Federico Poli

Ngoc-Lam Dinh

Valerian Mannoni

Benoît Denis

Evaluation of 5G-NR V2N Connectivity in a Centralized Cooperative Lane Change Scenario

By means of system-level simulations, we analyze in this paper the performance of Vehicle-to-Network (V2N) connectivity based on the 5th Generation -New Radio (5G-NR) as a support to Cooperative, Connected and Automated Mobility (CCAM), in light of both network and Multi-access Edge Computing (MEC) deployments. Focusing on a canonical centralized Cooperative Lane Change (CLC) use case that involves three vehicles in a cross-border highway environment, we assess the link reliability and the End-to-End (E2E) latency of all the messages involved in the CLC negotiation phase (from/to interconnected MECs hosting the centralized maneuvering application), while assuming different deployment configurations and the coexistence with a second demanding vehicular service running over the same radio resources. On this occasion, we illustrate possible benefits from Bandwidth Partitioning (BWP) on Uplink (UL) latency, as well as from an hypothetically tight cooperation between Mobile Network Operators (MNOs) on reliability and continuity, leveraging low-latency inter-MEC transactions and seamless cross-border handover capabilities.

I. INTRODUCTION

Vehicle-to-Everything (V2X) connectivity is seen as a key enabler to guarantee the seamless continuity of critical CCAM services along continental road corridors and beyond, allow higher and higher levels of driving automation (typically, up to levels 4 or 5). Related stakes concern the coexistenceand even ideally, the combination-of suitable radio access technologies (e.g., C-V2X sidelink a.k.a. PC5-Mode 4 or the cellular 5G-NR, covering respectively short-range and longrange communication needs), the definition of both network and overall system architectures and eventually, the deployment of new supportive elements of infrastructure [START_REF] Kousaridas | 5G Cross-Border Operation for Connected and Automated Mobility: Challenges and Solutions[END_REF].

Most of the research contributions reported in the recent literature address the previous problems from two rather decorrelated perspectives. On the one hand, some studies focus on the performance assessment of radio technologies on their own, for instance C-V2X sidelink based Vehicle-to-Vehicle (V2V) communications, most often at link level and regardless of any concrete application (e.g., [START_REF] Mannoni | A Comparison of the V2X Communication Systems: ITS-G5 and C-V2X[END_REF]- [START_REF] Gonzalez-Martín | Analytical Models of the Performance of C-V2X Mode 4 Vehicular Communications[END_REF]), or V2I communications with respect to Road Side Units (RSUs) (e.g., [START_REF] Poli | Evaluation of C-V2X Sidelink for Cooperative Lane Merging in a Cross-Border Highway Scenario[END_REF]) under various road infrastructure deployment assumptions (e.g., [START_REF] Chiha Ep | Techno-economic and Simulation Study of a V2I-based Cooperative Manoeuvring Case in a Cross-border Scenario[END_REF]). On the other hand, a second major research direction concerns the higher-level orchestration of networks, services and/or of MEC resources [START_REF] Coronado | Enabling Computation Offloading for Autonomous and Assisted Driving in 5G Networks[END_REF]- [START_REF] Slamnik-Krijestorac | Collaborative Orchestration of Multidomain Edges from a Connected, Cooperative and Automated Mobility (CCAM) Perspective[END_REF]. These analyses are usually intended in a global and generic 5G ecosystem, with only limited considerations with respect to the underlying radio access performance.

In contrast to the previous contributions, we herein propose a simulation-based study that aims at analyzing the performance of 5G-NR in a representative CCAM context, in light of both network and MEC deployments. More particularly, as a canonical example, we focus on a centralized CLC use case in a cross-border highway environment, assuming the support of Vehicle-to-Network (V2N) connectivity with respect to gNodeBs, as well as inter-connected MECs hosting the centralized maneuvering application. These simulations assess primarily link reliability and E2E latency performance indicators at the radio access level, while relying on simpler abstract assumptions for higher-level segments of the system architecture. Given that the CLC service is supposed to coexist with another demanding vehicular service involving a variable amount of active users, we benchmark the performances in both a baseline single-gNodeB scenario and after gradually increasing the number of gNodeBs. Finally, we illustrate the effect of 5G bandwidth partitioning, as a physical slicing means, onto the UL radio access latency, which is also shown here to be the dominating factor in the overall latency budget.

II. USE CASE AND MESSAGES SEQUENCE

We consider the CLC use case illustrated in Fig. 1 (also referred to a CLC as planned in [START_REF] Apruzzese | 5G-CARMEN Pilot Plan (Revised)[END_REF]). Accordingly, one vehicle (A) intends to insert from the overtaking lane onto the first lane where two other vehicles (B and C) are already running. These side vehicles must hence create a gap to allow the initiator (A) to insert between them, depending on the three respective positions. In a 5G-based centralized CLC, a manoeuvring service hosted in a MEC system first interprets the intention of vehicle A and therefore advises vehicle C to increase its headway from vehicle B, before informing vehicle A that the intended lane change is possible. Vehicles typically communicate with the centralized manoeuvring service via a Uu interface relying on 5G-NR. Recommendations from the manoeuvring service are also forwarded through a Uu interface to the respective engine control units (ECUs) of all relevant vehicles so that they can act upon. The time elapsed between the initiator's request (i.e., Vehicle A's updated status) and the actual lane merge realization is hereafter referred to as the CLC negotiation phase. In case of automated driving, the vehicles can execute the necessary actions directly. In manual driving mode, the ECU alerts the driver and provides recommendations for the necessary actions. In a specific cross-border context, the CLC procedure can be initiated and completed before, while or after crossing the border. In this case, vehicles geographically close to each other and participating to the same manoeuvre procedure may be connected to different MNOs, and hence, to different MEC instances. Hence, we assume herein that low-latency interfaces should be available in a reasonably near future to support information exchanges between services running on neighbouring MECs [START_REF] Georgakopoulos | 5G-CARMEN Preliminary System Architecture and Interfaces Specifications (Revised)[END_REF].

In Fig. 2, we show the corresponding messages sequence chart [START_REF] Visintainer | 5G CARMEN Use Cases and Requirements[END_REF], where regular and updated status messages rely on Cooperative Awareness Messages (CAMs), whereas recommendations (incl. both slowing down and safe-to-merge messages, resp. to vehicles A and C) rely on Decentralized Environmental Notification Messages (DENMs). Further details of all MEC-internal information flow, which is out of scope of this paper, are available in [START_REF] Georgakopoulos | 5G-CARMEN Preliminary System Architecture and Interfaces Specifications (Revised)[END_REF].

III. SIMULATION FRAMEWORK

The system-level simulation flow originally developed to evaluate PC5-based V2V/Vehicle-to-Infrastructure (V2I) [START_REF] Poli | Evaluation of C-V2X Sidelink for Cooperative Lane Merging in a Cross-Border Highway Scenario[END_REF], [START_REF] Da Silva | System-level Simulation of Cooperative Sensor Data Fusion Strategies for Improved Vulnerable Road Users Safety[END_REF] and LTE-based V2N [START_REF] Hillebrand | Current 4G Networks Limitations Pleading for 5G in Cross-border CAM Services[END_REF] connectivity in cooperative manoeuvring and data fusion contexts, has been expanded for the evaluation of 5G-NR V2N connectivity. Accordingly, mobility traces (incl. time-stamped vehicles' 2D coordinates, 2D speed and absolute heading) are first generated offline using the Simulation of Urban Mobility (SUMO) tool [START_REF] Krajzewicz | SUMO (Simulation of Urban MObility) -An Open-Source Traffic Simulation[END_REF], under various road densities based on real traffic data [START_REF] Poli | Evaluation of C-V2X Sidelink for Cooperative Lane Merging in a Cross-Border Highway Scenario[END_REF]. The previous mobility traces are then used for the generation of time-stamped 5G-NR connectivity traces via ns-3 simulations, accounting for the success/failure and latency of CLC transactions at both link and system levels. Mobility and connectivity simulations are deliberately decoupled here, since we are mostly interested by the information flow prior to CLC execution.

