Butadiene emulsion polymerization

Polychloroprene is the polymer of 2-chloro-l,3 butadiene. Emulsion polymerization produces an almost entirely trans-1,4 polymer, which is highly crystalline. Less crystalline polychloroprenes are produced by incorporating several wt.% of 2,3-dichloro-l,3 butadiene into the polymer to break up crystalline sequences. 

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Almost all synthetic binders are prepared by an emulsion polymerization process and are suppHed as latexes which consist of 48—52 wt % polymer dispersed in water (101). The largest-volume binder is styrene—butadiene copolymer [9003-55-8] (SBR) latex. Most SBRlatexes are carboxylated, ie, they contain copolymerized acidic monomers. Other latex binders are based on poly(vinyl acetate) [9003-20-7] and on polymers of acrylate esters. Poly(vinyl alcohol) is a water-soluble, synthetic biader which is prepared by the hydrolysis of poly(viayl acetate) (see Latex technology Vinyl polymers). 

The reactions of alkyl hydroperoxides with ferrous ion (eq. 11) generate alkoxy radicals. These free-radical initiator systems are used industrially for the emulsion polymerization and copolymerization of vinyl monomers, eg, butadiene—styrene. The use of hydroperoxides in the presence of transition-metal ions to synthesize a large variety of products has been reviewed (48,51). 

Synthetic. The main types of elastomeric polymers commercially available in latex form from emulsion polymerization are butadiene—styrene, butadiene—acrylonitrile, and chloroprene (neoprene). There are also a number of specialty latices that contain polymers that are basically variations of the above polymers, eg, those to which a third monomer has been added to provide a polymer that performs a specific function. The most important of these are products that contain either a basic, eg, vinylpyridine, or an acidic monomer, eg, methacrylic acid. These latices are specifically designed for tire cord solutioning, papercoating, and carpet back-sizing. 

Copolymers with butadiene, ie, those containing at least 60 wt % butadiene, are an important family of mbbers. In addition to synthetic mbber, these compositions have extensive uses as paper coatings, water-based paints, and carpet backing. Because of unfavorable reaction kinetics in a mass system, these copolymers are made in an emulsion polymerization system, which favors chain propagation but not termination (199). The result is economically acceptable rates with desirable chain lengths. Usually such processes are mn batchwise in order to achieve satisfactory particle size distribution. 

In addition to appHcations in dyeing, sodium formaldehyde sulfoxylate is used as a component of the redox system in emulsion polymerization of styrene—butadiene mbber recipes. 

The original SBR process is carried out at. 50° C and is referred to as hot polymerization. It accounts for only about 5% of aU the mbber produced today. The dominant cold polymerization technology today employs more active initiators to effect polymerization at about 5°C. It accounts for about 85% of the products manufactured. Typical emulsion polymerization processes incorporate about 75% butadiene. The initiators are based on persulfate in conjunction with mercaptans (197), or organic hydroperoxide in conjunction with ferrous ion (198). The rest of SBR is produced by anionic solution polymerization. The density of unvulcanized SBR is 0.933 (199). The T ranges from —59" C to —64 C (199). 

Acrylonitrile—butadiene copolymers (nitrile—butadiene mbber, NBR) are also produced via emulsion polymerization of butadiene with acrylonitrile,... 

ABS (acrylonitrile—butadiene-styrene) resins are two-phase blends. These are prepared by emulsion polymerization or suspension grafting polymerization. Products from the former process contain 20—22% butadiene those from the latter, 12—16%. 

Polymerization-grade chloroprene is typically at least 99.5% pure, excluding inert solvents that may be present. It must be substantially free of peroxides, polymer [9010-98-4], and inhibitors. A low, controlled concentration of inhibitor is sometimes specified. It must also be free of impurities that are acidic or that will generate additional acidity during emulsion polymerization. Typical impurities are 1-chlorobutadiene [627-22-5] and traces of chlorobutenes (from dehydrochlorination of dichlorobutanes produced from butenes in butadiene [106-99-0]), 3,4-dichlorobutene [760-23-6], and dimers of both chloroprene and butadiene. Gas chromatography is used for analysis of volatile impurities. Dissolved polymer can be detected by turbidity after precipitation with alcohol or determined gravimetrically. Inhibitors and dimers can interfere with quantitative determination of polymer either by precipitation or evaporation if significant amounts are present. 

