PRESSURE SENSITIVE ADHESIVE COMPOSITION

[0001] The present invention refers to a pressure sensitive adhesive composition, the use of said composition in re-closable packaging units, an article comprising the pressure sensitive adhesive composition and a method for manufacturing said article.

[0002] Pressure sensitive adhesives are a type of non-reactive adhesives which form a bond when pressure is applied to bond the adhesive with a surface. They are commonly employed in pressure-sensitive tapes, labels, glue dots, stickers, sticky note pads, automobile trimmings and a variety of other products.

[0003] EP 2 630 211 describes a multilayer easy peel re-closable film structure for heat seal application wherein at least an inner layer is made of or substantially consists of a pressure-sensitive adhesive comprising a butene-1 (co)polymer having a content of butene-1 derived units of 80 wt.-% or more, a flexural modulus (MEF) of 80 MPa or less, and a melting temperature DSC (Tml) equal to or lower than 50° C. and at least one tackifier.

[0004] EP 3 323 866 refers to a pressure sensitive adhesive composition comprising i) at least one butene-1 (co)polymer, ii) at least one liquid tackifier, iii) at least one solid tackifier and iv) at least one styrene copolymer, with the butene-1 (co)polymer having a butene-1 content of 85 to 100% by mol, based on the butene-1 (co)polymer and the styrene copolymer being selected from Styrene-Butadiene-Styrene (SBS), Styrene-Ethylene-Propylene (SEP), Styrene-Isoprene-Styrene (SIS), Styrene-Ethylene/Butylene-Styrene (SEBS) and Styrene-Isoprene/Butadiene-Styrene (SIBS) copolymers.

[0005] EP 3 260 512 discloses pressure sensitive adhesive compositions comprising i) at least one butene-1 (co)polymer, ii) at least one liquid tackifier, iii) at least one solid tackifier and iv) at least one styrene copolymer, with the butene-1 (co)polymer having a butene-1 content of 85 to 100% by mol, based on the butene-1 (co)polymer, and with the at least one liquid tackifier being selected from liquid hydrocarbon resins and hydrogenated hydrocarbon resins, liquid polystyrenes, liquid rosin esters, liquid polyterpenes, liquid resins from polymerized and hydrogenated C9 hydrocarbons streams, liquid resins from polymerized and hydrogenated dicyclopentadiene species; liquid resins from polymerized and hydrogenated pure monomer selected from styrene, vinyl toluene or alpha-methyl styrene.

[0006] Although a large number of pressure sensitive adhesive is adhesives are available for a variety of applications, most of the common compositions comprise styrene block copolymers or related compounds. However, styrene-based compounds are not easily recycled, therefore there is still a need for pressure sensitive adhesives which offer improved re-closable characteristics, in particular for re-closable food packaging, and allow easy and effective recycling of the packaging. This need is address addressed by the present invention which provides a pressure sensitive adhesive composition especially useful in re-closable food packaging.

[0007] In this regard, a first object of the present invention is a pressure sensitive adhesive composition comprising:

• • [0008] a) at least one butene-1 (co)polymer;
  • [0009] b) at least one solid tackifier; and
  • [0010] c) at least one polyisobutylene polymer.

[0011] It was surprisingly found that the pressure sensitive adhesive composition of the present invention shows excellent re-closable characteristics while at the same time possessing a high transparency and low melt flow index (MFI) which is especially important for good processability, in particular by extrusion or melt blowing, typical techniques of manufacturing re-closable food packaging. In addition, the pressure sensitive adhesive of the present invention exhibits only a slight odor which makes it especially suitable for food packaging.

[0012] The butene-1 (co)polymer contained in the pressure sensitive adhesive composition according to the present invention is typically exhibiting a behavior ranging from elastomeric to plastomeric and can be a homopolymer or a copolymer of butene-1 with one or more α-olefins different from butene-1. Preferred are α-olefins, which may be present as comonomers in the butene-1 (co)polymer, are those of formula H₂CH═CHR, where R is H or methyl or a C3-6 linear or branched alkyl, in particular ethylene, propylene, pentene-1, hexene-1,4-methylpentene and octene-1. Particularly preferred as comonomer is ethylene.

[0013] In a preferred embodiment, the butene-1 (co)polymer of the pressure sensitive adhesive composition of the present invention has a content of butene-1 derived units of 80 wt.-% or more, preferably 90 to 99 wt.-%, based on the weight of the (co)polymer.

