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Rubber Compounding

Rubber compounding is often described as a black art by non-rubber people ...... but is in fact very far from it. It is (un)fortunate that rubber cannot be used in it's standard form, and must be mixed with other ingredients in order to produce a workable material. It is this mixing done by all and sundry in the industry and the fact that rubber in general is quite tolerant of many cheaper ingredients, that has probably by rights lead to people being somewhat suspicious of compounding.


Historically, had rubber compounding developed for the simple reason that a curative had to be added to convert the material from a processible form to a more stable form suitable for the various applications that rubber is used in.
From there developed the practice of extensively mixing other ingredients into rubber, most of which we shall see serve a very important purpose......

Compounding of rubber is done for the following reasons....
-As stated above, to add the various curatives
-To make it more processible, by adding ingredients that essentially reduce the nerve of the rubber
-To achieve the physical properties required by a particular application
-Last but not least, to cheapen the material, especially since many applications do not necessarily require a top class rubber.

Compounding is largely the domain of conventional rubbers. More modern rubbers like Silicone Rubber, PolyUrethane Rubbers and ThermoPlastic rubbers are not as a rule mixed / extended as extensively as Conventional Rubbers, and is mixing if at all done limited to the addition of small special purpose ingredients.

Compounding involves the following main groups of ingredients ....
-Polymer, being the basic material
-Curatives, being the curing agent, accelerators and activators
-Fillers, including the reinforcing fillers, non-reinforcing fillers, oils, process aids, and tackifiers
-Anti degradents, which include anti oxidants and anti ozonants

The ingredients and quantities to be mixed into a rubber is expressed in a formula, and is based on 100 parts of rubber (phr). All other ingredients are also expressed in phr (and not percentage as is the common practise with many other formulas).... this is because the other ingredients are very much related to the amount of rubber polymer in the mix.
The drawing up of the formula is largely the domain of the Rubber Technologist. Although there are many off the shelf formulas available, is extensive knowledde and experience is required to oversee the mixing process since there are so many variables that can change and requires adjustments in the formula.
One very important objective is to formulate to obtain the properties required for a particular application, at the lowets practical cost. I say practical, because although one should strive for the lowest ingredient cost, are mixing and processing costs similar irregardless of the ingredients, and are there sometimes practical reasons for not necessarily going for the cheapest ingredient cost

It is an important requirement that all ingredients mixed into a rubber be either super fine particulate solids, or a liquid or melting at or below the mixing temeparture and super soluble in rubber

-Mixing of rubber requires robust machinery, and is still very much done on a batch basis, and relies heavily on high shear forces not just because rubber is a very tough and viscous material, but also to disperse the ingredients evenly throughout the material.
Originally was mixing done on what is called a Two Roll Mill. Much small scale and custom mixing is still done on Two Roll Mills as this is practically and economically still a suitable option. The mixing time on a Two Roll Mill varies between 30 and 50 minutes.
Around the 1950's were Internal Mixers developed (first by Farrel, later Banbury, etc.) which essentially mixes the rubber inside a sealed and slightly pressurized cavity. This not only hugely reduced mixing times, but also allowed for an increased batch size, as well as increased quality and material consistancy. The mixing time on an Internal Mixer ranges between 3 and 6 minutes!


What makes rubber so different is that different rubbers are obtained by compounding various ingredients into the basic polymer to give it all the required properties

Unlike many other raw materials which you buy in a ready to use form (like steel, plastics, etc.), is this process often done by the rubber products manufacturer, so you as the user do not always quite know what you are getting.
Although you might have specified Neoprene, do you not know what other (cheaper) polymers are added to the mix, or what quantities of filler is added not only to make the rubber processible and give it the required physical properties, but also to cheapen
It is for this reason that various standards had been drawn up, to control if not the the content, then the quality of a rubber product

Since rubber is often used in a physical capacity, did certain physical properties develop to test rubber compounds by.

The most common and important physical properties are .....

