EP0136344A1 - Phenolic type resin containing formamide for low pressure lamination - Google Patents

Abstract

Fast-curing phenolic rolling resin comprising a resol resin in an organic solvent, approximately 2 to 15% of a material selected from the group consisting of formamide, N-methyl formamide, N, N-dimethyl formamide, N-ethyl formamide, N, N-diethyl formamide, N, N-diphenyl formamide and N-methyl formanilide, as well as a catalytic amount of an inorganic alkaline material. The laminating resin has sufficiently high penetration and rapid polymerization characteristics, allowing it to be used in a continuous rolling process at low pressure.

Description

PHENOLIC TYPE RESIN CONTAINING FORMAMIDE FOR LOW PRESSURE LAMINATION

BACKGROUND OF THE INVENTION

This application is a continuation-in-part of application Serial No. 468,547 filed on February 22, 1983,

1. Field of the Invention

This invention relates to a new resole-type phenolic laminating resin having excellent penetration and rapid cure suitable for making high-grade decorative laminates at high speed and low pressure.

2. Description of the Prior Art

Conventional decorative laminates are typically produced in a batch wise procedure by curing a plurali ty of t her m os et ti n g resin-impregnated fibrous substrate layers in a press at high pressures (e.g., 800-1500 psi) and elevated temperatures (e.g., 250-350 ^βF). The laminates generally consist of three elements: a core section consisting of several sheets of an inexpensive substrate, e.g., kraft paper, satu¬ rated or impregnated with a resole-type phenolic resin, a^j print sheet of alpha cellulose saturated with an expensive amino resin, e.g., a melamine resin and a translucent overlay of rayon or alpha cellulose, also saturated with an expensive amino resin.

When preparing the laminate, the paper substrate is first passed through a dip tank filled with resin. Resin penetration of the substrate is substantially accomplished during immersion in the tank. Excess resin is generally removed from the substrate by opposed scraper bars or blades as the substrate leaves the dip tank. Residual

O PI penetration must be completed as the substrate is thereafter passed to the drying zone. The drying zone generally comprises a long oven where the resin solvent is evaporated and the resin is substantially advanced to an infusible stage (B-Stage). The advanced resin removed from the drying oven is then cut to size, stacked and consolidated in the hot, high pressure press. Generally, the laminate is cured in the press for about 15 to ^' 60 minutes.

Because the laminate is consolidated in a batchwise fashion, the conventional high pressure laminating procedure is rather slow. This necessitates substantial capital investment in order to maintain high production rates, and consequently results in high production costs for the finished laminate.

In an attempt to improve the economics of the conventional laminating process, a continuous laminating procedure has recently been developed. In this process, the fibrous, resin-impregnated substrate layers are consolidated at an elevated temperature by passing them between the nips of serially arranged pressure rolls. The cured and consolidated sheets may then be cut to the desired size at any time prior to use. Because of the continuous operation, high production rates are possible. In this process, substrate penetration, drying and curing (consolidation) are all typically completed in a continuous manner in less than about 5 minutes, e.g., about 1 minute. Consolida¬ tion pressures in this arrangement are normally limited, however, to below about 400 psi, e.g., about 200 psi. Quite understandably, such processing conditions, i.e., complete saturation, high speeds and low pressures, impose stringent requirements on the laminating resin. Not only must the resin quickly penetrate the substrate and flow under a low consolidatio pressure, but it must also cure in an extremely short tim e to yield a product with properties at least comparable to conventional laminates.

It is, therefore, a principal object of the present invention to provide a laminating resin that satisfies the requirements of the continuous laminating process. It is also an object of this invention to provide a phenolic lami¬ nating resin that has been modified to enhance its penetration.

It is a further object of this invention to provide a modified phenolic laminating resin that exhibits excellent flow under low pres¬ sures.

It is still another object of this invention to provide a modified phenolic laminating resin that is completely and rapidly cured. SUMMARY OF THE INVENTION

These and other objects, which will be apparent from the following description, are satisfied by a laminating resin composition comprising a resole-type phenolic resin in an organic solvent having about 2 to about 15%, by weight of a material selected from the group consisting of formamide, N-methyl formamide, N,N-dim ethyl formamide, N-ethyl formamide, N,N-diethyl formamide, N,N-diphenyl formamide and N-methyl formanilide, and a catalytic amount of an inorganic alkaline material.

