Polyurethane belongs to the group of chemicals called reaction polymers, which also includes different types of epoxies. Some types of PUs are catalysts, while others have insulating properties. In this guide we will focus on the catalysts that aid in making chemical reactions faster. A polymer is made by joining organic materials with urethane (carbamate). Most PUs are thermosetting and do not melt upon heating, which is why they are used in PU adhesives.
Polyurethane as a material has a wide range of uses and applications, including insulation, wheels and tires, suspension bushings surface coatings, hard plastic and high-performance adhesives, which is our topic of discussion. The history of PU goes back to 1937 when it first appeared in the EU. Some common uses of PU adhesives include:
- Home repair & other DIY projects that require strong, long-lasting and weather resistant bonds
- Car interior
- Home construction
- Building boats
- Bonding panels
- Installing sub-floors
- Professional carpentry and hobbyist woodworks
Advantages of Polyurethane Adhesives
Being chemical cousins of epoxies, PU adhesives are structural adhesives and are extremely chemical and heat resistant, and made for jobs that require high-strength bonds. Because of their ability to form strong and long-lasting bonds, they are suitable for a variety of materials and projects ranging from DIY woodworking to home construction.
When used in woodworks, it’s the wood that usually gives up first than the adhesive, which also shows that PU adhesives are more long-lasting and stronger than white and yellow glues (PVA glues).
However, PVA glues are more suitable when gluing long grains together. PU adhesives are considered a better option (at least theoretically) when gluing wood with other materials like metal or gluing long-grain to end-grain and end-grain to end-grain.
PU adhesives are also a recommended option when moisture is high in the environment as they are weather resistant and work well in outdoor projects. That’s the reason PU adhesives and sealants are often the preferred adhesive for boats because they can easily tolerate some splashed and sunlight. Polyurethane glues can also be scraped and sanded easily for a smoother and neat finish.
Another advantage of PU adhesives is that specialized PU adhesives are available for different jobs. This is also what makes selecting the right PU adhesive somewhat difficult and can overwhelm beginners. Its chemical properties make is a multi-purpose, strong, weather-resistant and flexible adhesive for projects that require non-porous adhesive, which also cures quickly.
We have already covered some of the best PU adhesives as well different types of adhesives and top mixing equipment in separate posts. You can refer to these posts for more information and product recommendations.
Disadvantages of PU Adhesives
Despite its chemical properties and strength, there are certain disadvantages of PU adhesives that make other adhesives a better choice in certain jobs. As mentioned earlier, PVA adhesives form stronger bonds when you are gluing long wood grain with long grain.
Another disadvantage of PU adhesives is related to health and safety. Although for the most part PU adhesives are safe to use, users need to be careful during certain stages of adhesive application. In such cases epoxies are considered a safer alternative, but you still need to be careful when using any adhesive.
The ability to cure in a moist environment is an advantage as well as a disadvantage of PU adhesives. Leaving the cap off the adhesive container for too long can harden it, resulting in a complete waste. PU adhesive containers need to be properly sealed and usually remain useful for about a year.
PU Adhesive Application and Tips
Safety
It’s recommended to always wear a pair of quality protective gloves when using a PU adhesive as they can damage or irritate bare skin. Users also need to be careful about not breathing too much air when working with these adhesives and make sure the adhesive never gets in contact with their eyes.
