The plastic carbon arc lamp aging test is a laboratory accelerated weathering method that evaluates the durability of plastic materials by exposing them to intense carbon arc radiation, heat, and moisture. This article explains the testing principles, applicable standards such as ISO 4892-4, ASTM G152, and JIS B 7753, specimen requirements, test conditions, and result evaluation. It is a practical reference for engineers, quality managers, and material developers who need to validate the long-term performance of plastic products.
Technical Overview
The carbon arc lamp aging test is one of the earliest laboratory accelerated weathering techniques used to assess how plastic materials degrade when exposed to light, heat, and moisture. In this method, an electric arc is struck between a pair of carbon electrodes, generating an intense light source with strong ultraviolet output that simulates and greatly accelerates the photochemical degradation caused by natural sunlight. Test specimens are mounted on a rotating drum surrounding the arc lamp and are exposed to controlled cycles of light, water spray, temperature, and humidity until a defined endpoint is reached.
Compared with natural outdoor exposure, which may require months or years, carbon arc testing can produce measurable degradation such as discoloration, gloss loss, chalking, and loss of mechanical strength within hundreds to a few thousand hours. Although xenon arc and fluorescent UV devices have become more common in modern weathering laboratories, carbon arc testing remains mandatory in many legacy specifications, particularly in the Japanese automotive and electrical industries, and continues to be performed by accredited third-party laboratories worldwide.
Three main configurations of carbon arc equipment are in use:
- Open-flame carbon arc: the arc burns in open air, producing a very high ultraviolet output; filters are normally applied to remove excessively short wavelengths below those found in terrestrial sunlight.
- Enclosed carbon arc: the carbon electrodes burn inside a borosilicate glass globe, which automatically filters out the shortest ultraviolet wavelengths; historically used for textile lightfastness and general material screening.
- Sunshine carbon arc: a filtered, daylight-type carbon arc widely specified in Japanese industrial standards, especially for automotive component weathering.
Testing Samples
Molded Plastic Specimens
Thermoplastics such as ABS, PP, PE, PS, PVC, PC, PA, POM, and PMMA are typically injection molded or compression molded into standard plaques or dumbbell-shaped tensile bars before testing. Using specimens prepared under identical molding conditions is essential because processing history, residual stress, and surface texture all influence weathering behavior. Specimens should be conditioned in a standard atmosphere prior to initial measurements.
Films, Sheets, and Coated Panels
Plastic films, rigid sheets, profiles, and plastic substrates with decorative or functional coatings can be tested either as flat coupons or as finished surface panels. For materials whose appearance is the primary concern, specimens are prepared with the intended production surface finish, since gloss and color measurements are highly sensitive to surface condition.
Finished Products and Components
Automotive interior and exterior trim, lamp housings, instrument panels, electrical enclosures, cable sheaths, and household appliance parts may be tested in their end-use form whenever they can be mounted on the specimen holder. Testing actual components provides the most realistic assessment of geometry effects, rib shadows, and molded-in stress.
Specimen Mounting and Preparation
Specimens are fixed to the rotating specimen drum using inert holders that do not contaminate or shield the exposed face. Minimum recommended exposure areas are defined in the relevant standards so that color, gloss, and mechanical evaluations can be performed after aging. Because the radiation field around the arc is not perfectly uniform, specimens should be periodically repositioned according to the repositioning practice described in ASTM G151 to ensure even exposure.
Testing Items
The degradation of plastics under carbon arc exposure is quantified through a combination of visual, optical, and mechanical evaluations performed before and after aging. Commonly assessed items include:
- Color change: color difference (delta E) and color coordinates measured with a spectrophotometer in accordance with ISO 7724 or ASTM D2244.
- Gloss retention: surface gloss measured at specified angles, typically 60 degrees, per ISO 2813 or ASTM D523.
- Yellowing index: particularly important for white and light-colored plastics and optical parts, per ASTM E313.
