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The relation between wear and the thermochemical stability of polymers
THE RELATION
BETWEEN WEAR AND THE THERMOCHEMICAL STABILITY OF POLYMERS * S. B. RATNER and YE. G. LUR’IE Scientific
Research
Institute
(Received 16 Februnry
of Plastics
1965)
WEAR is a complex process of surface degradation which, according to [l, 21, can be divided into the following stages: 1) contact deformation of the surface; 2) repeated redeformation of the surface layer by projections on the counter body, resulting in a change in the properties of this layer; 3) failure of the layer followed by detachment of the abrased particles. While the first stage is mainly a mechanical process, in the second and third physicochemical effects must be very important, particularly for polymeric materials, which have a tendency to mechanochemical degradation. The present article considers the wear of polymeric materials in the light of those physicochemical changes with which the process and rate of wear is connected. Mechanism of we& and mechunochemical degradation. As with other solids [l], the wear of polymeric materials [3-51 is usually the result of repeated surface deformation by projections on the counterbody. This we call fatigue wear. In the particular case where the number of such acts is not very great (one or more), it is called abrasive wear. A tendency to fatigue or abrasive wear may appear under a variable load. The relation between wear 1 and load P is given by Cf3,71: I=I,P’, (1) where 1, is the wear at the load taken as the unit (P= 1 kg/cm2), a is a constant of the material in friction with a given counterbody, and is specific for fatigue wear. It is symbatic with the number of acts required for the surface to fail as a result of friction: cr=l+pG=1+3/3 log ni,s. (2) where p is the surface characteristic of the counterbody; 6 characterizes the resistance of the material to fatigue; it is the same for volume or contact fatigue [5] and is definitely related with n,,s the number of fatigue cycles required to halve the original strength [3]. The minimum value a= 1 is observed in abrasive wear [8]. In [9] it was found that in elastic polymers the tendency to fatigue wear was aggravated by an increase in the interaction forces in the material for which the characteristic is the value nllz. Moreover, in agreement with formula (2), a rise in a is always observed [3, lo]. * Vysokomol.
soyed. 8: No:l,
88-93,
1966. 93
S. B. RATNER and YE. G. LUR’IE
94
One way of extending the fatigue life of polymers is by means of special additives, antifatigue agents. These are antioxident in nature, and are found to prolong the fatigue life. of a polymer [ll]. In fact, in [lo] it was found that the addition of stabilizers to rubber lead to a rise in the characteristic 6. Then, according to formula (2), the OLvalue should also increase. This can be seen from Table 1, which compares 6 and LXfor the vulcsnizate SKN-26 with different stabilizers. * TABLE
1.
INFLUENOE
OF
STABILIZA-
TIONONTEEFATIGUIZFAILURE ACTERISTIO ISTIO
6 AND
a FOR
CEAR-
WEAR
CHARACTER-
CARBON-FILLED
ON
SKN-26
RUBBER
BASE
Stabilizer No stabilizer 4010 NONOX
a
6
2.1 2.9 3.4
29 42 49
Unfortunately, the effect of stabilizers on fatigue of plastics has not been studied. But antifatigue additives must be sought for them among the usual thermo-stabilizers, bearing in mind the following arguments. According to the fluctuation theory of strength [12], failure is due to thermal movement and load only reduces the activation barrier, lowering the actual lifetime to a value commensurate with the experimental time; then failure is mainly due to the breakdown of chemical bonds. It is suggested that the same TABLE
2.
