Test Methods

Compression set is defined as the permanent height reduction caused by compression under specified conditions of load, temperature, and time. The parts are measured before and after the test period to ascertain compression set, which is calculated as a percentage of the original thickness, or as a percentage of the compressed distance. The two equations are given below.

Compression as a percentage of the original thickness:

% = [(t_o – t_f)/t_o] x 100,

Compression as a percentage of the compressed distance:

The two ASTM tests commonly used for FOF gaskets are ASTM D3574-95 and ASTM D1056-98. ASTM D3574 is the standard method for testing open-cell cores, such as urethane foams, and prescribes 50% compression of the gasket for 22 hours at 70°C, followed by a 30-minute uncompressed cooling period. Compression set is reported as a percentage of the original thickness. ASTM D1056 is the standard method for testing closed-cell cores, such as neoprene, elastomer, and silicone foams and rubbers, and likewise prescribes 50% compression of the gasket for 22 hours, but at 23°C, followed by a 24-hour uncompressed cooling period. The set is reported as a percentage of the compressed distance.

Because FOF gaskets usually fall below the minimum sample dimensions specified for these tests, a modified test using the same setup conditions can determine compression set based on the true dimensions of the gaskets rather than the required ASTM sizes.

All pliable materials have compression set values. The objective in constructing an FOF gasket is to minimize compression set to optimize long-term performance. Type of foam, gasket size and shape, test temperature, duration of the test and cooling period, and calculation method all need to be explored to understand compression set in FOF gaskets.

Dimensional Impact

There is an inverse relationship between gasket dimensions and compression set. As a result, a gasket with a height and width of 0.5 in. will take a lower set than one with 0.125-in. dimensions. But which dimension is the best predictor of compression set—height, width, or a cross-sectional combination of the two? A recent study of 15 gasket samples with heights and widths ranging from 0.04 to 0.75 in. provided some interesting insights into the phenomenon of compression set. All parts were made with flame-retardant 4# urethane foam, which is specified by Underwriters Laboratories for V0-rated FOF gaskets. The 15 parts were tested for compression set using modified ASTM D3574 test conditions.

Figure 1 shows compression set results of each of the 15 samples plotted against its height, width, and cross-sectional area (cross-sectional area should be plotted in square inches, but was plotted on the same scale for comparative purposes). A logarithmic best-fit curve was then added to each of the three data sets. To better understand the success of the fit, a correlation coefficient (R²) was then calculated and placed next to each curve. An R² value of 1 is defined as a perfect fit, whereas a value of 0 shows no similarity between the data and the best-fit curve.

Figure 1. Modified ASTM D3574 22-hour compression set versus gasket height, width, and cross-sectional area (V0 4# urethane foam).

The dimensional measurements all gave a similar response related to their impact on compression set. As the parts got taller and wider, the resulting compression set got smaller.

Interestingly, height was the least reliable indicator of compression set, with an R² value of 0.09. At lower thicknesses, for example, compression set values range from 5 to 25%. The data do not consistently adhere to the trend line.

Gasket width resulted in an R² value of 0.79, a much better fit. The data consistently fall close to the trend line, making width a much better predictor of compression set. The cross-sectional area resulted in an R² value of 0.56, predictably falling between the values for height and width.

Gasket Shape

The same data shown in Figure 1 were separated by gasket shape and plotted versus the gasket width in Figure 2. Squares, rectangles, and Ds are three of the most common shapes of FOF gaskets. A best-fit logarithmic curve was again superimposed on the data. All three shapes displayed the same relationship, with compression set decreasing as the width increased. Note that the square parts, which get taller at the same rate they get wider, have the steepest slope. This is not necessarily the case with the Ds and rectangles. As shown in Figure 1, as the parts get both wider and thicker, compression set on average improves. A rectangle and a D can get wider without getting thicker, resulting in a smaller slope to the curve.

Figure 2. Modified ASTM D3574 22-hour compression set versus width by gasket shape (V0 4# urethane foam).

Test Duration

ASTM prescribes 22 hours as the normal period for gasket compression, followed by a timed cooling-off period. But FOF gaskets must be able to withstand compression for more than 22 hours. Longer-term compression set can impair electrical contact between the gasket and the mating surfaces, adversely affecting the z-axis conductivity of the gasketed seam.

To explore the effects of additional time in compression, the test was extended on several parts. Following the initial 22 hours, samples were remeasured at 2-day intervals, with the resulting long-term compression set plotted against time in Figure 3. After 7 days, the slope of the graph had approached zero, ending the study. With the urethane foam, only 60% of the long-term compression set occurs within the first 22 hours. All foam types do not exhibit the same trends, so foam selection is critical to the long-term performance of a gasket.

