Adhesives, 1969 C. CLEMENT ANDERSON
TABLE I .
CRITICAL SURFACE TENSIONS O F REACTIVE SILANES Polyriloxane monomer
C, dynes/cm
33.5 38.5 40.5 35 41 25 28
n-(Trimethoxysilylpropyl)ethylenediamine y-Glycidoxypropyltrimethoxysilane y-Chloropropyltrimethoxysilane y-Aminopropyltricthoxysilane yMercaptopropyltrimethoxysilane Vinyltrimethoxysilane y-Methacryloxypropyltrimethoxysilane
TABLE II. STRENGTH O F BONDS FORMED BY ULTRASONIC BONDING Weld
tima, Adhesive
Adherends
3 M , 1357 Elmer Glue-All Versalon 1175 Phenoxy PRDA 8080 Polyvinyl butyral Shell Epon 927
Plywood/plywood Plywood/plywood Phenolic/phenolic Plywood/plywood Aluminum/glass Aluminum/aluminum
sec
4
6 8 4 12 8
Shear strength,
psi
480 1970 790 2210 840 1460
the last two reviews (2, 3 ) , the subject matter has been almost Inentirely ‘ devoted to the chemistry and performance of new adhesives disclosed in patents and publications during that particular year. This review will not only mention the latest developments in adhesive materials, but will also briefly mention new bonding processes and several papers which further elucidate the important aspects of adhesion, such as wetting, contact angle, and other interfacial phenomena. The past year has been marked by the appearance of a quarterly journal which is wholly devoted to adhesion and adhesives. The Journal of Adhesion used much of its first edition for a discussion of interfacial contact and bonding in autohesion. Several books on adhesion also appeared, the foremost of which, I believe, is Volume 2 of the “Treatise on Adhesion and Adhesives,” a fine addition to Volume 1. An excellent review of “Elastomers and Their Adhesion” (72) covered the last eight years of work in the investigations of interfacial phenomena, bulk phenomena, and the interpretation of adhesion measurements. An empirical relationship has been established between the glass temperature ( T g ) and the critical surface tension (yc) of a polymer (26) * ycO.86 =
(0.03RTg
- 1.5)(n@2/Vm0.71)
where n = degree of freedom, Vm = molar volume, and 4, = ratio between reversible work of adhesion and geometrical mean of the work of cohesion. I n the models used, the scheme was effective for predicting critical surface tensions of polymers; however, it was not accurate where hydrogen bonding was a factor. Through the kinetics of wetting of surfaces by polymer melts, namely polyethylene and an ethylene-vinyl acetate copolymer, it was shown that the cosine of the contact angle attains its final value according to apparent second-order kinetics (24). The wettabilities and conformations of reactive polysiloxanes were studied, and their critical surface tensions were determined (Table I ) (27). The role of the interface in glass-epoxy composites was investigated as a chemical function of the coupling agent (22). Fracture occurred a t the glass-resin interface when the adhesion was poor, but deep in the resin matrix when good adhesion existed. The good adhesion was observed when a coupling agent, such as yglycidoxypropyltrimethoxysilane, capable of coreacting with the epoxy resin, was used, and a covalent bond formed. Bonding Processes
C. Clement Anderson is Research Associate in the Coatings and Resins Division of PPG Industries, Research and Development Center, Springdale, Pa. 15744. This is the third adhesives review that Dr. Anderson has prepared. AUTHOR
48
INDUSTRIAL A N D ENGINEERING C H E M I S T R Y
Several new methods for bonding have recently gained acceptance, particularly in the bonding of thermoplastics and in bonding materials which are not easily joined by conventional processes. The technique is known as ultrasonic bonding. The ultrasonic process is a means of heating the adhesive at the interface between the two adherends. The ultrasonic method permits heat activation of the adhesive. The technique has been used successfully in thermoplastic welding where the ultrasonic vibrations produce local heat so that a bond can be formed on the outside layers of the plastic pieces. This method produces no depression marks at
