Background Of The Invention

This invention relates to the field of thermally expandable materials for actuators, especially to thermally expandable wax compositions containing one or more additives to increase thermooxidative stability, increase viscosity, and/or decrease hardness of the composition.

Actuators that use a thermally expandable material are well-known in the art. A commonly used thermally expandable material is a type of wax, although plastics and metals have also been employed in actuators and similar devices that use thermally expandable materials. Waxes have the advantage of a low melting point, so that the large expansion that occurs when the solid melts into liquid occurs at relatively low operating temperatures.

The stability of a wax composition, e.g. resistance to oxidation and thermal breakdown, and the viscosity and hardness of the composition are significant factors in the performance of a wax composition in an actuator.

Repetitive heating and cooling cycles in an oxidative environment are a typical part of most actuator performance; for this reason, thermooxidative stability is very desirable. Increased viscosity is highly desirable where additives are used, e.g., relatively dense metal powders added to increase thermal conductivity may tend to settle in a low viscosity medium. Furthermore, decreased hardness is desirable to avoid the formation of hard plugs in the actuator.

The term wax refers to a substance that is a plastic solid at room temperature and melts at a relatively moderate temperature to form a relatively low viscosity liquid. Waxes are generally contain a complex combination of organic compounds, especially long-chained organic acids, esters and hydrocarbons. Waxes include beeswax, waxes taken

from plants (e.g., carnauba wax, bayberry wax, and the like), and mineral waxes derived from petroleum or coal. Montan wax is an example of the latter, being derived by solvent extraction of lignite. Paraffin is a well- known type of petroleum wax, obtained by crude oil distillation/separation. Low molecular weight (about 10,000 g/mole or less) hydrocarbon polymers also form waxes, especially polyethylene and polypropylene waxes; these waxes may be made by polymerization or obtained by thermally degrading higher molecular weight polymers. Unlike other waxes, these polymers tend to contain molecules that are of the same type, although as in all waxes the molecular weights of the molecules vary.

The exact composition of any type of wax varies based on the origin of the wax and the treatment it has undergone. Waxes of the same type may vary in purity, color, melting point, hardness, and other properties and characteristics.

U.S. Patent Number 3,194,009 issued to Baker discloses thermal actuators having a composition prepared from a molten wax and a liquid monomer of elastomeric material, which composition is vulcanized or cured to a dry semi-solid elastomeric material.

Summary Of The Invention

The present invention is a thermally expandable wax composition for use in actuators comprising a wax and a thermooxidative stabilizer. Such stabilizers include tetrakis(3,5-di-tert-butyl-4-hydroxycinnamate)) methane, calcium bis (monethyl (3,5-di-tert-butyl-4-hydroxybenzyl) phosphonate), bis(2,4-di-tert-butylphenyl) pentaerythritol diphosphite, diestearyl pentaerythritol diphosphite, and similar compounds, and combinations thereof. Optionally, the composition may also include a viscosity enhancer, e.g. ethylene-vinyl acetate copolymer, and/or an oil,

e.g. paraffin oil, added to reduce hardness. The stabilizer typically comprises about 0.1 - 2.0% by weight of the total composition, preferably about 0.1 - 0.5%.

It is an object of the present invention to provide a wax composition having improved resistance to thermooxidative decomposition.

It is another object of the present invention to provide a wax composition having increased viscosity and decreased hardness.

Other objects and advantages of the present invention will be apparent to those skilled in the art from the following description and the appended claims.

Detailed Description Of The Preferred Embodiments

In one preferred embodiment of the present invention, a polyethylene wax (e.g., PE-190 polyethylene wax available from Hoechst

A.G. of Germany) is blended with tetrakis (methylene (3,5-di-tert-butyl-4- hydroxycinnamate)) methane and calcium bis (monethyl (3,5-di-tert-butyl- 4-hydroxybenzyl) phosphonate) which provide thermooxidative stability to the wax. Preferably, each stabilizer comprises about 0.5% by weight of the wax composition. In an alternate preferred embodiment, the calcium bis (monethyl (3,5-di-tert-butyl-4-hydroxybenzyl) phosphonate) may be replaced by 0.5% diestearyl pentaerythritol diphosphite.

