Definition and basic concept of heat sealing strength
Heat sealing strength refers to the size of the force required to peel off two or more layers of thermoplastic materials under certain temperature, pressure and time conditions, and is the core index for evaluating the quality of heat sealing and sealing reliability. It reflects the ability of the heat-sealed area to resist external forces and separate, usually expressed as a force value per unit width, in Newtons per millimeter or Newtons per centimeter. Heat seal strength is widely used in the flexible packaging industry, including food packaging, pharmaceutical packaging, daily chemical packaging, and other fields, and is a key parameter to ensure that the packaging contents are not leaked or contaminated during storage, transportation, and sales. The higher the heat sealing strength, the better, and the appropriate range needs to be determined based on factors such as packaging material characteristics, content requirements, and opening convenience.
Physical principles of heat seal strength
The formation of heat seal strength is essentially the process of physical state change and interfacial molecular chain diffusion and entanglement of thermoplastic materials under the action of heat and pressure. When the two layers of thermoplastic material are heated above their melting temperature or viscous flow temperature, the molecular chain segments on the surface of the material gain enough energy to start moving, and the molecular chains at the interface diffuse, penetrate and entangle with each other to form an interface layer. During the subsequent cooling process, the molecular chain's ability to move decreases, eventually forming a bonding layer with some strength at the interface.
From the perspective of polymer physics, the heat sealing process involves several interrelated physical stages. The first is the heat transfer stage, where heat is transferred from the heating element through the packaging material to the interface to be sealed, so that the interface temperature rises to the sealing temperature. The temperature distribution is affected by the material's thermal conductivity, thickness, and heating time. The second is the wetting and diffusion stage, when the interface temperature reaches above the melting temperature of the material, the surface of the material melts, and the molecular chain segments diffuse each other across the original interface. The depth and degree of diffusion depend on temperature, time, and material compatibility, following the reptation model or more complex molecular dynamics model. The third is the entanglement and homogenization stage, where the diffused molecular chains form a new entanglement network in the interface area, making the original interface gradually blurred and forming an interface layer with a certain cohesive strength. Finally, there is the cooling and setting stage, where as the temperature decreases, the molecular chain activity decreases and the interface structure is frozen, forming the final heat seal strength.
The formation of heat seal strength can be described by a simplified diffusion model:
σs ∝ t1/4 · exp(-Ea/RT)
In the formula, σsIt represents heat sealing strength, t represents heat sealing time, and EaIt represents the diffusion activation energy, R represents the gas constant, and T represents the heat sealing temperature. This relationship shows that the heat sealing strength increases with the increase of heat sealing time, but the growth rate gradually slows down. It increases exponentially with the increase of heat sealing temperature, but too high a temperature may cause the material to degrade or collapse.
The measurement of heat seal strength is based on the peel test principle. The heat-sealed specimen is peeled at the specified angle and speed, the force value during the peeling process is recorded, and the average peeling force per unit width is calculated. Its basic calculation formula is:
Ts = F / b
In the formula, TsIt represents the heat sealing strength, F represents the average force value during the peeling process, and b represents the width of the specimen. In the actual test, the average value of the stable peeling stage is usually taken as F, and the rationality of the result is judged according to the failure mode.
From the perspective of fracture mechanics, the failure of the heat sealing area can be divided into three modes: interface failure, cohesive failure and substrate failure. The interface failure occurs on the original interface, reflecting the insufficient bonding force of the heat sealing interface. Cohesive failure occurs inside the heat sealing layer, reflecting that the cohesion strength of the heat sealing layer is lower than the interfacial bonding force. Substrate failure occurs on the substrate outside the heat sealing area, reflecting that the heat sealing strength has exceeded the strength of the substrate itself, and the measured value is actually the substrate strength rather than the heat sealing strength. The ideal heat seal should be cohesive failure or substrate failure, indicating a strong bond at the heat sealing interface.
Measurement method of heat seal strength
Laboratory measurements of heat seal strength follow standardized testing procedures, mainly referring to GB/T 2358, ASTM F88, QB/T 2358 and other standards according to different material types and application fields. The measurement process includes sample preparation, heat sealing condition setting, heat sealing operation, state adjustment, tensile testing, and result processing.
