Low–interfacial toughness materials for effective large-scale deicing

The accretion of ice on surfaces can have a severe detrimental impact on a range of commercial and residential activities (1). Consequently, there has been an effort to create coatings that protect against the buildup of ice. Typically, the efficacy of these coatings has been evaluated by measuring the force, F, to debond a specified area, A, of ice, and defining an ice adhesion strength τ_ice = F/A as the characteristic property for the system (2). The term icephobic is generally used to describe surfaces for which τ_ice < 100 kPa (3, 4), in comparison to structural materials such as aluminum and steel, for which τ_ice > 1000 kPa (2, 5, 6).

Using τ_ice to characterize an interface inevitably requires that the force necessary to remove ice scales with the iced area. Many engineering structures susceptible to icing, such as airplane wings, wind turbine blades (7), and boat hulls (8), have surface areas that can approach thousands of square meters. Consequently, even with extremely icephobic coatings, structures with large surface areas would require prohibitively high forces to detach entire sheets of ice from the surface. In this work, we developed low–interfacial toughness (LIT; interfacial toughness Γ < 1 J/m²) materials for which the force required to remove adhered ice from large areas (few cm² or greater) is both low and independent of interfacial area.

An interfacial cohesive strength, as represented by the ice adhesion strength, is one way to describe the bonding across an interface (9). A countervailing perspective on fracture (10, 11) is that an interface should be described in terms of its bonding energy (or, more correctly, its toughness). Further, although the work of adhesion is often discussed in connection to ice adhesion, it is the strength that is generally used to describe failure (1, 2, 4). This is true whether the adhesion is viewed in terms of surface energy (3, 12, 13), interfacial cavitation (14), or lubrication (15–17).

The two competing perspectives for delamination, strength and toughness, can be rationalized by means of cohesive zone models of fracture (18–22). Simple analytical models (23) can be used to demonstrate that the shear strength of the interface, τ^, controls delamination when the length of the interface is relatively small, so that τice=τ^. This is manifested by a spontaneous rupture along the entire interface (movie S1). Conversely, Γ controls delamination when the length of the interface is relatively large. This is manifested by the propagation of an interfacial crack (movie S1). The analysis shows that there is a critical bonded length at which a transition between the two modes of failure occurs, given by Lc=2EiceΓh/τ^2, where E_ice is the modulus of ice (~8.5 GPa) of thickness h. In this context, ice is not a ductile material even close to its melting temperature, and treating it as an elastic solid, at the engineering strain rates and time scales relevant here, is a reasonable approach (11) provided that 2EiceΓ/σY2<h<L, where σ_Y is the yield strength of ice. Note that when L > L_c, the force required to delaminate the ice is constant, no matter how large the interface may be.

We first verified the concept that the force required to remove an ice layer reaches an asymptotic value if the interface is long enough. This was done using substrates made from common plastics such as polyethylene, polypropylene, and polystyrene (substrate thickness t = 1.6 mm; see Table 1) without any additional modification. We used a setup similar to those reported previously (2, 14, 24) but instead of using relatively short lengths corresponding to a few millimeters of bonded ice (2, 14, 24–26), we designed our apparatus so that much longer interfaces could be evaluated (Fig. 1, C and D, insets) (23). Plots showing the force (per unit width) necessary to detach the ice, F˜ice, as a function of the bonded length, L, are shown in figs. S7 to S12. For the most part, we observed that F˜ice increased proportionally to L only when L was small. Beyond the transition length L_c, no additional force was necessary to dislodge the ice. The corresponding asymptotic force, F˜icecr, can be used to determine the interfacial toughness, Γ, from Γ=(F˜icecr)2/2Eiceh (27–29). Specific examples of this behavior are shown in Fig. 1A for ice bonded to four different plastic substrates, each of which has a transition length less than 10 cm.

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We repeated this experiment for aluminum substrates coated with different icephobic coatings (t ≈ 1 mm; Fig. 1B). These coatings were all based on polydimethylsiloxane (PDMS) rubber, which has been studied for its low–ice adhesion properties enabled via lubrication (25), interfacial cavitation (14, 24), low surface energy (30, 31), and interfacial slippage (32, 33). In contrast to the plastic substrates, these materials did not exhibit a toughness-controlled regime of delamination within the range of bonded lengths studied.

