Acrylonitrile butadiene styrene (ABS) is an opaque, two-phase engineering thermoplastic produced by grafting styrene-acrylonitrile copolymer onto polybutadiene latex or by mass polymerization to create discrete polybutadiene elastomer particles dispersed in a continuous styrene-acrylonitrile (SAN) matrix. Commercial polymerization lines operate with acrylonitrile between 15 wt% and 35 wt%, butadiene between 5 wt% and 30 wt%, and styrene between 40 wt% and 75 wt%; density typically falls in the range 1.03 g/cm³ to 1.07 g/cm³ when measured per ISO 1183-1. The material is classified as amorphous because the SAN continuous phase does not organize into a crystalline lattice on cooling from the melt. That structural characteristic is the central reason ABS does not exhibit a true melting point: no first-order solid-to-melt enthalpy transition exists in the DSC trace. Instead, ABS passes through a glass transition in which molecular segmental mobility in the SAN matrix becomes active over a temperature interval.

Why ABS Has No True Melting Point: Amorphous Structure and the Glass Transition

At the molecular scale, the SAN matrix of ABS retains random-coil conformations. A semicrystalline polymer such as polypropylene stores long-range three-dimensional order in lamellar crystallites; the melting of that order generates a sharp endothermic peak and a latent heat of fusion. ABS does not form such a crystalline phase. During a heat scan at 20 K/min under ISO 11357-2, a general-purpose ABS shows only a step change in specific heat capacity near 105 °C to 110 °C and no endothermic melting peak before thermal degradation begins above approximately 260 °C. The step is the glass transition of the SAN continuous phase. Heating from room temperature therefore produces a glassy plateau, then a leathery viscoelastic region, then viscous flow as temperature is raised above 220 °C to 250 °C in melt-processing equipment. The absence of a crystalline lattice means there is no thermodynamic melting point and no latent heat barrier; process energy demand is dominated by sensible heat and viscous dissipation rather than crystallite enthalpy.

The polybutadiene phase has a separate glass transition near -85 °C to -80 °C. It remains as discrete rubber particles grafted with SAN. These particles are responsible for impact energy absorption and whitening under multiaxial loading, while the SAN matrix governs the upper service temperature and creep resistance. Because the matrix is amorphous, the transition from solid to melt is broad and rate-dependent. Thermal expansion and modulus reduction take place over a wide interval rather than at a single melting temperature.

In injection moulding, barrel temperature profiles for general-purpose ABS are typically set from 200 °C at the feed throat to 230 °C at the nozzle, with a reciprocating screw of 20:1 to 25:1 L/D ratio and a compression ratio of 2.0:1 to 2.5:1. Mould temperatures are held between 40 °C and 80 °C, and injection pressures from 70 MPa to 140 MPa are usual. Because no crystalline solidification front exists, the hold-pressure time is set by the time required for the melt to cool below the glass transition rather than by a crystallisation plateau. Shrinkage after moulding is anisotropic and in the range 0.4% to 0.7% per ISO 294-4. Pellets exposed to relative humidity above 60% require drying at 80 °C for 3 h to 4 h to reduce moisture below 0.1 wt%; otherwise steam-induced splay and internal voids appear at the moulded part surface. On production lines, melt temperatures above 260 °C for residence times exceeding 30 min degrade the polybutadiene phase, producing yellow-brown discoloration, gas bubbles, and a measurable loss of Charpy notched impact strength per ISO 179-1.

Mould Filling and Solidification Without a Latent Heat Barrier

Flow behaviour during mould filling follows shear-thinning characteristics. Spiral flow length is not governed by a crystallisation arrest; instead, filling length depends on melt viscosity, mould-wall heat transfer, and the temperature at which the material drops below its glass transition. Capillary rheometry at 230 °C and shear rates between 100 s⁻¹ and 1,000 s⁻¹ shows pseudoplastic behaviour; the melt elasticity of the SAN continuous phase contributes to die swell in extrusion and to gate freeze-off in cold-runner injection tools.

On production-scale twin-screw extrusion lines with 32:1 to 44:1 L/D ratio and side-fed glass-fibre or flame-retardant packages, ABS is processed at melt temperatures from 200 °C to 240 °C. The broad softening window means barrel set points can be adjusted across a 40 K band without crossing a melt fracture boundary, but the same breadth causes a less distinct screw-back pressure response at the transition from solids conveying to melt conveying. Operators observing the screw torque curve do not see a sharp melt peak; they see a gradual torque reduction as the resin moves through the compression zone.

