The Science Behind PET Appearance: Transitioning from Milky White to Transparent

Why does bottle-grade PET initially appear milky white?

The production of bottle-grade PET resin involves a Solid State Polymerization (SSP) process. During this process, amorphous PET resin generated in the Continuous Poly-condensation (CP) stage are heated above their glass transition temperature (Tg) to induce crystallization. As PET resin undergoes 50-70% thermal crystallization, its intrinsic viscosity (IV) increases. This transition from an amorphous state to a partially crystalline state causes the resin to appear milky white due to light scattering within the crystalline regions.

Additionally, in the production of bottle-grade PET, the color transformation occurs during the manufacturing process.

Color Transformation in Bottle-Grade PET Resin: Transparent to Milky White

The production of bottle-grade PET resin involves two key stages: continuous poly-condensation (CP) and solid state polymerization (SSP). 

In the CP stage, bottle-grade PET base resin are transparent. This transparency results from the high-temperature molten polymer reaction, where PET's molecular chains exhibit high mobility and fluidity. Upon rapid cooling, the amorphous regions dominate, and the lack of pronounced crystalline structures allows light to pass through, making the material appear transparent.

In the SSP stage, the initially transparent bottle-grade PET base resin undergo 50-70% thermal crystallization, with an increase in intrinsic viscosity (IV). Specifically, these amorphous PET slices are heated above their glass transition temperature in a crystallizer to initiate crystallization. The SSP process involves extended high-temperature treatment, typically lasting 8 to 12 hours. This prolonged heat treatment further enhances crystallinity. During this process, PET molecules form spherulites—large, radial crystalline structures that cause light scattering, resulting in the milky white appearance of PET.

Transparency of Preforms and Bottles

PET undergoes several transformations from PET resin pellets to preforms and finally to bottles.

During the injection molding stage, milk white bottle grade PET crystals melt to form an amorphous melt that is injected into preform molds. To prevent re-crystallization, PET preforms are rapidly cooled after molding, maintaining their amorphous structure. At this stage, PET molecular chains are unoriented and non-crystalline, similar to a bowl of unstructured spaghetti. The absence of crystalline structure allows light to pass through, resulting in transparent preforms. However, PET in this state has minimal strength and barrier properties.

In the stretch-blow molding process, PET preforms are heated and subjected to stretching and blowing, which orients the material both axially and radially. This process induces the formation of small strain-induced crystals. Due to their tiny size and specific alignment, these crystals do not significantly scatter light, allowing PET bottles to remain transparent. Moreover, this orientation significantly enhances the strength and barrier properties of PET bottles. With proper preform design and processing conditions, the crystallinity of the bottle walls can reach approximately 25%, improving mechanical performance and gas barrier capability.

Appearance Differences in Specific Areas

The neck and gate areas of the finished bottles often exhibit different structural characteristics. In the neck and gate regions, parts that are not sufficiently stretched may remain amorphous, appearing transparent or slightly cloudy. In contrast, the sidewalls of the bottle typically exhibit oriented regions where thermal stretching has induced an ordered crystalline structure. These fine and uniform crystals generally do not affect light transmission significantly, thus maintaining the transparency of the sidewalls.

Occasionally, localized overheating during blow molding can lead to thermal crystallization around the gate area. This can cause the appearance of faint white regions, which are common defects in blow molding processes but do not significantly affect the bottle's basic functionality.

How to Identify Bottle-Grade PET Resin by Appearance

One direct method for identifying bottle-grade PET resin is through its appearance and physical characteristics.

Transparency is a key indicator. Initial bottle-grade PET resin usually appear milky white due to their partially crystalline structure. In contrast, low intrinsic viscosity non-bottle-grade PET resin that have only undergone CP but not SSP processes are generally transparent. Thus, when assessing PET resin, attention should be paid to its purity, transparency, and absence of impurities.

Additionally, high-quality bottle-grade PET resin should exhibit good gloss, with no noticeable impurities or discoloration. The color and morphology of the slices should be uniform, with no significant color variation or particle size differences, and the resin should not stick together.

Conclusion

Overall, PET can exhibit three primary states: amorphous (e.g., preforms and molten resin), thermally crystallized (e.g., resin pellets), and strain-induced crystalline (e.g., bottle sidewalls). The production process for PET products involves a complex transition from initially transparent base PET resin, to milky white bottle-grade PET resin after solid-state polymerization, and finally to transparent bottles. By examining PET’s transparency, gloss, and uniformity, one can preliminarily assess its quality and processing state.