A low interfacial shear strength does not necessarily imply a low toughness. Thus, a material that debonds from ice more readily if the interface is short does not necessarily debond more readily if the interface is long. This can be seen by comparing the results for polyvinyl chloride (PVC) and polyamide (Fig. 1A) or the results for polypropylene and silicone B (Fig. 1, A and B). The shear strength of the interface between ice and polypropylene can be calculated from the initial slope of the line in Fig. 1A as τ^ = 320 ± 40 kPa. The shear strength of the interface between ice and the silicone B coating is an order of magnitude lower, being equal to τ^ = 29 ± 2 kPa. However, the force for detachment continually increases for silicone B, even out to 100 cm (Fig. 1C). For interfaces longer than 50 cm, the ice is removed more easily from polypropylene (Γ = 1.9 J/m²) than it is from the silicone B coating (Γ > 9 J/m²).

These data can be reexpressed in terms of the apparent ice adhesion strengths for the two interfaces by dividing the force by the initial bonded area (Fig. 1D). As such, the apparent ice adhesion strength, for a length of 100 cm for polypropylene (τ_ice ≈ 12 kPa), was less than half that of the icephobic PDMS (τ_ice ≈ 29 kPa), although the true ice adhesion strength τ^ was an order of magnitude greater (Fig. 1D). Over the past decade, achieving τ_ice < 15 kPa has necessitated the use of either soft rubbers (14, 24) or highly lubricated systems (25, 26, 32), which can suffer from poor durability (14). However, here we show that one can obtain much lower values of apparent ice adhesion strength for large structures by selecting a material with a low toughness.

There are several contributions to interfacial toughness. One is associated with the bonding energy between the ice and the coating. A lower bound on this energy would be ~0.1 J/m², corresponding to van der Waals interactions (5, 34)_. An additional contribution could come from localized losses within the coating, associated with the high-stress region at the crack tip. These two effects would be classically considered to be contributing to interfacial toughness. However, if the process of delamination causes deformation of the coating, then the strain energy associated with this deformation must also be considered as a contribution to the effective toughness between the ice and substrate. From a cohesive zone perspective, one can consider the toughness of an interface to be given by the area under the force displacement curve of the entire interface, including the coating (18, 19). Therefore, assuming linear elasticity, this contribution to the toughness can be estimated as Γ≈τ^2t/2G, where G is the shear modulus and t is the thickness of the coating (23) (fig. S6). Consequently, it should be possible to minimize the effective toughness by minimizing the thickness of a polymeric coating.

To investigate this concept, we varied the thickness of PVC films and confirmed that Γ scaled with the coating thickness t (Fig. 2A). Lowering t from 150 μm to 2 μm (23) reduced Γ from ~3 J/m² to 2 J/m². Previous work (14, 35) has shown that for icephobic elastomers, τ^ ∝ t^1/2. Therefore, the design of LIT (Γ < 1 J/m²) materials can be substantially different from the design of icephobic materials. Icephobic surfaces can be made more effective as t increases, whereas LIT surfaces become more effective as t decreases.

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To further reduce Γ, we explored the effects of plasticizing PVC with medium-chain triglyceride oil (MCT) (23). Figure 2A shows the general drop in toughness observed with increased plasticization. As shown in Fig. 2B, the additional drop in shear strength associated with the addition of 50 weight percent (wt %) MCT was large enough for the transition length to become too long for the toughness to be measured for the thicker coatings. The general trends among strength, toughness, coating thickness, and level of plasticization can be seen in Fig. 2, C and D.

By optimizing the thickness and plasticizer content within the PVC, we fabricated LIT materials exhibiting Γ as low as 0.27 J/m² (F˜icecr = 52 ± 7 N/cm). Similarly, by optimizing the thickness and plasticizer content, we fabricated LIT polystyrene (20 wt %) diisodecyl adipate; Table 1 and fig. S18] (23) with Γ ≈ 0.43 J/m² (F˜icecr = 66 ± 6 N/cm) and LIT PDMS (40 wt % silicone oil; Table 1) that displayed an even lower Γ ≈ 0.12 J/m² (F˜icecr= 35 ± 4 N/cm).

