Aerosol containers
- Can-making technology
- Tinplate
- Aluminum
- Other
- Tinplate options
- Linings
- Size
- Seams
- Description
- Can-making trends
In 1946–1947, Harry E. Peterson developed such a can in the laboratories of the Continental Can Corporation in Chicago. It consisted of a 2.68-in.-diameter solder side seamed can body, to which were seamed a pair of concave end sections. The top section, assembled to the body by the canmaker, carried a small valve, soldered at the centerline. The can was designed to be filled upside-down with highly refrigerated (451F or 431C) aerosol concentrates and propellants, after which the end was double-seamed to the body. The final unit stood 4.8 in. tall and had a capacity of about 12.2 fluid oz (361 mL). In an almost concurrent but independent development, Earl Graham of Crown Cork & Seal Company developed a higher-strength modification of the ‘‘Crowntainer’’ beer can. This was a two-piece container. The base was double-seamed onto a drawn steel shell that contained a soldered valve. The early valves were manufactured by Bridgeport Brass Company, Continental Can Corporation, and many other firms. Most were outrageously costly and inefficient. Slightly later, pioneers such as Robert Alplanalp (Precision Valve Corp.) and Edward Green, Sr. (Newman-Green, Inc.) patented more efficient types.
A major innovation occurred about 1951, when Crown Cork & Seal Company engineers developed the ‘‘1-in.’’ (‘‘25.4-mm’’) hole, along with a corresponding valve cup. The cup could carry the valve components within the central pedestal (except for actuator and dip tube), and it could be crimped (swaged) onto the curl or bead that surrounded the ‘‘one-inch’’ (‘‘1-in.’’) hole. This secondary plug-type closure rather quickly displaced the soldered valve units, the last of which were filled during October 1953. The Continental Can Company developed a can dome—called a ‘‘cone’’ in the United Kingdom—to provide the needed ‘‘1-in.’’ hole, while at the same time enlarging their can somewhat and making it taller.
The budding aerosol industry had to make do with these two nominal 12-oz cans until 1953, when the 2.12- in.-diameter can size was developed as a nominal 6-oz container.
At the insistance of a fast-growing industry, the two can companies introduced some additional can heights. They were joined in 1955 by the American Can Company, who entered the market with their ‘‘Regency’’ line of 2.47-in.- diameter cans in four heights. The National Can Corporation followed soon afterward.
The impact extrusion technology for drawing aluminum beer and beverage cans was well developed by the 1950s. The Peerless Tube Company produced aerosol units as early as 1952 and perhaps even earlier. The American Can Company made a unique two-piece aluminum can (Mira-Spray and Mira-Flo) in just two 6-oz sizes. Other very early entrants were Victor Tube, White Metal, and Hunter-Douglas Corporation (see also Cans, aluminum; Cans, fabrication).
Glass aerosols were made, first by the Wheaton Glass Company (Mays Landing, NJ) about 1953 and then by Ball Brothers and several other firms. The valve was incorporated into a ferrule by Risdon Manufacturing Corporation, Emson Research, Inc., and Precision Valve Corporation, and the ferrule was sealed to the glass finish by means of clinching—a method then also used to seal metal caps on beer bottles.
During 1954, Wheaton developed a method for encasing their bottles in a heavy skin of PVC. The plastic envelope helped the bottle withstand minor falls to hard surfaces, but if breakage should occur, it contained the glass shards and kept them from flying outward and possibly injuring persons nearby. Later on, a bonded film was developed and made an unofficial industry standard on glass bottles of W1-oz (30-mL) capacity.
The packaging process also underwent dramatic changes. Several equipment suppliers are now specialized in the manufacture of aerosol filling and packaging equipment. As aerosol volume grew, higher speed lines were needed. Even double-indexing, two-lane-in-line equipment could not produce more than 125 cans/minute. Reengineering resulted in a production rate of 216 cans/ minute. This rate was important to contract fillers because it related directly to competitivety and profitability. Today, very complex rotary machines are available with rated capacities of 480 cans/minute.
In recent history, the aerosol industry has had to contend with many obstacles: the ban on the popular CFC propellants, the global warming potential of various propellants, and, in 2007, legislation that curtails the use of volatile organic chemicals (see Propellants, aerosol).
