An acrylic resin, a preparation method and application thereof, and an acrylic powder coating

Abstract

The application provides an acrylic resin, a preparation method and application thereof, and an acrylic powder coating, and belongs to the technical field of photovoltaic packaging materials. The acrylic resin is prepared from raw materials including the following components in mass fractions: 10-60 parts of soft monomers, 30-70 parts of hard monomers, 20-50 parts of first crosslinking monomers, 0.2-0.5 parts of second crosslinking monomers, 1-5 parts of initiators and 60-300 parts of solvents; the first crosslinking monomers contain epoxy groups; the second crosslinking monomers contain at least two unsaturated bonds, and the at least two unsaturated bonds include terminal carbon-carbon double bonds; and the soft monomers are alkyl acrylate compounds. The second crosslinking monomers are used, the second crosslinking monomers are limited to contain terminal carbon-carbon double bonds, the second crosslinking monomers can realize self-crosslinking of the carbon-carbon double bonds, the crosslinking degree is improved, and the thermal stability of the acrylic resin is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of photovoltaic packaging materials, and particularly relates to an acrylic resin, a preparation method and application thereof, and an acrylic powder coating. BACKGROUND

[0002] Photovoltaic power generation is a technology for directly converting light energy into electric energy by using the photovoltaic effect of a semiconductor interface, and mainly comprises a solar cell panel (assembly), a controller and an inverter. A traditional photovoltaic assembly uses glass for packaging, and the weight of the glass is relatively large, which is more than 10 kg per square meter, which brings difficulties to installation and construction. Therefore, researchers seek new packaging materials to replace glass to reduce weight and meet the technical standards of the photovoltaic industry, such as ultraviolet resistance, aging resistance, impact resistance and fire resistance.

[0003] The related art discloses that an acrylic powder coating is used as a new type of packaging material with light weight, high transparency and strong weather resistance. For example, an epoxy type acrylic powder coating resin is prepared by using a crosslinking monomer containing an epoxy group and / or a crosslinking monomer containing a hydroxyl group, so that the obtained acrylic resin has excellent light transmittance and good flexibility and impact resistance. A high-transparency acrylic resin prepared by using methacrylic acid, methyl methacrylate, n-butyl methacrylate, glycidyl methacrylate, a functional monomer (including one or more of hexafluorobutyl methacrylate, n-dodecyl acrylate and polyethylene glycol methacrylate) and styrene has good mechanical strength. However, the acrylic resin in the related art still has the problem of poor thermal stability. SUMMARY

[0004] Therefore, the present application aims to provide an acrylic resin, a preparation method and application thereof, and an acrylic powder coating. The acrylic resin provided by the present application uses a second crosslinking monomer containing at least two unsaturated bonds and containing an end carbon-carbon double bond, thereby improving the thermal stability of the acrylic resin.

[0005] In order to achieve the above-mentioned application purposes, the present application provides the following technical solutions.

[0006] The present application provides an acrylic resin prepared from raw materials including the following mass fractions:

[0007] 10-60 parts of a soft monomer, 30-70 parts of a hard monomer, 20-50 parts of a first crosslinking monomer, 0.2-0.5 parts of a second crosslinking monomer, 1-5 parts of an initiator and 60-300 parts of a solvent;

[0008] The first crosslinking monomer contains an epoxy group; the second crosslinking monomer contains at least two unsaturated bonds, and the at least two unsaturated bonds include an end carbon-carbon double bond;

[0009] The soft monomer is an alkyl acrylate compound.

[0010] Preferably, the second crosslinking monomer comprises allyl methacrylate and / or allyl acrylate.

[0011] Preferably, the mass fraction of the second crosslinking monomer in the raw material is 0.3-0.4 parts.

[0012] Preferably, the first crosslinking monomer comprises one or more of glycidyl methacrylate, glycidyl acrylate, 4-hydroxybutyl acrylate glycidyl ether and glycidyl versatate.

[0013] The application also provides a preparation method of the acrylic resin as described in the technical solution above, comprising the following steps:

[0014] Mixing the soft monomer, the hard monomer, the first crosslinking monomer, the second crosslinking monomer and the initiator to obtain a mixed solution;

[0015] Dropping the mixed solution into a solvent to perform a polymerization reaction to obtain the acrylic resin.

