Chemistry and technology of synthetic rubber. Textbook

2.2 Chain Propagation

The radical formed at the initiation stage (e.g., RO•) involves in the reaction with the monomer only once. It originates the chain propagation:

RO• + M → RO-M•.

Further the propagation of polymer chain occurs only as the addition of monomer to the active macroradical:

~~M• + M → ~~M-M•.

2.3 Chain Termination and Transfer

Two principal modes of termination are combination and disproportionation, occurring by the interaction of two growing

In principle, the transfer of the active center can occur to any molecule in the polymerization system.

Chain transfer to monomer proceeds according to the scheme:

~~M• + M ~~M-H + •M-H,

where M-N is the monomer molecule from which a hydrogen atom is abstracted homolytically.

When transferring to polymer, hydrogen atom is abstracted from any part of the polymer chain:

~~M• + ~~MM~~ ~~MH + ~~M-•M-H~~

This gives rise to the growth of new polymer chains, which leads to the branching of the macromolecule.

In addition, chain transfer can occur to any molecule of another substance (solvent, special additives, etc.). In general, the reaction can be written as:

~~M• + A-H ~~M-H + A•.

If the radical A• is active enough to monomer addition, a new polymer chain is formed:

A • + M M • A, etc.

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Such substances are called chain transfer agents. One of the most effective industry chain transfer agent is tert-dodecyl mercaptan (S12N25SH).

If the radical A• is insufficiently active and unable to bind the monomer, it will only react with the growing polymer chains, stopping their growth:

~~M• + A• ~~M-A.

Therefore, these substances are the inhibitors of radical reactions.

2.4 Components of Emulsion Polymerization

The main components of emulsion polymerization are the dispersed phase (monomer or monomers mixture in the copolymerization), dispersing medium (aqueous phase), emulsifier and polymerization initiator.

Monomers. In the manufacture of synthetic rubber there are used: butadiene, chloroprene, styrene, -methyl styrene, acrylonitrile, 2-methyl- 5-vinyl pyridine, vinylidene chloride, methacrylic acid, etc.

Emulsifiers. In the production of the majority of emulsion rubbers and latexes there are used anionic surfactants like alkaline or ammonium salts of carboxylic acids RC(O)OMt (where Mt = Na, K), sulfonic acids (RSO3Mt - sulfonates) and sulfoesters (ROSO3Mt - alkylsulphates).

One of the main industrial emulsifiers is rosin soaps, containing about 90% resin acids based on abietic acid and its derivatives.

Н3ССООН

Abietic acid

Абиетиновая кислота

СН3

Abietic acid usually is exposed to disproportionation.

As emulsifiers there can be used carboxylic acetates (for example, laurates, stearates, oleates); synthetic fatty acids (SFA) of C10÷C13-, C10÷C18-fractions (paraffinates), C12÷C14-fractions; tall oil soaps; potassium alkilbenzylbenzoates of C14÷C18-fraction (ABBP); fluorinated aliphatic acids of general formula H(SF2)nCOOH, where n = 4 ÷ 10.

Another type of anion-active emulsifiers for the synthetic rubber industry are alkylarylsulfonates. They is successfully used the nekal

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emulsifier, a mixture of sodium salts of mono-, diand tributylnaphthalenesulfonic acid.

Among the group of alkylarylsulfonates is also leikanol (dispersing agent NF), a sodium salt of the β-naphthalenesulfonic acid and formaldehyde condensation product, corresponding to the formula:

Н

,

n

where n = 1÷9.

Electrolytes and addition agents. Electrolytes are used to reduce the surface tension and the viscosity of latex. Potassium chloride or sodium sulfates are the most widely employed electrolytes. Buffering agent is used to maintain pH during the synthesis of rubber. Sodium carbonate and trisodium phosphate are the most widely used buffer additives, which maintain the pH in the range 10.2±1.0 and 12.4±1.0, respectively. Their content is 2-4% of the polymer weight.

Initiators. As a water-soluble initiator is used potassium persulfate for the SR industry. As oil-soluble initiators are used hydroperoxides in combination with the oxidation-reduction systems.

Polymers chain transfer agents. In the SR industry mercaptans are the most general as chain-terminating agents, particularly, dodecyl mercaptan (S12N25SH), mostly tertiary; as well as organic disulfides, thioethers (for example, diisopropylxanthogen disulfide (diproxyd) and bis- ethylxanthogen disulfide). The amount of agent in the emulsion is 2-5% by weight of the polymer.

Stoppers (short-stopping agent). Emulsion polymerization has rather higher rate up to 60% conversion of monomers. In case then conversion exceeds 60%, secondary processes as cross-linking and isomerization start playing a significant role. The monomer conversion can reach 70% using active peroxides.

The time of 60-62% conversion depends on the mass ratio of the aqueous and hydrocarbon phases. The reaction time for standard system of butadiene- -methylstyrene at ratios of 200:100 and 133:100 is 16 and 20 hours, respectively.

Hydroquinone and sodium dimethyldithiocarbamate are used as stopper in the industry.

