Thermoplastic Elastomers for Fluid Resistance Applications
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[edit] Abstract
The chemistry and morphology of all plastic and rubber polymers, including Thermoplastic Elastomers (TPEs), largely dictate their respective resistances to changes in dimensions and other physical properties on exposure to various fluids. Therefore, great care must be exercised in selecting an appropriate TPE for applications involving contact with fluids, to ensure that the required performance and durability will be achieved.
This paper discusses the relative native resistances of various commodity and specialty TPEs to selected petroleum and aqueous based fluids. It also demonstrates that these native resistances to a specific fluid or combination of fluids may be altered by alloying compatible polymers, producing specialty TPE compositions with unique dimensional and property-retention behavior, which may be optimized for a given application.
[edit] Introduction
As a rule, rigid thermoplastics only have useful properties at temperatures below either their glass transition temperatures or their crystalline melting points. The extremely dense frozen crystalline melt or glass structure of rigid plastics effectively prevents penetration by most fluids that would otherwise solvate these materials. Hence, ordinary high-density polyethylene is typically used for gasoline and oil containers and isotactic polypropylene can be used for chemical beakers. With the exception of some very aggressive organic solvents, those specifying most rigid plastics for applications, which may involve fluid exposure, can often safely ignore fluid resistance as a specified requirement.
Thermoset rubbers are largely amorphous homo-polymers or random copolymers. They only function as rubbers above their glass transition temperatures. Thermoplastic elastomers are similar to their thermoset counterparts in that they contain large amorphous rubbery components, which are the soft segments or phases of block copolymer and two-phase systems, respectively. These non-crystalline elastomers and elastomeric components are much more vulnerable to fluid penetration and attack than crystalline, rigid plastics.
Failure to consider fluid resistance when specifying a TPE for a fluid-contact application can have serious consequences. The TPE part having a perfect match to the required appearance and initial physical properties, quickly and dramatically may sustain unacceptable changes in dimensions and physical properties after fluid exposure, leading to total part failure (Figures 1 & 2).
The first photo shows the effect of immersion in IRM 903 Oil for 70 hours at 100°C on the dimensions of three test specimens. The center specimen showed no change, the left specimen exhibited significant shrinkage and the specimen on the right showed severe swelling and distortion. The photo in Figure 2 shows an extreme reaction to the same fluid exposure, comparing a specimen unchanged by the exposure with one that was completely solvated, leaving nothing but a precipitate in the immersion tube.
[edit] Theory and Definitions
Fluid resistance is often grouped with chemical resistance. This paper focuses on the physical effects of various fluids on the properties of TPEs. No intentional chemical degradation is involved. The most end-use significant of these effects include volume change and related changes in hardness and stress/stain properties. Fluid resistance is strongly related to solubility, with classes of fluids having commensurately greater effects on chemically similar polymers.
Most TPEs derive their rubber properties from a combination of one or more hard plastic phases with one or more soft elastic phases. Only one of those components need be vulnerable to a particular fluid to render it unsuitable for service. For example, for Thermoplastic Vulcanizates (TPVs), which are typically comprised of a crystalline, isotactic polypropylene hard phase, and a highly-crosslinked, amorphous, thermoset rubber soft phase, the latter phase normally determines fluid resistance. In block copolymer TPEs, such as thermoplastic polyurethane (TPU) and hydrogenated styreneolefin-styrene tri-block TPEs (HSOS), the amorphous olefinic soft segment usually is the more vulnerable component.
In fully compounded TPEs, the presence of certain compounding ingredients also may impact resistance to specific fluids. For example, highly water-soluble inorganic additives and hygroscopic fillers may influence dimensional change in aqueous fluids. Conversely, organic-fluid-soluble plasticizers may be extracted and/or replaced by some non-aqueous immersion fluids.
As shown in the equations at the right, starting with all of the plasticizer in the TPE (1), there are a number of possibilities for physical interaction with the immersion fluid. Where the TPE has very high resistance to swelling in the immersion fluid (2), virtually all of the soluble plasticizer may be extracted, resulting in significant shrinkage of the TPE specimen and attendant increases in stiffness and hardness.
In cases where the TPE has a tendency to swell in the immersion fluid, there are two options. Equation (3) applies when the plasticizer has nearly equal solubility in the TPE and the immersion fluid. An equilibrium may be established, wherein some of the immersion fluid is exchanged for some of the extracted plasticizer. Equation (4) illustrates the instance where the plasticizer is more soluble in the immersion fluid and is completely replaced by it in the TPE.
The latter two cases will typically show increases in volume after immersion, however, the resultant effects on changes in physical properties will depend upon the relative ability of the plasticizer and the immersion fluid to plasticize the TPE.