A. Environment, Mobility and Technology Penetration

The tested environment is the same as that considered in [START_REF] Poli | Evaluation of C-V2X Sidelink for Cooperative Lane Merging in a Cross-Border Highway Scenario[END_REF] and [START_REF] Hillebrand | Current 4G Networks Limitations Pleading for 5G in Cross-border CAM Services[END_REF]. It is composed of a 4.5 km-long portion of the A22 highway located at the border between Italy and Austria. Based on real traffic measurement data collected in 2019 at this border, macroscopic mobility parameters have been derived to configure our SUMO simulations. These measurements reveal an upper (resp. lower) limit on the traffic flow in each driving direction at approximately 2500 (resp. 750) vehicles per hour, corresponding to an average traffic density of 83 (resp. 28) vehicles per kilometre. Among the simulated flow of vehicles, we also consider a variable ratio of active connected vehicles (i.e., spanning from 1% up to 25%), which somehow reflects the technology penetration in very first approximation. Besides the vehicles involved in the CLC under test, all these other connected vehicles are supposed to transmit their own packets (e.g., regular CAMs) and hence contribute to load the network.

B. 5G-NR Radio Parameters

In our ns-3 simulations, the frequency domain granularity is the Resource Block (RB) and the time domain granularity corresponds to one Orthogonal Frequency Division Multiple (OFDM) symbol, while 14 symbols are assumed per slot. The numerology is systematically set to 2, which offers a reasonable trade-off between low latency and link robustness1 , given the imposed CLC requirements. The addressable Modulation and Coding Schemes (MCSs), which span from 5 to 25, are automatically and contextually optimized in each simulation realization on a case-by-case basis. Time Division Duplex (TDD) slots are dedicated either to Downlink (DL), UL, or both. We use a default pattern of 10 repeated slots (all flexible), each of them containing both UL and DL OFDM symbols, as well as a guard band of 2 symbols. The actual number of UL or DL OFDM symbols per slot is not pre-defined, even though higher priority is given to UL over DL. Regarding propagation modelling, a synthetic propagation channel suitable to motorway environments is generated with both large scale (path loss and shadowing) and small scale (fast fading) statistics according to the standardized models described in [START_REF] Gpp | Study on Channel Model for Frequencies from 0.5 to 100 GHz[END_REF], regardless of deterministic in-site propagation effects.

The implemented Medium Access Control (MAC) supports both Orthogonal Frequency Division Multiple Access (OFDMA) and Time Division Multiple Access (TDMA) in UL and DL, where a single User Equipment (UE) is scheduled per Transmission Time Interval (TTI) in TDMA while multiple UEs are scheduled per TTI in OFDMA, using all symbols but different Physical Resource Blocks (PRBs). A mixture of TDMA and OFDMA is also feasible. In terms of scheduling, we have mainly considered a basic Round Robin scheme for simplicity to derive nominal performances as a baseline, keeping in mind that future enhancements of this scheduler would lead to substantial gains in terms of latency. Finally, Automatic Repeat Request (HARQ) is also activated with systematic prioritized data retransmission. The maximum number of retransmissions is set to 4, again as a trade-off between latency and reliability. Table I summarizes the main radio parameters above.

C. Network Deployment and Handover Assessment

The deployment of a gradually increasing number of 5G-NR gNodeBs, all operating at 3.5 GHz, is considered along the 4.5 km of motorway around the Brenner pass. In our experiments, without loss of generality, the CLC vehicles are arbitrarily supposed to drive north from Italy to Austria, thus being primarily associated with the Italian MNO. The idea is to identify the minimum amount of infrastructure density to make sure that reliability requirements (at both radio link and overall CLC transaction levels) could be satisfied. The considered deployment configurations are summarized as follows:

• 1 gNodeB (reference/baseline for benchmark only): The location corresponds to that of the closest LTE eNodeB site currently deployed on the Italian side (See Fig. 3-a)). • 2 gNodeBs: The first gNodeB is the same as previously, while the location of the second corresponds to that of the closest LTE eNodeB site currently deployed on the Austrian side (See Fig. 3-b)). • 3 gNodeBs: The two first gNodeBs are the same as previously, while the 3rd one is added "manually" on the Italian side, approximately halfway between the two former to bridge possible coverage gap (See Fig. 3-c)). Besides, handover is not explicitly simulated but treated primarily through coverage assessment, by verifying if the resulting link quality experienced by the UE (i.e., given a perfect handover) would enable the right level of link reliability, and hence ultimately, of service availability. As regards to the induced latency (i.e., beyond reliability considerations), two simplified handover hypotheses are then accounted by means of extra penalty terms in the overall latency budget:

• 5G-NR Non Standalone (NSA), as it is: Here, we consider 5G-NR NSA without any specific improvement, neither with respect to inter-MNO handover, nor with respect to LTE core network. Since standard cross-border roaming in this situation could lead to interruptions on the order of several tens of seconds (See e.g., field measurements in [START_REF] Hillebrand | Current 4G Networks Limitations Pleading for 5G in Cross-border CAM Services[END_REF]), CLC service continuity would simply not be ensured in this case2 . This is also somehow reflected in our single-gNodeB evaluations, by the fact that an UEs at the cell border could experience relatively poor link quality at 3.5 GHz, while waiting for association with the next network. • Fully-fledged 5G-NR with tight inter-MNO coordination:

Just like in [START_REF] Hillebrand | Current 4G Networks Limitations Pleading for 5G in Cross-border CAM Services[END_REF], we also assume a more futuristicand somehow idealized-case, where inter-MNO handover would be performed transparently and almost instantaneously at the border (say, with a delay on the order of a standard intra-MNO handover), between the last serving gNodeB on the Italian and the first available gNodeB on the Austrian side. This somehow prefigures higher levels of inter-MNO cooperation (e.g., through slice-aware handover) and anticipated/non-postponed handover decisions (i.e., contrarily to current field observations [START_REF] Hillebrand | Current 4G Networks Limitations Pleading for 5G in Cross-border CAM Services[END_REF]).

D. MEC Deployment and Latency Assessment

We also consider several MEC deployment configurations, including fully decentralized (i.e., 1 MEC per gNodeB), clustered/semi-centralized (i.e., 1 per regional concentration site), and highly centralized (i.e., one per MNO data center) schemes, as schematically represented in Fig. 4.

