Environmental Stress Crack Resistance ESCR

The ESCR performance of a resin is not easily modeled. A laboratory technique for the preparation of thin films of HIPS materials for the study of deformation processes by microscopy allows the deformation process to be better understood. The transmission electron microscope (TEM) allows direct visualization of the crazes themselves in thin films. For good contrast between the crazes and the bulk polystyrene, thin, cast films from 0.5 to 2 p,m are required, and also staining of the rubber phase with a heavy atomic species to provide contrast between the rubber and the polystyrene. Another intricacy of this method requires a solution of the HIPS material in a 65 35 methyl ethyl ketone-toluene solution to prevent significant swelling of the rubber particles during the preparation process. 

Heat aged (2 h, 200 °C, nitrogen-purged vacuum oven) 2.1 8.1 113 

Aggression of a liquid upon a rubber-modified polymer under stress is well studied and depends upon capillary transport of the liquid to the crack tip. Crack propagation is resisted by the molecular weight of the continuous phase and aided by the plasticization effect of the aggressive liquid or plasticizer in the polymer matrix. High molecular weight, unplasticized products can be expected to have enhanced ESCR to aggressive liquids. 

Polymer additives that can be incorporated in the polymerization process or post-compounded into the HIPS product can add significant enhanced functionality. The use of 2 % polyisobutene (PIB), for instance, in the HIPS process feed solution in place of the usual plasticizer can dramatically increase ESCR by a factor of 10 for both standard- and ESCR-grade HIPS. It is interesting that even in the presence of PIB, RPS still has a significant effect on ESCR, as seen in a comparison of extrusion-grade with ESCR-grade HIPS in Table 12.12. 

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Resistance to Chemical Environments and Solubility. As a rule, amorphous plastics are susceptible, to various degrees, to cracking by certain chemical environments when the plastic material is placed under stress. The phenomenon is referred to as environmental stress cracking (ESC) and the resistance of the polymer to failure by this mode is known as environmental stress cracking resistance (ESCR). The tendency of a polymer to undergo ESC depends on several factors, the most important of which are appHed stress, temperature, and the concentration of the aggressive species. 

Environmental standards, 10 158 Environmental stress crack resistance (ESCR), 20 166... 

High-impact grades present better impact resistances even at low temperature, higher flexibility and environmental stress cracking resistance (ESCR). The butadiene-styrene block copolymers are transparent but the alloys made of polystyrene and polybutadiene are not. 

An example of the dependence of polymer properties on catalyst activation temperature is shown in Table 5. Polymers of various MW were made with a Cr/silica catalyst activated at various temperatures. The polymers were then tested for environmental stress crack resistance (ESCR), a measure of their ability to resist chemical attack when polymer strips are placed under stress (bent). The test is conducted in the presence of hot surfactant, and the time to failure is monitored. Like most other polyethylene physical properties, ESCR is usually improved when the MW is increased. Therefore, comparisons of catalysts are usually made at similar melt indices (a surrogate for MW). A broader MW distribution... 

TABLE 5 Environmental Stress Crack Resistance (ESCR) of Polymers Made with Cr/Silica, Activated as Indicated... 

FIGURE 45 Environmental stress crack resistance (ESCR) of polymers made with various comonomers, showing that longer branches tend to enhance the physical properties of the polymer. (Cr/silica, 815 °C, 0.2-0.3 Ml, 0.940 g mL density). 

FIGURE 210 The influence of cocatalyst type and amount on the environmental stress crack resistance (ESCR) of polymers made with Cr/silica activated at 540 °C. The reaction temperature (135-160 °C) was adjusted to maintain a constant HLMI, and with 1-butene added to reach a constant 0.950 g mL-1 polymer density. 

The chemical resistance, especially environmental stress cracking resistance (ESCR), is more affected by processing than by chemical composition. As a general mle, the materials that have greater orientation exhibit higher resistance to environmental stress cracking. This illustrates how important is the temperature control and the molding rate. 

The molecular construction of Dow Primacor copolymers creates internal toughness, abrasion resistance, puncture resistance, and high environmental stress-crack resistance (ESCR). Flexible coatings with good flex-crack resistance and exceptional elongation are possible. 

Even if carbon black does allow some surface oxidation, this damage may be permitted by a specification, as long as pressure resistance elongation at break, and environmental stress cracking resistance (ESCR) are maintained. 

Ree van Reenen, A. J., Shebani, A. N. The effect of molecular composition and heterogeneity on the environmental stress cracking resistance (ESCR) of propylene impact copolymers. Polym. Degr. Stab. 94 (2009) 1558-1563. 

ROLE OF AN ACTIVE ENVIRONMENT OF USE IN AN ENVIRONMENTAL STRESS CRACK RESISTANCE (ESCR) TEST IN STRETCHED POLYETHYLENE A VIBRATIONAL SPECTROSCOPY AND SEM STUDY Lagaron J M Pastor J M Kip B J DSM Research Valladolid,Universidad... 

