Rheonics Support

What is the purpose of this article?
Review the industry of adhesives and sealants, common types of products, their production and applications challenges and how Rheonics inline viscosity and density sensors allow high-quality end products.

What products are involved?
SRV - Inline Viscosity Meter and SRD - Inline Density and Viscosity meter

TABLE OF CONTENTS

• What are Adhesives and Sealants?
• Why are Adhesives used?
• Adhesive Market
• Types of Adhesives - Production and Applications
  • 1. Natural Adhesives
  • 2. Synthetic Adhesives
• Viscosity for Adhesives and Sealants
• Density for Adhesives and Sealants
• Viscosity and Density monitoring and control in a production process of adhesives
• Installation of the SRV and SRD in adhesive processes
  • In tanks
  • In pipelines

What are Adhesives and Sealants?

Adhesives and Sealants are two related terms when it comes to gluing and bonding two or more parts together. This joint usually requires the use of a paste or liquid that has been chemically processed to create a strong bond at the surface it is applied.  

The actual origins of adhesives and sealants come from nature itself and have been around us since the beginning of time. Nowadays adhesives and sealants are required in all we use and see around, from home workshops to high-technology products. Some example industries are packaging, paper production, aircraft manufacturing, aerospace, construction, footwear, automotive, electronic devices, medical, etc. 

This article explains how viscosity and density are of relevance for the production and application of adhesives, and how Rheonics SRV and SRD inline viscosity and density sensors can be used to monitor these variables.

Figure 1: Adhesives and Sealants types and applications

Learn more on the SRV Inline Process Viscometer

Learn more on the SRD Inline Process Density and Viscosity Meter

Monitoring and controlling viscosity in formulation, testing, and application of adhesives and sealants

Why are Adhesives used?

Adhesives create a joint or bonding between two or more objects or substrates (material to be bonded).  The joint is determined by the inner strength of a material (i.e., by molecules in the bulk), based on chemical bonds between molecules [1].

Many product designs and manufacturing processes use adhesives as preferred joint methods, with alternatives being welding, soldering, brazing and fasteners like bolts. Some design scenarios favor the use of adhesives, while others are not suitable for them. Advantages of using adhesives over other joint methods can include:

• Simplicity of application
  
• Various curing methods, each suited for specific industries and uses
  
• Smoother surface compared to other joint methods. Joint lines can be invisible 
  
• Creates joints without affecting the mechanical properties of the substrate (common in welding)
  
• Favors a better distribution of stresses
  
• Can reduce vibration in the assembly
  
• Good shock and impact resistance
  
• Allows the joining of different materials
  
• Easier production
  
• Can also act as sealing
  
• Generally lower cost 

Adhesives and Sealants - Differences and Similarities
These two terms are used a lot and can be thought to be very similar or even interchangeable, but there are actually differences between them in ther purrpose and final use. The main definitions are [1]:

Adhesive: A substance capable of holding at least two surfaces together in a strong and permanent manner. 
Sealant: A substance capable of attaching at least two surfaces, thereby filling the space between them to provide a barrier or protective coating. 

From the definitions, it can be said that an adhesive is used when a long-lasting and solid union is required, while a union by a sealant is mainly to avoid fluid or gas leakage, but is not intended to be permanent. This doesn’t necessarily mean that a sealant provides a “weaker” union compared to an adhesive as forces sustained and thermic characteristics vary between types and their intended final use.
Some similarities between adhesives and sealants are the way they behave to achieve the union, for example:
Fluidity: Both substances should behave as a fluid at some point of application to make contact with the surfaces or substrates and fill any possible gap.
Solidification: Both substances should harden to a solid or a semisolid state to transfer and sustain the variable loads the union can be exposed to.

Adhesive Market

The adhesives and sealants market runs an estimated value of USD 76.1 billion in the global market in 2024 and is projected to achieve almost 90 billion by the year 2029 [2]. The growth in the market can be put in context considering that it closed the 20th century with a value around USD 10 billion [3].

The leading countries in the adhesive and sealant industry are China, Japan, the USA, the UK, and Germany. This multi-billion dollar industry is comprised of approximately 750 companies with the largest companies, listed below, running almost 50% of the global market for adhesives and sealants.

