Background
Barriers to microbial penetration are an important and essential property of materials used to package medical devices and other items that must be sterilized. Materials currently used for medical packaging include various films, flash spun polyolefin nonwovens, and medical grade paper. For sterilization with gases or plasmas (e.g. ethylene oxide,Etc.) the contents of a package, which typically comprises a film (e.g., a thermoformed film), a substrate (bottomweb) forming a heat-seal to a porous and breathable closure (e.g., paper or flash-spun polyolefin sheet). Alternatively, the package may be in the form of a pouch comprising a porous layer heat sealed to a film. The porous cover or layer must allow the sterilant gas or plasma to enter and exit the package to sterilize its contents while providing a barrier to microbial penetration to maintain the sterility of the medical device or other item packaged therein until use.
The microbial barrier properties of porous fibrous sheets depend on average pore size, sheet thickness, fiber size, fiber morphology, and the like. The porous microbial barrier sheet prevents the penetration of microbial spores and particles ranging in size from submicron to several microns. The ability of the porous sheet to prevent bacterial penetration was measured by its Log Reduction Value (LRV). The higher the LRV value, the stronger the material's ability to prevent microorganisms from penetrating the package. For example, when the quantification (BW) is from about 1.65oz/yd2Increased to 2.2oz/yd2(55.9g/m2To 74.6g/m2) The LRV of the flash spun polyolefin sheet for medical packaging is about 3.2-5.5 or higher. Medical grade papers known in the art typically have an LRV of about 1 to 3, depending on their basis weight, pore size, additive treatment, etc., and are much less effective as a microbial barrier than flash spun materials. One unit increase in LRV corresponds to a 10-fold increase in microbial barrier efficacy. Although natural cellulose pulp papers have improved over the years of use in medical packaging, their barrier and mechanical properties are still insufficient compared to papers made of synthetic materials.
U.S. patent application publication No. 2003/0177909 (Koslow) describes an air filtration media containing nanofibers. Nanofiber coatings may be used to enhance the performance of the filter media. The nanofibers are preferably fibrillated nanofibers. In one embodiment, the filter media is prepared from a blend of fibrillated fibers and glass microfibers.
U.S. patent application publication No. 2005/0142973 (Bletsos et al) describes a porous fibrous sheet comprising nanofibers or nanofibers and wood pulp, which has excellent microbial barrier properties. Some such sheets have a LVR of at least 5.5.
Generally, the use of nanofibers and increasing their content can improve the barrier properties of nonwoven fabrics. It is desirable to improve barrier performance in a cost effective manner, for example by using only cheap and naturally renewable raw materials. There remains a need for wood pulp paper for medical packaging that has improved microbial barrier properties that is economical compared to microbial barrier sheets constructed primarily from synthetic materials.
Summary of The Invention
One embodiment of the invention is a paper having a Log Reduction Value (LRV) of at least about 3.6 and a density of at least about 0.80g/cm3A basis weight of at least about 55g/m2The paper is composed of wood pulp fiber componentAnd wherein the wood pulp is a refined pulp having a Canadian Standard freeness of less than about 360ml and a weight weighted average length of less than about 3 mm.
Another embodiment of the present invention is a multi-ply paper structure wherein at least one ply is the above-described paper.
Detailed Description
The present invention relates to paper composed of a wood pulp fiber component. The paper has improved barrier properties under substantially the same conditions as the basis weight of known wood pulp paper. Certain wood pulp papers of the present invention are useful as microbial barrier materials, such as closures (lidding) for medical packaging. Certain wood pulp papers of the present invention are also suitable for use in air filtration and liquid filtration.
As used herein, "wood pulp" refers to the product obtained by boiling wood chips with lye or solutions of acid or neutral salts and then bleaching with chloride, the treatment aiming to more or less completely remove the hemicelluloses and lignin encrustations from the wood.
The paper type used in medical packaging varies with fiber density, porosity, various treatments, additives, and basis weight. Medical paper is bleached and highly refined, and is prepared by a conventional wet-laid (wet-laid) process using raw wood pulp. The basis weight of the pre-formed paper used in the art is about 1.4oz/yd2(49g/m2)～2.9oz/yd2(98g/m2). Kraft paper is a special type of paper commonly used for medical packaging. It is made from kraft pulp by a process that involves cooking (digesting) wood chips in an alkaline solution for several hours, during which the chemicals attack the lignin in the wood. The dissolved lignin is subsequently removed, leaving the cellulose fibers. Unbleached kraft pulp is brownish black in color and therefore must be subjected to a series of bleaching processes before it can be used in a variety of papermaking applications.
