Bags and Foams Dr. Muhammad Farooq Bulk and Heavy-Duty Bags • Bulk bags and heavy-duty bags are designed for packaging large quantities of solid or liquid product. They can contain as much as 5000 kg (11,000 lbs) and, therefore, must have high tensile strength. • Woven PP fabric is usually the material of choice, although HDPE, PVC, and polyester fabric are also used. Some bags, especially in smaller sizes, are made of film, usually LLDPE or HDPE, rather than fabric. • Other bags include a plastic film layer in a construction that is mostly paper. An alternative is to use the plastic as a coating on the paper. HDPE, LDPE, PVDC, PP, and combinations are all used in such applications. • Bags made of woven fabric do not generally provide sufficient containment for liquid products, so such bags often incorporate a film liner. • Polyethylene is used most often, but aluminum foil or PVDC copolymers can be used to provide improved barrier ability. The liner may be a single material, or have a multilayer construction. • Alternatively, the bags can be coated with PVC or latex to make them waterproof. Liners can also be used in bags for solid products. Often the liner is designed to be disposable to facilitate reuse of the bag. The bag seams can be heat-sealed or sewn, depending on the material and the application. Some bags are made from tubular material, so sealing is required only at the top and bottom. Others have seams at the back, bottom, and top; or they can be made like three-side seal pouches, with a fold forming the bottom of the bag. • Large bulk bags are most often designed to be filled from the top and emptied from the bottom. • Filling and discharge openings may be sealed, or may simply be tied shut. Spouts or other dispensing devices may be included, or the bags may be cut to open. • Handling devices, such as loops of fabric, are often incorporated. Ultraviolet light stabilizer must be incorporated into the resin formulation if the bags will be stored and used outdoors Bag-in-Box • Bag-in-box structures of various kinds are very common. One of the common ways to add rigidity to a flexible package, for stacking and other purposes, is to combine it with a folding carton. • The most familiar example is breakfast cereal. The product is contained in a pouch, but that pouch is itself contained in a folding carton. In addition to rigidity that facilitates shipping, stacking, and display in the store, along with storage in the consumer’s kitchen cabinet, the paperboard provides an excellent surface for printing, adding to the sales appeal of the package. • Bags can also be contained in corrugated boxes, when additional rigidity is desired. This is the approach taken for flexible pouches containing wine. Bag-in-box wine pouches are multilayer structures, typically containing a foil or metallized film layer for oxygen protection. • The pouch has a dispensing orifice and closure attached that protrudes through a special opening in the corrugated box, allowing for ease of use. • Such packages provide much greater efficiency in terms of weight and use of space than glass bottles. They also provide excellent product quality over an extended period after opening, since they prevent oxygen from reaching the product and causing deterioration. • However, they do not have the same consumer appeal as glass bottles. They are used almost exclusively for inexpensive brands in large quantities. Foams • • • • • Plastic foams are used in packaging both as cushioning materials and as containers. Polystyrene is used most often, but there is also considerable use of polyethylene, polypropylene, and urethane foams, along with some other materials. Foams are characterized by very light weight, good insulating capacity, and the ability to absorb shocks and protect the enclosed product. The most common use of packaging foams is as cushioning materials, to protect products during the distribution cycle. Foam cushioning materials can be used as small shapes piled loosely around the object, loosefill, or as molded pieces designed for a particular application. Loosefill is used most often for light weight objects (used to prevent damage to fragile objects during shipping) and molded pieces are used most often for large heavy objects. A variant of molded foam is foam-in-place systems, where the foam is formed within the shipping container, using the container and the product as molds, and producing a foam that fits tightly around the product. Foam containers are most often produced by thermoforming extruded foam sheet. They can also be produced by molding. Foams types • Foams can be divided into two groups. • Open cell foams have communicating channels between adjacent cells, got low compression set. • In closed cell foams, the cells are completely surrounded by the polymer. • In an open cell foam, liquids or gases can travel through the foam by traversing the channels between the cells. Therefore these foams tend to be absorbent; a sponge is a common example. • Closed cell foams provide much better barrier and are