1. Introduction

2. Materials and Methods

Materials:

The cryogenic delamination was done on different kinds of finished leather. They were three samples of commercial leather, with three kinds of colour and finished layer, for a method versatility demonstration. They were all tanned by chromium, because it was the most common strategy [24]. This choice was done to discover if the new potential recycling system that was descripted below, was suitable for ordinary leather. They had distinct characteristics, reported on Table 1:

Table 1. This table reports main general characteristics of leather samples.

Table 1. This table reports main general characteristics of leather samples.

Sample	Thickness before (b) and after (a) (mm)	Look	Color	Photo before	Photo after
Entry 1	1,64 (b) 1,54 (a)	Full hand	black
Entry 2	1,56 (b) 1,36 (a)	Compact touch	brown
Entry 3	1,29 (b) 1,11 (a)	Soft hand	Light brown

They were analysed by some techniques, for a better characterization of samples and comparison with products.

3. Results and discussion

Cryogenic removal

In base on the principle revealed before, the contraction of polymeric layer permits an easy separation of it form the underside leather. This is possible due to the different glass transition temperature between the two substances and a very efficient resistance of leather.

A representative sketch of the principle of this process is illustrated in Figure 1.

Figure 1. Schematic illustration of the cryogenic delamination process.

Figure 1. Schematic illustration of the cryogenic delamination process.

This is done immersing the sample into liquid nitrogen [28]. Some other techniques were tried, like cotton immersion and application on leather (to avoid the possible leather damages) or nitrogen spilling on the leather surface [29] but the first one rests the most efficient. Another possibility could be the ΔT extremization, by the nitrogen affixation on a 40°C leather samples, to check if polymer will be removed in less time on a hotter support. Anyway, all these methods weren’t useful and fast as the first.

An example of the frozen leather aspect is illustrated in Figure 2, where the cracks produced by different contraction are evidenced.

Figure 2. finished layer break due to N₂ liquid and slow folding.

Figure 2. finished layer break due to N₂ liquid and slow folding.

One of the main advantage of this idea is about the aswer to thickness, it has remained the same in case of leather, with only a difference of tenths of millimetres. These data demonstated the high innovation and quality of this method, much better than the shaving recycle system. Also, in Table 1, the cryogenic polymer removal, was easily observable. It allowed to know which was the best way, to discover some interest effects about this method.

Dissolution

For analysing the chemical properties of these layers, as molecular structure (NMR) or polymeric status (GPC), it was important to find a good solvent for them. It also permits to separate some components with liquid chromatography and permits to know the mixture composition, very important for some properties. Infact, the contracting temperature (T_c) depends by compounds; if these substances have a particular T_c, it could be applicated a better temperature control for the cryogenic remotion.

Common solvents were tried, used during the finishing step in tanning industries [30]: cyclohexanone, acetone, butanone, methyl acetate, ethylene glycol and dioxane. Ethanol was used, to represent alcohols commonly used in the same field said before. Common diluent was tried to apply, normally used in the finishing polymer mixture in tanning industry, n-hexane. Methyl tert-butyl ether was tried to represent ethers commonly used in the same field said before. Solvents, normally used in deuterated form for nuclear magnetic resonance (NMR) analysis to know the exact structures, were tried: distilled water, DMSO, DCM, and chloroform [31]. THF, normally used in Gel Permeation Chromatography (GPC) analysis to know and characterize different polymers, was tried. Acetonitrile, generally used in high permeation liquid chromatography (HPLC), was tried. Toluene, to check the non-polar solvent dissolution activity, was tried. DMS, to know the polymers behaviour with useful or reactive substances [32], were tried and finally DMF: generally used for polyurethanes dissolution [33]. All these compounds didn’t dissolute completely the polymeric layer, like it could be seen on Table 2:

Table 2. solubility in different solvents of several samples.

Table 2. solubility in different solvents of several samples.