1. Initial Equilibrium Between Immersion Fluid and Plasticizer: TPE Compound + Plasticizer ↔ Immersion Fluid
2. Total Extraction of Plasticizer into Immersion Fluid: TPE Compound → Immersion Fluid + Plasticizer
3. Partial Exchange of Immersion Fluid for Plasticizer in TPE: TPE Compound + Plasticizer + Immersion Fluid ↔ Immersion Fluid + Plasticizer
4. Total Exchange of Immersion Fluid for Plasticizer in TPE: TPE Compound + Immersion Fluid ↔ Immersion Fluid + Plasticizer[edit] Description and Application of Equipment and Processes
The scope of our full study determined the effects of total immersion on the original physical properties of nineteen commercial and developmental thermoplastic elastomers in eight aqueous and petroleum-based fluids, all of which are routinely encountered in typical service environments.
After completing separate determinations of the effects of immersion time and of immersion temperature on the physical effects of selected TPEs in IRM 903 Oil, 70 hours was selected as the immersion time to generate the bulk of our data. This time interval permitted two sets of immersion tests per workweek, with results entirely consistent with longer time exposures. The 70-hour immersion time is also in agreement with that of the ASTM D-2000 system for establishing the fluid resistance of commercial thermoset rubbers. An immersion temperature of 100°C was chosen as the best compromise to accelerate results for all nonvolatile fluids, considering the wide range of heat resistance among the TPEs tested. Specimens were immersed in the highly volatile and flammable fuels at 23°C.
The process used to evaluate TPE fluid resistance followed procedures set forth in ASTM D-471, “Standard Test Method for Rubber Property – Effect of Liquids.” ASTM D-471 specifies test specimen dimensions, the conditions of immersion and the methodology for preparing the immersed specimens for subsequent measurement of changes in volume, mass, 100% modulus, tensile strength, ultimate elongation and hardness, all performed at 23°C and 50% R.H. on specimens cooled to 23°C.
Measures are provided to ensure that these tests are completed without compromising the results through volatile losses of the immersion fluids from the test specimens. For example, specimens immersed at elevated temperatures in non-volatile fluids are allowed to drip, while being cooled to room temperature in a draft-free enclosure for 30 minutes and are then blotted dry before further physical testing.
For specimens immersed in volatile fluids at room temperature, weight and volume change measurements (ASTM D-471) must begin no more than 30 seconds after removal from the fluid. Stress-strain (ASTM D-412 – Die C specimens) and Durometer Hardness (D-2240) measurements must be initiated not less than 2 or more than 3 minutes after removal from the fluid.
The full study generated far more data than can be addressed in this paper. To capture the essence of this work, the list of TPEs was pared down to ten (Table I), the list of fluids to six (Table II) and the monitored properties to four (Table III).
The first tested material in Table I — CEAA — is a fully compounded, Chlorinated Ethylene Acrylic Alloy based meltprocessible rubber (MPR). Invented by Du-Pont, it is now commercially available from Advanced Polymer Alloys (http://www.APAinfo.com) under the trademark Alcryn®. CEAA is a highly-polar, specialty thermoplastic rubber, generally regarded as having broad fluid resistance.
The next four TPEs are alloys of CEAA with compatible polar TPEs and TSEs (A1, A2, and A3 on Table I), to demonstrate the degree to which inherent further alloying can modify fluid resistance. FPVC-A3 is a flexible plastic with rubbery properties. It was included in the test to examine the impact of the A3 TSE alloying resin on the fluid resistance of second plasticized polar polymer.
The remaining four TPEs are commercial representatives of TPU, TPV, HSOS, and the high-temperature-resistant ETPV.
Thermoset elastomers in general use, experiencing volume increases no more than 10% to 20% after elevated temperature immersion in IRM 903 Oil (ASTM D-2000 Classes J and K) are considered to have very-good to excellent oil resistance. Shrinkage of greater than 20% can seldom be tolerated for most practical elastomer applications. Consequently, in this paper, the fluid resistances of the tested TPEs were rated versus an arbitrary target of limiting changes in their dimensions and key physical properties after immersion to within ± 20% of original values.
[edit] Data and Results
[edit] Relative TPE Performance in Various Fluids
Following the list of property changes (Table III) resulting from immersion in each test fluid, the first bar on the graph indicates volume change, which reflects swelling for some materials and shrinkage for others. The remaining bars show changes in stiffness, tensile and hardness, respectively.
The content and analysis for the first fluid will be gone over in detail, and serve as a template for each of the remaining five fluids. Subsequent fluid data will have the same content and will follow the same bargraph format. For those fluids, observations and conclusions will simply be summarized, with reference to the associated Figure number.
