Abstract:
A high-opacity cellulose-free synthetic paper is formed from a wet-laid nonwoven web of thermoplastic fibers, all or most of which fibers are made of a predetermined polymeric material. The wet-laid web is dried to remove excess water, drying being carried cut at temperatures below the melting temperature of the predetermined polymeric material. The dried nonwoven web is saturated on at least one side with a pigmented binder forming a continuous coating thereon. The binder is cured at temperatures below the melting temperature of the predetermined polymeric material.

Description:
This is a continuation of application Ser. No. 08/004,881 filed on Jan. 19, 1993 abandoned which is a C-I-P of Ser. No. 07/823,525 filed Jan. 21, 1992 abandoned and a C-I-P of Ser. No. 07/916,819 filed Jul. 20, 1992 now U.S. Pat. No. 5,403,444 which is a C-I-P of Ser. No. 07/489,427 filed Mar. 5, 1990 now U.S. Pat. No. 5,133,835. 
    
    
     FIELD OF THE INVENTION 
     This invention generally relates to synthetic paper made on conventional continuous wet-lay papermaking equipment. In particular, the invention relates to recyclable polymeric synthetic paper made of 100% polymeric material. 
     The invention also relates to labels, especially to labels adapted for use in labeling of blow-molded plastic containers. In particular, the label comprises a coated 100% synthetic web prepared by a wet-lay process. The label may be applied either in-mold or post-mold to a blow-molded container made of the same synthetic material as the main synthetic fiber component (for example, polyethylene, polyester or polypropylene) of the label with or without the use of an adhesive material and may be recycled along with the container. 
     BACKGROUND OF THE INVENTION 
     It is conventional practice to make synthetic paper using synthetic pulp comprising short fibers of polyethylene. Such synthetic paper is made using polyethylene pulp with or without cellulose fibers. Such flexible polymeric synthetic substrates are used to make water-resistant cardboard, embossed paper, heat-sealing paper, battery separators, felt mats, hygienic absorbents and building materials. To meet the demands of various applications, many grades of polyethylene have become commercially available. These synthetic pulp products use polyethylenes of different physical properties. Polypropylene and polypropylene/polyethylene products are also known. 
     U.S. Pat. No. 5,047,121 to Kochar discloses a process for making synthetic paper containing at least 97 wt. % polyethylene on conventional continuous wet-lay papermaking equipment. The process includes the steps of: (1) preparing a pulp furnish comprising 97-99.5 wt. % polyethylene fibers and 0.5-3.0 wt. % polyvinyl alcohol binder fibers; (2) depositing the pulp furnish on the screen of a wet-lay papermaking machine to form a waterleaf sheet; (3) drying the resulting waterleaf sheet on heated drying cans having a drying profile wherein an initial drying phase is provided at a temperature between 200° F. and 270° F. to melt the polyvinyl alcohol fibers and a second drying phase is provided at a temperature between 190° F. and 240° F. to control stretch and elongation of the sheets; and (4) thermally bonding the dried sheet at a temperature between 250° F. and 315° F. to provide polyethylene paper. The thermal bonding can be accomplished with a calendar roll. The Kochar patent teaches that: (1) the strength of the synthetic paper can be tailored by varying the amount of polyvinyl alcohol fibers mixed into the polyethylene pulp; and (2) the porosity of the synthetic paper can be tailored by varying the bonding temperature. 
     In accordance with the teaching of the Kochar patent, the polyethylene pulp is fused to a degree dependent on the thermal bonding temperature. This results in a polyethylene paper which is suitable for the specific applications identified in that patent, i.e., filtration applications (e.g., vacuum cleaner bags) and battery separators. However, the low opacity of the resulting paper makes it unsuitable for use in high-quality printing. This is because the application of too much heat for a long duration causes the polyethylene pulp to flow to such a degree that it becomes increasingly translucent as it approaches a polyethylene film in structure. 
     Paper made of 100% synthetic fibers is useful as label paper. For example, the in-mold labeling of blow-molded plastic containers is less costly than conventional labeling methods in which labels with adhesive backing are adhered to the container in a separate step subsequent to blow molding. In-mold labeling eliminates this separate step, thereby reducing labor costs associated with handling of the adhesive-backed labels and capital costs associated with the equipment used to handle and apply adhesive-backed labels. 
     In accordance with conventional in-mold labeling of blow-molded plastic containers, labels are sequentially supplied from a magazine and positioned inside the mold by, for example, a vacuum-operated device. Plastic material is then extruded from a die to form a parison as depicted in FIG. 6 of U.S. Pat. No. 4,986,866 to Ohba et al., the description of which is specifically incorporated by reference herein. The mold is locked to seal the parison and then compressed air is fed from a nozzle to the inside of the parison to perform blow molding wherein the parison is expanded to conform to the inner surface of the mold. Simultaneously with the blow molding, the heat-sealable layer of the label of Ohba et al. is pressed by the outer side of the parison and fused thereto. Finally, the mold is cooled to solidify the molded container and opened to obtain a labeled hollow container. 
     For the sake of efficiency, it is desirable that the labeling of blow-molded containers be conducted continuously and rapidly. Also the labels to be applied during in-mold labeling should be sufficiently stiff that the automatic equipment used to handle the labels does not cause wrinkling or folding thereof. Conversely, the labels must be sufficiently elastic that they neither tear nor separate from the plastic container during flexing or squeezing of the latter. 
