What is Vacuum Adhesive or Membrane Adhesive?
In recent generations, vacuum or membrane adhesive is a water-based adhesive to produce MDF with opaque and glossy PVC coating with flat press. It is used for the production of grooved, convex and curved MDF sheets using a vacuum machine and is marketed as one and two components. Recently, the single component type of this adhesive has been noticed in the market because it is easy to operate. The market can be greater by enabling wood industry to produce a variety of PVC laminates which are closer and closer to natural wood. At the same time, the cost of PVC laminated MDFs are much higher than that of natural wood, allowing the general public to provide wood shaped goods and furniture.
Types of coatings
Vacuum technology is not the name of a particular product or object, but is a process in which is used to compress work pieces and create vacuum. Due to the compactness of the work pieces and the created vacuum, this technology helps us to create the strongest coatings with the least possible thickness on the surfaces. That is why vacuum coating is very popular among the industrial manufacturers. In addition to beauty, these surfaces are also highly resistant to damage. Vacuum coating is also used to coat the surface of cabinets. In the following, WPU membrane vacuum press adhesive as one of the most important elements for the vacuum veneering in the furniture and interior decoration industry is described.
PU Membrane Vacuum Press Adhesive
This product is specially designed for coating wood products in vacuum presses. All its properties are optimized and selected for this operation. One of the main features of this type of adhesive is its adhesion strength. This product is ideal for ABS, PVC or PET coatings on MDF, wood panels and a variety of products. As mentioned, this product is a single component and does not contain any organic solvents which preventing any shrinkage or opaque. This defect is clearly visible in non-standard adhesives available in the market, and by mistake, sometimes it is attributed to vacuum press machines. A non-standard vacuum adhesive will include organic solvent materials that can easily be negatively impacted with reducing the quality and clarity of the coating. This product is easily sprayed on the workpiece by a 1.8 mm to 2 mm adhesive nozzle and subjected to vacuum pressure in the machine at a temperature of at least 60 ̊C for 20 to 40 seconds. The color of this adhesive is white, fluorescence and will have a medium viscosity.
What could be the reason for PVC separation from the edges of the work piece after the press operation?
In the new generation presses, the PVC is heated by the high temperature elements. The thermometers which are installed on the press machine shows the surface temperature of the MDF. While there is a temperature difference between the edges and the surface of the MDF. For example, in some cases the machine temperature indicators show the temperature of the MDF surface 90 °C, while the edges temperature is 40°C. In fact, the edges temperature is lower than the adhesive activation temperature that causes the PVC separation from the edges. It is recommended to use a laser thermometer to measure the temperature of the edges and set the elements temperature according to it.
Why does the workpiece surface become inflate after vacuum pressing?
The main cause of the inflating is the presence of water molecules on the surface of the MDF. Therefore, before spraying the adhesive it must be shaken several times to obtain a homogeneous solution.
The using pistol nozzle should be 1.2 to 1.8 mm in order to ensure uniform spraying and preventing adhesive loss. The edges should be sprayed twice and make sure that the adhesive thickness won’t be excessive.
The waiting time for drying the adhesive varies in different areas, but it should be such that no moisture felt by touching the surface.
What should be done to improve adhesion quality?
It is best to sand the MDF surfaces before gluing, and then use air pressure to clean the surface and parts of the tool from the wood pollen.
In some cases, in the preheating operation, the PVC veneer becomes flexible which causes wrinkles on the surface of the workpiece. As a matter of fact, it should be considered and it is better to blow air by reversing the pump operation under the PVC veneer to prevent covering the workpiece and enhance the quality of coating.
In vacuum machines there is no compressed air pressure from above and only vacuum is provided from under the tray. After vacuuming the PVC, the surface of the veneer must be cooled. Because the memory effect of the veneer makes it to expand as a result it will return to its original state. In professional machines cold air blows between the veneer and the upper section, and in the workshop or vacuum machines usually a fan has been embedded above the drain tray. In the summer, the condition of the veneer is stabilized by a wet wipe and fan on the work surface.
One of the most important factors is an optimal PVC laminate adhesion which is provided with a primer coating. The primer is usually white in color on the back of the PVC coating that is applied by the manufacturer and acts as the intermediate ring between the adhesive and the PVC. It gives the polarized state to the coating to create suitable and long-lasting bond between the work pieces..
- Spray the edges and work surface carefully (usually gluing the edges twice.)
- Allow the adhered surfaces to dry completely. Examine the glued areas with a UV lamp.
- The using amount of the past generation of adhesives has decreased from 130 to 90 g/m2 and in single component adhesives this amount has decreased to 60 to 80 g/m2.
Discharge conditions after press:
- Separate panels to cool.
- Carefully clean the corners of the overlay.
- Cut boards from the inside
- Clean the glue sprayed on the lower part of the panel with a damp cloth.
Adhesives require heat to activate:
- PVC > 60°C
- Polyolefin Coated (PP) > 90°C
- Press time between 60 and 90 seconds
- Press cycle time between 150 and 190 seconds