Review
Stress and embrittlement in organic coatings during general weathering exposure: A review

https://doi.org/10.1016/j.porgcoat.2022.107085Get rights and content

Highlights

  • Sources of tensile, cracking-opening stress

  • Polymer embrittlement during degradation

  • Fracture energy changes during degradation.

Abstract

Failure is defined by a coating's purpose, but a crack is a failure in almost every application. Cracks cause problems in service when they first arise, well before they are widespread. By itself, degradation of a polymer coating can cause cracks or adhesive failure, but several other sources of stress are possible. How do these stresses arise and how do coating properties change during service resulting in cracks? Moisture can produce swelling or blistering, entailing compressive stresses, but often tensile stresses also result at the same time. Environmental stress cracking causes embrittlement in polymers due to a combination of plasticisation and swelling caused by moisture. Seemingly small stresses, from thermal mismatch or bending, can initiate failure by repetition or due to a stress concentration. Polymers become brittle, manifested in several ways. Modulus and tensile strength may increase, but elongation and fracture surface energy diminish considerably. The Lake-Thomas theory allows estimates of how breaking bonds contributes to fracture surface energy but currently there is no settled model for calculating how mechanical dissipation contributes or changes. Degradation causes small defects to become more likely to initiate failure than in unweathered coatings and Poisson's ratio provides insight how 3-dimensional structure relates to potential cavitation. Not only do all these factors affect the overall coating film, but they also affect the internal integrity between the binder and other ingredients and thus failure can be initiated internally before it become apparent externally.

Introduction

Weathering exposure, depending on where or how it occurs, can provide several factors that cause stress in a coating, and, over the same period lead to a reduction in the coating's ability to resist stress by embrittling the polymer binder. Natural weather varies immensely from one geographical region to another but ultraviolet (UV) photons from the sun, moisture and oxygen are the factors that lead to chemical change, i.e. degradation, in nearly all circumstances. In many places, pollution, which is often acidic, either in the atmosphere or below, can cause or accelerate chemical damage. In hotter climates, higher temperatures accelerate chemical change. Chemical attack may manifest first as a colour change, usually yellowing, caused by oxidation but usually it is accompanied by gloss loss and changes in mechanical properties which are both manifestations of chemical bond scission. Stresses that cause failure can arise outside the coating but this review includes how the same factors that cause chemical degradation of the polymer binder also cause stresses via their physical actions. For example, UV will cause chain scission and create small molecular fragments, which are lost, creating a shrinkage stress within the coating but also leaving a damaged polymer network with reduced mechanical properties; water may cause hydrolysis and thus change the chemistry of the polymer, but it will also swell the polymer as it is imbibed, thus causing a swelling stress. Generally, the term “degradation” used here will imply bond scission rather than gentler chemical changes.

Laboratory studies of degradation usually expose coating samples to more UV, more moisture and pollution, at higher temperatures and in much more frequent cycles than would occur in the natural world because product development and academic studies demand rapid results. Laboratory, accelerated studies often lead to unrealistic results not only because the chemical changes caused are different to those caused in the natural world, but also because the stresses arising in the coatings may also be different.

For coatings that are providing corrosion, or other, protection failure implies an increased amount of corrosive water and electrolyte etc. reaching the substrate. Degradation increases the permeability of a coating; it causes defects, and it provokes cracks by causing embrittlement and stresses to arise. There are several mechanisms discussed in various disciplines that can cause stresses in exposed coatings and how they can lead to cracks, so this review attempts to collect them together with how the properties of a coating change so that cracking becomes more likely.

Weathering exposure attacks the exterior side of a coating first but until sufficient stress occurs, even a fragile material will remain intact, so determining the likelihood of cracking becomes one of assessing likely stresses and the ability of the material to resist them, i.e., its toughness. For a protective coating, a loss of gloss or colour are tolerated because these are surface effects, and a coating should be thick enough that there is enough protective material remaining despite changes at its surface. However, these are signs of degradation and future trouble. Any defect that is large enough may initiate failure and it is more than possible to apply a coating badly, e.g., badly mixed, under-cured, too thick, bubbles etc., but failure eventually occurs in coatings that have been applied perfectly.

A coating formulation is rarely homogenous. Not only may the binder system have several components, but also crosslinking naturally produces a physically heterogenous structure [1]. Pigment or extender particles have much different properties to typical organic binders so there are many discontinuities and interfaces where there may be stress concentrations and the impact of mechanical stresses can initiate cracks. There are also cases where pigment chemistry affects the binder, especially over the long term [2].

