JPS6125667B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing alkali-resistant glass fibers for reinforcing cement and calcium silicate materials, particularly alkali-resistant glass fibers that can withstand high temperature curing. Glass fiber has excellent tensile strength and high elastic modulus, so it is used as FRP (Fiber Reinforced
It is currently widely used as a suitable reinforcing agent for the production of fiber-reinforced composite materials such as plastics. In particular, in recent years there has been a strong demand for higher strength cement products, and research on glass fiber reinforced cement has been considered in various fields. That is _, research and development efforts have been carried out in various fields to develop alkali-resistant glass fibers that are not easily corroded by the alkali components in cement. It is described that glass and glass fibers with excellent alkali resistance can be produced. When these glass fibers are made into fibers, a sizing agent is applied to prevent scratches on the fiber surface and to improve workability in post-processing steps.
Usually, a sizing agent whose main component is vinyl acetate polymer is used. By cutting the obtained glass fiber strands and introducing them into cement paste and curing them at room temperature, the flexural strength is approximately 3 to 5 times higher than that without fibers in the early stage of cement hardening. It is possible to create cement products. However, it is very difficult to maintain the reinforcing effect of glass fibers in cement over a long period of time, and therefore, the durability of the obtained cement products is insufficient when used in the building materials sector. In addition, in order to shorten the curing time when making the above cement products, generally 60 ~
Heat curing is performed at a temperature of 120°C for about 10 to 48 hours. In this case, the high-temperature curing performance of the alkali-resistant glass fiber described above is insufficient, and a rapid deterioration in fiber strength that is not seen when curing at room temperature occurs.
At present, the obtained fiber-reinforced cement products do not even satisfy initial strength. In addition, when creating a glass fiber reinforced calcium silicate product using the glass fiber, the CaO component and
It is necessary to generate calcium silicate crystals by reacting with SiO ₂ component at high temperature, and generally an autoclave curing treatment is performed at a temperature of 150 to 200°C for 6 to 24 hours under saturated steam. Therefore, it is difficult to produce a glass fiber-reinforced calcium silicate product having desired physical properties due to significant deterioration of the glass fibers. As a result of experiments conducted by the present inventors, it has been found that the durability of glass fibers in cement or calcium silicate can be improved by adding a furan resin to the sizing agent. However, when the content ratio of furan resin in the sizing agent is increased, the brittleness of the obtained glass fiber strand increases, and the reinforcing effect as a reinforcing material decreases. For this reason, the strength and physical properties of cement products or calcium silicate products made using the chopped strands of glass fibers are reduced. An object of the present invention is to provide an alkali-resistant glass fiber that has a good reinforcing effect on cement products and can withstand high temperature curing. Another object of the present invention is to provide a method for industrially easily and inexpensively manufacturing alkali-resistant glass fibers that have a good reinforcing effect on cement products and can withstand high temperature curing. In the method of the present invention, when alkali-resistant glass containing ZrO ₂ with an alkali solubility of 4% or less is melt-spun and made into fibers, in the first step, a resin having a film-forming ability is added as a solid content of 40 to 90% by weight. %,
Smoothing, softener 2-50%, silane coupling agent 0
After the glass fibers are bundled into strands by treatment with a sizing agent containing ~15 wt. It is characterized in that two types of sizing agents are sequentially deposited on glass fibers by treatment with sizing agents containing %. Since the glass fiber produced by the method of the present invention uses cement or calcium silicate as a matrix, it is necessary to have alkali resistance. From this point of view, it is necessary to use ZrO ₂ -containing alkali-resistant glass fibers whose alkali solubility is 4% or less. Here, alkali solubility means fiber diameter
The figure shows the degree of weight loss of the glass fiber as a percentage when 2 g of glass fiber of 13±0.2μ was immersed in 100 g of 10% NaOH aqueous solution at 95°C for 1 hour. The glass that can be used in the method of the present invention must contain at least 5 mol% of ZrO ₂ .
In general, the alkali resistance tends to increase with the ZrO ₂ content, so the glass fiber in the method of the present invention
_ZrO2 of 10 mol or more, preferably 11 mol% or more,
Most preferably, the content is 11.5 mol% or more. Glass compositions that satisfy these conditions include, for example, _SiO2 : 50 to 69% and _ZrO2 : 8 to 13.5% by mole.
