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A publication on trends and applications
from Battenfeld Gloucester Engineering
TechLiner
December 2001
HEAVY DUTY SACKS
by German Laverde, Director of Marketing
Characteristics & Applications
Welcome to the TechLiner...a publication from Battenfeld
Gloucester Engineering that shares trends and applications in
the plastics industry. This newsletter is published 3 times per
year. We’d love to hear your comments and input! Please
contact Loretta Cobb, Technical Writer at (978)282-9396 or
[email protected].
Plastic shipping sacks, part of the industrial packaging sector
of the polyolefin film market, are used to transport bulky
materials from one place to another. Based on the final
applications and the market demands, it is common to classify
them according to the volume or weight they are able to hold,
which determines the film thickness.
Abstract
Heavy duty sacks had an estimated polyethylene
consumption of 670 million pounds in 2001. Very important
in the growth of the market are the price changes for the
resins involved, and the development of new technologies
and structures. This article will review the typical
applications, product requirements, and production
techniques for the preferred process to produce heavy duty
sacks. The types of materials used and typical structures
for monolayer and coextrusion films will also be examined.
The two main categories are:
Consumer sacks, designed to carry products weighing
less than or equal to 20 pounds, with thicknesses between 0.5
and 3.5 mils (12.7 and 89 microns).
Heavy duty sacks, designed to carry products weighing
more than 20 pounds, with thickness between 3.6 and 8 mils
(91.4 and 203 microns)
In this article, we will discuss only heavy duty sacks, which
have an estimated polyethylene consumption of 670 million
pounds for the year 2001.
Applications
Typical Applications
As mentioned previously, very often the products packaged in
heavy duty sacks are bulky and heavy.
A typical application is to package raw materials including:
• Plastic resins in pellet or powder form - polyethylene,
polypropylene, polystyrene
• Chemical products - sulfur, caprolactams, anhydride phtalic
• Food - salt, sugar
Advantages
Advantages of heavy duty sacks made of
film vs. those made from polypropylene
raffia or multi-wall paper:
• Protection from short term exposure to
outside weather conditions
• Better and more reliable sealability
• Product visibility where desirable
• Better accessibility to products
• Better environmental stress cracking
resistance (ESCR)
In these applications, a very common weight for the filled sack
is 25 kg or 55 lb.
Applications
Applications
Product requirements
Product requirements can be divided into film mechanical
properties, physical properties, behavior expected once the
sack is stored, and dimensional requirements.
Film mechanical properties:
The tensile properties of the film must resist the forces and
loads during the filling operation, plus the loads resulting from
handling and storage.
Other applications for heavy duty sacks
include:
•
•
•
•
•
•
•
Carbon black
De-icing salt
Explosives
Garden compost
Sand
Pet food
Fertilizers
•
•
•
•
•
•
Potting soil
Wood pellets
Seeds
Hazardous waste
Asbestos debris
Insulation
Different things can affect the sack design
and the properties needed:
Typical Structures for Monolayer Film
• Product weight and temperature at time
of filling
Composition
Comments
• Type of filling system
70% LDPE + 30% LLDPE butene
LDPE improves clarity with
acceptable tear and sealing
properties.
60% LDPE + 40% LLDPE butene
Better tear resistance and
sealing behavior with good
transparency.
60% LDPE + 20% LLDPE octene
+ 20% LLDPE butene
Very good sealing properties
tear resistance with equilibrium
in price. Hexene can be a good
option to replace the octene,
which has a better raw
materials price.
60% LLDPE butene + 40% LDPE
Excellent tear and sealing
properties with good
processability.
40% LLDPE butene + 20% LLDPE
octene or hexene + 40% LDPE
Much better sealing properties,
especially in contaminated
environments (dust, grease,
etc.).
50% LDPE butene + 30% LLDPE
+ 20% MDPE
Improved stiffness allowing
thickness reduction.
Heterophasic Copolymer PP
+ Copolymer PP flexible + LLDPE
Very good creep resistance.
Allows thickness reduction.
60% LLDPE butene + 20% HDPE
+ 20% LDPE
Increase in the tensile
properties and film stiffness.
However, adding HDPE reduces
tear resistance.
• Handling and storage conditions
• Seal type at the bottom and top of the
sack
The first design consideration for the
plastic sack is the weight of the product
being packaged. Normal thickness for
weights around 20 - 30 lb. (approx. 10 15 kg) is 3.8 or 4.0 mils (96.5 or 100
microns). If the contents’ weight is around
50 lb. (25 kg), which is the case of resin
sacks, the thickness goes from 5.5 to 7
mils (140 to 180 microns). New materials
and technologies are allowing thicknesses
to be reduced to around 4 mils.
