Chemistry and technology of synthetic rubber. Textbook

The solution of titanium tetrachloride is taken from bottle by means of syringe in an amount calculated in advance, for example, 5.8 ml and then is injected into a bottle for the complex preparation placed in a beaker for cooling, which is put on a magnetic stirrer and the stirrer is turned on. Another syringe with 5.6 ml of TIBA solution or TIBA etherate, prepared in advance, is injected to the bottle for the complex preparation and the etherate is poured dropwise, freely draining, without pushing the hub. When the dropwise addition is over, it is stirred on cooling at least 5 minutes for the maturation of the catalytic complex or the complex is kept on cooling for 2 hours.

Polymerization

Magnetic stirrer is placed in dilatometer and tested, the tube is closed with stopper with sleeve nut and joint to a vacuum line (residual pressure is 1-3 mm Hg) by needle and rubber tube with valve for vacuum treatment at 180-200°C for one hour in a cylindrical furnace. When preparing several dilatometers the attachment is carried out through a manifold, and heating - at special electric furnace. After the dilatometer cooling to room temperature the vacuum is released with inert gas, the dilatometer is removed from the needle and filled with the batch using appropriate syringes.

Assembled syringe is purged with an inert gas on the device, needling the stopper, fixed by sleeve nut and lowering the needle end to the bend. Slowly raising the hub syringe is filled with an inert gas, then it quickly pulled down and raised again, and repeat this operation 2-3 times. Syringe filled with inert gas (the hub is in the up position) with the needle is attached to the vessel with a pre-prepared isoprene, the hub is pulled down and 3 ml of isoprene is taken to the syringe. If the vessel is under pressure it is released through the valve. Isoprene-filled syringe is removed and is needled with the free needle for excess inert gas release when pouring isoprene from syringe to the dilatometer.

Isopentane (25-300 ml) is injected to the dilatometer similarly with a syringe of larger volume and in such amount that the level of the liquid stood at 8-10 devision above. After the isopentane addition the dilatometer is placed to the water bath at 20°C, the stirrer is turned on and the content is stirred for 5-10 min.

For the catalyst take-off 2 ml syringe is used, which is "trained" as described above, then the calculated amount of the catalyst complex (1.6- 1.7 ml) is taken to a syringe, holding the bottle upside down by the sleeve

41

nut to prevent chilled bottle from the hands warming. Carefully removed the needle is injected into the dilatometer to the end, the catalyst solution is injected to the batch, then a stopwatch is turned on and the initial level of the reaction mass is written down. The catalyst syringe is immediately washed with ethanol. The mixing process goes until 30-40% conversion, noting the time of in the level decline at each division. The reaction is completed when the polymerizate volume reduction is stopped.

At the end of polymerization the dilatometer is revealed and the polymer solution is poured into a porcelain cup with alcohol. To facilitate pouring the dilatometer is fixed tube down under the lamp, and then it is filled with alcohol, the film of polyisoprene is removed by wire and combined with a polymer, swaged in a porcelain cup. Polymer is washed 2- 3 times with alcohol, cut into pieces and dried in a vacuum oven to constant weight, the yield of the polymer is determined.

The resulting polymer, according to the instructions of the teacher is

Determination of 1,4-trans and 1,2-units content in the diene rubbers

1 g of rubber is dissolved in a glass-stoppered flask in 100 ml of benzene and precipitated in 200 ml of ethanol. Reprecipitated rubber is dried in a thermostat at 50°C for constant weight and dissolved in carbon tetrachloride. Concentration of solution depending on the expected ratio of units of various configurations is 0.5-2%.

Part of the solution is used to determine the exact concentration of the solution by the method presented in the paper.

Optical density is determined in a cell with a layer thickness of 0.05 cm at 910 and 957 cm-1.

The content of 1,4-trans-units xtrans and 1,2-units x1,2 is calculated as:

where D-optical density; d - thickness of the layer in the cell, cm; 203 and 224 - the extinction coefficients;

c - concentration of the rubber solution, g/l

Determination of the polymer content in solution

This determination is important in controlling the solution polymerization. It consists of the precipitation of the polymer from solution by ethanol and drying the separated polymer until constant weight.

