2. Principal Chemical and Analytical Methods Used in Reverse Engineering
2.2 Infrared Spectroscopy
2.2.1 Introduction
instrumental methods, the spectroscopic, chromatographic, and thermal techniques are gaining wide acceptance as analytical tools for qualitative and quantitative analysis. An overview of some principles and applications of different analytical instruments are tabu- lated in Table 2.4.
2.2 Infrared Spectroscopy
TABLE 2.2
Chemical Analysis: Definition, Significance, and Procedure of Measurement Sl
Number
Chemical
Analysis Definition, Significance, and Measurement Procedure
1 Specific gravity • The ratio of the weight of unit volume of a material and the weight of the same volume of water at a given temperature.
• It is an important parameter for checking accuracy in compounding and serves as a guide in comparing relative compound costs.
• Three major ways of determining specific gravity are : i) Hydrometer method: used for liquid materials ii) Direct weight method: used for solid materials
iii) Liquid displacement method: used for polymeric materials 2 Ash content • Residue obtained after ignition of materials at high temperature.
• Ash is mainly inorganic impurities like copper, manganese, etc. These impurities can have serious implications on the aging properties of final products.
• A weighed amount of sample is ignited in a muffle furnace at 900°C.
Residual material is calculated as ash percentage.
3 Heat loss • Weight loss of materials measured at around 60 to 70°C for rubber chemicals and at 125oC for carbon black.
• Heat loss of rubber chemicals indicates low boiling organic materials.
Carbon black heating loss consists primarily of moisture, but other volatile matter may also be lost.
• A weighed amount of sample is heated under controlled temperature;
weight loss is the measure of heat loss.
4 Melting point • Temperature at which the material changes its phase from solid to liquid, under specified conditions.
• Melting point is an important tool to identify the materials and to check the purity of the materials. This also helps in predicting the processing characteristics of materials.
• Two methods—capillary method and differential scanning calorimetry—
are mainly used for the determination of melting point of the raw materials.
5 Sieve residue • The sieve residue test is used to perform the qualitative evaluation of aggregate size of particulate material, namely, different fillers, and distribution of aggregate size of powdery chemicals.
• This test generally analyzes suitability of the use of the powdered rubber chemicals in rubber compounds, where fine distribution of the small particle clusters is needed to achieve a uniform crosslinked network. This test is also used to ascertain that there will be no excessively large particles present in the powdery rubber chemicals which would result in “physical flaws” in the rubber matrix.
• Powdered rubber chemicals are wetted with a dilute aqueous solution containing defoamer. The sample is passed through stacked sieves arranged in order of decreasing mesh size with the help of water flow. The residue retained on each sieve is dried in an oven. The dried mass of residue is obtained for each sieve. The percent residue is calculated on the basis of the original sample mass.
6 Acidity • Determines the presence of acid material, which dissociates in distilled water.
• Acidity indicates the effect of raw materials on the vulcanization system and on the rate of vulcanization.
• Determined by titration.
(Continued)
TABLE 2.2
Chemical Analysis: Definition, Significance, and Procedure of Measurement (Continued) Sl
Number
Chemical
Analysis Definition, Significance, and Measurement Procedure
7 Insolubility • Generally determines the insoluble impurities present in rubber chemicals with the use of suitable organic solvents.
• Solubility is an important parameter to characterize sulfenamide-type accelerators. Since MBTS is a primary degradation product of
sulfenamides, the determination of MBTS is a means of assessing possible degradation of sulfenamides. Insolubles are a means of MBTS content of sulfenamide. This also indicates the purity of the materials.
• A specimen is dissolved in a prescribed solvent, stirred, and filtered through a gooch crucible. The insoluble content is calculated from the amount of residue.
8 Oil content • The oil content test determines the amount of hydrocarbon oils present in any oil extended materials.
• This parameter helps to control fly loss and proper dispersibility of materials in the rubber matrix.
• Measured by extracting the oil by suitable solvents.
9 Bulk attrition • Determines the pellet attrition of pelleted carbon black.
• This attrition of carbon black gives some indication as to the amount of fines that may be expected to be created by pellet breakdown in conveying and handling or in a bulk shipment while in transit.
• A test sample of carbon black is placed on a 120 mesh sieve and shaken in a mechanical sieve shaker for 5 min to remove the fines. The same test sample is shaken for an additional 15 min to determine the amount of pellet breakdown created during this additional shake interval. The attrition is expressed in percent.
10 Softening point • Softening point is the temperature at which a solid sample gets softened, when the sample is heated under constant atmospheric pressure and at a particular heating rate, generally at 5°C/min in a suitable heating media.
The commonly used heating media is a glycerin bath.
