text: referring to the drawings , and initially to fig1 and 2 , can top 10 is provided with aperture 12 , tape closure 14 overlying can top 10 and sealing aperture 12 . closure 14 comprises film backing sheet 26 , adhesive 20 being applied over the lower surface thereof and serving to bond closure 14 , to can top 10 . at the distal portion of closure 14 , adhesive 20 is covered by a small piece of film 19 , thereby forming grip tap 18 to aid in removal . on the upper surface of film backing layer 26 is a layer of binder material 23 , throughout which are dispersed capsules 24 filled with liquid dye precursor and finely divided particles of material capable of reacting with the dye precursor to form a dye . when tab 18 is gripped and pulled upward to remove closure 14 from top 10 , capsules 24 will rupture , thereby releasing their liquid contents , which will react with the solid coreactant to form a dye and thus show that closure 14 has been tampered with . even if closure 14 is thereafter reapplied to can top 10 , the evidence of the prior opening of the seal remains . fig3 shows another embodiment of this invention , where the indicator layer is incorporated as part of a separate tape construction 21 . layer 22 , comprising liquid - containing capsules 24 in binder 23 , is coated on and bonded to film substrate 27 to form a laminate . the resulting laminate is then adhered to the top surface of film 26 by adhesive layer 28 . a roll 30 of material suitable for forming the can closure of this invention is shown in fig4 . a film 26 suitable for forming can closures has a strip of indicator material 16 longitudinally adhered to the film . can closures having the desired configuration can be die - cut from roll 30 and the resulting closure mounted on a can top as shown in fig1 or 2 . the invention is further described with reference to the following examples . all parts are by weight unless otherwise noted . example 1______________________________________a mixture comprising : ______________________________________9 . 0 parts of the condensed formaldehyde capsules containing derivatives of dithiooxamide described in u . s . pat . no . 3 , 516 , 941 , example 18 , having a diameter of about 15 microns ; 9 . 0 parts nickel rosinate13 . 2 parts isophthalic - terephthalic acid / ethylene glycol copolyester resin having a ring and ball softening point of 155 ° c . (&# 34 ; vitel &# 34 ; pe - 200 , available from goodyear chemical co . ), diluted to 40 % solids with a 1 : 2 toluol - methyl ethyl ketone solvent ; 13 . 5 parts methyl ethyl ketone______________________________________ was gently blended , to prevent capsule rupture , to a uniform consistency with an air driven propeller mixer and knife coated at a wet thickness of 3 mils on 1 mil biaxially oriented polyethylene terephthalate film . the coating was dried at 65 ° c . for 20 minutes to form a dry indicator layer between 1 and 2 mils ( 0 . 025 - 0 . 05 mm .) thick . a pressure sensitive adhesive was made by milling 100 parts &# 34 ; kraton &# 34 ; 1101 block copolymer ( an aba block copolymer where a is 15 , 000 molecular weight polystyrene and b is 30 , 000 molecular weight polybutadiene ) and 85 parts &# 34 ; hercules &# 34 ; s1010 , a β - terpene . the blend was diluted with toluene to 40 % solids and knife coated on the film surface opposite the dry indicator layer . the film was dried at 65 ° c . for 20 minutes to evaporate the toluene , leaving 15 grains of adhesive per 24 square inches . the coated film was adhesively bonded to the upper surface of a can closure similar to that disclosed in example ii of u . s . pat . no . 3 , 389 , 827 . the indicator layer turned deep purple when the closure was removed from a can top even at shallow angles . a closure like that of example 1 was made using as the coating mixture 18 parts of the capsules described in example 1 , 9 parts nickel rosinate , 13 . 2 parts 40 % solids &# 34 ; vitel &# 34 ; pe200 in a 1 : 2 toluene : methyl ethyl ketone solvent , and 21 . 6 parts of methyl ethyl ketone . the resulting indicator closures give a slightly deeper purple color than those of example 1 when the closures were removed from can tops at shallow angles of about 45 °. a closure like that of example 1 was made using a coating comprising 18 parts capsules of example 118 parts nickel rosinate15 parts 35 % solids &# 34 ; vitel &# 34 ; pe - 200 in cyclohexanone ; and28 . 