Sample preparation is the basis for reliable results. Cut the specified size of the sample strip from the packaging material to be tested, usually 15 mm or 25 mm wide, long enough for heat sealing and subsequent testing, generally not less than 150 mm. When sampling, attention should be paid to directionality, and for oriented materials such as bidirectional stretch film, the longitudinal and transverse directions should be clarified and marked on the specimen. At least 5 parallel specimens were prepared for each combination of heat sealing conditions to ensure statistical reliability. The edge of the specimen should be flat, without creases and no pollution to avoid the introduction of artificial defects affecting the heat seal quality.
The choice of heat sealing conditions directly affects the heat sealing quality and needs to be determined according to the material characteristics and standard requirements. The heat sealing temperature is the core parameter, which is usually set in a certain range above the melting temperature of the material, and the appropriate temperature range is determined by temperature gradient test. The heat seal pressure affects the tightness of the interface contact, too low pressure may lead to poor interface contact, too high pressure may cause excessive deformation of the material or extruded of the melt. The heat sealing time determines the degree of heat transfer and molecular diffusion, and if the time is too short, the heat sealing is incomplete, and the time is too long may cause thermal degradation of the material. In actual testing, two parameters are usually fixed, one parameter is changed, and the heat sealing strength is determined by studying the change law of the parameterSuitableProcess window.
The heat sealing operation is carried out on the heat sealing testing machine. Align and stack the two layers of specimens, place them between the heat sealing heads, set the temperature, pressure and time before starting the heat sealing program. The heat sealing head should be parallel and the surface should be flat to ensure uniform pressure distribution. After the heat sealing is completed, the specimen is taken out to cool naturally or cooled in the prescribed manner. For multi-layer composite films, attention should be paid to the selection of heat covers, usually choose the surfaces with heat sealing layers for heat sealing. After heat sealing, the specimen should have no bubbles, no melting, no offset, and the width of the sealing area should be uniform and consistent.
State adjustment is a necessary step to ensure the comparability of test results. The heat-sealed specimen needs to be placed in a standard temperature and humidity environment for a certain period of time, usually more than 24 hours, so that the specimen can reach a moisture and stress balance state, and eliminate the influence of residual stress and ambient humidity after heat sealing. The standard environmental conditions are generally a temperature of 23±2 degrees Celsius and a relative humidity of 50%±5%.
Tensile testing is performed on a tensile testing machine. The state-adjusted specimen is installed on the fixture, so that the unsealed ends are clamped into the upper and lower fixtures respectively, and the sealing area is located in the middle of the two fixtures. The initial distance of the fixture should ensure that the sealing area is free from additional stress. Stretch at a constant speed, usually set at 300 mm per minute or as specified by the standard, until the sealing area is completely separated. Record the force-displacement curve during the stripping process. For the 180-degree peel test, the two ends of the specimen should be bent in opposite directions and clamped; For 90-degree peeling or T-peeling, the corresponding clamp and installation method are used.
Data processing includes reading of peel forces and calculation of heat seal strength. From the recorded force-displacement curves, the initial peak area and the final area are identified and excluded, and the average value is taken as the average peel force in the stable peeling area. For curves with large fluctuations, the method of integral averaging can be used. The heat sealing strength is calculated according to the formula, and the result is adjusted to the specified number of digits. Failure modes such as interface failure, cohesive failure, or substrate failure are also recorded, which is important for analyzing heat seal quality issues. Multiple specimens are tested under the same condition, and their arithmetic mean and standard deviation are calculated as the heat sealing strength value under that condition.
The relationship between heat sealing strength and heat sealing temperature usually shows S-shaped characteristics, which can be divided into three stages: low strength in the low temperature region, rapid rise in the medium temperature region, and plateau or decline in the high temperature region. Through this curve, the material can be determinedSuitableHeat sealing temperature range and heat sealing process window. The relationship between heat sealing strength and heat sealing pressure and time usually shows a stable trend after rising, and the process parameters can be optimized accordingly.
Key factors that affect heat seal strength measurements
The measurement results of heat sealing strength are influenced by a combination of factors, from material properties to heat sealing conditions, from sample preparation to test parameters, each of which can have a significant impact on the final result.
