Starch-Based Polymers in Waterborne Coatings Raise the Sustainability Bar

Bio-based coatings offer a cost-effective opportunity to replace petroleum-based coatings with recyclable, compostable materials to reduce carbon footprint.

June 3, 2024

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Global manufacturing has benefitted from the use of petroleum-based materials (PBMs) in thousands of applications since the Industrial Revolution. PBMs have not only revolutionized processing efficiencies and product capabilities but have also lowered the cost of many finished products. Water bottles, computer hardware, automotive products, food wrappers, grocery bags, and many more products that incorporate fossil fuel-based materials are commonly produced at price points that make them readily available for the average consumer.

In recent decades, the negative impact of PBMs on the environment and human health has become a great concern for regulatory bodies and consumers. The coatings industry has come under fire for its use of various PBM-based materials that have contributed to environmental concerns. In response, the industry has been transitioning to more sustainable options.

Advances in material science technology present opportunities to reformulate packaging solutions using more functional and bio-based chemistries. Bio-based coatings offer the potential to reduce carbon emissions, reduce usage of natural resources, and transition away from petroleum products.

Based on our application experience, starch-based biopolymers are a solution to replacing a variety of synthetic chemicals in waterborne coatings. Today, starch is an underexplored biomaterial in waterborne coatings outside of paper-based packaging.

Over the last decade, many waterborne coating formulations have been compounded to reduce or eliminate volatile organic compound (VOC) levels. These significantly reduce environmental impact and potential health risks to industry workers and consumers. Additionally, they have eliminated or significantly reduced the presence of many neurotoxins common to coatings, including benzene, ethylene glycol, styrene, and xylene.

Many manufacturers have already implemented natural materials in their formulations using carboxymethyl cellulose, soy proteins, and sodium alginate. These natural materials provide new challenges to manufacturers because their performance or cost may not be as competitive in the current market. Finding the most sustainable formulation, at the right cost, with the best performance, continues to be the challenge.

Starch as a carbohydrate can be obtained from sources including corn, maize, tapioca, or potato, which contain linear amylose molecules and/or branched amylopectin molecules. The quantity of these molecules within the starch source defines the functionality of the starch product. Modern plant science works with industry and university partners to develop crop varieties that provide both natural molecular differences and performance advantages within various applications. Starch is a valued renewable resource in industries such as food manufacturing, brewing, and package manufacturing. Due to the widespread availability of the crop sources, starch is also one of the least expensive biopolymers on the market. Additionally, starch-based products are inherently biodegradable and are compostable.

Native starch is typically chemically modified (after extraction) to impart the performance benefits. The most common modified starch in the coating space is hydroxyethylated, which is widely known for providing strong rheological stability to formulations, including water retention, coating stability, and superior runnability on application equipment in surface/coating applications.

Hydroxyethylated starches are commonplace in paper-based packaging coatings through size press, off-machine, and on-machine coating applications. Other modified starches used in coating applications include oxidized, acetylated, acid-thinned, or starch esters. With the variety of options, starch selection relies on a deep understanding of process needs and capabilities, as different modifications impart different properties.

In paper/packaging coating applications, starch is a preferred naturally sourced binder to implement into a formulation for either base or topcoats. In base coat applications, starch-based products have been shown to potentially eliminate thickeners and reduce latex content, ultimately improving board quality. In topcoat applications, starch-based products increase water retention of the coating resulting in better structure, reduced binder migration, and improved glueability with less blocking.

Below is a case history showing the successful use of a starch-based binder in coating on coated recycled boxboard (CRB) to improve performance:

The solutions used in the above case history are from Ingredion. The company offers a variety of modified starches for coating applications with various viscosities and molecular weights to provide unique benefits. Products are summarized below, however, selection is often customized to specific application goals and key performance indicators.

Liquid natural polymers (LNPs) are ready-for-use starch products that are designed for use in coating formulations where the desired solids content of the total formulation limits the use of a dry product. Liquid coating starches are designed with high solids content, like those of common latex resin materials. At equivalent material solids, the utilization of these LNP allows the formulator to closely maintain the overall formulation’s solids content. Formulation viscosity will change with the substitution of starch for latex due to the influence of molecular weight upon coating rheology. (REDIFILMTM is the name for our LNP solutions.)

Hydrophobic starch esters (HSEs) are modified starches that are generated with an octenyl succinic anhydride (OSA) derivative that is substituted onto the starch molecule. HSE products have been demonstrated to exhibit similar properties to polymeric surfactants in some formulations. When dispersed with latex resin in waterborne formulations, HSEs generate a weak ionic network comparable to that of associative thickeners. (FILMKOTE® is the name for our HSE solutions.)

Coating binders: Pre-gelled, or stir-in starch materials, are unique in that they enable the use of a high molecular weight product in powder form, whereas standard liquid starches would not be sufficiently suitable due to the higher water content of the cooked paste. Since pre-gelled starch products have been spray dried after cooking, they have high solubility. This allows them to be dry blended into a formulation pigment slurry or, depending on the application, into the latex resin liquid. Due to their application method, these starches contribute to higher binding strength per dosage. (PEN-COTE® is the name for our pre-gelled starch coating binders.)

Scott Hyland

Project Leader, Coatings Applications, Customer Solutions Product Innovation, Ingredion

Scott Hyland is Project Leader, Coatings Applications, Customer Solutions Product Innovation with Ingredion Incorporated. He is a 38-year industry veteran in paper/packaging with technical roles in research, product development, coating applications, and more

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Starch-Based Polymers in Waterborne Coatings Raise the Sustainability Bar