Troubleshooting Shrinkage in Urethane Adhesives and Encapsulants

In our last blog, the team at Resin Designs walked you through how to troubleshoot voids in urethane adhesives and encapsulants. This week, let’s explore how to properly troubleshoot shrinkage.

How Shrinkage Occurs

Shrinkage takes place in elastomers when polymers and catalysts react. Under normal processing conditions, shrinkage rates remain at 1-2%. Warping will occur with excessive shrinking which can lead to cracking – these cracks can grow into large surface cracks.

Causes of Shrinkage

Mismatched Resin & Mold Temperatures

The urethane reaction is exothermic which means once the material mixture starts to react the blend will heat up. Differences between the mixture and mold temperature will cause the material to cure at difference speeds depending on which interface the material is touching. This dissimilar cure speed leads stresses on the material. As the mixture changes from liquid to solid, it’s crucial that the two temperatures remain in balance. Gelling reactant will shrink if the mold temperatures are too low, often leading to cracking. When the temperatures are too high, there can be warping.

Localized Temperature Disparities

Internal stresses can occur due to localized hot and cold spots in the mold. This often leads to shrinkage, cracking and warping.

Solutions

Exotherm & Mold Temperatures Should Be Balanced

Peak exotherms are the highest reaction temperatures reached by mixtures and each polyurethane system has a different peak exotherm. Mixtures of a larger mass typically have a higher peak exotherm than smaller masses. The goal is to have the mold temperatures balanced within ±5°C of the peak exotherm.

Thoroughly Heat the Mold

Air in the oven must circulate so that the temperature is consistent throughout. Make sure that the mold remains in the oven until there is a uniform temperature.

Keep an Eye on Raw Material Temperatures

When degassed and mixed, raw materials tend to lose heat. They can also lose heat when sitting before mixing which means they need to be reheated so the correct temperature is maintained. Cracking can occur if the temperature is too high. It’s advised to use lower system temperatures and high mold temperatures.

 

If you need custom urethane adhesives and encapsulants, contact Resin Design at 781.935.3133. Our elite team of Polymer Scientists have years of experience designing and developing adhesives and encapsulants for a wide number of industries including aerospace, automotive and electronics. Entex 52101 and Entex 53060 are both made with aliphatic isocyanate backbones. Urethanes made from this chemistry are slower to cure, and therefore reach lower peak exothermic temperatures during cure which minimizes shrinkage. Additional benefits of using urethanes based on aliphatic structures are clear final products and UV stability.

 

Troubleshooting Voids in Urethane Adhesives and Encapsulants

In our last blog, you gained insight into troubleshooting polyurethane air entrapment. This week let’s look at troubleshooting voids in urethane adhesives.

Causes of Voids

Before de-molding, there could potentially be huge voids in the part or on the surface. The voids are visible with the naked eye and they occur at thin-walled sections.

Pot Life is Too Short – Air can be trapped for a short period of time when the batch is poured into the mold and displaces the air. Air then moves to the upper surface of the part. This process can take some time, and when the pot life is too short, the air bubbles don’t have reach the surface to self-release.

Mold Leaks – When mold leaks into the adhesive a void can easily be formed within this area. This typically occurs when the system has an extended pot life or a low viscosity.

Air Entrapment – Air is displaced from the mold when the batch is poured into the mold.  A void will form when the air is trapped in different cavities and there are no mechanisms to allow the air to escape. This typically occurs in thin-walled areas when the viscosity is too high.

How to Fix These Voids

Mold & Vents Should be Redesigned – Since large voids occur when there isn’t proper ventilation, a new mold design may be necessary to improve ventilation.

Utilize Low Viscosity Urethanes – Air can easily escape when a low viscosity material is used. A system with an extended pot life and lower viscosity can decrease the void formation.

Reduce Splashing  – Specifically when using large rollers, it’s best to tilt the mold before filling. You can also limit the amount of splashing when you pour down the side of the mold.

Use a Degassing Agent – The surface tension of the system can be reduced when a degassing aid is used. This allows air to escape with more ease. This will significantly reduce the formation of voids when higher viscosity materials are used.

