FI64182C - BELAEGGNINGSFOERFARANDE - Google Patents

Description

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Patent- och registerstyrelsan '· Amafcmuti ^ dodincUkrtHtwybHcftd 30.06.83 (32) (33) (31) Pyrdwy we «kw» -S «glrd prtoriut 13.03.72 Great Britain-Storbritannien (GB) ll6l7 / 72 (71) Donald MacPherson Group Limited, Three Quays, Tower Hill, London EC3 ·, United Kingdom-UK (GB) (72) Stanley Edward Young, Hornchurch, Essex, UK-UK (GB) (71 *) Berggren Oy Ab (5M Coating method - Beläggningsförfarande This applies to the coating method and in particular to the coating method in which the coating is dried by electromagnetic radiation.

It is known that coatings formed by varnishes, paints, alkyd varnishes and adhesives on porous or non-porous substrates can be dried by high-cycle heating. As the intensity of heating increases, the tendency of the coating to form bubbles and blisters increases greatly, especially when the coating composition is applied to a porous substrate such as wood, hardboard, fiberboard, asbestos board, or a plastic material such as foamed polystyrene. The formation of bubbles and vesicles in these cases may be due to the release of air and / or water from the substrate under the influence of heat or, in the case of materials with a low "loss factor", due to distortion or decomposition of the substrate, which damage or pull the coating in a twist. This difficulty can in some cases be avoided by preheating the substrate just before applying the paint or varnish layer. However, methods using preheating are not desirable because they involve an additional work step. Moreover, when using the preheating step, the method of applying the coating composition is generally limited to the curtain coating method, since other commonly used methods cannot be used due to the limitations of flow properties, stability, etc. due to direct application of the coating composition to the heated substrate.

U.S. Patent No. 3,356,637 describes coating compositions intended to improve the performance of the paints formed in the present invention. of the ingredients used in the case, the adhesion properties. The said publication speaks of the Ί \ temperature, the so-called a second-order transition temperature, defined in the literature as a film drying temperature with a film torsion modulus of 2 ....

300 kg / cm. The quantity is conceptually similar to the mime film formation temperature or the glass transition temperature, but nevertheless there is no direct relationship between these three temperatures. If the temperature Th and the minimum film formation temperature are sometimes replaced, the above definition for 1h may no longer be correct.

It has now surprisingly been found that certain dispersion, emulsion or latex coating compositions can be dried using electromagnetic radiation without having to preheat the surface of the substrate, resulting in coatings without substantial surface defects.

The invention therefore relates to a process for coating a substrate surface, comprising applying to the substrate a coating composition consisting essentially of a mixture of an aqueous dispersion of at least two polymers or copolymers, characterized in that the substrate is a porous substrate, the first aqueous dispersion is a is at least 5 ° C above ambient temperature and that the second aqueous dispersion is a polymer or copolymer dispersion having a minimum film-forming temperature at or below ambient temperature, the ratios of the mixture dispersions being selected to give a coating composition having a minimum film-forming temperature above ambient temperature. and that the coating is dried by heating it with electromagnetic radiation having a wavelength greater than 780 nm.

In general, the diameter of the particles in the dispersion is between 0.01 and 6 μm, and coating compositions are most commonly used to form particles with a diameter between 0.1 and 1.0 μm. , 3 64182

By the phrase "minimum film formation temperature" as used herein is meant the temperature at which an emulsion or latex forms a continuous polymer layer upon drying. This temperature must be at or above the temperature at which the polymer particles become soft enough to fuse together and is therefore sometimes referred to in the art as the "fusion temperature".

The term "ambient temperature" as used herein means the temperature in the immediate vicinity of the surface of the substrate to which the coating composition is applied. The ambient temperature in the United Kingdom for carrying out the process of the invention is generally between 15 and 30 ° C.

t “64182

When a dispersion, emulsion or latex coating is applied to the surface of the substrate, water is lost by evaporation and also, in the case of application to absorbent substrates, by absorption into the substrate. The polymer particles of the coating mixture then become immobile and the energy barriers between the particles that resist coalescence in the diluted state weaken and the particles then come into contact with each other. If the polymer particles are soft and deformable in this state, fusion of the particles occurs and a uniform film is obtained. However, if the particles obtained do not have flow properties at that temperature, no film formation will occur unless the temperature is raised to a level where "softening and deformation of the particles take place.

