Post on 11-Mar-2018
Filler Treatment with Silanes and Titanates – Famas Technology Version 04/07
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Fil ler Treatment with Famasil Silanes and Titanates Introduction:
The global market of silane surface treatments for fillers and reinforcements has been growing significantly during the past 20 years, Reasons for employing silanes
were first to improve abrasion resistance and electrical properties of synthetic
rubbers filled with kaolins and clays, later to improve stiffness and heat resistance
of engineering thermoplastics with fillers like talc, wollastonite and mica. In recent years, it has been the use of precipitated silica to improve dynamic
properties and rolling resistance of automotive tires while replacing carbon black.
This latter segment alone generated silane sales of over !uro 100 MM in 2006.
All these developments were only possible, due to silanes ability to react with the
surface of most mineral fillers to impart designed properties. One design feature is also the fact that the organic part of the silane is reacted with the polymer and
that this covalent bridging mechanism controls surface properties like adhesion.
A large proportion of all thermoplastics, thermosets and elastomers worldwide are
compounded and reinforced with fillers and fibers. Their function is to provide
specific properties to the final product and to reduce costs. The majority of these fillers are incompatible with the polymer matrix, particularly the inorganic minerals
that account for a large majority of the fillers used. Embrittlement, degradation of
mechanical properties and increased moisture pickup of the composite can result.""To overcome this problem, coupling agents and other surface modifications
have been developed.
Coupling agents tend to be bifunctional molecules able to bond chemically with both the filler surface and the polymer matrix, which forms a 'molecular bridge'
between the two. The strong interfacial bond not only aids the mixing of the two
phases but also benefits the overall properties of the composite. The most
commonly used coupling agents are organotrialkoxysilanes, organotitanates and functionalized (especially acid functionalized) polymers. A worldwide total of
about 16,000 tonnes - worth around !uro 300 millions - of coupling agents are
consumed annually in the treatment of around 3 million tonnes of fillers.
Other surface modifiers that provide a physical rather than a chemical bond
between filler and resin are also available for applications where the performance
levels provided by coupling agents are not necessary. These are mostly waxes or fatty acids such as stearic acid. These yield improved filler incorporation and
dispersion for around a tenth of the cost of the average silane. Their role in
polymer composites is far from trivial: of the 3 million or so tonnes of fillers
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surface treated each year some 1.75 million tonnes are calcium carbonates
treated with fatty acids for incorporation in PVC, PP and various elastomers.
Usage of treated fi l lers
Mineral fillers, such as calcium carbonate (CaCO3), clays, silicas, mica, talc, alumina
trihydrate (ATH) and titanium dioxide, account for the lion's share (about 90%) of
the demand for fillers and extenders, with CaCO3 being by far the most commonly used filler.
Non-mineral fillers and extenders include carbon black, glass beads and various
organic materials.""Among the thermoplastics, PVC accounts for about 70% of the
demand for fillers, with PP, nylon and polyester together accounting for another 20%. Natural calcium carbonate (treated with fatty acid) accounts for more than
80% of that total.
In terms of volume, the main fillers treated with coupling agents for incorporation in thermoplastics are ATH, calcined clays, wollastonite (calcium metasilicate) and
mica, together consuming about 2,500 tonnes of coupling agents per year.""The
annual requirement for treated fillers in elastomers is reported to be in excess of 500,000 tonnes, of which about 70% employ coupling agents. More than 10,000
tonnes / year of coupling agents of various types are used, with silanes
predominating. "
For thermosets the picture is slightly more complicated due to in-situ coatings and the use of mineral fillers in conjunction with glass fiber.
Silanes
Organofunctional silanes were introduced over 50 years ago as coupling agents
for fibreglass and have proved equally successful in treating mineral fillers. They are the dominant materials in the coupling agent market, their success is due to
their ability to react with a broad range of fillers and resins, the fact that they can
be produced in readily dispersible form, with stably attached organic functionalities.""
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Coupling mechanism
Silanes have the generic structure:
Y-R-Si-X3,
where X is a hydrolysable alkoxy group (methoxy or ethoxy) and Y an organofunctional group (amino-, vinyl-, epoxy-, methacryl- etc.) attached to the
silicon by an alkyl bridge, R.""The alkoxy groups are able to react with the surface
groups of many inorganic fillers.
