Occupational exposure to epoxy resins

Introduction

Products based on epoxy resins as a binder have become popular in various settings, among which the construction industry and in windmill blade production, as a result of their excellent technical properties. However, due to the same properties epoxy products are a notorious cause of allergic skin disease among workers. In addition, constituents of epoxy products may cause airway and eye irritation, and airway allergies or even more serious diseases such as cancers or diseases of the reproductive system. Epoxy products that contain volatile solvents may give rise to neurotoxic effects. This article focuses on the use phase of epoxy products, and describes the various types of epoxy products, areas of application, health hazards, professions at risk, developments in the composition and risks of products, the development of classification systems to assess these risks, and control measures, following the hierarchy of controls.

What are epoxy resins?

Epoxy resins are a class of synthetic resins (polymers) that are chemically characterised by the very reactive ‘epoxide group’ (figure 1).

Figure 1: The ‘epoxide’ group, responsible for the specific properties of epoxy resins

Source^[1]

Most commonly, the epoxy resin prepolymer is constituted of 1 to 3 monomers, each made up of one molecule of bisphenol-A and one molecule of epichlorohydrin^{[2] [3]}(figure 2). Resins with only one unit are liquid, and these are generally used at ambient temperatures (e.g. in construction; figure 3). Resins with more than one unit are solid and are either used in a solvent, or at elevated temperatures, e.g. in industry. Generally, the epoxy resin is mixed on-site with a curing agent or hardener (generally a polyamine, figure 2, sometimes anhydrids). The chemical reaction between the resin and the curing agent is an exothermic polymerisation reaction, where the curing agent reacts with the free epoxyde groups, to form a  three-dimensional network. Once fully cured, epoxies generally turn into much less toxic polymers. The cured epoxy forms a solid, high-strength material. 

This provides epoxy products their favourable properties, such as^{[2] [3] [4]}:

• excellent adhesion to many substrates;
• resistance to mechanical wear;
• corrosion protection
• liquid tightness and chemical resistance;
• thermal stability and mechanical strength;
• fast curing;
• no shrink at curing.

Figure 2: Building blocks of epoxy products: Bisphenol-A (a), Epichlorohydrin (b), example of a polyamine hardener: diethylene triamine (c)

Figure 2a: Bisphenol-A

Figure 2b: Epichlorohydrin 

Figure 2c: diethylenetriamine

Source^[5]

Figure 3: Application of an epoxy-reinforced flooring material

Source: copyright by Stichting Arbouw, reproduced with permission by the copyright holder

Products based on epoxy resins

Products based on epoxy resins generally are multi-component products. Usually, the worker has to mix at least two components prior to applying the product: the epoxy resin and the (polyamine) hardener. In some cases, a third component is added, e.g. a filling material such as quartz sand in epoxy-reinforced flooring.

Furthermore, several ‘subtypes’ of epoxy products may be distinguished:

• Solvent based products, in which the epoxy resin is dissolved in an organic solvent. In this case, higher molecular weight epoxy resins, made up of 2 or 3 units, are used. This is common in metal coatings for corrosion protection.
• High solids products: these are similar to solvent-based products, but they have a reduced solvent content, and therefore a higher content of ‘solids’;
• Solvent free products: products with no or only a very low amount of organic solvent.

In this case, so-called ‘reactive diluents’ are added - generally glycidylethers - which react with the hardener and become part of the cured resin. Most epoxy products – except metal coatings – are solvent free products nowadays (see 'Major applications' as well).

Major applications

Epoxy resins are used in a large variety of applications and are used primarily as coatings for consumer and industrial applications. Products based on epoxy resins include:

• windmill blades
• protective coatings for automotive and marine uses
• food and drinks cans
• electric isolators
• electronic components
• models in museums and exhibitions

In addition, the components of epoxy products are frequently mixed and applied on-site. Typical applications include:

• Adhesives for tiles and for metal or concrete construction parts
• Flooring materials and floor coatings, such as industrial floors
• Corrosion protective metal coatings for steel constructions, ships and vehicles
• Concrete repair agents and wood repair agents.

