IE60045B1

IE60045B1 - Manufacture of fibreboard

Info

Publication number: IE60045B1
Application number: IE274787A
Authority: IE
Original assignee: Borden Uk Ltd
Priority date: 1986-10-14
Filing date: 1987-10-13
Publication date: 1994-05-18
Also published as: GB2196342B; GB8624557D0; GB8724097D0; GB2196342A; ES2005034A6; IE872747L

Abstract

A cellulose fibre composite is manufactured by a method comprising blending together a urea- formaldehyde pre-polymer resin having a high formaldehyde:urea mole ratio, a particulate cellulose fibre material and a urea solution. The blend is then subjected to hot pressing to give fibreboard with low free formaldehyde content yet good physical strength.

Description

The present invention relates to the manufacture of cellulose fibre-based composites, e.g. particleboard and medium density fibreboard, using urea-formaldehyde resins as binders.

In the production of such compositions using urea-formaldehyde resin binders, the small amounts of formaldehyde retained in the board after manufacture can cause environmental problems at some later date. It is therefore desirable to minimise this free formaldehyde content as much as possible without sacrificing the advantages that are achieved from the use of urea-formaldehyde resin binders. Many ways of solving the problem have been proposed and the approach most commonly used in the industry is to reduce the formaldehyde: urea mole ratio in the resin. Although this solves the particular problem of free formaldehyde in the product, it unfortunately causes reduced reactivity, reduced bonding efficiency and reduced storage stability of the resin. Reduced bonding efficiency in the product is displayed as reduced physical strengths in the board and as increased swelling of the board on being immersed in water or on being subjected to conditions of high humidity.

The present invention is based on the discovery that the above problems of the prior art can be avoided by the addition of a urea solution to the ureaformaldehyde resin binder prior to the hot pressing stage of the resin/fibre mixture. Accordingly, the present invention provides a method of manufacturing a cellulose fibre composite comprising blending together a urea-formaldehvde pre-polymer resin having a high formaldehyde:urea mole ratio, a particulate cellulose fibre material and a urea solution, and subjecting the mixture obtained to hot pressing. fibre Typically the particulate cellulose material used in the present invention will comprise any of the materials conventionally used in the manufacture of cellulose fibre composites. In particular, we have achieved good results using wood chips and medium density wood fibres. The use of a high formaldehyde ratio resin blended simultaneously with urea solution and wood fibres has been found to give the following advantages over the use of a resin of equivalent final molar ratio:(a) improved storage life of resin pre-polymer compared with conventionally used resins; (b) board; (c) lower formaldehyde emission for finished improved tensile strengths perpendicular to the plane of the board (i.e. known as internal bond).

The urea-formaldehyde pre-polymer resins useful in the present invention have a high formaldehyde : urea mole ratio. By high formaldehyde : urea mole ratio we mean a formaldehyde: urea mole ratio in the range of from 2.5:1 to 3*5:1. Preferably, the formaldehyde: urea mole ratio will be about 2.5:1. The pre-polymer resin will generally have a pH value in the 0 in order that it has good storage the pH of the pre-polymer resin will have a value in the range of .from 7.0 to 8.0. The solids content and the viscosity of the pre-polymer resin are not critical to the working of the present invention. However, the pre-polymer resin will generally have a solids content of from 55 to 70% by weight and a viscosity in the range of from 500 to 5000 cP at 25°C.

According to the present invention urea, as an aqueous solution, is added to the high formaldehyde resin in an amount sufficient to give a final formaldehyde : urea molar ratio of typically from range of 6.5 to 9 stability. Preferably 1.1 - 1.6:1, preferably about 1.2:1. The concentration of urea in the aqueous urea solution is not critical to the working of the invention although in practice it 1 will be adjusted to give a convenient quantity of ( 5 addition. A 50% (by weight) concentration of urea will normally be convenient. For a 50% urea solution, a typical quantity of addition is from 20 to 50% based on the amount of pre-polymer resin and depending on the mole ratio of the pre-polymer and the final mole ratio required. Other additives, such as wax emulsion conventionally used to improve hydrophobicity, may also be incorporated with the components mentioned above.

In general, the resin will be used in an amount to give a % resin solids (based on the weight of dry fibre) in the range of from 8 to 10%, preferably 6 to 8%.

The method of the invention is particularly suitable for the manufacture of medium density fibre (MDF) board where the resin is added to the wood fibre by ’’blow line blending. Preferably the pre-polymer solution, the urea solution and a wax emulsion are metered separately through metering pumps and introduced simultaneously into a static mixer. The mixture is then flowed to the chip blender or, if appropriate, the blow line. In the case of blow line blending, it is also possible to meter the urea components directly into the blow line without the use of a static mixer.