     A further disadvantage of conventional in-mold labels prepared from paper is that prior to recycling of the plastic container, the paper label must be removed using either solvent or mechanical means to avoid contamination of the recycled plastic material by small pieces of paper. 
     One prior art attempt to grapple with this recycling problem is disclosed in U.S. Pat. No. 4,837,075 to Dudley, which teaches a coextruded plastic film label for in-mold labeling of blow-molded polyethylene containers. The label comprises a heat-activatable ethylene polymer adhesive layer and a surface printable layer comprising polystyrene. The heat activatable adhesive substrate layer comprises a polyethylene polymer. Pigment or fillers are incorporated in the polystyrene layer to provide a suitable background for printing. An example of a suitable pigment is titanium dioxide and an example of a suitable filler is calcium carbonate. Preferably a layer is interposed between the adhesive substrate and the surface printable layer that comprises reground and recycled thermoplastic material used to prepare such labels. The label stock is prepared by coextrusion of the various label layers utilizing conventional coextrusion techniques. Separately applied adhesive is not employed. 
     The aforementioned patent to Ohba et al. teaches a synthetic label for in-mold labeling of blow-molded resin containers comprising a thermoplastic resin film base layer and a heat-sealable resin layer having a melting point lower than that of the thermoplastic resin base layer. The base layer has an inorganic filler, such as titanium dioxide or calcium carbonate, incorporated therein or incorporated in a latex coating thereon. The base layer may, for example, be high-density polyethylene or polyethylene terephthalate. The heat-sealable resin layer may, for example, be low-density polyethylene. The heat-sealable resin layer serves to firmly adhere the label to a resin container. In accordance with the preferred embodiment of the Ohba et al. label material for use on a blow-molded container made of polyethylene, four separate layers are joined together by coextrusion. 
     U.S. Pat. No. 5,006,394 to Baird teaches a polymeric film structure having a high percentage of fillers, for example, opacifying or whitening agents such as titanium dioxide and calcium carbonate. The fillers are concentrated in a separate filler containing layer coextruded with a base layer. The base layer may comprise polyolefins (for example, polyethylenes), polyesters or nylons. The filler-containing layer may comprise any of the same polymeric materials, but preferably comprises ethylene vinyl acetate coploymer. However, this film material is intended for use in disposable consumer products such as diapers. 
     In addition, U.S. Pat. No. 4,941,947 to Guckert et al. discloses a thermally bonded composite sheet comprising a layer of flash-spun polyethylene plexifilamentary film-fibril strand sheet in face-to-face contact with a layer of polyethylene synthetic pulp suitable for use in bar code printing. The layer of polyethylene synthetic pulp is formed by conventional wet-lay papermaking techniques. 
     The Dudley and Ohba et al. patents both disclose an in-mold label having a multiplicity of layers coextruded together. This complexity of structure raises the costs of manufacturing the respective in-mold label materials. Although there is no suggestion in the Baird patent that the film material disclosed therein would be suitable for use as in-mold label paper, if it were usable for that purpose it would suffer from the same disadvantage of being a relatively complex laminated structure and therefore relatively costly to manufacture. Likewise the patent to Guckert et al. discloses a laminated structure. 
     SUMMARY OF THE INVENTION 
     The present invention improves upon the prior art by providing a flexible polymeric synthetic nonwoven substrate which is suitable for use as lint-free writing paper, labels on plastic bottles, release liner, specialty packaging paper or filter paper. In particular, one preferred embodiment of the invention is a high-opacity polymeric synthetic nonwoven substrate suitable for use in high-quality printing applications. 
     In addition, the polymeric synthetic paper of the invention contains no cellulosic fibers and therefore can be easily recycled without costly procedures for separating polymeric and cellulosic materials. In particular, it is an object of the invention to provide a synthetic paper which does not leave behind any foreign material to be screened out when the paper is melted. 
     The synthetic paper in accordance with the invention can be used as labels on polymeric containers, for example, labels for blow-molded polymeric containers, which need not be removed prior to recycling of the polymeric containers. Such labels sufficiently elastic to withstand flexing and squeezing of the plastic container without tearing or separating therefrom. Also the nonwoven label of the invention is more porous than film labels, which enhances the printability of the label, and is cheaper to manufacture. 
     In accordance with the invention, the synthetic paper comprises a nonwoven web of fibers, at least one side of which has a pigmented coating, e.g., a pigment-containing latex. The paper is manufactured from commercially available fibers. The components may be combined in water into a homogeneous mixture and then formed into a web employing a wet-lay process. 
     In accordance with a first preferred embodiment, the fiber composition of the web is 88-100% polyethylene pulp and 0-12% polyvinyl alcohol binder fibers. In a variation of this embodiment, the web comprises 70-100% polyethylene pulp, 0-12% polyvinyl alcohol binder fibers and 0-30% polypropylene fibers. Polypropylene pulp can be substituted for all or any portion of the polyethylene pulp. 
