Composite materials consist of two or more materials that remain distinguishable in the finished product and do not dissolve into one another. At least one forms the continuous matrix, the other the reinforcement. Materials science distinguishes four types by the shape of the reinforcement: particle composite, fibre composite, layered composite and interpenetrating composite. GRP, CFRP and aramid composites are fibre composites, sandwich panels and insulation laminates such as NMN are layered composites. Electrical insulation relies mainly on layered composites (NMN, DMD, NKN, mica tape, aluminium-PET films) and fibre composites (laminated paper, glass-epoxy laminate), while CFRP is ruled out as an insulator because carbon fibre conducts at 10⁻³ to 10⁻⁴ Ω·cm. This article draws the line between a composite, an alloy and a material assembly, classifies the four types with property values and shows what changes when a composite is slit or punched: GOBA slits fabrics, prepregs, laminates and composite films in its composite slitting service into widths from 5 to 1,600 mm at plus minus 0.1 mm and punches insulation composites from 0.023 to 3.0 mm thick.
Composite, alloy, material assembly: where the lines run
What separates a composite from an alloy is the interface. In an alloy the metals are mixed at the atomic level, in a composite the constituents survive as separate phases, divided by an interface that shows up under the microscope. Concrete is a composite, brass is not.
That interface also decides whether the composite works as a material at all. The matrix surrounds the reinforcement, holds the material in shape, transfers loads from one reinforcement to the next and protects it from moisture, media and abrasion. The reinforcement carries strength and stiffness, or in insulation work the electrical barrier. The interface itself is the third, often overlooked constituent. If the matrix does not bond to the fibre, or the adhesive between two plies lets go, the composite is just a stack of parts. Anyone assessing a composite therefore checks the bond first, not the data of the constituents.
Composite material, composite and compound material describe the same thing. What needs separating is the material assembly, or hybrid component: manufacturing engineering uses that term for a part in which different materials are joined, such as an aluminium housing with a bonded-in steel bush. A composite is a material, a material assembly is a construction. Composite packaging such as the beverage carton made of board, polyethylene and aluminium belongs to the same term but is a separate field in which GOBA does not work.
The four types of composite materials
The classification follows the geometry of the reinforcement. It also predicts how the material behaves when slit, punched and bent, which is why it is worth knowing in purchasing too.
| Composite type | Structure | General examples | Examples in electrical insulation |
|---|---|---|---|
| Particle composite | particles in a matrix, direction-independent (isotropic) | cemented carbide, concrete, filled plastics | mineral-filled impregnating and potting resins, thermally conductive filled epoxy |
| Fibre composite | short, long or continuous fibres in a matrix, direction-dependent (anisotropic) | GRP, CFRP, aramid fibre composite, reinforced concrete | laminated paper, laminated fabric, glass-epoxy laminate (FR4), mica-glass composite |
| Layered composite | flat plies, bonded, laminated or pressed | plywood, sandwich panel, laminated glass, beverage carton | NMN, DMD, NKN, aluminium-PET film, self-adhesive laminated insulation film |
| Interpenetrating composite | two continuous phases intertwined with each other | tungsten-copper, metal foam filled with polymer | contact materials in switchgear |
The difference between isotropic and anisotropic is the most important one in this table for practical work. A particle composite behaves the same in every direction. A fibre composite is many times stronger along the fibre than across it, and a layered composite has different values in the plane than perpendicular to it. Anyone designing a punched part from a fabric laminate therefore also fixes the fibre direction relative to the part. And anyone creasing an insulation laminate loads exactly the adhesive layer that holds the composite together.
