Patent Application: US-76999596-A

Abstract:
a method for controlling the viscosity of molten polymers , such as polycarbonate , takes place prior to a molding operation such as injection molding , extrusion , thermoforming or blow molding . when a significant reduction of viscosity is desirable , the plastic melt is submitted , at constant temperature , to the action of a vigorous mechanical extensional shear vibration , with minimum or no external pressure , at a constant amplitude and frequency , causing the melt to become highly elastic , and simultaneously it is fatigued , for a certain time at that temperature , maintaining this high elastic state , until the macromolecules have partially or totally disentangled , in a controllable manner , at which stage the melt is ready for a molding operation such as a simple quenching operation or an extrusion process followed by quenching to produce pellets or compounds with a better mix or a lower viscosity when remelted , or an injection molding operation where the melt viscosity has been greatly reduced allowing a better processability of the injected part , for instance allowing the use of a lower temperature of injection , a lower pressure of injection or both .

Description:
fig1 displays the log of viscosity of three polycarbonate grades as a function of temperature . the grades have different molecular weight average and polydispersity ratio . this figure demonstrates how the resin manufacturers cope with providing the industry with means to modify the melt viscosity or elasticity of a resin , by changing the molecular weight characteristics and the level of branching . fig2 displays the variation of the ratio ( g &# 39 ;/ g *) during a shear vibration treatment of a general purpose polycarbonate performed at 220 ° c . for 600 seconds at various frequencies designated on the figure ( expressed in rad / s ). the melt is shear vibrated in torsion between two parallel plates separated by a 2 mm gap . the top plate is oscillating , the bottom plate is fixed . the strain amplitude of vibration is 25 %. the temperature in the vibration chamber is maintained constant within 1 ° c . the ratio ( g &# 39 ;/ g *) is related to the amount of elasticity stored in the plastic at the corresponding temperature . the closer this ratio is to 1 the more elastic energy is stored at each cycle . the process according to the present invention is optimized for ( g &# 39 ;/ g *) ratios between 0 . 76 and 0 . 93 . these high values of melt elasticity are obtained by combining relatively high shear vibration frequencies , up to 100 hz , lower melt temperatures and high strain amplitudes . for example , the efficiency of the invention in reducing the melt viscosity of polymers in pure torsional shear , according to the present invention , does require not only a high melt elastic state , but also a high strain amplitude to put the melt in extension during fatigue . depending on temperature and frequency , the strain amplitude can be chosen between 10 % and 90 %, which corresponds to the domain of non - linear viscoelastic behavior . however , the vibration parameters should be chosen and adjusted during fatigue to avoid melt instability to occur , which is generally observed by a sudden drop of the ratio ( g &# 39 ;/ g *). according to one embodiment of the invention , the strain amplitude is increased stepwisely from an initial low value to the high value required for the process to be successful , letting at each step some shear - thinning to take place , which eases the value of the torque required to maintain a given strain amplitude . this adjustment of the amplitude at the initiation of the fatigue process also prevents the melt to slip at the contact with the top and bottom surfaces . fig2 demonstrates clearly that the ratio ( g &# 39 ;/ g *) increases significantly with the frequency of shear vibration at constant strain amplitude of vibration and temperature . the ratio ( g &# 39 ;/ g *) also increases as temperature decreases , at constant frequency and strain amplitude of vibration , until it reaches a maximum and starts to decrease . the temperature for the maximum varies with frequency but is generally found 20 to 25 ° c . above tg for a frequency of 1 hz . fig2 also shows that the ratio ( g &# 39 ;/ g *) remains practically constant during the 10 minutes of fatigue which occurs at this temperature . the strain amplitude of shear vibration may be adjusted upwardly to compensate for a slight loss of the elasticity of the melt during the fatigue process . fig3 represents a plot of the log of the complex viscosity , eta *, measured at decreasing temperature , every 5 degrees , in a dynamic mechanical analyzer ( rheometrics rdaii ), working in the linear region of viscoelasticity , e . g . at a constant frequency of oscillation , 1 hz , and low strain amplitude . the numbers near the curves refer to the value of the frequency in rad / sec which was used during the melt vibration fatigue treatment at 220 ° c ., according to the present invention . the top curve ( 4 . 39 ) of fig3 corresponds to the viscosity of the melt of fig2 after it has been fatigued at a frequency of 4 . 