It is known that the specific terms of aging of tires are almost never disclosed by their manufacturers. It is believed that for 2-3 years, aging processes do not lead to catastrophic changes in rubber compound tires, and after this time, almost every motorist will definitely change the set of tires to a new one. But possible different situations- these 2-3 years of tires can simply be spent in the warehouse of an unscrupulous seller or in a wholesale warehouse, tires can be used on cars with low annual mileage - various campers, etc. As a result, quite often tires are used even after 5 or even 10 years from the date of their release. What does it threaten? Let's try to figure it out.
There are two main factors leading to age-related destruction of tires - ozone from the atmosphere, which leads to a violation of molecular bonds between rubber molecules and, as a fact, to a loss of elasticity, and age cracks arising from the contact of tires with fats and oils, as well as simply from long-term operation. As a result tires "tan", which leads to a sharp deterioration in all their qualities without exception. Especially dangerous deterioration driving performance on the wet road. ADAC's research on spinning old tires has shown an increased risk of tire "explosion". A few years later, the analysis of severe accidents associated with tire bursts on high speed, conducted by DEKRA, revealed that in 100 (!!!) percent of cases, the age of the tires was to blame. Bottom line - recommendation: the maximum service life of conventional medium-speed road tires operating in standard conditions- six years . But this is only if the tires are not tested high loads. If tested, then the maximum is 4 years. And no means to give "blackness".
For winter tires, the situation is even more complicated. low temperatures the destruction of intermolecular bonds is faster, therefore, already in the 2nd or 3rd season, tires, even with careful operation, "glass" and lose some of their qualities due to aging. ADAC states that after 2 years winter tire cannot be considered new. and 100 percent usable.
The designation of the date of manufacture of the tire can be found after DOT inscriptions on the sidewall. The four digits indicate the week and year of manufacture. For example, the designation 1105 indicates that the tire was released in week 11, 2005. Remember that if the storage conditions of the tire were not observed, then its aging will occur even ahead of schedule specified by ADAC. Therefore, it is better to shop in reputable stores with a good reputation - such as the AUTOEXPERT company. Buying tires in our store, you can be sure that you are buying really new tires stored in the right conditions.
And most importantly - remember that if your tires are older than 4 years, then it's time to think about replacing them, even if physical wear has not come. Such tires can be dangerous, especially at high speeds.
Rubbers and their vulcanizates, like any unsaturated compounds, are capable of various kinds of chemical transformations. The most important reaction that continuously occurs during the storage and operation of rubber products is the oxidation of rubber, leading to a change in its chemical, physical and mechanical properties. Only ebonite, which turns into a fully saturated compound by adding the maximum possible amount of sulfur to the rubber macromolecules, is a chemically inert material. The totality of all the changes that occur in rubber during long-term oxidation is called it aging.
Aging belongs to the category of complex multi-stage transformations, at certain stages of which the elasticity, wear resistance and, to some extent, strength of rubber are significantly reduced. In other words, over time, the performance of rubber products, and consequently, the reliability of the car is reduced. The category of the most unfavorable changes in rubber resulting from aging includes an irreversible decrease in its elasticity. As a result, the increased brittleness of rubber, primarily its surface layers, causes the appearance of cracks in the deformable parts, which gradually deepen and eventually lead to the destruction of the product.
The effects of rubber aging are similar to those of a decrease in temperature, with the only difference that the latter are temporary in nature and partially or completely removable by heating, while the former cannot be weakened by any means, let alone eliminated.
The fight against aging is underway various methods. The supplement is very effective. antioxidants(inhibitors), 1 ... 2% of which, in relation to the rubber contained in rubber, slows down the oxidation process by hundreds and thousands of times. For the same purpose, some rubber products are produced from factories in sealed packaging (in polyethylene cases).
However technological means is insufficient, therefore, a number of operational measures have to be additionally applied. With increasing temperature, aging intensifies, and from heating for every 10 ° C, the aging rate doubles. It is also noticed that the oxidation of rubber is more intense in those areas that experience greater stress. Therefore, it is necessary to keep the rubber products as undeformed as possible.
Wheels and tires
Car wheel are distinguished by their purpose, type of tires used, design and manufacturing technology.
The main parameters of the wheels of some domestic cars are given in Table. 11.2.
Pneumatic tires passenger cars are divided according to the method of sealing the internal volume, the location of the cords in the frame, the ratio of the height to the width of the profile, the type of tread and a number of others. specific features caused by their purpose and operating conditions.
