Mixing in an Internal Mixer

Internal mixer has a fixed volume mixing chamber and therefore only one particular total amount of ingredients is the correct one.

Thus we need to calculate the correct batch weight, based on the density or the specific gravity of the ingredients and volume of the mixing chamber. This step involves the multiplication of formulation quantity in phr by a fill factor, which is a ratio of optimum mix volume to formulation volume.

CALCULATING OPTIMUM BATCH WEIGHT

Internal mixer chamber volume  =   50 L

Optimum mix volume (80%)  =   40 L

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Compounding	Parts	Density	Volume	Batch weight
ingredients	(phr)	kg/L	(L)	(kg)
SMR 10	100	0.94	106.4	25.810
Zinc oxide	10	5.55	1.8	2.581
Stearic acid	2	0.92	2.2	0.516
N550 Carbon Black	50	1.80	27.8	12.910
Oil (naphtenic)	10	0.92	10.9	2.581
Antioxidant TMQ	2	1.08	1.9	0.516
Antiozonant DPPD	2	1.22	1.6	0.516
Sulphur	0.25	2.07	0.1	0.065
TBBS	2.10	1.29	1.6	0.542
TMTD	1	1.35	0.7	0.258
Total	179.35	1.16	155.0	46.290

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Total formulation phr =   179.35 phr

Total formulation volume =   155 L

Formulation density (ρ = m/V) =   179.35 kg/155 L   =   1.16 kg/L

Fill factor  =  (optimum volume)/(formulation volume) =   40 L/155 L

∴   fill factor =   0.258

Each Ingredient phr x fill factor   =   weight of each ingredient in a batch

Eg:  Rubber             →         100 x 0.258  =   25.80 kg

Carbon Black          →           50 x 0.258   =   12.91 kg

TBBS                    →           2.1 x 0.258   =     0.54 kg

Total wt of ingredients  =  Batch weight =   46.29 kg

Batch volume  =  Batch weight/density  =   46.29/1.16  =   40 L

This confirms that the calculated batch weight of 46.29 kg will occupy exactly 40 L volume which is 80% of chamber full capacity (50 L).

OPEN MILL MIXING ON TWO-ROLL MIL

Mixing on 2-roll mill

The mixing is often done on a two-roll mill (open mill).  A mill consists of two horizontally placed hollow metal cylinders (rolls) rotating towards each other.  The distance/gap between the mill rolls (nip) can be varied, typically between 2 to 20 mm.

Rolls Friction Ratio

The speeds of the two rolls are often different, the back roll rotating faster than the front. The difference in speed between the two rolls is called the friction ratio. The friction ratio allows a shearing action (friction) at the nip to disperse the ingredients and to force the compound to stay on one roll, preferably the front one. A friction ratio of 1.25:1 is common.

STEPS IN MASTICATION AND MIXING

Mastication Operation

Set the roll nip opening to 2 mm. Adjust and maintain roll temperature at 70 ± 5 degrees C. Add rubber into the mill nip and band, as a continuous sheet, onto the front roll. Using a hand knife, make two ¾-cuts from each side and allow the rubber to move through the nip quite a few times until a smooth rolling bank is formed on the nip. Now the rubber is sufficiently masticated and thus becomes softer at a reduced viscosity.

Initial Addition of Ingredients

Set the nip opening to 2.5 mm. Add activators (and antidegradants if synthetic rubber) to the rolling bank. Make two ¾-cuts from each side. Cut through the rubber at one end of the roll, remove it, and place it in the nip at the other end, to ensure a homogeneous end-to-end blend.

Addition of Fillers

Increase the nip size to 3 mm. Add fillers. If large amount of fillers is to be added, divide it into 2 or 3 equal parts and add sequentially. For large amount of filler formulation pour on processing aids/plasticisers (oils) incrementally (as are the fillers), after most of the fillers have been mixed in. After each addition of filler portion make three ¾-cuts from each side and again cut through the rubber at one end of the roll, remove it, and place it in the nip at the other end.

Note well that antioxidants may inhibits oxidative breakdown of rubber, they should be added late during mixing of natural rubber. It is advisable to add antioxidant just before the addition of sulphur and accelerator. But with synthetic rubbers an early antioxidant addition can avoid cyclization. (Reference: Moneypenny H, Menting K, Grag F. General Compounding, Rubber Compounding Chemistry and Application 2004 (373); Ed., Brendan Rodgers.)

Addition of Curatives

Add sulfur and accelerator toward the end of the mixing process to avoid premature cross-linking reaction. Make two ¾-cuts from each side and again cut through the rubber at one end of the roll, remove it, and place it in the nip at the other end.

