Elastic moduli and damping vary with crosslink density in thermosetting elastomers (e.g. rubber). Crosslink density is varied by adjusting material (resin type, hardener/initiator concentration, etc.) and process conditions (e.g. temperature).

If you are making sanctioned paddles, you are limited to a rather thin thickness for the striking surface: http://www.natabletennis.com/index.php?option=com_... http://www.ittf.com/ittf_handbook/ittf_hb.html

I think that you'll find that there are:
> A LOT of material choices https://www.megaspin.net/store/Default.asp?cid=rub...
> all of them have some compromise somewhere to achieve the totality of performance required

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Of course I can. I can do anything. I can do absolutely anything. I'm an expert!

There are the basic rubber properties. Also, most of the rubber has "pips" on the inside, size/spacing affect the springiness. Then there is the sponge layer, thickness and hardness. Then there is the paddle (aka blade) which has any number of factors that will affect the bounce; thickness, composition, type of wood, number of plies, composite reinforcement.

If you look at the ratings for table tennis equipment, you will be absolutely baffled at how many parameters there are. Most of them subjective, no idea how they claim to test them.

If you really want to get confused, look up "speed glue." Ignoring the context and just answering the initial question, speed glue makes table tennis rubber more springy. It has been banned for competition.

I am an club TT player. I know about the different types of rubbers one can buy. Yes, I have used speed glue too. I am looking for more of an engineering type of answer as to what makes a rubber springy.

[quote=CoryPad]
Elastic moduli and damping vary with crosslink density in thermosetting elastomers (e.g. rubber). Crosslink density is varied by adjusting material (resin type, hardener/initiator concentration, etc.) and process conditions (e.g. temperature).
[/quote]
Would I be correct in assuming that those polymers with fewer crosslinks would have a lower internal resistance?

If I drop a ball on a rubber the ball will only bounce up a fraction of the original height because of the coefficient of restitution. Clearly the rubber absorbs some of the impact energy. This must be due to some form of internal resistance in the rubber. Are these crosslinks responsible for the internal resistance?

Thanks

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com

When discussing coefficient of restitution (CoR), a lower numbers means inelastic energy, e.g. through damping. Damping is caused by motion of the polymer chains. Crosslinking reduces the motion, reduces damping, and increases CoR.

Thanks, this seems intuitive to me but I wanted someone to confirm my intuition.

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com

Wait, CoryPad, don't you mean the crosslinking reduces motion and INCREASES damping and DECREASES COR?

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com

I'm not suggesting you re- or double-post, but, FYI, there is a Rubber Engineering Forum (http://www.eng-tips.com/threadminder.cfm?pid=335) that is monitored by rubber industry experts. This question might have been better posted there.

Increasing crosslinking means the chains are covalently bonded to each other in more locations, which decreases damping. If you increase crosslinking enough, the part moves from a flexible elastomer to a stiff, brittle thermoset.

Isn't table tennis, a ping pong table which is some type of wood composite for a surface?

I don't want to go over old ground but much will depend on the relative thicknesses of the rubber cover and, where it exists, the underlying sponge layer. Increasing the crosslink density will certainly raise the resilience of the rubber, but not that dramatically once optimum cure for other key properties has been exceeded. But a thin rubber layer, however well cured, will have limited effect against the damping behaviour of the sponge. And, of course, we all have to comply with the requirements of the ITTF.

Have a look at the article Guide to Choosing Table Tennis Rubber on mega.spin.net

https://www.megaspin.net/store/extra/rubber-guide....

CoryPad, damping is the d term in m*x"+d*x'+k*x=F(t)
basically damping is a frictional force proportional to the velocity x'. The higher the damping the more the rubber or sponge will absorb energy and not return it to the ball. Assume the reference is the paddle. On impact the ball will compress the rubber converting all the kinetic energy into potential energy and heat. When the rubber accelerates the ball in the opposite direction some of the potential is lost again due to damping but also the some of the energy must move the sponge as well as the ball.

[quote=CoryPad]
Increasing crosslinking means the chains are covalently bonded to each other in more locations, which decreases damping. If you increase crosslinking enough, the part moves from a flexible elastomer to a stiff, brittle thermoset.
[/quote]
Doesn't increasing the number of cross links make it more difficult for the polymers to slide by each other. This would increase internal friction or damping.

To get maximum speed after impact one would want to reduced the damping d to as low as possible and also reduce the mass of the spring ( rubber ) so less energy is used to accelerate the rubber and more can accelerate the ball.

Reducing the d term, damping, is part of making rubbers more efficient ( faster ). My question is how does one reduce the d term at the polymer level. It seems that more cross links is not the answer. Few cross links may let the polymer chains slide by each other more easily and therefore have a lower internal resistance.
Longer cross links may also allow the polymers to move relative to each other more freely.

I said above, I am a mid level club player and an engineer.
I am aware of megaspin.net. Most of the information there is marketing hype and opinion.
I have bought many rubbers but playing with them doesn't tell me anything about how a manufacture makes one rubber "faster", springier or more efficient than another rubber.
I am looking into what rubbers would be the most efficient and how are they cross linked that makes them so special.

Thanks again for any useful information

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com

To ensure we’re on the same wavelength, let’s review some rubber chemistry. Natural rubber has the highest rebound resilience and the lowest internal damping. In principle polybutadiene should match or better NR but in practice falls short and has inferior vulcanizate properties. NR is usually crosslinked with 2 -4 % elemental sulfur assisted by one or more accelerators. Elasticity is highest in the ‘gum’ state, ie with antioxidants and pigments but without fillers. The resulting hardness is 30 to 40 Shore A units but can be raised a little by a small amount of filler such as calcium carbonate. Using sulfur levels above normal can raise elasticity but at about 10 % the vulcanizate enters the ‘loggy’ phase between soft rubber and ebonite. The increase in hysteresis is caused not by the change in crosslink density but by the substantial increase in main chain modification, usually in the form of cyclic sulfides. That causes an increase in the glass transition temperature and the closer this gets to room temperature the higher the damping. I have not seen any evidence that crosslink length has any bearing on viscoelastic behaviour. Even in a well cured rubber there are relatively few crosslinks within the network – and the network is the key feature governing recovery. As far as ease of polymer chain slippage is concerned, don’t forget that leads to creep and set if the crosslink density is below optimum, thus increasing energy loss.
My company looked at compound development many years ago, but if my memory serves me there was nothing worth pursuing. Which isn’t really a surprise given the influence of pimples, surface tack, thickness, tensioning, glue etc. Hence the wide selection of rubber covers now available, including the anti-spin harder sheets and the use of synthetic elastomers such as neoprene. Sometimes I think it comes down to personal preference and a question of faith rather than science, and naturally we can’t forget the skill factor. However, good luck with your investigation

According to this article, speed glue is though to work by creating a trampoline effect. The solvent in the glue makes the foam layer swell and that stretches the rubber out layer, giving the trampoline effect (just like a drum with the skin pulled tightly would be more bouncy).

If this is true then it's nothing to do with cross-linking or the rubber material. You could test the theory by first stretching a rubber then gluing it in place. That would get you around the rules against speed glue as well.

https://en.wikipedia.org/wiki/Speed_glue

Chris DeArmitt PhD
President - Phantom Plastics LLC

Trusted adviser to leading companies around the world
www.phantomplastics.com