Tensile test methods for polymers. Polymer Testing. The mechanical properties of polymers in an amorphous state

Test methods for 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 the material is information about how the material responds to any load. Knowing the amount of deformation created by a given load (voltage), the designer can predict the response of a particular product to its operating conditions. The dependences of tensile stress and strain are the most widely published mechanical properties for comparing materials or designing specific products.

Mechanical Testing Laboratory

Test speeds:
   Speed \u200b\u200bA - 1 mm / min - tensile modulus.
   Speed \u200b\u200bB - 5 mm / min - tensile stress diagram for resins with fiberglass filler.
   Speed \u200b\u200bC - 50 mm / min - diagram of tensile stresses for resins without filler.

The dependences of stress-strain under tension are determined as follows. A double-blade specimen is stretched at a constant speed and the applied load and elongation are recorded. After that, stress and strain are calculated:

Universal test specimen ISO R527

Stress diagram
   A: The limit of proportionality.
   B: Yield strength.
   C: tensile strength
   X: Destruction.
   0-A: Yield area, elastic properties.
   After A: Plastic properties.

2. Bending strength and modulus of elasticity ISO 178 (DIN 53452, ASTM D790)

Modern installation for bending tests: "Flexometer"

Bending strength is a measure of how well the material resists bending, or "what is the stiffness of the material." Unlike tensile stress, in bending tests, all forces act in the same direction. An ordinary, freely supported rod is loaded in the middle of the span: thereby creating a three-point loading. On a standard testing machine, a loading tip presses a sample at a constant speed of 2 mm / min.

To calculate the modulus of elasticity in bending, the curve of the dependence of the deflection on the load is constructed from the recorded data. Starting from the initial linear part of the curve, at least five values \u200b\u200bof load and deflection are used.

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

The values \u200b\u200bof stress and modulus of elasticity in bending are measured in MPa.

Bending test

3. Endurance tests on Taber machine ISO 3537 (DIN 52347, ASTM D1044)

Wear test on Taber machine

In these tests, the amount of attrition loss is measured by abrasion of the sample on a Taber machine. The sample is mounted on a disk rotating at a frequency of 60 rpm. The forces created by the weights press the abrasive wheels to the sample. After a given number of cycles, the tests are stopped. The mass of attrition losses 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 grinding stones in the shape of a circle. Various types of circles are used.

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

Using the ISO method not only changes the test conditions and dimensions of the test mandrel (compared to the ASTM method), but also requires standardized mold designs and molding conditions in accordance with ISO 294. This can lead to differences in published values \u200b\u200b- not due to changes in material properties, and due to changes in test methods. According to ASTM, the test sample has a thickness of 3 mm, while ISO chose samples with a thickness of 4 mm.

Hardness test

1. Comparison of Brinell, Rockwell and Shore hardnesses

Hardness Ratio Ratio

The Rockwell test determines the hardness of plastics after the elastic restoration of sample deformation during testing. This is the difference between this method and Brinell and Shore hardness tests: in these tests, hardness is determined by the penetration depth under load and, therefore, any elastic restoration of deformation of the material is excluded. Therefore, Rockwell values \u200b\u200bcannot be directly correlated with Brinell or Shore hardness values.

The ranges of values \u200b\u200bon the Shore A and D scales can be compared with the ranges of hardness values \u200b\u200bfor prints obtained by the Brinell method. However, there is no linear correlation.

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

Brinell hardness test

A polished hardened steel ball with a diameter of 5 mm is pressed into the surface of the test sample (at least 4 mm thick) with a force of 358 N. 30 seconds after the application of the load, the imprint depth is measured. Brinell hardness N 358/30 is calculated as the "applied load" divided by the "surface area of \u200b\u200bthe print."

The result is expressed in N / mm2.

3. Rockwell hardness ISO 2039-2

Rockwell hardness test

The Rockwell hardness number directly relates to the hardness of the print on the plastic: the higher this number, the harder the material. Due to the small overlap of the Rockwell hardness scales for the same material, you can get two different numbers on two different scales, both of which can be technically correct.

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

Rockwell Portable Hardness Tester Rockwell Laboratory Hardness Tester

Rockwell hardness numbers must be between 50 and 115. Values \u200b\u200bbeyond these limits are considered inaccurate: the measurement must be repeated again using the next harder scale. Scales increase in rigidity from R through L to M (with increasing material hardness). The loads and diameters of the indenters are shown in more detail in the table.

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 suitable. Then you can use the method of determining shore hardness (ISO 868), which is used for low-modulus materials.

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

Shore hardness test

Shore hardness values \u200b\u200bare scale readings obtained as a result of the penetration of a certain steel rod into the plastic. This hardness is determined by scleroscopes of two types, both of which have calibrated springs for applying a load to the indenter. Scleroscope A is used for softer materials, and scleroscope D is for harder materials.

