Hot melt film spacer

Abstract

The hot melt printed spacer membrane element provides the unique advantage of applying any pattern on the membrane surface to act as a feed spacer material. This technique also eliminates damage to the active surface of the film by avoiding photocuring (UV, light, or energy of other wavelengths). By printing the narrow features, the bending moment at the film surface imparted by the printing features will be less than the bending moment imparted by the wider printing features, thereby minimizing damage to the sensitive film coating.

Description

Technical Field

[0001] The present invention relates to membrane systems for separating fluid components, in particular spiral wound membrane elements or flat sheet membrane systems. Background Art

[0002] In cross-flow filtration, the feed fluid flows through the filter and is released at the other end, while some of the fluid is filtered and removed by the membrane surface parallel to the direction of fluid flow. There are various forms of cross-flow filtration, including plate and frame systems, cassette systems, hollow fiber systems, radial systems, or spiral wound systems. Plate and frame filter modules, cassette filter modules, radial filter modules, and spiral wound filter modules generally rely on stacked membrane layers that provide spacing between adjacent layers of the filter membrane. The present invention generally relates to, but is not limited to, spiral wound membrane elements.

[0003] Spiral wound membrane filter elements are well known in the art and comprise a laminate structure having a membrane sheet sealed to or around a porous permeate carrier that creates a path longitudinal to the axis of a central tube for removing fluids that pass through the membrane to the central tube, while the laminate is spirally wound around the central tube and separated from itself by a porous feed spacer to allow fluid to flow axially through the element from the feed end to the retentate end of the element. Conventionally, the feed spacer mesh is used to allow feed water (some portion of which will pass through the membrane) to flow into the spiral wound element and to allow the retentate fluid to exit the element in a direction parallel to the central tube and axial to the element construction.

[0004] Improvements to spiral wound element designs are disclosed in the following patents: U.S. Patent 6,632,357 to Barger et al., U.S. Patent 7,311,831 to Bradford et al., and Roderick et al., entitled "Improved Spiral Wound Element Construction," in Australia (2014223490), Japan (6499089), China (CN105163834B), Israel (240883), and South Korea (10-2196776), which replace feed spacers with islands or protrusions printed, deposited, or embossed directly onto the active or inactive surface of the membrane or onto the permeate support. U.S. Patent 11,090,612 to Roderick et al., entitled "Graded spacers for filtration wound elements," describes the use of highly graded spacer features that are used to modify feed flow characteristics in spiral wound elements. U.S. Patent No. 11,040,311 to Roderick et al., entitled “Interference Patterns for Spiral Wound Elements,” describes patterns in spirally wound elements that keep the membrane feed space open but also provide support for the membrane envelope glue area during winding. U.S. Patent Application PCT / US 18 / 55671 to Roderick et al., entitled “Bridge Support and Reduced Feed Spacers for Spiral-Wound Elements,” describes support features applied to the distal end (the end farthest from the center tube) of the membrane envelope to provide support during gluing and winding of the spirally wound element. U.S. Provisional Application No. PCT / US21 / 40353, entitled “Variable Velocity Patterns in Cross Flow Filteration,” by Herrington et al., describes a support pattern of varying dimensions from the feed end to the retentate end of the membrane feed space in a feed flow path parallel to a center tube to control the velocity of the feed solution as the concentration of the feed solution increases from the feed end to the retentate end of a spirally wound element.U.S. Patent No. 11,083,997 to Roderick et al., entitled “Non Nesting Patterns,” describes denser patterns in the feed and rejection ends of a membrane feed space, and a more open pattern in between, to avoid nesting of printed patterns during component manufacturing, particularly during the membrane envelope gluing process to support the glue line. PCT Application No. PCT / US21 / 26030 to Herrington et al., entitled “Independent Spacers and Methods,” describes various methods for applying spacers to a membrane surface that do not expose the membrane surface to UV or visible light from an inkjet, stencil, or screen printing process to be photocured. U.S. Patent Application No. 63294377 to Herrington et al., entitled “High Rejection Element,” describes a membrane printing and assembly process that provides support to a membrane sheet in areas of high stress concentration to avoid damage to the membrane active layer. US Patent Application No. 63294378 to Kurth et al., entitled "Spiral Element Enhanced Capacity," describes a membrane printing and assembly process that provides improved membrane design features to increase the permeate flow capacity of spirally wound elements.

[0005] To date, many spacer printing technologies have used multi-pass UV or light-cured inkjet processes that build up the pattern's height in layers. Alternatively, single-layer stencil printing processes using epoxy resins or UV or light-cured urethanes have been used. Stencil printing offers the potential for faster printing compared to light or UV-cured photopolymer inkjet processes because the stencil can provide the desired spacer height in a single pass, whereas inkjet printing typically requires application in multiple passes. Summary of the Invention

[0006] The present invention describes novel methods of applying and configuring printed spacer features that avoid damage to the polyamide coating of thin film composite (TFC) films due to the bending effect of printed features adhering to the TFC surface, and also facilitates maximum speed of printing patterns on the film surface, as well as innovative applications of heat curing wax and hot melt materials.

[0007] An exemplary embodiment of the present invention provides a membrane assembly for a spirally wound filter element, the membrane assembly comprising: (a) a membrane sheet comprising a thin film composite structure comprising a porous structure layer, a support layer, and an active membrane layer; (b) a plurality of edge spacers comprising a hot melt line segment arranged on the active membrane layer, wherein the length of each edge spacer is less than one-quarter of the distance between the opposite first edge and the second edge of the membrane sheet, the edge spacers are positioned near the opposite first edge and the second edge and arranged with the long axis perpendicular to the first edge, wherein adjacent edge spacers are separated by a first distance in a direction parallel to the first edge; and (c) a plurality of intermediate spacers comprising a hot melt line segment arranged on the active membrane layer, wherein the length of each intermediate spacer is less than one-quarter of the distance between the opposite first edge and the second edge of the membrane sheet, the intermediate spacers are positioned between the opposite first edge and the second edge and arranged with the long axis perpendicular to the first edge, wherein adjacent intermediate spacers are separated by a second distance in a direction parallel to the first edge, wherein the second distance is greater than the first distance.

