Note: Descriptions are shown in the official language in which they were submitted.
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TITLE
METHOD OF MAKING A WATER-SOLUBLE POLYMER, WATER-SOLUBLE
POLYMER SO PRODUCED, AND MELT-PROCESSABLE WATER-SOLUBLE
POLYMER COMPOSITION
FIELD OF THE INVENTION
The present invention relates to a polymer composition, particularly but not
exclusively
to polyvinyl alcohol polymers, and to methods for their production.
BACKGROUND OF THE INVENTION
There is currently an increasing demand for water soluble, biodegradable
polymers to
replace the substantial amount of non-biodegradable polymers that are in the
marketplace.
Non-biodegradable polymers place a significant demand on resource due to the
requirement for their disposal in landfill sites or by incineration.
Poly vinyl alcohol (PVA) is recognized as one of the very few vinyl polymers
soluble in
water that is also susceptible of ultimate biodegradation in the presence of
suitably
acclimated microorganisms. Accordingly, increasing attention is being devoted
to the
preparation of environmentally compatible PVA-based materials for a wide range
of
applications. PVA has excellent film and thin-walled container forming
properties,
demonstrating a high degree of impermeability to a number of gases, making it
highly
suitable for use in packaging products for release in an aqueous environment.
It also has
high adhesive strength, and is nontoxic. However, these properties are
dependent on
humidity due to the polymer absorbing water which reduces its tensile strength
but
increases its elongation and tear strength. It is also difficult to
successfully extrude PVA
or PVA-containing compositions which further limits its potential use. In
particular, there
are currently no formulations that can be readily moulded to a minimum
thickness
required (less than 200 microns) to ensure product release at low aqueous
temperatures,
typically at or below 5 C in less than 2 minutes. Such a film is desirable for
applications
such as packaging of laundry products which require release of the laundry
detergent at
low temperatures and on a short wash cycle.
One of the properties of the polymer that it is desirable to improve is the
Melt Flow Index.
This relates to the ease of flow of the melt of the polymer, defined as the
mass of the
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polymer, in grams, flowing in ten minutes through a capillary of a specific
diameter and
length by a pressure applied via prescribed alternative gravimetric weights
for alternative
prescribed temperatures. One such method is described in the standard ISO
1133. A
higher MFI is required to provide a thin walled moulding.
It is known in the art to use internal lubricants within the PVA to increase
its melt flow
index. For example, EP1112316 B1 (PVAXX Technologies Limited) includes a fatty
acid amide in an amount up to 5% by weight. The fatty acid provides
lubrication between
the polymer chains thereby increasing the melt flow of the polymer. However,
they are
insoluble, melting and coating the polymer during processing, which may impede
dissolution of the polymer. Furthermore, the amount of lubricant that can be
used is
limited due to excess lubricant (generally above 5% by weight of the PVA)
separating
out of the blend, thereby restricting their ability to improve the MFI of a
polymer.
The polymer is produced by polymerisation from vinyl acetate and subsequent
hydrolysis
of the polyvinyl acetate (PVAc) which is formed. PVA and PVA-derived polymers
are
soluble in water with their solubility being determined by the molecular
weight of the
polymer and the degree of hydrolysis, i.e. the percentage of acetate groups of
the starting
polymer (PVAc) that has been replaced with OH groups. The higher the degree,
the lower
the solubility and the speed of solution. The differences are much more marked
at low
dissolving temperatures than at high ones due to the formation of crystalline
zones within
the polymer.
It is also known in the art that PVA requires the removal of volatiles from
the composition
before melt processing as without such removal processing is difficult due to
the
formation of steam and subsequent foaming of the polymer. Drying is generally
achieved
using standard drying equipment at a temperature of 90 C for a period of 4-9
hours,
depending on make, model and formulation.
Despite attempts to mould articles from PVA or PVA derived compositions, the
desired
solubility characteristics have not been achieved due to the lack of melt flow
required to
mould a thin walled article that will dissolve in an aqueous solution within a
desired time
frame.
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It is an aim of the present invention to provide water soluble polymer
compositions,
particularly but not exclusively polyvinyl alcohol compositions, that address
or at least
alleviate the aforementioned problems experienced with the polymer
compositions of the
prior art.
Further aims of the present invention are to provide methods for the
production and
extrusion and/or moulding of water-soluble polymer compositions.
BRIEF SUMMARY OF THE INVENTION
According to the invention, there is provided a method of creating a water
soluble
polymer, the method comprising the steps of: extruding a water soluble polymer
composition from an extruder barrel without the use of a die, wherein the
extruder barrel
is not vented other than via its extruder outlet, to produce an irregularly
shaped polymer
extrudate; directing the irregularly shaped polymer extrudate onto a chilled
conveyor to
rapidly cool the irregularly shaped polymer extrudate to below 60 C;
granulating the
irregularly shaped polymer extrudate to form a granulate.
Optionally, the granulation of the irregularly shaped polymer extrudate may
occur
immediately following the chilled conveyor without a further drying step.
Preferably, the method may further comprise the step of forming the granulate
into a
water-soluble polymer product.
