Tensile test methods for polymers. Polymer testing. Mechanical properties of polymers in the amorphous state

Methods for testing polymeric materials

Mechanical tests. Hardness tests. Impact strength tests. Thermal tests. Electrical tests. Optical tests. Physical tests. Rheological tests. Flammability tests.

Mechanical Tests

1. Strength, deformation and tensile modulus ISO R527 (DIN 53455, DIN 53457, ASTM D638M)

The basis for understanding the properties of a material is knowing how the material reacts to any load. Knowing the amount of deformation created by a given load (stress), the designer can predict the response of a particular product to its operating conditions. Tensile stress-strain relationships are the most widely published mechanical properties to compare materials or design specific products.

Laboratory installation for mechanical testing

Test speeds:
Speed A - 1 mm/min - tensile modulus.
Speed B - 5 mm/min - tensile stress diagram for resins with glass fiber filler.
Speed C - 50 mm/min - tensile stress diagram for resins without filler.

Tensile stress-strain relationships are determined as follows. The double spade specimen is stretched at a constant rate and the applied load and elongation are recorded. After that, stresses and strains are calculated:

Universal test piece ISO R527

Stress diagram
A: Limit of proportionality.
B: Yield strength.
C: Tensile strength.
X: Destruction.
0-A: Yield strength region, elastic properties.
After A: Plastic properties.

2. Strength and flexural modulus ISO 178 (DIN 53452, ASTM D790)

Modern Bend Testing Machine: "Flexometer"

Flexural strength is a measure of how well a material resists bending, or "how stiff the material is". Unlike tensile loading, in bending tests all forces act in the same direction. An ordinary, freely supported rod is loaded in the middle of the span: thus, a three-point loading is created. On a standard testing machine, the loading tip presses on the specimen at a constant rate of 2 mm/min.

To calculate the modulus of elasticity in bending, a deflection-load curve is plotted from the recorded data. Starting from the original linear part of the curve, a minimum of five load and deflection values are used.

Flexural modulus (the ratio of stress to strain) is most often referred to when referring to elastic properties. The flexural modulus is equivalent to the slope of the line tangent to the stress/strain curve in that part of that curve where the plastic has not yet deformed.

The values of stresses and modulus of elasticity in bending are measured in MPa.

Bending tests

3. ISO 3537 Taber wear test (DIN 52347, ASTM D1044)

Wear test on the Taber machine

In these tests, the amount of attrition loss is measured by abrading the sample on a Taber machine. The sample is fixed on a disk rotating at 60 rpm. The forces created by the weights press the abrasive wheels against the sample. After a predetermined number of cycles, the test is terminated. The mass of attrition loss is defined as the mass of particles that have been removed from the sample: this mass is expressed in mg/1000 cycles. Abrasive wheels are actually whetstones in the shape of a circle. Various types of these circles are used.

4. Comparison of ISO (International Standards Organization) and ASTM (American Society for Testing and Materials) methods.

The application of the ISO method not only changes the test conditions and test mandrel dimensions (compared to the ASTM method), but also requires standardized mold designs and molding conditions in accordance with ISO 294. This may lead to differences in published values - not due to a change in material properties, but due to a change in the test method. According to the ASTM method, the test specimen is 3 mm thick, while ISO has chosen specimens with a thickness of 4 mm.

Hardness tests

1. Comparison of Brinell, Rockwell and Shore hardness

Ratio of hardness scales

The Rockwell test determines the hardness of plastics after elastic recovery of the test specimen. This is the difference between this method and the Brinell and Shore hardness tests: in these tests, the hardness is determined by the depth of penetration under load and, therefore, any elastic recovery of the material deformation is excluded. Therefore, Rockwell values cannot be directly correlated with Brinell or Shore hardness values.

Shore A and D ranges can be compared with Brinell indent hardness ranges. However, there is no linear correlation.

2. Brinell hardness ISO 2039-1 (DIN 53456)

Determination of Brinell hardness

A polished hardened steel ball 5 mm in diameter is pressed into the surface of the test specimen (not less than 4 mm thick) with a force of 358 N. 30 s after the load is applied, the depth of the indentation is measured. Brinell hardness H 358/30 is calculated as "applied load" divided by "surface area of the impression".

The result is expressed in N/mm2.

3. Rockwell hardness ISO 2039-2

Rockwell hardness determination

The Rockwell hardness number directly relates to the hardness of the print on the plastic: the higher the number, the harder the material. Due to the slight overlap in the Rockwell hardness scales for the same material, it is possible to obtain two different numbers on two different scales, and both of these numbers may be technically correct.

The indenter, which is a polished hardened steel ball, is pressed into the surface of the test specimen. The ball diameter depends on the applied Rockwell scale. The specimen is loaded with a "light load", then with a "main load" and then with the same "light load" again. The actual measurement is based on the total penetration depth, which is calculated as the total depth after the main load is removed minus the elastic recovery after the main load is removed and minus the penetration depth at light load. The Rockwell hardness number is calculated as "130 minus the penetration depth in units of 0.002 mm".

Portable Rockwell Hardness Tester Lab Rockwell Hardness Tester

Rockwell hardness numbers must be between 50 and 115. Values outside these limits are considered inaccurate: the measurement must be repeated again using the next harder scale. The scales increase in hardness from R through L to M (with increasing material hardness). Loads and diameters of indenters are specified in the table in more detail.

Hardness scale	Rockwell indenter ball diameter, mm
R	98,07	588,4	12,7
L	98,07	588,4	6,35
M	98,07	980,7	6,35

If a softer material requires a less rigid scale than the R scale, then Rockwell hardness testing is not appropriate. The Shore hardness method (ISO 868), which is used for low modulus materials, can then be used.

4. Shore hardness ISO 868 (DIN 53505, ASTM D2240)

Shore hardness determination

Shore hardness values are scale readings obtained by penetrating plastic with a specific steel rod. This hardness is determined by two types of scleroscopes, both of which have calibrated springs to apply a load to the indenter. Scleroscope A is used for softer materials and scleroscope D for harder materials.

Indenters for scleroscopes

Shore hardness values change:
from 10 to 90 for Shor scleroscope type A - soft materials,
20 to 90 for Shore type D scleroscope - hard materials.
If the measured values are >90A, then the material is too hard and the scleroscope D must be used.
If the measured values<20D, то материал слишком мягок, и должен применяться склероскоп А.

There is no simple relationship between the hardness measured by this test method and other basic properties of the material being tested.

Impact test

1. The concept of impact strength

In standard tests, such as tensile and bending tests, the material absorbs energy slowly. In reality, materials very often quickly absorb the energy of an applied force, such as forces from falling objects, impacts, collisions, falls, etc. The purpose of impact testing is to simulate such conditions.

Izod and Charpy methods are used to study the properties of certain samples at given impact stresses and to evaluate the brittleness or toughness of samples. Test results from these methods should not be used as a source of data for component design calculations. Information about typical material properties can be obtained by testing different types of test specimens prepared under different conditions, varying the notch radius and testing temperature.

