A process for producing scratch-resistant coatings, encompassing the following steps:

The present invention relates to a process for producing scratch-resistant coatings on the basis of radiation-curable coating compositions.
Coating compositions which cure by UV radiation are used in industry to produce high-quality coatings. Radiation-curable coating compositions are generally flowable formulations based on polymers or oligomers containing crosslinking-active groups which on exposure to UV radiation undergo a crosslinking reaction with one another. This results in the formation of a high molecular mass network and thus in the development of a solid polymeric film. Unlike the heat-curable coating compositions often used to date, radiation-curable coating compositions may be used free from solvents or dispersants. They are further notable for very short curing times, which is particularly advantageous in the case of continuous processing on coating lines.
Coating compositions curable by UV radiation generally give high surface hardness and good chemical resistance. For some time there has been a desire for coatings which possess high scratch resistance, so that when it is cleaned, for example, the coating is not damaged and does not lose its gloss. At the same time, the coatings should retain the properties normally achieved with radiation-cured coatings.
In the literature there have been various descriptions of the physical processes involved in the appearance of scratches and the relationships between scratch resistance and other physical parameters of the coating (on scratch-resistant coatings cf., e.g., J. L. Courter, 23rd Annual International Waterborne, High-Solids and Powder Coatings Symposium, New Orleans 1996).
A variety of test methods have been described to quantify the scratch resistance of a coating. Examples include testing by means of the BASF brush test (P. Betz and A. Bartelt, Progress in Organic Coatings 22 (1993) 27-37), by means of the AMTEC wash brush installation, or various test methods analogous to scratch hardness measurements, as described for example by G. Jxc3xcttner, F. Meyer, G. Menning, Kunststoffe 88 (1988) 2038-42. A further test to determine scratch resistance is described in European Coatings Journal 4/99, 100 to 106.
In accordance with the present state of development, three routes to scratch-resistant surfaces are being discussed, which in principle may also be transferred to UV-curing systems.
The first route is based on increasing the hardness of the coating material. For example, EP-A 544 465 describes a coating composition for scratch-resistant coatings which comprises colloidal silica and alkolysis products of alkoxysilyl acrylates. The increase in hardness is based here on the incorporation of the silica into the polymer matrix of the coating. However, the high level of hardness is at the expense of other properties, such as the penetration hardness or the adhesion, which are vital to coating materials.
The second route is based on selecting the coating material such that on scratching it is stressed in the reversible deformation range. The materials involved are those which permit high reversible deformation. However, there are limits on the use of elastomers as coating materials. Coatings of this kind usually exhibit poor chemical stability.
A third approach attempts to produce coatings having a ductile, i.e., plastic deformation behavior and at the same time to minimize the shear stress within the coating material that occurs in scratching. This is done by reducing the friction coefficient, using waxes or slip additives, for example. Coatings additives for UV-curing systems are described, for example, in B. Hackl, J. Dauth, M. Dreyer; Farbe and Lack 103 (1997) 32-36.
U.S. Pat. No. 5,700,576 describes a UV-curing, scratch-resistant coating which comprises 1-30% by weight of a prepolymeric thickener containing thiol groups and 20-80% by weight of one or more polyfunctional acrylates or methacrylates, and also diluents, especially reactive diluents containing a free-radically polymerizable group, free-radical initiators, and further customary additives for producing coatings. The polymerization and thus curing of the coating is initiated by irradiation with UV light, under inert gas, for example.
However, the solutions proposed for producing scratch-resistant coatings are unsatisfactory because they are comparatively expensive and because the other coating properties are not satisfactory.
In another invention, which is the subject of a parallel application, it has been found that scratch-resistant coatings having a balanced profile of properties can be produced if a radiation-curable coating based on urethane acrylates is cured under inert gas conditions. Inert gases generally contain not more than 500 ppm of oxygen, which under standard conditions corresponds to an oxygen partial pressure of less than 0.05 kPa. The substantial exclusion of oxygen requires an expensive technology. In order to exclude oxygen, the curing of the coating on structures, i.e., nonplanar articles having a three-dimensional form, has to be carried out in chambers closed off to the outside and maintained strictly under an inert gas atmosphere. Especially in the case of continuous coating lines, this would necessitate an expensive airlock technology and would therefore be uneconomic.
It is an object of the present invention to provide a simple process for producing scratch-resistant coatings which overcomes the disadvantages of the prior art.
We have found that this object is achieved if a conventional radiation-curable coating composition is cured by exposure to ultraviolet radiation in an oxygen-containing, protective-gas atmosphere having a oxygen partial pressure of not more than 18 kPa, without the need to observe strict inert gas conditions.
The present invention accordingly provides a process for producing scratch-resistant coatings, encompassing the following steps:
applying at least one UV-curable coating composition to at least one surface of an article to be coated, said coating composition comprising at least one polymer and/or oligomer P1 containing on average at least one ethylenically unsaturated double bond per molecule, and
curing the coating composition by exposure to UV radiation,
which comprises conducting the curing of the coating composition under an oxygen-containing protective gas which has an oxygen partial pressure in the range from 0.2 to 18 kPa.
In the case of a protective gas under atmospheric pressure, an oxygen partial pressure of 18 kPa corresponds to an oxygen fraction of about 20% by volume. Under the same conditions, an oxygen partial pressure of 0.2 kPa corresponds to a volume fraction of 2200 ppm of oxygen in the protective gas (cf. also E. W. Bader [Ed.], Handbuch der gesamten Arbeitsmedizin [Handbook of complete occupational hygiene], Vol. 1, Urban und Schwarzenberg, Berlin, Munich, Vienna, 1961, p. 665). An oxygen partial pressure of 9 kPa corresponds to 10% by volume of oxygen in the protective gas.
For the process of the invention all that is necessary is for the coating compositions to be subject to an oxygen concentration of less than 18 kPa in the regions where curing takes place at the time of their exposure to UV radiation. The relevant regions are the surface regions of the article to be costed which have been provided with the radiation-curable coating compositions, at the time of their exposure to UV radiation. In order to attain optimum scratch resistance, the oxygen partial pressure is preferably not more than 17 kPa (≈19% by volume), in particular not more than 15.3 kPa (≈17% by volume), and with particular preference not more than 13.5 kPa (≈15% by volume). Optimum curing results are generally obtained at oxygen partial pressures in the range from 0.5 kPa to 10 kPa (≈5500 ppmxe2x88x9211% by volume), in particular in the range from 0.5 to 6.3 kPa (≈5500 ppmxe2x88x927% by volume). Typically, the oxygen partial pressure will not be below a level of 0.5 kPa, especially 0.9 kPa (≈1% by volume), 1.8 kPa (≈2% by volume), or 2.5 kPa (≈3% by volume).
Suitable protective gases include inert gases such as nitrogen, carbon monoxide, carbon dioxide and noble gases, e.g., argon, and mixtures thereof with air or oxygen, preferred inert gases being argon and nitrogen, especially nitrogen.
Suitable polymers P1 for the radiation-curable formulations of the invention are in principle all polymers and/or oligomers having on average at least one ethylenically unsaturated double bond per polymer or oligomer molecule, which may be free-radically polymerized under the action of electromagnetic radiation, such as UV radiation.
In general, the amount of ethylenically unsaturated double bonds in P1 will be situated within the range from 0.01 to 1.0 mol/100 g of P1, preferably in the range from 0.05 to 0.8 mol/100 g of P1, and with very particular preference from 0.1 to 0.6 mol/100 g of P1. The terms polymer and oligomer as used here and below embrace addition polymers, polycondensates and polyaddition products, chemically modified polymers, and prepolymers. Suitable prepolymers are obtainable, for example, by reacting polyfunctional compounds having at least two reactive groups with monofunctional or polyfunctional compounds having at least one ethylenically unsaturated double bond and at least one reactive group which is able to react with the reactive groups of the abovementioned polyfunctional compounds with formation of bonds.
The polymers and/or oligomers generally have a number-average molecular weight Mn of at least 400 g/mol. Preferably, Mn is not more than 50,000 and in particular is situated within the range from 500 to 5000.
In the process of the invention it is preferred to use coating compositions whose polymers or oligomers P1 contain per molecule on average at least 2 and with particular preference from 3 to 6 double bonds.
The polymers or oligomers P1 preferably have a double bond equivalent weight of from 400 to 2000, with particular preference from 500 to 900.
Furthermore, the radiation-curable coating compositions preferably have a viscosity of from 250 to 11,000 mPas (as determined by means of a rotational viscometer in accordance with DIN EN ISO 3319).
Radiation-curable polymers and/or oligomers P1 of this kind are sufficiently well known to the skilled worker. An overview of such coating compositions is given, for example, in P. K. T. Oldring (editor) Chemistry and Technology of UV and EB Formulations for Coatings and Paints, Vol. II, SITA Technology, London, 1991. The full content of said work insofar as it describes radiation-curable coating compositions is hereby incorporated by reference.
In the polymers or oligomers P1, the double bonds generally have a vinylidene structure (CH2=CR structure where R=H or CH3) which is derived from vinyl, allyl or methallyl esters, ethers or amines or from xcex1,xcex2-ethylenically unsaturated monocarboxylic acids such as acrylic acid, methacrylic acid or their amides. In the process of the invention, preference is given to polymers and/or oligomers P1 whose double bonds are in the form of acrylate, methacrylate, acrylamide or methacrylamide groups. Examples thereof are polyether acrylates, polyester acrylates, unsaturated polyesters, epoxy acrylates, urethane acrylates, amino acrylates, melamine acrylates, silicone acrylates, and the corresponding methacrylates. Particularly preferred polymers and/or oligomers P1 are selected from urethane (meth)acrylates, polyester (meth)acrylates, oligoether (meth)acrylates, and epoxy (meth)acrylates, particular preference being given, with regard to weathering stability of the coatings, to urethane (meth)acrylates and polyester (meth)acrylates, especially aliphatic urethane acrylates.
