How Glass is Made

8/11/2019 How Glass is Made

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FST 649, Packaging Materials and Methodology Wednesday, April 1, 1998

T.H. Shellhammer Packaging Materials

Glass Containers for Food Packaging

The goals for this lecture are for you to understand:

(1) the chemical composition and manufacturing of glass,

(1) the chemical composition and properties of glass,

(2) terminology used to describe glass packages and their closures,

(3) advantages and disadvantages of glass packaging materials.

Reading Assignment:

Chapter 9, pages 232 - 250 - Glass Packaging Materials

Chapter 20, pages 658 - 659 - Safety Concerns of Glass Packaging

A. What are glass containers made of?

Main ingredient in glass is silica- derived from sand, flint, or quartz

Silica is combined with other minerals to produce glass

alkali salts (Na and K) -

Ca and Mg -

Alumina -

Lead -Boron -

Typical formula for soda-lime glass:

Silica, SiO268 - 73%

Calcia, CaO 10 - 13%

Soda, Na2O 12 - 15%

Magnesia, MgO 0.3 - 3%

Alumina, Al2O 1.5 - 2%

Ferric oxide, Fe2O30.05 - 0.25%

Sulfur trioxide, SO30.05 - 0.2%

Physical Properties of Glass

Material k(W/mC) r(kg/m3) Cp(kJ/kgC) a( 10-7m2/s) Tensile Strength (MPa)

Glass

Steel 54 7830 0.465 1.474 350


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Tin 64 7300 0.227 3.884

Aluminum 177 2700 0.892 7.311 90

Note, freshly drawn glass fibers have strength of 0.7 - 2.1 GPa.

Handled fibers, strength = 350 - 700 MPa.

Viscosity of Glass

Operation Viscosity, Poise

Melting 102

Gather when placed in mold 104.5

Ware removed from mold 107

Annealing 1013- 1013.5

At max service temperature 1014.6- 1015.5

INSERT Viscosity DIAGRAM FROM Varshenya, A.K. 1994.Fundamentals of Inorganic Glasses,

Academic Press, Inc., New York, NY. Page 186.

Brief discussion of the glassy state

Glass definition 1:

1. A glass is an amorphous solid which exhibits a glass transition2. A solid is a material whose shear viscosity exceeds 1014.6poise

from Elliot, S.R. 1983.Physics of Amorphous Materials, London: Longman Group Ltd ISBN 0-582-44636-8.

Glass definition 2:

Although glass has many p roperties of a solid, it is really a highly viscous liquid . . . at ambient temperatures

(glass) has the characteristics of a solid, it is a supercooled liquid and will flow even at ambient temperatures

over long periods of time

From Robertson, G.L. 1993.Food Packaging, Principles and Practice, Marcel Dekker, Inc., New York, NY.Page 234.

Glass definition 3:

A glass is a solid with liquid like structure, a noncrystalline solid, an amorphous solid.

From Varshenya, A.K. 1994.Fundamentals of Inorganic Glasses, Academic Press, Inc., New York, NY. Page

14.

Phenomena observed with cooling or heating materials so that they pass in and out of the glassy state:


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(1) There is no phase transition

(2) Molecules exist in an amorphous state - non crystalline

(3) In the range of the glass transition temperature there is a shift in:

(i) thermal expansion -

(ii) mechanical properties -

(iii) permeability -

INSERT V -T DIAGRAM FROM Varshenya, A.K. 1994.Fundamentals of Inorganic Glasses, Academic

Press, Inc., New York, NY. Page 15.

Structure of glass

INSERT STRUCTURE DIAGRAM FROM Shand, E.B. 1958. Glass Engineering Handbook, McGraw-Hill

Book Company, Inc., New York, NY. Page 16.

Types of glass

Soda - Lime Glass

The least expensive, most popularly used type of glass. Soda, Na2O, and Potash, K2O, reduce viscosity

greatly below that of the silica and permit use of lower melting temperatures and improve fining qualities of

the glass. Good chemical durability. Broad spectral transmission in visible range.

Borosilicate Glass

Boron oxide, B2O3, p roduces a glass with very low thermal expansion coefficient and high resistance to

chemical attack. But B2O3, does not lower viscosity as much as soda, thus furnace temps must be higher(higher cost).

