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The Best Adhesive For Composites?

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General information on fibre composites

  • General information on fibre composites
    • Fibre composites are mixed or multiphase materials that essentially consist of two main components: the surrounding matrix (plastic, synthetic resins) and the reinforcing fibres (e.g. glass, carbon, polymers or ceramics).
      The fibre bundles are surrounded by the matrix-like an elastically enclosed beam.

      The combination of these two components gives this material higher-quality properties than either of the two components involved individually.

      Advantage: in principle, fibre composites are more stable than monocomponent materials made of plastics for the same weight.

      However, since the fibres transmit the main load in the component, the fibre must be aligned according to the load paths in the component.


The following fibre types are most used in fibre composites:

  • Glass fibres (GRP)
  • Carbon fibres (CFRP)
  • Ceramic or mineral fibres (e.g. from aluminium oxide, basalt fibres)
  • Nylon fibres
  • Aramide fibres
  • Natural fibres
  • Steel fibres
  • Boron fibres

Glass fibres are also the most widely used fibre types with a share of over 90 %. Depending on the application, the length of typical reinforcing glass fibres is between 10 and 300 µm. Fibres longer than 1 mm are already considered "long" in the field of plastics processing.

The following polymers are the most used as embedding matrix material:

Thermosetting: Phenolic resins, Polyester resins, Epoxy resins, Polyimide resins

Thermoplastics: Polypropylene, Polyamide, Polyphenylsulfide, Polyetheretherketone, Polysulfone, Polyetherimide, Polyphenylsulfone, Polyethersulfone, Polyamideimide

Polyamide 6.6 is widely used as the matrix material with an admixture of 20 to 50 percent by weight of glass fibres.

The following processes exist for the production of customised fibre composite components:

  • Hand lay-up process
  • Hand lay-up with vacuum presses
  • Fibre spraying
  • Fibre winding
  • Vacuum infusion
  • Prepreg technology (e.g. in aircraft construction)
  • Injection moulding
  • Extrusion
  • Sheet Moulding Compound (SMC)
  • Fibre-reinforced concrete

However, most parts made of fibre-reinforced plastics are produced cost-effectively by injection moulding.


Areas of application

Recent trends and advancements in the manufacturing and cost reduction of composite materials has increased their usage in the transportation, industrial, and many other markets in addition to their traditional use in the aerospace field. Driven by increased government regulations on vehicle emissions, the need for light-weighting, and increased end consumer demand for higher performance products, composite materials and parts are increasingly becoming part of an engineer’s day to day design specification. Composites are used in a wide variety of applications to reduce weight, provide improved environmental resistance, improved aesthetics, greater design options and increased stiffness to weight ratio. 


Adhesive bonding for fibre composites in the material mix

Composites require new methods of bonding or joining (beyond traditional mechanical and thermal methods) to allow for design and performance optimization. Fortunately, advances in structural adhesives (such as epoxies, acrylics, and urethanes) have enabled designers to create products meeting structural integrity requirements without the use of mechanical fasteners, rivets, or welding. Additionally, these structural adhesives work well with multiple substrates including plastics, metals, and composites without sacrificing performance properties.

Even Low Surface Energy (LSE) plastics, such as thermoplastic polyolefin (TPO), polypropylene (PP), and polyethylene (e.g. HDPE), which in the past had to be mechanically attached or heat welded, can now be bonded with speciality structural adhesives.

To join composites or mixed materials, mechanical attachments (such as clips, screws, etc.) can be used with virtually any surface, but they require additional steps to mould or create features for the attachment. This can lead to stress concentrations, which may result in plastic cracking and premature failures. Also, drilling holes into composite materials will result in reduced strength due to the introduction of discontinuities in the matrix and reinforcing fibres. All mechanical attachment methods will result in increased weight and often a poorer aesthetic finish.

Heat and friction welding is a common alternative for certain composites. However, these welding techniques are energy and tooling-intensive and limited in the geometries and substrate combinations that can be addressed. In addition to forming strong bonds, structural adhesives can lower overall costs while increasing the durability of products; and are typically lighter weight than mechanical fasteners. Durability is improved because adhesives distribute stress across the entire bonded area, whereas mechanical fasteners, rivets, and spot welding can create stress concentration leading to weak points across the substrates. Furthermore, the use of adhesives provides a way to seal the entire bonding area while also providing a high strength joint. Another huge consideration and advantage for adhesive bonding is the ease in which it allows different materials to be combined – compared to conventional mechanical methods. For example, structural adhesives prevent galvanic corrosion between dissimilar metals. Finally, the cleaner look of bonded joints versus mechanical fasteners allows for better looking, more efficient product builds without additional finishing work. Thus, adhesive bonding could be the best option for joining the next generation of engineering composites and plastics. 

