Hot compaction and consolidation of polycarbonate powder.
This paper is the second in a series of studies on the understanding of compaction and solid state processing. The first paper (2) describes how solid-state processing of high performance thermoplastic polymers through compaction and sintering can offer some processing advantages over conventional polymer processing methods (1-3). Compaction involves the application of a large pressure, usually at room temperature, to form contact between particles. Bonding between particles is promoted by the application of heat. in order for polymers to achieve strengths greater than the as-compacted, or green strength, the sintering temperature must be high enough to allow the reptation of polymer chains across the particle interfaces. A companion paper (3) described pressureless sintering of polycarbonate powder compacted at room temperature and presented data that suggest that recovery is driven by entropic factors.
From a review of the literature (1) and results presented in a previous paper (3), it was apparent that pressureless sintering of room temperature compacted polymers just above the [T.sub.g] of amorphous polymers or melting temperature of semi-crystalline polymers was not feasible. When thermoplastic polymers are compacted at room temperature, they deform both physically and viscoelastically, with dimensional recovery observed upon removal of the compaction pressure or upon heating of the compact to relatively low temperatures.
Possible methods of reducing or preventing recovery during sintering include hot compaction (compacting the powder at elevated temperatures) prior to sintering and consolidation of room temperature compacted powders (heating just above the glass transition temperature under applied pressure). Hot compaction has been investigated by several groups of researchers (4-7) and was found to improve green strength and density. However, hot compaction's effect on post-sintering properties was not studied. Consolidation of room temperature compacted polymer powders has not yet been discussed in the literature.
Based on previously discussed arguments (1, 3), hot compaction was not expected to improve sintered properties unless it was performed at temperatures at or above the glass transition region, as low configurational entropy resulting from residual stresses would continue to drive recovery upon heating above [T.sub.g]. If such high compaction temperatures were required, then hot compaction would offer little advantage over conventional hot pressing of polymer powders. Therefore, the experimental studies on hot compaction were confined to compaction temperatures below [T.sub.g].
Consolidation of room temperature compacted polymers could be shown to offer processing advantages if it was found that relatively low consolidation temperatures and pressures eliminated recovery. Since dimensional recovery occurs in a short period of time relative to the amount of time needed for reptation and healing to occur between particles (3), a short consolidation step just above [T.sub.g] followed by a higher temperature, pressureless "annealing" stage would make a reasonable processing method.
This paper describes results of compaction and consolidation experiments performed on the LPC3000 polycarbonate powder used in the previous studies (1-3).
The LPC3000 (8) polycarbonate powder used in this study was donated by DTM Corporation of Austin, Texas. Polycarbonate is an amorphous polymer with a [T.sub.g] of 142 [degrees] C, as characterized by differential scanning calorimetry at a scan rate of 10 [degrees] C/min. The specific gravity of the LPC3000 polycarbonate powder was reported to be 1.20 g/[cm.sup.3] (8). Additional properties of this material have been published elsewhere (2, 8).
Compaction was performed using a Carver laboratory Press (Model C) with 153 mm (6 inch) square platens. The applied pressure was controlled manually with the aid of a 5000 lb or a 24,000 lb force gauge. Room temperature and elevated temperature compaction experiments were performed using a 28.6 mm (1.13 inch) diameter, single-action stainless steel cylindrical die commonly used for powder metallurgy. A light coat of Chem-Pak Shop Silicone mold release was applied to the mold prior to compaction to reduce the friction between the powder and the die.
Room temperature compaction was achieved by pouring the pre-weighed powder into the mold, manually applying the desired pressure for 5 min, then quickly releasing the pressure. The procedure for cold compaction is more thoroughly described in previous work (2).
For hot compaction, the hot press and cylinder die were preheated to the desired temperature prior to pouring the pre-weighed powder into the die. The loaded die was then placed in the preheated press, with the upper platen very close to, but not touching, the die. A fiberglass cloth was draped around the platens to reduce convective heat loss. Note that the large thermal mass of the die prevented significant temperature fluctuations during hot compaction.
