Dr. Pinaki Ranjan Samanta*, Dr. Manisha, Mr. Kaushal Kishor Chaudhary, Ms. Kahkashan Bano and Mr. Prem Kumar Nautiyal.

Regulatory Affairs and Analytical R&D Dept.

Uflex Ltd** (Chemicals Division), C-5-8, Sector-57, Noida, Uttar Pradesh, India.

Abstract

Flexible plastic products represent a major challenge for recycling due to presence of printing Inks and laminating adhesives. However, during the mechanical recycling of post-consumer waste, these inks act as contaminants, subsequently compromising the quality and usability of recycled material.

Nitrocellulose (NC)-based inks create significant recycling challenges, especially when used on polyolefin laminated substrates. Nitrocellulose, used as a binder, degrades during reprocessing, results in film discoloration and the emission of potentially odorous compounds.

Mechanical recycling of flexible plastics is a challenging task, due to the degradation of polymer chains and alterations in properties induced by contamination from other polymers and additives, subsequently limiting the quality and applicability of recyclates.

This review covers the decomposition mechanisms of printing ink binder resins, herein nitrocellulose (NC), with a particular focus on NC’s behavior under extrusion conditions in the mechanical recycling process of polyolefin (PO) based plastic packaging.

Keywords:  Nitrocellulose Resin, Flexible packaging Recycling, Printing Inks, Sustainability, regulations etc.

 * Corresponding author (E mail: pinaki.chem@uflexltd.com)

** www.uflexltd.com

Introduction

The common treatment for plastic packaging waste involves mechanical recycling, energy recovery, and landfilling. Recycling, according to the waste hierarchy, should be the preferred treatment route as it reduces greenhouse gas emissions and the use of fossil resources (Volks et al, 2021)

A further challenge is the presence of printing ink on the surface of the packaging. The state-of-the-art recycling process begins with collection and sorting. During sorting, plastic packaging is segregated based on polymer types using near-infrared spectroscopy (NIR) or physical properties, and in some cases, a combination of both.

Waste packaging is transported to the recycling plant for mechanical recycling. This process includes bale opening, pre-washing, shredding, friction washing, swim-sink separation, drying, and extrusion [1- 9].

 A further challenge is the presence of printing ink on the surface of the packaging. Although printing ink is essential for providing necessary information and marketing appeal, it is a source of contamination in the mechanical recycling process (Gecol et al., 2001; Guo et al., 2022).

The ink content in mono-layer flexible packaging can range from 0.5–5 wt.%, and for full-area printing the ink content can take up to 10 wt.% (Guo et al., 2023a). Some components of printing ink are not suited to the extrusion process in plastic recycling due to their low stability under extrusion temperatures and potential side reactions (EuPIA, 2021).

These conditions can lead to degradation and side reaction products, altering the thermal and mechanical properties (Gecol et al., 2002; Guo et al., 2022), discoloration, and increasing the volatile organic content of the recycled plastic (Kaedin-Koppers, 2023).

Certain printing inks contain an additional layer of varnish, known as the overprint varnish (OPV), on the top of the printing layer to enhance abrasion resistance and optical effects (Flick, 1999; Kol et al., 2021a). The OPV, essentially a pigment-free binder resin, may be identical to or different from the binder that carries the colorants (Guo et al., 2023b; Kol et al., 2023).

Nitrocellulose is the most prevalent binder resin for flexographic printing, holding approximately an 80% share in the EMEA (European, Middle East, and African) market (Guo et al., 2023b).

Degradation Mechanism of Nitrocellulose

NC is the nitro-derivative of cellulose, which is obtained through a reaction with sulfuric and nitric acids. This reaction replaces the hydroxyl groups (OH^–) in cellulose with nitro groups (NO₂^–), as shown in Eq. (A) (McKeen, 2017).

 (C₆ H₁₀   O ₅) + n HNO₃,H₂SO₄ −−−−−−−→ (C₆ H₇ (ONO₂ )x O₂ +YH₂ O      ………..(A)

It is noteworthy that nitro groups replace not all hydroxyl groups in the cellulose. The specific nitrogen content of the nitrocellulose determines its applications.

In the context of printing ink binders, nitrocellulose typically has a nitrogen content ranging from 10.9% to 11.2% and is soluble in ethanol and isopropanol  (Heinze et al., 2012). Nitrocellulose binders are popular due to their low cost and high gloss (Dave, 2020). However, their susceptibility to thermal degradation is a significant disadvantage, with degradation commencing between 160 and 185 °C and complete degradation occurring at 210 °C (Pourmortazavi et al., 2009; Abd El-Wahab et al., 2020; Devra et al., 2022).

The decomposition of nitrocellulose in the solid phase takes place at temperatures between 140 and 190 °C, slightly below the extrusion temperature of polyolefins This thermal degradation largely involves the dissociation of the CO–NO₂ bond.

The degradation process begins with the separation of the O–NO2 bond, leading to the formation of NO₂. Subsequently, nitroxylated groups are formed as oxidation products and concurrently produce peroxyl groups (Dauerman and Taijima, 1968; He et al.,2017)

Two primary degradation mechanisms come into play during the recycling process. The first is direct thermal degradation, a known phenomenon concerning nitrocellulose (NC) binder. This pathway is notorious for its gas generation and browning during the extrusion process (Kaedin-Koppers, 2023).

