CH621563A5 - Process for the preparation of a phenol-formaldehyde resin and its use - Google Patents

Description

The present invention relates to a process for the preparation of a thermosetting phenol-formaldehyde resin, as well as its use.

It has been discovered that resins formed from mole ratios of formaldehyde to phenol greater than about 1.7: 1 have unique properties and a characteristic infrared spectrum which is not exhibited by resins formed from mole ratios formaldehyde to phenol less than about 1.7: 1.

Thus, while the resins all have a high absorption at 1230 cm-1, the resins prepared from a mole ratio greater than 1.7: 1 have a strong absorption at 1030 cm-1, this peak being absent from the spectrum of resins formed from mole ratios less than 1.7: 1.

In addition, the ratio of absorption at 1030 cm-1, measured from a baseline drawn between 1130 cm-1 and 950 cm-1, to absorption at 1230 cm-1, measured using d 'a baseline drawn between 1130 cm-1 and 1310 cm-1 is greater than 0.6, in the spectrum of the new resin.

The new resin prepared according to the invention has properties leading to new, unexpected and advantageous uses.

These new properties come from the difference in structure of the resins prepared for the different molar ratios. Thus, for a given solids load, a given catalyst content, with respect to phenol, and a given reflux time, the viscosity of the resin decreases when the molar ratio of formaldehyde / phenol increases, and the resin hardens rapidly to high temperature (around 200 ° C). However, the stability of the resins, after addition of the acid catalyst, at room temperature or close to room temperature, about 20 to 25 ° C, increases with an increasing molar formaldehyde-phenol ratio.

In the conventional fields of application of phe-nol-formaldehyde resins of the resol type, it is common to accelerate the rate of hardening or thermosetting of the resin by adding small amounts of acid catalysts, such as benzenesulfonic acid and acid. p-toluenesulfonic.

Phenol-formaldehyde resins of the resol type, non-caustic, having an infrared spectrum having a high absorption at 1010, 1060 and 1230 cnr1 are used in certain fields of application such as the preparation of rice bran panels, and have a viscosity high at room temperature. These resins require heating to lower their viscosity, before application to a substrate. Since such resins harden quickly in the presence of an acid at the high temperature necessary for application to the substrate, it is necessary to apply the resin and the acid separately to the substrate. Reference is made, for example, to United States patent no. 3,850,677.

This mode of application is completely hazardous, since it absolutely does not ensure proper contact of the acid and the resin on the surface of the substrate, and leads to the use of higher amounts of acid, compared to those necessary in another mode of application, which results in an uneconomic use of the chemicals, and an often poor appearance of the product.

Depending, for mole ratios of formaldehyde to phenol greater than 1.7: 1, the viscosity of the resin is such that it allows application at a temperature close to room temperature, and, after addition of acid, by example up to about 2 to 3% of p-toluenesulfonic acid, the resin has a stability which allows prolonged storage before use, and the application of the resin to the substrate in the acid catalyzed state, which avoids the need for a separate application of the catalyst.

Since it is possible to obtain, according to this embodiment of the invention, a rapidly hardenable resin, stable at room temperature, acid catalyzed, and single component, intended for application on a substrate, the quantity of acid catalyst required for the proper hardening of the resin is considerably reduced, and conventionally represents approximately 1/6 to 1/10 of the quantity required when the resin and the catalyst are applied separately, which is necessary in the prior art, and the invention therefore leads to more economical use

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substances and a very improved appearance of the product, in particular in the form of rice bran panels.

In addition, resins prepared by choosing mole ratios of formaldehyde to phenol greater than 1.7: 1 require a viscosity of about 1000 cP, or more, at 23.9 cC, to behave satisfactorily as adhesives for rice bran panels.