In ns-3, the Radio Access Network (RAN) latency between a source and a destination (e.g., typically, for an UL CAM message from a vehicle up to the gNodeB) can be simulated up to the application level, meaning that it covers contributions from lower layers (i.e., physical layer, MAC/scheduling, Radio Link Control (RLC). . . ) but also further delays related to higher layers of the protocol (i.e., IP, TCP/UDP encapsulation and transfer. . . ), while accounting for jitter at the radio level, depending on link quality, resource scheduling and network load. Beyond the RAN aspects mentioned above, other latency components on top are not explicitly simulated either but represented in as abstract bounds. For instance, under a highly centralized MEC hypothesis, for the same UL CAM example as previously (i.e., from a vehicle up to the MEC in charge of elaborating the notifications in the centralized CLC mode), the overall E2E latency integrates extra delays related to backhaul, core network, and possibly inter-MEC connections (typically, in case of border crossing). Table II shows the mapping between the considered MEC deployment hypotheses, the situation of the vehicles involved in CLC transactions, and finally, the resulting latency terms that must be accounted in the overall E2E latency budget. For backhaul latency (e.g., between a gNobeB and a regional concentration site) and core network latency (i.e., between a radio access regional aggregation site and a data center), which are mainly distancedriven, we assume 2 ms for each component. This corresponds to lower bounds (typically, under the coarse assumption of 1ms/100km for fiber transport). Besides, although ultra-fast inter-MEC connections are mandatory (i.e., typically 1-2 ms per MEC switch for transferring service-related data/sessions, whatever the MEC deployment hypothesis), this term is left as an input parameter in the study.

E. Multi-service Assessment

Finally, we consider three different BWP schemes to illustrate possible benefits from flexible 5G slicing at radio physical level with respect to multi-service coexistence issues. The first scheme consists in allocating the full bandwidth to both the primary CLC service (1st service) and another vehicular service (2nd service) running in the background (typically with UL transmission of regular CAMs at 20 Hz), while relying on nominal TDD with a default flexible pattern that automatically gives high priority to UL OFDM symbols. The second scheme based on BWP consists in allocating half of the available bandwidth to each service separately. The third scheme, which also uses BWP, relies on a mixed TDD/FDD scheme where each of the UL and DL flows is assigned to half of the overall bandwidth. Unless specified, the first scheme is applied by default.

IV. SIMULATION RESULTS

A. Latency in a Single-gNodeB Case Fig. 6 shows the average UL radio access latency (averaged over simulation trials, vehicles and simulation time), as an integral component of a parametric E2E overall latency budget between the source vehicle and the MEC, depending on the different hypotheses introduced in Table II, as well as on the percentage of active users (i.e., with 1% to 25% of the simulated vehicles besides the CLC transmitting their own CAM messages at 20 Hz over V2N, while sharing the same spectrum resource), under both low (i.e., 30 veh./km, Fig. 6a)) and high (i.e., 80 veh./km, Fig. 6-b)) traffic conditions. As expected, this overall E2E latency increases as a function of the percentage of active users in the "background" of CLC (typically from 10 ms at 1% up to 12ms at 25% in the most favourable low traffic case). In addition, the order of magnitude imputable to radio access seems to dominate by far other higher level latency components present in the budget, whatever the considered hypothesis. In line with the previous remark (i.e., since architectural aspects seem relatively more marginal), the fully decentralized MEC option could hence be only marginally advantageous at very first sight, so that the clustered MEC deployment may represent a good tradeoff between low latency and reasonable deployment costs. All in all, even with a very basic scheduling policy at the radio access level, as that implemented in our simulations, a typical a priori requirement of 10 ms looks close to be achievable in most of the tested settings, thus suggesting significant margins of improvement ahead with even smarter scheduling policies. Under high traffic conditions (Fig. 6-b)), UL latency seems to increase by a few milliseconds only in comparison with the low traffic density case (typically, spanning from 12 ms at 1% of active users up to 14 ms at 25% of active users). Other E2E latency evaluations regarding DL DENMs (not shown here for brevity) also confirm that the DL flow is subject to much lower latency (typically bounded down to a few ms at most), regardless of both the road traffic density and the amount of active users.

B. Link Reliability under Gradual gNodeBs Deployment

Tables III and IV summarize the link reliability results, expressed as the percentage of non-corrupted Transport Blocks (TBs) over all the received TBs, accounting for the correct reception of UL CAMs and DL DENMs, as a function of both the percentage of users active in transmission and the number of deployed gNodeBs (given that seamless handover is performed), under high and low traffic conditions respectively. As expected, UL reliability decreases by far more rapidly than that of DL as a function of the percentage of users active. As expected too, in comparison with the single-gNodeB case evaluated here mostly for benchmark 3 , the link reliability on both UL and DL is significantly improved while gradually adding a second gNodeB (necessitating tight inter-MNO coordination though) and beyond, a third gNodeB (thus, bridging further the "coverage gap" at 3.xGHz between the two first deployed gNodeBs). These gains are even more noticeable under high road traffic and high percentage of active users transmitting in the background of CLC, as shown in Table III. Note that the latter percentage is however likely over-pessimistic from a network load perspective in this evaluation. Much lower rates on the order of a few percents are indeed practically expected in more conservative scenarios regarding the technology penetration in the short/medium terms (typically for the 5 upcoming years or so). In case of low road traffic density and/or low penetration rate (typically less than 5%), if gNodeBs deployment is sufficient to ensure good coverage, the link reliability is even superior to 97% in all the tested configurations, which is already acceptable in comparison with standard requirements, without any further optimization.

C. Multi-service Coexistence through Bandwidth Partitioning

Finally, we address the issue of multi-service coexistence by illustrating the impact of several bandwidth partitioning schemes on performances. In a first bunch of results (not shown here for length considerations), partitioning the overall available bandwidth is not shown to significantly improve link reliability for both DL and UL, since the main dominating parameters (i.e., propagation model, transmitted power and MCS) are invariant in comparison with the previous study. Similar trends as that of the full bandwidth case (See Tables III and Tables IV) can thus be observed. This was also foreseen in the sense the designed partitioning strategies (i.e., Schemes 2 and 3) are mostly intended to improve UL E2E latency (without degrading reliability though).

In Table V, we thus compare the impact on RAN latency of both the first and third bandwidth partitioning schemes (See Fig. 5). This table reports both DL and UL average latency, still as a function of the percentage of equipped vehicles transmitting 20Hz CAMs (with respect to all the simulated vehicles). According to the third BWP scheme, half of the overall bandwidth is thus reserved to the DL flow of the CLC service, while performing mixed TDD/Frequency Division Duplex (FDD). In comparison with Fig. 6, one can thus see that DL latency remains extremely low (slightly improved and systematically lower than 2 ms in all tested configurations), while some milliseconds can also be saved on the most critical UL latency in comparison with the first BWP scheme, where the full available bandwidth of 100 MHz is occupied by the UL and DL flows of both services. This shows that the impact onto the first CLC service of the extra network load generated by the background second service can be somehow mitigated and overcome through adequate physical slicing, even if mainly in terms of latency. Beyond, similar results have been obtained for the second bandwidth partitioning scheme, where half of the overall bandwidth is reserved to the first CLC service, including both of its UL and DL flows, whereas the second service is assigned to the other spectrum half. In this case however, we have considered only a worst case scenario of 25% of active users under the highest traffic conditions. In this situation, the second BWP strategy seems mainly relevant whenever the number of users belonging to simultaneous CLC procedures in a given environment remains relatively low, what is by the way expected in most practical real life situations (e.g., Scheme 2 achieving less than the required 10 ms for a percentage of CLC users lower than 5% among connected/active users, in comparison with Scheme 1, which was leading to 14 ms in the same operating conditions).

V. CONCLUSIONS

In this paper, we have provided and discussed a variety of simulation results that aim at illustrating the potential and limitations of 5G-NR connectivity to support CCAM services, through the example of a centralized CLC scenario.

