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, SiO2 68 - 73%
Calcia, CaO 10 - 13%
Soda, Na2O 12 - 15%
Magnesia, MgO 0.3 - 3%
Alumina, Al2O 1.5 - 2%
Ferric oxide, Fe2O3 0.05 - 0.25%
Sulfur trioxide, SO3 0.05 - 0.2%
Physical Properties of Glass
| Material | k(W/m·ºC) | r(kg/m3) | Cp(kJ/kg·ºC) | a(× 10-7 m2/s) | Tensile Strength (MPa) |
| Glass | |||||
| Steel | 54 | 7830 | 0.465 | 1.474 | 350 |
| 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
transition
2. A solid is a material whose shear viscosity exceeds 1014.6
poise
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 properties 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:
(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, produces 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 SiO2 as 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 -
Headspace -
INSERT FIG 9-4
C. Properties of glass containers
Mechanical Strength and Failure
Stress -
Tensile vs. compressive stress
Affect of temperature 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?
• Heating the material results in
material expansion and thus
compressive stress
The heated material would like to assume this position illustrated below; however, it can't since its ends are fixed in space. As a result, compressive stress is built up in a similar manner to compressing the 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?
• Cooling the material results in
material contraction and 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, tensile stress is built up in a similar manner to stretching the 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.
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?
(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 - steel
Roll-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 1500ºC until mixture fuses together
Viscosity decreases
Mixtures degasses and clarifies - fining
3. Molten glass is formed into container
Blow and Blow (B&B)
Press and Blow (P&B)
Narrow Neck Press and Blow (NNPB)
4. Containers are annealed to reduce internal stresses from
nonuniform cooling
Annealing lehr is an oven which starts at 540ºC 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 coatings
tin 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. Portrays an upscale image (in some cases) as with wine, beer, soda.
Disadvantages