Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With chemical or physical modification of liberated fiber
C162S100000, C162S149000, C162S181200, C162S182000
The present invention relates to a filler for use in paper manufacture as defined in the preamble of claim
. Moreover, the invention relates to a procedure for producing said filler.
In the present application, ‘paper’ refers to various kinds of paper and cardboard, manufactured with paper and cardboard machines, coated or uncoated.
Today, the direction of development of paper products is to an increasing degree determined by customers and legislative measures. The buyers of printing paper want to reduce the postage expenses and the amount of waste produced. Further, packages are subject to waste processing charges dependent on weight. Generally, it seems that energy taxes and environmental protection taxes are being imposed on the price of paper products as a permanent extra encumbrance. For these reasons, paper buyers want products which have a lower grammage while still meeting high quality standards.
Specification FI 931584 presents a composite product based on chemical pulp fibre or mechanical pulp fibre, with calcium carbonate crystals precipitated onto the surface of the product. Further, specification FI 944355 presents a precipitated calcium carbonate, which is in the form of calcite particle aggregates where at least 25% of the particles are of a prismatic shape. Precipitation is performed using a seed material. Further, specification EP 0604095 presents a procedure for the processing of waste material whereby calcium carbonate is precipitated onto the surface of waste material containing inorganic matter; the waste material may contain organic waste fibre, e.g. waste fibre contained in the effluent of a paper mill, with fibre length below 75 &mgr;m. The calcium carbonate products described in the specifications referred to are intended to be used as fillers in paper manufacture.
In the manufacture of high-quality paper, the aim is to produce the paper with a minimum amount of raw material. When the grammage of the paper is reduced, its opacity becomes a critical factor. The opacity can be increased by increasing the filler content of the paper, which, however, generally reduces its strength. Therefore, the aim is to alter the structure of the paper while at the same time preserving the important good product qualities. For paper based communication to remain competitive in relation to electric communication, the printing quality of paper products has to be further improved. These general development trends impose very high requirements on the raw materials and manufacturing processes used in paper production. To meet the requirements, very intensive efforts have been made in recent times to develop paper raw materials and manufacturing processes.
The object of the present invention is to produce a new kind of calcium carbonate based filler for paper manufacture that meets the criteria described above.
A further object of the invention is to produce a new kind of calcium carbonate based filler which has better optic properties than earlier calcium carbonate based fillers.
A further object of the invention is to produce a new kind of calcium carbonate based filler which gives the paper better strength properties, especially a better tensile strength, than earlier calcium carbonate based fillers.
A further object of the invention is to produce a new kind of calcium carbonate based filler which gives the paper a lower grammage than earlier calcium carbonate based fillers.
A further object of the invention is to produce a new kind of calcium carbonate based filler which has a higher retention than earlier calcium carbonate based fillers.
A further object of the invention is to produce a new kind of calcium carbonate based filler which reduces the overall costs of paper manufacture.
An additional object of the invention is to present a procedure for the manufacture of said filler.
As for the features characteristic of the invention, reference is made to the claims.
The invention is based, among other things, on the fact, established via corresponding investigations, that calcium carbonate can be precipitated in a way that causes it to effectively adhere to fibres and noil fibrils. The precipitation can be so performed that porous calcium carbonate aggregates held together by fibrils, i.e. fine fibres, are formed, which aggregates contain plenty of empty space and in which the calcium carbonate particles have precipitated onto the noil fibrils, adhering to them. The noil fibrils with calcium carbonate particles precipitated on them form fibres resembling pearl necklaces, and the calcium carbonate aggregates resemble clusters of pearl necklaces. The aggregates have a very large ratio of effective volume to mass as compared with the corresponding ratio of conventional calcium carbonate used as filler; effective volume here means the volume taken up by pigment in the paper.
The noil fibrils used in the filler of the invention are obtained from cellulose fibre and/or mechanical pulp fibre. The fibrils are produced from cellulose fibre and/or mechanical pulp fibre by refining. Furthermore, the noil fibrils are preferably divided into fractions, thickness 0.1-2 &mgr;m, length mainly 10-400 &mgr;m, suitably 10-300 &mgr;m, preferably 10-150 &mgr;m. Thus, the noil fibrils consist of cellulose fibre and/or mechanical pulp fibre, which means that they contain no significant amounts of inorganic matter, preferably no inorganic matter at all.
The diameter of the calcium carbonate particles in the aggregate is of the order of about 0.2-3 &mgr;m, preferably about 0.3-1.5 &mgr;m.
The diameter of the CaCO
crystal aggregates is of the order of about 2-10 &mgr;m.
Cellulose based noil also contains roundish noil particles which, after the precipitation process, are covered with calcium carbonate particles. In this case, as to its properties, a particle of calcium carbonate filler corresponds in the first place to a hollow filler particle having a small unit weight. In reality, the pigment is not completely hollow, because it contains noil; however, the noil has a lower unit weight than calcium carbonate, therefore the particle has a very low unit weight.
The new precipitated, calcium carbonate based filler of the invention bestows paper better optic properties and a clearly greater strength than prior-art calcium carbonate based fillers do. Furthermore, the filler of the invention allows the filler content of paper to be increased without impairing its other properties, e.g. the aforementioned strength properties, such as tensile strength. This is a significant contribution towards lowering the grammage of paper.
Further, the new filler of the invention has a clearly better retention in paper manufacture than prior-art calcium carbonate based fillers.
In consequence of the aforesaid factors, it is generally possible to achieve cost savings in paper manufacture by using the filler of the invention.
In prior art, light filler pigments are known, e.g. hollow plastic pigments, which are supposed to provide the same advantages as the calcium carbonate based filler of the present invention. However, plastic pigments are expensive, which restricts their use. When the filler of the invention is compared with pore filled or lumen filled fibre, it is to be noted that, unlike in the case of aforesaid fibres, the calcium carbonate in the filler of the invention is not inside individual noil fibres but on the surface of the noil. In addition, the mass ratio of calcium carbonate and fibrous matter is much larger in the filler of the invention than in pore filled or lumen filled fibre. Thus, the filler of the invention is a completely new product, and it should not be confused with prior-art pore filled or lumen filled fibre.
The filler of the invention and the procedure for its manufacture differ from the specification FI 931584 referred to in the introduction especially on the basis of the thickness and length of the noil fibrils, i.e. in the present application, the noil fibrils have been refined with a pulp refiner. From the filler and manufacturing method described in specification FI 944355, th
Merchant & Gould P.C.
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