42

Dry weighing bottle with a glass rod is weighed on analytical balance with an accuracy of 0.0002 g. Then 2 g of polymer solution is loaded to the weighing bottle with a glass rod, which is quickly and tightly closed with a cap and weighed again with the same accuracy. Then it is poured 6-8 ml of ethyl alcohol to the weighing bottle and thoroughly stirred with a glass rod, the polymer is collected in a total mass, which is moved to the metal plate previously weighed with an accuracy of 0.0002 g. The plate is placed on the closed electric hot plate and the polymer is dried. The drying is considered to be completed by the color (light brown) and smell (smell of burning oil). At the end of drying the plate with the polymer is cooled to room temperature and weighed (with the same accuracy).

The polymer content in the solution x (wt%) is calculated by the

formula х а1 а 100 , b1 b

where a - the weight of empty plate, g;

a1 - the weight of plate with the dried polymer, g; b - the weight of empty weighing bottle, g;

b1 - the weight of the weighing bottle with polymer sample, g; Duration of analysis 1 hour.

For method of determining the viscosity, average molar mass of the polymer, see Lab 2. The coefficients K and for polyisoprene rubber, produced with heterogeneous catalyst based on titanium chloride are shown in Table 2.4

Table 2.4

The coefficients K and

Safety Information

Toluene - a liquid with a characteristic odor, mp -95°C, bp 110°C. It is highly soluble in organic solvents, poorly soluble in water. It is absorbed by intact skin; it has general toxic, irritant, narcotic, mutagenic and carcinogenic effects. MPC = 50 mg/m3, hazard class III.

Isopentane - a colorless liquid bp 16.55°C, mp -159.89°C. It is miscible with organic solvents, water-insoluble. In the air it is in the form of vapors. It has a narcotic effect. It causes cough, dizziness, drowsiness,

43

headache, short breath, sore throat, irregular heart rhythm. Hazard Class IV, MPC = 300 mg/m3. Use safety glasses, in combination with breathing protection in a fume hood.

Isoprene - a colorless flammable liquid. Isoprene is polymerized and readily oxidized when storing to form explosive peroxides. Therefore, it is stored in the presence of inhibitors (eg, hydroquinone). Before using isoprene is released from inhibitors by rectification. In high concentrations it acts like a drug, inhibits the formation of blood, in low concentrations irritates the mucous membranes, MPC = 0.04 mg/l. Use under a hood, prevent skin contact.

4 Production of Synthetic Rubber by Polycondensation

Polycondensation (or step-growth polymerization) is the formation of high molecular compounds as the result of molecular reactions of functional groups of monomers, in which along with the polymer formation usually, the low molecular weight substances release (water, alcohol, hydrogen chloride).

Polycondensation reaction can be divided into two groups: equilibrium and non-equilibrium polycondensation.

Upon the equilibrium polycondensation all reactions occurring in the system are reversible, so along with the chain growth, the reversible reactions of polymer degradation are possible as well. The scheme shows an example of the reversible equilibrium polycondensation of dicarboxylic acid and dihydric alcohol (diol):

The first stage of the reaction is an interaction of the functional groups of the initial reagents, leading to the formation of dimers. The next stage - is the interaction of the functional groups of dimers and the formation of tetramers. Then there is the interaction of the functional groups of tetramers. Thus, the polycondensation is a stepwise process in which the chain growth goes through the interaction of n-mers.

For more high molecular weight polymer it is necessary to shift the equilibrium to the right, and it's easier to make it by the removal of the formed low molecular weight product with typically low boiling point from the reaction zone.

44

Upon non-equilibrium polycondensation all reactions occurring in the system are irreversible.

Equilibrium and non-equilibrium polycondensation reactions differ in kinetic characteristics. The rate constants of equilibrium processes are small (10-3-10-5 l/(mol•s)), and their activation energies are rather high (84167 kJ/mol), they can be exoand endothermic. Non-equilibrium polycondensation reactions are characterized by high rate constants (up to 105 l/(mol s)) and low activation energy (8-42 kJ/mol), they are usually highly exothermic.

In the synthetic rubber industry polysulfide polymers and polyurethane elastomers are obtained using non-equilibrium polycondensation. Polyurethanes are produced by reacting of dior polyisocyanates with dior polyhydric alcohols:

nOCN R NCO + nHO R' OH

OCN [ R NHC(O)O R' OC(O)NH ] n-1 R NHC(O)O R' OH

Urethane formation is not accompanied by the formation of low molecular weight products and therefore in literature this process is sometimes called migration polymerization.

Most solid polyurethane elastomers are produced by reacting oligoetherdiol or oligoesterdiol with diisocyanates.

