Chemistry and technology of synthetic rubber. Textbook

After 30 minutes, the stopper is taken out, the solution is washed off with water and is boiled for 3-5 minutes till the hydrogen peroxide destruction and then is neutralized by 0.5 N solution of nitric acid by phenolphthalein and 1 ml of the same acid is added. The solution is cooled and titrated with 0.01 N solution of mercury nitrate till it becomes of light purple color. The indicator is diphenylcarbazone (3-4 drops).

Simultaneously, the blank experiment is carried out.

The halogen content in the polymer is calculated by the formula:

X a b K m 100, g

where a and b - amounts of 0.01 N solution of mercury nitrate, spent on the titration of test and control samples, ml; K - the correction factor of 0.01 N solution of mercury nitrate (the value K is given by laboratory assistant); m - the amount of halogen, corresponding to 1 ml of exactly 0.01 N solution of mercury nitrate, which equals to 0.35457 and 0.7992 mg for chlorine and bromine, respectively; g - the sample of analyzable compound, mg.

Safety information

All the IIR halogenation works are performed in a fume hood at the operating ventilation. Keep the exposed body surfaces away from the solution of halogenating agent. It's obligatory to wear glasses when mixing a solution of chlorine and bromine.

6 Production of Liquid Rubbers

One of the interesting trends in the chemistry and technology of synthetic rubber are liquid rubbers, which unlike its high molecular analogs have high fluidity at room temperature.

Liquid rubbers - synthetic oligomers that form rubber-like material when curing (vulcanizing). The molecular weight of liquid rubbers is 50010000, the viscosity is 0.5-5·102 Pa·sec.

The progress in chemistry and technology makes it possible to synthesize liquid rubbers with predetermined molecular mass characteristics: microstructure, composition, molecular weight and molecular weight distribution.

The most promising application of liquid rubber is the possibility of transferring the production of rubber products (including tires) to automated, continuous molding technology, which significantly reduces the

61

energy and metal intensity of equipment, and increases the labor productivity.

The most common types of liquid rubbers are hydrocarbon (diene and olefin), silicone, urethane, polysulfide rubbers.

Liquid rubbers are divided into two large groups. The first group includes liquid rubbers without the terminal functional groups and rubbers with functional groups statistically located along the chain. The second group includes liquid rubbers, which contain the functional groups at the ends of the chain (the last are called in the literature as "telechelate", "polymerizable", "prepolymers" or "reactive oligomers").

Hydrocarbon liquid rubbers without functional groups and having the functional groups statistically located along the chain, are cured in the oxidative polymerization or vulcanization with sulfur and organic peroxides at the double C=C-bonds.

Silicones, urethane, polysulfide and hydrocarbon liquid rubbers with terminal functional groups are cured in reaction of terminal functional groups with bior polyfunctional compounds (alcohols, amines, epoxides, isocyanates, etc.).

Liquid rubbers are produced by polymerization, polycondensation, telomerization, polymer-analogous transformations at the terminal functional groups and controlled destruction of high molecular polysulfide and diene rubbers (polyisoprene, polybutadiene, polychloroprene, etc.).

The main advantage of liquid rubbers is the ease of their processing (compared to their high molecular weight analogs) to the finished product. Products made of liquid rubbers are formed by the gravity, vacuum or centrifugal casting. Liquid rubbers are widely used in the manufacture of rubber products, tires, shoes, sealants, plasticizers, synthetic drying oils, adhesives, insulating and corrosion resistant coatings.

Synthesis of liquid rubbers containing and not containing functional groups which are statistically distributed along the chain, is carried out by the usual methods of radical, anionic, cationic, stereospecific polymerization, polymerization of cycloolefins with ring opening and destruction of high molecular rubbers. The liquid hydrocarbon rubbers are mainly synthesized with functional groups or without them and which are statistically placed along the chain.

In industry the liquid polyisoprene, polybutadiene and copolymers of isoprene and butadiene with acrylonitrile, styrene, acrylic acid, 2-methyl-5- vinylpyridine and other monomers are produced.

