A novel method for monitoring chemical degradation of crosslinked rubber by stress relaxation under tension

doi:10.1016/j.polymdegradstab.2006.03.002

Polymer Degradation and Stability

Volume 91, Issue 10, October 2006, Pages 2520-2526

https://doi.org/10.1016/j.polymdegradstab.2006.03.002 Get rights and content

Abstract

This paper describes a novel test method for monitoring chemical degradation of a crosslinked rubber by stress relaxation under tension. An accelerated sulphur cured ethylene propylene diene rubber (EPDM) was subjected to stress relaxation under tension while exposing to 50% aqueous solution of nitric acid (HNO₃). An experimental set up was designed and built in-house for this purpose. The tensile test specimen was stretched to a constant elongation. The stress decay was monitored upon exposure to the harsh chemical. Stress decay was found much faster in the exposure media than in air. Decrosslinking was the main reason for quicker stress decay as observed by decrease in crosslink density. To compare, experiments were conducted by exposing similar specimens in the same exposure media under unstressed conditions. It was found that monitoring chemical degradation under given stressed condition yielded relatively quicker yet reliable and reproducible results.

Introduction

The importance of chemical degradation and its impact on formulating a rubber vulcanizate is evident from our previous studies [1], [2], [3] and literature [4]. Industries that manufacture rubber products for chemical applications require a reliable yet easy test methodology to ascertain durability in service conditions. This, therefore, necessitates accelerated degradation tests on a laboratory scale that can monitor the performance of the rubber product in a harsh chemical environment on a relatively shorter time scale. Rubber products, in general, are designed to withstand stresses of external and internal origin depending on the service conditions. The external factor arises from the application condition whereas the internal stress may arise either during processing and/or degradation. Thus, the chemical reactions leading to degradation of rubbers usually take place in stressed condition and the extent of degradation is aggravated by the external stress in combination with temperature and harsh chemicals. Several new test methods for adjudging durability of rubber products by monitoring stress relaxation are proposed in the literature [5], [6], [7], [8], [9], [10], [11]. However, no report is available on the test method vis-a-vis test equipment for observing chemical degradation in situ during stress relaxation of rubber particularly in harsh acidic or alkaline chemical environments.

Over the years, rubber products (e.g., seals, gaskets, o-rings, hoses and cable insulation) are being used in various chemical environments that could potentially undergo degradation. In practice, service conditions vary widely and are often complex in the presence of both harsh chemicals and varying temperatures. Since “maintenance free” products are gaining popularity in the market, the demand for durability is also becoming stringent. In this situation, a reliable short time testing for estimating lifetime is becoming essential. The laboratory tests are usually being done on a relatively faster time scale using an exaggerated condition in order to extrapolate over the lifetime of the material in actual application. Several such standardized test methods are available in the form of ASTM, ISO and BS Specifications. In this study the rubber sample is strained in situ in harsh chemical environments and stress decay is measured as a function of exposure time. New equipment is indigenously built for this purpose and a new test methodology is proposed.

Section snippets

Equipment concept

The concept of stress relaxation under tension during a chemical exposure condition is applied for fabricating the test equipment. There are some distinct advantages in this approach. First, the equipment is fabricated in such a manner that a standard dumbbell-shape specimen commonly used for routine tensile testing could be employed. This avoids extra effort for special sample preparation. The sample could be stretched to any level up to 200% elongation. More importantly, the chemical attack

Stress relaxation of rubber

When a constant strain is applied to a rubber sample crosslinked well above its gel point, the force required for maintaining the strain decreases with time. This behaviour is called “stress relaxation”. Both physical and chemical processes can cause the stress relaxation depending upon the time scale. At ambient conditions and/or short times, the stress relaxation predominantly results from physical processes. At highertemperatures or in degradative environments, and/or for long time

Materials

EPDM rubber used in this study (Nordel^® IP 4520) contained 5-ethylidene-2-norbornene (ENB) as diene with a typical monomer composition of ethylene/propylene/ENB as 50/45/5 by weight percentage. The pure rubber was characterized for monomer compositions (ethylene/propylene/ENB, wt%), mass average molar mass (133 kg/mol) and molar mass distribution (2.74), which is described in details elsewhere [1], [2].