[0008] In some embodiments, the second distance is an integer multiple of the first distance. In some embodiments, the length of each edge spacer is at least one inch and no more than four inches. In some embodiments, the plurality of intermediate spacers comprises a plurality of groups of intermediate spacers, wherein each group of intermediate spacers comprises a plurality of intermediate spacers, each of the plurality of intermediate spacers being arranged along a single line perpendicular to the first edge, wherein the single line coincides with the edge spacers, and wherein the intermediate spacers in a group comprise line segments separated from each other by a third distance in a direction along the single line, wherein the third distance is greater than the first distance.

[0009] In some embodiments, each group of intermediate spacers is separated from the adjacent group of intermediate spacers by a third distance, wherein the third distance is an integer multiple of the first distance. In some embodiments, the cross-section of each intermediate spacer is rounded at the end facing away from the film sheet. In some embodiments, the cross-section of each intermediate spacer has a convex side. In some embodiments, the cross-section of each intermediate spacer has a concave side. In some embodiments, the hot melt is a liquid at a temperature higher than 100°C. In some embodiments, the hot melt is a liquid at a temperature higher than 170°C. In some embodiments, the hot melt has low viscosity. In some embodiments, the second distance is 3 times the first distance. In some embodiments, the intermediate spacer is 100 to 1500 microns wide, 250 to 5000 microns long, and 75 to 1200 microns high.

[0010] An exemplary embodiment of the present invention provides a method for manufacturing a membrane assembly, the method comprising: (a) providing a membrane sheet, the membrane sheet comprising a thin film composite structure, the thin film composite structure comprising a porous structure layer, a support layer, and an active membrane layer; (b) forming a plurality of edge spacers by depositing a hot melt in a plurality of line segments on the active membrane layer, wherein the length of each edge spacer is less than one-quarter of the distance between opposite first and second edges of the membrane sheet, the edge spacers are positioned near the opposite first and second edges and arranged with the long axis perpendicular to the first edge, wherein adjacent edge spacers are separated by a first distance in a direction parallel to the first edge; (c) forming a plurality of intermediate spacers by depositing a hot melt in a plurality of line segments on the active membrane layer, wherein the length of each intermediate spacer is less than one-quarter of the distance between opposite first and second edges of the membrane sheet, the intermediate spacers are positioned between the opposite first and second edges and arranged with the long axis perpendicular to the first edge, wherein adjacent intermediate spacers are separated by a second distance in a direction parallel to the first edge, wherein the second distance is greater than the first distance.

[0011] In some embodiments, the second distance is an integer multiple of the first distance. In some embodiments, the length of each edge spacer is at least one inch and no more than three inches. In some embodiments, the plurality of intermediate spacers comprises a plurality of groups of intermediate spacers, wherein each group of intermediate spacers comprises a plurality of intermediate spacers, each of the plurality of intermediate spacers being arranged along a single line perpendicular to the first edge, wherein the single line coincides with the edge spacers, and wherein the intermediate spacers in a group comprise line segments separated from each other by a third distance in a direction along the single line, wherein the third distance is greater than the first distance. In some embodiments, each group of intermediate spacers is separated from an adjacent group of intermediate spacers by a third distance, wherein the third distance is an integer multiple of the first distance.

[0012] In some embodiments, the second distance is 3 times the first distance. In some embodiments, the cross-section of each intermediate spacer is rounded at the end facing away from the film sheet. In some embodiments, the cross-section of each intermediate spacer has a convex side. In some embodiments, the cross-section of each intermediate spacer has a concave side. In some embodiments, the hot melt is liquid at a temperature above 170°C.

[0013] In some embodiments, forming a plurality of edge spacers comprises: conveying a hot melt dispenser from near the first edge to near the second edge in a direction perpendicular to the first edge; and applying hot melt via the dispenser while the dispenser travels from near the first edge to a position a distance from the first edge equal to the length of the edge spacer, and applying hot melt via the dispenser while the dispenser travels from a position a distance equal to the length of the edge spacer to near the second edge. In some embodiments, forming a plurality of intermediate spacers comprises: applying hot melt via the dispenser while the dispenser travels from a position at the beginning of the intermediate spacers to a position at the end of the intermediate spacers while conveying the dispenser. In some embodiments, forming a plurality of intermediate spacers comprises: applying hot melt via the dispenser over less than the entire distance of the dispenser from the first edge to the second edge. In some embodiments, the dispenser dispenses the hot melt at a rate of less than 100 nanoliters per drop. In some embodiments, the dispenser dispenses the hot melt at a rate of less than 50 nanoliters per drop. In some embodiments, the dispenser dispenses the hot melt at a rate of less than 10 nanoliters per drop.

[0014] An exemplary embodiment provides a method for producing a recycled spiral wound element, the method comprising: (a) providing one or more initial spiral wound elements, the one or more initial spiral wound elements having a spacing feature including a hot melt; (b) disassembling the one or more initial spiral wound elements; (c) recovering some or all of the hot melt from the initial spiral wound elements; (d) using the hot melt to produce one or more membrane modules as described herein; and (e) winding the one or more membrane modules into a recycled spiral wound element. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is an exploded view of a spiral wound membrane element.

[0016] Figure 2 is an exploded view of a partially assembled spiral wound membrane element.

[0017] Figure 3 is a view of the printed film surface showing damage to the active film layer due to widely spaced features in the longitudinal direction of the component winding.

[0018] Figure 4 is an end view of a spirally wound element showing the geometry of the printed features and damage to the membrane surface.

[0019] Figure 5 is an end view of a spiral wound element showing the narrow printed features to avoid damage to the membrane surface as the membrane sheet is wrapped around the center tube.

[0020] Figure 6is a view of a printed pattern on a film sheet that facilitates high-speed printing using single or multiple passes of heat-curable materials.

[0021] Figure 7 It is a schematic diagram of the layout of a hot melt production plotter.

[0022] Figure 8 are side views of printed spacers with and without sharp edges on opposing film surfaces.

[0023] Figure 9 is a side view of a concave hot melt spacer showing the accumulation of droplets of hot melt material.

[0024] Figure 10 is a side view of a hot melt spacer having concave features.

