Ageing of polyamide film materials in the stressed state

Polymer Science U.S.S.R. Vol. 30, No. 10, pp. 2286--2292, 1988 Printed in Poland

0032-3950/88 $10.00+.00 © 1990 Pergamon Press plc

AGEING OF POLYAMIDE FILM MATERIALS IN THE STRESSED STATE* O. YE. KUZOVLEVA, L. YU. KABAL'NOVA, L. M. YARYSHEVA, A. A. PED' a n d P. V. KOZLOV M. V. Lomonosov State University, Moscow Central Scientific Research Institute of Material Science

(Received 12 May 1987) The ageing of stressed polyamide films is studied over a wide range of aggressive factors (temperature, atmospheric moisture, stretching loads), and the main kinetic laws of the physical and chemical processes taking place on ageing polyamide under load are determined. A connection is established between the kinetics of change of the physicomechanical criteria of the materials studied and the nature of the processes taking place. IT is k n o w n [1] that the application o f loads to polymer materials can significantly affect the kinetics o f the physical a n d chemical processes taking place in the material. However, so far studies in this direction have been restricted to a n a r r o w range o f polymers (PE, PP, a n d PA-fibre), a n d for a limited range o f chemical processes (oxidation reactions have been especially studied) [2, 4]. The materials generally studied have been previously oriented polymers, subjected to oxidation at increased temperature. O n the other hand, investigation o f the kinetics o f the ageing process u n d e r the simultaneous action o f a n u m b e r o f different factors, including deformational loads, is of great imp o r t a n c e for solving the problem o f predicting the properties o f materials subjected to the complex action o f external factors. The object o f this w o r k was to study the effect o f external stretching loads acting on the material during ageing, on the kinetic laws governing the ageing o f P A films subjected to thermal oxidation and the c o m b i n e d action o f heat and moisture. Two industrial PA films were used: a biaxially oriented PA-6 film of thickness 23 + 2/tin and a three-layer film of thickness 60_+5 /tm, the internal layer of which was identical to the first film, the external layer consisting of P-548 amide copolymer. The crystalline structure of the original films comprised a mixture of ~t- and ;,-modifications; the original degree of crystallinity was about 33 ~o for the PA-6 film and about 21 ~ for the laminated film. The film specimens for the ageing tests were selected in the form of a two-sided slice with a working part of dimensions 22 x 6 mm along one of the mutually perpendicular orientation directions. Thermal-oxidation ageing in air under load was studied at 80, 100, 120, and 140°C in ovens under creep conditions at stretching loads of a---(0.06-0.6) at2°" in the case of the laminated film (for which at 20°C and a relative air humidity of 45 ~ at a stretching rate 50 mm/min at2°'= 110 MPa), and over the range o=(0-1-0.8) crt2°" in the case of PA-6 film, for which at 20°C and a relative air humidity of 45 ~o, at a stretching rate 50 mm/min at2a'= 160 MPa. The relative error in determining the mechanical properties never exceeded 5 ~. * Vysokomol. soyed. A30: No. 10, 2141-2146, 1988. 2286

