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Social scientists have opined that social behavior is influenced both by environment and genetic makeup, the latter relating to our body chemistry. Interestingly, many polymers' resistance to cracking follows these same two influences. Whereas some cracking results from excessive loads, or from thermal or UV degradation, a large percentage of plastic part failure results from a phenomenon termed environmental stress cracking (ESC), which is the result of an external chemical acting on a plastic part that contains internal stresses. The stress at which ESC cracking occurs is often well below the nominal fracture stress of the material.

ENVIRONMENTAL STRESS CRACKING EXAMPLES

We often see examples of ESC in components that are subjected to regular exposure to cleaning agents. Hospital equipment, which will often see alcohols, surfactants, and disinfectants during reprocessing, often experience ESC after multiple cleaning cycles. Whereas ESC may not lead to parts failure or loss of functionality, they do result in a marred appearance, and may also lead to disinfection issues due to the generation of internal surfaces that may harbor microorganisms.

Polycarbonate is another thermoplastic that is susceptible to environmental stress cracking. Windows made from polycarbonate often show evidence of crazing (fine, silvery cracks) over time, which is the result of ESC (look at an airplane window next time you fly; you will likely see some crazing). Similar to the description above, crazing does not necessarily mean that the window’s integrity is compromised. Rather, the crazes have relieved localized stresses and may have stopped growing.

ESC PROCESS

ESC occurs when a polymer component is exposed to a chemical when under stress. The stress may be residual stress resulting from a molding or machining process, or can be an externally applied stress during its application use. The chemical, almost always a liquid, does not result in bulk solvation or chemical degradation of the polymer. Rather, typical ESC chemicals are weak solvents for the polymer in question, and result in partial chain disentanglement in the regions of high stress. This chain disentanglement leads to localized plastic flow at stresses below the normal yield or fracture stresses, resulting in craze formation followed by cracking. The kinetics of ESC depend on the rates of the ESC chemical absorption and the relative rates of craze and crack formation, which are also influenced by the mechanical stresses in the samples.

ESC MORPHOLOGY

ESC is characterized by brittle fracture surfaces, even occurring in polymers that normally exhibit ductile behavior. The crack morphology usually shows a smooth fracture plane, indicating a slow growing crack, as opposed to a striated surface indicating fatigue behavior. Since ESC often results in craze formation ahead of the crack, residual craze fibrils are sometimes evident on the fracture surface. ESC cracks often initiate from the surface, which is the source of the chemical cracking agent.

NATURE VS. NURTURE

So what materials (nature) and what environmental conditions (nurture) lead to situations where ESC is more likely? On the nature side, lower molecular weight polymers are more susceptible, since chain disentanglement is more likely. Additionally, polymers that have a lower amount of crystallinity, or that are amorphous, are more likely to exhibit ESC behavior, since crystalline domains have less free volume available for an ESC chemical to occupy. For this reason, polystyrene and polymethyl methacrylate (amorphous) are more susceptible to ESC than polyethylene (semicrystalline).

On the environmental side, the chemical makeup of the polymer is the primary determinant of which chemicals may act as ESC agents for a given polymer. Potential ESC agents include chlorinated hydrocarbons, aromatics, carbonyls, fatty acid esters, alcohols and aliphatic hydrocarbons, to name a few. Additionally, lower molecular weight ESC agents are more active than higher molecular weight due to improved ability to diffuse into the polymer structure.

Also on the environmental side, the nature of the loading is important. Tensile stresses are required to induce ESC; compression will not cause ESC, unless they lead to a component of tensile orientation in another plane. Additionally, residual stresses resulting from material orientation during injection molding can result in ESC.

