Mounting Compounds Information
Mounting compounds are used to encapsulate specimens of a materials such as metal, ceramic, minerals, or biomaterials for sample preparation. Examples of sample preparation include grinding and polishing for analysis using microscopes, hardness testers or spectrometers. Mounting compounds are in the form of compression mounting resins (powders, pellets or preforms) or liquid resins. Mounting pressing, vacuum impregnation, and cold casting are techniques used in mounting metallographic or material samples.
Edge retention is an important consideration when selecting a mounting resin. Hot mounting compounds tend to provide better edge retention compared to cold mounting resins, but they require investment in a mounting press. Samples can be mounted in cold mounting resins with loss cost mold or mold cups. Some microscopy, spectroscopy, and metallography require a conductive mount. For instance, samples examined in electron microscopy need to be grounded or electrically bonded to the sample holder or stage to avoid charging during examination and analysis. Clear or transparent mounting compounds are useful in application because the metallographer or materialographer can visually identify a feature of interest (weld, crack, inclusion, interface, etc.) and then grind down to expose the feature for analysis.
Types of Mounting Compounds
Cold mounting resins are typically two or three part systems (resin(s) and hardener), which are mixed together and castable. The two parts can be in the form of two liquids or a liquid and powder. A sample is place in a cold mounting ring mold or dam, mold cup, or rubber molds and then the mixed cold mounting compound is poured around the sample and cured. Some mold rings remain with the sample and cannot be reused. Silicone rubber molds are reusable because the mounting sample can be removed. Curing typically occurs at ambient or room temperatures. The lower curing temperatures and the absence of applied pressure can be useful in some biological and materialography applications with heat sensitive or delicate samples. In some applications, a vacuum degasser is used to remove entrapped air from the resin or sample to avoid the formation of air bubble around the sample. The void formed from the air bubble would hinder sample grinding and polishing because coarser grit abrasive will collect in the pocket.
Compression mounting, also called hot mounting, compounds are formed and cured around a sample using a compression or hot mounting press. The pressure and heat applied is typically not a problem with most metal alloy, ceramic, glass, or inorganic samples. Compression mounting resins are typically powdered or granular forms of thermoset resins such as diallyl phthalates, phenolics, melamines and epoxies. Some thermoplastic resins such as acrylics are used to form a clear mount around the sample, which may be useful if a specific part of the sample needs to examined.
Embedding compound are typically used for preparation and embedding of biological samples such as bone, tissue, plants, or other organic matter from living or expired organisms. Embedding compounds often consist of a cold mounting compound.
Mounting waxes, blocking compounds and mounting adhesives are used to bond a sample to a holder for subsequent sample preparation and/or analysis. The melted adhesive, wax, blocking resin, or mounting compound is poured around a sample or used to bond a sample to holder, which allow a larger sample to be sectioned to produce a sample small enough for mounting or microscopy analysis. Mounting adhesives are used in electron microscopy for holding sample during dimpling to produce a sample thin enough for TEM analysis. Conductive adhesives are useful to bond a sample to a metal plug, which can then be held securely on an SEM stage. Blocking compounds are commonly used to hold lens or optical components during grinding and polishing operations.
Pellet or briquetting powders are used to prepare powdered samples for spectroscopy or chemical analysis in a spectrometer. Potassium bromide (KBr) powder is used to make FTIR pellets because KBr is transparent in from the near UV to the infrared wavelength ranges with very minimal optical absorption. XRF samples are prepared by mixing the powdered sample with 1 to 3 weight percent of boric acid or a proprietary grinding or briquetting additive. The sample and pelletizing powder mixture are pressed into a pellet in a pellet press.
Curing time - The curing or setting time (minutes or hours) required during the cold mounting process. Cure time can range from a few minutes to several hours.
Peak temperature / exotherm – The peak temperature applied or generated during the mounting process to form or cure the mounting compound. The peak temperature can be a selection concern for analysis of biological samples, organic samples, fusible or low melting alloys, and certain polymer materials, which are heat sensitive.
Pot life – The amount of time (minutes) the mounting compound mix can remain before thickening and curing renders the compound un-castable or unsuitable for mounting samples.
Based Resin System
Acrylic – Acrylic resin systems are available for hot mounting and cold mounting. Acrylics are typically low cost systems. Cold mounting acrylic compounds cure quickly, but they typically do not have the hardness or durability compared to other compounds. Acrylics are typically thermoplastic. Clear or transparent unfilled hot or cold mounting compounds are often acrylic based.
