The present invention relates to a method of treating glass fibers by which chemical resistance is imparted to the glass fibers.
Recently, long glass fibers or short glass fibers (hereinafter referred to as glass fibers) are incorporated into mortars, concretes and the like to increase bending strength of cement products. According as diameter of the glass fibers decreases, tensile strength of the glass fibers increases. Use of glass fibers having high tensile strength affords cement products having high bending strength, so that it is recommended to use glass fibers having small diameter.
As major components of the glass fibers are silicon dioxide SiO.sub.2, calcium oxide CaO, Magnesium oxide MgO, diboron trioxide B.sub.2 O.sub.3, sodium oxide Na.sub.2 O, potassium oxide K.sub.2 O, aluminum oxide Al.sub.2 O.sub.3 and lithium oxide Li.sub.2 O, the glass fibers have acid resistance at broad ranges of pH2-6. However, no glass fibers have alkali resistance at alkaline regions of pH9-14, particularly pH12-14. Alkaline earth metal oxides such as calcium oxide and magnesium oxide as well as alkali metal oxides such as sodium oxide, potassium oxide and lithium oxide are leached out from the glass fibers in the form of Ca(OH).sub.2, Mg(OH).sub.2, NaOH, KOH and LiOH respectively. Therefore, cement products having good bending strength can not be obtained in the prior art. In particular, when glass fibers having small diameter are used, deterioration of the glass fibers is remarkable because large surface areas of the glass fibers contact with alkali components present in cement.
It has been proposed that glass fibers for use in reinforcing cements should be alkali resistant and it has been proposed to use glass composition containing 5-25% by weight of zirconium oxide ZrO.sub.2. However, when glass composition containing less than 10% by weight of zirconium oxide are used, glass fibers having sufficient alkali resistance can not be obtained. When glass composition containing 10-25% by weight of zirconium oxide are used, glass fibers having slight alkali resistance can be obtained. However, it is difficult to fuse the glass composition on producing glass fibers. Also, if fused, a temperature of liquid phase elevates, so that a good spinning efficiency can not be obtained. Further, glass fibers can not be formed efficiently. Furthermore, it is necessary to use a heavy-duty equipment. To overcome the drawbacks accompanied with more than 10% by weight of zirconium oxide, it has been proposed that alkali metal oxides such as Na.sub.2 O, K.sub.2 O and Li.sub.2 O (hereinafter referred to as R.sub.2 O) are used as flux such that the total amount of R.sub.2 O is 14-18% by weight. Alternatively, it has been proposed that alkaline earth metal oxides such as CaO, MgO, BaO and/or boron family element oxides such as B.sub.2 O.sub.3 are added to decrease a melting point of the glass composition. However, when R.sub.2 O is added in an amount of 14-18% by weight, weathering occurs in an atmosphere or an aqueous solution. Also, R.sub.2 O is leached out from the glass fibers by alkali components present in cement, so that strength of the glass fibers decreases. For example, after one year, bending strength of cement products decreases to 60% of cement products immediately after preparation. Therefore, it has been hesitated to use the glass composition containing 14-18% by weight of R.sub.2 O. On the other hand, when alkaline earth metal oxides and the like are incorporated into the glass composition, acid resistance of glass fibers therefrom decreases remarkably.