MXPA97009017A - Procedure for the preparation of polyethyl tereftalate foam - Google Patents

Abstract

A process is described for the preparation of polyethylene terephthalate foam which includes the steps of feeding in a controlled manner to an extruder machine, a resin of polymeric material to be foamed, adding foaming agents to the resin to initiate and accelerate the foaming, as well as to modify the softening point of the material, to heat the polymeric material until it melts, to inject a blowing agent into the molten material, to form a uniform and controlled mixture, and finally to extrude the material. In the process of the present invention, the blowing agent is pure nitrogen, in such a way that a foamed polymer product is obtained, without contaminating the environment.

Description

"PROCEDURE FOR THE PREPARATION OF POLIETI ENO TERTHTALATE FOAM" FIELD OF THE INVENTION The present invention is related to the processes for the transformation of polymers within the Chemical Industry, and more particularly, is related to a process for the preparation of polyethylene terephthalate foam.

BACKGROUND OF THE INVENTION Foaming processes have traditionally been restricted to polyolefins or polyurethanes. It is important to mention that, for a foam to form, it is necessary to introduce a gas or blowing agent into a liquid. In the case of plastics such as polyethylene terephthalate, until prior to the present invention, no foaming process had been developed for this polymer, due to complications during the procedure. The process for the foaming of polymers currently used, basically comprises the following steps: a first step of feeding the polymeric resin to a conventional extruder machine; subsequently, foaming agents are added which serve to initiate and accelerate the foaming, and to modify the softening point of the material, allowing a better flow of the same through the machine; a next step of heating the polymeric material to melt it; to subsequently inject a blowing agent into the molten material, under certain operating conditions so as to form a uniform and controlled mixture; and, finally, extruding the material, obtaining a foamed polymer product. The foaming processes of plastics in general, have been carried out with various blowing agents, among which polyhalogenated hydrocarbons (freons), such as dichloro-fluoro-methane (Freon 12) and dichloro-tetrafluoro-ethane stand out (Freon 114). However, the use of polyhalogenated hydrocarbons as blowing agents for the preparation of foams has been limited to such a degree that, at present, they are practically no longer used, due to the disadvantage of being very expensive gases and limited production. Additionally, polyhalogenated hydrocarbons have the great disadvantage of being toxic to humans, in addition to having a negative effect on the atmosphere, since they greatly affect the ozone layer, which is why environmental laws have restricted their use and have has been replaced in the industry by low molecular weight hydrocarbons such as isobutane. The use of low molecular weight hydrocarbons as blowing agents is described in Mexican Patent Application No. 9200422, in which a process for the manufacture of polyethylene foams using low molecular weight hydrocarbons as blowing agents is claimed. However, the use of such hydrocarbons is highly risky due to their nature, since they are highly flammable and explosive, which represents a great inconvenience to carry out the process of making foams. Additionally, polyethylene terephthalate requires low humidity conditions for processing; therefore, to eliminate this moisture, heat must be supplied, which increases the risk that the blowing agent will ignite and explode. Another disadvantage presented by the use of polyhalogenated hydrocarbons or low molecular weight hydrocarbons as blowing agents is that they can be trapped inside the material, which can be harmful to health, as it is in the case of foams that can to be in contact with man. As a consequence of the foregoing, it has been sought to eliminate the disadvantages presented by the blowing agents for the preparation of foams of the current technique and to provide a method for the preparation of polyethylene terephthalate foam, which makes it possible to eliminate the disadvantages previously established, offering a procedure that is economical, practical and functional, and at the same time reduce the risk of accidents during processing.

OBJECTIVES OF THE INVENTION Taking into account the defects of the prior art, it is an object of the present invention to provide a process for the preparation of polyethylene terephthalate foam, which is practical, simple and economical; and, nevertheless, of high efficiency for the foaming of polyethylene terephthalate. It is a further object of the present invention to provide a process for the preparation of polyethylene terephthalate foam, which makes it possible to eliminate the use of toxic and harmful blowing agents both for the human being and for the environment. It is still a further object of the present invention to provide a process for the preparation of polyethylene terephthalate foam, which makes it possible to obtain a high quality polyethylene terephthalate foam, which does not have interstitially trapped blowing agents toxic to humans and the environment . The above objects, as well as other objects and advantages of the invention, are achieved by a process for the preparation of polymer foams of the type comprising the steps of feeding in a controlled manner the resin of polymeric material to be foamed; adding foaming agents that serve to initiate and accelerate foaming, as well as to modify the softening point of the material; heat the polymeric material; injecting a blowing agent into the molten material to form a uniform and controlled mixture; and, finally, extruding the material, obtaining a foamed polymeric product, wherein the process for foaming of the present invention includes the use of pure nitrogen as a blowing agent at certain operating conditions.

