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 thermots?
Polyesters and epoxies are both families of thermotting 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 u 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 becau the molder can lect curing conditions and properties that are just right for the product and manufacturing application. Bad becau the system can become
1
so complex that specialists are required to choo the proper components of the resin mixture, the type of curative, and the molding conditions. The specialists are called epoxy resin formulators. The purpo 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 e what the formulators are doing and, perhaps, do some simple formulating yourlf.
The formulation of epoxy resins requires the optimization of cost, performance, and processing aspects. Typically ingredients are lected 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 asmbled and ud affects the consistency of properties, the processing characteristics, safety in scale up, and full-scale production
practice. The discussion of fundamentals and the ca studies in this article illustrate the principles which, we believe, will give the results desired. The ca studies link the chemical and physical properties desired for a part with common n ways to mix and u resins in actual plant conditions. This article explains the "why" behind the mixicology (our new word) of epoxy resins.
粘土层Epoxy resins formulating
柳行Thermot 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 the can be available should the supplier decide the market justifies it. This vast number of compounds enables the formulator to devi mixtures that cloly 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. Compromis will
语文题目花菜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 lect 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 ud 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 process the resin is heated to reduce its viscosity during the pumping and wetout steps, but then must become a high viscosity liquid or mi-solid for subquent processing; thus, both high and low viscosity requirements must be met.
分享妈妈吧All thermot 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 parate until just before u when they are combined and all
owed to react, usually just before molding. Their viscosity rapidly builds until they are no longer processable. Converly, 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. Converly resins formulated for high temperature capability will cure very slowly or not at all at room temperatures.国庆自驾游
General Rules for Mixing
Epoxy resins themlves, that is, without the curative, are relatively unreactive. They can be subjected to elevated temperatures to dissolve additives and, if required, conduct reactions with tho 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 prent. 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.2028年奥运会在哪个国家举办
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 the categories and are easily handled. However, the physical form of many epoxy resins can make them difficult to measure and mix. The resins range from honey-like consistency to materials that are mi-solids. Unlike crystalline solids which can require a high temperature to
4
become molten, resins demonstrate high nsitivity to moderate temperatures. For instance, the viscosity of some resins can drop dramatically with relatively modest increas in temperature. This high thermal nsitivity 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 a
nd their high viscosity makes convection inefficient for heat transfer so heating times might be quite long. The fully preheated resin can be dispend 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 misolids 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 houkeeping 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 ud. The u of disposable containers can go a long way towards keeping the workplace free from contamination.吞噬拼音
The mixing of resins must overcome veral impediments associated with resin viscosities. As mentioned above, just as mi-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 otherwi 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 mi
xer first and brought up to the mix temperature. It
5