Sample	Solvent	Dissolution activity
Sample 1 finished layer	Distilled water	Negative
	DMSO	Partially
	THF	Negative
	Chloroform	Polymer separation
	Acetonitrile	Negative
	Ethanol	Negative
	n-hexane	Negative
	DCM	Negative
	Cyclohexanone	Polymer separation
	Acetone	Negative
	Methyl tert-butyl ether	Negative
	Toluene	Negative
	Butanone	Negative
	Methyl acetate	Negative
	Ethylene glycol	Negative
	DMS	Negative
	Dioxane	Negative
	DMF	Partially
Sample 1 uncolored finished layer	THF	Negative
	Acetone	Negative
	Acetonitrile	Negative
	Methanol	Negative
	Distiller water	Negative
	Toluene	Negative
	DCM	Negative
	DMF (100°C)	Negative
	Dioxane	Negative
Sample 2 finished layer	DMSO	Partially
Sample 3 finished layer	DMSO	Negative

DMSO and DMF were the only solvents that could dissolute certainly one layer (the colored) and only in two samples (not in sample 3). After that, some lipophilic and hydrophilic common solvents were tried, to check which direction must be followed. But, also in these cases, there weren’t exhaustive results. It could be possible that PU were particularly substituted and functionalized, so they weren’t reactive like the simplest and normal forms [34]. The polymers separation seen in Figure 3, by cyclohexanone and chloroform, permitted to analyze better the uncolored layer, without the presence of the colored one. Furthermore, thanks to these solvents, samples were prepare for other analysis. The uncolored finished layer was tested with, mainly, solvents seemed unactive. Them were used because, if the uncoloured part was dissolved, it become undetectable on the colored finished layer of sample 1.

Figure 3. separation of polymeric finished layers.

Figure 3. separation of polymeric finished layers.

After this first assay, it was decided to analyse samples in solid state, thanks to the poor solubility.

FTIR

In case of infrared spectroscopy, it was used for the identification of the polymeric layer. The finished layers of all three main samples and the separated pieces of sample 1 were analyzed; they belonged to the same polymer family: polyurethanes (PU). In fact, the substance that was made up, was possible to determine, with a good approximation. This was done by the amount of infrared radiation absorbed analysis, or transmitted, by a sample.

Figure 4. FTIR representative spectra, here it is showed the case of sample 1, on the top outside finished layer and down the underside leather layer.

Figure 4. FTIR representative spectra, here it is showed the case of sample 1, on the top outside finished layer and down the underside leather layer.

There were some differences in substituents, in quantity of CO and NH isocyanides. It confirmed the mixture of different polymers and the variety of them in similar samples. Where they were a mixture of several substances, the general characterization, became more complex. Fortunately, the resulting spectra were very similar. In particular: at 3300-3500 cm^-1 there was the normal NH stretching of amines, at 2850-3000 cm^-1 there was the usual CH stretching of alkanes, at 1630-1750 there was the ordinary CO stretching of amides and esters, at 1500-1690 cm^-1 there was the classic NH scissoring of amines and amides, at 1350-1440 cm^-1 there was the usual CH deformation of alkanes, at 1000-1300 there was the common CO stretching of esters, at 1000-1250 cm^-1 there was the ordinary CN stretching of amines, at 720-725 cm^-1 there was CH rocking of alkanes and at 660-900 cm^-1 there was the ordinary NH wagging of amines. It has to bring in mind that 900-600 cm^-1 area is the fingerprints, characteristic area of everly particular compound. All these signals represented functional groups of polyurethanes. An overlapping of the polymer species spectrum with a general PU (aliphatic PU from amine exended polyesther ATR ZnSe) in Figure 5 was also done, to be sure of in case of uncolored finished layer. It confirmed the polymeric main structure, so these spectra were compared to find more characteristics of analyzed polymers. An accurate spectra analyses, arrived from a peak interpretation and a comparison with polymer libraries reveals that the outside layer, coloured and soluble on DMSO and DMF, was PU with aliphatic chains. The inside layer in front of leather, maybe the uncoloured, insoluble and shiny one was mainly ether-based PU [35].

Figure 5. uncolored layer (blue, down line) and a classic polyurethane (red, upper line) overlay.

Figure 5. uncolored layer (blue, down line) and a classic polyurethane (red, upper line) overlay.

These results confirmed also that the polyurethane used is functionalized and different from the normal one and it could permit an explatation of why it was so difficult to dissolve these layers. Also, generally in the finishing step, leather surface changes some characteristics thanks to not only one product, but more and usually they are a mixed products [36]. Another interest confirmation, is the completely absence of collagen [37]. It was an important result to exclude the leather damages or cryogenic substrate removal. This was a further confirmation of method effectiveness and validation.

