Plastics analysis - improved characterization of polymer behavior and composition.
Plastics analysis plays an important role in several key areas of the polymer industry. This article focuses on new techniques for laboratory and on-line characterization of plastics in the areas of thermal analysis, flow characterization, mechanical testing and chemical analysis.
New developments in chemical analysis permit faster and more precise evaluations of polymers, enabling the design of better materials and promoting resin quality. With the ever widening use of computer simulation programs for plastics part design, new protocols are now available to produce the specialized material data needed by these applications. These simulation tools are a tremendous design aid since they permit the engineer to evaluate potential designs in the conceptual stages, resulting in well-designed and functional products. At the other end of the process, the ability to monitor the production process in real time using on-line methods provides a means to ensure that the parts being manufactured conform to design specifications. As an aid to the reader, a partial listing of vendors of the instruments described in the article is provided. Additionally, several commercial testing laboratories (see ASTM International Directory of Testing Laboratories) and universities are already offering such characterizations.
Advances in Laboratory Techniques
The differential scanning calorimeter (DSC) is perhaps the instrument that has dominated the field of thermal analysis in the past decade. The ease with which important properties such as transitions, heat capacity, reaction kinetics, and crystallization kinetics are characterized have made the DSC widely used in the plastics laboratory. Significant efforts to simplify the technique have put this form of analysis within the reach of most plastics analysts. In fact, several DSC manufacturers have introduced robust, low-end instruments for routine, nonresearch applications At the other end of the spectrum, the new Pyris 1 from Perkin-Elmer represents a new generation in DSC technology. Using a combination of high-tech insulation and a laminar flow air curtain [ILLUSTRATION FOR FIGURE 1 OMITTED], the instrument virtually eliminates the icing problems that used to plague DSC users operating at cryogenic temperatures. A new Windows interface includes, among other features, on-line multimedia instructions to enable operators to learn on the job.
An important technological development is the appearance of the modulated DSC from TA Instruments and the Dynamic DSC from Perkin-Elmer. In a manner similar to dynamic mechanical analysis, these new techniques supply a dynamic heat signal to the specimen instead of the constant heating or cooling ramps of the conventional DSC. An important difference in the approaches used by these manufacturers lies in the mode of data interpretation and analysis. While the modulated DSC [ILLUSTRATION FOR FIGURE 2 OMITTED] deconvolutes the resulting data into kinetic and heat-capacity related components, Perkin-Elmer's methods utilize DMA analogs of storage heat capacities (Cp[prime]) and loss heat capacities (Cp[double prime]). These new techniques permit the separation of complex overlapping transitions into more easily interpreted components. They are very useful in the analysis of crystallization kinetics, in the characterization of complex blends, in the resolution of reversible and irreversible thermodynamic phenomena, as well as in the direct measurement of heat capacity.
High-resolution thermogravimetric analysis, an enhanced TGA technology from TA Instruments that utilizes high heating rates when no weight loss is occurring, but which sows to as low as [less than]1 [degree] C/min during a weight loss, optimizes both resolution and repeatability in compositional analysis. Further, by combining TGA with an evolved gas analysis system such as mass spectrometry (MS), an analysis of the composition of the evolved gaseous product can now be obtained. This enhancement, called TGA-MS, provides quantitative as well as qualitative analysis of the decomposition products.
The concept of utilizing multiple measurement techniques for simultaneous analysis is being applied in several areas An important application is in the rheokinetic studies of thermoset reactions. Simultaneous DMA-DEA (dynamic mechanical analysis-dielectric analysis), using the Perkin-Elmer 7e DMA and Micromet's Eumetric System III Micro-dielectrometer, is being used for the characterization of epoxies, polyesters, and other industrial thermosets. The power of this combined technique lies in its ability to attribute the theological changes due to a chemical reaction directly to the extent of the reaction measured using the DEA component of the test method. Similar systems are available from Rheometrics and TA Instruments.
