Acrylic Acid-Free Adhesives for Motor Assembly

Transcription

1 Acrylic Acid-Free Adhesives for Motor Assembly New Technology Makes Cold Bonding and Other Motor Assembly Applications More Reliable and Regulatory Compliant Tom Charlton Dymax Corporation Torrington, CT Introduction Cold Bonding is a popular process for the easy automation of magnet bonding and other DC motor assembly operations. However, cold bonding depends upon one of the several types of acrylic-based adhesives. Because cold bonding adhesives contain either acrylic or methacrylic acid, this method of rapid bonding is now being threatened by regulatory issues and worker safety concerns. In addition, even small amounts of uncured acrylic or methacrylic acid have been found to be causative agents in the corrosion of commutators and brushes, especially in sealed-can motors. This paper briefly reviews why Aerobic Acrylic adhesives became the industry standard for cold bonding in DC motor magnet assembly, and how new technology aerobic acrylic adhesives overcome the regulatory concerns. New advances in both adhesives and dispensing equipment are also highlighted. New patented technology is now available which increases worker safety and minimizes: Corrosion issues, especially in closed-can motors Methacrylic acid HazMat issues Acrylic Acid HazMat issues DC Motor Assembly Adhesive Dispensed on Magnet OD Magnet-to-Can Assembly

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3 History Cold Bonding found general acceptance for automotive DC motor bonding in the 1980 s. Dymax s Aerobic Acrylic Adhesives solved flammability and other safety concerns, which were significant issues for second-generation acrylics. Alternative acrylic adhesive technologies offered at that time included structural anaerobic adhesives and second-generation acrylics. Types Of Acrylic Adhesives Available For Magnet Bonding For DC Motors Critical Adhesive Characteristics Aerobic Acrylic Anaerobic 2 nd Generation Acrylic Fixture Speed seconds 1-5 minutes seconds High Flash Point Yes No No Gap Cure 5-20 mils 3-5 mils 5-20 mils Surface Preparation Some Scrupulous Cleaning Some Of these adhesive choices for magnet-to-can assembly, the Aerobic Acrylic s superior fixture rate, combined with better gap cure and low flammability (200 C closed cup), won the day and they became widely utilized for magnet-to-can bonding in DC motor assembly. This automated assembly came to be called cold bonding by the industry. More types of cold bonding adhesives became available in the 1990s, as well as better heat curing epoxy adhesives. Each of these types of magnet attachment has their respective advantages and disadvantages. Heat-Cured Epoxy Assembly Method Steps with nine workers: 1. One or two workers bring parts to conveyor belt 2. Two workers apply epoxy adhesive 3. Four workers assemble magnets into housing, attach a stainless steel fixturing clamp, and them place the clamped can onto the conveyor belt 4. Parts are heated to 300 F for 30 minutes to cure the epoxy 5. Two workers remove hot parts to cool down stack. Return fixtures to beginning of line -3-

4 6. After 24 hour cool down, parts go into the main assembly process Cold Bonding Increases Productivity Cold Bonding Method 1. Worker loads magnets to automatic feeder/adhesive dispenser and also brings motor cans to second worker station 2. Second worker applies activator to can housing with a semi-automatic dispenser 3. Second worker than places can over magnets assembly (Note: This step can be automated. 4. Turntable cycles, engaging fixture pressure mechanism for about 30 seconds. 5. Bonded parts are automatically off-loaded to continue in production Magnet Bonding Adhesives FeaturesOther Considerations Aerobic Acrylics* Rapid in line curing 100% in line inspection High impact strength Rejects can be repaired Lower capital cost to automate Provide highest automation speed Lower labor content Gap cure to 20 mils* 2 part application process Precision application Epoxy High tensile strength unlimited gap cure One part Slow cure High temperature cures are frequently off line Require cleaner surfaces Can be brittle High capital cost to automate Limited pot life *An attempt was made to overcome the gap curing limitations with two part mixing acrylics or bead on bead acrylics. While useful on certain plastics and automotive SMC, they have not found acceptance for durable magnet bonding. -4-

