CS258251B1 - A method for conducting thermal oxidation and atmospheric aging tests of polymers - Google Patents

Abstract

Method of performing tests of thermal-oxidative and atmospheric aging of polymers, corrosion tests and other external environmental influences on samples subjected to impact tests under multiaxial tension using parachutes to cause a mandrel impact on the wall of the tested samples. From the tested injection-molded polymer types, model products in the form of plates with a film gate measuring 50 x 50 mm are injected or larger extruded plates are cut into 50 x 50 mm plates, then these model products are exposed in series of 5 to 25 pieces, always on the same side, to the action of different external environments at different time intervals, after exposure, the plates are placed ■on a support with a circular hole of at least 40 mm diameter and subjected to a mandrel impact on the center of the exposed surface and the energy to failure due to fracture or cracks in the tested plate is determined. The average energy to failure of individual series of exposed plates is evaluated in % relative to the value of the average energy to failure of the unexposed series of plates.

Description

The invention relates to a method of performing thermal-oxidative aging tests of polymer materials in hot-air dryers, atmospheric aging tests of polymers in climate stations, corrosion tests of materials in various environments, tests of the influence of ionizing radiation and other influences of the external environment on various non-metallic, especially polymer materials.

The purpose of these tests is to determine changes in the mechanical behavior of polymers depending on the time and conditions of exposure to external influences, to determine the resistance and service life of products made of these materials in various environments and to determine the conditions of processing and use of materials. The vast majority of tests of thermal-oxidative and atmospheric aging, corrosion tests and other effects of the external environment on various, especially polymer materials, used so far are based on monitoring irreversible changes in basic mechanical and chemical-physical properties, measured on test specimens for tensile testing and for determining impact or notch toughness, e.g. by the Charpy, Izod, Dynstat, or impact tensile methods depending on the time and conditions of exposure to the external environment.

When measuring standard mechanical properties, the relevant test specimens are subjected to uniaxial tension, i.e. uniaxial tensile or bending stresses arise in the specimens. It is also known that in the case of aging, corrosion and other environmental influences, the determination of ductility changes and changes in notch and impact toughness or changes in fracture energy are among the most sensitive test methods, which provide basic criteria for assessing the service life and resistance of materials to various environmental influences.

The lifetime of a material in a given environment is most often defined as the time in hours or years after which a defined change occurs, e.g. a 50% decrease in ductility, notch or impact toughness or other properties of exposed materials relative to the initial unexposed materials. These lifetime values are most often determined from graphical dependences of changes in the above-mentioned mechanical properties of materials on the exposure time in the given external environment.

The basic shortcoming of the methods used so far for testing aging, corrosion, the effects of ionizing radiation and other external environmental influences on polymer materials is the fact that the most frequently used monitoring of the mechanical behavior of materials and their service life in various external environments, corresponding to the environments of application practice, is based on the performance of standard tests, in which the material in the form of a test specimen is subjected to uniaxial mechanical stress. In practice, products made of polymer materials are most often found in the form of thin-walled shells subjected to multiaxial stress. Stressing of such products under uniaxial stress occurs only exceptionally in practice.

The above-mentioned deficiency is eliminated by the method of performing tests of thermal-oxidative and atmospheric aging of polymers, corrosion tests and other environmental influences on samples subjected to impact tests under multiaxial tension using parachutes and high-speed tearing machines to cause a mandrel impact on the wall of the tested samples according to the invention. The essence of the method lies in the fact that model products in the form of plates with a film gate of minimum dimensions of 50 x 50 mm are injected from injection polymers and plates are extruded from extrusion polymers, which are also cut into plates of dimensions of 50 x 50 mm. These model products are then exposed in series of 5 to 25 pieces, always on the same side, to the action of various external environments at different time intervals.

After exposure, the model products are placed on a support with a circular hole of a minimum diameter of 40 mm and subjected to impact with a mandrel on the center of the exposed side. This determines the energy required to break due to fracture or cracks in the tested plate. The average energy to break of the individual series of exposed plates is then evaluated as a % relative to the value of the average energy to break of the unexposed series of plates tested on the same side as the exposed plates.

The method for performing tests of thermal-oxidative and atmospheric aging and other effects of the external environment on polymers was developed and verified based on tests of various polymers, e.g. polyethylene, polypropylene, polystyrene, high-impact polystyrene, ABS polymer, structurally lightweight polypropylene filled with calcium carbonate, polyester glass laminates, etc. The tests performed showed that the method according to the invention is also suitable for testing corrosion, the effects of ionizing radiation and other external effects on various metallic and non-metallic materials, e.g. based on stainless steel, aluminum, duralumin, cladding materials, roofing materials, etc.