Another means of examining fundamental thermodynamic phenomena is the use of high-pressure dilatometry to measure the pressure-volume-temperature dependence of polymers. This results in the development of an equation of state describing the variation of specific volume with temperature and pressure. As with DSC, these curves show thermodynamic as well as kinetic phenomena. The data yield three important pieces of information: the volumetric thermal expansion coefficient, the compressibility, [TABULAR DATA FOR TABLE A OMITTED] and the pressure dependence of transitions. Data are primarily utilized for mold-filling simulations. Two measurement methods exist: a mercury bellows-based technique from Gnomix Inc.; and a piston-cylinder instrument, pvT-100, available through SWO Polymertechnik, GmbH.
Thermal conductivity is an important thermal transport property that characterizes the rate of heat transfer through a polymer. The data are crucial for heat-transfer calculations and design analysis. The Thermoflixer from SWO Polymertechnik utilizes a transient-line-source method to perform very fast measurements on plastics in the melt and solid states. In combination with their pvT-100, this instrument is also capable of measuring thermal conductivity at high pressures. Another new instrument, the TC Probe, available through GRC Instruments, is a nondestructive, nonintrusive technique, usable on a wide variety of materials.
The melt-flow index has been by far the most common measure of flow characteristics of plastics. This simple test has grown in sophistication with recent additions such as automation and multiple-measurement capability. More important, the capillary rheometer is rapidly gaining wider utilization in the industry as greater understanding and new applications increase its popularity. Low-cost, Windows-based units from Goettfert and Kayeness are now available. In an effort to simplify data analysis and interpretation, both Goettfert's WinRheo and Kayeness's KARS software packages fit a number of common rheological models, which can be used to offer greater insights into the behavior of the polymer. The WinRheo software also provides a macro programming capability, called Absolute Rheometer Control (ARC), to allow users to create custom test methods This powerful tool can enhance productivity and enable the creation of new test scenarios
There is an increasing interest in the study of melt elasticity. Such data are used in the study of die swell in extrusion and blow molding, factors that can affect finished part appearance and quality. In general, such characterizations are fairly complex. Maxwell's melt-elasticity tester represents one of the simplest approaches, characterizing the elastic recovery of a melt that is subjected to an instantaneous step-rotational shear.
Both Rheometrics and Goettfert offer elongational melt rheometers. Goettfert's Rheotens is an attachment to their capillary rheometer and uses two counter-rotating wheels to draw down a vertical polymer melt strand at constant velocity or at constant acceleration. The Rheometrics instrument, which is based on the Meissner Rheometer [ILLUSTRATION FOR FIGURE 3 OMITTED], utilizes a horizontal sample supported on a gas cushion in a temperature-controlled environment, which is drawn horizontally by metal belts moving in opposing directions.
The personal computer has revolutionized the universal testing machine to the point that several companies now offer "no-dials" direct interface of older machines to slick new Windows-based PC front ends that completely control the instrument and also perform data analysis and reduction. With real-time graphics and automatic calculations, these technologies greatly enhance productivity and flexibility, permitting a much wider range of test programs at the click of a mouse.
With the need for improved measurement accuracy, new extensometers such as the non-contact laser extensometer have begun to replace the use of crosshead travel as a means of displacement measurement, dramatically improving the quality of tensile-test data. While suitable for elongation measurements, laser extensometers do not have the resolution required for modulus measurements. Strain-gauge extensometers remain the mainstay for such tests New higher resolution measuring devices such as video noncontact extensometers have entered the market, offering convenient biaxial measurement capability and other imaging capabilities.
In the impact testing area, Dynatup has introduced highly instrumented equipment to permit users to learn more about the manner in which plastics break. With their equipment, tests can now be performed destructively to fracture or fatigue, or nondestructively [ILLUSTRATION FOR FIGURE 4 OMITTED] on specimens instrumented with multiple sensors to evaluate the effect of impact on different locations of the test specimen. Data can be analyzed and interpreted using Windows-based structural analysis software packages, which provide 3-D dynamic animation of the impact event.
