Review pubs.acs.org/IECR
Membrane Adhesives Colin A. Scholes,*,† Julius Motuzas,‡ Simon Smart,‡ and Sandra E. Kentish† †
Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), Department of Chemical and Biomolecular Engineering, University of Melbourne, Parkville, Victoria 3010, Australia ‡ FIMLab − Films and Inorganic Membrane Laboratory, School of Chemical Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia S Supporting Information *
ABSTRACT: The adhesive seal between membranes and their housing has a vital role to play in any membrane application, as it ensures the product and feed streams do not mix and that pressure can be maintained. In many applications, the adhesive seal is a major source of membrane module failure and can dictate the operational life of a module. Hence, understanding adhesives in membrane systems is fundamental in ensuring both separation performance and durability; however, this field has been widely overlooked in the literature. This paper attempts to rectify this by discussing in depth adhesive theory and factors that will maximize the adhesion strength, relative to membrane technology. Also highlighted are specific membrane factors that lead to adhesive failure, important when designing a module. The performance of different adhesives is then presented, based on their ability to adhere to different substrates and their resistance to environmental factors. They are discussed and compared relative to the wide range of polymeric and inorganic membrane systems that are currently commercialized or under research. The conclusion raises the possibility of future research in membrane adhesives, as membrane specific developments in the adhesion field have the potential to increase the durability and environmental resistance of membrane modules.
1. INTRODUCTION Membrane technology is a rapidly growing field that has been commercialized for a number of applications. The best examples are reverse osmosis and ultra- and nanofiltration, as well as acid gas removal.1 In all of these applications the adhesive seal between membranes and with the membrane housing is critical to the overall performance of the membrane operation. In spiral wound membrane modules, adhesive is used to glue two flat membrane sheets back to back on three sides, with a permeate spacer between them, to form an envelope. The open end of each envelope is then glued to a central permeate tube. These envelopes are then rolled up with a feed spacer between each one. The glue lines are typically 4 cm wide in a full scale module.2 In hollow fiber modules, adhesive is used to form a seal around both ends of the fiber bundle. The seal forms an adhesive “tubesheet” in a method often described as “potting”. Adhesive can also be used to attach this tubesheet to the membrane housing. In larger hollow fiber modules, a supportive tube can also be used in the center of the bundle, and this tube must also be incorporated into the adhesive tubesheet. Potting can be achieved by inserting the fibers into a mold already containing the adhesive resin. Another approach is to pour a liquid adhesive into a tubesheet mold into which the ends of the fibers have already been placed and allowing the adhesive to cure or harden around these fibers. This is often done under a centrifugal force field to prevent wicking of the adhesive back up the fiber length and to ensure an even distribution of adhesive. Finally, the adhesive can be injected into a mold, from the bottom of which the ends of the fibers extend.3 In this case, the mold is often the pressure housing which will be used to retain the bundle in operation. If a two © XXXX American Chemical Society
component adhesive system is used, the second component is added to the system during the mold compression to ensure the seal is formed. After curing, the adhesive resin is machined to size, ensuring that the bores of each hollow fiber are unobstructed (Figure 1).4
Figure 1. Epoxy sealed hollow fiber bundles.
