Hot-Dip PVC-Based Polymeric Composite Coating for Advanced Electrical Insulation of Electric Vehicle Battery Systems

Abstract

Polyvinyl chloride (PVC) is a widely used polymer in composite systems due to its versatility and processability, with growing use in advanced engineering applications. This study presents the formulation, processing optimisation, and detailed characterisation of a hot-dip PVC-based plastisol composite coating developed for electrical insulation in electric vehicle (EV) battery systems. A series of plastisol formulations with varying filler contents were prepared and applied via dip-coating at withdrawal speeds of 5, 10, and 15 mm s⁻¹. The 5 mm s⁻¹ withdrawal speed resulted in the most uniform coatings with thicknesses of 890–2100 µm. Mechanical testing showed that lower filler content significantly improved performance: Group 1 (lowest filler) exhibited the highest tensile strength (11.9 N mm⁻²), elongation at break (465%), tear strength (92 N mm⁻¹), and abrasion resistance. SEM and EDX analyses confirmed more homogeneous filler dispersion in Group 1, while FTIR spectra indicated stronger polymer–plasticiser interactions. Contact-angle measurements showed an increase of ${38}^{\circ }$ in low-filler samples, indicating enhanced surface hydrophobicity. Furthermore, Group 1 coatings demonstrated superior dielectric strength (22.1 kV mm⁻¹) and excellent corrosion resistance, maintaining integrity for over 2000 h in salt-spray testing. These findings highlight the importance of filler optimisation in balancing mechanical, electrical, and environmental performance. The proposed PVC-based composite coating offers a durable, cost-effective solution for next-generation EV battery insulation systems and has potential applicability in other high-performance engineering applications.

1. Introduction

Polyvinyl chloride (PVC) is among the world’s most widely produced polymers. It is inexpensive, versatile, and durable, and, when plasticised, exhibits excellent mechanical properties [1]. Owing to this combination of performance and cost-effectiveness, PVC is used across a wide range of sectors, including automotive and transportation, construction, healthcare, food packaging, electrical equipment, playground and recreational structures, art and design installations, and agriculture.

PVC combines mechanical strength and durability with remarkable design flexibility, allowing the production of aesthetically rich parts in virtually any colour or surface texture. Its outstanding performance–cost ratio has led to its adoption across diverse sectors. Technically, PVC is valued for its intrinsic resistance to water, UV radiation, chemicals, and abrasion; low raw-material and processing costs; lightweight nature that eases transport and installation; and versatile processability, including extrusion, injection moulding, and hot-dip coating. The polymer offers excellent electrical insulation, underpinning its extensive use in cable jackets, wire insulation, and, more recently, EV battery housings. Being a thermoplastic, it can be reprocessed and thus supports recycling initiatives, while its chemical inertness enables service in harsh industrial environments. These combined advantages continue to secure PVC’s position as one of the most widely employed polymers worldwide [2,3].

The rigidity of poly(vinyl chloride) originates from restricted chain mobility caused by strong dipole interactions along the C–Cl bonds. Processability and flexibility are therefore improved by incorporating plasticisers, which enlarge the intermolecular free volume and weaken interchain attractions through secondary dipole–dipole interactions [4]. Nearly one-third of global PVC production is now sold as flexible PVC, appearing in everyday products such as food-packaging films, cable insulation, playground equipment, art and design installations, and outdoor sculptures (Figure 1) [5].

Plastics are broadly prone to ageing, and plasticised PVC is a notable case because plasticiser migration and dehydrochlorination can undermine mechanical and dielectric performance. Consistent with this general vulnerability, survey data reported that while most plastic artefacts required no or only minor intervention, a small high-priority fraction (0.6%) was dominated by deteriorating plasticised PVC (with aggregate distributions of 27.5%, 60%, and <12% across intervention categories) [8]. These observations motivate our EV-focused evaluation of DOTP (dioctyl terephthalate)—plasticised PVC plastisols. Accordingly, we concentrate on EV service conditions—thermal-oxidative stress, moisture/salt exposure, vibration, and sustained electric fields—which define the dielectric and durability targets pursued in this work.

A broad spectrum of plasticisers such as adipates, citrates, benzoates, azelates, and phosphates is available for the manufacture of flexible PVC [9]. Bocqué et al. (2016) systematically correlated plasticiser molecular structure with performance, underscoring the importance of side-chain geometry and polarity [10]. Among commercial options, di-2-ethylhexyl phthalate (DEHP, also marketed as dioctyl phthalate, DOP) has long served as the industry benchmark, accounting for roughly 75% of the global plasticiser market owing to its favourable cost to performance ratio and well-balanced mechanical and rheological properties [11,12].

Toxicological evidence has linked DEHP to adverse developmental and reproductive outcomes. The compound is now classified in the European Union as a reproductive toxicant, Category 1B (H360), and is also suspected of being carcinogenic to humans and of causing ecological harm. Consequently, DEHP and several related phthalates have been restricted in the European Union, including bans in toys and childcare articles [5]. Current mitigation strategies focus on (i) substituting PVC plasticisers with safer analogues and/or (ii) minimising plasticiser migration from finished products [13]. Dioctyl terephthalate (DOTP) has emerged as a leading replacement: it delivers mechanical performance comparable to DEHP, exhibits lower volatility, and shows no evidence of carcinogenicity, genotoxicity, or developmental toxicity. DOTP is therefore increasingly adopted in applications such as electrical connectors and other PVC formulations where reduced health and environmental risk is paramount [13,14].

