Extended Pressurization with N2
Day Pressure (psi) Day Pressure (psi) Day Pressure (psi)
1 34.5 16 34.5 31 34.5
2 34 17 34.5 32 34.5
3 34.5 18 34 33
4 34.5 19 34 34
5 34.5 20 35 34
6 21 34 36 34
7 34 22 34 37 34.5
8 34.5 23 34.5 38 34.5
9 34.5 24 34.5 39 34.5
10 34 25 34.5 40 34
11 34.5 26 41 34.5
12 34.5 27 42
13 28 34.5 43
14 34 29 34.5 44 34
15 34.5 30 34.5 45 34.5
Miscellaneous Cryogenic Test Results
Pressurization with N2
Pressurization with LN2
Thermal Cycling (1000x +)
34 psi 34 psi
no crack formation
SEM Micrographs of Cryogenically Cycled Surface Internal Surface
External Surface
Appendix F. Publications
Sintering of Thermotropic Liquid Crystalline Polymers
Eric Scribben and Donald Baird, Department of Chemical Engineering, Virginia Tech
Abstract
Two thermotropic liquid crystalline polymers were evaluated for use in rotational molding: Vectra A 950 and Vectra B 950. Material is typically prepared by grinding.
When ground, even under cryogenic conditions, TLCPs tend to yield particles of high aspect ratio translating to low bulk density, poor granular flow, and incomplete densification. A process was developed to generate spherical particles of controlled size and distribution.
Particle coalescence and densification characteristics were determined and compared against model predictions.
Introduction and Background
Rotational molding is composed of four distinct steps. First the material is ground then mixed to ensure a homogenous powder exists. Next the powder is loaded into a mold, which is rotated biaxially. In this phase, a solid granular flow problem exists. As the material tumbles, the mold is heated eventually causing the particles to coalesce and adhere to the mold wall. Finally, once densification is complete, the mold is cooled and the solidified part is removed.
Coalescence can be divided into two steps: initial interface bridge formation between particles, where there is little or no change in density, and densification in which the inter-particle cavities are eliminated [1].
Densification is a bulk effect and is modeled to account for pore closing. Neck formation is not dependent on the bulk powder because individual necks do not influence each other until pores form. Hence, it is typically modeled as the coalescence of two particles. This will be referred to as sintering in this paper, although it is technically the sintering of two particles.
Frenkel [2] first explored viscous sintering by deriving an expression for the coalescence rate of two identical spherical particles, which was corrected for continuity by Eshelby [3]. Their model, Eq. 1, is limited to Newtonian materials at the beginning of sintering where the particle radius has not changed due to neck growth. It is the result of a mechanical energy balance between the work done by viscous forces to that presented by the reduction of surface area due to surface tension (see equations 2 and 3).
12
2 0
3
= a t a
x
η
Γ (1)
where x is the radius of the neck between the drops, a is the drop radius, t is time, Γ is the surface tension, and η is the zero shear viscosity.
∫∫∫
=
V
v dV
W τ :γ& (2)
dt
Ws =−ΓdS (3)
where t is stress, g is the rate of strain tensor, S is surface area, and V is volume.
Some progress has been made since Frenkel proposed his viscous sintering model. Pokluda [4] extended Frenkel’s expression to apply to the entire sintering process by using geometrical arguments and applying the conservation of mass with constant density to account for changing drop radius. Bellehumeur [5] used those geometrical arguments and adopted the upper convected Maxwell constitutive model (UCM) to incorporate viscoelastic effects. However, it is doubtful that this could capture the anisotropic nature of TLCPs.
TLCPs have numerous benefits over traditional polymers used in processes involving sintering. They typically possess superior mechanical properties, are chemically resistant to corrosive liquids, and have good barrier properties. Exploiting these advantages could result in high performance products capable of new applications.
