Optimization of mixing quality by use of pre-dispersed additives.

One of the characteristics of the rubber industry is the need for a wide range of very different raw materials for production of rubber mixtures. Even simple formulations for sulfur-crosslinking mixtures will easily comprise eight or more components. The chemical activity and consistency of the raw materials may span a very wide range, from inert solid fillers to reactive liquid co-crosslinking agents.

The formulation shown in table I could easily be expanded to include many more items if the mixture requires dispersing agents, silane primers, propellants, flame resistant components or haptic and olfactory enhancers. Though only a small proportion in terms of quantity, the chemical additives from zinc oxide through sulfur have an important role to play alongside filler/plasticizer systems, to achieve the desired properties of the vulcanizate. The "small chemicals" can have a decisive influence on processing behavior, crosslinking density, aging stability, specific gravity, sensoric properties and overall performance of the rubber compound.

Mixing quality

The term mixing quality comprises two aspects. The first is homogeneous distribution of all components, in particular the small quantities of additives, throughout the entire mixing volume; the second is reproducibility in discontinuous production processes.

Optimal physical and chemical interaction of all components of a mixture requires effective distribution down to the very small, molecular dimensions. However, the standard commercial forms of delivery of most raw materials are not sufficient for this purpose. Rubber materials are de livered in bales; carbon black filler particles are delivered in agglomerated form; and additives are not normally supplied as nano-scale fine dusts, but rather as agglomerates with a more or less wide range of particle sizes.

What does dispersal mean?

So these solids, and also liquids, have to be dispersed. Dispersal means that the initial forms of the agglomerate particles are reduced in size, until they take the form of aggregates or primary particles that can no longer be reduced any further.

The quality of dispersion is the critical factor so that the desired material characteristics are present at every point in the matrix, and that there are no defects in the material.

The photos in figure 1 show damaged parts, where the cause of the damage is definitely due to poor distribution of raw materials. The following are sensitive areas:

[FIGURE 1 OMITTED]

* Rubber products in areas which are visible in use, or which need to have specified surface qualities for proper functioning, e.g., seals or printing rolls;

* rubber articles with specified physical properties such as density, buoyancy (for example cleaning balls for heat exchanger pipes);

* low-viscosity mixtures;

* thin-walled articles; these include various assemblies for tires, for example the inner liners that have to withstand a certain air pressure, with layer thickness of 0.7 to 1 mm. Any impurities in this thin matrix are pre-programmed defect positions for a flat tire. Other thin-walled constructions are cable jackets, seals and compressed air or liquid carrying pipe systems that, like vehicle tires, have essential safety functions.

The mixing process

Let us take a brief look at the basic steps in the mixing process. The diagram that we are going to discuss is universally applicable for both open and closed mixing systems, for filler and additive solids, for elastomer compounding and for the preparation of whipped cream or shaving foam.

After plasticizing of the polymer and addition of the filler, the next step is incorporation, whereby polymer and solid merge to form a new phase. That time is characterized by the highest possible input of shear forces (which is desirable) and very high load on the mixing equipment (which is not desirable). Shortly after that, there is a drop in energy input, which can be explained by the reduction in viscosity due to the rapid rise in mixing temperature and the sliding effects because the mixing components are not yet optimally distributed.

The progressive reduction in size of the agglomerates increases the available surface and hence the filler/polymer interaction, so that mixing viscosity (energy input) rises to a local maximum. The time from the first to the second maximum is also known as the black incorporation time. The plateau indicates that no further dispersive mixing is taking place. From now on, the aggregates generated are only dispersed within the compound matrix.

The mixing process is decisive for quality, because the degree of dispersion achieved there can hardly be improved by subsequent work.

An impression of dispersive and distributive mixing is given in figures 3 and 4. Two different types of flow occur in parallel (figure 3) in the mixing operation--elongation flow, shown by the blue arrows in the top part, and shear flow, shown in the bottom part. Depending on the type of flow and the forces acting on them, agglomerates can either be eroded or be split apart. The first demonstration and characterization of these degradation processes was achieved recently by a French researcher group, using individual carbon black pellets in plasticizer oil (ref. 1).

[FIGURES 3-4 OMITTED]

The mechanisms shown are degradation processes of the first order, and do not take account of the additional possibilities of agglomerate collision.

