The New Glues.
Nowadays, adhesives are easier to use. Acrylics were first to be offered in convenient cartridge form, and similar offerings for urethane and epoxy adhesives have followed in the last few years. Cartridge development has been accompanied by easier-to-use dispensing guns and disposable static mixers (for two-component adhesives) that can plug into cartridges. Nowadays, bulk dispensing equipment is also much easier to use and lower in cost.
Newer adhesives are themselves more economical and more consistent from batch to batch, says Don Bladecki, business director at National Starch & Chemical Co. And with the increasing focus on environmental issues, most suppliers have moved toward water-based or 100%-solids formulations.
Most recently, adhesives that can handle hard-to-bond polyolefins without surface treatment are making news. As are new reactive urethane hot-melt adhesives, which reportedly surpass the performance of thermoplastic hot melts and conventional urethane adhesives. A number of other new adhesives have been introduced recently, including more versatile urethane types and new methacrylates said to go neck-to-neck with urethanes in automotive and marine applications.
Three Key Types
Epoxy-based structural adhesives are primarily used in aerospace components, electronic parts, and medical devices, as well as in automotive applications for metal/plastic assemblies that go through hot-oven metal processes. They are typically used to bond SMC and other thermoset composites as well as high-performance engineering resins such as PEEK and nylons, which require high heat resistance. They are stable up to 700-750 F.
Structural epoxy adhesives are very rigid and generally have higher strength than urethanes or methacrylates. They are applied in much thinner coats in either spray or bead form. But because they are brittle, they are not good for parts requiring flexibility because of their end use or the difference in the coefficient of thermal expansion (CTE) of the two substrates, which can result in warping. The ideal application for epoxies is two rigid plastics that are fairly similar in CTE.
For more than 25 years, urethane adhesives have been used successfully in automotive applications such as fascia bonding and in large-volume marine applications. A key advantage of urethane adhesives is their greater flexibility--lower modulus and greater tensile elongation. They typically have elongations of 200-250% vs. about 8% for epoxies. Like epoxies, urethanes can also be formulated to be clear.
Urethanes are used for bonding SMC, FRP, ABS, polycarbonate, acrylic sheet, and polyurethanes. They can also be used for bonding polyolefins but require a primer. Curing time can range from 15 min to 2 hr, although some require as much as 24 hr. Some single-component urethanes can take up to a week to cure. Urethane adhesives generally withstand temperatures up to 350-375 F. Urethanes are best suited to bonding plastic to metal or bonding plastics that are dissimilar in CTE, says Bladecki at National Starch.
Acrylic-based adhesives, particularly methacrylate structural adhesives, are noted for their especially good adhesion at room temperature. Curing cycles can range from as little as 3 min to 1 hr. Methacrylates are water resistant and particularly well suited to heat-sensitive materials, including ABS, PVC, polycarbonate, acrylic, SMC, and other FRP composites. They generally perform at up to 425 F. Flammability and bad odor are two disadvantages. However, suppliers have come up with products that have no offending odor.
Methacrylates used to be brittle, but newer formulations have better elasticity--some nearing the tensile elongation of urethanes (100-150%). They have found use in marine applications, particularly with small-volume custom boat builders seeking room-temperature cure. They perform well on unsaturated polyester gel coats, which are difficult to bond due to the mold releases used. Methacrylates are also seeing use in some automotive interior trim.
Bill Loven, marketing manager for ITW Plexus, a leading supplier of methacrylates, says there is a move toward room-temperature-curing methacrylates to eliminate the cost of ovens for heat-curing urethane adhesives. Within the last few years, methacrylates have begun to compete with urethane adhesives such as Ashland Chemical's Pliogrip and Lord's Tyrite in markets such as transportation and marine.
However, a big plus for urethanes is their "deropability"--as it's squeezed off, it can be cut like a strip of tape. Methacrylates are messy and must be cleaned up after squeeze-off before they cure. Some suppliers are working to even the score there. ITW Plexus, for one, plans to introduce a deropable methacrylate adhesive this year.
Plastics with low surface energy are difficult to bond. Widely used examples are polypropylene, polyethylene, PTFE, silicone-coated plastics, and some grades of ABS. Modifying the surface energy of the plastic to create receptor sites to which an adhesive can bond is a way to overcome the problem. In Europe, flame treating is used for this purpose, whereas corona treatment, electrostatic treatment, or plasma treatment are favored by processors in the U.S.