[0016] Preferably, the dropping time is 3-6 h.

[0017] Preferably, the temperature of the solvent is 130-150℃.

[0018] Preferably, the temperature of the polymerization reaction is 130-150℃, and the holding time is 1-4 h, the holding time being calculated from the completion of dropping.

[0019] The application also provides an application of the acrylic resin as described in the technical solution above or the acrylic resin prepared by the preparation method as described in the technical solution above as a photovoltaic module packaging material.

[0020] The application also provides an acrylic powder coating, comprising the following mass fractions of components: resin 100-140 parts, curing agent 12-40 parts and accelerator 0.2-2 parts, the resin being the acrylic resin as described in the technical solution above or the acrylic resin prepared by the preparation method as described in the technical solution above.

[0021] The application provides an acrylic resin prepared from raw materials comprising the following mass fractions: soft monomer 10-60 parts, hard monomer 30-70 parts, first crosslinking monomer 20-50 parts, second crosslinking monomer 0.2-0.5 parts, initiator 1-5 parts and solvent 60-300 parts; the first crosslinking monomer contains an epoxy group; the second crosslinking monomer contains at least two unsaturated bonds, the at least two unsaturated bonds including terminal carbon-carbon double bonds; and the soft monomer is an alkyl acrylate compound.

[0022] Compared with the prior art, the application has the following beneficial effects:

[0023] The present application uses a second crosslinking monomer, which contains at least two unsaturated bonds, and the at least two unsaturated bonds include terminal carbon-carbon double bonds, the second crosslinking monomer can realize carbon-carbon double bond self-crosslinking, improve the crosslinking degree, make the epoxy value be 0.24-0.3 mol / 100g, and further improve the thermal stability of the acrylic resin; meanwhile, through the limitation of the amount of each raw material, the weather resistance and thermal stability of the acrylic resin are improved while the light transmittance is maintained. Meanwhile, the raw materials used in the present application are common raw materials, and the cost is low.

[0024] The present application also provides a preparation method of the acrylic resin described in the above technical solution, which has a simple polymerization process, low requirement on production equipment, and saves equipment cost.

[0025] Further, the present application controls the dropping time and the holding time of the polymerization reaction, so that the number average molecular weight of the prepared acrylic resin is 2000-4000, and the molecular weight distribution is 1.6-1.8, thereby ensuring the thermal stability of the acrylic resin.

[0026] Still further, the present application also limits the temperature of the solvent to be 130-150℃, which can control the solvent reflux state, thereby controlling the reaction heat, stabilizing the polymerization reaction temperature, and further ensuring the stable performance of the resin. DETAILED DESCRIPTION

[0027] The present application provides an acrylic resin prepared from raw materials including the following mass fractions:

[0028] 10-60 parts of soft monomers, 30-70 parts of hard monomers, 20-50 parts of first crosslinking monomers, 0.2-0.5 parts of second crosslinking monomers, 1-5 parts of initiators, and 60-300 parts of solvents;

[0029] The first crosslinking monomer contains an epoxy group; the second crosslinking monomer contains at least two unsaturated bonds, and the at least two unsaturated bonds include terminal carbon-carbon double bonds;

[0030] The soft monomer is an alkyl acrylate compound.

[0031] In the present application, the raw materials used are commercially available products in the art, unless otherwise specified.

[0032] In the present application, the mass fraction of the soft monomer in the raw materials is preferably 15-55 parts, and specifically can be 10, 15, 20, 25, 30, 35, 40, 45, 50, 55 or 60 parts. The soft monomer is an alkyl acrylate compound, and the soft monomer provides the resin toughness and corresponding adhesion.

[0033] In the present application, the alkyl acrylate compound preferably includes one or more of methyl acrylate, ethyl acrylate, propyl acrylate and butyl acrylate (BA).

[0034] The mass fraction of the hard monomer in the raw material is preferably 35-65 parts, and specifically can be 30, 35, 40, 45, 50, 55, 60, 65 or 70 parts, based on the mass fraction of the soft monomer, and the hard monomer serves to provide a certain degree of hardness.

[0035] In the present application, the hard monomer preferably includes one or more of methyl methacrylate, ethyl methacrylate, butyl methacrylate, isobornyl methacrylate, dicyclopentadiene acrylate, dicyclopentadiene ethoxy acrylate, styrene, methyl styrene and acrylonitrile.

