Hot Melt Film Spacer

Abstract

Hot melt printed spacer film elements offer the unique advantage of applying arbitrary patterns onto the film surface to act as a feed spacer material. This technology also eliminates damage to the working surface of the film by avoiding either photocuring, UV, light, or other wavelengths of energy. By printing narrow features, the bending moment on the film surface induced by the printed feature is less than that induced by a wider printed feature, thereby minimizing damage to the sensitive film coating.

Description

[Technical Field]

[0001] Technical Field The present invention relates to membrane systems utilized in the separation of fluid components, and in particular to spiral wound membrane elements or flat sheet membrane systems. [Background technology]

[0002] Background technology In cross-flow filtration, a feed fluid flows through the filter and is discharged at the other end, while a portion of the fluid is removed by filtration through membrane surfaces parallel to the direction of fluid flow. Cross-flow filtration exists in various forms, including plate-and-frame systems, cassette systems, hollow fiber systems, radial systems, or spiral wound systems. Filtration modules in plate-and-frame, cassette, radial, and spiral wound systems often rely on stacked membrane layers to provide spacing between adjacent filtration membrane layers. The present invention is primarily, but not exclusively, directed to spiral wound membrane elements.

[0003] Spiral-wound membrane filtration 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 forms a path for fluid removal through the membrane to a central tube, longitudinally relative to the axis of the central tube, the laminate structure being spirally wrapped around the central tube and spaced from itself by a porous feed spacer to allow axial flow of fluid through the element from the feed end to the reject end of the element. Conventionally, a feed spacer mesh is used to allow the flow of feed water (a portion of which passes through the membrane) into the spiral-wound element and to allow reject fluid to exit the element in a direction parallel to the central tube and along the axis of the element structure.

[0004] Improvements to spiral wound element design are disclosed in U.S. Patent No. 6,632,357 to Barger et al., U.S. Patent No. 7,311,831 to Bradford et al., and in Australian (2014223490), Japanese (6499089), Chinese (CN 105163834B), Israeli (240883), and Korean (10-2196776) patents to Roderick et al. entitled "Improved Spiral Wound Element Construction," which replace feed spacers with islands or protrusions printed, deposited, or embossed directly onto the active or non-active surfaces of the membrane or onto the permeate carrier. U.S. Patent No. 11,090,612 to Roderick et al. entitled "Graded spacers for filtration wound elements" describes the use of height-graded spacer features used to modify the 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 for spiral wound elements that keep the membrane feed space open but provide support for the membrane envelope adhesive area during winding. U.S. Patent Application No. PCT / US18 / 55671 to Roderick et al., entitled "Bridge Support and Reduced Feed Spacers for Spiral-Wound Elements," describes support features applied to the distal end (end farthest from the central tube) of the membrane envelope to provide support during adhesive bonding and winding of the spiral wound element. U.S. Patent Application PCT / US21 / 40353 to Herrington et al., entitled "Variable Velocity Patterns in Cross Flow Filtration," describes support patterns that vary in size from the feed end to the reject end of the membrane feed space within a feed flow channel parallel to a central tube to control the velocity of the feed solution as its concentration increases from the feed end to the reject end of a spiral wound element.U.S. Patent No. 11,083,997 to Roderick et al., entitled "Non-Nesting Patterns," describes providing denser patterns at the feed and rejection ends of a membrane feed space and more open patterns in the middle to prevent nesting of printed patterns during element fabrication, particularly during the membrane envelope bonding process that supports glue lines. PCT application PCT / US21 / 26030 to Herrington et al., entitled "Independent Spacers and Methods," describes various methods for applying spacers to membrane surfaces that do not expose the membrane surface to UV or visible light from photocured inkjet, stencil, or screen printing processes. U.S. Patent Application No. 63,294,377 to Herrington et al., entitled "High Rejection Element," describes a membrane printing and assembly process that provides support to membrane sheets in areas of high stress concentration to avoid damage to the membrane active layer. U.S. Patent Application No. 63,294,378 to Kurth et al., entitled "Spiral Element Enhanced Capacity," describes a membrane printing and assembly process that provides improved membrane design features to increase permeate flux in spiral wound elements.

[0005] To date, most printed spacer technologies have used multi-pass UV- or light-cured inkjet processes to increase the pattern height within a layer. Alternatively, single-layer stencil printing processes utilizing epoxies 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, as the stencil can provide the desired spacer height in a single pass, whereby inkjet printing is typically applied in multiple passes. Summary of the Invention [Means for solving the problem]

[0006] Summary of the Invention The present invention describes a novel method of applying and configuring printed spacer features that avoids damage to the polyamide coating of thin film composite (TFC) membranes due to the bending action of printed features adhered to the TFC surface, and further, maximizes the printing speed of patterns onto the membrane surface, as well as facilitating the innovative application of thermally cured waxes and hot melt materials.

[0007] An exemplary embodiment of the present invention provides a membrane assembly for a spiral-wound filtration element, the membrane assembly comprising: (a) a membrane sheet comprising a thin film composite structure including a porous structural layer, a support layer, and an active membrane layer; and (b) a plurality of edge spacers comprising hot melt line segments disposed on the active membrane layer, each edge spacer having a length less than 1 / 4 of the distance between first and second opposing edges of the membrane sheet, positioned near the first and second opposing edges, and disposed with a major axis perpendicular to the first edge, wherein adjacent edge spacers are spaced apart from the first edge by a distance less than 1 / 4 of the distance between the first and second opposing edges of the membrane sheet. and (c) a plurality of intermediate spacers comprising line segments of hot melt disposed on the active membrane layer, each intermediate spacer having a length less than 1 / 4 of the distance between the first and second opposing edges of the membrane sheet, positioned between the first and second opposing edges and arranged with a major axis perpendicular to the first edge, adjacent intermediate spacers being separated by a second distance in the direction parallel to the first edge, the second distance being greater than the first distance.

