In Focus: Can sunshine melt cars? Testing

Sunlight cannot melt a car, but sustained solar radiation raises surface and cabin temperatures high enough to deform plastics, soften adhesives, fade coatings, and speed material failure. Sun-heat testing (solar simulation or weathering testing) confirms whether exterior and interior materials will survive long outdoor exposure. It evaluates polymer trim, dashboard skins, coatings, sealants, and glazing under laboratory conditions that reproduce full-spectrum sunlight and elevated temperature.

Principle & mechanism

Solar-heat testing reproduces the coupled energy inputs that drive material degradation outdoors. These inputs are ultraviolet photons, visible and infrared radiation, temperature, and humidity cycling. Ultraviolet photons carry enough energy to break polymer chains through photo-oxidation. This generates free radicals that spread in the presence of oxygen and moisture. The result is embrittlement, chalking, discoloration, and loss of mechanical strength in plastics, paints, and elastomers. Infrared radiation acts differently. It raises surface and cabin temperature, which speeds thermal oxidation, softens thermoplastics, and relaxes molded-in stresses. This is when warpage, sagging, or gloss loss appears. Laboratory methods combine a xenon-arc or metal-halide light source with filtered radiation that matches terrestrial sunlight, plus programmed temperature and humidity control. By separating and then combining photonic and thermal action, the test distinguishes purely photochemical damage from heat-driven deformation. It also provides measurable endpoints such as color difference, gloss retention, and dimensional change.

Test objects and sample preparation

Test objects cover nearly all polymeric and coated components of a vehicle. Interior parts include instrument panels, door trims, steering wheel skins, seat materials, and headliners. Exterior items include bumper fascias, mirror housings, grilles, badges, wheel covers, and body paint systems. Functional materials such as adhesives, sealants, wiring insulation, and glazing interlayers are also tested. Specimens are prepared as flat plaques whenever possible, since curved parts give uneven irradiance across the surface. Recommended practice is to cut or mold panels of a size that fits the exposure rack of the chamber. Edges should be deburred and the exposure face clearly marked. BefOre testing, samples are conditioned in a standard atmosphere, typically room temperature and controlled humidity. Initial color, gloss, and dimensions are then measured as a baseline. Thickness, curvature, surface texture, and pigment must be recorded because they all influence heat absorption and radiation response. Retained control samples are stored in the dark for later side-by-side comparison.

Solar

simulation and heat aging test methods
Two complementary approaches are used. Outdoor natural exposure places racks at fixed-angle, equator-facing orientations in high-irradiance climates. It measures cumulative radiant energy over months to years and remains the reference condition for correlation. Laboratory accelerated testing uses xenon-arc lamp systems, commonly run to recognized procedures such as the xenon-arc family of plastics and paint weathering methods. Metal-halide solar simulators are used for whole-component cabin heating studies. A typical xenon cycle alternates irradiance, water spray, and dark humid phases. Black panel or black standard thermometers control surface temperature. Heat aging runs in parallel using forced-ventilation ovens at several elevated temperatures for graded durations. This allows thermal endurance to be estimated by comparing property retention across the temperature ladder. Fluorescent UV lamp devices offer a lower-cost option focused on the ultraviolet portion of the spectrum. The choice among these methods depends on which failure mode, photochemical or thermal, dominates the component in service.

Thermal

deformation and temperature measurement
The practical question is whether sun-loaded parts warp or sag, so dimensional stability is tracked alongside visual change. Heat deflection and Vicat softening measurements on molded plaques give the temperature at which a material loses rigidity. Load-deflection testing under heat quantifies sag of horizontal trim. During simulation, surface and internal temperatures are measured with attached thermocouples, infrared thermography, or radiometer-monitored black panel sensors. Infrared cameras are useful for locating hot spots on complex geometry such as dashboard tops and rear shelves. These areas often run far hotter than ambient because of greenhouse trapping inside a closed cabin. Cabin-level testing with full solar simulator arrays records air temperature curves, component surface peaks, and time-to-steady-state. Dimensional measurements before and after exposure, taken with calipers or coordinate measurement, convert deformation into quantifiable values. These include warp height, shrinkage percentage, or gap change.

Sensitivity,

repeatability and acceptance criteria
Measurement sensitivity depends on instrument resolution and procedural discipline. Color change is assessed with a spectrophotometer reporting the delta E value against the archived baseline. Gloss is measured with a glossmeter at defined geometry. These methods detect changes long before they become visually obvious to an unaided observer. Repeatability requires stable irradiance calibration using radiometric reference devices. It also requires verified chamber uniformity, controlled specimen rotation, and consistent measurement locations and operator. Acceptance criteria are usually set in material or component specifications. They are expressed as property retention after a stated exposure dose. Examples include a maximum allowed color difference, minimum gloss or tensile strength retention percentage, no cracking or blistering under magnification, and dimensional change within a stated tolerance. Statistical treatment of replicate specimens, rather than single readings, separates real degradation from measurement scatter. When results fall near a limit, extended exposure or repeat testing clarifies whether the margin is genuine.

Co-testable

parameters and application scenarios
Sun-heat testing is rarely run alone, because the same exposed specimens support multiple measurements. A single campaign can combine colorimetry, gloss, visual defect grading, tensile and flexural property retention, and hardness change. It can also include Fourier-transform infrared spectroscopy for oxidation depth and surface morphology by scanning electron microscopy to document microcracking. Volatile emissions from interior materials after heat exposure can be sampled with thermal desorption and analyzed by gas chromatography-mass spectrometry. This links cabin odor and fogging to solar loading. Application scenarios include new-material qualification for interior and exterior trim, validation of paint and coating systems, and root-cause investigation of field complaints such as warped panels or faded trim. Other scenarios are quality control for incoming resins and comparative aging of alternative suppliers. Data from these campaigns feed durability predictions. They also support internal quality decisions, supplier comparison, and product documentation for market entry.