JPH07100152B2 - How to paint synthetic resin moldings - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for coating a synthetic resin molded product, particularly a molded product made of a polyurethane resin for automobiles.

(Prior Art) In recent years, parts made of a synthetic resin material are often used for the purpose of weight reduction, high performance, etc., and a front bumper 1 and a rear bumper 2 as shown in FIG. 8 are one of them. Is one. The bumpers 1 and 2 are mainly modified polypropylene (polypropylene modified with ethylene propylene rubber; hereinafter referred to as modified PP) or polyurethane (hereinafter referred to as PU).
It is often applied to the outer panel color to improve the appearance of the automobile.

Molding of such a PU resin molded product is usually performed by an injection molding method, and a release agent is sprayed onto the upper and lower molds having the inner shape corresponding to the outer shapes of the bumpers 1 and 2 in advance. With the upper and lower molds clamped, the PU resin melted to a predetermined temperature is injected into the cavity formed between the upper mold and the lower mold at a predetermined pressure. After this,
The injection resin is cooled and hardened for a certain period of time, and both molds are opened to take out the work. However, the bumpers 1 and 2 used for the outer panels of the vehicle body, etc.
The appearance of pores called d) is not desirable in terms of appearance. Further, the step of applying the release agent is also a bottleneck in improving the molding productivity.

Therefore, in the molten resin injected into the mold, accelerate the crosslinking reaction of the PU resin, which is the main component, at an appropriate reaction rate,
Mixing a highly active cross-linking agent to suppress voids generated during molding and a mold release agent (internal mold release agent, Inner Mold Release, hereafter referred to as IMR) that improves mold release from the molten resin itself. However, technology has been developed to improve molding quality and productivity.

As a highly active cross-linking agent suitable for being mixed with the PU resin, diethyltoluene diamine,
(Hereinafter referred to as DETDA), and this DETDA is an additive in which 3,5-diethyltoluene-2,4-diamine is present at about 80% and 3,5-diethyltoluene-2,6-diamine is present at about 20%. It is characterized by its excellent solubility with polyols.

On the other hand, suitable materials for use as the IMR are metal soaps such as zinc stearate. This IMR is PU
It has a feature that it acts as a lubricant inside the resin and is excellent in mold release when the mold is opened.

Further, when a molded product of PU resin molded in this way is applied, it is conventionally performed by a coating process as shown in FIG. That is, a drying step 3 in which voids formed in the PU resin are removed in advance before the primer coating, a cleaning step 4 in which IMR deposited on the surface of the work, dust and the like adhering to the work surface are cleaned, and will be described later. The primer coating step 5 applied to secure the adhesion between the top coating material and the work and the electrical conductivity of the top coating material, the drying step 6 of the primer coating, and the top coating material mixed in a predetermined color on the work surface. It is composed of a top-coating step 7 of applying the top-coating material and a step 8 of drying the top-coat paint. In the cleaning step 4, an aqueous solution having a pH of 2.4 and a water temperature of 60 ° C. is used, and the discharge pressure is 3
The workpiece is cleaned under the conditions of kg / cm ² , discharge rate of 4 to 10 / min, and nozzle. As the primer paint, a paint using a polyester melamine resin as a base material is used, and it is heat-cured by being heated in the subsequent drying step 6 under the condition of holding at 80 ° C. for 20 minutes, whereby the undercoating film is formed. To form. In this primer coating, conductive carbon or the like for reducing the volume resistivity of the resin may be mixed in, which allows electrostatic coating in the top coating step 7. The drying conditions in the above-mentioned top coating drying step 8 were 120 ° C. × 30 minutes.

(Problems to be Solved by the Invention) The above-mentioned primer coating is based on a polyester melamine resin, and the etherification reaction, which is the main reaction of the curing reaction, proceeds in an acidic atmosphere but is suppressed in an alkaline atmosphere. It has the property of being done.

However, in such a conventional coating method, in both cases of mixing the highly active cross-linking agent into the PU resin and mixing the release agent into the PU resin, the primer coating film There is a problem that defects occur.