     In accordance with another preferred embodiment, the fiber composition of the web is 50-90% chopped polyester staple fibers, 10-40% bicomponent polyester/co-polyester core/sheath binder fibers and 0-10% polyvinyl alcohol binder fibers bonded together. Each bicomponent binder fiber comprises a core of polyester surrounded by a co-polyester sheath. 
     In both preferred embodiments, the nonwoven web of fibers is made more printable by saturation with a binder material, for example, with an ethylene vinyl acetate latex or other suitable latex having a glass transition temperature (T g ) of 0-30° C. The latex is preferably compounded to contain pigment such as calcium carbonate, titanium dioxide or both at pigment/binder ratios of 0.5/1 to 8/1, resulting in a synthetic paper having a surface suitable for high-quality printing thereon. However, the use of a latex binder, as opposed to other conventional binders, is not required to practice the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing the production line for making up the stock for use in manufacturing the synthetic paper in accordance with the invention; 
     FIG. 2 is a diagram showing the production line for making synthetic paper in accordance with the invention from the stock make-up output by the apparatus of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In accordance with the invention, synthetic paper is formed from a web of synthetic fibers with no cellulosic fibers. The synthetic fibers may be made of polyethylene, polyester, polypropylene or any other polymeric material suitable for use in high-opacity paper. 
     In accordance with a first preferred embodiment, the web comprises 88-100% polyethylene fibers and 0-12% polyvinyl alcohol fibers and is coated with an ethylene vinyl acetate latex or other suitable latex having a glass transition temperature (T g ) of 0-30° C. and compounded to contain pigment such as calcium carbonate, titanium dioxide, clay, talc or other inorganic pigments as known to those skilled in the art. The coating may contain any conventional binder other than latex. 
     The synthetic paper in accordance with the invention is manufactured from commercially available fibers such as polyethylene pulp, polypropylene pulp, chopped polyester staple fibers and polyvinyl alcohol binder fibers. The components may be combined in water into a homogeneous mixture and then formed into a mat employing a wet-lay process. 
     In accordance with a first example of a polyethylene-based synthetic paper, the starting fiber materials consist of 90 wt. % Mitsui 9400 Fybrel™ polyethylene pulp commercially available in the United States from Minifibers, Route 14, Box 11, Johnson City, Tenn. 37615 and 10 wt. % Kuraray 105-2 polyvinyl alcohol (PVA) binder fibers commercially available in the United States from Itochu Corp., 335 Madison Avenue, New York, N.Y. 10017. In Mitsui 9400 Fybrel™ polyethylene pulp the polyethylene fibers have an average length of 0.90 mm and a diameter of 15 microns. Kuraray 105-2 PVA binder fibers have an average length of 5 mm and a denier of 2.0. 
     In accordance with a second example of a polyethylene-based synthetic paper, the starting fiber material may be 100 wt. % Mitsui 9400 Fybrel™ polyethylene pulp, that is, PVA binder fibers are not essential to practice of the invention. In this embodiment, the polyethylene pulp is entangled during the wet lay process to form the base sheet. Optionally, the base sheet may thereafter be coated with the pigmented binder—avoiding thermal fusion of the polyethylene pulp—to produce a high-opacity synthetic paper having excellent printability. 
     Alternatively, in accordance with a variation of the polyethylene-based synthetic paper, some of the Kuraray 105-2 PVA binder fibers are replaced by 10 mm×2.2 denier Hercules Herculon™ polypropylene staple fibers. These polypropylene staple fibers are commercially available in the United States from Hercules, Inc., 3169 Holcomb Bridge Road, Suite 301, Norcross, Ga. 30071. In accordance with this variation the web is comprised of 70-100% polyethylene fibers, 0-12% PVA fibers and 0-30% polypropylene fibers. One example of this variation successfully made by the inventors had 85% polyethylene fibers, 7.5% PVA fibers and 7.5% polypropylene fibers. 
     In all of the foregoing variations, polypropylene pulp can be substituted for the polyethylene pulp. 
     After the base mat has been dried, it is preferably treated with a coating comprised of a binder, e.g., latex, pigmented with calcium carbonate, titanium dioxide, clay, talc or other inorganic pigment to enhance the printability of the paper. The surface treatment may be applied with any commercially available coater, treater or size press. Thereafter the web can be machine calendared to give the coating a predetermined surface smoothness. 
     In accordance with the preferred embodiment of the coating applied to the above-described webs, the starting coating materials are 50 wt. % Vinac 884 ethylene vinyl acetate latex and 50 wt. % Albagloss calcium carbonate. Alternatively, Airflex 4514 ethylene vinyl acetate/ethylene vinyl chloride copolymer latex can be used in place of the Vinac 884 ethylene vinyl acetate latex, although the latter is preferred. The Vinac 884 and Airflex 4514 latexes are commercially available in the United States from Air Products and Chemicals, Polymers and Chemicals Division, 5100 Tilghman Street, Allentown, Pa. 18104. The Albagloss calcium carbonate is commercially available in the United States from Pfizer, Inc., Minerals, Pigments and Metals Division, 640 North 13th Street, Easton, Pa. 18042-1497.The range of calcium carbonate incorporated in the coating can be varied from a pigment/binder ratio of 0.5/1 to 8/1, although the preferred ratio is 1/1. 
     The synthetic paper in accordance with the invention can be made on standard papermaking equipment. The process for making label paper prepared from a web of polyethylene pulp, PVA binder fibers and polypropylene staple fibers is described hereinafter with reference to FIGS. 1 and 2, which show the stock make-up equipment  8  and the papermaking equipment  10 , respectively. 