First, in case other background vehicular services must coexist with the CLC over the same V2N 5G-NR resource, the E2E latency of the UL messages involved in a typical latency-critical service such as CLC may be problematic under high traffic density, a fortiori in case of high penetration of the 5G-NR technology on the road (typically, with up to 25% of the cars active in transmission in our evaluations). However, as a possible mitigation solution offered by 5G-NR technology (physical slicing), BWP can positively and largely contribute to relax this latency constraints at a level lower than 10 ms, in compliance with typical requirements. This kind of approach looks even more relevant when the latencycritical service to guarantee concerns a fairly low number of vehicles at a time. This eventually opens the floor to promising context-aware physical slicing in future 5G V2N connectivity to guarantee latency-critical services. Beyond, the radio access latency has been shown to dominate by far other higher-level latency components present in the budget, whatever the considered architecture hypothesis. In light of these observations, we have sorted out the clustered MEC deployment option, which may represent a good trade-off between low latency and deployment costs. All in all, even with the very basic scheduling policy that we have considered at radio access level in the previous evaluations, the typical requirement of 10 ms could be achieved in a significant number of tested configurations. Finally, regarding specifically cross-border latency issues, beyond the fine FNR capabilities already demonstrated on the field (i.e., a few seconds of interruption, as per recent field measurements carried out in a 5G NSA context), ultra-low latency inter-MNO cooperation seems almost inevitable, at least in highly localized crossborder portions of the corridor (e.g., slice-aware seamless handover), so as to keep this nominal RAN latency of 10 ms achievable, whatever the operating conditions, as already emphasized in [START_REF] Hillebrand | Current 4G Networks Limitations Pleading for 5G in Cross-border CAM Services[END_REF].

Then, in terms of link reliability and ultimately, in terms of overall successful CLC messages exchange sequence (thus, impacting service availability), a network densification in localized cross-border areas looks also mandatory while operating at 3.x GHz (typically, up to 3 gNodeB over the tested 4.5 km of motorway), so as to bridge potential coverage gaps, given that low-latency inter-MNO handover can be performed. Overall, under such dense deployment, the demonstrated level of link reliability is also fairly close from the target KPI (i.e., typically larger than 97% even under high traffic density and for up to 5% of connected users transmitting at 20 Hz, what represents a quite aggressive operating context). Finally, these results still advocate the combination of PC5-Mode4 and V2N 5G-NR to achieve even better redundancy and hence, better robustness. Relying on the same evaluation framework, possible extensions of this simulation work will the evaluation of decentralized CLC and in-lane cooperative maneuvering scenarios, supported by the combination of both PC5 and 5G-NR technologies.

Fig. 1 :

 1 Fig. 1: Illustration of a 3-phase V2X-assisted CLC use case [10].

Fig. 2 :

 2 Fig.2: Typical message sequence in a centralized CLC[START_REF] Visintainer | 5G CARMEN Use Cases and Requirements[END_REF].

Fig. 3 :

 3 Fig. 3: Locations of gNodeB sites considered for the evaluation of the centralized CLC scenario under gradual network deployment at the Brenner Pass, along with their respective RMa path loss maps..

Fig. 4 :

 4 Fig. 4: Decentralized (a), clustered (b) and centralized (c) MEC deployment hypotheses, where coloured arrows represent the unilateral E2E information path from one vehicle up to the MEC (red: RAN, orange: backhaul, violet: core network, green: inter-MEC).

Fig. 5 :

 5 Fig. 5: Schematic representation of the different multi-service management schemes at radio physical level.

  Fig. 6: Average E2E latency budget for typical UL CAMs between a source vehicle and the MEC, as a function of MEC deployment hypotheses and CLC vehicles situations under low (a) and high (b) traffic conditions.

TABLE I :

 I Main 5G-NR radio parameters used in simulations

	Bandwidth	100 MHz
	Channel access scheme	TDMA
	Scheduler policy	Round Robin
	Numerology	2
	Tx power at gNB	36 dBm
	Tx power at UE	26 dBm
	gNodeB antenna	8x16 Planar Linear Array (PLA)
	UE antenna	4x8 PLA with BF
	Channel model	RMa (Rural Macrocell)
	MCS	from 5 to 25
	Duplexing scheme	TDD with flexible pattern
	BLER threshold	0.0001
	PHY packet payload	100 bytes (DENM) / 500 bytes (CAM)
	Regular CAM Tx frequency	20 Hz

TABLE II :

 II Mapping between MEC deployment hypotheses, CLC vehicles situations and latency budget components (UL message illustration)

	MEC Deployment	CLC Vehicles Situation	E2E Latency Components
	Decentralized (1 MEC / gNodeB)	1.1) same cell	RAN (UL)
	Decentralized (1 MEC / gNodeB)	1.2) different cells	RAN (UL) + Inter-MEC
	Clustered (1 MEC / concentration site)	2.1) same cell	RAN (UL) + Backhaul
	Clustered (1 MEC / concentration site)	2.2) different cells, same cluster	RAN (UL) + Backhaul
	Clustered (1 MEC / concentration site) 2.

TABLE III :

 III

	BWP1 / High Traffic	1%	%age of active UEs 5% 10%	25%
		1 gNodeB	96.67%	94.33%	90.99%	87.72%
	UL	2 gNodeBs	97.99%	96.96%	91.12%	89.11%
		3 gNodeBs	98.98%	96.97%	93.86%	90.74%
		1 gNodeB	96.61%	95.81%	94.63%	92.01%
	DL	2 gNodeBs	98.93%	97.72%	97.64%	94.88%
		3 gNodeBs	100%	98.83%	98.67%	96.48%

Link reliability as a function of the number of gNodeBs and percentage of active UEs for BWP Scheme 1 under high traffic density.

TABLE IV :

 IV Link reliability as a function of the number of gNodeBs and percentage of active UEs for BWP Scheme 1 under low traffic density.

	BWP1 / Low Traffic	1%	%age of active UEs 5% 10%	25%
		1 gNodeB	98.71%	96.82%	95.78%	93.85%
	UL	2 gNodeBs	99.96%	97.99%	96.01%	91.00%
		3 gNodeBs	100%	99.99%	97.52%	93.99%
		1 gNodeB	99.43%	97.42%	96.34%	95.79%
	DL	2 gNodeBs	99.98%	98.72%	97.99%	96.87%
		3 gNodeBs	100%	100%	99.87%	98.02%

TABLE V :

 V Average E2E radio access latency of UL CAM with BWP Schemes 1 and 3, as a function of the percentage of active users.

				%age of active UEs
			1%	5%	10%	25%
		BWP3 & Low traffic	6.90ms	7.01ms	7.03ms	7.18ms
	UL	BWP3 & High traffic 7.03ms	7.04ms	7.11ms	9.23ms
		BWP1 & High traffic 9.41ms	9.73ms 10.31ms	12.01ms
		BWP3 & Low traffic	0.85ms	0.87ms	0.91ms	0.95ms
	DL	BWP3 & High traffic 0.89ms	0.95ms	1.11ms	1.68ms
		BWP1 & High traffic 0.87ms	0.92ms	0.99ms	1.67ms

Among other suitable configurations, a numerology of 1 was for instance considered in 5G-CARMEN field trials (e.g., for a subcarrier spacing of 30 kHz in the n78 frequency band), thus relaxing synchronization constraints for implementation practicability.

Recent experiments[START_REF] Casetti | Final Report on 5G Technological Enablers for CCAM[END_REF] suggest that the network re-selection time could be considerably accelerated (typically down to a few seconds) through advanced Fast Network Reselection (FNR) mechanisms, which do not fall in the scope of this paper.

This scenario is just intended to reflect the lack of coverage at the cell edge of the first serving network while approaching/crossing the border, hence prefiguring service shortage in case seamless low-latency handover to another network cannot be feasible.

