Effect of Pigments on Laser Beam Transmission in Diode Laser Transmission Welding of Poly(propylene)

Abstract: Welding technologies are state of the art for joining polymer composites, with one
of the two joining parts considered laser transmissive (LT) and the other laser absorptive (LA).
Pigments are often added to LT to enhance the crystallinity of the polymer matrix. However,
pigments lead to internal scattering of the laser beam and the rate of transmission or the laser
energy density decreases. Depending upon the type and amount of pigments added in the
formulation of LT, the percentage of the laser beam transmitted, absorbed, or scattered differs.
Laser welding performance depends on the laser energy available for welding after considering
the losses. In the present study, optical transmission of injection molded isotactic polypropylene
(iPP) samples were analyzed with a varying dosage of organic pigment (neat PP, 2%, 3%, 4%,
5%, 6%, 8%, and 10%) using a LPKF TMG 3 transmission tester. The device uses a wavelength
of 980 nm and simulates the optical radiation conditions of diode laser transmission welding
(LTW). The percentage transmission varied with the sample thickness and the composition
percentage of pigment. The modified Bouguer-Lambert law described the transmission energy
and apparent extinction coefficient. The model was validated with the experimental value of
transmittances of the samples with varying sample thicknesses of iPP. There was a decrease in
the percentage of laser transmission with an increase in the pigment content of the samples. It
was found that the apparent extinction coefficient is a function of the pigment levels.