Abstract:
A method of forming a composite seal structure having great resistance to rupture, rare generation of pinholes is provided which comprises using a heat sealer having two blocks which presses sheets to be heat-sealed and at least one of which is a heating block provided with a seat which renders its surface at a temperature forming peel seal, on a part of its surface pressing the sheets, heating the heating block at a temperature forming tear seat, and pressing the sheets to form a heat seal having the composite seal structure of peel seal and tear seal.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a method of forming a composite seal structure of peel seal and tear seal. 
       BACKGROUND OF THE INVENTION 
       [0002]    Generally, heat sealing is applied to the production and sealing of bags and containers using plastic films or sheets. 
         [0003]    In conventional heat sealing, heating is carried out at a high temperature region where thermal adhesive layer is melted, and bonded area becomes tear seal uniformly. 
         [0004]    As a type of heat seal, peel seal is also known as well as tear seal. The inventor has investigated as to peel seal, patent applications were carried out (Japanese Patent Nos. 381145 and 3876990 corresponding to U.S. Pat. No. 6,952,959 B2). 
         [0005]    However, heating temperature range for forming peel seal is narrow and delicate compared with tear seal. Heretofore, there is no case of combining peel seal with tear seal, and moreover, it is not known any necessity for combining them. 
       SUMMARY OF THE INVENTION 
       [0006]    The inventor continues investigating heat sealing for a long period, and noted that, in the case of conventional seal which is tear seal formed by heating up to melted, when force is added to seal line, bonded area does not separate, but rupture occurs at the edge. That is, in the case of tear seal, stress of rupture is undertaken not by whole bonded area but only by the edge of heat seal line. Under high temperature conditions where adhesive layer falls into liquid state, the melted adhesive layer is pressed out of heat seal line by the pressure of heating body to form polymer bead. 
         [0007]    When a sealed bag or container is pressed from the outside, it is deformed unevenly to form tucks at portions where stress is concentrated. In the case that the end of tucks meets the portion of polymer bead, stress is further concentrated on minor areas causing bag rupture or generation of pinholes. 
         [0008]    Heretofore, countermeasures are to thicken the adhesive layer or to employ strong materials. However, they are not radical measures to solve the problems of bag rupture and pinholes because of increasing material cost, lengthening heating period or elevating heating temperature. 
         [0009]    The bonded area formed by conventional heat sealing heated in tear seal region is bonded through cohesive adhesion of which tensile strength by JIS Z 0238 is great. However, since there is no cushioning action against breaking force, breaking stress is concentrated on minor portions to induce bag rupture or generation of pinholes. Thus, thickness of the adhesive layer is increased, or strong materials are employed, without utilizing average resistance to bag rupture of packaging materials to increase cost. 
         [0010]    Turning to the peel seal, although tensile strength is less than the tear seal, peeling occurs when breaking stress is loaded. Since energy is consumed corresponding to the product of the multiplication of peel strength by peel area during peeling, the peel seal has buffering action which absorbs breaking stress. However, it is not easy to form peel seal, because temperature range where peel seal is generated is narrow within several degrees. 
         [0011]    It is an object of the invention to provide a method of forming a heat seal structure which is resistant to bag rupture and generation of pinholes by dispersing bag rupture stress, without formation of polymer bead, and which can be formed by using cheap packaging materials with high reliability. 
         [0012]    The inventor investigated in order to achieve the above object, and conceived a combination of tear seal and peel seal located on articles to be packaged. However, this method requires two steps heat sealing which is practically not realized. Then, the inventor further investigated to develop a method of concurrently forming tear seal and peel seal by one step. First, I examined to form temperature difference on heating face by using two heaters, but this is not successful because of failing to form temperature boundary strictly. Subsequently, I tried a method of forming tear seal and peel seal by difference of heat sealing pressure generated by mounting a seat with slope on the heating body. This means is successful, but having a problem in practical viewpoint due to unstable in reproduction. 
         [0013]    Thereupon, I further investigated and conceived to mount seats having different thermal conductivity on the surface of heating body. As a result of repeating experimentations, I found that this method can form tear seal and peel seal each at designed positions exactly, and complete the invention based on this finding. 
         [0014]    Thus, this invention provides a method of forming a composite seal structure of peel seal and tear seal which comprises: 
         [0015]    using a heat sealer having two blocks which presses sheets to be heat-sealed and at least one of which is a heating block provided with a seat which renders its surface at a temperature forming peel seal, on a part of its surface pressing the sheets, 
         [0016]    heating the heating block at a temperature forming tear seal, and 
