Rheometry Is More Than Just Viscosity

Rheometers are instruments that impose a highly controlled deformation to a fluid while measuring the force required to maintain that deformation (or vice versa). Viscosity, a parameter that indicates a fluid’s resistance to flow, is normally the main property that people think of when using a rheometer. However, rheometers can provide a great more information than simply viscosity.

Rotational shear rheometers confine a fluid between a top geometry, either a flat plate or a cone, and a fixed flat platen. The instrument then either rotates the geometry at a series of specified velocities (shear rates), providing shear viscosity as a function of shear rate, or oscillates the top geometry at a series of specified rotational frequencies, providing the elastic and viscous modulus as a function of frequency. The latter test is particularly useful for probing the viscoelastic properties of materials as a function of deformation rate, such as relaxation times, moduli, dynamic viscosity, and normal force, or tracking time-related phenomena, such as gelation and curing times.

Extensional rheometers act more like load frames, pulling a fluid in a tensile deformation while measuring force and cross-sectional area. These instruments report the extensional viscosity as a function of strain and strain rate, which can vary by orders of magnitude for non-Newtonian fluids and polymer melts depending on the molecular weight, solution concentration, temperature, and strain rate.

This extensional viscosity can be markedly higher than the shear viscosity for the same fluid and is therefore important for filling and pumping of these complex fluids. Often, the relaxation time of the material is also determined, which can dictate if the material will behave more like a solid or a liquid in response to the deformation rate.

These properties can be used to determine optimal process conditions, such as extrusion rates, fiber spinning rates, and mixing behavior. Additionally, these properties influence the consumer perception of products that are eaten, smoothed on, or otherwise applied in a tactile fashion.

A priori characterization of these products by rheometry can screen out products that have viscoelastic behaviors that are known to have poor responses in consumer test panels.

Three case studies were presented at a rheology meeting by CPG scientists that explore how rheometry can be used to assess consumer perception of materials.
Link to presentation

On The Subject of Tires

Ever wonder what your tires are made of? Tires these days are highly formulated composite structures encompassing several types of rubber compounds, crosslinking agents, plasticizers, stabilizers, and fillers, all designed to provide durability, low wear, and traction. Around WWII, butyl rubber was in short supply, causing rubber scientists to try crosslinking silicone elastomers. The result from one set of tests was Silly Putty. Silly Putty was not terribly successful as a tire material, but made a great children’s toy and demonstration tool for polymer scientists. Since WWII, tire compositions have become much more complex as polymer formulations have become more sophisticated.

CPG performed a deformulation analysis of a commercial automobile tire using some common techniques for deformulation analysis, including TGA-FTIR, GC-MS, and SEM-EDS. Read more about this analysis in this application note.

US Patent Issued to CPG

CPG was issued US Patent 8728379 in May 2014. This patent describes methods of making wear resistance, oxidatively stable polyethylene for orthopedic implants. The technology involves irradiating ultra high molecular weight polyethylene (UHMWPE) which contains Vitamin E as an antioxidant. This patent is available for license. For more information, please visit our web site.

Vitamin E Content in UHMWPE

In a recently published article in the Journal of Biomedical Materials Research, CPG scientists describe a new method of quantifying the amount of Vitamin E, a naturally-occurring antioxidant, in ultra high molecular weight polyethylene (UHMWPE). This technique uses a thermal approach to measure the oxidation resistance of the material via an oxidation induction time measurement. The results are compared to a calibration curve, which then allows determination of the effective Vitamin E concentration in the material following any processing step (e.g. molding, irradiation, sterilization). The technique has better sensitivity than other published techniques. For more information, contact Cambridge Polymer Group or view the publication.

Filler Content in Plastics

Not the night sky, but rather an elemental map of filler in a polymer matrix. The bright spots are zirconium oxide in PMMA.

Inorganic fillers are often added to thermoplastics to provide increased rigidity, hardness, impact strength, thermal conductivity, radiopacity, as well as reduced mold shrinkage. Filler, in the form of a powder, is normally compounded into the thermoplastic resin with an extruder, with filler contents ranging up to 60 wt.% depending on the application.