ESC TESTING

There are multiple approaches to testing materials and components for their propensity to exhibit ESC. ASTM F484 “Standard Test Method for Stress Crazing of Acrylic Plastics in Contact with Liquid or Semi-Liquid Compounds" describes a method where plaques of acrylic materials are flexed to specific levels of strain, and hence stress, while exposed to potential ESC agents for periods of time, while examining the plaques for evidence of craze or crack formation. Strain (stress) can be applied to samples in many ways, including a circular strain jig (detailed in ASTM D543 “Evaluating the Resistance of Plastics to Chemical Reagents”) or an elliptical Bergen jig, which applies a range of strains to a single sample bar.  Chemical exposures can also be performed in several ways including immersion, wet patch, wipe, or spray.

CPG has many years of experience investigating cracks in polymeric devices, and can assist in determining the root cause and preventing their appearance. 

Some of Cambridge Polymer Group's staff spent last Friday’s lunch hour working with polymers for a good cause. Unlike our usual analytical testing and contract R&D, this particular work did not require a lab. We braided polyethylene terephthalate (PET) into dog toys for PetsEmpower, a non-profit that fosters and reunites pets with survivors of domestic violence. The polar fleece toys will be given to animals during and after their foster experience. 

PET - A Warm and Fuzzy Material 

From dog toys to astronaut underwear, polar fleece has infiltrated every aspect of our textile lives. It's hard to believe this ubiquitous fabric was invented as recently as 1979. Two factors contributed to polar fleece's widespread adoption. Firstly, extruded and then woven PET is a good replacement for wool; it is soft and warm, but not as scratchy as wool and certainly not as smelly when wet. Secondly, Malden Mills, the fabric's inventor and manufacturer, chose not to patent the new technology, allowing other vendors to make the wool alternative. Although originally seen as an outdoor material, garment makers quickly recognized polar fleece's potential benefits in applications beyond backpacking gear.

Polyethylene terephthalate is made by heating terephthalic acid with ethylene glycol (a.k.a. antifreeze). As the PET cools, it forms a viscous liquid. That liquid is then extruded through a nozzle, dried, and cut into plastic flakes. To make polar fleece, those flakes are heated and extruded through fine holes. As the liquid sprays out of the holes, it hardens into a fiber that is wound onto heated spools. In addition to making polar fleece out of virgin PET, it is also possible to use chopped up soda bottles.

PET Toys for PetsEmpower

The polar fleece toy is soft on the dog's mouth, yet strong enough to withstand some pulling. Since the fabric is machine washable, it doesn't matter how dirty it gets. These toys provide enrichment and chewing stimulation during and after the dog's foster stay.

Why is the foster stay necessary? Some domestic violence survivors are reluctant to leave abusive living situations because of what will happen to their pets; most human shelters do not allow animal family members. PetsEmpower finds foster families for those animals so that domestic violence survivors can access resources without worrying about the safety and comfort of their pets. Once survivors have reestablished themselves in new environments, they are reunited with their pets.

Cambridge Polymer Group is proud to support PetsEmpower. We are grateful to Polartec (previously known as Malden Mills) for providing the navy polar fleece for the dog toys and to Building Impact for supplying the other colors and for facilitating this warm and fuzzy volunteer opportunity.

Warming from the sun occurs through the absorption of visible light and infrared light into materials whose molecular groups become energized at specific wavelengths, and the material warms. These molecules then can lose the energy by emitting it at different wavelengths, often in the infrared spectrum, cooling the material.

Passive Cooling

Creating surfaces that do not heat up as much has been a goal of material scientists interested in passive cooling systems, which are more cost and energy effective than active cooling systems like air conditioning and thermoelectric coolers.

Researchers have tried to make materials that do not absorb much infrared light, but are capable of absorbing heat from adjacent surfaces and then emitting the absorbed energy in the form of infrared radiation. Films of materials like this could be put on solar panels to drop their temperature to a level where they operate more efficiently, or could be used on the outside walls of buildings to reduce the internal temperature by a few degrees.