PU adhesives are subject to CPO (Chemicals Prohibition Ordinance) and in many regions and cannot be sold to under 18s. Users also need to take certain safety measures into consideration when using these adhesive including:
- Wear safety glasses, protective gloves and a breathing mask when working with PU adhesives
- Protect your skin and avoid working with parts of your skin exposed
- Workers need to be extra careful to avoid contact with mucous membranes
- Some ingredients of PU adhesives are considered to be carcinogenic, so work in a well ventilated or if possible in an open space. Air the space after finishing the job
- Always read safety instructions and contact the manufacturer if you have any queries
- Remove the adhesive residue with warm water and soap immediately if it contacts any part of your skin, while moist adhesives can simply be wiped off using a cloth
- Hardened adhesives can be removed using a sharp object such as a razor blade, but extra care is needed when working with such objects
Storage
PU adhesives have to be stored in cool/dark places and in temperatures ranging from 50-77 °F (10-25 ° C). The containers/cartridges also need to be protected from cold, wet and moist environments. It’s recommended to process old adhesives first before moving on to new cartridges and periodically check the packaging for any damage
The cartridges must be sealed well otherwise the adhesive inside can harden over time and become unusable. Some regions require you to dispose off PU containers separately, so make sure to understand the local laws before working with these adhesives. Instead of disposing it off with the household waste, it’s recommended to deliver adhesive containers to a collection point
Clamping
In most projects you need to clamp the objects together for a strong bond otherwise the adhesive might expand and create foam. This foam can create weak spots because of which and you might be able to force the bonded materials apart by using some force. To overcome this drawback, you can either chose foam-free PU adhesives (expensive and rare) or clamp the objects together tightly
Single vs Two-component PU Adhesives
PU adhesives are available as single as well as two-component systems. Applying them involves a catalyst such as heat, light or moisture. PU glues can compensate tiny unevenness and can be painted after curation or sanded for a smoother finish. Two-component adhesives are further categorized according to the hardness level, but they all share some common characteristics including:
- Highly temperature and weathering resistant
- Shear strength, high lap and impact
- Highly resistant to aging and provide superior structural strength
- Can be used to join large gaps and provide excellent bridging capabilities
- Can be used to join large areas
- Simple to use and clean
- Work well for a variety of outdoor and indoor projects
- Can also be used to join natural stone & fire-protected elements such as gypsum plasterboards
Two-component aka 2K PU adhesives create a strong bond through reaction of two different chemicals or components, which in most cases is resin and hardener. Users need to be careful when mixing these two components as they have to be mixed in a specific ratio, which is usually 100:30 or 100:15 (follow the product instructions for mixing in the recommended ratio).
Single component or 1K adhesives usually come in a paste-like form and use moisture from air to cure. The curation happens pretty fast, usually within a few minutes. It is mostly used for joining leather, plastics, fabric and resin.
It comes premixed with the adhesive components so users don’t have to mix anything or take into account the mixing ratios. For small projects, workers can use a hand pistol to apply 1K adhesives. Advantages of 1K PU adhesives include:
- Simple and easier application
- Elastic and soft bonding
- High tensile strength
- Most 1K adhesives remain elastic even after curation and are suitable even for large areas
- Some specialized types can be used in projects in which contact with food is required
- Useful in a variety of applications, including flooring, parquet floors, wood, plastics, metal and gypsum boards
PU Adhesive Application
PU adhesives can be used for joining metal, wood, cork, plaster, plastic, cork and many other materials. But they work best when bonding different materials such as bonding parquet flooring or gluing floorboards. Time is an important factor to consider when using PU adhesives. Users have to work fast for better adhesion and cannot afford to lose too much time, which is something that requires some experience and expertise.
Applying a single component adhesive is fairly straight forward, but a few extra steps are involved in preparing and mixing two-component adhesives. The user first has to measure and mix the two components according to instructions by the manufacturer. Even tiny air bubbles can create weak points, so care is needed when adding air. Double cartridges make the process easier, but the total cost also goes up.
In most projects, applying a PU adhesive on one side in enough if the surfaces are brushed. Using too much of an adhesive can cause leakage at the joints. If that happens, you can quickly use a cloth to apply it evenly or you can use a sharp blade to remove extra adhesive after curation. PU adhesives cannot be used once their pot life is over so you might want to add an inhibitor to prolong their pot life.
The objects to bond should be free of dust and grease, and preferably slightly roughened with sandpaper. The adhesive has to be applied evenly and its recommended to start on one side before spreading it to the whole surface. Use screw clamps to tightly fix the surfaces and give the glue ample time to completely dry.
Polyurethane PUR Hot Melts
As the name suggests, PUR hot melt is a type of PU adhesive that is heated and applied using a dispensing equipment, usually a heated glue gun. They are solid at room temperature and return to this state after being heated and cooled.