Figure 3. Modified ASTM D3574, average compression set versus time (average taken on gasket widths of 0.125, 0.25, and 0.5 in.).

Temperature

Many electronic devices generate heat, so for these applications, gaskets must be able to perform at elevated temperatures. What effect, if any, do these higher temperatures have on both short- and long-term compression set? Two groups of gasket samples with urethane foam cores in widths of 0.125 and 0.375 in. were compressed 50%, one group using the modified ASTM D3574 test at a temperature of 70°C, the other at ambient conditions of 23°C. The data in Figure 4 plot the effects of temperature.

Figure 4. Modified ASTM D3574, average compression set versus time by temperature (average taken on gasket widths of 0.125 and 0.375 in. using V0 4# urethane foam).

After the standard 22-hour period, the samples at 70°C had taken a set of more than 4.5 times that of the samples at ambient temperature. However, when left compressed for 7 days, the ambient samples took a set of approximately 65% of the samples compressed at 70°C. Elevated temperatures affect both the magnitude and rate of compression set, with the greatest difference noted after just 22 hours. Based on these observations, it is clear that temperature must be taken into account when evaluating the compression set performance of the foam.

Compression Force

Several types of foam are used as cores in FOF gaskets, including urethanes, elastomers, silicones, and neoprenes. Each type offers different characteristics in terms of stiffness and resistance to compression set. These characteristics are based on the material's structural members, cell geometry, and type of cell window. The nature of the structural members includes the foam chemistry, polymer arrangement, density, and thickness. Cell geometry refers to the cell size, usually expressed as pores per inch (ppi), and cell size distribution. In terms of cell windows, foams are classified as open cell (urethanes) or closed cell (neoprenes and elastomers).

Often, foam is selected strictly on the basis of stiffness, also referred to as compression force, compression load deflection, or force displacement. Typically, this parameter is expressed either as the force required to compress a unit length of gasket to a specific compression height (e.g., 44 lb/ft at 0.150 in.), or as the force to compress it to a specific percentage of its free height (e.g., 10 lb/in. at 20% compression). In application, it can be described as the force that the gasket exerts on the enclosure to make electrical contact.

Figure 5 is a compression force plot for a sample of four different foams used in FOF gaskets. The curves show the force required to compress the foams a given percentage. All of the samples were 0.25 x 0.25 inch square profiles. They were made of 4# urethane, two varieties of thermoplastic elastomer (TPE), and a neoprene.

Figure 5. Force displacement graph of 0.25-in square profile by foam construction.

Two key differences emerged between these foams. The first was compressibility. The open-cell urethane foam compressed 80% of its original height, whereas the closed-cell neoprene compressed only 50%. This difference has significant design implications for an EMC engineer.

The second difference was the force required to compress the gaskets a given percentage. A common compression target in FOF design is 40%. With the urethane foam gasket, a force of 1 lb/in. was required to compress the part to the target. To achieve the same compression on the neoprene part, a force of 4 lb/in. was required. FOF parts come in all shapes, sizes, and lengths, so the compression factor must be considered when selecting the best foam core for a given application.

Compression Set by Foam Type

As shown in Figure 5, the nature of the structural members, the cell geometry, and type of cell wall all affect compression force. These properties also significantly affect compression set. Three gasket samples of the four foam types in widths of 0.125, 0.25, and 0.5 in. were tested, again using the modified ASTM D3574 parameters. The average compression set was taken on each group and plotted over time for 1 week.

From the data in Figure 6, it is clear that different types of foam range widely in terms of compression set performance. All three of the closed-cell foams took significantly higher sets than the open-cell urethane. The neoprene samples, which had the worst performance of the four, took a 45% long-term set. With the gaskets being compressed only 50%, the neoprene core lost 90% of its compressed thickness.

Figure 6. Modified ASTM D3574, average compression set (average taken on gasket widths of 0.125, 0.250, and 0.5 in.).

Another point to note on the closed-cell foams is the amount of set taken in just 22 hours. In looking at the neoprene sample again, 94% of the long-term set was taken in the first 22 hours. So not only do the closed-cell foams take a higher compression set, they take it much more quickly. Because of the performance of the closed-cell foams in the ASTM D3574 test, their compression set values are often listed as a result of ASTM D1056, the standard compression-set test for closed-cell foams.

