Completely cured adhesives and sealants are generally considered safe and non-toxic, although caution must be exercised when machining or grinding joints due to airborne particles. Also, decomposition products may exhibit substantial vapor pressures that present health and safety issues. However, it is exposure to uncured components that can be the most harmful, and that is the subject of this article.
Four primary factors must be considered in all adhesive bonding or sealing operations: toxicity; flammability; hazardous incompatibility; and equipment. Resins and catalysts must be carefully considered because they not only present health and safety issues within the factory or worksite, but they also present issues within the community relative to the release of volatiles and waste disposal. These materials could be toxic or flammable, or they could be skin irritants. Formulators should ensure that these raw materials are compounded in a safe environment. Once formulated, the end user also has responsibilities with regard to safe handling and disposal of adhesive and sealant products.
Some materials are more dangerous than others, and some affect certain parts of the body while others do not. Amine curing agents, for example, generally irritate the skin and eyes. They are also considered to be strong caustics and will produce serious local injury on short exposure. Their fumes pose additional hazards in hot work environments.
In a two-component product, the resin and curing agent are packaged separately and must be mixed together just before use. Each component can be hazardous. In a single-component product, the resin and curing agent are supplied in pre-mixed form. Single-component systems are generally safer for end users because the hazardous chemicals are already partly combined into less-toxic mixtures, and because they do not evaporate as readily into the air.
Solid adhesives, such as powder, preforms or film, are the least toxic because they have very low vapor pressures. However, they may still present a hazard when heated to a more volatile state to cure.
Accessory materials and equipment used to process and apply adhesives or sealants can also be hazardous. Solvents that are used to either dilute the product or to clean equipment after use can be both dangerous and toxic. Mixing equipment can run at very fast speeds, and caution should be used when operating such equipment. Curing equipment often runs at high temperatures, which can create a fire or explosion hazard.
Because of the variety of materials, equipment and processes used with adhesive and sealant systems, the following information provides only a general guide. For more detailed information, contact the manufacturer of each material used.
Effects of Exposure to Adhesive and Sealant MaterialsAll product components, solvents, chemical treatments and the like must be handled in a manner that prevents toxic exposure to the workforce. Methods and facilities must be provided to ensure that the maximum acceptable concentrations of hazardous materials are never exceeded. These values are prominently displayed on the material’s Material Safety Data Sheet (MSDS), which must be maintained and readily available for the workforce.
There is no medical or laboratory test that can quickly measure the amount of a foreign material in the human body, and most of the chemicals found in adhesive and sealant systems are not stored in the body. Regional environmental regulators will set and enforce workplace exposure limits. These limits are generally set for some, but not all, of the components that one may come into contact with.
Table 1 shows legally permissible exposure limits (PEL) for some chemicals commonly found in epoxy adhesive systems. These particular levels have been set by the California Division of Occupational Safety and Health (Cal/OSHA), though it bears mentioning that maximum permissible exposure limits may change from region to region. Also, an individual’s exposure may be above the PEL at times, though requirements can be met if the individual’s exposure level is below the PEL value at other times, thus ensuring that the average exposure for any eight-hour work shift is not greater than the PEL for the specific chemical.
PELs are measured as parts of chemical per million parts of air (ppm). The chemicals in adhesive and sealant systems can affect health when they come into contact with the skin, or if they evaporate and form a mist or vapor in the air. The main effects of overexposure are irritation of the eyes, nose, throat, and skin; skin allergies; and asthma. Solvent additives and other high-vapor-pressure materials can cause other effects, such has headaches, dizziness, and confusion. The effects of these chemicals on various parts of the human body are summarized in the Chart.
Materials Used and Their Effect on Health and SafetyThe main components of adhesive and sealant materials may be hazardous and/or affect the health of those who come into contact with them. In general, these products have hazardous properties but can be handled safely. The hazards associated with the specific product being handled depend upon the nature of the components.
Because different material components have different properties and health and safety characteristics, one should always try to find out what chemicals are in the products that are being used. This can be done by consulting the specific manufacturer’s MSDS.
An MSDS lists the hazardous chemical contents of a product, describes its health and safety hazards, and details methods for its safe use, storage, and disposal. It also includes information on fire and explosion hazards, reactivity, and first aid, and lists procedures for handling leaks and spills.
