Improving mixing, quality with dispersions.

Mixing operations and product quality are often improved by using dispersions of additives. The role of chemical dispersions and the values they provide are classified into:

* Product quality;

* manufacturing economics;

* environmental responsibility.

The role of chemical dispersions and their impact on these valued improvements will be discussed in detail.

A dispersion is a two-phase system in which one phase (the dispersed phase) is homogeneously distributed as small particles within a second phase called the continuous phase. This yields complete encapsulation of an ingredient or additive as the dispersed phase. When the dispersed phase is formulated at a high concentration (15 to 95% active) the product is often called a masterbatch (MB).

Theory and basic principles

Dispersions or MBs eliminate or reduce a potentially serious problem associated with many of the powdered additives used in polymeric products. Many powders may form agglomerates due to attractive forces between primary particles. Agglomerates may be very large (> 50 microns), difficult to disperse and may yield serious product quality problems. Agglomerates have an apparent surface area which is considerably less than the total surface area of the primary particles due to voids' between primary particles within the agglomerate.

During the production of a dispersion, three stages are encountered. First, incorporation of the large agglomerates occurs as they are encapsulated by the polymer or liquid continuous phase. Second, the agglomerates are distributed randomly throughout the system. And finally, the actual dispersion process begins with the agglomerates being reduced into smaller ones and/or to their basic particle size due to internal shear stress caused by viscous drag within the system. For this process to occur, wetting or replacement of the solid-air interface by a solid-liquid interface is necessary. Variables which affect this process include: the nature of the continuous (liquid) phase, the chemical/electrical nature of the powder, dimensions of the voids between particles within the agglomerate, primary particle size and size distribution and the mechanical process used to perform the dispersion. Displacement of absorbed material, such as water, oxygen, nitrogen or other atmospheric gases, from the surface of the particle may also be necessary. If the chemical and/or electrical nature of the particle surface interferes with the wetting/dispersion process, it is common for manufacturers of the powder material to perform surface coating to facilitate processing. Wetting agents or dispersion aids may also be added by the dispersion producer. Due to the high surface area of very fine particles (often in the 1 micron range), complete wetting and high particulate concentrations are often difficult to attain.

Often, considerable energy is needed to reduce agglomerates to their primary particles. The dispersion process equipment, procedure and formulation must be designed to impart sufficient shear to efficiently and effectively break down agglomerates.

Polymer based dispersions

Equipment used to manufacture polymer (rubber or thermoplastic) based MBs include the two-roll rubber mill, internal mixers and compounding extruders.

The primary particle shape can influence the system rheology and, therefore, the dispersion process. Products with low melt points can be difficult to pre-disperse at high concentrations due to the melt influence on the system rheology and processing shear. Similarly, the continuous polymer phase should be of sufficient viscosity at processing temperatures to insure adequate shear input to attain good dispersion of the additive.

The resulting MB viscosity should be similar to the viscosity of the compound into which it is being incorporated to insure good mixing. The continuous phase of the MB must also be compatible with the final compound. Single chemical and chemically compatible (stable) blends based on NR, SBR, NBR, EPR, EPDM, EPDM/EVA, CPE, CR, PVC, various other thermoplastic elastomers or customer specified polymer grades and/or sources are available. Additionally, it is frequently advantageous to use a binder system which is non-reactive with the chemical(s) in the dispersion. Often, saturated polymers (EPR and CPE) are used for vulcanizing agents or cure system blends.

Polymer based chemical or color MBs may be supplied in slab form, cut into cubes or extruded into pellets. The cube/pellet form is preferred for automatic weighing operations. Extruded pellet forms may also be strained to eliminate contaminants and/or undispersed particles. Slab forms or mill strips are preferred for economics reasons. PVC-based MBs may also be micro-pulverized. The rate of MB distribution in a compound is influenced by its form. A small cube or pellet may be homogeneously distributed more rapidly than a slab or mill strip.

