Colloidal Silica Additives for Flexible Packaging Coatings
At the time of writing, flexible packaging is the second largest packaging segment in the United States, with around $41 billion in sales annually. Since its inception in the mid-1900s, the flexible packaging industry has grown to such heights owing to its advantages in sustainability and cost; it requires relatively less material mass and is lighter in weight compared to other forms of packaging.
While it inherently offers these advantages, flexible packaging is highly engineered to optimally fulfill other performance requirements of food and consumer goods packaging. These include providing an effective barrier between the environment and product, facilitating print such that brand owners can drive growth and recognition, and enabling high-throughput production and finishing of the package. Since the plastic films that constitute flexible packaging are, in some cases, detrimental to achieving the functionality mentioned, specialized coating formulations are required to apply on these films. LUDOX® colloidal silica is a performance additive widely used in various coating formulations. It provides significant benefits to packaging coatings.
LUDOX® Colloidal Silica for Flexible Packaging Coatings
Though silica is one of the most abundant chemical compounds on Earth, it can fulfill a variety of functionalities that improve chemical formulations. LUDOX® colloidal silica, which contains nano-sized silica particles electrostatically dispersed and stabilized in water, is one of the most effective silica products for improving the physical properties and functionality of coatings. The silica particles in LUDOX® contain an abundance of silanol groups on their surface, which are highly reactive towards polar functional groups present on organic polymers in coating formulations and hydroxyl groups present on coating substrates. Through this, LUDOX® provides the following benefits to flexible packaging coatings and film substrates.
Decreased oxygen permeability
Improved ink receptivity for printing
Enhanced antiblocking properties
The specific mechanisms by which LUDOX® achieves these benefits are explored in further depth below.
Reduced Oxygen Permeability
One of the most common types of plastic films used to construct flexible packaging, whether these are multilayer flexible packages or single-layer flexible packages, are based on polyolefins like polypropylene or polyethylene. While these materials provide a high degree of transparency and a good barrier to moisture, they have relatively high oxygen permeability. One method that multilayer flexible packaging is constructed to overcome this is to bond polyolefin films with foils or metallized plastic layers, which have minimal oxygen permeability. However, this is incompatible with trends surrounding the generation of a circular economy through recyclable mono-material flexible packaging. Barrier coatings formulated with LUDOX® can have decreased oxygen permeability due to the silica particles’ ability to act as physical barriers to oxygen by occupying voids within the coating’s polymer matrix. In addition, as the silica particles interact with the coating polymers, they provide an interfacial surface that can densify the polymers in a way that reduces the diffusivity of gas.
Increased Ink Receptivity
Another characteristic of plastic packaging films that presents a manufacturing challenge is their tendency to have low surface energies. This means that water-based ink formulations, which have surface tensions higher than the surface energy of the packaging films, will not wet and level easily. Wetting and leveling are essential to creating a good appearance on films, such that brand information, designs, and nutritional information are appealing and presented clearly. To overcome this, primers or coatings are typically applied to the films to promote adhesion. LUDOX® can be used to formulate primers and coatings, which help increase ink receptivity when applied to packaging films. The mechanism by which this is achieved is thought to come from interactions of the silanol groups on the silica particles and the hydroxyl groups or polar moieties on dyes and pigments.
Improved Antiblocking Performance
LUDOX® can even provide benefits to the flexible packaging manufacturing process far upstream of the steps described so far. When packaging films are freshly manufactured, they have a propensity to block or stick together when wound up into rolls for shipping. Simply put, this occurs because the newly made film layers are chemically identical, quite warm, and experience high amounts of pressure. Prior to winding, LUDOX® helps reduce blocking by being applied as a coating. While imperceptible to the naked eye, the nano-sized silica particles impart a degree of surface roughness that reduces the surface area of the films’ interface. This mechanism is very similar to the one in which LUDOX® is used as an anti-slip coating for paper packaging. The surface roughness applied to these films allows them to be easily unwound without sacrificing visual quality. When these films are used to manufacture packaging, this surface roughness also helps to provide easier separation between the film and heated plates used to seal films together.
LUDOX® Colloidal Silica Grades
The LUDOX® portfolio contains several options that vary in particle size, stabilizing counterion type, and pH. Grades are generally selected based on the characteristics and functionality of the formulations they will be incorporated into. For example, larger particle sizes can function as more effective antiblocking solutions, while smaller particle sizes, which have larger surface areas, will provide a greater surface area to react with functional groups of coating resin polymers and substrates. When used in coating formulations, LUDOX® will begin to coalesce and form gels once the formulation is brought outside of the grade’s stabilized pH range. Thus, grades used in coatings should be selected based on the formulation pH. Below is a list of options packaging coating formulators and film manufacturers may consider.
*LUDOX® AM has been formulated for stability within pH 4–10, a broader range than other grades.
Summary
Flexible packaging is constantly subject to evolution and improvements through engineering that improves its aesthetics, barrier properties, and sustainability. One of the main ways flexible packaging films are improved is through the use of coatings, which impart functional properties that help packaging performance. LUDOX® colloidal silica is an ideal ingredient used in packaging coatings because it can help reduce oxygen permeability, increase ink receptivity, and improve antiblocking properties. Contact us below to speak to one of our formulation experts about LUDOX® and request a sample.
