3
Additives and Compounding
The market for plastics additives is expected to grow 4.5% per year
through to 2010 [1]. The additives market in the USA is worth $32
billion, and usage of additives in poly(vinyl chloride) (PVC) accounts
for 85% of the total market [2].
Additives are necessary to modify and improve the properties of
plastics. To make the processability of PVC easier, it has been
necessary to add heat stabilisers, lubricants, pigments, and fillers.
In PVC, heat stabilisers and lubricants mitigate properties such
as degradation and adhesion to equipment, which are undesirable
features.
From the beginning of the PVC industry, much has been learnt about
processing difficulties. This has led to studies into the development of
additives. Even though there is growing concern about the safety of
additives such as phthalate plasticisers, halogenated flame retardants,
and lead-based heat stabilisers used for PVC production, usage of
these additives continues in many parts of the world.
3.1 Poly (Vinyl Chloride) Formulation
PVC requires stabilisers along with certain types of lubrications
to prevent PVC from adhering to the surface and altering the
flowability of the melt. The lubrication used is dependent upon the
application (pipe, profile, or sheet extrusion). Wax-based or stearic
acid-based lubricants as used in all three systems (injection, extrusion,
calendaring) are similar. However, they are used in different quantities
and combinations according to the final application (pipe, profile, or
23
Update on Troubleshooting the PVC Extrusion Process
sheet extrusion). Pigmentation and ultraviolet (UV) stabilisation is
provided in most cases by the highly abrasive material TiO2. Calcium
carbonate has always been considered to be another important
ingredient.
The additives used in PVC formulations are mainly plasticisers,
stabilisers, lubricants and fillers. Some additives can migrate to
the surface during use, where they are lost by volatilisation or
diffusion upon contact with other surfaces. Stabilisers for PVC
have low mobility but can change their function by consumption or
degradation. Fillers usually remain in their initial form and quantity.
Thus, to determine if a used PVC product can be reprocessed, an
essential step before reprocessing is the determination of the degree
of deterioration of the chemical structure of the base polymer as well
as the loss of additives and their functionality [3].
At the heart of a dynamically processed PVC component is a
formulation containing lubricants and heat stabilisers. These
key ingredients are the cornerstone of every PVC formulation
[4]. However, in PVC formulations or products, additives alone
cannot provide a balance of final product properties and processing
characteristics.
Tin-based stabilisers are used in the USA for PVC formulations.
Tin-based stabilisers have a lower lubricating effect. Lead-based
stabilisers are very common in Europe and Asia. They have fairly
good heat-stabilising effects but few lubricating effects. Relatively
non-toxic calcium–zinc stabilisers are used in many countries.
Calcium–zinc stabilisers have poor thermal stability and there are
concerns regarding their toxicity [5].
3.2 Role of Additives
Additives should:
• Achieve an optimum balance of ecological and economical
benefits
24
Additives and Compounding
• Enable tailor-made systems to be made
• Be able to be used for product engineering
• Have strong influences in applications
• Perform in the polymer
• Enhance the essential key properties of plastics
• Ensure efficient processing without sacrificing physical properties
• Provide higher performance
• Provide the intended functions
• Be thermodynamically constrained from free migration to the
surface
Additives belong to a broad category of essential components in PVC
formulations. It is difficult to define all the functions of additives in
PVC formulations. Additives are chemicals used in plastics with or
without reactions. Inorganic as well as organic substances termed
‘PVC additives’ can be used to suppress the undesirable effects which
coincide with plate-out phenomena [6]. Additives make PVC useful
and versatile. In PVC products, inappropriate processing results in
poor quality of the end product. However, additives are defined by
different sources in different contexts [7].
Additives are required in minimum quantities during the processing
of PVC and its end-product applications. Additives such as phthalate
plasticisers may lead to leaching and can change surface properties.
The additives used in PVC formulations are mainly stabilisers,
plasticisers, lubricants, processing aids and fillers. Coupling agents
and antimicrobial agents are used in PVC production if necessary.
