Stock material or miscellaneous articles – Composite – Of carbohydrate
Reexamination Certificate
2001-02-27
2003-03-25
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Composite
Of carbohydrate
C428S533000, C428S534000, C428S535000, C428S536000, C428S537500, C162S123000, C162S125000, C162S129000, C162S132000, C162S135000
Reexamination Certificate
active
06537680
ABSTRACT:
TECHNICAL FIELD
The invention relates to a paper or paperboard laminate composed of at least one bulk-promoting layer, here termed bulk layer, and on at least one side of the bulk layer at least a secondary layer, the secondary layer and bulk layer being joined to one another directly or indirectly over basically the whole of their surfaces facing one another. The invention also relates to a method for producing such a laminate.
PRIOR ART
One of the most important attributes of paperboard material when used as packaging material is its stiffness. The stiffness of a paper or paperboard laminate is proportional to its thickness raised to the third power. This ratio means that a considerable saving in material can be achieved by reducing the density of the less loaded middle layers in a laminate. The ratio has long been known, but one difficulty has been to produce sufficiently stiff and strong middle layers which at the same time are of a low density.
Corrugated paperboard is a classic example of a paperboard laminate with good flexural rigidity in relation to the density of the laminate. Due to microcorrugation of the bulk-promoting middle layer, relatively thin laminates can also be produced, which are not however regarded as satisfying the maximum demands made on packaging material. Thus the wave-shaped pattern can often be discerned, which reduces the aesthetic value of the material.
In “Weyerhaeuser Paper Company introduces HBA (High Bulk Additive)”, Elston and Graef describe the possibility of using chemically cross-linked fibres in paperboard material. By adding 10% HBA (High Bulk Additive) to the stock, the basis weight of the paperboard material can be reduced by 25%, with a sheet of the same flexural rigidity as a control sample without the addition of HBA. The thickness of the sheet can be retained, the density being reduced instead in one example from 705 to 500 kg/m
3
. Taber stiffness is shown to increase by approx. 40% with the addition of 15% HBA. However, this results in reduced tensile strength, approx. −25%. Admixing has been performed inter alia on a three-layer laminate, all the HBA being put into the middle layer.
WO95/26441 likewise describes the use of a chemically cross-linked fibre in paper laminate with two or more layers. The object of using the cross-linked fibre (HBA) is to achieve a construction of increased bulk while retaining the tensile strength. Paper material of low density (high bulk) normally gives lower tensile strengths. To reduce this negative effect of low density, the use is proposed of waterborne binders sirch as starch, modified starch, polyvinyl acetate and polyvinyl alcohol etc. It is proposed to use these binders in percentages of between 0.1 and 6% of the material's weight. The flexural rigidity achieved is expressed in Taber units. If the same method is used for converting stiffness as described below under test methods, then the result in WO95/26441, Example 5, corresponds to a bending stiffness index of approx. 1.6 Nm
7
/kg
3
.
Dry forming in the manufacture of paper has been described in literature in a large number of articles and patents. In “An introduction to dry forming of paper”, Tappi, 1978, pp. 3-6, amongst others, Swenson describes various techniques for forming a web using air as a dispersing medium for wood fibres. Here examples are given of products which are manufactured by dry forming, e.g. soft hand towels, stiff paperboard and masonite.
In GB 1,430,760 and GB 1,435,703 a forming technique is described for producing paper material with several layers. It is proposed inter alia to combine dry- or wet-formed layers with one another. It is proposed that consolidation of the sheet (consisting of several layers) is done by using binders, moisture and pressing at high temperature. Product attributes for dried-out products are characterized by high bulk, squareness (i.e. same properties in different directions of the sheet in a plane) and good dimensional stability. Furthermore, it is considered possible to achieve product attributes similar to conventionally formed paperboard. The manufacturing technique is considered to reduce investment costs, water and energy consumption.
In “Where research pays off”, PPI, March 1977, pp. 42-26, Haas describes certain important product attributes for conventionally wet-formed and dry-formed paperboard. Haas describes the attributes of the dry-formed sheets as having an even surface with a lack of felt and wire markings and an approved tear strength. Stiffness is reported using numerical values for the various manufacturing techniques, but not commented on in the text. The dry-formed multilayer materials have not produced increased stiffness. In interpreting the document here it has been assumed that “stiffness %” or “stiffness X” means the stiffness of the sheets in a transverse or longitudinal direction (TR or MR). In the event of conversion for better comparison between different materials, the bending stiffness index can be calculated as the geometric mean value of MR and TR (the square root of MR*TR), a maximum bending stiffness index achieved according to the values reported by Haas being approx. 1.2 Nm
7
/kg
3
. It is thus perceived here that dry forming techniques such as have been applied have not contributed to increased flexural rigidity. Haas also reports the basis weight and thickness of the different paper constructions, 550 k/m
3
appearing to be the lowest density produced for the wholly or partly dry-formed constructions.
In “Dry forming of paperboard: a look at its history and technology”, Pulp and Paper, 54, 1980:4, pp. 120-123 Attwood reports on experiments inter alia with paper constructions which combine dry-formed and wet-formed layers. The results reported with regard to stiffness and thickness (at the same basis weight) point to great differences in stiffness in machine (MR) and cross machine direction (TR). The maximum stiffness converted as the square root of stiffness MR*TR was obtained for material which had been produced with wet-formed outer layers and dry-formed middle layers, no values in excess of 1 Nm
7
/kg
3
having been achieved, however. Furthermore, Attwood reports various proposals for methods of designing a process which combines dry-formed middle layers with wet-formed outer layers. Attwood also reports the basis weight and thickness of the different paper constructions, approx. 600 kg/m
3
appearing to be the lowest density produced for the wholly or partly dry-formed constructions.
U.S. Pat. No. 4,914,773 reports methods of producing stiff paperboard material by using dryly exposed fibres with a freeness of 500 CSF. The fibres which are to be formed into the middle layer in a sheet are to be dispersed in foam. This has the object of preventing them from being wetted with water to too great an extent. The addition of different types of binder such as latex, starch, gums etc. is specified as necessary preconditions for achieving adequate strength of the sheet. When the flexural rigidities reported are converted it is clear that the maximum bending stiffness index achieved is approx. 1.8 Nm
7
/kg
3
.
DESCRIPTION OF THE INVENTION
It has turned out surprisingly to be the case that by using fibres with a freeness of 550-950 ml CSF, preferably fibres with a freeness value higher than 600 ml CSF, at best higher than 650 but less than 850 ml CSF, and best of all higher than 700 ml CSF, in a bulk-promoting layer in the laminate, termed bulk layer below, in combination with a secondary layer on one or both sides of the bulk layer, a laminate can be obtained which exhibits very great stiffness. The advantage is also hereby achieved that the laminate has a lower density, and thereby lower material consumption compared with previously known paperboard laminates intended for the same type of use as the laminate according to the invention, such as material for packaging of liquid and solid foodstuffs and also for wrapping and packing industrial goods and other goods, or as an intermediate product for the manufacture of such material or other
Fredlund Mats
Karlsson Annika
Norlander Leif
Kiliman Leszek
Nixon & Vanderhye
Stora Kopparbergs Bergslags Aktiebolag (publ)
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