Method of manufacturing a foam fibrillated fibrous web from an isotactic polypropylene, polystyrene and α-methylstrene blend
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of Ser. No. 320,356, filed Jan. 2, 1973 by Gary L. Driscoll and now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     In the past there has been considerable effort to find a way of forming fabric-like materials by means other than weaving or knitting. Weaving fabrics is an expensive operation, particularly when the woven material is made of fiber slivers. Woven slit film eliminates the carding or garneting of fibers but still involves the expensive weaving operation. Needle punching of layers of fibrillated films is used for some purposes, but for many purposes the layers are not sufficiently unitized. 
     SUMMARY OF THE INVENTION 
     The present invention relates to forming non-woven fabrics from a novel foam fibrillated web. This web is formed of a blend of polypropylene and polystyrene. The polypropylene supplies the strength and backbone of the web while the polystyrene serves to cause the individual polypropylene fibrils within the web to be finer which causes their surface area with respect to their weight to increase which helps in bonding the individual fibrils together when a plurality of layers of such web are formed into a non-woven fabric. Additionally the polystyrene serves to cause the webs to become more susceptible to orientation by drawing which increases their strength considerably. The presence of the polystyrene further serves to improve the bonding of the webs together as compared with the relatively poor bonding obtained with pure polypropylene webs. This function can be utilized in two ways. First the amount of polystyrene used can be increased above the optimum amount for improving orientability and then used as the bonding agent alone. Secondly, the amount of polystyrene present can be kept fairly small and an adhesive used to bond the layers. In this case polystyrene serves as a bonding agent on the surface of the polypropylene fibrils within the web. The webs are assembled into a plurality of layers by any suitable means such as simply unrolling some webs onto a carrier belt and cross-lapping some other layers to provide strength across the machine direction of the final non-woven fabric. Alternatively the webs may be broken up into fibers and air laid in a random disposition to form a bat which is then laminated to form a non-woven fabric. If desired such a random laid bat can be used as the inner layer replacing the cross-lapped layers. For many uses it is preferred that one or both surfaces of the final non-woven fabric be formed from a layer of one web laid in the machine direction. When such a non-woven fabric is embossed to appear like a woven fabric the effect is more realistic because of the parallel fibers on the surface. The assembled layers are finally laminated together using a combination of heat and pressure. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic side view of the foam extrusion and fibrillation apparatus. FIG. 2 is a schematic side view of the laminating apparatus. 
    
    
     In FIG. 1 a blend of polypropylene and polystyrene is fed to hopper 1 of feed meterer 2, along with whatever blowing agent is required. The blend is fed at a controlled rate from feed meterer 2 to the feed hopper 3 of extruder 4 as free falling pellets 5. Extruder 4 is equipped with a slit die 6 the slit of which is offset from the extruder feed port so as to build up sufficient back pressure to provide for a uniform feed rate across the width of the die. The extrudate is taken up and attenuated by first pair of nip rolls 7, 7&#39;. As the extrudate leaves the die it is air quenched by means of an air quench manifold 8 which contains ports directed at the extrudate. A hood 9 is provided to remove the gaseous blowing agent which may contain noxious fumes from the atmosphere. First nip rolls 7,7&#39; are operated at a rate from two to 25 times the rate at which the polymer blend is supplied to the lips of die 6 by extruder 4. This serves to break the foam bubbles as they approach the lips of die 6 within the die or immediately as they leave die 6, whereby a foam fibrillated web 10 of the polymer blend is formed. The foam fibrillated web is fed over heated shoe 11 and drawn by second pair of nip rolls 12, 12&#39;. Generally the second pair of nip rolls 12, 12&#39; are driven at a surface speed rate of from two to 10 times the surface speed rate of first pair of nip rolls 7,7&#39; to orient and thereby strengthen foam fibrillated web 10. The thus oriented foam fibrillated web 10 is then taken up on take-up reel 13. 
