Flame Treating Coroplast for Adhesion
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Flame Treating Coroplast for Adhesion
I have become a coro addict, and I would like to take this opportunity to offer some insight as to what is happening chemically during the flashing, or flame treating of the surface of coroplast in order to get good adhesion with various types of adhesives.
Back in 1966, immediately after obtaining my mechanical engineering degree, I accepted a position with one of the largest producers of high density polyethylene and in the world. My first assignment was in their plastics development laboratory working on the development of a flame treatment process for a proposed high density polyethylene automotive fuel tank to facilitate the application of an epoxy barrier coating to prevent the transmission of gasoline vapors through the fuel tank walls into the atmosphere. Automotive emissions legislation was just beginning to be discussed at that time.
Polyethylene and Coroplast (which is made from polypropylene) are first cousins chemically, and are members of a family of polymers known as polyolefins. Olefin means "lack of affinity," meaning that these materials are, for all practical purposes, chemically inert. Nothing will adhere to them in their natural state, because nearly all of their covalent bonds are saturated.
In order to get anything to adhere to a polyolefin, you need to create chemically active bond sites on the surface of the polymer that the adhesive can chemically bond to. One way to do this is to flame treat the surface with a stoichiometric air/fuel mixture. This means that there is sufficient oxygen present in in mixture to completely burn all of the fuel molecules present. We explored many different fuel/air mixtures and found the stoichiometric ratio to be optimal. If you are using a Bernz-O-Matic propane torch, simply adjust the flame to get a good blue cone, with no orange flame present. The outer cone should be almost clear. Go to your local hardware store and pick up a fan-shaped flame spreader used for burning off house paint to give you a nice 2-inch wide flame pattern.
Hold the torch about 4 inches above the surface to be treated and move steadily at about one to one and one half feet per second. You will see a blush pattern spread out about two inches to either side of the flame in the reflected light from your shop lights. Wait about 10 seconds for the surface to cool and make a second pass to the side of the first pass, if the area you are gluing is wider than the flame pattern.
One test to see if the flame treatment has worked is to "water wet" the surface. Untreated polyolefins have little effect on the surface tension of water, therefore, water will bead up on untreated surfaces. Treated surfaces, with their available covalent bond sites, will attract the polar water molecules and the water will spread out evenly on treated areas. Try it. Hold the untreated surface under a faucet and see how the water beads up. Then dry the surface and flame treat it. Repeat, holding the test piece under the faucet and notice the difference. In the lab we used a carbol fuscia dye to see the results. The treated areas would turn purple because the dye molecules had bonded to the treated areas.
Do not touch the treated area. The oils present in your skin will destroy the monomolecular thickness of the treated surface. Chemically, what you are doing is forming carbonyl (pronounced "carbon-eel") groups on the surface. These groups contain a hydroxyl radical, which forms the bond with the adhesive molecules.
During my employment in this capacity, I searched the organic chemical library for an organic compound containing metallic atoms in the polymer that could be blended into the base polyolefin polymer to create a bondable polymer that would eliminate the need for flame treating, but I couln't find anything at that time. Perhaps such a material exists now, but flame treating still works.
Hope this helps. One last thing. There are no manufacturing oils used in the extrusion process. The material is nothing but polypropylene polymer, which possesses a natural "oily" feeling to the touch, a hallmark of all of the polyolefin family of polymers. When extruding Coroplast, the only thing the polymer sees is the pressure exerted on the liquified polymer from the extrusion screw which rotates in an electrically-heated cylinder plasticizing the resin pellets before the melt passes through the die, and the cooling temperature of the extrusion die which brings the polymer back to the solid state in the form of Coroplast sheet.
Please accept my sincere apologies if my remarks have offended anyone by this. Tatoo and Kraut and all of the other SPADers have done all of us a tremendous service by selflessly sharing their wealth of knowledge and their designs. I just wanted to set the record straight as to what is actually happening chemically when we flash this material. Additional comments or questions are always welcome.