Of the tested basic types of model products in the form of plates of various thicknesses, U-profiles and hollow cylinders, the most successful were plates measuring 50 x 50 mm and 4 mm thick, produced by injecting polymers through a film gate into an injection mould. This type of model product has the advantage that it best reproduces the complex supramolecular structure of injected polymers, which occurs in a completely similar state in most differently shaped injected polymer products. Plates with an optimal thickness of 4 mm allow, in addition to modelling the structure of real products, an easy comparison of the ageing and corrosion of polymers using standard tensile, notch or impact test specimens, the standard thickness of which is 4 mm.

The above-mentioned model products in the form of plates, which are prepared under similar technological conditions as during the injection molding of real products, are then subjected in series of about 25 pieces to the action of the relevant external environment. At pre-selected time intervals, individual series of exposed model products are removed from the tested environment and, after conditioning, e.g. at a temperature of 50 °C for 48 hours, they are subjected to impact tests under multiaxial tension. The principle of such a test consists in the impact of a mandrel on the product and the evaluation of the energy required for its failure by fracture, the formation of a crack, etc.

The impact of the mandrel is directed perpendicular to the surface of the product, in the direction of the normal at the point of impact. During the impact, multiaxial tension is created in the wall of the product around the point of impact. The test is performed as a passive drop test, in which a mandrel with a hemispherical impact working surface falls on the product in free fall. The test is performed on a vertical or pendulum fall arrester. A high-speed tearing machine or other device can also be used. The fall arrester or similar device ensures the mutual position of the product and mandrel during impact. The product is suitably placed or clamped in a fixture on the base plate of the fall arrester in advance of the impact. The amount of energy of the mandrel hitting the product is regulated by changing the weight of the mandrel or changing its fall height, or a combination of both.

Various methods described in the literature can be used to determine the energy to failure. Products can be tested at normal or other suitable temperature, consistent with practice.

Although impact tests under multiaxial tension have been used for many years to test variously shaped products, the use of these tests to evaluate the durability and resistance of materials to external environmental influences, e.g. in aging tests, has not yet been described. The advantage of the new method of performing these tests according to the invention is significantly greater sensitivity than in the standard aging test methods used so far. Changes in the material of the model product resulting from exposure to external environments are reflected much more significantly in the results of impact tests under multiaxial tension than in standard tests under uniaxial tension.

Another advantage is that the testing method according to the invention provides an approximation of the actual service life, i.e. the useful life of real products under conditions similar to external environments.

In practice, real products are in most cases subjected to multiaxial stress. Another advantage is that the magnitude of the coefficient of variation of up to about 20% in the test according to the invention is comparable to the coefficient of variation in standard tests of impact and notch toughness of exposed test specimens under uniaxial stress.

The method of performing thermal-oxidative and atmospheric aging tests of polymers according to the invention is illustrated in more detail in the following examples.

Example 1

Thermal-oxidative aging of polypropylene. The tests were carried out on heat-stabilized polypropylene MOSTEN 52 522 in the form of granules with a flow index of 1.98 g/10 min and an induction period of 207 min. Model products in the form of plates 50 x 50 mm with a nominal thickness of 1, 2 and 4 mm were produced from polypropylene by injection molding with a film gate. Technological injection molding conditions: melt temperature 230 °C, mold temperature 60 °C, injection pressure 80 MPa, contact pressure 70 MPa. Test specimens for the type-II tensile test according to ČSN 64 0605 and for determining the notch toughness by the Charpy method No. 2 according to ČSN 64 0612 were also manufactured from the same material under similar conditions.

The manufactured plates in the number of 25 pieces for each of the planned samplings for different time intervals were placed in hot-air dryers together with standard specimens for tensile testing and determination of notch toughness, 10 pieces for each sampling, and subjected to aging at temperatures of 130, 110 and 90 °C. After the selected time intervals in multiples of thousands of hours, the exposed plates and exposed test specimens were removed from the individual dryers. After they had cooled in a desiccator over silica gel, their basic mechanical and chemical-physical properties were measured at normal temperature.

Tensile tests were carried out at a speed of 5 m.min ^- ''·. Aged and unaged plates of all thicknesses were subjected to a passive drop test on a vertical drop machine. The test arrangement is shown in Fig. la, lb and lc. The test plate 2 was placed on a steel prism 2 with a circular hole é 40 mm. The position of the prism 2 ^on the base plate 2 was fixed by a pin 2» the position of the plate 2 ^on the steel prism 2 was fixed by pins 2· Thus it was guaranteed that the bumper of the mandrel 2 fell on the center of the plate 2i ^{and the axis} of the hole passed through the point of impact of the bumper of the mandrel 2 on the plate and the center of gravity of the mandrel. The mandrel 2 was provided with a bumper with a hemispherical impact surface of radius 5 mm with a dull surface, so that it would not be permanently deformed during the tests. The energy of the falling mandrel was changed by changing the drop height.

