Paper making and fiber liberation – Processes and products – Plural fiber containing
Reexamination Certificate
2002-01-30
2003-03-04
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Plural fiber containing
C162S150000, C162S162000, C162S158000, C162S160000, C162S164600
Reexamination Certificate
active
06527914
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to methods and chemical compositions used to enhance brightness and brightness stability of paper made with Mechanical Pulp.
BACKGROUND OF THE INVENTION
There are three major types of pulping methods known in the Pulp and Paper Industry. The first is Chemical and the second is Mechanical and the third is a Combination of Chemical and Mechanical.
Because Chemical pulps are not the subject of the instant claimed invention, there will be no extensive discussion of them in this patent application.
The first step in the Mechanical pulping process is the grinding or refining of wood.
The Stone Groundwood (SGW) process involves making pulp by pressing logs and chips against an abrasive rotating surface. Many years ago the grinding surface used was an actual stone. In current practice specifically designed “artificial pulp stones” are available for the grinding.
A Pressurized GroundWood (PGW) process is where the grinding operation is completely pressurized.
Another type of Mechanical pulping is Refiner Mechanical Pulp (RMP) featuring atmospheric refining with no pretreatment of the wood chips. This process is one of the main mechanical pulping operations.
Thermo Mechanical Pulping (TMP) is a Mechanical pulping process that evolved from RMP and a high temperature process known as the Apslund process. Thermo Refiner Mechanical Pulping (TRMP) is a variation in Thermo Mechanical Pulping. In this case, the chips are preheated under pressure and refining is carried out at atmospheric pressure. TMP and TRMP pulps are stronger than either SCW or RMP pulps.
The third type of pulping process is a Combination of Chemical and Mechanical pulping processes. Two types of Combination processes are ChemiMechanical Pulping and SemiMechanical Pulping. There is little difference between ChemiMechanical Pulping (CMP) and SemiChemical Mechanical Pulping (SCMP). Both processes involve pretreatment of chips with chemicals, followed by mechanical refining. Four different chemical treatments are associated with these processes. These chemical treatments are: sodium hydroxide, sodium bisulfite, sodium sulfite, and acid sulfite treatment. These processes are generally used on hardwoods. Chemical treatment weakens the fiber structure allowing fibers to rupture similarly to softwood that is mechanically pulped.
ChemiThermoMechanical Pulping (CTMP) appears to be a full evolution of all Mechanical pulping methods. It includes chemical treatment at elevated temperature steaming followed by mechanical refining. This process can produce fibrous raw materials that vary considerably in properties depending upon process conditions such as sodium sulfite concentration, pH, temperature, etc.
With all paper, “paper brightness” is a measurement of the ability of a sample to reflect monochromatic (457 nm) light as compared to a known standard, using magnesium oxide (MgO). Since cellulose and hemicellulose are white, they do not contribute to paper color. It is generally agreed that the lignin present in the paper is responsible for any color of the paper. The chromophores are believed to be quinone-like materials formed from the lignin's phenolic groups through an oxidative mechanism. Additionally, heavy metal ions, especially iron and copper, can form colored complexes with the phenolic groups.
There are generally two approaches to removing color. The first uses a selective chemical to destroy the chromophores but not the lignin. The other approach is to use a bleaching system to remove the residual lignin. The bleaching of pulp is the standard method of removing color from pulp. It is current state of the art technology for all Chemical and Mechanical pulps to be bleached. Even with bleaching it is common for paper made with Mechanical Pulp to have unwanted color present.
Mechanical pulps can be used in furnishes for the manufacture of business forms, writing papers, and high grade publication papers for books; which are all long-life uses requiring paper that does not yellow with age. However, papers made with Mechanical pulps are known to turn yellow during use. This yellowing restricts their use to applications requiring only a short-life for the paper. If the time taken before yellowing of these papers begins could be increased, the potential market for bleached TMP and CTMP would be expanded significantly. If the tendency to yellow, also known as “brightness reversion”, could be prevented, bleached TMP and CTMP could be included in furnishes used to manufacture high brightness papers. Displacing significant amounts of more expensive fully bleached, low yield chemical pulps with less expensive high yield Mechanical pulps promises significant economical benefits. On the other hand, even simple sustainable increase of brightness resulting from an inexpensive chemical treatment may be of a significant commercial value for paper mills.
Photoyellowing occurs primarily in finished paper while thermal aging occurs in both pulp and finished paper. It is thought that photoyellowing results mainly from radical photochemical reactions of residual lignin in pulp. Therefore, high-lignin pulps and products containing such pulps are more susceptible to brightness loss than more expensive, low-lignin pulps. Phenoxyl, hydroxyl, alkoxyl and peroxyl radicals are likely intermediates in the process. Consequently, radical scavengers and hydrogen donors provide protection against photoyellowing.
The mechanism of thermal aging is much less understood. From a practical point of view, thermal aging results in two separate problems. First, it is a slow brightness loss in finished paper. Second, it is a fast brightness loss in pulp itself that occurs at a pulp and paper mill during the storage and processing, and also during formation of paper (especially in the dryer).
What has been determined is that these two unwanted processes occur simultaneously in finished paper. Therefore, any effective composition designed to provide stabilization and increase in brightness must positively impact both of these unwanted processes. This is a major challenge in developing remedies against the brightness loss because the two processes develop along different chemical mechanisms. Thermal aging becomes a more important process in a common situation when paper or paper products are stored without extensive exposure to light. However, there is general understanding of importance for both thermal aging and photoyellowing of some common factors such as the pH of the paper as well as the content and state of transient metal ions in them.
The known classes of chemicals that provide limited protection against yellowing of mechanical pulps include radical scavengers and antioxidants, phosphites, dienes, aliphatic aldehydes, UV screens, chelating agents, and polymeric inhibitors. However, usually the amounts of chemicals required for adequate protection are unrealistically high (on the order of 5 percent) and, besides, these compounds carry other undesirable traits, such as high toxicity and some of them have unpleasant odors. Examples of chemicals with these undesirable effects are low-molecular-weight and polymeric thiols such as 1-thioglycerol, glycol dimercaptoacetate, polyethyleneglycol dithiolactate, which do inhibit photoyellowing, however, such chemicals are usually malodorous. Furthermore, these types of chemical typically have to be applied in quantities that are not economically feasible.
A synergistic mixture of a radical scavenger 4-hydroxy-2,2,6,6-tetramethylpiperidine N-oxyl (HO-TEMPO) and 2-(2-hydroxy-3,5-di-tert-amylphenyl)benzotriazole was proposed as an effective inhibitor against both photoyellowing and thermal aging. However, the toxicity of these compounds is known and must be taken into account when using them.
It would be very desirable to discover new compositions that would effectively enhance the initial brightness and brightness stability of paper made with Mechanical Pulp.
SUMMARY OF THE INVENTION
The first aspect of the instant claimed invention is in a method of making paper wi
Duggirala Prasad Y.
Shevchenko Sergey M.
Breininger Thomas M.
Brumm Margaret M.
Chin Peter
Ondeo Nalco Company
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