However, another often-overlooked mechanism is the hydrolysis degradation of binder resins under extrusion. Under an industrial recycling process, the input material retains 3–4 wt.% water after the mechanical and thermal drying from the upstream cleaning process (COWI, 2019; Berkane et al., 2023).

Nitrocellulose degradation begins with the homolytic scission of nitrate groups, releasing – NO₂^• radicals, which then form nitrous and nitric acids via H-abstraction or reaction with water. This de-nitration process is followed by cellulosic backbone scission, primarily at the glycosidic bond, triggered by hydroperoxides formed during photo-oxidation. The result is the formation of hydroxyl groups, gluconolactone, and ultimately, acid powder, leading to a decrease in the material’s nitrogen content and molecular weight. 

Initial Stage: De-nitration

1.  Homolytic Scission:

The degradation process starts with the breaking of the C-O bond in the nitrate ester group, a process called homolytic scission. 

•  Radical Formation:

This scission releases a nitrogen dioxide radical (^•NO₂). 

•  Acid Formation:

The ^•NO₂ radical can then abstract a hydrogen atom (^•H) or react with water to form nitrous acid (HNO₂) and nitric acid (HNO₃). 

• Degree of Substitution (DS) Reduction:

These reactions result in the removal of nitrate groups and their replacement with hydroxyl (–OH) groups, leading to a decrease in the material’s degree of substitution. 

Second Stage: Backbone Scission

1. Hydroperoxide Formation: Excited states generated from photo-oxidation can lead to the formation of hydroperoxides.
2. Glycosidic Bond Cleavage: These hydroperoxides cause the cleavage of the glycosidic bond, which forms the backbone of the cellulose chain.
3. Gluconolactone Formation: This cleavage produces a gluconolactone as a primary degradation product.
4. Further Reactions: The gluconolactone can then be further transformed.

Overall Effect

• Loss of Nitrogen: The cumulative effect is a loss of nitrate groups, which reduces the nitrogen content of the nitrocellulose. 

• Increase in Hydroxyl Groups: Simultaneously, the number of hydroxyl groups increases. 

• Cellulose Breakdown: The cellulosic backbone breaks down, leading to a decrease in molecular weight and the eventual formation of acid powder. 

Schematically Mechanism as Per Followings: [First Stage]

Nitrocellulose –

Schematically Mechanism as Per Followings: [Second Stage]

Why Minimize Nitrocellulose (NC) for Sustainability

• Recycling Contamination: 

NC-based inks can prevent the proper recycling of plastic films, especially those with polyolefin laminations. 

• Decomposition During Recycling: 

When printed films are mechanically recycled, the NC binder can degrade, leading to discoloration, odor, and reduced mechanical properties of the recycled plastic. 

• Environmental Impact: 

While not directly addressed in the same way as health hazards, the challenges in recycling contribute to a less sustainable packaging system. 

• Focus on a Circular Economy: 

EuPIA supports initiatives that promote a circular economy for printed packaging, which includes reducing problematic substances like those in NC-based inks. 

• Product Stewardship: 

EuPIA provides guidance for the risk and safe handling of NC inks to protect workers and ensure compliance with safety regulations. This is more about managing risks than advocating for substitution for environmental reasons. 

Conclusion

The contaminants from inks are recognized: those undergoing major degradation like NC and those (partially) persisting in the recycled material such as PU, pigments, affecting its properties.

Both types of contaminants, regardless of degradation, compromise recyclate quality, depending on degradation more to VOCs, mechanical properties or aesthetics. Any contaminant, regardless of degradation that compromises recyclate quality and safety, is a concern for plastic circularity. Nevertheless, the significance of small molecular weight components and the potential formation of secondary by-products underscore the necessity for thorough investigations into the degradation profiles of these chemicals.

Nitrocellulose (NC) is not fully banned in flexible packaging, but its use is increasingly restricted by organizations like EuPIA and by other regulatory bodies in Europe due to significant problems in the polyolefin recycling process. The German Central Office for Packaging Recycling introduced a new minimum standard to assess packaging recyclability. This regulation, passed in June 2023, classifies packaging containing NC-based inks on polyolefin laminated structures as non-recyclable.

Acknowledgement

The authors are overwhelmed in all humbleness and gratefulness to acknowledge for writing this review article to Uflex management, who helped to put these ideas well above the level of simplicity and something concrete. It is a genuine pleasure to express our deep sense of thanks and gratitude to our colleagues for their timely advice, meticulous scrutiny, scholastic advice and scientific approach to a very great extent.

Authors

• Dr. Pinaki Ranjan Samanta; M.Sc, Ph.D, PG diploma in paint technology, has more than 32 years of experience in the areas associated to industrial research of coating chemistry.

• Dr. Manisha, M. Sc, Ph.D, has more than 17 years of experience in the field of polymers and analytical R&D.

• Kaushal Kishore Chaudhary, M.Sc in chemistry, has more than 12 years of experience in analytical R&D.

• Kahkashan Bano, M. Sc in chemistry, has more than five years of experience..

• Prem Kumar Nautiyal, M.Sc, M. Phil in chemistry along with a Certified Black Belt and PGDMM, has 36 years of industrial experience in various industries.

Reference

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Disclaimer: The views expressed are solely the corresponding author’s personal opinion. Uflex Ltd will not accept any liability for any loss or damage that may occur from the use of this information nor do we offer a warranty against patent infringement.