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Dry-blend rotational molding: dispersible carbon black is key: dry blending offers benefits over masterbatches, but requires careful selection of the filler grade.

Rotational molding is a process of making hollow plastic parts by rotating melted resin along two axes inside a hot mold. To maintain uniform thickness throughout the part, the mold continues to rotate at all times during the heating phase. To avoid sagging or deformation it also rotates during the cooling phase.

Rotomolding often is used to create products for outdoor use, such as water tanks, kayaks, playground sets, street lamps, and traffic and other outdoor signs. These products require good resistance both to weather and to ultraviolet (UV) rays. Carbon black, particularly grades with smaller particle sizes < 25 nm), provides these properties in rotomolded parts and products.

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Carbon black proyides excellent light absorption, thus imparting UV resistance. UV resistance prevents photooxidative degradation of polymers and extends product life. In addition to particle size and surface area, the efficiency of carbon black as a UV absorber depends on its loading levels in polymer compounds and on level of dispersion. In addition to its use for UV resistance, carbon black also offers important colorization attributes.

To start the rotomolding process, manufacturers charge the mold with plastics material in the form of ground beads or micro-pellets created via masterbatch or in the form of dry blended powder.

The masterbatch option

Masterbatch producers mix carbon black, resin and other additives to make the compound and extrude it in the form of pellets. They mechanically grind the pellets to produce ground beads or powder. The grinding process creates additional surface area on the product. More surface area reduces heat transfer rates and causes the material to melt more uniformly, which is beneficial to the rotational molding process.

The masterbatch technique allows compounders to use carbon black beads, which are much easier to handle than the material in powder form. Rotomolding via the masterbatch technique is known for producing products with better color properties and dispersion.

Dry blending is a simpler, more cost-effective alternative to masterbatch technology. In dry blending, rotomolders directly mix easy-dispersing carbon black powder with ground beads or micro-pellets of polymer resin, and pour the dry blended materials into the mold cavity. The resin and carbon black are heated to a molten condition, in which the two components will mix. The dry blending process eliminates the extrusion/compounding and mechanical grinding steps.

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Orion Engineered Carbons' studies show that the masterbatch technique is more widely used than dry blending. However, the reduced steps in dry blending serve to reduce production costs, time, energy, and human involvement, resulting in lower end-product cost.

Rotomolders choosing the dry-blending route should consider dispersion properties, cleanliness, surface area .and structure of carbon black when selecting the best grade for a particular application.

Dispersibility is key

The first requirement for dry blending is an easy-dispersing carbon black powder. The rotomolding process does not generate a shear force high enough to break the carbon black pellets and agglomerates and disperse them well in a polymer system if the carbon black itself is not easily dispersible.

By contrast, the extruder in the masterbatch process generates a very high shear force during mixing of the polymer and carbon black. This force breaks carbon black pellets and agglomerates, and easily disperses them in a polymer system.

A microscopic image can be used to check dispersion quality over time. It is best to use a laboratory-scale rotomolding machine to create the test sample.

Cleanliness is a virtue

The extruder in the masterbatch technique has a screen pack located just before the die and breaker plate that filters the melt to eliminate contamination and carbon black grit. The dry blending process has no similar screening steps to filter carbon black, making it important to use a cleaner grade for dry blending than for masterbatch. Otherwise, the contamination or unwanted material in carbon black can cause defects in the final rotomolded product.

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Structure: low or high?

Structure relates to the degree of aggregation of carbon black particles. Carbon black pigments with a high degree of aggregation are called high-structured, while those with low aggregation are called low-structured.

Structure relates to the degree of aggregation of carbon black particles. Carbon black pigments with a high degree of aggregation are called high-structured, while those with low aggregation are called low-structured.

Along with the melt flow rate of the polymer, carbon black structure influences the melt flow rate of the compound. Low-structured carbon black (Oil Absorption Number, or OAN: <70 cc/1 OOg) in dry blending generates low compound viscosity and offers better flow in rotomolding. So, it is important to use low-structured carbon black in the dry blending process rather than higher structured (OAN: >100 cc/100g).

