How to Choose the Right Polyurethane Hot Melts for Electronics Assemblies


The #electronics market is rapidly expanding, and there is a high demand for adhesives for bonding different parts and components during the electronic assembly process.  #Structural_adhesives are being utilized more and more in place of mechanical fastening methods and tapes due to their high strength, ease of use and low total cost of ownership.  In particular, Polyurethane hot melts are becoming one of the most popular adhesives selected by electronics manufacturers, owing to their wide range of benefits.  These products can be found in almost every consumer electronics product today including smartphones, wearable devices, automotive touch panels, tablets and notebook computers.  Among many other benefits, PUR hot melts provide: 

  • Very fast fixation for high speed production
  • Single component formulations allow for flexible dispensing methods
  • Excellent adhesion to variety of porous and non-porous surfaces
  • Good gap filling capability for greater tolerances in part design
  • Ability for thin bond-line or dot dispensing in micro applications
  • Excellent moisture and chemical resistance
  • Excellent durability and impact resistance for tough yet flexible bonds
  • Stable formulations with good shelf life

Applied in a molten state, these solvent-free, 100% solid adhesives solidify and crystallize on cooling, providing green strength and instant fixation on the parts being assembled.  Over the course of the next several days, PUR hot melts further react with ambient moisture to increase in strength and environmental resistance until a fully cured, robust bond-line is created. Since they are available in different viscosities, colors, adhesion packages and impact properties, among several other specifications, choosing the right polyurethane hot melt holds a great significance. This blog lists a few things to consider when choosing polyurethane hot melt adhesives for electronic assemblies.

Factors to Consider When Choosing Polyurethane Hot Melt Adhesives 

Although most types of polyurethane hot melt adhesives share similar qualities, they differ between one another in curing time, viscosity, cost and flexibility, among others. The following are some of the things to consider during the selection process. 

  • Colors: Do you know hot melt adhesives featuring polyurethane are available in various colors to match the application requirements? White, transparent, black, gold, yellow, silver, gray, and blue are common colors. However, you need to check with the manufacturer, if these adhesives can be customized in any other color of your choice. Many manufacturers also provide satin and glossy finishes of these adhesives, which are widely preferred in wearable devices for clean aesthetics and bond-lines.
  • Check Whether the Chosen Product Adheres to your Substrates: Every hot melt adhesive is not made the same and they may not fit well with your application. Products have different adhesion to porous or non-porous surfaces, various plastics, glass, metals ceramics or composites. Choosing the best product that adheres well to your surfaces should be your primary concern. Hence, before making a purchase, it’s always a good idea to contact an application specialist for testing and validation.
  • Electrical and Thermal Properties: Basic adhesive formulations are generally considered to be electrically and thermally insulative.  Some applications, however, require increased thermal conductivity to remove heat away from sensitive components or especially insulative electrical properties to not interfere with performance of transmitting devices. Chemence® has developed several specialty PUR hot melts that can provide thermal conductivity or low dielectric constants to suit your application, but some performance tradeoffs may exist such as longer open time or lower overall adhesion. 
PUR Hot Melt adhesives are typically dispensed using precise valves for electronics applications in highly automated production lines.  The fast fixture time of PUR Hot Melts allows for high production throughput and enables mass production of consumer electronics devices
  • Application Methods: As mentioned earlier, hot melt polyurethane adhesives are available in a varied range of rheologies, making the application method and processing parameters different from one application to another. Once an adhesive is selected that will work from a performance perspective, it is important to fine tune the dispensing and curing process to optimize long term performance. Will the adhesive be sprayed? Dispensed as a bead?  Jetted through a specialty valve?  Working with our technical experts to optimize rheology and dispensing parameters is critical for success in your dispensing system and ensure that stringing, clogging and other issues do not arise.
  • Processing Requirements: The recommended application temperature for polyurethane hot melt adhesives usually falls between 85°C to 140°C (185°F to 284°F). Also, the adhesives are manufactured with open times varying from 10 seconds to over 10 minutes. Some substrates such as aluminum have high thermal conductivity and act as a heat sink when a PUR hot melt is applied – decreasing open times significantly.  All these process requirements must be closely examined before making a final adhesive selection. 
  • Green Strength and Curing Times: PUR adhesives have different stages of curing. Once dispensed the adhesive has a specific open time that allows for mating of parts.  Upon cooling a certain amount of green strength is obtained that fixtures parts together.  Over the next several days the PUR hot melt will react with ambient moisture to further crosslink and build final adhesive properties such as environmental resistance, impact strength and full adhesion strength. Some adhesive formulations may form strong green strength bonds in a few seconds, whereas some act more as gasket materials and remain soft, reworkable and tacky similar to a pressure sensitive adhesive. Hence, it is important to concentrate on their curing times while selecting the adhesives for your application. 

