Stress-cracking Agent Research Articles

On the environmental stress cracking of a vibration-welded polycarbonate (PC)-based nanocomposite

In the present study, the environmental stress cracking resistance (ESCR) of a vibration-welded polycarbonate (PC) nanocomposite filled with 1 vol.-% nanosilica was comprehensively investigated with the help of an online optical crack measurement approach. The experimental results reveal that the improvement of the ESCR strongly depended on the stress cracking agents (SCAs) used. In deionized (DI) water, addition of nanosilica leads to a significant increase in the ESCR of neat PC. In contrast, the enhancement of the ESCR in isopropanol after incorporating nanosilica is marginal. This observed improvement is attributed to the much more plastic deformations and micro cavities induced by the nano-sized particles during fracture of the materials, which are elucidated through the optical analysis.

Molecular dynamics study of the environmental stress cracking agent assisted cavitation in linear and branched polyethylene

We used molecular dynamics (MD) simulation to investigate whether an environmental stress cracking (ESC) agent would help the cavitation process in linear polyethylene (LPE) and branched polyethylene (BPE) by examining the free volume hole size and spatial distributions at the [PE]-[ESC agent] interfaces. The BPE models used contained 10 and 82 butyl/hexyl branches/1000 backbone carbons and the ESC agent used was nonyl ethoxylate (NE) at concentrations 0.001–1.4 wt%. The results show that there exist no significant changes in the packings of LPE and BPE chains, as characterized by their radial distribution functions (RDFs), in the presence of NE regardless its concentration. However, molecular packings at the [PE]-[NE] interfaces vary with NE concentration. In particular, ethylene (E) segments of NE, compared to the ethylene oxide (EO) segments, exhibit stronger association with LPE and BPE chains. We used the Voronoi tessellation method to determine the size and spatial distributions of free volume holes at the [PE]-[NE] interfaces and found that more and larger free volume holes formed around the E-segments than the EO-segments. And such effect was found to be more pronounced in the case of LPE chains than the BPE chains. We further investigated the free volume holes coalescence dynamics using the Fourier mode analysis. The number of frequency modes and the corresponding intensities of the E-segments were significantly higher than those of EO-segments, suggesting that E-segments exhibited a higher tendency to form more and larger free volume holes. This suggests that [PE]-[E-segments] interfaces have a higher chance to form larger voids (i.e., cavitation, a feature of brittle failure that is experimentally observed in ESC). The tendency intensified with increasing NE concentration but was reduced by the presence of short chain branches. The high mobility of terminal carbons in the short chain branches is believed to suppress the formation of large free volume holes at the [PE]-[E-segments] interfaces by absorbing the newly formed free volume by that of the terminal carbons.

Influence of Sample Wetting Method on ESC-Behavior of PMMA under Dynamic Fatigue Crack Propagation

Environmental stress cracking (ESC) is one of the most prominent failure mechanisms for polymer components. The high sensitivity of plastics in regard to environmental influences has always meant that plastics as materials have been viewed very critically in outdoor applications. Recently, the massive occurrence of microplastics in the environment means that questions about the long-term stability of plastic parts and the studies of plastic fragmentation are of great scientific interest. ESC behavior also plays an important role in connection with the formation of microplastics. In this work, the influence of two different sample wetting methods on ESC behavior was investigated. In case A, the sample was in situ wetted with the medium during the measurement by using a sponge. In case B, the sample was wetted by storage in the medium prior to measurement. Different stress cracking agents (SCA) were examined for polymethylmethacrylate (PMMA). Fracture-mechanical fatigue crack propagation (FCP) tests were carried out to quantitatively determine the sensitivity to ESC. Correlations between the absorption behavior and the ESC behavior of the SCA and the resulting morphological phenomena were established. Depending on the wetting method, significant differences in FCP were observed. The in situ wetting of the samples (case A) during the FCP measurement with ethylene glycol (EG) and with deionized water (DI) led to a significant shift in the crack propagation curves to higher ∆K—compared to the PMMA reference. In the case of n-heptane (NH), a more brittle crack propagation behavior was observed due to the chemical interaction with PMMA. The previously immersed samples (case B) give different results. Storage in NH and EG showed no influence on the crack propagation behavior. Samples immersed in DI showed a completely different course of crack growth. At a certain load, a sudden deceleration of the crack propagation and thus a horizontal curve could be seen. Above a certain ∆K value, crack growth began again. Depending on the immersion time (14, 30, or 60 days), this so-called stepped behavior shifted to lower da/dN values.

