ADHESIVES, GLUES AND SEALANTS


Polyurethane adhesives and glue - definition and properties of PUR

What are polyurethane adhesives?

Did you know that your car windows are probably bonded with a polyurethane adhesive?, Surely many of the things around you are joined by these so special adhesives commonly called as elastic adhesives.

Polyurethane adhesives are those glues that are made of urethane polymers with chemical based of isocyanate group, polyurethane adhesives are also known as elastic adhesives, due they have an extraordinary elasticity and elongation before the fracture occurs (up to 600%), and also like epoxy adhesives exist rigid polyurethane adhesives, due to the high fracture resistance presented by these adhesives about 25 MPa.

The elastic polyurethane adhesives are cured by polyaddition reactions, taking on a slightly reticulated structure with properties of an elastomer material; rigid polyurethane adhesives are as well as cure by polyaddition but they also acquire a high crosslink structure of thermoset material.

There are 3 different types of polyurethane adhesives or glues:

Monocomponent polyurethane adhesives or adhesive 1 component can be elastic or rigid adhesives, depending on the structure of thermoset or elastomer that acquires the adhesive once it has been cured.

The elastic polyurethane adhesives are those cured by the action of moisture; however the rigid polyurethane adhesives are those that are cured by heat input.

1 component polyurethane adhesives moisture curing.

Note - All information described on 1 component polyurethane adhesives moisture curing are referenced to the basic principles. For each adhesive the valid information is contained in the technical data sheet do it by the manufacturers, due that the formulation of the adhesive changes its properties.

Basic Components of 1C Pur moisture curing and polyreaction

Resin / Hardener - prepolymer with isocyanate groups endings.

The polyreaction is by polyaddition, resulting in a lattice structure of elastomer polymers.

The reactions in which the polyurethane is formed by the action of moisture are the following:

Considerations of 1C Pur moisture curing

With this type of glue you must take into account the skin formation time, equivalent to the concept of Pot -life in 2 component adhesives.

The skin formation time is due to the action of moisture in the atmosphere, which reacts with the resin to form the polyurethane.

The skin formation time is the maximum time in which we have to apply the adhesive and it has to join the substrates, once you have exceeded the skin formation time it worsens adhesion to the substrates, due a thin layer of adhesive which has been already cured, becoming impossible the wetting of the adhesive on the substrate.

One trick that tells you if the skin is formed is to press lightly the first bead of adhesive you applied, if you spot your finger you can join the other piece, if you do not stain so it is best to remove all the adhesive and reapply or find another adhesive with more tack-free time.

talk free test polyurethane adhesive

The curing time depends strongly on the concentration of moisture, the curing can be accelerated through the use of "booster", which is a product containing mostly water and that it joins with the adhesive in the mixing nozzle, resulting in a homogeneous and fast cure.

Typical properties of 1C Pur moisture curing

Mechanical properties

Chemical properties

1-component polyurethane adhesive cured by heating

Note - All information described on 1 component polyurethane adhesives curing by heat are referenced to the basic principles. For each adhesive the valid information is contained in the technical data sheet do it by the manufacturers, due that the formulation of the adhesive changes its properties.

glue point Basic Components of 1C Pur heat curing and polyreaction

Resin - Polyols.

Hardener - isocyanate prepolymer with chemical blocked terminations, which are released by the addition heat energy.

The polyreaction is by polyaddition resulting in a lattice structure of highly crosslink thermoset polymers

glue point Considerations of 1C Pur cure by heat

With this type of adhesive we need to apply temperatures about 100ºC and 200ºC, in order to unlock the isocyanate group necessary to produce the polyurethane.

The contribution of heat energy causes a lower curing rate compared to 1C Pur adhesive cure by moisture, while producing a highly crosslinked polymer structures or bonded with thermoset material properties, that is the reason why these adhesives are considered as rigid glues, due they have a high resistance against stress or loads, thanks to the thermoset structure of the main polymers.

Typical properties of 1C Pur cure by heat.

Mechanical properties

Chemical properties

2 component Polyurethane adhesives.

Note - All information described on polyurethane adhesives 2 components are referenced to the basic principles. For each adhesive the valid information is contained in the technical data sheet do it by the manufacturers, due that the formulation of the adhesive changes its properties.

Polyurethane adhesives 2 components can be elastic or rigid adhesive, depending on the structure of thermoset or elastomer which acquires the adhesive once it has solidified or cured.

glue point Basic Components of 2C Pur and polyreaction

Resin – Polyols with low molecular weight.

Hardener - low molecular weight prepolymers with isocyanate finishes.

The polyreaction conducted is by polyaddition, resulting in a lattice structure of thermoset polymer or elastomer polymer, producing rigid polyurethane adhesives for the first case and elastic polyurethane adhesives for second case.

Considerations of 2C Pur adhesives

Keep in mind the pot-life, due that it is a 2-component adhesive, after mixing the 2 components in the right proportions, it begins the curing process of the adhesive, an increase in temperature in the environment will also reduce the pot-life of the mixture.

Typical Properties of 2 components polyurethane adhesives

Mechanical characteristics

Elastic polyurethane adhesive:

Rigid polyurethane adhesive:

Chemical properties

Applications and examples of polyurethane adhesives

Due 2 components polyurethane adhesives can be rigid or elastic adhesives, its use widespread applications in a wide range of products and sectors, because they cover the requirements of both rigid and elastic joints.

One of the most common applications of elastic polyurethane adhesive is the bonding of glass, such as the adhesive of the front and rear windows of cars, buses and airplanes.

The polyurethane adhesive is also used to form sandwich structures, construction of doors, container manufacturing and joining plastic components.

After meeting polyurethane adhesives, do you use to bond a broken wine glass?


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