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Why is the oil stain on plastic tableware difficult to clean?

Update:11 Apr 2022

1. From the perspective of interfacial chemistry PP (po […]

1. From the perspective of interfacial chemistry
PP (polypropylene) is a typical non-polar material and is hydrophobic; ceramics (silicates) have strong hydrophilic properties because of a large number of hydroxyl groups (hydrophilic functional groups). Most contaminants on tableware (such as oil stains, etc.) contain a large number of non-polar groups (long-chain aliphatic hydrocarbons). According to the "structural similarity and mutual solubility principle", water molecules with strong polarity have affinity for ceramic materials with a large proportion of surface polar components. Strong, anti-pollution ability, non-polar oil has strong affinity to polypropylene, weak anti-pollution ability.
Further explanation is given with the help of the "adhesion work" theory proposed by Harkins (that is, the work done to pull the two-phase interface apart). The greater the work of adhesion, the firmer the solid-liquid interface is. The adhesion work of oil in water on the surface of the material is (oil oil, water water): Wows=γsw+γow-γso, and the surface tension of the material can be regarded as composed of polar components and dispersion components, γ=γd+γp.
Owens and Wendt used the geometric mean method to combine polarity and dispersion, and obtained the interfacial tension between the two phases as:
According to the above formula, the adhesion work of underwater oil stains and tableware can be obtained as:
Taking salad oil as an example and referring to the corresponding literature, its surface tension is 33mN/m, polar 9mN/m, and non-polar 24mN/m. PP polypropylene, the surface energy is about 30mN/m, and the polarity is 0. The surface energy of ceramics is between 40-60mN/m, mainly polar, and the dispersion part is very small. The surface tension of water is 72.8 mN/m, the polarity is 51 mN/m, and the dispersive part is 21.8 mN/m. According to the above formula, the adhesion work of salad oil in water on PP tableware is about 62mN/m, while the adhesion work of salad oil in water and ceramic tableware is about 7mN/m (taking the ceramic surface energy as 40mN/m, ignoring the dispersion component ). The adhesion work between salad oil and ceramics in water is far less than the adhesion work between salad oil and PP polypropylene, and the oil stains are more easily detached from the ceramic surface.
For example, when a drop of conventional salad oil drops on the ceramic surface, due to the presence of a large amount of water, the water will quickly spread on the ceramic surface, replacing the original solid-liquid interface between the oil droplet and the ceramic, and forcing the oil droplet to aggregate, with the continuous entry of water into the oil droplet and the ceramic surface. Between the ceramic interfaces, the contact area between the oil droplets and the ceramics decreases continuously. Since water is a polar liquid, it has a greater interaction with the highly polar ceramic surface, which directly leads to the gradual decrease of the force between the oil droplets and the ceramics, and the spreading of water on the ceramic surface becomes easier. Easier to displace oil stains from ceramic surfaces. And PP tableware is more difficult.
2. Compatibility
Polypropylene itself is a semi-crystalline long-chain aliphatic hydrocarbon with super large molecular weight. The long-chain fatty acid triglycerides in meals have good compatibility with PP, and can easily penetrate into the amorphous parts between PP crystals to cause surface swelling, and these oils that penetrate into the surface of PP cannot be washed off. High temperature will increase the rate of oil penetration into plastic, so PP tableware often used to heat milk and hot dishes will soon turn from transparent to white and foggy.
3. Cleaning method
The use of dish soap (a surfactant) reduces the surface tension of all liquids and provides partial relief from cleaning conditions. A good way to clean oily PP tableware is to use a dry method - paper towel (napkin) to wipe the bowl. Because paper towels have many microporous structures, rough surface structure and large surface area, paper towels can better absorb oil stains and absorb oil stains. The ability is several orders of magnitude stronger than PP. After wiping with a paper towel, wash it with water, add some detergent, and the PP tableware will be easier to clean.
If you want PP tableware to last for a long time, you should try to avoid using it to hold food rich in fat, especially hot food. Of course, some foods that seem harmless to humans and animals are not necessarily safe. For example, β-carotene (C40H56) in carrot juice can penetrate into the surface of PP containers at a speed visible to the naked eye. Can't be washed off.

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