Mark J. Kirwan Paper and Paperboard Packaging Technology
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217
Polyethylene tubular sleeve valve
A paper strip is attached to a tubular polyethylene sleeve and positioned in the
sack aperture (Fig. 8.4f). The width of the sleeve may be less than or equal to the
width of the bottom.
8.2.3 Sewn closures
There is a choice for the user of the type of sewing and the materials used in sewn
closures. Stitching can be with either one or two sewing threads and may be in
combination with other ancillary crepe paper tape and cords.
8.2.3.1 Single sewing or chain stitch
Single sewing, Figure 8.5, can be readily unravelled and is a feature that may be
used effectively as an easy-opening device by incorporating a ripcord.
8.2.3.2 Double sewing
With double sewing, Figure 8.6, a second thread loops through and round the
stitches on the underside during the sewing process and is very effective in increasing
resistance to unravelling. This stitch is not used for easy opening.
8.2.3.3 Sewn closure constructions
In addition to sewing threads, a number of other materials may be used in forming
the sewn closure.
Crepe tape may be folded over the end of the flattened multi-ply tube and sewn
through. This has the effect of cushioning the stitching and assisting in preventing
the sifting out of fine powdery contents. Different colours of crepe tape may be
used for product identification.
Figure 8.5 Single sewing or chain stitch.
Figure 8.6 Double sewing.
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PAPER AND PAPERBOARD PACKAGING TECHNOLOGY
In addition to the crepe tape, sewing may be made through a jute string generally
called a filler cord, a soft cotton string called a filter cord, or a double-folded paper
tape. These cushion and reinforce the stitches and will also tend to block up the
sewing holes and resist sifting.
An additional crepe tape may be applied over the stitching and adhered with
a quick-setting latex adhesive to fully seal the sewing holes. Latexed closures are
used as a means of preventing the entry of contamination and sifting of contents.
For users who require a carry-home pack, it is possible to incorporate a carrying
handle into the manufacturer’s sewing line. One design uses a thick polypropylene
string, in a similar manner to the jute filler cord, which is left unsewn at the centre
portion of the face. An alternative is a pre-made plastic handle, generally of low density
polyethylene (LDPE), which is inserted under the sewing tape and sewn in.
8.3 Sack materials
The materials used in multiwall paper sacks are most easily divided into those of
the sack body and the ancillary materials employed elsewhere.
8.3.1 Sack body material
The construction of a multiwall paper sack is based on the use of sack kraft –
a strong, durable and versatile paper that can be coated, laminated with other materials,
modified to give it additional strength properties, printed, glued and sewn in the
converting operations to produce a product of high strength and quality.
8.3.1.1 Sack krafts
The word ‘kraft’ means ‘strong’ in a number of languages and sack kraft is so named
as it is one of the strongest papers made. It is manufactured using woodpulp fibres
made by the sulphate papermaking process. This uses sodium sulphate as the main
chemical, which produces strong, undegraded cellulose fibres.
The sheet is formed by flowing a suspension of wood-pulp fibres in water onto
a moving wire mesh. Most of the water drains away through the wire to leave a con-
solidated layer of intermeshed fibres. This is then pressed by mechanical presses,
dried by steam-heated rolls and then wound into reels.
This mode of manufacture imparts different properties to the paper in the direction
of movement, compared to the properties across the sheet. For example, sack kraft
has higher tensile and lower stretch properties in the machine direction (MD)
compared with those in the cross direction (CD). This arises from the random
lattice-work orientation of the fibres as they form a sheet on the moving wire in
the papermaking process. Both the tensile and the stretch properties contribute to
strength, and the overall effect is high sheet toughness in both directions.
Paper produced from cellulose fibres will readily absorb water unless the sheet
is treated by ‘sizing’ with an additive to inhibit ingress of water into the dry sheet.
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All standard sack kraft is sized on the paper machine, generally with rosin. The
degree of sizing is measured by the Cobb test (ISO 535: 1991).
Sizing does not impart strength-when-wet properties to the sheet because it only
slows down the rate at which the paper absorbs water. Wet strength is achieved by
the addition of a liquid resin during the papermaking process. This locks the fibres
together at their points of contact so that they are not readily pulled apart when
the sheet is soaking wet. Normal wet-strength treatment will allow approximately
20–30% of the strength of the dry sheet to be retained after the sheet has been
thoroughly soaked in water.
