Injection-stretch blow molding lines processing the copolyester grade designated YS-W100R exhibit a characteristic shift in preform reheat behavior that is more reliably interpreted through thermal profile mapping than through single-point surface temperature measurements. A preform of wall thickness 3.0 mm to 4.8 mm and mass 18.0 g to 32.0 g is heated in a quartz-infrared oven, and the surface temperature at the exit of each heating zone is recorded with an infrared pyrometer calibrated against a blackbody source. The reheat profile is generated by plotting surface temperature against lamp zone position or residence time, allowing calculation of the heating rate in °C/s and the equilibration plateau before the preform enters the conditioning zone at 105°C to 115°C. In unmodified poly(ethylene terephthalate), this profile is controlled principally by pellet intrinsic viscosity, preform crystallinity developed during injection molding, pellet moisture, and the wavelength distribution of the IR emitters. The addition of a comonomer such as isophthalic acid or diethylene glycol disrupts the regularity of the polymer backbone, lowers the glass transition temperature by approximately 2°C to 4°C per mol% of semiaromatic comonomer, and broadens the cold crystallization exotherm observed by differential scanning calorimetry according to ISO 11357-3:2018. Under production conditions, the direct consequence is that preforms containing YS-W100R often require a reduction in total IR exposure of 8% to 12% relative to a homopolymer reference to reach the same stretch-rod force signature and container base clearance. Published data for this specific YS-W100R formulation is limited; the quantitative relationship must be established on the specific blow molder using design-of-experiment calibration with resins of known copolymer concentration and controlled intrinsic viscosity.

What Does the Container Sidewall Haze Ratio Reveal About YS-W100R Comonomer Localization?

The finished container produced from a reheat-conditioned YS-W100R preform carries additional information in its sidewall haze ratio and in its thermal shrinkage response when the article is reheated to hot-fill conditions. A container blown from a homopolymer PET preform typically develops strain-induced crystallinity in the sidewall of 20% to 30% at blow ratios between 8.0 and 12.0, whereas a YS-W100R-grade preform with higher comonomer content may reach comparable sidewall crystallinity only after a longer stretch-rod stroke or a higher final blow pressure due to reduced strain-hardening modulus. Haze measured according to ASTM D1003-21 on the container sidewall frequently increases by 1.5% to 4.0% if the preform surface is overheated by as little as 5°C above the upper limit of the orientation window, because surface spherulite formation competes with strain-induced orientation. Container reheat profiling, in which the blown article is exposed to 85°C water or air for 30 min and volumetric shrinkage is measured by a calibrated burette method adapted from ASTM D2732-20, discriminates between comonomer-rich and comonomer-lean regions because amorphous copolymer segments relax at lower temperatures than oriented homopolymer segments. A sidewall shrinkage value above 3.0% after 85°C exposure may indicate a localized comonomer excess that lowers the onset of thermal relaxation below the hot-fill threshold. This measure is operationally relevant because hot-fill containers must retain dimensional stability at 85°C to 95°C, and the preform reheat recipe can be adjusted only if the copolymer distribution is known from thermal profile data.

On a production line using a 72-cavity injection molding cell and a 48-cavity reheat blow molder, the reheat profile is not a fixed recipe but a closed-loop adjustment driven by preform thickness, resin lot, and ambient humidity. A change in YS-W100R copolymer content of 0.5 mol% can produce a measurable shift in the heating rate at the neck ring of 0.4°C/s to 0.8°C/s when measured with a fixed pyrometer at the third heating zone. This sensitivity is due to the lower cold crystallization temperature of the copolymer, which releases exothermic heat within the preform wall during heating and creates a nonlinear temperature rise that is absent in homopolymer grades. The process window for final preform surface temperature is narrow, often ±5°C, with a lower bound defined by neck stretch failure and an upper bound defined by surface haze and base clearance loss. If the reheat profile shifts by more than 7% in total IR energy, operators may observe batch-to-batch variation in container base clearance of 1.0 mm to 2.5 mm, or a rise in leakage failures in 24-hour production lots. These failure signatures are not specific to comonomer content by themselves; however, when combined with a shift in the measured preform surface plateau below 112°C and a simultaneous reduction in stretch-rod force of 8% to 12%, they indicate a higher copolymer fraction. Published data for YS-W100R reheat force signatures on specific OEM blow molders is limited, so a calibration set of preforms with known comonomer content and controlled intrinsic viscosity is required. The calibration set should be dried to 50 ppm moisture or lower and molded at a barrel temperature between 275°C and 290°C to minimize hydrolysis-induced molar mass loss.