The powder was heated for 5 min in the preheated die before the desired pressure was manually applied and held for 5 min. The compacts were removed from the die and allowed to air cool for at least 10 min prior to being measured and weighed. Compacts were stored at room temperature in a dry box and allowed to recover for at least 24 h prior to subsequent testing. As was the case for cold compaction, it was found that changes in the dimensions of the compacts after 24 h were within experimental error for the polycarbonate powder. Compacts were stored at room temperature in a dry box and allowed to recover for at least 24 h prior to subsequent testing.
Consolidation was performed on room temperature compacted polycarbonate approximately 24 h following compaction, after the thickness, diameter, and mass of the compacts were measured and recorded. Two consolidation methods were employed, isothermal and dynamic consolidation. The density calculated based on the geometric technique was used to estimate the initial density.
* Isothermal Consolidation. The Carver laboratory hot press was used for a series of short term consolidation tests at 100 [degrees] C and 125 [degrees] C, as the press could be used to quickly apply and hold a constant temperature. A sheet of 0.05 mm (0.002 inch) thick DuPont Kapton film coated with Frekote 800-NC mold release was placed on the bottom platen of the preheated hot press. Consolidation was accomplished by placing the compacts on the preheated platens and immediately applying a preheated block of steel of known mass, whose smooth contact surface was covered with a second Kapton film coated with mold release. After the designated consolidation time had elapsed, the compacts were removed from the hot press and allowed to air cool in the absence of an applied pressure.
Although the average consolidation temperature could be fairly well controlled in the hot press, a temperature gradient of [approximately]10 [degrees] C was believed to exist between the top and bottom of the compacts. This was evident by the variation in color, morphology, and strength across the compacts (delamination of the layers sometimes occurred when the compacts were sliced with a razor blade). A much better consolidation test would involve using the upper platen, instead of a preheated mass to apply the desired pressure. However, very sensitive pressure control and perfect platen alignment would be required. The maximum applied pressure during the consolidation tests performed in this study was less than 10 kPa (1.5 psi). The hot presses available for use in this study could not accurately control this pressure.
* Dynamic Consolidation. A DuPont Thermal Analyst 2100 system equipped with a TMA 943 thermomechanical analyzer was used to measure the dimensional changes during a simulated "dynamic" consolidation process. Heating and cooling was performed at a rate of 5 [degrees] C/min. over temperatures ranging from 20 [degrees] C to 150 [degrees] C. A 2.5 mm (0.1 inch) diameter quartz probe was placed in direct contact with the test specimen, and various masses were used to apply pressures ranging from 0.04 to 100 kPa during testing.
The Geometric and Archimedes techniques were used to measure the bulk density of the compacted powder. These two techniques have been previously described in detail in previous work (2). For both methods, the density data is reported as a normalized density, or the actual density divided by the theoretical density of polycarbonate, 1.2 g/[cm.sup.3] (8).
A DuPont Thermal Analyst 2100 system equipped with a TMA 943 thermomechanical analyzer was used to measure the dimensional changes of the hot compacted and consolidated powder as a function of time and temperature. Details of this test method have been described in previous work (3).
Environmental Scanning Electron Microscopy (ESEM)
Microstructures of cryogenically fractured compacts were evaluated using an ElectroScan environmental scanning electron microscope (ESEM). An operating environment of 5.0 torr of water vapor allowed the non-conductive polycarbonate samples to be examined without the application of a conductive coating, such as gold or carbon.
Tensile properties were measured by performing microtensile tests with a Polymer Laboratories Minimat tensile tester. Tests on bulk polycarbonate films were conducted at a crosshead speed of 10 mm/minute. A speed of i mm/minute was required to increase the time of failure of polycarbonate compacts above the 30 second minimum suggested by ASTM D-638, since they failed at a much lower strain. Test specimens were punched from polycarbonate films and hot compacted polycarbonate powder using a self ejecting dogbone-shaped microtensile die having a gauge length of 10 mm and a width of 2.72 mm shaped per ASTM Standard D-638M. Because of their fragile nature, tensile strips approximately 6 mm wide with a 10 mm gauge length were cut from the other polycarbonate samples (cold compacts, consolidated compacts, and compacts that had experienced pressureless sintering) using a single-edged razor blade.