Preferably, the water soluble polymer may comprise a water soluble polymer
composition
with at least 15% by weight of the total weight of the composition of a
hygroscopic salt
to act as a lubricant to render the polymer extrudable and/or mouldable and
with a solvent
polymer plasticizer, wherein the solvent polymer plasticizer is mono-propylene
glycol or
di-propylene glycol, and wherein the water content of the composition is of
less than 10%
by weight of the total weight of the composition.
Preferably, the hygroscopic salt may be provided in a higher amount by weight
than the
solvent polymer plasticizer.
Preferably, the hygroscopic salt may be an anhydrous or hydrated salt selected
from the
group consisting of: sodium chloride, sodium citrate, magnesium chloride,
calcium
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chloride, potassium chloride, sodium sulphate, sodium carbonate, potassium
carbonate,
and ammonium carbonate.
Preferably, the hygroscopic salt may be a water-soluble salt which will
dissolve at a rate
of at least 10% by weight in 90 C water in ten minutes.
Preferably, the hygroscopic salt may have a water content of less than 10% by
weight of
the total weight of the composition.
Preferably, the hygroscopic salt may be included in an amount of at least 25%
by weight
of the total weight of the composition.
Preferably, the hygroscopic salt may be included in an amount of at least 30%
by weight
of the total weight of the composition.
Preferably, the hygroscopic salt may be in the anhydrous form.
Preferably, the water-soluble polymer and hygroscopic salt may be provided in
solid
form.
Preferably, the polymer may comprise a polyvinyl alcohol polymer.
Preferably, the method may further comprise the step of extruding the water-
soluble
polymer composition without using a die.
Preferably, the method may further comprise the step of extruding the water-
soluble
polymer composition onto a chilled conveyor.
Preferably, the chilled conveyor may be at or below a temperature of between
10 C and
30 C.
Preferably, the method may further comprise the step of granulating the water-
soluble
polymer composition immediately following the chilled conveyor without a
further
drying step.
Preferably, the water-soluble polymer may comprise a wax to improve humidity
resistance.
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Accordingly, the present invention provides a melt-processable water soluble
polymer
composition comprising a blend of a water soluble polymer with at least 15% by
weight
of the total weight of the composition of a hygroscopic salt to act as a
lubricant to render
the polymer extrudable and/or mouldable, wherein the composition has a water
content
5 of less than 10% by weight.
Preferably, there is provided at least 20% by weight of the total weight of
the composition
of the hygroscopic salt. Furthermore, the water-soluble polymer may be solid
at ambient
temperature. More preferably, the polymer comprises a polyvinyl alcohol
polymer. The
PVA used in the present invention is not limited to any particular degree of
hydrolysis.
Partially or fully hydrolysed PVA may be used in the present invention.
Similarly, the
PVA is not limited to a particular molecular weight. The PVA may have a
relatively low
molecular weight of around 20,000 up to and beyond a molecular weight of
150,000.
The PVA preferably has a maximum water content of 5% by weight. It has been
surprisingly found that the hygroscopic salt not only draws water from the PVA
but also
acts as an internal lubricant for the PVA increasing its Melt Flow Index.
Preferably, the hygroscopic salt is an anhydrous or hydrated salt selected
from an alkaline
or alkaline earth metal salt. Depending upon the end use of the composition,
it may be
beneficial to use a salt that is approved for food and/or pharmaceutical use
and/or has
other properties that may impart additional benefits in the end product, such
as a water
softener. More preferably, the salt is selected from the group consisting of
sodium
chloride, sodium citrate, magnesium chloride, calcium chloride, potassium
chloride,
sodium sulphate, sodium carbonate, potassium carbonate, and ammonium carbonate
especially being sodium chloride or citrate.
A preferred embodiment of the first aspect of the invention comprises a blend
of a water-
soluble polymer and an amount of sodium chloride effective to render the blend
extrudable.
The salt has a water content of less than 10% by weight, preferably less than
1% by
weight, more preferably less than 0.5% by weight, especially less than 0.2%.
Ideally, the
salt is in the anhydrous form. The salt may be micronized, for example the
particles
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having a mean size of less than 100 microns, preferably being in the range
0.03-75 p.m,
especially 60-70 p.m. The salt may also be coated to improve its properties,
for example
with sodium aluminosilicate, silicon dioxide and/or sodium hexacyanoferrate.
One such
example is sodium chloride salt coated with Sodium Aluminosilicate 0.5%
(E554),
Silicon Dioxide 0.75% (E551) and Sodium hexacyanoferrate (E535) as an anti-
caking
agent, available from Custom Powders (www.custompowders.co.uk).
The salt may be included in the composition in an amount up to 75% by weight
of the
total weight of the formulation. The salt is included in an amount of at least
15%, and
preferably at least 20%, by weight of the total weight of the formulation,
more preferably
at least 25% by weight, more preferably at least 30% by weight, more
preferably at least
35% by weight, more preferably at least 40% by weight, more preferably 45% by
weight,
and especially at least 50% by weight.
The composition may be compounded with optional additives to improve
processability
of the composition, such as plasticizers to enhance flexibility and/or lower
the melt
temperature of the polymer under extrusion or moulding, stabilizers to
increase heat
resistance and/or pigments to add colour. Preferably, thermal stabilizers,
such as metal
stearates, are included in an amount up to 0.5% by weight, preferably up to
0.3% by
weight. However, the composition is preferably free from any fatty acid amides
or esters.