[0036] In the present application, the mass fraction of the first crosslinking monomer in the raw material is preferably 25-45 parts, and specifically can be 20, 25, 30, 35, 40, 45 or 50 parts.

[0037] The first crosslinking monomer preferably includes one or more of glycidyl methacrylate, glycidyl acrylate, 4-hydroxybutyl acrylate glycidyl ether and glycidyl versatate, based on the mass fraction of the soft monomer.

[0038] The mass fraction of the second crosslinking monomer in the raw material is preferably 0.3-0.4 parts, and specifically can be 0.2, 0.3, 0.4 or 0.5 parts, based on the mass fraction of the soft monomer, and the terminal carbon-carbon double bond contained in the second crosslinking monomer can achieve carbon-carbon double bond self-crosslinking, improve the crosslinking degree, and thus improve the thermal stability of the acrylic resin.

[0039] In the present application, the second crosslinking monomer preferably includes allyl methacrylate and / or allyl acrylate.

[0040] The mass fraction of the initiator in the raw material is preferably 2-4 parts, and specifically can be 1, 2, 3, 4 or 5 parts, based on the mass fraction of the soft monomer.

[0041] In the present application, the initiator is preferably an azo initiator and / or a peroxide initiator; the azo initiator preferably includes one or more of azobisisobutyronitrile and / or azobisisoheptyl nitrile; and the peroxide initiator preferably includes one or more of benzoyl peroxide, tert-butyl hydroperoxide, cumyl hydroperoxide, diisopropylbenzene hydroperoxide, di-tert-butyl peroxide, tert-butyl benzoyl peroxide, tert-butyl tert-amyl peroxide, di-perfuryl peroxide, tert-butyl peroxy-2-ethylhexanoate and dicumyl peroxide.

[0042] The mass fraction of the solvent in the raw material is preferably 80-250 parts based on the mass fraction of the soft monomer, and can be 60, 80, 100, 120, 150, 200, 250 or 300 parts.

[0043] In the present application, the solvent preferably includes one or more of toluene, xylene, trimethylbenzene, ethyl acetate, butyl acetate, tetrahydrofuran, ethylene glycol methyl ether and acetone.

[0044] In the present application, the mass ratio of the soft monomer, the hard monomer, the first cross-linking monomer and the second cross-linking monomer is preferably 10:40:40:0.4, and the acrylic resin prepared under this raw material ratio has the best performance, weather resistance and thermal stability.

[0045] In the present application, the mass ratio of the solvent to the monomer is preferably 1:1, and the monomer includes the soft monomer, the hard monomer, the first cross-linking monomer and the second cross-linking monomer. Controlling this ratio makes the Tg of the obtained acrylic resin suitable and the cross-linking degree suitable.

[0046] In the present application, the epoxy value of the acrylic resin is preferably 0.24-0.3 mol / 100g, the light transmittance is preferably 92-94%, the number average molecular weight is preferably 2000-4000, and the molecular weight distribution is preferably 1.6-1.8.

[0047] The present application also provides a preparation method of the acrylic resin described in the above technical solution, which comprises the following steps:

[0048] The soft monomer, the hard monomer, the first cross-linking monomer, the second cross-linking monomer and the initiator are mixed to obtain a mixed solution.

[0049] The mixed solution is added dropwise into the solvent to perform a polymerization reaction, and the acrylic resin is obtained.

[0050] In the present application, the soft monomer, the hard monomer, the first cross-linking monomer, the second cross-linking monomer and the initiator are mixed to obtain a mixed solution. The present application does not have a special way for the mixing, and a way known to those skilled in the art can be used.

[0051] After obtaining the mixed solution, the mixed solution is added dropwise into the solvent to perform a polymerization reaction, and the acrylic resin is obtained.

[0052] In the present application, the dropping time is preferably 3-6h, and can be 3, 4, 5 or 6h. By controlling the dropping time, the number average molecular weight of the prepared acrylic resin can be 2000-4000, and the molecular weight distribution can be 1.6-1.8, thereby ensuring the thermal stability of the acrylic resin. If the dropping time is too long, the molecular weight distribution will be wide, and if the dropping time is too short, the reaction will be incomplete.