Indentors for scleroscopes

Shore hardness values \u200b\u200bvary:
   from 10 to 90 for a Shore type A scleroscope - soft materials,
   20 to 90 for a Shore D-type scleroscope - solid materials.
   If the measured values \u200b\u200bare\u003e 90A, then the material is too hard and a D. scleroscope should be used.
   If the measured values<20D, то материал слишком мягок, и должен применяться склероскоп А.

There is no simple correlation between the hardness measured using this test method and other basic properties of the test material.

Impact Strength Testing

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 the applied force, for example, the force from falling objects, impacts, collisions, falls, etc. The purpose of impact strength tests is to simulate such conditions.

The Izod and Charpy methods are used to study the properties of certain samples at given shock stresses and to assess the fragility or toughness of samples. Test results for these methods should not be used as a data source for design calculations of components. Information on the typical properties of the material can be obtained by testing different types of test samples prepared under different conditions, with a change in the notch radius and test temperature.

Tests by both methods are carried out on a shock pendulum. The sample is clamped in a vice, and the pendulum head with a hardened steel impact surface of a certain radius is released from a given height, which causes the sample to be cut off from a sharp load. The residual energy of the pendulum hoist lifts it up. The difference between the drop height and the return height determines the energy spent on the destruction of the test sample. These tests can be carried out at room temperature or at low temperatures to determine cold brittleness. Test specimens may vary in type and size of incisions.

The results of impact tests with a falling load, for example, according to the Gardner method or a curved plate, depend on the geometry of the falling load and the support. They can only be used to determine the relative ranking of materials. Impact test results cannot be considered absolute, unless the geometry of the test equipment and specimen meets end-use requirements. It can be expected that the relative ranking of the materials according to the two test methods will coincide 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

Impact characteristics can to a large extent depend on the thickness of the sample and the orientation of the molecules. Different thicknesses of the samples used in the ISO and ASTM methods can very significantly affect the impact strength. A change in thickness from 3 mm to 4 mm can even lead to a transition of the fracture pattern from viscous to brittle due to the influence of the molecular weight and thickness of the notched specimen using the Izod method, as demonstrated for polycarbonate resins. For materials already showing the brittle nature of fracture at a thickness of 3 mm, for example, materials with mineral and fiberglass fillers, a change in the thickness of the sample is not affected. The same properties are possessed by materials with modifying additives that increase impact strength.

The influence of the thickness and molecular weight of the notched sample on the results of impact tests of polycarbonate resins according to Izod

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

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

3. Izod Impact Strength ISO 180 (ASTM D256)

Izod Impact Strength Laboratory

Izod notch impact testing of specimens has become the standard method for comparing the impact strength of plastics. However, the results of this test method are little consistent with the reaction of the molded product to impact in a real environment. Due to the different sensitivity of the materials to the notch, this test method allows you to reject some materials. Although the results of these tests were often requested as significant measures of impact strength, these tests tend to measure the sensitivity of the material to notching, rather than the ability of a plastic to withstand impact.

The results of these tests are widely used as reference for comparing the toughness of materials. Izod impact test notches are best used to determine the impact strength of products having many sharp angles, such as ribs, intersecting walls and other places of stress concentration. When testing the Izod impact strength of specimens without a notch, the same loading geometry is used, except that the specimen has no notch (or is clamped in a vice in an inverted position). Tests of this type always give better results than tests on Izod notched specimens due to the lack of stress concentration.

The impact strength of notched specimens 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 \u200b\u200bthe specimen at the notch. This strength is expressed in kilojoules per square meter: kJ / m2. The specimen is vertically clamped in the vice of a hammer.

ISO designations indicate specimen type and incision type:
   ISO 180 / 1A denotes the type of specimen 1 and the type of notch A. As can be seen in the figure below, the type 1 specimen has a length of 80 mm, a height of 10 mm and a thickness of 4 mm.
   ISO 180 / 1O denotes the same specimen 1, but clamped in an inverted position (indicated as “not cut”).
   The samples used by the ASTM method have similar sizes: the same rounding radius at the base of the notch and the same height, but differ in length - 63.5 mm and, more importantly, in thickness - 3.2 mm.

ISO test results are defined as the impact energy in joules spent on the destruction of the test specimen divided by the cross-sectional area of \u200b\u200bthe specimen at the incision site. The result is expressed in bajoules per square meter: kJ / m2.

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

Different thicknesses of samples may affect different interpretations of “impact strength,” as shown separately.

Impact strength samples

Izod Impact Measurement Method

4. Charpy Impact Strength ISO 179 (ASTM D256)

Charpy Strength Tester

The main difference between Charpy and Izod methods is the method of installing the test sample. When tested using the Charpy method, the sample is not clamped, but freely mounted on a support in a horizontal position.

ISO designations indicate specimen type and incision type:
ISO 179 / 1C denotes a sample of type 2 and a notch of type CI;
ISO 179 / 2D refers to a Type 2 specimen, but not cut.

Charpy Impact Measurement Method

Samples used according to DIN 53453 have similar dimensions. 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 \u200b\u200bthe specimen at the incision site. These results are expressed in kilojoules per square meter: kJ / m2.