Lead Silicate Glass

Contains PbO and SiO2as the principal components - PbO replaces lime (CaO) in the soda-lime glass. Lead

increases brilliance, expands the working range (useful to make art objects and intricate shapes without

frequently reheating), but decreases hardness.

B. Glass container terminology

Important components for discussing glass container design

Finish -

Neck -

Shoulder -

Heel -

Body -


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Headspace -

INSERT FIG 9-4

C. Properties of glass containers

Mechanical Strength and Failure

Stress -

Tensile vs. compressive stress

Affect of t emperature change on material stress, thermal expansion

Given a material, what happens to it when you heat it or cool it?

If the ends of the material are fixed during heating, what happens?

Heatingthe material results in

material expansionand thus

compressive stress

The heated material would like to assume this p osition illustrated below; however, it can't since its ends are

fixed in space. As a result, compressivestress is built up in a similar manner to compressingthe length of the

rod (in its heated state) to its original unheated length.

If the ends of the material are fixed during cooling, what happens?

Coolingthe material results in

material contractionand thus

Tensile stress

In an opposite fashion to the heated case, the cooled material would like to assume the position illustrated

below; however, it can't since its ends are fixed in space - the material is in a state of tension. As a result,

tensilestress is built up in a similar manner tostretchingthe length of the rod (in its heated state) to its

original unheated length.

Glass fails under tension, not under compression

glass has tremendous compressive strength but poor tensile strength.

Failure is

Stress Concentrators

small crack, scratch, bruise.


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Four factors leading to failure to consider when designing/selecting a glass container:

1. Internal pressure resistance

2. Vertical load strength

3. Resistance to impact

4. Resistance to scratches and abrasions

Surface treatments help - such as tin compounds to increase scuff resistance

Lubricant coatings to help containers slip against each other

Side wall design to address point of contact.

INSERT FIG 9-1

CROSS SECTION OF CONTAINER - STRESSES

Optical Properties - Can you see the product? Do you want to see the product?

Optical properties of glass containers

Silica absorbs below 150 nm (UV) and above 6000 nm (IR)

Determined by transition metal oxides added to glass mixture.

Refresh your memory on the electromagnetic spectrum from UV to IR

Far-Ultraviolet =

Near-Ultraviolet =

OR

UV-C=

UV-B=

UV-A =

Visible spectrum =

Infrared =

What part of the spectrum should we be concerned with in food packaging?


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(1) stimulate singlet oxygen production - oxidation

(2) break covalent bonds - off flavor production

INSERT FIG 9-2 RADIATION TRANSMISSION

Closures for glass packaging.

Nomenclature

Panel -

Radius or shoulder -

Skirt -

Lug -

Liner -

(1) Those designed for internal pressure

Crown cap - steel

Twist-off crown cap - steelRoll-on or spun-on aluminum closure

Internal components consist of cap/compressible layer/food contact-seal layer

(2) Those designed for vacuum seal

Lug-type

Twist cap

Press-on twist-off cap

Pry off

(3) Those designed to contain materials in package

Screw top with minimum thread engagement.

D. How is glass manufactured?

1. Combined materials

Silica

Cullet - scrap or recycled glass - less energy to melt, economical

Other components

2. Heat in furnace at 1500C until mixture fuses together

Viscosity decreases

Mixtures degasses and clarifies - fining

3. M olten glass is formed into container

Blow and Blow (B&B)

Press and Blow (P&B)

Narrow Neck Press and Blow (NNPB)


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4. Containers are annealed to reduce internal stresses from nonuniform cooling

Annealing lehr is an oven which starts at 540C and slowly cools under controlled conditions

5. Surface treatments applied

Inner surface may be treated with fluorocarbon gas to stabilize surface sodium ions - increases durability of

surface.

Outer surface coatingstin or titanium vapor to form a metal oxide layer.

improves cold-end coating with waxes, silicone or polyethylene

These coatings add lubricity to outer surface.

Health concerns with glass packaging

D. What are the advantages and disadvantages of glass packaging?

Advantages

1. Glass containers provide absolute barrier to mass transport between package and environment

2. Consumer can (in many cases)

3.

4. Glass packages provide

5. Delivers the product well, i.e., ease or pouring product out of package.

6.

7. Port rays an upscale image (in some cases) as with wine, beer, soda.

Disadvantages

1.

2.

Multi-step manufacturing, including annealing and coating.3.

4.

Cylindrical shape occupies excess space in storage.5.

How Glass is Made

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