Polyurethane compounds developed by Crosslink Technology Inc. can be easily processed by hand, without the necessity of automated equipment. Many of these urethane systems are suitable for processing at room temperature due to their low inherent viscosities.
The following are typical processing instructions covering two and three component urethane systems:
Read more: Three Component variable hardness Polyurethane
Read more: Two part Polyurethane systemsEquipment Required
- A pair of disposable Nitrile gloves
- Safety glasses
- A clean plastic or glass container sufficiently large enough to hold the total amount of desired mix. (Waxed paper cups are not recommended.)
- Steel or plastic stir stick. (Wooden stir stick are not recommended because the moisture contained in the wood may contaminate some products and interfere with the curing process)
- An accurate weigh scale. (Gram scales are best.)
- Technical data sheet for the product being used
- Safety data sheet for the product being used
- Read and observe all safety instructions indicated on the safety data sheet for the product.
- Look up the “pot life” specification of the mixed material on the Technical Data Sheet to estimate the maximum amount of epoxy that should be mixed at a time. The shorter the “pot life” the smaller the amount that may be mixed at a time. Mixing excessive amounts of material can result in dangerous heat generated by the reaction between the resin and hardener.
- Prepare all work surfaces and/or tooling for the application. (Do not forget to apply mould release if the finished part is to be removed from a tool after casting)
- Place the container on the scale and tare.
- Thoroughly mix the epoxy resin and epoxy hardener in their original containers to eliminate any separation and disperse any settled fillers. (1) (2 – 3 minutes of mixing, while scraping the sides and bottom of the container is recommended.)
- Accurately weigh the required amount of epoxy resin, according to the mix ratio, into the container. (3)
- Tare the scale again.
- Add the required amount of epoxy hardener, according to the mix ratio, on top of the resin. (3)
- Thoroughly mix the resin/hardener together; scraping the sides of the container to make certain that all the hardener is thoroughly and evenly dispersed in the resin. Mix carefully to minimize air entrapment due to turbulence in the material. Try to angle the edge of the stir stick rather than using the flat surface which creates the most turbulence.
- If possible, let the mix stand for a few minutes to release the surface bubbles.
- Pour or use the material as intended.
- Follow the curing instructions on the Technical Data Sheet. (2)
- Some highly filled materials may have to be slightly warmed to allow for thorough mixing. Do not heat excessively and allow both resin and hardener to cool to the recommended processing temperature before mixing the components together.
- Do not subject the epoxy to mechanical stress until it is fully cured. Do not subject the epoxy to significant temperature changes prior to full cure. (This would be like thermal cycling the uncured epoxy and could cause cracking).
- Be careful when using percentages in calculating mix ratios. It is easy to make a mistake as shown in the following example: Assume that you are using an epoxy system with a mix ratio of 100:25 on the technical data sheet and you need 125 grams or oz of mixed material for you part.
Urethanes
As a general rule, Polyurethane compounds are moisture sensitive and will react with moisture in the air or with moisture in the substrate they come in contact with during cure. Depending on the chemistry, some products actually require the presence of moisture in the air to cure while others require the evaporation of a solvent carrier to solidify. Urethanes may be formulated as 2 component or single component systems.
Surface bubbles are most likely caused by one or more of the following:
- High ambient temperature causing the product to skin over and preventing proper solvent evaporation below
- Material reacting with moisture introduced during mixing component A with component B (2 component systems)
- Excessive moisture present in either the substrate or in the air during application
- Do not mix violently. De-air the mix under vacuum before use
- Apply at the recommended relative humidity
- Do not apply on very humid days
- Apply at the recommended temperature
- Once the bubbles have appeared, let the material fully cure as recommended, sand the surface and re-apply
- Excessive air introduced during mixing.
- Improper surface tension preventing bubbles from self-releasing.
- Index of thixotrophy too high.
- Mix slowly to avoid air entrapment
- Warm the mix if possible to reduce the viscosity
- Spray surface with mould release (usually works)
- Pour slowly in one corner and let the material rise pushing the air ahead of it
- Select a different product for the application
Automated mix, dispense equipment is highly desirable especially in high volume production environments. There are some very important basic items to be considered, both from the material and the equipment reliability stand point before purchasing such equipment. There are many prominent manufacturers who are able to supply high quality reliable equipment to dispense all types of compounds.
