Paper making and fiber liberation – Processes and products – Conditioning – preparing or repairing of apparatus
Reexamination Certificate
2001-05-18
2002-05-14
Nguyen, Dean T. (Department: 1731)
Paper making and fiber liberation
Processes and products
Conditioning, preparing or repairing of apparatus
C162S168300, C162SDIG004
Reexamination Certificate
active
06387215
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a process for controlling the deposition of sticky impurities from paper stock suspensions in paper manufacture.
Even when paper was invented in the second century, the use of waste material, i.e. the technique of at least partial recycling, played a certain role. Nowadays, considerable significance is attributed to recycling technology through increasing ecological awareness. In view of the increasing production of paper, therefore, the supply of raw materials and the avoidance of waste are acquiring increasing significance.
By using secondary fiber stock from the recycling of waste paper, savings can now be made in raw materials required, waste-disposal space, and the energy required for paper manufacture. Unfortunately, the technology still involves specific difficulties.
In the processing of waste paper, sticky impurities, normally known as “stickies,” can seriously disrupt the production process and adversely affect the quality of the paper produced. Stickies enter the papermaking process when the waste paper used contains adhesive bonds, adhesive tapes or refined products, such as coated or laminated papers and paperboards. In addition sticky impurities can be formed by the resin in wood and through its interaction with paper auxiliaries.
The present invention is primarily directed to a problem associated with the secondary fiber industry. This problem is found in all stages of the industry, including the repulping stage, the paper or paper product formation stage, paper or paper product finishing stage, and the handling, storage, and usage stages. The secondary fiber industry utilizes waste paper or paper products as a source of paper fiber (commonly referred to as “secondary fiber”) to produce finished paper products. Waste paper and paper products which are the source of the secondary fiber may include any waste paper product such as newspapers, books, magazines, waste bags and boxes, waste ledgers, files, waste cuttings or trimmings from pressure sensitive adhesive backed labels, decals, stickers (i.e., bumper), etc.
In the repulping of the secondary fibers, a problem is encountered due to the adhesives, ink and coating binders (primarily those composed of synthetic polymers) that are found on some of the waste paper being utilized. More specifically packaging tapes, labels, decals, stickers, stamps, envelopes, book bindings, etc., each have adhesives associated therewith. Thus when waste products are recycled, the adhesives give rise to the stickies problem. Stickies are the remainder of the adhesives, ink and coating binders which are not solubilized in the process water and cause many problems ranging from machine operating efficiency through to finished product quality.
Where the stickies are present in large enough form, they can be mechanically removed relatively easily by means of screening equipment. In general, however, the stickies are present not only as large pieces, but also as small dispersed particles in the pulp stock which are very difficult to remove. Recently, therefore, the increasing use of waste paper in paper manufacture and the restriction of the water circuits has increasingly resulted in larger quantities of stickies remaining in the pulp furnish and in the recirculated process water.
As indicated, the stickies cause a number of problems and disruptions not only in the paper-making process, but also in the processing of the product paper. On account of their stickiness, deposits are formed on machine parts, tube walls, screens, forming wires, wet felts, dry felts, drying cylinders, press rolls, calender rollers, and in addition, even on the paper itself, resulting in web tears in the papermaking machine and in a deterioration in paper quality through holes, stains and marks (cf. H. L. Baumgarten, Das Papier, 1984, 38, No. 10A, pages V121-V125). According to Baumgarten, stickies in industrial and institutional publications have for years been the biggest problem in the recycling of waste paper. Even minimum quantities of adhesive can cause tears in papermaking and printing machines, so that the machines have to be stopped for cleaning purposes. Baumgarten states: “2 g of adhesive at the right place in the papermaking machine can turn several hundred kg of paper into waste” (loc. cit., page V122, right-hand column).
Stickies have various origins. Essentially, they emanate from the residual wood resin in pulp, from auxiliaries involved in paper manufacture, from binders for the coating of paper and cardboard, from packaging tape and label adhesives, from printing ink binders and from materials involved in the processing of paper. Stickies emanating from the residual wood resin and from the adhesives found in recycled paper and paperboard are particularly important in the context of the problem addressed by the present invention. The resins present in chemical wood pulp and mechanical wood pulp contain around 1 to 5% by weight of so-called harmful resins, depending on the type of wood. These resins may be present in colloidal, unbound form or may adhere to the paper fibers.
The adhesives commonly associated with recycled paper and paperboard may be divided into three main groups, namely: contact adhesives, dispersion-based adhesives, and hot-melt adhesives. Contact adhesives are permanently tacky and permanently bondable products. Adhesion is achieved by application of pressure to the surfaces of the parts to be bonded. The basic polymers may be any of various key chemicals in combination with corresponding additives, for example tackifying resins, plasticizers or antioxidants. Typical basic polymers are inter alia natural rubber, butyl rubber, styrene/butadiene copolymers (SBR rubber), acrylonitrile copolymers, polychloroprene, polyisobutylene, polyvinyl ether, acrylates, polyesters, polyurethanes, and silicones.
Dispersion-based adhesives have the adhesive layer polymers in the form of solid particles in an aqueous dispersant. In the production process, the basic monomers are first emulsified in an aqueous phase and then polymerized therein—a technique known as emulsion polymerization. The polymer is then present in the form of small particles with varying particle sizes ranging from molecularly disperse to coarsely disperse. In general, agglomeration and hence sedimentation of the polymer particles is counteracted by adding protective colloids or emulsifiers to the system.
The so-called hot-melt adhesives, also known as “hotmelts,” belong to the group of thermoplastics. These materials have the property of softening on heat-ing, so that they become fluid. On cooling, they solidify again. Examples of polymers used as hot-melt adhesives include polyamides, copolyamides, poly-aminoamines, saturated polyesters and ethylene/vinyl acetate copolymers.
The stickies generated during the recycling of waste paper often have undergone a change in their particle size brought about by thermal, chemical and mechanical influences. This means that even impurities which are still present in extremely coarse form at the beginning of recycling can undergo more or less considerable size reduction in the recycling process. In particular, stickies are dispersed by the processes taking place in the repulping and refining equipment used in the recycling of waste paper. For example, stickies with a low melting point can be liquefied and then very finely dispersed. Crumbly or fragile stickies also dis-integrate into very small particles. The particle size of the dispersed stickies thus ranges from coarsely disperse through colloidally disperse to molecularly disperse.
In other words, many stickies are readily dispersible with the result that, after the repulping step, they are present in finely divided form and are not removed by the pulp screening equipment. These substances are in danger of forming agglomerates—also known as secondary stickies—in the papermaking machine under thermal, mechanical or chemical influences. It is precisely these secondary stickies which cause problems in the further processing of pape
Kurowsky Stephen R.
Schellhamer Alan J.
Spence Gavin G.
Callaway Chemical Corporation
Jacobs Bruce F.
Nguyen Dean T.
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