Influence of environmental conditions on the degradation of rubber compounds

Rubber compounds are complex, chemically active and viscoelastic materials. In this material type, transient and transient changes in the individual rubber properties occur during the manufacturing processes. A large number of rubber compounds consist of a large number of elastomers and virtually all of them are made up of fillers such as (carbon black, silica, kaolin, calcium carbonate, etc.). A very important element of which rubber compounds are composed is sulphur. Its important function is to ensure the cross-linking process during vulcanisation. Lubricants, plasticizers and various organic substances used to modify the properties are also essential components of rubber compounds. The aim of this work is to assess the influence of natural environmental conditions (temperature, time, humidity, microorganisms, UV radiation) on the modification of the properties of rubber composites.


Introduction
A rubber compound refers to a mixture or blend of various ingredients that are combined to produce rubber with specific properties.Rubber compounds are typically made by mixing rubber polymers with various additives, fillers, vulcanizing agents, and other chemicals [1,2].
The rubber polymer forms the base of the compound and provides the material with its rubbery characteristics, such as flexibility and resilience.Different types of rubber polymers can be used, such as natural rubber (derived from the latex of rubber trees) or synthetic rubbers (e.g., styrene-butadiene rubber, nitrile rubber, or silicone rubber) [3 -6].
Additives are incorporated into the rubber compound to enhance or modify its properties.For example, accelerators and activators are added to speed up the vulcanization process, which is a chemical reaction that transforms the rubber from a soft and pliable material into a more durable and heat-resistant state.Other additives may include antioxidants to prevent degradation due to exposure to oxygen or UV light, plasticizers to improve flexibility, and fillers (e.g., carbon black or silica) to enhance strength, abrasion resistance, or reduce cost (Figure 1) [7,8].The exact composition and proportions of the rubber compound depend on the desired properties and the intended application of the final rubber product.Rubber compounds are extensively used in various industries, including automotive, aerospace, construction, electronics, and many others.
Rubber and tires can undergo degradation in natural conditions due to various factors, including environmental exposure, aging, and physical stress.Here are some key aspects of degradation that can occur:  Oxidation: Rubber materials, including those used in tires, can undergo oxidation when exposed to oxygen in the air.This can lead to the formation of free radicals, which initiate chemical reactions that degrade the rubber.Oxidation can cause the rubber to become brittle, hard, and more prone to cracking and degradation.
 UV radiation: Ultraviolet (UV) radiation from sunlight can accelerate the degradation of rubber.
UV rays can break chemical bonds in the rubber, leading to the formation of free radicals and causing the material to deteriorate.This degradation is often observed as discoloration, surface cracking, and loss of elasticity.
 Heat: Elevated temperatures can accelerate the aging process of rubber.Heat can cause the rubber molecules to move more quickly, leading to increased chemical reactions and degradation.High temperatures can result in the loss of elasticity, increased hardness, and reduced strength of the rubber or tire.
 Ozone: Ozone, present in the atmosphere, can cause rubber degradation.Ozone exposure can lead to the formation of ozone cracks or checking on the surface of the rubber.These cracks can weaken the rubber and reduce its overall performance and lifespan.
 Moisture and Humidity: Moisture and humidity can contribute to rubber degradation, especially when combined with heat and oxygen exposure.Moisture can penetrate the rubber structure, causing it to swell, become soft, and lose its mechanical properties.It can also accelerate chemical reactions and promote the growth of microorganisms, leading to further degradation.
 Mechanical Stress: Repeated mechanical stress, such as flexing, stretching, and impact forces, can contribute to rubber degradation over time.The repetitive loading and unloading cycles can lead to fatigue, cracking, and degradation of the rubber structure [9 -12].
Rubber degradation can also occur due to the activity of micro-organisms.Certain species of bacteria and fungi are able to degrade rubber materials through a process known as biodegradation.These microorganisms have enzymes that can hydrolyse the chemical bonds in rubber polymers, leading to their degradation.Microbial degradation of rubber can occur in a variety of environments including soil, water and certain industrial environments.It is particularly prevalent in environments where rubber waste or products come into contact with moisture, nutrients and temperatures suitable for the growth of microorganisms.
The degradation of rubber by micro-organisms is a complex process influenced by factors such as the type of rubber, the microbial species involved, the environmental conditions and the presence of other organic compounds.The specific mechanisms and pathways of rubber degradation by microorganisms are still being studied and understood [13][14][15].