• Generally measured for polymeric materials. It indicates the average molecular weight as well as processing temperature. It also indicates the dispersibility of the materials at processing temperature.
• Measured by ring and ball method.
11 Acid number • The number of milligrams of KOH required to neutralize the fatty acids in 1 gm of sample.
• This analysis is required for determining the acidity of certain rubber chemicals which may affect the vulcanizing reaction.
• This analysis is done by titrimetric method.
12 Saponification
number • The number of milligrams of KOH to react with 1 gram of sample.
• The measure of the alkali reactive group. A higher saponification number from the normal indicates the fatty acid has been oxidized.
• Measured by titrating with KOH.
13 Iodine number • The number of milligrams of iodine required for 1 gram of sample.
• Iodine number is used to measure the unsaturation in organic material which indicates stability of materials.
• The determination of iodine value is based on the absorption of iodine under suitable conditions selected to promote stoichiometrical relations.
TABLE 2.2
Chemical Analysis: Definition, Significance, and Procedure of Measurement (Continued) Sl
Number
Chemical
Analysis Definition, Significance, and Measurement Procedure
14 Toluene
discoloration • This test method measures the amount of toluene discoloration when the carbon black is extracted using toluene.
• Toluene discoloration value obtained by UV-Visible spectrophotometer gives an estimate of toluene-soluble discoloring materials present in carbon black. These are generally low boiling organic materials.
• Measured by the UV-Visible spectrophotometer at 425 nm wavelength.
The percent transmittance of the toluene extract of carbon black is determined.
15 Pour density • The pour density of carbon black is defined as the mass per unit volume of pelleted carbon black.
• The pour density of carbon black is used to determine the weight-volume relationship for certain applications like automatic batch loading systems, and for estimating total weights permissible for bulk shipments.
• Measured by estimating the amount of carbon black (in gm) in a fixed- capacity (624 cm3) cylindrical container.
16 Iodine
adsorbtion • The number of grams of iodine adsorbed per kilogram of carbon black under specified conditions.
• Measures the surface area and is generally in agreement with multipoint nitrogen surface area. The presence of impurities like volatiles, surface porosity, or extractables affects the iodine adsorption number.
• A weighed amount of carbon black is treated with a portion of standard iodine solution and the mixture is shaken and centrifuged. During this process, a certain amount of iodine is adsorbed by the surface of carbon black and the rest is evaporated off. The excess iodine was treated with standard sodium thiosulfate solution, and the adsorbed iodine is expressed as a fraction of the total mass of carbon black.
17 DBP absorption • This test method determines the n-dibutyl phthalate absorption number of carbon black.
• The DBP absorption number is related to the processing and cured properties of rubber compounds containing carbon black. It also indicates the structure of carbon black.
• In this test method, DBP is added in a controlled way through a burette to a sample of carbon black in the mixer chamber of an absorptometer. As the sample absorbs the DBP, the mixture changes its physical form, from powder to semi-plastic agglomeration. As a result the viscosity of the mixture increases. The increased viscosity is sensed through the torque- sensing system of the absorptometer. When the viscosity reaches a torque level, the absorptometer and burette will stop immediately. The volume of DBP per unit mass of carbon black is the DBP absorption number.
• The DBP has been listed as SVHC (substance of very high concern) under REACH (Registration, Evaluation, Authorisation, and Restriction of Chemical substances). As a result the DBP absorption test is replaced by the OAN (oil absorption number) test.
(Continued)
TABLE 2.2
Chemical Analysis: Definition, Significance, and Procedure of Measurement (Continued) Sl
Number
Chemical
Analysis Definition, Significance, and Measurement Procedure
18 Aniline point • This is the minimum temperature at which equal volumes of aniline and the desired oil should be completely miscible.
• This is an important characterizing tool for rubber processing oils. A high aniline point oil indicates lower aromaticity and vice versa. The aniline point of aromatic oil is 25 to 50°C, naphthenic oil is 55 to 75°C, and paraffinic oil is 100 to 150°C. Aniline point also indicates the compatibility of oils with different types of polymer.
• Sample oil and aniline are taken in a U-tube where two separate layers are observed. Both the materials are then heated, and at a particular
temperature, both the materials are miscible to each other. That particular temperature is expressed as the aniline point.
19 Pellet hardness • The force required to fracture or crush a carbon black pellet.
• Pellet hardness is related to the mass strength and attrition characteristics of the carbon black. Pellet hardness also plays an important role in obtaining proper dispersion of carbon black in a rubber matrix. High pellet hardness black needs higher shearing energy for better dispersion. Also, high pellet hardness black acts as physical flaws in the rubber network.
• Measured by a pellet hardness tester.
