A theory of the environmental stress cracking of polyethylene

It is proposed that the environmental stress cracking of polyethylene is caused by stress-induced swelling and plasticization of certain favourably oriented amorphous regions in polyethylene. The criteria for stress-induced swelling together with the criteria for little swelling at zero stress show that a vigorous stress cracking agent will have a solubility parameter close to that of polyethylene and a large molar volume. Some detergents fit into this category.     

Kinetics of environmental stress cracking in high density polyethylene

The observation of environmental stress crack (ESC) growth in high density polyethylene (HDPE) in a 10% lgepal CO-630 solution is reported using double-edge- notched specimens, which allow a fracture mechanics approach. Below the initial stress intensity factor K₁ value of 0.4 MPa m^1/2, the cracking process consisted of both an incubation time for cracking, t_d′ and a crack growth stage. The incubation time is stress dependent (decreasing with increasing stress), while the crack growth exhibits a root time $\text{(t}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}$ dependence and is relatively stress independent. The incubation time is the time necessary to generate a dry void craze structure sufficient to allow the PE to absorb the aggressive liquid. As a consequence of the liquid transport in the craze structure, the crack growth is believed to be controlled by the velocity of the liquid entering the void/fibril structure where capillary pressure is the driving force. The incubation times were determined to be more significant than the actual average crack growth rates for the PE samples tested. Injection moulding orientation increases the average crack growth rate without significantly changing the incubation time.     

Environmental stress cracking of low molecular weight high density polyethylene

Stress intensity factor (K) — crack speed ($\text{a}\text{?}$) relationships have been obtained for environmental stress cracking (ESC) of specimens of a high density polyethylene having different thermal treatments. Also, scanning electron microscope examination of ESC fracture surfaces has been carried out and correlation between K and the fracture surface appearance has been established. It appears that at low values of K the failure takes place by interlamellar crack propagation; as K increases the mechanism undergoes a transition to void formation and growth, with voids beginning to appear within the spherulites. At high K the failure is entirely by void formation and growth.     

Environmental stress cracking of polyethylene

Deformation of polyethylene in environmental stress cracking (ESC) agents results in changes in both the mechanism of deformation and structure of the resulting drawn material. Stress-cracked failure surfaces are highly fibrillar, the fibrils having less elastic recovery than those in samples drawn in air. In thin films drawn in ESC agents, small blocks of the lamellae remain undrawn and attached to the fibrils drawn across micronecks. The ESC agents are suggested to weaken the cohesion between the fibrils in samples drawn beyond yield as well as the cohesion between mosaic blocks or similar structural elements in the original lamellae as they are being reoriented to form the fibrils. The stress is thus supported by a number of independent, nonuniform fibrils rather than a coherent structure; the weakest of these fibrils fail in turn as the crack propagates through the sample.     

Prediction of environmental stress cracking resistance in linear low density polyethylenes

Recently it has been shown that the strain hardening modulus (G_p) at 80°C can be used to predict the environmental stress cracking (ESC) resistance of polyethylenes. The advantage of using strain hardening to determine ESC resistance is that the data may be obtained relatively easily and quickly using simple tensile test equipment. In this article, the strain hardening modulus has been used to predict the ESC resistance of three grades of linear low‐density polyethylene. Unlike in the previous research, the measurements were conducted at room temperature enabling tests to be performed without the need of a temperature‐controlled oven. This was achieved by reducing the strain rate to increase the sensitivity of the technique and increasing the thickness of the specimens to improve the repeatability. The strain hardening modulus data were found to correlate well with the ESC results obtained from long‐term full notch creep tests and are consistent with the known molecular structure of the polyethylene grades. POLYM. ENG. SCI., 2008. © 2007 Society of Plastics Engineers     

Environmental stress cracking and morphology of polyethylene

Studies have been made of the fracture surfaces of high density polyethylene which failed by environmental stress cracking at very low stress. The failure appears to be almost entirely brittle, contrary to the evidence of plastic deformation and void formation reported by other workers. The failure occurred either in an interlamellar manner within a spherulite, or, when the lamellae matched poorly across an interspherulitic boundary, by cracking along that boundary. Because of the brittle nature of the failure, the fracture surface provides information on spherulite morphology in bulk crystallized material.     

The role of intercrystalline links in the environmental stress cracking of high density polyethylene

Results of dynamic mechanical spectroscopy, infrared spectroscopy, and tensile stress-strain data show that the non-ionic surfactant Igepal CO-630, often used as a stress cracking agent, and water are absorbed by high density polyethylene to cause an internal stress relaxation of the intercrystalline tie molecules. The resulting molecular rearrangements produce changes in both the crystalline and amorphous regions. Thus, a molecular mechanism is proposed for the long-term aging process based on the results of accelerated aging in the presence of an environmental stress cracking agent.     