The present invention also relates to a process for producing a phenolic laminate comprising the steps of saturating a plurality of continuous, fibrous substrate sheets with a fast-curing phenolic lami¬ nating resin, said resin comprising a resole-type phenolic resin in an organic solvent, about 2 to about 15% by weight of a material selected from the group consisting of formamide, N-methyl formamide, N,N-dimethyl formamide, N-ethyl formamide, N,N-diethyl formamide, N,N-diphenyl formamide and N-methyl formanilide, and a catalytic amount of an inorganic alkaline material; drying the resin-saturated continuous sheets so as to remove the organic solvent; consolidating the dried, resin-saturated continuous sheets at a pressure of between about 140 to 400 psi and at a temperature sufficient to cure the resin. DETAILED DESCRIPTION OF THE INVENTION

The present invention specifically relates to a fast curing phenolic laminating resin, modified with a material selected from the group consisting of formamide N-methyl formamide, N,N-dimethyl - 4 -

formamide, N-ethyl formamide, N,N-diethyl formamide, N,N-diphenyl formamide and N-methyl formanilide, to enhance its penetration.

Phenolic resins useful for preparing laminating resins of the present invention are produced in accordance with reaction conditions normally observed for manufacturing phenolic resoles, i.e., by reacting a molar excess of an aldehyde with a phenolic- component in the presence of alkaline catalytic material under moderately elevated reaction temperature conditions. The reaction is typically conducted under reflux at atmospheric pressure.

In the broad practice of this invention aldehyde components such as formaldehyde, acetaldehyde, propionaldehyde, furfural or the like can be used. Formaldehyde is generally preferred because of its low cost and high reactivity. The formaldehyde is generally added as 50% aqueous solution (formalin), although anhydrous para-formaldehyde can also be used. Similarly, examples of suitable phenolic components include phenol, substituted phenols, e-g.- cresol, and phenol homologs, which are generally employed for forming phenolic resole resin. Phenol is generally preferred. Preferably, about 1.5 to 3.0 mols of formalde¬ hyde are used for each mol of phenol. Most preferably, the formal¬ dehyde to phenol molar ratio is about 2.0.

As noted, the phenol and formaldehyde are reacted in the presence of an alkaline catalyst. In order to produce a resin with desired properties an inorganic alkaline material is typically employed as the catalyst. Preferably an alkali metal hydroxide or an alkali metal salt of a weak acid is used as the alkaline material. Suitable alkaline materials include sodium and potassium hydroxide and potassium and lithium carbonate. Potassium carbonate is particularly preferred because it yields a resin having . a reactivity at least equal to and gen¬ erally higher than a resin prepared using sodium hydroxide as the cat¬ alyst, and it does not undesirably increase the molecular weight of the resin to a point which interferes with penetration. For optimum high speed, low pressure continuous lamination processes, a resin catalyzed with potassium carbonate unexpectedly can be employed without the

JO PI ^~ use of additional curing catalysts as described below, because such a resin can be advanced to the point where rapid cure is possible without undesirable increases in viscosity.

The phenol- formaldehyde reaction is preferably advanced under carefully controlled conditions. After blending the phenol and formal¬ dehyde components, a first quantity of alkaline catalyst material is added to the reaction mixture. The mixture is then subjected to mild heating, e.g., up to atmospheric reflux, for a short time, e.g., about 15 minutes. The mixture is then partially cooled, additional alkaline material is added and the reaction mixture is again subjected to mild heating. This procedure is preferably repeated one more time before the resin is vacuum dehydrated with mild heating to reduce its water content. Preferably, a weak organic acid, e.g., lactic, citric, boric, propionic, butyric, etc. is added to neutralize the reaction mixture before the vacuum distillation step so as to inhibit excessive resin advancement during this stage. Thereafter, the resin is cooled to ambient conditions.

The laminating resin of the present invention is then prepared by adding a material selected from the group consisting of formamide, N-methyl formamide, N,N-dimethyl formamide, N-ethyl formamide, N,N-diethyl formamide, N,N-diphenyl for ma mide and N-m ethyl formanilide, an organic solvent and additional alkaline material to the resin. The formamide or disclosed formamide equivalent, solvent and alkaline material can be added to the resin at any time prior to the step of .saturating the fibrous substrate sheets. These components are preferably added at the end of the resin manufacturing proces, i.e., as the resin is being cooled.