The invention provides a process for the preparation of a fluid thermosetting phenol-formaldehyde resin catalyzed by acid, stable at room temperature, capable of being rapidly cured at high temperature, characterized in that the phenol is reacted and formaldehyde at a mole formaldehyde / phenol ratio greater than 1.7: 1, in an aqueous medium in the presence of a water-soluble metal carboxylate to form a first thermosetting phenol-formaldehyde resin having a viscosity of 1000 to 40,000 cP at 23.9 ° C, this resin comprising benzyl ether bonds in ortho of the hydroxyphenolic group and an infrared spectrum having peaks of strong absorption at 1230.1060 and 1030 cm-1, and which the first resin thus formed is reacted with at least one strong acid to cause a decrease of at least 5% of absorption at 1060 cnr1 and an increase of at least 5% of absorption at 1030 cm-1, absorption at 1230 cm-1 remaining practically unaltered .

Resins formed from a mole ratio of formaldehyde to phenol greater than about 1.7: 1 can be used in many fields of application, as adhesives. For example, the resins can be used for different spray applications, for example in the field of rice bran and particle board. The resins can also be used for the manufacture of paper laminates, foundry core applications, the manufacture of plywood and chipboard.

In cold molding applications, phenolic resins are commonly used, in particular for the manufacture of core elements. However, the resins used are not of the conventional resol type, but are generally modified by expensive and hardly available chemicals, such as furfuryl alcohol and silanes. Surprisingly, the resins according to the invention, while being very stable in the presence of small amounts of acid catalysts at room temperature, react rather quickly for acid contents of about 20% or higher, and are quite suitable. for applications of the curing type at room temperature (of the cold type).

The resins obtained according to the process can be used for the formation of a molded article consisting of a set of individual elements, in particular rice bran, grains of molding sand and wood chips, linked together by a phenol-formaldehyde resin. In this case, the resin is applied to the individual elements to form a coating on at least part of the surface of these elements, the elements thus coated are placed in a mold and the resin forming the coating of the elements is subjected to conditions of hardening capable of forming the molded article.

Resins corresponding to a mole ratio of formaldehyde to phenol greater than 1.7: 1 can also be easily emulsified to give aqueous dispersions in the presence of protective colloids. When the resins are used in the form of emulsions, the acid catalyst can either be added after the emulsion has been formed or added to the phenolic resin before the emulsification. When the resins are thus emulsified after receiving the catalyst, they retain their stability at room temperature, around 20-25 ° C, very long, up to 6 weeks.

Furthermore, when the precatalysis technique is chosen, the quantities of acid used are much lower than the quantities normally used when a post-analysis of the resin emulsions is carried out, ie by separate addition of the acid to the substrate, either by adding the acid to the emulsion. When using resin emulsions such as those described here for the manufacture of panels, the moisture content of the resin forming the final coating is a factor of critical importance and very significantly affects the formation cycles of press panels. For example, for the manufacture of agglomerates, very short press cycles are conventionally used, and, to keep the economic data of production at an attractive level, reasonably high catalyst contents are used in the emulsions, when the moisture content of the particles or chips is high. In some cases, it becomes necessary to use quantities of acid catalyst which are impossible to use in practice and which are accompanied by adverse effects on the quality of the panels, for example very dark coloring and acid degradation of the cellulose.

However, when emulsifying pre-catalyzed resins according to the invention, quality agglomerates are obtained for much lower acid contents, which is accompanied by economical manufacture of the agglomerates.

Another surprising property is the ability of these resins to harden under the influence of a radio frequency. For example, a radio frequency is commonly used when faster manufacturing cycles are required. Typical examples are mid-wood assembly and end assembly, as well as preheating of masts in the field of panel manufacturing. However, conventional phenolic resins, containing a caustic element, are not suitable for these applications because they very easily form an arc under the influence of high frequency energy, and tend to undergo pre-hardening. More complicated resins, such as modified resorcinol resins, catalyzed by paraformaldehyde can be used at radio frequency. Such resins are not only excessively expensive, but also have a very poor pot life.

However, when the resins according to the invention, produced from mole ratios of formaldehyde to phenol at least equal to 1.7: 1, are used in the field of plywood manufacturing, with preheating by radio frequency, it is arrives at considerable advantages with regard to manufacturing cycles.