%, R ₂ O: 10-22%, K ₂ O: 0-7%, R'C: 0
~10%, _B2O2 : 0-5%, _{Al2O2 :} 0-5%, and _R2O represents _Na2O , up to 3% _of which can be replaced with _Li2O . R' ₂ O is an alkali-resistant glass composition such as one representing an alkaline earth metal oxide. Hereinafter, a method for producing glass fiber according to the method of the present invention will be explained. Fiberization of glass follows a conventional method. First, a glass raw material is heated for 3 to 60 hours at 1300 to 1500°C in a mullite crucible to form a homogeneous glass, which is then molded into a rod or marble shape. The glass is introduced into a melt spinning process. The molten glass is drawn out from 200 to 2000 holes (chips) using a melter generally called a platinum alloy bushing, and the glass fibers are made into a monofilament using an apron type or roller type oiling device. A first step sizing agent is applied. Subsequently, the filament is formed into a strand consisting of 20 to 400 filaments using a focusing guide, and then heat treated at a temperature of 60 to 250°C, preferably 80 to 220°C, most preferably 100 to 200°C. Ru. It is best to carry out the heat treatment continuously by passing the strand through a heating cylinder or by bringing it into contact with a heating roller, etc., and then leading it to the second step. It is also possible to use a batch type in which winding and heat treatment are performed. By applying the sizing agent of the second step to the obtained glass fiber strand after the heat treatment using a spray method, a drip method, or an oiling device similar to the first step,
A glass fiber strand can be produced in which the sizing agent according to the present invention is attached in a two-layer structure. In the method of the present invention, the sizing agent in the first step has a solid content of 40 to 90% resin having film-forming ability.
Contains 2-50% by weight of smoothing agent and 0-15% by weight of silane coupling agent. Examples of resins having film-forming ability include polyvinyl acetate, ethylene-vinyl acetate copolymer, acrylic ester polymers such as polyacrylic acid methyl ester and polymethacrylic acid methyl ester, and methacrylic acid methyl ester-styrene copolymer. and copolymers mainly composed of acrylic esters such as methyl methacrylate-butyl methacrylate ester copolymers, styrene-butadiene copolymers, acrylonitrile-butadiene copolymers, polyester resins, epoxy resins, and polyvinyl alcohols. Among these, polymers mainly composed of vinyl acetate and polymers mainly composed of acrylic esters are particularly suitable. The solid content of the resin in the sizing agent in the first step is 40 to 90% by weight, preferably 50 to 90% by weight.
Contains 80% by weight. The smoothing and softening agents used in the first step include paraffin wax, polyethylene oxide, various pace oils such as vegetable oil and mineral oil, and various surfactants such as nonionic surfactants, cationic surfactants, and anionic surfactants. The content is preferably 2 to 50% by weight, particularly 5 to 40% by weight based on the solid content of the sizing agent. Examples of the silane coupling agent used in the first step include aminosilane coupling agents, epoxysilane coupling agents, vinylsilane coupling agents, etc.
The content is preferably less than 15% by weight, particularly 3 to 10% by weight. The sizing agent used in the first step includes the above-mentioned resin having film-forming ability,
In addition to the smoothing/softening agent and the silane coupling agent, a plasticizer, glucose, sucrose, mannose, dextrin, soluble starch, etc. may be contained in an amount of less than 5% by weight based on the solid content. 1st
In the process, the sizing agent is used at a solid content concentration of 3 to 40% by weight, preferably 5 to 30% by weight. The amount of sizing agent attached to glass fiber in the first step is 0.2 to 4.0
The weight percent is preferably 0.5 to 3.0 weight percent. If the amount of adhesion is less than 0.2% by weight, the cohesiveness of the glass fiber strands will be insufficient, and if it exceeds 4.0% by weight, gum build-up at guides, traverses, etc. during spinning will be unfavorable, resulting in poor spinability. After the sizing agent is applied in the first step, the glass fibers bundled into strands are heat-treated to form a film on the surface of the glass fibers. Here, in order to shorten the time required for heat treatment, it is preferable to perform the treatment at as high a temperature as possible, but on the other hand, in order to form a good film, the heat treatment temperature must be 60 to 250°C. Also, the processing time is generally 0.2 seconds ~
It takes about 10 seconds, but in the case of a batch type that is rolled up into a cake shape and heat-treated, it takes 0.5 to 2 hours.
