Synthetic polymers such as those based on polyvinyl acetate and acrylic offer a more versatile range of adhesives for packaging applications than do their natural counterparts. These adhesives can be used on a wider range of surfaces including the more difficult to bond "low surface energy" polymer films, such as polyethylene and polypropylene.Common synthetic adhesives that are used in the packaging industry are:
Below Table 3.5 provides common applications and forms for these Synthetic adhesives.
Table 3.5: Packaging Adhesives of Synthetic Origin
Acrylic and Polyurethane Dispersions
Water based dispersion based on acrylic and polyurethane based polymers are popular adhesives for laminating various packaging constructions. Waterborne laminating adhesives are commonly formulated from natural occurring materials, such as:
- sodium silicates,
- natural rubber,
as well as synthetic organic polymer emulsions based on:
- vinyl acetate,
- polyurethane, etc.
The naturally occurring adhesives are more commonly employed for labeling and other packaging applications; whereas the synthetic emulsions are mostly used for either wet or dry laminating. Waterborne laminating adhesives have become popular mainly due to governmental pressure for laminating converters to reduce VOCs.
Although waterborne adhesives generally have poorer moisture and thermal resistance than their solvent-based counterparts, the introduction of crosslinkers into the formulation has enabled waterborne adhesive to meet many of the performance criteria required and narrowed the performance gap between solvent-based and waterborne adhesives.
Acrylic emulsions offer a low cost adhesive with moderate performance properties. They are tremendously versatile due to the large number of different monomers and resins available. Acrylic waterborne laminating adhesives can provide bonds ranging from flexible and tough to hard and rigid depending on the formulation. Acrylic adhesives inherently have very good UV and oxidative stability and are generally preferred for outdoor applications.
Although acrylic emulsions can be used as laminating adhesives directly, they are often formulated for specific applications. Tackifiers, for example, are added to provide improved peel strength especially to low energy surfaces as shown in Figure 3.1.
Figure 3.1 : Relative peel strength of an acrylic latex adhesive that is unmodified
and modified with tackifier
Typically, waterborne polyurethane adhesives also have high performance properties. Waterborne polyurethane adhesives have been developed for standard laminating equipment and fast line speeds. They generally provide excellent adhesion to a wide range of flexible substrates and, when crosslinked, have strength greater than the substrates to which they are attached. The primary disadvantage of polyurethane adhesives is their relatively high cost.
Polyurethane dispersions formed mainly of anionic polyesters and aliphatic isocyanates are preferable for adhesives because of their higher degree of adhesion and resistance to UV light. By varying the product's molecular weight, one can produce a wide range of end-properties. Low molecular weight polyurethane dispersions yield adhesive with high hot tack and very fast crystallization. The crystallization rate is an important parameter for thermoplastic polyurethanes. It directly relates to the green strength and strength development rate.
Since waterborne polyurethanes are primarily linear polymers, their performance can be greatly improved by the addition of crosslinking agents. A variety of crosslinkers are available, including polyaziridenes, polyisocyanates, carbodiimides, epoxies, and epoxy silanes. These systems differ significantly in:
- pot life,
- the effect on final properties.
Although waterborne acrylic and polyurethane adhesives are often dried at temperatures up to 95°C, the majority of the crosslinking reaction takes place in the finished laminate at ambient temperatures. There is generally sufficient integrity after the laminating process so that slitting or die cutting can be accomplished immediately after lamination even though the crosslinkers have not fully reacted.
|| Polyvinyl Acetate Emulsions
Polyvinyl acetate (PVAc) is one of the most versatile of the vinyl adhesives. This polymer is perhaps best noted as an emulsion stabilized with a few percent of polyvinyl alcohol. Such a polyvinyl acetate latex is the basis for the common household "white glue".
PVAc's are normally not suitable for use as an adhesive in their basic unmodified state since they form very brittle films and have limited adhesion capabilities. They are, however, compatible with a wide range of modifiers. Plasticizers (e.g., dibutyl phthalate) and thickeners (methyl cellulose) are two additives commonly used in PVAc based adhesives. PVAc resins are often copolymerized with polyethylene or blended with polyvinyl alcohol. Their formulations may contain fillers and pigments, depending on the application.
The adhesive is relatively flexible especially at low temperatures. Resistance to most solvents and moisture is poor although these adhesives will withstand contact with grease,
The cured films tend to soften at temperatures approaching 45°C. With proper formulation, these can be low cost adhesives with high initial tack properties and good gap filling properties. Polyvinyl acetates tend to creep under substantial load. Assemblies bonded with polyvinyl acetate adhesive can be disassembled and then reactivated by heat or solvent.
The adhesives should be applied at 15°-32°C. The latex can be ruined by repeated freeze-thaw cycles and shelf life is generally limited. The low viscosity of the PVAc water emulsion and its fast setting capability provide an excellent adhesive for paper, cardboard, and other porous substrates.