Both methods are tested on a pendulum impact tester. The sample is clamped in a vice, and the pendulum hammer with a hardened steel impact surface of a certain radius is released from a given height, which causes the sample to be sheared from a sharp load. The residual energy of the pendulum impact driver lifts it up. The difference between the drop height and the return height determines the energy expended to break the test specimen. These tests may be carried out at room temperature or at reduced temperatures to determine cold brittleness. The test specimens may vary in the type and size of the notches.

The results of drop weight tests, such as the Gardner method or curved plate, depend on the geometry of the drop weight and support. They can only be used to determine the relative ranking of materials. The results of impact tests cannot be considered absolute unless the geometry of the test equipment and test piece is consistent with the requirements of the end application. It can be expected that the relative ranking of materials by the two test methods will be the same if the nature of the destruction and the speed of impact are the same.

2. Interpretation of impact test results - comparison of ISO and ASTM methods

The impact characteristics can depend to a large extent on the thickness of the sample and the orientation of the molecules. Different sample thicknesses used in ISO and ASTM methods can have a very significant effect on impact strength values. A change in thickness from 3mm to 4mm can even change the fracture behavior from ductile to brittle due to the effect of molecular weight and thickness of the notched sample using the Izod method, as demonstrated for polycarbonate resins. Materials already exhibiting brittle fracture behavior at 3 mm thickness, such as materials with mineral and glass fiber fillers, are not affected by a change in sample thickness. The same properties are possessed by materials with modifying additives that increase impact strength.

Influence of Notched Specimen Thickness and Molecular Weight on Izod Impact Test Results for Polycarbonate Resins

It must be clearly understood that:
It was not the materials that changed, but only the test methods;

The aforementioned transition from ductile to brittle fracture plays an insignificant role in reality: the majority of designed products have a thickness of 3 mm or less.

3. Impact strength according to Izod ISO 180 (ASTM D256)

Izod Impact Tester

Notched Izod impact testing has become the standard method for comparing the impact strength of plastics. However, the results of this test method bear little resemblance to the response of a molded product to impact in a real environment. Due to the different notch sensitivity of materials, this test method allows some materials to be rejected. Although the results of these tests have often been requested as meaningful measures of impact strength, these tests tend to measure the notch sensitivity of a material rather than the ability of the plastic to withstand impact.

The results of these tests are widely used as a reference for comparing the impact strengths of materials. Notched Izod impact tests are best used to determine the impact strength of products that have many sharp corners, such as ribs, intersecting walls, and other stress concentrations. For Izod impact testing of unnotched specimens, the same loading geometry is applied, except that the specimen is not notched (or clamped in an inverted vice). Tests of this type always give better results than Izod notched specimens due to the lack of stress concentration.

The impact strength of specimens with a notch according to the Izod method is the impact energy spent on the destruction of the notched specimen, divided by the initial cross-sectional area of the specimen at the notch. This strength is expressed in kilojoules per square meter: kJ/m2. The sample is vertically clamped in the vise of an impact copra.

ISO designations reflect sample type and notch type:
ISO 180/1A designates specimen type 1 and notch type A. As can be seen in the figure below, specimen type 1 is 80 mm long, 10 mm high and 4 mm thick.
ISO 180/1O designates the same sample 1 but clamped upside down (referred to as "uncut").
The ASTM specimens are similar in size, with the same radius at the base of the notch and the same height, but differ in length at 63.5 mm and, more importantly, in thickness at 3.2 mm.

ISO test results are defined as the impact energy, in joules, expended to break the test specimen divided by the cross-sectional area of the specimen at the notch. The result is expressed in colojoules per square meter: kJ/m2.

ASTM test results are defined as impact energy in joules divided by the notch length (i.e. specimen thickness). They are expressed in joules per meter: J / m. The practical conversion factor is 10: i.e. 100 J/m equals approximately 10 kJ/m2.

Different sample thicknesses can result in different interpretations of "impact strength" as shown separately.

Impact Test Specimens

Method for measuring impact strength according to Izod

4. Charpy impact strength ISO 179 (ASTM D256)

Charpy strength tester

The main difference between the Charpy and Izod methods is the method of mounting the test sample. When testing according to the Charpy method, the sample is not clamped, but freely placed on a support in a horizontal position.

ISO designations reflect sample type and notch type:
ISO 179/1C designates specimen type 2 and notch type CI;
ISO 179/2D designates a Type 2 specimen, but not notched.

Charpy impact strength method

Samples used according to the DIN 53453 method have similar dimensions. The results for both ISO and DIN methods are defined as the impact energy in joules absorbed by the test specimen divided by the cross-sectional area of the specimen at the notched point. These results are expressed in kilojoules per square meter: kJ/m2.

Thermal testing

1. Heat resistance according to Vika ISO 306 (DIN 53460, ASTM D1525)

Laboratory Vicat Heat Resistance Tester

These tests give the temperature at which the plastic begins to soften rapidly. A round, flat-tipped needle having a cross-sectional area of 1 mm² is inserted into the surface of a plastic test specimen under a certain load, and the temperature rises at a uniform rate. Vicat's heat resistance (VST - Vicat softening point) is the temperature at which penetration reaches 1 mm.

Determination of heat resistance according to Vika

The ISO 306 standard describes two methods:
Method A - load 10 N;
Method B - load 50 N.
... with two possible temperature rise rates:

50 °C/hour;
120 °C/hour.
ISO test results are reported as A50, A120, B50 or B120. The test assembly is immersed in a heating bath with an initial temperature of 23 °C. After 5 minutes, a load of 10 or 50 N is applied. The temperature of the bath at which the indenter tip penetrates to a depth of 1 mm + 0.01 mm is recorded as the Vicat heat resistance of the material at the selected load and temperature rise rate.

2. Interpretation of thermal characteristics comparison of ISO and ASTM methods

Some differences can be found in the published results according to the ISO method compared to the ASTM standards due to the different sizes of the test specimens: the values of deformation heat resistance measured according to the ISO methods may be lower.

3. Deformation heat resistance and deformation heat resistance under load ISO 75 (DIN 53461, ASTM D648)

Heat deformation resistance is a relative measure of a material's ability to withstand a load for a short period of time at elevated temperatures. These tests measure the effect of temperature on stiffness: a standard test piece is subjected to certain surface stresses and the temperature is raised at a uniform rate.

The samples used in the tests are tempered (annealed) and unreleased (unannealed). Tempering is a process in which the sample is heated to a certain temperature, kept at that temperature for a while, and then gradually lowered to ambient temperature. Such actions make it possible to reduce or completely remove internal stresses in the body of the sample, which have arisen, for example, at the time of accelerated polymerization in an injection molding machine.

In both ISO and ASTM standards, the loaded test specimen is immersed in a heating bath filled with silicone oil.