62

Liquid rubbers without terminal functional groups are produced using oligomerization of dienes and their cooligomerization with vinyl monomers at a concentration of catalyst or initiator of 1-2 orders of magnitude greater than in the synthesis of high molecular analogues.

Liquid rubbers with terminal functional groups have a number of advantages over its high molecular analogues when processing into the finished product:

-Liquid rubbers can be poured into molds and get ready products for any purpose, directly curing them in form under mild conditions (low temperature and pressure);

-Due to the low specific concentration of functional groups the heating and shrinkage of polymers when curing are small;

-The ability to pre-generate macromonomers of any structure in order to provide necessary properties of the material and product;

-Linear, branched or three-dimensional polymer with a given, including the regular structure can be obtained based on liquid rubbers with a known structure, molecular weight distribution, functionality and functionality type distribution;

-Combining liquid rubbers of different nature with interacting functional groups you can obtain copolymers with any ratio and sequence of comonomers.

There are the following methods to produce liquid rubbers with terminal functional groups: radical and ionic polymerization, polycondensation, destruction of high molecular rubbers and chemical modification of the liquid rubbers.

6.1 Obtaining Liquid Rubbers by Free Radical Polymerization

To obtain liquid rubbers with terminal functional groups by free radical polymerization it is required to use a bifunctional initiator such as hydrogen peroxide H2O2. The chemistry of the process is discussed in Chapter 1.

Liquid hydrocarbon (isoprene and butadiene) rubbers are synthesized by radical polymerization which are used as the hydroxylcontaining component during the production of polyurethane elastomers.

Other compounds may be used as difunctional initiators:

НО(О)С-RО-ОR-С(О)ОН Н2N-RО-ОR-NН2 Н2N-RN=NR-NН2, etc.

63

Strictly bifunctional liquid rubbers cannot be obtained by radical polymerization. There are at least four reasons of monofunctional macromolecules occurrence during radical polymerization:

-Mono functional and functionless initiator's impurities;

-Impurities with labile hydrogen or halogen atom in a solvent, and impurities of antioxidants;

-Impurities arising from side reactions of the initiator with the components of the reaction mixture, for example during the peroxide oxidation of the monomer;

-chain termination under disproportionation.

6.2 Obtaining Liquid Rubbers via Anionic Polymerization

Anionic polymerization on alkali metals or organometallic compounds is only used for the synthesis of the liquid hydrocarbon rubbers. The synthesis consists of the following steps.

1. Obtaining bifunctional organometallic initiator: XRX + 4Li → LiRLi + 2LiX,

where X - a halogen atom.

The reaction takes place in an organic solvent at reduced temperatures.

2.Getting a living polymer on bifunctional organometallic catalyst: LiRLi + 2nM → LiMnRMnLi.

3.Replacing the terminal metal atoms:

a)to carboxyl end groups by using a large excess of CO2;

b)to hydroxyl groups by adding ethylene oxide and formaldehyde;

c)to mercaptan groups by treating living oligomers by sulfur thiooxide, cyclic disulfides;

g)to end-secondary amino groups by adding Schiff bases;

d)to terminal epoxy groups by the reaction with the epoxy aldehydes, epoxyketones, epichlorohydrins.

In all cases, the formation of monofunctional macromolecules and macromolecular and not containing functional groups.

Cationic and anionic polymerization are used to prepare liquid silicone rubber. The raw material is dimethyldichlorosilane (DDS). The content of the basic substance is not less than 99.96% by weight. The chemistry of the process is presented in Chapter 2. The difference in technology - increased catalyst concentration.

64

Liquid silicone rubbers (MQ) are used for the production of castings, coatings, sealants for various purposes.

6.3 Obtaining Liquid Rubbers by Destruction of the High Molecular Rubbers

Chemical destruction of the double bonds produces liquid rubbers maintaining microstructure starting rubber (e.g. cis-1,4-polyisoprene or cis- 1,4-polybutadiene). Most often as decomposition agent used ozone, less p- nitrosodiphenylamine and ruthenium tetroxide.

The liquid polysulfide rubber is obtained via controlled destruction of high thiokols.

