This article was updated May 23, 2023. It was originally published June 15, 2000.
Plastics are an integral part of everyday life. They can be found in everything from packaging machine and cars to consumer electronics and medical devices. Engineers appreciate them for their durability, pleasing appearance, and the design flexibility they offer.
But to get the most from plastics, manufacturers and engineers must know how to efficiently and permanently turn plastic components into completed assemblies. More often than not, that means turning to adhesives.
Whether bonding plastic to plastic or other materials, adhesives have several advantages over other joining methods. They distribute loads evenly over a broad area, reducing stress on the joint. They are applied inside the joint, so they cannot be seen if used properly. Adhesives resist flex and vibration stresses and form a seal as well as a bond, which protects the joint from corrosion.
Adhesives also join irregularly shaped surfaces more easily than mechanical or thermal fastening; barely change assembly weight, part dimensions or geometry; and quickly and easily bond dissimilar substrates, as well as heat-sensitive materials. They can also be applied using automated equipment.
They do have limitations. Adhesives require setting and curing time—the time it takes for the adhesive to fixture and strengthen fully. They also need some surface preparation prior to assembly. And they may not be the best fastening and joining option for putting together a multi-component assembly of plastic parts if it will be repeatedly taken apart and reassembled.
When determining the best adhesive for an application, a lot depends on the substrate. Plastics are broadly characterized as thermoset materials or thermoplastics. Once polymerized, thermoset plastics (such as polyester) and phenolic and epoxy resins cannot be melted or reformed. Thermoplastics, which re-flow when heated after final processing, include many common materials such as acrylonitrile butadiene styrene (ABS), polyamide (nylon), polycarbonate and polyolefins.
While each family of plastics has different bond-strength performance characteristics, several are designated as “difficult-to-bond.” These plastics are usually linear or branched carbon-chain polymers with low surface energies and porosities and have nonpolar/nonfunctional surfaces. Difficult-to-bond plastics include polyolefins (such as polyethylene and polypropylene), fluoropolymers (such as Teflon), acetal resins and thermoplastic vulcanizates (such as Santoprene).
The Right Adhesive
Out of the many adhesives currently available, seven types are commonly used to bond plastics. Each offers a different combination of performance and processing benefits.
Cyanoacrylates cure rapidly at room temperature to form thermoplastic resins when placed between two substrates that contain trace amounts of surface moisture. Curing starts on the substrate surface, so these adhesives have a cure-through gap limited to only 0.010 in. Cyanoacrylates reach fixture strength in seconds and full strength within 24 hr, making them a good fit for automated production.
Early cyanoacrylates had low impact, peel strengths, and solvent resistance, and operating temperatures only as high as 160 to 180°F. Today, thanks to advances in material science, there are a wide variety of cyanoacrylates with varying viscosities, cure times, strengths and temperature resistance that surpass previous limitations.
For example, rubber-toughening cyanoacrylates (which embed nanoparticles of rubber in them) gives them more peel and impact strength. If polyolefin primers are put on substrates before the adhesive, it increases the strength of the bond on difficult-to-bond plastics. Adding accelerators helps the adhesives cure rapidly in low-humidity environments.
Companies also make surface-insensitive cyanoacrylates that cure quickly in low-humidity environments as well as on acidic surfaces. In addition, there are also non-blooming cyanoacrylates that minimize frosting (a white haze around the bond line). And thermally resistant cyanoacrylates withstand continuous exposure to temperatures up to 250°F.
Newer cyanoacrylates have solved many of the adhesive’s shortcomings, but not all. Fortunately, proper bond design and processing techniques can mitigate some of these. For example, the bond strength of cyanoacrylates on plasticized PVC assemblies can decay over time. To prevent this, some firms age and test PVC assemblies to ensure the bond will withstand the effects of plasticizer leaching onto the substrate.
Another problem is that cyanoacrylates cause stress cracking in some thermoplastics if left uncured. Minimizing the bond gap and limiting the adhesive dispensed generally controls this problem. Plastics highly prone to stress cracking may need a surface-insensitive cyanoacrylate.
Light-curing acrylics (thermoset plastics) are solvent-free liquids with typical cure times of 2 to 60 sec. and cure depths exceeding 0.5 in. They resist wind, water and sunlight; have good gap-filling capabilities; and leave clear bond lines forbetter appearance. They come in viscosities ranging from thin liquids with viscosities of about 50 cP to thixotropic gels.
The acrylics remain liquid until exposed to light of a specific wavelength and strength makes fixture rapidly and cure. Curing is controlled, so technicians have time to align and position parts. Secondary cure mechanisms, such as heat and chemical activators, can be used to cure these adhesives in shadowed areas.
Light-curing acrylics create strong bonds on a variety of plastics and can have flexibilities ranging from soft elastomers to glassy plastics.
Light-cure cyanoacrylate combines the benefits of cyanoacrylates and light-curing acrylics. They fixture quickly and cure in shadowed areas, thanks to a secondary moisture cure mechanism. They offer minimal blooming and frosting, strong bonds, increased cure depths and rapid dry-surface curing.