Why do additives have to be modified?

It was about 40 years ago that the first indications were published showing that a rubber chemical in its original form was not ideal for industrial use. It was the time when the rubber industry was faced with a need to act and with challenges from various raw material innovations in the elastomer and plastics sector. And it was the eve of the race for steel cord radial tires. So it was no coincidence that the first preparations arrived (ref. 2) to help the rubber industry in that turbulent period when the industry depended for its very survival on moving from the status of a generalist to that of a specialist. Constant increases in technical requirements meant that elastomers had to be transformed into high-tech materials.

But before we can answer the question posed at the beginning, we have to define what a preparation is. A preparation is a physical mixture of individual substances or a number of substances and carrier materials. The purpose of creating such a preparation, that is modifying application forms or changing morphology, is to optimize the materials for their specific application.

Tailor-made preparations with the inclusion of additives can solve many problems, including:

* They prevent direct contact of the workers with hazardous materials;

* they simplify handling and facilitate mixing to make the work easier, to save time and energy and thus help to reduce production cost;

* pre-dispersed preparations increase the distribution degree of the active substance; improved handling and avoidance of adhesion ensure the reproducibility of the mixing quality, and reduces the rejection rate;

* preparations with encapsulated active substances have increased shelf life. Some additives are approved for putting into circulation only in inert form, in order to reduce hazard potential (e.g., peroxides, propellant formulations).

There is one point that has not been explicitly mentioned, but which is readily understandable--preparations give increased efficiency and reduced costs over the whole of the mixing process. They are an option that helps to regain flexibility in static production systems and to increase productivity.

Technical capabilities of modification

What technical capabilities are available for modification of a chemical? First, let us look at a group of preparations that have one thing in common--their preparation does not ensure pre-dispersal of the active agent particles.

* Powder cocktails are mixtures and combinations of different active substances, practically the precursor of sachets;

* sachets are tailor-made, pre-weighed powders or powder cocktails in low-melting-point PE/EVA bags;

* dust forming powders are encapsulated and avoid internal weighing operations. Powders that are compacted to pellets likewise improve health and safety by reducing the proportion of fine dust.

* Oil coated pastes are powder-form free-flowing particles which are coated with a plasticizer liquid. The coating ensures easier uptake of the paste in a mixture matrix; degrading of the agglomerates has to take place in the production of the compound itself.

Unlike the first group, pre-dispersion was effected during the manufacturing process with the preparations indicated below:

* Pastes are likewise a powder-plasticizer mixture, but they are mixed more thoroughly in a Z-kneader with powerful shear force effect;

* wax-bonded liquids--these are organosilanes that are homogeneously distributed in the wax matrix, and can also be prevented from migrating out by appropriate choice of the carrier wax.

Dry liquids, polymer bonded and carbon black bonded preparations, are considered in greater detail below.

Selected examples of preparations

Dry liquids

Dry liquids are low-dust powder-form preparations of liquid additives on inert mineral carriers. The active substance load is 70 to 75%, and 50% mixtures are also in common use. Suitable carriers are precipitated silicic acids and calcium silicates with an oil absorption number (OAN) of 350-400 ml/100 g.

The dispersion effect comes from the size reduction of large droplets into smaller droplets at the dry liquid mixing stage. Large liquid droplets impact the carrier particles and are partially adsorbed by the capillary forces of the carrier particles--first in the inter-particulate voids, and then comes migration into the intra-particulate caverns.

Use of the dry liquid technique makes it possible to handle and weigh liquids like solids. In the case of high-viscosity liquid additives, the user of dry liquids avoids the need to keep melting them or warming them up.

In the weighing and material feed area, there is no need to remove stubborn liquid residues sticking to the equipment, but only small amounts of powder.

Apart from the improvement in handling characteristics, there is also an improvement in weighing accuracy of the formulation.

Important quality parameters in dry liquid production

The first parameter is selection of the right carrier. Its adsorption capability is decisive for the feasibility of the process of dry liquid production, and the carrier also determines the release rate of the active substance under the influence of shear forces. Excessive absorption of the carrier is just as great a disadvantage as insufficient activity.

The second parameter is grain size distribution of both the carrier and the end product, where freedom from impurities is essential.

