Environmental stress cracking

Last updated October 14, 2023

Crazes (surface cracks) produced by ESC in PMMA drinking beaker Crazes1.jpg — Crazes (surface cracks) produced by ESC in PMMA drinking beaker

Environmental Stress Cracking (ESC) is one of the most common causes of unexpected brittle failure of thermoplastic (especially amorphous) polymers known at present. According to ASTM D883, stress cracking is defined as "an external or internal crack in a plastic caused by tensile stresses less than its short-term mechanical strength". This type of cracking typically involves brittle cracking, with little or no ductile drawing of the material from its adjacent failure surfaces.^[1] Environmental stress cracking may account for around 15-30% of all plastic component failures in service.^[2] This behavior is especially prevalent in glassy, amorphous thermoplastics.^[3] Amorphous polymers exhibit ESC because of their loose structure which makes it easier for the fluid to permeate into the polymer. Amorphous polymers are more prone to ESC at temperature higher than their glass transition temperature (T_g) due to the increased free volume. When T_g is approached, more fluid can permeate into the polymer chains.^[4]

Exposure of polymers to solvents
Factors influencing ESC
Mechanisms of ESC
ESC mechanism in polyethylene
Characterizing ESC
Strain hardening as a measure of environmental stress cracking resistance (ESCR)
Examples
Styrene acrylonitrile susceptibility to ketone solvent
Polymer formworks susceptibility to concrete paste
See also
References
Further reading

Exposure of polymers to solvents

ESC and polymer resistance to ESC (ESCR) have been studied for several decades.^[5] Research shows that the exposure of polymers to liquid chemicals tends to accelerate the crazing process, initiating crazes at stresses that are much lower than the stress causing crazing in air.^[5]^[6] The action of either a tensile stress or a corrosive liquid alone would not be enough to cause failure, but in ESC the initiation and growth of a crack is caused by the combined action of the stress and a corrosive environmental liquid. These corrosive environmental liquids are called 'secondary chemical agents', are often organic, and are defined as solvents not anticipated to come into contact with the plastic during its lifetime of use. Failure is rarely associated with primary chemical agents, as these materials are anticipated to come into contact with the polymer during its lifetime, and thus compatibility is ensured prior to use. In air, failure due to creep is known as creep rupture, as the air acts as a plasticizer, and this acts in parallel to environmental stress cracking.^[7]

It is somewhat different from polymer degradation in that stress cracking does not break polymer bonds. Instead, it breaks the secondary linkages between polymers. These are broken when the mechanical stresses cause minute cracks in the polymer and they propagate rapidly under the harsh environmental conditions.^[8] It has also been seen that catastrophic failure under stress can occur due to the attack of a reagent that would not attack the polymer in an unstressed state. Environmental stress cracking is accelerated due to higher temperatures, cyclic loading, increased stress concentrations, and fatigue.^[7]

Metallurgists typically use the term Stress corrosion cracking or Environmental stress fracture to describe this type of failure in metals.

Factors influencing ESC

Although the phenomenon of ESC has been known for a number of decades, research has not yet enabled prediction of this type of failure for all environments and for every type of polymer. Some scenarios are well known, documented or are able to be predicted, but there is no complete reference for all combinations of stress, polymer and environment. The rate of ESC is dependent on many factors including the polymer's chemical makeup, bonding, crystallinity, surface roughness, molecular weight and residual stress. It also depends on the liquid reagent's chemical nature and concentration, the temperature of the system and the strain rate.