Highly elastic cross-linked polyurethanes are obtained by thermoset molding according mainly to the two-stage scheme. At the first stage the oligomer with terminal isocyanate groups (pre-polymer) is synthesized, which at the second stage is mixed with hardener containing groups reactive with isocyanate groups, and the composition is poured into molds for curing; such elastomers are called molded.

Solid polyurethane elastomers, obtained by reaction molding are applied in areas requiring high wear resistance and good mechanical properties, combined with long-term chemical resistance to oils and solvents.

In the industry there are high molecular linear or slightly branched polyurethanes containing no free isocyanate groups and sufficiently stable during storage. Such urethane rubber is processed and cured by the usual methods used in traditional rubber industry and it's called milled.

In addition, the large group of urethane elastomers is thermoplastic elastomers in which spatial network is formed by intense physical interactions or chemical bonds reversibly dissociating when heated. Thermoplastic elastomers do not require curing, and they are processed by

45

methods specific to the plastics processing technology. These materials are based on polyesters and (rarely) polyethers. Manufacturing is as follows: the components are mixed in a certain ratio by prepolymeric or one-step method, and then the mixture is poured into the open mold, where it cures.

Thermoplastic polyurethanes are widely used in the automotive industry, for electrical insulation, hoses, outside television cable sheath, and medicine.

Special additives can be included to the polyurethane elastomers: polycarbodiimides (antioxidants), UV stabilizers, "internal" lubricants (release agents), pigments, fillers, plasticizers.

Lab 5. Linear Polycondensation of Ethylene Glycol and Dicarboxylic Acid, Catalyzed by n-Toluenesulfonic Acid Melt

One of the initial components for solid polyurethane elastomers are oligoesterdiols (in the industry they are called polyesters), synthesized on the base of dicarboxylic acids and dihydric alcohols.

Linear polycondensation of dicarboxylic acid and glycol in the presence of a catalyst is described by a kinetic equation of the third order.

d[COOH ] d[OH] k[X][COOH ][OH] dt dt

where k - the rate constant of the third order (l2/(mol2•sec));

[X] - the catalyst concentration, which in the case of strong acid is constant; [COON] is the concentration of carboxyl groups;

[OH] is the concentration of hydroxyl groups.

If the initial reactants (the concentration of carboxyl and hydroxyl groups, respectively) are taken in equimolar amounts, i.e. [COOH] = [OH] = C, then

dC kxC2 , dτ

where kx = k[X] - the second order reaction rate constant (l/(mol•sec)), depending on the concentration of the catalyst. After integrating the equation becomes:

46

where Pn is the degree of polymerization, C0 and C are the initial and current concentration of one of the functional groups, q=(C0-C)/2 - the conversion of functional groups.

The equation of the degree of polymerization is valid for nonequilibrium processes and for the initial stage of equilibrium processes, when we can neglect the inverse reaction rate.

To determine the kinetic parameters of the linear polycondensation during the reaction the sequence of samples of the reaction mixture is taken and the content of unreacted carboxyl groups is determined by titration. The degree of polymerization, the degree of conversion and reaction rate constant are calculated. Activation energy of the polycondensation reaction is estimated on the basis of the temperature dependence of the rate constant.

Objective: To study the kinetics of catalytic linear polycondensation of ethylene glycol and dicarboxylic acid in the melt. Determination of the activation energy of the process.

Materials:

1)ethylene glycol, MM = 62.07 g/mol, mp -12.6 C, bp 10925°C, d=1.1155;

2)diethylene glycol MM = 106 g/mol, mp -8.0 C, bp 244.8 C, d=1.1177204;

3)adipic acid, MM = 146.15 g/mol, mp. = 153 C, bp. = 265100 C,

d=1.36025;

4)0.02 N KOH solution in 60% ethanol;

5)n-toluenesulfonic acid MM = 172.20 g/mol;

6)chloroform MM = 119.38 g/mol, mp -63.5 C, bp 61.15 C, d=1.48820;

7)ethanol MM = 46.07 g/mol, mp -114,15 C, bp 78,39 C, d=0,7851325;

8)1% alcoholic solution of phenolphthalein.

Apparatus: hot plate connected to the relay and contact thermometer; glass stirrer; laboratory jack; bath with Wood alloy; thermometer; two-neck flask, 150 ml; conical flask, 100 ml (6 pcs.); rubber bulb; measuring cylinders, 10 and 25 ml; 5 ml pipette; 25 ml burette; watch glasses; analytical balance.

