Engineers and maintenance personnel often are faced with the question of elastomeric compatibility when heat transfer fluids are considered. This consideration may arise during the design of new equipment, or while selecting a thermal fluid or new seal for existing plant equipment. Material compatibility also may become critical in the investigation of leaks and other equipment failures.
The overarching concerns are maintaining equipment integrity and system performance over the applicable operating temperature ranges. Ultimately, these relate to personnel safety as well as potential environmental liabilities: Improperly selected elastomers can lead to equipment failure, bodily harm and environmental liability.
The multiplicity of chemistries and thermal fluid types in the market complicates decision-making with respect to elastomeric compatibility. Even though the mechanics of materials and polymer science are mature disciplines, there is a dearth of literature directed specifically at the applicability of commonly available elastomeric materials in heat transfer fluid service. Where it exists, the available resource is sometimes generic or targeted at industrial lubricants.
This article contributes to bridging the existing gap. To do justice to this subject, a brief overview of thermal fluid typology is necessary for a deeper appreciation of the complexities that heat transfer fluids present with respect to material compatibility.
Reviewing Heat Transfer Fluid Types
In general terms, heat transfer fluids can be categorized based on a few operational and compositional criteria. These include base chemistry, operating temperature range and application type. As will be shown in the subsequent sections of this article, the first two criteria have the most influence on the suitability of elastomeric materials for a particular heat transfer fluid service.
Table 1 shows a general classification of thermal fluids. Base chemistry refers to the main componentry of a heat transfer fluid. The operating temperature range speaks to the maximum and minimum use temperatures of a thermal fluid.
Generally speaking, heat transfer fluids can be formulated from relatively few base chemistries. These include:
- Refined petroleum-based mineral oils.
- Aromatic compounds such as partially hydrogenated terphenyls and alkyl benzene, di-phenyl/biphenyl oxide.
- Glycols such as ethylene glycol and propylene glycol.
- Polyalkylene glycol.
- Silicone fluids (polydimethylsiloxane).
- Molten salts (e.g., sodium nitrate or potassium nitrate), though these are used to a lesser extent.
The variety of fluid types translates into varying levels of permeation (or attack) when various elastomers are deployed. This diversity necessitates a knowledge of the applicable material-compatibility requirements for commercially available elastomers.
Furthermore, chemical resistance data for common materials of construction often is reported at ambient temperature. Operating conditions, however, deviate from this baseline temperature in most cases.
Elastomeric swelling and loss of strength can be exacerbated at high temperatures. Temperature-sensitivity considerations, therefore, must be kept in mind when heat transfer fluids are deployed in high temperature or ultra-low temperature applications.
Selecting an Elastomer for Your Heat Transfer Fluid
Elastomers are polymers that exhibit viscoelastic properties. They are flexible, resilient and can convert mechanical energy to thermal energy under deflection. By virtue of these properties, elastomers often are used as seals — in the form of O-rings and gaskets — in process and manufacturing equipment such as pumps, valves, air compressors and piping.
Typical elastomers include tetrafluoroethylene propylene, nitrile rubber, silicone, neoprene and fluorosilicone. Often, seals made from these materials come in direct contact with thermal fluids in static and dynamic applications. The polymeric materials from which they are made can be susceptible to permeation and attack in heat transfer fluids service, however. Incompatible chemistries can cause seals to swell, shrink, blister or delaminate, leading to leaks.
Engineers and maintenance personnel also must keep thermal fluid degradation and the resultant byproducts in mind when making compatibility determinations. With air exposure, many thermal fluids oxidize and eventually form a dark carbonaceous sludge — a mixture of aldehydes and peroxides. In other scenarios, the overheating of a thermal fluid beyond its maximum bulk temperature results in thermal cracking — the splitting of the fluid composition into fractions of low boilers and high boilers. The high boilers are high molecular weight compounds. In dynamic applications where there is repetitive or fluctuating loading, these adverse reactions can affect elastomer performance and, eventually, result in seal failure.
This discourse on the material compatibility of thermal fluids would be incomplete without a commentary on the typical mechanism of attack in elastomeric seals. Degradation and depletion occur in accordance with the solution-diffusion model. This model borders on a well-established mechanism of polymer membrane solvent transport. Permeants — in this case, heat transfer fluid molecules — dissolve in the membrane material and then diffuse through the membrane. In other words, the thermal fluid is adsorbed on the surface of an elastomeric seal and absorbed based on selective solubility. The permeants diffuse through polymeric channels and the interstitial spaces in the direction of a concentration gradient. As permeation progresses, the swelling of the elastomer and its volume change begin. This is followed by the plasticization of the elastomer’s polymer strands, which causes the elastomer to eventually lose its viscoelastic and damping properties.
A suitable elastomer for any particular heat transfer fluid type therefore must be able to mitigate the adsorption and absorption of solvent molecules. The elastomer also must be able to limit permeation through its interstitial spaces. To provide decision-makers with an easily understandable qualification rubric, seal manufacturers often rate the performance of elastomers based on their ability to perform these two functions. In table 2, the compatibilities of several elastomers with different heat transfer fluid types are presented.
- An ‘A’ ranking represents excellent resistance to the thermal fluid and minimal volume change (less than 10 percent). An elastomer with that ranking will be considered an optimal choice for the corresponding heat transfer fluid type.
- A ‘B’ rating is interpreted as good resistance and a minor swelling effect (between 10 and 20 percent).
- A ‘C’ ranking represents fair resistance and significant swelling/volume change (between 20 and 40 percent).
- A ‘D’ rating is given to elastomers that offer poor resistance to thermal fluid attack. A severe effect or degradation is expected, and the volume change is likely to be greater than 40 percent. Because of this significant swelling and change in physical properties, the elastomer in question is considered unsuitable for the specified heat transfer fluid service.
As shown in table 2, synthetic heat transfer fluids are quite constrictive. Severe swelling and degradation can occur if a poor material selection is made. Perfluoroelastomer (FFKM) and fluorocarbon (FKM/FPM) are the most forgiving with synthetic heat transfer fluids. In the same vein, ethylene propylene (EPDM) performs poorly when exposed to mineral oil and PAO heat transfer fluids.
Furthermore, silicone heat transfer fluids have an adverse reaction when in contact with silicone elastomers. As such, silicone seals should never be used in a heat transfer system that utilizes silicone thermal fluids. As counterintuitive as this sounds, this solvent-polymer interaction is well understood.
Silicone heat transfer fluids and silicone elastomers have a similar molecular composition, with the crosslink between polymers strands in the elastomer being the main difference. For this reason, there is a similarity in polarity or solubility parameter. Thermal fluid attack on the elastomer occurs according to the solution-diffusion model, causing the solvent molecules to be inserted preferentially into the polymer network until an equilibrium saturation point is reached. This causes deterioration in hardness, tensile strength and elongation.
In conclusion, the importance of ensuring suitable elastomeric compatibility in heat transfer fluid service has been duly emphasized. This case is made based on the practical implications of poorly selected elastomers. Different heat transfer fluid types are considered with corresponding compatibility rankings provided for common elastomeric materials. By providing specific guidance on the match or mismatch between commercially available elastomers and different thermal fluid types, a relevant addition is made to the existing body of knowledge.
Mistakes can be costly. Decision-makers might be wise to consult with elastomer manufacturers and thermal fluid experts when in doubt.