第5、6讲
Unit 3 Packaging Materials and Containers
（第三单元 包装材料和容器）
Lesson 1 Paper and Paperboard（第1课 纸与纸板）
（概述，典型造纸机，纸板的加工方向与横向，表面处理与涂覆，纸特性，纸/纸板等级，纸盒）
1. Introduction (概述)
Definition of paper: paper is defined as a matted or felted sheet usually composed of plant fiber. Paper has been commercially made from such fiber sources as rags (linen), bagasse (sugar cane), cotton, and straw. Modern paper is made almost exclusively from cellulose fiber derived from wood.
Although the word “paper” is derived from the Egyptian term, “papyrus” was not a true paper in the modern sense.
Invention of paper: the invention of paper by blending cellulose fibers didn’t occur until the beginning of the second century A.D. Ts’ai Lun, a member of the court of the later Han Dynasty, is generally credited with developing the first real papermaking process in 105 A.D.
The “Fourdrinier machine” was the first on the market and produced a homogenous (single-ply) sheet of boxboard in various thicknesses. It was soon joined by the “Cylinder machine” which formed a multi-layered (multi-ply) type of paperboard. These machines were first installed in the United States around 1830.
· Paperboard, boxboard, cardboard, and carton board are all terms used to describe heavier paper stock. The International Organization for Standardization (ISO) states that material weighing more than 250 grams per square metre (511b per 1,000 sq. ft.) shall be known as paperboard. U.S. practice calls material that is more than 0.3mm(0.012 in.) thick paperboard.
2. Representative Papermaking Machines (典型造纸机)
(1)Fourdrinier Machines
Fourdrinier machines (Figure 3.1) pump furnish from a headbox directly onto a moving wire screen through which the water is continuously drained. Fourdrinier machines may have a second headbox (Figure 3.2)situated downstream of the first headbox to add further quantities of furnish onto the partially dewatered initial lay-down.
Figure 3.1 Furnish pours out of the headbox of a fourdrinier machine and onto an endless wire or screen where excess water can be drained. The fibers remain trapped on the screen
Figure 3.2 Paper is dewatered at the wet end of a fourdrinier machine
(2)Cylinder Machines
A cylinder machine (Figure 3.3) rotates a screen drum in a vat of furnish (The paper is sometimes called vat paper). As the water pours through the screen, fiber accumulates on the outside of the screen. This thin layer of matted fiber is transferred onto a moving felt belt that passes sequentially over further rotating cylinders, each of which deposits another fiber layer.
Figure 3.3 A single cylinder station on a cylinder-type machine
Cylinder machines dewater furnish at the cylinder and paste a thin layer of fiber against the felt (Figure 3.4). The fibers of subsequent layers do not intermingle, and therefore the bond between the layers is weak. The dry end is similar to that of the fourdrinier machine.
Figure 3.4 A cylinder machine with six cylinders at which a paper layer can be formed
Cylinder machines do not have the fourdrinier machine's limitation on the number of stations, and six-or seven-station machines are common. Higher-caliper boards for folding and setup cartons are usually cylinder boards. An advantage of cylinder machines is that low-quality fiber can be used to fill or bulk the middle of a board, while higher quality bleached fibers can be used on one or both liners.
Cylinder board has definite layers, or plies, and individual plies can often be easily separated. Generally, papers are made on fourdrinier or twin-wire formers, whereas heavier paperboard products are made on cylinder-type machines. Extremely heavy boards are made by laminating several thinner sheets.
A typical cylinder board construction (Figure 3.5) may have a top liner composed of good-quality bleached pulp with some short fibers, possibly sized and clay coated to produce a smooth, attractive printing surface. The underliner may also be composed to a good-quality stock, possibly bleached to provide a smooth, opaque base for the top liner. Filler plies use the most economical recycled pulps, since they have little impact on properties such as stiffness. The bottom liner is a better quality pulp to add stiffness. If appearance is not a factor, the liner may be good-quality recycle pulp. If appearance is critical or if the paperboard will be printed on both sides, the bottom liner will also be bleached stock.
Figure 3.5 Cylinder boards are multiply boards.
An advantage is that the plies can all be different
(3)Twin-Wire Machines
Vertiformers and twin-wire formers (Figure 3.6) inject the furnish between two moving wire screens. The advantage is that dewatering takes place on both sides of the paper and is therefore fast. These machines can produce single and multi-ply sheets with identical formation at both faces.
