Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2000-11-01
2001-10-23
Foelak, Morton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C264S126000, C264S127000, C521S138000
Reexamination Certificate
active
06306921
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to foamed beads of polyester resin, to their method of preparation and to their use as loose fillers or for preparing molded foamed articles.
BACKGROUND ART
Foamed beads of various polymers, such as polystyrene, polyethylene and polypropylene, and their use to prepare foamed articles, are known.
It is known that foamed beads and articles obtained from polypropylene have far higher characteristics of chemical inertness, heat-resistance, resilience and impact-resistance than foamed polystyrene.
In the case of polypropylene, however, the process for preparing the beads is either complicated or requires the use of a polymer which is modified by linking side chains in order to improve its melt strength.
In the case of polyester resins, particularly polyethylene terephthalate (PET), the possibility to obtain foamed beads requires the use of resins which have appropriate rheological characteristics in order to be able to foam the particles.
The resins furthermore have relatively high melting temperatures and an equally high crystallinity under the temperature conditions in which it is necessary to work in order to allow the sintering of the beads.
This makes it troublesome to use pre-foamed beads of polyester resin in preparing molded foamed articles.
The production of foamed materials having a complex geometric shape and/or a considerable thickness directly by extrusion-foaming of the polyester resin is difficult to perform.
The need is therefore felt to have foamed beads of polyester resin which can be sinterized easily and effectively in order to be able to prepare said foamed articles.
BRIEF DESCRIPTION OF THE INVENTION
It has now been found unexpectedly that it is possible to obtain foamed beads of aromatic polyester resin which have mechanical and morphologic properties which make them usable not only as loose fillers in applications such as packaging and heat insulation, but also for preparing molded foamed articles having any geometric shape.
DETAILED DESCRIPTION OF THE INVENTION
The beads according to the invention are obtained from aromatic polyester resins having a melt strength of 1 cN at 280° C., a melt viscosity of more than 1500 Pa.s at 280° C. and with shear rate tending to zero, a die-swell index of more than 150%, an intrinsic viscosity of more than 0.8 dl/g and a crystallization rate by heating at 120° C. for 5 minutes such that the resulting crystallinity is not higher than 15%.
The above characteristics are necessary in order to obtain beads which can be easily sinterized without the premature crystallization of the resin on the surface preventing the mutual adhesion of the beads and/or without the beads collapsing when they are subjected to hot compression in the mold used to form the foamed article.
Resins having the above indicated characteristics can be obtained, by way of example and preferably, from copolyethylene terephthalates containing 2 to 20% of isophthalic acid units, by solid-state regrading of the polymer having an intrinsic viscosity generally below 0.7 dl/g in the presence of a dianhydride of a tetracarboxylic acid, preferably pyromellitic dianhydride (PMDA).
Temperature, treatment duration and dianhydride concentration are chosen so as to achieve the intended increase in intrinsic viscosity and in the rheological characteristics of the starting resin.
The regrading temperatures are generally between 150 and 210° C., according to the content of isophthalic acid units present in the resin.
In addition to copolyethylene terephthalates isophthalates, it is possible to use any other aromatic polyester resin which has the above specified rheological and crystallizability characteristics.
For example, it is possible to use polyester resins modified according to known methods with branching agents such as pentaerythritol and mellitic anhydride, added during the molten-state polycondensation of the resin.
It is also possible, again starting from resins that have the above specified crystallizability characteristics, to obtain the required rheological properties directly during extrusion foaming by working in the presence of PMDA and under appropriate extrusion-foaming conditions.
The preparation of the spheroidal foamed beads according to the invention is performed by cutting the foamed threads, by means of rotating blades at the output of an extrusion head with multiple holes, to a length which is a function of the die-swell of the resin.
It is also possible to obtain, by using extrusion heads which have holes of appropriate geometric shape and according to the end-applications, foamed beads which are non-spheroidal, such as C-shaped, T-shaped, L-shaped beads or other letter-like shapes.
The diameter of the spheroidal beads is generally between 1 and 15 mm or more.
The non-spheroidal shapes can have even larger dimensions.
The apparent density of the beads in their various shapes can vary within wide limits according to the conditions used in the extrusion-foaming process and particularly according to the amount of foaming agent used.
Densities between 30 and 500 kg/m
3
and preferably between 50 and 200 kgm
3
are representative.
The foaming agent is generally an inert gas, such as nitrogen and carbon dioxide, or a hydrocarbon, such as butane or isobutane and mixtures thereof
The amount used is conveniently between 1 and 10% by weight on the resin.
Nucleating agents, such as for example talc and similar compounds, can be used in order to control the microstructure of the cells.
The beads, directly after being cut, are propelled away by the rotating blades due to the centrifugal force they apply and are collected in a cooling bath of water kept at relatively low temperatures (10-15° C.).
Rapid cooling of the beads allows to maintain surface crystallinity at relatively low values.
The crystallinity of the beads is generally lower than 10%.
The resulting beads have the characteristics of a fine and uniform microcellular structure (cell size between 50 and 500 microns) in the outermost layer and a macrocellular structure, with cell sizes of a few millimeters, in the central part.
The macrocellular structure disappears when the beads, during sintering, are subjected to compression in the mold for forming the foamed articles.
The outermost microcellular structure remains practically unchanged.
Sintering of the beads is performed in a mold provided with holes for the passage of steam or other heating gas.
Superheated steam at a temperature between approximately 130 and 180° C., or air or an inert gas likewise heated to this temperature range, is used.
The residence times vary from a few seconds to a few minutes.
When using the beads according to the invention, it is not necessary to use pressurized steam to achieve adhesion between said beads.
The beads mutually sinter stably without forming undesired voids between them.
The resulting foamed articles have good resistance to compression and good elastic recovery; their density is slightly higher than the bulk density of the pre-foamed beads.
Density is generally between 50 and 600 g/dm
3
.
Crystallinity is high, generally between 20 and 40%.
The foamed articles, by virtue of their high mechanical and heat-resistance properties, are advantageously applied in sectors in which said properties are required, and particularly in the field of thermal and acoustic insulation, as structural elements, as barriers for acoustic insulation on highways, as simulated wood decorative elements, as dishes and trays to be subjected to microwave heating, as floating elements, et cetera.
The following examples are given to illustrate but not to limit the scope of the invention.
Measurement Methods
Intrinsic viscosity was measured in a 60/40 solution by weight of phenol and tetrachloroethane at 25° C. according to ASTM D 4603-96.
Rheological measurements were made according to the ASTM D 3835 standard, using a Goettfert rheometer at the temperature of 280° C.
Melt strength was determined by measuring the force in cN (centinewtons) required to stretch the material extruded from t
Al Ghatta Hussein
Pulcini Luigi
Severini Tonino
Foelak Morton
Josif Albert
Modiano Guido
O'Byrne Daniel
Sinco Ricerche S.p.A.
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