Calendering system using hard and soft nips

Abstract: This invention relates to calendering systems. Such structures of this type, generally, employ the use of hard and soft nips to provide excellent smoothness without gloss mottle.

Description: Field of the Invention This invention relates to calendering systems. Such structures of this type, generally, employ the use of hard or soft nips to provide excellent smoothness without gloss mottle. Description of the Related Art The US Patent 3,982,056 discloses an apparatus for producing a calendered paper with increased smoothness by calendering a coated paper, wherein said apparatus comprises a first deformable calendering roll, a heated metal roll located adjacent to said first calendering roll so as to form a first nip with the first calendering roll, and a second deformable calendering roll located adjacent the heated roll so as to form a second nip with the heated roll. This document further discloses a method producing a calendered paper with increased smoothness, wherein the method comprises the steps of: coating a paper web, passing that coated paper web through a first nip formed between a first deformable calendering roll and a heated metal roll and then passing said coated paper web through a second nip formed between a second deformable calendering roll and said heated roll. According to this document, the first calendering roll is designed as a soft roll and the second calendering roll is designed as a hard roll. US Patent No. 3,124,480 discloses a hot pressure finishing apparatus for web materials. Uncoated or previously coated paper may be conveyed over a plurality of guide rolls to a feed roll and thence in contact with a furnishing platen or backing roll for the application of the particular coating employed. In the arrangement the glossing apparatus may take the form of a driven roll having a highly polished finishing surface and a pair of hard surfaced but resilient backing rolls. The backing rolls may be driven by contact with the web and finishing roll or driven means may be provided. In order to minimize operating pressure loading of the respective rolls, it is desirable that the resilient rolls be quiet hard, i.e. less than 30 P.&J. The transversely and uniformly distributed high intensity unit pressure at the nip is from about 1724 kPa to about 16547 kPa (from about 250 to about 2400 pounds per inch), depending primarily upon the particular coating and the paper web involved. US Patent No. 2,395,992 relates to improved mineral-coated paper, and particularly to high-grate printing paper having high brightness, high gloss and a good affinity for printing ink. The coated web is fed to a supercalendering treatment, using a supercalender composed of alternating metal rolls and yielding rolls whose yielding rolls have a hardness intermediate a metal roll and a cotton roll, of a severity equivalent to the imposition on the paper of a pressure of about 350 kN/m (about 2000 pounds per linear inch) at the top nip of the super calender. US Patent No. 5,237,915 discloses a calender stack having a plurality of hard and soft calender rolls for use in calendering of coated or uncoated paper web which comprises employing a plurality of soft rolls exhibiting at least to distinct levels of hardness and arranging the rolls in a calender such that the level of hardness is progressivly harder in the direction of travel of the coated paper web. It is well known in calendering systems, particularly heated soft roll calendering systems, to employ a soft roll at high pressures. Exemplary of such prior art is U.S. Patent No. 4,624,744 ('744) to J. H. Vreeland, entitled "Method of Finishing Paper Utilizing Substrata Thermal Molding". While the '744 patent does achieve calendering, the use of the high nip pressures, namely, pressures above 13,793 kPa (2000 psi), reduce the bulk of the paper. Consequently, such use of a calendering device is, typically, employed when calendering fine papers. Consequently, a more advantageous calendering system, then, would be employed if calendering could be done at lower nip pressures in order to reduce bulk loss. It is apparent from the above that there exists a need in the art for a calendering system which is able to calender as well as the known calendering systems, while providing excellent smoothness without gloss mottle (an uneven pattern of gloss or reflectance), but at the same time is able to calender at lower nip pressures. It is a purpose of this invention to fulfil this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure. SUMMARY OF THE INVENTION Generally speaking, this invention fulfils these needs by a method of producing a gloss mottle-free calendered paper having increased smoothness, wherein said method comprises the steps of coating a paper web or sheet, calendering said coated paper by passing said coated paper through only two nips, said coated paper being passed first through a first nip formed between a first calendering roll with a hard resiliently yieldable surface and a heated metal roll, and then through a second nip formed between said heated metal roll and a second calendering roll with a soft resiliently yieldable surface, wherein said first calendering roll has a Shore D surface hardness rating of greater than 80 and said second calendering roll has a Shore D surface hardness rating of less than or equal to 80 and operating said method at nip pressures between said first and second nips of substantially less than 13,793 kPa (2000 psi). The apparatus according to the invention comprises a first calendering roll having a hard resiliently