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Polyurethanes and Epoxies are excellent bonding materials with a vast range of applications, however at times it can be difficult to understand the differences between each, especially when browsing our extensive catalogue. This blog post will outline the key differences between Polyurethanes and Epoxies with explanations of the varying product types related to them and their typical applications.

Epoxies are found in resin form with high mechanical and thermal properties. One of the most impressive features of epoxies is their versatility – a quality which has created a niche for this material in a number of industrial applications. Epoxies provide high strength bonds on a wide variety of substrates and work most effectively bonding glass, metal and wood. As such, characteristics include: a slow cure, honey-like consistency, the ability to hold outdoors, strong bond strength including high water resistance. In addition to this, the presence of thermal and electrical properties create an ability to be exposed to chemicals. Overall, this creates low cure shrinkage and temperature resistance up to 140°C wet and 220°C dry.

In contrast to epoxies, polyurethanes are polymers composed of organic units joined by carbamate links. Polyurethanes create superior bonds which require minimal surface preparation for high performance thermoplastics. As such, curing is achieved via a catalyst, through heat or by air evaporation. Polyurethanes work best bonding glass, metal, plastic and wood and have a number of key characteristics. These include a slow cure, honey-like consistency, the ability to hold in varied temperatures as well as strong bond strength. Finally, Polyurethanes can be exposed to chemicals.

Polyurethanes and Epoxies are commonly used as adhesives, encapsulants and conformal coatings, product groups that we will now further explore.

 

Adhesives

Polyurethane adhesives can be altered depending on the need, ensuring they are a versatile choice. Polyurethanes may be solid with little flexibility or entirely the opposite. As such, they provide an extremely strong bond ensuring the manufacturing process is quick and simple. Epoxy adhesives also provide extremely high strength bonds, but an advantage they have over polyurethanes is that they possess excellent water, chemical and heat resistance.

Henkel Loctite manufactures both polyurethane and epoxy adhesives. Their polyurethane adhesives provide a high strength two component solution with excellent chemical resistance and gap filling, as such they can be adapted to meet the needs of the industry they will be applied in. Henkel Loctite’s epoxy adhesives are extremely high strength creating rigid bonding, excellent gap filling and incredible chemical resistance. Manufacturing Brand Robnor ResinLab also offers an excellent range of electrical and electronic polyurethanes and epoxies, available from Ellsworth Adhesives Europe. 

 

Encapsulants

Polyurethane encapsulants consist of a balance of heavy-duty protection and suppleness, ensuring they are the material of choice for applications requiring expansion and abrasion resistance. Epoxy encapsulants can also be used for a vast variety of applications due to their ability to conform unaided, to a range of substrates.

Robnor Resinlab manufactures a series of Polyurethane and Epoxy encapsulants. Polyurethane encapsulants can cure in a wide variety of gel times and have high performance in low temperatures i.e. (-40C) which allows them to maintain a high degree of elasticity. On the other hand, theirEpoxy encapsulants offer exceptional resistance to high temperatures and work well when exposed to chemicals without causing harm to the environment. Resinlab manufactures a variety of standard potting epoxies and can create custom formulated epoxies to meet individual needs. Standard offerings include thermally conductive and flame retardant encapsulants.  

 

Conformal Coatings

Polyurethane conformal coatings offer excellent dielectric properties, with outstanding abrasion and chemical resistance to solvents. As such, Polyurethane conformal coatings are the most cost-effective and widely used coatings and are well suited to work within harsh chemical environments.  Epoxy conformal coatings are available as a single part or two-part compound. Single part compounds are cured by UV exposure or thermally. The two-part compounds curing process begins as soon as they are mixed. Epoxy Conformal Coatings provide an excellent thin protective film which creates resistance to humidity, chemicals and abrasives. If applied correctly their properties include excellent moisture, dielectric, temperature and chemical resistance. Ellsworth Adhesives Europe supplies a wide range of Polyurethane conformal coatings from Humiseal with excellent resistance to chemicals and humidity. Single component Polyurethanes are simple to apply and long lasting but may require very lengthy cure cycles to achieve full or optimum cure. Two component formulations can reach optimum cure properties in a mere one to three hours, however heat assistance is required. It must be noted that, when compared to single component formulations, two-component formulas can have a relatively short pot life sometimes making them strenuous to work with.