Experimental material and methods
Textile carcass components for truck tyres were used as experimental material (Figure 2).This type of material is used exclusively in the automotive industry.The films consist of two layers of rubber with textile fibres between them, which provide the required improvement in the tyre carcass properties and, when the films are assembled, the tyre properties are also improved.The method of producing the carcass liner consists of introducing the textile cord onto a rubberising line where it is rubberised with a thin layer of rubber.The rubber itself is a polymer which is made up of the following types of rubber: Styrene Butadiene Rubber (SBR), Natural Rubber (NR), Isobutylene Isoprene Rubber (IIR) and Butadiene Rubber (BR).
In formula (1), m is the original mass of the body [g] and m1 is the mass of the body after the test [g].After soil sampling, the pH of the soil extract was measured.
The degradation effect of the environment was verified by tensile testing according to STN ISO 37 (62 1436).For the tensile test for each degradation environment, 5 test samples were used.The tensile strength of the original specimens without exposure was compared with the specimens after exposure.All samples were cleaned with distilled water after exposure.

Experimental results and discussion
At the beginning of the experiments, all samples were weighed and measured.The experiment monitored the weight change (weight loss) of the samples after exposure in three different natural environments.The exposure was every fourth day during the exposure: -4.6 °C controlled ambient temperature after the first month of exposure, 8.4 °C after the second month, and 12.1 °C after the third month, -WS -exposed samples were flooded with natural water and WS -B -exposed samples were flooded with natural water to which probiotic bacteria were added to treat the septic tanks [16].Table 1 shows the weight changes of samples in dry soil at different time intervals.Based on the values obtained in Tables 1 to 3, we see that the mass of the samples varied between the different exposure environments.Already after the first month of exposure, changes in the weight of the samples were observed in all environments.After the first month, a slight increase in weight was observed for all samples.From the second month onwards, weight loss was observed in all samples.The greatest loss in sample weight was in the moist soil with bacteria after three months of exposure in the moist soil with bacteria.There are many studies testing the effect of different types of bacteria on the breakdown of rubber.For example, in Nguyen et al. [17] tested the potential of Bacteroidetes and Proteobacteria to degrade natural rubber.The researchers recorded a 48.37 percent weight loss in the rubber samples after just 14 days of exposure.In Nawong et al. [18], the authors found an 18.38% reduction in weight after 30 days of exposure to the bacterial environment.Rubber is a highly durable material composed mainly of polymers, such as polyisoprene, which are challenging to degrade naturally.However, certain bacteria have evolved the ability to break down rubber and utilize it as a carbon source.This process is known as rubber degradation or rubber biodegradation.Several studies have investigated the microbial degradation of rubber compounds, particularly focusing on rubber products like tires [19,20].
Another part of the experiment was to compare the pH of the soil before and after exposure.The measurement found that the soil pH changed at all times and environments.Before exposure, the soil was slightly acidic (pH = 5.89 -6.45).After 3 months of exposure, the pH in both dry and moist soil changed to alkaline (pH = 7.01 -7.32).
The next part of the experiment monitored the change in the mechanical properties of the rubber compound.The INSTRON tensile tester was used to perform the tensile test.The test monitored the magnitude of force required to rupture individual specimens before and after their exposure to the degradation environment.The resulting ultimate strength (Rm) and elongation (ε) values obtained from the test are shown in The tensile test results show that in all environments there was an increase in the tensile strength of the rubber film after 1 month of exposure (Figure 3).After reaching a maximum, the strength began to decrease in all environments.The greatest decrease was observed in the dry soil after 3 months of exposure.Ozone in the atmosphere can attack the double bonds present in rubber molecules, leading to cracking and deterioration of mechanical properties, particularly in outdoor applications.Similarly, oxygen can promote oxidation reactions in rubber.This can result in hardening and embrittlement of the material [21].Moisture or humidity can facilitate the degradation of rubber, especially in the presence of oxygen, leading to swelling and weakening of the material [22].Rubbers can be subject to hydrolysis under certain conditions.Hydrolysis is a chemical reaction where a compound reacts with water, leading to the breaking of chemical bonds and the decomposition of the material.While natural rubber is generally resistant to hydrolysis, some synthetic rubbers are more susceptible to this process.For example, certain types of synthetic rubbers, can undergo hydrolysis under specific environmental conditions.NBR contains nitrile groups that are vulnerable to hydrolytic attack, particularly in the presence of acids or bases.Similarly, EPDM rubber, although known for its excellent weathering can experience hydrolysis in the presence of strong acids or bases at elevated temperatures.The susceptibility of rubbers to hydrolysis depends on factors such as the chemical composition, the presence of functional groups, the of the environment, temperature, and the duration of exposure, stabilizers or protective coatings [23].

