The rotational molding of a thermotropic liquid crystalline polymer.

INTRODUCTION

Thermotropic liquid crystalline polymers (TLCPs) exhibit a number of mechanical and physical properties that make them desirable for use in storage vessels for cryogenic or corrosive fluids. TLCPs have demonstrated exceptionally high values of tensile strength and modulus (strengths in excess of 1000 MPa and moduli near 100 GPa from fiber spinning and approaching 200 MPa and 20 GPa, respectively, in injection molding) [1]. They possess low permeability to gases, which is essential for the containment of gases [2]. They exhibit excellent resistance to acidic or basic environments and a wide range of organic solvents, prerequisites for storing corrosive fluids [3]. Finally, they have low coefficients of linear thermal expansion (CLTE) and, therefore, may be useful for storage of cryogenic fluids such as liquid hydrogen and oxygen as they may be less prone to failure due to thermally induced stresses [1].

Rotational molding is a convenient processing method for manufacturing large storage vessels from thermoplastics [4]. In the process, polymer powder is loaded into a hollow mold that is simultaneously rotated about two principal axes. Heat is applied to the external surface of the mold and is conducted to the tumbling powder, which melts and adheres to the mold surface. As heating continues, the powder coalesces as a result of the surface tension and densifies into an evenly distributed layer that coats the internal surface of the mold. The mold is cooled, and once the plastic has solidified, the product is removed [5].

Unfortunately, there are no reports in the technical literature on the rotational molding of TLCPs. The only work that was presented involved the static coalescence of two TLCP powders [6]. Three key results were reported. The first result was that powder generated by cryogenically grinding TLCP pellets was composed of high aspect ratio particles. Secondly, it was reported that coalescence was either slow or incomplete and speculated that the observed difficulties with coalescence may be due to large values of the shear viscosity at low deformation rates. Finally, complete densification was not observed for the high aspect ratio particles. Hence, it is most likely that the problems encountered in the static coalescence of TLCPs would preclude rotational molding of these materials.

High aspect ratio particles are not desirable for rotational molding because they typically have low apparent (or bulk) densities and tend to agglomerate. Free flow of granular solids, also referred to as powder flow, is essential for material distribution during mold rotation and is greatly dependent upon powder properties such as size, shape, and density [4]. In general, a desirable powder for rotational molding is one with a diameter that is approximately between 150 to 500 [micro]m, gives good packing density, possesses reasonable surface area to volume ratio, and is not fibrous or thread-like [7]. Therefore, it is expected that a free-flowing powder is not readily available for use in the rotational molding process.

Complete coalescence is crucial for successful rotational molding. In its simplest form, coalescence describes the merging of two particles into a single, homogeneous drop. When two fluid-like particles are brought into contact, surface tension, being opposed by the fluid's viscosity, acts to minimize surface area by expanding the contact interface between the particles. It has been shown that the rate of interfacial growth is proportional to surface tension and inversely proportional to the shear viscosity [8, 9]. Unlike Newtonian fluids, the viscosity of a polymeric fluid is dependent on the rate of deformation. The deformation rates that occur during coalescence are estimated to be less than approximately 1 X [10.sup.-2] [sec.sup.-1] [10]. These low deformation rates suggest that the zero shear viscosity of a polymer melt represents the resistance to flow during this process. It has been shown that some TLCPs do not have a well defined zero shear viscosity [11]. Instead, they exhibit a yield stress-like behavior that is part of what is referred to as a three-region flow curve, in which the magnitude of the steady shear viscosity is related to different stages of the destruction of a polydomain texture into a single nematic phase by the imposed shear deformation [11]. If the selected TLCP exhibits this behavior the large value of viscosity at low deformation rates may inhibit coalescence. Hence, identifying conditions (i.e., temperature and environmental) at which the zero shear viscosity behavior is conducive to coalescence is a prerequisite for successful rotational molding.

Densification refers to the stage in rotational molding by which the coalescing powder bed consolidates into a homogeneous pore-free layer. Once the contact interfaces between coalescing particles become large enough, the structure of the powder bed begins to resemble a continuous, three-dimensional lattice [7]. As the particles continue to melt, the height of the powder bed decreases and the lattice gradually collapses upon itself. Unfortunately, not all of the air that was once surrounding the lattice structure is immediately expelled during the collapse and a portion becomes encapsulated within the melt. The encapsulated bubbles are not drawn to the free surface by buoyancy or capillary forces [12]. Instead, the gas in the bubbles is removed by dissolution into and diffusion through the surrounding polymer melt [13, 14]. Hence, the same factor that makes TLCPs useful for gas storage vessels may also hinder the densification process. It is noted that TLCPs have permeability coefficients for gases such as He, [H.sub.2], Ar, [N.sub.2], and C[O.sub.2] that are comparable to or less than those for polyacrylonitrile (PAN), one of the least permeable polymers known [2].

As there are no reports of successful rotational molding of TLCPs, obviously, the mechanical properties of rotational molded TLCP structures have not been reported. While TLCPs have demonstrated exceptionally high values of tensile strength and modulus, the magnitude of these quantities is dependent upon the degree of mesophase orientation introduced during processing [1]. Processing methods that introduce high deformation rates increase orientation and produce structures with higher values of tensile strength and modulus. Compression molding, which generates deformation similar to those that occur during rotational molding, produces an unoriented TLCP that has mechanical properties similar to those of a conventional isotropic polymer [1]. Furthermore, the strength may also be limited by the presence of weld lines, which form at the contact interface between coalescing particles [1]. Hence, the order of magnitude of mechanical properties that might be possible in the rotational molding of TLCPs is unknown.

