Methods of preparing molded thermoplastic articles wherein radio frequency energy is utilized to heat the thermoplastic material
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to methods of preparing molded thermoplastic articles, and more particularly, but not by way of limitation, to methods of preparing molded thermoplastic articles wherein radio frequency energy is utilized. 
     2. Description of the Prior Art 
     Methods of preparing molded articles from thermoplastic materials using radio frequency energy to heat the materials are well known. For example, U.S. Pat. No. 4,288,399 issued Sept. 8, 1981 discloses a method of preparing molded articles wherein an amine polarizing agent is utilized to sensitize a thermoplastic composition to microwave energy followed by the application of such microwave energy to the composition for a time sufficient to produce a moldable consistency therein. The polarizing or radio frequency energy sensitizing agent, when mixed with a thermoplastic composition, functions to render the resulting mixture susceptible, or at least more susceptible, to the heating effects of radio frequency energy. 
     Heretofore, it has generally been necessary to ball mill or otherwise intimately mix the radio frequency energy sensitizing agent used with the thermoplastic material for a prolonged period of time prior to subjecting the resulting mixture to radio frequency energy. Also, it has generally been necessary to utilize separate sensitizing, plasticizing and lubricating agents in thermoplastic compositions. Plasticizing agents improve the low temperature flexibility and other properties of the molded product, and lubricating agents improve extrusion and molding properties. 
     Foamed thermoplastic products have heretofore been produced by mixing a chemical blowing agent with the thermoplastic composition at a temperature below the decomposition temperature of the blowing agent, and subsequently heating the mixture by conductance, convection or mechanical work to a temperature high enough for the blowing agent to decompose and release gases. Because the thermoplastic composition with blowing agent is often heated prior to being placed in a mold, the process involves the risk that the blowing agent will prematurely decompose. When reinforcing fibers, filaments or mats formed from fibers or filaments (all referred to herein as fibrous materials) are incorporated in the thermoplastic material to improve the strength and other physical properties of the resulting foamed product, the often used initial heating of the thermoplastic composition to facilitate combining the blowing agent and fibrous materials therewith risks premature foaming and often results in non-uniform cellular structure or the collapse of the cellular structure due to wicking action by the fibrous materials. 
     By the present invention improved methods of preparing molded thermoplastic articles, both solid and foamed and with and without fibrous materials, are provided which obviate the above problems. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to methods of preparing molded thermoplastic articles which basically comprise the steps of admixing a radio frequency energy sensitizing agent comprised of N-ethyl toluenesulfonamide with a thermoplastic polymer, applying radio frequency energy to the mixture for a time sufficient to produce a moldable consistency in the mixture, and then compression molding the mixture to produce a molded article therefrom. The radio frequency energy can be applied to the mixture after it is introduced into a mold, or the energy can be applied just prior to transferring the mixture into a mold. 
     The methods are particularly suitable for use with polyolefin and polyarylene sulfides, and the N-ethyl toluenesulfonamide sensitizing agent also functions to plasticize and provide lubrication to the thermoplastic material. While intimate mixing such as by ball milling for a period of time can be utilized for increasing the plasticizing and lubricating functions of the N-ethyl toluenesulfonamide, thermoplastic polymers such as polyolefins and polyarylene sulfides only require mixing to the extent necessary to bring about the surface wetting of the polymer particles by the liquid sensitizing agent at room temperature in order to adequately sensitize the polymer particles to radio frequency energy. 
     In one aspect, the present invention is directed to methods of preparing molded thermoplastic articles including composites or laminates reinforced with fibrous materials. 
     In another aspect of the present invention, methods are provided for preparing molded articles formed of foamed thermoplastic with or without reinforcing fibrous materials. The reinforced composites produced from foamed thermoplastic can be porous or non-porous. 
     It is, therefore, a general object of the present invention to provide methods of preparing molded thermoplastic articles. 
     A further object of the present invention is the provision of methods of preparing molded thermoplastic articles utilizing radio frequency energy for heating wherein a single component requiring minimum mixing time at low temperature is utilized as the radio frequency energy sensitizing agent. 
     Still a further object of the present invention is the provision of methods for preparing molded thermoplastic articles, both solid and foamed and with and without fibrous reinforcing materials, wherein the thermoplastic polymer used is efficiently sensitized to radio frequency energy by combining a single component therewith. 
     Other aspects, objects and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In accordance with the present invention, methods are provided for preparing molded thermoplastic articles from thermoplastic polymers. The polymers are made radio frequency energy sensitive by admixing a sensitizing agent comprised of N-ethyl toluenesulfonamide therewith. Radio frequency energy is then applied to the composition for a time sufficient to produce a moldable consistency in the composition, and the composition is compression molded to produce a molded article therefrom. 