Unfortunately there are several disadvantages inhibiting their processing via sintering. Generally
particle size thereby increasing the driving force for coalescence and decreasing the void fraction. Grinding TLCPs, even under cryogenic conditions, results in high aspect ratio particles with low bulk density. This material cannot completely sinter under pressureless sintering conditions. Fig. 1 is an SEM of cryogenically ground Vectra B 950. These particles tend to aggregate causing poor solid granular flow, which makes it nearly impossible to rotationally mold them.
Typical deformation rates obtained from sintering experiments are in the range of 10-2 s-1, during the early stages, down to approximately 10-4 s-1, nearing complete densification. This implies that the zero shear viscosity is the relevant viscosity in this process. Some TLCPs under appropriate conditions have been observed to contain a viscosity increase at low deformation rates that could effectively slow or even halt sintering prematurely.
In this paper the sintering problems associated with TLCPs are addressed for the two selected resins. The low shear rate rheology was evaluated to ensure that a zero shear viscosity exists. A drop deformation technique was used to generate spherical particles to eliminate the low bulk density problem. These spheres were sintered and compared to several models to assess their applicability to the TLCP sintering process.
Experimental
Materials
To address the processing concerns, two readily available commercial resins were evaluated: Vectra A 950 (hydroxy benzoic acid/ 2,6 hydroxynapthoic acid) and Vectra B 950 a polyesteramide (60 hydroxy naphthoic acid/ 20 terephthalic acid/ 20 aminophenol).
Apparatus
Steady stress growth measurements were performed on a Rheometrics RMS 800 and were complemented with creep measurements obtained from a stress controlled Rheometrics RSR 8600. A 25 mm diameter cone and plate geometry with a 0.1 radian cone angle was used in an inert (N2) atmosphere. Higher rates were obtained with the RMS in small strain dynamic mode.
A 25.4 mm Killion extruder was used to disperse the TLCP in a low molecular weight polypropylene. The TLCP was then extracted from the polypropylene matrix to obtain the spherical TLCP particles.
Sintering experiments were performed in a Linkam THM 600 heating stage equipped with an optical microscope and a camcorder to record high resolution video. The hot stage was capable of achieving a heating rate of 90°C per minute and could maintain temperature within 0.1°C making it safe to assume the experiments were isothermal. Once again nitrogen was used to ensure an inert atmosphere.
Results and Discussion Rheology
Rheological results can be seen in Figures 2 and 3.
Vectra B 950 displays a definite zero shear viscosity at both temperatures. Vectra B reached a plateau of approximately 400 Pa s at 320°C and 600 Pa s at 330°C.
Vectra A 950 exhibits a low rate viscosity increase at 320°C, reaching values above 10,000 Pa s. It has been suggested that this behavior is the result of residual crystallinity [6]. At 330°C, Vectra A appeared to achieve a zero shear value of around 4000 Pa s. As mentioned, the viscosity increase at 320°C could translate to incomplete sintering.
Spherical Particles
As previously mentioned, the spherical particles were generated by blending the TLCPs with polypropylene and subjecting the mixture to high deformation rates in the extruder. Fig. 4 is an SEM of the spherical drops as formed in the polypropylene matrix. The TLCP was then extracted, and the size distribution was measured. As seen in Fig. 5, a fairly wide range of particle sizes is obtainable. It should also be mentioned that mean particle size can be manipulated through extrusion residence time and cooling rate.
Sintering
A 250 àm radius was used in every trial. At 320°C Vectra B 950 appeared to coalesce within approximately 20 seconds. Vectra A 950 was much slower and appeared to stop after two minutes at around 75% of complete neck growth. Sintering results and model predictions are
pictured in Figures 6 and 7. For Vecra B 950 the modified Frenkel expression underpredicted the sintering time by almost 10 seconds. The experimental points appear to initially increase at a constant rate whereas the models proceed with a decaying rate. The UCM based expression, with an approximate terminal relaxation time of 50 seconds, grossly overpredicts the sintering time. In the case of Vectra A 950, both models overestimate sintering time but this could be influenced by the fact that the true zero shear behavior was not observed.