Products of this type are good adhesives for applications where at least one substrate is porous. Paper, plastics, metal foil, leather and cloth are bonded well with polyvinyl acetate emulsions. The major use is in packaging.
Polyvinyl acetates are also used in hot melt adhesive formulations. In this form they are used for lamination of foils. Organic solvent solutions are also used, although not as much as the latex form.
In the tapes and labels industry PVAc's are commonly used for remoistenable labels and gummed tapes. They are also widely used as additives to other adhesives. The use of polyvinyl acetate emulsions for PSA applications includes remoistenable adhesives on:
- postage stamps,
- sealing tapes,
- permanent labels,
- decorative ribbons.
|| Polyvinyl Alcohol
Polyvinyl alcohol is a water soluble thermoplastic synthetic resin that is supplied generally as a formulated water solution. They have very good wet tack properties for laminating. Polyvinyl alcohol adhesives are commonly used for applications such as laminating and bonding or binding of porous materials.
With cardboard and paper, polyvinyl alcohol adhesives lose water rapidly and consequently set quickly, allowing pressure release within minutes after assembly. Wet tack properties, combined with quick set, make them quite useful in automatic labeling operations, packaging, and continuous laminating of foil and paper. Vinyl alcohol adhesives can also be heat-sealed.
Water resistance is only fair but sufficient for most interior applications. Resistance to grease and oil is excellent. The adhesives are also odorless and tasteless. They consequently find application in the packaging of food.
Polyvinyl alcohol solutions are generally added to polyvinyl acetate emulsions to slow down the speed of set with PVAc and to improve flexibility and tack. However, the heat resistance and resistance to long-term creep of the pure polyvinyl acetate resin is generally lowered. Polyvinyl alcohol adhesives are also compatible with starches and dextrins. Fillers such as wood flour, china clay or whiting can be added to reduce costs and provide viscosity control.
|| Vinyl Acetate Ethylene Copolymers
Vinyl acetate ethylene (VAE) copolymers in which the ethylene content is less than 40% by weight are often used as waterborne adhesives. They are similar to polyvinyl acetate emulsions except the ethylene constituent contributes to increased flexibility and enhances adhesion to low energy surfaces.
Generally VAE copolymer emulsions are used without additives or modifiers. The primary factor determining the properties of these adhesives is the percentage of ethylene. Adding ethylene (for example from 10 to 25%) will reduce Tg and generally improve peel strength. However, if higher viscosity is required, thickening is readily accomplished through plasticizer (e.g., dibutyl phthalate) or solvent (e.g., toluene) addition. Tackifiers and antioxidants are sometimes added to the base emulsion to suit specific applications. Inorganic fillers are used to increase viscosity and lower cost. EVA can be crosslinked through the addition of peroxide or by irradiation.
Another common laminating adhesive is a vinyl acetate / ethylene (VAE) copolymer that is stabilized with polyvinyl alcohol. VAEs offer significant improvement over polyvinyl acetate. Incorporation of ethylene into the vinyl acetate backbone decreases glass transition temperature and provides superior flexibility and adhesion.
A more recent development has been vinyl acetate acrylic (VAA) copolymers. VAA adhesives have excellent strength, setting speed, compatibility with common formulating raw materials, and emulsion stability (some are even freeze / thaw stable). VAA copolymers can be made to be soft and pressure sensitive or hard and tack free. VAAs generally offer lower glass transition temperature than VAEs. The incorporation of carboxyl functionality provides reactive groups for crosslinking.
|| Hot Melts
There are several types of polymers that are used to formulate hot melt adhesives. In the packaging industry the most common are those based on:
- ethylene vinyl acetate (EVA)
- styrene block copolymers (SBC).
Ethylene vinyl acetate (EVA) copolymers are used in hot melt pressure sensitive adhesives (10-40% vinyl acetate) and in emulsions (> 40% vinyl acetate). The high VA content broadens their solubility and compatibility with polar modifying resins. When used as hot melts the ethylene content is greater so they are often referred to as EVA hot melts; when used as emulsions the vinyl content is greater and they are referred to as VAE adhesives. Hot melt EVA adhesives are often used for packaging applications such as forming and sealing of:
- cardboard boxes
- heat sealing of plastic packaging.
As with all hot melt formulations, melt viscosity is very dependent on molecular weight of the base resin. Tackifiers are added to:
- reduce viscosity
- improve wetting and adhesion.
Waxes are added to:
- lower cost
- reduce viscosity.
Inorganic fillers are used to:
- lower cost
- increase viscosity.
Antioxidants are required to protect the adhesive during applications and during its service life. For hot melt pressure sensitive adhesives, the low vinyl acetate content EVAs are commonly blended with hydrocarbon waxes and tackifying resins, such as esterified wood resins.
Thermoplastic elastomers are block copolymers. The best known materials are styrene-butadiene copolymers (SBCs). They do not have the good adhesion characteristics of nitrile, neoprene, or natural rubber, but these properties can be improved by using tackifiers in their formulation. However, they are lower in cost and have better heat aging resistance than natural rubber.