Major health risks

Practically all major ingredients of epoxy products are moderate to strong irritants as well as moderate to strong occupational allergens. This holds true for the epoxy resin itself as well as the polyamine or anhydride hardeners and the reactive diluents^{[6] [7]}.

Epoxy products are potent skin sensitizers (allergens). Frequently, skin contact with the individual components or the ready-for-use mixed product gives rise to allergic contact dermatitis (eczema). The contact dermatitis generally expresses at the hands or fore-arms, and sometimes in the face. Workers that have acquired an epoxy-allergy will be faced with an ever stronger skin reaction after each contact with the products. Avoiding every contact is the only option left in that case.

If relatively volatile polyamine hardeners are still used (e.g. ethylenediamine), or if the product is applied by spraying, the airways may be affected as well. Workers may be at risk of strong airway irritation as well as (allergic) occupational asthma.

Epichlorohydrin, one of the constituents of the epoxy resin monomer (figure 2) is a skin sensitizer. In addition, epichlorohydrin is classified as carcinogenic in the category 1B, ‘presumed human carcinogen’, according to the EU classification^[8]. In the epoxy resin, prior to curing generally only a very low amount of residual epichlorohydrin is present. Manufacturers that are member of Epoxy Europe a sector group of Cefic, the European Chemical Industry Council have voluntary committed to a limit concentration of 0.001% of epichlorohydrin in the resins^[9]. 

The other constituent of the epoxy resin monomer, bisphenol-A (figure 2), is a skin sensitizer too, as well as endocrine disruptive and it is also with prostate and breast cancer, neurobehavioral deficits, heart disease, and obesity^[10]. Based on the CLP Regulation bisphenol-A is classified in the EU as a substance that^[11].

• causes toxic effects on our ability to reproduce (Repr. 1B),
• causes serious eye damage (eye dam. 1),
• may cause respiratory irritation (STOT SE 3); and
• may cause skin allergies (skin sens. 1). 

Studies have shown that under certain conditions, such as exposure to heat, acidic or basic foods, or prolonged storage, traces of bisphenol-A can migrate from the epoxy coating into food or beverages^[12]. This has led to consumer concern and regulatory restrictions. Due to its endocrine disrupting properties, Bisphenol-A has been restricted under the REACH^[13] as a substance on its own and in mixtures intended for consumer use in the EU since March 2018. Its use in thermal paper has been restricted since January 2020^[11].

Professions at risk

At construction sites and in the manufacturing of wings for wind turbines or cabins of large aircraft the use of epoxy products is most widespread. In the manufacturing of other reinforced composite structures such as ships, yachts, cars and trucks its use is common as well^{[3] [7] [14]}.

Reliable data on the number of workers at risk at a European level, or the number of occupational diseases arising from exposure to epoxy products are not available. In the Dutch construction industry it has been estimated that one out of every five workers that frequently use epoxies (e.g. floor layers) develops an epoxy-allergy at a certain moment in their career^[3]. Data from the Finnish Register of Occupational Diseases 2005–2016. show that exposure to epoxy compounds is the leading cause of occupational skin diseases among construction workers (37%)^[15].  In Germany, it has been estimated that around 10% of construction workers' occupational allergies are due to epoxy systems^[16].

Over the past few decades, the wind energy sector has grown in importance, leading to an increase in the number of workers exposed to epoxy resins. A study among workers in a Danish wind turbine blade factory during 2017–2022 showed that showed that workers exposed to epoxy resins had a significantly higher risk of dermatitis compared to non-exposed workers^[17].

Risk assessment at the workplace

Council Directive 98/24/EG (‘Chemical Agents Directive’) contains minimum requirements concerning the assessment of risks due to exposure to hazardous chemicals at work^[18]. Employers have to make sure that the hazards of substances and products used are identified, that the exposure is assessed and that these two aspects are combined into a risk assessment, which enables the selection of the proper risk management measures.

