Sofia Martinez
When plastic is bent, it often turns white at the point of stress. This phenomenon, known as stress-whitening, is caused by changes in the way light is reflected and scattered off the plastic due to molecular changes in the material. Specifically, bending can force the amorphous regions of the polymer chains to align and crystallize, causing light to be scattered and the plastic to turn opaque or white. Additionally, heating a polymer can also induce crystallization in the amorphous sections, leading to similar whitening effects.
| Characteristics | Values |
|---|---|
| Phenomenon | Stress-whitening |
| Cause | Bending or punching operations |
| Effect | White line appears along a bend or curve |
| Material | Amorphous materials, brittle polymers like PS, PMMA and Polycarbonate |
| Mineral fillers | Talc, calcium carbonate, feldspar, calcined clays |
| Polymer testing | Differential scanning calorimetry (DSC), optical birefringence |
| Thermal transitions | Residual reactivity, evaporation of solvents, melting, glass transition temperature, crystallization |
| Light | Scattering, refraction |
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What You'll Learn
Stress-whitening
When plastic is bent, it often turns white at the point of stress—this phenomenon is called stress-whitening. It is more commonly observed in amorphous materials and some brittle polymers like PS, PMMA, and polycarbonate.
The colour change is particularly noticeable in plastic items containing mineral fillers such as talc, calcium carbonate, feldspar, or calcined clays. These fillers have a bright appearance and can easily scatter light, making the whitening effect more prominent.
Optical birefringence testing can be employed to examine how a material refracts light, helping determine whether stress factors have induced whitening in a plastic sample. Additionally, differential scanning calorimetry (DSC) can be used to analyse thermal transitions, including crystallisation, to understand if heat is the cause of whitening in a given plastic sample.
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Stress-induced crystallisation
The whitening of plastic when bent is due to a phenomenon known as stress-induced crystallisation. This occurs when the amorphous regions of polymers are forced to align with the axis of strain, causing the plastic to become opaque or white.
Polymers, such as plastics, often contain both crystalline (ordered) and amorphous (unordered) regions along their molecular chains. When stress is applied to these polymers, the amorphous regions are stretched and realigned, forming a more ordered structure. This process is known as stress-induced crystallisation and can be influenced by factors such as the temperature and bending force applied.
During stress-induced crystallisation, the refractive index of the plastic is altered. The refractive index refers to how light is reflected and scattered by a material. When the plastic is bent, the newly formed crystalline regions scatter light differently, causing the plastic to appear white or opaque. This is because the crystalline region size becomes similar to the wavelength of light, allowing it to scatter and reflect light in a way that eliminates the original colour of the plastic.
Additionally, the mineral fillers commonly used in plastic items, such as talc or calcium carbonate, can contribute to the whitening effect. As the plastic is stretched or thinned, the difference in the refractive indices of the mineral fillers and the plastic resin becomes more noticeable. This can further scatter light and enhance the whitening effect.
To analyse and confirm the occurrence of stress-induced crystallisation, polymer testing methods such as differential scanning calorimetry (DSC) and optical birefringence can be employed. DSC helps in understanding the thermal transitions and potential heat-induced crystallisation, while optical birefringence examines how the material refracts light to determine the impact of stress factors.
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Crazing
When amorphous polymers are bent or stressed, the tangled polymer chains are forced to align with the axis of strain, leading to stress-induced crystallisation. This process alters the refractive index of the plastic, causing light to be scattered and resulting in the whitening effect.
The whitening effect due to crazing is distinct from that caused by heat, which can also turn plastics white. Heating a polymer can cause the amorphous sections of its molecular chains to crystallise, changing how the molecules scatter light. This is one reason why plastics can turn yellow when exposed to UV radiation, as the heat causes a similar molecular change.
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Polymer analysis
DSC is a useful technique for understanding the thermal transitions that occur within a material, including crystallization. By subjecting the plastic to extreme temperatures ranging from -90 to 725 degrees centigrade, DSC can help determine whether heat is the cause of whitening in a given plastic sample. This is particularly relevant as heating a polymer can cause its amorphous sections to crystallize, which can result in whitening.
Optical birefringence testing examines how a material refracts light. This test can help determine whether stress factors have caused whitening in a plastic sample. When plastic is bent, the polymer chains are forced to align in the axis of strain, inducing crystallization. This can scatter light and turn the plastic opaque or white. The refractive index of the plastic is altered, resulting in a colour change from its original colour to a whitish colour.
Additionally, mineral fillers commonly found in plastic items, such as talc or calcium carbonate, can also contribute to the whitening effect when the plastic is stretched or thinned, as they can easily scatter light and appear white. This is known as "crazing", which is the formation of a fine system of voids in the material in response to applied stress. Crazing can halt the crack propagation of a material by increasing its internal surface energy.
By employing these polymer analysis techniques, we can gain a deeper understanding of the factors contributing to the whitening of plastic when bent, including the role of heat, stress, and the presence of mineral fillers.
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Thermal transitions
The whitening of plastic when bent is due to stress-induced crystallisation, which is observed in semi-crystalline polymers. This phenomenon is known as stress-whitening and is characterised by the appearance of a white line along a bend or curve, indicating incipient failure of the material. Differential scanning calorimetry (DSC) is a useful technique for understanding the thermal transitions that underlie this phenomenon.
DSC involves subjecting a material to extreme heating and cooling, ranging from −90 to 725 degrees Celsius, to define its thermal transitions. This technique is particularly useful for determining whether heat is the cause of whitening in a given plastic sample. By analysing the thermal transitions, researchers can gain insights into the underlying mechanisms of stress-whitening.
During DSC analysis, the material undergoes distinct thermal transitions, such as residual reactivity, solvent evaporation, melting, and glass transition temperature. These transitions provide valuable information about the behaviour of the plastic at different temperatures. For instance, the glass transition temperature represents the point at which the amorphous regions of the polymer chains transition from a rigid, glassy state to a more flexible, rubbery state.
By studying these thermal transitions, scientists can better understand the relationship between temperature, molecular structure, and the whitening effect in plastics. It helps in identifying the critical temperatures at which these transitions occur, providing a basis for developing strategies to mitigate or control the stress-whitening phenomenon in specific plastic materials.
Additionally, DSC can be used in conjunction with other analytical techniques, such as optical birefringence, to comprehensively study the behaviour of plastics under stress. Optical birefringence focuses on how a material refracts light, providing insights into the stress-induced whitening mechanism, where the refractive index of the plastic is altered, leading to the scattering of light and the resulting colour change.
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Frequently asked questions
When plastic is bent, the amorphous (unordered) regions of its molecular chains are forced to align with the axis of strain, inducing crystallization. This crystallization scatters light and changes the way it is reflected, making the plastic appear white.
This phenomenon is called "stress-whitening" or "stress-induced crystallization".
Two types of polymer analysis can be used to test for stress-whitening: differential scanning calorimetry (DSC) and optical birefringence. DSC helps us understand the thermal transitions that occur within a material, while optical birefringence testing looks at how a material refracts light to determine whether stress factors have caused whitening.
