Exploring the Properties of Fiberglass Reinforced Plastic (FRP)


Fiberglass boasts a low density, excellent dielectric insulation properties, superior thermal insulation, as well as absorbency and thermal expansion characteristics.

I. Density

The density of fiberglass ranges from 1.5 to 2.0, merely a quarter to a fifth of that of ordinary carbon steel, and about two-thirds lighter than lightweight aluminum. Despite its lightness, its mechanical strength is impressively high.

In some respects, it can even approach the level of ordinary carbon steel. For instance, certain epoxy fiberglass materials achieve tensile, bending, and compressive strengths exceeding 400MPa. When considering relative strength, fiberglass not only significantly outperforms ordinary carbon steel, but it can also match and even surpass the level of some special alloy steels.

A comparison of the density, tensile strength, and relative strength of fiberglass and several metals is presented in Table 1.

Table 1

Material NamesDensity Tensile Strength
Specific Strength 
Advanced Alloy Steel8.01280160
A3 Steel78540050
LY12 Aluminum Alloy2.8420160
Cast Iron7.424032
Epoxy Fiberglass1.73500280
Polyester Fiberglass1.8290160
Phenolic Fiberglass1.8290160

Strength-to-Weight Ratio: This refers to the tensile strength per unit of density, that is, the ratio of a material’s tensile strength to its density, indicating the extent of its lightweight and high-strength properties.

II. Electrical Properties

Fiberglass has excellent electrical insulation properties, making it suitable as an insulation component in instruments, motors, and electrical appliances. It maintains good dielectric properties even under high-frequency conditions. Replacing paper and cotton cloth with fiberglass cloth in insulation materials can enhance the insulation grade of these materials.

Using the same resin, it can improve at least one grade. Fiberglass constitutes one-third to one-half of the total amount of insulation materials. In some large motors, such as 125,000 KW motors, hundreds of kilograms of fiberglass are used as insulation material.

Furthermore, fiberglass is not affected by electromagnetism and has good microwave transparency. Table 2 introduces the dielectric properties of a few types of fiberglass.

Table 2

Types of FiberglassDielectric ConstantDielectric Loss Tangent
Styrene Butadiene Fiberglass3.5~4.0(3.5~5.0)*10-3
DAP Fiberglass4.0~4.8(0.9~105)*10-2
Polybutadiene Fiberglass3.54.0(4.5~5.5)*10-3
307 Polyvinyl Acetate Fiberglass4.0~4.8(0.9~1.5)*10-3
6101 Epoxy Fiberglass 4.7~5.2(1.7~2.5)*10-2

III. Thermal Properties

Fiberglass exhibits excellent thermal characteristics, with a specific heat capacity 2-3 times greater than that of metals, and a lower thermal conductivity, which is only 1/100 to 1/1000 of that of metallic materials.

Additionally, certain varieties of fiberglass possess remarkable resistance to instantaneous high temperatures. For instance, phenolic-based high silica cloth fiberglass forms a carbonized layer under extremely high temperatures, effectively protecting rockets, missiles, and spacecraft from the 5000 to 10000 K high temperatures and high-speed airflows they must endure while passing through the atmosphere. Table 3 outlines the thermal properties of a few materials.

Table 3

MaterialsSpecific Heat
[KJ (Kg·K)] 
Thermal Conductivity
Linear Expansion Coefficient
Polyvinyl Castings0.170.176~13

As illustrated by Table 3, fiberglass possesses exceptional thermal insulation properties, an advantage metallic materials simply cannot compete with.

IV. Aging Resistance

All materials confront the issue of aging, and fiberglass is no exception; it merely varies in rate and severity. Under exposure to atmospheric conditions, humid heat, water immersion, and corrosive media, the performance of fiberglass declines. Long-term use can result in diminished gloss, color changes, resin shedding, fiber exposure, and delamination, among other phenomena.

However, with advancements in science and technology, necessary anti-aging measures can be taken to improve its performance and extend the product’s lifespan.

For instance, when fiberglass was subjected to natural aging tests in Harbin, the least decline was observed in tensile strength of the panel, less than 20%; followed by bending strength, generally not exceeding 30%; the greatest decrease was seen in compressive strength, which also showed the most fluctuation, usually between 25% and 30%. Refer to Table 4 below.

Table 4

Mechanical PropertiesTypes of Fiberglass Initial Strength
Strength after 10 Years Strength after 10 Years 
Residual Strength
Strength Decline Rate

Tensile Strength
Bending StrengthEpoxy330.06260.6821
Compressive StrengthEpoxy216.58160.2326
Modulus of CurvatureEpoxy1.73*1041.11*10436

Moreover, exposure to outdoor elements such as wind, rain, and sunlight can lead to the shedding of the resin layer on fiberglass surfaces. Regular maintenance is necessary to prevent this.

V. Long-term Heat Resistance and Flame Resistance

The heat resistance and flame resistance of fiberglass depends on the type of resin used. The continuous operating temperature cannot exceed the heat distortion temperature of the resin. Commonly used epoxy and polyester fiberglass are flammable. For structures that require fire resistance, flame-retardant resins or retardants should be used. Therefore, caution is necessary when using fiberglass.

Typically, fiberglass cannot be used for extended periods under high temperatures. For instance, the strength of polyester fiberglass begins to decline at temperatures above 40°C to 45°C, and that of epoxy fiberglass starts to decrease above 60°C.