Tensile stresses in the edges of injection moldings: Roles of packing pressure, machine compliance, and resin compression

AbstractThe slow spontaneous development of cracks in the edges of injection moldings under “field” conditions has been observed for 30 years or more. While environmental stress cracking agents have long been implicated, the magnitude and distribution of the stresses associated with cracking have been obscure. The current study of these stresses involved polycarbonate as a model test material that was molded under systematically varied molding conditions. Surface tensile stresses, though rarely great enough alone to cause “dry” crazing or cracking were revealed through exposure to environmental stress crazing and cracking (ESC) agents. Using an old technique involving a set of calibrated ESC liquids, edge tensile stresses as great as 18 MPa were found in the edges of the moldings. Other, independent methods of stress assessment gave results in semiquantitative agreement with those of the ESC tests. Packing force, machine compliance, injection hold time, and mold flashing emerged as major variables either raising or mitigating stress levels. The root cause of the edge tensions is the mismatch in the times and pressures at which the skins and cores of moldings solidify. In short, skins quench at low pressure first, while cores solidify later during the packing stage. Upon release from the mold, elastic recovery of the core stretches the skin. More importantly, machine and mold compliances allow expansion of the part in the packing stage, during which certain areas of the skin are stretched. Solidifying the core during the packing preserves part of the skin extension as elastic strain. These effects are capable of outweighing the classical tendency of quenching to generate skin compression and core tension. A number of other effects, including release from the mold before the core has solidified, and flashing of the mold, have been found to limit the rise of skin tension.

Effects of Environmental Stress Cracking Agents on the Failure Mechanisms of Commercial Gas Pipe Resins

Abstract Failure processes for two linear medium density polyethylene resins commonly used for producing piping for natural gas distribution systems have been compared in the absence and presence of environmental stress cracking (ESC) agents. A clear decrease in cohesion of the crazed, drawn material is seen in the presence of the ESC agents for both compact tension pieces and notched tensile bars. Creep experiments suggest the ESC agents only have an effect on polyethylene that has undergone micro-necking.

Stress-cracking of nylons induced by zinc chloride solutions

Swelling, orientation, and stress-cracking experiments with nylons in various zinc chloride solutions show that a low degree of solubility of the polymer in the stress-cracking agent is a prerequisite for cracking. As demonstrated by scanning electron microscopy, dissolving, yielding, and tearing occur during the solvent stress-cracking process.

The environmental stress-crazing and cracking in polystyrene under high pressure

Abstract Tensile experiments on polystyrene (PS) in various pressure-transmitting fluids were performed up to 4.0 kbar to study the effects of test media on the mechanical behavior of amorphous polymers under pressure. The fracture stress of PS in silicon oil environment was found to depend strongly on the viscosity of oil and the strain rate of tensile tests. As the strain rate increases, the fracture mode at 2.5 kbar was observed to change from brittle to ductile at some critical strain rate, a rate which shifts to a higher value with decreasing viscosity of silicon oil. In addition, creep fracture experiments conducted at 2.5 kbar in silicon oil with different viscosities suggested that the environmental effects are governed by the rate of diffusion of the environmental liquid into the tensile specimen. Fourier transform infrared measurements performed on specimens fractured in silicon oil environment under pressure proved that the penetration of liquid takes place mainly through the porous structure of the crazes. It was also found, surprisingly, that water, which hardly affects the mechanical behavior of PS at atmospheric pressure, becomes a significant stress-cracking agent under higher pressure.

