What gives PVC its weather resistant qualities?

January 27, 2010 by VEKA Inc

Due to its chemical composition, PVC alone is insufficient to produce a quality profile. It is necessary for PVC to be protected against deterioration during high temperature thermoplastic processing (e.g. extrusion and frame welding). In the U.S., tin-derivatives are typically added to enhance the performance of the PVC.

In addition, lubricants are required in order to prevent PVC-melt from sticking to the hot extruder surfaces and to ensure a good heat transfer within the melt. During this time the lubricant plays an important role in the necessary disintegration of the PVC-particle into its sub-segments, which ultimately is important for maintaining the physical properties of the final profile.

TABLE I - Lubricant Examples

Chemical Base

Function

Paraffin wax

external

PE wax

external

Stearic acid

external

Fatty alcohol

internal

Phtalic acid esters

internal

PVC itself has remarkable impact resistant properties. However, for window and door and fencing/railing profiles, this is not enough, especially in severe weather conditions. Acrylic-based impact modifiers are usually added in the range of 6% – 10% to achieve the superior performance that is required.

Acrylate based flow modifiers and processing aids are also used to adjust the melt flow in the extruder to the required needs. Additional fillers are added for the perfecting of surface gloss, mechanical profile properties, color, weathering properties and so on.

TABLE II - Filler Examples

Type

Effect

Calcium Carbonate

gloss, stiffness

Aluminum hydroxide

fire rating

Dolomite, Talcum

gloss, stiffness

Carbon black

weathering

Pigments and colorants are used to achieve a variety of colored profiles. The predominant pigment in PVC window and door, fence/railing profiles is Titanium Dioxide. It is also used in a wide variety of paints for other applications. Titanium Dioxide provides both high reflective properties in the visible light and UV-absorption. Titanium Dioxide will cause the profile to be more resistant to weather and UV light. Depending on the desired final properties, further additives may be used.

TABLE III - Additional Additives

Type

Effect

Anti-oxidants

stabilize PVC against degradation

UV-stabilizers

additional protection against sunlight

Optical brighteners

compensates absorption of blue light by the polymer

Flame retardants

rarely used, as PVC is flame retardant itself

As you can see, several additional components are added into the PVC to produce high performance profiles. Rather than adding every single component separately during the extrusion process, it has become common practice in the industry to produce a compound (typically called dryblend). Dryblend is a result of mixing the PVC and all the other components together prior to entering the extrusion process.

Over the years, it was discovered that only by hot mixing are the dry components ensured a good homogenous blend. Such a blend remains stable and does not separate when it is transported to various locations using trucks or railcars. The hot mixing process typically is conducted in vertical mixers using specially designed mixing blades.

This process utilizes the friction created by the rotation of the mix. The friction increases the temperature of the mix to a point where lower melting components and waxes become liquid. This allows the other additives to adhere to the PVC-particles and partially enter it. Subsequently, the hot mix is conveyed into another mixer that is rotating at lower speed. This process slowly cools the mix in order to avoid clumping.
After cooling, the produced dry blend is now homogenous, stable and has an ideal particle size and shape. Now it can be easily transported to extrusion lines, e.g. via piping, or conveyed into a storage silo for future use.




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