PC/PMMA Bonding

• Building machines.
  
• Special-purpose vehicles (Border control, Police).
  
• Light-weight construction vehicles.
  
• Race cars.

Typical materials are PMMA (Polymethylmethacrylate) and PC (Polycarbonate). In the manufacturing process, many polycarbonate sheets are coated with a scratch resistant surface. Unlike standard float glass, plastic parts have a much higher coefficient of thermal expansion, resulting in greater movement (expansion and contraction of the window).
In addition, plastic parts do not have the ceramic coating of glass windscreens, which provides UV-protection for the adhesive bond-line. For these reasons, the following procedures must be followed when bonding plastic parts.

Pre-treatment of PC or PMMA; If there is an unknown coating, the surface should be tested before bonding (contact Sika); otherwise, use the following procedure:

Sika® Primer-207 must be used for bonding plastic parts. Black primerless bonding is not possible. 

Due to the high thermal expansion specified a low modulus adhesive/sealant should be used. The adhesive accommodates the movements and the part can expand without causing too much stress. A standard windscreen adhesive is usually too stiff and could lead to detachments in the long run. Curing should be done in a controlled environment as for example a temperature change outside in winter could already shrink the part size and increase the risk of detachments in warm conditions.

A small and high triangular bead should be applied and a minimum of 5mm thickness in compressed stage should be secured. If the bead is compressed too much it loses its elastic behaviour and does not compensate thermal expansion.
For elastic bonded parts the following urethane is recommended:

• Sikaflex®-223
  
• Sikaflex®-223 PowerCure (for accelerated curing)

Plastic parts have a very high thermal expansion factor which means every degree of temperature change the size of the plastic part changes. Therefore the installation and curing has to be carried out in-house at 15 - 25 °C.  

Any anti-scratch hard coating can be difficult to be removed. The removal of coating damages the original design and protection of the plastic. Application of primer and adhesives (solvents, plasticizers) and stresses (internal/external) could lead to ESC (Environmental Stress Cracking) as known from other unprotected plastic parts.

Using a low modulus adhesive can deviate from the car design (if designed for mineral glass and all in one modulus) and needs to be communicated and carried out in accordance to OE instructions. It must be considered that a car body structure is calculated based on its standard design. If mineral glasses are replaced by organic parts the body stiffness cannot be guaranteed any longer. (E.g. Police car or similar applications) For example drive comfort or crash behaviour (roll over) can be different and are in Installers responsibility.

Sikaflex®-223 is recommended for the durable bonding of plastic windscreens. This adhesive has a low modulus (soft) and can handle the thermal expansion of windscreen without resulting in mechanical overload. Since PMMA is particularly sensitive to stress cracks, windscreens should remain flat during the bonding process. If there are larger-than-specified dimensions of PC or PMMA to be bonded, please contact Sika before beginning work.

Temperature changes during curing can cause stresses and increase the risk of detachments.
Engine covers (Porsche, Ferrari, etc.) are highly sensitive applications as high temperatures are causing big changes of dimensions.

Depending on the type of organic glass, UV protection is provided, but not given in all cases. To ensure a durable bond, transparent windscreens must be surface-protected against UV. Sika® UV Shielding Tape or trims can be used in cases where no protection is given. Sika Primer (and most glass primers from other manufacturers) are organic materials and will be destroyed by UV radiation within a very short time and therefor provide no durable protection.
Any anti-scratch hard coating can be difficult to be removed. The removal of coating damages the original design and protection of the plastic. Application of primer and adhesives (solvents, plasticizers) and stresses (internal/external) could lead to ESC (Environmental Stress Cracking) as known from other unprotected plastic parts.

Possible solutions are:

• Suitable print color
  
• Protective metal or opaque plastic trim
  
• Sika® UV Shielding Tape*
  
  

Without this protection, Sika® Primer-207 will eventually detach from the plastic windscreen. The exact period until failure depends largely on the weather, the synthetic material quality, and the general load.
* Availability depends on the local Sika Organisation.