         [0017]    pressing the sheets to form a heat seal having the composite seal structure of peel seal and tear seal. 
         [0018]    Typical composite seal structure is band shaped having a peel seal zone 2 to 20 mm in width and a tear seal zone 1 to 10 mm in width in the longitudinal direction of the composite seal, and a width ratio of the peel seal zone/tear seal zone being 0.2 to 20. 
         [0019]    The region increasing heat seal strength with raising temperature is called peel seal (definition: ASTM F88-00) where bonding state is interfacial bonding by intermolecular force. When the non-heat seal end of a piece with peel seal is grasped and pulled, the bonded area of the heat-sealed piece is peeled without rupture as shown in  FIG. 1(   a ). 
         [0020]    The region where tensile strength exceeds the maximum and becomes flat is called tear seal (definition: ASTM F88-00). In this temperature range, the adhesive layers faced to each other are fused. 
         [0021]    In the tear seal, the bonded portion becomes integral without bonding interface in cooled conditions after heating, and heated portion becomes thicker than before bonding. Accordingly, tensile strength of the bonded portion is stronger than the other portions. However, the adhesive layer was easily pressed out in melted state unevenly by the pressing while heat sealing to the edge of heat seal to form polymer bead. When the non-heat seal end of a piece with tear seal is grasped and pulled, the bonded area of the heat-sealed piece is torn just in the vicinity of the heat seal line as shown in  FIG. 1(   b ). 
         [0022]    In tensile test, since breaking strength of tear seal is greater than peel seal, heretofore, it has been evaluated favorable. However, as shown in  FIG. 2 , when a sealed bag or container is pressed from the outside, it is deformed unevenly to form tucks at portions where stress is concentrated. In the case that the end of tucks meets the position of polymer bead, stress in further concentrated on minor areas of 1 mm or less. Since the tear seal is in cohesive adhesion, the minor area is broken to act as a starting point of bag rupture or to generate pinholes. 
         [0023]    Since volume of bag or container made of film or sheet is increased by charging an article to be packaged, generation of tucks cannot be avoided. In this invention, breaking energy caused by the concentration of bag rupture stress on heat seal portion is absorbed by peeling at interfacial bonding of peel seal. Thereby, the concentrated stress is dispersed by lengthening pressure receive line against bag rupture stress into semicircular shape, and progress of peel is stopped by decreasing load per unit length. The conditions are shown in  FIG. 3 . 
         [0024]    When peel progresses to reach near to outer periphery of the heat seal, bag rupture caused by the peel is avoided by stopping to peel by the tear seal zone having strong adhesive force. Energy consumed in the meantime is several times as much as the load bearing ability of the tear seal. In the method of forming a composite seal structure of the invention, the peel seal is followed by the tear seal. Since the formation can be conducted at an optimum temperature for the material to be heat-sealed, formation of polymer bead can be avoided. 
         [0025]    According to the method of the invention, since heating can be conducted surely in a boundary region of peel seal and tear seal, optimum adhesion conditions for the material to be heat-sealed can be obtained in heat seal region. Since a bag rupture stress greater than the load bearing ability loaded to the heat seal line can be buffered by the consumption of the bag rupture energy by the peel energy, bag rupture and generation of pinholes can be avoided. Since temperature gradient is formed on heat seal area to form peel seal and tear seal continuously from the inside of bag or container, formation of polymer bead can be controlled. Since bag rupture stress can be dispersed in bonding area, generation of troubles can be prevented even by using cheap materials without increasing thickness or choosing strong materials. Since heat sealing can be controlled by reasonable method, reliability of heat seal can be ensured and improved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a schematic illustration of (a) peel seal and (b) tear seal conditions in a tensile test. 
           [0027]      FIG. 2  is a schematic view illustrating a state of a pouch to which external force is loaded. 
           [0028]      FIG. 3  is a schematic illustration of a state of the heat seal structure of the invention loaded with peel stress. 
           [0029]      FIG. 4  shows forms of heat seal. 
           [0030]      FIG. 5  is a schematic illustration of a heat sealing apparatus for carrying out the method of the invention. 
           [0031]      FIG. 6  is a partially enlarged view of  FIG. 5 . 
           [0032]      FIG. 7  is a schematic illustration of heat sealing portion to form continuous bags with the composite seal structure of the invention. 
           [0033]      FIG. 8  is a schematic illustration of heat sealing portion of impulse heat sealing apparatus applicable to the invention. 
           [0034]      FIG. 9  is a graph showing a relationship between the temperature of heat sealing surface and tensile strength. 
           [0035]      FIG. 10  is a graph showing a relationship between the temperature of heat sealing surface and pressing time with varying the thickness of a heat flow control seat. 
           [0036]      FIG. 11  is a graph showing a relationship between tensile strength and peeled length with respect to the composite seal structure and conventional tear seal. 
           [0000]    
         
