In the Bag

Less is more in today's heavy-duty sacks. New developments in production and materials keep profits.... In the Bag

The heavy-duty sack industry has been showing some changes based on the prices of raw materials as well as the production technologies. The effect of those variables will determine the growth of this sector in the near future.

Heavy-duty sacks, with thicknesses between 3.6 and 8 mils (91.4 and 203 microns), are used to transport bulky materials from one place to another and are designed to carry products weighing more than 20 lb. This sector of the market has an estimated polyethylene (PE) consumption of 670 million lb for the year 2001.

The market size was estimated to be 524.8 million lb (262,400 tons) for 1998 and 670 million lb (335,000 tons) for 2001, with an average annual growth rate of 8.4% into the year 2003.

Very important in the growth of the market are the price changes for the resins and the development of new technologies and structures. Depending on these advancements and price fluctuations, consumers will be changing from multiwall paper sacks to heavy-duty plastic sacks.

As mentioned previously, very often the products packaged in these sacks are bulky and heavy. A typical application is to package raw materials, including plastic resins in pellet or powder form such as PE, polypropylene (PP), polystyrene, and chemical products such as sulfur, caprolactams, and anhydride phtalic. Also using this type of packaging are food items such as salt and sugar and items including pet food, sand, de-icing salt, fertilizers, and potting soil, among others. A common weight for a filled sack of these applications is 25 kg or 55 lb.

Determining the Specs
The product specifications will be determined by the film's mechanical properties, physical properties, behavior expected once the sack is stored, and dimensional requirements.

The tensile properties of the film must resist the forces and loads during the filling operation, plus the loads resulting from handling and storage. The following aspects must be considered during sack design to achieve the properties needed:

  • Product weight and temperature at time of filling

  • Type of filling system

  • Handling and storage conditions

  • Seal type at the bottom and top of the sack.

The weight of the product being packaged is extremely important for the design. Normal thickness for weights around 20-30 lb (approx. 10-15 kg) is 3.8 or 4.0 mils (96.5 or 100 microns). If the contents' weight is around 50 lb (25 kg), which is the case of resin sacks, the thickness goes from 5.5-7 mils (140-180 microns). New materials and technologies are allowing thicknesses to be reduced to about 4 mils. Also very important is the sack's resistance to the load occurring in storage. Due to the methods used to store or palletize the sacks, sometimes they must resist 10 or even 20 times the weight of the product packaged (see photo above left).

As a consequence, the sack must be rigid enough to avoid deformation in the transversal direction. This problem becomes more critical when the temperatures in the storage place are higher than 87 deg F (30 deg C). The resin selection for the film structure must consider this problem.

Automatic filling machines requiring a valve in the top of the sack are more demanding. The air used to transport and feed the material tends to inflate the sack. As a result, the film must be rigid enough to avoid deformations and thickness reduction. Small perforations in the film could be used to help in the air evacuation.

Tear resistance of the film is very important because of increased risk of puncture and damage to the sack in the storage place and during transport. The integrity of the sack under critical conditions such as pressure, impact, and mishandling is guaranteed by good sealing properties. Although impact could occur during transport and handling, more important is the resistance to the impact generated when the sack falls from heights around 9 ft or more at the storage place.

The film's slipping characteristics are very important for this application. The sack should have a high coefficient of friction (COF) on the outside to prevent sack slippage once it is palletized. Some modifications to the film surface can be made to increase the COF, including embossing or by introducing additives into the resin.

Also important is the ability to print the product description and manufacturer information on the sack surface. The sack requires a surface treatment to permit a good ink adhesion, and the inks must have abrasion resistance.

Dimensional requirements must be considered, including method of transportation, common dimensions of the pallets, and number of units per pallet to ensure good pallet stability and reduction of the slippage risk.

The Material End
The traditional materials used for the production of heavy-duty sacks are PE [including low-density (LD), linear LD, ultra-LD, and high-density] and sometimes ethylene vinyl acetate. Depending on the final application and the mechanical properties required, LDPE is used alone or in blends with the other materials. Butene LLDPE, among the different LLDPE types available, is used most widely due to the balance between price and properties obtained.

LLDPE provides very good tear resistance, sealing properties, and puncture resistance. When it is blended with LDPE, a typical blending ratio is 60%-75% LLDPE and 40%-25% LDPE. It is important to select the appropriate LLDPE grade, because too much flexibility will be detrimental to the dimensional stability of the sack under load and temperatures.

