Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From aldehyde or derivative thereof as reactant
Patent
1988-09-19
1990-11-06
Kight, III, John
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From aldehyde or derivative thereof as reactant
528232, 528239, 528242, 528259, C08G 1212
Patent
active
049687727
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a new process for the preparation of urea-formaldehyde resins having a low extractable formaldehyde content.
Urea-formaldehyde resins are widely used in industry as binders for the production of wood-based materials, particularly chipboard, plywood, and various fibreboards. They are usually employed as colloidal aqueous solutions having a 60-70% solids content, the solids content being the percentage weight left when a 2 g sample of the resin is heated at atmospheric pressure at 120.degree. C. for 3 hours.
The resins themselves are complex mixtures of various condensation products, ranging rom simple methylolated ureas to fairly high molecular weight materials, and the precise properties of any particular resin will depend upon its molar ratio (i.e. the ratio of formaldehyde to urea used in its manufacture), the type and proportion of the various linkages within the resin structure, and the proportions of high and low molecular weight material. These last two factors are dependent upon the precise manufacturing method that is used.
Conventional commercial resins all liberate a certain amount of formaldehyde during hot curing and after cure has taken place. The latter liberation of formaldehyde can be a serious problem since it limits the use to which, for example, fibreboards and particle boards may be put. A board that liberates formaldehyde above a specified level cannot be used in unventilated areas. The measurement of the total extractable formaldehyde is therefore an important test that is carried out on fibreboard and particle board samples, and is usually effected using the method of the Federation Europeenne des Syndicates des Fabricants des Panneaux de Particules (FESYP) and described in British Standard 1811. In this test small rectangular blocks sawn from air-dried fibreboard or particle board are extracted with toluene, the toluene is washed with water, and the water measured iodometrically for formaldehyde. A need exists for resins that have a very low content of extractable formaldehyde but which, in all their performance properties, behave like a conventional urea-formaldehyde resin.
The conventional method of manufacturing urea-formaldehyde resins comprises precondensing urea with formaldehyde in aqueous solution at the reflux at pH 7-9, and heating the mixture at pH 5-6 until the desired degree of water insolubility or viscosity is reached. The product is neutralised and may then be evaporated and blended with a further quantity of urea if desired. Such conventional resins usually have a F:U molar ratio within the range 1.3-2.3:1.
Resins having a F:U molar ratio as low as 1:1 have been described in British Patent Specification No. 1 420 017. These resins are said to be suitable for binding wood-based materials and are prepared by
(i) condensing formaldehyde and urea at a F:U molar ratio of at least 3:1 and at pH 3 or less,
(ii) adding urea to bring the molar ratio down to 2-2.75:1, and continuing to heat at pH 3 or less
(iii) optionally, adding more urea so that the final molar ratio is within the range 1-2.75:1,
(iv) adjusting the pH to 5-6.5 or to 8-10, and continuing the heating until the desired product is formed. This product may then be concentrated or dried, if desired. A serious drawback with this process is that, on an industrial scale, it is difficult to obtain a consistent product and there is a serious risk of the product forming an intractable gel during the process, such formation then requiring dismantling of the apparatus in order for it to be cleaned. Further, when the product has been made successfully, it has poor `wash down` properties. This means that pipework, pumps etc. through which the resin has been passed are difficult to clean, due to the inherent stickiness of the resin.
Another process for preparing adhesive resins with low formaldehyde content is described in U.S. Pat. No. 4,410,685. In this process resins having a F:U molar ratio within the range 1.0 to 1.2:1 are prepared as follows:
(i) formaldehyde solution is acidified to pH 0.5-
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Acquah Sam A.
Dyno Industrier AS
Kight III John
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