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Healing of Stress-Whitening in Polyethylene and Polypropylene at or Below Room Temperature.

K. D. PAE [*]

H.-C. CHU [*]

J. K. LEE [**]

J.-H. KIM [**]

Stress-whitening occurs in many crystalline polymers when they are subjected to cold-drawing at atmospheric pressure and at room temperature. The exact structure and morphology of stress-whitening are still in contention, although it is widely believed that stress-whitening is a combination of crazes and voids. The healing of stress-whitening at room temperture may be divided into two stages: transient, which is a short time-dependent and post-transient. The post-transient healing phenomenon involves a long time-dependent or time-independent healing. Stress-whitening in the post-transient stage reaches a steady or permanent state. Healing of stress-whitening in the post-transient stage would occur when acted upon by an external agency, such as stress and temperature, or a combination thereof. A methodology has been developed to heal the permanent stress-whitening at or below room temperature in HDPE and PP. The study involved the following procudure: tensile samples of HDPE and PP were cold-drawn in an Insto n testing machine at atmospheric pressure and room temperature to develop stress-whitening; the stress-whitened samples were subsequently, after days of rest, pulled in hydrostatic pressure environment at room temperature; the stress-whitened section was partially or completely healed depending on the magnitude of the applied stress (or strain) and the intensity of hydrostatic pressure.

I. INTRODUCTION

Stress-whitening has been known to occur in many polymers, copolymers. and blends when they are subjected to tensile deformation at certain ranges of strain-rates and temperatures [1-8]. Stress-whitening normally begins to form when applied stress exceeds the yield point of crystalline polymers, or is associated with cold drawing. Although it is widely believed that stress-whitening is some combinations of crazes and voids, the microstructure of stress-whitening depends on polymer systems: a single or multiple phase systems.

Liu et al. [1-3] studied the microstructure of tensile samples of polypropylene by means of SEM, DSC, and XRD. They concluded from the study that stress-whitening had "craze-lime" structure, parallel to the draw direction, although no evidence was shown. DSC results showed that stress-whitened regions exhibited a single normal melting peak, while transparent samples (drawn at high temperatures 40 and 56[degrees]C) produced two peaks. The XRD analysis showed that the crystallites were broken into finer pieces in the stress-whitened than the transparent samples.

Yoon et al. [4] and Sauer [5] showed that stress-whitening in tensile samples of PE did not form when the tests were conducted under high pressure environment. Lee and Kung [6] showed that biaxial cold rolling reduced or eliminated formation of stress-whitening in HMWPE. They also found that stress-whitened samples had a significantly lower hardness and density owing to the presence of voids. They were able to remove stress-whitening in HMWPE by heating it to 130[degrees]C. They observed that the voids were elliptical on the surface of the samples and their size varied from 20 [micro]m to 60 [micro]m.

Gotham [7] observed that void was responsible for stress-whitening in a PVC sample undergoing tensile deformation; while Vincent et al. [8] found crazing followed by stress-whitening in PVC. The crazing appeared on the surface of the samples and stress-whitening occupied the entire body of the samples. The stess-whitening was caused by 200 nm to 2.0 [micro]m voids, originating from inhomogeniety of PVC.

Breuer et al [9] showed that, in rubber-modified PVC, stress-whitening was due primarily to a large number of cavities formed by rupture of rubber particles and crazing did not significantly contributed to volume dilation or stress-whitening. The cavities were aligned in shear bands at an angle of 55-64[degrees] to the stress direction. They correlated the density fluctuation to void content. They also advocated that formation of voids was not limited to a few exotic polymer blends, but might be a general process for shear deformation of polymers.

Brown [10] induced a craze in a notched sample of PE. The craze at the end of a notch was well formed and the presence of fibrils across the craze was clearly visible. The craze was perpendicular to and the fibrils were parallel to the direction of the applied stress. He also measured strains and stresses around the craze. The nature and properties of craze in amorphous polymers have been studied extensively and are known to be well understood. Two comprehensive review papers are cited to represent numerous references [11, 12]. Steger and Nielson [13] determined the formation of microvoids in HIPS by SAXS. They attributed the increase of scattering to the formation, the relative size, and the concentration of microvoids. The spherical voids deformed into ellipsoids by the tensile stress up to the yield point. The elongation ratio remaind constant at approximately five. If the sample was unloaded, the voids relaxed back toward spherical shape and about half of them closed. No mention of reapperance of voids w as made upon reloading.