Most often liquid hydrocarbon rubbers prepared by ozonolysis of high molecular cis-1,4-polydienes (SBR rubbers and BR) in solution at room temperature followed by conversion to the target rings of ozonid functional groups:

The molecular weight of liquid diene rubbers depends on the amount of ozone greater than ozone fed into the reaction mixture, the lower the molecular weight. The molecular mass distribution of a liquid rubbers in a ozone-oxygen mixture expands with the increase in the concentration of ozone.

For liquid olefin rubbers as the feedback high molecular weight polymer is used butyl rubber (copolymers of isobutylene with isoprene or piperilen). The reaction proceeds as follows:

65

CH3

The molecular weight of the functional oligoizobutilen depends on the concentration of diene butyl rubber. The higher diene content, the lower the molecular weight of the liquid rubber.

Liquid polysulfide rubbers (thiokols) terminated with SH-groups prepared controlled degradation of high molecular disulfide bonds polysulfide polymer (obtained by polycondensation) of sodium hydrosulfide in the presence of sodium sulfite in an aqueous dispersion of:

The basic reagent is sodium hydrosulfide, sodium sulfite, and the role is to suppress the reverse reaction and a conversion of the very reactive thiosulfenic terminal groups in the mercaptan. Liquid polysulfide rubbers recovered from the aqueous dispersions of the dispersant destruction magnesium hydroxide mineral acids, followed by thorough washing with water. The molecular weight of liquid rubbers depends on the amount fed to the reaction of sodium hydrosulfide and the ratio of NaHS and Na2SO3.

6.4 Obtaining Liquid Rubbers by Polycondensation

Polycondensation is the most common process of synthesis, modifying and converting oligomers with reactive functional groups.

66

The greatest practical importance are urethane and silicone liquid rubbers (the latter is obtained by the polycondensation reaction is much less common). It should be noted that the raw material for synthesis of liquid polysulfide rubbers, polysulfide rubber, high molecular weight is obtained by a polycondensation reaction.

Liquid urethane rubbers (cast polyurethanes) contain isocyanate terminal groups. They are prepared by reacting diisocyanate with oligodiol at twofold molar excess of the latter.

To structure of liquid urethane rubbers multifunctional hydroxyl compounds or bifunctional amines are used.

6.5 Chemical Modification of Liquid Rubbers

The term "chemical modification" polymers means purposeful change of their chemical and physical properties as a result of chemical reactions of the macromolecule. Chemical reaction liquid rubbers can be divided into two types:

-Polymer-transformation is a change in the chemical nature of the links oligomer molecules, and the nature of the end groups remains unchanged;

-Reactions of terminal functional groups.

In both cases, the properties of liquid rubbers can change dramatically. Polymer-analogous transformations are used in the case of hydrocarbon diene rubbers. The modification is carried out at double bonds of the oligomer by hydrogenation, epoxidation, oxidation, hydroxylation, chlorination, grafting vinyl monomers etc.

Reaction of the terminal functional groups produce more convenient to use functional groups. Widely used reaction of reactive end groups with mono-and diamines, alcohols, isocyanates, halides, etc.

6.6 Molecular Parameters of Liquid Rubbers with Terminal Functional Groups

Molecular parameters rubbers include the following parameters: number average and weight average molecular weight distribution (MWD), the microstructure of the main chain, the chemical nature of the functional end groups, the number average and weight average functionality distribution type functionality (RTF).Functionality and distribution of specific types of functionality are molecular parameters terminated liquid rubbers with

67

functional groups. No functional group on the end of the oligomer chain leads to the defect mesh elastomeric material based liquid rubber and dramatically reduces the quality of the finished product.

By understanding the functionality of the average number of functional groups per one molecule of oligomer.

The experimentally determined average functionality of the ratio average molecular weight measured by physical or chemical method, and is equivalent to the molecular weight:

Fn = Mn / Mэ .

In turn, the equivalent molecular weight found from the relationship:

Me = Equivalent weight of functional groups * 100 / concentration of functional groups in the polymer

Weight average functionality determined by the method based on the setting gel point of the oligomer by reaction with a polyfunctional agent, the functionality is known.