The resins according to the invention are preferably produced by a one-step process in which phenol and formaldehyde are reacted, in amounts such that the mole ratio of formaldehyde to phenol exceeds about 1.7: 1, in a aqueous reaction medium containing a catalyst of the metal carboxylate type, for example zinc acetate, the reaction developing entirely at a temperature above about 90 ° C.

The resin prepared by this procedure has significant absorptions at 1230 cm-1.1060 cm-1 and 1030 cm-1. By adding to this resin at least one strong acid, a reduction of at least 5% in the absorption at 1060 cm-1 is obtained, and an increase of at least 5% in the absorption at 1030 cm- 1, while absorption at 1230 cm-1 is practically unaffected. This thermosetting phenol-formaldehyde resin can be isolated.

The following examples illustrate the invention.

For these examples, reference is made to the appended drawings, in which FIGS. 1 to 12 represent the infrared spectra of a certain number of thermosettable thermoset phenol-formaldehyde resins.

Example 1

This example illustrates the preparation of a phenol-formaldehyde resin according to the invention.

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1794.1 g (19.09 moles) of phenol, 2788.3 g (34.35 moles) of formaldehyde are introduced into a glass reactor equipped with a stirrer, a reflux condenser and a thermometer. having a methanol content of less than 1.5% by weight (which corresponds to a formaldehyde / phenol ratio of 1.8: 1), 144.8 g (0.034 mole, and approximately 8% by weight relative to the phenol) zinc acetate dihydrate, and 772.8 g of water, which makes it possible to obtain a reaction mixture having a total load of reagents of 54.01%. The mixture is quickly heated to 90 ° ± 2 ° C, in 40 + 5 min. The temperature is further raised to the reflux temperature, approximately 100 ° C., during the 15 min. following, by adjusting the heating speed. The reaction mixture is kept under constant reflux for 210 min. in total. About 105 min. after the start of reflux, a distinct phase separation is observed.

After completion of the reflux period, the reaction mixture is cooled to 35 ° C and stirring is stopped. The cooling is then continued to approximately 25 ° C. The liquid resin phase is separated from the aqueous phase. The resin appears to have a Brookfield viscosity of 2000 to 2300 cP at 48.9 ° C and a non-volatile solid content of 75 to 80%.

Example 2

This example illustrates the different structures obtained when resins are prepared from different mole ratios of formaldehyde to phenol.

A series of phenol-formaldehyde resins is prepared, following the procedure of Example 1, and choosing different mole ratios of formaldehyde to phenol. An infrared spectrum is plotted for each resin.

To a sample of each resin, 1% paratoluene sulfonic acid (as a 50% aqueous solution) is added, and the infrared spectrum is plotted again. The two series of spectra can be found in Figs. 1 to 12, with reference to the following:

Figure

1

1.4 mole ratio:

: 1

no acid

Figure

2

1.6 mole ratio:

1

no acid

Figure

3

1.8 mole ratio:

: 1

no acid

Figure

4

2.0 mole ratio:

: 1

no acid

Figure

5

2.5 mole ratio:

: 1

no acid

Figure

6

3.0 mole ratio:

: 1

no acid

Figure

7

1.4 mole ratio:

: 1

added acid

Figure

8

1.6 mole ratio:

: 1

added acid

Figure

9

1.8 mole ratio

: 1

added acid

Figure 10

2.0 mole ratio:

: 1

added acid

Figure 11

2.5 mole ratio:

: 1

added acid

Figure 12

3.0 mole ratio:

: 1

added acid

We see, from a comparison of the spectrum of resins before and after addition of acid, that there is a sharp and strong absorption peak at 1030 cnr1 in the spectrum of resins corresponding to an equal or higher molar ratio at 1.8, while this peak is absent from the spectrum of resins corresponding to the mole ratios of 1.4 and 1.6.

