Thermo-Responsive Wax Millicapsules as Lubricating Agents Carriers

Abstract

Encapsulation of lubricating agents has many advantages, as it helps to protect them from external factors, oxidation and degradation, can support their controlled and prolonged release, and also preserves the environment from accidental contamination with these substances. In our experiments various types of thermo-responsive, paraffin wax capsules capable of safely transporting liquid and semi-solid lubricants were designed, fabricated and tested. Lubricating oils were primarily encapsulated inside hemispherical wax shells closed with special caps, but also in wax spherocylinders and two-compartment structures. Greases were protected with wax coatings with the thickness ranging from 0.187 to 0.774 mm. The payload release from our core–shell capsules occurred not only due to the exerted mechanical force but also in a controlled manner upon prolonged contact with a heated surface. The wax shells of the capsules lying on the plate, whose temperature was increased at a rate of 0.025°C/s, began to melt gradually, starting from ≈55.5 °C. This temperature-triggered lubricant liberation can be useful when, for example, a machine element becomes excessively hot due to friction. The wax itself also has lubricating properties, so the crushed or melted coating cannot be treated as waste, but only as an additional factor supporting lubrication. The practical applications of our wax capsules were demonstrated with five examples.

1. Introduction

In the industry and various everyday applications lubricants fulfill many functions. Their primary task is the reduction in friction and wear, which, of course, translates into decreased energy losses. There is a wide range of lubrication systems available on the market. One can mention ionic liquids and bio-based vegetable oil-derived lubricants, which play an increasingly important role in the era of promoting environmentally friendly solutions [1]. Silicone lubricants are suitable for use on surfaces made of rubber, plastic, metal and ceramics. Another group consists of viscoelastic machine greases containing refined mineral oils and thickeners such as calcium or lithium metal soaps [2]. A representative example is multifunctional Tovotte grease that is based on a lithium thickener and used to lubricate slide bearings and machine surfaces. It has a form of a dense paste and, after application, creates a thick layer on the surface, not only minimizing friction but also protecting against moisture and penetration of pollutants from the environment. Furthermore, grease is easier to use than lubricating oil because, thanks to its consistency, it does not flow out of the bearing [3]. The lack of grease due to its limited durability can cause damage to machinery, which should be prevented by occasional re-lubrication. The use of standard lubricants can also be problematic due to the fact that they can accidentally contaminate various places during application. Oil-free environment is often required in the case of optical instruments, electronic devices or medical equipment [4].

It is believed that intelligent lubricating materials will play an increasingly important role in future technologies. This includes stimuli-responsive capsules, which are capable of controlled release of lubricating agents when external force or heat is applied to their surface [4]. In recent years, there has been a growing interest in enclosing lubricants inside capsules. The release of such a cargo can occur at the interface between the contacting bodies when the capsule walls rupture, for example, due to mechanical stress [5]. Spherical, oil-carrying capsules can be fabricated based on Pickering droplets formed in a liquid medium using electrohydrodynamically controlled assembling [6]. However, transferring capsules with the shell made of a monolayer of jammed polymer microparticles outside the liquid environment may be difficult. In the case of capsules intended to perform their tasks in the air, the use of liquid marble-type structures seems to be an alternative solution. They are usually formed by the mechanical rolling of droplets on a bed of various solid particles [7]. Unfortunately, the core of marbles is usually composed of a hydrophilic material, and the pores between particles that make up the shell are responsible for cargo evaporation. Therefore, structures of this type do not seem to be an optimal carrier for hydrophobic lubricants.

Among the substances insoluble in water, various types of liquid lubricants are very often enclosed in protective shells. To preserve against degradation and oxidation, oils are encapsulated using a number of methods, such as simple extrusion and coextrusion with gravitational or electrostatic dripping, millifluidic, jet cutting and vibrating jet techniques [8]. These procedures are not perfect; their major drawbacks may include: frequent clogging of nozzles by viscous material, excessive wastage of material or relatively high costs of equipment. Therefore, new and simple methods of capsule synthesis are constantly being sought, including the use of thermo-responsive materials such as various types of waxes for the production of shells.