ACKNOWLEDGMENT

This work has been carried out in the frame of the 5G-CARMEN project, which is partly funded by the European Commission (H2020 GA No.825012).


Ayoub Ouarga

Hassan Noukrati

Itziar Iraola-Arregui

Hicham Allal Barroug

Ben Youcef

Abdelhamid Elaïssari

Abdelhamid Elaissari

Hicham Ben Youcef
email: hicham.benyoucef@um6p.ma

Development of anti-corrosion coating based on phosphorylated ethyl cellulose microcapsules

Keywords: Anti-corrosion coating, biopolymers, ethyl cellulose, microencapsulation, phosphorylation, self-healing Highlights

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Corrosion is a widespread thermodynamically governed natural phenomenon that can deteriorate materials generally, and metals specifically. It is a chemical process that breaks down materials gradually by electron exchange and new molecules formation [START_REF] Katoueizadeh | Investigating the effect of synthesis conditions on the formation of urea-formaldehyde microcapsules[END_REF]. At present, enormous investment is allocated all over the world to protect materials from corrosion, especially metals. In 2016, the global cost of damage caused by corrosion was estimated to be US $2.5 trillion [START_REF] Koch | 1 -Cost of corrosion[END_REF]. To prevent and protect metals against corrosion, many methods exist and are used commonly. For instance, the cathodic protection and the anticorrosion coating methods. However, these methods are costly and need periodic replacement of the sacrificial anode or renewal of the coating, implying the use of chemicals with high toxicity and harmful effect on the environment [START_REF] Popoola | Corrosion problems during oil and gas production and its mitigation[END_REF][START_REF] Shchukin | Layer-by-Layer Assembled Nanocontainers for Self-Healing Corrosion Protection[END_REF][START_REF] Brooman | Modifying organic coatings to provide corrosion resistance-Part I: Background and general principles[END_REF][START_REF] Zvonkina | 5 -Strategies for developing multi-functional, self-healing coatings for corrosion prevention and other functions[END_REF]. So far, new approaches have been adopted to protect materials from corrosion, especially self-healing techniques, which allow cost-effective corrosion protection in more environmentally friendly approaches [START_REF] Makhlouf | Self-healing anti-corrosion coatings for applications in structural and petrochemical engineering[END_REF].

The self-healing phenomenon can be classified into two types: intrinsic and extrinsic, depending on the way of healing. The first type is based on special characteristics of the polymer that allow repairing under certain stimuli. In the second type, extrinsic self-healing, the polymeric matrix is not healable. Thus, the healing agent must be encapsulated and preembedded in the bulk material [START_REF] An | A review on corrosion-protective extrinsic selfhealing: Comparison of microcapsule-based systems and those based on core-shell vascular networks[END_REF][START_REF] Trask | Self-healing polymer composites: mimicking nature to enhance performance[END_REF]. The microencapsulation of natural self-healing agents is an active research area. A significant part of these investigations is dedicated to specific materials that protect and cover the defects of metal coatings caused by several surrounding conditions and stimuli [START_REF] Ataei | Biofriendly vegetable oil healing agents used for developing self-healing coatings: A review[END_REF][START_REF] Montemor | Functional and smart coatings for corrosion protection: A review of recent advances[END_REF][START_REF] Koh | Self-anticorrosion performance efficiency of renewable dimer-acidbased polyol microcapsules containing corrosion inhibitors with two triazole groups[END_REF][START_REF] Matsuda | Corrosion protection of epoxy coating with pH sensitive microcapsules encapsulating cerium nitrate[END_REF][START_REF] Li | Fabrication of microcapsules containing dualfunctional tung oil and properties suitable for self-healing and self-lubricating coatings[END_REF][START_REF] Montemor | Functional self-healing coatings: A new trend in corrosion protection by organic coatings[END_REF][START_REF] Samadzadeh | A review on selfhealing coatings based on micro/nanocapsules[END_REF]. Microencapsulation technology has been considered a relevant method to develop new and versatile smart materials for different applications, especially in the field of self-healing composite materials. Within the coating application, microencapsulation allows the protection of solid, liquid or gas materials using polymeric shells. This technology is considered as the first and primordial step in the preparation process of extrinsic self-healing materials [START_REF] Samadzadeh | A review on selfhealing coatings based on micro/nanocapsules[END_REF][START_REF] Onder | 2 -Thermal regulation finishes for textiles[END_REF][START_REF] Brown | Retardation and repair of fatigue cracks in a microcapsule toughened epoxy composite-Part II: In situ self-healing[END_REF][START_REF] Suryanarayana | Preparation and characterization of microcapsules containing linseed oil and its use in self-healing coatings[END_REF].

Microcapsules for anti-corrosion coating applications are based on core materials containing an anti-corrosion healing agent. Then, microcapsules must be embedded in paints as part of the anti-corrosion coating [START_REF] Koh | Self-anticorrosion performance efficiency of renewable dimer-acidbased polyol microcapsules containing corrosion inhibitors with two triazole groups[END_REF][START_REF] Bogdanowicz | Preparation and Characterization of Light-Sensitive Microcapsules Based on a Liquid Crystalline Polyester[END_REF]. Among shell materials for microencapsulation, cellulose and derivatives were widely used for diverse applications. In fact, cellulose and derivative materials are sustainable, biocompatible, non-toxic and possess good chemical and mechanical properties, that promote their use in the microcapsules preparation [START_REF] Carrick | Hollow cellulose capsules from CO2 saturated cellulose solutions-their preparation and characterization[END_REF][START_REF] Prasertmanakit | Ethyl Cellulose Microcapsules for Protecting and Controlled Release of Folic Acid[END_REF][START_REF] Mirabedini | Preparation of selfhealing acrylic latex coatings using novel oil-filled ethyl cellulose microcapsules[END_REF]. The main approaches for the preparation of these materials for microencapsulation include emulsion-based methods such as temperature-induced phase separation, emulsion solvent evaporation and solvent evaporation from oil-in-water emulsion [START_REF] Rogers | Reviewing the use of ethylcellulose, methylcellulose and hypromellose in microencapsulation. Part 2: Techniques used to make microcapsules[END_REF][START_REF] Mirabedini | Preparation and characterization of ethyl cellulose-based core-shell microcapsules containing plant oils[END_REF].

Vegetable oils have received recurrent attention in the last decades due to their ecofriendliness, low cost and availability which make them suitable materials for several purposes, including: food industry, paints and anticorrosion applications [START_REF] Dabi | Application potential of vegetable oils as alternative to diesel fuels in compression ignition engines: A review[END_REF][START_REF] Sharmin | Recent advances in vegetable oils based environment friendly coatings: A review[END_REF]. Linseed, soybean and tung, are the most studied vegetable oils due to their abundance and ease of extraction from renewable and sustainable resources [START_REF] Behzadnasab | Evaluation of corrosion performance of a self-healing epoxy-based coating containing linseed oil-filled microcapsules via electrochemical impedance spectroscopy[END_REF]. They are used in edible applications such as food industry, as well as non-edible applications such as self -healing materials [START_REF] Ataei | Biofriendly vegetable oil healing agents used for developing self-healing coatings: A review[END_REF][START_REF] Li | Fabrication of microcapsules containing dualfunctional tung oil and properties suitable for self-healing and self-lubricating coatings[END_REF][START_REF] Behzadnasab | Evaluation of corrosion performance of a self-healing epoxy-based coating containing linseed oil-filled microcapsules via electrochemical impedance spectroscopy[END_REF][START_REF] Wool | 4 -Polymers and composite resins from plant oils[END_REF]. In this context, we report hereby almond oil as a promising green corrosion inhibitor. The chemical structure of vegetable oils is generally composed of 90 to 98 % of triglycerides, which are esters containing multiple fatty acids, that comprise oleic, linoleic, alpha linoleic, palmitic, palmitoleic and stearic acids [START_REF] Karatay | Major Fatty Acids Composition of 32 Almond (Prunus dulcis (Mill.) D. A. Webb.) Genotypes Distributed in East and Southeast of Anatolia[END_REF]. Almond oil is generally a combination of 68 % oleic acid, 25 % linolenic acid and 4 % palmitic acid. High content of double bonds-rich fatty acid as oleic acid offers this material the ability to protect metals against corrosion by free radical auto-oxidation reaction mechanism of the double bonds with oxygen [START_REF] Abdallah | Corrosion Behavior of Nickel Electrode in NaOH Solution and Its Inhibition by Some Natural Oils Authors[END_REF][START_REF] Ahmad | The uses and properties of almond oil[END_REF][START_REF] García-Cáceres | Evaluation of the lubricating power of sweet almond oil without additives[END_REF][START_REF] Abdallah | Natural oils as corrosion inhibitors for stainless steel in sodium hydroxide solutions[END_REF].

