Determination of Structural and Dimensional
Changes of O-ring Polymer/Rubber Seals Immersed
in Oils
A.A. Roslaili
1, A.S. Nor Amirah
1, S. Mohd. Nazry
2, K. Ain Nihla
11School of Environmental Engineering, Universiti Malaysia Perlis, 02600 Jejawi, Perlis, Malaysia. 2School of Materials Engineering, Universiti Malaysia Perlis, 02600 Jejawi, Perlis, Malaysia.
Email: roslaili@unimap.edu.my
Abstract-- The purpose of this work is to investigate the suitability use of vegetable-based oil in hydraulic system and the compatibility between the rubber seals and lubricant extracted from vegetable-based oil in hydraulic system. Two types of o-ring rubber seals were used which are VITON and NBR. These rubber seals were fully immersed in two different types of oil, Seri Murni Palm Olein and Pennzoil 68 Hydraulic Oil for two months. Detailed analysis of the rubber seals mass, thickness and perimeter, swelling test, SEM and oxidation test were done during the period in order to investigate the dimensional and structural changes of rubber seals. The viscosities of immersed oil were also tested to analyze its impact and influence on the physical changes of seals. The analysis was done using the ASTM standard method. Result shows that the Seri Murni Palm Olein has the potential to be used as hydraulic fluid especially when using with VITON seal. However, some of physical and its chemical properties need to be enhanced first such adding additives to the olein in order to improve the effectiveness of the vegetable-based oil as hydraulic fluid.
Index Term-- VITON, NBR, palm olein and mineral-based
hydraulic oil.
1. INTRODUCTION
Hydraulic fluids or hydraulic liquids are the medium by which power is transferred in hydraulic machinery. Examples of equipment that might use hydraulic fluids include excavators, brakes, power steering systems, transmissions, backhoes, garbage trucks, aircraft flight control systems and industrial machinery. Reports indicated that nearly 38 million metric tonnes of lubricants were used globally in 2005, with a projected increase of 1.2 percent over the next decade (Kline, 2004–2014). Approximately 85% of lubricants being used around the world are petroleum-based oils (Loredana, 2008). Use of hydraulics is expanding, and consumption of hydraulic fluids today constitutes a significant part of the world’s total consumption of defined mineral oils, approximately 1 million tonnes per annum or around 10%. Continuing efforts to achieve improved efficiency resulted in development of fluids with higher quality, displaying longer life and providing better protection for hydraulic components under operating conditions (Shashidhara & Jayaram 2009). Plus, stronger environmental concerns and growing regulations over contamination and pollution will increase the need for renewable and biodegradable lubricants.
Vegetable oils, especially palm oil was considered the most likely candidate for a fully biodegradable hydraulic
fluid. Plant oil is a natural resource available in abundance. Vegetable oils have already been considered as potential industrial fluids as early as the 1900s (Oommen & Claiborne,1999).Vegetable oils with high oleic content are considered to be potential candidates as substitutes for conventional mineral oil-based lubricating oils and synthetic esters (Randles and Wright, 1992; Asadauskas et al., 1996). The use of vegetable oils as hydraulic fluid would help to minimize hazardous pollution caused by accidental spillage, lower disposal costs of the used fluid, and help the user industry to comply with environmental safety regulations (Nik et al., 2005). Due to the advance of petrochemical industry development, the readily available of petroleum oils replaced vegetable oils for reasons of lubricity, stability, and economics. Recently, environmentally related issues that include biodegradability, toxicity, occupational health and safety, and emissions have created important issues to be revealed and reconsidered especially the use of mineral oils in environmental sensitive areas.
2.0 SAMPLE PREPARATION
Two types of o-ring seal investigated are Fluoroelastomers (VITON) and Acrylonitrile-Butadiene Rubber (NBR).While for oil samples, two types of oils were used which are Seri Murni Palm Olein and Pennzoil 68 Hydraulic Oil. The oils were filled in the boiling tube about 50 mL and immersed in an oil bath at 70oC and pressure at 1 atm. VITON and NBR
were centred cut, tied with the black threads, labelled, and put into boiling tube containing oil samples according to the labels (as explained in Table I) for about two months. The dimensional changes tests were carried out for every two weeks.
