TWO PART EPOXY RESIN SYSTEMS
Chemistry of Epoxy Resins - two part EPOXY
How Epoxy Resins Work, Chemical Reactions, Polyurethanes 101
Your Host and Tour Guide:
Paul Oman, MS, MBA - Progressive Epoxy Polymers, Inc. (floor epoxies, marine epoxies, underwater epoxies, repair epoxies)
Member: NACE (National Assoc. of Corrosion Engineers), SSPC (Soc. of Protective Coatings)
"Professionals helping Professionals since 1994"
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PART 2: Underwater Epoxy
PART 3: Polyurethane Coatings
PART 4: What You Can Tell from an MSDS Doc.
Epoxies consist of two components that react with each other forming a hard, inert material. Part A consists of an epoxy resin and Part B is the epoxy curing agent, sometimes called hardener. Let's begin with taking a closer look at the epoxy resin.
Epoxy resin begins with the reaction of two compounds, bisphenol A - Bis A -(or bisphenol F -Bis F- and/or 'Novolac' -(visit our novolac web page) used for superior temperature and chemical resistance) and epichlorohydrin. Bisphenol A is the chemical product of combining one acetone unit with two phenol groups. Phenol is a man-made chemical, although it is also found naturally in animal waste and decomposing organic material. It was originally produced from coal tar and was named carbolic acid. Structurally it contains a benzene ring with an attached hydroxyl (a carbon ring with an attached OH). Acetone is a organic ketone (.i.e. it contains a carbonyl C=O group attached to two organic methyl groups) primarily used as a solvent or chemical intermediate or raw material for many other products.
Nearly 70% of all epichlorohydrin is used in the production of epoxy resin. This colorless liquid with its irritating chloroform like odor finds it way into the production process of various synthetic materials. Leading producers are Dow Chemical (Texas) and Shell Chemical (Texas and Louisiana).
The reaction between bisphenol A and epichlorohydrin removes unreacted phenol and acetone and attaches two glycidyl groups to the ends of the bisphenol A, creating a 'diglycidyl ether of bisphenol A (called DGEBA)', which is standard epoxy resin. The glycidyl group on both ends of the bisphenol A are also referred to as an oxirane or ‘epoxy group'. The size of the resulting molecule (and hence its molecular weight) depends upon the ratio of epichlorohydrin to bisphenol A.
The ‘amine' curing agent has a molecular structure which typically consists of four hydrogen ‘arms and legs'. These hydrogens react with the oxirane (epoxy group) ring unit on the ends of the epoxy resin. The result is a new carbon-hydrogen bond, this time using the hydrogen from the curing agent and freeing the epoxy group's hydrogen to unit with the group's oxygen to form an OH (hydroxyl) pendant. This hydroxyl group contributes to the epoxy's outstanding adhesion to may substrates. The aromatic ring unit, which the hydroxyls attach to, helps provide the epoxies positive thermal and corrosion properties.
Because there are at least four hydrogens on the curing agent they can react with four epoxy resin groups, which causes giant interlocking structures (in a process known as ‘cross linking').
The curing agent selection plays the major role in determining many of the properties of the final cured epoxy. These properties include pot life, dry time, penetration and wetting ability. Curing agents come in many different chemical flavors, generally based upon amines or amides. Some of the more common amines and amides often listed in Material Data Safety Sheets (MSDS) include:
A) Aliphatic (carbon atoms forming open chains) and cycloaliphatic (ring structured aliphatics) amines and polyamines. Amines are basically ammonia with one or more hydrogen atoms replaced by organic groups;
B) Amides and polyamides. Amides are basically ammonia with a hydrogen atom replaced by a carbon/oxygen and organic group.
Amine based curing agents are considered to more durable and chemical resistant than amide based curing agents but most have a tendency to ‘blush' in moist conditions. Blushing produces a waxy surface layer on actively curing epoxy, the result a reaction with the curing agent and moisture in the air. Other potentially toxic chemicals within the curing agent can also be released in the same manner, thus amines are often viewed in light of these potential shortcomings. Amides, on the other hand, are more surface tolerant and less troubled by moisture. More on Blushing.
Fortunately for epoxy end-users involved with underwater applications, there is a small subgroup of non-benzene ring structured amines that maintain all the benefits of amines while removing the toxic leachability and moisture attracting properties of typical amines. These special polyamines form the basis for today's cutting edge underwater epoxies.
C) Cycloaliphatic curing agents. These curatives generally provide better water/moisture resistance, weatherability, low blush and water spotting, and better chemical resistance. Cycloaliphatics there own web page - CLICK HERE.