RESULTS AND DISCUSSION
I. Hot Compaction
In Fig. 1, normalized green density is plotted as a function of compaction pressure at 100 [degrees] C and 125 [degrees] C, along with the room temperature data from previous work (2). The density data presented in Fig. 1 were obtained using the Archimedes technique, and each data point represents the average of three samples.
From Fig. 1 it can be seen that compaction temperature strongly influenced green density. The largest effects were observed at the small compaction pressures, where differences of up to 20% of the theoretical density were measured between the compacts formed at room temperature and those compacts formed at 125 [degrees] C. As the compaction pressure was increased, differences between the green density obtained at room temperature and elevated temperatures decreased. At 160 MPa, hot compaction (at either 100 [degrees] C or 125 [degrees] C) yielded a normalized density [approximately]5% of the theoretical density higher than did room temperature compaction. Also noted was that at compaction pressures greater than 60 MPa, the compacts formed at 100 [degrees] C and 125 [degrees] C samples had very similar green densities.
Increasing the compaction temperature lowered the particle yield strength, and therefore, affected the relative amounts of particle rearrangement and deformation that occurred during compaction. Differences in density brought about by changing the compaction temperature were similar to those observed by altering the state of physical aging discussed in previous work (2). As was shown there (2), the green density of compacts made from unaged polycarbonate powder was higher at all compaction pressures than the density of compacts made from aged powder. This difference in densities was attributed to the lower yield strength of the unaged particles, which allowed more particle deformation to occur at a given compaction pressure. When comparing ESEM micrographs of the fracture surfaces of room temperature compacted polycarbonate from the previous paper (2), and (unaged) polycarbonate compacted at 125 [degrees] C, shown in Fig. 2, it was clear that more particle deformation took place in the hot compacted (unaged) polycarbonate.
The green density of the hot compacted polycarbonate was observed to peak at moderate pressures (40 MPa for 125 [degrees] C and 60 MPa for 100 [degrees] C) and decreases very slightly at higher pressures. This small decrease in density is not a result of increased closed porosity, as might be expected. All compacts were found to have closed porosity values of less than 0.3% by volume. Similar decreases in the density of hot compacted polyphenylene sulfide were reported by Mokashi and Jog (6) at high compaction pressures.
A possible explanation for the slight decrease in density with hot compaction pressure is that the densification of polycarbonate compacts is more time-dependent at 100 [degrees] C and 125 [degrees] C than at room temperature. The density at the moderate pressures may be increasing with time as the particles flow to fill in voids that are present after the compaction pressure has been applied. At the highest compaction pressures, a larger component of hydrostatic compression may be preventing some of this flow from occurring once the compaction pressure has been applied. This suggests that the plastic deformation mechanism discussed in previous work (2) contributed more to densification at moderate compaction pressures, while bulk compression (due to hydrostatic forces) dominated the plastic deformation mechanism at the higher compaction pressures. To determine if this is true, one could change the rate at which the pressure is applied at the highest compaction pressures. If the density increased as the loading rate was decreased, this would indicate that the above explanation is correct.
Isothermal Pressureless Sintering of Hot Compacted Polycarbonate
The normalized density of hot compacted polycarbonate compacts formed at 80 MPa was calculated as a function of pressureless sintering time at 165 [degrees] C ([approximately]20 [degrees] C above [T.sub.g]), for the 100 [degrees] C and 125 [degrees] C compaction temperatures, to examine the effect of compaction temperature on recovery. These experiments were repeated for pressures of 40 MPa and 160 MPa for the 125 [degrees] C compaction temperature to study the effect of compaction pressure on recovery.