COMPMtX3ON
OF
THE
BEHAVIOUR
OF
VISCOSITY
Viscosity Polymer
before wear, fl0
IN
WEAR
WITH
VARIATION
OF
characteristic after wear on abraon mesh, sive cloth,
‘lsc--flm am-%2 VO
rim ‘taC
Polyamide 68 High-pressure polyethylene Polyvinyl chloride Polycarbonate Polystyrene
0.9 1.2 0.8 0.6 0.85
0.6 1.0 0.7 0.5 0.5
0.4 0.9 0.6 0.5 0.5
0.2 0.1 0.1 0 0
mechanism also holds for wear, although it is a more complex form dation. The following experimental results support this proposition. * Results
obtained
in cooperation
with
G. S. Klitenik:
1.0 0.9 0.6 0.2 0.1
of degra-
c(
Thormomechanical
stability
of polymers
a) In wear the temperature dependence of the degradation to a relation [13, 141 analogous to that of static failure [12]: I= col~st. ,-cu.-BP)lRT,
96
time is subject
(3)
where RT is the energy of thermal movement, U, and B are constants. b) During wear there is mechanochemical degradation of the polymer. This can be seen from Table 2, which shows that the viscosity of the products of wear (crumbs) is less than for the initial specimen. Molecular breakdown certainly takes place in both abrasive wear (on an abrasive cloth, i.e. as a result of sharp projections), and in fatigue wear (on metal mesh, i.e. by blunt projections). Moreover, in the case of fatigue wear mechanochemical processes must be the more important since they are operative not only in the detachment of particles, but also in the surface fatigue which promotes failure. With friction against blunt projections the viscosity of the polymer crumbs is usually less than with sharp projections. But if the material is brittle (polystyrene, polycarbonate) then its wear is mainly abrasive, whether abrasive cloth or mesh is in question [7, 151, which means that there is only a very small difference ~,-a,, for the two surfaces (Table 2). For these two polymers therefore, we find no difference in the viscosities of crumbs after wear against mesh or abrasive cloth. Not only this, the opposite effect may even arise, since the size of the detached particles will affect the molecular weight of the polymer for the same mechanism; the larger the wear particles, the smaller their surface and consequently, the lower the degree of mechanical degradation. This is what we found in an earlier work [16] for the two polymers mentioned. Thus wear is an activation-kinetic process in which degradation is the result of the breakdown of chemical bonds activated by mechanical energy. Ageing also involves the breakdown of chemical bonds, but due to other factors (light, heat, 0, ans so on). In a mechanical field therefore, the ageing of elastomers has lower activation energy [17]. But in this case, the enhanced chemical stability of the polymer should also affect its lifetime, particularly its wear resistance. The first thing to be expected here is an increase in the a value, because it reflects the tendency to a fatigue mechanism of wear. The existence of this mechanism opens up the possibility of affecting its characteristics by the addition of antifatigue agents, while the mechanochemical nature of fatigue seggests that such additions should be sought among the usual thermo-stabilizers. Let us consider the experimental results. Stabilization and wear of plastics. The role of stabilization has been studied for a number of crystalline antifriction materials in wear against a metal mesh. Under these conditions the tendency to a fatigue mechanism is greatest, which means that the OLvalue is much more than unity for all the materials. In view of the non-linear 1 vs. P relation in fatigue wear (see formula (l)), in Table 3 we have used the two values I1 and u as the wear characteristics of
S. B. RATNER and YE. G. LUE’IE
96 TABLE
3. WEAR CHARACTERISTICS
-
FOR
PLASTICS
WITH
.
Polymer
Stabilizer
a
-
DIFFERENT
T Aa
Wear on -7
mesh, mm31 /m. cma
I’,
Polyformaldehyde
No stabilizer Santovar 0 Nitroxide radical Two-ring phenol Diphenylamine Polypropylene No stabilizer Topanol Topanol +pigment Polyamide AK-7 r No stabilizer NFD ,?-Naphthylpyrocatechinphosphite Polyamide 68 No stabilizer NFD Neozone D Antracene I l-Naphthylpyrocatechinphosphitc Ionolpyrocatechinphosphite 4,6-Salicyl-di-tert.butyl-4-methylphenylphosphite 2,4,6-Salicyl-tri-tert.bulylphenyl phosphite
-
STABILIZERS
abrasive cloth, mm3/ /m f cm2
3.1 4.9 4.4 3.1 3.1 1.4 2.0 2.6 1.3 2.2
-
0.1
1.8 1.3 0 0 OTI 1.2 0.9
0.001 0.004 0.04 0.1 3.2 3.0 1.0 0.9 0.09
2.2 2.0 2.4 2.2 2.0 2.0 2.0
0.9 0.4 0.2 0 0 0
0.07 0.2 0.004 0.04 0.12 0.2 0.2
3 5 1.8 1.4 3 5 5
1.1 1.1 1.2 1.1 -
2.0
0
0.2
5
-
2-o
0
0.2
5
-
10 2 7 5 10 27 80 GG 10 4
4.3 4.4 4.5 4.4 4.0 1.3 1,1
--
plastics. In contrast to OL,which is a constant of the material, I, is a conditional characteristic; the comparative wear of different materials differs under different loads; for comparison, Table 3 gives the value I, (wear at P=5 kg/cm2). Besides this, to show the role of stabilizers* in abrasive wear (CI= 1) we have given the value I,, for wear on an abrasive cloth at P=l kg/cm2. Polyformaldehyde. Polyformaldehyde (PFA) is usually stabilized by the addition of polyamide and radical-type additives. It is practically impossible to prepare specimens from completely unstabilized PFA, for which reason the control batch in our experiments is semi-stabilized PFA in which the only additive is polyamide. It can be seen from Table 3 that the addition of 2,5-di-tert. butylhydroquinone (santovar 0) and the stable nitroxide radical, which are the * The following assisted formaldehyde, B. B. Pashenin the polyamide.
