## 4 Results and Discussion
### Material Development Experiment: Strapping Tape Recycling  
Based on the types and quantities identified, as detailed in Table 2, different strategies for the recycling of these wastes have been analyzed. Among the three main waste types, polyolefins are semi-crystalline polymers prone to crystallization. This is a drawback when referring to 3D printing, since these materials will warp significantly on cooling. Among them, PP has more potential to be modified for 3D printing applications, and filaments can be commercially found. An experiment to evaluate the novel strategy using polypropylene strapping tape waste (see Table 2) is presented in this section.  
There is limited data regarding the amount of recycled content of current products on the market. Most of the recycling done at present involves reintroducing post-industrial wastes (scrap, noncompliance components, etc.) into the process to produce components with the same material. In this case, the wastes are well identified and clean. However, the strapping tape waste is the post-consumer type, in contact with different environments such as marine water, and in most cases it is contaminated. In these cases, apart from the need for sorting, cleaning, and shredding of the wastes, the waste usually shows some degree of degradation and lower molecular weight due to chain scission, which increases the degree of crystallinity [43].  
This variability of the properties of post-industrial wastes, such as strapping tape, has led to a common strategy in different industries to incorporate these materials: the use of low recycled contents so as not to disrupt production lines. It has been estimated that less than 5% recycled content is used in automotive, electrical, and agricultural sectors; other sectors, such as packaging, fabricate some products with higher recycled contents of 20% [76]. In this scenario, a novel system to print components with high recycled content based on maritime waste is proposed to the maritime sector.  
Experiments were carried out by producing filament with recycled strapping tape in order to be able to make an assessment of the properties of this post-consumer waste. The strapping tape was cleaned only with water, due to minor impurities, and subsequently shredded (Figure 7a). The outcome is a material with a very low apparent density, which is difficult to be properly fed into the extruder. For this reason, it is difficult to produce a filament with a constant diameter. In this case, the first filament is shredded again to get a better granulate to feed the filament line, as can be seen in Figure 7b.  
Printing trials were carried out with a standard geometry (central part of the ISO 527 testing specimen). A strong warping effect was detected (Figure 7d). However, this is an already-known effect for standard polypropylene due to the intrinsic thermal contraction properties of this semicrystalline material. Therefore, it is not an effect that can be attributed to the recycled material. Using additives to enhance the flexibility of the material and reduce its overall shrinkage coefficient can improve the printability of PP, as for instance using fillers such as calcium carbonate or blending with more flexible polymers. Here, process parameters have to also be adapted to the modified material.  
These trials indicate the potential of using strapping-tape-based PP materials with high recycling content for 3D printing, as long as adequate material developments are made and process parameters are adjusted to the material, as is proposed in the integrated 3D printing framework.