Our research work aimed to incorporate almond oil in the core of microcapsules based on ethyl cellulose and phosphorylated ethyl cellulose as shell materials, and their subsequent use in a self-healing anti-corrosion coating. For this purpose, ethyl cellulose bearing phosphonate groups was prepared by phosphorylation reaction under specific conditions. We report for the first time, microcapsules combining phosphorylated ethyl cellulose with almond oil in self-healing coating. Almond oil was selected due to its composition and corrosion inhibition abilities. The resulting modified ethyl cellulose and microcapsules were subjected to several characterization techniques. The effect of the phosphonate group in combination with the almond oil was investigated as anti-corrosion additives. Thus, metallic samples having a coating loaded with microcapsules were subjected to a mechanical scratch and their corrosion resistance was monitored via salt spray tests.

Experimental

Materials

The reagents used for the materials preparation were ethyl cellulose powder (viscosity 10 cP, measured as a 5 % vol in toluene/ethanol 80:20 (vol), extent of labeling: 48 % ethoxyl), urea powder and sodium dodecyl sulfate (SDS) (Sigma Aldrich), ethyl acetate (EA) (Somaprol), orthophosphoric acid (85 %), methanol (Merck KGaA) and Sodium hydroxide (VWR). Sweet almond oil was purchased from a drugstore and used as a core material for microcapsules. All chemicals were analytical reagents grade and were used as received.

Ethyl Cellulose phosphorylation

The ethyl cellulose phosphorylation was performed under homogeneous conditions using phosphoric acid in molten urea [START_REF] Kokol | Characterisation and properties of homo-and heterogenously phosphorylated nanocellulose[END_REF][START_REF] Inagaki | Phosphorylation of cellulose with phosphorous acid and thermal degradation of the product[END_REF]. 5 g of ethyl cellulose were dispersed under stirring in 25.7 g of phosphoric acid H 3 PO 4 and 7 g of distilled water. 16 g of urea were melted in a two-necked flask attached to a condenser at 140 °C. The dispersion was added to the molten urea while the temperature was adjusted to 150 °C, and the reaction was carried out for 30 minutes. The obtained combination was then dissolved in a solution of NaOH, precipitated in methanol, filtered, washed and purified via membrane-based dialysis process in distilled water. Finally, the phosphorylated ethyl cellulose was freeze dried for 72 hours.

Preparation of microcapsules

In order to prepare almond oil-filled microcapsules using ethyl cellulose (EC) or phosphorylated ethyl cellulose (P-EC) as shell material, we followed the procedure described elsewhere [START_REF] Mirabedini | Preparation and characterization of ethyl cellulose-based core-shell microcapsules containing plant oils[END_REF]. The selected microencapsulation method is considered as a two-stage solvent evaporation method. The first preparation, oil phase, consists on dissolving the ethyl cellulose powder or the phosphorylated ethyl cellulose in ethyl acetate (90 wt.%) under magnetic stirring of 650 rpm at ambient temperature. The dissolution was done in 2 hours.

Almond oil was then added gradually to the above-mentioned solution. The system was kept under magnetic stirring at room temperature for further 30 minutes. For the second preparation, water phase, an aqueous solution of 1 wt.% sodium dodecyl sulfate (SDS) was prepared. The oil phase was then added drop wise to the water phase under mechanical mixing at 60 °C for 90 minutes. The mixture was freeze-dried then washed with distilled water and filtered to remove the remaining surfactant and generate the final microcapsules.

Obtained microcapsules based on EC and P-EC are designated by MC and P-MC, respectively.

Characterization

The surface morphology of ethyl cellulose (EC) and phosphorylated cellulose (P-EC) were observed by scanning electron microscopy (SEM) using a Quanta FEG 250. The presence of phosphorus (P) in the P-EC was examined by Energy Dispersive X-ray spectroscopy (EDX) using IDFix software by SAMx attached to the Quanta FEG 250 electron microscope.

Fourier Transform Infrared (FTIR) Spectra of initial raw materials, phosphorylated ethyl cellulose and prepared microcapsules were recorded by a PerkinElmer FTIR spectrophotometer (Wellesley, MA, United States), collecting 16 scans in the 4000-600 cm -1 range with 4 cm -1 resolution.

The phosphorus (P) content of the modified ethyl cellulose was measured using the inductively coupled plasma optical emission spectrometry method (ICP-OES), on Agilent 5110 ICP-OES Instrument. The specific surface area of the EC and P-EC powders was determined by the BET method, using nitrogen adsorption measurements (Micromeritics Gemini VII 1014 Instrument).

The prepared microcapsules were observed by an optical microscope (Nikon metallographic microscope LV100ND) under 20-100 X magnification. The microcapsules morphology and shell thickness were examined using a scanning electron microscope (Philips XL 40 ESEM).

The microcapsules size was determined by a laser particle size analyzer (Malvern, Mastersizer 2000).

Microcapsules thermal stability was evaluated by means of thermogravimetric analysis (TGA) technique using a thermogravimetric analyzer (Model -TGA 5500, Discovery series, TA Instruments, United States). The tests were performed under a nitrogen atmosphere and heating from room temperature up to 700 °C using a heating rate of 10 °C/min. The oil content in the microcapsules was determined by solvent extraction. Weighed mass of microcapsules (m1) was mixed with 200 mL of n-hexane and the mixture was sonicated for 10 min. The suspension was then filtrated, and the solid filtrate was then collected and dried in an oven at 50 °C until a constant weight is measured (m2). The oil content encapsulated in the microcapsules can be determined using the following equation:

Preparation of coating samples containing microcapsules

Mild steel substrates with a dimension of 100 × 45 × 1 mm were ground to 1000-grits with abrasive paper, degreased in acetone and dried in air. 15 wt.% of microcapsules were gently dispersed into the epoxy resin under manual mixing using spatula. A stoichiometric amount of the hardener was then added to the mixture. The coatings were applied on degreased and polished steel substrates using a manual film applicator. The coated steel panels were dried overnight in a fume hood at room temperature. The average thickness of dry film samples was measured as 500 ± 15 µm.