It takes about time. The heat treatment allows the glass fiber strand to maintain high cohesiveness, and has the effect of protecting the surface of the glass fiber strand with a two-layer structure, thereby making it possible to create the alkali-resistant glass fiber according to the present invention. Next, the sizing agent in the second step will be explained in detail. The sizing agent of the second step comprises at least 30% by weight, preferably at least 35% by weight, especially 40-90% by weight,
In particular, it contains 45 to 80% by weight of furan resin as a solid content. Here, the furan resin includes furfuryl alcohol resin, furfural alcohol/furfural cocondensation resin, furfural/ketone cocondensation resin, phenol/furfural cocondensation resin, and the like. The amount of furan resin deposited on the glass fibers in the second step of the method of the present invention is usually 0.5 to 50% by weight, preferably 1.0 to 40% by weight, particularly 3.0 to 30% by weight. The attached furan resin has high alkali resistance and is considered to protect the glass fiber strands when the glass fiber strands are used as a reinforcing material for cement or the like. As a result, high-humidity curing of glass fiber-reinforced cement products and the like is possible, and the resulting products have excellent durability for long-term use. If the solid content of the sizing agent is less than 30% by weight of the furan resin, the resulting glass fiber strands will have low durability, which is not preferred. On the other hand, if the amount of furan resin attached to the glass fibers increases more than necessary, the brittleness of the glass fiber strands increases, which is not preferable. The solid content concentration of the sizing agent in the second step is usually 3 to 70.
% by weight, preferably 5-60% by weight. Although furan resin alone can be used as the sizing agent, it is preferable to use other synthetic resins, synthetic rubbers, etc. in combination, since the method of the present invention can be carried out more smoothly. Incidentally, the furan resin is usually dissolved in furfural, benzene, acetone, trichlene, etc., and an inorganic or organic acid is added thereto as a curing agent to be used as a sizing agent. The synthetic resins and synthetic rubbers used in the second step include acrylic ester polymers such as polymethyl acrylate and polymethyl methacrylate, methyl methacrylate-styrene copolymers, and methyl methacrylate-methacrylic acid. Examples include copolymers mainly composed of acrylic esters such as butyl esters, styrene-bradiene copolymers, and acrylonitrile-butadiene copolymers. The synthetic resin and synthetic rubber are preferably contained in an amount of 10 to 60% by weight, particularly 17 to 53% by weight, based on the solid content of the sizing agent. A plasticizer such as dimethyl phthalate, dibutyl phthalate, or phenols may be added in an amount of 10% by weight or less based on the solid content of the sizing agent. The amount of the sizing agent adhered to the glass fiber in the second step is 0.5 to 50% by weight as furan resin, but when other synthetic resins, synthetic rubbers, etc. are used in combination, the amount adhered to the glass fiber is preferably 0.7 to 80% by weight.
1.0-70% by weight, most preferably 1.5-50% by weight. In the method of the present invention, glass fibers are treated with a sizing agent containing a resin having film-forming ability in the first step, bundled into strands, and then heat-treated to form a film made of the sizing agent on the glass fibers. , and then a second one containing furan resin.
Because these two types of sizing agents are attached to the glass fibers in a two-layered structure, they are not only able to stably form fibers, but also maintain good cohesiveness of the glass fibers while also maintaining the outer layer. It is believed that the furan resin in this case greatly improves the durability of glass fibers in cement or calcium silicate. Therefore, the cement product reinforced with glass fiber according to the present invention can be cured at high temperatures, and the curing time can be shortened. Furthermore, it is a safe material with extremely little decline in material strength during long-term use. The glass fibers obtained by the method of the present invention can be used to produce cement products and calcium silicate products by conventional methods. For example, for cement products, the spray method, premix method, paper-making method, etc. are used, and for calcium silicate products, the paper-making method, press molding method, semi-dry press molding method, etc. are used. The method of the present invention will be explained below with reference to Examples. In the Examples, the characteristics of the glass fiber strands were evaluated according to the following method. 1 Evaluation of convergence A glass fiber strand with a fiber diameter of 13.5±0.2μ (number of filaments = 200) was released from the cake,
A chopped strand was created by cutting it to a length of 12.5 mm using an electric cutter. Here, 5 points are given when there is no occurrence of fluff or strand cracking, 4 points are given when no fluff occurs but some strand cracking occurs, 3 points are given when there is considerable strand cracking, and 3 points are given when there is severe strand cracking and some strand cracking. A score of 2 points was given for cases in which fluffing occurred, and a score of 1 point was given for cases in which a large amount of fluff was generated. The evaluation was repeated 10 times, and the convergence was evaluated based on the average score. 2 Amount of sizing agent adhered The glass cake was dried at 125°C for 8 hours in a hot air circulation dryer. Approximately 30 g of the fiber bundle sample was placed in a porcelain crucible, and then fired at 750°C for 3 hours in an electric furnace to burn off the organic matter and reduce its weight. Find the amount,
Based on this value, the total amount of sizing agent adhered was calculated. (We thought that the Si atoms in the silane coupling agent would remain as SiO ₂ even after the organic matter was removed by firing.)