According to the method of sealing the internal volume, they distinguish chamber And tubeless tires.
Tube tires consist of a tire, a chamber with a valve and a rim tape that is put on the rim. The size of the chamber is always slightly smaller than the inner cavity of the tire in order to avoid wrinkling when inflated. The valve is check valve, which allows air to be forced into the tire and prevents it from escaping. The rim tape protects the tube from damage and friction against the wheel and tire bead.
Table 11.2
The main parameters of the wheels of some domestic passenger cars
Cars
Rice. 11.9. Tubeless car tire:
1 - protector; 2 - sealing airtight rubber layer; 3 - frame; 4 - valve; 5 - deep rim
Tubeless tires (Fig. 11.9) are distinguished by the presence of an airtight rubber layer superimposed on the first layer of the carcass (instead of the tube), and have the following advantages (compared to chambered ones):
less weight and better heat exchange with the wheels;
increased safety when driving the car, since when punctured, air escapes only at the puncture site (with a small puncture, rather slowly);
Simplified repair in the event of a puncture (no need for disassembly).
At the same time, the mounting and dismounting of tubeless tires is complicated and requires more skill, and is often only possible on a special tire changer.
Tubeless tires are used for wheels with rims of a special profile and high precision manufacturing.
Chamber and tubeless tires according to the location of the cord threads in the carcass, the tires can be either diagonal or radial.
Tire marking
Bias and radial tires differ not only in design, but also in marking.
For example, in the designation bias tire 6,15-13/155-13:
6.15 - conditional tire profile width (IN) in inches;
13 - landing diameter (d) tires (and wheels) in inches;
155 - conditional tire profile width in mm.
Instead of the last number 13, the bore diameter in mm (330) can be indicated.
Radial tires have a single mixed millimeter-inch designation. For example, in the marking 165/70R13 78S Steel Radial Tubeless:
165 - conditional tire profile width (IN) in mm;
70 - the ratio of the height of the tire profile (R) to its width (IN) in percentages;
R - radial;
13 - landing diameter in inches;
78 - conditional tire load capacity index;
8 - speed index tires (maximum permissible vehicle speed) in km/h.
For everyday driving Russian roads appropriate to confine N/A not lower than 0.65, and this applies to quite big tires, i.e. tires for cars of the GAZ-3110 Volga type. On VAZ models, it is better not to use tires with N/A below 0.70, and on a VAZ-111 Oka car, it is completely inappropriate to install any other tires other than the factory size 135R12.
Modern high-speed ultra-low profile tires with N/V== 0.30...0.60 only suitable for operation on smooth highways with good quality coverage, which in our country is practically non-existent.
Each Russian tire manufacturer has its own brand name or, for example, Moscow tire factory, badge of the TAGANKA model.
The tire marking includes a letter (or letters) encoding the manufacturer (for example, K - Kirov Tire Plant; Ya - Yaroslavl Tire Plant, etc.) and numbers (numbers) of the internal factory index of this tire.
It is placed on the sidewall of the tire serial number and other, quite useful (in case of a complaint) information is encoded (Table 11.3).
RTI or rubber products have special characteristics, thanks to which they remain very popular. Especially modern ones. They have improved indicators of elasticity, impermeability to other materials and substances. Also possess high rates electrical insulating and other qualities. It is not surprising that RTIs are increasingly being used not only in the automotive industry, but also in aviation.
When the vehicle is actively used and has high mileage, the technical condition of RTI is significantly reduced.
A little about the features of RTI wear
The aging of rubber and some types of polymers occurs under conditions that are affected by:
- warm;
- light;
- oxygen;
- ozone;
- stress/compression/tension;
- friction;
- working environment;
- operational period.
A sharp drop in conditions, especially climatic ones, has a direct impact on the state of rubber goods. Their quality is deteriorating. Therefore, polymer alloys are increasingly used, which are not afraid of lowering degrees and increasing them.
With a decrease in the quality of rubber products, they quickly fail. Often it is the spring-summer period, after the winter cold, that is a turning point. With an increase in temperature on a thermometer, the rate of aging of rubber goods increases by 2 times.
To ensure the loss of elasticity, it is enough for rubber products to survive a significant and sharp cold snap. But if the linings and bushings change their geometric shapes, small gusts and cracks appear, this will lead to a lack of tightness, which, in turn, leads to breakdowns of systems and connections in the car. The least that can appear is a leak.