Conclusions
The aim of the experiment was to verify the possibility of biodegradation of rubber in natural conditions.One way to control rubber degradation was to monitor the weight loss of experimental samples in different environments and over different times.Commonly available soil was used as the exposure medium.Dry soil, wet soil and wet soil with bacteria were used.The duration of exposure was set at 1, 2 and 3 months.The weight loss results of the samples after exposure were compared with the weight of the samples before exposure.Based on the results, we can conclude that in all environments and at all exposure times, a change in the weight of the samples, namely weight loss, was registered.The most significant weight loss was observed in rubber samples exposed to wet soil with bacteria after 3 months exposure (3 %).
The degradation of rubber compounds in different environments was also confirmed by comparing the mechanical properties of the samples before and after exposure.In all environments, an increase in tensile strength was observed after the first month of exposure.The increase in the Rm value of polymers Tensile test dry soil wet soil wet soil with bacteria in environment after one month can be attributed to the fact that in certain cases, the movement of water molecules into the polymer matrix can create an osmotic pressure, leading to an influx of water molecules.This can further contribute to an increase in the value of Rm.Another factor may be that the rate of water absorption can vary between different polymers and is affected by factors such as temperature, humidity and the molecular structure of the polymer.In some cases, it take some time for the polymer to reach equilibrium with respect to water absorption, and the increase in Rm value may continue for some time before it stabilizes.Thereafter, the tensile strength decreased, with the greatest decrease in tensile strength recorded in the dry soil (approximately 20 MPa).The change in weight (loss) also corresponded to a change in the pH of the exposure medium.The pH values showed that even after a short exposure time, the substances from the exposed samples leached into the soil and there was a gradual change in the pH value from acidic through neutral to weakly basic pH (7.32).The components of the rubber additives were probably partially released from the samples into the soil.
The experiments conclude that the environment (soil, water, change of temperature, bacteria) in which bacteria are found degrades rubber the most.This option seems to be an appropriate solution to the ever-growing problem of polymer pollution in the ecosystem.The resulting amounts of waste produced by the excessive consumption of rubber products are so great that the pollution of our planet is alarming.
For this reason, the use of bacteria causing the decomposition of individual types of rubbers is a positive factor for the environment.However, this problem is associated with a large number of disadvantages, such as the slow rate of the biodegradation process of rubber, but also the growth rate of the initiators (bacteria) of this process is very slow.At present, biodegradation is a major subject of interest research and development.
It is important to note that rubber degradation is a complex phenomenon, and the specific mechanisms and bacteria involved may vary depending on the conditions and type of rubber being studied.Nevertheless, research in this area provides valuable information on the potential for bacterial biodegradation of rubber compounds and may have positive implications for waste management and environmental protection.

Table 1 .
Weight change of samples in a dry soil (DS).Table2shows the weight changes of samples in dry soil at different time intervals.

Table 2 .
Weight change of samples in a wet soil (WS).Table3shows the weight changes of samples in wet soil with bacteria at different time intervals.

Table 3 .
Weight change of samples in a wet soil with bacteria (WS -B) at different time intervals.