The properties of the material itself are intrinsic factors that affect the heat seal strength. The chemical structure of heat-sealing resin determines its melting temperature, melt flow rate, and molecular diffusion capacity. The heat sealing behavior of different materials such as polyethylene, polypropylene, ethylene-vinyl acetate copolymer, and ionic resin was significantly different. The density and crystallinity of the resin affect the melting temperature and heat sealing window, with low-density polyethylene having a wider heat sealing window and high-density polyethylene having a narrower heat sealing window. The melt flow rate of the resin reflects the fluidity in the molten state, and an appropriately high fluidity is conducive to molecular diffusion and interfacial binding, but too high may lead to melting and thinning of the heat-sealed area. In the structural design of multi-layer composite films, resins with excellent heat sealing properties are usually selected for the heat sealing layer, while the outer layer provides mechanical strength and barrier properties, and the matching between the layers affects the overall heat sealing performance.
The setting of heat sealing conditions directly determines the quality of heat sealing. The heat sealing temperature is the most sensitive parameter, when the temperature is too low, the material is not fully melted, the molecular diffusion is insufficient, and the interface bonding is weak. Thermal degradation of the material may occur when the temperature is too high, or excessive extrusion of the melt will cause the heat seal area to thin and reduce strength. The heat sealing pressure affects the tightness of the interface contact, and when the pressure is too low, the two layers of materials cannot fully contact, and there are microscopic voids. Excessive pressure can cause excessive deformation of the material and even damage to the film structure. The heat sealing time determines the degree of heat transfer and molecular diffusion, and if the time is too short, the heat sealing is incomplete, and the time is too long may cause changes in the structure and properties of the material. There is a coupling relationship between these three parameters, which needs to be comprehensively optimized according to the material properties.
The condition of the heat sealing equipment is as critical as the accuracy of its operation. The parallelism and surface flatness of the heat sealing head directly affect the pressure distribution, and the non-parallel heat sealing head will lead to insufficient or excessive local pressure, resulting in uneven heat sealing strength. The uniformity of the temperature distribution of the heat sealing head affects the consistency of the sealing area, and the edge temperature is usually lower than the center temperature, which needs to be calibrated and compensated to ensure the accuracy of the set temperature. The heat seal pressure is applied in a way that is pneumatic, hydraulic, or servo controlled, affecting the stability and repeatability of the pressure. The control accuracy of heat sealing time determines the reproducibility of the heat sealing process, especially for the rapid heat sealing process.
The standardization of sample preparation has an important impact on the results. The quality of the edge when the specimen is cut directly affects the stress distribution, and the presence of chips or burrs at the edges will become a source of stress concentration when stretched, leading to premature failure. The cleanliness of the specimen surface affects the binding of the heat-sealed interface, and contaminants such as oil, dust, and anti-adhesion agents can hinder direct contact and diffusion of molecular chains. The thickness uniformity of the specimen affects heat transfer and pressure distribution, and films with uneven thickness may be partially unsealed or oversealed during heat sealing. The pretreatment of the specimen before heat sealing, such as static elimination and preheating, also has a certain impact on the quality of heat sealing.
The control of state conditioning conditions cannot be ignored. The change of temperature and humidity affects the moisture content and internal stress distribution of the film, which in turn affects the mechanical behavior of the heat sealing area. When the adjustment time is insufficient, the specimen does not reach the equilibrium state, and the stability of the test results is poor. Especially for hygroscopic materials such as nylon and ethylene-vinyl alcohol copolymers, the moisture content has a significant impact on the heat sealing performance, and the adjustment conditions need to be strictly controlled and the test must be completed within the specified time.
The choice of test parameters directly affects the measured values. For viscoelastic materials, the increase in speed usually increases the peel strength, and the uniform speed needs to be specified according to the standard to ensure the comparability of the results. The clamping state and peeling angle of the fixture affect the stress distribution, and deviating from the specified angle will deviate the result from the true value. The specimen should be installed in the fixture to ensure that it is centered and there is no torsion to avoid additional stress during the stretching process. The frequency of data acquisition and the choice of analysis method affect the accuracy of the average peel force, especially for highly fluctuating peel curves.
The role of environmental factors cannot be ignored equally. Changes in temperature and humidity in the test environment can affect the mechanical properties and viscoelastic behavior of the material, and should be tested under standard environmental conditions. For packaging that needs special treatment after heat sealing, such as high-temperature sterilization and frozen storage, it is also necessary to consider the impact of these subsequent treatments on the heat seal strength, and conduct simulation tests under corresponding conditions.
The experience and standardization of the operator are the basis for ensuring the reliability of the results. The setting and calibration of heat sealing parameters, the proficiency of heat sealing operations, the identification and handling of abnormal conditions, and the correct judgment of failure modes all require standardized training and rich practice accumulation.