nonabsorbent. The cells sometimes contain substantial amounts of residual blowing agent. Most packaging foams are closed cell. A closed cell foam can be converted to an open cell foam by subjecting it to enough force to rupture polymer membranes between the cells, providing channels between them. Polystyrene Foam • Polystyrene, as mentioned, is the most common packaging foam. Polystyrene foam is the material of choice if it can perform acceptably, since it is typically the least expensive packaging foam available. It is used extensively for containers as well as for cushioning material, in molded shapes and in extruded form. • Molded shapes are commonly termed expanded polystyrene foam (EPS), while the extruded material is called simply extruded PS foam. • The term “styrofoam” is often incorrectly used for these materials, but Styrofoam is a Dow Chemical Company trademarked extruded polystyrene foam used primarily for building insulation and not found in packaging. PS foam is relatively chemically inert, and is acceptable for use in food packaging. The ubiquitous coffee cup utilizes the light weight of PS foam combined with its insulating ability to provide an inexpensive container for hot drinks. • Fast food outlets often use PS foam in the shape of bowls or clamshell for products that need to be kept either hot or cold. Shippers of sensitive products ranging from fish to pharmaceuticals often rely on PS foam containers to keep products cold during distribution, especially if the products are shipped by air freight. • When insulation is not required, PS foam is still frequently used as part of distribution packaging systems due to its excellent shock-absorbing capabilities. • In a very different application, the cushioning ability of PS foam is used in the form of labels on single-serving glass bottles for carbonated beverages to reduce surface abrasion of the glass, permitting significant light-weighting that makes the glass bottles more competitive with aluminum cans and PET bottles. Expanded Polystyrene Foam • To make expanded polystyrene foam, PS granules or beads are impregnated with a hydrocarbon blowing agent. The blowing agent produces bubbles in the plastic, forming the cells, as it vaporizes. Pentane is the most common choice, and is used in amounts up to 8% of the polymer by weight. Both the amount of blowing agent and the processing conditions influence the properties of the foam. • The PS beads are pre-expanded by heating them to 85 to 96°C (185 to 205°F)to vaporize the pentane. Typically the beads reach 25 to 40 times their original size, resulting in a foam density of 16 to 26 lb/m3 (1 to 1.6 lb/ft3). • These pre-expanded beads are aged so that they reach equilibrium. Next, they are packed into a mold, where they are held under several tons of pressure while steam is introduced directly into the mold. The heat and pressure cause the beads to fuse together, producing a semirigid closed cell foam. The mold is overfilled at the beginning of the cycle, since space is occupied by the voids between the beads, as well as by the foam itself. If the mold were not overfilled, gaps in the finished product would result. • The product is held in the mold during a cooling cycle, and then released when it is dimensionally stable. The mold can directly produce the final shape desired, or it can simply produce a block of material that is subsequently cut into the final shape. • Additional aging and curing steps may be employed to further reduce the density of the material. Expandable PS beads are manufactured in three sizes: small, medium, and large. • The bead size required depends on the wall thickness of the molded part. Large beads can be used for thick walls, while small beads are necessary for products with thin walls. Extruded Polystyrene Foam • Extruded PS foam is produced using extrusion, rather than molding. The PS resin is melted in an extruder, the blowing agent and a nucleator mixed in, and the blend extruded. • As with molded foam, the blowing agent creates the cells as it vaporizes. The purpose of the nucleator is to aid in obtaining the desired cell size and uniformity by providing sites for bubble growth. • Talc, citric acid, and blends of citric acid with sodium bicarbonate are often used as nucleators. The blowing agent is usually a hydrocarbon or hydrocarbon blend, which is injected into the melt as a liquid, or sometimes as a pressurized gas. In recent years, use of carbon dioxide as a blowing agent, either in place of or blended with hydrocarbons, has increased substantially. • Chlorofluorocarbons (CFCs) were common blowing agents in the past, but are no longer allowed due to their ozone depleting activity; hydrochlorofluorocarbons (HCFCs), which are less active in ozone depletion, cannot be used as blowing agents in the U.S., and are also being phased out worldwide. • The melt is kept under pressure until it leaves the die, preventing vaporization of the blowing agent. When the melt exits the die, the pressure is released and the blowing agent immediately vaporizes, expanding the