Characterizing Silicone Elastomers for Pump Tubing Applications

Medical Device & Diagnostic Industry Magazine
MDDI Article Index

An MD&DI  January 1998 Column

TUBING APPLICATIONS

Knowing the pros and cons of peroxide-cured and platinum-cured silicone elastomers can help manufacturers select the one that best fits their tubing needs.

Tubing has a wide range of health-care applications. It serves as a conduit for blood, chemotherapy drugs, nutrients, fluids, and electrolytes into the body; as a medium for extracorporeal circulation of blood during surgery; and as a means of removing fluids from the body. The most demanding application is peristaltic pump tubing used for circulation of blood, drug filling, and dialysis. Because the pumps subject the tubing to an ongoing endurance test, pump manufacturers require strong, flexible tubing that resists compression set and that offers the necessary level of purity. The tubing must be biocompatible and consistent in size, and it must not become distorted in use, which could cause inconsistent delivery. It must be bubble-free so that flaws in the tubing are not confused with bubbles in the blood. In addition, it must have low surface tension and high resilience. Because silicone tubing meets these requirements, it represents a viable choice for peristaltic pumping applications.

Strong, flexible tubing, such as the items shown here, meets the requirements of pump manufacturers.

This article describes an approach for characterizing both peroxide-cured and platinum-cured silicone elastomers in order to determine their various performance characteristics and physical properties. In using this approach, manufacturers of peristaltic pump tubing will be able to select a formulation that will meet all the key requirements.

SILICONE-CURE SYSTEMS

Silicone elastomers are cured by either of two methods: peroxide curing and platinum curing. Peroxide-cured silicones have the physical characteristics necessary for long life in pumping applications: they hold their shape after repeated compression and thus maintain the consistent diameter needed to deliver a precise measurement of fluid. However, peroxide-cured tubing requires an additional postcure step to remove by-products such as PCBs generated by the decomposition of the peroxide. In contrast, platinum curing provides very low levels of extractables without the postcure step. But the resulting material typically has not stood up to the rigors of pump action as well as peroxide-cured material does.

In the case of peroxide-cured elastomers, when a non-vinyl-specific peroxide is used, the following free-radical cure reactions occur:

(1) 

(2) 

(3) 

In the presence of vinyl-functional species in the polymer, reaction (3) is replaced with reactions (4) and (5):

(4) 

(5) 

For platinum-cured elastomers, a hydrosilylation reaction occurs, which involves the addition of a silicon-hydrogen bond to an unsaturated C-C bond in the presence of platinum (Pt):

(6) 

Both platinum- and peroxide-catalyzed reactions are accelerated by elevated temperatures.

PHYSICAL PROPERTIES OF SILICONE ELASTOMERS

Tables I through III show the typical physical properties for peroxide- and platinum-cured systems at various durometers. For pump tubing applications, the most important physical properties are softness or hardness of the material (durometer), tear strength, and pump life. In general, both peroxide- and platinum-catalyzed elastomers have good tear strength, but platinum-catalyzed elastomers have higher tear strength overall. However, because platinum-cured tubing lacks resilience and loses its configuration after repeated compression by a pump head, it must be used for shorter periods in order to maintain consistent flow and to avoid premature rupture. An ideal elastomer for tubing applications would combine the best properties of each cure type: high tear strength and long pump life at various durometers.

Table I. Peroxide versus platinum cure at 35A durometer.

Table II. Peroxide versus platinum cure at 50A durometer.

Table III. Peroxide versus platinum cure at 65A durometer.

SELECTING THE TEST METHOD

Results of the typical physical tests done on elastomers (durometer, tensile, elongation, and tear), though important, do not translate well to actual pump life. To determine whether the elastomer is resilient under repeated compression, one would have to produce a large batch of the developmental elastomer (typically on a manufacturing line), extrude tubing, and conduct pump-life testing. This approach is not cost-effective because a significant amount of tubing must be scrapped. When selecting a test, it is important to understand what happens to the elastomer when it is used in pumping applications—in other words, its rheology.

A peristaltic pump pushes fluid along by means of repeated contractions produced by the pump head on a flexible piece of tubing. These contractions result in stretching and compression of the tubing. Attention typically focuses on the compression portion of this process. Over time, the repeated compression causes deformation and a resulting change in the inner diameter of the tubing. The once-circular orifice becomes flattened, and the initial flow rate decreases—a condition typically described as "the tubing taking a set in the pump head." This response can also be characterized as elastic deformation, which is influenced by time, force, and temperature. The slow recovery of the tubing after deformation is called the elastic aftereffect. One way to measure this effect on the elastomer is to conduct a compression test. Compression testing is useful for comparison studies within a sample set, but it is very important to maintain the same time, temperature, and deflection force for each sample within the study. These three factors have an effect on recovery results, and they make comparisons invalid if test conditions are not controlled.

