How Critical is the Mix Ratio?
The answer is; it depends on the system. Epoxies are not like polyester resins where basically any amount of hardener will allow the laminate to cure. Epoxy resins and hardeners have calculated “fixed” ratios, which allow their individual molecule groups to cross-link completely with one another. Careful measurement of the two ensures that ultimate properties for a given system are achieved on a repeatable basis.

A high performance laminating resin and hardener such as PR2032 and PH3660 with a mix ratio of 100:27 parts by weight (pbw), require that level of accuracy to obtain the physical properties we state on our technical data sheet. Generally we like to see that within a range of 100:25-100:29 pbw. Our lab worked hard to arrive at a volumetric mix ratio of 3:1 for builders without scales to help eliminate “guesswork.”

It is important to note that the target mix ratio should be the one that is published. The tolerance range allows for slight errors in measurement. To intentionally measure off ratio is never advised. Some epoxy hardeners are formulated to be “variable-ratio hardeners”. The published ratios for these hardeners will show a range of acceptable mix ratios.

Exceptions to this rule are the Aeropoxy adhesives and Lightweight filler. These are 1:1 and 2:1 respectively by weight or volume. 1:1 and 2:1 systems have inherent “fudge factors” built in due to the higher mix ratios. Obviously anytime you are bonding critical structural components it’s advised to be as accurate as possible. As a rule of thumb remember the lower the mix ratio (100:10 for example) the higher the required degree of accuracy.

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Is Aeropoxy Compatible With Other Cured Epoxies?
Yes. Epoxies are inherently good adhesives. They will bond together quite well if the surface is prepared properly, i.e. a clean, scuffed or sanded surface.

The exception to this would be some sort of specialty epoxy, such as one that had unusual additives or fillers to enhance particular properties, chemical resistance and/or finish. If that is the case, you would need to consult with the manufacturer of the specialty system for their recommendation.

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How Well Does it Bond to Metal? Wood? Styrofoam?
The AEROPOXY product line was developed with all of these materials in mind. Choosing the proper system for the job at hand being the main consideration. As in any bonding application, surface preparation is the most important aspect of the process. If you are unsure of the proper AEROPOXY system for your bonding project, consult with a PTM&W representative.

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Why is the Back of the Laminate Tacky the Next Day?
There are several reasons why this can occur. First, the mix ratio could be off. Under or over catalyzed epoxy resins can have the same net effect – rubbery and/or tacky laminate. Second, if the temperature is below 65°F, chances are the laminate has not fully cured, leaving a tacky finish. Third, in extremely humid conditions, moisture can react with the uncured surface forming a tacky compound, called carbamate. This carbamate will not affect the cured properties of the laminate, but will affect adhesion of subsequent layers of epoxy if not removed prior to laminating. The easiest way to prevent this potential problem is to peel ply the final layer of your laminate, protecting the epoxy from the moisture.

It should be noted; laminating in high humidity conditions will not affect the cured properties of the laminate. The tacky condition described above will only occur while the laminate is gelling or curing.

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Can AEROPOXY be Used for Major Structural Areas?
Yes! The AEROPOXY PR2032 / PH3660 laminating system was developed for tough, high performance structural applications. You can expect high physical properties along with excellent heat resistance in your well-designed, properly laminated parts or tooling. For areas where a structural bond is required, we would recommend either ES6228 or ES6279 paste adhesives in conjunction with the laminating resin.

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Will AEROPOXY Bond to Carbon Graphite and Kevlar Fabrics?
Yes!  The PR2032 / PH3660 laminating system has been used extensively in the fabrication of high performance carbon graphite and Kevlar parts.  Test results show excellent physically properties can be achieved when using these fabrics.  Refer to the technical data sheet for physical properties.

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Can PR2032 / PH3660 be Used with Fillers?
The answer is yes.  The AEROPOXY system works very well with the addition of microballons, cotton floc, milled fibers, and many other fillers adding excellent diversification.  Having said that, PTM&W developed the AEROPOXY line of resins with specific uses in mind, i.e., laminating resins for composite laminates, pourable and paste adhesives for structural bonding and a lightweight filler for finish sanding, lightweight fillets and build-up of non-structural areas.

