the hook strip 10 shown in fig1 has a substantially continuous backing 12 of thermoplastic resin . integral with the backing is an array of - mushroom - shaped projections or hooks 14 , each having a molecularly oriented stem 16 , a mushroom head 18 , and a fillet 20 at the base of the stem . a piece of the hook strip 10 can be the hook portion of a hook - and - loop mechanical fastener , or it can be releasably fastened to a fabric which is penetratable by the mushroom - shaped hooks 14 , or it can be configured such that two pieces of the hook strip 10 can be interengaged to form a hermaphroditic mechanical fastener as shown in fig2 . in fig3 a a feed stream 30 of thermoplastic resin is fed into an extruder 32 from which a heated resin melt is fed through a die 34 to a rotating cylindrical mold 36 . cavities 38 in the cylindrical surface of the mold 36 are evacuated by an external vacuum - system 40 . the die 34 has an output radius equal to that of the mold 36 in order to provide a seal between the die and the mold . rapid flow of the resin into the mold cavities 38 induces molecular orientation parallel to the direction of flow , and the mold is water - cooled ( cooling means not shown ) to provide rapid quenching to freeze this orientation in place . the solidified resin is stripped from the mold 36 by a stripper roll 44 as a web 42 that has an array of upstanding stems 48 . this web can either be wound into a roll for storage or fed directly into the mushroom forming apparatus of fig3 b . in fig3 b , the web 42 is fed between two calendar rolls 52 , and the roll that contacts the stems 48 heats the tips of the stems to a temperature exceeding the flow point of the thermoplastic resin . briefly , maintaining the tips at this temperature allows molecular disorientation to take place . during this time and upon subsequent cooling , the tips draw back to form uniformly shaped mushroom heads 14 as shown in fig1 each having an upper , substantially convex surface and a larger cross section than the original stem 48 . birefringence can be measured by any of several different optical techniques , such as by using standard fluids with different indices of refraction , the becke line technique , dispersion staining , or a compensator . the compensator technique is used to obtain the birefringence measurements reported below . using an &# 34 ; ortholux 2 pol &# 34 ; polarized light microscope with a berek compensator from e . leitz company , covington , ky ., a hook strip is placed under crossed polarized light with its z - axis oriented north - south . the microscope stage is rotated 45 degrees . a compensator is rotated in each direction until a black fringe appears ; at this point retardations are equal and opposite . compensator readings are recorded and the birefringence of the sample is calculated according to the equation : where r = retardation , c = compensator constant , and t = sample thickness . the retardation r , is defined as the phase difference between the two components in numbers of waves . the &# 34 ; break force &# 34 ; of a hook of a novel mushroom - type hook strip can be measured using an instron model 1122 tensile tester . the hook strip is folded in half , thus exposing one row of mushroom heads . the folded web is placed in the lower grips of the tensile tester with the stems of the exposed mushroom heads extending in a vertical direction . one end of a nylon thread is looped around one of the exposed mushroom heads , and the other end is secured to a 500 gram loadcell . the crossbar holding the loadcell is then moved in an upward direction at a rate of 12 . 7 mm per minute . the shear strength of the hooks of a mushroom - type hook strip can be measured using a tensile tester ( instron model 1122 ) equipped with a &# 34 ; ct &# 34 ; load cell . the hook strip to be tested and a mating loop strip of a hook - and - loop fastener are each cut to a length of 102 mm and a width was 51 mm . the mating loop strip can be scotchmate ™ woven loop , style no . sj 3401 from 3m co . the samples are conditioned under a bell jar by immersion for 24 hours in water supersaturated with magnesium acetate . the conditions under the jar are 21 ° c . (± 1 ° c .) and 65 % (± 2 %) relative humidity . the loop strip is laid loop side up on a flat surface and adhered to the surface by pressure - sensitive adhesive tape . the hook strip is intermeshed with the loop strip , and a roller weighing 5 kg is rolled over the back of the hook strip three times in each direction at a rate of 305 mm / min . the sample is placed in the jaws of the tensile tester with a free end of the hook strip gripped in the upper jaws and a free end of loop strip gripped in the lower jaws with the shear line centered . at a crosshead speed of 305 mm / min , a pen and chart records the maximum value obtained during the complete separation . four specimens are tested , and an average is taken . a mushroom - type hook strip and a mating loop strip of a hook - and - loop fastener are each cut to 178 mm in length and 51 mm in width . the strips are then conditioned for 24 hours under a bell jar by immersion in a supersaturated solution of magnesium acetate and water for 24 hours at 21 . 