In an aspect of the present invention, the inbound water from other
constituents within
the composition surface, such as the polymer itself, treat the salt to enable
it to act as an
internal lubricant. An "internal" lubricant functions to improve lubrication
between
polymer chains. The use of a salt as a lubricant removes the need to use other
types of
prior art internal lubricant, such as fatty acid amides or esters.
An example composition without solvent plasticizer may be as follows:
PVOH (80% hydrolyzed) 69.7%
Sorbitol 10.0%
NaCl 20.0%
Thermal stabilizers 0.3%
Total 100%
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Melt Flow Index tested 190 C using a 2.16 kg weight grams in 10 minutes = 21.5
The sodium chloride is more readily soluble in water than the polymer, but is
not a
thermoplastic resin. Therefore, it would be expected that the angular
crystalline structure
of the salt would not mix with the polymer and would actually impede melt
flow.
Surprisingly, this is not found to be the case.
Preferably, the salt is provided in a higher percentage by weight than the
solvent polymer
plasticizer. More preferably, the ratio of salt to solvent polymer plasticizer
is 1.25-12:1,
especially 1.25-7:1, ideally 4-5:1.
Alternatively, the composition may include a solvent polymer plasticizer,
preferably
.. being a hygroscopic organic solvent, more preferably being selected from
glycerine (also
known as glycerin or glycerol) and propylene glycol. Preferably, the solvent
polymer
plasticizer is mono-propylene glycol, and in an even more preferable
embodiment, the
solvent polymer plasticizer is di-propylene glycol.
According to the invention, there is provided a melt-processable water soluble
polymer
composition comprising a blend of a water soluble polymer with at least 15% by
weight
of a hygroscopic salt to act as a lubricant to render the polymer extrudable
and/or
mouldable.
Another aspect of the present invention provides a soluble polymer internal
lubricant
comprising a blend of a hygroscopic salt and solvent polymer plasticizer of
the salt.
The lubricant according to the present invention may be blended with a water-
soluble
polymer for processing thereof. In this respect, a further aspect of the
present invention
provides a melt-processable water soluble polymer composition comprising a
blend of a
water soluble polymer with an internal lubricant to render the polymer
extrudable and/or
mouldable, the lubricant comprising a hygroscopic salt, preferably an
anhydrous or
hydrated metal salt, blended with a solvent polymer plasticizer. Preferably,
the lubricant
is in accordance with the preceding aspect of the invention.
It is preferable for the salt to have minimal water content, preferably the
salt having a
water content of less than 10% by weight, the salt comprising at least 15% by
weight of
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the total weight of the formulation, more preferably at least 40% by weight,
especially at
least 50% by weight.
The lubricant according to the invention when mixed with the water-soluble
polymer
preferably has a Melt Flow Index of at least 20g (10 mins/190 C/2.16kg, under
ISO 1133),
more preferably at least 40g, especially 60g.
The compositions of the present invention may be used in foodstuffs and/or
pharmaceuticals. Therefore, it is to be appreciated that, if possible, the
lubricant and other
constituents of the composition have been approved for food and/or
pharmaceutical use.
The composition according to the present invention may be provided in any
suitable form
for further processing but preferably is provided in the form of a powder,
tablet or pellets
for use in extrusion and/or moulding of an extruded and/or moulded product,
such as an
extruded filament, containing the soluble polymer. The composition may be
manufactured by any conventional method, such as by melt compounding or cold
processing, which in this latter case may include calendaring, adapted
calendaring, and/or
compaction. Cold pressing, and more preferably adapted calendaring, may be the
technique of choice.
The compositions of the present invention may further include a plasticizer to
lower the
melt temperature of the polymer under extrusion and/or moulding. The
plasticizer may
be selected from the group consisting of glycerine, ethylene glycol,
triethylene glycol,
low molecular weight polyethylene glycols and low molecular weight amides. A
preferred plasticizer is glycerine, and more preferably mom-propylene glycol
or most
preferably di-propylene glycol. The plasticizer may also function as the
solvent polymer
plasticizer of the salt internal lubricant.
A further aspect of the invention provides a method of making a water-soluble
polymer
composition comprising blending a water soluble polymer with at least 15% by
weight of
the total weight of the composition of a hygroscopic salt to act as a
lubricant to render the
polymer extrudable and/or mouldable, wherein the water content of the
composition is
less than 10% by weight, the method optionally including adding a solvent
polymer
plasticizer.
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A further aspect of the invention provides a method of extruding and/or
moulding a water-
soluble polymer composition comprising softening a composition according to
the
preceding aspects of the present invention to form a melt flow. Preferably,
heat and/or
pressure soften the composition to create a melt flow.
The melt flow preferably has a Melt Flow Index of at least 20g (10 mins/190
C/2.16kg,
under ISO 1133), more preferably at least 40g, and especially 60g. Preferably,
the
polymer composition is moulded into a form having a thickness of less than 200
microns,
preferably less than 100 microns thereby enabling the form to dissolve within
80 seconds
at 5 C in aqueous solution. The moulded form may be any thin walled moulding,
such as
a container, or a film. Extrusion is also possible, although the composition
of the present
invention is especially beneficial for mouldings.