That is, in the former case, the aromatic amine DE
When TDA remains in the PU resin in an unreacted state, the surface of the PU resin becomes an alkaline atmosphere in the primer drying step, which hinders the progress of the curing reaction of the primer coating, resulting in poor curing of the primer coating film. It will cause it. As a result, the adhesion between the surface of the work and the primer coating film is reduced, and peeling of the coating film results in defective primer coating film.

On the other hand, in the latter case, when the primer coating is applied in a state where the release agent is deposited on the surface of the work, poor primer wetting occurs in the primer coating film. And
To completely remove the release agent that has precipitated,
As described above, a high shower pressure of about 3 kg / cm ² is required, and the equipment for the cleaning process becomes large, which is disadvantageous in terms of space and cost.

The present invention has been made in view of the problem of poor primer coating film inherent in the above-mentioned prior art, and in coating a molded product in which a highly active cross-linking agent is mixed in PU resin, a primer coating film defect does not occur. The first purpose is to obtain a coating method. A second object of the present invention is to obtain a coating method in which a primer coating film defect does not occur in the coating of a molded product in which a release agent is mixed in PU resin.

(Means for Solving the Problems) The present invention for achieving the above object includes a forming step of forming and processing a polyurethane resin mixed with a release agent, and a drying step of heating a formed product formed in the forming step. And a washing step of washing the formed product with a chlorine-based solvent after the drying step, and a temperature range of 80 to 130 ° C. for 10 to 3 after the washing step.
Within the range of 0 minutes, a cleaning / drying step of heating and removing the chlorine-based solvent from the formed article within a time range in which no void failure and primer coating failure occur, and a primer is applied to the formed article after the cleaning / drying step. A method for coating a synthetic resin-formed article, comprising: a primer coating step of applying a coating material and drying; and a top coating coating step of applying a top coating material and drying after the primer coating step.

(Operation) In the present invention configured as described above, after the synthetic resin mixed with the release agent is formed, the synthetic resin-formed product is heated in the drying step to remove the voids remaining in the formed product resin. At the same time, the unreacted release agent remaining in the molded product resin is deposited on the surface of the molded product. The deposited release agent is removed by a cleaning process using a chlorine-based solvent, and the chlorine-based solvent used for cleaning in the cleaning / drying process is removed from the formed product, so that the chlorine-based solvent in each subsequent coating process is removed. Prevents coating problems due to chlorine. In this washing and drying process, the temperature range is 80-130.
By heating within the range of 10 to 30 minutes at a temperature of 10 to 30 minutes for which poor primer coating does not occur, the chlorine-based solvent can be efficiently removed without damaging the synthetic resin-formed product. In addition, in each of the drying steps of the primer coating and the top coating, good coating can be performed without causing defective primer even when directly heated and dried in a furnace.

(Example) Hereinafter, one example of the coating method according to the first invention and the coating method according to the present invention will be described with reference to the drawings.

FIG. 1 is a process drawing showing a method for coating a synthetic resin molded product according to an embodiment of the present invention.

Further, FIG. 2 is experimental data supporting the suitability of the drying conditions in the post-molding drying step shown in FIG. 1, and is an evaluation table showing the experimental results of observing the occurrence of voids after PU resin molding. In this experiment, a PU resin using DETDA as a cross-linking agent and a conventional PU resin using ethylene glycol (EG) as a cross-linking agent were tested, evaluated and compared under the same conditions. After treating the molded product under each condition, each sample was immersed in glycerin heated to about 80 ° C. for 5 minutes, taken out, and its surface condition was visually observed. In addition, the evaluation results are indicated by ∘ indicating a good surface condition and × indicating a poor surface condition.

FIGS. 3 to 6 are experimental data supporting the drying conditions of the washing and drying process set in the coating method according to the embodiment of the second invention, and FIG. 3 shows the PU resin of this embodiment washed with trichloroethane. Fig. 3 is a graph showing the experimental results of how the amount of trichloroethane impregnated on the resin surface changes depending on the drying time in the subsequent drying step when the drying temperature is left at room temperature (about 20 ° C). The results were measured at 80 ° C., and the case of PU resin using conventional EG as a cross-linking agent is shown as a comparative example.