In ASTM D1056, samples are allowed to sit uncompressed for 24 hours prior to taking the final thickness measurements, compared with just 30 minutes in ASTM D3574. Figure 7 plots the data by foam type, listing the compression set after a 30-minute and a 24-hour cooling period in each of the ASTM tests. The compression set values are averages for gaskets with widths of 0.25 and 0.5 in. Although temperature had a greater effect on compression, the extended cooling period also had an impact, reducing compression set by as much as 8% in the standard 22-hour tests.

Figure 7. Extended-cool effects on compression set (average taken on gasket widths of 0.25 and 0.5 in.).

The one difference that actually has a negative effect on compression set values in the ASTM D1056 test method is the calculation formula. As noted, compression set is the percentage of original thickness in ASTM D3574 and the percentage of the compressed distance in ASTM D1056. Typical 50% deflection during testing basically doubles the calculated values. But as Figures 8 and 9 show, the lower temperature and extended cooling period more than offset the increase due to the calculation method.

Figure 8. Modified ASTM D3574 22-hour compression set results versus width by foam type.

Three samples of each foam type with common widths of 0.125, 0.25, and 0.5 in. were tested using both methods. Figure 8 shows the compression set results of the four foams by width in the ASTM D3574 test. Figure 9 shows the results when the same-size samples were run using the setup conditions in ASTM D1056.

By using the same y-axis scales, it becomes evident that the results in ASTM D1056 compare quite favorably with those in ASTM D3574. On average, there is a 2x–4x-factor reduction in compression set when comparing identical parts from both tests. When reporting results based on ASTM D1056, closed-cell foams gain the most in terms of magnitude of compression set improvement. By switching to the ASTM D1056 test conditions, the urethane foam samples improved an average of 7%, the TPEs 12%, and the neoprene samples 25%.

Figure 9. Modified ASTM D1056 22-hour compression set results versus width by foam type.

Because of the significant difference of these results, it is important to identify the test method when comparing compression set values. A quoted 5% compression set performance on a wide part tested using the ASTM D1056 method may not be that impressive.

Indeed, it would be more appropriate to select the test method based on the application rather than the type of foam. Some shielding applications are run at elevated temperatures and require a quick recovery when uncompressed. The closed-cell foams may not be the best option for high-temperature applications.

It should also be noted that all urethane foams used in V0 FOF gaskets do not perform the same. To obtain the UL V0 rating, flame retardant (FR) characteristics must be built into the gaskets. These additions are often made to the foam itself. FR additives in the foam typically have a negative effect on its natural compression-set properties.

Suppliers will even add metallic particulate to their foams to increase the FR properties. Figure 10 shows the different compression-set performance between FR urethane foams both with and without the particulate additives. The average compression set is plotted versus time for a group of five gaskets of various sizes. The negative effect the additives have on the long-term performance of the foam can clearly be seen.

Figure 10. Modified ASTM D3574 average compression set versus time by urethane foam type.

Compression Set in Application

It is vital to quantify the effects of compression set in FOF gaskets in real-life performance tests. A 500-hour z-axis resistivity test, for example, can yield valuable projected data. In the test, 1-in. gaskets in widths of 0.5 in. were compressed to 50% of their original heights in a fixture between two stainless-steel plates. Inside the fixture, the gaskets were the only electrical contact between the plates. The test fixture was held at 65°C for a period of 500 hours. The z-axis resistance between the plates was measured at approximately 50-hour intervals over the duration of the test.

Figure 11. The 500-hour z-axis resistivity test (50% compression at 65°C on 0.2 x 0.5 in. rectangular gaskets).

Figure 11 shows a plot of two gasket samples, one with a urethane core, and the other with a neoprene core. As the foam begins to take a set, electrical contact between the gasket and the mating surfaces is reduced. This negative effect on the z-axis conductivity of the gasketed seam can lower the shielding effectiveness of the gasket. The deficiencies of the neoprene core led to a significant and steady increase in resistance measured between the plates. The urethane core, however, allowed the gasket to maintain steady contact.

Conclusion

Notwithstanding inherently strong electrical performance, FOF gaskets must use cores properly suited for their applications. The compression set characteristics of the foam core have been shown to be equally important in sustaining effective long-term shielding performance. There is more to understanding compression set than just the number itself. Foam type, gasket size and shape, test temperature, duration of the test and cooling period, and calculation method all must be taken into account when assessing compression set in FOF EMI gaskets.

Kevin Hug is a technology manager for Laird Technologies (St. Louis, MO). He can be reached at khug@lairdtech.com.