The silicone (meth)acrylates are generally linear or cyclic polydimethylsiloxanes having acrylic and/or methacrylic groups which are connected via an oxygen atom or via an alkylene group to the silicon atoms of the polydimethylsiloxane parent structure. Silicone acrylates are described, for example, in P. K. T. Oldring (see above), pp. 135 to 152. The disclosure made therein is hereby incorporated fully by reference.
Suitable ethylenically unsaturated epoxy acrylates are, in particular, the reaction products of oligomers or compounds containing epoxy groups with acrylic acid or methacrylic acid. Typical compounds containing epoxy groups are the polyglycidyl ethers of polyhydric alcohols. These include the diglycidyl ethers of bisphenol A and of its derivatives, and also the diglycidyl ethers of oligomers of bisphenol A, as obtainable by reacting bisphenol A with the diglycidyl ether of bisphenol A, and, further, the polyglycidyl ethers of novolaks. The reaction products of acrylic acid and/or methacrylic acid with the abovementioned epoxides may further be modified with primary or secondary amines. In addition, further ethylenically unsaturated groups may be introduced into the epoxy (meth)acrylates by reacting OH groups in epoxy resins with suitable derivatives of ethylenically unsaturated carboxylic acids, examples being the acid chlorides. Epoxy (meth)acrylates are sufficiently well known to the skilled worker and are available commercially. For further details, reference is made to P. K. T. Oldring, pages 37 to 68, and the literature cited therein.
Melamine acrylates are understood to be the reaction products of melamine/formaldehyde condensation products with hydroxyalkyl esters of acrylic acid or of methacrylic acid, and also with acrylic acid, methacrylic acid or with their ester-forming derivatives. Examples of suitable melamine/formaldehyde condensation products are hexamethylol melamine (HMM) and hexamethoxymethylolmelamine (HMMM). Furthermore, both HMM and HMMM may be modified with the amides of ethylenically unsaturated carboxylic acids, an example being acrylamide or methacrylamide, to give ethylenically unsaturated melamine (meth)acrylates. Melamine (meth)acrylates are known to the skilled worker and are described, for example, in P. K. T. Oldring, pp. 208 to 214, and also in EP-A 464 466 and DE-A 25 50 740, to which reference is made for further details. Polyester (meth)acrylates are likewise known to the skilled worker. They are obtainable by a variety of methods. For example, acrylic acid and/or methacrylic acid may be used directly as the acid component when synthesizing the polyesters. A further possibility is to use hydroxyalkyl esters of (meth)acrylic acid as the alcohol component, directly, when synthesizing the polyesters.
The polyester (meth)acrylates are preferably prepared by reacting hydroxyl-containing polyesters with acrylic or methacrylic acid or their ester-forming derivatives. It is also possible to start from carboxyl-containing polyesters, which are then reacted with a hydroxyalkyl ester of acrylic or methacrylic acid. Unreacted (meth)acrylic acid may be removed from the reaction mixture by washing, distillation or, preferably, by reaction with an equivalent amount of a monoepoxide or diepoxide compound with the use of suitable catalysts, such as triphenylphosphine, for example. The products of this reaction generally remain in the radiation-curable coating composition and are incorporated into the polymer network in the course of curing. For further details, reference may be made to P. K. T. Oldring, pp. 123 to 135. Their number-average molecular weight is generally in the range from 500 to 10,000 and preferably in the range from 800 to 3000.
Suitable polyesters containing hydroxyl groups for the preparation of polyester (meth)acrylates may be prepared in conventional manner by polycondensing dibasic or polybasic carboxylic acids with diols and/or polyols, the OH-containing component being used in excess. Accordingly, polyesters containing carboxyl groups are prepared by employing the carboxyl-containing component in excess. Suitable carboxylic acid components in this case include aliphatic and/or aromatic C3-C36 carboxylic acids, their esters and anhydrides. They include maleic acid, maleic anhydride, succinic acid, succinic anhydride, glutaric acid, glutaric anhydride, adipic acid, pimellic acid, suberic acid, azelaic acid, sebacic acid, phthalic acid, phthalic anhydride, isophthalic acid, terephthalic acid, tetrahydrophthalic acid, tetrahydrophthalic anhydride, trimellitic acid, trimellitic anhydride, pyromellitic acid, and pyromellitic anhydride. Examples of suitable diol components are ethylene glycol, propylene glycol, 1,4-butanediol, 1,5-pentanediol, neopentyl glycol, 1,6-hexanediol, 2-methyl-1,5-pentanediol, 2-ethyl-1,4-butanediol, dimethylolcyclohexane, diethylene glycol, triethylene glycol, mixtures thereof, and also polyaddition polymers of cyclic ethers, such as polytetrahydrofuran, polyethylene glycol, and polypropylene glycol. Higher polyfunctional alcohols that are suitable include in particular trihydric to hexahydric alcohols, such as glycerol, trimethylolethane, trimethylolpropane, trimethylolbutane, pentaerythritol, dipentaerythritol, ditrimethylolpropane, sorbitol, erythritol, and 1,3,5-trihydroxybenzene, and also the alkoxylated derivatives of the abovementioned polyfunctional alcohols. Polyether (meth)acrylates are likewise known in principle to the skilled worker. Polyether (meth)acrylates are composed of a polyether base unit having acrylate and/or methacrylate groups at its ends. The polyether base unit is obtainable, for example, by controlled. polymerization of epoxides such as ethylene oxide or propylene oxide or by reacting a polyhydric alcohol, for example, an alcohol specified above as the polyol component for preparing polyesters, with epoxides such as ethylene oxide and/or propylene oxide. This polyether base unit further contains free OH groups which may in accordance with known methods be esterified with acrylic acid and/or methacrylic acid, or ester-forming derivatives such as acid chlorides, C1-C4 alkyl esters, or anhydrides (cf., e.g., Houben-weyl, Volume XIV, 2, Makromolekulare Stoffe II (1963)). Suitable polyethers also include polymerization products of tetrahydrofuran and of oxetane.
Flexibilization of the polyether (meth)acrylates and of the polyester (meth)acrylates is possible, for example, by reacting corresponding OH-functional prepolymers and/or oligomers (based on polyether or polyester) with relatively long-chain aliphatic dicarboxylic acids, especially aliphatic dicarboxylic acids having at least 6 carbon atoms, such as adipic acid, sebacic acid, dodecanedioic acid, and/or dimeric fatty acids, for example. This flexibilization reaction may be conducted either before or after the addition of acrylic and/or methacrylic acid to the oligomers and/or prepolymers.
The invention""s preferred urethane (meth)acrylates generally comprise oligomeric compounds containing urethane groups and acryloxyalkyl and/or methacryloxyalkyl groups or (meth)acrylamidoalkyl groups. Urethane (meth)acrylates normally have a number-average molecular weight Mn in the range from 500 to 5000 daltons, preferably in the range from 500 to 2000 daltons (determined by means of GPC on the basis of authentic comparison samples). Preferred in accordance with the invention are urethane (meth)acrylates having on average at least two double bonds, especially those having on average from three to six double bonds per molecule. The aliphatic urethane (meth)acrylate prepolymers PU which are particularly preferred in accordance with the invention are essentially free from aromatic structural elements, such as phenylene or naphthylene or substituted phenylene or naphthylene groups.
The urethane (meth)acrylates or mixtures thereof with a reactive diluent that are employed in accordance with the invention preferably have a viscosity (as determined using a rotational viscometer in accordance with DIN EN ISO 3319) in the range from 250 to 11,000 mPa.s, in particular in the range from 2000 to 7000 mPa.s.
The aliphatic urethane (meth)acrylates are known in principle to the skilled worker and may be prepared, for example, as described in EP-A-203 161. The content of that document, insofar as it relates to urethane (meth)acrylates and their preparation, is hereby incorporated fully by reference.
Urethane (meth)acrylates preferred in accordance with the invention are obtainable by reacting at least 25% of the isocyanate groups of a compound containing isocyanate groups (component A) with at least one hydroxyalkyl ester of acrylic acid and/or of methacrylic acid (component B) and, if desired, with at least one further compound having at least one functional group which is reactive toward isocyanate groups (component C), examples being chain extenders C1.
The relative amounts of components A, B and C are preferably chosen such that
1. the ratio of equivalents of the isocyanate groups in component A to the reactive groups in component C is between 3:1 and 1:2, preferably between 3:1 and 1.1:1, and in particular about 2:1, and
2. the hydroxyl groups of component B correspond to the stoichiometric amount of the free isocyanate groups of component A, i.e., to the difference between the total number of isocyanate groups of component A minus the reactive groups of component C (or minus the reacted reactive groups of component C if only partial reaction of the reactive groups is intended).
Preferably, the urethane (meth)acrylate contains no free isocyanate groups. In one advantageous embodiment, therefore, component B is reacted in a stoichiometric ratio with the free isocyanate groups of the reaction product of component A and component C.
The urethane (meth)acrylates may also be prepared by first reacting some of the isocyanate groups of a low molecular mass diisocyanate or polyisocyanate, as component A, with at least one hydroxyalkyl ester of an ethylenically unsaturated carboxylic acid, as component B, and subsequently reacting the remaining isocyanate groups with component C, e.g., with a chain extender C1. In this case it is also possible to use mixtures of chain extenders.
In this case also, the relative amounts of components A, B and C are chosen such that the ratio of equivalents of the isocyanate groups to the reactive groups of the chain extender is between 3:1 and 1:2, preferably 2:1, and the ratio of equivalents of the remaining isocyanate groups to the hydroxyl groups of the hydroxyalkyl ester is 1:1.
Compounds A containing isocyanate groups are understood, hereinbelow, to be low molecular mass, aliphatic or aromatic diisocyanates or polyisocyanates and also aliphatic or aromatic polymers or oligomers containing isocyanate groups (prepolymers) having at least two and preferably from three to six free isocyanate groups per molecule. The boundary between the low molecular mass diisocyanates or polyisocyanates and the prepolymers containing isocyanate groups is fluid. Typical prepolymers containing isocyanate groups generally have a number-average molecular weight Mn in the range from 500 to 5000 daltons, preferably in the range from 500 to 2000 daltons. The low molecular mass diisocyanates or polyisocyanates preferably have a molecular weight of less than 500 daltons, in particular of less than 300 daltons.
