Why is it difficult to clean oil stains on plastic tableware?

Update:08 Mar 2021

Why is it difficult to clean oil stains on plastic tabl […]

Why is it difficult to clean oil stains on plastic tableware?

   1. From the perspective of interface chemistry

  PP (polypropylene) is a typical non-polar substance, hydrophobic; ceramic (silicate) has a strong hydrophilicity due to a large number of hydroxyl groups (hydrophilic functional groups). Most of the contaminants in tableware (such as oil stains) contain a large number of non-polar groups (long-chain aliphatic hydrocarbons). According to the principle of "similar structure and mutual solubility", water molecules with strong polarity have an affinity for ceramic materials with a large proportion of surface polar components. Strong, strong anti-pollution ability, non-polar oil pollution has a strong affinity for polypropylene, and weak anti-pollution ability.

   With the help of the theory of "adhesion work" proposed by Harkins (the work done by pulling apart the two-phase interface) for further explanation. The greater the work of adhesion, the stronger the solid-liquid interface bond. The adhesion work of oil in water on the surface of the material is (oil, 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 average method to combine polarity and dispersion to obtain the interfacial tension between the two phases:

  According to the above formula, the adhesion work of underwater oil and tableware can be obtained as:

   Take salad oil as an example, check the corresponding literature, its surface tension is 33mN/m, polarity is 9mN/m, non-polarity is 24mN/m. PP polypropylene has a surface energy of about 30mN/m and a polarity of 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 dispersion 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, and the adhesion work of salad oil in water with ceramic tableware is about 7mN/m (the ceramic surface energy is 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 likely to be separated from the ceramic surface.

For example, a drop of conventional salad oil drips onto 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 oil droplet and the solid-liquid interface of the ceramic, and forcing the oil droplets to polymerize. Between the ceramic interface, the contact area between the oil droplet and the ceramic continues to decrease. Since water is a polar liquid, it has a greater interaction with the ceramic surface with high polarity, which directly causes the force between the oil drop and the ceramic to gradually become smaller, and the spreading of water on the ceramic surface becomes easier. It is easier to replace oil stains from the ceramic surface. The PP tableware is more difficult.

   2. Compatibility

  Polypropylene itself is a semi-crystalline long-chain aliphatic hydrocarbon with a super large molecular weight. The long-chain fatty acid triglycerides in the food have good compatibility with PP, and it is easy to penetrate into the amorphous part between the PP crystals to cause surface swelling, and these oils that penetrate the surface of the PP cannot be washed away. High temperature will increase the rate of oil penetration into the plastic, so PP tableware often used to heat milk and hot dishes will quickly turn from transparent to white and misty.

   3. Cleaning method

  Using detergent (a type of surfactant) can reduce the surface tension of all liquids and partially relieve the cleaning condition. A good way to clean the oil stains of PP tableware is to use the dry method-paper towels (napkins) to wipe the dishes. Because the paper towels have a lot of microporous structure, the surface structure is rough, and the surface area is large, so that the paper towels can better absorb oil stains and absorb oil stains. Ability is several orders of magnitude stronger than PP. After wiping it with a paper towel, wash it with water and add some detergent to make it easier to clean the PP tableware.

   If you want PP tableware to last longer, you must try to avoid using it to store fat-rich foods, especially hot foods. Of course, some foods that seem harmless to humans and animals are not necessarily safe. For example, the β-carotene (C40H56) in carrot juice can penetrate into the surface of PP containers at a speed visible to the naked eye. The coloring power is super strong, ensuring that the skin of the hand is broken. Can't wash off.