The significance of binders in compaction and wet granulation processes cannot be overstated, as they play a crucial role in ensuring the success of material formulation. Their importance lies primarily in imparting superior compactibility and binding ability to the granules of the material being processed. Recent advancements in particle engineering and materials science have illuminated the significance of careful binder selection in optimizing wet granulation processes.
In this article, we will cover the following points regarding binders in compaction and wet granulation:
- What is a binder and how do they work?
- Why use a binder?
- Additional considerations when selecting a binder
- Binder types
- Step-by-step binder process
What is a binder and how do binders work?
Simply put, a binder is a substance used to make other substances or materials stick or mix together, with different binders having different mechanisms that cause the bulk material to stick together. During compaction and wet granulation processes, binders are critical for achieving appropriate surface wetting, which is necessary for fostering adhesion between different particulate surfaces amid the wet phase.
In the pharmaceutical industry, for example, the judicious choice of binders is required to strike a balance between promoting granule formation and maintaining the desired drug release properties, as well as ensuring compatibility with downstream processes like tablet compression. Beyond pharmaceuticals, wet granulation processes that involve the use of binders are also widely employed in various industries such as the food, fertilizer, and detergent manufacturing. In food production, they are utilized to create granulated forms of ingredients for convenient processing, while in the fertilizer and detergent sectors, wet granulation is employed to enhance the physical properties and handling characteristics of the final products. The essential role that binders play in wet granulation underscores their significance in a spectrum of industries where the precise control of material properties is paramount.
Displayed below are the many different types of binders that cover a wide range of processes, however, we will be primarily focusing on the following three binder types in this article:
- Matrix
- Film (Liquid)
- Chemical
Before we dive deeper into those three binder types, a couple of notes to consider in relation to the list of binders pictured above:
In dry compaction/dry granulation processes, you are typically dealing with interlocking, molecular forces, or partial melting in some rarer cases, but with binders we are just going to focus on chemical, liquid, and matrix binders. For example, sinter bridges and partial melting can come into play with compaction or briquetting when the parent material melts in localized surfaces because of frictional heat developed during agglomeration. Since the material that melts is still part of the raw feed, we don’t consider it a traditional binder since no additional substance is added during compaction.
Additionally, some binders will work as both a binder and a lubricant. The difference between the two is that a lubricant decreases the coefficient of friction between the individual particle in the agglomerate, or between the surface of the agglomerate and the die or rolls. This can be advantageous when material is sticking to the rolls, but we won't be covering lubricants in detail in this article.
When are binders needed?
There are many factors to consider when deciding whether or not to use a binder, but assessing the specific materials that you need to process is generally the best place to start. The use of a binder can lower the pressure-levels needed to make a material compact, so it can be advantageous for when material does not compact well by itself, or to enhance the performance of the end product. You will also want to consider using a binder if the material you are processing doesn’t like to stick together, or the product is very weak. Sometimes, with certain materials, high pressure can have detrimental effects on the materials themselves, so you would want to use a binder to decrease the amount of pressure required for compaction.
To simplify your decision-making process, here are some general rules of thumb around binders:
- With enough binder you can virtually compact anything
- Minimizing the amount of binder needed (if at all) minimizes the cost of creating the final product
- The amount of binder that is needed is material dependent
- Ensure that the binder you choose is not going to interact with the material in a detrimental way
- Ensure compatibility with end use
- Evaluate cost constraints/material value to narrow down your options
A few examples for when you absolutely should not use a binder are:
1) If a binder reacts with the material and it becomes insoluble as a product, but the end-use for that product requires dissolution, it would not be the correct binder to use.
2) If you're going to be burning the briquettes (the end-product), like with charcoal production, you don't want to add a toxic binder to that material.
Also, binders are typically going to add cost to the material you are processing, so if using a binder leads to doubling the cost of a final product, it likely won’t make financial sense to use one.
Additional considerations for using binders
Even after making the above assessments for whether or not you need a binder for your product, there are additional considerations you will need to keep in mind before coming to a decision:
- Is a binder acceptable? Regardless of what you decide in terms of using a binder or not, you must also find out if the customer will even accept a binder. Some customers don't want to use a binder at all since their product can't have any sort of contaminants.
- Project economics. Again, if the binder is going to add significant cost to the final product, it is likely not going to make sense to use one.
- Binder availability. Is the binder material even available in your area? And, are you able to purchase the material at a price that makes sense for the project?
- Product requirements. You’ll also need to consider the strength requirements for the end product, storage/weather conditions, downstream compatibility, and moisture percentage (usually 10% is the upper limit, but this is product dependent).
- Contaminants. The final consideration when selecting binders is contamination. Some binders can add silica, carbon, or Sulphur to the product. In some cases, contamination of the product by binders of any kind is objectionable, so water or some other solvent that will evaporate on drying may be the only choice.
To expand on point #4 - Product requirements, you will need to be able to quantify how strong the product is after a binder is added by conducting either a drop test, attrition test, crush test or tumble test. For example, the picture below is of a tumble drum, where the material can be taken after it's been compacted. The material is then left inside the drum for a specific amount of time in order to measure the attrition.