Thermal test

1. Wick heat resistance ISO 306 (DIN 53460, ASTM D1525)

Vick Laboratory Heat Resistance Tester

These tests give the temperature at which the plastic begins to soften quickly. A round needle with a flat end having a cross-sectional area of \u200b\u200b1 mm² is inserted into the surface of the plastic test specimen at a certain load, and the temperature rises at a uniform rate. Wick heat resistance (VST - Wick softening temperature) is the temperature at which penetration reaches 1 mm.

Determination of heat resistance by Wick

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

50 ° C / hour;
120 ° C / hour.
ISO test results are indicated 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 is introduced to a depth of 1 + 0.01 mm, is recorded as the Wick heat resistance of the material at the selected load and rate of temperature increase.

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

It is possible to detect some differences in published ISO results compared to ASTM standards due to the different sizes of the test samples: the values \u200b\u200bof deformation resistance measured by ISO methods may be lower.

3. Strain resistance and strain resistance under load ISO 75 (DIN 53461, ASTM D648)

Heat resistance is a relative measure of the ability of a material to withstand stress for a short period of time at elevated temperatures. In these tests, the effect of temperature on stiffness is measured: certain surface stresses are created on a standard test sample, and the temperature is raised at a uniform rate.

The samples used in the tests are annealed and unannealed. Vacation is a process in which a sample is heated to a certain temperature, kept at it for some time, and then the temperature is gradually lowered to ambient level. Such actions can reduce or completely remove the internal stresses in the body of the sample, which occurred, for example, at the time of accelerated polymerization in the injection molding machine.

According to both ISO and ASTM standards, the loaded test sample 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.
A force is allowed for 5 minutes, but this holding period may be skipped if the test materials do not show noticeable creep within the first 5 minutes. 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 sample 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 both test standards, two abbreviations are usually used:

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

Determination of heat resistance

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

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

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

For amorphous polymers, the HDT values \u200b\u200bapproximately coincide with the glass transition temperatures Tg of the material.

Since amorphous polymers do not have a specific melting point, they are processed in their highly elastic state at temperatures above Tg.

Crystalline polymers can have low HDT values \u200b\u200band still have structural utility at higher temperatures: the HDT determination method is more reproducible with amorphous plastics than with crystalline ones. For some polymers, tempering (annealing) of the test samples may be required to obtain reliable results.

When glass fibers are added to the polymer, its modulus increases. Since HDT is the temperature at which a material has a certain modulus, an increase in modulus also increases the value of HDT. Fiberglass has a greater effect on the HDT of crystalline polymers than amorphous polymers.

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

5. Indentation of the ball EC335-1

These are heat resistance tests similar to the Wick test. The sample is horizontally mounted on a support in the heating chamber and a ball with a diameter of 5 mm 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 print diameter is less than 2 mm, then it is believed that the material has passed the ball indentation test at a given temperature.

Ball Indentation Test

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

6. Thermal conductivity ASTM C 177

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

7. Relative heat conductive index, RTI UL 746B

The Relative Temperature Index (RTI), previously called Permissible Continuous Use Temperature (CUTR), is the maximum operating temperature at which all critical material properties remain within acceptable limits for a long period of time.

According to UL 746B, three independent RTI indices can be assigned to one material:

Electric - by measuring the dielectric strength.
   Mechanical shock - by measuring the impact strength in tension.
   Unstressed mechanical - by measuring tensile strength.
   These three properties were selected as critical values \u200b\u200bin the tests because of their sensitivity to high temperatures when used.

The thermal characteristics of the material are tested for a long time in comparison with the second control material, for which the RTI index has already been determined and which has shown good characteristics.

Based on the term "relative temperature index", the control material is used because characteristics that deteriorate with increasing temperature have an inherent sensitivity to the variables of the test program itself. The control material is influenced by the same specific combinations of these factors during the test, which provides a reliable basis for comparison with the test material.

Ideally, the thermal characteristics measured over a long period of time could be evaluated by aging the test material at normal temperature for a long period of time. However, this is impractical for most applications. Therefore, accelerated aging occurs at significantly higher temperatures. During the aging process, samples of the test and control materials are placed in an oven in which a predetermined constant temperature is maintained. Samples of the test and control materials are removed at predetermined points in time, and then tested to maintain the basic properties. By measuring the three mentioned properties as a function of time and temperature, the “end of life” for each temperature can be mathematically calculated. This "end of life" is defined as the time during which the properties of the material have deteriorated by 50% compared to the baseline. By substituting the test data into the Arrhenius equation, one can determine the maximum temperature at which the test material will have a satisfactory life. This calculated temperature is the RTI index for each material property.

Understanding the methodology for determining the RTI index allows the designer to use this index to predict how parts formed from this material will work in real life under the influence of elevated temperatures.