Poly(butadiene- (9-acrylonitrile) [9008-18-3] NBR (64), is another commercially significant random copolymer. This mbber is manufactured by free-radical emulsion polymerization. Important producers include Copolymer Rubber and Chemical (Nysyn), B. F. Goodrich (Hycar), Goodyear (Chemigum), and Uninoyal (Paracdl). The total U.S. production of nitrile mbber (NBR) in 1990 was 95.6 t (65). The most important property of NBR mbber is its oil resistance. It is used in oil well parts, fuels, oil, and solvents (64) (see Elastomers, synthetic— nitrile rubber). 

At the same time, however, considerable research was being done, especially in Germany, on a novel process called emulsion polymerization, in which the monomer was polymerized as an emulsion in the presence of water and soap. This seemed advantageous since the product appeared as a latex, just like natural mbber, leading to low viscosity even at high soHds content, while the presence of the water assured better temperature control. The final result, based mainly on work at the LG. Farbenindustrie (IGF) (10), was the development of a butadiene—styrene copolymer prepared by emulsion polymerization, the foremnner of the present-day leading synthetic mbber, SBR. 

Distribution of the monomer units in the polymer is dictated by the reactivity ratios of the two monomers. In emulsion polymerization, which is the only commercially significant process, reactivity ratios have been reported (4). IfMj = butadiene andM2 = acrylonitrile, then = 0.28, and r2 =0.02 at 5°C. At 50°C, Tj = 0.42 and = 0.04. As would be expected for a combination where = near zero, this monomer pair has a strong tendency toward alternation. The degree of alternation of the two monomers increases as the composition of the polymer approaches the 50/50 molar ratio that alternation dictates (5,6). Another complicating factor in defining chemical stmcture is the fact that butadiene can enter the polymer chains in the cis (1), trans (2), or vinyl(l,2) (3) configuration ... 

Third Monomers. In order to achieve certain property improvements, nitrile mbber producers add a third monomer to the emulsion polymerization process. When methacrylic acid is added to the polymer stmcture, a carboxylated nitrile mbber with greatly enhanced abrasion properties is achieved (9). Carboxylated nitrile mbber carries the ASTM designation of XNBR. Cross-linking monomers, eg, divinylbenzene or ethylene glycol dimethacrylate, produce precross-linked mbbers with low nerve and die swell. To avoid extraction losses of antioxidant as a result of contact with fluids duriag service, grades of NBR are available that have utilized a special third monomer that contains an antioxidant moiety (10). FiaaHy, terpolymers prepared from 1,3-butadiene, acrylonitrile, and isoprene are also commercially available. 

The preparation and characterization of 1,3-butadiene monomer is discussed extensively elsewhere (1 4) (see Butadiene). Butadiene monomer can be purified by a variety of techniques. The technique used depends on the source of the butadiene and on the polymerization technique to be employed. Emulsion polymerization, which is used to make amorphous /n j -l,4-polybutadiene (75% trans-1 4 , 5% kj -l,4 20% 1,2), is unaffected by impurities during polymerization. However, both anionic and Ziegler polymerizations, which are used to prepare kj -l,4-polybutadiene, mixed cis-1 4 and... 

Other polymers used in the PSA industry include synthetic polyisoprenes and polybutadienes, styrene-butadiene rubbers, butadiene-acrylonitrile rubbers, polychloroprenes, and some polyisobutylenes. With the exception of pure polyisobutylenes, these polymer backbones retain some unsaturation, which makes them susceptible to oxidation and UV degradation. The rubbers require compounding with tackifiers and, if desired, plasticizers or oils to make them tacky. To improve performance and to make them more processible, diene-based polymers are typically compounded with additional stabilizers, chemical crosslinkers, and solvents for coating. Emulsion polymerized styrene butadiene rubbers (SBRs) are a common basis for PSA formulation [121]. The tackified SBR PSAs show improved cohesive strength as the Mooney viscosity and percent bound styrene in the rubber increases. The peel performance typically is best with 24—40% bound styrene in the rubber. To increase adhesion to polar surfaces, carboxylated SBRs have been used for PSA formulation. Blends of SBR and natural rubber are commonly used to improve long-term stability of the adhesives. 

