EP0000920B1

EP0000920B1 - Molding compositions comprising thermoplastic elastomers, method for preparing molded articles, and molded articles

Info

Publication number: EP0000920B1
Application number: EP78100674A
Authority: EP
Inventors: Roy William Siedenstrang; William Henry Cornell
Original assignee: Phillips Petroleum Co
Current assignee: Phillips Petroleum Co
Priority date: 1977-08-17
Filing date: 1978-08-16
Publication date: 1982-11-10
Also published as: ES472566A1; IT7826775A0; EP0000920A1; IT1098055B; DK362678A; DE2862084D1; CA1109979A; US4288399A

Description

The invention relates to molding compositions on the basis of thermoplastic elastomers useful for low pressure processing utilizing microwave energy.
In the past, microwave energy has been used in vulcanizing techniques to raise the temperature of vulcanizable compositions containing polar compounds to a level at which vulcanization can be accelerated by conventional means. It has now been discovered that microwave energy can be used to increase the temperature of thermoplastic elastomers-containing molding compositions above the softening point to permit low pressure molding, e.g. below about 1.4 MPa (200 psig), by incorporating certain polar compounds into said compositions. Thermoplastic elastomers exhibit the stress-strain properties of vulcanized rubbers without having been chemically vulcanized. A number of polar compounds and polymers that work well to promote the start of vulcanization using microwave energy are not, however, useful in promoting softening of thermoplastic compositions. Temperatures obtained within practical time cycles are not enough to overcome the flow deficiency caused by the addition of solid non-thermoplastic polar polymers such as halogenated rubbers, nitrile rubbers, and/or fillers such as carbon black. It should also be pointed out that highly filled resins do not react well to microwave energy so that highly filled thermoplastic elastomer based stocks result in lower melt flow rates because of lesser percentages of polymer in the total composition. Limited compatibility with thermoplastic elastomers also contributes to lack of efficiency of some ingredients which proved useful in promoting vulcanization, but are not useful in promoting softening of thermoplastic compositions. Overall, it has been found that polar compounds useful in thermoplastic compositions used in low pressure molding must be selectively chosen.
Thus, the invention relates to a molding composition comprising a thermoplastic elastomer and, optionally, a solid resinous polymer which composition is characterized by containing, as polar compounds, one or more of simple and polymeric alkylene glycols, their mono and dialkyl ethers; ethanolamines, isopropanolamines, and their hydrocarbyl-substituted derivatives; liquid acrylonitrile/butadiene polymers; acrylonitrile/butadiene polymer blends with homopolymers of polyvinyl chloride and styrene/acrylonitrile copolymers.
In a specific embodiment the invention relates to a molding composition wherein said thermoplastic elastomer is a normally solid linear or radial teleblock copolymer being present in an amount of 30 to 100 weight percent, and said resinous polymer is a polymer of a vinyl-substituted aromatic compound being present in an amount of 0 to 70 weight percent, each based on the total polymeric content of the composition.
In a further specific embodiment the invention relates to a molding composition wherein said thermoplastic elastomer is a radial teleblock butadiene/styrene (52/48) copolymer extended with 60 parts by weight of naphthenic oil, said resinous polymer comprises poly-a-methylstyrene and polystyrene, and said polar compound is chosen from triethanolamine, a mixture of triethanolamine and diethylene glycol, butadiene/acrylonitrile rubber, and a mixture of triethanolamine and polyethylene glycol, said polyethylene glycol having an average molecular weight of 4000 to 6000.
In a further specific embodiment the invention relates to a molding composition wherein said thermoplastic elastomer is a radial teleblock butadiene/styrene (52/48) copolymer extended with 60 parts by weight of naphthenic oil, said resinous polymer is crystalline polystyrene, and said polar compound is chosen from a mixture of triethanolamine and diethylene glycol and a 50/50 blend of butadiene/acrylonitrile rubber and polyvinyl chloride.
The composition of the invention comprise a major amount of thermoplastic elastomer and, optionally, minor amounts of solid resinous polymers. They contain one or more of the aforesaid polar compounds so that the resulting compositions can be readily preheated by exposure to energy at microwave frequency to a moldable consistency that can be molded at a pressure of 1.4 MPa or less.
In one embodiment of the invention a method for molding articles is provided in which molding composition solids as described above are introduced into a mold and enclosed therein, with the application of microwave energy to the composition for a time sufficient to produce a moldable consistency in the composition. Thereafter, molding pressure, up to about 1.4 MPa is applied to the molding composition for a time sufficient to produce a molded article after which the mold is cooled and the cooled molded article is unmolded.
In a further embodiment the invention relates to a method for preparing molded articles by

