Past, present and future of organosilane treatments for fillers.

From their first development, over 50 years ago, organosilanes have been used to provide a variety of performance and process benefits to mineral fillers. Interest in current and newly developed molecules has never been keener as filler manufacturers seek to differentiate their products by modifying them with silanes. Organosilanes provide better bonding at the polymer/filler interface, improved filler dispersibility and increased composite durability.

This overview will consider the wealth of technology and its application to a variety of substrates that include mineral fillers, nano-technology, flame retardant fillers and natural origin fillers. Trends in the silane market, applications and patent literature will also be discussed in light of environmental and health considerations.

Organofunctional silanes have been commercially available since the early 1940s when they were first used as coupling agents to improve the reinforcement of glass fibers in organic resins. The silanes were found to be particularly useful under conditions where exposure to moisture was likely. There is now about 60 years of history on the use of these materials to make many of the composite products we rely on today. Over this time, the technology has evolved to address the ever changing needs of the users of organosilanes and composites. From the early chloro-functional silanes to today's blends and hydrolyzates, the silane producers have been striving to provide the materials needed by the composites industry. Now, there is more emphasis on mineral fillers and improving them to be reinforcements. This article will review the fundamentals of organofunctional silane technology and how these materials are, and may be, used in mineral-filled products.

Being a hybrid of inorganic (silica-like) and organic constituents, the organosilane shows affinity for both inorganic and organic surfaces. The diagram in figure 1 shows this bi-functionality, so-called because of the dual reactivity of the silane molecule. The silane functions by chemically "bridging the gap" between the inorganic and organic materials. The silane forms a durable chemical bond that improves many desirable properties of a mineral-filled composite. While there are many other materials that can be called a coupling agent, it is this unique dual reactivity that differentiates silanes from all the other surface treatments.

[FIGURE 1 OMITTED]

Silanes used for treating mineral fillers typically have a structure represented as [R.sub.n]Si[(OR').sub.4-n]. The organo group, R, is a functional group chosen to match compatibility or reactivity with the organic polymer. The hydrolyzable group, OR', may be chloro-, but usually has been methoxy or ethoxy for easier handling. Plus, in the most common products, the subscript n equals one, making a trialkoxysilane. The trialkoxysilanes have been the workhorses of the industry from the beginning to now. As we will see later, there have been advances towards using di- and mono- alkoxy materials, as well as moves to reduce their VOC potential. Currently, the number of silanes available From the various suppliers covers a large range of organo-reactivities. From Dow Corning alone, there are approaching 90 products, split fairly evenly between commercial, specialty and developmental types.

There are several aspects to the function of silane coupling agents. As stated the first is chemical interaction and, preferably, reactivity. It is important for the organo group on the silane to be compatible with the organic polymer. However, this doesn't necessarily provide a chemical link between the mineral filler and the polymer. For this to occur, the chemical reactivity of the silane should be chosen to provide the desired level of chemical bonding to the polymer. A second factor in the performance of silanes is that of the Interpenetrating Network Theory. This is a widely accepted model that helps to explain the mechanism of coupling of the inorganic and organic/polymer surfaces (ref. 1). Figure 2 provides a representation of the IPN theory. The essence of the theory is that the silane provides a three-dimensional network into which the polymer can diffuse to give both an increase in the surface area of contact and molecular level entanglement. To form this three-dimensional network, the silane must first react with the surface of the filler, and then it must react with other silane molecules to form a siloxane network. In this network are many organoreactive points brought by the "R" group of the alkoxysilane.

[FIGURE 2 OMITTED]

Experimental studies have provided an estimate of the extent of the siloxane network formed by treating a surface with an alkoxysilane. As represented in figure 2, at the surface of the mineral filler there is true covalent bonding existing for one molecular layer (ref. 2). This is the anchor for the rest of the alkoxysilane network. The terms chemisorption or chemisorbed layer are often used to describe the section remaining dominant for the next 10 to 20 molecular layers. The ideal is a loose siloxane network that allows a significant amount of interpenetration by the organic polymer. However, the chemisorbed layers are chemically bound to one another to form a durable layer of silane. Finally, there can be a significant amount of silane on top of the chemisorbed layers that is merely associated with the surface and not reacted to it. This is called physisorption or physisorbed layer, which can extend up to 200 layers from the surface.

Fundamental to all of this is the reaction of the hydrolyzable alkoxy portion of the silane with the mineral surface. The alkoxysilanes want to form siloxane bonds once they have been hydrolyzed or are in a moist or aqueous environment. Consider that it is siloxane bonds that form the basis of the majority of the earth's crust-sand. Fortunately, many naturally occurring, and synthetic, mineral fillers have some amount of silicate or siloxane character. This is commonly the point of reaction between the silane and a mineral surface. Surface moisture on the mineral helps to facilitate the reaction of the silane, as well. So, it is important that the mineral surface provides some level of reaction with the silane in order for the silane to form the basis of the network. Also, the reaction with the surface must be reasonably competitive with the intramolecular silane condensation reaction, or the silane will be consumed in forming the siloxane network without the anchor to the surface. Figure 3 shows the two most important condensation reactions for alkoxysilane, "with silane" or intermolecular, and "with surface" to treat the surface.

