JEC COMPOSITES MAGAZINE - Issue #119 - March 2018 - 45

GRP composite manufacturing optimization: opportunities with UV curing

An important limitation, on the other hand,
ties to shadow areas unexposed to UV light,
leaving the resin uncured. When opaque
fibres are used, such as carbon fibres, or when
the composite part is very thick or with a
complex shape, it becomes difficult to install
a lamp set-up that helps avoid shadow areas.
Introducing a secondary curing mechanism
can be a solution in such cases, even if this
implies a trade-off on some of the benefits.
Hence, while non-UV technologies work
with all fibre types, UV curing adoption is
specifically developing in Glass-Reinforced
Plastic (GRP) composites.
While the above benefits and limitations
are relevant to all manufacturing processes,
particular features of some of them are highlighted in this article.
Prepregs: stable storage and fast,
controlled curing
The prepreg process consists of two steps.
First, the fabric is pre-impregnated with a
reactive resin that is only partially cured (B
stage curing).
This prepreg is then stored in a freezer to
prevent full curing. In a second step, the composite is manufactured and the resin is cured
completely.
With a UV system, B stage curing can be
accomplished by using either a low-intensity
UV LED light, thixotropic agents or reaction
on pending acid groups. As the resins will not
react without exposure to UV light, long shelf
lives are obtained and freezer storage is not
needed.
A well-known use of UV curing is Cured In
Place Piping (CIPP), i.e. the trenchless repair
of pipes. After a prepreg is drawn into the pipe,
and pressed against the walls of the old pipe
using air pressure, UV light rides through and
cures the pipe instantly.
Pultrusion: increased line speed, reduced
energy consumption and cleaning labour
With pultrusion, continuous lengths of
reinforced composites with a constant
cross-section are manufactured by pulling
fibres through a resin bath and a dye that is
heated and shaped according to the cross-section. A UV curing process can be installed by
replacing the dye with a preshaping form and
a UV lamp.
Often implemented by manufacturers to

Tab.1: Viscosity, glass transition temperature (T(tan dmax) determined with dynamic mechanical
analysis) and flexural properties of three typical UV-cured systems (Photo: allnex)

Tough

High Tg

Strong

Viscosity (mPa s, 23°C)

450

490

500

Tg (°C)

2.1
157

Flexural modulus (23°C, GPa)

3.3

Flexural strain (23°C, %)

Flexural strength (23°C, MPa)

110

increase the line speed, the UV set-up is
cost-effective thanks to the generic benefits
of UV: reduced cleaning labour, waste, energy
consumption and solvent processing.
Filament winding: reduced cycle time,
energy cost and number of mandrels in
use
In the filament winding process, fibres are
pulled through a resin bath and wrapped
around a mandrel.
The simple, often cylindrical, geometries
created in this process provide an excellent
match with UV curing.
A possible implementation of a UV process
is shown in Figure 1, where the UV lamp is
installed on the winding line.
In practice, the UV lamp will typically be
separated from the winding line in order to
free up the spindle.
Parts usually requiring one to three hours
of curing at 100°C are cured in less than five
minutes by exposure to a UV lamp of 80-200
W/cm. This results not only in short cycle
times, but also in a lower number of mandrels
used. The reduced cleaning labour, waste,

3.4

energy consumption and solvent processing
are again key contributors to the savings that
can be achieved.
In-field repair: fast return to service and
high process control
The introduction of UV curing technology
outside of the factory is increasing as well.
When repairing composite parts using a nonUV wet lay-up process, return to service can
take several days. When two-component systems are used to shorten this time, the balance
between curing time and pot life will depend
on the environmental conditions on-site and
can be difficult to control.
UV-curable repair systems, typically using
prepregs, represent an alternative in which
curing happens fast and pot life limitations are
avoided.
Furthermore, as curing will only be triggered
upon exposure to UV light, contractors are
able to inspect for defects before proceeding
with the curing.
UV systems also eliminate the need for heat
blankets, which are used when thermally
curing prepregs for in-field repair.

Fig. 1: Possible implementation scheme of a filament winding process with UV curing (Photo: allnex)

N°119 March 2018

/ jec composites magazine

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #119 - March 2018