8 parts cyclohexanone the resulting indicator closures changed color when removed from can lids . when the closures were removed at 45 °, the color was light purple , lighter than in previous examples , although still acceptable . a family of blends of &# 34 ; vitel &# 34 ; pe200 and &# 34 ; vitel &# 34 ; pe207 ( the latter being an isophthalic - terephthalic acid / ethylene glycol copolymer having a ring and ball softening point of 127 ° c . ), each weighing 200 grams , were prepared by milling on a two roll rubber mill at about 310 ° f . for about 10 minutes and allowed to cool . the blends were hot pressed at 375 ° f . between two sheets of release paper , using a force of 10 , 000 pounds , to form sheets about 50 to 65 mils thick . test pieces , conforming to astm test standard d1708 - 66 , were stamped from the sheets using a standard die . the test pieces were dumbbell - shaped , approximately 1 . 5 inches long × 0 . 625 inch wide in their greatest dimension , with a reduced center section 0 . 876 inch long and 0 . 187 inch wide . samples h - k were brittle and heated with a warm air gun until the polymer started to soften to facilitate stamping . the die - cut pieces were stress - relieved in a warm air oven at 150 ° f ., for a sufficient time to insure that no residual stress remained from stamping and then conditioned for 24 hours at 72 ° f . and 50 % relative humidity . the samples were mounted in an &# 34 ; instron &# 34 ; tensile machine and the jaws separated at a rate of 1 inch / minute until break or a 10 % elongation was reached , whichever occurred first . the average young &# 39 ; s moduli of 3 tensile specimens of each blend are given in table i . blends of resins , corresponding to the samples , were made and used in place of the &# 34 ; vitel &# 34 ; pe200 as a binder resin in making tape closures which were otherwise identical to that of example i . the resulting closures were mounted on can lids and removed at three different angles . first , the closures were pulled away at a shallow angle , where the angle between the adhesive - coated surface of the tape closure and the can lid was about 45 °- 60 °. second , they were pulled vertically away from the can top at a removal angle of about 90 °. third , the closures were doubled back upon theselves and pulled away at a removal angle of about 180 °. table i______________________________________ %&# 34 ; vitel &# 34 ; pe200 %&# 34 ; vitel &# 34 ; pe207 young &# 39 ; s mod . at 2 % sample by weight by weight elongation______________________________________a 0 100 810b 10 90 3 , 360c 20 80 12 , 421d 30 70 23 , 966e 40 60 48 , 358f 50 50 71 , 906g 60 40 94 , 302h 70 30 139 , 168i 80 20 156 , 667j 90 10 194 , 770k 100 0 174 , 496______________________________________ closures made with binder compositions h - k produced a deep purple color change , even when removed at shallow angles . on the other hand , closures made with binder compositions a - g did not give an acceptable color change when removed even at angles of 90 °- 180 °. further testing of blends in the composition range lying between samples g and h established that a ratio of 2 : 1 &# 34 ; vitel &# 34 ; pe200 : pe207 gave a binder composition which gave a light color change when the tape closure was removed at a shallow angle and deeper color at 90 °- 180 ° removal angles . this blend corresponds to a binder having a young &# 39 ; s modulus at 2 % of about 125 , 000 psi . ratios of 3 : 1 gave a good color change even when removed at shallow angles and have a young &# 39 ; s modulus at 2 % of about 150 , 000 psi . example 1 was repeated , employing the same amounts of capsules and nickel rosinate but varying the binder system . a . 31 . 5 parts of a 16 . 8 weight percent solids vyns ( polyvinylchloride available from union carbide ) resin in methyl ethyl ketone , 6 . 0 parts mek , having a young &# 39 ; s modulus of 280 , 000 psi ; b . 35 . 1 parts of 15 weight percent solids polyvinyl butyral in toluol ( available as &# 34 ; butvar &# 34 ; b - 76 from shawinigan chemicals ), 21 parts toluol and 6 parts methyl ethyl ketone , having a young &# 39 ; s modulus of 227 , 000 psi ; and c . 23 . 4 parts of 22 . 5 weight percent solids polycarbonate in dichloromethane ( available as &# 34 ; rowlux &# 34 ; from rowland products ), and 18 parts dichloromethane , having a young &# 39 ; s modulus of 207 , 000 psi . tape closures made with binders a and c produced a faint lavender when pulled from can tops at a shallow angle , the depth and amount of color increasing when the closure is pulled off at 90 ° or more . tape closures made using binder b produced a light purple color when removed at 90 ° and deep color when pulled back upon itself at 180 °.