Reheat Rate Variability as a Diagnostic for Copolymer Distribution

The local reheat rate within a preform wall is governed by the absorbed IR flux, the density of the polymer, and the effective thermal conductivity, all of which change when comonomer units are inserted into the poly(ethylene terephthalate) chain. YS-W100R-grade preforms typically exhibit a density of 1.36 g/cm³ to 1.37 g/cm³ after injection molding, compared with 1.37 g/cm³ to 1.38 g/cm³ for homopolymer PET molded under the same conditions. This small density difference reflects reduced crystallinity and alters the thermal diffusivity by approximately 1% to 3%. The reheat profile therefore shows a lower initial surface temperature rise when the preform enters the IR oven, followed by a steeper rise once the core passes the cold crystallization onset. A diagnostic index can be defined as the ratio of the heating rate in the second lamp zone to the heating rate in the fourth lamp zone; for homopolymer preforms this ratio remains between 0.85 and 0.90, whereas for increased YS-W100R copolymer content the ratio may fall below 0.80. This index is insensitive to lamp aging and reflector fouling because both zones are affected by the same IR source degradation. It is not suitable as a stand-alone release criterion because it does not separate copolymer content from moisture, pellet size distribution, or preform wall thickness variation. The index must be combined with intrinsic viscosity measured by ASTM D4603-18, melt mass-flow rate measured by ISO 1133-1:2022 at 285°C with 2.16 kg load, and density measured by ASTM D792-20.

When the Preform Reheat Plateau Shifts Below 112°C

A shift in the preform reheating plateau to below 112°C is one of the more direct thermal indicators of elevated YS-W100R copolymer content because the plateau temperature reflects the balance between absorbed IR energy and the latent heat effects associated with cold crystallization. In a homopolymer PET preform dried to 50 ppm moisture, the plateau typically stabilizes between 114°C and 118°C before the conditioning zone. In a YS-W100R preform with a comonomer content above a resin-specific threshold, the plateau may drop to 108°C to 111°C under the same lamp settings, and the time spent at the plateau increases by 0.3 s to 0.8 s because the broader crystallization exotherm dissipates heat over a wider temperature interval. The reheat profile should be interpreted in conjunction with the preform axial temperature gradient; a plateau shift at the neck ring only, without a corresponding shift in the sidewall, points to injection molding inhomogeneity rather than a change in copolymer content. A plateau shift across the entire preform body, accompanied by a lower stretch-rod force and a reduction in the required IR energy, indicates a higher average comonomer content. The boundary condition is critical because a preform surface temperature below 110°C may still be sufficient for blowing but may reduce the orientation-induced crystallization in the container sidewall, resulting in a softer container with lower top-load capacity. Top-load capacity measured on the finished container according to ISO 8113:2004 can fall below the specification limit if the preform reheat profile is not corrected for the lower plateau associated with higher copolymer content.

Analytical confirmation of the comonomer content inferred from reheat profiles requires a combination of thermal, rheological, and spectroscopic measurements. The following table summarizes the standard methods used to anchor the process-level observations to polymer-level properties in YS-W100R grades.

Operational boundaries for YS-W100R preforms must account for the fact that reheat profile shifts are not exclusively caused by copolymer content. Pre-drying is required when the storage relative humidity exceeds 60%, because moisture absorbed in the pellet is hydrolyzed at melt processing temperatures and lowers intrinsic viscosity, which in turn changes the preform heating curve. The resin should not be combined with amine-based additives or certain color concentrates that are known to promote premature crosslinking or chain scission, because these reactions can produce reheat profile changes resembling comonomer fluctuations. The processing window at the blow molder narrows further when the preform surface temperature rises above 118°C; at this condition, surface spherulite formation increases haze and initiates orange-peel defects on the container sidewall. If the preform reheat profile indicates a shift below 110°C, the blow molder must verify that the IR lamp voltage and reflector condition are within specification before attributing the change to YS-W100R copolymer content. Published data for the specific reheat behavior of YS-W100R on all OEM blow molding platforms is limited, so each manufacturing site must validate the relationship between reheat profile indices and comonomer content using calibrated resin lots and standardized preform geometry.