[0008] In some embodiments, the second distance is an integer multiple of the first distance. In some embodiments, each edge spacer has a length of at least 1 inch and not more than 4 inches. In some embodiments, the plurality of intermediate spacers comprises a plurality of sets of intermediate spacers, each set of intermediate spacers comprising a plurality of intermediate spacers respectively arranged along a single line perpendicular to the first edge, the single line coinciding with the edge spacers, wherein the intermediate spacers of a set comprise line segments spaced a third distance apart from each other in a direction along the single line, the third distance being greater than the first distance.

[0009] In some embodiments, each set of intermediate spacers is separated from an adjacent set of intermediate spacers by a third distance, the third distance being an integer multiple of the first distance. In some embodiments, each intermediate spacer has a rounded cross-section at the end away from the membrane sheet. In some embodiments, each intermediate spacer has a cross-section with convex sides. In some embodiments, each intermediate spacer has a cross-section with concave sides. In some embodiments, the hot melt is liquid at temperatures above 100°C. In some embodiments, the hot melt is liquid at temperatures above 170°C. In some embodiments, the hot melt has low tack. In some embodiments, the second distance is three times the first distance. In some embodiments, the intermediate spacers have a width of 100 to 1500 micrometers, a length of 250 to 5000 micrometers, and a height of 75 to 1200 micrometers.

[0010] An exemplary embodiment of the present invention provides a method of making a membrane assembly, the method comprising: (a) providing a membrane sheet comprising a thin film composite structure comprising a porous structural layer, a support layer, and an active membrane layer; and (b) depositing a hot melt as a plurality of line segments onto the active membrane layer to form a plurality of edge spacers, each edge spacer having a length less than ¼ of the distance between first and second opposing edges of the membrane sheet, positioned near the first and second opposing edges, and arranged with a major axis perpendicular to the first edge, and adjacent edge spacers arranged parallel to the first edge. (c) forming a plurality of intermediate spacers by depositing hot melt onto the active membrane layer as a plurality of line segments, each intermediate spacer having a length less than 1 / 4 of the distance between the first and second opposing edges of the membrane sheet, positioned between the first and second opposing edges, and arranged with a major axis perpendicular to the first edge, with adjacent intermediate spacers separated by a second distance in a direction parallel to the first edge, the second distance being greater than the first distance.

[0011] In some embodiments, the second distance is an integer multiple of the first distance. In some embodiments, each edge spacer has a length of at least 1 inch and no more than 3 inches. In some embodiments, the plurality of intermediate spacers comprises multiple sets of intermediate spacers, each set of intermediate spacers comprising a plurality of intermediate spacers respectively arranged along a single line perpendicular to the first edge, the single line coinciding with the edge spacers, wherein the intermediate spacers of a set comprise line segments spaced a third distance apart from each other in a direction along the single line, the third distance being greater than the first distance. In some embodiments, each set of intermediate spacers is spaced a third distance apart from an adjacent set of intermediate spacers, the third distance being an integer multiple of the first distance.

[0012] In some embodiments, the second distance is three times the first distance. In some embodiments, each intermediate spacer has a rounded cross-section at the end away from the membrane sheet. In some embodiments, each intermediate spacer has a cross-section with convex sides. In some embodiments, each intermediate spacer has a cross-section with concave sides. In some embodiments, the hot melt is liquid at temperatures above 170°C.

[0013] In some embodiments, forming the plurality of edge spacers includes conveying a hot melt dispenser along a direction perpendicular to the first edge from near the first edge to near the second edge, and dispensing hot melt through the dispenser while the dispenser moves from near the first edge to a position at a distance from the first edge equal to the length of the edge spacer, and while the dispenser moves from a position at a distance from the second edge equal to the length of the edge spacer to near the second edge. In some embodiments, forming the plurality of intermediate spacers includes dispensing hot melt through the dispenser while the dispenser moves from a position at which the intermediate spacer starts to a position at which the intermediate spacer ends, while conveying the dispenser. In some embodiments, forming the plurality of intermediate spacers includes dispensing hot melt through the dispenser for less than all of the stroke of the dispenser from the first edge to the second edge. In some embodiments, the dispenser dispenses 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 of manufacturing a regenerated spiral wound element, the method including: (a) providing one or more initial spiral wound elements having spacing features comprising 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 manufacture one or more membrane assemblies described herein; and (e) winding the one or more membrane assemblies into the regenerated spiral wound element. [Brief explanation of the drawings]

[0015] BRIEF DESCRIPTION OF THE DRAWINGS [Figure 1] FIG. 2 is an exploded view of a spiral-wound membrane element. [Figure 2] FIG. 2 is an exploded view of a partially assembled spiral wound membrane element. [Figure 3] FIG. 10 is a diagram of a printed film surface showing damage to the active film layer from a wide longitudinal spacing feature of the rolling element. [Figure 4] FIG. 10 is an end view of a spiral wound element showing the geometry of the printed features and damage to the film surface. [Figure 5] FIG. 10 is an end view of a spiral wound element showing thin printed features that avoid damaging the membrane surface as the membrane sheet is wrapped around the center tube. [Figure 6] FIG. 1 is a diagram of a printing pattern on a film sheet that facilitates high speed printing using single or multiple pass thermosetting materials. [Figure 7] 1 is a schematic diagram of a hot melt production plotter layout. [Figure 8] FIG. 10 is a side view of printed spacers with and without acute angles on opposing film surfaces. [Figure 9] FIG. 10 is a side view of a concave hot melt spacer showing the accumulation of hot melt material droplets. [Figure 10] FIG. 1 is a side view of a hot melt spacer having recessed features. [Figure 11] FIG. 1 is a side view of a hot melt spacer having raised features. DETAILED DESCRIPTION OF THE INVENTION

[0016] MODES FOR CARRYING OUT THE INVENTION AND INDUSTRIAL APPLICABILITY Hot melt material. Hot melt technology, such as that of the present invention, can have significant advantages over alternative spacer application techniques. As used herein, "hot melt" refers to any polymer-based adhesive that is applied in a molten state. Hot melts can be applied by glue guns, high-frequency tappet-type nozzle heads, heated stencils, and other techniques. Due to their versatility, hot melts can be used in a variety of settings, including packaging, bookbinding, boxbinding, graphic arts, tape and labeling, product assembly, and spacers for spiral wound, flat plate, and pleated filtration and membrane systems.