The natural colour of sack kraft varies from dark to light brown, a shade which
is suitable for many applications. By bleaching the wood-pulp fibres during manu-
facture, a pure white sack kraft can be made. When used on the outside of a paper
sack, the white appearance enhances presentation and provides an improved
surface for printing. In addition to the natural and bleached colours, a range of
coloured sack krafts can be made by the addition of dyes during the papermaking
process or by surface-colouring the paper after it has been manufactured. The
range of colours available is normally restricted to a number of standard shades
with any one manufacturer, and users should consult sack supplier to obtain details
and samples.
8.3.1.2 Extensible sack krafts
The term ‘extensible’ is used to describe those sack krafts which have been given
enhanced MD stretch properties, either in the papermaking process or as a subse-
quent operation. This increase of stretch is generally associated with a slightly
lower corresponding tensile but the overall effect is for most extensible krafts to
be tougher than normal sack kraft. All are available in normal and wet-strength
grades, either natural or bleached.
Low-stretch creped (LSC) krafts
These are sack krafts that have been subjected to a wet-creping process, usually on
the papermaking machine, to give a greater MD stretch. The creping process
permanently wrinkles the sheet so it is rougher in appearance, more porous and
more flexible than normal kraft.
Microcreped krafts
These sheets are mechanically crimped, or compacted, with a barely visible creping
in the MD during papermaking, to give greater MD stretch. Typical of a number of
systems for producing such paper is the ‘Clupak’ process.
8.3.1.3 Coated sack krafts
Certain commodities packaged in paper sacks require some form of barrier protection
to restrict either the ingress or the egress of vapours or liquids. For this purpose,
sack krafts can be coated with a range of materials to give strong, flexible barrier
sheets for inclusion in paper-sack constructions.
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Protection of the packaged material from the water vapour in the atmosphere is
a common user requirement, and the barrier generally used is polyethylene coated
sack kraft. This is readily available in a range of grades to suit most requirements.
8.3.1.4 Laminated sack krafts
For greater strength or barrier protection of sensitive commodities, laminates of
sack kraft with foil, film, woven or non-woven plastics maybe utilised. These
materials are relatively more expensive than normal and coated sack krafts, and
the choice would depend on the barrier requirements and/or the hazard level of the
distribution system.
8.3.1.5 Non-paper materials
A number of non-paper materials, such as plastic film, layflat plastic-film tubing,
woven or non-woven plastics, may be included in paper-sack constructions where
high barrier or high strength is a prime requirement. The use of thermoplastic film
can make it possible to include a heat seal in the closure of some types of sack.
There are limitations to the use of these materials in the manufacture of certain
types of paper sacks. For example, it may not be possible to include some of the
reinforced and high-strength materials into stepped-end sacks, as these are not
compatible with the normal manufacturing process.
8.3.1.6 Special purpose sack krafts
Although the majority of user requirements can be met by paper-sack constructions
using combinations of materials described in the previous subsections, there is
occasionally a need for a special material for a specific requirement. A number of
such special purpose materials which meet particular requirements are discussed
in Section 8.3.1.7. Any such need should be discussed with sack manufacturers for
advice on the most appropriate material that can be used in specific situations.
8.3.1.7 Summary of sack body materials
The following notes give information on most sack-body materials in general use,
but are not intended to be fully comprehensive. Where applicable, abbreviations in
general use have been included in brackets after the name of the material. There is
no international agreement on names and abbreviations, and where there is more
than one alternative in general use, all are included.
Natural sack kraft (NK or K)
Generally in grammages (substances) of 70g/m
2
, 80g/m
2
, 90g/m
2
and 100g/m
2
.
Higher, lower and intermediate grammages are obtainable but not usually held as
stock by sack manufacturers. It may be used generally throughout a sack in all plies.
Wet-strength kraft (WIS or WS)
The most generally used grammages are 70 g/m
2
, 80 g/m
2
and 100g/m
2
but other
grammages are available, although not as normal stock items. Widely used for the
outer plies of sacks which maybe exposed to rain.
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Bleached kraft (BK or BIK or FB)
Available mainly in 80g/m
2
and 90 g/m
2
grades, but other grammages are available
if required. Mainly used as the outside ply to enhance presentation.