Additives do not change the PVC particles, but could change the
volume of the external voids between them. This is attributable to the
efficiency of each additive, and its structure and polarity. However,
25
Update on Troubleshooting the PVC Extrusion Process
the additives lead to denser packing of the PVC powder. This reduces
its inter-particle interaction at the expense of interaction between the
additive and PVC particle [8].
3.3 Classification of Poly (Vinyl Chloride) Additives
Economic cost and processing by injection molding, extrusion and
calendering make PVC a universal polymer with many applications
[9]. Such applications include pipes, profiles, floor coverings, cable
insulation, roofing sheets, packaging foils, bottles, and medical
products.
The PVC industry uses a large amount of additives. PVC is an
important thermoplastic with low stability among carbon-chain
polymers, and undergoes severe degradation with elimination of
HCl below its melting temperature [10–12]. Therefore, in the PVC
industry, the significance of additives increases as the formulations
become more precise and sophisticated [13]. The amount of additive
added in the formulation varies with respect to the processing
technique used in the manufacture of products. Additives are usually
employed in small quantities to improve the processing, performance,
appearance and use. The major factor responsible is the ability to
compound with many additives to a wide range of flexible and rigid
products.
The additives used in PVC can be classified as:
• Heat stabilisers
• Lubricants
• Impact modifiers
• Plasticisers
• Fillers
• Flame retardants
26
Additives and Compounding
• Blowing agents
• Pigments
• Coupling agents
• Smoke suppressants
3.3.1 Heat Stabilisers
Pure PVC is a rigid polymer at room temperature with low thermal
stability. Hence, PVC requires heat stabilisers during processing at
high temperatures. The stability of PVC can be readily modified by
using heat stabilisers. Many metallic compounds have been proposed
and used as thermal stabilisers to protect PVC during processing
and shaping.
The main functions of stabilisers are to:
• Prevent degradation during processing
• React with HCl when it is liberated from PVC
• Replace labile chlorine atoms (this may initiate the dehydrochlorination of more stable groups)
• Enhance heat stability
Thermal stabilisers based on compounds with lead, tin, barium,
calcium, and zinc have been employed for decades to improve the
stability of PVC during processing. Common thermal stabilisers in
use for the stabilisation of PVC are usually basic lead salt, metallic
soaps, as well as esters or mercaptides of dialkyltin. These stabilisers
seem to be established in many parts of the world. These metal
stabilisers are utilised in PVC compounding and processing because
PVC catalyses its own decomposition. Heat stabilisers are used in
PVC to construct and extend the life of the end-product. Addition of
27
Update on Troubleshooting the PVC Extrusion Process
a sufficient quantity of heat stabilisers prevents dehydrochlorination
and discoloration during processing and application [14–16].
The poor stability of PVC requires heat stabilisers during processing.
PVC has been used in the study of various aspects of stabilisation.
Thermal stabilisers based on lead, tin, and calcium–zinc are
established in certain parts of the world. Several organometallic
compounds and inorganic salts have also been used as heat stabilizers
for many years. After stabiliser addition, part of the stabiliser will be
consumed during processing and sometimes during the application
period. Currently, obtaining increased stability with low metalcontent stabilisation is the key research area.
Stabilisers must (i) breakdown the polyene sequences which cause
discoloration of degraded PVC, and (ii) inhibit dehydrochlorination
[17]. Different kinds of stabilisers are used to inhibit the release of
hydrogen chloride from PVC to ensure adequate processing.
Thermal stability can be improved by:
• Using highly specific material for particular applications
• Understanding the degradation and stabilisation of PVC
• Formulating with stabilisers
Due to the requirement of heat stabilisers to stop the thermal
degradation of PVC, different types of metal soap (e.g., stearates of
lead, cadmium, barium, calcium, and zinc) are used. Metal soaps of
dicarboxylic acids are fairly heat-stable and may be suitable as PVC
stabilisers. Thermal stabilisers of lead and tin are the most effective.
