NR Compounds for Latex Tubing 2

[Christopher F]

Greetings, rubber gods.

Firstly, with risk of getting auto-banned before I even get to "Hello"... Apologies as I may have ignored the sign-up requirements of being directly involved in engineering. I'm a hobbyist come minor-retailer with minimal knowledge of engineering. I can make a Lego model great though, so that's something I guess. Plus, you're the only active rubber engineering forum I could find. And Coruba sales reps are driving me nuts.


Anyway, I'm currently attempting to have some small rubber parts manufactured, with material mirroring that of standard extruded latex tubing. You know the type; NR, stretchy, medical/slingshot/spearfishing use. I'm not too fussy on properties, mainly just hardness, and to a degree the elasticity. Any "latex tubing" will do, so any similar material will do.

My technical knowledge stretches as far as the correct material being Natural Rubber (NR), usually of varying grades (SVR-3L, RSS1-5, whatever), and Shore A 35+/-5 hardness (generally though, mostly 40A). Which was enough for me to class it as simply generic NR material, bar one small issue....

Most, if not all latex tubing, floats. And, from my limited knowlege, standard NR does not. Which means "latex tubing" isn't standard NR, may have additional chemicals, may have been mixed with BR (Butadine), etc. And this may or may not affect the one or two minor properties that are actually important to me (namely, elasticity). So now I'm attempting to figure out if there's any real difference between NR "latex tubing" and generic NR compounds that will have any major affect on the handful of properties I'm interested in.


TLR; Does anyone know the exact NR compound Latex Tubing is made from? Anything specific I might want to know? If I ask a manufacturer to mold parts in NR, will it differ significantly from regular tubing? Am I spending far too much of my life obsessing over rubber? When will it ever stop snowing?


Thanks

Christopher

Attached: It probably floats; far more of a conundrum than it should be.

 

[btrueblood]

A pretty good overview in wikipedia,

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

, might help you.

Natural rubber is not much of a specification, but NR usually refers to compounded (fillers and other stuff added) and vulcanized (cross-linked by action of sulfur or peroxides). Latex, like the tubing you show, is less vulcanized or not vulcanized at all, and has fairly low levels of compounding or none at all, and may or may not float depending on the type of raw stock and degree of processing. The NR compounds can be made to float if you are ok with foam (you want your stuff to float, or not?).

Bottom line, it's complicated. A good rubber chemist I worked with described the process as being much like making a cake - you mix up the stuff you think will give you what you want, based on prior recipes that have been written down, and what you get...varies.

Find a shop near you that makes/molds rubber parts and go talk to them about what you want (I would personally avoid the unvulcanized stuff, as it's not predictable enough, but then i'm an engineer and like to predict things).

 

[Christopher F]

Thanks, clears things up a little.

My understanding was Latex (or any other term containing the word Latex) is the raw sap, and when refined and dried is then termed Natural Rubber (available in raw sheets or bundles), with other terms just varying depending on whether you speak American or not Eg. Gum Rubber, Natural Gum Rubber, Latex, Latex Rubber, Natrual Latex Rubber, Latex Natural Rubber, Natural Rubber, Dry Natural Rubber...

The vast majority of manufacturers I've spoken to only list NR as a production material and never mention latex specifically, with few exceptions (eg. these guys

http://www.hygenic.com/news/the-differences-between-latex-and-natural-rubber.html

). And even then everyone seems to have a different idea as to which term applies to what ("natural rubber is the sap", "no, latex is the sap", "no latex rubber is the sap"...). Eg.

https://www.westpharma.com/blog/201...r-Natural-Rubber-Latex-and-Dry-Natural-Rubber

which immediately contradicts the terms in the previous link

On the plus side, looks like tubing is made from both though -

http://www.hygenic.com/products/tubing/hytone-natural-rubber-latex-tubing.html

(With Natural Rubber Latex - raw Latex?)

http://www.hygenic.com/products/tubing/hygenicr-extruded-natural-rubber-tubing.html

(With Dry Natural Rubber; aka standard NR)
And... looks like both could either sink or float, too. Which, fwiw, I couldn't care less about. The elongation / elasticity however...

So I'm thinking I wing it and go with any NR I can get hold of and hope for the best (I'm a comp sci guy; we just hack stuff - whatever's easy and works )

 

[btrueblood]

Yeah, it's terminology. There are/were MIL standards and the IUPAC naming conventions that came up with the terminology that most engineers use.

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

But natural rubber has been around long before things like MIL standards and the IUPAC, and so a lot of traditional lingo is used.

 

[GrahamBennett]

btrueblood has given you a good start there Christopher.

The first thing you need to know is that the specific gravity of uncompounded natural rubber (be it in the dry or latex form) is 0.92-0.93, i.e. it will float on water.

Latices (or latexes) are either from natural sources (NR latex) (from the hevea Braziliensis tree), or synthetic sources (usually NBR latex) for use in products (usually examination gloves) that can't be used by people that have an allergy to NR latex.

Dry NR rubber is commonly sold in 33kg bales and is graded according to it's dirt content. TC (technically classified)5 NR contains <0.05% dirt, TC20 <0.16% dirt, etc. SMR grades indicate NR is from Malaysia, SVR - rubber is from Vietnam, SIR - Indonesia. TC graded rubber could come from anywhere in the tropics.

As a basic starting point for rubber technology I recommend you Google John Loadman bouncing-balls. His site is full of very entertaining, knowledgable and very accurate information.

forumtowers.com

 

[btrueblood]

Graham, that's a very cool website, I like reading history.

 

[Metten]

Give the rubber molder a specification to which the rubber needs to comply and request a small vulcanised sample of the type they offer (test slab, not your required shape).

example spec:
NR or Latex based (the exact name really doesn't matter, if it works, it works)
30-40ShA
density <1 or >1 kg/L (which you prefer)
elongation at break >500%
modulus at 100% <1 MPa
tensile strength >6 MPa
colour:

usefull info for compounder/molder: will be used at ...°C, in sunlight yes/no, comes in contact with ... (grease, oil, chemicals or cleaning agents)