Generally, environmental regulations require that an employer have all MSDSs for any workplace product that contains hazardous substances, and these must be made available to employees on request. MSDSs can also be obtained from the manufacturer of the material, or sometimes over the Internet.
Most liquid resins can be mild to moderate irritants to the skin, eyes and mucous membranes. The irritant potential is increased by their sticky nature, which tends to lead to prolonged skin contact. These resins are generally mild-to-moderate dermal (skin) sensitizers in susceptible individuals.
Solid resins are not readily absorbed through the skin and present a low risk of skin irritation. Direct contact with solutions of these resins can cause mild-to-moderate irritation of the skin and eyes, principally because the solvents remove the protective layer of fat on the skin’s surface. When crushed into a fine powder, most resins should be considered an irritant dust, and inhalation and skin contact should be avoided. Solid resins are generally considered to be low-to-mild sensitizers.
Resins that are modified by the addition of reactive diluents or solvents can be more serious irritants. These resins should be handled with the same precautions used with chemical solvents. Their sensitizing potential tends to increase with decreasing molecular weight. Components with significant volatility could cause irritation to skin, eyes and the respiratory tract, but inhalation usually is not a hazard, except under certain conditions of use, such as heating, spraying, or applications with large surface areas.
Reactive curing agents are generally more of a health hazard than unmodified resins. Catalytic-type curing agents are difficult to generalize because of differences in chemical makeup. Specific information on these, as well as all materials used in adhesives and sealants, should be requested from the manufacturer prior to use.
Certain curing agents, such as aliphatic amines, cycloaliphatic amines and anhydrides, may cause irritation or damage to the skin, eyes, and lungs. Other curing agents may be considered to be skin sensitizers. Certain curing agents may absorb through the skin and cause damage to organs, such as the liver, and interfere with the blood’s ability to carry oxygen.
Solvents commonly used in adhesive or sealant applications present a flammability hazard. In addition, these solvents present other special health hazards. Contact with solvents will cause “de-fatting” and drying of the skin, which increases the chance of skin irritation. Some solvents are absorbed directly though the skin, and absorption may be increased if the skin is abraded or irritated. These solvents also have the ability to dissolve other epoxy resin system chemicals and carry them through the skin. The inhalation of solvent vapors or mists may cause respiratory irritation and depression of the central nervous system.
The hazardous nature of fillers depends on the type of filler and how it’s handled in production. Some inorganic fillers are considered to be essentially nonhazardous. However, dusts of glass, silica-bearing powders and powdered metals may present a serious risk of inhalation and/or explosion. The explosion hazard is due to the high surface areas of finely divided fillers.
Fillers present a potential inhalation and dermal contact hazard. They can cause mechanical damage to the skin, which may aggravate the irritant effect of other chemicals and additives. When fillers are handled in a liquid matrix or in a cured matrix, their inhalation hazard is low. However, inhalation exposure to fillers can occur when they are handled in the dry state or when machining or grinding cured products. Inhalation exposure to fillers such as crystalline silica or fiberglass may result in delayed lung injury. Asbestos fillers have long been abandoned from use for these reasons.
Processes EmployedPotential exposure to chemicals will vary with the type of process or task. Closed systems with engineering controls are used to prevent workers from overexposure. However, occasionally open areas with limited controls are encountered, and the potential for exposure increases.
Dermal exposure is the most likely route of exposure; this generally occurs in tasks involving hand contact with the adhesive or sealant. However, if certain volatile components or solvents are being used, inhalation exposure may also be a problem. Inhalation and dermal risks are most significant with any task involving the use of solvents or curing agents. The risk of exposure from the ingestion of materials is generally minimal.
Several of the more common processes that occur with the formulation and end-use of adhesives and sealants are described in Table 2. Potential exposure risks for these processes are characterized by dermal, inhalation or ingestion. Comments regarding the effect of a given process on potential exposure are also included.
Workplace Practices to Limit ExposureA number of practices can be used to limit the exposure to chemicals commonly used in formulating and processing these products. Generally, employers are required to protect workers from exposure to any hazardous chemical over the permissible exposure level.
