Volume 9, Issue 8 - September 2008

 

Customers WantColor
Delivering This Means Paying Close Attention to the Extrusion Process
by Paul Adams

Designing vinyl products such as a window that resists color fade and exhibits the same magnitude of physical integrity compared to the original birth or installation of the window is paramount to all in the vinyl industry. From the development of a vinyl formula additive to the final steps of window fabrication and installation, each phase of the process shares the burden of color fastness. This topic is critical to the vinyl industry. So what are the key essentials for the development of a vinyl extrudate that will resist changing color?

An Exacting Science
First, the processes of extrusion and compounding can never be overlooked or considered secondary. Formulations are designed to address the limitations and strengths of the extrusion process, which relates to extruder type, screw and tool designs. Optimization of the extrusion process relates to obtaining the necessary conditions that will provide supreme melt homogeneity indicated via a high degree of fusion (near 90 percent or better).

The degree of fusion can be measured indirectly through many common techniques involving solvent exposure of the profile, differential scanning calorimetry and impact measurements. In the literature there are many other analytical techniques described for the purpose of measuring the relative degree of fusion but the point is this: formulations can be advanced continually but without the process of extrusion optimization then superior performance will be compromised. Like all performance attributes, color hold properties are influenced strongly by achieving excellent fusion. In addition, the compounding step prepares for a successful extrusion process. Complete material mixing, accompanied by the adsorption of critical performance additives into the PVC polymer, is imperative for subsequent product color properties.

Color is achieved by careful manipulation of additives such as titanium dioxide and organic/inorganic pigments; in addition, numerous other processes and additives play critical roles in color. There are many essential objectives that must be met through various formula technologies.

Pigment Chemistry
It is paramount that the PVC producer develops strong relations with competent and discreet suppliers. A good example of this type of partnership exists between our company and Holland Colours America (HCA). “Deceuninck and HCA work together to ensure that they are offering the most weatherfast colored vinyl window profiles available,” says Rob Leversege, technical manager for HCA. “By evaluating colorant formulations in Deceuninck’s proprietary compounds, we work to ensure that the total compound formulation is optimized for color retention. Accelerated and real-time weathering tests are conducted to characterize total compound/color performance. Additives are also evaluated for their impact on color retention. Testing is conducted in Arizona, Florida and Indiana in order to evaluate weatherfastness in a variety of climates. Individual pigments are selected based on their durability and ability to reflect heat. When possible, special complex inorganic pigments, which have both excellent weatherfastness and IR (heat) reflectance are used. These formulations are tightly controlled to ensure that every profile is the same color. Deceuninck blends color and additives into its compounds via a state-of-the-art blending system  which delivers consistent color and performance.”

Heat Stabilizer
Materials such as PVC will often change color due to heat and outdoor weather instability. Stabilizer chemistries are designed to function in the polymer system through many different mechanisms—color depends on a stable system. Primarily, in North America, the common heat stabilizer for rigid PVC is an organotin mercaptide though there are other additives (such as secondary stabilizers) that can greatly enhance the color stability of the profile. The mercaptide group bound to the alkyltin is designed to react with the PVC backbone; however, the same reactive molecule can also be involved in side reactions that are not wanted or foreseen. Sometimes these reactions can be catalyzed by trace impurities carried into the system from an additive such as an inorganic pigment. Extreme care and energy should be placed on identifying formula additive purities in order for the PVC extruder to prevent any surprises that can disastrously alter color. Though color formation often indicates polymer degradation, it can also be the symptom of other chemistries occurring on the extruded surface.

Titanium Dioxide
Many products fall into this category and the role of this ingredient is critical for achieving superior PVC outdoor weatherability. As already emphasized in this article, there are many factors contributing to improved color stability. The idea of dosing greater amounts of titanium dioxide into the formula until a stable white color has been attained is not a good choice. Often this approach is an example of insufficient research and data; furthermore, this practice increases formula costs driving prices up in a market where profit margins are sharply decreasing. If the manufacturer is using at least eight to nine phr titanium dioxide in a rigid outdoor PVC formula, then it should be possible to attain sufficient color fastness.

There are other factors, such as the issue of chalking compared to non-chalking products, that should be considered. The choice of titanium dioxide grade depends on many objectives such as product color space, product exposure to harsh environment conditions, and profile surface appearance. Ample experimental data designed to answer these questions are part of an evolving partnership—the importance of which was highlighted previously. New technologies keep the PVC manufacturer constantly improving performance and showing the market products with superior color hold properties. Identifying suppliers that are innovative and discreet during product development are keys to success.

One final subject that is critical before and during product development is the identification of useful measurement tools. Discover where the product will be used and identify what environmental conditions the profile will experience during its service. There are questions that must be answered for the purpose of structuring a meaningful set of test protocol.

Color fade or change is an issue for PVC and all outdoor building products. It is essential to optimize process with formulation while developing key partnerships in order for the PVC manufacturer to deliver products continually with greater color hold and in a variety of color choices.



DWM

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