The degree of dispersion, identity, and quantity of filler can be determined with scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) and thermogravimetric analysis (TGA). SEM-EDS provides structural and elemental information about the polymer resin. TGA provides mass change in the sample as it is heated to high temperatures.

In the application note below, this analysis was performed on a filler in polymethyl methacrylate. The results shows that the resin contained approximately 10 wt.% zirconium oxide.

Link to the full application note.

Monomer Analysis in Bone Cement

Polymer methyl methacrylate-based bone cement is commonly used in some hip and knee replacement arthroplasty surgeries to fix the metal components in place in the joint space. These cements are normally provided as two components. The first component, a powder, contains pre-polymerized PMMA powder along with some initiator (usually benzoyl peroxide) and a radiopacifier (usually barium sulfate). The second component, a liquid, is methyl methacrylate monomer along with some stabilizer. In the surgery, the operating staff will mix the two components together to form a viscous liquid, which is then injected or packed into the cavity behind or surrounding the metal implant. In the course of 5-15 minutes, the monomer cures after contact with the benzoyl peroxide.

As the residual unreacted monomer may leach out of the cement over time in the body, manufacturers and regulators are interested in knowing the amount of unreacted monomer present in polymerized PMMA. ASTM F451 describes a two methods for determining residual methyl methacrylate monomer in curing and cured bone cement, both based on gas chromatography with mass spectroscopy. In the first method, aliquots of freshly prepared cement are placed into vials containing water, and the monomer amounts that are elutable are quantified with GC-MS at time points up to around 30 minutes after start of mixing. In the second method, fully cured cement is exposed to water for a period of time up to 30 days, and the residual methyl methacrylate monomer is quantified in the water by comparison of peak heights to a methyl methacrylate monomer calibration curve.

The general method for GC-MS analysis of bone cement is discussed in this application note.

What’s in Your Coffee?

Coffee is prepared by steeping roasted ground coffee beans in hot water, and then removing the grounds. Caffeine, a naturally occurring stimulant found in coffee, can be removed from the coffee bean by a variety of methods. Benzene was originally used to extract caffeine from coffee in the early 1900s, but its toxicity resulted in this process being abandoned. Water extraction, or the Swiss Water Process, is sometimes used, whereby the water is infused with desirable oils found in the coffee to prevent their extraction, and the unroasted beans (green coffee beans) are repeatedly extracted until the desired level of caffeine is achieved. Dichloromethane or ethyl acetate are sometimes used to extract the caffeine from the beans. Super critical carbon dioxide can also be used to extract caffeine. Caffeine levels in coffee vary according to the bean and the decaffeination process. Decaf will typically contain around 20 ppm of caffeine, while regular coffee may contain around 800 ppm. The decaffeination process may remove or alter desirable aromatics in the coffee that impart its flavor and aroma, hence processors are concerned not only with caffeine levels, but also other properties of the coffee following decaffeination.

Cambridge Polymer Group tested decaffeinated and regular coffee with a variety of techniques to allow assessment of the chemicals that lead to its aroma and flavor, caffeine content, impurities, and shelf-life stability, using gas chromatography, mass spectroscopy, infrared spectroscopy, oxidation induction time testing, rheology, electron spin resonance spectroscopy,sol/gel, and UV spectroscopy.

Read the full white paper here.

Set Free the Radicals!

Free radicals are unpaired electrons found on molecules, and can be the result of incomplete chemical reaction, radiation exposure, oxidation, or mechanical stress. Normally, free radicals are highly reactive and immediately react with other free radicals, oxygen, or other available chemical species. In some materials, however, free radicals can be temporarily stable, sometimes for years, waiting for the appropriate conditions to react. Knowing the free radical content of a material can sometimes be used to predict long term oxidative stability of the material. Identifying the type and location of the free radical on the material can help determine how to stabilize it and know how it was formed.