The surface stresses of the sample are:

Low - for ISO and ASTM methods - 0.45 MPa;
High - for the ISO method - 1.80 MPa, and for the ASTM method - 1.82 MPa.
The force is allowed for 5 min, but this holding period may be omitted if the test materials show no appreciable creep during the first 5 min. After 5 minutes, the initial bath temperature of 23 °C is increased at a uniform rate of 2 °C/min.

The deformation of the test specimen is continuously monitored:

the temperature at which the deflection reaches 0.32 mm (ISO) and 0.25 mm (ASTM) is recorded as "deformation heat resistance under load" or simply "deformation heat resistance" (thermal deformation temperature).

Although not mentioned in either test standard, two abbreviations are commonly used:

DTUL- Deformation heat resistance under load
HDT- Deformation heat resistance or bending heat resistance

Determination of deformation heat resistance

In general practice, the abbreviation DTIL is used for ASTM results and HDT for ISO results.
Depending on the surface stress created, the letters A or B are added to the HDT abbreviation:

HDT/A for load 1.80 MPa
HDT/B for load 0.45 MPa

4. Deformation heat resistance (HDT) and amorphous and semi-crystalline plastics

For amorphous polymers, the HDT values approximately coincide with the glass transition temperature Tg values of the material.

Because amorphous polymers do not have a defined melting point, they are processed in their highly elastic state above Tg.

Crystalline polymers can have low HDT values and still have constructive utility at higher temperatures: the HDT method is more reproducible with amorphous plastics than with crystalline ones. For some polymers, tempering (annealing) of the test specimens may be required to obtain reliable results.

When glass fibers are added to a polymer, its modulus increases. Since HDT is the temperature at which a material has a certain modulus, increasing the modulus also increases the HDT value. Glass fiber has a greater effect on the HDT of crystalline polymers compared to amorphous polymers.

Although widely used to indicate performance at high temperature, HDT tests only simulate a narrow range of conditions. In many high temperature applications, products operate at higher temperatures, higher loads, and without support. Therefore, the results obtained with this test method do not represent the maximum application temperature, since in reality such significant factors as time, load and nominal surface stresses may differ from the test conditions.

5. Ball indentation EC335-1

These are heat resistance tests similar to the Wick test. The sample is placed horizontally on a support in the heating chamber and a ball 5 mm in diameter is pressed into it with a force of 20 N. After one hour, the ball is removed, the sample is cooled in water for 10 seconds, and the imprint left by the ball is measured. If the indentation diameter is less than 2 mm, then the material is considered to have passed the ball indentation test at that temperature.

Ball indentation test

Depending on the application, the test temperature may vary:
75 °C for non-live parts,
125 °C for live parts.

6. Thermal conductivity ASTM C 177

The thermal insulation properties of plastics are determined by measuring thermal conductivity. Wide plates of plastic are placed on both sides of a small heated plate, and heat sinks are attached to the free surfaces of the plates. Thermal isolators located around the test chamber prevent radial heat loss. The axial heat flux through the plastic plates can then be measured. The results are recorded in W/m°C.

7. Relative thermal conductivity index, RTI UL 746B

Formerly referred to as the Continuous Use Temperature Tolerance (CUTR), the Relative Temperature Index (RTI) is the maximum operating temperature at which all critical material properties remain within acceptable limits for an extended period of time.

Three independent RTIs can be assigned to the same material according to UL 746B:

Electrical - by measuring the electrical strength of the dielectric.
Impact mechanical - by measuring impact tensile strength.
Impactless mechanical - by measuring the tensile strength.
These three properties were selected as critical in testing due to their sensitivity to high temperatures in use.

The thermal performance of the material is tested over a long period of time in comparison with a second control material for which the RTI has already been determined and which has shown good performance.

Based on the term "Relative Temperature Index", the control material is used because the characteristics that deteriorate with increasing temperature are inherently sensitive to the variables of the test program itself. The control material is affected by the same specific combinations of these factors during the test, providing a valid basis for comparison with the test material.

Ideally, long-term measurable thermal performance could be evaluated by aging the test material at normal temperature for an extended period of time. However, this is impractical for most applications. Therefore, accelerated aging occurs at much higher temperatures. During the aging process, samples of the test and control materials are placed in ovens that maintain a predetermined constant temperature. Samples of test and control materials are removed at predetermined times and then tested for the retention of basic properties. By measuring the three mentioned properties as a function of time and temperature, it is possible to mathematically calculate the "end of life" for each temperature. This "end of life" is defined as the time during which the properties of the material have deteriorated by 50% compared to the initial values. By substituting the test data into the Arrhenius equation, the maximum temperature at which the tested material will have a satisfactory service life can be determined. This design temperature is the RTI for each material property.

Understanding how the RTI is determined allows the designer to use the RTI to predict how parts formed from a given material will perform in real-life elevated temperatures.

8. Coefficient of linear thermal expansion ASTM D696, DIN 53752

Every material expands when heated. Injection-molded polymer parts expand and change dimensions in proportion to the increase in temperature. To evaluate this expansion, designers use the coefficient of linear thermal expansion (CLTE), which determines changes in the length, width, and thickness of the molded part. Amorphous polymers are generally characterized by consistent expansion rates over their practically usable temperature range. Crystalline polymers generally exhibit increased expansion rates at temperatures above their glass transition temperature.

The addition of fillers that create anisotropy significantly affects the CLTE of the polymer. Glass fibers are usually oriented in the direction of the flow front: when the polymer is heated, the fibers prevent expansion along their axis and reduce the CLTE coefficient. In directions perpendicular to the flow direction and thickness, the CLTE will be higher.

The polymers can be formulated with a CLTE corresponding to the coefficients of thermal expansion of metals or other materials used in composite structures such as automotive parts.

Electrical Tests

1. Dielectric strength IEC 243-1

Laboratory installation for measuring dielectric strength

Dielectric strength reflects the dielectric strength of insulating materials at different power supply frequencies (from 48 Hz to 62 Hz) or is a measure of the breakdown resistance of a dielectric material under applied voltage. The applied voltage just prior to breakdown is divided by the sample thickness to give a result in kV/mm.

The environment can be air or oil. The thickness dependence can be significant and therefore all results are recorded at a given sample thickness.

Many factors influence the results:

Thickness, uniformity and moisture content of the test specimen;
Dimensions and thermal conductivity of test electrodes;
Frequency and shape of the applied voltage curve;
Temperature, pressure and humidity of the environment;
Electrical and thermal characteristics of the environment.
2. Surface resistivity IEC 93 (ASTM D257)

When an insulating plastic is energized, a portion of the total current flows along the surface of the plastic if there is another conductor or ground wire connected to this product. Surface resistivity is a measure of the ability to resist this surface current.

It is measured as resistance when a constant current flows between electrodes mounted on a surface of unit width with a unit distance between them. This resistance is measured in ohms, sometimes referred to as "ohms per square".

3. Volume resistivity IEC 93 (ASTM D257)

When an electrical potential is applied across an insulator, current flow will be limited by the resistance properties of the material. Volume resistivity is the electrical resistance when an electrical voltage is applied to opposite faces of a unit cube.

