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Water based binders SBR, CMC Na:

1) Introduction: SBR is the most widely used water-based adhesive. SBR stands for styrene butadiene rubber, which is highly soluble in water and polar solvents. It has high bonding strength, good mechanical stability, and operability. When used as an adhesive in the battery industry, it has good bonding effects and stable quality.

CMC Na: Sodium carboxymethyl cellulose, is the most widely used and widely used type of cellulose in the world today. It is a cellulose derivative with a glucose polymerization degree of 100-2000 and a relative molecular weight of 242.16. White fibrous or granular powder. Odorless, odorless, tasteless, hygroscopic, insoluble in organic solvents.

2) Bonding mechanism: The groups on the surface of SBR react with the groups on the surface of copper foil to form chemical bonds. SBR lotion itself is a product of the balance of hydrophilicity and hydrophobicity. On the one hand, it organically combines graphite through hydrophobicity, and on the other hand, it condenses with the surface groups of copper foil through hydrophilic groups. As a stabilizer and suspension dispersant, CMC Na has an auxiliary bonding effect on SBR, and can also make SBR more evenly dispersed. At the same time, the repulsive effect of space charge is utilized to ensure the stability of the entire system.

3) Synthesis method and detection parameters

Needle piercing low-temperature explosion-proof 18650 2200mah

Needle piercing low-temperature explosion-proof 18650 2200mah

Complies with Exic IIB T4 Gc explosion-proof standard

Charging temperature: 0-45 ℃

-Discharge temperature: -40~+55 ℃

-Maximum discharge rate at 40 ℃: 1C

-40 ℃ 0.5 discharge capacity retention rate ≥ 70%

Synthesis method of SBR: SBR is synthesized from 1,3-butadiene (CH2=CH-CH=CH2) and styrene (C6H5-CH=CH2). Generally, there are two methods: lotion polymerization and solution polymerization; By adjusting the ratio of the two, a series of products with different crosslinking degrees and glass transition temperatures (one is the temperature at which deformation occurs, and the other is the temperature at which the state transitions from a glassy state to a high elastic state) can be obtained.

Lotion polymerization: In the early stage, potassium persulfate was used as initiator to polymerize butadiene and styrene in free radical lotion at 50 ℃, commonly known as hot glue. At present, there is still a small amount of production. Starting from the 1950s, the polymerization temperature was generally lowered to 5 ℃ in industrial production, and the basic product was SBR1500, commonly known as cold glue. The production of cold glue is to disperse butadiene monomer in water lotion with rosin soap or fatty acid soap as emulsifier, use mercaptan as molecular weight regulator, and add redox initiator system consisting of organic peroxide, ferrous salt and activator for free radical polymerization. The lotion polymerization is carried out continuously in several series reactors, and the conversion rate is controlled at about 65%. Unreacted butadiene and styrene are successively removed by horizontal flash tanks and distillation towers, refined, and reused. The copolymer lotion with unreacted monomers removed is coagulated with sodium chloride, calcium chloride and acid, and the rubber generated is separated from whey by vibrating screen, and then dehydrated and dried to get the finished product. Compared with the hot glue, the cold glue has a lower degree of branching and cross-linking, the content of gel and low molecular weight is greatly reduced, and the performance is significantly improved, so it basically replaces the hot glue.

Solution polymerization: butadiene styrene rubber synthesized in non-polar solvents using butyl lithium as a catalyst Solution polymerized styrene butadiene rubber is divided into two types: block copolymers (i.e. thermoplastic rubber) and random copolymers. During the copolymerization process of solution polymerized butadiene styrene rubber, there is a tendency to spontaneously form polystyrene blocks. In order to synthesize copolymers with random distribution of styrene on the main chain (i.e. without polystyrene blocks), measures such as continuous addition of monomers, high-temperature polymerization at 90-150 ℃, and addition of ether, tertiary amine, phosphite, sulfide or surfactant as random agents can be taken. The molecular weight distribution of solution polymerized random SBR is narrower than that of lotion polymerized SBR, and the degree of branching is also lower. In order to reduce the cold flow tendency of raw rubber, it is necessary to add divinylbenzene or tin tetrachloride as a crosslinking agent during the copolymerization process to create a small amount of crosslinking between polymer molecules. Copolymers with different molecular weights can also be mixed to widen the molecular weight distribution. Solution polymerized random styrene butadiene rubber contains 35%~40% of the top isomer -1,4. Its wear resistance, flexibility, resilience, heat generation and other properties are better than lotion polymerized styrene butadiene rubber, and its shrinkage after extrusion is small. It can replace lotion styrene butadiene rubber in general occasions, especially suitable for making light colored or transparent products, and can also be made into oil filled rubber.

Substitution degree (DS) refers to the average hydroxyl value on the dehydrated glucose unit. If all three hydroxyl groups are replaced, the theoretical maximum value of DS is 3.0. The above figures show the states at a substitution degree of 0.7 and 1.2, respectively. The higher the substitution degree, the stronger the hydrophilicity, and it is also easy to absorb water. Choose appropriate CMC as the bonding agent and dispersant for the negative electrode as needed.

It should be noted that the joint use of SBR and CMC is a long-term accumulation result, and their effects are complementary and indispensable.

CMC and SBR are complementary and indispensable in the actual graphite negative electrode of lithium batteries, which is the result of long-term practice and accumulation in the industry. If only CMC is used as the adhesive, the condition is that the thickness of the electrode is relatively thin, no rolling process is carried out, or the compaction density of the electrode is not high; In actual polarizers, due to the requirement of energy density, graphite polarizers must be rolled and compacted with high density. In this case, CMC adhesive cannot be used alone because CMC is brittle and the structure collapses after rolling, causing severe powder loss and rendering the polarizers unusable; In addition, SBR cannot be used alone as a binder because it is difficult to prepare slurry. SBR does not have suspension and dispersion functions, and will the slurry settle? At the same time, too much SBR will also cause the electrode to swell in the electrolyte; The above mentioned problems can be basically solved by using CMC and SBR at the same time. Because graphite material itself is not hydrophilic, it is difficult to disperse in the water system. One of the functions of using CMC is to act as a dispersant, disperse graphite and conductive additive. In addition, CMC will form gel in water, making the slurry thicker. When large-scale coating, because of the existence of gel structure, it can not only keep moisture but also stabilize the slurry, and can maintain the uniformity of the slurry within a certain period of time, which is conducive to large-scale production; At the same time, SBR is introduced because SBR lotion is soluble in water, SBR itself is flexible material, and has good bonding performance. In this way, under the condition of high pressure compaction, the pole piece will not drop powder, and the bonding strength of the rolled pole piece is also high.

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