Polymer tensile performance

The labor-intensive nature of polymer tensile and flexure tests makes them logical candidates for automation. We have developed a fully automated instrument for performing these tests on rigid materials. The instrument is comprised of an Instron universal tester, a Zymark laboratory robot, a Digital Equipment Corporation minicomputer, and custom-made accessories to manipulate the specimens and measure their dimensions automatically. Our system allows us to determine the tensile or flexural properties of over one hundred specimens without human intervention, and it has significantly improved the productivity of our laboratory. This paper describes the structure and performance of our system, and it compares the relative costs of manual versus automated testing. 

B) Tensile and Impact Performance Table I presents a summary of tensile impact and ultimate tensile data for the three sets of composites. For ease of comparison both sets of data are normalized to the reference tensile impact or ultimate tensile performance of the unfilled polymer matrix. 

Static and dynamic thermomechanical analysis of the commercial polymer separators (PP and PE) were made to understand their properties (Love, 2011). Anisotropic separators manufactured from a dry process showed limited tensile performance in the transverse direction. The separators prepared from a wet process displayed a uniform and biaxial structure and nearly showed identical mechanical strength on both directions. It was also found that small losses in mechanical integrity were observed after the separators were exposed to various electrolytes. 

In Fig. 24.6 we show several types of behavior seen in tensile testing of polymers. For performing a specific test, consult one of the standards listed in Table 24.2. 

Polymer Tensile strength (MPa) Flexural modulus (GPa) Elongation of break (%) Strain at yield (%) Notched Izod (kj/m) Excellent or very good performance Poor performance... 

The finer the dispersion and the better the incorporation, the lower is the impact on a polymer s performance. Properties like impact and tensile strength are lowered when agglomerates or aggregates are present. Surface problems such as specks in film and injection-molded parts are also a result of poor dispersion. Poor incorporation can lead to other typical color problems of low strength and inconsistency. Processing issues such as screen pack plugging and low throughput can also be avoided if a robust process is employed. 

The hyperbranched polymer used in Ref. 19 had little beneficial effect on the tensile performance. The HSP of this polymer are not known but are likely to be a significant distance from PLA. But by in situ anhydride reactions (which, incidentally, would be expected to reduce the incompatibility with PLA), the hyperbranched polymer becomes cross-linked into a matrix, which gives big increases in toughness and elongation at break. 

One of the initial motivations for developing thermotropic LCPs was the search for high-tensile-performance fibres with properties similar to those exhibited by DuPont s Kevlar , a lyotropic LCP [3]. As polymer developments continued, the wholly aromatic, thermotropic polyesters were found to offer useful properties in addition to excellent tensile capabilities, especially as engineering thermoplastics [4]. 

Samples used for mechanical testing were prepared via molding at a temperature of 190°C and a pressure of 150 atm. The pressurized samples were cooled to room temperature atarate of20 K/min. Irganox 1010 (-0.8 wt.%) was added to stabilize the nascent polymer. Tensile tests of the polymers were performed at a rate of 50 mm/min on an Instron 1122 tensile machine using trowel-shaped samples with a cross-sectional area of 0.75 mm, 0.5 X 5.0 mm, and a base length of 35 mm. 

Returning to the Maxwell element, suppose we rapidly deform the system to some state of strain and secure it in such a way that it retains the initial deformation. Because the material possesses the capability to flow, some internal relaxation will occur such that less force will be required with the passage of time to sustain the deformation. Our goal with the Maxwell model is to calculate how the stress varies with time, or, expressing the stress relative to the constant strain, to describe the time-dependent modulus. Such an experiment can readily be performed on a polymer sample, the results yielding a time-dependent stress relaxation modulus. In principle, the experiment could be conducted in either a tensile or shear mode measuring E(t) or G(t), respectively. We shall discuss the Maxwell model in terms of shear. 

Biaxial Orientation. Many polymer films require orientation to achieve commercially acceptable performance (10). Orientation may be uniaxial (generally in the machine direction [MD]) or biaxial where the web is stretched or oriented in the two perpendicular planar axes. The biaxial orientation may be balanced or unbalanced depending on use, but most preferably is balanced. Further, this balance of properties may relate particularly to tensile properties, tear properties, optical birefringence, thermal shrinkage, or a combination of properties. A balanced film should be anisotropic, although this is difficult to achieve across the web of a flat oriented film. 

Applied Sciences, Inc. has, in the past few years, used the fixed catalyst fiber to fabricate and analyze VGCF-reinforced composites which could be candidate materials for thermal management substrates in high density, high power electronic devices and space power system radiator fins and high performance applications such as plasma facing components in experimental nuclear fusion reactors. These composites include carbon/carbon (CC) composites, polymer matrix composites, and metal matrix composites (MMC). Measurements have been made of thermal conductivity, coefficient of thermal expansion (CTE), tensile strength, and tensile modulus. Representative results are described below. 

Extruded composites of plasticized PVC and short cellulose fibers have been investigated by Goettler [103]. Pronounced increases in tensile modulus, yield, and ultimate tensile strength are observed. Single step processing of reinforcement and polymer with good product performance are key characteristics of the material whose field of application lies in the vinyl hose industry. 

We perform flexural testing on polymer rods or beams in the same basic apparatus that we use for tensile or compressive testing. Figure 8.6 illustrates two of the most common flexural testing configurations. In two-point bending, shown in Fig. 8,6 a), we clamp the sample by one end and apply a flexural load to the other. In three-point bending, shown in Fig. 8.6 b), we place the sample across two parallel supports and apply a flexural load to its center. 

An interesting feature of polarized IR spectroscopy is that rapid measurements can be performed while preserving molecular information (in contrast with birefringence) and without the need for a synchrotron source (X-ray diffraction). Time-resolved IRLD studies are almost exclusively realized in transmission because of its compatibility with various types of tensile testing devices. In the simplest implementation, p- and s-polarized spectra are sequentially acquired while the sample is deformed and/or relaxing. The time resolution is generally limited to several seconds per spectrum by the acquisition time of two spectra and by the speed at which the polarizer can be rotated. Siesler et al. have used such a rheo-optical technique to study the dynamics of multiple polymers and copolymers [40]. 

Physical characterization of polymers is a common activity that research and development technologists at the Dow Chemical Company perform. A material property evaluation that is critical for most polymer systems is a tensile test. Many instruments such as an Instron test frame can perform a tensile test and, by using specialized software, can acquire and process data. Use of an extensometer eliminates calibration errors and allows the console to display strain and deformation in engineering units. Some common results from a tensile test are modulus, percent elongation, stress at break, and strain at yield. These data are then used to better understand the capabilities of the polymer system and in what end-use applications it may be used. 

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