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To establish the conditions for ageing under the combined action of heat and moisture "Faetron" and "Gidrostat" type climatic chambers were used; the specimens were aged under the same loading conditions as in thermal oxidation ageing. The same absolute atmospheric moisture content was used (7'8× 10a mole H20 I.): T=60°C, ¢=95%, and T=80°C, ¢a=430/o (~ is is the relative air humidity). Before structural studies and mechanical tests the aged films were conditioned at room temperature and ~= 45 %. The changes in structure of the materials during ageing were followed by means of large angle X-ray light scattering (URS-55 apparatus with a RKSO chamber, characteristic radiatiou Cu K~), differential scanning colorimetry ("Dupont" calorimeter, model 1091; healing rate 20 deg/ /min) and IR spectroscopy (UR-20 spectrometer, spectra recording rate 160 cm-l/min). In using: IR spectroscopy the relative values of the optical density of the ":t-structure bands" D93o/D112swere calculated, where D93o and Dl125 are the optical densities of the or-structure bands at ,u=930 c m - t (planar skeletal vibrations of the N H - C O fragment) and the bands for the skeletal C - C vibrations) (internal standard at v= 1125 cm- 1) respectively, as evaluated from the base line method [5]. The kinetics of the chemical processes (oxidation, hydrolysis) were studied by IR spectroscopy from the accumulation of carbonyl groups formed, with calculation of the optical density at the band maximum at Vc=o= 1720-1740 cm-~ as the difference between the IR spectrum of the aged and original films (when there are appreciable changes in film thickness during ageing under load, a specimen of the original film stretched to the same thickness is placed in the reference beam). The dynamometric studies were carried out on an 'qnstron" instrument at 20°C and a rate of 50 mm/min the weight changes of the specimens during ageing were evaluated by a weight method. It was shown previously that in thermal ageing o f unstressed PA-films in moist air [6] the total chemical ageing process is complicated c o m p a r e d with oxidation in dry air [7], and furthermore, restructuring o f the crystalline structure o f the PA, stimulated by moisture absorption, takes place even before the chemical processes start. It is thus reasonable in this work to give separate consideration to the results o f the action o f stretching loads on thermal-oxidation (dry) ageing and ageing under the combined action o f moisture and heat. Figure 1 shows some kinetic laws for the thermal-oxidation ageing o f a laminated PA-film at 100°C under various loads: these curves show the accumulation of carbonyl groups formed during oxidation (a), the change in optical density of the b a n d for the crystalline or-structure at p = 9 3 0 cm -1 (b) and the change in mechanical properties o f the film during ageing, i.e. the tensile strength (e). It can be easily seen that the overall oxidation rate (Fig. la) is almost independent o f the stress applied to the specimen (it is maintained the same as in the unstressed state, the duration o f the " i n d u c t i o n " period z i = 2 - 3 days, and the oxidation depth is equal to Dc=o/D~tz5=0.3-0"5). Oxidative degradation under stress leads to an increase in the content of the crystalline a-form of the P A (growth o f the b a n d at p = 9 3 0 cm -~, Fig. lb). The rate o f f o r m a t i o n o f a-structure is directly related to the load (at constant temperature): the higher the value o f ~r, the earlier the oxidation stages at which it produces a noticeable increase in the degree o f ordering o f the structure. The depth o f " a c c u m u l a t i o n " o f the ~-form is also directly related to the stress (at the same degree o f oxidation). The X-ray diffraction analysis d a t a also indicate an increase in the degree of ordering during the earlier stages o f oxidative degradation: the intensity o f the crystalline reflections o f ~t-structure on the X-ray patterns at large angles is increased.

O. YE. KUZOVLEVAe t al.

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Irt principle, both additional crystallization and also a structural transition of one crystalline modification into another (7~-transition) can lead to an increase in content of the ~-form in the PA under the action of external factors. The IR spectroscopy and X-ray diffraction methods do not provide for calculating these processes in PA, but the differential scanning calorimetry (DSC) method for this stage indicated an increase in heat of melting of PA, corresponding to an increase in the degree of erystallinity of DG=O Din5

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FIG. 1. Kinetic relations for the accumulation of carbonyl groups (a), changes in the optical density of the band for ~-structure at p = 9 3 0 cm-1 (b) and changes in the fracture stress (c) during ageing of laminated PA-fiim at 100°C in air at a/tr2°'=0 (1), 0.06 (2), 0.12 (3), 0.2 (4), 0.25 (5), 0"3 (6), 0"35 (7), and 0"4 (8),

Ageing of polyamidefilm materials in stressed state

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7-8 ~ (assuming that the heats of melting of both crystalline modifications are approximately identical: ,dHr~ (a-form)= 241 J/g, AHmc (y-form)=239 J/g [8]), which indicates additional crystallization in the PA under load. Since these structural changes are observed only at the beginning of oxidative degradation (r> ri), it cart be assumed that the degraded parts of the PA chains oriented in the stretching direction are crystallized in the 0~-form. The change in strength characteristics of the material (Fig. l c) on thermal oxidation ageing under load is a result of three processes: orientation, growth of ordered structure (crystallization), and oxidative degradation. The first two processes result in an increase in the ultimate tensile stress (U.T.S.), and as a result of degradation the value of at is decreased. The domination of crystallization and orientation even at the stage of oxidation during ageing of the material under load ( r > q ) results in a situation where with increase in the applied stress the values of o t are not decreased (Fig. lc, curves 4-8), but continue to increase. During the stage of the "induction" oxidation period (z
O. YE. KUZOVLEVA et at.

2290

ing of the chains should be accompanied by additional crystallization with the formation of the ~-form. It can thus be assumed that the tensile stresses have no effect on the rate and degree of oxidation because of the special structural features of the PA studied. The mechanism of the effect of tensile loads on the ageing of PA materials in moist air at increased temperatures will now be considered. In the case of PA films in the unloaded state, two stages of the ageing process are possible [6]. In the first stage, before the onset of chemical degradation processes, structural reorganizations stimulated by moisture absorption occur (a transition from the less ordered crystalline y-modification to the more ordered ~-modification), the rate of which is directly related to the temperature and the moisture content of the material (at constant temperature). At the second ageing stage strictly chemical ageing processes occur (oxidation and hydrolysis), which lead to an irreversible decrease in the strength characteristics of the PAmaterials. O't,MPa

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FIG. 2. Kinetic curves for the change in optical density of the band for the ~t-structure at g = 930 c m - t (a) and changes in the fracture stress (b) during ageing of laminated PA-film at a/trt2°'= 0 (1), 0.12 (2), 0'2 (3) T = 60°C, tp = 95-98 ~o. The arrow shows the value of rl.

