CAUSES OF POOR ADHESION Weak Boundary Layers
I
40
INDUSTRIAL A N D E N G I NEERING CHEMISTRY
layer is the real event when failure seems to occur in the adhesi,an,. ,’
JACOE J. BIKERMAN
ceording to the rheological theory of adhesion, A rupture of an adhint (this is an abbreviation for “adhesive joinf’) practically always’ proceeds in a material rather than between two materials. The rupture is initiated at a spot where the local stress exceeds the local strength. This spot may be situated in one of the adherends, in the adhesive film, or in one of the boundary strata which usually exist along the interface adhexendadhesive. I n the last named alternative, a weak boundary layer is present. When failure seems to be in adhesion-i.e., the separation appears to be exactly along the adherend-adhesive interfaccusually a cohesive break of a weak boundary layer is the real event. Only a few years ago, the existence of these layers was not suspected, and incorrect explanations were given for the common observation that many adhints failed at stresses far below the breaking stresses of the adherends and the adhesive. Three phases are present when an adhint is being formed, namely the environment (usually, air), the adhesive, and the adherends. I t is convenient to classify the weak boundary layers according to the phase (or phases) in which they originated. Thus, seven claws are differentiated, namely those caused by: 1.
Air
2. The adhesive 9. The adherend 4. Air and the adhesive 5. Air and the adherend 6. The adhesive and the adherend 7. Allthree 1. Weak boundary layers of the first class are very common. They always occur when the adhesive does tlot wet the adherend and air pockets remain between
Jacob J . B i k m n is Scnim Ruemch Assaciatc far H m i m , Znc., Cleveland, Ohio. This paga was presenkd at a summer session an “Adhesion of Polymers” a8 Wayne Sfatc Unimsi@,J m 1967. AUTHOR
the two after the setting of the adhesive. Figure 1 is a schematic representation of such a boundary; each air pocket causes a stress concentration and, when a tensile force is applied, fracture advances from one to the next void. Incomplete wetting is often encountered when aqueous adhesives are used because water wets fewer substances than do customary organic liquids; thus paper labels glued to an oily metal with a starch or dextrin solution do not remain attached long. Some solids, such as poly(tetrafluoroethylene), are poorly wetted by almost all liquids; consequently, it is difficult to find an adhesive suitable for them. In some instances, the weak boundary layer of the first class consists of an impurity present in air rather than of its main ingredients (nitrogen, oxygen, and so on). Thus, an attempt to produce strong joints by making them in a vacuum may prove unsuccessful because the vapor pressure of most pump fluids is high enough to contaminate the adherend and to create a zone of weakness.
2. If a material “does not stick to anything” and “nothing sticks to it,” it is very likely to contain a n impurity which concentrates near the surface and thus forms a n interfacial weak zone of the eecond class. Rupture takes place in this zone and still is a cohesional NPtUE. Commercial polyethylenes are typical examples of these materials. When a pellet of pplyethylene is melted on a clean solid and permitted to solidify, the resulting knob, as a rule, can be dislodged with a fingernail. When a drop of a solvent cement is placed on polyethylene, the residue after evaporation generally can be readily thrown off. This lack of adherence was attributed to the inactive chemical nature of polyethylene molecules; it was believed that the field of force around the -CH-CHchain was so feeble that no other material could be strongly attached to a polyethylene. This explanation is not acceptable by the rheological theory. Since separation proceeds in a material, the forces between two materials cannot determine the strength of an adhint. Y O L 5 9 NO. 9 S E P T E M B E R 1 9 6 7
41
