Adhesive Selection for Effective Plastic Bonding

by Anne Forcum, Christine Marotta, Mike Williams, and Nicole Laput, Henkel Corporation

As OEM and subcomponent manufacturers endeavor to design lower cost, lighter weight, more durable products, they have begun to replace metal and glass components with plastic components. As the number of plastic components continues to increase, it becomes more important for the manufacturers to be able to effectively and efficiently join these components for a complete assembly.

With the wide variety of adhesives currently available on the market, it can be challenging to select the correct adhesive for a specific plastic bonding application. Seven families of adhesives have proven to be effective in bonding a variety of plastic substrates. Each family of adhesives offers a unique combination of performance and processing properties.

Many modern plastics used in a variety of industries are formulated specifically for their resistance to harsh chemical and environmental conditions. As a result, these substrates also tend to be difficult to chemically bond. Fortunately, advances in adhesive chemistries and surface preparation techniques have yielded the tools necessary to achieve strong bonds to even the most difficult-to-bond polymer substrates.

Adhesives for Plastic Bonding

The adhesive selection process must include many different factors, including viscosity, strength of the bond joint, temperature resistance, the bond gap between the two substrates, and various additional factors. One of the most important properties in adhesive selection is the strength of the bond between the adhesive and substrate surface, which is dependent upon the surface energy of the substrate. Cyanoacrylate, light-cure acrylic, light-cure cyanoacrylate, hot melt, epoxy, polyurethane, and two-part acrylic adhesives have proven to be effective in bonding a wide variety of plastic substrates. Each family of adhesives offers a unique combination of performance and processing properties.

Cyanoacrylates 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″. A wide variety of cyanoacrylate formulations are available with varying viscosities, cure times, strength properties, and temperature resistance. Cyanoacrylates achieve fixture strength in seconds and full strength within 24 hours, which makes them well suited for automated production.

Early cyanoacrylates exhibited low impact and peel strength, low-to-moderate solvent resistance, and operating temperatures only as high as 160 to 180°F. However, newer formulations have been developed to address many prior limitations. For instance, rubber-toughened cyanoacrylates feature greater peel and impact strength and special non-blooming formulations have been developed to minimize frosting: the presence of a white haze around the bond line. Manufacturers also have developed thermally resistant cyanoacrylates that can withstand continuous exposure to temperatures up to 250°F as well as two-part cyanoacrylates that can fill gaps up to 0.080″ (2mm).

Light-cure acrylics are one-part, solvent-free liquids with typical cure times of 2 to 60 seconds and cure depths exceeding 0.5″. Available in various formulations, light-curing acrylics provide good environmental resistance, superior gap filling, and clear bond lines that improve aesthetics. Like cyanoacrylates, light-curing acrylic adhesives 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. Because cured acrylic adhesives are thermoset plastics, they offer superior thermal, chemical, and environmental resistance.

Light-curing cyanoacrylates are fast-fixturing light-cure adhesives that also 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 blooming/frosting, increased cure depth, rapid dry-surface cure, high-bond strength, and compatibility with primers. These adhesives limit stress cracking on sensitive substrates, such as polycarbonate and acrylic, by curing the adhesive before it has the ability to diffuse into the plastic surface and attack the material. 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 percent of their final strength after only five seconds of exposure to light.

Hot-melt adhesives have been used for decades to assemble industrial and consumer products. 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.

Epoxies 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 also can fill large volumes and gaps. The major disadvantage, however, is that epoxies tend to cure much slower than other adhesive families, with typical fixture times between 15 minutes and 2 hours. 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.

Polyurethanes 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 allows manufacturers to 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 minutes to 2 hours), which can require racking of parts and substantial work-in-progress. 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.

Two-part acrylics are similar to 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 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.

(from The Loctite Design Guide for Bonding Plastics)

Table 1: Typical Bond Strengths of
Light-Curing Adhesives

Adhesives for Hard-To-Bond Plastics

Hard-to-bond plastics are most often assembled using adhesives1. While adhesives are the most versatile assembly method for plastics, capable of joining 36 types of plastics, only a few industrial adhesives offer consistently high bond strengths on hard-to-bond plastics. Cyanoacrylate, light-curing cyanoacrylate, hot melt, and occasionally light-curing acrylic adhesives have exhibited high bond strengths on typical difficult-to-bond substrates, including PE, PP, acetal, fluoropolymers, and TPVs. A new breed of acrylic adhesive also is designed for use with hard-to-bond plastics.

Although light-curing acrylics do not offer the significant performance strength of cyanoacrylates on hard-to-bond substrates, they do offer moderate performance on several of the cited plastics, including acetal and polyethylene. Table 1 shows typical bond strengths on hard-to-bond substrates using a standard ultraviolet and/or visible curing adhesive.

Cyanoacrylate adhesives (both the room temperature and light-curing variations) are compatible with primers for hard-to-bond plastics. The bond performance of light-curing cyanoacrylates on difficult-to-bond substrates is similar to that of traditional cyanoacrylates. Without primers, typical shear strengths of 50-300 psi can be realized. The addition of a primer results in typical shear strengths of 200-2000 psi.

(from The Loctite Design Guide for Bonding Plastics)

Table 2: Hot Melt Adhesive Performance on Select Hard-to-Bond Plastics

The polyolefin family of hot melts offers superior adhesion to polypropylene when compared to the other types of hot melts. Reactive urethane hot melts also perform well on hard-to-bond plastics. In addition to offering good bond strengths to difficult-to-bond materials, reactive urethanes process at temperatures of approximately 250°F, as much as 200°F cooler than EVA, polyamide, and polyolefin hot melts.