Cool as a Glass-Polymer Hybrid

Researchers at the University of Colorado-Boulder recently developed a composite structure made from a thin sheet of transparent plastic (polymethylpentene) which was infused with 8 micron glass beads. The sheet was coated on the back with silver. When put on another surface to be cooled, the silver reflected most of the visible light that contacted the sheet. The film also removed the heat from the underlying layer and reflected it in the mid-IR range. The glass beads act as IR resonators  to make them efficient IR emitters. The Colorado researchers estimate that the films can reduce the surface temperatures by 10°C and can be made cost-efficiently.

Biosafety Level 4 labs are used to study the deadliest pathogens known to humankind. In these labs, the scientists are garbed in protective suits that provide an external air supply and positive pressure throughout, to prevent inhalation and permeation of any airborne pathogens that may be present in the lab. The NY Times reported that the Center for Disease Control and Prevention recently discovered that the air hoses, composed of nylon, had never been properly tested to ensure that they were not releasing chemicals into the air stream that could potentially affect the scientist wearing the hazmat suit.

Lack of Cleaning Validation

To date, no one has gotten sick from the untested air hoses; the CDC discovered this validation omission as a result of a procedures and safety equipment audit. The current hoses were not designed for air transmission in humans, but were instead meant for carrying compressed air for power tools. Hoses designed for breathing equipment are tested for volatile components by the hose manufacturer or equipment assembly manufacturer.

Headspace GC-MS for Volatile Residues

Volatile residues from polymeric materials are often tested by head space gas chromatography with mass spectroscopy. CPG often performs this test to provide a risk assessment of potential elutable compounds from devices, particularly those devices that will be in contact with living tissue.

Material Selection in Dental Fillings

So you indulged in a little too much holiday candy, and now need a cavity filled? There are several material choices, including amalgam alloy and composite resin.

Metal Amalgam vs. Polymeric Composite

If you had a filling prior to 1986, it was likely composed of an amalgam alloy, which is an alloy of mercury, silver, tin, and copper. The composition is carefully regulated by ISO 1559, to account for corrosion and dimensional changes. First used in the 1500s, amalgam remains a very successful material to this day, although concerns about its appearance and perceived issues with mercury toxicity have diminished enthusiasm for its use. Polymeric composite materials based on methacrylate chemistries have found greater use in restorative dental procedures, as these resins can be color matched to the native tooth. 

Put On the Gloves 

Earlier in the days of dentistry, some dentists would mix the amalgam in the palms of their hands, without gloves. Multiple studies have shown that dentists had higher levels of mercury in their bodies than the regular population. Mercury poisoning affects the central nervous system and the kidneys, resulting in tremors in the face and eyelids, and then affecting the limbs and handwriting. The regular use of gloves in dentistry is believed to have reduced issues with mercury accumulation in dentists, along with the increased use of non-mercury alloys and dental composites.

The Mad Hatter, by the way, is a reference to chronic mercury poisoning found in Victorian-aged hatmakers, which was used in the manufacturing of felt hats. Hatmakers of that era often had tremors, paranoia, shyness, and irritability.

Sweetheart Conversation Hearts are an iconic symbol of Valentine's Day. First created by the New England Confectionery Company in 1902, Sweethearts are Necco Wafers cut in heart shapes and stamped with romantic messages. The recipe hasn't changed much since the early 20th century, but the messages are updated as popular vernacular evolves, and now include "Text Me" and "Tweet Me." Although Necco makes more than 8 billion hearts a year, some candy aficionados aren't impressed.

Complaints about Sweetheart's chalk-like texture abound throughout popular culture and the blogosphere. CPG scientists decided to characterize candy hearts to see if they deserve their chalky reputation. We examined the chemical composition, surface topography, flavor and odor of candy hearts, using SEM, EDS, and HS-GC-MS.

Not surprisingly, EDS analysis (see Figure 2) showed the candy consisted of carbon and oxygen, the two main elements in sugar (aside from hydrogen, which is not detectable by EDS). The spectrum showed a complete lack of calcium signal, indicating the absence of calcium carbonate (chalk) in the candy.

Read more in our Material Characterization of Candy Hearts application note.