SBC adhesives have very good toughness and low water absorption. Formulated compositions have good tensile strength characteristics. They are used commonly as pressure sensitive adhesives, hot melt adhesives, and sealants.
Hot melt adhesives can also be used in the laminating process. This hot melts are 100% solid:
- thermoplastic and reactive urethane adhesives
They are applied at ambient temperature to a substrate and activated using heat. A second substrate is introduced for laminating to the first after the material is activated.
Hot melt adhesive can also be applied directly to the substrate. The adhesive (pellets, pillows, granules, or bulk filled drums) is melted and applied to the substrate via rotogravure, spray, or extrusion coating technology. Significant material savings and line speeds can result by direct application of hot melt adhesives.
|| Solvent-Based Laminating
Solvent-borne laminating adhesives can be formulated from many of the same polymers described above for waterborne systems. Acrylic solvent solutions, for example, have especially found application when a high degree of environmental resistance and non-yellowing properties are required. They can be used for wet laminating, but are generally found in dry laminating applications. The speed of which the solvent evaporates is a significant benefit in most production operations. Because of the lack of emulsifiers and surfactants, the moisture resistance of solvent-based adhesive is generally superior to waterborne systems.
One of the most popular solvent-based laminating adhesives is polyester. Introduced in the 1950s for laminating polyester film, solvent-borne polyester resins contain relatively low solids (20-30%). These materials provide excellent adhesion to polyester film and very good adhesion to many other polymeric films and metal foils.
The reaction of the hydroxyl of the polyester with a polyisocyanate produces a crosslinked adhesive network with very good thermal and chemical resistance. The polyester laminating adhesives:
- have high green strength,
- are fairly fast curing,
- are well suited to fast production processes.
Due to environmental regulations, polyester adhesive systems with reduced VOCs and solvent recovery and incineration systems have been developed. Polyester solution adhesive of 40-50% solids content were developed to provide excellent adhesion properties, but they have lower green strength than the low solids products.
|| Solventless Laminating Adhesives
Major changes have occurred in 100% reactive solventless adhesives over the last decade. The first solventless laminating adhesives were primarily moisture-cured polyurethanes. The adhesive is coated onto a substrate and atmospheric moisture reacts with excess isocyanate groups to crosslink the adhesive after the secondary film has been joined. Slitting of the laminate generally can occur in 24-72 hours.
While water based systems have been developed as an economic alternative to overcome some of the drawbacks associated with solvent, in practice the performance of water based adhesives, especially in wet environments, is not as good as their solvent counterparts. The most important solventless and high solids adhesives used in laminating flexible packaging are those belonging to the polyurethane family (Table 3.6).
Table 3.6: Polyurethane Laminating Adhesive
Two-part solventless polyurethanes were also developed to negate some of the disadvantages of the moisture cured type, such as bubbling, inconsistent cure rate, and cloudiness. These problems were generally associated with variations in the ambient moisture content. The two-part polyurethane laminating adhesives require a mixing and metering unit since pot life is limited. High residual monomers and low initial bond strengths somewhat limit the application of these adhesives
An improved polyurethane adhesive has been developed based on moderately high viscosity polyurethane polymers that require a 50-70°C application temperature. The increased viscosity reduces the cure time to 12-24 hours before slitting. This generation of adhesive is made from a process that removes nearly all of the excess isocyanate monomer from the prepolymer.
|| UV Curable Laminating Adhesives
Ultraviolet light (UV) or electron beam (EB) curing laminating adhesives are creating significant interest for flexible packaging. Acrylate / methacrylate monomers and oligomers as well as photoinitiators are essential components of UV adhesives. Similar monomers and oligomers are used in EB adhesives, but they do not require photoinitiators. Aliphatic urethane acrylates are commonly used in laminations since they have good adhesion to most films and are non-yellowing. These adhesives are also characterized by a very low application viscosity, on the order of 350-450 cps.
UV/EB laminating adhesives are directly coated on the surface of the film, nipped and cured as shown in Figure 3.2. Line speed can be adjusted by using UV lamps of different intensity. These adhesives were developed primarily as low VOC laminating systems but offer a number of potential advantages over solvent-based, waterborne, and solventless adhesive systems. These advantages include:
- bond strength is achieved immediately on cure
- the adhesive is a one-component system with long shelf life
- the adhesive remains unchanged until cured and no viscosity adjustments are required
- UV/EB systems can be cured at room temperature
- significant number of raw materials with broad formulation range to meet many widely differing performance requirements.
Upon curing, these adhesives offer a non-tacky film that provides excellent adhesive strength to a number of different substrates. However, when a specific application requires laminating opaque substrates, EB-curing adhesives may be necessary, since EB cure is not negatively affected by an opaque substrate.
Figure 3.2 : UV cured adhesives can be applied as a solventless, low viscosity liquid and then converted to a solid or semi-solid state by radiation crosslinking.