Further, FIG. 4 shows the relationship between the amount of internally added release agent reprecipitated on the resin surface and the heating time when the PU resin of this example was washed with trichloroethane and then heated at 120 ° C. and dried. It is a graph which shows the experimental result which measured.

Furthermore, FIG. 5 shows that after molding the PU resin of this example, 120 ° C. × 60
Is a graph showing the experimental results of measuring the relationship between the drying temperature (drying time is 15 minutes) and the surface tension of the resin surface in the drying process after drying under the condition of minutes, and then performing steam cleaning with trichloroethane for 2 minutes. is there.

Furthermore, FIG. 6 is a graph showing the experimental results of measuring the drying conditions after the cleaning step in the case where the primer dryer and the top coat dryer are so-called direct furnace type, and the relationship between the drying temperature and the drying time. Shows the range of establishment of the drying condition.

FIG. 7 is a composition table showing the coating composition of the primer coating used in the primer coating step shown in FIG. 1 in the coating method according to the embodiment of the present invention.

As shown in FIG. 1, the coating process of this embodiment includes a post-molding drying process 11, a cleaning process 12, a cleaning and drying process 13, a primer coating process 14, a primer drying process 15, a top coating process 16, and a top coating drying process 17. Each of the steps will be described together with the resin molding step 10 before the coating step.

Resin molding step 10 First, in the resin molding step 10, the highly active crosslinking agent DE
TDA and internal addition type release agent such as zinc stearate I
A PU resin containing MR is molded into a predetermined shape by injection molding. The DETDA promotes the reaction of the PU resin moderately, and the generation of voids is extremely reduced. Further, since the zinc stearate acts in the PU resin as a lubricant for the resin itself, when the work is taken out after injection molding, the mold release of the work from the mold is improved and the release before the mold clamping is performed. There is an advantage that the work of applying the mold agent can be omitted.

Post-Molding Drying Step 11 The PU resin molded product thus molded is heated for a predetermined time while maintaining a predetermined temperature (post-molding drying step 11). This treatment 11 is carried out in order to remove the air bubbles remaining in the work. As a condition for the post-molding drying step 11, the experimental result shown in FIG. In other words, it is preferable to leave it at about 20 ° C. for about 2 hours and then to dry at about 120 ° C. for about 15 minutes or more. Here, the drying conditions in the present invention are not particularly limited to the above-mentioned values, and it is sufficient that bubbles in the PU resin can be suitably removed.

Thus, when DETDA was used as the cross-linking agent, as is clear from the experimental results shown in FIG. 2, the ethylene glycol (hereinafter referred to as EG
It can be seen that a high drying treatment is required as compared with the case (1).

Cleaning Step 12 Next, on the surface of the molded article that has been subjected to the post-molding drying step 11, dust at the time of molding, zinc stearate precipitated by the treatment of the post-molding drying step 11, and the like are attached. In order to remove the impurities cleanly, cleaning is performed with a chlorine-based solvent such as trichloroethane (cleaning step 12). As a result, it is possible to remove the mold release agent that previously required a high pressure shower.
Since this trichloroethane easily dissolves zinc stearate, the release agent can be removed accurately without increasing the size of equipment.

The washing with trichloroethane may be either vapor washing for washing the solvent in a vapor state, shower washing in a liquid state, or a step combining both methods. However, in the case of an automobile bumper, the cleaning treatment time by the steam cleaning may be about 1 to 2 minutes, which is an advantage that the productivity can be significantly improved. However,
What must be noted in this washing step is that it is not preferable that the treatment time with the trichloroethane is too long. This is because a coating defect called "gas checking" occurs depending on the type of the drying device in the primer drying step 15 and the top coating drying step 17, which are the subsequent steps, which will be described later.