The third parameter is viscosity. It is possible to work with low-viscosity to high-viscosity liquids, following careful thermal conditioning, in order to achieve a processing viscosity of max. 600 mPa x s.

The fourth parameter is temperature equalization. When processing high-viscosity liquids, temperature equalization of the mixing equipment is necessary. That prevents adhesion of pasty substance to the centrifugal components.

The fifth parameter is process control--the rate of addition of the liquid, the shear force input and the configuration of the mixing tools (centrifugal system and chopper) are decisive for the dispersion of the liquid and destroy pasty dry-liquid agglomerates before they are established.

And finally, the duration of shear force application is important, so that release of the liquid active substance does not take place shortly after its uptake.

Dry liquid application example

The advantage in application is shown with the example of the co-crosslinking agent TAC (triallyl cyanurate). TAC is used as co-agent in the radical crosslinking of polyolefins, EVA, HNBR and EPDM rubbers, and is used in cable mixtures and high-temperature resistant mixtures. TAC melts at 27[degrees]C and has an SADT of 89[degrees]C; exothermal auto-polymerization may occur from 160[degrees]C.

Using the EPDM test formulation shown in table 2, the mixing behavior of pure TAC was compared with that of TAC as a 70% preparation. The formulation used 1 phr TAC instead of 1.43 phr TAC DL 70 (70% preparation).

Figure 6 shows the comparison of the torque curves for the two mixing tests. The mixing procedure was a conventional method, whereby first the polymer was plasticized, then carbon black N-772 and plasticizer oil were added. After closing of the ram, filler and plasticizer were incorporated into the polymer, and the mixing phase was formed. After stabilization of the torque values, the peroxide masterbatch was added and the TAC co-crosslinking agent was added in its respective application form.

[FIGURE 6 OMITTED]

Whereas the TAC as a melt required an extensive period of addition, the mixing procedure operated at relatively low force shortly after closing of the mixing chamber. Evidently, a sliding film had been formed between the rotors and the mixture, and incorporation and dispersal of the polar co-crosslinking agent in the non-polar EPDM matrix got started only slowly. It should be borne in mind that only 1 phr of the active substance is enough to cause this "process delay" effect.

The behavior of the TAC dry liquid was quite different. The good flow characteristics of the powder mean that addition could be effected relatively fast. The rotors had positive force again immediately after the ram has been lowered, and could distribute the active substance more intensively. The conclusion from this is that the dry liquid was closely unified with the elastomer matrix at once, despite the polarity differences. The active substance was exposed better to the distributing shear forces.

The test results of the two vulcanized mixtures (table 3) indicate that the mechanical properties tend to be improved in mixture 2 made with TAC DL70. These findings suggest that the pre-dispersed TAC dry liquid increases the number of polar TAC domains in the non-polar EPDM matrix.

Polymer linked preparations

A second example of pre-dispersed additives is the polymer-bonded preparations. These are additives mixed into elastomer binders, mostly organic or inorganic powders, and in some cases also liquids.

Active substance concentrations are normally about 70-90% by weight. 50% active substance concentration or less may also be appropriate in specific cases where small formulation proportions make it hard to achieve accurate dosing and distribution.

A specific aspect in the polymer-bonded active substance batch is the close relationship with rubber mixtures. With dry liquids, it is always important to use absorbent mineral carriers. These are sometimes undesirable as mixture components. However, the polymer binder may be optimally adapted to the elastomer type of the target compound - the preparation becomes a full formulation component of the compound.

The material characteristics of the relatively soft, ductile additive masterbatches also permit filtering operations. Typical filtration degrees are 250-140 microns. Individual suppliers are in the process of getting below the 100 micron limit.

Important factors for product quality are:

* Particle size profile of the respective active substance, alongside inadequate mixing equipment, high agglomeration contents are the main reason for defects arising later;

* freedom from impurities of the active substances;

* the viscosity of the masterbatch should be adapted to subsequent application conditions. If high-viscosity granulates meet up with a soft mixing mass, they will float around undivided, like a cork in water. If granules that are too soft meet with high-viscosity mixtures, incorporation will be delayed by lubricant and sliding effects, or the mixing band will be partially separated from the roller.

