Automobile assembly applications require the use of specific 2K adhesives for bonding substrates without resulting in read-through. Bond line read-through (BLRT) is the visible distortion (surface waviness) of a bonded substrate caused by stresses that occur during adhesive curing. Primary areas where the elimination of bond line read-through is important are tailgates, spoilers, hoods and external trim.
Adhesives that eliminate BLRT allow for the use of alternative materials, such as plastic, which can help lightweight the entire assembly of a vehicle. These lighter weight materials require an adhesive that can adapt to the thin materials yet provide adhesion with the same unvarnished visual quality. Eliminating BLRT is important since these areas are prominently visible on a vehicle meaning distortions and deformations are unacceptable to end users.
What kind of adhesive should I use?
Various types of adhesives are available, so what is best to eliminate BLRT and promote automotive lightweighting? In general, pure epoxies are much thicker and increase the potential for defects upon curing. Polyurethanes are often considered but may require surface treatment prior to application. In addition, they contain isocyanates that are becoming less accepted by regional environmental, health and safety (EHS) regulations, especially in Europe. Softer acrylics have many desirable characteristics, yet present challenges such as lower resistance to metal corrosion and lower shear strength. Therefore, the combination of acrylic and epoxy (acrylic modified epoxy) is the best adhesive for no read through on lightweight materials. It delivers a low modulus, which provides better performance. This type of adhesive is flexible enough to behave like a plastic and eliminates read-through.
Advantages of Acrylic Modified Epoxies
Examples of low bond line read-through adhesives are LORD 800 series of acrylic modified adhesives. These flexible adhesive systems are optimized for fast bonding of engineered plastics, such as PC-ABS, PC-PET, PC-ASA. They can also be used with critical metals, such as aluminum. A popular use for the 800 family adhesives is high throughput spoiler bonding. LORD 810 Acrylic Adhesive is optimized for applications that allow for little to no bond-line read through. It can provide a fast cure system, even at room temperature when combined with LORD Accelerator 20GB.
Another perk of using a LORD adhesive is that plasma or flame treatments may even not be required on ABS-PC substrates, which makes LORD 800 series superior in process & cost when compared to other external trim bonding solutions.
Important factors to remember about the LORD 800 family of adhesives is that softer material improves BLRT performance. This perfect chemical combination of acrylic and epoxy allows the adhesive to stretch in order to compensate for thermal expansion and shrinkage during the curing process. The high elongation improves BLRT and improves peel performance. Additionally, these adhesives provide excellent sag resistance and allow for application on critical and difficult elements.
A 2:1 mix ratio helps to reduce shrinkage during cure and improves BLRT performance. Initial room temperature-cured properties deliver reduced BLRT, while a secondary cure over time accelerated with heat, delivers increased bond strength.
Equally important, these adhesives are user friendly with low, matched viscosities of A- and B-sides for effortless pump-ability through meter-mixed dispense equipment.
While several properties affect BLRT, such as substrate stiffness and thickness, cure shrinkage, bond line thickness and uniformity, and coefficient of thermal expansion, by using our LORD® 800 series of acrylic adhesives you can easily assemble spoilers, hoods, tailgates and exterior trim to adhere to vehicles without deformity.