Washing / Drying Step 13 The molded product which has finished the washing step 12 is sent to a washing / drying step 13 in order to remove trichloroethane attached to the surface or impregnated on the surface. As is clear from the measurement results shown in FIG. 3, this is because in the PU resin molded product containing DETDA and IMR, the PU containing EG which was conventionally used is mixed.
This is because the impregnated amount of trichloroethane is larger than that of resin.
What was left for 15 minutes at 20 ℃) is the DE used in this example.
The amount of trichloroethane impregnated is the same as that obtained by heating TDA PU resin at room temperature for 48 hours or at 80 ° C for 15 minutes (see state E in Fig. 3). Since this trichloroethane causes a coating defect called gas checking in the primer coating step 14 and the top coating step 16 in the subsequent step, it must be removed almost completely in the washing / drying step 13.

The drying conditions are preferably heating at about 80 ° C. for about 15 minutes, but it is also possible to leave it at room temperature (about 20 ° C.) for about 48 hours or more to dry. Further, although the drying step 13 can be omitted, in this case, a primer drying device (primer drying step 15) and a top coating drying device (top coating drying step 17) described later are so-called indirect furnaces (burners which are heat sources). It is necessary to be a drying device of a system in which the flame from No. 1 is not exposed in the furnace and the work is heated by radiant heat through a shield plate or the like. Because, the purpose of the washing and drying step 13 is
This is to prevent a painting defect called gas checking.
When the trichloroethane impregnated on the surface of the work by the cleaning 12 is heated at a high temperature in the primer drier or the overcoat drier, it causes thermal decomposition and generates hydrochloric acid gas, and this hydrochloric acid gas causes the overcoating. It is generated by contributing to the melamine reaction which is a curing reaction of the paint and adhering to the surface of the top coat film. Therefore, when either the primer dryer or the top coat dryer is a direct furnace, it is not preferable to omit the washing / drying step 13 in terms of coating quality.

In this embodiment, the primer drying device and the top coat drying device are constituted by a direct furnace, and the drying conditions of this washing and drying step will be described below with reference to FIGS.

First, the range A shown in FIG. 6 is a temperature range in which the heat resistance of the PU resin, which is the work, is inferior, and heating the work above 130 ° C. will cause problems such as deformation, which is not suitable as a drying condition. It is an appropriate range.

Further, a range B in the figure is a range in which the surface tension of the work surface is lowered by heating at a high temperature and a poor wetness of the primer coating occurs, and this range is also unsuitable as a drying condition in the cleaning / drying step 13. .

More specifically, the amount of zinc stearate, which is a mold release agent, deposited in the washing / drying step 13 correlates with the drying treatment temperature and time in the drying step 13, as shown in the experimental results of FIG. According to the experimental results shown in FIG. 5, the PU resin heated once at 120 ° C. for 60 minutes in the drying step 11 after molding was used to remove the zinc stearate deposited on the surface of the PU resin, and then again at high temperature. When heated, zinc stearate precipitates again on the PU resin surface, lowering the surface tension. The value of the surface tension that does not affect the primer coating applied in the subsequent step is 25 dyn / cm ² or more, and when the value is less than this value, poor wetting of the primer coating (so-called repelling). Etc.). Therefore, as is clear from the experimental results shown in FIG. 5, the drying conditions of the washing / drying step 13 are that heating at a temperature of 80 ° C. for 15 minutes is effective in preventing the decrease in surface tension. ,
Further, for the drying after washing with the washing liquid, it is advantageous to provide the washing / drying step 13 in consideration of productivity, and the conditions of the washing / drying step 13 are heating at 80 ° C. for 15 minutes. Re is also preferable.

Further, ranges C and D shown in FIG. 6 are regions where void defects occur even after the cleaning when the conditions of the post-molding drying step 11 are 120 ° C. × 30 minutes holding and 120 ° C. × 15 minutes holding, respectively. There is an unsuitable range for the drying conditions of the washing and drying step 13.

Therefore, in the area excluding the defective areas A, B, C, and D for these coatings, that is, in the temperature range 80 to 130 ° C, the drying time is about 10
Within the range of up to 30 minutes, the range F that does not cause resin deformation failure (area A portion), primer coating failure (area B portion), and void failure (area D portion) is the cleaning and drying step.
13 is a suitable part as a drying condition, and particularly 80 ° C.
In this state, the heating time of 15 minutes or more is most preferable.

