Why Use a Polyurethane Sealant over Silicone Sealant? Full Guide

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Polyurethane Sealant VS Silicone Sealant - Difference Between Silicone and Polyurethene Sealant

A sealant is a versatile material, whether it's silicone sealant or polyurethane sealant.

Either way it forms an integral part of most people's toolkit - DIY or Professional.

If you're interested in doing jobs around the house or you're part-way through a Professional renovation project such as bridge building or even Coach Building panels.

Whether you're planning to modernise a property or revamp the bathroom, here's one thing we know.

It's highly likely that in selecting the best sealant for the job, you're given it some serious thought.

Which is no easy job.

There are various forms of sealant or mastic sealant available, so which is the best option for you.

Here's why it is so important to spend time weighing up the options.

Hopefully, this guide will provide the regarding the difference between silicone and polyurethane sealant.


A brief introduction to mastic sealants

A sealant is a form of material, which is used to seal gaps and holes to make something airtight or watertight.

When you use sealant, you effectively form a seal, which prevents the movement of fluids and liquids.

Sealant is sometimes compared to caulking.

The main difference between building caulking and sealing is the flexibility of the seal. If you include mastic sealing in this equation sealants can not only seal but can also be used as an effective adhesive.

Sealants tend to be made from much more flexible materials.

This makes them useful for areas and joints where a degree of contraction or expansion may occur.


Polyurethene Sealant Properties

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Mastic sealants such as polyurethane tends to be especially used in professional industries such as bridge building or automotive coach building. It's hard wearing approach and exceptional adhesive properties means it works well for professional applications where strength and speed are crucial.

Polyurethene sealant can also be used effectively in joinery because the sealant cures quickly and exceptionally hard into wood joints. These joints can then be sanded and painted over for a top quality finish.

They are not the same as adhesives, but it is possible to utilise sealants that have adhesive properties.

Sealants are used widely, and may come in handy for those working in the following industries:

  • construction
  • automotive
  • engineering
  • aerospace
  • marine
  • joinery
  • road building
  • bridge building

As well as those with an interest in DIY and property maintenance. There are many different forms of sealant both silicone and polyurethane sealants. This guide will focus on silicone and polyurethane sealant although these sealants have similarities, there are some significant chemical differences.

One of the principal differences between silicone and polyurethane sealants is the chemical composition. Silicone is an inorganic substance, while polyurethane is organic.

The primary difference between organic and inorganic substances is the presence of carbon. Organic compounds contain a carbon atom. In most cases, a hydrogen atom is also present. With an inorganic substance, it's uncommon for either atom to be present.

Polyurethane is a polymer, which comprises of a series of organic atoms connected by carbamate. Silicone is a unique material because it contains both inorganic and organic compounds.

These synthetic polymers comprise silicon and oxygen atoms, which are joined using C-Si bonds. The makeup of the chain exposes the organic elements of the structure to external influences.

This is why silicone makes such a versatile sealant for DIY as it can do most jobs satisfactorily - it's non organic composition makes it fairly dependable across the board and long lasting. Silicone is commonly used in construction and the development of paint and food products.

Chemical Breakdown

The chemical makeup of both silicone and polyurethane sealant are important. This is because the composition affects the breakdown of the materials. When organic and inorganic substances are exposed to UV light from the sun, the way they react differs.

Inorganic materials are less responsive to UV rays than organic chemicals. In the long-term, polyurethane, an organic material, will revert back to its natural state in response to UV exposure.This affects both the lifespan and versatility of polyurethane sealant. Polyurethane sealants set exceptionally fast and exceptionally hard as the organic compounds react faster with UV light.

This makes them especially useful for professionals working outside in industries such as road and bridge building. Polyurethane still has sealant and adhesive properties that exceed silicone and the fast acting nature helps project planning.

Paint and Sealants

silicone over polyurethane

(image courtesy of House Painting Tips with Silicone)

A key player in the discussion between silicone sealant and polyurethane sealant is paint...If you use paint (spraying, roller or brush) and need an adhesive or sealant in close proximity to your finished paint work. You will have come up against the common issue of silicone!

Silicone cannot be painted over its inorganic compounds will not allow paint to adhere or absorb into its top layer surface. The paint will simply peel off or in most cases simply run off during painting.

Polyurethane sealant is organic and after curing time it can be painted over and even sanded successfully to the same finish as other surfaces. This provides a huge advantage over silicone for both the DIY'er and professional alike.


Time Debate - Will my Mastic Gun be out sooner with Polyurethane?

silicone over polyurethane


It could be argued that silicone lasts longer (being non organic) than polyurethane (organic). Typically, a polyurethane sealant will produce effects that last between 5 and 10 years. While a silicone joint could last more than 20 years.

Most professionals however would argue that the quality and harder nature of Polyurethane sealants slightly edge out the longer lasting nature of Silicone. It's potentially why silicone is used more frequently in the home - where traffic and use is reduced. While polyurethane sealants are more prevalent in professional situations where drying time and hardening qualities are more crucial.


Which Polyurethane Sealant?

We always advise the Soudaflex 40FC range which is a high quality polyurethane sealant and adhesive perfect for a wide number of applications.

By far the biggest market for Soudaflex 40FC is the Coach Building Industry. It is perfect for bonding and sealing panels. It is also a high quality joint sealant, being used in road and bridge building, as well as industrial flooring applications.

Sold by the by the box (12) we have large stocks of all colours.

Colours White, Black and Grey Tube size is 310ml

Advantages & Benefits of Soudaflex 40FC Adhesive and Sealant:

  • Very good adhesion on many materials
  • Stays Elastic after curing
  • Over Paintable**
  • Very easy application
  • Excellent resistance to many chemicals
  • Developed for concrete applications
  • Structural bonding in vibrating constructions

** Whilst Soudaflex 40FC is over paintable, a test area is recommended. Also 40FC is a flexible sealant and adhesive where the paint generally will not be. Cracks may appear in time as the sealant moves and the paint does not.

Buy Ultrimax supplies the full polyurethane sealant and mastic gun


silicone soudal tube





Soudaflex 40FC Polyurethane Sealant

310ml Black, Grey or White

Need More Help Selecting The Correct Silicone Or Polyurethane?

If you can’t find a fix for the issue you’re experiencing or you need more advice, drop us a line and we’ll get your paint job on track.


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