[edit] IRM 903 Oil
IRM 903 Oil is classified as a naphthenic oil and is a laboratory reference standard for automotive engine lubricant Figure 3 shows the effects of immersion in this oil, for 70 hours at 100°C, on the dimensions, modulus, tensile strength and hardness of our ten TPE compositions. (Note that hardness change here is expressed in percent, not points, to be consistent with the Y-axis scale for all other property changes.)
Using the above criteria, all of the compositions tested based on CEAA are shown to have good resistance to swelling in lube oil, as do the other polar materials – the PVC–A3 and TPU. The less-polar ETPV and the non-polar TPV and HSOS TPEs show much greater tendencies to swell on exposure to IRM 903 Oil.
After immersion, the base CEAA volume increased <20% and all measured properties showed changes below ± 20%. The A1 alloy showed slightly lower swelling and hardness loss than the base CEAA, but was not a significant improvement.
However, the A2 alloy represented a major reduction in change in volume and every other measured physical property. The CEAA-A3 alloy demonstrates the effects of alloying with a material that is so oil resistant that all of its plasticizer is extracted and not replaced by the immersion fluid. (See equation (2) above.) The results were shrinkage and attendant undesirable increases in stiffness and hardness.
The A3A alloy showed that the shrinkage can be reduced to less than 10% by further compounding, but property changes still were not improved over the basic CEAA.
The FPVC-A3 showed much higher plasticizer extraction than the CEAA-A3, accompanied by a huge increase in stiffness and hardness, providing strong evidence that the A3 additive will follow the same mechanism for all plasticized polar polymers.
For non-homogeneous systems, like ETPV, TPV and HSOS, property changes other than volume appear to depend on the fluid resistance of the hard phase or block. The highly crystalline polypropylene hard phase of the TPV is likely responsible for the observed low percentage changes in stiffness and strength properties, despite a relatively high volume increase.
The ETPV, with an undisclosed hard phase, showed lower swelling than conventional TPV, but a greater loss of tensile strength. Conversely, the more weakly associated polystyrene hard domains of the HSOS appeared to be largely destroyed by high absorption of the oil, as reflected in almost total loss of all desirable physical properties.
[edit] Diesel Fuel
Diesel fuel is more aromatic than IRM 903, resulting in all ten TPEs showing increases in volume after the 70-hour immersion (Figure 4). The volatility and flammability of this fluid dictates that the immersion be at room temperature.
This time the basic CEAA showed acceptable volume change, but stiffness dropped below the ±20% goal. Alloys of CEAA with polymers A1, A2, and A3 demonstrated increasing incremental improvement in diesel fuel resistance, however, modification A3A did not outperform A3, as it did in IRM 903 Oil.
The FPVC-A3 flexible plastic also performed well, but the TPU performance was outstanding; providing virtually no change in volume or any of the tested physical properties after immersion.
The volume and property changes of the final three TPEs all fell outside the ± 20% goals for volume change and tensile strength. The HSOS also suffered major losses in stiffness and hardness.
[edit] Kerosene
Kerosene is less aromatic than diesel fuel, resulting in a similar post immersion picture, but with all TPEs exhibiting lower volume increases and the two CEAA-A3 alloys showing very slight shrinkage (Figure 5).
Again, all of the CEAA alloys, the PVCA3 and the TPU were highly resistant to this volatile fuel, while the two TPVs and the HSOS exhibited unacceptable changes in all properties.
[edit] Gasoline
Gasoline, as defined in this study was ASTM Reference Fuel A (isooctane), which is a pure branched aliphatic hydrocarbon, and therefore not aromatic by definition. As seen in Figure 6, volume increase values are generally lower than for kerosene, with the two CEAA-A3 alloys showing shrinkage. Again, the CEAA and its A1 and A2 alloys met the target limit of ± 20% for changes in volume and all physical properties.The TPU showed the least effects of the immersion on original volume or physical properties. The HSOS again suffered high swelling and catastrophic losses in all properties. The two TPVs behaved differently, but both exemplified poor gasoline resistance. The conventional TPV was far above the target volume increase and change in tensile strength of the ETPV fell well below -20%.
[edit] Gasohol
Gasohol is created by adding 10% ethanol to gasoline. This blend is mandated for auto fuels in many states for a number of environmental and political reasons, therefore, any TPE likely to encounter gasoline in-service must also show adequate resistance to gasohol.
The ethanol addition resulted in major shifts in fuel resistance among the test subjects, as shown in Figure 7.While ethanol is a clean-burning fuel, it is highly polar and chemically quite unlike isooctane. It had a profoundly negative effect on the properties of the TPU, which was outstanding in straight gasoline, rendering it a questionable candidate for all applications requiring general resistance to gasoline.