Plasticizer/oil based dispersions

Chemical and color paste dispersions, often called PDs to distinguished them from the polymer based MBs, may be designed with a wide viscosity range for handling and mixing considerations. Very high viscosity PDs are dry in character and have often been called cuttable pastes. Viscous PDs may be difficult to handle and may leave considerable residual material in their container. Low viscosity PDs may be pumped, but can have a tendency to settle. Often, paste dispersions are packaged in low melt bags of customer specified weight. The low melt bag incorporates into the final compound during mixing. This packaging overcomes the handling difficulties expressed above.

The quality or degree of dispersion attained depends on the processing equipment and procedures. Simple wetting of the powder, without significant agglomerate reduction, is obtained with low shear equipment such as Pony and sigma blade mixers. A high speed disperser may provide greater agglomerate reduction. The greatest agglomerate reduction can be obtained with high shear equipment such as the three-roll mill (high viscosity pastes) or horizontal pebble mill (moderate viscosity pastes).

Dispersion quality or de-agglomeration is often measured by the Hegman NS grind test (ASTM D-1210). This simple test measures the maximum residual particle size and expresses it as a number between 0 (100 microns) to 8 (0 microns). Dispersion grind should be specified to ensure adequate particle size reduction for the application. For optimum color pigment efficiency, a Hegman grind of 6 (24 microns) or better is recommended. Generally, dispersion cost increases significantly with the quality of dispersion required.

PDs incorporate more readily into very low viscosity compounds (like sponge or soft roll covers) than polymer (elastomeric or thermoplastic) binder system MBs. PDs do have the tendency to increase mixing time due to their lubricating affect on compounds. This reduces the shear input during mixing.

Since the dispersion's continuous phase should be compatible with the final compound, many different fluids are available for PDs. Most of the fluids used for chemical PDs are the common oils and plasticizers used in rubber compounds. For colors, the fluids may vary from liquid epoxy resins, silicone fluids, urethane polyols to liquid thermoset polyesters and the common plasticizers used in vinyl compounding.

Pre-wet and encapsulated powders

Another form of dispersion is the wetted or oiled powder or powder blend. As noted above, chemical compatibility is a necessity for blends. For oiled products, the use of 4 to 20% oil is common to properly wet the powder or powder blend. Oil addition provides the benefits of dust reduction, surface wetting of the powder(s) and more rapid incorporation during mixing.

Encapsulation by a polymer/plasticizer system is the preferred method if a small bead or crumb form is desired. Several basic systems are used in the industry. Some dispersion producers use systems based on thermoplastic materials such as wax or low softening point resins. An elastomer/plasticizer system for chemical dispersions has been developed. Several key improvements have been achieved with this technology. Storage stability and handling are improved by elimination of the remassing tendencies of thermoplastics. Mixing efficiency is also improved. Dispersions with thermoplastic binders are prone to mill roll coating and batch lubrication. Elastomeric based binder systems yield dispersions which rapidly incorporate into elastomer based compounds and they do not have an affinity for metal surfaces of processing equipment. Proper encapsulation eliminates dust problems.

A common application of the treated powder blend product is the complete, formulated cure system for tire retread mixing. In this case, the formulation is customer specified. The pre-blend assures consistent product cure and physical properties with accurate weighing of only one item, the cure blend. Another application of this product form is blended color pigments to form a customer specified or matched color. Single powder based products may include accelerators, zinc oxide, sulfur, magnesium oxide, calcium oxide or other difficult to disperse, dusty products.

Oil treated magnesium oxide and calcium oxide products have a longer shelf life than the untreated chemicals since hydration is reduced.

Many types of powder blending equipment can be used. Some agglomerate size reduction may occur during processing, however, high shear input is not achieved by most of the process equipment commonly used. Examples of low intensity mixers include ribbon blenders, conical blenders, V-blenders and double cone blenders. Typical examples of high intensity equipment include the intensive vertical mixer and the horizontal plow mixer which may be equipped with several high speed de-agglomeration blades. Powders may also be oil wetted to form a paste which can be extruded into a bar form. The pastes are typically produced in a sigma blade or dough mixer. Short barrel extruders are used to form the rectangular extrudate which is cut into bars. Zinc oxide and magnesium oxide are frequently used in this form.

