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Rheology and processing of BaS[O.sub.4]-filled medical-grade thermoplastic polyurethane.


Barium sulfate (BaS[O.sub.4])-filled thermoplastic polyurethane is widely used in the medical device industry. Barium sulfate is radiopaque and is usually given to patients in a "barium meal" as a means of imaging the digestive tract. In the application of intravenous catheters, barium sulfate is added to bulk polymers to make the catheter inside the blood vessels visible by X-ray. In this investigation the rheological behavior as well as the extrusion processability of a BaS[O.sub.4]-filled TPU are studied and compared with the behavior of the unfilled TPU. The use of various additives (including stabilizers, antioxidants, lubricants etc.) in medical-grade polymers in general and in medical-grade TPU specifically is discouraged. The medical-grade BaS[O.sub.4]-incorporated TPU used in this study is processed without any additives.

Rheological behavior and processability of filled polymeric materials have been the subjects of numerous investigations. The influences of filler content, particle size, particle size distribution, and particle shape on suspension rheology were described in various reviews (1-6). The shear viscosity of a suspension increases with increasing volume loading concentration of the filler particles (7-9). The ratio of the volume loading level to the maximum packing fraction of the filler is generally employed for correlations with the ratio of the shear viscosity of the suspension over the shear viscosity of the binder-polymer at the same shear rate. The shear viscosity of the suspension increases with increasing concentration of the particles and decreasing maximum packing fraction of the particles. Thus, the shear viscosity of the suspension increases with asymmetric particles (high aspect ratio) in comparison to low aspect ratio (especially spherical) particles. Outside the colloidal size regime, the use of a multimodal particle size distribution increases the maximum packing fraction and hence gives rise to a reduction in the shear viscosity of the suspension in comparison to the shear viscosity of a suspension containing unimodal particles.

In our extrusion experiments, however, the shear viscosity of the suspension decreased with the incorporation of the filler. In these experiments the processing characteristics of the pure TPU versus the filled TPU were compared using the flow rate versus the pressure loss and generation characteristics of the flood-fed extrusion process. Figure 1 shows the extrusion die and screw characteristic curves for unfilled and 20 vol% BaS[O.sub.4]-filled TPU. The intersection of the two characteristic curves defines the "operating point." The operating point specifies the extrusion output dependence on pressure for a given extruder and die, operating at a particular screw speed and pumping a melt with a particular viscosity (10). It is interesting to note in Fig. 1 that the pressure rise in the single-screw extruder and the pressure drop at the extrusion die were significantly affected by the presence of the BaS[O.sub.4] filled, but in an unexpected direction. The pressure drop required to extrude the BaS[O.sub.4]-filled TPU is significantly smaller than the pressure required for the extrusion of the unfilled TPU under similar operating conditions. This indicates that the shear viscosity of the 20 vol% filled TPU is smaller than the shear viscosity of the unfilled TPU under similar conditions. As a result, the operating point is shifted from B to A, suggesting that the production rate does increase with the incorporation of the filler. The underlying reasons for this "anomalous" finding were not clear and formed the impetus for this detailed study, which is reported here.


In spite of its commercial importance, there are no detailed reports on the rheological behavior and processing of BaS[O.sub.4]-filled TPU in the literature. Here the processability and the rheological behavior of the BaS[O.sub.4] suspensions of TPU are reported along with various factors, which can play a role including the moisture and the air incorporation effect.


Materials of the Study

The TPU used in this study is a medical grade thermoplastic polyurethane, based on 4,4'-diphenylmethanedi-isocyanate (MDI), a polyether glycol and 1,4-butanediol. Its solid density of 1200 kg/[m.sup.3] reflects its relatively high hard segment content. The TPU is polymerized in a batch reactor and granulated into chips and then fed into a twin-screw extruder for conversion into pellets.

When polymeric resins are utilized in complicated industrial applications, their formulations include various types of stabilizers, such as antioxidants, lubricants, pigments, etc. However, the medical use mandated here precluded the use of any additives and the TPU was thus unusually pure. The results of a comprehensive study on the TPU (without the presence of the filler) can be found elsewhere (11).

