APPLICATION AND PASTING GUIDELINES OF LAMINATES
CHARACTERISTICS OF LAMINATES
Like Wood, a High Pressure Decorative Laminate (HPDL) has grain direction and its dimensional behaviour is similar to that of wood. When the humidity changes, the width of the laminate undergoes greater dimensional change than the length by a ratio of approximately 2:1. In brief, as the humidity decreases, the laminate sheet contracts and when the humidity increases, the laminate sheet expands.
ADHESIVE APPLICATION GUIDELINES FOR LAMINATES
A Variety of adhesive have been found satisfactory for bonding decorative to core materials. The choice of adhesive is based upon the service for which the assembly is intended and upon the bonding facilities available. In all cases, the adhesive manufacturer's including M/s. Pidilite (Fevicol) instructions for application should adhered to:
|Fevicol SH* or Equivalent *Synthetic resin adhesive water based white glue.||Fevicol speedx* or Equivalent *Synthetic resin adhesive||Fevicol SR 998* or Equivalent *Synthetic rubber based adhesive contains solvent.||Fevicol SRXPRESS* or Equivalent *Synthetic rubber based adhesive contains solvent|
|1) Surface||Should be properly leveled||Should be properly leveled||Should be properly leveled||Should be properly leveled|
|2) Coating||Should be on both the surfaces||Should be on both the surfaces||Should be on both the surfaces||Should be on both the surfaces|
|3) a) Humidity||Should be used in normal humidity||Normal humidity||Can be used in higher humidity||Can be used in higher humidity|
|3) b) Humidity more than 80%||Should not be used||Should not be used||Can be used||Can be used|
|4) Pressure used||Under Pressure||Under Pressure||Under Pressure||Under Pressure|
|6 – 8 Hours||2 Hours||30 Minutes||20 Minutes|
In case of highly porous surface like particle board it is advised that a thin coat of Fevicol SH is applied on the surface with pressure and allowed to dry. After drying light sanding is to be done to level the surface. Then apply second coat for bonding the material. In case of Marine/Treated Plywood care should be taken to remove any excess deposition of treatment chemical on the surface. This can be done by light sanding and wiping with a damp cloth.
The time to achieve handling strength and complete curing varies with moisture content of wood, relative humidity in atmosphere and ambient temperature. In case of very high humidity, i.e. more than 80%, it is advisable not to do any bonding job. However, if you have to BOND, please use SRX PRESS.
CAUSES AND RECOMMENDATION FOR AVOIDING FAILURES ARE:
- A) The surfaces to be bonded should be properly leveled, free from oil/grease/dust and excess moisture. For best results roughen the surfaces to be bonded.
- B) INSUFFICEIENT AMOUNT OF ADHESIVE ON EITHER OR BOTH OF THE SURFACE TO BE BONDED:
When ready for bonding, the spread film of most contact adhesive will exhibit a uniform semi-gloss appearance over the entire surface of the material to be bonded. Marked variation in appearance will generally indicate an improper or non-uniform adhesive spread. The substrate can generally be seen more readily through those areas where insufficient adhesive has been applied. If this occurs, recoating the surfaces should achieve a uniform coating. Double coating the edges with adhesive is advisable because of the higher porosity of the substrate edge.
- C) INSUFFICEINT BONDING PRESSURE
To insure intimate contact necessary for an adequate bond, sufficient pressure should be applied over the entire area using as much pressure as possible without damaging the assembly .Pinch rollers (rotary press) and heavy weighted rollers are ideal. Hand pressure rolling is adequate provided the operator exerts maximum pressure by means of a two handled or single long handled roller. The rollers should be of steel or hard solid rubber (50 – 80 durometer) and not over 75 mm (3 inches) wide. Hand rolling should be done from the center to the edges to ensure the removal of all air bubbles. The edges should be rolled twice.
- D) BONDING WHEN ADHESIVE SURFACE ARE OVER-DRY OR UNDER-DRY
Care should be taken to follow the manufacturer's recommendations concerning the allowable tack range of adhesive. Of assembly's made before the adhesive is dry or after the allowable open tie is exceeded, an unsatisfactory bond may result.
- E) BONDING OF SURFACE UNDER 21 deg. C (70 deg F)
Unless otherwise indicated by the manufacturer, the temperature of the gluing area and all materials should be 21 deg. C (70 deg. F) or above.
- F) BONDING OF SURFACE WHERE HUMIDITY IS TOO HIGH
Experience has shown that when the relative humidity is above 80 percent at temperatures of 21 deg. C (70 deg. F) or lower, moisture may condense on the surface during drying (known as "blushing") and will prevent an acceptable bond. Hot spray or forced air drying may be used to help prevent this condition.
- G) IMPROPERLY PREPARED OR DIRTY GLUING SURFACES:
The surfaces should be clear, dry and free of oils or other contaminants, such as dust, loose paint particles, and so forth. The adhesive film should have full contact with surface to which it is applied in order to give maximum adhesion.
- H) ADHESIVE NOT AGITATED OR STIRRED THOROUGHLY
The adhesive should be stirred or agitated before use. This is particularly necessary with large containers.
- I) FIELD BONDING OF OVERSIZED SHEETS
It is recommended that the maximum sheet size used for vertical field application be limited to 610 X 2400 mm (2 X 8 feet). If larger panels are required, these should be fabricated in the shop.
- J) STRESS CRACKS
When cut-outs are to be made for switchboards, plug points, eyeholes, etc. at 90 degree, the normal practice is to make the cut-out by just cutting the sheet. But this is not the right method. If cut-outs at 90 degree are to be made, a 3 mm diameter hole should be drilled at the four corners of the cut-out area before cutting the sheet. If this method is not followed, stress cracks will develop from each corner of the cut-outs. These cracks will gradually spread in various directions.
Similar care should be taken while converting the sheets in various sizes where there is a cut angle of 90 degree. Ensure that the cut edges do not have any corner cracks. Please also ensure that there are no hair line cracks at the corners of the balance portion before its use. Any hairline cracks at the corners will lead to surface cracks later on.
Cracks in the center of the laminate can be caused due to bending of the substrate when it spans a wide distance or by spot gluing. Wide spans require sturdy framework and all glue line should be uniform.
If any foreign objects are trapped in the glue line between the laminate and the core, cracks could develop when pressure is applied.
During extreme climate, there is a slight expansion of the core material. If the ratio of the expansion of the core material is not identical to the expansion of laminate, again the stress cracks will appear on the laminate. To avoid this, good quality of the plywood should be used.