Progress in the development of synthetic polymer adhesives is reviewed. Other aspects of adhesion-new
bonding methods and interfacial phenomena-are
also covered
the bond site (7). For this bonding process, in which adhesives are employed, a n ultrasonic horn, which produces about 20,000 Hz, is used. Many commercial adhesives are appropriate for the ultrasonic process. They may be hot melt, thermosetting, or thermoplastic adhesives and may be applied from solution or placed between the adherends as a film. I n all cases, the ultrasonic vibrations cause heat activation of the adhesive so that a bond is formed in a matter of seconds (20). Table I1 shows the shear strengths obtained and the weld time required. Because ultrasonic vibrations travel from the horn into the plastic and to the interface, the adhesive should melt a t a temperature slightly lower than the adherend. The harder or higher melting adherend is placed adjacent to the horn with the softer adherend on the other side (79). This new method provides bonds with adequate strength for many applications a t great speed and efficiency. Another new bonding method has been developed which might find great utilization in the bonding of metal to glass. This process is anodic bonding ( 3 7 ) and involves heating the two components up to 200' to 4 O O O C below the softening point of the glass to produce a more intimate contact of the two surfaces and to increase the conductivity of the glass. At this point a direct current potential across the interface is applied resulting in a strong bond. A bond with higher tensile strength than either of the adherends is produced (70). The inventor believes an oxide is formed at the interface as a reaction product resulting from the electric current. However, because of the minute thickness of this interfacial layer, its chemical and physical properties are undetermined, except that it is distinctly different in electrical properties from the two materials being bonded. The process is particularly suitable for electronic components such as vacuum tubes and integrated circuits. Pressure-Sensitive Adhesives
Pressure-sensitive adhesives are characterized by their stickiness or tackiness. These adhesives form a bond to many substrates by the use of only a light pressure, but they exhibit low strength and low service temperatures. Despite these deficiencies, pressuresensitive adhesives are being called upon to bond in areas where heretofore only contact adhesives had been used, and much research has been directed toward this end. Pressure-sensitive adhesives may be prepared from any number of organic materials, such as styrene-butadiene rubber, natural rubber, polyesters, and polyacrylates. I n this paper we investigate the means employed over the past two or three years to broaden the scope of pressuresensitive adhesives. The main component of a pressure-sensitive adhesive is often an acrylate with an alkyl chain from four to eight carbon atoms long. Copolymers of 2-ethylhexyl acrylate, where this monomer consists of at least 50% of the weight of the polymer, are tacky a t room temperature and exhibit good adhesion. I n many cases, the 2-ethylhexyl acrylate is copolymerized with a harder monomer to attain the desired specific properties of hardness, tack, and adVOL. 6 1
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AUGUST 1969
49
Figure 7.
Figure 21
hesion required for a particular application (78, 39). Various methods of crosslinking have appeared in the patent literature which are attempts to increase the strength and service temperature of acrylic pressure-sensitive adhesives. Many of these methods are chemical reactions used in other fields. For example, latently crosslinkable pressure sensitive adhesives were prepared by polymerizing the usual acrylic esters, ethyl, butyl, and 2-ethylhexyl acrylate with a maleamic acid monomer formed by the reaction of maleic anhydride and a mixture of C I I - C ~branched ~ alkyl amines and a difunctional unsaturated monomer (34). Table I11 shows the effect of curing at 350'F in the creep resistance of the adhesive.
