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Eingeladener Vortrag
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A phenomenological criterion for an optical assessment of PE-HD fracture surfaces obtained from FNCT
(2021)
The full-notch creep test (FNCT) is a common test method to evaluate the environmental stress cracking (ESC) behavior of high-density polyethylene (PE-HD), e.g. for container materials. The test procedure as specified in ISO 16770 provides a comparative measure of the resistance against ESC using the time to failure of PE-HD specimens under constant mechanical load in a well-defined liquid test environment. Since the craze-crack damage mechanism underlying the ESC phenomenon is associated with brittle failure, the occurrence of a predominantly brittle fracture surface is a prerequisite to consider an FNCT measurement as representative for ESC, i.e. a time to failure dominated by craze-crack propagation.
The craze-crack propagation continuously reduces the effective residual cross-sectional area of the specimen during the test, which results in a corresponding increase of the effective mechanical stress. Thus, a transition to ductile shear deformation is inevitable at later stages of the test, leading usually to a pronounced central ligament.
Therefore, an optical evaluation of FNCT fracture surfaces concerning their brittleness is essential. An enhanced imaging analysis of FNCT fracture surfaces enables a detailed assessment of craze-crack Propagation during ESC. In this study, laser scanning microscopy (LSM) was employed to evaluate whether FNCT fracture surfaces are representative with respect to craze-crack propagation and ESC. Based on LSM height data, a phenomenological criterion is proposed to assess the validity of distinct FNCT measurements. This criterion is
supposed to facilitate a quick evaluation of FNCT results in practical routine testing. Its applicability is verified on a sample basis for seven different commercial PE-HD container materials.
The material parameters melt flow rate (MFR), density (D), notched impact strength (NIS) at –30°C, stress crack resistance [determined with the Full Notch Creep Test (FNCT)] and resistance to oxidative degradation (Ox) have been selected for a comparison of polyethylene grades in the European standard EN 15507. These parameters have a relationship with design type tests of packagings for the transport of dangerous goods.
This paper presents the results of additional investigations to determine the impact of the pre-storage time of jerricans with 55% nitric acid at 40°C and 23°C on the marginal drop heights in drop tests at –18°C, on the MFR and on the tensile properties. Jerricans made of different polyethylene grades were pre-stored with 55% nitric acid for 21, 42 and 84 days at 40°C and for six months at 23°C.
The tests showed that the NIS values at –30°C of polyethylene grades were not comparable with marginal drop heights determined in drop tests at –18°C.
Pre-damage with 55% nitric acid for 21 days at 40°C led to an increase in the marginal drop heights of the jerricans. Altering the test regulations by increasing pre-storage time with 55% nitric acid to 42 days would be an alternative.
The test results demonstrated clearly that pre-storage of the jerricans for six months at 23°C caused a higher increase in the MFR and lower marginal drop heights for the jerricans when compared with pre-stored jerricans for 21 days at 40°C.
The material parameters melt flow rate MFR, density D, notched impact strength N.I.S. at -30 °C, stress crack resistance FNCT and resistance to oxidative degradation Ox (determined by the increase of MFR) have been selected for a comparison of polyethylene grades for one design type in the “European Standard EN 15507 - Transport packaging for dangerous goods - Comparative material testing of polyethylene grades”. The parameters have a systematic relationship with design type tests. The notched impact strength at -30 °C and the resistance to oxidative degradation of polyethylene grades determined with test specimens of compression moulded sheets are directly comparable with drop tests at -18 °C of design types produced of these grades after pre-storage with 55 % nitric acid for 21 days at 40 °C. The impact of the pre-storage time of jerricans with 55 % nitric acid at 40 °C and 23 °C on the marginal drop heights in drop tests at -18 °C, on the MFR and on the tensile properties tensile strength, breaking elongation and the elastic modulus were investigated. Packaging design types (jerricans) made of three polyethylene grades from different producers were pre-stored with 55 % nitric acid for 21,42 and 84 days at 40 °C as well as for six months at 23 °C. The MFR and tensile properties of test specimens taken from the design type side walls as well as the marginal drop heights of the jerricans in drop tests at -18 °C were determined.