[FIGURE 1 OMITTED]

Based on the fact that rotational molding of TLCPs may beset by several problems, the objectives of this work are to: 1) develop a method to generate free flowing powders; 2) establish the thermal and environmental conditions required for the successful coalescence of TLCP particles; 3) determine if the conditions for coalescence can be translated to appropriate conditions for rotational molding of the bulk powder; and 4) evaluate the mechanical properties of a rotationally molded tube.

ANALYTICAL METHODS

Material

Vectra B 950, a nematic TLCP available from Ticona (Summit, NJ), was selected for this evaluation. The primary motivation for selecting this particular material is because it has demonstrated superior mechanical properties when compared to other commercial TLCPs [15]. Vectra B is a wholly aromatic polyesteramide, randomly copolymerized from 60 mole percent hydroxynaphthoic acid, 20 mole percent terephthalic acid, and 20 mole percent aminophenol. The chemical structure is shown in Fig. 1. The melt temperature, as determined by the peak value from differential scanning calorimetry (DSC), is 279[degrees]C [16]. When the melt is cooled quiescently, the crystallization temperature is 226[degrees]C, as defined by the beginning of the exothermic crystallization peak measured by DSC. However, the resin may crystallize in shear flow at higher temperatures, up to approximately 300[degrees]C [16]. The glass transition temperature, as measured by the peak value from dynamic mechanical thermal analysis (DMTA), is 147[degrees]C [17]. The nematic to isotropic transition temperature is unknown because the degradation occurs at temperatures below the transition. The weight average molecular weight and polydispersity index are thought to be around 30,000 and 2, respectively [18].

Generation and Characterization of Powders

Vectra B 950 is available as a pellet, which is too large for rotational molding. Thus two powder generation methods were evaluated. The first method was the approach used to make powders from conventional rotational molding polymers. The pellets were milled with a Vortec M-1 impact mill at room temperatures and under cryogenic conditions by combining liquid nitrogen to the feed. The second method for generating powders was based on the blending of a TLCP with an incompatible polymer in a 1-inch Killion single screw extruder. When two immiscible phases are mixed, the minor phase is dispersed with a droplet size determined by its properties (i.e., viscosity and interfacial tension) and the shear stress [5]. The TLCP was blended at up to 40 wt% with a low molecular weight polypropylene (melt index = 400) at 340[degrees]C. The extruded blend was quenched below the melt temperature of both components, effectively freezing the dispersed blend morphology. The cooling rate played an important role in determining the size of the dispersed TLCP phase. If the mixture was not quenched quickly enough, the TLCP phase would continue to separate from the continuous phase and coalesce with itself, increasing the dimensions of the TLCP phase. The dispersed TLCP phase was retrieved by fracturing the polypropylene matrix with a mill and separating the components in water (the specific gravity of polypropylene is approximately 0.9 and it will float in water, while the TLCP has a specific gravity of 1.4 and it sinks). Residual polypropylene on the TLCP particles (less than 1 wt% as measured by thermal gravimetric analysis) was removed by dissolving it in light mineral oil at 170[degrees]C. The mineral oil mixture was then washed from the TLCP particles using a biodegradable degreaser and water. The TLCP powder was then dried in accordance to manufacturer specifications in vacuum oven at 150[degrees]C for between 12 and 24 hours [17].

After the particles were retrieved from the polypropylene matrix, they were separated into discrete groups according to their size with a series of U.S. standard sieves and a Rotap shaker. Separation was performed not only to measure the size distribution of the accumulated powder but also to allow for the evaluation of particular sizes and distributions in later experiments. The procedure for separating the powder was to sift 100-gram samples for 10 minutes as described in ASTM test D 3451 for testing polymeric powder and powder coatings. Shaking for 10 minutes delivered consistent results while minimizing static charge buildup in the extremely fine particles (less than 149 microns).

The apparent density of all powders was measured according to ASTM test D 1895, which describes powder property measurements for plastic materials. Test method A was used for the particles obtained from the melt blending process and test method C was used for the powder acquired from the milling process. Test method A was designed for fine granular materials that can be readily poured through a standardized funnel. Test method C was applicable to materials that cannot be poured through the funnel because they are compressible, usually composed of coarse flakes, chips, cut fibers, or strands. Two values are reported for the apparent density measured according to test method C: one for the initial density of loosely packed material and one with the powder under a prescribed compressive load.

[FIGURE 2 OMITTED]

The dynamic angle of repose is the angle of the surface of a flowing powder relative to horizontal, as shown in Fig. 2. It was measured in a 100 mL graduated cylinder. A total of 50 mL of the powder was poured into the cylinder. The open end of the cylinder was plugged with a rubber stopper that had been placed on a shaft, driven by a variable speed motor. The cylinder was supported at the opposite end so that it would remain horizontal during rotation. Rotation rates from approximately 1-10 rpm were used during the measurement of each sample. The rates were selected because they were comparable to what was introduced to the powder during rotational molding. A digital image of the tumbling powder was taken at each rotation rate and the angle was determined by analyzing the image in Adobe PhotoShop.

