Formulation of polymeric mixtures for the production of cross-linked expanded PVC foams and process for producing said foams

Abstract

A formulation of polymeric mixtures for the production of cross-linked expanded PVC foams, of the type comprising PVC, isocyanates, anhydrides and one or more nucleating agents, wherein the nucleating agents are composed of nucleating materials having a porosity of 1-100 nm, preferably 2-50 nm. With respect to known formulations for the production of cross-linked PVC foams, a formulation according to the invention offers the advantage of obtaining the desired degrees of stabilization, nucleation and expansion, even without the use of diazocompounds.

Claims

1. A method of producing a cross-linked expanded PVC foam, comprising: providing a polymeric mixture comprising an isocyanate, an anhydride, a stabilizing and nucleating agent having a porous surface, and PVC (polyvinyl chloride), wherein the anhydride is liquid at room temperature, and wherein the polymeric mixture contains no diazocompounds; mixing the polymeric mixture under vacuum, so as to strip air from the polymeric mixture; immediately after the mixing, pouring the polymeric mixture into a mold; heating the mold in a press at a temperature of 160-180 C. and a pressure of 80-180 bars for a time sufficient for the PVC to melt, the isocyanate and the anhydride to begin cross-linking, and CO.sub.2 to form; allowing the polymeric mixture to expand and the cross-linking to complete by exposing the polymeric mixture to a temperature between 45 and 99 C. in presence of water vapor.

2. The method according to claim 1, wherein the cross-linking is a reaction consisting of: ##STR00004## and wherein: R1=C.sub.6H.sub.12; C.sub.9H.sub.18; C.sub.12H.sub.24; C.sub.10H.sub.18; C.sub.13H.sub.10; C.sub.6H.sub.4; C.sub.7H.sub.5; C.sub.10H.sub.6; C.sub.16H.sub.11; or C.sub.7H.sub.6; and R2=C.sub.6H.sub.6; C.sub.7H.sub.6; C.sub.6H.sub.8; C.sub.6H.sub.10; C.sub.14H.sub.26; C.sub.3H.sub.4; C.sub.3H.sub.6; C.sub.5H.sub.10; C.sub.10H.sub.18; C.sub.4H.sub.8; C.sub.7H.sub.10; C.sub.7H.sub.8; C.sub.4H.sub.4O.sub.2; C.sub.6H.sub.12; or C.sub.7H.sub.12.

3. The method according to claim 1, wherein the cross-linking is a reaction consisting of: ##STR00005## and wherein: R1=C.sub.6H.sub.4, or C.sub.7H.sub.6; R2=C.sub.6H.sub.12; C.sub.9H.sub.18; C.sub.12H.sub.24; C.sub.10H.sub.18; C.sub.13H.sub.10; C.sub.6H.sub.4; C.sub.7H.sub.5; C.sub.10H.sub.6; C.sub.16H.sub.11; or C.sub.7H.sub.6; and R3=C.sub.6H.sub.6; C.sub.7H.sub.6; C.sub.6H.sub.8; C.sub.6H.sub.10; C.sub.14H.sub.26; C.sub.3H.sub.4; C.sub.3H.sub.6; C.sub.5H.sub.10; C.sub.10H.sub.18; C.sub.4H.sub.8; C.sub.7H.sub.10; C.sub.7H.sub.8; C.sub.4H.sub.4O.sub.2; C.sub.6H.sub.12; or C.sub.7H.sub.12.

4. The method according to claim 1, wherein the porous surface has pores with a size of 1-100 nm.

5. The method according to claim 1, wherein the nucleating agent is a porous zeolite, composed of an aluminosilicate having a formulation:
xMO.ySiO.sub.2.zAl.sub.2O.sub.3 wherein: x=0-0.5; y=0-0.5; z=0.5-1; and M=Na, K, Ca, NH.sub.4, Fe.

6. The method according to claim 5, wherein the porous zeolite is less than 3% by weight of the polymeric mixture.

7. The method according to claim 6, wherein the polymeric mixture further comprises sodium bicarbonate in quantity of less than 3% by weight.

8. The method according to claim 1, wherein exposing the polymeric mixture to a temperature between 45 and 99 C. comprises exposing the polymeric mixture to temperatures of 80-99 C. and 45-70 C.

9. The method according to claim 1, wherein allowing the polymeric mixture to expand and the cross-linking to complete comprises causing the cross-linking to form a cross-linked structure around chains of the PVC and create an interpenetrating polymer network.

Description

(1) These and other objectives, advantages and characteristics appear evident from the following description of some preferred embodiments of the formulation and process of the invention, illustrated, for purely exemplificative and non-limiting purposes, in the figures of the enclosed drawings.