     In FIG. 2 a reel 14 of foam fibrillated web 15 is fed onto carrier belt 16. A layer of bonding film 17 is fed on top of foam fibrillated web 15 from reel 18. An additional layer of foam fibrillated web 19 is fed from reel 20, supported overhead by means not shown, to lapper 21. Lapper 21 contains a pair of driven nip rolls mounted in a carriage. The nip rolls feed the foam fibrillated web onto bonding film 17 while being moved back and forth across bonding film 17 in the carriage. This results in the foam fibrillated web being laid down at a 45° angle to the machine direction in a double thickness. A second reel 22 feeds a foam fibrillated web 23 through lapper 24 onto lapped foam fibrillated web 21 to form two layers of foam fibrillated web 23 each disposed at 45° to the machine direction. An additional layer of bonding film 25 is laid on top of foam fibrillated web 23 from reel 26. A final layer of foam fibrillated web 27 is fed from reel 28 on top of bonding film 25. The entire lay-up of foam fibrillated webs and bonding film is then removed from carrier belt 16 and fed through heated laminating rolls 29, 29&#39; to form non-woven fabric 30 which is taken up on take-up reel 31. 
     DETAILED DESCRIPTION 
     In preparing the foam fibrillated webs of the present invention several extrusion and drawing techniques may be used. The drawings show the preferred technique. However for instance the extruder may be fed by any of a large number of alternate means including manually from sacks of preblended polystyrene and polypropylene. For small runs a ram-type extruder can be used but obviously it is desired to operate more or less continuously and for this a screw-type extruder is preferred. A slit die has been shown and has been found most convenient for forming relatively narrow width webs of from say 6 inches to 5 feet. For wider webs of say 3 to 20 feet an annular die has obvious advantages. When using such an annular die the web is drawn over a mandrel to maintain or slightly increase its circumference, during orientation. 
     The extruder used may be equipped with a port to inject the blowing agent. If this is done, various blowing agents may be used such as the various Freons, methylene chloride, nitrogen, carbon dioxide, etc. If the extruder is not equipped with a port to inject the blowing agent the blowing agent is fed into the extruder along with the polymer blend. While this can be done by coating the polymer pellets with a low boiling liquid such as pentane which becomes a gas in the extruder it is preferred to blend a solid physically or chemically decomposable blowing agent with the polymers and then to feed the resulting blend into the extruder. Exemplary chemical agents include but are not limited to azobisformamide, azobisisobutyronitrile, diazoaminobenzene, 4,4&#39;-oxybis(benzenesulfonylhydrazide), benzenesulfonylhydrazide, N,N&#39;-dinitrosopentamethylenetetramine, trihydrazinosymtriazine, p,p&#39;-oxybis(benzenesulfonylsemicarbazide)-4-nitrobenzene sulfonic acid hydrazide, beta-naphthalene sulfonic acid hydrazide, diphenyl-4,4&#39;-di(sulfonylazide) sodium bicarbonate and mixtures of materials such as sodium bicarbonate or sodium carbonate with a solid acid such as tartaric acid. The amount of foaming agent to be used in the process generally is in the range of from 0.1 to 20 wt. % of the polymer blend is being extruded with from 0.1 to 5.0 wt. % being the preferred range. 
     The polypropylene used in the present process is isotactic polypropylene having a melt index of below 30 g. Almost any commercial polypropylene plastic is suitable whether it be molding, film or fiber grade. 
     The polystyrene used in the present invention is substantially homopolystyrene although polystyrene containing minor amounts of up to about 10 wt. % of α-methylstyrene is also suitable. The high impact polystyrenes such as those which are a blend of styrene grafted on polybutadiene or the ABS resins are not satisfactory for the present purpose. Generally the polystyrene should have a melt index of from 1 to 30 g. 