Rust or cracks are usually noted during periodic examinations. Unfortunately, continuously observing the whole coating for incipient problems is too difficult, time-consuming, and uneconomic so failures usually occur without observation of how they arose. The mechanism of failure is then open to conjecture. This review will focus on sources of stress that may arise during typical exposure but does not include environments that intrinsically produce large mechanical stresses, e.g., cavitation erosion, abrasion, forming operations or impacts. Stress concentrations amplify the effect of what would be minor stresses into dangerous threats, as does fatigue, by repeating stress many times and thus gradually increasing the damage. Stress concentrations occur at corners, holes, edges, and other profile changes [3], [4], [5], but here stresses will be discussed that might occur within the main, flat, continuous area. Final failure may well result from a combination of stresses [6], [7], but they will be examined individually. Materials with extensive crack patterns are studied not only in art where the craquelure is important for appearance, provenance and conservation [8] but also in the drying of a variety of materials [9]. Poor control during drying can lead to a phenomenon known as mudcracking in paint, but that involves different mechanisms to those of brittle fracture in a continuous polymer network, although the crack pattern may be very similar. Such patterns are complex, and depend on the layer thicknesses, material properties and environmental conditions. A coating has obviously failed when it has an extensive pattern of cracks, but for most technical, protective coatings, failure occurs when the first few cracks appear.

Inevitably, failure occurs first where material is weakest or where it has suffered the greatest assault from the environment, thus lifetime is determined by extreme values in distributions of material properties or distributions of environmental factors [10], [11]. However, this review will examine mechanisms, not the statistics of material composition or exposure variations.

It has long been realised that stress accelerates molecular motion within glasses [12]; overall, while stress does not seem to affect the type of chemistry directly, it can have an indirect effect via a tensile strain permitting a little more moisture to enter the coating, for example. There are many articles on chemical change during polymer degradation and there are many empirical studies following how polymers become brittle. Unfortunately, there is no general model for how molecular changes cause embrittlement, a situation that has been noted before [13], although there are many studies on individual polymer compositions. Progress has been made recently in observing molecular fracture in situ [14] but the techniques have not yet been applied to coatings in service. This review attempts to bring together ideas from disparate areas of literature on how stresses might arise and how embrittlement might occur to any coating polymer, regardless of specific chemical composition.

Section snippets

Tensile fracture

Fracture in a coating film occurs to relieve the stress when its tensile strength is exceeded. In a ductile film, considerable elongation causes a progressive failure with yielding and distortion that are distributed over a substantial volume. Ductile materials require much more mechanical energy input before failure than do brittle materials.

Brittle fracture is localised, sudden cracking at a critical value of stress. The brittle-ductile transition happens when conditions cause yield and

Embrittlement

Exposure in service can include a period when the polymer is above its Tg, either because of the service temperature, or it has become plasticised. When the service temperature becomes cooler, or the material loses the plasticiser, it will become brittle and stress results in cracks. This can occur without chemical changes due to degradation, but it is important to consider how the response of a polymeric coating to stress is affected because the material, itself, has been changed.

Environmental

Summary

At failure, cracks occupy only a small fraction of a coating's surface area, and we know that they start from small defects, so it is the first, most degraded regions of the coating that are crucial. In general, cracks form because a material is brittle and is under tension. Any cause of a tensile stress, or strain, in a coating is dangerous. Since coatings are usually thin films adhering to a more rigid substrate, even situations that, at first sight, cause a compressive stress in one

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

I would like to thank Marion Mecklenburg for reading my efforts and for his suggestions and all my colleagues, students and others who have raised provocative questions over the years. I would like to acknowledge the Syilx people of the Okanagan Nation who allow later arrivals to live within their unceded, traditional territory.

References (135)

  • N. Pugno et al.

    New unified laws in fatigue: from the Wöhlers to the Paris' regime

    Eng. Fract. Mech.

    (2007)
  • Z. Tianyu et al.

    Probabilistic model of the fatigue life of epoxy-coated aluminum alloys considering atmospheric exposure

    Int. J. Fatigue

    (2022)
  • G. Allegri

    A unified formulation for fatigue crack onset and growth via cohesive zone modelling

    J. Mech. Phys. Solids

    (2020)
  • T. Wu et al.

    Fatigue cracking behaviour of epoxy-based marine coatings on steel substrate under cyclic tension

    Int. J. Fatigue

    (2017)
  • F.S. Sorce et al.

    The effect of structure-property relationships on the formability of pigmented polyester coatings

    Prog. Org. Coat.

    (2021)
  • T. Prosek et al.

    The role of stress and topcoat properties in blistering of coil-coated materials

    Prog. Org. Coat.

    (2010)
  • C.M. Harvey et al.

    Determination of mode I and II adhesion toughness of monolayer thin films by circular blister tests

    Theor. Appl. Fract. Mech.

    (2018)
  • Z. Cao et al.

    “A blister test for interfacial adhesion of large-scale transferred graphene

    Carbon

    (2014)
  • M.W. Moon et al.

    The characterization of telephone cord buckling of compressed thin films on substrates

    J. Mech. Phys.Solids

    (2002)
  • S.G. Croll et al.

    The interplay of physical aging and degradation during weathering for two crosslinked coatings

    Prog. Org. Coat.

