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The adhesion of elastomers to powder fillers and reinforcement of filled systems
Adhesion of elastomers to powder fillers
1643
14. E. IWATSUKI, N. TAKIKAWA, M. OKADA, J. JAMASHITA and Y. ISHH, Kogyo kagaku zasshi 67: 1236, 1964 15. D. E. MeLAUGHI,IN, M. TAMRES and S. SEARLES, J. Amer. Chem. Soc. 82:5621 1960 16. T. J. CIOFFI and S. ZENCHELSKI, J. Phys. Chem. 67: 357, 1963 17. V. I. VEDENEYEV, L. V. GURVICH, V. N. KONDRAT'EV, V. A. MEDVEDEV and Ye. L. FRANKEVICH, Energiya razryva khimicheskikh svyazei. Spravoehnik. (Energy of Rupture of Chemical Bonds. Handbook.) Izd. Akad. Nauk SSSR, 1962 18. V. N. KONDRAT'EV and D. N. SOKOLOV, Zh. fiz. khim. 29: 1265, 1955 19. W. KERN, Chemiker-Ztg. 88: 623, 1964 20. A. ISHIGAKI, T. SHONO and J. HASHINOMA, Makromol. Chem. 79: 170, 1964 21. B. A. ROZENBERG, Dissertation, 1964 22. G. V. RAKOVA, L. M. ROMANOV and N. S. YENIKOLOPYAN, Vysokomol. soyed. 6: 2178, 1964 (Translated in Polymer Science U.S.R.R. 6: 12, 2410, 1965)
THE ADHESION OF ELASTOMERS TO POWDER FILLERS AND REINFORCEMENT OF FILLED SYSTEMS*t V. G. I~AYEVSKII, S. )/I. YAGNYATINSKAYA and S. S. VOYUTSKII M. V. Lomonosov Institute of Fine Chemical Technology, Moscow, Moscow Technological Institute of the Meat and Milk Industry (Received 5 June 1965)
CORRELATION between adhesion and reinforcement in rubber mixtures has been found previously with respect to change in the conditions of interaction of the rubber with the filler [1], increase in the degree of structure formation in the rubber [2] and chemical modification of the surface of the filler [3]. The present communication presents data on the effect on reinforcement of additives that increase or decrease the adhesion of the elastomer to the filler (adhesives or antiadhesives). The effect of the molecular weight of the polymer on the reinforcement of filled systems is also discussed. EFFECT ON REINFORCEMENT OF THE ADDITION OF SUBSTANCES ALTERING THE RUBBER-FILLER ADHESION
The a d d i t i o n to r u b b e r m i x t u r e s o f certain organic substances is m a d e use o f t o increase reinforcement, b u t m o s t often t h e y are used in c o n j u n c t i o n with mineral fillers. A n i m p o r t a n t p o i n t is t h a t w h e n these additives are used t o g e t h e r with fillers the reinforcing effect is greater t h a n w h e n t h e y are used s e p a r a t e l y [4]. * Vysokomol. soyed. 8: No. 9, 1493-1500, 1966. t 3rd Communication in the series "Reinforcement of polymers".
1644
V. G. RAYEVSKIIet al.
So far no explanation of this has been given. I t m a y be supposed however t h a t the positive role of these additives consists in increase in the adhesion of the polymer to the particles of the filler. In this investigation the materials studied were vulcanizates of the butadiene-acrylonitrile elastomer SKN-40, either unfilled or containing 20 parts by volume of chalk (per 100 parts by volume of rubber). Five parts by volume of one or other of the following substances were added to the mixtures from which the vulcanizates were prepared: stearic acid, Rubrax, rosin, an indene-coumarone resin or a 1 : 1 mixture of rosin and the indene-coumarone resin. The vulcanization temperature was 143 ° and the time from 45 to 60 minutes, depending on the type of additive. The physieo-mechanical characteristics of the filled and unfilled mixtures, with and without the additives and vulcanizated to the opitmal degree, are presented in the Table. I t is seen from the Table t h a t the strength of unfilled vulcanizates containing an additive is always lower t h a n the strength of the vulcanizates without these additives. I n contrast to this the strength of filled vulcanizates is increased when adhesives are added to the system and decreased by the addition of anti-adhesives (stearic acid). The positive effect of adhesives on the strength of filled vulcanizates together with their negative effect in unfilled rubbers can be attributed onE F F E C T OF A D D I T I O N OF A D H E S I V E AND A N T I - A D H E S I V E S ON T H E PHYSICOM~CHAN-ICAL CHARACTERISTICS OF S K ~ - 4 0
Additive
Without additive Stearic acid Rubrax Rosin Indene-coumarone resinrosin (1 : 1) Indene-coumarone resin Without additive Stearic acid Rubrax Rosin : 1) Indene-cotunarone resin-rosin (1 Indene-coumarone resin
Tensile strength, kg/cmz
VULCA.~IZATES
Relative elongation,
Residual elongation,
%
%
kg/cm
Unfilled vuleanizates 140 780 120 770 112 700 112 780
12 16 15 22
13'0 12"8 13"1 13"1
18
105 110
Tear resistance,
750 740
16
13"1 13"2
FiHed vulcanizates 95 800 85 700 95 700 113 790 130 790 140 750
24 22 24 30 26 20
17"0 17"0 17"7 20"5 23"0 26"0
ly to increase in the elastomer-fiUer adhesion. I f this assumption is correct the increase in strength of filled vulcanizates caused by adhesive additives should be greater in proportion to increase in the extent to which the additive improve
Adhesion of elastomers to powder fillers
1645
the adhesion of the elastomer to the filler. I n other words there must be a correlation between the efficiency of additives promoting reinforcement and their ability to increase the elastomer-filler adhesion. I n order to confirm this supposition experiments were set up to determine the resistance of unfilled vuleanizates (with and without additive) to peeling from a substrate of chalk, b y the m et hod developed and described in reference [5]. I n these experiments the conditions for formation of the adhesive bond were identical with the conditions of preparation of specimens for physicomeehanical tests. As can be seen from Fig. 1, correlation was in fact found between the adhesion of vulcanizates containing an additive to the filler (chalk) and their tensile strength and tear resistance. Moreover the relationship is represented satisfactorily b y a straight line regardless of the t y p e of test (tensile or tear). Despite the correlation between adhesion and the absolute values of t he tensile strength and tear resistance it was felt t h a t it would be of interest to find the correlation between adhesion and relative reinforcement. This correlation is 2
