The effect of molecular weight on slow crack growth in linear polyethylene homopolymers

Huang, You–Min; Brown, Norman

doi:10.1007/bf00540508

Cited by 181 publications

(162 citation statements)

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“…18 Research has shown that the ESCR of HDPE increases with increasing MW. [19][20][21] When the DCP content increases, both the degree of cross-linking and the MW increase. A high MW indicates the presence of long polymer chains that can crystallize into two or more lamellae and form interlamellar connections that improve the ESCR.…”

Section: Resultsmentioning

confidence: 99%

The effect of using the two-step extrusion method on the oxidation induction time value of recycled high density polyethylene

2012

Polym J

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The two-step extrusion method was used to manufacture pipe-grade high density polyethylene (HDPE) material from recycled HDPE. With the two-step extrusion method, dicumyl peroxide (DCP) can completely react after reactive extrusion without adding antioxidants in the first step, which increased the degree of cross-linking and the molecular weight of the recycled HDPE. Then, adding the antioxidant in the second extrusion step can avoid the conflict between the two additives. As a result, this method improved the oxidation induction time (OIT) and the environmental stress cracking resistance (ESCR) value of HDPE compared with the traditional one-step extrusion method. For both Irganox 1035 and Irganox 300, the OIT using the two-step extrusion method is much higher than that using the one-step method. In addition, introducing carbon black (CB) into the black masterbatch affected the OIT. When the CB content was 0.8%, the OIT was at its maximum value. Polymer Journal (2012) 44, 421-426; doi:10.1038/pj.2012.9; published online 7 March 2012Keywords: oxidative induction time (OIT); reactive extrusion; recycled HDPE; two-step extrusion method. INTRODUCTIONPlastics account for an increasing fraction of the municipal solid waste worldwide. Low density polyethylene, high density polyethylene (HDPE) and polypropylene are the three largest components of plastic municipal waste. 1 Recycling contributes to the reduction in resource consumption and pollution. Fortunately, HDPE is an easily recyclable material. When HDPE is broken down for reuse, many materials can be made, such as recycled plastic lumber, toiletry containers, recycling bins, outdoor furniture, liquid containers and pipes. 2 The typical lifetime requirement for gas and water pipes is B50 years, but the pipes can fail in only 1 year because of the environmental stress cracking. 3 In addition, the oxidation induction time (OIT) is widely used for the determination of the thermal oxidative resistance of polyethylene materials. 4 The requirement on the OIT in pipe use is that the OIT value must be 420 min. 5 Therefore, both the environmental stress cracking resistance (ESCR) and the OIT of polyethylene are of particular interest to manufacturers and researchers.Currently, recycled HDPE materials cannot be directly used for producing pipes, because the melt flow index (MFI) of these materials is too high to be extruded for pipes. The cross-linking of PE is an effective way to solve this problem. The most common method of cross-linking is the chemical method, particularly using peroxides as an initiator. Peroxide cross-linking by reactive extrusion is used to cause chain extension or long chain branch, even the cross-linking of HDPE chains, by which the molecular weight (MW) increases,

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Section: Resultsmentioning

confidence: 99%

The effect of using the two-step extrusion method on the oxidation induction time value of recycled high density polyethylene

2012

Polym J

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“…0.959 g/cm 3 and 0.954 g/cm 3 , ESCR increases in the order Cr13 < Cr21 < ZN1 and Cr15 < Cr23 < ZN2, respectively. It is obvious that the greater ESCR of the Cr2 copolymers compared with the Cr1 ones is mainly due to the presence of longer chains (see M w values of Table 1) which improves the build up of tie chain [8] for reasons of chain dimensions with respect to crystal dimensions. Regarding the comparison between Cr2 and ZN copolymers, the preferred distribution of the co-units in the longer chains for the latter ones is a major factor of their excellent ESCR performances owing to the combined benefiting effect on tie chain density, in spite of lacking very long chains (see M w values of Table 1).…”