The inventors have surprisingly found that the use of wood pulp having a specific combination of wood pulp freeness and pulp fiber length to produce paper having a defined apparent density and basis weight results in a paper with a significantly higher LRV than wood pulp papers known in the art. The wood pulp based paper of the present invention has an LRV of at least 3.6, or at least 4, and can be up to at least 5.
The wood pulp used to make the paper of the present invention has a Canadian Standard Freeness (CSF) of less than about 360ml, preferably less than about 300ml, more preferably less than about 250ml, and a weight weighted average length of less than about 3mm, preferably less than about 1.2 mm. The paper of the present invention has an apparent density of at least about 0.80g/cm3. In one embodiment of the invention, the paper has a density greater than about 0.80g/cm3And less than about 0.98g/cm3. The paper of the invention is prepared by methods known in the art, for example by wet-laid processes to prepare an as-formed paper, preferably followed by calendering of the as-formed paper. The paper generally has a Gurley Air Resistance (Gurley) of no more than about 250 seconds, and even no more than about 50 seconds.
The wood pulp used to make the paper of the present invention may be unbleached or bleached, hardwood pulp or a combination of hardwood and softwood pulps. In a preferred embodiment of the invention, the wood pulp is bleached.
The raw formed paper of the present invention can be made by methods known in the art, for example, on a fourdrinier, inclined wire, or cylinder machine. Preferably, the dry raw formed paper of the present invention has a basis weight of at least about 55g/m2Preferably about 60g/m2～120g/m2More preferably about 80g/m2～100g/m2。
In one embodiment, the paper of the present invention may comprise one or more layers, each layer having the same or different wood pulp characteristics. For example, one layer may be formed from hardwood pulp and another layer may be formed from softwood pulp. Alternatively, the CSF and/or weight weighted average length of the pulp in one layer may be different from the pulp used in another layer. The fibrous or pulp component of the paper of the present invention can consist entirely of wood pulp, or consist essentially of wood pulp. The term "consisting essentially of" as used herein means that a small amount (less than about 10% by weight) of additional components is allowed to be added. These additional components may include secondary fibers, such as organic or inorganic staple fibers of at least 0.7 denier, which may be blended with wood pulp. Examples of suitable fibers that can be blended with wood pulp include viscose, polyester, polyamide, carbon, and the like. Other additional components, including but not limited to powders, flakes, and pigments, can be added to the paper composition using methods known in the art, typically in an amount of about 1 to 10 weight percent.
To achieve the desired density, the paper of the present invention may be calendered after it is formed by methods known in the art. Calendering can be carried out in-line immediately after the paper forming step, or as a separate step, at room or elevated temperature, with or without steam or other plasticizers. In the calendering process a hard nip (metal-metal) calender or a soft nip (metal-paper pulp roll or metal-coated roll) calender with one or several nips can be used. The calendering conditions (nip pressure, line speed, temperature, etc.) are selected to achieve the desired paper density and LRV using methods known in the art. Typically, the paper is smooth calendered, but an engraved or other roll may be used in the calendering process.
Multi-ply paper can be prepared in which the high LRV paper of the invention is compounded with other plies. For example, in a three-ply paper, an inner layer of the high LRV paper of the present invention is sandwiched between two outer layers of plain paper. For example, the outer layer may comprise wood pulp paper made from relatively long fiber pulp (e.g., fibers having a weight weighted average length of at least about 3mm, or from about 3mm to 6 mm). One or more of the layers comprising longer pulp fibers may further contain up to about 70 weight percent of synthetic fibers, such as polyamide fibers (e.g., nylon 6, 6) or polyester (e.g., polyethylene terephthalate) fibers. The synthetic fiber may have a denier per filament of about 0.7 or more. The layers of a multi-layer paper can be deposited sequentially during the paper forming process and then calendered. Alternatively, individual paper layers that have been previously formed are combined to form a layered structure, which can subsequently be calendered. In the latter embodiment, the paper plies may be calendered separately prior to forming the layered structure and then after combining the multi-layer structure. The use of longer fibrous layers in a multilayer structure can improve the mechanical properties of the paper, such as tear strength, etc.