Suitable adhesives

For the joining of lightweight materials, numerous products have been developed, including adhesives by 3M, which are perfectly suited for the efficient filling or joining of fibre composites, multi-material systems and low-energy plastics.

These include, for example:

  • polyurethanes Structural 1-K and 2-K acrylic or epoxy resin-based construction adhesives.
  • Structural adhesive films for composites as well as high-performance epoxy resin-based plastics.
  • Low-density core and edge fillers are suitable for reinforcing highly rigid honeycomb structures, e.g. in vehicle or aircraft construction.
  • Acrylic foam adhesive tapes can be used to replace rivets or clips.
SUBSTRATE 2
Metals
  • Aluminum
  • Colled Rolled Steel
  • Galvanized Steel
Fiber- Reinforced Epoxy
  • Carbon Fiber (CFRP)
  • Glass Fiber
Fiber- Reinforced Thermosets
  • Polyester (FRP)
  • Phenolic
  • SMC
Thermoplastics
  • Polyeolofin
  • PET
Other Thermoplastics
  • Acrylic/PMMA
  • Polycarbonate (PC)
  • Rigid PVC and HIPS
Fiber- Reinforced Nylon

SUBSTRATE 1

Metals DP420NS
DP125 Gray
DP420NS
DP6310NS
DP6310NS
DP8410NS
DP8010 Blue DP8410NS
DP6310NS
DP6310NS
  • Aluminum
  • Colled Rolled Steel
  • Galvanized Steel
FiberReinforced Epoxy DP420NS
DP6310NS
760
DP6310NS
DP8410NS
760
DP8010 Blue DP8410NS
DP6310NS
DP6310NS
  • Carbon Fiber (CFRP)
  • Glass Fiber
FiberReinforced Thermosets DP6310NS
DP8410NS
760
DP8010 Blue DP8410NS
DP6310NS
DP6310NS
  • Polyester (FRP)
  • Phenolic
  • SMC
Thermoplastics DP8010 Blue DP8010 Blue DP8010 Blue
  • Polyeolofin
  • PET
  • HDPE
Other Thermoplastics DP8010 Blue DP8010 Blue
  • Acrylic/PMMA
  • Polycarbonate (PC)
  • Rigid PVC and HIPS
FiberReinforced Nylon DP6310NS

One example is the 3M™ Scotch-Weld™ Multi-Material Composite Urethane Adhesive DP6330NS. It is a green, non-sag, two-component urethane paste for bonding a variety of composites, plastics, metals and wood. This flexible adhesive has good energy absorption and fatigue properties for durable bonding of composite parts and multi-material assemblies.

When using a Duo-Pak (DP) size adhesive, rely on 3M dispensing equipment for convenient and accurate metering, mixing and dispensing. Designed specifically for multi-material and composite assemblies, our 3M™ Scotch-Weld™ Multi-Material Composite Urethane Adhesive DP6330NS delivers outstanding strength and performance. This adhesive has excellent elongation and stresses strain properties for durable bonding of composite parts and multi-material assemblies, including plastics, metals and wood. With a 1:1 mix ratio, this green adhesive has a 30-minute open time and reaches handling strength in approximately 2 hours. Adhesive features excellent water and humidity resistance with very good chemical resistance. Recommended Applications Bonding composite or plastic panels to metal frames Bonding composites to each other 3M™ Scotch-Weld™ Multi-Material Composite Urethane Adhesives DP6330NS can replace rivets and screws in attaching composites to other substrates, providing a more aesthetically pleasing, fatigue-resistant bond line. It also bonds well to most metals without requiring priming.



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When designing adhesive joints, the following must be taken into account:

  • Stresses caused by different thermal expansions must be avoided,
  • Jumps in the thickness of the adhesive layer should be avoided,
  • Use of low-shrinkage adhesives,
  • Avoidance of excess adhesive residues,
  • Maintain constant curing conditions. Outer skin must not be damaged due to excessive curing temperature.
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