Performance of Hybrid Adhesives vs. Traditional Structural Adhesives

by Timothy Holmes, senior application engineer, and Andrew Paluch, applications engineer, Henkel Corporation

Newly introduced hybrid adhesives represent a patented technological breakthrough. These universal bonders are a patented combination of proven adhesive technologies: instant and structural adhesives. This creates a new adhesive system with unique attributes of fast cure, high-strength with multiple substrates, temperature and environmental durability. This article will review the performance characteristics of several new hybrid adhesives and compare them to traditional structural adhesive systems. It will explore the process and performance capabilities this new technology provides to engineers, as well as contrast these characteristics to the inherent shortcomings of customary structural adhesives.

Hybrid adhesive technology

Henkel Corporation has developed a portfolio of patented technology structural hybrid adhesives that combine benefits and advantages of multiple adhesive chemistries into two-component hybrid adhesives.

LOCTITE® HY 4090™ GY and LOCTITE® HY 4092™ are made up of a two-part curable composition comprising (1) a cyanoacrylate curing component and a cationic catalyst; and (2) a cationic curable epoxy. When mixed together, the cationic catalyst initiates cure of the epoxy component.1 This two-component, room-temperature curable system bonds to a multitude of substrates, including plastics, metals and elastomers.

LOCTITE® HY 4090™ GY is a high-viscosity adhesive with a 1:1 mix ratio. Because it is a two-component mixture, the risk of blooming that can occur with a cyanoacrylate is greatly reduced. It has a three- to seven-minute fixture time at a 1 mm gap. This hybrid formulation combines the most critical attributes of a cyanoacrylate – fast fixture time and substrate versatility – with advantages of using a structural epoxy – high bond strength, temperature, environmental and impact resistance.2 LOCTITE® HY 4092™ is a low-viscosity hybrid adhesive designed to be flowable and self-leveling, ideal for potting applications.

Hybrid adhesives LOCTITE® HY 4070™ and LOCTITE® HY 4080™ are comprised of cyanoacrylate and acrylic (methacrylate) technologies. These two-component, room-temperature curable hybrid adhesives combine the benefits of the cyanoacrylate component, which provides cure speed, multi-substrate versatility and good plastic adhesion, with that of the methacrylate component, which provides toughness, strength and environmental resistance. LOCTITE® HY 4070™, for example, maintains a less than one minute fixture time at 0 mm gap (aluminum substrates), and obtains 85 percent of full strength after one hour on grit-blasted mild steel.

Traditional epoxy technology

Epoxies are two-part structural adhesives that cure at room temperature or a one-part product that cures at elevated temperatures. Two-part epoxy systems contain a resin – typically designated part A – and a hardener – typically designated part B – that, once mixed, form a thermoset polymer. Epoxies can often bond a variety of substrates and maintain a small degree of linear and volumetric shrinkage upon cure. They provide very good environmental and thermal resistance, retaining high cohesive bond strength and toughness. Epoxies exhibit excellent gap fill and depth of cure capabilities.

Traditional acrylic technology

Acrylics are traditionally two-component adhesives that can be either two-part or two-step. Two-step acrylics require an activator on one surface and resin on the mating surface, and cure is propagated when they are mated. In structural applications, two-part methyl methacrylates combine a resin and activator through a static mix nozzle. Structural acrylics offer great heat and peel resistance in a strong and durable bond. Acrylics are more flexible than epoxies but give off a much stronger odor and have a higher peak exotherm temperature.

Structural assembly

Structural and hybrid adhesives can be utilized in a variety of applications, but the requirements of each application dictate which adhesive would be most applicable for that scenario. Although hybrid adhesives may be compared to traditional epoxies over various categories of applications – such as structural assembly, potting and encapsulation, coatings and maintenance, repair and overhaul (MRO) applications – this paper focuses exclusively on structural assembly applications for original equipment manufacturing (OEM).

Bonding is a joining/assembly method gaining popularity from traditional methods, such as welding or mechanical fastening. Joining/assembly methods are generally chosen by analysis of the part design, the forces on the part (particularly permanent force) and whether there are large gaps. In recent years, adhesives have become widely used in all industries for bonding structural elements. Structural bonding is a common technical term used in the past for classification of very high bond strength assemblies.

Today’s terms depict structural bonding as securing a joint construction in which the bond line is a stress-transporting bridge from one substrate to another. Thus, the bond line requires creep resistance and durability in terms of fatigue and aging. The modulus of elasticity of the adhesive should be in the range of the modulus of elasticity of the bonded substrates.

Figure 2 indicates an even distribution of force throughout the bond area using an adhesive assembly method compared to mechanical fastening (bolting or riveting) or welding (MIG, TIG or arc). One primary advantage of structural bonding using an adhesive vs. bolting or welding is the even stress distributed along the bond line. This allows for the most even transmittance of loads while maintaining the integrity of the structure with no material property changes from melting.