Natural waxes constitute long-chain fatty acids esterified to fatty alcohols and containing varying proportions of other components, e.g., aldehydes or triacylglycerols [9]. Among mineral waxes, the most well-known is paraffin, obtained from the crude petroleum distillate fraction. It consists of saturated hydrocarbons with more than 15 carbon atoms in a molecule, being a waxy solid with a melting point of up to 67 degrees [10]. Paraffin wax is most often used to produce candles, crayons and as a lubricant. The use of paraffin wax for bicycle chain lubrication is particularly popular because this material provides better performance and resistance to dirt and water [11]. Wax is insoluble in water, and at room temperature, it makes up a ductile material. Due to its beneficial properties, such as low cost, lack of toxicity, chemical inertness and high storage energy capacity, paraffin was used for the production of phase-change capsule cores [12]. Interiors containing N-eicosane were also coated with a poly-siloxanethe, forming core–shell microcapsules with the shape of spheres or distorted spheres [13]. Nowadays, waxes are also increasingly used to create thermo-responsive shells. For example, wax coatings have many applications in the food industry. In this case, structures of various sizes have already been produced. Cinnamaldehyde-carrying beeswax and propolis wax-based nanoparticles were fabricated by means of the antisolvent precipitation method [14]. Larger wax capsules intended for the transport of water-soluble substances were obtained by the injection of hot carnauba wax with a functional ingredient into cold sunflower oil while stirring and then isolating the structures by centrifugation [15]. Multiple methods for manufacturing wax-based delivery systems utilize hot homogenization in which the active compound is dispersed into the melted wax [9]. This solution is not always optimal, because the transported substance during prolonged exposure to elevated temperature or rapid mixing may lose its properties or degrade. Therefore, it is worth developing a technique for filling already formed hollow spheres at room temperature.

The aim of our experiments was to design different types of thermo-responsive wax capsules capable of safely transporting liquid and semi-solid lubricating agents and releasing them not only as a result of the exerted mechanical pressure, but also in a controlled and gradual manner upon prolonged contact with a heated surface.

2. Materials and Methods

2.1. Materials

Olive oil was purchased from Gatt-Koller (Absam, Austria) and rapeseed oil from Princes Food, B.V. (Warsaw, Poland). Silicone grease was ordered from Avantor Performance Materials Poland POCH (Gliwice, Poland). It has NLGI (National Lubricating Grease Institute) No. 2, worked penetration at 25 °C about 230–270 mm/10, specific heat 1200–1300 J/(kg·K) and thermal conductivity 4–4.5 J/(m·s·K). Machine (Tovotte) grease ŁT-43 came from two manufacturers: dark brown colored from Refinery Plant (Jasło, Poland) and green colored from Orlen Oil (Kraków, Poland). According to the product specification, ŁT-43 greases are characterized by the following parameters: NLGI class 3, worked penetration at 25 °C about 237 mm/10, dropping point ≈ 192°C and weld load ≈ 160 daN. The operating range of these greases is from −30° to 130 °C.

Paraffin wax was recycled from white candles manufactured by Bispol (Głuchów, Poland). Laboratory-grade normal paraffin was ordered from WarChem (Warsaw, Poland). Ethylene glycol (spectrophotometric grade ≥ 99%) was bought from Sigma-Aldrich (St. Louis, MO, USA).

2.2. Methods

Wax from white candles was scraped and melted in a cylinder placed in a laboratory water bath LW RS-485 (Laboratory Equipment Factory Bytom, Poland) with a thermostat set to 70 °C. The melted wax was kept at a constant temperature. The density of molten wax was determined at 70 °C based on the measurement of the mass of 10 mL of this substance, repeated five times. The densities of the rapeseed and olive oils were determined experimentally using a calibrated 25 mL glass pycnometer at 24 °C. The hardness of solid wax at 24 °C was tested using the digital durometer 3130 (Zwick/Roell, Ulm, Germany).

2.2.1. Encapsulation of Liquid Lubricants

Shells in the shape of a hemisphere or an elongated hemisphere were produced on a matrix. Such templates were usually rounded ends of rods of various diameters. The use of cylindrical PTFE-coated magnetic stirring bars with various diameters (e.g., 6 mm or 4.4 mm) for this purpose also turned out to be a good solution, due to their appropriate shape and the possibility of easier manipulation when using ferromagnetic tools. Before the first dip in liquid wax, the rod was lubricated with silicone grease to make the coating easier to remove. Depending on the planned shell thickness, different numbers of cycles of immersion in molten wax (for time shorter than 1 s) and air cooling were applied. The solidified wax coating in the form of a half-sphere or hollow cylinder with a hemispherical end was removed from the matrix. In the next step, the structure was positioned with the opening upwards by means of the cylindrical guide and trimmed, if necessary, using heated tungsten wire.