           
                 
                 
               
             
                 
                     
                 
               
               
                 
                   1-1, 1-2 
                   Heating Body 
                 
                 
                   2-1, 2-2 
                   Electric Heater 
                 
                 
                   3-1, 3-2 
                   Temperature Sensor 
                 
                 
                    4 
                   Temperature Controller 
                 
                 
                   5-1, 5-2 
                   Heat Flow Control Seat 
                 
                 
                    6 
                   Material to be Heat-Sealed 
                 
                 
                   7-1, 7-2 
                   Planar Seat 
                 
                 
                   8, 9 
                   Heating Wire 
                 
                 
                   10 
                   Heat Flow Control Sheet 
                 
                 
                   11 
                   Fixed Block 
                 
                 
                   12 
                   Vertically Moving Block 
                 
                 
                   13, 14 
                   Heat-Resistant Cover 
                 
                 
                     
                 
               
            
           
         
       
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0037]    Examples of the form of heat seal applicable to the invention is shown in  FIG. 4 . Illustrated on the left side is a four-sided fin seal bag, and on the right side is a cup container. Besides, other containers, such as three-sided fin seal bag, two-sided fin seal bag, other heat seal bags, etc. are also applicable. 
         [0038]    The composite seal structure of the invention is band shaped having a band shaped peel seal zone and a band shaped tear seal zone in the longitudinal direction, and the peel seal zone is arranged on the inside (the side of article to be packaged) and the tear seal zone is arranged on the outside. Both of the seal zones are arranged, in general, continuously, and have almost uniform width (in  FIG. 3 , figured exaggeratively). 
         [0039]    A suitable total heat seal width (the width of the heat seal area in  FIG. 4 ) varies depending on the capacity of bag or container, volume of article to be packaged, accordingly material, thickness and the like of film or sheet to be packaged, and in general, is about 3 to 30 mm, usually about 2 to 20 mm, particularly about 7 to 15 mm. The width of the peel seal zone is about 2 to 20 mm, preferably 5 to 10 mm, more preferably 5 to 8 mm. This width is set by considering that peel energy is almost equivalent to twice as much as the rupture energy of the tear seal but that the width does not exceed conventional heat seal width. 
         [0040]    On the other hand, the width of the tear seal zone is about 1 to 10 mm, preferably 2 to 5 mm, more preferably 2 to 3 mm. As the role of the tear seal, 1 mm in width is enough therefor. However, in order to execute the tear seal exactly, it is preferable to be at least 2 mm for positioning performance of apparatus. 
         [0041]    A suitable width ratio of peel seal zone/tear seal zone is about 0.2 to 20, preferably about 1 to 5, more preferably about 2 to 5. The ratio is set, as set forth, by considering the width of the peel seal which peels by 5 to 10 mm to consume rupture energy equivalent to twice as much as the rupture energy of the tear seal 2 mm in width. 
         [0042]    The width of peel seal and tear seal can be confirmed by utilizing the method of designing a heat seal width developed by the inventor (U.S. Pat. No. 6,952,959 B2). 
         [0043]    The composite seal structure can be formed by mounting seats having different thermal conductivity on the surface of heating body of a conventional heat seal apparatus. That is, a high thermal conductivity material is attached to the portion for forming tear seal, and a low thermal conductivity material is attached to the portion for forming peel seal. However, heating body is, in general made of a high heat conductivity material, the seat to be attached to the portion for forming tear seal may be the same material, although may be a material having a thermal conductivity higher than the conventional heating body. 
         [0044]    Accordingly, a seat made of a material having a low thermal conductivity (hereinafter referred to as “heat flow control seat”). The material is not thermally denatured at heat sealing temperature, for example, fluoro resins, such as polytetrafluoroethylene (trade name: “Teflon”), glass fiber sheet impregnated with fluoro resin, carbon fiber sheet, ceramic plate or the like. A suitable thickness of the seat varies according to the material of the seat and the material to be heat sealed, and the like, and in general, is about 0.1 to 2 mm, usually about 0.1 to 1 mm. Between the seat for forming tear seal and the seat for forming peel seal is, in general, flat without space and step. 
