Adhesives Undergo a Revolution in Materials ApplicationsKevin J. Harrigan | December 01, 2015
Increasingly, design engineers are implanting industrial adhesives into product design. For decades, mechanical fastening--mostly threaded hardware and welding--was the preferred assembly method for manufacturing. This was the result in part of immature adhesive materials technology, as well as the processing challenges presented by integrating adhesives into an assembly line.
Adhesives formerly had to be mixed and applied by hand on the production line, a time-consuming task. This labor was also expensive, as employees had to be properly trained and outfitted to work with chemicals. The adhesives also needed a chance to cure, in some cases for many days, which created production bottlenecks and backlogs.
(Search products and services related to adhesives at Engineering360.)
However, recent materials advances have improved the overall processability of industrial adhesives. These chemical compounds can be precision-measured and mixed by on-floor dispensing equipment, or even thawed from frozen or repackaged for one-time application. Innovation in software and automation equipment means that human workers can be removed from most adhesives processing tasks. Instant and near-instant curing adhesives mean assembled substrates can be immediately sent to the next manufacturing step.
Perhaps most importantly, manufacturers have recognized applications where adhesives are all but unrivaled. This includes structural assembly applications as well as applications that require protecting sensitive instruments used in everyday devices. By incorporating adhesives into more product designs, engineers are benefitting from shorter lead times, improved product quality and more flexible manufacturing solutions.
Adhesives suppliers typically can deliver an adhesive for almost any bonding challenge. While there are other chemistries, the majority of industrial adhesives belong to one of these four categories and exhibit specific characteristics:
- One-part resin, or two-part resin that must be mixed
- Excellent for bonding similar materials, especially woods, metals, glass, ceramics, hard, rubbers and some plastics
- Strong—shear rates of 5,000 psi are common
- Exhibits good heat and chemical resistance
- Difficult to process: resins might need to be mixed; curing rates vary, but are often longer than other chemistries
- Bonds both similar and dissimilar materials (except some plastics)
- Relatively inexpensive
- Moisture resistant
- Vulnerable to chemicals (easy to rework/remove)
- Exceptional processability: no mixing; minimal preparation; quick curing
- Can bond dirty and oily workpieces
- Shear strength on the order of 3,500 psi (but can vary)
- Maximum temperature limit: less than 100° C
- More susceptible to dynamic load fatigue than epoxies and urethanes
- Usually contains volatile organic compounds (VOCs)
- Bonds plastic and dissimilar materials
- One-part resin, or two-part resin that must be mixed
- Resilient to dynamic loads and shock
- Wide variety of shear strengths (up to 5,000 psi)
- Excellent chemical and abrasion resistance
- Difficult to process: might need mixing; long fixating and curing times; begins to cure if exposed humidity/moisture
- Highly popular
- Bonds dissimilar materials
- UV stable
- Often cures are room temperature (room temperature vulcanization—RTV)
- Temperature resistant, often from -40° C to 200° C.
- Chemically resistant
- Impermeable liquid water (water vapor may get through)
- Excellent processability.
Occasionally, hybrid chemistries are available. The most innovative is likely ultraviolet-curing urethane-acrylic compounds. These adhesives begin to cure once exposed to UV sources and are often completely cured in 60 seconds.
Although chemistry usually determines adhesion performance, filler materials are what determine active functional performance. This includes endowing the adhesive with thermal or electrical conductive properties.
- Metal powders: Powders suspended in the base material that are thermally and electrically conductive include solder, aluminum, copper, zinc, silver and gold.
- Metal oxides (ceramics) and metal nitrides (inorganics): Materials that typically promote thermal conductance but are dielectric are beryllium oxide, aluminum oxide, zinc oxide, silica/silicon dioxide, boron nitride, aluminum nitride and mica, among others.
- Carbon based: These materials are rare due to expense and include diamonds, graphene, carbon fiber and graphite.
