Effect of a thermal treatment on thermoplastic polyurethane– metal hybrids

Abstract

The effect of a thermal treatment, i.e. annealing, on the adhesion of a thermoplastic polyurethane overmolded on a precoated steel substrate was evaluated. The hybrid part was subjected to three different annealing temperatures after the overmolding process. Adhesion of the thermoplastic elastomer was achieved by using an in-house-developed, two-step curable organic powder coating, which was selectively crosslinked to a polyallophanate state. The powder material was applied to the steel substrate by electrostatic charging followed by molecular crosslinking in an oven. The influence of the metal surface pretreatment was also evaluated regarding adhesion. The effects of the postthermal treatment were assessed through mechanical peel test, differential scanning calorimetry, atomic force microscopy, and scanning electron microscopy. Interesting results were obtained regarding strong adhesion between the precoated metal sheets and thermoplastic polyurethane after injection molding, especially in hybrids that were annealed at higher temperatures with the in-house adhesive coating.

Influence of annealing on the permeation properties of a thermoplastic elastomer

“…In Fig. , the general “fingerprint” represented by the black spectrum, and a description of spectral bands of freshly overmolded TPU are shown, similar to previous work done by the authors . The red spectrum represents the annealed sample at 100°C for 20 h; spectra at lower annealing temperatures did not vary when compared to the untreated TPU.…”

“…Figure also displays the stress versus strain behavior of freshly injected and thermal treated TPU strips at 100°C for 20 h. Annealing increased the strength of the material, from an average value of 35–46 MPa, which translates to a 20% increase. As reported in the literature , the increase in strength is caused by the molecular rearrangement of hard domains which favors stronger and more uniform intermolecular hydrogen bonds in the urethane groups. Contrarily, elongation at break (mm/mm) remained nearly the same for both the fresh and the annealed specimens with 6.8 and 6.5, respectively.…”

“…For the thermal treated polymer, the melting peak of the short‐range crystalline segments is not easily recognizable. It was demonstrated in a previous publication of the authors that T m 1 shifted progressively to higher temperatures every time that the annealing temperature was increased. For the thermally treated specimens at 100°C, the T m 1 overlaps the melting regions T m 2 and T m 3 as encircled in the respective thermograph.…”

“…The hard blocks consist of isocyanate and chain extender, which provide the toughness and mechanical performance, while the soft segments are made of polyols that contribute to material flexibility and elastomeric properties . These specific characteristics may be influenced with a postthermal treatment after processing, as it was presented in a previous work of the authors , and also treated in detailed by Petrović and Ferguson .…”

“…However, there is still little literature available about the influence of a postthermal treatment on diffusion and permeation parameters of diverse media such gases or water . In this work, the permeation and diffusion properties of TPU after annealing were investigated.…”