Why are PET plastic bottles easily damaged by heat?

Why are PET plastic bottles easily damaged by heat?

PET (polyethylene terephthalate) is one of the most widely used materials in cosmetic and personal care product packaging due to its high transparency, light weight, and low cost. However, it has a well-known weakness: low heat resistance. Standard PET bottles begin to soften and warp at 60–70°C (140–158°F). Under prolonged heat exposure-common during sea transport to tropical regions such as the Philippines, Vietnam, and Indonesia-even moderate temperatures can cause irreversible deformation of the packaging, including bottle twisting, label detachment, neck bending, and capacity changes, ultimately rendering the product unusable or non-compliant.

 

PET Bottle Temperature Resistance Range Reference Table

Temperature range	Impact on conventional cold-fill PET bottles	Impact on heat-filled/heat-set PET	Industry assessment
≤30°C	Basically stable	Basically stable	Standard security zone
30~40°C	Generally acceptable, but prolonged sun exposure begins to accumulate risks.	Stablize	Tropical warehouses/docks may enter the warning zone
40~50°C	Lightweight thin-walled bottles, pressure bottles, and irregularly shaped bottles may experience stress release and slight deformation.	Generally still under control	It is not recommended to expose the object to direct sunlight during long-term transportation.
50~60°C	High-risk areas may exhibit shrinkage, ellipticization, shoulder collapse, and panel deformation.	It is relatively more stable, but this still needs to be verified.	The most common risk areas for shipping containers exposed to direct sunlight
67~81°C	Approaching/entering the glass transition region of PET, the material stiffness decreases significantly.	It still depends on the specific process and bottle shape.	Ordinary PET should not be used in this range.
Approximately 76°C	Common industry-standard PET softening threshold empirical values	Heat-set processes are not suitable for ordinary PET.	High-risk points of ordinary PET bottles
82~85°C	Ordinary PET typically cannot maintain its shape	Some heat-set PET can withstand	Typical application areas for hot-fill PET
Temperatures above 85°C and sustained for several minutes	Ordinary PET is basically not applicable	Available only with specific hot-fill designs and resin systems	Requires specialized design verification
Liquid filling temperature: 82~95°C	Not applicable to ordinary PET	Hot filling systems can be used within the process window.	This is the "process temperature," not the ordinary storage and transportation temperature.

 

The root cause of deformation during tropical shipping

1. Extremely high temperatures inside the container

Sealed shipping containers are essentially metal ovens. The steel material is extremely efficient at absorbing and retaining solar radiation heat. In tropical regions like the Philippines, where ambient temperatures typically reach 35–38°C, the internal temperature of a sealed shipping container can soar to 65–80°C after 2–4 hours of direct sunlight. If containers are stacked on top of a ship's deck, the measured peak temperature can even exceed 85°C. This is far higher than the critical temperature of approximately 55°C at which PET begins to deform. In other words, every hour a container remains in the sun is a process of continuous thermal stress accumulation on every PET bottle inside.

2. Greenhouse heat concentration effect

Standard pallet wrapping uses transparent LLDPE stretch film. This material is almost transparent to incident sunlight, but it acts like a greenhouse, blocking infrared radiation emitted by heated goods and preventing heat loss. When sunlight enters through container ventilation gaps, or during brief openings at port, the heat is locked in by the layers of stretch film. Studies have shown that the internal temperature of a tightly wrapped pallet can be 10–15°C higher than the air temperature inside the container. For bottles deeply embedded in the center of the pallet, there is virtually no path for heat to escape.

3. Long port delays

Tropical ports such as Manila (Philippines), Bangkok (Thailand), and Ho Chi Minh City (Vietnam) commonly experience congestion. Containers often remain in open port yards under the equatorial sun for 1–5 days while waiting to be loaded onto ships or clear customs. Unlike sea transport, where ship movement and sea breezes provide some cooling, containers parked in yards face continuous and uninterrupted heat accumulation. PET deformation depends not only on peak temperature but also on the combined effect of temperature and duration. Deformation caused by exposure to 58°C for more than 48 hours can far exceed the effect of 70°C for 30 minutes.

