III. EMULSION POLYMERIZATION AND EMULSION POLYMERS

Table 2 GR-S Recipe for Emulsion SBRa

Component

Styrene

Butadiene

Water (deoxygenated)

Fatty acid soap

Dodecyl mercaptan

Potassium persulfate



Parts by weight

25

75

180

5

0.5

0.3



a

Polymerization conducted at 50jC.

Source: Ref. 15.



are approximately 1017–1018 micelles per milliliter of emulsion (15). Most of

the monomer is contained in monomer droplets, which are in lower concentration (1010–1011 monomer droplets per milliliter emulsion) and much larger

than the micelles (15). When the mixture is heated to 50jC, the potassium

persulfate decomposes into radicals in the aqueous phase. Because the surface

area of the micelles is much greater than that of monomer droplets, the

radicals are more likely to inoculate the micelles to begin the polymerization.

A representation of this is shown in Figure 5.

As the polymerization proceeds, monomer migrates from the monomer

droplets to the micelles until the monomer droplets are gone. Chain transfer

to the mercaptan controls polymer molecular weight. Conversion is stopped

at approximately 70% by addition of a radical trap such as the salt of a

dithiocarbamate or hydroquinone. The latex is stabilized, then coagulated to

give crumb rubber.

A major improvement in this process was the development of the redox

initiation system shortly after World War II (16) (Table 3). With this recipe,

the polymerization could be conducted at 5jC by changing the initiator

system from potassium persulfate to cumene hydroperoxide. The iron(II) salt

lowers the activation energy for the decomposition of the cumene hydroperoxide and is oxidized to iron(III) during the process. The dextrose is present to

reduce the iron(III) back to iron(II) so more peroxide can be decomposed.

The importance of the lower polymerization temperature is shown in

Figure 6. As the polymerization temperature is decreased, the ultimate tensile

strength of cured rubber increases dramatically (17). This is because there is

less low molecular weight material and less branching at the lower polymerization temperature (18).

There is little control over butadiene polymer microstructure in the

emulsion process. It remains fairly constant at 12–18% cis, 72–65% trans, and

16–17% vinyl as the polymerization temperature is increased from 5jC to



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Figure 5 Species present during emulsion polymerization. (From Ref. 15. Reprinted by permission.)



50jC. Butadiene microstructure does not vary significantly as the styrene

content is changed (19). The glass transition temperature of emulsion SBR is

controlled by the amount of styrene in the polymer.



B.



Functional Emulsion Polymers



It is easy to incorporate a functional monomer into an emulsion polymer as

long as there is some water solubility. Emulsion butadiene or styrene



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Table 3 ‘‘Custom’’ Recipe for Emulsion SBR

Component

Styrene

Butadiene

Water

Potassium soap of rosin acid

Mixed tertiary mercaptans

Cumene hydroperoxide

Dextrose

Iron(II) sulfate heptahydrate

Potassium pyrophosphate

Potassium chloride

Potassium hydroxide



Parts by weight

28

72

180

4.7

0.24

0.1

1.0

0.14

0.177

0.5

0.1



Source: Ref. 16.



Figure 6 Effect of polymerization temperature on mechanical properties of ESBR.

(From Ref. 18. Reproduced with permission.)



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butadiene rubbers containing acrylate, amine, cyano, and hydroxyl groups

have been made. Although some recent work has been done in exploring the

interaction of functional emulsion rubbers with fillers, more work could be

done. Emulsion SBR containing 3–5% acrylonitrile displays better abrasion

resistance than the corresponding unfunctionalized rubber in carbon black

compounds (20). Emulsion SBRs containing one to four parts of copolymerized amines were compounded into silica-containing stocks and showed

good processability, improved tensile strength, lower hysteresis, and better

abrasion resistance than a corresponding emulsion SBR control (21).



C.



Oil-Extended Emulsion Polymers



A substantial percentage of the rubber used in tire compounds is oil-extended

emulsion SBR, which is prepared by adding an emulsion of oil to SBR latex

prior to coagulation. Oil extension allows higher molecular weight elastomers

to be used without processing problems, and incorporating the oil into the

latex is much easier than putting it in the compound at the mixer. The oils used

in compounding rubber are classified as paraffinic, naphthenic, and aromatic

depending on the aromatic content of the oil. The different types of oils affect

rubber compounds differently, and they cannot be directly substituted for

each other without compounding changes. The more paraffinic the oil is, the

lower its Tg, which will lead to different compound properties than a higher Tg

naphthenic or aromatic oil. Direct comparison of SBR 1712 (37.5 phr

aromatic oil) with SBR 1778 (37.5 phr of naphthenic oil) in a sulfurvulcanized stock showed that the 1778 stock had a six point higher room

temperature rebound and a higher 300% modulus but poorer wet traction

(22). Schneider et al. suggested using a higher surface area black and adding

small amounts of a higher Tg SBR to match the 1712 performance. Since the

late 1980s the aromatic oil used in SBR 1712 has come under fire for

containing polycyclic aromatics that may be a factor in causing cancer.

Compounders must be ready to make the necessary changes to eliminate

the high aromatic oil if necessary.



D.



Emulsion–Filler Masterbatches



Carbon black and carbon black–oil masterbatches of emulsion SBR have

been used commercially for a long time. They are prepared by blending a

dispersion of carbon black and oil with latex followed by coagulation.

Masterbatching offers the advantages of improved black dispersion and



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shorter mix times. A major problem with masterbatching is that it limits

compound flexibility to compounds that contain the type of black that is in

the masterbatch. There can also be unexpected effects on the vulcanization

rate (23). Surprisingly, there is no commercial counterpart in an emulsion

SBR silica masterbatch, although there have been a number of patents on the

subject (24–27). In most of these patents, a dispersion of silica and some material to reduce the filler–filler interaction is blended with the latex prior to

coagulation. The problems encountered with carbon black masterbatch are

also expected in silica masterbatches.



E.



Commercial Emulsion Polymers and Process



The International Institute of Synthetic Rubber Producers (IISRP) classifies

commercial emulsion polymers as shown in Table 4. Specifics (soap type,

Mooney viscosity, coagulation, and supplier) for different grades of polymers

are provided in the detailed section of the IISRP Synthetic Rubber Manual

(28).

A schematic representation of a commercial continuous emulsion SBR

process is shown in Figures 7 and 8. Most of the ingredients are mixed and

cooled, then combined with a solution of initiator immediately before they

enter the first reactor. The number of reactors is chosen to control the

residence time to reach 60–65% conversion in 10–12 hr. The polymerization

is shortstopped, and the latex is pumped to a blowdown tank and flash tanks

to remove most of the residual butadiene. A dispersion of an antioxidant is

added to protect the polymer through the subsequent processing steps and



Table 4 Numbering System for Commercial Emulsion

Polymers

Series no.

1000

1500

1600

1700

1800

1900



Description

Hot nonpigmented rubbers

Cold nonpigmented rubbers

Cold black masterbatch with 14 or

less parts of oil per 100 parts SBR

Cold oil masterbatch

Cold oil black masterbatch with more

than 14 parts of oil per 100 parts SBR

Emulsion resin rubber masterbatches



Source: Ref. 28.



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Figure 7 Emulsion polymer process—polymerization. (Courtesy of G. Rogerson, Goodyear Tire & Rubber Co.,

Akron, OH.)



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