General
Cure Kinetics
Cure Related
Physical Properties
Mechanical Properties
There are many advantages structural adhesives offer over other fastening methods. First, structural adhesives provide processing convenience and overall assembly cost savings. Also, structural adhesives:
Design freedom is another advantage of adhesives. Adhesives allow for the easy joining of complicated shapes and can join dissimilar substrates (dissimilar metals or metals to composites or thermoplastics). Assembly adhesives are also multi-functional to allow for:
Structural adhesives are thermoset polymers, commonly available in three main types, or chemistries: Acrylic Adhesives, Epoxy Adhesives, and Urethane Adhesives. They will not melt or change with environmental exposure, temperature or time. Acrylics and epoxies can withstand temperatures from -40°F to +400°F. Most urethanes are good up to 250°F with a low end slightly better than the others. Exposure to water, humidity, oil, gasoline, solvents, and other environmental factors won’t weaken bond strength in properly designed joints. Two-component adhesives cure at room temperature, most within 24 hours. Multiple cure speeds are available (Slow, Medium and Fast Speeds).
In general, acrylic adhesives excel at bonding unprepared metals, composites, and thermoplastics. Urethane adhesives offer resiliency and flexibility and are candidates for joining composites, thermoplastics, natural materials, and prepared metals. Epoxy adhesives give the highest strengths when bonding prepared metals, composites, thermoplastics, and natural substrates such as wood. Continue reading the sections below for more details on the properties and differences between the three main structural adhesive chemistries.
The viscosity of acrylic adhesives allows for very easy dispensing by hand while maintaining a non-sag characteristic. Also, acrylic adhesives change color during cure, giving a visual indication that the adhesives are cured. Another key property of acrylic adhesives is that they can have a long room-temperature open time but once they begin to react, they quickly reach handling strength. Other adhesive chemistries are not able to match this cure profile.
Epoxy adhesives generally tend to be viscous liquids or pastes, although self-leveling liquids are available. In general, epoxies are less sensitive to slight variations in mix ratio. Also, epoxy adhesives offer long open times, allowing for the repositioning and assembly of complex shapes. Final properties develop within 24 hours at room temperature.
Urethane adhesives are offered as either one and two-component systems. Also, urethane adhesives are formulated with much lower viscosity compared to epoxy or acrylic adhesives. Urethanes are inherently sensitive to moisture which reacts with the isocyanate in the system. Care should be taken to avoid high humidity conditions and moisture in the substrates.
General Structural Adhesive Process Guidelines
General Adhesive Joint Design Guidelines
Further Guidelines for Adhesive-Bonded Joint Design
Joint configuration should be designed so that the basic stress is primarily shear. Tensile, or compressive with cleavage, and peel stresses should be minimized on the bond line. Joints should be designed so that all of the bonded areas equally share the load. The illustrations below depict both recommended joint design alternatives and joint designs to be avoided.
Lap Joints - Lap joints are the most practical design and are applicable in bonding thin materials. Lap joints are used to enhance joint strength by reducing their potential to peel stress.
Butt Joints - In tension, the straight butt joint is impractical for load-bearing assemblies. To minimize this stress, the angle design applies compression. Compressive loading will not affect the joint unless bucking of the vertical component occurs.
Joint Design Alternatives
Substrate Surface Preparation
The amount of surface preparation required for good bonding will depend upon both the substrate and the adhesive that is used. In general, obvious dirt and loose particles should be removed from the bond surface with a clean, dry rag. Using compressed shop air to blow off parts is not recommended, since shop air usually contains water from condensation and oil from the compressor that can contaminate the bond surface. Avoid handling the bond area after the surface has been prepared. Dirty hands/gloves, soap, mold release, grease, etc. can contaminate the surface and potentially lead to poor adhesion.
Before adhesive application, remove soils, greases, oils, dust, mold release agents, rust, and other contaminants from the substrate surface with the use of a vapor-free solvent, such as MEK, acetone, or isopropyl alcohol.
Preparing Convenience Cartridges for Use - Purge and Run Adhesive
Below are best practices for preparing two-part adhesive and seam sealer cartridges for optimum bonding results (refer to Figures 2-7). The static mix tip may generally be left attached to the cartridge if the entire cartridge is not used. The cured adhesive in the tip will act as a cap. However, it is possible in some cases that mixed, the cured adhesive will block the nose of the cartridge, so the best practice is to remove the static mix tip and replace the original plastic plug(s) — taking care to match the proper sides — for longer-term storage.
Understanding Meter Mix Dispensing (MMD) Equipment
NOTE: If you are using manual cartridge dispensing as described above, the following section on MMD equipment can be skipped, you can proceed directly to the "Preparing Bond Area" section.