They first react with water to produce the silane triol, releasing alcohol as a by-
product. The silanol groups then condense with oxide or hydroxyl groups on the filler surface. Neighboring siloxane chains can interact further to produce a
polysiloxane layer at the surface.
Silanes require active sites, preferably hydroxyl groups, on the filler surface for
reaction to occur. They can therefore be used to treat all silicate-type fillers,
inorganic metal oxides and hydroxides. Materials successfully treated with these coupling agents include: ATH, alumina, chrome oxide, hydrous and calcined clays,
glass fiber, magnesium hydroxides, mica, mineral wool, oxide pigments, quartz,
silicas, talc, titanium dioxide.
However, silanes do not interact to any significant degree with calcium carbonate,
or with barium sulphate or carbon black.""Once coupled at the filler interface the
reactive Y component allows bonding to the polymer matrix by chemical reaction (grafting, addition, substitution) with active groups on the polymer and / or by
physicochemical interactions.
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Y groups are selected to maximize compatibility with particular resin formulations.
For example, methacrylate-functional silanes are most used in
unsaturated polyesters, while amino-functional silanes are found more frequently in polyamides as well as epoxys.
In general, silanes are highly effective coupling agents for polar thermoplastics, thermosets and rubbers but have only a slight interaction with non-polar
polymers such as polyolefins (where titanates are of bigger interest).
Organic Group - Silane
Base Polymer Amino Epoxy Sulfur Mercapto Methacryl Vinyl Ureido Fluorine
Acrylic
Acrylic latex
Butyl
Cellulose
Epoxy
Fluoro
Melamine
Neoprene
Nitrile
Phenolic
Polyamide
Polyester
Polyether
Polyolefin
Polysulfide
Polyurethane
PUD
Silicone
SBR emulsion
SBS
Vinyl
Generally Effective / Best Choice
Alternative / 2nd Choice
Only effective with specific silanes
Unsuitable
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Fil ler treatment
There are several commercially established methods of treating fillers with silane
coupling agents.
1. Fillers can be pre-treated before compounding with the polymer by spraying
neat silane onto dry, well-agitated filler. Moisture on the filler surface is
typically sufficient to initiate the hydrolysis reaction.
2. Integral blending methods can also be used. The silane can be blended with the polymer and filler during compounding either in the form of a dry
concentrate (absorbed on a carrier), or as neat silane added before or
together with the filler during compounding, followed by intensive mixing. This in-situ method is widely use in resin-filler systems because of its
simplicity.""
Silane loading depends on surface area of the mineral filler usually about 1 wt.% of silane on filler is required for fillers with a surface area of less than 20 m2/g.
Higher surface area fillers require a higher dosage.
Property enhancement
Silane coupling agents provide a strong, stable, water- and chemical-
resistant bond between fi l ler and resin, typically improving:
- mechanical and electrical properties,
- reduces shrinkage, - increases weather resistance, and lessens or eliminates surface or internal
defects,
- surface appearance of processed parts.
The immediate benefit that can be observed when blending a silane-treated filler
with a polymer is usually improved wettabil ity - the filler adsorbs the polymer more completely because the silane treatment reduces the interfacial
tension with organic l iquids.
Another result of improving the interfacial tension with the polymer, are reductions in viscosity of the fi l ler / polymer system.
Silane treatments make their surface hydrophobic and the interfacial tension is increased. As a result of increased interfacial tension with water, the penetration
of the liquid into the interphase is hampered and a better resistance
against humidity is achieved.
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Note that this is almost independent of the inorganic group of the silane, as long as the treatment conditions are set to ensure complete reaction between the
silane and the polymer. The major factor governing the hydrophobic
effect is thus the silane organic group - best results are usually obtained with octyl silanes - however, the benefit would disappear with time if the silane
treatment itself was not chemically stable in a wet environment.
Chemical interaction between fi l ler and polymer.
The reactions between the silane, the filler and the polymer take place in presence of surface water : The first reaction is the hydrolysis reaction of the
silane. For example the reaction of a trimethoxysilane releases methanol and
consumes water:
R-Si(OMe)3 + 3/2 H2O ! R-Si (OH)3 + 3 MeOH
Then the resulting silanol reacts with a surface hydroxyl group of the filler surface
(condensation).
In the equilibrium state, the amount of OH-groups on the filler surface is the most
important parameter governing the quantity of silane-filler covalent bonds.
Separately, the reaction kinetics are related to surface catalysis.
