PU Sealant, Hybrid Sealant, Silicone Sealant and Acrylic Sealant: The Complete Guide to Sealants in Construction

Antonio Neves
Written by
Antonio Neves
Published in
6/2/2020
Updated
20/4/2021
PU Sealant, Hybrid Sealant, Silicone Sealant and Acrylic Sealant: The Complete Guide to Sealants in Construction

PU Sealant, Hybrid Sealant, Silicone Sealant and Acrylic Sealant: The Complete Guide to Sealants in Construction

You may have already bought a tube of silicone sealant, pu sealant, hybrid sealant or acrylic sealant to make a repair or for some stage of your job site, right?

But when you bought it, did you know why you were choosing specifically that type of sealant?

Ever been in doubt like, what is the difference between a pu sealant, acetic silicone sealant, neutral silicone sealant, acrylic sealant or hybrid sealant?

There are several factors that can influence the purchase of a sealant and more importantly, the specification of the correct product for each stage of the job.

Not all sealants are the same! Didn't you know?!

So come along with us in this Complete Guide to Sealants where you'll learn all about it and choose the right sealants for your job site!

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What is Sealant?

You may have heard about sealants, but you may be wondering what's so special about knowing what a sealant is.

Precisely this definition will guide this article to talk about products that seal structures and not to get into another subject very close, which are the adhesives.

So let's understand about sealants?

As I said, the definition of sealing in construction can be said as, leak prevention work, in this way, sealants are used to seal joints or openings between two or more substrates.

The main role of a sealant is to prevent water, air or other substances from entering or leaving the structure, even with some movement of the substrates where it has been applied.

The distinction between adhesives and sealants is not always very clear, but it is worth defining that line. Sealant is intended to seal a joint, and the material chosen for the sealant has the ability to move and adhere to the substrate. Adhesives, on the other hand, are materials that are designed to hold substrates together by bonding surfaces and are often used as an alternative to mechanical fasteners such as screws and nails.

Some materials can be considered both sealants and adhesives and fulfill the roles of sealing and bonding between surfaces. These types of materials are known as structural sealants.

All sealants must fulfil at least three main functions:

  1. Sufficiently fill an expansion or movement joint to create a seal between substrates;
  2. Form a barrier against gases and liquids;
  3. Maintain sealing between the structures, allowing a certain amount of movement between the parts.

History of Sealants (Brief History)

I do not want to prolong the subject too much, but I believe it is interesting to present, at least, a historical line of the sealants developments. From the first technologies that were adopted in construction, until the most recent ones.

The first sealant technology, marketed on an industrial scale, was introduced in the early 1940s with polyisobutylene. The main applications were in construction, automotive and electrical.

In the late 1940s, the first polysulfide-based sealants were introduced to the construction and aerospace markets.

A decade later, in the late 1950s, solvent-based acrylic sealants entered the construction market. This type of sealant, had a thermoplastic characteristic, unlike newer acrylic technologies with elastomeric characteristics.

Silicone sealants entered the market in the early 1960s. This type of sealant, with its elastomeric characteristic, was and still is used in construction, aerospace, electrical, electronics and automotive applications.

As early as the 1960s, water-based acrylic sealants and the first polyurethane sealants began to be used in construction and polyurethane sealant gained ground primarily in the automotive industry.

In the mid-1980s, silicon acrylic sealants emerged, which here in Brazil is popularly known as water-based silicone sealant. At the end of the 80s, the technology of polyether-based sealants began, known in our market as modified silicone sealant, or MS sealants.

The polyurethane sealants most used in Brazil today is the polyurethane technology known as hybrid, because it closes the chain through a tip with silicone. This sealant technology emerged in the early 1990s. At the end of that decade, polyisobutylene, also hybrid, appeared on the market.

Here in Brazil, currently, the main chemical bases used to manufacture sealants for civil construction are polyurethane (PU), acrylic, hybrid polyurethane, polyether and silicones, both neutral cure and acetic cure.

What Types of Sealants in Brazilian Civil Construction?

Sealants in construction can be used extensively in a variety of industrial, commercial and residential applications. These applications can be in places such as glazing, sealing windows and doors, finishing sanitary ware in bathrooms, expansion joints in floors, facades and fire seals.