melt. • If the melt strength of the polymer is not high enough, this abrupt expansion will fracture the melt. It must be able to withstand the pressure exerted by the blowing agent, forming a uniform network of cells. For polystyrene, this requires that the melt be cooled after the blowing agent has been introduced and before the pressure is released. • Two methods are commonly used for cooling the melt to the required temperature. One method is to incorporate a cooling zone in the extruder, after the blowing agent has been blended in. • The more common approach is to use a two-extruder system, or tandem system. The polymer is melted and the blowing agent and nucleator added in the first extruder. The melt is then introduced, still under pressure, to the second extruder, where it is cooled and then released through the die. • • • • The foam can be produced as a flat sheet, using a slit die as is done for cast film. Most often, however, a tubular film is produced, using an annular die. In this process, a compressed air ring around the circumference of the die forms a thin skin on the foam surface as it exits the die and begins to foam. The tube of foamed PS passes over a water-cooled mandrel, which provides additional cooling. Next the foam is slit and wound into rolls. The amount of blowing agent introduced is the primary determinant of the final density of the foam. The size and number of cells are controlled by the amount of nucleating agent. Extruded PS foams used for packaging typically have densities of 64 to 96 kg/m3 (4 to 6 lb/ft3). As mentioned previously, the sheet is commonly thermoformed, often using matched molds, to produce the desired package shapes. Matched mold forming is used to minimize the distortion that would otherwise likely result when the sheet is heated. For best results, the sheet should be aged for 3 to 5 days before thermoforming, so the gas pressure in the cells can equilibrate. The scrap produced during thermoforming can be ground and densified in an extruder for recycling. Used foams can also be recycled. Styrene Copolymer Foams • Copolymers of styrene and other monomers are also used in packaging foams. The most common is styrene-acrylonitrile (SAN) foam, which is a semirigid foam and offers better performance than PS foam for heavy products (high static loads). • The density of SAN foam is usually about 16 lb/m3 (1 lb/ft3). Polyolefin Foams • Polyolefin foams are somewhat higher in cost than PS foams. They are also more flexible and better able to provide protection from multiple impacts. • Typical densities are 16 to 32 kg/m3 (1 to 2 lb/ft3). Polypropylene foams have somewhat greater rigidity than polyethylene foams. Polyolefin foams, like PS foams, are available in two varieties; expanded (generally termed moldable) and extruded. PE/PS copolymer foams are also available, with characteristics generally intermediate between PS and PE foams, but with outstanding toughness. The manufacture of polyolefin foams is very similar to manufacture of extruded PS foam. Hydrocarbons or blends of hydrocarbons and carbon dioxide are used as blowing agents. An annular die and forming mandrel is used to produce extruded sheet products. Extrusion through rectangular slit dies onto a conveyor belt is used to produce planks. • PE foams are also available in cross-linked grades. These are manufactured in a much different manner. • Rather than physical blowing agents, like hydrocarbons and carbon dioxide, chemical blowing agents such as azodicarbonamide are used. These agents produce a gas as a result of a chemical reaction. The resin, additives, cross-linking agents, and blowing agents are mixed together at temperatures below the activation temperature of the blowing agent, and then extruded into a flat sheet or other profile. Next, the material is crosslinked, either chemically or by radiation. Radiation is typically used for thin materials, and chemical crosslinking for thick profiles. The final step is to expand the foam by exposing it to hot air (about 200°C) to activate the blowing agent. Polyurethane Foams and Foam-inPlace Systems • Polyurethane foams are not widely used in packaging, but they do have significance in systems that rely on foam-in place operations, rather than on molded cushions or loosefill. In these systems, a mixture of a polyol and an isocyanate, with other ingredients, is injected into the erected shipping container (usually a corrugated box). As the foaming reaction begins, a PE film is placed over the top of the mixture, the product is added, and a second layer of film placed over the product. Next another shot of the polyol/isocyanate mixture is added, and the case is quickly sealed. As the foaming action continues, the product, protected by the polyethylene sheets, is tightly encapsulated within the foam (Fig. 13.1). Alternatively, the foam ingredients can be dispensed into a plastic bag within the cart • Foam-in-place systems are not usually as cost-effective as molded cushioning or loosefill in high volume packaging operations. However, in low