Zhurkov et al. [14], using light scattering and LAXS, determined that the microcavities appeared to be disc-shaped with their planes perpendicular to the stress axis and that the concentration of microvoids was approximately 1015-1016 [cm.sup.-3]. Microvoids were more numerous close to the sample surface. They also estimated that the microvoids had an average size of 300A and separated by a distance of 0.25 to 0.5 of their size. Zhurkov [15] also measured the size and concentration of microvoid, and the concentration of bond rupture of polyethylene and polypropylene. It was found that the diameter of the disc-shaped microvoid of PE and PP was 400A and 320A, respectively; the bond rupture concentration was 6 x [10.sup.18] and 1.4 x [10.sup.18] [cm.sup.-3], respectively; the microvoid concentration was 6 x [10.sup.14] and 5 x [10.sup.14] [cm.sup.-3], respectively. They then calculated the number of molecular fracture event per cavity to be 10,000 and 2800 for PE and PP, respectively; when the number of aligned molecules that intersected a microvoid was 11,000 for PE and 2600 for PP.

A series of papers have been published by Wool and O'Connor [16-21] on healing of crazes in glassy polymers at elevated temperatures. They dealt with transient healing process and thus found it to be time-and temperature-dependent. The healing process was partial or complete, and the possibility of reopening of the original craze depended on time-temperature history. Wool and O'Connor proposed a theoretical model for healing of crazes [21]. It is not surprising that healing would occur at elevated temperatures near [T.sub.g] at which molecular chains would be highly mobil. Wool [18] studied the process of healing of cracks or voids in the transient time regime, in crystalline polymers, block copolymers, and filled elastomers by means of recovery of stress-strain curves. He stated that the healing process involved restortion of secondary bonding between chains or microstructural components. He also stated that the severed chains were transported by a diffusional process through Brownian motion in the healing process. It is widely known that a complete recovery of plastically deformed (highly cold-drawn) crystalline polymers would occur at elevated temperatures.

II. MATERIALS

The sample materials used were high density polyethylene (HDPE) and polypropylene (PP), purchased from commercial sources in 1/2 inch rods. The rods were cut and machined into 2 inches long with both ends threaded and the reduced gauge diameter of 1/4 inch and length of 1 inch. A schematic diagram of a sample is shown in Fig. 1.

III. EXPERIMENTAL

3.1 Sample Preparation

The stress-whitened samples were prepared for various tasks by testing the samples shown in Fig. 1 in tension at atmospheric pressure and room temperature in a Instron Testing Machine (Model 4411). The tensile tests were carried out with a cross-head speed of 3 in/min for PE and 1 in/min for PP.

3.2 Healing Stream-Whitening

The stress-whitened samples were subsequently tested, after an extended period of time (days), under elevated hydrostatic pressure in a high pressure testing apparatus to heal stress-whitening. The cross-head speed under elevated pressures was set at 0.5 in/min for all samples.

A schematic diagram of the high pressure tension/compression test apparatus is shown in Fig. 2. The apparatus consisted of two thick-walled pressure vessels (one for testing and the other for pressure compensation), made of maraging steel, and a double-ended hydraulic cylinder for loading and unloading samples. The two interconnected pressure vessels were so arranged that the volume of pressure transmitting medium (silicon oil, Dow Corning 200, 5 cs) in the vessels remained constant during testing; thereby the pressure in the system was kept constant during testing. A sample was loaded by pushing the loading piston, which was attached to the hydraulic cylinder, into the test vessel. The apparatus is capable of containing pressure to 7 kbar.

The data acquisition system consisted of an LVDT; a pressure transducer (Autoclave Engineering), a load cell, made of a full bridge strain gauge; two amplifier/signal conditioners (Daytronics, Models 3130 and 3170); Labtech software; an analog/digital converter (Data Translation Inc., Model DT 2805), and a PC (Gateway 2000 Model 4DX2-66V). The bridge was mounted at the inside tip of the loading piston, so that its function was unaffected by hydrostatic pressure.

The stress-whitened sample in a sample holder was then placed in the test chamber, the pressure was increased, and a tensile load was applied. The load/deformation data were stored in the computer for analyses.

3.3 DSC and Laser Analysis

DSC (Perkin Elmer, Pyris-1) analysis with a scan rate 10[degrees]C/min was made to determine any changes that might be associated with melting characteristics of stress-whitened and healed samples.