yieldable surface, a heated metal roll located adjacent said first calendering roll, a first nip formed between said first calendering roll and said heated metal roll, a second calendering roll located adjacent said heated metal roll, said second calendering roll having a soft resiliently yieldable surface, and a second nip formed between said second calendering roll and said heated metal roll, wherein the apparatus comprises only two nips, the coated paper to be calendered being passed first through the first nip and then through the second nip, and wherein said first calendering roll has a Shore D surface hardness rating of greater than 80 and said second calendering roll has a Shore D surface hardness rating of less than or equal to 80. In certain preferred embodiments, calcium carbonate (CaCO 3 ) is added to the coating placed upon the paper. The coating is applied at a coat weight of approximately 3.63-10.89 kg/278.7m 2 (8-24 lbs/3000 ft 2 ). The coating contains at least 40% solids and at least 30% CaCO 3 . In another further preferred embodiment, the use of the harder-softer roll combination allows one to produce a paper which is substantially gloss mottle-free and has a significantly increased smoothness. The preferred calendering system, according to this invention, offers the following advantages: good stability; good durability; substantially reduced gloss mottle; significantly increased smoothness; reduced operating nip pressures; increased operating capacity; reduced converting problems; and excellent economy. In fact, in many preferred embodiments, these factors of improved gloss mottle, improved smoothness, reduced nip pressures, increased capacity, and reduced converting problems are optimized to an extent that is considerably higher than heretofore achieved in prior, known calendering systems. BRIEF DESCRIPTION OF THE DRAWING The above and other features of the present invention, which will become more apparent as the description proceeds, are best understood by considering the following detailed description in conjunction with the accompanying FIGURE, in which the FIGURE is a schematic illustration of a calendering system using hard and soft rolls, according to the present invention. DETAILED DESCRIPTION OF THE INVENTION As discussed earlier, the '744 patent adequately calenders fine papers, but at higher nip pressures. Typically, these nip pressures are greater than 13,793 kPa (2000 psi) as measured by Equation (1) below as set forth by H. L. Schmidt, Rubber Roll Hardness-Another Look, Pulp and Paper, March 18, 1968, pp 30-32. The Equation (1) is: nip width, n = 4 LTD 1 D 2 E ( D 1 + D 2 ) 1 / m m = exponent which is dependent on roll diameter L = line load (pli) T = thickness of cover cm's (inches) D 1 = diameter of harder roll cm's (inches) D 2 = diameter of softer roll cm's (inches) E = elastic modulus However, in today's modern paper manufacturing machines, it is desirable to run at lower nip pressures, i.e., substantially less than 13,793 kPa (2000 psi). These lower nip pressures reduce bulk loss of the calendered paper and allow paper with greater caliper or thickness to be produced. Using Equation (1), nip pressures in the present invention have been measured from 6,210-9,600 kPa (900 to 1400 psi). Along with reducing bulk loss, there are several other desired qualities that a paper manufacturer wants the paper to achieve after calendering. From past studies, it has been determined that a Parker Print-Surf (a measurement of surface roughness) of 1.0 or less and a gloss (or reflectance) of greater than or equal to 60 based upon a 75° Hunter gloss are currently acceptable parameters for determining whether or not a paper is calendered to achieve the best results. With reference first to the FIGURE, there is illustrated an advantageous environment for use of the concepts of the invention. In particular, as shown in the FIGURE, there is illustrated calendering system 2. System 2, includes in part, harder or backing roll 4 having a hard resiliently yieldable surface, conventionally treated, polished metal roll 6, softer or backing roll 8 having a soft resiliently yieldable surface, conventional paper 10, coating 12, and nips 14 and 16. Harder roll 4, preferably, is any roll constructed of natural or synthetic materials having a surface hardness of greater than 80 shore D measured by conventional techniques. Softer roll 8, preferably, is any suitable roll constructed of natural or synthetic materials having a surface hardness of less than or equal to 80 shore D. Paper substrate 10 of the present invention is coated by coating 12 on at least one side surface and frequently on both sides. The paper trade characterizes a paper web or sheet that has been coated on one side as C1S and C2S if sheet coated on both sides. Compositionally, coating 12 is a fluidized blend of coating clay, calcium carbonate (CaCO 3 ), and/or titanium dioxide with binders and additives which is smoothly applied to the traveling web surface. In particular, CaCO 3 is added to the fluidized blend of minerals such that the CaCO 3 comprises greater than 30% by weight of the minerals. Also, the mixture includes at least 40% by weight of solids in order to reduce gloss mottle and increase smoothness. Coating 12 is applied to paper 10 at a rate of 3.63-10.89 kg/278.7m 2 (8-24 lbs/3000 ft 2 ) by conventional techniques. Preferably, coating 12 is applied by a means of a rod coater, air knife or blade by conventional techniques. The following test results prove the novelty of the present invention and its application as a desired calendering system. Using coated basestock