In addition to the basic function of sealing and accommodating the movements of structures, some specific applications may also impose additional requirements on sealants such as vibration dampening, fire fighting, electrical, acoustic and thermal insulation.

In construction, sealants can be classified by their shape, cure and cureability, chemical basis, performance or the end market in which they will be used. In the case of shape classification, sealants can be divided into two groups, preformed sealants and cast-in-place sealants.

In this article we will focus on cast-in-place sealants. These sealants are applied in liquid or paste form and cure or cure in place.

I will introduce basic concepts about the main sealants on the market. I will better detail each of them in separate articles, not to overextend the subject only in this one.

The sealants I will introduce are, acetic silicone sealants, neutral silicone sealant, pu sealant, hybrid polyurethane sealant, ms sealant, crack sealant, acrylic sealant and water-based silicone.

Silicone Sealant

Silicone construction sealants were introduced in the mid-1960s and many of these sealants developed at that time are still in use today.

These sealants are available in various forms of application, from thixotropic pastes, to self-leveling forms. We can also find it in one-component and two-component form.

Most sealants that are used in Brazil are applied and cured under ambient conditions, so they are referred to as room temperature vulcanizing, or RTV(Room Temperature Vulcanizing) products. There are other special sealants, which cure when melted, known as hot melts and aqueous dispersion, but these are subjects for other matters.

So, let's learn more about silicone sealants?

In the Brazilian market, silicone sealants can be divided into two major classes, acetic cure silicone sealants and neutral cure silicone sealants.

Acetic Silicone Sealant

Acetic Silicone Sealant, was the first one-part silicone sealant developed and I am sure you must have seen or used this sealant, am I right?

This sealant is known primarily for its characteristic vinegar odor. Because the condensation curing process releases acetic acid, the smell of this product during the curing process is very noticeable.

Further implications of this type of cure are the applications in which acetic silicone sealant can be used. In applications such as glass-to-glass, glass-to-metal and metal-to-metal, this type of sealant is highly recommended because of its high adhesion strength.
On the other hand, if the metal surface is not finished for protection, the acetic acid can attack the metal, causing corrosion and adhesive failure between the sealant and the surface.

The same adhesive failure occurs if acetic silicone sealant is applied to cementitious surfaces in general. The acetic acid released during the cure of the sealant will react with free calcium and magnesium hydroxides on the cement surface, causing an acid-base reaction, forming a salt and water. This salt formed at the interface between the sealant and substrate causes the sealant to bond to the surface.

Neutral Silicone Sealant

After acetic cure sealants were developed, other crosslinkers were used for the development of new sealants with different characteristics and new applications where acetic silicone sealant was not suitable for the application.

Oxygen Curing Neutral Silicone Sealant

Neutral sealant of oxime cure, are the neutral sealants most commonly found in building materials stores. This type of cure has as characteristic smell the oxime released in the process.

In applications where acetic silicone sealant is not recommended, such as unprotected metal surfaces and cementitious substrates, neutral silicone sealant is the best alternative.

Alkoxi Neutral curing silicone sealant

Another, slightly less common type of neutral cure is alkoxy, or alcohol-cure silicone sealant. As the name implies, during the curing process, the sealant releases alcohol, specifically methanol.

This type of silicone sealant is not found in building material stores for sale, only in specialized distributors.

The main advantage of this silicone sealant is its high adhesion to metal surfaces such as aluminum. The market for frames and ACM facades end up being the markets where this type of sealant is used.

PU Sealant - Polyurethane Sealant

The properties of polyurethane sealants are determined by the type of cure and polymers used in the formulation. Unlike silicone sealants, which use the same type of polymer, pu sealant can be formulated with a wide variety of polymers.

The standard curing mechanism is via isocyanates, however, other types of urethane polymers have been developed using different functional groups at the end of the chains, which provide other types of curing for the material.

This type of formulation was most commonly found in the Brazilian market about 5 years ago or more. This is because this formulation requires the use of solvents such as xylene, heptane or acetone to adjust viscosity and increase the extrusion rate of the sealant.

You may have seen or still see some polyurethane sealants being sold in metal containers. One of the most striking features of this type of sealant is the smell of the solvents used, in addition to the risk they offer with regard to flammability.

This type of sealant has been less widely used in recent years due to the industry-wide move towards solvent-free, low-VOC products.

Hybrid Sealant

Some types of sealants, newer ones, are based on a technology called hybrid polymers.