In recent years, high-temperature-resistant fiberglass varieties have emerged, such as cycloaliphatic epoxy fiberglass and polyimide fiberglass. However, their long-term operating temperature is only within 200-300°C, which is significantly lower than the long-term operating temperature of metals.

Given these five aspects of physical properties, it’s clear that fiberglass differs from materials like metals and ceramics. Therefore, to maximize its advantages, it needs to be used properly. For example, fiberglass has excellent low-temperature performance as its strength doesn’t decrease.

Thus, even when outdoor temperatures drop to -45°C to -50°C in northern winters, fiberglass doesn’t become brittle. Outdoor structures like cooling towers and rain shelters remain safe to use in northern winters.

Conversely, in high-temperature environments, specific resins and formulas are needed for fiberglass. For continuous use at 100°C, a high-temperature-resistant formula and specific process conditions for molding are required. Otherwise, fiberglass can be damaged under continuous operation at temperatures above 100°C.

VI. Chemical Properties of Fiberglass

The primary chemical property of fiberglass is its outstanding corrosion resistance. It doesn’t rust or corrode like metal materials or rot like wood. It’s almost immune to erosion by mediums like water and oil. It can substitute stainless steel in chemical factories for the manufacturing of tanks, pipes, pumps, valves, etc.

Not only does it have a long service life, but it also doesn’t require protective measures against corrosion, rust, or insects, reducing maintenance costs. Fiberglass is widely used for its corrosion resistance. In some major industrial countries, over 13% of corrosion-resistant products are made from fiberglass, with usage increasing annually. It’s also commonly used in our country, mostly for lining metal equipment to protect the metal.

The corrosion resistance of fiberglass mainly depends on the resin used. Although the resin used for fiberglass is corrosion-resistant, if applied directly onto metal surfaces, it can cause severe cracking and won’t prevent leaks or protect the metal.

Adding a certain amount of glass fiber to the resin can transform potential severe cracking into numerous minor cracks. The chance of these small cracks forming a continuous crack is minimal, and they can also serve to halt further cracking. This helps prevent the penetration and corrosion by chemical solutions.

Fiberglass is not only stable against a variety of low-concentration acids, alkalis, salts, and solvents but also resistant to atmospheric, seawater, and microbial effects.

However, for different corrosive mediums, the appropriate resin and glass fiber and their products should be selected. The use of fiberglass for anti-corrosion has become increasingly popular in recent years, demonstrating the advantages of low anti-corrosion investment, long service life, and substantial savings in stainless steel materials, leading to significant economic benefits.

Typically, the corrosion resistance of fiberglass is evaluated by measuring its mass change when placed in different corrosive mediums. A smaller mass change indicates better corrosion resistance, and a larger mass change indicates poorer corrosion resistance.

Table 5 lists the mass ratios of several types of fiberglass in different concentrations of acid and alkali solutions, while Table 6 shows the retention rate of the bending strength of polyester fiberglass in acid, alkali, and other mediums.

Table 5

MediumMedium ConcentrationAge 307 Polyester Fiberglass Styrene FiberglassFuran-Epoxy Fiberglass 634 Epoxy 193 Polyester FiberglassDAP Fiberglass197 Polyester FiberglassPolybutadiene Fiberglass
Sodium Hydroxide5.2%366-5.426+0.5091+0.7122+10.85+1.023+9744+0.531
Sodium Hydroxide29.2%366-17.21+0.103-0.49+12.07+2.301+0.522+0.174
Sodium Hydroxide48.3%386-8.85-1.432-1.28-0.604+8.34-1.84-1.78
Sulfuric Acid5.6%365+0.472-0.155+4.74-0.0371-0.012-0.212
Sulfuric Acid28.8%365+5.855+1.199+17.38+0.032+1.795+1.217+4.338
Sulfuric Acid48.3%365+1.565+0.115+6.193+0.321+0.434+0.339+0.428
Hydrochloric Acid4.7%365-0.6762-3.350+3.987+0.044-0.7414-2.083
Hydrochloric Acid15.2%365-6.254-6.74+0.7428+3.878-8.371-7.211

Table 6

Resin Grade 191#189#196#197#198#199#
Original Strength (MPa) 259267278295337290
Steel Hydroxide5%8.755.9612.1020.306.2427.10
Steel Hydroxide30%22.60
Sulfuric Acid5%50.655.545.543.447.069.8
Sulfuric Acid30%58.545.138.640.064.5
Hydrochloric Acid5%70.555.368.546.849.269.8
Hydrochloric Acid30%50.645.245.039.728.171.0
Nitric Acid5%69.850.359.556.252.275.0
Hydrochloric Acid30%50.645.245.039.728.171.0
Nitric Acid5%69.850.359.556.252.275.0
Nitric Acid30%5740.25339.636.664.6
Benzene 21.924.42128.855.288
Transformer Oil 81.57475.166.569.484.8
Gasoline 85.575.774.879.674.089.6

* The soaking time is one year.

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Founder of MachineMFG

As the founder of MachineMFG, I have dedicated over a decade of my career to the metalworking industry. My extensive experience has allowed me to become an expert in the fields of sheet metal fabrication, machining, mechanical engineering, and machine tools for metals. I am constantly thinking, reading, and writing about these subjects, constantly striving to stay at the forefront of my field. Let my knowledge and expertise be an asset to your business.

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