The effect of varying molecular weight on the performance of HMMM-crosslinked polyester coatings
File(s)
Author(s)
Sorce, Fabian
Ngo, Sonny
Lowe, Chris
Taylor, Ambrose
Type
Journal Article
Abstract
Thermosetting polyester coatings crosslinked with hexa(methoxymethyl)melamine (HMMM) are ubiquitous for the pre-painted metal sheet used in white goods and architectural cladding. The coatings are typically 20 μm thick and must have superior resistance to cracking during the forming process to maintain their excellent aesthetics and corrosion resistance. Hence, understanding their structure-property relationships is key to design durable coatings with good formability. The thermo-mechanical properties of clear and TiO2-pigmented polyester-HMMM free-films with varying number average molecular weight (MW) from Mn =1500 g/mol to 3300 g/mol and a constant crosslinker content of 20 % have been determined, and this work provides a fundamental investigation into the effects of varying the MW for the first time. Increasing the MW decreases the glass transition temperature (Tg) as the crosslink density reduces due to fewer functional chain ends. The Young’s modulus and yield stress decrease with an increase in MW at low temperatures, and the strain to failure increases around Tg. The TiO2 pigment increases the stiffness of the coatings and reduces the strain to failure around Tg, but has a toughening effect at higher temperatures. All the coatings show comparable behaviour in the Erichsen cupping test; however, increasing the MW reduces the damage during the T-bend test as the coatings are able to withstand higher applied strains. Thus controlling the MW allows a balance of properties to be achieved, as increasing the MW reduces the Tg and modulus while increasing the strain to failure and formability of the coating. Such an understanding of the structure-property relationships allows for better formulating and targeted coating design, reducing cost and increasing performance in the coil coating industry.
Date Issued
2020-12-01
Date Acceptance
2020-08-13
Citation
Progress in Organic Coatings, 2020, 149
ISSN
0300-9440
Publisher
Elsevier
Journal / Book Title
Progress in Organic Coatings
Volume
149
Copyright Statement
© 2020 Elsevier B.V. All rights reserved. . This manuscript is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International Licence http://creativecommons.org/licenses/by-nc-nd/4.0/
Subjects
Polymers
0912 Materials Engineering
0913 Mechanical Engineering
Publication Status
Published
Article Number
ARTN 105920
Date Publish Online
2020-09-01