[0017] Hot melts, such as those of the present invention, have several advantageous properties. They are fast-acting and can be applied in one or more layers, depending on the desired height of the pattern. The curing or setting time can be adjusted based on the needs of the application. Hot melts are generally safe to use and environmentally friendly. They can be used to bond difficult-to-bond surfaces. They are inherently safe and easy to transport and store, with a long shelf life. Being polymer-based, hot melts are more durable, more economical, and more tacky, producing fewer volatile organic compounds than solvent-based adhesives. Therefore, hot melt materials produce fewer undesirable volatile substances that can contaminate drinking water during reverse osmosis (RO) processes. This property helps ensure compliance with toxicology protocols when tested against NSF International requirements. NSF International ensures that water-contacting materials do not extract substances that could contaminate fluids (such as water) and cause adverse health effects for consumers. Due to their chemical nature, hot melts can be in any number of forms. This includes granules, pellets, bags, cakes, drums, bricks, slats, and pillows. Hot melts can also be applied in several ways, including high-frequency printhead nozzles, extrusion, meltblowing, spiral spraying, screen printing, stencil printing, and slot-die coating. Supply equipment for hot melts can be in the form of melt reservoirs, vacuum conveying, drum or pail unloaders, and pre-melters.

[0018] Generally, hot melts are composed of a polymer (which can be in a variety of forms) and several additives. These additives include resins, waxes, antioxidants, and plasticizers. Other chemicals can be added to give the hot melt more properties.

[0019] polymer. The basic component of hot melts (and many types of adhesives) is a polymer. These are long, repeating chains of specific molecules that have different properties based on the length of the chain and the type of molecule. The main polymers used in hot melts are ethylene vinyl acetate (EVA), polyolefins, polyamides and polyesters, styrenic thermoplastic elastomers, polyethylene, and ethylene methyl acrylate (EMA) or ethylene n-butyl acrylate (EnBA).

[0020] The polymer gives the hot melt its strength and flexibility, heat resistance, impact resistance, and shear properties. These properties are primarily driven by the type of polymer, its molecular weight, and its amount. Higher polymer content results in higher viscosity and greater flexibility and toughness. Lower polymer content typically results in lower viscosity.

[0021] Tackifying resin. The resin defines the tack of the hot melt. Tack is a measure of the adhesive's stickiness, essentially how well the adhesive stays attached after application. The resin determines the adhesive's wetting (i.e., how long it remains liquid while 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 to only one-half of the membrane sheet. The sheet is folded in half so that the unprinted side faces the printed side. It is important that the hot melt material does not have a sticky surface. The spacer that contacts the unprinted side of the membrane should not stick to the unprinted surface. During rolling, the membrane sheets shear slightly past each other, and the hot melt spacer should not stick to the opposing surface (as the two surfaces move relative to each other during rolling, potentially damaging the opposing surface). The active membrane layer is very thin, and any damage to the active layer will compromise the salt rejection properties of the membrane assembly.

[0022] The resin also has an effect on the tack of the hot melt. The choice of resin is determined by its compatibility with the base polymer, its softening point, and its inherent adhesion. The main types of tackifying resins used in hot melts are rosin and hydrogenated rosin, C9, hydrogenated hydrocarbon, terpene phenolic resins, rosin esters, and C5. Pure aromatic monomers are also used.

[0023] wax. The wax in a hot melt primarily controls the setting speed and open time. Open time is the time it takes for the hot melt to bond. This can range from a few seconds to an open period. The setting speed measures how quickly the hot melt can form a bond of acceptable strength. In addition to setting speed and open time, wax also affects the hot melt's heat resistance and subambient (i.e., below application temperature) adhesion. The main types of wax used in hot melts are natural wax, microcrystalline wax, and synthetic wax. Wax properties are determined by percent crystallinity, melting point, and molecular weight. With lower wax content, the hot melt has higher viscosity and greater flexibility, resulting in a stronger bond. With less wax, the hot melt has lower viscosity, solidifies faster, and bonds less strongly.

[0024] Antioxidant. Antioxidants are primarily used in hot melts to protect the material from degradation over its shelf life. Some of the antioxidants commonly used in hot melts include phenols, aromatic amines, phosphates, phosphites, and BHT. In addition to stabilizers, antioxidants are added in small amounts and do not affect the physical properties of the hot melt. They protect the hot melt not only during shelf life, but also during the molten state when added and mixed.

[0025] Plasticizers. Besides the base polymer and tackifying resin, plasticizers are the most common additives in hot melts. In fact, they are used as a kind of second base polymer to give the hot melt greater flexibility and toughness. Plasticizers are often hydrocarbon oils with low aromatic content and paraffinic chemical properties. Ideally, plasticizers are low volatility, transparent, and odorless. Using plasticizers, hot melts can achieve lower melt viscosities and faster wetting.

[0026] In addition to these primary components, hot melts can have any number of other additives and combinations of additives to impart specific desired properties. Biocides can be added to prevent bacterial growth. Fillers add bulk and strength while reducing cost. Hot melts may also be doped with flame retardants, as well as various pigments or materials such as copper, silver, carbon, and other conductive materials, as well as nanoparticles that can provide electrical properties to printed spacers and allow for the reduction of scale and biofilm.

[0027] Important defining properties of hot melts are viscosity, melt color, failure temperature (for shear and peel adhesion), softening point, substrate specific adhesion, thermal stability, cold crack formation, loop tack, and various mechanical properties.