Three specimens were tested at each time and compaction pressure. Sintering was performed using the hot press for test times less than one hour and the vacuum oven for sintering times of one hour and greater, possibly causing a shift in the data between the 10 min and 60 min data points.
Effect of Compaction Temperature on Sintered Density
Normalized densities versus sintering time for compacts formed at 80 MPa at 100 [degrees] C and 125 [degrees] C are plotted in Fig. 3. Also included in Fig. 3 are the results previously reported for polycarbonate compacted at room temperature at the same compaction pressure (3). As was true in the case of the room temperature compacted polycarbonate, the density of the hot compacted polycarbonate quickly dropped upon heating above the [T.sub.g] of polycarbonate. Within the first few minutes at 165 [degrees] C, the density of all compacts reached a minimum, with higher compaction temperatures resulting in higher densities measured after 5 min at 165 [degrees] C.
The sintered densities of compacts formed at 125 [degrees] C were over 10% of the theoretical density larger than those formed at room temperature, demonstrating the effect compaction temperature has on density. After sintering, the compacts formed at 125 [degrees] C had normalized densities up to 5% of the theoretical density greater than those formed at 100 [degrees] C after sintering, even though the initial green densities of the compacts were almost identical. This indicates that the morphologies and/or states of residual stress of the compacts formed at the different temperatures were not the same.
Despite the fact that increasing the compaction temperature resulted in improvements in sintered density, compacting at elevated temperatures below [T.sub.g] did not entirely eliminate dimensional recovery. As the polymer molecules still existed in non-equilibrium, low configurational entropy conformation following hot compaction. As discussed earlier in this paper, it was concluded that to entirely eliminate recovery, hot compaction would have to be performed above [T.sub.g].
Effect of Compaction Pressure on Sintered Density
In Fig. 4 normalized density is plotted as a function of sintering time for compacts formed at 125 [degrees] C at pressures of 40, 80, and 160 MPa. From this figure, it can be seen that the hot compaction pressure does not significantly affect sintered density for compaction pressures of 40 MPa or larger. These results suggest that by 40 MPa, the majority of deformation that can occur during hot compaction at 125 [degrees] C has taken place. Further increases in pressure led to bulk compression of the compact rather than localized yielding at the contact points.
Microstructural Changes During Pressureless Sintering
To examine the effect of pressureless sintering on the microstructure of hot compacted polycarbonate, a compact comparable to the one shown in the ESEM micrograph in Fig. 2b was formed by hot compacting at 125 [degrees] C and 80 MPa. It was then isothermally sintered at 165 [degrees] C in the hot press for 20 min with no applied pressure. The ESEM micrograph in Fig. 5 shows the resulting microstructure of the cryogenically fractured surface.
As was the case with room temperature compacted polycarbonate (3), mechanical interlocking between the particles has been lost during sintering, and the particles have recovered much of their original shape. A major difference between the room temperature and 125 [degrees] C compacts is the much larger number of contact points evident in the hot compacted polycarbonate [ILLUSTRATION FOR FIGURE 5 OMITTED] following sintering. From these micrographs and the data from Fig. 3, one might conclude that the improvements in sintered density in the hot compacted polycarbonate resulted from strength developed at the necks between the particles prior to recovery, as opposed to differences in the configurational entropy of the room temperature and elevated temperature compacted polycarbonate. To examine this hypothesis, microtensfie strength tests were performed on polycarbonate compacts subjected to various processes.
Effect of Processing on Microtensile Strength
Although the green strength of the polycarbonate compacts was fairly low, by carefully preparing specimens from the compacts, tensile strengths could be estimated at various stages of processing. A summary of the average yield strength for 5 specimens at each stage of processing is presented in Fig. 6. For comparison, the average strength of bulk polycarbonate film made from the same powder was 60 MPa as-processed (unaged) and 75 MPa following physical aging.