SEM

Despite directly by a visual analysis, it could be possible to see if leather turned in crust state; for a better comprehension, this kind of microscopy was done [38]. This analysis was done for a better visual of top, down and side of leather: before and after finished layer remotion. Them reveals an important leather surface modification, a witness of an high changing on the collagen fibres status. It permitted to observe the collagen fibers disposition and confirmed the completely polymer removal (Table 3).

Table 3. SEM images, before and after nitrogen treatment collagen status.

Table 3. SEM images, before and after nitrogen treatment collagen status.

Sample	Before	After
leather side
leather top
leather down

These fibers changed status, from more compact and tidier to free and messy. Normally, with the polymeric layer, these are more orderly and bonded. If this part will be removed, these become more free and superficially untied, ready to a new finishing treatment, centring perfectly the objective of this innovative recycling method [39]. This could demonstrate the complete polymer removal. In fact the finished layer deposition is the step that could start to take a correct disposition, according with fashion and customer taste.

A possible problem could be the modification of all leather layers in case of stability and collagen disposition, maybe for the contact with very low temperature. But a reassurance about that arrived from SEM images “leather down”, In particular the collagen fibers not directly interested of remotion. It permitted to demonstrate if there could be a leather modification, after this treatment at -200°C. With a careful observation, collagen fibers stayed in similar disposition and conformation, without any particular changes.

So, these images demonstrated the innovative capacity of this new removal method and differentiated it from the others partial disruptive methods to recycle leather [40].

Tensile strength according to UNI EN ISO 3376:2020

This normative was applied to check if there were differentiations in terms of qualities on leather and permitted to be sure about the crust leather resistance, after nitrogen treatment. Samples were compared with and without treatment and generally, leather with finished layer had more resistance than without. The polymers structure reinforcement thanks the more contribute. At the same time, these results were expected to be really like the normal resistance values. So, it could be possible to demonstrate the good resistance of leather, also after nitrogen treatment.

Figure 6. stress/deformation chart obtained by tensile tests at 23°C. As representative case, it was used sample 2, green line with finished layer and yellow line without by the application of cryogenic method.

Figure 6. stress/deformation chart obtained by tensile tests at 23°C. As representative case, it was used sample 2, green line with finished layer and yellow line without by the application of cryogenic method.

Surprisingly, leather with this treatment not only remained with similar resistance, but in one case over three, it improved its strength characteristics (Table 4) [41]. This is because, generally, the finishing operations are done to take mainly some visual effects, touch effects and permeation resistance [42]. It is important and in base of which kind of polymers are applied, it could change lots characteristic quatities. So, the tensile resistance isn’t the main discriminative characteristic in this kind of leather. Or to say better, it has already high resistance and a little decrease does not pay high attention. It could be due to the collagen fibres compression and less elasticity of polymers applied on this substrate. Anyway, these results were quite similar between the same sample, so these were a very positive answers for this new method. Interest was about the brown sample (sample 2) that apparently was like sample 1, but experimentally it had a stronger resistance and a different profile. Before there was a high stress resistance and after that the slop was less, remaining high. Another way to see these important results is Table 4.

Table 4. main values evaluated on tensile strength assay; the entries represented 1: blanck sample 1; 2: sample 1 with cryogenic method; 3: blanck sample 2; 4: sample 2 with cryogenic method; 5: sample 3; 6: sample 3 with cryogenic method (with cryogenic method means without finished layer).

Table 4. main values evaluated on tensile strength assay; the entries represented 1: blanck sample 1; 2: sample 1 with cryogenic method; 3: blanck sample 2; 4: sample 2 with cryogenic method; 5: sample 3; 6: sample 3 with cryogenic method (with cryogenic method means without finished layer).

Entry	Deformation at maximum stress (%)	Maximum stress (MPa)
1	36.7	20.6
2	40.4	23.4
3	31.6	22.3
4	29.8	18.7
5	35.1	22.3
6	34.1	20.4

These results opened doors to the possible real affirmation of this new method, in terms of performances and results qualities. In fact, products had very high performances and it permitted to be the best substrate for another finishing step finally.

