Omar Khalid
There are several reasons why the surface of plastic may turn white. The most common cause is stress, which can lead to crystallization and cause the plastic to reflect all visible light wavelengths, making it appear white. This can happen when the plastic is bent or exposed to high temperatures. In addition, white spots on plastic can also be caused by the manufacturing process, normal wear and tear, or the use of hard water.
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What You'll Learn
- Plastic whitens due to stress-induced crystallization and crazing
- The temperature difference between the melt and the mould can cause white spots
- Hard water can cause a white film on plastic
- White spots can be caused by a rough surface during the moulding process
- Boiling water can repair whitened plastic
Plastic whitens due to stress-induced crystallization and crazing
Plastic is a polymer that can be classified as either crystalline or amorphous based on its molecular chain structure. Amorphous plastics have a disordered structure, which makes them transparent or translucent, while crystalline plastics have an orderly structure, resulting in higher density and opacity. When amorphous plastics are heated, they can undergo crystallization, leading to whitening as the molecules scatter light differently. Similarly, bending or applying stress to plastics can induce molecular changes that result in whitening.
This whitening phenomenon is known as "stress-whitening" or "crazing." Crazing occurs when localized deformation creates voids and aligns the molecular chains, leading to permanent deformation. The affected areas appear white due to light scattering by the crazes and a decrease in the refractive index, causing the reflection of all visible light wavelengths.
Stress-whitening is more common in amorphous materials and brittle polymers, indicating the onset of failure. The whitening can be repaired by submerging the plastic in boiling water, although this should be done with care to avoid distortion. However, if the plastic has started to turn white due to crazing, it may not recover its original shape.
In addition to stress and bending, other factors can contribute to whitening in plastics. Normal wear and tear, such as scrubbing or dishwasher use, can cause white spots or films on plastic surfaces. Hard water is a common culprit for the formation of white films. Higher temperatures can also induce crystallization and whitening in plastics, as observed in vacuum forming processes.
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The temperature difference between the melt and the mould can cause white spots
Plastic turning white is a result of crazing, which is the addition of variously sized holes that reduces the refractive index of the stressed area, causing it to scatter and reflect all visible light wavelengths. This phenomenon is influenced by temperature and strain rate, with high strain rates and low temperatures causing brittleness in polymers due to the reduced ability of molecules to accommodate the load.
When it comes to the temperature difference between the melt and the mould, a higher disparity can lead to increased residual stress in the plastic part. Consequently, when subjected to local forces, white spots are more likely to appear. This issue can be mitigated by reducing the melt temperature and minimising the temperature gap between the melt and the mould.
Additionally, a shorter cooling time can contribute to the occurrence of white spots. With insufficient cooling, the stress in the plastic part cannot fully balance, and when subjected to a large local force, white spots emerge. Extending the cooling duration helps alleviate this problem.
The injection pressure and holding pressure also play a role in the formation of white spots. Higher injection pressure results in more significant residual stress in the plastic part, leading to the appearance of white spots. Similarly, elevated holding pressure increases the stress in the plastic part, making it more susceptible to white spots when exposed to a substantial ejector force. Adjusting these parameters can help prevent the development of white spots.
Furthermore, a prolonged holding time can induce residual stress and the formation of white spots. Reducing the holding time is recommended to avoid this issue. By optimising these various factors, manufacturers can effectively minimise the occurrence of white spots on plastic products.
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Hard water can cause a white film on plastic
Hard water can cause a white film to appear on plastic items, leaving them looking dull and unclean. This is due to the high concentration of minerals, such as calcium and magnesium, in hard water. When hard water comes into contact with plastic and dries, it leaves behind chalky, cloudy residue and white spots. This can occur when plastic items are washed in a dishwasher, or when water droplets dry and leave behind mineral deposits.
The longer the exposure to hard water, the more likely it is that stains will develop. Hard water spots are the result of water with high iron content, and the limescale buildup is visible where the water is distributed. This can be removed with a number of household products. For example, white vinegar can be used to dissolve the marks without damaging the plastic. A 50/50 mixture of vinegar and water can be used for particularly tough stains, and the plastic can then be scrubbed with a cloth and left to air dry. Baking soda paste can also be applied to discoloured containers and left for 20-30 minutes before being washed, rinsed, and dried. Lemon juice can be used in a similar way, and the item can be left in the sun for 1-2 days to remove discolouration.
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White spots can be caused by a rough surface during the moulding process
The appearance of white spots on plastic products during the manufacturing process is known as a whitening phenomenon. This occurs due to a variety of reasons during the moulding process, one of which is a rough surface on the mould cavity.
When the surface of the mould cavity is rough, it increases the demoulding resistance, making it harder to remove the plastic product from the mould. This increased resistance can cause the formation of white spots on the plastic surface. To prevent this, it is important to reduce the surface roughness of the mould cavity.
Additionally, other factors during the moulding process can contribute to the development of white spots. These include:
- A small demoulding angle, which increases the force required to remove the plastic part and can lead to white spots.
- High injection pressure, resulting in large residual stress in the plastic and causing white spots when subjected to local force.
- Short cooling times, preventing the stress in the plastic from fully balancing and leading to white spots when subjected to a large force.
- Placement of the ejector pin on the weak part of the plastic, aggravating the occurrence of white spots.
By understanding and addressing these factors, manufacturers can minimise the occurrence of white spots on plastic products during the moulding process.
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Boiling water can repair whitened plastic
Plastic turning white is usually due to stress on the material. This can be caused by raising the temperature of the plastic above its glass transition temperature, allowing for crystallisation to occur. It can also be caused by normal wear and tear, such as scrubbing and washing plastic items.
Boiling water can be used to repair whitened plastic without distorting it. However, this method must be applied with care. A recommended method for whitening plastic is to submerge the plastic in hydrogen peroxide and place it in direct sunlight. This method is not reasonable for large items.
It is important to note that the effectiveness of boiling water in repairing whitened plastic may vary depending on the type of plastic and the cause of the discolouration. While boiling water can repair some types of plastic, it is not a guaranteed solution for all cases of whitened plastic.
Additionally, the presence of hard water, which contains high levels of calcium, can impact the effectiveness of using boiling water to repair whitened plastic. Boiling hard water yields calcium carbonate, which can help to ensnare and remove plastic particles. As a result, boiling water may be more effective in removing microplastics from hard water than from soft water.
In conclusion, boiling water can be a potential solution for repairing whitened plastic, but it should be approached with caution and an understanding of the specific plastic and discolouration involved.
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Frequently asked questions
Your plastic surface is white because it has been exposed to high temperatures or stress.
When the temperature of plastic rises above its glass transition temperature, it undergoes crystallization, which causes the plastic to turn white.
When stress is applied to plastic, it undergoes stress-induced crystallization, which causes the plastic to turn white.
Crazing is a series of very fine fractures that cause a white spiderweb pattern on the plastic. It occurs when the plastic is under stress or chemically attacked, and it scatters light, making the plastic appear white.