Mechanisms of ESC

There are a number of opinions on how certain reagents act on polymers under stress. Because ESC is often seen in amorphous polymers rather than in semicrystalline polymers, theories regarding the mechanism of ESC often revolve around liquid interactions with the amorphous regions of polymers. One such theory is that the liquid can diffuse into the polymer, causing swelling which increases the polymer's chain mobility. The result is a decrease in the yield stress and glass transition temperature (T_g), as well as a plasticisation of the material which leads to crazing at lower stresses and strains.^[2]^[6] A second view is that the liquid can reduce the energy required to create new surfaces in the polymer by wetting the polymer's surface and hence aid the formation of voids, which is thought to be very important in the early stages of craze formation.^[2] ESC may occur continuously, or a piece-wise start and stop mechanism

There is an array of experimentally derived evidence to support the above theories:

Once a craze is formed in a polymer this creates an easy diffusion path so that the environmental attack can continue and the crazing process can accelerate.
Chemical compatibility between the environment and the polymer govern the amount in which the environment can swell and plasticise the polymer.^[2]
The effects of ESC are reduced when crack growth rate is high. This is primarily due to the inability of the liquid to keep up with the growth of the crack.^[2]
Once separated from the other chains, the polymers align, thus allowing embrittlement.

ESC generally occurs at the surface of a plastic and doesn't require the secondary chemical agent to penetrate the material significantly, which leaves the bulk properties unmodified.^[7]

Another theory for the mechanism of craze propagation in amorphous polymers is proposed by Kramer. According to his theory, the formation of internal surfaces in polymers is facilitated by polymeric surface tension that is determined by both secondary interactions and the contribution of load-bearing chains that must undergo fracture or slippage to form a surface. This theory provides an explanation for the decrease in the stress needed to propagate the craze in the presence of surface-active reagents such as detergents and high temperature.^[9]

ESC mechanism in polyethylene

Semi-crystalline polymers such as polyethylene show brittle fracture under stress if exposed to stress cracking agents. In such polymers, the crystallites are connected by the tie molecules through the amorphous phase. The tie molecules play an important role in the mechanical properties of the polymer through the transferring of load. Stress cracking agents, such as detergents, act to lower the cohesive forces which maintain the tie molecules in the crystallites, thus facilitating their "pull-out" and disentanglement from the lamellae.^[10] As a result, cracking is initiated at stress values lower than the critical stress level of the material.

In general, the mechanism of environmental stress cracking in polyethylene involves the disentanglement of the tie molecules from the crystals. The number of tie molecules and the strength of the crystals that anchor them are considered the controlling factors in determining the polymer resistance to ESC.^[11]

Characterizing ESC

A number of different methods are used to evaluate a polymer's resistance to environmental stress cracking. A common method in the polymer industry is use of the Bergen jig, which subjects the sample to variable strain during a single test. The results of this test indicate the critical strain to cracking, using only one sample.^[5] Another widely used test is the Bell Telephone test where bent strips are exposed to fluids of interest under controlled conditions.^[12] Further, new tests have been developed where the time for crack initiation under transverse loading and an aggressive solvent (10% Igepal CO-630 solution) is evaluated. These methods rely on an indentor to stress the material biaxially, while preventing a radial stress concentration. The stressed polymer sits in the aggressive agent and the stressed plastic around the indentor is watched to evaluate the time to crack formation, which is the way that ESC resistance is quantified. A testing apparatus for this method is known as the Telecom and is commercially available; initial experiments have shown that this testing gives equivalent results to ASTM D1693, but at a much shorter time scale.^[13] Current research deals with the application of fracture mechanics to the study of ESC phenomena.^[14]^[15] In summary, though, there is not a singular descriptor that is applicable to ESC—rather, the specific fracture is dependent on the material, conditions, and secondary chemical agents present .

Scanning electron microscopy and fractographic methods have historically been used to analyze the failure mechanism, particularly in high density polyethylene (HDPE). Freeze fracture has proved particularly useful for examining the kinetics of ESC, as they provide a snapshot in time of the crack propagation process.^[1]

Strain hardening as a measure of environmental stress cracking resistance (ESCR)