in stabilizing the materials: V. V. Gur’yanova for the polythe polypropylene, I. I. Levantovskaya and P. M. Tanunina
Thormomechanical stability of polymors
97
most effective stabilizers [18, 191, gives a sudden rise in u. Although it does not alter rx, the addition of 2-ring phenol improves the wear resistance of PFA, and only diphenylamine has practically no effect on the wear characteristic.
,x-x-. 00,?5x!
4Lx, ---x-
-1
I
OW Effect of ageing PFA in W
600
77me, days
light (I, 2) and with heat (3, 4) on the wear (I, 3) and tensile strength (2, 4).
PFA with the addition of the more effective antifatigue agent santovar (4 was also subjected to wear after heat and light ageing. The a value showed no change as compared with the initial specimen before ageing, and therefore the materials must be compared from the I, value. It is evident from the Figure that on exposure to UV radiation wear increases with the ageing time. But in the case of heat ageing there is little change in the wear, and it even diminishes in a certain range (curve 3). Approximately the same kind of relations were obtained for simple mechanical properties also, particularly tensile strength (curves 2, 4).* We can see that, although it is not effective for light ageing, santovar 0 is a good thermo-stabilizer and simultaneously an antifatigue agent. Polypropylene. The addition of the stabilizer topanol to polypropylene also raises the a value. It is interesting to note that an ever greater effect (da=l.2) is obtained if not only a radical-type stabilizer but also a pigment is added, the latter stabilizing the supermolecular structure of polypropylene. Polyamide AK-7. Thermo-stabilizers added to polyamide AK-7 also cause a considerable rise in the CIvalue; Dora 1. PoZyamide 68. Of the number of stabilizers added to polyamide 68, which are shown in Table 3, three stand out; these are neozone D, di-P-naphthyl-pphenylenediamine (NPD) and heat-treated anthracene, while the rest have no effect in wear. The stabilizers indicated have an action which consists mainly * Results obtained in collaboration with V. V. Gur’yanove.
98
S.B.
RATNER and
YE. G.LUR'IE
in raising the wear resistance of polyamide 68, the a value showing little change. In the case of the first two stabilizers, which are particularly effective in thermooxidative degradation [20], there is also a slight rise in cx;Aa=O*2-0.4. Of course, if the wear is abrasive then the addition of stabilizers will have no effect because the process of the surface layer degradation is such a short one that the preparation stage practically disappears, contact deformation (the first stage) going over directly to microcutting (the third stage). When additives are used, control of the radical processes is achieved during the intermediate stage, the fatigue of the surface layer. The absence of this stage in abrasive wear thus rules out the possibility of this type of additive affecting the wear process. This can also be seen from Table 3, which shows that wear on an abrasive cloth (a=l) does not depend on the presence or absence of additions, although mechanochemical degradation takes place in this kind of wear (Table 2). The authors are grateful to M. S. Akutin, K. N. Vlasova, V. V. Gur’yanova, V. V. Kovrige, G. S. Klitenik, B. M. Kovarskaya, I. I. Levantovskaya, B. I. Pashenin and P. M. Tanunina for presenting the materials and discussing the results. CONCLUSIONS
(1) Mechanochemical degradation always takes place in the wear of polymers. (2) If the wear is abrasive it cannot be reduced by adding stabilizers since the surface layer fatigue stage is absent. (3) Only in fatigue wear does the addition of thermo-stabilizers raise the characteristic CC,which gives the fatigue resistance of the material or, if CIremains unchanged, increase the wear resistance. (4) Together with the experimental data, the analysis of wear as a fatigue process indicates the possibility of simultaneously a.djusting the fatigue life and wear resistance of polymeric materials by increasing their chemical stability. Translated
by
V.
