Improved UV stabilization expands uses for polyester elastomers.
Photodegradation of polyester elastomers based on polybutylene terephthalate (PBT) is different from that of PBT alone. PBT photodegradation immediately produces a gray color, while elastomers degradation produces yellow. Generally, polyester elastomers (COPEs) initially undergo a rapid color change followed by a steady color change at a reduced rate. A second characteristic of elastomer degradation is surface erosion. Surface cracks (crazing) develop, and eventually, pits form on the exposed surface. This erosion can be so severe that portions of the surface may actually flake off.
In this article, a new two-component UV-stabilizer system for polyester elastomers is shown to provide improved UV resistance over conventional formulations. This article also discusses uses for the improved elastomer compounds in interior and exterior applications.
Test plaques were exposed to UV light and heat in a water-cooled, controlled-irradiance-type xenon-arc weatherometer, according to automotive test procedures SAEJ 1885. Primary test conditions were:
Light Dark Condition cycle cycle Irradiance, W/[m sup.2] 0.55 - Black panel 89 38
temperature, [degree] C
Relative humidity, % 50 100 Cycle time, hrs 3.8 1.0
The amount of exposure is the total irradiation or received by the test plaques, expressed in kilojoules per square meter [kJ/m sup.2).
The degree of color change - determined by measuring the color of an exposed sample versus that of an unexposed sample - is quantified as the total color difference (Delta E) calculated for illuminant "D-65," 10-degree observer, expressed in CIELab units, in accordance with ASTM D-2244. Samples were measured at various of accumulated irradiation.
An HPUV device - an instrument used to predict the UV stability of parts exposed to the fluorescent lighting of an office environment - was used for some interior applications. The parameters of this test, according to ASTM D-4674, were:
Light source: 11 Coolwhite s FS-40 UVB Temperature : 42 [degree] C Equivalence : 100 HP hrs simulates a one year office
The UV resistance afforded by the new two-component UV-stabilizer system is shown in Table 1, where color changes are compared for unstabilized COPE A, a conventional polyester elastomers, and two COPE B formulations. COPE B, a preferred elastomer for pigmented applications, was stabilized first with one component of the new system and then both components. The value 315 kJ/ [m sup.2] very approximately simulates one year's exposure in Florida under glass. COPE A demonstrated the typical elastomer UV degradation pattern - an initial burst of degradation leading to a large color change at 25 kJ/ [m sup.2], followed by a steady rate of degradation resulting in a smaller but still substantial color change from 25 to 315 kJ/ [m sup.2].
COPE B stabilized with one component showed a significantly reduced initial color change, but a sizable long-term color change still remained. The two-component-stabilized COPE B formula showed significant color change improvements in both phases - an 86% reduction at 25 kJ/ [m sup.2] and a 75% reduction at 315 kJ/ [m sup.2] In addition, this system exhibited none of the surface attack that the conventional elastomer experienced in the form of microcracks, which appeared as a cobweb pattern. Because surface erosion also causes whitening, Delta E values at 315 kJ/ [m sup.2] may be artificially lowered.
Color changes for COPE B stablized with the two-component system are compared in Table 2 with those for two commercial products: COPE C, a conventional elastomer stabilized with a three-component system; and COPE D, an elastomer recommended for UV applications. The COPE B formulation clearly shows the best UV resistance for both phases of degradation.
TABLE : TABLE 1. Weatherometer Color Changes of UV-Stabilized COPEs.
Elastomer/stabilizer Color COPE A/ COPE B/on COPE B/two- change none component component
Delta E at:
25 kJ/ [m sup.2] 21.7 13.1 3.1 315 kJ/ [m sup.2] 33.4 22.2 8.6
Table : TABLE 2. Color Changes With the New Two-Component System and Commercial UV Systems.
Elastomer/stabilizer Color COPE B/two COPE C/three COPE D [sup a]/ Change component component none
Delta E at:
25 kJ/ [m sup.2] 3.1 3.5 14.8 315 kJ/ [m sup.2] 8.6 13.4 [NA.sup.b] Surface Smooth Smooth Severe erosion
(a)Recommended by the manufacturer for UV-sensitive applications. (b)Not available - severe erosion and chalking precluded an accurate measurement
Office furniture. Blends of PBT and COPE B in ivory and charcoal gray - colors representing the extremes found in office environments - were evaluated by the Atlas-HPUV exposure test. Color changes after 300 HP hrs, a three-year simulation, were:
Stabilizer system Ivory gray One-component 3.4 - New Two-component 1.7 0.2 Three-component 2.6 -
The three-component system is recommended by other COPE and additive suppliers. The superiority of the new two-component system is again demonstrated. While this level of UV protection is acceptable for this application, the two-component system can be further optimized to provide additional protection in more demanding applications.
