Sofia Martinez
Have you ever noticed how plastic changes colour when it's bent? This phenomenon, known as whitening, is caused by the formation of tiny cracks or crazes in the plastic that scatter light, giving it a milky white appearance. The colour change is particularly noticeable in coloured plastic because the light no longer reaches the pigments. Additionally, the bending motion can induce crystallization in the plastic, further altering the way light is reflected off its surface.
| Characteristics | Values |
|---|---|
| Phenomenon | Plastic changes colour to white in areas where stress is applied |
| Cause | The way light is reflected from the plastic changes as the strands of plastic material, the polymers in the plastic, start to stretch when twisted |
| Testing | Differential scanning calorimetry (DSC) and optical birefringence |
| Factors | Heat (molecular energy) and stress |
| Composition | Most polymers contain both crystalline (ordered) and amorphous (unordered) areas on their molecular chains |
| Effect of heat | Heating a polymer can cause the amorphous sections of the chain to crystallize, changing the way molecules scatter light and turning the plastic white |
| Effect of bending | Bending can cause molecular changes that lead to whitening |
| Other factors | Exposure to UV radiation can cause plastic to turn yellow |
| Microstructural features | Light can scatter off of spherulites or crazes (cracks unique to polymers) |
| Whitening cause | Formation of crazes rather than stress-induced crystallization |
| Polymer behaviour | At high strain rates and low temperatures, polymers are brittle because there is less time for molecules to yield to accommodate the load |
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What You'll Learn
The whitening effect occurs due to light scattering
The whitening effect observed in plastics when they are bent is a result of light scattering. This phenomenon occurs due to the structural changes that take place within the plastic's polymers.
Polymers, such as plastics, often exhibit a colour change when subjected to stress, turning white at the point of stress. This colour change is a result of the molecular alterations that occur within the plastic's structure. When a plastic object is bent, the outside of the curve experiences tension, causing some of the polymer strands to break. As a result, tiny air pockets are created, providing a surface for light to refract.
The formation of these micro-cracks or "crazes" is known as crazing, a type of localized deformation. These cracks scatter light as it passes through or reflects off the surface, resulting in a milky white colour. The process of bending the plastic forces the polymer chains to align along the axis of strain, leading to crystallization in that region. This crystallization further contributes to the scattering of light, causing the plastic to appear opaque or white.
Additionally, heating a polymer can also induce crystallization. The amorphous sections of the molecular chains can crystallize upon heating, altering the way molecules scatter light and resulting in the plastic turning white. This phenomenon is not limited to bending but can also occur due to repeated bending motions or other forms of stress.
To determine whether the whitening is caused by heat or stress, differential scanning calorimetry (DSC) and optical birefringence tests can be employed. DSC helps analyze thermal transitions, including crystallization, while optical birefringence examines how a material refracts light, providing insights into whether stress factors have induced whitening.
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Crazing and cracks cause light refraction
When plastic is bent, it experiences stress, which often results in a colour change—the plastic turns white at the point of stress. This whitening effect occurs when the strands of plastic material, or polymers, stretch and break, creating tiny air pockets that refract light.
The phenomenon is caused by the formation of crazes, a special case of cracks unique to polymers, which scatter light and turn the plastic opaque or white. This is known as crazing, a type of localized deformation that leads to the creation of voids and the alignment of molecular chains. The affected area undergoes permanent deformation and will not return to its original position, resulting in a highly strained area that appears permanently white.
Optical birefringence testing can be used to determine whether stress factors have caused plastic to turn white by examining how a material refracts light. This analytical technique involves heating and cooling the material to extreme temperatures to define its thermal transitions.
The whitening effect can also be influenced by the rate at which strain is applied to the plastic. At high strain rates and low temperatures, polymers are more brittle, resulting in less yielding and consequently, less whitening. Additionally, rapid forced motion creates internal friction, requiring higher stress to deform the material.
While heating a polymer can induce crystallization, not all polymers crystallize under stress. The whitening observed in polymers after straining them is often caused by the formation of crazes rather than stress-induced crystallization.
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Heat and stress cause crystallisation
The whitening of plastic due to bending is a well-known phenomenon. This occurs due to the semi-crystalline nature of polymers, which contain both crystalline (ordered) and amorphous (unordered) regions. When a plastic item is bent, the amorphous regions become ordered, leading to crystallization. This change in molecular structure affects how light is scattered, causing the plastic to turn white.
Heat and stress are the primary factors contributing to this crystallization process. Firstly, let's examine the role of heat. When a polymer is heated, the amorphous sections of its molecular chains can undergo crystallization. This phenomenon is known as thermal crystallization and can be studied through a technique called differential scanning calorimetry (DSC). DSC analyzes the thermal transitions of materials, including crystallization, by subjecting them to extreme temperatures ranging from -90°C to 725°C. By using DSC, scientists can determine the specific temperatures at which amorphous polymers crystallize.
