Published: May 26, 2011

Adhesives

INTRODUCTION


The packaging industry represents a significant market for adhesives materials. Some applications include coatings, which act as adhesive layers for bonding to other forms of packaging, such as labels or tapes, or are used in the actual assembly of a carton, or to prepare a laminate.
Adhesives make up more than 80% of the adhesive and sealants market. In 2002, the global market was about 16.7 million tons and was expected to rise about 3–4% per year. In the United States, the market was estimated to be 8.89 million tons in 2004; as a comparison, the market in 1999 was 6.89 million tons (1).
The principal uses of adhesves include: the forming and sealing of corrugated cases and folding cartons; the forming and sealing of bags; the winding of tubes for cores, composite cans, and fiber drums; the labeling of bottles, jars, drums, and cases; the lamination of paper to paper, paperboard, and foil; and the lamination of plastic films for flexible packaging. The markets for packaging materials are diverse, mainly concerned with food, beverage, medical, and heavy-duty industrial applications, each of which can bring stringent performance requirements and impose harsh environments under which the adhesive layer is expected to function.
There is a strong movement to replace solvents and solvent-based adhesives in both the United States and Europe, in order to minimize volatile organic emissions. This has led to a growth in water-based and hot-melt adhesives at the expense of solvented types. Also, there has been a strong interest in recyclable adhesives based on requirements for recycled paper content in the United States and Europe. There is a desire to develop single material packaging to facilitate recycling and minimize sorting. A problem is that adhesive residues left in board can produce difficulties in the subsequent handling of the regenerated material. Under consideration is the use of bioadhesives to assist in the material breakdown (2).
There are many types of packaging adhesives, frequently for the same end-use applications, with the choice dictated by cost, productivity factors, the particular substrates involved, special end-use requirements, and environmental considerations (see also Adhesives, extrudable). To help clarify this complex picture involving many different chemical types, it is useful to classify packaging adhesives into three physical forms: water-based solutions and dispersions, solvent-based systems, and solvent-free 100% solids and hot melts.

WATER-BASED SYSTEMS


This is the oldest—and still, by far, the largest-volume—class of adhesive used in packaging. These adhesives share the general advantages of ease and safety of handling, energy efficiency, low cost, and high strength. Waterbased adhesives can be further divided into two categories, natural and synthetic.

Natural Water-Based Adhesives


The earliest packaging adhesives were based on naturally derived materials—indeed, almost all were until the 1940s—and they still constitute a large segment of the market. However, they have seen their gradual replacement in many applications. Starch and dextrin have been replaced by synthetic poly(vinyl acetate) emulsions. However, starch-based adhesives are still being investigated (3).

Starch

. The largest class of natural adhesives is based on starch; in the United States, this means corn starch. Some potato starch is used in Europe primarily because of economics, since they are heavily subsidized. Adhesives are produced from raw flour or starch, but more frequently the starch molecule is broken down into smaller chain segments by acid hydrolysis. Depending on the conditions of that reaction, the resulting material can be a fluidity or thin-boiling starch or a dextrin. These can then be further compounded with alkaline tackifiers such as borate salts, sodium silicate, or sodium hydroxide, with added plasticizers or fillers.
A principal use of starch adhesives is in the manufacture of corrugated board for shipping cases. The standard process involves suspending ungelatinized cornstarch in a thin carrier-starch cook as a vehicle. When the bond line is subjected to heat and pressure, the cornstarch gelatinizes almost instantly, forming a bond between the flutes and the linerboard at very high rates of production. Additives are usually used to improve adhesion, lower gel temperature, increase water resistance, and further increase speed of bond formation.
Other important uses of modified starches and dextrins are in the sealing of cases and cartons, winding of spiral or convolute tubes, seaming and forming of bags, and adhering the seams on can labels. Glass bottles are frequently labeled with a special class of alkaline-treated starch adhesive called a ‘‘jelly gum.’’ These have the special tacky, cohesive consistency required on some moderate-speed bottle labeling equipment. Specially modified starches based on genetically bred high-amylose strains are also used as primary ingredients in the remoistening adhesive on gummed tape used for box sealing.
There are many strong points to recommend starchbased adhesives. They are regarded as very easy to handle, clean-machining, easy to clean up, and, above all, inexpensive. Starch has excellent adhesion to paper; and, being nonthermoplastic, it has outstanding heat resistance. Starch also has a green, environmentally friendly image, being ‘‘natural’’ and based on a renewable resource. The negatives are the relatively slow rate of bond formation, the limited adhesion to coatings and plastics, and poor water resistance.

