# Analysis of the Delamination Process with Nitric Acid in Multilayer Composite Food Packaging
## 3 Results  
### Packaging Structure and Products after Delamination  
Recovering valuable materials from polymer and aluminum composites presents a challenge due to the need for a delicate balance between preserving the integrity of the materials and maximizing value extraction. The delamination principle has been identified as an effective means of achieving this goal. Utilizing this principle enables the recovery of all polymer layers without causing any damage. Multilayer plastic composites require adhesive layers for their formation. Two types of adhesives are commonly employed in polymer and aluminum composites: acrylics and polyurethane (PUs). Acrylics are suitable for bonding polyethylene to aluminum, while PU is more versatile and widely used, even functioning as plasticizers in polymer packaging. PU is composed of a hard segment and a soft segment. The hard segment consists of isocyanates, and the soft segment comprises polyols. During the delamination process, hydrolysis breaks the isocyanate bonds, facilitated by the addition of water. This process results in the polyols' recovery and the polymer layers' separation. Additionally, PUs are vulnerable to oxidative degradation in acidic or alkaline media [11]. The kinetics of this process depend on several variables, such as moisture, heat, and UV light. Aqueous solutions and temperatures are carefully controlled during recovery to mitigate these risks. As the analyzed packages contain an aluminum layer, an oxidizing agent like nitric acid can be employed to separate the layers. Although aluminum does not dissolve in oxidizing acids, such as concentrated nitric and sulfuric acids, due to their passivating effect, it does dissolve in dilute HNO\({}_{3}\), being reduced to N\({}_{2}\)O and partially to NH\({}_{4}\)+ [12]. \[8\text{Al}+30\text{HNO}_{3}\to 8\text{Al}(\text{NO}_{3})_{3}+3\text{NH}_{4} \text{NO}_{3}+9\text{H}_{2}\text{O} \tag{1}\]  
The boiling temperature of nitric acid is 83 \({}^{\circ}\)C. At a temperature of 75 \({}^{\circ}\)C, the release of nitrous dioxide gas has been observed [10]. \[4\text{HNO}_{3}\to 4\text{NO}_{2}+2\text{H}_{2}\text{O}+\text{O}_{2} \tag{2}\]  
Likewise, ammonium nitrate can split into dinitro oxide and water, but this necessitates a high temperature [13]. \[\text{NH}_{4}\text{NO}_{3}\to\text{N}_{2}\text{O}+\text{H}_{2}\text{O} \tag{3}\]  
In the context of our material recovery process, we assume that the adhesive layers in the packaging consist of PU, as these are the most commonly used adhesive materials. As a strong oxidizing agent, nitric acid is also suitable for breaking down PU structures. PU is a segmented co-polymer, composed of soft and hard segments. The soft segments provide elasticity to the co-polymer while the hard segments contribute to the co-polymer's strength [14]. When the PU enters a solvent or solution, the hard segments swell first. During swelling, the solvent penetrates deeply into the polymer and disrupts its structure (Figure 2) [15,16]. In the analysis of packaging materials, the process entails the dissolution of adhesive layers, ink, and aluminum, which result in the recovery of three distinct polymers. Upon inspecting the polymer strips, ink residues are observable, and their presence assists in visually distinguishing the individual polymer layers. Nonetheless, more advanced methods of polymer recognition, such as optical methods, are necessary to eliminate all color residues. The study of packaging composition and its impact on food safety has garnered significant attention in recent years. Comprehending the composition of packaging materials is vital, as it can offer insights into material properties and their interaction with food. In this context, our research team examined the packaging material of a food product, and based on the available data, we devised a preliminary structure for the packaging composition. Our analysis disclosed that the packaging material comprises three polymers.