It is also possible to make a comparison between the absorption ratios at 1030 cm-1, measured from a baseline drawn between 1130 cm-1 and 950 cm-1, and the absorption at 1230 cm-1, measured from a baseline drawn between 1130 cm-1 and 1310 cnr1.

Analysis of the spectrum of resins corresponding to mole ratios greater than or equal to 1.8: 1 reveals that, before the addition of acid, there is also a significant absorption at 1060 cm-1, and that after the addition of acid, there is a decrease in absorption at 1060 cm-1 and an increase in absorption at 1030 cm-1. In the case of resins corresponding to mole ratios less than or equal to 1.6: 1, an increase in absorption is noted at 1060 cm-1.

Appropriate comparisons are found in Table I below. We see that the resins corresponding to a mole ratio greater than or equal to 1.8: 1 not only have a distinct peak at 1030 cm-1, but also have an absorption ratio, relative to the absorption at 1230 cm -1, greater than about 0.6, both before and after addition of acid, while the resins corresponding to a mole ratio less than or equal to 1.6: 1 not only do not have such a peak at 1030 cm-1, but also have an absorption ratio, compared to the absorption at 1230 cm-1 less than 0.6, both before and after addition of acid.

Example 3

This example illustrates the physical properties of phenol-formaldehyde resins prepared for different mole ratios of phenol to formaldehyde.

A series of phenol-formaldehyde resins is produced by following the procedure of Example 1. Measurements are carried out on different properties of the resins, and the results obtained are collated in Table II below.

The results of Table 2 above show the decrease in the viscosity of the resins for mole ratios greater than or equal to 1.8: 1, the decrease in the electrical resistance of the resins for mole ratios greater than or equal to 1, 8: 1, the rapid reduction in hardening times during acid catalyzed hardening, when the mole ratio increases, as well as the ability of the resins to be emulsified, for mole ratios greater than or equal to 1.5 : 1.

The results of Table II below also show that, contrary to what one skilled in the art could expect, the electrical properties of the resins and the modifications of the electrical properties of the resins after addition of acid are independent of the viscosity of resin.

Example 4

This example illustrates the formation of a rice bran panel from acid catalyzed resins.

A resin containing 75% of solids produced by the procedure of Example 1, and corresponding to a mole ratio of formaldehyde to phenol of 1.6: 1, is catalyzed by 0.5% by weight of p- acid. toluenesulfonic in the form of a 50% solution, and appears to be stable at room temperature for more than 24 hours. However, it is not possible to spray the resin at room temperature, due to its inherent viscosity greater than 50,000 cPo at room temperature, and therefore must be heated to approximately 71.1 ° C before use. At this temperature, the viscosity of the resin increases rapidly, and can therefore no longer be applied by spraying on rice bran panels.

It is however possible to apply the resin and the acid catalyst separately, in a satisfactory manner, on the rice panels. The acid is first sprayed on samples of clean rice panels, at a rate of 4 and 8% of the weight of the resin. Then, the resin is sprayed hot in the molten state, at 65.6 ° C., on the samples of rice bran panels, and in both cases, at a rate of 10% of solids of the resin type relative to to the panels. The resin-coated panels are placed on mats, consolidated, then undergo a hot press treatment at 199 ° C for approximately 11 min.

A resin equivalent to 75% solids, produced by the procedure of Example 1, and corresponding to a mole formaldehyde-phenol ratio of 1.8: 1, after catalysis with 0.5% by weight of acid paratoluenesulfonic, used under

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the form of a 50% aqueous solution appears to be stable at room temperature for an extended period of more than a week. The catalyzed resin appears to be sprayable at room temperature and at a temperature close to room temperature, from 26.7 to 32.2 ° C, and rice bran panels are made by spraying the panels clean of 10% of the acid catalyzed resin followed by the formation of mats, of consolidation and of a treatment with the hot press described above. The resulting panel, 12.7 mm thick and 0.8 kg / dm3 in density, has a very light coloring, and is thus opposed to the typical rice bran panels formed from a two-component system, and which are very dark in color.

Similar rice bran panels are made from one-component systems, using resins with other mole ratios greater than 1.7: 1.