The other process that occurs during contraction is stretching. When a rubber is stretched, the molecules slip alongside each other, distributing the stress evenly among themselves. Energy is absorbed and only partially regained upon retraction. This loss in energy is measured as hysteresis, which can be correlated to the resilience of the rubber. The higher the resilience, the lower the hysteresis.

Hysteresis is calculated from a stress-strain curve, as shown in Figure 1. The marked area represents the hysteresis, or the measure of work lost. Percent hysteresis is the difference between the loading energy and the unloading energy, expressed as a percentage of the loading energy. Energy values are determined by calculating the area under the test curve.

Figure 1. Hysteresis stress-strain curve.

Our testing at Dow Corning is based on an internal test method. Three bars are cut from a cured elastomer slab 0.075 in. (±0.010 in.) thick using an ASTM D 412, die D. The test is conducted using a tensiometer, and the test bar is cycled once to 100% elongation and back. Hysteresis calculations are performed as described above. The test is repeated two more times, and the average of the three results is reported (repeatability of the value reported does not exceed 6% of the hysteresis loss at a 95% confidence level).

Compression testing is more commonly used to evaluate the elasticity of an elastomer. Our test method is based on ASTM D 395, method B. Compression is made with two highly polished chromium-plated steel plates with four holes at the corners. Four hexagon screws are used to clamp the plates together, and shims of varying thickness are used as spacers to construct the desired height for each clamp used. The force is a constant deflection at specified conditions and 25% compression. Three samples are tested, and the average is reported. (Precision of the reported value is within ±5%.)

Compression and hysteresis tests are both good measures for resilience, and we used both methods for our evaluation of elastomers during our studies. We eventually moved away from using compression testing because we found more discrepancies in the test results from the same elastomer formulations tested at different time intervals and by different testers than we did in the results we obtained from hysteresis testing. Also, hysteresis testing requires less time and is easy to run. Table IV compares compression and hysteresis test results for peroxide- and platinum-cured samples. The lower the values for compression or hysteresis, the more resilient the elastomer. Both compression and hysteresis tests show that peroxide-cured elastomers are more resilient than platinum-cured elastomers.

Table IV. Compression and hysteresis test method comparison for an elastomer sample.

Our next step was to correlate the hysteresis data for the elastomer to pump-life data on a tubing of the same formulation as the elastomer.

PUMP TEST CORRELATION

For the evaluations in this study, elastomer formulations were optimized based on durometer, tear, and hysteresis. Based on typical peroxide formulations, hysteresis values of 45% were chosen as a target (Table IV). To verify that hysteresis correlates to pump-life testing, selected formulations were scaled up to meet the hysteresis target, and tubing was extruded. We expected to see the formulations with lower hysteresis loss (i.e., those that were more resilient) have longer tubing life in a pump.

Table V shows the relationship between the elastomer properties (tear, hysteresis) and tubing (1/8 x 1/4 in.) tested at 0 psi, 600 rpm, under a continuous pumping cycle until rupture. The test procedure for pump life was obtained from the manufacturer of the pump used. Pump-life hours reported are median values; a minimum of three samples were run for each tubing. The table shows pump-life results for a typical peroxide formulation (control), a typical platinum formulation (control), and a new elastomer formulation based on platinum cure at 50A durometer. The pump life of the new elastomer tubing is much better than that of the platinum control (which has higher hysteresis) and approaches that of the peroxide control (which has a comparable hysteresis value). The lower the hysteresis of the elastomer, the longer the pump life of the elastomer tubing. Similar testing on several other elastomer formulations and on tubing samples of various sizes from these elastomers confirms the relationship between hysteresis and pump life.

Table V. Pump life of tubing in relation to hysteresis of elastomer.

CONCLUSION

Combining hysteresis with durometer and tear is essential to developing an elastomer that, when extruded and used in pumping applications, is resilient and has good pump life. By selecting the appropriate test method, evaluation of new formulations for pump tubing applications is easier and more cost-efficient. Establishing a correlation between hysteresis and pump-life testing allows better prediction of the effect of formulation changes, which allows faster response to the needs of the health-care industry and better all-around material for use in peristaltic pumps.

Judith Fairclough Baity is a development specialist and project leader for Dow Corning Corp. (Midland, MI).

Photos courtesy of DOW CORNING CORP. (Midland, MI)

Copyright ©1998 Medical Device & Diagnostic Industry