8. The coefficient of linear thermal expansion of ASTM D696, DIN 53752

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

The addition of anisotropic fillers significantly affects the CLTE coefficient of the polymer. Glass fiber is 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 direction of flow and thickness, the CLTE coefficient will be higher.

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

Electrical test

1. Dielectric strength IEC 243-1

Laboratory installation for measuring electrical strength

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

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

Many factors influence the results:

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

When insulating plastic is exposed to voltage, part 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 of resistance to this surface current.

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

3. Volume resistivity IEC 93 (ASTM D257)

When an electric potential is applied across the insulator, the flow of current 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 environmental conditions acting on the material. It changes back to temperature and decreases slightly in a humid environment. Materials with a volume resistivity of more than 108 Ohm * cm are considered insulators. Partial conductors have a volume resistivity of 103 to 108 Ohm * cm.

4. Relative dielectric constant IEC 250

As stated in IEC 250, "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 electrode configuration in a vacuum."

In alternating current dielectric applications, the required characteristics are good resistivity and low energy dissipation. The dissipation of electricity leads to ineffective 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 is no energy loss due to the dipole movement of molecules. In solid materials, such as plastics, dipole displacement becomes one of the influential factors. A measure of this inefficiency is the relative dielectric constant (previously called the dielectric constant).

This is a dimensionless coefficient obtained by dividing the parallel capacity of the system with a plastic dielectric element by the capacity of the system with vacuum as the dielectric. The lower the number, the better the material characteristics as an insulator.

5. Dissipation factor IEC 250

As stated in IEC 250, “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 the Pi / 2 radian when the capacitor’s dielectric consists solely of the dielectric under test. Dissipation coefficient tg d of the dielectric insulating material is the tangent of the loss angle d ".

In an ideal dielectric, the voltage and current curves do not exactly coincide in phase exactly 90 °. When the dielectric becomes less than 100% effective, the current wave begins to lag behind the voltage in direct proportion. The magnitude of the current wave, which deviates from the mismatch of 90 ° in phase with the voltage, is defined as the "dielectric loss angle". The tangent of this angle is called the "loss tangent" or "scattering coefficient".

A low scattering coefficient is very important for plastic insulators in high-frequency applications, for example, in radar equipment and parts operating under microwave conditions: lower values \u200b\u200bcorrespond to better dielectric materials. A high dissipation factor is essential for welding performance.

The relative dielectric constant and the dispersion coefficient 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 the insulator, this surface is damaged after some time and becomes conductive.

Arc Resistance is the amount of time, in seconds, required to create the conductivity of an insulating surface at high voltage and low ampere arc. In another embodiment, arc resistance is called 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 ampere 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 path when a surface under electrostatic stress is exposed to contaminants containing water. The Comparative Tracking Index (CTI) and CTI-M tests are performed to evaluate the safety of components that carry live parts: the insulating material between the live parts must be resistant to dielectric tracking. The CTI index is defined as the maximum voltage at which insulation does not fail after exposure to 50 drops of an aqueous solution of ammonium chloride. High CTI values \u200b\u200bare desirable. Materials meeting the CTI index requirements of 600 V are referred to as high tracking resins.

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

Results can be reduced by adding additives, for example:

Pigments, in particular carbon black,
   Antipyrins,
   Fiberglass.
   Therefore, in the general case, it is not recommended to use materials with antipyrins, carbon black and fiberglass, when the determination of resistance to dielectric tracking is a basic requirement.

Minerals (TiO2) tend to increase CTI values.

8. CTI Tests

CTI tests are carried out using two platinum electrodes with a given size, evenly resting slightly rounded "chisel" edges on the test sample.

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

The surface of the test material is moistened with 50 drops of a 0.1% solution of ammonium chloride in distilled water (the so-called solution A) falling in the center between the two electrodes. The sizes and frequency of falling electrolyte drops are regulated. If there is no current at the selected voltage, then the test is repeated with a voltage increased by 25 V until a current appears. This voltage, reduced by one step of 25 V, is called the CTI index. After that, the test is repeated with a voltage of 25 V below the CTI voltage, but with 100 drops of electrolyte instead of 50. The voltage is determined at which 100 drops do not cause current. This value can be indicated 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 short for the French word “mouille” - “moistened”). Solution B contains 0.1% ammonium chloride and 0.5% alkylnaphthalenesulfonate. Holes created by erosion can also be measured and their depth recorded.
Registration Example: CTI 375 (300) M-0.8 means:

50 drops of solution B do not create current at a voltage of 375 V.
100 drops do not generate current at a voltage of 300 V.
The depth of the erosion holes in the surface of the sample can be 0.8 mm.

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

According to the results of these tests, the materials are divided into PLC (Performance Level Categories) 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 PLC category
120 <= NA 0
60 <= NA < 120 1
30 <= NA < 60 2
15 <= NA < 30 3
0 <= NA < 15 4

Optical tests

1. Turbidity and light transmission ASTM D1003

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

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 does not meet obstacles. 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.
The following two values \u200b\u200bcan be calculated from these two quantities:

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

2. Gloss DIN 67530, ASTM D523

Gloss is associated with the ability of a surface to reflect more light in a certain direction compared to 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 by a photo detector. The most commonly used angle is 60 °. More shiny materials can be measured at an angle of 20 °, and matte surfaces at an angle of 85 °. The gloss meter is calibrated using a black glass reference having a gloss value of 100.