Results and Discussion

Phosphorylated Ethyl-Cellulose characteristics

Phosphorylated polysaccharides are a very interesting class of materials that can exhibit practical properties (e.g., biocompatibility, bactericidal activity, bio absorbability, and metalchelating). In this study, P-EC was prepared by the phosphorylation of the EC using the procedure depicted in the scheme 1. The phosphorylation protocol was performed at 150 °C to activate the attachment of phosphonate groups in order to obtain a high degree of substitution [START_REF] Illy | Phosphorylation of biobased compounds: the state of the art[END_REF]. Urea was used in excess to fulfil a dual role in the functionalization procedure. On one hand it promotes the phosphorylation reaction, and on the other hand it protects the polymeric matrix at high temperature from degradation in the aggressive acidic conditions (10.7 M) [START_REF] Ghanadpour | Phosphorylated Cellulose Nanofibrils: A Renewable Nanomaterial for the Preparation of Intrinsically Flame-Retardant Materials[END_REF].

Scheme 1. a) Schematic of the phosphorylation reaction of the ethyl cellulose (EC). b)

Visual aspect of ethyl cellulose (EC) and phosphorylated ethyl cellulose (P-EC).

These conditions and the drying technique did not affect the visual powder like aspect of the obtained P-EC final product (scheme 1). Interestingly, the specific surface area of P-EC adsorbent, determined by the N 2 -BET method, was highly increased compared with that of the native ethyl cellulose. In fact, the obtained results were 1.1 m 2 g -1 for EC and 21.4 m 2 g -1 for P-EC.

It was expected that the phosphorylation reaction will induce a change in the intermolecular interactions between the P-EC chains, thus, modifying the compactness properties due to steric effect and surface charge modification. The polysaccharide samples were sputter coated with copper (Cu) before SEM-EDX analysis. The SEM images of the lyophilized P-EC powder showed a spongy aspect after freeze drying, as can be depicted in (Fig. 1 (a-b)). This new roughness may be the reason for the measured specific surface area increase. The phosphorus element (P) mapping image showed almost uniform distribution in the characterized sample (Fig. 1(d)). Even if the mapped sample is rough, the obtained result supports the effective chemical modification and the phosphorylation of the ethyl cellulose.

The ethyl cellulose phosphorylation was confirmed by FTIR measurements (Fig. 2). The FTIR spectrum of the modified ethyl cellulose showed the characteristic peaks of both native ethyl cellulose and new peaks related to phosphorus groups. The peak at 800 cm -1 is assigned to P-O-C groups, and another broad peak at 1630 cm -1 is related to O=P-OH deformation vibrations. Moreover, the phosphorus content was confirmed by inductively coupled plasma -optical emission spectrometry (ICP-OES) analysis and the modified EC contains 6.26 wt.% P.

Fig. 2. FTIR spectra of ethyl cellulose (EC) and phosphorylated ethyl cellulose (P-EC).

Figure 3 shows the main characteristics of thermal degradation of native and phosphorylated ethyl cellulose, determined by thermogravimetric analysis.

As seen from (Fig. 3), native ethyl cellulose shows a maximum weight loss rate at 362 °C, whereas this appeared at 304 °C for the phosphorylated ethyl cellulose. Thus, the introduced structures containing phosphorus catalyze the polymer's thermal degradation and carbonization, so that the main degradation process starts for phosphorylated ethyl cellulose at a temperature of 213 °C compared to the corresponding native ethyl cellulose which starts at 273 °C (Table 1). The phosphorylated ethyl cellulose loses 20 wt.% at 281 °C, whereas for the native ethyl cellulose, the same weight loss is observed at a higher temperature of 338 °С. It was noticed that, the weight loss of the P-EC is considerably lower compared to the native EC, and a difference of 17 wt.% was observed at 400 °C. The residue reaches 11.7 wt.% for the P-EC and 4.3 wt.% for native EC at 700 °C, which mean a difference of 7.4 wt.% (Table 1).

The native EC undergoes complete decomposition at 406 °С whereas the phosphorylated one underwent it at 350 °C. The observed thermal behavior of the P-EC is a clear indicator of a flame retardancy polymer material, where the phosphonate groups act as a promoter for more char residue at high temperature [START_REF] Ghanadpour | Phosphorylated Cellulose Nanofibrils: A Renewable Nanomaterial for the Preparation of Intrinsically Flame-Retardant Materials[END_REF][START_REF] Malucelli | Surface-Engineered Fire Protective Coatings for Fabrics through Sol-Gel and Layer-by-Layer Methods: An Overview[END_REF]. Only 6.26 % weight amount of phosphorus in the P-EC induced a clear modification of its pyrolysis process reducing the flammable volatiles amount and increasing the carbonaceous residue formation which indicates its higher heat resistance. The absence of almond oil around the microcapsules on the optical micrographs confirms the good oil encapsulation inside the shell material in both ethyl cellulose and phosphorylated ethyl cellulose. SEM micrographs did not show significant dissimilarity of surface morphology for microcapsules prepared using different shell materials, which makes clear the non-dependence of microcapsules features on shell material nature. Still, both nonphosphorylated and phosphorylated microcapsules have crater features on their surfaces and a golf ball-like morphology. The microcapsule morphology and final shape are depending on several parameters, e.g., core-shell materials, temperature, mixing mode, emulsifier and drying method. The lyophilization (freeze-drying) procedure is suspected to be the main factor responsible for the crater features observed on the surface of the obtained microcapsules. In fact, this drying method was previously pointed out to be the main cause of the porous structure on the surface of the obtained microcapsules [START_REF] Corrêa-Filho | Advances in the Application of Microcapsules as Carriers of Functional Compounds for Food Products[END_REF]. This shape might affect self-healing efficiency and microcapsule's long-term stability. Hence, the craters will increase the exposition of the active ingredient encapsulated to the atmosphere, EC as shell material. SEM micrographs of almond oil-filled microcapsules using (c, e) EC and (d, f) P-EC as shell material.

Table 1. Specific surface area and thermal degradation characteristic properties of ethyl cellulose (EC) and phosphorylated cellulose (P-EC).

thus lowering the protection of the core material. Interestingly, the remaining microcapsules (i.e., MC and P-MC) were stable over several months inside a desiccator without any noticeable changes. The same microcapsules were prone to agglomeration when left for a prolonged shelf exposure under ambient air humidity conditions. In fact, the hydrophilicity of the shell materials is promoting the increase of water content, hence the observed tendency to agglomeration.

Core-Shell Microcapsules FTIR Analysis

For chemical characterization of the core, shell and microcapsules, attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy was used (Fig. 6). The infrared spectra of almond oil showed sharp peaks at 2921 cm -1 , 2849 cm -1 , 1462 cm -1 and 723 cm -1 related to the methylene groups. Peaks corresponding to the methyl group vibration were observed at 1464 cm -1 and 1375 cm -1 . The characteristic peak of olefinic double bonds is seen at 3009 cm -1 . The triglyceride ester groups appeared at 1743 cm -1 , 1235 cm -1 , 1159 cm -1 and 1095 cm -1 [START_REF] Cortés | Discrimination of intact almonds according to their bitterness and prediction of amygdalin concentration by Fourier transform infrared spectroscopy[END_REF][START_REF] Sharma | FTIR Spectroscopic Characterisation of Almond Varieties (Prunus dulcis) from Himachal Pradesh (India)[END_REF][START_REF] Hernández | Sweet almond oil: Extraction, characterization and application[END_REF]. The phosphorylated microcapsules FTIR spectra reveal the appearance of new small peaks at 810 cm -1 and 1630 cm -1 corresponding respectively to P-O-C and O=P-OH group, due to the presence of the phosphorus in the P-EC shell.

