JEC COMPOSITES MAGAZINE - Issue #99 - September 2015 - 50

heating
More information
Specific case of carbon or conductive composites
In the case of carbon composites, which of course are inherently conductive, Tibtech developed a heterogeneous double insulation for the highest voltages: Thermotech IWg wires.
The core generally consists of the Thermotech I filaments with the previously mentioned
advantages, plus a flexible overbraiding that can be impregnated. The filaments are flexible
to facilitate placement in complex shapes. Then, once in place (in the form of Tibgrid mats
or wires) and impregnated with the resin or thermoplastic in the composite, they become
stiff, establishing the complementary dielectric material, which becomes integral with the
composite itself. After hardening, a dielectric channel is created in which the standard
Thermotech I filaments can expand.
Note: Care is required in the case of doped resins or thermoplastics/elastomers (e.g. carbon black or conductive fillers, aluminium or other oxides): the stresses on the dielectric
material are even higher, as the reinforcement material is inherently conductive, and the
conductive fillers create valence zones that turn the structure into an actual semiconductor within the composite material. Unlike the carbon reinforcement, this type of impregnation could penetrate the sheath and compromise the double insulation properties of
the Thermotech IWg filaments - so be on the lookout for short circuits. In this specific,
highly restrictive case, Tibtech can propose solutions whereby the outer braiding is already preimpregnated with a flexible insulating material that is high-temperature resistant.

heating yarn in a standard environment (in
the open air). The table below (see Figure
3) shows the evolution of the surface temperature for some insulated Thermotech
yarns as a function of the intensity.
What we are talking about, then, is the
capacity of each of these wire to dissipate
the Joule effect heat into the air at ambient
temperature.
The equilibrium between the heat added
and the heat dissipated depends on the
exchange surface between the filament and
its environment. Of course, the capacity of
the environment to rapidly dissipate this
heat is essential. The insulating material's
specific exchange surface and thermal
conductivity will therefore have a strong
influence on this capacity to rapidly dissipate the added heat into the environment.
The equilibrium is established more or less
rapidly as a function of the insulation. Of
course, each heating yarn is characterized
by a maximum operating temperature.
Note that the design surface effect for
Thermotech "Nu " yarns makes it possible
to almost double the specific exchange
surface compared to an equivalent round
filament with the same cross-sectional
surface (corresponding to the sum of the
cross-sectional surfaces of the fibres).

Fig. 3: Thermal behavior of Thermotech yarns related to current intensity

the heating filaments. There are standard
solutions, of course, but they often result
in thickes wires and successive insulating
layers that make the whole thing stiffer,
limiting the drapability on complex shapes
and the heat conductivity. This in turn
leads to overheating and weakening of the
filaments inside their insulation. With its

range of Thermotech yarns specifically designed for this type of application, Tibtech
can offer a broad range of flexible, resistive
wires with single or double insulation (see
Figure 2).
- heat exchange: First of all, it is important to know the characteristics of the

50 jec composites magazine / No99 September 2015

In this example of Thermotech I filaments,
the maximum operating temperature of
240°C is due to the high-temperature resistance of its core material. So, to continue
with our example, the Thermotech I 3.6
yarn (the brown line in the diagram) will
have a theoretical maximum heat capacity
in open air at ambient temperature that
corresponds to a maximum intensity of
3.8A, lowered to 3.6A for safety reasons.
When the wire is inserted into a composite
material of a given thermal resistivity (and
this is something that needs to be factored
in), the higher the temperature of the
composite that constitutes the filament's
environment, the lower the heat diffusion
will be. Therefore, it will be advisable to
progressively limit the amperage when
working at temperatures that are close to
the maximum heat capacity.

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #99 - September 2015