Sealing Microporous Materials

There are a variety of factors that must be considered when handling and sealing microporous materials into a device. Note: The following information is intended to serve only as a guide. Users must verify conditions appropriate to their specific use.

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 the handling and placement of die cut membranes and processing of membrane ribbons, especially in automated operations.
• The relative humidity in the membrane processing area can also be a factor. We recommend processing the membrane in a climate controlled environment.
• Tension control is important to ensure proper tracking of roll stock membrane and membrane ribbons. Care must be taken not to over-stress and stretch the membrane, which can result in membrane breakage or altered membrane performance.
• 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 almost always the weakest area in the device. It is desirable to radius all edges in contact with the membrane so damage to, or puncture of, the membrane does not occur. It is also preferable to have the fluid path of the finished device enter through the sealed side so that the membrane is supported in the device during flow. Flowing fluid 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 for uniform integral seals.
• Molded parts should be designed to eliminate or minimize stressed areas or sinks.

Material Properties

• Microporous media is available on a variety of support materials. The material 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. The properties of various resins lend themselves to different sealing mechanisms.
• 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 needs 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 will probably cause by-pass of particulates into the downstream filtrate.
• Surface treatment such as plasma or chemical treatment of the membrane 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 polyethersulfone membranes as it will dissolve the membrane.

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. The first occurs if the membrane and housing materials have the proper thermal characteristics, the media and housing can be melted together to form a secure seal. The second 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 filter medium, forming a seal. Many of Pall's materials form this type of seal, including our glass fiber composite media.

General heat sealing 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 for uniform heat distribution. Round or square 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 puckering of the membrane or a "domed" membrane 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 of the membrane and the plastic part.
• 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.
• Some membranes, such as nitrocellulose, require layering with a secondary material for heat sealing.

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.

General ultrasonic welding guidelines:

• 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 as it will put holes in and damage the membrane.
• Some single and most multiple ultrasonic welds may cause damage to membranes.
• Avoid secondary ultrasonic weld cycles to prevent damage to the membrane.
• 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.

General heated die 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 simple for uniform heat distribution; round or square membrane coupons provide the best results.
• To minimize plastic build-up 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; 0.127-0.318 cm (0.05-0.125 in.) 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 of supported membranes.
• 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.

Radio frequency 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.

General radio frequency guidelines:

• 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. Almost all microporous membranes can be mechanically sealed.

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.

General insert molding guidelines:

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

Bulk membrane rolls can be further processed and converted into more usable formats for device assembly. Care must be taken when handling microporous membranes to prevent damage during the converting stage. At Pall, we offer a variety of filter cutting options that can eliminate the need for membrane conversion prior to assembly into your device. Contact your local sales representative for details on slitting, die cutting, and other conversion options.

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.

For best results, protect membrane from pleater blade damage by sandwiching it between spun bond or highly calendered non-woven fabrics. The use of warming lamps or heated pleater platens softens thermoplastic membranes yielding higher quality and better-defined pleats.

Potting is a manufacturing step when sealing capsule endcaps to pleat packs. It is desirable to use adhesives with viscosities that will wick slightly into the pack for a bypass-free seal. Refer to adhesive sealing guidelines.

Use our Sealing Compatibility Guide to find the compatibility of Pall membranes with various housing materials and sealing methods.

Reference used:

• Handbook of plastics joining: A practical guide. (1997). Norwich, N.Y.: Plastic Design Library.
• Modern plastics encyclopedia. (1999). New York, NY: McGraw-Hill.