TABLE 1 1 1 . Adhesion
No cure
53 Cure 81 10 min at 350°F.
EFFECT OF CUREa ON CREEP RESISTANCE Tack
Creep
844
1/2 in./1.5 min 1/8 in./80 hr
820
Q
This same improvement in creep resistance can be accomplished a t room temperature by copolymerizing the acrylic esters with glycidyl methacrylate and an unsaturated acid (23). The curing characteristics of this type of polymer can be improved by the incorporation of titanium-containing amine compounds such as triethanolamine titanate (7). Often the crosslinkable polymers, such as those mentioned above, are formulated with polyvinyl ethers, phenolic resins, and epoxy resins to optimize the combination of tack, heat resistance, and solvent resistance (25). Several acrylic copolymers, with good resistance to ultraviolet radiation, can be applied directly to polyvinylchloride without the need of a primer. They consist of the usual acrylic monomers copolymerized with fumaric and itaconate half esters. Such monomers supply both the polarity of the carboxycylic acid function and the long chain alkyd group (33). Pressure-sensitive adhesives containing these half esters often have a combination of adhesive and cohesive strength unattainable by the use of acrylic or methacrylic acid. Vinyl acetate is often copolymerized with acrylic monomers to reduce the raw material cost and to produce a harder adhesive mass; occasionally fumaric and maleic diesters are also copolymerized with acrylates and vinyl acetate to achieve a particular balance of properties ( 1 ) . Maleic anhydride has also been copolymerized with octyl acrylate and vinyl acetate to improve the strength of the adhesive (8). Most of the adhesives mentioned claim to have a good combination of tack, adhesion, cohesion, and heat resistance. However, an adhesive that performs well in one application does not necessarily work well in another application. Drying cycles, viscosity, service temperatures, coating weight, and economics must all be considered in the selection of the adhesive. 50
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
Rubber-Based Adhesives
A large number of adhesives contain elastomeric components such as styrene-butadicnc rubber, an acrylonitrile-butadiene rubber or neoprene, resinous components, and a tackifier. These types of adhesives have been used for the past 25 years and improvements have been continually made. One of the more significant contributions has been the development of block copolymers (Figure 1 ) of styrene and butadiene and of styrene and isoprene. These block copolymers are particularly suited for pressure-sensitive applications ( 4 ) and as contact adhesives (73), where they exhibit excellent tack properties. These block copolymers have also been hydrogenated for increased stability. The block copolymers have the unique feature of attaining stressstrain properties of an elastomer without being subjected to a cure or vulcanization (Figure 1 ). Along with the new type styrene-butadiene rubbers, new tackifier systems have emerged for styrene-butadiene rubber. 01Pinene polymers have been developed to be formulated with SBR. They produce systems competitive with natural rubber adhesives with regard to tack, peel, and cohesive strength (74). The CYpinene polymers offer a high degree of tack without the usual loss of cohesive strength. The a-pinene tackifier was compared with the pentaerythritol ester of hydrogenated rosin and the glycerol ester of hydrogenated rosin and was shown to be superior in quick stick and peel adhesion when formulated with SBR (Figure 2). Styrene-butadiene rubber has also been formulated with an ethylene, propylene, and a 1,4-hexadiene terpolymer to produce an adhesive which has an exceptional cohesive tack before and after curing (38). This adhesive is particularly useful in bonding rubbers. Carboxylaied Adhesives
There has been increasing interest in carboxylated polymers for use in adhesives. Styrene-butadiene rubber, neoprene, ethylenevinyl acetate copolymers, polyethylene, and acrylic polymers have all been prepared with carboxylic acid groups interspersed along the polymer backbone. These acid groups often remain as the free acid or can be reacted with metal ions to alter the properties of the polymer. I n the case of ethylene methacrylic acid copolymers, polymers with low acid content exhibit an increase in melt viscosity and ultimate tensile strength while the degree of crystallinity is reduced (32). The interchain hydrogen bonding (Figure 3) accounts for the change in properties of ethylene-methacrylic acid copolymers as shown in Table IV. The hydrogen bonding equilibrium persists in the molten state and, even a t 21OoC, there are 10 to 12% of the carboxyl groups associated by hydrogen bonds (5). Recent investigation elucidates the structure of ethylene methacrylic acid copolymers and their salts (30). The investigators used branched polyethylene containing 4.1 mol 70methacrylic acid (Figure 4), and ionized the carboxyl function with lithium, sodium, and calcium. They determined that the acid copolymer
Figure 3.
Figure 4.
TABLE IV. CHANGE I N PROPERTIESOF ETHYLENEMETHACRYLIC ACID COPOLYMERS W I T H ACID CONTENT (32) % Acid, molal
Yield point, psi
Ultimate tenrile strength,p s i
950 1440 5500
2560 5000 5000
1.4 5.9 13.0
TABLE V.
PROPERTIES OF ETHYLENE ACRYLIC ACID COPOLYMER SALTS ( 6 )
Conlrol, 74.870A A
Sodium salt Lithium salt Calcium salt Magnesium salt Sodium salt Potassium salt Lithium salt Calcium salt Magnesium salt
%
Tensile strength (Ultimate),pst
Elongation
0 12 12 11 12 66 63 67.5 63 5 64
2150 3150 3150 2850 2400 4800 5000 4600 4500 4500
470 420 410 465 400 280 390 250 200 220
Ionized
%
TABLEVI. EFFECT OF IONICALLY BOUND SODIUM I N PRESSURE-SENSITIVE ADH ESIVES Bound Nu,
%
Tack, g/cm 2
Peel g/i;.