If you compare rubber products, neoprene is better. Rubber RTIs are more subject to changes. If you do not protect both of them from the sun, fuel, acidic or corrosive liquids, mechanical damage, they will not be able to pass even the minimum operational period specified by the manufacturer.
Features of different RTI
The properties of polyurethane and rubber rubber products are completely different. Therefore, storage conditions will be different.
Polyurethane is different in that it:
- plastic;
- elastic;
- not subject to crumbling (unlike rubber products);
- does not harden, like rubber, when the temperature drops;
- does not lose geometric shapes;
- with elasticity, hard enough;
- resistant to abrasive substances and aggressive environments.
Obtained by liquid mixing, this material is widely used in the automotive industry. Synthetic polymer is stronger than rubber. With a homogeneous composition, polyurethane retains its properties in different conditions, which simplifies the conditions and characteristics of its use.
As can be seen from the above material, polyurethane outperforms rubber products in terms of properties. But it doesn't apply everywhere. In addition, silicone alloys are emerging. And what is better - not every driver understands.
Polyurethane is technologically produced longer. 20 minutes are spent on the release of rubber RTI. And 32 hours for polyurethane. But rubber is a material born by mechanical mixing. This affects its heterogeneity of composition. And also entails a loss of elasticity and uniformity of the components. It is rubber hoses and sealed linings that harden and become stiffer during storage, crack on the surface and become soft inside. Their term is only 2-3 years.
Care and storage
The state and quality of RTI depends very much important process- management control. To understand the importance of rubber products, you need to know that violations in their structure lead to the following consequences:
- increased tire wear under heavy load due to incorrect operation some systems and connections;
- irregularities in the way of braking;
- perceptible violations in feedback with steering wheel control;
- destruction of neighboring parts or in nearby nodes.
RTI must be stored:
- Fold freely so that there is no excessive load or compaction;
- control the necessary temperature regime ranging from zero to plus 25 degrees Celsius;
- In conditions where there is no high humidity, above 65%;
- In rooms where there are no fluorescent lamps (it is better to replace them with incandescent lighting devices);
- In conditions where there is no ozone in large quantities or devices that produce it;
- Paying attention to the presence / absence of direct rays of the sun (no direct UV exposure can be the same as the conditions that create thermal overheating for rubber products).
With fluctuations in temperature during the cold period and the hot season, it must be understood that guarantee period RTI storage is narrowed to a figure equal to 2 months.
Moscow Aviation Institute
(Technical University)
Department of Materials Science
Course work
in materials science
on the topic of:
"Rubber resistant to aging"
Checked by: Vishnevsky G.E.
Completed by: Pavlyuk D.V.
Introduction
Atmospheric aging of rubber
Protecting rubber from atmospheric aging
Changes in the mechanical properties of rubber during thermal aging
Thermal aging of rubber under compression
Protecting rubber from radiation aging
Bibliography
INTRODUCTION
Rubber is a product of special processing (vulcanization) of rubber and sulfur with various additives.
Rubber differs from other materials in high elastic properties, which are inherent in rubber - the main raw material of rubber. Rubber materials are characterized by high abrasion resistance, gas and water resistance, chemical resistance, electrical insulating properties and low density.
Different requirements are imposed on rubber in terms of operating conditions. The rubber lining of conveyor belts conveying ore or coal must be frost-resistant at low temperatures and resist abrasion well;
the rubber chamber in sleeves for oil products must be resistant to swelling; rubber lining of railway tanks for the transportation of hydrochloric acid, resistant to its chemical action, etc.
Special requirements are placed on rubber products used in aircraft, which contain hundreds of different rubber parts. Such products, along with compactness and low weight, must be elastic and durable. It is very important that the parts retain their properties over a wide range of temperatures and, in some cases, when exposed to various liquid and gaseous media. When flying at a speed of 3600 km / h, even at an altitude of 5000 m, the heating temperature of the skin reaches +400 ° C; the parts located in the engine assemblies must retain their properties at temperatures up to +500 ˚С. At the same time, a number of parts are exposed to temperatures of the order of minus 60 ° C and below. Since the dimensions of aircraft parts remain practically constant throughout the entire service life, small residual compression deformations are a necessary quality of such rubbers. Even greater demands are placed on rubbers for rocket science.