The application of stresses affects both stages of thermal oxidation of PA in the presence of moisture. Kinetic relations for the change in intensity of the band for the ~-structure ( D 9 3 o ) in ageing of a laminated film at T=60°C, ~0=95-98% under load and in the unloaded state are shown in Fig. 2a as an example. Figure 2 shows that on applying a stress the rate and depth of structural reorganization both increase, as shown by an increase in the content of the crystalline ~-form in the PA. It was established that at the same stress level the rate and extent of the structural reorganizations are directly related to the moisture content of the material. For example, at T=60°C and ~=95~o (maximum moisture content of specimens in the unloaded state is about 7"07o), after 50 days ageing at a=0.2t2°° the relative intensity of the band for the ~-structure is higher by a factor of 1.6 than for the same stresses and ageing time when T= 80°C and ~0=43 9/00(maximum moisture absorption for the specimens in the unloaded state is about 0.8 ~). These results show that the moisture content is the factor having the dominating effect on the rate and extent of structural reorganizations in the PA materials under load. The mobility of the macromolecules resulting from

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plastification is directly related to the moisture content, which finally, when tensile loads are applied, results in an increase in the degree of orientation and ordering of the structure. A 10-12 % increase in the degree of crystallinity can be demonstrated by DSC at this stage, which indicates further increase in the structural reorganizations during ageing under load compared with ageing of these PA in the unstressed state [6]. Accordingly, during the first stage of thermal PA ageing in the presence of moisture in the stressed state a combination of the effects of orientation and additional crystallization in the or-form and a 7~a~transition is observed. These physical processes have a significant effect on the kinetics of the chemical processes have a significant effect on the kinetics of the chemical processes during subsequent ageing stages. In the first place, the "sorption capacity" of the PA matelials is decreased as a result of the structural reorganizations, i.e. water is displaced from the bulk of the material during ageing. For example, on ageing at 60°C and fp=95%, the maximum moisture absorption (7-7.5 %) is attained rapidly (after 10-15 min), after which the weight of the specimens is gradually decreased. In the second place, the accessibility to atmospheric oxygen [10] must be decreased, which finally has a significant retarding effect on the beginning of the chemical processes. At high moisture contents in the PA material (for example, < 1% at 80°C and q~= 43 %) no appreciable retarding action of the tensile loads on the kinetics of the chemical processes can be observed, i.e. behaviour characteristic of thermal oxidation ageing under load in dry air is observed. The manifestation of chemical degradation processes in PA materials on thermal ageing in the presence of moisture under load results in significant stabilization of their strength processes (in particular, at). As can be seen from Fig. 2b, on applying a load the fracture stress is not only not decreased after 20 days (curves 2 and 3), as occurs in the unstressed state, when the chemical processes begin (curve 1), but actually increases because of orientation and ordering of the structure. It must be noted that in studying the ageing of highly oriented PA-6 (degree of stretching 5.5-6) in moist air under a tensile load [11 ], mechanical activation of hydrolysis reactions was established: under load the hydrolytic degradation of PA (accumulation of COOH groups) was observed even at room temperature. The results of investigating the thermal ageing of PA-films based on biaxially oriented PA-6 in the presence of moisture, which indicate a tow initial degree of crystallinity, show that the potential possibility of an increase in the degree of orientation and reorganization of the structure (degree of stretching of the original PA-6, as determined by a linear dilatometry method, is not high: 1.5-2.0 in both directions) in the direction of ordering under the action of tensile loads under conditions which do not produce "over stressing" of the bonds in the stretched conformations, not only does not accelerate the degradation processes, but appreciably suppresses them. Accordingly, the ageing of PA materials based on biaxially oriented PA-6 under conditions where external tensile loads are applied differs both qualitatively and quantitatively from ageing in the stressed state. On thermal ageing in the presence of moisture the application of external stresses does not affect the beginning and extent of oxidation of the material, but produces, partial structural reorganization of the material (increase

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O. YE. KUZOVLEVAet al.

in the content of the crystalline a-form). Moreover, orientation and crystallization of P A materials prevents a decrease in its strength properties at the oxidative degradation stage. In thermal ageing of P A materials under load in the presence of moisture even at the early ageing stages the structural reorganizations are significantly more fundamental than in " d r y " ageing (additional crystallization, 7 ~ transition), found on more strongly orientated materials because of the plastifying effect of moisture. The result of this is a significant retardation of chemical degradation processes and an increase in the strength criteria of the material. Translated by N. Sa'ANDEN

REFERENCES

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