Measured in Ohm * cm. The volume resistivity is influenced by the environmental conditions acting on the material. It varies inversely with temperature and decreases slightly in humid environments. Materials with a volume resistivity greater than 108 ohm*cm are considered insulators. Partial conductors have volume resistivity values from 103 to 108 Ohm*cm.

4. Relative dielectric constant IEC 250

As stated in the IEC 250 standard, "The relative dielectric constant of an insulating material is the ratio of the capacitance of a capacitor in which the space between and around the electrodes is filled with insulating material to the capacitance of a capacitor with the same configuration of electrodes in vacuum."

In AC dielectric applications, good resistivity and low energy dissipation are required characteristics. The dissipation of electricity leads to inefficiency in the functioning of electronic components and causes an increase in the temperature of the plastic part, which serves as an insulator. In an ideal dielectric, for example, in a vacuum, there are no energy losses due to the dipole movement of molecules. In solid materials, such as plastics, dipole displacement becomes one of the influencing factors. A measure of such inefficiency is the relative dielectric constant (formerly called the dielectric constant).

This is a dimensionless coefficient obtained by dividing the parallel capacitance of a system with a plastic dielectric element by the capacitance of a system with vacuum as a dielectric. The smaller this number, the better the material's performance as an insulator.

5. Dissipation factor IEC 250

As stated in the IEC 250 standard, "the dielectric loss angle of an insulating material is the angle by which the phase difference between the applied voltage and the received current deviates from Pi/2 radians when the dielectric of a capacitor consists solely of the dielectric material under test. The dissipation factor tg d of the dielectric insulating material is the loss tangent d".

In an ideal dielectric, the voltage and current curves are exactly 90° out of phase. When the dielectric becomes less than 100% efficient, the current wave begins to lag the voltage in direct proportion. The amount of current wave that deviates from 90° out of phase with the voltage is defined as the "dielectric loss angle". The tangent of this angle is called the "loss tangent" or "scattering factor".

A low dissipation factor is very important for plastic insulators in high frequency applications such as radar equipment and microwave parts: lower values correspond to better dielectric materials. A high dissipation factor is essential for welding performance.

The relative dielectric constant and the dissipation factor are measured on the same test equipment. The test results obtained are highly dependent on temperature, moisture content, frequency and voltage.

6. Arc resistance ASTM D495

In cases where an electric current is allowed to pass through the surface of an insulator, this surface is damaged after a while and becomes conductive.

Arc Resistance is the amount of time, in seconds, required to conduct the insulating surface at high voltage and low current arc. In another embodiment, arc resistance refers to the time during which the surface of the plastic can resist the formation of a continuous conductive path under the influence of high voltage with a low current arc under special conditions.

7. Comparative Tracking Index (Comparative Breakdown Index) IEC 112

The tracking index is the relative resistance of electrical insulating materials to the formation of a conductive track when an electrostatically charged surface is exposed to contaminants containing water. Comparative Tracking Index (CTI) determination and CTI-M tests are carried out to assess the safety of components containing live parts: the insulating material between live parts must be resistant to dielectric tracking. The CTI index is defined as the maximum voltage at which no insulation failure occurs after exposure to 50 drops of an aqueous ammonium chloride solution. High CTI values are desirable. Materials that meet the CTI requirements at 600V are referred to as "high tracking" resins.

The test procedure for determining the CTI index is complex. Influencing factors are the state of the electrodes, electrolyte and sample surface, as well as the applied voltage.

Results can be reduced by adding additives such as:

Pigments, in particular carbon black,
Antipyrines,
Fiberglass.
Therefore, flame retardant, carbon black, and glass fiber materials are generally not recommended when dielectric tracking resistance is a primary requirement.

Minerals (TiO2) tend to increase CTI values.

8. CTI testing

The CTI test is carried out using two sized platinum electrodes with slightly rounded "chisel" edges resting evenly on the test specimen.

The minimum voltage applied to the electrodes is usually 175 V. If the parts are under high electrostatic voltage, then a potential difference of 250 V is set. The voltage is applied in steps of 25 V: the maximum voltage is 600 V.

The surface of the material to be tested is moistened with 50 drops of a 0.1% ammonium chloride solution in distilled water (so-called solution A), falling centrally between the two electrodes. The size and frequency of falling electrolyte drops are regulated. If there is no current at the selected voltage, then the test is repeated with the voltage increased by 25 V until a current appears. This voltage, reduced by one step of 25 V, is called the CTI index. The test is then repeated with a voltage 25 V below the CTI voltage, but with 100 drops of electrolyte instead of 50. Determine the voltage at which 100 drops do not induce current. This value can be given in brackets () in addition to the CTI value when exposed to 50 drops of electrolyte.

CTI test

9. CTI-M tests

The CTI-M tests are similar to the CTI tests, except that they use a more aggressive wetting agent (M stands for the French word "mouille" - "wetted"). Solution B contains 0.1% ammonium chloride and 0.5% alkylnaphthalenesulfonate. Erosion holes can also be measured and their depth recorded.
Registration example: CTI 375 (300) M-0.8 means:

50 drops of solution B produce no current at 375 V.
100 drops do not produce current at 300 V.
The depth of erosion holes in the sample surface can be 0.8 mm.

In accordance with the UL94 standard, to classify the safety of materials used for the components of electrical appliances, test sets have been developed for the resistance of the polymer to electrical current and fire.

According to the results of these tests, the materials are divided into PLC (Performance Level Categories):

Comparative tracking index

Arc resistance, D495

High Voltage Arc Breakdown Index (HVTR)

Hot wire flammability test (HWI)

High current arc ignition (HAI)

NA - Number of discharges before ignition Category PLC
120 <= NA 0
60 <= NA < 120 1
30 <= NA < 60 2
15 <= NA < 30 3
0 <= NA < 15 4

Optical Tests

1. Haze and light transmission ASTM D1003

Haze is caused by light scattering in the material and may be due to the influence of molecular structure, degree of crystallization, or foreign inclusions on the surface or inside the polymer sample. Haze is only characteristic of translucent or transparent materials and does not apply to opaque materials. Haze is sometimes considered the opposite of gloss, which may actually be the absorption of an incident beam of light. However, according to the turbidity test method, the absorption, transmission and deflection of a light beam by a translucent material is actually measured.

The sample is placed in the path of a narrow beam of light so that part of the light passes through the sample and the other part is unobstructed. Both parts of the beam pass into a sphere equipped with a photodetector.

Two quantities can be defined:

The total intensity of the light beam;
The amount of light deflected by more than 2.5° from the original beam.
From these two quantities, the following two values can be calculated:

Haze, or the percentage of the supply light scattered by more than 2.5°,
The light transmittance, or the percentage of incident light that passes through the sample.