The ratios of the polymer or copolymer dispersions present in the dispersion or latex coating compositions vary with their minimum film-forming temperature. The following equation (Fox's equation, LESCHAEK & FOX, Bull. Div. Phys. Soc. 1965, 3, 123) is often useful in estimating the ratios of polymer or copolymer dispersions required to obtain the resulting dispersion, emulsion or latex coating composition. having a minimum film-forming temperature of ambient temperature, i.e. generally above about 15 ° C: 1 = Wa + Wb TF TFa TFb where Tp (° K) is the minimum film formation temperature of the emulsion or latex coating composition, Tpa and Tpfe are the minimum film formation temperatures of the polymer or copolymer dispersions a and b In degrees K and Wa and are the weight percentages of dispersions a and b. It should be noted that this equation is intended to be used only for estimating the approximate ratios of polymer or copolymer dispersions used, and the actual ratios required must be confirmed by an experimental assay.

It is recommended that the difference between the lower film-forming temperatures of the polymer or copolymer dispersions be at least 10-15 ° C. Thus, if the ambient temperature is 15 ° C, a polymer or copolymer dispersion having a minimum film-forming temperature of 25 ° C can be mixed in a suitable ratio with a polymer or copolymer dispersion having a minimum film-forming temperature of 10 ° C to obtain a coating composition having the minimum film formation temperature is above 15 ° C. In a preferred aspect of the present invention, the second film of the mixture has a minimum film-forming temperature of less than 15 ° C and m.p. Il 5 64182 The second minimum film formation temperature is above 30 ° C.

Types of monomers that can be used to prepare copolymer dispersions in your polymer with a high minimum film-forming temperature include, for example, styrene, methyl methacrylate, and vinyl acetate. Monomers that can be used to prepare polymer or copolymer dispersions with a low minimum film-forming temperature include, for example, higher esters of acrylic and methacrylic acid, such as m-butyl methacrylate, isobutyl acrylate, etc., butadiene, ethylene ester, fumarene, fuma or vinyl esters.

In addition, reactive or polyvalent monomers such as acrylamide, aminomethacrylate, hydroxyacrylates, hydroxymethacrylates, N-methylolacrylamides, acrylic and methacrylic acid can be used to crosslink or polymerize with other polymers or copolymers capable of crosslinking. Thus, it may be possible to crosslink the polymeric or cormormer components of the coating composition, e.g. either by adding acid or by heat alone, leading to internal crosslinking and the loss of thermoplastic properties of the resulting film, increased solvent and water resistance and improved hardness and strength.

High molecular weight polymers generally do not have clearly defined melting points, as is the case with crystalline compounds, but a rubbery intermediate zone exists between the hard, brittle state and the liquid state. The temperature at which the transition from a hard, brittle state to a rubbery intermediate state occurs is called the glass transition temperature, which is usually represented by the symbol Tg and is defined as the temperature at which there is a change in the volume-temperature curve.

The glass transition temperature varies greatly with the type of polymer, but is also affected by the presence of plasticizers and other additives.

Typical Tg values for different polymers are:

methyl methacrylate - 105 ° C

polystyrene - 100 ° C

polyvinyl acetate - 28 ° C

butyl methacrylate - 20 ° C

methyl acrylate - 6 ° C

sec-butyl acrylate - -20 ° C

n-butyl acrylate - -55 ° C

n-hexyl acrylate - -57 ° C

Thus, it can be appreciated that by polymerizing pure monomers and copolymerizing mixtures of monomers, it is possible to produce a wide variety of polymers with T 1 values ranging from 105 ° C to at least -60 ° C.

Although there is a close relationship between the minimum film-forming temperature and the glass transition temperature, it is not possible to directly contact them because the glass transition temperature is determined for a pure, dry polymer, while the minimum film-forming temperature is determined as a result of aqueous dispersion experiments.

However, it has been found that a mixture of a dispersion of a polymer or copolymer having a T above * 0 ° C and

S

a dispersion of the polymer or copolymer below 20 ° C

so that the effective T of the mixture is between 20- * 40 ° C, ε can be used in the process of this invention.

