Why the choice of adhesive or sealant is so decisive in industry
In a workshop, on an assembly line or during industrial maintenance, choosing between an epoxy adhesive, a polyurethane sealant and an MS polymer sealant (also called silane hybrid) is never trivial. A poor choice can lead to premature joint failure, incompatibility with the substrate, accelerated degradation from UV or moisture, or even a risk to the safety of operators and equipment.
Yet these three families of products meet very different needs. They share neither the same chemistry, nor the same mechanical properties, nor the same application conditions. Before picking up a cartridge gun or ordering drums in quantity, it is essential to understand what fundamentally separates these technologies and in which contexts each one excels.
This guide is aimed at technical buyers, maintenance managers, project managers and all professionals who need to justify a product choice against a precise specification.
Epoxy adhesive: the benchmark for rigid structural assemblies
How it works
Epoxy adhesives are two-component systems (resin + hardener) that cure by chemical reaction. Once cross-linked, the joint is extremely hard and rigid, with excellent tensile and shear strength. It is the reference technology when you want maximum structural strength on fixed assemblies.
Strengths of epoxy
- High mechanical strength: ideal for bonding metals, composites, ceramics or concrete together.
- Chemical resistance: withstands oils, solvents, fuels and many aggressive agents.
- Heat resistance: some formulations withstand high temperatures in continuous service.
- Low creep: the joint does not deform under prolonged static load.
Limits to be aware of
Epoxy is rigid and brittle once cured. It copes poorly with vibration, repeated impacts and differential thermal expansion between two dissimilar substrates (steel and plastic, for example). On assemblies subject to movement, epoxy can crack and lose adhesion.
In addition, surface preparation is critical: substrates must be perfectly degreased, often sandblasted or shot-blasted. Mixing resin and hardener requires a precise ratio and controlled working time. Finally, most conventional epoxies yellow and degrade under prolonged UV exposure.
Typical industrial uses
- Structural bonding of metal parts in the workshop (fixing inserts, repairing cast iron).
- Assembling high-performance composites (aerospace, marine, sports equipment).
- Setting anchors in concrete.
- Filling and rebuilding worn or damaged parts.
Polyurethane sealant: flexibility and sealing under stress
How it works
Polyurethane (PU) sealants and adhesives cure by reacting with ambient moisture. They form an elastic, tough joint able to absorb movement, vibration and dimensional changes. It is a very widespread technology in the automotive industry, transport, construction and marine sectors.
Strengths of polyurethane
- Excellent elasticity: the joint deforms without breaking, which is valuable on assemblies subject to vibration or expansion.
- Very good adhesion on many substrates: steel, aluminium, glass, wood, concrete, some plastics.
- Abrasion resistance: PUs are often chosen for joints exposed to friction.
- Versatility: available in ready-to-use one-component formulations that are easy to apply.
3M™ 540 Polyurethane Sealant is a representative example of this family: formulated for demanding marine and industrial applications, it offers robust adhesion on varied substrates while keeping lasting flexibility, even when immersed or subject to significant thermal cycles.
In the assembly of vehicle bodies, structures or cabs, Sikaflex 252 polyurethane adhesive sealant is a recognised reference. It combines structural bonding and sealing, with enough mechanical strength for load-bearing joints subject to continuous vibration.
Limits to be aware of
Polyurethanes are sensitive to moisture during application: excessive humidity can cause bubbling in the joint. They often require an adhesion primer on some substrates (glass, some plastics). Once a cartridge has been opened, its life is limited after contact with air. Finally, conventional PUs can yellow and become brittle under UV if they are not formulated with suitable stabilisers.
Typical industrial uses
- Bonding and sealing glazing in commercial vehicles, buses and trains.
- Assembling body panels or light metal structures.
- Deck, hull or superstructure joints in marine applications.
- Fixing and sealing components subject to engine vibration.
MS polymer (silane hybrid): the best of both worlds?
What is an MS polymer?