    (2008)
  • J.R. White

    Polymer ageing: physics, chemistry or engineering? Time to reflect

    C. R. Chim.

    (2006)
  • J. Decelle et al.

    Oxidation induced shrinkage for thermally aged epoxy networks

    Polym. Degrad. Stab.

    (2003)
  • J.-F. Larché et al.

    How to reveal latent degradation of coatings provoked by UV-light

    Polym. Degrad. Stab.

    (2010)
  • K. Derrien et al.

    The effect of moisture-induced swelling on the absorption capacity of transversely isotropic elastic polymer–matrix composites

    Int. J. Solids Struct.

    (2009)
  • N.L. Thomas et al.

    A deformation model for case II diffusion

    Polymer

    (1980)
  • X. Mao et al.

    Hydration and swelling of dry polymers for wet adhesion

    J. Mech. Phys. Solids

    (2020)
  • E. Bosco et al.

    Moisture-induced cracking in a flexural bilayer with application to historical paintings

    Theor. Appl. Fract. Mech.

    (2021)
  • J.W. Kim et al.

    The response of a glassy polymer in a loading/unloading deformation: the stress memory experiment

    Polymer

    (2013)
  • M.G. Brereton et al.

    Non-linear viscoelastic behaviour of polymers: an implicit equation approach

    J. Mech. Phys. Solids

    (1974)
  • A.E. Mayr et al.

    Yielding behaviour in model epoxy thermosets—I. Effect of strain rate and composition

    Polymer

    (1998)
  • A.R. Sousa et al.

    The combined effect of photodegradation and stress cracking in polystyrene

    Polym. Degrad. Stab.

    (2006)
  • H. Mohammadi et al.

    Constitutive modeling of elastomers during photo- and thermo-oxidative aging

    Polym. Degrad. Stab.

    (2021)
  • E. Ernault et al.

    Prediction of stress induced by heterogeneous oxidation: case of epoxy/amine networks

    Polym. Degrad. Stab.

    (2019)
  • Q. Deshoulles et al.

    Origin of embrittlement in polyamide 6 induced by chemical degradations: mechanisms and governing factors

    Polym. Degrad. Stab.

    (2021)
  • L. Mishnaevsky et al.

    Micromechanical model of surface erosion of polyurethane coatings on wind turbine blades

    Polym. Degrad. Stab.

    (2019)
  • A.R. Sousa et al.

    The combined effect of photodegradation and stress cracking in polystyrene

    Polym. Degrad. Stab.

    (2006)
  • M. Celina et al.

    Chemiluminescence imaging of the oxidation of polypropylene powder

    Polym. Degrad. Stab.

    (1995)
  • M. Zee et al.

    Cavitation in crosslinked polymers: molecular dynamics simulations of network formation

    Prog. Org. Coat.

    (2015)
  • E. Chandrathilaka et al.

    Mechanical characterization of CFRP/steel bond cured and tested at elevated temperature

    Compos. Struct.

    (2019)
  • D. Erhardt et al.

    Long-term chemical and physical processes in oil paint films

    Stud. Conserv.

    (2005)
  • S. Timoshenko et al.

    Theory of Elasticity

    (1970)
  • W.C. Young et al.

    Roark's Formulas for Stress And Strain

    (2002)
  • D.Y. Perera et al.

    “Stress development and weathering of organic coatings,” Chapter 21

  • M.E. Nichols et al.

    Effect of weathering on the stress distribution and mechanical performance of automotive paint systems

    J. Coat.Technol. Res.

    (1998)
  • F. Giorgiutti-Dauphiné et al.

    Painting cracks: a way to investigate the pictorial matter

    J. Appl. Phys.

    (2016)
  • X. Ma et al.

    Universal scaling of polygonal desiccation crack patterns

    Phys. Rev. E

    (2019)
  • A. Rinaldi et al.

    Statistical damage mechanics and extreme value theory

    Int. J.Damage Mech.

    (2007)
  • H. Eyring

    Viscosity, plasticity, and diffusion as examples of absolute reaction rates

    J. Chem. Phys.

    (1936)
  • X. Zhao

    Multi-scale multi-mechanism design of tough hydrogels: building dissipation into stretchy networks

    Soft Matter

    (2014)
  • J. Slootman et al.

    Quantifying rate- and temperature-dependent molecular damage in elastomer fracture

    Phys. Rev. X

    (2020)
  • Cited by (11)

    • Performance development of polyurethane elastomer composites in different construction and curing environments

      2023, Construction and Building Materials
      Citation Excerpt :

      It is widely used in civil engineering, automobiles, ships and other fields [2,3]. With the enhancement of social awareness of environmental protection, it is necessary to develop an environment-friendly polymer coatings with fast curing speed, low environmental impact and significant mechanical properties [4,5,6]. It will accelerate the green and sustainable development of society.

    View all citing articles on Scopus
    View full text