(a) introducing the aforesaid molding composition solids into a mold and enclosing said composition in the mold;
(b) applying microwave energy to said composition for a time sufficient to produce a moldable consistency in said molding composition;
(c) applying sufficient pressure up to 1380 kPa to said molding composition for a time sufficient to produce a molded article;
(d) cooling said mold and molded article; and
(e) umolding said cool molded article.

In a further specific embodiment the invention relates to a method wherein said microwave energy is applied for a time range of 4 seconds to 4 minutes and said pressure is from 69 to 1380 kPa and is applied for a period of time ranging from 0.1 to 10 times the length of time that said microwave energy was applied.
The thermoplastic elastomers useful in practicing this invention are normally solid linear and radial teleblock copolymers which characteristically exhibit high tensile strength and elongation in their natural condition, i.e. nonvulcanized state. The copolymers are described in more detail in US Patents 3 823 109; 3 826 776 and 3 959 545. Particularly suitable copolymers are those derived from butadiene and styrene in which the butadiene to styrene ratio can vary from about 85/15 to 45/55 parts by weight and the rubber will contain from about 10 to 55 wt.% of the styrene incorporated as terminal polystyrene blocks.
The amount of the thermoplastic elastomer employed in the compositions constitutes from about 30 to 100 wt.% of the total polymers utilized in preparing the compositions.
The solid resinous polymers employed in forming the compositions, from 0 to about 70 wt.% of total polymer utilized, are generally polymers of vinyl-substituted aromatic monomers, e.g., styrene or alpha-methyl styrene, alone or copolymerized with a monomer such as acrylonitrile or a conjugated diene such as butadiene. Such homopolymers and copolymers generally have densities in the range from about 1.04 to 1.10 g/ml (ASTM D 792), a tensile strength in the range from about 34.5 to 82.7 MPa (5,000 to 12,000 psi), ASTM D 638, and a Shore A Hardness ranging from about 35 to 95 (ASTM D 2240) at 23°C.
The polar compounds, also referred as polarizing compounds hereinafter, employed in the invention compositions are normally liquid or solid in nature. Exemplary compounds include ethylene glycol, 1,2-propylene glycol, 1,3-butylene glycol, 1,4-butylene glycol, 1,6-hexylene glycol, diethylene glycol, triethylene glycol, dipropylene glycol and thiodiethylene glycol; polyethylene glycols having average molecular weights ranging from about 200 to 6,000; polypropylene glycols having average molecular weights averaging from about 400 to 2,000; mixed poly(ethylene)-poly(propylene) glycols having average molecular weights ranging up to about 6,000 and containing from about 30 to 90 wt.% ethylene oxide; the monomethyl, monoethyl and monobutyl ethers of ethylene glycol, propylene glycol and diethylene glycol; the monomethyl and monoethyl ethers of triethylene glycol; the dimethyl and diethyl ethers of diethylene glycol, dipropylene glycol and trimethylene glycol; the alkanolamines and substituted alkanolamines based on ethanol and isopropanol such as mono-, di- and triethanolamine, mono-, di- and triisopropylamine, methylethanolamine, dibutylethanolamine, phenyldiethanolamine, di-(2-ethylhexyl)ethanolamine, dimethylisopropanolamine and dibutylisopropanolamine; and mixtures thereof. Other polar compounds such as liquid acrylonitrile-butadiene polymers acrylonitrile-butadiene polymer blends with homopolymers of polyvinyl chloride and styrene/acrylonitrile copolymers are effective. Mixtures of the polar compounds can be used. Presently preferred compounds include diethylene glycol and triethanolamine and mixtures thereof.
Polarizing compounds selected from solid nitrile rubbers and polychloroprene polymers, or carbon black, are not suitable for use in the applications envisioned in the instant invention because compositions containing such materials do not flow readily under the low molding pressures utilized in the process of this invention.