Plastics have lower values \u200b\u200b- they strictly depend on the molding method.

Gloss Measurement Method

3. Turbidity and gloss

Turbidity and gloss tests measure how well a material reflects or transmits light. These methods quantify the classification of the material, for example, “transparent” or “glossy”. While turbidity is characteristic only of transparent or translucent materials, gloss can be measured for any material. Both types of turbidity and gloss tests are accurate. But they are often used to assess the appearance, which is more subjective. The correlation between the values \u200b\u200bof turbidity and gloss, as well as how people evaluate the "transparency" or "gloss" of plastic, are uncertain.

4. Refractive index DIN 53491, ASTM D542

Refractive index determination

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

Physical tests

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

Density is the mass divided by the unit volume of material at 23 ° C, and is usually expressed in grams per centimeter cubic (g / cm3) or in 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 by several methods, as described in ISO 1183:

The method of immersion plastics in the finished state.

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

A titration method for plastics of a mold similar to the molds required for method A.

Gradient density bars for plastics similar to those required for method A.

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

2. Water absorption of ISO 62 (ASTM D570)

Plastics absorb water. Moisture content can lead to dimensional changes or properties such as electrical insulation resistance, dielectric loss, mechanical strength and appearance.

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

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

Water absorption can be measured:

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

After 24 hours, the samples are dried and weighed.

Samples are placed in boiling water for 30 minutes, cooled for 15 minutes in water at 23 ° C and weighed again.

Before 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:

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

Rheological tests

1. Shrinkage during molding ISO 2577 (ASTM D955)

Shrinkage during molding is the difference between the dimensions of the mold and the molded part obtained in this mold. It is recorded in% or in millimeters per millimeter.

Shrink values \u200b\u200bduring molding are recorded both parallel to the flow of the material ("in the direction of flow") and perpendicular to the flow ("in the direction of transverse flow"). For fiberglass materials, these values \u200b\u200bcan vary significantly. Shrinkage during molding can also vary from other parameters: for example, part design, mold design, mold temperature, specific injection pressure and molding cycle time.

Shrinkage values \u200b\u200bduring molding (when measured on simple parts such as a tensile test specimen or disk) are only typical data for material selection. They cannot be applied to parts or tool designs.

2. Melt Flow Rate / Melt Index ISO 1133 (DIN 53735, ASTM D 1238)

In tests for melt flow rate (MFR) or melt index (MFI), the flow of molten polymer through an extrusion plastometer is measured under specified temperature and load conditions. The extruding plastometer consists of a vertical cylinder with a small head with a diameter of 2 mm in the lower part and a removable piston in the upper part. The charge of the material is placed in a cylinder and preheated for several minutes. The piston is mounted on the upper surface of the molten polymer, and its weight pushes the polymer through the head onto the collection plate. The test period varies from 15 s to 6 min, depending on the viscosity of the plastics. Used temperature values: 220, 250 and 300 ° С. The masses of the 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 squeezed out after 10 minutes. The melt flow rate is expressed in grams per reference time.

Example: MFR (220/10) \u003d xx g / 10 min - means the melt flow rate at a test temperature of 220 ° C and a nominal load weight of 10 kg.

Method for measuring the melt index

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

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

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

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

Verfahren A method is to measure the mass when extruding plastic through a given head.

Verfahren B's method is to measure piston displacement and material density under similar conditions.

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

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

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

At the top of the descriptions of these various methods, the volume flow index (MVI) is described in DIN 53735. (The ISO 1133 standard does not mention the MVI index.)

The MVI index is defined as the volume of plastic that extrudes through the head for 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 used temperature values \u200b\u200bare 220, 250, 260, 265, 280, 300, 320 and 360 ° С. The mass of the used freights - 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

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

5. The practical application of the characteristics of MV, MFR / MFI, MVI in production

The MV method with measurement in a capillary viscometer bears great resemblance 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, in which the shear rate is too low, are not suitable for use in the injection molding process. They are good reference information for control by the manufacturer and the processor, obtained easily, quickly and inexpensively, but are not suitable for selecting the material in terms of its expected flow during molding.

Flammability Testing

1. UL94 Flammability Overview

The most common flammability performance standards are standards for UL94 (research laboratories of insurance companies) categories for plastics. These categories determine the ability of a material to extinguish a flame after ignition. Several categories can be assigned based on burning rate, quenching time, resistance to droplet formation and depending on whether flammable or non-combustible droplets form. Several categories can be assigned to each test material, depending on color and / or thickness. For a specific material choice for use, the UL category should be determined by the thinnest wall of the plastic part. The UL category should always be indicated with the thickness: simply specifying the UL category without thickness is not enough.