Wet-strength bleached kraft (B/WS or FBWS)
The general trade standard is 80g/m
2
but higher and lower grammages can be
obtained if required. Used for the outside plies of sacks which are liable to become
exposed to rain.
Coloured krafts
Generally available in the 80g/m
2
grade with or without wet-strength additive.
The colour required should be written in full to prevent confusion with paper grade
abbreviations. Bleached or natural kraft may be used as the base paper, each of
which will give a distinctly different shade to the imparted colour.
Low-stretch crepe (LSC)
Available as natural or bleached, and generally as the 80g/m
2
grade, with or without
wet-strength additive. The normal level of creping is between 5 and 9% for MD
stretch. The creping process also imparts higher air porosity to the sheet.
Microcreped fully extensible kraft (EK or ExtK)
Such krafts are defined as having an MD stretch greater than 7%. It is a tough
paper that can be very similar in appearance to standard sack kraft as the microcre-
ping is barely apparent. It is available in both natural and bleached grades with or
without wet strength, and in the same range of grammages as normal kraft.
Microcreped semi-extensible kraft (S/EK or SExtK)
The shortened name in general usage for this material is ‘semi-extensible kraft’.
It is defined as having the MD stretch within the range 4–7%. The same grades
are available as for fully extensible kraft. The stiffness of semi-extensible kraft
allows good runnability on converting machinery, similar to normal sack kraft, and
high field performance in the finished sacks, making it an ideal material for many
applications.
Polyethylene-coated kraft (polykraft or PEK)
Low density polyethylene has reasonable moisture and water vapour barrier
properties, but its grease and odour barrier properties are not good. It is generally
available with 80g/m
2
natural kraft as the base paper and 15 g/m
2
, 23 g/m
2
or
34g/m
2
of coated polyethylene. The most popular and the most general purpose
coating weight is 23g/m
2
. A coating weight of 34 g/m
2
is one which is relatively
free from pinholes and film rupture caused by surface fibres. The 15g/m
2
coating
weight is not a good barrier to water vapour but does resist the ingress of liquid
moisture and may also be used as a means of preventing paper fibre contamination
of the packaged product.
Coating weights above 34 g/m
2
cause the paper to curl and give serious
problems in the converting operation and are not normally recommended.
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Silicone coated kraft
The silicone coating is not a barrier ply for liquids or vapours. It is a release coating
used on the inner surface of the innermost ply to enable sack kraft to be easily
stripped away from sticky or mastic contents.
Waxed kraft
The wax coating will partially or totally impregnate the sheet. It is used only to
a small extent, mainly as a grease or odour barrier. Although it has some moisture
and moisture vapour barrier properties, these are inferior to those of other materials
available.
Polyvinylidene dichloride coated kraft
Polyvinylidene dichloride is an excellent barrier for moisture, moisture vapour,
grease, solvents, oil and odours. It is appreciably more costly than polyethylene coated
kraft and its use is justified where a good odour or grease barrier is a requirement.
Foil laminated kraft
Aluminium foil is virtually a complete barrier for odours, grease and water
vapour. In paper sacks, it is generally used at thicknesses between 15 and 25 µ as
a laminate with sack kraft. At 25 µ thickness, it is virtually pinhole-free. Below
this thickness, the effect of the presence of pinholes is virtually eliminated by
the use of a plastic coating, usually polyethylene or ionomer. Either of these
two materials may also be used to laminate the foil to the sack kraft. The plastic
coating will also allow a sack closure to include a heat seal with certain designs
of sacks.
Bitumen laminates
Coatings of bitumen on sack kraft and laminates of two krafts or LSC kraft sheets,
using bitumen as a laminant, have been used in paper sack manufacture since the
early years. It is a general purpose barrier against moisture and water vapour, but has
been largely superseded by polyethylene coated kraft. It is not a high performance
barrier because a totally continuous coating of bitumen is not easy to achieve in
practice. This barrier is also not resistant to creasing and the fold lines in a paper
sack are weak areas of the barrier.
Vegetable parchment
This is a traditional grease-resistant material but its use has been largely superseded
by plastic film or coated kraft.