The formed hemispherical shells were filled with olive or rapeseed oil using automatic pipettes and closed with wax caps. These caps were synthesized by dripping small droplets of molten wax into room-temperature (24 °C) water without stirring or adding surfactant. The size of each cap was determined by the amount of substance used and the diameter of the opening of the tip through which the droplet was administered. In order to permanently attach the cap to the payload-filled hemisphere, a heated steel blade was touched to the contact area, which allowed the creation of a connection of melted wax. Wax capsules having two separate compartments and thus capable of carrying two different cargoes were also fabricated. Their production involved hot joining of two oil-filled and closed hemispheres with caps flattened with a heated blade.

Capsules in the form of spherocylinders were created by connecting two wax cylinders, each with one end closed with a hemisphere. For their production, matrices in the form of rods of different diameters of 6 mm and 5 mm were used. A cylindrical shell of smaller diameter was filled with oil using a Pasteur pipette (the structure was stabilized vertically by a guide), and then a second cylinder of larger diameter was pressed into it.

2.2.2. Encapsulation of Greases

The densities of greases at room temperature were determined by measuring the masses of four precisely defined volumes of a given semi-solid lubricant. The mass measurements were performed using a WAS 100/C/2 analytical balance (Radwag, Radom, Poland) with a readability of 0.1 mg.

In order to prepare capsules, machine grease was extruded directly from a syringe cone or through a pipette tip attachment. The diameter of the cone opening was selected depending on the planned capsule diameter (e.g., 4.15 mm, 2.15 mm). The semi-solid lubricant dose was then repeatedly immersed in molten wax (maintained at a constant temperature of 70 °C) for less than 1 s. The wax hardened in the air, forming a protective coating around the portion of lubricant. The thickness of the shell was determined by the number of times the dip and cooling cycle was repeated. Finally, the capsule was cut off with steel mercury-collecting tongs to seal its end. There was also an option where the filled shell was mechanically removed directly from the syringe tip, but in this case, the wax collar remained on the capsule. The distal opening in the capsule was then sealed with this collar material. Alternatively, after cutting the collar off, the uncapped part of the capsule was reinserted into the liquid wax to close the shell. Very small beads of semi-solid greases were coated by dipping them in liquid wax on the tip of a very fine pin with a diameter of 0.5 mm.

2.2.3. Imaging of Capsules and Studying Their Properties

Photos of all fabricated capsules were taken with a digital microscope Dino-Lite AM73115 MZT with magnification 10–220×, an integrated polarizer and a professional stand RK-06A (Dino-Lite Europe, Almere, The Netherlands). An advanced XY table MS15X with a rotating plate was used to position the samples. The experimental area was lighted by the two-arm gooseneck LED illuminator (Delta Optical, Nowe Osiny, Poland).

Measurements of the thermo-responsive properties of wax capsules were performed using a precision hotplate (Präzitherm, Düsseldorf, Germany), which allowed temperature regulation with an accuracy of 0.1 °C. Initially, the capsule was placed on a plate at room temperature. Then, during the experiment, the plate temperature was automatically raised by 0.5 °C every 20 s. Images of the heated capsule throughout the period of the wax melting process were recorded every 0.5 °C using the microscope positioned centrally above the capsule. Image analysis was performed in ImageJ 1.54p software. Termio-1 temperature recorder (resolution 0.01 °C) with immersion sensor PT1000 was used to measure the temperature of various liquids (e.g., water, ethylene glycol, liquid paraffin wax) during experiments.

The coating thickness tests were conducted for various numbers of immersions in liquid wax (from one to eight immersion and cooling cycles). A cylindrical PTFE-coated bar with a diameter of 6 mm was used as a matrix for preparing wax shells in the shape of a cylinder with a hemispherical end. The thickness measurements of the wax-coated rod were performed with a resolution of 0.001 mm using a digital micrometer (Mitutoyo, Kawasaki, Japan). The mensuration for each number of cycles was repeated 5 times and averaged. The shell thickness was estimated as half of the difference in the diameter of the coated and uncoated matrix.

