UNROLLING LOW-NOISE SELF-ADHESIVE TAPE AND PROCESS FOR THE PREPARATION THEREOF

Abstract: The present invention refers to a self-adhesive tape comprising an olephinic polymer or copolymer support, the support has an adhesive side and a back side, the adhesive side supports an adhesive substance layer possibly with an interposed anchoring layer. The self-adhesive tape according to the present invention does neither comprise a releasing layer applied on the back side thereof, nor has been treated on the back side and has a low noise while being quickly unrolled.

Description: The present invention refers to a self-adhesive tape in particular comprising a bistretched extruded polypropylene support and to the process for the preparation thereof. The support is provided with a main side and a back side, the main side supports an adhesive substance layer with an interposed anchoring layer. The adhesive substance layer comprises an adhesive mass coated in form of a solution in organic solvents. Adhesive tapes of this kind are known. For instance French patent application S.N. 79 19719 and German patent application S.N. 2,239,665 describe self-adhesive tapes wherein the support consists of polypropylene. In practice, such known polypropylene tapes however proved to have a high noise while being unrolled, in particular under quick unrolling, such as to cause serious problems of acoustic environmental pollution, both in the spool cutting step to obtain various width and length rolls, and in the use of the self-adhesive tape during its final employment, e.g. to close corrugated board boxes by means of applying machines. In fact, the noise caused while unrolling the adhesive tape is so high to he physically dangerous for the operators. European patent application S.N. 0096841 describes a polypropylene self-adhesive tape having a relatively low unrolling noise, which makes use of a suitable releasing layer coated on the back side of the tape and formed generally from polyvinyl-carbamate, polyvinyl-benenate, polyvinyl-stearylisocyanate aromatic organic solvent solutions. Such releasing layer involves complicated problems both for the cost of the releasing product itself, and the complexity of the applying machine which shall in any case comprise at least one additional applying head and a solvent drying stove. Moreover, the result is dubious, since small changes in the applying process may annul or reduce the releasing effect. The adhesive substance used in such known tapes in some cases contains a cross linking agent in a quantity lower than 0.24% by weight. By increasing such quantities of cross linking agent, the adhesive substance proved to have a low sticking power on rough surfaces, such as the corrugated board of packaging boxes, with a consequent poor seal and opening or the boxes. In such case, furthermore, the tape results to be particularly sensitive to ageing and presents unrolling problems, above all at low temperatures, with frequent breakings of the tape itself while being applied. It is therefore an object of the present invention to avoid the above said drawbacks by realizing an extruded, bistretched polypropylene self-adhesive tape which allows a particularly low unrolling noise to be obtained and to be prepared with simple and conventional applying machines. It is a further object of the invention to maintain a good sticking capability on a corrugated board, even after ageing of the tape and at low temperatures. This and other objects are achieved with the tape of the present invention, which comprises an extruded bistretched propylene polymer support, having a main side and a back side, an anchoring layer provided on the main side of the support, an adhesive layer provided on the anchoring layer, characterized in that the back side does not comprise a releasing layer and has a critical surface tension equal to the original extrusion values and in that a cross linking agent in a quantity from 0.5 to 2.5% by weight is comprised in the anchoring layer. Preferably, such self-adhesive tape comprises the cross linking agent in a quantity In the range from 1 to 1.5% by weight. Such cross linking agent may comprise e.g. a diisocyan product, preferably a methyldiphenylisocyanate compound. Preferably, the adhesive tape support has a thickness in the range from 30 to 50 micrometers. Such a support furthermore has a lenghtwise and crosswise stretching ratio in the range from 0.6 to 1.0. It has furthermore a lengthwise and crosswise tensile stress ratio in the range from 1.2 to 1.8. On the side which shall support the adhesive substance layer, such a support is furthermore subjected to a preferably flame-process such as to raise the critical surface tension to a value not lower than 38 dine/cm. According to the invention, the cross linking agent is dispersed in the anchoring layer such that the anchoring layer is used as a vehicle for the cross linking agent and allows its migration in the adhesive substance layer. Such an embodiment is preferred since it allows to avoid a pre-linking step of the adhesive substance prior to being coated onto the tape, which step should give rise to stoppages of the applying machines due to the formation of cross linked thick-material. Preferably, the adhesive substance layer has a adhesiveness (Peel Adhesion) in the range from 120 to 180 g/cm and preferably from 140 to 160 g/cm. Such adhesiveness is measured according to the PSTC 1 test method. Preferably the self-adhesive tape according to the invention has an adhesive substance layer which has a Stickiness/wettability value (tack) in the range from 0 to 3 cm. Such a Stickiness/wettability value (tack) is measured according to the PSTC 6 test method. Moreover, the adhesive tape has a tangent slipping resistance (Holding power or Shear adhesion) higher than 180 min., according to the PSTC 7 test method. The adhesive substance layer of the self-adhesive