Abstract:
An apparatus and method for determining the standardized melt elasticity (sME) force of a thermoplastic polymer needed to stretch a strand of melted polymer at a speed about thirty three times faster than the speed of strand formation from a body of melted polymer under constant stress from an unencumbered dead weighted piston.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Application Ser. No. 61/190,850 filed Sep. 3, 2008 (Attorney Docket No. 67401 US). For purposes of United States patent practice, the contents of this application is herein incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The instant invention is in the field of methods and apparatus for the measurement of viscoelastic properties of molten thermoplastic polymers. More specifically, the instant invention relates to methods and apparatus for the determination of melt elasticity. 
         [0003]    As stated in Wagner and Bernnat, J. Rheol. 42(4), July/August 1998, p917-928, the melt elasticity of polymer melts is of great importance for many polymer processes like fiber spinning, film blowing, blow molding, high-speed coating, and sheet casting. A measure of melt elasticity can be made by the use of an “extension diagram,” where the drawdown force needed for elongation of an extruded strand of melted polymer is measured as a function of a increasing drawdown speed. For this purpose, a tensile tester, the so-called “Rheotens,” was developed (e.g., see Meissner, Rheol. Acta 10, 1971, p230-242). The Rheotens test is readily performed, shows excellent reproducibility, and models industrial polymer processes like fiber spinning or film casting. Therefore, the Rheotens test has found widespread application. 
         [0004]    In a Rheotens test a strand of polymer melt extruded by a polymer melt viscometer is elongated under the action of rotating wheels which have gripped the meltstrand when the velocity of the wheels is larger than the output velocity of the strand. Normally the wheels are accelerated till the strand breaks or the maximum rotational speed of the wheels is obtained. A direct conversion of the tensile-force/drawdown-speed diagram into a relation between elongational viscosity and elongation rate is not possible. However, a considerable simplification in the analysis of constant force extension resulted from the discovery of “Rheotens mastercurves” for thermorheologically simple polymer melts (Wagner, et al., Polym. Eng. Sci. 36, 1996, p925-935). Rheotens mastercurves provide a basis for a direct and quantitative comparison of the elasticity of polymer melts under processing conditions. 
         [0005]    Surprisingly, even for Rheotens experiments performed at different extrusion pressures (termed extrusion “stress” in the art), Rheotens mastercurves can be found, if force and draw ratio are scaled appropriately as reported by Wagner, et al. Such mastercurves, which represent mastercurves of mastercurves, are termed “Rheotens supermastercurves.” Wagner and Bernnat showed that the concept of Rheotens mastercurves could be generalized to experiments with extrusion dies and spinlines of different length, and that information on the elongational viscosity of polymer melts could be extracted from Rheotens mastercurves by use of an analytic rheological model. 
         [0006]    Instruments for performing the Rheotens test are commercially available from Goettfert Inc., Parkway Rock Hill, S.C. However, the commercially available instrument for performing the Rheotens test is relatively expensive because it employs a polymer viscometer to generate the meltstrand. Furthermore, such instruments are considered by the art to be more useful in a research laboratory than in a quality control laboratory of a polymer production facility due to the expense of the instrument and the degree of skill needed to operate the instrument. Thus, there is a need for the development of a less expensive and more readily operated instrument for the determination of melt elasticity of a thermoplastic polymer. 
       SUMMARY OF THE INVENTION 
       [0007]    This disclosure provides a method and apparatus for the determination of a standardized melt elasticity of a thermoplastic polymer which are less expensive and more readily operated than the Rheotens test and instrument of the prior art. More specifically, the disclosed invention is a method for determining a standardized melt elasticity value for a thermoplastic polymer, comprising the steps of: (a) heating the thermoplastic polymer to melt the thermoplastic polymer; (b) subjecting the melted thermoplastic polymer to a selected constant stress so that the melted thermoplastic polymer flows through a channel to form a strand of melted thermoplastic polymer leaving the channel at a velocity V 0 , the selected constant stress and melted polymer temperature being in a range so that V 0  is greater than 1.27 millimeters per second, the channel having a diameter of about 2.1 millimeters and a length of about 8 millimeters; and (c) stretching the strand of melted thermoplastic polymer by applying a force to the strand of melted thermoplastic polymer to produce a stretched strand of thermoplastic polymer traveling at a velocity of about 33V 0 , the standardized melt elasticity value of the thermoplastic polymer being equal to the force. 