     In a further embodiment of the invention, improved methods of preparing molded reinforced thermoplastic articles are provided. That is, in addition to the above-described radio frequency energy sensitizing agent, a fibrous material is combined with the thermoplastic polymer composition prior to molding. The term &#34;fibrous material&#34; is used herein to mean fibers and filaments such as glass fibers, polyaramid fibers and carbon filaments as well as one or more fibrous or filamentous mats which are surrounded by a matrix of the thermoplastic composition. 
     In preparing molded reinforced thermoplastic articles in accordance with this invention, the fibrous material utilized is preferably combined with the mixture of thermoplastic polymer and radio frequency energy sensitizing agent at low temperature. When one or more fibrous material mats are utilized, the mats are surrounded with a matrix of the thermoplastic polymer and sensitizing agent mixture in a known manner, preferably while in the solid unmelted state. The thermoplastic mixture surrounded or impregnated mats are then compressed and irradiated with radio frequency energy. The resulting mixture of thermoplastic polymer, sensitizing agent and fibrous material is subjected to radio frequency energy to produce a moldable consistency therein and compression molded. As indicated above, the mixture can be introduced into a mold and the radio frequency energy applied thereto, or the radio frequency energy can be applied to the mixture just prior to introduction into a mold in a transfer molding process. 
     In yet another embodiment of the invention, a blowing agent is combined with the mixture of thermoplastic polymer and radio frequency energy sensitizing agent, either with or without fibrous material, whereby a foamed porous or nonporous thermoplastic reinforced or non-reinforced article results. The blowing agent decomposes to produce gases which foam the thermoplastic polymer composition only after the radio frequency energy is applied to the composition. That is, the radio frequency energy is applied to the composition for a period of time such that the composition is heated to a temperature above the decomposition temperature of the blowing agent. The gases formed by the decomposition cause the thermoplastic polymer composition to be foamed just prior to or as the composition is compression molded. 
     While various thermoplastic polymers which are well known to those skilled in the art can be utilized in accordance with the present invention, preferred polymers are selected from the group consisting of polyolefins and polyarylene sulfides. Of the various polyolefins, polyethylene is preferred. The term &#34;polyarylene sulfide&#34; includes homopolymers and the normally solid arylene sulfide copolymers, terpolymers and the like. Examples of polyarylene sulfides, often abbreviated PAS, are polybiphenylene sulfide, polytoluenesulfide and a copolymer prepared from dichlorobenzene, dichlorotoluene and sodium sulfide. Of these, polyphenylene sulfide is preferred. 
     The radio frequency energy sensitizing agent of this invention is comprised of N-ethyl toluenesulfonamide. A liquid mixture of the ortho- and para- isomers is particularly preferred. Such a mixture is commercially available from Monsanto Industrial Chemicals Company of St. Louis, Mo. under the trademark SANTICIZER®8. The liquid mixture of N-ethyl-ortho- and para-toluenesulfonamide is highly polar, has a very high boiling point, has a low viscosity, is non-hygroscopic, is non-toxic, and provides superior radio frequency sensitizing to thermoplastic polymers when mixed therewith in relatively small quantities. When used solely as a sensitizing agent, the liquid mixture is preferably mixed at room temperature with the thermoplastic polymer powder for a sufficient time to surface wet the polymer. The sensitizing agent is mixed with the polymer in an amount to provide the required sensitivity to radio frequency energy, generally in an amount in the range of from about 1% to about 10% by weight of the polymer utilized. When used as a radio frequency energy sensitizer and also as a plasticizer and/or lubricant, the mixture of N-ethyl-ortho- and para-toluenesulfonamide is preferably combined with the polymer in an amount in the range of from about 2% to about 15% by weight of the polymer and ball milled therewith. When the polymer used is polyethylene, the sensitizing agent is preferably admixed therewith in an amount of about 4%-6% by weight of the polymer. When the polymer is polyphenylene sulfide, the sensitizing agent is preferably admixed therewith in an amount of about 2%-10% by weight of the polymer. 
     In producing foamed thermoplastic articles, various well known blowing agents can be utilized such as the reaction product of carboxylic acid and urea absorbed on silica and phenyl tetrazole. Of the foregoing, a phenyl tetrazole blowing agent is preferred. A suitable such phenyl tetrazole blowing agent is commercially available from Olin Chemicals Company of Stamford, Conn. under the trade name EXPANDEX®5PT. 
     The particular amount of blowing agent utilized in the thermoplastic polymer composition to foam the composition varies depending upon the particular blowing agent used, the desired density of the foam produced thereby, etc. Generally, phenyl tetrazole is combined with the mixture of thermoplastic polymer and radio frequency energy sensitizing agent mixture in an amount of about 0.5% to about 2.5% by weight of the polymer used. In order to insure that the blowing agent is uniformly distributed throughout the mixture, the mixture including blowing agent is preferably ball milled for a period of time prior to applying the radio frequency energy thereto. 
     The reinforcing fibrous material which can be utilized in accordance with the present invention can be continuous filaments or the filaments can be cut or chopped (staple fibers). Chopped glass fibers are particularly preferred for reinforcing the foamed articles produced in accordance with this invention. When reinforcing mats are utilized, they can be formed of continuous filaments or staple fibers and can be bonded by means of a binder, by heating, by needle punching, etc. In addition to glass, the fibrous materials can be formed of carbon, aromatic polyamides or other fiber-forming inorganic materials and mixtures. 
    