         [0045]    When the seat for forming tear seal is made of the same material as the heating body, the portion to attach the heat flow control seat may be shaved so that the surface after attaching the seat becomes flat. 
         [0046]    As to the blocks pressing the material to be heat-sealed, there is a type of moving one of the blocks or a type of moving both blocks, and further, a type that one of the blocks is heating body and a type that both blocks are heating bodies. In the case that both blocks are heating bodies, the heat flow control seat is, in general, attached to both blocks, but, it is possible to attach it to only one of them. 
         [0047]    A schematic construction of an example of such a heat sealing apparatus is shown in  FIG. 5 . In this apparatus, both blocks for pressing the material to be heat-sealed are heating bodies  1 - 1 ,  1 - 2 , and electric heaters  2 - 1 ,  2 - 2  are embeded. Temperature sensors  3 - 1 ,  3 - 2  are mounted near the heating surface. Both electric heaters  2 - 1 ,  2 - 2  are connected to commercial electric source each through a switch. The temperature controllers  4 , and temperature of respective heating bodies  1 - 1 ,  1 - 2  can be controlled by the temperature sensor  3 - 1 ,  3 - 2  (as to heating body  1 - 2 , not illustrated.). As shown on the right side of  FIG. 5 , the left side of the heating surface of both heating bodies  1 - 1 ,  1 - 2  is provided with heat flow seat  5 - 1 ,  5 - 2 , and the right side is provided with planar seat  7 - 1 ,  7 - 2 . 
         [0048]    An enlarged heating and pressing portion is shown in  FIG. 6 . In the figure,  1  designates the thickness of the planar seat  7 - 2  and the heat flow control seat  5 - 2 ,  2  designates the width of tear seal, and  3  designates the width of peel seal. The thickness of the planar seat  7 - 1  and the heat flow control seat  5 - 1  is, in general, the same as that of the planar seat  7 - 2  and the heat flow control seat  5 - 2 . 
         [0049]    When this invention is applied to making connected plural, i.e. two or more bags, as shown in  FIG. 7 , planar seats  7 - 1 ,  7 - 2  are provided at the central portion, and heat flow control seats  5 - 1 ,  5 - 2  are provided on both sides, and cutting line is provided at the center. 
         [0050]    An example of applying the invention to impulse sealing is shown in  FIG. 8 . This apparatus is composed of a fixed block  11  and a vertically moving block  12  which descends to press a material to be heat-sealed  6 . Heat-resistant cover  13  is mounted on the press face of both blocks  11 ,  12 . Two (in conventional, only one) heating wires  8 ,  9  are set on the fixed block  11 , and the two heating wires  8 ,  9  have identical thickness and arranged close to each other. The aforementioned low thermal conductivity material, such as a Teflon (trade name) sheet, is laid so as to pass through the upper side of one heating wire  9  and through the underside of the other heating wire  8 . By arranging like this, heat flow difference can be generated without forming a step on the surface, while two heating wires  8 ,  9  can be insulated. The heat flow can be controlled by varying the thickness of the heat flow control sheet. However, since two heating wires  8 ,  9  are connected to separate electric source as shown in  FIG. 8  (B), the heat flow can be controlled sensitively by controlling each electric source or the time of applying electric current. 
         [0051]    In addition, composite seal structure of peel seal and tear seal can be formed by providing controllable heating portions in two rows in ultrasonic sealing or electric field sealing, similar to the above impulse sealing. 
         [0052]    For cup containers as shown on the right side in  FIG. 4 , heating is carried out from one side. In this case, the composite seal structure can be formed by arranging a planar seat and a heat flow control seat on heating face, and temperature of heating body is set higher that the case of heating from both sides. 
         [0053]    Subsequently, a method of forming the composite seal structure of the invention will be explained. 
         [0054]    Heat seal conditions vary according to material to be heat-sealed. Thereupon, a relationship between temperature of heat sealing surface of the material to be heat-sealed and tensile strength of the heat-sealed material is determined. A typical example of the relationship is shown in  FIG. 9 . 