Al Mangani
JG 26
AMA 22336
Back in 1966, immediately after obtaining my mechanical engineering degree, I accepted a position with one of the largest producers of high density polyethylene and in the world. My first assignment was in their plastics development laboratory working on the development of a flame treatment process for a proposed high density polyethylene automotive fuel tank to facilitate the application of an epoxy barrier coating to prevent the transmission of gasoline vapors through the fuel tank walls into the atmosphere. Automotive emissions legislation was just beginning to be discussed at that time.
Polyethylene and Coroplast (which is made from polypropylene) are first cousins chemically, and are members of a family of polymers known as polyolefins. Olefin means "lack of affinity," meaning that these materials are, for all practical purposes, chemically inert. Nothing will adhere to them in their natural state, because nearly all of their covalent bonds are saturated.
In order to get anything to adhere to a polyolefin, you need to create chemically active bond sites on the surface of the polymer that the adhesive can chemically bond to. One way to do this is to flame treat the surface with a stoichiometric air/fuel mixture. This means that there is sufficient oxygen present in in mixture to completely burn all of the fuel molecules present. We explored many different fuel/air mixtures and found the stoichiometric ratio to be optimal. If you are using a Bernz-O-Matic propane torch, simply adjust the flame to get a good blue cone, with no orange flame present. The outer cone should be almost clear. Go to your local hardware store and pick up a fan-shaped flame spreader used for burning off house paint to give you a nice 2-inch wide flame pattern.
Hold the torch about 4 inches above the surface to be treated and move steadily at about one to one and one half feet per second. You will see a blush pattern spread out about two inches to either side of the flame in the reflected light from your shop lights. Wait about 10 seconds for the surface to cool and make a second pass to the side of the first pass, if the area you are gluing is wider than the flame pattern.
One test to see if the flame treatment has worked is to "water wet" the surface. Untreated polyolefins have little effect on the surface tension of water, therefore, water will bead up on untreated surfaces. Treated surfaces, with their available covalent bond sites, will attract the polar water molecules and the water will spread out evenly on treated areas. Try it. Hold the untreated surface under a faucet and see how the water beads up. Then dry the surface and flame treat it. Repeat, holding the test piece under the faucet and notice the difference. In the lab we used a carbol fuscia dye to see the results. The treated areas would turn purple because the dye molecules had bonded to the treated areas.
Do not touch the treated area. The oils present in your skin will destroy the monomolecular thickness of the treated surface. Chemically, what you are doing is forming carbonyl (pronounced "carbon-eel") groups on the surface. These groups contain a hydroxyl radical, which forms the bond with the adhesive molecules.
During my employment in this capacity, I searched the organic chemical library for an organic compound containing metallic atoms in the polymer that could be blended into the base polyolefin polymer to create a bondable polymer that would eliminate the need for flame treating, but I couln't find anything at that time. Perhaps such a material exists now, but flame treating still works.
Hope this helps. One last thing. There are no manufacturing oils used in the extrusion process. The material is nothing but polypropylene polymer, which possesses a natural "oily" feeling to the touch, a hallmark of all of the polyolefin family of polymers. When extruding Coroplast, the only thing the polymer sees is the pressure exerted on the liquified polymer from the extrusion screw which rotates in an electrically-heated cylinder plasticizing the resin pellets before the melt passes through the die, and the cooling temperature of the extrusion die which brings the polymer back to the solid state in the form of Coroplast sheet.
Please accept my sincere apologies if my remarks have offended anyone by this. Tatoo and Kraut and all of the other SPADers have done all of us a tremendous service by selflessly sharing their wealth of knowledge and their designs. I just wanted to set the record straight as to what is actually happening chemically when we flash this material. Additional comments or questions are always welcome.
