Langue

Curing with UV-LED

The experiment

The prepregs and composites that are discussed in this article have been prepared by impregnating UD glass fibers with 3 types of UV curable resin formulations via padding followed by UVA-LED curing.
The 3 types of formulations were all 100 % systems and polyurethane, epoxy and polyester based. We determined the flexural modulus and strength of the composites and compared the results and we examined the influence of the monomer concentration on the flexural properties of the epoxy composite.

Experimental setup

  • Resins, photoinitiators and viscosity modifiers.
  • UV- LED lamp with a maximal output of 3.2W/cm² and an emission wavelength of 395 nm.
  • Glass UD fabrics as fibre source.

Synthesis

We have prepared three types of UV matrix formulations:

  • The first one contains a polyester acrylate binder. The viscosity was adjusted (lowered) by adding 30% of a di-acrylate monomer and 4% of photo-initiator was added to allow photopolymerization.
  • The second formulation contains a flexible polyurethane binder with 30% mono-acrylate thinner and 4% photoinitiator.
  • The third is epoxy-based containing 4% of photo-initiator, but the concentration of diluent (tri-acrylate) was varied (0, 10, 20, 30%) to investigate the influence on the mechanical properties.

For each formulation, 8 sheets of glass fabric were impregnated by padding and stacked in the same fibre orientation.
To obtain prepregs, the stacked layers were only exposed to a low dose of UV light (0.5s; 50% of total power).
To obtain composites the layers were illuminated by UV-LED light for 1 minute at maximum power.

Characterisation

The flexural modulus and strength of the obtained composite samples was determined by the 3-point bending test (ISO14125).

Results

The polyester composite showed the highest flexural modulus and strength whereas the polyurethane-based composite has the lowest values.
UV-LED cured epoxy and polyester composites are therefore interesting to create stiff (not bendable) materials. For more flexible composites with no requirements about strength, flexible polyurethane curable resins are an option.

Conclusion

It is possible to prepare prepregs and fully cured composites by UV-LED curing even with thicknesses in the mm range.
Stiff materials can be obtained by using epoxy or polyester based formulations.
Finally, the project showed that the concentration of viscosity modifier in the formulation has a significant influence on the bending properties of the UV-LED composite.

Further reading

Acknowlegdement

Research performed in the framework of Cornet and funded by IWT (IWT 120626)