Whether bonding plastic to plastic or other materials, adhesives offer several benefits over other joining methods. Adhesives distribute loads evenly over a broad area, reducing stress on the joint. Since adhesives are applied inside the joint, they are invisible within the assembly. Adhesives resist flex and vibration stresses and form a seal as well as a bond, which can protect the joint from corrosion. They join irregularly shaped surfaces more easily than mechanical or thermal fastening, barely increase assembly weight, create virtually no change in part dimensions or geometry, and quickly and easily bond dissimilar substrates and heat-sensitive materials. However, they have certain limitations : they require setting and curing time (the time it takes for the adhesive to fixture and strengthen fully), they need some surface preparation prior to assembly, and they may not be the best solution when there is a need to repeatedly assemble and disassemble the joint.
According to Patrick J Courtney, following are the various adhesive types:
are high-strength, one-part adhesives that cure rapidly at room temperature to form thermoplastic resins when confined between two substrates that contain trace amounts of surface moisture. Because cure initiates at the substrate surface, these adhesives have a limited cure-through gap of about 0.010 inches. A wide variety of cyanoacrylate formulations are available with varying viscosities, cure times, strength properties and temperature resistance. They achieve fixture strength in seconds and full strength within 24 hr, which makes them suited for automated production. Newer formulations address many prior limitations, such as: Polyolefin primers applied to substrates before the adhesive, enhance bond strength on difficult-to-bond plastics, and accelerators ensure rapid cure in low-humidity environments. Surface-insensitive cyanoacrylates also cure quickly in low-humidity environments and on acidic surfaces. Since strength of cyanoacrylates can reduce over time on plasticized PVC assemblies, heat aging and testing PVC assemblies to ensure that the bond withstands the effects of plasticizer leaching to the substrate surface is recommended.
are one-part, solvent-free liquids with typical cure times of 2 to 60 sec and cure depths exceeding 0.5 inches. They provide good environmental resistance, superior gap filling, clear bond lines that improve aesthetics, and come in a wide range of viscosities from thin liquids (≈50 cP) to thixotropic gels. The adhesives remain liquid until exposure to light of a specific wavelength and irradiance causes them to fixture rapidly and cure. Secondary cure mechanisms, such as heat or chemical activators, completely cures adhesives in shadowed areas. Because cured acrylic adhesives are thermoset plastics, they offer superior thermal, chemical, and environmental resistance. As curing is on demand, light-cure acrylics allow ample time for aligning and repositioning parts. The adhesives offer high bond strength to a wide variety of plastics, and are available with flexibility ranging from soft elastomers to glassy plastics.
is an adhesive technology that combines the benefits of cyanoacrylates and light-curing acrylics. Light-curing cyanoacrylates are fast-fixturing adhesives that cure naturally in shadowed areas due to a secondary moisture cure mechanism. This hybrid technology overcomes many limitations of cyanoacrylates and light-cure acrylics, offering minimal bloom-ing/frosting, increased cure depth, rapid dry-surface cure, high-bond strength, and compatibility with primers for difficult-to-bond plastics. Light-cure cyanoacrylates also minimize vapor emissions, surface cure immediately when exposed to light, adapt easily into production lines and require no second-step accelerators or activators. The adhesives are surface insensitive and versatile, offering excellent adhesion to numerous substrates, including rubber and plastic. These adhesives limit stress cracking on sensitive substrates such as polycarbonate and acrylic, and will bond polyolefin plastics when used with special adhesion promoters compounded into the molded parts or applied to the part surface. Ideal for high-volume bonding applications, they are increasingly used for bonding medical devices, cosmetic packaging, speakers, electronic assemblies and small plastic parts. Rapid cure speed allows parts to be processed in seconds rather than minutes, often delivering 60% of their final strength after only5 seconds of exposure to light. Light-cure cyanoacrylates are especially recommended for bonding overlap-ping, nontransparent parts.