CAN-MAKING TECHNOLOGY
Tinplate
The three-piece tinplate can still command about 82% of the U.S. aerosol market. This translates to 3.65109 cans in 2006.
Tinplate, in a number of thicknesses of steel and tin, is routinely delivered to the can-making plant in the form of large rolls typically 39.4 in. (1 m) wide and 48 in. (1.22 m) in diameter. A roll may weigh 15,000 lb (6800 kg) and contain about 4.5 mi (7.25 km) of tinplate, depending on the thickness. After sending the roll through a straightener, a slitter cuts it into sheets best suited for making can bodies with a minimum of waste. At the same time, a scroll cutter produces strips from which dome and base section circles can be cut—again with a minimum of waste (see also Cans, steel).
The body sheets are first lined with epoxy–phenolics or other materials, baked in huge ovens, and then lithographed. At this point the individual can bodies are cut apart. A bare metal fringe is allocated for the area to be welded. The WIMA or other type of bodymaker acts to roll up the can body and tackweld it every inch or two to hold it in place with just the right lapover. After this the welding process produces either a ‘‘standard’’ or ‘‘full’’ (tin-free) weld line. Weld nuggets (B31 in.1) form the basic structure. While still extremely hot, the overlap thickness is reduced by heavy compression to only B1.4 times the average plate thickness. The cylindrical can body is then flanged top and bottom for a ‘‘standard’’ or straight-wall can, or is both necked-in and flanged for the ‘‘necked-in’’ containers. The latter have several advantages and are increasingly popular.
Meanwhile, can domes and bases are being formed in multistage presses. They either are used directly or are ‘‘sleeved’’ into long paper tubes for later fabrication. Quite often, one large plant will make sleeve packs of can ends, for shipment to satellite locations where the assembly process is undertaken. The same is true for can bodies, which can be easily shipped in the flat to other facilities.
The final can is assembled using double seamers that typically operate at about 350 cans/minute. As a rule, the finished cans are then tested using a large wheel-like device that pumps a significant air pressure into each one and then checks for pressure leakage, if any. Such cans are automatically shunted aside and scrapped. Finally, the cans are tiered onto pallets of about 40-in.44-in. size, strapped, plastic shrink-wrapped, and warehoused for delivery to fillers.
A recent innovation in tinplate can manufacturing is simply known as shaping. It was developed in the 1970s. The process consists of placing a cylindrical can in a metal mold and then pressurizing it with about 1160 psig (80 bars) of nitrogen (or purified compressed air) to create a can with the shape of the mold. This process remained dormant for 20 years, to some extent because the process was expensive. The method has been improved upon and is now commercialized. Today a relatively large number of aerosol cans are shaped, especially those made of aluminum. One reason for shaping relates to the increasing duplication of U.S. product labels by firms in foreign countries. The use of shaped cans has corrected the problem for now since forgien can-makers do not have the technology. As an example, the WD-40 Company has collected 380 cans of foreign-made specialty lubricants, many of which have patterned their labels and product claims on WD-40 aerosols, They have now changed to shaped cans.
Another innovation dating back to the 1970s involves the development of a compartmentalized tinplate aerosol. The initial product consisted of a 52-mm149-mm aerosol can into which a fluted collapsible polyethylene bag was inserted before the base section was attached. The tubular chimney of the bag protruded through the nominal 1-in. can opening and then it was gently heated and flared around the can bead. The aeosol filler poured the concentrate into the bag, almost filling it but leaving a little space for the insertion of the valve. After the valve was crimped into place, isobutane was added through the bottom hole and into the space between the can and the bag. With the isobutane exerting a constant squeezing pressure in the bag of the concentrate, the product would be dispensed when the valve was actuated regardless of the can position. In this fashion the dispenser could extrude such products as honey, jellies, ointments, and so on. This process was expensive and languished until S. C. Johnson & Sons, Inc. developed a unique gelled shave cream concentrate that contained 2–3% of a weak propellent blend. When the mixture was dispensed into the hand and touched with the fingertips, the consumers had the pleasing experience of generating their own foam. The company patented their invention and marketed it under the ‘‘Edge’’ brand. By 2006, the compartmentalized shave creams captured over 62% of the market despite their higher price.
