Polyurethane (PU) is a versatile polymer created through a reaction between a polyol and an isocyanate, forming strong urethane bonds. The question of whether polyurethane is chemical resistant does not have a simple yes or no answer because performance varies widely. A PU’s resistance depends almost entirely on its specific chemical formulation, particularly the type of polyol used. For instance, polyester polyols behave differently under chemical attack than polyether polyols.
The Chemistry Behind Polyurethane Resistance
The resistance of polyurethane is rooted in its molecular structure, which consists of alternating soft segments and hard segments. The hard segments, formed by the isocyanate component, contain the strong urethane linkages that act as the polymer’s backbone. The soft segment, derived from the polyol component, largely determines the material’s specific chemical compatibility. Polyether-based polyols offer enhanced resistance to hydrolysis, while polyester-based polyols often provide better resistance against many common solvents and oils. Selecting the right polyol type customizes the finished material’s resistance profile.
Chemicals Polyurethane Handles Effectively
Polyurethane generally demonstrates good resistance to non-polar substances like most pure mineral oils, greases, and petroleum-based fuels such as diesel and kerosene. This makes it a preferred material for seals and gaskets in machinery where oil exposure is constant. The material also holds up well against common water exposure and high humidity, particularly polyether formulations.
Most polyurethanes can tolerate short-term or splash exposure to dilute acids and dilute bases. Low concentrations of household cleaners, mild acids like vinegar, and weak alkaline solutions typically do not cause immediate degradation. However, this resistance is limited and depends heavily on the concentration and duration of contact.
Common Agents That Cause Degradation
A number of chemical groups severely attack and degrade polyurethane. Strong polar solvents, such as ketones like acetone and methyl ethyl ketone (MEK), are highly aggressive and act as partial solvents, causing significant swelling and a drastic loss of tensile strength. Aromatic solvents like toluene and xylene can cause the material to swell by up to 50%, severely compromising its mechanical properties.
Concentrated acids and strong alkalis also cause rapid chemical degradation by attacking the polymer chains. Strong acids and bases cause chain scission or hydrolysis, where the chemical bonds within the polymer are broken. Polyester-based polyurethanes are particularly susceptible to hydrolysis when exposed to warm water or steam at temperatures above 60°C over time, which cleaves the ester linkages and results in a loss of strength and elasticity.
Variables That Determine Resistance Performance
Chemical type is only one factor; external conditions and material formulation significantly modify polyurethane’s resistance performance. Temperature is a major variable, as higher temperatures accelerate nearly all chemical reactions, causing faster degradation upon contact with an aggressive chemical. What a PU resists at room temperature may fail quickly at an elevated temperature.
The concentration of the chemical agent and the duration of exposure also play a crucial role in performance. A brief splash of a strong chemical might only cause surface changes, but continuous submersion or prolonged contact will likely result in swelling and irreversible material breakdown.
The manufacturing process, particularly the curing stage, must be properly controlled. An improperly cured polyurethane will exhibit significantly weaker resistance to chemicals than a fully cross-linked material. Due to these many variables, engineers rely on specific product chemical resistance charts.