Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
2000-08-11
2004-01-13
Medley, Margaret (Department: 1714)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
C423S636000, C423S286000, C423S331000, C423S335000
Reexamination Certificate
active
06676920
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to magnesium hydroxide particles having a novel form each and a process for the production thereof. More specifically, the present invention relates to magnesium hydroxide particles having a hexagonal crystal form each and having a specific aspect ratio, and a method of the production thereof. Further, the present invention also relates to a flame retardant and a resin composition containing the above magnesium hydroxide particles.
PRIOR ART OF THE INVENTION
Magnesium hydroxide particles have been known for a long time and are used for medical and industrial products in broad fields. For example, the medical products include an antacid, an evacuant, medicaments for animals, etc., and the industrial products include a flame retardant which imparts a thermoplastic resin flame resistance when incorporated into the resin, an adsorbent for an oil-containing effluent, a soot desulfurizer, a waste water neutralizer and a soil improver.
A variety of methods of producing magnesium hydroxide particles have been hitherto available, such as a method in which ion bitten, seawater or dolomite is used as an Mg source and lime or sodium-hydroxide is used as an alkali source, a method which is based on the hydration of magnesium oxide and a method in which an Mg salt and ammonia are allowed to react deposit a magnesium hydroxide crystal. In conventional magnesium hydroxide, however, the form of magnesium hydroxide particles is determined depending upon a production method, so that a different production method has been required for obtaining a magnesium hydroxide particles having a different form.
Meanwhile, an organohalogen compound or antimony trioxide is widely used as a flame retardant for synthetic resins.
A halogen-containing flame retardant comprising an organohalogen compound, antimony trioxide or a combination of these generates a large quantity of smoke and toxic gases in fire, and it has therefore socially caused problems. Flame retardants have been therefore studied for avoiding the use of the halogen-containing flame retardant so as to make its amount as small as possible. As a result, magnesium hydroxide particles have come to be evaluated as an effective flame retardant. When magnesium hydroxide particles are incorporated into a resin, the amount of smoke during combustion is small, and they are non-toxic. Further, magnesium hydroxide particles can be applied to a broad range of resins since they are free from a phenomenon that those like aluminum hydroxide particles are dehydrated and decomposed at a resin processing temperature to foam a resin molded article.
Synthetic resins containing a high concentration of magnesium hydroxide particles as a flame retardant have come to be widely used in electric cables for use in/for an atomic power plant, vessel, an automobile, a subway and communications in an underground or tunnel passage, in a part for an electric home appliance and an electronic machine or equipment and in a construction material.
PROBLEMS TO BE SOLVED BY THE INVENTION
When a synthetic resin is flame-retarded with magnesium hydroxide particles, the synthetic resin is required to contain a high concentration of magnesium hydroxide particles, and there is caused a problem that a poor appearance is formed or that properties of a composition of the synthetic resin are deteriorated. For overcoming the above problems, there have been proposed flame retardant of magnesium hydroxide particles which are surface-treated with a higher fatty acid or various coupling agents. However, the above problems have not yet essentially solved.
The present inventors have therefore made diligent studies for obtaining novel magnesium hydroxide particles having an unconventional specific form. It has been therefore found that magnesium hydroxide particles having a relatively large aspect ratio unlike conventional one and having a hexagonal crystal form can be obtained by adding a specific acid or a salt thereof in the step of producing magnesium hydroxide particles from magnesium chloride or magnesium oxide as a raw material. According to the present invention, particularly, there are provided magnesium hydroxide particles having an aspect ratio of a specific value with regard to a specific surface area and an average particle diameter.
In the present invention, studies have been further made concerning use of the above magnesium hydroxide particles having a specific form, and it has been found that the magnesium hydroxide particles have excellent properties as a flame retardant for synthetic resins.
MEANS TO SOLVE THE PROBLEMS
According to the present invention, there is provided magnesium hydroxide particles having a hexagonal crystal form and having an aspect ratio (H) which satisfies the following expression (I),
0.45·
A·B<H
<1.1
·A·B
(I)
wherein H is an aspect ratio, A is an average secondary particle diameter (&mgr;m) of all of the particles measured by a laser diffraction scattering method and B is a specific surface area (m
2
/g) of all of the particles measured by a BET method, and a flame retardant comprising the magnesium hydroxide particles.
According to the present invention, further, there are provided a flame-retardant resin composition comprising 100 parts by weight of a synthetic resin and 5 to 300 parts by weight of the above magnesium hydroxide particles, and a molded article produced therefrom.
The present invention will be explained further in detail hereinafter. First, the magnesium hydroxide particles having a hexagonal crystal form and the process for the production thereof, provided by the present invention, will be explained.
The magnesium hydroxide particles of the present invention are characterized by a hexagonal crystal form and a specific aspect ratio. The hexagonal crystal form is observed, for example, in a microscopic photograph of the particles taken at a magnification of 5,000 to 10,000 diameters. The magnesium hydroxide particles of the present invention are hexagonal single crystals, and the hexagonal form is not required to be of a regular hexagonal form.
FIG. 1
shows a board having the form of a regular hexagon as a model for calculation of an aspect ratio. However, the hexagonal form may be a form having six sides forming the board as a total, and for example, may be a hexagon that the length of each of two sides opposed to each other is larger than the length of each of the other sides. Further, angles (interior angles) forming apexes of corners (six corners) formed by combinations of two sides in contact with each other may be 100° to 130° each. Further, the corners (six corners) formed by combinations of two sides in contact with each other may be partly rounded. When observed in an enlarged photograph, most (at least 90%, preferably at least 95%) of the magnesium hydroxide particles of the present invention have the above hexagonal crystal form, and the sizes of them are not necessarily uniform. However, the sizes (particles diameters) of the particles have a certain constant distribution width as will be described later, and one of the features of the magnesium hydroxide particles of the present invention is that the sizes of the particles are relatively uniform. The distribution of the particle diameters will be further explained later.
The magnesium hydroxide particles of the present invention have characteristic features that the single crystal thereof is hexagonal and that the aspect ratio (H) thereof is relatively large as compared with conventional ones. The range of the aspect ratio (H) is determined in correlation with values of an average secondary particle diameter (A) and a BET specific surface area (B) of the magnesium hydroxide particles. That is, the aspect ratio (H) is in the range which satisfies the following expression (I) with regard to a product (A×B) of the average secondary particle diameter (A) and the BET specific surface area (B).
0.45·
A·B<H
<1.1
·A·B
(I)
The range of the aspect ratio (H) is preferably a range which satisfies
Ando Taro
Hiraishi Wataru
Oishi Shunji
Yoshii Makoto
Kyowa Chemical Industry Co. Ltd.
Medley Margaret
Sherman & Shalloway
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