The PVC used to make vinyl windows is actually a cake mix of ingredients. What follows is a list of the basic ingredients used in the manufacturing of vinyl windows and doors in the United States. It is made up of the following:
- PVC resin
- External Lubricants
- Internal Lubricants
- Heat Stabilizers
- Processing aids
- Impact modifiers
- Titanium Dioxide
This is the raw PolyVinyl Chloride molecule. This is a powder when it is made and is the basic ingredient to the PVC compound that is used to manufacture today’s vinyl windows.
The PVC in your window is not just PolyVinyl Chloride. It has two different types of lubricants in it the first of which is an external lubricant (e.g., Paraffin wax, PE wax or Stearic acid.) The external lubricant is added only so the PVC can be processed during the extrusion cycle of the manufacturing process. Without this lubricant the PVC would stick to the metal in the barrel and screws of the extruder and burn up. The external lubricants also allow the PVC extrudate to “slip” through the “calibrators” which size the part. External lubricants do not like the PVC molecule and want to get away from those molecules, (kin’a like oil and water don’t mix). Due to this dislike, when these lubricants melt in the extrusion phase, they go to the surface and form a thin boundary layer between the PVC and the metal parts of the extruder and calibrator. Each time that PVC compound is run through an extruder this external lubricant diminishes. Typically, companies that make PVC compound, put enough external lubricant into the compound so that the material can be run through the extrusion process at least five times before it is depleted. The amount of external lubricant placed in PVC compound is critical to the recoverability of PVC.
FYI, if you want to paint your windows and they are brand new from the factory, the external lubricants are still on the PVC. I’d say within about 6 months the lubricants on the surface of the PVC are effectively removed by the UV of the Sun. This lubricants reduce the surface energy PVC which make paintablility of PVC an issue. I will write an article about the technical aspects of painting PVC in the future. And believe me, there are a lot of things to write about. For now, if you want to paint your windows, I’d recommend scuffing up the window with ScotchBrite (the green version has about the right effective grit) and then wiping it down with Acetone. Use only one wipe (not a back and forth action–with back and forth action all your doing is smearing the external lubricant back and forth across the surface). I would use this technique even if the windows have weathered in the field for six months. And make sure you use reflective pigments in the paint, otherwise the windows will warp due to excessive heat build up.
Internal lubricants are the opposite of external lubricants: they like the PVC molecule (they are made up of Fatty alcohol or ester waxes). When these lubricants melt they coat the PVC molecule. Without these lubricants the PVC would stick to itself and burn up in the extruder. Both external and internal lubricants are only put in the compound for processability. They serve no other purpose for the ultimate vinyl window that is to be produced.
I’d like to continue the cake mix analogy and extend it to making bread dough. You know how dough will get really sticky and stick to your hands and stick to the counter top, and is generally a real mess. Well…PVC “dough” is a lot like that, without the lubricant packages it would be unmanageable, because unlike bread dough which is at room temperatures, PVC dough is at 360o Fahrenheit–you just can’t work it with your hands until it gets past the sticky stage, so chemists and engineers have experimented with lubricants until the PVC mix is juuust… right.
Even with all the lubricants added to PVC, PVC would burn and char through the intense temperatures induced in the twin screw extruders if it were not for heat stabilizers. The use of heat stabilizers ensures that the performance properties of this mixture will not be affected by the severe thermal changes that occur during processing. In the United States it is common practice to use the chemical Tin (Sn) as the basis to create the heat stabilization required in the extrusion process. Tin is included in the PVC formulation primarily to aid the material in the extrusion phase; however, Tin serves as a benefit to the finished product by increasing the weatherability of the extrusion.
These ingredients affect the the consistency of the compound as it changes into a plastic. Proper amounts of these polymers are necessary to insure the plastic melts and cools at the same consistent rate. Without the processing aids, an extrusion would be inconsistent in quality and non-predictable in fabrication. Acrylic is a polymer that is used as a processing aid in PVC compound. In addition to making the hot plastic melt become more stable at temperatures above 350 Fahrenheit, these same processing aids also add to the weatherability of the product in the field.
Without impact modifiers, the final window product would be very brittle. Great care would be required in handling the window during fabrication, shipping, and installation to prevent serious breaks or cracks from occurring in the vinyl. Impact modifiers create ductility to the final product. Windows without sufficient impact modifiers could be damaged by blows from installation tools, would show cracks or breaks from environmental hazards as hail or sleet, and may even show damage caused by the contact of tilt latches when the operable sashes are returned into the frame. The impact modifiers are extremely important when it comes to new construction windows. The nailing fin, a single wall of vinyl that protrudes from the window frame with only one end of support, may be exposed to various forces during the fabrication process as well as the shipping process. Also, framers and carpenters installing these windows in all types of weather, will find that errant blows from hammers or nailing guns may damage the nailing fins in windows that use a lower quality vinyl. Acrylic is used as a processing aid and it is also used as an impact modifier for the reasons described. Acrylic is “soluble” in PVC. Which means that the molecules intersperse into a nearly homogenius amalgamation of PVC and Acrylic. The Acrylic increases the impact resistance and will reduce cracking issues that were once a problem in formulations 30 and 40 years ago. Today’s PVC formulations help insure that the product will look new after more than 20 or 30 years of exposure to the sun.
Titanium Dioxide (TiO2) is a common chemical used as a pigment in many industries. It is the basic pigment to create the color white in PVC as well as in paints. It is mined from the earth as a mineral or rock and is also a very good resistor to the sun’s UV light which is the main cause for pigments to fade. Because the TiO2 is mixed into the compound, the color is retained throughout the profile. Since TiO2 is a white pigment, it reflects the sun’s rays away from the window unit (it has a relatively low emissivity, i.e., high reflectivity) preventing unwanted heat build up on the exterior surfaces and inside the internal chambers. TiO2 is typically is roughly 10% of the finished compound by weight. It is a very expensive ingredient but is essential in order for the the PVC window to perform for a long period of time.
That’s a short primer on the subject. Beyond that you going to have to work on that polymer science degree!