Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
1999-08-23
2001-06-05
Foelak, Morton (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S079000, C521S081000, C521S139000, C521S140000
Reexamination Certificate
active
06242502
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for preparing a foamed thermoplastic article from a thermoplastic elastomer.
PRIOR ART
Such a process is known from EP-A-40,934, in which a blend of a polyolefin resin and a rubber is foamed by the use of either chemical or physical blowing agents.
A drawback of such a process is that the proposed blowing agents (e.g. chemical blowing agents, (such as azodicarbonamides), or physical blowing agents, (such as chlorofluorocarbons or low boiling hydrocarbons)) have all their disadvantages, like inefficiency, and more in particular the detrimental influence on the ozon-layer of the earth-atmosphere.
It has been proposed to use gases, like N
2
or CO
2
, as an alternative, but they are difficult to be mixed in thermoplastic elstomers.
In the prior at the use of liquid water as a blowing agent is known. WO-92/18326 as well as EP-A-503,220 describe a process for the preparation of foamed thermoplastic elastomers in which the thermoplastic elastomer is heated to a temperature above its melting point, after which water is added to the melted thermoplastic elastomer and the mixture is released to atmospheric pressure.
Such a process has a lot of drawbacks: the liquid water is injected and mixed with the thermoplastic elastomer; the process results in a non-uniformity of the resulting foam: large and irregular shape of the foam, as well as surface roughness. Such a process also requires special equipment for the mixing of the ingredients and the metering of the water.
SUMMARY OF THE INVENTION
The process of the present invention provides a solution for the above problems. The invention concerns the preparation of a foamed thermoplastic article from a thermoplastic elastomer (TPE), in which the thermoplastic elastomer is selected from the group comprising styrene based thermoplastic elastomers (hereinafter also abbreviated as “STPE”) and thermoplastic polyolefin elastomers (hereinafter also abbreviated as TPOE).
The present process comprises the following steps:
a) heating a mixture of the TPE and an effective amount of a water releasing chemical compound (WCC), to at least a temperature at which the WCC releases water, which is above the melting point of the TPE, and
b) releasing the resulting heated mixture to atmospheric conditions.
Here and hereinafter the term “WCC” means any chemical compound or a mixture of chemical compounds that are capable of releasing water at elevated temperatures. In that sense different types of WCC are applicable according to the present invention; examples thereof are:
1) metal salts of groups 1 or 2 of the Periodic Table (CRC Handbook of Chemistry & Physics, 1990), wherein the anion is a phosphate, chromate, sulfate, borate or the like, said salts containing hydrate water. Suitable salts include, for instance, magnesium sulfate dihydrate, magnesium sulfate heptahydrate, calcium sulfate dihydrate, potassium citrate monohydrate, tricalcium phosphate monohydrate, sodium perborate tetrahydrate, barium acetate monohydrate and barium borate heptahydrate, among others;
2) metal hydroxides which decompose at elevated temperature, releasing water. Suitable, water-releasing metal hydroxides include, among others, aluminum trihydrate (ATH), also known as aluminum trihydroxide (Al(OH)
3
), and magnesium hydroxide (Mg(OH
2
);
3) organic di-acids, which can be transformed to their anhydride equivalent. An example of this are succinic acids, having the formula
which can be converted to the anhydride
In these formulae (1) and (2) R
1
-R
4
can, each independently, be hydrogen, an alkyl group having 1-20 C-atoms or a halogen group.
4) a mixture of ingredients that generate water through a polycondensation reaction. Suitable water-releasing polycondensable mixtures include
a mixture of an organic &agr;-&ohgr; diamide and an organic &agr;-&ohgr;-diacid which react during the polycondensation to form a polyamide. For instance, the reaction of 1,4 diamino butane with adipic acid under the formation of Nylon 4,6; and the reaction of 1,6 diaminohexane with adipic acid under the formation of Nylon 6.6, will release water.
Another suitable form of polycondensation reaction is the formation of a polyester.
The use of such a type of WCC not only results, during the foaming process of the TPE, in a release of water, but also in the formation of a nylon or an polyester phase in the TPE and therewith influencing the properties of the foamed article by their presence in the final article.
In the process of the present invention the WCC may only release its water (at least in substantial amounts) at a temperature at or above the melting point of the TPE, as a result of which the TPE is melt processable. The water release temperature may on the other hand be not so high, that such temperature is detrimental for the thermoplastic elastomer to be foamed. The appropriate WCC can therefor be selected upon choosing the TPE to be foamed (and therefor knowing the range of the temperature in which the TPE can be processed (for instance the melting point and the decomposition temperature of that TPE)). Analytical instruments like DTA (differential thermal analysis), DSC (differential scanning calorimetry) or TGA (thermogravimetric analysis) can be used to determine the suitability of a potential WCC for the intended purpose.
DETAILED DESCRIPTION OF THE INVENTION
The mixture of the TPE with the WCC, as used in the present invention, can be made according to several processes:
a) the TPE and the WCC can be dry blended, and added as a blend to the equipment in which the foaming takes place;
b) the WCC and the TPE can be mixed at a temperature above the melting point of the TPE, but below the temperature at which the WCC releases its water. Thereafter this melt mixture can either be processed to pellets (by for example extrusion and cooling), or directly processed into a foaming equipment. The melt-mixing of the WCC and TPE as well as the foaming of the TPE can take place in one apparatus, provided that the melt mixing and the foaming take place under different temperature conditions to avoid premature release of water before a thorough mixing and dispersion of the WCC in the TPE has taken place.
The thermoplastic elastomer (TPE), applicable in the process of the present invention, can be either a styrene based thermoplastic elastomer (STPE) or a thermoplastic polyolefin elastomer (TPOE), or, especially for low hardness products, mixtures thereof.
The STPE is a styrene block copolymer of the form A-B-A, in which A is a polystyrene chain and B a diene chain, such as polybutadiene or polyisopropene. The polystyrene may also be based on substituted styrenes, like &agr;-methylstyrene. The styrene/diene molar ratio generally ranges from 50/50 to 15/85.
A preferred form of STPE is at least one of styrene-butadiene-styrene blockcopolymers (SBS) and their partially or fully hydrogenated derivatives (SEBS).
The TPOE is a family of thermoplastic elastomers comprising a blend of a (semi-)crystalline polyolefin resin and a rubber dispersed in said resin. In general these blends comprise from 15-85 parts by weight of polyolefin resin and correspondingly from 85-15 parts by weight of rubber.
The polyolefin in such a TPOE can be one or more polyolefins originating from a (co-)polymerisation of &agr;-olefins, such as ethylene, propylene, butene-1 and otaers, as well the crystalline polycycloolefins. They have to behave like a thermoplastic and have a DSC crystallinity of at least 15%. A preference is present for homo- and copolymers of polyethylene and polypropylene; in the case of copolymers of said polyolefins the content of ethylene resp. propylene in said copolymer is at least 75 wt. %.
The rubber in the TPOE used according to the invention can be any rubber known in the art, provided that the rubber is stable under the foaming conditions. Rubbers useful are butylrubber (copolymer as well as terpolymers, and also in its halogenated form); ethylene/&agr;-olefin copolymer rubber (EAM) as well as ethylene/&agr;-olefin/diene terpolymer r
Brzoskowski Ryszard
Dion Robert
Hua Cai
La Tulippe Christopher
Wang Yundong
DSM N.V.
Foelak Morton
Pillsbury & Winthrop LLP
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