CS217056B1 - A method for producing internal press films for molded burrs - Google Patents

Abstract

The invention allows for shortening the pressing time and reducing the amount of resin in the final product while maintaining its properties. At the same time, it also reduces the amount of phenol and formaldehyde exhalations during production. This is achieved by impregnating layers of cellulose fiber material with an aqueous solution of phenol-formaldehyde resins prepared by the described procedure and exposing the impregnated material to heat at 120-180 °C for a period of less than 10 8 minutes. The impregnation solution has a viscosity of less than 800 mPa.s and contains no more than 30 wt.% water.

Description

The invention relates to a method of producing internal pressing foils for pressed laminates, impregnated with phenolic resins, by a technological process that significantly improves the previously known processes both from a technical and economic point of view and especially from a point of view of preserving the cleanliness of the environment.

The production of phenolic pressed laminates has been known for more than 50 years. It is carried out in principle in such a way that a flat material, the essence of which is most often fibrous cellulose, such as, for example, paper in the form of strips, boards, sheets, etc., is first impregnated with a solution, dispersion or suspension of a phenolic thermosetting resin, possibly containing plasticizers, wetting agents, curing accelerators, dyes or other additives, and the resin solvent is removed by subsequent drying, usually at a higher temperature than room temperature, if necessary with a simultaneous partial condensation reaction, the so-called pre-curing. Then, several layers of the material impregnated in the above-mentioned way are stacked on top of each other and the whole is hot-pressed; the action of heat and pressure achieves the hardening of the resin and good bonding of the individual layers into a homogeneous mass, insensitive to the effects of higher temperatures, water and common solvents.

Phenolic laminated molded materials are produced in various designs according to their intended use, especially as electrical insulating materials, as structural materials or in combination with surface decorative layers as so-called decorative laminated materials.

For the production of internal pressing foils for pressed materials, various types of phenolic thermoactive resins are applied, most often they are resoles based on phenol, cresols or xylenols, formaldehyde is used as the aldehyde component. Phenolic novolaks are less commonly spliced in combination with hexamethylenetetramine. Phenolic resins contain a greater or lesser amount of unreacted components, up to 20% by weight of free phenol or brominated substances and up to 5% by weight of free formaldehyde.

As a solvent for phenolic resin for the preparation of the impregnation solution itself, methanol or ethanol is usually used, and water for water-soluble types of resin or water-alcohol mixtures. The use of alcoholic solutions has the advantage of lower heat consumption when drying the impregnated material compared to aqueous solutions due to lower heat of vaporization, on the other hand, their disadvantage is higher raw material costs and the need for expensive disposal of harmful exhalations. , ^And technological methods for producing laminated materials by hot pressing of phenolic resin-impregnated sheets of cellulose material have a number of variants, each of which may from one point of view represent certain advantages over other methods, but from another point of view is less advantageous.

For example, according to US Patent No. 2,801,198, impregnation is carried out with a mixture of a water-soluble phenolic resole with an alcoholic solution of a water-insoluble ammonia-catalyzed resole in order to achieve sufficient impregnation of the cellulose material and thus good properties of the laminate. According to US Patent No. 2,670,316, impregnation is carried out first with a water-soluble resin and then with an alcoholic solution of a water-insoluble resin, according to US Patent 2,579,637, impregnation is carried out with diluted aqueous solutions of the resin with a viscosity below 200 mPa.s/20 °C. According to other patents (US patents 2,190,672 82,834,755; British patent no. 1,011,369), it is impregnated with aqueous solutions of water-soluble resins, from which, after the addition of an acid neutralizing the alkaline catalyst, the precipitated salts were removed by filtration. According to British patent no. 581,127, metallic soaps are added to the impregnation solutions of phenolic resins to increase their water solubility. US patent no. 2,889,241 protects a method of adjusting the viscosity of impregnation solutions by adding boron compounds, Canadian patent no. 571,702 protects a method of stabilizing impregnation solutions by adding polyemines, etc.

As can be stated, none of the methods known to date satisfactorily solves the problem of producing inner foils for laminated materials in such a way that technical, economic and environmental cleanliness requirements are simultaneously met.

In the professional patent literature it is stated that impregnation solutions are used in various concentrations, in the range of 30 to 75% by weight of the solids content, depending on the properties - especially absorbency - of the material intended for impregnation and the requirement for the degree of its saturation, and depending on the properties of the resin, i.e. especially the molecular distribution and average molecular weight, determining the viscosity of the impregnation solution at a given concentration.

