Relationship of fracture behavior and morphology in polyolefin blends.
Inexpensive combined thermoplastics are of great interest for technical applications. In particular, blended materials of increased fracture toughness combined with balanced stiffness and strength as well as appropriate processing and production properties are highly desirable. Currently, this goal is reached by methods such as copolymerization or modification of the corresponding elastomer.
Another possible method of improving the performance of thermoplastics is mixing two components with and without compatibilizer (CP) (1). However, to our knowledge, mechanical properties of polyolefin blends, especially the modulus, found in the literature mostly follow the rule of mixture Voigt model (2-4), and morphologies show coarse dispersion; but there are also antagonistic (3, 5, 6) and synergistic effects reported (7-9).
Production of blend materials with toughness and flow properties beyond the mixing rule level is highly desirable. To optimize the toughness properties it is necessary to know the relationship of fracture behavior and morphology.
The isotactic polypropylene (PP) and high-density polyethylene (HDPE) used are products of Hoechst AG. HDPE and PP were mixed with addition of a stabilizer and with addition of a stabilizer and a compatibilizer in different mixing ratios ranging from 5wt% HDPE/95wt% PP (5HDPE/95PP) to 95wt% HDPE/5wt% PP (10). The blend samples were prepared on a single-screw extruder.
Injection molding of test pieces was performed according to ISO/DIS 3167 Type A. Processing was carried out under typical conditions used for PP. Microtome sectioned injection molding test pieces of the pure materials and of the blends were stained by Ru[O.sub.4] and imaged by transmission electron microscopy (TEM). The fractured surfaces were imaged by scanning electron microscopy (SEM).
The morphology imaged by TEM was analyzed by determination of particle size and distance of the embedded phase as well as the specific interphasial area. Notched impact testing was done according to ISO 179, tensile testing according to DIN 54455, and Young's modulus was determined according to DIN 53457. For the fracture mechanic measurements, an instrumented Charpy impact tester with 4J work capacity was used and load (F) - deflection (t) - diagrams were recorded (11, 12). Single-edge notched (SENB) specimens with the dimensions 80 x 10 x 4 [mm.sup.3] were used for this study. These test conditions made it possible to determine geometry-independent fracture mechanics values against unstable crack growth (13, 14). J-integral values were determined according to the approximation method from Sumpter and Turner (15), and the crack opening displacement (COD) values were calculated based on the plastic hinge model (16, 17).
During the tensile test, the deformation behavior was investigated by acoustic emission analysis. Combined with data extracted from SEM images of stretched test specimens, this gave important information about mechanisms of toughness. From characteristic properties derived by acoustic emission analysis, the rate and the energy of AE signals were chosen.
We focus on the TEM images [ILLUSTRATION FOR FIGURE 1 OMITTED] of the blend material (50HDPE/50PP) with and without compatibilizer to get information about the morphology, since this specific content shows a significantly higher impact strength. A very detailed report about this topic can be found elsewhere (18).
All images display heterogenity of the blends. The 50HDPE/50PP blend system can be described as a fine bicontinuous network-like matrix filled with larger HDPE particles. Addition of a compatibilizer reduces the particle size in the dispersed phase by a factor of 1.7. The averaged diameter of the HDPE particles in the pure blend was about 3.0 x [10.sup.-3] mm, whereas in the blend with compatibilizer it was reduced to 1.8 x [10.sup.-3] mm. The images taken at highest magnification show lamellae of the dispersed phase. The HDPE phase consists of thick lamellae; PP lamellae are very short and thin (18).
In the following text the effect of the morphology of the blend with respect to the mechanical properties will be presented.
The Young's modulus [ILLUSTRATION FOR FIGURE 2 OMITTED] for a HDPE content up to 70% is higher than calculated by the rule of mixture (Eq 1):
[E.sub.blend] = [[Phi].sub.1] [multiplied by] [E.sub.1] + [[Phi].sub.2] [multiplied by] [E.sub.2] (1)
where E is the Young's modulus and [Phi] the contents of components I and 2, respectively. At high PP content, a pronounced stiffening effect is found, which is caused by interactions of the structures of HDPE and PP.
The yield stress as a function of the content of each component changes within the limits given by the pure materials. The influence of the compatibilizer is small.
The elongation at yield point exhibits a small maximum. The same effect, but much more significant, is mirrored by the values of the impact strength [ILLUSTRATION FOR FIGURE 3 OMITTED].
Maximum impact strength is detected at 50HDPE/50PP plus compatibilizer. The impact strength is five times higher than that of HDPE and eight times higher than that of PP. In the blend without compatibilizer, the maximum is found for 60HDPE/40PP. Both maxima correlate with the dependence of the elongation at yield point.
Additionally, fracture mechanical investigations were performed by the instrumented Charpy impact test to determine the instable crack growth [ILLUSTRATION FOR FIGURE 4 OMITTED]. PP behaves almost linearly; the crack growth is instable. The fracture of HDPE can be characterized as elastic-plastic behavior possessing a high percentage of stable crack growth. Samples containing 40% and more HDPE indicate elastic-plastic behavior; crack growth changes at 50% HDPE content from dominant instable to dominant stable.
The compatibilizer causes elastic-plastic fracture of the blend over the whole concentration range; the crack growth is dominant stable.
