Chemistry and technology of synthetic rubber. Textbook

Lab 6. Obtaining Cast Polyurethane

At the basis of the obtaining of any type of polyurethane is the reaction between aromatic or aliphatic diisocyanates with hydroxyl compounds.

RNCO + R OH RNHC(O)OR

Basic compounds for urethane elastomers are oligomers with terminal hydroxyl groups (diols) with molecular weight from 700 to 3000, and aromatic diisocyanates. Based on diols and 2,4-toluene diisocyanate (TDI) the prepolymer can be pre-obtained, which is an oligomer chain having terminal isocyanate groups.

For the subsequent structuring of the prepolymer extending and crosslinking agents are used - the low molecular weight polyols and diamines.

Objective: Synthesis of cast polyurethane rubber based on the oligoesterdiol and 2,4-toluene diisocyanate.

Materials:

1) polyethylene glycol adipate (PEA), MM = 1800-2000, mp = 40°C;

HO(CH2)2 O C (CH2)4 C O n (CH2)2 OH

OO

2)) polyethylene butylene glycol adipate (PEBA), MM = 1800-2000, mp 40°C;

3) 2,4-toluene diisocyanate (TDI), MM = 174, bp 220 C, mp 19-21 C;

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CH3

NCO

NCO

4) 1,1,1-trimethylolpropane (TMP) MM = 134, mp 40 C.

CH2OH

CH3 CH2 C CH2OH

CH2OH

Apparatus: three-neck round bottom flask, 200 ml; thermometer to 100°C; hot plate; glass stirrer; 2 glass rods; water-jet or vacuum pump.

Procedure

The required amount of oligoesterdiol (50 g) is loaded in a threenecked flask with thermometer and stirrer, and the flask is heated by electric hotplate. After oligoesterdiol melting and reaching the temperature of 100°C the water-jet or oil pump is connected to the flask. Upon establishing the residual pressure of less than 10 mm Hg in a system, the oligoesterdiol is drying from residual moisture for one hour, at 100°C and stirring.

After one hour the hotplate is turned off, and still vacuuming and stirring, the temperature of the oligoesterdiol is lowered to 60°C. Then the vacuum is released, the stirring is stopped and the calculated amount of TDI, as in the following method, is poured into the neck of the flask. After adding TDI the system is reconnected to the vacuum pump and the synthesis of prepolymer (the oligomer which is a reaction product of 1.0 mol of oligoesterdiol and 2.0 mol of TDI, and having terminal reactive isocyanate groups) is carried out at 80°C. Immediately after the addition of TDI, and then at 15, 30, 45 and 60 minutes, counting from the time of adding TDI, several drops (0.3-0.5 g) of prepolymer is taken with a clean glass rod in a 100 ml flat-bottomed flask pre-weighed on an analytical balance with an accuracy of four decimal digits, and is analyzed for the content of isocyanate groups.

Calculating TDI

2 moles of TDI is taken to 1 mole of oligoesterdiol. The estimated weight of TDI is calculated in accordance with this ratio:

1800 g - 2•174 g

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50 g - x g, where x = 9.67 g of TDI,

where 50 g - weight of oligoesterdiol.

In case of changing of oligoesterdiol molar mass (MM) the value of 1800 is replaced by corrected value of MM. Subject to the specific density of TDI of 1.22 g/ml the required volume of TDI is calculated.

VTDI = 9.67 g / 1.22 g/ml = 7.92 ml

Theoretical calculation of the percentage of isocyanate groups

The molecular weight of two isocyanate groups is 84, and MM of TDI is 174, hence the sample of TDI contains the following amount of isocyanate groups:

174 g - 84 g

9.67g - x g

x= 4.67 g of NCO-groups.

The weight of oligoesterdiol and 2,4-TDI mixture is 50 + 9.67 = 59.67 g. Concentration of isocyanate groups in the mixture is calculated as follows:

59.67 g - 4.67 g

100 g - x g

x = 7.80 g (at 100 g), or 7.80% of NCO groups.