Molecular weight dependence of melt crystallization behavior and crystal morphology of low molecular weight linear polyethylene fractions

Melt crystallization behavior and corresponding crystal morphology of five low molecular weight (3,900 ≤ MW ≤ 20,800) linear polyethylene (PE) fractions have been investigated. The overall crystallization data indicate that the lower molecular weight (MW) fraction possesses a higher crystallization rate at the same undercooling (ΔT). On the contrary, at the same crystallization temperature (T_c) the rate increases with MW. The Avrami exponent (n) varies from ca. 3 to 4 with decreasing ΔT for the fractions studied, which implies the nucleation process changes from athermal type to thermal type as T_c increases. For the low MW PE’s, the different crystal growth regimes (regime I and II) have been first time identified via linear crystal growth rate (G) measurements. The regime I/II transition temperatures are close to previously reported data, which were obtained through a different method. As reported for intermediate MW PE’s, the transitions occur at an almost constant ΔT of 17.5±1 °C for each fraction studied. Morphological study shows that single crystals could be formed isothermally at low ΔT’s. Typical banded spherulites and axialites, which are MW and ΔT dependent, are also observed. Orthorhombic structure is ascertained to be the dominant crystal structure that exists irrespective of MW and crystal growth regime.     

Molecular weight distribution of polyethylene terephthalate in homogeneous,continuous-flow-stirred tank reactors

Poly(ethylene terephthalate) (PET) formation in homogeneous, continuous-flow-stirred tank reactors (HCSTRs) operating at steady state has been simulated. The feed to the reactor is assumed to consist of the monomer bis-(hydroxyethyl) terephthalate and monofunctional compound (MF₁) cetyl alcohol. The overall polymerization is assumed to consist of the polycondensation, reaction with monofunctional compounds, redistribution, and cyclization reactions. At a given time, the reaction mass consists of polyester molecules (P_n), polyester molecules with an ending of molecules of monofunctional compound (MF_n), and cyclic polymers (C_n). A mass balance for each of these species in the reactor gives rise to a set of algebraic equations to be solved simultaneously. The MWD calculations show that the redistribution reaction plays a major role and cannot be ignored, This result is in contrast lo the observation for semi-batch reactors, for which redistribution becomes important when the cyclization reaction is included. For the same residence times of semi-batch and HCSTRs, the latter gives considerably lower-number average molecular weight, N_av, and polydispersity index, ρ. However, for the same conversions, the ρ for CSTR is higher. The concentration of the monofurctional compound, [MF₁]₀, in the feed and the reactor temperature both influence ρ, but the effect is small within the range studied.     

Environmental stress cracking of polyethylene: Temperature effect

Polyethylene was drawn at temperatures ranging from 30° to 60°C in aggressive and nonaggressive environments. Fibrillation was found to occur in the aggressive environment, and this effect increased with temperature. The temperature effect was more prominent at lower strain rates. Thin films drawn in aggressive environments deformed inhomogeneously. Again, this effect was found to increase with increasing temperatures. Single crystal deformation was also found to be inhomogeneous, and “solvation” of the amorphous surface layer occurred in the presence of the aggressive environment. Infrared measurements of sorption under different loads indicated that there is an increase in the amount of sorbed materials with increasing load. Dynamic mechanical studies revealed the intracrystalline regions to be affected preferentially.     

Embrittlement and stress cracking of polyethylene by fuming nitric acid

Oxidative embrittlement and stress cracking of high-density polyethylene has been studied by using fuming nitric acid at 60°C as the oxidative agent. Oxidative stress cracking was found to be insensitive to changes in molecular weight in contrast to environmental stress cracking in a surface-active medium. The oxidative attack was found to be influenced by the surface crystalline texture of the polyethylene. Oriented polyethylene showed an increased resistance to oxidative embrittlement and stress cracking, the mode of failure being dependent upon the prior annealing treatment.     

