Nonhygroscopic thermally stable aluminum hydroxide

Details Nonhygroscopic thermally stable aluminum hydroxide Nonhygroscopic thermally stable aluminum hydroxide

1999-05-21

2001-08-28

Le, Hoa T. (Department: 1773)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Including a second component containing structurally defined...

C106S018120, C106S018260, C428S329000, C428S405000

Reexamination Certificate

active

06280839

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a nonhygroscopic thermally stable aluminum hydroxide and its application in thermally stable fire retardant laminates for printed circuit boards.
2. Background Art
Printed circuit board laminate types are defined by the American NEMA (National Electrical Manufacturers Association) standard; the terminology is accepted world-wide. Generally, laminates are categorized according to the type of reinforcement used, i.e., cellulose paper or glass. Typically, the type FR-2 uses paper only, and CEM-1 uses paper and woven glass, while type CEM-3 contains both woven and non-woven glass. The type FR-4 contains woven glass only.
To achieve the required V0 level of fire retardancy according to American Underwriters Laboratories' standard UL-94, it is necessary to add either fire retardant chemicals to the polymer system or build halogens or phosphorous into the backbone of the polymer itself. On combustion, these additives help to extinguish the fire. However, in the process of doing so, they produce toxic and corrosive gases. In the case of the combustion of FR-2 laminates, which contain phosphorous compounds, phosphoric acid is formed. CEM-1, CEM-3 and FR4 laminates, which contain brominated epoxy resin, produce corrosive and toxic hydrogen bromide on combustion.
It is well known in the art that aluminum hydroxide may be used to improve the fire retardancy of synthetic polymer systems based on, for example, epoxy, polyester, and vinyl ester, because the polymers decompose in the same temperature range as the aluminum hydroxide. However, the thermal stability of the gibbsite form of aluminum hydroxide [Al(OH)
3
or sometimes represented as Al
2
O
3
.3H
2
O] is insufficient at the temperatures used to solder components to a printed circuit board laminate. This can result in blistering of the laminate, thereby rendering it unusable.
It is known that when gibbsite type aluminum hydroxide is heat treated in air, it is partially converted to the monohydrate form, boehmite (AlOOH or Al
2
O
3
.H
2
O), which improves thermal stability but to the detriment of fire retardancy.
Japanese Published Patent Application No. A 60/203438 describes a CEM-3 laminate containing heat treated gibbsite aluminum hydroxide which has improved thermal stability over standard aluminum trihydroxide, but which does not give the required level of fire retardancy so that brominated epoxy resin has to be used to achieve the UL 94 V0 rating. In such circumstances, and in the absence of superior fire properties, other inorganic materials, such as, talc or clay, could also be used.
British Published Patent Application No. 9700708.2 describes superior CEM-3 laminates having good thermal stability, being halogen/phosphorus free and meeting the UL 94 V0 requirement by introducing a thermally stable aluminum hydroxide to the laminate.
However, it has been found that these thermally stable aluminum hydroxides have a tendency to absorb moisture. This moisture significantly reduces solder resistance of the laminates. The thermally stable aluminum hydroxide needs accordingly and optionally to be applied more or less immediately after production. It has been found that the moisture may become firmly bound on the surface with the effect that the techniques commonly applied to remove surface moisture, such as, prior heat treatment at 110° C. or azeotropic removal with the acetone used in the laminating system, may not satisfactorily work.
BROAD DESCRIPTION OF THE INVENTION
An object of the invention is to further improve the application and characteristics of the thermally stable aluminum hydroxide towards a significant lower tendency to absorb moisture. A further object of the invention is to produce a CEM-3 laminate which has good thermal stability, is halogen/phosphorus free, meets the UL 94 V0 requirement and, finally, shows excellent solder and moisture resistance of the laminate produced. These objects have been achieved with the nonhygroscopic thermally stable aluminum hydroxide of the invention, by a method for its preparation, the laminate containing the new and inventive nonhygroscopic thermally stable aluminum hydroxide and with a method for producing the laminate.
The nonhygroscopic thermally stable aluminum hydroxide is characterized in that it has the molecular formula Al
2
O
3
.nH
2
O, wherein n has a value of >2.6 to <2.9, and in that it is surface treated with a silane.
The basis for the nonhygroscopic thermally stable aluminum hydroxide according to the invention is the thermally stable aluminum hydroxide as described in detail in British Published Patent Application No. 9700708.2. This thermally stable aluminum hydroxide can, as described therein, be created by a size reduction of an aluminum hydroxide agglomerate with an average particle size D50% of 40 to 80 &mgr;m, preferably of 50 to 70 &mgr;m, crystallized out from a typical Bayer process—sodium aluminate solution.
Any size reduction technique, e.g., ball milling, which separates the crystals present within coarser agglomerates with little or no gross fracture of the single crystals can be applied. A simultaneous broadening of the particle size distribution due to attrition and the absence of crystal breakage by gross fracture improves the processability of the thermally stable aluminum hydroxide in the respective resin system.
The subsequent thermal stabilization can be achieved by simply heating the material recovered from the size reduction treatment at a temperature and during a time sufficient to reduce the water loss on ignition from 34.5 weight percent (n=3) to a level corresponding to the n value as mentioned above.
In the molecular formula, Al
2
O
3
.nH
2
O, n preferably has a value of 2.7 to 2.8.
A very essential point is the particle size distribution which combines a relatively fine average size with a breadth of particle sizes. This improves the dispersion of the thermally stable aluminum hydroxide in the resin while at the same time minimizing the tendency of the coarser particles to sediment out during processing and avoiding any filtering effects by the non-woven glass.
An optimum particle size distribution allows minimum viscosity of the resin/filler mix at a filling level sufficiently high to achieve the desired flame retardant properties of the laminate without the need to add any other flame retardants.
The average particle size D50% of the nonhygroscopic thermally stable aluminum hydroxide is 5 to 10 &mgr;m. The breadth of the particle size distribution is indicated by the D10% range, i.e., 10 percent by weight of the particles are smaller than 0.5 to 1.5&mgr;m, and the D90% range, i.e., 90 percent by weight of the particles are less than 20 to 35 &mgr;m in size.
The silane treatment is, as a rule, achieved by first dispersing the thermally stable aluminum hydroxide in a diluent commonly used in the production of laminates for printed circuit boards, preferably a ketone and, most preferably, acetone.
Before the silane is added, it is advantageous to vigorously mix the dispersion with an equipment allowing the application of heavy shearing forces to the mixture like a shearing head mixer or the like.
Thereafter, a suitable silane, preferably in liquid form, is added in such an amount that the content onto the thermally stable aluminum hydroxide ranges between 0.1 and 2 weight percent.
Suitable silanes are commercially available, e.g., from Hüls under the trade name Dynasilan® (Hüls-brochure “Anwendung von organofunktionellen Silanen, Dynasilan®”of October, 1989). The preferred silane is, as a rule, an aminoalkylsilane, an epoxyalkylsilane or a vinylsilane. Particularly well suited to epoxy applications is an aminoalkylsilane.
The resulting nonhygroscopic thermally stable aluminum hydroxide has, after the silane treatment, a hydrophobic surface free of any adsorbed moisture.
As outlined in British Published Patent Application No. 9700708.2, the CEM-3 type laminates are, as a rule, constructed from two layers of woven fiberglass on the ou

Affiliated with

Also associated with

Rating

Nonhygroscopic thermally stable aluminum hydroxide does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Nonhygroscopic thermally stable aluminum hydroxide, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Nonhygroscopic thermally stable aluminum hydroxide will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2466034