Scrim and cloth laminates
Scrim laminates, comprising an open mesh woven cloth adhered to a single layer
of sack kraft or sandwiched between two layers of sack kraft, will give a very high
resistance to sack burstage and tear propagation. The degree of toughness of the
laminate will be governed by the quality of the scrim cloth used; very open mesh
cloth mainly giving resistance to tear propagation, and closer mesh, resistance
to burstage.
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Woven plastics/sack kraft laminates
Laminates of sack kraft and woven plastics are very tough and resistant to tear,
burst and puncture. Sack kraft contributes stiffness to the laminate and the sack.
8.3.2 Ancillary materials
In addition to the materials comprising the plies of the sack, other ancillary materials
are also included, being either necessary to the construction or optional, at the
request of the user.
8.3.2.1 Sewing tapes
The most important requirements for sewing tapes are softness and bulk to assist
the blocking of the sewing holes, and strength, flexibility and extensibility to suit
the intermittent action of the sewing head. Sewing tapes are most generally made
from crepe paper, with a creping level of the order of 10–12%.
Overtaping the stitching in the same operation as sewing is generally made
using a wider version of the same crepe tape in conjunction with a quick-setting
latex or hotmelt adhesive. Overtaping with a polyethylene coated kraft tape, either
creped or non-creped, may also be used in conjunction with a heat sealing unit.
This is most generally undertaken as an additional operation.
8.3.2.2 Sewing threads
Sewing threads are either made of natural cotton, synthetic filaments or a mixture
of the two. Each thread is comprised of a number of strands or plies. Three and
four ply threads are used for sacks containing up to five plies of paper and five-ply
threads for sack constructions of six plies.
8.3.2.3 Filler (filter) cords
A cord of jute, double folded crepe tape or fibrillated plastic string may be used
in the sewing line to cushion the stitches and block the sewing holes to resist the
sifting of fine powders, etc. Jute and cotton cords may be applied on one or both
sides of the sewing line.
A single cord or double folded crepe tape may be used in conjunction with
single sewing for use as an easy-opening device.
8.3.2.4 Plastic handles
Injection-moulded LDPE carrying handles may be included in the manufacturer’s
sewn closure. These are inserted under the tape and sewn through during the
closing operation and are available in a range of colours.
A simple carrying handle may be made using a heavy-duty polypropylene
string, which is not sewn at the centre of the sack face.
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8.3.2.5 Adhesives
Starch and modified starch-based adhesives are generally used for the side seams,
cross pasting and end closures, generally with a wet-strength additive included.
When special coatings or laminates are employed in the sack construction, synthetic
emulsion adhesives may be necessary. Hotmelt adhesives may also be used with
certain designs of sacks as a quick acting adhesive or as a pre-applied hotmelt
adhesive which can be reactivated for the user’s closure.
8.3.2.6 Printing inks
Paper sacks are generally printed with flexographic water-based printing inks
which are formulations of pigments, resins and other additives for colour fastness,
printing opacity, etc. Lead pigments are no longer used in these formulations.
Over-varnishes are also available to give an enhanced finish.
8.3.2.7 Slip-resistant agents
There are a number of proprietary materials which may be applied to the outside
of paper sacks to impart resistance to slipping during transportation. These are
generally colloidal suspensions of a finely divided solid material, such as silica, in
water. Such applications are reasonably effective but operators will need to exercise
care in handling such sacks, as skin irritation may occur. High application levels
may also give difficulties with the sliding of sacks in chutes.
8.4 Testing and test methods
In present-day papermaking, automatic controls are incorporated to check sheet
properties and adjust the machine settings to ensure consistent and high quality. In
addition, all finished paper coming from the machine is sampled and tested on
a frequent and regular basis for quality control and quality assurance purposes.
Sack manufacturers may also test the incoming consignments of sack kraft and the
other materials used in their production.
Specialist equipment is required for paper testing, which must be undertaken in
an atmosphere which is controlled for temperature and humidity. Paper testing is
not normally undertaken by users, but a sufficient knowledge of testing and
quality is needed when changing specification levels or establishing new sack
specifications.
Testing encompasses the two broad areas of the materials and the finished
sacks, each of which requires the application of quite different techniques and
principles.
8.4.1 Sack materials
A multiwall paper sack is a flexible form of packaging and is required to resist the
stresses arising from the contents as well as those of the filling, handling and
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225
distribution systems. Testing should measure properties which best relate to sack
performance in these areas.
