The handling of adhesives in a responsible way throughout their lifecycle, namely from their manufacture via the usage stage right through to recycling and disposal, is a generally recognized principle. The development and manufacture of adhesives is carried out following the principle of Responsible Care® and the subsequent rules for sustainable development defined by the International Council of Chemical Associations. This specifically means that health protection and environmental compatibility considerations are taken into account when developing and manufacturing new adhesives. This has consequences for the composition of the adhesives, the product design, the recommendations for application of adhesives, and the purpose of use and for the recycling of bonded products after they have been used. This chapter deals with the different aspects of consumer, work, and environmental protection, including health and safety information, related to the use of adhesives in industrial and private areas.
KeywordsAdhesives waste allergies consumer protection emission of organic solvents environmental assessment environmental compatibility hazard potential health protection personal protection equipment predicted Environmental Concentration (PEC) predicted No-Effect Concentration (PNEC) protective measures recyclable design responsible Care safety Data Sheet skin irritation sustainable Development toxicity work protection workplace limit value
The aim of health protection is to protect people against hazards and exposure. “Work protection” is used to describe protection in commercial and industrial work environments while “consumer protection” refers to private users.
Industrial and commercial users of adhesives require special protection because they work on a daily basis with adhesives, often for many hours a day. In addition to hazards associated with the physical properties of adhesives such as flammability, explosiveness, and burns when using hot melts, it is in particular chemical effects such as toxicity, skin irritation, acid burns, and allergies that have to be avoided. This is achieved by equipping the workplace (workplace guidelines) with air replenishment systems and local extractors, personal protection equipment such as work clothing, gloves, and safety glasses and by ensuring that hygiene regulations are adhered to, for example, thorough washing of the skin before breaks and at the end of work and refraining from eating, drinking, and smoking in the workplace. Safety officers monitor compliance with these safety measures. Personnel who work with adhesives undergo regular training on matters relating to work safety and environmental protection. If special risks are involved, then personnel undergo regular medical examinations.
Although in a professional work environment possible hazards can be efficiently managed using suitable protective measures, even in situations where there is long-term use, the situation is different for private individuals using adhesives at home where no special protective measures are taken. Different requirements are hence put on these adhesives. This is also so because private users, unlike industrial users, generally have no knowledge of the properties and potential hazards of products. That is why only relatively few of all the known types of adhesives are available to private users, and even then only small amounts are made available in the form of tubes, cartridges, and tins. On the other hand, private users do not use adhesives every day, but rather only occasionally, and even then only in limited quantities and for limited periods of time. As such, the protective measures described for industrial users are generally neither possible nor necessary. It is nevertheless essential that the safety information given on the small packages is observed, as well as basic principles of work hygiene.
Health Protection When Using Adhesives – Examples of Toxicological Assessment
Of the many different types of adhesives, those discussed below are those that are also commonly used in the household:
Physically Hardening Adhesives
The active components are mainly solid polymers and resins. For application these must be converted to a liquid form. This can either be carried out by users by means of heating (hot melts) or can be carried out by adhesive manufacturers by dissolving the components in organic solvents or dispersing the components in water. The dry, fully aired adhesive is generally unreactive and biologically inert. There is hence usually no health hazard. In contrast, there is a potential health hazard from auxiliary components, such as organic solvents, that are present in some of these adhesives. These can make up as much as 80% of the weight of the adhesive product.
These adhesives contain largely polymers and resins and only small amounts of auxiliary materials. There is virtually never a health hazard. When applying these adhesives manually, there is the risk of burns and users must protect themselves against this. During heating, small amounts of auxiliary materials, contaminants, and cleavage products can be liberated, but these are insignificant when small amounts of adhesive are being processed. In an industrial or commercial environment, an extraction system is recommended due to the larger quantities being used and the longer working times with the adhesives.
Solvent Containing Adhesives (e.g., Contact Adhesives)
In these adhesives, the polymers and resins are dissolved in organic solvents. The hazard potential is determined by the nature of the solvent (e.g., flammability, irritation potential). Due to the high volatility of the solvents, exposure by inhalation of the vapors is the biggest problem. For most solvents, the maximum concentration at the workplace and limiting factors are laid down (workplace limit value). Due to the small amounts of adhesive used by private users, these limit values are generally not reached or are only exceeded for a very short time.