• Henkel
  
• Sika
  
• 3M
  
• H.B. Fuller
  
• Huntsman
  
• ITW Performance Polymer
  
• RPM International
  
• Avery Dennison
  
• DOW

Current growth of the adhesives and sealants industry is based primarily on applications in building and construction, as well as on increasing demand in medical areas. The industry is closely regulated to achieve environmental standards, which encourages producers to find new technology to use resources efficiently and reduce waste. 

Types of Adhesives - Production and Applications

Adhesives can be generally classified by their origin, by the type of application (curing method), or by their chemical structure. 

The adhesive may originate from natural resources (animal, vegetable, or mineral) or synthetic materials (elastomer, thermoplastic, or thermosetting).

Table 1: Adhesive Types by origin [1]

1. Natural Adhesives

Natural adhesives are obtained from organic sources where the bonding is achieved by natural substances like protein, starch, and cellulose. An advantage of natural adhesives is they can be considered a more eco-friendly compared to the alternatives. Some natural adhesives, like gelatin glue, can also be 100% biodegradable and recyclable, even after being processed.

The most common natural adhesives are the starch adhesives and dextrin glues mainly used in the Paper and Packaging industries. These are relatively easy to produce so they are easy to find in the market at low prices.

Glue is a type of adhesive formed from many natural substances, including proteins like gelatin, starch, and cellulose [4].

Starch adhesives are based on starch, a natural carbohydrate or polymer, found normally in plants. The most common sources are Corn, Wheat, Potato, and Pea.

Starch adhesives have found rapid growth in the industry due to the ability to enhance their bonding and resistance properties through chemical processes. For example, borax (sodium tetraborate decahydrate) and sodium metaborate (borax and sodium hydroxide) can be added to change starch adhesive properties including viscosity, tack, and cohesiveness [3].

Figure 2: Water-based starch adhesive [4]

2. Synthetic Adhesives

Synthetic adhesives are based on prepolymers or polymers. The polymers used can be classified as thermoplastics or thermosets. 

The advances in the plastic industry allow these types of adhesives to be mass-produced with modified characteristics for each application. Synthetic adhesives are used in the construction of various objects, from furniture to airplanes. 

The main examples of synthetic adhesives are epoxies, urethanes, and cyanoacrylates. Some of these adhesives are two-part adhesives, meaning that they are applied as two or more components that, when mixed, react chemically to form a cross‐linked joint.

Synthetic adhesives are the most common adhesive type used in the world. Despite this, it also faces many challenges due to the limited raw materials (reserve of oil) and the negative impact of synthetic compounds on human health and the environment. 

Figure 3: Hexane adhesive

Next table shows a list of adhesive types, briefly specifying the production - how are they made? the application - how or where are they used? and use cases.

Table 2: Adhesive types, production and application

Viscosity for Adhesives and Sealants

In simple terms, viscosity can be defined as the resistant of a fluid to flow. Adhesives and Sealants in their fluid state are classified as non-Newtonian fluids, this means that their viscosity readings are dependent on the shear rate at which they are measured.

The viscosity is of great relevance during the production and application of the adhesive, and it can also give insights on changes of density, stability, solvent content, mixing rate, molecular weight, etc. The viscosity is an effective indicator of the adhesive consistency or particle size distribution.

The viscosity of adhesives can vary greatly depending on the final use case (e.g. sealing, bonding, etc.). Low, medium and high viscosity adhesives can be found.

• Low viscosity adhesives: Used for encapsulation, potting, and impregnation.
  
• Medium viscosity adhesives: Mostly used for bonding and sealing applications.
  
• High viscosity adhesives: Used for non-drip or non-sagging requirements. Common for some epoxies.

Traditionally the viscosity measurement technologies required manual sampling and laboratory readings. This process requires both time and man hours, and are not suited for real-time control of the process. The properties read on the laboratory are not representative of the fluid in the lines, due to the elapsed time, sedimentation, or aging of the fluid.

The Rheonics SRV is an inline process viscometer suited for inline readings of viscosity and temperature. The SRV reads the product of viscosity and density of the fluid and outputs the result as dynamic viscosity.

The SRV is suited for installations in different pipeline sizes, reactors, mixing and storage tanks, allowing full traceability of the adhesive fluid process. The SRV sensor enables a fully automated control process of adhesives, by sending viscosity readings to a controller to handle the addition of solvents or diluents to achieve the desired result. The SRV readings are also used as a real-time monitoring of the mixing ratio or consistency of the adhesive in the reactor, to take necessary actions when the mixing ratio is reached.