A plastisol is a suspension of PVC resin particles dispersed in a liquid phase composed mainly of plasticisers; colourants, thermal stabilisers, and fillers may be added as required [15]. When heat-cured, the dispersion forms a solid PVC-based composite coating that combines decorative appeal with corrosion and chemical protection for metal substrates. The principal advantages of such PVC plastisol coatings can be summarised as follows:

1.

Ease of application: plastisols can be dip, spray, or screen-coated and even room temperature cast because solidification occurs only on subsequent heating.

2.

Mechanical robustness: cured films resist impact, abrasion, and deformation, and withstand many chemicals, solvents, and weathering agents.

3.

Tunability: formulation adjustments allow hardness from soft and flexible to rigid, in both thin and thick builds.

4.

Aesthetic versatility: available in virtually any colour, with matte, glossy or textured finishes and high UV stability that minimises fading.

5.

Cost-effectiveness: raw materials are inexpensive, and the process is energy-efficient, requiring only a heat-curing step.

6.

Moisture resistance: excellent barrier performance makes plastisols suitable for outdoor or wet service environments.

7.

Electrical insulation: high volume resistivity enables long-term dielectric protection.

8.

Corrosion and thermal stability: coatings protect underlying metals and maintain performance up to moderate service temperatures [16,17,18].

Reliable electrical insulation is critical to the safety and performance of high-voltage electric vehicles (EVs). Adequate insulation protects passengers and technicians from electric shock, prevents short circuits that could trigger fires or component failure [19], and diminishes resistive energy losses so that battery charge is utilised more efficiently. By limiting Joule heating and shielding battery packs from external chemical or atmospheric attack, properly insulated housings help maintain cell chemistry and extend service life [20]. Robust inter-cell insulation also suppresses leakage currents and secures reliable feedback to the battery-management system (BMS). Conversely, the ingress of water or moisture into a high voltage stack can induce short circuits and catastrophic malfunctions [21]. Effective insulation, therefore, hinges on judicious design choices, the selection of advanced materials or coatings, and purpose-formulated polymer structures [22].

The battery housing and ancillary components that require electrical insulation within an EV are illustrated in Figure 2. To date, polymeric candidates such as polypropylene (PP), polycarbonate (PC), polyphenylene sulfide (PPS), polytetrafluoroethylene (PTFE), poly(vinylidene fluoride) (PVDF), and their nanocomposites have all been explored for this role; yet none have fully met the combined targets of process efficiency, cost, and performance [23]. PVC plastisols, by contrast, offer an attractive route to mitigate these risks through a single coating step [24]. When properly formulated, plastisol films deliver not only high dielectric strength for tools, enclosures, and battery cases, but also resistance to scratch, abrasion, impact, fire, and flammability in metallic transport and power transmission hardware [17]. In PVC systems, appropriately designed compound flame-retardant (FR) packages can increase the limiting oxygen index (LOI) and markedly suppress smoke density while maintaining acceptable properties [25]. Application techniques include casting, hydrodynamic spraying, and dip coating, the latter being preferred for its ability to build the required thickness rapidly and uniformly [26]. In practice, the combination of a tailored plastisol mixture and controlled dip curing confers robust protection, extended service life, noise suppression, electrical insulation, and improved safety to a wide range of industrial parts [27].

In EV battery housings, achieving reliable insulation on metallic enclosures further imposes practical constraints that favour mm-scale coatings. First, sharp edges, welds, fasteners, and surface roughness require a pinhole-free dielectric barrier with sufficient overbuild to maintain creepage/clearance and suppress local field intensification. Second, the insulation must simultaneously act as a corrosion and wear barrier under moisture/salt exposure and mechanical abrasion; thin films are more vulnerable to defect formation and moisture ingress over large areas. Third, a single-step application that rapidly produces a uniform, thick layer on complex geometries is operationally attractive—criteria that align with hot-dip PVC plastisols.

The literature contains numerous reports on how plasticiser and filler selection governs the performance of PVC plastisol coatings. Siekierka et al. formulated plastisols from emulsion PVC plasticised with bis(2 ethylhexyl) adipate, stabilised with octyltin mercaptide, and filled with either fine or coarse wood flour; hydraulic-press films were evaluated for density, hardness, thermal stability, and mechanical/thermomechanical behaviour. SEM revealed that a formulation containing 20 wt% fine wood flour and gelled at 150 °C delivered the most favourable balance of properties [26]. Tüzüm and Ergin addressed plasticiser migration in cable-sheath compositions, which compromises mechanical integrity and causes environmental release. They prepared a series of plastigel films by blending PVC with plasticiser, thermal stabiliser, and various inorganic fillers (boric acid, boron clay, sintered–calcined boron-waste clay, talc, and zircon oxide) using a four-sided film applicator, and correlated filler type with the diffusion rate of plasticiser and consequent property retention [18].

Tüzüm and Ergin further showed that the diffusion coefficients of their plastigel films decreased in the order boric-acid > unfilled > talc ≈ sintered-boron clay > zircon > boron clay, demonstrating that suitable inorganic fillers can simultaneously enhance mechanical/thermal performance and suppress plasticiser migration in cable sheaths [18]. Ji et al. compared unfilled PVC plastisols with formulations containing calcium carbonate (CaCO₃) and measured viscosities with a vibrational viscometer; they attributed the rheology primarily to plasticiser–particle electrostatic interactions as well as resin morphology, although no additional characterisation was reported [19]. Bahloul et al. investigated the spectral response of PVC plastisol films loaded (2–10 wt%) with various near infrared (NIR) barrier fillers including nacre, mica/TiO₂, glass beads, alumina, boehmite, ZnO, MgO, and rutile TiO₂. Filler geometry and loading markedly altered NIR reflectance while maintaining high transmittance in the visible region, highlighting the tunability of plastisol optical properties [20]. Further experiments showed that nacre-based lamellar fillers produced little benefit, whereas TiO₂ particles markedly lowered visible light transmittance and increased NIR reflectance, making them attractive for thermal barrier coatings [17]. Caturla et al. investigated ethyl cinnamate, a bio-based plasticiser derived from cinnamic acid at 70 phr in PVC plastisols and systematically mapped curing windows. Optimal properties were obtained after curing at 190 °C for $11.5$ $\min$, yielding a tensile strength of 6.4 N mm⁻² and an elongation at break of nearly 570%, values that meet or exceed those of conventional phthalate plasticised PVC [21].