Aluminum
Aluminum cans, which constituted about a 18% of the U.S. market in 2006 (and growing), are made quite differently. Pure (99.70%) aluminum slugs or pucks are lubricated with zinc stearate in a tumbler and are then conveyed to an extruder, where they are formed into a ‘‘cup.’’ The cup may be drawn and ironed (draw–ironed), in some advanced operations, to obtain a more uniform wall thickness and lighter structure. After trimming and vigorous cleaning, the lining and exterior decoration coatings are applied. The top is then formed in a number of stages (the number increasing with can diameter) and finally convoluted into either an outside or inside curl configuration. The outside curl is more common. Both have different advantages. Curl machining is sometimes done to smooth the curl surface of larger-diameter cans. Finally, they are strapped into typically 96-pack hexagon shapes and loaded onto 40-in.44-in. wood pallets. After strapping and shrink-wrapping, they are ready for delivery. Table 1 gives some details on aluminum aerosol cans.
General Information on Aluminum Aerosol Cans Table 1.
- Most common diameters: 22, 25,35, 38, 45,50, 55, 59, and 66mm.
- Largest size is 66235mm. Overflow capacity is 709 mL. (Typical use: Saline rinsing solution—sterile.)
- The 22- and 35-mm cans require valves with 20-mm-diameter ferrules.
- Since aluminum cans are nonmagnetic, most production lines have ‘‘puckers’’ and ‘‘de-puckers.’’ The pucks must be ordered for each can diameter.
- Aluminum cans of 45- to 66-mm diameters must use valve with ‘‘lathe-cut’’ rubber cup gaskets, generally Buna-N. (With over 10% DME, use butyl or cholorbutyl.)
- The 3897mm is (more or less) standard for laboratory samples. (Fills can be 2 oz.)
- Tare weights in a lot of aluminum cans are quite constant as compared to tinplate or steel.
- Aluminum can corrosion can create hydrogen, but very rarely. This overpressurizes the can.
- The passivity range for aluminum is about pH= 4.3 to 8.2 (251C).
Other
Very small numbers of cans are made by other methods. The Sexton Can Company produces one-piece steel cans by an extrusion process. For larger diameters, they extrude a steel shell, and then double-seam a can bottom to it. The company is able to make very strong cans by this process, able to withstand pressures up to 650 psig (lb/in.2 gauge) (45 bars). They are used to pack such higherpressure products as HCFC-22 refrigerant, which generates 302 psig at 1301F (21 bars at 54.41C). A special permit from the U.S. DOT (Department of Transportation) is required for such high pressures. As part of the development process, Sexton learned how to produce these cans with a bottom indentation, able to open at about 425 psig (30 bars) and thus long before heating could cause bursting. The orifice lets the product come out with a fair degree of control; otherwise, the dispenser might eventually burst with the brissance and concussive effects of a grenade.
TINPLATE OPTIONS
The electrotinplated steel sheet stock from the tin mill is available in a modest variety of plate thicknesses and tin coating weights. The steel for aerosol cans will typically be B0.007–0.015 in. (0.18–0.38mm) thick, according to intended use. The thinnest plate is used for bodies of small (45 to 52-mm)-diameter cans. The bodies of larger cans (57–76mm in diameter) will typically be made from 75 to 85-lb ETP, which is a can-making term for stock of B0.0083–0.0094 in. (0.21–0.24mm) thick. Tops and bottoms require still heavier plate, to prevent premature buckling (eversion) and subsequent unwrapping of the top or bottom double seam, leading to a burst event. End sections of the smaller cans typically use 112-lb ETP, or plate that is 0.0123 in. (0.31mm) thick. The largest-diameter aerosol can is a nominal 3.00-in. (76-mm) size and requires 135-lb ETP, or 0.015-in. (0.38-mm)-thick plate. Valve cups are almost always made of 95-lb ETP [e.g., 0.0105-in. (0.266-mm) plate].
Since aluminum is notably softer and more deformable than steel, these cans are extruded to have thicker metal. The thickness must be increased as diameters are made larger. The thinnest part of a typical 52-mm-diameter aluminum aerosol can will be about 25% up on the body wall, measuring about 0.017 in. (0.43 mm). The base of the largest aluminum can (66mm) may easily get to 0.080 in. (2.0 mm). Aluminum valve cups average 0.016 in. (0.41mm) thick.