This converted value was defined as the true sizing agent adhesion amount. Further, the amount of the second step sizing agent adhered was determined from the difference between the total amount of the sizing agent adhered and the amount of the first step sizing agent adhered. 3. Durability of glass fiber in cement Glass fiber strands cut into approximately 10 cm length are fixed in a stainless steel frame. then cement
Apply cement paste obtained by mixing 100 g of cement paste and 35 g of water to a width of about 10 mm in the center of the strand. Leave the obtained sample in a constant temperature and humidity chamber at 80°C and 100% RH for 48 hours. After treatment, the strand is removed from the form and its tensile strength is measured using a tensilon under the conditions that the sample length including the cement paste applied part is 2 cm and the tensile speed is 0.2 cm/min. Number of samples
The tensile strength (Kg/mm ² ) was determined using the average value of 20 pieces. This process is called cement paste application process. 4 Durability of glass fiber in calcium silicate In the above 4, instead of cement paste,
A paste obtained by uniformly stirring and mixing cement, finely powdered silica sand (passed through 325 mesh), and water in a ratio of 60:40:100 parts by weight, respectively, and then proceeding with a gelation reaction of the slurry in a water bath at 95°C for 2 hours. After applying to the glass fiber strand using 20℃, 100%RH
Leave it under these conditions for 24 hours. The obtained sample was then treated in an autoclave at 180° C. for 24 hours under steam. After treatment, the fiber tensile strength (Kg/mm ² ) was determined according to Section 4. This treatment is referred to as a Cical coating treatment. 5 Durability of Glass Fiber Reinforced Cement Products Roving is created by doubling 32 glass fiber strands (consisting of 200 filaments). Next, in order to create a glass fiber reinforced cement product using the roving, 100 parts by weight of Portland cement was mixed with river sand (particle size 1 mmφ).
Below) 85 parts by weight and 45 parts by weight of water are mixed uniformly in a mixer to form a slurry. Spray the slurry onto the wooden formwork (1m x 1m) with a spray gun and at the same time cut the roving with a hand cutter.
Cut into 32mm length and mix into cement slurry. However, the amount of fiber mixed in is 6.5% by weight based on cement. After being left for one day, it was removed from the mold and cured in water at 20°C for 28 days. The bending strength of the glass fiber reinforced cement product obtained here is measured. (Measurement sample: 40 x 100 mm x 10 mm thickness) Next, the cement product was left in hot water adjusted to 60°C for 7 days, and the bending strength was measured after treatment to determine the durability of the glass fiber reinforced cement product. Evaluate. 6 Physical properties of glass fiber reinforced calcium silicate product After uniformly stirring and mixing 54 parts by weight of Portland cement, 36 parts by weight of finely divided silica sand (passed through 360 mesh), and 10 parts by weight of pentonite with 100 parts by weight of water, 95%
Heat to gel for 1.5 hours at °C. Next, chopped strands of glass fiber (12.5 mm long) were added to the gelled slurry in an amount of 10% by weight based on the solid content, mixed uniformly, poured into an acrylic resin mold, and
Leave at room temperature under 100% relative humidity for several days. The mold is then demolded and the molded body is steam-cured in an autoclave at 180°C for 12 hours. The bending strength of the glass fiber reinforced calcium silicate product obtained here was measured. (Measurement sample: 40 x 70 mm x 5 mm thickness) 7 Evaluation of fragility of glass fiber strand A glass fiber strand with a fiber diameter of 13.5 ± 0.2 μ (number of filaments = 200) was released from the cake,
Chopped strands were created by cutting to a length of 25 mm using an electric cutter. The chopped strand is bent so that the radius of curvature is approximately 4 mm, and the score is 5 points if there is no strand cracking or filament breakage, 3 points if there is considerable strand cracking or filament breakage, and 3 points if there is no strand cracking or filament breakage. Significant cases were given 1 point. This evaluation was performed on 100 chopped strands, and the average score was used to evaluate the brittleness of the glass fiber strand. Example 1 Polyvinyl acetate resin (VQ-553; manufactured by Kanebo NSC Co., Ltd.) 65% by weight, cationic surfactant as a smoothing and softening agent (Cation Softener X; manufactured by Onyx Co., Ltd.)