The term MS polymer refers to a family of binders based on silane-modified polyether (or silane-terminated polymer). This technology, developed in Japan in the 1980s, has gradually won over European industry thanks to its balanced performance. It is also called a hybrid sealant, because it combines the elasticity of silicones and the toughness of polyurethanes, without their main drawbacks.
Like PU, it cures with ambient moisture, but without free isocyanates, which is a significant advantage in terms of operator safety and regulatory compliance (REACH, CMR).
Strengths of MS polymer
- Free of isocyanates and solvents: a safer formulation for operators and the environment.
- Excellent resistance to UV and weather: does not yellow or become brittle outdoors.
- Adhesion on difficult substrates: bonds to many materials without primer, including some plastics and damp surfaces.
- Paintable: accepts paint after curing, unlike conventional silicones.
- Good mechanical strength combined with real elasticity: ideal for assemblies subject to moderate movement.
3M™ 780 Hybrid Sealant illustrates these characteristics perfectly. Designed for marine, industrial and construction applications, it offers versatile adhesion on steel, aluminium, GRP, wood and concrete, with excellent outdoor durability and no UV degradation. It is a product of choice for professionals looking for a durable adhesive sealant on exposed assemblies.
For applications requiring a more general-purpose hybrid adhesive sealant, Stik MS60 hybrid adhesive sealant is a practical, effective solution. Its MS polymer formulation gives it good elasticity, primerless adhesion on many substrates and an ease of application appreciated on site and in the workshop.
Limits to be aware of
MS polymers do not rival epoxy in pure mechanical strength on heavily loaded rigid assemblies. Their chemical resistance is also narrower than that of epoxies against aggressive solvents. Finally, like any one-component moisture-curing product, they need a minimum of ambient humidity to cure properly, and curing can be slow in cold, dry weather.
Typical industrial uses
- Assembling and sealing facades, sandwich panels and industrial joinery.
- Bonding components on leisure vehicles, motorhomes and caravans.
- Fixing decorative or functional elements on structures exposed to UV.
- Expansion and sealing joints on outdoor structures.
- Industrial maintenance on varied substrates where surface preparation is limited.
Comparison: epoxy, polyurethane and MS polymer side by side
Mechanical strength
Epoxy largely dominates this criterion for rigid assemblies. Polyurethane offers mechanical strength combined with real elasticity, which makes it superior to epoxy on dynamic assemblies. MS polymer sits in a similar range to PU, with a profile slightly more oriented towards flexibility than raw strength.
Resistance to UV and weather
MS polymer is clearly superior on this point. Conventional PUs yellow and can become brittle without a specific anti-UV formulation. Epoxy is generally not recommended for direct UV exposure without protection.
Safety and regulations
Isocyanate-free MS polymer is the most favourable technology from a regulatory point of view. One-component polyurethanes contain residual monomeric isocyanates, subject to growing restrictions (EU Regulation 2020/1149). Two-component epoxy involves handling potentially sensitising resins and hardeners.
Ease of application
One-component PU and MS are the simplest to apply: no mixing, direct application with a gun. Two-component epoxy requires precise dosing and mixing, with a limited pot life once opened.
Adhesion without primer
MS polymer generally adheres to the widest range of substrates without primer. PU often needs a primer on glass or some plastics. Epoxy requires rigorous surface preparation in every case.
The most common mistakes when choosing an industrial adhesive sealant
Choosing epoxy by default "because it is the strongest"
This is a classic mistake. A rigid epoxy joint on an assembly subject to vibration or thermal movement will crack, and adhesion will be lost. In that case, an elastic PU or MS polymer will perform much better in the long term, even if its pure tensile strength is lower.
Ignoring compatibility with the substrate
Some plastics (PE, PP, PTFE) are very hard to bond without surface treatment or a specific primer. Applying any adhesive sealant without checking compatibility with the substrate inevitably leads to debonding. Always consult the product data sheets and, if necessary, carry out a preliminary adhesion test.