volume applications where relatively heavy products of many different shapes and sizes are packaged, they can be advantageous. In these situations, molded cushions to fit all the various packages and products are not economical, both in terms of initial investment and in terms of the logistics involved in getting the correct cushion for the correct product. Loosefill works well in such situations for light products, but for heavy products it tends to become displaced within the package, allowing the product to settle to the bottom, and perhaps become damaged. Starch-Based Foams • Starch-based foam loosefill packaging shapes are also available. While starch was not traditionally considered a plastic, the technique for producing these foams involves plasticizing the starch with water (and sometimes additional plasticizing agents) in an extruder and molding it. Starch foam shapes and cushions compete with loosefill PS and molded PS cushions. The major advantage of starchbased foams is that they are considered by many to be more environmentally friendly, in part due to their water solubility. A related disadvantage is their tendency to be susceptible to moisture sorption, which may even result in collapse in environments with very high humidity. There have also been concerns, for some starch-based cushioning materials, about rodent or insect infestation Nonfoam Plastic Cushioning Systems • Not all plastic cushioning systems rely on the use of foam. Two major alternatives use air to provide resiliency, but are not foams. In bubble wrap, air bubbles of a defined size and pressure are sealed between two plastic sheets. These materials can be used as wrapping material, or as bags or envelopes. Several different sizes of bubbles are available. The larger sizes are intended for heavier duty applications, and the smaller ones for lighter duty. Bubble wrap is an exceptionally light weight packaging material, with a density as low as 11 kg/m3 (0.7 lb/ft3). Another alternative is the use of transparent inflatable bags to provide a cushion of air around the product. A major advantage of these systems is that they occupy minimal space during shipping and storage prior to use, since they are inflated at the point of use. Some are designed to be collapsible and reinflatable, permitting reuse. Cushioning • To select a cushioning system that will protect a product during transportation, it is necessary to know how fragile the product is and the distribution hazards it is likely to be exposed to, as well as the performance characteristics of the cushioning material. Product fragility is generally determined by actual testing of the product. It is usually reported in “G-levels,” the number of multiples of normal gravitational force that results in product damage. For analysis of distribution hazards, it is common to use characteristic drop heights that depend on the product weight and the type of handling system used. Then a calculation can be made, determined by the characteristics of the cushioning material, of the G-level a product would be subjected to in falling from the designated drop height, protected by the cushion under consideration • If this G-level is lower than that which causes product damage, the cushion would be expected to successfully protect the product. If it is higher, a different cushion needs to be chosen. The information about cushion performance is most often transmitted in the form of characteristic “cushion curves” that let us determine the suitability of foams as cushioning materials. Computer programs that contain mathematical models of cushion curves for various types of foams, sometimes along with cost information, are also available. Relatively simple calculations then permit selection of an appropriate cushion thickness and loadbearing area that can reasonably be expected to provide adequate protection for a given product. • In addition to impact forces, transmission of vibrations to the product can result in damage. Characteristic vibration transmissibility plots for foams are also available, which can be matched with the product’s susceptibility to damage. Of course, the design process should be followed by the preparation of a model package and suitable testing of the product/package system, or in case of a very expensive product, a mock-up used with accelerometers to determine actual performance. It must be kept in mind that the fragility of the product may be different in different parts. For example, if the product falls on its side as opposed to its bottom, the result may be either greater or less damage. In general, at least for products contained in corrugated fiberboard boxes, flat drops are more damaging than corner drops, because when the box deforms during a corner drop, it absorbs some of the impact energy. Example • For the cushion with a characteristic cushion curve shown in Fig. 13.2, calculate the loadbearing area that will provide adequate protection for a product measuring 20 cm by 30 cm, with a mass of 10 kg, and with a fragility of 50 G. The maximum expected