The nature of the adhesion of polymers

Adhesion of polymers

45

REFERENCES 1. G. A. RAZUVAYEV, K. S. MINSKER and R. P. CHERNOVSKAYA, Dokl. Akad. Nauk SSSR 147: 636, 1962 2. K; S. MINSKER, R. P. CHERNOVSKAYA and A. S. ZAKHAROVA, Vysokomol. soyed. 5: 1627, 1963 3. G. A. RAZUVAYEV, K. S. MINSKER,

G. T. FEDOSEYEVA and L. N. SAVEL'EV,

Vysokomol. soyed. 1: 1691, 1959 4. G. A. RAZUVAYEV, K. S. MINSKER, G. T. FEDOSEYEVA and V. N. BYgHOVSKII,

Vysokomol. soyed. 2: 404, 1960 5. G. A. RAZUVAYEV, K. S. MINSKER and I. Z. SHAPIRO, Vysokomol. soyed. 4: 1833, 1962 6. R. P. CHERNOVSKAYA, K. S. MINSKER and G. A. RAZUVAYEV, Tezisy dokladov na Vsesoyuznom nauch.-tekhn, soveshchanii po aluminiiorganicheskim soyedineniyam. (Summaries of Reports of the All-Union Conference on Organoaluminium Compounds.), p. 25, Izd. VKHO im. D. I. Mendeleyeva, Moscow, 1963 7. S. Z. ROGINSKII, Adsorptsiya i kataliz na neodnorodnykh poverkhnostyakh. (Adsorption and Catalysis on Heterogeneous Surface.) Izd. Akad. Nauk SSSR, 1948 8. R. P. CHERNOVSKAYA, K. S. MINSKER and G. A. RAZUVAYEV, Vysokomo]. soyed. 6: 1656, 1964 9. G. NATTA and I. PASQUON, sb. Kataliz. Voprosy izbiratel'nosti i stereospetsifichnosti katalizatorov. (Collected papers. Catalysis. Problems in the Selectivity and Stereospecificity of Catalysts.) p. 27, Foreign Literature Publishing House (Russian translation), 1963 1O. M. I. MOSEVITSKII, Uspekhi khimii 28: 465, 1959

THE NATURE OF THE ADHESION OF POLYMERS* V. YE. GUL' and L. L. F O M I N A Moscow Technological Institute of the Meat and Milk Industry

(Received 28 February 1964) STUDY of t h e d e p e n d e n c e o f a d h e s i o n on t e m p e r a t u r e a n d r a t e is o f i n t e r e s t in c o n n e c t i o n w i t h t h e e l u c i d a t i o n of t h e n a t u r e of adhesion. T h e v a l i d i t y of e x i s t i n g theories is s o m e t i m e s j u d g e d f r o m t h e n a t u r e o f these relationships. T h e a d s o r p t i o n t h e o r y o f a d h e s i o n p o s t u l a t e s t h a t t h e first s t a g e of a d h e s i o n consists in m i g r a t i o n o f t h e large p o l y m e r molecules f r o m solution or f r o m t h e m e l t i n t o t h e surface o f t h e s u b s t r a t e , a n d t h e second s t a g e is e s t a b l i s h m e n t o f a d s o r p t i o n equilibrium. I n t h e second s t a g e b o n d s a r e f o r m e d b y i n t e r m o l e c u ]ar i n t e r a c t i o n . I t is t h o u g h t t h a t it should n o t b e possible t o e x p l a i n t h e d e p e n dence o f t h e w o r k o f a d h e s i o n o n t h e r a t e o f peeling of t h e a d h e s i v e layer, w i t h i u * Vysokomol. soyed. 7: No. 1, 45-49, 1965.

46

V. YE. GUT,' and L. L. FOMINA

the framework of the adsorption theory of adhesion. Any intermolecular interaction bonds, formed by the adsorption mechanism, become ruptured by the fluctuation of thermal motion and again re-form. The higher the rate of peeling (the shorter the time of action of the force causing peeling) the less the extent to which thermal motion can rupture the bonds preventing peeling. In this respect, regardless of the nature of the forces on which the strength of the adhesive bond is dependent, the dependences of adhesion on rate and temperature are similar to the rate and temperature dependences of cohesive strength [1]. In a number of cases the work done in the peeling of polymer films is much greater than is required for overcoming only intermoleeular interaction forces. In some cases this is due to the formation of chemical bonds between the molecules of the adhesive and substrate, and in others with the formation of an electrical double layer [2]. In the separation of the adhesive from the substrate work is done on separation of the electrical charges forming the electrical double layer. In certain cases it is possible for intermolecular bonds to be formed between two neutral molecules, a donor (hydrocarbon, amine, ether etc.) and an acceptor (halogen, nitrile, qninone etc.), as a result of partial transfer of an electron without formation of a chemical bond [3]. In all the cases considered above the factor determining the strength of the adhesive bond is the energy of interaction between the molecules of ~he adhesive and substrate, U12. The value of UI~ can vary from 3-7 keal/mole for intermolecular interaction to tens of kilocalories per mole for typical cases of chemical interaction. All other conditions being equal an increase in UI~ must be accompanied by an increase in the strength of the adhesive bond. However, regardless of the value of U12 the bonds between the adhesive and substrate molecules will rupture with greater or lesser frequency depending on the temperature. The higher the peeling temperature, Tp, the greater will be the frequency with which the bonds resisting peeling will rupture as a result of the thermal motion of the kinetic units. Figure 1 shows the dependence of the strength of the bond between polyethylene and Cellophane on the temperature of contact. Films of polyethylene and Cellophane were used for making the adhesive joints. The polyethylene films were obtained by means of a "Troster" extruder ~ith a slit head, at extrusion temperatures of 250 and 280% The films were then compressed at a specific pressure of 20 kg/cm ~ for 5 minutes at chosen temperatures of contact, and subsequently cooled in the press under pressure. The stress required to separate the 6Ires was determined in a tensile testing machine with stress recording and thermostatting systems. The rate of peeling chosen was 150 ram/rain. Weak oxidation of the surface of polyethylene gives rise to too slight interaction with the polar, oxygen-containing groups of cellulose, of which Cellophane is made. I t is evident that the oxidation of polyethylene occurring at a higher extrusion temperature (280 °) causes formation of oxygen-containing groups in the polyethylene molecule giving a fairly high