BaS[O.sub.4] with a density of 4250 kg/[m.sup.3] was obtained from J.T. Baker under the product code of 1030. This is a fine and white powder with a melting point of 1580[degrees]C and a decomposition temperature of 1620[degrees]C. A typical scanning electron micrograph of the BaS[O.sub.4] is shown in Fig. 2. The shape of particles is approximately elliptical with the major axis of the particles ranging from 0.1 to 2.0 microns. The major axis distribution of the particles is also shown in Fig. 2. About 50% of the particles have major to minor axis ratio values falling between 1.1 and 1.7. The maximum packing fraction, [[phi].sub.max], of the BaS[O.sub.4] particle was determined to be 0.64 using the method of Ouchiyama and Tanaka (12, 13). Since TPUs absorb moisture rapidly upon exposure to atmospheric conditions, they are dried before processing in the extruder. BaS[O.sub.4] powder, however, is usually not dried.

Experimental Procedures

Drying of the TPU

Prior to extrusion, the TPU chips and pellets were dried in a hopper dryer, which makes use of hot air (80[degrees]C-90[degrees]C) having a dew point below -8[degrees]C for 4 to 6 hours. For other lab experiments, the TPU samples were dried in a vacuum oven at 80[degrees]C-90[degrees]C for 4 to 6 hours to achieve a moisture content of less than 0.03 wt% as determined by a thermogravimetric method.

Thermogravimetric Analysis (TGA)

TGA analysis of the BaS[O.sub.4] filler particles was obtained on a Shimadzu TGA 50H and a TA Instruments TGA 2950 with Thermal Advantage Software. The experiments were carried out under dry air, nitrogen or oxygen atmosphere. The heating rate was 10[degrees]C per minute and the scans were made between room temperature and 900[degrees]C.


Rheological Characterization

The linear viscoelastic dynamic properties upon small amplitude oscillatory shear and the relaxation modulus as a function of time and strain upon a step strain material functions of the TPU and filled TPU were characterized using a rotational rheometer. The Advanced Rheological Extended System (ARES) of Rheometric Scientific (currently TA Instruments) was used in conjunction with the 8-mm parallel-disk fixtures.

Processing of the TPU and Filled TPU

Twin-Screw Extrusion

TPU chips from the reactor were converted into pellets in a twin-screw extruder. A Leistritz fully intermeshing, co-rotating twin-screw extruder with 18-mm screw diameter and a length over diameter ratio, L/D, of 35 was used. TPU chips were also compounded with the BaS[O.sub.4] filler using the same twin-screw extruder. The compounding was carried out at the screw speed of 200 rpm and in the temperature range of 180[degrees]C to 200[degrees]C. The die of the extruder was a rod die with a diameter of 0.0025 m and a length-to-diameter (L/D) ratio of 10.

Keeping the operating conditions in the twin-screw extruder the same for both the unfilled TPU and BaS[O.sub.4]-filled TPU ensured that the differences in the rheological behavior are associated with the incorporation of the filler alone.

Single-Screw Extrusion

The TPU pellets and the compounded TPU obtained upon twin-screw extrusion were also extruded using a Harrel single-screw extruder with a 0.0254-m barrel diameter and an L/D ratio of 24. A strand die with a diameter of 0.0036 m and an L/D ratio of 4.0 was used. All three barrel-zone temperatures and the temperature of the die were kept at 200[degrees]C. This is a flood-fed extruder, necessitating that the flow rate be determined as a function of time by weighing the extrudate emerging from the die at 1-minute intervals. The data reported in Fig. 1 were obtained with this single-screw extrusion process.

Microstructural Analysis

A LEO 435 variable-pressure scanning electron microscope was used for microstructural characterization. The electron beam was varied between 10 and 25 kV. For the entire voltage range a tungsten emitter was used. An Olympus Vanox LE 8080-006 microscope was also employed.