Morphological examination confirmed complete fusion of the PVC microcrystals, yielding a cohesive matrix with minimal plasticiser migration; thermogravimetric analysis corroborated this result by showing a higher onset temperature for dehydrochlorination, indicative of efficient plasticiser absorption and gelation [2]. Perito et al. compared six plasticisers—dioctyl phthalate (DOP), dioctyl adipate (DOA), Lestarflex^® (polymeric polyester), polycaprolactone, polyester polyol, and 1,2,3-propanetriol triacetate—in PVC plastisols formulated for shoes and toys. Mechanical properties were measured before and after accelerated ageing, complemented by DSC and small-angle X-ray scattering to track structural changes. The authors demonstrated that DOP can be substituted by DOA or Lestarflex without appreciable loss of performance, even after ageing tests [28]. Collectively, these findings highlight how judicious plasticiser and filler selection can be used to tailor the rheological, mechanical, and thermal response of PVC plastisols.

This work develops a purpose-formulated PVC plastisol for high voltage components in electric vehicles (EVs) and systematically maps the influence of filler loading on its structure property profile. Composite films containing graded filler contents were prepared by dip-coating and assessed through a comprehensive suite of mechanical, thermal, dielectric, and morphological tests. The resulting dataset pinpoints the filler range that maximises toughness and dielectric strength while maintaining processability knowledge that can be directly applied to the design of durable, energy-efficient insulation coatings for next-generation EV battery packs. The study, therefore, advances ongoing efforts to couple enhanced functionality with long-term sustainability in e-mobility materials.

Despite extensive studies on PVC plastisols—especially for cable sheaths and sub-millimetre films, including how plasticiser and filler choices tune rheology and properties, most reports examine cast/pressed laminates or low-thickness coatings [17,18,26,28]. Consequently, there is limited evidence on hot-dip plastisols delivering thick (0.8–2.0 mm) builds that simultaneously satisfy dielectric-strength, corrosion-resistance, and mechanical-robustness targets under EV battery-housing service. Here, we address this gap by: (i) formulating DOTP-plasticised PVC plastisols with controlled inert-filler loadings (BaSO₄ + CaCO₃), (ii) defining a robust hot-dip process window that yields uniform 0.9–2.1 mm films, and (iii) mapping how filler content governs mechanical, morphological, FTIR, wetting, corrosion, and dielectric responses relevant to high-voltage EV operation [2,17,18,23,26,28]. We selected barium sulfate (${\mathrm{BaSO}}_4$) and calcium carbonate (${\mathrm{CaCO}}_3$) as inert, electrically insulating, and chemically stable fillers that are widely used in PVC plastisols to control viscosity/flow for hot-dip, improve hardness/abrasion, and support uniform, pinhole-free mm-scale builds on metallic housings. Using a binary filler blend enables practical tuning of bath rheology and particle packing at the targeted 0.8–2.0 mm thickness while keeping the formulation simple and cost-effective for EV applications. Accordingly, our study varies the total ${\mathrm{BaSO}}_4+{\mathrm{CaCO}}_3$ fraction at fixed PVC/DOTP/stabiliser/${\mathrm{SiO}}_2$ levels to map the composition–process–property space. Collectively, these results delineate a practical composition–process–property map for durable, cost-effective battery-case insulation.

To contextualise prior PVC plastisol/coating studies against the present hot-dip, mm-scale approach, a concise comparison is provided in

Table 1. Summary of cited PVC plastisol/coating studies and comparison with the present hot-dip, mm-scale coating.

Study (Ref.)	System & Process	Target Properties	Key Variables	Main Outcome (One Line)
Ji et al. (2018) [15]	PVC plastisols with CaCO3; films (thickness not stated)	Viscosity/aging behaviour	CaCO3 loading; resin morphology	Rheology governed by plasticiser–particle electrostatics + resin morphology.
Bahloul et al. (2019) [17]	PVC plastisol films with NIR-barrier fillers; thin films (thickness not stated)	Optical (NIR/visible)	Nacre; mica/TiO2; glass beads; Al2O3; boehmite; ZnO; MgO; rutile TiO2	Geometry/loading tunes NIR; TiO2: decreased visible and increased NIR reflectance.
Perito et al. (2022) [28]	PVC plastisol (shoes/toys); films (thickness not stated)	Mechanical (pre/post aging), DSC, SAXS	DOP, DOA, polymeric (Lestarflex), etc.	DOP → DOA/Lestarflex feasible without loss.
Siekierka et al. (2023) [26]	Emulsion PVC plastisols; hydraulic-pressed films (numeric thickness not stated)	Density, hardness, thermal, mech/thermomech	Wood flour (fine/coarse); gel 150 °C	Best balance at 20wt% fine flour (SEM-supported).
Tüzüm & Ergin (2023) [18]	PVC plastigel for cable sheaths; thin applicator films (thickness not stated)	Plasticiser migration; property retention	Boric acid, boron clay, sintered boron waste, talc, ZrO2	Migration decreases: boric-acid > unfilled > talc ≈ sintered-boron clay > zircon > boron clay.
Caturla et al. (2024) [2]	PVC plastisols plasticised with ethyl cinnamate (70 phr); films (thickness not stated)	Curing window; mechanical	Cure 190 °C/11.5 min	Tensile 6.4  N mm−2; elongation ∼570%.
Badi et al. (2024) [29]	PVC/CNC nanocomposite films (thickness not stated)	Dielectric response; conductivity (HV insulation potential)	CNC content (casting method)	Bio-based CNC can enhance dielectric performance (complementary route).
Kowalik et al. (2025) [30]	PVC plastisol coatings; AgNP@silica; gelation + pressing; films (thickness not stated)	Mechanical, thermal, antibacterial (Shore, TMA, TGA)	AgNP (1–2wt%), DEHA plasticiser	1–1.5wt% AgNP gives balanced mech/thermal + antibacterial.
This work	Hot-dip PVC plastisol (DOTP); mm-scale builds (0.9–2.1 mm)	Mechanical, dielectric, corrosion (EV housing)	BaSO4 + CaCO3 loading; withdrawal rate	Tensile 11.9  N mm−2; dielectric 22.1 kV mm−1; salt-spray ≥2000 h.

