapes), adhesive can also reduce mechanical stresses caused by different expansion coefficients. Another joining technique, welding, places considerable strain on the joining partners due to the temperatures generated during the welding process. Bonding, on the other hand, is usually carried out at room temperature or only at a slightly higher temperature. At the same time, the adhesive can also serve as a seal.

This article will focus on adhesive tapes that are used in electrical engineering (IEC 60454 Technical adhesive tapes; IEC 60085 Classification according to thermal classes). In most cases, the function of the adhesive is that of an assembly aid, e.g. to attach the insulation film to the winding of a coil.

1 Electrical insulation foils and tapes 

(IEC 60674 Insulating films for electrotechnical insulating films; IEC 60454 Electrotechnical adhesive tapes; IEC 60626 Multilayer insulating materials)

A sensible solution to an insulation task in electrical engineering requires knowledge of the basic properties of the usual insulation film. The following is therefore an (incomplete) overview:

1.1 Polyvinyl chloride film (PVC)

Acetate silk, crepe paper

Flexible PVC film is often used for simple insulation tasks with low temperature resistance requirements. Thanks to its suppleness, it clings well to the surface to be insulated. It also has good UV resistance, which makes it suitable for outdoor use.

However, the temperature resistance of even filled PVC tapes is rarely higher than approx. 90°C, which is why PVC is often used as cable insulation, for example, without high temperature loads. As the material is stretchable, the dielectric strength may drop significantly.

In the event of fire, the degradation products produce hydrochloric acid, which can cause considerable damage to buildings. Thanks to its very favorable general properties and low price, PVC continues to be used despite long-standing reservations.

Acetate silk fabric tapes, which can be used up to 120°C, are another inexpensive alternative. Thanks to their good printability, they are ideal as e.g. end bandages for coils with imprints.

Crepe paper is suitable for simple fixing tasks during installation. Thanks to its special structure, it lays well over unevenness. Assemblies that have been subsequently reinforced with impregnating resin (e.g. field coils of electric motors) can thus be prepared for easy assembly. Specially coated crepe paper adhesive tapes are also suitable for covering powder coatings.

1.2 Polyester film (PETF)

(IEC 60454-3-2, polyester adhesive tape)

For use at continuous temperatures of up to 130°C (class B material, IEC 60085), the inexpensive standard "polyester" film is used. This film has very good chemical and physical properties and offers insulation strengths of 5000Vrms at a thickness of just 23µm. Due to its excellent price/performance ratio, the film is used in most common insulation applications today, e.g. intermediate layers in transformer coils.

Equipped with different adhesives, these films can also be used well in conjunction with impregnating resins thanks to their chemical resistance.

Polyester is highly resistant to ageing and tearing. It can be printed on for labeling purposes. Many different colors are possible by dyeing the adhesive.

However, due to the slow embrittlement of the (standard) film under the influence of UV light, permanent outdoor use is only possible to a very limited extent.

1.3 Polyethylene naphthalate (PEN)

PEN films are polyester films with a modified chemical structure. This film has significantly improved heat resistance and can be used as a Class-F material (IEC 60085) up to 155°C continuous operating temperature in motors or transformers. In addition, the film shrinks only half as much as conventional PETF film when exposed to high temperatures and has significantly better chemical resistance. However, the film also has a significantly higher price. Nevertheless, in view of the increased thermal class requirements, it is now considered the more favorable solution compared to polyimides.

1.4 Polyimide film (Kapton®) 

(Polyimide adhesive tape IEC 60454-3-7)

This brown-transparent film combines outstanding properties. So is Kapton® can be used in a range from  -269°C to 360°C (temperature class H = 180°C continuous use and more, IEC 60085). The film is flame-retardant and self-extinguishing (UL 94 V0 or IEC 60674).

The advantageous properties are largely constant over the entire temperature range. 

Kapton® is used in the electrical industry in particular as space-saving insulation with high dielectric strength ( 10KVeff at 50µ film thickness). In combination with a high-performance adhesive (polysiloxane), it can be used at temperatures well above 200°C. Kapton enables insulation systems (DIN EN 61857) for the highest demands in thermal class H (180°C continuous temperature). 

[Nomex®Teflon® and Kapton® are registered trademarks of DuPont]

Due to the high mechanical stability even at very high (e.g. fault) temperatures and the consistently good dielectric strength, a very high energy density can be realized with Kapton insulation. This means that smaller designs are possible than with other insulation materials. In addition, the positive properties are retained for considerably longer (longer service life of the devices). 

Variants of Kapton, for example, are thermally conductive or withstand partial discharge phenomena for considerably longer. There have only been alternatives to polyimide film from DuPont for a few years.

1.5 Polyamide paper (Nomex®)

This material combines similar properties to Kapton®. One major difference is the manufacturing process.

Nomex is made from long fibers by felting, similar to the way paper is made. 

The uncalendered form is relatively loose and therefore very easy to impregnate with impregnating resins. The compacted, calendered Nomex can be used as insulation, similar to a film. 

Thanks to its good cushioning effect, Nomex® is often used as a groove lining in electrical engineering. 