Fatigue testing instruments, such as the GRC SP1 servo-pneumatic fatigue tester, permit high-frequency cyclic testing in a wide array of modes - tensile, compressive, and flexural. Again, additional instrumentation can be applied to the test specimen to gain deeper insights into the long-term behavior of the part.
In the areas of chemical analysis, significant breakthroughs have been experienced in the fields of mass spectrometry and gas chromatography. From the practical viewpoint, there are basically no limitations to the type of sample that can be characterized by mass spectrometry - all the way from highly volatile, low-molecular-weight gases to very high-molecular-weight polymers - thanks to recent novel developments in the interface between liquid chromatography and mass-spectrometry. These are the particle-beam and electrospray interfaces The former presents multiple advantages and has been very well proven and established.
The electrospray (atmospheric pressure ionization) interface [ILLUSTRATION FOR FIGURE 5 OMITTED], developed by Analytica and now generally available through most MS vendors, presents additional advantages in the analysis of a much broader range of compounds in terms of polarity and molecular weight. It provides a liquid interface to either HPLC or GPC systems that acts as a highly specific and sensitive (picomole levels) detector. It is a soft ionization technique that yields molecular ions (singly or multiply charged), and is ideal for analysis of large molecules, with a mass range up to 100,000 daltons.
"Matrix Assisted Laser Desorption Time of Flight Mass Spectroscopy" (MALDI-TOF), a new technique available from Hewlett-Packard and other suppliers, is a valuable tool now accessible to the polymer scientist. It is a highly sensitive (picomole to femtomole levels), fast technique, requiring only a few minutes per analysis vs. 20 to 40 minutes for gel permeation chromatography (GPC), while dispensing with the need for the large amounts of solvent required for GPC analysis. Most significant, the true molecular weight of the polymer is obtained, obviating the need for a reference standard, as required for GPC. With its high mass range of 500,000 daltons, a major application of this technique is the analysis of molecular weight and molecular-weight distribution of polymers [ILLUSTRATION FOR FIGURE 6 OMITTED], which has traditionally been done by GPC The technique has been used extensively in the characterization of biopolymers. A broad application of this approach is yet to be demonstrated for all types of synthetic polymers, but significant work has been devoted recently toward this end. The method is also amenable to mixture analysis.
Two recent developments in chromatography deserve special attention: high-temperature gas chromatography (GC), and Rapid GC analysis. The former has significantly extended the capability of GC analysis to include samples previously not suitable for this technique. Hewlett-Packard's new equipment can reach oven temperatures up to 450 [degrees] C. A cool, temperature-programmable, on-column injector is normally required for these applications, plus high-temperature aluminum-clad capillary columns that can take these temperatures without column bleeding. Most conventional GC detectors are usable for these applications. While analytes of molecular weight of about 800 amu (atomic mass units) represented the upper limit for conventional GC, the upper limit for high temperature GC is about 1200-1400 amu, depending on the nature of the analytes of interest. This means that many chemical compounds previously analyzed solely by liquid chromatography can now be easily analyzed by high-temperature GC.
Rapid GC analysis allows rapid heating of the GC oven as well as precise control of the carrier-gas pressure. Although this is not a widely used approach in chromatography, it has already been demonstrated that this new technique can reduce analysts time by a factor of 5 or better, compared with conventional GC analysis. The advent of high-frequency sampling devices has been crucial in the development of this new technology.
Other techniques such as capillary electrophoresis, which have found extensive application in biopolymers and pharmaceutical compounds, are beginning to be applied to polymer analysis. Although some work has been done in the analysis of polymer additives, broad applicability of this approach in the plastics area remains to be demonstrated.