Diallyl Phthalate - Diallyl Phthalate is a common thermosetting resin used in hot mounting compounds. Thermoset diallyl phthalate resins form hard temperature resistance mounting compounds.
Epoxy – Epoxy resin systems are available for hot mounting and cold mounting. Epoxies are typically higher cost systems. Cold mounting epoxy compounds cure times range from minutes to several hours with low to high shrinkage. Epoxies typically have higher hardness, strength and durability compared to other acrylics. Epoxies are typically thermosetting. Liquid epoxy mounting compounds are used with vacuum infiltration methods for the preparation of porous material samples or samples with cracks, pores, voids, cavities or other defects. Some epoxy mounting resins are relatively transparent, exhibit low shrinkage and have excellent adhesion.
Fusible Alloy – Fusible alloys are used as mounting adhesives or blocking compounds to temporarily bond a sample to a holder for sectioning, cutting, grinding or polishing. Temperature must be kept low during processing.
Melamine- Melamine is a common thermosetting resin used in hot mounting compounds. Thermoset melamines form hard temperature resistance mounting compounds.
Pitch / rosin – Pitch and rosin based compounds are used as mounting adhesives or blocking compounds to temporarily bond a sample to a holder for sectioning, cutting, grinding or polishing. Temperature must be kept low during processing.
Phenolic- Phenolic is a common thermosetting resin used in hot mounting compounds. Thermoset phenolics form hard temperature resistance mounting compounds.
Polyester – Acrylic resin systems are available for hot mounting and cold mounting. Acrylics are typically low cost systems. Cold mounting acrylic compounds cure quickly, but they typically do not have the hardness or durability compared to other compounds. Polyester mounting compounds can produce water clear transparent mounts. Cure time can take several hours.
Wax – Wax compounds are used as mounting adhesives or blocking compounds to temporarily bond a sample to a holder for sectioning, cutting, grinding or polishing. Temperature must be kept low during processing.
- Copper Filled – Mounting compound contains copper powder fillers, which can improve conductivity. Copper filled mounting compounds should not be used with aluminum samples in order to avoid galvanic corrosion, which might occur during wet grinding and polishing or etching steps.
- Iron Filled – Mounting compound contains iron or iron based alloy powders fillers. Iron fillers are chosen to avoid galvanic corrosion problems during sample etching or wet processing and for electrolytic polishing.
- Glass Fiber Filled – Mounting compound contains glass fiber fillers, which can improve mount strength and edge retention.
- Graphite / Carbon Filled – Mounting compound contains graphite or carbon fillers, which can improve conductivity.
- Mineral Filled – Mounting compound contains mineral fillers. The hardest of the mineral filler can enhance edge retention.
Clear / transparent – The mounting compound is clear or transparent size, which allows the materialographer or metallographer to grind and polish down to the plane or point of interest in the sample.
Conductive – The mounting compound is electrical conductive, which is useful in some microscopy, spectroscopy and metallography applications such as electron microscopy or electropolishing of samples. Conductive mounting compounds are typically filled with copper, iron, or carbon powdered fillers to provide the required electrical conductivity.
Edge retention – The mounting compound is designed for enhanced edge retention, which can be useful during examination of plating, coatings, and corrosion or reaction layers on surfaces of samples. An alternative for enhanced edge retention is the application of a hard nickel plating or coating layer to the sample’s surface before mounting. However, plating or coating is not always a viable option.
Fluorescent dye – The mounting compound contains a fluorescent dye, which helps highlight or improve the visibility of cracks, voids or defects within a sample.
Low shrinkage – The mounting compound exhibits low shrinkage during curing, which can be a selection consideration for soft or deformable samples.
Preform – Hot mounting compounds are sometimes available in a perform plug shape, which eliminates the need to measure and handle powdered compound.
Vacuum impregnation – The mounting compound is suitable for vacuum impregnation processing, which allow the mounting compound to infiltrate into pores, cracks or other voids within the sample. Typically, low viscosity epoxy cold mounting compounds are used for vacuum impregnation applications.
- Biomaterials (Tissue, Bones, Cells, etc.)
- Electronics (PCB, SMT)
- Failure Analysis
- Mineralogy / Petrology
- Optics / Lens
- Porous Samples
- Sectioning / Cutting
- Semiconductor / Microelectronics
- Spectrographic / Spectrometry
- Wafer / Substrates