2. Gloss DIN 67530, ASTM D523

Gloss refers to the ability of a surface to reflect more light in one direction than in other directions. Gloss can be measured with a gloss meter. Bright light is reflected from the sample at an angle, and the brightness of the reflected light is measured with a photodetector. The most commonly used angle is 60°. Higher gloss materials can be measured at 20° and matt surfaces at 85°. The glossmeter is calibrated with a black glass standard having a gloss value of 100.

Plastics are of less importance - they are strictly dependent on the molding method.

Gloss measurement method

3. Haze and gloss

Haze and gloss test methods measure how well a material reflects or transmits light. These methods quantify the material's classification, such as "clear" or "shiny". While haze is only characteristic of transparent or translucent materials, gloss can be measured for any material. Both haze and gloss tests are accurate. But they are often used to evaluate appearance, which is more subjective. The correlation between haze and gloss values, and how people rate the "clarity" or "shine" of a plastic, is uncertain.

4. Refractive index DIN 53491, ASTM D542

Determination of the refractive index

A beam of light is passed through a transparent sample at a certain angle. The deflection of the beam caused by the material as the beam passes through the sample is the refractive index, which is determined by dividing sin a by sin b.

Physical testing

1. Density ISO 1183 (DIN 53479, ASTM D792)

Density is mass divided per unit volume of material at 23°C and is usually expressed in grams per cubic centimeter (g/cm3) or grams per milliliter (g/ml). "Specific gravity" is the ratio of the mass of a given volume of material to the mass of the same volume of water at a specified temperature.

Density can be measured in several ways, as described in ISO 1183:

The method of immersing plastics in the finished state.

Pycnometric method for plastics in the form of powders, granules, tablets or molded products reduced to small particles.

Titration method for plastics of a shape similar to those required for method A.

Density gradient column method for plastics like those required for method A.

Gradient density columns are liquid columns whose density increases uniformly from top to bottom. They are particularly suitable for measuring the density of small product samples and for comparing densities.

2. Water absorption ISO 62 (ASTM D570)

Plastics absorb water. Moisture content can lead to changes in dimensions or properties such as electrical insulation resistance, electrical loss of dielectrics, mechanical strength and appearance.

Determination of the water absorption of a plastic sample of certain dimensions is carried out by immersing the sample in water for a given period of time and at a given temperature. The results of the measurements are expressed either in milligrams of absorbed water or as a percentage increase in mass. Comparison of water absorption between different plastics is possible only when the test samples are identical in size and in the same physical state.

The test specimens are pre-dried at 50°C for 24 hours, cooled to room temperature and weighed before being immersed in water of a given temperature for a given period of time.

Water absorption can be measured:

Samples are placed in a vessel with distilled water at 23°C.

After 24 hours, the samples are dried and weighed.

The samples are placed in boiling water for 30 minutes, cooled for 15 minutes in water at a temperature of 23°C and weighed again.

Up to saturation

Samples are immersed in water at a temperature of 23°C until they are completely saturated with water.

Water absorption can be expressed as:

The mass of absorbed water
The mass of absorbed water per unit surface area,
Percentage of absorbed water relative to the mass of the test sample.

Rheological testing

1. Mold shrinkage ISO 2577 (ASTM D955)

Mold shrinkage is the difference between the dimensions of a mold and the molded part produced in that mold. It is recorded in % or in millimeters per millimeter.

Molding shrinkage values are recorded both parallel to the flow of the material ("in the direction of the flow") and perpendicular to the flow ("in the direction transverse to the flow"). For fiberglass materials, these values can vary significantly. Mold shrinkage can also vary with other parameters such as part design, mold design, mold temperature, injection pressure, and mold cycle time.

Mold shrinkage values (when measured on simple parts such as a tensile test piece or disc) are only typical data for material selection. They cannot be applied to part or tool designs.

2. Melt flow rate / Melt index ISO 1133 (DIN 53735, ASTM D 1238)

Melt flow rate (MFR) or melt index (MFI) tests measure the flow of molten polymer through an extrusion plastometer under specified temperature and load conditions. The extrusion plastometer consists of a vertical cylinder with a small head 2 mm in diameter at the bottom and a removable piston at the top. A charge of material is placed in the cylinder and preheated for several minutes. The piston is mounted on the top surface of the molten polymer and its weight forces the polymer through the die and onto the build plate. The test time period varies from 15 s to 6 min depending on the viscosity of the plastics. Used temperatures: 220, 250 and 300°C. The masses of applied loads are 1.2, 5 and 10 kg.

The amount of polymer collected after a given test period is weighed and converted to the number of grams that could be extruded after 10 minutes. The melt flow rate is expressed in grams per reference time.

Example: MFR (220/10)=xx g/10 min means the melt flow rate at a test temperature of 220°C and a nominal load mass of 10 kg.

Melt index measurement method

The flow rate of the polymer melt depends on the shear rate. The shear rates in these tests are significantly less than the rates used under normal manufacturing conditions. Therefore, the data obtained by this method may not always match their properties in actual use.

3. Melt volume flow / Melt volume index ISO 1133 (DIN 53735, ASTM D 1238)

DIN 53735 describes three methods for measuring current:
"Verfahren A"

"Verfahren B", which in turn includes two methods:

The Verfahren A method consists in measuring the mass while extruding plastic through a given die.

The Verfahren B method consists in measuring the displacement of the piston and the density of the material under similar conditions.

The Verfahren B/Mebprinzip 1 method measures the distance the piston travels.

The Verfahren B/Mebprinzip 2 method measures the time during which the piston moves.

Summarizing these methods, we can say that the flow index according to Verfahren A according to DIN 53735 is equal to the flow rate MFR according to ISO 1133.

At the top of the description of these various methods, DIN 53735 describes the Volumetric Flow Index (MVI). (ISO 1133 does not mention the MVI index.)

The MVI index is defined as the volume of plastic that is extruded through the die during a given time.

The MFI index is defined as the mass of plastic extruded through the die for a given time. The MVI index is expressed in cm³/10 min and the MFI index in g/10 min.

The temperatures used are 220, 250, 260, 265, 280, 300, 320 and 360°C. Weight of cargo used - 1.2; 2.16; 3.8; 5; 10 and 21 kg.

Example: MVI (250/5) means volumetric flow index in cm³/10 min for a test temperature of 250°C and a nominal load mass of 5 kg.

4. Melt viscosity DIN 54811

Melt properties are determined in a capillary viscometer. Measure either the pressure at a given volumetric flow rate and a given temperature, or the volumetric flow rate at a given pressure. The melt viscosity (MV) is a ratio of the actual shear stress t and the actual shear stress f. It is expressed in Pa*s.

5. Practical application of MV, MFR/MFI, MVI characteristics in production

The MV method with capillary viscometer measurement is very similar to the normal extrusion process. As such, the MV method is a good basis for comparing the flow of injection molded materials: it represents the viscosity as the melt passes through the nozzle. The MFR/MFI and MVI methods, where the shear rate is too low, are not suitable for use in the injection molding process. They are a good reference for control by the manufacturer and processor and are easily, quickly and inexpensively obtained, but are not suitable for material selection in terms of its expected mold flow.