Along with general-purpose rubbers widely used in rubber production - natural (NK) and butadiene-styrene (SKS-ZOA, SKS-30, SKMS-30, etc.), special ones are also used:
chloroprene rubbers (A, B, C, NT), nitrile butadiene rubbers (SKN-18, SKN-26, SKN-40, SKN-40T), butyl rubber, chemically resistant fluororubbers (SKF-32-12, SKF-62-13 ), heat-resistant organosilicon polymers (SKT). Stereoregular rubbers are mastered: polybutadiene (SKD) and isoprene (SKI). Searches are underway for new rubbers based on compounds containing boron, phosphorus, nitrogen and other elements.
Rubber as a structural material in a number of its properties is significantly different from metals and other materials. Its most important feature is the ability to transfer significant deformations under the action of an external load without destruction. The main features of rubber also include: small moduli in shear, tension and compression; the great influence of the duration of the applied load and the temperature factor on the stress-strain dependence; almost constant volume during deformation; almost complete reversibility of deformation; significant mechanical losses during cyclic deformations.
Soft rubber vulcanizates, under the influence of a number of storage or operational factors, acting in isolation or more often in combination, change their technically valuable properties. The change is reduced to a decrease in elasticity and strength, to the appearance of hardening, brittleness, cracks, discoloration, an increase in gas permeability, i.e., to a greater or lesser loss of their technical value by products. The influence of atmospheric oxygen, and in particular ozone, leads to aging and fatigue of rubber. This is facilitated by: heat and light, stresses arising from dynamic or static loading, including irrational storage, the influence of aggressive environments or the catalytic effect of metal salts.
Low temperatures lead to a decrease in the elasticity of rubber, to an increase in its fragility. These changes deepen with the duration of cooling. However, with a return to normal temperatures, the original properties are restored. The influence of the dimensions and features of the shape of the product in rubber is much greater than in other structural materials. The stabilization of its technically valuable properties in rubber, the fight against the phenomena of aging, fatigue and freezing are currently one of the important tasks modern rubber technology.
ATMOSPHERIC AGING AND RUBBER PROTECTION
The problem of increasing the durability of rubber products is directly related to increasing the resistance of carnage to various types of aging. One of the most common and destructive types of aging is the atmospheric aging of rubber, which affects almost all products that come into contact with air during operation or storage.
Atmospheric aging is a complex of physical and chemical transformations of carnage occurring under the influence of atmospheric ozone and oxygen, solar radiation and heat.
Changes in the physical and mechanical properties of rubber
Under atmospheric conditions, as well as during thermal aging, rubbers gradually lose their elastic properties, regardless of whether they are in a stressed or unstressed state. Rubbers based on NK with light-colored fillers age especially intensively. Quickly (after 1-2 years) a noticeable change in the properties of rubbers from butadiene-nitrile, butadiene-styrene rubbers and from nairite occurs. The most resistant are rubbers based on SKF-26, SKEP, SKTV and butyl rubber.
Solar radiation significantly affects the rate of change in the properties of rubber under atmospheric conditions, accelerating the process in some cases by five or more times.
In carbon black-filled rubbers, this difference in aging rate is primarily the result of a strong heating of the rubber surface under direct sunlight. Since temperature turns out to be the most important parameter influencing all ongoing processes, it seemed necessary to create a reliable method for its experimental determination.
A study of the temperature of rubber in the open air showed that its daily change, as well as the change in air temperature (in the absence of clouds), is approximately described by sinusoidal curves. Overheating compared to air (at an air temperature of 26 ° C) reaches 22 ° C for black and 13 ° With white rubber.
The course of change in the temperature of rubber during the day follows the course of change in the magnitude of solar radiation, and overheating of the rubber is a function of the latter. Along with this, overheating depends on the heat exchange between rubber and air. This allows, based on the solar radiation flux and using the heat transfer equation for the flat plate - gas system, to determine the temperature of the rubber surface by calculation. In particular, knowing the absolute temperature maxima at different geographical points, it is possible to calculate the maximum temperature to which the rubber surface will heat up in these places. For Moscow, this temperature is 60 °C (absolute maximum 37 °C), for Tashkent 81 °C (absolute maximum 45 °C).
Increasing the surface temperature of rubber even by 20-25°C can cause a sharp change in the aging rate. Thus, this parameter must be taken into account when assessing the aging time of rubber under atmospheric conditions.