They can be substituted by non-toxic calcium–zinc stabilisers due to
the toxicity of heavy metals. Mono- and di- alkyl compounds (e.g.,
maleates, carboxylates, mercaptides) of tin are also used as heat
stabilisers during PVC processing [18]. These stabilisers retard the
appearance of discoloration during processing by accepting liberated
HCl from PVC [19–23]. The efficiency of the stabiliser decreases after
compounding with PVC with the necessary additives. The residual
28
Additives and Compounding
stability of the PVC products is a useful feature before they are
recycled [24]. Heat stabilisers of PVC include metal salts of organic
acids [25–28], organometallic compounds, and inhibitors of radical
chain reactions.
Increased addition of heat stabilisers in the PVC formulation decreases
the maximum concentration of HCl, and efficiency is increased as
the calcium-to-chloride molar ratio increases. However, the efficiency
is not proportional to the increase in the amount of stabiliser [29].
PVC catalyses its own decomposition, so metal stabilisers are added to
vinyl for construction and other extended-life applications. Common
PVC additives that are particularly hazardous are lead, cadmium,
and organotins, with global consumption of each by vinyl estimated
to be in the thousands of tonnes per year.
3.3.1.1 Lead Stabilisers
In PVC, lead stabilisers have proved to be very successful. In the
formulations, lead stabilisers containing lead octate, lead stearate,
tribasic lead sulphate, dibasic lead phthalate, and dibasic lead
phosphate are used for different applications. Lead stabilisers provide
heat-ageing resistance, prevent discoloration, and steady process
viscosity. Other non-lead additives have been suggested to supplement
lead stabilisers [30]. Heavy metal stabilisers have become less popular
due to environmental concerns [31]. Lead stabilisers have proved
to be very successful in stopping HCl from being released during
processing. Such stabilisers start from lead sulfates to the higherperformance phthalate and fumarate-based systems. In many PVC
products, lead replacement has made significant inroads.
3.3.1.2 Secondary Heat Stabilisers
In PVC formulations, along with lead compounds, secondary organic
stabilisers such as epoxides, polyols, ß-diketones, and dihydropyridine
29
Update on Troubleshooting the PVC Extrusion Process
can be used [19, 20, 32–35]. In secondary stabilisers, non-metallic
epoxy compounds enhance the effectiveness of metal soaps [36].
Highly basic calcium stearates can be superior to neutral or slightly
basic grades of calcium stearate for use as secondary heat stabilisers
for PVC. This allows for the use of lower levels of lead- or organotinbased heat stabilisers. This offers overall improved economics and
weathering performance, along with retention of the processing
characteristics and physical properties of rigid PVC compounds. The
synergistic effects of highly basic calcium stearates with low levels of
organotin stabilisers should allow for the cost-effective replacement
of other stabilisers with more environmentally acceptable stabilisers
[37].
We reported on the epoxidation of sunflower oil and the effects of
epoxidised sunflower oil (ESO) on the thermal degradation and
stabilisation of PVC in the presence of metal carboxylates (Ba/Cd
and Ca/Zn stearates) [38]. ESO showed excellent properties as a
secondary stabiliser for PVC. These marked effects of ESO were not
observed in the absence of metal carboxylates. Further investigations
on the thermal stabilisation of PVC by ESO in combination with Ca/
Zn stearates have also been reported. The influence of the amount of
oxirane oxygen in the ESO and of the ratio of Zn and Ca stearates
(1/1, 1/1 and 2/1) have also been considered.
Several inorganic lead compounds and organic secondary stabilisers
such epoxides, polyols, phosphites, b-diketones, and dihydropyridine
are also used in industrial recipes [19, 20, 32–35]. Epoxy compounds
are typical non-metallic stabilisers for PVC [36]. They are generally
regarded as secondary stabilisers and used to enhance the effectiveness
of metal soaps. They act as acceptors for the liberated HCl [21, 32]
and retardants for the appearance of discoloration [22, 23].