In contrast to the use of cartridges, meter mix dispensing (MMD) enables the use of bulk containers of adhesives. The MMD equipment is generally manufactured and dedicated to a specific volume mix ratio, covering a range from 1:1 by volume to 22:1 by volume. MMD equipment can be utilized by an operator in a totally manual fashion. However, if you anticipate the need for semi-automated or fully automated applications in the future, the equipment can be supplied with appropriate functionality that allows for totally robotic operation. Anticipating the need for this type of capability will save the cost of refit or replacement of the equipment as production volumes grow.
The type of MMD unit selected is based on the type of adhesive used for the application and the anticipated production volumes. To select the right piece of MMD equipment, one needs to know the details of the manufacturing process and possible future needs.
Reasons for using MMD equipment:
Types of MMD equipment:
Rod Meters and Piston Meters
Piston Meter
Rod Meter
Pictured below is a portable rod metering system designed for use with acrylic structural adhesives. The common air cylinder drive unit on the rod meters helps to ensure a constant mix ratio (strokes both rods at the same time). This unit also incorporates integrated transfer pumps that supply the adhesive to the rod meters.
Progressive Cavity Meters
Proportioner Systems (Double Acting Piston Pumps)
Gear Meters (Gear Metering Pumps)
Gerotors (Gear Rotor Metering Pumps)
Key Components of MMD Equipment
Selection of MMD Equipment for Lord Structural Adhesive Chemistries
Preparing Bond Area
Structural adhesives work best at a very thin, controlled bond thickness (10 to 20 mils), and they sometimes contain glass bead “spacers” to set this bond gap. Before dispensing the bead, attempt to remove any scrap material from the substrate such as protruding burrs, welds, or other irregularities that would prevent the two bonding surfaces from lying flat on top of one another. Apply adhesive in a continuous bead in the desired locations, taking care that the dispensed pattern will not cause air to be trapped in the bondline when the substrates are mated. A single adhesive bead dispensed in the center of the bond area is generally preferred.
Estimating Material Coverage
The bead diameter should be predetermined based on the desired final bondline width and thickness. Table 1 can be used as a guide for sizing the adhesive bead diameter.
Note: These bead diameters will yield an excess of 10 percent in case of irregularities in the surface.
The engagement area is critical to adhesive performance, so it is important to apply enough adhesive to fill the designed joint. Insufficient adhesive quantity, or introduction of air into the adhesive, will cause a reduction in bond strength and a characteristic pattern known as “spider-webbing” (the pattern is visible when parts are disassembled). This problem can also be caused by insufficient or ineffective clamping, as detailed in the section following.
Refer to Table 2 for estimated linear foot coverage based on the cartridge size and bead diameter.
Open Time / Working Time
Open Time is the amount of time from when the adhesive starts to travel down the static mix tip until the parts must be mated to deliver the specified bonding performance. Working Time is often used synonymously with Open Time, but working time can also refer to the time after the substrates are mated and can still be (slightly) re-positioned relative to each other.
It is important to work quickly to mate parts before the adhesive Open Time expires. Knowledge of the estimated Open Time or Working Time is particularly important when bonding large parts that have long adhesive bead lengths and during periods of higher-than-normal temperatures within the production facility. Higher temperatures will generally reduce Open Times due to the acceleration of the cure. In general, Open Time can be estimated by the hardness of the dispensed adhesive bead. If the adhesive bead cannot be readily compressed and spread, it has most likely passed beyond its Open Time. However, epoxy adhesives can have an additional condition referred to as “blushing,” which can limit their Open Time without any indication of bead hardening. When the working time is exceeded, the adhesive will no longer wet out on one of the surfaces to be bonded. This will generally cause a reduced bond strength and be visible as a shiny, very smooth surface on the adhesive after disassembling the bonded parts, in contrast to the rough surface generated with good, cohesive failure. The technical data sheet for each adhesive should contain specific information related to Open Time/Work Time.
Positioning Parts
Place parts in position as gently as possible, watching that the mating process works to eliminate trapping air in the bondline. Avoid applying pressure initially, allowing the clamping system to do this work. After a part has been mated and needs to be moved or repositioned, the substrates mustn't be pulled apart during the manipulation. This introduces air gaps into the adhesive that significantly weaken the bond, and may even prevent the adhesive from curing completely. If a part needs minor repositioning, ALWAYS SLIDE the part to the new position. If a part needs major repositioning, it may be better to separate the substrates, remove the adhesive, and begin the bonding process anew. Sliding the part over a long distance may scrape all of the adhesives out from the intended bond area and result in poor bonding.
Fixturing Methods
Clamping Parts
Parts should be positioned and clamped within the working time of the adhesive. Apply uniform pressure to the joint as soon as possible after mating the parts, spreading the adhesive bead, and compressing it to the desired thickness. While clamping, special care should be taken to avoid “levering” the parts, causing the bond to separate on the opposite end. Uniform pressure (pressure spread out over the length of the bondline) is very important, especially when working with a thin gauge or non-uniform parts. Effective methods for applying uniform pressure can include:
Immediately after the parts are positioned correctly, they must be weighted with even pressure until handling strength is achieved. Some of the means typically used to accomplish this are clamps, boards/stiffeners, weights, mechanical fasteners, or braces.