[FIGURE 3 OMITTED]

The affinity of silanes to various minerals can be quite different across the range of minerals. Table 1 (ref. 3) gives the relative bond durability of a methacrylate functional silane with the surface of a variety of mineral fillers. To some extent, the pH bias of the mineral affects the stability of the siloxane bonds to the surface. The ionic species present in acidic or basic minerals can catalyze cleavage of the bond between the silanes and the mineral. Clearly, this does not explain the entire picture of the silane. Some minerals, such as calcium carbonate, have very little reactivity with alkoxysilanes.

So far, we have covered the how of using silanes with mineral fillers; now we would like to discuss the why. By way of illustration, property improvements include increased dry and wet strength, easier filler dispersibility and increased durability of the composite (ref. 4). Table 2 shows a specific silica filled polyester composite where several coupling agents have been evaluated. These data illustrate that the performance, both processing, as indicated by viscosity, and of the final product, as indicated by flexural strength, depend on the specific coupling agent used. Clearly, the optimum silane in this instance would be Silane H, the cationic styryl amine which gives a low viscosity mixture, as well as excellent final strength properties, in this case, using no coupling agent compromises the properties possible in the composite, and the processing would likely be more difficult. This also shows the behavior of an alternate coupling, a titanate. While this material does help with dispersion of the silica into the resin, the final properties are not much better than having no treatment at all.

There are other properties in mineral-filled composites that can benefit from the addition of an alkoxysilane. These include electrical (insulation/dielectric) and cure rate benefits that are a function of the coating of the surface of the mineral and isolation of the mineral chemistry from the resin. They also include environmental durability and overall strength of the composite that benefit from chemically linking the filler into the resin matrix (ref. 5).

Turning to the past, present and future activities relating to silanes and fillers, we will analyze the available patent database using the computer-based SciFinder research and development tool. It allows access to over 35 million scientific and patent references, and has capability to analyze data into a visually effective form. By analyzing the data in this way, we can see what technologies have the most attention and what are the trends in the technologies. It is possible to categorize the information using three different groupings, including time, application and technology.

For the purpose of this analysis, arbitrary definitions of the timeline are: Past = 1955 to 1985 (not much relevant work before 1955); present = 1986 to 1999; and future = 2000 to 2005.

The applications can also be arbitrarily assigned: Past = mineral fillers, including glass fiber; present = flame retardant materials; and future = nano-fillers and natural organic fillers.

Finally, the development of silane types can also receive an arbitrary classification: Past = alkyl and organoreactive trimethoxy types; present = trimethoxy, triethoxy; and future = mixtures, hydrolyzates and bis-silanes.

SciFinder database searching gave the summary results listed in table 3.

The tabulated results are pretty much as expected, with the silane and nano showing the biggest increase in the 21st century followed closely by silane and flame retardant filler patents. The results for silane and wood filler may have a significant degree of error since there seemed to be no common term such as biocomposite, natural origin filler or organic filler, that would give a greater number of hits. Lastly, the general category of silane treated fillers, although still with the most activity, looks to have leveled-off.

Given this information on the patent activities through the years, now we will focus on the broad silane developments over these same time periods.

Past (1955-1985)

During this period, the alkyl and organofunctional silanes were very limited in number. The first were methyl, vinyl and amino functional, with the epoxy, methacryloxy and mercapto following. The alkoxy group was for the most part methoxy, although the aminosilane (Dow Corning Z-6011/Silquest A-1100) did offer ethoxy. Plus, the products were predominantly trialkoxyfunctional. There was even a vinyl silane with beta-methoxyethoxy groups developed to treat fillers in peroxide cured rubbers.

Applications included usage as coupling agents for fiberglass and mineral fillers. These materials were then used to reinforce resins and plastics to make composites. Much of the development and empirical understanding efforts were focused on fiberglass materials. Also, silanes were used as a treatment for fillers, such as clays, in reinforced rubber cable insulations where both coupling between the filler and polymer together with improved hydrophobicity were required. The other major application area was for the alkyl alkoxy silanes used as masonry water repellents.

