Formulating (Meth)Acrylic Structural Adhesives for Modern Vehicle Assembly

Transcription

1 Formulating (Meth)Acrylic Structural Adhesives for Modern Vehicle Assembly Al DeCato Technical Manager TT-Automotive Product Development Henkel Corp Trout Brook Crossing Rocky Hill, CT September 12, 2006 Presented at a meeting of the Thermoset Resin Formulators Association at the Hyatt Regency Montreal in Montreal, Quebec, Canada, September 11 through 12, This paper is presented by invitation of TRFA. It is publicly distributed upon request by the TRFA to assist in the communication of information and viewpoints relevant to the thermoset industry. The paper and its contents have not been reviewed or evaluated by the TRFA and should not be construed as having been adopted or endorsed by the TRFA. Page 1 of 6

2 How much data did it take to convince GM management that they could bond a composite pickup box onto the 2001 Chevrolet Silverado? How did a design engineer at Thomas Built Bus build a case to replace rivets and screws on the body of the new Saf-T-Liner school bus with a structural adhesive? What did it take for Group Hesse, the oldest beverage trailer and truck body manufacturer in North America, to use adhesives in place of traditional fasteners and welding? There were the sensible reasons of improved aesthetics, reduced assembly time, and lower production costs. More importantly there were the practical reasons of weight reduction, distribution of stress along the length of the joint, less corrosion, reduced wind drag, reduction of secondary sealing processes, improved stiffness and less loss of stiffness over vehicle life, resistance to vibration fatigue, and better crash performance. History of Formulating One of the first patents that describe a fast curing adhesive demonstrates the change from monomer in polymer in the late 60 s to polymer in monomer in the early 70 s. This transformation from rubber cement to a room temperature curing adhesive with structural strength is the beginning of what today is being used to replace welding, rivets, and screws. The two most significant developments of this change are 1) the implementation of a redox catalyst/curing system, and 2) the compositional ratio of polymer and monomer. The structural strength is achieved by the use of a monomer, typically methyl methacrylate, that can be polymerized by a free radical mechanism. The monomer is also the solvent for dissolving the liquid rubber that provides the flexibility. The monomer is also effective in swelling the coreshell polymer filler that provides the impact resistance. The system is cured through a free radical mechanism that can be effected at room temperature. The exotherm of this curing reaction accelerates the cure as the temperature of the adhesive rises. The redox catalyst system is storage stable when the reducing agent and free radical initiator are not combined with the monomer. The adhesive composition is thus formulated in two components that are mixed together at the time of application. A significant part of the historical development of these adhesives has been the optimization of the curing system and stabilization of the two components for storage stability. Goals of Formulating Ideally the adhesive would be formulated in a one to one mixing ratio, which would be the most robust to off-ratio mixing. However in order to keep the three reactive ingredients separated, it is necessary to formulate the components to be mixed in higher ratios such as 3:1 or even 10:1. Commercial packaging items are available in 1:1, 3:1, 4:1 and 10:1 mixing ratios. A formulator may choose to use a non-reactive carrier or plasticizer as the liquid ingredient of one component. This does, however, impact the overall performance and curing of the mixed adhesive. It is also preferred to make the two components different colors to visually insure uniform mixing. Page 2 of 6

3 The major ingredient of the formulation is the monomer, which is typically a (meth)acrylic ester. Monomers such as methyl methacrylate, ethyl methacrylate, 2-ethylhexyl methacrylate, cyclohexyl methacrylate, ethyl acrylate can be used. Methyl methacrylate is the most cost effective choice. Many of the properties; odor, tensile strength, bulk modulus, hydrolytic stability, are directly related to the choice of monomer. Longer chain monomers generally increase the flexibility and toughness of the cured adhesive. Anti-sliding, the ability of the adhesive not to slide off of the applied surface, is related to the solvating power of the monomer to the substrate surface 29. Boiling of the adhesive during the curing is related to materials with low boiling point and higher exothermic heat 29. One of the most common complaints of these adhesives is the odor. Odor can be improved by the choice of monomer and can be reduced by the addition of a paraffinic wax to reduce the volatility. The wax can also extend open time by occluding atmospheric ozone. Another complaint of these adhesives is the flammability associated with the monomer. This, too, is related to the choice of monomer. One of the first steps of compounding the adhesive is to dissolve the polymeric elastomer or liquid rubber into the monomer. These ingredients provide flexibility, toughness, vibration damping, and peel strength to the adhesive. Polychloroprene, polybutadiene, chlorosulfonated polyethylene, copolymers of butadiene or isoprene with styrene, acrylonitrile, copolymers of ethylene and acrylate esters, homopolymers of epichlorohydrin, nitrile, and copolymers of epichlorohydrin and ethylene oxide have all been used effectively. The choice of elastomer is typically governed by its glass transition temperature, T g, and its solubility in the monomer. Polymers can be combined to improve both the low temperature flexibility and the cured adhesive s hot strength. Depending on their solubility, these elastomers can flexibilize the continuous phase or create discreet rubbery domains in the matrix. The addition of graft copolymers or core shell fillers will improve the impact resistance of the cured adhesive. Methyl Methacrylate/1,3-butadiene/styrene (MBS) 14, Acrylonitrile/butadiene/styrene (ABS), or styrene/butadiene/styrene (SBS) 34 swell in the monomer and also provide thixotropy to the adhesive. The core polymer has a glass transition temperature substantially below ambient temperature, while the shell polymer has a T g substantially above ambient temperature. Since the inhibitors or stabilizers are mostly solids, it is advantageous to add them early in the mixing cycle to allow them to completely dissolve. Free radical stabilizers such as 2,6-di-tbutyl-4-methylphenol (BHT) 6, hydroquinone (HQ), or methylhydroquinone (MHQ) are very effective. A metal ion scavenger such as EDTA salt is often added to the component without the metal salt catalyst to improve storage stability. These stabilizers will impact the cure speed at elevated concentrations, since they must be consumed before effective polymerization can take place. The adhesion of these systems can be improved by the addition of carboxylic acids 18 and some phosphorus compounds 11. The adhesion promoters work well with certain substrates and are chosen based on whether the application is bonding plastic, metal, wood, or primed metal surfaces. The carboxylic acids can also increase the cure speed of the system 3. Page 3 of 6