Table I
Label of Rubber Seals and Oils
Label of Boiling Tube Types of Rubber Seal Types of Oil
VIT 1 VITON Seri Murni Palm Olein
VIT 2 VITON Seri Murni Palm Olein
VIT 3 VITON Pennzoil 68 Hydraulic Oil
VIT 4 VITON Pennzoil 68 Hydraulic Oil
NBR 1 NBR Seri Murni Palm Olein
NBR 2 NBR Seri Murni Palm Olein
NBR 3 NBR Pennzoil 68 Hydraulic Oil
NBR 4 NBR Pennzoil 68 Hydraulic Oil
Fig. 1. Schematic Diagram of Boiling Tubes in Oil Bath
3.0 TEST EQUIPMENT AND PROCEDURE
3.1 Dimensional Changes of Seal (VITON and NBR) Test
The rubber seals dimension was measured using Vernier Caliper. Dimensional changes of the seals were measured based on its perimeter and width of the seals, as according to the ASTM D471: Liquid Immersed Properties Test to measure the changes in weight and dimension (depth and perimeter) of materials immersed. Readings for the seals dimension were taken four times in every two weeks
3.2 Changes in Mass Test
The mass of seals rubber were weighed using Digital Analytical Balance after taking out from the boiling tube containing oil samples. Before weighing, the seals were cleaned with dilution water and dried properly using filter paper. All the measurement was recorded and changes in
mass were determined four times along the immersion period.
3.3 Swelling Test
Swelling test was performed according to ASTM D3616: Swelling Test by immersing the rubber seals in those oils at 25ºC for 72 hours. The rubber seals were weighted before and after removed from the oils.
The swelling ratio (Q) was calculated according to the following equation:
Where;
Wd = Seals weight before swelling
A6
Stainless Steel Tube Rack Test
Boiling Tube
Q = 1+ (Ws-Wd)ρp
Wd (ρs)
A1
A8
A7
A2
A6
A5
A4
A3
A1
000
0
A9
A10
Ws = Seals weight after swelling
ρs = is the density of the solvent, ρp = is the seal density.
3.4 Viscosity Test
A Brookfield (Viscometer model DV-I+) rotational type viscometer was used to measure the viscosity of oil samples. S61 spindle has chosen and was operated at different speeds between 10 and 100 rpm. For both oil samples the viscosity and percentage of torque were manually recorded when the viscosity reading reached apparent equilibrium (appears relatively constant for reasonable time).
3.5 Oxidation Test
The oxidation performance of test oils is when the test oils are aged for three days in beaker at temperature of 95 C while ambient air is introduced and a copper wire is immersed periodically. At the end of the test viscosity of test oils is tested and the viscosity increase by oxidation must not exceed 20%.
3.6 Scanning Electron Microscope (SEM) Test
The scanning electron microscope (SEM) model was used to observe the morphology of VITON and NBR before and after the immersion test.
4.0 RESULTS AND DISCUSSION
a) Structural and Dimensional Changes of Rubber Analysis.
Figure 2 shows the seal deformation before and after immersion process. Both of seals NBR and VITON tend to swollen and shrunken after immersion in both oils.