The mixture of prepolymer, particulate cellulose fibre material and urea solution obtained by blending the components together is subjected to hot ' pressing. Hot pressing techniques are, of course, well known in the art and do not require detailed discussion here. Generally, however, pressing temperatures in the range of from 150° to 220°C, preferably from 170° to 200°C, will be used. The moisture content of the material prior to pressing will usually be in the range of from 7 to 14% and more usually from 9 to 12%.

Pressure will be varied throughout the pressing cycle, r up to a maximum of about 400 psi, to produce fibreboard of a typical density, of 600 - 800 Kg/m3, preferably h 680-750 Kg/m3.

EXAMPLE I A pre-polymer resin I was prepared by acidifying 2000g of a 50% (by volume) aqueous solution of formaldehyde with a 50% (by volume) sulphuric acid solution to give a pH of 1.8, This solution was heated to 60°C and 780g of urea was added slowly allowing temperature to rise to 85°C. After neutralising with 32% (by weight) caustic soda solution to a pH of 7.0 7.4 the resin was distilled under vacuum at 60°C to remove 400g distillate, finally cooling to 25°C or below. This pre-polymer solution had a viscosity of approximately 200cP at 25°C and a solids content after drying for 3 hours at 120°C of 60% by weight and a molar ratio of formaldehyde to urea of 2.55:1.

A resin II was made using the same quantities and process as above but after distillation the resin was cooled to 50°C and a further 886g urea were added and allowed to react for 1 hour at 40°C prior to cooling to 25°C.

MDF boards were made using blow line mixing of resin and fibre. In the case of pre-polymer I, the flow rates of fibre, pre-polvmer I and 50% urea solution were set to give the ratios:fibre 90 parts by weight, c pre-polymer I 10.9 parts by weight, 50% (by weight) urea solution 8.2 parts by weight - equivalent to 10% binder solids on dry fibre. In the case of resin II, the flow rates were set to give the ratios: fibre 90 parts by weight and resin II 15 parts by weight again fibre. In both cases giving 10% binder solids on dry fibres were dried to 10% moisture content, formed into a mat and hot pressed for 3 minutes at 170°C to give a 12mm thick MDF board of about 750Kg/m density.

Duplicate boards were tested and the following results obtained : Density Internal Bond MPa Total Extractable Formaldehyde mg/1OOg Pre-polymer I urea 760 0.66 Pre-polymer I -ι- urea 750 0.61 10 Resin II 750 0.60 20 Resin II 7*0 0.52 16 EXAMPLE II A pre-polymer III was made by adding 910g urea to 2000g 50% (by volume) formaldehyde solution at a pH of 7.0 - 7.2, heating for 10 minutes at 100 C, dropping pH to 5 - 6 and reacting until a viscosity of 80cSt § 25°C was reached, neutralising to pH 7.0 - 7.5 and distilling at 60°C to remove 300g of water. The resulting pre-polvmer resin solution had a viscosity of 200cPs @ 20°C and a solids content of 60% by weight. A similar resin was made but after distillation and cooling to 50°C a further 620g urea was added and reacted at 40°C for one hour before finally cooling to l80cP at 20°C and a solids content of 65% by weight.

MDF boards were made by simultaneously mechanically blending dried wood fibres with prepolymer III and a 50% (by weight) urea solution in the ratio'.wood fibre 90 parts by weight, pre-polymer III 11 parts by weight and 50% urea solution 8 parts by weight *7 to and similar boards were made using wood fibre 90 parts by weight and resin IV 15 parts by weight. In both cases, a total binder content of 10% was used and boards were pressed at 170°C for three minutes. The following results were obtained :Internal Density Bond kg/m^MPa Total extractable formaldehyde mg/1OOg Board made with 10 pre-polymer III + urea 760 Board made with resin IV. 740 0.52 48 0.43 77

Claims

1. A method of manufacturing a cellulose fibre composite comprising blending together a urea-formaldehyde pre-polymer resin having a .high formaldehyde: urea mole ratio in the range of from 2.5:1 to 3-5:1, a particulate cellulose fibre material and a urea solution, and subjecting the mixture obtained to hot pressing.

2. A method according to claim 1, wherein the urea-formaldehyde pre-polymer resin has a formaldehyde: urea molar ratio of about 2.5:1.

3. - A method according to either claim 1 or claim 2, wherein the urea solution is added in an amount sufficient to give a final formaldehyde: urea molar ratio in the range of from 1.1:1 to 1.6:1.

4. A method according to claim 3» wherein the urea solution is added in an amount sufficient to give a final formaldehyde: urea molar ratio of about 1.2:1.

5. A method according to any one of claims 1 to 4, wherein the urea solution has a urea concentration of about 50% by weight.

6. A method according to any one of claims 1 to 5, wherein the particulate cellulose fibre material is selected from wood chips and medium density wood fibres.

7. A method according to claim 1 of manufacturing a cellulose fibre composite s substantially as hereinbefore described and exemplified.

8. A cellulose fibre composite, whenever manufactured by a method claimed in a preceding claim.