The polycarbonate compacts formed from aged and unaged polycarbonate powder at room temperature had very low strengths of 0.7 MPa and 1.3 MPa, respectively. The green strength of the compacts formed at room temperature resulted from mechanical Interlocking between the particles caused by non-elastic deformation. Because physical aging increases the yield strength of polycarbonate, less particle deformation occurs for a given compaction pressure In aged powder, and as expected, the green strength is lower than in compacts formed from unaged powder.
When the compaction temperature was raised to 125 [degrees] C, the tensile strength of the compacts increased by an order of magnitude, over data collected on compacts formed at room temperature (2). Since 125 [degrees] C is too low to cause diffusion of chains across particle interfaces during the 5 min compaction time, the increase in strength was attributed to an increase in particle deformation and mechanical interlocking. This is consistent with the findings for the room temperature compacted aged and unaged powder (3), with an increase in yield strength and modulus due to physical aging resulting in less deformation and lower green strength. For the hot compacted polycarbonate, raising the compaction temperature had a similar effect on deformation and green strength due to the lowering of the yield strength and modulus with temperature.
Although pressureless sintering (for 20 min at 165 [degrees] C) was found to greatly decrease density, microtensile tests showed that it increased tensile strength. In contrast with the hot compacted polycarbonate, this increase in strength as opposed to room temperature compacted polycarbonate was a result of the formation of necks at particle interfaces and subsequent neck growth during pressureless sintering. These necks were observed in the ESEM micrographs of the fracture surface of room temperature compacted polycarbonate following pressureless sintering (3). Note that in the sintering of metallic and ceramic powders, higher density is generally an indication of higher strength. based on results from this study, this is definitely not true of polymers.
If neck formation and growth were responsible for increases in strength, then the more necks present, the higher the sintered strength should be. Since a much larger number of necks was evident in the hot compacted polycarbonate [ILLUSTRATION FOR FIGURE 5 OMITTED] than the room temperature compacted polycarbonate (3) following pressureless sintering, the hot compacted/pressureless sintered compacts should have an even higher strength. From Fig. 6, it can be seen that this is indeed true. Increasing the compaction temperature to 125 [degrees] C increased the sintered strength by a factor of three.
Dynamic Sintering of Hot Compacted Polycarbonate
Thermomechanical analysis was performed on hot compacted polycarbonate formed under the same compaction conditions as the compacts that were isothermally sintered at 165 [degrees] C. As with the previous TMA tests on room temperature compacted polycarbonate, the samples tested using the procedure described in the experimental section.
Effect of Compaction Temperature on Recovery
Figure 7 plots the change in estimated normalized density versus temperature for (unaged) polycarbonate powder compacted at room temperature, 100 [degrees] C, and 125 [degrees] C at a pressure of 80 MPa. An isothermal hold at 150 [degrees] C was used for all three specimens. In Fig. 7 it can be seen that compaction temperature greatly affects the temperature at which recovery becomes irreversible (evident by a change in the slope of the density versus temperature curves). The transition temperature was [approximately]50 [degrees] C for the compact formed at room temperature, 100 [degrees] C for compacts formed at 100 [degrees] C, and 125 [degrees] C for compacts formed at 125 [degrees] C.
As noted earlier, dynamic sintering tests were performed [approximately] 24 h after the compaction pressure was removed from each specimen. It is possible that if the room temperature compact was tested immediately after the pressure was removed, the transition to irreversible recovery may have been much closer to room temperature. In fact, significant decreases in the transition temperature (up to 10 [degrees] C) were observed when the rest time prior to testing was shortened to under an hour. This findIng supports the idea that although the glass transition temperature of the bulk compact is unchanged following compaction, very small, localized areas of high deformation have locally reduced "effective" glass transition temperatures. This allows the samples to recover at temperatures well below the bulk glass transition temperature range in relatively short periods of time (hours at room temperature).
When the compacts were heated well above room temperature, the compacts formed at 100 [degrees] C and 125 [degrees] C followed very similar density versus temperature curves, exhibiting large-scale recovery just above the glass transition (142 [degrees] C). Final densities for the two were very similar and were estimated to be [approximately]63% of the theoretical density of bulk polycarbonate.