3. Results and Discussion

In our experiments, different types of thermo-responsive capsules with wax coatings were produced. Depending on the design and manufacturing method, the capsules carried liquid or semi-solid lubricants. Each of the developed lubricant encapsulation methods utilized immersions in liquid wax. Paraffin wax recycled from candles was used. The average hardness value of solid wax, obtained on the basis of 10 measurements at 24 °C using a digital durometer and expressed in Shore A hardness scale, was 58.8 ± 0.9 ShA. The experimentally determined density of molten wax at 70 °C was 0.861 ± 0.003 g/cm³. The methods of capsule preparation were also verified to work with paraffin purchased as a laboratory reagent.

3.1. Capsules Filled with Liquid Lubricants

Capsules closed with caps proved to be the most optimal solution for transporting liquid lubricants, such as various oils. The process of manufacturing these types of capsules had four main stages. In the first part of the experiment, a matrix was used to produce the shell in the form of a hemisphere. The size of the capsule was determined by the diameter of the matrix rod, while the wall thickness was determined by the number of dips in liquid paraffin wax. It was determined how the number of immersion and cooling cycles affected the obtained wax coating thickness. The results are presented in Figure 1.

After a single immersion, the coating had an average thickness of about 0.187 mm, and the thickness increased with subsequent dips, giving a linear relationship.

The shell thickness can therefore be easily adjusted. In most capsule formation experiments, three immersion-cooling cycles were used, resulting in a coating layer of approximately 0.345 mm. An example of hollow hemispheres created by dipping the matrix three times in liquid wax is shown in Figure 2a.

The wax hemispheres, thanks to the opening on one side, were filled with the appropriate amount (depending on the size of their interior, e.g., 20 or 30 μL) of olive or rapeseed oil (Figure 2b) using an automatic pipette. The densities of these oils, determined experimentally using a calibrated pycnometer, were: 0.9263 ± 0.0003 g/cm³ for rapeseed oil and 0.9197 ± 0.0003 g/cm³ for olive oil, both at 24 °C.

Due to the fact that in our experiments the oil was poured into already formed shells, it was not heated, which prevented negative changes in its properties.

Oil-filled hemispherical shells were closed using wax caps. These caps, produced in large numbers by successively dripping portions of molten wax onto the surface of room-temperature water, had a practically regular, circular cross-section when viewed under a microscope (Figure 2c). On the water side, their surface was spherical, while on the side where they came into contact with the air, they had a concavity resembling an erythrocyte.

In our case, the optimal distance between the pipette tip and the bath interface was 2 cm. Based on this fall height h, the impact velocity v₀ can be calculated, using the formula that takes into account air resistance and the gravitational acceleration g [16]:

$$v_0=\sqrt{\left({\displaystyle \frac{g\left(1-e^{-2Ah}\right)}{A}}\right)},$$

(1)

$$A={\displaystyle \frac{3f_c{\rho }_{air}}{{\rho }_w{r}_0}},$$

(2)

where f_c = 0.7796 is a friction coefficient, r₀ represents the radius of the drop, ρ_air and ρ_w denote densities of the air and the molten wax, respectively. The experimentally determined density of molten wax at 70 °C was ρ_w = 0.861g/cm³. According to the literature, the air density at 24 °C is 1.188 kg/m³ [17]. Calculations indicated that for our droplets impact velocities were close to 0.61 m/s. For example, for drop radii of 1 mm and 2.5 mm, these values were 0.607 m/s and 0.618 m/s, respectively.

Interestingly, the deformation and solidification of millimeter-sized molten wax droplets at the interface of an immiscible cooling liquid medium of higher density are complex phenomena that have received a separate scientific study [18]. In general, the morphology of the created wax structures depends on the fine balance between inertial, interfacial, viscous and thermal forces. In order to form appropriate caps, the kinetic energy of the paraffin drop before contact with water cannot be too high. Our tests showed that caps of the desired shape were formed when liquid paraffin portions were dropped from a height of 2–5 cm. When the fall height was 10 cm, elongated structures resembling the shape of a boat were created. The situation was even worse when the distance was 20 cm, then the flake-like or flower shapes were obtained. Example patterns of solidified wax are shown in Figure 3.

