FIBROUS TAPE BASE MATERIAL

Abstract: A method for manufacturing a stretchable fibrous tape base material having surfaces with improved smoothness comprises the steps of forming a wet fibrous base (24), partially dewatering the web (24) to between about 30% and 50% solids, conveying the partially dewatered web (24) on a smooth-surfaced roll (40, 140, 24) to a compression nip defined by the smooth-surfaced roll (40, 140, 240) and a smooth-surfaced fabric material (55, 155, 255), moving the fabric material (55, 155, 255) at a speed of about 10% to about 20% less than the surface speed of the smooth-surfaced roll (40, 140, 240), compressing the web (24) in the nip by forcing the fabric material (55, 155, 255) against the web (24) on the smooth-­ surfaced roll (40, 140, 240) to directly transfer the web (24) from the roll (40, 140, 240) to the material (55, 155, 255), the fabric material (55, 155, 255) being compressed against the web (24) with a compression force of less than 15 lbs/linear inch or an average pressure of less than about 50 psi in the compression nip, the web (24) being also compressed in the machine direction by the difference in speed between the fabric material (55, 155, 255) and the sur­ face of the roll (40, 140, 240), and drying the web (24), the dried web (24) having an apparent bulk less than .20 caliper pts/lb ream of 3000 sq. ft. Preferably, the dried web (24) has a caliper of less than .006 inch.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a fibrous sheet material having improved surface smoothness and improved stretch characteristics and a method and apparatus for manufacturing it. Description of the Related Art Fibrous sheet base materials are conventionally used in tapes such as masking tape. In such tapes, an adhesive is applied to one side of the sheet of fibrous base material while a nonstick coating is applied to the opposite side. The coated sheet is then wound and cut in rolls. In many applications, it is important that the tape be stretchable. For example, when masking tape is used to mask portions of an automobile being painted, the tape must be capable of stretching so that the tape may be smoothly applied along curved or irregular edges. It is also important that such tapes have relatively smooth surfaces so as to minimize the amounts of adhesive and nonstick coating required to cover the tape, and to provide improved tape performance. For example, when tape is used for masking during painting operations, it is desirable that the side of the tape adhering to the object being painted be as smooth as possible in order to prevent paint seepage beneath the edge of the tape. Making the opposite nonstick surface smooth facilitates prolonged use of the tape because a smooth outer surface will not irritate the fingers and thumbs of workmen applying the tape for prolonged periods, thus helping prevent the "sore thumb" problem experienced by many car painters. Stretchability in tapes has been conventionally achieved by creping the tape base material while it is still in a wet state as it is being produced on a paper machine. Conventional creping of masking tapes is described in United States Patent No. 2,941,661 issued to Picard et al. In such creped tapes, creping is performed on a wet press roll with a thin and sharp creping blade so as to impart about 16 to 24 crepe lines per cm (40 to 60 crepe lines per linear inch) and to provide rounded crepe ridges on one side and a flat appearing reverse side. This creped structure is retained during subsequent drying of the tape base. However, this creping process is difficult to control and presents many problems in maintaining product uniformity and productivity. Additionally, a creped sheet by its very nature is not smooth and the best that can be hoped for is to make the crepes as fine, uniform and even as possible. Improvement in stretch of paper webs has been achieved with the apparatus and process for treating web material disclosed in U.S. Patent No. 4,551,199 issued to Weldon . According to Weldon , bulking and creping of a paper web is achieved by transporting a web into a differential velocity nip defined by a web support surface and an open mesh fabric pick-up member having voids therein. The pick-up member has a relative velocity slower than that of the support surface at the nip location. When the web is applied to the fabric pick-up member, the web is impressed into the voids of the fabric so as to emboss the web. Also, as the web approaches the nip, a deceleration of the web occurs due to the slower moving fabric filaments of the pick-up member causing the web to collapse on itself one or more times to form crepe folds. The succeeding folds in the web press against the earlier folds, pushing them into the voids of the fabric. The size and number of folds are determined by the flexibility of the web and the magnitude of the relative velocity differential between the pick-up fabric and the transport member support surface. Web material produced as described in Weldon has surface undulations and a caliper greater than is desirable for tape base materials. For fibre web sheets of a given thickness, as the amplitude of surface undulations decreases, the density of the sheet increases. Accordingly, apparent bulk, which is the inverse of sheet density, decreases as the amplitude of the undulations decreases. For a tape base material, it is advantageous for the