tape of the invention comprises at least a gum in a quantity from 49 to 70% and a tackifier resin in a quantity from 30 to 41% with respect to the total dry adhesive substance weight. The gum can be a natural gum and have preferably a constant viscosity in the range from 50 to 60° mooney, and the tackifier resin can be a hydrocarbon resin and have e.g. a melting point of 100°C The natural gum is preferably contained in a quantity from 40 to 70% with respect to the total dry adhesive substance weight and the tackifier hydrocarbon resin is preferably contained in a quantity from 20 to 40% by weight. Furthermore, the adhesive substance layer can comprise a polyisoprene synthetic gum in a quantity from 10 to 14% with respect to the dry adhesive substance weight. The adhesive substance layer furthermore comprises preferably a colophonic tackifier resin having e.g. a melting point of 100°C, preferably in a quantity from 5 to 11% by weight. The adhesive substance layer furthermore comprises preferably a polyterpene tackifier resin ,having e.g. a melting point of 110°C in a quantity from 5 to 9 % with respect to the total dry adhesive substance weight. The adhesive substance layer furthermore comprises preferably a naphthene mineral oil in a quantity from 2 to 9% with respect to the total dry adhesive substance weight. The adhesive substance layer furthermore comprises preferably an alkylbenzene synthetic oil in a quantity from 2 to 10% by weight. Additives, e.g. antioxidants, ballasts or pigments may be furthermore added thereto. All components of the adhesive substance layer are preferably dissolved and/or dispersed in an organic solvent in a quantity from 25 to 35% by weight with respect to the wet total weight. The solution/suspension in an organic solvent of the above mentioned adhesive agents is then spread on the support, preferably after having properly treated the support itself to increase the surface tension thereof and having spread a cross linking agent-containing anchoring layer. The organic solvent is then evaporated. The back side of the support does not undergo any treatment to increase the critical surface tension. Further characteristics and advantages of the present invention will better result from the description of some preferred realization forms of the self-adhesive tape which are reported as mere indicative and non-limiting examples, wherein examples 1-9 refer to a self-adhesive tape according to the present invention and examples 10-13 are comparison examples which are not a part of the present invention. Examples 1-6 in particular describe some compositions of the adhesive substance layer according to the invention to which the cross linking agent has not yet been added. Such examples 1-6 are outlined in the following Table 1. EXAMPLE NR IR HR GR TR MO ABO 1 69 0 26 0 5 0 0 2 56 14 20 10 0 0 0 3 58 0 40 0 0 2 0 4 47 11 35 5 0 0 2 5 49 0 32 0 9 0 10 6 40 10 25 11 5 9 0 wherein: NR = constant viscosity natural gum; IR = polyisoprene synthetic gum; HR = hydrocarbon tackifier resin; GR = colophonic tackifier resin; TR = polyterpene tackifier resin; MO = naphthene mineral oil; ABO = alkyl-benzene synthetic oil. Making reference to examples from 1 to 6, a good sticking capability on corrugated board and a good unrolling capability even at low temperatures are obtained in all examples from 1 to 6 by using an anchoring layer containing 1% of the cross linking agent formed e.g. from diisocyanate. Noise is also very low, lower than that of the so-called "no-noise" BOPP tapes which can be found at present on the market. With reference to examples 7-9, the adhesiveness values have been evaluated according to PSTC 1 standards, the tack values according to PSPC 6 standards, the shear adhesion values according to PSTC 7 Kg 1 standards and noise in function of different quantities of cross linking agent contained in the adhesive substance layer. As regards the above mentioned parameters, the samples have been measured twice, i.e. after 72 hours from manufacture and after an accelerated ageing process. The accelerated ageing process is made in the following way: the sample is conditioned for 72 hours in a climatic chamber at 60°C and 50% R.H. As shown by comparison tests, such conditioning practically corresponds to a natural ageing of about 12 months. As regards the noise measures, a microphone is placed at a distance of one meter from the reflecting plane of the tape-unrolling machine, on the same plane of the unrolling axis of the roll under examination. The decibel value reported in the tables represents the average of the values taken in 15 sec. Ground noise has also been measured, under the same test conditions, with the unrolling machine under working status but without unrolling the adhesive tape. In particular, when the machine works at a speed of 52 m/min, a ground noise of 65 dB is obtained and when the spool moves at a speed of 103 m/min a ground noise of 68.6 dB is obtained. Examples 7-9 are shown in the following table 2. With reference to comparison examples 10-13, example 10 regards a PVC tape to which a releasing layer has not been applied, example 11 regards a PVC tape to which a releasing layer has been applied, example 12 regards a bistretched polypropylene tape with a releasing layer and example 13 regards a so-called "no-noise" bistretched polypropylene tape which can be found on the market. Samples of the tapes described in such examples have been submitted to unrolling noise tests in the same way as that described in the previous examples. The results are reported in the following Table 3. The self-adhesive tape of the present invention in practice has been found to be capable of giving particularly low noise levels. Furthermore, in all examples of the invention the tape has good characteristics of adhesion on corrugated board and does not cause unrolling or breaking problems even at low temperatures.