     The Fybrel™ 9400 polyethylene pulp is loaded in a fiber opening chest 12 at consistencies between 2% and 5% solids. The pulp is agitated until it is completely dispersed in water and no fiber bundles are apparent. This mixture is then pumped to a blend chest  18  via a deflaker  16 . In the deflaker the fibers are subjected to fiber-to-fiber agitation which removes any fiber bundles or unopened clumps. The def laker is preferable to a disk refiner in that no cutting or shortening of the fibers occurs. 
     At the same time a predetermined amount of Kuraray 105-2 PVA binder fibers and, optionally, a predetermined amount of polypropylene staple fibers are loaded in a fiber opening chest  14  at consistencies between 0.5% and 5% solids in hot water. The PVA binder fibers become gelatinous in hot water. The dispersion is agitated until the staple fibers are completely dispersed in water and no fiber bundles are apparent. This mixture is then pumped into blend chest  18 . Alternatively, no pump is needed if the mixture is dropped by gravity into blend chest  18 . The binder and staple fiber dispersion is added to the furnish so that the PVA binder fibers and the staple fibers make up 0-12 wt. % and 0-30 wt. % of the furnish solids, respectively. The mixture is agitated to achieve a uniform dispersion of the polyethylene pulp, staple fibers and gelatinous PVA having a consistency between 1% and 5% solids. 
     The furnish is then pumped by pump  20  to the refiner  22 , which beats the fibers as needed to reduce their average length. The refined furnish then enters a surge chest  24 , where it is mixed with the broke furnish from broke pulper  26 . 
     Broke is synthetic paper that has been rejected during the process of manufacture. Broke may take the form of either “wet” broke or “dry” broke. Wet broke is synthetic paper taken off the wet press of the paper machine. Dry broke is paper spoiled when passing through the dryers or the calendar, trimmed off in the rewinding of rolls, trimmed from sheet being prepared for shipping or rejected for manufacturing defects. 
     In accordance with the process of the invention, the broke is loaded in the broke pulper  26  at consistencies between 1% and 5% solids. The broke furnish is agitated by high-shear agitator  28  until the broke fibers are completely dispersed in water and no fiber bundles are apparent. The broke furnish is then pumped to surge chest  24  via a deflaker  30  in a controlled manner to maintain consistency and limit the percent of broke addition to not exceed 20% of the total volume. The refined furnish and the broke furnish are mixed in surge chest  24  until a uniform dispersion is achieved. 
     The furnish in surge chest  24  is then pumped via pump  32  into machine chest  34 , which feeds its contents into the forming section while maintaining a constant level in the chest to reduce variation in product weight. The final stock is pumped to the papermaking machine (see FIG. 2) by pump  36 . 
     Before the stock is made into synthetic paper, large contaminants (such as dirt, gravel, pieces of kraft bags, sand and grit) and fiber bundles are removed from the stock by screening in primary and secondary cleaners  38  and  40 . Material containing rejected debris is fed to the secondary cleaners from the primary stage. Rejects from the secondary stage are sewered while accepts are sent back to the main feed stream. This is a way to concentrate the rejects and save fiber. 
     The furnish is supplied to the headbox  42  at consistencies between 0.1% and 1% solids. A web of synthetic fibers is then formed on standard wet-lay papermaking equipment by forming wire  44 . Excess water is removed by gravity and vacuum devices. The formed web is wet-pressed in press section  46  and then dried in the first dryer section  48  at a temperature in the range of 140° F. to 260° F. to remove more water. 
     During drying, the polymeric fibers are not fused, but rather the gelatinous PVA becomes a glue which pre-bonds the polyethylene pulp and staple fibers into a web. (For applications where high strength is not a requirement, PVA is unnecessary. For example, 100% polyethylene pulp entangled by the wet-lay process has adequate strength to be fed to the saturator/coater.) When drying the web, care must be taken to ensure that the web and dryer can temperatures remain below the melting point of the polyethylene fibers, that is, below 269° F. (132° C.). Otherwise the opacity of the synthetic paper will be degraded. The use of release coating on the dryer cans was found to be beneficial in preventing buildup or sticking that will eventually cause web defects and/or breaks. 
     Thereafter the dried web is saturated with ethylene vinyl acetate latex solution containing calcium carbonate pigment. This treatment may be performed on a paper machine size press or any type of off-line coater or treater  50  which is supplied with saturant from mixing chest  52 . The coating is applied to the web in an amount that achieves a 10 wt. % add-on of dried coating solids, that is, 200 lbs/ton, although it will be recognized by the person skilled in the art that the weight percentage of dried coating solids can be varied over a wide range. The coating is then dried in the second dryer section  54 , again at a temperature in the range of 140° F. to 260° F., whereby the ethylene vinyl acetate bonds the fibers to each other and bonds the pigment to the fibers. Excessive heat is to be avoided during saturation because the latex coagulates when exposed to excessive heat, leading to latex build-up on the rolls. After the coating is dried, the coated web is machine calendared in calendar  56  to attain a surface smoothness (Sheffield) of 125-250 units and is then wound on winding reel  58 . 
     The physical properties of synthetic paper made from 90% polyethylene pulp and 10% PVA binder fibers in accordance with the invention are listed in Table I. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Physical Property Test Data 
               