C?ecpl zn.
0 0.34 0.50 0.59 0.69
607 247 142 92 0
87 46 45 25 25
0.2 4.2 7.5 68 64
Wt
existed as a n amorphous phase crosslinked by hydrogen bonded dimers as in Figure 3 and a crystalline polyethylene phase. The salts exist as a three-phase system: a crystalline polyethylene phase, an amorphous polyethylene phase, and a n ionic domain. The effect of the metallic ion was examined in a polyethylene acrylic acid copolymer (Table V). At the same level of neutralization there appears to be little difference in the ultimate tensile strength and % elongation except where divalent ions are used. The environmental stability of ethylene-acrylic acid adhesive copolymers has been investigated with respect to bonds made to copper and aluminum (40). In high humidity an ethylene acrylic acid adhesive bonded to copper develops a blue-green color and a carboxylate ion adsorption a t 1600 and 1400 cm-1. The bond strength a t this point has deteriorated, and the polymer film can easily be removed. This was not true of bonds made to aluminum. The mechanism of failure hypothesized by the investigators is that water vapor and oxygen permeate the polymer film and react with the copper or copper oxide layer to solubilize it. The rate of oxidation of the copper is increased by the acid groups on the copolymer. The acid functions form ionic complexes with the cupric ion a t the interphase; a weak boundary layer develops. The acid groups also increase the susceptibility of the polymer to oxidative degradation. I n this respect, methacrylic acid copolymers are more stable than the acrylic acid copolymers. A study has also been made on the effect of ionic bonds of pressure-sensitive adhesives on their tack, peel, and creep properties (35). The investigators prepared pressure-sensitive adhesives containing carboxyl groups which were then reacted with metal ions. The amount of bonding was followed by the increase in infrared absorption a t 6 . 2 8 ~ . The metal ion used was sodium and it was introduced into the system by the addition of sodium hydroxide. The effect of the metallic ion in the adhesive is shown in Table VI. An increase in the bound sodium decreases the tack and peel values of the adhesive and increases the creep resistance. This type of behavior is not unexpected and the forming of ionic bonds can be a useful tool in increasing the cohesive strength of a pressuresensitive adhesive to a point where one has a good combination of tack, peel, and creep. Chloroprene-methacrylic acid copolymers have improved solution stability when formulated into a n adhesive cement by the addition of small amounts of water (16). When the chloroprene methacrylic acid copolymer is mixed with heat reactive phenolic resins and magnesium oxide, the formulation is susceptible to gelation; however, with the addition of water, a maximum viscosity stability can be realized. Polyurethanes
Adhesives which contain isocyanates are well known. Like many epoxy adhesives, they are quite often two-package systems which cure upon the reaction of the isocyanate with a reactive species such as an alcohol or an amine. Several new “twists” have been applied to the urethane technology to develop new materials for specific bonding purposes. Polyurethanes, prepared from polytetramethylene ether glycol, 1,4-butanediol, and piperazine have been blended with oilsoluble phenolic resins in ratios from 4 to 1 to 1 to 4 and have formed products. A 1-to-1 ratio of polyurethane to a terpene modified phenolic resin produced the peel values shown in Table VI1 and the shear values in Table VI11 (37). Recent work has developed one-package urethane adhesives. A binder for nonwoven fabrics (36) utilizes a n acrylic emulsion polymer composed of ethyl acrylate, acrylic acid, and 2-hydroxyethyl methacrylate crosslinked with a phenolic blocked isocyanate derived from trimethylol propane and toluene diisocyanate. The binder withstands dry cleaning and has a long pot life. Acrylic monomers, such as 2-hydroxyethyl acrylate and t-butylaminoethyl methacrylate, have been reacted with diisocyanates, s ch as toluene diisocyanates and 4,4’-diphenylenemethanediisoc&ate, VOL. 6 1
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51
TABLE V I I . PEEL STRENGTH VALUES OF POLY U RETHAN E-P H ENOL I C AD H ES I VES Bond
Peel strength, lb/in.