Contact Resin Designs for Urethane Adhesives & Encapsulants

Resin Designs offers a range of products for various industries including aerospace, automotive and electronics. Entex 53051 is a clear, rigid polycarbonate bonder and encapsulant. This urethane is formulated, by use of two of the methods mentioned above, to minimize void generation. This material is low in viscosity, 400-900 cps mix viscosity, and contains a degassing agent which reduces surface tension allowing trapped bubbles to escape.

Troubleshooting Polyurethane Air Entrapment

Urethanes are commonly used to encapsulate electronic circuits because of their moisture resistance and other positive attributes. These benefits are important, however sometimes issues arise that must be remedied immediately to achieve the best results.

Bubbles Appearing in Parts

On occasion, small bubbles do become visible throughout the parts and may not be localized.

Causes of Bubbles

  1. Moisture Contamination of Components

Moisture reacts with the isocyanate in a polymer, producing carbon dioxide gas. This carbon dioxide gas accrues as small bubbles. Moisture can come from a variety of sources such as the prepolymer, catalyst, adhesive and primer. When you have moisture contamination of either the adhesive or primer, bubbles are visible only at the bond line.

  1. Moisture Contamination of Mold

Molds that aren’t heated for an extended period over the boiling point of water may experience atmospheric moisture on the surface. In humid atmospheres, molds can have water condensation if they are cooled and not reheated. When water-based mold release agents are applied improperly, this can lead to water on the mold. When this occurs, bubbles typically appear on the surface.

  1. Insufficient Degassing

When the prepolymer isn’t degassed before mixing AND the batch isn’t degassed before pouring, small bubbles will form.

  1. Incorrect Pouring Technique

If there is a lot of splashing while pouring, bubbles will form in the batch. This is a major problem when it comes to low-viscosity systems where splashing occurs frequently.

  1. Seal in the Processing Machine is Not Working

Degassing happens in the component storage tanks and when using a meter-mix dispensing machine, components are pumped from the storage tanks to the mixing head. Since they are pumped directly into the mixing head, it’s highly unlikely that air will end up into the mixture.

Fixing Air Bubbles

Place Materials Under Dry Nitrogen

Atmospheric moisture is much less likely to be present when nitrogen gas replaces the air in container holding the material. If you can’t get a hold of nitrogen, use a heat gun or hairdryer for roughly 10 seconds prior to putting the lid on.

Degas Components

In the components and the mixed system there could be entrained air so it’s crucial to degas components prior to and after mixing. Moisture can also lead to foaming so again, degas the raw materials.

Heat Mold Efficiently

Any moisture that has condensed on the mold will be removed when the mold is heated above the boiling point of water.

About Resin Designs Urethanes

Resin Designs creates several custom products including the Nexus 52101 which is ideal for automotives, bonding plastics, encapsulants and potting. The Entex 51011 is both soft and compliant, and suitable for high vibration environments. Fill out our contact form to get in touch with us or give us a call at 781-935-3133.

Urethanes in LED Encapsulation – PT 2

In our previous blog “Urethanes” we discussed the advantages of urethane adhesives and applications when they are advantageous. One particular application that urethanes excel in is LED encapsulation. When high temperature resistance is not necessary, urethanes can be implemented in place of silicones. Urethanes work more efficiently in low temperature applications and are easily utilized for electronic packaging. Another major benefit when working with urethanes is that they protect stress sensitive electronic devices, as well as act as a barricade against water.

Polyurethanes are often utilized for their flexibility, shore hardness variances and short pot life. Polyurethanes are also perfect for delicate electronic components as they can act as shock absorbers. They are water resistant which makes them great resources for marine applications.

Polyurethanes & LEDs

LED lighting has become increasingly common, you may find them in illuminated furniture, surrounding cabinetry or concealed in room to produce indirect lighting. Two-component potting solutions are used to protect LEDs (polyurethane or silicone).

