Composite Materials
In composites the colour paste becomes part of the matrix, so it influences wet-out, gel time and the exotherm of a laminate that may be several millimetres thick. The paste must be compatible with the resin and the fibre sizing, must not retard cure at the moulding temperature, and in high-temperature processes must survive a thermal cycle that would decompose an organic carrier. Because a laminate cannot be reworked once cured, a dispersion fault or a cure deviation becomes scrap rather than a repairable defect.
Key Requirements in This Application
Fibre wet-out and surface quality
Pigment agglomerates filter out at the fibre surface and leave dry patches, pinholes or a visible print-through on gel-coated parts. Dispersion must be fine enough that the resin flows and wets the reinforcement uniformly.
Cure compatibility and gel time
Some pigments and carriers adsorb the curing agent or slow the reaction, which shows up as under-cured areas in thick laminates. The paste must leave gel time and peak exotherm within the process window.
Exotherm control in thick sections
Thick laminates generate heat faster than they can dissipate it. A paste that accelerates cure or insulates the section can push the exotherm high enough to crack the part or distort the moulding.
Thermal stability for high-temperature processing
Where the composite is processed or serviced at elevated temperature, an organic carrier would decompose and outgas. A resin-free high-temperature paste is required so that the colour survives the cycle.
Mechanical neutrality
The paste must not plasticise the matrix or create interfaces that reduce interlaminar strength. Loading has to be kept as low as the colour allows.
Gelcoat and cosmetic surface
On gel-coated parts the colour is often judged on the outer surface, where print-through, mottling and shade variation are immediately visible. The paste has to disperse finely enough for the gelcoat film and must not cure at a different rate from the backing laminate, or the surface will show a texture that cannot be polished out.
Batch consistency across resin lots
Composite shops often change resin lots between projects, and a repair or a matching part may be produced months after the original. The paste must reproduce the same cured colour on each lot, so that a replacement panel does not stand out against the rest of the assembly.
Secondary bonding and repair
Composite parts are often joined or repaired by secondary bonding after cure. A carrier that blooms to the surface leaves a weak boundary layer that must be removed by abrasion before bonding, and if it is not removed the joint fails at the interface rather than in the laminate. The paste should leave a surface that can be prepared predictably, and any residue should be easy to detect and remove rather than spreading across the bond area.
Recommended DENSON Series
Solvent-free epoxy colour pastes that become part of the epoxy matrix without carrying solvent into a laminate, so no volatile is trapped in a thick section. Because the carrier is solvent-free, the laminate does not need an extended post-cure to drive off residual solvent.
View seriesResin-free high-temperature colour pastes that do not rely on an organic carrier and are intended for processes where a conventional resin would decompose, so the colour survives a thermal cycle that an ordinary paste could not.
View seriesTechnical Considerations
- Keep pigment loading low and check interlaminar strength, because every percent of paste displaces matrix resin.
- Measure gel time and peak exotherm on the actual laminate thickness, not on a thin laboratory casting.
- For infusion processes, confirm that the paste does not raise viscosity enough to slow wet-out of the reinforcement.
- For high-temperature cycles, verify that the paste contains no organic carrier that would outgas and leave porosity.
- Check colour after cure rather than in the wet mix, since several pigments shift shade on heating.
- Confirm that the paste is compatible with the fibre sizing, because a carrier that wets the fibre poorly shows up as dry spots that only appear after demoulding.
Testing and Compliance
Cure and exotherm
Gel time and peak exotherm are recorded on the production resin with and without paste, and thick-section castings are sectioned to confirm that no under-cure or voiding is present.
Laminate quality
Fibre wet-out, void content and interlaminar properties are inspected on a representative laminate, because dispersion faults show first as dry spots and print-through.
Thermal behaviour
For high-temperature systems, the cured part is cycled to the service temperature and checked for colour change, outgassing and cracking rather than assumed from the pigment datasheet.
Mechanical retention
Flexural and interlaminar properties are compared with and without the paste at the intended loading, so that the colour is proven not to trade away the structural performance the part was designed for.
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