Light-cure cyanoacrylates also emit few vapors, 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 adhere well to numerous substrates, including rubber and plastic. They limit stress cracking on sensitive substrates, such as polycarbonate and acrylic, and will bond polyolefin plastics if adhesion promoters are added into the molded parts or applied to the part surface.
Ideal for high-volume bonding applications, light-cure cyanoacrylates 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 only 5 sec. of exposure to light. Light-cure cyanoacrylates are especially recommended for bonding overlap-ping, nontransparent parts.
Hot-melt adhesives have been used for decades to assemble industrial and consumer products. Traditional hot melts are thermoplastic resins that, once cooled, hold components together. Many hot melts are available, but the higher performing ones include ethyl vinyl acetate (EVA), polyamide, polyolefin and reactive urethane. Hot melts can fill large gaps and create strong bonds as soon as they cool.
EVA hot melts are typically used for low-cost potting, while polyamide hot melts are used in similar applications that have more stringent temperature and environmental demands. Polyolefin hot melts resist moisture—as well as polar solvents, acids, bases and alcohols—and adhere well to polypropylenes.
Another category of hot melt is reactive urethanes, which are good options on difficult-to-bond plastics. Whiles most traditional hot melts are thermoplastic resins that can be repeatedly reheated, reactive urethanes form thermoset plastics. Their strength develops more slowly than traditional thermoplastic hot melts. Still, for structural bonding, reactive urethanes outperform all other hot melts. And they melt at 250°F, as much as 200°F cooler than other hot-melt adhesives.
Epoxies are one- or two-part structural adhesives that bond well to many substrates. They do not outgas byproducts and shrink very little when curing. Epoxies typically have excellent cohesive strength and good chemical and heat resistance. They can fill large volumes and gaps.
The major disadvantage of epoxies, however, is their tendency to cure more slowly than other adhesives; typical fixture times range between 15 min. and 2 hr. Heat accelerates curing, but the plastic substrate might not withstand the higher temperatures. In fact, epoxies generate considerable heat as they cure, which lead to temperatures high enough to damage certain plastic substrates.
Polyurethanes adhesives are tough polymers that offer more flexibility and peel strength, and lower modulus than epoxies. They can be one- or two-part adhesives and, when cured, contain soft regions that add flexibility to the joint as well as hard regions that contribute strength, in addition to temperature and chemical resistance. Varying the ratio of hard and soft regions lets manufacturers tailor physical properties to an application’s needs.
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 parts and having room for substantial work-in-progress.
Although polyurethanes do not present much of a stress-cracking hazard, the solvents used in primers do. Polyurethanes resist chemicals and temperatures. However, long-term exposure to high temperatures degrades polyurethanes more rapidly than epoxies. When bonding with polyurethanes, moisture can damage both performance and appearance, and therefore must be kept out of the adhesive.
Two-part acrylics are like epoxies and polyurethanes in that they offer good gap-filling abilities, along with good environmental and thermal resistance. Two-part acrylics can be formulated to fixture faster than epoxy and polyurethane adhesives or to 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.
The Right Joint
The most frequent reasons for adhesive joint failure do not involve adhesive strength. They typically fail due to poor design, inadequate surface preparation or if the adhesive wasn’t compatible with the substrates and operating conditions.
To design a strong, long-lasting bond, engineers must fully understand the stress distribution across two mating substrates, because it plays a significant role in the success or failure of a plastic joint bonded with adhesives.
Five types of stress commonly affect assemblies bonded with adhesives:
- Tensile stresses tend to elongate and pull assemblies apart.
- Compressive stresses squeeze assemblies together.
- Shear stresses pull parallel objects apart lengthwise, creating a sliding motion in opposite directions of each object.
- Peel stress arise when a flexible substrate lifts or peels away from the substrate to which it is bonded.
- Cleavage stress is similar to peel stress, but it is generated in inflexible substrates when a joint is forced open at one end.
In use, most joints experience a combination of these forces.
Most adhesives have excellent resistance to tensile, shear and compressive stresses, but are weak in cleavage and peel strength. For the strongest possible adhesive joints, distribute loads as evenly as possible over the entire joint area. The goal should be to maximize tensile and compressive stresses, minimize shear stress, and avoid cleavage and peel forces.
The best joint designs maximize the bond area and rely on mechanical locking and adhesive bond strength. The ends of a bond resist more stress than the middle, so joint width is more important than substrate overlap for a successful joint design. By increasing the width, the bond area at each end increases, along with the overall joint strength.
Regardless of the adhesive, surface preparation is critical. The adhesion between the substrate and adhesive to a great extent determines the bond’s strength. To prepare the surface of the plastic substrate, remove unwanted films; use primers to create active surfaces; and apply plasma treatments, corona discharge or chemical etching techniques to the substrate.
Patrick J. Courtney was an engineering project manager at the Loctite Corp. when this article was originally published.