In the rotomolding process, the two-axis mold rotation allows resin to flow against all mold walls. Low-viscosity material will flow easily inside the mold and will form uniformly thick end products. By contrast, a higher-viscosity polymer melt will make it difficult to create a uniform layer inside the mold.

Selecting the right viscosity

In selecting the best carbon black for a particular dry-blend rotomolding application, consider first the type of resin to be used. A low-viscosity resin (with a high melt index) may be able to handle a high-structured carbon black to obtain a desired melt flow rate of the compound. It can also accommodate a higher surface area or smaller-particle-size carbon black, which influences the viscosity of the compound. A medium-viscosity resin will require a medium particle size and structure carbon black. A higher-viscosity resin, when melted, requires a lower-structured carbon black.

To achieve uniform thickness in the final product, rotomolders can adjust the viscosity or flow rate of the compound by selecting the right combination of polymer and carbon black. If the polymer melt flow rate is too high, a higher-structured carbon black will reduce the flow rate. Conversely, if a high-structured material is not enabling a uniform thickness --the viscosity being too high--then a lower-structured carbon black will lower the compound melt viscosity and facilitate a uniform thickness throughout the mold.

Orion generally recommends a lower-structured carbon black for uniform flow and thickness inside the mold and in the parts'being produced.

Carbon black grades with low structure and low surface area generate very low viscosity, so high viscosity resin will be helpful in the dry blending process. While rotomolders often select the resin system first and match the carbon black to the polymer, they also can reverse the order and select their resin systems based on carbon black colloidal properties.

Proper carbon black selection balances structure, particle size and surface area to achieve UV resistance and conductivity if desired. Selection also entails a balance of properties to achieve optimum viscosity and consequently uniform thickness throughout the mold.

Rotomolders looking for high jetness and UV resistance will require a carbon black with a very small particle size, or one with high surface area. However, a small-particle-size carbon black is difficult to disperse. So, the filler selected should have a particle size large enough to disperse well while also delivering both high jetness and good UV resistance.

Orion Engineered Carbons will work with rotomolders to determine through trials the grade that best balances these properties in their particular operation.

Conductive parts

Many rotomolded parts--for example, water tanks, kayaks and toys--don't require any conductivity nor a conductive carbon black. Products or parts requiring conductivity, such as intermediate bulk containers, can use a material such as Printex L6. This grade features a high structure (126 cc/100g OAN) and very high surface area (270 m2/g)--both of which are required parameters to achieve conductivity.

Summing up

Rotomolders shifting to the dry blending technique will eliminate production steps, thereby reducing costs and production time, but will need to carefully select a more dispersible, cleaner carbon black than they would use for the masterbatch technique.

Easy dispersibility, low structure and powdered form are key parameters for carbon black in dry blending applications. Good dispersibility is important for obtaining uniform properties in the final product. Carbon black grades with smaller particle sizes will enhance UV- and weather-resistance properties.

By Purnatosh Saha

Orion Engineered Carbons

ABOUT THE AUTHOR

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Purnatosh Saha is technical market manager for polymers at Orion Engineered Carbons LLC (www.orioncarbons.com) in Kingwood, Texas. Prior to joining Orion in November 2014, Saha held technical positions at SABIC Innovative Plastics in Indiana. Apollo Tires Ltd. in India. and j.K. Tire & Industries in India. He earned a Master of Science degree in plastics engineering from the University of Massachusetts Lowell. and Master and Bachelor of Technology degrees in polymer science and technology as well as a bachelor's degree in chemistry from the University of Calcutta.
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Title Annotation:TECHNICAL ARTICLE
Author:Grace, Robert
Publication:Plastics Engineering
Geographic Code:1USA
Date:Sep 1, 2017
Words:1440
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