The old explanation was refuted (7) by purifying several commercial polyethylenes and testing the adherence of the products. After purification, the polyethylenes gave joints stronger than the polyethylenes themselves. Infrared analysis showed that the impurities removed included oxygenated compounds or low molecular weight hydrocarbons, or both; thus, composition of the purified polymer was nearer to the ideal picture of long parafftnic chains than was that of the original substance. The d e c t of purification could be reversed by adding small amounts of suitable low molecular weight compounds to the adhesionable polyethylene. Oleic acid proved to be such a compound. Its molecule is polar; according to the molecular theory of adhesion it would be expected to raise the strength of polyethylene adhints. In reality, it lowered the peeling tension and the breaking stress f, of butt joints (3) as soon as its concentration exceeded about 0.1%; when the concentration was 1%, the adhints were as weak as those with nonpdfied polymer. This is shown in Figure 2. When a thin aluminum foil was attached to a glass plate with molten polyethylene (free of, or containing mme oleic acid) and, after setting, was peeled off, there was always some organic material remaining on the foil. Its thickness was about 4 p in the absence of oleic acid, about 19 p (equal to about half the total thickness of the adhesive film) at 0.4% oleic acid, and less than 1 p at the impurity concentration of 1%. Apparently, at 0.4%, the amount of impurity was not large enough to form a zone of weakness all along both interfaces (polyethylentglass and polyethylene-aluminum); patches of acid r a n d o d y dotted these interfaces, and the polyethylene film broke sometimes in one and sometimes in the other boundary layer; consequently, the average residue on the ribbon was almost equal to that on glass. At 1%, the zone of weakness was complete, and rupture proceeded in the area of the highest stress concentration -i.e., at theknee of the ribbon. Accumulation of oleic acid in the surface layer WBP c o n h e d (2) also by measuring electrical surface conductance of contaminated polyethylenes in a humid ammonia atmosphere. The electrical resistance of the superlicial stratum decreased when the oleic acid conemtration increased along a curve which was almost parallel to that of peeling tension us. concentration (Figure 2). Evidently, oleic acid was concentrated on the surface; this dect is known in colloid chemistry
.
a8 EyllCSeSk.
42
INDUSTRIAL A N D ENGINEERING CHEMISTRY
To avoid syneresis, a compound miscible with both polyethylene and oleic acid can be added. Its function would be analogous to that of, say, acetone whose addition renders water and benzene mutually miscible. Ethyl palmitate is such a compound. It is soluble in solid polyethylene and readily dissolves in oleic acid. The breaking stress of adhints, in which a mixture of polyethylene 99 parts and oleic acid 1 part was the adhesive between hvo steel cylinders, was below 1 bar. When a mixture of polyethylene 94, ethyl palmitate 5, and oleic acid 1 was used instead, the breaking stress was in the range of 73 to 94 bars-i.e., about 100 times as high. The old theory expected polar molecules always to increase the strength; in reality, they may lower it (oleic acid as the sole addition), or may have almost no effect (ethyl palmitate as the only addition), or may raise it (addition of ethyl palmitate to polyethylene containing oleic acid) ; solubility is more important than polarity. Low molecular weight materials accumulated in the surface layer can be polymerized or made to react with the bulk of the polyethylene; this treatment also results (0) in “adhesionable” polymers. Thus, three methods are known at present to overcome the apparent inertness of these materials. The low molecular weight ingredients which give rise to weak boundary layers can be (a) removed by fractional precipitation or extraction with solvents, (b) prevented from going to the surface by increasing the dipsolving power of the substrate, or (c) chemically transformed into high molecular weight and thus innocuous substances. In industry, commercial polyethylenes are treated with
TABLE I . Contaminant
I
EFFECT O F STEARIC ACID ON A D H I N T STRENGTH
I
Alkyd Paint Before
Vinyl Resin Lacquer
After
Before
G./Sq. Cm.