39 rad / s for 10 minutes ( bottom curve of fig2 ). in other words , fig3 provides the temperature dependent viscosity of the melts after treatment at a given frequency and elasticity , as provided in fig2 . the top two curves of fig3 ( corresponding to melt fatigue treatments at respectively 4 . 39 and 6 . 28 rad / s ) coincide almost entirely with the flow curve obtained for a reference melt , for which there has been no shear vibration treatment at 220 ° c . in these cases , the shear vibration treatment does not modify the subsequent viscosity behavior of the melt . the treatment is not successful in reducing the viscosity of the polymer . however , fig3 shows that for a melt fatigue frequency starting at 12 . 56 rad / s , i . e . for 31 . 4 or even more obvious for 157 rad / s , the viscosity behavior is significantly different from the reference sample viscosity , which coincides with the top curve . in fact , the viscosity for the sample fatigued at 157 rad / s for 10 minutes is approximately 5 times lower than the reference viscosity at all temperatures . this clearly demonstrates the benefit of the treatment . the following factors have been shown to regulate the efficiency of the vibration treatment in viscosity reduction per the present invention : the temperature of the melt , the frequency of the vibration , the strain amplitude of the vibration , the time the vibration is effective , the surface friction coefficient of the plates or tubes confining the melt , the gap dimension between the plates or tubes , the amount of stored elasticity in the sample and the amount of extensional shear during the treatment . any person skilled in the art of plastic processing and polymer rheology would know how to adjust the respective value of these variables to obtain the same level of efficiency in viscosity reduction . for example , temperature can be lowered or increased by a few degrees and adjustments could be made to the frequency and / or the strain amplitude to obtain the same ( g &# 39 ;/ g *) ratio , characteristic of the rheological state of the material . when a torsional shear mode is used , the frequency of vibration and strain amplitude play essentially the same two roles of : 1 . increasing the amount of elasticity in the melt and 2 . increasing the level of extensional shear resulting from a centrifugation effect . the melt , which assumes the theological state of a rubber under these vibration conditions , is fatigued in extensional shear , with , as a result , the slow separation of the entangled macromolecules and the reduction of the area of their interaction . the net result is a decrease of the melt viscosity without the breakage of the macromolecules . any person skilled in the art of stress analysis and mold design would know how to combine the relative motion of parallel plates or concentric tubes to submit a melt to a vigorous shear vibration , under no or minimum compressive force , and its simultaneous extension by application of a complementary motion . fig4 provides an example of such design in the case of parallel disks . in fig4 the bottom disk is oscillated with frequency w and strain amplitude a around a fixed axis . the top disk is &# 34 ; rubbing the melt &# 34 ; in a small independent circular motion with rotation speed w &# 39 ; and eccentricity e . the gap thickness is d , typically 1 to 2 mm . in this configuration of two parallel disks confining the melt in the gap in between , two coordinated shear motions of the melt are available to bring the melt into a high elastic state and fatiguing it in extension . the amount of eccentricity determines the amount of shear extension during fatigue . in a simplified embodiment of the present invention , the bottom plate is fixed and the vibrating shear comes solely from the rubbing motion of the top disk . for such an embodiment the speed of rotation must be greatly increased , since the single motion must fulfill both roles of creating a high elastic state and fatiguing the melt in extension . in the similar case of concentric tubes , the relative rotation of the tubes at a certain vibration frequency and strain amplitude in the hoop direction can be used to determine the rheological state of the melt , whereas the coordinated motion on the perpendicular longitudinal direction determines the level of extensional shear during fatigue . conversely , of course , the vibration parameters for the motion along the longitudinal axis could be the ones which determine the theological state , while the shear motion in the hoop direction could be programmed to determine the amount of extensional shear . any person skilled in the art of plastic processing design would know how to use and adapt the apparatus described above and build machines which would also include heaters to heat the disks or the tubes , heat exchangers to maintain the melt temperature constant or to allow a quench of the plastic at any particular time , or would combine the use of gear pumps and melt conveyers to feed the melt in and out of the melt vibrating area , whether annular or slit dies are used , or a compressive chamber in the case of packing under vibrating pressure . for example , molten polymer can be conveyed through an external flexible hose feeding the vibrating plates from the center of the top disk , or from the side of a vibrating pressure chamber . the treated melt can be scratched off the disk plates at the end