Gel Permeation Chromatography (GPC)

A Waters 717 plus GPC coupled to a Waters 2410 refractive index detector was used to determine the molecular weight distribution of TPU. The absolute-weight average molecular weight values of TPU were also determined using a Wyatt Technology Minidawn static three-angle laser light scattering detector (MALLS). GPC measurements were performed at 40[degrees]C in THF. For all samples, it was assumed that 100% of the polymer eluted from the column during the measurement.


The major objective of this investigation was to understand the important factors governing the development of the rheological and the processability behavior of BaS[O.sub.4]-filled TPU. It was especially important to analyze the role played by the BaS[O.sub.4] in generating the unexpected processing behavior summarized in Fig. 1, where BaS[O.sub.4] incorporated into the medical-grade TPU gave rise to a decreased pressure drop at the die and a decreased pressurization rate in the extruder, in comparison with the unfilled TPU under the same operating conditions.

The first obvious factor explaining the processing observations summarized in Fig. 1 is the possible hydrolysis of the binder TPU upon reactions with the moisture. Moisture can be carried into the extruder with the filler, since the TPU itself was very carefully dried prior to extrusion and the BaS[O.sub.4] is not dried. Upon contact with moisture and especially under typical processing conditions, the urethane linkages in thermoplastic polyurethane will be hydrolyzed (14, 15). As shown in the reaction scheme below, the hydrolysis reaction involving the urethane linkages yields polymer fragments with amine and carbonic acid end groups. The carbonic acid formed is very unstable and is decarboxylated immediately. As a result, the hydrolytic degradation products of a TPU include amine and hydroxy-terminated species and carbon dioxide (14, 15).


Let us illustrate the effects of the hydrolysis reaction on the rheological material functions of the TPU binder. This is done by side-by-side comparisons of the rheological behavior of "wet" and dried TPU specimens. The magnitude of the complex viscosity values of "wet" TPU with 0.65% moisture versus "dried" TPU with 0.02% moisture as a function of frequency at 1% of strain and at 200[degrees]C are shown in Fig. 3. The effect of the presence of a relatively large concentration of moisture, i.e., 0.65%, is to reduce the molecular weight of the TPU and thus give rise to a modest reduction of the magnitude of complex viscosity (20% to 30%).

Storage modulus and loss modulus values of the TPU specimens with 0.65% and 0.02% moisture content as a function of the frequency at 200[degrees]C are shown in Fig. 4. At the low-frequency range, the loss modulus values are greater than the storage modulus values, suggesting that the TPU is behaving more fluid-like versus solid-like (more energy is dissipated instead of being stored in this range). With increasing frequency, the storage and loss moduli values increase for both TPU samples with differing concentrations of moisture. At a certain material-dependent crossover frequency, the storage and loss modulus values become equal to each other. For both TPUs the storage modulus values become equal to the loss modulus values at 100 rps.


Overall, the storage and the loss modulus values of the TPU with 0.02% moisture are 10%-30% greater than the values for the TPU with 0.65% moisture. The hydrolysis of the specimens when additional moisture is present results not only in a reduction of the viscosity but also in the reduction of the elasticity of the specimens, as manifested by the lowering of the storage modulus values.

Figure 5 shows the stress relaxation behavior at 10% of strain and 180[degrees]C. TPU with 0.65% moisture exhibits lower values of stress relaxation modulus than TPU with 0.02% moisture, consistent with the hydrolysis reaction. In addition, TPU with the additional moisture relaxes faster, reflecting the reduction in the relaxation times upon a reduction of the molecular weight and hence the entanglement density of TPU.

