The task of wood floor adhesives is to firmly bond the organic wood floor elements to the generally inorganic screed. The adhesive must hold the wood in place, connect it to the underlying screed and transfer stress originating from the natural expansion and contraction of the wood to the generally rigid screed. The undesired but natural movement of the wood can become visible in cupping, joint formation, or in extreme situations failure of the adhesive or screed.
Classes of Adhesives for Wood Floor Bonding
Wood floor adhesives are characterized by their performance under shear stress which closest corresponds to the situation when they are installed and connecting the wood to the screed. Lap shear values as described in European Standard EN 14293 are generally referred to classify this for the respective adhesive in Product Data Sheets. There are 3 classes of adhesives for wood floor bonding:
Hard Adhesives
(defined as having lap shear ≥ 3 MPa)
Generally these can only partially accommodate movement of the wood, therefore the stress of this is transferred directly to the screed. If the screed is not sufficiently strong, it will fail and break within the screed. If the adhesion is not strong enough there will be adhesive failure at the interface.
Hard-Elastic Adhesives
(defined as having lap shear 2 – < 3 MPa)
These are a compromise with some of the advantages of both hard and elastic adhesives. Strong enough to limit movement of the wood and elastic enough to absorb and accommodate stress evenly to the screed. This type of hard-elastic adhesives are recommended for bonding large massive wood plank flooring.
Elastic Adhesives
(defined as having lap shear 1 – < 2 MPa)
These elastic adhesives are now very commonly used to bond engineered wood floors, as well as smaller solid natural woods with limited movement. Elastic adhesives are the best solution to absorb and accommodate stress from the wood to the screed, whilst only partially restricting the natural movement of the wood.
From the chemistry side there are now several different adhesive polymer technology classes:
| Technology | Adhesive classification [Lap Shear] | Advantages | Drawbacks |
|---|---|---|---|
| Solvent based | Hard adhesive | Rapid Economic | High VOC emissions in application and service Limited application range |
| Water based | Hard adhesive | Low EHS impact Low cost | Moisture sensitive (not suitable in higher RH%) Very limited applications |
| 2 component PU | Hard-elastic Hard adhesive | Very wide application range Good cost performance ratio | Two component Rapid curing EHS issues during installation |
| 2 component Epoxy-PU | Hard-elastic Hard adhesive | Long open time Good cost performance ratio | Two component EHS issues during installation |
| 1 component PU | Elastic | High elongation at break Stable and robust | Limited application range Difficult to remove any residual adhesive from wood |
| 1 component SMP (Silane Modified Polymer) | Elastic Hard-elastic Hard | The most recent technology platform now widely available With a wide range of adhesive products for a wide range of applications. | Big quality differences – you get what you pay for |
The lap shear value is determined by bonding two overlapping wooden pieces that are then tensile strength tested in a standard test rig after curing.
The application range of the adhesives has to be given and confirmed by the manufacturer i.e. the types of substrates, screeds, woods, plank dimensions and environmental conditions, together with the limitations and necessary installation procedures.
Today in Europe one component SMP based adhesives dominate the wood floor bonding market due to their versatility, resulting in a wide assortments ranging from basic solutions for engineered wood to premium solutions for large solid wood planks. Additionally SMP based adhesives can be supplied in plastic pails and residual adhesives or stains are easy to clean-up and remove.
Non-Reactive Wood Floor Adhesives
Water based (also called dispersions), or solvent based adhesives cure to rigid thin films by losing their liquid carrier medium. Water is released into the screed and the wood, whereas solvents mostly evaporate into the air. Water and solvent based adhesives are older technologies and seldom used commercially to any extent today due to their limitations with wood and screeds, plus their EHS/VOC issues.
However these types of adhesives were in widespread use before modern reactive adhesives became better known and well proven, but they are still available, though apart from their low cost there are no real benefits in using them today.
Reactive Wood Floor Adhesives
Wood floors are generally installed using reactive adhesives today because these technologies have several important advantages:
- Ease of use, characterized by optimum open times, pourand spread-ability, trowel stability, odor, clean-ability and others.
- Thick film application, making it possible to compensate unevenness like with a levelling mortar and thereby saving time.