The so-called step test method was used, which consists in the fact that a series of 25 plates are gradually exposed to the impact of a mandrel falling in free fall. The energy of the mandrel is given by the product of the mandrel weight and the fall height. In the event of breakage of the first plate by fracture or the formation of a crack, the energy of the mandrel hitting the next plate is reduced by a predetermined amount, e.g. by one step; if the plate does not fail, the energy is increased by one step. This procedure is repeated for the entire series of plates. Each plate is subjected to the impact only once. The size of the step does not change during the test.

By mathematically processing the results of the drop test, e.g. according to DIN 53 443, etc., the energy to failure in joules was then calculated. This is the kinetic energy of the falling mandrel required to break the test plate. At the same time, the standard deviation and the coefficient of variation of this energy were calculated. To compare the energy to failure of plates of different thicknesses, the values found were divided by the average thickness of the plates, and the energy to failure per 1 mm of thickness was thus calculated in J.mm ^- ''.

The measured results are shown in the graphs in Fig. 2 to Fig. 4. The values of the changes in the energy to break the exposed plates are expressed here in % relative to the energy to break the initial unexposed plates, the value of which is set equal to 100 4. Fig. 2 shows the results of aging at 130 °C, Fig. 3 at 110 °C and Fig. 4 at 90 °C. For comparison, the same graphs also show the percentage changes in the tensile properties of standard test specimens on the aging time, the dependence of the density h and the solution viscosity eta of the polymer. It is clear from the graphs that the changes detected by the method according to the invention are characterized by greater sensitivity than standard tensile tests, because the changes in the energy to failure of the model products were about 1,200 4 at an aging temperature of 130 °C, about 900 4 at 110 °C and about 500 4 at 90 °C. In contrast, the changes in the tensile properties: yield strength elongation to yield strength delta, stress to yield

2204 elongations t>2204 ^{and mo<4u4u E for} standard test specimens tested in a standard manner under uniaxial tension do not exceed about 150 4 at an aging temperature of 130 °C, 70 4 at 110 °C and 30 4 at 90 °C.

The higher sensitivity of the method according to the invention is further documented by the graphs in Fig. 5, which compares the results of measurements on model products using the method according to the invention and on standard bodies under uniaxial tension.

Fig. 6 graphically shows the dependence of the energy to failure of aged plates of different thicknesses on the aging time at different aging temperatures, converted to 1 mm thickness.

The graph shows that the largest changes in the energy to failure are shown by plates with the greatest thickness, i.e. 4 mm, which is related to the identified failure mechanism of aged plates.

From the graphs in Fig. 2 to 4 it can be shown that the initial increase in the energy to break the aged model plates with a thickness of 4 mm to a maximum lying around 4,000 hours is caused by the prevailing strengthening of polypropylene due to its additional crystallization at increased aging temperatures. This is indicated by a similar increase in density with a similarly situated maximum. With further aging over 4,000 to 8,000 hours, there is a gradual decrease in the energy to damage the aged samples, and this decrease is caused by the prevailing oxidative cleavage of the polymer associated with the formation of pores and cracks. This is also indicated by the gradual decrease in the density of the polymer below its initial value and a further decrease in the logarithm of the viscosity number of the polymer, which retains a linear unthickened structure throughout the aging process.

Example 2

Thermal-oxidative aging tests of ABS-polymer. ABS-polymer FORSAN 548 from the production of k. p. Chemopetrol with a flow index of 1.99 g/10 min was used. The procedure for manufacturing test plates was the same as in example 1. Technological conditions: melt temperature 212 °C, mold temperature 66 °C, injection pressure 80 MPa, downforce 80 MPa. The samples were aged for 20,000 hours at 70 °C.

The measurement results are graphically shown in Fig. 7 as a function of the percentage changes in the energy to failure of model products as a function of the aging time in thousands of hours. For comparison, the changes in mechanical properties measured on standard test specimens are also shown in the graph:

yield strength b _R ^., elongation at yield c _Kfc , tahrfbp^., ductility delta, modulus E, notch toughness a„, Brinell hardness T„ and density h.

Λ V

The lower horizontal dashed line indicates a half, i.e. 50% decrease in the observed property. The intersection of this line with the time dependence curve of the relevant property then indicates on the time axis the service life at a given aging temperature in terms of that property. For example, from Fig. 7 it follows that the service life 05/2' ^time P ^r ° half decrease in ductility delta of standard test specimens is about 1,500 h, the service life θ _Ε /2' Li · ûoba P ^r ° half decrease in energy to failure of model products according to the invention is about 13,000 h, the service life ^D aK/2' time for half decrease in notch toughness aK of standard test specimens is about 17,000 h.

