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Compostable Packaging Adhesives – Challenges, Types, Testing and the Eco Footprint

Posted on 9/9/2020 9:31:44 AM By David Speth
  

With the advent of the Covid 19 pandemic, more consumers are shopping from home or using delivery services to get products they used to pick up at brick and mortar stores. This has increased the need for shipping boxes and other forms of packaging. While major e-tailers are working to reduce or even eliminate packaging, for the moment, disposable packaging to protect goods during shipping is still a necessity. There is even a commercial about the importance of boxes on TV! Reducing the eco footprint of single use packages is something we all should consider.

Recently one of our customers asked me whether we could make hot melt adhesives compostable to make their products more ecologically friendly. (Whether the small amount of adhesive really matters is an open question.) Composting is one way to ‘recycle’ articles back to the environment. Compostability is not a simple concept. There are several challenges that need to be met to develop and evaluate products that are bio-degradable. The first is a clear definition of success. There are at least four ASTM tests intended to evaluate bio-degradability of plastic materials.

  • ASTM D5511 - 18 Standard Test Method for Determining Anaerobic Biodegradation of Plastic Materials Under High-Solids Anaerobic-Digestion Conditions
  • ASTM D7475 - 20 Standard Test Method for Determining the Aerobic Degradation and Anaerobic Biodegradation of Plastic Materials under Accelerated Bioreactor Landfill Conditions
  • ASTM D5526 - 18 Standard Test Method for Determining Anaerobic Biodegradation of Plastic Materials under Accelerated Landfill Conditions
  • ASTM D5338 - 15 Standard Test Method for Determining Aerobic Biodegradation of Plastic Materials under Controlled Composting Conditions, Incorporating Thermophilic Temperatures

Each of these evaluates performance under different exposure conditions. To evaluate existing products, or develop new products, a clear definition the exposure conditions that will be evaluated (aerobic, anaerobic, industrial, residential) must be created. It will be particularly important to determine how much degradation is necessary for success (50%, 100%?) and over what period of time that would be measured (months, years?). It will also be critical to define what degradation products are acceptable (does success require complete degradation to carbon dioxide and water, or some intermediate level).

If complete degradation is the goal, there are a couple of adhesive families that can be used. Protein based glues are one option. These gelatin relatives are derived from animal by-products like hoofs and hides. They can provide good strength and they are used today for woodworking, especially for restoration of musical instruments and other antiques. They do not however provide the processability or level of performance of the synthetic adhesives as they have limited temperature resistance, high moisture sensitivity and microbial sensitivity (potential contamination and early degradation).  For these reasons, they are now specialty products and are available in limited supply. For commodity applications further development would be required.

Starch based adhesives are being used for paper substrates. These adhesives based on corn, potato, tapioca, or other starches are prominent in the corrugated cardboard industry. They are used to bond the face sheets to the core. Since they are lightly processed natural products, these adhesives have a small ecological footprint and degrade readily when exposed to the natural environment. Since most are water-based however, they present some challenges when used for packaging closures produced at high speed.

If partial degradation is sufficient, there arecommon adhesive products that allow a level of bio degradation. Traditional hot melt packaging adhesives are comprised of three elements: a polymer to provide strength, a tackifier to make the polymer sticky, and a wax to control viscosity and help with a fast set that is required for high speed production lines. Many of the tackifiers used are based on natural raw materials like rosins, terpenes and citrus oils. Natural waxes can also be used. These allow us to make products that are 35-60% bio-based and subject to bio-degradation on some time scale. If this is sufficient, then products already exist to satisfy this need. If not, new options will need to be developed. To get to 100% degradation, the polymer base will need to be addressed perhaps by using a polymer like poly lactic acid. This will require additional development.
Compostability is one approach to improving the eco footprint of adhesives used for packaging. Another would be to use non petroleum sources for the polymer bases of these adhesives. I will discuss these in a future blog.



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