In the past, tinplate could be ordered with very heavy tin coatings: up to 1.35 lb of total tin weight per basis box area of 31,360 in.2 on each side. This is equivalent to a tin coating of 15.1 g/m2 on each side. But today, with economic considerations forcing lower inventories, plus improvements in the tinplating process, it is rare to see tinplate of greater than 0.50 lb—that is, 5.6 g/m2 per side. Some tinplates are made with the so-called ‘‘kiss of tin’’ (0.05-lb ETP) having a nominal coating weight of only 0.56 g/m2 per side. The thickness then averages only 0.00000303 in. (0.077 mm) per side. The dark gray color of the steel and FeSn2 alloy layer can be easily seen through this ultrathin coating.
With electrotinplating methods it has been possible to obtain differentially coated tinplates; that is, plate having coatings of different thickness on each side. For example, D50/25-lb ETP (more accurately noted as D0.50/0.25-lb ETP) will carry 0.25 lb of tin on one face and 0.125 lb of tin on the other (5.6 + 2.8 g/m2). This type of plate is generally used with the heavier tin-coated area turned toward the aerosol product, to provide corrosion protection.
Linings
Over half of all tinplate aerosol cans and virtually 100% of all aluminum cans have organic linings. Single linings are the most common, but double linings and (for a few tinplate can bodies) even triple linings can be ordered. A variety of lining materials are used. The most common are the epoxyphenolics, used for about 70% of tinplate cans and around 78% of aluminum cans. Other options include the vinyl organosols, polyamideimide (PAM), and now the polyimideimide (PIM). The PAM and PIM coatings are relatively costly and often more difficult to apply, especially on tinplate. They are extremely resistant to permeation. Finally, there are the pigmented epoxyphenolics and the vinyls. The latter do not adhere well to metal substrates, and are used as a second or top coating, when extra performance is needed. They are unaffected by water, but quickly dissolved by methylene chloride, oxygenated solvents, and certain other solvents.
Corrosion inhibitors should be considered whenever aqueous solutions or dispersions are packed in tinplate. Typical inhibitors include sodium nitrite, sodium benzoate, and various amines.
Can decorations are a very valuable sales tool. They must always be the correct color and not affected by body contouring, consumer use or product spillages. The lining is often critical as a means of assuring product purity and dispenser shelf life. Table 2 gives data on the surface coatings (or linings) process for aluminum aerosol cans.
Surface Coatings (or Linings) for Aluminum Aerosol Cans Table 2.
Exterior coatings are applied by offset printing.
The Base Coat
- Selected for adherence to the metal, absence of crazing, radia fracturing, and blistering during the ovencuring process.
- Appearance aspects:
High-opacity white or colored enamel.
Clear or clear-tinted lacquer.
Pearlized lacquer.
Metallic lacquer. - Selected for a relatively low curing temperature.
- May be applied, up to 7 or 8 colors, in a single operation.
- Special effects include:
Half tones.
Color gradation. - Transparent lacquers, often showing brushed metal.
- Hard, protective varnish, usually glossy but sometimes matte.
- Epon-phenolic are the most common.
- Organosols (as ‘‘Microflex’’) are preferred for mousse.
- P.A.M. (polyimideamides) are very resistant.
Size
The can-making industry has strived for uniform dimensions of cans, both from different plants of a given supplier and between suppliers. This acts to save fillers from making time-consuming adjustments to crimping and gassing machines when moving from one lot of cans to the next. The CSMA (Aerosol Division) Commercial Standards Committee has now developed about 13 key dimensions and their tolerances for tinplate cans—both standard and necked-in—and about 10 more for aluminum cans (see Figures 1 and 2).
The smallest tinplate can is 112x214 (45x72mm) size, holding 101 mL. Aluminum aerosol cans are known in sizes down to 13x26mm and posibly smaller. The 13- mm size is used for such products as metered dose inhalants (MDIs), breath fresheners, and pepper sprays (which attach to key chains). These tiny aerosols require a 13-mm ferrule-type aerosol valve.