The values of the notched impact strength at -30 °C of polyethylene grades are not comparable with the marginal drop heights determined in drop tests at -18 °C. The oxidative damaging impact of nitric acid has a greater effect on the grade with lower resistance to oxidative degradation by reducing more significantly the marginal drop heights with increasing pre-storage time.
Pre-damage with 55 % nitric acid for 21 days at 40 °C, as recommended in ISO 16101 and EN 15507 caused an increase in the marginal drop heights of most design types in drop tests when compared with design types without pre-damage. The test results demonstrated clearly that pre-storage of the jerricans for six months at 23 °C caused a higher increase in the MFR and lower marginal drop heights of the jerricans in drop tests at -18 °C compared with pre-storage of the jerricans for 21 days at40 °C.
The mechanical properties change as a result of diffusion of nitric acid into the polyethylene grades and the reaction of the nitric acid with the polymers. The percentage decrease and increase in the tensile strength, breaking elongation and elasticity modulus of test specimens taken from the design type side walls after different pre-storage times were only partly comparable with the marginal drop heights in drop tests at -18 °C.
Resistance to stress cracking by wetting solution is one of the selected properties together with the corresponding test method FNCT and tolerances for a comparison of polyethylene grades of one design type in the procedural rule on suitability proof for alternative plastic resins used for packagings and intermediate bulk Containers (IBCs) for the transport of dangerous goods.
The environmental stress crack resistance determined by Full Notch Creep Test (FNCT) represents the impacts from the stacking tests at 40 °C performed with Standard liquid wetting solution (without pre-storage) and with normal butyl acetate saturated wetting solution (pre-storage with normal butyl acetate). Environmental stress crack resistance determined with FNCT for the polyethylene grades was related to the times to failure of different jerrican samples made of these grades in stacking tests with 5 % wetting solution and a n-butyl acetate saturated wetting solution (pre-storage with n-butyl acetate) at 40 °C. The FNCT is suitable for comparison of polyethylene grades in relation to environmental stress crack resistance independent of their use as drum, IBC or jerrican material. The stiffness of polyethylene grades should be taken into account to the comparison as jerricans are not only chemically but also mechanically stressed.
Resistance to stress cracking by wetting solution is one of the selected properties together with the corresponding test method FNCT and tolerances for a comparison of polyethylene grades of one design type in the procedural rule on suitability proof for alternative plastic resins used for packagings and intermediate bulk containers (IBCs) for the transport of dangerous goods.
The environmental stress crack resistance determined by Full Notch Creep Test (FNCT) represents the impacts from the stacking tests at 40 °C performed with standard liquid wetting solution (without pre-storage) and with normal butyl acetate saturated wetting solution (pre-storage with normal butyl acetate).
Environmental stress crack resistance determined with FNCT for the polyethylene grades was related to the times to failure of different jerrican samples made of these grades in stacking tests with 5% wetting solution and a n-butyl acetate saturated wetting solution (pre-storage with n-butyl acetate) at 40 °C. The FNCT is suitable for comparison of polyethylene grades in relation to environmental stress crack resistance independent of their use as drum, IBC or jerrican material. The stiffness of polyethylene grades should be taken into account to the comparison as jerricans are not only chemically but also mechanically stressed.
Technically high-quality hollow articles made of polyethylene have proved many times their value as dangerous goods packagings. Design type tests according to the international regulations for the transport of dangerous goods - drop test, stacking test, hydraulic pressure test and leakproofness test must be performed for every design type and manufacturer. The properties melt flow rate (MFR), relative density (D), low temperature notched Impact strength (N.I.S.) at -30 °C, environmental stress crack resistance (FNCT) and resistance to oxidative degradation Ox by nitric acid (determined by the increase of MFR) have been selected together with the corresponding test methods and tolerances for a comparison of polyethylene grades of one design type in the procedural rule on suitability proof for alternative plastic resins used for dangerous goods packagings and intermediate bulk containers (IBCs) – EN 15507. We wanted to proof the practical relevance of this standard by comparing the relationship between the results of the FNCT and the times to failure of design types in stacking tests with wetting solution and normal butyl acetate saturated wetting solution.