Canvas/plasticized PVC Canvas/aluminum Canvas/steel Canvas/wood
68 25
35 50
TABLE V I I I . SHEAR STRENGTH VALUES 0 F POLY U RETHAN E-PH ENOL I C AD H ES IVES Shear strength, psi
Bond
Steel/steel Aluminum/aluminum
880 822
to form various compounds (Figure 5 ) (77). These have been used in anaerobic adhesive formulations to obtain improvements in the cured adhesive, particularly the reduction of brittleness. By varying the diisocyanate, a wide range of properties may be obtained. Curable one-package polyurethane systems have also been prepared by incorporating an alkoxy methyl group on to the polymer chain (75). By the reaction of an alkoxymethyl isocyanate with a diamine or diol, a compound is formed which readily reacts only a t one end, and a stable urethane polymer can be prepared by the standard procedures. The resulting solution has its pH adjusted to 3 to 5 and in this condition, a shelf life of over a year can be realized. O n drying, a solvent-resistant material is formed. Tensile strengths of 1700 psi have been recorded for the cured polyurethane. The capsule technique has also been employed to prepare onepackage urethane adhesives (27). Diamines have been encapsulated by the reaction of formaldehyde on the surfaces of small particles of each diamine in a nonsolvent. The encapsulated diamines, such as o-tolidine and 2,4,5,6-tetrachloro-m-xylenea,a’diamine, are mixed with isocyanate prepolymers, and, on the application of mild heat, a cured adhesive is produced. Lap shear strengths of 2000 psi have been reported. Finally, the incorporation of epoxy silanes into two-package urethane adhesives which are then cured by diamines, significantly increases the water resistance of the adhesive (28). r-Glycidoxypropyltrimethoxysilane in a concentration of 0.5 to 5y0 was particularly useful in this respect. New Methods of Analysis
Lately, several new methods of analysis have been employed in adhesion studies. T o determine whether a bond failure was cohesive or adhesive, evaporative rate analysis has been employed
(47). In this technique, a measured quantity of radioactive liquid is deposited on the surface of the broken bond. If cohesive failure has occurred, the organic residue on the substrate retains the liquid resulting in a slower rate of evaporation. If adhesive failure has occurred, the evaporation from the metal is more rapid. In such a manner, the extent of cohesive failure can be investigated. The scanning electron microscope has also been utilized to study the nature of surfaces in adhesion investigations ( 7 7 ) . Due to the depth of field and ease of sample preparation, the instrument is very useful in the examination of bonding surfaces both before bonding and after a bond has failed. Teflon, epoxy, and polyurethane polymers have been studied as well as glass fiber-reinforced structural composites. A new method of determining the tack of pressure-sensitive adhesive tapes has been developed known as the rotating drum technique (9). I t is particularly useful for the determination of tack of pressure-sensitive tapes. The method involves the firm placement of the tape on an aluminum drum. Then a freely rotating light aluminum alloy wheel is placed in contact with the adhesive. This wheel is counterbalanced by a load normal to the drum surface. The drum is rotated a t a known angular velocity, and the force required to restrain the wheel axis from movement is measured and called the tack. The developers of the method state that the major attributes of the rotating drum tack tester is its low cost and the large area of adhesive which may rapidly be investigated. The tack value is taken from a recorder only during the first rotation of the wheel since contamination of the surface is quite possible. REFERENCES (1) Alexander, R . R., and Vigil, A. J . , (to W. R . Grace Co.), U. S. Patent 3,275,589