of the vibration treatment by opening the die gap to let use of an automated scratching tool collecting the treated resin into a secondary tank from which it is quickly gear - pumped to the next molding stage . in another embodiment of this invention , the two facing disks are motioned with respect to one another in a way which slowly expands ( bulges ) the melt towards the periphery where it is cut and conveyed away , or cut and quenched simultaneously , by a rotating knife coming in proximity , but not in contact , with the edge of the disks . new molten polymer is continuously fed from the center and is sheared away to the periphery in exactly the time required for the fatigue process to be successfully disentangling the macromolecules to the desired level . as another example , several of the concentric tubes operating as per the requirements of the invention can be positioned parallel to each other with their longitudinal axis vertical . the inside tube is fixed and the outside tubes are geared to rotate and translate together , powered from common sources for each motion - transverse rotation and longitudinal extension . the top of the tubes is caped and communicates with a common feeder , such as an extruder line or a gear pump , working under no pressure conditions . at the bottom of the tubes , the fatigued melt budges out and is cut by a moving knife . the pieces from the several tubes are collected continuously and gathered by gravity to a common tank from which it is pumped away to the next molding station . any person skilled in the art of low frequency vibration ( 1 to 100 hz ) would know how to design electrical motors , or hydraulic systems operating with actuators and servo - valves , capable of providing the shear or packing forces required to rotate tubes periodically , move parallel disks and plates in torsion and in eccentric motion , or pack a confined melt between one or two vibrating pistons . any person skilled in the art of controls , pid loops , and computer aided controller design , would know how to build the controller to drive the actuators , heat and cool the tubes and plates , monitor and record the melt temperature , the torque value , or the pressure value , the frequency of the vibrations , and the dynamic parameters g &# 39 ;, g * and k &# 39 ;, k *. fig5 provides the viscosity curve ( as a function of temperature ) for five different melts which have been shear melt fatigued in pure torsion and / or allowed to recover from melt fatigue in a way which is explained below : the resin is grade 3 polycarbonate , which is a branched polycarbonate with a high elasticity level , ideal for blow - molding applications . trace 1 is the viscosity curve of a reference sample , which has not been fatigued , and held without any mechanical constraint at 230 ° c . for 1800 seconds . the viscosity curve is obtained at the end of the 1800 seconds with a dma apparatus working at 16 hz . traces 2 and 3 are the viscosity curves for the two melt fatigue treatments shown in fig6 . trace 2 corresponds to fatigue - melt 12 of fig6 and trace 3 to fatigue - melt 13 . the viscosity curves are obtained at the end of the treatment with the same dma rheometer also working at 16 hz . trace 4 is a viscosity curve performed on the melt labelled &# 34 ; 2 &# 34 ; in fig5 after it is cooled in the dma instrument down to 138 ° c . ( below tg ), and then reheated for one minute , under no constraint , to 327 ° c . the dma measurements start at 327 ° c . trace 5 is a viscosity curve performed on the melt labelled &# 34 ; 3 &# 34 ; in fig5 after it is cooled in the dma instrument down to 138 ° c . ( below tg ), and then reheated for one minute , under no constraint , to 327 ° c . the sample is cooled again and dma measurements are performed starting at 230 ° c . fig6 displays the two melt fatigue treatments 12 and 13 which provide the two melts analyzed by dma in fig5 respectively 2 and 3 . it is clear from fig5 that the viscosity curve of both melt 2 and 3 is located far below the reference viscosity curve 1 , showing the significant benefit of the present invention . however , there is a significant difference between melt 2 and melt 3 in terms of the efficiency of their respective treatment in reducing the melt viscosity . the two melt treatments 12 and 13 have most of the vibration parameters identical ( 157 rad / s frequency , 230 ° c .) except for the strain amplitude history - to reach 50 % strain - which was built up a little bit faster for treatment 13 . the temperature of melt 12 was also slightly lower at the beginning . one sees that ( g &# 39 ;/ g *) is almost identical for the two treatments up to approximately 1200 seconds , where the ( g &# 39 ;/ g *) of treatment 12 sharply drop down to lower values ( final value : 0 . 6 ). this sharp change of melt behavior is attributed to either slippage at the surface or melt instability . this behavior is not observed for treatment 12 ( final value of g &# 34 ;/ g * : 0 . 