The paper is particularly suitable for use in medical packaging. For example, after medical devices or other items to be sterilized are placed in the cavity formed by the thermoformed film, a closure assembly comprising the paper of the present invention can be heat sealed to the second component of the thermoformed film. The heat-seal layer may be extruded or coated over the entire heat-seal surface of the closure, or it may be extruded or coated only onto the areas where sealing with the thermoformed film is desired (known in the art as zone coating), or it may be extruded or coated onto the thermoformed film.
Test method
In the following non-limiting examples, the following test methods were employed to determine various reported characteristics and properties. ASTM refers to the American society for testing and materials. TAPPI refers to the pulp and paper technology association.
The caliper and basis weight of the paper were determined according to ASTM D645 and ASTM D646, respectively. Caliper measurements were used to calculate the apparent density of the paper.
The apparent density of the paper was measured according to ASTM D202.
The Gurley permeability (Gurley) of the paper was determined by the following method: air permeability in seconds (in cm) of a circular area of about 6.4 square centimeters of paper per 100 ml of cylinder displacement was measured according to TAPPI T460 using a pressure differential of 1.22 kPa.
The Barrier (Barrier) Log Reduction Value (LRV) of paper is a measure of the bacterial Barrier properties of paper sheets, as determined by ASTM F1608. In the test, the front side of the paper sheet was contacted with a fine mist of an aerosol containing spores of the black variety of Bacillus subtilis. The pressure on the back side of the paper web was reduced to achieve a flow of air through the paper web of 2.8 lpm. The number of spores reaching both sides of the paper sheet in 15 minutes was counted. LRV ═ Log (number of spores on the front face) -Log (number of spores on the back face). The number of spores on the front face may vary with the particular test, but must be at least 1 million.
Canadian Standard Freeness (CSF) of the pulp is a measure of the rate at which a dilute suspension of pulp can be discharged, as measured by TAPPI test method T227.
The fiber lengths (arithmetic mean length, weight-weighted mean length, and length-weighted mean length) were measured according to TAPPI test method T271 using a fiber mass analyzer (manufactured by OpTest Equipment Inc).
Examples
Comparative example A and example 1
These examples demonstrate the effect of calendering on the LRV of paper
For comparative example a and example 1, 5.0g (dry weight basis) of hardwood Pulp and about 1600g of water were placed in a laboratory mixer (British push Evaluation Apparatus, available from Mavis engineering ltd., london, england) and stirred for 3 minutes. The hardwood pulp had a CSF of 351ml, an arithmetic mean length of 0.36mm, a length weighted mean length of 0.83mm and a weight weighted mean length of 1.25 mm.
The dispersion and 8 liters of water were poured into a handsheet mold of about 21cm by 21cm to form a wet laid web.
The wet-laid web was placed between two blotters, hand-rolled with a rolling pin, and dried in a handsheet dryer at 150 ℃.
The Hardwood pulp used was Hawesville Hardwood, which is a fully bleached kraft pulp composed of southern Hardwood (supplied by Weyerhaeuser).
Comparative example a is raw formed paper. The paper of example 1 was made by passing the as-formed paper additionally through the nip of a metal-metal calender having a roll diameter of about 20cm (herein referred to as "hard calendering" or "hard") at a temperature of about 23 ℃ and a linear pressure of about 2600N/cm.
The properties of the paper are shown in table 1 below. Comparative example a shows that the apparent density of the as-formed paper is too low to achieve the desired LRV, while hard calendering increases the apparent density of the paper of example 1 to achieve the desired LRV increase.
Comparative examples B to C and example 2
These papers were prepared as described in comparative example A and example 1 above, except that hardwood pulp having a CSF of 246ml, an arithmetic mean length of 0.36mm, a length weighted mean length of 0.80mm and a weight weighted mean length of 1.08mm was used.
Comparative example B is raw formed paper. Example 2 was made from raw formed paper by hard calendering.
Comparative example C was prepared by passing a sample of the as-formed paper through the nip (referred to herein as "soft calendering" or "soft") of a BF Perkins soft nip calender (metal roll diameter of about 28cm, encapsulated soft roll diameter of about 23cm) at a temperature of about 23 ℃ and a line pressure of about 2000N/cm.