.

2. Materials and Methods

Plastisols are suspensions of poly(vinyl chloride) (PVC) particles in a plasticiser medium that fuse upon heating to form a continuous, tough film. During dip-coating, the workpiece is heated in a furnace at a set temperature and dwell time; furnace temperature strongly dictates the final coating thickness, while uniformity depends on part geometry, size, and internal air flow distribution. Immersion and withdrawal speeds must be matched to the viscoelastic properties of the plastisol: slower rates promote smooth flow and a defect-free surface, whereas excessively slow or fast motion can, respectively, produce over-thick layers or surface irregularities. Formulation also governs mechanical response; higher plasticiser levels reduce hardness, whereas added filler increases hardness but can diminish tensile strength, elongation, and flexibility. Optimisation therefore seeks a balance between these opposing effects [31,32]. The step-wise fusion mechanism from room temperature to $T_4$ is illustrated schematically in Figure 3.

Plastisols are viscous dispersions of fine PVC particles in a liquid plasticiser, typically prepared at an equal mass ratio of resin to plasticiser (50:50). Formulations may also include low levels of extenders, stabilisers, pigments, and fillers to tailor performance. Heat stabilisers are indispensable because they scavenge the HCl liberated during PVC degradation, thereby protecting the polymer backbone. Dimensional stability, softening point, and hardness are governed by both the plasticiser content, most commonly dioctyl terephthalate (DOTP), and the molecular weight of the base PVC [34]. To reduce cost and adjust rheology, inert fillers such as silicon dioxide (SiO₂), barium sulfate (BaSO₄), and calcium carbonate (CaCO₃) are frequently incorporated, particularly in electrical insulation plastisols [15,35,36]. After compounding, the plastisol was degassed in a planetary mixer under vacuum at 1200 rpm for 30 $\min$ to obtain the target application viscosity. Reliable adhesion requires a contaminant-free metal surface; therefore, the substrates were acid cleaned to remove grease, oxides, and residual coatings, then flash dried at 120 °C for 30 min to eliminate moisture [37]. Where necessary, a thin primer was applied to further promote adhesion. Immediately upon removal from the furnace, each substrate was immersed in the pre-heated plastisol bath at 5 mm/s. Coating thickness was controlled by the dwell time kept at 60 s within a practical 30–120 s and by the part temperature. The workpiece was then withdrawn slowly until dripping ceased, after which it was cured in a hot air oven at 200 °C for 40 min. Required cure time scales with oven efficiency, part mass, and coating thickness; complete fusion is evidenced by a uniform glossy surface [2]. A schematic of the dip-coating workflow used in this study is shown in Figure 4.

2.1. Materials

Poly(vinyl chloride) emulsion resin (E-PVC 6834, K-value 67–69) was supplied by Eymen Petrokimya (Istanbul, Türkiye). Dioctyl terephthalate (DOTP, purity 99.5%, density $0.98$ g cm⁻³, viscosity 88 cP, measured according to ASTM D1045 [38], M = 390.5 g mol⁻¹, CAS 6422-86-2) was purchased from Ataman Kimya A.Ş. (Istanbul, Türkiye). Antimony–tin oxide (ATO, purity 99.5%, 22–44 nm mean particle size) served as the heat stabiliser and was obtained from Labor Teknik (Istanbul, Türkiye). Paraffinic oil (Octopus PW522; purity 99%, refractive index 1.480, density $0.82$ g cm⁻³) was sourced from Petroyağ Lubricants (Kirazpınar, Türkiye). Silicon dioxide (SiO₂, 10 nm average size, purity 99%, density $2.65$ g cm⁻³) was procured from Nanokar Kimyevi Maddeler (Pendik, Türkiye). Calcium carbonate (CaCO₃, 15–40 nm, purity 98%, density $2.80$ g cm⁻³) came from Edukim (Türkiye), whereas barium sulfate (BaSO₄, 10 nm, purity 96%, density $4.30$ g cm⁻³) was supplied by Bereket Kimya (Türkiye). Inorganic red and blue pigments were obtained from Betek Boya. Analytical grade hydrochloric acid (37%) for surface cleaning was purchased from Merck. All reagents were used as received without further purification.

Six candidate plastisol formulations were initially screened. Three exhibited acceptable processing behaviour and preliminary properties, and these became the focus of this study. In all cases, the PVC resin (100 phr), the plasticiser DOTP (50 phr), and a stabiliser/SiO₂ package were held constant; only the total fraction of inert fillers (BaSO₄ + CaCO₃) was varied to map the process–property window. The three formulations listed in

Table 2. Formulations of PVC-based plastisol composite coatings.