DSC

This kind of analysis was used to characterize and for a better understanding of thermal behavior of leather, after nitrogen method and polymeric layer [43]. It represented if there were differentiation in terms of exo and endothermic transformation, between -60 and 180°C. To be sure of that, two completely cycles (from cold to hot and from hot to cold) were done. It could confirm the results and said what was the materials attitude. The thermograms were grouped in terms of samples, charts were done like heat flow referred on temperature or time. In this article, it could be seen the black sample thermogram, (Figure 7) representative of the others samples.

Figure 7. DSC charts (temperature vs heat flow) : a) leather thermograms, from below: orange line was for sample 1 without finished layer and blue line was for blanck sample 1 and; b) it represented the overlay of all finished layers, from below, grey line for light brown (from sample 3), orange line for black (from sample 1) and blue line for brown (from sample 2).

Figure 7. DSC charts (temperature vs heat flow) : a) leather thermograms, from below: orange line was for sample 1 without finished layer and blue line was for blanck sample 1 and; b) it represented the overlay of all finished layers, from below, grey line for light brown (from sample 3), orange line for black (from sample 1) and blue line for brown (from sample 2).

From leather graphs, the first interesting phenomenon was about the great endothermic curve during the first heat cycle. It seems to correspond to water evaporation, an endothermal transformation that after the first cycle. But in reality it was a glass transition [44], assessable also in four cycle charts, thanks to the partial reversibilty of collagen; for this reason the third cycle was different from the first one. The profile was similar of all the kinds of leather, with a marked difference only on the relaxing trend. It was interest the exothermic curve at 60-80°C, maybe due to a possible mechanism that could happened only in case of leather plus polymer. The comparison of leather with and without finished layer, represented the protection polymer property to the heat flux. It was useful for a better determination of samples characteristics. At the same time, the changing on them after and before nitrogen, generally respected the normal thermal behaviour without so strong differences. It was a further confirmation, of the high qualities remaining inside treated leather. In b it was possible to see a regular thermal profile, without any great changes. The more defined and regular curve demonstrated: a similar physical status of all PU on leather and similarity on thermal characteristics in this range of temperature. The first endotermal curve, that represented the glass transition was around -50°C and the first exotermal curve that represented the cristallization process was around 50°C [45]. Anyway, the substantial difference of b with the others, attested another time the absence of leather with finished layer.

TGA-TMA

This analysis was done for a comparison with DSC, to know what happened at high temperature on substrate samples and to check if it was possible to do a weight loss analysis [46]. The behavior, was studied between 30 and 900°C. It permitted to have a complete panoramic of all the main characteristics of these materials, as well as better understand of polymeric role [47]. Results could be well explicated by sample 1 thermogram (Figure 8).

Figure 8. a) blank test sample 1; b) leather of sample 1; c) finished layer of sample 1.

Figure 8. a) blank test sample 1; b) leather of sample 1; c) finished layer of sample 1.

It was possible to see similar transition peaks and weight loss range in a and b. At same time, there were high differences in c, with one main transition in a short temperature range. Unfortunately, the decomposition of polymer and leather started at the same temperature. For this reason, it wasn’t possible to separate and quantify them by an isotherm. Another interest aspect was the protection of leather thanks to this polymeric layer at high temperature, in fact all three samples had represented this situation. A particular characteristic, like on tensile strength analysis, is the high resistance of sample 2. It wasn’t thanks only to mechanical properties, but also in thermal qualities. temperature It happened some phenomena: in case of leather, the remaining weight is the inorganic part, chromium mainly (judging by the colour). Another is the indirectly leather weight diminution, in line with the DSC behaviour, to attest the undefined and different status of collagenic structures. Similar was the case of polymers. Another result that confirmed the DSC discussions, was the major temperature resistance of blank protected leather [48]. About polymers, they had all a similar behavior, they started a fast decomposition. For the cryogenic delamination method, TGA-TMA represented the possibility to attest the absence of leather in polymer, as in DSC. It was a useful analysis for a comparison of these techniques, for a thermal results validation and to have a complete idea of thermal behavior first and after nitrogen.

By all these analyses it was possible to check the potentialities of nitrogen finished layer removal method. It has to be better developed, but these data encourage this new principle, that could change the recycling field. Costs are the limiting factor, but with the unique versatility and the new improving leather value, it will be possible to be an important opportunity.

4. Conclusions

5. Patents

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