Clamping Time
Bonded parts should remain clamped until the Handling Time of the adhesive has passed. Handling Time is an estimate of the amount of time required from when the adhesive starts to travel down the static mixing tip until the adhesive has cured enough to ensure the bonded parts will not shift when handled (roughly 50-100 psi bond strength). Handling Time is usually dependent upon cure temperature, and can also vary based on factors such as the amount of adhesive applied, the bondline thickness, the type of substrates being bonded, and environmental factors such as humidity. The technical data sheet for each adhesive contains specific information related to Handling Time.
De-Roping Adhesive
The excess adhesive that is squeezed out at the seams between mated parts may be removed (after it has gelled or partially cured at room temperature) by scraping it off with a putty knife. This de-roping process can also be used with a heat curing process, scraping off the excess adhesive while it is still hot enough to remain soft. Solvents such as isopropyl alcohol or acetone can be used to remove smears or adhesive residue left behind by the de-roping process.
Adhesive Removal
If substrates are accidentally pulled apart or need major repositioning after the adhesive is applied, the adhesive should be removed and the bonding process restarted. The process for doing this is outlined to the right.
Bonded Part Removal
If for some reason, bonded parts must be separated after the adhesive has fully cured, a heat gun may be used to soften the adhesive. It may be helpful to remember that adhesives are generally stronger in tension or shear, and parts can be separated more easily if pulled apart with a peel load.
Ideal Failure
When bonded parts are separated, the residue pattern from the failure of the cured adhesive, in addition to the bond strength, can be used to diagnose the quality of the bond. The preferred method of failure for adhesive joints is a cohesive failure (COH), which can be identified by the residue of adhesive left on both substrates. Cohesive failure mode indicates excellent adhesion to the substrates, with the residue divided evenly (COH) or unevenly (Thin Layer Cohesive, or TLC) between the failed surfaces. This can be contrasted with undesirable adhesive failure (ADH), which is characterized by a clean, usually shiny surface exposed on one of the two separated substrates. The ability to gain both high strength and ideal failure mode will depend upon appropriately matching the correct adhesive with the substrate to be bonded, and following the user guidelines detailed above.
Powder Coating after Bonding with Lord Acrylic Adhesives
Note: Clamp or fixture the assembly before powder coating to avoid slippage during the powder coating process. The assembly should remain fixtured until the adhesive returns to room temperature and re-hardens.
The Lord® 400, MaxlokTM, and 800 series acrylic adhesives have excellent heat resistance characteristics up to 400°F (204°C), thus reducing the concern of possible degradation of the cured adhesive during the high heat associated with the powder coating process.
Lord acrylic adhesives will not degrade at the higher temperatures associated with powder coating. However, the hot tear strengths will be very low, causing the assembly to possibly sag and slide apart — especially if the assemblies are heavy. The lower strength values make it essential that the assembly is properly fixtured or placed to avoid slippage of the bonded pieces.
Spot welds or another type of mechanical fixturing are frequently used in the industry to aid in holding the assembly in place. The area to be bonded can also be masked off the assembly before powder coating with bonding done after the process.
The integrity of the bond will remain unchanged after powder coating, and greater strength is often seen after exposure to heat once the assemblies have been returned to ambient temperature.
How to Avoid Bondline Read-Through
Read-through is a condition where you can see the footprint of the adhesive through the material. This is caused by shrinkage that results in a pull on the bonded materials. Read-through can occur on surfaces that are high gloss, high polish, or have a mirrored finish. Thin gauge metals less than 0.030 inches are more susceptible to read-through.
Lord 810./20GB Low Read-Through (LRT) acrylic adhesive is a flexible adhesive system specifically designed for bonding metals, such as aluminum, galvanized steel, and CRS, and engineered plastics, such as PC-ABS and ASA. Lord 810./20GB adhesive delivers fast cure speed and strong bonding with minimal bondline read-through (BLRT).
The following are some application tips to help you avoid read-through:
Structural Adhesive Applications - Common Substrates & Joining Method Comparisons
Metal Bonding - typically replacing conventional joining methods, such as welding and mechanical fastening, in structural applications for reasons of cost, performance, and aesthetics.
Composite Bonding - typically replacing conventional joining methods such as tabbing and mechanical fastening in structural applications for reasons of cost, performance, and aesthetics.
Plastic Bonding - many plastics are only bonded by a specific adhesive or type of adhesive:
Natural Substrate Bonding
Difficult-to-Bond Materials
Structural Adhesive Bond Performance Tests
The four test methods below are commonly used to evaluate the bond performance of structural adhesives on assembly substrates:
Other Structural Adhesive Performance Tests
Structural Adhesive Bonded Assembly Trials
Structural Adhesive Bond Failure Modes