Composite material examples: data for GRP, CFRP and aramid compared
Fibre composites form the largest and technically most important group, and they are what most readers mean by composites. The table sets three fibre composites against steel and aluminium. The right-hand column decides whether a composite can be considered as an insulating material in electrical engineering at all.
| Material | Density | Tensile strength | Modulus | Resistivity of the fibre |
|---|---|---|---|---|
| GRP, quasi-isotropic laminate | 2.1 g/cm³ | 720 MPa | 30,000 MPa | E-glass around 10¹⁵ Ω·cm, insulating |
| CFRP, quasi-isotropic laminate | 1.5 g/cm³ | 900 MPa | 88,000 MPa | carbon HT 10⁻³ to 10⁻⁴ Ω·cm, conductive |
| Aramid fibre composite, fabric laminate | fibre 1.45 g/cm³ | 460 to 540 MPa | 22,000 to 27,000 MPa | aramid around 10¹⁵ Ω·cm, insulating |
| Steel | 7.8 g/cm³ | 1,100 MPa | 210,000 MPa | conductive |
| Duralumin | 2.8 g/cm³ | 350 MPa | 75,000 MPa | conductive |
The values come from the materials data collection of Suter Kunststoffe AG (swiss-composite) and apply to an epoxy matrix, the fabric laminates at 43 percent fibre volume. Fibre composites scatter considerably with fibre content, fabric type and resin, so design work follows the datasheet of the semi-finished product. Breaking length makes the lightweight advantage tangible: a steel cable carries its own weight over 14 km, GRP over 34 km, CFRP over 60 km.
The last column separates two worlds. Carbon fibre conducts electricity, its resistivity sits around 18 orders of magnitude below that of glass fibre. CFRP is therefore a lightweight structural material, not an insulator, and along the fibre it is even a good thermal conductor at 15 to 40 W/(m·K), against 0.8 W/(m·K) for GRP. Glass and aramid fibre sit at 10¹⁵ Ω·cm and are for that reason the reinforcement of choice in insulation composites. In classical composites the matrix is almost always a resin, in other words a thermoset: epoxy withstands 45 to 230 °C depending on the system, polyester resin 50 to 160 °C. Organosheets and thermoplastic UD tapes use a thermoplastic matrix instead, which can be reshaped and welded afterwards. What these bonding types mean in practice is explained in the articles on thermosets and thermoplastics.
The applications of fibre composites range from rotor blades for wind turbines and fuselage and wing structures in aerospace to body parts in automotive lightweight construction and sports equipment. In that chain GOBA works at the semi-finished stage: we slit carbon, glass and aramid fabrics, hybrid fabrics and prepregs in contract slitting for composites to the width that the lay-up fixture, infusion process or tape-laying head requires. The finished component is made at the customer, the width accuracy of the reinforcement is made at our plant.
Composite materials in electrical insulation
An insulating part in a stator slot has to do four things at once: block voltage, survive the edge stress of insertion without tearing, absorb the impregnating resin and hold the thermal class of the winding. No single material manages all of that at 0.25 mm thickness. Film provides the dielectric strength but is fragile at the edge and, as PET, only holds class B. Aramid paper carries class H and protects mechanically but costs too much in the required thickness. Nonwoven soaks up resin but blocks no voltage. That is exactly why most sheet insulation materials above class B are composites.
Layered composites: flexible insulation laminates and composite films
Insulation laminates are layered composites of paper, film and nonwoven, bonded across the full area. The abbreviation names the order of the plies. The depth on each type sits in the articles on laminate composites, DMD laminate and NMN laminate, this is the overview.
| Composite | Structure | Thermal class | Typical thickness | What the composite delivers |
|---|---|---|---|---|
| DMD | PET nonwoven, PET film, PET nonwoven | B to F (130 to 155 °C) | 0.15 to 0.40 mm | lowest-cost composite, absorbent for impregnating resin, film blocks the voltage |
| NMN | Nomex, PET film, Nomex | F (155 °C) | 0.15 to 0.50 mm, Nomex plies 37 to 130 µm | aramid protects the edge and carries the class, film provides the strength |
| NKN | Nomex, Kapton, Nomex | H (180 °C), up to C (200 °C) with a suitable resin | up to 0.55 mm | polyimide core lifts the class, aramid remains the protective ply |
| Mica tape on glass fabric | mica paper, resin-bonded to glass fabric, PET or polyimide | F to H/C depending on backing | 0.10 to 0.14 mm, mica content 110 to 160 g/m² | mica blocks partial discharge, the backing makes the brittle mineral windable |
| Aluminium-PET composite film | aluminium laminated to PET, also aluminium-copper-PET | per film | 12 to 50 µm for data cables, 50 to 125 µm for power cables | metal shields, film carries and insulates towards the inside |
| Copper foil with conductive adhesive, copper-nickel composite | metal foil, adhesive layer, liner | per adhesive | copper 6 to 200 µm | copper drains, nickel shields magnetically, the adhesive makes contact |
Thermal classes follow the class table on our insulation materials page and the manufacturer data of Karl Schupp AG and Dr. Dietrich Müller GmbH, laminate thickness range follows Schupp, mica tape follows Dexlu, cable films follow Bleher, copper foil follows our page on slitting EMC shielding film. The breakdown voltage of flexible laminates sits at 5 to 30 kV in the composite depending on thickness, roll stock is available up to 2,000 mm untrimmed, tapes from 4 mm.