A beam of He-Ne laser (Uniphase, Model 1124) with 10 mW and [lamda] = 633 nm was used to quantify clarity due to healing of stress-whitening. A convex lens was placed between the laser source and the sample to focus the beam on the sample. The beam diameter was 0.25 mm. The beam intensity through a sample and after passing through a pinhole (diameter = 0.5 mm) was measured by a photosensor and read by a digital voltmeter. A schematic diagram of the test arrangement is shown in Fig. 3. For the clarity measurement, stress-whitened and healed samples were sliced across their diameter to the thickness of 0.42 + 0.02 mm. The diameters of the samples were [sim]2.4 mm for HDPE and [sim]2.6 mm for PP.

A sliced sample was placed on a clean glass slide and a solvent was dropped on the sample to prevent interference of the beam due to surface scratches on the samples caused by slicing. The solvent used was decahydronaphthalene, which is inert to and has nearly the same reflex index (1.48 at 20[degrees]C) as HDPE (1.54 at 25[degrees]C) and pp (1.5 at 20[degrees]C). The solvent worked satisfactorily in elimnating the interference. Another clean glass slide was placed on the top of the sample. The sandwiched sample was fixed on a sample holder, which can be moved horizontally and vertically by two micrometers. The intensity profile across the diameter of sliced samples was obtained by moving the sample horizontally at 0.1 mm intervals.

Optical microscopy with an Olympus microscope (BF 60F-3) in the tramission mode at 50X was used on the sliced samples to determine global geometry and morphology of stress-whitening.

IV. RESULTS

The healing methodology involved subjecting the stress-whitened samples to combinations of tensile stress and hydrostatic pressure. The two variables, namely tensile stress and hydrostatic pressure, could be independently varied for healing. Application of hydrostatic pressure alone did not heal stress-whitening regardless of its intensity.

4.1 Effect of Hydrostatic Pressure on Healing of Stress-Whitening at/Below Room Temperature

This experiment was conducted to determine the role hydrostatic pressure played on healing of stress-whitening in HDPE and PP. Tensile tests were conducted on stress-whitened samples at various pressures.

Stress-whitened samples of HDPE were tested in tension under P = 44.8 MPa (6,500 psi) and P = 89.7 MPa (13,000 psi), respectively, until cold-drawing occurred. The former showed a partial healing of stress-whitening; while the latter showed complete healing as shown in Fig. 4. For PP samples, stress-whitened samples were tested in tension under P = 44.8 MPa (6,500 psi) and P = 89.7 MPa (13,000 psi), respectively. The sample tested at the lower pressure showed a partial healing and those tested at the higher pressure appeared to be nearly completely healed (Fig. 5).

It has been shown that hydrostatic pressure raised various transition temperatures, such as [T.sub.g] and [T.sub.m]. For PE and PP. 1 kbar pressure raises [T.sub.g] and [T.sub.m] by 15-20[degrees]C [22, 23]. Therefore, the thermodynamic equivalent test temperature on PE and PP at P = 89.7 MPa corresponds to 13.5 - 18[degrees]0 below room temperature.

4.2 Effects of Applied Stress (or Strain) on Healing of Stress-Whitening at/Below Room Temperature

The second experimental variable of the two is the magnitude of the applied tensile stress; that is, the effect of the magnitude of stress (or strain) on healing of stress-whitening under a constant hydrostatic pressure environment. The first part of this investigation dealt with whether the applied stress was in the elastic range (initial linear region of the stress-strain curve) or plastic range (beyond the yield point) for healing in HDPE and PP.

Tensile tests on four separate stress-whitened samples were can-led out with predetermined levels of tensile stress under a constant hydrostatic pressure of P = 89.7 MPa (13,000 psi). This pressure level was chosen because it was known from the earlier experiments that complete healing of stress-whitening was possible at that pressure for HDPE.

Full stress-strain curves of the stress-whitened samples under P = 89.7 MPa for HDPE and PP are shown in Fig. 6. The predetermined values of stresses were [sigma] = 13.0 MPa (1880 psi) in the elastic range and a = 21.2 MPa (3060 psi) in the plastic range for HDPE and a 18.5 MPa (2688 psi) in the elastic range and [sigma] = 29.3 MPa (4250 psi) in the plastic range for PP. The samples were loaded to the predetermined elastic limits and unloaded immediately. The dimensions of the samples were measured before and after the tests. No dimensional change was observed when the stress was limited to the elastic range, which was an indicative of the fact that the applied stresses were indeed elastic.