[0028] viscosity. Viscosity is a measure of a liquid's thickness or how much it resists flow. High viscosity liquids move very slowly (think thick oil). Lower viscosity liquids, like water, flow more easily. Hot melt viscosity is not just a single value, but can depend on the application temperature (which can range from 250-400 degrees Fahrenheit). Higher viscosities are usually better for applying hot melts for print spacers because they can build height at faster travel speeds.

[0029] Melting color. The color of hot melts is rated on a numerical scale using both subjective and quantitative methods. These include the Gardner, Hunter, and Saybolt methods, and the Yellowness Index. Color is usually not an important criterion for printed spacer applications, since the spacers are usually out of the field of view.

[0030] Peeling. Peel is a measure of how much force is required to break the bond between two bonded surfaces. Peel is expressed in pounds per inch and can be measured at different angles (right angles and 180 degrees are most common) and for different surfaces. Peel is an important property for printed spacer applications because printed spacers are discrete components and should not move in spiral-wound applications. Acceptable adhesion for printed spacers is demonstrated when the features are removed from the polyamide surface, and the polymeric film material remains attached to the hot melt features, exposing the polysulfone support layer underneath.

[0031] Destruction temperature. As the name suggests, the failure temperature is the temperature at which the hot melt stops working. There are two types of failure temperature measurements used to characterize hot melts.

[0032] Peel Adhesion Failure Temperature (PAFT). At higher temperatures, it becomes easier to peel two surfaces joined with a hot melt. PAFT measures how well a hot melt resists peeling at higher temperatures.

[0033] Shear Adhesion Failure Temperature (SAFT). Shear is the force present when one surface slides over another. In a shear test, a specimen is mounted vertically and a weight is attached. How long it takes to pull the surfaces away from each other indicates how strong the hot melt is.

[0034] Softening point. The softening point of a hot melt (or any adhesive) is the temperature at which the adhesive begins to flow. The primary determinants of the softening point of a hot melt are the melting point of the wax being used and the transition temperature of the base polymer. The application temperature of hot melt materials for printed spacers is preferably in the range of 170°C, but can vary widely from 100°C to 200°C, depending on the ingredients of the hot melt material.

[0035] Base material specific adhesion. This measure is dependent on the type of material and is used with hot melts. Properties such as bond strength can be determined using the actual substrate. For printed spacer technology, where the substrate is a film, good adhesion is characterized by the ability of the hot melt to adhere to the thin film coating and separate it from the underlying polysulfone layer.

[0036] Thermal stability. How stable a hot melt is under temperature changes indicates how durable it will be overall. Hot melts with good pot life will not char or decompose at higher temperatures.

[0037] Mechanical properties. There are various mechanical properties associated with hot melts. These include tensile strength (how much force is required to break a specimen), yield point (how much stress can be applied to the hot melt before it permanently deforms), elongation at break (how much the specimen can stretch before breaking), and Young's modulus (the ratio of stress to strain). Other properties are combinations of these.

[0038] The variety of materials used to make hot melts and the properties of the resulting adhesives means that there is great variability in the function, performance, and cost of hot melt products.

[0039] In part, hot melts are attractive for nanomembrane filtration fabrication in accordance with the present invention because, when printing photopolymers, UV curing is not part of the process. In UV or photocuring applications, the energy imparted by the UV or light source can damage the polysulfone substrate, which can affect flux and salt rejection properties in nanomembrane applications.

[0040] Feed spacers and spiral wound elements. Feed spacers in spiral-wound filtration elements are necessary to maintain a fluid flow channel from the feed end to the reject end of the feed channel, and the spacer design also affects local flow rates, turbulence, stagnation zones, and other fluid flow conditions. Extruded mesh feed spacers have traditionally been used in membrane manufacturing due to their ease of integration into the manufacturing process, but by the nature of their design, many of their hydrodynamic properties depend on the spacer thickness. Traditional mesh spacers also provide uniform support characteristics in the feed space, from the distal end from the central tube to the proximal end of the membrane sheet near the central tube. Printed feed spacers offer unique design features not available with traditional extruded or woven mesh spacers, and their thickness and shape can be independently varied, resulting in a wide range of configurations that can be tailored to specific applications or the specific challenges encountered in constructing spiral-wound membrane elements.

[0041] Crossflow filtration, by its very nature, relies on a portion of the feed fluid passing through the membrane to become part of the permeate (product fluid), creating a situation in which the amount of feed fluid is constantly decreasing as it passes through the membrane. The greater the portion of permeate that is produced, the smaller the portion of the feed / concentrate fluid that continues to flow through the membrane element. As fluid passes through the element, a portion of the fluid passes through the membrane. A simple model suggests that for a constant flux through the membrane, the flow of the feed solution will gradually decrease as it flows through the element from the feed end of the feed space to the reject end. In practice, the amount of fluid passing at any point along the feed flow path depends on the local flow conditions, the local concentration of solutes or suspended solids, and the local pressure, which in turn depends on the backpressure within the feed space and locally on the permeate side of the element.

[0042] During the manufacture of spiral-wound elements, the permeate carrier material is attached to the central tube by tape or adhesive, the membrane envelope is placed adjacent to the permeate carrier, the flat sheet assembly is glued (to seal the permeate carrier envelope), and the envelope is rotated around the central tube by a rotating mechanism such as a lathe. The central tube is captured or keyed to a lathe so that the lathe can rotate the central tube and wrap the membrane envelope and permeate carrier around it. The torque on the central tube must be sufficient to wrap the envelope until the entire envelope is wrapped around the central tube. Sufficient tension must be maintained on the membrane envelope so that the glue penetrates completely through the permeate carrier and contacts both membrane leaves, completely sealing the membrane envelope. As the membrane envelope is wrapped around the central tube, the diameter of the element increases. However, the torque and force on the membrane envelope are greatest at the smallest diameter central tube. Higher forces proximal to the central tube result in higher forces on the membrane envelope, especially the feed space, during winding. A key advantage of the printed spacer technique is the ability to create a more open feed spacer channel, allowing for closer spacing of the printed spacers in the central tube where stress concentrations are higher.