This is the theoretical content of the NCO-groups in unreacted mixture at the initial time.

At 50% conversion of the NCO groups the reaction mixture should

contain

7.80% - 100% x% - 50%

x = 3.90% unreacted isocyanate groups or 7.80% - 3.90% = 3.90% of free NCO-groups.

Determination of the percentage of isocyanate groups

The method is based on the interaction of the isocyanate groups with a secondary amine according to the following reaction:

Excess of the secondary amine is titrimetrically determined by the following reaction:

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Methods of analysis

A clean, dry conical flask, weighed with an accuracy of 0.0001 g, is added with 0.3-0.4 g of the analyzed product using a glass rod and then it is weighed with the sample with an accuracy of 0.0001 g. Then 10.0 ml of 0.2 N solution of diethylamine in acetone is injected into the flask by a pipette, and the time of injecting is fixed. The flask content is shaken to dissolve the sample and then is titrated by 0.1 N solution of hydrochloric acid in the presence of bromphenol blue indicator until the blue color changes to the greenish-yellow. Simultaneously, the blank experiment is carried out.

The content of NCO-groups (X) in% by weight is calculated using the formula:

X (a b) F 0.0042 100 , g

where a- the amount of 0.1 N hydrochloric acid solution, which used for the titration in the blank experiment, ml; b-the amount of 0.1 N hydrochloric acid solution, which used for the titration of the sample, ml; F - correction factor of 0.1 N hydrochloric acid solution; 0.0042 - the amount of NCOgroups corresponding to 1 ml of 0.1 N hydrochloric acid solution, g; g - the weight of the analyzable substance in the sample from the solution, g.

5 Production of Synthetic Rubber by Polymer-Analogous Transformations

Polymer-analogous transformations are the chemical reactions of polymers, in which the polymerization degree (and thus the molecular weight) of polymer does not change (or changes very little). Each elementary unit of the macromolecule appears in these reactions as an independent reactive group. Thus, as a result of polymer-analogous transformations the chemical nature of the elementary unit changes, meanwhile the degree of polymerization does not change.

The most reactive rubbers are the high molecular unsaturated polymers of dienes (isoprene, butadiene, chloroprene) and their copolymers with vinyl monomers (styrene, acrylonitrile, isobutylene, etc.) due to the

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high reactivity of the C=C bond (isoprene, butadiene, butadiene styrene, butadiene nitrile and other diene rubbers). High molecular saturated rubbers (urethane, polysulfide, silicone) enter into chemical reactions mainly at the terminal functional groups, the content of which is negligible and they are usually not used for polymer-analogous transformations (chemical reactions of functional liquid rubbers of mentioned types are discussed in the relevant section).

Polymer-analogous transformations of unsaturated rubbers include the addition reactions, substitution, cis-trans-isomerization reactions. The degree of polymerization does not change or changes only slightly (it should be remembered that the chemical reactions of polymers in general, and particularly rubbers never occur in the same direction).

Unsaturated rubbers easily interact via double C=C-bond with sulfur-and nitrogen-containing compounds, hydrogen halides, maleic anhydride, halogens, azodicarboxylic acid esters, nitriles, peracids, hydrogen, nitrosocompounds, etc. Herewith the rubber properties vary widely.

Two types of rubber are produced in an industrial scale using polymer-analogous transformations: chlorosulphonated polyethylene and halogenated butyl rubbers.

Chlorosulphonated polyethylene is prepared by treating the solution of polyethylene (MM 18000 - 30000) with chlorine and sulfur dioxide. Sulphochlorination proceeds by a radical mechanism (peroxides are the most commonly used for the formation of free radicals). During sulphochlorinating the following reactions occur:

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The final product contains 26-29% of the chlorine and 1.3-1.7% of sulfur, which corresponds to the following structure of chlorosulphonated polyethylene:

Cl

One of the most important processes of the synthetic rubber production using polymer-analogous transformations is halogenation reactions.