[0025] Figure 11 is a side view of a hot melt spacer having convex features. DETAILED DESCRIPTION

[0026] Hot melt materials, such as the hot melt technology used in the present invention, can offer significant advantages over alternative spacer application techniques. As used herein, "hot melt" refers to any polymer-based glue that is applied in a molten state. Application of hot melt can be performed using glue guns, high-frequency tappet-type nozzle tips, heated stencils, and other techniques. Due to its versatility, hot melts can be used in a variety of applications, including packaging, bookbinding, carton making, graphic arts, tapes and labels, product assembly, and as spacers in spiral-wound, flat-sheet, and pleated filtration systems and membrane systems.

[0027] Hot melts as in the present invention have several beneficial properties. They are fast-acting and can be applied in one or more layers depending on the desired height of the pattern. The time for curing or setting can be adjusted according to the needs of the application. Hot melts are generally safe and environmentally friendly to use. Hot melts can be used to bond difficult surfaces. They are inherently safe, easy to transport and store, and have a long lifespan. Compared to solvent-based adhesives, polymer-based hot melts can be faster, more cost-effective, more adhesive, and produce less volatile organic compounds. In this way, hot melt materials will produce fewer undesirable volatiles that may contaminate drinking water during the reverse osmosis (RO) process. This feature helps ensure compliance with toxicology protocols when tested according to NSF International requirements. NSF International verifies that water-contact materials do not extract materials that may contaminate fluids (such as water) and have adverse effects on consumer health. Due to its chemical properties, hot melts can be in any number of forms. This includes granules, pellets, bags, lumps, barrels, bricks, slats, and pillows. Hot melt can also be applied in several ways, including through high frequency print head nozzles, extrusion, meltblowing, spiral spraying, screen printing, stencil printing, and slot die coating. The dispensing equipment for hot melt can be in the form of melt reservoirs, vacuum conveying, drum or barrel unloaders, and pre-melters.

[0028] Typically, a hot melt is made up of a polymer (which can be in various forms) and several additives. These additives include resins, waxes, antioxidants, and plasticizers. Other chemicals can be added to give the hot melt more properties.

[0029] Polymers. The basic components of hot melts (and many types of adhesives) are polymers. These are long, repeating chains of certain molecules, with different properties based on the chain length and type of molecule. The main polymers used in hot melts are ethylene vinyl acetate (EVA), polyolefins, polyamides and polyesters, styrene block copolymers, polyethylene, and ethylene methyl acrylate (EMA) or ethylene n-butyl acrylate (EnBA).

[0030] Polymers impart strength and flexibility, as well as heat resistance, impact resistance, and shear properties to hot melts. These properties depend largely on the type of polymer, its molecular weight, and its amount. A higher polymer content provides higher viscosity, as well as greater flexibility and toughness. Lower polymer content generally results in lower viscosity.

[0031] Tackifying resin. The resin defines the tack of the hot melt. Tack is a measure of the adhesive's cohesive force, essentially how long the adhesive remains adhered after application. The resin determines the adhesive's wettability (i.e., how long the adhesive remains liquid when in contact with the substrate surface). Low-tack resins are preferred for printed spacer technology on membranes. Typically, the printed pattern on the membrane is applied only to half of the membrane sheet. The sheet is folded in half with the unprinted side facing the printed side. It is important that the hot melt material does not have a sticky surface. The spacer in contact with the unprinted side of the membrane should not stick to the unprinted surface. During winding, the membrane sheets will shear slightly across each other, and the hot melt spacer should not adhere to the opposing surface (which could damage the opposing surface as the two surfaces move relative to each other during winding). The active membrane layer is extremely thin, and any damage to the active layer will damage the salt retention properties of the membrane assembly.

[0032] The resin also affects the adhesive properties of the hot melt. The choice of resin is determined by its compatibility with the primary polymer, its softening point, and specific adhesion. The main types of tackifying resins used in hot melts are rosin and hydrogenated rosin, C9, hydrogenated hydrocarbons, terpene phenols, rosin esters, and C5. Pure aromatic monomers are also used.

[0033] Wax. Hot melt wax mainly controls the setting speed and open time. Open time refers to the amount of time required to bond. This can range from a few seconds to an infinite duration. The setting speed measures the speed at which the hot melt can form a bond with a certain acceptable strength. In addition to the setting speed and open time, wax also affects the heat resistance and sub-ambient (i.e., below the application temperature) adhesion of the hot melt. The main types of wax used in hot melts are natural wax, microcrystalline wax, and synthetic wax. The characteristics of wax are determined by percentage crystallinity, melting point, and molecular weight. When the wax content is low, the hot melt will have a higher viscosity and greater flexibility, and the bond strength will be higher (aggressively). When the wax content is less, the hot melt will have a lower viscosity, solidify faster, and the bond strength will be lower.

[0034] Antioxidants. Antioxidants are primarily used in hot melts to protect the material from degradation during its lifespan. Some common antioxidants used in hot melts include phenols, aromatic amines, phosphates, phosphites, and BHT. Antioxidants are added in small quantities along with stabilizers and do not affect the physical properties of the hot melt. They protect the hot melt not only during its lifespan but also during its molten state during application and compounding.

[0035] Plasticizers. Besides the base polymer and tackifying resin, plasticizers are the most common additives in hot melts. In fact, they are used as a second base polymer to give the hot melt greater flexibility and toughness. Plasticizers are typically hydrocarbon oils with low aromatic content and the chemical properties of paraffin wax. Ideally, plasticizers have low volatility, are transparent, and have no odor. Using plasticizers, hot melts can achieve lower melt viscosities and wet out faster.

[0036] In addition to these main components, the hot melt can have any of several other additives, as well as combinations of additives, to impart certain desirable properties. Biocides can be added to prevent bacterial growth. Fillers increase bulk and strength while reducing cost. The hot melt can also have flame retardants and various pigments or materials, such as copper, silver, carbon, and other conductive materials, as well as nanoparticles, which can provide electrical properties to the printed spacers, enabling the reduction of scaling and biofilm.

[0037] Important defining properties of hot melts are viscosity, melt color, failure temperature (for shear adhesion and peel adhesion), softening point, substrate specific adhesion, thermal stability, cold crack formation, loop tack, and various mechanical characteristics.