Note that the dynamic sintering results in Fig. 7 do not exactly match the "isothermal" sintering results in Fig. 3, in part because the heating rates were much slower (and more controlled) in the dynamic sintering experiments and the hold temperature was 150 [degrees] C for the data in Fig. 7 instead of 165 [degrees] C. However, both the isothermal and dynamic experiments did show a higher final density when the compaction temperature was raised to 100 [degrees] C or 125 [degrees] C compared to that measured for room temperature compacted polycarbonate. In both cases, the hot compacts had sintered densities [approximately] 10% higher than the room temperature compacts formed at the same compaction pressure.
Effect of Compaction Pressure on Recovery
Figure 8 plots the change in estimated normalized density versus temperature for (unaged) polycarbonate powder compacted at 100 [degrees] C at compaction pressures of 40, 80, and 160 MPa. The isothermal hold temperature for these tests was once again 150 [degrees] C. Unlike the compaction temperature, the compaction pressure did not have a large effect on recovery during dynamic sintering for the range of pressures studied. Small differences in initial density persisted up to the glass transition region, when large-scale recovery began to occur. The compacts formed at the different pressures followed almost identical cooling curves, with no significant variations in final density observed between the specimens.
Results obtained for polycarbonate compacted at 125 [degrees] C at 40, 80, and 160 MPa (which are not shown) were very similar, with compaction pressure having an insignificant effect on the cooling curve and final density. These findings support the conclusion previously made (based on isothermal sintering results) that the majority of particle deformation takes place in polycarbonate by 40 MPa at the elevated compaction temperatures. Above this pressure, bulk (elastic) compression of the compact is the major source of deformation received.
II. Consolidation of Room Temperature Compacted Polycarbonate
Polycarbonate compacts formed at room temperature were consolidated (sintered) while pressure was applied, in an attempt to reduce dimensional recovery upon heating above the glass transition temperature. One set of consolidation experiments was performed on compacts in the Carver hot press (similar to the "isothermal" sintering tests) to examine the effect of consolidation pressure on compact density and recovery. A second set of "dynamic" consolidation experiments was performed using thermomechanical analysis (TMA) to simulate consolidation with known applied pressures and controlled temperature profiles. Although both of these experimental techniques have advantages and disadvantages, they were both useful in predicting how the compacts would respond to sintering when a small pressure was applied. For example, the hot press allows for longer consolidation times, while the TMA allows for higher precision.
Isothermal Consolidation of Polycarbonate Compacts
Polycarbonate compacts formed at room temperature at a compaction pressure of 80 MPa were consolidated for 15 min at an average temperature of [approximately] 165 [degrees] C using the preheated bottom platen of the Carver hot press. After the compact was placed on the hot platen a preheated steel block of known mass was quickly placed on top of the compact to provide even heating and a uniform consolidation pressure. As discussed in the Experimental Methods section, neither variations in the temperature across the thickness of the compacts nor variations in the consolidation pressure could be totally eliminated and were responsible for some non-uniformities in the consolidated compacts.
Figure 9 shows (a) the normalized density (measured using the Archimedes technique) and (b) the increase in compact thickness as a function of consolidation pressure. At the smallest consolidation pressures, less than 5 kPa (0.7 psi), almost no reduction in the amount of recovery occurred compared to the results obtained by pressureless sintering (3). However, as consolidation pressure was raised to [approximately] 22 kPa (3.2 psi), dramatic increases in the final densities and reductions in the amount of recovery were observed.
The diameter of each compact was measured before and after consolidation. Nominal increases in diameter of between 2% and 5% were observed for all compacts, with a 2% increase in diameter common for compacts consolidated with no load or a very small load. These small diameter increases indicated that the consolidation pressures used in these experiments were not large enough to cause significant bulk flow of the compacts.