Any holes or gaps between adjacent membranes or between the membranes and their housing will lead to mixing of feed and product streams and a loss of pressure control, reducing the membrane performance. Indeed, in many applications the point of failure of the membrane module is the point of contact between the adhesive seal or gaskets and the membrane or membrane housing, rather than the membrane itself. This failure can come about for a number of reasons; the adhesive Received: March 11, 2014 Revised: May 14, 2014 Accepted: May 16, 2014
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substrate provides the mechanical strength of the adhesion process. Physical bonding, also called dispersive adhesion, is primarily through intermolecular bonding, such as van der Waals forces and hydrogen bonding, between the adhesive and the substrate.13,15 This form of bonding is weak, but has the advantage of occurring across the entire interfacial area between the adhesive and the substrate. Hence, collectively the dispersive adhesive strength can be substantial. However, if the primary source of adhesion is van der Waals forces, then the adhesive strength can be considerably weakened by exposure to permanent dipoles, such as water under humid conditions.16 The forces acting in physical adhesion are very short ranged, and as a consequence only a small separation between adhesive and substrate needs to occur before adhesion is lost. Hence cracks between the adhesive−substrate can easily propagate along the interface. Chemical bonding includes the formation of covalent, ionic, metallic, and chelating bonds between the adhesive and substrate surface.17,18 These bonds are substantially stronger than physical bonding and are desirable for long-term adhesion durability. For most adhesive systems the difficulty is in creating the chemical bond between unlike materials, such as polymer− metal and polymer−polymer. Developing adhesives with strong chemical bonding to various substrates is a very active area of research.5,13 If the degree of chemical bonding is limited because of poor adhesive−substrate matching, then actual adhesive strength can be very low and separation of adhesive and substrate occurs easily. Diffusive adhesion involves molecules within the two materials being mobile and soluble within each other, and hence molecules of one material diffuse into the other material forming the adhesive bond.5,13 This is common for metal or ceramic materials, where diffusive adhesion is known as sintering.19 The metal or ceramic materials are sealed together through pressure and heating, which forces the atoms to diffuse from one material to the other and merge to form one material. Polymers can display diffusive adhesion when operating above their glass transition temperature. Essentially the chains of one polymer diffuse into the other, and adhesion is through the formation of one blended material. Diffusive adhesion is generally not observed if the polymers are different in nature and is mostly observed for adhesion of polymers to themselves, known as autohesion.10 For polymeric membrane systems, the most common adhesive mechanisms are mechanical and chemical bonding, while for inorganic and metallic systems adhesion is achieved through mechanical, chemical, and diffusive adhesion. 2.1. Criteria for Strong Adhesion. There are a range of factors that can be used to maximize the bond strength between the adhesive, membrane, and its housing. First, the membrane and housing surfaces must be very clean and free of any foreign material. This includes films of water, hydrocarbons, microorganisms, organic residue, and dust.14 This is to ensure that the substrate surface is in a high energy state, to maximize the wetting of the adhesive and strengthen the adhesive bond. In addition, this prevents the adhesive from bonding with foreign matter present instead of the substrate,5 which otherwise would become a localized source for adhesive failure. The contact between the adhesive and the materials being joined must also be maximized. The objective is for the adhesive to spread across and envelop the topology of both substrate surfaces, while remaining viscous enough to form a
may not have good contact between adjacent membrane sheets or fibers or the housing and, hence, during operation easily separate. The adhesive may degrade because it cannot withstand the chemical, thermal, or mechanical operating conditions, or the adhesive may degrade over time because of aging, resulting in a loss of its adhesion properties. As a result, choosing the correct adhesive for a membrane module is critical to ensure maximum performance. However, given the importance of adhesives in membrane technology, the lack of information in the literature on membranes and adhesives is surprising. Here we provide a detailed and in depth analysis of the various adhesive types available and their compatibility with membranes and modules. Importantly, the discussion focuses around those parameters that are needed for an effective membrane adhesive. The key criteria are • Chemical compatibility between the adhesive, membrane layers and housing. This ensures that the adhesive does not chemically degrade the substrate surfaces. • An appropriate adhesive bonding mechanism for the process temperature, fluid composition, and pressure, as well as ensuring the adhesive can withstand these process conditions. For example, corrosive conditions can chemically degrade the adhesive bond as well as the cross-linking within the resin structure.5 In liquid based applications, such as reverse osmosis or microfiltration, the solvent may swell the adhesive, causing it to lose adhesive strength or even to dissolve over long periods of time.6 High temperature applications can speed up aging and degradation, especially for those adhesives operating near their design limit. • Maximum adhesive wettability on the membrane and housing surfaces to allow them to be joined. • An appropriate method of application of the adhesive, in particular ensuring that the adhesive curing conditions are appropriate for the fabrication approach. This paper is divided into three sections; the first focuses on adhesive theory, which leads into a discussion on the characteristics that make a good adhesive for membranes, as well as the common causes of membrane adhesive failure. The types of adhesives available for polymeric membranes and their compatibility are then discussed. The final section of this review focuses on providing high temperature seals for metallic, inorganic, and ceramic membranes.