Determining the temperature of rubbers in air under various light filters showed that the heating of rubber occurs almost entirely due to the infrared part of solar radiation, which has a decisive influence on the aging rate of carbon black-filled rubbers. So, for 140 days of exposure of rubber from NK in Batumi, the tear resistance drops on average (in%): in the open air - by 34, under a filter that transmits 70% of infrared and does not transmit ultraviolet rays, by 32, under a filter, transmitting 40% of infrared rays, as well as a small amount of ultraviolet, - by 24, under the foil - by 20.
Based on the foregoing, it can be concluded that the change in the physical and mechanical properties of rubber under atmospheric aging conditions is mainly due to the process of thermal aging, which occurs under the action of heat and atmospheric oxygen. In accordance with this, an effective reduction in the rate of change in the physical and mechanical properties of rubber during atmospheric aging, as well as during thermal aging, can be achieved with the help of antioxidants, mainly for NR-based rubbers.
Changes in the physical and mechanical properties of rubber under atmospheric conditions can affect the durability of rubber products in the case of their long exposure to air in an unstressed state or at sufficiently low stresses. This process is also essential for deformed rubbers that are well protected from the action of ozone or are made from ozone-resistant rubbers that are exposed to air for a long time.
Changing the surface of rubber
Under atmospheric conditions, the surface of rubbers undergoes significant changes, and first of all, the surface of light-colored rubbers from NK. In addition to the relatively rapid color change, the surface layer first softens and then gradually becomes hard and takes on the appearance of embossed leather. At the same time, the surface is covered with a network of cracks.
The process of surface destruction proceeds mainly under the influence of photochemical reactions caused by the action of ultraviolet rays. This is proved, in particular, by comparing the change in the rubber surface under atmospheric conditions under different light filters: in the absence of UV rays (rays with λ< < 0,39 mk) surface change is incomparably smaller than under the action of rays with wavelengths up to 0.32 mk.
This phenomenon is typical for rubbers with light-colored fillers, because the latter (zinc, titanium, magnesium oxides, lithopon, etc.), unlike carbon blacks, are capable of absorbing UV rays and, as a result, are sensitizers of chemical reactions in rubber.
Cracking and degradation of rubber
Cracking of rubber in atmospheric conditions proceeds at a relatively high rate and, as a result, is the most dangerous type of aging.
The main condition for the formation of cracks in rubber is the simultaneous effect of ozone and tensile forces on it. In practice, such conditions are created to some extent during the operation of almost all rubber products. According to modern concepts, the formation of nucleating ozone cracks on the rubber surface is associated either with the simultaneous rupture of several macromolecules oriented in the same direction under the action of ozone, or with the rupture of a structured brittle ozonide film under the influence of stresses. The penetration of ozone into the depth of microcracks leads to their further growth and rupture of rubber.
A study of the kinetics of rubber cracking in the open air at constant tensile strain (the intensity of cracking was estimated in arbitrary units on a nine-point system) shows that different rubbers differ not only in the time of appearance of visible cracks τ y and rupture time τ p, but also in the ratio of velocities processes of formation and growth of cracks.
The most important factors determining the weather resistance of rubber, as well as the entire course of the cracking process, are:
reactivity of rubbers in relation to ozone;
magnitude of tensile stresses;
exposure to solar radiation.
Protecting rubber from cracking
To protect rubber from cracking, two types are used. protective equipment: antiozonants and waxes.
Unlike antioxidants, which have a moderate protective effect on the thermal aging of rubber, the effectiveness of the effect of antiozonants and waxes on ozone aging is very high.
Antiozonants.
Among the typical and most effective antiozonants are compounds of the class N,N "-substituted-n-phenylenediamine and dihydroquinoline derivatives. Protection from the action of ozone is also carried out by some dithiocarbamates, urea and thiourea derivatives, n-alkoxy-N-alkylaniline, etc.
The mechanism of action of antiozonants in last years attracts the attention of many scientists. As a result of studying the effect of antiozonants on the kinetic patterns of ozonation and cracking of rubbers and rubbers. There are several different views on this issue.
The formation of a continuous protective layer on the rubber surface due to a migrating antiozonant, its reaction products with ozone, and the reaction products of ozone with rubber, in which the antiozonant is involved, is widely discussed.
It is assumed that the latter type of reactions leads either to the elimination of the breakage of macromolecules or to the crosslinking of their fragments.