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Stabilizers (other terms are also used: antioxidants, antiaging agents, oxidation preventives). Stabilizer is a substance that protects the polymer in the latex and commercial rubber from oxidation and thereby maintaining the latex and rubber properties during storage and processing. Secondary aromatic amines (naphtham-2 and diphenyl-para- phenylendiamine (DPPD)), VTS-60 and VTS-61, DPPD derivatives; secondary amines VTS-120, VTS-150, VTS-200; VS-1, a condensation product of alkyl phenols with urotropin are used as stabilizers. There are also used phenolic compounds such as VS-30 (dioktylbutylphenol), different grades of Agidols; resorcinformaldehyde, alkylresorcinformaldehyde, aminophenol oligomers (resins) as stabilizers.

2.5 Polymerization Rate

The polymerization rate (W) is proportional to the concentration of emulsifier raised to the 0.5-th power for systems with water-soluble initiator:

W = k2 [M] [I]0,5 [E]0,5,

where k2 - the effective polymerization rate constant, [M], [I] and [E] the concentration of monomer, initiator and emulsifier, respectively.

For the systems with water-insoluble initiators the polymerization rate is proportional to the concentration of emulsifier raised to the first power:

W = k2 [M] [I]0,5 [E].

Thus, an emulsifier stabilize the drops of the monomer and polymer-monomeric particles and plays significant role for the kinetics of the polymerization process.

Lab 1. Preparation of Isoprene-Styrene Rubber via Emulsion Polymerization

Objective: observe the method of the isoprene and styrene emulsion copolymerization by addition of redox initiation system – [peroxide-iron- trilon-rongalite].

Materials:

1)isoprene d = 0.6849, bp 34°C;

2)styrene d = 0.906, bp 145°C;

3)water;

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4)potassium paraffinate;

5)leikanol;

6)potassium chloride, KCl;

7)ferrous sulfate FeSO4∙7 H2O;

8)rongalite HOCH2OSONa;

9)Trilon-B - disodium ethylenediaminetetraacetate dihydrate (NaOS(O)CH2)2NCH2CH2N(CH2C(O)ONa)2∙2H2O;

10)cumene hydroperoxide (HPIPB) S6H5C(CH3)2OOH, bp 50°C / 0.01 mmHg, d = 1.06;

11)"vistex-solvent", a mixture of toluene, isopropyl alcohol and water in the ratio 80:20:0.5 (volume ratio).

Apparatus:

1)glass vials, 50 ml;

2)Ubbelohde viscometer;

3)Schott filter №2;

4)isoprene and styrene distillation unit.

Procedure

Preparation of monomers

Before polymerization isoprene is distilled from impurities and inhibitor. Distillation unit consists of round-bottomed flask with capacity 250 ml, fish-bone reflux condenser, Liebig condenser with adapter and collection receiver. The flask, filled by 2/3 with isoprene, is attached to the unit. 3-5 pieces of porous porcelain are placed into the flask. The flask is heated in hot water bath, the temperature of the bath should not be higher than 45°C. Reagents should be heated in a hood with a steam bath, not a hot plate. The first fraction of distilled isoprene is poured into the sink of organic wastes, the fraction boiling at 34.5±0.5°C is run off for the polymerization. Distilled isoprene is stored when cooling until it is used, the storage life is no more than 6 hours. The distillation is carried out in a fume hood, and it is not allowed to have hooked up electrical appliances and open flame. Before distillation styrene is purified from inhibitor by polymerization. For this purpose, it is washed three times in a separating funnel with 5% sodium hydroxide solution and then with water until neutral (by phenolphthalein). Washed styrene is distilled under reduced pressure.

Preparation of the components of aqueous phase

The composition of the aqueous phase in the emulsion polymerization includes:

- surfactants (potassium paraffinate, leikanol)

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-buffer salts (potassium chloride)

-components of the redox system (ferrous sulphate, rongalite, Trilon B).

All reagents are easier to load in the form of aqueous solutions, so the components of the aqueous solutions are prepared immediately before the polymerization in separate beakers. The reagents concentrations are given in Table 1.1.

Table 1.1

Composition of the aqueous phase for emulsion polymerization

* The paraffinate aqueous solution is prepared by training staff beforehand and its concentration is varied in the range of 15-25%. Prior to the work execution its particular concentration should be clarified.

Calculating the number of reagents

The copolymerization is carried out at three different weight ratios of isoprene:styrene (90:10, 70:30, 50:50). The ratio of monomers:water phase and the composition of all the components of emulsion copolymerization remain constant.

Each vial is filled by 10 g of monomers for copolymerization. The aqueous phase is prepared once for all the experiments with the excess per 40 g of monomers. The copolymerization composition with the necessary quantities of reagents (g) is shown in Table 1.2.

The required amounts (ml) of solutions of the aqueous phase components from Table 1.3 are calculated using the formula (1.1):

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where A - the amount of reagent, g; C - concentration of reagent solutions in % wt., prepared in accordance with Table 1.1.