The terminolgy used to designate tinplate can sizes is never used for aluminum aerosol cans, which are always described in the metric system. For tinplate, the long-established practice of describing cans in English measurements is now more or less unique to North America, but it is well understood in Western Europe. For instance, 112 signifies 1 and 12/16 in. in diameter, while 214 indicates 2 and 14/16 in. for the height from the base of the bottom double seam to the top of the top double seam. For tinplate (or steel) cans not having a top double seam, the height measurement is the total height to the top of the can curl.
Straight-wall aerosol can (standard tinplate). Figure 1.
Well-known examples of can dimensions are A, which is 1.00070.004 in. (25.470.1mm) for all can sizes and metals, and B, which is 1.23270.010 in. (31.370.25mm) for tinplate cans of all sizes. These particular dimensions are quite critical because the valve cup must fit rather perfectly into the ‘‘1-in.’’ can opening to avoid jamming or scraping and to allow a good hermetic seal when crimped. The outer wall diameter of the valve cup is B0.99270.003 in. (25.270.08 mm), and this leaves a contingency clearance of only 0.001 in. (0.025mm) between the largest cup and the smallest hole. This is important not only for fit, but to anticipate traces of out-of-round, metal dimpling and other factors. The B dimension is important in making the top rim and shirt of the standard valve cup fit snugly to the can bead, increasing the statistical probability of a good seal. See the article ‘‘Pressure containers’’ for more information.
Seams
The tinplate can has three seams, which can occasionally become matters of concern. Aside from aesthetics, if can leakage occurs, it will often be at the side seam and will more rarely be at the top or bottom double seams. The side seam is also a favored site for can corrosion, due to exposed iron and the relatively poor coverage of sideseam enamel stripes, if applied. The dimensions of the top and bottom double seams are about the same, for a given can diameter. Small-diameter side seamed cans (35, 38, and most importantly 45mm) have smaller-size double seams than the larger cans. In fact, the 35- and 38-mm cans actually have no top double seam; the metal (side seam and all) is smoothly formed into the ‘‘inch–inch’’ curl. Figure 3 can be used to illustrate the general shape and important elements of a typical side seam. In general, these seams are about 0.125 in. (3.2mm) high, from sealing wall radius to end hook radius, if the can is 52mm or larger in diameter.
Description
The United States and Canada use a ‘‘sales description’’ method for indicating can overall dimensions that is based on the English (inch) measurement system. England is changing over to the metric system to conform to the ISO descriptions used in the rest of Europe and generally throughout the rest of the world. Australia and Canada have taken steps in the same direction.
The U.S. ‘‘can description’’ system can be best described by illustrations. The 202 can is one with a body diameter of 2 2 16 in., or 2.125 in. The ISO diameter (actually the inside diameter of the body) would be 52 mm.The U.S. can height is dimension D (see Figure 1 or 2), measured as total body height over the two seams. A 612 can would then have a body height of 6 4 16 in., or 6.250mm. The system extends to such descriptions as 2111208 and 207.5605. Seeds of change are being sown in the United States by the United Nations and other international groups, but the system is deeply ingrained and is likely to persist.
Necked-in aerosol can (tinplate). Figure 2.
Anatomy of a double seam. Figure 3.
CAN-MAKING TRENDS
The three-piece aerosol can is highly serviceable and is produced by the daily use of heavy equipment valued in the multi-billion-dollar range. There are no massive changes predicted during the next decade. The aerosol dispenser has been accused of having about the same cylindrical shape it had over 40 years ago. Minor improvements—such as necking-in, welding side seams, plastic labeling (to cover the side seam scar), and the ‘‘ecogorge’’ indentation on some aluminum cans, for the attachment of full-diameter caps—have been relatively unnoticed by consumers. To them the aerosol is a cylindrical package, although sometimes with a variously convoluted top portion.
The cylindrical image is an architectural necessity for a pressure-resistant dispenser (see also Pressure containers). Even though glass aerosols had somewhat wider limits, the underlying shape limitations have been a factor in having nearly all the perfume and cologne business transfer to nonpressurized glass pump sprayers and other containers. However, some advances have been offered recently, in the metal can area.
