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Polymers
Offered service
Chemical and mechanical specification of polymeric material components (plastics and gums).
Different methods are applied. The following are among the most important ones:
» Chemical and thermal analysis
» Mechanical Tests
» Tests of functionality
» Colour
Chemical analysis
The chemical analysis, necessary for the polymer identification, is carried out by FTIR spectrophotometry.
By the FTIR spectrophotometer analysis, the infrared polymer spectrum is obtained; this spectrum is compared with the spectrums inside the data bank: at the end of the research, the spectra with the highest correlation to the one of the analysed sample are highlighted and the correlation index to the unknown spectrum is given for each spectrum. This technique, together with the operator experience, leads to the correct polymer identification.
Other methods allow to determine characteristics such as: density, inorganic charge, water absorption, chemical resistance etc.
Differential scanning calorimetry DSC
This technique is based on the detection of exothermic and endothermic phenomena undergone by the material in the calorimeter; its most common applications concern the determination of melting and vitreous transition temperatures, of specific heat and the study of crystallization kinetics of polymeric matrix materials.
The data obtained are recorded in a characteristic curve called thermogram.
Thermogravimetric analysis TGA
These analytic techniques are particularly suitable for determining the specific heat, melting temperature, crystallization and related heat content, vitreous transition temperature (Tg), thermic stability (weight loss according to temperature). It can thus be useful to assess the purity degree of a product or the filler percentage in polymers.
VICAT softening temperature
This test allows to determine the behaviour of thermoplastic materials in case of softening due to heating.
The polymer mechanical characteristics have to be determined on standard test samples.
Resistance to tensile stress, compressive stress and bending stress
The characteristic values obtained by these tests give important clues about the polymeric material behaviour in case of mechanical strength and elongation. For this purpose, we use material testing machines capable of covering a measuring range between 10 N and 600 kN, with adjustable testing speed and fitted with electronic extensometers.
The following data can be obtained:
· resistance to tensile stress;
· elastic modulus;
· elongation;
· resistance to compressive stress;
· resistance to bending stress
Sometimes, the customer is interested both in the polymer mechanical characteristics and in the finished product characteristics. In this case the resistance to tensile stress, to compressive stress and to bending stress of the sample as it is can be determined. These calculations can be explained by a load-strain diagram and can concern different products; here are some examples:
· rubber O' Ring;
· chains with plastic links;
· supports;
· hinges;
· containers;
· any object that can reasonably be fitted in the testing machine.
These tests can also be used to check the homogeneity of a sample lot.
Hardness
The methods for testing hardness are based on the penetration strength. Contrary to what happens for metals, polymer strains are measured under load, according to set times, due to the high elastic recovery. Depending on the polymer kind, several hardness scales can be used. In TEC Eurolab S.r.l. , we are able to carry out hardness measurements using the following scales:
· Brinell;
· Shore A;
· Shore D;
In particular cases hardness measurements can be carried out also using the following scales:
· Rockwell;
· Vickers .
Resistance to fluid
This test applies above all to gum and consists in detecting the changes in some sample characteristics (weight, hardness, volume) when the sample touches the testing fluid and proved to be particularly useful to assess the strength of O-rings, gaskets and other elements that come into contact with lubricating or hydraulic oils, fuels etc. This method makes use of a procedure to expose test samples to the effects of fluids in definite conditions of:
· fluid type (an ASTM fluid or a fluid suggestd by the customer);
· fluid temperature;
· dipping time.
The possible sample deterioration is determined by measuring the changes in its physical properties before and after the dipping in the testing fluid. The test can be carried out according to standard ASTM D 471-98 or to other methods agreed with the customer.
In practice, the characteristics are determined on the sample as it is, then the sample is dipped in the testing fluid at a set temperature controlled by a thermostat. The sample is left in the fluid for a set number of hours, then it is pulled out and dried. Now, the same tests carried out before are repeated in order to detect the changes in the various characteristics.
Compression Set
This kind of test applies especially to rubber O-rings and generally when you need to evaluate components subject to compressive stress inside a fluid or in air.
This test is carried out according to standard ASTM D395.
The initial sample thickness is measured by a digital micrometer with 0.001-mm units.
The sample is then fitted between the plates of a press, between suitable spacers shorter than the sample, according to the desired compression ratio.
The press is blocked in that position and, in case of air test, it is put in a thermostat-controlled chamber at the test temperature for a certain number of hours.
In case of fluid test, the press in inserted in a container with the test fluid and the whole unit is put in the thermostat-controlled chamber.
At the end of the test, the press is taken out from the thermostat-controlled chamber, the sample is released, dried if required, and left at room temperature for 30 minutes.
When the 30 minutes have elapsed, the sample thickness is measured again in order to calculate the compression set value: in practice, the datum is expressed as percentage of residual bending to induced bending.
The absolute colour and/or the colour difference of polymeric samples can be determined through the parameters L*a*b* of CIELab colour space. This is carried out by means of a spectrophotometer.
This test can be particularly useful to evaluate sample colour changes caused by accelerated aging processes due to exposure to light and/or to environments simulating various conditions.
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