    
     In order to further illustrate the methods of the present invention, the following examples are given. 
     EXAMPLE 1 
     A non-crosslinked polyphenylene sulfide polymer produced by Phillips Petroleum Company under the trade designation &#34;PR-09&#34; is used to test a mixture of N-ethyl-ortho- and para-toluenesulfonamide as a radio frequency energy sensitizer. The N-ethyl-ortho- and para-toluenesulfonamide used is Monsanto&#39;s SANTICIZER®8 having the specifications and typical properties given in Table I below. 
     
                       TABLE I                                                     
______________________________________                                    
 Properties of N--Ethyl-o- and p-Toluenesulfonamide.sup.1                 
______________________________________                                    
Appearance           Light-yellow, viscous                                
                     liquid                                               
Color (molten, APHA) 200 maximum                                          
Odor                 Slight, characteristic                               
Free amide, %        3-13                                                 
Acidity, meq./100 g  0.80 maximum                                         
Chlorine, %          0.01 maximum                                         
Structural formula   CH.sub.3.C.sub.6 H.sub.4.SO.sub.2 NH.C.sub.2         
                     H.sub.5                                              
Molecular weight     199.22                                               
Specific gravity, 25°/25° C.                                
                     1.190                                                
Density, at 25° C., pounds per gallon                              
                     9.92                                                 
Refractive index, at 25° C.                                        
                     1.540                                                
Solidification temperature, °C.                                    
                     0                                                    
Crystallizing point, °C.                                           
                     &lt;40                                                  
Boiling point (760 mmHg), °C.                                      
                     340                                                  
Vapor pressure, mmHg                                                      
150° C.       0.45                                                 
200° C.       5.5                                                  
Surface tension, at 25° C., dynes/cm                               
                     44.5                                                 
______________________________________                                    
 .sup.1 Monsanto Industrial Chemicals Co. Specifications and Typical      
 Properties (Technical Bulletin IC/PL339)                                 
 
    
     The polyphenylene sulfide polymer (35 mesh, U.S. Sieve Series) is ball milled for one-half to one hour at room temperature with 0, 1, 2, 5, 10, 20 and 40 parts N-ethyl-o- and p-toluenesulfonamide sensitizing agent per 100 parts of polymer. At 20 parts sensitizing agent per 100 parts polymer the mixture is still free flowing. With 40 parts sensitizing agent per 100 parts polymer, the mixture has a dry consistency, but in order to restore free flowing properties 10 parts per hour of a radio frequency energy unresponsive silica are added to that mixture. 
     The various mixtures are each placed in a TEFLON® sample holder, heated in an Ultramic Industries 9 KW, 100 MHz DIATRON® 97001 radio frequency generator, and subsequently transfer molded in a GIVEN-PHI® press with the mold maintained at a temperature of 230° F. Each of the mixtures are heated in the radio frequency generator for the time required for the polymer to be melted and achieve good flow characteristics. Such times and the characteristics of the thermoplastic parts produced are given in Table II below. 
     