2. ADHESIVE THEORY There is currently no general theory for adhesion, with different mechanisms postulated for different adhesive systems.7−11 Essentially adhesion can be associated with four processes; mechanical, physical, or chemical bonding, and diffusive adhesion, of which one or more can be present in a membrane module situation. Mechanical bonding is essentially the penetration of adhesive into and around the irregular surface features of the substrate, so that it becomes mechanically interlocked at some level.12−14 For most membrane systems this mechanical interlocking occurs on the microscopic level and involves the topology of the substrate surface being irregular. Initially the liquid adhesive wets the substrate surface and penetrates any microscopic pores that are present, as well as encloses any substrate extensions protruding from the surface. Upon solidifying, the adhesive becomes interlocked around the substrate surface, creating an interphase region. This interlock between the adhesive and the B
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Industrial & Engineering Chemistry Research
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seal.13,14 Contact angle measurements and surface tension are used to describe the adhesive wettability of substrates.20 Solid substrates are considered to be either high energy surfaces or low energy surfaces dependent upon their critical surface tension (Supporting Information Table S1). This critical surface tension is extrapolated from contact angle measurements of multiple liquids on the solid surface.21 Polymeric substrates are generally low energy surfaces and often have surface tensions similar to that of organic liquids (Supporting Information Table S1). As a result, many adhesives have difficulty in wetting these substrates, and therefore adhesion strength is reduced. The surface energy can be increased by generating new functional groups on the membrane surface.8,22 This is achieved primarily through chemical or plasma treatment. Chemical treatment involves exposing the membrane adhesive area to various chemical agents, often known as primers.23 For most polymeric systems this increases the surface polarity and surface energy of the membrane, which increases the chemical bonding potential with the adhesive. Alternatively, the primer can swell the polymeric material, enabling the adhesive to have better traction and mechanical interlocking, such as with fluoropolymers (e.g., poly(tetrafluoroethylene) (PTFE) and poly(vinylidene fluoride) (PVDF).24 Plasma treatment is an alternative approach that is very effective at modifying polymeric membrane surfaces.25−27 Plasma treatment induces the formation of oxygen containing functional groups, such as hydroxyl groups, on the polymer surface, which increases the surface wettability and improves adhesion. Both chemical and plasma treatment can have the drawback of inducing aging effects in the adhesive system, since the surface functionality differs from the bulk polymeric material.27 Hence, further chemical reactions and/or reorientation of functional groups on the surface of the polymer can weaken the bond with the adhesive over time. Finally, mechanical methods or primers can be used to increase surface roughness, which increases the adhesive strength through mechanical interlocking. During membrane module fabrication, critical criteria for strong, long lasting adhesion include the width and thickness of the adhesive layer applied, the viscoelastic properties of the adhesive, the curing temperature, and the time allowed for adhesive bonding. The viscosity of the adhesive must be low enough to ensure that it properly wets the membrane surface and housing. However, too low a viscosity may lead to runoff into areas where adhesive is not desirable. For example, wicking from the tubesheet of a hollow fiber bundle into the fiber length can result in stiffening of the surface around the individual fibers, causing cracking or breakage.3 In this case, the best viscoelastic behavior can be achieved by using a thixotropic adhesive, i.e., one that becomes less viscous under the applied stress within the tubesheet mold but does not flow under capillary action.28 Such thixotropic adhesives also find application in spiral wound applications,29,30 presumably because they allow some movement in the seal as the glued flat sheets are rolled into the spiral format. The curing time of the adhesive (pot life) must be greater than the time it takes to apply the glue lines and form the membrane module. The curing temperature is also an important parameter, as this controls the viscosity of the adhesive, the reactivity of the adhesive, and the curing time. Curing is often carried out at elevated temperature to both increase the curing rate and also to drive the cross-linking reaction to as close to 100% complete as possible.31 Too cool
conditions leads to long curing times which can mean poor adhesive sealing during large scale manufacturing, while high temperature conditions can lead to faster curing times but the potential for the adhesive to dissolve the membrane material. Adhesive curing is generally exothermic, and it is important to control this release of heat during curing to ensure that the localized temperature rise does not damage the membrane or housing. Almost all adhesives used in membrane systems undergo some volumetric shrinkage during the curing process, because of cross-linking reactions and loss of solvent, as well as thermal contraction as they cool.13 It is therefore important that the adhesive does not shrink to the extent that the solid adhesive separates from the substrate. Reported shrinkages are provided in Table 1 for common membrane adhesives. This can be overcome through choosing the correct adhesive or compensating for shrinkage during membrane module fabrication. Table 1. Percentage Volumetric Shrinkage of Adhesive during Curing13 shrinkage (%) acrylic epoxy polyurethane silicone
5−10 6−9 3−5 1000 °C. However, the development of the membrane materials is not the only limiting parameter for their widespread industrial implementation. The development of appropriate membrane sealing technology, particularly at high temperatures, also features prominently in this challenge. Yet, like their polymeric counterparts, there is a dearth of literature regarding appropriate membrane sealing technologies for specific applications. This section attempts to bridge that gap by providing an overview of the main types of sealants and the challenges arising with their implementation. 5.1. Polymeric and Graphite Seals. Polymeric. For low temperature applications (i.e.,