Fig. 7 further shows that in the case of thermal-oxidative aging of ABS polymer at 70 °C, relatively rapid embrittlement of the polymer occurs from the very beginning of the aging tests, which is manifested by a sudden decrease in the ductility delta of standard bodies, a gradual decrease in the energy to damage model products and a slower decrease in the notch toughness and _R of standard bodies. This embrittlement of ABS polymer, which is accompanied by the formation of cracks and pores in the polymer, as shown in the figure by the decrease in polymer density with aging time, is caused by oxidative cleavage of the polybutadiene component in ABS polymer, which was proven from parallel measurements of infrared spectra of aged samples.

AND

Example 3

Comparison of atmospheric aging of various polyolefin and styrene compositions and copolymers.

The following polymers were used as starting materials:

1. -Polyethylene PE brand LITEN MT 62, light stabilized, melt index 5.62 g/10 min.

2. Polypropylene PP brand MOSTEN 52 532, light stabilized, melt index 1.71 g/10 min.

3. Propylene-ethylene copolymer CEP brand MOSTEN 55 235, stabilized with carbon black, melt index 0.36 g/10 min.

4. Polystyrene PS brand KRASTEN 127, clear, flow index 4.63 g/10 min.

5. High-density polystyrene hPS brand KRASREN 552, white, flow index 1.76 g/10 min.

6. ABS polymer brand FORSAN 548, stabilized with carbon black, melt index 1.47 g/10 min.

Test plates were prepared from the above polymers using the method described in Example 1.

standard samples. The technological conditions of injection molding are as follows a b u 1 k a 1 Injection pressure MPa No. Polymer type Temperature of melting forms 1 PE 206 61 80 2 PP 212 61 80 3 CPE 218 62 80 4 P.S. 211 62 80 5 hPS 215 64 80 6 ABS 219 63 80

Model products - plates - and standard test specimens were clamped in special holders, in aluminum fixation panels according to Czechoslovak AO 220 633 with dimensions of 550 x 330 x 2 mm. These panels were then fixed in exposure stands in the climatic station in Prague-Letňany and in a standardized position, at an inclination of 135° to the south at a height of 1 m above the ground, they were exposed to the effects of weathering. Individual fixation panels were then removed at 3-month intervals, the plates and test specimens were released, washed with water, wiped and dried at 50 °C for 48 h. Then the properties were measured on the samples as in example 2. The results of the effects of weathering after 3 summer months in Ϊ changes are given in table 2. The value of 100% corresponds to the initial unexposed samples.

Table 2

Measured % change after 3 months of atmospheres, aging

property PE PP CPE P.S. hPS ABS breakthrough energy x/i 66.3 73.1 81.3 10.7 20.7 yield strength 1> _Kt 102.4 112.3 102.8 - 95.3 100.4 elongation to yield point 109.3 83.7 106.8 101.5 108.6 tensile strength - - - 101.9 79.5 91.3 ductility δ x/2 x/2 x/2 112.5 32.7 43.7 module E 112.7 103.8 93.2 207.1 99.6 105.2 notch toughness- 88.9 102.7 97.2 102.6 90.8 91.9

tost and _R

Note: x/1 without fracture at 23 and at »-40 °C x/2 = elongation greater than 220% lying outside the measuring range of the tearing machine

The table shows that after 3 months of atmospheric aging of light-stabilized polyolefins and polystyrenes, it is possible to detect significant changes using the method according to the invention, i.e. a decrease in the energy to break the model products from the original 100% for the initial, unaged state of the samples to, for example, 66.3% for polypropylene or 20.7% for ABS polymer. Only in the case of polyethylene PE, which is characterized by exceptionally high ductility and toughness, could the relevant values of the energy to break the plates be measured even after 3 months of aging, both at normal temperature and at -40 °C, because the tested plates only deform plastically during drop tests without the formation of brittle cracks and without fracture. Only after a longer period of atmospheric aging does embrittlement of this type of polyethylene occur and the aged plates break brittlely during drop tests. Their energy to break gradually decreases with increasing atmospheric aging time.

Table 2 also shows the high sensitivity of the atmospheric aging tests according to the invention, significantly exceeding the sensitivity of standard aging tests.

Claims (1)

object of the invention Method for carrying out thermo-oxidative and atmospheric aging tests for polymers, corrosion tests and other environmental influences on specimens subjected to impact tests at multiaxial stress using paddles and high-speed blasting machines to induce a thrust impact on the wall of test specimens, characterized in that they are injection molded of the polymer types inject model sheet-shaped film inlets having a minimum size of 50 x 50 mm, and extruded polymers are extruded into sheets which are also cut into plates of at least 50 x 50 mm, and then the model products are exposed in series from 5 to 50 mm. 25 pieces always on the same side of exposure to different external environments at different time intervals. the failure energy due to fracture or cracks in the test plate is determined, whereby the average failure energy of the individual series of exposed plates is evaluated in% relative to the average failure energy of the unexposed series of plates tested on the same side as the exposed plates.