Static mixing has long been used in the chemical process industry for blending low-viscosity fluids or treating wastewater. Its use within the adhesive field gained popularity about 10 to 12 years ago and has been increasing ever since.
Prior to this time, two–component mixing was carried out by the end user in a more manual fashion, by measuring out each component by weight or volume, and stirring for some amount of time. This could obviously lead to off-ratio blends, poor mixing, significant air entrapment or particulate contamination, and ultimately poor product. Static mixers offer substantial advantages by allowing a repeatable, reliable mixture with significantly less mess or possibility of operator error.
The majority of static mixers used in the adhesives industry are disposable, allowing for quick clean up and easy changeover from one mixer to the next. This article intends to provide an understanding of how these tools achieve their goal, and what factors need to be considered to enable selection of an optimum disposable static mixer. Innovative methods of using static mixers with problematic materials will also be discussed.
The importance of proper mixing at the point of use for two-component adhesives and sealants cannot be overstated. The end user ultimately determines the success of a product, and easy, repeatable, reliable mixing is critical to the success of two-part products. Improper mixing results in poor adhesion, low-joint reliability, or unsatisfactory aesthetic quality. Inappropriate static mixer selection by the adhesive supplier may cause excess waste and disposal costs for the customer or difficulties in dispensing and application of the product.
Visual examination alone may not be sufficient to determine complete mixing. All of these factors may ultimately affect the market demand for a particular material. In order to properly select a mixer, the method by which mixing is achieved and the factors affecting the final mix quality must be properly understood.
Unlike many dynamic mixing methods, the fluid flow through a static mixer is laminar rather than turbulent. The method by which material is mixed is therefore different in a static mixer. Figure 1 shows four elements of a typical spiral static mixer. The elements are stacked atop one another, each rotated 90 degrees relative to the next. As material enters the first element, the fluid stream is split into two layers. Upon entering the second element the stream is again split into two layers, for a total of four layers.
Layering continues in this manner through the mixer, as depicted in Figure 2. At the outlet of the mixer, the number of layers should be sufficiently high, and each layer sufficiently thin, that the material is considered mixed. The number of layers present is calculated as the number of layers = 2 N , where N is equal to the number of elements.
The downward curving shape of the spiral elements is critical, as it contributes to mixing to a smaller but critical extent. As the new material flows into the element, it causes a sweeping motion around the walls of the housing, generating radial mixing along the axis of the element. This subtle motion enhances mixing efficiently and may help to reduce the number of elements necessary to achieve a complete mix.
The process of selecting an appropriate mixer is not trivial. The needs of the end user must be considered as well as the properties of the mixer and adhesive or sealant being mixed. The dispensing method of the end user may significantly affect the acceptable parameters of the mixer. For example, a pneumatic or meter/mix/dispense applicator would tolerate a greater resistance to flow, and therefore a smaller diameter or longer mixer than a user with a manual dispenser.
When applying adhesive into holes for anchor bolts, a certain length may be very desirable, while for automotive repair applications it may be critical to be as close as possible to the work being performed. Such considerations must be balanced with the other factors discussed below.
Typically, two properties of the static mixer are of primary concern. These are the retained volume and backpressure, or pressure drop, across the mixer. The retained volume of a mixer refers to the amount of adhesive, both cured and uncured, that will be retained in a mixer and must be discarded. This typically includes the entire empty volume of the mixer not in contact with the cartridge (Figure 3).
While some mixers are available with stainless steel housings or elements, the need for solvent rinsing does not obviate disposal costs or environmental issues. In addition to the retained volume, the backpressure or resistance to flow of a mixer must be considered. While higher backpressures are more acceptable to pneumatic or meter/mix dispensing situation, the material throughput will be reduced as backpressure increases. The limits of pumps or air supply will determine the allowable back pressure limit and hence provide guidance for sizing mixers in these situations. The backpressure of a particular mixer is a function of the diameter, length and geometry as well as the flow rate and material viscosity. An estimate of the backpressure may be calculated as follows:
D P = Q* m *L where D P is the pressure drop across a mixer, or back pressure, Q is the material flow rate, m is the viscosity of the mixed material, and L is the pressure drop factor for a particular mixer.
This pressure drop factor is specific to a given mixer, and takes into account the length, geometry, diameter and surface finish of the elements and housing. This is a conservative method of calculating backpressure and often tends to overstate the pressure drop.
Certain aspects of the adhesive or sealant to be mixed can also play a role in the selection of an appropriate static mixer. The ratio of components A and B as well as the ratio of viscosities between the two parts are very important considerations. The closer a material is to a 1:1 mix ratio, the easier it will be to mix. Viscosity extremes are very difficult to mix, and may cause problems with pumps in a meter/mix/dispense application.
In a manual dispensing operation, such viscosity extremes may challenge either the ability of the operator to dispense material or result in off ratio dispensing as the less viscous component “drools” down the mixer ahead of the other. It has been observed that if a significant viscosity difference exists between parts A and B, there is a tendency for the lower viscosity material to furrow, or streak down a mixer rather than blending with the other component. The miscibility of the components may be affected by fillers or additives, which can contribute to mixing difficulties, requiring longer mixers.