Coalescence Experiments

Thermal and environmental conditions for the successful coalescence of TLCP particles were identified by performing neck growth experiments. Two spherical particles with a diameter of 500 [micro]m were placed in contact inside a Linkam THM 600 hot stage set at one of two potential operating temperatures, as identified by DSC, surface tension, and shear viscosity measurements. The tests were performed in an inert, nitrogen atmosphere to assist in eliminating thermooxidative degradation during the experiment. The test at the lower temperature was also performed in air to determine the significance of the inert atmosphere during neck growth. The heating rate for the neck growth experiments was 90[degrees]C per minute and the test temperature was maintained at the set point to within 0.1[degrees]C, which provided nearly isothermal conditions. The neck growth process was observed in the hot stage with a Zeiss Axioskop equipped with a color CCD camera. The video feed was recorded to high resolution digital video. The neck growth between the two particles was identical to that shown in Fig. 3. Still images from the digital video were extracted at prescribed intervals, and the neck and particle radii, x and a, were measured using Scion Image, a digital image analysis software available from Scion Corporation. Each neck growth experiment was conducted three times to ensure reproducibility, and the reported neck radius vs. time data were the average of the three runs.

[FIGURE 3 OMITTED]

Thermal Behavior

Dynamic scanning calorimetry (DSC) was used to identify a potential minimum temperature for the neck growth experiments by measuring the end of the melt transition. This analysis can only identify a potential minimum temperature because it has been shown that TLCPs can recrystallize at temperatures above their melt point [19]. Also, the presence of a small amount of crystallinity, undetectable by DSC, can still greatly affect the viscosity [18]. In both of these cases, care should be taken in assuming that complete melting has occurred. The thermal analysis was performed at a heating rate of 10[degrees]C/minute with a Seiko Instruments SSC/5200 series auto cooling DSC-220C. The sample was exposed to both a heating and a cooling cycle before the reported measurement to ensure the material had been properly dried and to impose a known thermal history.

Surface Tension

The surface tension was measured at two potential operating temperatures to determine the magnitude of the driving force during neck growth. The surface tension was determined by fitting the Bashforth and Adams equation to a sessile drop profile of the molten polymer at the particular test conditions [20]. This method was selected because it presents a noninvasive means of measuring the surface tension of the TLCP as a melt with the identical geometry, thermal history, and deformation history used in the neck growth experiments. A single, 500 [micro]m diameter sphere, identical to those used in the coalescence experiments, was placed on a glass slide in the hot stage, where it was melted into a sessile drop. The sample was quenched and the glass slide was rotated to allow a profile view of the drop from above. The sample was reheated to the test temperature and a digital image of the profile was recorded with an optical microscope equipped with a camcorder. The Bashforth and Adams equation was fit to data points representing the profile shape that were extracted from the digital image of the profile with Scion Image. The accuracy of this technique (0.1%) demands that the particle radii must be small so that gravitational forces cannot influence the shape of the profile. The absence of gravitational forces was verified by calculating the Bond number Bo = [[[rho][r.sup.2]g]/[GAMMA]](Bo = 0.030) and was supported by the observation that the profile shape did not change upon rotating the glass slide.

Rheology

The shear viscosity was measured at the two potential operating temperatures to determine the magnitude during coalescence and to verify the absence of the three-region viscosity behavior. All rheological characterization was performed with a Rheometrics Mechanical Spectrometer Model 800 (RMS-800). The instrument test geometry was a 25 mm diameter cone and plate with a 0.1 radian cone angle. The magnitude of the complex viscosity, |[eta]*| vs. angular frequency, [omega], and shear viscosity, [eta], vs. shear rate, [dot.[gamma]], data were measured in the presence of an inert nitrogen atmosphere to prevent thermooxidative degradation. Test specimens were prepared by compression molding preforms at 320[degrees]C under nominal pressure and allowing them to quiescently cool without applied pressure. This method produces homogeneous samples with minimal residual stress that were the desired dimensions for the test geometry. Reported rheological results represent the average of at least three runs using different samples for each run. Small amplitude dynamic oscillatory shear measurements were performed for frequencies of 0.1-100 rad/sec at 10% strain for both 320 and 330[degrees]C. The steady shear viscosity was measured by recording the steady state value of a stress growth upon inception of steady shear flow experiment. The stress growth experiments were performed at low shear rates because of the lengthy times required to obtain data at low angular frequencies from dynamic oscillatory measurements. Viscosity values at shear rates less than 0.1 [sec.sup.-1] were thought to be the most pertinent to the coalescence process.

The measured rheological response of TLCPs is strongly dependent upon thermal and deformation history. Therefore, a pretest procedure was devised to introduce reproducible shear and thermal histories to minimize variation in rheological data. The cone and plate preform was placed in the rheometer and while the sample was heated to 340[degrees]C, the cone was brought to 0.05 mm from the plate. Once the temperature reached 340[degrees]C, a steady shear deformation was applied at a shear rate of 0.1 [sec.sup.-1] for 10 sec. After the preshear was complete the sample was cooled to the test temperature, where it was given 5 minutes to reach a stress free state before beginning the test. The 340[degrees]C preheat temperature was selected because the sample would not relax to a stress-free state within the allotted time if lower temperatures were used.