Typical aliphatic diisocyanates or polyisocyanates of low molecular mass are tetramethylene diisocyanate, hexamethylene diisocyanate, octamethylene diisocyanate, decamethylene diisocyanate, dodecamethylene diisocyanate, tetradecamethylene diisocyanate, 1,6-diisocyanato-2,2,4-trimethylhexane, 1,6-diisocyanato-2,2,4,4-tetramethylhexane, 1,2-, 1,3- or 1,4-diisocyanatocyclohexane, 4,4xe2x80x2-di(isocyanatocyclohexyl)methane, 1-isocyanato-3,3,5-trimethyl-5-(isocyanatomethyl)cyclohexane (isophorone diisocyanate), 2,4- or 2,6-diisocyanato-1-methylcyclohexane, and also the uretdiones, biurets, cyanurates and allophanates of the abovementioned diisocyanates. Examples of aromatic diisocyanates and polyisocyanates are diisocyanates, such as 2,4-diisocyanatotoluene, 2,6-diisocyanatotoluene, tetramethylxylylene diisocyanate, 1,4-diisocyanatobenzene, 4,4xe2x80x2- and 2,4-diisocyanatodiphenylmethane, p-xylylene diisocyanate, and also isopropenyldimethyltolylene diisocyanate and also the uretdiones, biurets, cyanurates and allophanates of the abovementioned aromatic diisocyanates.
The polyisocyanates containing isocyanurate groups comprise, in particular, simple triisocyanato isocyanurates, which represent cyclic trimers of the diisocyanates, or comprise mixtures with their higher homologs having more than one isocyanurate ring. Mention may be made here by way of example of the isocyanurate of hexamethylene diisocyanate and of the cyanurate of toluene diisocyanate, which are available commercially. Cyanurates are used preferably in preparing urethane (meth)acrylates.
Uretdione diisocyanates comprise cyclic dimerization products of diisocyanates. The uretdione diisocyanates may be used, for example, as sole component or in a mixture with other polyisocyanates, especially with the polyisocyanates containing isocyanurate groups, to prepare urethane (meth)acrylates. Suitable polyisocyanates containing biuret groups preferably have an NCO content of from 18 to 22% by weight and an average NCO functionality of from 3 to 4.5.
Allophanates of the diisocyanates may be obtained, for example, by reacting excess amounts of diisocyanates with simple, polyhydric alcohols, such as, for example, trimethylolpropane, glycerol, 1,2-dihydroxypropane, or mixtures thereof. Polyisocyanates containing allophanate groups that are suitable for preparing urethane (meth)acrylates generally have an NCO content of from 12 to 20% by weight and an average NCO functionality of from 2.5 to 3.
Suitable hydroxyalkyl esters of acrylic acid and of methacrylic acid (component B) are the monoesters of acrylic acid and, respectively, of methacrylic acid with C2-C10 alkanediols, such as 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 3-hydroxypropyl acrylate, 3-hydroxypropyl methacrylate, 4-hydroxybutyl acrylate, and 4-hydroxybutyl methacrylate. As well as or in addition to the hydroxyalkyl esters of acrylic acid and/or of methacrylic acid it is also possible to use other hydroxyl-containing esters of acrylic acid and/or of methacrylic acid in order to introduce double bonds into the urethane (meth)acrylate prepolymer, such as trimethylolpropane diacrylate or dimethacrylate, and also hydroxyl-carrying amides of acrylic acid and of methacrylic acid, such as 2-hydroxyethylacrylamide and 2-hydroxyethylmethacrylamide.
Suitable chain extenders (component C1) are aliphatic diols or polyols having up to 20 carbon atoms, such as ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, 1,4-butanediol, 1,5-pentanediol, neopentyl glycol, 1,6-hexanediol, 2-methyl-1,5-pentanediol, 2-ethyl-1,4-butanediol, 2,2-bis(4xe2x80x2-hydroxycyclohexyl)propane, dimethylolcyclohexane, glycerol, trimethylolethane, trimethylolpropane, trimethylolbutane, pentaerythritol, ditrimethylolpropane, erythritol and sorbitol; diamines or polyamines having up to 20 carbon atoms, such as ethylenediamine, 1,3-propanediamine, 1,2-propanediamine, neopentanediamine, hexamethylenediamine, octamethylenediamine, isophoronediamine, 4,4xe2x80x2-diaminodicyclohexylmethane, 3,3xe2x80x2-dimethyl-4,4xe2x80x2-diaminodicyclohexylmethane, 4,7-dioxadecane-1,10-diamine (3,3xe2x80x2-bis[1,2-ethanediylbis(oxy)]-1-propanamine), 4,9-dioxadodecane-1,12-diamine (3,3xe2x80x2-bis[1,3-butanediylbis(oxy)]-1-propanamine), 4,7,10-trioxatridecane-1,13-diamine (3,3xe2x80x2-bis[oxybis(2,1-ethanediyloxy)]-1-propanamine), 2-(ethylamino)ethylamine, 3-(methylamino)propylamine, diethylenetriamine, N3 Amine (N-(2-aminoethyl)-1,3-propylenediamine), dipropylenetriamine or N4 Amine (N,Nxe2x80x2-bis(3-aminopropyl)ethylenediamine); alkanolamines having up to 20 carbon atoms, such as monoethanolamine, 2-amino-1-propanol, 3-amino-1-propanol, 2-amino-1-butanol, isopropanolamine, 2-amino-2-methyl-1-propanol, 5-amino-1-pentanol, 2-amino-1-pentanol, 6-aminohexanol, methylaminoethanol, 2-(2-aminoethoxy)ethanol, N-(2-aminoethyl)ethanolamine, N-methylethanolamine, N-ethylethanolamine, N-butylethanolamine, diethanolamine, 3-(2-hydroxyethylamino)-1-propanol or diisopropanolamine; and dimercaptans or polymercaptans having up to 20 carbon atoms, such as 1,2-ethanedithiol, 1,3-propanedithiol, 1,4-butanedithiol, 2,3-butanedithiol, 1,5-pentanedithiol, 1,6-hexanedithiol, 1,8-octanedithiol, 1,9-nonanedithiol, 2,3-dimercapto-1-propanol, dithiothreitol, dithioerythritol, 2-mercaptoethyl ether or 2-mercaptoethyl sulfide. Further suitable chain extenders include oligomeric compounds having two or more of the abovementioned reactive functional groups, examples being hydroxyl-containing oligomers, such as polyethers, polyesters or hydroxyl-containing acrylate/methacrylate copolymers. Oligomeric chain extenders are extensively described in the literature and generally have molecular weights in the range from 200 to 2000 daltons.
Preferred chain extenders are the diols or polyols having up to 20 carbon atoms, especially the aliphatic diols having 2 to 20 carbon atoms, examples being ethylene glycol, diethylene glycol, neopentyl glycol, and 1,6-hexanediol.
It is preferred in the process of the invention to employ urethane (meth)acrylates obtainable by reacting the component B with at least one isocyanato-containing prepolymer having at least two isocyanate groups per molecule as component A. In this case, preference is given to those isocyanato-containing prepolymers which are obtainable by reacting one of the abovementioned low molecular mass diisocyanates or polyisocyanates with at least one of the compounds of component C1, the ratio of equivalents of the isocyanate groups to the reactive groups of component C1 being in particular about 2:1. Preference is further given to those compounds containing isocyanate groups that are selected from the isocyanurates and biurets of aliphatic or aromatic diisocyanates.
Component C further includes compounds C2 which flexibilize the UV-cured coating. Flexibilization can be achieved, inter alia, by reacting at least some of the free isocyanate groups of the binder with hydroxyalkyl esters and/or alkylamine amides of relatively long-chain dicarboxylic acids, preferably aliphatic dicarboxylic acids having at least 6 carbon atoms. Examples of suitable dicarboxylic acids are adipic acid, sebacic acid, dodecanedioc acid, and/or dimeric fatty acids. The flexibilization reactions may in each case be carried out before or after the addition of component B onto the isocyanato-containing prepolymers. Flexibilization is also achieved by using relatively long-chain aliphatic diols and/or diamines, especially aliphatic diols and/or diamines having at least 6 carbon atoms, as chain extenders C1.
In addition to the polymers and/or oligomers P1, the coating compositions may comprise one or more reactive diluents. Reactive diluents are liquid compounds of low molecular mass which have at least one, polymerizable, ethylenically unsaturated double bond. An overview of reactive diluents can be found, for example, in J. P. Fouassier (ed.), Radiation Curing in Polymer Science and Technology, Elsevier Science Publisher Ltd., 1993, Vol. 1, pp. 237-240. They are used usually to influence the viscosity and the technical properties of the coating, such as the crosslinking density, for example.
The coating compositions used in accordance with the invention contain reactive diluents preferably in an amount of up to 70% by weight, with particular preference from 15 to 65% by weight, based on the overall weight of P1 and reactive diluent in the coating composition.