drop height for the product is 1 m. Solution: • We can eliminate the 2.5-cm cushion immediately since it does not provide decelerations below 50 G at any static loading. The 5-cm and 7.5-cm cushions are both possibilities. To determine under what conditions they would be usable, we need to calculate the static loading. The static loading must be between 7000 and 27,500 nt/m2 to keep the G under 50 for the 5-cm cushion. The static loading must be above 4000 nt/m2 and can be as high as 45,000 nt/m2, the maximum extent of our information, for the 7.5 cm cushion. With a 10 kg object, the loading will be 7000 nt/m2 with a cushion area of 140 cm2 (see sample calculation next). The loading will be 27,500 nt/m2 with a cushion area of 35.7 cm2. • Since the footprint of our item is 600 cm2, either of these or anything in between is feasible. Thus our cushion can provide any contact area between 35.7 and 140 cm2, if we are using a 5-cm cushion, but should not provide more or less than this to get adequate performance. If we choose to use a 7.5-cm cushion, by the same process we determine that the cushion area should be at least 21.8 cm2 and not more than 245 cm2 to ensure adequate performance. The precise cushion geometry can be anything that is practical for the particular product. Static loading = force/area = (mass × g)/area So, area = (mass × g)/static loading ,area = (10 kg × 980.665 cm/s2 × m/100 cm × nt/(kg m/s2))/7000 nt/m2) = 0.014 m2 0.014 m2 × (100)2 cm2/m2 = 140 cm2 Thermal Insulation Using Foams • Foams, in addition to being useful as cushioning, can be used to provide thermal insulation for products. A frozen product, for example, might be packaged with ice (or dry ice or gel packs) to provide cooling, and encased in a foam container to help reduce the conduction of heat from the surroundings into the container. Often the temperature inside and outside the container can be regarded as relatively constant, and the heat transfer process can considered essentially one-dimensional. In such cases, Fourier’s law of heat conduction reduces to its one-dimensional steady state form: • where q is the rate of heat conduction, T is temperature on the two sides of the material, A is the area available for heat transfer, x is the thickness of the material in the direction of heat transfer, and k is the thermal conductivity of the material. As can be seen, the higher the thermal conductivity of a material, the more heat will be transferred through it in a given amount of time, at a specific temperature difference. Plastics are, in general, good insulators. Gases are much better insulators than solids, so in a foam, the combination of the plastic and the gas pockets within it provides very low thermal conductivity. Each phase, gas and solid, contributes an amount roughly proportional to its volume fraction. Thermal conductivities of some common packaging foams are given in Table 13.1. • These values can be used to calculate the length of time that a product in a given package can be expected to remain at a safe temperature, as shown in the following example. Example: • A 1 kg product that must be kept at a temperature of 0°C is packaged with 2 kg of ice in a 1.5 cm thick expanded polystyrene container, with inside dimensions measuring 0.4 m × 0.4 m × 0.6 m. How long will the product be protected? The thermal conductivity of the polystyrene foam is 0.030 w/m K. The outside temperature is constant at 25°C. To simplify the problem, we will assume the heat conduction can be represented with reasonable accuracy by one-directional heat transfer through the total inside area of the container, and that the container provides the only resistance to heat transfer. We will also assume the product and the ice start out at a temperature of 0°C. They will then remain at 0°C until all the ice is melted. Thus we need consider only the heat of fusion of the ice, 6.01 kJ/mol. • The total amount of heat which can be conducted without harming the product, then, is (6.01 kJ/mol)(2 kg)(mol/18 g)(1000 g/kg) = 668 kJ The rate of heat transfer can be calculated from Equation 13.1: • One of our assumptions in this problem, that the foam container provides the only resistance to heat transfer, is, in many cases, a poor assumption. We have also assumed that conduction is the only mechanism for heat transfer—that radiation and convection do not play a role. This assumption is accurate in many instances, but inaccurate in others. We must, therefore, regard this calculation as only a rough approximation to the actual performance of the package. Plastic Pallets • Increasingly, plastic pallets are being used in place of wooden pallets for product distribution. Although the cost of a plastic pallet can be four times the cost of a wooden pallet, its lifetime is usually much longer, typically five to nine years. Repair of broken plastic pallets is not usually feasible, but they are recyclable. Consequently, there is frequently an overall economic advantage to using plastic pallets. Because plastic pallets represent a substantial investment, their