Fracture response of polyimide and polysulfone under hydrostatic pressure

Fracture response of polyimide and polysulfone in tension under superimposed hydrostatic pressures up to 100,000 psi is presented. The influence of hydrostatic pressure and the pressure medium on the specific surface energy γ is determined by utilizing the Griffith theory of brittle fracture. For polyimide as well as for polysulfone, γ is found to increase with increasing pressure. Furthermore, for polysulfone at any given pressure level, the value of γ is found to be lower in heptane medium than in kerosene. Heptane is known to be a stress-cracking agent for polysulfone. Fracture surface of the tested tensile specimens is examined by using a scanning electron microscope. The observed fracture features are correlated with the macroscopic deformation behavior and also with the effects of the pressure medium used. In polyimide, the region of crack initiation narrows down from a very broad region at atmospheric pressure to almost a point source along the outer periphery of the specimen at 100,000 psi. In addition, polyimide undergoes a transition in the nature of fracture response between 80,000 to 100,000 psi, and this is clearly indicated in the scanning electron micrographs. In polysulfone, the crack propagation appears to be faster when heptane is used as the pressure medium than when kerosene is used. The penetration of the medium into the specimens can be observed on the micrographs. Several scanning electron micrographs of the fracture surface suggest the possibility of a significant temperature rise in the specimen during fracture. This increase in the specimen temperature is roughly estimated from the stored elastic energy released upon fracture.

The stress cracking of polyamides by metal salts. Part I. Metal halides

AbstractThe stress cracking of polyamides (nylons) by a number of metal salts, in aqueous and nonaqueous solutions, has been investigated. Many metal halides and halide‐like salts were found to be active stress‐cracking agents, while metal acetates and sulfates were inactive. Zinc chloride was found to be most active, and its activity was compared with other metal halides. Using a recording tensometer, time to crack initiation, time to crack‐through and time to rupture were determined. These parameters were found to be dependent on temperature, moisture content of the nylon, concentration of the cracking agent and level of stress. High values of all these factors favored rapid cracking. Cracking parameters were shown to be mainly independent of the surface geometry of the nylon and the hydrogen ion concentration of the cracking agent. Metal halides did not appear to cause any chain scission in the nylon and stress cracking was not due to hydrolysis or metal‐ion catalysed hydrolysis.

Mechanism of environmental stress cracking in linear polyethylene

By means of a constant stress test, the environmental stress-cracking behavior of linear polyethylene has been studied on a macro and micro scale in an effort to determine the mechanism of the process. Upon the application of stress, linear polyethylene develops a network of very fine, elliptical fissures, the edges of which are connected by cold-drawn material. In the absence of an active environment, these fissures slowly grow and interconnect, resulting ultimately in the formation of a “neck.” When exposed to an active environment, however, the cold-drawn material ruptures as it is formed at the tips of the fissures. Unsupported, these fissures grow rapidly and interconnect resulting in sample failure. Fissures form both around and through the centers of spherulites with less cold drawing occurring at the interspherulite boundaries. Macroscopic studies confirmed the observation that active environments attack stressed polyethylene specifically at microzones of cold drawing. The effect of low molecular weight hydrocarbon species on the stress-crack resistance of linear polyethylene was evaluated. The role of flaws in the process is also discussed. Attempts have been made to establish a criterion of environmental activity. All of the active stress-cracking agents studied were found to reach similar levels of absorption in polyethylene; however, the specific chemical nature of the environment and not merely its level of absorption determines its ability to cause stress cracking.

Environmental stress cracking of polyethylene

AbstractThe stress cracking of polyethylene was studied under various experimental conditions, and the importance of the effect of both crystallinity and molecular weight of the polymer was established. Many of the variables were investigated, such as the thermal pretreatment of the sample, temperature of the sample, surface tension, and concentration of the stress‐cracking agent, and the magnitude of the stress. The importance of this test procedure and the relationship that exists between the stress cracking of polymers and between similar problems affecting nonpolymeric systems have been analyzed. Some microscopic studies of stress cracking have also been made.