             
          
           
               
                 TAPPI 
                 Physical 
                 Uncoated 
                 Finished 
               
               
                 No. 
                 Property 
                 Base Sheet 
                 Coated Sheet 
               
               
                   
               
               
                 410 
                 Basis Weight (3300 ft 2 ) 
                 45.0 
                 50.0 
               
               
                   
                 (oz./yd 2 ) 
                  2.2 
                 2.4 
               
               
                 411 
                 Caliper (mils) 
                  8.8 
                 8.0 
               
               
                 251 
                 Porosity-Permeability 
                 &lt;0   
                 &lt;0 
               
               
                   
                 Frazier Air (cfm) 
               
               
                 460 
                 Gurley Porosity (sec/100 cc) 
                 10   
                 22 
               
               
                 538 
                 Sheffield Smoothness (T/W) 
                 — 
                 200/260 
               
               
                 403 
                 Mullen Burst (psi) 
                 — 
                 5 
               
               
                 414 
                 Elmendorf Tear (g) (MD/CD) 
                 — 
                 25/31 
               
               
                 511 
                 MIT Fold (MD/CD) 
                 — 
                 2/0 
               
               
                 494 
                 Tensile (lbs/in.) (MD/CD) 
                 4.1/2.4 
                 5.6/2.8 
               
               
                 494 
                 Elongation (%) (MD/CD) 
                 — 
                 4.3/6.5 
               
               
                 494 
                 TEA (ft-lb/ft 2 ) (MD/CD) 
                 — 
                 2.1/1.6 
               
               
                 452 
                 GE Brightness (%) 
                 93.3 
                 93.9 
               
               
                 425 
                 Opacity (%) 
                 97.1 
                 96.6 
               
               
                 413 
                 Ash (%) (500° C.) 
                  0.0 
                 3.0 
               
               
                   
               
             
          
         
       
     