Dispersion Adhesives (e.g., PVAc – Polyvinylacetate/Wood Adhesives)
In the adhesives, the organic solvents are replaced by water and suitable polymers are dispersed in the water. There are hence no potential health hazards from organic solvents. However, water-based adhesives are sensitive to attack by microorganisms (e.g., mold formation). For that reason, dispersion adhesives contain small amounts of preservatives for protection purposes. The potential health hazard is the triggering of allergic skin reactions, for example, allergic reactions triggered by natural polymers such as natural rubber and non-modified colophony resins. The risk of sensitization in nonallergic people is generally extremely low due to the very small amounts of preservatives in the adhesives. Skin contact is here the exposure issue. Depending on the mode of application, skin contact may be unavoidable, as, for example, when using wallpaper pastes. However, here the concentration of preservatives is reduced as a result of mixing with water. As even wearing protective gloves for a long time can lead to skin irritation (e.g., caused by constant sweating), it is worth considering whether the very low risk of direct skin contact causing an allergic reaction justifies wearing protective gloves.
Pressure Sensitive Adhesives
Private users only come into contact with these adhesives in the form of self-adhesive articles such as labels, adhesive tape, etc. As such, these adhesives represent no hazard to private users in practice. Such articles are manufactured industrially using adhesives in the form of solutions, but mostly using dispersions and melts.
Chemically Curing Adhesives
The chemically reactive monomers/oligomers and hardeners and cross-linking agents determine the potential health hazard of these products. Once fully cured, the adhesive polymers are in general nonhazardous. Exposure and risk considerations hence only apply for the time period up until the adhesives have fully cured.
Cyanoacrylate Adhesives (Superglues)
This group of adhesives reacts with water from the surroundings or water on the substrate. As private individuals only usually use small drops when applying the adhesive, the potential health hazard here is that if there is contact with the eyes or splashes of adhesive enter the eye then undesired bonding can take place (can gradually be dissolved using a soap solution). When being used industrially and commercially, possible irritation caused by the cyanoacrylate monomer, thermal effects, and the rapid polymerization reaction have to be taken into consideration. Increasing the humidity prevents irritation of the respiratory tract. Therefore, cyanoacrylate adhesives must be specifically labeled. When carrying out major bonding tasks, not only is it recommended to adjust the humidity of the air but also to wear safety glasses and protective gloves.
Silicones react with water. Depending on the type of silicone, this reaction releases either acetic acid or alcohols. The acetic acid can be clearly sensed by the nose before any irritation begins. In general, the slowly released amounts are so small that they present no health risk, especially in the case of private users. Old formulations of neutral silicones that release butanone oxime must be labeled, but they are only used nowadays for special applications.
Epoxy resin Adhesives
Epoxy resins are widely used by hobbyists and in home do-it-yourself applications because of their good bonding properties, for example, for bonding glass, ceramics, or metals. This is particularly so because the supplied twin-nozzle container allows even the smallest quantities to be dispensed and then mixed. Liquid epoxies resins, and also cross-linking agents, irritate the skin and eyes and cause skin hypersensitivity. As such, hobbyists should avoid epoxy resin adhesives contacting the skin. For the industrial application of epoxy resins, safety glasses and suitable gloves must be worn. Epoxy resins having a molecular weight up to 700 are irritating to eyes and skin and they bear the potential risk of causing skin hypersensitivity. Therefore, epoxy resin systems must be labeled accordingly.
For the private use of adhesives, there are no legal requirements. The selection of an adhesive is left largely to the experience of users and the recommendations of manufacturers. For commercial users, and in particular for industry, the selection of a suitable adhesive system for a particular application must be based on the technical requirements and the potential health hazard. In a situation where two adhesives are equally suitable, the one providing the lower potential health hazard must be chosen. If using an adhesive that does represent a potential health hazard is unavoidable for technical reasons, the exposure must be limited by taking suitable measures, which may even include automating the process, such that a risk to health is prevented. Safe working with adhesives is guaranteed by observing this regulation and the other regulations described in this section. Adhesive manufacturers provide support here by providing information in the form of technical data sheets and safety data sheets. If necessary, users should contact the adhesive manufacturers directly. As is clear from the iterations in this section, adhesive manufacturers make considerable efforts to protect the health of both private and professional users of their products. It must be mentioned here that from a quantity point of view, only a small number of adhesives have to be classified as hazardous formulations.
Private users complying with the safety instructions given on containers and industrial workers complying with relevant company regulations in their production facilities serve to protect people in accordance with the motto: “prevention is better than cure.”