use is most suitable in situations where the distribution environment is relatively controlled, so that loss can be minimized. • HDPE is used most often in pallet manufacture, with PS, PP, and fiberglass-reinforced plastics also being used. Steel reinforcements can be added to increase the pallet’s loadbearing capacity. Structural foam molding is common, with low-pressure injection molding producing pallets with a solid skin and a foamed core. Wall thickness is generally 0.5 to 2.5 cm (0.19 to 1.0 in). Plastic Drums and Other Shipping Containers • High molecular weight HDPE is increasingly being used for drums, pails, totes, and similar shipping containers, as an alternative to steel and fiberboard. Plastic drums are often used for chemicals, and are also used frequently in the food-processing industry. Standard drum sizes in North America are 57, 76, 114, 132, 208, and 216 L (15, 20, 30, 35, 55, and 57 gal), with 208 L (55 gal) used most often. Drums are categorized as closed-head (or tight-head) or open-top. Open-top drums are used mostly for water-based products, and are easier to clean and reuse. Plastic drums are permissible for shipment of hazardous materials provided the appropriate testing has been done and the containers are properly labeled. • Another use of plastic in drums is as a polyethylene liner in a steel drum. The liner adds chemical resistance and simplifies cleaning and reuse of the drum, since it can be discarded and replaced with a new liner. • Plastic pails, usually produced by injection molding from HDPE that contains a butene comonomer, are available in sizes from 4 to 23 L (1 to 6 gal). Open-top containers, which account for about 75% of the market, are referred to as pails. Closed-head containers are called either pails or jerrycans (also written jerry can or jerrican). Pails generally include a handle. Pails or other containers used for shipping hazardous materials must have passed required performance tests. • Plastic crates and boxes are usually injection molded from HDPE or PP. They are available in a variety of sizes and designs. Most of these are designed to be reusable, and often will either collapse or nest to minimize space requirements when they are not in use. • Plastic boxes can also be made from plastic corrugated, usually a PP copolymer or HDPE. Polycarbonate is used in some special applications. The corrugated board can be made directly using extruded profiles, or it can be assembled by laminating together three separate sheets: the two liners and the inner fluted layer. The boxes can then be made by die-cutting, scoring, and folding the board, similar to the process used for making corrugated card board. The main difference is in joining the seams. High frequency welding generally gives the best results. Silicone-based or hot-melt adhesives can be used if the board has been corona-treated to enhance adhesion. Metal staples (stitching) can be used, but are not as desirable since they create weak spots in the container around the staples. Plastic corrugated is particularly suited for manufacture of conductive containers for electronic parts that require protection against electrostatic discharge (ESD). Carbon films printed onto corrugated paperboard can be used, but there are problems with wear and fiber generation. These are avoided with conductive plastic corrugated, since the conductive material can be introduced into the polymer before extrusion. Packaging for Electrostatic Discharge Protection • As mentioned above, sensitive electronic products require protection from electrostatic discharge (ESD). In unmodified plastics, ESD is a significant problem, since the nonconductive nature of plastics causes them to easily accumulate static charges. Consequently, when plastic packaging is used for sensitive devices, it must be modified to make it less susceptible to charge buildup, and to provide for dissipation of any charges that are created. Both of these are accomplished by providing a conductive path for electrons. • The most common way to alter the susceptibility of the plastic to static charge accumulation is to add to it some component that attracts and retains a thin layer of surface moisture, and at the same time weakly ionizes it. The ionized water provides the needed conductive path so that static charges will dissipate. The additive can be applied to a surface layer in the material, can be distributed through the bulk of the material, or can even be confined in an inner layer of a coextruded material. Antistatic additives that are commonly used include ethoxylated amines, quaternary amines, and ethoxylated amides. • Antistatic packaging materials come in a variety of forms, including film, foam, bubble wrap, and containers. These materials are often tinted pink as a visible signal of their antistatic properties. An alternative to the use of additives is selection of plastics that are inherently static-dissipative. While such materials are still largely experimental, one promising alternative is polyamide ethylene oxide block copolymers.