Traditionally, PUR hot melt adhesives have been used in high volume, low cost applications such as panel laminating, book binding, woodworking and furniture manufacture and even kitchen sponge assembly. These products have stood the test of time because of their ease of use and durability in these applications.  Chemence® has taken the formulation of PUR hot melts a step further by customizing these products to fit the rigorous requirements of electronics manufacturers while maintaining the important benefits that have been outlined above with regards to performance and ease of processing. Our Polyurethane hot melts for electronics are available in syringes and cartridges to fit many existing electronics production lines.  Get in touch with Chemence®, a market leader in the development of polyurethane hot melts for the electronics industry.  Our KRYLEX® polyurethane hot melts come in a variety of formulations to meet most of the requirements discussed in this article. Contact us and we will be more than willing to work on customizing a formula specifically for your needs.

Growing Significance of Adhesives in Wearable Devices

With the advent of wearable technology, devices such as smart watches, sleep trackers and heartrate sensors have become a part of our life. In addition to fitness and wellbeing, these wearable electronics find many applications in medicine and healthcare, and they have tremendously helped in improving the quality of life for many people globally. Adhesives are integral to wearable technology and we can find them in almost every device on the market including medical patches, smart watches, activity trackers, emergency call buttons, virtual reality (VR) devices, health monitoring devices and more. Because of the critical nature of assembly adhesives and sealants in these devices, research and development has grown significantly in this area in the recent past to ensure that products not only perform well over their lifespan but that they also don’t negatively interact with the body wearing the device. This post focuses on the uses of adhesives and sealants in wearable devices, their significance in the medical and healthcare industries, and what kind of materials are used to make them to ensure safe user interaction.

The use of adhesives in consumer electronics – wearable technology.

Use of Adhesives and Sealants in Wearable Devices

Adhesives and sealants are used in wearable devices in many ways – on printed circuit boards to fix components, for environmental protection and waterproofing, in assembly of sensors componentry, for thermal and electrical conductivity and/or insulation and even fixturing onto human skin.

  • Adhesives Used in Wearable Device Assembly:The PCB assemblies for wearable devices have many small, intricate components. These components are fixed using adhesives and protected from the environment with coatings.  Board level adhesives tend to fall into epoxy, silicone, acrylate or urethane chemistries and are formulated based on the requirements of the specific application. Some of the board level applications could include micro-electro-mechanical systems (MEMS), electromagnetic interference (EMI) shielding, die and chip attachment and flexible printed circuit (FPC) reinforcement.  Usually these adhesives can offer some or all the following benefits:
    • Heat dissipation
    • Stress absorption/reduction
    • Vibration, shock and impact resistance
    • Electrical insulation
    • Chemical resistance
    • Structural adhesion

Assembling and sealing the enclosures within wearables and the device itself is also generally accomplished with adhesives and sealants. Cover glass bonding and enclosure/lid attachment is generally accomplished with structural adhesives that form strong, structural bonds between substrates while also providing excellent chemical resistance, superior shock and impact resistance and often these adhesives and sealants are developed to be reworkable for ease of repair should components break from wear and tear in the device.

Adhesives used in the assembly of wearable devices generally can be manually or automatically dispensed and curing can be accomplished via moisture, UV light, thermal exposure, mixing of two components or any combination of the above.  Many adhesives developed for these applications have superior adhesion to common substrates such as FR4, FPC, metals, glass, ceramics and plastics. 

  • Adhesives Used for Fitting on Human Skin: These adhesives come in intimate contact with skin and are formulated and tested to be extremely safe for human use. They are formulated to be moisture, sebum, saliva and sweat resistant, so that these fluids do not come in the way of health monitoring such as pulse rate and activity level calculation. It is very common that these adhesives are latex free as some people may be allergic to it. Rubber-based adhesives tend to be used if the device is meant to be worn for a short period of time. Acrylic-based adhesives are generally meant for longer-term use as they tend to have higher adhesion and environmental resistance properties. These adhesives are generally formulated to be dispersed onto a film backing creating a tape which in turn is used to adhere to the body.