caliper of the tape base material to be relatively thin, that is less than 0.2 mm (0.008"). Sheets of this thickness produced according to the process disclosed in Weldon have an apparent bulk which will give a thickness of 0.01 mm to a sheet having a weight of 1.63 g/m² (that is greater than 0.4 cal pts/lb ream) which is indicative of folding and bulking much greater than what is desirable for tape base materials. Accordingly, it is an object of the present invention to improve upon known apparatus and methods of manufacturing fibrous web products to produce a tape base material having significantly improved smoothness, which results in a lower apparent bulk, while at the same time maintaining the desired stretch characteristics and thin caliper that are important for tape base materials. The above-discussed Weldon US-A-4 551 199 provides the basis for the prior art portions of claims 1 and 10. The method of the invention for manufacturing a stretchable fibrous tape base material having surfaces with improved smoothness is defined in claim 1 and comprises the steps of forming a wet fibrous base, partially dewatering the web to between about 30% and 50% solids, conveying the partially dewatered web on a smooth-surfaced roll to a compression nip defined by the smooth-surfaced roll and a smooth-surfaced fabric material, moving the fabric material at a speed of about 10 to 20% less than the surface speed of the smooth-surfaced roll, compressing the web in the nip by forcing the fabric material against the web on the smooth-surfaced roll to transfer directly the web from the roll to the material, the fabric material being compressed against the web with a compression force of less than 2.68 kg/cm (15 lbs/linear in) or an average pressure of less than 3.4 bar (50 psi) in the compression nip, the web being also compressed in the machine direction by the difference in speed between the fabric material and the surface of the roll, and drying the web, the dried web having an apparent bulk giving a thickness of less than 0.005 mm to a sheet having a weight of 1.63 g/m² an apparent bulk less than 0.20 caliper pts/lb ream of 3000 ft²). Preferably, the dried web has a caliper of less than 0.15 mm (0.006"). The method may further include the step of passing the dried web through a one nip calender with a nip pressure in the range of 7.15 to 10.7 kg/cm (40 to 60 lbs/linear in). In the method of the preferred embodiment, the fabric material has a first surface facing the smooth-surfaced roll, the first surface being shaved to reduce roughness. It is preferred that the fabric be a 5-shed (1 x 4) broken twill weave with a 33 x 33 /cm² (84 x 84 /in²) mesh. The apparatus of the invention is defined in claim 10 and comprises means for forming a wet fibrous web, dewatering means for partially dewatering the web to between about 30 and 50% solids, a smooth-surfaced roll having an outer circumferential surface for conveying the dewatering web at a predetermined speed to a transfer station, a sheet of fabric material for receiving the web at the web transfer station and for conveying the web from the transfer station at a speed less than the predetermined speed, the sheet of fabric material being substantially tangential to a point on the outer circumferential surface of the smooth surfaced roll at the transfer station, compression means acting with a compression force of less than 0.98 kg/cm (5.5 lbs/linear in) on the web through the sheet of fabric material for transferring the web from the smooth-surfaced roll to the sheet of fabric material, and for compressing the web to adjust the apparent bulk of the web, and means for drying the web after compression, the dried web having an apparent bulk giving a thickness of less than 0.005 mm to a sheet having a weight of 1.63 g/m² (an apparent bulk of less than 0.20 caliper pts/lb ream). The apparatus may further comprise a one nip calender for receiving the web from the drying means, the one nip calendar having a nip pressure in the range of 7.15 to 10.7 kg/cm (40 to 60 lbs/linear in). Preferably, the dewatering means comprises felt-loop means for receiving the wet web from the forming means and the generally smooth-surfaced roll defines a first compression nip with the felt-loop means. In the preferred embodiment, the smooth-surfaced roll defines a second compression nip with the sheet of fabric material at the web transfer station. The fabric may be a 5-shed (1 x 4) broken twill weave with a 33 x 33 /cm² (84 x 84 /in²) mesh. It is preferred that the compression means comprise a back-up roller covered with a soft elastic material, the back-up roller abutting a second surface of the sheet of fabric material opposite from the first surface at the second compression nip. The preferred embodiment may further comprise back-up roller adjusting means for moving the back-up roller toward and away from the smooth-surfaced roll to adjust the compression of the fabric material against the web and smooth-surfaced roll, the first surface of the sheet of fabric material being tangential to the surface of the smooth-surfaced roll at the second compression nip when the back-up roller is adjusted away from the smooth-surfaced roll. The fabric material may be supported between first and second support rollers on opposite sides of the transfer station, the first and second support rollers being aligned to support the fabric material at a tangent