Polymerization Additives

Polymer additives refer to a class of auxiliary substances added during the polymerization of low-molecular-weight monomers into high-molecular-weight polymers. Their functions include initiating polymerization, improving polymerization speed, adjusting system viscosity, controlling branching reaction and controlling molecular weight, improving polymer performance and other aspects. For example, the addition of plasticizers can lower the molding temperature of polyvinyl chloride resin and make the product softer. Adding a foaming agent to the polymer can produce a lightweight, vibration-proof, heat-insulating, and sound-insulating foam. There are many kinds of polymer additives, including plasticizing agents, pigments, UV Stabilizers and antioxidants and formability additives and so on.

Classification:

• Plasticizer: Although some polymers (such as rubber) are naturally flexible, others (such as lignin or nitrocellulose) are relatively hard and cannot be softened by exposure to non-solvent materials. Therefore, plasticized compounds can be added to the polymer to reduce its stiffness and increase its formability. Plasticizers gather in molecular groups between different polymer chains without changing the volume of the polymer. The solubility of the plasticizer should usually be close to that of the polymer itself, and a variety of plasticizing additives can be used in a single mixture, as long as they are compatible with each other and with the polymer. When plasticizers (such as dioctyl phthalate) are introduced into polyvinyl chloride (PVC) polymer, it reduces its melt viscosity and improves its photostability.

Figure 1. Plasticizer dioctyl phthalate

• Pigment additives: Pigment additives can be used to change the color of polymer materials, which is beneficial to a range of consumer products. However, some pigments increase the sensitivity of polymers to chemical reactions, so careful selection is an important step in the process of pigment deposition. For example, carbonate pigments (such as limestone) make materials vulnerable to inorganic acids. These effects can be mitigated by the addition of adhesives (such as organosilanes), which help compensate for the lack of bonds between pigments and polymer molecules.

Figure 2. Silane polymer-stabilized super-hydrophobic fabric

• Ultraviolet stabilizers : Many unsaturated polymers will be degraded for a variety of reasons, such as the absorption of oxygen, resulting in the release of organic peroxides. To reduce the risk of mechanical and chemical degradation, phenolic or amine antioxidants can be added to these polymers. Some common additive groups used for these purposes include: (1) blocked phenol: phenol consists of benzene rings connected to hydroxyl groups. When the bonded hydrogen of adjacent carbon atoms is replaced by heavier elements, the result is a blocked phenolic compound that prevents the oxidative degradation of polymers such as rubber. (2) antioxidants: secondary amines, which include aromatic hydrocarbons (such as benzene) in their composition, can be used as antioxidants to protect polymers from oxidative deterioration.
• Antioxidants: Some organic polymers are degraded by photooxidation, which is caused by oxygen irradiated by ultraviolet rays. Pigments such as carbon black and ultraviolet stabilizers such as hydroxy benzophenone can be added to inhibit the rate of photooxidation.
• Molding additive: Polymers with poor formability can be improved with lubricants or processing aids. These auxiliaries prevent the material from sticking to the metal surface and reduce the energy required to make the material. Some additives, such as acrylic polymers, can improve impact resistance and are easier to process. PVC materials used for molding or extrusion are usually treated with lubricants, processing aids and impact additives.

Figure 3. Molding additive polyacrylic acid.

References:

1. H. Nakata, A. Nishidome, N. Shikata. (2013) “Benzoriazole UV stabilizers in road dust and estimation for their amounts on the surface of the road.”J. Environ. Chem. 23:37-42.
2. Lingxiao Li, Bucheng Li. (2016) “Roles of silanes and silicones in forming superhydrophobic and superoleophobic materials.” J. Mater. Chem. A, 4, 13677-13725.