The second consideration is the sack’s
resistance to the load occurring in
storage. Due to the methods used to store
or palletize the sacks, sometimes they
must resist 10 or even 20 times the
weight of the product packaged. See
photo below.
As a consequence of the higher load, the
sack must be rigid enough to avoid
deformation in the transversal direction.
This problem becomes more critical when
the temperatures in the storage place are
higher than 87°F (30°C). The resin
selection for the film structure must
consider this problem.
The sack stiffness is also important if it is
filled using an automatic machine and
includes a valve in the top of the sack. The
air used to transport and feed the material
tends to inflate the sack. As a result, the
film must be rigid enough to avoid
deformations and thickness reduction.
Sometimes small perforations in the film
are made to help in the air evacuation.
9 feet (2.7 mts.) or more at the storage
place.
Tear resistance of the film is very
important because of increased risk of
puncture and damage to the sack in the
storage place and during transport. The
tear propagation characteristics should be
reduced to avoid spillage.
Physical properties of the film:
The most important physical property in
this application is the slipping
characteristics of the film. The sack should
have a high coefficient of friction (COF) on
the outside to prevent sack slippage once
they are palletized. At the same time,
depending on the filling procedure or the
product packaged, a low coefficient of
friction on the inside could be required. It
is helpful during the packaging process.
Sealing properties are extremely important
to guarantee the quality and performance
not only of the seal, but the whole sack
under critical conditions such as pressure,
impact and mishandling.
The possibility of impact occurs during
transport and handling. Most important is
the resistance to the impact generated
when the sack falls from heights around
Some modifications to the film surface can
be done to increase the COF by
embossing or by introducing additives into
the resin. The first method must be
performed without reduction to the
mechanical properties or film thickness.
Another important consideration is the
ability to print the product description and
manufacturer information on the sack
surface. Normally this is done using
flexography, and the sack requires a
surface treatment to permit a good ink
adhesion. The ink types are important
because they must have abrasion
resistance. However, inks containing
excessive additives such as waxes, can
negatively affect the COF on the film.
Dimensional requirements:
When the sack is being designed,
dimensional requirements must be
considered including method of
transportation, common dimensions of the
pallets and number of units per pallet. This
will ensure good pallet stability and
reduction of the slippage risk. Some
market standards exist depending on the
type of product and weight packaged.
Characteristics
Materials used, typical structures
and reasons
The traditional material used for the
production of heavy-duty sacks has been
polyethylene, including LDPE, LLDPE,
ULDPE, HDPE and sometimes EVA.
Depending on the final application and the
mechanical properties required, LDPE is
used alone or in blends with the other
materials.
Butene, hexene and octene LLDPE are
included in the recipes. Butene is most
widely used due to the balance between
price and properties obtained.
LLDPE provides very good tear resistance,
sealing properties and puncture
resistance. When it is blended with LDPE,
a typical blending ratio is 60-75% LLDPE,
plus 40-25% LDPE. It is important to
select the appropriate LLDPE grade,
because too much flexibility will be
detrimental to the dimensional stability of
the sack under load and temperatures.
Alternatively, LDPE can be the major
component. Fractional melt resins help
with bubble stability and have better melt
strength. These factors are important
when thick films are being produced.
Additionally, these resins provide a better
impact resistance to the film.
During the last few years, polypropylene
(PP) has been participating in applications
that were exclusive for PE. Heavy duty
sacks are not an exception. Some resin
companies such as Borealis, Basell and
Dow have developed PP grades to be
used in coextrusion or even mono-layer
structures. PP will improve stiffness and
tensile properties and increase the creep
resistance, allowing, in some cases,
thickness reduction. One of the most
important properties achieved using PP is
higher temperature resistance of the
sacks, permitting their use in hot-filling
operations. Cement and other industrial
products require hot filling because of the
characteristics of the production process.
A packaging material that is able to resist
higher temperatures helps reduce costs,
increases plant throughputs and efficiency,
and decreases the risk of pallet instability
after filling and during storage.
When ULDPE is used it can produce
excessive flexibility, which can cause
deformation under high loads. HDPE in the
core layer or thinner layers of ULDPE can
be used to diminish these problems.
Typical densities for these resins are
0.918 to 0.925 gr/cc in the case of LDPE
and LLDPE; 0.895 to 0.915 gr/cc for
ULDPE. Melt index values for LDPE are
Typical Structures for Coextrusion
Layer
A:
% Final
Thickness
25
Composition
Comments
100% LDPE
LDPE provides processability
and clarity while LLDPE
provides puncture and tear
resistance properties with a
good balance in raw
materials cost.