The abrasive nature of inorganic fillers or any tendency for them to agglomerate can damage a mixer or dispensing system or lead to potential blockage. All of these considerations result in a general rule of thumb for determining the number of mixing elements required for a particular chemistry. Generally, adhesives become harder to mix and require more mixing elements in the following order:
Acrylic < Epoxy = Silicone < Urethane
Case Study Examples
The discussion points covered above indicate the selection of an appropriate static mixer requires balancing the needs of the user with the characteristics of the material and mixer in order to arrive at an acceptable solution. This can best be exemplified by a sample case study.
For example, a customer may have an application for a 1:1 two-component structural epoxy with component viscosities of 150,000 and 300,000 cps. The material will be dispensed from 400-ml cartridges on a construction site in small volumes to bond steel anchor bolts into a concrete block.
As mentioned previously, epoxy materials are typically not difficult to mix, often requiring 15 to 24 elements, making 20 or 24 elements a reasonable number to begin with. Since this material is viscous and will be used on a construction site where pneumatic dispensing may not be an option, selection of the diameter should avoid creating significant backpressure that would make manual dispensing difficult. However, since the material will be used with 400-ml cartridges, the mixer size must avoid excessive waste. Hence, 3/8- and ½-inch diameters may be investigated for difficulty of dispensing.
Initial testing may involve only the dispensing of beads of material to visually evaluate the mix quality and determine the difficulty of manual dispensing. If the mix quality appears to be acceptable, as determined by visual analysis of the surface and cross-sectional area of the bead, 15- and 18-element mixers may be investigated. However, even if the mix quality appears acceptable, adhesive bond testing should be performed to determine that the desired properties (strength, cure time, hardness, etc.) are present. This could lead to the determination that while the material appears to be mixed with only 15 elements, achieving the target tensile strength requires a minimum of 18 elements.
A similar approach would be adopted for a meter/mix/dispense application. Typical data necessary to identify the appropriate mixer are the type of material, the flow rate and viscosity of each component, and the pump discharge pressure or pressure of the material at the inlet of the mixer. For example, a customer may have a urethane in a 2:1 mix ratio, with a mixed viscosity of 10,000 cps and a flow rate of 0.5 gallons/minute. Urethanes often require 24 to 36 elements to mix.
Based on this information, one may select a 24-element mixer of 3/8-inch diameter and calculate the pressure drop across the mixer using the equation provided previously. This results in a pressure drop across the mixer of 480 psi, which could be close to the upper limit of the pumping system capability. Repeating this calculation with a 24-element 1/2 –inch diameter mixer yields a pressure drop of 190 psi, and this is the mixer that would be recommended for initial testing. As in the previous example, mix quality must be determined visually and based on the desired properties to ensure the correct number of elements have been selected.
Static Mixing Technologies
Variations in the specific geometry of the mixer element have been available for some time, although new modifications have been developed. The layering method of mixing discussed earlier applies to many of these geometries. There are a few element geometries that cause different fluid flow patterns to achieve mixing. In most instances, the difference in mix quality from one mixer geometry to another is slight
Occasionally, with a particular material, geometry differences may provide a small but critical increase in mixing efficiency due to minor flow path variations. This concept is similar to the enhanced mixing efficiency provided by the angled elements in the spiral element shown in Figure 1. It is difficult at this point to predict which materials will perform best with any particular geometry. However, it may be more confidently stated that no one mixer would be the answer to every material-mixing question.
New and innovative designs and uses for static mixers are appearing in the marketplace. Many foaming materials are used in automotive repair and can be challenging to mix with a static mixer. Designs that include a hollow tube attached to the outlet of the mixer tip can allow some foaming action to occur prior to dispensing. Other users have found that a gap between sections of the mixing elements provides the correct balance of residence time and mixing prior to dispensing.
Every foam material and application requires investigation into mixer options that will achieve this delicate balance. Examples of such mixers are shown in Figure 4.
This disposable static mixer has also been used in conjunction with a dynamic dispensing method to create a hybrid-style mixing method. While this technology has been available for several years, it has been a significant increase in importance during the past 12 months with a variety of adhesives. During the past year, many meter/mix/dispense users encountered the need for a significant number of mixing elements, and hence very long mixers. Where this length has been found to hinder material application, the hybrid static-dynamic mixing method has often provided a satisfactory alternative.
The mixer itself is installed onto a special manifold equipped with a bar to grasp onto the element and rotate it within the housing. An example of this is provided in Figure 5. TAH Industries' dynamic valve uses an air driven motor spinning between 500 and 4,000 rpm. This rotation incorporates shearing action in addition to the layering previously described for static mixers, and may allow for a satisfactory mix while using fewer elements.
Disposable mixers are useful with viscosities up to about 25,000 cps. For more viscous materials, other dynamic mixing methods are available, but these do not use disposable static mixers and require significant investment.
Selection of a static mixer to allow correct, efficient mixing of adhesives and sealants is a critical aspect of packaging and supplying a two-component reactive material. The needs and requirements of the end user must be considered at all times, along with characteristics of the mixers and materials in question. The advantages offered by static mixers in terms of the ease and repeatability of mixing make this selection process worth the effort in many instances.
This paper was presented at the Adhesive and Sealant council's Fall Conference and Exposition in New Orleans.