Sheet Out

Sheet out the compound by pressing it with your hands against the roll and allow it to roll back into itself to become a long cylindrical roll. Set the nip size to 8 mm and maintain the roll temperature at 70 ± 5 degrees C . Pass the rolled batch endwise through the mill nip 6 times. This is a homogenization step.

Weigh the rolled batch to measure actual batch weight and % weight loss.

% wt loss = {(theoretical batch wt – actual batch wt) /theoretical batch wt} x 100

Adjust the nip size to give a sheet with minimum thickness of 6 mm when the compound is sheeted out of the mill.
Allow, at least 24 hours, for maturation after which the compounded sheet is ready for subsequent process (moulding /curing).

MASTICATION OF RUBBER

Before we can add compounding ingredients, such as activator (Zinc Oxide and Stearic Acid), accelerator and sulphur, into the rubber for vulcanization purposes, we need to soften the high molecular weight rubber, especially natural rubber (NR), so that a homogeneous dispersion of all ingredients into the rubber matrix becomes possible.

This process of softening by means of mechanical shearing is known as mastication. Sufficiently masticated rubber will form a rolling bank on top of a nip in between two rotating rollers. This is the case if the mixing step is carried out on an open mill (2-roll mill).

In its raw form, NR is a very high molecular weight (MW) rubber naturally polymerised in hevea trees. Thus man cannot control NR MW as it is always the case for synthetic rubbers. High MW means high viscosity thus renders it hard. It is almost impossible to mix any powder or liquid ingredients into NR at its original viscosity. Thus Natural rubber requires controlled reduction in molecular weight, this molecular weight reduction process is actually a mastication process.

Mastication shortens rubber molecular chains, resulting in a reduced molecular weight and hence reduces its viscosity. These allow compounding ingredients to be homogeneously distributed into the rubber. Masticated rubber is softer and flows more easily than in its original state. However, if over masticated the rubber becomes tacky.

In short, mastication is a preliminary stage to processing the raw rubber. This process involves the use of special mechanical equipment and additives (e.g. aromatic mercaptans – sulfur-containing compounds) at low temperatures to shred the rubber molecules into smaller units.  This improves the plasticity and reduces the viscosity.

BASIC RUBBER COMPOUNDING

Introduction

One of the crucial tasks in rubber industry is to select the basic raw materials for the preparation of a specific commercial product. This requires a personnel with necessary technical background since the processes involve complicated chemical reactions such as vulcanization.  

The technical personnel, say a rubber technologist, may be required to do chemical analysis of the raw materials and of the finished rubber products. Thus he or she should have some background knowledge in order for him or her to make a right decision to select the most appropriate test methods.

As a rubber technologist, one must be capable of describing these processes and the problems involved to those concerned with the production of  a serviceable product at a reasonable cost.  Thus a rubber technologist must have an understanding of various types of raw materials used in making the rubber articles since raw rubber on its own is almost useless. 

In developing a rubber compound, it is  essential to mix the raw dry rubber with various compounding ingredients.  The process of sequentially adding the ingredients into the raw rubber is termed ‘compounding’ and the resulting final homogeneous mix is referred to as the ‘rubber compound’. 

The most common  compounding ingredients  added into the rubber are vulcanizing agents, vulcanization accelerators, activators for the accelerators, fillers, processing aids, softeners, antidegradants and other miscellaneous ingredients for specific purposes.  Each compounding ingredient has specific role in the compound and some give significant effects on the final properties of the resulting products.

Therefore a rubber compound has to be formulated with an expectation to incorporate the desired properties required in the finished rubber product as well as in the production process.  In view of this, the important factors that need to be considered when designing a rubber compound are price, processability and functional properties. 

Compounding Recipes

How much the quantity of each ingredients that is required to be added into the rubber must be specifically prescribed prior to the mixing process. This prescription is a compilation of various ingredients into a “recipe” (formulation).  In the recipe the quantity of each ingredient is quoted as an amount based on a total of 100 parts of the rubber or combinations of rubbers (or masterbatches) used. This notation is generally listed as phr (parts per hundred of rubber). 

Hence if we are to compare the outcome of two or more different recipes with respect to the processing characteristics or the physical properties of the final vulcanizate, the effects of varying any particular ingredient, from one recipe to another, can be easily identified.

Below is an example of a compounding recipe.

INGREDIENTS            phr

SMR 10 (NR)              100

Zinc Oxide                       5

Stearic Acid                     2

MBT                                0.5

Sulphur                           3.0

It is a common practice to list the materials in the general order that they are mixed into the rubber during processing. This will help the mill room workforce in setting up their mixing schedules for processing various compounds and for the preparation of special masterbatches which may be used in many different products.

SINCE JAN 2012