Al Mangani
JG 26
AMA 22336
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Thanks a Million!!! Very interesting! We learned about flashing from a guy who works at Coleman that flies with us...we tried it...it worked... and we've never looked back It's sort of like the airfoils we use...they work great, but the explinations why are still a little over my head As for the manufacturing oils...simply going off of other's explinations...sounded correct...and unresearched and unconfirmed...more or less the TSAR method (That Sounds About Right)...but to get more accurate info from someone in the educated know really rocks!!!
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Jagdgeschwader 26
Glad to see the scientifically accurate description appear here. I considered making a similar post some time ago when I first read about manufacturing oils etc. I decided that most people care more that it works rather than why it works! The "wave" that is supposedly oils burning off is actually moisture condensing out of the flame (water as steam is a major combustion product from propane). The flame treatment (as well as the commercial corona discharge treatment) actually cause a change in the surface chemistry of the polymer; creating chemically active sites - forming a bonding layer.
I don't think that anyone would be offended by your post and it serves as a good explaination to those who might want to use a heat gun or a "cool" butane lighter flame. (these won't work)
Any idea why the mineral spirits works for some people and acetone doesn't?
Ross
Glad to see the scientifically accurate description appear here. I considered making a similar post some time ago when I first read about manufacturing oils etc. I decided that most people care more that it works rather than why it works! The "wave" that is supposedly oils burning off is actually moisture condensing out of the flame (water as steam is a major combustion product from propane). The flame treatment (as well as the commercial corona discharge treatment) actually cause a change in the surface chemistry of the polymer; creating chemically active sites - forming a bonding layer.
I don't think that anyone would be offended by your post and it serves as a good explaination to those who might want to use a heat gun or a "cool" butane lighter flame. (these won't work)
Any idea why the mineral spirits works for some people and acetone doesn't?
Ross
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Flame Treating Coroplast for Adhesion
This also tells me why my 'Flashing' never really worked well...
All Yellow Flame!?!
I will do some more experimenting with all blue flame now!
So what does this tell us about 'old' coro and 'fresh' coro as far as flashing goes?
All Yellow Flame!?!
I will do some more experimenting with all blue flame now!
So what does this tell us about 'old' coro and 'fresh' coro as far as flashing goes?
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Flame Treating Coroplast for Adhesion
Any idea why the mineral spirits works for some people and acetone doesn't?
I am just curious. Is there some other type of reaction happening with mineral spirits that makes the coroplast bondable?
Frank
P.S. thanks for that awesome description of how that works. I really dig that stuff and truly admire the education it takes to understand that type of thing. That is something that I still sometimes wish I had taken the time to get. Thanks
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Please understand that I am a mechanical engineer, not a polymer chemist. I have enough knowledge to be dangerous
It has been nearly forty years since my stint in the laboratory, but I think what happens with a yellow flame is there is an excess of fuel to oxygen in the flame. Kinda like your engine running rich. The excess hydrocarbons in the combustion products react instantly with any freshly-created bond sites, re-saturing the surface with all kinds of mean and ugly molecules that are still chemically inert. Welders call this is called a carburizing flame. Conversely, going in the other direction, that is, too little fuel to air, or lean, you get an oxydizing flame, and I can't remember exactly why that is detrimental to the process. Suffice it to say that a neutral flame, one in which all of the oxygen and all of the hydrocarbons are converted to carbon dioxide and water vapor gives the best results.
Hope this answers your question.
Al Mangani
JG 26
AMA 22336
It has been nearly forty years since my stint in the laboratory, but I think what happens with a yellow flame is there is an excess of fuel to oxygen in the flame. Kinda like your engine running rich. The excess hydrocarbons in the combustion products react instantly with any freshly-created bond sites, re-saturing the surface with all kinds of mean and ugly molecules that are still chemically inert. Welders call this is called a carburizing flame. Conversely, going in the other direction, that is, too little fuel to air, or lean, you get an oxydizing flame, and I can't remember exactly why that is detrimental to the process. Suffice it to say that a neutral flame, one in which all of the oxygen and all of the hydrocarbons are converted to carbon dioxide and water vapor gives the best results.