 

Final Thoughts

Polyurethanes and Epoxies are both excellent material choices, with each material boasting an attractive list of features and benefits. The choice between materials must therefore be based upon the user’s individual requirements.

Epoxy adhesives are generally the strongest category of adhesive. Epoxies come in the widest range of properties and can have the best overall properties on metals. On the other hand, polyurethanes require some amount of time for the cure to start after the parts are mixed. Room temperature storage is the shortest for Polyurethanes which are moisture sensitive. Polyurethanes are quite flexible but have lower strength in general meaning they are lower priced than epoxies and other categories of structural adhesives.

When choosing a suitable adhesive, consider end user requirements as well as the requirements of the process environment, for final selection testing and validation is always recommended. It must be noted that the bonding surfaces should be properly prepared to allow suitable application performance.

If you are still unsure if an epoxy or polyurethane is best suited to your application, don’t hesitate to contact ‘The Glue Doctor” who will be happy to analyse your application needs, end product and process requirements and recommend the most appropriate material.

 

Structural adhesives are of the highest strength, making them capable of holding and bonding parts together with chemical and mechanical surface attachment forces. Having the ability to withstand the toughest of conditions, structural adhesives have become increasingly popular with manufacturers who are shunning traditional fasteners in favour of the durable structural adhesive.

When compared with traditional fastening methods, such as rivets, welds and tapes, structural adhesives are more cost effective. Manufacturer’s using Structural adhesives eliminate the costs associated with metal preparation and finishing operations. Furthermore, structural adhesives have been formulated to improve manufacturing processes and final products for a variety of composite, metal and plastic assemblies.

Ellsworth Adhesives Europe supplies a range of structural adhesives manufactured by the Lord Corporation. The Lord structural adhesives range is diverse and consists of acrylic, epoxy and urethane adhesives with each category catering to different application requirements.

Chemistry Acrylic-based adhesives are very ‘aggressive’ and are primarily used to bond metals but also work very well on plastics. Acrylics require minimum surface preparation but should be tested for compatibility.

Urethane-based adhesives are another good choice for bonding plastics and are also very compatible with composites, wood and foam. It’s worth noting that urethane structural adhesives do not adhere well to metal without a primer or other coating, so they are generally used for metal bonding applications where the metal is primed, painted, powder-coated or e-coated.

Epoxy-based structural are generally very strong and chemically resistant and can be used
on metals, plastics, composites, concrete, wood and foam.

For the convenience of users, Lord structural adhesives are available in multiple quantity sizes – from convenience cartridge packaging to bulk packaging that includes gallons, pails, drums and totes for higher volume applications.

To make an enquiry about Lord Structural Adhesives, contact official Lord Distributor, Ellsworth Adhesives Europe via email: infoeurope@ellsworth.com or alternatively call 01355 321122.

Shop Structural Adhesives here. 

 

Modern technological advances have seen electronic systems integrated into nearly every facet of life and environment. Delicate electronics are no longer simply working in dedicated machine rooms, but are instead being carried around in our pockets, through deserts, and oceans. These environments present potential threats to the backbone of modern computing – the printed circuit board. Conformal coatings have been developed to combat exactly the kinds of threats most frequently experienced in different situations. They also add a level of protection to circuitry in critical applications like power plants or emergency services.

Conformal coatings – so named because they conform to the physical structure of their substrate, can be made out of a number of different materials including acrylic, silicone, urethanes, epoxies as well as parylene. New types of coatings, like synthetic rubber, are also being developed to meet specific demands. These coatings offer different types of protection as well as application methods/cure and ease of rework. A number of factors need to be considered when finding the right conformal coating. The first factor to take into account is the type of environment the compone
nt will be used in. Most components benefit from some coating because moisture and dust can affect systems in almost all environments.

