On creep test loading for soft polymers

The dead-weight loading speed in the creep test for soft polymers was investigated. The optimum dead-weight loading speed can be determined so as to assure instantaneous elastic deformation and, at the same time, also avoid dynamic effects. It is found that small dynamic effects are inevitable for soft polymers such as polypropylene even at optimum speed. The experimentally determined optimum dead-weight loading speed for polypropylene was 300 mm/min for 30 kg dead-weight at 16°C.     

Surface embrittlement of ductile polymers: A fracture mechanics analysis

The embrittlement of ductile polymers resulting from outdoor weathering or aging or from the application of brittle surface coatings is explained using fracture mechanics principles. It is shown that in order for surface embrittlement to occur, the brittle surface should have a dynamic stress intensity factor at the brittle/ductile interface that exceeds the arrest toughness of the ductile polymer. This phenomenon is modeled using duplex tensile specimens fabricated from poly(methylmethacrylate) (brittle layer) and polycarbonate (ductile substrate) as well as from styrene-acrylonitrile and acrylonitrile-butadiene-styrene copolymers.     

Measurement of aging effects of ABS polymers

ABS (acrylonitrile-butadiene-styrene) plastics are one of the most common two-phase commercial polymer systems. They consist of a continuous rigid phase (styrene-acrylonitrile co-polymer) in which the elastomer phase (polybutadiene grafted with styrene and acrylonitrile) is finely dispersed in the form of spherical particles. Because of their properties and relatively low cost compared to other engineering thermoplastics, ABS resins are now being used increasingly in fields of application involving severe aging. The polybutadiene content, however, poses a problem in relation to ABS aging resistance, since it is a prime site for degradative attack at double bonds and tertiary carbon atoms. The present paper presents a concise account of the methods used in our labortory for measuring ABS aging. As an example, ABS degradation during processing and during natural and artificial light aging are discussed in more detail.     

Structure and impact strength of ABS polymers

This paper has dealt with the phenomenal theory describing impact strength as the result of the competition of breaking and yield processes inside the material. Because the breaking processes are not too temperature and deformation speed dependent, they were roughly considered as the one-point constant. The yielding processes were described by means of the EYRING's theory of the absolute reaction rates. From this viewpoint were discussed relations between some structural features of ABS polymers: the chemical composition of rubber, the molecular weight of the S-AN copolymer, the content and degree of the crosslinking of rubber and the degree of grafting. In all cases was achieved a satisfactory agreement between the proposed theory and the experiment.     

Interpretation of the tensile creep response of an ABS polymer

Tensile creep measurements were carried out on a commercial ABS polymer over a temperature range from 40 to 100°C at stress levels from 0.6 to 1.8 × 10⁸ dynes/cm². Experiments were conducted in a prototype of an apparatus designed to be compatible with digital acquisition systems. Analysis of the data indicated that application of the time-temperature super-position principle was of limited value due to the use of test temperatures near and below the effective glass transition temperature of the acrylonitrile-styrene component of the polymer. A strong stress dependence of the compliance was observed, even at relatively short times after loading. This was analyzed in terms of a model in which the height of the potential energy barrier to motion of the molecular flow unit is lowered by the application of stress. Analysis of the temperature dependence of the compliance at low stress levels indicated that the effective T_g of the acrylonitrile-styrene phase is about 85°C. The temperature dependence of the magnitude of the activation energy is considered as is the stress dependence of the glass transition temperature. Implications of short-time response in creep with regard to response under impact loading are pointed out. Practical application of results to the prediction of dimensional stability of molded parts is discussed as well as the limitations involved in extrapolation of experimental data to long times and high stress levels.     