(a) VITON Before Immersion (b) VITON Minor Changes After Immersion
(a) NBR Before Immersion (b) NBR Harden After Immersion
Fig. 2. Seal Deformation After and Before Immersion
VIT 1
VIT 4 111
VIT 2 111 VIT 3
NBR 1
NBR 4
Fig. 3. Changes in Mass with Time
Figure 3shows changes in mass of VITON and NBR o-ring seals before and after immersed in oil bath with different types of oil. It shows the mass of VIT 1 & 2 (immersed in Seri Murni Palm Olein Oil) were significantly increased with increasing time. The results of VIT 1 & 2 was started with increased of mass value from range 0.05 g up to 0.3 g and raised until 0.4 g. Same thing goes happened to VIT 3 & 4 which were immersed in Pennzoil 68 Hydraulic Oil. The results value at the beginning of experiment was 0.05 g then increased to the 0.3 g and rose up until 0.3 g. However, results of NBR 1 & NBR 2, which immersed in vegetable oil shows the fluctuation reading. The mass of both seals were first begin to increase but then decreased and rose again. It shows that the NBR 1 & 2 was shrunken at the beginning of the experiment conducted due to the mass value which indicates negative sign (0.07 g decreased to -0.08 g) and tend to swollen at the final stage up to 0.21 g). While NBR 3 & NBR 4 (immersed in Pennzoil 68 Hydraulic Oil) shows the result were quite fluctuated but unobvious. There is not much different values for NBR 3 & 4 which were swollen ( -0.01g to -0.04 g) and increased for about 0.16 g at the final experiment. This proven that VITON is more compatible to be use with vegetable oil.
The effect of increased of mass on their physical properties deserves careful consideration. The enhancement of rubber properties with increased molecular weight has been known for many years but development has been limited, because of the difficulty of processing these high molecular weight rubbers. However, for high pressure and high temperature sealing applications, as in the oil industry, only high molecular weight rubbers are suitable, since they
possess low compression set along with other desirable functional properties.
Fig. 4. Changes in Width with Time
Fig. 5. Changes in Perimeter with Time
The more serious cause of deterioration in rubbers is its reaction with atmospheric oxygen. This is possible because rubber is a diene polymer and some; NBR and VITON have olefinic double bonds in their structure. Oxidative
cross-link produced varies considerably, and this can affect the balance of chemical and particularly of physical properties of the vulcanizates.O-Ring surfaces contains many flaws, where cracks can be initiated via ozone attack. Increasing stress will increase the number of flaws, which leads to a larger number of cracks. The depth of the cracks is inversely related to their number, and so, low stresses that produce long deep cracks are more damaging to elastomer seals than high stresses.
Furthermore, low molecular weight compounds (non-polar) have sharply defined maximum levels that will dissolve in oil. Elastomer on the other hand, first swells, absorbing the fluid without true solvation occurring. An increasing amount of fluid is absorbed, leading first to the formation of a gel and finally a true solution (Abu Abdeen & Elamer, 2009). Firstly, a rapid uptake of fluids occurs, reaching a fairly well defined equilibrium state, and then secondly fluid is absorbed slowly at an approximately constant rate. This second stage does not take place in the absence of air, and it is therefore assumed that it is related to irreversible breakdown of the rubber.
Cross linking is a process of forming a three-dimensional network structure from a linear polymer by a chemical and physical method a cross linking process can be classified into addition, substitution, and elimination reactions, or it can involve two or even all of these. Elastomers can degrade in chemical seal environments through reactions with the polymer backbone and crosslink system, or by reactions with the filler system. Presence of the polar side-groups in the backbone chain increases the oil resistance of the polymer (Patil & Coolbaugh, 2005). Cross linking also limits the degree of polymer swelling by providing tie-points (constrains) that limit the amount of
solvent that can be absorbed into the polymer (Hoffman, 2001 ). NBR is a copolymer of acrylonitrile and butadiene. NBR is a low-cost elastomer with good mechanical properties. The concentration of acrylonitrile in the copolymer has a considerable influence on the polarity and swell resistance of the vulcanizates in non-polar solvents. The greater the acrylonitrile content, the lesst he swell in motor fuels, oils, fats and other (Hoffman, 2001) However, the elasticity and low temperature flexibility also become poorer. NBR seals in both oils remain almost unchanged compared to VITON seals. This added by the cross-linking of polymers in VITON seals (which are synthetic seal) that reduced the capability of the elastomer in palm olein. This may be influenced by the higher fat content in palm olein.
b) Swelling Analysis
Swelling ratio of NBR and VITON shows VITON is relatively good in term of swelling for both different oils compared with NBR immersed in Seri Murni Palm Olein was relatively high (1.63) compared to NBR immersed in Pennzoil 68 Hydraulic Oil (0.431).The analysis of swelling is represented in Table II. This phenomenon is referred as swelling, which normally takes place as liquid is absorbed. The diffusion rate of liquid into a seal test will determine the time taken to reach equilibrium. After that, the rate of absorption of liquid slows. The lower the viscosity of the liquid, the higher the diffusion rate. (Challapa Chandsekaran ,2010).