20% by weight, polyethylene glycol (molecular weight
2000) to prepare a water-soluble emulsion solution having a solid content of 5% by weight, 5% by weight of γ-glycidoxypropyltrimethoxysilane (A187; manufactured by Nippon Unica), and 5% by weight of dibutyl phthalate.
This was prepared so that the solid content concentration was 20% by weight.
It was used as a sizing agent in the process. In addition, furan resin (Hitafuran 303; manufactured by Hitachi Chemical Co., Ltd.), polyacrylic acid ester resin (S-1006; manufactured by Toagosei Chemical Industry Co., Ltd.), curing agent (A3; manufactured by Hitachi Chemical Co., Ltd.), and dibutyl phthalate as a plasticizer. A trichlene solution having a solid content of various composition ratios was prepared using each of these, and the solution was adjusted to a solid content concentration of 25% by weight, and used as a sizing agent in the second step. On the other hand, in mol% SiO ₂ 67%, ZrO ₂ 12%, Na ₂ O 15.5
%, K ₂ O 3%, Li ₂ O 2%, Al ₂ O ₃ 0.5% in a mullite crucible (inner volume 200 l).
Created by melting at 1350℃ for 48 hours, 0.8mm
It was molded into a rod shape with a length of φ x 25 mm. Using a platinum-rhodium (80/20% ratio) alloy bushing having 200 chips with an inner diameter of 2 mmφ, it was extruded into a fiber at 1290°C and wound at a speed of 300 m/min. (fiber diameter
(13.5 μ, alkaline solubility 1.7%) Here, the sizing agent of the first step was applied using an apron type oiling device, and then the fiber bundle was formed into one fiber bundle using a convergence guide, and then the length was heated at 180℃. It was passed through a 3m heating tube and subjected to heat treatment. Subsequently, a sizing agent for the second step was applied using a roller type oiling device. The obtained glass fiber strand was wound into a cake shape, and the cake was dried in a hot air circulating dryer at 130° C. for 12 hours. Various properties were evaluated and the results shown in Table 1 were obtained.

[Table] From Table 1, it can be seen that by adding furan resin to the sizing agent in the second step, the durability of glass fibers in cement is improved, and as a result, the durability of cement products is also improved. However, adding too much will increase the brittleness of the glass fiber strands,
The reinforcing effect as a reinforcing material tends to decrease.
Therefore, the size material in the second step preferably contains at least 30% by weight, preferably at least 35% by weight of furan resin as a solid content. In addition, considering the brittleness of the glass fiber strands, it is preferable to add other synthetic resins or synthetic rubber to the sizing agent.
From this point of view, the sizing agent contains furan resin as a solid content.
The content is preferably 40 to 90% by weight, especially 45 to 80% by weight. Example 2 Polyacrylic ester resin (32A-514; manufactured by Kanebo NSC), polyvinyl acetate resin (V-4250;
Kanebo NSC), N-β-(aminoethyl)-γ
-Aminopropyltrimethoxysilane (A-
1100; manufactured by Nippon Unica Co., Ltd.), a cationic surfactant (Cation Softener Aqueous emulsion solutions with solid content in various composition ratios were prepared using
It was adjusted to % by weight and used as a first step sizing agent. In addition, a 50% by weight acetone solution of furan resin (Hitafuran 303; manufactured by Hitachi Chemical Co., Ltd.) was prepared, and a nonionic active surfactant (POE-nonyl phenyl ether (n = 16)) An aqueous emulsion solution was prepared by dropping an aqueous solution until the solid content of the nonionic active surfactant was 20% by weight based on the furan resin. Subsequently, using the aqueous furan resin emulsion, 73% by weight of furan resin and polyacrylic acid ester resin (32A-514;
Kanebo NSC) 23.2% by weight, dibutyl phthalate
An aqueous emulsion solution having a solid content of 3.0% by weight and 0.8% by weight of a curing agent (A3; manufactured by Hitachi Chemical Co., Ltd.) was prepared, and this was adjusted to a solid content concentration of 40%. Used as a sizing agent. The sizing agent in the first step was applied according to Example 1, collected into a strand shape using a focusing guide, and then heated at 200°C.