Accordingly, in a specific part, the invention provides a method of coating a substrate surface comprising applying to the surface of a substrate a layer of a dispersion, emulsion or latex coating composition comprising a mixture of at least one polymer or copolymer of a polymer or copolymer having above M0 ° C, and at least

one polymer or copolymer dispersion with a T_ of less than 20 ° C

ε in the mixture of said polymer or copolymer dispersions in a mixture so selected as to obtain a dispersion, emulsion or latex coating mixture having an effective Tg of between 20 and 40 ° C and said coating being dried by heating with electromagnetic radiation having a wavelength of more than 780 nm.

It is to be understood that the method defined above is within the scope of the method of this invention, although a different method has been used to express the properties of the polymer or copolymer.

The emulsion or latex coating compositions used in the process of this invention are dried by heating with radiant energy having a wavelength greater than that of visible light, i. greater than 780 nm, -9 (1 nm = 10 m). The preferred type of heating is large cycle heating.

By international agreement, only certain frequencies are available for industrial purposes in the high-frequency area. These are 13.56, 27.12, 38 and * 40, 65 MHz. The frequencies used for industrial purposes in the microwave range are 300, 915 »2 *» 50, 5800 and 22125 MHz. The entire infrared range in the wavelength range of 780 n'm to 1 mm is beautifully usable.

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Large cycle equipment has the advantage of being lower cost and simpler to operate and maintain than microwave equipment. Besides, it is much more adaptable to changes in production requirements. Both high-cycle and microwave equipment have the advantage over infrared equipment in that they operate with fast start and low idle power requirements. In addition, very precise adjustment can be achieved, either by automatic or manual tuning of the power supplied to the transmitter.

It is possible to control the relative heating of the paint film and the substrate by varying these materials. Thus, by applying a highly polar water-borne emulsion coating mixture to a substrate having a low water content and a low loss coefficient, almost all the heating effect can be limited to the paint film; an aspect of particular interest when crosslinking coatings to thermally sensitive substrates. Conversely, by applying a low polarity coating composition to a substrate having a high water content and a high loss factor, the heat generated can be largely limited to the substrate. In addition, the heating effect can be adjusted by performing a small calibration with respect to the air gap between the radiating plate and the object.

When using infrared heating, the installation costs are significantly lower, but a certain time is required to heat the oven. The heat efficiency is much lower and the adjustment is more difficult to perform. The polarity of the coating and the substrate is no longer the main factor and therefore the control of the relative heating of the substrate and the lacquer layer is much more difficult to achieve.

Several methods for transmitting large period radiation to a workpiece are in common use. The first, in which electrodes are placed on both sides of the workpiece and high-cycle energy is transmitted between them, causing the polar substances in the path to heat up, is generally known as "permeable high-cycle heating." Another method in which a slab consists of a plurality of rods or rods spaced at suitably calculated distances from each other according to the frequency of use and connected in turn on each side of the generator is generally known as "scattered field moon". The air gap, which usually varies between 1 and 20 mm, is between the slab and the workpiece to be dried as it passes under the slab. Another method is the preferred method for use in this invention.

Direct measurement of high-cycle energy transferred to a surface is a difficult operation. Therefore, it is more common to place an ampere meter in the anode circuit to obtain a direct reading of the anode current flow. Thus, with an Intertherm DH 83 generator, a water-cooled 6 kV / 38 MHz generator, in this case connected to a 0.6 mx 0.6 m stray field plate, a constant current of 0.6 A was recorded in the anode circuit when the plate was connected. a generator. When passing the fiberboards through the equipment and tuning the system to a maximum power supply corresponding to 6 kW of output power from the generator, a current of 1.65 A was recorded in the anode circuit. The procedure used in the examples detailed below was to record the area of the fiberboard and the total anode current. The tiles are generally designed to have a length-to-width ratio in the range of 1/1 to 2/1 and to transmit a maximum power of 12-16 kW / m ^.

In carrying out the method of the present invention, it may sometimes be advantageous to use a two-stage drying process using, for example, a pre-drying step, such as drying with warm air or convection heating before heating with electromagnetic radiation. It may be possible to use the waste heat from the large cycle generator in the pre-drying phase.

A variety of modifiers can be added to the emulsion or latex coating compositions used in this invention. Such additives are well known in the art and include pigments and colorants to provide color and opacity, additives to improve fill properties and reduce cost and gloss, and wetting and dispersing agents. Colloids such as methylcellulose and ethylhydroxyethylcellulose may also be added.

