Plastics Bonding Design Considerations for Processors
by Andrew Scott
Henkel Corporation
Today's production world constantly is looking to streamline production and create more reliable assemblies with faster and easier methods. Adhesives successfully have been used to displace solvent welding in the plastics industry. The most common bonding applications using plastics range from delicate medical components to heavy industrial equipment.
Adhesives are being recognized in the production market to help diminish the cost and time accompanying the usage of traditional mechanical fasteners and welding to provide structural reliability to assemblies. As with any material, mechanical fasteners tend to weaken the structure of the material at the mating surface due to the need to perforate the material with holes. The intermittent nature of the clamping pressure also is known to concentrate the loads acting on a joint in the area of one or a few fasteners. Adhesive bonding of plastics in general commonly is recognized as the ideal method.
Design considerations
When bonded appropriately, the design and bond configuration can result in substrate failure of an assembly prior to an adhesive bond joint failure. There are five main types of forces that can be applied to joints: tensile, shear, compressive, peel and cleavage.
Tensile force is the force applied to a bonded joint when pulling the assembly apart perpendicularly to the bond line and adjoining substrate. Many perform well when tested in the tension plane. Shear force is the force applied when substrates in a bonded joint are pulled parallel from one another along a plane.
Compression is the optimal force to place on an adhesively bonded assembly. Compressive force is the force applied to an assembly when the bonded substrates are pushed together perpendicularly by an outside force.
Peel and Cleavage forces are similar to each other and are the least desirable forces to apply to any bonded assembly. Peel and cleavage forces are applied to the leading edge of an assembly. These forces apply an uneven distribution of stress to the edge of the bonded materials. Once the adhesive begins to pull apart along the leading edge, the fractures being created in the adhesive begin to spread through the bond line.
Improve bonding through surface preparation
Surface preparation and pretreatment methods have been accepted to allow adhesives to bond well to plastics, such as polyolefins which are commonly labeled as difficult-to-bond. There are five common types of surface preparation to improve bond capabilities: abrasion, acid etching, flame treatment, corona treatment and plasma.
Abrasion is a process of mechanically roughening the surface of a material to enlarge the surface area at the bond line. A surface cleaning should be performed after the abrasion to remove any debris from the process.
Acid etching is an effective method of increasing surface area in a manner similar to abrasion. Acid etching can be hazardous due to the chemicals required to perform the process, and it also can be time consuming.
Flame treatment is the process of passing a flame over the surface of a plastic. This process causes oxidation on the surface, which improves chemical functionality of the surface, as well as the surface energy. This process presents concerns when dealing with thermoplastic substrates. Although the flame may only briefly pass over the surface, there is a possibility of warping the substrate.
Corona treatment is an electrical discharge that forms oxygenated functional groups for improving surface energy. A corona application is accomplished by applying a source of high-voltage, high-frequency over the parts or passing the parts over the electrode.
Plasma, which is the ionizing of a particular gas, commonly is used to increase the surface energy of small complex surfaces which are difficult to abrade or evenly accept flame treatment.
Available adhesive technologies
Currently, there are a multitude of adhesives available, which fall into six families that most commonly are used in manufacturing environments. Each of these families offers a unique combination of performance and processing benefits. Manufacturers that dedicate significant up-front time to research and select the proper adhesive for an application will save significant time and expense later in manufacturing and reliability.
Choosing which type of adhesive is appropriate for a manufacturing system depends on the materials being bonded, joint design and the projected end-use conditions of the assembly.
Epoxy adhesives
Epoxies are one- or two-part structural adhesives that bond very well to a wide variety of substrates, have excellent toughness and offer superior environmental resistance. The major disadvantage of epoxies is that they tend to cure much slower than other adhesive families, with typical fixture times between five minutes and two hours. Slow cure also may be beneficial though in a situation where it takes extended time to join parts after adhesive application or if parts need to be repositioned after being mated.
Cyanoacrylates
Cyanoacrylates (or instant adhesives) are a versatile single-component, quick-fixturing, room-temperature curing adhesive. They commonly are used to bond elastomeric substrates to metal or plastic and for bonding/sealing plastic components together. These adhesives achieve fixture strength in just seconds and full strength within 24 hours, making them ideally suited for high-speed production.
Cyanoacrylates do have some limitations. In particular, cyanoacrylates bond to skin rapidly, have limited gap filling and curing capabilities, have poor polar solvent-resistance (isopropanol, acetone, methylene chloride) and exhibit poor long-term durability on glass substrates.
Many cyanoacrylates contain rubber toughening agents that enhance peel and impact strengths of cyanoacrylates on bonded assemblies.
Hybrid cyanoacrylates
The patented hybrid two-part cyanoacrylates are two-component, room-temperature curing adhesives, which provide excellent adhesion to a wide variety of substrates. The Hybrid technology combines the benefits of cyanoacrylates and epoxy adhesives. Unlike epoxies, the hybrid formulation allows for a typical fixture time between 60 and 90 seconds.
Two-part cyanoacrylate adhesive systems only are available in dual cartridges with a disposable mix nozzle. One side of the cartridge contains the cyanoacrylate monomer resin and the other side of the cartridge contains a proprietary catalyst that promotes the cure of the adhesive.
Since the two-part cyanoacrylates are not limited to a moisture cure, the adhesive is capable of large gap filling and curing, unlike the traditional one-part cyanoacrylate adhesives. The Hybrid technology provides better bonding performance on acidic surfaces and in low-humidity curing environments.
Methyl methacrylate adhesives
Methyl Methacrylate (MMA) adhesives are two-component products consisting of a resin and hardener combination. MMAs cure at room temperature when the resin and hardener are mixed at the correct ratio.
MMA systems can develop strength in as little as two minutes and have outstanding environmental and impact resistance, which make them well-suited for weld replacement or structural bonding of both plastics and metals. MMAs have the ability to cut through a variety of surface contaminations and provide reliable bonds.
Elastomeric adhesives
When bonding dissimilar substrates like glass to metal, the best option to ensure a robust assembly is silicone technology. Silicones are flexible, rubber-like materials that cure at room temperature, exhibit excellent resistance to heat and moisture and bond a variety of substrates. The flexibility of silicones over a broad temperature range makes them an ideal stress absorber. Today, there are UV/visible light cure, dual UV/moisture cure, heat cure and extremely fast two-part silicone technologies to tribute the older RTV chemistry.
When a silicone is not an option due to the contamination concerns of a paint line, silane modified polymers (SMP) can be substituted. SMPs offer high strength and elongation while having the ability to be painted. The trade-off to using a SMP is the reduced temperature resistance when compared to silicones.
UV-cure acrylic adhesives
One-part, solvent-free UV-curable acrylics offer performance benefits comparable to epoxies. While early UV-curable acrylic adhesives relied on high doses of ultraviolet energy, advances in the technology allow for dual cure (UV and visible light) or only visible light energy to cure the adhesive. Due to the cured acrylic adhesives being thermoset plastics, they offer superior thermal, chemical and environmental resistance.
As cure is on demand, light cure acrylics offer extended open times for positioning and repositioning of parts. These adhesives provide high strength bonds to a variety of substrates and are available in ranging degrees of flexibility, from soft elastomers to glass plastics. All this, coupled with cure times of only two to 60 seconds, makes UV-curable acrylics an attractive manufacturing option.
Adhesives and the technology that has developed over the years have provided cost and time savings, offered reduced waste and allowed for the building of better assemblies. Adhesives have helped to eliminate the need for screws, clips and welding of assemblies. When an adhesive is properly selected, it can provide years of predictable reliability on an assembly.