The formation of a surface layer of an antiozonant or products of its interaction with ozone, which provides effective protection of rubber, can be expected only if they are in a resinous state and can form a continuous uniform layer during migration. Indeed, according to experiments, the ozone resistance of NR rubber containing crystalline antiozonant N-phenyl-N"-isopropyl-p-phenylenediamine (FPPD) in some cases turns out to be even slightly higher before the antiozonant migration to the surface than after the formation of a faded FPPD layer. This is apparently due to the fact that, although individual crystalline formations of antiozonant can have some protective effect on rubber, in the intervals between such formations, “weak” spots should appear on the rubber, due to the depletion of the surface layer of rubber in antiozonant due to its fading. and the absence of purely mechanical protection due to antiozonant crystals.
The decisive importance of the migration of antiozonants of the crystal structure to the surface in terms of the effectiveness of their protective action can be questioned, since the protective effect of antiozonants usually manifests itself even at dosages not exceeding the limit of their solubility in rubber. Thus, N-phenyl-.N "-isopropyl-n-phenylenediamine is effective in rubbers from NK and other non-polar rubbers at a concentration of 1-2 parts by weight per rubber. Probably, the antiozonant dissolved in the surface rubber layer.
The protective action mechanism based on the cross-linking of fragments of macromolecules or on the elimination of their decay seems likely, but requires further experimental confirmation.
A very common concept is that antiozonants on the surface of rubbers bind ozone, preventing it from interacting with the rubber.
Our studies of the effect of antiozonants on the reaction of rubber with ozone (in CCl4 solution) showed that antiozonants do not affect the nature of the kinetic curve of rubber ozonation and practically do not change the activation energy of the process. In the presence of an antiozonant, only the total amount of absorbed ozone increases. However, as follows from the data on the accumulation of oxygen-containing groups, the reaction rate of the rubber itself with ozone decreases in this case. At the same time, the rate of destruction of macromolecules also decreases. Under these conditions, simultaneous ozonation of rubber and antiozonant occurs.
Studies of the ozonation kinetics of the antiozonant itself (in solution) showed that the activation energy of this reaction for FPPD is slightly higher than for rubber (1.4 kcal/mol), and the rate of interaction of this antiozonant with ozone in the entire temperature range of interest exceeds the rate of ozonation of rubber (when the weight ratio of rubber and antiozonant is 100: 5).
All this suggests that the reaction of the antiozonant with ozone on the rubber surface plays a certain role in protecting rubber from ozone aging. However, the reaction rate for various antiozonants does not correlate with their effectiveness in rubber cracking, so the process is not decisive in the protective effect of various compounds.
The foregoing allows us to conclude that at present there is no generally accepted and sufficiently substantiated point of view on the mechanism of action of antiozonants. This issue requires serious study. However, this mechanism seems to be different for different types compounds, and, probably, one type of antiozonants acts not according to one, but according to different mechanisms.
The protective effect of antiozonants increases with their concentration. However, in practice, the use of antiozonants in concentrations significantly exceeding their solubility limit is not possible, therefore, combinations consisting of two antiozonants with predominantly different chemical structures. The most effective systems of antiozonants, consisting of FPFD, paraoxyneozone (PON), acetonanil, and a number of other products, increase τ u several times under atmospheric conditions.
Waxes.
Some mixtures of hydrocarbons of the paraffinic, isoparaffinic and naphthenic series, which are products similar in properties to waxes, provide physical protection of rubber from atmospheric aging. Waxes with a molecular chain length of 20-50 carbon atoms have optimal protective properties. Waxes are effective mainly only in statically stressed rubbers. The protective effect of waxes is based on their ability to form a continuous film on the rubber surface, which prevents the interaction of rubber with ozone. The essence of the phenomenon of film formation is as follows: when the rubber is cooled after the vulcanization process, the wax introduced into the rubber mixture forms a supersaturated solution in the rubber, from which it subsequently crystallizes. Crystallization of a substance from a supersaturated solution in a polymer can occur both in bulk and on its surface (“fading”). The latter leads to the formation of a protective film.
