VINACHEM

Rubber raw materials’ checking includes essential chemical-analytic and physicalmechanic measuring, the results of which are mostly physical parameters. On the base of these physical parameters it is possible to assume the raw material purity. The parameters are such as: determination of the melting point, moisture volume, dry mass volume, percentage of ash or volatile substances, determination of liquid samples viscosity, specification of effective substance volume, specification of heavy metals (of rubber toxicants), refractive index, acid value, and others. From the methods used for entry raw material quality control it is required to be quick, exact and their results should be in correlation with material purity.

Melting point [°C] – it is temperature at which the sample change of consistency takes place; it is from the solid state into a liquid state (the liquid phase and the solid phase are in dynamic balance). It is one of the compound purity criteria. The temperature is dropping down at the moment when small dirt content is present. With impure substances the transmission is not so sharp; therefore there is a temperature melting point interval expressed along with it. Determination of mixture melting point is based on an assumption that the mixture of two different non-isomorphic substances is melting at considerably lower temperature than respective pure compounds. Therefore if two substances have the same melting point and they are same then their melting point will also be the same. If they are different their melting point will be lower. Melting point is determined at accelerators, antidegradants and others.

Dropping point – it is temperature at which the substance, while being heated by certain rate, will be softened up to such a point that originated drop will drop down by its weight. It is similar to determine dropping point by a circle and a ball, where generated drop will drop down by gravity force effect of the steel ball. Dropping point is determined at resins, colophony and asphalt.

Volatile substances– they are compounds that are being released from a sample during temperature increasing (the temperature of 105°C is mostly used at determination of compounds). In most cases it is water or volatile organic compounds expressed to the original sample backfill in %. It is determined at carbon black, silica, organic chemicals (accelerators, antidegradants) and others.

Dry mass – it is the rest of the sample after defining of volatile substances or, if appropriate humidity. It is stated in % to the original sample backfill. It is determined at liquid samples of cements and separating agents.

Ash – it is composed of organic, at a certain temperature non-volatile and further indecomposable compounds, which are parts of the sample. Due to decomposition of some inorganic compounds (e.g. carbonates, sulphides) the ash volume does not have to define inorganic parts share in a very exact way. It is stated in % to the original sample backfill and it is necessary to indicate temperature to the result (usual execution of ash volume determination is at 550°C or, if appropriate 950°C). It is defined at carbon blacks, organic chemicals, fillers, auxiliary rubber aditives.

Refractive index (refraction) – it is ratio between speed of propagation of light in vacuum and in the sample. Speed of propagation of light reduction is caused by induction of dipole moment in environment molecules by electro-magnetic field around a light beam. It depends on the exposure length (light dispersion – the base of light decomposition into a spectrum) as well as on relative dielectric constant factor ε_r, therefore it is indicated for sodium discharge lamp and defined temperature (subsequently being marked as n_D – D is marking of a characteristic lines doublet in sodium emission spectrum). Specific refraction r is:

n - is is absolute refractive index

ρ- is density of material

R length and its density: Mol refraction Rm is weight

Ratio between compound refraction index at defined temperature and constant wave length and its density:

Mol refraction Rm is composition of specific refraction and compound M mol weight:

Mol refraction off a certain substance does not depend on temperature and pressure, at the same time it is only slightly influenced by a change of consistency. Mol refraction of organic compounds can be figured out as a sum of atom and bonding refractions that are tabulated. Comparison of experimental and calculated refraction values is used for confirmation of compound structure.

Refractive index is characteristic magnitude that defines purity of rubber raw materials and solvents.

Viscosity – it is a measure of inner liqid friction. Newton’s equation determines viscosity as a constant of proportionality η between tangential stress τ and speed gradient (dυ/dy).

The measure of dynamic viscosity is Pa.s; kinematic viscosity is a ratio of dynamic viscosity and liquid density with the measure m2 .s -1. In practice, different relative viscosity measures are still often used, they define viscosity value of a sample in the ratio toward standard fluid; such as water (Engler’s °E, Redwood’s °R and Seybold’s degrees °S)

With oils a viscosity – density constant is used. Higher value of viscosity – density constant shows higher portion of aromates in oil. It can be figured out in according to the relation:

ρ15 is density at 15 °C in g/cm3

η50 is cinematic viscosity at 50 °C in mm2 /sec.

Viscosity is an important constant, similar to other physical constants because it is closely related to purity of substances and thus also related to their quality. It is determined at liquid softeners and oils.

Acid number – it is amount of alkali hydroxide necessary for neutralisation in the sample of present compounds. It is defined in mg KOH/g of the sample. It is determined by volumetric methods at samples which contain free acids such as resins and some softeners.

Active substance determination – ordinary industrial chemicals are mostly not used as pure ones, but they form a mixture with various additives which make processing easier, in some cases they may improve hygiene of work (dust nuisance). Due to appropriate quantization it is necessary to know the content of substance active of the industrial chemical. For determination of substance active different analysing methods are used, beginning with volumetric analysis (redox, neutralisation, coagulating or complex-forming titrations) up to modern instrumental methods (GC, HPLC).

Carbon Blacks testing – it covers essential determinations, accessible in each laboratory. They are such as determination of pH of carbon black (boiling slurry and sonic slurry), determination of volatile substances, ash content, sieve residue analysis, oil absorption number (dibutylphthalate absorption), iodine adsorption number, tint strength, transmittance of toluene extract, powder density, individual pellet hardness, dust content.

PRI (Plasticity Retention Index) – this mechanical testing substitutes more time demanding and more expensive analysing determination of heavy metals in rubbers. It is based on the matter of fact that some heavy metals present in the rubber sample at increased temperatures cause destruction of rubber polymer string (polymer molecular weight is dropping down as well as its physical-mechanical properties – viscosity, plasticity). Raw rubber sample is recalendered and divided into two parts. One of them is thermally loaded with 140°C for 30minutes. Plasticity value (plasticity deformation measure) will be determined for both samples.

PRI is figured out as follows:

P₃₀ is plasticity after thermal loading

P₀ is plasticity of a sample that was not thermally loaded. - Source: laroverket.com -