Many different methods exist for measuring ESCR. However, the long testing time and high costs associated with these methods slow down the R&D activities for designing materials with higher resistance to stress cracking. To overcome these challenges, a new simpler and faster method was developed by SABIC to assess ESCR for high density polyethylene (HDPE) materials. In this method, the resistance of slow crack growth or environmental stress cracking is predicted from simple tensile measurement at a temperature of 80 °C.^[9] When polyethylene is deformed under a uniaxiial tension, before yield, the stiff crystalline phase of the polymer undergoes small deformation while the amorphous domains deforms significantly. After the yield point but before the material undergoes strain hardening, the crystalline lamellae slips where both the crystalline phase and the amorphous domains contribute to load bearing and straining. At some point, the amorphous domains will stretch fully at which the strain hardening begin. In the strain hardening region, the elongated amorphous domains become the loading bearing phase whereas the crystalline lamellae undergoes fracture and unfold to adjust for the change in strain. The load-bearing chains in the amorphous domains in polyethylene are made of tie-molecules and entangles chains. Because of the key role of tie-molecules and entanglements in resisting environmental stress cracking in polyethylene, it follows that ESCR and strain hardening behaviors can very well be correlated.^[16]

In the strain hardening method, the slope of strain hardening region (above the natural draw ratio) in the true stress-strain curves is calculated and used as a measure of ESCR. This slope is called the strain hardening modulus (G_p). The strain hardening modulus is calculated over the entire strain hardening region in the true stress strain curve. The strain hardening region of the stress-strain curve is considered to be the homogeneously deforming part well above the natural draw ratio, which is determined by presence of the neck propagation, and below the maximum elongation.^[9] The strain hardening modulus when measured at 80 °C is sensitive to the same molecular factors that govern slow crack resistance in HDPE as measured by an accelerated ESCR test where a surface active agent is used.^[9] The strain hardening modulus and ESCR values for polyethylene have been found to be strongly correlated with each others.

Examples

An obvious example of the need to resist ESC in everyday life is the automotive industry, in which a number of different polymers are subjected to a number of fluids. Some of the chemicals involved in these interactions include petrol, brake fluid and windscreen cleaning solution.^[6] Plasticisers leaching from PVC can also cause ESC over an extended period of time, for example. One of the first examples of the problem concerned ESC of LDPE. The material was initially used in insulating electric cables, and cracking occurred due to the interaction of the insulation with oils. The solution to the problem lay in increasing the molecular weight of the polymer. A test of exposure to a strong detergent such as Igepal was developed to give a warning of ESC.

Styrene acrylonitrile susceptibility to ketone solvent

A more specific example comes in the form of a piano key made from injection moulded styrene acrylonitrile (SAN). The key has a hook end which connects it to a metal spring, which causes the key to spring back into position after being struck. During assembly of the piano an adhesive was used, and excess adhesive which had spilled onto areas where it was not required was removed using a ketone solvent. Some vapour from this solvent condensed on the internal surface of the piano keys. Some time after this cleaning, fracture occurred at the junction where the hook end meets the spring.^[17]

To determine the cause of the fracture, the SAN piano key was heated above its glass transition temperature for a short time. If there is residual stress within the polymer, the piece will shrink when held at such a temperature. Results showed that there was significant shrinkage, particularly at the hook end-spring junction. This indicates stress concentration, possibly the combination of residual stress from forming and the action of the spring. It was concluded that although there was residual stress, the fracture was due to a combination of the tensile stress from the spring action and the presence of the ketone solvent.^[17]

Polymer formworks susceptibility to concrete paste

Polymer formworks can suffer from sudden failures during casting, which are generally associated with the pressure exerted by the wet concrete on thin plastic formworks. Such failures can be substantially accelerated by the corrosive effects of the wet concrete paste, which has a pH of circa 13. Certain thermoplastics are more severely affected, especially those in amorphous form, such as PLA, PET and PC. This phenomenon is even more pronounced in 3D-printed polymer formworks, where there is a correlation between the environmental stress cracking mechanism and layer interface grooves, where stresses concentrate.^[18]

Related Research Articles

A polymer is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic and natural polymers play essential and ubiquitous roles in everyday life. Polymers range from familiar synthetic plastics such as polystyrene to natural biopolymers such as DNA and proteins that are fundamental to biological structure and function. Polymers, both natural and synthetic, are created via polymerization of many small molecules, known as monomers. Their consequently large molecular mass, relative to small molecule compounds, produces unique physical properties including toughness, high elasticity, viscoelasticity, and a tendency to form amorphous and semicrystalline structures rather than crystals.

Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and containers including bottles, etc.). As of 2017, over 100 million tonnes of polyethylene resins are being produced annually, accounting for 34% of the total plastics market.

Ductility is a mechanical property commonly described as a material's amenability to drawing. In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stress before failure. Ductility is an important consideration in engineering and manufacturing. It defines a material's suitability for certain manufacturing operations and its capacity to absorb mechanical overload. Some metals that are generally described as ductile include gold and copper, while platinum is the most ductile of all metals in pure form. However, not all metals experience ductile failure as some can be characterized with brittle failure like cast iron. Polymers generally can be viewed as ductile materials as they typically allow for plastic deformation.

<span class="mw-page-title-main">Thermoplastic</span> Plastic that softens with heat and hardens on cooling

A thermoplastic, or thermosoftening plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling.

Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene.

A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a sharp snapping sound.

Crazing is the phenomenon that produces a network of fine cracks on the surface of a material, for example in a glaze layer. Crazing frequently precedes fracture in some glassy thermoplastic polymers. As it only takes place under tensile stress, the plane of the crazing corresponds to the stress direction. The effect is visibly distinguishable from other types of fine cracking because the crazing region has different refractive indices from surrounding material. Crazing occurs in regions of high hydrostatic tension, or in regions of very localized yielding, which leads to the formation of interpenetrating microvoids and small fibrils. If an applied tensile load is sufficient, these bridges elongate and break, causing the microvoids to grow and coalesce; as microvoids coalesce, cracks begin to form.

Low-density polyethylene (LDPE) is a thermoplastic made from the monomer ethylene. It was the first grade of polyethylene, produced in 1933 by Dr John C. Swallow and M.W Perrin who were working for Imperial Chemical Industries (ICI) using a high pressure process via free radical polymerization. Its manufacture employs the same method today. The EPA estimates 5.7% of LDPE is recycled in the United States. Despite competition from more modern polymers, LDPE continues to be an important plastic grade. In 2013 the worldwide LDPE market reached a volume of about US$33 billion.

Embrittlement is a significant decrease of ductility of a material, which makes the material brittle. Embrittlement is used to describe any phenomena where the environment compromises a stressed material's mechanical performance, such as temperature or environmental composition. This is oftentimes undesirable as brittle fracture occurs quicker and can much more easily propagate than ductile fracture, leading to complete failure of the equipment. Various materials have different mechanisms of embrittlement, therefore it can manifest in a variety of ways, from slow crack growth to a reduction of tensile ductility and toughness.

A geomembrane is very low permeability synthetic membrane liner or barrier used with any geotechnical engineering related material so as to control fluid migration in a human-made project, structure, or system. Geomembranes are made from relatively thin continuous polymeric sheets, but they can also be made from the impregnation of geotextiles with asphalt, elastomer or polymer sprays, or as multilayered bitumen geocomposites. Continuous polymer sheet geomembranes are, by far, the most common.

A landfill liner, or composite liner, is intended to be a low permeable barrier, which is laid down under engineered landfill sites. Until it deteriorates, the liner retards migration of leachate, and its toxic constituents, into underlying aquifers or nearby rivers, causing potentially irreversible contamination of the local waterway and its sediments.

Polymer engineering is generally an engineering field that designs, analyses, and modifies polymer materials. Polymer engineering covers aspects of the petrochemical industry, polymerization, structure and characterization of polymers, properties of polymers, compounding and processing of polymers and description of major polymers, structure property relations and applications.

Rubber toughening is a process in which rubber nanoparticles are interspersed within a polymer matrix to increase the mechanical robustness, or toughness, of the material. By "toughening" a polymer it is meant that the ability of the polymeric substance to absorb energy and plastically deform without fracture is increased. Considering the significant advantages in mechanical properties that rubber toughening offers, most major thermoplastics are available in rubber-toughened versions; for many engineering applications, material toughness is a deciding factor in final material selection.

