The Polycarbonate Part Crazed in Storage: Solvent Exposure on the Floor

In production storage areas where floor-level solvent residues remain after maintenance shifts, transparent molded polycarbonate components occasionally develop dense networks of fine surface fissures after direct contact with the concrete floor. The polycarbonate part crazed in storage: solvent exposure on the floor represents an environmental stress cracking response rather than simple chemical dissolution. Typical injection-molding grades of polycarbonate exhibit a tensile yield strength near 60–70 MPa and elongation at break in the range 80–150% when tested under ISO 527-2:2012. These air-conditioned short-term properties do not predict storage behavior because environmental stress cracking occurs below the yield point, frequently at applied or residual stress levels below 10 MPa, when a surface-active organic liquid is present. Polycarbonate is an amorphous thermoplastic with a glass transition temperature near 147 °C and an unfilled density near 1.20 g/cm³, but the storage failure initiates at ambient temperature because solvent uptake depresses local segmental mobility and reduces the stress required for craze nucleation.

Manufacturing environments introduce a broad range of low-molecular-weight organic compounds onto concrete floors. Hydraulic fluids, cutting oils, degreasers, floor strippers, and cleaning solvents adsorb into the cementitious surface and can transfer to polymer parts by wicking, direct contact, or vapor-phase adsorption. For polycarbonate, the most damaging species are those whose solubility parameters lie close to the polymer solubility parameter range of approximately 19.7–20.3 (J/cm³)^0.5. Aromatic hydrocarbons, ketones, esters, and chlorinated solvents are particularly aggressive because they penetrate the amorphous matrix and lower the local yield stress. This combination of molded-in stress and solvent plasticization creates the branched craze features observed on parts removed from floor storage, often concentrated near gates, weld lines, holes, and sharp internal corners.

What Mechanisms Link Residual Molding Stress to Solvent-Induced Crazing?

Residual stress controls the spatial distribution of storage crazing. In a cold-runner injection mold operated at a melt temperature of 290–310 °C and a mold temperature of 80–110 °C, the surface skin solidifies rapidly while the core continues to cool and densify. This differential shrinkage produces a triaxial tensile stress state near gates, thick-to-thin transitions, and metal-in-plastic interfaces. When the ejected part is placed on a solvent-wet floor, the existing stress can exceed the critical strain threshold for environmental stress cracking even without an external mechanical load. Published environmental stress cracking data for polycarbonate under constant tensile load in aggressive solvents such as toluene or methyl ethyl ketone indicate that threshold strain values for medium-viscosity molding grades may fall below 0.5%; however, published data for this specific floor-contact configuration is limited. Craze initiation begins at surface flaws where localized strain is magnified. The resulting craze structure consists of oriented fibrils and microvoids, commonly reported with void spacings in the range 5–30 nm, which scatter light and produce the characteristic stressed-whitened appearance.

On production-scale injection molding lines with clamp forces between 1,200 kN and 4,500 kN, residual stress is frequently highest in thick-walled bosses, threaded inserts, and areas machined after molding because these features maximize differential cooling and orientation. Field observations on such lines show that floor-contact crazing is not uniformly distributed across a part. It initiates preferentially around gates, the perimeter of holes, and molded-in sharp radii where photoelastic stress measurement can reveal tensile strain concentrations. Annealing at 120 °C for 2–4 h relieves a portion of the frozen-in stress, but it does not eliminate environmental stress cracking susceptibility if subsequent solvent contact occurs. General-purpose polycarbonate molding grades with a melt volume rate of 9–12 cm³/10 min at 300 °C under 1.2 kg load according to ISO 1133-1:2022 provide higher flow and often lower molded-in stress than high-viscosity grades, but the lower molecular weight may reduce resistance to craze propagation once a solvent front has entered the surface.

Solvent Classes and Concrete Floor Carryover Risks

Concrete floors in manufacturing and warehouse facilities are rarely inert. Residual solvent can remain within floor pores after visible evaporation and may be released later when humidity rises or when cleaning water is applied. A polycarbonate part placed directly on that surface creates a localized contact zone where the polymer acts as a sink for residual solvent. The severity of the resulting crazing depends on the solvent class, contact duration, part temperature, residual stress, and the molecular weight of the resin. Small polar molecules such as acetone and methyl ethyl ketone diffuse rapidly through amorphous polycarbonate and produce early visible crazing under molded-in strain. Aromatic hydrocarbons and chlorinated solvents also pose severe risk because of favorable solubility-parameter matching and high plasticizing efficiency. Alcohols generally produce milder effects but can still craze highly stressed parts after prolonged contact.