"Our aim is to develop improved adhesives that eliminate the need for these pretreatments," says Bladecki. National Starch has some urethane formulations now under development that look promising, he reports.
Meanwhile, 3M Co. has introduced Scotch-Weld DP 8005, a methacrylate-based structural adhesive that is said to bond many low-surface-energy plastics to themselves and to many other plastics, as well as to steel, aluminum, concrete, glass, and wood. Surface preparation is reduced to simply wiping off dust or oil. Marketing manager Joe Stroh says DP 8005 has been shown to work well with PP, PS, and PP-based TPEs such as Santoprene. It has also performed well with some HDPE and LLDPE grades. DP-8005 reportedly has generated a lot of interest for both automotive and non-automotive uses, such as recreational vehicles, marine, and household goods.
This two-part, solvent-free product is said to provide structural bonds with 1000 psi overlap shear on low-surface-energy plastics. After full curing, it is said to have a bond-line strength often greater than that of the substrate.
3M's EPX hand-held applicator dispenses the adhesive from a DuoPak cartridge. This room-temperature-curing product has up to 3-min open time, allowing easy positioning and repositioning of multiple or large components.
Also new for bonding low-surface-energy plastics is Polybond33 from Nbond Adhesives International. This two-part epoxy reportedly bonds PE or PE to wood, metals, foams, PVC, and PS.
Company president and CEO Steve Harris says what makes this product different from other epoxy adhesives is the use of a patented process that provides both molecular and mechanical bonds to the polyolefin. To obtain a secure structural bond with this adhesive, a pretreatment of the polyolefin is required. Within the normal operating temperatures of PP and PE (-40 to 180 F), PB33 shows no loss of bond strength or change in physical properties. Testing has shown that its bond strength remains strong above the working temperature of PE and PP, failing only at temperatures high enough to incinerate the adhesive--above 350 F. The product cures in 24 hr and is packaged in cartridges, pails , and drums.
Best of two worlds
New reactive urethane hot-melt adhesives offer performance benefits formerly unobtainable with conventional thermoplastic hot melts. They also reportedly have more flexibility than conventional two-part urethanes. The latter must be processed at temperatures over 350-400 F, which can cause softening of some substrates--particularly in thin-wall plastics.
Reactive hot-melt adhesives are 100% solids, one-part systems that cure by absorbing atmospheric moisture at room temperature. They must be melted at 250 F so they can be dispensed through a gun like thermoplastic hot melts. Then they cool and cure in 24-48 hr.
The cured material reportedly has a higher bond strength than a typical hot melt. What's more, they have the rapid set-up of a thermoplastic hot melt plus the bond strength of a structural thermoset adhesive, says Bladecki of National Starch.
Reactive urethane hot melts can provide bond strengths of 300-1500 psi, depending on the substrate, vs. 100-500 psi for typical hot melts, explains Chris Verosky, applications engineer at Loctite. Major applications for the reactives are engineering plastics such as PBT and polycarbonate, but these products can also adhere to PP and PE. These new adhesives are getting a lot of play in automotive manufacturing like instrument-panel assembly and under-hood applications.
The latest reactive hot melts are faster curing and more economical, says Bladecki. Examples are National Starch's PurFect Lok line. Also, Loctite recently introduced three new reactive urethane hot melts to its Proform line. Grade 3630 has an open time of 30 sec. Grades 3631 and 3632 are both blends of polyester and polyether urethane. Grade 3631 boasts good chemical resistance and an open time of 60 sec. Grade 3632 resists water and low temperatures and has a long open time of 180 sec.
3M Co. offers a new line of moisture-cure urethanes trade-named JetWeld. They are said to achieve bond strength of 1000 psi in 10 min. The company views these adhesives as an excellent option for high-production environments. Applications include marine parts, interior vehicle trim, PVC window profiles, and appliance housings. The 12 products currently in the JetWeld line have very fast set times of 15 to 30 sec and open times of 1.5 to 15 min. Average cure time is 2 hr or less.
New Methacrylate, Urethane, and Hot-Melt Adhesives
Several recently developed methacrylates for structural bonding show improved performance and low ador. Also new is a urethane that comes in different formulations to accommodate a range of working times. There are also new polyolefin hot melts for PP and other hard-to-bond plastics.