Procedure

Assemble a device for the reaction performing, which is a two-neck flask equipped with a glass stirrer.

One neck remains open and is used for sampling the reaction mixture. The hot plate with oil bath is plugged in, but first the desired temperature of the contact thermometer must be set.

47

Polycondensation reaction is carried out at two work places simultaneously: one at 130°C and the other at 150°C. The reaction flasks are charged with reagents according to the Table 3.1:

Dry reagents are weighed on an analytical balance. The concentration of carboxyl groups and the catalyst in the initial mixture are calculated. The stirrer is turned on. In each experiment 6 samples are taken to the numbered flasks, which must be pre-weighted on analytical balance. The first sample is taken after 30 minutes, after the melting of the reaction mixture; the next 5 samples are taken at intervals of 15 minutes.

Before sampling the stirrer is turned off, the pipette is introduced into the reactor, warmed up for a few seconds, and 1-2 ml of the reaction mixture is taken using a rubber bulb. To prevent freezing the mass is quickly blown to the weighted flask. The flask with the sample is cooled and weighed. Then, each sample is analyzed for a content of carboxyl groups, and the results are listed in the Table 3.1.

Determination of carboxyl groups in polyesters

The determination is in the direct titration of the carboxyl groups with an alcoholic solution of alkali by the reaction:

C(O)OH + NaOH C(O)ONa + HOH

Method of analysis

0.5-1.0 g of polymer is placed with the help of glass tube into a flask weighted on an analytical balance with an accuracy of 0.0005 g and the flask is weighed again with the same accuracy.

The flask is poured with 5 ml of the solvent mixture (ethanol: chloroform = 1:1) using a pipette and the mixture is stirred until dissolving the sample. Then, 10 ml of distilled water, 2-3 drops of phenolphthalein are

48

added and the content of the flask is titrated with the alkali solution until permanent pinking.

At the same time the blank experiment (without polymer) is carried out: 5 ml of solvent mixture and 10 ml of distilled water are titrated with the same alkaline solution until discoloring.

The content of carboxyl groups X (in %) is calculated by the formula:

X (V V1) F 0.0009 100 , g

where V - volume of 0.02 N alkaline solution used for titration of the analyzed product, ml;

V1 - volume of 0.02 N alkaline solution, used for the blank experiment, ml; F - correction factor of 0.02 N alkaline solution;

0.0009 - the number of carboxyl groups, corresponding to 1 ml of exactly 0.02 N alkaline solution, g;

g - the sample of the analyzable product, g.

Results processing

Each sample taken at time τ, contains both reacted and unreacted molecules of monomer and catalyst. Therefore the material balance equation for the sample is:

g = Mlink No + (1-q) Mw No + Mx No. α,

where g - weight of sample, g;

Mlink - molecular weight of the repeating unit in the polymer chain per one ether bond, respectively;

Mw, Mx - molecular weight of water and catalyst;

No - moles number of carboxyl groups in the initial mixture;

α=Nx/No - the ratio of the moles number of the catalyst to the moles number of carboxyl groups in the initial mixture.

When sample titrating, the alkali is spent to neutralize the molecules, unreacted carboxyl groups of adipinic acid and to neutralize the catalyst. Thus,

b = V•T•10-3 = (1-q)•No+α•No,

where b and V - the amount of alkali used for the titration of the sample in (gE) and in ml, respectively, T - titer of alkaline solution (gE/l).

49

Let's introduce a neutral equivalent of a sample: Ge = g/b. Solving the overall equation relative to 1/(1-q), we obtain the basic equation for the construction of the kinetic curve of the polycondensation reaction:

1 q Mзвена Mx Ge

For a system of ethylene glycol - adipic acid the equation becomes:

For the system diethylene glycol - adipic acid, the equation becomes:

With the same coordinate axes two plots of 1/(1-q) versus time for two temperatures are built. The values of conversion degrees (q), corresponding to the analyzed samples are drown on the coordinate axis. Knowing the initial concentration of the carboxyl groups, on the base of the inclination of lines the rate constant (kx) is found which is calculated in the coordinates of the second-order reaction. Then, knowing the concentration of catalyst, the rate constant (k) is determined. The constants values are included in Table 3.3.

Table 3.3

The measurement results

Analyze the kinetic data of the polycondensation reaction. From the obtained value Ea draw a conclusion on the type of the process (equilibrium or non-equilibrium).

50