Densification Experiments

Eight samples of the spherical particles were used to evaluate particle size and size distribution, each sample is described in Table 1. The first four samples, identified as [S.sub.1] through [S.sub.4], represent particular sizes, and samples [D.sub.1] through [D.sub.4] represent various distributions. [D.sub.1] is skewed toward smaller particles, [D.sub.2] is skewed toward larger particles, [D.sub.3] is a normal distribution, and [D.sub.4] represents a traditional rotational molding distribution, in which the majority of its content is between approximately 300 and 600 microns and contains a small fraction of fine particles. The samples were poured into a 1.27 X 6.35 cm rectangular bar mold with an exposed top surface and a thermocouple fixed in the center of one of the sides of the mold. A nitrogen purge (powders were not under pressure) was supplied through a chamber that covered the mold. The entire unit was placed on a preheated hot plate that simulated conductive heating from one side as occurs in rotational molding. The total heating time was 40 minutes and began once the set temperature, as measured at the mold, was reached. After 40 minutes the apparatus was removed from the hot plate and allowed to cool in ambient temperature with continued nitrogen purge.

In addition to the 40 minute experiments, sample [S.sub.3] was used in three other tests. In the first test, the heating time was extended to 80 minutes to check for improved densification with increased time. The other two tests were to evaluate the possibility of increasing strength by an oxidative effect that had been observed during the neck growth tests. The first heating cycle for those experiments was designed to determine if introducing air to the sample at the end of the formerly described 40-minute cycle could improve properties. For this case, the sample was exposed to 20 minutes of heating in nitrogen followed by 20 minutes in air. The last test was to verify that any differences observed in the 20/20 test were not due to reducing the time the sample was heated in nitrogen. For this, the sample was exposed to an additional 20 minutes of heating in air after spending 40 minutes heating in nitrogen.

Properties of Densification Samples

Several properties of the bars molded in the densification experiments were evaluated. The density of the molded specimen, which is defined as the average density and differs from the material density by the amount of voids in the molded sample, was measured to determine the extent of densification. The average density was measured according to method A in ASTM test D 792, for testing solid plastics by displacement in water. It should be noted that the average density measured by this method is not affected by surface porosity, but only by differences in the amount of encapsulated gas. The test also requires the results to be corrected for variation in the test temperature and reported at 23[degrees]C.

Tensile tests were performed on the molded bars with a model 4202 Instron tensile testing machine to determine the ultimate tensile strength and Young's modulus. The cross-head speed was 1.27 mm/minute and the gauge length was 30.5 mm, according to ASTM test D 638. The samples fracture surfaces were inspected for uniform pore distribution and size. All reported results for average density and mechanical properties were the average from at least three samples.

Single-Axis Rotational Molding Experiments

The laboratory-scale rotational molding device consisted of a cylindrical, stainless steel mold, with an inside diameter of 1.59 cm and 7.62 cm in length, was used as the rotational mold. For each test, the mold was filled with 30 grams of [D.sub.4] powder, this distribution was selected because it produced the greatest average density during the densification study. Both ends of the mold were capped. One cap was threaded onto a shaft that was driven by a variable speed motor. A fitting was installed in the center of the opposite cap so that a nitrogen purge could be delivered directly into the mold cavity. The mold was placed in a forced convection oven that could heat at up to 60[degrees]C/minute and maintain the set temperature to within 1[degrees]C. The heating cycle was designed to mimic the conditions used in the densification experiments. The heating stage began once the mold reached the set temperature and after 40 minutes, the heat was stopped and the oven was opened. Rotation and nitrogen purge continued as the mold was allowed to cool to room temperature with the assistance of the oven fan.

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Properties of the Rotational Molded Samples

The molded product was visually inspected before testing the density, tensile strength and modulus, and burst pressure. The tensile properties were measured by the same method that was used in the densification study. Rectangular strips 5-mm-wide were cut axially from the molded cylinders. The specified width was found to sufficiently minimize radial curvature and allow the sample to be clamped into the Instron test fixture. Each of the reported results for average density and mechanical properties were the average of at least three runs.

One rotational molding sample was prepared from 50 grams of sample [D.sub.4] to be tested for its burst strength. The sample was clamped between the two plates as shown in the diagram of the test fixture in Fig. 4. The sample was then pressurized with water that was delivered through the fitting that was installed on the one plate until it burst.

RESULTS AND DISCUSSION

Powder Flow Characteristics

Vectra B 950 was obtained in pellet form, which was too large to successfully use in the rotational molding process. An attempt was made to generate powder by methods used for conventional polymers. Visual evaluation showed that the milled TLCP was composed of high aspect ratio particles that agglomerate, which is illustrated in Fig. 5. The fibrillar particles obtained from the milled pellets arise from the unique morphology generated during the extrusion of TLCPs. Shear deformation introduced during extrusion orients the liquid crystalline domains into a hierarchical fibrillar texture [21]. The macroscopic fiber is a collection of many microscopic fibrils that separate during milling to produce the undesirable fibrillar particles. The apparent densities of the cryogenically ground pellets, before and after applying the compacting load, were 111.34 [+ or -] 1.60 and 188.83 [+ or -] 1.96 kg/[m.sup.3], which were extremely low considering the material density was 1400 kg/[m.sup.3]. An attempt was made to measure the dynamic angle of repose for this material. The ground material was of such low bulk density that the mold could not be filled to the extent prescribed by Brown and Richards [22]. Despite the reduced loading, the powder clumped together into a single mass that tumbled as a rigid body within the rotating cylinder. The low apparent density and inability to freely flow gave conclusive evidence that the cryogenically ground particles were unacceptable for use in rotational molding and an alternative method to produce a powder was required.