3.3.1.3 Tin Stabilisers
Tin stabilisers inhibit the degradation of PVC during processing
by co-ordination with the labile chlorine sites of PVC. This is
30
Additives and Compounding
particularly true in the case of carboxylate or thiolate groups, or
esters or mercaptides of dialkyltin [39–43]. Tin stabilisers form three
types of bonds: SnC, SnO and SnS. This is in comparison with metal
soaps, which form only MO (where M is the metal). The stability
of the bond between tin and carbon is a critical factor because any
further reaction with HCl leads to the formation of a Lewis acid,
RSnCl3 or SnCl4.
In practice, organotin stabilisers can decompose peroxides and exhibit
retarding effects [44]. The higher the content of dibutyl tin dilaurate,
the greater is the concentration of shorter polyene sequences [45].
3.3.1.4 Calcium–zinc Stabilisers
Basic calcium- and zinc-based products have initial heat stability.
However, they are susceptible to inferior long-term heat ageing along
with water/moisture access over relatively short time periods. Hence
the insulation properties of PVC are affected [46].
Mixtures of calcium/zinc carboxylates are becoming important heat
stabilisers due to their lack of toxicity. These are some of the oldest
stabiliser systems [47, 48]. In such mixtures, calcium carboxylate
reduces the rate of elimination of HCl from PVC, and the calcium
soap reduces the rate of dehydrochlorination by avoiding the
1,3-rearrangements and controlling the propagation of the HClelimination reaction. Such mixtures act mainly as HCl scavengers
[50]. Calcium and zinc carboxylates can react with labile chlorine
atoms in PVC.
Mixtures of Ca/Zn carboxylates are becoming important again due to
their lack of toxicity. Metal carboxylates were considered to be HCl
scavengers until Frye and Horst [49] demonstrated the esterification
reaction with the polymer by substituting with allylic chlorides. It is
well known that the Zn carboxylate is the most active and that the
Ca carboxylate acts mainly as an HCl scavenger.
Zinc stearate has undesirable effects on stabilisation and
31
Update on Troubleshooting the PVC Extrusion Process
promotes a sudden dehydrochlorination of PVC. Increase in the
concentration of zinc chloride as the reactive product induces sudden
dehydrochlorination [51]. Hence, zinc chloride is inactivated and
cannot catalyse the dehydrochlorination. However, calcium chloride
does not promote the sudden dehydrochloination. Pentaerythirtol is
used to hinder the detrimental effect of zinc chloride. Pentaerythritol
is used widely to considerably delay the degradation time for PVC.
3.3.1.5 Other Heat Stabilisers
Compounds of barium and cadmium are used in flexible and
calendering applications. In Ba–Cd compounds, barium acts as a
scavenger of HCl and inhibits further degradation, and cadmium is
used to provide colour retention. However, cadmium-free products
are currently in demand (particularly for PVC compounds made in
the USA) [31].
Phosphate esters have an expanding role in the development of
environmentally friendly vinyl stabiliser systems in the stabilisation
of flexible PVC compounds [52]. Quinone tin polymers act as
stabilisers through intervention in the radical process of degradation
and through effective absorption of the degradation products [46].
Plasticised PVC compounds with various stabiliser systems show a
colour change at 180 °C. In comparison with the Ba–Cd stearate
system, the colour change occurs with various stearates and salts
as well as with octyl tin mercaptide. The Ba–Cd stearate system
has better stabilization, and the Ca–Ba–Zn system has a similar
performance [53].
3.3.1.6 One-pack Stabilisers
‘One-pack stabiliser systems’ are simple to use as a concentrate. They
may contain one or more other ingredient along with the stabiliser at
a much higher concentration. As the number of chemicals involved in
the heat stabilisation of PVC increases, compounding becomes more
32
Additives and Compounding
difficult. Hence, an alternate procedure with many heat-stabilising
additives has been developed as a single composite compound called
the one-pack system. Varieties of one-pack stabilisers are available
which combine heat stabilisers, lubricants and other ingredients.