Also, if the product requirements dictate that it needs to be able to be left outside, you’ll need to find a binder that is weatherproof. For the weather considerations, you can wet the material and see if it absorbs water or not.
As for downstream considerations with the binder that you're selecting, if there's going to be a drying step after the compaction, you don't want to use a binder which will melt or stick in the dryer, or possibly off-gas any substances that can’t be released into the air.
Lastly, a rule of thumb for compaction is that the upper limit on moisture for compaction is going to be around 10%. This is very material dependent, and the upper limit needs to be determined by bench testing, but 10% is a good starting point.
Remember that the overall goal of adding a binder during compaction is to enhance the performance of the final product or make the material compact. You will want to ensure this by using a standard test method similar to what we’ve outlined above. The end-product must also have adequate “green strength” (the strength of the material right out of the compactor, before any curing is done to it) for handling and enough “dry strength” for its intended purpose.
Binder types
As we mentioned previously, there are many binders that exist and the one you need will be material-dependent, along with the other considerations we’ve outlined. However, for the purposes of this article, we are going to focus on the most common binders we interact with here at Bepex, broken down into three types: matrix, film and chemical. Within these three types, below you will see the most common binders bolded.
Matrix Type
Coal tar pitch
Petroleum asphalt
Portland cement
Carnauba wax
Paraffin
Clay
Dry starch
Dry sugars
Wood tars
Gilsonite
Film Type
Water
Sodium silicate
Plastic resins Glues
Starch
Gums
Bentonite
Tapioca
Glucose
Sucrose
Dextrin
Lignosulfonates
Molasses
Alginates
Chemical Binders
Ca(OH)2 + C02
Ca(OH)2 + Molasses
MgO + Fe S04 MgO + MgCl2
Sodium silicate + CaCl2
Sodium silicate + C02
Matrix-Type Binders
Matrix binder effectiveness depends on embedding the particles in a more-or-less continuous matrix of the binding material. Rather substantial quantities of these binders are required, because their film strength tends to be low, and because briquette strength depends upon the presence of a continuous phase of binder surrounding the individual particles, much in the nature of an asphalt or concrete pavement. Strength, therefore, depends, to a great extent, on the relative density or the absence of voids in the briquette structure. Thus, the void fraction of the feed material is going to dictate the amount of matrix binder required.
Film-Type Binders
Film binders, or liquid bridges, are generally, but not necessarily, used as solutions or dispersions. Water is the most common solvent, but other solvents have been used in some instances. The green or wet strength of the briquettes made with these binders may be low compared to that of matrix-type binders, but film binder strength increases as the briquettes are dried/cured after compaction. The film strength is ultimately going to dictate the green strength of the material.
The quantity of binder required in this case depends upon the density of the particles and their specific surface area in the raw feed (particle size), rather than their void fraction. Typically, the finer the materials, the more binder that's going to be required. Since the binder is in the form of a solution or dispersion, the quantity also depends on the extent of dilution. The film strength of the binder, in turn, determines how much dilution can be tolerated.
Satisfactory wetting of the surfaces of many particulate materials can be achieved with 1-2% of a liquid or binder, but for fine materials up to 10% binder may be required. Porous materials also require high percentages of binder, for instance, charcoal may need up to 30% of a starch paste binder.
Chemical Binders
Chemical binder effectiveness depends on a chemical reaction between the components of the binder or, in other cases, on a chemical reaction between the binder and the material being agglomerated. A chemical binder may also consist of adding multiple different binders to a raw feed, for example, that will react with one another to bind the material together. These types of binders depend on the chemical reaction between the binders, or the binder and the feed, for their green strength and their final product strength.
Step-by-step binder process
Finally, let’s take a look at step-by-step visualizations of the binder process, both with and without the curing phase.
Binder with curing
Pictured above is a briquetting system with a binder addition that requires a drying step. The flashing in this case is ground before going back to the compactor. The binder used in this installation is a starch slurry and the briquettes are screened before going to a tray dryer. Curing is necessary when the green strength of the briquette is not sufficient for the end use.
Binder with no curing
This diagram shows a binder addition without curing, which can be used when the green strength of the product is high enough that the material can survive handling, or a chemical binder is used. For example, liquid pitch can be used as a binder that does not require curing. In this example, caustic magnesia is used as a binder for magnesite with added water for green strength. The reaction with the caustic dries and cures the material.
Choosing the right binder
The process of selecting and using binders is one that takes significant consideration, both due to the considerations we’ve outlined above and their importance in compaction and wet granulation processes. However, leveraging the constraints of your next project such as budget, end-product requirements and binder availability, should allow you to narrow the options at your disposal. From there, using some of the testing methods we outlined under “Additional Considerations” will help you discover which binder (if any) will lead to the best final product possible.
If you have any questions about binders or could use our help with your next process, please don’t hesitate to contact us!