Basically, hybrid polymers, combine the backbone of one family of sealants, with reactive groups, usually positioned at the ends of the chains, of another type of polymer.

Two types of hybrid sealants used here in Brazil are, hybrid polyurethane sealant and ms sealant.

If you still do not know the difference between them, I have separated below in separate topics. See here!

Hybrid Polyurethane Sealant

Hybrid polyurethane sealant has a different type of cure than traditional pu sealant. Two advantages of this type of molecule are greater stability in relation to exposure to weathering and also the elimination of the formation of carbon dioxide bubbles inside the cured sealant, a result of the reaction between polyol and isocyanate.

MS Sealant

MS sealant, unlike what is said in our market, it is not a sealant derived from the combination of polyurethane and silicone, but a polymer known as polyether.

Just as hybrid polyurethane is a polyurethane modified with a reactive functional group, polyether, it is also modified with a reactive silicone functional group, hence MS, "Modified Silicone".

Acrylic Sealant

The acrylic sealants have been gaining space in our market, with increasingly technical applications and, in some cases, replacing other sealants in applications with little movement.

There are two major classes of acrylic sealants, water-based acrylic sealants and solvent-based acrylic sealants. The solvent-based products, which are dispersed in organic solvents, while the other is dispersed in water. In both cases, we cannot say that there is curing of the materials, since what occurs is the evaporation of the solvents present in them.

What we will deal with here, will be only the water-based acrylic sealants, since they are the most marketed products in our market and with the highest number of applications in our constructions.

Within the acrylic sealants sold here, we can separate them into three groups, crack sealant, acrylic sealant itself and silicon acrylic sealant or water-based silicone.

Each of these materials meets a specific standard, unfortunately none ABNT for now, but with clear definitions of movement capacity of each of these sealants.

Sela Trinca

As the name of the sealant says, the crack sealant is used to seal cracks and fissures on facades. You don't know the difference between cracks and fissures? Take a look at this material that explains everything about this subject.

This type of material is also an acrylic sealant, usually with a more rigid formulation than other sealants. But don't take this as a negative point, it brings several benefits to this application.

Of course, it is a material that, being rigid, does not follow the movement of structures like other sealants, but for the purpose of closing or sealing cracks, it is perfect.

For having a formulation, in which after drying, will be more rigid in relation to other products, it will present a smaller retraction. Consequently, when you apply the product, you will not have to go back to the site and spend a second or third coat of product, as happens in pastes or other acrylic sealants.

Acrylic Sealant

Different from the crack sealant, acrylic sealant is an elastic material that follows certain movements of structures and can be used for sealing door jambs, meeting between furniture and walls, sealing planned furniture, sealing door frames from the inside and some external cases.

There are on the market, from sealants for internal application of buildings, even high-performance acrylic sealants for outdoor application. We can classify acrylic sealants for internal use, those that meet the standards ASTM C 834 and ASTM C 920, class 12.5.

The highest performance acrylic sealants found in the market meet the ASTM C 920 standard, classes 25 and 35. These classes already allow their use in external applications, such as sealing windows and frames.

Silicone Acrylic Sealant - Water Based Silicone

Silicone acrylic sealant, also known as water-based silicone, is a high-performance acrylic sealant with silane additives, which help promote greater adhesion to substrates such as concrete and metals, in addition to increasing the moisture resistance of the product.

Thus, it is possible to use water-based silicone in applications where previously only silicone sealants or pu sealants were used, such as sealing gutters, downspouts, downpipes and also sealing windows and window frames in buildings.

Sealant Properties

Cured-in-place sealants rely on their adhesion properties to maintain a durable seal and perform required functions during their service life. They must accommodate joint movement without failing between interfaces or cohesively or causing substrate failure. Based on these requirements, the most important performance properties in construction sealants are movement capability, modulus, adhesion and life expectancy (durability).

Structure seals are directly dependent on the properties of the sealants chosen for the application to maintain a perfect seal over their lifetime.

The sealant chosen for the job must accommodate the movements of the structures without causing the three main types of failures of a sealant which are:

  1. Adhesive Failure: pathology in which the sealant completely detaches from the substrate, leaving no part of it in the structure and neither bringing part of the structure with it.
  2. Cohesive Failure: Type of failure in which the sealant remains adhered to the substrates but breaks off in between.
  3. Substrate Failure: A less common failure to see, but occurs when the sealant pulls away part of the substrate during movement of the structure.