Examples of reactive diluent classes include (meth)acrylic acid and esters thereof with diols, polyols and amino alcohols, maleic acid and its esters and monoesters, vinyl esters of saturated and unsaturated carboxylic acids, vinyl ethers, and vinylureas. Examples that may be mentioned include C2-C12 alkylene glycol di(meth)acrylates such as 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate and 1,12-dodecyl diacrylate, esters of acrylic acid or of methacrylic acid with (poly)ether diols, such as dipropylene or tripropylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate and polyethylene glycol di(meth)acrylate, esters of acrylic acid or of methacrylic acid with olefinically unsaturated alcohols, such as vinyl (meth)acrylate, allyl (meth)acrylate and dicyclopentadienyl acrylate, esters of acrylic acid or of methacrylic acid with higher polyhydric alcohols such as glycerol tri(meth)acrylate, trimethylolpropane tri(meth)acrylate, trimethylolpropane di(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, and also monounsaturated compounds such as vinyl acetate, styrene, vinyltoluene, ethoxy(ethoxy)ethyl acrylate, N-vinylpyrrolidone, phenoxyethyl acrylate, dimethylaminoethyl acrylate, hydroxyethyl (meth)acrylate, butoxyethyl (meth)acrylate, and isobornyl (meth)acrylate, and also diunsaturated or polyunsaturated compounds such as divinylbenzene and dimethylacrylamide. Furthermore, it is also possible to use the reaction product of two moles of acrylic acid with one mole of a dimeric fatty alcohol having generally 36 carbon atoms. Mixtures of said reactive diluents are also suitable.
Preference is given to reactive diluents based on esters of acrylic acid and/or of methacrylic acid, among which monoacrylates and diacrylates and also monomethacrylates and dimethacrylates are preferred, especially isobornyl acrylate, hexanediol diacrylate, dipropylene glycol diacrylate, tripropylene glycol diacrylate, and Laromer(copyright) 8887 from BASF AG. Very particular preference is given to isobornyl acrylate, hexanediol diacrylate, dipropylene glycol diacrylate, and tripropylene glycol diacrylate.
The coating compositions of the invention comprise photoinitiators or photoinitiator combinations as commonly used in radiation-curable coating compositions and able to initiate the polymerization of ethylenically unsaturated double bonds on exposure to UV radiation. Radiation-curable coating compositions generally contain, based on the overall weight of P1 and, if present, of the reactive diluents, at least 0.1% by weight, preferably at least 0.5% by weight and up to 10% by weight, more preferably from 0.5 to 6% by weight, in particular from 1 to 4% by weight, of at least one photoinitiator. Suitable photoinitiators are, for example, benzophenone and derivatives of benzophenone, such as 4-phenylbenzophenone and 4-chlorobenzophenone, Michler""s ketone, anthrone, acetophenone derivatives, such as 1-benzoylcyclohexan-1-ol, 2-hydroxy-2,2-dimethylacetophenone and 2,2-dimethoxy-2-phenylacetophenone, benzoin and benzoin ethers, such as methyl, ethyl and butyl benzoin ether, benzil ketals, such as benzil dimethyl ketal, 2-methyl-1-(4-methylthiophenyl)-2-morpholinopropan-1-one, anthraquinone and its derivatives, such as xcex2-methylanthraquinone and tert-butylanthraquinone, acylphosphine oxides, such as 2,4,6-trimethylbenzoyldiphenylphosphine oxide, ethyl 2,4,6-trimethylbenzoylphenylphosphinate, and bisacylphosphine oxides. Initiators of this kind are, for example, the products available commercially under the brand names Irgacure(copyright) 184, Darocure(copyright) 1173 from Ciba Geigy, Genocure(copyright) from Rahn, or Lucirin(copyright) TPO from BASF AG. Preferred photoinitiators also include phenylglyoxalic acid, its esters and its salts, which may also be used in combination with one of the abovementioned photoinitiators. For further details reference may hereby be made to German Patent Application P 198 267 12.6 in its entirety.
Furthermore, the coating compositions may additionally comprise customary auxiliaries and/or additives, examples being light stabilizers (for example, HALS compounds, benzotriazoles, oxalanilides, and the like), slip additives, polymerization inhibitors, flatting agents, defoamers, leveling agents, and film-forming auxiliaries, e.g., cellulose derivatives, or other additives commonly used in topcoats. These customary auxiliaries and/or additives are commonly used in an amount of up to 15% by weight, preferably from 2 to 9% by weight, based on the overall weight of P1 and, if present, of the reactive diluents.
In the process of the invention it is preferred to use flowable or liquid coating compositions. These may be applied by the standard methodsxe2x80x94for example, by dip coating, spraying or knife coatingxe2x80x94to the surfaces of the article that is to be coated.
If desired, the still wet coating may be subjected to a drying step prior to curing with UV radiation. Alternatively, the still wet coating may first be partly crosslinked and then cured with UV radiation.
In general, the coating composition of the invention is applied in an amount of from 3 to 200 g/m2, preferably from 5 to 150 g/m2. This produces coat thicknesses in the cured state of from 3 to 200 xcexcm, preferably from 5 to 150 xcexcm.
In the process of the invention, the coating compositions are frequently used in the form of clearcoats, so that they normally contain no fillers, or only transparent fillers, and no hiding pigments. Use in the form of pigmented coating compositions is, however, also possible. In that case the coating compositions contain from 2 to 40% by weight, based on the overall weight of the coating composition, of one or more pigments. Furthermore, in this case the coating compositions may contain from 1 to 30% by weight, based on the overall weight of the coating composition, of one or more fillers.
Moreover, it is also possible to use the UV-curable coating compositions in the process of the invention in the form of aqueous formulations. Binder dispersions or emulsions of this kind are virtually free from environmentally harmful, volatile constituents, such as monomers or cosolvents. Crosslinking in accordance with the process described here under a protective-gas atmosphere takes place following complete evaporation of the water and, in the case of spray application, after a complete escape of the included air as well. With regard to the preparation and processing of radiation-curable aqueous binder dispersions and emulsions, exemplary reference is made at this point to EP-A 12 339.
A very wide variety of substrates may be coated by means of the process of the invention, examples being glass, metal substrates, such as aluminum, steel and other ferrous alloys, and also wood, paper, plastics, and mineral substrates, e.g., concrete roof tiles and fiber cement slabs. The process of the invention is also suited to the coating of packaging containers and to the coating of thin sheets for the furniture industry. A further feature of the process of the invention is that not only planar or substantially planar articles but also structures, i.e., articles having a three-dimensional form, may be provided with scratch-resistant coatings.
To produce coatings on metal substrates, the coating compositions of the invention are applied preferably to primed or basecoated metal surfaces, e.g. metal sheets or metal strips, three-dimensionally formed metal articles, e.g., shaped parts made from metal sheets, such as bodywork parts, casings, frame profiles for windows or the like. The commonly used basecoat materials may be used as primers. Both conventional and aqueous basecoat materials are employed. Further, it is also possible to apply the coating compositions of the invention to metal substrates which are first coated with an electrodeposition coating and subsequently coated with a functional coat and wet-on-wet with a basecoat material. In the case of said processes it is generally necessary for the basecoat material and the surfacer and/or the functional coat to be baked before the coating composition of the invention is applied.
Installations for the curing of radiation-curable coatings under standard atmospheric conditions and under strict exclusion of oxygen are known to the skilled worker (cf., e.g., R. Holmes, U.V. and E.B. Curing Formulations for Printing Inks, Coatings and Paints, SITA Technology, Academic Press, London, United Kingdom 1984). The process of the invention may in principle be carried out in both types of installation. The installations for curing under standard atmospheric conditions are then provided with additional devices by means of which those regions of the installation within which the coating is curedxe2x80x94for example, the curing unit in a coating linexe2x80x94are flushed with an inert gas or with a mixture of inert gas and oxygen or air in order to achieve the desired oxygen concentration at the site of curing. For example, one or more nozzles or nozzle arrays for the supply of protective gas may be provided in the openings of the installation through which the substrate provided with the wet coating is supplied to the UV source, for example, a high-pressure mercury lamp. Additionally it is advisable to provide further facilities for the supply of protective gas in the region of the UV source. In the case of customary UV curing apparatus, which provide a UV curing unit with an entry and an exit opening and a conveyor belt which transports the still-wet, coated article through the entry opening into the curing unit, past the UV source, and subsequently through the exit opening out of the curing unit, it is common to provide at least one device each for flushing with protective gas, e.g., a nozzle array, in the entry opening and in the exit opening, and also, if desired, further devices for flushing with inert gas in the interior of the curing unit, e.g., in the vicinity of the UV source. The surfaces of uniformly shaped structures, e.g., vehicle bodies and bodywork parts, may be guided past a UV source through a region enriched with protective gas, similar to the drying zone of a car wash line. It is likewise possible to move a mobile UV source over the contour of a structure that is present within the region enriched with protective gas. Installations for the UV curing of structures, especially structures having a complex three-dimensional form, are known, for example, from U.S. Pat. No. 4,208,587 and from WO 98/53008. The types of installation described therein may be converted in the manner described above for use in the process of the invention with suitable flushing devices for protective gas.
The UV source used for curing may be provided with nozzles or slots through which protective gas flows continuously in the course of curing, i.e., in the course of exposure of the article provided with the wet coating composition, so that at the site of radiation curing the oxygen concentration is reduced to the value in accordance with the invention. The nozzles or slots are preferably arranged in the form of a ring or crown around the UV source. For curing the complete surface of a structure, a UV source equipped in this manner may also be guided over the structure by means of suitable devicesxe2x80x94for example, by means of a robot arm (cf. also WO 98/53008).
The curing of the coated surfaces by means of UV radiation may of course also take place in outwardly closed-off rooms or chambers having a reduced oxygen content in the atmosphere.
One advantage of the process of the invention is that the desired oxygen concentrations may be realized without great technical expenditure. Moreover, the amount of inert gas used is lower than the amount normally necessary to achieve strict exclusion of oxygen, since the oxygen concentrations in accordance with the invention may be established just by flushing with an amount of inert gas insufficient to displace the oxygen completely from the atmosphere present in the curing zone. To this extent, the process of the invention may also be classified as a process for UV curing of UV-curable coatings under a reduced or restricted protective-gas atmosphere.
These advantages are manifested in particular in the case of structures of complex design. With structures of this kind there is, fundamentally, the problem that complete exclusion of oxygen in the surface region of the structure is not possible by flushing with inert gas. Consequently, UV curing of structures provided with UV-curable coatings was hitherto considered possible only in outwardly closed-off curing units, and thus was considered uneconomic. In contrast, the process of the invention permits simple curing of the surfaces provided with a radiation-curable coating on articles of any desired form, owing to its tolerance for residual amounts of oxygen in the surface regions of the coated article. A further advantage is that the ambient air of the actual curing unit, in a coating line, for instance, still contains sufficient oxygen and so there is not the danger of asphyxiation which exists for closed-off rooms with a protective-gas atmosphere.