Different imaging techniques were employed to monitor Full Notch Creep Test (FNCT) experiments addressing environmental stress cracking in more detail. The FNCT is a well-established test method to assess slow crack growth and environmental stress cracking of polymer materials, especially polyethylene. The standard test procedure, as specified in ISO 16770, provides a simple comparative measure of the resistance to crack growth of a certain material based on the overall time to failure when loaded with a well-defined mechanical stress and immersed in a liquid medium promoting crack propagation.
Destructive techniques which require a direct view on the free fracture surface, such as light microscopy and laser scanning microscopy, are compared to non-destructive techniques, i.e. scanning acoustic microscopy and xray micro computed tomography. All methods allow the determination of an effective crack length. Based on a series of FNCT specimens progressively damaged for varied Durations under standard test conditions, the estimation of crack propagation rates is also enabled. Despite systematic deviations related to the respective Imaging techniques, this nevertheless provides a valuable tool for the detailed evaluation of the FNCT and its further development.
The aim of this research is to study the influence of moisture absorption at low moisture contents on the creep behaviour of an epoxy adhesive in steel bonded joints. Single lap joints were manufactured using high strength steel adherends and a two-component epoxy adhesive. The single lap joints were tested at load Levels corresponding to average lap shear stresses of±5%, 15%, 30% and 45% of the dry lap shear strength in both 40 °C air and 40 °C distilled water. Specimens were not pre-aged to be able to analyse the coupled effect of moisture and loading. The test results show that an increase in the load level resulted in an increase in the instantaneous strain and in the creep strain rate. The creep strain of single lap joints loaded in water was generally larger than for the ones loaded in air. For joints loaded in water the creep behaviour was found to be dependent on the moisture concentration in the adhesive. At low moisture percentages creep was suppressed, resulting in a lower instantaneous strain. At higher moisture percentages creep was promoted, resulting in a larger strain rate. The suppression of creep at low moisture percentages is attributed to water molecules bonding to the epoxy macromolecules, resulting in a reduction in molecular mobility and a smaller creep strain. At higher moisture percentages the plasticizing effect of the water dominates, resulting in a larger creep strain. The Maxwell threeelement solid model and Kelvin-Voigt three-element solid model were used to simulate the creep behaviour of the single lap joints loaded in air and water. The models gave good representations of the creep Response across the different load levels in both water and air, they were however unable to give a correct representation of the instantaneous strain of the single lap joints loaded in water. This is attributed to the models being unable to account for the present short-term relaxation process that is dependent on the moisture concentration.
The phenomenon of environmental stress cracking is still a major issue in materials engineering as well as from a scientific perspective. Especially in the case of packaging materials made of high-density polyethylene, assessing the potential of premature damage due to environmental stress cracking is complex because of the large number of components in possible liquid filling goods. As a first guideline, the well-known effect of detergents and dispersants is usually considered, but the interplay with other components, such as organic solvents is often unknown. Particularly challenging in this respect are crop protection products, consisting of several different admixtures in addition to the biologically active ingredients. This study is based on two model liquids representing these admixtures that were established for testing in this context. The model liquids as well as their constituting components were used as liquid media in the Full Notch Creep Test to characterize the environmental stress cracking behavior of a typical high-density polyethylene. Complementary to the time-to failure of a notched specimen obtained from these tests, the crack opening was monitored optically, and the fracture surface was analyzed post-failure by laser scanning microscopy as well as scanning electron microscopy. Based on the results, the effect of various surfactants as well as the influence of organic solvents on environmental stress cracking are discussed.