Absorbed liquids or dry liquid concentrates (DLC)

Many of the liquids and low softening point resins used in rubber compounding are difficult to handle and mix. Such materials may be absorbed on high surface area siliceous carriers which will typically yield 60 to 75 percent active products DLC). The DLCs are free flowing, non-dusting powders. Typical materials which are handled in a DLC form include petroleum oils, ester plasticizers, low softening point resins (pine tar, rosin, coumarone-indene, phenolic, etc.), waxes, low molecular weight liquid elastomers, adhesion promoters, coupling agents, co-agents and peptizers. Among the advantages of using DLCs are the following.

* Elimination of raw material loss due to viscous material residues in the original package. This could also reduce packaging disposal costs.

* More precise weighing and transfer of material to the batch which improves batch composition uniformity.

* Elimination of hot boxes and the energy and time consumption associated with their use.

* Better worker safety by eliminating handling of hot materials.

* Reduced compounding and mixing time. DLC's can be added to a batch rapidly without the lubrication problems associated with liquids.

* Improved housekeeping and reduced clean up time.

The functionality of the absorbed material is retained in the DLC form since the carrier releases the material into the rubber compound by deabsorption and migration.

Product quality issues

The word quality has many different definitions, but it can be simply stated as consistent conformance to (or exceeding) the customer's specified expectations. In many cases, poor dispersion of compounding ingredients can yield poor product quality. Some of the common problems associated with poor dispersion include the following.

* Reduced product life;

* poor performance in service;

* poor product appearance;

* poor processing characteristics;

* poor product uniformity;

* raw material waste;

* excessive energy use during processing;

* high finished product rejection rate.

The above potential quality problems are due to large agglomerates which can act as failure initiating flaws in mechanical applications. Poorly dispersed materials can reduce ultimate tensile, elongation, tear strength and flex fatigue. High local stress promotes ozone attack. These effects may be due to the physical presence of a large agglomerate or the affect of a localized high cure state due to poorly dispersed curative(s). Electrical properties can also be affected by dispersion quality. Thus, product life and service performance can be reduced.

Product appearance can be affected by dispersion quality. This ranges from surface quality to actual color. If the agglomerate is a color pigment, uneven color or a spotty appearance may result from poor dispersion of the colorant.

Many chemicals commonly used in the rubber and plastics industries are heat and/or environment sensitive. Their performance and the resulting quality of the finished product can be diminished by exposure to excessive heat during compound processing. Carefully prepared MBs of these materials can preserve them and allow their introduction into the compound at the end of a mix, thus reducing their heat history. The following are typical products which are often used as MBs for this reason.

* Sulfenamide accelerators - premature decomposition yields-reduced scorch delay.

* Insoluble sulfur - reverts to rhombic sulfur which will bloom.

* Substituted para - phenylenediamine antiozonants - direct oxidation reduces their efficiency.

* Peroxide curatives - must be processed below decomposition temperature to avoid premature vulcanization.

* Blowing agents - must be processed below their decomposition temperature to avoid premature gas evolution.

* Certain organic color pigments - Pyrazolone orange and others change color with excess heat.

* MgO and CaO - their performance is diminished by hydration and dispersion encapsulates the material.

Manufacturing economics

Economics can not be separated from quality in practice. Lack of quality always affects the profitability of a business. However, some MB economic advantages can be directly measured. Some of the more obvious ones are the following.

* Less scrap and/or re-work due to better dispersion.

* Reduced incorporation time and power consumption.

* Reduced loss due to airborne dust and/or spillage.

* Potential one pass mixing instead of two passes.

* Reduced worker exposure and probable elimination of special protective equipment required for handling hazardous chemicals.

* Reduced contamination of dust collector fines and plant clean up with materials which may increase the disposal cost.

* Reduced housekeeping costs and potential for cross contamination via airborne dust.

Greater uniformity of chemical/color blends due to scale of weighing and blending = improved product uniformity.

* Improved handling and suitability for automated weighing equipment. Example: Cubes or pellets versus powders.

* Improved worker moral due to cleaner work environment. Often yields, improved quality/productivity due to better attitude.

* Reduced packaging "contamination" with materials which could increase the cost of packaging disposal.

* Greater chemical/color efficiency due to fine dispersion (small particle size and high surface area) of products which are surface area dependant for performance.

