DESCRIPTION OF WOOD PULP PRODUCTS
Mechanical Pulp
Mechanical pulps are characterized by the fact that a very high percentage of the original wood components are retained in the finished product. For this reason they are often termed high yield pulps, with yields usually in the 85-95% range.
Due to the high yield, mechanical pulps contain a number of different types of wood particles of various shapes and sizes, resulting in highly opaque products with good printing properties. However, the high yield also results in the presence of large percentages of lignin in the pulp, which yellows when exposed to heat or ultraviolet light. Given this, products which contain mechanical pulps tend to discolor over time.
Also, mechanical processes cause considerable damage to wood fibers and result in a relatively weak pulp. In order to achieve adequate sheet strength it is often necessary to add a longer fiber chemical pulp to the mechanical pulp. Following is a list of the major mechanical pulping processes:
1) Stone Groundwood Pulp (SGW) (roundwood)
2) Pressurized Groundwood (PGW) (roundwood)
3) Refiner Mechanical Pulp (RMP) (chip)
4) Thermomechariical Pulp (TMP) (chip)
5) Chemi-Thermomechariical Pulp (CTMP) (chip)
Each of these pulps is produced at relatively low cost when compared to chemical pulps. Typical end uses include newsprint, directory paper, supercalendered papers and lightweight coated paper (LWC).
The CTMP process can also produce a pulp of high bulk and absorbency and is used in products such as tissue, toweling, and disposable diapers. CTMP is also used in the production of some paperboard grades such as those used in liquid packaging. Bleached CTMP is believed to have significant potential in printing and writing grades such as forms paper, uncoated bond and some book grades.
Chemical Pulp
The main grades of chemical pulp are kraft and sulfite. Chemical pulping removes fibers from wood by dissolving the lignin which holds them together. In contrast, mechanical pulping separates the fibers by a grinding action, leaving much of the lignin in the pulp. Chemical pulps are much stronger since individual fibers are not damaged to the extent that they are with mechanical pulping methods.
The removal of lignin results in lower yields, and is typically between 40-55%, therefore leading to higher wood consumptions per unit and higher wood costs. In addition to resulting in a relatively stronger pulp, the removal of lignin makes chemical pulp less likely to lose its brightness over time as compared to mechanical pulp.
Kraft Pulp (Sulphate):
Kraft pulp is noted for its superior strength characteristics and can be used in virtually all paper and paperboard grades in order to improve strength properties. In fact, the word kraft is the Swedish and German word for strength.
Kraft pulp can be produced from both hardwood and softwood species and is used in bleached, semi-bleached and unbleached forms. Unbleached kraft is usually made with softwood and is used primarily in the furnish of kraft linerboard, wrapping paper and bag papers such as grocery bags.
Semi-bleached kraft is used in the furnish of grades which do not require high brightness, such as newsprint and other groundwood based papers.
Bleached kraft is used in a much wider range of products than either unbleached or semi-bleached. Its greatest importance is in the printing and writing grades. In these grades softwood kraft is used for its strength characteristics, while hardwood kraft, having shorter fibers, is used for its superior printing properties.
Sulphite Pulp:
Sulphite was the most important pulping method until the 1930's, when kraft pulp began to dominate because it was able to utilize a wide variety of wood species not suitable for sulphite pulping. Today, sulphite is used much less than kraft.
Sulphite is not as strong as kraft and is typically used in products which require good sheet formation and moderate strength. Historically, sulphite has been most widely used in newsprint furnish. However, its importance in newsprint has been declining in recent years with the increasing use of stronger mechanical pulps such as TMP and CTMP.
Semi-Chemical Pulp:
There are several types of semi-chemical pulps in production, but the most important of these is Neutral Sulfite Semi-Chemical (NSSC). NSSC is made primarily from hardwood species and is noted for its exceptional stiffness. Its primary use is for the production of corrugating medium.
Dissolving Pulp:
Not used in papermaking, dissolving pulp is chemically converted for use in such products as rayon, cellophane, cellulose acetate, cellulose nitrate and carboxymethyl cellulose. Softwoods are the major raw material for dissolving pulps, but some hardwood is used.
DESCRIPTION OF PAPER AND PAPERBOARD PRODUCTS
Kraft Paper
Kraft papers can be grouped into three general categories:
1) Unbleached Packaging
2) Bleached Packaging
3) Special Industrial Papers
Unbleached and bleached packaging papers are used for products such as wrapping, grocery bag, shipping sacks and other converted papers. Basis weights for these grades depend greatly on the end use and can range from 30 to 80 lbs per 3000 ft2 (49 to 130 g/m2).
Special industrial papers include papers used in the manufacture of abrasives, electrical insulators, gaskets, filters and many other products. Bleached specialty papers include grades such as glassine, greaseproof and vegetable parchment
Linerboard
Linerboard is a relatively light weight board used for the outer plies of corrugated box stock and also as wrapping paper. The furnish for linerboard can be either kraft pulp or recycled material, usually old corrugated containers (OCC) or double lined kraft (DLK). The standard basis weight for linerboard is 42 lbs per 1000 ft2 (205 g/m2), but can range from 26 to 90 lbs.
Linerboard is made with two layers and is typically manufactured on a two-headbox fourdrinier paper machines. In kraftliner, a lower quality high yield, unbleached kraft base sheet is formed from the first headbox. A better quality, lower yield layer is formed on top of the base sheet to provide a good printing surface.