A further aspect of the invention provides for a water-soluble polymer product
formed in
accordance with the preceding aspect of the invention.
A further aspect of the invention provides for a melt-processable water
soluble polymer
composition comprising: a blend of a water soluble polymer with a hygroscopic
salt to
act as a lubricant to render the polymer extrudable and/or mouldable, a
solvent polymer
plasticizer; and a wax, wherein the wax content of the composition is at least
0.3% by
weight of the total weight of the composition; wherein the water content of
the
composition is of less than 10% by weight of the total weight of the
composition; and the
hygroscopic salt is included in the composition in an amount of at least 15%
by weight
of the total weight of the composition.
Preferably, the melt-processable water soluble polymer composition may have a
wax
content of at least 1.0% by weight of the total weight of the composition.
Preferably, the wax may be glycerol mono stearate.
DETAILED DESCRIPTION OF THE INVENTION
The melt-processable compositions of the present invention may be processed by
any
known thermoprocessing method, including but not limited to, injection
moulding,
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compression moulding, rotational moulding and film extrusion. The compositions
are
particularly suitable for thin-walled mouldings.
The melt-processable compositions of the invention are suitable for the
manufacture of
any article currently made from extrudable and/or mouldable polymers,
including films,
5 containers and bottles. The compositions are suited to the manufacture of
filaments and
fibre, for use in spunbond, non-woven and melt-blowing applications. The
composition
is suitable also for manufacture of such articles as detergent and agro
chemical sachets
and containers, mulch films, plant pots, domestic bags, diapers, drinking
straws, fem care
products, hangers, incontinence pads, sachets, six pack rings, disposable
clothing,
10 expanded foams, gloves, film canisters, golf tees, shot gun cartridges,
bed pans, bottles,
bowls, cotton buds, hospital curtains, " one-use " sterile products and
packaging materials.
PVA generally has moisture content up to 5% by weight. This has to be reduced
to below
1% to avoid processing issues on standard thermoplastic equipment, such as the
generation of volatiles which causes foaming. Conventionally, the polymer is
dried in a
standard polymer dryer for 4-8 hours at 90 C. It has been found that the
addition of a
hygroscopic salt in a particular amount, preferably having a low water content
or being
anhydrous, removes water from the surrounding PVA. More surprisingly, the
absorption
of water by the salt provides a self-lubricating coating on the salt which is
then able to act
as an internal lubricant for the PVA. The desiccant effect of the salt reduces
drying time
to 2-4 hours enabling a substantial energy saving in the polymer production
and
furthermore, the lubrication brought about by the salt greatly increases the
Melt Flow
Index of composition such that the PVA may be readily extruded and/or moulded
into
products and, in particular to thin forms of less than 200 microns making it
suitable for
applications where dissolution of the film and/or moulding is required at
temperatures as
low as 5 C within a short time period (under 2 minutes). The self-lubricating
effect is
still active at low moisture levels of less than 1%, i.e. even during the
drying process. It
is feasible that the temperature may be decreased further and therefore closer
to 0 C, and
the time may also be decreased.
The hygroscopic salt, such as sodium chloride, has been found to have an
enhanced
lubricating effect if polymer plasticizers are included in the composition,
such as
hygroscopic salt solvents including glycerine or propylene glycol, and
particularly mom-
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propylene glycol or most preferably di-propylene glycol. Absorption of water
by the salt
appears to act as a surface treatment enabling non solvent plasticizers to be
used in the
formulation, if desired. Anhydrous salts would not normally be considered as
suitable for
internal lubrication of water-soluble polymers. In this respect, precipitated
calcium
carbonate (PCC) has been used in small amounts (2-3%). Alcohol plasticizers
are not
solvents of PCC so high loadings have to be used to produce high melt flow
resulting in
very ductile products as the PVA becomes encapsulated around the PCC
particles. Melt
flow indexes are also disappointing. In comparison, the present invention uses
glycerine,
mono-propylene glycol, or di-propylene glycol, which are solvents of anhydrous
sodium
chloride, to partially dissolve the outer surface of the sodium chloride to
provide a
lubricant within the polymer chains. This produces a comparatively high MFI
and
increases the solubility of the polymer, whilst decreasing its ductility (a
desirable trait).
Sodium chloride is one possible hygroscopic salt which can be used here.
Sodium citrate
and magnesium chloride are other possible alternatives, as are calcium
chloride,
potassium chloride, sodium sulphate, sodium carbonate, potassium carbonate,
and
ammonium carbonate. The requisite characteristics of the hygroscopic salt for
the present
invention are that the hygroscopic salt is water-soluble, having the ability
to absorb
atmospheric water, and/or which will achieve a dissolution rate of at least
10% by weight
in 90 C water in 10 minutes.
It is to be appreciated that including high levels of water in the composition
(either as a
plasticizer or binder) even with a hygroscopic salt would not achieve the
benefits of the
present invention. The water would result in reversible dissolution of some or
all of the
salt present. Furthermore, reduction of the water content for successful
processing would
result in removal of the self-lubricating layer and the reformation of salt
crystals of
unpredictable size and shape. This hampers melt flow and would add to the
drying time.
Thus, it is a preference of the present invention that water is not added to
the formulation
beyond the low water content contained in the various constituents, such as
the PVA,
making up the composition.