2. Brief description of the classification categories of UL94

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

Burning rate less than 38 mm / min with a thickness of more than 3 mm.

V-0
The burning of a vertical sample ceases within 10 s;

V-1

drops are not allowed.

V-2
The burning of a vertical sample ceases within 30 s;

droplets of burning particles are allowed.

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

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

5VA
Samples in the form of wide plates should not burn through (i.e. do not form holes) - this is the most stringent UL category.

If flammability is a safety requirement, the use of HB materials is generally not permitted. In general, HB materials are not recommended for use in electrical engineering, with the exception of mechanical and / or decorative products. Sometimes a misunderstanding arises: non-flame retardant materials (or materials that are not referred to as flame retardant) do not automatically meet the requirements for category HB. The UL94HB category, although the least stringent, is a flammability category and should be verified by testing.

Horizontal specimen ignition test

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

Vertical specimen ignition test

1st Test Stage 5V

Standard samples for determining flammability are fixed vertically and each sample is exposed five times to a flame with a torch height of 127 mm each time for 5 s. To comply with the test conditions, no sample shall burn with a flame or smoldering for more than 60 s after the fifth flame exposure. In addition, burning droplets that ignite the cotton lining under the specimens are 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 samples is tested horizontally with the same flame. The whole procedure is repeated with three plates.
Based on these horizontal tests, two classification categories are determined: 5VB and 5VA.

Category 5VB allows through burning (with holes).
Category 5VA does not allow hole formation.
UL94-5VA tests are the most stringent compared to all UL tests. Materials of this category are used for fire protection covers of large-sized office equipment. In these applications with an expected wall thickness of less than 1.5 mm, grades of fiberglass material should be used.

6. Flammability according to the CSA standard (CSA C22.2 No. 0,6, test A)

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

The purpose of determining the limited oxygen content flammability index (LOI) is to measure the relative flammability of materials when they are burned in a controlled environment. The LOI is the minimum oxygen content in the atmosphere that a flame can hold on a thermoplastic material.
The test atmosphere is an externally controlled mixture of nitrogen and oxygen. The fixed sample is ignited with an auxiliary flame, which is then quenched. In successive test cycles, the oxygen concentration is reduced until the sample can no longer sustain combustion.

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

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

Oxygen Index Test

8. Tests with glow wire IEC 695-2-1

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

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

The sample is considered to have passed the test with hot wire in the event of one of the following situations:

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

With or without supervision
   With or without continuous load
   Located close to or away from the central power point,
   It is in contact with a live part or used as a casing or cover,
   In less or more stringent conditions.

Glow wire test

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

Flammability test bench

9. Needle flame tests IEC 695-2-2

Needle flame test

Needle flame tests simulate the effects of small flames that may occur due to a malfunction inside the electrical equipment. To assess the likely spread of flame (burning or smoldering particles), either a layer of the test material, or components usually surrounding the sample, or one layer of thin paper are placed under the sample. The test flame acts on the sample for a certain period of time: usually 5, 10, 20, 30, 60 or 120 seconds. For specific requirements, other levels of stringency can be accepted.

Unless otherwise specified in appropriate specifications, the specimen is considered 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 smoldering particles falling from the sample cause the fire to spread to surrounding parts or to the layer under the sample, and if there is no flame or smoldering on the sample at the end of exposure to the test flame.
   If the duration of combustion does not exceed 30 seconds.
   If the combustion spread specified in the relevant specifications has not been exceeded.