1 ° c . (± 1 . 1 ° c .) and 65 % (± 2 %) relative humidity . the hook strip is then carefully aligned with the loop strip and intermeshed with light finger pressure . the loop strip can be scotchmate ™ woven loop , style no . sj 3401 from 3m co . a roller weighing 5 kg and 76 mm in width is rolled over the strips making three passes in each direction at a rate of 305 mm / min . at one end , the strips are peeled apart to free approximately 70 mm in the lengthwise direction . the freed end of the hook strip is gripped in the upper jaws of a tensile tester ( instron model 1122 ) and the freed end of the loop strip is gripped in the lower jaws . the tensile tester has a &# 34 ; ct &# 34 ; loadcell and a 305 mm / min crosshead speed . the gauge length is 6 . 2 mm (± 3 . 2 mm ), and the load range is 44 . 5n for a full scale load . the peel distance is 76 . 22 mm . six specimens are tested with three specimens having their marked ends at corresponding ends of the specimen and three specimens having their marked ends at opposite ends of the specimen . the peaks are recorded , with the first peak being ignored and the remaining five largest peaks being averaged to provide the &# 34 ; t - peel strength &# 34 ;. the invention will be more readily understood by the following examples . to make apparatus as illustrated in fig3 a , mold cavities 38 were formed in the cylindrical face of a steel mold 36 by drilling a series of frustro - conical holes using bits having a tip width of 0 . 15 mm and a draft angle of 6 degrees . the resulting square array of holes had a uniform depth of 1 . 27 mm and uniform center - to - center hole spacing of 1 . 27 mm . while maintaining a vacuum at 46 . 8 mm of mercury and cooling the mold 36 internally with water at 35 ° c . low density polyethylene 1017 from norchem , rolling meadows , ill ., was extruded at a temperature of 235 ° c . into the cavities while rotating the mold at a surface speed of 7 . 7 m / min . the thickness of the layer of resin overlying the cavities was 0 . 2 mm . the solidified resin was stripped from the mold as a web having upstanding stems . using apparatus as illustrated in fig3 b , the web was run through a gap between two calender rolls 52 at a linespeed of 3 . 7 m / min while maintaining the top roll at 130 ° c . and the gap between the two rolls 52 at 0 . 9 mm . this produced a mushroom - type hook strip as illustrated in fig1 which hook strip showed good engagement with typical knitted and stitchbonded loop systems . the stems of the hooks , as measured using photomicrographic analysis , exhibited a birefringence value of 0 . 003 . the mushroom heads were about 0 . 8 mm in diameter . the mushroom - type hook strip was tested in comparison to a hook strip made as described in comparative example a . a mold was made by drilling a metal plate with bits as described in example 1 to form a series of frustro - conical holes similar to the array of holes of example 1 . while maintaining a vacuum , a 0 . 2 mm sheet of the polyethylene resin of example 1 was compressed at a temperature of 235 ° c . into the array of holes . after cooling the mold with water to solidify the resin , the sheet which was stripped away had upstanding stems similar in appearance to those of the web that had been stripped from the cylindrical mold in example 1 . when the sheet was run through a gap between calendar rolls 52 as in example 1 , the tips of the stems formed nondescript heads which were of less uniform shape than were the mushrooom heads produced in example 1 . table i______________________________________ birefringence break force ( n ) ______________________________________example 1 0 . 003 1 . 46comparative a 0 . 000 0 . 85______________________________________ the hook strip of example 1 was also tested with two different loop strips as reported in table ii . table ii______________________________________ shear strength t - peel ( n / cm ) ( n / cm ) ______________________________________with scotchmate ™ 83 . 1 1 . 225woven loop styleno . sj 3401with scotchmate ™ 86 . 6 0 . 61tricot knit loopstyle no . sj 3491______________________________________
a mushroom - type hook strip for a hook - and - loop fastener has a flexible backing of thermoplastic resin and , integral with backing , an array of upstanding stems distributed across at least one face of the backing , each having a mushroom head . the stems have a molecular orientation as evidenced by a birefringence value of at least 0 . 001 . the mushroom - type hook strip can be produced continuously by injecting resin into cavities of a cylindrical mold while evacuating and cooling the cavities so that the cooled resin becomes molecularly oriented , thus affording to the stems excellent stiffness , durability , and tensile and flexural strength .