Cycloaliphatic chemistry explained by "The Chemist" - the hands-down expert on epoxies - in a 2/18/03 post on the Wooden Boat Forum --
"The cycloaliphatic structure refers to a six-member carbon-atom
ring in the backbone of the curing agent instead of carbon-carbon linkages between amines in some other curing
agents such as West or Sys. 3. The cycloaliphatic curing agents have the amine groups connected to the rings. Both
use Diglycidyl ether of Bisphenol-A as the major component of the epoxide curing agent, and both contain benzyl
alcohol, a volatile plasticizer which acts as a molecular lubricant and facilitates curing [although systems with
cycloaliphatic curing agents contain much more, typically 20-30%, as they simply do not cure more than about half-way
at room temperature without it.]
While the cycloaliphatic ring is resistant somewhat to UV degradation, more that the carbon-carbon linkages between amines in some other types of curing agents, both contain the aromatic ring structure of diglycidyl ether of Bisphenol-A, which breaks down fairly readily on UV exposure, and both contain amines; these give both molecular breakdown and yellowing all by themselves.
None of these structures should be considered UV-stable, even though the cycloaliphatic structures are better than the aliphatic amine-structures in that regard."
Amines vs. amides in coal tar epoxies - CLICK HERE
The well known adhesion of epoxies is due to the strong polar bonds it forms with the surfaces it comes in contact with. On dry surfaces the bond between the surface and the epoxy displaces the air, which is a fluid. The same is true underwater. As on dry surfaces, the polar bond attraction is strong enough to displace the fluid, in this case the water, and produce an strong bond even underwater. Thus, painting underwater is, in theory, no different that painting above the water. The cross linking reaction of epoxies should be independent of the surrounding environment. Still, as mentioned above, many or most curing agents will react with water molecules rather than the epoxy base, resulting in a waxy layer, also mentioned above, known as amine blush. This makes them unsuitable for underwater application.
At least one modern hydrophobic, underwater epoxy goes one step farther to ensure a strong underwater polar bond with the introduction of a proprietary ‘bond enhancer'. This is important because many marine structures are subjected to active cathodic protection systems. Such systems place an electrical charge on the structure's surface that will literally and actively repel the epoxy's existing polar bonding surfaces. The enhancer provides additional polar bond surfaces that are also firmly anchored into the cross linking epoxy structure.
By: A. SEN. Consultant, Protective & Functional Coatings Bombay, INDIA.
Epoxy resins must be cross linked in order to develop the coating's required characteristics. This cross linking process is achieved by chemically reacting the resin with a suitable curing agent or hardener. The reactive groups of molecules in the epoxy resin formulations are the terminal epoxide groups and the hydroxyl groups. For protective coatings, the principle cross linking reaction is between the epoxide group and the curing agent. Amine curing agents are the most common type used in epoxy formulations.
Primary amines are organic materials containing a nitrogen atom linked to two hydrogen atoms (-NH2). In epoxy formulations, the active hydrogen of the amine is what reacts with the epoxide group of the resin. The structure of the amine-containing organic compound and the number and type of amine groups in the compound is what determine the rate of cross linking and the coating's properties.
There are different types of polyamine curing agents: aliphatic, cycloaliphatic, aromatic, polyamine adduct, etc.
The aliphatic amines like EDA, DTA, & TETA contain short, linear chemical chain between amine groups. Coatings produced with them tend to have highly cross linked layers with good resistance to heat and chemicals, including solvents. However, they are rather brittle and possess poor flexibility & impact resistance. Because of their reaction with moisture, they are not suitable for use under damp conditions.
Modified cycloaliphatic amines from IPDA are probably the most used curing agents for epoxy resins today. Because of their low viscosities, they can be used in low VOC coatings. They produce coatings with a fast cure rate, short pot life and are also suitable for low temperature cure. They provide very good resistance to chemicals, solvents & water, which makes them suitable for use in portable water tanks.
In the aromatic amines, the amine group is separated by rigid benzene rings rather than flexible chains of molecules as in the aliphatic amines. Coatings produced with them have good physical properties like impact resistance as well as high resistance to heat and chemicals. But being aromatic in nature, they produce dark coatings. They are used to produce chemical & solvent resistant coatings. And particularly when they are formulated with epoxy novolac & phenolic epoxy resins, they produce coatings that can resist high temperatures. Aromatic amines are generally modified for use as curing agents, which although reduces their heat resistance are still good for chemical resistance. They have good resistance to water and hence work well in damp conditions & low temperatures.
One common modification to aliphatic amines is to form a polyamine adduct by reacting the curing agent with a small amount of epoxy resin. This gives high molecular weight polyamines that produce coatings with low vapor pressure, with more practical mixing ratios and less formation of amine bloom than the simple aliphatic amines. Adducting has little effect on other properties. Polyamine adducts can be prepared from either aliphatic or aromatic polyamines.