[0043] The present invention addresses the problems of flux reduction and salt rejection at the membrane surface by using a hot-melt material to create spacers on the membrane sheet. The hot-melt material is applied as a hot liquid that solidifies as the temperature decreases; therefore, no UV or light is required to drive the chemical reaction that leads to the solidification of the liquid resin in the form of a printed pattern. Therefore, no damaging UV or other wavelengths of energy are imparted to the membrane's working surface, which could damage flux or rejection at the membrane surface. Hot-melt materials also offer significant cost advantages over light-cured adhesives. Furthermore, in contrast to UV- or light-cured photopolymers, hot-melt materials can be recovered and reused at the end of the membrane element's life.

[0044] Another problem with previously used printing methods is that, prior to solidification, the low surface tension and / or viscosity of the printing material allows the photopolymer material to spread, i.e., overcoat and cover more of the membrane surface than intended. This additional coverage can impede membrane flow, leading to reduced efficiency. The higher viscosity of the hot melt material and the fact that the hot melt material cools as it reaches the surface reduces its ability to spread over the membrane.

[0045] Another aspect of the present invention is the ability to rapidly apply hot melt materials by printing a pattern in one direction with a printhead or multiple printheads. The pattern first lays a continuous line pattern along the edge of the membrane, typically 3 inches long but can be longer or shorter, to provide support for the glue line when the membrane envelope is glued to seal the permeate carrier between two membrane sheets. The printhead continues printing along the entire length of the membrane sheet, laying a series of short dashed or curved segments, or spacers, which create a less dense support pattern in the middle of the membrane sheet than along the edges. Then, on the opposite side of the sheet, the printhead lays support lines on the reject end of the membrane sheet in the same manner as the inlet feed support lines. The printhead can then index a preferred distance of 0.100 inches down the longitudinal length of the membrane sheet, and then return in the opposite direction of the first printed line. Alternative line spacings can be between 0.040 inches and 1.00 inches. In this second pass, a high-density pattern is printed on the edge of the membrane, but short patterns in the open areas of the membrane may or may not be printed based on the desired longitudinal spacing of the short patterns in the middle of the membrane sheet. For example, the printed pattern may omit printing short dashed lines or segments during the second pass of the printhead. This results in high-density line patterns on the feed and reject ends of the membrane sheet being printed every 0.100 inches down the longitudinal length of the membrane, but short dashed line patterns being printed only every 0.300 inches down the longitudinal length of the membrane sheet. Of course, many variations in the spacing and length of the edge printed patterns and the spacing of the shorter straight or curved patterns in the middle of the membrane can be made in the longitudinal length.

[0046] The system for applying hot melt to the film surface can include a two-axis gantry system and an adjustment mechanism for varying the height of the print head above the film sheet. This system can be controlled by a programmable logic controller (PLC), and the print pattern can be loaded into a program using conventional software designed to control plotters. A high-frequency print head is utilized to apply the hot melt material to the film sheet. The film sheet can be held in place by a vacuum table, ensuring it does not move and is held at a fixed height in the printer. Individual sheets can be printed, or a roll-to-roll system can deliver the film to the print area. The hot melt delivery system can consist of a conventional bulk hot melt delivery system with a heated delivery path to the print head. A pressure control system can be integrated into the hot melt delivery system to ensure a constant and stable supply of hot melt material to the print head. While a single print head can print a film sheet, multiple print heads working together can reduce the time required to print a full sheet.

[0047] Droplet volume, melt temperature, feature size, support damage (set speed vs. print speed). Hot melt printing onto thin films requires that the applied droplets have a sufficiently small amount of thermal energy to avoid damaging the 30 nm to 1 micrometer separation layer on the coated membrane. This results in a preferred process using resins with a freezing temperature of 45°C to 200°C. Materials with lower freezing temperatures cannot withstand typical cleaning. Materials with higher freezing temperatures can lead to membrane damage due to deposition of hot material on the membrane. The droplet volume also affects the amount of thermal energy applied, and as a result, it has been found that materials with higher application temperatures are better applied in multiple droplets. Preferred features are typically dashed line segments with diameters or lengths of 100 to 1500 micrometers, preferably 2500 micrometers, but also as little as 250 micrometers and up to 5000 micrometers or more, and heights of 75 to 1200 micrometers or more. The preferred drop volume from the printhead per strike is less than 100 nanoliters, more preferably less than 50 nanoliters, and even more preferably less than 10 nanoliters.

[0048] The surface can be oleophobic or chemically modified, or can have microfeatures that minimize diffusion.

[0049] Preferred printed features can also have a section in the middle of the feature (midway between the membrane base and the top of the feature), where the diameter of the feature is wider or narrower than the base. This geometry can improve mixing performance.

[0050] Sharp corners at the top of the feature can cause damage to the opposing film surface with which the feature is in contact. Preferred features can have a dome-like profile and are largely free of sharp angles. For the cooling and cohesive function of the hot melt material, the surface of the feature is typically rounded and free of sharp spikes or edges.

[0051] A typical durometer range for the hot melt material applied to the membrane surface is preferably about 70 Shore A durometer. Alternative durometers can be as low as 80 to 40 Shore A hardness.

[0052] A preferred pattern has a repeating pattern in the central region (i.e., not a denser edge pattern) that is divisible by an integer ratio. More specifically, the central section may have features that line up in a row from edge to edge, with the feature positions being the same in the left and right halves, or in the left third, middle third, and right third, etc., up to the point where all consecutive features from edge to edge have the same spacing. The positions from one row to the next may be aligned or offset, but the symmetry of the row positions is preferably the same. This results in more effective mixing and improves the application process.