Flammability Tests

1. General information about UL94 flammability

The most widely used flammability standards are UL94 (Insurance Research Laboratories) category standards for plastics. These categories determine the ability of a material to extinguish a flame once ignited. Several categories can be assigned based on burning rate, extinguishing time, resistance to droplet formation, and depending on whether the resulting droplets are combustible or non-combustible. Each material tested can be assigned several categories depending on color and/or thickness. For a specific choice of material for an application, the UL category must be determined by the thinnest wall of the plastic part. The UL category must always be listed with the thickness: simply listing the UL category without the thickness is not sufficient.

2. Summary of UL94 classification categories

HB
Slow burning of a horizontal sample.
Burning rate less than 76 mm/min at thickness less than 3 mm.

The burning rate is less than 38 mm/min for thicknesses greater than 3 mm.

V-0
The burning of the vertical sample stops within 10 s;

V-1

dripping is not allowed.

V-2
The burning of the vertical sample stops within 30 s;

drops of burning particles are allowed.

5V
The burning of the vertical sample stops within 60 s after five exposures to the flame with the duration of each exposure to the test sample for 5 s.

5VB
Samples in the form of wide plates can burn through with the formation of holes.

5VA
Wide plate samples must not burn through (i.e., not form holes) - this is the most stringent UL category.

If flammability is a safety requirement, then HB materials are generally not allowed. In general, HB category materials are not recommended for electrical applications, except for mechanical and/or decorative applications. A misunderstanding sometimes arises: materials that are not fire resistant (or materials that are not listed as fire resistant) do not automatically meet the requirements for category HB. Category UL94HB, although the least severe, is a flammability category and must be verified by testing.

Ignition test of a horizontal sample

When testing vertical specimens, the same specimens are used as in the HB tests. All parameters are recorded: Burning time, smoldering time, droplet appearance and ignition (or non-ignition) of the cotton lining. The difference between V1 and V2 is burning droplets, which are the main source of flame propagation or fire.

Ignition test of a vertical specimen

1st stage test 5V

Flammability standards are fixed vertically and each sample is exposed to a flame five times with a flame height of 127 mm each time for 5 s. To meet the test conditions, no sample shall burn with the appearance of a flame or smolder for more than 60 s after the fifth exposure to a flame. In addition, the formation of burning droplets that ignite the cotton lining under the samples is not allowed. The whole procedure is repeated with five samples.

2nd Test Stage 5VA and 5VB

A wide plate of the same thickness as the plate specimens is tested in a horizontal position with the same flame. The whole procedure is repeated with three plates.
These horizontal tests determine two classification categories: 5VB and 5VA.

Category 5VB allows burn through (with holes).
Category 5VA does not allow holes.
The UL94-5VA category is the most stringent of any UL test. Materials in this category are used for fire enclosures of large office machines. For these applications with expected wall thicknesses of less than 1.5 mm, fiberglass-filled material grades must be used.

6. CSA Flammability (CSA C22.2 No. 0.6 Test A)

These Canadian Standards Association (CSA) flammability tests are performed similarly to UL94-5V tests. But the conditions of these tests are stricter: each exposure to the flame lasts 15 seconds. In addition, the sample must be extinguished within 30 seconds during the first four exposures, and within 60 seconds after the fifth exposure (compare test method UL94-5V with five exposures of five seconds each).
The results of these CSA tests are to be considered as corresponding to the UL94-5V test results.

The purpose of the Limited Oxygen Flammability Index (LOI) is to measure the relative flammability of materials when burned in a controlled environment. The LOI index represents the minimum amount of oxygen in the atmosphere that a thermoplastic material can sustain a flame.
The test atmosphere is an externally controlled mixture of nitrogen and oxygen. The fixed sample is ignited with an auxiliary flame, which is then extinguished. In successive test cycles, the oxygen concentration is reduced until the sample can no longer sustain combustion.

The LOI is defined as the minimum oxygen concentration at which a material can burn for three minutes, or can keep a sample burning for 50 mm.

The higher the LOI, the lower the likelihood of combustion.

Oxygen Index Test

8. Glow wire test IEC 695-2-1

Hot Wire Ignition (HWI) tests simulate thermal stresses that can be caused by a source of heat or ignition, such as overloaded resistors or hot elements.

A sample of insulating material is pressed for 30 seconds with a force of 1 N to the end of the electrically heated glow wire. The introduction of the end part of the hot wire into the sample is limited. After the wire is removed from the sample, the time to extinguish the flame and the presence of any burning droplets are recorded.

The sample is considered to have passed the glow-wire test if one of the following situations occurs:

In the absence of flame or smoldering;
If the flame or smoldering of the sample, the surrounding parts and the lower layer goes out within 30 seconds after the removal of the hot wire, and also if the surrounding parts and the lower layer are not completely burned out. In the case of using thin paper as the bottom layer, this paper should not catch fire, or there should be no scorching of the pine board, if used as a substrate.
Real live parts or enclosures are tested in a similar way. The temperature level of the hot end of the wire depends on how the finished part is used:

Supervised or unsupervised
With continuous load or without,
Located near or away from a central power point,
Makes contact with a live part or is used as a shroud or cover,
Under less or more stringent conditions.

Glow wire test

Depending on the required level of severity of the conditions surrounding the finished part, the following temperatures are preferred: 550, 650, 750, 850 or 960 °C. The appropriate test temperature shall be selected by assessing the risk of failure due to unacceptable heating, ignition and flame propagation.

Flammability test bench

9. Needle flame test IEC 695-2-2

Needle flame test

The needle flame test simulates the effects of small flames that can occur due to a malfunction inside electrical equipment. To assess the likely spread of flame (burning or glowing particles), either a layer of the test material, or components that usually surround the sample, or one layer of thin paper is placed under the sample. The test flame is exposed to the sample for a specified period of time: typically 5, 10, 20, 30, 60 or 120 seconds. For special requirements, other stringency levels may be accepted.

Unless otherwise stated in the relevant specification, a sample is deemed to have passed the needle flame test if one of the following four situations occurs:

If the sample does not ignite.
If the flame or burning or glowing particles falling from the specimen cause the fire to spread to the surrounding parts or to the layer placed under the specimen, and if there is no flame or glow on the specimen at the end of exposure to the test flame.
If the duration of burning does not exceed 30 sec.
If the spread of fire given in the relevant specifications has not been exceeded.