B:
50
100% LLDPE
C:
25
100% LDPE
A:
25
B:
50
C:
25
70% LDPE + 30% LLDPE HDPE is included to increase
+ slip additive
the tensile properties without
70% LLDPE + 30% HDPE big change in the tear
strength. Additionally, a
70% LDPE + 30% LLDPE different effect is achieved in
+ anti-slip additive
the outer layers, helping in
the packaging process and
reducing the COF.
A:
10
100% coPP
B:
80
100% LLDPE
C:
10
80% coPP + 20% LLDPE
A:
20
B:
65
100% LLDPE with or
without additives
100% LDPE + color
C:
15
100% LLDPE + additives
to increase COF, or
100% ULDPE
Good sealing, very good
puncture and temperature
resistance. Can be used for
hot-fill applications. Good
creep resistance. Allows
thickness reduction.
Thin outside layers minimize
the effect of expensive
materials with a good
combination of special
effects.
ULDPE can be used in the
inner layer also, to improve
the sealing properties
especially in presence of
grease. At the same time it
gives very good tear
resistance, and its tacky
nature helps to increase the
COF.
Photo #2
Photo #4
Photo #3
Photo #1
better around 0.25 to 0.8 gr/10 min;
lower values are preferred because of
better impact resistance properties. Melt
index values for LLDPE generally are around
0.8 to 1.5 gr/10 min.
Extruded Product: Generally the film is
produced as flat non-gusseted tubing.
Printed, bottom sealed, stacked, gusseted,
and non-gusseted bags are also produced
in-line with a blown film process. Single
wound sheet is produced specifically for
form/fill/seal (FFS) machines or for sacks
with a back seam. This type of film is more
popular in Europe where there is a trend
toward using automatic filling machines.
Good sealability, toughness and stiffness
are important properties for good film
performance in FFS machines with speeds
up to 1500 sacks/hr (25 sacks/min).
Occasionally tubing is produced with
gussets to get a better sack shape once it
is filled. Some problems can arise due to
excessive pressure on the edges of the
gussets by the haul-off nip rolls.
Modifications are implemented to this part
of the process to override these problems,
For HDPE, densities of 0.945 to 0.96 gr/cc
are used, corresponding to medium or high
molecular weight materials.
Production Techniques
Film Production
Process: Blown film extrusion is the
preferred process to produce heavy duty
sacks. Tear strength and the impact
resistance of this type of film is far superior
than cast extruded film. The estimated
market share for monolayer films is around
55% versus 45% for coextruded structures.
Coextrusion allows more material
combinations, special effects on the outside
surface, and downgauging.
Hot-fill Heavy Duty Bags - 2 BIG Differences
1% Secant Modulus
Tear Strength
100
1200
80
1000
grams
kpsi
60
40
800
400
200
0
0
MD
TD
MD
TD
Machine Direction
Traversal Direction
Machine Direction
Traversal Direction
Coex
Blend
BUR: The best mechanical properties are
achieved in the range of values from 2.0:1
to 2.6:1. Very high blow up ratios (BUR)
values can cause retraction problems in the
seals, while very low values can affect the
tear properties.
Thickness: Typical values are in the range
of 3.6 to 8 mils (91.4 to 203 microns) with
gauges of 4 to 6 mils (100 to 150 microns)
being the most common.
Width: This depends on the type of product
and the final use of the sack. A typical
example is resin sacks with widths of 16 to
22 inches (40 to 55 cm).
Converting
Once the tubing or the single sheet is
produced, it needs to be converted into the
final sack.
Single wound sheet is used in a
form/fill/seal machine in a single operation
to produce sacks filled with the product.
One disadvantage is the sharp corners on
the top and bottom ends of the sack which
can make the palletizing process difficult.
Another disadvantage is the COF needed for
the forming operation, which can cause bag
slippage in shipping and storage.
In the case of tubing, there are several
different forming and filling techniques.
Some examples are:
600
20
including s-wrap rolls or even cooling rolls
before the nip.
Coex
Blend
source: Basell Polyolefins
Open mouth layflat sack: the tubing is
sealed at one end and cut; the customer
fills and seals the sack at the top. Tapered
sides and ends are obtained making the
storage difficult.
Open mouth side gusseted sack: the
tubing is gusseted when extruded or postgusseted when sealed in the factory. The
gusseted tubing is sealed only at one end
and then cut. The customer fills and seals
the sack at the top. The final shape is
square at the sides when filled, for easier
stacking. See photo #1.
Hot-fill
Hot-fillHeavy
HeavyDuty
DutyBags
Bags- 2 BIG Differences
2 potential solutions for hot-fill heavy duty bags
2 potential solutions for hot-fill heavy duty bags
Std Coex
20% Blend of Terpolymers
Std Coex
60% LLDPE
Squared bottom open mouth: special
shape for the bottom seal. Uses flat tubing.