[0053] In the present application, the mixed liquid is preferably added at a constant rate.

[0054] In the present application, the temperature of the solvent is preferably 130-150℃, and can be 130℃, 138℃, 140℃ or 150℃, which can control the reflux state of the solvent, thereby controlling the heat of reaction, so that the polymerization temperature is stable, and the resin performance is stable.

[0055] In the present application, the temperature of the solvent is preferably 130-150℃, and can be 130℃, 138℃, 140℃ or 150℃, which can control the reflux state of the solvent, thereby controlling the heat of reaction, so that the polymerization temperature is stable, and the resin performance is stable.

[0056] In the present application, the temperature of the solvent is preferably 130-150℃, and can be 130℃, 138℃, 140℃ or 150℃, which can control the reflux state of the solvent, thereby controlling the heat of reaction, so that the polymerization temperature is stable, and the resin performance is stable.

[0057] In the present application, the temperature of the solvent is preferably 130-150℃, and can be 130℃, 138℃, 140℃ or 150℃, which can control the reflux state of the solvent, thereby controlling the heat of reaction, so that the polymerization temperature is stable, and the resin performance is stable.

[0058] In the present application, the polymerization reaction is preferably carried out in a protective atmosphere, and the protective atmosphere is preferably nitrogen.

[0059] After the polymerization reaction is completed, the present application preferably carries out vacuum distillation on the obtained polymerization product to obtain the acrylic resin.

[0060] In the present application, the vacuum degree of the vacuum distillation is preferably 0.098 MPa, and the temperature is preferably 100℃, and the present application does not have special limitations on the time of the vacuum distillation, and the vacuum distillation is stopped until no distillate is distilled out.

[0061] The present application also provides the application of the acrylic resin in the above technical solution or the acrylic resin prepared by the preparation method in the above technical solution as a photovoltaic module packaging material.

[0062] The present application also provides an acrylic powder coating, which comprises the following components in mass fraction: 100-140 parts of resin, 12-40 parts of curing agent and 0.2-2 parts of accelerator, and the resin is the acrylic resin in the above technical solution or the acrylic resin prepared by the preparation method in the above technical solution.

[0063] The mass fraction of the resin in the acrylic powder coating provided by the present application can be 100 parts, 120 parts or 140 parts.

[0064] The mass fraction of the curing agent in the acrylic powder paint is preferably 15-35 parts, and can be specifically 12, 15, 20, 25, 30, 35 or 40 parts.

[0065] In the present application, the curing agent preferably comprises one or more of undecanedioic acid, dodecanedioic acid, tridecanedioic acid and tetradecanedioic acid.

[0066] The mass fraction of the accelerator in the acrylic powder paint is preferably 0.5-15 parts, and can be specifically 0.2, 0.5, 1, 1.5 or 2 parts.

[0067] In the present application, the accelerator preferably comprises one or more of 2-methylimidazole, zinc acetylacetonate, 2-mercaptobenzothiazole zinc salt (zinc salt MZ), cetyltrimethylammonium bromide and benzyltriethylammonium chloride.

[0068] The present application also provides a preparation method of the acrylic powder paint, comprising the following steps: mixing resin, a curing agent and an accelerator to obtain the acrylic powder paint.

[0069] In the present application, the mixing temperature is preferably 80-120 DEG C, and can be specifically 80, 100 or 120 DEG C, and the mixing time is not particularly limited in the present application, and the raw materials can be mixed uniformly.

[0070] In the present application, the mixing preferably further comprises sequentially crushing and grinding to obtain the acrylic powder paint.

[0071] The present application does not particularly limit the specific parameters of the crushing and grinding, and the acrylic powder paint can pass through a 120-mesh sieve.

[0072] The present application also provides an application of the acrylic powder paint, preferably comprising the following steps: spraying the acrylic powder paint to the surface of a substrate, and then curing, wherein the spraying is electrostatic spraying.

[0073] In the present application, the substrate preferably comprises an aluminum plate.

[0074] In the present application, the aluminum plate is preferably subjected to surface treatment, and the surface treatment preferably comprises wiping dust and impurities on the surface of the aluminum plate with ethanol.