Viscosity, flow properties and spreading properties can be varied by adding resins, e.g. styrene-maleic acid * copolymers or dissolved alkyd resins. The storage properties of the coating compositions can be improved by adding fungicides and bactericides and corrosion inhibitors.

To reduce the minimum film-forming temperature of the emulsion or latex coating composition, various amounts of various solvents may be added to the mixture. For example, high boiling alcohols, glycol ethers, esters and aromatic hydrocarbons may be used provided that the minimum film-forming temperature of the emulsion or latex coating mixture thus modified is above ambient temperature.

Determination of the minimum film formation temperature

The chrome-plated rod, fitted at one end for cold water recycling and at the other end for heating with 9 641 82 low-power electric heaters, has drilled small holes below the rod at 25 mm intervals, each with a thermistor sensor connected to the electric thermometer. The rod is enclosed in an insulated box with a transparent viewing window through which a flow of dry air is maintained at a rate of 25 l / min.

In the preliminary test, a large temperature difference is maintained in the rod by balancing the mutual cooling and heating effects at both ends. The polymer or copolymer dispersion is applied as a 0.75 mm thick wet film to the surface of the rod and the box is closed and the air flow is applied. After one hour, it is observed that the film has dried and at a certain point on the rod shows a change from a continuous film to a discontinuous film. The temperature at this point can be determined.

The rod is cleaned and the test is repeated so that the temperature gradient along the rod is adjusted to about 0.5 ° C / cm and that the temperature recorded above is recorded in the middle of the rod. The value obtained for the transition point from continuous film to discontinuous film is the minimum film-forming temperature of the emulsion.

One of the disadvantages of the concept of the lowest film formation temperature of a polymer or copolymer dispersion is that at 0 ° C and below, ice formation occurs in the aqueous phase, resulting in agglomeration of polymer or copolymer particles and thus impossibility of film formation. In such cases, it is therefore possible to report only the minimum film-forming temperature below 0 ° C.

The method described above essentially corresponds to the standard method described in ASTM Standard D-235-68, modified for temperatures above 25 ° C.

The invention is further illustrated by the following examples. (Examples 1-3 are comparative examples).

Example 1

Methyl methacrylate latex having a resin content of 50% by weight and a minimum film-forming temperature above 90 ° C was applied by spraying onto a veneered glass bead. After 30 seconds, the panel was routed under an R.F. plate connected to a TNTERTHERM DH83 generator. The distance of the plate from the panel was mm, the speed of the conveyor p was 2 m / min and the anode current was 1.5 A for a panel surface of 0.2 m. The panel reached an approximate surface temperature of 70 ° C. After cooling, the film was dry but very weak and brittle, with the failure of a suitable fusion. However, there were no bubbles, blisters, and similar defects in the membrane at all.

Example 2

The procedure of Example 1 was repeated using a latex having a minimum film-forming temperature of 10 ° C and for which a reduction in the minimum film-forming temperature was achieved by replacing part of the latex used in Example 1 with methyl methacrylate with ethyl methacrylate. The resulting film was continuous but soft and very badly vesicular. Example 3

The procedure of Example 1 was repeated using a latex having a minimum film formation temperature below 0 ° C and in which a decrease in the minimum film formation temperature was achieved by replacing a portion of the methyl methacrylate of the latex used in Example 1 with n-butyl methacrylate. The resulting film was continuous but soft and very badly vesicular. Example 4

The procedure of Example 1 was repeated using mixtures of 75 x 20 wt.% Of the latex used in Example 1 mixed with 25 x 80 wt.% Of the latex used in Example 2, the minimum film-forming temperature of said latex mixture being above ambient temperature of 15 ° C. The resulting films were very strong and had no bubbles, blisters, or other surface defects.

Example 5

The procedure of Example 1 was repeated using mixtures of 78-40% by weight of the latex used in Example 1 mixed with 22-60% of the latex used in Example 3, with the minimum film-forming temperature of said latex mixture being above ambient temperature of 15 ° C. The resulting films were very strong and did not show bubbles, blisters or other surface defects.

