The effect of HMMM crosslinker content on the thermal-mechanical properties of polyester coil coatings

• Published in: Progress in organic coatings Vol. 137; p. 105338
• Main Authors: Sorce, Fabian S., Ngo, Sonny, Lowe, Chris, Taylor, Ambrose C.
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 01.12.2019; Elsevier BV
• Subjects: Coatings; Continuous coating; Covalent bonds; Crosslinking; Density; Differential scanning calorimetry; Ductile-brittle transition; Dynamic mechanical analysis; Failure mechanisms; Glass transition temperature; Household appliances; Intermolecular forces; Low temperature; Mechanical properties; Melamine; Metal sheets; Modulus of elasticity; Organic chemistry; Polyesters; Polymer; Strain; Substrates; Tensile tests; Thermal response; Thin-film coatings; Titanium dioxide; Yield strength; Yield stress; Mechanical properties; Polymer; Thin-film coatings
• ISSN: 0300-9440; 1873-331X
• DOI: 10.1016/j.porgcoat.2019.105338

[Display omitted] •Increasing HMMM increases Tg but causes self-condensation at high concentrations.•Failure strain decreases with increasing HMMM with highest strains around Tg.•TiO2 increases dynamic modulus by ∼100% and causes toughening at high temperatures.•Normalised failure envelopes are chemistry dependent at low temperatures only. The thermosetting polyester-based coatings crosslinked with hexa(methylmethoxy)melamine (HMMM) used for coil coating sheet metal experience large deformations when formed into architectural cladding and white goods. Cracking of the 20-μm-thick coatings must not occur during forming, to prevent corrosion of the steel substrate, so the relationship between the composition and the thermal-mechanical properties is critical to develop highly formable and durable coatings, and to choose suitable forming conditions. Free films of coatings with 5% to 30% crosslinker content have been analysed. Dynamic mechanical analysis (DMA) showed that the glass transition temperature (Tg) and crosslink density increase with crosslinker content. Differential scanning calorimetry (DSC) has been used to measure the Tg from the thermal response, based solely on the chemical structure, and agrees well with the DMA. Tensile tests were performed at temperatures as a function of DSC Tg (Tg − 40 °C to Tg + 50 °C). There was little variation in Young’s modulus and strain to failure in the glassy region where the intermolecular forces dominate, but in the rubbery region governed by the covalent bonds a lower crosslinker content gave lower values. This indicates that the failure mechanism undergoes a transition with increasing temperature from being controlled by the brittle fracture stress to the yield stress. The addition of TiO2 pigment increased the modulus and apparent yield stress at low temperatures in the glassy region, and increased the strain to failure and failure stress in the rubbery region. Failure envelopes, normalising the tensile data with the DSC Tg and the crosslink density, show the dependence on crosslinker content and pigmentation. This allows the behaviour of coatings to be predicted from their structure, and enhanced coatings to be developed based on the required mechanical properties.