Due to experimental difficulties encountered in applying larger heated masses, the determination of the load necessary to eliminate recovery in the consolidated compacts could not be investigated. However, based on the data in Fig. 9, thickness increases should certainly be insignificant at a consolidation pressure of 50 kPa (7 psi), a very low pressure.
Dynamic Consolidation of Polycarbonate Compacts
Polycarbonate compacts formed at room temperature at a compaction pressure of 80 MPa were dynamically consolidated using a controlled temperature profile with various applied pressures. As was the case with previous TMA tests, a heating rate of 5 [degrees] C/min was used to ramp up to the isothermal hold temperature, which was held for 20 min. Rather than using glass coverslips to distribute the load over the entire specimen, the probe alone was used to apply relatively large loads to an area of known, constant cross section in the center of each specimen. The data presented here are changes in thickness versus time plots at specific isothermal temperatures, where the data have been shifted to zero at a temperature of 100 [degrees] C in the interest of easy presentation.
Figure 10 shows the effect of consolidation pressure on the thickness increase during thermomechanical testing of compacts held isothermally at 152 [degrees] C, which is [approximately] 10 [degrees] C above the glass transition temperature. The actual temperature profile was the same for all scans, and is included in this Figure for reference. Note that large-scale increases in thickness (otherwise referred to as recovery) began when the specimens reached the glass transition temperature of polycarbonate, at an adjusted time of [approximately]8 min. The maximum thickness, decreased with consolidation pressure from [approximately]37% to 14%, for consolidation pressures of 4 kPa (0.6 psi) to 100 kPa (14.5 psi), respectively.
At the highest consolidation pressure of 100 kPa, the thickness change becomes negative and large (-35%), much higher than would be necessary to achieve 100% of the theoretical density. This indicates that in addition to densifying the sample, large consolidation pressures can cause flow of the particles perpendicular to the direction of the applied load if no side support is provided. For dynamic consolidation tests, it is unclear at what load significant particle flow began to occur, however, in the isothermal consolidation tests previously described, very little flow was observed at consolidation pressures up to 22 kPa.
Note that each isothermal consolidation data point in Fig. 9 is the equivalent of a point on a dynamic sintering curve in Fig. 10 at an adjusted time of [approximately]27 min (or 15 min after the isothermal hold temperature was reached). Although the isothermal data in Fig. 9 suggest that expansion at [T.sub.g] is proportional to the final thickness change, Fig. 10 shows that the maximum expansion does in fact occur near [T.sub.g] and that the final dimensions are affected by time-dependent deformation, which occurs during the isothermal hold at temperatures above [T.sub.g].
Figure 11 shows data from dynamic sintering tests performed on polycarbonate compacts at an isothermal hold temperature of 162 [degrees] C, which was 20 [degrees] C above the glass transition temperature and 10 [degrees] C higher than the isothermal hold temperature used to obtain the data in Fig. 10. Note that increasing the hold temperature by 10 [degrees] C did not affect the maximum thickness change, which occurred near [T.sub.g] (below the isothermal hold temperatures), but it did greatly affect dimensional changes due to time-dependent deformation during the isothermal hold.
Figure 12 shows a plot of the maximum thickness change (which occurred near [T.sub.g]) versus consolidation load for the dynamic consolidation tests performed with the two different hold temperatures. Increasing the load decreased the maximum thickness change, but even a consolidation pressure of 100 kPa (14.7 psi) did not entirely eliminate the dimensional recovery that occurs at the glass transition temperature. Since the 100 kPa load was shown to be high enough to cause flow of the particles (as was evident by the large, negative thickness change in [ILLUSTRATION FOR FIGURE 10 OMITTED]), it appears that to eliminate dimensional recovery at [T.sub.g], a mold capable of providing side support is required during consolidation.