[0014] Preferably, the butene-1 (co)polymer has a low crystallinity of less than 40%, preferably less than 30%, even more preferably lower than 20%, measured via X-ray.

[0015] Preferably, the butene-1 (co)polymer has a density of 0.899 g/cm³ or less, more preferably of 0.895 g/cm³ or less, even more preferably lower than 0.875 g/cm³, determined according to ISO 1183.

[0016] Preferably, the butene-1 (co)polymer has hardness shore A lower than 90, more preferably lower than 70, determined according to ISO 868.

[0017] Preferably, the butene-1 (co)polymer has a content of xylene insoluble fraction at 0° C. from 1 to 60 wt.-%, preferably from 1 to 20 wt.-%, more preferably less than 15 wt.-%.

[0018] Preferably, the butene-1 (co)polymer has an intrinsic viscosity [η] measured in tetralin at 135° C. from 1 to 4 dL/g, preferably from 1.5 to 2.5 dL/g.

[0019] Preferably, the butene-1 (co)polymer has a distribution of molecular weights (Mw/Mn) measured by GPC lower than 4.

[0020] In a preferred embodiment, the at least one butene-1 (co)polymer is comprised in the pressure sensitive adhesive composition in an amount of 30 to 80 wt.-%, preferably 35 to 75 wt.-%, based on the total weight of the composition, respectively.

[0021] The pressure sensitive adhesive comprises at least one solid tackifier. The presence of solid tackifiers was found to increase adhesive strength to different substrates, especially to low-energy foils like polyethylene or oriented polypropylene films.

[0022] In a preferred embodiment, the at least one solid tackifier is selected from the group consisting of hydrocarbon resins, preferably hydrogenated hydrocarbon resins; aromatic modified resins, terpen phenol resins, piperylene, 2-methyl-2-butene copolymers, cyclopentadiene resins or mixtures thereof and/or the at least one solid tackifier has a softening point (measured according to ASTM E28-99) of 80 to 150° C.; more preferably 85 to 130° C., most preferably 90 to 115° C.

[0023] The at least one solid tackifier employed according to the present invention can be selected from aliphatic hydrocarbon resins, aromatic modified aliphatic hydrocarbon resins, hydrogenated polycyclopentadiene resins, polycyclopentadiene resins, gum rosins, gum rosin esters, wood rosins, wood rosin esters, tall oil rosins, tall oil rosin esters, polyterpenes, aromatic modified polyterpenes, terpene phenolics, aromatic modified hydrogenated polycyclopentadiene resins, hydrogenated aliphatic resin, hydrogenated aliphatic aromatic resins, hydrogenated terpenes and modified terpenes, hydrogenated rosin acids, and hydrogenated rosin esters. In some embodiments the tackifier is hydrogenated.

[0024] The term “hydrogenated” as used herein refers to hydrocarbon resins which are at least 95%, preferably at least 98%, more preferably at least 99%, and in particular fully hydrogenated.

[0025] Preferred hydrocarbon resins for use as tackifiers or modifiers include: Resins such as C5/C6 terpene resins, styrene terpenes, alpha-methyl styrene terpene resins, C9 terpene resins, aromatic modified C5/C6, aromatic modified cyclic resins, aromatic modified dicyclopentadiene based resins or mixtures thereof. Typically these resins are obtained from the cationic polymerization of compositions containing one or more of the following monomers: C5 diolefins, such as 1,3-pentadiene, isoprene; C5 olefins, such as 2-methylbutenes, cyclopentene; C6 olefins, such as hexene; C9 vinylaromatics, such as styrene, alpha methyl styrene, vinyltoluene, indene, methyl indene; cyclics, such as dicyclopentadiene, methyldicyclopentadiene; and/or terpenes, such as limonene, carene.

[0026] Another class of preferred solid tackifiers are resins obtained by the thermal polymerization of dicyclopentadiene and/or the thermal polymerization of dimers or oligomers of cyclopentadiene and/or methylcyclopentadiene, optionally with vinylaromatics such as styrene, alpha-methyl styrene, vinyl toluene, indene or methyl indene. The resins obtained after polymerization and separation of unreacted materials are preferably hydrogenated.