                       TABLE II                                                    
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Polyphenylene Sulfide-                                                    
N--Ethyl-o- and p-Toluenesulfonamide                                      
Mixture Heating Times and Molded Part Properties                          
Quantity of                                                               
N--Ethyl-o and                                                            
            Heating                                                       
pToluenesul-                                                              
            Time,       Part                                              
fonamide, phr                                                             
            Seconds     Properties                                        
______________________________________                                    
0           240         Shiny brittle break;                              
                        brown color                                       
1           180         Shiny brittle break;                              
                        brown color                                       
2           120         Shiny brittle break;                              
                        brown color                                       
5            40         Dull break; tan color                             
10           30         Dull break; dark grey                             
                        color                                             
20           20         Dull break; grey                                  
                        color                                             
40           5          Dull break; ivory                                 
                        color                                             
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     As can be seen from Table II, the amount of N-ethyl-o- and p-toluenesulfonamide in admixture with polyphenylene sulfide polymers has a dramatic effect on the radio frequency energy heating time, the brittleness of the molded parts and the color of the molded parts. 
     EXAMPLE 2 
     Various test mixtures of the polyphenylene sulfide and N-ethyl-o- and p-toluenesulfonamide described in Example 1 are dry blended in a high speed mixer at 900 rpm. The mixtures are thereafter extruded at 600° F. (nozzle temperature) in a Davis Standard Thermatic III extruder, and subsequently chopped and injected molded in a four-zone injection molding machine. The injection molding machine temperatures are 550° F. in zone 1, 550° F. in zone 2, 530° F. in zone 3 and 400° F. in zone 4. The molded parts produced are annealed 2 hours at 400° F., and then tested for the physical properties listed in Table III below. The procedures followed in the physical properties tests are ASTM standard, as noted in Table III. The results of the physical property tests are given in Table III. 
     
                                           TABLE III                               
__________________________________________________________________________
Properties of Molded Non-Reinforced Mixtures                              
of Polyphenylene Sulfide and N--Ethyl-o-                                  
and p-Toluenesulfonamide                                                  
             Mixture                                                      
                  Mixture                                                 
                       Mixture                                            
                             Mixture                                      
                                  Mixture                                 
             1    2    3     4    5                                       
__________________________________________________________________________
Quantity of PPS                                                           
             100  100  100   100  100                                     
Polymer,                                                                  
parts by weight                                                           
Quantity of N--ethyl-o-                                                   
             0    1    2.5   5    10                                      
and p-toluenesulfon-                                                      
amide, parts by weight                                                    
Flexural Modulus, MPa                                                     
             3,543                                                        
                  3,751                                                   
                       3,754 3,685                                        
                                  3,225                                   
(ASTM D790)                                                               
Flexural Strength, MPa                                                    
             129  138  137   131  127                                     
(ASTM D790)                                                               
Tensile Yield, MPa                                                        
             82.9 75.0 83.5  93.0 66.6                                    
(ASTM D638)                                                               
Izod Impact                                                               
Notched, Joule/m                                                          
             34.6 35.1 35.1  37.1 34.9                                    
Unnotched, Joule/m                                                        
             294  222  188   202  204                                     
(ASTM D256)                                                               
Heat Distortion, °C.                                               
             119  112  106   98   92                                      
at 1820 KPa                                                               
(ASTM D1637)                                                              
Exudation    None None None  None Very                                    
                                  slight                                  
__________________________________________________________________________
 
    
     EXAMPLE 3 
     Various test mixtures of the polyphenylene sulfide and N-ethyl-O- and p-toluenesulfonamide described in Example 1 above are prepared by dry blending in a high speed mixer at 900 rpm. 11/2 ounce glass fiber mats are impregnated with each mixture. The resulting composites are compression molded in a 9&#34;×10&#34; positive pressure mold at a temperature of 600° F., and then annealed for 2 hours at 400° F. The molded parts are tested for physical properties using standard test procedures. The results of the tests are set forth in Table IV below. 
     