         [0008]    The disclosed invention is also an apparatus for determining a standardized melt elasticity value for a thermoplastic polymer, comprising: (a) a frame; (b) a body defining a cylindrical barrel shaped depression in the body from the top of the body, the body defining a channel at the bottom of the body in communication with the cylindrical barrel shaped depression, the body attached to the frame; (c) a heater in thermal communication with the body for heating the body; (d) a dead weighted cylindrical piston dimensioned to fit into the cylindrical barrel shaped depression; (e) a position sensor for sensing the dynamic vertical position of the dead weighted cylindrical piston in the cylindrical barrel shaped depression; (f) a tension roller; (g) a load cell, the tension roller connected to the load cell, the load cell attached to the frame; (h) a guide roller attached to the frame; (i) an electrical speed controlled motor attached to the frame; (j) a pull roller driven by the motor, so that when a thermoplastic polymer is placed in the cylindrical barrel shaped depression followed by the dead weighted piston, the polymer melts and flows through the channel by the force of the constant stress of the dead weighted piston on the melted polymer to form a strand of melted polymer leaving the channel at a velocity V 0  determined from the dimensions of the channel and the rate of piston movement, the weight of the dead weighted piston and the temperature of the melted polymer being in a range so that V 0  is greater than 1.27 millimeters per second, so that the strand of polymer can be passed under the tension roller, over the guide roller and onto the pull roller, the pull roller being driven at a rate so that the strand of melted thermoplastic polymer is stretched to form a solidified stretched strand of thermoplastic polymer having a velocity of about 33V 0 , so that the melt elasticity value of the thermoplastic polymer can be standardized as being equal to the stretching force measured by the load cell. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a front view, part in full and part in cross section of an apparatus of this disclosure; 
           [0010]      FIG. 2  is an upper view in full of a tool of this disclosure; 
           [0011]      FIG. 3  is a perspective view in full of a weight of this disclosure having handles; 
           [0012]      FIG. 4  is a side cross-sectional view of a weight guide of this disclosure; 
           [0013]      FIG. 5  is a side view of a melt strand length adjustment gauge used for initial installation and set-up of the apparatus shown in  FIG. 1 ; 
           [0014]      FIG. 6  is a side view, part in full and part hidden, of a laser alignment tool used to set-up the apparatus shown in  FIG. 1 ; 
           [0015]      FIG. 7  is a perspective view in full of a tool used to confirm the temperature recovery response of the heated body of the apparatus shown in  FIG. 1 ; and 
           [0016]      FIG. 8  is a perspective view in full of a tool used to set the horizontal position of the piston position sensor of the apparatus shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Referring now to  FIG. 1 , therein is depicted an apparatus  10  for determining a standardized melt elasticity value for a thermoplastic polymer according to the instant disclosure. The apparatus  10  is based on a frame having an upper top section  11   a , a lower top section  11   b , a bottom  12 , a sliding base plate  12   a , a left side  13 , a right side  14  and a back  15 . The upper top section  11   a  is attached to lower top section  11   b  by leveling bolts  11   c  and leveled using top two dimensional bubble level  11   d . An electrically heated body  16  defining a cylindrical barrel shaped depression  17  is attached to the upper top section  11   a . The operating temperature of the body  16  preferably ranges from 125-390° C. as prescribed by ASTM test method D 1238-4 based on the specific melting point for each polymer type. Although each polymer type may use a different body temperature (for example, 190° C. for polyethylene), the Melt Flow Rate method specifies a constant preheat time of 7 minutes±30 seconds for all polymer types in order to minimize the impact of degradation and cross-linking on measurement results. However, it is understood by those skilled in the art that different preheat times may be required to perform non-standardized melt flow tests or for polymers that are more or less sensitive to thermal influence. 