Structural assemblies often require not only high tensile strength but high impact resistance as well. Solenoid pumps, power brake systems and prosthetics are just a few examples where there are high-impact joints.2

Structural bonding can help to achieve weight reduction, improve aesthetics, create faster production processes and increase reliability and durability vs. other joining/assembly methods. Structural bonding also allows for the use of a wider range of materials for various applications (i.e. dissimilar plastics, pre-painted material and advanced materials such as carbon fiber, engineering plastics and new metals) and compensates for inaccuracies or roughness in the substrates. Structural assembly allows for invisible joints with high bond strengths upon selecting the correct choice of adhesive.

Analysis

Structural adhesive considerations
Though epoxies and acrylics are excellent choices for many structural applications, they still have their disadvantages. Mainly, they have long fixture times that tend to be much slower than other chemistries. Typical fixture times for epoxies range from 15 minutes to two hours,2 which can be problematic for many applications requiring high throughput and low work-in-progress (WIP). Though heat can be used to accelerate the cure, the temperature limitations of the substrates – such as plastics – must be considered.2 Epoxy adhesives also provide poor adhesion to a variety of plastics, specifically those considered hard-to-bond plastics, such as polyolefin plastics. Polyolefin plastics are the most common type of hard-to-bond plastics because of their low surface energy. Examples of polyolefin plastics are polyethylene (HDPE & LDPE) and polypropylene.3 During the cure of a two-part system the reaction can also exotherm, which again can be problematic for highly sensitive parts.2 Mix nozzle life also may be a concern for many of the faster curing epoxies of the five-minute variety. Though these adhesives maintain fast fixture time, i.e., 15 minutes, the life of the adhesive located in the static mix nozzle is often a challenge for customers who have long periods of time between dispense cycles.

Hybrid vs. structural adhesives

Figure 3 presents a chart comparing general properties of hybrid adhesives vs. traditional two-component (2K) epoxy adhesives, both five-minute and high-performance variety. Note: Green = Excellent, Yellow = Good, Red = Fair.

A critical takeaway from this chart is the advantage of hybrid technologies when bonding plastics and elastomers (rubber). Rubber is commonly used, and the inability of traditional structural adhesives to adhere to it caused issues for designers. Now one adhesive can handle virtually all substrates.

Hybrid adhesives maintain an advantage when comparing EH&S ratings to both five-minute and high-performance epoxies. Hybrid adhesives maintain a significant advantage over high-performance epoxies when comparing fixture time, yet are generally comparable to the fixture times of various five-minute epoxies. Hybrid technology creates patented universal bonders that offer robust performance across many key performance attributes.

Figure 4 presents a deeper look at the EH&S advantages of hybrid adhesives over traditional epoxy and methyl methacrylate (MMA) adhesives. Hybrid technology is safer to handle than traditional structural bonders.

The following graphs will look at each hybrid adhesive’s shear strength on a variety of metal and plastic substrates compared to a Henkel five-minute epoxy and MMA, along with a Henkel high-performance toughened epoxy and MMA.

All strengths are reported on a scale per MPa. The metal comparison figures are on a scale of 0-30 MPa. The plastic comparison figures are on a scale of 0-8 MPa or 0-10 MPa depending on the graph.

LOCTITE® HY 4070™ shows excellent performance on all the metal substrates it was tested on, even having the top strength on galvanized steel. All the comparison products are used on a variety of metal assemblies. LOCTITE® HY 4070™ adhesive’s higher performance on plastics is what sets it apart from the other adhesives. It has substantial increase in performance on polycarbonate compared to these traditional structural adhesives.

LOCTITE® HY 4080™, another CA-acrylic hybrid adhesive, shows comparable performance on a variety of metals, and value can be seen with its exceedingly quicker fixture time than the toughened structural adhesives.

LOCTITE® HY 4080™ does very well on a variety of plastics, only being outperformed by a five-minute MMA on a couple substrates. This product shows its versatility and innovation as top structural performer on PVC plastic.

LOCTITE® HY 4090™ GY, a CA-epoxy hybrid adhesive, shows good strength on metals, outperforming a five-minute epoxy.

LOCTITE® HY 4090™ GY shows top of the line performance on a variety of common plastics, with major improvements over a traditional epoxy adhesive. LOCTITE® HY 4092™, another CA-epoxy hybrid adhesive, performs similar to a five-minute epoxy on metals and would be an excellent product for potting applications where a quick fixture is needed.

LOCTITE® HY 4092™ bonds well as all products besides an MMA on plastics, but, unlike an MMA, this product can be used to pot and encapsulate components without the risk of an exceedingly high exotherm. With an MMA, the exotherm temperature increases when the amount of product increases; therefore, it is not ideal to be used in most potting applications.