Density for Adhesives and Sealants

The density of a fluid is the relation of its mass per volume. The density of adhesives is used to monitor the content of compounds added in the production or the content of solids and particle sizes within the fluid before and after mixing. So density of an adhesive in a mixing process can be used as an indicator of the correct final composition.

The Rheonics SRD is an inline process density and viscosity meter suited for inline readings of density, viscosity and temperature. The SRD can also output kinematic viscosity, concentration percentages, etc. Rheonics offers the SRD in different variants and sizes to suit the installation requirement.

SRV Inline Viscometer	SRD Inline Density and Viscosity meter

Viscosity and Density monitoring and control in a production process of adhesives

The production of an adhesive is usually based on the mixing or dispersion of different materials to obtain the desired chemical resistance, thermal characteristics, shock resistance, shrinkage, flexibility, serviceability, and strength in the final product.

The Rheonics inline viscometer SRV and inline density and viscosity meter SRD are suitable for installation in different points of the adhesives, glues, or starch production processes. The Type-SR sensors allow the inline monitoring of viscosity, density and temperature, as well as derivative parameters such as concentration and mixing ratio. Installation can be directly in mixing tank to understand the evolution of viscosity and determine when the required mixing is reached; in storage tanks to verify the fluid properties are maintained; or in pipelines, as the fluid flows between units. 

Figure 4: SRV and SRD installation in Adhesive Production

Installation of the SRV and SRD in adhesive processes

In tanks

Viscosity measurements inside a mixing tank of an adhesive fluid allows quick action on the control of the fluid’s consistency. This results in more production and less waste of resources. 

The SRV viscometer can be installed in a mixing tank from the top lid, wall or bottom. If installed from the top, an -X5 long insertion probe is recommended. Learn more on the SRV-X5 and SRD-X5. The insertion length (A) should be long enough for the sensing element to be in contact with the fluid. Installation on the tank wall and bottom can use a short probe variant like SRV-X1 or SRV-X3. If existing ports are available on the tank, the client can consider reusing them for the SRV installation.

The SRD density and viscosity meter is not recommended for mixing processes as the mixing action can add too much noise to the readings. If the mixing tank has a recirculation pump line, the SRD can be installed there easily as explained in the next section.

Contact the Rheonics Support Team at [email protected] for recommendation on the installation. The client should share drawings or pictures of the tank, specifying any existing available ports and the operating conditions (Temperature, Pressure, Expected Viscosity, etc.).

In pipelines

The best possible location of the SRV and SRD in pipelines of adhesive fluids is in an elbow installation. This is an axial installation with the probe’s sensing element pointing into the fluid flow. This solution usually requires a long insertion -X5 sensor. Learn more on the SRV-X5 and SRD-X5.

Using a long insertion -X5 probe allows the client to have a custom insertion length (A) and process connection, depending on the installation requirements, like the line size. The insertion length A should be enough for the sensing element of the probe (in blue zone in next Figure) to be in contact with flowing fluid, avoiding dead or stagnation zones near the installation port. Placing the sensing element in the straight section of the pipe has the additional advantage of keeping the sensing element clean as the fluid flows over the streamlined design of the probe.

Figure 5: SRV Installation in elbow

If the line is small (less than 1.5”) the client can consider the use of Rheonics flow cells or the Slimline -X6 probe variants with a smaller diameter. Learn more:

• SRV-X6 Slimline probe
• SRD-X6 Slimline probe
• SRV flow cells
• SRD flow cells

Consider that high-viscosity adhesives can easily create deposits on the surfaces of the probe and pipe. Here, avoiding dead zones around the sensing element is key to reliable readings. If probe needs manual cleaning, review the article: How to clean the Type-SR probes? 

References

[1]: Handbook of Adhesives and Sealant 

[2]: Adhesives & Sealants Market Size, Industry Share Growth Forecast, Global Trends Report

[3]: "Boring" Boron and Adhesives | 2014-09-02 | Adhesives & Sealants Industry

[4]: Natural Glues for Industrial Applications

[5]: Monitoring and controlling viscosity in formulation, testing, and application of adhesives and sealants