Chemical manufacturers, formulators and distributors must make health, safety, and environmental protection an integral part of the product lifecycle. Several guides have been created to help develop and implement policies and practices that ensure protection through the product lifecycle. For example, the Epoxy Resin Formulators Group of the Society of Plastics Industry has developed an excellent guide.2
The workplace processes that can be used to limit or prevent potential exposure to hazardous chemicals usually include the following.
- Engineering controls
- Protective equipment and clothing
- Good housekeeping
Companies that formulate or use adhesives or sealants should conduct continuing training programs for all personnel involved in the handling of epoxy materials or those who may come in contact with them. Planning for employee and plant safety has value only when it is interpreted and practiced by the people involved.
Continued instruction of all employees must be given concerning the consequences of contact, as well as the precautions necessary for safe operation (storage, handling, compounding, packaging and disposal). The training program should address the following items at minimum.
- Labels, MSDSs and product information bulletins
- Health and safety hazards
- Emergency procedures
- First aid
- Workplace controls
- Personal protective equipment
- Safe handling procedures
When flammable products or solvents are used, they must be stored, handled and used in a manner preventing any possibility of ignition. Proper safety containers, storage areas and well-ventilated workplaces are required.
Certain adhesive materials are hazardous when mixed together. Epoxy and polyester catalysts, in particular, must be well understood, and the user should follow the manufacturers’ recommended procedure for handling and mixing. Certain unstabilized solvents, such as trichloroethylene and perchloroethylene, are subject to chemical reaction on contact with oxygen or moisture. Only stabilized grades of solvents should be used.
Certain adhesive systems, such as heat-curing epoxy and room-temperature curing polyester, can develop very large exothermic reactions upon mixing. The temperature generated during this exotherm is dependent on the mass of the materials being mixed. Exotherm temperatures can get so high that the adhesive will catch fire and burn. Adhesive products should always be applied in thin bond lines to minimize the exotherm until the chemistry of the product is well understood. Never use elevated-temperature-curing sealants or adhesives for casting or for application in excessively thick cross-sections without first consulting the manufacturer. These materials are formulated to be applied and cured in thin cross-sections so that the heat generated by exotherm can easily be dissipated.
Safe equipment and proper operation are crucial to a workplace. Sufficient training and safety precautions must be established in the factory before any bonding process is activated.
The most effective way to reduce hazardous chemical exposure is to use a safer chemical, if available. Unfortunately, this is also one of the most difficult methods of control.
Substitution is a difficult task because the specific materials will control the application and end-use properties of the adhesive or sealant. Any substitution of material generally requires that the complete product be re-verified with production and prototype service testing. Of course, the health and safety issues of any alternative material must be carefully considered to ensure that it is actually safer.
Several elements should be considered when looking for a substitute material or when selecting materials for original formulation. For example, one may be able to choose an alternative system that:
- Contains higher-molecular-weight resins and has a lower vapor pressure. High-molecular-weight resins are less likely to be vaporized and transported through air streams, thus causing inhalation problems.
- Has a reduced solvent content or is solvent-free to minimize health effects due to solvents. There have been significant recent efforts in developing waterborne and UV/EB-cured adhesives for the purpose of reducing or eliminating solvents.
- Does not contain fillers in a dry form.
In addition to training, engineering controls are the most effective way of minimizing exposure to harmful chemicals. Engineering controls include process or equipment modifications that reduce the amount of potentially hazardous materials to which an employee may be exposed. Isolation and ventilation are the primary methods of control. Engineering controls also include maintenance, policing and changing when necessary for work practices.
Isolation, or enclosure of a process or work operation to reduce exposure, is a standard industrial hygiene method. The use of suspect resins, for example, could be isolated to designated areas that are separate from the remainder of the plant. Examples of isolation include spray booths, enclosed curing and mixing rooms, and glove-bag systems. Isolation is an ideal method for use with adhesives systems since the isolation can also prevent unwanted materials (generally contamination [e.g., moisture, mold release, dust]) from getting to the work piece and degrading the strength of the finished joint.
Ventilation is the standard method of controlling exposure to airborne vapors of resins and solvents. Ventilation involves controlling air flow to reduce exposure. Local exhaust ventilation systems capture the contaminant at the source and either filter or remove it from the work area. The ventilation system needs to be designed so vapors, aerosol and dusts are pulled away from - and not into - workers’ breathing zones. A constant supply of fresh, non-contaminated air should be available at all times.