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Can PR2032 / PH3660 be Thinned to Reduce its Viscosity?
No.  This is a high performance laminating resin system designed for specific uses.  The viscosity, handling and cured properties are balanced and optimized to provide the best results possible.  The resin and hardener were developed as a “tuned system” and should not be altered in any way.  Special additives were incorporated to enhance the “wet-out” of all fabrics readily.  The addition of solvents will not only change the viscosity but could alter the pot life, physical properties, Tg, and bring into play a host of other problems both known and unknown. 

The vast majority of people complaining about the viscosities of epoxies are normally former polyester users.  Polyesters are generally in the 300-500cps ranges.  The main problem with materials too low in viscosity is the tendency for them to run or slump.  The accumulation of resin in the low areas can result in unbalanced, resin-rich laminates.  This fact is accentuated even more-so with the longer pot life resin systems, since they stay liquid longer.

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Will the Resin or Hardener Freeze?

Epoxy resins and hardeners do not “freeze” to a solid at a certain low temperature, as does water.  Some epoxy hardeners can crystallize at low temperatures, and some epoxy resins will do the same, to a lesser extent.  The consistency of these products can range from hazy liquid to a loose, slushy semi-solid, even, in some extreme cases to a waxy solid.  This is a general statement about epoxies, and does not refer to the Aeropoxy materials.  The AEROPOXY product line was formulated not to crystallize in normal shipping and storage conditions, and we have not been able to cause any of the Aeropoxy product line to crystallize in the lab.  Since there is rarely such a thing as an absolute certainty, we want to make you aware of what crystallization entails and what can be done if in fact it occurs. 

First, it should be noted that a crystallized material is not ruined, or even changed.  The condition is easily reversed.  If a resin or hardener has crystallized, it can be returned to normal by heating it slightly.  Warm the material over low heat.  A hot plate on low works well, as well as a warm water bath that you’d set the can into.  A temperature of 100°F-120°F is sufficient to dissolve the crystals.  The material can then be used after it has cooled to room temperature. 

It is fact, once a material has crystallized it is impossible to redissolve every single tiny crystal and some will remain in the material, although you may not see them.  These will not impede the reaction when mixed, or change the cured properties.  If however, all the material is not used, these seed crystals will reinitiate the solidification process again rather rapidly. Therefore any remaining material that is stored should be checked closely before being used again, if it looks hazy the safe bet would be to warm it again.

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None of the AEROPOXY products are flammable materials, so there are no fire hazards associated with them, as there would be with solvents, for example.  Being organic materials, they will support combustion, but they will never be the ignition source. 

Epoxy materials can be made in fire retardant or self-extinguishing versions.  These materials are designed to stop burning very soon after the fire source is removed.  They are not non-burning materials.  With the AEROPOXY line, we have developed products that have very high performance and high heat resistance.  The use of fire retardant additives would diminish the performance and lower the heat resistance of the AEROPOXY product line.  Should future regulations or market conditions require fire retardant products, we can provide them.

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What Finish is Recommended for the Fiberglass Reinforcing Fabric?

When glass fabrics are manufactured, the individual warp and fill strands are coated with various binders and sizings to improve handling during construction.  Since these binders can detract from the adhesion of the resin system and lower performance of the structure, it is best to use fabric that has had the sizing removed.  Fabric manufacturers typically remove binders and sizing by heat or scouring, and glass fabric thus cleaned is called greige goods.  Glass fabric is sold as greige goods, or with a coupling agent applied.  Coupling agents, or finishes, are chosen based on compatibility with the polymeric resins.  Finished fabrics generally provide improved wet-out and higher performance in the part or structure.  The original finish for glass fabrics was Volan, developed by Dupont.  Volan is still popular because it gives good performance with both epoxy and polyester resins.  More recent technology finishes include various silane coupling agents designed for specific resins, which give higher physical properties because they are “tuned” to that specific resin. 

Often, individual fabric weavers have their own number for the different finishes, so it is impossible to list all of them here.  The important thing to remember when ordering fabric for your AEROPOXY project is to specify either a Volan finish or a silane that is designed for the best performance with epoxy resins.

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When Preparing Surfaces for Bonding, Should You Solvent Clean, or is Air Blow-off Sufficient?