to the outer circumferential surface of the smooth-surfaced roll at the second compression nip. In another preferred embodiment of the invention, the apparatus may comprise means for forming a wet fibrous web, dewatering means for partially dewatering the web to between about 30% and 50% solids, smooth-surfaced means for conveying the dewatered web at a predetermined speed to an embossing station, fabric conveying means for supporting the web at the embossing station and for conveying the web from the embossing station at a speed less than the predetermined speed, embossing means acting on the web through the fabric conveying means at the embossing station, for forming a three-dimensional surface on one face of the web, and means for drying the web after embossing. The product of the invention is a sheet of fibrous web material comprising contacting fibres bonded together, the sheet having opposite smooth surfaces, an apparent bulk with a caliper reading in the range of about 0.0025 mm (0.10 mil) to about 0.005 mm (0.20 mil) where the sheet has a weight of 1.63 g/m² (1 lb/3000 ft²) and a machine direction stretch of at least 8%, and wherein surface undulations on the opposite smooth surfaces of the sheets have an amplitude variance of less than 0.005 mm (0.0002"). It is preferred that the sheet has a machine direction tensile strength of at least 2.86 kg/cm (16 lbs/in) and that the sheet has a caliper of less than 0.15 mm (0.006"). More particularly, the product of the invention may be in a masking tape comprising a sheet of fibrous material having first and second surfaces, the first surface having an adhesive coating and the second surface having a nonstick coating, the improvement comprising a fibrous sheet having an apparent bulk giving a caliper reading in the range of about 0.0025 mm (0.10 mil) to about 0.005 mm (0.20 mil) where the sheet has a weight of 1.63 g/m² (1 lb/3000 ft²) and a machine direction stretch of at least 8%, and wherein surface undulations on the first and second surfaces of the sheet have an amplitude variance of less than 0.005 mm (0.0002"). Brief Description of the Drawings The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate the embodiments of the invention, and together with the description, serve to explain the principles of the invention. Fig. 1 is a schematic side elevation view of a portion of an apparatus of the invention used for forming a wet fibrous web. Fig. 2 is a schematic side elevation view of one embodiment of the apparatus of the invention used for forming the fibrous web of the invention. Fig. 3 is a schematic side elevation view of another embodiment of the apparatus of the invention used for forming the fibrous web of the invention. Fig. 4 is an enlarged view of the transfer station portion of the embodiment shown in Fig. 2. Fig. 5 is an enlarged view of the transfer station portion of the embodiment shown in Fig. 3. Fig. 6 is a photo-microtome of the fabric material used in the embodiments of the invention shown in Figs. 4 and 5. Fig. 7 is a cross section of the fabric material shown in Fig. 6. Fig. 8 is a schematic side elevation view of a portion of the apparatus of the invention for treating the dried fibrous web. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. The Apparatus The apparatus of the invention for manufacturing a stretchable fibrous base web having opposite surfaces with improved surface smoothness and reduced apparent bulk comprises means for forming a wet fibrous web. As depicted schematically in Fig. 1, the forming means includes a head box 10 for containing a supply of fiber furnish 12 which generally comprises a dilute slurry of fiber and water. Head box 10 includes slice 14 disposed over the moving surface of a condenser 16, which in this embodiment is a foraminous woven wire such as a Fourdrinier wire. The fiber furnish in head box 10 issues from slice 14 onto the upper surface or flight 17 of wire 16. Wire 16 moves in a continuous loop around breast roll 18 and wire turning rolls 20 and 22. One or more rolls including rolls 18, 20 and 22, are driven to move wire 16 around its loop. Additional guide rolls (not shown) may be used to support and/or drive wire 16. Water is removed from the furnish disposed on wire 16 forming a wet web 24. One or more vacumm boxes, deflectors and hydro-foils (not shown) may be employed along upper flight 17 of wire 16 to assist in removal of water from web 24 during its formation. Wet web 24, proximate the end of upper flight 17 of wire 16, is transferred to pick-up felt 26 which is lightly pressed into engagement with web 24 on wire 16 by means of upper couch roll 28. Transfer of web 24 to felt 26 may be accomplished or assisted by other means such as an air knife or vacumm box (not shown), both means being well known. Felt 26 moves in the same direction as wire 16 in a continuous path around upper couch roll 28 and press roll 30. One or more guide rolls (not shown) may also be used to support felt 26. At least one roll supporting felt 26 is driven to move felt 26 at a speed preferably substantially the same as wire 16. A guide board and showers (not shown) may be employed adjacent the surface of felt 26 to clean and condition the felt prior to pick up of web 24 as is well known. The apparatus includes dewatering means for partially dewatering the web to between about 30% and about 50% solids. As embodied and depicted in Fig. 2, the