The present invention will now be described with reference to the following
non-limiting
examples. The examples illustrate the high melt flow values achieved with the
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compositions according to the present invention, their reduced drying times
and compare
these properties with compositions falling outside the scope of protection.
Method of Production
PVA (polymer), sodium chloride (lubricant), glycerin (plasticizer) and thermal
plasticizers were mixed in a bell tub, low-shear mixer for 3 minutes. The mix
was then
fed into a compounder via a screw and formed into pellets via an adapted
calendaring
process. The adapted calendaring process causes partial or complete melting of
the PVA
as a result of frictional shear as it is passed between the roller and die,
causing
agglomeration before extrusion through the die. The temperature of the pre-
extrudate
varied from 110 C to 140 C and the formed pellets were then placed in a tray
polymer
dryer for 3 hours at 90 C.
Melt Flow Analysis
5 gram samples of the formulations prepared according to the invention were
tested for
MFI at 190 C using a 2.16 kg weight. Each sample was tested and compared for
MFI
according to ISO 1133. The test was repeated by a factor of 10 and the mean
result was
recorded.
The samples were moulded using a 50 tonne moulding press in automatic mode
with a
cycle time of 7-10 seconds using a mould with a hot runner system at 180 C to
200 C.
The screw temperature profile (in C) from the hopper to tip was 160, 170,
180, 180-190.
The part wall section was measured between 600 ¨ 350 microns.
Example 1
Formulations were made according to the method above having the ingredients
shown in
Table 1 below mixed in the given percentage by weight. 88% hydrolysed PVA was
used
in each formulation and the thermal stabiliser was calcium stearate. The Melt
Flow Index
(MFI) was determined according to the analysis given above.
Table 1
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Formulation PVA (% Glycerin NaCl Thermal Ratio of MFI/g
by (% by (%by stabilizers salt:
weight) weight) weight) (% by glycerin
minutes
weight)
1 84.7 10.0 5.0 0.3 0.5:1 7.0
2 39.7 8.0 52.0 0.3 6.5:1 44.0
3 38.7 10.0 51.0 0.3 5.1:1 56.0
4 36.7 12.0 51.0 0.3 4.25:1 77.0
5 25.7 14.0 60.0 0.3 4.28:1 78.0
Formulation 2 shown in Table 1 above was found to have a white/cream colour
with the
following properties:
Density 1.68g/cm
Melt density 1.52g/cm at 200 C (under ISO 1183).
5 These results illustrate the importance of having a high ratio of salt to
plasticizer in the
formulation to achieve the desired high MFI but that peak MFI values are
obtained in the
above formulations where the salt to plasticizer ratio is 3.5-5:1, more
preferably 4-4.4:1.
Example 2
Formulations were made according to the method above having the ingredients
shown in
10 Table 2 below mixed in the given percentages by weight. 88% hydrolysed
PVA was used
in each formulation. The Melt Flow Index (MFI) was determined according to the
analysis given above. The part wall section of the moulded formulations was
measured
from 600 microns to 100 microns.
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Table 2
Formulation PVA Propylene NaCl Thermal Ratio of MFUg
(% by glycol (% (%by stabilizers salt:
weight) by weight) (% by propylene
weight) weight) glycol
6 84.7 10.0 5.0 0.3 0.5:1 9.0
7 39.7 8.0 52.0 0.3 6.5:1 40.0
8 36.7 10.0 51.0 0.3 5.1:1 51.0
9 38.7 12.0 51.0 0.3 4.25:1 75.0
Table 2 demonstrates that the type of plasticizer does not have a significant
effect on the
WI achieved
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Example 3
Formulations were made according to the method above having the ingredients
mixed in
the percent by weights shown in Table 3 below. 98% hydrolysed PVA was used in
formulation 10-13 and 80% hydrolysed PVA was used in formulations 14-16. The
5 formulations were moulded using a Boy 50 tonne moulding process in
automatic mode
with a cycle time of 20 seconds using a mould with a cold runner system. The
screw
temperature profile (in C) from the hopper to tip was 160, 170, 180, 180,
220. The part
wall section was measured from 600 to 2000 microns.
Table 3
Formulation PVA (% Glycerin NaCl Thermal Ratio of MFUg
by (% by (%by stabilizers salt:
weight) weight) weight) (% by glycerin
weight)
10 89.0 10.0 0.0 0.3 0:1 1.9
11 84.5 9.5 5.0 0.3 0.52:1 3.48
12 80.1 9.0 10.0 0.3 1.1:1 2.75
13 67.2 7.5 25.0 0.3 3.33:1 1.91
14 20.0 15.7 64.0 0.3 4:1 73.5
15 15.0 11.7 73.0 0.3 6:1 1.9
16 12.0 9.3 78.4 0.3 8:1 0.00
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Example 4
Formulation 17 was prepared as a blend in a similar way to Formulation 2 of
Example 1
but having sodium citrate in place of sodium chloride, as follows:
PVA (88% hydrolysed) 39.0 % by wt.
Sodium citrate 51.0 % by wt.
Glycerol 9.70 % by wt.
Calcium stearate 0.3% by wt
This formulation was found to have the following properties:
Density 1.67 g/cm
Melt density 1.40-1.42 g/cm at 190 C (under ISO 1183)
MFI 38g.
