Catalysts, Additives and Auxiliaries for Polymers

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Polymerization Aids

Auxiliary substances added during the polymerization process, such as initiators, catalysts, emulsifiers, dispersants, molecular weight regulators and terminators. Its functions include initiating polymerization reactions, improving polymerization speed, adjusting system viscosity, controlling branching reactions and molecular weight, and improving polymer properties. Inhibitors added during monomer storage to prevent monomer polymerization are also polymerization aids.

Efficiency

polymers

Application areas include film, blow molding, extrusion, monofilament, fiber, pipe, wood plastic, plate, wire and cable, decoration and other fields.

  1. Reduce surface defects, such as common melt fracture, and improve product surface brightness and smoothness.
  2. Make color dispersion more uniform and glossier.
  3. Reduce mold wear during processing and eliminate die accumulation.
  4. Reduce gel produced during extrusion.
  5. Reduce processing temperature and extend continuous processing time.
  6. Improve product molding rate and dimensional stability and reduce scrap rate.

Working Principle

When the formula with polymer processing aid is added in the plasticizing extrusion process of the extruder, under the action of potential difference, the fluorine-containing polymer tends to move to the outer layer of the melt and adhere to the metal surface, forming a lubricating layer between the metal wall and the polymer melt (the formation of the lubricating layer takes 1-3 hours, depending on the size of the machine). During the continuous extrusion process, this coating layer is in dynamic equilibrium. When the dynamic equilibrium is stable, the extrusion process and product quality will be stable.

Raw Materials

Initiator (Chemical) - an overview | ScienceDirect Topics

Initiator

Substances that are easily decomposed into free radicals and used to initiate the polymerization of olefin or diene monomers. There are mainly azo compounds and peroxides, as well as oxidation-reduction initiation systems. When selecting an initiator, in addition to considering safety, toxicity, and coloring of polymers, the most important issue is activity. The activity of an initiator is often expressed in terms of half-life. The so-called half-life refers to the time required for the initiator to decompose to half of the initial concentration at a certain temperature, in hours. The shorter the half-life, the higher the activity. It is required to select an initiator with an appropriate half-life to ensure a certain polymerization rate. The polymerization temperature is different for different monomer properties, and commonly used initiators can only be used within a certain temperature range.

Polymer materials as catalysts for medical, environmental, and energy applications - ScienceDirect

Polymerization Catalysts

There are two types:

  1. Catalysts for ionic polymerization and coordination polymerization, which generally cannot be regenerated after the reaction, should also be called initiators in most cases according to their role.

    Cationic polymerization catalysts are electrophilic reagents and belong to the acid class, mainly Lewis acids, hydrogen-containing acids and substances that can produce cations, such as iodine. Lewis acids need substances that can precipitate protons (such as water) or substances that can precipitate carbon cations (such as RX) as co-catalysts.

    Anionic polymerization catalysts are nucleophilic reagents and belong to the base class, mainly alkali metals and their organic compounds. The activities of different catalysts vary greatly and have strong selectivity for monomers.

    Generally, monomers with poor ionic polymerization activity need to use catalysts with high activity. Coordination polymerization catalysts mainly include Ziegler-Natta catalysts, alkyl lithium, π-allyl transition metal compounds, etc.

    Ziegler-Natta catalysts are a general term for a large class of catalysts, mainly used for stereoregular polymerization of α-olefins, but also commonly used for stereoregular polymerization of dienes and cycloolefins. Ziegler-Natta catalysts are mainly composed of two components. The main catalyst is a transition metal compound of Group IV to VII in the periodic table, such as TiCl4, TiCl3, VOCl3, etc., and the co-catalyst is an organic metal compound of Group I to III, such as AlR2Cl, AlR3, RLi, R2Mg, etc. These two components can be matched in many ways, so there are many types and quantities. In addition, there are three-component, four-component and supported catalysts.