Component	Group 1	Group 2 Amount (phr) a	Group 3
PVC emulsion	100	100	100
Dioctyl terephthalate (DOTP)	50	50	50
Antimony tin oxide (ATO)	5	5	5
Paraffinic oil	2	2	2
Silicon dioxide (SiO2)	5	5	5
Barium sulfate (Ba2SO4)	10	10	15
Calcium carbonate (CaCO3)	10	25	30
Pigment (red/blue)	0.5	0.5	0.5
Total	182.5	196.5	207.5

^a Parts per hundred parts of PVC resin (phr).

bracket low/medium/high filler loadings that maintained stable dip-coating viscosity and defect-free builds at the target thickness (within our withdrawal-rate window) and provided sufficient contrast to elucidate mechanical and dielectric trends. Component loadings (phr; parts per hundred resin) are given in Table 2.

2.2. Characterisation

Hardness was measured with a Shore A durometer (ZwickRoell GmbH & Co. KG, Ulm, Germany) in accordance with ISO 48-4:2018 [39]. Tensile strength and elongation at break were obtained using a Lloyd LRX Plus universal testing machine (AMETEK Lloyd Instruments, Bognor Regis, UK) at a cross-head speed of 500 mm min-1 in accordance with ISO 37:2017 [40]. Tear strength was measured according to ASTM D1004-13 [41], and Taber abrasion resistance according to ISO 4649:2017 [42]. Each mechanical test was performed on six replicates, and the mean values are reported. Microstructure and elemental distribution were examined by scanning electron microscopy (TESCAN VEGA3, TESCAN ORSAY HOLDING, Brno, Czech Republic) coupled with energy-dispersive X-ray spectroscopy (EDX). The chemical structure was analysed using Fourier-transform infrared spectroscopy (Shimadzu IRAffinity-1S, Shimadzu Corporation, Kyoto, Japan) in the range 4000–600 cm⁻¹. Surface wettability was evaluated at room temperature with a Krüss Drop Shape Analyser (Krüss GmbH, Hamburg, Germany) using 5 $\mathrm{\mu }$$\mathrm{l}$ ultrapure-water droplets; contact angles were measured at five positions per specimen and averaged with their standard deviations.

Corrosion resistance was evaluated by salt-spray exposure according to ASTM B117 [43]: specimens were placed in a chamber operating at 35 °C and sprayed with a 5 wt% NaCl solution (pH 6.5–7.2, fog rate 1–2 mL h⁻¹). After the designated test period, the panels were rinsed, air-dried, and inspected for blistering, rust, and coating delamination.

Dielectric breakdown strength was measured in accordance with ASTM D149 [44] and IEC 60243-1 [45]: flat specimens of known thickness were clamped between parallel-plate electrodes, and the voltage was increased at a uniform rate until electrical failure; the breakdown field was reported in ${\mathrm{kVmm}}^{-1}$.

Thermal stability was characterised by thermogravimetric analysis (Shimadzu DTG-60H, Shimadzu Corporation, Kyoto, Japan) under nitrogen, heating 5–7 mg samples from 30 °C to 400 °C at 10 °C/min.

The study was conceived as a targeted mapping of inert–filler loading on the multi–functional performance of a DOTP–plasticised PVC plastisol for EV insulation. Three formulations (low/medium/high total BaSO₄ + CaCO₃) were compared at constant PVC/DOTP/stabiliser/SiO₂ levels; dip–coating withdrawal speed was evaluated at 5, 10, and 15 mm s⁻¹. The uncoated metal substrate served as a negative control for corrosion and dielectric tests. Primary endpoints were mechanical strength/ductility (ISO 37; ASTM D1004–13; ISO 4649), dielectric breakdown (ASTM D149; IEC 60243) and salt-spray durability (ASTM B117). Unless noted, $n=5$ replicates were measured for mechanical and dielectric tests and $n=3$ for salt-spray; the results are reported as mean ± SD. Specimens were randomly allocated from each batch, and all testing followed the indicated international standards.

3. Results and Discussion

3.1. Withdrawing Rate and Thickness Relation Results

The thickness of PVC-based plastisol coatings is governed by the application method, formulation viscosity, processing conditions, and substrate topology. In dip coating, layer thicknesses from 0.5–6 mm are readily achievable; greater build can be obtained by extending dwell time or applying multiple passes. Film thickness scales with resin to plasticiser ratio, total viscosity, and bath temperature, while immersion time and withdrawal speed provide additional control [46,47]. The deposition rate determines both surface coverage and the ability to form successive layers: a longer bath residence deposits more plastisol, and a slower, steady withdrawal minimises run-off, yielding thicker coatings [48]. Accordingly, withdrawal velocities of 5 mm s⁻¹, 10 mm s⁻¹ and 15 mm s⁻¹ were examined. As shown in Figure 5, coating thickness increases monotonically as the withdrawal speed is reduced, with all formulations attaining their maximum build at 5 mm s⁻¹ consistent with the expected inverse relationship between deposition rate and film thickness.

3.2. Mechanical Results

Hardness, tensile strength, elongation at break, tear strength, Taber abrasion, and salt spray data are compiled in

Table 3. Mechanical properties of PVC-based plastisol coatings (mean ± SD, $n=5$).