5 Corrosion and Acrylic Adhesives All types of acrylic adhesives originally contained substantial quantities of either acrylic acid and/or methacrylic acid, which can be written collectively as (meth)acrylic acid. Recently however, patents have been issued for volatile acid free Advanced Acrylic Adhesives. (A) (B) How Acrylic Adhesives Lead To Internal Corrosion When a bead of activator-cured adhesive is put onto a surface and the mating part is put in place, it is typical for adhesive at the edge of the joint to be exposed to oxygen. That edge, and any squeeze-out or fillet, can be the source of vaporization, possibly causing odor and health concerns in an open-architecture product, or corrosion in a closed cavity. Aerobic acrylic adhesives cure hard and dry between surfaces, leaving a film at the joint edge that is exposed to air. Evolving from that film will be acrylic acid. Removing (meth)acrylic from the formula eliminates both the potential concerns of workers and corrosion problems on the finished part. Type (Meth)acrylic Acid Content Acrylic Acid Free Aerobic Adhesives Shown at left are commutators exposed to uncured adhesive in a closed environment for 500 hours at 200ºF. The clear, bright copper of the bottom commutator stands in sharp contract to the one at top blackened with corrosion. (A) The commutator at top was exposed to a popular anaerobic adhesive. (B) The commutator below was exposed to the new (meth) acrylic acid free advanced aerobic acrylic adhesive. (Meth)acrylic Acid None Anaerobic Adhesives 4-6% Second Generation Acrylics 5-30% Toughened Acrylics 5-30% Methacrylic Adhesives 5-30% Aerobic Adhesives 2-5% -5-

6 Advanced Aerobic Adhesives Overcome Both And Corrosion and HazMat Issues Retaining their original toughness and very fast bonding capability, the new grades of Aerobic Acrylic adhesives are as strong and fast as their predecessors. Regulatory Issues With (Meth)acrylic Acid No volatile acids Bond strengths up to 3, 500 psi Fixtures in seconds Thermal Ranges from -55 to 150 C Non-Flammable No ozone depleting chemicals Acrylic acid is a colorless, corrosive liquid, some 1.1 billions being produced in the U.S., mainly as an intermediate for the manufacture of acrylic esters and resins. Human health effects associated with breathing or otherwise consuming small amounts of acrylic acid over long periods of time are not known. Laboratory studies report that exposure to acrylic acid vapor damages the lining of the nose of animals. The acrylic acid industry has recently completed several animal studies in response to an EPA request for testing. Acrylic acid causes reduced birth weight in pups of animals consuming acrylic acid in drinking water. These studies report no other adverse effects on the reproductive system or on the development of the fetus of animals. Industry HazMat Requirements Both acrylic acid and (meth)acrylic acid identified as potential environmental hazards. It is also found on the list of regulated chemicals issued by most automotive manufacturers and other industries. Some companies have begun requiring that their vendors to certify that no products containing acrylic acid have been used to manufacture their components. Other companies, simply concerned over the safety of the workplace, have instituted similar controls. Many of these requirements have been extended even to international subsidiaries and suppliers. The Ford Motor Company, for example, cites regulates and restricted substances as any chemical on any other standard. Other automobile companies have similar requirements. The DaimlerChrysler Corporation lists acrylic acid as a regulated substance. The Standard is partially reproduced below: DaimlerChrysler Corporation Standard CS Appendix I is a partial reproduction of the Chrysler Standard. DaimlerChrysler Corporation Characteristics Standard Vehicle Engineering -6-

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8 1.2 Scope of Application This standard applies specifically to Chrysler Corporation and its suppliers and subsuppliers for materials and parts for vehicles designed or engineered for North American production or export. ENVIRONMENAL, HEALTH, AND OCCUPATIONAL SAFETY REQUIREMENTS FOR REGULATED There may, in SUBSTANCES addition, be country OR PROCESSES or region specific AND requirements PRODUCT RECYCLING for dismantling and/or REPORTING handling of end-of-life REQUIREMENTS vehicles. In these cases, specific accountability is as follows: 1.0 Specific GENERAL area or country standards will be addressed by Chrysler International Operations. 1.1 Purpose of the Standard Homologation issues relative to obtaining certificates for the sales of vehicles are This addressed standard by the provides Corporation vehicle Vehicle environmental Homologation and occupational staff. This safety group requirements will provide as found separate in Chrysler s standards Material, to meet whole Process vehicle and Performance type and/or part Standards. approval. It establishes, consistent and coordinated product recyclability guidelines and targets to support: Chrysler Parts and Service Division will develop a separate system to track recyclability, recycled content and regulated Employees substances Health required by this standard, while utilizing this standard for all replacement parts. Customers Health Suppliers Employee Health 1.3 Vehicle Paints The Environment For Vehicle production body paints materials applied and by processes the car assembly plants, as well as those utilized on supplier painted parts shall conform to the requirements established by Paint and Energy This Management. standard was updated in August, 1997 to include policy direction or objectives relative to ISO 1402X and 1404X draft requirements addressing Life Cycle Assessment, as well as EU directives dated April, CS-9003, 1997 Change relative E, Page to vehicle 1 recyclability Regulated Substances Chrysler will also screen the regulated substances, listed in Table 6, before allowing them to be used in new or carryover materials, processes or parts. These substances are targeted for reduction or elimination, and if they are present Life Cycle Management analysis may be requested to evaluate the selected material and possible alternatives. Table 6 Regulated Substances Substance CAS Number 1,1 Dichloroethylene ,2 Dichloroethane ,4 Dioxane Acetylaminofluorene Nitropropane ,3 Dichlorobenzidine ,3 Dichlorobenzidine Salts Multiple 4-Dimethylaminpazobenzene ,4 Methylene Dianiline (MDA) ,4 Methylene Bis-2-Chloroaniline Acetaldehyde Acetone Acrylic Acid Alpha-Naphthylamine Aluminum (Fume or Dust) (1) Antimony Compounds Multiple Arsenic

