Paper making and fiber liberation – Processes and products – Non-fiber additive
Patent
1997-10-27
1998-12-08
Silverman, Stanley S.
Paper making and fiber liberation
Processes and products
Non-fiber additive
164135, 1641641, 1641643, 427391, D21H 1708, D21H 2120
Patent
active
058463836
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 of PCT/EP96/01670, filed Apr. 22, 1996.
BACKGROUND OF THE INVENTION
Customary products available on the market for increasing the wet and dry strength of paper are either condensates of epichlorohydrin and polyamines and comprise chlorine bonded organically and in salt form, or polycondensates of formaldehyde which can release formaldehyde again under the influence of heat and action of moisture. Both classes of product therefore present environmental problems since they pollute the waste water with chlorine or formaldehyde.
The use of products containing isocyanate groups for increasing the wet or dry strength of paper is known from EP-A 564 912.
EP-A 582 166 describes the use of polyisocyanates which have been rendered cationic, DE-A 4 319 571 describes polyisocyanates which have been rendered cationic and optionally hydrophilic for this purpose, and German Patent Applications P 4 418 836.6 and P 4 419 572.9 describe the use of such products in multi-component mixtures.
The content of organically bonded halogen (OX content) of the products described in the Applications cited is extremely low; consequently, the AOX pollution (AOX=adsorbable organically bonded halogen) of the waste water of paper mills from this content is very low.
When such products are used industrially, the strengthening of the paper is very high and is also stable under hydrolytic conditions. Recycling of these papers by digestion in dilute sodium hydroxide solution or aqueous ammonia is therefore achieved only with difficulty or not at all.
There was therefore the need for strengthening agents which impart to the paper--without surrendering the advantages of isocyanates--a "terminable" strengthening; i.e. a strengthening which can be cancelled out during breakdown or deinking.
It has now been found that water-emulsifiable compounds which contain isocyanate groups and have on average at least 2 isocyanate groups bridged via ester and/or amide groups per mol are outstandingly suitable as dry and wet strength agents with terminable strengthening for paper, it being possible for these compounds to be employed before sheet formation (use in the pulp), i.e. as an additive to the fibrous substance suspension, or in the surface, i.e. as an application to a sheet of paper which has already been formed.
It has furthermore been found that strengthened papers which can be digested again to a desired extent can be produced if isocyanate mixtures in which at least 20% by weight of the isocyanates contain isocyanate groups bonded via ester and/or amide groups are employed for the strengthening.
SUMMARY OF THE INVENTION
The invention relates to a process for the production of structurally strong papers (or paper-like materials) which have a terminable strength by treatment of paper in the pulp or of finished paper in the surface with 0.0001 to 50% by weight, preferably 0.01-25% by weight (based on the pulp), of a mixture of isocyanates which contain on average at least two isocyanate groups and of which at least 20% by weight (preferably at least 50%) are those in which the NCO groups are bonded via an organic radical which contains at least one ester and/or amide group in the main chain.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Ester groups include carbonate and allophanate groups, but not the urethane group itself. In addition to the polyisocyanates containing ester and/or amide groups, customary polyisocyanates known for strengthening in paper or else customary wet strength agents or retention agents can be present.
The invention also relates to the use of such isocyanate mixtures of the type defined above which contain emulsifiers for better emulsification in water, or in which the emulsifiability has been brought about by reaction of some (5-50%, preferably 8-30%) of the NCO groups with compounds which are capable of salt formation (for example dimethylolpropionic acid or N,N-dimethylethanolamine; (cf. DE-OS 4 319 571 or P 4418836.6)) and/or hydrophilic, preferably monofuctional polyethers (according to DE-OS 4
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H.L. Baumgarten, et al., Deinking--Entwicklungsstand einer Schlusseltechnologie fur die Altpapierverwertung, 42, Jahrgang, Heft 10A, pp. 166 -177, (1988).
W. Berndt, Die Chemikalien der Deinking-Prozesse, Wochenblatt fur Papierfabrikation, 15, pp. 539 -541, (1982).
Konig Klaus
Traubel Harro
Bayer Aktiengesellschaft
Fortuna Jos'e A.
Silverman Stanley S.
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