dewatering means includes felt loop 32 for receiving the wet web 24 from felt 26. Felt loop 32 is supported on a plurality of turning rolls 34 so as to rotate the felt in the direction indicated by arrow 36. Felt 32 passes through a first compression nip formed between press roll 38 and smooth-surfaced transfer roll 40. Web 24 passes through a nip 42 formed between press roll 38 and transfer roll 40. In nip 42, web 24 is compressed between the felt on press roll 38 and smooth-surfaced transfer roll 40 so as to dewater web 24 to between 30% and 50% solids. Preferably, web 24 is dewatered to about 40% solids. Moisture removed from web 24 at nip 42 is transferred by felt loop 32 and is removed from the felt by a ringer (not shown) or other well known conventional means. The apparatus includes a smooth-surfaced roll having an outer circumferential surface for conveying the dewatered web at a predetermined speed to a transfer station. Preferably, smooth-surfaced roll 40 has an outer circumferential surface 44 for transporting dewatered web 24 to transfer station 46, as depicted in Fig. 2. Smooth-surfaced transfer roll 40 is driven at a fixed predetermined speed. The dewatering means may be otherwise embodied as shown in Fig. 3. In this embodiment web 24 is transferred on a felt or wire loop 116 from a web forming tank similar to head box 10 of Fig. 1. Rolls 122 and 124 press the web to dewater it to between 30% and 50% solids. Dewatered web 24 then is drawn between a rubber roll 126 and a smooth-surfaced roll 140 whereby web 24 conforms to the surface of roll 140 to be conveyed by rotation to transfer station 146. The apparatus includes a sheet of fabric material for receiving the web at the web transfer station and for conveying the web from the transfer station at a speed less than the predetermined speed, the sheet of the fabric material being substantially tangential to a point on the outer circumferential surface of the smooth-surfaced roll at the transfer station. As embodied herein, a sheet of fabric material 55 is part of a continuous fabric loop 56 mounted on support rollers 52 and 54 on opposite sides of transfer station 46 and on a plurality of guide rolls 62 and a dryer 60. Stretch rollers 58 may be employed to maintain the desired tension in fabric loop 56. Showers 64 may be employed to clean fabric 55 of fabric loop 56 upon each rotation of the loop and vacumm 66 may be employed for removing excess shower fluid from fabric 55. In the embodiment of the invention shown in Fig. 3, fabric material 155 is part of a fabric loop 156 mounted on turning rollers 152 and 154 on opposite sides of transfer station 146 and on guide rollers 162. A stretch roller 158 may be employed to maintain a desired tension in fabric loop 156. According to a preferred embodiment of the apparatus the sheet of fabric material has a first smooth and porous surface facing the smooth-surfaced roll at the transfer station. Preferably, the fabric material is comprised of a 5-shed (1x4) broken twill weave with an 33x33 per square cm (84x84 per square inch) mesh of polyester threads 65 having a diameter less than 0.18 mm (.007 inch) wherein each of the threads is spaced within 0.15 mm (.006) of a generally parallel thread. A photo-microtome of the fabric material described above is shown in Fig. 6. Fig. 7 shows a cross section of the fabric shown in Fig. 6. In a preferred embodiment of the apparatus of the invention, the fabric material moves at a speed of about 10% to about 20% less than the surface speed of the smooth-surfaced roll. The fabric material is subjected to a machine direction tension in which the tension in the fabric entering the transfer station is greater than the tension in the fabric leaving the transfer station. Preferably the speed differential between the fabric and the surface of the smooth surfaced roll is about 14% to 16% where Because fabric loop 56 moves at a slower speed than surface 44 of smooth-surfaced roll 40, tension in the portion of the fabric loop 70 between the turning roller 54 and transfer station 46, as shown in Fig. 4, will be greater than tension in the portion of the fabric loop 72 between turning roller 52 and transfer station 46. Depending on the elasticity of fabric 55 in fabric loop 56, fabric 55 undergoes delongation as it passes through transfer station 46 because of the differential tensions. Specifically, the length of voids 63 in fabric 55 are shortened in the machine direction as fabric 55 passes through transfer station 46. Such delongation has a "pinching" effect on web 24 as the web exits from transfer station 46. This pinching may contribute to the unique stretch characteristics of the finished web material while also helping fabric 55 of fabric loop 56 to firmly hold web 24 as it exits from the transfer station. The apparatus includes compression means acting with a compression force of less than 2.68 kg/cm (15 lbs/linear inch) with an average pressure of less than 3.4 bar (50 psi) in the compression nip on the web through the sheet of fabric material for transferring the web from the smooth-surfaced roll to the sheet of fabric material. An enlarged view of the transfer station portion of the apparatus of Fig. 2 is shown in Fig. 4 and a similar enlarged view of the transfer station portion of the apparatus of Fig. 3 is shown in Fig. 5. Because the apparatus shown in Fig. 5 is very similar to the apparatus shown in Fig. 