[FIGURE 6 OMITTED]

The novel process that was developed for generating particles via melt blending with a low molecular weight polypropylene produced powder that was better suited for rotational molding than the particles generated by cryogenic grinding. A scanning electron microscope (SEM) image of a fracture surface of the extruded blend is shown in Fig. 6; the figure shows that the dispersed TLCP phase is spherical in shape and present in a range of sizes on the order of 10-100 [micro]m, which is similar to particle sizes customarily used in rotational molding. The apparent density of the generated powder was much greater than what had been measured for the milled pellets. It has been suggested that spherical particles are not ideal for the rotational molding process because the point contacts between spherical particles reduce heat transfer relative to line and surface contacts in other shapes [23]. However, the current goal is to improve powder flow and spherical particles perform very well in that respect [4]. The results of sieving and measuring the apparent density are shown in Table 2. The apparent density increased as the nominal particle size as decreased. This trend suggests that the packing efficiency increases with decreasing particle size. The apparent density of all sizes of spherical particles was much greater than the milled material and well over the suggested ratio of apparent to material density for linear low density polyethylene (LLDPE), which is approximately 0.35 [4].

The dynamic angle of repose of the generated powder was measured. An example of an image used during this measurement is shown in Fig. 7 for sieve number 30. Powder flow occurred in a steady state fashion and the dynamic angle of repose was found to be relatively constant (32-35[degrees]) for all of the sizes over the tested range of rotation rates (approximately 1-10 rpm). Powders commonly used in rotational molding have dynamic angles of repose between 25 and 50[degrees] [7]. The fact that the powder had the ability to flow was a tremendous improvement over the particles generated by cryogenic grinding.

[FIGURE 7 OMITTED]

The evaluation of the powder flow characteristics for cryogenically ground pellets and the spherical particles produced several important results. The poor performance of milled TLCP pellets in the selected tests suggested that an alternative approach was necessary to produce a powder that was acceptable for rotational molding. A technique was devised that produced spherical particles with a range of particle sizes that are commonly used in rotational molding. The apparent density was much higher for the spherical powder than for the milled pellets and steady state powder flow was observed in a horizontal rotating cylinder. These observations and measurements suggest the generated powder should possess acceptable powder flow characteristics for the rotational molding process.

Coalescence

DSC was used to identify the end of the melt transition, and the measured DSC thermogram is shown in Fig. 8. The peak of the melt transition occurred at 284[degrees]C and the melt transition was complete by 310[degrees]C; both are represented as stars in Fig. 8. In an attempt to ensure that complete melting would occur, 320 and 330[degrees]C were selected for the coalescence experiments.

The surface tension and shear viscosity were measured at 320 and 330[degrees]C, to verify their relative magnitudes and the absence of a three-region viscosity behavior. The surface tension was measured as 0.029 [+ or -] 0.002 J/[m.sup.2] and was independent of temperature and the surrounding atmospheric composition. This demonstrates that the driving force for coalescence is constant for the two temperatures. The shear flow curves at the two temperatures in nitrogen are shown in Fig. 9. As shown in the figure, the material does not exhibit the three-region viscosity behavior at either temperature, but, instead, a fairly well defined zero shear viscosity that is similar in magnitude at both temperatures. With the driving force and the resistance to flow equal at the two temperatures, coalescence was expected to progress at similar rates.

[FIGURE 8 OMITTED]

In addition to the complex and steady shear viscosity values, two stress growth upon inception of steady shear flow experiments were conducted in the zero shear limit (0.01 [sec.sup.-1]) at 320[degrees]C, one in the presence of nitrogen the other in air, the results are shown in Fig. 10. The transient viscosity of the sample measured in nitrogen reaches a steady state value, while the sample that was measured in the presence of air increases in an unbounded fashion. It is doubtful that this behavior is the result of recrystallization during shear because it did not occur in the sample measured in nitrogen at the same temperature. However, a possible explanation is that the molecular weight was increasing, as has been reported for a similar wholly aromatic TLCP, Vectra A 950. It was shown, for Vectra A, that the melt was polymerized by interchain transesterification that begins at temperatures approximately 35[degrees]C above the melt temperature [24]. Polymerization increases the molecular weight by one liquid crystal molecule forming a covalent bond with one of its neighbors. Regardless of the reason for the increase in viscosity, the measured behavior suggests that the presence of an inert atmosphere will be essential to obtaining complete neck growth.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

Coalescence experiments were carried out to confirm that the conditions identified by rheological characterization were appropriate. A representative example of the images recorded during the coalescence experiments is shown in Fig. 11. Initially, there was a finite contact area and the neck radius increased with time until the two drops converged. In Fig. 11, the two particles nearly reached a dimensionless neck radius, x/a, of 1 within 20 sec. Although the test was stopped shortly thereafter because the magnitude of the change in the dimensionless neck radius becomes comparable to the magnitude of the error in the measurement, the two particles do appear to continue to coalesce towards a single, nearly spherical drop.

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

The coalescence experiments confirm the observations made from rheological characterization and the measurement of surface tension. The results from all three sets of experiments, 320 and 330[degrees]C in nitrogen and 320[degrees]C in air, are shown in Fig. 12. For the neck growth experiment at 320[degrees]C in the presence of air, the two particles began to coalesce at nearly the same rate as the samples at the other conditions but stopped prematurely at a dimensionless neck radius of approximately 0.6. This result was anticipated by the large increase in viscosity at low shear rates in the presence of air. In nitrogen, the neck growth rate at 330[degrees]C was marginally faster than was measured at 320[degrees]C. This was in agreement with what was anticipated from the similarity in the relative magnitudes of the surface tension and viscosity at the two temperatures. Because there was no advantage in using the higher temperature, 320[degrees]C was selected for the single-axis rotational molding experiments.