Alternatively, LDPE can be the major component. Fractional melt resins help with bubble stability, have better melt strength, and provide a better impact resistance to the film. Typical melt index values for LDPE are better around 0.25-0.8 g/10 min. Melt index values for LLDPE generally are around 0.8-1.5 g/10 min.

Lately, PP has become another option to produce film for heavy-duty sacks. Some resin companies, including Borealis, Basell, Propilco, and Dow, have developed PP grades to be used in coextrusion or even monolayer structures.

PP will improve stiffness and tensile properties and increase the creep resistance, allowing, in some cases, thickness reduction. One of the most important properties achieved using PP is higher sack temperature resistance, permitting their use in hot-filling operations such as cement packaging.

Processing for Success
Blown film extrusion is the preferred process to produce heavy-duty sacks. Tear strength and the impact resistance of this type of film is far superior than cast extruded film. The estimated market share for monolayer films is approximately 55% versus 45% for coextruded structures. Coextrusion allows more material combinations, special effects on the outside surface, and downgauging.

Flat, non-gusseted tubing is a popular film presentation. Printed, bottom sealed, stacked, gusseted, and non-gusseted bags also are produced in-line with a blown film process. Single wound sheet is produced specifically for form/fill/seal machines or for sacks with a back seam. This type of film is more popular in Europe where there is a trend toward using automatic filling machines running at speeds to 1,500 sacks/hr (25 sacks/min).

Occasionally tubing is produced with gussets to get a better sack shape once it is filled. Some modifications are implemented to the haul-off nip rolls section of the line to override problems due to excessive pressure on the edges of the gussets.

Blow-up ratios from 2.0-2.6 are used frequently, because the best mechanical properties are achieved in that range of values.

Typical thickness values are in the range of 3.6-8 mils (91.4-203 microns) with gauges of 4-6 mils (100-150 microns) being the most common. The width of the film depends on the type of product and the final use of the sack. A typical example is resin sacks with widths of 16-22 in. (40-55 cm).

On to Converting
Once the tubing or the single sheet is produced, it needs to be converted into the final sack.

One disadvantage of using single-wound sheet is the sharp corners on the top and bottom ends of the sack that can make the palletizing process difficult. Another disadvantage is the COF needed for the machine operation can cause bag slippage in shipping and storage.

In the case of tubing, there are several different forming and filling techniques. Some examples are as follows:

Open-mouth layflat sack: The tubing is sealed at one end and cut; the customer fills and seals the sack at the top. Tapered sides and ends are obtained making storage difficult.

Open-mouth side gusseted sack: The tubing is gusseted when extruded or post-gusseted when sealed in the factory. The gusseted tubing is sealed only at one end and then cut. The customer fills and seals the sack at the top. The final shape is square at the sides when filled for easier stacking.

Squared-bottom open mouth: Special shape for the bottom seal. Uses flat tubing. The customer fills and seals the sack at the top.

Valve sack with flat bottom: Squared seal in the bottom or gusseted tubing includes a valve in the mouth. Perfect shape, easy to stack. The valve avoids spillage and permits faster filling. The customer fills the sack without post-sealing operations.

Valve sack with simple seal at bottom: Includes a valve in the top to make the filling process easier. The customer fills the sack without post-sealing operations. Stacking is difficult.

Other processes performed on the film are flexographic printing, embossing, and handle sealing. Embossing is a secondary process used to modify the film surface to increase the COF and prevent the slipping, sliding, and falling of filled palletized bags once stored. The process embosses the tubing from inside, burls to the exterior of the film, and produces an obstacle to sliding. This increases the friction between one sack and another tremendously.

The world of heavy-duty sacks is complex and dynamic. Changing times, technologies, and economics are key to its future.

German V. Laverde is director of marketing at Battenfeld Gloucester Engineering, Gloucester, MA. He has ten years' experience in the plastics industry in Bogota and five years supporting Battenfeld Gloucester's product line globally. With a degree in mechanical engineering and an M.S. degree in plastics engineering, Laverde has worked as a professor and has published papers at Antec, TAPPI, LatinPack, and other conferences. Contact him at 978/282 9268;

The views and opinions expressed in Technical Reports are those of the author(s), not those of the editors of PFFC. Please address comments to author(s).

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