Hope this answers your question.
Al Mangani
JG 26
AMA 22336
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Mineral spirits
In response to the question as to why does wiping Coroplast with mineral spirits seem to improve the bond strength, I really cannot say for sure. Again, I am not a chemist, but an engineer, and the chemistry involved is somewhat outside my field of endeavor. One thing for sure...the organic solvency of mineral spirits has practically no effect in terms of dissolving the surface of a polyolefin polymer. Cases of dissolving a polymer are present in the solvent joining of PVC pipe fittings, or acetone on polystyrene "plastic" models.
Based on the development work I did back in the mid-sixties, I'd say that polypropylene (i.e., Coroplast) is not QUITE as chemically inert as high density polyethylene....there are SOME available chemically active bond sites...not a whole lot, but some. So maybe there is an organic molecule present in mineral spirits that bonds to the available site, providing a branch on the polymer chain that offers more available sites. I'm really reaching here, but I can't think of any other explanation.
Here's a crude example...
Polymer chain of carbon and hydrogen...all four bond sites on each carbon atom are satisfied with single covalent bonds as indicated by the single lines, there are no double carbon bonds, and no free radicals present...this is an example of saturation.
H H H H H H
| | | | | |
H-C-C-C-C-C-C- AND SO ON FOR THOUSANDS OF REPETITIONS
| | | | | | IN LINEAR POLYMERS SUCH AS THOSE IN
H H H H H H THIS DISCUSSION.
Here's what happens when you flame treat...
H H H H H OH-
| | | | | |
H-C-C-C-C-C-C- etc.
| | | | |
H H H H H
Below is a polymer with a double carbon bond present: See the equals sign? That is a double carbon covalent bond. This is a condition where the carbon atom is unsaturated. It can use a hydrogen atom to go to its lowest possible energy state and share all of its four outer shell electrons.
H H H H H OH-
| | | | | |
H-C-C=C-C-C-C- etc.
| | | |
H H H H
I think that's how it went...see the minus sign after the hydrogen atom on the hydroxyl radical? That's an active bond site...unsatisfied and looking for something to attach to...this is a chemically weak bond, yes...but a bond nonetheless. The double carbon bond is stronger than the single carbon bond, and takes more energy to break. The only example of a triple carbon bond is acetylene and offers the greatest energy liberation of any covalent bond in the field of organic chemistry. That is why acetylene is used for welding and cutting of steel....it is hotter than any other organic fuel.
Golly, I'm really in deep trouble now...all you PhD's in polymer chemistry can take over the class now...aaauuugghhhh!
Now back to the mineral spirits....add a hydrocarbon branch to the site where the hydroxyl group is in the example above and that is what may be happening...just a hunch. Assume there are additional active sites in the new branch to permit additional branching and bonding. Also, with increased branching, you get mechanical resistance by the simple mechanical interference between adjacent polymer chains...kinda like trying to pull one branch out of a brush pile...there is resistance between adjacent branches...but if all the chains are linear, without much branching, then there is greater opportunity for the adjacent chains to physically slip relative to one another in the axial direction. Like taking one strand of spaghetti out of the box...
Branching (i.e., the mineral spirits example, and also with paint, which is also organic) may be causing as much mechanical "bonding," as chemical bonding, but that is just a hunch. PhD's...the class is yours!!
Maybe I shoulda kept my fingers off the keyboard...
later,
Al
Based on the development work I did back in the mid-sixties, I'd say that polypropylene (i.e., Coroplast) is not QUITE as chemically inert as high density polyethylene....there are SOME available chemically active bond sites...not a whole lot, but some. So maybe there is an organic molecule present in mineral spirits that bonds to the available site, providing a branch on the polymer chain that offers more available sites. I'm really reaching here, but I can't think of any other explanation.
Here's a crude example...