[0075] In the present application, the parameters of the electrostatic spraying preferably comprise: a voltage of 60-90 kV, which can be specifically 60, 70, 80 or 90 kV, and an electrostatic current of 10-20 mu A, which can be specifically 10, 15 or 20 mu A.

[0076] In the present application, the temperature for the solidification is preferably 160-180℃, and can be specifically 160, 170 or 180℃, and the time is preferably 10-40min, and can be specifically 10, 20, 30 or 40min.

[0077] The technical solutions in the present application will be clearly and completely described below in combination with the embodiments in the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all the other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0078] Embodiment 1

[0079] The raw materials for preparing the acrylic resin include the following components in mass fraction: 10 parts of soft monomer butyl acrylate, 40 parts of hard monomer methyl methacrylate, 10 parts of styrene, 35 parts of first crosslinking monomer glycidyl methacrylate, 0.2 parts of second crosslinking monomer allyl methacrylate, 1 part of initiator tert-butyl hydroperoxide and 120 parts of solvent dimethylbenzene, and the preparation process includes the following steps:

[0080] The soft monomer, the hard monomer, the first crosslinking monomer, the second crosslinking monomer and the initiator are mixed uniformly according to the formula amount, so that the initiator is fully dissolved, to obtain a mixed solution; the solvent is added into a reactor provided with heating and stirring and condensation reflux, and after nitrogen is passed for 30min, the stirring is started and the temperature is raised to 138℃ (the heating table temperature is set to 80℃, and the temperature is raised by 20℃ every 15min), then the mixed solution is added at a constant speed, the adding is completed in 3h, and the temperature is kept for 2h, then the solvent is distilled off under reduced pressure at a vacuum degree of 0.098MPa and a temperature of 100℃ until basically no distillate is distilled off, to obtain the acrylic resin.

[0081] Embodiment 2

[0082] The same as Embodiment 1, except that the raw materials include the following components in mass fraction: 10 parts of soft monomer butyl acrylate, 70 parts of hard monomer methyl methacrylate, 50 parts of first crosslinking monomer glycidyl methacrylate, 0.5 parts of second crosslinking monomer allyl methacrylate, 5 parts of initiator tert-butyl hydroperoxide and 300 parts of solvent dimethylbenzene.

[0083] Embodiment 3

[0084] The same as Embodiment 1, except that the raw materials include the following components in mass fraction: 60 parts of soft monomer butyl acrylate, 70 parts of hard monomer methyl methacrylate, 50 parts of first crosslinking monomer glycidyl methacrylate, 0.5 parts of second crosslinking monomer allyl methacrylate, 5 parts of initiator tert-butyl hydroperoxide and 300 parts of solvent dimethylbenzene.

[0085] Example 4

[0086] The same as Example 1, except that the raw materials include the following components in mass parts: soft monomer butyl acrylate 60 parts, hard monomer methyl methacrylate 30 parts, first crosslinking monomer glycidyl methacrylate 20 parts, second crosslinking monomer allyl methacrylate 0.5 parts, initiator t-butyl hydroperoxide 1 part, and solvent dimethylbenzene 60 parts.

[0087] Example 5

[0088] The same as Example 1, except that the raw materials include the following components in mass parts: soft monomer butyl acrylate 10 parts, hard monomer methyl methacrylate 40 parts, first crosslinking monomer glycidyl methacrylate 40 parts, second crosslinking monomer allyl methacrylate 0.4 parts, initiator t-butyl hydroperoxide 5 parts, and solvent dimethylbenzene 90.4 parts.

[0089] Example 6

[0090] The same as Example 1, except that the mixed solution is added dropwise in 4 h.

[0091] Example 7

[0092] The same as Example 1, except that the mixed solution is added dropwise in 5 h.

[0093] Example 8

[0094] The same as Example 1, except that the mixed solution is added dropwise in 6 h.

[0095] Example 9

[0096] The same as Example 1, except that after the temperature is raised to 130°C (the heating platform temperature is set to 80°C, and the temperature is raised by 20°C every 15 min) and kept constant, the mixed solution is added dropwise at a constant speed to perform the polymerization reaction.

[0097] Example 10

[0098] The same as Example 1, except that after the temperature is raised to 150°C (the heating platform temperature is set to 80°C, and the temperature is raised by 20°C every 15 min) and kept constant, the mixed solution is added dropwise at a constant speed to perform the polymerization reaction.