4, except that the two apparatuses operate in opposite directions, Figs. 4 and 5 will be described together. Preferably, the compression means comprises a back-up roller 74, 174 covered with a soft eleastic material 76, 176. Back-up roller 76, 176 abuts a second surface 73, 173 of the sheet of fabric material opposite from the first surface 75, 175 and a second compression nip defined by surface 44, 144 of smooth-surfaced roll 40, 140 and first surface 75, 175 of fabric material 55, 155. The compression menad preferably includes back-up roller adjusting means for moving the back-up roller toward and away from the smooth-surfaced roll to adjust the compression of the fabric material against the web and smooth-surfaced roll. Preferably, the first surface of the fabric material is tangential to the surface of the smooth-surfaced roll at the second compression nip when the back-up roller is adjusted away from the smooth-surfaced roll. As embodied herein, the adjusting means comprises a powered cylinder 78, 178 for moving back-up roll 74, 174 toward or away from smooth-surfaced roll 40, 140. Turning rollers 52, 152 and 54, 154 support fabric 55, 155 on opposite sides of back-up roll 74, 174 so that the surfaces of back-up rolls 52, 152 and 54, 154 and surface 44, 144 of smooth-surfaced roll 40, 140 are aligned in a straight line at points A, B and C, as shown in Figs. 4 and 5, to support fabric 55, 155 at a tanget to outer surface 44, 144 of smooth-surfaced roll 40, 140 at second compression nip 80, 180. Because points A, B and C are aligned, turning rolls 52, 152 and 54, 154 do not add to the compression force applied to smooth-surfaced roll 40. Similarly, because of the alignment of points A, B, and C, back-up roll 76, 176 does not have to overcome any tension or forces in the fabric 55, 155 acting against back-up roll 76, 176. Accordingly, back-up roll 76, 176 can be controlled to apply the desired compression force of less than 2.68 kg/cm (15 lbs/linear inch) against the smooth-surfaced roll without having to overcome opposing forces in fabric 55, 155. Preferably, the fabric is pressed against the web with a compression force of less than 0.89 kg/cm (5 lbs/linear inch) with an average pressure of less than 1.4 bar (20 psi) in the compression nip. Because the compression force can be so exactly controlled, a compression force can be applied to the web that is sufficient to cause the web to transfer from smooth-surfaced roll 40, 140 to fabric 55, 155 without causing web 24 to be embossed by fabric 56, 156. According to preferred embodiments of the apparatus of the invention, the back-up roller has an outer circumferential surface pattern with raised cross hatching. As embodied herein the surface of the back-up roller is made of a soft rubber material so that it compresses easily and applies only a minimal pressure to the fabric and web so as to provide a nip with a maximum area of contact between the fabric and the fibrous web. It is preferred that the cover has a raised cross hatch pattern on it in which the area of the raised pattern is about 35% of the total area of outer circumferential surface 77,177 of roller 76,176. This cross hatch pattern makes the roller surface more readily compressible. However, a smooth surfaced roll of suitable softness can also be employed. The apparatus further includes means for drying the web after compression. Preferably the drying means results in a dried web having an apparent bulk giving a caliper thickness of less than 0.005 mm (0.20 mil) where the sheet has a weight of 1.63 g/m² (1 lb/3000 ft²). As shown in Figures 2 and 3, drying means may comprise conventional dryers 60,160 as are well known. In Figure 2, fabric loop 56 is shown transporting the wet web to the first of the dryers 60, while in the embodiment of Figure 3, the wet web is removed from fabric loop 56 by an air knife 153 and a sheet threader 151, as are well known, so that the web can be conveyed to the first of dryers 160. In accordance with a preferred embodiment of the apparatus of the invention, the apparatus may include a one nip calender for receiving the web from the drying means, with the one nip calendar having a nip pressure in the range of 7.15 to 10.7 kg/cm (40 to 60 lbs/linear in). As shown in Figure 8, dried web 24 is passed through a light one nip calendar 90, having press rolls 92 and 94, too provide for smoothness and caliper uniformity. Preferably, the calendar applies a nip pressure of about 8.9 kg/cm (50 lbs/linear in). The Method The method of the invention comprises a series of steps including the step of forming a wet fibrous web. Preferably, a dilute slurry of fiber and water is deposited on a flat, moving, foraminous surface, such as a felt or Fourdriner wire, to form a wet web of fibers which is subsequently transferred to another moving felt. The fibers are preferably lignocellulosic but may also be other known synthetic fibers. In accordance with the method of the invention, the wet fibrous web is partially dewatered to between about 30% and about 50% solids. In the preferred embodiment, a lignocellulosic web is pressed in a nip defined by rotating rolls. Water removed from the web is retained by the felt on which the web is conveyed. Other known methods may be used for forming a wet fibrous web and for partially dewatering the wet web to the required percentage of solids. Conventional wet compressing techniques are well