     In accordance with another preferred embodiment of the invention, the web comprises chopped polyester staple fibers, bicomponent polyester/co-polyester core/sheath binder fibers and PVA binder fibers. Each bicomponent binder fiber comprises a core of polyester surrounded by a co-polyester sheath. After the wet-laid sheet has been dried, the dried base sheet is thermal-bonded at a predetermined temperature and a predetermined pressure to bond the fibers on both surfaces of the sheet and impart strength. The sheet is then coated with an ethylene vinyl acetate latex having a glass transition temperature (T g ) of 0-30° C. Again the latex may be compounded to contain pigment such as calcium carbonate, titanium dioxide, clay, talc or other inoraganic pigments at pigment/binder ratios of 0.5/1 to 8/1. Because synthetic paper in accordance with these embodiments has no cellulosic fibers, the synthetic paper may be recycled without going through a separation process. 
     In accordance with a first example of the polyester-based synthetic paper of the invention, the starting fiber materials are 77 wt. % Kuraray polyester chopped strand, 19 wt. % Kuraray N-720 polyester/co-polyester core/sheath binder fibers and 4 wt. % Kuraray 105-2 PVA binder fibers. All of these fibers are commercially available in the United States from Itochu Corp., 335 Madison Avenue, New York, N.Y. 10017. The Kuraray chopped polyester staple fibers have an average length of 10 mm and a denier of 0.4. Kuraray N-720 polyester/co-polyester core/sheath binder fibers have an average length of 10 mm and a denier of 2.0. Kuraray 105-2 PVA binder fibers have an average length of 5 mm and a denier of 2.0. 
     In accordance with a second example of the polyester-based synthetic paper of the invention, the starting fiber materials are 80 wt. % Kuraray polyester chopped strand and 20 wt. % Kuraray N-720 polyester/co-polyester core/sheath binder fibers. No Kuraray 105-2 PVA binder fibers are used. 
     Alternatively, an equal weight percent of Teijin polyester staple fibers having an average length of 5 mm and a denier of 0.5 can be substituted for the Kuraray chopped polyester staple fibers in the polyester-based synthetic paper. In accordance with other variations, an equal weight percent of polyethylene pulp can be substituted for the PVA binder fibers. 
     In accordance with yet another variation, the polyester chopped staple fibers can be combined with either PVA binder fibers or polyester/co-polyester core/sheath binder fibers or with both, but only in an amount sufficient to hold the web together as it is fed to a thermal calendar. The thermal calendar then fuses the polyester chopped staple fibers using rolls heated to temperatures of 360-410° F. (preferably 390° F.) and nip pressures of 40 psi or greater (preferably 50 psi). The resulting base sheet may be optionally coated with pigmented binder as disclosed above. 
     The fiber composition of the polyester-based synthetic paper is not limited to the specific weight percentages of the examples described above. The amount of PVA binder fibers may be varied from 0 to 10 wt. %; the amount of co-polyester/polyester sheath/core binder fibers may be varied from 0 to 40 wt. %; and the amount of polyester staple fibers may be varied from 50 to 90 wt. %. Furthermore, the average length and the denier of the chopped polyester staple fibers may vary from 5 to 12 mm and from 0.4 to 1.5 denier respectively; and the average length and the denier of the co-polyester/polyester sheath/core binder fibers may vary from 5 to 12 mm and from 2.0 to 6.0 denier respectively. 
     In accordance with the coated versions of the second preferred embodiment, the starting coating materials are 50 wt. % Vinac 884 ethylene vinyl acetate latex and 50 wt. % Albagloss calcium carbonate. Alternatively, Airflex 4514 ethylene vinyl acetate/ethylene vinyl chloride copolymer latex can be used in place of the Vinac 884 ethylene vinyl acetate latex, although the latter is preferred. The range of calcium carbonate incorporated in the coating can be varied from a pigment/binder ratio of 0.5/1 to 8/1, although the preferred ratio is 1/1. The glass transition temperature T g  of the ethylene vinyl acetate latex may vary from 0° C. to 30° C. 
     The web material in accordance with the second preferred embodiment can be made on standard papermaking or nonwoven fabric equipment. The polyester cut staple fibers, the polyester/co-polyester core/sheath binder fibers and the polyvinyl alcohol binder fibers are added to water undergoing agitation and containing a predissolved surfactant material, such as Milease T, at a level of 0.5% based on polyester fiber weight. Milease T is commercially available from I.C.I. Americas, Inc. 
     The foregoing fiber components should be added to the blend chest in the following sequence: (1) polyvinyl alcohol binder fibers, (2) polyester/co-polyester core/sheath binder fibers and (3) chopped polyester staple fibers. The consistency of the mixture in the blend chest should be between 0.5 and 2.5% solids. An anionic polyacrylamide such as 87P061 may be added at levels in the range 0.5-8.0 lbs/ton based on fiber weight to aid in fiber dispersion. 87P061 is commercially available from Nalco Chemical. The mixture is then agitated to attain a uniform dispersion of all materials. The refining step and broke recovery can be bypassed for the second preferred embodiment. 
     The resulting furnish is then formed on standard wet-lay papermaking equipment at headbox consistencies of 0.7-0.01%. The wet-laid material is then dried in the dryer section. 
     The dried web is calendared between smooth metal rolls heated to a temperature of 196° C. The web is calendared at minimal pressure, that is, 50-150 PLI, to achieve bonding of the surface fibers while maintaining the degree of opacity of the original sheet. 
     This material is then ready to be treated with the ethylene vinyl acetate latex solution pigmented with calcium carbonate. As noted above, the treatment may be applied on a paper machine size press or any type of off-line coater or saturator. The coating is applied in a manner that results in a 10 wt. % add-on of dried coating solids, that is, 200 lbs/ton. The coating is then dried. After the coating is dried, the coated web is supercalendared to attain a surface smoothness (Sheffield) of 125-250 units. 
     The physical properties of the label paper in accordance with the first example of the second preferred embodiment of the invention are listed in Table II. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 Physical Property Test Data 
               
