EPOXY RESIN FORMULATION MADE
SIMPLE: CASE STUDIES
By William T. McCarvill and A. Brent Strong
What are epoxies and how do they differ from polyester thermosets?
Polyesters and epoxies are both families of thermosetting resins. The fundamental difference is the type of reactive chemical group that is involved in the crosslinking reaction. For polyesters, the reactive group is a double bond between two carbon atoms. For epoxies, the reactive group is a small 3-membered ring of two carbon atoms and an oxygen atom – called the epoxy ring. We can appreciate the differences in crosslinking reactions and conditions without delving into the chemistries involved. Polyesters are crosslinked by adding a small amount of peroxide initiator (catalyst) to a solvent system of the resin. Then, either with heat or at room temperature, and perhaps with the addition of additional chemicals, the resin cures. Epoxies do not use peroxides and are often not solvated. Curing an epoxy is accomplished by adding a curative (hardener), but the conditions for curing can vary widely depending on the natures of resin and the curative, as well as the requirements of the manufacturing operation and the properties of the final product.
When compared with the polyester crosslinking process and the properties of polyester products, the epoxy system and product properties are far more versatile. This greater versatility is both good and bad. Good because the molder can select curing conditions and properties that are just right for the product and manufacturing application. Bad because the system can become
1
so complex that specialists are required to choose the proper components of the resin mixture, the type of curative, and the molding conditions. These specialists are called epoxy resin formulators. The purpose of this article is to give you insight into the capabilities of epoxy resin systems and to simplify the concepts of formulation so that you can see what the formulators are doing and, perhaps, do some simple formulating yourself.
The formulation of epoxy resins requires the optimization of cost, performance, and processing aspects. Typically ingredients are selected for their chemical and physical attributes and then various formulations are tried in small test mixes until the desired properties are achieved. But it is not just what is put together that matters, how the ingredients are assembled and used affects the consistency of properties, the processing characteristics, safety in scale up, and full-scale production practice. The
discussion of fundamentals and the case studies in this article illustrate the principles which, we believe, will give the results desired. The case studies link the chemical and physical properties desired for a part with common sense ways to mix and use resins in actual plant conditions. This article explains the "why" behind the mixicology (our new word) of epoxy resins.
Epoxy resins formulating
Thermoset chemistries such as epoxy resin technologies are generally mature and well understood. A whole industry has developed in supplying resins, additives, curatives, thickeners, diluents, wetting agents, impact modifiers, fillers and resins pre-dissolved in a variety of solvents. Custom versions of any of these can be available should the supplier decide the market justifies it. This vast number of compounds enables the formulator to devise mixtures that closely fit process and cured material requirements (and bewilder the non-formulator). Various
2
philosophical approaches have been taken to expedite the development of the optimum mixture but, for all of them, the key is to reach the combination of ingredients that will meet all requirements in an efficient manner. As with all engineering endeavors, perfection is never achieved. Compromises will be
made.
It is necessary to have a modest level of fundamental understanding of the chemical and physical attributes of epoxy systems for successful formulation. To select ingredients at random and put them together in a haphazard manner will lead to dissatisfaction with the end product, and a risk of injury and property damage. Here are a few general fundamentals.
A given manufacturing/molding process has physical and chemical requirements. For instance, highly fluid resins are needed for wet layup, filament winding and resin transfer molding. High viscosity, solid resins are needed for powder coating. A resin to be used for a painted or a trowelled application will have an intermediate viscosity and must be easily spread to wet the surface, but must not run off the surface. In some processes the resin is heated to reduce its viscosity during the pumping and wetout steps, but then must become a high viscosity liquid or semi-solid for subsequent processing; thus, both high and low viscosity requirements must be met.
All thermoset resins are reactive and exposure to time and temperature will result in their thickening and solidification. Resins designed to cure at room temperature have very short pot lives. The various reactive components are kept separate until just before use when they are combined and allowed to re
act, usually just before molding. Their viscosity rapidly builds until they are no longer processable. Conversely, resins that require elevated temperatures to reach full properties will have long pot lives and long storage times at lower temperatures. Sometimes they
3
are supplied as a one-part mixture. Incomplete and inadequate cure conditions will result in less that anticipated performance.
Resins designed for cure at low temperatures will not have the thermal resistance of resins formulated for high temperature capability, even if cured at high temperature. Conversely resins formulated for high temperature capability will cure very slowly or not at all at room temperatures.
General Rules for Mixing
Epoxy resins themselves, that is, without the curative, are relatively unreactive. They can be subjected to elevated temperatures to dissolve additives and, if required, conduct reactions with those additives. This is not to say that long times at elevated temperatures are always allowable and are safe. Care must be taken to ensure that the conditions are acceptable from a quality and safety aspect. However,
epoxy resins without the curative can be mixed under conditions that would be unsafe if the curative was present. When the epoxy resin is mixed with the curative, chemical reactions begin when subjected to time and temperature.  Therefore, it is necessary that the blending of epoxy resins, dissolution of thermoplastic components, pre-reactions with rubber, and other additive reactions be done before the curative is added to the mix.
If a material is a solid in the form of a powder, transfer and weighing is straight forward. This is also true if the material is a pourable liquid.  Some resins, curatives and modifiers fall into these categories and are easily handled.  However, the physical form of many epoxy resins can make them difficult to measure and mix. These resins range from honey-like consistency to materials that are semi-solids. Unlike crystalline solids which can require a high temperature to
4
become molten, resins demonstrate high sensitivity to moderate temperatures. For instance, the viscosity of some resins can drop dramatically with relatively modest increases in temperature. This high thermal sensitivity means that many resins can be easily heated by placing them in a hot box or using a drum heater. However, it should be remembered that resins have poor thermal conductivity an
d their high viscosity makes convection inefficient for heat transfer so heating times might be quite long. The fully preheated resin can be dispensed from the drum as a liquid and weighed and transferred as needed. A safe effective temperature and time for heating can usually be recommended by the supplier.
One side effect of resins that are semisolids is that it is very difficult to keep the work environment clean. The tacky nature will result in resin transferring onto whatever surface the resin might contact. Therefore, strict housekeeping rules should be followed. Once transferred to a surface, solvent will likely be required to remove the resin. Low toxicity, low volatility solvents such as n-methyl pyrrolidinone (NMP) or γ-butyrolactone (BLO) are generally used. The use of disposable containers can go a long way towards keeping the workplace free from contamination.
The mixing of resins must overcome several impediments associated with resin viscosities. As mentioned above, just as semi-solid resins are preheated to lower their viscosity for transfer and weighing, they must be heated for mixing. Although solid resins in powder or pellet form can be easily weighed and transferred, they must be converted to a liquid by heat, or added carefully to a low viscosity resin otherwise they will go through a high viscosity unstirrable condition. If a mixture of resins is to be blended, it is preferable to start with the lowest viscosity component charged to the mixe
r first and brought up to the mix temperature. It
5
>reactive to

版权声明：本站内容均来自互联网，仅供演示用，请勿用于商业和其他非法用途。如果侵犯了您的权益请与我们联系QQ:729038198，我们将在24小时内删除。