Scientific selection strategy for thickness and applicable temperature range of polyolefin shrink film

I. Overview of material properties and shrinkage mechanism of polyolefin shrink film
Polyolefin shrink film is a functional packaging material with heat shrinkage performance prepared by extrusion blow molding, cast film or biaxial stretching process with polyethylene (PE), polypropylene (PP) or their copolymers as the main raw materials. Its shrinkage principle originates from the oriented structure formed by the polymer molecular chain during the melting process. When the temperature rises to a specific range, the oriented molecular chain restores the disordered state through thermal motion, thereby generating shrinkage force.
The molecular structure and crystallinity of polyolefin materials directly affect the shrinkage performance. PE shrink film has good flexibility and low-temperature shrinkage characteristics, and its melting temperature (Tm) is usually 105-135℃, while PP shrink film has higher crystallinity, and the melting temperature can reach 160-175℃, with stronger mechanical strength and high temperature resistance. The thickness of the shrink film is usually in the range of 15-200μm. At different thicknesses, the thermal conductivity efficiency, molecular chain orientation density and crystallization behavior of the material are significantly different, which in turn determines its applicable temperature range.

II. The mechanism of the influence of thickness on the physical properties of polyolefin shrink film
1. The correlation between thickness and heat conduction efficiency
The increase in the thickness of the shrink film will lead to a decrease in the heat conduction rate. Taking PE shrink film as an example, the heat conduction time constant of the ultra-thin film layer of 15-30μm is about 0.2-0.5s/mm, while the value of the thick film above 100μm can be increased to 1.5-3s/mm. This is because when the thinner film layer is heated, the heat can penetrate into the material faster, so that the molecular chain quickly reaches the activation energy threshold for orientation recovery; while the thick film requires a higher external temperature or a longer heating time to compensate for the energy loss during heat conduction.

2. Thickness regulation of molecular orientation and crystal structure
The film thickness affects the degree of stretching orientation during processing. In the biaxial stretching process, thinner film layers (such as 20-50μm) are easy to obtain higher orientation, and their glass transition temperature (Tg) shifts to low temperature by about 5-10℃, and the shrinkage start temperature decreases accordingly; while thick films (such as 80-200μm) have decreased orientation uniformity during stretching, and the number of interfaces between the crystalline and amorphous regions increases, resulting in a wider shrinkage temperature range and a shift of the shrinkage peak temperature to high temperature.

3. Synergistic effect of thickness and mechanical strength
Increasing thickness will significantly improve the tensile strength and puncture resistance of the film, but it will also reduce its shrinkage rate and shrinkage stress. For example, the longitudinal shrinkage rate of a 30μm PE shrink film at 120℃ can reach 45%-55%, and the shrinkage stress is about 1.2-1.8MPa; while the shrinkage rate of a 100μm film layer of the same material at the same temperature is reduced to 30%-40%, and the shrinkage stress increases to 2.5-3.5MPa. This difference in mechanical properties directly affects the temperature selection - thick films require higher temperatures to overcome intermolecular forces and achieve effective shrinkage.

III.Principles for selecting temperature ranges for polyolefin shrink films of different thicknesses
1. Ultra-thin shrink film (15-30μm)
Material type: mainly low-density polyethylene (LDPE) or linear low-density polyethylene (LLDPE), some containing EVA copolymers.
Thickness range: 15-30μm
Applicable temperature range: 60-110℃
Selection basis:
The film in this thickness range has low heat capacity and high molecular chain orientation, and can start shrinking in the low temperature zone. For example, a 20μm LLDPE shrink film starts to shrink slightly at 60-70℃, and the shrinkage rate can reach 30%-40% at 80-100℃, which is suitable for packaging of temperature-sensitive items (such as fresh fruits and vegetables, chocolate). The upper temperature limit is controlled within 110℃ to avoid melting and rupture of the film layer due to overheating, and to prevent deformation of the packaging caused by excessive shrinkage stress.

2. Thin shrink film (30-60μm)
Material type: LDPE/LLDPE blend system, or modified PE with metallocene catalyst added.
Thickness range: 30-60μm
Applicable temperature range: 80-130℃

Selection basis:
This thickness film has both shrinkage performance and operational adaptability. Taking 50μm metallocene PE shrink film as an example, it begins to shrink significantly at 80-90℃, and the shrinkage rate can reach 45%-50% at 120-130℃, and the shrinkage stress is stable at 1.5-2.0MPa. This temperature range is suitable for conventional food packaging (such as bottled beverages and boxed snacks). It can achieve rapid shrinkage through medium-temperature heating and avoid the impact of high temperature on the quality of the packaging contents.