The adhesive layer is the weakest point in every one of these composites. It has to hold the same thermal class as the plies, otherwise the adhesive sets the class of the laminate, not the aramid. A good composite cannot be split by hand and shows no open gap between paper and film at the cut. Layered composites also include the self-adhesive laminated films, in which adhesive and liner form the additional plies, described in the article on laminating film, and the metal-laminated films for EMC shielding. The carrier ply of many laminates is explained in the article on nonwovens, the mica composite for high voltage in the articles on mica tape and micanite.
Fibre composites as insulating material: laminated paper, laminated fabric, glass-epoxy
The rigid insulating materials are fibre composites with a thermoset matrix, and at the same time layered composites, because they are pressed from many resin-impregnated plies of paper or fabric. Datasheets therefore distinguish between values parallel and perpendicular to the ply direction.
| Material | Carrier and resin | Density | Tensile strength | Temperature limit | Electrical |
|---|---|---|---|---|---|
| Laminated paper HP 2061 (PF CP 201) | cellulose paper, phenolic resin | 1.35 g/cm³ | 120 MPa | 110 °C long term | good insulation, brittle, moisture-sensitive |
| Glass-epoxy laminate HGW 2372.1 (FR-4, EP GC 202) | glass fabric, epoxy resin | 1.9 g/cm³ | 220 MPa, flexural 340 MPa | 120 °C to VDE 0304/2, other grades up to 155 °C | 10.2 kV/mm perpendicular, CTI 200, UL 94 V-0 |
Laminated paper per the datasheet of noltewerk GmbH, glass-epoxy per the datasheet of Amsler & Frey AG (dielectric strength at 90 °C in oil to IEC 60243-1). In between sits laminated fabric HGW 2082 made of cotton fabric and phenolic resin, tougher than laminated paper but no better electrically. Across all three groups the DuroBest grade series of AGK quotes dielectric strengths of 2.7 to 18 kV/mm and continuous service temperatures of 120 to 280 °C. These boards come in 0.5 to 60 mm. Thin grades can be punched, thick ones are sawn and milled, and laminated paper chips easily in the process because the phenolic resin is brittle. For slot wedges, spacers and support parts inside the motor, glass-epoxy is the better choice, for simple covers in a dry environment laminated paper is enough.
Composite or single material: when the laminate pays off
Our recommendation starts with the single material. A biaxially oriented PET film withstands 130 °C to IEC 216, blocks more than 70 kV/mm, costs a fraction of a laminate and can be returned as single-grade production offcut. As long as one ply meets the requirement, it stays one ply. A laminate only becomes the better choice when two requirements apply at the same time that no single material meets at the available thickness: voltage withstand and edge protection in the tight slot (NMN instead of bare film), resin uptake and barrier in the VPI process (DMD instead of film), or a thermal class above B with a PET core that only the lamination with aramid paper lifts into class F. Where one of these conditions is missing, the composite only makes the part more expensive and adds a weak point in the form of the adhesive.