Polyamides are formed by the reaction of aliphatic polyamines and dimer acids of either tall oil fatty acids or from soya or castor oil. Here again adducting polyamides is common and produce coatings with good low temperature curing and reduced tendency for amine bloom. Good color and good chemical resistance can be achieved using these adducted polyamides. They generally produce coatings with excellent adhesion, water resistance & flexibility. Unmodified polyamides produce coating layers that are much more open in terms of their chemical structure because of their large distances between amine groups in the chemical chain. Consequently they are more flexible. This open structure of the curing agents results in coatings with low resistance to chemicals, solvents and acids. However, their resistance to water & corrosion are enhanced because of their surface wetting and adhesion properties.
When an aliphatic polyamine is reacted with a monofunctional fatty acid rather than a dimer acid, then an amidoamide is formed. These curing agents are less volatile and have less irritation potential than polyamines, and they have properties that although are similar but inferior to polyamides. For instance, polyamides are better in water resistance and provide better adhesion than the amidoamides.
In addition to the above types of curing agents there are many other curing agents based on polyamines, and there are also non-amine based curing agents. Finally, either aromatic or aliphatic isocyanates may also be used as curing agents for epoxy resins. The isocyanates react through the hydroxyl groups of the epoxy resin and provide very good low temperature curing, good flexibility, good impact & abrasion resistance as well as good adhesion.
I trust this brief discussion helps. The "World of Epoxy Curing Agents" is so large and varied that it is impossible to explain them in a forum of this nature. Over the years, both JPCL & PCE have carried several articles and several viewpoints on the different classes of epoxy hardeners. Perhaps a search through them will yield lot more. Reference may also be made to "Protective Coatings - Fundamentals of Chemistry & Composition" by Clive H. Hare.
Epoxies made with Bis F, a Bis F and novolac mixture, or Novolac resin exhibit greatly improved chemical and heat resistance compared to the much more common Bis A epoxies. Probably 98% of all epoxies are 'regular' Bis A epoxies. Assume Bis A epoxy unless specifically told otherwise. Novolac epoxies are more expensive than regular epoxies. They exhibit higher 'heat distortion temperatures', higher 'T sub G temperatures' (both of these are measures of when the epoxy begins to soften with heat). The values for these measurements vary slight with the different resins, and by whether the vendor reports conservative values or optimistic numbers. Generally Bis A epoxies will begin to soften in the 120-160 degree F range. Novolac epoxies initially raise this value by about 25 degrees F. More important is what happens above this temperature. All epoxies will reharden when the elevated temperatures fall below this transition temperature. However, Bis Novolac epoxies will continue to cure when exposed to temperatures of about 150 degrees F for a few hours. After this 'additional curing' they generally can withstand about 300 degrees F (dry environment) without problems. An exception to this is our non-hazmat novolac epoxy (FC 2100 N). The non-hazmat curing agents used greatly reduce the temperature resistance (but not the chemical resistance). For 'true' novolac temperature resistance use our Nova Clear hazmat novolac epoxy.
Chemical Resistance: A good quality Bis A epoxy will handle 70% sulfuric acid. A novolac epoxy will handle 97 or 98% sulfuric acid.
The bad news: You get good chemical resistance or good heat resistance, generally not both.
A more technical third-party article: click here
We are the supplier of an inexpensive marine and industrial clear novolac epoxy as well as a pigmented industrial novolac floor epoxy.
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This current page is all about:
Chemistry of Epoxy Resins - two
part EPOXY systems
How Epoxy Resins Work, Chemical Reactions, Polyurethanes 101
In general terms, three generations of apply underwater epoxies have emerged over the years. Each has pushed the technology window forward. The success of first generation epoxies seems to be in their ability to be applied and cured underwater. The next generation moved these epoxies into true coating status, albeit with issues of user friendliness and chemical safety issues still to be addressed. The new third generation epoxies have addressed those issues successfully.
* Sticky, like ‘Bubble Gum' - similar to a thick putty
* Hard to mix and apply - knead the two parts together in hand-sized amounts and literally push on to the surface
* Expensive on a ‘cost per square foot' - @ 1/4 thick and $50 gallon = $7.80 per square foot material cost
* Potentially difficult to ship and/or store - may require hazmat shipping (Corrosive Liquid - N.O.S.), may have short shelf life
* Good underwater adhesion - true bonding instead of ‘sticking'
* Poor storage stability (heating required) - products tended to crystallize over time
* Toxic - MDA and possible solvents (see footnote)
* Hazmat shipping required - keep away from foodstuffs
* Problem with cathodically protected surfaces - interference with the polar bonding
* Low cost - $2.50 per sq. ft. for 40 mils, $5.00 per sq. ft. for 80 mils ($100 per gallon)
* Stable storage - will not crystallize over time
* Basically Non-toxic, 100% solids (0% VOC), no MDA
* Non-hazmat - unregulated shipping
* Good application on cathodically protected surfaces - additives to overcome polar bonding interference, easy application results in productivity increase
NOTE: We at Progressive Epoxy Polymers, Inc. offer several underwater epoxies for waste water, nuclear, marine (boating and commercial). See links at bottom of page.