The rate-determining step for the chemisorption process is under most conditions
the condensation reaction. Hydrolysis and condensation reactions are pH-
dependant and are catalyzed by both acidic and basic conditions.
2 3 4 5 6 7 8 9 10
Rate of Condensation Rate of Hydrolysis
-1
-5
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Table below shows the improvement in flexural strength of filled polyester with
0.5% of Methacryl silane (Famasil ME-TMO) after 4 hours in boiling water.
Filler
OH
Groups
Filler
pH
Filler
loading, phr
Flex Str.
MPa, Untreated
Flex. Str.
MPa, Silane-
treated
% Chg.
ATH high 10 160 30 60 +100%
Amorphous Silica high 7 120 60 100 + 67%
Hydrous Clay high 9 90 30 45 + 50%
Calcined Clay low 5 100 60 100 + 67%
Mica low 8 50 20 30 + 50%
Talc low 9 100 30 50 + 67%
To increase the reactivity of silanes one may also use the inherent basic
nature of aminosilanes. Combining aminosilanes with other compatible silanes or titanates can help achieving faster chemisorption on difficult substrates.
After silanes built covalent bridges between filler and polymer, this bridge needs to be resistant and stable.
The data below illustrates the stability of silylated surfaces towards hydrolysis.
One reason for this is that the silane / water reaction is reversible, allowing rearrangements with the overall effect, that the trifunctional silanes are able to
re-adsorb on the filler surface after partial hydrolysis.
50% Kaolin-filled PA 6. Effect of AM-TEO silane and AM-TITAN blend treatment
after wet ageing
Unfilled
Untreated
kaolin
AM-TEO
Treated
AM-TITAN
Treated
Flexural strength,
MPainitial 85 120 150 155
16 hours in 50°C
water 39 69 100 138
Flexural Modulus,
MPaInitial 1800 5730 6050 6240
16 hrs in 50°C
water 570 2350 2620 2970
Tensile Strength,
MPaInitial 60 70 80 90
16 hrs in 50°C
water 40 40 60 80
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On the other hand the silane organic group must react with the polymer to complete the bridging process.
Guidelines for determining silane amount to be applied on fi l lers
Average particle size of mineral filler (microns)
Amount of silane (%w/w)
< 1 1.0 - 5.0
1 to 10 1.0 – 2.0
10 to 20 0.75 – 1.0
20 to 100 < 0.1
Solvents can be used in the preparation of a suitable silane solution (ask your local Famas representative for guidelines). The benefit is that the process allows for
complete silane reaction and elimination of alcohols as hydrolysis by-products. In
some cases water can be used, but only aminopropylsilanes can be dispersed well in water at elevated concentrations.
Titanates
Organotitanates overcome many of the limitations of silanes as coupling agents
for fillers. Like silanes they have four functional groups, but where silanes have only one pendant organic functional Y group, titanates have three, providing more
effective coupling to the resin. In addition, the mechanism by which they couple
to inorganic surfaces differs, which means that they are suitable not only for fillers with surface hydroxyl groups but also for carbonates, carbon black and other
fillers that do not respond to silanes.
In addition to improving filler dispersion and enhancing the properties and
processing of the composite as with silanes, titanate couplers also act as
plasticizers facilitating higher filler loadings, and as catalysts for a number of reactions in the polymer matrix. Costs for titanate treatment are slightly lower
than silanes.""
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Structures and mechanisms
Titanates have the general structure:
XO-Ti-(OY)3,
where XO- can be a monoalkoxy or neoalkoxy group capable of reacting with the inorganic substrate, and -OY is the organofunctional fragment.
The Y portion can typically contain several different groups to provide interactions between polar and non-polar thermoplastics (e.g. benzyl, butyl) and thermosets
(e.g. amino, methacryl), as well as pyrophosphato or carboxylic groups that can
introduce additional functions.
Unlike silanes titanates do not require the presence of water to react." "Titanates
fall into several classes. The simplest are the monoalkoxy (e.g. isopropoxy) titanates introduced in the Mid 70s. These react with the filler surface via
solvolysis generating an alcohol by-product.""
The neoalkoxy titanates have a more complex but more thermally stable
structure. They were developed for high-temperature applications (above 200°C in
the absence of water) such as in-situ addition during thermoplastics compounding
and the production of urethane composites. They react via a coordination mechanism with free protons on the filler surface, generating no by-product or
leaving group.