known in the paper making industry. Partially dewatering the web enables the compression step to provide the desired final web characteristics and makes for more energy efficient drying of the prepared web material. However, it is important to note that the web remains 70-50% wet after dewatering. Over-drying of the web through pressing could substantially reduce the effectiveness of the subsequent manufacturing steps for producing a stretchable tape base material. In accordance with the invention, the method includes conveying the partially dewatered web on a smooth-surfaced roll to a compression nip defined by the smooth-surfaced roll and a smooth-surfaced fabric material. The fabric material is moved at a speed about 10% to about 20% less than the surface speed of the smooth-surfaced roll. In the presently preferred embodiment, the speed differential is between about 14 and 16% where The speed differential causes microcrimping within the web fibers without causing the exterior surfaces of the web to become rough or undulated. This microcrimping and microkinking of the fabric fibres contributes to the stretch characteristics of the finished web product. In the embodiment of the invention shown in Fig. 2, the web is conveyed directly from felt 32 to smooth-surfaced roll 40 and then to fabric 56. Direct transfer of the web permits constant support of the web throughout the forming process. In the embodiment of the invention shown in Fig. 3, the web is self-supported beween the dewatering rollers 122, 124 and smooth-surfaced roll 140. The method includes the step of compressing the web in the compression nip by forcing the fabric material against the web on the smooth-surfaced roll to directly transfer the web from the roll to the material, the fabric material being compressed against the web with a compression force of less than 2.68 kg/cm (15 lbs/linear inch), or an average pressure of less than about 3.4 bar (50 psi) in the compression nip, the web being further compressed by the difference in speed between the fabric material and the surfaces of the roll. The smooth-surfaced fabric material has a first surface facing the smooth-surfaced roll at the compression nip, that is preferably shaved to reduce roughness. Because the fabric material is comprised of fine woven polyester threads, and because the compression force is less than 2.68 kg/cm (15 lbs/linear inch), the surface of the web is not embossed in the compression nip. The applied compression force of 2.68 kg/cm (15 lbs/linear inch) or average pressure of 3.4 bar (50 psi) in the compression nip is great enough to smooth the surfaces of the web and the compression nip and to cause the web to be transferred from the transfer roller to the fabric material, but not so great as to compress the web to an extent that might reduce stretchability in the finished web product. Further, only selected areas of the fabric material are forced against the web by the compression force in order to minimize the pressure of the fabric against the web so as to minimize fabric embedment into the web which in turn helps achieve a smoother web surface. According to a preferred method of the invention, the fabric material is subjected to a tension on opposite sides of the compression nip, the tension of the fabric material entering the nip being greater than the tension of the fabric material exiting the nip. As embodied herein, the greater speed of the surface of the smooth-surfaced roll as compared to the fabric causes the tension in the fabric entering the nip to be greater than the tension in the fabric leaving the nip. Depending on the elasticity of the fabric, the length of the voids in the fabric are shortened in the machine direction as the fabric passes through the nip. This shortening has a "pinching" effect on the web which may contribute to the properties of the fibrous web product produced by this method. The "pinching" effect also helps the fabric to hold onto the web. The method further includes the step of drying the web, the dried web having an apparent bulk giving a caliper thickness of less than 0.005 mm (0.20 mil) where the sheet has a weight of 1.63 g/m² (1 lb/3000 ft²). Preferably, the web is dried with a Yankee dryer to which it is transferred and dried in a known manner. According to a preferred embodiment of the invention, the method may include the step of passing the dried web through a one nip calendar with a nip pressure in the range of 7.15 to 10.7 kg/cm (40 to 60 lbs/linear in). It is preferred that the dried web be passed through a light one nip calendar with a nip pressure of 8.9 kg/cm (50 lbs/linear in) to provide web smoothness and caliper uniformity. The finished dried web product should have a caliper of less than 0.15 mm (0.006"). The Product The method and apparatus of the invention produces a fibrous web material having unique physical characteristics that are advantageous for use as a base material for a masking tape. Specifically, the product comprises a sheet of fibrous material comprising contacting fibres bonded together, a sheet having opposite smooth surfaces, an apparent bulk giving a caliper thickness in the range of about 0.0025 mm (0.10 mil) to about 0.005 mm (0.20 mil) where the sheet has a weight of 1.63 g/m² (1 lb/3000 ft²) and a machine direction stretch of at least 8%, wherein surface undulations on the opposite smooth surfaces of the sheet have an amplitude