4. The stacking pressure increases dramatically after softening due to heat.

At room temperature, PET bottles have good compressive strength and can withstand normal stacking loads. However, when the temperature rises to around 55–65°C, the glass transition temperature (Tg) of amorphous PET decreases, and the polymer enters a viscoelastic state-at which point the material is neither a pure solid nor a pure liquid, and will creep rather than elastically recover under sustained load. A standard tray of cosmetic bottles can withstand a cumulative stacking pressure of 200–500 kg on the bottom layer. As the bottles soften, the bottom bottles will buckle outwards, be flattened, or develop permanent side bulges under this pressure, which cannot be restored to their original shape after cooling.

 

Prevention strategies

1. Use refrigerated containers (freezers)

This is currently the most effective single protective measure. Refrigerated containers can maintain the internal temperature within a set range-typically 18–22°C for cosmetic packaging-completely unaffected by external tropical high temperatures. While refrigerated containers cost approximately 20–40% more than regular dry containers, this additional cost must be weighed against the risk of loss due to the entire shipment becoming unsaleable due to deformation. For high-value cosmetic brands, or large orders exported to tropical markets such as the Philippines, refrigerated containers should be considered a non-negotiable logistics standard, not an option. It is essential to confirm with the freight forwarder that the container has been pre-cooled before loading and to obtain complete temperature records upon arrival of the goods.

2. Insulated tray cover and thermal protection blanket

When freezers are not economically feasible, insulated pallet covers are a practical secondary protection measure. These covers are typically made of multi-layered aluminum foil-bubble-aluminum foil (FBF) composite material, which reflects solar radiation heat, traps cool air around the pallet, and significantly slows the rate at which the internal temperature of the pallet rises. Studies have shown that high-quality thermal protection pallet covers can reduce peak internal temperatures of the pallet by 8–15°C. It is recommended to use desiccants in conjunction with the cover to address moisture issues. During installation, ensure the bottom of the pallet cover is sealed, leaving no gaps that allow hot air convection to enter.

3. Requirement for stowage within the ship's hold (loading below deck).

When placing an order with a freight forwarder or shipping company, explicitly request in writing that the containers be **"stowed in the hold"**. Containers stowed on the top deck of the ship are exposed to direct sunlight throughout the entire voyage without any shade or thermal buffer. Stowing in the hold places the containers inside the hull, where the natural heat dissipation from the hull's steel structure in contact with seawater effectively regulates the temperature inside the hold. Although shipping companies cannot always guarantee stowage positions, making this request in writing provides documentary evidence for claims in case of heat damage to the cargo-especially if it is later determined that the containers were placed on deck.

4. Strengthen the structure of inner and outer packaging.

Strengthening the internal structure of the container can directly reduce compressive damage under stacking pressure after the bottle softens. Specific measures include:

• Corrugated cardboard compartments/honeycomb dividers: Independent compartments prevent the bottles from tilting and squeezing together after softening.

• Sturdy outer box: Using double-walled or triple-walled cardboard boxes instead of single-walled boxes ensures the overall shape of the box is maintained even if the bottles inside are slightly deformed.

• Reduce pallet stacking: For PET bottle shipments to tropical markets, it is recommended to reduce the number of layers from the standard 5-6 layers to 3-4 layers, directly reducing the pressure on the bottom bottles.

• Corner guards and top pallet cover: Reinforce the top and four corners of the pallet to distribute pressure evenly and prevent collapse caused by dynamic loads during transport.

• Interlayer rigid cardboard dividers: Place a rigid cardboard divider every two layers of carton to distribute the vertical load across the entire surface, rather than concentrating it on the bottleneck area.

5. Conduct thermal stress simulation tests before shipment.

Before officially exporting new PET packaging SKUs to tropical markets, it is recommended to conduct thermal stress simulation testing in a controlled environment: place the filled and sealed finished bottles in an oven or climate test chamber and apply simulated stacking loads at 60°C for 24–48 hours. Check for neck deformation, cap torque retention, label adhesion, and bottle bottom stability. This test is extremely low-cost but effectively prevents large-volume shipment losses. Any SKU that fails this test should undergo material replacement or structural redesign before being approved for export to tropical markets.