     As the polypropylene-polystyrene blend is extruded it is taken up by a take-up means such as a first pair of nip rolls and attenuated about two to 25 times. This attenuation serves to cause the foam bubbles forming within the die to break as the blend approaches the die resulting in a network or web of intertwined and connected fibrils. The temperature of the blend within the extruder is generally maintained at from 175° to 260°C. As the blend approaches the die lips it should be in the range of from 220° to 260°C. As the blend leaves the die lips it is quenched as with an air quench which serves to insure that the polymer blend is below 150°C which causes the foam bubbles which were forming as the pressure imposed on the polymer blend drops as the polymer blend approaches the lips of the die to rupture and form fibrils rather than merely to expand into larger bubbles. After this foam fibrillated web has been formed it is then stretched to orient the individual fibrils which make up the web thereby strengthening the web. When the web is stretched it tends to break at a given torque applied to it. It has now been found that when the polypropylene contains from 2 to 50 wt. % and particularly from 5 to 15 wt. % of polystyrene a substantial increase in the amount of stretching the web undergoes at a given torque occurs. Yet the amount of torque at which the web breaks remains substantially constant (other factors being equal) whether the polystyrene is present or not. This results in enabling a substantial increase in the number of times the web may be drawn or stretched and results in a web having substantially increased strength on a grams/denier basis. Generally the webs are drawn at a moderately elevated temperature. Suitable temperatures are from 90° to 150°C. The webs formed of the polystyrene-polypropylene blend are also considerably superior to webs formed of polypropylene alone with respect to their ability to be bonded to each other. When the blend contains about 15 wt. % polystyrene this bond strength is generally adequate without requiring the presence of additional adhesive. However the strongest webs are formed using from 5 to 15 wt. % polystyrene and 95 to 85 wt. % polypropylene. Therefore it is preferred to use an adhesive. The adhesive can be a liquid which is sprayed, doctored or otherwise applied to whatever webs are to be assembled into a non-woven fabric. Any thermoplastic type adhesive or cross-linkable formulation which softens in the range of from 100° to 175°C can be used. The commercially available ethylene-vinyl acetate copolymer emulsions are particularly satisfactory adhesives which can be applied. The assembly of webs is then laminated together by application of heat and pressure. In an especially preferred aspect of the invention the foam fibrillated webs are adhered together into a non-woven fabric by means of a film of thermoplastic having a softening point in the range of from 100° to 175°C. Particularly satisfactory films are polyethylene films and ethylene-vinyl acetate copolymer films wherein the copolymer contains from 2 to 40 wt. % vinyl acetate. Generally the die used has an opening from 15 to 25 mils in the thickness direction of the extrudate which results in the final oriented foam fibrillated webs weighing from 0.2 to 0.8 ounces per square yard. Generally the total thickness of however many adhesive films are used should be equal to from 0.1 to 1 mil per ounce per square yard of total foam fibrillated webs used in the final non-woven fabric. 
     The final non-woven fabric will normally contain from three to 20 layers. However in the case of a random laid bat a single bat can be used. For most uses such as industrial bagging, primary carpet backing, secondary carpet backing, wallpaper, upholstery backing from five to 10 layers are used and the non-woven fabric product has a weight of from 2.5 to 10 ounces per square yard. There are a plurality of ways in which the layers of webs with or without the adhesive film can be assembled. Often the way in which the webs are assembled is dependent on the use to which the non-woven product is to be put. Usually this involves two to four layers in the machine direction and two to four lapped layers at an angle thereto. However the webs can be run through a tenter frame to increase their width and impart a biaxial disposition to the direction of the individual fibrils within the web in which case all of the webs can be laid down in the machine direction and laminated. 
     For individual laminates of from say 6 inches square up to about 4 ft. × 8 ft. a press can be used to laminate the foam fibrillated webs together. Generally such a press is operated at from 10 to 500 p.s.i. and at 100° to 160°C. For long rolls of the non-woven product heated pressure rolls are used. Generally these are heated metal rolls using steel or coated steel operated at from 2 to 200 lbs. per linear inch pressure, from 100° to 170°C and the material being laminated is fed at a rate of from 10 to 300 feet per minute. The hand, appearance, porosity and other physical characteristics of the non-woven product can be varied considerably by varying the severity of the laminating conditions within the parameters set forth above. Further these characteristics of the product non-woven fabric can be varied by using embossed or textured laminating rolls. If one (or if desired both) laminating rolls (or one surface of a press if such is being used) are covered with burlap or a screen of the appropriate size a non-woven fabric which looks like burlap can readily be obtained. This is a distinct advantage over other non-woven fabrics or even woven slit film in the production of secondary carpet backing where asthetics are important and burlap, which is now in short supply, has been the traditional material used. 