The chemical structure of SBR is given in Fig. 4. Because butadiene has two carbon-carbon double bonds, 1,2 and 1,4 addition reactions can be produced. The 1,2 addition provides a pendant vinyl group on the copolymer chain, leading to an increase in Tg. The 1,4 addition may occur in cis or trans. In free radical emulsion polymerization, the cis to trans ratio can be varied by changing the temperature (at low temperature, the trans form is favoured), and about 20% of the vinyl pendant group remains in both isomers. In solution polymerization the pendant vinyl group can be varied from 10 to 90% by choosing the adequate solvent and catalyst system. 

Acrylonitrile-butadiene rubber (also called nitrile or nitrile butadiene rubber) was commercially available in 1936 under the name Buna-N. It was obtained by emulsion polymerization of acrylonitrile and butadiene. During World War II, NBR was used to replace natural rubber. After World War II, NBR was still used due to its excellent properties, such as high oil and plasticizer resistance, excellent heat resistance, good adhesion to metallic substrates, and good compatibility with several compounding ingredients. 

Chemistry of polychloroprene rubber. Polychloroprene elastomers are produced by free-radical emulsion polymerization of the 2-chloro-1,3-butadiene monomer. The monomer is prepared by either addition of hydrogen chloride to monovinyl acetylene or by the vapour phase chlorination of butadiene at 290-300°C. This latter process was developed in 1960 and produces a mixture of 3,4-dichlorobut-l-ene and 1,4-dichlorobut-2-ene, which has to be dehydrochlorinated with alkali to produce chloroprene. 

The free radical initiators are more suitable for the monomers having electron-withdrawing substituents directed to the ethylene nucleus. The monomers having electron-supplying groups can be polymerized better with the ionic initiators. The water solubility of the monomer is another important consideration. Highly water-soluble (relatively polar) monomers are not suitable for the emulsion polymerization process since most of the monomer polymerizes within the continuous medium, The detailed emulsion polymerization procedures for various monomers, including styrene [59-64], butadiene [61,63,64], vinyl acetate [62,64], vinyl chloride [62,64,65], alkyl acrylates [61-63,65], alkyl methacrylates [62,64], chloroprene [63], and isoprene [61,63] are available in the literature. 

Butadiene could be polymerized using free radical initiators or ionic or coordination catalysts. When butadiene is polymerized in emulsion using a free radical initiator such as cumene hydroperoxide, a random polymer is obtained with three isomeric configurations, the 1,4-addition configuration dominating ... 

A modified latex composition contains a phosphorus surface group. Such a latex is formed by emulsion polymerization of unsaturated synthetic monomers in the presence of a phosponate or a phosphate which is intimately bound to the surface of the latex. Thus, a modified latex containing 46% solids was prepared by emulsion polymerization of butadiene, styrene, acrylic acid-styrene seed latex, and a phosphonate comonomer in H20 in the presence of phosphated alkylphenol ethoxylate at 90°C. The modified latex is useful as a coating for substrates and as a binder in aqueous systems containing inorganic fillers employed in paper coatings, carpet backings, and wallboards [119]. 

Emulsion polymerization is the most important process for production of elastic polymers based on butadiene. Copolymers of butadiene with styrene and acrylonitrile have attained particular significance. Polymerized 2-chlorobutadiene is known as chloroprene rubber. Emulsion polymerization provides the advantage of running a low viscosity during the entire time of polymerization. Hence the temperature can easily be controlled. The polymerizate is formed as a latex similar to natural rubber latex. In this way the production of mixed lattices is relieved. The temperature of polymerization is usually 50°C. Low-temperature polymerization is carried out by the help of redox systems at a temperature of 5°C. This kind of polymerization leads to a higher amount of desired trans-1,4 structures instead of cis-1,4 structures. Chloroprene rubber from poly-2-chlorbutadiene is equally formed by emulsion polymerization. Chloroprene polymerizes considerably more rapidly than butadiene and isoprene. Especially in low-temperature polymerization emulsifiers must show good solubility and... 

Propagation constants for butadiene and isoprene were determined from rate of polymerization per particle in emulsion polymerization. 

We have considerable latitude when it comes to choosing the chemical composition of rubber toughened polystyrene. Suitable unsaturated rubbers include styrene-butadiene copolymers, cis 1,4 polybutadiene, and ethylene-propylene-diene copolymers. Acrylonitrile-butadiene-styrene is a more complex type of block copolymer. It is made by swelling polybutadiene with styrene and acrylonitrile, then initiating copolymerization. This typically takes place in an emulsion polymerization process. 

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