fm
A,
fm
A,
fm
none
210
10-6
150
55% 90%
120
x x
100
95% 90%
10-6
110
100%
60
100%
4 4
a flame, a corona discharge, an oxidizing solution, and so forth, to enhance their adhesiveness. Any of the three above-mentioned mechanisms, or a combination of these, may be responsible for the improvement achieved. I t may be pointed out that the cause of the poor adherence of commercial polyolefins was found by measuring not only the breaking stress of their adhints but also the tensile strength of the adhesive material in bulk, the distribution of the adhesive residue between the two adherends, and the electrical surface conductance. I t is almost impossible to derive generally valid conclusions from tests on the breaking stress alone. 3. Removal of weak boundary layers of the third class is the task of the pretreatment of adherends or of the preparation of the solid for gluing. Several books and a host of papers are devoted to this problem but the scientific value of the information published is not impressive because, above all, of an insufficient characterization of the ingredients to be eliminated. A surface component which, chemically, is an impurity may cause no deterioration of the adhint and, consequently, does not need to be removed when readying a solid for a gluing operation; this is the case when this impurity is stronger than the adherend or the adhesive. Aluminum oxide on aluminum metal is a clear example. Dangerous contamination of adherend surfaces often is caused by an unsuitable pretreatment. For instance, when a magnesium alloy containing zinc and cerium was treated with a hot alkaline chromate solution, it became covered with a solid chromate layer of a low strength; and adhints made with this alloy broke cohesively in the coating (70). A particularly unexpected example was observed when brass was electrolytically deposited on another metal to improve its bonding to rubber (6). Some specimens contained a foreign matter on the surface and the metalrubber adhints easily broke in this substance. Its analysis was made, and it proved to be the salt [Zn(”3)6]-
After A,
f m
A,
100
75%
120
100%
60
95%
100%
50
1 OO(30
80 70
100%
{Zna[Fe(CN)e]z). Zinc and cyan radicals were present in large amounts in the plating bath, but ferrous iron originated from the tools used around the bath. I n several series of experiments, the amounts needed to affect the performance of an adhint were determined. The breaking stress of butt joints “steel-poly(viny1 acetate)-steel” was about 500 bars. When gram of decanoic acid was deposited on 1 sq. cm. of the steel surface before the formation of the adhint, the breaking stress was near 250 bars; and it was about 170 bars when gram/sq. cm. (72). the acid concentration was 9 X Decanoic acid seems to be soluble in both molten and solid poly(viny1acetate). When steel contaminated with 9 X 10-7 gram/sq. cm. of decanoic acid was kept in contact with molten polymer for 0, 1, and 2 hours, the breaking stress was 207, 268, and 321 bars. Table I contains similar data (7). A steel sheet was contaminated with stearic acid, coated with an alkyd paint or a vinyl resin lacquer, and the butt joint “steel coating’’ was broken in tension; fm is the breaking stress in bars. After the fracture, a part of the steel surface appeared uncoated; the ratio of this bare area to the total adhesive-adherend interface is shown as A , (yo)in the table. The rupture tests were performed soon after the adhint formation (before in the table) and after an accelerated weathering for 500 hours. I t is clear that very small amounts of stearic acid markedly lower f m and markedly raise A,. Metals encountered in daily life contain much greater amounts of impurities in their surface layers. Steel, clean in the customary sense of the word, was rubbed with a powder of potassium bromide, the powder collected, and its infrared spectrum determined. In the spectrum, lines characteristic for a phthalate were distinctly visible ( 7 7). Dibutyl phthalate is a common lubricant for steel in rolling mills; apparently, it was pressed into the surface layer of the steel sheet so well that ordinary washing procedures did not dislodge it. When the upper crust of VOL. 5 9
NO. 9
SEPTEMBER 1967
43
the sheet was abraded with KBr, the lubricant spread over the crystals and was removed. 4. Weak boundary layers of the fourth class have not been deliberately studied. An instance was observed during the above-mentioned study of polyethylene adhints. When these were prepared in air and the heating period was too long, discoloration of the polymer occurred. The yellow layer formed at the polymer-air boundary was weak and lowered the failing stress. To avoid this complication, the adhints for measurements were prepared in a nitrogen atmosphere. 