Cambridge Polymer Group offers electron spin resonance spectroscopy (ESR), also known as electron paramagnetic resonance spectroscopy (EPR), to identify and quantify free radical content in materials. This technique is useful for evaluating antioxidants, shelf-life stability, and the effect of chemicals on materials.

Visit our site for more information on electron spin resonance spectroscopy.

Is Your Liquid Tense?

Surface tension is the property that allows water striders to glide across the surface of a pond, for razor blades to float in a glass, and for water to stream from a hose for a certain length before breaking up into droplets. It is caused by intermolecular forces holding molecules in a liquid together, resisting an externally applied force. In water, the strong hydrogen bonds give water a high surface tension relative to other liquids (72.8 mN/m vs. ~30 mN/m for many organic solvents); this hydrogen bonding also results in water having a higher boiling point than many organic solvents. Mercury is also known for having a high surface tension (480 mN/m), resulting in its propensities for forming balls when placed on a substrate. Surface tension and energy are sometimes used interchangably, although tension is only applied to liquids. High surface energy liquids will not wet, or spread out, onto substrate that has a lower surface energy. Surfactants reduce the surface tension of liquids, allowing them to wet substrates that would normally not be wet by the unmodified liquid.

Surface tension in liquids is commonly measured by the Du Nuoy ring method. In this approach, a precision platinum ring of known diameter is attached to a precision balance, or tensiometer, and is slowly lowered into the liquid with a vertical stage (see image below). The direction is then reversed, and a ring of fluid is slowly pulled up out of the liquid. The tensiometer measures the force required to break free of the liquid, which can be easily converted into a surface tension by knowning the diameter of the ring.

For solids, the surface energy is usually required. A single surface energy term is a bit more challenging to report. The contact angle is normally reported, either from the sessile drop method, in which a liquid of known surface energy is placed on the substrate and the degree of wetting is assessed by its contact angle (see below), or from a Wilhelmy plate approach, in which a piece of the substrate is lowered into a vessel of a liquid of known surface energy, and the force required to lower and raise the liquid is monitored as a function of the perimeter length of the substrate. This latter test gives the advancing and receding contact angle.

By making measurements of contact angle on a single substrate with multiple liquids of different surface tensions, the critical surface energy of a substrate can be determined where the contact angle goes to zero, indicating good wetting of the liquid on the substrate. All liquids with a surface tension below this critical surface energy will wet the substrate.

Visit our website for more information on surface energy and surface tension measurements.

Thermal and Infrared Characterization of Materials

The increasing interest in material deformulation analysis and quantification of trace compounds in materials has led to the development of coupled analytical techniques. One of these techniques is TGA-FTIR, which combines the mass sensitivity of thermogravimetric analysis (TGA) with the compound identification ability of Fourier transform infrared spectroscopy (FTIR). In TGA-FTIR, the sample is placed in a conventional TGA, and the mass loss is monitored as a function of temperature. As materials evaporate or are combusted, the mass of the sample will change accordingly and is monitored quantitatively by the TGA. A gas transfer line connects the TGA to the FTIR, which reports the chemical signature of the volatile components as they leave the TGA. In this manner, the identity of the volatile species can often be determined, in addition to their concentration based on the TGA mass change.

In an example experiment, we ran a 60:40 mixture of water and glycerol in our TGA-FTIR. Water has a boiling point of 100C, whereas glycerol has a boiling point around 290C. In an experiment run in nitrogen, the mass change of the sample as a function of time is shown below, indicating 60% loss of water by the time the tempererature reaches 100C. The remaining glycerol starts evaporating, and is gone by the time the system reaches the boiling point of glycerol.

The FTIR spectrum as a function of time is shown below (the lower curve is the Gram Schmidt signal, showing overall absorption intensity). The slice taken at 5.8 minutes shown as Extraction 1 in the upper absorption spectra indicates that the material coming out of the TGA at this time point is indeed water.

The slice taken around 24 minutes below shows that the spectrum is glycerol, matching the TGA results.

For an unknown material, this technique would allow quantification and identification, which would be useful for polymer additives such as antioxidants, organic colorants, and stabilizers. Contact us for more information about this procedure.