e,l
~
m
~26 ~ 22
d
14
400 600 Resi~anee to peeling, 9s/crn I
200
FIG. 1
I
I
I
o
200
I
I
4~
600 Re~i~tancetopeel/ng , gs/cm FIO. 2
FIG. 1. Dependence of tensile strength (1) and tear resistance (2) of SKN-40 vulcaniza~es with chalk filler on their adhesion to the filler. Additives used: a--stearie acid, b--no additive, c -- Rubrax, d-- rosin, e -- rosin ~ indene -coumarone resin, f-- indene -coumarone resin. Fro. 2. Dependence of the reinforcement of SKN-40 vulcanizates by chalk according to tensile strength (1, 1') and tear resistance (2) on their adhesion to the filler: /--without allowance for the effect of the additive on the strength of the unfilled vulcanizate; 1--with allowance for this effect. Lettering as in Fig. 1. illustrated in Fig. 2. Here curve 2, relating adhesion to reinforcement as measured b y tear resistance shows r at her less scatter of the points and a greater slope t h a n curve 1, which relates adhesion to reinforcement according to tensile strength. I n view of the fact t h a t adhesive and anti-adhesives alter the strength of unfilled vulcanizates it was of interest to eliminate this effect from the relationship. This can be done b y put t i ng the breaking stress of the unfilled vulcanizates
1646
V.G.
RAYEVSKII et al.
containing the adhesive or anti-adhesive into the formula for calculating relative reinforcement, instead of the breaking stress of the vulcanizates containing neither a filler nor additive. The correlation found in this was is represented in Fig. 2 by curve 1. The slope of this line is a little greater than the slope of curve 1, and differs little from that of curve 2, relating adhesion to relative reinforcement according to tear resistance. A similar curve relating to tear resistance coincides with curve 2, since the additives have practically no effect on the tear resistance of unfilled vulcanizates. Thus the addition of adhesive additives brings about an increase in the strength of filled rubbers. The reinforcing effect is dependent on the nature of the additive used. I t is evident that only a substance t h a t has some compatibility with the polymer and creates between the polymer and filler a stronger bond than that existing in this absence, increases the strength of a filled system. From the example of the substances used in this work one can find how these requirements are fulfilled. The similarity of the solubility parameters of the rubber (6sx~.40~9.83 [kcal/cm3]½) and the above additives can be used as a measure of compatibility. The solubility parameters of stearic acid, Rubrax, rosin and the indene-coumarone resin can be found from the solubility parameters of solvents in which these substances have good solubility [6]. From our comparison of the solubility parameters of SKN-40 and the given additives it was found that the indene-coumarone resin should have the greatest compatibility with the rubber. This is in complete accord with the positive effect of the resin on the adhesion of the rubber to chalk and on the strength of the filled vulcanizate. Stearic acid has the least compatibility with the rubber of all the additives, as comparison of its solubility parameter with that of SKN-40 shows. I t is very probable that the excess fatty acid accumulates at the rubber-filler interfacial boundary. Since the quantity of the acid added is sufficient to form a polymolecular layer on the surface of the filler particles the existence of this intermediate layer must have a negative effect on the strength of the polymerfiller bond, and consequently on the strength of the filled system. This was in fact found in the present work and is in good agreement with the results of a number of authors, which show that stearic acid is not an active filler [7, 8]. The efficiency of additives as activators for fillers depends of course not only on the nature of the additive, but also on the type of filler, the quantity of the latter present in the polymer, and also on the nature of the polymer. I t m a y be assumed, a priori, that the effectiveness of adhesive additives will be greater the lower the strength of the polymer-filler bond in the original system. EFFECT OF THE MOLECULAR WEIGHT OF THE POLYMER ON REINFORCEMENT OF RUBBER MIXTURES
The depedence of the strength of filled systems on t h e m o l e c u l a r weight of the polymer has been studied in a number of papers [9-11]. Unfortunately, in the papers cited the curves expressing the dependence of strength on molecular
Adhesion of elastomers to powder fillers
1647
weight for filled systems were not compared with corresponding curves for the unfilled systems. In this section of the work this omission was rectified and in addition an attempt was made to find the relationship between the molecular weight of the polymer and its adhesion to the filler. The materials used for this investigation were the linear, nonpolar polymers, polyisobutylenes P-85, P-118 and P-200, with molecular weights in the region of 85, 118 and 200 thousand, and butyl rubber of molecular weight 36 thousand and a degree of unsaturation of 0.99%, and also unvulcanized mixtures of these with 20 parts by volume of carbon black DG-100 per 100 parts by volume of polymer. The tear resistance of plates made from the untreated rubbers and the carbon-black mixtures was measured according to the State Standard Specification GOST 263-53, because this characteristic should correspond best to the adhesion of the polymer found by the peeling method. In the tear resistance tests the stress and deformation during stretching were recorded up to the moment of breakdown of the specimen. The dependence of the nominal stress on the elongation of the polymers and their carbon-black mixtures is shown in Fig. 3. 8 CI
6
cvj
4
I
0
200
I~1 0
200
I 400
6'00
II :800
Defo/,mation , %
FIG. 3. Stress-strain curves of polymers (a) and their mixtures with I)G-100 carbon black (b): l - - P - 2 0 0 , 2--P-118, 3--P-85, 4 - - b u t y l rubber.
