The investigation of functions of heat seal strength and heat sealing testing method

Transcription

1 The investigation of functions of heat seal strength and heat sealing testing method Chana Yiangkamolsing (1), Kazuo Hishinuma (2) (1) Department of Industrial Engineering. The University of the Thai Chamber of Commerce 126/1 Vibhavadee-Randsit Rd. Huaykwang, Dindaeng, Bangkok, 125 Thailand (2) Hishinuma Consulting Engineering Office, 1232 Ogura Saiwai-Ku, Kawasaki city, Japan Abstract: Heat sealing is an important technique of bonding thermoplastic materials. However, only heat seal strength has been used to evaluate the bonding of thermoplastic material for a long time. The customers of the packaging products hope for both easiness of opening and completed seal. Heat sealing technology is the most economical for easy opening the package. Manufacturers could not serve customers requirements although they apply the conventional standards such as ASTM F88, F229, JIS Z 238. Indeed, the heating temperature at the interface of bonding affect mainly to heat seal strength. The objectives of this research are proposed testing method and analysis of the Openability and Closability of heat seal strength. The methodology used in this research is divided into 2 testing methods; the testing method of heat seal strength with accurate heating temperature, and the testing method of elements generating heat sealing. The first testing method is used for identify the accurate heat seal strength by regarding peculiar thermobonding performance. The second testing method is used to identify the close functions and comprehend the peculiar characteristics of materials. With this proposed methodology, it is possible to determine the closing function of heat sealing based on reliable characteristics of material. The analysis of the peeling energy of adhesive bonding of heat sealing will be discussed for Closability in this research. Functional Heat Seal Strength (FHSS) is introduced to determine the generation of pinhole and polyballs by observation the failure of heat seal testing. Experiments in this research are conducted by using propose innovative testing method with MTMS (The Measurement of temperature at melting surface). PE/7 material widely used in local market is used in the experiment. Also, this research challenge evaluation method for Closability of heat sealing of thermoplastic materials. The result shows that Peel seal can guarantee closing ability of material for packaging. Keywords: Heat sealing; Temperature at the interface; Peel seal, Tear seal; heat seal strength; MTMS; Openability; Closability 1. Introduction 1.1. Background of heat sealing technique Heat seal strength is used to measure the bonding strength of thermoplastic material. It is a result of bonding phenomena. Conventionally, there are two standards related to measurement of heat seal strength. ASTM F88-7 [1] and JIS Z 238 [2] are widely used in manufacturers all over the world. ASTM F88-7 defined about heat sealing function as following; Seal strength is a quantitative measure for use in process validation, process control, and capability. Seal strength is not only relevant to opening force and package integrity, but to measuring the package processes ability to produce consistent seals. Seal strength at some minimum level is a necessary package requirement, and at times it is desirable to limit the strength of the seal to facilitate opening For controlling heat seal strength, temperature is confirmed as the most important factor for controlling heat seal strength. A graph of heat seal strength characteristics corresponding with various temperatures is widely used by industry and research shown in figure 1. High reliable heat sealing characteristic is important to analyze the coexistence of Openability and Closablity of flexible packaging. Many customers want to open easily flexible package by pinching, and peeling method. Some people feel irritate and think pinching method is not good then do slapping. Many packages are applied strong bonding so they need to open the package by pinching and tearing method. However, the package is very difficult to reseal for the next consumption. It makes the small pieces of garbage also. Expectation of customers for thermoplastics packaging are such as 1)Strong bonding, 17