Hazard identification

In order to enable proper risk assessment and control, companies and workers will have to recognise the products they work with as being epoxy products. Whenever a two- of three-component product is used, one should be aware that it might be an epoxy product. In that case, the product label will state “This product contains epoxies". Technical data sheets and the safety data sheets should be consulted as well. Generally, the product labelling may warn the worker as well (figure 4).

Figure 4a: Labelling of epoxy products	Most common labelling of the epoxy resin component: H319 Causes serious eye irritation, H315 Causes skin irritation, H317 May cause an allergic skin reaction.
Figure 4b: Labelling of epoxy products	Most common labelling of the hardener (in most cases a polyamine) component: H314 Causes severe skin burns and eye damage, H317 May cause an allergic skin reaction.

In addition, when relatively volatile polyamine or acid anhydride hardeners are still used, products may be classified with H335 (May cause respiratory irritation) and H334 (May cause allergy or asthma symptoms of breathing difficulties if inhaled).

Exposure of the skin

As dermal (skin) exposure usually is a major risk when epoxy products are used, the employer should carefully consider all tasks that may bring about skin contact and implement adequate measure to eliminate or, if not possible, minimise exposure (see below). Measures at source and measures that enable a safe use such as safer packaging and proper mixing aids should be implemented whenever possible. In practice, workers appear to find it hard to fully avoid skin contact with epoxies during work. In the construction industry as well as in ship, yacht, windmill blade and car manufacturing, large quantities of epoxy products may be applied to large surfaces. Obvious moments of skin contact include:

• Weighing and mixing the two components: splashes and spillages, contaminated packaging, mixing at a too high rotation speed, in too small vessels etc.;
• Manual transportation of (open) cans or vessels with epoxy products;
• Application: direct contact during application by means of spatulas, trowels, glue spreaders etc., as well as spatters during application of coatings by means of rollers.
• Cleaning of tools, and disposal of empty packaging;
• Continued wearing of contaminated or even soaked clothes or shoes.

Exposure through inhalation

Even though modern epoxy products contain low-volatile diluents and hardeners in most cases, employers and workers should be aware of situations in which exposure through inhalation may be relevant, such as:

• Situations in which solvent-based epoxy products are still used;
• The use of epoxy products that (still) contain hardeners or reactive diluents that are relatively volatile. As a ‘rule-of-thump’, substances that have a boiling point lower than 250°C are relatively volatile. Consult the Safety Data Sheet;
• The use of epoxy products in confined spaces;
• Spraying of epoxy products.

Indicative measurements for comparison with Occupational Exposure Limits or the use of risk assessment tools may support the assessment and management of risks through inhalation.

Prevention

Industrial products that contain epoxy resins – such as car parts – may be produced in closed systems that largely prevent exposure to the non-cured product. Obviously, leakages of the closed-system, maintenance or cleaning of closed systems may give rise to exposure. During such activities, personal protective equipment and protective clothing may be needed. Because these activities occur irregularly and are generally of relatively short duration, these measures may be appropriate in this case.

However, in many ‘open’ applications, such as those on construction sites, windmill blade production and the manufacture of yachts, frequent exposure may occur if adequate prevention measures are not in place. Measures to prevent negative consequences such as allergic contact dermatitis and airway disease should be implemented according to the ‘prevention hierarchy’. The following paragraphs follow this hierarchy of measures.

Measures at source

Before deciding to use an epoxy resin-based product one should consider whether it is possible to apply alternative materials and techniques.

When epoxy products cannot be substituted, it may be possible to apply an epoxy product that is relatively less harmful. Ingredients that are less volatile or less capable of penetrating the skin may reduce the risk of harmful effects. In order to identify relatively less harmful epoxy products, one should consult the Safety Data Sheet of the supplier. 