3. Medium-thickness shrink film (60-100μm)
Material type: high-density polyethylene (HDPE), copolymer polypropylene (CPP) or PE/PP blend system.
Thickness range: 60-100μm
Applicable temperature range: 100-150℃

Selection basis:
The thermal conductivity of thick film layer is reduced, and the heating temperature needs to be increased to ensure the full movement of internal molecular chains. For example, 80μm CPP shrink film starts to shrink at 100-110℃, reaches the maximum shrinkage rate (about 35%-40%) at 140-150℃, and the shrinkage stress can reach more than 2.5MPa, which is suitable for packaging of heavy industrial products (such as barreled chemical raw materials and pallet cargo bundling). The lower temperature limit is set to 100℃ to break through the glass transition temperature of PP (about 90℃), and the upper limit is controlled within 150℃ to prevent thermal oxidation degradation of PP.

4. Thick shrink film (100-200μm)
Material type: high crystallinity HDPE, homopolymer polypropylene (HPP) or multi-layer co-extrusion structure (such as PE/PP/PE).
Thickness range: 100-200μm
Applicable temperature range: 120-180℃
Selection basis:
This type of film is mainly used for heavy packaging or scenes that require high temperature resistance. Taking 150μm HPP shrink film as an example, its melting temperature is about 165℃, and it needs to start shrinking above 120-130℃. The shrinkage rate can reach 30%-35% at 160-180℃, and the shrinkage stress exceeds 3.0MPa, which can meet the fastening packaging needs of heavy objects such as metal products and building materials. Temperature control must strictly follow the melting characteristics of the material - incomplete shrinkage below 120℃, and above 180℃ may cause the film layer to melt and flow or produce toxic volatiles.

IV. Application scenario adaptation strategy for temperature selection
1. Food packaging field
Low temperature scenario (≤100℃): select 30-60μm PE shrink film, such as the shrinking temperature of cold drink bottle label is controlled at 80-95℃ to avoid the influence of high temperature on the taste of beverage; fresh meat vacuum packaging film uses 50μm PE/EVA blended film, which shrinks at 90-100℃ to maintain the sealing of the package.
Medium temperature scenario (100-130℃): 60-80μm CPP shrink film is suitable for instant noodle bowl lid sealing, and the heating temperature of 120-130℃ can achieve good edge shrinkage and heat sealing strength.

2. Industrial packaging field
Conventional heavy object packaging: 80-100μm HDPE shrink film is used with 130-150℃ heating for corrugated box cluster packaging, which can provide sufficient shrinkage stress to fix the goods and resist friction and impact during transportation.
High temperature resistant scenario: 150-200μm HPP shrink film shrinks at 160-180℃, which is suitable for the outer packaging of high temperature sterilized medical devices. Its heat resistance can withstand 121℃ steam sterilization without failure.

3. Special function packaging
Shrink label: 20-30μm PETG/PE composite film shrinks at 70-90℃, which is suitable for labeling of special-shaped bottles to avoid the deformation of the bottle body (such as PVC bottles) caused by high temperature.
Shockproof cushioning packaging: 100-150μm cross-linked PE shrink film shrinks by heating at 140-160℃, which can form an elastic buffer layer around the electronic product and absorb the impact energy by using the toughness of the thick film.

V. Key factors affecting temperature selection and optimization direction
Adaptability of heating method:
Hot air heating: good temperature uniformity, suitable for the full thickness range of 60-180℃, thick film needs to extend the heating time (such as 100μm film heating time at 150℃ is about 5-8s).
Infrared heating: Short-wave infrared has stronger penetration into thick films. Films above 100μm can be quickly heated at 140-180℃ (3-5s), but care should be taken to avoid local overheating.
Shrinkage equipment parameter coordination:
Temperature needs to be adjusted in conjunction with conveying speed and heating zone length. For example, the conveying speed of 30μm film at 100℃ can be set to 10-15m/min, while the conveying speed of 100μm film at 150℃ needs to be reduced to 5-8m/min to ensure sufficient heat conduction.

Future technology trends:
The thermal conductivity efficiency of polyolefin films can be improved through nanocomposite modification (such as adding montmorillonite and graphene), which can shift the applicable temperature range of thick films to the low temperature zone by 10-20℃ while maintaining shrinkage performance, which provides a development direction for energy-saving packaging processes.
The selection of the thickness and applicable temperature range of polyolefin shrink film is a system engineering based on the thermophysical properties of the material, processing technology and application requirements. The temperature range of ultra-thin to thick film layers extends from 60℃ to 180℃. Its core logic is to adjust the molecular orientation and thermal conductivity efficiency through thickness to match the temperature requirements of different scenarios. In practical applications, it is necessary to combine the characteristics of the packaged items, the parameters of the heating equipment and the shrinkage performance indicators, and optimize the temperature selection to achieve a balance between efficiency and cost while ensuring the packaging effect.