Parameter	Unit	Group 1	Group 2	Group 3
Hardness	Shore A	$65\pm 2$	$72\pm 3$	$76\pm 2$
Tensile strength	N mm−2	$11.9\pm 2.1$	$5.1\pm 1.3$	$4.3\pm 1.1$
Elongation at break	%	$465\pm 23$	$261\pm 14$	$197\pm 13$
Tear strength	N mm−1	$92.09\pm 5.11$	$37.11\pm 4.24$	$29.24\pm 2.46$
Taber abrasion a	%	$0.68\pm 0.12$	$0.02\pm 0.003$	$0.03\pm 0.001$
Acceptance b	—	OK	OK	NOK

^a Weight loss after 1000 cycles, CS-10 wheel, 1 kg load. ^b Internal specification: OK = all criteria met; NOK = criterion failed.

and illustrated in Figure 6. Hardness, the resistance to local indentation, increases monotonically with filler loading: Group 1 recorded $65\pm 2$ Shore A, Group 2 $72\pm 3$ Shore A and Group 3 $76\pm 2$ Shore A, corresponding to a rise of ∼15% from the lowest to the highest filled formulation (Figure 6a). This trend is attributed to reduced chain mobility as rigid particulates occupy free volume in the PVC matrix. Similar filler-induced hardening has been observed in PVC plastisols containing almond husk and wood flour additives [26,49]. Tensile strength, the maximum uniaxial load a polymer can sustain before rupture, decreased sharply with increasing filler content (Figure 6b, Table 3). Group 1 reached $11.9$ N mm⁻², whereas Group 2 and Group 3 yielded only $5.1$ N mm⁻² and $4.3$ N mm⁻², respectively, i.e., a drop of roughly $55\%$. The loss in strength is ascribed to stress concentration around filler particles caused by weak interfacial adhesion with the PVC matrix [50]. Elongation at break followed the same trend (Figure 6c): Group 1 elongated to $465\%$, Group 2 to $261\%$ and Group 3 to $197\%$. Because elongation is expressed as a percentage of original gauge length, it is independent of specimen cross-section and directly reflects ductility reduction with filler loading. A clear declining trend is also evident in the elongation at break data. The elongation of Group 1 (465%) is roughly 50% higher than those of Group 2 (261%) and Group 3 (197%), which correlates with their greater filler contents. This loss of ductility is attributed to the increased stiffness imparted by the fillers, which restricts chain mobility and weakens matrix filler interfacial interactions [51]. Tear strength, defined as a material’s resistance to the propagation of a crack once a tear has initiated, follows the same tendency. Group 1 reaches $92.1$ Nmm⁻¹, whereas Groups 2 and 3 fall to $37.1$ Nmm⁻¹ and $29.2$ Nmm⁻¹, respectively (≈65% lower; see Figure 6d and Table 3). Perito et al. showed that tear behaviour depends strongly on the chemical affinity between plasticiser and matrix [28]; here, the higher filler loadings evidently disturb that interaction.

Taber abrasion, a rapid indicator of surface wear resistance, mirrors the previous results. An increase in filler loading cuts the abrasion resistance by ≈90%, as illustrated in Figure 6e and Table 3. Such sensitivity to filler level is consistent with earlier reports that abrasion in PVC composites scales with the extent of polymer filler bonding [52].

Salt-spray testing confirmed the trend: only Groups 1 and 2 satisfied the corrosion requirement, consistent with their superior mechanical performance and the more favourable polymer plasticiser filler interactions observed earlier. Because Group 3 failed the salt-spray test, the detailed corrosion analyses are limited to Groups 1 and 2 and are discussed in the following section.

3.3. Morphological Results

Figure 7 compares the macroscopic appearance and SEM micrographs of the coatings. In Group 1, the inorganic fillers are finely and uniformly dispersed within the PVC matrix, whereas Group 2 shows pronounced agglomerates and inter-particle voids. This segregation is attributed to the higher overall filler loading in Group 2, which weakens polymer–filler interfacial interactions and favours filler–filler attraction [26]. The morphology, therefore, mirrors the mechanical data: the more homogeneous microstructure of Group 1 underpins its superior strength, toughness, and abrasion resistance.

Elemental EDX mapping (C, O, Cl) of the Group 1 cross-section (Figure 8 and Figure 9) further verifies the absence of large agglomerates and confirms that the fillers are evenly embedded throughout the plastisol network. Figure 9 documents matrix homogeneity (C/O/Cl maps) and the uniform spatial distribution of high-Z filler particles evident in the BSE image; direct Si/Ba/Ca maps were not reliable under our mapping conditions and are therefore not shown.

3.4. Fourier Transform Infrared Spectroscopy (FTIR) Results

The FTIR spectra of the coatings are shown in Figure 10; the band assignments for PVC are summarised in Table 4. The peaks at 2911 cm⁻¹ correspond to CH stretching, 1330 cm⁻¹ to CH₂ deformation, 1253 cm⁻¹ to CH rocking, 961 cm⁻¹ to trans CH wagging, 835 cm⁻¹ to C–Cl stretching, and 613 cm⁻¹ to cis CH wagging.

Two spectral windows exhibit formulation-dependent differences between Group 1 and Group 2: 1500–1750 cm⁻¹ (aromatic C=C/plasticiser carbonyl region) and 750–900 cm⁻¹ (PVC skeletal modes, including C–Cl and CH wagging). In Group 1, several minor features in these ranges are markedly attenuated, and some bands appear broadened or merged into the baseline, whereas they are more distinct in Group 2. This behaviour is consistent with stronger polymer–plasticiser/filler interactions and microstructural differences that modify band intensity and width through overlap and scattering effects [53,54]. These formulation-dependent differences are consistent with the microstructural interpretation discussed in Section 3.3.