In accordance with this invention about 2 to 15% of a material selected from the group consisting of formamide, N-methyl formamide, N,N-dimethyl formamide, N-ethyl formamide, N,N-diethyl formamide, N,N-diphenyl formamide and N-methyl formanilide, is added to the resin. Testing has shown that adding a small amount of formamide or disclosed formamide equivalent to a fast curing phenolic resole resin produces a laminating resin having a high penetration. Importantly, this enhanced penetration is achieved without having to increase the relative amount of organic solvent in the laminating resin as is presently practiced. The ability to achieve a balanced ratio of water to organic solvent is very important since higher solvent content detri¬ mentally lowers the resin flash point while higher water content (which raises the flash point) is detrimental to saturation and cure speed. Indeed, a reduction in the amount of organic solvent relative to water is actually per mitted as a result of the formamide or disclosed formamide equivalent addition to the resin. Consequently, an enhanced penetration may be provided without an undesirable lowering of the flash point of the laminating resin.

This result is particularly surprising. Generally, in order to maximize the cure rate of a laminating resin, the resin's penetration must to some extent be undesirably compromised. Indeed, it is not obvious that a fast curing laminating resin suitable for use in the continuous, low pressure laminating process outlined above could be prepared having a high penetration as well as a suitably high flash point. The present invention is based in part on the discovery that by using formamide or disclosed formamide equivalent in formulating the laminating resin, the amount of organic solvent otherwise required to appropriately enhance the penetration of a fast curing resin is advan¬ tageously reduced. Consequently, the laminating resin has a lower organic to aqueous solvent ratio and accordingly a higher flash point.

Peterson et al U.S. 2,981,652 discloses an aqueous phenolic adhe¬ sive, i.e., a phenolic condensation product or resin dissolved in a highly alkaline aqueous alkali - metal hydroxide solution, which employs formamide as a setting accelerator. Since the formamide is used as a setting accelerator by means of the reaction with excess caustic, it is not blended with the alkaline phenolic solution until the point of use. The adhesive described by Peterson is obviously not suitable as a lami¬ nating resin because of high molecular weight and insufficient satu¬ ration essential for high speed operation. A sufficient amount of organic solvent is also added to the phenolic resin so as to provide the laminating resin with a viscosity between about 80-150 cps. As noted above, the amount of organic solvent required is less than would otherwise be needed in the absence of formamide. Suitable solvents include alcohols, e.g., methanol, ethanol, and isopropyl alcohol, and ketones, e.g., acetone and methyl ethyl ketone. Methanol is the preferred solvent.

Additional alkaline material is also added to the resin at this time. About 0.1 to 2% alkaline material based on the weight of the laminating resin, is added to the laminating resin composition for neu¬ tralizing acid conditions after distillation and for accelerating its cure rate during the lamination process. In this way, the laminating resin can be fully cured to a solid infusible product in as little as 20 seconds. Importantly, the storage life of the laminating resin is not severely curtailed. The resin is stable for about 2 to 4 weeks at room temperature.

While the phenolic resin is being cooled, or up to the time the fibrous substrate is saturated, other laminating resin modifiers may optionally be added to the laminating resin composition for further enhancing specific properties of the laminating resin. For example, urea is generally added to reduce the free formaldehyde content to below about 2%. A small amount of a polyamide resin and/or a polyol, e.g., a polyglycol or a polyglycerine, may also be added to aid penetration and enhance resin flow at low consolidation pressures. A suitable lubricant, such as oleic acid, may also be added to ease the release of the fully cured laminate from the rolls.

In order to achieve sufficiently rapid cure for the preferred high speed low pressure continuous lamination process, it may be necessary to add an additional curing agent to the laminating resin just prior to use, specifically where the resin was originally catalyzed with caustic rather than potassium carbonate, since a caustic catalyzed resin cannot be advanced as far without undesirable viscosity increases. Among the known suitable curing agents are resorcinol and resorcinol-type resins. - 8 -

The additional curing agent can be used in amounts of from about 2 to 10% by weight depending on the type of cure desired.