Primer coating step 14 A primer coating is applied to the entire surface of the work that has been subjected to various treatments as described above (primer coating step 14). As this coating method, a method such as air spray coating or electrostatic coating is preferable, and unmanned by an industrial robot or the like can be sufficiently achieved. As a work transfer device, each work may be transferred by fixedly mounting the work on a hanger fixed to a chain conveyor. From the post-molding drying process 11 to the after-coating drying process 17, which will be described later, it is transferred by one conveyor. It is also possible to do so. In addition, the coating booth is formed to have a predetermined length and size according to the production volume of the work, and it supplies a constant wind speed from the upper part to the lower part in order to prevent the spray paint from scattering and maintain the environment. It is preferable to provide an air supply / exhaust device.

The primer paint used in this example is a paint formulated with the composition shown in FIG. 7, and is a room temperature curable paint. As shown in FIG. 7, the resin base is a polyurethane resin, and other pigments exhibiting a gray color, hydrocarbons and ketones as solvents for dissolving these resins and pigments, and additions for ensuring storage stability and the like. It is composed of agents. The specific gravity of the paint is 1.04 and the solid content is 25.51%.
Is.

In the present example, the primer coating was constituted with the composition as described above, but the present invention is not limited to this, and suitably exhibits the coating performance without reacting with the DETDA mixed in the PU resin. Any primer paint can be used.

Primer drying step 15 After applying such primer coating on the work surface,
The work is conveyed to the primer drying step 15 having a drying device. Here, as described above, since the primer coating material of the present embodiment is a room temperature drying type, the coating film performance can be sufficiently obtained even if it is dried by leaving it at room temperature.
In other words, considering productivity, it is more advantageous to provide a primer drying step having a drying device. This drying device burns fuel such as butane gas in the burner section and blows this heat into the furnace by a blower.As described above, the direct furnace in which the burner section is exposed in the furnace and the burner section are There is an indirect furnace that cuts off with a shield plate etc. and does not expose it in the furnace and heats the work by its radiant heat.The direct furnace is advantageous from the viewpoint of energy efficiency, but from the viewpoint of the effect on the painted surface. Indirect furnaces are better. only,
In the present embodiment, even if the drying device is not an indirect furnace, the washing and drying step 13 can be carried out under the predetermined conditions as described above.

In the primer drying step 15, even if the DETDA, which is a cross-linking agent, remains in the PU resin, there is no reaction with the primer coating and no curing failure of the primer coating occurs. Therefore, this primer coating is suitably cured to form an undercoat coating film.

Topcoating Step 16 The work on which the undercoat coating film is formed by the primer coating is conveyed to the topcoating step 16 and the topcoat paint mixed in a predetermined color is applied to the work. For the coating equipment, such as a conveying device, a coating booth, a coating gun, etc., the same device as in the primer coating step 14 may be used, and it can be shared with the primer coating booth. Further, in the case of automating an industrial robot or an automatic painting gun, the automatic device may be provided with a color changing device for changing the top coating material for each work. This color changing device is a device having a valve that operates each paint sent from each paint tank through a paint hose by an air signal or the like (in the case of an electric signal, an explosion-proof device is provided), and is selected by the valve. The predetermined paint is pumped to the painting gun.

This topcoat paint has one coat and one bake type and two
There is a coat 1 beta type, and when applying the latter 2 coat 1 bake type paint, apply the base paint at the entrance side of the booth, let it air dry for a short time, and then near the exit of the booth. Apply clear paint. This embodiment can be carried out regardless of the type and material of the top coating material.

Topcoat drying process 17 Workpieces coated with topcoat paint are 70 to 150 ° C x 30 to 60
It is dried and cured under the condition of holding for minutes.

Next, the operation of this embodiment will be described.