Protein

. Another class of natural adhesives is based on animal or plant protein. Proteins are highly susceptible to changes in their structure through changes in pH, the process of denaturation, and breakdown in molecular weight to effect solubilization.

Soybeans

. Soybeans are important sources of both proteins and trigylceride oils. Proteins for adhesives are obtained from harvested soybeans by extracting and pressing out the oils and then heating. Soy in combination with blood or casein seems to exhibit the most water resistance. Protein-based adhesives are of interest to the medical industry; however, blood contamination is an issue (1). Considerable work is being done in this area. Stable adhesives from urea-denatured soy flour have been reported (4).

Animal Glue

. This is one of the earliest types of adhesive. It is derived from collagen extracted from animal skin and bone by alkaline hydrolysis. When used as a heated colloidal suspension in water, animal glues have an unusual level of hot tack and long, gummy tack range. However, because of fluctuating availability and cost, along with the development of improved synthetics, there are only two significant uses of animal glue in packaging: (1) as a preferred ingredient in the remoistening adhesive on reinforced gummed tape use for box sealing and (2) as the standard adhesive used in forming rigid setup boxes.

Casein

. This is produced by the acidification of skimmed cow’s milk. The precipitated curds thus produced form the basis of casein adhesives. There is a lack of suppliers of casein in the United States. The main sources for casein are Australia, New Zealand, and Poland, but it is also produced in Argentina and the Scandinavian countries.
There are two packaging applications where casein is used in large volume. One is in adhesives for labeling glass bottles, particularly on newer high-speed labelers where they outperform starch-based adhesives. They are especially favored for beer bottles, where casein provides the resistance to cold-water immersion required by brewers, together with removability in alkaline wash when the bottles are returned. The second use is as an ingredient in adhesives used to laminate aluminum foil to paper. Combined with synthetic elastomers such as polychloroprene or styrene-butadiene lattices, casein provides a unique balance of adhesion and heat resistance (5).

Natural Rubber Latex

. This is extracted from the rubber tree, Hevea brasiliensis, and is available in several variations of concentration and stabilization. One major use in packaging is as a principal ingredient in adhesives for laminating polyethylene film to paper, as in the construction of multiwall bags. Natural rubber latex also finds use in a variety of self-seal applications, since it is the only adhesive system that will form bonds only to itself with pressure. This property is used in self-seal candy wraps (where it is called cold-seal) and in press-to-seal cases, as well as on envelopes.