The extent of healing of stress-whitening of the HDPE samples depended on the magitude of the applied stress under that pressure (P = 89.7 MPa). At [sigma] = 13.0 MPa (the corresponding strain [epsilon] = 8.4%), only a partial healing occured while at [sigma] = 21.2 MPa (the corresponding strain [epsilon] = 17%), the healing was complete and a transparent gage section was observed, as shown in Fig. 7. For PP samples, no complete healing was achieved. The cross-sectional view of the unhealed, partially healed (at [sigma] 18.6 MPa and corresponding [sigma] = 8.4%), and nearly completely healed (at [sigma] = 29.0 MPa and corresponding [epsilon] = 20.0%) samples are shown in Fig. 8.

4.3 Morphology of Stress-Whitening on Sample Cross Sections

Macro-morphology of the stress-whitened samples of HDPE and PP was examined by an optical microscope with the magification of 50X. Light and dark regions in the photographs, shown in Fig. 9(a) for HDPE and 9(b) for PP, represent clear (or unwhitened) and opaque (whitened) regions, respectively. The photographs show a two layer structure: the opaque core and a translucent outer shell. It is worth noting that the surface morphology displays a "single-eye" pattern for PP and a "twin-eye" morphology for HDPE. It is also observed that the darkness or stress-whitening was not uniform throughout the core region.

4.4 DSC Study

Virgin samples of HDPE and PP show a single melting peak at 130[degrees]C and 146[degrees]C, respectively. A stress-whitened HDPE sample, which was deformed at P = atm shows a lower melting shoulder. The main melting peak increased its height and sharpness with increasing degree of healing [Fig. 10(a)]. For PP, a similar result could be observed in its melting behavior except that the shoulder was small and the change of the shoulder with healing was also negligible. [Fig. 11(b)].

4.5 Degree of Healing Determined by Laser Beam Transmission

The healing may be quantified by means of laser light transmission through a thin layer (0.42 mm thick) of a cross-sectional area of stress-whitened and healed samples.

When the laser beam passed through two microscope glass slides without a sample, the intensity was about 435 mV. As the beam moved from one side of a sample to the other across a diametrical line on a sliced, stress-whitened sample of HDPE, the intensity decreased because of scattering from stress-whitening to level [I.sub.ave] = 315 mV). The same test was carried out on a sliced sample of partially healed and completely healed HDPE and the average of transmitted intensities were about [I.sub.ave] = 380 mV and [I.sub.ave] = 410 mV, respectively. It can be seen in Fig. 11(a) that the complete healing in HDPE sample tested at P = 89.7 MPa (13,000 psi), as seen in Fig. 4, is confirmed, that is the sample is nearly transparent. However, the degree of healing is not uniform across the diametrical lines.

For PP, partially healed patterns were obtained for both samples tested at P = 44.8 MPa and P = 89.7 MPa. When the laser light moved in and out the edges of the sample, it scattered badly. The non-uniform healing across the diametrical lines are quite apparent. Unlike that in Fig. 5, which appeared to be completely healed at P = 89.7 MPa, some light scattering is observed in this test. It could be that the healing is non-uniform along the length as well.

IV. DISCUSSION

Stress-whitening is known as a dilatational process (24). If dilatation of a sample is suppressed during a tensile test, stress-whitening will not occur. This hypothesis was tested by superimposing hydrostatic pressure on tensile stress. We obtained cold-drawn samples that were free of stress-whitening.

The test result led us to the idea that the permanently set stress-whitening might be healed or reversed. First, we found that application of hydrostatic pressure alone to the stress-whitened samples, regardless of its intensity, did not heal it. However, healing could be accomplished by subjecting the samples simultaneously to tensile stress and hydrostatic pressure. What it suggests is that normal stresses alone cannot heal stress-whitening. Rather, the shear component of the applied tensile stress superimposed on hydrostatic pressure is responsible for healing stress-whitening.

The maximum shear stress, [[tau].sub.max] = [sigma]/2, occurs on the planes which make 45[degrees] to the direction of the applied tensile stress [sigma]. Therefore, the macroscopic mechanism for healing appears to be as follows. The stress element (or stress plane) corresponding to tensile load superimposed on hydrostatic pressure containing an ellipsoidal void is shown in Fig. 12(a). The equivalent state of stress to the above with the maximum shear stress superimposed on hydrostatic pressure that contains an ellipsoidal void can be shown as in Fig. 12(b). When a finite shear deformation occurs, the deformed stress plane would appear to be distorted owing to shear stress and decreased in size owing to hydrostatic pressure [Fig. 12(c)]. Consequently, the ellipsoid is now flattened or collapsed.