[0038] Viscosity is a measure of the thickness of a liquid or how much it resists flowing. High-viscosity liquids move very slowly (think thick oil). Lower-viscosity liquids, like water, flow more easily. The viscosity of a hot melt is not just a single value but can depend on the application temperature (which can range from 250 degrees Fahrenheit to 400 degrees Fahrenheit). For hot melt applications like printed spacers, higher viscosities are typically better because they can build height at faster travel speeds.

[0039] Melt Color. The color of the hot melt is measured on a numerical scale using both subjective and quantitative methods. These methods include the Gardner, Hunter, and Saybolt methods, as well as the Yellowness Index. Color is not typically a critical criterion for printed spacer applications, as the spacers are typically not visible.

[0040] Peelability. Peelability is a measure of how much force is required to break the bond between two bonding surfaces. Peelability is expressed in pounds per inch and can be measured for different surfaces at different angles (right angle and 180 degrees are the most common). Peelability is an important property for printed spacer applications because printed spacers are discrete components and should not move in spiral wrap applications. Acceptable adhesion of the printed spacer is demonstrated when the feature is removed from the polyamide surface and the polymer film material remains attached to the hot melt feature, exposing the underlying polysulfone support layer.

[0041] Failure Temperature. As the name implies, failure temperature is the temperature at which a hot melt stops functioning. There are two failure temperature metrics used to characterize hot melts.

[0042] Peel Adhesion Failure Temperature (PAFT). At higher temperatures, it becomes easier to peel two surfaces bonded with a hot melt. PAFT measures how well a hot melt resists peeling at higher temperatures.

[0043] Shear Adhesion Failure Temperature (SAFT). Shear is the force that exists when one surface slides against another. In a shear test, a specimen is mounted vertically and a weight is attached. The time it takes for the surfaces to separate indicates the strength of the hot melt.

[0044] Softening point. The softening point of a hot melt (or any glue) is the temperature at which the glue begins to flow. The primary determinants of the hot melt softening point are the melting point of the wax used and the transition temperature of the base polymer. The application temperature of the hot melt material used to print spacers is preferably in the range of 170°C, but can vary widely from 100°C to 200°C depending on the components in the hot melt material.

[0045] Substrate specific adhesion. This metric depends on the type of material used for the hot melt. Properties such as bond strength can be determined using the actual substrate. For printed spacer technologies where the substrate is a film, good adhesion is characterized by the hot melt's ability to adhere to the film coating and separate it from the underlying polysulfone layer.

[0046] Thermal stability. The stability of a hot melt under varying temperature changes indicates its overall durability. A hot melt with good pot stability will not burn or decompose at higher temperatures.

[0047] Mechanical properties. There are various mechanical properties associated with hot melts. These include tensile strength (how much force is required to break the sample), yield point (how much stress can be applied to the hot melt before it permanently deforms), elongation at break (how long the sample will stretch before breaking), and Young's modulus (stress-strain ratio). Other properties are combinations of these properties.

[0048] The resulting characteristics of different materials and adhesives used to make hot melts mean that there is great diversity in the function, performance, and cost of hot melt products.

[0049] Hot melts are attractive for nanomembrane filtration fabrication according to the present invention, in part because UV curing is not part of the process as in photopolymer printing. In UV or light-curing applications, the energy imparted by the UV or light source can damage the polysulfone substrate, which affects the flux and salt rejection properties in nanomembrane applications.

[0050] Feed Spacers and Spiral Wound Elements Feed spacers in spiral wound filter elements are required to maintain a path for fluid to flow from the feed end to the interception end of the feed channel, and the spacer design also affects local flow velocities, turbulence, stagnation zones, and other fluid flow conditions. Extruded mesh feed spacers are conventionally used in membrane manufacturing due to their ease of integration into the production process, but due to the nature of their design, many of their fluid dynamic properties are dependent on the thickness of the spacer. Conventional mesh spacers also provide uniform support characteristics in the feed space from the distal end of the center tube all the way to the proximal end of the membrane sheet near the center tube. Printed feed spacers allow for unique design characteristics that are not available with conventional extruded or woven mesh spacers because their thickness and geometry can be independently varied to produce a wide range of configurations that can be tailored to specific applications or specific challenges found in spiral wound membrane element construction.

[0051] Cross-flow filtration, by its nature, relies on some portion of the feed fluid passing through the membrane and becoming part of the permeate (product fluid), thus creating a situation where the amount of feed fluid is constantly decreasing as the feed fluid passes through the membrane. The higher the portion of permeate produced, the lower the portion of feed / concentrate fluid that remains flowing through the membrane element. As the fluid flows through the element, a portion of the fluid passes through the membrane. Simply modeled, a constant flux through the membrane produces a gradually decreasing flow of feed solution as the feed solution flows from the feed end of the feed space in the element to the interception end. In reality, the amount of fluid passing through any position along the feed flow path depends on the local flow conditions and the local concentration of solute or suspended material, as well as the local pressure, which also depends on any back pressure in the feed space and locally from the permeate side of the element.

[0052] During the manufacture of spiral wound elements, the permeate carrier material is attached to the central tube by adhesive tape or bonding, the membrane envelope is placed adjacent to the permeate carrier, and the flat sheet assembly is glued to seal the permeate carrier envelope, and the envelope rotates around the central tube with a rotary mechanism (such as a lathe). The central tube is captured or bonded to a lathe so that the lathe can rotate the central tube and wind the membrane envelope and the permeate carrier around the central tube. The torque on the central tube must be enough to wind the envelope until the entire envelope is wound around the central tube. Sufficient tension must be maintained in the membrane envelope to ensure that the glue fully penetrates through the permeate carrier and contacts two membrane leaves to ensure that the membrane envelope is fully sealed. As the membrane envelope is wound around the central tube, the diameter of the element increases. However, the torque and force on the membrane envelope are maximum at the central tube with the smallest diameter. During winding, the larger force near the central tube produces a larger force on the membrane envelope, particularly on the feed space. The important advantage of printed spacer technology is that a more open feed spacer channel can be produced, and a tighter printed spacer spacing can be provided at the central tube where stress concentration is higher.