The effectiveness of the protective action of waxes is primarily related to the ozone permeability of this film, which is determined by the film thickness and the basic physicochemical characteristics of the wax. Along with this, the effectiveness of the wax depends to a large extent on the operating temperature of the rubber; usually with increasing operating temperature, the protective effect of the wax deteriorates. The higher the melting point of the wax (within certain limits), the greater the temperature range, all other things being equal, it can work. With an increase in the operating temperature of rubber, it is necessary to use waxes with a higher melting point. There is data showing that effective protection carried out under the condition that the operating temperature of the rubber is 15-20 °C below the melting point of the wax. This value decreases with increasing wax dosages and the use of mixed waxes.
Taking into account the fact that the melting temperature cannot serve as an unambiguous characteristic of a specific waxy state of a substance with a wide softening temperature range, new characteristics of waxes have been proposed - the onset temperature and the complete softening temperature, which are determined by studying the thermomechanical properties of waxes. The use of these parameters made it possible to establish that, in contrast to the above, according to the data of accelerated laboratory tests, the protective effect of a number of waxes increases with increasing temperature (from 25 to 57 ° C).
The dependence of the effectiveness of the protective action of a number of waxes on their dosage during atmospheric aging of statically stressed rubber is described either by a saturation curve or an extreme curve.
The limit of the effective wax concentration is apparently associated with a high degree of supersaturation of the wax solution in the rubber, which contributes to the intensive crystallization of the wax in the volume, which can only have a negative effect on the uniformity and, consequently, on the resistance of rubber to atmospheric cracking. Taking into account the data on the effectiveness of protective waxes, as well as their negative impact on a number of technological properties of rubber, it is recommended to use waxes in amounts not exceeding three parts by weight. The greatest effect of rubber protection is achieved by the combined use of antiozonants and waxes, and the effect of such compositions is greater than the additive effect of both components. This can be explained by the fact that in the presence of a wax film on the rubber surface, the antiozonant diffuses into it at any content of it in the rubber. The amount of antiozonant transferred into the film will be determined by the distribution law. The calculation shows that when introduced into the rubber 2 wt. h. FPPD (less than the solubility limit), its content in the monomolecular surface layer of rubber will be two orders of magnitude less than in the wax film formed on the rubber with a thickness of 10 mk(the solubility of this antiozonant in paraffin is about 0.1%). Thus, wax contributes to a sharp increase in the content of antiozonant on the rubber surface, evenly distributed in a continuous film.
Peculiarities of rubber aging in the tropics
The main features of the tropical climate, characteristic of low geographical latitudes (from 0 to 30 °), are:
high overall level of solar radiation, which changes little during the year. A large amount of direct solar radiation and a high content of ultraviolet rays in the solar spectrum; higher average annual temperature compared to other climatic zones. Especially characteristic is the large fluctuation in daily temperatures. In this regard, in the dry tropics, there is also a higher average maximum annual temperature (the average of the maximum temperatures in each month); high value of relative humidity (in the humid tropics), which plays a role mainly for rubbers made from polar rubbers. The consequence of high humidity is the presence of various microorganisms, causing in some cases the appearance of mold on rubber.
Although the concentration of ozone in the tropics is lower than in other climatic zones, as a result of its combination with intense solar radiation and high air temperature, the aging of rubber in the tropics proceeds much faster than in temperate climates. Rubbers made from unstable rubbers that do not contain special protective agents crack in a tropical climate within 2-3 months, and sometimes after several days. The same rubbers protected by effective antiozonants and waxes do not undergo changes for several years. A comparison of rubber aging rates in some climatic zones shows that the aging rate consistently increases with exposure to the following points: Moscow, Batumi, Tashkent, Indonesia. The acceleration of the process depends on the type of rubber and varies widely, for example, in Indonesia, compared to Batumi, aging accelerates by 2.7-8 times, and compared to Moscow, by 25 times.
CHANGES IN THE MECHANICAL PROPERTIES OF RUBBER DURING THERMAL AGING
Heat resistance - the ability of rubber to maintain properties when exposed to elevated temperatures. Usually, this term refers to the resistance to thermal aging, during which the chemical structure of the elastomer changes. The change in the properties of rubber during thermal aging is irreversible.
The temperature dependence of the aging rate often formally obeys the Arrhenius equation, which makes it possible to predict the degree of change in property indicators. The maximum allowable temperature for long-term (more than 1000 h) and short-term (168 h) use of rubbers based on various rubbers in air (reduction in tensile strength up to 3.5 MPa or relative elongation at break up to 70%) is (°C): AC - more than 149 and 177, FC (amine vulcanization) -177 and more than 177, BNK (peroxide vulcanization) - more than 107 and 149, BNK ("cadmat" vulcanization) -135 and 149, EHGK-121 and 149, BBK-121 and 149, BK (resin vulcanization) -135 and 149, EPT (peroxide vulcanization) -149 and more than 149, respectively.