Toughness behavior of the blends is described well by the [J.sub.id]-integral [ILLUSTRATION FOR FIGURE 5 OMITTED] and the ([[Delta].sub.Id]) crack opening displacement value [ILLUSTRATION FOR FIGURE 6 OMITTED].
The influence of the compatibilizer on the fracture toughness with respect to initiation of instable crack growth [ILLUSTRATION FOR FIGURES 5, 6 OMITTED] is clearly much less than with respect to the impact strength. Nevertheless, there is some effect caused by the compatibilizer; the material stores energy, detected by comparatively large crack propagation energies [ILLUSTRATION FOR FIGURE 7 OMITTED].
Acoustic Emission Analysis
The results presented before let us conclude that the most important property of the investigated blend is the high toughness, especially with respect to technical applications.
To detect the mechanism causing the increased toughness, acoustic emission analysis (19, 20) was performed. We mainly attempted to answer the questions: In which stage of deformation does acoustic emission happen in the test sample? Which phase in the polymer blend is responsible and how can this mechanism be described?
Tensile tests (100 mm/min crosshead velocity) combined with acoustic emission analysis were performed. Under these conditions HDPE and PP emitted only few signals [ILLUSTRATION FOR FIGURES 8, 9 OMITTED].
Pure HDPE gave some signals beyond the yield point, e.g. in the flow region [ILLUSTRATION FOR FIGURE 9 OMITTED]. It is known that in semicrystalline materials, processes such as orientation and fibrilation, splitting of fibrils, and rupture of fibrils possibly occur (21). Since acoustic emission is due to elastic recovery during the fracture process, the measured signals are caused by fibrilation and fracture of fibrils. The primary process detected by AE is splitting of fibrils.
Compared with the pure materials, the blends with and without compatibilizer [ILLUSTRATION FOR FIGURES 10, 11 OMITTED] exhibit much more pronounced AE signals. In the blend with compatibilizer, the highest acoustic emission energy is detected.
SEM images taken from stretched tensile test samples verify the above-described results [ILLUSTRATION FOR FIGURE 12 OMITTED]. The pure PP displays only very low elongation; no fibrilation is evident. Fibrilation in the pure HDPE is slightly higher: single fibrillations and a few fractured fibrils are recognized. In the 50HDPE/50PP blend with compatibilizer, strong elongation and fibrilation are recorded by SEM. The number of fractures of fibrils is strongly increased compared with the pure materials.
Similar mechanisms are found for the dynamic fracture processes [ILLUSTRATION FOR FIGURE 13 OMITTED]. Fracture surfaces of unnotched test pieces imaged by SEM proved the effect of the compatibilizer even under these extreme relaxation conditions (T = -50 [degrees] C). Adding the compatibilizer improves the plastic deformation performance for both components in the blend. This process is considered the most important mechanism for measuring the toughness.
The highest emission energies were detected for the blend with compatibilizer. This was correlated with a very pronounced fibrilation in the material imaged by [TABULAR DATA FOR TABLE 1 OMITTED] SEM. In the following a correlation between specific particle sizes and the impact strength was set up.
To interprete the toughness quantitatively, the presented mechanisms were modeled according to (22) (Eq 2).
[Mathematical Expression Omitted], (2)
[A.sub.spec] specific interphasial area
[r.sub.p] radius of the fracture process zone
[G.sub.Ic] energy release rate
[[Phi].sub.M] matrix volume
[[Phi].sub.I] inclusion volume
[[Phi][prime].sub.I] inclusion volume which causes additional plastic deformation
[a.sub.cN] impact strength modified phase
The dimension of the fracture process zone [r.sub.P] was calculated by the critical crack opening [[Delta].sub.Id] and the strain at rupture of fibrils.
The specific interphasial area [A.sub.[spec.sub.Int]] is determined by computerized image analysis of the TEM image [ILLUSTRATION FOR FIGURE 1 OMITTED]. In principle, this value changes as function of the concentration in the blend. [G.sub.[Ic.sub.Int]] is determined by the double cantilever beam (DCB) test. By this experiment the order of magnitude of adhesion in the blend with and without compatibilizer was estimated. The compatibilizer causes the energy release rate at the interphase to increase by a factor of 10. According to Table 1, [[Phi][prime].sub.I] is equal to the value of [[Phi].sub.I]. According to the model, the contribution to the energy of the detected maximum of the impact strength consists of three parts: 60.3% due to the interphase, 13.8% due to the matrix, and 25.9% due to the inclusion effects.
Figure 14 presents the calculated as well as the experimental values. According to the model, the maximum of toughness is mainly caused by interphase and embedding effects. The matrix component presents a minimum in this region of concentration. Although Eq 2 is just a model, the agreement between experiment and calculation is quite good.
The above-described investigations show that the compatibilizer has a strong effect on the toughness of the polyolefin blend within the analyzed concentrations. There are mainly two significant reasons that lead to the high impact strength:
1. The high phase dispersion and the compatibilizer causes strong interphasial interaction. Results are increased impact toughness of both phases and a large amount of debonding energy.
2. Embedding of small inclusions initiates additional fibrilation process in the surrounding matrix, which dissipates impact energy.
We are grateful to Dr. Frechen and Dr. Heckmann of BASF AG for providing the TEM images.
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|Author:||Niebergall, U.; Bohse, J.; Seidler, s.; Grellmann, W.; Schurmann, B.L.|
|Publication:||Polymer Engineering and Science|
|Date:||Jun 1, 1999|
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