The microcapsules infrared spectra contain all the characteristic peaks of the core material (Almond oil) and shell material (in yellow in fig. 7). This gives a clear confirmation that the microencapsulation was correctly carried out using both ethyl cellulose and phosphorylated ethyl cellulose as shell materials. Moreover, no bands related to SDS were observed either for the MC and P-MC based microcapsules, hence, the efficiency of the cleaning procedure during the preparation step is proved.

Thermal Analysis

The thermal stability of the prepared microcapsules was investigated by thermogravimetric analysis (TGA) and the obtained results are presented in Fig.

Salt spray test results

To evaluate the self-healing abilities and corrosion resistance of the prepared microcapsules, a salt spray test was performed on metallic substrates. The visual aspect progression during the study of the scratched areas on coated blank, MC and P-MC based samples is shown in (Fig. 8). To begin with, both scratched microcapsules loaded coatings showed a marked effectiveness in preventing corrosion compared to blank epoxy coating during the first week of test. However, all samples showed corroded zones at different stages across the scratched region after 28 days of the spray test using the 3.5 wt.% NaCl solution. In fact, the corroded zone was more evident on the pure epoxy resin coated sample compared to samples coated with epoxy resin loaded with microcapsules. In the case of blank samples, the rust was already propagated upon most of the scratch area after only 7 days of salt spray exposure and continued to develop until the end of the test. Moreover, it was clearly visible that the corrosion was spreading even more in between the coating and the metal surface at the end of the test. While in the case of samples containing non-phosphorylated microcapsules (MC), the corrosion emerged on the scratch center starting from day 7 and continued to progress showing a better anti-corrosion resistance compared to the blank samples. The study showed also that, the corrosion process did not visibly occur in the sample containing phosphorylated microcapsules (P-MC) until day 28, which reveals a better anti-corrosion resistance in comparison to both blank and MC containing samples. Considering the component of the P-MC compared with the MC, the phosphonate group is playing a crucial role in the observed anticorrosion performances. It is well known that phosphoruscontaining compounds promote the anti-corrosion effects owing mostly to their excellent adhesion ability [44]. In addition to the strong interfacial interactions of phosphonate groups with the metal substrate, additional interactions are expected with the functional groups of the fatty acids of the core materials. Thus, there is a synergic effect between the phosphonate groups and the fatty acids of the almond oil integrated within the prepared coatings. In optimal conditions, a strong interaction between the P-O -functional groups and the metal ions M n+ from the metal substrate is occurring [44]. Furthermore, the hydroxyl and phosphonate groups on the P-EC can interact via hydrogen bonding with the triglycerides constituted of fatty acids.

Conclusions

Phosphorylated ethyl cellulose was successfully obtained through a phosphorylation reaction using phosphoric acid in the presence of molten urea. ICP-OES analysis confirm a content of 6.26 wt.% of phosphorus in the structure of P-EC. The functionalization of the EC promoted its flame retardancy behavior as revealed by the TGA analysis. Moreover, the specific surface area of P-EC showed an increase by a factor of 20 compare to that of native EC. The almond oil encapsulation was carried out successfully and the microcapsules showed a practical thermal stability window for the targeted application. The self-healing and anticorrosion properties of prepared microcapsules were evaluated by a salt spray test on scratched epoxy-based coatings. The results revealed that the coating containing phosphorylated microcapsules had a higher corrosion resistance compared to both blank coating and the coating loaded with non-phosphorylated microcapsules over a 4 weeks test.

The phosphorylated microcapsules formulated in this work are presented as a proof of concept. To the best of our knowledge, this is the first attempt to prepare almond oil microcapsules using ethyl cellulose and phosphorylated ethyl cellulose as shell materials.

These microcontainers approach seems to be promising and could potentially be used in different green and sustainable applications to enhance materials properties such as durability, biodegradability, metal chelating and flame retardancy. Further investigations and tests on the influence of the degree of phosphorylation on the performances of these materials are currently in progress and should be the subject of a forthcoming publication.

CRediT author statement

Fig. 1 .

 1 Fig. 1. SEM micrographs of (a) ethyl cellulose (EC) and (b-c) phosphorylated cellulose (P-EC). (d) Mapping of phosphorus element for phosphorylated ethyl cellulose (c).

Fig. 3 :

 3 Fig. 3: Thermal degradation of native EC and P-EC. (a) TGA weight loss curves, and (b) weight loss first derivative curves.

3 . 2

 32 Microcapsules morphology and size distributionMicrocapsules based on ethyl cellulose (MC) and phosphorylated ethyl cellulose (P-MC) were prepared successfully. The content of the almond oil in the microcapsules was determined by extraction in n-hexane. The average oil content in the microcapsules was evaluated to be 58% and 61% for the MC and P-MC, respectively. Optical microscope images and SEM micrographs show that the obtained microcapsules have a spherical shape without any physical and strong inter-capsule bonding (Fig.4).

Fig. 4 .

 4 Fig. 4. Optical microscope images of almond oil-filled microcapsules using (a) EC and (b) P-

Fig. 5 .

 5 Fig. 5. (a) Size distribution of ethyl cellulose-based microcapsules (MC) containing Almond oil, and (b) Size distribution of phosphorylated ethyl cellulose-based microcapsules (P-MC) containing Almond oil.

Fig. 6 .

 6 Fig. 6. (a) FTIR spectra of ethyl cellulose microcapsules (MC), almond oil and ethyl cellulose (EC). (b) FTIR spectra of phosphorylated ethyl cellulose microcapsules (P-MC), almond oil and phosphorylated ethyl cellulose (P-EC).

7 .

 7 The prepared microcapsules (phosphorylated and non-phosphorylated) exhibit two stages of weight loss. The almond oil microcapsules show two stages at 206-290 °C and 290-450 °C, which correspond to the decompositions of ethyl cellulose shell and the almond oil core, respectively. The thermal degradation shows an overlapping temperature range of the core and shell materials which limits the accurate determination of the percentage of each component.

Fig. 7 .

 7 Fig. 7. (a) TGA weight loss curves of native ethyl cellulose (EC), almond oil and full microcapsules (MC). (b) TGA weight loss curves of phosphorylated ethyl cellulose (P-EC), almond oil and full phosphorylated microcapsules (P-MC). The phosphorylated microcapsules thermogram shows an initial stage weight loss at 180-290 °C related to the decomposition of phosphorylated ethyl cellulose. The second stage weight loss supporting the decomposition of almond oil was observed between 290 °C and 500 °C. It is worth to note that the thermal degradation of the MC and P-MC microcapsules resulted in a final residue of 0.5 % and 5.2%, respectively. The TGA study confirmed the absence of the SDS material within the MC and P-MC microcapsules, which is typically characterized by an onset degradation temperature around 162 °C and a maximum degradation temperature around 218°C. It is worth commenting that the flame retardancy character of the used P-EC

Fig. 8 .

 8 Fig. 8. Optical photographs of scratched coated mild steel samples (a,b,c) after various exposure times (1, 7, 14 and 28 days) to salt spray with 3.5 wt.% NaCl solution. a) blank, without microcapsules b) coating with ethyl cellulose based microcapsule (MC) c) coating with phosphorylated ethyl cellulose microcapsules (P-MC).

  Ouarga: conceptualisation, Writing-original draft preparation. Hassan Noukrati: investigation, Supervision. Itziar Iraola-Arreguia: Formal analysis, review. Abdelhamid Elaissari: Supervision, review & editing. Allal Barroug: Supervision, review & editing. Hicham Ben Youcef: Writing -original draft -review & editing.

Química Nova, 32 (2008)