Plastic pipe is a tubular section, or hollow cylinder, made of plastic. It is usually, but not necessarily, of circular cross-section, used mainly to convey substances which can flow—liquids and gases (fluids), slurries, powders and masses of small solids. It can also be used for structural applications; hollow pipes are far stiffer per unit weight than solid members.

Polymer characterization is the analytical branch of polymer science.

<span class="mw-page-title-main">Solid</span> State of matter

Solid is one of the four fundamental states of matter along with liquid, gas, and plasma. The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structural rigidity and resistance to a force applied to the surface. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire available volume like a gas. The atoms in a solid are bound to each other, either in a regular geometric lattice, or irregularly. Solids cannot be compressed with little pressure whereas gases can be compressed with little pressure because the molecules in a gas are loosely packed.

Twin-wall plastic, specifically twin-wall polycarbonate, is an extruded multi-wall polymer product created for applications where its strength, thermally insulative properties, and moderate cost are ideal. Polycarbonate, which is most commonly formed through the reaction of Bisphenol A and Carbonyl Chloride, is an extremely versatile material. It is significantly lighter than glass, while managing to be stronger, more flexible, and more impact resistant. Twin-wall polycarbonate is used most commonly for green houses, where it can support itself in a structurally sound configuration, limit the amount of UV light due to its nominal translucence, and can withstand the rigors of daily abuse in an outdoor environment. The stagnant air in the cellular space between sheets provides insulation, and additional cell layers can be extruded to enhance insulative properties at the cost of light transmission.

Crystallization of polymers is a process associated with partial alignment of their molecular chains. These chains fold together and form ordered regions called lamellae, which compose larger spheroidal structures named spherulites. Polymers can crystallize upon cooling from melting, mechanical stretching or solvent evaporation. Crystallization affects optical, mechanical, thermal and chemical properties of the polymer. The degree of crystallinity is estimated by different analytical methods and it typically ranges between 10 and 80%, with crystallized polymers often called "semi-crystalline". The properties of semi-crystalline polymers are determined not only by the degree of crystallinity, but also by the size and orientation of the molecular chains.

Polymer fracture is the study of the fracture surface of an already failed material to determine the method of crack formation and extension in polymers both fiber reinforced and otherwise. Failure in polymer components can occur at relatively low stress levels, far below the tensile strength because of four major reasons: long term stress or creep rupture, cyclic stresses or fatigue, the presence of structural flaws and stress-cracking agents. Formations of submicroscopic cracks in polymers under load have been studied by x ray scattering techniques and the main regularities of crack formation under different loading conditions have been analyzed. The low strength of polymers compared to theoretically predicted values are mainly due to the many microscopic imperfections found in the material. These defects namely dislocations, crystalline boundaries, amorphous interlayers and block structure can all lead to the non-uniform distribution of mechanical stress.

Materials that are used for biomedical or clinical applications are known as biomaterials. The following article deals with fifth generation biomaterials that are used for bone structure replacement. For any material to be classified for biomedical applications, three requirements must be met. The first requirement is that the material must be biocompatible; it means that the organism should not treat it as a foreign object. Secondly, the material should be biodegradable ; the material should harmlessly degrade or dissolve in the body of the organism to allow it to resume natural functioning. Thirdly, the material should be mechanically sound; for the replacement of load-bearing structures, the material should possess equivalent or greater mechanical stability to ensure high reliability of the graft.