Traditional hot melts are thermoplastic resins that essentially reflow onto a bonding surface. Once cooled, the adhesive holds the components together. While many types of hot melts are available, higher performance varieties include ethyl vinyl acetate (EVA), polyamide, polyolefin and reactive urethane. Hot melts have the ability to fill large gaps and provide high bond strength as soon as they cool. EVA hot melts are typically used for low-cost potting, while polyamide hot melts are used in similar applications with more stringent temperature and environmental demands. Polyolefin hot melts provide good moisture resistance, superior adhesion to polypropylene substrates and excellent resistance to polar solvents, acids, bases and alcohols. The newest hot-melt classification, reactive urethanes, performs well on difficult-to-bond plastics. Whereas most traditional hot melts are thermoplastic resins that can be repeatedly reheated, reactive urethanes form thermoset plastics when fully processed. Initial strength develops a bit slower than traditional thermoplastic hot melts; however, for structural bonding, reactive urethanes are generally the hot-melt adhesive of choice. Process temperatures are approximately 250°F, as much as 200°F cooler than other hot-melt chemistries.
are common one or two-part structural adhesives that bond well to many substrates, give off no by-products, and shrink minimally upon cure. Cured epoxies typically have excellent cohesive strength and good chemical and heat resistance. The adhesives can also fill large volumes and gaps. The major disadvantage, however, is epoxies tend to cure much slower than other adhesive families, with typical fixture times between 15 min and 2 hr. While heat can accelerate curing, temperature limits of plastic substrates often prevent heat curing. In addition, epoxies generate considerable heat as they cure, which may result in high temperatures that can damage certain plastic substrates.
are tough polymers that offer greater flexibility, better peel strength, and lower modulus than epoxies. Available as one or two-part systems these adhesives contain soft regions that add flexibility to the joint and rigid regions that contribute cohesive strength, temperature resistance and chemical resistance. Varying the ratio of hard and soft regions lets manufacturers tailor physical properties to a designer's specific application. Like epoxies, polyurethanes bond well to many substrates, including heavily plasticized PVC, although a surface primer is sometimes required. Polyurethanes also have fixture times similar to epoxies (15 min to 2 hr) which can require racking of parts and substantial work-in-progress. Although polyurethanes do not present much of a stress-cracking hazard, the solvents used in primers do. Polyurethanes offer good chemical and temperature resistance. However, long-term exposure to high temperatures degrades polyurethanes more rapidly than epoxies. When bonding with polyurethanes, moisture can impair both performance and appearance and must be excluded from adhesive components.
are similar to epoxies and polyurethanes in that they offer good gap-filling abilities, along with good environmental and thermal resistance. They can be formulated to fixture faster than epoxy and polyurethane adhesives and improve adhesion to many plastics. Acrylics are highly flexible and bond well to many metals and plastics, which makes them a good choice for applications that require long-term fatigue resistance and durability.
Determining the best adhesive for an application depends on the substrate. Plastics are broadly characterized as thermoset materials or thermoplastics. Once polymerized, thermoset plastics such as polyester, phenolic, and epoxy resins cannot be melted or reformed. Thermoplastics, which will re-flow when heated after final processing, include a large number of common materials such as acrylonitrile butadiene styrene (ABS), polyamide (nylon), polycarbonate, and polyolefins. While each family of plastics has unique bond-strength performance characteristics, several families are designated as "difficult-to-bond." Linear or branched carbon-chain polymers, low surface energies, low porosity, and nonpolar/nonfunctional surfaces characterize these substrates.
: The use of Teflon (polytetrafluoroethylene-PTFE) as non-stick and chemical resistant material has become well known. Low-surface energy plastics like polyethylene (PE) and polypropylene (PP) are inherently difficult to bond. The surfaces usually demand priming prior to joining and often they demand much more energy-intensive surface treatment. Now, an unusual hybrid class of adhesives is set to make the need for extensive pre-treatment a thing of the past. Acrylic adhesive can bond surface of PE and PP but it has lower strength at high temperatures. When formulated into acrylic/epoxy hybrid adhesive for improved high temperature adhesive performance, it increases brittleness. It requires further modifications which increases cost. Silicone polymers are highly flexible and have excellent thermal stability, but this usually comes at a high monetary cost, and often with the drawback of low tear strengths. Therefore incorporating polysiloxane polymers with radical-curing acrylics intuitively seems like a great way to make a more versatile adhesive. It provides advantage of the flexibility and temperature stability of the polydimethylsiloxane (PDMS) homopolymers, the mechanical strength and cohesiveness of the acrylic matrix, plus the aggressive radical bonding mechanism. Commercially available amino-functional silicones are therefore useful in performing this role in a silicone/acrylate hybrid system, catalyzing acrylic polymerization and promoting adhesion. Borane is highly reactive with PTFE.
Silicone/acrylate hybrid have been around since the1970s, but normally comprise a shell/core system rather than a single continuous phase. However, different groups have shown that one way of simultaneously polymerizing acrylic and silicone without phase separation is to use (acryloxypropyl) trimethoxysilane as a phase crosslinker. For better stability until put into use silicone-acrylate adhesives are produced as two-part adhesives.