Compounding was done using rubber grade chemicals as per the recipe given in Table 1. Carbon

Stress relaxation in situ during chemical degradation

A typical calibration curve for converting the data from volts to load is made and obtained a linear equation (Eq. (3)) $Load = (- 0.1611) + (0.4269) \times volt$

The correlation coefficient (R² = 0.99976) indicates the sensitivity and the accuracy of the calibration method used in this study.

Fig. 2 represents the normalised stress decay curve (both in air and in exposure media) for the cured EPDM rubber after a constant stretching of 50% elongation. A rubber product like seal, gasket or hoses which requires the

Conclusions

Monitoring chemical degradation of a crosslinked rubber upon exposure to harsh chemicals in situ stress relaxation by our indigenous fabricated equipment and techniques seems to be reliable for the shorter time scale of the experiments. The repeatability and reproducibility of the equipment and data are also demonstrated. Accelerated sulphur cured EPDM is used because our previous studies show the resistance of this sample under aqueous acidic environment in unstressed conditions. Teflon

Acknowledgement

Financial support from MONEPOL (Danish Centre for the Study of Polymer Degradation and Stability) a Danish Research Agency funded centre contract (J.nr. 2000-603/4001-51) is gratefully acknowledged.

References (15)

S. Mitra et al.
Chemical degradation of crosslinked ethylene–propylene–diene rubber in acidic environment. Part I. Effect on accelerated sulphur crosslinks
Polym Degrad Stab
(2006)
S. Mitra et al.
Chemical degradation of crosslinked ethylene–propylene–diene rubber in acidic environment. Part II. Effect of peroxide crosslinking in presence of a coagent
Polym Degrad Stab
(2006)
R.P. Brown et al.
Compression stress relaxation
Polym Test
(1981)
R.P. Brown
Stress relaxation of rubbers in tension – the simple approach
Polym Test
(1980)
A.B. Othman et al.
Stress relaxation measurement of rubber in tension – a new technique
Polym Test
(1992)
M. Akiba et al.
Vulcanization and crosslinking in elastomers
Prog Polym Sci
(1997)
Mitra S. Chemical degradation of specific uncrosslinked and crosslinked rubbers upon exposure to specific non-absorbing...

There are more references available in the full text version of this article.

Cited by (38)