[0053] The present invention solves the problem that using hot melt materials to create spacers on membrane sheets reduces the flux and salt retention rate of the membrane surface. The hot melt material is applied as a liquid at an elevated temperature, and the liquid hardens as the temperature decreases, and therefore does not require UV or light to drive the chemical reaction that causes the liquid resin in the form of a printed pattern to harden. Therefore, there is no UV or other wavelength energy damage applied to the active surface of the membrane, which may damage the flux or retention of the membrane surface. Compared with light-cured adhesives, hot melt materials also have significant cost advantages. In addition, compared with UV or light-cured photopolymers, hot melt materials can be recovered and recycled at the end of the life of the membrane element.

[0054] Another problem with previously used printing methods is that the photopolymer material can spread or overspray before the printed material hardens (due to the low surface tension and / or viscosity of the printed material) and cover more of the film surface than intended. This excess coverage can block film flow, resulting in reduced efficiency. Due to the higher viscosity of the hot melt material and because the hot melt material cools when it hits the surface, the ability to spread on the film is reduced.

[0055] Another aspect of the present invention is that hot melt material can be quickly applied by printing a pattern in one direction with one or more print heads. The pattern first lays a continuous line pattern at the edge of the membrane, typically 3 inches (although it can be greater than or less than 3 inches), which provides support for the glue line when the membrane envelope is glued to seal the permeate carrier between the two membrane sheets. The print head continues to print on the length of the membrane sheet, laying a series of short dotted or curved line segments or spacers, which produce a support pattern with a smaller density in the middle of the membrane sheet than at the edge. On the opposite side of the sheet, the print head then lays the support line on the retained end of the membrane sheet in the same manner as the inlet feed support line. The print head can then be indexed downward along the longitudinal length of the membrane sheet for a preferred distance of 0.100 inches, and the print head then moves backward in the direction opposite to the first printed line. Alternative line spacing can be in the range of 0.040 inches to 1.00 inches. In this second pass, the dense pattern at the edge of the membrane is printed, but the short pattern in the open area of ​​the membrane may or may not be printed, depending on the desired longitudinal spacing of the short pattern in the middle of the membrane sheet. For example, the printed pattern may skip printing short dashes or line segments in two passes of the print head. Thus, the dense line pattern at the feed end and the intercept end of the membrane sheet will be printed every 0.100 inches down the longitudinal length of the membrane, but the short dashed line pattern will only be printed every 0.300 inches down the longitudinal length of the membrane sheet. Of course, many variations can be made in the longitudinal length of the spacing and the length of the edge printed pattern, as well as the spacing of the shorter straight or curved patterns in the middle of the membrane.

[0056] The system for applying hot melt to the film surface can include a dual-axis frame system and an adjustment mechanism for changing the height of the print head above the film sheet. The system can be controlled by a programmable logic controller (PLC), and the printing pattern can be loaded into the program using conventional software designed for controlling a plotter. A high-frequency print head is used to apply the hot melt material to the film sheet. The film sheet can be held in place with a vacuum table to ensure that the film sheet does not move and is held at a fixed height in the printing press. A single sheet can be printed, or a roll-to-roll system can feed the film to the printing area. The hot melt feed system can be configured by a conventional main body hot melt feed system, which has a heated feed line to the print head. A pressure control system can be integrated into the hot melt feed system to ensure that a constant and stable supply of hot melt material is delivered to the print head. Although a single print head can print a film sheet, multiple print heads operating together can reduce the time required to print an entire sheet.

[0057] Droplet volume, melting temperature, feature size, support damage (solidification speed vs. printing speed). Printing with hot melt on a film requires that the applied droplet have a sufficiently low amount of thermal energy to avoid damaging the 30nm to 1 micron separation layer on the coated film. This leads to a preferred process using resins with a curing temperature between 45°C and 20°C. Materials with lower curing temperatures cannot withstand typical cleaning. Materials with higher curing temperatures may cause damage to the film due to hot material deposited on the film. Droplet volume also affects the amount of thermal energy applied, and therefore, it has been found that applying multiple small droplets is better for materials with higher application temperatures. Preferred features are typically a diameter of 100-1500 microns or the length of the dash segment, preferably 2500 microns, but as small as 250 microns and up to 5000 microns or longer, and a height preferably from 75 microns to 1200 microns or greater. The preferred droplet volume per impact from the print head is less than 100 nanoliters, but more preferably less than 50 nanoliters, and even more preferably less than 10 nanoliters.

[0058] The surface can be oleophobic or chemically modified, or have microfeatures that minimize spreading.

[0059] Preferred printed features may also have a section in the middle of the feature (midway between the base of the film and the top of the feature) where the diameter of the feature is wider or narrower than the base. This geometry may give improved mixing properties.

[0060] Sharp edges at the top of a feature can cause damage to the opposing film surface in contact with the feature. Preferably, the feature can have a profile similar to a dome, with essentially no sharp corners. Due to the cooling and coalescing characteristics of the hot melt material, the surface of the feature is typically rounded and does not have sharp peaks or edges.

[0061] A typical hardness range for hot melt materials applied to the film surface is preferably about 70 Shore A. Alternative durometers may range from 80 to as low as 40 Shore A.

[0062] The preferred pattern has a repeating pattern that is divisible into integer ratios in the center region (i.e., not the denser edge pattern). More specifically, the center section can have features in a row from edge to edge, where the feature positions are the same for the left and right halves, or the same for the left third, center third, and right third, and so on, until each feature in a row has the same spacing from edge to edge. The positions between adjacent rows can be aligned or offset, but the row position symmetry is preferably the same. This results in more efficient mixing and an improved application process.

[0063] Hot melt application is a non-contact process, whereby the print head is preferably positioned 0.2 inches above the film surface, but this can range from 0.02 to 0.5 inches above the film surface. Non-contact printing avoids damage to the film surface, which can be a problem with application techniques such as screen printing or stencil printing. Hot melt dispensing also has the advantage of using fewer print heads than photopolymer inkjet printing. Fewer print heads means faster maintenance for cleaning heads and less downtime.