(2) In these:

(3) FIGS. 1 to 4 illustrate, by means of an optical microscope, the cellular structure of a cross-linked expanded PVC foam, according to the known art and according to three examples of the invention, respectively;

(4) FIG. 5 illustrates the trend of the thermal conductivity values of the foams of the previous figures;

(5) FIG. 6 illustrates the trend of the resin absorption values of the foams of FIGS. 1 to 4;

(6) FIG. 7 illustrates the graph of the trend of the Storage Modulus in the Dynamic Mechanical Analysis (DMA) as a function of the temperature, in the foams of FIGS. 1 to 4;

(7) FIGS. 8 and 9 illustrate microscope magnifications of the porous structure, of a zeolite and sodium carbonate of the invention, respectively;

(8) FIGS. 10 and 11 schematically illustrate a nucleation example of a nucleation agent with a non-porous surface and porous surface, respectively.

(9) The polymeric mixture of the invention for the production of cross-linked expanded PVC foams has the objective of providing an expanded material without diazo-compounds, characterized by the presence of closed cells having a diameter smaller than or equal to 0.6 mm, i.e. sufficiently small to confer a homogeneous structure to the expanded product, having a low thermal conductivity, a reduced resin absorption and a higher glass transition temperature.

(10) To enable the gas to generate a stable core, capable of forming closed cells having a small diameter, a certain energy threshold must be exceeded, which, for this reason, must be as low as possible. This energy threshold depends, in particular, on the critical radius of said core, which in turn depends on the interfacial tension between the gas bubbles and the polymeric mass. In the system of the polymeric mixture of the invention, which has become heterogeneous due to the presence of nucleating agents, the free energy Get to be exceeded is expressed by the formula:
Get=Gom.Math.f()

(11) wherein:

(12) Get=heterogeneous free energy, i.e. in the presence of nucleating agents

(13) Gom=homogeneous free energy, i.e. in the absence of nucleating agents

(14) f()= (2+cos ).Math.(1cos ).sup.2 wherein is the wettability angle.

(15) Furthermore, as 0<f()<1, in order to obtain a low Get value, f() must approach zero, i.e. the angle must be high. For this reason, according to the invention, materials characterized by having a porous surface are used as nucleating agents for the production of cross-linked expanded PVC foams.

(16) In this case, in fact, and as better illustrated in FIGS. 10 and 11, the angle 1 formed between the tangent t1 at the surface of the gas bubble 1 and the corresponding wall 2 of the porous nucleation site 3, is greater than the angle 2 formed between the tangent t2 at the surface of the gas bubble 4 and the corresponding wall 5 of the non-porous nucleation site 6.

(17) Nucleating agents suitable for the purposes of the invention are, for example, the zeolite of FIG. 8, with pores ranging from 2-50 nm, and the sodium bicarbonate of FIG. 9, with pores of about 100 nm. The porosity of the nucleating agent for the purposes of the invention is 1-100 nm, more preferably 2-50 nm. Furthermore, the desired porosity degree of the sodium bicarbonate is advantageously obtained, according to the invention, in the hot pressing phase of the formation process of cross-linked PVC foams.

(18) According to the invention, the CO.sub.2 gas phase is given by the following reaction between isocyanate and anhydride:

(19) ##STR00001##
wherein
R1=C6H12; C9H18; C12H24; C10H18; C13H10; C6H4; C7H5; C10H6; C16H11; C7H6
R2=C6H6; C7H6; C6H8; C6H10; C14H26; C3H4; C3H6; C5H10; C10H18; C4H8; C7H10; C7H8; C4H402; C6H12; C7H12.

(20) According to a preferred variant of the invention, the CO.sub.2 gas phase is given by the following reaction between isocyanate and anhydride:

(21) ##STR00002##
wherein:
R1=C6H4, C7H6
R2=C6H12; C9H18; C12H24; C10H18; C13H10; C6H4; C7H5; C10H6; C16H11; C7H6;
R3=C6H6; C7H6; C6H8; C6H10; C14H26; C3H4; C3H6; C5H10; C10H18; C4H8; C7H10; C7H8; C4H402; C6H12; C7H12.

(22) As can be observed, in these reactions, use is no longer made of the traditional diazo-compounds, with the function of expanding agents for the production of N.sub.2.

(23) The zeolites suitable for the invention are of the aluminosilicate type:
xMO.ySiO.sub.2.zAl.sub.2O.sub.3

(24) wherein:

(25) x=0-0.5

(26) y=0-0.5

(27) z=0.5-1

(28) M=Na, K, Ca, NH.sub.4, Fe.