Figure 3.6 Water can be simultaneously removed from both sides of the paper on a twin-wire paper former
3. Machine Direction and Cross Direction (纸板的加工方向与横向)
Depositing a fiber-and-water slurry onto a moving wire belt tends to align fibers in the direction of travel, known as the machine direction (MD). The direction across the papermaking machine and across the fiber alignment is the cross direction (CD) (Figure 3.7). Because of this fiber alignment, paper is an anisotropic material; measured properties differ depending on the direction in which the property is measured.
Figure 3.7 Fibers in a manufactured paper sheet tend to align themselves in the machine direction
Figure 3.8 shows the relationship of tear, stiffness, and fold endurance to machine direction. Paper specification sheets normally show physical values measured in both directions. Package designers need to be aware of paper's directionality.
Figure 3.8 The relationship between MD and tear, stiffness, and fold endurance properties
Cylinder machines tend to align fibers more than fourdrinier machines. Tensile strength ratios in MD and CD for a typical fourdrinier board are about 2:1, whereas for a cylinder board the ratio might be 4:1 or higher, meaning that the MD tensile strength is four times greater than the CD tensile strength. The greater the degree of fiber alignment, the greater the difference in a given property when measured in MD and CD. The ratio of a property in the two directions is often used as a gauge of fiber alignment.
4. Surface or Dry-End, Treatments and Coatings (表面处理与涂覆)
After the paper is formed and dried, it is usually passed between multiple sets of heavy rolls (Figure 3.9). This "calendering" operation has many variations, but the prime objective is to iron and smooth out the surface of the paper stock to make it more suitable for printing. Calendering also compresses the paper sheet, giving a denser product and a glossier surface.
Starch is a typical surface sizing used to fill surface voids and reduce liquid penetration rate.
Figure 3.9 Calendering consists of passing the formed dried paper between sets of heavy rolls.
The paper surface may be dampened to help in smoothing it. To meet the highest opacity, gloss, brightness, and printing-detail requirements, papers are coated with pigments such as clay, calcium carbonate, and titanium dioxide. Coated papers are usually called "clay-coated" regardless of the actual formulation. Coated papers are calendered to maintain a high-quality, smooth surface. In addition, highly sized and clay-coated boards can be difficult to bond with water-based adhesive because of poor liquid penetration and the inability of the adhesive to bond to the underlying fibers. Where necessary, coated boards should have perforations in the adhesive-bond areas so that adhesive can penetrate to the body of the paper.
5. Paper Characterization (纸特性)
(1)Caliper and Weight
In inch/pound units:
·Caliper is expressed in thousandths of an inch or in "points." One thousandth of an inch is 1 point. (For example, a 0.020-in. board would be 20 points.)
·Containerboard for the corrugated board industry and most paperboards are specified by the weight in pounds per 1,000 sq. ft., the "basis weight."
·Fine papers can be specified by the weight in pounds per ream. A ream is 500 sheets, but the actual sheet size can vary depending on the product. In most instances a ream is taken to be 3,000 sq. ft..
In metric units:
·Caliper is expressed in "m" or micrometres(μm).
·Paper mass/unit area relationship is reported as "grammage", defined as being the mass (weight) of paper in 1 square metre(m2).
The metric conversion factors are
lbs./l,000 sq. ft.= 4.88grams/m2
0.001 inch = 25.4 (usually rounded to 25μm)
1 mm = 1,000μm
(2)Brightness
Brightness is a measure of the total reflectance of white light. Values are expressed on a scale of 1 to 100, with 100 being the brightness of pure magnesium oxide. Most quality grades have reflectance values in the mid-80s. The brighter the board, the more brilliant the graphic possibilities.
(3)Paper and Moisture Content
Paper is hygroscopic and absorbs and loses moisture according to the ambient relative humidity (R.H.) and temperature. Paper at 20% R.H. will contain about 4% moisture, while at 80% R.H., it will contain about 15% moisture.
The physical properties of paper vary dramatically with moisture content, and in some applications the moisture content of the paper during processing must be controlled. Because physical characterization values depend on moisture content, all paper testing must be done at a precisely controlled temperature and humidity. Internationally, the standard conditions are specified as 23°C and 50% R.H.