Densification

Several sizes and distributions of the spherical powder were created to evaluate the effect that particle size and distribution had on densification. The apparent density of each sample is shown in Table 3. As was observed for the individual particle sizes, the apparent density of the distributions increased as the mass average particle size was decreased. In fact, the apparent density of all of the samples, except [D.sub.4], increased as the particle size decreased. Although distribution [D.sub.4] has a larger average particle size than sample [S.sub.4], the apparent density was greater, demonstrating that a small mass fraction of fine particles significantly increased the apparent density.

The particle sizes and distributions were molded by static coalescence of the bulk powder into rectangular bars using a 40-minute heating cycle in nitrogen. The bars were tested to determine their tensile properties and the fracture surfaces were examined to confirm that densification was incomplete. The fracture surface of sample [D.sub.1], which was representative of all samples, is shown in Fig. 13. The trapped gas bubbles varied slightly in size but appeared to be uniformly distributed throughout the sample cross section.

[FIGURE 13 OMITTED]

The measured values of the average density at 23[degrees]C, tensile modulus, and the ultimate tensile strength are all reported in Table 4. Of the molded samples that consisted of a particular particle size ([S.sub.1]-[S.sub.4]), it is shown that the gas content in the molded sample, as determined by measuring the average density, was slightly reduced as the apparent density of the powder was increased. This trend was also observed by the increase in the ratio of average to apparent density for these samples. It was also found that the extended 80-minute heating cycle was ineffective at increasing the average density of sample [S.sub.3], proving that densification could not be increased by simply increasing the cycle time, as is usually done in rotational molding. A similar relationship to that observed for samples [S.sub.1] through [S.sub.4] was found for the distributions, [D.sub.1] through [D.sub.4]. The average density of the molded samples increased as the apparent density of the powder increased. Although the extent of densification appears to be correlated to the apparent density, complete densification was not attained by increasing the apparent density.

Several interesting results were shown by the measured tensile properties. The tensile modulus was near 1 GPa for all samples except [S.sub.1]. It is possible that the results for [S.sub.1] were not representative because of the low average density in that sample. The ultimate tensile strength of samples [S.sub.1] through [S.sub.4] increased with the average density, indicating that strength was dependent on the extent of densification. The values for strength were normalized for differences in their average density to determine if the relationship between strength and average density was solely due to variation in the extent of densification or if there was also a change in adhesion between particles. Because the strength of the normalized results was not all equal it was concluded that adhesion had been increased. A clear relationship between tensile strength and average density was not observed for the sample distributions. Sample [D.sub.1] delivered the highest tensile strength, but [D.sub.4] possessed the highest average density. Perhaps the small fraction of extremely fine powders in distribution [D.sub.4] actually reduces the strength. If this were true, it could be rationalized by the distribution possessing a greater number of particles, and, therefore particle interfaces.

The two trials that were used to evaluate the possibility of increasing the strength by introducing the oxidative effect were unsuccessful. The results are shown in Table 5 with the results for the sample molded in nitrogen. It was found that the strength of the samples that were exposed to air was less than the sample molded in nitrogen. Not only does this imply that the speculated molecular weight increase by transesterification was not occurring, it also demonstrates the importance of supplying an inert atmosphere throughout the entire molding process and not only during the initial stages when neck growth occurs.

The results from the densification experiments identify that gas removal represents a major obstacle to rotational molding TLCPs. It was not possible to completely densify the bulk powder as evaluated in static coalescence. The average density of the eight selected samples increased as the apparent density was increased. The particle sizes and size distributions did not affect the tensile modulus, but did influence strength, with an increase in the ultimate tensile strength as the particle size was decreased. By normalizing the tensile strength for differences in the average density, it was discovered that the increase in strength could not be accounted for solely by the increase in density and, thus, adhesion between particles was improved. Finally, the tensile strength was decreased by exposing the sample to air during the heating cycle. Unfortunately, this implies that the molded product cannot be strengthened by this technique, but it does demonstrate the importance of supplying an inert atmosphere throughout the entire molding cycle.

Single-Axis Rotational Molding

The conditions identified from evaluating the powder characteristics, neck growth, and densification were tested in a single-axis rotational mold to determine if they could be translated to the rotational molding of the bulk powder. The powder successfully rotationally molded using the previously identified conditions. Pictures of the internal and external surfaces of the rotationally molded sample are shown in Figs. 14 and 15. The internal surface of the molded cylinder was smooth, but the external surface contained a fair amount of surface pores. The surface pores did not extend all the way through the sample wall, which demonstrates that sufficient coalescence was achieved during rotational molding.

The results from the measured average density, ultimate tensile strength, and tensile modulus for the rotationally molded cylinder are shown in Table 6. The results for the same distribution from the densification study are included for comparison. Surprisingly, all quantities were increased relative to what had been measured for static coalescence. The increase in density suggests that further improvement may be possible in the rotational molding experiment. Although the tensile modulus was over twice that measured in the densification study, it was still only a fraction of the 20 GPa that can be obtained with proper molecular alignment and implies that further increase may still be possible. The measured increase in the tensile strength was quite significant because it is very close to the nominal strength (17.9 MPa) required for industrial cross-linked high density polyethylene tanks. This was sufficient to pressurize the molded sample to 1.59 MPa before rupture occurred (50-gram sample with 3.8-mm wall thickness).