         [0055]    In the case of  FIG. 9 , peel seal zone having a heat seal strength in the range from 10 N/15 mm to 50 N/15 mm can be obtained by heat sealing at a temperature of heat sealing surface in the range from 144° C. to 150° C. at the finish of heating. A peel seal having heat seal strength which varies from 10 N/15 mm to 50 N/15 mm continuously can be obtained by adjusting the temperature at the boundary of the planar seat and heat flow control seat to 156° C. and the temperature of the other end of the heat flow control seat to 144° C. Since the temperature of the planar seat is rendered at 160° C., tear seal is formed there. 
         [0056]    Then, the heat flow control seat is designed. The surface temperature of the heating bodies  1 - 1 ,  1 - 2  are set at a temperature higher than the temperature at the point (a) in  FIG. 9  by 3 to 5° C. Two temperature sensors are interposed between materials to be heat-sealed, one is located at a distance of 1 mm from the periphery of the heat flow control seat, and the other is located at the center of the planar seat. Then, a relationship between temperature of heat sealing surface and pressing time with varying the thickness of the heat flow control seat to be employed is determined. A typical example of the relationship is shown in  FIG. 10 . From the results, a time that the temperature of heat sealing surface reaches the temperature (a) (see  FIG. 9 ) at the planar seat, and the time is set as pressing time (see Reference Point in  FIG. 10 ). Then, a suitable thickness of the heat flow control seat capable of forming peel seal is designed with reference to  FIGS. 9 and 10 . In this example, since the temperature range where peel seal is formed is 144 to 153° C. as shown in  FIG. 9 , the maximum thickness for forming peel seal is about 0.3 mm which renders the temperature of heat sealing surface to the lower limit temperature of 144° C. as can be seen from the results of  FIG. 10 . The minimum thickness can be determined by further varying the thickness or interpolating the results. 
         [0057]    Thus, after the heat flow control seat has been designed, the heat flow control seat and a planar seat having the same thickness are mounted on the heating surface of the heating bodies  1 - 1 ,  1 - 2 , and heat sealing is carried out to form the heat seal having the composite seal structure. At that time, the setting temperature and pressing time are set those obtained in the above designing pressing force may be conventional, and the same pressing force as used in the above designing is employed. 
       EXAMPLES 
       [0058]    A commercial aluminum laminated packaging material 0.09 mm in thickness being used for retort package was used as the material to be heat-sealed  6 , and a relationship between temperature of heat-sealing surface and tensile strength was measured by the method described in U.S. Pat. No. 6,952,959 B2 to obtain the results shown in  FIG. 9 . From the results, the temperature of heat sealing surface was set at 160° C. ((a) in  FIG. 9 ) as a minimum temperature capable of forming tear seal surely. 
         [0059]    The heat sealing apparatus employed had the construction as shown in  FIG. 5 . The heating bodies  1 - 1 ,  1 - 2  had a size of 3.5 cm in longitudinal width, 3 cm in lateral width, 20 cm in length and heating width of 1.7 cm, and electric heaters  2 - 1 ,  2 - 2  were mounted thereon with 400 W, 0.8 cm in diameter and 20 cm in length. In order to decrease temperature unevenness in the longitudinal direction, heat pipes (not illustrated) were mounted between the electric heaters  2 - 1 ,  2 - 2  and the surface of the heating bodies  1 - 1 ,  1 - 2 . As the temperature sensors  3 - 1 ,  3 - 2 , minute sensors 0.2 mm in wire diameters were used. The electric heaters  2 - 1 ,  2 - 2  were controlled by an on-off system with PID function. Temperature of heat sealing surface was measured by the apparatus employed in U.S. Pat. No. 6,952,959 B2. 
         [0060]    The temperature of the heating bodies  1 - 1 ,  1 - 2  were set at 165° C. As the heat flow control seat, Teflon (trade name) sheet 0.2 mm, 0.3 mm or 0.4 mm in thickness was attached to the heating bodies. Two sensors  3 - 1 ,  3 - 2  were inserted between the material pieces to be heat-sealed  6 , and temperatures of heat sealing surface were measured at a position with a distance of 1 mm from the outer periphery of the heat flow control seat and at the center of the planar seat. The results are shown in  FIG. 10 . From the results, thickness of the heat flow control seat was set 0.3 mm, and pressing time was set 0.38 second. 
         [0061]    Then, to each heating face of the heating bodies  1 - 1 ,  1 - 2 , a Teflon sheet 0.3 mm in thickness and 9 mm in width ((3) in  FIG. 6 ) was attached, and a stainless steel sheet 0.3 mm in thickness and 5 mm in width ((2) in  FIG. 6 ) made of the same material as the heating bodies was attached to the remaining area. 
         [0062]    Using the heat sealing apparatus, heat sealing was carried out under the following conditions; 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Surface temperature of heating body: 
                 165° C. 
               