Finally, two sample preparation and delivery systems have received significant attention: supercritical fluid extraction and micro-extraction. These two sample preparation methods present an excellent alternative for the analysis of volatiles and semivolatiles in plastics resins and other matrices. Their main advantages over conventional methods are fast sample preparation without the need for solvents and the ease of automation for analysis of multiple samples. From the environmental and productivity points of view, these offer excellent alternatives for the analytical chemist.
Advances in On-Line Techniques
The potential of on-line analytical instrumentation is just beginning to be felt on the plastics production floor. The primary factor that has spurred this technology has been the need to determine the physical characteristics of plastic feedstocks during the production process. This requires the ability to sample the process stream and monitor a representative characteristic that would then provide an insight into the consistency and quality of the feed stream. In order to be effective, the measurement technique used would need to be capable of a relatively fast response, typically of the order of a few minutes, and also be sensitive to the fundamental characteristic being monitored. Rheological characterization remains the single most popular method in use today. The success of on-line rheology has prompted new technologies such as on-line nuclear magnetic resonance (NMR), and even Raman spectroscopy, to perform on-line evaluation of polymerization feedstocks.
Several types of on-line instruments are being used today. The Goettfert Real Time Rheometer (RTR), a closed-loop instrument, diverts a portion of the process stream to a measurement head and then returns it to the process [ILLUSTRATION FOR FIGURE 7 OMITTED]. Within the measurement head, most of the material flows through a bypass channel and a fraction of the stream is pumped across the capillary die, using precision gear pumps upstream and downstream of the die. This effectively isolates the die from pressure variations in the process stream. Goettfert reports good agreement between the on-line data and the data from a laboratory instrument. By removal of the restriction to flow that the capillary would normally place, the melt stream is sampled as much as 20 times faster than in conventional on-line rheometers, greatly improving the response time of their instrument.
Rheometrics' Process Control Rheometer (PCR) uses a similar concept but makes use of a slit die to measure the viscosity. The PCR differs from the Goettfert RTR in the manner in which the bypass stream operates in that it is used intermittently as a purge line that is closed during the measurement process. Window cartridges can be installed, which allow the melt to be examined using near infrared, ultraviolet, or visible light, so that morphological and compositional studies can be carried out simultaneously. Additionally, they offer NMR to monitor the density and the tacticity on-line.
Kayeness's Dual Stream Process Rheometer FCR-100 is an open-loop rheometer where the diverted melt stream is not returned to the process after measurement [ILLUSTRATION FOR FIGURE 8 OMITTED]. Here, the diverted melt stream is split to two capillary dies of different L/D ratios to enable simultaneous determination of viscosity at two different shear rates spanning a range as high as 1000:1. They show that the data correlates well to that determined from a laboratory capillary rheometer. They also point to die configurations that would permit the application of Bagley corrections to the data, and also to the use of an orifice die to determine extensional viscosity using Cogswell's scheme.
In addition, multiple extruder lines can be monitored on a single measurement system by means of Melt Delivery Platform (MDP 1000), which uses a pneumatic conveying system to capture and convey samples from each process stream. An off-line extruder produces a melt for subsequent analysis by a series of instruments to monitor process-stream quality. Available instruments include the FCR-100 capillary system and IROS 100, an on-line FTIR system, providing real-time compositional and additives analysis of the polymer melt.
Dramatic new technologies are emerging in plastics analysis that have changed the face of laboratories and the kind of information that they produce. The arrival of the personal computer into the laboratory has revolutionized both the measurement and the analysis of material property data. The other major factor that has influenced this change is the demand for data that are more representative, and fundamental in nature. The ability to produce representative physical properties with a strong fundamental basis makes such polymer characterizations extremely useful in the design of plastic parts. Further, the domain of plastics analysis now extends beyond the laboratory to the production floor itself. Powerful on-line methods now provide means for closed-loop control of polymer processing applications. Thus, plastics analysis forms a fundamental bridge to all the other aspects of the production and processing of plastics.
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|Author:||Bobo, Hubert; Bonilla, Jose; Riley, David W.|
|Date:||Nov 1, 1996|
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