Synthetic Water-Based Adhesives


Synthetic water-based adhesives are the most broadly used class of adhesives in general packaging. Almost all are resin emulsions, specifically poly(vinyl acetate) emulsion, which is a stable suspension of poly(vinyl acetate) particles in water. These systems usually contain watersoluble protective colloids such as poly(vinyl alcohol) or 2- hydroxyethyl cellulose ether and may be further compounded by the addition of plasticizers, fillers, solvents, defoamers, and preservatives.
These emulsions are supplied in liquid form (the ubiquitous ‘‘white glue’’) in a range of consistencies from thin milky fluids to thick, nonflowing pastes. They are used in a broad range of packaging applications, to form, seal, or label cases, cartons, tubes, bags, and bottles. In most of these uses they have replaced natural adhesives because of their greater versatility. They can be compounded to have a broad range of adhesion not only to paper and glass but also to most plastics and metals. They can be rendered very water-insensitive for immersion resistance, or very water-sensitive to promote ease of cleanup and good machining. They are the fastest-setting class of waterbased adhesives, facilitating increased production speeds. They are low in odor, taste, color, and toxicity and have excellent long-term aging stability. They are tough, with an excellent balance of heat and cold resistance. The equipment used to apply them is relatively simple and inexpensive to purchase and to operate. Finally, they are economical and reasonably stable in cost.
The utility of these emulsion systems has broadened in recent years with the greater use of copolymers of vinyl acetate. Copolymerizing vinyl acetate with ethylene or acrylic esters in particular has greatly improved the adhesion capabilities of these emulsions, particularly where adhesion to plastics or high-gloss coatings is required. For example, crosslinking acrylic–vinyl acetate copolymer emulsions have replaced polyurethane solution systems for laminating plastic films for snack packages. The largest areas of use for vinyl emulsions, however, are in case and carton sealing, forming the manufacturers joint on cases and cartons, and the spiral winding of composite cans.
All acrylic emulsion pressure-sensitive adhesives have to a significant degree replaced acrylic or rubber solution products in the manufacture of pressure-sensitive labels. The development of water-based acrylics eliminates a source of solvent vapors. The development of pressuresensitive adhesives compared to other tapes is convenience of use. There are no storage problems, and no mixing or activation is necessary. No waiting is involved. Often the bond is reversible. Disadvantages are that adhesive strength is low and they are not suitable for rough surfaces.
Polyurethane dispersions have found acceptance in medium-performance flexible packaging applications laminating plastic films together where some chemical resistance is required.
The use of other synthetic water-based systems is quite minor and specialized. Some synthetic rubber dispersions are used in film adhesives and, in conjunction with casein, for the lamination of aluminum foil to paper. There is some use of tackified rubber dispersions as pressuresensitive masses on tapes and labels replacing solventbased rubber–resin systems.
Sodium silicate was once widely used in many paper packaging applications, ranging from corrugating to case sealing, but today the primary use of silicate adhesives is in tube winding, especially in the convolute winding of large drums or cores where it produces a high degree of stiffness.