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TABLE OF CONTENTS 1 Mechanical tests 1.1. Tensile strength, deformation and tensile modulus ISO R527 (DIN 53455, DIN 53457, ASTM D638M) 1.2. Bending strength and tensile modulus ISO 178 (DIN 53452, ASTM D790) 1.3. Tensile tests on Taber machine ISO 3537 (DIN 52347, ASTM D1044) 1.4 Comparison of ISO (International Organization for Standardization) and ASTM (American Society for Testing Materials) methods 2 Hardness tests 2.1 Comparison of Brinell, Rockwell and Shore hardnesses 2.2 Brinell hardness ISO 2039-1 (DIN 53456 ) 2.3 Rockwell hardness ISO 2039-2 2.4 Shore hardness ISO 868 (D IN 53505, ASTM D2240) 3 Impact strength tests 3.1 Impact strength concept 3.2 Interpretation of impact test results - comparison of ISO and AST methods 3.3 ISO 180 impact strength (ASTM D256) 3.4 Charpy impact strength ISO 179 (ASTM D256 ) 4 Thermal tests 4.1 Vick heat resistance ISO 306 (DIN 53460, ASTM D1525) 4.2 Heat and heat resistance under stress ISO 75 (DIN 53461, ASTM D648) 4.3 Heat resistance (HDT) and amorphous and semi-crystalline plastics 4.4 Ball indentation EC335- 1 4.5 Thermal Conductivity ASTM C177 4.6 Relative heat conductive index, RTI UL 746V 4.7 Linear thermal expansion coefficient ASTM D696, DIN 53752 5. Flammability tests 5.1 General information on flammability according to UL94 5.2 Classification categories classification UL94 5.3 Category UL94HB 5.4 Category UL94V0, V1, V2 5.5 Category UL94V0 -5V 5.6 Limited Oxygen Flammability Index ISO 4589 (ASTM D2863) 5.7 Glow wire tests IEC 695-2-1 5.8 Needle flame tests IEC 695-2-2 6 Electrical tests 6.1 Dielectric strength IEC 243-1 6.2 Surfaces specific resistivity IEC 93 (ASTM D257) 6.3 Volume resistivity IEC 93 (ASTM D257) 6.4 Relative dielectric 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 tests 6.9 CTI tests -M 6.10 Categories PLC UL746A 7 Optical tests 7.1 Turbidity and light transmission (ASTM D1003) 7.2 Gloss (DIN 67530, ASTM D523) 7.3 Turbidity 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 tests 9.1 Shrinkage during molding ISO 2577 (ASTM D955) 9.2 Melt flow rate / Melt index ISO 1133 (DIN 53735, ASTM D1238) 9.3 Melt flow rate / Melt volume index ISO 1133 (DIN 53735 , ASTM D 1238) 9.4 Melt viscosity DIN 54811 9.5 Practical application of the characteristics MV, MFR / MFI, MVI in production

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

General laws of the mechanical properties of polymers

The mechanical properties of polymers are characterized by:

Ability to develop under the influence of external mechanical forces large reversible (highly elastic) deformationsreaching tens, hundreds and even thousands of percent. This ability is characteristic only of polymeric materials.
The relaxation nature of the reaction of the body to mechanical stress, i.e. the dependence of deformations 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).
The dependence of mechanical polymer on the conditions for its preparation, processing method and pre-treatment. This is due to the existence in polymeric bodies of various forms of a supramolecular structure, the restructuring times of which can be so long that the polymer under the same conditions can stably exist in states with different morphologies.
Ability under the influence of anisotropic mechanical stress to acquire sharp mechanical properties and maintain it after termination of exposure.
The ability to undergo chemical transformations under the influence 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 fluid;

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

Many polymers may also be present. in crystalline state, an essential feature of which is that almost always in the 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 in which both amorphous and crystalline polymers can be located and for which it is characteristic anisotropy of mechanical properties.

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

Polymers in this range in a highly elastic stateare called elastomers. Of elastomers, rubber is widely used in technology. Polymeric materials that are in use in a glassy or crystalline state are called plastics. 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 under sufficiently strong or prolonged exposure, they are destroyed. In accordance with this distinguish deformation and strength properties. In a separate group of mechanical properties emit frictional properties, manifested by the movement of a solid polymer body on 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.

Tests differ in the type of deformation:

uniaxial and biaxial tension and compression,
comprehensive compression
bend
shift,
torsion,
indentation, etc.

and loading mode:

load providing linear strain growth or its constancy,

blow and others

The choice of test method is defined as their goalsso 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-polymer materials. However, the behavior of polymers requires the introduction of new concepts, and sometimes some change in the meaning adopted.

The deformation properties of polymers:

Strength properties of polymers:

Frictional properties of polymers

To quantify these properties using coefficient of friction - 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

The mechanical properties of polymers in an amorphous state

The difference between the individual physical states of amorphous polymers is in different polymer reactionsin these conditions on mechanical stress:

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

Due to the relaxation nature of the highly elastic deformation and viscous flow, the nature of the reaction to mechanical stress depends significantly from exposure duration. In a certain temperature range, the body can respond to short-term exposure. resilientlyand for long-term (of the order of the relaxation time of highly elastic deformation or greater) to manifest 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 separation into a glassy, \u200b\u200bhighly elastic and viscous flow state is associated with a temporary exposure regime.

To give certainty to the separation of states, when finding the transition temperatures, some heating rate (for example, 1 ⁰С / sec) and transition temperatures are determined by a sharp change in the strain value. Since elastic and highly elastic deformations have characteristic, very different module values, division into states spend also by module value, measured in dynamic mode or in stress relaxation mode.

The glass state corresponds to the module values 10 3 -10 4 Mn / m 2 (10 4 -10 5 kgf / cm 2),
Highly elastic - order 10 -1 Mn / m 2 (10 kgf / cm 2),

transition to a viscous flow state (pour point) fixed by module drop to values \u200b\u200bless 10 -1.5 Mn / m 2 (10 -0.5 kgf / cm 2). With this method of separation in a special physical state (viscoelastic) sometimes emit transition region between the glassy and highly elastic stateswhich corresponds to the intermediate values \u200b\u200bof the module. This area can span tens of degrees.