* Elimination of color adjustment due to pigment changes yielding better uniformity.

Environmental responsibility

Some essential components of rubber and plastic products are considered hazardous and can cause serious environmental problems. The use, handling, exposure and disposal of these materials and often the products containing them are usually highly regulated by governmental agencies. Powders are the most difficult to handle due to their fugitive nature. Therefore, encapsulation in an dispersion is common. Liquids can be a problem too. Complete removal of a viscous liquid from its container is difficult and the remaining material may render the drum to be classified as hazardous waste. Packaging disposal and the related costs have become serious problems for industry. Worker exposure is an environmental problem of even more serious proportion. Some powders, such as sulfur may be explosive if airborne. In these cases, the consequence of improper handling and use of chemicals can be serious.

New concepts

The long list of quality, economic and environmental issues has lead to new concepts in handling and using materials in the rubber and plastics industry. This concept can be best described by the following discussion. Powders or powder blends, MBs or PDs are formulated for a specific compound and pre-weighed to correspond to the batch size. Typically, the packaging is a sealed, low melt bag which disperses into the final compound during mixing. Thus, the mixing operator adds the bag(s) to the batch without weighing or exposure. This concept can be carried to the point of shipping the pre-weighed bags in a returnable container, thus all packaging waste is eliminated.

Although there are costs associated with pre-blending, pre-weighing and packaging compounding ingredients, the cost is usually off-set by the advantages listed below.

* Reduced packaging disposal costs - land fill costs are high and increasing; regulations are reducing the disposal options eliminating the land fill option; risk costs can be very high. New regulations can require land and/or water clean up which is very expensive.

* World wide increase in quality and uniformity standards. QS 9000 family of standards is becoming required to conduct market segments and locations. Note: Automotive component suppliers for OEM are under extreme pressure to meet these new quality levels.

* Reduced purchase order and inventory control costs are noted for pre-blends of two or more raw materials.

* Better controlled inventory levels and faster inventory turns (lower days inventory ahead costs).

* Reduced cost of testing/approving incoming raw materials for blends. Blend composition is the responsibility of the producer.

* Reduced labor costs of weighing the individual components of the blend.

* Reduced weighing equipment maintenance and calibration costs.

* Elimination of paper bags, tubs or other container costs associated with the compounding area operation.

* Reduced worker and environmental exposure to the chemicals since the pre-weighed bag is added unopened to the mixer.

* Increased productivity due to improved uniformity. Rework and scrap are reduced.

* Reduced laboratory expenses in re-testing and adjustment or work away determination due to improved uniformity.

* Reduced raw material costs by efficient volume purchases by the producer of the MBs or blend. Often materials are purchased in returnable super-size containers.

* Verified individual components by the producer of the MB or blend.

* Improved uniformity of the MB or blend (such as cure systems) due to the larger quantities weighed to a make preblend or MB. A preblend or MB may be large enough for several hundred final batches. Thus, final product uniformity is improved. Accurate weighing of small quantities of individual blend components is difficult and time consuming.

* Improved precision of automated pre-weighing of a blend since the whole is being weighed in place of each component. With modern equipment interfaced with microprocessor, the equipment can be continuously adjusted to deliver the specified weight. Using a check scale interfaced with a computer, statistical process control data is available to verify the product weight uniformity and process capability. Generally, accuracy is at [+ or -]1% or better.

* Verified batch composition and testing by the producer.

* Traceable raw material lot numbers for each batch.

* Separation of chemicals which may react with other components of a blend, Reactive chemicals are packaged in separate bags, but may be supplied as a single, bag in a bag unit.

* Since the chemicals are packaged in a sealed and identified bag, the bags are loose loaded into a large container which will be clean for disposal. Collapsible and returnable containers are also an option.

Conclusions

Suppliers of chemical dispersion to the rubber and plastics industry have developed many options to address the three prime concerns with using their products: product quality, manufacturing economics and environmental responsibility. Each chemical dispersion option has its place in the industry. The user must weigh the benefits and costs of each option to make the most cost effective choice. The characteristics and benefits of these options have been briefly discussed in this article and only the customer can make the right selection for the specific application under consideration. In many plants, all of these options are being used.