Optimal process operation for the production of linear polyethylene resins with tailored molecular weight distribution

An optimization model is presented to determine optimal operating policies for tailoring high density polyethylene in a continuous polymerization process. Shaping the whole molecular weight distribution (MWD) by adopting an appropriate choice of operating conditions is of great interest when designing new polymers or when improving quality. The continuous tubular and stirred tank reactors are modeled in steady state by a set of differential‐algebraic equations with the spatial coordinate as independent variable. A novel formulation of the optimization problem is introduced. It comprises a multi‐stage optimization model with differential‐algebraic equality constraints along the process path and inequality end‐point constraints on product quality. The resulting optimal control problem is solved at high computational efficiency by means of a shooting method. The results show the efficiency of the proposed approach and the benefit of predicting and controlling the complete MWD as well as the interplay between operating conditions and polymer properties. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Ultra-high-modulus linear polyethylene through controlled molecular weight and drawing

Recent work is presented on the plastic deformation of linear polyethylenes. This work demonstrates the importance of molecular weight and initial morphology in determining the drawing behavior, and shows that by appropriate choice of these two factors a substantial increase in draw ratio and hence stiffness can be achieved over conventionally-oriented polyethylenes. Under optimum conditions a modulus of approximately 700 Kbar was obtained at a draw ratio of ～30. These very high modulus materials displayed an extensibility to break of at least 3 percent and a strength of about 4 Kbar. In some cases they also exhibited very high melting points (～139°C) and exceptionally good thermal stability.     

Direct evidence for the existence of a craze at the crack tip in environmental stress cracking of polyethylene

Liquid nitrogen fracture tests have been carried out to produce direct evidence of the existence of a voided region, the craze, ahead of the crack in environmental stress cracking of polyethylene. Evidence of crazing is presented for both low and high density polyethylenes.     

Radiation-induced polymerization of ethylene in pilot plant. V. Molecular weight distribution of polyethylene

The molecular weight distribution of the polyethylene produced by radiation in a large-scale pilot plant at pressures of 105–395 kg/cm² and temperatures of 30–80°C was determined by gel permeation chromatography and discussed in connection with the polymerization conditions. The bimodal molecular weight distribution was observed in most of polymers. The number-average molecular weight at two peaks are 10⁴, and 10⁵, respectively. The fraction of the peak at higher molecular weight increased with pressure and mean residence time, and with decreasing dose rate and temperature. The distribution was unimodal in the early stage of the operation and became bimodal, remaining unchanged in the later stage. The distribution also changed from bimodal to unimodal with rising temperature. These results were consistent with those in static batch experiments and well explained by the polymerization scheme assuming two physical states to be different in polymer mobility.     

Branching and molecular weight distribution of polyethylene SRM 1476

A method of determining the distribution of branching in a polymer is developed employing limiting viscosity numbers (intrinsic viscosity), gel permeation chromatography (GPC), and absolute molecular weight determinations of fractions of the whole polymer. A molecular weight calibration of the GPC column set is first determined empolying these fractions. From the limiting viscosity number measurements of these fractions and their molecular weight distribution determined from the GPC chromatogram, the viscosity–molecular weight relationship is determined by a nonlinear least-squares fitting procedure. For the same molecular weight, the limiting viscosity number of the branched polymer is less than the limiting viscosity number of the linear polymer. From the ratio of the two, the number of branches per unit molecular weight of the branched polymer is calculated. The method was applied to SRM 1476, the standard reference branched polyethylene issued by the National Bureau of Standards. The branching density for the constituents of SRM 1476 rise from zero at molecular weights less than 10,000 to about 6 to 8×10^?5 at molecular weights of 50,000 and above. The branching of SRM 1476 was also determined by the method of Drott and Mendelson, giving a result in fair agreement with the above method.     

Resistance of a polyetherimide to environmental stress crazing and cracking

The environmental stress crazing and cracking (ESC) behavior of an aromatic polyetherimide (PEI) has been characterized in a wide spectrum of organic liquids and compared to the behavior of several other glassy thermoplastics. PEI's response is qualitatively similar to that of the other resins for each of which ESC resistance reaches a minimum in solvents having solubility parameters close to that of the resin. Taken as a whole, the ESC resistance of PEI is found to be quantitatively superior to that of any other glassy resin for which similar data are available for comparison.     

宽峰聚乙烯／蒙脱土纳米复合材料的制备

利用2种不同的方法制备了2种氢调敏感性不同的蒙脱土／氯化镁／四氯化钛（MMT／MgCl2／TiCl4）催化剂,利用这2种催化剂及其混配催化剂,通过原位聚合法,制备出一系列宽峰聚乙烯纳米复合材料,采用X_射线衍射仪（XRD）、凝胶色谱测试分析（GPC）及力学性能测试等方法对催化剂及聚合产物进行分析,结果表明,2种催化剂以及按照不同比例混合的混配催化剂均表现出较高的聚合活性,XRD测试结果表明,蒙脱土片层在乙烯聚合过程中发生了插层及剥离,以单片层或几层共存的形式分散于聚乙烯基质中;用混配催化剂可制得宽峰聚乙烯纳米复合材料,Mw／Mn=7．23,并且聚合物的堆积密度达到工业生产的标准,宽峰聚乙烯纳米复合材料的综合力学性能较工业产品5000S及工业上应用的管材料有很大的提高。