2 2)Control of the generation of the pinhole, 3)Easy peel, 4)Easy open, and 5)Easy cut. (1) and (2) provide guarantee of the closing function. (3), (4), and (5) can provide convenience but they are excluded from this research because they need other material and other processes. This research focuses on (1) and (2). However, a reasonably completed corrective strategy has not yet presented. First of all, it reports on logical development of Closability indispensable in this study Problem of heat seal technique in Thailand In Thailand, Many manufacturing apply strong bonding. Therefore the package is breaking at the edge of heat seal when customer using pinching and peeling method. Strong bonding leads to cohesion and it is overheat. Therefore the generation of pinhole and polyball occur at the edge of heat seal. Manufacturers solve these problems by increasing the material thickness. Consequently, increasing the thickness of material, packaging material is not economical. Also, the circle of problems is occurred repeatedly. The method to control of the generation of the pinhole can be applied by adhesion. The benefit of adhesion is that the packages can absorb the impact load, prevention of the poly ball and pinhole, also utilization of the peeling energy. [3] Heat sealing technology is the most economical for easy opening the packaging. The benefit of heat sealing technique is that people can peel the package easily and can reseal it again when the product is not finished and customers want to keep it for the next consumption. Also the surface of heat seal area is smooth and clean which is neater. Peeling the flexible packaging does not make small pieces of garbage. Moreover, material thickness can be decrease and then material cost decrease by using heat sealing technology. Tensile strength (N/15mm) Figure 1: Heat seal strength characteristic Tear sealing zone (cohesive bonding ) Peel sealing Zone (Adhesive bonding) Temperature 1.3. Problem of coexistence of open function and close function of heat sealing operation Figure 1 shows the heat steal strength and boundary temperature determines. The coexistence of open and close function of thermoplastic packaging is very The authors would like to gratefully thank the following organization, university, and companies: HISHINUMA Fund for heat sealing research support us as main sponsor. The University of the Thai Chamber of Commerce supports us for many contributions. The Thai Packaging Association (TPA) supported us with valuable information and access to member companies in Thailand. important for packaging. Many packages in the markets are strong bonding. It has been applied for the package for a long time. Peel seal zone can guarantee both opening./closing functions of material. Although easy opening is very important for customers, heat sealing stakeholders do not know basic information of material and how to control functions of material. Therefore, they could not guarantee the quality of heat sealing. The coexistence of heat sealing functions between open and close function of the package need to consider carefully but there is no standard mentioned about the method for such testing functions. The conventional standard is no indication about evaluation of packaging function between easy opening and intimate closing in the standards. Also, scientific testing for the completed product has not conducted. The stakeholders solve it by trial and error method Problems of current standard ASTM F229-, is a recommendation for preparing heating method.[4] This standard presented wide step of increasing heat sealing temperature. It is difficult to identify peel sealing zone and tear sealing zone with wide step of heating temperature. ASTM F229 recommends heating temperature step 1 C approximately. For an accurate graph of heat seal strength, temperatures should increase 1-2 C in each step in peel seal range for the accurate analysis. Hence, this research challenges evaluation technique for testing of Openability and Closeability of heat sealing Procedure of testing method for identification of expected function of heat sealing A novel testing method for open function and close function comprise of two sections as figure 2. Firstly, measurement method for special characteristics of thermo-bonding and tensile strength of material is used to determine basic information of material. It refers to figure 3. Item (1) and (5) is important and supported by the conventional standard. Secondly, bonding condition and testing method for the expected function is used to determine Openability and Closeability of the packaging material. It refers to figure 4. Item (1), (2), and (4) are important items. The requirements of high accuracy heating temperature and supporting techniques are to obtain accurate characteristics of material and to describe the specific characteristics of material. The procedure of testing method is described roughly as following; 1) Accuracy of the heating temperature, 2) Generation of temperature gradient of material in single side or double side heating method [5], 3) Generation of temperature distribution of heating material [6], 4) Heat seal area of materials of pressing, 5) Load before the cooling down preventing hot tack, 6) Shape of the tensile test sample. The heat conduction of item (2), (3) is not an object of the control, because it is the phenomenon which depends on the material characteristic. Item (1) and item (4)-(6) 18