A.WORD
REFERENCES 1. I. V. KRAGEL’SKII, Trenie i iznos. (Friction and Wear.) Mashgiz, 1962 2. S. B. RATNER, Friktsionnyi iznos rezin. (Friction Wear of Rubbers.) Izd. “Khimiya”, 31, 1964 3. S. B. RATNER, Dokl. Akad. Nauk SSSR 150: 848, 1963 4. M. M. REZNIKOVSKII, Rauchuk i rezina, 9, 33, 1960 5. I. V. KRAGEL’SW and Ye. F. NEPOMNYASHCHII, Izv. Akad. Nauk SSSR, Mekhanika: 5, 190, 1963 6. S. B. RATNER and G. S. KLITENIK, Zavodskaya lab., 11, 1375, 1959 7. S. B. RATNER and I. I. FARBEROVA, Plast. 9, 61, 1960 massy, 8. S. B. RATNER, Dokl. Akad. Nauk SSSR 144: 327, 1962 9. G. S. KLITENIK and S. B. RATNER, Kauchuk i rezina 3, 19 1960 10. S. B. RATNER, G. S. KLITENIK and Ye. 6. LUR’IE, Teoriya treniya i iznosa. (Theory Friction and Weas.) Izd. “Nauka”, 156, 1965
of
Macromolecular
orientation
indices
99
11. I,. G. ANGERT and A. S. KUZ’MINSKII, Rol’ i primen. antioksidantov v kauchukakh i rezinakh. (Role and Application of Antioxidants in Rubbers and Vulcanizates.) Goskhimizdat, 1957 12. S. N. ZHURKOV, Vestnik Akad. Nauk SSSR, 11, 78, 1957 13. S. B. RATNER, Dissertation, 1964 14. S. B. RATNER and Ye. G. LUR’IE, Dokl. Akad. Nauk SSSR, 166, 151, 1966 15. S. B. RATNER, Dokl. Akad. Nauk SSSR 135: 294, 1960 16. E. G. LUR’IE and S. B. RATNER, Plast. massy, 11, 47, 1962 17. A. S. KUZ’MINSKII, N. N. LEZHNEV and Yu. S. ZUYEV, Okislenie kauchukov i rezin. (Oxidation of Rubbers and Vulcanizates.) Goskhimizdat, 1957 18. V. R. ALISHOYEV and M. B. NEIMAN et al., Vysokomol. soyed. 5: 644, 1963 19. B. M. KOVARSKAYA, M. B. NEIMAN and V. V. GUR’YANOVA el al., Vysokomol. soyed. 6: 1737, 1964 20. I. I. LEVANTOVSKAYA, Dissertation, 1964
CORRELATION OF THE MACROMOLECULAR INDICES IN FIBRES ACCORDING TO X-RAY MEASUREMENT * A. SH. Go-, Kiev
Branch
L. A. OSININA, S. G. Osm of the All-Union
Scientific
Research
(Received 17 February
Institute
ORIENTATION AND ACOUSTIC
and M. P. Nosov of Synthetic
Fibres
1965)
V~IOUS methods are used to assess the molecular chain orientation in fibres; these are birefringence, IR-dichroism, and X-ray diffraction analysis. Recently it has been found possible to evaluate the mean molecular orientation from the velocity of a sound wave passing through a fibre. It is interesting to establish the correlation between the orientation indices determined by different methods. This is quite a complicated problem since it is difficult to find a single measure of orientation for all these methods. By X-ray diEraction analysis it is possible to assess the spread in the orientation of crystallites by measuring the azimuthal half-width of reflections. The birefringence method assesses the mean statistical orientation of macromolecules in amorphous and crystalline regions of the polymer, so long as the latter is a two-phase system. The criterion of orientation used is the factor introduced by Hermans [l]: 3&=ly
* Vysokomol.
soyed. 8: No. 1, 94-97,
sin2p.