Purchasing mater/mix/dispense equipment usually represents a major investment and it is worth while to spend some time in considering the overall process at hand, how the equipment will be implemented and the possibilities of future changes in the product line. One critical process consideration would be the manpower required to run the system in production. In some cases, if the process is too fast, it will require two people to run it but the total output may be less than the two people might produce individually, thereby negating the planned savings in labour. The following suggestions are based on our experience as a material supplier, having dealt with both problems and successes.
The following are the suggested steps and considerations in this process:
Test and approve the material in terms of performance. Get all the necessary material modifications completed and prototype parts tested and approved
Allow the formulator and the equipment manufacturer to work together in developing the complete system
Arrange for an extensive trial run at the premises of the equipment manufacturer utilizing the finalized material. This is important since minor problems are easily corrected in house without the requirement for service calls and possibly having to ship components to the installation site. We recommend at least a full day's uninterrupted production for final approval to delivery.
Arrange for training the maintenance personnel so that they are fully aware of the critical maintenance items and trouble shooting procedures.
Order and stock the recommended spare parts. The equipment supplier can recommend a list of parts to keep on hand.
NOTES:
- It is critical to have the equipment and material supplier work together from the start of the project. This will permit minor adjustments prior to final installation. It is always more difficult to schedule travel in order to correct problems at the installation site.
- Short trial runs, prior to equipment delivery, may not show all the potential problems. Our suggestion is a full day uninterrupted production.
- Select the material supplier for the Epoxy or Urethane compound.
- Select the equipment supplier and describe the intended application in detail. Obtain any pertinent information regarding the general limitations of the equipment if any.
- Discuss the material requirements with the formulator and be certain to indicate that the product is to be dispensed using automated equipment. This is extremely important from the formulating stand point. The formulation to be used can be selected or developed to favour the equipment by;
- Selecting or formulating products that incorporate "non-abrasive" fillers
- Selecting or formulating products with the desired pot life or gel time
- Selecting or formulating products that include resin and hardener components that are easily mixed together
- Selecting or formulating material components to yield the best viscosity
- Selecting or developing a material with the most suitable mix ratio
In some cases adding more hardener will speed up the reaction because more reactive hardener sites are available for the resin to react with. This may result in a shorter time to achieve gel but the excess un-reacted hardener remaining after gel point, will reduce the integrity of the final cured product. Mild warming is recommended to speed up the reaction, not altering the mix ratio.
Fillers need to be incorporated by mixing under high shear with vacuum applied. This is required to remove trapped air and break up agglomerations to properly wet out filler with the resin. If filler is not properly wet out and air is trapped in the mix, physical performance of the cured system will be compromised. If fillers are to be incorporated on the shop floor, it is best to add them to the resin that is already mixed with the hardener in the correct ratio. Using this method will eliminate the necessity of having to re-calculate the mix ratio based on the extra fillers.
A mould or part should first be designed to allow air to escape easily when mixed epoxy or urethane is poured into the part. If this is not feasible, mixed product may be brushed or vacuumed into intricate areas prior to topping up with the remaining mixture. If vacuum or brushing is not possible, warming the mixed product or preheating the part will help reduce the viscosity of the mixed liquid, making it easier for the trapped air to escape.
The cure reaction can be made quicker by introducing mild warming to the mixed epoxy or urethane system prior to pouring, or pouring the mixture into a preheated unit or mould. The reaction can be slowed down by reducing the temperature of the mixed product or by mixing up a smaller batch size.
There are 3 basic methods to extend the pot life of mixed material. Since most Epoxy and Polyurethane compounds generate heat (exotherm) during the reaction controlling the heat will have an effect on pot life...
Epoxies and Polyurethanes generate heat during the reaction.
Since these materials are very poor in conducting heat while in liquid form this energy is retained within the mass of mixed product. This retained energy intern will accelerate the reaction further resulting in the generation of even more heat. The more reactive the system the more heat it will generate during the curing process.
Hardeners, catalysts and similar components in a given formulation are designed to yield a certain pot life at a specified temperature for a specified mass of material. The best way to extend pot life is to control the amount of energy (heat) that is available for the curing process.
Trade Off: The lower the temperature the higher the viscosity. Pouring and de-airing are more difficult at lower temperatures.
Two Component Materials (Manual mix):
- Reduce the amount of heat generated while mixing the components together.*
- Reduce the amount mixed at one time.
- Keep the mix cool.
- Select a slower hardener.
- Lower the resin and hardener holding tank temperatures.*
- Lower the temperature on heated dispense lines.*
- Select a slower hardener.