3 are controlled objects, as it is related to the operation of the researcher. Accurate heating with the setting temperature is required for the measurement of the precise heat sealing strength. [7] Putting specimens across the heat jaw is avoided because heat transfer to ambient both sides of specimen cause to temperature at interface slightly decrease. This research applying putting the specimen on the heat seal jaw. The detection point of temperature sensor for measuring melting surface temperature is 2mm from the edge of material. [8] The setting of each heating temperature was decided 1-2 C in peel seal range. Specimens are immediately cooled after heated with pressure about.5 MPa to avoid hot tack. Specimens cut dumbbell shape to exclude the influence of the elongation. [9] Heating temperature was controlled by following method; [1] 1) Temperature of heat Jaws of heating unit is adjusted nearby the preset temperature., 2) A set value of the temperature controllers are adjusted respectively by using the surface temperature monitor so that the surface temperature of two jaws may become differ only within.1 C., 3) Response of melting surface temperature is measured by instrument of melting surface temperature., 4) When melting surface temperature of the attainment is different from a setting value, it is readjusted by the method (2)., 5) An obtained arrival time and isochronal heating is applied to the sample making of the tensile test. [heating time and 15s (PE) was used in the actual experiment.] With the proposed method, temperature at heating surface fluctuates slightly and uniformly, 6) Specimens after heated at prescribed time promptly did the contact cooling to the room temperature at.5mpa., 7) Pressure when heating it gave.35mpa. This pressure generates the "Polyball" in the range of the cohesion bonding. Testing method for important function [Open and Close] of heat sealing Measurement method for special characteristic of thermo-bonding and tensile strength of material [refer to Fig3] Bonding condition and testing method for the expected function (Open & Close) [refer to Fig 4] Figure 2: Testing methods for open and close function of heat sealing Measurement method for special characteristics of thermo-bonding and tensile strength of material Measurement method for special characteristics of thermo-bonding and seal strength measurement of each material 1)Standardization 2)Method for 3)Measurement 4)Manifestation 5)Testing 6)Setting of heating way preparing method for characteristic of method for heat pressure method samples melting surface heat sealing seal strength Supported by temperature Supported by ASTM F229 ASTM F88-xx JIS Z 238 Figure 3: Measurement method for special characteristics of thermo-bonding and seal strength measurement of each material Identification method of peculiar characteristic and functions of the material Testing method for function elements of heat sealing 1)Detection method of overheating 2)Testing method for peel sealing 3)Testing method for easy peel Figure 4: Identification method of special characteristic and functions of the material 3. Results and Discussions This section will be discussed about; 1) Definition of Peeling Energy and Computational method of Peeling Energy, 2) Definition of Functional heat seal strength 4)Testing method for peeling energy 5)Detecting method for volatile component 6)load test of actual package (FHSS) As can be seen from the pattern of the heat sealing, the difference of pattern of heat sealing depends on bonding condition. The bonding line of the cohesion bonding is broken when tensile load reaches to yield point. The integrated value to yield point is generally 19

4 called breaking energy. On the other hand, it peeled in adhesion that decided at the heating temperature. The patterns of seal strength of peel seal and tear seal are shown in Figure 5. Peeling energy has defined the integrated value of the peeling condition. [11] Peeling energy is calculated from seal strength corresponding to peeling length or elongation length. Basic idea of peeling energy calculation is integral the small area of seal strength pattern. The small area of heat seal strength depends upon analog to digital converter. Calculation of peel energy from seal strength pattern of both tear seal and peel seal is similar but the differences are the end point of calculation. For tear seal, it selects the position that the pinhole was generated in the heat sealant and the whole of the material cut. For peel seal, it needs to t L L 11

