Effect of Urethane Crosslinking by Blocked Isocyanates with Pyrazole-Based Blocking Agents on Rheological and Mechanical Performance of Clearcoats

Abstract

A novel blocked isocyanate crosslinker was synthesized, and its applicability was investigated for the low-temperature curing of automotive clearcoats. Various pyrazole derivatives were prepared as blocking agents in isocyanate crosslinkers, which strongly affect the deblocking and curing properties of the urethane-bonded coating systems. The thermal curing properties of clearcoat samples containing a pyrazole-based blocked isocyanate crosslinker and polyol resin were characterized under two different temperature conditions (120 and 150 °C). The decrease in the amount of hydroxyl groups in the polyol before and after curing was expressed by the change in OH stretching frequency in the Fourier transform infrared (FT-IR) spectra. The real-time rheological storage moduli of the bulk clearcoat mixtures were measured via a rotational rheometer to determine the effect of pyrazole-based blocking agents on the curing dynamics. In addition, a rigid-body pendulum tester (RPT) was employed to investigate the curing behavior in the thin film form. The nano-indentation and the nano-scratch tests were conducted to examine the surface hardness and scratch resistance characteristics of the cured clearcoat films. The results show that a low-temperature curing system of clearcoats can be realized by tuning the curing temperature and reactivity of isocyanate crosslinkers blocked with pyrazole-based substituents.

Curing Behavior of Waterborne Paint Containing Catalyst Encapsulated in Micelle

“…However, since lowering the baking temperature would also increase reactivity at room temperature, this would likely have an adverse effect on the storage stability of the paint. To help prevent this issue, research has examined the application of masking the curing agent in advance using an isocyanate blocker such as dimethyl pyrazole or methyl ketoxime [8]. However, as the blocker is detached from the isocyanate during the baking process and evaporates in the air, this is not an optimum solution due to concerns over possible adverse environmental effects.…”

Special Issue “Recent Developments on Functional Coatings for Industrial Applications”

“…In particular, this Special Issue contains six research articles and two review articles. A novel blocked isocyanate crosslinker for the low-temperature curing of automotive clearcoats was reported by June et al [6]. The results evidenced that a low-temperature curing clearcoat can be tailored by tuning the curing temperature and reactivity of an isocyanate crosslinkers blocked with pyrazole-based substituents.…”

Optimal combination of UV‐induced cationic ring opening polymerization monomers for light‐weight automotive plastic coating

“…34 The point of the first fracture, which was regarded as the scratch resistance indicator such as plastic or elastic deformation of the coating film against external stresses, was determined as the critical load (Lc1) during the scratch test. 35,36 Finally, crosslinking density or hardness at a specific height within the photocured films were determined using SAICAS (Daipla Wintes, Japan), which could practically screen specifications required for coating films, for example, the adhesion between a film and a substrate. 37 This test consists of two stages: cutting and peeling.…”

“…During the test, the normal force was progressively increased from 0.1 to 30 mN at a rate of 59.8 mN/min to capture the actual penetration depth according to ASTM D 7187 34 . The point of the first fracture, which was regarded as the scratch resistance indicator such as plastic or elastic deformation of the coating film against external stresses, was determined as the critical load (Lc1) during the scratch test 35,36 …”