20 Organic acids • This method is intended to determine the organic acid remaining in a synthetic rubber.
• Organic acids in the polymer may affect the cure rate of compounded stock.
• Thin, narrow strips of dried rubber are extracted twice in hot extraction solvent. The solvent extracts are titrated against sodium hydroxide solution using a chosen indicator. The titration and sample mass are used to calculate the organic acids.
21 Nitrogen
content • This test method determines the total nitrogen in natural and synthetic rubbers and other nitrogen containing raw materials.
• The determination of nitrogen in natural rubber is usually carried out in order to estimate the protein content. This is required to assess the quality of the material and its processing characteristics.
• The common technique for determination of total nitrogen content is the Kjeldahl method. In this method rubber is digested with catalytic mixture followed by distillation from a strong alkaline solution. The distillate is absorbed in a boric acid solution, and the excess base is titrated against the acid.
22 Assay of
accelerators • Percent active content is known as assay.
• This test method is designed to assess the purity of accelerators. These products are used in combination with sulfur for the vulcanization of rubber.
• For most commonly used sulfenamide accelerators, a weighed specimen is dissolved in the appropriate solvent, the “free amine” blank is titrated with standard acid, and the sulfenamide is reduced with H2S. That is, BtSNR2 + H2S → BtSH + HNR2 + S
where Bt is benzothiazole radical; BtSH is 2-mercaptobenzothiazole; HNR2
is free amine; and BtSNR2 is sulfenamide accelerator.
TABLE 2.2
Chemical Analysis: Definition, Significance, and Procedure of Measurement (Continued) Sl
Number
Chemical
Analysis Definition, Significance, and Measurement Procedure
23 Pour point • The temperature at which the oil starts flowing is known as pour point.
• Pour point identifies the processing characteristics of the rubber processing oils.
• Measured by pour point apparatus under specified conditions.
24 Solvent
extractable • The amount of material extracted from raw materials by organic solvents.
• Organic solvent extractables of different raw materials used in the rubber industry are measured by this method. The actual chemical composition of the extractables can vary significantly with the nature of raw materials to be tested. This is an important parameter to use as a quality check of the raw materials. These extractables may affect the processing, storing, as well as curing reaction.
• A specimen of carbon black is extracted with a suitable solvent in a soxhlet extractor to obtain the equilibrium extraction. The solvent is removed by controlled temperature evaporation, and the extracted residue is determined gravimetrically.
25 Tint strength • The ratio expressed as tint units, of the reflectance of a standard paste to a sample paste, both prepared and tested under specified conditions.
• Tint strength of the carbon black depends on the particle size of the black.
Tint strength can be used as an indication of particle size. However, tint strength is also dependent on structure and aggregate size distribution of carbon black.
• A carbon black sample is mixed with a white powder (ZnO) and soyabean oil epoxide to produce a black gray paste. This paste is spread to produce a surface suitable for measuring the reflectance of the mixture by means of a photo-electric reflectance meter. The reflectance of the tested sample is compared to the reflectance of the Industry Reference Black (IRB) prepared in the same manner. The tint strength of the tested sample is expressed as units of the reflectance of the IRB divided by the reflectance of the sample multiplied by 100.
26 Flash and fire
point • The flash point of a volatile material is the lowest temperature at which it can vaporize to form an ignitable mixer in air. Measuring a flash point requires an ignition source. At the flash point, the vapor may cease to burn when the source of ignition is removed.
• Fire point is the temperature at which the vapor continues to burn even after the source of ignition is removed.
• Flash and fire point indicates the process safety of the materials. It also indicates the nature of the processing oils.
• Measured by Cleveland open cup apparatus.
27 pH • The concentration of hydrogen ions in a medium. In short it indicates the acidity and basicity of the materials.
• Measurement of pH value of a material determines whether it is acidic or basic in nature. It is better to be aware of the pH since basic material may accelerate vulcanization while acidic material may delay the vulcanization time of a rubber compound.
• Solid sample is taken in a beaker and stirred with distilled water. The whole mixture is then allowed to settle, and the filtered pH of the filtrate is determined by using glass electrode.
(Continued)
TABLE 2.2
Chemical Analysis: Definition, Significance, and Procedure of Measurement (Continued) Sl
Number
Chemical
Analysis Definition, Significance, and Measurement Procedure
28 Dip pick-up • This method is used to determine the amount of adhesive present on the fabric.
• Textile yarns and cords are treated with an adhesive dip to improve the adhesion of elastomer to textile materials. The amount of dip solids pick-up on the yarns or cords is used for process control.
• The fiber is dissolved in an appropriate solvent and the residue of dip solids is recovered by filtration after which the residue is dried and weighed. The amount of adhesive solid on a sample is reported as percentage dip pick-up based on the oven dry weight of the dip free sample.