Application of heat sealing strength in the industrial field
Heat seal strength testing has a wide range of application value in many industrial fields, and is an important technical means for packaging material development, process optimization, quality control and product design.
In the flexible packaging industry, heat sealing strength is the core indicator for evaluating packaging sealing performance. The sealing strength of flexible food packaging such as instant noodle bags, snack bags, frozen food bags, etc., is directly related to the freshness and leakage prevention of the contents. Optimize material structure design and heat sealing process parameters through heat sealing strength testing to ensure that the packaging is securely sealed on the high-speed packaging line and does not crack during transportation and sales. Retort bags, high-temperature sterilization bags, etc. need to withstand high temperature and high pressure treatment, and the change of heat sealing strength before and after heat treatment is an important basis for evaluating heat resistance. Inflatable packaging and vacuum packaging have higher requirements for the airtightness of the seal, and the heat sealing strength needs to be considered in conjunction with the barrier performance. Liquid packaging such as soy sauce bags, laundry detergent bags, etc., needs to withstand liquid impact and static pressure, and the requirements for heat sealing strength are more stringent.
In the field of pharmaceutical packaging, heat seal strength is directly related to drug safety and expiration date. In pharmaceutical blister packaging, the heat sealing strength of aluminum foil and PVC or PP substrates determines the moisture protection and child safety of the drug. Medical device packaging, such as surgical instrument bags and syringe packaging, needs to ensure the integrity of the sterile barrier, and the heat seal strength needs to meet the requirements of sterilization treatment and long-term storage. For packaging that is in direct contact with the drug solution, the heat seal strength is not only related to the sealing, but also needs to consider the compatibility with the drug solution and the safety of the patient. Pharmaceutical packaging regulations such as GMP and ISO 11607 clearly stipulate heat seal strength, and pharmaceutical companies need to regularly verify heat seal strength to ensure that the packaging process meets the specification requirements.
In the field of daily chemical packaging, heat seal strength affects product experience and shelf performance. The sealing of shampoo and shower gel bagged products needs to withstand the impact of crushing and dropping during transportation, and at the same time is easy for consumers to tear and use, and the heat seal strength needs to balance firmness and ease of opening. The sealing of the tail of the toothpaste tube and the sealing of cosmetic samples, the heat sealing strength determines the sealing reliability of the product during the shelf life. The sealing of wet wipes packaging needs to be opened and closed repeatedly, and the heat sealing strength and repeated sealing performance together constitute the evaluation system. Daily chemical enterprises screen packaging material suppliers through heat sealing strength testing, optimize packaging design and filling processes, and ensure stable product quality.
In the field of material research and development, heat sealing strength is an important parameter for evaluating new heat sealing materials and optimizing formulations. When developing new heat sealing resins, resin suppliers systematically test the heat sealing strength of resins with different comonomer content and different molecular weight distributions, establish the relationship between heat sealing performance and molecular structure, and guide product design. In the research and development of multilayer co-extruded films, the heat sealing strength under different heat sealing layer thicknesses and core layer materials is tested to optimize the interlayer ratio and structural design. The development of functional packaging materials such as easy-to-remove film, antistatic film, high barrier film, etc., the balance between heat sealing strength and functionality is a key issue. In the research and development of degradable packaging materials, the coordination of heat seal strength and biodegradability is a technical difficulty, and it is necessary to optimize the formulation and process through a large number of tests.
In the field of packaging machinery and automation, heat seal strength data is used for the commissioning and validation of packaging machines. The heat sealing system of different types of equipment, such as continuous packaging machines, rotary packaging machines, and vacuum packaging machines, has different designs and different adaptability to materials. Evaluate the heat sealing performance of packaging materials on specific equipment through heat seal strength testing, optimize equipment parameters such as heat sealing temperature, pressure, and time to ensure packaging quality and production efficiency. The development of online monitoring technology for heat sealing strength provides a means for real-time quality control of the packaging process. The development of new packaging forms such as stand-up bags, nozzle bags, special-shaped bags, etc., needs to verify the rationality of the sealing design through heat seal strength testing.
In the field of quality supervision and product inspection, heat seal strength testing is an important means to ensure that packaged products meet standards and specifications. Packaging material suppliers need to provide heat seal strength test reports to prove that the products meet the requirements of the purchase contract. Food and pharmaceutical manufacturers regard heat sealing strength as a key item in incoming material inspection and process control to ensure the stability of packaging materials and packaging processes. Third-party testing institutions conduct heat seal strength tests according to national or international standards to provide a technical basis for market supervision and quality disputes. In the inspection of import and export commodities, heat sealing strength is a routine test item used to verify whether imported packaging materials meet the requirements of China's standards and trade contracts.