The properties of the paper are shown in table 1 below. Although the CSF of the papers of comparative examples B to C falls within the scope of the present invention, the density of the paper is low and thus the desired LRV level cannot be achieved. Even in comparative example C, which was soft calendered.
Comparative examples D to E
Samples of comparative examples D to E, comparative example D being virgin formed paper and comparative example E being hard calendered paper, were prepared as described in comparative example A and example 1 above, except that softwood pulp having a CSF of 360ml, an arithmetic mean length of 1.02mm, a length weighted mean length of 2.85mm and a weight weighted mean length of 3.74mm was used.
Softwood pulp is Port Wentworth Softwood, a southern bleached Softwood kraft pulp supplied by Weyerhaeuser.
The properties of the paper are shown in table 1 below. These examples describe the use of softwood pulp (rather than hardwood pulp) for making wood pulp paper having a CFS that falls within the scope of the invention, but a weight weighted average length that is outside the scope of the invention. In addition, even though comparative example E was "hard", the desired LRV was not achieved.
Comparative examples F to G
These examples describe the preparation of wood pulp paper using softwood pulp having a weight weighted average length outside the scope of the present invention.
Samples of comparative examples F-G were prepared as described above for comparative examples D-E, respectively, using the same softwood pulp but refined to a CSF of 210ml, an arithmetic average length of 0.84mm, a length weighted average length of 2.74mm, and a weight weighted average length of 3.74 mm.
The properties of the paper are shown in table 1 below.
Example 3
This example describes the preparation of a hardnip calendered paper of the present invention from hardwood pulp.
A Paper sample of example 3 was prepared as described in example 1, but southern bleached hardwood kraft pulp (supplied by International Paper Company) was used refined to a CSF of 104ml, an arithmetic average length of 0.45mm, a length weighted average length of 0.66mm, and a weight weighted average length of 0.80 mm.
The properties of the paper are shown in table 1 below.
Comparative example H
This example describes the preparation of wood pulp paper using a blend of hardwood pulp and softwood pulp in a weight ratio of 50/50.
This example was prepared as described in comparative example A and example 1 by placing 2.5G (dry weight basis) of hardwood pulp (from comparative examples B-C) and 2.5G of softwood pulp (from comparative examples F-G) together in a laboratory mixer. This example was then hard calendered.
The properties of the paper are shown in table 1 below.
Example 4
This example describes the preparation of a two ply wood pulp paper using a combination of hardwood pulp and softwood pulp in one ply and softwood pulp in the second ply, wherein the ratio of hardwood pulp to softwood pulp is 50/50 by weight.
Using the method described in the previous examples, hardwood layers were made from 2.5G (dry basis) of hardwood pulp as used in example 2 and softwood layers were made from 2.5G of softwood pulp from examples F-G.
The two wet laid webs were placed together between two blotters, hand rolled with a rolling roll, and dried in a handsheet dryer at 150 ℃. The two-ply paper structure is then hard calendered.
The properties of the paper are shown in table 1 below.
Example 5
This example describes the preparation of a two-ply wood pulp paper using a combination of two plies, hardwood pulp in one ply and softwood pulp in the second ply, prepared in the same manner as in example 4 except that the hardwood ply was made from 3.75g (dry basis) hardwood pulp and the softwood ply was made from 2.5g softwood pulp, resulting in a hardwood to softwood pulp ratio of 75/25 by weight.
The properties of the paper are shown in table 1 below.
Comparative examples J and K
Comparative examples J and K are commercially available wood pulp type medical paper. Comparative example J isMedical paper, comparative example KMedical papers, both available from Kimberly-Clark Corporation (Neenah, Wis.).
The properties of the paper are shown in table 1 below.
From the above data, it can be seen that only the combination of refined pulp and short fibers (examples 1-4) that have been densified to high apparent densities provides significantly higher LRV than paper based on known wood pulp.
Densifying to about 0.98g/cm3(example 3) shows a relatively high air permeability (Gurley). In some embodiments, the apparent density is less than 0.98g/cm3But greater than 0.80g/cm3。
The blend of hardwood pulp with short fibers and the blend of softwood pulp with long fibers (comparative example H) did not have barrier properties falling within the desired range of the present invention. However, by distributing each type of pulp in a single layer, a high LRV is obtained, which is superior to prior art samples if the basis weight of the short fiber containing layer is at least 55g/m2(example 4).