As is evident in Fig. 11, increasing the consolidation temperature by only 10 [degrees] C greatly enhanced the ability of the load to decrease the thickness change while the compacts were held above the glass transition temperature. For example, the curve for the specimen subjected to a consolidation pressure of 40 kPa [ILLUSTRATION FOR FIGURE 10 OMITTED] is similar to that obtained by increasing the hold temperature by 10 [degrees] C and decreasing the pressure to 4 kPa [ILLUSTRATION FOR FIGURE 11 OMITTED]. This data serves as a warning that increasing the consolidation temperature increases the likelihood that distortion of the compact will occur if proper side support is not provided.
Figure 13 provides a comparison of data obtained by isothermally consolidating compacts in the hot press and dynamically consolidating the TMA samples. In this plot, the percent change in thickness measured after 15 min at the isothermal hold temperature (at an adjusted time of 27 min in the dynamic tests) is plotted versus the consolidation pressure. At low consolidation loads, the two techniques yielded similar results. As the pressure was increased, the isothermal tests resulted in a desirable leveling off of the thickness change of the bulk compacts. However, the dynamic TMA tests resulted in penetration of the probe into the specimens, as was evident by a negative thickness change at higher pressures. The differences in consolidation behavior at higher pressures reflect differences in the consolidation conditions, such as temperature profile, load distribution, boundary conditions and friction, which appear to be very important variables in consolidation.
Hot compaction was found to greatly increase the green density of polycarbonate compacts by allowing more particle deformation to take place at a given compaction pressure. Raising the compaction temperature to 100 [degrees] C or 125 [degrees] C increased the density of compacts subjected to pressureless sintering at 165 [degrees] C; however, it did not increase dimensional recovery in the compacts. Furthermore, hot compaction pressure did not have a large effect on green or sintered density above 40 MPa. It was suggested that bulk compression of the compact, rather than localized yielding, occurs above this pressure.
Microtensile strength tests showed that cold compacted polycarbonate had the lowest tensile yield strength. Raising the compaction temperature of 125 [degrees] C or performing pressureless sintering for 20 min at 165 [degrees] C raised the strength of cold compacted polycarbonate by an order of magnitude, but for different reasons. Increasing the compaction temperature resulted in more particle deformation and mechanical interlocking, while performing pressureless sintering allowed the development of necks between particles. The highest strength was obtained by sintering the hot compacted polycarbonate.
It was concluded that the small improvements in sintered density through hot compaction resulted from a greater number of necks being formed between particles during sintering. However, to completely eliminate dimensional recovery, the compaction temperature would have to be increased to the glass transition temperature so that the polymer molecules would remain in their high entropy, equilibrium states following compaction.
Isothermal consolidation tests on bulk polycarbonate compacts showed that small pressures (22 kPa or 3.2 psi) could be used to limit the dimensional recovery during consolidation at 165 [degrees] C. without causing significant distortion of the compacts. Dynamic consolidation tests revealed that although consolidation pressures can be used to reduce the maximum thickness increase experienced by the compact at the glass transition temperature, they cannot be used to entirely eliminate this expansion without causing significant flow of the compact perpendicular to the load direction. It was concluded that molds capable of exerting small pressures (less than 50 kPa or 7.3 psi) and capable of providing side support to the compacts would be desirable to use during consolidation. Steps that might be taken to model the consolidation process were outlined.
The authors would like to acknowledge the financial support of the Charles E. Minor Fellowship Program at Virginia Tech. They would also like to thank the DTM Corporation for donating the polycarbonate powder used in this study.
1. L. W. Vick, PhD Dissertation, Virginia Polytechnic Institute and State University (1996).
2. L. W. Vick and R. G. Kander, Polym. Eng. Sci., 37, 120 (1997).
3. L. W. Vick and R. G. Kander, "Pressureless Sintering of Polycarbonate Powder Compacted at Ambient Temperature," Polymer Engineering and Science, in press.
4. T. Maeda and S. Matsuoka, J. Fac. Eng., Univ. of Tokyo, XXXII(3), 191-226 (1975).
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|Author:||Vick, Linda W.; Kander, Ronald G.|
|Publication:||Polymer Engineering and Science|
|Date:||Nov 1, 1998|
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