Effect of ion penetration on the aging of EPDM components used in caustic liquid transfer lines by microscopic analysis
2024, International Journal of Pressure Vessels and Piping
The waste transfer lines at the United States Department of Energy's Hanford Site Tank Farm in Benton County, Washington, USA have flexible sections that are fabricated from ethylene propylene diene monomer (EPDM) hoses. The hoses are of a hose-in-hose design with an inner hose that carries the waste and an outer hose that acts as containment in the event of an inner hose failure. During waste transport, the inner hoses are exposed to several stressors including caustic solutions at high temperatures and high pressures. These hose-in-hose transfer lines (HIHTL) are exposed to several stressors including caustic solutions at high temperatures and high pressures during the waste transport. The aging behavior of the HIHTL inner hose were evaluated by exposure to solutions at 77 °C containing 6.25, 12.50, and 25.00% (v/v) sodium hydroxide (NaOH) for 12-months. After the exposure, the burst pressure of the HIHTL specimens were measured and compared to the unaged samples.
The burst pressure of the HIHTL specimens exposed to the 6.25% NaOH solution exhibited the most significant deterioration and those exposed to 25.00% NaOH solution had the least deterioration. Examination of the inside surface of the HIHTL specimens with scanning electron microscopy showed that the surface deterioration for the specimens exposed to 6.25% NaOH solution were the most severe and the samples exposed to 25.00% NaOH solution had the least deterioration. Scanning Electron Microscope with Energy Dispersive Spectroscopy (SEM-EDS) analysis of the specimens showed that the specimen aged with the 6.25% NaOH solution had that greatest sodium ion penetration into the material with the sodium concentration being significantly higher than those seen with the other specimens. In addition, when the specimens aged with both the 12.50% and the 25.00% NaOH solutions were examined, a white crystalline coating was observed on the inside surfaces of the specimens which appears to have acted as a barrier that protected the EPDM material from attack by the NaOH solutions.
Ageing behaviour and molecular/network structure evolution of EPDM/carbon black composites under compression and in thermal-oxidative environments
2023, Polymer Degradation and Stability
EPDM/carbon black (CB) composites were first prepared using sulfur as vulcanizing system, and the mechanical properties of EPDM/CB composites gradually increased with increasing CB content. Compressive stress-thermo oxidative ageing was performed in terms of three compressive stress levels and temperature conditions. During ageing process, two typical reactions of chains scission and crosslinking were deduced. On the one hand, degradation reaction of EPDM molecules occurred and unstable free radicals were produced under thermo/stress effect, which were oxidized to generate oxygenated compounds, leading to the increase of dangling chains ratio and the breakage of CB network with two-phase separation. On the other hand, the active free radicals were further consumed to initiate secondary crosslinking reaction, resulting in increase of crosslinking density and restriction of chains mobility. During the whole ageing process, the crosslinking reaction was dominant over chain scission reaction, resulting in the increase of crosslinking density and poor uniformity of network structure. Consequently, the surface/fractured surface morphologies of aged EPDM showed coarse features with obvious CB aggregates and the rupture of continuous CB network due to interfacial debonding, eventually leading to the remarkable deterioration of mechanical properties and decline of sealing resilience.
Effect of solution concentration on ethylene propylene diene monomer (EPDM) nonmetallic components used in caustic liquid waste transfer lines
2023, Engineering Failure Analysis
The hose-in-hose transfer lines (HIHTL) used to transfer waste at the United States Department of Energy’s Hanford Site Tank Farm are fabricated from an ethylene propylene diene monomer (EPDM) synthetic rubber. During waste transport, these hoses are exposed to several stressors including caustic solutions at high temperatures and high pressures, as well as β and γ radiation. The aging behavior of HIHTL and dog-bone shaped EPDM specimens were evaluated by exposure to solutions at 77 °C containing 0.00, 6.25, 12.50, and 25.00 % (v/v) sodium hydroxide (NaOH) for 12-months. After the exposure, the material properties of the specimens were measured and compared to the unaged samples. The material properties evaluated included the tensile strength of the EPDM dog-bone samples and the burst pressure of the HIHTL. Both the tensile strength of the EPDM dog-bone samples and the burst pressure of the HIHTL specimens exposed to the 6.25 % NaOH solution exhibited the most significant deterioration and those exposed to 25.00 % NaOH solution had the least deterioration. Examination of the HIHTL and the EPDM dog-bone specimens with scanning electron microscopy showed that the deterioration for the specimens exposed to 6.25 % NaOH solution were the most severe and the samples exposed to 25.00 % NaOH solution had the least deterioration.
Effect of temperature and aging duration on ethylene propylene diene monomer (EPDM) nonmetallic components used in caustic liquid waste transfer lines