                       TABLE IV                                                    
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Properties of Molded Reinforced Mixtures                                  
of Polyphenylene Sulfide and N--Ethyl-o-                                  
and p-Toluenesulfonamide                                                  
            Mix-                                                          
            ture  Mixture  Mixture  Mixture                               
            1     2        3        4                                     
______________________________________                                    
Quantity of PPS                                                           
              100     100      100    100                                 
Polymer,                                                                  
parts by weight                                                           
Quantity of N--ethyl-o-                                                   
              0       2.5      5.0    7.5                                 
and p-toluenesulfon-                                                      
amide, parts by weight                                                    
Weight of PPS, g                                                          
              187     155      151    149                                 
Weight of Glass                                                           
              130     130      121    110                                 
Fiber Mat, g                                                              
Flexural Modulus, MPa                                                     
              9,648   10,713   9,271  9,143                               
(ASTM D790)                                                               
Flexural Strength, MPa                                                    
              285     299      229    217                                 
(ASTM D790)                                                               
Tensile Break, MPa                                                        
              192     213      169    148                                 
(ASTM D638)                                                               
Elongation, % 4.88    5.19     4.21   4.15                                
(ASTM D638)                                                               
Heat Distortion, °C.                                               
              279     277      277    272                                 
at 1820 KPa                                                               
(ASTM D1637)                                                              
Exudation     None    None     None   None                                
Surface Appearance                                                        
              Rough   Slightly Smooth Very                                
                      Rough           Smooth                              
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     From Tables III and IV above it can be seen that both non-reinforced and glass fiber reinforced molded polyphenylene sulfide parts containing N-ethyl-o- and p-toluenesulfonamide have excellent physical properties, particularly at a N-ethyl-o- and p-toluenesulfonamide concentration of 2.5% by weight of the polymer used. 
     EXAMPLE 4 
     Ultrahigh molecular weight polyethylene polymer manufactured by Hercules, Inc. of Wilmington, Del. (now Himont Corp., Wilmington, Del.) under the trade designation UHMWPE 1900 having an intrinsic viscosity of 27 is used to prepare test mixtures containing a radio frequency energy sensitizing agent and various lubricants. 100 parts by weight portions of the polyethylene polymer are dry blended with 4 parts by weight sodium aluminum silica sensitizing agent manufactured by the J. M. Huber Corp., of Havre DeGrace, Md., after having first been wetted with 1 part by weight of the various lubricants set forth in Table V below. The resulting mixtures are each heated in a 9 KW, 100 MHz radio frequency generator [Ultramic Industries, Inc. of Albany, N.Y. (now NTRONIX of Nashville, Tennessee) Model 9700L] in a TEFLON® sample holder, and transfer molded into a 1/8 inch spiral flow mold, under the following conditions: 
     
         ______________________________________                                    
air gap on RF-generator between                                           
                     21/2     inches                                      
electrodes:                                                               
infrared temperature controller                                           
                     340°                                          
                              F.                                          
setting:                                                                  
amount of each sample:                                                    
                     16       grams                                       
mold temperature:    220°                                          
                              F.                                          
ram pressure (1-inch ram):                                                
                     2900     psi                                         
______________________________________                                    
 
    
     The flow length of each molded sample produced is determined and the molded samples are observed for color change. The results of these tests are given in Table V. 
     
                       TABLE V                                                     
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Flow Lengths of Flow Molded                                               
Samples Using Various Lubricants                                          
              Flow                                                        
              Length,                                                     
Lubricant Used                                                            
              in.        Observations                                     
______________________________________                                    
None           1.25      No color change                                  
Silicone Fluid                                                            
              2.0        Slight grey color                                
(Dow-Corning SF1080)                                                      
Glycerin      5.0        Tan color                                        
Triethanolamine                                                           
              less than 1                                                 
                         Yellow color                                     
Diethylene glycol                                                         
               5.75      Slight yellow color                              
N--ethyl-o- and p-                                                        
              9.5        No color change.                                 
toluenesulfonamide       (More uniform melt)                              
______________________________________                                    
 
    
     A direct comparison of the mixtures containing N-ethyl-o- and p-toluenesulfonamide and the Dow-Corning silicone fluid lubricants with unmodified polyethylene polymer is made by preheating for 30 minutes at 350° F., then compression molding at 4000 psi followed by 10 minutes of cooling. The molded samples produced are tested for physical properties using standard procedures as indicated in Table VI below. 
     