Present (1986-1999)

The size of applications grew significantly during this period. Total European annual consumption of alkoxysilanes was about 8,000 metric tons/year in 1999. The market segments included tire/rubber (56%), glass fiber reinforced (GFR) plastics (23%) and filler coupling-plastics (21%) (ref. 6). As the relative market shares indicate, there was major growth in the treatment of silica with bis-sulfidosilanes for use in the so-called green or environmentally friendly tires. The technology in use here enables the production of tires with better traction, fuel economy and durability than the standard tires (ref. 7). Silane producers also sought to provide safer products by switching more organofunctionality into ethoxy-silanes which are inherently less toxic and less flammable than the methoxy equivalents. While this improves the situation of handling the silane, the ethoxy groups react more slowly than the comparable methoxy groups. So, using an ethoxysilane will likely slow down processing. However, once the silanes have been hydrolyzed or reacted to the mineral surface, the nature of the alkoxy group is no longer an issue.

Future (2000-2005)

Now, in the time period referred to as the future, organosilanes have become big business for all the major silane producers, including the Japanese and Chinese who are also present in the global market. Competition is fierce for the older, standard grade materials on the market. Alternatively, new developments such as oligomeric silanes can provide a handling benefit for the user. These materials are formed by pre-hydrolysis of the silane, where some or all of the alcohol had been removed. The result, hopefully, is a stable aqueous silane. Mixed alkyl and vinyl silanes offer an alternative for the wire and cable applications. And new organofunctionalities have been offered to better interact with the new polymer developments. These include isocyanato, allyl, ureido and benzylamino, as well as variations in sulfido, epoxy, vinyl and alkyl silanes.

Apart from the continued expansion of activities for the already mentioned silanes, there are some indications in the patent and other literature of where we might look for further improvements. These include:

* di- and mono-alkoxy silanes;

* mixed silanes;

* reduced and VOC-free silanes;

* oligomeric silanes;

* other organofunctionalities; and

* surface active silanes.

In conclusion, we believe that there are five trends and needs that customers involved in mineral surface treatment have prioritized:

* The elimination of ethanol or methanol emissions in the processing of alkoxysilanes and fillers. This is a stated need for tires, robber, silicone rubber and other users of fillers requiring silane surface treatment.

* Flame retardants--the use of halogen-based flame retardants is being regulated out and companies are looking for new options based on aluminum trihydrate and magnesium hydroxide. To make these fillers compatible with the polymer matrix and allow them to be used in large percentages, they need to be surface treated, particularly for plastics.

* Current users of mineral fillers are looking for new surface treatments that will enable them to have higher performance in several areas.

a. reinforcing effect;

b. easier dispersion (low viscosity, high filler loading);

c. better compatibility with the matrix;

d. ability to replace high value fillers with upgraded, lower cost minerals; and

* Manufacturers of fillers are looking to develop the next generation of filler technology and they consider that will be driven by upgrades in surface treatment chemistry.

* Environmental--natural origin and waste powders or fibers (glass, wood) being used as alternative fillers.

The case for nano-fillers needs to be closely monitored as they become more cost-effective.

References

1. K.L. Mittal, ed., Silanes and Other Coupling Agents, p.13. VSP Publications, Netherlands, 1992.

2. E.P Plueddemann, Silane Coupling Agents, 2nd Ed., Plenum Press, 1991.

3. K. Nagata and Y. Nakamura, Nippon Gomu Kyokaishi, 75 (5), p. 183 (2002).

4. Ibid ref. 2, p. 201.

5. Ibid ref 2.

6. Data from in-house report.

7. B. Schwaiger and M. Haddeman, "Progress in rubber reinforcement through advanced filler systems--challenges and opportunities," The 9th International Tyre Technology Conference, TyreTech, 2000.

Table 1--effect of mineral filler type on durability of silane finish

      Acidic fillers                 Neutral fillers

[Fe.sub.2][O.sub.3]    100    Ti[O.sub.2]               84
Zr[O.sub.2]             78    [Al.sub.2]Si[O.sub.5]     87
Clay                    66    Kaolin                    96
Sn[O.sub.2]             77    ZnO                      100

  Alkaline fillers

Ca[(OH).sub.2]    57
MgO               98
E-Glass           50
Wollastonite      21
CaC[O.sub.3]      19
Mica              55

Table 2--performance of silane coupling
agents in a silica-filled polyester composite

                                  Flexural strength, Mpa
Additive (0.3%)     Viscosity
based on filler    of mix, cp.    Dry    2 hr. water boil

       None          24,500       115           70
   Silane D          22,000       163          143
   Silane H           8,700       156          139
     TTM-33          10,000       135           72

50% Minusil (5 micron) in Rohm & Haas Paraplex P-43
Silane H--cationic styryl silane
Silane D--methacryloxpropyltrimethoxysilane
TTM-33--isopropyltrimethacroyltitanate

Table 3--the average number of patents per
year for the applications and technologies
during the indicated time periods

                             Past          Present        Future
Search term               (1955-1985)    (1986-1999)    (2000-2005)

Silane treated fillers        5             18              21
Silane and flame              1.2            5.4            14
  retardant filler
Silane and wood filler        0.3            1.8             2.2
Silane and nano               0.2            1               6.7