How about the water-based binder SBR and CMC-Na for lithium-ion battery auxiliary materials?

Water-based adhesive SBR, CMC Na:

1) Introduction: SBR is the most widely used water-based adhesive, the SBR is the abbreviation of styrene-butadiene rubber, easily soluble in water and in polar solvent, has the very high bond strength and good mechanical stability and maneuverability, used in the battery industry as a binder, the binder has good effectiveness, stable quality.

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

2) bond mechanism: SBR groups on the surface of the condensation reaction with copper foil surface groups form chemical bonds.SBR latex itself is a product of hydrophilic and hydrophobic balance, on the one hand, through the hydrophobic organic combination of graphite, on the other hand through the hydrophilic groups and the copper foil surface groups in condensation reaction. And CMC - Na as a stabilizer, suspended dispersant, the SBR has assisted bonding effect, at the same time also can let the SBR dispersed more uniform, at the same time make use of the space charge repulsion to ensure the stability of the system.

3) Synthesis method and testing parameters

Synthesis methods of SBR, SBR by 1, 3 - butadiene (CH2 = CH - CH = CH2), styrene (C6H5 - CH = CH2) synthesis, emulsion polymerization and solution polymerization generally have two kinds of methods. By adjusting the proportion of the two, you can get a series of different degree of cross linking and the glass transition temperature (one is in the temperature of deformation, a state is from the glass state to the high elastic state transition temperature) of the product.

Emulsion polymerization: early is using potassium persul fate as initiator, styrene butadiene and under 50 ℃ for free radical emulsion polymerization system, commonly known as hot glue, there is still a small amount of production. In the 1950 s, in industrial production will be reduced to 5 ℃, polymerization temperature basic product is SBR1500, commonly known as the cold glue. Cold glue production is dispersed in the butadiene monomer rosin soap or water emulsion of fatty acid soap as emulsifier, using mercaptan as molecular weight regulator, joined by organic peroxide, ferrous salt and activator of oxidation - reduction initiator system composed of radical polymerization. Emulsion polymerization is continuous in several series of kettle, conversion rate control about 65%.Unreacted styrene butadiene and subsequently using horizontal flash tank and distillation tower, after removing the refined using again. Removal without reaction monomer copolymer emulsion with sodium chloride, calcium chloride and acid, such as cohesion, generated by the rubber by the vibrating screen and whey separation, dehydration, drying and then, for a quick finished product. Compared with hot glue, glue the branched cold and low degree of cross linking, gel and greatly reduce the content of low molecular weight, performance is improved significantly, so basically replaced the hot glue.

Solution polymerization: butyl lithium as the catalyst, in nonpolar solvent in the synthesis of styrene-butadiene rubber. And solution polymerization styrene butadiene rubber block copolymer and random copolymer (thermoplastic rubber) two kinds. Solution polymerization styrene butadiene rubber in the process of copolymerization, there is a tendency of spontaneous polystyrene block, in order to synthesize styrene random distribution in the main chain (i.e., do not contain polystyrene block) copolymer, can adopt continuous adding monomer, 90 ~ 150 ℃ high temperature polymer, as well as add ether, tertiary amine, phosphate, sulphur or surfactant as random agent, etc. Solution polymerization random styrene butadiene rubber narrow molecular weight distribution than the emulsion polymerization of styrene butadiene rubber, branched degree is low. In order to reduce the cold flow tendency of raw rubber, it is necessary to add in the process of copolymerization divinyl benzene or stannic chloride as cross linking agent, to produce a small amount of cross linking between polymer molecules. Can also be different molecular weight copolymer blending, widen the molecular weight distribution. Solution polymerization random styrene butadiene rubber cap type - 1, 4 isomer content is 35% ~ 35%, wear-resisting, flexure, the spring back, such as heating performance is better than the emulsion polymerization of styrene butadiene rubber, extruding the shrink, in general can be instead of emulsion styrene butadiene rubber, especially suitable for light and transparent products, also can be made into oil filled rubber.

Raw materials mainly come from the nature of the cotton, straw, wood, etc., the key parameters such as viscosity, degree of substitution and purity.

Degree of substitution (DS) refers to the sugar dehydration units on average hydroxyl value. If three hydroxyl groups were replaced, the maximum of 3.0 DS theory, above is the substitution degree was 0.7 and 0.7 respectively, the higher the degree of substitution, the stronger the hydrophilicity, also easy to water at the same time, according to the need to choose the appropriate CMC as anode bonding agent and dispersant.

Here to be sure, SBR and the CMC jointly use is the result of a long-term accumulation, is used to complement each other, be short of one cannot.

CMC and SBR in actual the lithium battery Graphite anode cathode is complementary to each other, be short of one cannot, is the result of the industry long-term practice. If simply using CMC as bonding agent on the condition that the thickness of the sheet is thinner, not on pole piece rolling process "or the compaction density is not high; In actual production, because of the requirement of the energy density, the graphite piece must roll, and compaction density is big, this case can't be used alone CMC binder, because the CMC is brittle, structure collapse, after rolling sheet off powder is serious, cannot use. In addition, also cannot separately using SBR as bonding agent, because it is hard to the preparation of slurry, SBR no have suspended function, slurry settlement will happen, also will make too much of a SBR sheet was swelling in the electrolyte; And CMC and SBR used at the same time can basically solve the problem mentioned above, because the graphite material itself is not water, it is difficult to disperse in water system, using a function of CMC is as dispersant, dispersed graphite and conductive additives, other CMC can form gel in water, makes the slurry thickening and large-scale coating, because of the existence of the gel structure, both water and stability of the slurry, in a certain period of time to maintain uniformity of slurry, is conducive to mass production. Because at the same time the introduction of SBR, SBR latex is soluble in water, the SBR is itself a flexible material, adhesive has good performance, this piece in the case of high pressure solid, do not lose powder, pole piece rolling sheet after the bonding strength is high.

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