5. The fifth class embraces weak boundary layers originating from the environment and the adherend together. All examples known concern heating or aging metals in air. Lap joints of “aluminum-epoxy adhesive-aluminum” were prepared either in dry argon or in a mixture of argon 80, oxygen 20, and enough water vapor to achieve a relative humidity of 50% (74). The preparation included the following steps: abrasion of the aluminum with silicon carbide; no aging or aging for 60 minutes; application of the adhesive; curing. The breaking stress was 205 and 190 bars without and with aging, if everything was done in dry argon; evidently, the argon atmosphere had almost no effect. In the other medium, the breaking stress was lowered from 200 to 105 bars by an hour of waiting. If a wire of particularly pure copper is heated in dry air, it becomes coated with a thin crust of cuprous oxide with some cupric oxide next to the air phase. Such a wire can be twisted without losing any of its scale at all temperatures between 400’ and 900’ C. If, however, an analogous experiment is performed with a wire of copper containing 0.03 to 0.04% phosphorus, a twist causes the wire to shed its coating at both 400’ and 500’ C. (73). This coating contains some cuprous phosphate which lowers the strength or increases the brittleness of the oxide layer. 6 . The sixth class of weak boundary layers is represented by many examples of which only a small selection can be recalled here. An interesting type of weakness sometimes occurs when both adherend and adhesive are metals. When metal A is heated in contact with metal B , the atoms of A may diffuse into the B phase more rapidly than B diffuses into A . Consequently, the stratum of A next to B contains many vacancies in the lattice. Usually, vacancies coalesce and thus form a porous layer of reduced strength. This process was observed, for instance, at the interface of gold and aluminum (5). Gold wires were bonded by compression at a high temperature to an aluminum slab. When a tensile force was applied to such a wire, the adhint usually failed “at the interface.” To see what actually happened, the joints were aged for 2 hours at 450’ C., and a beveled cross section of the interfacial region was made. Eight distinct strata were visible after etching, namely Al, AuA12, AuA1, Au2A1, AusA12(?), an area of pores, AudAI, and finally Au. The voids were so numerous that the small strength of the joints was easy to account for. 44
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
In the above example, the weak boundary layer formed as a consequence of diffusion. Chemical reactions also can be the culprits. At the boundary of copper and molten tin solder, two intermetallic compounds, CusSn and Cu&3nh, gradually grow. The crystals of CusSnE are brittle, and a brittle zone results when their concentration a t the interface is considerable. Thus, the peeling tension of copper-solder-copper adhints was 900 kilodynes/cm. after heating for 1 sec. at 400’ C. and only 200 kilodynes/cm. after 30 sec. of heating; and the average thickness of the Cu&nZ interphase grew simultaneously from about 1 to about 6 p (8). When a rubber mix-Le., unvulcanized rubber sulfur other ingredients-was heated with brass, a mechanical separation of vulcanized rubber from brass was easy as long as the sulfur content was above, say, 5%. I n this instance, a powdery deposit of metal sulfide was detectable along the interface, and rupture took place in it (6). 7. Weak boundary layers of the seventh class were encountered (4) when some metals were glued together with an experimental adhesive consisting of a phenolic resin and an admixture of an epoxy polymer. When aluminum was the adherend, the breaking stress of the adhints, when fresh, was about 122 bars. When copper was employed rather than aluminum, this stress was 96 bars. The difference was even greater when the adhints were kept in air at 288’ C. for 100 hours; after this treatment, the strength of aluminum joints was still 65 bars while that of the copper adhints was immeasurably small. Copper acted as an oxidation catalyst. At the temperature of curing (and during the subsequent heating, if any), the adhesive was oxidized by the atmospheric oxygen at the copper-air-adhesive boundary, and the zone of weakness gradually expanded along the copperadhesive interface. All three phases were essential for this process. If nitrogen was substituted for air, or if aluminum, silver, or zinc was substituted for copper, or another adhesive was employed, the deterioration did not occur. The above examples demonstrate that weak boundary layers can form as a result of various and often unexpected processes. They may be denoted as “diseases of the joints.’’ Whenever an apparent failure in adhesion is observed, a weak boundary layer must be suspected and a diagnosis of its origin should be attempted. Only then can a rational cure be prescribed.
+
+
REFERENCES (1) Bikerman, J. J., “ T h e Science of Adhesive Joints,” Academic Press, New York, 1961. (2) Bikerman, J. J., SPE Trans. 2, 213 (1962). Marshall, , D. W., J. Appi. Polymer Sci. 7, 1031 (1963). (3) Bikerman, J. .I. (4) Black, J. M . , Blomquist, R. F., IKD.Exo. CHEM.50, 918 (1958). (5) Blech, I. A., Sello, H., J.Electrochem. Soc. 113, 1052 (1966). (6) Buchan, S., “Rubber t o Metal Bonding,” 2nd ed., Crosby Lockwood, London, 1959. (7) Bullett, T. R., Prosser, J. L., Trans. Inst. M e t a l Finish. 41, 112 (1964). (8) Chadwick, R., J . Inrt. Metnlr 62,277 (1938). (9) Hansen, R. H . , Schonhorn, H., J.PoiyrnerSci. PI. B 4, 203 (1966). (10) Hunter, R. J. E., Can. Aeronaut. J . 3, 161 (1957). (11) Johnson, W. T. M . , Ofic.Dig. Federation Sot. Paint Technol. 33, 1489 (1961). (12) Lasoski, S. W., Kraus, G., J. PolymerSci. 18, 359 (1955). (13) Tylecote, R. F., J . Inst. iMetais78, 301 (1950). (14) Wegman, R . F., Bdhesiuer Age 10, No. 1, 20 (1967).