4 One of the cure system components added later in the mixing process is the free radical source. Organic peroxides, organic hydroperoxide, peresters, peracids, and azo compounds can be formulated in these systems. Many of these are relatively stable at room temperature and can be easily initiated at ambient temperatures. Thus, they can be added to the monomer with the proper stabilizer levels. To safely combine them with the activators, it is best to use a non-reactive carrier liquid such as DGEBA or an organic plasticizer. Activators are the reducing agent of the redox cure mechanism. Some examples are N,Ndimethylaniline, diethyl aniline, N,N-dimethyl-p-toluidine, aldehyde-amine reaction products, and dihydropyridine. The efficacy and concentration of these will impact the cure speed, heat of exotherm, and stability of the adhesive components. Catalysts can be incorporated to accelerate the cure at ambient temperatures. These are oxidizable heavy metal salts, such as the naphthenates, octoates, acetylacetonates, and hexoates of transition metals such as copper, cobalt, nickel, and iron. Again, the efficacy of these will impact the cure speed, heat of exotherm, and stability of the adhesive. To those experienced with formulating it should be apparent that the science and expertise of formulating these products is a balance between stability and cure activity, toughness versus strength. The number of ingredients within the adhesive and activator components of the formulation can be twenty or thirty, with each having its own primary effect and interactions with other ingredients. Nevertheless, forty years of formulation development has turned rubber cement into one of the highest performance adhesives sold today. Shear strengths of 6000 psi (41 MPa), peel strengths of 100 pli (17.5 kn/m), impact peel energy of 18 J, fracture toughness of 7000 J/m gap are all possible. These adhesives can also be formulated to be resistant to salt fog, humidity, and temperatures from 40 to 220 F. Page 4 of 6

5 Acknowledgements To Henkel Corp. for the opportunity, encouragement, and support to develop this chemistry To my family which has supported me, always To Paul Rachielles who has always had the faith that I could succeed Page 5 of 6

6 Relative US Patents 1 3,300,545 Monsanto Methacrylate monomer and thermoplastic polymer blends 2 3,333,025 E. Bader Polychloroprene adhesives 3 3,725,504 Lord Corp Faster curing with methacrylic acid 4 3,832,274 Lord Corp Fast cure and flexibility from addition of polybutadiene elastomers 5 3,890,407 DuPont 2K adhesive w/ chlorosulfonated polyethylene w/ high bond strengths 6 3,962,372 DuPont Improved shelf life of acrylic and methacrylic adhesives w/ 2,6-di-t-butyl-4- methylphenol 7 3,994,764 Pratt& Polyurethane polymer and peroxide in ester monomer Lambert 8 4,009,150 DuPont Time lapse catalysis through the use of a chelating agent 9 4,126,504 Pratt&Lam. 2K thermoplastic based adhesive w/ benzoyl peroxide with improved storage stability 10 4,182,644 DuPont Adhesives w/ acrylic monomer with organic sulfonyl chloride 11 4,223,115 Lord Environmentally resistant adhesive w/ phosphorous compounds capable or bonding untreated metal surfaces 12 4,287,106 Ciba-Geigy Vinyl adhesive resistant to oil, water and heat with very high shear strength 13 4,293,665 Lord Continuation of 4,223, ,304,709 Hooker Chem Addition of MBS for hydrolytic stability 15 4,451,615 Henkel 2K adhesive with a polyisoprene elastomer in polymerizable acrylate ester monomer 16 4,452,944 Lord RTV polymerizable adhesive that is both effective and resistant to degradation at elevated temperatures 17 4,536,546 ITW Methacrylate adhesive including chlorosulfonated polyethylene resin to improve adhesion 18 4,714,730 ITW Actylate and Methacrylate adhesives can be improved with the addition of carboxylic acids 19 4,773,957 ITW Improved adhesion from chlorosulfonated polyethylene and graft polymers 20 4,942,201 ITW Methacrylate adhesive composition with high impact strength and elongation at low temperatures 21 4,959,405 ITW Continuation of 4,714, ,112,691 ITW Methacrylate adhesive with high impact strength and elongation at 10 o F for use on plastic or metal substrates 23 5,206,288 ITW Methacrylate adhesive with same properties as 5,112, ,656,345 ITW Adhesive to join PVC, CPVC or ABS pipe without use of volatile organics 25 5,859,160 Lord Control of cure speed or a free radical catalyst with vinyl aromatic compounds 26 6,291,593 Henkel Retarding cure w/ non-protonic Lewis acids and zinc salts 27 6,433,091 Henkel 2K composition w/liquid rubber & polymeric elastomer 28 6,462,126 ITW Structural adhesive composition 29 6,512,043 ITW Longer working time w/ anilines and toluidines 30 6,602,958 IPS Adhesive combinations to bond composites w/o surf. prep. High elasticity. 31 6,730,411 ITW Improved fixture and open times 32 6,852,801 IPS Highly flexible. Divisional appl. of 6,602, ,869,497 ITW 2K w/ and w/o metal molybdate for improved green strength 34 6,894,113 Atofina Thermosets with improved impact resistance from copolymers Page 6 of 6