[FIGURE 14 OMITTED]

When comparing the results from the densification study with the results from rotational molding, it appears that the dynamics introduced by rotation increased the average density. The increase must occur by reducing the amount of gas that initially gets trapped during coalescence because it was demonstrated in the densification study that once gas is encapsulated, it cannot be removed. Understanding the mechanism that controls the relationship between rotation and the amount of gas that is encapsulated during coalescence may lead to a method to optimize the rotation rate.

[FIGURE 15 OMITTED]

To explain the increased density the static densification and dynamic rotational molding processes were reconsidered. The encapsulation of bubbles in the static scenario may be described as shown in Fig. 16. The powder is heated by conduction from the mold wall. As the powder melts, neck growth occurs and a network of connected particles is formed. Eventually the network collapses, and bubbles are encapsulated, which are removed by dissolving and diffusing through the surrounding melt. However, the permeability of a gas in the TLCP is extremely low so any bubbles that are formed remain in the melt, as shown by path 1 in Fig. 16.

In the case of the rotating system, as in the static example, the powder is heated by conduction from the mold wall. As the powder melts, neck growth occurs and a network of connected particles begins to form. However, in the rotational molding case, the coalescing particles are attached to the rotating mold wall and a layer of particles is removed from the tumbling powder bed, as shown by path 2 in Fig. 16. Assuming the layer of particles is only one particle thick, a three-dimensional network does not exist and, therefore, cannot collapse and encapsulate gas. The validity of the layer thickness assumption depends upon the rate of conduction from the molten layer to the powder bed and the amount of time a particular position on the mold surface stays in the powder bed, which is proportional to the rotation rate. The optimal rotation rate should be slow enough for a single layer of particles to coalesce in one rotation. If rotation is too slow, more that one particle will be attached to the mold wall as it passes through the powder bed. If it is too fast, the layer will not completely coalesce before the next layer of particles attach.

[FIGURE 16 OMITTED]

The single axis rotational molding experiment was repeated to test the proposed explanation of the observed difference in average density between the static and dynamic cases. The rotation rate used in this experiment was determined by using the coalescence time measured in the neck growth experiments. The particles coalesced at 320[degrees]C within approximately 20 sec. To promote the formation of a single particle layer, the rotation rate was set at 20 sec per revolution or 3 rpm.

Several of the measured properties were improved by reducing the rotation rate. The density of the sample that was rotationally molded at 3 rpm was increased to 1392 [+ or -] 6 kg/[m.sup.3]. This is a significant improvement because it is essentially the material density and, therefore, complete densification was achieved by the reduction in rotation rate. Unfortunately, an increase comparable to what was measured in density was not measured in the ultimate tensile strength and tensile modulus. The measured values were 17.63 [+ or -] 0.019 MPa and 2.010 [+ or -] 0.014 GPa for strength and modulus, respectively, which were within the experimental error of the sample molded at 10 rpm. The surface pores in the exterior surface were reduced as shown in Fig. 17. Although some surface pores still exist, this represents a dramatic improvement over what had been found at 10 rpm, shown in Fig. 15.

CONCLUSIONS

A commercially available TLCP, Vectra B 950, was evaluated for use in rotational molding by separately investigating the phenomenological steps of powder flow, neck growth, and densification and applying the identified conditions to rotational molding. Several important conclusions can be drawn from this work. The available pellets are not acceptable for use in rotational molding and they cannot be ground into a freely flowing powder by conventional means. It is possible to convert the pellets into an freely flowing powder by the described melt blending process. The particles can be rotationally molded at 320[degrees]C in nitrogen, as was identified by the neck growth experiments. Densification by dissolution and diffusion was not possible. However, the dynamics of rotational molding could be used to achieve complete densification and improve the surface appearance. Although the ultimate tensile strength was only a fraction of what can be obtained by this material it still exhibited a value that was approximately equal to the requirements for cross-linked high density polyethylene.

FUTURE WORK

Although this work has answered several questions about the rotational molding of TLCPs in a single axis rotational mold, further assessment of the performance such as: the structural integrity during thermal cycling, the mechanical performance at cryogenic conditions, and permeability to a variety of gases, would be invaluable in evaluating the molded structure for potential use as a cryogenic storage vessel. It would also be beneficial to rotational mold the material in a larger, possibly more complex, mold with biaxial rotation. Such an investigation would address the ability to effectively fill corners and allow for the evaluation of their mechanical performance. An additional area of interest is to address the strength limitation by altering the TLCP chemical structure with the addition of telechelic ionomeric groups to attempt to increase the interfacial adhesion between particles.

[FIGURE 17 OMITTED]

TABLE 1. Descriptions of the powder samples used in the densification
study.