At 330°C, the sintering rates became indistinguishable between the two resins reaching complete sintering within 15 seconds. The modified Frenkel results initially diverge but achieve acceptable agreement again when the experimental rate begins to decline. The UCM values, with a relaxation time of approximately 100 seconds for Vectra B 950, again underestimated the sintering rate by approximately 35 seconds. For Vectra A 950, (λ= 2.5 s) both models were almost identical in underestimating the rate.
These models unsatisfactorily predicted sintering times for the selected TLCPs. This inadequacy could be attributed to several rheological phenomena. In both models zero shear behavior is assumed to exist and deformation rates are not great enough to cause shear thinning behavior. If deformation rates are great enough this could explain some of the discrepancies. Another explanation is that true steady state behavior has not been
reached. In order to obtain a steady value these resins must be deformed for 200 to 800 seconds. Fig. 8 shows the results of the stress growth experiments for Vectra A 950 at 0.1 and 0.01 s-1. Similar results are observed for Vectra B. These times vastly exceed the deformation time observed during sintering, 10 to 20 seconds. Within that time, the viscosity has only reached approximately half of the steady state value at the low rates observed during sintering.
Conclusions
Generating spherical particles not only increases bulk density and improves granular flow, but it provides an ideal system for studying the sintering of TLCPs. Both TLCPs exhibit zero shear viscosities at 330°C, but at 320°C Vectra A 950 viscosity increases for decreasing shear rates. Sintering kinetics were not accurately captured by either Newtonian or viscoelastic Maxwell models. This was attributed to the shear thinning and transient nature of TLCP viscosity, which might be addressed using Doi’s theory for rigid rod-like polymers.
References
1. Z. Tadmor and C. Gogos, Principles of Polymer Processing, Wiley, New York, 1979, pp. 305-307.
2. J. Frenkel, “Viscous Flow of Crystalline Bodies under the Action of Surface Tension,” J.Phys., 9, pp.
385-391, (1945)
3. J.D. Eshelby, “Discussion.” in A.J. Shuler, ‘Seminar on the Kinetics of Sintering,” Metals Trans., 185, pp.
806-807, (1949)
4. O. Pokluda, C.T. Bellehumeur, J. Vlachopoulos,
“Modification for Frenkel’s Model for Sintering,”
AIChE Journal, 43, 12, pp. 3253-3256, (1997)
5. C.T. Bellehumeur, M. Kontopoulou, J. Vlachopoulos,
“The Role of Viscoelasticity in Polymer Sintering,”
Rheologica Acta, 37, pp. 270-278, (1998)
6. K.F. Wissbrun, “Rheology of Rod-Like Polymers in the Liquid Crystalline State,” J. Rheol., 25, 6, pp.
619-662, (1981)
Figure 1. SEM of cryogenically Ground Vectra B 950
Figure 2. Rheology of Vectra at 320°C
Figure 3. Rheology of Vectra at 330°C
Figure 4. SEM of Spherical Drops of TLCP in Polypropylene Matrix
Figrue 5. Sphere Particle Size Distribution
Figrue 6. Sintering Experiments and Model Predictions at 320°C
Figure 7. Sintering Experiments and Model Predictions at 330°C
Figure 8. Vectra A 950 Stress Growth
Performance of a Rotationally Molded Thermotropic Liquid Crystalline Polymer Eric Scribben and Donald Baird, Department of Chemical Engineering, Virginia Tech
Abstract
Thermotropic liquid crystalline polymers (TLCPs) have a number of potentially useful physical properties for rotational molding: excellent chemical resistance, good barrier properties, low coefficient of thermal expansion, high tensile strength and modulus, and good impact resistance. However, it is possible that the nature of the molding process is such that full advantage of these properties cannot be obtained. To determine how well TLCPs perform when rotationally molded a commercially available TLCP, Vectra B 950, was studied under static conditions as well as with a single axis rotational molding unit capable of measuring the internal air temperature.
The processing temperature was determined by measuring shear viscosity at several temperatures. The tensile strength and modulus of both statically molded and rotationally molded samples were measured. Samples were evaluated for complete densification by inspecting the fractured surface.