The levels of addition of each ingredient in the formulation differ
depending upon the manufacturer. The advantage of the one-pack
system is that it is dust-free, pollution-free, and a composite. The
main disadvantage is that each one-pack system requires a separate
procedure to formulate and process in the equipment.
3.3.2 Lubricants
Lubricants are used to prevent PVC materials from adhering on
the barrel surface and thereby promoting degradation and altering
the flowability of the melt. The nature and usage of lubricants
is dependent upon the processing equipment. Wax (particularly
polyethylene wax and paraffin wax) is used in PVC processing as
an external lubricant. Wax is a non-polar material. Paraffin wax,
polyethylene wax, and long-chain fatty esters promote forward
movement of the material in the equipment due to film formation
on the barrel and screw surfaces.
Stearic acid is used as an internal lubricant in PVC products such
as pipes, profiles, and sheets. In general, stearic acid or polar
lubricants attract intermolecular particles and bring particles closer.
Therefore, such lubricants retard forward flow of the material
from the processing equipment. Quantity and combinations differ
to accommodate the various applications. Excess lubricants in
the formulation can create problems of output reduction during
extrusion. Excess lubricants form a layer on the barrel surface and
reduce the conveying efficiency of the PVC compound (and sometimes
discolour or degrade the material).
Lubricants are used to control fusion and reduce shear heating in
the extrusion processing of rigid PVC compounds. Lubricant systems
containing complex esters provide improved compound stability,
33
Update on Troubleshooting the PVC Extrusion Process
weatherability, and physical properties in comparison with ethylene
bis stearamide and paraffin. The complex esters result in lower
compound viscosity during processing [54].
Lubricants must be: compatible to reduce the tendency to plate-out;
more efficient; allow faster extrusion speeds. PVC formulations
require lubrications to prevent adhering of PVC to metal surfaces
during extrusion. The lubricants used are dependent upon processing
and the equipment. Wax and stearic acid are used as lubricants. They
are used in different proportions to other ingredients.
3.3.3 Impact Modifiers
The impact resistance of PVC is enhanced through the introduction
of rubbery dispersed-phase material such as acrylonitrile-butadienestyrene and nitrile rubber. This has been commercially exploited on
a large scale, and is one way to develop high-impact-strength PVC
compounds [55, 56].
To improve the formulation of PVC for impact properties, certain
features need to be considered [57]:
• PVC is a ductile material
• PVC often fails prematurely through brittle fracture
• The viscoelastic nature of PVC
• Parameters such as temperature and strain rate under impact
loading with high velocity
3.3.4 Plasticisers
To improve the flexibility and softness of PVC, plasticisers such
as phthalates, phosphates, trimellitates, adipates, and citrates are
employed. Global plasticiser demand was 4,647,000 tonnes in 2000,
34
Additives and Compounding
and shows an annual growth of 2.1%. In the past, this slow growth
stagnation could have reflected the enduring argument over the use
of phthalates in vinyl [58]. Plasticisers are considered to represent
~58% of the total additives market, most of which accounts for
flexible PVC manufacturing [59].
The usage of plasticisers is dependent upon:
• The type of PVC, its molecular weight and compatibility
• The type and concentration of plasticiser, its molecular weight,
branching and polarity
• Homogeneity during compounding
• Processing method
Once PVC have been blended and processed with additives, these
additives should remain in the end-products obtained. However,
plasticisers can be released from flexible PVC.
Phthalate plasticisers such as bis-(2-ethylhexyl) phthalate (DEHP) and
di-(isononyl) phthalate (DINP) are used for medical applications due
to their high compatibility with PVC as well as their softening ability
with important increases in the flexibility of PVC formulations [60].