During the tests, it also turned out that mixing the liquid in the beaker deteriorated the shape of the created caps; in particular, they clumped together and lost their regularity. The use of a surfactant was also undesirable. However, suitable caps were also created by replacing pure water with ethylene glycol:H₂O 50:50 at a temperature of 24 °C. Examples of such caps are shown in Figure 2d. The capsules could be closed with a cap by mechanically attaching it and using the adhesive forces of the oil. Structures fabricated in this manner (an example is shown in Figure 2e) were not fully durable, because the cap could fall off due to mechanical force or prematurely during melting. Therefore, it was decided to permanently attach the caps to the payload-filled hemispheres. It was performed by creating a connection of melted wax between the cap and the shell by external contact with a heated steel blade. A capsule, such as the rapeseed oil-filled one shown in Figure 2f, remained tight for at least four months of storage at room temperature.

The release of substances from capsules closed with caps may occur as a result of mechanical force or heat. A microscope with a polarizer was used to study the melting process of the coatings. The use of optical microscopy is an inexpensive and widely accepted practice when studying the behavior of materials as a function of temperature and time, e.g., the melting point of different substances [19].

The thermo-responsive behavior of the capsules turned out to be particularly interesting. It varied depending on the way the cap was attached and the position of the capsule in relation to the heated surface. Figure 4a shows the process of olive oil release when the capsule was lying on its side and the cap was attached only by adhesive forces. From a temperature of about 50 °C, an expanding spot of melted wax was observed, and between the temperature values of 54–55 °C the cap opened from the side of the heating plate (resembling a movement on a hinge). The hemispherical shell melted completely at 57 °C and the cap at 58 °C. Different behavior was observed for the capsule with the cap permanently attached and positioned towards the heating plate, as depicted in Figure 4b. Here, the shell gradually shrank and starting from a temperature of 55 °C, a stain of liquid wax and oil spread, lubricating an increasingly larger area. After reaching 64 °C the entire wax from the coating was completely melted.

Another capsules produced in our experiments were wax spherocylinders (Figure 2g,h) composed of two parts that slid into each other. These structures resembled telescopic hard capsules made of starch or gelatin and known from pharmacy. The medical drugs are often encapsulated inside the hard capsules, consisting of a body into which the substance is filled and the upper part for closing [20]. However, in the case of medical applications, both the material and the method of release are different. As mentioned before, in our case, the lubricant was released by mechanically crushing the capsule or by exposure to heat. The force required to crush the capsule (wax spherocylinder filled with oil) was approximately 1.4–1.5 N when acting on the largest surface from the top.

In the case of mechanical devices, we also deal with heat generation. The thermal decomposition process of a hollow, two-part, spherocylindrical wax capsule lying on a surface whose temperature was systematically increasing is illustrated in Figure 4c. The liquid wax stain spread starting from a temperature of 58 °C, and at 72 °C, all the wax from the coating has already been melted. During the melting process, the capsule changed shape several times, sometimes violently.

It is also worth mentioning that two capsules with permanently attached caps can be heat-bonded to form a single structure with two distinct compartments, as shown in Figure 2i. Such a solution made it possible to safely encapsulate two different oils and avoid mixing these liquid lubricants during storage and transport.

3.2. Capsules with Grease as Payload

A separate issue in our research was the encapsulation of semi-solid lubricants in wax capsules. The experimentally determined densities of these lubricants at room temperature were as follows: grease ŁT-43 (green): 0.915 ± 0.006 [g/cm³]; ŁT-43 (brown): 0.96 ± 0.02 [g/cm³]; silicone grease: 1.112 ±0.022 [g/cm³]. The thickness of the coating formed around a grease depended on the number of dip-and-pull cycles in liquid wax, similar to the case of capsules formed using a matrix. Typically, three repetitions were used. Due to the fact that the wax created a transparent white shell, the color of the grease was visible through it, making it easy to identify the transported lubricant. The examples of fabricated structures are presented in Figure 5.