In cleaning metal surfaces for subsequent bonding, it is best to first solvent clean the metal, then abrade the surface to be bonded, followed by a final solvent clean to remove loose particles generated by the abrasion.  Additionally, there are safe metal etching products (phosphoric acid) available at local hardware or home improvement stores that not only etch but clean the surface as well.  The Aladine or uredine metal treatment systems available are designed to protect and prevent corrosion of the metal but may not be practical to use for the home-builder.  You should consult with the applicable supplier if you are considering using these products. 

In cases of metal bonding when solvents are used to clean the surface, always make sure you use new, clean solvent to prevent contamination and bond failure.  Avoid reclaimed solvents, as their exact makeup cannot be determined and the possibility of contamination or adverse reactions are too high.

For composites surfaces, an initial solvent wipe of the surface to remove any grease or oils is acceptable.  After the solvent dries, the area to be bonded should be sanded to roughen it up and increase the surface area.  Then, the sanded surface should be blown off with compressed air to remove loose particles and dust from sanding.  Since there is a possibility that a solvent might wick into the composite through the sanded area, solvent wiping after sanding is not recommended.  While the use of a 40 grit sandpaper will increase the overall surface area you are bonding to, such a rough paper can damage the fibers in a given cloth matrix and potentially weaken your laminate.  The question is whether you’re sanding to add additional layers of reinforcement or preparing for a finishing system.  Note: See “Why Use Peel Ply” for additional option.

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Why Use a Peel Ply? Does a Peel Ply Starve a Laminate?

The use of a peel ply, or tear ply, offers several benefits when constructing a laminate with epoxy resins.  It’s a fabric that is applied over the last layer of cloth before the laminate is allowed to begin its cure.  Since it does not bond to the laminate, it will peel off when and if, additional laminating or bonding is ready to continue.  It’s often used when the lamination process has to be halted before all the plies of cloth have been applied.  It leaves a roughened surface that does not require further preparation before laminating or bonding continues, i.e. sanding/scuffing the laminate.

The reasons to use a peel ply far out-way any objections not to.  First, as mentioned, there is virtually no preparation necessary before continuing laminating or bonding (with the exception of areas where the peel ply wrinkles and you’ll have resin rich ridges that are easily sanded or ground off).  Second and very important, peel ply protects your laminated parts from dirt, dust and contaminates that they may be exposed to over many days, months or even years before they’re needed.  Third and maybe the most beneficial, it leaves a much smoother finish on the laminate than you would otherwise have. It minimizes pinholes and virtually eliminates the weave of the cloth, which eventually has to be filled in during the “finishing” process. 

A number of types of peel ply materials are commercially available.  These include, Teflon coated fiberglass fabric, fiberglass fabric coated with other release agents, as well as coated and uncoated nylon fabrics.  Some AEROPOXY users have had good results with very tight weave Dacron fabric purchased from sewing centers and retail fabric shops. 

It is highly unlikely that the use of peel ply will starve a laminate.  The technique of applying a peel ply involves laying it on the back of a wet laminate and pressing it into intimate contact.  The goal is to lay it on as smooth and wrinkle free as possible, while avoiding trapping air pockets under it.  Peel ply fabrics are tightly woven, which minimizes the penetration and collection of resin, so the amount of resin removed with the peel ply is minimal when it is torn away.  The tendency for amateur builders is to use more resin than necessary, resulting in a resin-rich laminate.  The use of peel ply will help to control this common fault.

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Do I Have to Post Cure My AEROPOXY Laminate? Does Post Curing Improve the Performance?

The Aeropoxy laminating systems will cure completely at normal room temperature and do not need a post cure to achieve the full properties listed on the literature.  However, we always suggest that if you can, post cure the part. 

A beneficial aspect of post curing is that the laminate gets to full cure much faster than if allowed to room temperature cure alone.  This is especially important if you are on a tight schedule and want to move a laminate around, or remove it from a fixture to make another part.  If this were done too soon, with only a partial, insufficient room temperature cure, there could be distortion of the laminate.  On the other hand, if the laminate is post cured after a short room temperature cure, it has full properties and can be moved, worked on or assembled without worry of damage or distortion.

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