The customer fills and seals the sack at the
top. See photo #2.
Valve sack with simple seal at bottom:
includes a valve in the top to make the
filling process easier. The customer fills the
sack without post-sealing operations.
Stacking is difficult. See photo #4.
Valve sack with flat bottom: squared seal
in the bottom or gusseted tubing includes a
valve in the mouth. Perfect shape, easy to
stack. The valve avoids spillage and
permits faster filling. The customer fills the
sack without post-sealing operations.
20% Blend of Terpolymers
Hot-fill Coex
10% Copolymer PP
80% LLDPE
10% Copolymer PP
Hot-fill Blend
20% Copolymer PP
20% LLDPE
Thickness, mil
4.2
4.2
4.0
656
590
590
Tear Strength
MD
1600
1280
170
g
TD
1520
1280
330
Yield Strength
MD
1876
2025
3250
psi
TD
1911
2000
2900
Break Strengh
MD
6253
5015
6750
psi
TD
5764
4800
6100
1% Secant
MD
35
46.5
86.5
(kpsi)
TD
36
46.25
81.5
240
310
300
Heat Resistance
source: Basell Polyolefins
of filled palletized bags once stored.
The process embosses from inside the
tubing, burls to the exterior of the film and
produces an obstacle to sliding. This
tremendously increases the friction between
one sack and another. See photo left.
The embossing pattern can be modified
and chosen according to the specific
requirements and final results.
Market size
Embossed film offers advantages.
Me lt index
(gr/10 min)
Density
(gr/cc)
Type
Do w 123
0.25
0.921
LDP E
Do w 133
0.22
0.921
LDP E
Do w 2045A
1.0
0.920
LLDPE
Do w 4001
1.0
Marlex D252/D257
0.3
0.923
LDP E
P et rot hene NA 35 7
0.25
0.928
LDP E
P et rot hene NA 98 5
0.25
0.920
LDP E
Exxo nMobil LD 140
0.75
0.919
LDP E
Exxo nMobil LD 052/051
0.25
0.918
LDP E
Borstar FB223 0
0.9 (MFR5)
0.923
Bimodal LDPE
Mopl en EP310D
0.8 (MFR 2.1)
0.900
PP
0.5
0.905
VLDPE
UC Flexomer E TSE- NT7
(F)
60% Impact Copolymer PP
Examples of resins used to produce heavy duty sacks:
Producer/R eference
Hot-fill
Blend
Dart Impact, g
Other processes performed on the film are
flexographic printing, embossing and
handle sealing. See photo #3.
Embossing: Secondary process used to
modify the film surface to increase the COF
and prevent the slipping, sliding and falling
Hot-fill Coex
ULDPE
The market size was estimated to be 524.8
million pounds (262,400 Tons) for 1998
and 670 million pounds (335,000 Tons) for
2001 with an average annual growth rate of
8.4% into the year 2003.
Very important in the growth of the market
are the price changes for the resins, and
the development of new technologies and
structures. Depending on these
advancements and price fluctuations, the
consumers will be changing from multi-wall
paper sacks to heavy duty plastic sacks.
For more information, please contact
Battenfeld Gloucester Engineering at
978-281-1800.
German Laverde
Director of Marketing
The Story of Bakelite
The first completely synthetic man-made substance was
History of Plastics
discovered in 1907, when New York chemist, Leo Baekeland,
created a liquid resin that he named Bakelite. Baekeland had
developed an apparatus, which he called a Bakelizer, enabling
In today’s world, life without plastics is incomprehensible.
We all know the many ways that plastics contribute to
our health, safety and peace of mind. But how did the
material plastic come about? Who were the key
individuals in its development and use? In each
TechLiner issue we will feature one element of
plastic’s amazing history.
him to vary heat and pressure precisely controlling the reaction
of volatile chemicals. Using this apparatus, Baekeland
developed a new liquid (bakelite resin), which rapidly hardened
and took the shape of its container. This new material would
not burn, boil, melt, or dissolve in any commonly available acid
or solvent. Once it was firmly set, it would never change. This
one benefit made it stand out from previous "plastics”
produced. Previously, celluloid-based substances could be
melted down innumerable times and reformed. Bakelite was
the first thermoset plastic, which would retain its shape and
form under any circumstances.
Bakelite could be added to almost any material instantly
making it more durable and effective. The US government saw
Bakelite as opening the door to production of new weaponry
and lightweight war machinery. In fact, Bakelite was a key
ingredient in most of the weapons used in the Second World
War.
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