SiO₂ Glass

While silica alone can be used to form glass, it has some undesirable features, which make it unsuitable for use in making glass containers.

Advantages

• Inexpensive Raw Materials
• Single Component
• Low Expansion
• Excellent Durability

Disadvantages

• Hard to Fine
• Hard to Melt
• Hard to Form

Fining refers to the removal of the gas bubbles (seeds) from molten glass. Expansion refers to the "Thermal Expansion Coefficient",which is the relative change in volume per unit change in temperature. Chemical durability is the resistance of glass to be altered or harmed by solvents or products.

Na₂O - SiO₂ Glass

In order to overcome some of the disadvantages of a pure silica glass, other oxides are added. Among these are the fluxing agents, the most common of which is soda, (Na₂O). As with a pure silica glass, a soda-silica glass has advantages and disadvantages:

Advantages

• Low Melting
• Low Viscosity

Disadvantages

• Expensive Raw Materials
• High Expansion
• Poor Durability

Chief among the disadvantages is its poor chemical durability. Sodium silicate glass, in the proper combination, is actually soluble in water and is aptly called water glass.

A network modifier, Na₂O in this case, opens up the silica network by breaking some of the Si-O bonds in the network, which leads to low viscosity and poor chemical durability.

Na₂O - CaO - SiO₂ Glass

In order to overcome the poor chemical durability, another class of oxide is added to strengthen and rebuild the network. These are known as network stabilizers. The most common network stabilizer is calcia (CaO), the basic element in limestone. The results of adding CaO are as follows:

Advantages

• Low Expansion
• Good Durability

Disadvantages

• High Melting
• High Viscosity
• Tendency to Devitrify (process in which a glass, noncrystalline or vitreous solid, transforms to a crystalline solid)

Na₂O - CaO - Al₂O₃ - SiO₂ Glass

Advantages

• Resists Devitrification
• Good Durability

Disadvantages

• High Viscosity

The network formers, modifiers, stabilizers and intermediates comprise the major oxides used in typical container glasses and make up the bulk of the composition. A typical container glass has the following composition:

Minor ingredients such as fining agents, decolorizers, and colorizers are added to the typical container glass composition:

• The most common fining agents are sulfates in combination with carbon
• Of the sulfates used, sodium sulfate, or salt cake, is the most common
• Sodium sulfate acts as a wetting agent to aid in melting the silica source and also as a fining agent

When combined with carbon, sodium sulfate releases SO₂ and CO₂ gas by the following reaction:

2Na₂SO₄ + C → 2Na₂O + 2SO₂ + CO₂

These gases rise up through the melt and collect smaller bubbles of oxygen along the way. The oxygen reacts with the SO2 to form SO3, which is soluble and is absorbed in the glass. The CO2 rises to the top of the melt and is released into the atmosphere.

continue to Section 2.5: Composition Control

1. Glass composition can be controlled by analyzing each new shipment of raw materials and its respective glass chemistry, requiring an analytical laboratory at each glass plant
2. A far better method of composition control is to monitor certain easily measured physical properties of the glass that are influenced by the glass’ composition
   Two such properties are:
   a.Density
   b.Softening Point

Density

• Density is defined as weight per unit volume.
• It is measured by comparison to a known standard using a sink-float technique, which can determine the density to the nearest 0.0002 grams per cubic centimeter.

Softening Point

• The softening point for composition control is defined as the temperature at which a fiber of specified length and diameter will elongate at a rate of 1mm per minute, or the temperature at which the log of the viscosity equals 7.65.
• Using a fiber elongation technique, the softening point can readily be determined to the nearest 0.2 C.

Each oxide in the glass affects the Softening Point and Density differently:

• By measuring one of the physical properties daily, a change in glass composition can be detected by a change in that property.
• Since density is the easiest to measure and also the most accurate, this property is measured every day at all plants.
• Softening point is measured at certain plants as a second precautionary measure for ensuring composition stability.

By using density as the measured property, glass composition can be controlled by Statistical Process Control techniques. By using such techniques it is possible to limit glass composition variations within detectable limits of wet chemical analytical techniques.

continue to Section 2.6: Recycling Glass Containers

Glass Recycling Facts

• Glass is 100% recyclable with no loss in quality or purity, making it the only true “cradle-to-cradle” packaging material.
• Glass containers go from recycling bin to store shelf in as little as 30 days.
• An estimated 80% of recovered glass containers are made into new glass bottles.
• In 2007, 34.5% of glass beer and soft drink bottles were recycled, and 28.1% of all glass containers.
• Glass recycling rates are higher in some states, especially those with mandatory beverage container deposits. In California, for example, glass bottle recycling nears 70%.
(Sources: U.S. EPA, Municipal Solid Waste Generation, Recycling, and Disposal in the United States: Facts and Figures for 2007; and California Department of Conservation)

Environmental Benefits

Recycling glass containers provides for unmatched production efficiencies and significant environmental benefits:

• Saves raw materials - Over a ton of natural resources are conserved for every ton of glass recycled.
• Lessens the demand for energy - Energy costs drop about 2-3% for every 10% cullet used in the manufacturing process.
• Cuts CO₂ emissions - For every six tons of recycled container glass used, a ton of carbon dioxide, a greenhouse gas, is reduced.
• Extends furnace life - Including cullet in the manufacturing mix makes it less corrosive and lowers the melting temperature (from 2800 degrees F. to 2600 degrees F.), prolonging furnace life.
• No processing by-products - Glass recycling is a closed-loop system, creating no additional waste or by-products.

Glass Recycling and Quality

Bottle to bottle recycling is the highest and best use of recycled glass. But it requires a consistent supply of high-quality cullet—which can make up to 70% of the raw material mix.

Contamination from non-container glass, metal, gravel, and dirt can occur when glass containers are set out for recycling, during collection and processing, or in transit.

Unwanted items in cullet:

• decrease the value of recovered glass
• increase recycling costs
• slow container production
• reduce glass quality
• damage glass manufacturing equipment

Download GPI’s brochure on glass recycling and quality.

Sources for Recycled Glass Bottles and Jars

• Residential curbside collection
• Drop-off collection sites
• Mandatory beverage container deposits (currently in 11 states)
• Collection from bars, restaurants, and hotels
• Unused inventory
• In-house cullet at glass manufacturing plants

Glass Recycling and the Consumer

Today's consumers look for packaging that can be recycled—and is made from recycled material.

The Recycling “G” logo lets customers know that a product’s packaging is made from recycled glass-and can be recycled again.

It’s easy to use the Recycling “G” logo. There are no restrictions on the placement, size, or color. No permission is required for use.

continue to Section 3: Manufacturing