INSIGHTS ON DISPERSION

Transcription

1 INSIGHTS ON DISPERSION Overview Dispersion is the process of the spreading of mass from highly concentrated areas to lower concentrated areas. Achieving an adequate dispersion involves imparting enough energy in the system to overcome attractive forces between particles without putting so much energy in the system that you destroy or change the desired properties of the carrier resin. Dispersing particulate matter in a visco-elastic medium, such as a polyolefin, involves both science and art. Industries have emerged to service this need in the polymer market place and a body of scientific work has developed around the subject. However, achieving dispersions of particulate matter, and in particular carbon black dispersions, remains the domain of experience and know how. This Modern Dispersions Insight article will focus on the important attributes that must be considered in achieving dispersion of carbon black. Achieving a good dispersion has important commercial consequences. Good dispersion is needed to achieve some level of aesthetic appeal such as uniform color tone or high black jetness. Other markets need good dispersion to improve functionality of the end-use article such as electrostatic dissipative plastics and UV resistant articles. Dispersion is the key to delivering the value of the additive or pigment. Pigments, especially carbon blacks, present problems to customers trying to handle and disperse these products. Customers desire to have a product which is dust free, can be easily handled in conventional material transferring equipment and have the same product in a form that will easily disperse in the host matrix resin of choice. Additionally, customers desire a product that meets SH&E requirements for low dust levels in the work place. Masterbatches (20% - 50% pigment loading levels) were developed to address these requirements.

2 Carbon Black Fundamentals that Effect Dispersion Carbon black in its nascent form is fluffy powder. The particle size is roughly the size of virus. Consequently, the bulk density is very low and the material, as produced, is extremely difficult to convey in conventional material transfer equipment. Additionally, carbon black lacks significant surface functional groups, which means that the particles are hard to wet and therefore difficult to disperse. Manufacturers of carbon black solve some of these problems by pelletizing their products using low levels of wax or surfactants resulting in a product that has the form of small beads (prill) roughly 2-5mm in diameter. In forming the prill the carbon black particles are forced together to form aggregates. These aggregates must be broken down to achieve good dispersion. The energy that was used in making the prill must be matched by either chemical energy or mechanical energy to separate the carbon black particles back to their nascent non-aggolmerated state. Additionally, if the carbon black particles were forced into the van der Walls radii of adjacent particles this strong attractive energy barrier must also be overcome to achieve good dispersion. Carbon black particles are comprised of complex clusters of very small (~10nm) primary particles to form the primary particle - much like a cluster of grapes. (Please see the Modern Dispersion Insight article on Carbon Black Fundamentals.) The properties of carbon black that are critical to dispersion include the surface area and structure (complexity of shape) of the particle. Surface area is determined by primary particle size and to a lesser extent structure. Particle Size/Surface Area The primary particle size of a carbon black determines to a large extent, its degree of dispersability. The smaller the primary particle the higher the surface

3 area resulting in more area to wet and thus requiring more energy than a carbon black particle having lower surface area. Additionally, as the carbon black aggregate becomes smaller there is a higher volume concentration of carbon black at the same weight loading (compared to particles having larger aggregates) resulting in smaller interaggregate distances and, consequently, greater attractive forces to overcome. Structure High structure carbon blacks are easier to disperse than lower structure carbon blacks. High structure blacks exhibit less close packing than those of lower structure, thus, there is considerably more space accessible to the carrier resin. The entanglement between the carbon black particle and the carrier resin is the fundamental chemical process enabling dispersion. Inter-aggregate attractive forces are lower for higher structure blacks, consequently, less energy input is required to overcome the attractive forces which facilitates ease of dispersion. Carrier Resin Selection To achieve the desired dispersion in the end use application requires the masterbatch manufacturer to balance the visco-elastic properties of the carrier resin with those of the host polymer matrix. This balance requires that the carrier resin be similar in make-up, be from the same polymer family and have viscosity characteristics that ease dispersion of the pigment without causing deleterious effects on the end-use article. The viscosity requirements are such that the carrier resin disperses readily in the host matrix. Typically, masterbatches are made using lower viscosity resins that those of the intended host matrix but not so low of a viscosity so as to modify the desired viscoelastic properties of the final material. A rule of thumb is that the carrier resin should have a melt thinning profile similar to the host resin and have a melt-index within an

4 order of magnitude of the host resin. In selecting the carrier resin the masterbatch manufacturer must consider a myriad of compromises including; cost, degradation temperature, ultimate end-use, inertness, compatibility with the host resin and compatibility with the pigment. Choosing the appropriate carrier resin is an art and recipes are closely guarded secrets. The Dispersion Process The dispersion process can be viewed as sequence of events. These events include wetting and premixing of the pigment, breaking down the carbon black prills, deagglomeration and dispersion. Mechanical and thermal energy are used to break down the agglomerates, separate them into their fundamental aggregates and disperse the aggregates into the carrier resin. Chemical energy, in the form of dispersion aids, is used in difficult to disperse systems. Wetting and Premixing This step involves the premixing of the masterbatch components and occasionally wetting of the carbon black dispersion aids. The goal is to achieve displacement of occluded air and cover the surface of the carbon black agglomerates completely with vehicle yielding a workable dispersion mix. Prill Breakdown and De-Agglomeration Before melt compounding the dispersion mix is subjected to high energy mixers so as to breakdown the pigment prills and achieve a uniform material mix. Carbon black and the carrier resin are subjected to high energy mixers with the goal being to break down the aggregates of carbon black. This step is essential to achieve a good carbon black masterbatches. Melt-Compounding

5 Melt compounding is accomplished using a variety of equipment including Banburry mixers, single screw extruders and twin screw extruders. The goal of the melt-compounding step is to combine the dry mix and the carrier resin creating uniform pellets that have homogenous composition. Thickening of the melt and melt fracture is a frequent problem in producing the masterbatch because the mixture has between 20% and 50% active ingredients. Achieving the desired end-use viscosity, loading and performance envelope frequently requires significant trial and error. Production profiles are closely guarded secrets and key to the differentiation of masterbatch producers. Manufacturing know-how, intellectual property and trade secrets are key to the sustainable competitive advantage of most masterbatch manufacturer