Halogenation of unsaturated rubbers can occur both by the radical and ionic mechanisms:

Cl

etc.

The main reactions are usually accompanied by side reactions such as cyclization, crosslinking of the macromolecules, and the addition of hydrogen chloride to the double bonds.

Chlorinated 1,4-cis-polyisoprene containing 66-68% of chlorine, is used for lacquers, printer's inks, binders, etc.

Chloroand bromobutyl rubbers (CIIR and BIIR) are commercially produced by halogenating the copolymer of isobutylene and isoprene (butyl rubber).

Only isoprene links of copolymer are chlorinated. The hydrogen substitution reaction occurs, meanwhile up to 75% of unsaturation retains and the following chemical structures are formed:

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The chlorine content in the rubber reaches 1.1-1.3% by weight.

Unlike chlorine, bromine is more inclined to addition reactions, but under controlled conditions it is possible to save 90% of the initial unsaturation:

~ CH2 CCH CH2 ~ +Br2 -HBr ~CH2 C CH CH2~ Br

The bromine content in the rubber is 2-3% by weight.

Lab 7. Synthesis of Halogenated Butyl Rubber

More than 60% of butyl rubber produced in the world is subjected to halogenation. Halogenated butyl rubbers - are the products of the interaction of butyl rubber with chlorine or bromine containing about one halogen atom for one isoprene link. As a result of halogenation the butyl rubber has:

-the increased adhesion to metals;

-extended assortment of vulcanizing agents (the most effective curing agent is zinc oxide);

-the increased reactivity of the double bonds;

-the possibility to covulcanization of the mixtures of halogenated butyl rubber with highly unsaturated rubbers as Ti-BD, Ti-IR, etc.

Chlorobutyl contains 1.1-1.3% of the chlorine attached mainly in - position to the double bond of the isoprene units of macromolecules. Chlorobutyl has about 75% of the unsaturation of the original butyl rubber. Bromobutyl contains about 2% of bromine, 90% of which is in the - position to the double bonds of the isoprene units.

Bromobutyl has the best properties.

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Halogenation is usually carried out in solution, the solvent is an aliphatic or chlorinated hydrocarbons - hexane, gasoline, carbon tetrachloride.

The amount of halogen which should be introduced into the polymer depends on the unsaturation of initial butyl rubber (IIR, brand name BK) and is about one atom of chlorine for one double bond. Halogen concentration corresponding to one atom for the double bond is called "critical" and is calculated by the following formula:

a U 100

X 100 U M1 M2 a ,

where X - wt% of halogen in the halogenated BR; U - unsaturation of BR, % mol.; M1 - the molecular weight of isobutylene; M2 - the molecular weight of isoprene; a - the atomic weight of the halogen.

With the introduction of more than one atom of chlorine for the double bond, the molecular weight of the polymer significantly decreases. Usually for the reaction the halogen is taken twice as much as calculated since half of it is released as hydrogen halide.

Halogenation reaction time is determined by conditions of the full distribution of the halogen in the reaction volume and is practically no more than 2 minutes. The presence of free chlorine in the reaction mixture after the reaction is undesirable because the neutralization of this leads to the formation of hypochlorites, causing the destruction of the polymer.

Halogenation temperature has almost no effect on the process, the reaction is usually carried out at room temperature.

Objectives: practical introduction to the technology of polymeranalogous transformations for producing elastomers with specific properties in terms of preparation of halogenated butyl rubber.

Materials:

1)8% solution of butyl rubber (brand name BK-1675 or BK-2045);

2)Carbon tetrachloride (reagent grade or dried and distilled) mp -23°C, bp 76.8°C, nd = 1.4631, d20 = l.595 g/cm3;

3)Ethyl alcohol;

4)0.5% solution of bromine or chlorine in carbon tetrachloride;

5)10% aqueous sodium carbonate solution (Na2CO3);

6)Universal indicator paper.