The objection that now shows up regularly in tenders is recycling: composites are hard to separate. That is true. Laminated and bonded plies can only be split into single-grade fractions with effort, often not at all, and for packaging the EU packaging regulation demands at least 70 percent recyclability from 2030, which favours single materials. For insulating parts the sum is a different one. At end of life the part sits in a resin-impregnated winding, the stator is shredded, copper and electrical steel are recovered, and the insulation goes to thermal recovery. Whether the insulating part was a laminate does not decide that balance. What counts is service life: a composite that hits the thermal class correctly keeps the motor running longer than a film at the edge of its class. Production offcut, on the other hand, is very much decided by the material, PET offcut goes back as regranulate, laminate offcut does not. More on this in the article on film sustainability.
Slitting and punching composites: what runs differently from a single material
A datasheet describes the composite inside the component, not at the blade. At the blade, however, shear slitting separates exactly the interfaces that make up the composite. Five effects decide the result, and each has a countermeasure.
- Delamination. In shear slitting two circular blades run past each other and put the adhesive layer under shear. If the lamination is weak or the blade clearance too wide, the edge opens and the outer ply lifts off the core. We match blade clearance and winding tension to the build-up and check the first cut for a closed gap between paper and film.
- Fibre pull-out and fraying. Dry fabric has no matrix holding the fibres at the edge. If the fibres fray or the fibre orientation shifts, the later component lacks strength at that point. The cut edge therefore has to stay closed, over the full roll length.
- Blade wear. Glass and carbon fibre are abrasive and load the blades far harder than film or paper. We monitor tool condition during the order, because a dull blade costs edge quality first and only shows up afterwards.
- Resin condition in prepreg. Pre-impregnated fabrics on epoxy, phenolic or polyester resin must see neither heat nor mechanical crushing during slitting, otherwise the resin distribution shifts. Temperature-controlled storage is part of our process.
- Adhesive and burr on laminated and metallic composites. Self-adhesive plies clog the blade when adhesive squeezes out at the edge. On copper foil from 6 µm, even a slight burr causes a short circuit in downstream processing or reduces shielding performance, which is why freedom from burrs is the acceptance criterion on metal composites.
For these materials we run shear slitting with the following key figures, documented on the pages on contract slitting and on slitting EMC shielding film.
| Parameter | Value at GOBA |
|---|---|
| Parent roll width | up to 1,600 mm, parent rolls up to 1,000 kg |
| Slit width | from 5 mm |
| Width tolerance | plus minus 0.1 mm |
| Slitting speed | up to 400 m/min, depending on material |
| Core diameter | 25, 55, 76 or 152 mm |
| Punching | material thickness 0.023 to 3.0 mm, parts up to 1,000 x 2,000 mm |
On the composite side we slit carbon fibre fabric in twill, plain and satin weave from 80 to over 600 g/m², glass fibre fabric from E- and S-glass from 25 to over 1,200 g/m², aramid and hybrid fabrics, prepregs, glass, polyester and carbon veils, and thermoplastic UD tapes, for which the 5 mm minimum width and the 0.1 mm tolerance were made. Most composite fabrics arrive at 1,000 to 1,270 mm width, in other words 50 inches, and are brought to the width that overlap and butt gap in the component dictate. On the insulation side, NMN, DMD, NKN, mica tape, aluminium-PET and copper-nickel composite films come in as roll stock. The overview of fabrics and semi-finished products sits on the page composite slitting.
The insulation composites are punched into slot wedges, phase papers, slot covers and spacer plies to drawing, and we pre-emboss fold lines in the same stroke so the laminate yields along the intended line when bent and does not tear open in the adhesive layer. Whether the more economical steel rule die is enough or a solid steel tool is needed for tight tolerances and long tool life is decided by the contour. Processes and tooling are described on the page stamping and forming, the finished parts on the page on insulating and moulded parts.
GOBA Takeaway
Composite materials are the rule in electrical insulation above class B and the exception below it. The four composite types predict where the material gives way during slitting: at the adhesive in a layered composite, at the fibre edge in a fabric, at the blade with abrasive glass. Anyone specifying a laminate should therefore think about converting alongside thermal class and breakdown voltage, because the edge is made at the converter, not at the maker of the web.
We have been producing slit rolls and punched parts from the composites described here since 1959, the material overview sits on the page on our insulation materials. If you are unsure whether your application needs a laminate or a film is enough, send us thermal class, voltage and drawing. For special constructions and small quantities, talk to us about custom products.