To determine RTF using liquid chromatography. The carrier should be active with respect to the functional groups and is not enabled to the oligomer chain.

Functionality liquid rubbers depends on the method of preparation. The most uniform in RTF oligomers are obtained by radical polymerization using an azo dinitrile initiator (content of monofunctional fractions 1-6%). Oligomers polymerization catalyst contain up to 30% monofunctional fractions and, consequently, have lower viscosity. Oligomers synthesized by radical polymerization using a peroxide initiator, characterized by functionality dispersion (content of monofunctional fractions up to 20%) and molecular weight. Destructive method (under the influence of ozone) leads to oligomers obtaining, the molecular parameters are similar to ones of a liquid rubbers synthesized in the presence of peroxide initiators.

6.7 Rheological Properties of Liquid Rubbers

The rheological properties of liquid rubbers are determined primarily chemical nature of the main chain. In case of equal molecular weight the viscosity of rubber increases in the series polybutadiene oligomer <oligoizopren <oligobutadienakrilonitril <oligoizobutilen.

68

The chemical nature of the terminal functional groups impacts a lot to the viscosity of liquid rubbers. The polar groups favor the association of oligomer molecules.

Depending on the nature of the terminal functional groups of the viscosity of the liquid rubbers can vary by an order or more. For example, the viscosity of the oligomer (MW = 3000-4000) with terminal hydroxyl groups depending on the mechanism of fusion ranges 3-10 Pa·sec, and epoxide groups and acrylateurethane groups - 90 Pa·sec.

With a decrease in the molecular weight of the viscosity increases due to the increase in the concentration of terminal functional groups and an increase in the degree of association of oligomer molecules.

6.8 Features of the Technology of Liquid Rubber

For liquid rubbers used the same equipment as in the preparation of high molecular analogues. The synthesis was performed as in a periodic pattern in a polymerizer, in a continuous or in a cascade circuit of a polymerizer.

When formulating liquid rubbers by free radical polymerization process for producing a narrow MWD using fractional initiator feed. However, the washing step, separation, drying and degassing are specific differences. Features of the physical condition of liquid rubbers do not allow the use of these stages the equipment used for high-rubbers. By washing liquid hydrocarbon rubbers from the breakdown products of the catalyst or initiator decomposed, the formation of stable emulsions, the separation of which requires special techniques.

Liquid rubbers obtained by polymerization in solution can be recovered from solution in the rotary thin film evaporators.

Lab 8. Obtaining Liquid Isobutylene Rubber

Objectives: ozonolytic destruction of butyl rubber (copolymer of isobutylene with isoprene or piperilenom) in the solution for preparation of a functionalized oligoizobutilen and to determine its characteristics.

Materials:

1)Rubber IIR - 10 g;

2)pyridin - 2 g;

3)chloroform - 500 cm3;

4)0.1 N NaOH solution in alcohol - 100 cm3;

5)phenolphthalein.

69

Apparatus: unit for ozone producing (ozonizer), ozonation reactor of 500 cm3; round-bottomed flask of 500 cm3, five flat-bottomed flasks with a volume of 500 cm3, pipette of 5 cm3, measuring cylinder of 25 cm3, burette of 25 cm3, rotary vacuum evaporator.

Ozonolysis reaction proceeds according to the following scheme:

CH3

O CH3 CH3 O

HO C CHCH2 CnCH2 C OH

CH3

Procedure

1.Prepare the rubber solution.

2.Perform ozonation of rubber solution controlling the reaction.

3.Distill off the solvent and isolate the product of the reaction.

4.Record the infrared spectrum of the initial high molecular rubber and liquid rubber.

5.Plot the kinetic curve of carboxyl groups accumulation during the

reaction.

Setup for ozone obtaining and general ozonation technique

For ozone, and the reactions of ozonolysis, the following setup is

used (Figure 5.1).

Fig. 5.1. Setup for ozone producing: 1 - pressure relief valve; 2, 3 - drainage column; 4 - rotameter; 5 - discharge tube; 6 - high-voltage transformer (U =

70