Flexible switches. A flexible switch is actuated when an operator presses down a flexible membrane located over a switching device. The UV performance of several flexible materials and an ABS handset used as a control are shown in Table 3. The parts were tested in the HPUV device for 500 HP hrs - a five-year simulation of an office environment. Because the parts were too small to allow instrument readings, the AATCC Gray scale was used; this is a visual scale technique in which 5 = no change in color and 1 = sever color change. Color standards are provided for comparison and rating of tested specimens.
Good performance was shown by the copolyester elastomers with the two-component UV-stabilizer system. As expected, the light-colored samples did not test as well as the black COPE sample - the ivory COPE and TPO vulcanizate samples showed slight changes while the ABS handset exhibited a major change. A rating of 4 is the minimum accepted by flexible switch vendors.
Other interior applications being evaluated in a variety of colors include athletic shoes, eyeglass frames, and sporting goods.
Table : TABLE 3. UV Resistance of Flexible Switch Materials, 5-Yr Simulation
AATTCC Appearance Material Gray scale
COPE, 2-Component stabilized,
black 5 No change ivory 4 Slight yellowing
ivory 4 Slight yellowing
ivory 3-4 Moderately dark
The following cost distribution (without tooling) for manufacturing a typical fascia shows that painting accounts for the major share: raw materials, 35%; primer coat application, 10%; basecoat (color coat) application, 30%; and clearcoat application, 25%. Considerable cost savings can be achieved by simply clearcoating a pigmented, UV-stable, molded fascia. Not only would the primer and basecoat application costs be eliminated, but also the scrap and rework costs associated with these steps. The clearcoat cannot be eliminated because it is essential for meeting the high gloss or distinctness of image (DOI) requirements for high-end automotive models. The tools typically used to mold fascias (and fenders) do not produce shiny parts, resulting in out-of-the-mold mismatches in color and gloss with the rest of the vehicle. The clearcoat minimizes these mismatches in DOI and richness of color. Incorporating UV stabilizers in the clearcoat also improves total UV resistance.
A fascia can cost from $50 to $120, depending on model and styling options. Thus, a fascia requiring only a clearcoat would save manufacturers $20 to $50000 per part - a large savings for even a relatively small model year of 100,000 cars.
Hoechst Celanese has eliminated the primer through a line of proprietary polyester elastomer alloys that can be painted without priming. Prototype fascias and other parts have been molded from red and blue proprietary alloys, which were UV stabilized with the two-component system and then clearcoated. These parts have been assessed by a major automotive company and found to meet vehicle appearance requirements.
A short evaluation of the UV stability of the stabilized, clearcoated red substrate was conducted at high humidity in a xenon-arc weatherometer. This exposure cycle has been found suitable for formulation screening. Delta E values at several total irradiation increments are shown below.
Total Delta E, Delta E, irradiation, no with kJ/ [m sup.2] clearcoat clearcoat 193 2.1 0.7 331 5.2 1.6 598 7.4 3.5 800 8.9 4.6
The value 598 kJ/ [m sup.2] is roughly equivalent to two year's exposure in Florida under glass. The clearcoat is seen to significantly retard the onset of color change and reduce the loss of color. Improvement of Delta E values is still needed, but this first test of the pigmented fascia/clearcoat concept was encouraging.
An improved two-component UV-stabilization system was evaluated next. A natural formulation was used to eliminate any effect of the pigment, and all samples were clearcoated. Delta E values at 800 kJ/ [m sup.2] for samples with the initial UV stabilizer, with the improved UV stabilizer, and with no stabilizer wer 4.5, 1.9, and 6.6, respectively. These results, which show that the improved UV stabilizer reduced color change by 58%, establish that UV stabilization of both the substrate and the clearcoat is necessary to minimize color change.
Parts have been molded by the pigmented substrate/clearcoat process, and several cooperative evaluations are under way with automotive companies and suppliers.
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|Author:||Golder, Michael D.; Mulholland, Bruce|
|Date:||Jun 1, 1990|
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