Examples of local exhaust ventilation system include draw-down exhaust tables, slot hoods, dust-extraction systems and portable vapor and dust collectors. Care must be taken to ensure that the ventilation systems are used and maintained as designed. The condition of the filters and the air-flow rate should be checked periodically, as well as the condition of the duct work, motors, belts, etc.
Certain work processes, such as heating or curing, can be isolated, enclosed or automated to reduce exposure. Heating during compounding or cure can cause components to evaporate more quickly. The higher the temperature, the greater the amount of contaminant released into the air. Therefore, the lowest possible temperatures for performing the functions should be used, and adequate ventilation should be maintained around these areas. Electrostatic spray systems can reduce the amount of workplace contamination and waste from sprayed adhesives.
Protective Equipment and Clothing
When engineering controls cannot sufficiently reduce exposures, protective personal equipment and clothing must be used. An industrial hygienist or another knowledgeable person should be consulted to ensure that the equipment is appropriate and is used correctly. The most common forms of personal protective equipment include eye protection, gloves, clothing and respirators.
Eye protection can be provided by safety glasses with side shields, chemical goggles, full-face respirators and face shields with glasses or goggles. The type of eye protection required will depend on the hazard assessment of the specific application. When there is danger of vapor, aerosol or dust exposure, such as when mixing, spraying, or pumping materials under pressure, the eyes must be protected by chemical goggles at a minimum.
Cotton, leather or rubber gloves should be worn to protect the hands from repeated contact with the materials. There is not much acute danger with many of these systems, but repeated contact over long periods of time can sensitize the skin and produce unpleasant reactions such as itchiness, redness, swelling, and blisters.
Selecting the appropriate glove material involves matching the characteristics of the glove with the requirements of the production task. Glove resistance characteristics can be classified as either physical or chemical. Physical characteristics of production tasks are dexterity, wet grip and cut, tear, puncture, and abrasion resistance. Chemical characteristics depend upon the aggressiveness of the resins, solvents and other materials being used.
The most common chemical characteristics to consider when selecting protective gloves are permeation and degradation. For example, glove materials that provide excellent resistance to epoxy resins, curing agents and solvents commonly used with epoxies are ethyl vinyl alcohol laminate and butyl rubber. Nitrile and neoprene gloves have less resistance to solvents and certain curing agents. Gloves should be replaced whenever signs of degradation are noticed. Typical signs of chemical degradation include swelling, softening, cracking or discoloration of the glove material.
Similarly, protective clothing should be made of materials that will provide protection from the chemicals in the product or chemicals used in associated processing. The same chemical resistance characteristics that apply to gloves apply to chemically resistant clothing. An assortment of disposable aprons, coveralls, lab coats and sleeves is also available.
Respirators come in two primary types: air-supplied or air-purifying. Air-supplied respirators provide the user with an external supply of clean breathing air, while air-purifying respirators make use of adsorbents and fillers to remove chemical vapors and particulates from the air. Respirators are generally required where solvent or dust levels are high, where irritating odors are present, and where materials that are respiratory sensitizers are common.
Personal hygiene is also important in minimizing exposure levels to harmful chemicals. Contaminated clothing should be immediately removed and laundered or discarded. Absorbent articles of clothing, such as belts and shoes, are particularly troublesome. Provisions should be made for storage, laundering and disposal of contaminated clothing.
When one comes into contact with a resin or any of the chemicals used with adhesives or sealants, he or she should immediately wash off any components that touch the skin. Do not use solvents for personal cleanup; use soap and water or a commercial cleaner. Particular attention needs to paid to fingernails and the area around the nail bed. After washing, a skin conditioner or lotion should be used to help keep the skin in good condition.
Good housekeeping also dictates that eating, drinking and smoking facilities be maintained separate from the workplace. Refrigerators and freezers used for the storage of epoxy adhesive components must not be used to store food or drinks.
First aid should be an important component of any training program. First-aid materials and facilities should also be maintained in a nearby but isolated area, away from hazardous operations and materials. The specific type of first aid for a given problem is generally indicated on the MSDS.