[0027] In a preferred embodiment, the pressure sensitive adhesive composition comprises the at least one solid tackifier in an amount of 0.5 to 50 wt.-%, preferably 5 to 45 wt.-%, more preferably 10 to 40 wt.-%, based on the total weight of the composition, respectively.

[0028] The pressure sensitive adhesive may comprise a mixture of solid tackifiers. In a preferred embodiment, the pressure sensitive adhesive comprises at least two solid tackifiers. In an especially preferred embodiment, the first solid tackifier comprises a hydrogenated hydrocarbon C9 resin and the second solid tackifier comprises a C5 resin. The first and second solid tackifiers are preferably present in the pressure sensitive adhesive composition in a weight ratio of 1:1.

[0029] The pressure sensitive adhesive composition of the present invention comprises at least one polyisobutylene. In a preferred embodiment, the composition comprises the at least one polyisobutylene polymer in an amount of 0.5 to 30 wt.-%, preferably 0.5 to 15 wt.-%, more preferably 1 to 15 wt.-%, especially 0.5 to 7.5 wt.-%, based on the total weight of the composition, respectively.

[0030] In a further preferred embodiment, the at least one polyisobutylene has a molecular weight Mw of 40 000 to 1 110 000 g/mol, measured via GPC using polystyrene standards, and/or a Mw/Mn of lower than 5, measured via GPC using polystyrene standards. In an especially preferred embodiment, the polyisobutylene may be comprised in the pressure sensitive adhesive composition in form of medium-molecular polyisobutylene. The polyisobutylene preferably has a molecular weight Mw of 10{circumflex over ( )}4 to 5*10{circumflex over ( )}6 g/mol more preferable 5*10{circumflex over ( )}4 to 10{circumflex over ( )}6 g/mol even more preferable 10{circumflex over ( )}5 to 10{circumflex over ( )}6 g/mol.

[0031] Within the course of the present invention, it was surprisingly found that a pressure sensitive adhesive composition could be provided which shows excellent re-closable characteristics without the need of styrene block copolymers which are commonly employed. Therefore, in a preferred embodiment, the pressure sensitive adhesive composition is free of styrene block copolymers. Preferably, the content of styrene block copolymer in the composition is less than 1 wt.-%, more preferably less than 0.5 wt.-%, in particular less than 0.1 wt.-%, based on the total weight of the composition respectively.

[0032] The pressure sensitive adhesive composition according to the present invention is in particular designed for being processed via extrusion or melt blowing. In this regard, a low melt flow index was found to be of particular advantage. Whereas common pressure sensitive adhesive compositions show a melt flow index (MFI) of about 20 to 30 g/10 min, the pressure sensitive adhesive composition according to the invention surprisingly showed a reduced melt flow index. Thus, in a preferred embodiment, the pressure sensitive adhesive composition has a melt flow index (MFI) of 2 to 30 g/10 min, preferably 2 to 25 g/10 min, more preferably 2 to 20 g/10 min, in particular 2 to 18 g/10 min, measured according to DIN EN ISO 1133-1:2012-03; 2.16 kg, 190° C./10 min.

[0033] The pressure sensitive adhesive composition of the present invention may comprise further components. In a preferred embodiment, the pressure sensitive adhesive composition comprises at least one further polymer different from a) and c). The further polymer is preferably an olefin (co)polymer, such as an alpha-olefin (co)polymer; more preferably a metallocene catalyzed alpha-olefin (co)polymers, even more preferably an ethylene or octene alpha-olefin (co)polymer; most preferably an octene ethylene alpha-olefin copolymer.

[0034] The pressure sensitive adhesive composition according to the invention is especially suitable for use in food packaging, in particular re-closable food packaging. Therefore, a further object of the present invention is the use of a pressure sensitive adhesive composition according to the invention as closing means for re-closable packaging unit, preferably a packaging unit for foods, drugs, hygienic tissues, cleaning tissues or cosmetic tissues and/or for packaging units to be heated in a microwave or oven.

[0035] In another aspect, the present invention refers to an article comprising a pressure sensitive adhesive composition according to the invention as an adhesive layer between two substrates.

