Attività

Sandblasting

Air-abrasion (sandblasting) techniques have long been employed in restorative dentistry to enhance the mechanical adhesion between metals and adhesive resins. Sandblasting uses a high-speed stream of aluminum oxide particles propelled by compressed air to remove unfavorable oxides and contaminants, increase surface energy, bonding surface area, and surface roughness. Sandblasting in orthodontics has been implemented to improve the bond strength of new brackets/bands or to remove adhesive remnants from debonded brackets prior to rebonding.71 However, in vivo data from a randomized trial indicate that no significant difference exists in the bond failure rates of sandblasted and non-sandblasted brackets or in the ARI of debonded brackets.72 Finally, sandblaster has also been implemented directly on the tooth enamel surface as an alternative or adjunct to conventional acid-etching techniques, but as no randomized trial exists on this subject, the efficacy cannot be assessed in an evidence-based way at the present time.

Grit-blasting or sandblasting, if available, may also be used. A specially-trained operator is needed to produce uniformity within pieces by sand blasting tank. A clean, uniform-size grit is essential for proper surface preparation with sandblasting. This may be a problem when treating GPR surfaces, and for that reason this technique is rarely used.

After sanding or sandblasting, the surface is sometimes wiped with solvent such as MEK, acetone, toluene, trichloroethylene, Freon® TF, or Freon® TMC, depending on the known mold lubricants. In some cases, a solvent is used before and after abrading the surface. If water-break-free surfaces are not obtained, the procedure should be repeated.

Glass-reinforced plastic laminates prepared for bonding by hand- and machine-sanding can be stored to 30 days at 23°C and 50% RH with no adverse effect on bond strength. Machine sanding gave slightly better results than hand sanding. Tear ply and sanding gave about the same results, but the tear-ply method has less risk of surface contamination. Variations in bond strength are more likely to occur as a result of changes in sanding techniques than by the difference in methods.

In general, bond quality diminished with increased surface exposure time (SET). The best overall adhesive evaluated was epoxy film adhesive, which was found to be the least sensitive to the method of surface preparation. In general, the best result was obtained when GPR laminates were bonded within four hours after sanding. If absolutely necessary, bonding can be carried out after periods of time up to 14 days SET with only moderate strength loss. Brass is an alloy of copper and zinc. Sandblasting or other mechanical means of surface preparation may be used.

Shot peening (which is a similar process to sandblasting, but has more controlled peening power, intensity, and direction) is a cold working process in which the surface of a part is bombarded with small spherical media called shot. Each piece of shot striking the material acts as a tiny hammer, imparting to the surface small indentations or dimples. In order for a dimple to be created, the surface fibers of the material must be yielded in tension. Below the surface, the fibers try to restore the surface to its original shape, thereby producing below the dimple a hemisphere of cold-worked material highly stressed in compression.

Shot-peening (which is a similar process to sand blasting cabinet, but has more controlled peening power, intensity, and direction) is a cold working process in which the surface of a part is bombarded with small spherical media called shot. Each piece of shot striking the material acts as a tiny hammer, imparting to the surface small indentations or dimples. In order for the dimple to be created, the surface fibers of the material must be yielded in tension. Below the surface, the fibers try to restore the surface to its original shape, thereby producing below the dimple a hemisphere of cold-worked material highly stressed in compression.

Injection-molded samples of PP were exposed to oxygen plasma and SACO (SAndblasting and COating) treatments. The pretreated surfaces were successively adhesively bonded or lacquered.

Any successful rehabilitation measure demands proper preparation of the structural element. This includes surface preparation, such as dry sand blasting tank, of the concrete to a desired degree, storage of fiber and resin constituent materials, as well as mixing of the resin system. On-site processing, if performed incorrectly, contains a high potential for flaw introduction. It further necessitates assessments with regards to the integrity and bond capability of the concrete substrate. In some cases, cracked or split concrete sections may contain wide cracks that must be injected with resin prior to application of the composite-strengthening system. Defects induced by preparation and site processing are listed below.

Improper storage of the resin system, as well as the hardener/catalyst, can lead to significant moisture absorption. If stored under inappropriate conditions – such as extreme cold, heat, or humidity – resin properties may change dramatically with time. In addition, as discussed previously, shelf life must be monitored to assure sufficient reactivity and viscosity.

