Handling and Environmental Factors

• When handling membranes, individuals should wear gloves. Skin oils on the hands can adversely affect membrane properties.
• Static eliminators are helpful for handling and placement of die cut membrane and processing of membrane ribbons, especially for high speed operations. The relative humidity in the membrane processing area can also be a factor.
• Tension control is important to ensure proper tracking of roll stock membrane and membrane ribbons.
• Parts and work surfaces must be clean and free of particulates.
• Molded parts must be free of silicone-based mold-release agents.

Design Factors

• The design of the seal area in a membrane device is a critical factor for the finished seal integrity. The seal is nearly always the weakest area in the device. It is desirable to radius all edges in contact with the membrane. It is also preferable to have the flow in the finished device enter through the sealed side so that the membrane is supported in the device during flow. Flowing in the opposite direction can cause stress on the seal.
• Minimize excess flash around the seal area.
• Parallelism and alignment of tooling to part and fixture is critical.
• Molded parts should be designed to eliminate or minimize stressed areas or sinks.

Material Properties

• Media is available on a variety of support materials. The support determines many of the sealing and handling characteristics.
• For some supported membranes and composite media, the seal will actually be made between the support or composite material and the plastic part, not the membrane. It is important to keep this in mind when determining melting temperatures, dwell time, etc.
• Use the appropriate resin for the desired process.
• Parameters for each of the sealing methods will vary depending on both the media properties and those of the plastic to which it is being sealed.

A pressure-sensitive adhesive seal uses a thick layer of adhesive to seal the membrane to the housing. The adhesive can be applied in a peel-and-stick format. Check with a converting company for information on configurations of membranes, adhesives, support layers and release liners. UV sets can also be used.

When evaluating an adhesive seal, the compatibility of the adhesive with the housing, the membrane, and the intended application need to be considered. Compatibility of the adhesive with the membrane and housing polymers should be discussed with your adhesive supplier or media converter.

Industry standards for adhesives can be critical. Some industries have basic requirements for the adhesive used, specifically the electronics and medical industries. Most adhesive manufacturers are aware of these requirements and have adhesives that comply with the appropriate standards or regulations. If not, check with an adhesive manufacturer that specializes in your industry.

• Polyurethanes, epoxies, or similar chemically compatible pure polymers work well as adhesives.
• Avoid using adhesives containing suspended solids; particulate-laden adhesives will not penetrate the pores, causing poor adhesion and probably by-pass of particulates into the downstream filtrate.
• Surface treatment such as plasma or chemical treatment may increase adhesive wettability.
• Unfilled polymers are preferable regardless of the method used for sealing because they will wet the surface of the membrane better than filled polymers, creating a stronger bond.
• Do not use cyanoacrylate when sealing Supor® membranes.

Heat sealing uses a variety of heat sources and pressure to melt the housing and membrane together. Heat seals are typically classified according to the heat source; among these are ultrasonic welding, heated dies, and radio frequency welding. With heat seals, it is possible to form two different types of bonds. If the membrane and housing materials have the proper thermal characteristics, the media and housing can be melted together to form a secure seal. Another type of bond occurs when the housing material melts at a lower temperature than the membrane. The molten housing plastic penetrates into the structure of the medium, forming a seal. Many of Pall's materials form this type of seal, including Pallflex® composite media.

General guidelines:

• The plastic to which the microporous membrane is being sealed should have a melting temperature similar to or lower than the membrane.
• Keep geometry of the membrane coupon simple. Round is recommended.
• Adequate seal land width is important. Widths from 0.15875 - 0.3175 cm (0.0625 - 0.125 in.) are recommended. Inadequate seal land will result in a "domed" coupon.
• A high temperature, non-stick coating on the heat seal die helps to prevent membrane sticking.
• Transparency in the seal area is usually indicative of a seal.
• Relationships between time, temperature, and pressure must be optimized through experimentation. Larger coupon sizes and seal land widths will require higher pressures.
• Heat sealing is the recommended microporous membrane sealing technique in most cases.

Ultrasonic welding is based on generated heat, pressure and time. The heat is created by the use of high frequency mechanical motion (vibrations). The high frequency energy travels through the material and must be focused at the desired melt location. The greater the vibration, the hotter the material becomes.

• A high frequency, low amplitude setting is preferred.
• For weld areas greater than 3.8 cm (1.5 in.) diameter, use 20 kHz; for smaller diameters, use 40 kHz.
• Seal to an energy director rather than a wide, flat surface.
• Avoid long duration weld times at high amplitudes.
• Some single and most multiple ultrasonic welds may cause damage to membranes.
• Avoid secondary ultrasonic weld cycles.
• Provide a smooth transition from the seal to the membrane.
• Cutting, placement and sealing of membrane can be accomplished at one time.
• Cushion the parts to dampen vibrations throughout the entire part.
• Care must be taken to minimize excess ultrasonic vibrations to maintain pore integrity. Improper use of ultrasonics can lead to damage of the microporous membrane's pore structure.

Heat is transferred through a die under pressure directly to the materials to be joined for the appropriate time necessary to form an integral seal. The heated die melts the housing or the media at the point of contact which, in combination with the applied pressure, bonds the housing and membrane together.

• The plastic to which the microporous membrane is being sealed should have a melting temperature similar to or lower than the membrane.
• Keep geometry of the membrane simple; round membrane coupons provide the best results.
• To minimize plastic buildup on tooling, use lower temperatures and higher pressures for longer times.
• A high-temperature, non-stick coating on the heat seal die is recommended.
• Adequate seal land width is important; 50 to 125 mils is recommended.
• Transparency in the seal area is usually indicative of a complete seal.
• Short seal times may result in the membrane and its backing pulling apart when the seal die is removed. This is known as delamination.
• In some cases, sealing and cutting may be accomplished in a single step. This depends on the membrane type and the shape of the part and must be determined during the design phase.
• Relationships between time, temperature and pressure must be optimized through experimentation. Larger coupon sizes and seal land widths will require higher pressures.

RF uses heat, pressure and time to form a seal. The heat is generated from high-energy electromagnetic waves (27 to 31 MHz) that excite the molecules of the materials being bonded. The excitement of the molecules creates heat that combines with pressure and time to form an integral seal.

• RF sealing can only be used with plastics having the correct dielectric properties, such as PVC and acrylics.
• The most common method for RF sealing of microporous membranes is to encapsulate the membrane between two plastic components.
• Longer seal times are preferred for better control of the sealing process.
• Avoid potential arcing, which can cause seal failures.

Membranes can be sealed in place by mechanical means such as a filter support that is clamped in place using an O-ring or gasket, or by insert molding the membrane into plastic components.

In this technique, the media is held in place or placed into a mold, while molten plastic is forced into the mold, forming an integral piece containing the membrane. Insert molding is a good choice for producing high volume/low cost components containing membranes.

• Minimal pinch force should be applied to prevent membrane damage.
• Avoid venting hot gases through the membrane.

Conventional steel rule, rotary, impact or male/female dies work well with fabric-reinforced and unsupported membranes. To avoid thread pulls and resulting membrane failures when cutting reinforced fabric, dies must be kept sharp and strike through the membrane to a hard surface. The use of interleafing layers may be necessary.

Male/female die sets must fit tightly; ideally the gap should be no more than half the membrane thickness. Static eliminators are helpful when cutting membrane ribbons in high-speed operations.