[0036] The substrates preferably contain or are made of a material selected from the group consisting of PE (polyethylene), LDPE (low density polyethylene), PP (polypropylene), OPP (oriented polypropylene), PET (polyethylene terephthalate), APET (amorphous PET), PET-G (glycol modified polyethyleneterephthalate), recycled PET (rPET) or CPET (crystalline PET). Those are typical substrates as used in the production of re-closable food packaging units. The pressure sensitive adhesive composition of the present invention not only showed good compatibility with these substrates but was also found to be easily formed into a film suitable to be processed in the manufacturing of packaging units, for example by extrusion or melt blowing.

[0037] The article is preferably one selected from the group consisting of packaging unit for foods, drugs, hygienic tissues, cleaning tissues or cosmetic tissues and/or packaging units to be heated in a microwave or oven.

[0038] The article comprising the pressure sensitive adhesive composition of the present invention may be manufactured according to methods known to the person skilled in the art. Accordingly, a further object of the present invention is a method for manufacturing an article according to the present invention, the method comprising the following steps:

• • [0039] i) forming a film of the pressure sensitive adhesive composition according to the invention through a blow film head and/or nozzle;
  • [0040] ii) bringing the formed film of step i) as middle layer in contact with two further layers.

[0041] The present invention will be explained in more detail with reference to the following examples, which by no means are to be understood as limiting the scope and spirit of the invention.

[0042] Following compounds were employed:

• • [0043] Koattro KT MR 06: Polybutene-1 polymer from Lyondell Basell
  • [0044] Oppanol N80: polyisobutylene Mn=8*10{circumflex over ( )}5 g/mol from BASF
  • [0045] Oppanol N50: polyisobutylene Mn=5*10{circumflex over ( )}5 g/mol from BASF
  • [0046] Regalite R1010: hydrogenated hydrocarbon resin from Eastman
  • [0047] Kraton 1200 HT: styrene-isoprene-styrene star shaped blockcopolymer
  • [0048] Escorez 2203 LC: tackifying resin from ExxonMobil
  • [0049] Escorez 5380: cycloaliphatic hydrocarbon resin from ExxonMobil,
  • [0050] Irganox 565: Stabilizer from BASF
  • [0051] Irganox B 225: Stabilizer from BASF

[0052] Several pressure sensitive adhesive compositions were prepared as summarized in Table 1 with the content given in wt.-% based on the total weight of the respective composition.

TABLE 1
Component	Com.-Example 1	Example 2	Example 3
Koattro KT MR 06		60	63.7
Koattro KT MR 05	40
Oppanol N50			2.5
Oppanol N80		10
Regalite 1010	9.5
Kraton 1200 HAT	20
Escorez 2203 LC	30	14.6	14.8
Escorez 5380		14.7	18.4
Irganox 565		0.2	0.2
Irganox B 225	0.5	0.5	0.5
Add	100	100	100

[0053] Example 1, which serves as a comparative example, contains a styrene block copolymer, while in Example 2 and Example 3 according to the invention no styrene block copolymers are used.

[0054] The compositions were analyzed with regard to their re-closable properties, viscosity and MFI according to the following test methods:

• • [0055] Viscosity: Rheometer TA DHR2; ASTM D 3236; 200° C.

[0056] Test method for re-closable characteristics: The pressure sensitive adhesive was coated on PE with a coating knife (300 μm) and dried for 1 day at room temperature, followed by 1 hour at 90° C. The dried layer (thickness of dried layer about 20 μm) was adhered on PE and a 25 mm thick strip was rolled up. The adhesion was measured right after production and again measured after 24 hours. Immediately after the measurement the strip was rolled up again (with a 2 kg roller) and the adhesion was measured again. This step was repeated three times. The adhesion was measured according to DIN EN ISO 11339:2010-06 (unit N/25 mm; measurement 300 mm/Min). The re-closability given in Table 2 corresponds to the one measured after the fourth time.

[0057] Melt flow index: DIN EN ISO 1133-1:2012-03; 2.16 kg, 190° C./10 min

[0058] Results are given as −− (bad); − (sufficient); 0 (acceptable); + (good); ++ (very good)

[0059] The results are summarized in Table 2:

	Composition	Example 1	Example 2	Example 3
	Viscosity (200° C.)	300 Pa*s	1700 Pa*s	800 Pa*s
	Re-closable	+	+	++
	characteristics
	MFI	20	8	10

[0060] As can be depicted from the data provided in Table 2, the pressure sensitive adhesive compositions showed excellent viscosity while at the same time having more than satisfying re-closable characteristics and MFI.