Processing temperature was 190-200 C with a residence time of up to 30
minutes. Drying
time was 4 hours at 90 C. The MFI is again substantially higher with the salt
included in
the composition.
This formulation and that of Formulation 2 were examined for their
extrudability in
injection moulding machines made by Boy, Demag and Arburg. Extrusion
processing
was carried out using a single full flight screw with constant pitch. The
barrel temperature
had a profile of 160-200 C and the screw speed varied typically between 20-
150rpm.
Shut down of the apparatus was carried out by maintaining the temperature at
100 C with
screw rotation stopped. Complete shutdown was then carried out by switching
off the
machine.
Formulations 2 and 17 were capable of being moulded into a range of containers
of
various sizes and colours and were suitable for injection moulding. The use of
sodium
citrate as the polymer lubricant provides additional advantages if it is used
for packaging
laundry products as it acts as a water softener.
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Example 5
A study was carried out to investigate the requirement to have a low water
content in the
formulations of the present invention.
Table 4 below sets out the composition of the formulations, together with
their MFI and
drying time.
Table 4
Formulatior PVA CY Glycerin NaCl Thermal Water
Moisture Moisture Drying MFI/g
by CY by (%by (%by content content
time/hr
weight) weight) weight) Stabilizer weight) pre-
post at 90 C
s(Yoby drying drying
weight)
A 73.0 11.7 15.0 0.3 0.00 2.4 0.9 5.0 22.0
B 37.0 11.7 51.0 0.3 0.00 2.0 0.9 3.0 77.0
C 71.3 10.2 5.2 0.3 13.0 24 0.9 9.0 10.0
D 60.5 9.7 12.5 0.3 17.0 23 0.9 13.0 7.0
Table 4 clearly shows the importance of the amount of salt and water contained
within
the formulation on drying time and on MFI. The formulation has a higher
percentage of
salt (of at least 15%, preferably at least 20%, and more preferably at least
40%) with
minimal or no water content. Formulations C and D which contained 13 and 17%
water
respectively were very sticky formulations that were not free-flowing making
them
unsuitable for compounding. Additionally, the excess drying times resulted in
undesirable
glycerine vapour loss.
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Example 6
The ratio (typically expressed as a percentage) of partial pressure for water
vapor in a
given environment compared to the equilibrium vapor pressure of pure water is
known as
relative humidity (RH). The RH at which a given material begins to deliquesce,
that is,
when water adsorbed onto the material starts to solvate molecules of the
material, is
termed the material's deliquescence point (RHo; also referred to in the art as
"critical
RH"), and is an important parameter characterizing structural stability of a
polymer
material. The RHo is temperature-dependent and is typically expressed as a RHo
value at
a given temperature. By way of example, the RHo of crystalline NaCl at 20
degrees
Celsius (deg. C) is approximately 77% RH ¨ that is, when ambient temperature
is at 20
deg. C, water vapor from the atmosphere will spontaneously be adsorbed onto
and solvate
NaCl crystals when ambient RH is at or above 77%.
A material with a low RHo value tends to absorb moisture more readily and may
serve
well as desiccants. By contrast, a material with a high RHo tends to absorb
moisture less
readily. In the case of polymer compositions, having a higher RHo
advantageously
reduces requirements for secondary packaging, and provides a more structurally
stable
product when opened to an unprotected environment.
One issue with products comprising water soluble polymers is that they tend to
be
hygroscopic and structurally unstable in environments with a high RH. By way
of
example, at an ambient temperature of 20 deg. C, PVA begins to deliquesce when
RH
reaches above 50%.
One issue found with formulations comprising PVA, sodium chloride and
glycerine as
outlined above is that, although the lubricating effect of the glycerine and
salt
combination aids melt processing, the resultant mouldings are susceptible to
moisture
uptake, and secondary packaging is required with high moisture- and gas-
barrier
properties.
Various water-soluble polymer compositions comprising 88% hydrolyzed Polyvinyl
alcohol (PVA) and NaCl were produced using one of three different solvent
polymer
plasticizers: glycerine, mono-propylene glycol and di-propylene glycol. Table
5, provided
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herein below, shows the formulation of nine different formulations, formulas 1-
3 with
glycerine, formulas 4-6 with mono-propylene glycol and formulas 7-9 with di-
propylene
glycol:
Table 5
PVA PVA Di- Mono- Glycerine NaCl Deliquescent
hydrolysi (% by Propylene Propylen (% by (%by Point (RHo at
s (%) weight) Glycol (% e Glycol weight) weight) 20 C)
by weight) (% by
weight)
88.0 74.0 8.0 15.0 83%
88.0 62.0 10.0 25.0 82%
88.0 37.0 10.0 50.0 80%
88.0 74.0 8.0 15.0 77%
88.0 62.0 10.0 25.0 77%
88.0 37.0 10.0 50.0 76%
88.0 74.0 8.0 15.0 66%
88.0 62.0 10.0 25.0 66%
88.0 37.0 10.0 50.0 62%
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As shown in Table 5, it was found that using propylene glycol as the solvent
polymer
plasticizer surprisingly resulted in a less water-absorbent and thus more
stable PVA-based
polymer product relative to comparable formulations using glycerine as the
solvent
polymer plasticizer. Whereas formulations 7-9 with glycerine demonstrated an
RH0 at
5 20 C of 62% to 66%, all formulations with propylene glycol had a
substantially higher
RH0 at 20 C, between 76% and 83%.