         [0018]    The body  16  comprises a die  19  defining a channel  20  with a diameter of 2.095±0.0051 millimeters and a length of 8.000±0.025 millimeters at the bottom  21  of the body  16 , the channel  20  in communication with the cylindrical barrel shaped depression  17 . A piston  22 , weighted by an unencumbered dead weight  23  and connecting rod  24 , is dimensioned to fit into the cylindrical barrel shaped depression  17 . The unencumbered dead weight  23  can alternatively be configured as shown in  FIG. 3  to include handles  61  and  62  connected to weight  60 . The handles  61  and  62  can be used to facilitate the movement of the piston  22  to pre-defined barrel start positions for both the standardized melt elasticity and melt flow rate methods. A position sensor  25  for sensing the dynamic vertical position of the piston  22  relative to the top of the die  19  by way of a feeler rod  26  is supported on the upper top section  11   a  by support  27 .  FIG. 8  shows a perspective view in full of a tool  100  made of stainless steel that is used to set the horizontal position of the position sensor  25  by setting the tool  100  on top of the piston  22  in place of the weight  23 . Since the elements  16 - 27  are commercially available as the Tinius Olsen (Horsham, Pa.) Model MP600 Plastometer, which is designed to conform to the requirements of ASTM test method D1238-04, the Melt Flow Rate of a melted polymer  39  can be determined after the standardized melt elasticity is determined. 
         [0019]    Referring to  FIG. 1 , a tension roller  28  is connected to platform  29  by way of mount  30 . The platform  29  weighs about 175 grams and is attached to a load cell  32   a  (For example, the Mettler Toledo (Columbus, Ohio) Model X52035 electronic balance) having hemisphere ended leveling bolts  32   b  resting in corresponding depressions in sliding base plate  12   a , so that the load cell  32   a  can be leveled using two dimensional level  32   c . The tension roller  28  incorporates a low friction instrument grade ball bearing  31 . The tension roller  28  is made of anodized aluminum, has a diameter of twenty five millimeters, a thickness of three millimeters and a circumferential one millimeter deep v-groove. A guide roller  33  is attached to base plate  12   a  by mount  34 . The guide roller  33  is made of anodized aluminum, has a diameter of twenty five millimeters, a thickness of three millimeters and a circumferential one millimeter deep v-groove. The guide roller  33  incorporates a low friction instrument grade ball bearing  35 . The use of the instrument grade ball bearings  31  and  35  and the careful alignment of the mounts  30  and  34  reduce to a minimum any frictional error in the final method result. An electrical motor  36  (For example, the Oriental Motor (Torrance, Calif.) Model RK564AA-T7.2) is attached to base plate  12   a  by mount  37 . A pull roller  38  is driven by the motor  36 . The pull roller  38  has the shape of a truncated cone having a circumference at its large end of about one hundred and twenty five millimeters, a width of about fifty millimeters and a slope of about 0.5 degrees so that the solidified stretched strand  41  winds up onto the pull roller  38  in a single layer. 
         [0020]    Referring to  FIG. 1 , when a thermoplastic polymer is placed into the cylindrical barrel shaped depression  17  followed by the dead weighted piston  22 , the polymer melts to form melted polymer  39  which flows through the channel  20  by the force of the constant stress of the dead weighted piston  22  on the melted polymer  39  to form a strand of melted polymer  40  exiting from channel  20  at a velocity V 0  determined from the dimensions of the channel  20  and the rate of piston  22  movement with the weight of the dead weighted piston  22  and the temperature of the melted polymer  39  being in a range so that V 0  is greater than 1.27 millimeters per second. The strand of melted polymer cools while being stretched to form a solidified stretched polymer  41  which is passed under the tension roller  28 , over the guide roller  33  and onto the pull roller  38 . The pull roller  38  is driven at a rate so that the strand of melted thermoplastic polymer  40  is stretched to form the strand of solidified stretched polymer  41  having a velocity in the range of 32.6-33.6V 0 , so that the melt elasticity value of the thermoplastic polymer can be standardized as being equal to the drawdown force measured in centi-Newtons (cN) by way of the absolute value of the gram-force measurement made by load cell  32  (the conversion factor is 0.980665 centi-Newtons per gram). Preferably, the velocity of the stretched strand of thermoplastic polymer is linearly ramped from about 25V 0  to about 40V 0  over a period of time of about one minute, the standardized melt elasticity of the thermoplastic polymer being equal to the drawdown force when the velocity of the solidified stretched strand of thermoplastic polymer is in the range of 32.6-33.6V 0 . Such ramping protocol facilitates analysis even when the melt flow rate of a polymer sample deviates somewhat from its expected value. 