1966.
(1)
BETWEEN WEAR AND THE THERMOCHEMICAL STABILITY OF POLYMERS * S. B. RATNER and YE. G. LUR’IE Scientific
Research
Institute
(Received 16 Februnry
of Plastics
1965)
WEAR is a complex process of surface degradation which, according to [l, 21, can be divided into the following stages: 1) contact deformation of the surface; 2) repeated redeformation of the surface layer by projections on the counter body, resulting in a change in the properties of this layer; 3) failure of the layer followed by detachment of the abrased particles. While the first stage is mainly a mechanical process, in the second and third physicochemical effects must be very important, particularly for polymeric materials, which have a tendency to mechanochemical degradation. The present article considers the wear of polymeric materials in the light of those physicochemical changes with which the process and rate of wear is connected. Mechanism of we& and mechunochemical degradation. As with other solids [l], the wear of polymeric materials [3-51 is usually the result of repeated surface deformation by projections on the counterbody. This we call fatigue wear. In the particular case where the number of such acts is not very great (one or more), it is called abrasive wear. A tendency to fatigue or abrasive wear may appear under a variable load. The relation between wear 1 and load P is given by Cf3,71: I=I,P’, (1) where 1, is the wear at the load taken as the unit (P= 1 kg/cm2), a is a constant of the material in friction with a given counterbody, and is specific for fatigue wear. It is symbatic with the number of acts required for the surface to fail as a result of friction: cr=l+pG=1+3/3 log ni,s. (2) where p is the surface characteristic of the counterbody; 6 characterizes the resistance of the material to fatigue; it is the same for volume or contact fatigue [5] and is definitely related with n,,s the number of fatigue cycles required to halve the original strength [3]. The minimum value a= 1 is observed in abrasive wear [8]. In [9] it was found that in elastic polymers the tendency to fatigue wear was aggravated by an increase in the interaction forces in the material for which the characteristic is the value nllz. Moreover, in agreement with formula (2), a rise in a is always observed [3, lo]. * Vysokomol.
soyed. 8: No:l,
88-93,
1966. 93
S. B. RATNER and YE. G. LUR’IE
94
One way of extending the fatigue life of polymers is by means of special additives, antifatigue agents. These are antioxident in nature, and are found to prolong the fatigue life. of a polymer [ll]. In fact, in [lo] it was found that the addition of stabilizers to rubber lead to a rise in the characteristic 6. Then, according to formula (2), the OLvalue should also increase. This can be seen from Table 1, which compares 6 and LXfor the vulcsnizate SKN-26 with different stabilizers. * TABLE
1.
INFLUENOE
OF
STABILIZA-
TIONONTEEFATIGUIZFAILURE ACTERISTIO ISTIO
6 AND
a FOR
CEAR-
WEAR
CHARACTER-
CARBON-FILLED
ON
SKN-26
RUBBER
BASE
Stabilizer No stabilizer 4010 NONOX
a
6
2.1 2.9 3.4
29 42 49
Unfortunately, the effect of stabilizers on fatigue of plastics has not been studied. But antifatigue additives must be sought for them among the usual thermo-stabilizers, bearing in mind the following arguments. According to the fluctuation theory of strength [12], failure is due to thermal movement and load only reduces the activation barrier, lowering the actual lifetime to a value commensurate with the experimental time; then failure is mainly due to the breakdown of chemical bonds. It is suggested that the same TABLE
2.