Indentation creep of polymers. I. Experimental

An experimental setup is proposed to perform creep indentation tests on thermoplastics. An infrared (IR) lamp is used to locally heat the sample, and a flat punch made of tungsten carbide is loaded on the sample surface. In order to show the capability of this test in evaluating the creep properties, creep indentation tests were carried out on PA66 and HDPE sheets at different temperatures and creep loads. Dynamic mechanical analyses (DMA) and uniaxial creep tests were also performed at different temperatures to have a comparison with the indentation test results. Master curves were built for all the test results, and time shift factors were extracted. In all cases, the logarithm values of the shift factor were linearly dependent on the temperature. Small differences were found in the shift factors of the DMA and uniaxial test results, whereas high differences were observed in the case of the indentation results due to the nonuniformity of the temperature in the sample. However, indentation creep compliance is strictly correlated with tensile creep compliance, and equivalent tensile creep properties can be extracted. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

On the damage energy of polymers in creep

Creep experiments carried out on transparent and translucent polymers permit the observation of irreversible material damage in the form of crazes or microcracks. The formation of such damage is strongly dependent on the applied stress, temperature and environmental conditions. The first appearance of observable material damage seems to be explicable by an energy criterion. The energy supplied by the external load can be divided into conserved and dissipated parts, each of them causing volume (isotropic) and shape (deviatoric) changes. The different parts of the energy can be computed if the creep-compliance is approximated by a Prony-Dirichlet series with a finite number of terms. Computations carried out for experiments with air as the environmental medium under isothermal conditions show the dependence between the appearance of first visible material damage (crazes or microcracks) and the conserved energy.     

Characterization of the thermomechanical behaviour of polymers: non-isothermal creep deflection processes

A non-isothermal creep process of a thermorheologically simple polymer is considered. For such a material, if the results of isothermal creep experiments are known, the creep compliance can be calculated as a function of time for any given thermal history by a suitable method. The results of creep deflection measurements on a sample of commercial polystyrene are reported. The data isothermally obtained at different temperatures are determined so as to obtain the master curve of the compliance as a function of time and the shift factor as a function of temperature. As an example, the above method is applied to predict the course of a laboratory test which is commonly used to determine the heat deflection temperature of thermoplastic polymers. The experimental results agree satisfactorily with the predicted values.     

A novel methodology to model the creep behavior of polymers at different temperatures

A new methodology for modeling the creep behavior of polymers at different temperatures, by using phenomenological constitutive models, is presented in this paper. The viscoelastic model is given by a combination of springs and dashpots and is used to describe the nonlinear response of polymers, and the viscoplastic formulation is given by a power-law equation. The approach proposed in this work is based on building master curves for different stress levels, and finding the dependency of the constitutive parameters with the temperature. After fitting the equations to the tensile creep tests at different temperatures, the final constitutive formulation is capable of modeling the behavior of polymers at any stress level and temperatures. Poly methyl metacrytale (PMMA) was used to investigate the accuracy of this proposal, and the results showed good agreement with the experimental data.     

The effect of surface embrittlement on the mechanical behavior of rubber-modified polymers

An experimental study was conducted to better understand the mechanism of surface embrittlement in rubber-modified thermoplastics. Brittle polymer films were laminated onto acryionitnie-Duraaiene-styrene terpolymer and high-impact polystyrene to simulate either embrittlement due to environmental degradation or brittle paint systems. The effect of varying coating thickness and molecular weight was studied in tensile and impact testing. Results of this study suggest that the multiple crazing mechanism normally associated with the energy-absorbing capability of rubber-toughened polymers is severely restricted due to the formation of multiple surface cracks resulting from coating failure. This is particularly true when the coating thickness reaches a critical value at which a single surface crack is able to propagate across the coating/substrate interface in an unstable manner.     

Crazing limit of polymers in creep and stress relaxation

The appearance of the first visible damage in polymers in the form of crazes or microcracks may be assumed to be a sign of failure. Experimental investigations have shown that in uniaxial creep and stress relaxation experiments, under isothermal conditions, a certain time between the quasi-spontaneous loading and visible crazing is needed. This ‘incubation time’ is very strongly dependent on the magnitude of the quasi-spontaneously applied stress or induced strain. Based on the Reiner-Weissenberg theory of strength, simple relations allowing the prediction of crazing are developed. The agreement between theoretical computation and experiment is very good.     