  2. Many polycondensation reactions require catalysts, such as inorganic acids, esters and salts for polyesterification reactions, acids or bases in phenolic and urea-formaldehyde polycondensation, and amines for synthesizing polycarbonates.
Emulsion Polymerization - an overview | ScienceDirect Topics

Emulsifier

An important auxiliary agent for emulsion polymerization, belonging to the surfactant class, there are cationic, nonionic and anionic emulsifiers. The role of emulsifiers is to reduce surface tension, form micelles to solubilize monomers, and stabilize polymer emulsions. The hydrophilic-lipophilic balance value (HLB value), critical micelle concentration (CMC value), and three-phase equilibrium point are important indicators for selecting emulsifiers.

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Dispersant

The auxiliary agent used in suspension polymerization plays a role in preventing polymer particles from sticking to each other. It can be divided into two categories: inorganic powder and water-soluble organic polymer.

  • Inorganic dispersants include carbonates, phosphates or hydroxides of calcium, magnesium and barium. They mainly play a mechanical isolation role and are relatively easy to remove with acid, so they are often used to make polystyrene transparent polymers.
  • Organic dispersants include natural polymers such as gelatin, alginate, and protein, cellulose derivatives such as methyl cellulose and hydroxypropyl methyl cellulose, partially alcoholyzed polyvinyl alcohol, sodium salts of copolymers of maleic anhydride and styrene or vinyl acetate, and synthetic polymers or copolymers such as polyacrylates.

They adsorb on the surface of droplets to form a protective film, while increasing the viscosity of the medium to prevent the two droplets from sticking together. The type and amount of dispersant have a great influence on the particle size and morphology of polymer particles. For example, when vinyl chloride is suspended and polymerized, loose polyvinyl chloride can be obtained by using polyvinyl alcohol or cellulose derivatives as dispersants, and compact resin can be obtained by using gelatin as a dispersant.

Recent developments in tubulin polymerization inhibitors: An overview - ScienceDirect

Inhibitors

Substances that prevent monomer polymerization. Inhibitors are often added during the storage and transportation of monomers, and they need to be removed before polymerization to avoid an excessively long induction period. Inhibitors include polyphenols, quinones, nitroso and nitro compounds, aromatic amines, organic sulfides and elemental sulfur. Stable free radicals can also be used as inhibitors, and oxygen also has an inhibitory effect.

Various inhibitors have different inhibitory effects on different monomers. In addition to referring to the inhibition constant, for monomers with electron-donating substituents such as styrene and vinyl acetate, quinones, aromatic nitro compounds, variable valence metal salts and other electrophilic polymerization agents are usually preferred. For monomers with electron-withdrawing substituents such as acrylonitrile, acrylic acid and its esters, phenols, amines and other inhibitors are preferred.

Terminator

Substances added before the end of polymerization to terminate the polymerization to prevent the quality of the polymer from varying. In essence, it is a highly efficient inhibitor, such as sodium dimethyl dithiocarbamate used in the production of styrene-butadiene rubber and bisphenol A used in the production of polyvinyl chloride. Ionic polymerization often uses water, alcohol or acid as a terminator.

Coagulant

Polyaluminum chloride is an inorganic polymer coagulant. It is an inorganic polymer water treatment agent with a large molecular weight and a high charge produced by the bridging effect of hydroxide ions and the polymerization of multivalent anions.

The basic component that causes the changeable form of polyaluminum chloride is OH ions. The indicator for measuring OH ions in polyaluminum chloride is called basicity (abbreviated as B). The basicity is usually defined as the equivalent percentage of OH and Al in the polyaluminum chloride molecule (〔OH〕/〔Al〕×100(%)).

The basicity of polyaluminum chloride is a relatively important indicator in polyaluminum, especially for drinking water grade polyaluminum products. This standard is one of the important indicators for Lebang polyaluminum production line production control. Lebang’s basicity is controlled between 40%-120%. The lower the basicity, the higher the price. Each purchaser can operate according to the actual situation of the factory. In addition, the basicity of polyaluminum chloride products produced and processed with different raw materials and different processes is also different, which requires manufacturers to make adjustments. Increasing the basicity of polyaluminium chloride products can greatly improve the economic benefits of production and use. Increasing the basicity from 65% to 92% can reduce the cost of raw materials by 20% and the cost of use by 40%.

References

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