Conformal coatings also offer varying levels of dielectric insulation to protect from internal or external shock. Aside from these everyday concerns, components in high-stress environments need special consideration. Determining the direct exposure to abrasion, heat and cold, and amount of mechanical stress and vibration will help narrow down the possible coatings. Reworking is important to plan for if the underlying components will need to be repaired. Coatings are usually removed for repair by abrasion, burning through, or use of chemical strippers – depending on the type.

 

Types of Conformal Coating

Acrylic conformal coatings are one of the most commonly used conformal coating bases for general use. They are relatively easy to apply, quick to cure without heat or shrinkage and are easy to rework when repairs are needed.

Urethane conformal coatings generally provide a better chemical resistance than acrylics, but that chemical resistance can also make them more difficult to remove for repairs. Cure time for urethanes are also longer, particularly with one-part formulations. Two-part formulations can reduce cure-time at the expense of shorter pot-time and the added necessity of mixing.

Epoxy Conformal Coatings offer excellent abrasion and chemical resistance, but can also be difficult to remove for repairs. They have a relatively short cure time, but care must be taken since shrinkage can occur with some formulations that could potentially damage delicate parts.

Silicone conformal coatings usually need to be removed by abrasion for repairs, but offer some unique advantages. These coatings are highly flexible, which makes them ideal in situations where operating temperatures are expected to reach up to 200C. They can also provide mechanical shock resistance for the same reason. They are ideally suited to the oil and gas industry because of their resistance to specific chemicals, as well as other high-temperature applications like automobiles.

Parylene is the widely used name for poly para-xylene. This coating offers exceptional protection from a number of elements, and provides the most uniform board coating. Parylene has also been FDA approved for use in medical applications. However, these advantages come at a cost – parylene coating requires specialty application using vapor deposition polymerization, and surfaces must be impeccably clean before coating. This coating is most often used in harsh environments or military applications.

 

Cure-types

Many of the types of coating can be cured with different methods, which offer different advantages and disadvantages. Solvent-evaporation curing is the simplest cure for acrylic, urethanes and epoxies, but can take a longer time – particularly with urethanes (upward of one week). These solvent-borne coatings can also be subject to VOC regulations. Water-based and other low-VOC or non-VOC coatings are available but are also more costly.

Heat curing, which can greatly shorten cure-times, is an option for some coatings, but requires a specialized setup for temperature control and is more prone to shrinkage that can damage delicate assemblies.

UV-curing conformal coatings afford the quickest cure times – in a matter of seconds – and can be a viable option when production speed is paramount. However, this method requires other factors to be taken into consideration, since shadowed areas of the PCB may not cure fully, and shrinkage can be an issue.

 

Other Considerations

Most conformal coatings are applied in one of two methods – dipping or spraying. When applying a coating, masking must be applied to the PCB where the coating is not desired. Dipping and spraying with selective robotic coating can lower the amount of masking required by utilizing specific component placement or designed application. Spraying by hand or over the entire board requires more extensive masking – as does the Vapor Deposition Polymerization (VDP) process. Brushing is usually not used for production, but can be used to repair some coatings. Altering the viscosity of a coating by the use of thinners can allow precise control over the thickness of coating.

Boards with intricate geometries or sharp edges need special consideration since coating these areas can be difficult. Dipping can provide adequate coverage with special care, but parylene also excels at coating these types of boards.

While conformal coatings protect against many kind of contamination and environmental risks, they can make components less resistant to fire. UL standard 764E measures the resistance of coated boards to fire and there are a number of coatings which meet the highest (V-0) standard.

Adequately checking that a coating has been applied in the desired thickness and coverage is critical. To make this task feasible, many coatings contain tracers that are visible only under UV light for inspection.

These coatings provide excellent general protection from a number of environmental factors, and individual formulations can provide specific safeguards against other stresses. However, they may be inadequate to protect some components. Potting with other compounds in conjunction with a conformal coating may be required. As always, determining what the components will encounter throughout their working life, as well as production needs, will help determine exactly what kind of protection is needed.

The content for this month’s blog post was provided by Gluespec®  – a partner of Ellsworth Adhesives.

Gluespec helps engineers to discover adhesives and other materials for their industrial applications.

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