U.S. standard  Opening      Size and distribution mass fractions
sieve no.      (micron)  [S.sub.1]  [S.sub.2]  [S.sub.3]  [S.sub.4]

 20            840       1.0        -          -          -
 30            595       -          1.0        -          -
 40            420       -          -          1.0        -
 50            297       -          -          -          1.0
 60            250       -          -          -          -
 70            210       -          -          -          -
100            149       -          -          -          -

U.S. standard    Size and distribution mass fractions
sieve no.      [D.sub.1]  [D.sub.2]  [D.sub.3]  [D.sub.4]

 20            0.07       0.53       0.10       -
 30            0.13       0.27       0.40       0.08
 40            0.27       0.13       0.40       0.48
 50            0.53       0.07       0.10       0.39
 60            -          -          -          0.02
 70            -          -          -          0.02
100            -          -          -          0.01

TABLE 2. Apparent density of the sieved TLCP particles.

U.S. standard  Opening   Apparent density  Apparent density;
sieve no.      (micron)  (kg/[m.sup.3]     material density

    20           840         812.36              0.58
    30           595         826.02              0.59
    40           420         833.27              0.60
    50           297         834.64              0.60
    60           250         836.33              0.60
    70           210         847.85              0.61
   100           149         906.61              0.65

TABLE 3. The apparent density of the samples used in the densification
study.

U.S. standard     Opening      Size and distribution mass fractions
sieve no.         (micron)  [S.sub.1]  [S.sub.2]  [S.sub.3]  [S.sub.4]

     20             840       1.0          -         -          -
     30             595       -            1.0       -          -
     40             420       -            -         1.0        -
     50             297       -            -         -          1.0
     60             250       -            -         -          -
     70             210       -            -         -          -
    100             149       -            -         -          -
Apparent density            812.36       826.02    833.27     834.64
  (kg/[m.sup.3])

U.S. standard       Size and distribution mass fractions
sieve no.         [D.sub.1]  [D.sub.2]  [D.sub.3]   [D.sub.4]

     20             0.07       0.53       0.10        -
     30             0.13       0.27       0.40        0.08
     40             0.27       0.13       0.40        0.48
     50             0.53       0.07       0.10        0.39
     60             -          -          -           0.02
     70             -          -          -           0.02
    100             -          -          -           0.01
Apparent density  834.14     825.04     825.87      841.71
  (kg/[m.sup.3])

TABLE 4. Results of the density and tensile measurements for the
densification study, [s*.sub.3] represents the results from the extended
cycle time.

            Average density
            [D.sup.23[degrees]C]  [rho]/           Tensile modulus
            (kg/[m.sup.3]         [[rho].sub.app]  (GPa)

[S.sub.1]   1022 [+ or -] 7       1.26             0.598 [+ or -] 0.008
[S.sub.2]   1148 [+ or -] 8       1.39             1.093 [+ or -] 0.015
[S.sub.3]   1189 [+ or -] 8       1.43             1.140 [+ or -] 0.015
[S*.sub.3]  1175 [+ or -] 8       1.41             1.147 [+ or -] 0.015
[S.sub.4]   1193 [+ or -] 8       1.43             0.964 [+ or -] 0.013
[D.sub.1]   1176 [+ or -] 8       1.41             1.040 [+ or -] 0.014
[D.sub.2]   1106 [+ or -] 7       1.34             1.008 [+ or -] 0.014
[D.sub.3]   1155 [+ or -] 8       1.40             1.066 [+ or -] 0.014
[D.sub.4]   1218 [+ or -] 8       1.45             0.930 [+ or -] 0.012

                                       Ultimate tensile strength
            Ultimate tensile strength  normalized by
            (MPa)                      [D.sup.23[degrees]C] (MPa)

[S.sub.1]    7.18 [+ or -] 0.10         9.84
[S.sub.2]   10.08 [+ or -] 0.14        12.29
[S.sub.3]   11.00 [+ or -] 0.15        12.96
[S*.sub.3]  10.92 [+ or -] 0.15        13.01
[S.sub.4]   12.78 [+ or -] 0.17        15.00
[D.sub.1]   13.48 [+ or -] 0.18        16.05
[D.sub.2]    9.61 [+ or -] 0.13        12.16
[D.sub.3]    9.11 [+ or -] 0.12        11.04
[D.sub.4]   10.51 [+ or -] 0.14        12.08

TABLE 5. Comparison of the tensile strength and modulus of sample
[S.sub.3] when molded in the presence of air.

Cycle           40 min in [N.sub.2]    20 min [N.sub.2]/20 min air

Strength (MPa)  11.00 [+ or -] 0.15    9.49 [+ or -] 0.16
Modulus (GPa)    1.140 [+ or -] 0.015  1.109 [+ or -] 0.014

Cycle           40 min [N.sub.2]/20 min air

Strength (MPa)  9.83 [+ or -] 0.15
Modulus (GPa)   1.126 [+ or -] 0.016

TABLE 6. The average density, tensile strength, and tensile modulus from
the rotationally molded distribution [D.sub.4] compared the results for
the distribution from the densification study.

                                                 Sample [D.sub.4]
                  Sample [D.sub.4] results from  results from rotational
                  the densification study        molding

Average density
  (kg/[m.sup.3])  1218 [+ or -] 8                1288 [+ or -] 7
Ultimate tensile
  strength (MPa)    10.51 [+ or -] 0.14            17.49 [+ or -] 0.16
Tensile modulus
  (GPa)              0.930 [+ or -] 0.012           2.022 [+ or -] 0.013

Contract grant sponsor: National Aeronautics and Space Administration (NASA) (Phase II SBIR, Managed by Luna Innovations); contract grant number: NAS-2S-4018-285.

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Eric Scribben, Donald Baird

Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061

Correspondence to: D. Baird; e-mail: [e-mail]