Adhesives for Structural Assemblies
It used to be that mechanical assembly methods were the predominant assembly technique with adhesives relegated to niche applications. Slowly, these roles are reversing. Manufacturers have become increasingly aware of the flaws of mechanical assembly. Foremost, assemblies completed with fasteners or welds are more susceptible to fatigue than those completed by adhesives. This is because joint stress is concentrated near the weld or fastener; on workpieces that might experience constant vibration, deflection, or thermal expansion the effects are realized quicker. Meanwhile, threaded fasteners require the workpieces to make use of a reciprocating clamp force that may not be possible depending on the application or material. Additional, over- and under-torqued fasteners will overstress the fastener, hastening joint failure. Welds also have inherent faults; defects in the weld job create weak spots in the seam. Welding also requires pre- and post-processing.
Welds and fasteners are also prone to oxidation, and coefficient of thermal expansion (CTE) mismatches will lead to additional joint stress. Material choices also play a key role; galvanic corrosion is all but inevitable if fastener and substrate materials are mismatched. Joining brittle materials such as glass or ceramics is only possible with adhesives. Finally, welding and fasteners create protrusions, recesses, or seams in material surfaces.
Adhesives solutions have almost none of the limitations of mechanical solutions. Adhesives that have high elongation and a low modulus of elasticity create resilient bonds with good flexibility and ductility. As long as the joint isn’t strained beyond its elastic limit it will almost always rebound. Adhesives also distribute stress across the entire bond surface and are resistant to vibration and shock. Many times an adhesive with the same CTE as the workpieces can be used, and if not, the adhesive’s flexibility at least reduces the innate friction and heat created from expansion and contraction. Since adhesives are so adaptable, bonds often last 20 years or more.
Adhesives can bond most industrial materials. This includes glass and ceramics; high surface-energy materials, such as metals and ceramics; and low surface-energy materials like rubbers and plastics. Surface cleaning is easily completed by wiping with isopropyl alcohol. Occasionally, materials may need pre-processing in order to improve adhesion. For example, gold sometimes needs to be abraded before an adhesive can thoroughly wet its surface. Some plastics require abrasion, a primer or plasma or corona flame treatment. Primers and anodization also help prevent rusting in rapidly oxidizing materials such as aluminum.
Downsides to adhesives also exist. To achieve the most efficiency out of adhesive bonding techniques, a capital investment in equipment often is required. Most bonds are meant to be permanent, so it can be impossible to rework defective products or upgrade obsolescent components. Adhesives dissolved in solvent need proper ventilation during application and curing. Finally, some products may irritate a worker’s skin. This can be prevented, however, with correct personal protection equipment and training.
Adhesives Protect LED
Light emitting diodes (LEDs) are as much as 75% more energy efficient and last 50 times longer than a comparable incandescent bulb. Similarly, they last twice as long as compact fluorescent lamp (CFL) technologies, and don’t have inefficient warm-up or cool-down periods and temperature and disposal restrictions. LED technologies are now being integrated wherever possible.
Even though they are more efficient than incandescent bulbs, 85% of LED energy is expended as waste heat. Overheating is the most common cause of electronics failure, and LEDs with temperatures in excess of 60° C deteriorate quickly. Thermal interface materials (TIMs) can be used to attach heat sinks to the circuit, or as pottants, encapsulants or glob tops that exhaust heat while also protecting the circuit from contaminants. Clear adhesives enable inspection for quality control and maintenance, and an adhesive with minimal CTE prevents the adhesive from damaging the circuit due to cyclic expansion and contraction.
An adhesive is the only solution that can overcome all of the environmental challenges faced by outdoor LEDs. This includes bonding and sealing polycarbonate or glass domes and lenses to the enclosure. These adhesives are non-discoloring and non-degassing to ensure LED clarity in extreme temperatures and after nonstop UV exposure. Additional sealing and gasketing prevents the ingress of all types of precipitation, humidity, and dirt and debris. A properly sealed luminaire is essential for LEDs placed near water, such as those near boat docks or swimming pools. Since adhesives accommodate deflections from loads so well, outdoor luminaires easily withstand typical wind, vibration and dynamic loads. Even when there is no alternative to threaded fasteners, they’re commonly reinforced with threadlockers.