The table summarizes commonly observed behavior under laboratory screening; however, production floor exposure is more complex because concrete additives, detergent residues, hydraulic fluid oxidation products, and water can alter solvent activity. A part that fails in storage is often extracted with a solvent wipe for Fourier-transform infrared spectroscopy analysis, but the resulting spectrum may contain overlapping peaks from concrete floor compounds and cleaning agents. Quantitative assignment of a single causative solvent is therefore difficult without controlled exposure testing using the actual degreaser lot and the actual molded part.

If a Part Is Exposed to Ketone-Based Floor Degreaser Without Post-Mold Annealing

Ketone-based floor degreaser contact under production conditions is associated with a characteristic craze pattern that begins at the floor contact line and propagates along part geometry or radial stress contours. The failure is sometimes misclassified as brittle fracture during handling, but scanning electron microscopy of the affected surface reveals craze fibrils and microvoids rather than a smooth cleavage surface. The onset time for visible crazing depends on solvent concentration, part thickness, initial stress, and polycarbonate molecular weight. For medium-viscosity injection grades with weight-average molecular weight near 30,000 g/mol, continuous wet contact with methyl ethyl ketone can produce visible crazing within 10–60 min when surface tensile stress is elevated. This range is not a universal release limit and should not be used as a production acceptance criterion. A more reliable assessment requires controlled environmental stress cracking testing under ISO 22088-2:2006 or ISO 22088-3:2006 using the specific solvent formulation and the specific part geometry.

Vapor exposure can also initiate crazing before direct liquid contact is noticed. The vapor pressure of methyl ethyl ketone near 20 °C is approximately 10.5 kPa, sufficient to create an enriched boundary layer above a recently cleaned floor. Plastic crates, closed bins, and shrink wrapping can trap solvent vapor and intensify exposure. Because polycarbonate is amorphous and uncrosslinked, absorbed solvent may desorb if the part is removed from the source early, but the craze voids remain. The damaged zone acts as a stress concentrator during subsequent assembly, ultrasonic welding, or mechanical loading. For this reason, visible crazing after floor contact is considered a functional rejection criterion rather than a cosmetic surface defect. Amine-based floor stripping agents present an additional incompatibility because they can cause alkaline hydrolysis and chain scission of the polycarbonate backbone, producing a degradation pattern distinct from physical environmental stress cracking.

Test Methods and Threshold Strain Measurement Without Immersion

Qualitative screening according to ASTM D543-20 is a common starting point, but it does not reproduce the geometry-specific residual stress of a molded part. Environmental stress cracking resistance is more appropriately measured with ISO 22088-2:2006, which applies a constant tensile load in a controlled chemical environment, or with ISO 22088-3:2006, which fixes a bent strip at a known surface strain. These methods allow time-to-craze or time-to-failure to be plotted against applied strain for a particular solvent. Polycarbonate tensile data generated under ISO 527-2:2012 show yield stresses near 60 MPa, but environmental stress cracking tests in aggressive solvents may show craze initiation at applied stress below 10 MPa. Notched Izod impact values of 600–850 J/m under ASTM D256 are frequently cited as evidence of overall toughness, but they do not predict storage crazing because the impact test is performed in air at high strain rate, whereas solvent craze formation is a low-strain, time-dependent phenomenon.

Residual stress in transparent polycarbonate parts can be evaluated qualitatively by photoelastic inspection under crossed polarizers, but quantitative assessment for failure analysis may require layer-removal strain measurement or hole-drilling strain gage methods. The highest-risk areas identified by these measurements do not always coincide with the thinnest wall sections; gate regions and abrupt transitions from thick to thin sections typically show higher tensile residual stress and therefore earlier solvent crazing. When a floor-contact incident is investigated, tensile testing alone of exposed parts may not detect localized craze damage because the crazed layer may represent only a small fraction of the total cross-sectional area.

Storage controls for molded polycarbonate parts in production facilities include raised dunnage made of polyethylene or polypropylene, sealed conductive bins, and a designated quarantine area positioned at least 10 m from solvent use points. Concrete floors should be dried after cleaning and pass a visual absence-of-residue check using a clean white cloth wipe. When floor contact is suspected, inspection under 10× magnification with polarized light can reveal craze networks before they become visible to the unaided eye. Parts exposed to ketone or aromatic solvent residues should be isolated and dispositioned by material engineering rather than returned to finished stock without examination.