To bond polypropylene and other resistant plastics, engineers need to raise the material’s surface energy. One way to do that is by exposing the surface of the plastic to plasma, the so-called fourth state of matter according to an article by John Sprovieri.
Plasma is a mixture of free electrons, ions, radicals and molecular fragments created when energy, such as electricity or microwaves, is applied to a gas. Treating plastic with plasma improves bondability in several ways. It removes grease and other organic contaminants that inhibit adhesion. It etches the surface of the plastic at a microscopic level, which improves the bond’s mechanical strength. And, most importantly, plasma chemically activates the surface of the plastic, making it more wettable and more likely to react with an adhesive. Oxygen molecules from the ionized gas are inserted into the hydrocarbon substrate to produce hydroxyl and carbonyl functional groups. It is through these water-soluble functional groups that a polar surface with high energy is produced.
Besides polypropylene, plasma treatment is effective on such polymers as ABS, PVC, polycarbonate, polyethylene, polystyrene, Santoprene, silicone, Teflon, Mylar and nylon. However, energy, exposure time and other parameters vary depending on the material, part size and bonding objectives. No special part designs are required—complex shapes and continuous films can be treated. Plasma can be directed at areas as large as a dashboard or as small as the inside of a needle hub on a syringe.
Plasma is isotropic and nondirectional, it conforms to any three-dimensional object, and gets into nooks and crannies. It can get under silicon wafers on chip carriers and into fine polymer capillaries. It doesn’t suffer from line-of-sight problems, and it doesn’t cast a shadow. Despite the energy applied to the gas, plasma treatment is a relatively low-temperature process. It is environmentally friendly, and it’s safe for operators. A small amount of nitric oxide is emitted during treatment, and this may need to be vented depending on the layout of the system. The treatment does not produce ozone. Plasma treatment does not change how the plastic looks or feels, nor does it alter the material’s inherent properties. In fact, it’s impossible to distinguish between treated and untreated parts except by checking how well liquids wet their surfaces.
Plasma treatment can be done as a batch process in a chamber with a low-pressure atmosphere, or it can be done as a continuous process, on-line, at atmospheric pressure.
With the former, engineers have the advantage of controlling every variable of process: the composition, flow rate, pressure and concentration of the gas, as well as the frequency and wattage of the electrical energy. This can be important if engineers want to produce a specific chemical composition on the part’s surface. Another advantage is that any object inside the vacuum chamber is treated on all sides by the plasma. The disadvantage of such a system is low throughput. The size of the vacuum chamber is limited, and only so many parts will fit inside at one time.
The chief advantage of plasma treatment at atmospheric pressure is that it can be done on the assembly line just before bonding. This greatly improves throughput compared with batch processing. In this case, a jet of plasma is generated from clean, dry shop air and directed at the parts through a nozzle. A plasma nozzle can be mounted to a six-axis or Cartesian robot, or a stationary unit can be integrated into a multistation automated assembly system.
Another advantage of atmospheric plasma treatment is that it can target specific areas of a part. The plasma jet can cover widths ranging from 3 to 25 millimeters, depending on the power and the geometry of the nozzle. With multiple nozzles, web materials several meters wide can be treated. The nozzle is located 10 to 40 millimeters above the parts, which can move past the plasma stream at a rate of 6 to 600 meters per minute.
The shelf life of treated parts ranges from minutes to years, depending on the plastic, its formulation, how it was treated, and its exposure to heat following treatment. Shelf life is also limited by the presence of compounds, such as plasticizers and mold release agents. These compounds eventually migrate to the surface of treated plastic. Teflon will lose bondability within 8 hours of plasma treatment. Polystyrene, polypropylene and polyethylene are activated for weeks or months. Some treated polystyrene has lasted six years without losing its activation. Once the plastic has been bonded, the surface activation becomes permanent, and the treated area will not degrade.
|Some Hard-to-Bond Plastics
|Polyvinyl chloride, rigid
|Acrylonitrile butadiene styrene
|Polyvinyl chloride, plasticized
The strength of a bonded joint is primarily determined by how well an adhesive flows across the substrate. When a drop of liquid is applied to a surface, it will rest at equilibrium based on the surface tension of the liquid and the surface energy of the substrate. To gauge the substrate’s surface energy, measure the angle between the horizontal line of the substrate and the edge of the droplet where it contacts the substrate. For optimal bonding, the droplet should have a contact angle of less than 45 degrees.