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Case Study - Acrylic Bulkhead Light Product Failure

A manufacturer of bulkhead lights approached Smithers Rapra following the failure of its product in use. The product was becoming discoloured & loosing its’ transparent properties.

History

bulklight failure
(Figure 2.20 A cracked & discoloured bulkhead light cover)

A manufacturer of bulkhead lights decided to change from their traditional glass to plastic covers. The lights were predominantly being installed outdoors to illuminate passages & porches but were susceptible to impact damage. The use of plastic covers would allow the product to be promoted as ‘ vandal-proof’. However, improved aesthetic appeal derived from the design freedom offered by injection moulded plastic, was the manufacturers main incentive for change.

The technical development personnel had limited knowledge of plastics & other polymers but managed through discussions with various material suppliers to recommend a material. Polycarbonate was chosen mainly because it would best satisfy the vandal-proof claim. The marketing staff chose polymethyl methacrylate (PMMA) because of its superior aesthetics. An unequal struggle resulted in the selection of high impact PMMA (an acrylic toughened by acrylate rubber). Two years after the new product was launched complaints were received with increasing frequency.

Inspection

The consultants at Smithers Rapra examined the light covers & noted they suffered from discolouration, reduction in transparency, & embrittlement. The discolouration as shown in the photograph was not uniformly distributed. It maximised in areas of the cover that were closest to both the tungsten filament light source & rising hot air. This suggested degradation via thermo-oxidation &/or photo-oxidation.

Failure Diagnosis

The radiation from tungsten light bulbs is characteristically low in UV light & high in infrared intensity. Acrylate-modified PMMA has excellent resistance to photo-oxidation but modest resistance to thermo-oxidation. Therefore excessive heat was the main suspect.

A thermocouple was embedded into the inside upper surface of a cover & the near surface temperature was measured with the resealed unit exposed to a light source at the maximum recommended wattage rating. The surface temperature of the casing equilibriated at 55 °C above the ambient temperature. During its operation in a temperate climate the actual temperature would range from about 55°C to about 75°C. This compares with a 100,000 hour maximum continuous use temperature for an acrylic of only 50 °C.

The pattern of discolouration was replicated by continuous exposure to the light source for 3 months at an ambient temperature of 25 °C (an inner surface temperature of 80 °C). This was deemed to be reasonably equivalent to two years of normal use.

The discolouration, loss of transparency, & embrittlement were primarily due to thermo-oxidation. Secondary or subsequent additional degradation due to photo-oxidation would contribute to the chain reaction by way of UV light induced reactions with the by-products of thermo-oxidation.

Lessons & Consequences

  1. Polycarbonate should have been selected. It offers a maximum continuous use temperature in excess of 100 °C. The light source should not have been positioned near to the upper surface of the cover.
  2. All products that involve exposure in service to heat or light should be tested for durability prior to product launch.
  3. Marketing may be the modern route to success but it should not be allowed precedence over product fundamentals.