In other preferred embodiments of the present invention, an oxidized polyethylene wax or a Montan wax (e.g., PED-521 oxidized polyethylene wax or Montan wax E, both available from Hoechst A.G. of

Germany) is blended with tetrakis (methylene (3,5-di-tert-butyl-4- hydroxycinnamate)) methane and calcium bis (monethyl (3,5-di-tert-butyl- 4-hydroxybenzyl) phosphonate) which provide thermooxidative stability to the wax. Preferably, each stabilizer comprises about 0.1 - 0.5% by weight of each wax composition. Alternatively, the calcium bis (monethyl (3, 5-di-

tert-butyl-4-hydroxybenzyl) phosphonate) may be replaced by 0.1 - 0.5% diestearyl pentaerythritol diphosphite in each composition.

In another preferred embodiment, a polypropylene wax (e.g., Hoechst A.G.'s PP-230 polypropylene wax) is blended with tetrakis (methylene (3,5-di-tert-butyl-4-hydroxycinnamate)) methane and bis(2,4- di-tert-butylphenyl) pentaerythritol diphosphite. Preferably, each stabilizer comprises about 0.1 - 0.5% by weight of the wax composition. Alternatively, the bis(2,4-di-tert-butylρhenyl) pentaerythritol diphosphite may be replaced by 0.1 - 0.5% diestearyl pentaerythritol diphosphite. The following Examples are presented to illustrate the present invention, but should not be construed as limiting the scope of this invention.

EXAMPLES

To test the effectiveness of the thermooxidative stabilizers, four different waxes were tested, both with stabilizers according to the present invention and without stabilizers (control). The tests consisted of measuring weight loss for wax compositions held at 200EC or 225EC for one hour, both in a nitrogen atmosphere and in air. The weight loss in the nitrogen atmosphere is believed to be the result of evaporation of the more volatile components of the composition, whereas the weight loss in air includes the evaporation of additional volatile species that are generated by the oxidative degradation of the wax. The results are shown in Table 1 , below, in which: stabilizer A is tetrakis (methylene (3,5-di-tert- butyl-4-hydroxycinnamate)) methane; stabilizer B is calcium bis (monethyl

(3,5-di-tert-butyl-4-hydroxybenzyl) phosphonate); stabilizer C is bis(2,4-di- tert-butylphenyl) pentaerythritol diphosphite; and the waxes are identified by the Hoechst A.G. names introduced above.

Table 1

Wax Stabilizers EC Wt loss in Wt loss in air N ₂

Montan Wax E None (Control) 200 7.05% 3.23% Montan Wax E 0.5% A + 0.5% B 200 3.49% 2.43%

PP-230 None (Control) 225 58.85% 2.21% PP-230 0.5% A + 0.5% C 225 2.62% 2.29%

PED-521 None (Control) 200 11.42% 3.28% PED-521 0.5% A + 0.5% B 200 7.43% 3.24%

PE-190 None (Control) 225 10.33% 0.61% PE-190 0.5% A + 0.5% B 225 0.85% 0.67%

The data in table 1 shows that weight loss in a non-oxidative environment, i.e. nitrogen, is not affected much by the presence of the stabilizers, but that in an oxidative environment, i.e. air, these waxes suffer a much greater weight loss without the stabilizers. The stabilized wax compositions according to the present invention had dramatically reduced weight losses in air; in most cases, the weight losses were reduced almost to the level of the weight losses in the non-oxidative environment. These data clearly show the effectiveness of the stabilizers in reducing the thermooxidative decomposition of these waxes.

Many variations of the present invention not illustrated herein will occur to those skilled in the art. The present invention is not limited to the embodiments illustrated and described herein, but encompasses all the subject matter within the scope of the appended claims.