The resistance of the panels is determined and the results are collated in Table III below.

The results in Table III below clearly show that the ability of a resin corresponding to a mole ratio of 1.8: 1 to be used in the context of a spray application by means of a one-shot system. component leads to obtaining rice bran panels of good quality, having an improved appearance, and corresponding to quantities of catalyst considerably lower than those required when applying a resin corresponding to a mole ratio of 1.6 : 1 by means of a two-component spraying system, the latter method being adopted due to the inability of the resin corresponding to a mole ratio of 1.6: 1 to be sprayed by means of a system unique, cost-effective.

The results in Table III below also show that it is possible to obtain good quality rice bran panels even when using very low viscosity resins for a molar ratio of 3.0: 1. .

United States patent no. 3,850,677 has already demonstrated that resins corresponding to a 1.6: 1 mole ratio and of low viscosity at room temperature are not suitable for the manufacture of rice bran panels. These latter low viscosity resins are typically prepared by the process described in Canadian patent no. 927 041, for a mole ratio close to 1.5: 1.

Example 5

This example illustrates the use of different phenol-formaldehyde resins in sand applications for cold molding.

The sand is coated with 1.5% by weight of resin, and different amounts of catalyst, by passing the mixture through the barrel mixer. Cylindrical molds are prepared in a mold and allowed to dry at room temperature. The molds are tested at different time intervals for their strength properties. The results are collated in Table IV below, as are the results for commercially available resins.

The results of Table IV below show that the resins having a molar ratio of 1.8: 1 are quite suitable for use with cold molding sand. These results are in contradiction with the tests carried out with resins formed at a molar ratio of 1.6: 1 which have shown

that similar molds could not be obtained for catalyst contents of 20% or more as shown in Table IV. There is considerable pre-hardening and the resulting molds do not have sufficient strength and homogeneity.

Example 6

This example illustrates the use of phenol formaldehyde resins in the production of chipboards.

A phenol-formaldehyde resin with a mole ratio of 1.8: 1 (about 75% non-volatile solids), cured according to the procedure of Example 1, is emulsified by dispersing equal amounts of resin and solution aqueous at 1% of Natrosol HXR 250 (hydroxyethylcellulose) to obtain a stable emulsion having approximately 35% of non-volatile solids.

Samples of this emulsion are catalyzed with various quantities of para-toluenesulfonic acid (in the form of a 50% aqueous solution) and pulverized on poplar shavings with a moisture content of 4 and 6%, at a rate of 2, 5% by weight of resin relative to the final panel. The shavings coated with resin are placed in the form of a mat and hot pressed at 210 ° C. to the desired thickness, at a density of 0.636 g / cm 3 for the duration of the necessary cycle, cooled and subjected to trials.

The results are shown in Table V below.

The results of Table V below show that, although a thin panel of satisfactory strength can be formed from catalyzed emulsions having a moisture content of 6%, thicker panels cannot be formed for this content in humidity even when the catalyst content is high. In addition, although thicker panels satisfactory for a moisture content of 4% are obtained, from the point of view of commercial practice, it is difficult to maintain these low moisture contents.

In comparative tests, the resin is first catalyzed with 2% para-toluenesulfonic acid (in the form of a 50% aqueous solution) and emulsified as described above, at least 3 hours after addition of the catalyst . The emulsion is then used to form agglomerated panels with a density of 0.636 g / cm3 and a thickness of 7.94 mm at 210 ° C.,

as shown above, using poplar shavings with a moisture content of 6%.

The resulting panels have the properties indicated in Table VI below.

It is evident from the results of Table VI that if the resin is catalyzed before emulsifying, thick agglomerated panels with satisfactory strength can be formed from chips having a moisture content of 6%.

It has also been found that emulsions prepared from pre-catalyzed resins as indicated above are very stable, even after 6 weeks while emulsions prepared from non-catalyzed resins form a deposit after only 1 or 2 days and must be shaken before use.