The major changes of volume noticed after immersion of a NBR in a vegetable oil for a specified period. This shows that NBR seals absorbed more vegetable oil rather than mineral oil.
Table II
Swelling Ratios of NBR and VITON
O-Ring Seals Oil Swelling Ratio
VITON Pennzoil 68 Hydraulic Oil 1.0058
Seri Murni Olein Palm Oil 0.9750
NBR Pennzoil 68 Hydraulic Oil 0.4319
Fig. 6. SEM images VITON and NBR
SEM images of NBR (a) before immersing in oil, followed by after immersion in Seri Murni Olein Oil (b) and (c) in Pennzoil 68 Hydraulic Oil. SEM images of VITON before immersion (d ) followed by immersion in Seri Murni Olein Oil (e) and in Pennzoil 68 Hydraulic Oil (f) shows in Figure 6.NBR relatively undergone a huge cracking effect and it’s swollen drastically after immersed in Seri Murni Palm Olein. VITON however shows small cracking and unobvious swelling effects when immersed in palm olein. The seals deteriorates because of oxidation at an elevated temperature; as it tooks up large quantity of oxygen. This led to the increase in weight of seals. Oxidation of rubber may take place in three ways; (i) deterioration throughout the rubber, (ii) formation of a film on the surface of the rubber, and (iii) ozone cracking. Ozone cracking is considered unlikely to be happened but because only very small traces of gas are needed to initiate the cracking, have succumbed to the problem. Ozone attack will occurs at the most sensitive zones in a seal, especially the sharp corners where the strain is greatest when the seal is flexing in use. The corners represent stress concentrations, so the tension is at a maximum when the diaphragm of the seal is bent under air pressure (Asadauskas, 2007).
In an atmosphere, stretched samples of VITON and NBR has developed surface cracks which grows in length and depth until they eventually breakdown the test piece. Even when they are quite small, they can cause a serious reduction in strength and fatigue life (Gent, 2005). The aging of rubber is caused by oxidative degradation in the physical and mechanical properties of vulcanized rubbers (Li-Gui & Koenig, 2005). This has be seen the SEM images clearly proved through the SEM images where that, VITON
is suitable to be used with vegetable oil due to the minor deteriorations.
c) Viscosity Analysis
Figure 7,8,9,10,11 and 12 shows the results of viscosities for both oils. In this study the speed used were 10 RPM, 20 RPM, 50 RPM and 100 RPM. As speed increase, the velocity will decrease. The velocity of Pennzoil 68 Hydraulic Oil is higher than Seri Murni Palm Olein Oil. The viscosity was decreased with increasing time. It happens for both samples Pennzoil 68 Hydraulic Oil and Seri Murni Palm Olein.