Sack kraft is made from natural fibres which will readily take up or lose moisture,
depending on the ambient temperature and humidity of the air. If there is no
control of this, the absorbed moisture can vary widely and will have an appreciable
affect on the physical properties and test results. For this reason, it is necessary to
keep sack kraft for 24 h in a controlled standard atmosphere of temperature and
relative humidity (RH) before any physical testing is undertaken.
In the United Kingdom and Ireland, the standard atmosphere used in the
paper-sack industry is 23 °C and 50% RH, which gives an equilibrium moisture
content of approximately 9.5% to sack kraft.
Sack kraft, like all other paper materials, is composed of a mass of intermeshed
fibres and any testing will show a degree of variation of properties both across
and along the sheet, i.e. in the CD and MD. Because of this, it is always necessary
to undertake a number of replicate tests on each sample of paper and sufficient
samples to be taken from each consignment, for meaningful average results to be
obtained.
There are a number of ISO, EN and British Standards (BS) on sampling, con-
ditioning and testing, details of which are given in Table 8.5.
8.4.1.1 Strength tests
The more important of the tests for strength are those which measure the resistance
to breaking or the energy needed to cause rupture.
Tensile strength
The tensile strength of paper is the maximum pulling force that a narrow strip will
stand before it breaks. Generally sack kraft has a higher tensile strength in the
MD compared with the CD.
It is measured by an instrument in which a strip of sack kraft is clamped between
two jaws which are gradually moved apart until the strip breaks. The machine records
the final force before breakage as well as the amount of stretch that has occurred.
Stretch
This is determined at the same time as the tensile strength, and is a measure of the
elongation that will occur before a sheet is ruptured. The product of the tensile and
stretch is an indication of the energy-absorbing ability or toughness of the sheet
and may be used for comparison between materials.
Tensile energy absorption (TEA)
As the name implies, this is a test which gives a direct measurement of the energy
needed to rupture a sheet. A special type of tensile tester is used, which not only
measures the tensile and stretch but will integrate the two results and work out the
total energy the paper strip absorbs before rupture occurs.
It is a test which is regarded as giving a better relationship to performance
of finished sacks than all other tests and may therefore be used as an index
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when adjusting sack constructions or drawing up specifications. It is normally
performed on two sets of sample strips, cut in the MD and in the CD of the sheet.
The average between the two directions is normally used for comparing two or
more materials.
Wet strength
Wet strength is defined as the level of strength that is retained by a paper sheet
after it has become thoroughly wet, i.e. after a sample has been immersed in water
for at least half an hour.
Traditionally the pneumatic burst test was used to establish the wet strength of
sack kraft but tensile testers based on load cells are now used more generally for
this purpose.
Normal sack kraft will possess little or no strength after immersing for half an
hour in water, but wet-strength sack kraft should retain an agreed percentage of its
dry strength after soaking. Approximately 30% wet-strength retention is the level
most generally used within the paper sack industry.
Burst strength
The burst test is one that has largely gone out of favour, mainly because of the
extreme variability of the results. In the test, a sample of paper is clamped above
a rubber diaphragm covering an orifice of approximately 30mm diameter. Air or
hydraulic pressure is gradually increased under the diaphragm which expands until
the paper sheet becomes ruptured. Hydraulic burst testers are not sufficiently sensitive
for sack-kraft testing, and pneumatic instruments are more generally employed.
Although it is regarded as an unsuitable test for assessing paper quality, the test
may be used as a quick check for wet strength by burst testing a conditioned
sample and another that has been soaked in water for half an hour. It may also
be used as a quick check for confirming weakness in a paper sheet, such as that
caused by over-creasing, abrasion, etc.
Tear strength
The tear strength is a measure of the resistance to tear propagation after puncture
or rupture has occurred. In the test, a partially slit rectangular sample is held by
two adjacent clamps, on either side of the slit, and the force to tear the sample fully
apart is measured.
There is an inverse relationship between tear strength and tensile strength over
which the paper-maker can exert a large degree of control. To obtain maximum
tensile strength would result in a paper which possesses a relatively low tear strength.
Good tear strength and good tensile strength are important in sack kraft, and the
paper-maker will aim for a reasonable balance between the two.
8.4.1.2 Other physical properties/tests
There are a number of generally accepted testing methods for assessing other
physical properties in addition to those for strength.