+Measured using an indentation gauge, essentially measuring the material's resistance to indentation.
+For softer rubbers using a Shore A or IRHD gauge, (very similar scales) and expressed in those units, for harder rubbers using a shore D gauge and expressed in those units.
+A hardness of 100 degrees indicates no indentation, meaning the rubber is "rock" hard. A reading of 0 indicates full indentation, meaning that the rubber is "totally" soft.
+Although gauges should be calibrated, do they not "go out" for no reason (unless dropped, especially a problem with the handheld gauges). A simple test to ensure that it is not way out, is to check..... with no pressure on the indentor must the reading stand at 0, and when applied to a very hard and smooth surface must the reading come to (as near as dammit) to 100.
+Some hardness gauges have a mechanism for exerting a constant pressure on the rubber sample, but many types are handheld for practical purposes, in which case the reading obtained is suject to the amount of force exerted..... but it doesn't take much effort to develop a skill whereby a constant pressure is exerted and fairly consistant readings obtained.
+There is quite a bit more on the subject of hardness measurement and recording, in order to ensure that consistant and realistic results are obtained....... see full procedure on the subject.

-Tensile Strength and Elongation at Break.
+Measured using a Tensiometer (or Tensile Tester), essentially a device that stretches a rubber sample, and recording the force applied.
*A Tensile Tester suitable for rubber must have a travel of at least 1.5 meters, and capable of applying a force of at least 1000 Newtons
+The sample is obtained by punching, using a standard dumbell punch, from a rubber sheet between 3 and 6mm thick.
+The sample is stretched at a constant rate, and at the point of breaking, is the Force and the extension recorded.
+Using the prescribed formulas, is the Tensile Strength calculated (in Mpa), as well as the Elongation at Break (in %)

The above 3 properties are considered the most important for describing the physical strength of a rubber in general. Although other properties might be as or even more important (as listed below), can Tensile Strength and Elongation at Break give a very good first indication of the quality of the rubber.

Other more speciality properties are.....

-SG: Specific Density; ranges between 0.9 and 1.3; largely used for control and other purposes, not much of an indicator of quality.
-Compression Set: the extent to which a rubber un-recovers after being compressed and relaxed; expressed as a %; the lower the reading generally the better the quality of the rubber.
-Resilience: the extent to which a rubber is able to rebound; normally determined by measuring the bounce of a ball when dropped from a certain height; expressed as a %; the higher the reading generally the better the quality of the rubber.
-Tear Resistance: force required to tear a sheet of rubber, not a very repeatable test as the tearing of rubber is affected by many uncontrollable factors, one being the direction of the grain.
-Resistance to heat: simply the temperature at which the rubber shows a marked deterioration, often coupled to a certain elasped time.
-Resistance to abrasion: the extent to which a rubber will wear well, particularly relevant to the tyre industry; but all tests merely a ________ emperical
-Resistance to chemicals: measured in terms of time subjected to a certain chemical, before the rubber shows a marked deterioration. Must be done and reported for a specified temperatures, since the deterioration increases dramatically with temperature as well.
-Resistance to ..... you name them, there are many custom tests that tests rubbers for different properties, specific to certain products and applications.

A very interesting subject, yet one that often confuses people and causes them pay more for a poorer quality rubber......
-Rubber is often sold by weight, and as you can imagine must density play some role in the scheme of things.
+when rubber is sold (or you are clever enough to insist that it be sold) by volume, ie. by item, is this discussion not particularly relevent and can you relax since you are now able to compare "apples with apples"
-Firstly realise that because of compounding, can various ingredients be added that affect the SG of the material dramatically, ie. fillers like Barytes with and SG of 2.8 will increase it, and Oil having an SG of 0.8 will reduce it, which is why a low SG does not necessarily imply a high quality.....
-If you are buying your material by weight, and a lot of high SG ingredients had been used, will you needless to say get less material for your money. You in turn will use or sell the rubber in terms of it's volume, meaning you get the same value out of it irregardless of the SG.
-So when buying rubber, insist that you pay by volume or article, and if you can't that you are given the (correct) SG so that you can convert the weight cost to a volume cost, and be able to compare it to similar products from other suppliers.
-In conclusion, beware that even having done the above, are you only able to compare price, and still not quality. For this you need to compare the various properties of the products on offer.

Good luck!