In order for an LED encapsulation to be working at an optimum performance, it should be potted with polyurethane or silicone products. This also protects the LED from mechanical damage, moisture or any other environmental disturbance. Two part potting systems allows the adhesive to be processed on fully automatic mixing machines.

light diffusing urethane

Top LED Light: Potted with Entex 53071 – Light Diffusing Bottom LED Light: Potted with Entex 53061 – Not Light Diffusing

Both space and time are saved when LEDs are casted with potting and silicone materials. The chosen sealant can greatly influence the properties of the emitted light. There will be a greater light dispersion and clear casting that does not interfere with the emitted light. Polyurethane-based optical casting protects from yellow which is generated from UV radiation by aliphatic isocyanate.

Urethane LED encapsulants can be used as a tool to control light emitting from the LED. These products can be made to be light diffusing if the application calls for it. See the video (or photos) below for the difference between a light diffusing and not diffusing urethane.

When designing LED lights and lighting systems, the casting material should first be determined. Resin Designs offers clear, ridged or flexible, urethane LED Encapsulants that can be light diffusing:

  • Entex 53051: Crystal Clear, UV stable, encapsulants. This material is ridge (Shore D75) and non-light diffusing.
  • Entex 53061: Flexible version of Entex 53051.
  • Entex 53071: A light diffusing version of Entex 53061.

Contact us to create custom LED encapsulants for your application.

Urethanes

Understanding Urethanes

Urethanes offer an array of benefits including flexibility, water white clarity, low temperature functionality, and elongation as well as abrasion resistance. Due to these properties, urethanes are at an advantage compared to other adhesives in devices that are undergoing stress due to discrepancies of component composition.

Numerous urethanes consist of low glass transition temperatures so they have the ability to protect components in consumer electronics which operate at a range of -40C to 105C. Several urethanes can also be used at temperatures of -70C and others as high as 130C.

Chemists can manipulate urethane formulations to have either an extremely quick gel time at room temperature or a slow gel time with low exotherms. In fact, urethanes can be created to a range of hardness from soft gels to high Shore D.

Generally, urethane chemistry consists of a two-component system; one side is polyol and the other side is isocyanate. When moisture is in the air, the isocyanate will react so it’s encouraged to protect unused materials with a layer of nitrogen gas during storage. This will remove an air that is in the container.

Polyurethane Adhesives

Commonly referred to as “elastic adhesives,” polyurethane adhesives are glues which consist of urethane polymers; these polymers contain chemical based of isocyanate group. Typically you’ll find polyurethane adhesives in the following application:

  • Encapsulation/Potting/Sealing
  • Medical grade adhesives
  • Automotive applications
  • Plastic bonding
  • Application that require low temperature capabilities
  • Applications that require strong water/moisture resistance
  • Applications that are exposed to the outside elements (weather/sun radiation)

Cured by polyaddition reactions, polyurethane adhesives take on a reticulated structure. This structure can be ridged or flexible, elastomer like, dependent on the level of crosslinking.

Advantages of Polyurethane Adhesives

Due to the hydroxyl functionality of the polyol portion of polyurethane these adhesives are completely water resistant. Additionally, polyurethane adhesives can be formulated to be at a low viscosity, cure within seconds, and cure to a pliable state at room temperature conditions. As an adhesive, the cure time is determined by the length of time required before the adhesive has bonded two materials together.

Three Types of Polyurethane Adhesives

  • Two component polyurethane adhesives
  • One component polyurethane adhesives cured by heat
  • One component polyurethane adhesives cured by moisture

Polyurethane Adhesive Applications

Resin Designs has urethanes products that cover the majority of the described applications:

  • Entex 51021: A general purpose electronic encapsulate with success in underwater applications. Features include low Tg (-65 °C) and medium hardness (Shore A90).
  • Nexus 52101: Clear encapsulate and plastic bonder, especially to polycarbonate. Features include medium hardness (Shore 45D) and good elongation (170%). Axis M-100N is the medical grade version of this product.
  • Entex 53051: Crystal clear, UV stable, adhesive with success in lens coating. This material is more ridge (Shore D75) than other urethane products.
  • Nexus 51031: Strong adhesive bonder. Works well with thermoplastics, wood, metal, ceramic, and stone. This adhesive has a fast work life (3 minutes) and is ridged (Shore D80).

If you need a custom urethane adhesive please contact us. We look forward to work with you to help deliver the product you need.