First, injection molding processing (resin molding step 10) of PU resin mixed with DETDA which is a highly active cross-linking agent and IMR which is a release agent,
By heating in the drying step 11 after molding, voids remaining in the resin will be removed. Then, on the surface of the work that has passed this post-molding step 11, unreacted IMR remaining in the resin will be deposited, but this deposited IMR is easily removed by trichloroethane in the washing step 12. Further, after this cleaning, by drying at a predetermined temperature (cleaning / drying step 13), a coating failure due to the trichloroethane impregnated on the resin surface, and
It is possible to prevent coating defects due to re-deposition of IMR. Furthermore, when a lacquer paint containing polyurethane is applied as an undercoat paint from above, the paint is mixed in the PU resin as the work and remains in an unreacted state.
Since it does not react with DETDA at all and is suitably cured, it is possible to carry out a suitable coating without causing a defective primer coating film.

As described above, in this embodiment, since DETDA as a highly active cross-linking agent and IMR as a mold release agent for the molding die are mixed in the PU resin and injection molding is performed, the occurrence of voids occurs. Is significantly reduced and molding and coating quality are improved,
The release agent coating operation can be omitted, and thus the productivity in the molding process is improved. Furthermore, such DETDA
When coating PU resin containing IMR and IMR, prevention of re-generation of voids, coating defects due to precipitation of IMR, or curing failure of primer coating due to DETDA, which has been a problem in the past. The coating quality can be improved.

(Effects of the Invention) As described above, according to the present invention, even in the case of a molded product obtained by molding and processing a polyurethane resin mixed with a release agent, a coating film defect of a primer coating film that forms an undercoat coating film It is possible to provide a coating method that does not occur and is excellent in coating quality and productivity.

[Brief description of drawings]

FIG. 1 is a process diagram showing a method for coating a synthetic resin molded product according to an embodiment of the present invention, and FIG. 2 shows experimental results for observing the occurrence of voids after PU resin molding of the same embodiment. The evaluation table, FIG. 3 shows the experimental results of measuring how the impregnated amount of trichloroethane impregnated on the resin surface when the PU resin of the same example is washed with trichloroethane changes depending on the drying time of the subsequent washing and drying step. FIG. 4 is a graph showing the internal addition type release agent reprecipitated on the surface of the resin when the PU resin of the same example was washed with trichloroethane and then heated at 120 ° C. for drying (washing and drying step). FIG. 5 is a graph showing the experimental results of measuring the relationship between the amount and the heating time, and FIG. 5 shows that the PU resin of the same example was dried at 120 ° C. for 60 minutes after molding (drying step after molding), and then trichloroethane. And steam clean for 2 minutes Fig. 6 is a graph showing the experimental results of measuring the relationship between the drying temperature (drying time is 15 minutes) and the surface tension of the resin surface in the washing and drying step after the (washing step). Is a graph showing the experimental results of measuring the drying conditions after the cleaning step in the case of a so-called direct furnace type in terms of the relationship between the drying temperature and the drying time, and FIG. 7 is used in the primer coating step of FIG. FIG. 8 is a perspective view showing the appearance of an automobile, and FIG. 9 is a process diagram showing a conventional PU resin coating method. 10 …… Resin molding process, 11 …… Drying process after molding, 12 …… Washing process, 13 …… Cleaning and drying process, 14 …… Primer coating process, 15 …… Primer drying process, 16 …… Top coat coating process, 17 …… Overcoat drying process.

Claims (1)

[Claims] 1. A forming step of forming and processing a polyurethane resin containing a release agent, a drying step of heating a formed article formed in the forming step, and a chlorine-based solvent for the formed article after the drying step. A washing step of washing, and after the washing step, the formed article is heated in a temperature range of 80 to 130 ° C. and a time range of 10 to 30 minutes in a range in which void defects and primer coating defects do not occur, and the chlorine-based solvent is added to A washing / drying step of removing from the formed article, a primer coating step of applying the primer coating to the formed article after the washing / drying step and directly drying in an oven, and a top coating material after the primer coating step. And a method of coating a synthetic resin-formed product, which comprises a top coating process of directly drying in an oven.