30X 500X
(a)
(b)
(c)
500X
30X 500X 500X
Fig. 7. Viscosity of Pennzoil 68 Hydraulic Oil at Speed 10 RPM
Fig. 8. Viscosity Seri Murni Palm Olein versus Time at 10 RPM
Fig. 10. Viscosity Seri Murni Olein Palm Oil versus Time at 20 RPM
Fig. 11. Viscosity Pennzoil 68 Hydraulic Oil versus Time at 20 RPM
Fig. 13. Viscosity Pennzoil 68 Hydraulic Oil versus Time at 50 RPM
Fig. 14. Viscosity Seri Murni Palm Olein versus Time at 100 RPM
The range of viscosity is unstable especially when the speed is lower. At lower speed of 10 RPM, A4 and A8 shows the lowest reduction of viscosity. The results varied from 33 cP to 62 cP and from 44 cP to 115 cP. Viscosity is tending to increase at speed of 50 RPM and above and achieved the highest stability at 100 RPM. This can be seen when NBR seals immersed in both oils they shows the viscosity of the oils were much higher and reduced unstable at lower speed compared to when immersed with VITON. At lower speed of 10 RPM, the lowest reduction viscosities were shown by A5 and A7 where the results ranged from 40 cP to 67 cP and from 54 cP to 113. At speed of 20 RPM, A4 shows the lowest reading for vegetable oil (33 cP to 62 cP) while A8 shows the lowest reading for Pennzoil oil (43 cP to 114 cP).
The viscosity also affected by immersion of seals in both oils. Viscosities tend to be stable at speed of 50 RPM and above and achieved the highest stability at 100 RPM. A3 and A8 at speed of 50 RPM show the highest reduction result of viscosities. A3 (NBR immersed in vegetable oil) shows the viscosity range from 26 cP to 58 cP .While for A8 (NBR immersed in Pennzoil Oil) shows the results from 29 cP to 109 cP . The highest stability was of achieved at speed of 100 RPM which performed by A5 and A9, that are VITON immersed in both samples. Both show the best
performance of all by undergone the smallest reduction reading of viscosities. Final stage of oxidation resulted in more changes that are significant in viscosity. Thus, we can see the different ranges of viscosity at initial and after completing the experiment.
Moreover, the changes of viscosity can be seen in their colour .At the beginning of study the colour of the oil was thick and more viscous. After two months, the colours seem to be lighter and less viscous. Figure 16 represents the colour of the oils at beginning of the project and after two months. This is because oils will decay during the lifetime of the lubricant either; in storage or during the application. The physical and chemical changes that occur within the oil during oxidation are likely to have an impact on the lubrication performance, which shows on the colour changes of both oils. Moreover the viscosities for both oils also being affected by oxidation of the seals and oil. Heat is one of the factors of oxidation, which for every 8ºC of increasing temperature, the rate of oxidation will be twice. Oxidation will attack the elastomers and this has lead to “dehydrofluorination” and the degradation of the seal itself. Vegetable oils also show poor corrosion protection (Ohkawa et al., 1995). The presence of ester functionality renders these oils susceptible to hydrolytic breakdown (Rhodes et al., 1999).
Fig. 16. Colour Changes of the Oils:
(A) At the Beginning of the Project and, (B) After Two Months Project Completed
5.0 CONCLUSIOS
It can be concluded that Seri Murni Palm Olein is actually has a big potential to be used as hydraulic fluid. There are few unique characteristics and advantages of palm olein, as such it is a potential candidate to replace the function of conventional mineral-based lubricating oil. Moreover, palm olein is highly non-toxic and exhibits a ready biodegradability, good lubricity and cause fewer health problems (Vizintin et al., 2002). It even possible to provide satisfactory high performance as a functional fluiddue to its good resistance to oxidation and formation of breakdown products at high temperatures and longer shelf life of
finished products. Moreover, products derived from them
are generally environmentally friendly (Gryglewicz et al., 2003). Meanwhile VITON shows minimum changes of structural and dimensional in both oils and successfully to be applying together with vegetable-based hydraulic oil. Its synthetic characteristics are more compatible with Seri Murni Palm Olein, as it is easily oxidized and quickly react compared to NBR seals. However due to some constraints
during their usages in hydraulic system such as oxygen, water, air pressure and heat, suitable additives should be added in order to improve the effectiveness of the palm olein as hydraulic fluid. Seals’ durability also can be enhanced by different additives, as example antioxidants from phenolic and aminic as they offer flex fatigue and ozone protection as well.
6.0 RECOMMENDATIONS
This improves the thermal, oxidative andhydrolytic stability of the oil significantly without affecting much on the biodegradability.
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