[0064] There are many characteristics that affect the deposition rate of the material on the substrate in hot melt materials. These include viscosity, temperature, print head travel speed, the frequency of each drop of the print head, nozzle size, the pressure of the hot melt on the nozzle, and other factors. In an exemplary embodiment, for a pattern 0.02 inch (0.5mm) wide and 0.02 inch (0.5mm) high, when the print head speed is 15mm / s, there will be a print head frequency of about 400Hz and a drop size of 10 nanoliters. One of the advantages of a stream of small droplet size is that, for example, the hot melt applied from the print head at a nominal temperature of 170 degrees Celsius will have a Joule heat content lower than that of larger droplets, and will cool at a rate faster than larger droplets, because smaller droplets have an advantage in terms of surface area versus heat dissipation rate compared to larger droplets. Compared with the continuous hot melt stream applied from a syringe-type nozzle, a series of small droplets will also give less heating to the substrate. Faster cooling will minimize the damage to the active polymer film coating or support layer below, and will minimize the damage to the membrane salt interception. The minimal overspray from high-frequency hot melt tappet-type printheads also minimizes flux loss (the volume of fluid passing through a given surface area) due to overspray, a characteristic of inkjet-type printing. Hot melt printing does not overspray due to rapid cooling and does not spread the way adhesive does in stencil-type systems. Before stencil-type adhesives are UV or light-cured, the adhesive tends to spread around the printed spacers and reduce the active membrane surface area, thereby reducing the flux and productivity of the membrane element.

[0065] A variety of properties will determine build height and printing speed. Various manufacturers offer hot melt materials that are well suited for producing printed spacers on film substrates. Through these different properties and precise control of the X and Y directions of the gantry system, unlimited configurations of printed spacers can be produced at practical heights for feed spacer applications.

[0066] One aspect of a more open feed space is that the forces applied to the membrane envelope and thus to the feed spacer are concentrated resulting in higher forces applied to the feed spacer elements, particularly near the central tube.Hot melt printing of the spacer according to the invention can also solve these problems.

[0067] The feed forming features employed can have any of a variety of shapes, including dots, ovals, strips with rounded ends, lens forms, stretched polygons, lines, or other geometric shapes. Due to the shape of the features and the fact that the fluid must traverse around the outside of the features, the fluid flow rate will locally change in the region between the feed spacer features from the feed end to the retentate end of the membrane element. An efficient printed spacer pattern will allow maximum flow from the feed end to the retentate end of the membrane, will provide very little flow resistance from the feed to the retentate end, will minimize the formation of stagnation points on the leading and trailing edges of the spacers, and will help promote mixing of the feed solution to reduce concentration polarization in the feed space.

[0068] In a spiral wound element, the membrane leaf is folded at the centerline where it contacts the permeate carrier at the center tube prior to winding. Fold protection is described in the prior art. Fold protection typically consists of a tape applied along the width of the membrane sheet at the fold of the membrane sheet. The prior art also discusses fold protection applied by printing or otherwise applying a polymer or other resin as a fold protection material. Fold protection is used to protect the membrane leaf that is creased when folded to avoid damage from the crease. Without fold protection, damage in the fold can result in retention and flux loss in the finished membrane element. Fold protection can be uniquely utilized in printed spacer technology by extending the fold protection above the top of the printed spacer feature near the center tube to help avoid stress concentration from the printed spacer feature damaging the active surface of the membrane on the unprinted side of the membrane leaf.

[0069] Figure 11 is a schematic diagram of a conventional spiral wound membrane element before winding, showing the important elements of a conventional spiral wound membrane element 100. The permeate collection tube 12 has a hole 14 in the collection tube 12 for collecting the permeate fluid from the permeate carrier 22. In manufacture, the membrane sheet 36 is a single continuous sheet folded at the center line 30, consisting of an inactive porous support layer (e.g., polysulfone) on one side 28 and an active polymer membrane layer bonded or cast onto the support layer on the other side 24. A porous polyester structural layer can be between the support layer and the active layer. In the assembled element, the active polymer membrane surface 24 is adjacent to the feed spacer web 26, and the inactive support layer 28 is adjacent to the permeate carrier 22. The feed solution 16 enters between the active polymer membrane surfaces 24 and flows through the open space in the feed spacer web 26. As the feed solution 16 flows through the feed spacer web 26, particles, ions, or chemicals rejected by the membrane are retained at the active polymer membrane surface 24, and permeate fluid molecules (e.g., water molecules) pass through the active polymer membrane surface 24 and into the porous permeate carrier 22. As the feed solution 16 passes along the active polymer membrane surface 24, the concentration of the material rejected by the membrane increases due to the loss of permeate fluid from the bulk feed solution 16, and this concentrated fluid exits the retentate end of the active polymer membrane sheet 24 as retentate solution 18. The permeate fluid in the permeate carrier 22 flows from the distal end 34 of the permeate carrier 22 in the direction of the central tube 12, where the permeate fluid enters the central tube 12 through the central tube inlet aperture 14 and exits the central tube 12 as permeate solution 20. To prevent contamination of the permeate fluid with the feed solution 16, the non-active polymer membrane layer 28 is sealed with adhesive through the permeate carrier 22 along the adhesive line 32, thereby creating a sealed membrane envelope in which the only exit path for the permeate solution 20 is through the central tube 12. Typically, the width of adhesive line 32 is 1" to 3" after the adhesive has been compressed during the winding process.

[0070] Figure 2 A partially assembled spiral wound membrane element 200 is shown in FIG. Figure 1 As shown, the membrane envelope 40 comprises membrane sheets 36 folded at one end with the permeate carrier 22 disposed between the membrane sheets, which are sealed along the edges with a suitable adhesive line 32 ( Figure 1). In conventional designs of membrane elements, once wound, a feed spacer web 26 is placed adjacent to the membrane envelope 40 to allow flow of the feed fluid 16 between the layers of the membrane envelope 40 and expose all of the active polymer surface 24 of the membrane sheet to the feed fluid. The permeate or product fluid is collected in the permeate carrier 22 within the membrane envelope 40 and travels spirally down to the center tube 12 where it is collected while the retentate stream 18 exits the element. A single spirally wound element may include a single membrane envelope and feed spacer layer, or may include multiple membrane envelope and feed spacer layers stacked and wound together to form the element.