In failure analysis and quality improvement, heat seal strength testing is an effective tool for finding the root cause of problems. When the packaging product has quality problems such as leakage and bag bursting, by testing the heat sealing strength of the suspicious batch and comparing it with normal samples, it is determined whether it is a problem caused by fluctuations in the heat sealing process, degradation of material properties or equipment failure. The combined analysis of heat sealing strength and failure mode can further clarify the cause of failure, weakened cohesion of heat sealing layer or decreased strength of the substrate, and point out the direction for improvement measures. Through the systematic test of heat sealing strength and heat sealing temperature, pressure and time, the process window is optimized and the stability and fault tolerance of packaging quality are improved.
Summary and outlook
As the core index for evaluating heat sealing quality and sealing reliability, heat sealing strength reveals the internal law of interfacial bonding formed by thermoplastic materials under the action of heat and pressure from the physical nature of polymer diffusion and interfacial bonding. Through standardized heat seal testing and peel test methods, heat seal strength is quantified as a foundational data for process optimization and quality control. From material properties to heat sealing conditions, from sample preparation to test parameters, the combined influence of many factors requires testers to have a deep theoretical foundation and rigorous practical skills. In a wide range of fields such as flexible packaging, pharmaceutical packaging, daily chemical packaging, material research and development, packaging machinery and quality supervision, heat sealing strength testing has become an important technical means to ensure packaging integrity, optimize packaging processes, and develop new materials.
Looking ahead, heat sealing strength testing technology is moving towards higher accuracy, more modes, and deeper mechanisms. The high-precision heat sealing testing machine is equipped with a closed-loop control system for temperature and pressure, which realizes the accurate setting and real-time monitoring of heat sealing parameters, and improves the stability and reproducibility of the heat sealing process. The thermal imaging analysis technology of the heat sealing process can monitor the temperature distribution of the heat sealing area in real time, which provides an experimental basis for understanding the heat transfer law and optimizing the design of the heat sealing head. Multi-mode heat sealing strength tests, such as tests at different peel angles, different peeling speeds, and different environmental conditions, can more comprehensively evaluate the performance of heat sealing areas under different service conditions.
The research on heat sealing mechanism continues to deepen. The application of molecular simulation technology in the study of heat sealing process reveals the diffusion behavior and interface structure evolution between different materials from the molecular level, and provides theoretical guidance for the design of new heat sealing materials. The rheological analysis of the heat sealing process combines the melt flow with the interface, which provides a scientific basis for the optimization of the heat sealing process. The microstructure characterization of the heat sealing area, such as interface layer thickness, crystal morphology, orientation distribution, etc., is correlated with the macroscopic heat sealing strength, promoting the transformation from empirical process to scientific design.
With the development of sustainable packaging, new environmentally friendly materials pose new challenges to heat sealing performance. The heat sealing behavior of biodegradable plastics such as PLA, PBAT, PHA, etc. is different from traditional polyolefin materials, with higher sensitivity to temperature and time, and a narrower heat sealing window, requiring the development of specialized heat sealing testing and evaluation methods. The heat sealing technology of paper packaging has developed rapidly, and new technologies such as paper-plastic composite and all-paper heat sealing have put forward new requirements for heat sealing strength testing. The promotion of single-material recyclable packaging needs to meet the recycling requirements while maintaining heat sealing performance, and put forward higher standards for the material selection and structural design of the heat sealing layer.
The rise of intelligent packaging proposes new application scenarios for heat sealing strength. The integration of electronic components such as smart tags and RFID antennas with the heat sealing of packaging needs to consider the impact of the heat sealing process on electronic components and the impact of components on the quality of heat sealing. The introduction of functional components in active packaging and intelligent packaging may change the local heat sealing performance, and it is necessary to develop micro-area heat sealing strength testing technology. The correlation between heat sealing strength and other packaging properties such as barrier, chemical resistance, impact resistance, etc. is increasingly deepened, and the establishment of a comprehensive evaluation system for packaging performance is promoted. It is foreseeable that the classic test index of heat sealing strength will continue to play an irreplaceable role in packaging technology innovation and sustainable development.