The Beginning Point
Paper is composed primarily of cellulose fibers. These fibers are obtained from the processing of trees raised and cultivated on mostly private forested land. Both softwood (pine, spruce, fir, etc.) and hardwood (poplar, ash, maple, etc.) varieties are used depending on the properties desired. Softwood trees generally provide longer fibers and stronger paper, while hardwoods have shorter fibers and improved formation and printability.
The wood from trees is composed of 50 percent cellulose walls, 30 percent lignin that bonds the cellulose together and 20 percent carbohydrates, protein, fats and resins. The trees are debarked, cut into small chips and cooked to remove the lignin and the remaining ingredients.
The wood fibers remaining in the form of pulp slurry are used to make paper. This pulp, consisting mainly of softwood for kraft paper (the type used for most sacks), is then further refined and treated to arrive at the desired consistency. The refined pulp slurry now consists of fibers plus additives and is about 99 percent water as it moves into the paper machine.
A typical paper machine is a Fourdrinier on which paper is formed as the pulp slurry flows onto a moving wire cloth, allowing some of the water to drain through. As this occurs, the fibers unite by matting or intertwining. This damp mat of paper fibers then leaves the wire and is supported by a felt blanket that moves through a series of roller presses to squeeze out more water. It then passes through a dryer section which removes all but a small percentage of moisture necessary for paper to perform properly. The paper is finally rolled, slit and distributed for use or additional processing.
Paper is usually specified according to basis weight. A ream of paper is the number of sheets for a given unit, usually 500 sheets in packaging paper.
Basis weight in the multiwall industry is the weight of five hundred 24 by 36 inch sheets of paper or pounds per 3,000 square feet of paper. The most common basis weights of paper used in paper shipping sacks run between 40 and 60 pounds.
Kraft paper is the common term used to designate paper made from a sulfate process which is the alkaline treatment of softwood pulp. The term comes from the German word “kraft,” meaning strength, which is its distinguishing characteristic. Natural kraft is a brown color and is commonly abbreviated NK. It is characterized by long fibers for superior pliability and tensile strength properties. It can be bleached to a white color at the expense of some strength and opacity.
Machine Direction (MD) is the direction of the paper fibers when running parallel to the forward movement of the paper machine wires. The machine direction runs the length of a paper shipping sack. Because of the paper fibers’ orientation, the tensile strength is generally highest in the machine direction.
Cross Direction (CD) of paper is at right angles to the machine direction. The cross direction of paper runs the width of a paper sack. Tear strength is generally highest in this direction.
Paper Finishes, Additives and Treatments
The amount of desired polishing that can be applied to the surface finish of paper – a rough surface – gives better porosity and a non-skid surface, while a highly polished finish yields a denser sheet with better print qualities.
- Resin Sizing
An internal or surface treatment applied to provide better printing, some water resistance and scuff resistance of the paper sheet.
- Wet Strength Resin
A treatment of internal additives applied to allow a paper sheet to retain a substantial degree of its original strength after saturation with water. Wet strength sheets are usually marked with colored stripes.
- Silicone Resin
A coating which imparts release properties when applied to paper used in the packaging of tacky products.
- Water Repellency
A special chemical treatment applied internally or to the surface of paper to provide water repellency. It may also offer release properties and grease resistance.
- Insect Resistant Treatment (IRT)
A treatment used to make paper insect-repellent.
A treatment employed to resist the growth of fungus or mold.
- Volatile Corrosion Inhibitor
A coating used for rust prevention.
- Fluoropolymer Treatment
A coating or impregnation of a fluoropolymer agent to make paper repel oil or grease. It can be furnished on regular or supercalendered paper and in the base stock of clay-coated paper.
Major Paper Properties
The ability of 100 cubic centimeters of air to pass through one square inch of paper as measured by time (in seconds). For example, paper that withstood the passage of air for a high number of seconds (indicating high resistance) would be termed a low-porosity sheet (15-30+ seconds); a low number of seconds (indicating little resistance) would indicate a high-porosity sheet (4-15 seconds).
Porosity becomes important in the printing or coating of a sheet or in the filling of a paper sack with a powdery product.