If skin or clothing becomes contaminated with chemical components, remove the contaminated clothing and wash the exposed area with soap and water for at least 15 minutes. Seek medical attention immediately if irritation or other complications develop.
If the eyes become contaminated, they should be washed out with copious quantities of clean water, and medical advice should be sought. Eye-wash facilities should be provided in areas of the workplace where such accidents have a potential to occur. Medical advice should be sought immediately after washing the eyes.
If respiratory distress is experienced, immediately remove the individual from the contaminated area to fresh air. If the person is not breathing, artificial respiration should be provided. Again, seek immediate medical attention. If breathing is difficult, transport the individual to a medical care facility for treatment and, if available, give the individual supplemental oxygen.
If a chemical is accidentally ingested, seek immediate medical attention. If the victim is conscious, give them water. Do not induce vomiting unless directed to do so by a physician, or as directed by the MSDS.
Emergency procedures are generally required with respect to flammability, spill contamination and cleanup of adhesives and sealants.
Most resinous materials are organic and will burn when sufficient heat and oxygen are supplied. A common measure of a material’s flammability is its flash-point temperature. This value indicates the minimum temperature at which flammable conditions are produced in controlled laboratory experiments at atmospheric pressure. Solvents, diluents and other materials used with adhesives and sealants commonly increase the hazard of flammability and/or explosion.
Fires involving adhesives and sealants can generally be extinguished with foam, dry powder or carbon dioxide. Water is not normally an effective extinguishing agent for organic resins. When burning, some resins give off toxic byproducts, such as carbon monoxide gas. Therefore, avoid breathing fumes or gases resulting from a fire. Firefighters should use an organic vapor respirator or self-contained breathing apparatus.
Any spills should be cleaned up immediately. The immediate concern in any spill is to protect personnel, prevent a possible fire hazard and contain the spill until it is cleaned up. Persons engaged in spill cleanup should be protected from vapors and from skin contact by wearing the appropriate protective clothing and equipment. Persons engaged in spill cleanup should also be aware of proper disposal techniques for the materials in question.
For small spills, apply an absorbent material or a high-surface-area material such as sand to the spill. Shovel the mass with absorbed epoxy into a suitable container. The residue should then be cleaned with hot, soapy water or steam. For material spills containing solvent, keep spark-producing equipment away form the spill site and shut off or remove all potential sources of ignition.
For larger spills, employees should stay upwind of the spill to avoid inhalation of components. Evacuate and rope off the spill area, and shut off all potential sources of ignition. The spill should be contained with a dike, and excess material should be collected in suitable containers for final disposal. Hot, soapy water or steam may be used for cleaning up the residue from floors or equipment. The use of solvents during cleanup should be avoided because of their hazardous nature.
The spilled material must be prevented from entering sewers or drains, or any body of water, including rivers, streams, or lakes. Flexible emergency dikes, sometimes known as “pigs,” can be used to prevent spills from entering sewers and drains. If spilled materials do enter drains or waterways, notify local authorities immediately.
Dealing with Environmental RegulationsIn the adhesives and sealants industry, which has historically trailed other industries in regulation, environmental regulations have spurred significant development on several fronts. Some, however, see this as a plague of regulations and added cost.3Whatever your position on the matter, your strategic business plan should be adjusted to respond to these factors.
When setting strategy, it is important to look at the big picture. A number of alternative materials and processes should be considered when trying to reduce emissions or waste. However, there are several erroneous assumptions made by companies when using government regulations as the sole basis for strategy development (see Table 3).
Traditional factors must also be considered when setting business strategy. These include assessment of competencies relative to growth opportunities. A “SWOT” (strength-weakness-opportunities-threats) analysis will aid in this activity. Above all, the strategy must be aimed at providing a sustainable profit.
Strategic plans must be routinely evaluated to address the best opportunities and market segments for growth. For those with the highest priorities, one must identify: 1.) the potential to deliver and capture value; 2.) alternative paths to market; 3.) alternatives or the elimination of processing steps and the reduction in scrap or waste (both for the supplier and the customer); and 4.) a complete value chain solution that takes into consideration how one can provide value to both suppliers and customers that benefits everyone. In most cases, this traditional approach to developing a strategic plan will, out of necessity, also incorporate environmental factors.