Some time ago, a customer reported experiencing a very pungent odor while using a resinoid diamond grinding wheel. They said the smell was unusually strong and irritating, unlike previous uses, and described feeling like they were about to be poisoned.
Upon investigation, we learned that the customer had always used wet grinding to process tungsten carbide workpieces. After ruling out issues with the cutting fluid, we found that the first use was in a spacious workshop, while the second use, with a different resinoid diamond grinding wheel, was in a relatively confined space.
The composition of resinoid diamond grinding wheels
We know that, in addition to diamond raw materials, resinoid diamond grinding wheels mainly consist of phenolic resin and other auxiliary materials. Phenolic resin is made from phenol and formaldehyde. Thermosetting phenolic resin is a small-molecule resin, and a large excess of formaldehyde is used during synthesis. If not properly processed, the resin will contain a large amount of formaldehyde. To make a thermosetting resin, phenol and other phenolic compounds are added as curing agents. These components will volatilize and form harmful gases during the heating process.
Phenolic resin, also known as Bakelite or Bakelite powder, is originally a colorless or yellowish-brown transparent substance. However, it is often sold commercially with added colorants, resulting in red, yellow, black, green, brown, blue, and other colors. It is available in granular and powder forms. It is resistant to weak acids and bases, but decomposes in strong acids and corrodes in strong bases. It is insoluble in water but soluble in organic solvents such as acetone and alcohol. It is obtained by the condensation polymerization of phenol or its derivatives.
Solid phenolic resin is a yellow, transparent, amorphous solid. It appears slightly reddish due to the presence of free phenol. Its specific gravity is approximately 1.7. It is easily soluble in alcohol but insoluble in water, and stable in water, weak acid, and weak alkali solutions. This resin is produced by the polycondensation of phenol and formaldehyde under catalytic conditions, followed by neutralization and washing. Depending on the catalyst used, it can be classified into two types: thermosetting and thermoplastic. Phenolic resin possesses excellent acid resistance, mechanical properties, and heat resistance, and is widely used in anticorrosion engineering, adhesives, flame-retardant materials, and grinding wheel manufacturing, among other industries.
Liquid phenolic resins are yellow to dark brown liquids; for example, alkaline phenolic resins are mainly used as casting binders.
Summary of the main properties of phenolic resins
1. High-Temperature Performance
The most important characteristic of phenolic resin is its high-temperature resistance. Even at very high temperatures, it maintains its structural integrity and dimensional stability. For this reason, phenolic resin is used in high-temperature applications such as refractory materials, friction materials, adhesives, and in the foundry industry.
2. Bonding Strength
One important application of phenolic resin is as an adhesive. Phenolic resin is a versatile material that is compatible with a wide variety of organic and inorganic fillers. Properly designed phenolic resins exhibit exceptionally fast wetting speeds. After cross-linking, they can provide the necessary mechanical strength, heat resistance, and electrical properties for abrasives, refractories, friction materials, and bakelite powder.
Water-soluble or alcohol-soluble phenolic resins are used to impregnate paper, cotton cloth, glass, asbestos, and other similar materials to provide them with mechanical strength, electrical properties, etc. Typical examples include electrical insulation and mechanical laminate manufacturing, clutch plates, and filter paper for automotive filters.
3. High Carbon Residue Rate
Under inert gas conditions at approximately 1000°C, phenolic resins produce a high carbon residue, which helps maintain the structural stability of the phenolic resin. This characteristic of phenolic resin is also an important reason why it can be used in the field of refractory materials.
4. Low Smoke and Low Toxicity
Compared with other resin systems, phenolic resin systems have the advantage of low smoke and low toxicity. Under combustion conditions, phenolic resin systems produced with scientific formulations will slowly decompose to produce hydrogen, hydrocarbons, water vapor, and carbon oxides. The smoke produced during the decomposition process is relatively low, and the toxicity is also relatively low. These characteristics make phenolic resins suitable for public transportation and areas with very strict safety requirements, such as mines, guardrails, and the construction industry.
5. Chemical Resistance
Cross-linked phenolic resins can resist decomposition by various chemical substances, such as gasoline, petroleum, alcohols, ethylene glycol, oils, and various hydrocarbons. Due to its chemical stability, it is suitable for making kitchen and bathroom utensils, drinking water purification equipment (phenolic carbon fiber), bakelite tea trays and tea sets, and is widely used in food and beverage packaging materials such as cans and easy-open cans (National Standard GB 05009.069-2003) and liquid containers.
6. Heat Treatment
Heat treatment increases the glass transition temperature of the cured resin, which can further improve the various properties of the resin. The glass transition temperature is similar to the melting state of crystalline solids such as polypropylene. The initial glass transition temperature of phenolic resin is related to the curing temperature used in the initial curing stage. The heat treatment process can increase the fluidity of the cross-linked resin, promoting further reaction, and can also remove residual volatile phenols, reduce shrinkage, and enhance dimensional stability, hardness, and high-temperature strength. At the same time, the resin also tends to shrink and become brittle. The post-treatment heating curve of the resin will depend on the initial curing conditions and the resin system.
7. Foaming Properties
Phenolic foam is a type of foamed plastic obtained by foaming phenolic resin. Compared with early materials such as polystyrene foam, polyvinyl chloride foam, and polyurethane foam, which dominated the market, it has particularly excellent flame-retardant properties. It is lightweight, highly rigid, has good dimensional stability, is resistant to chemical corrosion, has good heat resistance, is flame-retardant, self-extinguishing, produces low smoke, resists flame penetration, does not drip when exposed to fire, and is inexpensive. Therefore, it is an ideal insulation and heat preservation material for industries such as electrical appliances, instruments, construction, and petrochemicals, and has received widespread attention.
Phenolic foam has become one of the fastest-growing types of foamed plastics. Consumption is constantly increasing, the application range is continuously expanding, and research and development are quite active both domestically and internationally. However, the biggest weakness of phenolic foam is its high brittleness and high open-cell ratio; therefore, improving its toughness is a key technology for improving the performance of phenolic foam.
In summary, it is normal for some odors to be produced during the grinding process using resinoid diamond grinding wheels. It is best to use wet grinding methods during processing to lower the temperature of the resinoid diamond grinding wheel, reducing reactions and volatilization. Dry grinding will produce more volatile gases, so it is essential to ensure proper ventilation in the workshop to guarantee the safety of the processing personnel!