For example, cellulose materials with common properties, such as paper with a basis weight of around 100 g/m, are most often used, according to literature data, with impregnation solutions either thin or medium-thick or thick, for which the approximate relationship between concentration and viscosity is impregnation solutions thin medium-thick thick solids content (mass.) 40 sz 50 5^ to 60 60 to 70 viscosity (mPa.s/20 *C)_50 to 100_100 to 250_250 to 400

The technical and economic advantages or disadvantages of applying thin, medium-thick or thick impregnation solutions cannot be well assessed against each other, because the decisive criteria, i.e. the content and method of resin deposition in the impregnated material, are not only a function of the properties of the cellulose material and the properties of the resin impregnation solution, but also depend on the technological process of impregnation and the conditions of subsequent drying.

The amount of resin in the cellulose material and the way it is distributed in the inter-fibrous space largely affect all the properties of the laminated material, i.e. mechanical, electrical, chemical resistance, moisture resistance, etc. It is known that a material consisting of cellulose fibers, such as paper, is a porous medium; when wetted with a liquid, the liquid penetrates the fibrous space by adsorption forces, where the liquid components are separated. In the given case - when impregnating cellulose paper with a resin solution - there is a selective separation of the resin components according to their molecular mass based on their different adsorbability. Smaller and more mobile molecules run in front of bulky macromolecules, which are slowed down by physical obstacles; in particular, by intermolecular attractive forces. According to this mechanism, the impregnation process is determined in a qualitative and quantitative sense both by the properties of the cellulose material and the properties of the impregnation solution. *roto The concentration of the resin component in the impregnation solution and the value of its viscosity and their mutual relationship represent the parameters of the impregnation process that most influence the properties of the laminated material.

The viscosity of a solution for a given concentration of a dissolved macromolecule is a function of its molecular weight. A phenolic resole resin is a system of molecules with a wide molecular weight distribution, which, in addition to the monomer, contains dimers, trimers, tetramers, etc., which are more or less branched, with a greater or lesser amount of reactive methylol groups attached.

The mutual ratio of the content of molecules of individual sizes, expressed by the respective distribution curve, to the amount of reactive groups bound to them determines the qualitative properties of the impregnation solution and, in essence, determines the properties of the layered material; with a high content of molecules with a high molecular weight in the impregnation solution, the applied resin remains longer on the surface of the fibrous material than in the case of a higher content of molecules with a low molecular weight in the impregnation solution.

Laminated material made of cellulose-based material insufficiently saturated with resin shows insufficient resistance to the effects of water and chemicals, does not withstand weathering. On the other hand, excessive oversaturation of cellulose fibrous material with low-molecular resin products prevents a certain freedom of movement of individual fibers or their parts in the laminated material, which is necessary to equalize the tension during mechanical stress of the material, resulting in impaired mechanical properties of the laminated material, especially impact strength.

A serious current problem in the production technology of pressed layered phenolic materials is the large amount of harmful exhalations produced during drying of the impregnated material and containing mainly phenol, formaldehyde and organic solvent in the case of application of alcoholic impregnation solutions.

Since the amount of harmful exhalations is limited by law, laminated material production facilities must be equipped with expensive equipment for their disposal, usually carried out by appeal with significant operating costs. Conventional methods of solvent recovery cannot be used well due to the presence of monomeric substances, leading to the formation of resins and thus to clogging and disruption of the function of the recovery equipment.

T8k, for example, for impregnation of paper for the inner component of decorative laminates, an impregnation solution of phenyl formaldehyde resole is used with a content of 50% by weight of solid resin, 9.45% by weight of free formaldehyde and approx. 38% by weight of methanol. When operating a conventional impregnation machine with a capacity of 0.4 t of impregnated paper per hour, the corresponding emission of pollutants in 24 hours is approx. 300 kg of methanol, 480 kg of phenol and 110 kg of formaldehyde.

From the above example it is clear that the largest share of harmful exhalations is methanol and it is therefore easy to apply a type of phenolic resin that would allow replacing part or all of the methanol with water. However, replacing the organic solvent with water leads to a significant and economically undesirable reduction in the performance of the impregnation machine due to the higher heat of vaporization of water compared to methanol.

As is known, water-soluble types of phenolic resins are the so-called simple resoles, which are essentially low-molecular addition products of phenol with formaldonide - methylolphenols. They are prepared by the reaction of phenol and a significant molar excess of formaldehyde in a strongly alkaline environment. They contain a large amount of unreacted starting components, phenol and formaldehyde, and during their hardening, a large amount of formaldehyde is released. Therefore, the use of aqueous impregnation solutions of low-molecular phenolic resin can solve the problem of harmful exhalations only partially and at the cost of increased production costs and deteriorated properties of the laminate, mainly deterioration of impact strength.