The amount of distilled water needed for one ampoule is reducing, because of addition of some amount of the water with the solutions of the components of the aqueous phase. Potassium chloride, ferrous sulfate, Rongalite and Trilon B solutions have a low concentrations, that’s why the amount of water in their solution are in round numbers equals to the volumes of the solutions.

The amount of water 2.0 - 0.45 = 1.55 ml is added with a solution of potassium paraffinate. The amount of water 1.0 - 0.03 = 0.97 ml is added with a solution of leikanol. Total amount of water injected with the reagents solutions: 1.55 + 0.97 + 4∙1.0 = 6.52 ml.

As a result, the preparation of the aqueous phase requires injecting 15 - 6.52 = 8.48 ml of water.

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The polymerization procedure

The components of the aqueous phase are prepared with an excess for 4 vials. The aqueous phase is prepared in bottles of a capacity of 100-250 ml. The calculated amounts of water and solutions are injected to the aqueous phase by using cylinder with a pipette. The amounts of the components are given in accordance with Table 1.3.

Table 1.3

Calculation of the components of the aqueous phase

The mixture in the bottle is thoroughly stirred with a stirring rod. The required amount of isoprene, styrene, cumene hydroperoxide in accordance with Table 1.4 and 15.5 ml of the aqueous phase are poured into three clean and dry bottles. Bottles are closed with rubber stoppers, fastened with a wire on the bottle's neck and placed in a thermostat. The polymerization is carried out at ambient temperature and continuous stirring for several hours.

The bottle should be opened at the end of the polymerization (in a fume hood!). A bit of hydroquinone is added into the synthesized latex to

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stop the polymerization process. Latex from the bottle is transferred to a rotary-film evaporator for uncured monomers recovery. The recovery is carried out at the bath temperature 60°C, isoprene is distilled at atmospheric pressure. The condensate receiver is changed and the distillation unit is connected to the water-jet pump. Unpolymerized styrene is distilled off under reduced pressure. The styrene distillation goes until the disappearance of large bubbles in a flask with latex.

The amount of the obtained latex (G, g) and its solids content (C1, wt%) should be measured at the end of degassing. The solids content is determined by weighing a cup of aluminum foil (Po) on an analytical balance to within 0.0001 g, small amount of latex is placed in and weighed again (P1). Then latex is evaporated in a special heater and the obtained polymer is dried until constant weight (P2). The solids content is calculated by the formula (1.2):

The polymer yield (include all nonvolatile substances in the latex) Gc is calculated by the formula (1.3):

The amount of polymer (Gn) is subtracted from the value of Gc, the weight of all nonvolatile products loaded into the vial:

Gn=Gc-(0.45+0.03+0.02+0.001+0.01+0.002+0.03)=Gc-0.543. The degree of conversion (K) can be found from the equation (1.4) (for the amount of monomers equal to10 g):

K Gn 100% (1.4) 10

Determination of the molecular weight of the polymer

The molecular weight of the polymer is determined by viscometric method using so-called "vistex-solvent". This method allows investigating the polymer without separating it from the latex. In this case the mixture of toluene, isopropyl alcohol and water in ratio of 80:20:0.5 (by volume) is used as the "vistex-solvent". The polymer solution is obtained by addition of 0.3 ml of latex to 30 ml of "vistex-solvent", further stirring until complete dissolution. The obtained solution is filtered through the Shott filter №2.

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The concentration of the filtrate (C2, %) is determined as a solids content of latex.

Concentration is calculated using the formula (1.5):

where A - the weight of solids content in the vial, g; B - the amount of filtrate, g; 0.0024 and 0.85 - adjustments for the salt content in the trilonrongalite recipe.

A specific viscosity (ηsp) of filtrate is determined at different concentrations of the polymer in solution (see measuring method in lab 2) .

sp c 0 ,

0

where с - flow time of the solution, sec; τ0 - flow time of the solvent, sec. The molecular weight of the polymer (M) is calculated by the Mark-

Kuhn-Houwink formula:

[ ] KM

For isoprene-styrene copolymers in "vistex-solvent" K=0.000535, = -0.658 (c).

The value of [η] is defined as the length of the ordinate cutted off by straight line of ηsp/C dependence on C.

A graph of the dependence of the conversion degree of the monomers and the polymer molecular weight on the styrene content in the monomer mixture is plotted on the base of data and then its impact on the process is revealed.

Safety Information

Styrene - is a flammable, colorless liquid. Exposure of humans to styrene may result in temporary irritation and narcotic effect. Long-term exposure to styrene can damage the kidneys, liver and central nervous system. Work under the hood, avoid contact of liquid styrene with skin.

Isoprene - is a colorless flammable liquid. Isoprene oxidizes readily in air to form unstable peroxides that may explode spontaneously and polymerize (uncontroled reaction) with heating. Therefore, it is stored in the presence of inhibitors (e.g., hydroquinone). Before using, isoprene is purified from inhibitors by rectification. In high concentrations, it acts like a drug, inhibits the blood formation, in low concentrations irritates the mucous membranes. MPC=0.04 mg/l.

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