Benefits of Adhesives in Wearable Devices

The medical industry today heavily relies on electronics and technology for improved, quick, and non-invasive healthcare. This calls for the use of safe, biocompatible adhesives to fit these devices on human skin for various purposes such as continuous monitoring of certain physical parameters, generation of medication alerts, and disease prevention. Thus, the demand for wearable adhesives has surged over the last few years and they have special importance in the medical and fitness industry. These adhesives have transformed the devices in the said industry in the following ways:

  • Adhesives are being formulated to be safe and biocompatible for use on a human body.
  • They are crucial to the effectiveness and accuracy of the device to give the right output – especially in sensor assembly.
  • Adhesives used in attachment of medical devices are thin and breathable and can easily hide behind clothes.
  • Replacement of mechanical fasteners helps devices become more lightweight and slimmer.
  • Replacement of welding removes possibility of damaging sensitive componentry.
  • Replacement of solvents allows for more environmentally friendly assembly methods.

Applications of Adhesives in the Medical and Fitness Industry

Adhesives are widely used in the following applications of medical and fitness industry:

  • Heart monitoring devices
  • Temperature monitoring patches
  • Glucose monitoring devices
  • Transcutaneous electrical nerve stimulation (TENS) electrode for muscular pain relief
  • Arm bands
  • Special shoes that help prevent injury
  • Smart watches
  • Fitness trackers

For over 30 years, Chemence® has formulated and manufactured adhesives and sealants for industrial use, electronics assembly and consumer use.  Chemence® Medical also develops and manufactures Class 2 and Class 3 adhesive-based medical devices in its ISO13485 accredited facility in Georgia. Chemence’s line of KRYLEX® adhesives and sealants have been developed leveraging our years of experience in electronics and medical adhesives.  If you are a manufacturer of wearable devices and you require assistance in designing and manufacturing of your device, Chemence® is here to help.

Beginners Guide to Biocompatibility for Medical Device Adhesives

Adhesives are used in most industries, including the medical industry, to assemble components of finished devices.  Adhesives can provide value over traditional joining methods such as mechanical fasteners, ultrasonic welding and solvent welding by reducing hazardous solvent usage, distributing stresses more evenly or even increasing production throughput.  In the medical device industry, particular care is taken to formulate adhesives for biocompatibility – to make them safe for use on or in the human body. 

As defined by the FDA, a medical device is an instrument, machine, device, implant, or contrivance that is used to diagnose or treat a disease. Given the critical nature of medical devices, the FDA requires these devices to meet strict biocompatibility standards. Generally speaking, the FDA and other regulatory bodies do not require individual components of the finished device to carry any specific approvals. However, medical device manufacturers choose to use various biocompatible components including adhesives during the assembly for increased confidence that their products are reliable and safe. What are biocompatible adhesives and how do they differ from regular industrial adhesives? Read this post to find the answers. 

biocompatible adhesives, Medical Adhesives, KRYLEX, needle bonding
Needle Bonding – biocompatible adhesives

A Brief Introduction to Biocompatibility 

Biocompatibility refers to the compatibility of a material with a living system or living tissue, as many medical devices are used for internal and external examination and treatment. Biocompatible materials produce no toxic response when in contact with any body part or tissue. These materials are subjected to a variety of rigorous tests to confirm their compatibility towards bodily fluids or body parts. 

A Discussion on Various Types of Biocompatibility Tests Performed on Adhesives

A medical adhesive differs from other industrial adhesives because it has been tested to certain standards and deemed as biocompatible at the end of the tests.  The two main testing standards used today include USP Class VI Biocompatibility Testing and ISO 10993 Biocompatibility Testing. 

  • USP Class VI Biocompatibility Testing: This testing is devised by the U.S. Pharmacopeial Convention (USP) that regulates standards for healthcare technologies, medications, food ingredients, polymers, as well as plastics used for building medical devices. The materials used in a product are distinguished into classes on the basis of their proximity with the human body and contact time. This contact time is taken as limited, prolonged and permanent on the basis of the product used. Adhesives tested by verified third parties and found to adhere to this classification are considered safe to use for medical device assembly. 