A characteristic of paper which affects many important properties. Below 3 to 4 percent, paper tends to become brittle and rapidly loses strength. Paper exposed to the air, depending upon the humidity, quickly picks up or loses temperature and moisture.
The force (as measured in grams) required to continue a tear that has been previously started in a sheet of paper. Good tear strength is important to paper sack performance in packing, handling and shipping. Tearing strength is measured in both MD and CD and is expressed as MD tear and total tear (MD + CD) in shipping sack specifications.
The force (measured in pounds) required to break a one-inch wide strip of paper by a constant rate of elongation instrument. It is measured in MD and CD and is a part of most shipping sack specifications. The increased tear and tensile strengths of shipping sack grades differentiates them from sack, grocery and industrial wrap grades of natural Kraft papers.
The effort (measured in pounds per square inch) required to rupture a sheet of paper when a rubber diaphragm is pressed against the flat surface of the sample. Although this test is an indication of the overall strength of a sheet, it is not part of a paper shipping sack specification.
The measure of elongation of paper before it ruptures during a tensile strength test. To a great degree, stretch indicates the resistance to shock in paper sacks and accounts for much of the difference in performance from various types of kraft paper.
Tensile Energy Absorption (TEA)
A sheet’s toughness, encompassing both tensile strength and stretch, thus measuring the amount of work (force times distance) performed by a sheet. TEA is measured in both MD and CD and is expressed in CD TEA and total TEA (MD + CD) in shipping sack specifications for extensible-type paper.
Types of Paper Used in Shipping Sacks
A strong paper made from long fibers of unbleached wood pulp.
Wet Strength Kraft
The same as natural kraft but with additives which enable it to retain about 20-25 percent of its dry tensile strength when saturated with water. Ideal as an outer ply for moisture resistance.
Any kraft sheet can be fully or semi-bleached to attain a white or off-white color. However, bleaching lowers the strength of kraft paper to some degree and must be considered in sack performance.
In making this sheet, the paper fibers are mechanically compacted, mainly in the MD direction, yielding a paper with a greater ability to absorb more impact than natural kraft. Extensible paper will do more “work” because of its higher stretch than natural kraft. Less total basis weight and/or fewer paper plies can be used in such sacks with little or no loss of strength. There is also a modified extensible kraft which makes the bunching or compacting of fibers more balanced in both directions.
High Performance Papers
A unique process that allows unrestricted natural shrinkage during the drying process. This provides high cross-direction stretch for improved sack performance. The high performance can also be achieved with high consistency refining in the paper making process.
Clay-Coated Bleached Kraft
A bleached sheet coated with three to six pounds of clay coating to improve opacity and increase smoothness and printability.
A coating that is used to provide release properties.
An extrusion coating of plastic resin to provide moisture protection.
A sheet with its surface either saturated or coated to provide moisture protection.
Polyvinylidene Chloride Coated Kraft (PVDC)
An extrusion or emulsion coating of plastic resin for grease and odor protection.
Greaseproof, Glassine or Parchment Sheet
A specially treated, very dense and hydrated sheet which provides a barrier to grease or oily ingredients.
Barrier Films And Coatings Used In Paper Shipping Sacks
Although there is no universal, economical barrier possessing all the properties required for all products, there are many material options available which, when combined with kraft paper, offer a balanced, cost/performance ratio package.
Paper can be treated, coated, laminated or combined with other materials to meet the unique requirements of a specific end use.
The barrier properties of coatings or films tend to be straight-line relationships to density and gauge and will improve as density and gauge increase. The material properties as well as physical location or position of the barrier ply help determine the overall effectiveness of a paper sack construction.
Barrier plies do not usually contribute significantly to the overall strength of a paper sack. However, there are a few strength plies that can be used in a sack which may or may not also offer barrier properties. Such plies permit the reduction of paper plies or provide a super strong, reinforced sack with maximum protection for special products or requirements.
|Common Barrier Materials Used in Paper Shipping Sacks
|Type of Protection
|moisture and grease
|Low-density or linear low-density polyethylene
|Greaseproof or glassine paper
|moisture, odor, grease
|grease, odor, moisture
|moisture, odor, grease