[0071] Figure 3 Depicts a membrane element disassembled for inspection. Figure 3 The element is prior art, which shows a membrane sheet damaged by a rigid circular spacer printed on the membrane sheet. Figure 4 As shown, a membrane sheet 54 having an active membrane polymer layer 52 is wound around the central tube 12. The spacer features 50 can be rigid and will not bend when printed via inkjet printing, screen printing, stencil printing, or other printing methods. Thus, the polymer layer 52 is lifted from the polysulfone support layer at breakpoints 56a and 56b, allowing ions to enter the damaged area and reduce the retention characteristics of the assembled element. The hot melt material is flexible in nature, with a Shore A hardness of approximately 70, and will conform to the membrane surface and will not cause lifting at the edges of the printed pattern.

[0072] Likewise, from a plotter's perspective, printing straight lines is more efficient than printing lines at the edges of the sheet and dots in the middle. Figure 5 A cross section of a narrow spacer 50 is shown. In one embodiment, the spacer may be a 0.050 inch diameter dot. However, the same support area can be provided by a printed dashed line 0.020 inch wide and 0.100 inch long. The printed spacer 50 is also more flexible and will conform to curved surfaces when wrapped around the base tube 12. This avoids cracks 58a and 58b in the polymer coating 52 when the membrane sheet 54 is wrapped around the base tube 12.

[0073] Figure 6, which illustrates an efficient printing pattern for a hot melt printer. For a hot melt printer, a 0.050-inch diameter dot requires the print head to print a circle and fill the dot. This slows the print head and increases the time required to print the entire pattern on the membrane sheet 36, starting from the inlet edge 66 and continuing to the outlet edge 64. However, the print head can travel in one direction and print a longer support line 76 at the inlet end 16 of the membrane sheet, print spacers 68, and continue to the other end of the membrane sheet 36 to print the outlet line 72 at the discharge end of the membrane. In the exemplary embodiment, it should be noted that every other edge support line 72 or 76 does not have a printed spacer 68 aligned with the inlet and outlet edge support lines 72 and 76. When the spacers 68 are printed in the same line as the edge support lines 72 and 76, the print head can travel at the same speed from the inlet to the outlet end of the membrane sheet 36, such as the less dense spaces 74. When spacer 68 is not printed in line with support lines 72 and 76, the printing head can be accelerated at high speed to the other end of the membrane sheet 36 in a less dense space 74, and then slowed down to print support lines 76 or support lines 72 (depending on the direction of travel of the print head). This process can reduce the time of printing a complete pattern on the membrane sheet 36. The time required to print a complete membrane spacer pattern on half of the membrane sheet 36 will determine the final efficiency and cost of printing the membrane sheet. The shorter the printing time, the better. The spacing X of the printed lines 72, 74, 76 is preferably 0.10 inches, but the range of the spacing can be from 0.20 to 1.0 inches. It should also be noted that only half of the membrane sheet 36 is printed and folded at the fold line 30. This also speeds up the printing process by printing only half of the membrane sheet 36. Spacer 68 can be printed high enough to create the necessary space between the membrane sheets, thereby creating a fluid feed space.

[0074] Figure 7 8. A system 800 is shown for printing a pattern 92 on a film sheet 88. The film sheet 88 can be cut individually and placed in the printer system 800, or the film sheet 88 can be rolled up and down from the printer system 800. The film sheet 88 is placed on the printer system 800 so that the left end of the film sheet 88 is positioned above the vacuum table 90 to secure the film sheet 88 in place and ensure that the film sheet 88 is flat and maintains a consistent vertical space between the film sheet 88 and the print head 94. The right end of the film sheet 88 can be placed on a flat support table 86. The print head 94 is mounted on a print head support 84, which is mounted to one or more X-axis tables 82. The print head support 84 can also be mounted on a Y-axis table 80, depending on the design of the frame system. Assuming that the print heads 94 are less than or equal to 2.0 inches wide, they can be offset from each other and face each other to maximize the number of print heads 94 in the system. The more print heads 94 there are in the system, the faster the film sheets 88 can be printed.

[0075] Figure 8 Shown are a hot melt spacer feature 104 and an inkjet or stencil spacer feature 106 between film sheets 100 and 102. Due to the melting and cooling properties of the hot melt spacer, the hot melt spacer 104 has a domed top that does not damage the active surface of the film sheet 102. In contrast, the spacer feature 106 applied with an inkjet or stencil process may have sharp points 110 at the top of the spacer, which may damage the sensitive polymer surface of the film sheet 102. This characteristic of the hot melt spacer makes it much less likely to damage the surface of the film sheet 102. The hot melt spacer 104 can also be made of a compound with very low viscosity. When the film element winding process causes the film sheets 100 and 102 to slide adjacent to each other during the film element winding process, this feature of the hot melt spacer 104 will help avoid damage to the sensitive polymer surface of the film sheet 102.

[0076] Figure 9 The formation of a spacer feature is shown, which is constructed from small hot melt droplets 120 leaving the hot melt print head. In actual practice, these individual droplets will coalesce into a uniform form. In this way, the various cross-sections of the spacers can be configured to help create turbulence in the feed fluid flow due to the shape of the spacers. Figure 9 In an embodiment, the concave spacer can be configured so that it is positioned between the membrane sheets 100 and 102. Figure 10 is a representation of a concave spacer feature where all of the hot melt droplets have coalesced and cooled to form a solid spacer feature 124 . Figure 11 is an embodiment of a convex spacer feature 128 that hardens and forms a uniform spacer feature 124 .

[0077] The present invention has been described in conjunction with various exemplary embodiments. It should be understood that the above description is merely illustrative of the application of the principles of the present invention, the scope of which will be determined by the claims read in light of the specification. Other variations and modifications of the present invention will be apparent to those skilled in the art.