Now, let's turn our attention to the role of stress. When a plastic item is bent, the amorphous regions experience stress, which forces the polymer chains to align and become ordered. This stress-induced ordering results in the crystallization of these regions. The bending action induces localized deformation, creating voids and leading to the alignment of molecular chains. This process is known as "crazing," and it causes permanent deformation, resulting in a highly strained area that appears white.
The combination of heat and stress can also contribute to crystallization. For example, when a plastic item is subjected to rapid forced motion, internal friction generates heat within the material. This heat, coupled with the stress of deformation, can accelerate the crystallization process. Additionally, the rate at which strain is applied (strain rate) influences the whitening effect. At high strain rates and low temperatures, polymers become brittle, leaving less time for the molecules to accommodate the load. This reduced molecular yielding results in decreased whitening. Conversely, when strain is applied slowly, plastics will gradually deform, even under small loads, potentially leading to more pronounced crystallization and whitening.
In summary, the whitening of plastic due to bending is a result of heat and stress-induced crystallization. The amorphous regions of polymers transition from an unordered state to an ordered, crystalline structure, altering the way light is scattered and causing the plastic to appear white in the areas of stress. The interplay between heat, stress, and strain rate determines the extent and rate of crystallization, providing insight into the fascinating behavior of plastics under mechanical stress.
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Bending induces molecular changes
The whitening observed in polymers after straining them (e.g. by bending) is generally caused by the formation of crazes—a special case of cracks unique to polymers—rather than stress-induced crystallization. When plastic is bent, the outside of the bend is put under tension and some polymer strands break, creating tiny air pockets. These air pockets give light a surface to refract off of, causing it to scatter. When light scatters, the plastic loses its original colour and appears white. This phenomenon is called crazing, a localized deformation that leads to the creation of voids and the alignment of molecular chains.
The colour change effect occurs when the strands of plastic material, the polymers, start to stretch as the plastic is twisted. This changes the way light is reflected, or refracted, by the plastic. The refractive index of the plastic is altered, from its original colour to a whitish colour.
The mechanical properties of plastics vary with temperature and strain rate. At high strain rates and low temperatures, polymers are brittle because there is less time for molecules to yield to accommodate the load. Less yielding means less whitening. Rapid forced motion also causes internal friction, so a higher stress is required to deform the material.
Polymer testing can be used to understand the whitening phenomenon. Differential scanning calorimetry (DSC) helps to understand the thermal transitions that occur within a material, including melting and crystallization. Optical birefringence testing looks at how a material refracts light and can help determine whether stress factors have caused whitening.
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Strain rate sensitivity affects whitening
When polymers like plastics experience stress, they often exhibit a colour change, turning white at the point of stress. This phenomenon, known as stress whitening, is a milky-white appearance that occurs during mechanical loading and plastic deformation. It is caused by the formation of tiny cracks or crazes, which scatter light as it passes through or reflects off the surface, resulting in a white colour.
Strain rate sensitivity is a fundamental principle of viscoelastic behaviour, where the effects of increased temperature are equivalent to the effects of extended time without a temperature increase. In other words, the mechanical properties of plastics are influenced by both time and temperature. For example, at high strain rates and low temperatures, polymers become brittle because the molecules have less time to yield and accommodate the load. This reduced yielding results in less whitening.
Additionally, rapid forced motion at high strain rates can cause internal friction, requiring higher stress to deform the material. As a result, plastics become measurably stronger as loads are applied more quickly. This phenomenon is also known as strain rate sensitivity. However, when given time, plastics will slowly deform under very small loads, a process called plastic creep. While plastics can recover from small creep deformations, they are typically unable to return to their original state once they have started to turn white.
The strain rate sensitivity of plastics can be observed through tensile tests, where the speed, strain rate, and temperature are varied to study their impact on the material's behaviour. By increasing the strain rate or decreasing the temperature, the yield strength and modulus increase, making the material appear stronger and stiffer. Conversely, reducing the temperature or increasing the strain rate has a similar effect.
Overall, the strain rate sensitivity of plastics plays a crucial role in understanding their mechanical properties and behaviour under stress. It influences the whitening process by affecting the time and temperature conditions under which whitening occurs, as well as the strength and stiffness of the plastic material.
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Frequently asked questions
Plastic changes color when bent due to stress. This phenomenon causes the plastic to turn white at the point of stress.
When plastic is bent, the strands of plastic material, or polymers, start to stretch and break, creating tiny air pockets. These air pockets give light a surface to refract off of, scattering it and giving the plastic a milky white color.
Yes, heating a polymer can cause the amorphous sections of its molecular chains to crystallize. This changes the way molecules scatter light, resulting in the plastic turning white.
Yes, in addition to heat and stress, exposure to UV radiation can cause plastic to turn yellow.
Differential scanning calorimetry (DSC) and optical birefringence are two types of polymer testing that can help determine the cause of whitening in plastic. DSC analyzes thermal transitions, while optical birefringence examines how a material refracts light.
