Processing, June 2020

JUNE 2020 www processingmagazine com 21 geenan gelatin pectin xanthan etc acrylate based polymers starches stearates fatty acid alcohols waxes and oils Inorganic thickeners include clays silicas and salts In particular high molecular weight polyacrylic acids also known as carbomers are commonly used in all manner of personal care products including toothpaste shampoo lotions and hand sanitizers to name a few is class of thickeners requires neutralization to achieve maximum viscosity Rheology modifiers alter a systems viscosity through a combination of mechanisms In the case of carbomers adding an alkali to an emulsion containing these tightly coiled polymers generates anionic charges along the chains like charges repel causing the polymers to uncoil and swell to many times their volume Another thickening mechanism is the interaction among the hydrophobic groups of the thickener Particle to particle interactions between thickener and other components in the formulation also contribute to changes in rheological properties The actual mechanisms and combinations thereof are complex and vary from one application to another but a common denominator is the need for proper dispersion or dissolution of the thickening agent at the right shear levels The primary objectives of the mixing step are to provide a homogenous mixture and to expose as much surface area of the additive particles as possible To achieve these goals mechanically the batch must be subjected to ample shearing energy as the thickeners are being incorporated Processing pitfalls e rst apparent challenge is handling thickeners that come in the form of u y and dusty ne powders To reduce the number of airborne particles in the mixing area operators must carefully transfer the solids from their original container and into the mixing vessel Adding powders right on top of an agitated batch poses limitations Most thickeners due to their hydrophobic groups resist wet out upon contact with water form lumps and oat on the surface for long periods This forces operators to sift and add powders only as fast as the liquid will take them In lab scale research and development charging powders slowly into vigorously stirred liquid is not hard to accomplish in small quantities But in a full scale production setting this method of addition can be very costly and time consuming Moreover if powders are charged too slowly an uncontrolled viscosity buildup can occur mid processing and prevent the rest of the solids from being fully incorporated In contrast manually adding the powders too fast can cause particles to clump up as mentioned ese clumps so called sh eyes form a tough outer layer that prevents complete wetting of the interior particles The high shear and turbulent flow conditions usually needed to break up these agglomerations can potentially overshear the already hydrated particles Prolonged intense mixing may in fact damage the polymers and lead to permanent viscosity loss or other undesired rheological properties In extreme cases to shorten cycle times operators resort to intentionally overdosing thickeners and ltering out the resulting sh eyes e above establishes that it is easy to mix thickeners ine ciently Mixing intensity rate and method of powder addition cycle time batch temperature viscosity pro le and raw material quality these factors contribute to operational productivity batch tobatch consistency and revealing the full functionality of each ingredient Some solutions One processing technique is to disperse carbomer powders into water under high shear in a dedicated mixing tank e resulting dispersion is later blended with the alcohol phase and the rest of the ingredients in a main tank equipped with a low speed propeller or anchor sweep In the main tank one of the final ingredients to be mixed in is the neutralizing agent sodium hydroxide potassium hydroxide or an amine base which then initiates a rapid increase in gel viscosity Dispersing the thickeners in a smaller tank eliminates the need for a large high shear mixer to t the main tank considering that the other product components do not require intense mixing If thickeners are added directly to the main tank operators must be mindful to switch to gentler agitation as soon as the powders are fully incorporated High shear mixers characterized by a rotor stator mixing head are widely used for wetting out thickeners and preparing a ne dispersion e classic design is characterized by a four blade rotor running at tip speeds in the range of 3000 4000 ft min within a close tolerance fixed stator It creates mechanical and hydraulic shear by continuously drawing product components into the rotor and expelling them radially through the openings in the stator High shear mixers are available in both batch and inline designs In batch operations the top entering rotor stator mixer is sized according to tank volume or range of operating volumes if intended for use on multiple vessels For example a 4 diameter rotor driven by a 10HP motor can handle low viscosity batches 20000 cps as small as 20 gallons and as large as difference would simply be the total time required to achieve X number of passes through the mixing head To ensure batch uniformity recirculating for no less than ten theoretical passes is a good rule of thumb in addition to applying some form of in tank agitation as needed In the past and even in recent years some manufacturers have adopted eductor based powder Figure 2 Close up of a batch style rotor stator mixer 150 gallons Within this approximate volume range and viscosity the 10HP batch high shear mixer is able to produce adequate flow in a vessel with reasonable L D ratio 12 to 08 ideally As batch volumes become larger bigger rotor stator heads are required and it is not uncommon to install an auxiliary propeller or anchor agitator to supplement bulk flow Eventually large batch high shear mixers demand horsepowers that are prohibitive for many operations In such cases moving to a recirculating inline mixer set up is more practical Externally installed inline high shear mixers are less sensitive to tank size and geometry compared to their batch counterparts For instance a 10HP inline rotor stator mixer set up for recirculation will pump around 240 gallons of process fluid per minute based on water like viscosity It can shear material from a 1500 gallon stirred tank as easily as it can shear product from a 150 gallon tote e dif induction techniques in inline operations to combat excessive dusting and oating powder issues ese systems operate based on the Venturi principle a pump accelerates liquid into an eductor creating a vacuum powder fed through an overhead tube is drawn by the vacuum into the educ Figure 3 Inline high shear mixer on a mobile cart