29 Titer value • The temperature at which the stearic acid starts solidifying. It is also identified as solidification temperature.
• Titer value indicates the purity of the materials and also the processing characteristics of stearic acid.
• Measured by monitoring the temperature during cooling of the melted stearic acid.
30 Spin finish • Spin finish determines the extractable materials on most fibers, yarns, and fabrics.
• This test method is used for the determination of oily or waxy impurities that were applied to yarns or fabric during manufacturing. These are generally the impurities that generate dip fumes and dip dipositions.
• The specimen is extracted with appropriate solvent. The solvent is evaporated, and residue and the specimen are dried and weighed separately. The amount of extracted material is reported as percent extractables or percent spin finish.
31 Soaps • This method is intended to measure the soap content in emulsion polymer (SBR).
• Since soap is a by-product of the emulsion process, it may affect the cure rate of compounded stock, tack, and adhesion properties.
• One portion of the solvent extract is titrated with hydrochloric acid, using the chosen indicator. The percentage of soap is calculated from the weight of the original sample and the titrations.
32 Viscosity • The internal friction between layers of fluid as they pass over each other while moving with different velocities. Viscosity is defined as the internal force per unit area required to maintain a unit velocity gradient.
• Viscosity signifies the processing characteristics of different liquid materials used along with rubber. It also helps to identify the flowability of the materials.
• A number of methods are available to determine the viscosity of a liquid.
Of these, the Saybolt viscometer and Brookfield viscometer are widely used for viscosity measurement.
33 American
Petroleum Institute (API) gravity
• This is a special way of representing relative density, represented by deg = (141.5/5p.gr.60/60oF) – 131.5.
• API gravity governs the quality of the crude petroleum. It is also necessary for conversion of measured volumes to volumes at standard temperatures.
• API gravity is calculated after measuring the specific gravity of oils by the hydrometer method.
TABLE 2.2
Chemical Analysis: Definition, Significance, and Procedure of Measurement (Continued) Sl
Number
Chemical
Analysis Definition, Significance, and Measurement Procedure
34 Viscosity
gravity constant (VGC)
• VGC is a function which is calculated
• From kinematic viscosity at 40°C and density at 15°C:
VGC =
G - 0.0664 – 0.1154Log(V-5.5) 0.94-0.1099Log(V-5.5)
where G is the density of oil at 15°C, g/mL and V is the kinematic viscosity of oil at 40°C, cSt.
• From kinematic viscosity at 100°C and density at 15°C:
VGC =
G-0.108-0.1255Log(V -0.8) 0.90 – 0.097Log(V -0.8)
′
′
where G is the density at 15.5°C, g/mL and V′ is the kinematic viscosity at 100°C, cSt.
• VGC is an indication of the nature of oils. If VGC is around 0.8, oils are classified as paraffinic; those with VGC above 1 are classified as extremely aromatic.
• VGC is calculated after measuring the specific gravity by hydrometer and viscosity by Saybolt viscometer.
35 Surface tension • Expressed as the force in dynes per unit length acting at right angles to the line along the surface of the liquid.
• It signifies the stability of the liquid or liquid mixtures. In the rubber industry, surface tension is generally an important characteristic for latex. It indicates the storage and handling characteristics of the latex.
• Measured by Torsion ring method using du Nouy principle.
36 Mechanical
stability • The amount of material creamed off during high-speed stirring of the latex.
• Latices which are mainly used in dip units are kept under constant stirring condition. This test helps the user to determine whether the latex will bear the high speed without generating excess cream. It is an important processing parameter.
• Measured by a mechanical stability tester.
37 Surface area • The square meter area per gram.
• Surface area indicates the effective area available for interaction with rubber. This is generally measured for fine particles, viz., carbon black, ZnO, precipitated silica, etc. The higher the surface area, the higher will be the efficiency of the materials.
• Measured by BET method.
38 Clay-gel
analysis • This analysis determines the % n-pentane insolubles, % polar compounds,
% aromatics, and % saturates.
• The concentrations of the characteristics of hydrocarbon groups as determined by this method are used to classify petroleum oil types as used for extending and processing rubbers. Compatibility and certain finished product properties can often be correlated with the composition as determined by this method.
• The sample is diluted with n-pentane and purged to a glass percolation column containing clay in the upper section and silica gel in the lower section. The n-pentane is then passed to the double column until a definite quantity of effluent has been collected. The upper section is removed from the lower section and washed with n-pentane which is discarded. A benzene-acetone (50:50) mixture is then charged to the clay section, and a specified volume of effluent is collected. From this collection, saturates and polars are measured. Aromatics are calculated by differences.