2021, Engineering Failure Analysis
Nonmetallic materials are used in waste transfer lines at the United States Department of Energy’s Hanford Site Tank Farm in Benton County, Washington, USA. During use, the ethylene propylene diene monomer (EPDM) inner hose of the hose-in-hose transfer lines (HIHTLs) are exposed to β and γ radiation, caustic solutions as well as high temperatures and high pressures. Aging behavior of specimens of EPDM HIHTLs and dog-bone shaped specimens were evaluated by exposing to a solution of 25% sodium hydroxide (NaOH) at 38 °C, 54 °C and 77 °C for 6 and 12-months as well as water only at 77 °C for 12-months. Tensile strength and burst pressure of the specimens were characterized and compared with the unexposed (baseline) specimens. Both the tensile strength of the EPDM material and the burst pressure of the HIHTLs significantly decreased with the higher temperatures and longer exposure times. Analyses using scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) of the HIHTLs and the dog-bone specimens were conducted. Results show that the surface degradation increased with increasing exposure temperature. Penetration of NaOH into the EPDM material also increased with increasing temperature and exposure duration.
Vibrations characterization in milling of low stiffness parts with a rubber-based vacuum fixture
2021, Chinese Journal of Aeronautics
Citation Excerpt :
Nevertheless, this pin-array solution is employed for the machining process of high stiffness parts and it is not adaptable for complex geometries. The proposed solution combines the damping capacity of the rubbers with the outstanding sealing properties of the nitrile butadiene rubber (NBR)25–27 to transform a flexible plate into a adaptable vacuum table. Thus, the process vibrations are mitigated, and the part clamping can be made uniformly by the opposite side of the machined zone.
Fixtures are a critical element in machining operations as they are the interface between the part and the machine. These components are responsible for the precise part location on the machine table and for the proper dynamic stability maintenance during the manufacturing operations. Although these two features are deeply related, they are usually studied separately. On the one hand, diverse adaptable solutions have been developed for the clamping of different variable geometries. Parallelly, the stability of the part has been long studied to reduce the forced vibration and the chatter effects, especially on thin parts machining operations typically performed in the aeronautic field, such as the skin panels milling. The present work proposes a commitment between both features by the presentation of an innovative vacuum fixture based on the use of a vulcanized rubber layer. This solution presents high flexibility as it can be adapted to different geometries while providing a proper damping capacity due to the viscoelastic and elastoplastic behaviour of these compounds. Moreover, the sealing properties of these elastomers provide the perfect combination to transform a rubber layer into a flexible vacuum table. Therefore, in order to validate the suitability of this fixture, a test bench is manufactured and tested under uniaxial compression loads and under real finish milling conditions over AA2024 part samples. Finally, a roughness model is proposed and analysed in order to characterize the part vibration sources.
Improved mechanical and rheological behavior of nitrile rubber reinforced with multi-walled carbon nanotubes and carbon black dual-filler system
2021, Materials Today Communications
Citation Excerpt :
Thus, it can be concluded that the highest stress relaxation occur at the beginning of the process, although they occur almost instantly, given the low values of τ1, τ2 and τ3 as expected [39,40]. According to Mitra et al. [40], the pair A1 and τ1 correspond to the breakage of the filler-filler interactions of compounds. As can be seen from the Table 4, compound 40C/0 N has greater A1 values only when compared to neat NBR (0C/0 N).
The use of carbon nanotubes at low content in the production of rubber nanocomposites has already been proven promising, leading to significant improvements in overall mechanical properties of these materials. The addition of dual-filler systems in rubber compounds can be advantageous, leading to synergistic improvements. In this work, the effect of dual-filler system containing carbon black and multi-walled carbon nanotubes on mechanical and rheological properties of nitrile rubber matrices was evaluated. Increasing nanotube content has been proven effective in optimizing mechanical performance of nitriles. Based on a 40 phr carbon black reference compound, the dual-filler system containing 10 phr of nanotubes and 40 phr of carbon black presented noticeable results in tear and tensile strenght, hardness, besides higher stress relaxation. Formation of agglomerates are highlighted herein as a point of caution.

View all citing articles on Scopus

¹: Present address: NKT Flexibles I/S, Priorparken 510, DK-2605 Broendby, Denmark.

View full text

A novel method for monitoring chemical degradation of crosslinked rubber by stress relaxation under tension

Abstract

Introduction

Section snippets

Equipment concept

Stress relaxation of rubber

Materials

Stress relaxation in situ during chemical degradation

Conclusions

Acknowledgement

Polym Degrad Stab

Polym Degrad Stab

Polym Test

Polym Test

Polym Test

Prog Polym Sci