Environmental issues have gained increasing importance over recent decades. The following is an example of a positive contribution to the environment: The car manufacturing industry uses up to 40 kg of adhesive sealants in each vehicle. The main objective of this is to save energy in the form of fuel by means of so-called “lightweight design,” whereby less weight has to be moved over the lifetime of the car and so resources are saved (e.g., 3-l car). On the other hand, the adhesives in this same example could burden the environment if there are residues left over from the application and any cleaning. At the end of the lifecycle of the car, the adhesive that was used must not adversely affect the recycling process and must be disposed of in accordance with regulations. In order to determine the effects on the environment, an environmental assessment is carried out. The expected concentration in the environment is calculated using a model. Simultaneously, the concentration at which no harm is expected to environmental organisms is determined. The calculation models and determination methods are laid down in international standards (European Chemicals Bureau 2003), (European Chemicals Agency 2008). An adverse effect on or harm to the environment can be excluded with certainty if the Predicted Environmental Concentration (PEC) is less than the predicted concentration for which no harm to environmental organisms occurs (Predicted No-Effect Concentration – PNEC), taking into account safety factors.
The emission of organic solvents is detrimental to air quality. As such, considerable efforts have been made by the adhesives industry over many years to switch to low-solvent adhesives and where possible to solvent-free adhesive systems. There has been considerable success, and this has also been beneficial for health protection. Today, adhesive bonding is only responsible for less than 3% of all the solvent emissions in Europe. The few large-scale industrial applications operate with solvent-recovery systems. One typical example for a significant reduction of the use of solvent-based adhesives is the construction industry: While, for example, in Germany in 1985 the fraction of solvent-containing adhesives was still about 50%, it is in 2010 less than 10% with a still decreasing tendency. Dispersion adhesives and reactive systems have largely replaced these solvent-containing adhesives (Adhesion KLEBEN and DICHTEN 2010).
With dispersion adhesives there is a particular risk of contaminating surface waters. Organic polymers and pre-polymers, although not easy to biodegrade, can be removed in biological treatment plants with the excess sludge. The use of these adhesives in handicraft work, in the home, and for DIY work normally results in only relatively small amounts of adhesive ending up in large amounts of wastewater (communal treatment works). As polymers usually have low toxicity for water organisms and due to the very low concentrations of adhesives in the wastewater, an adverse effect on the treatment plant and adjacent surface waters (outfall) is not expected in this case. In industry, residual adhesive and rinse water must be disposed of in accordance with waste legislation.
When used by private individuals, liquid adhesive residues are usually collected in the hazardous waste collection boxes, but are also often disposed of with the rest of the household waste. The amounts involved here are, however, small. For industrial and commercial users, disposal must be carried out in accordance with the so-called waste code numbers (to be found on the safety data sheets), which determine how the waste is disposed. Adhesive residues are normally disposed of as landfill or incinerated. Regarding the latter, specific contamination of the air by the incineration plants is not expected. The energy used in the manufacture of the adhesives can be partially recovered here.
Examples of Assessing the Environmental Impact of Adhesives
Production and Industrial Use of Adhesives
In industry, residues of cured adhesives and also non-cross-linked and liquid adhesives occur as waste. The former, as solid materials are either directly passed on as waste for disposal, or are preferably passed on for recycling. In the least favorable environmental scenario, they are disposed of as landfill. They are, however, usually recycled, either via composting or for energy recovery via incineration. Depending on how they have been treated, liquid adhesives are discharged with the wastewater and are either directly passed to a treatment plant or, if necessary, passed to a treatment plant after undergoing a specific pretreatment. There they are biodegraded or removed.
Do-It-Yourself/Private Use of Adhesives
Solid adhesive waste from the home such as hardened paper or wood glue is usually disposed of with the rest of the household waste.
Water is usually used to remove water-soluble, hardened adhesives in the home, for example, for removing wallpaper paste. This adhesive-containing water is disposed of with the household wastewater via the water treatment plant. The adhesive components, for example, cellulose derivatives, which are dissolved to varying extents, can easily be eliminated under real environmental conditions, meaning that only very small amounts enter the outfall (river). When composting is used for recycling, cellulose decomposes slowly but thoroughly – as known for plant materials.
Adhesives used in the home, for example, roofing adhesives, or around the home, for example, for the car, are exposed to processes that can wash out the adhesive, for example, rain. This means that a small amount of adhesive ends up in drain water and then directly enters surface waters.
Adhesives as a whole do not represent a major environmental problem. Nevertheless, cured and nonrecyclable residues, as well as excess adhesive from applications are waste materials. These materials not only have to be disposed of, but also represent an unnecessary use of materials and resources. The principle of recyclable design, made possible by having detachable bonded joints, will be important in the future. Besides technologies for separating bonded materials, a further challenge of bonding technology will be resource-friendly optimization of production and application processes. It will be necessary to incorporate non-removable adhesive into future recycling processes. This will mean that the adhesive to be used for manufacturing a component will have to be customized to the recycling process at the end of the component’s lifetime.