Polymer chain of carbon and hydrogen...all four bond sites on each carbon atom are satisfied with single covalent bonds as indicated by the single lines, there are no double carbon bonds, and no free radicals present...this is an example of saturation.
H H H H H H
| | | | | |
H-C-C-C-C-C-C- AND SO ON FOR THOUSANDS OF REPETITIONS
| | | | | | IN LINEAR POLYMERS SUCH AS THOSE IN
H H H H H H THIS DISCUSSION.
Here's what happens when you flame treat...
H H H H H OH-
| | | | | |
H-C-C-C-C-C-C- etc.
| | | | |
H H H H H
Below is a polymer with a double carbon bond present: See the equals sign? That is a double carbon covalent bond. This is a condition where the carbon atom is unsaturated. It can use a hydrogen atom to go to its lowest possible energy state and share all of its four outer shell electrons.
H H H H H OH-
| | | | | |
H-C-C=C-C-C-C- etc.
| | | |
H H H H
I think that's how it went...see the minus sign after the hydrogen atom on the hydroxyl radical? That's an active bond site...unsatisfied and looking for something to attach to...this is a chemically weak bond, yes...but a bond nonetheless. The double carbon bond is stronger than the single carbon bond, and takes more energy to break. The only example of a triple carbon bond is acetylene and offers the greatest energy liberation of any covalent bond in the field of organic chemistry. That is why acetylene is used for welding and cutting of steel....it is hotter than any other organic fuel.
Golly, I'm really in deep trouble now...all you PhD's in polymer chemistry can take over the class now...aaauuugghhhh!
Now back to the mineral spirits....add a hydrocarbon branch to the site where the hydroxyl group is in the example above and that is what may be happening...just a hunch. Assume there are additional active sites in the new branch to permit additional branching and bonding. Also, with increased branching, you get mechanical resistance by the simple mechanical interference between adjacent polymer chains...kinda like trying to pull one branch out of a brush pile...there is resistance between adjacent branches...but if all the chains are linear, without much branching, then there is greater opportunity for the adjacent chains to physically slip relative to one another in the axial direction. Like taking one strand of spaghetti out of the box...
Branching (i.e., the mineral spirits example, and also with paint, which is also organic) may be causing as much mechanical "bonding," as chemical bonding, but that is just a hunch. PhD's...the class is yours!!
Maybe I shoulda kept my fingers off the keyboard...
later,
Al
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Thanks for all the nice comments, but the thanks really should go to others
I am heartened by all the kind remarks you guys have sent me regarding this thread.
But I really think we need to give a special thank-you to persons like Kraut, Tattoo, and Dave McDonald, who, in my opinion, are to inexpensive RC aircraft what the Wright brothers were to aviation.
Having a lot of technical knowledge can be of use. But when you combine the technical data with the creativity that these men have brought to the table, then collectively we have a brand new aspect to the hobby that opens it up for many new people to enjoy who wouldn't have done so otherwise. Look at all the newbie questions that come up on this forum. That's growth!!Our hobby has changed dramatically recently. Look at all of the facets of RC modelling that are available to us today that weren't even in existance just a few years ago.
Now the Home Depot's and Ace hardware stores become the new "hobby shops." Additionally, my enthusiasm for combat has been rekindled with the advent of SSC and their coro designs. I no longer spend 40-plus hours on a foamy only to have it obliterated in the first round. Now I have a ship I can assemble in little more than a weekend that will last much of the season. And my piggybank is happier to boot.
So the next time you pick up piece of coro, or a piece of extruded aluminum channel, think of the guys who have the innate creativity to come up with these wonderful, and sometimes outrageous ideas. I admire those who possess such talent and only wish that I were one-tenth as creative as they are.
Al
But I really think we need to give a special thank-you to persons like Kraut, Tattoo, and Dave McDonald, who, in my opinion, are to inexpensive RC aircraft what the Wright brothers were to aviation.