- Switch to a static mix head.**
- Click here for trouble shooting and more tips
There are ways to reduce the viscosity of a mix but all the methods involve some trade-off either in the handling or the cured properties of the epoxy or urethane material.
It is best to purchase products that are formulated to yield the required viscosities to accommodate the processing. In certain cases however, it may be desirable to lower the viscosity of a given epoxy or polyurethane system to improve its handling characteristics. It must be considered that all deviations from the recommended handling will result in some form of trade-off.
The following are ways to reduce viscosity:
- Warming the mixed material. Viscosities decrease quickly with the application of heat. Two of the most significant trade-offs will be shorter pot life and faster set up time.
- Warming the resin only. This will lower the resin viscosity and make mixing easier. The resultant trade-offs will be the same as above, only to a lesser extent, since the hardener is cooler.
- Warm the hardener only. This is the least desirable since many hardeners contain at least some volatile components and the volatility of these components will become more severe with the application of heat. The same trade-offs will apply complicated by possible cosmetic defects caused by the volatile ingredients.
It is very difficult to remove cured epoxy or polyurethane without damaging the encapsulated components. In certain cases however there are ways to remove at least part of the encapsulant.
At best it is difficult to remove cured material without damaging the encapsulated components. In certain instances however, where the embedded component is reusable, such as an encapsulated conductor, it is worth while to remove the encapsulant and reclaim the part.
The easiest method to remove epoxy or polyurethane is to heat it above its glass transition (softening point) temperature. At this point the encapsulant will be soft enough to cut or penetrate with a chisel like device and break it away from the embedded component. This method is more convenient for recovering heavier solid components than reclaiming delicate electronic circuits. Epoxies and polyurethanes will smoke and give off odours when heated to high temperatures. Provide an appropriate face mask and good ventilation while removing the encapsulant using this method.
The other method is to immerse the encapsulated component in Methylene Chloride. This solvent is hard to obtain as it has been declared a carcinogen. The user must take all precautions to prevent contact with skin and not to inhale the vapours while using this product. Methylene Chloride will cause the encapsulant to swell and become soft for removal. Unfortunately, since printed circuit boards and many electronic components are also encapsulated with epoxy, they will also be damaged or destroyed by this solvent.
There has been some success reported in using the commonly available paint stripper sold under the trade name Circa 1850. Immersing the encapsulated component in this material will also soften the encapsulant the same as Methylene Chloride and of course, it will also attack and destroy any embedded epoxy devices. The Circa 1850 Heavy Body paint & Varnish Remover can be used for spot removal or on large surfaces that cannot be dipped.
Epoxy and Urethane chemistry is a crosslinking reaction between resin molecules and hardener molecules. The stated mix ratio produces enough reactive sites in both the resin and hardener to crosslink the system for optimum performance. Adding less or more hardener will compromise the performance of the cured product. Two mixes, one low in hardener, and the other high in hardener, will produce a cured product with excess un-reacted resin or excess un-reacted hardener respectively. The physical performance of these two castings would be compromised due to incomplete crosslinking.
Depending on the chemistry employed, some epoxy and urethane products are more tolerant to slight variations in mix ratio than others. Generally speaking, ± 2% by weight accuracy should yield satisfactory results in most cases. Since the mix ratio is determined by product chemistry, based on the number of desired reactive sights contained in each component, it is best to be as close as possible to the stated ratio in order to obtain the optimum cured properties.
It sound like your epoxy system contains what is called “blushing”. This is usually caused by high humidity (moisture content in the air) when the coating was applied or your substrate was not totally dry at the time of application. Some hardeners are more moisture sensitive than others and this is worth investigating with your supplier.
Assuming that the coating has fully cured, the only way to fix the problem is to sand it aggressively and apply another coat on top. Make sure that your hardener is not too moisture sensitive and everything is absolutely dry before applying the last coat. Use a dehumidifier or use a heater to increase the air temperature (which lowers the relative humidity level) at the time of application.
The most common cause of “water stains” is moisture trapped in or under the coating. This could be due to the floor not being completely dry when the coating was applied or excess humidity at the time of application. Be sure you are not confusing water stains with “blushing”.
Some times “water stains” are actually not stains but areas of poor adhesion (separation from the substrate) due to insufficient surface preparation. Another issue might be that the Hardener used is highly moisture sensitive. This is worthwhile checking out with your supplier and changing to a different hardener.
Assuming that your coating has fully cured, the only way to fix the problem is to thoroughly sand the surface, make sure it is absolutely dry and apply a fresh coat.