         [0021]    Referring to  FIG. 1 , leveling screws  44  are adjusted to support the bottom  12  on support  42  in a level position as determined by an integrated two dimensional bubble level  43 . Adjusting screws  45  are used to adjust slidable base plate  12   a  so that the v-groove of the tension roller  28  is directly below the channel  20  using a laser bore scope system carefully aligned with the longitudinal axis of the depression  17  when the body  16  is at its operational temperature so that a laser beam directed through the channel  20  just strikes the v-groove of the tension roller  28 .  FIG. 6  shows a suitable laser bore scope system  90  consisting of a stainless steel heat sink  93 , a heat resistant polyamide-imide polymer adapter  92  made of TORLON brand engineering polymer (Parkway Products Inc., Florence Ky.) and a laser bore scope  91 . The adapter  92  protects the laser bore scope  91  from overheating when the laser bore scope system  90  is inserted into the depression  17 . The laser bore scope  91  is adjusted to center the laser beam from the laser bore scope  91  through bore  95  and the center of the channel  20  by adjustment of alignment screws  94  of the laser bore scope  91 . The laser bore scope  91  is commercially available from Midway USA, Columbia, Mo. Not shown in  FIG. 1  is a Plexiglas shield positioned across the front of the apparatus  10  so that vagrant air currents do not interfere with the analysis. 
         [0022]    Referring to  FIG. 5 , the distance between the bottom of the body  21  and the platform  29  is preferably carefully adjusted to be four hundred and forty five millimeters resulting in the distance between the bottom of the body  21  and the horizontal center line of the bearing  31  preferably being four hundred and twenty four millimeters to further standardize the apparatus and promote solidification of the polymer prior to contact with the tension roller  28  by inserting the melt strand length adjustment gauge  80  into an empty depression  17  and adjusting leveling bolts  11   c  until the load cell  32   a  just indicates that platform  29  is being depressed. The melt strand length adjustment gauge  80  shown in  FIG. 5  consists of a stainless steel rod  84  having a brass handle  81  at one end thereof and a twenty five millimeter long half round portion  85  at the other end. A stainless steel sleeve  82  is held in place on the rod  84  by a set screw  83 . The half round portion  85  allows the tool  80  to slide down past the tension roller  28  without vertical deviation.  FIG. 7  shows a solid brass temperature recovery standard  110  dimensioned to fit into the depression  17  for defining and validating control parameters governing the rate of temperature recovery by the electrically heated body  16  of  FIG. 1 . 
         [0023]      FIG. 2  shows an upper view in full of a tool  50  that can be used to safely secure the hot die  19  when it is removed from the electrically heated body  16  of  FIG. 1  for cleaning following the measurement of standardized Melt Elasticity and Melt Flow Rate. The tool  50  possesses modified clamp tips  51  and  52  specifically designed to securely grasp the hot die  19  with the top and bottom surfaces exposed as well as the entrance and exit of channel  20 . The tool  50  is fabricated by modifying a commercially available hemostat (Lakeside Scissor Sales, Sacramento, Calif.) and eliminates the need for thermal resistant gloves to directly handle the hot die  19  while removing residual sample material between analyses. 
         [0024]      FIG. 4  shows a cross-sectional side view of a modified dual weight management restraint system  70  for use with the above disclosed Tinius Olsen plastometer. The individual brackets  71 ,  72  and  73  replace the original guide rods situated on the automatic weight lifter platform  77 . The modified dual weight management restraint system  70  includes a fourth identical bracket not shown in  FIG. 3 , which is symmetrically positioned directly in front of bracket  73 . While space  74  is dimensioned to hold a smaller weight, Space  75  is dimensioned to hold a larger weight which can be combined with the smaller weight when the weight lifter platform  77  is sufficiently lowered. 
         [0025]    Referring to  FIG. 1 , all electric components comprising apparatus  10  are preferably programmed to control the parameters disclosed above using a general purpose digital computer (Dell Model GX520) configured with four serial ports, 512 MB of RAM, a 3.4 GHZ processor and an 80 GB hard drive. In addition, apparatus  10  is significantly less expensive than an apparatus for performing the Rheotens test primarily because the polymer viscometer required in the Rheotens test instrument is much more expensive than the elements  16 - 27 . 
       Example 
       [0026]    The origin of the six different samples (Samples 1-6) of commercial grade low density polyethylene having melt flow rates (MFR) tested using ASTM D 1238, condition 190° C./2.16 kg in the range of from about 0.7 to about 8.7 dg/min used to validate the performance of apparatus  10  are described in Table 1. 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Sample # 
                 ~MFR 
                 Source 
                 Country of Production 
               