(Scpt 27, 1966). (2) Anderson, C. C., IND.END.CHEM.,59 (a), 91-6 (1967). (3) Anderson, C. C., ibid., 60 (a), 80-7 (1968). (4) Baily, J. T., J. Elastoplastics, 1, 2-15 (1769). (5) Blyer, L. L., Jr., and Haas, T. \V.! ACS Preprint, Div. Polymer Chem., 10, 72-9 (1969). (6) Bonotto, S., and Bonner, E. F., ibid., 9, ( I ) , 537-46 (1968). (.7.) British Plastics,. 42., 88-93 (1969). (8) Brooks, B. A. and Jubilee, B. D., (to National Starch & Chemical Corp.), U. S. Patent 3.;71.071 (Feb 27. 1968). (9) Bull, R . F., Martin, C. N., and Vale, R . L., Adhesives Age, 11, No. 5, 20-4 (1968). (IO) Cham. Eng. News, 46 (361, 14 (1968). (11) Chem. Ens. N e w , ibid., (41) pp 60-1. (12) Crocker, G. J., Rubber Reviews, 42 (l), 30-70 (1969). (13) Davis, F. C., Luther, W. B., and Martinson: D . L. (to Shell Oil Corp.), U. S. Patent 3,427,269 (Feb 11, 1969). (14) DeBeradinis, M . , Rubber World, 159, 45-50 (1969). (15) Dieterich, D., Keberle, W., and Muller, E., (to Farbenfabriken Bayer Akt.), U.S. Patent 3,415,768 (Dec 10, 1968). (16) Geschwind, D . H., (to E. I . du PontdeNemours & Co.), U. S. Patent 3,361,693 (Jan2, 1968). (17) Gorman, J. W., and Toback, A. S. (to Loctite Corp.), ibid., 3,425,988 (Feb 4, 1969). (18) Hart, D. P., and Plasynski, J. E., (to PPG Indusrries, Inc.), ibid., 3,268,357 (Aug 23,1966). (19) Hauser, R . L., Adheriver A g e , 12, 26-7 (1969). (20) Hauser, R . L., and Fisher, R . E., SPE ANTEC, Tech. Papers, XII, 335-7 (1967). (21) Ilkka, G. A , , and Slates, R. (to General Motors Corp.), U. S. Patent 3,426,097 (Feb 4, 1969). (22) Kenyon, A. S., J. ColloidInrarJnceSci., 27 (4), 761-71 (1968). (23) Knapp, E. C . (to Monsanto Corp.), C . S. Patent 3,284,423 (Nov 8, 1966). (24) Kuei, T., Schonhorn, H., and Frisch, H . L., J . Colloid Interfac. Sci., 28 (34), 543-6 (1968). (25) Lader, W.,and Zang, D . (to PPG Industries, Inc.), U. S. Patent 3,280,217 (Oct18, 1966). (26) Lee, L-H., J . Appl. Poiyrn. Sci., 12, 719-30 (1968). (27) Lee, L-H., J , Colloid Interfac. Sci., 27 (4), 751-60 (1968). (28) Lewis, A. F., and Zaccardo, L. M. (to American Cyanamid Co.), U. S. Patent 3,391,054 (July 2, 1968). (29) MacKnight, W . J., Kajiyama, T., and McKenna, L., Polym. E n s . Sci., 8,267-71 (1968). (30) Monsanto Co., Brit. Patent 1,076,051 (July 19, 1767). (31) Pomeranty, D. I . (to P. R. Mallory and Co.), U. S. Patent 3,397,278 (Aug 13, ,
I
.
1968).
Figure 5. 52
I N D U S T R I A L AND ENGINEERING C H E M I S T R Y
(32) Rees, R . W., and Vaughn, D. J., ACS Preprints, Div. of Polymer Chem., 6, ( l ) ,296-303 (1965). (33) Samour, C. M . (to Kendall Co.), U. S. Patent 3,299,010 (Jan 17, 1967). (34) Samour, C. M,, and Satas, D . (to Kendall Co.), ibid., 3,400,103 (Sept 3, 1968). (35) Satas, D., and Mihalik, R., J . Appl. Polym. Sci., 12, 2371-9 (1968). (36) Sato, Y . (to Takeda Chem. Co.), ibid., 3,401,135 (Sept 10, 1968). (37) Shaw, F. D. (to E. I. d u Pont de Nemours & Co.), ibid., 3,354,237 (Nov 21, 1967). (38) Souffie, R. D., ibid., 3,364,155 (Jan 16, 1968). (39) Ulrich, E. W. (to 3M Co.), ibid., 2,884,126 (April 28, 1959). (40) Wargoty, B., J . Appl. Polym. S c i . , 12, 1873-88 (1968). (41) Wegman, R., Tech. Rept. PA-TR-3746, “Study of Adhesive Joint Failures by Evaporative Rate Analysis,” (Oct 1968).