The coatings obtained by the process of the invention have a considerably improved scratch resistance. High scratch resistance is interpreted in this case as a good performance in the Scotch-Brite test. Thus the coatings obtainable in accordance with the invention frequently have delta gloss values in accordance with the Scotch-Brite test of not more than 30, with values of not more than 20 or not more than 10 also being achieved, without the need for complete exclusion of oxygen.
Below, the invention is illustrated with reference to working examples. All parts therein are by weight unless expressly stated otherwise.
Unless expressly stated otherwise, the coating compositions were prepared from the components indicated in the working examples by intensive stirring with a dissolver or stirrer.
To produce the scratch-resistant coatings, the coating compositions described in the working examples were applied in the form of a film to cleaned, blackened glass plates using a box-type coating bar with a gap size of 200 xcexcm. The films were cured in a IST coating unit M 40 2xc3x971-R-IR-SLC-So inert with devices for the supply of protective gas in the region of the entry opening and exit opening, with two UV lamps (wavelength range, high-pressure mercury lamps type M 400 U2H and M 400 U2HC), and with a conveyor belt speed of 10 m/min. The radiation dose was approximately 1800 mJ/cm2. The oxygen content in the curing zone was adjusted by throttling the nitrogen supply. The oxygen content in the curing region was measured between the two UV lamps with the aid of a Galvanoflux probe (electrochemical cell based on a lead/lead oxide redox couple having three measurement ranges: 0-1000 ppm, 0-5%, and 0-25%). Prior to each curing, the oxygen concentration was adjusted and a time of 20 minutes was left for the atmosphere to equilibrate.
The mechanical stability of the coatings cured at different oxygen concentrations was examined by determining the Kxc3x6nig pendulum hardness, DIN 53157, ISO 1522, and by determining the scratch resistance by the Scotch-Brite test following storage for 24 hours in a climate-controlled chamber.
In the Scotch-Brite test, the test element used is a 3xc3x973 cm silicon carbide-modified fiber web (Scotch Brite SUFN, 3M Deutschland, 41453 Neuss, Germany) mounted on a cylinder. This cylinder presses the fiber web against the coating under a weight of 750 g and is moved pneumatically over the coating. The path length of the deflection is 7 cm. After 10 or 50 double strokes (DS), the gloss (sixfold determination) is measured in the central stress region in analogy to DIN 67530, ISO 2813, at an incident angle of 60xc2x0. The difference (delta gloss value) is formed from the gloss values of the coatings before and after the mechanical stressing. The loss of gloss, i.e., the delta gloss values, is inversely proportional to the scratch resistance.

Method for producing scratch-resistant coatings


A target tissue localization device (28) has outer and inner members (30, 32) which are longitudinally movable relative to one another. An anchor (42) is secured to the outer and inner members and is movable from a radially-contracted configuration to a radially-expanded configuration. The anchor may be a tubular mesh anchor. The anchor may have a sufficiently large cross-sectional area when in the radially-expanded configuration to substantially eliminate inadvertent dislodgment of the anchor once radially-expanded. In use, the location of the target tissue (56) is determined and the distal end of the localization device is passed through the patient""s skin (58) and to the target tissue to create a tissue track (60) between the patient""s skin and the target tissue. The target tissue includes near and far sides (62, 64). The anchor element, located at the target tissue is expanded to an expanded configuration. The method the anchor element may be located on the far side of the target tissue. The localization device may be selected with an anchor which is palpable for relocation of the target tissue.

In the U.S. alone approximately one million women will have breast biopsies because of irregular mammograms and palpable abnormalities. Biopsies can be done in a number of different ways for non-palpable lesions, including surgical excisional biopsies and stereotactic and ultrasound guided needle breast biopsies. In the case of image directed biopsy, the radiologist or other physician takes a small sample of the irregular tissue for laboratory analysis. If the biopsy proves to be malignant, additional surgery (typically a lumpectomy or a mastectomy) is required. In the case of needle biopsies, the patient then returns to the radiologist a day or two later where the biopsy site (the site of the lesion) is relocated by method called needle localization, a preoperative localization in preparation for the surgery.
Locating the previously biopsied area after surgical excision type of biopsy is usually not a problem because of the deformity caused by the surgery. However, if the biopsy had been done with an image directed needle technique, as is common, help in relocating the biopsy site is needed. One procedure to permit the biopsy site to be relocated by the radiologist during preoperative localization is to leave some of the suspicious calcifications; this has its drawbacks.
Another way to help the radiologist relocate the biopsy site involves the use of a small metallic surgical clip, such as those made by Biopsys. The metallic clip can be deployed through the biopsy needle, and is left at the biopsy site at the time of the original biopsy. With the metallic clip as a guide, the radiologist typically inserts a barbed or hooked wire, such as the Hawkins, Kopans, Homer, Sadowski, and other needles, back into the patient""s breast and positions the tip of the wire at the biopsy site using mammography to document the placement. The patient is then taken to the operating room with the needle apparatus sticking out of the patient""s breast. While the clip provides a good indication of the biopsy site to the radiologist during preoperative localization, the clip remains permanently within the 80% of patients with benign diagnoses. Also, because the clip is necessarily attached to a single position at the periphery of the biopsy site, rather than the center of the biopsy site, its location may provide a misleading indication of the location of diseased tissue during any subsequent medical intervention. The clip is also relatively expensive. In addition, the soft nature of breast tissue permits the tip of the barbed or hooked needle to be relatively easily dislodged from the biopsy site. The surgeon typically pulls on the needle to help locate the area to be removed. This pulling motion during the excision may be responsible more than any other single factor for the movement of the needle. Additionally, these devices are not easily felt by external palpation and the tip is difficult to locate without dissection into the tissues; this prevents a surgical approach which may be more cosmetically advantageous and surgically appropriate than dissecting along the tract of the needle.
Another localization method involves the use of laser light from the tip of a optical fiber connected to a laser. A pair of hooks at the tip of the optical fiber secures the tip at the biopsy site; the glow indicates the position of the tip through several centimeters of breast tissue. This procedure suffers from some of the same problems associated with the use of barbed or hooked wires. Another preoperative localization procedure injects medical-grade powdered carbon suspension from the lesion to the skin surface. This procedure also has certain problems, including the creation of discontinuities along the carbon trail.
The present invention is directed to a target tissue localization device including an elongate placement device having first and second members being longitudinally movable relative to one another. An anchor is secured to the first and second members and is movable from a first, radially contracted configuration to a second, radially expanded configuration. The placement device and anchor are sized and configured for use in localization of target tissue. According to one aspect of the invention the anchor is a tubular mesh anchor. According to another aspect of the invention the anchor has a sufficiently large cross-sectional area when in the second, radially expanded configuration to substantially eliminate inadvertent dislodgment of the anchor once radially-expanded.
A further aspect of the invention relates to a method for localizing target tissue including determining the location of the target tissue and passing the distal end of an elongate localization device through the patient""s skin and to the target tissue to create a tissue track between the patient""s skin and the target tissue. The target tissue includes a near side toward the patient""s skin and a far side opposite the near side. The localization device includes an anchor element secured to a placement device, the placement device extending from the anchor element along the tissue track and through the patient""s skin. The anchor element is located at the target tissue and expanded to an expanded configuration. According to one aspect of the method the anchor element is located on the far side of the target tissue. According to another aspect of the method, the localization device is selected with an anchor which is palpable at the patient""s target tissue so that the target tissue can be relocated at least in part by palpation of the patient to locate the anchor; according to this aspect of the invention, at least a sample of the relocated target tissue is removed. The elongate localization device can be used as a guide or guide wire for further diagnostic of therapeutic procedures.
In preferred embodiment of the invention the anchor is a tubular mesh anchor which can be placed into a disk-like shape when in the fully expanded configuration or in a more flattened spherical shape, that is not as flat as the disk-like shape, which may aid in palpability. The anchor may also be in other forms such as a Malecot-type of radially expandable anchor. Another anchor embodiment could be deployed from the end of a hollow sheath and include numerous resilient curved projections which extend generally radially outwardly to fully surround the sheath.
The invention may find particular utility as an aid in the removal of small, difficult-to-grasp body structures, such as lymph nodes or other tissue that is difficult to grasp. The anchor could be positioned behind or inside a lymph node targeted for removal; lymph node removal can be aided by leaving the anchor in its expanded position so that after the surgeon cuts down to the lymph node, the lymph node, some surrounding tissue and the entire localization device can be removed for further analysis.
In some situations the anchor may be left in a partially or fully radially-expanded condition and used to pull the target tissue from the patient, typically with the aid of the surgeon opening the tissue track. For example, for tumor removal of the breast, the anchor may be used in a partially or fully expanded condition so that the tumor and surrounding margins can be removed during lumpectomy. Similar procedures can be used in regions other than the breast.
The invention may also include structure which may enhance ultrasound visibility. Invention may include certain treatment, grooves, slits, or other structure to enhance the ultrasonic visibility of the placement device or the delivery needle, or both. Also, the mesh anchor may provide sufficient reflectivity so that treatment of the placement device and delivery needle is not required. Other techniques for enhancing remote visualization, such as radiopaque markers, can also be used.
The invention is primarily related to mammographic localization, but may find utility with other areas of imaging including ultrasound, thoracoscopy, nuclear medicine, MRI, computed tomography, plain film and any other image guided technology.
The present invention provides a simple, relatively inexpensive, very stable localization device to locate a lesion or other target tissue, typically within the breast of a patient, for subsequent biopsy, excision, intervention, or other purpose. The localization device is easy to insert, deploy, reposition if needed, and remains anchored at the target site substantially better than conventional localization needles. Further, the invention aids relocation of the target tissue by being configured to permit location by palpation by the surgeon in the operating suite. This permits the surgeon to proceed using much less radical techniques, which may be both cosmetically and surgically advantageous.