ITW Plexus was among the first suppliers to introduce low-ador methacrylate adhesives two years ago. The company's 900 series boast 90% less ador than standard methacrylates Recently, the company launched the MA 1000 series adhesives, which feature very low shrinkage. They are said to be recommended for bonding thin-wall thermoplastics, including ABS, and PVC, as well as composites where other adhesives or assembly methods show read-through of the bond lines.
The MA 1000 series boasts low ador, as well as unmatched toughness and excellent fatigue endurance. Plexus MA 1020 has a working time of 4-6 min. and achieves 75% of ultimate strength in 15-20 min. Plexus MA 1025 has a working time of 20-25 min and achieves 75% of ultimate strength in 40-45 min.
Last month, Loctite introduced seven new adhesives to its speedbonder line of two-part, structure acrylic adhesives. Four are methyl methacrylates for bonding PVC, ABS, polycarbonate, nylon, and composities. One is a hema-methacrylate said to be a non-flammable, low-ador adhesive for general bonding. The remaining two are methacrylate/epoxy blends for bonding epoxy to glass, ABS, rigid PVC, and acrylic resins. They are acid-free and have lower ador and lower flammability than typical methacrylates, Locatite says.
Dexter Corp has developed methacrylates adhesives that can bond plastics to metals without the use of primer Hysol H4500 has an internal proprietary primer that eliminates need for a primer coat on stainless steel. Steel, and aluminum. Heat and humidity testing has shown the adhesive to retain bond strength and inhibit corrosion.
Also in development at Dexter is technology that expands the working time of methacrylates to as much as 2 hr--a development that has been driven primarily by boat builders in Florida. New Hysol H4900 gives boat builder 40-60 builders 40-60 min working times at 95-100 F vs. 15-20 min in the same conditions with competitive products.
Hysol H6000 is a new white methacrylate that boasts a significant improvement uv resistance. It has been tested for up to 20,000 hr with essentially no color change, unlike most methacrylates, which tend to yellow. White boats and outdoor signs are target applications.
New Lord 7545 urethane adhesive from Lord Corp is a two-component system formulated for easy dispensing from cartridges or automated equipment. It is said to produce exceptional bonds to composites, including SMC. The product is aimed at small-volume users (e.g., 20 gal a week). It is offered in five different formulas to accommodate working times from 4 to 65 min. It takes 30 min to 5 hr to reach handling strength. All achieve full strength in 24 hr.
Loctite has added three new polyolefin blends to its line of hot melts for general bonding or potting. Proform 3650. 3651, and 3652 are said to bond well to PP and other difficult-to-bond plastics.
New Epoxy-Based Adhesives
Master Bond, Inc. has two new epoxy adhesives:
* EP3HT Medical is said to be the first one-component epoxy that meets USP Class VI requirements. It is designed for bonding medical devices made of engineering plastics such as PBT, PEI, PEEK, and some nylons. It is resistant to repeated sterilization, has an unlimited working life at room temperature, and cures quickly at elevated temperatures. Advantages over conventional medical-grade epoxies are said to include extreme durability and excellent adhesion. It is serviceable at up to 400 F and boasts superior dimensional stability.
* EP65HT-1 is said to represent a breakthrough in epoxy adhesive technology due to its combination of ultra-fast cures and high-temperature resistance. It is said to bond to a variety of substrates including many plastics such as ABS, PS, PC, and acrylic. The product sets up in 35 min and fully cures within a few hours. It shows exceptionally high tensile shear strength that approaches 3000 psi. It is targeted for the appliance, automotive, computer, optical, electrical, and electronic industries.
Both Master Bond and Tra-Con, Inc., a National Starch & Chemical Co., recently introduced low-viscosity, optically clear, two-component epoxy adhesives. Both are said to bond well to ABS, PS, polycarbonate, and acrylic. Both are said to be electrical insulators with good chemical resistance.
* Master Bond's new EP37-3FLF can be used in medical and consumer applications where clarity is required. It cures at room temperature or within a few hours at elevated temperatures. It has a Shore D hardness of 30 and tensile strength of 4000 psi at 75F.
* Tra-Bond 2113 is being used for assembly and repair of fiber optic and other high-performance electronic and aerospace components as well as for structural laminating applications. It cures at room temperature, but cure can be accelerated to 4 hr at 149 F, providing a Shore D hardness of 88 and an ultimate Tg of 194 F.