Having a lot of technical knowledge can be of use. But when you combine the technical data with the creativity that these men have brought to the table, then collectively we have a brand new aspect to the hobby that opens it up for many new people to enjoy who wouldn't have done so otherwise. Look at all the newbie questions that come up on this forum. That's growth!!Our hobby has changed dramatically recently. Look at all of the facets of RC modelling that are available to us today that weren't even in existance just a few years ago.
Now the Home Depot's and Ace hardware stores become the new "hobby shops." Additionally, my enthusiasm for combat has been rekindled with the advent of SSC and their coro designs. I no longer spend 40-plus hours on a foamy only to have it obliterated in the first round. Now I have a ship I can assemble in little more than a weekend that will last much of the season. And my piggybank is happier to boot.
So the next time you pick up piece of coro, or a piece of extruded aluminum channel, think of the guys who have the innate creativity to come up with these wonderful, and sometimes outrageous ideas. I admire those who possess such talent and only wish that I were one-tenth as creative as they are.
Al
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Loctite primers
Egad!!
I worked for Loctite later in my career!!
Loctite anaerobic thread lockers, sealants and adhesives are polyester polymers that cure (i.e., harden or solidify) in the absence of air. That is why when you buy a new bottle of Loctite it is only half full. The air in the bottle keeps it from curing.
Another thing that helps this chemical reaction to kick off is the presence of "active" metals such as copper or brass. Less active metals are like aluminum, and the ferrous metals - iron and steel fall somewhere in between.
The first generation Loctite products would not "kick off" with just the absence of air...they needed a catalyst...i.e., the presence of an active metal. Now if you happened to be joining a steel part with a steel fastener, it might kick off, but it took forever to happen. So Loctite developed a family of "primers" which had copper salts in them, to apply to the mating members first before applying the adhesive and subsequent assembly. The adhesive saw the absence of air and was fooled by the primer into thinking it was joining two pieces of copper together and the solidification took place.
Later generations of adhesives were developed that did not require the primer, just the absence of air. But the use a primer will still accelerate the reaction.
Hope this helps.
Al
I worked for Loctite later in my career!!
Loctite anaerobic thread lockers, sealants and adhesives are polyester polymers that cure (i.e., harden or solidify) in the absence of air. That is why when you buy a new bottle of Loctite it is only half full. The air in the bottle keeps it from curing.
Another thing that helps this chemical reaction to kick off is the presence of "active" metals such as copper or brass. Less active metals are like aluminum, and the ferrous metals - iron and steel fall somewhere in between.
The first generation Loctite products would not "kick off" with just the absence of air...they needed a catalyst...i.e., the presence of an active metal. Now if you happened to be joining a steel part with a steel fastener, it might kick off, but it took forever to happen. So Loctite developed a family of "primers" which had copper salts in them, to apply to the mating members first before applying the adhesive and subsequent assembly. The adhesive saw the absence of air and was fooled by the primer into thinking it was joining two pieces of copper together and the solidification took place.
Later generations of adhesives were developed that did not require the primer, just the absence of air. But the use a primer will still accelerate the reaction.
Hope this helps.
Al
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Flame Treating Coroplast for Adhesion
I used to paint cars for about 5 years and plastics used a special primer to get the paint to stick to it.Of course you had to wet sand the surface to be primed,or the primer would not stick.I think the same principle with mineral spirits and a scrub pad.You are scratching and cleaning the surface at the sametime.I think the scratching is more relevant than the spirits.Just my $.02
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Flame Treating Coroplast for Adhesion
Two things happen when you scratch the surface with a Scotchbrite pad or sandpaper:
1. You provide a tooth for greater mechanical adhesion.
2. Microscopically you are increasing the surface area and exposing more potential bond sites for chemical bonding, though their numbers are relatively few with regard to polyolefins.