The Group 1 formulation—our lowest total inert-filler level (BaSO₄ + CaCO₃ = 20 phr; Table 2)—delivered the most favourable balance of toughness, dielectric strength, and corrosion endurance within the dip-coating window reported here (0.9–2.1 mm builds at controlled withdrawal rates). Further reductions below 20 phr were screened in pilot trials but led to bath-viscosity/flow instabilities (run-off/edge sag) at the target thickness and to a hardness/abrasion penalty, indicating a practical lower bound under the present processing conditions. We define a stepwise follow-up (≤20 phr → 15 phr → 10 phr) with rheology control (viscosity modifiers and anti-sag measures) and full re-qualification of mechanical (ISO 37; ASTM D1004; ISO 4649), dielectric (ASTM D149; IEC 60243), and corrosion (ASTM B117) performance to determine whether additional filler reductions can be realised without compromising property targets.

Figure 10. ATR–FTIR spectra of the PVC-based plastisol coatings measured in this study: Group 1 (red, low filler) and Group 2 (black, high filler). Measurements were recorded at room temperature in the 4000–600 cm⁻¹ range with a 4 cm⁻¹ resolution (32 scans; baseline corrected). Labeled peaks correspond to characteristic PVC backbone vibrations—–CH stretching (2911 cm⁻¹), –CH₂ deformation (1330 cm⁻¹), C–H rocking (1253 cm⁻¹), trans C–H wagging (961 cm⁻¹), C–Cl stretching (835 cm⁻¹), and cis C–H wagging (613 cm⁻¹); assignments are consistent with standard PVC literature [55]. Dashed windows highlight regions where Group 2 shows broader peaks, indicating reduced molecular order compared with Group 1.

Figure 10. ATR–FTIR spectra of the PVC-based plastisol coatings measured in this study: Group 1 (red, low filler) and Group 2 (black, high filler). Measurements were recorded at room temperature in the 4000–600 cm⁻¹ range with a 4 cm⁻¹ resolution (32 scans; baseline corrected). Labeled peaks correspond to characteristic PVC backbone vibrations—–CH stretching (2911 cm⁻¹), –CH₂ deformation (1330 cm⁻¹), C–H rocking (1253 cm⁻¹), trans C–H wagging (961 cm⁻¹), C–Cl stretching (835 cm⁻¹), and cis C–H wagging (613 cm⁻¹); assignments are consistent with standard PVC literature [55]. Dashed windows highlight regions where Group 2 shows broader peaks, indicating reduced molecular order compared with Group 1.

Table 4. Characteristic ATR–FTIR absorption bands of the PVC plastisol coatings measured in this study (25 µm, 25 ℃); band assignments were cross-checked with Ref. [55].

Vibrational Mode a	Wavenumber (cm−1)
–CH stretching	2911
–CH2 deformation	1330
CH rocking	1253
trans CH wagging	961
C–Cl stretching	835
cis CH wagging	613

^a Peak positions were read from our ATR–FTIR spectra in Figure 10 (rounded to the nearest cm⁻1; instrument resolution 4 cm⁻). The “–” prefix denotes a pendant group attached to the PVC backbone.

Table 4. Characteristic ATR–FTIR absorption bands of the PVC plastisol coatings measured in this study (25 µm, 25 ℃); band assignments were cross-checked with Ref. [55].

Vibrational Mode a	Wavenumber (cm−1)
–CH stretching	2911
–CH2 deformation	1330
CH rocking	1253
trans CH wagging	961
C–Cl stretching	835
cis CH wagging	613

^a Peak positions were read from our ATR–FTIR spectra in Figure 10 (rounded to the nearest cm⁻1; instrument resolution 4 cm⁻). The “–” prefix denotes a pendant group attached to the PVC backbone.

3.5. Contact Angle Results

Surface wettability was assessed by static water-contact angle, which reflects the balance of interfacial tensions between liquid, solid, and vapour phases and hence the hydrophobic or hydrophilic character of a coating [56]. The uncoated steel substrate exhibited a contact angle of ${69}^{\circ }$, whereas the values for Group 1 and Group 2 coatings were ${119}^{\circ }$ and ${81}^{\circ }$, respectively (Figure 11). Thus, applying the PVC plastisol markedly increased water repellency, and the formulation with the lower filler loading (Group 1) showed a further ${38}^{\circ }$ rise relative to Group 2. According to Young’s equation, the contact angle depends on the solid–liquid interfacial tension, which in polymer coatings is governed by polar and non-polar interactions, hydrogen bonding, van-der-Waals forces, surface roughness, and porosity [57,58]. Increasing the filler content reduces polymer plasticiser interaction and promotes particle agglomeration, thereby altering surface chemistry and topography; the resulting change in solid–liquid interfacial tension explains the lower contact angle observed for Group 2. These findings are consistent with the morphological differences identified earlier. Mechanistically, the higher filler fraction increases the probability that BaSO₄/CaCO₃ domains intersect the free surface and introduces micro-roughness (consistent with the SEM evidence in Figure 7), which lowers the apparent contact angle through a Wenzel-type response [56]. In addition, adsorption/immobilisation of the DOTP plasticiser at filler interfaces reduces the effective polymer–plasticiser interaction near the surface, exposing more polar sites and thereby increasing the solid–liquid interfacial tension; such filler–polymer interfacial effects are well documented for CaCO₃–PVC systems [57,58].

3.6. Corrosion Performance Results of Coatings

Corrosion is a physico-chemical interaction between a metal and its environment that degrades the metal’s properties and, ultimately, the technical system in which it operates [59]. A 1 mm thick PVC plastisol layer offers multiple advantages when applied to the steel or aluminium profiles used in EV battery cases, providing both chemical isolation and mechanical wear resistance [60]. In salt-spray testing (ASTM B117), the Group 1 coating protected carbon-steel panels for more than 2300 h, whereas uncoated steel failed in less than 24 h (Figure 12). The intermediate formulation (Group 2) withstood approximately 1850 h, i.e., about 30% less than Group 1. The superior performance of Group 1 is attributed to a more homogeneous microstructure, stronger polymer–filler interactions, and better adhesion to the metal substrate, all of which minimise permeation pathways and defect sites [61,62].