Evaluation of UV-induced embrittlement of PE-HD by Charpy impact test

The impact fracture behavior of two common high-density polyethylene grades for container applications were intensively studied by the instrumented Charpy impact test after well-defined exposure to UV-irradiation. Individual stages of the impact event, such as crack initiation and crack propagation energy as well as maximum impact load, were investigated from the recorded load–deflection curves. UV-induced material property changes were further investigated by infrared spectroscopy, differential scanning calorimetry, and dynamic-mechanical analysis as well as density measurements. Based on the results of the Charpy impact test, three indicators were identified to describe the extend of photooxidation on high-density polyethylene: (a) a reduced Charpy impact strength—at least to half of its initial value for a distinctly brittle impact fracture, (b) a marked decrease in the crack propagation contribution to the impact strength, and (c) an increase of the brittle features of the fracture surface.     

有效消除ABS塑料产品素材应力的涂装工艺

针对ABS塑料产品按正常工艺喷涂后出现应力吸漆导致铝粉漆排列不均的外观缺陷,介绍了有效消除应力、避免涂膜缺陷的涂装工艺。     

ABS树脂冲击强度测试的影响因素

对比了4个牌号ABS树脂ASTM标准和国标悬臂梁冲击强度,剖析了ABS树脂橡胶相组成对测试结果的影响,并运用moldflow软件模拟冲击样件注塑成型过程,分析引起冲击强度测试结果差异的原因.结果表明,通过降低冲击测试样条注塑成型注塑速度减小样件表面剪切应力的方法,能够提高冲击强度测试结果,较高的橡胶含量可以减弱注塑速度...     

Description of creep of complex fibres made from rigid-chain polymers

The possibility of applying the temperature—time analogy principle for quantitatively describing creep of SVM fibre was demonstrated. An increase in the modulus of elasticity of SVM fibre was found in the region of nondestructive stress.     

Non-linear creep of polymers under superimposed static and dynamic stress

The non-linear creep of polymeric materials under super-imposed static and dynamic stress is considered theoretically. The equation of state due to Green, Rivlin, and Spencer to-gether with the power law of time dependence for the kernel functions as suggested by Nakada is assumed to characterize the non-linear materials. Expressions for creep strain under constant static, oscillatory dynamic and superimposed static and dynamic stress are obtained in terms of the material constants and time dependent functions, called dynamic creep functions. It is shown that the creep strain due to dynamic stressing is damped as the number of stress cycles is increased. Damping is faster if the power law of time dependence is high. Expressions for the cumulative creep strain after multiple stress cycles are also obtained in terms of cumulative strain functions. All these functions are evaluated numerically at one thousand stress cycles. Finally, a special case of stress history is considered where the stress periodically reaches zero. It is shown that the ratio of the strains due to dynamic and static stressing can be characterized by the power law parameter when the mean stress is either very high or very low. Due to the slow damping when the power law parameter is small, the decrease of the strain ratio with number of cycles is slow compared to higher power law parameters.     

Viscoelastic properties of acrylonitrile-butadiene-styrene (ABS) polymers in the molten state

The viscoelastic behavior of acrylonitrile-butadiene-styrene (ABS) terpolymers is studied in the molten state. First, the behavior of the styrene-acrylonitrile (SAN) matrix is determined. Then, the effect of the degree of grafting on ABS rheological properties is emphasized. A critical degree of grafting (DGc), corresponding to a minimum of the viscoelastic functions in the low frequency region, is determined. It defines a critical thickness of the grafted layer, and is related to the conformation of the grafted chains at the surface of the rubber particle. The DGc affects the morphology of the ABS and also affects the appearance of a secondary plateau at low frequencies. The effects of the rubber particles, of the grafted chains, and of the morphology on the appearance of the secondary plateau at low frequencies are clearly dissociated.