USP Class VI testing is comprised of the following three evaluations: 

  1. Implantation Test: In this testing, the material is implanted into the intramuscular tissue of the specimen. Toxicity, irritation, and infection response caused by this implantation is then measured over five days. 
  2. Acute Systemic Toxicity Test: The material or compound is orally administered, inhaled, or applied to the outer skin of the specimen. Toxicity and irritation response on the specimen is measured over several days.  
  3. Intracutaneous Test: The material is kept closer to subdermal tissue or the tissue to be accessed by the medical device. Toxicity and localized irritation produced by this contact in the specimen is measured and observed for several days.

The material extracts under evaluation for Toxicity and Intracutaneous testing are fixed at several time and temperature conditions.  The tests are administered and measured at 122°F for 72 hours, 158°F for 24 hours and finally at 250°F for one hour.

Adhesives that qualify USP Class VI biocompatibility testing are considered safe for bonding and assembly of medical devices.

  • ISO 10993 Biocompatibility Testing: In recent years USP testing has been superceded by the more robust ISO 10993 testing standards. These testing standards are developed by the International Standards Organization (ISO), which aims to standardize the evaluation of materials worldwide.  Like the USP standard, the ISO standard also distinguishes medical devices on the basis of their body contact and contact duration. The ISO 10993 series consists of over 20 standards that guide the biocompatibility testing of medical devices and components. ISO 10993 biocompatibility testing that is generally applicable to adhesives includes the following:
    • ISO 10993-4 Hemolysis
      • Selection of tests for interactions with blood
    • ISO 10993-5 Cytotoxicity
      • Tests for in vitro cytotoxicity
    • ISO 10993-6 Implantation
      • Tests for local effects after implantation
    • ISO 10993-10 Intracutaneous: Sensitization and Irritation
      • Tests for irritation and delayed-type hypersensitivity
    • ISO 10993-11 Systemic Toxicity
      • Tests for systemic toxicity

Generally, the minimum test method used in determining biocompatibility of an adhesive is ISO 10993-5 Cytotoxicity test. This test helps confirm if the adhesive has any negative impact on cells of mammals. It is important to note that the ISO 10993 standard is similar in many aspects to the methods described in the USP standard. However, the main difference is these methods from the USP methodology is the rigorous testing strategy. Medical adhesives testing to and found to pass ISO 10993 methodology are used in a wide range of medical devices including catheters, needles and syringes, connectors and tubes, imaging equipment, blood filtration equipment, smart health devices and wearable technologies. 

If you are a medical device manufacturer, it is important that you source biocompatible adhesives that are USP VI or ISO 10993 certified from a trusted supplier or manufacturer. The KRYLEX® range of medical adhesives have been developed specifically with biocompatibility in mind while maintaining the robust performance and quality associated with traditional industrial adhesives. Owing to their strong and reliable bonding characteristics, speed of cure, exceptional tensile properties and strict quality standards, these adhesives have emerged as an efficient, user-friendly, and easier alternative to traditional fasteners and welding.


KRYLEX distributor's logo

Chemence® Ltd is pleased to announce the appointment of Premier Farnell plc as stockist & distributor for 28 carefully selected products from our KRYLEX® range.

Premier Farnell (part of Avnet, Inc) is a leading high service, multi-channel distributor of electronic and industrial products throughout the UK, Europe and Asia Pacific.

The listing of KRYLEX®products has been fully supported by onsite training of Premier Farnell’s technical sales team to guide and direct the selection of the most suitable KRYLEX® adhesive or sealant for any given application.

Premier Farnell aim to consolidate and reduce their supplier base for adhesives and sealants and were looking for a reputable manufacturer to replace several competing brands across a number of categories. They chose KRYLEX® due to the breadth of the range and the technical support that Chemence® can offer.

There are plans to further extend the range of KRYLEX® products offered throughout 2019.

Together, we are confident that the combination of the technical expertise of Chemence® combined with the global reach of Premier Farnell, will create a platform for further growth for our KRYLEX® adhesives and sealants range. 

UV-curing adhesives see the light

Adhesives require a chemical reaction to convert their structure from the liquid to the solid state. Once cured, the adhesive provides a high-strength bond between two substrates, which resists temperature or humidity changes and is impervious to many chemicals.