Sufficient polarizing compound is included in the compositions of the instant invention to insure that they will be heat-softened quickly when placed in a microwave field. Generally, the amount of polarizing compound utilized ranges from 0.5 to 20 parts by weight per 100 parts by weight thermoplastic elastomer and more preferably from 0.75 to 10 parts by weight for reasons of economy coupled with adequate response to the microwaves. Heating times employed are selected to achieve rapid softening of the compositions to moldable consistency without deleterious effects caused by local overheating. Generally, the heating times used range from 4 seconds to 4 minutes. From a commercial standpoint, however, heating times ranging from 4 to 55 seconds are employed to obtain favorable production rates and this is a preferred range.
Other components which can be contained in the molding compositions include stabilizers, plasticizers, odorants, colorants and fillers, e.g., silica or clay; silicates, e.g., Wollastonite; calcium carbonate, glass beads and fibers. Plasticizing agents compatible with the thermoplastic elastomer and the solid resinous polymers can be employed if desired. Examples of these include naphthenic petroleum oils, e.g., ASTM type 104A and esters of adipic acid or phthalic acid. Processing aids include the metal stearates, e.g., calcium stearate and zinc stearate, silicones, natural and synthetic waxes. Antioxidants and UV stabilizers can be added as desired from suitable commercially available materials. Exemplary of these include thiodipropionic esters, e.g., dilaurylthiodipropionate, hindered phenolic antioxidants, e.g., 2,6 - di - t - butyl - 4 - methylphenol, octadecyl[3 - (3,5 - di - t - butyl - 4 - hydroxyphenyl)jpropionate and thiodiethylene bis(3,5 - di - t - butyl - 4 - hydroxy) hydrocinnamate, and UV stabilizers such as 2(2'-hydroxy - 5' - methylphenyl) benzotriazole, 2 - hydroxy - 4 - n - octoxy- benzophenone and [2,2' - thiobis(4 - t - octyl - phenolato)] - n - butylamine - nickel(II). Generally, the amounts of the various components in parts by weight per 100 parts by weight thermoplastic elastomer (php) will be as follows: filler, 10 to 150 php, plasticizing agent 20 to 50 php; antioxidant, 0.1 to 1 php, and UV stabilizer, 0.1 to 3 php. The molding compositions can be molded into various useful articles, e.g. sheet stock, shoe soles, mats and watch bands.
The molds employed in shaping the invention compositions are generally constructed from low cost, relatively low strength materials including silicone rubber, polysulfide rubber, polyurethane, plaster and cast aluminum. The nature of the mold is dependent upon the molding process used. If the invention composition is placed within the mold and the entire assembly is preheated by microwave energy, it is preferred that the mold used be made of a material such as silicone rubber that has a lower dielectric loss factor than the invention composition. It is within the scope of this invention to preheat the composition in a low dielectric loss container such as glass or ceramic and transfer it to a mold constructed from metals for the actual shaping step. Generally, the composition is placed within a silicone rubber mold, the mold top is covered with a silicone sheet or a release paper, e.g., paper covered with a release agent such as a silicone, and the assembly is placed between the plates of a high frequency electrical field which form a part of commercially available molding machines. The top plate is lowered to contact the release paper covering the mold and the composition is preheated by application of microwave energy for a desired length of time. After the preheating sufficient pressure is employed to compression mold the composition, e.g., about 68.9-1380 kPa (10 to 200 psig), for a period of time generally ranging from 0.1 to 10 times the preheating time. The pressure is released, the assembly is preferably placed in a separate zone for cooling the mold and contents, after which the molded article is removed. A rotary table containing a plurality of molds can be employed to provide molded parts at commercially attractive rates. Separation of the heating and cooling zones speeds production and reduces consumption of power and water.