             
          
           
               
                   
                   
                 Un- 
                   
                   
               
               
                   
                   
                 coated 
                 Thermally 
                 Finished 
               
               
                 TAPPI 
                 Physical 
                 Base 
                 Bonded 
                 Coated 
               
               
                 No. 
                 Property 
                 Sheet 
                 Sheet 
                 Sheet 
               
               
                   
               
               
                 410 
                 Basis Weight (3300 ft 2 ) 
                 45.0 
                 45.0 
                 51.3 
               
               
                 411 
                 Caliper (mils) 
                 15.6 
                 4.8 
                 7.9 
               
               
                 251 
                 Porosity-Permeability 
                 192 
                 13 
                 38 
               
               
                   
                 Frazier Air (cfm) 
               
               
                 451 
                 Taber V-5 Stiffness 
                 1.9/1.4 
                 1.1/0.9 
                 4.2/2.5 
               
               
                   
                 (gcm) (MD/CD) 
               
               
                 403 
                 Mullen Burst (psi) 
                 13 
                 126 
                 183 
               
               
                 414 
                 Elmendorf Tear (g) 
                 233/261 
                 229/168 
                 184/138 
               
               
                   
                 (MD/CD) 
               
               
                 511 
                 MIT Fold (MD/CD) 
                 3/6 
                 2500+/2500+ 
                 2500+/2500+ 
               
               
                 494 
                 Tensile (lbs/in.) 
                 4.7/4.6 
                 25.0/25.0 
                 33.2/43.2 
               
               
                   
                 (MD/CD) 
               
               
                 494 
                 Elongation (%) (MD/CD) 
                 1.4/2.2 
                 11.2/10.7 
                 12.3/15.8 
               
               
                 494 
                 TEA (ft-lb/ft 2 ) (MD/CD) 
                 0.7/1.3 
                 32.9/32.1 
                 40.4/72.9 
               
               
                 452 
                 GE Brightness (%) 
                 82.5 
                 86.9 
                 85.6 
               
               
                 425 
                 Opacity (%) 
                 69.0 
                 74.2 
                 76.5 
               
               
                   
               
             
          
         
       
     
     Tests were conducted to determine the effect of PVA binder level on the strength of the synthetic paper made from polyethylene pulp. The results of those tests are shown in Table III. The results show that the tear and tensile strengths of the synthetic paper are better at a 7.5 wt. % PVA binder fiber level than at 4 or 11 wt. %. 
     
       
         
               
             
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE III 
               
             
             
               
                   
               
               
                 Effect of Polyvinyl Alcohol Level 
               
             
          
           
               
                   
                 Physical 
                 PVA Level 
               
             
          
           
               
                   
                 Property 
                 4% 
                 7.5% 
                 11% 
               
               
                   
                   
               
               
                   
                 Basis Weight (GMS/m 2 ) 
                 77 
                 78 
                 72 
               
               
                   
                 Caliper (mils) 
                 7.6 
                 7.8 
                 7.7 
               
               
                   
                 Gurley Porosity (sec/100 cc) 
                 24 
                 19 
                 16 
               
               
                   
                 Mullen Burst (psi) 
                 10 
                 11 
                 6 
               
               
                   
                 Elmendorf Tear (g) (MD/CD) 
                 39/51 
                 45/51 
                 37/45 
               
               
                   
                 MIT Fold (MD/CD) 
                 16/3  
                 23/10 
                 11/4  
               
               
                   
                 Tensile (lbs/in.) (MD/CD) 
                 5.3/3.8 
                 6.3/4.1 
                 5.3/3.1 
               
               
                   
                 GE Brightness (%) 
                 95.2 
                 95.0 
                 94.3 
               
               
                   
                 Opacity (%) 
                 93.9 
                 93.5 
                 91.9 
               
               
                   
                   
               
             
          
         
       
     
     Tabe IV shows the effect of adding a 10-mm-long polypropylene staple fiber to the furnish. The three samples tested had the following compositions: (A) 90% Mitsui 9400 polyethylene pulp, 10% PVA binder fiber and 0% staple fiber; (B) 90% Mitsui 9400 polyethylene pulp, 0% PVA binder fiber and 10% staple fiber; and (C) 85% Mitsui 9400 polyethylene pulp, 7.5% PVA binder fiber and 7.5% staple fiber. Tear strength is improved as the result of adding staple fiber and the improvement is maximized when a binder fiber is included. Porosity increases as the level of higher-diameter fiber (the binder fiber and the staple fiber) increases. This is one way in which sheet porosity can be controlled when designing synthetic papers for applications where either minimal porosity or a specific level of porosity is required. 
     