After heat treatment, the strand is passed through a temperature-controlled heating tube (same as in Example 1), and then guided to a roller oiling device in which a sizing agent for the second step is dripped. was attached. The obtained glass cake was dried at 135° C. for 10 hours in a hot air circulation dryer. Various properties were evaluated, and the results shown in Table 2 were obtained.

【table】

[Table] From Table 2, the cohesiveness of glass fibers obtained with increasing content of resin with film-forming ability (total amount of polyacrylic acid ester and polyvinyl acetate) relative to the solid content of the sizing agent in the first step It can be seen that the flexural strength of the calcium silicate plate is improved, and the deterioration of the glass fibers in calcium silicate is reduced, resulting in an increase in the bending strength of the calcium silicate plate. However, it has been found that if the content of the resin is too large, the cohesiveness of the glass fiber strands will be reduced and the deterioration of the glass fibers will also be increased. Therefore, the content of the sizing agent based on the solid content of the sizing agent in the first step is 40 to 90% by weight,
Preferably it is 50 to 80% by weight. Example 3 45% by weight of polyvinyl acetate resin (VQ-553; manufactured by Kanebo NSC) and polyacrylic acid ester resin (obtained by emulsion polymerization of 1 mol of methyl methacrylate, 1 mol of butyl acrylate and 0.5 mol of styrene according to a conventional method) Polyvinyl alcohol (SL-25; manufactured by Kanebo NSC) 7% by weight, Cation Softener-X (manufactured by Onyx) 7% by weight
Weight%, polyethylene glycol (molecular weight 1000)
An aqueous emulsion solution having a solid content of 6% by weight and 5% by weight of γ-aminopropyltriethoxysilane (A1100; manufactured by Nippon Unica Co., Ltd.) was prepared;
This was adjusted to have a solid content concentration of 22% by weight. In addition, furan resin (Hitafuran 303; manufactured by Hitachi Chemical Co., Ltd.) 78% by weight, polyacrylic acid ester resin (S-1006; manufactured by Toagosei Chemical Industry Co., Ltd.) 18.2% by weight, dibutyl phthalate 3.0% by weight, curing agent (A3; A sizing agent for the second step was prepared by preparing an acetone solution having a solid content of 0.8% by weight (manufactured by Hitachi Chemical Co., Ltd.) and increasing the solid content to 20% by weight. When spinning and forming fibers according to Example 1,
The temperature of the heating tube was varied. The obtained glass cake was dried and various properties were evaluated, and the results shown in Table 3 were obtained. The results are shown in Table 3.

【table】

[Table] From Table 3, it can be seen that there is a correlation between the heat treatment temperature after the sizing agent is attached in the first step and the cohesiveness and alkali resistance performance of the obtained glass fibers. In addition, the heat treatment temperature is 60 to 250°C, preferably 80 to 220°C,
It turns out that 100 to 200℃ is especially good.

Claims (1)

[Claims] 1. When melt-spinning alkali-resistant glass containing ZrO ₂ with an alkali solubility of 4% or less to form fibers, in the first step, the solid content of resin with film-forming ability is 40 to 90% by weight, smooth and flexible. Agent 2
~50% by weight, silane coupling agent 0-15% by weight
After the glass fibers are treated with a sizing agent containing a sizing agent to bundle them into strands, they are heat-treated at a temperature of 60 to 250°C, and then in a second step, a size containing at least 30% by weight of furan resin as a solid content is formed. A method for producing alkali-resistant glass fibers that can withstand high temperature curing, the method comprising sequentially attaching two types of sizing agents to glass fibers by treating them with a sizing agent.