FRP liners shall consist of:
(a) surface layer ("gel-coat"): adequate resin rich surface layer, reinforced with a veil, compatible
with the resin and contents. This layer shall have a fibre mass content of not more than 30%
and have a thickness between 0.25 and 0.60 mm;
(b) strengthening layer(s): layer or several layers with a minimum thickness of 2 mm, containing a
minimum of 900 g/m² of glass mat or chopped fibres with a mass content in glass of not less
than 30% unless equivalent safety is demonstrated for a lower glass content.

Thermoplastic liners shall consist of thermoplastic sheet material as referred to in 6.9.2.3.4, welded
together in the required shape, to which the structural layers are bonded. Durable bonding between
liners and the structural layer shall be achieved by the use of an appropriate adhesive.
NOTE: For the carriage of flammable liquids the internal layer may require additional measures in
accordance with 6.9.2.14, in order to prevent the accumulation of electrical charges.

The structural layer of the shell is the zone specially designed according to 6.9.2.4 to 6.9.2.6 to
withstand the mechanical stresses. This part normally consists of several fibre reinforced layers in
determined orientations.

The external layer is the part of the shell which is directly exposed to the atmosphere. It shall consist
of a resin rich layer with a thickness of at least 0.2 mm. For a thickness larger than 0.5 mm, a mat
shall be used. This layer shall have a mass content in glass of less than 30% and shall be capable of
withstanding exterior conditions, in particular the occasional contact with the substance to be carried.
The resin shall contain fillers or additives to provide protection against deterioration of the structural
layer of the shell by ultra-violet radiation.

Raw materials

All materials used for the manufacture of FRP tanks shall be of known origin and specifications.

Resins

The processing of the resin mixture shall be carried out in strict compliance with the recommendations
of the supplier. This concerns mainly the use of hardeners, initiators and accelerators. These resins can
be:
- unsaturated polyester resins;
- vinyl ester resins;
- epoxy resins;
- phenolic resins.
The heat distortion temperature (HDT) of the resin, determined in accordance with EN ISO 75-1:2013
shall be at least 20°C higher than the maximum service temperature of the tank, but shall in any case
not be lower than 70 °C.

Reinforcement fibres

The reinforcement material of the structural layers shall be a suitable grade of fibres such as glass
fibres of type E or ECR according to ISO 2078:1993. For the internal surface liner, glass fibres of type
C according to ISO 2078:1993 may be used. Thermoplastic veils may only be used for the internal
liner when their compatibility with the intended contents has been demonstrated.

Thermoplastic liner material

Thermoplastic liners, such as unplastified polyvinyl chloride (PVC-U), polypropylene (PP),
polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), etc. may be used as lining materials.

Additives

Additives necessary for the treatment of the resin, such as catalysts, accelerators, hardeners and
thixotropic substances as well as materials used to improve the tank, such as fillers, colours, pigments
etc. shall not cause weakening of the material, taking into account lifetime and temperature
expectancy of the design.

Shells, their attachments and their service and structural equipment shall be designed to withstand
without loss of contents (other than quantities of gas escaping through any degassing vents) during the
design lifetime:
- the static and dynamic loads in normal conditions of carriage;
- the prescribed minimum loads as defined in 6.9.2.5 to 6.9.2.10.

At the pressures as indicated in 6.8.2.1.14 (a) and (b), and under the static gravity forces caused by the
contents with maximum density specified for the design and at maximum filling degree, the design
stress σ in longitudinal and circumferential direction of any layer of the shell shall not exceed the
following value:
 

At the dynamic stresses, as indicated in 6.8.2.1.2 the design stress shall not exceed the value specified
in 6.9.2.5, divided by the factor α.

At any of the stresses as defined in 6.9.2.5 and 6.9.2.6, the resulting elongation in any direction shall
not exceed 0.2% or one tenth of the elongation at fracture of the resin, whichever is lower.

At the specified test pressure, which shall not be less than the relevant calculation pressure as
specified in 6.8.2.1.14 (a) and (b) the maximum strain in the shell shall not be greater than the
elongation at fracture of the resin.
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