The healed material resists reappearence of stress-whitening under reloading of tensile stress. The healed sample would not redevelop stress-whitening even when it was subjected to tensile stress either to the elastic or plastic ranges. Specifically, we carried out a tensile test in the following sequence: 1) applied tensile load to a stress-whitened sample under hydrostatic pressure (P = 89.7 MPa). The stress-whitened section was healed and became transparent, and 2) the same sample was again subjected to a tensile load at atmospheric pressure to the extent that additional cold drawing occurred at either side of it. The newly cold-drawn section was stress-whitened. The healed region, however, remained clear; that is, no stress-whitening reappear or reformed in this section. The sample with the above mentioned sequence of loading is shown in Fig. 13.

It seems that when a stress-whitening collapses, only van der Waals forces will be developed at the newly formed interface at or below room temperature; therefore, it would open up again when it was reloaded. But this was not the case. It is not likely also that diffusion of chain ends into the new interface would occur at that temperature, either. One possible explanation for non-reappearance of stress-whitening in the healed section is that the direction of the applied tensile load coincides with the the major axis of elongated (or collapsed) ellipsoid, such that the reopening of the elongated ellipsoid was not possible. However, if the applied tensile load is perpendicular to the major axis, it might be possible to reopen it.

Stress-whitening would not be expected to form within the crystallites that are not broken up during orientation as in the structural model of Peterlin [25], Ward [26], or Provorsek [27]. Therefore, since complete healing takes place above [T.sub.g] but far below [T.sub.m] for HDPE and PP, it is unlikely that molecular chain movement in the well defined or undamaged crystallites are involved in the healing process, but rather the chains in the amorphous regions and the interface between the crystallites and the amorphous regions would be involved in the healing process.

It is also found that a cold-drawn sample under hydrsotatic pressure resists stress-whitening in a subsequent drawing at atmospheric pressure. A sample was pulled under 100 MPa (1 kbar), which produced clear cold-drawn section. The sample was subsequently drawn again in atmospheric pressure, which produced additional stress-whitened sections on either side of the clear section. That is, the clear section remained clear after additional drawing. This result is shown in Fig. 14.

The changes observed in the melting behavior of stress-whitened and healed samples of HDPE (Fig. 11) are due probably to breakup of spherulites during cold drawing and the newly formed crystallites are with a narrow distribution in size and shape for increased sharpness of the melting peak.

V. CONCLUSIONS

(1) The permanently stress-whitened HDPE and PP due to prior drawing at atmospheric pressure can be healed when they are subjected to a tensile stress superimposed on hydrostatic pressure. The level of the applied tensile stress may be Within the elastic range or in the plastic range of the stress-strain curve of the materials. Hydrostatic pressure alone, that is, normal stress only, does not heal stress-whitening.

(2) The shear stress component of the tensile stress superimposed on hydrostatic pressure is responsible for healing stress-whitening.

(3) The healed material appears to be resistant to reformation of stress-whitening under tensile load. When a healed sample is resubjected to a tensile stress at atmospheric pressure to the extent that new cold drawing occurs in either side of it, the healed section remains free of stress-whitening, even though the newly formed regions on either sides of it stress-whitened. One possible explanation for non-reappearance of stress-whitening in the healed section is that the direction of the applied tensile stress coincides with the the major axis of elongated (or collapsed) ellipsoid, such that the reopening of the elongated ellipsoid would not be possible. That is, if the applied tenisle load is perpendicular to the the major axis, it might be possible to reopen it.

(4) Cold-drawn section of HDPE at high pressure, thus without stress-whitening, resists formation of stress-whitening on it in subsequent colddrawing.

(5) Healing of stress-whitening increased optical clarity. Thus the clarity can be used to determine the degree of healing. Healing appears to initiate at the outer surface and moves in toward the center region of the circular cross-section of the samples.

(6) Stress-whitening would not be expected to form within the crystallites that were not broken up during orientation process as in the structural model of Peterlin, Provorsek, or Ward. Therefore, since complete healing takes place above [T.sub.g] but far below [T.sub.m] for HDPE and PP, it is unlikely that molecular chain movement in the well defined or undamaged crystallites are involved in the healing process, but rather the chains in the amorphous regions and the interface between the crystallites and the amorphous regions would be invoked in the healing process.

(*.) Department of Mechanical & Aerospace Engineering Rutgers University 98 Brett Rd, Piscataway, NJ 08854-8058

(**.) Department of Polymer Science and Engineering Kumho National University of Technology Kumi, Kyungbuk 730-701, Korea

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Author:PAE, K. D.; CHE, H.-C.; LEE, J. K.; KIM, N.-H.
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Date:Aug 1, 2000
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