Stock material or miscellaneous articles – Composite – Of silicon containing
Patent
1986-06-30
1987-11-24
Van Balen, William J.
Stock material or miscellaneous articles
Composite
Of silicon containing
156 622, 264123, 428326, 4285371, 428697, 428921, B27N 900, B32B 2102
Patent
active
047089105
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to fire-resistant wood composites, in particular building boards such as wallboards, containing principally a wood component such as wood chips, wood fibres, sawdust and/or woodwool, and a bonding agent, a process for manufacture of said composites and a new hydraulic bonding agent for said manufacture.
The bonding agents currently used for wall boards based on wood fibre, sawdust and woodwool with improved resistance to fire, fungi and termites etc. are principally Portland cement magnesia or Sorel's cement and gypsum. Cement-bonded wood composites such as e.g. particle boards contain 30-70 wt. % wood fibre and particles and approx. 70-30 wt. % of an inorganic bonding agent. The bonding agent affects to a high degree the characteristics of both the process and the product.
Wood fibres which have undergone various preparatory stages are mixed together with a bonding agent, various chemicals and the required quantity of water. After the boards have been formed in one or more layers on a conveyor belt or on plates they normally undergo a preliminary compression in a press after which the boards are compressed at high temperature in a main press. The pressure of the press varies between 5-40 atm. and pressing is continued until the boards have attained sufficient strength for them to be released and transported. The press time for Portland cement bonded boards can amount to 8 h or more, whereas with magnesia cement the release strength can be attained already after 10-15 min.
Pressing is followed by storage, after which the boards are trimmed, sanded and packed.
Particularly for Portland cement bonded boards the long press time and the subsequent long storage involves a high cost and often constitutes the limiting factor in production.
The setting or hardening of Portland cement when it is mixed with the required quantity of water is readily affected by various chemicals. Substances which speed up the setting of the cement are called accelerators and those which delay the hydration reactions are called retarders. Accelerators consist mainly of simple readily-soluble salts of alkali or alkali-earth chlorides, sulphates, nitrates etc. which raise the ion concentrations in the aqueous phase and thereby the solubility for the hydration products of the cement.
Retarders consist of readily soluble metal salts which together with the lime in the cement form sparingly soluble or insoluble Ca salts or reaction products which are precipitated on the surface of the cement grains and hinder further hydration. Soluble organic substances with a high molecular weight also have a retarding effect on the setting of cement.
The most common retarders for Portland cement are carbohydrates and in particular sugar which even in concentrations as low as a few hundredths percent of the weight of cement can cause considerable retardation of the hydration of the cement.
Various species of wood contain many extractable components which have a strongly retarding effect on the setting of Portland cement. The most harmful components are various sugars and starch, tannins, certain phenolic components and decomposition products of hemicellulose. As a consequence in the manufacture of cement-bonded wood composites it is only possible to employ species of wood which have a low content of these components or in which said components can be reduced to an acceptable level without incurring excessive costs.
Hardwoods in particular have a high content of extractable sugar in the form of pentoses and as a consequence it has not normally been possible to employ hardwoods in wood composites containing Portland cement as a bonding agent.
In the hydration of Portland cement large amounts of Ca(OH).sub.2 are released and the pH of the pore solution is around 12.5. At this pH the hemicellulose of the wood is decomposed to smaller units which in part are water-soluble and give rise to retardation of the setting time.
The suitability of wood for purposes such as cement-bonded wood boards can be tested by means of mixing a know
Forss Consult Ky, Kb
Van Balen William J.
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