Fabricating indoor luminaires presents different issues for OEMs, but these also may be solved with chemical compounds. Indoor fixtures are primarily developed with safety, aesthetic, ambiance and price point in mind. Adhesives typically do not obscure light clarity, an important consideration for LEDs in commercial and industrial settings. Eliminating fastener heads, recesses or protrusions makes LED fixtures more appealing, which is important for residential décor. Manufacturing efficiencies leads to lower production costs and savings that are passed onto the consumer.
One of the more notable innovations for chemical compounds is the use of silicone as the LED secondary lens. Instead of adhesives bonding polycarbonate or glass lens to the luminaire, a two-part silicone resin can be injection-, casting-, cavity- or transfer-molded into a protective lens. The material is clear and has a similar refractive index (1.41) as many plastic LED domes (1.42).
Because silicone has better transmittance than acrylic or polycarbonate, an LED enclosure with a silicone secondary optic can emit the same lumens with less power. The silicone also remains supple in temperatures up to 200° C, doesn’t discolor from UV exposure and provides reliable impact resistance. Traditional lens materials are rigid so manufacturing shapes are limited, but silicone lenses can be molded into shapes with negative draft angles.
Adhesives Save LCD Panels
With growing device connectivity, an increasing number of objects are outfitted with screens. Such screens typically double as the user interface and must be able to capture data as well as present it. When idle, some screens are thrown into pockets, while others must remain on stand-by, perhaps even outdoors or in a vehicle. Each liquid crystal display (LCD) screen is provided additional mechanical protection that is made possible by liquid optically clear adhesives (LOCAs).
LCD modules and touch panels typically have a touch-capacitive lens placed over the main display. This prevents objects from reaching the LCD module itself. OEMs sandwich a layer if LOCA acrylic between the LDC and lens. LOCAs must be able to transmit the capacitive touch to the touch panel, while also maintaining 100% transparency. In many instances, LOCAs help improve screen contrast and optical quality, meaning the screen must be less bright and can save device battery power. LOCAs are needed in high-performance applications, such as military or aerospace LCD displays that must confront extreme temperatures, weather and shock. For the benefit of manufacturers, LOCAs have good processability, as they can be cured with UV, primers, heat, moisture or special tooling.
Adhesives Save PCBs and Sensors
Printed circuit boards (PCBs) and sensors are often the most delicate part of any electronic device. Chemical compounds help ensure reliable device operation by protecting PCBs and sensors from common risks.
The most common hazard to PCBs is overheating, but a thermal interface material (TIM) is an effective way of adhering heat sinks to boards without additional hardware, or they can even help exhaust heat in some instances. Each adhesive type has a different usage for thermal management.
- Epoxies: one- and two-part compounds that provide strong, permanent adhesion; small bond lines maintain dimensional tolerances; better conductance than tapes, less conductance than greases and pastes
- Greases and pastes: a non-curing compound that is drawn into component gaps via capillary action; susceptible to bleed-out and dry-out; supports PCB upgrades and repairs
- Pads and tapes: semi-permanent TIM with pressure-sensitive adhesive that conforms to component topography and reduces thermal resistance; can be pre-fabricated or customized into needed sizes; supports quick product assembly and throughput; some pads and tapes have no adhesive
- Encapsulants: one- and two-part potting compounds that cover whole circuit or glob-tops that cover specific components or areas; excellent shock, vibration, contaminant, and dielectric protection; prevents alterations; not preferred for high-heat electronics.
Encapsulants also play an integral role in protecting devices in challenging environments as the adhesive fillers dictate the utility of the compound. Sometimes encapsulants aren’t optimal for a particular application, such as when coefficient of thermal expansion (CTE) concerns arise or if the PCB would require an excessive amount of encapsulant material. Conformal coatings (CCs) are typically elected in these instances.
CCs can insulate on-board components from each other and also prevent solder from whiskering. They can help slow the degradation of PCB components from common hazards, such as oxidation from humidity and moisture, and fractures and breaks from shock loads and thermal effects. CCs are effective at keeping related components outside the PCB envelope that could interfere with board operation, such as radio frequency antennas and electromagnetic sources. While CCs cannot be used as a TIM, they’re typically thin enough so they don’t contribute to board heat retention.