Free protons, unlike the hydroxyl groups needed for silane reaction, are present on almost all three-dimensional particulates, which is claimed to make titanates
more universally reactive. The reaction with free protons generates an organic
monomolecular layer at the inorganic surface - in contrast to the polymolecular layers typical with other coupling agents - which in combination with the chemical
structure of the titanates creates surface energy modifications and polymer phase
interactions. In addition to their higher thermal stability, neoalkoxy titanates offer somewhat enhanced final properties compared to their monoalkoxy counterparts.
Other types of titanates are chelates (for greater stability in wet environments), and quats (water-soluble systems).
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Fil ler treatment
In-situ coupling is recommended but requires good compounding techniques to
avoid localization and inconsistent coupling. Uniform distribution of the titanate before the polymer melt phase is essential. The recommended dosage for most
fillers is typically 0.2-2% by weight of filler.
Applications
A large body of published research is available demonstrating the ability of titanate coupling agents to enhance the properties of composites when combining
a wide range of fillers and polymers.
In addition to silane-reactive fillers, titanates are effective with carbonates, carbon
black and other fillers. With appropriate organofunctional groups, titanates can
also bond successfully with polypropylene (and other polyolefins) as well as PVC, two of the largest consumers of fillers.
High loadings of other common mineral fillers such as wollastonite and talc have
also been achieved in PP with the aid of titanate coupling agents – especially organofunctional titanates such as NDZ 130 are suitable candidates in these
applications.
Several wollastonite producers report that both silanes and titanates are used to surface treat its products, though these markets for organotitanate-treated
wollastonite are relatively small and specialized.
The reactivity of the TiO bond can cause problems with discoloration in the
presence of phenols.
Zirconates
The chemical structure and applications of alkoxy zirconates are completely analogous to those of alkoxy titanates.
Zirconates' main advantage is their greater stability; unlike titanates they neither discolor in the presence of phenols nor do they interact with hindered amines
(HALS).
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Functionalized polymers
Functionilzed Polymers are the most class of coupling agents. They account for
about 6% of the market for coupling agents by value, or about $10 million / 3000
tonnes annually, and acid functional polyolefins are intermediate in price between fatty acids and silanes / titanates.""
The coupling concept here is to have substrate reactive groups on molecules of the host polymer itself, or of another polymer compatible with it. This removes
the problem of finding polymer reactive functionalities, and is particularly
attractive for thermoplastic polyolefins. The problem to date seems to have been in producing effective functionalized polymers. This is partly due to
the prevalence of siliceous fillers in composite materials. These are most
effectively bonded with alkoxysilanes, but such groups are difficult and expensive to introduce into polymer chains.""
The easiest materials to produce are probably acid functionalized polymers, especially those with grafted or copolymerized anhydride groups. Examples are
carboxylated polyethylene and polypropylene and maleinized polybutadienes.
The main l imitation of these acid functionalized additives is that they are most effective with basic or amphoteric substrates, while the
majority of substrates where true coupling is required are sil iceous in
nature and generally not directly responsive to them.
One option is to pre-treat the siliceous filler with an aminosilane, which can
then react with the acid functionalized polymer to form an amide
linkage. Where functionalized polymers can be used, the effects achieved are typically those of coupling (improved heat distortion temperature, strength,
stiffness and abrasion resistance, for example).
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Conclusion
Appropriately used, silane and titanate coupling agents create significant
improvements in the properties and processing of filled plastics. Silanes are well
established and enjoy widespread use. Titanates can have broader functionality but often suffer due to incorrect use.
Functionalized polymers occupy an intermediate position between fatty acid
surface modifiers and the silanes / titanates in terms of both cost and functionality."
Surface treatment of fillers using organofunctional silanes has a strong influence on several parameters :
- Physical interactions between filler and polymer can be modified to control processing properties like wetting speed and efficiency, viscosity during
compounding and filler dispersion.
- Chemical interactions between filler and polymer leading to the formation of
stable covalent bonds. The formation of a chemisorbed layers protect the
filler/polymer interface from hydrolysis and improves ultimate mechanical properties and ageing.
- Physical interaction between filler particles can be designed to control
rheological and dynamic mechanical properties.
- Chemical modification of the polymer in the interphase can be introduced
through silane crosslinkers or reactive plasticizers to minimize the impact of large property differences between filler and polymer.
The treatment of fillers is a relatively simple process if attention is payed on the reactivity between silane / titanate and filler. The surface reactivity should be
measured case by case.
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