Paper is hygroexpansive: when it absorbs moisture, it expands; when it dries out, it shrinks. Between 0 and 90% R.H., the dimensions can change 0.8% in the MD and 1.6% in the CD.
Whenever a paper sheet is laminated to or coated with a material that is not affected by moisture (for example, plastic film, aluminum foil, or heavy print or varnish), there is the potential for curling when the humidity changes. If the paper gains moisture and expands while the surfacing laminate or coating remains the same, the paper will curl toward the surfacing material. When the paper loses moisture, it will shrink and curl away from the surfacing material (Figure 3.10).
Paper/foil laminate Paper/foil laminate Paper/foil laminate
at 40% R.H. at 20% R.H. at 80% R.H. Figure 3.10 Paper's hygroexpansive nature can cause unwanted curling when paper is bonded to an environmentally stable surface
(4)Viscoelasticity
Paper is more or less viscoelastic, depending on the rate at which load is applied. Simply put, the faster a load is applied, the greater the apparent strength. Over long loading periods, paper fibers move and distort or "creep."
6. Paper Types (纸类型)
(1)Newsprint and Related Grades
(2)Book Papers
(3)Commercial Papers
(4)Greaseproof Papers
(5)Natural Kraft Paper
(6)Bleached Krafts and Sulfites
(7)Tissue Paper
(8)Label Paper
(9)Pouch Papers
(10)Containerboards (linerboard and medium)
7. Paperboard Grades (纸板类型)
(1)Chipboard, Cardboard, Newsboard
(2)Bending Chipboard
(3)Lined Chipboard
(4)Single White-Lined (SWL) Paperboard
(5)Clay-Coated Newsback (CCNB)
(6)Double White-Lined (DWL) Paperboard
(7)Solid Bleached Sulfate (SBS)
(8)Food Board
(9)Solid Unbleached Sulfate (SUS)
8. Paperboard Cartons(纸盒)
Paperboard provides a versatile and economical material not readily matched by other packaging mediums. One significant advantage is the low tooling cost compared with that for materials such as plastics. Effective paperboard package design is based partly on knowledge of paper and product properties and partly on craftsmanship and art. Paperboard packaging can be considered in a number of categories.
(1)Folding Cartons
Folding cartons are by far the largest and most important group in paperboard packaging. Folding cartons are made as flat blanks or as preglued forms that can be flattened for shipping. They can be made economically on high-speed production machinery. The majority of folding carton designs can be classified as falling into either the tube-style (Figure 3.11) or the tray-style design families (Figure 3.12).
Figure 3.11 A gable-top carton blank and an erected gable-top carton
Figure 3.12 A six-cornered Brightwood tray, flat and assembled
The gable-top container (Figure 3.11) is basically a tube-style carton that has found many applications, particularly for dairy products and fruit juices. The heavily sized and polyethylene-coated board is erected and heat-sealed at the point of fill. Combibloc and Tetra Pak are similar-appearing proprietary cartons made from complex paper/foil/poly laminates. A principal application is for aseptic beverage packaging such as juice boxes.
(2)Setup Boxes.
Setup boxes (Figure 3.13) are rigid cartons that are delivered erected and ready for filling. They need as much storage space empty as they do when full. Setup boxes are not as amenable to high-speed production as folding cartons, and their manufacture, by comparison, is slow and labor intensive. These factors add significantly to the cost of a setup box. The rigidity of setup boxes gives them an upscale image, a factor used to advantage by marketers of cosmetics, fancy stationery supplies, quality chocolates, jewelry, and other gift items. Setup boxes are typically constructed from a heavy, low-grade chipboard with no particular folding or printing qualities. In its most elementary form, the board is cut to shape, and the sides folded up and taped with stay tape to form a stayed box.
Figure 3.13 Examples of setup box designs
(3)Tubs, Trays, and Liquid-Resistant Boxes.
Paperboard can be formed into round or square tubs with paper end seals. Such forms, constructed from food board, are used to contain such items as ice cream and frozen foods. Flat sheets with gusseted corners can be folded to form food trays for frozen entrees or other food products. In most wet food applications, the board is coated with either polyethylene or wax. Dual-ovenable paperboard trays are coated or laminated with an oven-temperature-tolerant plastic such as polyethylene terephthalate (PET).
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