Page 202 - Materials Chemistry, Second Edition

Ch005-P373623.qxd 3/22/07 5:34 PM Page 181
Sustainability of Municipal Solid Waste Management
181
bottles. Accordingly, other materials needed consideration for milk packag-
ing for increasing efficiencies, transporting, and storing the milk products
(Suriyage, 2005).
Milk packaged in paper cartons was first introduced in San Francisco in
1906, but using paper to package fluids was a difficult challenge and was not
considered practical because of the perforated nature of paper and its absorp-
tion of moisture. To overcome this problem, milk cartons were coated with
microcrystalline and paraffin waxes. But another problem encountered was
to find a suitable bonding material for sealing the top, bottom, and sides of the
package to form a box. Animal glues were used with varying degrees of suc-
cess; they were not acceptable at first as they caused leaks and contamina-
tion of the contained milk. The paper cartons entirely disappeared from the
market soon after their introduction (Planet Ark, 2005; Suriyage, 2005).
Containers made out of polyethylene terephthalate, a widely used type
of plastic, became available to the market in 1977 as a packaging material
for beverages (Berger, 2005). However, the energy-efficient production, reduc-
tion of carton weight, and improved sealing and opening techniques have kept
the paper carton at the peak of packaging technology (Planet Ark, 2005).
Paper packaging products such as milk and beverage cartons are cur-
rently made of more than one material, where each layer is present for a spe-
cific reason. These multilayered cartons are generally classified into aseptic
cartons and non-aseptic cartons. Non-aseptic cartons are composed of 89%
paperboard and 11% polyethylene by weight (Charlier and Sjoberg, 1995),
while aseptic cartons have different compositions due to the presence of an
aluminum layer. Aseptic cartons are composed of several layers of three kinds
of materials: two layers of paperboard representing about 74% of the material,
four layers of polyethylene representing about 21% and a layer of aluminum
foil of about 6.5 microns in thickness representing about 5% by weight
(Charlier and Sjoberg, 1995; Olah, 2004).
Starting from the outside the first layer in non-aseptic cartons is made
of polyethylene to act as a barrier for moisture and bacteria followed by a layer
of paperboard to provide the carton with stiffness and strength. The last two
layers are both made of polyethylene; the third layer is present as an adhe-
sion layer while the fourth and last inner layer seals in the liquid contents.
The adhesion layer is used to ensure that the layer of polyethylene in con-
tact with the packed product remains intact. The first two layers in aseptic
cartons are the same as those present in non-aseptic ones and have the same
functions. The third layer is also made of polyethylene to act as an adhesion
layer but it is present to bind the paperboard to the fourth layer of alu-
minum. This aluminum layer is essential to prevent oxygen and light from
entering the carton. The last two layers are both made of polyethylene; the
fifth layer is present as an adhesion layer while the sixth and last inner layer
seals in the liquid contents. According to these structures, non-aseptic car-
tons are generally used for pasteurized products while aseptic cartons are
used for long-life products. These aseptic packages allow liquid products to