On examination of Fig. 3a one's attention is drawn to the different nature of the stress-deformation curve for the butyl rubber in comparison with the polyisobutylenes. The shape of the curves, and also the specimens themselves after rupture, lead to the conclusion that the specimens of the low molecular weight butyl break as a result of viscous flow, whereas elastic rupture mainly occurs in the polyisobutylene specimens. The polyisobutylenes, as would be expected, fall into the following order with respect to decrease in stress and relative elongation P-200>P-118>P-85.
i648
V.G. RAY~vsF~I~et al.
Addition of carbon black to the butyl rubber does not alter the nature of the stress-strain curve, though the maximal stress (yield point) is then somewhat higher (Fig. 3b). The stress-strain curves of the mixtures of polyisobutylene with carbon black are clearly S-shaped. This indicates that reinforcement occurs before breakdown of the specimens. I t is well k n o w n that reinforcement is characteristic also in the rupture of vulcanizates. The disposition of the curves with respect to breaking stress is similar to that for the unfilled systems. I t can be seen from l?ig. 4 that the elongation at the tear point for filled A
BI
5O 8001 ~6 400 i 3O
2OO o
I0 I
40
Fro. 4
I
I
120 200 MOI.wLI Mx lO'a
40
I
120 2O0 MoLwf., M,10"a F1o. 5
Fia. 4. Dependence of the relative elongation in the tear test on the molecular weight of the elastomer: /--carbon black mixtures, 2--unfilled elastom~ers. FIG. 5. Dependence of the true breaking stress (1, 2) and the relative reinforcement (3) by DG-100 carbon black on the molecular weight of the polymer: /--carbon black mixtures, 2--unfilled elastomers, 3--relative reinforcement (by tear resistance). Ordinates: A--tear resistance, kg/cm; B--relative reinforcement by tear resistance, ~o. systems based on polyisobutylenes is considerably greater than the corresponding value for the pure polymers, and it increases with decrease in molecular Weight. The difference between the elongations at break of filled and unfilled systems indicates the formation of a spatial network, the nodal points of which are carbon black particles. Judging b y the elongation this network is more deformable in mixtures based on polyisobutylene P-85. A continuous network is evidently not formed in the cause of the butyI rubber, because the short molecules of this elastomer are not in a state t o form a link between neighbouring carbon particles. Therefore the elongation at the tear point for the butyl rubber and mixtures of the latter with carbon black are practically the same, though it is possible that if the quantity of active filler were increased the effect of linking of its particles would then be observed.
Adhesion of elastomers to powder fillers
1649
Systems based on polymers of different molecular weight differ markedly in deformability and this complicates comparison of these with respect to mechanical properties. The fact t h a t the elongation of P-85 mixtures exceeds t h a t of P-200 systems by a factor of 2.3 is an example of a particularly large difference in carbon-black filled systems. The mechanical properties of these systems must therefore be characterized by the true stress, which takes account of the change in cross-sectional area of the specimens as a result of deformation, and not by the nominal stress. Figure 5 shows the molecular weight dependence of the true breaking stress and elongation at the tear point for filled and unfilled systems. The true tear resistance of unfilled polyisobutylenes increases uniformly with increase in molecular weight. I n contrast to this the true tear resistance of filled polyisobutylene systems falls steadily with increase in molecular weight. The relative elongation falls even more steeply with increase in molecular weight. The fall in tear resistance and reinforcement is probably associated with decrease in the adhesion of the polyisobutylenes to the carbon-black particles as the molecular weight increases. In order to confirm this hypothesis experiments were conducted to find the effect of the molecular weight of polyisobutylenes on their adhesion to DG-100 carbon black. The same polymers t h a t were used for determination of tear resistance were used as the adhesives. The polymers were deposited from solution on the reinforcing fabric. The method of preparation of the substrate of
A 500
~.~800
300
"~ 800 0
I
I
8O /80 Mol.~..,MxlO-a FIG. 6
Z00 400
l I 500 8OO B
Fro. 7
FIG. 6. Dependence of resistance to peeling on the molecular weight of the elastomers. FIG. 7. Dependence of the relative reinforcement on the adhesion of elastomers to DG-100 carbon black. A - relative reinforcement, ~o; B--resistance to peeling. filler and the method of determination of adhesion were as described in reference [5]. The adhesive bonds were produced under conditions similar to the conditions of preparation of specimens for tear resistance tests. The time of contact was varied between 5 and 240 minutes.