[0053] Hot melt application is a non-contact process whereby the print head is preferably positioned 0.2 inches above the film surface, but the range can be 0.02 to 0.5 inches above the film surface. Non-contact printing avoids damage to the film surface, which can be an issue with application techniques such as screen 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 to clean the heads and less downtime.

[0054] There are several properties of hot melt material that affect the rate at which the material is deposited on the substrate. These include viscosity, temperature, printhead travel speed, printhead frequency per drop, nozzle size, hot melt pressure on the nozzle, and other factors. In one exemplary embodiment, printing was performed at a speed of 15 mm / s, with a 0.02 inch (0.5 mm) wide and 0.02 inch (0.5 mm) high pattern, a printhead frequency of approximately 400 Hz, and a 10 nanoliter drop size. One advantage of a stream of small droplet sizes is that hot melt applied from a printhead at a nominal temperature of, for example, 170 degrees Celsius has a lower Joule heat content than larger droplets and cools at a faster rate than larger droplets due to its smaller surface area, unlike the heat dissipation rate advantage of smaller droplets over larger droplets. A series of small droplets also generates less heat on the substrate than a continuous stream of hot melt applied from a syringe-type nozzle. Faster cooling minimizes damage to the underlying active polymer membrane coating or support layer and minimizes membrane salt rejection damage. Minimal overcoating from high-frequency hot melt tappet-type printheads also minimizes loss of flux (volume of fluid passage through a given area of ​​surface) due to overcoating, which is a characteristic of inkjet-type printing. Hot melt printing does not overcoat due to rapid cooling, and the adhesive does not spread as it does in stencil-type systems. Prior to UV or light curing of stencil-type adhesives, the adhesive has a tendency to spread around the print spacers and reduce the active membrane surface area, thereby reducing the flux and productivity of the membrane element.

[0055] Various characteristics determine the build height and printing speed. Various manufacturers can provide hot melt materials that work well for creating printed spacers on film substrates. These various characteristics and precise control of the X and Y directions of the gantry system allow unlimited configurations of printed spacers to be fabricated at practical heights for feed spacer application.

[0056] One aspect of a more open feed space is that the concentration of forces applied to the membrane envelope, and therefore the feed spacer, results in greater forces being applied to the feed spacer elements, especially near the center tube. Hot melt printing of the spacers according to the present invention can also address these concerns.

[0057] The employed feed shaping features can be any of a number of shapes, including rounded dots, ellipses, rounded rods, lens shapes, elongated polygons, lines, or other geometric shapes. Due to the shape of the features and the fact that the fluid must move around the outside of the features, the fluid flow rate varies locally in the region between the feed spacing features from the feed end to the reject end of the membrane element. An efficient printed spacer pattern allows for maximum flow rate from the feed end to the reject end of the membrane, with negligible resistance to flow from the feed end to the reject end, and the features help promote mixing of the feed solution to minimize stagnation point formation at the leading and trailing edges of the spacers and reduce concentration polarization within the feed space.

[0058] In spiral-wound elements, the membrane leaves are folded at their centerlines, which contact the permeate carrier at the central tube before winding. Crease protection has been described in the prior art. Crease protection typically consists of tape applied along the width of the membrane sheet where it is folded. The prior art also discusses crease protection applied by printing or otherwise applying a polymer or other resin as a crease protection material. Crease protection is used to protect the membrane leaves at the crease line when folded to avoid damage from the crease. Without crease protection, damage to the crease line could result in rejection and flux loss in the finished membrane element. Crease protection can be uniquely utilized in printed spacer technology by extending the crease protection to the tops of the printed spacer features near the central tube to help prevent stress concentrations from the printed spacer features from damaging the active surface of the membrane on the unprinted side of the membrane leaf.

[0059] FIG. 1 is a schematic diagram of a conventional spiral-wound membrane element before winding, showing the key elements of a conventional spiral-wound membrane element 100. The permeate collection tube 12 has holes 14 therein through which permeate is collected from the permeate carrier 22. In manufacturing, the membrane sheet 36 is a single continuous sheet folded about the centerline 30 and is composed of an inactive porous support layer, such as polysulfone, on one side 28 and an active polymer membrane layer on the other side 24, bonded or cast onto the support layer. 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 mesh 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 spaces in the feed spacer mesh 26. As the feed solution 16 flows through the feed spacer mesh 26, particles, ions, or chemical species rejected by the membrane are rejected at the activated polymer membrane surface 24, and molecules of the permeate fluid, e.g., water molecules, pass through the activated polymer membrane surface 24 and enter the porous permeate carrier 22. As the feed solution 16 moves along the activated polymer membrane surface 24, the concentration of substances rejected by the membrane increases due to the loss of permeate fluid in the bulk feed solution 16, and this concentrated fluid exits the reject end of the activated polymer membrane sheet 24 as reject 18. The permeate fluid in the permeate carrier 22 flows from the distal end 34 of the permeate carrier 22 toward the central tube 12, with permeate entering the central tube 12 through the central tube inlet holes 14 and exiting the central tube 12 as permeate 20. To avoid contamination of the permeate fluid with the feed solution 16, the inactive polymer membrane layer 28 is adhesively sealed along a bond line 32 through the permeate carrier 22, thereby forming a sealed membrane envelope whose only exit path for the permeate 20 is through the central tube 12. Typically, the width of the bond line 32 is 1 to 3 inches after the adhesive is compressed during the wrapping process.

[0060] A partially assembled spiral-wound membrane element 200 is shown in FIG. 2. The membrane envelope 40 comprises membrane sheets 36 folded at one end, as described in connection with FIG. 1, with permeate carriers 22 placed between the membrane sheets and sealed along the edges with suitable adhesive lines 32 (FIG. 1). In conventional designs of membrane elements once wound, a feed spacer mesh 26 is placed adjacent to the envelope 40, allowing the flow of feed fluid 16 to flow between the layers of the membrane envelope 40, exposing all of the active polymer surfaces 24 of the membrane sheets to the feed fluid. The permeate, or product, fluid is collected on the permeate carriers 22 within the membrane envelope 40 and spirals to the central tube 12, where the product, or permeate, fluid is collected, while the reject stream 18 exits the element. A single spiral-wound element may comprise a single membrane envelope and feed spacer layer, or may comprise multiple membrane envelopes and feed spacer layers stacked and wound together to form the element.