Introduction and Background
Rotational molding is a process used to manufacture hollow plastic products [1]. The process begins by loading polymer powder into a hollow mold and then
simultaneously rotating it about two principal axes. Heat applied to the external surface conducts to the tumbling powder, which eventually exceeds its tack temperature and adheres to the mold surface. While heating continues, the powder sinters into an evenly distributed layer and densifies as the trapped air bubbles diffuse through the melt. The mold continues to rotate as it is cooled, and once the plastic is sufficiently rigid the product is removed [2].
Thermotropic liquid crystalline polymers (TLCPs) are a class of engineering resins that offer some unique and potentially useful properties to rotational molding.
They can have a relatively high resistance to solvents and excellent barrier properties because of their low gas solubility. They may be molded into structures with extremely accurate dimensions because of their low or negligible coefficient of thermal expansion. They are also capable of providing high tensile strength and modulus, which are on the order of 102MPa and 101GPa respectively [3].
Unfortunately, some of these properties may become a disadvantage in rotational molding. For example low gas solubility may inhibit bubble dissolution and a
negligible coefficient of thermal expansion could make it difficult to remove the molded product from a complicated mold. In addition some TLCPs do not have a well defined zero shear viscosity, which can inhibit coalescence. Traditionally prepared TLCP powders are fibular with poor powder flow characteristics and result in bridging and poor surface quality. It would be useful to evaluate these materials to determine the state of these issues and their implication on performance.
This work identifies problems associated with mechanical performance of rotationally molded TLCPs.
The low shear rate rheology was measured to ensure that a zero shear viscosity exists and identify an appropriate processing temperature. A drop deformation technique was used to generate spherical particles to eliminate the low bulk density problem. Various sizes and distributions of these spheres were sintered statically. A cylinder was rotationally molded from a distribution more representative of what is currently used in practice.
Tensile tests were performed on the specimens to determine what influence size and size distribution has on strength and modulus.
Experimental Materials
A commercially available resin was selected for this set of experiments: Vectra B 950 a polyesteramide (60 hydroxy naphthoic acid/ 20 terephthalic acid/ 20
aminophenol) with a melt temperature of approximately 280°C.
Apparatus
Stress growth measurements were performed with a Rheometrics RMS 800 and complemented by creep measurements obtained with a stress controlled Rheometrics RSR 8600. A 25mm diameter cone and plate geometry with a 0.1 radian cone angle was used in an inert (N2) atmosphere for both sets of measurements.
Higher rates were obtained with the RMS in small strain dynamic mode with a 25mm diameter parallel plate geometry.
A 25.4 mm Killion extruder was used to disperse the TLCP in a low molecular weight polypropylene. The TLCP was then extracted from the polypropylene matrix to obtain the spherical TLCP particles. The particles were sieved to determine their size. Four mesh sizes were selected along with four distributions. Table 1 contains sieve information used in size measurement as well as distribution information. The 20, 30, 40, 50 mesh sizes were selected because extremely fine powders typically have poor solid flow characteristics so a mixture containing a majority of fines is not normally used [2].
Distributions D1, D2, and D3 were created to investigate the effect of distribution type on mechanical properties and densification while RM is a more typical distribution for rotational molding with the majority of the material being between approximately 300 and 600 microns [2].
The eight samples were statically sintered into tensile bars and tested. All tensile bars were sintered in a 1.27cm
× 6.35cm mold with an exposed top surface and a thermocouple fixed in the center of the side wall.
Nitrogen was supplied through a chamber that covered the mold. The entire unit was placed in a pre-heated hot press. The heating soak time was 40 minutes and began once the maximum temperature (320°C) was reached.
The sintered bars were tested with an Instron 4202 using a crosshead speed of 1.27mm/min and a 30.5mm gauge length to determine strength and Young’s modulus according to ASTM Standard D638-01.