The force required to crush the lying, grease-containing wax capsule depended on its shape. When this force came from the plate pressing the capsule from above to the ground, the values were in the range of 0.9–2.4 N.

It is worth noting that our capsules demonstrated long-term stability necessary for industrial applications. This is consistent with literature data, which indicates that wax as a wall material ensures leakproofness even for many weeks [15].

The studies of the thermo-responsive behavior of capsules with wax covering protecting a grease core were also carried out on a heating plate whose temperature was gradually increased. The primary cross-sectional area of the tested capsule containing ŁT-43 (green) grease was ≈11 mm². The initial temperature of the plate surface and the capsule lying on it was 22 °C. Starting from a temperature of 55.5 °C, the process of melting of the wax shell began to be observed, which was manifested by a growing spot of liquid wax on the plate around the capsule. Figure 6a shows how the area covered by liquefied wax (including the surface occupied by the capsule itself) increased with rising temperature.

In the case of a structure containing ŁT-43 green grease, this increase had an exponential character up to a temperature of 66 °C. Above this value, the stain continued to spread slowly; the coating was already melted, while the grease remained in the form of a lump.

In the case of silicone grease enclosed in a capsule initially occupying a surface of ≈18 mm², the dependence was analogous, as shown in Figure 6b. The exponential growth of the surface covered with liquid wax continued up to 67.5 °C. The grease itself was still formed into a lump and had not melted over the entire temperature range tested (up to 72 °C).

In the literature one can find an example of thermo-responsive palm oil wax-based microcapsules produced using microfluidic technology [21]. Those structures were intended for cosmetic applications, providing a hermetic seal and releasing cargo when heated above the melting point. The paraffin melting process itself was investigated numerically and experimentally inside a plate heat exchanger [22] and in the case of precipitated paraffin in crude oil pipelines [23]. Our experiments determined how wax shells behave on a heating surface.

3.3. Lubricating Properties and Tribological Mechanisms

Our wax capsules are capable of transporting a variety of liquid and semi-solid lubricants. In case of many types of machines, when lubricating agent released from the wax capsules may be used to separate contacting elements, one can experience a simultaneous occurrence of a dry friction (when the surface irregularities of device parts are in direct contact), a boundary friction (for a very thin lubricant layer) and a liquid friction (hydrodynamic and elastohydrodynamic friction). Mixed friction theories, examples of which are presented in [24,25,26,27], are used to describe such situations. The experimental, tribological investigations of the lubrication efficiency of various greases (including ŁT-43) in steel sliding pairs working in mixed friction conditions can be found in [28]. The type of lubrication is determined by the thickness and properties of the lubricating layer and also by temperature [29]. In our case, the film formed by the cargo lubricant at room temperature is enriched with crushed wax, and at temperature values above 56 °C, it is a mixture of liquid wax and lubricant. Tests showed that liquid lubricants spread better over the surface. In the context of the rheological properties of vegetable oils, it is worth adding that the kinematic viscosity of rapeseed oil is 35.1 mm²/s at 40 °C and 8.03 mm²/s at 100 °C and the viscosity of olive oil is 38.7 mm²/s at 40 °C and 8.28 at 100 °C [30]. The viscosity at different temperatures can be predicted using the Arrhenius-type equation [31]. Pressure-viscosity coefficient values for rapeseed and olive oils at 20 °C were measured as 11.0 GPa⁻¹ and 11.8 GPa⁻¹, respectively [30]. The properties of greases and oils used in our experiments are summarized in

Table 1. Characteristics of semi-solid greases used as capsule payload.

Type of Grease	Density at 24 °C [g/cm3]	NLGI	Penetration at 25 °C [mm/10]
ŁT-43 (brown)	0.96 ± 0.02	3	237
ŁT-43 (green)	0.915 ± 0.006	3	237
silicone	1.112 ± 0.022	2	230–270

and

Table 2. The properties of vegetable oils used as cargo for hemispherical and spherocylindrical capsules.

Type of Oil	Density at 24 °C [g/cm3]	Kinematic Viscosity at 40 °C [mm2/s]	PRESSURE-VISCOSITY Coefficient at 20 °C [GPa−1]
rapeseed oil	0.9263 ± 0.0003	35.1	11.0
olive oil	0.9197 ± 0.0003	38.7	11.8

.