The high temperature resistance (temperature class H = 180°C continuous use and more; IEC 60085) in conjunction with consistent mechanical and electrical values also predestines the material for rod armature sheaths in traction machine construction.

Resistant to resins, transformer oils and a variety of other substances used in electrical engineering, Nomex® is a very good alternative to Kapton® due to its significantly lower price. However, its dielectric strength is noticeably lower due to its similarity to paper. Due to its paper structure (air permeability), non-flammability and high temperature resistance, Nomex® is also used as flame-resistant clothing by fire departments.

There are now also alternatives to Nomex®, e.g. glass fiber/synthetic fiber nonwovens.

1.6 Polyester fleece, strip-coated

This material is produced by felting polyester fibers and is very absorbent for impregnating agents, similar to Nomex paper. If the material is only strip-coated with adhesive, for example, the impregnating agent can penetrate the nonwoven very well and thus create an intimate bond.

Pre-treated in this way, polyester fleece can be used together with the right impregnating agent up to insulation class F=155°C.

The insulating effect without impregnating agent is low.

1.7 Polytetrafluoroethylene film (PTFE, Teflon®)

(PTFE adhesive tape IEC 60454-3-14)

PTFE is a soft, fairly pliable film which, depending on the film thickness, is produced either as a cast film or by peeling it off a solid block of material.

PTFE is extremely resistant to chemicals and temperatures. PTFE also has high corona resistance, making it suitable for use in high-voltage applications.

Due to its repellent properties, PTFE can be used very well where it is important that a surface remains free of adhering dirt (commonly known: frying pan with Teflon coating).

PTFE film, possibly reinforced with glass fibers, is often used as a sliding film (e.g. for the armature in pull magnets). Here, PTFE considerably reduces the frictional resistance. However, pure PTFE cannot be used under permanent pressure, as the material then begins to flow (cold flow). This leads to a reduction in tensile strength due to the reduction in film thickness.

1.8 Glass fabric adhesive tape

(Glass fabric adhesive tape IEC 60454-3-8)

Although not a film in the classic sense, glass fabric is often used as an adhesive tape where insulating films do not have sufficient tear resistance or where a very high reserve is required in the event of overheating. Although glass fabric has a relatively low insulating effect, it can be used as an excellent assembly aid when bundling cables or conductor rods in large motor construction, for example. In addition, glass fabric is printable in a similar way to acetate silk and is easy to impregnate.

1.9 Metal films (copper, aluminum)

These films are naturally extremely temperature-resistant. However, they are rarely used without further treatment. 

A common application for polyester insulated Copper film is the static shield in safety transformers in accordance with EN 61558. The proportion of insulated flat copper strip used as a current winding has also been increasing for years. The advantages are better current carrying capacity, better thermal management and lower reactance.

Generally speaking, adhesive copper foil is used as an EMC shield in electrical engineering. 

Aluminum film serves as a conductive film, either to dissipate electrostatic charges or as a heat conductor. Aluminum foil can also be used as a heat reflector. Another application of aluminum film is as a vapor barrier, as even the thinnest metal films have excellent vapor-blocking properties compared to plastic foils.

2 polypropylene films with flame protection

(Polypropylene adhesive tape IEC 60454-3-12)

Formex® is an inexpensive insulation film with good temperature resistance (up to 115°C). The additional flame protection ensures excellent self-extinguishing properties.

Due to the many different material thicknesses available, the film can be used in particular to form a sufficiently strong, flame-retardant insulation barrier between live and earthed (e.g. housing parts) components in accordance with standards. This is particularly important if the manufacturer of the device wants to supply the US market, which is regulated by UL standards.

2.1 Polyetherimide films

A less frequently used insulation film. Due to the technical disadvantages (soluble in certain solvents) and the relatively high price, the material is rarely used as an insulating film. Polyetherimide is resistant up to 180°C.

There are many other films in use that are more suitable for certain tasks than the examples listed here due to their special properties. These include EPDM, PEEK, PES, PPS, PPSU, polyethylene (available from approx. 2µm film thickness for capacitors) and polycarbonate. Many of the high-performance polymers are extremely resistant to hydrolysis.

3 Laminates

Usually the Electrical insulation films as they are supplied by the manufacturer. In the search for ever more efficient materials, the idea of combining materials came about early on. These laminates then combine the sum of the advantageous properties of the basic materials. 

Laminates are usually produced by permanent bonding with a fully setting adhesive or by means of heat-sealing adhesive.

3.1 Polyester/crepe laminate

The good cushioning effect of the crepe paper is combined here with the high tear resistance and puncture resistance of the polyester. Good impregnant absorption by the paper improves the integration of the laminate into the overall insulation system.

2.2 Polyester/glass fabric laminate

Tear and impact-resistant bandages with a good insulating effect are the main areas of application for this combination. The mechanical protection against bumps and burrs is very good thanks to the glass fabric.

2.3 Nomex/glass fabric laminate

High mechanical strength combined with very good insulation properties and temperature resistance make this laminate ideal for large electrical machines and traction machines. 

CMC Klebetechnik offers many other laminates with different combinations of the films mentioned. It depends on the application which combination is preferred.