The features of thermal aging and the effect of the composition of the rubber mixture on the change in the mechanical properties of rubbers based on various rubbers under static loading are considered below. To characterize the resistance to thermal aging, you can use the ratios (in%):
,
,
Where f 0 ε And f ε conditional stress at a given elongation in the process of stretching the sample at a given rate; f 0 p And f p – tensile strength; ε 0 r and ε r relative elongation at break before and after aging.
Rubber based on isoprene rubber. (PI)
With the same vulcanizing system, PI-based rubbers have the minimum resistance to thermal aging. At 80–140°C, the reactions of destruction of the spatial network of the vulcanizate usually proceed, and at 160°C, the reactions of crosslinking of rubber macromolecules. The change in mechanical properties is largely due to the destruction of macromolecules, the intensity of which increases in air. At the same time, the value f p And IN decreases more than ε p. Activation energy calculated from the rate of descent f p , ε p And IN thiuramic vulcanizate NC containing carbon black is 98-103 kJ/mol.
How long will they last car tires, depends on operation, technical condition car and your driving style. Professional maintenance and constant checks will ensure safe driving.
Tires are in direct contact with the road, so it is very important to maintain the quality of tires in normal condition, because safety, fuel efficiency and comfort depend on their quality. It is necessary not only to choose the right tires, but also to monitor their condition to prevent them premature aging and wear.
The main causes of damage and wear of car tires
There are always plenty of unpleasant surprises on the road, which eventually lead to tire damage and wear: stones, pits, glass. We can neither foresee nor prevent them. But here are the problems that arise from high speed, air pressure and overload, are completely dependent on the owner of the car and are completely solvable.
1. Driving at high speed
Keep a close eye on speed mode! When driving at high speed, the risk of damage and tire wear is most likely, because the tires heat up more and lose pressure in them faster.
2. Tire pressure
Excess and insufficient pressure in tires reduces the life of the tires and causes them to premature wear(tire overheating, loss of traction with pavement), therefore it is necessary to control sufficient tire pressure.
3. Overload
Follow the manufacturer's recommendations for loading! To avoid overloading tires, carefully read the load index on the sidewall of the tire. This is the maximum value and should not be exceeded. When overloaded, a strong overheating of the tire also occurs, and, accordingly, its premature aging and wear.
How to protect tires from premature aging and wear
Even the highest quality expensive tires short-lived. Tire wear and aging is only a matter of time, but it is in our power to increase tire life to the maximum. What can be done to extend the life of tires and protect them from wear? Here are some simple tips:
When should tires be changed?
Weekly tire inspection (check tread depth, tire pressure, sidewall damage, signs of uneven wear) allows you to realistically assess the degree of wear and aging of tires. If doubts about the safety of using tires have crept into your head, then contact an experienced specialist for advice on further operation.
The tire must be replaced if:
Tire life
Tire life varies greatly, so it is almost impossible to predict how long a particular tire will last. The composition of the tire includes various ingredients and materials of the rubber compound that affect the life of the tire. Weather, usage and storage conditions can also extend or shorten the life of tires. Therefore, in order to increase the life of tires, protect them from wear and aging, keep an eye on them. appearance, maintaining tire pressure, the appearance of the following effects: noise, vibration or pull towards the car when driving, and of course, store them properly.
Tire storage rules
Even if the tires lie and are not used or are rarely used, they age. It is advisable not to store uninflated or dismantled tires for a long time in piles. Also, do not store any foreign objects, especially heavy objects, on the tires. Avoid hot objects, flames, sparking sources and generators near the tires. When interacting with tires, it is recommended to use protective gloves.
Tires are stored in a dry, well-ventilated room at a constant temperature, protected from precipitation and direct sunlight. To avoid changing the structure of the rubber, do not store near the tires chemicals and solvents. Avoid storing sharp metal, wood or other objects near the tires that could damage them. Black rubber is afraid of an excess of heat and frost, and excessive moisture leads to its aging. Tires should not be washed under a strong water jet, soap or special detergent is sufficient.
From all of the above, the conclusion suggests itself that it will help to save tires from wear and aging. proper storage, operation and a comprehensive check of their condition.