9 Federal & State Regulations The EPA defines Acrylic Acid as a Hazardous Air Pollutant. (See Section 112 of the Clear Air Act for a list of regulated substances.) Most states have begun requiring special licenses and fees for facilities contemplating the use of products containing acrylic acid. State requirements are available by contacting the relevant Department of Environmental Protection. Examples of state information regarding acrylic acid: Clean Air Assistance Program Michigan Department of Environmental Quality P.O. Box Lansing, MI (800) Clean Air Act Section 112 Hazardous Air Pollutants (HAPs) See Table 3 Illinois EPA Division of Air Pollution Control Box Springfield, IL Clean Air Act Section112(b)1 Table Curing Chemistry Of The Acrylic Adhesives And Corrosion Formation Free-radical polymerization is common to all three types of acrylic adhesives anaerobic structural adhesives, SGAs and aerobic acrylic adhesives. Figure 1 shows that several mechanisms can terminate the growing polymer chain. Two radicals can join to form a molecular species or there can be an abstraction of hydrogen or a free-radical transfer. FIGURE 1 FREE RADICAL POLYMERIZATION > C = C < + R. R C C. Initiation Vinyl Compound Free Radical R - C - C + > C = C R - C - C - C - C. etc. Chain Extension R - (C - C) - C = C < Termination or n -9-

10 R - (C - C)n - C - C - R All acrylics have their surface cures inhibited by oxygen. This inhibition responsible for the familiar tacky or even the wet surface frequently observed by users of acrylic adhesives. Laboratory tests comparing the lack of corrosion causing capability of the new adhesives compared to the traditional types were conducted as follows with the results visible in Figure 2 below. FIGURE 2 Cold Bonding Limitations A difficulty when using the cold bonding adhesive types was (1) that some manufacturers were unable or unwilling to control gap tolerances, and (2) that imprecise application of adhesive and activator, would lead to poor quality bonds. These road blocks have been largely removed because the strong impetus to achieve the assembly benefits offered by this technology has lead to the development of dispensing and assembly systems that provide the process and product reliability and quality required. Solvent Free Activators Posed A Dispensing Challenge The implementation of the U.S. Clean Air Act of 1993 presented a challenge to the dedicated users of the cold bonding systems. Though solvent-free activators had been shown to provide as good or superior bonds, as the older solvent born activators, dispensing equipment in use at the time was less effective at applying the correct amount, in the correct area. In an effort to assure presence of the activator, instances of over activation occurred, which lead to poor bond strength. (Over activation was almost impossible to occur with the solvent born activators as ¾ of the amount deposited evaporated, leaving the desired thin film.) Since that time, advances in dispensing and the concomitant monitoring and feedback systems, has met the activator dispensing challenge. -10-

