Ethan Clarke
When plastic is bent or twisted, it often turns white at the point of stress. This phenomenon is called stress-whitening or stress-induced crystallization, and it occurs when the strands of plastic material, or polymers, start to stretch and align in the axis of strain, inducing crystallization. This crystallization scatters light, making the plastic appear opaque or white. The whitening effect is caused by light scattering from the crazes, or fine fractures, that form in the plastic due to stress. While this phenomenon is common in amorphous materials and some brittle polymers, not all subcategories of plastics turn white under stress.
| Characteristics | Values |
|---|---|
| Phenomenon | Stress-whitening |
| Cause | Stress-induced crystallization |
| Other causes | Heat (molecular energy) |
| Effect | Change in colour to white at the point of stress |
| Effect | Plastic becomes opaque |
| Effect | Plastic loses original colour |
| Effect | Plastic becomes measurably stronger |
| Effect | Plastic creep |
| Effect | Plastic failure |
| Effect | Crazing |
| Effect | Shear banding |
| Effect | Whitish colour caused by light scattering from the crazes |
| Effect | Formation of a fine system of voids in the material |
| Effect | Increase in volume |
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What You'll Learn
Stress-induced crystallization
The phenomenon of plastic turning white when stressed is called stress-whitening. This phenomenon is more common in amorphous materials and some brittle polymers like PS, PMMA, and Polycarbonate. The whitening of plastic is caused by stress-induced crystallization.
When the crystalline region size is similar to the wavelength of light, it can scatter light, making the plastic opaque or white. This light scattering is caused by the creation of crazes, which are fine fractures or voids in the material. These voids form in response to applied stress, increasing the material's internal surface energy and preventing the propagation of cracks. The stress-induced crystallization and the resulting crazes cause the plastic to reflect all visible light wavelengths, making it appear white.
The whitening effect in plastics can also be caused by heat (molecular energy) in addition to stress. Heating a polymer can cause the amorphous sections of the molecular chains to crystallize, changing the way molecules scatter light and turning the plastic white. Similarly, bending or twisting plastic can stretch the strands of plastic material, altering the refractive index and causing light scattering, resulting in the whitening effect.
Not all plastics turn white under stress, and the degree of whitening depends on the strain rate and temperature. At high strain rates and low temperatures, the whitening effect is reduced due to the reduced mobility of molecules. However, when given time, plastics can slowly deform under very small loads, a phenomenon known as plastic creep. While plastics can recover from small creep deformations, they may not return to their original state once they start to turn white.
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Crazing
The colour change observed in crazed plastics is due to the scattering of light. When light hits the surface of an object, it can be absorbed, penetrate, or be reflected. The sum of all the reflected wavelengths is what gives the object its colour. In the case of crazed plastics, the scattering of light causes all visible light wavelengths to be reflected, resulting in a whitish appearance.
The occurrence of crazing can be observed through optical birefringence testing, which examines how a material refracts light. This testing method can help determine whether stress factors have caused the plastic to turn white. Additionally, DSC (Dynamic Scanning Calorimetry) can be employed to understand the thermal transitions that occur within the material, including crystallization, to ascertain if heat is the cause of whitening in a given plastic sample.
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Light scattering
When plastic is subjected to stress, it often exhibits a peculiar phenomenon: it turns white at the point of stress. This colour change, known as stress-whitening, is the result of light scattering within the plastic material. But why does this happen?
To understand why plastic turns white when stressed, we need to delve into the microscopic structure of plastics, which are made up of polymers. Polymers are long chains of molecules that can be either crystalline (neatly ordered) or amorphous (unordered). Most polymers contain both crystalline and amorphous regions along their molecular chains.
When stress is applied to a plastic material, such as by bending or twisting it, the strands of plastic material, or polymers, start to stretch and deform. This deformation can occur in two main ways: crazing and shear banding. Crazing is the more common form of deformation, characterised by the creation of voids and the alignment of molecular chains in the plastic. These voids are tiny fractures that form a spiderweb-like pattern, scattering light and causing the plastic to appear white. Meanwhile, shear banding involves a contraction of volume, which is less common in amorphous polymers that typically exhibit an increase in volume under compression.
The colour change in plastics due to light scattering is closely linked to the presence of crystalline regions within the polymer chains. When the crystalline region size is similar to the wavelength of light, it can effectively scatter light, making the plastic appear opaque or white. This phenomenon is known as stress-induced crystallization, where bending or stressing the plastic forces the polymer chains to align and induce crystallization in that region.