1650
V.G.
RAYEVS~
e~ al.
The molecular weight dependence of the elastomers to DG-100 carbon black is shown in Fig. 6. As would be expected, increase in the molecular weight of the polymer brings about a marked reduction in its adhesion to carbon black. The fall in adhesion is probably due to decrease in the number of ends of molecules capable of diffusing from the bulk of the polymer to the interracial surfac~ and along the surface of the substrate. The curve of Fig. 6 gives a straight line when plotted on a logarithmic scale and this enables the analytical relationship between adhesion and molecular weight to be found: Ad=B.M
-k
where B and k are empirical constants (k=0.57). The identical nature of the influence of molecular weight on the reinforcing effect and on the adhesion of the polymer to the filler shows that there is some correlation between these characteristics. In order to find this correlation the relative reinforcement-adhesion relationship was plotted, and this is shown in Fig. 7 I t is seen from Fig. 7 that the dependence of the relative reinforcement on adhesion is represented by a straight line. Consequently there is a linear relationship between reinforcement and adhesion over the molecular weight range from 85 to 200 thousand. The polymer of low molecular weight butyl rubber, falls outside this relationship, which is of course a result of the different nature of the rupture, i.e. by flow of the polymer under an externM stress. Since flow in this system probably takes place in the polymer phase the polymer-filler adhesive bond is not affected and it does not affect the tear resistance. Therefore despite the high adhesion to carbon black the reinforcibility of this polymer is not high at the studied degree of filling. CONCLUSIONS
(1) A study has been made of the effect on the strength of SKN-40 vulcanizates, containing chalk as a filler, of addition to the rubber mixtures of various additives that alter the adhesion of this polymer to chalk. I t was found t h a t there is a correlation, graphically represented by a straight line, between the adhesion to chalk and reinforcement of the vulcanizates (as measured by tensile strength and tear resistance). (2) The effect of the molecular weight of the elastomer on the tearing of specimens of polyis0butylene and butyl rubber, and of mixtures of these with carbon black DG-100 was studied. I t is shown t h a t there is good correlation between reinforcement and adhesion to carbon black for polyisobutylenes of different molecular weight. (3) The relationships found in this work completely confirm the validity of the hypothesis that the adhesion of the elastomers to the surface of the filler particles determines the reinforcing effect in the rubbers. Translated by E . O. PHILLIPS
Copolymers of polyvinylalcohol
1651
REFERENCES 1. V. G. RAYEVSKII, S. M. YAGNYATINSKAYA, S. N. YEPISEYEVA a n d S. S. VOYUTSKII, Vysokomol. soyed. 7: 1504, 1965 (Translated in Polymer Science U.S.S.R. 7: 9, 1664, 1966) 2. S. M. YAGNYATINSKAYA, V. G. RAYEVSKII, L. S. FRUMKIN and S. S. VOYUTSKH, Vysokomol. soyed. 7: 1510, 1965 (Translated in Polymer Science U.S.S.R. 7: 9, 1671, 1966) 3. S. S. VOYUTSKH, V. G. RAYEVSKII and S. M. YAGNYATINSKAYA, K a u c h u k i rezina, No. 7, 16, 1964 4. G. S. WHITBY, C. C. DAVIS a n d R. F. DUNBROOK, Sinteticheskii kauchuk, gl. XV. (Synthetic Rubber, Chap. XV.) Goskhimizdat, Leningrad, 1957 (Russian translation) 5. S. S. VOYUTSKII, S. M. YAGNYATINSKAYA, L. S. FRU1KKIN, S. N. YEPISEYEVA and V. G. RAYEVSKII~ Zavod. lab., No. 10, 1222, 1964 6. A. TOBOLSKY, Svoistva i s t r n k t u r a polimerov. (The Properties and Structure of Polymers.) Izd. K h i m i y a , Moscow, 1964 (Russian translation) 7. A. B. TAUBMAN, S. N. TOLSTAYA, V. N. BORODINA and S. S. MIKHAILOV, Dokl. Akad. N a u k SSSR 142: 407, 1962 8. J. J. BIKERMAN and D. W. MARSHALL, J. Appl. Polymer Sci. 7: 1031, 1963; K h i m i y a tekhnol, polimerov, No. 5, 141, 1964 9. J. A. JANKO, J. P o l y m e r Sci. 3: 576, 1948 i0. A. S. NOV][KOV, M. B. K H A I K I N A , T. V. DOROKHINA and M. I. ARKHANGEL'SKAYA, Kolloid, zh. 15: 51, 1953 11. E. W A R R I C K and P. LAUTERBUR, Ind. Eng. Chem. 47: 486, 1955
SYNTHESIS OF COPOLYMERS OF POLYVINYLALCOHOL AND POLYMETHYLACRYLATE WITH GRAFTED CHAINS OF KNOWN LENGTH* t G. G. DANELYAN and R. M. LIVSHITS Moscow Textile Institute, All-Union External Institute of the Textile and Light Industries
(Received 25 June 1965)
ONE of the basic problems confronting research workers in the field of graft copolymers is the synthesis of graft copolymers in which the degree of polymerization (DP) and number of grafted chains are known, and study of the effect of these factors on the properties of the copolymers. * Vysokomol. soyed. 8: No. 9, 1501-1505, 1966. ~f 1st Communication in the series "Synthesis and s t u d y of the properties of block a n d graft copolymers of polyvinylalcohol".