COMPMtX3ON
OF
THE
BEHAVIOUR
OF
VISCOSITY
Viscosity Polymer
before wear, fl0
IN
WEAR
WITH
VARIATION
OF
characteristic after wear on abraon mesh, sive cloth,
‘lsc--flm am-%2 VO
rim ‘taC
Polyamide 68 High-pressure polyethylene Polyvinyl chloride Polycarbonate Polystyrene
0.9 1.2 0.8 0.6 0.85
0.6 1.0 0.7 0.5 0.5
0.4 0.9 0.6 0.5 0.5
0.2 0.1 0.1 0 0
mechanism also holds for wear, although it is a more complex form dation. The following experimental results support this proposition. * Results
obtained
in cooperation
with
G. S. Klitenik:
1.0 0.9 0.6 0.2 0.1
of degra-
c(
Thormomechanical
stability
of polymers
a) In wear the temperature dependence of the degradation to a relation [13, 141 analogous to that of static failure [12]: I= col~st. ,-cu.-BP)lRT,
96
time is subject
(3)
where RT is the energy of thermal movement, U, and B are constants. b) During wear there is mechanochemical degradation of the polymer. This can be seen from Table 2, which shows that the viscosity of the products of wear (crumbs) is less than for the initial specimen. Molecular breakdown certainly takes place in both abrasive wear (on an abrasive cloth, i.e. as a result of sharp projections), and in fatigue wear (on metal mesh, i.e. by blunt projections). Moreover, in the case of fatigue wear mechanochemical processes must be the more important since they are operative not only in the detachment of particles, but also in the surface fatigue which promotes failure. With friction against blunt projections the viscosity of the polymer crumbs is usually less than with sharp projections. But if the material is brittle (polystyrene, polycarbonate) then its wear is mainly abrasive, whether abrasive cloth or mesh is in question [7, 151, which means that there is only a very small difference ~,-a,, for the two surfaces (Table 2). For these two polymers therefore, we find no difference in the viscosities of crumbs after wear against mesh or abrasive cloth. Not only this, the opposite effect may even arise, since the size of the detached particles will affect the molecular weight of the polymer for the same mechanism; the larger the wear particles, the smaller their surface and consequently, the lower the degree of mechanical degradation. This is what we found in an earlier work [16] for the two polymers mentioned. Thus wear is an activation-kinetic process in which degradation is the result of the breakdown of chemical bonds activated by mechanical energy. Ageing also involves the breakdown of chemical bonds, but due to other factors (light, heat, 0, ans so on). In a mechanical field therefore, the ageing of elastomers has lower activation energy [17]. But in this case, the enhanced chemical stability of the polymer should also affect its lifetime, particularly its wear resistance. The first thing to be expected here is an increase in the a value, because it reflects the tendency to a fatigue mechanism of wear. The existence of this mechanism opens up the possibility of affecting its characteristics by the addition of antifatigue agents, while the mechanochemical nature of fatigue seggests that such additions should be sought among the usual thermo-stabilizers. Let us consider the experimental results. Stabilization and wear of plastics. The role of stabilization has been studied for a number of crystalline antifriction materials in wear against a metal mesh. Under these conditions the tendency to a fatigue mechanism is greatest, which means that the OLvalue is much more than unity for all the materials. In view of the non-linear 1 vs. P relation in fatigue wear (see formula (l)), in Table 3 we have used the two values I1 and u as the wear characteristics of
S. B. RATNER and YE. G. LUE’IE
96 TABLE
3. WEAR CHARACTERISTICS
-
FOR
PLASTICS
WITH
.
Polymer
Stabilizer
a
-
DIFFERENT
T Aa
Wear on -7
mesh, mm31 /m. cma
I’,
Polyformaldehyde
No stabilizer Santovar 0 Nitroxide radical Two-ring phenol Diphenylamine Polypropylene No stabilizer Topanol Topanol +pigment Polyamide AK-7 r No stabilizer NFD ,?-Naphthylpyrocatechinphosphite Polyamide 68 No stabilizer NFD Neozone D Antracene I l-Naphthylpyrocatechinphosphitc Ionolpyrocatechinphosphite 4,6-Salicyl-di-tert.butyl-4-methylphenylphosphite 2,4,6-Salicyl-tri-tert.bulylphenyl phosphite
-
STABILIZERS
abrasive cloth, mm3/ /m f cm2
3.1 4.9 4.4 3.1 3.1 1.4 2.0 2.6 1.3 2.2
-
0.1
1.8 1.3 0 0 OTI 1.2 0.9
0.001 0.004 0.04 0.1 3.2 3.0 1.0 0.9 0.09
2.2 2.0 2.4 2.2 2.0 2.0 2.0
0.9 0.4 0.2 0 0 0
0.07 0.2 0.004 0.04 0.12 0.2 0.2
3 5 1.8 1.4 3 5 5
1.1 1.1 1.2 1.1 -
2.0
0
0.2
5
-
2-o
0
0.2
5
-
10 2 7 5 10 27 80 GG 10 4
4.3 4.4 4.5 4.4 4.0 1.3 1,1
--
plastics. In contrast to OL,which is a constant of the material, I, is a conditional characteristic; the comparative wear of different materials differs under different loads; for comparison, Table 3 gives the value I, (wear at P=5 kg/cm2). Besides this, to show the role of stabilizers* in abrasive wear (CI= 1) we have given the value I,, for wear on an abrasive cloth at P=l kg/cm2. Polyformaldehyde. Polyformaldehyde (PFA) is usually stabilized by the addition of polyamide and radical-type additives. It is practically impossible to prepare specimens from completely unstabilized PFA, for which reason the control batch in our experiments is semi-stabilized PFA in which the only additive is polyamide. It can be seen from Table 3 that the addition of 2,5-di-tert. butylhydroquinone (santovar 0) and the stable nitroxide radical, which are the * The following assisted formaldehyde, B. B. Pashenin the polyamide.