In the United Kingdom the Wantage Research Center of CarnaudMetalBox, plc (a firm now being purchased by Crown Cork & Seal Co.) engineers have developed a process by which a finished (plain or lithographed) three-piece tinplate can is placed momentarily in a heavy steel mold cell and expanded against the contoured sidewalls of the cavity by pressurization with some 1200 psig (83 bars) of filtered dry air. The volume increase is limited to about 12– 18%, depending on relative can length. The emerging cans are necked-in and may have pleated, quilted, crestlike, ergonomic finger depressions or other debossings in the body wall. In general, these are never more than B0.15 in. (3.8mm) deep. Round-the-can lateral depressions must not be too sharply defined, or the can will increase in height when pressure-tested during later can-making checks or in filler hot tanking. The fact that this is presently an extra cost operation has thus far prevented marketer acceptance, but one is mindful that such innovations as welded seams and necked-in profiles were well-engineered decades before marketers paid them much attention.
The trend toward aluminum cans has been quite noticable. During 2006, the production of these cans grew at about 18%/year, which is much more than the more proseic 2–3% increase/year for the total aerosol market. This is thought to relate to the greater aesthetics of aluminum, more than anything else. Marketers who use tinplate cans are responding by increasing the trend toward necked-in types and by permanently covering at least the unsightly valve cup, and ideally the entire can dome with a spray cap or foam spout. In Europe and Japan, starch and fabric finish products are offered with ‘‘pistol-grip handles’’ that are integral with a full-diameter, nonremovable spray cap. The accoutrement not only provides aesthetics but also reduces hand and finger fatigue.
During 2006, the firm of DS Containers, Inc. began production of two large sizes of 65-mm steel cans using the ‘‘Protact’’ process of double PET lamination developed by Corus RD&Tunit, Hoogevans Division of British Steel and refined by the Japanese can-maker, Daiwa Can Company. The plate is delivered in huge rolls of chrome/chrome oxide-coated steel, optimized for adhesion of PET and to which the main layer of PET is attached with a special adhesive, followed by a top layer for gloss, printability, internal lubrication, and scratch resistance. These laminates, the same for both sides of the steel plate, are about 80 times thicker that the typical 9 mm (0.0004-in.) thickness of roller-coated or sprayed-on can linings, so they offer outstanding protection for the metal substrate. A few solvents, such as dimethyl ether (DME), are claimed to soften PET plastics, and DS Containers state that the hydroalcoholic hair sprays containing a much as 37% of DME propellant can be packaged with no adverse effects.
DC Containers has produced containers for at least 50 domestic and international marketers to date. They plan to install can-making equipment to produce various sizes of the popular 52-mm-diameter aerosols during 2008. With their nicely rounded tops and unobtrusive botton double seams, these cans have an appearance almost identical to that of the 66-mm aluminum aerosol cans. There is no information of their ability to be shaped, but this probably will not occur until a few years in the future. Table 3 gives some general data on these cans.
DC Containers: Steel Aerosol Cans Table 3.
- The sole U.S. supplier is DC Containers, Batavia, IL.
- They currently produce two sizes: 211604 and 211713. DC Containers will make 52-mm cans in 2008. All cans are ‘‘DOT-2Q.’’ List prices are about $0.26 to $0.29. Cans are ‘‘two-piece,’’ with rounded domes and bottoms necked in double seams.
- Using Corus Research technology (from Europe, but perfected in Japan), all inside and outside surfaces of tin-free steel are laminated with:
Chrome/chrome oxide (Cr/CrOx) optimized for PET.
Adhesive.
Main layer of PET. ‘‘Protact’’
Top layer of PET. ‘‘Protact’’ - The total thickness, on each side, is about 0.020 in. (0.5 mm). WACO testing shows 0-mA conductance—inside to outside. All cans are automatically pressure-tested to 120 psig.
- The main PET layer can be colorized, but printing is done on an eight-color offset machine on the top layer, then baked. The top layer also provides gloss and abrasion resistance.
- For comparison, the lining of tinplate and aluminum cans is typically 0.0004 in. (0.01mm or 10 mm), or 2% as thick.
- WACO readings on aluminum cans are typically 2–20 mA, while those on lined tinplate cans are typically 300–800 mA.
- Pure dimethyl ether (DME) greatly softens PET, but solution of up to 38% DME (as in some 55% VOC hair sprays) are said to have no effect.