Moreover, formulations with di-propylene glycol were found to be even more
stable than
comparable formulations with mono-propylene glycol; formulations 1-3 with di-
propylene glycol had an RH0 at 20 C of 80% to 83%, whereas formulations 4-6
with
10 mono-propylene glycol had a lower RH0 at 20 C of 76% to 77%.
The deliquescent point of sodium chloride is around 77% RH0 at 20 C; however,
the
resultant moldings made with formulations to include PVA, sodium chloride and
glycerine have a tendency to deliquesce over 60 RH0 at 20 C.
The surprising effect of substitution of di-propylene glycol for glycerine is
that the
15 deliquescent point of the sodium chloride is raised over 82 RH0 at 20 C.
This reduces the
specification requirement for secondary packaging, as well as producing a more
dimensionally stable product when opened to an unprotected environment.
Example 7
Formulations involving di-propylene glycol and sodium chloride were made to
compare
20 melt-flow indexes.
Table 6
PVA PVA (% by Di-Propylene NaCl (%by MFI/g
hydrolysi weight) Glycol (% by weight)
10 minutes
s (%) weight)
88.0 74.0 8.0 15.0 10.0
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88.0 69.0 8.0 20.0 19.0
88.0 64.0 8.0 25.0 20.0
88.0 57.0 10.0 30.0 33.0
88.0 52.0 10.0 35.0 33.0
88.0 47.0 10.0 50.0 34.0
88.0 42.0 10.0 45.0 40.0
88.0 45.0 12.0 50.0 44.0
Appreciable improvements in melt-flow index are demonstrated as the
hygroscopic salt
concentration increases.
Further Method of Production
Traditionally in the art, extrusion is achieved via a slot die or strand die
to manufacture
PVA resin. Both the slot die and strand die provide sufficient back-pressure
to create an
even flow of polymer melt from the extruder to where the extrudate must be
cooled. This
is typically performed by water immersion or by cooling with ambient or
chilled air.
The problem with water immersion is that the PVA is water-soluble, and
therefore further
drying would be required post-immersion.
When cooling with ambient air, the issue is the loss of plasticizer from the
formulation
until the extrudate is cooled below 60 C.
PVA blended with lubricating hygroscopic salts, as formed according to the
present
method is more soluble and more brittle, and therefore unsuited for strand
extrusion and
water cooling. When cooled in water, the pellets become slimy due to
dissolution, sticking
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together and therefore further drying is required. Strands will also become
brittle due to
the reduced polymer and high salt content, as well as loss of plasticizer by
up to 0.75%
during air cooling. The process is not stable due to strand breakage and loss
of material,
resulting in process down-time. The volatile loss of plasticizer also leads to
inconsistent
product results, dependent on the ambient conditions and time taken to cool.
Melt-flow
indexes can vary greatly
Slot dies produce a tape for extrusion, and does not form a pellet for
reprocessing.
Air cooling is preferred when compared to a traditional water bath as
described. It is,
however, possible to cool the extrudate with fan-assisted air supported on a
moving
conveyor. The extrudate strand leaves the strand die at 165 C to 200 C and
reaches a
stand pelletizer at a maximum temperature of 60 C. The plasticizer evaporation
described
above occurs on this moving conveyor. This reduction in mass results in
financial loss, in
addition to having an adverse effect on the melt-flow index.
A new method of cooling of the extrudate has therefore been developed to cope
with the
improved polymer composition. For convenience of presentation, the new method
may
be referred to as a chilled roller method
In the chilled roller method, the polymer is extruded without additional
venting in the
barrel, and without a slot or strand die. With no back-pressure, the barrel
vents steam and
produces an extrudate which is (1) irregular and inconsistent in size and
shape, and (2)
very low in water content, preferably below 1% (v/v). This is a significant
difference to
a conventional process in which the polymer is extruded into a slot or strand
die. Slot or
strand die methods require the extrudate to have a regular and consistent size
and shape
to remain operational, and since the presence of steam in the barrel causes
the
irregularities, the additional venting to the barrel must be provided for slot
or strand die
methods.
Irregular extrudate is permitted to fall onto a chilled roller or conveyor,
where the
temperature chills the extrudate quickly to below 100 C but above 10 C, and
preferably
below 60 C. The chilling process is near instantaneous, occurring in less than
30s, more
preferably less than 20s, even more preferably less than 10s, and generally in
less than
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5s. The chilled rollers are set to obtain instant cooling, to thereby avoid
plasticizer loss.
In practice, if the extrudate were to cool slightly before chilling in an
instantaneous
manner, it would no longer be sufficiently malleable to pass through the
chilled rollers.
Immediate passage must be made to maintain sufficient malleability for
rolling. The
irregular extrudate, once chilled, was directed into a commercial polymer
grinder to be
made into granules, which can be stored for onward processing, typically via
encapsulation into hermetically sealed vacuum bags.
Adapted calendaring with, preferably water-chilled, chilled rollers to
accommodate an
inconsistent extrudate permits fast cooling of the extrudate to 60 C and
below.
