Glass fiber Reinforcements

Glass is an amorphous material that consist of a silica (SiO2) backbone and several ox-ides. The main ingredients of glass fibers are silica sand, limestone, boron oxide and small quantities of other ingredients. Glass fiber production process consists of batching, melting, fiberization, coating and drying/packaging. Batching mean weighing and mixing raw materials. Composition defines properties of the fibers. Mixture is melted in a high temperature furnace. In fiberization molten glass is extruded into fibers through bushings or spinneret containing several holes. Extruded streams of molten glass are drawn into filaments and the filaments are cooled with water. Fibers are coated immediately after manufacturing to prevent surface scratches during spooling and weaving. After mechan-ical treatments coating can be removed and replaced with sizing that improves the fiber-matrix bonding. Sized filaments are gathered without twisting into a bundle which forms a strand. Strand is wound into a package which can be further processed into roving, yarn or chopped fiber. [9]

Reinforcing fibers used in composite insulators produced by pultrusion are typically in the form of continuous single-end rovings. Single-end rovings are produced by pulling glass fibers directly from the bushings and winding them into a roving package. Multi-end rovings consists of multiple untwisted filaments or strands bonded together with siz-ing. Multi-end rovings can have poor strand integrity, which is why they may not process efficiently in pultrusion. [12] Also, chopped fiberglass mats, veils and nonwovens can be incorporated. They can be used to provide smooth finished surface and improve corro-sion resistance. [9,12]

3.1.1 Fiber type

Composite insulators can be manufactured with different glass fiber types. There are several types of glass fibers with different compositions that affect to their chemical and physical properties [11]. Glass fiber types are named according to their specific proper-ties [12]. They are presented in Table 1.

Table 1. Glass fiber types (adapted from [12]).

Glass fibers can be divided in two categories, low-cost general-purpose fibers and pre-mium special-purpose fibers. Over 90% of glass fibers are general-purpose E-glass.

They offer strength at low cost. E-glass contains 4 to 6 wt% of boron oxide. Also, a boron-free E-glass variant is on the market nowadays. It is known by the trademark Advantex.

It is environmentally friendlier option because it does not emit boron. It has also seven times higher resistance to acids than incumbent boron containing E-glass. [12]

ECR-glass refers to corrosion resistant E-glass. ECR-glass is special-purpose glass fiber with high corrosion resistance. It has improved long-term acid resistance and short-term alkali resistance [12]. ECR-glass is boron free, but it does not necessarily meet the re-quirements and fall into the ASTM standard classification of E-glass [13]. ECR-glass was developed from E-glass by replacing boron oxide for titanium oxide. The addition of tita-nium and zinc oxides increases the Si-content and Si/O ratio, which have showed con-siderable improvement in resistance to acids and corrosion. [14] The raised amount of silica as a network former and addition of titanium oxide also increases the strength of the fibers [15]. Compositions of E-glass, boron free E-glass and ECR-glass are pre-sented in Table 2.

Letter designation Property

E, electrical Low electrical conductivity

S, strength High strength

C, chemical High chemical resistance

M, modulus High stiffness

A, alkali High alkali or soda lime glass

D, dielectric Low dielectric constant

Table 2. Compositions of different glass types (adapted from [12]).

Other special-purpose glass fiber types in the market are S- and R-glass. Their boron- and alkali-free compositions gives them high strength. At room temperature their strength is 10-15% higher than E-glass and they have the advantage of withstanding higher tem-peratures compared to E-glass. D-glass have low dielectric constant, which is achieved due to its very high, 20-26%, boron oxide levels. Pure silica or quartz fibers have in-creased silica content and they can be used in ultra-high temperatures. [12] Typical prop-erties of different glass fibers are compared in Table 3.

Table 3. Properties of glass fibers (adapted from [12]).

Property

3.1.2 Seed count

In addition to glass fiber type, the seed count is another factor that significantly affects the insulating properties. Seeds are small gaseous inclusions trapped in the glass fibers.

Schematic picture to illustrate the seeds is presented in Figure 3. Seeds are caused due to formation of gas from the raw ingredients in the glass and by the melting conditions in the furnace. Seeds in the molten glass can range from 10-400 microns in diameter and they can create hollows within the filaments from 1-20 microns in diameter. Larger diam-eter seeds cause longer hollows. [13] Seeds have remarkable effect on electrical prop-erties of composites because they are the sites for partial electrical discharges and re-duce the resistance to electrical energy [13,16]. High seed count was a problem espe-cially in the original boron-free glass fibers, which made them unsuitable for electrical insulators. Addition of TiO2 results in lower seed count in the boron-free glasses.

Figure 3. Schematic cross-sectional view of glass fiber reinforced composite with seeds in the fibers.

3.1.3 Sizing

Sizing is a surface coating applied to fibers during their manufacture. Fiber sizing signif-icantly influences to fiber and composite processability, performance and interphase ad-hesion. Although the sizing has so important role, the technology involved is confidential and not much information on the formulations used is revealed by glass fiber producers.

[17]

Sizing is dilute water-based emulsion or dispersion which can contain even ten or more components. Primary components are coupling and film former agents. Coupling agents are almost always organofunctional silane compounds and they are said to be the most important chemicals used in glass fiber and composite industry. They are used to im-prove the adhesion and bonding of the fiber to the resin and imim-prove the interphase strength. [17] Organofunctional silane compounds create hydrophobic surface by dis-placing adsorbed water on the glass surface. They improve the wetting of the fibers and

form a strong interfacial bond between the resin and fiber. [18] The silanol group (-SiOH) of the coupling agent reacts with the silanols on the surface of the glass fiber, and other resin-reactive end group connects the resin directly to the fiber [19].

Film former agents are water based polymeric materials that are selected to be well-matched to the resin, because the compatibility can influence to reinforcing properties of the composite. Frequently used film formers are polyesters, polyvinyl acetates, epoxies, polyurethanes and polyolefins. Purpose of the film formers is to protect the fibers during manufacturing process preventing the loss of chemicals during winding and other me-chanical treatments. Sizing is applied before gathering the glass filaments to the strand, because the filaments are also very abrasive to each other. In addition, the film formers keep the filaments together in a strand. [17,18]

Sizing can also include lubricant agents, antistatic agents, surfactants, antioxidizing agents, plastifying agents and biocides. Lubricating agents are used to prevent abrasion while glass fibers travel through metal or ceramic guides, bars and pipes. Surfactants lower the surface tension and improve the wetting of surface of the fiber. Antistatic agents are used to prevent glass fibers natural charging by static electricity, because charged fibers can cause problems during processing and manufacturing. Antioxidizing agents are used to prevent the oxidation of organic compounds of the sizing. Oxidation of these chemicals can cause discoloration and degradation of film formers and lubricants. Plas-tifying agents are used to alter properties of the film formers. PlasPlas-tifying agents are added to film former polymers to make them softer and more flexible and processable. Biocides are used to prevent and stop growth of microbials, which can damage the size. [18]