- Initial grab, needed to hold the wood in place until the adhesive is cured. Especially when laying larger wooden planks which are never perfectly plane this initial grab is essential.
- Wide application range with regard to overlaying different screeds and for laying different types of wood and different wood dimensions.
- Curing time after which the wood floor can be walked on and further treated if required (e.g. sanding, polishing or varnishing etc.).
- Security, that the bond and the system build-up (screed-adhesive-wood) are stable. This means that the movement of the wood is restricted, preventing cupping and gap formation; additionally stress is accommodated and absorbed in the adhesive layer, protecting the screed from stress cracking and preventing adhesive and cohesive failures in the system.
There are many different qualities of adhesives on the market that are sold for wood floor bonding. Generally in Europe a key selection criteria is the cost per weight (kg) of the adhesive, therefore it also always important to check the recommended adhesive weight per area, as the wood flooring overall is sold by the area (square metre), which means you cannot compare the cost per 10kg pail for example. It pays to check!
Reactive adhesives cure after application into a stable polymer network. The initially viscous liquid becomes a solid with elastic to hard mechanical properties, depending on the density of the polymer network. There are one and two component reactive adhesives:
One Component Adhesives
In one component adhesives the surrounding humidity drives the polymerisation of the adhesive. Therefore the setting times (skin time) and cure times depend on the humidity, the surrounding temperature and the adhesive thickness, as these adhesives cure from the outside to inside. Therefore at lower temperatures and correspondingly dryer air, the reaction is slower, whereas in hot and humid climates the reaction is faster and both skinning and cure times are therefore reduced/faster.
Two Component Adhesives
With two component adhesives the reaction starts when the two components are mixed together. The cure time is defined according to the type and nature of the hardener and the surrounding temperature. Once both components are thoroughly mixed together in the appropriate amounts, these systems are very robust and reliable.
Wood Floor Installation Methods - Sika® AcouBond-System
There are several ways to install a wood floor. The most common methods are traditional mechanical fixing, full surface bonding, cordon (strip) bonding, and the so-called floating installations. The Sika® AcouBond-System is another method combining advantages of floating installation with full surface bonding for more unique advantages detailed below.
Generally speaking today, the best way to install a wood floor is always going to be using full surface bonding of the wooden planks / elements to the screed or other substrate. The main advantages of this system are:
- Comfort and safety: We like to stand on solid and firm ground
- Sound dampening: Walking noises are not reflected into the room, but are absorbed by the mass of the floor
- Thermal conductivity: With subfloor heating wood floors must be bonded to allow efficient heat transfer.
- Durable: Bonding unites the wood, adhesive and screed as a composite unit, making it strong.
- Hygienic: Wood floors that are not bonded have many flexible joints and hollow spaces, ideal for dust accumulation and microbial growth
- Sustainable: Only firmly fully bonded wood floors can be evenly ground and varnished
- Universal and versatile: From a plain linear plank installation, to a parquet herringbone pattern and other wood floor art works are possible
Due to its apparent ease and simplicity, plus the extensive availability of engineered wood in retail there are a lot of do-it-yourself installations, but all of these have serious disadvantages and are probably more decorative than durable. This approach and these products are therefore not recommended for professionals, quality residential, public or commercial applications.
Sika® AcouBond-System is an interesting, cost effective and far higher performance alternative to floating installations, which uses some of this simplicity, combined with the advantages of full surface bonded solutions.
Sika® Acoubond System consists of a sound dampening and insulation mat with elongated perforations. This mat is spread on the floor and triangular beads of elastic wood floor adhesive are applied in the elongated perforations directly onto the prepared screed. Finally the engineered wood is placed in position over the mat and adhesive.
The advantages of the Sika® AcouBond-System are:
Easy to install:
- Works on uneven, rough and weak screeds.
- Limited skill needed, adhesive application with the gun is easy.
- Failsafe, the perforated mat is a simple guide to ensure that sufficient adhesive is applied and that the space distance between the wood and the floor is maintained.
Comfort and safety:
- Solid and firm ground that ideally is resilient to live and work on.
Sound dampening:
- For significantly improved acoustics.
Author
Dr. Dominik Huber
Corporate Product Engineer
Target Market Sealing & Bonding
Sika Services AG