Additionally, the high heat transfer properties of PVA with at least 15% by
weight of
hygroscopic salts results in further improved cooling via this system.
If no salt were used in the formulation, lower temperature rollers would be
required, that
is, below 10 C to obtain the same cooling rate to prevent loss of plasticizer.
This can
cause condensation onto the chilled rollers which can cause process issues and
extrudate
sticking to the rollers. This renders the process ineffective. Water
temperature of between
10 C and 30 C has been found to be most efficient for PVA and salt
compositions,
whereas below 10 C is required for saltless compositions, though the
condensation proves
to be an issue here.
Below 60 C, plasticizer becomes locked into the polymer composition, being
below the
plasticizer evaporation temperature, preventing losses of the total weight of
the polymer
composition in the range of 0.75% to 1.25% due to evaporation. The resulting
polymer
also has a very low water content, as most of the water present is lost during
the extrusion
process as steam. Water content can then be maintained below 1% by weight
which is the
maximum possible water content required for successful reprocessing of the
irregular
extrudate. This also removes the need for a further polymer drying step.
A comparison between examples products of embodiments of the chilled roller
method
described herein above with a conventional chilled air stand line process,
that is, the
process in which regular extrudate is extrudate directly into a strand for air
cooling, is
outlined in Tables 7 and 8 below.
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Table 7 ¨ Chilled Roller Method
PVA PVA Di- NaCl MFI/g Extrudate Extrudate
hydrolysi (% by Propylen (%by Temperature Temperature
s (%) weight) e Glycol weight at Extruder at Granulator
minutes
(% by ) ( C) ( C)
weight)
88.0 59.0 8.0 30.0 17.0 190 59
88.0 58.0 9.0 30.0 20.0 200 60
88.0 57.0 10.0 30.0 23.0 185 57
Table 8¨ Extruder and Stand Line
PVA PVA Di- NaCl MFI/g Extrudate Extrudate
hydrolysi (% by Propylen (%by Temperature Temperature
s (%) weight) e Glycol weight at Extruder at Pelletiser
minutes
(% by ) ( C) ( C)
weight)
88.0 59.0 8.0 30.0 14.0 190 60
88.0 58.0 9.0 30.0 17.0 200 65
88.0 57.0 10.0 30.0 21.0 185 62
As can be seen, the chilled roller method results in an extrudate which has a
higher melt-
flow index under otherwise identical conditions. In addition, extrudate
temperature is
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consistently lower at the point of reaching the granulator in embodiments of
the chilled
roller method, thus reducing plasticizer evaporation.
The irregular extrudate, once chilled, can then be directed into a commercial
polymer
grinder or granulator, and the polymer ground until the particulate matter
falls through a
5 sieve plate to determine maximum particulate size. The irregularity of the
extrudate
however creates a variable polymer grain size, typically in the range of 2mm
to 4mm.
Surprisingly, this non-uniformity of particular size aids polymer melt during
subsequent
reprocessing. As such, the combination of the chilled roller method is highly
suited
towards use with a granulator. Smaller particles melt more quickly, requiring
less energy,
10 with an observable 10 C reduction in barrel zone temperatures when compared
with a
strand die pellet process. Lower processing temperatures also result in lower
plasticizer
loss rates, which might otherwise turn to gas and cause gassing during
reprocessing,
usually via injection molding or extrusion. Gassing can lead to burn marks on
molding,
and part rejection and mold contamination during processing, so reduction in
gassing is a
15 noticeable improvement.
Vegetable wax is typically used to impart humidity resistance to a polymer.
Typically,
0.3% to 1.0% of glycerol mono stearate or a stearamide would be used as the
vegetable
was. However, usage of vegetable wax during the melt process also has a
disadvantageous
effect of inducing "screw slip". Screw slip is a situation when, during the
melt process,
20 the extruder machine or injection moulder is unable to convey molten
material due to
slipperiness of the polymer melt, with the vegetable wax coating the barrel as
a lubricant.
For PVA having no salt included, 0.3% wax content would typically be the
maximum
permissible amount due to screw slippage, though with small amounts of salts,
higher
wax content, up to 2% to 3% can be provided. The addition of these vegetable
waxes
25 decreases water absorption of PVOH formulations with mono and di
propylene glycol by
as much as 50% when compared with formulations without salts.
Surprisingly, irregularity of the particulate shape also advantageously
results in a
reduction of screw slip when formulating the polymer composition including
amounts of
vegetable wax that would typically be expected to result in screw slip.
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=
The irregular size and shape of the granules prior to melt forms a mixture
more gradually
in the present invention, allowing for better adhesion during polymer transfer
via the
screw. This allows for a larger wax component to be added.
The compositions of the present invention thus provide a melt-processable PVA
containing polymer typically having a flexural modulus similar to other
extrudable
polymers. This enables a soluble and biodegradable polymer to be used for the
processing
of a wide variety of articles without the processing problems experienced in
the prior art,
such as thermal degradation and high temperature cross-linking. The known
advantageous properties of PVA, such as its high tensile strength and good
barrier
characteristics, are retained in the melt-processable composition, which may
be extruded
on current extrusion lines, blow-moulders and injection moulders without
modification.
