A research team used chlorine dioxide to successfully oxidize polypropylene plastic
[Translation by Dr. Nachmani Moshe]
Under LED light radiation, radicals of the substance chlorine dioxide (ClO2) attack the methyl groups of polypropylene, converting them into groups of carboxylic acids. Breaking the carbon-hydrogen bond was selective for the end chain. The oxidized surface can be dyed by cationic dyes. Oxidation of the surface of plastics is an important step in the industry, but existing methods are polluting or have only low control. As part of this clean and convenient process, it will be possible to create functional plastic for medical uses and the printing field.
Polypropylene is a product found everywhere, and is one of the most common types of plastic in everyday life. As a versatile material, the surface of this material, which is inert by nature, can be modified for specific uses. Researchers at Osaka University have now developed a convenient radiation-induced process for the oxidation of polypropylene, without producing harmful waste.
As published in the scientific journal ChemComm, the process uses radicals to make the inert plastic chemically active. The surface of polypropylene includes methyl groups that make up the side chains of the polymer. The strong carbon-hydrogen bonds within the methyl groups make polypropylene an unattractive material, which for many uses this feature is an advantage. At the same time, these strong bonds can be broken by highly reactive radicals, radicals of chlorine dioxide (ClO2).
"In applications such as printing and medical materials, the original plastic must be modified," explains one of the authors of the article Tsuyoshi Inoue. "Oxidation of carbon-hydrogen bonds is a well-known issue in textbooks in the field of organic chemistry. However, as for polymers, the risk is that any substance that is strong enough to break these bonds may also break the carbon-carbon bonds in the main chains in the polymer, while completely disintegrating The polymer itself. Fortunately, chlorine dioxide radicals are selective in their reaction with the side chain of the polymer and not with the central part of it."
The highly reactive radical can be prepared simply by mixing sodium chlorite with hydrochloric acid. In the next step, it only needs to be photochemically activated - for this, the research team chose an LED lamp as the lighting source. The activated ClO2 radical breaks down at this stage into a chlorine radical, which in turn reacts with a hydrogen atom present in the side chain of the polypropylene; And molecular oxygen (O2) is formed which in the next step oxidizes the exposed methyl groups. As a result, while the core of the polymer remains intact, its surface now includes many carboxyl groups, which change the reactivity of the polymer itself. For example, the colorless plastic can now be dyed by cationic dyes, such as Rhodamine B or Brilliant green, which react with the anionic carboxylate ions. The surface, which is naturally water repellent, becomes more hydrophilic.
"It turns out that the response actually proved to be doubly selective for our purposes," says the lead researcher. "Not only does the reaction break the carbon-hydrogen bond instead of the carbon-carbon bond, but it causes a selective oxidation of those groups that are in the side chain, even though they are stronger than those in the central part of the polymer. The reason for this is the fact that the oxidation step involving the molecular oxygen Especially preferred when the oxidation target is a methylene group (CH2)".
Previous methods for oxidizing olefinic polymers such as polypropylene and polyethylene were either poorly controlled or extremely polluting. The new process, therefore, is the first ever clean and convenient solution to this problem, and may be a valuable industrial tool in turning synthetic plastics into everyday consumer goods.
Article Summary the scientific
