When it comes to sealing, food and beverage equipment manufacturers can’t rely upon one-size-fits-all solutions for even their most basic components. Extreme temperatures, aggressive sterilization practices, longer operating times and flavor transfer issues can wreak havoc on even simple static seals when they are not tested and designed to address such complex environmental factors. In the past, food and beverage companies have used basic change in weight after exposure to fluid testing to evaluate rubber material compatibility. But, that is no longer sufficient today.
Freudenberg-NOK Sealing Technologies isn’t exactly a household name, but it is a global manufacturer of sealing solutions for multiple industries—especially the automotive industry. The materials innovation company, however, is a seal supplier to major food and beverage equipment manufacturers such as Krones-Evoguard, KHS, Rieger and others.
By leveraging its knowledge of comprehensive test methodologies used to evaluate material compatibility and component function in—for example, automotive—the company is establishing new technical standards in the food and beverage industry that are producing successful results. Freudenberg-NOK is currently helping the International Society of Beverage Technologists (ISBT) in developing new guidelines for evaluating polymeric material compatibility in beverages and will be presenting the basis of these guidelines at the annual BevTech conference.
The use of compression stress relaxation (CSR) and dynamic mechanical analysis (DMA) testing practices has helped Freudenberg-NOK solve seal failure issues for clients where one size fits all has been the norm. In one case, the application of DMA testing recently allowed the company to develop highly successful dynamic sealing for a major dispensing equipment manufacturer that specializes in frozen foods and beverages.
To learn more about CSR and DMA testing techniques, FE asked Freudenberg-NOK’s Rory Pawl, PE, who has an intimate knowledge of these technologies, to fill in some details.
FE: What is CSR testing and how does it work?
Rory Pawl: CSR testing per ASTM D6147 (the standard compression test method for vulcanized rubber and thermoplastic elastomers) is the single most important test in determining how a material will perform long term in a static application. CSR shows how an elastomer’s sealing force decays from its original compressed state when exposed to air or a requested media type (e.g., soft drink syrup or CIP/SIP) at an application-specific temperature.
Typically, an elastomer’s retained force will rapidly change within the first 168 hours or so due to its mechanical structure, and then change thereafter at a slower rate due to material compatibility with the media and temperature. In the test, an ASTM Test Button is typically compressed 25 percent, and an initial resulting force from the elastomer to the fixture is taken. Results of the retained force are taken at intervals of 0, 72, 168, 336, 504 and 1,008 hours.
The resulting data points create a curve, which gives a good indication as to how a material will perform in the application, with ideal results being a high final sealing force as well as a consistent “plateauing” retained force over time. [Editor’s note: See graph.]
CSR differs from compression set testing in that compression set only provides feedback on the dimensional change of the elastomer, not the elastomer performance. CSR provides more meaningful data that can be used to predict long-term performance. As an example, if a decision for a particular material was made after only 72 hours, then Material 2 would be chosen over Material 1. But when reviewing the residual sealing force over 1,008 hours through CSR, it is clear that Material 1 is actually superior to Material 2’s performance.
If a 1,008-hour compression set test was completed, a similar shaped graph would be created. However, the data gathered on the Y-axis (percentage set) just could not be used to determine if the part will work long term. Hence, the value of CSR is that it provides sealing force measurements, which can be used to predict product function.
FE: How can CSR results be used in developing a new seal technology?
Pawl: Since CSR ensures that an elastomer will perform in the application as designed during its anticipated lifetime, it’s a critical factor in the development of new technologies. This method shows end users what retained force would be necessary to seal the application through the durability period of the application to help them more confidently select their sealing material. CSR results have also been used to improve the FEA (finite element analysis) models to ensure proper design for a particular material when used in the particular test fluid.
FE: What applications benefit from CSR testing?
Pawl: While CSR is beneficial for all static sealing applications (e.g., O-rings and gaskets), it can also be used to get a very good understanding of an elastomer’s long-term compatibility and performance in a particular medium at the needed temperature. As a result, it’s a good choice for a number of food and beverage applications.
FE: Where is CSR testing not the method of choice for food and beverage?
Pawl: The food and beverage industry has yet to adopt one standardized test methodology for analyzing how materials perform in food and beverage applications. Many of our customers are unsure how to test a material in their fluids and often rely upon what they have tried in the past, meaning they repeatedly follow the same legacy material testing methods (for example, volume swell, tensile and elongation tests). With CSR, companies gain more practical and meaningful methods that can dramatically increase their confidence in how the given material will correlate to the actual application.
FE: What is DMA testing, and where should it be used?
Pawl: Dynamic mechanical analysis allows for temperature, frequency and strain sweeps. Using a temperature sweep, we can understand how a material’s stiffness and functionality change as it cools down, with the goal of ensuring rubber stays “rubbery.” In the food and beverage industry, we use DMA for low-temperature dynamic applications to utilize better materials that not only are compatible with the fluids and meet the desired wear performance, but also perform as designed at the low temperatures (e.g., soda dispensers and slush machines operate at or near freezing).
FE: Are there other tests for cold temperature applications?
Pawl: There are other ways to measure an elastomer’s performance at cold temperatures, such as Tg by DSC (glass transition temperature by differential scanning calorimeter), tr10 (temperature retraction by 10 percent), Gehman testing (torsional stiffness measuring degrees of twist) and brittleness (impact testing). DSC is a common low-temperature tool in rubber/seal test labs, but the glass transition temperature value it provides can be difficult to correlate to real-world performance.