Adhesion of polymers

47

apparent energy of actlvation Uz~----4-5 kcal/mole, and consequently a fairly high adhesive bond strength. A simple physical action--swelling in water-results in screening of the polar, oxygen-cbntaining groups by molecules of water

~G

G ,#/~n

JO0 ~ 2O0

×

•

a

54

loo

O ~" 120 =

I 200 I 180

FIG. 1

J

I 2.5

l

L 2.B

I

I J.J

Tc,°C

FIG. 2

FIG. 1. Dependence of the stress, ap, for rupturing t h e adhesive b o n d between polyethylene and Cellophane, on contact temperature. Polyetylene ~lrn~ prepared a t extrusion t e m p e r . atures of: a - - 2 8 0 °, 5--250 °. FIG. 2. Dependence of In ap on 1/Tp for polyethylene-Cellophane.

and the adhesive bond between polyethylene and Cellophane is destroyed. This can be avoided by formation of a chemical bond between the two polymers. In cases where intermolecular interaction in the adhesive and substrate is so different that Ux~ is small and the formation of an electrical double layer does not give rise to a sufficiently strong adhesive joint, it is possible to increase the adhesive strength by use of a block copolymer as an intermediate component, prepared for example by radiation or ultrasonic treatment of an emulsion of adhesive polymers [4]. In the light of the above considerations determination of the energy of the bond between the molecules of the adhesive and substrate becomes particularly important. I t is possible to calculate the apparent energy of activation for adhesive rupture from the temperature dependence of adhesion. However the process of adhesive rupture differs markedly from previously studied processes consisting of elementary acts of transition from one state to another, l~or chemical reaction between two 'molecules the energy of activation is calculated per mole of compound formed. In the breakdown of an adhesive bond several bonds can be involved simultaneously in an elementary act of rupture, hence the kinetic unit is that portion of the surface of contact in which the elementary act of rupture occurs. In both cohesive and adhesive rupture it is possible to assess the nature of the bond (chemical or physical interaction) only by determining the volume in which an elementary act of rupture occurs.

48

V. YE. GUL' and L. L. l~om~A

As a criterion of whether the interaction between the adhesive and substrate is chemical or physical one can take the "specific energy of adhesion", i.e. the apparent energy of activation at a-~0, related to the unit of volume in which an elementary act of rupture occurs. The apparent energy of activation can be calculated from the results presented in Fig. 2. I t is obtained from the ratio of the increase in 111ap to the increase in 1/Tp*, and is 4.5 kcal/mole, which shows that the adhesive interaction is of a physical nature. According to the above considerations the peeling stress should be proportional to the rate of peeling, V, and inversely proportional to the frequency of bond rupture due to thermal motion:

ap= K V~e~/R~o

(1)

The coefficient K is dependent on a number of factors acting at the time of formation of the adhesive joint:

K=f(Tc.t¢.P )

(2)

where Ta, t e and P are temperature, time and pressure of contact. I t must be emphasized that for the adhesion of polymers this equation normally determines the strength of the adhesive bond. I n the overwhelming majority of cases the adhesion of polymers to various substrates conforms well with the well known diffusion theory of adhesion [5], according to which adhesion is brought about b y diffusion penetration of segments of the molecules of the adhesive i n t o the substrate and the converse. I n the case of formation of an adhesive joint when the adhesive is in the molten state and the substrate is a glass the equation:

% =/(To'to" P). Y~. ev,,/R~p

(3)

can be similar to that observed in the adhesion of polymers above Tg. This behaviour has been explained either b y diffusion of active (in the sense of intermolecular interaction) groups of the adhesive over the surface of the substrate, or b y flow of the adhesive into microdefects in the surface of the substrate [6]. I n the case of the adhesion of polyethylene to Cellophane when molten polyethylene is deposited on the Cellophane, which does not soften at the temperature of contact, we consider that it is very probable t h a t the strength of the adhesive bond is determined b y the development of microrheological processes. Flow of the polyethylene into mierodefects in the Cellophane causes increase in the number of contacts between active groups of the polyethylene and Cellophane. The theological processes also continue with time and are intensified b y increase in the temperature and pressure of contact. As the microdefects become filled flow becomes retarded and finally ceases. Increase in pressure should also cause an increase in the number of contacts due to flow, up to a cer*In accordance with equation (1), an analysis of which was given in reference [8].