Other features and advantages of the invention will appear from the following description in which the preferred the embodiments have been set forth in detail in conjunction with the accompanying drawings.

Target tissue localization device and method


The invention relates to a process for the preparation of terpolymers of ethylene and at least 2 further olefinically unsaturated compounds by polymerization in a tubular reactor fitted with at least one side branch, wherein the fresh monomer components, which are introduced into the tubular reactor via the reactor inlet (the primary stream) or via the side branch or side branches (secondary stream or secondary streams), in each of the streams contain ethylene and at most one further olefinically unsaturated compound.

The present invention relates to a process for the preparation of terpolymers of ethylene, one or more vinyl esters or acrylates, and further olefinically unsaturated compounds, and to the use of the resultant copolymers for improving the cold-flow properties of mineral oils and mineral oil distillates.
Crude oils and middle distillates obtained by distillation of crude oils, such as gas oil, diesel oil or heating oil, contain, depending on the origin of the crude oils, various amounts of n-paraffins, which, when the temperature is reduced, crystallize out as platelet-shaped crystals and in some cases agglomerate with inclusion of oil. This crystallization and agglomeration causes an impairment of the flow properties of the oils or distillates, which can result in problems during the recovery, transport, storage and/or use of the mineral oils and mineral oil distillates. During transport of mineral oils through pipelines, the crystallization phenomenon can cause deposits on the walls of the pipes, especially in winter, and in individual cases, for example during stoppage in a pipeline, can even cause complete blocking thereof. During storage and further processing of the mineral oils, it may furthermore be necessary in winter to store the mineral oils in heated tanks. In the case of mineral oil distillates, the crystallization may result in blockage of the filters in diesel engines and furnaces, preventing reliable metering of the fuels and in some cases causing complete interruption of the supply of the fuel or heating medium.
In addition to the classical methods of eliminating the crystallized paraffins (thermal, mechanical or using solvents), which merely involve removal of the precipitates which have already formed, recent years have seen the development of chemical additives (so-called flow improvers), which, by interacting physically with the precipitating paraffin crystals, result in their shape, size and adhesion properties being modified. The additives act as additional crystal nuclei and in some cases crystallize with the paraffins, resulting in an increased number of relatively small paraffin crystals having a modified crystal shape. The modified paraffin crystals have a lower tendency toward agglomeration, so that the oils to which these additives have been added can still be pumped and/or processed at temperatures which are frequently more than 20xc2x0 lower than in the case of oils containing no additives.
Typical flow improvers for mineral oils and mineral oil distillates are copolymers and terpolymers of ethylene with carboxylates of vinyl alcohol, esters of acrylic or methacrylic acid and/or olefins.
EP-A-0 493 796 discloses terpolymers consisting of ethylene, 5-35% by weight of vinyl acetate and 1-25% by weight of vinyl neononanoate or neodecanoate, a process for their preparation, and their use as flow improvers for middle distillates.
DE-A-19 620 118 discloses terpolymers of ethylene, vinyl esters and 4-methylpentene which have improved solubility, a process for their preparation, and additive concentrates having a lowered inherent pour point prepared therefrom.
EP-A-0 203 554 discloses the use of terpolymers comprising, in addition to ethylene, from 0.5 to 20% by weight of diisobutylene and from 20 to 35% by weight of vinyl acetate (based on the terpolymer) and having a mean molecular weight of from 500 to 10,000, as additives for mineral oils and mineral oil distillates.
EP-A-0 099 646 discloses terpolymers having molecular weights of from 1500 to 5500 which comprise ethylene, from 10 to 20% by weight of vinyl acetate and from 3 to 15% by weight of an isoolefin and which contain, per 100 methylene groups, from 6 to 15 methyl groups which do not originate from the vinyl acetate.
EP-A-0 648 257 discloses terpolymers made from ethylene and 2 vinyl esters carrying acid radicals having a maximum of 8 carbon atoms, as constituents of a fuel oil composition.
EP-A-0 649 445 discloses terpolymers made from ethylene and two vinyl esters and/or acrylates in a proportion of up to 10 mol%, where the esters carry side chains having a maximum of 8 carbon atoms, as a constituent of fuel oil compositions.
EP-A-0 271 738 discloses a process for the preparation of copolymers of ethylene and esters of vinyl alcohol, acrylic acid or methacrylic acid by polymerization of monomer streams of different quantitative composition in a tubular reactor, and the use of these copolymers as flow improvers.
However, the terpolymers prepared by the prior-art process (EP 271 738) do not have the properties that would be expected of them on the basis of their monomer composition. For example, the terpolymers prepared in this way frequently impair the filterability of the oils in which they are present or their effectiveness is in many cases unsatisfactory. For example, the terpolymers of EP-A-0 493 796 have very good solubility, but they are ineffective in certain oils. The products prepared by the process of EP-A-0 271 738 have improved effectiveness, but impair the filterability of the oils to which they have been added.
The object was therefore to find a new process for terpolymerization which gives terpolymers whose properties can be controlled better via the specific properties of the various monomers.
Surprisingly, it has been found that improved terpolymers made from ethylene and at least two further comonomers can be obtained if the terpolymerization is carried out in a tubular reactor with a side branch and the fresh comonomers are fed in separately from one another via different reactor inlets.
The olefinically unsaturated compounds which make up the further comonomers are preferably vinyl esters, acrylates, methacrylates, alkyl vinyl ethers and/or alkenes.
The vinyl esters are preferably those of the formula 1
CH2xe2x95x90CHxe2x80x94OCOR1xe2x80x83xe2x80x83(1)
in which R1 is C1- to C30-alkyl, preferably C1- to C16-alkyl, especially C1- to C12-alkyl.
In a further preferred embodiment, R1 is a neoalkyl radical having 7 to 11 carbon atoms, in particular having 8, 9 or 10 carbon atoms. Suitable vinyl esters include vinyl acetate, vinyl propionate, 2-ethylhexanoic acid vinyl ester, vinyl laurate, vinyl neononanoate, vinyl neodecanoate and vinyl neoundecanoate.
The acrylates are preferably those of the formula 2
CH2xe2x95x90CR2xe2x80x94COOR3xe2x80x83xe2x80x83(2)
in which R2 is hydrogen or methyl and R3 is C1- to C30-alkyl, preferably C1- to C16-alkyl, especially C1- to C12-alkyl. Suitable acrylates include methyl acrylate, methyl methacrylate, ethyl acrylate and 2-ethylhexyl acrylate.
The alkyl vinyl ethers are preferably compounds of the formula 3
CH2xe2x95x90CHxe2x80x94OR4xe2x80x83xe2x80x83(3)
in which R4 is C1- to C30-alkyl, preferably C1- to C16-alkyl, especially C1- to C12-alkyl.
The alkenes are preferably monounsaturated hydrocarbons having 3 to 30 carbon atoms, in particular 4 to 16 carbon atoms, especially 5 to 12 carbon atoms. Suitable alkenes include isobutylene, diisobutylene, 4-methylpentene, hexene, octene and norbornene.
Particular preference is given to terpolymers in which one of the radicals R1, R3 and R4 is C1- or C2-alkyl and the other radical R1, R3 or R4 is C4- to C16-alkyl, in particular C6- to C12-alkyl or an alkene.
In a preferred embodiment of the invention, the fresh monomer components are introduced in the main stream and in a secondary stream. In the main stream, ethylene is introduced as a mixture with one of the two further comonomers. In the secondary stream, ethylene is introduced as a mixture with the other comonomer.
In a further preferred embodiment of the invention, the fresh monomer components are introduced in the main stream and in two secondary streams. In this embodiment, there are various preferred variants:
A) In the main stream, ethylene is introduced as a mixture with one of the two further comonomers. In each of the secondary streams, ethylene is introduced as a mixture with the other of the further comonomers. The proportion of the further comonomers in the secondary streams can be the same or different.
B) In the main stream, ethylene is introduced, in the first secondary stream, one of the further comonomers is introduced, and in the second secondary stream, the second of the further comonomers is introduced.
In a further preferred embodiment of the invention, the fresh monomer components are introduced in the main stream and in three secondary streams. In these cases, it is preferred to introduce a mixture of ethylene and a further comonomer via the primary stream and a secondary stream, and to introduce a mixture of ethylene and the other of the comonomers via the two further secondary streams. A particularly preferred embodiment comprises introducing a mixture of ethylene and a further comonomer in the primary stream and introducing a mixture of ethylene and the other comonomer in all three secondary streams. In a further embodiment, ethylene is introduced in the primary stream, a further comonomer is introduced in the first secondary stream and the other comonomer is introduced in the two remaining secondary streams.
Correspondingly, the terpolymerization can also be carried out in tubular reactors having 4 or more secondary streams.
It is preferred to introduce the comonomer of lower copolymerization factor in the primary stream. It is furthermore preferred to introduce the comonomer of higher boiling point in the primary stream.
In general, the residual monomers which are not consumed are recycled and reintroduced into the reactor. The reintroduction can take place, after the residual monomer mixture has been separated off, in the primary and/or secondary stream. It is preferably carried out in the secondary stream.
The polymerization is preferably carried out at pressures of from 50 to 400 MPa, preferably from 120 to 300 MPa, and at temperatures of from 150 to 350xc2x0 C., preferably from 100 to 300xc2x0 C. The polymerization initiators used are advantageously oxygen, hydroperoxides, peroxides and azo compounds in the form of a 1 to 30% strength by weight solution. Suitable solvents are, in particular, aliphatic hydrocarbons and hydrocarbon mixtures.
In the process according to the invention, at least two monomer streams of different composition are fed into the reactor. The first monomer stream (primary stream) is introduced into the reactor via the reactor inlet. At least one second monomer stream (secondary stream) is introduced into the reactor via at least one side branch. The term side branch is taken to mean any introduction point along the reactor via which monomers and/or polymerization auxiliaries, namely initiators and regulators, can be fed into the reactor.