Let us now compare the linear viscoelastic properties of the vacuum-dried unfilled TPU and the TPU filled with 20 vol% (50 wt%) BaS[O.sub.4]. The linear viscoelastic ranges of unfilled and filled TPU were determined initially by carrying out strain sweeps at the frequency of 1 rps in the overall strain amplitude range of 0.01% to 100% at 180[degrees]C and 190[degrees]C. The span of the linear viscoelastic region, where the storage modulus is independent of the strain amplitude, decreases with the incorporation of the BaS[O.sub.4]. The linear region of the TPU binder persists up to a strain amplitude of 20%, while the linear viscoelastic range of 20% filled suspension samples is limited to smaller values, i.e., strain amplitude of 1% and less. The reduction of shear strain range over which linear viscoelastic behavior prevails with increasing concentration of rigid particles is consistent with earlier studies on rigid particle filled suspensions (16).



Figure 6 shows the magnitude of complex viscosity values of unfilled TPU. and 20 vol% of BaS[O.sub.4]-filled suspension of TPU at 200[degrees]C. As expected on the basis of the single-screw extrusion data (smaller pressure drop at the die and smaller pressurization rates for the filled TPU versus the unfilled TPU as shown in Fig. 1), the magnitude of the complex viscosity values (hence the shear viscosity) of the TPU compounded with 20 vol% of BaS[O.sub.4] are about one order of magnitude smaller than the magnitude of complex viscosity values of unfilled TPU at all frequencies. Additional shear viscosity measurements using an Instron capillary rheometer with multiple capillaries with differing diameters and L/D ratios also confirmed that the shear viscosity values of the 20% BaS[O.sub.4] filled TPU are indeed significantly smaller than the shear viscosity values of the unfilled TPU at temperatures greater than 180[degrees]C.



Figure 7 shows the storage modulus values of the unfilled and filled TPU at 200[degrees]C. The storage modulus values (indicative of the elasticity of the material as a function of the deformation rate in the linear viscoelastic region) of the 20 vol% of BaS[O.sub.4]-filled TPU are also smaller than those of the unfilled TPU. Furthermore, with increasing temperature, the storage modulus values of the filled TPU become significantly smaller than their loss modulus values, indicating a significant relative loss of elasticity with increasing temperature.

The temperature dependence of the linear viscoelastic properties was also investigated. The magnitudes of complex viscosity values of the filled TPU at various temperatures are shown in Fig. 8. There is a very significant drop with increasing temperature. For example, the magnitude of the complex viscosity decreases by four orders of magnitude over a 20[degrees]C temperature increase from 180[degrees]C to 200[degrees]C. If such a drastic sensitivity to the temperature is not observed for the pure TPU, why would it be present in the filled TPU?


Is it possible to explain these findings with the hydrolysis reaction occurring as a result of the presence of the moisture upon being entrained into the BaS[O.sub.4] and introduced into the extruder? If the hydrolysis is taking place during the processing operation, there should be a reduction in the molecular weight of the TPU upon processing. To check, gel permeation chromatography, GPC, studies were carried out. Measurements were performed in THF at 40[degrees]C with a flow rate of 1 mL/min. Indeed, a dramatic reduction in the molecular weight averages (in g/mole or Daltons) was observed, as shown below in terms of the number, weight and z average molecular weights, i.e., Mn, Mw, and Mz, and the dispersity index, Mw/Mn:

TPU: Mn = 66,500, Mw = 215,000, Mz = 900,000, Mw/Mn = 3.24.

TPU upon undergoing the compounding operation with 20% BaS[O.sub.4]: Mn = 9400, Mw = 16,000, Mz = 20,000, Mw/Mn = 1.70.

Thus, there is a significant reduction in the molecular weight averages of the TPU found in the suspensions of TPU with BaS[O.sub.4] in comparison to the TPU without the presence of the filler. Another interesting point is the reduction in the breadth of the molecular weight distribution, i.e., the polydispersity index, (Mw/Mn), from 3.24 to 1.70 upon processing with the filler present.

How much water is necessary to reduce the number average molecular weight. Mn, from 66,000 to 10,000? To reduce the molecular weight from 66,000 to 10,000, on the average, six urethane linkages must be hydrolyzed. This indicates that six moles (or 108 grams) of water are necessary for the hydrolysis of 66,000 g of TPU. This requires that 108 X 100/66000 = 0.16% (1600 ppm) water is available for the hydrolysis to take place. It is not clear what role the presence of the BaS[O.sub.4] plays (it may be acting as a catalyst) regarding the hydrolysis/chain scission in urethanes during processing.