The mentioned shortcomings of these known procedures are largely eliminated by the invention protected under Czechoslovak Patent No. 181,093, the subject of which is a method of producing laminated materials by impregnating layers of cellulose material with a solution of phenolic resin of a certain chemical and structural composition defined by the value of the ratio of the content of free phenol to the content of methylols.

217056 4 groups, viscosity and water content, and processing of impregnated layers into the final product.

According to this invention, the maximum water content in the impregnation solution is 25% by weight, which limits certain impregnation properties of the solution in the sense that in some special cases, whether it is the specific properties of the cellulose material or the given technological arrangement of the impregnation equipment, difficulties may arise in terms of achieving the required size or uniformity of the resin coating on the cellulose material.

A method for solving such difficulties, especially those arising from poor seeding of the cellulose material, has not yet been found. This method is given by a new determination of the relationship between the values of the water content 8 of the effective viscosity at a defined composition and the structural-chemical properties of the resin of the impregnation solution, which is the subject of the present invention. The invention relates to a method for producing inner films for layered molding materials, especially with a decorative surface, by impregnating a layer of cellulose fibrous material with a resin solution prepared by the reaction of phenol and/or its homologues with formaldehyde, with a ratio of the free phenol content determined by the bromometric method according to Koppenschaar to the content of methylol groups of less than 0.5 s at a temperature of 15 to 60 ° C, optionally containing up to 10 wt. % of substances from the group including plasticizers, stabilizers, curing accelerators, modifiers, surfactants, organic solvents, retarders, dyes and pigments, and exposing the impregnated material for a period of less than 10 s to the effect of heat at 120 to 180 ° C. The essence of the invention lies in the fact that a layer of cellulose fibrous material is impregnated with an aqueous resin solution with a viscosity of less than 800 mPa.s containing not more than 30 wt. % water.

This binding relationship between the water content and the effective viscosity of the applied impregnation solution with its above-defined chemical-structural properties and composition determines its theological properties and thus the optimal range of impregnation conditions, individually determined for a given quality of cellulose fibrous material or for given technical parameters of the impregnation equipment.

The results of experimental testing have shown that the specified range of properties of the impregnation solution allows achieving satisfactory conditions and impregnation results, resulting in optimal properties of the laminated material, both for all types and qualities of cellulose material found in practice, and for all common technological conditions of the impregnation process itself.

Achieving the required effective viscosity of the impregnation solution can be easily achieved either by selecting the water content or temperature within the specified limits, or by modifying it with some suitable additives, as mentioned above. By such modification of the properties of the impregnation solution, all the key advantages provided by the chemical-structural composition of the resin remain utilized.

Due to the high concentration of resinous substances in the impregnation solution, the amount of water that needs to be evaporated is relatively small, so the heat consumption for evaporation of the viscous phase is also small.

Due to the limitation of the amount of free phenol in the impregnation solution by its relationship to the content of reactive methylol groups, the content of phenol and formaldehyde in the vapors leaving the impregnation device at temperatures above 120 °C is negligibly small, significantly lower than the limit specified by the state standard.

Due to the relatively high content of methylol groups, the reactivity of the resin is high and allows for the shortening of the pressing time of layered materials commonly used today.

Due to the favorable distribution of the resin components according to the size of their molecules in the cellulose material in the direction perpendicular to the surface of its core, the desired complex of properties of the laminated material (i.e. mechanical strength, dimensional stability and chemical resistance) is achieved even with a lower resin content in the final product than in the previously known methods of laminated material production technology.

The exemplary embodiments demonstrate the variability of impregnation conditions within the scope of the invention, allowing the application of cellulose material with significantly different properties.

Example 1

A sheet of sulfonated paper measuring 30 x 30 cm with a weight of 90 g/m and a suction height in water of 30 mm (determined by the method according to ČSN 500334) with a moisture content of 6% by weight is immersed for 3 s in an impregnation bath consisting of an aqueous solution of phenol-formaldehyde resin β with the following properties:

water content 26.1 wt. % free phenol content 4.6 wt. % content of methylol groups 15.6 wt% temperature 25°C viscosity 760 mPa.a

After removal from the impregnation bath, the wet paper is immediately stretched between squeezing rollers with a spacing of 0.10 mm and, after 5 s, is hung in a jet dryer heated to 150 °C for 30 s.

