Thiago A. L. Burgo

Hydrofluoric acid concentrations: Effect on the cyclic load-to-failure of machined lithium disilicate restorations

OBJECTIVES To evaluate the effects of the etching with different hydrofluoric acid (HF) concentrations on the cyclic load-to-failure (CLf) of machined lithium disilicate crowns cemented to dentin analogue material. METHODS Pairs of dentin analogue prosthetic preparations and lithium disilicate ceramic crowns with simplified and standardized designs were machined (n=18). The preparations were etched with 10% HF (60s), followed by primer application. The intaglio surface of the ceramic crowns was treated as follows: non-etched (control, CTRL); or etched for 20s with different HF concentrations - 3% (HF3), or 5% (HF5), or 10% (HF10). A silane coating was then applied onto the treated ceramic surfaces, and they were adhesively cemented to the preparations. To perform the fatigue tests (staircase approach), a hemispheric stainless-steel piston (Ø=40mm) applied cyclic loads in the center of the crowns under water (initial load: 720N; step-size: 70N; cycles: 500,000; frequency: 20Hz). Additionally, topographic, fractographic, and fractal analyses were carried out. The fatigue data were analyzed using the Dixon and Mood method. RESULTS Although the topographic and fractal analyses depicted the action of HF etching altering the superficial complexity and topography, the preponderant topography pattern was established by machining on CAD/CAM. All groups showed similar CLf (in N) (CTRL=805.00±91.23; HF3=781.25±29.87; HF5=755.00±154.49; HF10=833.75±100.74). SIGNIFICANCE Etching with different HF acid concentrations did not promote a deleterious effect on the cyclic load-to-failure of machined lithium disilicate crowns.

Hygroelectricity: The Atmosphere as a Charge Reservoir

The adsorption of water vapor on solids contributes excess charge to them, depending on the hydrophobicity/hydrophilicity and on the acid–base properties of the solid surface. This is a complicating factor in the study of water adsorption that was observed in many different systems: cellulose, various metals and particulate solids including oxides, clay and surfactants. Another important consequence of water adsorption on solids is its effect in modifying charge patterns in thermoplastics and other hydrophobic solids, but under a slower kinetics than in hydrophilic solids. For this reason, atmospheric relative humidity is an important parameter in many experimental techniques, like Kelvin force microscopy. Water itself acquires net charge under different conditions: by flow electrification, or when it drops from a biased needle and also when bulk water is divided into fine aerosol particles. Each type of phenomena produces excess charge by a different mechanism.

Living in an Electrified Environment

The Earth surface is an electrified environment where charge separation takes place due to internal and external factors. The ionosphere with its overall positive charge and the Earth surface carrying negative charge are the charged plates of the Earth capacitor. Ionosphere is a plasma with excess positive charge arising due to the action of a huge and persistent flux of radiation from the Sun that ejects the mobile electrons, leaving behind positive ions. Since the exposure to the Sun follows circadian and annual rhythms, charge density in the ionosphere shows local and temporal changes. The Earth surface is largely covered by the conductive, electrolytic oceans and it is usually considered as conductive. The potential difference between ionosphere and surface is impressive but the distance is also large, imparting to Earth a small capacitance, only 12 nF. For this reason, charge stored in the Earth capacitor is too low to account for the impressive displays of electric activity observed during thunderstorms. Moreover, impressive lightning is also produced by sand storms and volcanic activity. Summing up, any phenomena taking place above the Earth surface and atmosphere is within an electrified environment. Moreover, this environment houses natural and anthropic phenomena that produce electricity. Consequently, any phenomena taking place in the environments used by humans are affected by the planetary electric field, locally modified by events producing charge separation. However, humans do not sense the surrounding electric fields because their organs are strongly shielded by liquid and absorbed water.

Accidents and Losses Caused by Electrostatic Discharge

Many accidents including large disasters are triggered by electrostatic discharge. Human lives and billions of dollars in property damage are lost every year, mainly due to the unobtrusive condition of electrostatic charge: it is not detected by human vision and other senses, as opposed for instance to mass and temperature. Accumulated charge in any portion of matter is hardly perceptible without suitable instrumentation. Electrified materials may appear harmless and safe to the naked eye, even when they are storing large amounts of charge, approaching the dielectric breakdown limits. For this reason, safety codes for the avoidance of hazards due to static electricity are very important and they are constantly updated in industrial environments. This chapter describes situations where electrostatic charge is a safety concern together with protective measures, discussing them with the help of recent knowledge on charging mechanisms and charge stability in various systems.