[0099] Example 11

[0100] The same as Example 1, except that the second crosslinking monomer allyl methacrylate is replaced by allyl acrylate.

[0101] Comparative Example 1

[0102] The same as example 1, the only difference is that the second crosslinking monomer allyl methacrylate is replaced by glycidyl methacrylate.

[0103] The epoxy value, number average molecular weight and molecular weight distribution of the acrylic resins prepared in the examples and comparative examples were tested, and the results are shown in Table 1.

[0104] Application Example

[0105] The acrylic resin prepared in the examples and comparative examples 100 parts by mass, solid agent undecanedioic acid 40 parts by mass and accelerator 2-methylimidazole 2 parts by mass were mixed uniformly at 120℃, then crushed, ground and passed through a 120 mesh screen to obtain an acrylic powder coating. The acrylic powder coating was sprayed onto an alcohol-wiped aluminum plate using electrostatic spraying technology, the electrostatic spraying voltage was 60kV, the electrostatic current was 10μA, then the paint film was obtained by curing at 180℃ for 10min.

[0106] Performance Test

[0107] (1) Appearance: whether the surface of the paint film obtained after curing is flat was observed by visual observation;

[0108] (2) Light transmittance: tested according to the method specified in GB / T 40415-2021;

[0109] (2) Flexibility: tested according to the method specified in GB / T 1731-1993;

[0110] (3) Impact resistance: tested according to the method specified in GB / T 1732-1993;

[0111] (4) Weather resistance: tested according to the method specified in GB 1767-79(89);

[0112] (5) Thermal stability test: tested according to the method for determining the shrinkage of plastic thermosetting molding materials in GB / T 39818-2021, the test condition is 150℃, 30min.

[0113] The test results are shown in Table 1. As can be seen from Table 1, the acrylic resin prepared in Example 5 has the best performance and the best thermal stability.

[0114] Table 1 Performance Test Results

[0115]

[0116] The above merely describes the preferred embodiments of the present application, and does not limit the present application in any form. It should be noted that, for those skilled in the art, several improvements and refinements can be made without departing from the principles of the present application, and these improvements and refinements should also be considered as the protection scope of the present application.

Claims

1. An acrylic resin, characterized by, Specifically, the raw materials are prepared in the following mass fractions: soft monomer 10-60 parts, hard monomer 30-70 parts, first crosslinking monomer 20-50 parts, second crosslinking monomer 0.2-0.5 parts, initiator 1-5 parts, and solvent 60-300 parts; the first crosslinking monomer includes glycidyl methacrylate and glycidyl acrylate; the soft monomer includes one or more of methyl acrylate, ethyl acrylate, propyl acrylate, and butyl acrylate; the hard monomer includes one or more of methyl methacrylate, ethyl methacrylate, butyl methacrylate, and isobornyl methacrylate; the second crosslinking monomer includes allyl methacrylate and / or allyl acrylate.

2. The acrylic resin according to claim 1, characterized in that, The mass fraction of the second crosslinking monomer in the raw materials is 0.3-0.4 parts.

3. The method of producing an acrylic resin as claimed in claim 1 or 2, characterized in that, comprising the following steps: mixing the soft monomer, the hard monomer, the first crosslinking monomer, the second crosslinking monomer, and the initiator to obtain a mixed solution; dropping the mixed solution into a solvent to perform a polymerization reaction to obtain the acrylic resin.

4. The production method according to claim 3, characterized by, The dropping time is 3-6 hours.

5. The preparation method according to claim 3, characterized in that, The temperature of the solvent is 130-150°C.

6. The preparation method according to claim 3, characterized in that, The temperature of the polymerization reaction is 130-150°C, and the holding time is 1-4 hours, the holding time being counted from the completion of dropping.

7. Use of the acrylic resin of claim 1 or 2 or the acrylic resin prepared by the preparation method of any one of claims 3-6 as a packaging material for a photovoltaic module.

8. An acrylic powder coating, characterized in that, comprising the following mass fractions of components: resin 100-140 parts, curing agent 12-40 parts, and accelerator 0.2-2 parts, the resin being the acrylic resin of claim 1 or 2 or the acrylic resin prepared by the preparation method of any one of claims 3-6.

Images