Sheet extrusion for thermoforming uses a single-screw extruder with a barrier screw of 30:1 to 36:1 L/D and a flex-lip die set to 0.2 mm to 8 mm sheet thickness. Because the amorphous sheet has no crystalline melting plateau, oven set temperatures for thermoforming are usually 130 °C to 160 °C on the sheet surface, monitored with infrared pyrometers. At these temperatures the sheet enters the rubbery region above the glass transition and can be drawn over a mould; it must be held in the mould until surface temperature drops below 100 °C to avoid dimensional recovery. Non-uniform oven temperatures in this range cause visible thickness variation because ABS modulus changes rapidly with temperature through the glass transition.

Across commercial grades, the glass transition temperature of the SAN matrix is varied by the acrylonitrile-to-styrene mass ratio. A grade at 20 wt% acrylonitrile in the SAN phase may show a DSC glass transition near 100 °C, whereas a high-acrylonitrile grade at 35 wt% may shift the glass transition to approximately 115 °C. The increase is attributed to stronger dipolar interactions and chain stiffening supplied by acrylonitrile units. The polybutadiene phase does not participate in this shift; its glass transition remains near -80 °C unless grafting density or cross-link density is changed. This separation of the continuous-matrix glass transition and the dispersed-phase glass transition gives ABS its toughness over a broad temperature range.

What Distinguishes the Glass Transition from a Melting Endotherm in DSC and DMTA Data?

In a differential scanning calorimetry run per ISO 11357-2, the glass transition is measured as a step change in baseline heat flow over a temperature interval, typically reported at the half-step point. A melting endotherm would appear as a positive heat-flow excursion with a finite enthalpy expressed in J/g. ABS does not generate that excursion before degradation. Dynamic mechanical thermal analysis per ISO 6721-11 or ASTM D4065 separates the glass transition more distinctly: the storage modulus drops from a glassy value near 2 GPa to 0.01 GPa to 0.1 GPa across the transition, while the loss modulus and tan δ pass through a maximum. The tan δ peak occurs several kelvin above the DSC half-step, typically in the range 110 °C to 125 °C at 1 Hz. The exact temperature depends on frequency and heating rate because the segmental motions are kinetically controlled; this is a defining feature of a glass transition rather than a first-order melting point. The absence of a crystalline unit cell also means X-ray diffraction shows only amorphous halos, not sharp Bragg reflections.

When Acrylonitrile Content Is Increased from 20 wt% to 34 wt%, the Glass Transition Moves Upward

The shift in the glass transition changes the practical upper service temperature. The Vicat softening temperature measured under ISO 306/B50 moves from about 95 °C at low acrylonitrile to approximately 110 °C at high acrylonitrile, while the heat deflection temperature under 1.8 MPa load per ISO 75-2/A typically rises from 85 °C to 100 °C. Melt viscosity also increases at fixed processing temperature because acrylonitrile raises the segmental relaxation time. Processors may raise melt temperature to 240 °C or 250 °C for high-acrylonitrile ABS, but degradation limits the upper boundary near 260 °C. This creates a narrow processing window for high-acrylonitrile ABS, particularly in thick-walled moulded parts where long residence time raises the risk of polybutadiene degradation. On a hydraulic injection moulding machine with 1,500 kN clamp force and a 40 mm diameter screw, high-acrylonitrile ABS requires careful control of screw speed because increased viscous dissipation raises melt temperature at the nozzle. Back pressure is limited to 0.5 MPa to 1.0 MPa to avoid excessive shear heating. Hot-runner manifolds should be evaluated for shear balance because local shear heating in small gates can generate material temperatures that locally exceed 260 °C, resulting in streaking and black specks.

ABS is incompatible with concentrated oxidizing acids, ketones, esters, and aromatic hydrocarbons. Moulded articles exposed to these fluids undergo environmental stress cracking even below the glass transition; ASTM D543 can be used to assess chemical resistance. Prolonged exposure to ultraviolet light degrades the polybutadiene phase unless carbon black or stabilizer packages are incorporated. Regulatory documentation for unfilled ABS is commonly maintained under RoHS Directive 2011/65/EU and REACH SVHC lists; food-contact or medical grades require additional compliance with FDA 21 CFR 180.22 or EU 10/2011 migration limits. These limitations do not arise from a melting point but from the chemistry of the polybutadiene phase and the SAN matrix.