1643
14. E. IWATSUKI, N. TAKIKAWA, M. OKADA, J. JAMASHITA and Y. ISHH, Kogyo kagaku zasshi 67: 1236, 1964 15. D. E. MeLAUGHI,IN, M. TAMRES and S. SEARLES, J. Amer. Chem. Soc. 82:5621 1960 16. T. J. CIOFFI and S. ZENCHELSKI, J. Phys. Chem. 67: 357, 1963 17. V. I. VEDENEYEV, L. V. GURVICH, V. N. KONDRAT'EV, V. A. MEDVEDEV and Ye. L. FRANKEVICH, Energiya razryva khimicheskikh svyazei. Spravoehnik. (Energy of Rupture of Chemical Bonds. Handbook.) Izd. Akad. Nauk SSSR, 1962 18. V. N. KONDRAT'EV and D. N. SOKOLOV, Zh. fiz. khim. 29: 1265, 1955 19. W. KERN, Chemiker-Ztg. 88: 623, 1964 20. A. ISHIGAKI, T. SHONO and J. HASHINOMA, Makromol. Chem. 79: 170, 1964 21. B. A. ROZENBERG, Dissertation, 1964 22. G. V. RAKOVA, L. M. ROMANOV and N. S. YENIKOLOPYAN, Vysokomol. soyed. 6: 2178, 1964 (Translated in Polymer Science U.S.R.R. 6: 12, 2410, 1965)
THE ADHESION OF ELASTOMERS TO POWDER FILLERS AND REINFORCEMENT OF FILLED SYSTEMS*t V. G. I~AYEVSKII, S. )/I. YAGNYATINSKAYA and S. S. VOYUTSKII M. V. Lomonosov Institute of Fine Chemical Technology, Moscow, Moscow Technological Institute of the Meat and Milk Industry (Received 5 June 1965)
CORRELATION between adhesion and reinforcement in rubber mixtures has been found previously with respect to change in the conditions of interaction of the rubber with the filler [1], increase in the degree of structure formation in the rubber [2] and chemical modification of the surface of the filler [3]. The present communication presents data on the effect on reinforcement of additives that increase or decrease the adhesion of the elastomer to the filler (adhesives or antiadhesives). The effect of the molecular weight of the polymer on the reinforcement of filled systems is also discussed. EFFECT ON REINFORCEMENT OF THE ADDITION OF SUBSTANCES ALTERING THE RUBBER-FILLER ADHESION
The a d d i t i o n to r u b b e r m i x t u r e s o f certain organic substances is m a d e use o f t o increase reinforcement, b u t m o s t often t h e y are used in c o n j u n c t i o n with mineral fillers. A n i m p o r t a n t p o i n t is t h a t w h e n these additives are used t o g e t h e r with fillers the reinforcing effect is greater t h a n w h e n t h e y are used s e p a r a t e l y [4]. * Vysokomol. soyed. 8: No. 9, 1493-1500, 1966. t 3rd Communication in the series "Reinforcement of polymers".
1644
V. G. RAYEVSKIIet al.
So far no explanation of this has been given. I t m a y be supposed however t h a t the positive role of these additives consists in increase in the adhesion of the polymer to the particles of the filler. In this investigation the materials studied were vulcanizates of the butadiene-acrylonitrile elastomer SKN-40, either unfilled or containing 20 parts by volume of chalk (per 100 parts by volume of rubber). Five parts by volume of one or other of the following substances were added to the mixtures from which the vulcanizates were prepared: stearic acid, Rubrax, rosin, an indene-coumarone resin or a 1 : 1 mixture of rosin and the indene-coumarone resin. The vulcanization temperature was 143 ° and the time from 45 to 60 minutes, depending on the type of additive. The physieo-mechanical characteristics of the filled and unfilled mixtures, with and without the additives and vulcanizated to the opitmal degree, are presented in the Table. I t is seen from the Table t h a t the strength of unfilled vulcanizates containing an additive is always lower t h a n the strength of the vulcanizates without these additives. I n contrast to this the strength of filled vulcanizates is increased when adhesives are added to the system and decreased by the addition of anti-adhesives (stearic acid). The positive effect of adhesives on the strength of filled vulcanizates together with their negative effect in unfilled rubbers can be attributed onE F F E C T OF A D D I T I O N OF A D H E S I V E AND A N T I - A D H E S I V E S ON T H E PHYSICOM~CHAN-ICAL CHARACTERISTICS OF S K ~ - 4 0
Additive
Without additive Stearic acid Rubrax Rosin Indene-coumarone resinrosin (1 : 1) Indene-coumarone resin Without additive Stearic acid Rubrax Rosin : 1) Indene-cotunarone resin-rosin (1 Indene-coumarone resin
Tensile strength, kg/cmz
VULCA.~IZATES
Relative elongation,
Residual elongation,
%
%
kg/cm
Unfilled vuleanizates 140 780 120 770 112 700 112 780
12 16 15 22
13'0 12"8 13"1 13"1
18
105 110
Tear resistance,
750 740
16
13"1 13"2
FiHed vulcanizates 95 800 85 700 95 700 113 790 130 790 140 750
24 22 24 30 26 20
17"0 17"0 17"7 20"5 23"0 26"0
ly to increase in the elastomer-fiUer adhesion. I f this assumption is correct the increase in strength of filled vulcanizates caused by adhesive additives should be greater in proportion to increase in the extent to which the additive improve
Adhesion of elastomers to powder fillers
1645
the adhesion of the elastomer to the filler. I n other words there must be a correlation between the efficiency of additives promoting reinforcement and their ability to increase the elastomer-filler adhesion. I n order to confirm this supposition experiments were set up to determine the resistance of unfilled vuleanizates (with and without additive) to peeling from a substrate of chalk, b y the m et hod developed and described in reference [5]. I n these experiments the conditions for formation of the adhesive bond were identical with the conditions of preparation of specimens for physicomeehanical tests. As can be seen from Fig. 1, correlation was in fact found between the adhesion of vulcanizates containing an additive to the filler (chalk) and their tensile strength and tear resistance. Moreover the relationship is represented satisfactorily b y a straight line regardless of the t y p e of test (tensile or tear). Despite the correlation between adhesion and the absolute values of t he tensile strength and tear resistance it was felt t h a t it would be of interest to find the correlation between adhesion and relative reinforcement. This correlation is 2
