Laminating Adhesives for Flexible Packaging

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Laminating Adhesives for Flexible Packaging

7 June 2018

1. Introduction
2. Wet and Dry Lamination
3. Substrate Characteristics
4. The Laminating Adhesive
The variety of modern flexible packaging products that are available today would
not be possible without modern adhesive systems. The evolution of the packaging
industry has closely matched the development of new adhesive materials and
production processes. These trends have led to high quality and technically
demanding packaging structures required by the consumer.

For many applications in flexible packaging, the use of a single material may
not satisfy all of the properties demanded of the product. In these cases, a
composite consisting of two or more layers of material may provide the desired
performance. A particularly common means of creating such a composite is to
laminate various polymeric films to other films, foils, papers, etc. with a polymeric
adhesive.

This production solution is commonly employed in the packaging industry
where the end- products require multi-functional properties, such as high
tensile strength and high gas permeability. These are generally referred to as
barrier films. The laminate construction can become rather complicated due to
the nature of the specific application. A typical laminate used in the medical
packaging industry, for example, may be a multi-layer composite containing
films of polyester / polyethylene / metal foil / polyethylene.

Laminating adhesives for flexible packaging are available in a variety of
technologies, viscosities, and solids concentrations. There are four basic
categories of laminating adhesive that are commonly used. These are:

waterborne, solvent based,
reactive 100% solid (solventless) liquid,
and hot melt.

Each category has a number of applicable base polymers and a wide variety of
formulation possibilities. The specific formulations will heavily depend on the
nature of the laminating process employed, the nature of the film substrate,
and the final physical properties desired.

This article will provide a primer on laminating adhesives and processes used
mainly for flexible packaging. Thicker laminates (e.g., decorative laminates,
wood laminates) can be produced by bonding substrates in a hot press. However,
these adhesives are outside the scope of this article. The principles of the wet and
dry laminating processes will be discussed first, followed by a general discussion
of the main adhesive systems that are usually employed.

2. Wet and Dry Lamination
The manufacture of film laminates is a relatively simple continuous process of
coating and bonding. Generic illustrations of typical flexible laminate production
lines are shown in Figure 1. Specific processes differ primarily by how the
adhesive is applied and converted from a liquid to a solid. There are several
laminating processes that can be easily adapted to production. These are
generally classified as either wet or dry laminating processes and they are
described in Table 1.

The main items in a lamination line are the film unwinders and winders, adhesive
coater, and laminator. There are a number of coating methods that can be used
depending on the nature of the adhesive. Specific applications may require varying
degrees of adhesive coating thickness. Table 2 shows a summary of some of the
capabilities and limitations of common coating methods that are used in producing
laminates.

With either wet or dry laminating, full bond strength generally occurs over about
a period of 24 hours. Therefore, initial tack or “green strength” is often an important
criterion for a laminating adhesive. The immediate bond strength must be
sufficient to hold the substrates together and resist relaxation of the web until
higher bond strengths can be achieved. Various technologies have been developed
to either speed the strength development or to provide improved properties in
laminated products. These include the application of UV or electron beam for
crosslinking and the addition of chemical crosslinking agents in the adhesive
formulation.
Wet Laminating
With wet laminations, the adhesive is applied to one substrate, usually by
roller coating or air knife. The coated substrate is then nipped with another
substrate, and the resulting laminate may then be left to air dry or passed
through a heated oven to remove solvent and build bond strength.
The types of adhesive used for wet lamination are:

waterborne natural products, such as starch and dextrin or
waterborne synthetic latex products, such as polyvinyl acetate, acrylic, etc.
100% reactive liquids, such as polyurethanes or polyesters.
Wet lamination via waterborne or solvent based adhesives is confined to
applications where at least one substrate is porous (e.g., paper, cardboard,
textiles) to facilitate drying. Once cured, bond strength is generally high
enough to cause failure or tearing of the porous substrate. Most often,
waterborne synthetic latex adhesives are utilized for wet bonding because
of their high initial strength and fast drying characteristics when applied to
porous substrates.
Dry Lamination
Dry laminations are those in which the liquid adhesive is first dried before
lamination. The adhesive can be either applied to one substrate and dried or it
can be applied as a hot melt type of film (essentially another film layer).
The adhesive is then in the dry solid or slightly tacky stage when joined with
the other substrate. The bonding is generally achieved during a high
temperature, high pressure nip. The temperature and pressure are sufficient
to cause theadhesive to flow and create an instantaneous bond when it cools
and gels. Dry lamination can be applied to a broader range of products such
as film-to-film and film-to-foil. Dry laminating adhesives are generally solvent
based although considerable development has taken place to reduce or
replace the solvent to meet environmental regulations. This has produced
several strong competitors to conventional solvent-based adhesives such as:
hot melts (e.g., ethylene vinyl acetate copolymers), 100% reactive solids
(e.g., two-part polyurethanes, one part moisture curing polyurethanes and
UV/EB curable acrylates), high solids solvent based (e.g., silicone), and
waterborne adhesives (e.g., acrylic emulsions). A significant advantage of
reactive 100% solids adhesive in addition to the reduction in possible VOCs
is the possible elimination of drying ovens and resulting energy cost.
Hot melt adhesives are applied by heating the hot melt formulation to a
closely controlled temperature and applying via extrusion or die coaters.
With hot melt adhesives, a pre-made film of hot melt material may also be
interleaved between two substrates at a high temperature nip to achieve
lamination. Another method of making a laminate, which is outside the
scope of this article, is coextrusion. Coextrusion allows the production of a
laminate in a single process. In this process two or more thermoplastic
materials are extruded separately and combined either internally in the die
or immediately after leaving the die. A separate adhesive is often not required;
however, certain film laminates may require the application of a “tie-coat” to
maximize adhesion of one film to the other. The coextrusion process is
generally used for very high volume laminate production.
Adhesive lamination is the preferred joining process when a specific film
composition cannot be effectively run in a coextrusion system due to
equipment limitations or when the high temperatures required in coextrusion