Apparatus: three-necked flask with ground glass joints, 100 ml - 5 pcs.; glass stirrer; electric motor with laboratory transformer; dropping

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funnel, 100 ml; glycerol gate; thermometer at 100°C; separating funnel (250-500 ml); beaker, 200 ml; hot air oven; Petri dish.

Procedure

Bromination of butyl rubber is carried out in a three-necked flask of 100 ml with ground glass joints, equipped with stirrer, thermometer and dropping funnel.

In 0.5 liter flask 8% solution of butyl rubber in carbon tetrachloride is prepared in 300 ml (this solution is prepared in advance). 4 flasks are filled with 25 ml of solution of butyl rubber.

The required amounts of 0.5% solution of brominating agent in carbon tetrachloride in five concentrations of the molar ratio of bromine: isoprene = 1:1, 1:2, 1:3, 1:4 and 1:5 are calculated (specific ratio of the reactants is get from the teacher).

Calculated amount of 0.5% bromine solution is poured into the separating funnel. The stirrer is switched on and the bromine solution is poured for 1-2 minutes.

The stirring is carried out for 20 minutes and then the reaction is terminated by addition of a neutralizing agent, such as 10% aqueous solution of sodium carbonate (Na2CO3). The flask content is stirred and poured into the separating funnel. After settling the bottom layer (solution BIIR in carbon tetrachloride) is poured into the beaker, the washings are poured from the funnel into the special sink, the content of the beaker is poured back into the separatory funnel and the BIIR solution is washed until neutral.

BIIR is separated from washed solution by precipitation in ethanol, rubber is washed with ethanol and dried under vacuum to constant weight. The bromine content and the intrinsic viscosity [η] are determined at the resulting rubber, and the latter is measured by the Ubbelohde viscometer, using tetrachloroethane as a solvent (the method for the intrinsic viscosity determining, see Lab 2).

The dependence of molecular weight of bromobutyl rubber and polymer on the molar ratio of bromine: isoprene is plotted.

Analyzing the data, determine the conditions to obtain BIIR with given content of bromine (1.8-2.4 wt%).

Example of calculation

Butyl rubber BK-675 containing 1.6% mol. of isoprene units is taken for bromination. At a concentration of bromine, not exceeding the "critical" value, one hydrogen atom in isoprene unit of BK macromolecule

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is replaced, as 1 mole of bromine (chlorine) to 1 mole of isoprene must be entered the reaction zone.

The content of isoprene units (g) in butyl rubber sample is calculated (e.g. in 7.5 g 2.54 g of BK):

1.6 - 100 X -2.54

X 1.5 2.54 0.041 g 100

The amount of bromine (chlorine) needed for the reaction is calculated from the condition: 1 mol of bromine to 1 mol of isoprene:

68 - 2∙79.91

0.041 - Y

Y 0.041 79.91 2 0.096g (Br). 68

Determination of bromine (chlorine) by Schoniger Method Materials:

1)oxygen cylinder;

2)0.01 N solution of mercury nitrate;

3)0.5 N solution of nitric acid;

4)2.0 N KOH solution;

5)1% alcohol solution of diphenylcarbazone (indicator);

6)phenolphthalein;

7)hydrogen peroxide;

8)distilled water.

Apparatus: Erlenmeyer flasks of 250 ml with ground glass stoppers for platinum mesh hanging; pipette of 1 ml for hydrogen peroxide; pipette of 1 ml for 2.0 N KOH; pipette of 5 ml for 0.5 N solution of nitric acid; washing bottle; rubber bulb.

Methods of analysis

Platinum wire with mesh is calcinated on the burner (oxidative flame). The sample of 0.003-0.005 g of polymer, taken on a specially cut piece of filter paper, is folded and placed in a platinum mesh so that a long strip is toward the top. The flask is filled up with 10 ml of distilled water, 2- 3 drops of hydrogen peroxide and 2-3 drops of 2N KOH solution and run the oxygen for 2-3 minutes at 100-120 ml/min. Then the overhung end of the filter paper is set on fire and the flask is quickly covered.

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