Example 1

A series of compositions was prepared containing one or more polarizing compounds by mixing the components in a Banbury mixer for 6 minutes at a temperature of about 138°C (280°F). Each composition was dumped from the mixer at about 121°C (250°F) and converted into pellets. The base formulations employed shown in Table 1 A are in parts by weight.
Each pelleted composition was used to fill a (7,9 mm) thick silicone rubber, 2-cavity shoe soling mold maintained at about 25°C. The average cavity depth was about 7,9 mm. After placing a· paper release sheet over the mold, the assembly was placed in a commercial molding machine employing microwave energy as the heating medium, the top molding plate was lowered to contact the release paper covering the mold at a pressure of about 276 kPa gage and the microwave unit was energized to preheat the composition for the designated time. The microwave field was turned off, the pressure against the mold was increased to about (827 kPa gage and held for 45 seconds at that pressure. The pressure was released, the mold assembly was transferred to a cooling press through which water at a temperature of about 10°C was being circulated and the assembly was cooled for 2 minutes by contact with the platens of the press. The molded product was stripped from the mold at about 49°C.
The preheat times employed and results obtained are presented in Table 1 B.
Inspection of the results presented in Table 1 B shows the most effective polarizing compounds are TEA/DEG (run 2) followed by Paracril OZO (run 8) and Hycar 1312 (run 6). These runs show the TEA/DEG mixture is the most efficient based on quantity employed since 6 parts by weight of the mixture is equivalent in performance to 20 parts by weight Paracril OZO, both compositions requiring only 25 seconds preheat time. It required 10 parts by weight Hycar 1312 to give good results but 60 seconds is required to obtain desirable temperature properties of the composition. The compounds shown in runs 3, 4, 5 and 9.are deemed to be unsuitable either because of incompatibility with the thermoplastic elastomer or showing relatively low efficiency in bringing the compositions to a moldable condition in comparison to the TEA/DEG agent used in run 2. The relatively poor results shown in run 7, even using a favorable concentration of TEA/DEG, suggests that careful consideration needs to be given to the components making up the compositions in order to achieve rapid melting of the compositions in a microwave field.

Example 2

A series of compositions was prepared in which a polyethylene glycol was used alone or in combination with TEA as a polarizing compound with a thermoplastic elastomer composition. containing the following components in parts by weight as shown in Table 2A. The components employed are the same as described in Example 1 except when indicated otherwise.
The compositions were mixed as before and molded generally in the manner outlined previously. Specimens about 15x 15 cm and about 0.25 cm thick were molded in this series. Preheating time for each mold assembly was 12 seconds and each composition was molded under a pressure of 551 kPa gage and cooled 20 seconds as described before.
The polarizing compounds employed, quantities of each used and results obtained are given in Table 2B.
The good results in runs 1, 2 and 3 indicate that a polyethylene glycol is equivalent to diethylene glycol when used in combination with TEA in the compositions of the instant invention. The poor results shown in runs 4, 5 and 6 indicate that polyethylene glycol by itself is a less effective polarizing compound in this invention.

Example 3

A series of combinations was prepared as described before by admixing a thermoplastic elastomer with generally the same components used in the first 2 examples. However, a solid resinous polymer was also included in the formulations as well as varying amounts of TEA/DEG. Molded samples were prepared from each composition in the manner described in Example 2 except that a variable preheat time was employed. The compositions prepared and results obtained are presented in Tables 3A and 3B. Unless otherwise described, the components, in parts by weight, are the same as previously used.
The results show that compounds formulated according to the instant invention are easily moldable after preheat times ranging from 8 to 35 seconds in a microwave field. By adding from 0.5-3.0 parts by weight each of TEA and DEG, the preheat time is reduced from 35 seconds without the polarizing compound combination to 8 seconds with 6 parts by weight of the combination. Thus, excellent results are obtained by additionally employing a solid resinous polymer in the compositions and particularly in combination with the preferred polarizing compound.