Neglecting surface preparation
Whatever technology is chosen, surface preparation is the number one factor in bond quality. Degreasing, removing oxides, drying: these steps are not optional. A joint applied to a contaminated surface will never hold, even with the best product on the market.
Underestimating the importance of application conditions
One-component moisture-curing products (PU and MS) need a minimum relative humidity to cure. In very dry winter conditions, curing can be abnormally slow. Conversely, excessive humidity when applying a PU can cause bubbling. Always respect the temperature and humidity ranges given in the data sheets.
Confusing a sealing compound with a structural adhesive sealant
A flexible sealing compound is not designed to carry mechanical loads. A structural adhesive sealant is not always suited to wide expansion joints. Clearly define the main function of the joint (bond, seal, cushion, fill) before choosing your product.
How to structure your selection process
Step 1: Define the mechanical stresses
Will the joint be subject to static or dynamic loads, vibration, impacts? Should it be rigid or elastic? This first question immediately points towards epoxy (rigid, static) or towards PU/MS (elastic, dynamic).
Step 2: Identify the substrates
Which materials need to be joined? Are they similar or dissimilar? Some substrates impose specific constraints (mandatory primer, particular surface treatment). Check compatibility in the data sheets of the products being considered.
Step 3: Assess the service environment
Will the assembly be exposed to UV, moisture, chemicals or extreme temperatures? A permanently outdoor joint points towards an MS polymer. A joint in an aggressive chemical environment may require a specific epoxy.
Step 4: Take regulatory and safety constraints into account
If your company is subject to CMR reduction obligations or your operators work in confined spaces, isocyanate-free MS polymer technology deserves particular attention. Consult the safety data sheets (SDS) for each product.
Step 5: Validate with a field test
Before any large-scale roll-out, carry out a trial on representative parts in real application conditions. A simple pull-off or peel test can confirm or rule out a theoretical choice.
Summary: which technology for which need?
In short, each technology has its preferred field:
- Epoxy: rigid structural assemblies, high static loads, chemical resistance, controlled environments.
- Polyurethane: dynamic assemblies, sealing under stress, vibration, marine and transport, such as 3M™ 540 Polyurethane Sealant or Sikaflex 252.
- MS polymer / hybrid: versatility, UV exposure, varied substrates without primer, operator safety, such as 3M™ 780 Hybrid Sealant or Stik MS60 hybrid adhesive sealant.
The underlying trend in industry is towards growing adoption of MS polymers, driven by regulatory changes on isocyanates and demand for safer, more versatile products. That does not mean PU or epoxy are obsolete: they remain irreplaceable in their preferred fields. But for many everyday maintenance and industrial assembly applications, MS polymer today offers the best balance of performance, ease of use and regulatory compliance.
If in doubt about the best choice for your specific application, feel free to contact our technical team: personalised advice based on your specification will help you avoid costly mistakes and ensure the durability of your assemblies.
Frequently asked questions
What is the difference between epoxy adhesive and polyurethane sealant?
Epoxy is a two-component system (resin and hardener) that forms a hard, rigid joint with very high tensile and shear strength on fixed assemblies. Polyurethane is a one-component product that cures with ambient moisture and forms an elastic joint able to absorb vibration, movement and thermal expansion.
What is an MS polymer sealant?
It is a hybrid sealant based on silane-modified polyether. Like polyurethane it cures with moisture, but without isocyanates or solvents. It resists UV without yellowing, bonds to many substrates without primer, can be painted once cured and stays elastic.
Why avoid epoxy on an assembly exposed to vibration?
Once cured, epoxy is rigid and brittle. Under vibration, repeated shocks or different thermal expansion between two materials, such as steel and plastic, the joint can crack and lose adhesion. An elastic polyurethane or MS polymer will last longer.
Which adhesive sealant should I use for an outdoor joint?
MS polymer is the best fit: it does not yellow or become brittle under UV and weathering. Standard polyurethanes can yellow without an anti-UV formulation, and epoxy is not recommended for direct UV exposure without protection.