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CONTENTS 1 Mechanical testing 1.1 Strength, deformation and tensile modulus ISO R527 (DIN 53455, DIN 53457, ASTM D638M) 1.2 Strength and flexural modulus ISO 178 (DIN 53452, ASTM D790) 1.3 Taber wear test ISO 3537 (DIN 52347, ASTM D1044) 1.4 Comparison of ISO (International Standards Organization) and ASTM (American Society for Testing and Materials) methods 2 Hardness tests 2.1 Comparison of Brinell, Rockwell and Shore hardness 2.2 Brinell hardness ISO 2039-1 (DIN 53456 ) 2.3 Rockwell hardness ISO 2039-2 2.4 Shore hardness ISO 868 (DIN 53505, ASTM D2240) 3 Impact strength tests 3.1 Concept of impact strength 3.2 Interpretation of impact test results - comparison of ISO and AST methods 3.3 Izod impact strength ISO 180 (ASTM D256) 3.4 Charpy impact strength ISO 179 (ASTM D256) 4 Thermal tests 4.1 Vicat heat resistance ISO 306 (DIN 53460, ASTM D1525) 4.2 Deformation ionic heat resistance and deformation heat resistance under load ISO 75 (DIN 53461, ASTM D648) 4.3 HDT and amorphous and semi-crystalline plastics 4.4 Ball indentation EC335-1 4.5 Thermal conductivity ASTM C177 4.6 Relative thermal conductivity index, RTI UL 746B 4.7 Linear thermal expansion coefficient ASTM D696, DIN 53752 5. Flammability Tests 5.1 UL94 Flammability Overview 5.2 Summary of UL94 Classification Categories 5.3 UL94HB Category 5.4 UL94V0, V1, V2 Category 5.5 UL94-5V Category 5.6 ISO 4589 Limited Oxygen Flammability Index ( ASTM D2863) 5.7 Glow wire test IEC 695-2-1 5.8 Needle flame test IEC 695-2-2 6 Electrical test 6.1 Dielectric strength IEC 243-1 6.2 Surface resistivity IEC 93 (ASTM D257) 6.3 Volume resistivity IEC 93 (ASTM D257) 6.4 Relative di electrical constant IEC 250 6.5 Dissipation factor IEC 250 6.6 Arc resistance ASTM D495 6.7 Comparative training index (Comparative breakdown index) IEC112 6.8 CTI test 6.9 CTI-M test 6.10 PLC category UL746A 7 Optical test 7.1 Haze and light transmission (ASTM D1003) 7.2 Gloss (DIN 67530, ASTM D523) 7.3 Haze and gloss 7.4 Refractive index DIN 53491, ASTM D542 8 Physical tests 8.1 Density ISO 1183 (DIN 53479, ASTM D792) 8. 2 Water absorption ISO 62 (ASTM D570) 9 Rheological testing 9.1 Mold shrinkage ISO 2577 (ASTM D955) 9.2 Melt flow rate/Melt index ISO 1133 (DIN 53735, ASTM D1238) 9.3 Melt volume flow/Melt volume index ISO 1133 (DIN 53735) , ASTM D 1238) 9.4 Melt viscosity DIN 54811 9.5 Practical application of MV, MFR/MFI, MVI characteristics in production

Mechanical properties of polymers- this is a set of properties that determine the mechanical behavior when exposed to external forces.

General regularities of mechanical properties of polymers

The mechanical properties of polymers are characterized by:

Ability to develop under the action of external mechanical forces large reversible (highly elastic) deformations reaching tens, hundreds and even thousands of percent. This ability is characteristic only for polymeric materials.
The relaxation nature of the body's reaction to mechanical action, that is dependence of strains and stresses on the duration (frequency) of exposure. This dependence is due to the lag of strain from stress and can manifest itself in an extremely wide time range (from fractions of a second to many years).
Dependence of mechanical polymer on the conditions of its production, method of processing and pre-treatment. This is due to the existence in polymeric bodies of various forms of supramolecular structure, the rearrangement times of which can be so long that the polymer under the same conditions can stably exist in states with different morphologies.
The ability under the influence of anisotropic mechanical action to acquire sharp mechanical properties and retain it after the cessation of exposure.
The ability to undergo chemical transformations under the action of mechanical forces.

The general nature of the mechanical behavior of a particular polymer body is determined by the physical condition it is located.

Linear and branched polymers may be in three main amorphous states:

glassy;
highly elastic;
viscous;

three-dimensional (spatial, cross-linked) polymers- only in the first two of these states.

Many polymers can also be in a crystalline state, the essential feature of which is that almost always in a polymer body, along with strictly ordered crystalline regions, regions with an amorphous structure are preserved (therefore, this state is also called amorphous-crystalline, partially crystalline or semi-crystalline). A strictly crystalline state is realized only in polymer single crystals.

When considering the mechanical properties of polymers, a special group is distinguished oriented state which can contain both amorphous and crystalline polymers and which is characterized by anisotropy of mechanical properties.

The scope of the polymer is largely determined by the in what condition is it in the operating temperature range(usually from -40 to 40 ° C).

Polymers in this range in a highly elastic state, are called elastomers. Of the elastomers, rubbers are widely used technically. Polymeric materials that are in a glassy or crystalline state under operating conditions are called plastic masses. The latter are used in the form of bulk products and films. Uniaxially oriented polymers are widely used as fibers.

Classification and general characteristics of the mechanical properties of polymers

Under the action of mechanical forces, all bodies are deformed, and with sufficiently strong or prolonged impacts, they are destroyed. Accordingly, there are deformation and strength properties. In a separate group of mechanical properties, friction properties, manifested when a solid polymer body moves along the surface of another body.

To study the mechanical properties and determine the mechanical characteristics of materials, mechanical tests are carried out according to certain methods.

The tests differ in the type of deformation:

uniaxial and biaxial tension and compression,
all-round compression,
bend,
shift,
torsion,
indentation, etc.

and loading mode:

a load that ensures a linear increase in deformation or its constancy,

blow, etc.

The choice of test method is defined as their goals, and type of test material.

For the qualitative and quantitative description of the mechanical properties of polymers, the same concepts and characteristics are used as for the description of the mechanical properties of non-polymeric materials. At the same time, the features of the behavior of polymers require the introduction of new concepts, and sometimes some change in the meaning of the accepted ones.

Deformation properties of polymers:

Strength properties of polymers:

Friction properties of polymers

To quantify these properties, use coefficient of friction is the ratio of the tangential force to the normal force and wear resistance characterizing the rate of destruction of the material during friction.

The physical state of the polymer and its mechanical properties

Mechanical properties of polymers in the amorphous state

The difference between the individual physical states of amorphous polymers is in different reactions of polymers in these states, on mechanical impact:

elastic- in a glassy state,
mainly highly elastic- in highly elastic,
- in viscous.

Due to the relaxation nature of highly elastic deformation and viscous flow, the nature of the response to mechanical action depends significantly from the duration of exposure. In a certain range of temperatures, the body can respond to short-term exposure elastically, and for long-term (of the order of the relaxation time of highly elastic deformation or more) to show high elasticity.

At higher temperatures, due to a decrease in the relaxation time with increasing temperature, the body may exhibit high elasticity with short-term exposure, and with long-term behavior like a viscous liquid.

Thus, the division into glassy, highly elastic and viscous states is associated with the time regime of exposure.