Where appearance is particularly important, mottled white, white top, bleached or coated liner can be used, depending upon the desired printing surface. Mottled white and white top liner have a top layer which is bleached, the top layer of mottled white liner does not provide full coverage of the base sheet, causing a "mottled" appearance, whereas white top does. With bleached liner, both the base and the top sheet are bleached. Coated liner is typically an unbleached liner with a white pigment coating.
Corrugating Medium
Corrugating medium is a light weight board used for the fluted inner plies of corrugated box stock. The basis weight for corrugating medium range from 18 to 36 lbs per 1000 ft2, with the standard being 26 lbs.
Corrugating medium is typically made with semi-chemical pulp and or recycled material. About 75% of production utilizes a furnish containing about 80% semi-chemical pulp and 20% recycled fiber. The remainder of the production is made of 100% recycled material and is often termed "bogus medium".
Boxboard
Boxboard is a broad category encompassing coated and uncoated kraft paperboard, recycled paperboard and bleached paperboard. These boards have a wide range of basis weights, depending greatly upon the end use products.
Typical end use products for kraft and recycled paperboard include beverage carriers, folding cartons, setup boxes, as well as tube, can and drum stock.
Bleached kraft paperboard is commonly called solid bleached sulphate (SBS). The primary use for SBS is folding carton and milk carton. Other uses include disposable cups, plates and food containers.
Newsprint
Newsprint is an uncoated printing paper in which mechanical pulp and/or deinked recycled fiber is the main furnish component. The most common recycled furnish used is old news papers (ONP). Some chemical pulp is usually added to the sheet in order to increase strength, and this is usually unbleached sulphite or semi-bleached kraft. The standard basis weight for newsprint is 30 lb per 3000 ft2 (48.8 g/m2).
Newsprint comes close to being a true commodity item. Most newsprint is the similar in terms of basis weight, brightness and price. The primary end users of newsprint are newspaper publishers, although some production goes to cormnercial printers.
Uncoated Groundwood
Uncoated groundwood papers (UCGW) are the next step in the value-added chain relative to newsprint. As with newsprint, the main furnish component is mechanical pulp, with chemical pulp being added to increase strength. The amount of chemical pulp used in the furnish depends upon the end product as well as the type of mechanical pulp used. The basis weight ranges from 18-40 1b per 500 sheets (24 x 36 in.) or 30-66 g/m2.
UCGW papers are differentiated from newsprint with respect to printing properties and brightness, and are typically priced above standard newsprint.
Some of the main UCGW grades are supercalendered papers (SC), directory papers and roto-news. Typical end uses for UCGW grades include free-standing inserts, telephone and other directories, Sunday news magazines, direct mail and commercial printing. Some of the trade names are: rotogravure (standard, premium, high brightness), offset (premium, high brightness), magazine (gravure, offset), paperback/book, directory (white, yellow) and computer.
Coated Groundwood
Coated groundwood papers are often referred to as lightweight coated (LWC). However, basis weights for coated groundwood can range from 32-50 lbs per 3,300 ft2. As with UCGW, the furnish for coated groundwood papers is primarily mechanical pulp with varying amounts of chemical pulp added for strength.
The primary difference between coated and uncoated grades is the presence of a pigment layer which covers the base sheet. The main component of the coating is usually clay, however, calcium carbonate and titanium dioxide are also used in some formulations. Typically, the coating accounts for approximately 30% of the finished sheet by weight.
Coated papers are broken down into 5 grades based on brightness and basis weight (#1 through #5). On the spectrum of coated printing grades, coated groundwood is #5, while #1 through #4 are primarily freesheet based. However, coated #4 can contain relatively high amounts of mechanical pulp.
End uses for coated groundwood include magazines, Sunday news supplements, catalogs, newspaper inserts, directories, books and flyers.
Uncoated Freesheet
Uncoated freesheet papers (UCFS) are made primarily with chemical pulps and contain very little if any mechanical pulp. UCFS is produced in a wide range of basis weights and includes such end use products as personnel and business stationary, bond, ledger, envelopes, book paper, copy paper, laser printer papers, tablet paper, cigarette paper and base stock for products such as carbonless and thermal paper.
In order to fill void spaces between fibers and improve the printing properties of the sheet, UCFS grades typically have filler contents of 10-15%. Fillers are usually comprised of clay, calcium carbonate and/or titanium dioxide, depending upon the desired sheet properties. If an alkaline paper making system is used, filler contents can reach levels of up to 30%.
The use of calcium carbonate as a filler can also increase the permanence of the sheet, a quality that is becoming increasingly important to many publishers.
Coated Freesheet
Coated freesheet is the highest value-added grade of paper. As with uncoated freesheet, coated freesheet is made primarily with chemical pulp. The primary difference between the grades is the coating, which accounts for approximately 30% of the finished sheet weight.
Coated freesheet can be either coated one side (ClS) or coated two side (C2S); coated two side accounts for approximately 90% of production. As previously discussed, coated papers are further defined on the basis of brightness and basis weight. Coated freesheet grades range from #1 through #4, with #1 being the brightest and highest quality coated paper. The bulk of coated freesheet production is classified as coated #3.
Typical end uses for coated freesheet are high quality magazines, catalogs, advertising brochures, annual reports and various commercial printing applications. ClS papers are typically used for labels and other specialty products.
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