e,l
~
m
~26 ~ 22
d
14
400 600 Resi~anee to peeling, 9s/crn I
200
FIG. 1
I
I
I
o
200
I
I
4~
600 Re~i~tancetopeel/ng , gs/cm FIO. 2
FIG. 1. Dependence of tensile strength (1) and tear resistance (2) of SKN-40 vulcaniza~es with chalk filler on their adhesion to the filler. Additives used: a--stearie acid, b--no additive, c -- Rubrax, d-- rosin, e -- rosin ~ indene -coumarone resin, f-- indene -coumarone resin. Fro. 2. Dependence of the reinforcement of SKN-40 vulcanizates by chalk according to tensile strength (1, 1') and tear resistance (2) on their adhesion to the filler: /--without allowance for the effect of the additive on the strength of the unfilled vulcanizate; 1--with allowance for this effect. Lettering as in Fig. 1. illustrated in Fig. 2. Here curve 2, relating adhesion to reinforcement as measured b y tear resistance shows r at her less scatter of the points and a greater slope t h a n curve 1, which relates adhesion to reinforcement according to tensile strength. I n view of the fact t h a t adhesive and anti-adhesives alter the strength of unfilled vulcanizates it was of interest to eliminate this effect from the relationship. This can be done b y put t i ng the breaking stress of the unfilled vulcanizates
1646
V.G.
RAYEVSKII et al.
containing the adhesive or anti-adhesive into the formula for calculating relative reinforcement, instead of the breaking stress of the vulcanizates containing neither a filler nor additive. The correlation found in this was is represented in Fig. 2 by curve 1. The slope of this line is a little greater than the slope of curve 1, and differs little from that of curve 2, relating adhesion to relative reinforcement according to tear resistance. A similar curve relating to tear resistance coincides with curve 2, since the additives have practically no effect on the tear resistance of unfilled vulcanizates. Thus the addition of adhesive additives brings about an increase in the strength of filled rubbers. The reinforcing effect is dependent on the nature of the additive used. I t is evident that only a substance t h a t has some compatibility with the polymer and creates between the polymer and filler a stronger bond than that existing in this absence, increases the strength of a filled system. From the example of the substances used in this work one can find how these requirements are fulfilled. The similarity of the solubility parameters of the rubber (6sx~.40~9.83 [kcal/cm3]½) and the above additives can be used as a measure of compatibility. The solubility parameters of stearic acid, Rubrax, rosin and the indene-coumarone resin can be found from the solubility parameters of solvents in which these substances have good solubility [6]. From our comparison of the solubility parameters of SKN-40 and the given additives it was found that the indene-coumarone resin should have the greatest compatibility with the rubber. This is in complete accord with the positive effect of the resin on the adhesion of the rubber to chalk and on the strength of the filled vulcanizate. Stearic acid has the least compatibility with the rubber of all the additives, as comparison of its solubility parameter with that of SKN-40 shows. I t is very probable that the excess fatty acid accumulates at the rubber-filler interfacial boundary. Since the quantity of the acid added is sufficient to form a polymolecular layer on the surface of the filler particles the existence of this intermediate layer must have a negative effect on the strength of the polymerfiller bond, and consequently on the strength of the filled system. This was in fact found in the present work and is in good agreement with the results of a number of authors, which show that stearic acid is not an active filler [7, 8]. The efficiency of additives as activators for fillers depends of course not only on the nature of the additive, but also on the type of filler, the quantity of the latter present in the polymer, and also on the nature of the polymer. I t m a y be assumed, a priori, that the effectiveness of adhesive additives will be greater the lower the strength of the polymer-filler bond in the original system. EFFECT OF THE MOLECULAR WEIGHT OF THE POLYMER ON REINFORCEMENT OF RUBBER MIXTURES
The depedence of the strength of filled systems on t h e m o l e c u l a r weight of the polymer has been studied in a number of papers [9-11]. Unfortunately, in the papers cited the curves expressing the dependence of strength on molecular
Adhesion of elastomers to powder fillers
1647
weight for filled systems were not compared with corresponding curves for the unfilled systems. In this section of the work this omission was rectified and in addition an attempt was made to find the relationship between the molecular weight of the polymer and its adhesion to the filler. The materials used for this investigation were the linear, nonpolar polymers, polyisobutylenes P-85, P-118 and P-200, with molecular weights in the region of 85, 118 and 200 thousand, and butyl rubber of molecular weight 36 thousand and a degree of unsaturation of 0.99%, and also unvulcanized mixtures of these with 20 parts by volume of carbon black DG-100 per 100 parts by volume of polymer. The tear resistance of plates made from the untreated rubbers and the carbon-black mixtures was measured according to the State Standard Specification GOST 263-53, because this characteristic should correspond best to the adhesion of the polymer found by the peeling method. In the tear resistance tests the stress and deformation during stretching were recorded up to the moment of breakdown of the specimen. The dependence of the nominal stress on the elongation of the polymers and their carbon-black mixtures is shown in Fig. 3. 8 CI
6
cvj
4
I
0
200
I~1 0
200
I 400
6'00
II :800
Defo/,mation , %
FIG. 3. Stress-strain curves of polymers (a) and their mixtures with I)G-100 carbon black (b): l - - P - 2 0 0 , 2--P-118, 3--P-85, 4 - - b u t y l rubber.