There are several ways to achieve the reactive curing process, such as light, heat, moisture or the combining of two reactive components. A UV-curing adhesive contains photo-initiators that start the chemical reaction when exposed to light of the appropriate wavelength and intensity, usually wavelengths of 250 nm to 400 nm. The UV-curing mechanism can be applied to several types of chemistries; there are UV-curing acrylics, epoxies, silicones and cyanoacrylates to name a few.

Strengths and weaknesses of UV-curing adhesives

Like any curing process, UV curing has its advantages and disadvantages.

The UV-curing process is flexible. Since it depends on the intensity of the UV light, the curing time can be controlled and can be as fast as a few seconds at high intensity or much longer at lower intensities.

ASTM tensile testing dog bones of Krylex KU9980-M fluoresce under UV light.
Figure 1. ASTM tensile testing dog bones of Krylex KU9980-M fluoresce under UV light. Source: Chemence

UV-curing adhesives are typically single-component formulations requiring no mixing, and generally low-viscosity formulations with minimal solvent content. The uncured adhesive has a relatively long shelf life, sometimes up to 24 months, and is easy to handle as long as appropriate precautions are taken to minimize ambient light.

Some UV-curing adhesive formulations contain fluorescing agents that will only shine when the product is fully cured. These allow manufacturers to perform in-line visual inspections of their processes, ensuring that bondlines are completely sealed between two substrates.

After curing, UV-curing adhesives are non-toxic and suitable for a wide range of applications, such as medical-grade adhesives to assemble needles and catheters, conformal coatings to protect sensitive electronic components or deep curing formulations used for encapsulation and potting of components.

UV-curing adhesives can be formulated with a secondary moisture-curing mechanism for use in shadowed areas that aren’t reachable by the UV lamp. The moisture comes from the air and best performance is achieved by combining both methods to fully crosslink the adhesive matrix. Dual-curing adhesives fixture rapidly after UV exposure and continue to build strength and environmental resistance as the adhesive cures over the course of several days.

On the other hand, there are a few disadvantages and limitations. Curing requires a light source that emits UV light at a wavelength that matches absorbance of the photo-initiator of the adhesive. All adhesives do not operate at the same wavelength. The source can be a mercury vapor lamp, a UV fluorescent lamp or a UV LED, but it represents an additional cost compared to other adhesive technologies. The light output slowly degrades as the lamp ages, increasing the curing time and eventually requiring replacement of capital equipment.

Unless the adhesive includes a secondary curing mechanism, the UV light from the lamp must illuminate the entire bond area; shadowed areas will not cure. It is important to note that not all substrates are usable with UV-curing adhesives. It will only be possible to cure the adhesive through materials that transmit light at the specific wavelength of the lamp. UV and visible light can only reach a certain depth, with the limit generally around 10 mm.

It should also be noted that exposure to UV light can be harmful to human operators, who need appropriate eye and skin protection.

Chemence® has developed UV-curing formulations that combat a majority of these issues. Broad-spectrum cured adhesives contain photo-initiators that respond to a range of wavelengths with a bell-shaped response curve over the 250 nm to 400 nm range. A medium-pressure mercury arc or metal halide bulb lamp provides optimum curing. LED optimized formulations have also been developed to work specifically with curing systems utilizing LED light sources in the 365 nm to 405 nm range.

A variety of formulations is available to provide tack-free curing – eliminating the potential for oxygen inhibition, which causes a permanent surface tackiness on adhesive exposed to the atmosphere. Chemence® has also developed several dual-cure formulations that rely on light for initial cure and then achieve full cross-linking after exposure to heat or moisture.

About Chemence® UV-Curing Adhesives

Chemence® has optimized an extensive range of UV-curing adhesives for bonding, tacking, sealing, coating, encapsulating, laminating and potting. The KRYLEX® UV-curing product line includes both acrylic and cyanoacrylate adhesives, with formulations for high-strength, tack-free, dual-cure and fast-bonding applications.

Chemence® also has a full line of UV-curing adhesives for medical devices, including needle, catheter and tube set assemblies under its KRYLEX® brand.