Dispensing and Application
Advancements in mechanization, automation and software have eliminated the manufacturing limitations of adhesives, but create other challenges. Silicone is the most common adhesive used with dispensing systems, as it versatile, relatively higher viscosities of silicone may be dispensed compared with other chemistries and silicone is less wearing on equipment surfaces.
Mixing two-part resin systems manually is a common source of error. Modern meter-mix equipment will accurately measure resin components so a near-perfect adhesive ratio is created almost every time. This eliminates adhesion defects caused by mis-measured compounds that keep workers from completing a labor-demanding task. The compounds are mixed in a static mixing nozzle, a special nozzle with baffles that distributes the chemicals uniformly. The mixing nozzle prevents adhesive from beginning to cure in the machinery. Meter-mix dispensers can be programmed to perform a nozzle purge to prevent adhesive from curing in the mixing nozzle; nonetheless, mixing nozzles are meant to be disposable.
Fillers in adhesives might fall out of suspension if they remain in an equipment reservoir for too long. Additional heaters and agitators might be necessary. Since filled adhesives are more abrasive to wetted components, hardened materials must be used, and seals and o-rings will require regular maintenance. High-viscosity compounds might also need additional equipment. Gravity-feeding or shop air pressure typically is enough for low-viscosity adhesives. Sometimes a larger-diameter mixer nozzle with less back pressure is enough for medium and high viscosities. If not, then a more robust pump, such as a gear, piston, diaphragm or ram pump can be used. Highly viscous and abrasive compounds can be pumped with progressive cavity pump. Machines that hold isocyanate or other urethanes might also require an inert gas blanket to prevent curing before dispensing. Vacuum de-airing is effective at eliminating spatter from the nozzle tip created by air bubbles.
While automatic dispensers clearly have an advantage over humans, the advantage between robotic and human application techniques is less clear. The simplest, least expensive way to apply adhesives is manually with a brush or other tool. Doing so requires little workpiece preparation, compound changes are simple and repairs or touch-ups are easily ordered. However, this technique is completely dependent on the skill of the operator and proficiency of quality control. Operators can have trouble providing a uniform coating to components as well. Because this method is slow, it is usually only sufficient for small production volumes.
The primary advantage to dipping componentry is that many workpieces can have adhesive applied instantly. However, this technique requires masking components that must remain adhesive-free, such as ports. Dipping tends to run once removed from the reservoir, and maintaining a uniform thickness on corners and edges can be difficult. The adhesive reservoir must be covered when not in use to prevent contaminants, and some chemistries may require a gas blanket.
Spraying an adhesive is not possible with all chemistries, but provides good accuracy and rapid throughput when possible. Spraying also supports a variety of application patterns and resolutions, and changing adhesives is relatively easy. Manual spraying requires less masking than dipping, but still requires a skilled operator. The process is slowed if the operator needs to maneuver workpieces or equipment while spraying. Sometimes sprayed components need multiple coats as well.
In high-dollar, large-volume manufacturing, robots are increasingly used for adhesive application. Multi-axis robot arms or robotic tables provide reliable brushing, dipping or spraying application. Robots provide consistent, schedulable product throughput. As with manual operators, a variety of application patterns are available, including microdots, cavity fills, beading, underfilling and potting. Many times, masking can be eliminated as the robots layer adhesives with some precision. Robotic production lines can also be programmed to handle separate parts. Of course robotics adhesives application often requires what may be considered to be a significant capital expense, but manufacturers that are processing thousands or more parts per day may realize tangible efficiency gains from automated application technologies.
Ovens, humidifiers and other equipment to cure traditional adhesives remain an important part of adhesive technology. UV-curing chemistries require UV lamps to instigate curing. These adhesives begin to harden as soon as exposed to a UV light source and often cure completely in less than 60 seconds. UV lamps typically are more energy efficient than other curing types. This is helpful in eliminating production bottlenecks that might occur from adhesives that cure before the next step in the manufacturing process.
It’s true that adhesives won’t solve every structural assembly issue. Sometimes mechanical solutions are better for LED applications. And not every electronic device needs the excessive protection that begins with a chemical compound. However, the world of industrial adhesives has changed from one where they were impractical to one where they are so versatile and effective that they’ve become a go-to assembly solution. This change is nothing short of a materials revolution.