Section: Experimental Methodsmentioning

confidence: 99%

“…The resistance to stress cracking is generally ascribed to the tensile resistance of the microfibrils. It is mainly governed by the concentration of the load-bearing inter-crystalline tie molecules linking the crystal blocks in the microfibrils [8,9] that slow down the crack opening.…”

Section: Introductionmentioning

confidence: 99%

Structural Approaches of Polyethylene Environmental Stress-Crack Resistance

Cazenave¹,

Sixou²,

Séguéla³

2006

Oil & Gas Science and Technology - Rev. IFP

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Résumé -Approches structurales de la fissuration sous contrainte du polyéthylène en milieu tensio-actif -Ce travail est une contribution à la compréhension et la prédiction du comportement à long terme du polyéthylène en relation avec l'architecture moléculaire. L'exploration de la microstructure semi-cristalline et de la topologie moléculaire constituée de chaînes liantes intercristallines et de boucles de repliement constitue l'objectif majeur du travail. Une cristallisation fractionnée en deux étapes a été utilisée pour la séparation des espèces pouvant facilement cristalliser par repliement régulier de celles qui ne peuvent pas. Le taux de déformation naturel qui se développe au-delà du seuil d'écoulement plastique en traction uni-axiale s'est avéré très sensible à la topologie moléculaire par le biais de la fragmentation lamellaire et du dépliement de chaîne. La diffusion des rayons X aux petits angles a fourni de précieux renseignements semi-quantitatifs sur la régularité d'empilement de la microstructure lamellaire. Les données issues des trois approches révèlent des corrélations avec l'architecture moléculaire en bonne concordance avec l'évolution de la résistance à la fissuration sous contrainte. Ces corrélations sont discutées en termes de cinétique de cristallisation et de topologie moléculaire. Un résultat pratique de ce travail est l'accès à des outils prédictifs fiables pour la résistance à la fissuration sous contrainte. Abstract -Structural Approaches of Polyethylene Environmental Stress-Crack Resistance -This work was aimed at understanding and predicting of the long-term mechanical behavior of polyethylene in relation to molecular architecture. An insight into the semi-crystalline microstructure and the chain topology consisting of intercrystalline tie chains and loose folding loops was the main objective of the

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“…Several material parameters such as molecular weight [7][8][9][10][11] and branch frequency [12,7] have been found to have a beneficial influence on the density of tie molecules. In addition to material parameters, it is also well known that thermal history has a significant effect on the crystalline microstructure of melt-crystallized samples and samples crystallized on quenching rather than isothermally crystallized at higher temperatures are often associated with a higher tie molecule density (fT) [8][9][10][11].…”

Section: Introductionmentioning

confidence: 99%

Effect of branch frequency on dynamic fatigue behavior of slowly notched polyethylene polymers

Yeh¹,

Hong²

1994

J Polym Res

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Samples with the same weight average molecular weight and molecular weight distribution but different branch frequency were utilized to study the effects of branch frequency and thermal history on tie molecule density and their subsequent influence on the slow crack growth of short chain branched polyethylenes. The dynamic fatigue properties are improved significantly with increasing branch frequency and with samples crystalloid at fast cooling rate. However, at temperatures ranging from -20 to 80 °C, the amount of tile failure cycle (Nr) improved due to the slight increase in branch frequency is less than those of samples prepared by crystallization at fast cooling rate. Additionally, it is interesting to note that the drawn fibers observed on the fracture surfaces were larger and longer for samples associated with longer Nr. In fact, it is interesting to note that the average number of tie molecules formed per chain (T(M)) of samples associated with longer Nr is also larger. This increasing in T(M) is suggested to be responsible for the improved fatigue properties of samples associated with larger branch frequency and crystallized at fast cooling rate.

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The effect of molecular weight on slow crack growth in linear polyethylene homopolymers

Cited by 181 publications

References 12 publications

The effect of using the two-step extrusion method on the oxidation induction time value of recycled high density polyethylene

The effect of using the two-step extrusion method on the oxidation induction time value of recycled high density polyethylene

Structural Approaches of Polyethylene Environmental Stress-Crack Resistance

Effect of branch frequency on dynamic fatigue behavior of slowly notched polyethylene polymers

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