In Germany, a list has been compiled with possible ingredients of epoxy resin systems and their skin-sensitising properties^[19]. For each of the substances, the list indicates whether the substance is a strong sensitiser, a weak sensitiser or an unknown sensitiser. The list is therefore a useful guide to the selection of less harmful epoxy resins.

Technical measures

If elimination or substitution of epoxy products is not possible, technical measures should be implemented that enable safe handling and reduce skin contact as well as risks of inhalation. These include tools, packaging, machines, etc.:

• Preferably, choose ‘epoxy kits’ with a well-defined mixing ratio for the components, that are fully consumed during one application (‘one unit’ packaging). This prevents the risk of erroneous mixing, which increases health hazards (figure 5);
• Mixing of larger amounts of epoxy product should be done by means of closed, forced action mixing machines (figure 6);
• Hand mixing should be done with the help of a long-stemmed mixer with a continuously variable mixing speed. The maximum-diameter of the mixer blades is one-third of the diameter of the can or bucket in which mixing takes place. Mix with a low rotation speed (< 300 rpm) and cover the vessel while mixing;
• The transportation of mixed material from the mixing station to the application station can be best performed by means of a small trolley. This may be used for pouring the product (especially cast floors) as well;
• Apply floor coatings with long-stemmed rollers or with long-stemmed spreaders. These methods allow working ‘standing-up’;
• A splash protection shield (splash guard) attached to the roller prevents spattering;
• When epoxy resins are injected, such as in concrete repair, use two-component injection pumps (closed, automated mixing), take care when positioning filler necks so that they will not weep during application, provide ventilation openings, ensure packers are free of dust, debris, etc., and use couplings that cannot be pulled out;
• Use disposable tools (rollers etc.) where possible. This reduces the need for cleaning.

Figure 5: One-unit packaging that supports closed mixing

Source: Copyright by MC Bauchemie (Germany), reproduced with permission by the copyright holder

Figure 6: Closed mixing vessel for mortars or 2-pack products

Source: copyright by Carat-tools (the Netherlands), reproduced with permission by the copyright holder

Organisational measures

Prior to starting work with epoxies, the employer should provide training to workers on the hazards and risks and on control measures that should be taken. Workers should practice the use of protective clothing and personal protective devices.

The employer should provide for a separated mixing area which is clearly marked, so that co-workers not involved in the work involving epoxies can be protected from exposure. Mixing vessels should be placed on an even level surface. Contamination of the surrounding can be prevented by placing the bucket or container on a foil sheet on an even surface.

Personal protection

In addition to the measures mentioned above, the following personal protection equipment remains necessary when using epoxy products:

• Safety goggles. While spraying, or when splashes may occur, use a face shield;
• Protective gloves made of nitrile or butyl rubber. Change gloves at every end of shift, and immediately when the inside of the glove is contaminated.
• Respiratory protection when solvent based epoxy products are used and when spraying all types of epoxy products.
• Protective clothing. When mixing and spreading by long rollers, spatulas etc. a protective trouser leg may be sufficient. When spraying, use a protective suit.

Early detection and monitoring of work-related diseases

Early detection of skin or airway problems may prevent the development of more severe disorders such as allergic contact dermatitis or asthma. Irritant contact dermatitis may enhance the penetration of allergens through the skin, increasing the chance of developing an allergic contact dermatitis. For the early-detection of health disorders, one may use:

• standardised questionnaires or interviews (e.g.^[20]);
• clinical investigations of the airways (lung function tests) or the skin by the occupational physician, dermatologist or lung specialist.

In order to facilitate early detection of skin disease, so-called ‘pictionnaire’ questionnaires have been developed, which use photos of affected skin^[21].

After finding cases of sensitisation, skin or airway disease, the effectiveness of control measures in place should be evaluated. When the origin of the problem is found, adequate prevention measures should be implemented and their effectiveness should be evaluated. Reporting schemes of incidents and health problems within the company – in addition to any reporting obligation to national occupational disease registries – may increase the companies’ capacity to trace any remaining risks not tackled yet.