The adipate plasticisers start with di-2-ethylhexyl adipate and
increase in molecular weight up to polymeric plasticisers. As the
molecular weight of plasticisers increases, volatility and extraction
by various media improves, and UV light stability increases [61].
Plasticised PVC with di-(isodecyl)diphthalate (DIDP) requires more
energy than dihexylphthalate (DHP), and results in earlier fusion and
exhibits a slightly higher flow rate [62]. Neopentyl glycol diesters
with linear and C4–C12-branched saturated monocarboxylic acids
show good plasticising properties in PVC compounds (especially at
low temperatures) [63].
35
Update on Troubleshooting the PVC Extrusion Process
3.3.5 Fillers
PVC compounds often involve the careful specification and addition
of mineral fillers. Rigidity is sensitive to the size and shape of the
filler. Glass fibre is the most efficient filler. Talc is more efficient than
calcium carbonate. The impact performance is very sensitive to the
particle size, and precipitated calcium carbonate is the only filler to
act as an impact modifier [64].
Compared with PVC, CaCO3 is a ground mineral and has no melt
characteristics. Used in combination with PVC, it increases the
modulus of elasticity, reduces the tensile strength, and reduces the
cost of the formulation. This provides a resin price of PVC that is
at a good ratio to the cost/density of calcium carbonate. At higher
concentrations, it can add significantly to the wear of screws and
barrels of standard extrusion systems.
To keep the wear at a reasonable level with high-fill PVC formulations
fillers (particularly calcium carbonate) they should possess the
following properties:
• Surface area should be ≥5.17 m2/g
• Top cut should be >8 µm
• The acid insoluble should be higher than 47% acid insoluble
should be >47%
• The median on the top cut should be <2.6 µm
Calcium carbonate can be made hydrophobic by coating with stearic
acid or calcium stearate. As a result of this action, agglomeration,
the viscosity of the suspension in DOP, and absorption of plasticiser
are reduced [65].
Rigid PVC with higher filler content with a size range of 0.07–3 μm
and 0–8 phr acrylic impact modifier shows an increase in impact
properties by increasing the concentration of impact modifier and
the sub-micron level of calcium carbonate. The flexural modulus
36
Additives and Compounding
increases with increasing filler and decreasing impact modifier
contents. Mechanical properties such as notched Izod and falling
weight impact, low-temperature impact, and flexural modulus can
be enhanced. Using ultrafine fillers, the level of addition of impact
modifiers can be reduced [66].
Plasticised PVC compounds with lithium carbonate and various
calcium carbonates act as HCl absorbers. The synergistic effect of
the fillers on HCl uptake influences the mechanical properties and
oxygen index of the plasticised PVC. The fillers (particularly in
combination) are effective as HCl absorbers [67].
Precipitated silica is an effective additive for the reduction of plateout in PVC compounds. It is more effective than fumed silica with
respect to plate-out reduction. It reduces the plate-out without loss
in stress–strain and tear properties. The order of addition has a
significant effect on the amount of plate-out [68].
3.3.6 Flame Retardants
PVC is not considered to be particularly flammable. However,
with flexible PVC, the products can often contribute to smoke
hazards due to the aromatic volatiles produced. Char formation
is considered to be smoke suppression. Even though crosslinking
occurs during ignition, char formation is only part of the process.
Alumina trihydrate in PVC compounds acts as a flame retardant as
well as improving flammability. It also reduces smoke emission if
degradation occurs [69].
3.3.7 Blowing Agents
In PVC dry-blend formulations, blowing agents are used in the
processing of foams. The higher the decomposition temperatures,
the better is the blowing agent efficiency. Blowing agents such
as azodicarbonamide (AZO), dinitrosopentamethylenetetramine
37
Update on Troubleshooting the PVC Extrusion Process
(DNPT), and sodium bicarbonate (NaHCO3) are usually employed.
However, using liquid stabilisers, the stabiliser absorbed in the resin
particle is not in intimate contact with the blowing agent. Hence, the
blowing agent is not catalysed in the presence of liquid stabilisers
[70]. The specific gravity of PVC–wood powder can be reduced using
foaming agents.