The invention therefore provides new resins with superior tack characteristics for many applications.

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Table I

Mole ratio

Report -

Before adding acid

1060 -1

cm

1230

After addition of acid

Change (%)

Report

Before adding acid

1030 „„, -1

cm

1230

After addition of acid

Change (%)

1.4

1

0.572

0.642

+ 12.3

0.383

0.443

+ 15.7 (1)

1.6

1

0.684

0.724

+ 5.9

0.596

0.488

- 18.2 (1)

1.8

1

0.770

0.634

- 17.7

0.705

0.783

+ 11.1

2.0

1

0.814

0.733

- 10.0

0.797

• 0.899

+ 12.8

2.5

1

0.929

0.724

- 22.1

0.612

1.044

+70.6

3.0

1

1.043

0.690

- 33.8

1.077

1,146

+ 6.4

Note: (1) no peak such as 1030 cm 1 in these resins

Table II

Phenol / formaldehyde mole ratio at introduction

Solid content%

Viscosity

Brookfield

at 23.9 ± 1 ° C

cP

at 48.9 ± 1 ° C

Gardner tube at 23.9 ± 1 ° C

Electrical properties (£ 2) at 23.9 ± 2.3 ° C

Catalyzed Witness

Gel time 2

00

Emulsion formation4

1: 1.2

72.5

High viscosity

1880

> Z-6

13,000,000

300,000

12

No emulsion

1: 1.4

72.2

High viscosity

4350

> Z — 6

15,000,000

350,000

22

No emulsion

1: 1.5

62.5

High viscosity

8900

> Z — 5

2,100,000

550,000

40

Emulsified

1: 1.8

75.9

34,500

2300

> Z — 6

950,000

100,000

37

Emulsified

1: 2.0

80.5

11000

590

> Z — 6

370,000

18,000

22

Emulsified

1: 2.5

77.7

2,200

240

Z-Z +

130,000

4,000

13

Emulsified

1: 3.0

75.8

1400

130

X

80,000

1200

12

Emulsified

1: 3.0 introduction

progressive tion3

77.5

1260

195

X

12

Emulsified

Notes: 1. All resins are catalyzed by 1% p-toluene sulfonic acid (PTSA) (in the form of a 50% aqueous solution)

2. Resins catalyzed by 1% PTSA (in the form of a 50% aqueous solution)

Average over three values

Measurements with flat hot press at 204 ° C

3. Initial molar ratio 1.8, then 1.2 mol then added in 1 h, from 20 min after reflux

4. Emulsification tests for equal weights of resin and 1% thickening solution of hydroxyethylcellulose.

Table III

Application

Report

Content

Resistance

Module

in mole internal catalyst rupture

(%)

(kg / cm2)

(kg / cm2)

2 components

1.6

1

4

Of the mine

2 components

1.6

1

8

5.27

133.5

1 component

1.8

1

1

6.32

147.6

1 component

2.0

1

7.08

-

1 component

2.5

1

5.34

-

1 component

3.0

1

3.86

-

1 component

3.0

1 *

3.90

-

* Gradual introduction

Table IV

Resin type Example 1 D YNO S354 * DYNO L355 *

Sand 1000 1000 1000 1000 1000 1000 1000 1000

Resin 15 15 15 28.2 15 15 15 15

Catalyst 3.0 4.2 6.0 6.0 4.2 6.0 4.2 6.0

(85% PTSA) ** (85% PTSA) (85% PTSA) (85% PTSA) (85% H3PQ4) (85% H3PQ4)

Time (h)

1/4 0.29 0.64 0.44 0.20 7.0 6.1 0.25 0.75

1/2 1.58 2.8 2.20 1.45 14.5 8.85 0.95 8.00

1 6.30 13.2 4.20 8.0 18.5 12 3.50 15

2 13.50 15.8 14.50 16.00 25 18 13.50 30 4 20.00 26 18.50 33.0 28 34 29 34

24 29.00 43 34 54 66 36 39 52

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Table IV (continued)