variance of less than 0.005 mm (0.0002"). It is further preferred that the fibrous sheet has a machine direction tensile strength of at least 2.14 kg/cm (12 lbs/in) and that the caliper of the sheet be less than 0.015 mm (0.006"). The product of the present invention has the advantage that it is stretchable and that it has improved smoothness on its opposite surfaces. The fibrous web product is also strong and thin enough for use in a masking tape. The product of this embodiment has a smooth first surface to which adhesive can be affixed and an opposite smooth second surface to which a nonstick coating can be applied to produce a tape with better masking and sealing capabilities than conventional tapes. In Table I, a fibrous tape base product sample produced according to an embodiment of the invention is compared with fibrous tape base samples of conventional creped tape bases. Sample A is an example of a product produced with apparatus and method described with regard to Figures 2 to 5. Samples G and H are examples of conventional creped tape base webs. The unique balance of characteristics in the product of the present invention can be seen when this product is compared with the conventional tape base samples G and H in Table I. Characteristic A (2685-3C) G (2400-3) H (2539-2) P Hg" Vacuum NA NA NA % s Speed Differential 14 NA 20 (crepes) Basis Weight g/m² (lb/3000 ft²) 47.8 (29.4) 42.5 (26.1) 44.1 (27.1) Caliper mm (0.001") 0.099 (3.9) 0.17 (6.6) 0.17 (6.6) Apparent Bulk caliper pts mm (mil) 0.0034 (0.133) 0.0064 (0.252) 0.0062 (0.243) MD Tensile kg/cm (lb/in) 2.81 (15.7) 1.93 (10.8) 2.31 (12.9) CD Tensile kg/cm (lb/in) 1.16 (6.5) 1.23 (6.9) 1.41 (7.9) MD Stretch, % 8.0 13.9 16.5 Undulations Fabric Side mm (mil) 0.00376 (0.148) 0.0145 (0.571) 0.0128 (0.502) Undulations Opposite Side mm (mil) 0.00254 (0.100) 0.0131 (0.517) 0.0106 (0.416)

Claims: A method of manufacturing a stretchable fibrous tape base material, comprising: forming a wet fibrous web (24); partially dewatering said web; conveying the partially dewatered web on a smooth-surfaced roll (40;140) to a compression nip (46;146) defined by the smooth-surfaced roll and a fabric material (55;155); moving said fabric material at a speed less than the surface speed of said smooth-surfaced roll (40;140); compressing said web (24) in said nip (46;146) by forcing said fabric material (55;155) against said web (24) on said smooth-surfaced roll (40;140) to transfer directly said web from said roll to said fabric material, said web being further compressed in the machine direction by the difference in speed between said fabric material and the surface of said roll; and drying said web, characterised in that said web (24) is partially dewatered to between 30% and 50% solids, the fabric material (55;155) is smooth-surfaced, is moved with a surface speed of 10% to 20% less than the surface speed of the smooth-surfaced roll (40;140), and is compressed against the web (24) with a compression force of less than 2.68 kg/cm (15 lbs/in) and with an average pressure of less than 3.4 bar (50 psi) in the compression nip (46;146), to give the dried web an apparent bulk less than that giving a thickness of less than 0.005 mm to a sheet having a weight of 1.63 g/m² (0.20 caliper pts/lb ream). A method according to claim 1, wherein said fabric material (55) is passed linearly through said compression nip (46). A method according to claim 1 or 2, wherein said fabric material (55) is pressed against said web (24) with a compression force of less than 0.89 kg/cm (5 lbs/linear in) and with an average pressure of less than 1.4 bar (20 psi) in the compression nip; and wherein the fabric material (55) is subjected to a tension on opposite sides of the compression nip (46), the tension of the fabric material (55) entering said nip (46) being greater than the tension on the fabric material (55) exiting said nip (46). A method according to claim 1, 2 or 3, which is so carried out as to provide the dried web with a caliper thickness of less than 0.15 mm (0.006"). A method according to any preceding claim, wherein the dried web (24) is passed through a one nip calender (90) with a nip pressure in the range of 7.15 to 10.7 kg/cm (40 to 60 lbs/linear in). A method according to any preceding claim, wherein the fabric material comprises woven polyester threads having a diameter less than 0.18 mm (0.007"). A method according to claim 6, wherein the polyester threads are woven in the configuration in which each of the threads is spaced within 0.15 mm (0.006") of a generally parallel thread. A method according to any one of claims 1 to 6, wherein said fabric is a 5-shed (1 x 4) broken twill weave with a 33 x 33 /cm² (84 x 84 /in²) mesh. A method according to any preceding claim, wherein the fibrous web (24) comprises wood fibres. Apparatus for manufacturing a stretchable fibrous base web comprising means for forming a wet fibrous web (24), dewatering means (32) for partially dewatering said web (24), a smooth-surfaced roller (40) having an outer circumferential surface for conveying the dewatered web at a predetermined speed to a transfer station (46), means (52,54) supporting a sheet of fabric material (55) for receiving the web (24) at the web transfer station (46) and for conveying the web from the transfer station at a speed less than said predetermined speed, compression means (50) for applying a compression force to the transfer