In the primary stream, the comonomer content is preferably from 3 to 200 parts by weight, in particular from 7 to 100 parts by weight, per 100 parts by weight of ethylene.
The secondary stream preferably contains from 10 to 500 parts by weight, in particular from 20 to 300 parts by weight, of the further comonomer per 100 parts by weight of ethylene.
Preferred initiator contents in the primary stream are from 50 to 10,000 ppm by weight, preferably from 50 to 1000 ppm by weight, and those in the secondary stream are from 100 to 10,000 ppm by weight, preferably from 200 to 2000 ppm by weight, in each case based on the monomer mixture.
In order to set the molar mass, polymerization regulators are generally added to the primary and secondary streams. Depending on the type of regulator and the desired viscosity, which is a measure of the molar mass, the regulator concentration in the two streams is identical or different and is preferably from 0.05 to 20% by weight, based on the monomer mixture. Examples of regulators are hydrogen, saturated or unsaturated hydrocarbons, such as, for example, propane or propene, aldehydes, such as, for example, propionaldehyde, n-butyraldehyde or isobutyraldehyde, ketones such as, for example, acetone, methyl ethyl ketone, methyl isobutyl ketone or cyclohexanone, or alcohols, such as, for example, butanol. The primary stream preferably contains from 0.1 to 10% by weight of regulator, and the secondary stream from 0.1 to 5% by weight of regulator, based on the monomer mixture.
In general, approximately equal volumes of the monomer mixture are fed to the reactor inlet and the side branch or branches, i.e. the volume ratio between the primary and secondary stream is about 1:1. However, it is entirely possible to use different volume ratios. It has proven successful to maintain a ratio in the range from 1:0.2 to 5 (in parts by vol.), in particular from 1:0.3 to 3, between the primary and secondary streams.
The secondary stream can be introduced into the reactor via a single side branch or in sub-streams via a plurality of side branches. It is preferred to use from 1 to 3 sub-streams, in particular 3 sub-streams.
The amount of secondary stream component introduced into the reactor via the individual side branches depends on the temperature range, i.e. the maximum and minimum temperatures, to be observed in the reactor.
The ethylene terpolymers prepared by the process according to the invention contain from 15 to 55 parts by weight of comonomers per 100 parts by weight of ethylene. In order to achieve this composition in the polymer, it is necessary to employ, under the reaction conditions selected, comonomer mixtures which take into account the copolymerization parameters of the comonomers employed and contain in total (i.e. as the sum of the contents in the primary and secondary streams) from about 2 to 40% by weight of further comonomers.
The ethylene copolymers obtained in accordance with the invention generally have a viscosity, measured at 140xc2x0 C. using a rotational viscometer (for example from Haake) in accordance with EN 3219, of between 30 and 10,000 mPas, preferably from 50 to 5,000 mPas, in particular from 50 to 1000 mPas. The viscosity is adjusted in a known manner by adding polymerization regulators to the monomer mixture.
The terpolymers according to the invention contain methyl groups originating on the one hand from the structure of the comonomer molecules, such as, for example, CH3 groups from the vinyl neocarboxylates, terminal CH3 groups in the alkyl radicals or olefins, and on the other hand from the ethylene polymerization mechanism. The number of methyl groups is determined, for example, by 1H-NMR spectroscopy. The terpolymers preferably contain from 2 to 15, in particular from 3 to 11, CH3 groups per 100 CH2 groups with the exception of the CH3 groups originating from vinyl acetate as the vinyl ester.
The polymerization is preferably carried out in tubular reactors known per se having a length:diameter ratio in the range from about 10,000:1 to 100,000:1. The lower limit for this ratio is preferably about 20,000:1, in particular about 25,000:1. The upper limit is preferably about 80,000:1, in particular about 60,000:1. Tubular reactors usually have a length of from about 200 to 4000 m, the lower limit preferably being about 250 m and the upper limit preferably being about 3000 m, in particular 2000 m.
The temperature is regulated in accordance with the prior art by cooling the tubular reactor and through the secondary stream introduced into the reactor via at least one side branch. The temperature of the secondary stream is from 0 to 60 C. The points of introduction of the secondary stream into the tubular reactor are selected so as to form 2 to 4 reaction zones in which the reaction mixture has approximately the same residence time.
The polymers prepared according to the invention are added to mineral oils or mineral oil distillates in the form of solutions or dispersions comprising from 1 to 90% by weight, in particular from 10 to 80% by weight (based on the solution) of the polymers. Suitable solvents are aliphatic and/or aromatic hydrocarbons or hydrocarbon mixtures, for example gasoline fractions, kerosene, decane, pentadecane, toluene, xylene, ethylbenzene or commercial solvent mixtures, such as solvent naphtha, Shellsol AB, Solvesso 150, Solvesso 200 and Exxsol, Isopar and Shellsol D grades. Kerosene is particularly suitable. Mineral oils and mineral oil fractions whose rheological properties have been improved by the novel polymeric compounds preferably contain from 0.001 to 2% by weight, in particular from 0.005 to 0.5% by weight, of copolymer.
The copolymers according to the invention can furthermore be used, as flow improvers, in the form of mixtures consisting of polymers of the claimed type, but with different qualitative and/or quantitative composition and/or different viscosity (measured at 140xc2x0 C.). The mixing ratio (in parts by weight) of the copolymers can be varied over a broad range and can be, for example, from 20:1 to 1:20, preferably from 10:1 to 1:10. In this way, the flow improvers can be matched specifically to individual requirements.
In order to produce additive packages for specific problem solutions, the polymers prepared in accordance with the invention can also be employed together with one or more oil-soluble coadditives which, even on their own, improve the cold-flow properties of crude oils, lubricating oils or fuel oils. Examples of such coadditives are copolymers containing vinyl acetate or terpolymers of ethylene, polar compounds which effect paraffin dispersal (paraffin dispersants) and comb polymers. For example, mixtures of the terpolymers according to the invention with copolymers comprising from 10 to 40% by weight of vinyl acetate and from 60 to 90% by weight of ethylene have proven highly successful. In a further embodiment of the invention, the terpolymers prepared in accordance with the invention are employed, as a mixture with ethylene-vinyl acetate-vinyl neononanoate terpolymers or ethylene-vinyl acetate-vinyl neodecanoate terpolymers, for improving the flow properties of mineral oils or mineral oil distillates. The terpolymers of vinyl neononanoate or of vinyl neodecanoate contain, besides ethylene, from 10 to 35% by weight of vinyl acetate and from 1 to 25% by weight of the particular neo compound. The mixing ratio of the terpolymers prepared in accordance with the invention with the above-described ethylene-vinyl acetate copolymers or the terpolymers of ethylene, vinyl acetate and the vinyl esters of neononanoic or neodecanoic acid is (in parts by weight) from 20:1 to 1:20, preferably from 10:1 to 1:10.
For use as flow improvers, the terpolymers prepared in accordance with the invention can furthermore be employed as a mixture with paraffin dispersants. These additives reduce the size of the paraffin crystals and make sure that the paraffin particles do not separate out, but instead remain colloidally dispersed with significantly reduced sedimentation effort. Paraffin dispersants which have proven successful are oil-soluble polar compounds containing ionic or polar groups, for example amine salts and/or amides obtained by reaction of aliphatic or aromatic amines, preferably long-chain aliphatic amines, with aliphatic or aromatic mono-, di-, tri- or tetracarboxylic acids or their anhydrides (cf. U.S.Pat. No. 4,211,534). Other paraffin dispersants are copolymers of maleic anhydride and xcex1,xcex2-unsaturated compounds, which can, if desired, be reacted with primary monoalkylamines and/or aliphatic alcohols (cf. EP 0 154 177), the products of the reaction of alkenylspirobislactones with amines (cf. EP 0 413 279), and, according to EP 0 606 055, products of the reaction of terpolymers based on xcex1,xcex2-unsaturated dicarboxylic anhydrides, xcex1,xcex2-unsaturated compounds and polyoxyalkylene ethers of lower unsaturated alcohols.
Finally, in a further variant of the invention which has proven successful, the terpolymers prepared in accordance with the invention are used as flow improvers together with comb polymers. These are taken to mean polymers in which hydrocarbon radicals having at least 8, in particular at least 10, carbon atoms are bonded to a polymer backbone. These are preferably homopolymers whose alkyl side chains contain at least 8 and in particular at least 10 carbon atoms. In the case of copolymers, at least 20%, preferably at least 30%, of the monomers have side chains (cf. Comb-like Polymers-Structure and Properties; N.A. Platxc3xa9 and V.P. Shibaev, J. Polym. Sci. Macromolecular Revs. 1974, 8, 117 ff). Examples of suitable comb polymers are fumarate/vinyl acetate copolymers (cf. EP 0 153 176 Al), copolymers made from a C6- to C24-xcex1-olefin and a Nxe2x80x94C6- to C22-alkylmaleimide (cf. EP 0 320 766), further esterified olefin/maleic anhydride copolymers, polymers and copolymers of xcex1-olefins and esterified copolymers of styrene and maleic anhydride.
For example, comb polymers can be described by the formula
in which
A is Rxe2x80x2, COORxe2x80x2, OCORxe2x80x2, Rxe2x80x3xe2x80x94COORxe2x80x2 or ORxe2x80x2;
D is H, CH3, A or Rxe2x80x3;
E is H or A;
G is H, Rxe2x80x3, Rxe2x80x3xe2x80x94COORxe2x80x2, an aryl radical or a heterocyclic radical;
M is H, COORxe2x80x3, OCORxe2x80x3, ORxe2x80x3 or COOH;
N is H, Rxe2x80x3, COORxe2x80x3, OCOR, COOH or an aryl radical;
Rxe2x80x2 is a hydrocarbon chain having 8-50 carbon atoms;
Rxe2x80x3 is a hydrocarbon chain having 1 to 10 carbon atoms;
m is a number between 0.4 and 1.0, and
n is a number between 0 and 0.6.