                       TABLE VI                                                    
______________________________________                                    
Physical Properties of                                                    
Molded Samples Using Different Lubricants                                 
              N--Ethyl-o                                                  
              and p-            No                                        
              toluene- Silicone.sup.2                                     
                                Lubri-                                    
              sulfonamide.sup.1                                           
                       Fluid    cant                                      
______________________________________                                    
Yield, psi      2870       2787     2843                                  
(ASTM D638)                                                               
Tensile, psi    4440       3820     5520                                  
(ASTM D638)                                                               
Elongation, %   170        125      270                                   
(ASTM D638)                                                               
High Speed Impact:                                                        
Impact Rate, in/sec                                                       
                188        177      178                                   
Max. Load, lbs. 313        289      482                                   
Apparent Modulus, lbs/in                                                  
                1610       932      1380                                  
Energy Peak, in. lbs.                                                     
                35.6       32.4     82.4                                  
Energy Break, in. lbs.                                                    
                37.6       32.4     82.4                                  
______________________________________                                    
 .sup.1 Monsanto SANTICIZER®8                                         
 .sup.2 DowCorning SF1080                                                 
 
    
     From Tables V and VI, it can be seen that N-ethyl-o- and p-toluenesulfonamide functions as a very efficient lubricant in mold flow, and has less influence on the physical properties of the molded products than silicone fluid. Further, the N-ethyl-o- and p-toluenesulfonamide has a beneficial effect on radio frequency energy response, is non-toxic, has a very high boiling point (340° C.), is relatively low in viscosity and is non-hygroscopic making it ideal for use as a radio frequency energy sensitizer and lubricant. 
     EXAMPLE 5 
     40 grams of the polyphenylene sulfide polymer described in Example 1 are ball milled with 4 grams of the N-ethyl-o- and p-toluenesulfonamide described in Example 1 for a few minutes to uniformly wet the surface of the polymer with the sensitizing agent. 0.6 grams of a blowing agent, i.e., a phenyl tetrasole blowing agent manufactured by Olin Chemicals Company of Stamford, Conn. under the trade name EXPANDEX®5PT, is combined with the mixture and the mixture is ball milled for several additional minutes. The resulting free flowing powder is poured into a preheated (200° C.) cylindrical TEFLON® mold (outer diameter 51/2 inches, cavity diameter 41/4 inches). A preheated loose-fitting TEFLON® disc is placed on top of the powder. The mold is placed in an Ultramic Industries DIATRON®9700L 100 MHz, 9 KW radio frequency energy generator with electrode plate distance of 31/2 inches. After 3 minutes exposure time in the radio frequency energy field, the lid is removed and the foam produced in the mold is allowed to cool. The foamed product has a medium fine cellular structure and the density is 0.35 grams per cubic centimeter. 
     40 grams of the same polyphenylene sulfide polymer are ball milled with 2 grams of the N-ethyl-o- and p-toluenesulfonamide along with 2 grams of sodium aluminum silicate sensitizing agent manufactured by the J. M. Huber Corp. of Havre DeGrace, Md. 0.6 grams of a blowing agent manufactured by the Struktol Company of America under the trade name STRUKTOL® FA-541 (a reaction product of carboxylic acid and urea absorbed onto a silica filler) are added to the mixture, and the ball milling is continued for a total of 2 hours. The free flowing powder produced is poured into a preheated TEFLON® mold as described above, and covered with the same preheated, loose fitting TEFLON® disc as described above. The mold and powder are irradiated for 5 minutes in the same radio frequency energy generator described. The lid is removed from the mold and the foam allowed to cool. The resulting grey foam had a fine pore structure and a density of 0.58 grams per cubic centimeter. 
     From the above it can be seen that the N-ethyl-o- and p-toluenesulfonamide functions efficiently as a radio frequency energy sensitizer in the presence of blowing agents.