Claims

1. A membrane assembly for a spirally wound filter element, the membrane assembly comprising: (a) a membrane sheet, comprising a thin film composite structure comprising a porous structure layer, a support layer, and an active membrane layer; (b) a plurality of edge spacers comprising hot melt segments disposed on the active film layer, wherein the length of each edge spacer is less than one-quarter of the distance between opposing first and second edges of the film sheet, the edge spacers being positioned adjacent the opposing first and second edges and arranged with their major axes perpendicular to the first edge, wherein adjacent edge spacers are separated by a first distance in a direction parallel to the first edge; (c) a plurality of intermediate spacers comprising hot melt line segments arranged on the active film layer, wherein the length of each intermediate spacer is less than one quarter of the distance between the first and second opposite edges of the film sheet, the intermediate spacers are positioned between the first and second opposite edges and arranged with the major axis perpendicular to the first edge, wherein adjacent intermediate spacers are separated by a second distance in a direction parallel to the first edge, wherein the second distance is greater than the first distance.

2. The membrane module according to claim 1, wherein The second distance is an integer multiple of the first distance.

3. The membrane module according to claim 1, wherein Each edge spacer is at least one inch and no more than four inches in length.

4. The membrane module according to claim 1, wherein The plurality of intermediate spacers includes a plurality of groups of intermediate spacers, wherein each group of intermediate spacers includes a plurality of intermediate spacers, each of the plurality of intermediate spacers is arranged along a single line perpendicular to the first edge, wherein the single line coincides with the edge spacer, and wherein the intermediate spacers in a group include line segments separated from each other by a third distance in a direction along the single line, wherein the third distance is greater than the first distance.

5. The membrane module according to claim 4, wherein Each group of intermediate spacers is separated from an adjacent group of intermediate spacers by a third distance, wherein the third distance is an integer multiple of the first distance.

6. The membrane module according to claim 1, wherein The cross-section of each intermediate spacer is rounded at the end facing away from the membrane sheet.

7. The membrane module according to claim 1, wherein Each intermediate spacer has convex sides in cross-section.

8. The membrane module according to claim 1, wherein Each intermediate spacer has concave sides in cross-section.

9. The membrane module according to claim 1, wherein The hot melt is liquid at temperatures above 100C.

10. The membrane module according to claim 10, wherein The hot melt is liquid at temperatures above 170°C.

11. The membrane module according to claim 1, wherein The hot melt has low viscosity.

12. The membrane module according to claim 2, wherein The second distance is three times the first distance.

13. The membrane module according to claim 1, wherein The intermediate spacers are 100 to 1500 microns wide, 250 to 5000 microns long, and 75 to 1200 microns high.

14. A method for manufacturing a membrane module, the method comprising: (a) providing a membrane sheet, the membrane sheet comprising a thin film composite structure, the thin film composite structure comprising a porous structure layer, a support layer, and an active membrane layer; (b) forming a plurality of edge spacers by depositing a hot melt onto the active film layer in a plurality of line segments, wherein the length of each edge spacer is less than one-quarter of the distance between the first and second opposite edges of the film sheet, the edge spacers being positioned adjacent the first and second opposite edges and arranged with their major axes perpendicular to the first edge, wherein adjacent edge spacers are separated by a first distance in a direction parallel to the first edge; (c) forming a plurality of intermediate spacers by depositing a hot melt in a plurality of line segments on the active film layer, wherein the length of each intermediate spacer is less than one quarter of the distance between the opposite first edge and the second edge of the film sheet, the intermediate spacers are positioned between the opposite first edge and the second edge and arranged with the long axis perpendicular to the first edge, wherein adjacent intermediate spacers are separated by a second distance in a direction parallel to the first edge, wherein the second distance is greater than the first distance.

15. The membrane module according to claim 14, wherein The second distance is an integer multiple of the first distance.

16. The membrane module according to claim 14, wherein Each edge spacer is at least one inch and no more than four inches in length.

17. The membrane module according to claim 14, wherein The plurality of intermediate spacers includes a plurality of groups of intermediate spacers, wherein each group of intermediate spacers includes a plurality of intermediate spacers, each of the plurality of intermediate spacers is arranged along a single line perpendicular to the first edge, wherein the single line coincides with the edge spacer, and wherein the intermediate spacers in a group include line segments separated from each other by a third distance in a direction along the single line, wherein the third distance is greater than the first distance.

18. The membrane module according to claim 17, wherein Each group of intermediate spacers is separated from an adjacent group of intermediate spacers by a third distance, wherein the third distance is an integer multiple of the first distance.

19. The membrane module according to claim 15, wherein The second distance is three times the first distance.

20. The membrane module according to claim 14, wherein The cross-section of each intermediate spacer is rounded at the end facing away from the membrane sheet.

21. The membrane module according to claim 14, wherein Each intermediate spacer has convex sides in cross-section.

22. The membrane module according to claim 14, wherein Each intermediate spacer has concave sides in cross-section.

23. The membrane module according to claim 14, wherein The hot melt is liquid at temperatures above 170°C.

24. The method of claim 14, wherein: Forming the plurality of edge spacers includes conveying a hot melt dispenser from near the first edge to near the second edge in a direction perpendicular to the first edge; and applying hot melt through the dispenser while the dispenser travels from near the first edge to a position a distance from the first edge equal to the length of the edge spacer, and applying hot melt through the dispenser while the dispenser travels from a position a distance from the second edge equal to the length of the edge spacer to near the second edge.

25. The method of claim 14, wherein: Forming the plurality of intermediate spacers includes applying hot melt through the dispenser while the dispenser travels from a position of a starting point of the intermediate spacers to a position of an end point of the intermediate spacers while the dispenser is being conveyed.

26. The method of claim 25, wherein forming a plurality of intermediate spacers comprises: Hot melt is applied through the dispenser over less than the entire dispenser travel from the first edge to the second edge.

27. The method of claim 14, wherein the dispenser dispenses the hot melt at a rate of less than 100 nanoliters per drop.

28. The method of claim 27, wherein the dispenser dispenses the hot melt at a rate of less than 50 nanoliters per drop.

29. The method of claim 28, wherein the dispenser dispenses the hot melt at a rate of less than 10 nanoliters per drop.

30. A method of producing a recirculating spiral wound element, the method comprising: (a) providing one or more initial spirally wound elements having spacing features comprising a hot melt; (b) disassembling the one or more initial spirally wound elements; (c) recovering some or all of the hot melt from the initial spirally wound element; (d) using the hot melt to produce one or more membrane modules as claimed in claim 1; (e) winding the one or more membrane modules into a recirculating spiral wound element.

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