The present invention also comprises a continuous, low pressure laminating process for producing decorative laminates employing the fast-curing laminating resin of this invention. In the first step, a plu¬ rality of continuous, fibrous substrate sheets are saturated with the fast curing phenolic laminating resin described above. The fibrous substrate may be paper, e.g., heavy kraft paper, textiles, asbestos, glass fibers and the like. The substrate can typically be saturated in as little as 10- seconds. The saturated resin is then dried to evaporate the solvent and advance the resin. At temperatures between about 350° and 450 °F drying takes between about 5 and 15 seconds. Nor¬ mally, the resin content of the dried resin-saturated sheets will be on the order of 20 to 40% by weight. The continuous sheets are then consolidated, e.g., between the nips of opposing, serially positioned rollers, at a pressure of between 140 to 400 psi, e.g., 200 psi and at a temperature sufficient to cure the resin. Press temperatures are generally between about 350 ° to 450 °F. At these conditions, the consolidated laminate is fully cured in about 10 to 30 seconds. The fully cured laminate may be cut to size immediately after curing or stored in roll form for later use.

While this invention has been described primarily for use in pre¬ paring decorative laminates, the invention is broadly suited to the manufacture of other laminates which employ phenolic resins, e.g., lam¬ inates for electrical applications. Similarly, while the laminating resin of this invention is particularly suitable for use -in connection with the continuous low pressure laminating process, it can also be used to pre¬ pare high pressure laminates using the conventional batch process.

The following examples are included for illustrative purposes only and are not intended to limit the scope of this invention. EXAMPLE 1

A suitable phenolic resole resin for preparing the laminating resin of the present invention was produced by blending 42.82 parts by

O weight of 50% aqueous formaldehyde solution with 33.49 parts phenol and reacting the mixture under alkaline conditions. A small amount of triethanolamine (i.e., 0.04 parts) was also preferably added to neutralize formic acid present in the formaldehyde or formed during the conden¬ sation reaction. The addition of triethanolamfne to the reaction mix¬ ture maximizes the effectiveness of the alkaline catalyst.

Thereafter, the phenol-formaldehyde condensation reaction was controllably advanced by adding the alkaline material in a stepwise manner. Initially, 0.08 parts by weight of a 50% aqueous sodium hydroxide solution was added and the mixture was heated to about 96-100 °C and held for about 15 minutes. The mixture was then cooled to about 90 °C; 0.11 parts of the aqueous sodium hydroxide solution was added, and the mixture was again heated to about 96-100 °C and held for an additional 15 minutes. This procedure was repeated once more as 0.38 parts of the aqueous sodium hydroxide solution was added to the reaction mixture. The reaction mixture was thereafter cooled to about 80 °C and held at this temperature until the resin reached a viscosity in the range of 150-200 cps (at 25 °C) Lactic acid, about 0.73 parts by weight, was then added to neutralize the reaction mixture and the resin was vacuum dehydrated. After the water content of the resin had been reduced to the desired extent, the resin was cooled to ambient conditions.

While the above-described resin was undergoing final cooling, the modifiers listed below were added at the indicated level:

Parts by Weight Polyamide resin 0.66

Urea 1.32

Polyglycerine 0.78

Methanol 14.15

Formamide 3.96

Oleic acid 0.40

Sodium Hydroxide (50% aqueous solution) 0.16

y , nIinPOj In order to further reduce the curing speed of the above formulated laminating resin to the desired level, between about 5 to 10% by weight of a resorcinol-based resin curing catalyst (approx. 50% resorcinol content) was added to the laminating resin just prior to use, i.e., prior to saturating the fibrous substrate sheets with the laminating resin. The foregoing resin perform ed satisfactorily i n the above-described high speed, low pressure continuous laminating process. EXAMPLE 2

The Example 1 process for preparing a phenolic resole resin was repeated with the following formulation:

Parts by Weight Formaldehyde (50% aqueous solution) 39.57

Phenol 30.94

Triethanolamine 0.04

Potassium Carbonate - Stage 1 0.29

Potassium Carbonate - Stage 2 0.43

Potassium Carbonate - Stage 3 0.72

Lactic Acid 0.72

In the case of this potassium carbonate catalyzed resin, the reaction viscosity end point can go to 350-400 cps at 25 ^βC.