station (46) on said web (24) through said sheet (55) of fabric material for transferring the web from the smooth-surfaced roll (40) to said sheet (55) of fabric material and means (60) for drying said web (24) after compression, characterised in that the dewatering means (32) is adapted partially to dewater the web (24) to between 30% and 50% solids, the means for supporting the sheet of fabric material comprises first and second support rollers (52,54) positioned on opposite machine direction sides of said transfer station (46), said support rollers (52,54) being disposed along a linear path corresponding to a line passing through the transfer station (46), said line being tangential to the outer circumferential surface of the first and second support rollers (52,54) and the outer circumferential surface of the smooth surfaced roll (40), the sheet (55) of fabric material being supported along said linear path between the first and second support rollers (52,54) substantially coincident with said line, and the compression means (50) is adapted to apply compressive force at the transfer station of less than 2.68 kg/cm (15 lbs/in) on said web (24) through the sheet (55) of fabric material, the surface of the sheet (55) of fabric material engaging with the web (24) being substantially smooth. Apparatus according to claim 10, wherein a one nip calendar (90) is provided for receiving the web (24) from the drying means, said one nip calendar (90) having a nip pressure in the range of 7.15 to 10.7 kg/cm (40 to 60 lbs/linear in). Apparatus according to claim 10 or 11, wherein the dewatering means comprises a felt-loop (32) for receiving said wet web (24) from the forming means (10,12,14,17,26) with the generally smooth surfaced roll (40) defining a first compression nip with said felt loop (32). Apparatus according to claim 12, wherein the smooth-surfaced roll (40) defines a second compression nip with said sheet (55) of fabric material at said transfer station (46). Apparatus according to claim 13, wherein the sheet (55) of fabric material has a first smooth and porous surface facing the smooth surfaced roll (40) at said second compression nip. Apparatus according to claim 13 or 14, wherein said compression means comprises a back-up roller (74) covered with a soft elastic material (76), said back-up roller abutting a second surface of said sheet (55) of fabric material opposite from said first surface at the second compression nip. Apparatus according to claim 15, further comprising back-up roller adjusting means (78) for moving the back-up roller towards and away from said smooth-surfaced roll (40) to adjust the compression of said fabric material (55) against said web (24) and smooth-surfaced roll (40), said first surface of said sheet of fabric material being tangential to the surface of the smooth-surfaced roll (40) at the second compression nip when said back-up roller is adjusted away from said smooth surfaced roll. Apparatus according to claim 16, wherein said back-up roller (74,76) has an outer circumferential surface (77) patterned with raised cross-hatching, the area of said raised cross-hatched portions of said back-up roller outer circumferential surface including about 35% of the outer circumferential surface of said roller. Apparatus according to any one of claims 10 to 17, wherein the sheet (55) of fabric material is comprised of woven polyester threads having a diameter of less than 0.18 mm (0.007"). Apparatus according to claim 18, wherein the polyester threads are woven in the configuration in which each of said threads is spaced within 0.15 mm (0.006") of a generally parallel thread. Apparatus according to any one of claims 10 to 19, wherein said fabric is a 5-shed (1 x 4) broken twill weave with a 33 x 33 /cm² (84 x 84 /in²) mesh. Apparatus according to any one of claims 10 to 20, wherein means (52,54) are provided for subjecting said fabric material (55) to a machine direction tension, said tension being greater in the fabric material entering the second compression nip than in the fabric material leaving said second compression nip. Apparatus according to any one of claims 10 to 21, including means (74-78) for adjusting the compression of said fabric material (55) against said web (24) and smooth-surfaced roll (40) to a positive value of less than 0.89 kg/cm (5 lbs/linear in). A sheet of fibrous web material comprising contacting fibres bonded together, said sheet having opposite smooth surfaces, an apparent bulk in the range giving a thickness of between 0.0025 and 0.005 mm in a sheet having a weight of 1.63 g/m² (0.10 to about 0.20 caliper pts/lb ream) and a machine direction stretch of at least 8%, and wherein surface undulations on the opposite smooth surface of said sheet have an amplitude variance of less than 0.005 mm (0.0002 inch). A sheet according to claim 23, wherein said sheet has a machine direction tensile strength of at least 2.14 kg/cm (12 lb/inch). A sheet according to claim 23 or 24, wherein the caliper of said sheet is less than 0.15 mm (0.006 inch). A sheet according to claim 23, 24 or 25, which is in the form of masking tape with a first of said surfaces having an adhesive coating and the second surface having a nonstick coating.

Inventors: KLOWAK, BERNARD GEORGE
Applicants: JAMES RIVER CORPORATION
Application year: 1988
Main classification: D21H27
IPC codes: D21F9, D21H27, D21F1, D21F7, C09J7, D21H25, D21F11