On examination of Fig. 3a one's attention is drawn to the different nature of the stress-deformation curve for the butyl rubber in comparison with the polyisobutylenes. The shape of the curves, and also the specimens themselves after rupture, lead to the conclusion that the specimens of the low molecular weight butyl break as a result of viscous flow, whereas elastic rupture mainly occurs in the polyisobutylene specimens. The polyisobutylenes, as would be expected, fall into the following order with respect to decrease in stress and relative elongation P-200>P-118>P-85.
i648
V.G. RAY~vsF~I~et al.
Addition of carbon black to the butyl rubber does not alter the nature of the stress-strain curve, though the maximal stress (yield point) is then somewhat higher (Fig. 3b). The stress-strain curves of the mixtures of polyisobutylene with carbon black are clearly S-shaped. This indicates that reinforcement occurs before breakdown of the specimens. I t is well k n o w n that reinforcement is characteristic also in the rupture of vulcanizates. The disposition of the curves with respect to breaking stress is similar to that for the unfilled systems. I t can be seen from l?ig. 4 that the elongation at the tear point for filled A
BI
5O 8001 ~6 400 i 3O
2OO o
I0 I
40
Fro. 4
I
I
120 200 MOI.wLI Mx lO'a
40
I
120 2O0 MoLwf., M,10"a F1o. 5
Fia. 4. Dependence of the relative elongation in the tear test on the molecular weight of the elastomer: /--carbon black mixtures, 2--unfilled elastom~ers. FIG. 5. Dependence of the true breaking stress (1, 2) and the relative reinforcement (3) by DG-100 carbon black on the molecular weight of the polymer: /--carbon black mixtures, 2--unfilled elastomers, 3--relative reinforcement (by tear resistance). Ordinates: A--tear resistance, kg/cm; B--relative reinforcement by tear resistance, ~o. systems based on polyisobutylenes is considerably greater than the corresponding value for the pure polymers, and it increases with decrease in molecular Weight. The difference between the elongations at break of filled and unfilled systems indicates the formation of a spatial network, the nodal points of which are carbon black particles. Judging b y the elongation this network is more deformable in mixtures based on polyisobutylene P-85. A continuous network is evidently not formed in the cause of the butyI rubber, because the short molecules of this elastomer are not in a state t o form a link between neighbouring carbon particles. Therefore the elongation at the tear point for the butyl rubber and mixtures of the latter with carbon black are practically the same, though it is possible that if the quantity of active filler were increased the effect of linking of its particles would then be observed.
Adhesion of elastomers to powder fillers
1649
Systems based on polymers of different molecular weight differ markedly in deformability and this complicates comparison of these with respect to mechanical properties. The fact t h a t the elongation of P-85 mixtures exceeds t h a t of P-200 systems by a factor of 2.3 is an example of a particularly large difference in carbon-black filled systems. The mechanical properties of these systems must therefore be characterized by the true stress, which takes account of the change in cross-sectional area of the specimens as a result of deformation, and not by the nominal stress. Figure 5 shows the molecular weight dependence of the true breaking stress and elongation at the tear point for filled and unfilled systems. The true tear resistance of unfilled polyisobutylenes increases uniformly with increase in molecular weight. I n contrast to this the true tear resistance of filled polyisobutylene systems falls steadily with increase in molecular weight. The relative elongation falls even more steeply with increase in molecular weight. The fall in tear resistance and reinforcement is probably associated with decrease in the adhesion of the polyisobutylenes to the carbon-black particles as the molecular weight increases. In order to confirm this hypothesis experiments were conducted to find the effect of the molecular weight of polyisobutylenes on their adhesion to DG-100 carbon black. The same polymers t h a t were used for determination of tear resistance were used as the adhesives. The polymers were deposited from solution on the reinforcing fabric. The method of preparation of the substrate of
A 500
~.~800
300
"~ 800 0
I
I
8O /80 Mol.~..,MxlO-a FIG. 6
Z00 400
l I 500 8OO B
Fro. 7
FIG. 6. Dependence of resistance to peeling on the molecular weight of the elastomers. FIG. 7. Dependence of the relative reinforcement on the adhesion of elastomers to DG-100 carbon black. A - relative reinforcement, ~o; B--resistance to peeling. filler and the method of determination of adhesion were as described in reference [5]. The adhesive bonds were produced under conditions similar to the conditions of preparation of specimens for tear resistance tests. The time of contact was varied between 5 and 240 minutes.