There were subtle differences in behavior for gasohol versus gasoline for the CEAA and its four alloys, as well as for the PVC-A3 flexible plastic, but all property changes remained well within the ± 20% target.
For reasons not entirely understood, the blend of gasoline and ethanol has a more severe effect on many TPEs and thermoset rubbers than either component alone. That appeared to be the case here for ETPV and to a lesser extent HSOS. The TPV remained above 20% in volume change, but all physical property changes were well controlled.
[edit] Antifreeze
Antifreeze — 50:50 Water/Ethylene Glycol — is another common, highly-polar automotive fluid. With no safety issues, immersion was for 70 hrs at 100°C. This time, in addition to the same major effect on TPU seen when ethanol was added to gasoline, excessive property changes were seen in CEAA and CEAA-A2 suffered a severe loss of properties (Figure 8). However, the A1 and two A3 alloys of CEAA continued to fall into the target range for property change, as did the PVC-A3.
In this aqueous/alcohol environment, the excellent performance of both TPVs and the HSOS stand out, meeting all property change targets and exhibiting less property change than the rest of the test group.
[edit] Interpretation of Data
The significance of fluid-contact induced physical property changes depends a great deal on the exact exposure conditions of the intended fluid-contact application. Thus, the results seen in our immersion tests may not be valid for applications involving brief external contact with highly volatile fluids, because the fluid may simply evaporate without causing significant changes in dimensions or other physical properties.
However, our results would apply to applications involving similar contact with non-volatile fluids, wherein those TPEs, which were seriously softened and weakened in our study, should be avoided. This category would include ergonomic grips and other overmoldings encountering frequent contact with fuels and lubricants, found on factory equipment, mechanics power-tools, and all gasoline-powered tools and portable devices.
Clearly, applications for which dimensional change in contact with a specific fluid is critical to functionality, such as physically constrained compression seals, volume change is the single most important consideration. For such sealing applications, slight swelling is preferred over shrinkage, and may even be preferred over zero volume change. Rubber gaskets sealing wide-tolerance metal-to-metal components will better fill the void and ensure a liquid-tight seal if they swell slightly from absorbing some of the contacted fluid to create increased pressure on the sealing surfaces.
However, our study showed that attendant changes in physical integrity properties varied widely among TPEs meeting the same volume change criterion. For a host of other applications, including contact and lip seals and gaskets, dimensional change may be important, but less critical than changes in stiffness, strength, and hardness.
Retention of physical properties after immersion appears to depend largely on how much of the fluid is absorbed and/or how much plasticizer is extracted by the immersion fluid. Fluid gained usually acts as a plasticizer, resulting in losses in stiffness, strength, and hardness. Plasticizer extraction typically results in undesirable increases in stiffness and hardness. TPE parts which are stretched when installed could experience splitting or cracking in service, if shrinkage forces become very high. Increases in tensile strength are acceptable. Significant losses in Tensile signal a basic erosion of the integrity of the TPE structure by the immersion fluid.
[edit] Conclusions
- Using the rating criteria adopted for this study, the CEAA MPR appears to have broad fluid resistance. That intrinsic resistance can be further improved and tailored for specific fluids by knowledgeable alloying with appropriate compatible polymers.
- TPU performed very well in all of the non-polar fluids, but showed major losses of physical properties in polar fluids (gasohol and antifreeze).
- TPV performed very well in antifreeze, but exceeded the volume change target in all non-aqueous fluids.
- ETPV showed lower volume changes than TPV in most tested fluids, but with higher negative impact on physical properties (especially gasohol). On balance, it showed no advantage over conventional TPV for service in any of the fluids tested.
- HSOS only performed well in antifreeze. In non-aqueous fluids, high volume changes were accompanied by virtual total loss of physical integrity.
- It is probably safe to conclude that there is NO cost-effective, universally fluid resistant TPE
- The fluid resistance of TPEs is a complex, multi-dimensional property that cannot be defined solely by relative volume change values after immersion.
- Important strength and elastomeric properties are often dramatically impacted by fluid exposure and the correlation of the degree of that impact and volume change is often not linear
- Therefore, those specifying TPEs for any application, wherein the TPE part may be exposed to one or more fluids, must understand the nature of this contact, factor it into their TPE selection, and make properties after fluid exposure a critical part of the material specification.
[edit] Notes
Article written by: J. Eric Ingram and W. Robert Abell of Advanced Polymer Alloys - A Division of Ferro Corp.
Alcryn® is a registered trademark of the Ferro Corporation.
[edit] External Links
This article can be downloaded as a PDF at http://www.apainfo.com/pdf/APA_2005_TOPCON_paper.pdf
[edit] Internal Links