       
         
               
             
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE IV 
               
             
             
               
                   
               
               
                 Effect of Staple Fiber Addition 
               
             
          
           
               
                   
                 Physical 
                 Sample 
               
             
          
           
               
                   
                 Property 
                 A 
                 B 
                 C 
               
               
                   
                   
               
               
                   
                 Basis Weight (GMS/m 2 ) 
                 67 
                 85 
                 67 
               
               
                   
                 Caliper (mils) 
                  9 
                 11 
                  9 
               
               
                   
                 Porosity (sec/100 cc) 
                 18 
                 12 
                 13 
               
               
                   
                 Tear Strength (g) (MD/CD) 
                 26/30 
                 39/39 
                 51/55 
               
               
                   
                   
               
             
          
         
       
     
     Table V shows the effect of coating or size press applications of a binder. The main effect being designed to is the surface strength so that the web can be printed on without the surface being damaged from the tacky ink on the printing plate. The IGT number shows the improvement when a coating is applied. (IGT is a standard laboratory printing test wherein if the material is weak in the direction perpendicular to the sheet, it will pull apart or large sections of the surface will be pulled out.) A carefully formulated coating can also decrease porosity. Stiffness can be increased or left unchanged by careful selection of the binder. 
     Thus, in accordance with the invention the porosity of the synthetic paper can be controlled by carefully adjusting the coating formulation and by adjusting the amount of staple fibers. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE V 
               
             
             
               
                   
               
               
                 Effect of Coating 
               
             
          
           
               
                   
                 Physical 
                 Uncoated 
                 Finished 
               
               
                   
                 Property 
                 Base Sheet 
                 Coated Sheet 
               
               
                   
                   
               
               
                   
                 Basis Weight (GSM) 
                 60 
                  80 
               
               
                   
                 Caliper (mils) 
                  6 
                  7 
               
               
                   
                 Mullen Burst (psi) 
                  4 
                  9 
               
               
                   
                 Tensile (lbs/in.) (MD/CD) 
                 3.5/2.5 
                 6/5 
               
               
                   
                 Gurley Porosity (sec/100 cc) 
                 13 
                  22 
               
               
                   
                 Brightness (%) 
                 95 
                  95 
               
               
                   
                 Opacity (%) 
                 93 
                  93 
               
               
                   
                 IGT 
                  0 
                 115 
               
               
                   
                 Elongation (%) 
                 6/8 
                 13/15 
               
               
                   
                 Gurley Stiffness (mgf) (MD/CD) 
                 28/20 
                 35/35 
               
               
                   
                   
               
             
          
         
       
     
     The synthetic paper of the invention can be used in labeling of blow-molded plastic containers. In particular, the label may be applied either in-mold or post-mold to a blow-molded container made of the same synthetic material as the main synthetic fiber component (for example, polyethylene, polyester or polypropylene) of the label with or without the use of an adhesive material and may be recycled along with the container. 
     In accordance with conventional in-mold labeling of blow-molded plastic containers, labels are sequentially supplied from a magazine and positioned inside the mold by, for example, a vacuum-operated device. Plastic material is then extruded from a die to form a parison as depicted in FIG. 6 of U.S. Pat. No. 4,986,866 to Ohba et al., the description of which is specifically incorporated by reference herein. The mold is locked to seal the parison and then compressed air is fed from a nozzle to the inside of the parison to perform blow molding wherein the parison is expanded to conform to the inner surface of the mold. Simultaneously with the blow molding, the heat-sealable layer of the label of Ohba et al. is pressed by the outer side of the parison and fused thereto. Finally, the mold is cooled to solidify the molded container and opened to obtain a labeled hollow container. 
     A disadvantage of conventional in-mold labels prepared from paper is that prior to recycling of the plastic container, the paper label must be removed using either solvent or mechanical means to avoid contamination of the recycled plastic material by small pieces of paper. 
     Although the invention has been described with reference to certain preferred embodiments, it will be appreciated that it would be obvious to one of ordinary skill in the art of fiber technology and papermaking that other polymeric fibers could be used to achieve the same beneficial results. In particular, fibers other than polyethylene pulp and polyester chopped staple fibers can be used as the main fiber component. For example, polyester pulp could be used in place of polyester chopped staple fibers in the event that polyester pulp becomes commercially available. Further, suitable polymeric fibers having a melting point lower than that of the main fiber component can be substituted for PVA binder fibers. For example, polyethylene pulp could be used in place of PVA binder fibers in the polyester-based synthetic paper. Nor is the invention limited to the use of a specific coating binder: suitable coating binders other than ethylene vinyl acetate latex and ethylene vinyl acetate/ethylene vinyl chloride copolymer latex can be used. Also it would be obvious to one of ordinary skill that the preferred embodiments could be readily modified to meet specific conditions not disclosed here. All such variations and modifications are intended to be within the scope and spirit of the invention as defined in the claims appended hereto.