Holt melt adhesives have their own advantages and disadvantages and are mostly used in assembly and manufacturing assemblies. The main advantages include it’s easier to dispense, can strongly bond a variety of materials and is cost effective.
Workers can also add additives as required to achieve desired results, including wax, antioxidants and plasticizers. The two main types of hot melts include PUR hot melts and EVA hot melts (‘reactivatable’ and recyclable).
What differentiates hot melts from regular PU adhesives is that hot melts are not considered a good option for porous substrates, and the drying process of hot melts is physical rather than purely chemical. The drying process involves cooling instead of chemical reaction. Some major advantages of hot melts include:
- Available in different forms including films, sticks and granulates
- Solvent-free
- Quick solidification
- More economical production
- Become functional almost immediately
- Usable on porous surfaces as well as non-porous substrates
- EVA hot melts enable workers to pre-coat because of their reactivation properties
- Joints remain elastic even after curation
- Can be used on a wide range of materials
- Bonded objects can be separated by applying heat
- Strength of bases can be increased by adding additives
- Easier transport and storage, usually come in air tight containers
However, their application is rather limited to materials that are temperature-resistant. Since reactive hot melts can be reshaped after curation because of being thermoplastic. This can be both an advantage and a disadvantage depending on the project requirements.
Common applications of PUR hot melts include plastics, woods, fabrics, leather and almost any other project that require superior bonding strength. The adhesive is becoming increasingly popular because of its versatility as it can also be sprayed to cover large surfaces as well as applied as a bead. Hot melts are available in different viscosities for different application requirements.
Compared to regular hot melt adhesives, PUR hot melts form a stronger bond and are essentially hot melts on steroids. After curation, regular hot melts can liquidate again when melted, but PUR hot melt reacts with moisture, changes chemically and becomes super strong after 24-48 hours. The same advantage can also turn into a disadvantage as these adhesives have to be packed in air-tight containers that should not be opened right before use.
How to Work with PUR Hot Melts?
PUR hot melts have the advantage of being a single-system adhesive, allowing you to easily apply using hot melt application equipment. You don’t have to mix or meter when using this type of adhesive, which can be applied at 85-140 °C temperature range. This characteristic also makes it suitable for projects that involve heat-sensitive substrates.
The adhesive reacts with moisture present in the substrate and air, forming a secondary PU polymer bond as it cools, which adds to the flexibility and strength. High viscosity hot melts become structurally more rigid, so choosing the right type of hot melt comes down to the job at hand.
Hot melts are usually applied using specialized glue guns that heat and melt the adhesive and release it when pressure is applied by finger. Spiral syringes and extruders can also be used apply the adhesive. Users have to apply it and join the objects quickly for maximum efficiency. Hot melts cannot be processed once they have cooled down and solidified.
PUR Hot Melt Applications
Being a single-system strong adhesive means it can be used in variety of jobs ranging from industrial applications to medium to large scale home projects. It’s an affordable option for woodworkers who need a versatile and efficient solution.
It is also the adhesive of choice for construction workers and remodelers with applications ranging from kitchens, bathrooms, wall cladding, hard seaming and vertical applications. Bookbinding, packaging, labeling and cardboards are some other applications. Hot melts can essentially be used on almost any material.
Workers have to start cleaning the application equipment as soon as the job is done. The adhesive cools down quickly and forms a strong bond in little time. The equipment can get damaged if left uncleaned for too long after application.
Conclusion
PU and PUR hot melt adhesives remain the preferred adhesive for a wide range of applications because of their superior strength and versatility. However, proper care has to be taken when applying these adhesives.
Users need to make sure to use and store them properly and clean the application equipment as soon as possible. Users also need to carefully choose the most appropriate type of PU adhesive as they are available in both single and two-part systems.
Each type its own advantages and disadvantages. PU adhesives are a little more expensive than hot melts, but their positive attributes are worth the extra price, including more applications and easier handling. Users have a very narrow time window to apply hot melts and clean the equipment, but the bonds are stronger.
In the end, it comes down to your own preferences and choosing between the two depends on your experience, project requirements and the working environment.