     The foam fibrillated webs of the present invention find uses other than in making non-woven fabrics. For instance a web from one-quarter to ten inches in width can be either twisted or false-twisted to form bailing twine useful as such. Further if desired a plurality of such bailing twines can be twisted to form a rope which approaches a conventional polypropylene fiber rope in properties such as strength even though such rope produced from the foam fibrillated web is considerably less expensive. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A Killian one inch extruder having a 24:1 L/D screw is equipped with an 8 inch wide slit die (Johnson Flex Lip coat hanger type) having a 20 mil thick opening. The slit is offset from the screw by 10 inches and extrudes in the same direction the flow through the extruder barrel. The extruder hopper is continuously filled with the polymer blend reported in Table I. The extruder barrel is maintained at 170°- 230°C from feed end to die end and the die at 230°C. The screw is operated at 25 rpm. Immediately adjacent the die lips is an air quench which is a pair of 0.5 inch diameter pipes one located above the die lips and the other below the die lips containing air under 80 p.s.i. pressure. Each pipe contains a row of .030 inch diameter holes .125 inch apart directed at the die lips. The extrudate is withdrawn from the die lips by a first pair of 5 inch diameter nip rolls 8 inches in width driven at a surface speed of 15 ft./minute to form a foam fibrillated web. These rolls comprise a driven rubber covered roll and a stainless steel idler roll. The foam fibrillated web is then passed over a heated shoe eight inches wide and 36 inches long. The shoe is slightly arched in shape so as to maintain the foam fibrillated web in intimate contact with it. The shoe is maintained at 135°C. The foam fibrillated web is then passed between a second pair of nip rolls identical to the first pair of nip rolls and is then taken up by a take-up reel. In each of the examples reported in Table I the second pair of nip rolls are operated at a constant torque of 60% of full scale which results in the varying strength webs and varying stretch ratios reported. In each case the polypropylene is isotactic polypropylene having a melt index of 10 and the polystyrene has a melt index of 6. In each case the composition being extruded contains 1 wt. % as based on the resin Celogen AZ (azodicarbonamide) as a blowing agent. 
     
                                           TABLE I                                 
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                    Primary                                               
                         Secondary                                        
                               Secondary                                  
     Polypropylene                                                        
             Polystyrene                                                  
                    Speed                                                 
                         Speed Draw Ratio                                 
                                         Strength*                        
Example                                                                   
     %       %      ft./min.                                              
                         ft./min.                                         
                               (Stretch Ratio)                            
                                         g/d                              
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1    100     0      19.5 44    2.26      1.59                             
2    100     0      14.5 35    2.50      1.57                             
3    99      1      19.5 50    2.56      1.40                             
4    98      2      19.5 53    2.72      1.70                             
5    97      3      19.5 58    2.97      2.11                             
6    96      4      19.5 55    2.82      1.70                             
7    95      5      19.5 64    3.28      2.24                             
8    90      10     19.5 65    3.33      2.50                             
9    90      10     14   48    3.43      2.82                             
10   75      25     19.5 67    3.44      --                               
11   75      25     14   46    3.29      2.55                             
12   50      50     19.5 50    2.56      1.22                             
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 *Strength of sample collected at maximum stable stretch ratio under the  
 given conditions and reported in grams per denier                        
 
    
     EXAMPLES 13-17 
     Samples were fabricated from (Example 13) 100% polypropylene; (Example 14) 90% polypropylene-10% polystyrene; (Example 15) 25% polypropylene; (Example 16) 90% polypropylene-10% polystyrene plus one piece of clear 2-mil thick low density polyethylene film; and (Example 17) 90% polypropylene-10% polystyrene plus one piece of approximately 3-mil ethylene-vinyl acetate copolymer film. The samples were fabricated by cutting the stretched foam fibrillated webs into 8 inch lengths and laying two pieces side by side to form a layer, successive layers being in parallel planes and oriented with primary fiber direction at 90° to the adjacent layer. In the case of Example 16 and Example 17 the films were placed in the middle of the sample. The samples were tested by the grab tensile method (Federal Method 5100). Results are given in Table II. All are for six layers of foam fibrillated web. 
     
                       TABLE II                                                    
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            Basis Weight  Grab Tensile                                    
Example     oz./yd..sup.2 Strength/lbs.                                   
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13          2.40          35                                              
14          1.80          40                                              
15          1.86          52                                              
16          3.00          135                                             
17          3.65          174                                             
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