AZO is an odourless, non-toxic material that is chemically stable,
easy to handle, and which releases a large volume of gas. Therefore,
with AZO, the dry blend can be processed over a wide range of
concentrations, oven temperatures and processing times.
DNPT decomposes at a lower temperature than AZO. It has
slight residual odour and is less chemically stable. The exothermal
decomposition of DNPT can be utilised for the melting of PVC. At
higher dosages or higher processing temperatures, PVC degrades due
to the exothermic nature of DNPT.
NaHCO3 is non-toxic and inexpensive. It is odourless, and releases
water molecules as a decomposition byproduct during processing. It
produces a good cell structure at optimum concentration. However, at
higher concentration, the cell structure completely collapses, leading
to a rough surface to the end-products.
3.3.8 Pigments
Titanium dioxide (TiO2) is used as a pigment and UV stabiliser in
many PVC formulations. TiO2 is available in two forms: rutile and
anatase. The former has a specific shape and gives a better pigment
effect, whereas anatase has an irregular shape but is less expensive.
It is a highly abrasive material. It is used as much as 12 parts per 100
parts of PVC. TiO2 is preferably used in conjunction with calcium
carbonate to enhance the opacity of the product.
38
Additives and Compounding
3.3.9 Coupling Agents
Organometallic coupling agents can bring productivity improvements
in filled and unfilled as well as flexible and rigid PVC compounds.
Organometallic chemistry is specific and sensitive to factors such
as: heat stabilizers; dosage of coupling agent; surfaces of the
mixing equipment; distributive mixing and dispersion; sequence of
addition; and filler levels. The benefit from optimisation requires
an understanding of the appropriate usage of coupling agents and
addition to the PVC formulation [71].
Poly[methylene(polyphenyl isocyanate)] (PMPPIC), g-aminopropyltriethoxy-silane, and metallic copper complex have been proved to
be effective coupling agents for PVC–wood powder composites.
Wood flour with other fillers (e.g., mica, glass fibres) to form hybrid
reinforcements can enhance the mechanical properties of composites.
PVC compounds treated with coated xonotlite have shown improved
tensile strength but with reduced elongation compared with untreated
materials with little difference in impact strength. The coupling agent
appeared to improve the adhesion between the filler and matrix. The
treated material appeared to give rubber-like properties despite the
decrease in elongation [72].
3.3.10 Smoke Suppressants
Oxides of molybdenum, cerium, antimony and tin are used as
smoke suppressants. Among the oxides, antimony trioxide shows
significant synergistic activity along with moderate synergism even in
combination with other oxides. The mechanism of smoke suppression
is complex and does not conform with conventional usage of smoke
suppressants [73].
3.4 Migration of Additives
Change in weight or hardness indicates the migration of additives in
39
Update on Troubleshooting the PVC Extrusion Process
PVC products. The results are dependent upon the geometry of the
system employed. In terms of engineering, the diffusion coefficient
is an important parameter in utilisation of the migration process.
Particularly in PVC, predicting theoretically the degree of migration
is important. Heat stabilisers have low mobility but can change their
function by consumption or degradation. Fillers remain unchanged
in form and quantity below their decomposition temperature.
To summarise, additives are present in PVC formulations. They can
be stabilisers, plasticisers, lubricants, fillers, pigments, polymeric
modifiers, or colourants. Heat stabilisers have low mobility, but can
be utilised during processing. Additives with low melting points (e.g.,
lubricants) can migrate to the surface during processing or use. They
may volatilise or diffuse in contact with other surfaces. Fillers usually
remain in their initial form and quantity [3].
3.5 Compounding
The compounding of PVC is the combination of appropriate additives
with resin to regulate the behaviour of extrusion. It is one of the most
important phases in PVC processing. PVC products are produced
by mixing PVC powder with other additives aimed to improve
and control the properties of the end-product. Low K-value-resins
produce end products with poor physical properties. Suspension
resins are generally less expensive and easier to process.