Rankings of Polyurethanes (from weakest to strongest)
one part (polyurethanes: (sold at hardware stores, etc.)
moisture cured urethanes: (curing reaction takes place in presence of moisture - most not suitable for exterior applications)
acrylic aliphatic (linear) isocyanates polyurethanes: a two part epoxy (note not all these adjectives are always used)
polyester aliphatic isocyanates polyurethanes: the best of the polyurethanes. 'Awlgrip' by US Paint, and the 'LPU 100' Polyurethane, which we sell at both our marine and industrial sites, fall into this group. These paints are often used for aircraft finishes and aircraft hanger floors. Our LPU Marine has its own web page - CLICK HERE
Urethane resins are either aliphatic, aromatic or a combination of each. Aliphatic is straight chains (i.e. Linear) of
carbons in the backbone - perfect example is polyethylene.
Aromatic molecules are composed of resonant benzene rings. The resonance is closely tied to
energy absorption in the visible spectrum - all dyestuffs exploit resonance in particular areas of the
visible spectrum - as the molecule shakes at a certain frequency it absorbs energy in that frequency
thus changing color, shine white light on a surface, extract blue from it and you are left with red to
be reflected off. Aromatics do not have to change much to become similar to a dyestuff structure
and mess up the reflected light thus appearing heavily discolored. Because of their chemical
stability they tend to be excellent for applications such as heat resistance or chemical resistance
but they do suffer from poor light stability.
Epoxies are part aliphatic, part aromatic. They therefore have the best and worst properties of each.
Polyurethane curing agents are often aromatic - toluene diisocyanate (TDI) for example. Notice how
exposed PU foams often discolor and crumble in strong sunlight? When the curing agent is purely
aliphatic (linear) it has much better UV resistance. Classic PU curing agent is HMDI or hexamethylene
diisocyanate which is a ring but a saturated, non-resonant structure which does not turn into a
resonant ring upon exposure to UV. The "linear" term simply means a simple saturated line of
carbon atoms rather than the alternating unsaturated bonds characteristic of aromatic molecules.
Polyester polyurethanes improve upon the excellent properties of linear polyurethanes.
Most telling is the MSDS for the Part B curing agent (two part epoxies will have an MSDS for both the Part A and the Part B). The curing agent (Part B) MSDS that tells the best story. Unfortunately, the MSDS usually doesn't tell as much as you wish it would.
Section two in MSDS documents list the primary ingredients in the product. Some vendors proudly list their high end chemicals. Others decide to hide as best they can legally the ingredients of their products. Good or really good products will probably list: cycloaliphatic, or cycloxxx, or aliphatic followed by amine or polyamine. They also might use the words modified amine or modified aliphatic amine, yet they could all be the same thing. Again, not very useful for figuring out what you have or for comparing against another vendor's product.
The term Adduct is a good thing to see. Adducts use a little bit of the epoxy resin in the curing agent. This improves properties and blush resistance. Our Basic No Blush (tm) marine epoxy is an Adduct.
In very general terms, if the Part B uses the term cycloaliphatic and amine or polyamine and or adduct, you probably have a top tier quality epoxy. Seeing "aliphatic amine' might suggest a really good, albeit, second tier epoxy (or a top tier epoxy hiding its top tier roots). Clues, at best. Most, maybe all, underwater epoxies are use cycloaliphatic curing agents.
Even the Part A resin (Bis A generally) comes in flavors. Add that to the nearly endless blends of amines, polyamines, amides, in aliphatic or cycloaliphatic form, as well as other additives and a formulator can develop a product with just about any property desired (viscosity, blush or no blush, chemical or solvent resistance, flex, pot life, etc).
Perhaps more telling from an MSDS than the chemistry of the epoxy, is the red flag addition of excessively large amounts of nonyl phenol or solvents. Another red flag (not found in the MSDS sheet) is the requirement of letting the epoxy sit for some amount of time after mixing and before using. This is often called induction time or sweat in time. Generally only very low end epoxies have induction time requirements.
Technically, MSDS documents should be dated within the last 5 years. I've seen MSDS documents from major formulators that are dated well over 10 years ago. Even our products have outdated MSDS sheet that slip by our attention (where do the years go?). Read what you want from this oversight.
Chemistry of Epoxy Resins - two
part EPOXY systems
How Epoxy Resins Work, Chemical Reactions, Polyurethanes 101