The chemical primers I refer to above are used essentially to catalyze the polymerization process, and sometimes, they are applied to chemically etch the surface to be painted. Alodyne is used to prepare the surface of full-scale aluminum aircraft prior to the application of a zinc chromate primer coat. Aluminum rapidly oxidizes in the presence of air, and the Alodyne removes the oxidation layer and makes the surface microscopically etched and chemically oxide-free prior to the application of the zinc chromate.
With respect to Loctite 770 primer, most of Loctite's products employ similar reactions. The primer is used to initiate the cure on otherwise inert surfaces. I think it is used with one of their newer chemistries...perhaps methacrylate adhesives. I could find out what that system is if you would like. Lemme know.
Al
1. You provide a tooth for greater mechanical adhesion.
2. Microscopically you are increasing the surface area and exposing more potential bond sites for chemical bonding, though their numbers are relatively few with regard to polyolefins.
The chemical primers I refer to above are used essentially to catalyze the polymerization process, and sometimes, they are applied to chemically etch the surface to be painted. Alodyne is used to prepare the surface of full-scale aluminum aircraft prior to the application of a zinc chromate primer coat. Aluminum rapidly oxidizes in the presence of air, and the Alodyne removes the oxidation layer and makes the surface microscopically etched and chemically oxide-free prior to the application of the zinc chromate.
With respect to Loctite 770 primer, most of Loctite's products employ similar reactions. The primer is used to initiate the cure on otherwise inert surfaces. I think it is used with one of their newer chemistries...perhaps methacrylate adhesives. I could find out what that system is if you would like. Lemme know.
Al
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Flame Treating Coroplast for Adhesion
well, for well over a year before i began scotchbriting the coro with the m. spirits, i just wiped it down with a cotton rag, and mineral spirits. It held about as well. The mineral spirits defenitely have some kind of effect on it, as the coro loses its sheen once its been rubbed with it (with either scotchbrite, or just a cotton rag).
I think the 770 primer is mostly heptane. Which i've heard of people using with success as well.
Whatever it is, it defenitely has some effect on the coro. Not sure if it frees up some molecules or what, but, it does make a difference.
Most coroplast, if you buy it new, already has been corona treated. But, that wears off eventually, as i understand it. When i put control horns on, i always screw and glue them in place. I never prep them at all, in hopes that i can get them back off without damaging either the coro, or the horn. Most of the time i can, sometimes i can't. But, i can tell you that after they've set for 24 hours, its very hard to get them off. Its a very stong bond, even without prepping. If you prep it, forget it, its not coming off.
I think the 770 primer is mostly heptane. Which i've heard of people using with success as well.
Whatever it is, it defenitely has some effect on the coro. Not sure if it frees up some molecules or what, but, it does make a difference.
Most coroplast, if you buy it new, already has been corona treated. But, that wears off eventually, as i understand it. When i put control horns on, i always screw and glue them in place. I never prep them at all, in hopes that i can get them back off without damaging either the coro, or the horn. Most of the time i can, sometimes i can't. But, i can tell you that after they've set for 24 hours, its very hard to get them off. Its a very stong bond, even without prepping. If you prep it, forget it, its not coming off.
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They probably use the corona discharge for the benefit of the sign makers to enable their inks and paints to adhere. They probably never figured that a horde of free-spirited RC fanatics would change all that....
I put a call in to the local Loctite office here in Metro Detroit. I'll find out what's with their 770 primer and what's in it.
later,
Al
I put a call in to the local Loctite office here in Metro Detroit. I'll find out what's with their 770 primer and what's in it.
later,
Al
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Flame Treating Coroplast for Adhesion
Chris,
I found a copy of the Loctite design manual here in the office. You are correct - Primer 770 is heptane. I would caution anyone who uses it to use care...it has a flash point of -1 C or about 30 F. With a flash point that low it is very flammable.
The manual says:
"Used to bond otherwise unbondable plastics with cyanoacrylate adhesives."
That about says it all.
There is another CA primer they list in the same category - 793 which is water/glycol based. Flash point is much better: 65 C.