DETAILED DESCRIPTION OF THE INVENTION In the process for the preparation of polyethylene terephthalate foam of the present invention, it has been possible to use pure nitrogen directly as a blowing agent, without substantially contaminating the environment. As already mentioned in the previous art chapter, the procedure for the preparation of foams of the current technique, basically comprises the steps of feeding in a controlled manner to a conventional extruder machine, a resin of polymeric material to be foamed; add to the resin foaming agents that serve to initiate and accelerate foaming, as well as to modify the softening point of the material; heat the polymeric material until it melts; to subsequently inject a blowing agent into the molten material, to form a uniform and controlled mixture; and, finally, extruding the material thus formed, obtaining a foamed polymer product, wherein said foaming process employs as blowing agents polyhalogenated hydrocarbons such as Freon 12 or Freon 114, or low molecular weight hydrocarbons such as isobutane , which are injected under certain operating conditions, in such a way that a uniform and controlled mixture can be achieved. The innovative methodology of the present invention basically comprises the use of pure nitrogen as the blowing agent. To carry out the foaming process of the present invention, a plastic extruder machine of the conventional type is used, in which a specific amount of a polyethylene terephthalate resin is fed using a dosing device, either manual or automatic, so that a mixture is formed with complementary foaming agents that help to initiate and accelerate the foaming, as well as to modify the vitreous transition temperature of polyethylene terephthalate, which allows the flow of the same through the machine. distribute better. In a preferred embodiment of the present invention, an extruder machine with a capacity to produce between 200 and 270 kilograms per hour is used, when the motor thereof is operating at approximately 80% of its speed. The extruder machine comprises between 6 and 8 heating zones, which operate on a temperature scale from approximately 180 ° C to approximately 350 ° C, being able to have additional heating zones and tanks for sudden cooling of the material. In order to form the polyethylene terephthalate foam, during extrusion, a blowing agent is injected at about 15 to 30 liters per hour of a stream of pure nitrogen at a pressure of between approximately 250 kg / cm and approximately 300 kg / cm, in such a way that a uniform and controlled mixture is formed with the characteristics of specific thickness and density required for the final use that is intended to be given to the polyethylene terephthalate foam. The pressures used for the extrusion can vary between about 300 kg / cm2 and about 450 kg / cm to about 25 cm from the entrance of the polyethylene terephthalate to the extruder, and between about 400 kg / cm2 and about 500 kg / cm2 at the entrance of the polyethylene terephthalate to the extruder machine. The final product obtained is a polyethylene terephthalate foam, which takes the form of the outlet die of the extruder that is being used, being able to adopt, among others, the shape of a sheet or the shape of a tube. Likewise, the final product may contain trapped nitrogen, but nevertheless, because it is an inert gas, it does not represent a risk to health. It should be noted that nitrogen, by having an extremely low vapor pressure with respect to the blowing agents of the current technique, has a great advantage in terms of its volatility, and therefore, uniformity of the obtained foam, besides not containing humidity, fundamental situation for the processing of polyethylene terephthalate. The process for the preparation of polyethylene terephthalate foam of the present invention will be more clearly illustrated by means of the following example, which is presented solely for illustrative and non-limiting purposes.

EXAMPLE For purposes of exemplifying the use of pure nitrogen as a blowing agent in a process for the formation of foam, the example that follows is made in a plastic extruder machine with 8 heating zones, which had the following profile of temperatures in operation: TEMPERATURE TEMPERATURE ZONE OUTPUT INPUT 1 200 ° C 150 ° C 2 200 ° C 150 ° C 3 200 ° C 150 ° C 4 200 ° C 150 ° C 5 200 ° C 150 ° C 6 190 ° C 65 ° C 7 240 ° C 10 ° C 8 190 ° C 137 ° C Given 1 140 ° C Given 2 150 ° C Additionally, the extruder machine includes two drums with oil inside, which were kept at a constant temperature of 143 ° C in order to achieve a drastic cooling of the foam. The polyethylene terephthalate resin was fed in a controlled manner to the extruder by means of a hopper at a rate of 270 kg / h, with the motor operating at 80% of its speed and amperage. Some adjuvant blowing agents were added in a proportion of 10% by weight with respect to the resin that was fed, said commercial type foaming agents being such as SAFOAM P50, to initiate foaming, and SAFOAM RPC-40, to modify the glass transition temperature of polyethylene terephthalate. Once the mixture was formed, 20 liters per hour of pure nitrogen was injected at a pressure of 250 kg / cm between the heating zones 1 and 2 of the extruder. The extrusion pressures used were 400 kg / cm2 at 25 cm from the entrance of the material to the extruder, and 500 kg / cm2 at the entrance of the material to the extruder. The final product obtained was a foamed polyethylene terephthalate sheet, which was later coiled to form rolls. In accordance with the above description, it can be seen that the procedure in question has been devised so that by using pure nitrogen as a blowing agent in the manufacture of foams, a practical and functional solution to the problems presented in the present by the use of toxic and harmful blowing agents for the environment, in addition to allowing to offer a practical, economical and good quality product. Although certain embodiments of the invention have been illustrated and described, it should be emphasized that numerous modifications to them are possible. The present invention, therefore, should not be considered as restricted, except for what is required by the prior art and by the spirit of the appended claims.

Claims (4)

NOVELTY OF THE INVENTION CLAIMS

1. - A process for the preparation of polyethylene terephthalate foam of the type comprising the steps of feeding in a controlled manner to a conventional extruder, a resin of polymer material to be foamed; add to the resin foaming agents that serve to initiate and accelerate foaming, as well as to modify the softening point of the material; heat the polymeric material until it melts; to subsequently inject a blowing agent into the molten material, to form a uniform and controlled mixture; and, finally, extruding the material, characterized in that in order to obtain a foamed polymer product without contaminating the environment, the blowing agent is pure nitrogen.

2. - A process for the preparation of polyethylene terephthalate foam, according to claim 1, further characterized in that the pure nitrogen is injected into the machine as a current of between about 15 and 30 liters per hour and at a pressure of between approximately 250 kg / cm2 at approximately 300 kg / cm2.

3. A process for the preparation of polyethylene terephthalate foam, according to claim 1, further characterized in that the heating is carried out on a temperature scale of between about 180 ° C and about 350 ° C.

4. - A process for the preparation of polyethylene terephthalate foam, according to claim 1, further characterized in that the pressures used during the extrusion process can vary between about 300 kg / cm2 and about 450 kg / cm2 to about 25 cm from the entrance of the polyethylene terephthalate to the extruder, and from approximately 400 kg / c to approximately 500 kg / cm at the entrance of the polyethylene terephthalate to the extruder.