Making it stick: The many uses of cyanoacrylate adhesives

Krylex® Cyanoacrylate Adhesives blog
Figure 1. Cyanoacrylate: an adhesive for all seasons. Source:

Cyanoacrylate, commonly known as superglue, is a one-part adhesive that can form strong bonds with a wider variety of materials than most other types of adhesives. It can join plastics such as ABS, polyester, polycarbonate, PVC and nylon; metals such as zinc, brass, aluminum, permalloys and iron; woods such as maple, balsa, plywood and rosewood; rubbers; and many other materials.

In its liquid form, cyanoacrylate consists of monomers of cyanoacrylate: ethyl 2-cyanoacrylate methyl cyanoacrylate, n-butyl cyanoacrylate, and others. Each formulation has different properties to meet end users’ needs. After application, the adhesive cures through anionic addition polymerization: the monomers react to the presence of a weak base and form long chains that join the surfaces together. Moisture in the air initiates this reaction. 

Advantages and disadvantages of cyanoacrylate adhesives

There are many advantages to a cyanoacrylate adhesives. When applied between two surfaces as a thin film, curing takes place rapidly at room temperature, forming a rigid thermoplastic with excellent adhesion to most surfaces. Light handling is possible within seconds at room temperature; the bond is fully cured in 24 hours, with very high strength. 

The one-component formulation reduces manufacturing costs by eliminating a mixing operation. In addition, cyanoacrylates can easily be stored and transported in bulk since they contain no solvents. There are also two-component products that provide unique properties not available in single component products.

A cyanoacrylate adhesive does have a few weaknesses. It’s unsuitable for bridging large gaps or bonding large surfaces in a single application. Standard formulations have relatively low impact resistance, low elasticity, a distinct odor and relatively poor resistance to extended high temperatures. A cyanoacrylate has a limited shelf life after opening, and care must be taking in handling, since it almost instantly bonds to skin. 

Cyanoacrylates are available in a range of viscosities: from water-thin liquids to thixotropic gels. A wide variety of formulations has been developed for specific applications and to address drawbacks:

  • Cyanoacrylates with rubber added offer higher peel strength and impact resistance. 
  • Thermally resistant cyanoacrylates offer excellent bond strength retention; in some cases for thousands of hours after exposure to temperatures as high as 121° C.
  • Surface-insensitive cyanoacrylates offer rapid fixture times and cure speeds on acidic surfaces, such as wood or dichromated metals, which could prolong curing time.

Guidelines for effective use

Cyanoacrylates are quite effective in bonding small parts that fit well together. They are therefore widely used in the electronics industry and in the assembly of small mechanical components. 

Cyanoacrylates can bond a wide range of similar and dissimilar substrates, but achieving the best performance requires careful attention to detail. The surfaces to be bonded must be clean, although roughness is not as critical since cyanoacrylate adhesives form strong bonds even on smooth surfaces. 

The relative humidity is important and should be kept between 40% and 70%. A low relative humidity (less than 30%) increases the setting time; conversely a high relative humidity (greater than 80%) leads to extremely short setting time and causes shock polymerization that leads to shrinking of the adhesive layer, reducing the bond strength.Temperature also influences the time of the chemical reaction. The speed of many chemical reactions doubles for every 10° C increase in temperature, following the Arrhenius equation: for cyanoacrylate adhesives, this corresponds to a halving of the polymerization time. The optimal curing temperature is between 20 and 24° C.

 Applications for Krylex® Cyanoacrylate Adhesives
Figure 2. Applications for KRYLEX cyanoacrylate adhesives. Source:

Figure 2 illustrates some of the uses for KRYLEX® products. Chemence® also supplies consumer cyanoacrylates for multiple applications. Glues for DIY and home use are sold under the Dewalt®, Black and Decker® and Craftsman® brand names; liquid bandages are sold under the Liquidskin® brand; and a range of cosmetics cyanoacrylates are sold as fingernail glues and eyelash adhesives.

About Chemence®

Chemence® has developed well over 150 grades of cyanoacrylates, many under the industrial-grade KRYLEX® brand based on a variety of technologies that include ethyl, methyl, n-propyl, isopropyl, n-butyl, isobutyl and alkoxy-ethyl formulations. 

The Chemence® product family includes high-purity medical-grade adhesives, high-performance rubber-toughened impact-resistant products, surface-insensitive and high-temperature formulations, and glues with low-odor and low-bloom characteristics.