Powder blend compounds are usually employed as suspension
resins which possess specific particle characteristics, particle size,
and molecular-weight distribution. Compounds produced with
suspension resins are free-flowing powders, and therefore they do
not have to meet viscosity requirements. Dry blends can be kept
for indefinite periods (in simple fibre drums if necessary) without
changes in consistency. It is common to produce PVC products from
a powder blend or its compounds. Dry blend compounding is used
in the extrusion of flexible and rigid PVC products such as pipes,
profiles and film.
40
Additives and Compounding
In principle, the procedures for compounding of resins, additives
and fillers usually occur in hot -cool mixture equipment. PVC
compounding is a difficult task involving mixing of heat stabilisers,
lubricants, fillers and other additives. Mixing is a critical function
in most PVC processing operations. The resultant compound after
mixing is in powder form. PVC products from powder blend or dry
blend are used in processing. The dry blend is used in the extrusion
of shapes, blow moulding of bottles, and in the injection moulding
of rigid PVC or plasticised PVC.
With respect to mixing, powder is better than granulated or pelleted
PVC compound. It rapidly becomes extremely difficult as the number
of additives increases. Over time, mixing and compounding in PVC
processing has changed from the addition of individual ingredients
to a one-pack system which incorporates all the necessary chemicals.
Fine particles may lead to conveying and air entrapment.
3.5.1 Technology
Compounding involving resins along with pigments, fillers, and
other random-size solids such as one-pack heat stabilisers should be
pre-dispersed in a mixer. In PVC compounding, a high shear mixer
(for the dispersion) and a jacketed low shear mixer (for cooling) is
preferred. The capacity of the chamber of the high shear mixer is
critical. The size of the mixer is relative to the blade size. The mixing
cycle is 15–20 minutes.
For compounding vinyl dry blend formulations, the equipment
comprises a high shear mixer (for the dispersion) and a jacketed low
shear mixer (for cooling). The capacity of the chamber of the high
shear mixer is critical. For ultimate homogeneity, pigments, fillers
and any other large- or random-size solids should be pre-dispersed
and ground to uniform size in a plasticiser on suitable equipment
(e.g., three-roll paint mill).
Low or intermediate shear mixers require external heating and excess
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Update on Troubleshooting the PVC Extrusion Process
time for compounding. This gives a compound with less homogeneity,
which leads to variation in the extrusion process. Non-homogeneous
compounds lead to surging during extrusion. Without pre-dispersion,
more waste occurs due to particle agglomeration during feeding in the
extrusion machinery [70]. Low or intermediate shear mixers can be
employed but have several disadvantages: external heating is required;
extended compounding times are required; poorer compound
homogeneity results; more waste due to particle agglomeration; and
solids must be pre-dispersed. There have been developments in PVC
mix preparation with reference to system mixers [74].
The resin is charged to the shearing mixer and the cycle started at
1,500 rpm to 2,500 rpm. The plasticiser and pre-dispersed solids
are then added. Mixing is continued until 110 °C. The compound
is then cooled to ≥40 °C. The compound at this stage will be moist.
Compactable material with some agglomeration involves varying the
amount of plasticiser used.
PVC and the stabiliser should be blended for ~30 seconds before the
other ingredients are added to the mixture. The filler must be added
early to achieve good dispersion from the shear effect. The drop
temperature of the hot mixer should be ≥110 °C. Double-batching
does not benefit any extrusion process. Even though it is cheap, it
is an unqualified way to increase blending capacity and results in a
reduction of thermal stability and flowability.
In PVC compounding, double-batching permits a significant increase
in throughput in the heating and cooling mixer, with simultaneous
energy saving. The separation of the compound can be compensated
by homogenisation effects during processing, and also provides highquality extruded products [75]. However, double-batching results in
an expensive finished product.
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