Claims: A self adhesive tape comprising an extruded bistreched propylene polymer support having a thickness in the range of from 30 to 50 µm and a main side and a back side, a natural and/or modified gum anchoring layer provided on the main side of the support, an adhesive layer provided on the anchoring layer, characterized in that the back side does not comprise a releasing agent and has a critical surface tension equal to the original extrusion value the anchoring layer comprises a dillsocyanate cross linking agent in a quantity of from 0.5 to 2.5% by weight, and the adhesive layer comprises an adhesive mixture of at least a gum in a quantity of from 49 to 70% and at least a tackifier resin in a quantity of from 30 to 41% by weight with respect to the total dry adhesive substance weight. The self adhesive tape of claim 1, characterized in that said anchoring layer comprises said cross linking agent in a quantity of from 1 to 1.5% by weight. The self adhesive tape of claim 1, characterized in that said cross linking agent is methyldiphenylisocyanate. The self adhesive tape of claim 1, characterized in that said support has a lengthwise and crosswise stretching ratio in the range of from 0.6 to 1.0. The self adhesive tape of claim 1, characterized in that said support has a lengthwise and crosswise tensile stress ratio in the range of from 1.2 to 1.8. The self adhesive tape of claim 1, characterized in that said main side has a critical surface tension value higher than 38 dine/cm. The self adhesive tape of claim 1, characterized in that said gum comprises natural and/or polyisoprene synthetic gum. The self adhesive tape of claim 7 characterized in that said natural gum has a constant viscosity in the range of from 50° to 60° mooney and is contained in said adhesive layer in a quantity of from 40 to 70% by weight. The self adhesive tape of claim 7, characterized in that said polyisoprene synthetic gum is contained in said adhesive layer in a quantity of from 10 to 14% by weight. The self adhesive tape of claim 1, characterized in that said tackifier resin comprises a hydrocarbon and/or colophonic and/or polyterpene resin. The self adhesive tape of claim 10, characterized in that said hydrocarbon tackifier resin is contained in said adhesive layer in a quantity of from 20 to 40% by weight. The self adhesive tape of claim 10, characterized in that said colophonic tackifier resin is contained in said adhesive layer in a quantity of from 5 to 11% by weight. The self adhesive tape of claim 10, characterized in that said polyterpene tackifier resin is contained in said adhesive layer in a quantity of from 5 to 9% by weight. The self adhesive tape of claim 1, characterized in that said adhesive layer further comprises a naphthene and/or alkylbenzene mineral oil. The self adhesive tape of claim 14, characterized in that said naphthene mineral oil is contained in said adhesive layer in a quantity of from 2 to 9% by weight. The self adhesive tape of claim 14, characterized in that said alkylbenzene mineral oil is contained in said adhesive layer in a quantity of from 2 to 10% by weight. A process for the preparation of a self adhesive tape according to claim 1, characterized by comprising the following step of: a) corona or flame treating said main side of said support to raise the critical surface tension from an original value lower than 35 dine/cm to a final value not lower than 38 dine/cm, b) spreading a natural and/or modified gum anchoring layer containing of from 0.5 to 2.5% by weight of a cross linking agent selected in the class of diisocyan agents on said main side of said support, c) spreading an adhesive substance solution or suspension in an organic solvent on said anchoring layer, said ahdesive substance comprising a mixture of at least a gum in a quantity of from 49 to 70% and at least a tackifier resin in a quantity of from 30 to 41% by weight with respect to the total dry adhesive substance weight, and d) evaporating said organic solvent under formation of said dry adhesive substance.

Inventors: PELLEGRINI, GIUSEPPE
Applicants: MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1989
Main classification: C09J7
IPC codes: C09J7