[0061] Figure 3 depicts a membrane element opened for viewing. The element in Figure 3 is prior art, showing a membrane sheet damaged by rigid circular spacers printed on the membrane sheet. As shown in Figure 4, a membrane sheet 54 with an active membrane polymer layer 52 is wrapped around a central tube 12. The spacer features 50 can be rigid and, when printed by inkjet printing, screen or stencil printing, or other printing methods, do not bend. Consequently, the polymer layer 52 delaminates from the polysulfone support layer at fracture points 56a and 56b, allowing ions to enter the damaged area and reduce the rejection properties of the assembled element. The hot melt material is, by its nature, flexible, with a Shore A durometer of approximately 70, and conforms to the membrane surface without causing lifting at the edges of the printed pattern.

[0062] Similarly, printing straight lines rather than lines on the edges and dots in the middle of the sheet is more efficient from a plotter's perspective. Figure 5 shows a cross section of a thin spacer 50. In one embodiment, the spacer may be a 0.050 inch diameter dot. However, the same support area could 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 conforms to curved surfaces when wrapped around the central tube 12. This prevents cracks 58a and 58b in the polymer coating 52 when the membrane sheet 54 is wrapped around the central tube 12.

[0063] FIG. 6 represents an efficient printing pattern for a hot melt printer. In a hot melt printer, a 0.050-inch diameter dot requires the printhead to print a small circle to fill in the dot. This slows the printhead speed and increases the time required to print an entire sheet of pattern on the membrane sheet 36, starting at the entrance edge 66 and continuing to the exit edge 64. However, the printhead can move in one direction, printing longer support lines 76 on the entrance edge 16 of the membrane sheet, printing spacers 68, and continuing to the other end of the membrane sheet 36, printing exit lines 72 on the exit end of the membrane. Note that in one exemplary embodiment, every other edge support line 72 or 76 does not have a spacer 68 printed in line with the entrance edge support line 72 and the exit edge support line 76. If the spacers 68 are printed in the same line as the edge support lines 72 and 76, the print head can move at the same speed from the entrance end to the exit end of the film sheet 36, for example, through the low-density space 74. If the spacers 68 are not printed in line with the support lines 72 and 76, the print head can accelerate to the other end of the film sheet 36 at high speed in the low-density space 74 before decelerating to print either the support line 76 or the support line 72, depending on the direction of print head movement. This process can reduce the time to print the entire pattern on the film sheet 36. The time required to print the entire film spacer pattern on one half of the film sheet 36 determines the ultimate efficiency and cost of printing the film sheet. The shorter the printing time, the better. The spacing X between the print lines 72, 74, and 76 is preferably 0.10 inches, but can range from 0.20 to 1.0 inches. Also, note that only one half of the film 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. The spacers 68 can be printed tall enough to create the necessary space between the membrane sheets to create the fluid feed space.

[0064] FIG. 7 illustrates a system 800 for printing a pattern 92 on a film sheet 88. The film sheets 88 can be individually cut and placed in the printer system 800, or the film sheets 88 can be unwound and reeled out of the printer system 800. The film sheet 88 is placed on the printer system 800 with its left edge positioned on a vacuum table 90 to secure the film sheet 88 in place, ensuring that the film sheet 88 is flat and maintaining a constant vertical space between the film sheet 88 and the print heads 94. The right edge of the film sheet 88 can rest on a flat support table 86. The print heads 94 are mounted on print head supports 84 that are attached to the X-axis stage(s) 82. The print head supports 84 may also be mounted on the Y-axis stage 80, depending on the design of the gantry system. If the print heads 94 are 2.0 inches or less in width, they can be offset and facing each other to maximize the number of print heads 94 in the system. The more printheads 94 there are in the system, the faster the membrane sheets 88 can be printed.

[0065] FIG. 8 shows a hot melt spacer feature 104 and an inkjet or stencil spacer feature 106 between membrane sheets 100 and 102. Due to the melting and cooling characteristics of the hot melt spacer, the hot melt spacer 104 has a dome-shaped apex that does not damage the working surface of the membrane sheet 102. In contrast, spacer features 106 added with an inkjet or stencil process may have sharp points 110 at the top of the spacer that could damage the sensitive polymer surface of the membrane sheet 102. This characteristic of the hot melt spacer makes it much less likely to damage the surface of the membrane sheet 102. The hot melt spacer 104 may also be composed of a compound with very low tack. This characteristic of the hot melt spacer 104 helps prevent damage to the sensitive polymer surface of the membrane sheet 102 as the membrane element wrapping process slides the membrane sheets 100 and 102 adjacent to each other during the membrane element wrapping process.

[0066] FIG. 9 illustrates the formation of a spacer feature made up of small hot melt droplets 120 exiting a hot melt printhead. In effect, these individual droplets coalesce into a homogenous shape. In this manner, various cross sections of the spacer can be constructed to assist in creating turbulence in the feed fluid stream depending on the spacer shape. In the embodiment of FIG. 9, a concave spacer can be constructed to be positioned between film sheets 100 and 102. FIG. 10 is a representation of a concave spacer feature after all of the hot melt droplets have coalesced and cooled to create a solid spacer feature 124. FIG. 11 is an embodiment of a convex spacer feature 128 that solidifies to create the homogenous spacer feature 124.

[0067] The present invention has been described in connection with various exemplary embodiments. It is to be understood that the foregoing description is merely illustrative of the application of the principles of the invention, the scope of which is to be determined by the claims viewed in light of the specification. Other variations and modifications of the present invention will be apparent to those skilled in the art.