Rotational molding was done using the distribution RM with a single axis lab scale device. The mold was a stainless steel cylinder with a 3.81cm diameter and 7.62cm long. Both ends were capped. One end was fixed to a shaft that was driven by an electric motor rotating at 10rpm. The other cap had an opening so that a thermocouple could be installed to monitor the air temperature within the mold. Heat was provided by a convection oven equipped with a nitrogen purge and capable of heating rates up to 60°C per minute. The heating cycle was designed to mimic the static sintering conditions. The molded product was then sectioned and strips were used for tensile measurements.
Results and Discussion Rheology
Results from the rheological tests are shown in Figure 1. Vectra B 950 displays a zero shear viscosity at both temperatures, but rate independence occurs at 400 Pa sec at 320°C and 600 Pa sec at 330°C. Results for temperatures below 320°C are not reproducible, behavior that can be attributed to varying amounts of residual crystallinity [4]. Since the viscosity at 320°C is less than at 330°C and lower temperatures contain residual crystallinity 320°C was selected as the processing temperature.
Statically Sintered Tensile Properties
The results from the tensile measurements are summarized in Table 2. No clear relationship between size or size distribution and modulus was found.
Excluding the 20 mesh sample, all moduli were within standard deviation of each other. Therefore, it is reasonable to assume that the modulus remained constant with a mean of 1.03GPa. The 20 mesh sample should not be completely disregarded. It would be useful to understand why its modulus is almost half of the other samples. The 1.03GPa mean is also significantly below the 20GPa that is possible with good molecular alignment [3].
Possible causes for these discouraging results could be insufficient global molecular alignment. Although this may be part of the reason, it cannot completely account for the problem, since poorly oriented samples have moduli around 2.5GPa [3]. It could possibly be attributed
to poor interparticle adhesion due to the lack of molecular diffusion across contact boundaries. This possibility is reasonable since it is well documented that weld line strength is a problem in the injection molding of these materials. In addition, it is reasonable to speculate that entrapped bubbles were unable to completely densify, possibly the result of low gas solubility in TLCPs.
A trend was observed between strength and size.
Strength increased as particle size decreased. If densification was incomplete then it is reasonable to assume that voids become smaller in finer powders.
Decreased void size should mean increased structural continuity and strength. However, the results from samples D1, D2, and D3 do not support this because D3 is composed of a higher percentage of smaller particles than D2. Yet D2 was stronger than D3. To explain the results the fracture surfaces were inspected. (Refer to Figures 2 and 3 for the images).
Figure 2 shows that both weld lines and encapsulated bubbles may contribute to part failure. It is easier to identify the boundary of the larger particles in the 20 mesh sample and the concentration of large encapsulated bubbles decreases with particle size. The 50 mesh sample does appear to contain a large amount of small bubbles, but perhaps they fail to reduce strength because they do not disrupt structural continuity to the extent that the larger ones do. Figure 3 also shows that large bubble concentration and adhesion explain the strength results for
the distribution samples. Sample D2 has a higher concentration of large voids than D1, making D1 stronger.
D2 is stronger than D3 but it does not contain more voids.
However, it is easier to identify individual particles throughout the surface, the result of poor adhesion. It is apparent that a correlation between the fracture surface and strength exists, but it is not readily quantifiable.
Rotationally Molded Tensile Properties
It was found that the tensile modulus and strength of the rotationally molded product was higher than statically sintered material. The modulus was 2.022GPa and strength was 17.50MPa, which is notably higher than the modulus and strength (0.930GPa and 10.51MPa) of the static sample. This suggested that interparticle adhesion may have improved or the product contained less bubbles.
The fracture surfaces do not appear to reveal problems with adhesion but bubbles are distinguishable, as shown in Figure 4. Examination of the void area in the fractured surface can partially explain the improvement. The rotationally molded samples show that approximately 8%
of the total surface area is void while static conditions produced a sample with 13% void. After normalizing the strength and modulus for void content the values from static conditions were still not comparable to that prepared from rotational molding. Therefore, a change in adhesion did occur.
In addition to tensile measurements, the surfaces were inspected. The quality of the external surface,