Year

2013

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials and Mechatronic Engineering

Abstract

Paint coatings/films having pigments/filler particles, in general, are of technological importance because of their wide usage. Traditional quality control testing methods used in industry, such as tensile tests on bulk paint samples, and quick tests such as pencil scratch test, often could not predict coating performance during usage. Coating performance includes strength of adherence, scratch/mar resistance, erosion resistance, formability, colour fastness and gloss retention. Of particular interest in this thesis, is the determination of the elastic modulus of the coatings, since it can be linked to the cross-linking density and hence to the performance of the paint. The best way to evaluate performance are laboratory simulation tests and field exposure tests, but these tests often take weeks or years (in the latter case) to generate meaningful results, thus are not suitable for quality control (QC) purposes. It is therefore imperative to develop an improved quality control tool for quick assessment of pigmented paint coatings suitable for use in the industrial environment. Unlike unpigmented paint coatings, such as automotive top coats, pigmented coats have an inherent roughness imparted by the colour pigments and filler particles which makes determination of the paint matrix by indentation methods difficult. Three-body abrasion, caused by the dislodgment of these hard particles, also adds to the difficulties of interpreting scratch test results. The relationship between crosslink density in paint matrix and mechanical properties, such as ductility, is known [1], and can be correlated to scratch resistance. However, such correlation was difficult to establish in pigmented paint coatings as dislodgement of pigments during scratch tests sometimes led to accelerated wear. Indentation testing would yield information on paint properties, such as elastic modulus and hardness, without causing the dislodgement of pigments. Each indentation test typically takes a few minutes, making it an ideal candidate as a rapid quality control tool. The major drawback in using indentation techniques on soft, compliant materials such as polymers which make up the paint matrix are the timedependent response (creep at fixed load or stress relaxation on fixed displacement), leading to steeper and even negative unloading slopes and hence inaccurate modulus values. The literature review briefly covers some of the commonly used testing methods employed in industry for polymer coatings. Micro- and nano-indentation methods for the determination of elastic modulus are covered in detail. The effect of creep pertaining to indentation testing, and the treatment thereof, is also reviewed. The experimental work firstly examined the applicability of commercial microindentation equipment as QC tools. The results showed that these instruments could qualitatively differentiate the elastic modulus between paint coatings having different degrees of curing (hence differing crosslink densities and resultant mechanical properties), as well as different pigment/filler types and contents. However, creep affected the calculated values of the elastic modulus. Mechanical models using springs and dashpots to estimate elastic modulus values from the creep response were investigated, as were analytical methods to nullify the effect of creep in the unloading response. From this work it was proposed that the Boltzmann superposition principle (where strain response to a complex stress history for a linear viscoelastic material resulting from a complex loading history, is the algebraic sum of the strains due to each individual step in load) be used to extrapolate and then ‘subtract’ the creep displacement response during unloading to yield a more accurate value for the elastic modulus. This contribution provides the groundwork for possible future development of a rapid QC tool for industrial use.

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