A new two-part acrylic-based adhesive offers superior performance on materials such as polyethylene and polypropylene. The two-part acrylic offers tenacious bond strength without surface preparation, creating not only mechanical but also chemical attachment to mated surfaces. In addition to its performance on hard-to-bond plastics, the acrylic adhesive provides in excess of 1000 PSI on many traditional metal and plastic materials including polycarbonate, PVC, steel, and aluminum.

Surface Preparation Methods

In order to enhance adhesion, materials designated as hard-to-bond require surface preparation prior to joining. Surface preparation methods for hard-to-bond plastics include both chemical and physical treatments designed to increase reactivity and roughness on the surface of the substrate. Common methods include plasma or corona treatment, flame treatment, chemical etching, or surface priming.

Plasma treatment is used on a wide variety of substrates including polyolefins and polyester. A gas such as oxygen, argon, helium, or air is excited at low pressure, resulting in the production of free radicals. The ions generated bombard the substrate surface and form reactive groups that increase surface reactivity and wettability. One of the major drawbacks with plasma treatment is its potentially short shelf-life. Most substrates treated with plasma must be assembled within a very short period of time since the reactive surface is re-exposed to air and rapidly reverts back to its normal state. Plasma treatment is frequently used with cyanoacrylate, light-curing acrylic, and light-curing cyanoacrylate adhesive technologies. Since each plasma gas imparts different characteristics onto a substrate, end-users should consult their adhesive and substrate suppliers to determine the optimum gas for the materials used.

Similar to plasma treatment, corona discharge uses ionization of a gas to effect surface reactivity and roughness. Reactive groups such as carbonyls, hydroxyls, hydroperoxides, aldehydes, ethers or esters are introduced to the surface. Corona discharge has a limited shelf-life and requires parts to be assembled in a limited time period. Corona discharge is commonly used on polyolefin substrates. Both plasma and corona treatment require an investment in capital equipment or outsourcing of treatment. Like plasma exposure, corona treatment is most frequently used on polyolefin substrates bonded with cyanoacrylate or light-curing acrylic adhesives. The treatment generates reactive groups that serve as potential bonding sites for the adhesive, and result in a significant enhancement of bond strength.

Chemical treatment methods such as chromic acid etching are often used on polyolefins and acetals. Like other surface treatment methods, chromic acid etching adds reactive species to a surface. Because it can be hazardous, chemical treatment is used on a limited basis to treat a variety of substrates prior to bonding with cyanoacrylate and light-curing acrylic adhesives. Handling and storage of hazardous substances is one potential drawback of chemical treatments. Surface changes are controlled by two variables: the solvent selected and the overall exposure time.

In flame treatment, various reactive groups such as hydroxyls, carbonyls, and carboxyls are introduced to bonding surfaces through an oxidation reaction when the substrate is exposed to flame. In addition, flame treatment increases the surface energy of the substrate surface, allowing for better wetting. Flame treatment is commonly used on polyolefins and polyacetals, and is most frequently used when bonding with cyanoacrylate adhesives.

Surface roughening results in mechanical interlocking sites and causes bond strength to increase dramatically. A surface roughness of approximately 63-125 micro-inches is often used as a guideline for assemblies that are to be bonded with adhesives. Surface roughening will significantly increase the bond strength of most adhesive technologies, and is highly recommended for both hard-to-bond and traditional substrates.

Primers are solvent-based systems in which a reactive species is dissolved. Applied to a surface using a brush or spray, the primer’s solvent evaporates, leaving behind the reactive species on the substrate. The reactive species acts as a linking pin or bridge between an adhesive and the substrate.

A variety of substrates exhibit enhanced bond strengths following treatment with primers, including polyolefins, acetals, fluoropolymers, and TPVs. Although using a surface primer is often viewed as a two-step bonding process, primers can be applied with little to no capital expenditure and many remain active on substrates for more than eight hours. Polyolefin primers are frequently used on hard-to-bond substrates joined with traditional and/or light-curing cyanoacrylate technologies. Introducing the active species to the substrate surface generally increases bond strength three-fold for traditional ethyl cyanoacrylate adhesives.

(from The Loctite Design Guide for Bonding Plastics)

Table 3: A Comparison of Cyanoacrylate Performance on Select Hard-to-Bond Plastics with and without Primer

Whether bonding a “hard-to-bond” plastic such as the polyethylene or a plastic that bonds more readily such as polycarbonate, adhesive selection is very important to the design of an assembly. The adhesive must be compatible with the substrates and meet all performance and processing requirements. Each of the seven families of adhesives discussed has a unique set of properties that should be considered during adhesive selection.

Footnotes & References

“Engineer’s Guide to Plastics” published by Materials Engineering

The Loctite Design Guide for Bonding Plastics, Volume 2, LT-2197A

The Loctite Design Guide for Bonding Rubber and Thermoplastic Elastomers, Volume 1, LT-2662

Handbook of Plastics Joining, A Practical Guide, copyright 1997 by Plastics Design Library, published by Plastics Design Library, Norwich, NY

Plastics Decorating would like to thank Application Engineer Anne Forcum, Medical Focus Segment Manager Christine Marotta, Application Engineer Mike Williams, and Application Engineer Nicole Laput with Henkel Corporation for their input on this article. Henkel operates worldwide with leading brands and technologies in three business areas: Laundry & Home Care, Cosmetics/Toiletries, and Adhesive Technologies. Founded in 1876, Henkel holds globally leading market positions both in the consumer and industrial businesses with well-known brands such as Persil, Schwarzkopf, and Loctite. Henkel markets a wide range of well-known consumer and industrial brands in North America, including Dial® soaps, Purex® laundry detergents, Right Guard® antiperspirants, got2b® hair gels, and Loctite® adhesives. For more information, call (800) LOCTITE or visit www.henkelna.com.