The hybrid adhesive line has an increased peel strength compared to epoxies due to increased flexibility. Therefore, they can be used as flexible joints, as well as rigid joints. The hybrid products have lower T-peel strength than the acrylics, but in fairness acrylic adhesives are considered to have exceptionally high properties in this area. The hybrid adhesives, for most structural applications, have more than adequate peel strength for the applications, and, if the additional benefit of adhesion to rubber is needed, are the best choice.

Results and discussion

Hybrid adhesives in the structural market
Hybrid adhesives provide a patented technology solution to many unmet needs of the structural market through the following value drivers.

• Rising demand for weight reduction: Hybrid adhesives can be used as a universal bonding agent, allowing lighter substrates to be used and removing the need for mechanical fasteners.
• Rising demand for reduced manufacturing time and cost: Hybrid adhesives offer faster cure than traditional structural epoxies. Faster cure products result in faster production cycles, less down time, less storage space, etc.
• Increasing penetration of composites and other new enabling substrates: Excellent adhesion capabilities of hybrid adhesives and formulation flexibility can lead to new design options.
• Increasing demand for high-performance structural adhesives: Hybrid adhesives can help to improve overall performance due to well-balanced properties, even with impact resistance slightly lower than other structural bonding adhesives.
• Increasing demand for low hazardous and sustainable structural adhesives: Hybrid adhesives offer many benefits over traditional structural technologies when comparing environmental health and safety hazards.
• Rising demand from emerging markets: Hybrid adhesives have been launched globally within Henkel Adhesive Technologies and will be made available for sale in 50 countries. Many customers prefer the option of one product for multiple applications. Customers now may find it easier to serve multiple applications with a hybrid adhesive than with traditional structural adhesive products.

Conclusions

Modern designers utilize various substrates to achieve products that physically could not be produced in the past. Traditional mechanical joining techniques simply are no longer the best choice.

Many older structural adhesive technologies have been developed to join materials, but most had limitations that prevented joining commonly integrated materials and were not suitable for the fast cure required for high-volume production and modern manufacturing environments.

Henkel has developed a novel new set of adhesives that can bond structurally and, importantly, adhere to rubber – a material that was problematic for many older structural adhesives.

The new hybrid adhesives match older materials on metals, while offering substantially improved results on plastics. In some areas, certain older technologies may exceed their performance in a specific property, but they fail to offer the complete package of benefits hybrid adhesives bring to the market.

Hybrid adhesives are faster fixturing on a variety of substrates, i.e., LOCTITE® HY 4070™ as low as 20 seconds on grit-blasted mild steel (GBMS) with minimal gap.

Hybrid adhesives are universal bonding adhesives with high adhesion to metals, most plastics and elastomers, wood and many other substrates.

Hybrid adhesives are much safer to handle than traditional structural epoxies and MMAs, carrying fewer environmental and safety warnings.

Plastic bonding substrates index

PPS = Polyphenylene sulfide
PC = Polycarbonate
PVC = Rigid polyvinyl chloride
PMMA = Poly(methyl methacrylate)
ABS = Acrylonitrile butadiene styrene

References

1. Hersee, Rachel M., Barry N. Burns, Rory B. Barnes, Ray P. Tully, and John Guthrie. Two-part, Cyanoacrylate / Cationically Curable Adhesive Systems. Henkel Ireland Limited, assignee. Patent US20130178560 A1. 11 July 2013. Print
2. Lavoie, Nicole. INNOVATIONS IN HYBRID STRUCTURAL INSTANT ADHESIVE TECHNOLOGIES. Tech. N.p.:n.p., 2015. Print
3. Marotta, Christine; Mike Williams and Nicole Laput. ADHESIVES FOR THE ASSEMBLY OF HARD-TO-BOND PLASTICS. Tech. N.p.:n.p., Print
4. LOCTITE® HY 4070™ Technical Data Sheet. July 2017.
5. LOCTITE® HY 4080™ Technical Data Sheet. July 2017.
6. LOCTITE® HY 4090GY™ Technical Data Sheet. July 2017.
7. LOCTITE® HY 4082™ Technical Data Sheet. July 2017.
8. LOCTITE® EA E-05CL™ Technical Data Sheet. November 2014.
9. LOCTITE® EA E-20HP™ Technical Data Sheet. September 2015.
10. LOCTITE® AA H3300™ Technical Data Sheet. March 2014.
11. LOCTITE® AA H4800™ Technical Data Sheet. May 2015.

Acknowledgments

Special thanks to Matthew Miner, Christopher Burke, Zachary Bryan and Robert Dunkel for aiding in the review process. For more information, visit www.henkel.com.