References

1 2 Choi, Byoung-Ho; Weinhold, Jeffrey; Reuschle, David; Kapur, Mridula (2009). "Modeling of the fracture mechanism of HDPE subjected to environmental stress crack resistance test". Polymer Engineering & Science. 49 (11): 2085–2091. doi:10.1002/pen.21458. ISSN 1548-2634.
1 2 3 4 5 H. F. Mark (2004). Encyclopedia of Polymers Science and Technology – 3rd Ed. Vol 12. John Miley & Sons Inc.
↑ Henry, L. F. (1974). "Prediction and evaluation of the susceptibilities of glassy thermoplastics to environmental stress cracking". Polymer Engineering & Science. 14 (3): 167–176. doi:10.1002/pen.760140304. ISSN 1548-2634.
↑ J. Scheirs (2000). Compositional and Failure Analysis of Polymers. J. Wiley & Sons.
1 2 3 Xiangyang Li (2005). Environmental Stress Cracking Resistance of a New Copolymer of Bisphenol-A. Polymer Degradation and Stability. Volume 90, Issue 1, 44–52.
1 2 3 J. C. Arnold (1998). The Effect of Diffusion on Environmental Stress Crack Initiation in PMMA. Journal of Materials Science 33, 5193 – 5204.
1 2 3 "Plastics Engineering – November/December 2015 – Plastic Failure through Environmental Stress Cracking". read.nxtbook.com. Retrieved 23 May 2019.
↑ Michigan University – College of Engineering, Properties of Plastics Archived 6 May 2008 at the Wayback Machine . Accessed 22 April 2008.
1 2 3 4 Kureleca, L.; Teeuwenb, M.; Schoffeleersb, H.; Deblieckb, R. (2005). "Strain hardening modulus as a measure of environmental stress crack resistance of high density polyethylene". Polymer. 46 (17): 6369–6379. doi:10.1016/j.polymer.2005.05.061.
↑ Chen, Yang (2014). "Investigations of environmental stress cracking resistance of HDPE/EVA and LDPE/EVA blends". Journal of Applied Polymer Science. 131 (4): n/a. doi:10.1002/app.39880. ISSN 1097-4628.
↑ Ward, A. L.; Lu, X.; Huang, Y.; Brown, N. (1 January 1991). "The mechanism of slow crack growth in polyethylene by an environmental stress cracking agent". Polymer. 32 (12): 2172–2178. doi: 10.1016/0032-3861(91)90043-I . ISSN 0032-3861.
↑ "t4 uni-halle 3.1.2 Bell Telephone Test (BTT)" (PDF). Uni-halle Library: 2.
↑ Jar, Ben (2017). "A new method to characterize environmental stress cracking resistance (ESCR) of polyethylene pipes". SPE ANTEC Anaheim 2017: 1994–1998. S2CID 13879793.
↑ Andena, Luca; Castellani, Leonardo; Castiglioni, Andrea; Mendogni, Andrea; Rink, Marta; Sacchetti, Francisco (1 March 2013). "Determination of environmental stress cracking resistance of polymers: Effects of loading history and testing configuration". Engineering Fracture Mechanics. Fracture of Polymers, Composites and Adhesives. 101: 33–46. doi:10.1016/j.engfracmech.2012.09.004.
↑ Kamaludin, M.A.; Patel, Y.; Williams, J.G.; Blackman, B.R.K. (2017). "A fracture mechanics approach to characterising the environmental stress cracking behaviour of thermoplastics". Theoretical and Applied Fracture Mechanics. 92: 373–380. doi:10.1016/j.tafmec.2017.06.005. hdl: 10044/1/49864 .
↑ Cheng, Joy J.; Polak, Maria A.; Penlidis, Alexander (1 June 2008). "A Tensile Strain Hardening Test Indicator of Environmental Stress Cracking Resistance". Journal of Macromolecular Science, Part A. 45 (8): 599–611. doi:10.1080/10601320802168728. ISSN 1060-1325. S2CID 137204431.
1 2 Ezrin, M. & Lavigne, G. (2007). Unexpected and Unusual Failures of Polymeric Materials. Engineering Failure Analysis, Volume 14, 1153–1165.
↑ Jipa, Andrei; Reiter, Lex; Flatt, Robert J.; Dillenburger, Benjamin (1 October 2022). "Environmental stress cracking of 3D-printed polymers exposed to concrete". Additive Manufacturing. 58: 103026. doi:10.1016/j.addma.2022.103026. hdl: 20.500.11850/557858 . ISSN 2214-8604. S2CID 250379504.