New PTFE High-Performance Material Enables Single-Layered Diaphragm Seals

Diaphragms are used as hermetic seals for media separation and pumping in dynamic systems. The fields of application are wide and varied, ranging from pumps, compensators, pressure transmitters, pressure accumulators, drives and packaging equipment for foodstuffs through to use in diaphragm valves such as Saunders® diaphragms in high-purity applications etc.
In the past, the materials used for diaphragms were almost exclusively found in the family of elastomers. These materials are reinforced with fabric inserts to enable them to absorb higher media pressures. For applications in the chemical, pharmaceutical or food industries, PTFE-covered rubber diaphragms were the standard choices. The PTFE cover protects against chemical influences and is used due to the physiological harmlessness of the fluoro-plastic material and its anti-adhesive properties.
Diaphragms from HS22121 (Figure 1) now offer an optimized solution and alternative to the multi-composite diaphragms which have been the standard products used so far.

Most diaphragms made of a PTFE/elastomer/fabric composite can be replaced with thermally stamped or machined PTFE diaphragms using HS22121. As a modified PTFE, HS22121 offers the outstanding combination of high reverse bending strength and sealing action at low driving forces. System-related risks like separation, fiber cracks or crazing can be avoided, and development cycles and costs significantly reduced. The use of modified materials and the integration of secondary functions allow the development of demanding multifunctional diaphragm solutions.

High-Performance Plastic Offers Many Benefits
 
One of the valued properties of PTFE is its extreme resistance to frequent bending of its structure. No other thermoplastic material is known to exhibit this outstanding characteristic. As a result, PTFE is becoming the preferred material for diaphragms. In the past, applications requiring the use of modified PTFE inevitably implied the need to accept limitations in terms of reverse bending properties. Through its new HS22121 material ElringKlinger has managed to introduce a product on the market that combines all the advantages of modified PTFE.

Benefits:

• Excellent reverse bending properties
• Higher permeation density
• Lower cold flow
• Lower porosity
• Smoother surfaces
• Low stretch void index
• Weldability
• FDA / USP 6 / 3A approval
• Purity of the material / application possibilities in the semiconductor industry

In addition, HS 22121 surpasses the excellent reverse bending properties of non-modified standard PTFE types. In the past, it was not possible to combine these properties. The HS 22121 material is also available as an antistatic version specifically for use in applications involving contact with solvents.

Previous Fatigue Strength Values Have Been Surpassed

The results of the reverse bending test were obtained by bending a test rod with a thickness of 1 mm by 180° at a frequency of 4 Hz without media contact.
The HS 22121 material provides ElringKlinger Kunststofftechnik with a modified PTFE type which surpasses all previous fatigue strength values. The values obtained for this material surpass those of first-generation PTFE threefold, and those of second-generation PTFE even tenfold. Figures 2a) and 2b)

Figure 2: Fatigue strength of HS22121 compared to first- and second generation PTFE b) compared to alternative diaphragm materials

Reduced Cold Flow

In addition to its high reverse bending strength, the reduced cold flow of the material provides for better retention of the diaphragm in the clamping area, which is another advantage in terms of sealing effect and service life. (Figure 3)

Figure 3: Reduced cold flow of HS22121

Higher Barrier Effect
For long service life, diaphragms should be designed to have thin walls. Consequently, it is even more important that the material used has a high barrier effect against permeation. This is the case with modified PTFE materials and applies to aggressive, gaseous chemicals like SO2, HCl or Cl2 as well as to water (Figure 4a). Water, particularly at high temperatures or in the vapor phase (Figure 4b), or in the form of aqueous, aggressive chemicals, poses a challenge to fluoropolymers.

Figure 4: Higher barrier effect of HS22121 a) against aggressive chemicals (measurement method according to DIN 53380) b) against water/steam of 23 °C, c) against water/steam of 100 °C (sheet strength in each case: 1mm)

If extremely high durability is not required or other demands are more important, developers can choose from a wide range of different PTFE grades (see reverse bending material comparison b). In addition to FDA-conformant materials for the food industry and highly pure types for the semiconductor industry, ElringKlinger Kunststofftechnik also offers conductive materials which comply with the requirements of Atex 94/9/EC (European Standard EN 13436).

The HS22121 material brings its outstanding properties to bear in diaphragms and offers major advantages over the previously used multi-composite systems:

• PTFE diaphragms, through appropriate designs, achieve nearly identical mechanical properties as elastomer composite materials.

• Significantly enhanced diffusion properties through use of the HS22121 materials. Aggressive substances can diffuse through the thin standard PTFE layers and attack the back of the diaphragm.

• Lower driving forces due to the thinner and simpler structure.

• No application limitations since all approvals have been granted.

• The development of machined PTFE diaphragms offers cost benefits and can be achieved in shorter periods of time since no special tools or molds are required. Samples for testing can be made very quickly in various versions, resulting in less development loops.

• The designer enjoys a full range of design options to integrate several desired functions (flange areas, O-ring grooves, threads, pressure plates, reinforcement rings, shafts) into the PTFE diaphragm (Figure 5)

• PTFE diaphragms can be designed into the typically existing assembly space of multi-composite diaphragms.

Figure 5: The new material enables numerous shapes

• Machined PTFE diaphragms offer a simpler and clearly more reliable manufacturing process without quality variations.

• Longer service life since layers do not separate. The various materials of multilayered diaphragms have different elastic and shear moduli. This leads to non-homogenous stress distributions between the layers when the pressure and temperature conditions in the system vary, which exerts stress on the boundary layers. This specific characteristic limits the service life of a composite diaphragm.

• When using thermally stamped and machined diaphragms, even better surfaces can be achieved for high-purity applications.

• Stud pins or inserts may be welded or screwed in and thereby firmly connected to the PTFE diaphragm in order to achieve high mechanical loads in dynamic applications.

• Depending on the application, diameters from 5 to 800 mm and wall thicknesses of a few tenths to several millimeters can be realized.

• PTFE diaphragms can be manufactured as machined or thermally stamped versions, from small to large volumes.

HS22121 in Use as a Valve Diaphragm in High-Purity Applications in the Semiconductor Industry

The HS22121 material is used as a valve diaphragm (Figure 6) in so-called Saunders® diaphragm valves in high-purity applications where the material can fully exploit its advantages over multi-composite diaphragms. Purity of the material, smooth surfaces and chemical resistance are key requirements. A surface roughness of Ra= 0,3 µm can be easily achieved. The engineering design reduces dead spaces and contributes to optimal sealing of the system. The high reverse bending strength and the firmly welded stud pin extend the component’s service life and prolong scheduled service intervals. In case of pneumatic control systems the low drive force of the HS22121 diaphragm helps to reduce operating costs.

Figure 6: HS22121 as a
valve diaphragm for high-
purity applications