- Visual defects: chalking, cracking, crazing, blistering, warpage, and staining rated against standard scales or photographic references.
- Tensile properties: retention of tensile strength and elongation at break per ISO 527 or ASTM D638.
- Impact strength: retention of Izod or Charpy impact properties per ISO 179, ISO 180, or ASTM D256.
- Flexural properties: retention of flexural strength and modulus per ISO 178 or ASTM D790.
- Optical properties: light transmittance and haze for transparent materials per ISO 13468 and ASTM D1003.
- Surface morphology: examination of microcracks and surface erosion by optical microscopy or scanning electron microscopy.
- Mass and dimensional change: weight loss or gain and dimensional stability measured after conditioning.
Results are usually reported as measured values after exposure together with percentage retention relative to unexposed control specimens, which allows different materials or formulations to be ranked directly.
Testing Methods
Open-Flame Carbon Arc Method
The open-flame carbon arc method is specified in ISO 4892-4 and ASTM G152. The arc operates in open air, and daylight-type filters are fitted around the arc to attenuate the excessive short-wavelength ultraviolet radiation that does not occur in natural sunlight. This configuration delivers the highest acceleration factor among carbon arc systems and is suitable for screening the weather resistance of a wide range of plastics.
Filtered Open-Flame Carbon Arc for Coatings
ASTM D822/D822M defines filtered open-flame carbon arc exposure procedures for paints, coatings, and related materials applied to plastic or metal substrates. Prescribed exposure cycles combine dry light periods with periods of water spray to reproduce the combined effects of sunlight and rain.
Enclosed Carbon Arc Method
ASTM G153 describes the operation of enclosed carbon arc apparatus in which the electrodes burn within a borosilicate glass globe. The globe removes wavelengths below approximately 300 nanometers, producing a spectrum with less extreme short ultraviolet output. Enclosed carbon arc systems require less maintenance than open-flame units and were historically used extensively for textile and coating lightfastness evaluation.
Sunshine Carbon Arc Method
The sunshine carbon arc method is described in Japanese standards JIS B 7753 for the apparatus and is widely applied to automotive parts through JIS D 0205. The filtered sunshine carbon arc spectrum is designed to approximate daylight more closely than other carbon arc types. Many Japanese vehicle manufacturer specifications still require sunshine carbon arc exposure for interior and exterior plastic components, making this method essential for suppliers to that market.
Chinese National Standard Method
GB/T 16422.4, the Chinese adoption of ISO 4892-4, specifies open-flame carbon arc exposure procedures for plastics and is frequently requested for products certified or sold in China.
Typical Test Conditions and Cycles
Representative conditions used in carbon arc weathering tests include:
- Black panel or black standard temperature: commonly 63 ± 3 degrees Celsius, adjusted according to the standard and material.
- Chamber air temperature: typically between 40 and 60 degrees Celsius.
- Relative humidity: commonly controlled near 50 percent during dry periods, or as specified by the chosen cycle.
- Water spray cycles: a frequently used cycle consists of 102 minutes of dry light followed by 18 minutes of simultaneous light and water spray; alternative cycles use continuous dry exposure for lightfastness studies.
- Irradiance: monitored and recorded at a reference wavelength, typically 340 nanometers, and held within the tolerance stated by the standard.
- Exposure duration: defined either in accumulated radiant exposure or in hours, commonly ranging from 168 to 2000 hours depending on the specification and intended service environment.
- Electrode replacement: carbon electrodes are consumed during operation and must be replaced at the intervals prescribed by the equipment procedure; worn carbons alter the spectrum and invalidate results.
- Control specimens: unexposed reference specimens are retained under dark, cool conditions for comparison, and reference materials with known durability may be exposed alongside test specimens to verify machine performance.
All exposures should be conducted under the general quality framework of ASTM G151, which governs the use of laboratory light sources for nonmetallic materials and requires documentation of machine calibration, filter condition, and specimen repositioning.