3.7. Dielectric Strength Results of Coatings

Dielectric strength, the maximum electric field a material can sustain without breakdown, is usually reported in $\mathrm{kV}{\mathrm{mm}}^{-1}$ and serves as a primary measure of insulating capability [63]. Figure 13 compares the dielectric performance of the uncoated steel substrate with the two plastisol coatings. Bare steel exhibited only $0.36$ kV mm⁻¹, whereas the Group 1 formulation reached $22.1$ kV mm⁻¹ and Group 2 achieved $12.5$ kV mm⁻¹; thus the enhanced plastisol (Group 1) provides roughly 45% higher strength than Group 2 and more than two orders of magnitude improvement over unprotected steel. The lower value for Group 2 is attributed to filler agglomeration and weaker polymer filler interfacial bonding, which create local charge accumulation sites and initiate premature breakdown [23], a conclusion consistent with the SEM observations discussed earlier. All coated specimens satisfied the operational reliability criterion of the standard very low frequency (VLF) dielectric withstand test.

3.8. Thermogravimetric Analysis Results

Thermogravimetric analysis (TGA) was used to evaluate the thermal stability of the coatings; the thermograms and key parameters are presented in Figure 14 and

Table 5. Thermogravimetric analysis parameters for hot-dip PVC composite coatings. $T_{10\%}$ and $T_{50\%}$ are the temperatures at which 10% and 50% mass loss occur, respectively; residual mass was recorded at 750 °C.

Sample	${\mathit{T}}_{10\%}$ (°C)	${\mathit{T}}_{50\%}$ (°C)	Residual Mass (%)
Group 1	261.3	299.1	23.5
Group 2	239.2	263.8	19.8

. The onset temperatures for 10% and 50% mass loss is denoted $T_{10\%}$ and $T_{50\%}$, respectively, and the residual mass corresponds to the percentage weight remaining at 400 °C. As listed in Table 5, Group 1 exceeds Group 2 by $\Delta T_{10\%}=22.1°\mathrm{C}$ and $\Delta T_{50\%}=35.3°\mathrm{C}$, while leaving ∼4% more char at the end of the run. The superior thermal stability of the lower-filled formulation (Group 1) is consistent with the SEM evidence of a finer, more homogeneous microstructure: agglomeration in Group 2 can create local hot spots and disrupt matrix continuity, producing weak interfaces that accelerate thermal degradation [64,65,66]. Thus, the TGA trend parallels the mechanical, morphological, and dielectric results discussed earlier.

4. Conclusions

A process optimised PVC plastisol was formulated to meet the stringent mechanical, thermal, corrosion, and dielectric demands of electric vehicle battery enclosures. Dip-coating trials showed that a withdrawal rate of 5 mm s-1 produced the thickest films (890–2100 μm). Lower filler loading (Group 1) delivered superior multi-functionality: hardness $65\pm 2$ Shore A, tensile strength $11.9$ N mm⁻², elongation $465\%$, tear strength 92 N mm⁻¹ and minimal abrasion loss, whereas additional filler (Groups 2–3) systematically reduced toughness (−55% in strength, −50% in elongation) and wear resistance (−90%). Morphology, FTIR, and contact angle data confirmed that the enhanced performance of Group 1 arises from a finer filler dispersion and stronger polymer–plasticiser interactions. The same formulation sustained ≥ 2000 h salt-spray exposure and a dielectric strength of $22.1$ kV mm⁻¹, exceeding the requirements for high-voltage battery cases. While lowering the inert-filler loading can increase flexibility and impact resilience, it generally reduces hardness and dimensional stability and may weaken flame-retardant (FR) performance, potentially lowering the UL-94 classification unless compensated with dedicated FR packages. Within the present process window, the practical lower bound for total inert filler was ∼20 phr, set by dip-coating rheology and hardness/abrasion targets; further reduction will be examined with viscosity-control/anti-sag measures and full re-qualification.

These conclusions are directly supported by the reported data—mechanical metrics and abrasion in Section 3.2 (Figure 6 and Table 2); corrosion endurance in Section 3.6 (Figure 12); and dielectric breakdown in Section 3.7 (Figure 13). We explicitly confine our claims to these tested ranges and conditions. Fire/smoke regulations for aerospace (typically low-smoke/low-toxicity and often halogen-free) differ from those governing automotive EV battery enclosures; material selection is application-specific. The present study addresses the EV enclosure case. Consistent with the above, future work will systematically probe $<20$ phr total inert filler (15 phr → 10 phr) with rheology control and full mechanical/dielectric/corrosion re-qualification.We will also quantify fire performance under reduced-filler scenarios using UL-94, LOI ASTM D2863 [67] and cone calorimetry [68], and adjust the FR package if needed to maintain the target classification. This approach is consistent with reports that compound FR packages in PVC can raise LOI and markedly suppress smoke density while maintaining acceptable properties [25].

In practice, flexibility can be tuned by increasing plasticiser content, whereas hardness and thermal stability benefit from moderate filler levels. Accordingly, plastisol recipes should be tailored to the specific mechanical profile and service environment of the target component. Future work will investigate long-term thermal cycling and scale-up of the coating process for complex battery pack geometries. In addition, the re-qualification accompanying reduced-filler trials will extend dielectric testing beyond room temperature to elevated/lowered temperatures, humidity, and aging conditions, alongside the operational VLF withstand check, to reflect EV service environments.