in stabilizing the materials: V. V. Gur’yanova for the polythe polypropylene, I. I. Levantovskaya and P. M. Tanunina
Thormomechanical stability of polymors
97
most effective stabilizers [18, 191, gives a sudden rise in u. Although it does not alter rx, the addition of 2-ring phenol improves the wear resistance of PFA, and only diphenylamine has practically no effect on the wear characteristic.
,x-x-. 00,?5x!
4Lx, ---x-
-1
I
OW Effect of ageing PFA in W
600
77me, days
light (I, 2) and with heat (3, 4) on the wear (I, 3) and tensile strength (2, 4).
PFA with the addition of the more effective antifatigue agent santovar (4 was also subjected to wear after heat and light ageing. The a value showed no change as compared with the initial specimen before ageing, and therefore the materials must be compared from the I, value. It is evident from the Figure that on exposure to UV radiation wear increases with the ageing time. But in the case of heat ageing there is little change in the wear, and it even diminishes in a certain range (curve 3). Approximately the same kind of relations were obtained for simple mechanical properties also, particularly tensile strength (curves 2, 4).* We can see that, although it is not effective for light ageing, santovar 0 is a good thermo-stabilizer and simultaneously an antifatigue agent. Polypropylene. The addition of the stabilizer topanol to polypropylene also raises the a value. It is interesting to note that an ever greater effect (da=l.2) is obtained if not only a radical-type stabilizer but also a pigment is added, the latter stabilizing the supermolecular structure of polypropylene. Polyamide AK-7. Thermo-stabilizers added to polyamide AK-7 also cause a considerable rise in the CIvalue; Dora 1. PoZyamide 68. Of the number of stabilizers added to polyamide 68, which are shown in Table 3, three stand out; these are neozone D, di-P-naphthyl-pphenylenediamine (NPD) and heat-treated anthracene, while the rest have no effect in wear. The stabilizers indicated have an action which consists mainly * Results obtained in collaboration with V. V. Gur’yanove.
98
S.B.
RATNER and
YE. G.LUR'IE
in raising the wear resistance of polyamide 68, the a value showing little change. In the case of the first two stabilizers, which are particularly effective in thermooxidative degradation [20], there is also a slight rise in cx;Aa=O*2-0.4. Of course, if the wear is abrasive then the addition of stabilizers will have no effect because the process of the surface layer degradation is such a short one that the preparation stage practically disappears, contact deformation (the first stage) going over directly to microcutting (the third stage). When additives are used, control of the radical processes is achieved during the intermediate stage, the fatigue of the surface layer. The absence of this stage in abrasive wear thus rules out the possibility of this type of additive affecting the wear process. This can also be seen from Table 3, which shows that wear on an abrasive cloth (a=l) does not depend on the presence or absence of additions, although mechanochemical degradation takes place in this kind of wear (Table 2). The authors are grateful to M. S. Akutin, K. N. Vlasova, V. V. Gur’yanova, V. V. Kovrige, G. S. Klitenik, B. M. Kovarskaya, I. I. Levantovskaya, B. I. Pashenin and P. M. Tanunina for presenting the materials and discussing the results. CONCLUSIONS
(1) Mechanochemical degradation always takes place in the wear of polymers. (2) If the wear is abrasive it cannot be reduced by adding stabilizers since the surface layer fatigue stage is absent. (3) Only in fatigue wear does the addition of thermo-stabilizers raise the characteristic CC,which gives the fatigue resistance of the material or, if CIremains unchanged, increase the wear resistance. (4) Together with the experimental data, the analysis of wear as a fatigue process indicates the possibility of simultaneously a.djusting the fatigue life and wear resistance of polymeric materials by increasing their chemical stability. Translated
by
V.