SOLIDS/HOT-MELT ADHESIVES


Hot melts are the fastest-growing important class of adhesives in packaging. Most of their volume goes into high-speed large-volume case and carton sealing. Hot melts can be defined as 100% solids adhesives based on thermoplastic polymers, which are applied heated in the molten state and set to form a bond on cooling and solidification. Their chief attraction is the extremely rapid rate of bond formation, which can translate into high production rates on a packaging line.
The backbone of any hot melt is a thermoplastic polymer. Although almost any thermoplastic can be used, and most have been, the most widely used material by far is the copolymer of ethylene and vinyl acetate (EVA). These copolymers have an excellent balance of molten stability, adhesion, and toughness over a broad temperature range, as well as compatibility with many modifiers. The EVA polymers are further compounded with waxes and tackifying resins to convert them into useful adhesives. The function of the wax is to lower viscosity and control set speed. Paraffin, microcrystalline, and synthetic waxes are used, depending on the required speed, flexibility, and heat resistance. The tackifying resins also function to control viscosity, as well as wetting and adhesion. These are usually low-molecular-weight polymers based on aliphatic or aromatic hydrocarbons, rosins, rosin esters, terpenes, styrene or phenol derivatives, or any of these in combination. The formulations always include stabilizers and antioxidants to prevent premature viscosity change and char or gel formation that could lead to equipment stoppage.
Two variations on traditional EVA hot melts have recently become commercially significant. First, the recent availability of very low-molecular-weight EVA copolymers has made possible EVA hot melt that can be run at much lower temperatures, 2501F (1211C), rather than the traditional 3501F (1771C). This allows for much safer running conditions as well as energy savings. Second, an analog of EVA, ethylene-butyl acrylate, has been introduced as the backbone polymer in some packaging hot melts (6), providing advantages in both adhesion and in heat and cold resistance. Decrease in the application temperature has lessened safety concerns associated with this type of adhesive (1).
Another class of hot melts used in packaging is based on lower-molecular-weight polyethylene, compounded with natural or synthetic polyterpene tackifiers. These lack the broader adhesion capabilities of the EVA-based hot melts as well as their broader temperature resistance capabilities. However, they are economical and are adequate for many paper bonding constructions, and they find application in case sealing and bag seaming and sealing.
A third type of hot melt is based on amorphous-poly aolefin (APAO) polymers. Originally these were based on amorphous polypropylene, which was widely available as the byproduct of the polymerization of isotactic polypropylene plastics. As a byproduct, it was inexpensive, but it suffered from low strength and was limited to applications such as lamination of paper to paper to produce waterresistant wrapping material on two-ply reinforced shipping tape. Improvements in polypropylene polymerization catalysts have almost eliminated this byproduct, but several producers noted the market and began producing onpurpose APAO polymers, albeit at somewhat higher costs.
Another recent class of hot melt is based on thermoplastic elastomers: block copolymers of styrene and butadiene or isoprene. These find primary application in hot-melt pressure-sensitive adhesives for tapes and labels replacing solvent-borne rubber systems. More recently, their broad adhesion and low temperature and impact resistance are finding use such as (a) the attachment of polyethylene-base cups to polyester soft-drink bottles (7) and (b) the sealing of film laminated frozen-food cartons. Pressure-sensitive adhesives from high-molecular-weight block copolymers having a high diblock copolymer are suitable for PVC film labels and decals (8).
Even more specialized applications use hot melts based on polyamides or polyesters when specific chemical- or heat-resistance requirements have to be met, but their high cost and relatively poor molten stability have precluded their widespread use to date.
The most recent and highest-performance hot-melt technology is moisture curing polyurethane hot melts, but these have been limited to higher-requirement product assembly applications and have found few uses in packaging.
All hot melts share the same basic advantages, based on their mechanism of bond formation by simple cooling and solidification. They are the fastest-setting class of adhesive—indeed, preset before both substrates can be wet is the most frequent cause of poor bonds with hot melts. Because of the wide range of polymers and modifiers used, they can be formulated to adhere to almost any surface. With no solvent or vehicle to remove, they are generally safe and environmentally preferred. They are excellent at gap filling, since a relatively large mass of material can ‘‘freeze’’ in place, thus joining poorly mated surfaces with wide dimensional tolerances.
However, all hot melts share the same weakness, which is the rapid falloff in strength at elevated temperatures. They also have a tendency to damage substrates that cannot withstand their application temperature (1). Properly formulated hot melts can be suitable for almost all packaging applications, but they are not appropriate for very hot-fill or bake-and-serve applications.

SOLVENT-BASED ADHESIVES


Solvent-based adhesives, which are by far the smallest, and most rapidly declining, of the three classes of adhesive used in packaging, find use in specialized applications where water-based or hot-melt systems do not meet the technical requirements.
Rubber–resin solutions are still used as pressure-sensitive adhesives for labels and tapes. However, factors of cost, safety, productivity, and, above all, compliance with clean-air laws have led to a strong movement toward water-based or soilds/hot-melt alternatives. Such alternatives are available to meet most requirements and most knowledgeable observers predict an almost total disappearance of rubber–resin solvent-based pressure sensitives for packaging tapes and labels in the near future.
Solvented polyurethane adhesives are widely used in flexible packaging for the lamination of plastic films. These multilayer film constructions find application in bags, pouches, wraps for snack foods, meat and cheese packs, and boil-in-bag food pouches. They have the ideal properties of adhesion, toughness, flexibility, clarity, and resistance to heat required in this area. However, here, too, alternative systems are being introduced to eliminate the costs, hazards, and regulatory problems associated with solvent-based systems. Crosslinking waterborne acrylic polymers have gained acceptance in the large snack food laminating market for constructions such as potato chip bags. Polyurethane dispersions and (100% solids) ‘‘warm melt’’ systems are starting to find use in some of the more demanding food packaging applications.
Solvent-based ethylene–vinyl acetate systems found use in some heat-seal constructions, such as the thermal strip on form–fill–seal pouches, or on lidding stock for plastic food containers such as creamers or jelly packs.
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