Example 6

A white semi-emulsion paint was prepared by a very fast dispersion method using a cavitation dispersing device using a base-soluble acrylic copolymer emulsion together with ammonia to prepare an aqueous solution of a thickening and stabilizing acrylic resin. The following ingredients were charged to the tank: kg

Base-soluble acrylic copolymer emulsion (MO% dry matter) VINACRYL 4005® 7.6 * c, .λ Mixed

Water 6.41 0.880 ammonium hydroxide 1.9 g addition of carboxyl nolyte electrolyte sodium salt solution (QROTAN 731®-25%) 0.9 n 64182

Anti-foaming agent - NOPCO DNHI® 0.2

Rutile titanium dioxide 30.0

Micronized barite 10.8

Micronized talc 1.3

At the end of the dispersion process, the following addition was made:

Styrene-acrylic copolymer emulsion (50% dry matter)

Vinacryl 7170 © (minimum film formation temperature 28 ° C) 32.5

Styrene-acrylic copolymer emulsion (50% dry matter) Vinacryl $ 717 © (minimum film formation temperature 10 ° C) 5, M

100.0

The paint above was suitable for direct application by roller coating or could be sprayed after diluting with 15 5 & water.

Hardboard and plywood panels were coated at 16 ° C at room temperature by roller application using a wet film weight of 50 g / m 2 of the above paint applied in two coats without drying in between. The panels were then passed under the slab as in the previous examples.

When it came out, the panel had a flat, white surface that was hard, flexible, and free of blisters or other surface defects. Example 7 In this example, a mixture of self-crosslinking resins is used in contrast to the previous examples where thermoplastic resins were used. Although self-crosslinking resins vulcanize under the influence of heat alone, catalytic addition may be advantageous because a higher degree of insolubility can be achieved at lower temperatures.

Clear emulsion varnish was prepared by mixing together the following ingredients: kg ** 5% dry matter self-reactive acrylic copolymer emulsion Vinacryl $ 131 © (minimum film formation temperature 29 ° C) 79.3 5% dry matter self-reactive

VinylLyl acrylic copolymer emulsion NATIONAL X-

Linl (E) 2833 (minimum film formation temperature 70 ° C) 20.0

Anti-foaming agent NOPCO JMY® 0.2

Moisturizer and drain control agent TROY LATEX

ANTICRATEi ^ ° »‘ i ’W, T5“ Immediately before use, 20 kg of a 10 i aqueous solution of oxalic acid was added to the above mixture.

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After the addition of the acid solution, the stirred material was found to have a service life of a few hours.

The catalyzed emulsion was sprayed onto 50 x 10 mm veneered particle board panels. After allowing a few minutes for the resin to drain over the surface at room temperature of 15 ° C, the varnish was dried under the same R.F. plate as in the previous examples. In this case, the excitation was adjusted so that the maximum anode current became 1.2 A.

The varnish was flattened with 320 waxed carborundum paper and a second coat of varnish was applied and vulcanized in the same manner.

The clear, transparent varnish obtained was hard, strong and flexible and free of surface defects.

Claims (13)

A method of coating a substrate surface, comprising applying to the substrate surface a coating composition consisting essentially of a mixture of at least two polymer or copolymer water dispersions, characterized in that the substrate is a porous substrate, the first water dispersion being a polymer or copolymer dispersion having a lowest film-forming temperature of at least 5 ° C above ambient temperature, and the second water dispersion being a polymer or copolymer dispersion having the lowest film-forming temperature at or below ambient temperature, the ratios of the mixture dispersions being as follows; selected, that a coating composition is obtained with a lowest film-forming temperature above ambient temperature, and that the coating is dried by heating with electromagnetic radiation with a path length longer than 780 nm. Process according to claim 1, characterized in that the polymer or copolymer particles in the dispersion have a diameter of 0.01-6 µm. Method according to claim 2, characterized in that the polymer or copolymer particles have a diameter of 0.1-1.0 µm. Process according to any of the preceding claims, characterized in that the difference between the lowest film-forming temperatures of the polymer or copolymer dispersions is at least 10 ° C. Process according to any of claims 1-3, characterized in that the difference between the lowest film-forming temperatures of the polymer or copolymer dispersions is at least 15 ° C. Process according to any of the preceding claims, characterized in that one of the polymer or copolymer dispersions has a lowest film-forming temperature below 15 ° C and the other above 30 ° C. Method according to any of the preceding claims, characterized in that the coating is dried by heating with high frequency radiation at a frequency of 13.56, 27.12, 38 or * 10.65 MHz.