A.WORD
REFERENCES 1. I. V. KRAGEL’SKII, Trenie i iznos. (Friction and Wear.) Mashgiz, 1962 2. S. B. RATNER, Friktsionnyi iznos rezin. (Friction Wear of Rubbers.) Izd. “Khimiya”, 31, 1964 3. S. B. RATNER, Dokl. Akad. Nauk SSSR 150: 848, 1963 4. M. M. REZNIKOVSKII, Rauchuk i rezina, 9, 33, 1960 5. I. V. KRAGEL’SW and Ye. F. NEPOMNYASHCHII, Izv. Akad. Nauk SSSR, Mekhanika: 5, 190, 1963 6. S. B. RATNER and G. S. KLITENIK, Zavodskaya lab., 11, 1375, 1959 7. S. B. RATNER and I. I. FARBEROVA, Plast. 9, 61, 1960 massy, 8. S. B. RATNER, Dokl. Akad. Nauk SSSR 144: 327, 1962 9. G. S. KLITENIK and S. B. RATNER, Kauchuk i rezina 3, 19 1960 10. S. B. RATNER, G. S. KLITENIK and Ye. 6. LUR’IE, Teoriya treniya i iznosa. (Theory Friction and Weas.) Izd. “Nauka”, 156, 1965
of
Macromolecular
orientation
indices
99
11. I,. G. ANGERT and A. S. KUZ’MINSKII, Rol’ i primen. antioksidantov v kauchukakh i rezinakh. (Role and Application of Antioxidants in Rubbers and Vulcanizates.) Goskhimizdat, 1957 12. S. N. ZHURKOV, Vestnik Akad. Nauk SSSR, 11, 78, 1957 13. S. B. RATNER, Dissertation, 1964 14. S. B. RATNER and Ye. G. LUR’IE, Dokl. Akad. Nauk SSSR, 166, 151, 1966 15. S. B. RATNER, Dokl. Akad. Nauk SSSR 135: 294, 1960 16. E. G. LUR’IE and S. B. RATNER, Plast. massy, 11, 47, 1962 17. A. S. KUZ’MINSKII, N. N. LEZHNEV and Yu. S. ZUYEV, Okislenie kauchukov i rezin. (Oxidation of Rubbers and Vulcanizates.) Goskhimizdat, 1957 18. V. R. ALISHOYEV and M. B. NEIMAN et al., Vysokomol. soyed. 5: 644, 1963 19. B. M. KOVARSKAYA, M. B. NEIMAN and V. V. GUR’YANOVA el al., Vysokomol. soyed. 6: 1737, 1964 20. I. I. LEVANTOVSKAYA, Dissertation, 1964
CORRELATION OF THE MACROMOLECULAR INDICES IN FIBRES ACCORDING TO X-RAY MEASUREMENT * A. SH. Go-, Kiev
Branch
L. A. OSININA, S. G. Osm of the All-Union
Scientific
Research
(Received 17 February
Institute
ORIENTATION AND ACOUSTIC
and M. P. Nosov of Synthetic
Fibres
1965)
V~IOUS methods are used to assess the molecular chain orientation in fibres; these are birefringence, IR-dichroism, and X-ray diffraction analysis. Recently it has been found possible to evaluate the mean molecular orientation from the velocity of a sound wave passing through a fibre. It is interesting to establish the correlation between the orientation indices determined by different methods. This is quite a complicated problem since it is difficult to find a single measure of orientation for all these methods. By X-ray diEraction analysis it is possible to assess the spread in the orientation of crystallites by measuring the azimuthal half-width of reflections. The birefringence method assesses the mean statistical orientation of macromolecules in amorphous and crystalline regions of the polymer, so long as the latter is a two-phase system. The criterion of orientation used is the factor introduced by Hermans [l]: 3&=ly
* Vysokomol.
soyed. 8: No. 1, 94-97,
sin2p.
1966.
(1)