11 New Systems Offer Consistent Dispensing/Easier Automation For Cold Bonding For many years, the application of activators has been by inefficient rotary devices which fling uncontrolled amounts of activator onto the flux ring. This often caused an over activation condition causing a dramatically weakened bond line. New automated systems, called DADS for adhesive and activator applications have been developed so that questions of consistency of application are eliminated. Systems are available for bonding various sizes and configurations of motors. These range from simpler models for lower production situations to more automated models for high volume production. The pictures below show the two basic ways to dispense adhesive and activator: 1. Separate Application a) Activator is sprayed inside on the motor can b) Adhesive is applied to the magnets of fixture c) Can is placed over magnet fixture d) Timing table actuated, for 30 seconds e) Off load Activator Dispensing Applying Adhesive 2. Spray-On-Bead a) Adhesive applied to flat or curved surface b) Activator Sprayed on top of adhesive c) Parts mated within 10 seconds. d) Fixture for 30 seconds e) Offload Automated Assembly Adhesive and Activator Dispensing -11-

12 *DADS is a trademark of the Dymax Corporation These systems are PLC controlled and can be monitored by an array of optional sensors to optimize the application of activator and adhesive on the bond area of the flux ring and magnet. The systems utilize highly efficient rotary air atomized needle valves. The systems include all of the material supply equipment, fluid hoses and fittings to complete installation. They are available as off line dispensing stations, or in modular configurations for integration into automated production lines. Activator dispensers are designed to deposit a minimum amount of activator onto a substrate or the inside of a flux ring in the form of a controlled mist. They may be programmed to apply activator to the entire flux ring or to specific areas. Adhesive dispensers are designed to deposit a repeatable amount of adhesive onto a magnet in a bead type pattern to optimize the spreading and mixing action of the adhesive and activator when the parts are fixtured together, while minimizing squeeze out of excess adhesive. Pressure sensors and flow meters insure that the appropriate amount of activator is applied to the part monitor critical fluid pressure to insure that the appropriate amount of adhesive is applied to the part. Other Motor Assembly Applications Using Multi-Cure Grades Of Aerobic Acrylic Adhesives Multi-Cure Capability Multi-Cure adhesives are used through out the motor assembly industry. Multi-Cure combines the benefits of very fast UV curing (curing with Ultraviolet light) with activator (and/or heat) curing. The ability to achieve steel-bonding strengths to 3,500 psi whether cured by activator, UV light, or thermally, yields enhanced design options and process for improving productivity. commutator to shaft flange bonding sealing wire tacking magnet bonding bearing mount -12-

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14 Applications include: Armature wire strain relief Coil termination Sealing seam Flange to can attachment Commutator to shaft Bearing mount Curing Mechanisms UV cure only UV cure only UV cure only UV + activator cure Activator cure Activator cure Multiple Curing Options Eliminate Corrosion And Offer Additional Opportunities Adhesives can squeeze out from between metal or metal/ferrite surfaces. The adhesive between the two surfaces cures by action of the activator and the squeeze out may be cured by exposure to UV light. Where this happens and a fillet is UV cured, there is no outgassing or vapors to cause corrosion. Under high intensity light, curing occurs so rapidly, that the oxygen inhibition (discussed above) is overcome and no uncured residue is left. This use of Multi-Cure aerobic acrylic adhesives has allowed the UV cured fillet to serve as a replacement of a temporary fixture. This cured fillet serves as a tough plastic fixture holding parts in place for the 30 to 60 seconds necessary for activator to fixture down inside the joint where the light can t reach. This has worked successfully in can to flange attachment applications. The second use for Multi-Cure formulations can occur when most of the adhesive used in a shallow potting application can be cured but a small amount may not see the light and remain uncured. New two part UV curing resins have recently been developed that are ideal for shallow potting where some of the resin may run under wires and not see the light to cure. Some Uses of Multi-Cure Resins Tab sealing Wire tacking Wire strain relief, unitizing, coil terminating -14-

15 Potting * Multi-Cure is a registered trademark of the Dymax Corporation -15-

16 Summary New technology in the dispensing and chemistry of aerobic acrylic adhesives now allows consistent, reliable bonding for DC motor assembly. Acrylic acid free technology provides regulatory compliant, high performance adhesives to enhance productivity for a wide range of motor assembly applications. DYMAX CORPORATION * 51 GREENWOODS ROAD * TORRINGTON, CT * TEL: * FAX: DYMAX EUROPE GMBH * TRAKEHNER STRASSE 3 * D FRANKFURT a.m. GERMANY * TEL: * FAX: Dymax, Light Weld, Light-Welder, Ultra Light-Weld and Multi-Cure are registered trademarks of Dymax Corporation DYMAX USA info@dymaxcorp.com * Internet: * DYMAX EUROPE dymaxinfo@dymax.de