The mixing ratio (in parts by weight) of the terpolymers prepared in accordance with the invention with paraffin dispersants or comb polymers is in each case from 1:10 to 20:1, preferably from 1:1 to 10:1.
The polymers according to the invention improve the flow properties both of mineral oils, oils of vegetable or animal origin and of mineral oil distillates. They do not impair the filterability of the oils to which they have been added, even above the cloud point. The term mineral oils is taken to mean, in particular, crude oils, distillation residues and mineral oil distillates. The term mineral oil distillates is taken to mean hydrocarbon fractions having a boiling point of between about 150 and 450xc2x0 C. These include, for example, petroleum, light heating oils and diesel fuel. Of particular importance are the middle distillates such as heating oil EL and diesel fuel.
The polymers can be used alone or together with other additives, for example with other pour point depressants or dewaxing aids, with corrosion inhibitors, antioxidants, sludge inhibitors, lubricity additives, dehazers and additives for lowering the cloud point.
The effectiveness of the polymers according to the invention as flow improvers is illustrated in greater detail by the examples below.
The effectiveness of the copolymers investigated for improving the flow properties is described with reference to the xe2x80x9ccold filter plugging point testxe2x80x9d (CFPP test). The test is carried out in accordance with EN 116. It has also been published in J. Inst.
Petro., Vol. 52, June 1966, pages 173 to 185. Various mineral oil distillates, Test Oils 1 to 3, are used for the testing.

Process for the preparation of ethylene copolymers, and their use as additives to mineral oil and mineral oil distillates


A process for increasing the flame resistance of polyethylene-containing, single- or multilayer hollowware, in particular plastic fuel tanks, by heat treating the hollowware at 60-135xc2x0 C.

The present invention relates to a process for increasing the flame resistance of polyethylene-containing, single- or multilayer hollowware, in particular plastic fuel tanks, by heat-treating the hollowware at 60-135xc2x0 C.
Polyethylene (PE), in particular high-density polyethylene (HDPE), is highly suitable for extrusion blow molding of hollowware. A particular application of such hollowware is the storage and transport of hazardous liquid substances. A further application is in plastic fuel tanks (PFTs), which are used in automobiles for space- and weight-saving storage of liquid gasoline or diesel fuel. For safety reasons, such hollowware is subject to high requirements, in particular regarding its fracture resistance, impact strength and flame resistance. The flame resistance of such hollowware is determined by a fire test, i.e. the tank is treated with a flame under defined test conditions, and the time until leaks occur is determined.
The flame resistance of a PFT can be increased for a given shape by increasing the tank wall thickness, at least in the zones which are particularly at risk in the event of fire. However, this has the disadvantageous consequence of undesirably increasing the weight of the PFT. It may even have an adverse effect on the fracture behavior, since stresses concentrate locally to a greater extent. In addition, production costs increase with increased material consumption and the longer cooling times in the case of blow molding due to the greater wall thickness.
There has therefore been no lack of attempts to improve the flame resistance of polyethylene hollowware without increasing the wall thickness.
EP-A 645 400 discloses a specific ethylene polymer for the production of flame-resistant plastic fuel tanks. However, its production requires a two-step process consisting of prepolymerization and main polymerization and very specific catalysts. It is therefore inconvenient and expensive.
U.S. Pat. No. 5,020,687 discloses PFTs having reinforcing fabric inlays of flame-inhibiting, intumescent materials. However, these reinforcements must be introduced into the blow mold in advance.
This process too is inconvenient and expensive and can result in considerable reductions in strength in the components.
Furthermore, EP-A 106 099 and DE-A 196 17 592, for example, disclose providing PFTs with flame-inhibiting coatings. DE-19 50 662 discloses a PFT which is additionally coated with a metal foil. Coatings of this type are usually very expensive, since they require additional process steps. It is disadvantageous that the adhesion of such coatings to PE is low, and consequently they can become detached in the event of an accident, which is the very time when the full effectiveness of the coating is absolutely necessary.
It is an aim in automobile construction to reduce the vehicle weight. Although the outlined methods have already allowed PFTs with improved flame resistance to be obtained, there continues to be a demand for a further increase in the flame resistance. It is an object of the present invention to find a simple method of further improving the flame resistance of polyethylene-containing hollowware, enabling the weight of a PFT to be reduced without adversely affecting the flame resistance.
It is known that the rigidity and stress cracking resistance of polyethylene bottles can be improved by heat-treatment (see, for example, U.S. Pat. No. 3,293,341, U.S. Pat. No. 3,544,673, R. S. Schotland et al., Poly. Prep., Am. Chem. Soc., Div. Pol. Sci. (1968), 9(2), 1219-1226). Surprisingly, and also entirely unexpectedly to the person skilled in the art, it has been found that the flame resistance of polyethylene-containing hollowware can be considerably improved by a special heat-treatment.
Accordingly, we have found a process for increasing the flame resistance of polyethylene-containing single- or multilayer hollowware, in particular plastic fuel tanks, by heat-treating the hollowware at 60-135xc2x0 C.
The novel process is carried out using known single- or multilayer hollowware having at least one polyethylene-containing layer. In particular, the hollowware is the sort which can usually be used for the storage and transport of hazardous liquid materials, in particular flammable, liquid materials, such as, for example, chemical bottles, transport containers (IBCs), drums, tanks or canisters, particularly preferably plastic fuel tanks.
The walls of the hollowware can consist of one or more layers, where at least one layer comprises at least 90%, based on the total weight of the layer, of polyethylene homo- or copolymers.
The comonomer content of the polyethylene copolymers should preferably not exceed 1 mol % of the total amount of monomers employed Preferred comonomers are 1-olefins, and particularly preferred comonomers are 1-butene, 4-methyl-1-pentene, 1-hexene and 1-octene. The polyethylene may furthermore also be a blend of two or more polyethylene components. The polyethylene homopolymers or copolymers may furthermore contain conventional additives and auxiliaries, for example stabilizers against damaging processing influences, against thermal oxidation, ageing and the effect of UV, and dyes, organic and inorganic pigments or pigment preparations, for example carbon black dispersions in polyethylene, antistatics, waxes or special low-molecular-weight lubricants. However, the amount of additives and auxiliaries should not exceed 10% by weight, preferably 5% by weight, based on the total amount of material.
The density of the polyethylene homopolymer or copolymer is 0.94-0.97 g/cm3. The density is preferably 0.94-0.955 g/cm3. The melt flow rate MFR 190/21.6 of the polyethylene is 0.5-20 g/10 min. The melt flow rate MRF 190/21.6 is preferably 2-10 g/10 min. The polyethylene preferably has a weight average molar mass of 200-800 kg/mol and a polydispersity Mw/Mn of 5-80.
Further layers may be, for example, permeation barrier layers or adhesion promoter layers. Permeation barrier layers can be produced, for example, from ethylene-vinyl alcohol copolymers, polyamides or polybutylene terephthalate. Adhesion promoter layers can be, for example, layers essentially consisting of copolymers of ethylene and comonomers selected from the group consisting of vinyl alcohol esters, (meth)acrylic acids and esters thereof, and nitriles and amides of meth(acrylic) acid.
The hollowware may also have a number of polyethylene layers and a number of additional layers. An advantageous structure of a PFT consists, for example, of the following layer sequence: polyethylene layer-adhesive layer-permeation barrier layer-adhesive layer-regrind layer-polyethylene layer.
Multilayer hollowware of this type is known and can advantageously be produced by coextrusion (see, for example, W. Daubenbxc3xcchel, xe2x80x9cUse of coextrusion in extrusion blow moldingsxe2x80x9d, Kunststoffe 81, 894 (1991) and xe2x80x9cCoextruded plastic fuel tanksxe2x80x9d, ibidem, 82, 201 (1992).
Also suitable for the novel process is hollowware in which the barrier polymer is not in the form of a continuous layer, but instead forms labyrinth-like structures primarily intended to lengthen the diffusion path of the permeants through the PFT wall. Examples of these PFT types which may be mentioned here are Selar(copyright) PFTS.
It is also possible to use hollowware whose surface has been finished by methods known to the person skilled in the art, for example fluorination, sulfonation, plasma polymerization or surface coating. The hollowware described at the outset can also particularly advantageously be employed with flame-inhibiting coatings or flame-inhibiting inlays.
In the novel process for increasing the flame resistance, the hollowware described is heat-treated at 60-135xc2x0 C., preferably 80-120xc2x0 C. During the heat-treatment, the flame resistance of the hollowware increases significantly. The hollowware is advantageously heat-treated until an extension of the treatment time no longer produces any improvement in flame resistance. The heat-treatment duration depends on the treatment conditions. In general, the heat-treatment duration is from 0.25 to 30 h, preferably 1-20 h, in particular 2-15 h.
The warming of the hollowware to the heat-treatment temperature can be effected in any desired manner; the only important factor is that the warming takes place evenly. For example, the heat-treatment can be carried out in a fan-assisted drying cabinet. The heat-treatment step can advantageously be carried out in an oven directly downstream of the blow molding process, so that the energy of the hot-demolded hollowware can also be utilized for the heat-treatment. However, it can also be carried out in a separate step. For example, the heat-treatment can be carried out in a process step with any off-line fluorination which may be necessary. In this case, the tank is heat-treated directly in the fluorination chamber. It is also possible to heat-treat a PFTxe2x80x94incomplete, partly complete or fully completexe2x80x94on site just before it is installed in an automobile.
The flame resistance of hollowware treated in this way, which can be determined, for example, in accordance with DIN 54 837 by flame treatment in a combustion chamber, is, surprisingly, significantly increased compared to the untreated hollowware. A particular advantage of the novel process is that there is no need to use special PE polymers, but instead hollowware made from commercially available tough PE polymers.

Process for increasing the flame resistance of polyethylene-containing hollowware