What are the polymer additives?

Auxiliary substances added in the polymerization process, such as initiator, catalyst, emulsifier, dispersant, molecular weight regulator and terminator. Its functions include initiating polymerization, increasing polymerization speed, adjusting system viscosity, controlling branching reaction and molecular weight, and improving polymer properties. The polymerization inhibitor added to prevent monomer polymerization during storage is also a polymerization assistant.

A substance that easily decomposes into free radicals and is used to initiate the polymerization of ethylene or diene monomers. There are mainly azo compounds and peroxides, as well as redox initiation systems. When selecting initiator, besides safety, toxicity and color of polymer, the most important thing is activity. The activity of initiator is usually expressed by 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 necessary to select an initiator with a suitable half-life to ensure a certain polymerization rate. The polymerization temperature varies with the nature of monomers, and the commonly used initiators can only be used within a certain temperature range.

An important auxiliary agent of emulsion polymerization belongs to surfactants, including cationic, nonionic and anionic emulsifiers. The function of emulsifier is to reduce the surface tension, form micelles to dissolve monomers and stabilize polymer emulsion. Hydrophilic-lipophilic equilibrium value (HLB value), critical micelle concentration (CMC value) and three-phase equilibrium point are important indexes for selecting emulsifier.

The additives used in suspension polymerization can prevent polymer particles from sticking to each other. It is divided into two categories: inorganic powder and water-soluble organic polymer. Inorganic dispersant is carbonate, phosphate or hydroxide of calcium, magnesium and barium, which mainly plays the role of mechanical isolation and is easy to be removed by pickling, so it is often used to make transparent polystyrene polymers. Organic dispersants include natural polymers such as gelatin, seaweed gum and protein, cellulose derivatives such as methylcellulose and hydroxypropyl methylcellulose, partially alcoholized polyvinyl alcohol, sodium salt of copolymer of maleic anhydride with styrene or vinyl acetate, polyacrylate and other synthetic polymers or polymers. They are adsorbed on the surface of the droplets to form a protective film, and at the same time, the viscosity of the medium is increased to prevent the two droplets from sticking. The type and dosage of dispersant have great influence on the particle size and morphology of polymer particles. For example, in the suspension polymerization of vinyl chloride, loose polyvinyl chloride can be prepared by using polyvinyl alcohol or cellulose derivatives as dispersants, and dense resin can be prepared by using gelatin as dispersants.

The substance that controls the molecular weight of polymer is essentially a chain transfer agent. The chain transfer constant and the amount of molecular weight regulator are important factors to control the molecular weight, and it is best to choose a molecular weight regulator with a constant close to 1. Thiols, such as dodecyl mercaptan and ethyl mercaptan, are often used in free radical polymerization, and hydrogen is often used in coordination polymerization.

A substance that prevents the polymerization of monomers. Polymerization inhibitors are often added with monomers during storage and transportation, and need to be removed before polymerization to avoid too long induction period. Polymerization inhibitors include polyhydric phenols, quinones, nitroso and nitro compounds, aromatic amines, organic sulfides and elemental sulfur. Stable free radicals can also be used as polymerization inhibitors, and oxygen also plays a role in polymerization inhibition. Various polymerization inhibitors have different polymerization inhibition effects on different monomers. In addition to the mentioned polymerization inhibition constant, in general, electrophilic polymerization agents such as quinones, aromatic nitrates and valence-changing metal salts are preferred for monomers with electron-donating substituents such as styrene and vinyl acetate, while polymerization inhibitors such as phenols and amines are preferred for monomers with electron-withdrawing substituents such as acrylonitrile, acrylic acid and their esters.

The substance added before the end of polymerization is used to stop polymerization and prevent the quality of polymer from changing. Essentially, it is an efficient polymerization inhibitor, such as sodium dimethyl dithiocarbamate for the production of styrene-butadiene rubber and bisphenol A for the production of polyvinyl chloride. Ion polymerization usually uses water, alcohol or acid as terminator.

Polyaluminum chloride is an inorganic polymer coagulant, which is produced by hydroxyl ion bridging and polyvalent anion polymerization, with large molecular weight and high charge.

The basic component that causes the morphological change of polyaluminum chloride is OH ion. The index to measure the OH ion in polyaluminum chloride is called alkalinity (abbreviated as B), which is usually defined as the equivalent percentage of OH and Al in polyaluminum chloride molecule ([OH]/[Al] × 100 (%)).

The alkalinity of polyaluminum chloride is an important index in the production of polyaluminum chloride, especially for drinking water-grade polyaluminum chloride products. This standard is one of the important indexes to control the production of polyaluminum production line in Le Bang. At present, the alkalinity of Le Bang is controlled between 40%- 120%. The lower the alkalinity, the higher the price, and the buyer can operate according to the actual situation of the factory. In addition, the alkalinity of polyaluminum chloride products produced by different raw materials and processes is also different, which needs to be adjusted by manufacturers. Improving the alkalinity of polyaluminum chloride products can greatly improve the economic benefits of production and use. When the alkalinity is increased from 65% to 92%, the cost of raw materials can be reduced by 20% and the use cost can be reduced by 40%.