Adhesion of polymers

49

t~in limit. At higher pressures the so-called mechanical vitrification of the adhesive occurs and this retards rheological processes (Fig. 3). The results presented in Fig. 3 were obtained with adhesive joints prepared b y compression of Cellophane and polyethylene film extruded at 280 °. The compression routine was: time of contact 5 minutes, temperature of contact 140 °. We have previously proposed an equation in which the peeling stress is associated with irreversible.deformation, caused by flow of the molten adhesive into the glassy substrate [8]. I t is evident t h a t an equation relating the peeling stress to the area of contact between the adhesive and substrate, which increases as the microdefects become filled, would be more correct.

~ ,g/cm 400f ~

I

0

I

50

logG

2"6I ~

I

I

I50

f

k#/cm 2

~p ,

I

250

F~Q. 3

.2

{~ V

4

Fsz. 4

FIG. 3. Dependence of adhesion of polyethylene to Cellophane on contact pressure. Fin. 4. Dependence of ~p on rate of deformation for polyethylene-Cellophane film. Thus in the formation of an adhesive joint diffusion of segments of molecules of the softened adhesive into the substrate and flow of the adhesive into microdefects in the surface of the substrate are equally probable. A direct relationship between adhesion and flow into microdefects in silicate glass has been demonstrated experimentally [7]. After formation of the adhesive joint is complete its strength will depend on the peeling routine, i.e. the rate and temperature of peeling and the conditions of deformation, which determine the concentration of stresses over the adhesive-substrate inter£ace (equation (3)). Figure 4 shows ~esults illustrating the effect of the rate of peeling on the magnitude of the peeling stress, and these confirm the applicability of equation (3). Thus the rupture of an adhesive joint is similar to cohesive rupture. The formation of an adhesive joint is described by qualitative equations t h a t are equally applicable to diffusion and microrheological processes. CONCLUSIONS

(1) The relationship between the stress required to break the adhesive bond in polyethylene-Cellophane laminates and the temperature, time and

50

D . N . BORWe~ a/.

pressure of contact, a nd also t he r a t e and t e m p e r a t u r e of peeling have been studied experimentally. (2) Analysis of the results shows t h a t t h e y m a y be associated with microrheological processes. Trans~a~ b~/E. O. PHILLIPS REFERENCES

1. V. E. GUL', Dokl. ~ a d . Nauk SSSR, 96: 953, 1954; Sb. Uspekhi khimii i tehuologii pelimerov. (Collected papers. Advances in the Chemistry and Technology of Polymers.) p. 212, Goskhlmizdat, 1957 2. B. V. DERYAGIN and N. A. KROTOVA, Adgeziya. (Adhesion.) Izd. Akad. Nauk SSSR, 1949; Dokl. Akad. Nauk SSSR, 61: 849, 1948 3. G. BRIGLEB, Elektronen Donator-Akzeptor Komplexe, Springer Verlag, Berlin, 1961 4. V. E. GUL', GOL'DANSKII, B. V. DZANTIEV, E. V. YEGOROV, G. A. ZIL'BERG and V. G. RAYEVSKH. Biul. izobr. No. 2, 59, 1962 5. S. S. VOYUTSKII, Autogeziya i adgeziya vysokopolimerow. (The Auto-hesion and Adhesion of High Polymers.) Gostekhizdat, 1960 6. V. E. GUL', CHANG IN-SI, V. L. VAKULA and S. S. VOYUTSKH, Vysokomol. soyed. 4: 85, 1962 7. V. E. GUL', CHANG IN-SI, V. L. VAKULA and S. S. VOYUTSKII, Vysokomol. soyed. 4: 294, 1962 8. V. E. GUL' and L. L. KUDRYASHEVA, Sb. Adgeziya polimerov. (Collected papers. Adhesion of Polymers.), p. 134, Izd. Akad. Nauk SSSR, 1963

MORPHOLOGY OF BULK POLYVINYLCHLORIDE* D. N. BORT, YE. YE. RYLOV, N. A. OKLADNOV, B. P . SHTARKMAN and V. A. KARGIN (Received 2 March 1964)

THE formation of supramolecular structures in t he actual process of polymerization is a v e r y interesting problem, and also t h a t of t h e i r development as the conversion proceeds. This problem has been touched upon in previous papers [1, 2] for th e examples of polyethylene and crystalline polyvinylch]oride. I n the present work it was solved for bulk polyvinylchloride. EXPERIMENTAL

The polymerization was carried out in ampoules. The initiator concentration was 0.8% by wt. of monomer and the polymerization temperature was 18--22°. Depending on the degree of conversation the polymerizate was either a slightly opalescent liquid (traces of *Vysokomol. soyed. 7: No. 1, 50-54, 1965.