While this resin was undergoing final cooling, the modifiers listed below were added at the indicated level to produce a fast curing phenolic laminating resin in accordance with this invention:

— - Parts by Weight

Urea 1.44

Polyglycerine 1.44

Methanol 18.59

Formamide 3.81

Potassium Carbonate 0.79

The laminating resin of Example 2 performed satisfactorily in the above-described high speed, low pressure continuous lamination

OMPI process and exhibited the desired curing speed without the necessity of adding resorcinol-type curing agents. Example 3

Tests were conducted which compared the penetration character¬ istics of a laminating resin containing formamide with the same resin containing some of the other disclosed formamide equivalents. Assigning the laminating resin containing formamide a relative pene¬ tration value of 1, the other laminating resins tested containing the noted formamide equivalent had the following relative penetration values N-m ethyl formanilide (1.30); N-methyl formamide (0.82); N,N-diphenyl formamide (2.40) and N,N-diπτethyl formamide (0.38).

These tests show that better penetration results relative to a laminating resin containing formamide may be obtained using N-methyl formamide and N,N-dim ethyl formamide in the laminating resin instead. However, based on current commercial availability, cost and safety, formamide is the preferred material.

While preferred embodi m ents of this invention have been discussed herein, those skilled in the art will appreciate that changes and modifications may be made without departing from the spirit and scope of this invention, as defined in and limited only by the scope of the appended claims.

Claims

1. A fast-curing phenolic laminating resin comprising a resole-type phenolic resin in an organic solvent, about 2 to 15% by weight of a material selected from the group consisting of formamide, N-methyl formamide, N,N-dimethyl formamide, N-ethyl formamide, N,N-diethyl formamide, N,N-diphenyl formamide and N-methyl formanilide, and a catalytic amount of an inorganic alkaline material.

2. The laminating resin of claim 1 containing about 2 to 15% by weight formamide.

3. The laminating resin of claim 1 or 2 wherein said resole-type phenolic resin is prepared by reacting from about 1.5 to 3.0 mols of formaldehyde per mol of phenol in the presence of an alkaline catalyst.

4. The laminating resin of claim 1 or 2 wherein said inorganic alkaline material is selected from the group consisting of sodium hydroxide and potassium carbonate.

5. The laminating resin of claim 3 wherein said alkaline cat¬ alyst is selected from the group consisting of sodium hydroxide and potassium carbonate.

6. The laminating resin of claim 4 wherein said catalytic amount comprises between about 0.1 to 2.0% by weight of said lami¬ nating resin.

7. The laminating resin of claim 1 or 2 wherein the organic solvent is methanol.

8. The laminating resin of claim 3 wherein said alkaline cat¬ alyst is potassium carbonate.

9. ^' The laminating resin of claim 3 wherein said alkaline cat^ alyst is sodium hydroxide and said laminating resin additionally com¬ prises a curing agent added to said resin just prior to its use.

10. A process for producing a phenolic laminate comprising the steps of:

(a) saturating a plurality of continuous fibrous substrate sheets with a fast-curing phenolic laminating resin, said resin

OMP comprising a resole-type phenolic resin in an organic solvent, about 2 to 15% by weight of a material selected from the group consisting of formamide, N-methyl formamide, N,N-dimethyl formamide, N-ethyl formamide, N,N-diethyl formamide, N,N-diphenyl for mami de and N-methyl formanilide, and a catalytic amount of an inorganic alkaline material;

(b) continuously drying the resin saturated continuous sheets so as to remove the organic solvent;

(c) continuously consolidating the dried, resin-saturated continuous sheets at a pressure between about 140 to 400 psi and at a temperature sufficient to cure the resin.

11. The process of claim 10 wherein said resin contains about 2 to 15% by weight formamide.

12. The process of claim 10 or 11 wherein said resole-type phenolic resin is prepared by reacting from about 1.5 to 3.0 mols of formaldehyde per mol of phenol in the presence of an alkaline cata¬ lyst.

13. The process of claim 10 or 11 wherein said inorganic alkaline material is selected from the group consisting of sodium hydroxide and potassium carbonate.

14. The process of claim 12 wherein said alkaline catalyst is selected from the group consisting of sodium hydroxide and potassium carbonate.

15. The process of claim 13 wherein said catalytic amount comprises between about 0.1 to 2.0% by weight of said laminating resin.

16. The process of claim 10 or 11 wherein the organic solvent is methanol.

17. The process of claim 12 wherein said alkaline catalyst is potassium carbonate.

18. The process of claim 12 wherein said alkaline catalyst is sodium hydroxide and said laminating resin additionally comprises a curing agent added to said resin just prior to its use. - U -

19. The product produced by the process of claim 10 or 11.

20. The product produced by the process of claim 17.

21. The product produced by the process of claim 18.