               
                   
               
             
             
               
                 1 
                 0.7 dg/min 
                 TDCC 
                 Spain 
               
               
                 2 
                 1.0 dg/min 
                 TDCC 
                 The Netherlands 
               
               
                 3 
                 1.9 dg/min 
                 TDCC 
                 United States of America 
               
               
                 4 
                 7.0 dg/min 
                 TDCC 
                 The Netherlands 
               
               
                 5 
                 8.0 dg/min 
                 TDCC 
                 United States of America 
               
               
                 6 
                 8.7 dg/min 
                 TDCC 
                 The Netherlands 
               
               
                   
               
               
                 where TDCC is The Dow Chemical Company 
               
             
          
         
       
     
         [0027]    Samples 1-6 are analyzed for both standardized melt elasticity and melt flow rate using apparatus  10  shown in  FIG. 1  under the specific conditions described in Table 2. 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Sample  
                 Body  
                 Piston  
                 Pull Roller  
                 Die Speed  
               
               
                 Sample # 
                 wt. 
                 temp. 
                 wt. 
                 Speed 
                 (V 0 ) 
               
               
                   
               
             
             
               
                 1 
                 5 grams 
                 190° C. 
                 5.00 Kg. 
                 64.72 mm/sec. 
                 1.95 mm/sec 
               
               
                 2 
                 6 grams 
                 190° C. 
                 5.00 Kg. 
                 92.45 mm/sec. 
                 2.79 mm/sec 
               
               
                 3 
                 5 grams 
                 190° C. 
                 2.16 Kg. 
                 42.02 mm/sec. 
                 1.27 mm/sec 
               
               
                 4 
                 7 grams 
                 190° C. 
                 2.16 Kg. 
                 157.6 mm/sec. 
                 4.75 mm/sec 
               
               
                 5 
                 7 grams 
                 190° C. 
                 2.16 Kg. 
                 168.1 mm/sec. 
                 5.06 mm/sec 
               
               
                 6 
                 7 grams 
                 190° C. 
                 2.16 Kg. 
                 182.8 mm/sec. 
                 5.51 mm/sec 
               
               
                   
               
             
          
         
       
     
         [0028]    The analysis precision for standardized melt elasticity (sME) in centi-Newtons [cN] of each sample is determined from ten replicates and reported in Table 3 below. 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Sample # 
                 Average sME 
                 Relative STDEV 
               
               
                   
               
             
             
               
                 1 
                 9.05 cN 
                 0.72% 
               
               
                 2 
                 7.35 cN 
                 0.56% 
               
               
                 3 
                 3.36 cN 
                 0.68% 
               
               
                 4 
                 2.76 cN 
                 0.87% 
               
               
                 5 
                 2.44 cN 
                 1.07% 
               
               
                 6 
                 2.19 cN 
                 1.32%