5 119 C breaking energy. Peeling energy for 7.5 mm peel, temperature at C, and for 1 mm peel, temperature at C are showed high value from the breaking energy of tear seal at 119 C. The generation temperature of pinhole begins at 118 C. Higher temperature more than 118 C lead the seal strength to Strong bonding (more than 118 C). By using FHSS to detect the generation of pinhole and polyball, we can detect this critical temperature. Applying Peeling seal zone ( C) can control the generation of the pinhole. Also, Utilization of the peeling energy for open function can be used in Peel seal zone. In contrary, tearing energy to detect the yield point can be used in tear seal. 4. Conclusion The expected function in the heat sealing is established to Closeability first. It was proven that the acquisition of accurate heat sealing characteristic of Peel and Tear sealing by precise heat sealing testing method was possible. The measurements of peeling energy and breaking energy were possible from the accurate heat sealing characteristic. The detection of generation point of the edge cutting (pinhole) changed possible the accurate heat sealing characteristic. By changing heat sealing strength by this discovery until now, the definition of Functional Heat Sealing Strength [FHSS] was proposed. The usefulness of the peeling seal would be able to be quantitatively evaluated, because the peeling energy can be measured in quantitative. It was proven by the results of experiments. If heat sealing width is more than 5 mm, peeling energy is more than breaking energy. It means that the damage resistance of the Peeling seal is more than the breaking seal. It can guarantee Closability quantitatively. In addition, it is indicated that the thickness of the heat sealant layer can be decreased, as the peeling seal is the adhesion only of the surface of the sealant. [13] The basis is Strength, as the Cloasability is evaluated by the seal strength testing ASTM F88 instead of user feeling. It is necessary to continuously improve the adhesive strength in order to satisfy this demand. The peeling seal zone must be utilized in order to achieve this purpose. As the method in which Openability and Closeability was compatible, it was proven that the utilization of peeling seal zone of the packaging material is indispensable. The advanced technique for the temperature control is required in comparison with demand function and heat sealing characteristic of packaging material. Heat seal strength (N/15mm Generation point of [FHHS] Specimen: PE bag (PE/7) 13 C 125 C 12 C 118 C C Elongation length or Peeling length (mm) 119 C Peel/tear energy x 1-3 J/15m ) m ) Peel seal zone Peeling 1 mm Peeling 7.5 mm Peeling 5 mm Tearing energy Tear seal zone Heat seal strength by ASTM F88 Melting surface temperature ( o C) Figure 6: Load-displacement diagram of PE film Figure 7: Response of heat seal strength and energy of PE material FHHS Heat seal strength (N/15mm 4mm Yield point 1.5mm Yield point 117 C 118 C 122 C Figure 8: Peeling, elongation and tear surface of PE specimen 111

6 References [1] ASTM F88-7 (27) (Standard Test Method for Seal Strength of Flexible Barrier Materials), ASTM International, West Conshohocken PA, United States [2] JIS Z238 (22) (Testing Methods for Heat Sealed Flexible Packaging), Standardization promotion Department, Japanese Standard Association [3] K. Hishinuma, (21), The mutual map of heat sealing testing method for high reliability V.6, Hishinuma Consulting, Kawasaki, Japan [4] ASTM F229 (2) (Standard Practices for Making Heat seals for Determination of Hestsealability of Flexible Webs as Measured by Seal Strength), ASTM International, West Conshohocken PA, United States [5] K. Hishinuma,(29), Heat Sealing Technology and Engineering: Inc.pp.51 [6] K. Hishinuma,(29), Heat Sealing Technology and Engineering: Inc.pp.29 [7] K. Hishinuma,(29), Heat Sealing Technology and Engineering: Inc.pp.97 [8] K. Hishinuma, (29), Heat Sealing Technology and Engineering: Inc.pp [9] K. Hishinuma,(29), Heat Sealing Technology and Engineering: Inc.pp.23 [1] K. Hishinuma,(29), Heat Sealing Technology and Engineering: Inc.pp.96 [11] K. Hishinuma,(29), Heat Sealing Technology and Engineering: Inc.pp.116 [12] K. Hishinuma,(29), Heat Sealing Technology and Engineering: Inc.pp.13 [13] K. Hishinuma,(29), Heat Sealing Technology and Engineering: Inc.pp