1650
V.G.
RAYEVS~
e~ al.
The molecular weight dependence of the elastomers to DG-100 carbon black is shown in Fig. 6. As would be expected, increase in the molecular weight of the polymer brings about a marked reduction in its adhesion to carbon black. The fall in adhesion is probably due to decrease in the number of ends of molecules capable of diffusing from the bulk of the polymer to the interracial surfac~ and along the surface of the substrate. The curve of Fig. 6 gives a straight line when plotted on a logarithmic scale and this enables the analytical relationship between adhesion and molecular weight to be found: Ad=B.M
-k
where B and k are empirical constants (k=0.57). The identical nature of the influence of molecular weight on the reinforcing effect and on the adhesion of the polymer to the filler shows that there is some correlation between these characteristics. In order to find this correlation the relative reinforcement-adhesion relationship was plotted, and this is shown in Fig. 7 I t is seen from Fig. 7 that the dependence of the relative reinforcement on adhesion is represented by a straight line. Consequently there is a linear relationship between reinforcement and adhesion over the molecular weight range from 85 to 200 thousand. The polymer of low molecular weight butyl rubber, falls outside this relationship, which is of course a result of the different nature of the rupture, i.e. by flow of the polymer under an externM stress. Since flow in this system probably takes place in the polymer phase the polymer-filler adhesive bond is not affected and it does not affect the tear resistance. Therefore despite the high adhesion to carbon black the reinforcibility of this polymer is not high at the studied degree of filling. CONCLUSIONS
(1) A study has been made of the effect on the strength of SKN-40 vulcanizates, containing chalk as a filler, of addition to the rubber mixtures of various additives that alter the adhesion of this polymer to chalk. I t was found t h a t there is a correlation, graphically represented by a straight line, between the adhesion to chalk and reinforcement of the vulcanizates (as measured by tensile strength and tear resistance). (2) The effect of the molecular weight of the elastomer on the tearing of specimens of polyis0butylene and butyl rubber, and of mixtures of these with carbon black DG-100 was studied. I t is shown t h a t there is good correlation between reinforcement and adhesion to carbon black for polyisobutylenes of different molecular weight. (3) The relationships found in this work completely confirm the validity of the hypothesis that the adhesion of the elastomers to the surface of the filler particles determines the reinforcing effect in the rubbers. Translated by E . O. PHILLIPS
Copolymers of polyvinylalcohol
1651
REFERENCES 1. V. G. RAYEVSKII, S. M. YAGNYATINSKAYA, S. N. YEPISEYEVA a n d S. S. VOYUTSKII, Vysokomol. soyed. 7: 1504, 1965 (Translated in Polymer Science U.S.S.R. 7: 9, 1664, 1966) 2. S. M. YAGNYATINSKAYA, V. G. RAYEVSKII, L. S. FRUMKIN and S. S. VOYUTSKH, Vysokomol. soyed. 7: 1510, 1965 (Translated in Polymer Science U.S.S.R. 7: 9, 1671, 1966) 3. S. S. VOYUTSKH, V. G. RAYEVSKII and S. M. YAGNYATINSKAYA, K a u c h u k i rezina, No. 7, 16, 1964 4. G. S. WHITBY, C. C. DAVIS a n d R. F. DUNBROOK, Sinteticheskii kauchuk, gl. XV. (Synthetic Rubber, Chap. XV.) Goskhimizdat, Leningrad, 1957 (Russian translation) 5. S. S. VOYUTSKII, S. M. YAGNYATINSKAYA, L. S. FRU1KKIN, S. N. YEPISEYEVA and V. G. RAYEVSKII~ Zavod. lab., No. 10, 1222, 1964 6. A. TOBOLSKY, Svoistva i s t r n k t u r a polimerov. (The Properties and Structure of Polymers.) Izd. K h i m i y a , Moscow, 1964 (Russian translation) 7. A. B. TAUBMAN, S. N. TOLSTAYA, V. N. BORODINA and S. S. MIKHAILOV, Dokl. Akad. N a u k SSSR 142: 407, 1962 8. J. J. BIKERMAN and D. W. MARSHALL, J. Appl. Polymer Sci. 7: 1031, 1963; K h i m i y a tekhnol, polimerov, No. 5, 141, 1964 9. J. A. JANKO, J. P o l y m e r Sci. 3: 576, 1948 i0. A. S. NOV][KOV, M. B. K H A I K I N A , T. V. DOROKHINA and M. I. ARKHANGEL'SKAYA, Kolloid, zh. 15: 51, 1953 11. E. W A R R I C K and P. LAUTERBUR, Ind. Eng. Chem. 47: 486, 1955
SYNTHESIS OF COPOLYMERS OF POLYVINYLALCOHOL AND POLYMETHYLACRYLATE WITH GRAFTED CHAINS OF KNOWN LENGTH* t G. G. DANELYAN and R. M. LIVSHITS Moscow Textile Institute, All-Union External Institute of the Textile and Light Industries
(Received 25 June 1965)
ONE of the basic problems confronting research workers in the field of graft copolymers is the synthesis of graft copolymers in which the degree of polymerization (DP) and number of grafted chains are known, and study of the effect of these factors on the properties of the copolymers. * Vysokomol. soyed. 8: No. 9, 1501-1505, 1966. ~f 1st Communication in the series "Synthesis and s t u d y of the properties of block a n d graft copolymers of polyvinylalcohol".