Applications
Carbon arc lamp aging tests are applied across many industries where plastics must withstand sunlight during service:
- Automotive: weathering validation of bumper fascias, grilles, exterior trim, mirror housings, instrument panels, door trims, and lamp components, frequently against sunshine carbon arc requirements from vehicle manufacturers.
- Electrical and electronics: durability assessment of enclosure materials, switchgear parts, connector bodies, cable jackets, and appliance housings.
- Construction: evaluation of plastic pipes, fittings, profiles, roofing membranes, panels, and geomembranes intended for outdoor installation.
- Consumer goods: stability testing of garden tools, outdoor furniture, sporting goods, toys, and packaging that may be exposed to sunlight.
- Textiles and coated fabrics: lightfastness and weatherability assessment of fibers, films, and coated products.
- Materials research and development: comparison of stabilizer packages, UV absorbers, pigments, and resin formulations during new product development.
- Quality control and certification: routine verification that production batches meet specification limits for colorfastness and mechanical retention required by buyers and certification schemes.
Accredited third-party laboratories routinely perform carbon arc aging tests and issue reports that are accepted for supplier qualification, regulatory compliance, and marketing claims.
FAQ
What is the difference between carbon arc and xenon arc weathering tests?
Both methods accelerate sunlight-induced degradation, but they use different light sources. Carbon arc lamps produce a spectrum with intense ultraviolet emission, particularly below 350 nanometers, which is stronger than natural daylight even after filtering. Xenon arc lamps, when equipped with daylight filters, reproduce the solar spectrum more faithfully. Carbon arc testing is still required by certain standards and customer specifications, especially in the Japanese automotive sector, while xenon arc testing dominates modern international specifications. Results from the two methods are not directly interchangeable.
How is the test duration determined?
Duration may be specified as a fixed number of hours, such as 500, 1000, or 2000 hours, or as an accumulated radiant exposure in kilojoules or megajoules per square meter at a reference wavelength. The chosen duration should relate to the expected service life and geographic exposure conditions of the product. Specifications from end customers usually define the required duration and evaluation criteria explicitly.
Can carbon arc test results be correlated to outdoor weathering?
Carbon arc testing provides a comparative ranking of materials rather than an exact lifetime prediction. Because the unfiltered ultraviolet content differs from sunlight, degradation chemistry may differ from that of outdoor exposure. Reliable correlations must be established experimentally for each material family by comparing laboratory results with natural weathering data from outdoor exposure sites, such as those following ISO 877 or ASTM G7.
What specimen sizes and quantities are required?
Requirements depend on the evaluation items. For color and gloss, flat panels of approximately 50 by 100 millimeters or larger are typical. For mechanical testing, standard tensile dumbbells per ISO 527 or ASTM D638 are needed. A sufficient number of specimens should be prepared for baseline measurements, each exposure interval, and unexposed controls. Specific dimensional requirements are defined in the test standard or customer specification.
Why is carbon arc testing still performed when newer methods exist?
Many long-standing industry specifications, historical durability databases, and Japanese automotive requirements are built on carbon arc exposure, so maintaining comparability with legacy data requires continued use of the method. Regulatory documents and customer standards that explicitly cite ISO 4892-4, ASTM G152, ASTM G153, or JIS B 7753 keep the method in active commercial use.
What does a typical test report include?
A professional report identifies the test standard, equipment type, filters used, exposure cycle, black panel temperature, humidity, water quality, total exposure time or radiant dose, carbon electrode replacement records, and specimen positioning scheme. It presents before-and-after measurement data, retention percentages, photographic documentation of appearance changes, and a clear statement of conformity to the specified acceptance criteria.
Are there safety considerations for carbon arc testing?
Yes. Open-flame carbon arc operation generates ozone, ultraviolet radiation, and carbon dust. Laboratories must operate the equipment with proper ventilation, interlocked enclosures, and protective procedures. These requirements are normally handled by the testing laboratory, but clients should confirm that the facility operates the apparatus in accordance with the equipment standard and maintains current calibration records.