Resin type

Example 1

DYNO S354 »

DYNO L355 »

Sand

1000

1000 1000

1000

1000

1000

1000 1000

Resin

15

15 15

28.2

15

15

15 15

Catalyst

3.0

4.2 6.0

6.0

4.2

6.0

4.2 6.0

(85'ï PTSA) »»

(85r-r PTSA) (85 ^ PTSA)

(85 <: i PTSA)

(85 ci H, PO,) (85 rr H, PO,)

Material content

75.0

75.0 75.0

75.0

80%

80%

non-volatile solids

(Karl Fisher)

(Karl Fisher)

in resin (%)

(-20%

(-20%

F.F.A.)

F.F.A.)

* Dyno Industries Ltd., Oslo, Norway ** PTSA P-toluene sulfonic acid

Table V

Content

Thickness

Duration of

Content

Humidity resistance

nominal internal catalyst cycle (1)

(%)

(mm)

pressing the panel resin

(min.)

(%)

(kg / cm2)

6

6.35

3

5

4.43

6

7.94

3.5

5

Of the mine

6

7.94

5

5

Of the mine

6

7.94

3.5

8

Of the mine

6

7.94

3.5

12

2.18

6

7.94

3.5

20

2.11

4

7.94

3.5

5

3.86

Note: 1. An internal resistance of the panel exceeding 3.51 kg / cm2 is considered good

Table VI

Duration of the pressing cycle,

Internal resistance of the panel,

(min.)

(kg / cm2)

3.5

4.36

3

3.86

B

3 sheets of drawings

Claims (10)

621,563 1. A process for the preparation of a phenol-formaldehyde resin characterized in that the phenol and the formaldehyde are reacted at a mole formaldehyde / phenol ratio greater than 1.7: 1, in an aqueous medium. presence of a water-soluble metal carboxylate to form a first thermosetting phenol-formaldehyde resin having a viscosity of 1000 to 40,000 cP at 23.9 ° C., this resin comprising benzyl ether bonds ortho to the hydroxyphenol group and an infrared spectrum having peaks of high absorption at 1230, 1060 and 1030 cm −1, and the first resin thus formed is reacted with at least one strong acid to cause a reduction of at least 5% of the absorption at 1060 cm-1, and an increase of at least 5% of the absorption at 1030 cm-1 the absorption at 1230 cm-1 remaining practically unaltered. 2. Method according to claim 1, characterized in that the strong acid is para-toluenesulfonic acid. 2 3. Method according to claim 1, characterized in that the phenol and formaldehyde are reacted in a single step at a temperature above 90 ° C. 4. Method according to claim 1, characterized in that an acid catalyst is added to the first resin to obtain a thermosetting phenol-formaldehyde resin, stable at room temperature and capable of hardening quickly at high temperature. 5. Method according to claim 4, characterized in that the stable resin is emulsified at room temperature to form a stable resin-in-water emulsion. 6. Use of the resin obtained by the process according to claim 1 for the formation of a molded article consisting of a set of individual elements, chosen from rice bran, grains of molding sand and wood chips , bonded together by a thermoset phenol-formaldehyde resin, characterized in that said resin is applied to the individual elements to form a coating on at least part of the surface of these elements, the elements thus coated are placed in a mold and the resin forming the coating of the elements is subjected to curing conditions capable of forming the molded article. 7. Use according to claim 6, characterized in that the hardening conditions are achieved by heating the mold to a temperature higher than the thermosetting temperature of the resin forming the coating. 8. Use according to claim 6, characterized in that the curing conditions are achieved by subjecting the resin forming the coating to a radio frequency. 9. Use according to claim 6, characterized in that a resin-in-water emulsion of the resin is applied to the individual elements by spraying by applying the emulsion to a set of wood chips, arranged in a mold , the resin is hardened by heating the mold to a high temperature and the agglomerated panel obtained is removed from the mold. 10. Use according to claim 6, characterized in that the resin hardens in contact with the individual elements at a temperature of 20 to 25 ° C.