LLDPE is a substantially linear polymer with numerous short branches, primarily made by copolymerization of ethylene (C2) with short-chain alpha-olefins (e.g., 1-butene, 1-hexene, or 1-octene, i.e., C4, Ce, or Cg). LLDPE is more difficult to process than LDPE, but it has higher modulus, strength, impact, and puncture resistance than LDPE and better environmental stress cracking resistance (ESCR). Thinner films of LLDPE replace those of LDPE in packaging applications. 

Phillips Petroleum patented PE blends prepared by dry powder blending followed by compounding in an internal mixer or an extruder. The two PEs were 30-70 wt HDPE, narrow MWD LLDPE, and 30-70 wt% HDPE, having significantly different MW (Bailey and Whitte 1984). The former was Z-N-LLDPE [Ti catalyst p < 955 kg m . Ml = 0.1-50 g/10 min, and MJM = 2-10], while the second comprised low-MW HDPE [Cr catalyst p > 955 kg m, MI = 25 00 g/10 min, and M ,/M = 2-35]. The blends were used for the produced pipes, films, and bottles with good mechanical properties and environmental stress crack resistance, ESCR. 

LLDPE [made using a titanium catalyst, having p < 955 kg m, MI = 0.1-50 g/10 min, and Mw/M = 2-10], showed improved processability. The blends were used to produce pipes, films, and bottles with enhanced mechanical properties as well as the environmental stress crack resistance, ESCR. In the following patent 5 0 wt% low-MW HDPE [made using a titanium catalyst, with density p > 955 kg m, MI > 25 dg min , and MJMn = 2-8] blended with 60-95 wt% high-MW HDPE [from chromium catalyst, with density p > 930 kg m, MI = 1.5-15 dg min and M /Mn = 6-100] yielded blends with MI = 0.05 dg min and ESCR. In the last patent... 

An important measure of chemical resistance which is often used in household and industrial containers is environmental stress-crack resistance (ESCR). Many variations of the test are known and used, such as ASTM D1693, Conditions A and B. In these tests, the plastic part, a molded bar, a bottle, or a tube, etc, is placed under a standardized stress usually at elevated temperature, and exposed to a surface active agent. For example, one test uses 10% surfactant in aqueous solution at 60° C. The time needed for the sample to break is then recorded. The surfactant is thought to aid in the relaxation and disentanglement of chains. 

Environmental stress crack resistance (ESCR) is critical to many blow molded bottle applications. Changes in MW which may affect rheology may also affect ESCR, therefore it was a logical step to observe changes in ESCR with multiple extrusion passes. 

Environmental stress crack resistance (ESCR) is an important property for many applications because it estimates the service life of polyethylene in any particular application. As pointed out by A. Lustiger [32], the term Environmental Stress Cracking was defined in 1959 by J.R Howard to... 

Blends of two samples of polyethylene in which each blend component was prepared with a single-site catalyst has also been reported [38] to provide an injection molding grade of HDPE with greatly improved environmental stress crack resistance (ESCR). A commercial grade of HDPE suitable for injection molding and prepared with a Ziegler catalyst was used as a control material. This data is shown in Table 6.17. 

An alternative approach was adopted by Kurelec et al. [84] who determined true stress-true strain curves at 80°C for a range of polyethylenes. It was shown that the slope of the tensile curve above the natural draw ratio (called the strain-hardening modulus) correlated well with the measured stress crack resistance (Figure 13.31). These results are entirely consistent with those obtained by Capaccio and co-workers and Ward and co-workers described above. Kurelec et al. found similar effects on the environmental stress cracking resistance (ESCR) performance with regard to short chain branches, and elaborated these in terms of the exact nature of the branches, particularly with regard to bimodal molecular weight distribution polymers. 

Figure 13.31 Environmental stress cracking resistance (ESCR) plotted against strain-hardening modulus for a range of EIDPEs, both catalyst based with broad molecular weight distribution, and bimodal. Points labelled reproducibility are repeated tests on the same sample. (Reproduced from Kurelec, L, Teeuwen, M., Schoffeleers, El. et al. (2005) Strain hardening modulus as a measure of environmental stress crack resistance of high density polyethylene. Polymer, 46, 6369. Copyright (2005) Elsevier Ltd.)...

Figure 13.32 Environmental stress cracking resistance (ESCR) against natural draw ratio (NDR). Cri and Cr2 are high density polyethylenes with different catalysation regimes, and ZN is an ethylene-hexene copolymer. (Reproduced from Cazenave, j., Seguela, R., Sixou, B. et al. (2006) Short-term mechanical and structural approaches for the evaluation of polyethylene stress crack resistance. Polymer, 47, 3904. Copyright (2006) Elsevier Ltd.)...

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