               
                   
                 Pressing time: 
                 0.38 sec. 
               
               
                   
                 Material of heat flow control seat: 
                 Teflon (tradename) 
               
               
                   
                 Thickness of heat flow control seat: 
                 0.3 mm 
               
               
                   
                 Width for peel seal: 
                 9 mm 
               
               
                   
                 Width for tear seal: 
                 5 mm 
               
               
                   
                 Initial pressing force: 
                 0.1 Mpa 
               
               
                   
                   
               
             
          
         
       
     
         [0063]    Tensile test was carried out as to the composite seal samples thus obtained, and the results are shown in  FIG. 11  by a full line. Conventional heat seal samples having tear seal alone were prepared by the same heat sealing apparatus without the heat flow control seat and planar seat at a temperature of 170° C. determined by the results of heat analysis, and tensile test was carried out similarly. The results are shown in  FIG. 11  by a broken line. 
         [0064]    As shown in  FIG. 11 , in the case of the composite seal of the invention, rising was gentle, and peel of heat sealed area was clearly observed. Tensile strength was 57 N/15 mm at the maximum. Peeling was advanced, and ruptured at an advanced distance of about 0.85 cm which was entered in the tear seal region. Up to about 0.8 cm, good peel seal zone was formed in a boundary region of peel seal and tear seal. 
         [0065]    On the other hand, in the case of the conventional heat seal samples with tear seal alone, rising was fast, and reached yield point (c) at a peeled distance of 0.35 cm, and ruptured by a tensile strength of 51 N/15 mm. 
         [0066]    The tensile strength at each point is a response to minute tensile variation at each point. That is, workload at each measuring point is the sum of [(strength:N)×(pulling distance between sample)]/[(pulling speed)×15 mm] up to rupture. Protectability against bag rupture of the composite seal structure of the invention can be compared with that of conventional tear seal by the comparison of areas determined by operation, which have been converted to exponents, under the same pulling speed. From the results of  FIG. 9 , integration was carried out up to point (c) for conventional seal and up to point (d) for the composite seal in  FIG. 11 . The calculated results are 9.9 for the conventional seal and 41 for the composite seal of the invention. 
         [0067]    Since the numerical values can be substituted by ability of energy consumption against external stress, it is apparent that the composite seal structure of the invention exhibits great resistance to bag rupture compared with conventional heat seal. 
         [0068]    The composite seal structure of the invention has advantages in the prevention of torn at heat seal edge, no need to increase thickness and reasonable reliable security for heat seal, and therefore, it can substitute for conventional heat seal.