What are the possible sources of moisture to give rise to this hydrolysis effect? The TPU is fed dried into the extruder, and when it is processed alone without the presence of the filler, there is no significant hydrolysis effect. Is there sufficient amount of moisture in BaS[O.sub.4] (which is not dried prior to extrusion) to give rise to the observed extent of hydrolysis? To answer this question, we determined the moisture of BaS[O.sub.4] prior to compounding in the extrusion process and upon drying. For drying of the BaS[O.sub.4], the same parameters used for the drying of TPU were used, i.e., 5.5 hours under vacuum at 85[degrees]C. TGA analysis was also carried out to analyze the moisture content of the compounded and dried BaS[O.sub.4] filler as shown in Fig. 9. The TGA experiment was conducted under non-isothermal conditions in the temperature range of room temperature to 700[degrees]C. Nitrogen was used as the carrier gas for the different experiments. Weight losses were calculated at two temperature zones: 28[degrees]C to 240[degrees]C and 240[degrees]C to 700[degrees]C. The temperature 240[degrees]C is considered as possibly the highest for extrusion, and thus the weight percent loss in this temperature range is a good indicator of the amount of moisture that can be liberated during compounding.


In the temperature range of 28[degrees]C to 240[degrees]C, the weight percentage loss of moisture of "wet" BaS[O.sub.4] in nitrogen is 0.09%, while the value for vacuum-dried BaS[O.sub.4] is 0.07%, indicating that the maximum amount of water likely to be available for hydrolysis of the urethane linkages during compounding of TPU with the BaS[O.sub.4] is 0.02% of the weight of TPU. Since the filler constitutes only 50 wt% of the total suspension, then if all the moisture content of the BaS[O.sub.4] were transferred to TPU, the increase of the moisture content of TPU would only be 0.01% of the weight of TPU. Obviously, only a fraction of this moisture would be accessible during compounding. Thus, the drastic decreases of the molecular weight and the resulting decreases of the shear viscosity and the elasticity of the filled TPU cannot solely be explained on the basis of the minor concentration of water found in the BaS[O.sub.4] filler (see Fig. 3. which shows that a relatively high concentration of 0.65% moisture reduces the viscosity by only 20%-30%). Therefore, a significant amount of moisture in the air should be present during the processing operation to initiate the hydrolysis of TPU and give rise to the extent of hydrolysis observed here.

One hint to the probable cause of the source of moisture is obtained from the significant decrease in shear viscosity and elasticity of the TPU/BaS[O.sub.4] suspension with increasing temperature from 180[degrees]C to 190[degrees]C or 200[degrees]C, as shown in Fig. 8 for the magnitude of complex viscosity. Presumably the rate of the hydrolytic degradation should not be affected significantly as the temperature increases from 180[degrees]C to 200[degrees]C and that the available water would be consumed readily at the lower temperature also.

The micrograph shown in Fig. 10 demonstrates that a significant degree of porosity has formed in the TPU suspension upon being extruded. What is the source of the porosity? As shown in the TGA results reported in Fig. 9, the concentrations of moisture in the TPU or in BaS[O.sub.4] are not sufficient to cause the observed porosity. It is our hypothesis that the observed porosity is caused by the entrainment of relatively large quantities of air into the extrusion system. The single-screw extrusion system provides an ideal mechanism for the entrainment of the air under certain conditions. The air introduced into the extruder with the feed under some conditions forms a continuous channel during the compaction of the solid bed of TPU pellets and the filler powder and is squeezed out gradually. Under other conditions the air channel found in the compacted TPU/BaS[O.sub.4] bed collapses upon pressurization to give rise to the encapsulation of the air dragged into the extruder with the feeding of the TPU and the feeding of the filler. However, since the TPU processed alone does not generate the significant degree of hydrolysis observed here, the source of air into the extruder should be entrainment with the filler.