with a starting Parker Print-Surf value of 1.9 and a caliper value of 0.03 cm (.012"), the following results were achieved as shown below in TABLE 1: Load kglcm (pli) Roll Hardness Caliper cm (in) PPS Sheffield Gloss 767 (348) Softer 30.23 1.4 15 61 919/919(417/417) Harder/Softer 30.23 (11.9) 1.2 6 68 767 (348) Harder 3.05 (1.20) 1.1 8 68 ,where PPS = Parker Print-Surf, Softer = Softer roll 8, and Harder = Harder roll 4; where kglcm = kilograms per linear 2.54 cm; and where (pli) = pounds per linear inch. The above data demonstrate a more profound effect of the harder polymer roll (88 Shore D) on the larger scale roughness (Sheffield) than on the fine scale roughness (measured by PPS). There was an obvious visual improvement in surface uniformity of the harder/softer roll combination condition as compared to the harder roll only condition. Using coated basestock with a starting PPS value of 2.4 and a caliper value of 0.28 cm (0.11"), the following results were achieved as shown below in TABLE 2: Load kglcm (pli) Roll Hardness Caliper cm (in) PPS Sheffield Gloss 767 (348) Harder 27.69 (10.9) 1.9 10 64 919/919 (417/417) Harder/Harder 27.18 (10.7) 1.7 10 71 919/919 (417/417) Harder/Softer 27.43(10.8) 1.7 13 71 Again, the harder/softer roll combination provides reduced PPS values and higher gloss values than a single hard roll. Also, the harder/softer roll combination gives better gloss uniformity than the harder/harder roll combination. Based upon the favorable results from TABLE 1 and TABLE 2, calendering system 2 was placed on a conventional papermaking machine. The paper was calendered using a harder roll (Shore D hardness of greater than 80), two softer rolls (Shore D hardness of less than or equal to 80) and the harder/softer roll combination of the present invention. The results of the three runs are shown below in TABLE 3: Roll Hardness PPS Sheffield Gloss Mottle Harder 1.2 N/A 62 Unacceptable Gloss Uniformity Softer/Softer 1.3 6 56 Acceptable Gloss Uniformity Harder/Softer 0.8 4 68 Acceptable Gloss Uniformity Clearly, the use of the harder/softer calendering roll combination creates a paper having a Parker Print-Surf of 1.0 or less, a gloss of greater than or equal to 60, and reduced gloss mottle. Once given the above disclosure, many other features, modifications or improvements will become apparent to the skilled artisan. Such features, modifications or improvements are, therefore, considered to be a part of this invention, the scope of which is to be determined by the following claims.

Claims: A method of producing a gloss mottle-free calendered paper (10) having increased smoothness, wherein said method comprises the steps of: coating a paper web or sheet, calendering said coated paper (10) by passing said coated paper (10) through only two nips (14, 16), said coated paper (10) being passed first through a first nip (14) formed between a first calendering roll (4) with a hard resiliently yieldable surface and a heated metal roll (6), and then through a second nip (16) formed between said heated metal roll (6) and a second calendering roll (8) with a soft resiliently yieldable surface, wherein said first calendering roll (4) has a Shore D surface hardness rating of greater than 80 and said second calendering roll (8) has a Shore D surface hardness rating of less than or equal to 80, operating said method at nip pressures between said first and second nips (14, 16) of substantially less than 13,793 kPa (2000 psi). The method according to claim 1, characterized in, that said gloss mottle-free calendered paper (10) has a Parker Print-Surf rating less than or equal to 1.0. The method according to claim 1, characterized in, that the gloss mottle-free calendered paper (10) has a gloss rating of at least 60. The method according to claim 1, characterized in, that said paper coating (12) comprises particulate minerals. The method according to claim 4, characterized in, that said paper coating (12) comprises at least 40% by weight solids. The method according to claim 5, characterized in, that said particulate minerals comprises at least 30% by weight of calcium carbonate. The method according to claim 4, characterized in, that said paper coating (12) has a coat weight of approximately 3.63-10.89 kg/278.7m 2 (8-24 lbs/3000 ft 2 ). An apparatus for producing a gloss mottle-free calendered paper having increased smoothness by calendering a coated paper, wherein said apparatus comprises: a first calendering roll (4) having a hard resiliently yieldable surface, a heated metal roll (6) located adjacent said first calendering roll (4), a first nip (14) formed between said first calendering roll (4) and said heated metal roll (6), a second calendering roll (8) located adjacent said heated metal roll (6), said second calendering roll (8) having a soft resiliently yieldable surface, and a second nip (16) formed between said second calendering roll (8) and said heated metal roll (6), wherein the apparatus comprises only two nips (14, 16), the coated paper to be calendered being passed first through the first nip (14) and then through the second nip (16), and wherein said first calendering roll (4) has a Shore D surface hardness rating of greater than 80 and said second calendering röll (8) has a Shore D surface hardness rating of less than or equal to 80.

Inventors: NICHOLSON STEPHEN, PARKER STEVEN HERMAN, RASCHELLA DOMENICK LARRY, STAMPFL SHARON RAE, NICHOLSON, STEPHEN, PARKER, STEVEN HERMAN, RASCHELLA, DOMENICK LARRY, STAMPFL, SHARON RAE
Applicants: WESTVACO CORP, WESTVACO CORPORATION
Application year: 1998
Main classification: D21G1
IPC codes: D21G1, D21G9