Charge at Interfaces

Interfaces are important sites for charge accumulation, especially when water is one of the constituents of the interface. Charge patterns on interfaces have been extensively examined in electrochemistry and colloid chemistry where the concepts of double layer, triple layer, and ionic atmosphere are well established. A special case is the formation of excess charge at hydrophobic-aqueous interfaces, as the result of asymmetric partitioning of hydroxide and hydronium ions. An additional but complex issue is the effect of adjacent surfaces on the association of water molecules. Interfaces play important roles in any systems wherever they are found, since they differ from both joining phases and they are anisotropic, even when the two phases are isotropic. Interfaces are thus sites for unique phenomena and they play a leading role in charge accumulation and patterning, in solids and liquids.

Excess Charge in Solids: Electrets

Excess electric charge may be trapped within bulk solids during its synthesis and further transformation. This was definitely shown in polymer latexes, using analytical transmission electron and scanning probe microscopies. Moreover, charged species may be formed and trapped within solids following different events: charge injection, mechanochemical reactions, penetration of radiation and energetic particles, ion implantation or phase transitions. Slow-decaying charge is a characteristic of electrets that are now used in many technology areas. This chapter describes the formation and properties of electrified solids, the mechanisms for charge separation and dissipation and its implications to natural systems and technology.

Charge Patterns, Charge Separation

Excess charge is found under equilibrium and non-equilibrium conditions, in different systems. Charge patterns are observed in material systems in any size range, from molecules to bulk matter. Positive and negative charge separation, partition and/or segregation is caused by many agents: solvents, large electric fields, radiation, mechanical forces, Brownian motion/diffusion and gravity (sedimentation). It appears in transients like the liquid junction potentials or in equilibrium states, as in many soft matter self-assemblies. This chapter reviews the thermodynamic driving forces for the appearance of excess charge as well as non-equilibrium phenomena leading to metastable electrification, except triboelectricity phenomena and other newer mechanisms like hygroelectricity, that are treated in specific chapters.

Electrostatic Processes and Products

Electrostatic painting, copying, spraying and printing as well as electrostatic separation are well-established technologies widely used in the making of consumer goods and in various industrial transformation processes. Recent progress in understanding electrostatic phenomena is stimulating new applications of electrostatics in various industries and areas. This chapter presents a short review of the main processes and products and it discusses some new possibilities under investigation as well as others that can be conceived, considering especially the requirements for “green,” sustainable production.

Friction and Electrostatics

Electrostatic charging is a common outcome of friction but the resulting electrostatic interactions have not been considered in classical theories for contact mechanics such as Johnson-Kendall-Roberts (JKR) and Derjaguin-Muller-Toporov (DMT). On the other hand, tribocharge produced by friction has a strong effect on friction itself, modifying friction coefficients between the intervening surfaces. Friction and tribocharging are also associated with wear phenomena and the interplay between these three important concepts is mediated by tribochemical or mechanochemical reactions. This chapter presents experimental results on the interdependence between triboelectrification and friction coefficients, discussing the insertion of coulombic contributions in the theories for contact mechanics.

Electroneutrality: When and Where?

Electroneutrality is a widespread concept and expresses the fact that all pure substances carry a zero net charge. One of its roots is Pauling’s Electroneutrality Principle of molecular structure that is often unduly extrapolated to macroscopic matter in introductory science courses, implying that matter is intrinsically electroneutral. Other sources for the belief in electroneutrality derive from misunderstandings and insufficient knowledge on electrostatic phenomena, coupled to unwarranted but widespread assumptions. This chapter shows that charge partition and patterning within any system are expected, since any terrestrial phenomena takes place within the plates of a capacitor. Moreover, charge partition and accumulation are common features of most interfaces and excess charge accumulation is also detected in bulk solids. This chapter presents a critical re-examination of the basis for assuming that electroneutrality is a common feature of matter. Beyond, it shows that accepting that most systems on Earth are not electroneutral is bringing great progress to the understanding of electrified materials.