Claims

1. 1. A membrane assembly for a spiral wound filtration element, comprising: (a) a membrane sheet comprising a thin film composite structure comprising a porous structural layer, a support layer, and an active membrane layer; (b) a plurality of edge spacers comprising line segments of hot melt disposed on the active membrane layer, each edge spacer having a length less than one-quarter of the distance between first and second opposing edges of the membrane sheet, positioned near the first and second opposing edges and arranged with a major axis perpendicular to the first edge, adjacent edge spacers spaced a first distance apart in a direction parallel to the first edge; (c) a plurality of intermediate spacers comprising hot melt line segments disposed on the active membrane layer, each intermediate spacer having a length less than one-quarter of the distance between first and second opposing edges of the membrane sheet, positioned between the first and second opposing edges and disposed with a major axis perpendicular to the first edge, adjacent intermediate spacers spaced apart a second distance in a direction parallel to the first edge, the second distance being greater than the first distance; A membrane assembly comprising:

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

3. 10. The membrane assembly of claim 1, wherein each edge spacer has a length of at least 1 inch and no more than 4 inches.

4. 2. The membrane assembly of claim 1, wherein the plurality of intermediate spacers comprises a plurality of sets of intermediate spacers, each set of intermediate spacers comprising a plurality of intermediate spacers respectively arranged along a single line perpendicular to the first edge, the single line coinciding with the edge spacers, and the intermediate spacers of a set comprising line segments spaced a third distance apart from each other in a direction along the single line, the third distance being greater than the first distance.

5. each set of intermediate spacers is separated from an adjacent set of intermediate spacers by a third distance, the third distance being an integer multiple of the first distance; The membrane assembly of claim 4.

6. The membrane assembly of claim 1 , wherein each intermediate spacer has a rounded cross section at an end remote from the membrane sheet.

7. The membrane assembly of claim 1 , wherein each intermediate spacer has a cross section with convex sides.

8. The membrane assembly of claim 1 , wherein each intermediate spacer has a cross section with concave sides.

9. 10. The membrane assembly of claim 1, wherein the hot melt is liquid at temperatures above 100C.

10. 11. The membrane assembly of claim 10, wherein the hot melt is liquid at temperatures above 170C.

11. The membrane assembly of claim 1 , wherein the hot melt has low tack.

12. The membrane assembly of claim 2 , wherein the second distance is three times the first distance.

13. 10. The membrane assembly of claim 1, wherein the intermediate spacer has a width of 100 to 1500 micrometers, a length of 250 to 5000 micrometers, and a height of 75 to 1200 micrometers.

14. 1. A method of making a membrane assembly, comprising: (a) providing a membrane sheet comprising a thin film composite structure comprising a porous structural layer, a support layer, and an active membrane layer; (b) depositing hot melt onto the active membrane layer as a plurality of line segments to form a plurality of edge spacers, each edge spacer having a length less than one-quarter of the distance between first and second opposing edges of the membrane sheet, positioned near the first and second opposing edges, and arranged with a major axis perpendicular to the first edge, with adjacent edge spacers spaced a first distance apart in a direction parallel to the first edge; (c) depositing hot melt onto the active membrane layer as a plurality of line segments to form a plurality of intermediate spacers, each intermediate spacer having a length less than one-quarter of the distance between first and second opposing edges of the membrane sheet, positioned between the first and second opposing edges and arranged with a major axis perpendicular to the first edge, adjacent intermediate spacers spaced apart a second distance in a direction parallel to the first edge, the second distance being greater than the first distance; A method comprising:

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

16. 15. The membrane assembly of claim 14, wherein each edge spacer has a length of at least 1 inch and no more than 4 inches.

17. 15. The membrane assembly of claim 14, wherein the plurality of intermediate spacers comprises a plurality of sets of intermediate spacers, each set of intermediate spacers comprising a plurality of intermediate spacers respectively arranged along a single line perpendicular to the first edge, the single line coinciding with the edge spacers, and wherein a set of intermediate spacers comprises line segments spaced a third distance apart from each other in a direction along the single line, the third distance being greater than the first distance.

18. 18. The membrane assembly of claim 17, wherein each set of intermediate spacers is separated from an adjacent set of intermediate spacers by a third distance, said third distance being an integer multiple of said first distance.

19. 16. The membrane assembly of claim 15, wherein the second distance is three times the first distance.

20. 15. The membrane assembly of claim 14, wherein each intermediate spacer has a rounded cross section at an end remote from the membrane sheet.

21. The membrane assembly of claim 14 , wherein each intermediate spacer has a cross section with convex sides.

22. The membrane assembly of claim 14 , wherein each intermediate spacer has a cross section with concave sides.

23. 15. The membrane assembly of claim 14, wherein the hot melt is liquid at temperatures above 170C.

24. forming a plurality of edge spacers; conveying the hot melt dispenser from near the first edge to near the second edge along a direction perpendicular to the first edge; applying hot melt through the dispenser while the dispenser moves from near the first edge to a position at a distance from the first edge equal to the length of the edge spacer, and while the dispenser moves from a position at a distance from the second edge equal to the length of the edge spacer to near the second edge; 15. The method of claim 14, comprising:

25. 15. The method of claim 14, wherein forming a plurality of intermediate spacers includes applying hot melt through the dispenser while conveying the dispenser as the dispenser moves from a position where an intermediate spacer begins to a position where an intermediate spacer ends.

26. 26. The method of claim 25, wherein forming a plurality of intermediate spacers comprises applying hot melt through the dispenser for less than all of the strokes of the dispenser from the first edge to the second edge.

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

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

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

30. 1. A method for producing a regenerative spiral wound element, comprising: (a) providing one or more initial spiral wound elements having spacing features comprising 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 element; (d) using said hot melt to fabricate one or more membrane assemblies according to claim 1; and (e) winding said one or more membrane assemblies onto a regenerative spiral wound element; A method comprising:

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