The subject of air entrainment into suspensions of polymeric materials mixed with rigid particles during extrusion has been investigated by Kalyon and co-workers (17-19). Generally the air comes in through the hopper and is compressed, whenever there is pressurization in the extruder. Especially, if the solid bed being conveyed is prematurely broken up during the continuous process, the air channels are disrupted and the air becomes fully entrained into the suspension.

Why is the air introduced into the suspension through being entrained with BaS[O.sub.4] and not with TPU? The major factor should be the greater surface-to-volume ratio of the filler in comparison to the pellets of TPU. The characteristic lengths of TPU chips being fed into the extruder are 3-5 mm, whereas the characteristic length of BaS[O.sub.4] particles is only about 0.6 micron. During extrusion, air pockets found in the interstitial spaces between the BaS[O.sub.4] particles are encapsulated within the TPU melt. The moisture content of the incorporated air should then give rise to the hydrolysis of the TPU observed here. This suggests that the processing should have taken place with the hopper of the BaS[O.sub.4] filler flooded under a moisture-free inert gas during and prior to the compounding operation.


Besides resulting in the decrease of the molecular weight, the incorporation of the air itself provides an additional effect on the rheological behavior of concentrated suspensions. The incorporation of the air into the suspension leads to the decrease of the shear viscosity and the elasticity of the suspension and should increase the wall slip coefficient (18) even in the absence of a change in the molecular weight of the binder. Drastic changes in processability behavior were indeed shown to occur with the presence or the absence of air in concentrated suspensions (19).

The air incorporation effect was probed further by subjecting the specimens, which were extruded, to various temperatures. Moldings of the specimens upon drying were kept at several different temperatures in the 180[degrees]C-220[degrees]C range and were then cooled under similar cooling rates to ambient temperature and studied under a microscope. As shown in Fig. 11, the 20% filled samples exhibit visible bubbles under a microscope at temperatures of 200[degrees]C and 220[degrees]C. On the other hand, no bubbles are visible in the unfilled TPU subjected to the same thermal history by being kept at 180[degrees]C, 200[degrees]C and 220[degrees]C (20).

The presence of air should be an additional important factor in the structuring of the extrudates from the compounded TPU. For example, Fig. 12 shows cracks in the bulk of the filled TPU upon extrusion. The presence of this void fraction (which may be due partly to the degradation of the polymer) should introduce stress concentration points, which render the material more susceptible to forming, first, crazes and then cracks, and lead to immediate deterioration of the mechanical properties of the extruded articles.


The compounding of medical-grade polymers, which involves minimum or no additives, is a challenge. It is shown here that the compounding of a medical-grade TPU with the requisite BaS[O.sub.4] X-ray tracer for in vivo medical device applications leads to unexpected decreases in shear viscosity and elasticity in comparison to the rheological behavior of the pure TPU. The sources of this reduction in viscosity and elasticity can be associated with the hydrolytic degradation of the binder upon exposure to the moisture bound to the filler on one hand and the air introduced into the extrusion process through entrainment with the high surface-to-volumeratio BaS[O.sub.4] filler particles. The entrainment of the air into the suspension during the extrusion stage is the major factor giving rise to hydrolysis and the reduction in the elasticity and the shear viscosity of the filled TPU.




We thank Dr. B. Greenberg and Ms. M. Erol of the Highly Filled Materials Institute for the particle size analysis of the BaS[O.sub.4].


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(1) Highly Filled Materials Institute and Biochemical, Chemical and Material Engineering Stevens Institute of Technology Hoboken, NJ 07030

(2) Chemistry Department Koc University Istanbul, Turkey

* To whom correspondence should be addressed.
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Author:Lu, Guangyu; Kalyon, Dilhan M.; Yilgor, Iskender; Yilgor, Emel
Publication:Polymer Engineering and Science
Date:Oct 1, 2004
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