Example 4

A composition was prepared, molded and evaluated according to the procedure employed in Example 3. In this example, however, a thermoplastic elastomer consisting of a radial teleblock butadiene/styrene (70/30) copolymer having a molecular weight of 300,000 and Shore A Hardness of 84 and not containing extender oil was employed in place of the thermoplastic elastomer used previously. The composition employed was identical to composition A, Table 1A, except that the thermoplastic elastomer described above was used. A molding preheat time of 20 seconds was used.
The physical properties of the molded specimens were found to be as shown in Table 4.
The results show that excellent molding compositions can be prepared from thermoplastic elastomers consisting of teleblock butadiene/styrene copolymers of varying block styrene contents by employing the components described in this invention. When the compositions are heated by means of microwave energy for a period of time of less than 60 seconds, the compositions are softened sufficiently to be easily moldable under pressure of below about 1.4 MPa (200 psig). Thus, low cost, low strength molds can be employed in molding the compositions and very rapid cycle times result in practicing this invention.

Claims

1. A molding composition comprising a thermoplastic elastomer and, optionally, a solid resinous polymer characterized by containing, as polar compounds, one or more of simple and polymeric alkylene glycols, their mono and dialkyl ethers; ethanolamines, isopropanolamines, and their hydrocarbyl-substituted derivatives; liquid acrylonitrile/butadiene polymers; acrylonitrile/butadiene polymer blends with homopolymers of polyvinyl chloride and styrene/acrylonitrile copolymers.

2. A molding composition according to claim 1 characterized in that said thermoplastic elastomer is a normally solid linear or radial teleblock copolymer being present in an amount of 30 to 100 weight percent, and said resinous polymer is a polymer of a vinyl-substituted aromatic compound being present in an amount of 0 to 70 weight percent, each based on the total polymeric content of the composition.

3. A molding composition according to claim 1 or 2 characterized in that said thermoplastic elastomer is a radial teleblock butadiene/styrene (52/48) copolymer extended with 60 parts by weight of naphthenic oil, said resinous polymer comprises poly-a-methylstyrene and polystyrene, and said polar compound is chosen from triethanolamine, a mixture of triethanolamine and diethylene glycol, butadiene/acrylonitrile rubber, and a mixture of triethanolamine and polyethylene glycol, said polyethylene glycol having an average molecular weight of 4000 to 6000.

4. A molding composition according to claim 1 or 2 characterized in that said thermoplastic elastomer is a radial teleblock butadiene/styrene (52/48) copolymer extended with 60 parts by weight of naphthenic oil, said resinous polymer is crystalline polystyrene, and said polar compound is chosen from a mixture of triethanolamine and diethylene glycol and a 50/50 blend of butadiene/acrylonitrile rubber and polyvinyl chloride.

5. A method for preparing molded articles characterized by

(a) introducing molding composition solids of claim 2, 3 or 4 into a mold and enclosing said composition in the mold;

(b) applying microwave energy to said composition for a time sufficient to produce a moldable consistency in said molding composition;

(c) applying sufficient pressure up to 1380 kPa to said molding composition for a time sufficient to produce a molded article;

(d) cooling said mold and molded article; and

(e) unmolding said cool molded article.

6. A method according to claim 5 characterized in that said microwave energy is applied for a time range of 4 seconds to 4 minutes and said pressure is from 69 to 1380 kPa and is applied for a period of time ranging from 0.1 to 10 times the length of time that said microwave energy was applied.

7. A molded article made from the molding composition of one of claims 1 to 4.