Once the capsule melts and the lubricating oil is released, a two-component film is left on the surface. The properties of such a system can also be discussed by looking for certain analogies from the literature. It is known that crude oils, due to paraffin wax content, can exhibit non-Newtonian behavior below a certain temperature [32]. Such a behavior with a non-linear shear stress to shear rate relationship was more pronounced as the wax quantity increased. In case of a mixture of paraffin wax and oil, the shear stress rose when the temperature was lowered, and the viscosity η [Pa·s] of the system was described as a function of shear rate ẏ [1/s], temperature T [K], and wax concentration w [wt %] using the equation:

$$\eta =A{e}^{\left({\displaystyle \frac{B}{T}}+Cw\right)}\cdot {\dot{y}}^D$$

(3)

where A, B, C, and D are constants obtained from the analysis of experimental curves [32]. Due to the fact that the wax content has a noticeable influence on the oil rheology, other models were also created to predict viscosity [33]. In general, the apparent viscosity decreases significantly with rising temperature. Semi-solid lubricants also exhibit non-Newtonian behavior. Visco-elastic grease distribution can be predicted using numerical methods and the Herschel–Bulkley rheology model, which was demonstrated for ball bearings [34]. It was shown that the apparent viscosity of lithium grease decreases with increasing shear rate and rising temperature [35]. Theoretical description of the behavior of systems composed of two types of lubricants requires further research, which will also allow for a better understanding of the case of our thermo-responsive capsules.

3.4. Applications of Capsules with Wax Shells

The wax capsules containing liquid and semi-solid lubricants can find many potential implementations both in industry and household maintenance. In our experiments, several applications were tested in practice. Small capsules filled with silicone grease were used to lubricate zip fasteners. The wax coating proved to be very useful, as it protected the fabrics from getting dirty. The capsules were put inside the Y-shaped channel of a slider, and as it moved, they were crushed, which lubricated the interlocking teeth as shown in the microscopic photos in Figure 7a. After this treatment, the zipper worked more smoothly.

The second tested application was related to the lubrication of old window hinges. After slightly lifting the window sash, a capsule with grease was placed between the two parts of the hinge, as shown in two panels of Figure 7b. The window sash was then lowered to its original position and closed and opened several times, which caused the crushed capsule and its contents to lubricate the hinge (Figure 7c). The excess waxy coating material fell off without contaminating the surface or the experimenter’s hands.

Rapeseed oil-filled wax spherocylinders were used for the lubrication of a bicycle chain. As shown in Figure 7d, these millicapsules were fed to the moving chain from a dispenser attached to the bicycle frame. The wax shells of the capsules sequentially ejected from the feeder were abraded by contact with the moving chain, and the lubricant was released onto the subsequent links.

Lubrication of meshing gears was performed using wax spherocylinders with rapeseed oil (Figure 7e) or hemispherical capsules with a cap and two-compartment structures filled with olive and rapeseed oils, as shown in Figure 7f. Capsules placed between the gear teeth were gradually crushed, causing the lubricant to be liberated.

It should be borne in mind that, in addition to the simple applications presented in the article, millicapsules, depending on the needs, may have other interesting uses. Due to their size, they are not suitable for miniature and very precise devices, but they can be useful in the case of larger machines and objects.

4. Conclusions

In our studies, various thermo-responsive millicapsules with wax shells were developed, fabricated and tested. Hemispherical structures closed with caps, spherocylinders and two-compartment lubricating oil delivery systems were created. An efficient and fast method for coating semi-solid machine greases was also developed. The release of lubricating agents from our capsules was achieved both by applying mechanical force and also by prolonged melting of the wax coating due to the increasing temperature of the surface on which the capsule was placed. In practice, such a gradual rise in temperature can occur due to the friction between surfaces or machine parts. The wax itself also has lubricating properties, so the melted or crushed coating is not waste, but an element of the system.

Encapsulation of the lubricating agents in wax shells had a two-fold purpose. Firstly, it protected the lubricants against external factors and offered a more controlled and sustained release under the influence of the increasing temperature. Secondly, the capsules preserved the external environment, including human hands and clothing, from direct contact with lubricants that tend to stain surfaces. Sample experiments have shown that our capsules are suitable for practical use.