To give certainty to the division into states, when finding the transition temperatures, one chooses some heating rate(for example, 1 ⁰С / sec) and the transition temperatures are determined by a sharp change in the magnitude of the deformation. Since elastic and highly elastic deformations have characteristic values of moduli that differ greatly from each other, division into states also carry out by module value, measured in dynamic mode or in stress relaxation mode.

The glassy state corresponds to the values of the modulus 10 3 -10 4 MN / m 2 (10 4 -10 5 kgf / cm 2),
Elastic - order 10 -1 Mn / m 2 (10 kgf / cm 2),

transition to a viscous state (pour point) fixed by falling module up to values less than 10 -1.5 MN / m 2 (10 -0.5 kgf / cm 2). With this method of separation into a special physical state (viscoelastic) isolated sometimes transition region between glassy and highly elastic states, which corresponds to the intermediate values of the modulus. This area can span tens of degrees.

AT glassy state below brittle temperature T xp the polymer behaves like a brittle solid collapsing at small, up to several percent, relative deformations (Figure 1, curve 1). Above T xp, at stresses large σ in- yield strength (forced high elasticity), develops forced highly elastic deformation, which can reach tens and hundreds of percent; while it happens transition from brittle fracture to quasi-plastic, usually accompanied a sharp increase in impact strength(except for those cases when the drop in strength occurs faster than the increase in ultimate strain). Polymer stretching at temperatures higher than T xp(Figure 1, curve 2) for many polymers, it proceeds inhomogeneously over the sample, a local narrowing is formed - neck, in which the material is strongly oriented.

As it stretches, the neck extends over the entire specimen. As the temperature rises Young's modulus, strength, hardness fall, but their change does not exceed, as a rule, one order of magnitude. As the temperature increases, the values also decrease. yield strength, reaching zero at the glass transition temperature T s. Restoring the shape of the sample is achieved by heating to a temperature slightly higher than T s.

AT highly elastic state highly elastic deformation may develop at any voltage. The transition to this state T s is accompanied by a rapid change in some equilibrium physical properties, in particular coefficient of thermal expansion. The transition to the glassy state can also be effected by changing temporary factor of influence on the material, for example, the deformation frequency.

In this case, one speaks of mechanical glass transition. Each frequency corresponds to a certain temperature. T m, at which the development of deformations is accompanied by the greatest mechanical losses. The position of the maximum mechanical loss determines glass transition temperature value, and its dependence on frequency - kinetic (relaxation) nature of the glass transition.

close T m the increase in deformation with temperature occurs most sharply (Figure 2). This is due to the fact that in this area the time relaxation decreases with a linear increase in temperature(or rather, with a linear decrease in the reciprocal temperature) according to a law close to exponential. The superposition of principle of temperature-time (temperature-frequency), which quantifies the equivalence influence of temperature increase and reduction of exposure time(increasing the frequency, see also Alexandrova - Lazurkiia frequency-temperature method). As the temperature rises, reduction of internal friction leading to decrease in relaxation time, and at sufficiently high temperatures, the development of highly elastic deformation occurs in fractions of a second. This area is sometimes called high elasticity plateau. The stretching of the polymer in the highly elastic state is essentially non-linear and, at large deformations, is accompanied by orientation of macromolecules, which can lead to reversible crystallization. At large deformations, there is a significant difference in the behavior linear and spatial (crosslinked) polymers. If the deformation of cross-linked polymers is reversible, then in linear polymers the development of highly elastic deformation is also accompanied by development of irreversible deformations.

AT viscous state dominant is viscous flow, carried out as a result of the irreversible movement of whole macromolecules or even aggregates of macromolecules. Feature of the flow of polymer bodies is that at the same time it develops reversible highly elastic deformation. This leads to a series specific effects, in particular to jet swelling, flowing out of the pipe ( highly elastic recovery), Weissenberg effect and others. For polymers in a viscous state, the phenomenon is also characteristic thixotropy- reversible destruction of the structure during the flow, leading to a drop in viscosity.

The properties of polymers in a viscous-flow state are close to those of concentrated polymer solutions. The mechanical properties of dilute polymer solutions are close to the properties of viscous simple liquids, and with increasing polymer concentration, as well as the viscosity of the solutions increases. Even in very dilute polymer solutions, viscosity gradient.

Mechanical properties of polymers in the (amorphous-crystalline) state.

The mechanical properties of polymers in the amorphous-crystalline state are largely determined by the fact that in this state the polymers are peculiar microstructures, consisting of interconnected elements ( crystalline and amorphous regions) with different mechanical characteristics. Different regions of the polymer deform differently, and within the same region, different macromolecules are also stressed and deformed differently. Physical methods make it possible to establish the features of the reaction of individual structural elements to mechanical action. In particular, the study of the shift of reflections in wide-angle X-ray diffraction patterns of crystalline polymers during their stretching made it possible to calculate the strains and Young's moduli of crystalline regions. The calculated moduli for all polymers exceeded the Young's moduli determined from mechanical tests, and for tension, by about 10%, the fraction of crystalline regions accounted for the deformation of only 0,1% , and the Young's modulus of the crystal lattice has reached the value 25,000 MN/m2 (2500 kgf/mm2), exceeding the value of the mechanical Young's modulus by 2 orders of magnitude.

At low stresses and strains, due to the significant contribution to the total strain of the deformation of amorphous regions, mechanical properties of amorphous-crystalline polymers have similarities with mechanical properties of amorphous polymers. As the temperature rises, Young's modulus decreases, and when passing through the glass transition temperature in amorphous regions, a drop in the modulus is sometimes observed, but not by 4-5, as in the case of amorphous polymers, but in total by 1 - 2 orders. Below a certain temperature, amorphous-crystalline polymers, like amorphous ones, are usually brittle (with the exception of some polyimides, for example, polypyromellitimide, retaining the ability to large deformations up to a temperature -200 °С).

At high stresses, amorphous-crystalline polymers exhibit forced high elasticity. In this case, both amorphous and crystalline regions are deformed, some crystalline formations are destroyed and others arise. For many polymers, stretching in the crystalline state is with neck formation, in which the orientation of macromolecules occurs, usually accompanied by a transition from spherulitic crystal structure to fibrillar; in this case, a sharp change in the mechanical properties of the polymer occurs.

An increase in temperature causes a change in mechanical characteristics:

decrease in strength;
decrease in yield strength;
decrease in hardness;
increase in toughness.

At melting point i crystalline polymer passes into a viscous state. This transition is a phase transition, but the melting point depends on the crystallization conditions. The mechanical properties of amorphous-crystalline polymers depend on . So, with an increase in the degree of crystallinity, Young's modulus increases.

Mechanical properties of polymers in the oriented state.

AT uniaxial and biaxial oriented states both crystalline and amorphous polymers may be present. The mechanical properties of oriented polymers depend significantly on degree of orientation. As the degree of uniaxial orientation increases, strength(more than an order of magnitude), and deformability usually falls. The increase in strength is clearly anisotropic character and only occurs in the orientation direction; in the perpendicular direction, strength tends to drop, sometimes so much that delamination of the polymer (fiber) can occur.