Another type of failure that occurs frequently in markets where price dictates product choice is sealant degradation due to UV exposure, which is linked to product life.

Based on these requirements, we can determine the most important performance properties in construction sealants which are, movement ability, modulus, adhesion and life expectancy or durability.

Shore hardness

The Shore hardness in sealants for construction, represents the hardness of the material that will be used for a particular application. The Shore hardness has several scales, and the most common in the sealants in our market is the Shore A hardness.

You know when you go to buy a pu sealant, for example, and you see a package with the following text on the label, "Sealant PU 40"? Well, that means the following, that the product is a sealant, chemical-based polyurethane, with a Shore A hardness of 40.

Shore hardness is directly related to the material's modulus of elasticity and, consequently, to its ability to move.

Modulus of Elasticity

The modulus of elasticity of a sealant describes the force exerted per unit area of a sealant under tension.

Since the primary function of a sealant is to adhere to the substrates with which it is in contact, the forces generated by a joint opening or closing are transmitted by the sealant to the sealant-substrate interface.

For this reason, it is important to know the modulus of elasticity of the sealant as well as the strength of the substrate. For example, using a high modulus of elasticity sealant on a "weak" substrate such as asphalt may result in higher stresses than the substrate can support, resulting in substrate failure.

Handling Capacity

The movement capability of a sealant is the amount of repetitive displacement that a sealant can withstand, continuously, during its service life without failure.

The existing specific standards only allow an assessment of the initial handling capacity of the products, but do not include the handling capacity over the years.

This means that a sealant well sized today may not work in a few years due to its natural aging caused by exposure to weathering.

Adhesion to Substrates

Sealants, when applied, must develop adhesion to the substrate so that it can perform its sealing function.

Within this, three aspects are critical: the rate at which this adhesion is developed after the sealant is applied to the joint, the level and quality of adhesion achieved and the durability of the adhesion.

In many applications, sealants are exposed to the natural movement of structures, which can increase the chances that it will not develop rapid adhesion to the substrate and will fail early in its service life.

Durability

Once applied, sealants are exposed to a wide variety of environmental influences and stresses.

An important characteristic of a sealed joint is its tolerance to the environment in which it will be exposed. A primary factor in sealant durability is its ability to resist degradation introduced by environmental elements such as UVA and UVB, pH, oxygen and temperature.

Environmental and sealant service factors can cause degradation of the surface, bulk or substrate interface during the sealant's service life.

These environmental factors can cause degradation of color, gloss, shrinkage, crack formation and bleaching of sealants.

Performance Standard for Sealants

Performance standards for products are essential for the evolution of the quality of products used in the market, besides being documentation where the consumer himself can be based to choose the most suitable product for use.

In sealants it is no different, there are two standards that are the guide to other standards in various countries. These standards are restricted to the specifications and classifications of sealants.

ASTM C 920

ASTM C 920 - "Standard Specifications for Elastomeric Joint Sealants" is a North American standard that covers the specification of single-component and multi-component elastomeric gaskets for sealing, sealing, glazing caulking of buildings, plazing, flooring, construction, highways and airports.

ISO 11600

ISO 11600:2002 - "Building construction - Jointing products - Classification and requirements for sealants" is the standard that will most influence our sealant market. Brazil does not yet have its own performance standard for sealants, but this will change soon.

ABNT

As I said in the previous topic, for now, we do not have an ABNT standard for sealant performance. The elaboration of the Brazilian NBR is in the final stage of preparation, based on ISO 11600.


Want to know more about sealants?

In this article I gave an overview of sealants, going from basic definitions of what a sealant is and a brief history about them.

After that, I separated the main types of sealants found in the Brazilian market as, pu sealant, hybrid sealant, acrylic sealant, neutral silicone sealant and acetic silicone sealant.

On top of each of the sealants, I explained what each of them are and a brief explanation of advantages and disadvantages for each situation encountered on a construction site.

I also went through the essential properties you need to know to choose the best sealant for each stage of your job site and how to evaluate its performance over time.

Finally, I have separated the main performance standards used in the world and the current status of the Brazilian sealant performance standard.

If you have any questions regarding this subject, leave a message in the comment box below and don't forget, we are here for any challenge.

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