Additionally, the refractive index of the plastic also plays a role in light scattering. The refractive index determines how light is refracted or bent as it passes through a material. When crazing occurs, the addition of variously sized holes reduces the refractive index of the stressed area. This change in the refractive index, along with light scattering by the crazes, contributes to the whitening effect observed in stressed plastics.
While stress-whitening is a common occurrence in plastics, it is important to note that not all plastics exhibit this behaviour. The extent of whitening also depends on factors such as temperature, strain rate, and the type of plastic involved. Nonetheless, understanding the light scattering properties of plastics under stress provides valuable insights into their mechanical behaviour and potential failure modes.
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Heat as a factor
Heat is a significant factor in the whitening of plastic under stress. This phenomenon, called stress-whitening, occurs when a white line appears along a bend or curve in a material subjected to bending or punching operations. The colour change is due to light scattering by the crazes and changes in the refractive index of the stressed area.
When plastic is heated, its amorphous sections can crystallize. This crystallization causes the plastic to scatter light differently, resulting in a whitish colour. The degree of crystallization depends on the temperature and cooling rate of the plastic. DSC (Dynamic Scanning Calorimetry) is a technique used to understand the thermal transitions of materials, including the crystallization of plastics, by subjecting them to extreme heating and cooling cycles ranging from -90 to 725 degrees Celsius.
The mechanical properties of plastics are highly dependent on temperature and strain rate. At high strain rates and low temperatures, polymers become brittle because the molecules have less time to yield and accommodate the load. As a result, there is reduced whitening. Conversely, when given time to comply, plastics will slowly deform under very small loads, a phenomenon known as plastic creep.
The whitening effect of heat on plastics is not limited to bending or punching operations. For example, black plastic trash bags, when stretched, can also exhibit whitening due to the scattering of light. This is because the bags are usually filled with fine talc or calcium carbonate, which is bright and easily scatters light, causing the bag to appear white when stretched.
While heat can cause plastic to turn white, it is important to note that not all plastics will exhibit this behaviour. The whitening effect is more common in amorphous materials and some brittle polymers like PS, PMMA, and polycarbonate. Optical birefringence testing can be used to determine whether a plastic sample's colour change is due to heat or stress.
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Stress whitening in subcategories of plastics
Stress whitening, also known as stress-whitening, is a phenomenon where a white line appears on a material when it is stressed by bending or punching operations. This occurs in plastics due to the scattering of light caused by changes in the refractive index of the stressed area. This phenomenon is more commonly observed in amorphous plastics, which have unordered molecular chains. When stressed, the amorphous regions of these plastics can undergo crystallization, leading to light scattering and a whitish appearance.
Now, let's delve into the subcategories of plastics and their propensity for stress whitening:
Polypropylene (PP) Copolymers
PP copolymers are susceptible to stress whitening due to their unique composition. They are engineered to offer a balance between impact resistance and stiffness, which makes them prone to the development of microcrazes and microvoids when subjected to stress or impact. These microscopic flaws reflect light differently, resulting in a visible colour change.
Polymethyl Methacrylate (PMMA)
PMMA, also known as acrylic or plexiglass, is a type of amorphous polymer that is prone to stress whitening. When bent or stressed, the polymer chains in PMMA can align and induce crystallization, causing light scattering and a subsequent whitish appearance.
Polystyrene (PS)
PS is another brittle polymer that often exhibits stress whitening. Similar to other amorphous polymers, the bending or stressing of PS can lead to molecular changes and crystallization, resulting in light scattering and a whitish colour.
Polycarbonate
Polycarbonate is a polymer commonly used in various applications due to its strength and impact resistance. While it is known for its transparency, polycarbonate can also undergo stress whitening when subjected to significant bending or stress. The refractive index changes in the stressed areas, causing light scattering and a visible whitish line.
It is worth noting that the thickness of plastic parts also plays a role in stress whitening. Thinner sections generally exhibit more stress whitening and may require additional pigment to mask it. Additionally, certain pigments and resins can be strategically chosen to reduce the appearance of stress whitening in plastic products.
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Frequently asked questions
When plastic is stressed, the polymers within it start to stretch and change the way light is reflected, causing it to appear white. This phenomenon is known as "stress-whitening" and indicates the onset of failure of the material.
Stress-whitening is a phenomenon where a white line appears when a material is stressed by bending or punching operations. It is more common in amorphous materials and brittle polymers. The whitening is caused by light scattering by the crazes and changes in the refractive index of the stressed area.
Crazing refers to the formation of a fine system of voids or micro-fractures in the plastic due to applied stress. These voids scatter light, causing the plastic to appear white. Crazing is one of the two main failure modes of plastics, with the other being the rapid sliding of molecules.
