does the thickness of a rubber sheet affect the tensile strength and the elongation 2

[afx123]

Hi guys,

i would really appreciate it if someone can explain to me how to choose the right thickness of a rubber sheet and if the thickness would affect the tensile strength and the elongation?

thank you very much

 

[GrahamBennett]

It all depends on what you want to use the sheet for. For laboratory testing purposes the standard thickness is 2mm but it is possible to mould sheets from <0.5mm to >25mm thick.

 

[tom1953]

As thickness is force/cross-sectional area, it shouldn't matter matter whether you're testing a 1 mm or a 3 mm thick sample. Practically speaking, though, the thicker the sample, the more chance you'd have an inhomogeneity in the sample (undispersed ingredient, air bubble, cure variation, etc.) that may cause a premature break in the tensile test, so you may get a little lower tensile strength, and elongation, with thicker samples.

 

[asankaglove]

i think thickness should effect for the elongation. because crosslinking amount is higher in thicker product

 

[tom1953]

I see an error in my earlier post. In the first sentence, it should be "As tensile strength is force/cross-sectional area . . ."

Mea culpa.

 

[stancom]

I agree with Tom's explanation, and if you're testing using a standard dumbbell the quality of the cross section may be a factor if thickness results in a concave edge. With respect to asankaglove, I would have thought a thicker product would bring the risk of an undercure or a reverted surface, not an increase in crosslink density.

 

[JDLSHF]

Greetings.

There were different things written that are linked, but not the same.

TOM1953 explains correctly both.

Tensile strenght is affected by the samples' thickeness. For most of the properties (if not all) there are standard tests. For this case in particular, check ISO 372005). If you like to test this, just place a thicker sample for this test (if you can) and you will see this result.

A thick piece, is always in trouble for the reasons that TOM1953 said. Rubber does have air (beware of peroxide cures) trapped and air can get trapped inside your mould with the rubber. The smaller things are, the easy it is to not get air trapped, or to have all the air from inside the rubber released.

Asankaglove last doubt about the crosslinking and elongation is in fact not true. Elongation is not affected, for the stated principle of thickness, because of higher degree of crosslinking, but because there is more amount of material to distribute the aplied force. Of course that elongation is affected by crosslinking density! But not in terms of the same rubber but with different thicknesses.

Example: pick up an iron rod of the hardest you can find, with 1mm diameter, and say 1500mm length. Try to bend it by arm strenght only. You probably can. Now, for the same length and material, choose one with 100mm. Can you bend it (arm strenght again)? If you can, then superman is probably real. Chances are you can't.
Now, if the material is the same, the crystalline structure the same, with only changes in diameter, how can you bend one and not the other? Anwser: amount of material and not type of material through were your applied force is distributed. But you are right that elongation is affected by thickness. Not right as to the why.

Hope it helped!