IN glassy state lower brittle temperature T xp polymer behaves like a fragile solid collapsing with small, up to several percent, relative deformations (Figure 1, curve 1). Higher T xpat voltages large σ in - yield strength (forced high elasticity), develops forced high elastic deformation, which can reach tens and hundreds of percent; this happens transition from brittle fracture to quasi-plasticusually followed sharp increase in toughness (except in those cases when the drop in strength occurs faster than the growth of ultimate strain). Tensile polymer at temperature max T xp(Figure 1, curve 2) for many polymers flows non-uniformly along the sample, a local narrowing is formed - neckin which the material is strong oriented.

As it stretches, the neck extends to the entire sample. With increasing temperature young's modulus, strength, hardness fall, but their change does not exceed, as a rule, one order. With increasing temperature, the values \u200b\u200balso decrease yield strengthreaching zero at glass transition temperature T s. Sample shape recovery is achieved by heating to a temperature slightly higher T s

IN highly elastic state high elastic deformation may develop at any voltage. Transition to this state when T s 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 carried out by changing temporary impact factor on the material, for example, strain frequencies.

In this case, talk about mechanical glass transition. Each frequency corresponds to a specific temperature. T mat which the development of deformations is accompanied by the greatest mechanical losses. The position of the maximum mechanical loss determines glass transition temperature, and its dependence on frequency is kinetic (relaxation) nature of glass transition.

Near T mstrain growth with temperature occurs most sharply (Figure 2). This is due to the fact that in this area, 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. Superposition can describe in a unified way the polymer deformation in the transition region in a certain interval of times and frequencies of exposure the principle of temperature-time (temperature-frequency)quantifying equivalence the effects of temperature rise and reduced exposure time (increase in frequency, see also Aleksandrova-Lazurkia frequency-temperature method). As temperature rises reduced internal frictionleading to decrease relaxation time, and at sufficiently high temperatures, the development of highly elastic deformation occurs in a split second. This area is sometimes called high elasticity plateau. The stretching of the polymer in a highly elastic state is essentially nonlinear and is accompanied by large deformations orientation of macromolecules, which can lead to reversible crystallization. With large deformations, a significant difference in behavior is manifested. linear and spatial (stitched) polymers. If the deformation of crosslinked polymers is reversible, then in linear polymers the development of highly elastic deformation is also accompanied by development of irreversible deformations.

IN viscous flow dominant is viscous flowcarried out as a result of 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 high elastic deformation. This leads to a number specific effectsin particular to jet swellingflowing out of the pipe ( highly elastic recovery), Weissenberg effect etc. For polymers in a viscous flowing state, a phenomenon is also characteristic thixotropy - reversible destruction of the structure during the flow, leading to a decrease in viscosity.

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

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

The mechanical properties of polymers in an amorphous-crystalline state are largely determined by the fact that in this state polymers are unique microstructuresconsisting of interconnected elements ( crystallineand amorphous areas) with various mechanical characteristics. Different areas of the polymer are deformed differently, and within the same region, different macromolecules are strained and deformed also differently. Physical methods make it possible to establish the characteristics of the reaction of individual structural elements to mechanical stress. In particular, a study of the shift of reflections on wide-angle X-ray diffraction patterns of crystalline polymers under tension made it possible to calculate the strain and Young's modulus of the crystalline regions. The calculated moduli for all polymers exceeded Young's moduli determined by mechanical tests, and for tensile strain of approximately 10%, the crystalline regions accounted for only 0,1% and Young's modulus of the crystal lattice has reached 25 000 Mn / m 2 (2500 kgf / mm 2), 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 overall strain, the deformation of amorphous regions, mechanical properties of amorphous crystalline polymers resemble mechanical properties of amorphous polymers. With increasing temperature, the Young's modulus decreases, and when passing through the glass transition temperaturein amorphous regions, sometimes a drop in the module is observed, but not by 4-5, as in the case of amorphous polymers, but 1 - 2 orders. Below a certain temperature, amorphous-crystalline polymers, like amorphous ones, usually break brittle (an exception is made by some polyimides, for example, polypyromellitimideretaining the ability to large deformations to a temperature -200 ° C).

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 appear. Many polymers have crystalline stretching with the formation of the neckin which the orientation of macromolecules occurs, usually accompanied by a transition from spherulite crystal structure to fibrillar; this causes a sharp change in the mechanical properties of the polymer.

An increase in temperature causes a change in mechanical characteristics:

strength reduction;
yield stress reduction;
hardness reduction;
increase in toughness.

At melting pointi crystalline polymer goes into a viscous state. This transition is phase, 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.

The mechanical properties of polymers in an oriented state.

IN uniaxial and biaxial oriented states Both crystalline and amorphous polymers may be present. The mechanical properties of oriented polymers substantially depend on degree of orientation. With increasing degree of uniaxial orientation increases strength (more than an order of magnitude), and deformabilityusually falls. Strength increase is clearly defined anisotropic nature and occurs only in the direction of orientation; in the perpendicular direction, the strength, as a rule, decreases, and sometimes so much that delamination of the polymer (fiber) can occur.