After removal from the dryer, the dry impregnated film has a weight of 135 g/m ² , i.e. a resin deposition of 5<J wt. %. The surface of the impregnated film is matte, i.e. the resin is deposited inside the paper in the interfiber space.

From 9 pieces of impregnated foils made in the above-mentioned way, using standard melamine resin impregnated foils (pattern, covering and leveling foils), a decorative laminated material with the following properties is pressed under normal pressing conditions:

O / impact strength 0.55 J.cm mass gain during boiling test 3.1% thickness gain during boiling test 3.5%

Example 2

The impregnation solution given in Example 1 is modified by adding 5% by mass of urea, its temperature is increased by heating to 30 °C. The impregnation solution after dissolving the urea and homogenization has the following properties:

water content 26.1 wt. % free phenol content 4.5 wt. % content of methylol groups 14.0 wt. % temperature 30°C viscosity 230 mPa.a

By the method given in Example 1, using the same paper, an impregnated film is produced, showing a resin deposition of 52% by mass. From these impregnated films, a decorative laminate produced by the process according to Example 1 has the following properties:

impact strength 0.50 J.cm ¹ mass gain during boiling test 2.8% thickness gain during boiling test 2.1%

Example 3

An impregnation solution of phenol-cresol-formaldehyde resin prepared from a mixture of phenol and cresol in a ratio of 1:1, with the following properties:

water content 22.2 wt. % free phenol content 5.6 wt. % methylol group content 16.3 wt. % temperature 35 °C viscosity 790 m²/s

Kraft paper with a basis weight of 130 g/m2, a suction height in water of 50 mm and a moisture content of 6.8 & hmet is impregnated. The paper immersion time in the impregnation bath is 3 s, the gap of the squeezing rollers is 0.14 mm, the drying time in the air-jet dryer is 40 s/150 °C. The impregnated film produced has a resin content of 48% by weight. From 6 pieces of these impregnated films, using standard melamine resin-impregnated films (sample, cover and leveling) a decorative laminate is pressed under normal melamine conditions, which has the following properties:

impact strength 0.58 J.cm mass gain during boiling test-· j,l » thickness gain during boiling test. 2.6 #

Example 4

The impregnation solution given in Example 1 is prepared by adding 3 wt. % water, 2 wt. % methanol and 0.10 wt. % Olovasol EL. The impregnation solution has the following properties:

B water content 27.9 wt. % free phenol cycle é^á lsmot. % content of methylol groups 14.8 wt. % temperature 20°C viscosity 220 mPa.s

By the method given in Example 1, using the same kraft paper with a drying time of the wet film in a jet dryer of 35 s/150 °C, an impregnated film is produced, showing a resin deposition of 51 wt. %. The decorative laminate molded from these impregnated films, produced by the procedure according to Example 1, has the following properties:

p impact toughness > 0.48 J.cj mass gain in boiling test 2.7% thickness gain in VBurn test 2.0 á

Example 5

The impregnation solution given in Example 3 is modified by adding 3 wt.

λ> urea, 2 wt. % Impregnation solution of water and 4 wt. % hydrogen phosphate has the following properties:

water content free phenol content methylol group content temperature sodium viscosity, the temperature is adjusted to 25 °C.

22.2 wt. %

5.1 wt. %

14.8 wt.% °C 165 mPa.e

The impregnation foils produced by the procedure given in Example 3 have a resin content of 52% by weight, and the corresponding decorative laminates have the following properties:

shear toughness 0.48 J.cm mass gain during boiling test 3.2% thickness gain during boiling test 2.8%

Claims (1)

SUBJECT OF THE INVENTION Process for producing inner sheets for laminated composites, at least with a decorative surface, by impregnating a layer of cellulosic fibrous material with a resin solution prepared by reacting phenol and / or its homolegs with formaldehyde having a ratio of free phenol content determined by Koppenschsar bromometric method to less than 0 5 and at a temperature of 15 to 60 ° C, optionally containing up to 10 wt. % of the group consisting of emollients, stabilizers, accelerators, curing agents, modifiers, surfactants, organic solvents, flame retardants, dyes and pigments, and subjecting the impregnated material to less than 10 s of heat at 120 to 180 ° C, The composition of claim 1, wherein the layer of cellulosic fibrous material is impregnated with an aqueous resin solution having a viscosity of less than 800 mPa.s containing not more than 30 wt. % water.