We coated 1.2-m-long aluminum beams with these LIT PVC and LIT PDMS coatings (with a nominal thickness of t ≈ 1 to 2 μm) (23), and conducted large-scale testing inside a walk-in freezer at –10°C. Figure 2E shows that the force of detachment did not increase for L > L_c, even over 1 m of interfacial length (F˜icecr = 52 ± 7 N/cm for LIT PVC; F˜icecr = 35 ± 4 N/cm for LIT PDMS). These correspond to τ_ice values of <6 kPa and <4 kPa for the LIT PVC and LIT PDMS surfaces, respectively. In contrast, for aluminum coated with an extremely soft, icephobic PDMS rubber (plasticized silicone B, τ^ = 12 kPa), we measured F˜ice = 126 N/cm at L = 100 cm. Additional experiments confirmed that we were not observing delamination (fig. S3) or cohesive failure (fig. S5) of the LIT coatings during ice removal.

For a given ice thickness, there will always be an interfacial length beyond which LIT materials require less force than icephobic materials to remove the adhered ice (fig. S19). As an example, to mimic the deicing of power line cables, we conducted off-center loaded beam tests by flexing 1.2-m-long uncoated and coated (with either icephobic or LIT coatings) aluminum beams with ice adhered on one side (23). The icephobic (silicone B; t ≈ 1 mm) and LIT (silicone B + 40 wt % silicone oil; t ≈ 1 to 2 μm) coatings were fabricated using the same polymer, PDMS, but the icephobic PDMS system exhibited low interfacial strength (Γ > 8.8 J/m² and τ^ = 30 kPa), whereas the LIT PDMS exhibited low interfacial toughness (Γ = 0.12 J/m² and τ^ = 115 kPa). Upon flexing, ice fractured cleanly from the LIT PDMS at a low deflection of 2.4 cm from the center of the beam (Fig. 3A). Both the uncoated and icephobic-coated beams displayed limited signs of ice detachment even at an extreme deflection of ~35 cm (movie S2). To mimic the deicing of an airplane via wing tip deflection, we conducted end-loaded cantilever beam tests (fig. S20 and movie S3). The deflection necessary to remove the ice adhered to the LIT coating was an order of magnitude less than that of the icephobic or uncoated surfaces. The LIT coating also enabled the removal of ice from complex geometries, such as an ice cube tray (fig. S17 and movie S4), with little to no force.

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The isothermal freezing conditions within a freezer (Fig. 1, C and D, and Figs. 2E and 3A) differ from those experienced in Peltier plate–based systems (data shown in Figs. 1 and 2), in which ice is formed via unidirectional cooling from the surface. Ice formation conditions, particularly ambient temperature, can substantially affect the structure and interfacial properties of ice (1, 36–38). The similitude of our data for L ≤ 20 cm (Peltier) and L > 20 cm (freezer; see Fig. 1, C and D, and Figs. 2E and 3A) lengths of ice indicated that LIT materials can be effective in shedding ice in different ice formation conditions. Additionally, we measured τ^, F˜icecr, and Γ values for polypropylene and LIT-PDMS at –20°, –10°, and –5°C. The values of these interfacial properties (fig. S4), for both LIT PDMS and polypropylene, appear to be invariant with temperature and the different ice formation conditions within the ranges studied.

To evaluate a third ice formation condition, we coated a 1 m × 1 m aluminum panel with our LIT PDMS and allowed ice to form outdoors at –7°C overnight (23) (Fig. 3B). We observed that the weight of the ice at a thickness of 1 cm, once fully frozen, was enough to completely and cleanly remove the attached ice (movie S5). This yielded τ_ice = 0.09 kPa. Whereas varying icing conditions can lead to tensile cracking and subsequent fragmentation of the ice, as long as the fragmented length remains greater than L_c, LIT materials will remain an effective means of ice removal.