Good process control is the essential for dispersion quality of the polymer bonded master batches, and this has to be determined and optimized individually for each formulation. The mixing sequence, mixing duration, filling degree and machine parameters such as rotor speed, chamber temperature and ram control are the key factors which decide whether one gets a "constellation" of undispersed agglomerates (figure 7-top) or land exactly on target (figure 7--bottom). This shows the strainer residues of a filtration test of the accelerator problem case MBTS, where in each case a relatively large sample cross-section of the preparation is mixed into a block compound and then strained. The typical strainer residue at the top comes from a roller-manufactured batch, while the filter cake shown at the bottom comes from an optimized internal mixer batch.

[FIGURE 7 OMITTED]

Moving on to another point that is relevant for quality, the compatibility between the compound and the preparation will be examined. EPDM/EVA, the widespread standard binder system, already shows a strong temperature-dependent drop in viscosity at low shear forces. This system was originally designed as a universal binder for fast release of active substances in all usual applications. But in polar elastomers such as NBR and CR, this combination shows incompatibilities and detectable migration phenomena.

The photo on the left (figure 8) shows a mixing test of a masterbatch of 75% MBS EPM/EVA banded into a pure nitrile-PVC band. As is clearly evident here, there is no unification of the masterbatch; on the contrary, the nitrile matrix is cracked at many points and has been separated in some parts.

[FIGURE 8 OMITTED]

Another sample shown on the right (figure 8) was made by preparing the accelerator with an NBR binder and corresponding plasticizer. Under identical conditions, the mixing test showed close connection with the band after just a few revolutions.

This showed that it is an appropriate route to replace the universal system by a tailored binder system, which is capable of fulfilling its function without complications.

Carbon black bonded dry liquids

The last example of pre-dispersed preparations considers preparations with carbon blacks as the carriers. Most industrial carbon blacks have relatively high oil absorption numbers. Even semi-active carbon blacks such as N-550 and N-772 are capable of absorbing 40 to 50% of their own weight in liquids. The same applies to the mineral DL carriers--these carbon blacks likewise disperse the liquid droplets in the caverns of their aggregates.

Gentle mixing conditions are used to ensure that the carbon black beads are not destroyed, and maintain the same condition on delivery. A further development of this is carbon black dry liquids, in the form of rod-shaped granulates.

The use of carbon black-bonded additives involves three aspects. The pre-dispersal of the liquids is effected using the mineral/dry liquid principle. This is an alternative if mineral substances have to be kept out of the mixture. The pre-wetted carbon black is also distributed better compared with untreated carbon black in original condition. The addition of liquid almost completely binds dust-forming fines of the carbon black. The carbon black-bonded preparations significantly reduce contaminations that are otherwise characteristic of carbon black. This can be shown by a shaking test with conventional N-339 and a preparation based on that with naphthenic oil. After shaking, the top rim of a screw-top jar containing the carbon black-bonded dry liquid is not in the least contaminated, whereas the jar containing N-339 is contaminated with the typical fine-particle layer.

We have conducted a mixing test with a black SBR base mixture, comparing the incorporation behavior of the individual components with a carbon black-plasticizer oil preparation with an identical formulation comprising 60% N-339 and 40% naphthenic oil Nytex NY 810. Formulations and the respective mixing procedures are shown in table 4.

Figure 9 shows the reference kneader curves for both tests. The torque curves were different in particular after the addition of plasticizers/fillers. Formulation B, which gives simpler handling, permitted somewhat faster addition of the carbon black-plasticizer batch and somewhat earlier torque peak. Final torque level was also reached earlier with mixture B than with mixture A.

[FIGURE 9 OMITTED]

These curve characteristics permit the conclusion that the carbon black/plasticizer dry liquid permits faster incorporation of carbon black and plasticizer, making lower demands on the mixer and reaching a lower power peak, or else improvement of distribution for equal mixing time.

Summary and outlook

The following conclusions may be drawn in summary:

* The dispersion of fillers and in particular of additives is decisive for mixing quality;

* pre-dispersed additives give a technical improvement of the discontinuous mixing process in many respects. That gives cost benefits for the process as a whole.

The preparation techniques described provided a wide range of tools to master future technical and economic challenges in the rubber industry.

References

1. V. Collin, E. Peuvrel-Disdier, European Rubber Research Conference, Jan. 2005, D-Paderborn.

2. Lehmann & Voss & Co. brochure, Zinc Oxide Hansa Ultra, 1963.