72 ). the melt of treatment 13 is incoherently vibrated after 1200 seconds , which corresponds to a loss of internal elasticity and a slowly recovery process recreating entanglements . the result on the viscosity curve in fig5 is the partial loss of some of the viscosity reduction obtained during the first 1200 seconds of the treatment . as mentioned before , melt fatigue treatments produce only partially stable entanglement states , which , on reheating or annealing , kinetically loose the benefits of the treatment due to the alteration of the entanglement level . this is demonstrated in fig5 by the recovery of the viscosity curve of the reference sample ( no treatment ) after treated specimens 2 and 3 , which show a significant reduction of viscosity due to the treatment per the present invention , are reheated to 327 ° c ., producing traces 4 and 5 after reheating . these reheated samples have the same viscosity curve as the reference sample , demonstrating the reversibility of the process of entanglement manipulation . this also demonstrates that the melt fatigue samples can completely recover their viscosity of non - treated state , and therefore that there has not been any modification of molecular weight due to the vigorous extensional shear vibration treatment . fig7 shows two melt fatigue treatments for polycarbonate grade 3 : trace 1 : melt fatigue at 220 ° c ., 0 . 5 % strain amplitude , 157 rad / s frequency , during 600 seconds . trace 2 : melt fatigue at 230 ° c ., 30 % strain amplitude , 157 rad / s frequency , during 1800 seconds . the corresponding viscosity curves for treatment 1 and 2 of fig7 are already presented in fig5 . the viscosity curve for treatment 1 is not different from the reference curve ( no treatment ), demonstrating that , in torsion , if the strain amplitude is not large enough , even at very high level of elasticity , there is no effect on the melt viscosity when the treatment ceases . the viscosity curve for treatment 2 is trace 3 of fig5 which is found much lower than the reference viscosity curve and has already been commented upon for fig5 . fig8 is a side view in section of an attachment to an injection molding unit and a mold to perform packing / melt fatigue prior to injection . numeral 1 is an injection molding equipment of known design to which an hydraulic actuator equipped with servo - valve 2 is connected to a piston 3 which is movable toward a compression chamber closed at both ends when valves 6a and 6b are closed , to close access to a nozzle 5 . to operate , close valve 6a , open valve 6b to plasticate the melt into the packing chamber , and inject from the injection molding screw 1 . close 6b . perform the treatment on the plastic in chamber 4 using piston 3 activated by actuator 2 open 6a . push the treated plastic out of chamber 4 through nozzle 5 either by pushing down the piston 3 completely or by pushing up piston 3 entirely , opening valve 6b and plasticating new untreated plastic into chamber 4 , which has the effect of pushing down the treated plastic out of 4 . fig9 shows one example of equipment to melt fatigue a molten plastic under shear extension . the numerals identify parts as follows : 7 is a rotor which can either turn at specified speed or oscillate at given frequency and amplitude of strain . 8 is heaters elements , 9 a molten plastic reservoir at given temperature . 9a gear pump connected to new untreated plastic melt . 10 reservoir and controlled gate to outlet for treated plastic . 10a gear pump to return melt to 9 . in operation , this apparatus is designed to reduce the viscosity of melts . untreated melt is introduced in reservoir 9 and fills the cavity including reservoir 10 . in one embodiment of this invention gear pump 10a is not operational . rotor 7 is put in motion to shear and fatigue the melt temperature of the melt is controlled through heaters 8 . gate 10 is closed . the vigorously sheared melt is allowed to expand upward in 9 to avoid normal compression forces . the high rotation speed or oscillation frequency create centrifuge forces on the melt which put it in extension . the melt is purged out by opening the gate of reservoir 10 when the treatment is finished . in another embodiment of the present invention , the melt which is shear vibrated in the central section is further put into controlled elongation by having the reservoir 10 in communication with reservoir 9 through a gear pump 10a . this gear pump sucks the escaping melt from 10 back to 9 until the fatigue treatment is finished , at which time the gate in 10 is open allowing the purge of all or a part of the treated melt and new untreated melt is pumped into 9 through gear pump 9a . the extension of the melt is controlled by controlling the rate at which the melt in 10 is pumped back in 9 . fig1 shows an apparatus to melt - fatigue in shear extension prior to injection by plunger , with identified elements as follows : 15 tube rotated and translated to produce shear fatigue on molten plastic in 16 ; 17 reservoir of molten plastic with two inlets : either new untreated plastic or recycled molten plastic from gear - pump 14a ; 17a gear pump feeding plastic in reservoir 17 to fatigue chamber 16 ; and in operation , molten plastic is introduced in cavity 16 at the proper temperature and fills reservoirs 17 and 14 . from 14 it can be fed into injection molding chamber through gear pump 13 , or it can be returned to reservoir 17 by gear pump 14a . in one embodiment of this design , gear pump 14a is not operational and the melt is allowed to expand in reservoir 14 to avoid compressive forces due to normal stresses . melt 16 is highly sheared under constant controlled temperature environment by the motion of 15 to produce melt fatigue in extension . the treated melt comes out in 14 and is pumped away by 13 . through action of 11 the treated melt is injected out through 12 into a mold . in another embodiment of the present invention , the injection nozzle 12 is valve gated and can close , creating a compressive chamber between shooting plunger 11 and injection nozzle 12 , which is used to pack and fatigue the melt for a certain time according to one of the variations of this invention before opening of valve gate at 12 and injection . in another embodiment of this design , the treated melt at 14 is gear - pumped back ( gear pump 14a ) to the reservoir 17 at a certain chosen rate . the melt in reservoir 17 is gear pumped by pump 17a to cavity 16 at a slightly different rate than 14a , the difference being adjusted , along with the frequency of rotation and elongation of tube 15 , to cause a shear extensional flow in cavity 16 while the vibration fatigues the melt . the levels in the reservoirs are monitored to determine the time at which gear pump 13 is activated to suck the content of the reservoir , or part of it , into the shooting chamber of the injection system , 11 and 12 . new untreated molten plastic is simultaneously introduced in reservoir 17 . the degree of efficiency in lowering the viscosity by melt fatigue is determined by the length of the tube 15 , the time of the treatment before opening gear pump 13 , the amount of melt extension , which is driven by the difference in pumping rates between gear pumps 17a and 14a , and by the fatigue parameters , temperature , frequency and strain amplitude of the melt in cavity 16 . all parameters are monitored and controlled by computer . fig1 illustrates the oscillation of a coaxial cylindrical drum of the invention with elements as follows : 20 gear pump to control flow of plastic and quantity of melt treated ; 23 sucking device or gear pump to let plastic melt out at controlled time and rate ; 24 outlet connected to other processing operations such as pelletizers ; and in operation in the non - continuous mode of operation , untreated melt 21 is introduced at the desired temperature through inlet 19 and drum 22 is rotated to induce shear extension in the melt under oscillation . plastic 19 is allowed to expand in the direction of the outlet . this is achieved by not filling completely cavity 21 . when the fatigue treatment is completed , the treated melt is sucked out by action of a vacuum device which purges the remaining entrapped air ( or gas ) and plastic is gear pumped out to 24 . in another embodiment , once the untreated plastic melt is introduced through inlet 19 , valve 19 is closed , but valves 19a and 24a are communicating and open . gear pumps 20 and 23 are synchronized to continuously return plastic melt from 24a to 19a during the time of the treatment , and the rate difference between the pumps is controlled to cause a determined extension of the melt which is also sheared and fatigued in cavity 21 by vibration of drum 22 . fig1 illustrates the oscillation of an eccentric cylindrical drum . the two modes of operation are strictly identical to the case of fig1 . the only important difference here is the two - axis motion of the drum to allow a certain amount of melt extension or orientation which is not produced by the apparatus in fig1 . 25 is the frame of the shearing apparatus . 27 is a detail of the connection of the melt cavity to the gear pump 23 and outlet 24 . fig1 shows an apparatus with two pistons to perform packing / melt fatigue prior to another molding operation . numerals in the figure identify parts as follows : in operation , the plastic polymer is introduced in the compression chamber and two pistons clamp it from both sides . the two pistons are connected to hydraulic actuators activated by servo - valves . the hydrostatic pressure a5 is controlled by the relative position of the two pistons . in addition , a packing vibration of controlled frequency and strain amplitude is applied on both pistons at the same time or on only one piston as shown on the figure . the frequency and strain amplitude is commanded separately from the mean hydrostatic pressure . when the melt fatigue under vigorous packing pressure is completed , one of the two pistons retracts entirely and the other piston pushes the treated melt out of the compression chambers , where it is collected and pumped away to another molding operation . while certain preferred embodiments of the present invention have been disclosed in detail , it is to be understood that various modifications in its structure may be adapted without departing from the spirit of the invention or the scope of the claims annexed to and forming a part of this disclosure . 1 ! j . p . ibar , acs polym . prep ., 21 ( 1 ), 215 ( 1980 ), &# 34 ; vibro - 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1 from ikv , at the rhineland - westphalian technical university ( rwth ) in aachen ( 1976 ). 20 ! k . w . schramm , &# 34 ; injection molding of structural parts consisting of plastic molding materials utilizing forced oscillating flow processes &# 34 ;, doctor - engineer thesis , rhenish - westphalian college of technology ( 1976 ).