Yo Greg,
What does this term mean?
Why should we care to know what it means?
How does it apply to surfboard construction?
How does it affect that board that’s left in a closed carry bag or left in direct sun?
How will it vary in different Epoxy formulas?
How do it differ in Epoxy from Poly?
Gone Surfin’, Rich
HDT = heat deflection temp
http://en.wikipedia.org/wiki/Heat_Deflection_Temperature
Your other q’s are the proverbial can-o-worms. But in a nutshell, post curing epoxy has very positive affects on a surfboard laminate. But it is not always necessary, depending on various other factors.
Hey Rich,
I’ll expand a bit on what craftee had to say…
"What does this term mean? "
Craftee’s got it. Standardized test performed by plastics manufacturers to determine at what temperature a sample begins to lose it’s physical properties. Sample preparation, dimensions and testing methods are standardized.
“Why should we care to know what it means?”
Cause if you’re using a resin that has a low HDT, and you leave your board in a board bag, in the sun or your car, the plastic will soften. This will increase the likelihood of denting, etc. More or less answers your next question too. Also, think about sanding out a board; resin has a higher HDT than it might experience during “in-use” temperatures, but you’re not moving your sanding pad around enough to disperse heat buildup; the resin will soften and instead of abrading nicely, it’ll tear. Exposure of a part to temperatures exceeding the HDT of the matrix can lead to part deformation; especially for parts with “locked in” shear (think snowboard with the curves pressed in, rather than a surfboard with the rocker “cut” in).
“How will it vary in different Epoxy formulas?”
In general, higher modulus resins will have higher HDTs.
“How do it differ in Epoxy from Poly?”
You can formulate an epoxy to have a similar HDT to a poly; likewise, you can formulate a poly to have a similar HDT to an epoxy. Largely it has to do with space filling inside the resin matrix (less space between the molecules generally leads to higher HDTs), and degree of crosslinking (higher crosslinking = higher HDT; usually also higher modulus, and usually also more brittle).
Craftee’s suggestion in regards to post cure is good. Essentially, you want to insure that all possible chemical bonds that can form in a matrix, are fully formed before the part goes in to use. Taking a finished part and heating it to near it’s HDT insures that any remaining gross molecular motion within the matrix has taken place, allowing unreated monomers to get close enough to react. This essentially insures that all crosslinking that is GOING to take place HAS taken place, effectively maximizing the HDT while retaining the overall physical properties designed in to the resin.
Unlike polyester resin that will continue to crosslink over time (why it’s recommended that poly glassed boards sit for a period of time after glassing, before use), epoxy needs a bit of help getting the amines to come in contact with the epoxy portion of the resin; at the right angle, the right orientation; heating the part up allows this to happen. Epoxy will cure to a point and then stop; not enough molecular motion to get the rest of the job done; leave the part for years and it won’t cure any more; heat it up to close to the HDT and the rest of the reaction will occur, maximizing resin properties.
hth
g
Great explanation thanks.
So when you bake a board the temperature shouldn’t exceed the HDT for a given epoxy resin right?
Are there any rules of thumb for how long a board should be baked? Or does that also depend on the particular properties of a given epoxy resin?
Karl
I didn’t even have to answer … but I will. HDT heat deflection temp. This is a test and the number has to do with the stabuility of the parts made to heating. Many misinterperate this to mean that when a part gets to this temperature it will somehow disintegrate. This is just a test number that refers to when the deflection can be measured to 10% in the test. Post curing does raise HDT about 1 degree for every degree it’s raised in post cure above 77F. So if your HDT is 125F cured at RT then if you post cure to 127F your HDT will be about 175F.
It doesn’t apply much to surfboards because they don’t get very hot all that often and if they go any higher 160F then the foam begins to degrade (EPS or PU). In board bags your usually going to see foam problems. Different formulas have different HDT’s. Ours is rather high for a room temp system @ 125F. Polyesters do have slightly higher HDT at room temp cure but don’t increase much when post cured.
Greg, how long can i wait after lam, hot and gloss with RR to Cure it to bring the HDT up (hopefully thru the winter; two boards in my basement awaiting cure in May)