Plain water can be used to accelerate CA's.
Al
I found a copy of the Loctite design manual here in the office. You are correct - Primer 770 is heptane. I would caution anyone who uses it to use care...it has a flash point of -1 C or about 30 F. With a flash point that low it is very flammable.
The manual says:
"Used to bond otherwise unbondable plastics with cyanoacrylate adhesives."
That about says it all.
There is another CA primer they list in the same category - 793 which is water/glycol based. Flash point is much better: 65 C.
Plain water can be used to accelerate CA's.
Al
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Flame Treating Coroplast for Adhesion
The primers and kickers generally contain a small amount of an active organic compound dissolved in an organic solvent. The heptane in the Loctite primer is simply an inert solvent for an organic nitrogen compound that actually kicks the cyanoacrylate and may enhance bonding to the inert plastic surface.
The vast majority of the primer/kickers are solvent but the "amidine" present in the solution is what does the trick. Heptane is a simple seven carbon saturated hydrocarbon that does not interact chemically with the coroplast or the adhesive. It will not appreciably soften or dissolve the plastic.
By the way, even though the amount of active ingredient in CA kickers is quite small, you can dilute the kicker at least x4 with whatever common solvent is used in the particular formulation. Still works great for balsa or other common CA applications.
Ross
The vast majority of the primer/kickers are solvent but the "amidine" present in the solution is what does the trick. Heptane is a simple seven carbon saturated hydrocarbon that does not interact chemically with the coroplast or the adhesive. It will not appreciably soften or dissolve the plastic.
By the way, even though the amount of active ingredient in CA kickers is quite small, you can dilute the kicker at least x4 with whatever common solvent is used in the particular formulation. Still works great for balsa or other common CA applications.
Ross
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Flame Treating Coroplast for Adhesion
Thanks Ross, you are correct.
I spoke with a Loctite technical representative. The heptane is the volatile carrier with a tri-alkyl molecule dissolved in it that acts as the bonding agent.
The bond between two of the three alkyl groups with the polyolefin surface is essentially mechanical, with the remaining alkyl group and the central nitrogen atom providing the bonding site for the CA.
Reading Loctites Technical Data Sheet shows the shear strength on polypropylene to polypropylene joints to be upwards of 7 Newtons per square millimeter, depending on the CA product used and the thickness of the bond. This works fine on spar joints where the bond between the spar and the coro is subjected principally to shear loads.
However, they caution that they do not recommend this primer where high peel strength is required. The trailing edge joint is one such joint, until such time that the coro has undergone sufficient creep to reduce the residual stresses imposed by bending the coro over the spar and down to the trailing edge joint.
I think, in light of the flammability issues, the questionable peel strength issues, and the mechanical bond of the primer with the coro, I would lean toward flame treating as a more desirable method for bonding. However, if it works, and it aint broke...don't fix it.
Hope this helps,
Al
I spoke with a Loctite technical representative. The heptane is the volatile carrier with a tri-alkyl molecule dissolved in it that acts as the bonding agent.
The bond between two of the three alkyl groups with the polyolefin surface is essentially mechanical, with the remaining alkyl group and the central nitrogen atom providing the bonding site for the CA.
Reading Loctites Technical Data Sheet shows the shear strength on polypropylene to polypropylene joints to be upwards of 7 Newtons per square millimeter, depending on the CA product used and the thickness of the bond. This works fine on spar joints where the bond between the spar and the coro is subjected principally to shear loads.
However, they caution that they do not recommend this primer where high peel strength is required. The trailing edge joint is one such joint, until such time that the coro has undergone sufficient creep to reduce the residual stresses imposed by bending the coro over the spar and down to the trailing edge joint.
I think, in light of the flammability issues, the questionable peel strength issues, and the mechanical bond of the primer with the coro, I would lean toward flame treating as a more desirable method for bonding. However, if it works, and it aint broke...don't fix it.
Hope this helps,
Al