Difficulties may arise with systems that show inadequate stoichiometry. Resin and hardener/catalyst must be compatible and of adequate mechanical and chemical properties for the job at stake. For all resin systems, the hardener/catalyst ratio must be determined very carefully to prevent premature gelling or loss of matrix strength.

During mixing, several defects may be introduced to the system, and consequently to the laminate itself. Firstly, if using rotary mixers, air can be drawn into the resin and remain as small air bubbles, leading to laminate porosity. In some cases, this porosity may later result in the formation of air bubbles of much larger diameter (millimeter range). A high number of roller passes are thus required to remove porosity from laminates that have been infiltrated with air-rich resin systems, since it is known that in the range from zero to 5%, each 1% increase in void content decreases interlaminar shear strength by about 10% (Ghiorse, 1993). Consequently, mixing must be performed at a slow rate and without drawing an excessive amount of air into the matrix. In contrast, a low degree of mixing can result in chemical inconsistency, meaning that some regions contain high percentages of reactant, while others may contain no reactant at all. Secondly, the efficacy of the resin system depends on the appropriate use of mix ratio. Errors in mix ratio can result in under cure, extreme lack of even gel and vitrification, premature gelation, or local hot spots and runaway exotherms resulting in degradation.

Like resin, fibers are susceptible to moisture accumulation; however, accumulated moisture does not alter the performance of individual fiber tows. Instead, the bond to the surrounding matrix is severely weakened. Since fibers can be directly exposed to the environment, conditions must be monitored more closely than in the case of a sealed resin container. If visual detection shows a significant amount of moisture accumulation on the fiber surface, they must be discarded. When a moist fabric is infiltrated, it will experience a weak bond to the surrounding matrix. Debonding and subsequent delamination are likely to result.

To obtain adequate force transfer between the retrofit material and the concrete substrate, concrete preparation is essential. This includes thorough surface preparation to a specified degree and filling of concrete cracks. Large, deep cracks propagating into the concrete may contain water that can destroy the composite–concrete interface bond and should therefore be injected prior to rehabilitation. Cracks propagating at shallow depth can promote failure in the substrate. As such, the retrofit becomes ineffective.

Concrete is a porous material and hence absorbs liquids. Moreover, due to abrasion of cement paste during sand blasting room, a large number of small to medium diameter voids become exposed on the concrete surface. Prior to application of the composite overlay, regardless of type, a compatible primer coat should be applied. The role of this primer is to fill voids and quench the absorption so that the surface is prepared for the subsequently applied composite material. If primer coatings are omitted, the saturating resin would be to an extent absorbed. In addition, the primer presents a ‘bondable’ surface. To ensure an intimate bond between composite and concrete, the thickness of the coating should be kept as thin as possible. If excessive amounts of primer are used, low stress transfer capabilities and resin dripping can result.

Marking of areas that require strengthening is generally done using a chalk line. However, care must be taken not to cause separation between the layup and base material by applying a material that cannot be penetrated by the resin/adhesive, or that could cause edge debonding, which not only serves as a weak zone but also a potential initiator for bond degradation.

If the concrete substrate displays a high degree of microcracking at its surface, the composite overlay must bond to an initially weak base material. In extreme cases the degraded cover material may need to be completely removed and replaced by appropriate fillers, prior to rehabilitation.

Naturally, many structures in need of rehabilitation already show a large number of cracks, which may have opened to a significant degree and thus accumulated moisture, dirt, or other foreign material over time. By applying a primer coating, inclusions may become permanently encapsulated within the surface to serve as weak spots for future crack initiating and propagation at the interface level. As a preventive measure, cracks should be cleaned and injected with appropriate filler materials, depending on the depth and diameter of the crack.

To provide a smooth surface for bonding, any irregularities such as form lines or protruding aggregate should be ground down. If a composite laminate is applied to concrete surfaces that contain high spots, the laminate will tend to form an air pocket. Similarly, large, hollow regions, which may result from high spots in the formwork, must be filled prior to composite application.

Carbon fibers, when in direct contact with steel, cause the formation of a galvanic cell, which results in the accelerated corrosion of steel and degradation of the matrix in the composite (Woo et al., 1993). In spalled, or otherwise substantially degraded concrete components the loss of cover concrete can result in this interaction. Thus appropriate levels of concrete rebuilding are essential prior to the placement of the fiber.
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