(29) Furthermore, whereas the zeolites act exclusively as stabilizing and nucleating agents, the sodium bicarbonate of the invention also adds the effect of contributing to the cell expansion, producing CO.sub.2 through the following reaction with hydrochloric acid coming from PVC:
NaHCO.sub.3+HCl.fwdarw.NaCl+CO.sub.2+H.sub.2O

(30) In order to not subtract heat from the PVC melting process, the quantities of bicarbonate used are advantageously and preferably lower than 3% by weight. Furthermore, again with the same objective, the anhydrides used are those that are in the liquid state at room temperature, in particular anhydrides having formula:

(31) ##STR00003##
wherein
R3=C6H6; C7H6; C6H8; C6H10; C14H26; C3H4; C3H6; C5H10; C10H18; C4H8; C7H10; C7H8; C4H402; C6H12; C7H12.

(32) In this way, it is no longer necessary to have heat that serves for melting the traditional solid anhydrides. In addition, the use of the liquid anhydrides of the invention introduces imide structures into the reaction environment, necessary for the formation of an IPN-Inter Penetrating Network, having better thermal properties.

(33) The following formulations containing, by weight:

(34) TABLE-US-00001 PVC 30-60% Isocyanate 20-60% Liquid anhydrides 3-40%

(35) said formulations further comprising:

(36) TABLE-US-00002 Zeolites less than 3% Sodium bicarbonate less than 3%
are preferred for the invention.

(37) In the table, the following formulations are compared: A formulations of the known art, in which diazo-derivatives are used, with both a nucleating and stabilizing function (ADC), and also with the function of blowing agent (AZDN) (FIG. 1); B formulation of the invention, with sodium bicarbonate alone (FIG. 2); C formulation of the invention, with sodium bicarbonate and zeolite (FIG. 3); D formulation of the invention, with zeolite alone (FIG. 4).

(38) The foams obtained with these formulations were tested with respect to thermal conductivity, which must be as low as possible, the resin absorption quantity, which must also have minimum values, and the glass transition temperature, preferably high. The results of these tests are indicated in the following table, in which the quantities of compounds in the mixture are expressed as weight percentages:

(39) TABLE-US-00003 A B C D PVC 40 40 40 40 Isocyanate 51 51 51 51 Anhydride Phthalic Hexahydro- Hexahydro- Hexahydro- (solid) phthalic phthalic phthalic (liquid) (liquid) (liquid) 5 8 8 8 ADC 0.5 / / / AZDN 3.5 / / / Zeolite / / 0.5 3 Sodium- / 2 2 / bicarbonate Cell diameter 0.52 0.41 0.3 0.08 (mm) Thermal 0.031 0.03 0.029 0.026 conductivity (W/m .Math. K) Resinabsorption 389 280 199 0 (g/m.sup.2) Glass transition 91 101 105 109 temperature Tg( C.)

(40) From this table, it can be observed that the cross-linked expanded PVC foams have closed cells with a diameter smaller than or equal to 0.6 mm, i.e. sufficiently small to confer a homogeneous structure to the expanded product, with a thermal conductivity lower than 0.030 W/m K, a resin absorption lower than 300 g/m2 and a glass transition temperature higher than 100 C.

(41) The surface of the cross-linked expanded PVC foams of the previous table has the appearance illustrated in FIGS. 1 to 4, in which a decrease in the cell diameter in the foams of the invention, can be observed. The best results are obtained with formulation D of FIG. 4, which comprises high quantities of zeolite.

(42) In the process of the invention, the polymeric mixture is prepared by first introducing the liquid components (isocyanate and anhydride) into the mixer, followed by the powders of porous stabilizing and nucleating agents, and finally PVC. The mixing is effected under vacuum for stripping the air and the mixture thus obtained is poured into moulds which are heated under pressure in a press (80-180 bar) to a temperature of 160-180 C. for the time necessary for melting the PVC and for the isocyanate-anhydride cross-linking reactions and formation of the gas phase. It is during this hot compression that the sodium bicarbonate possibly present acquires the porous structure of the invention.

(43) The material obtained from the hot pressing, in which the cross-linking reactions between isocyanate and anhydride have partially taken place, and with the formation of imide and CO.sub.2, is then subjected to the expansion and cross-linking process in the presence of water vapour, at temperatures of 80-99 C. and 45-70 C., respectively. In this phase, the water reacts with the remaining isocyanate and anhydride reagents, thus completing the cross-linking reactions and producing additional quantities of CO.sub.2. The cross-linked expanded PVC foams of the invention (known as Inter Penetrating Network-IPN) are thus obtained, whose properties in terms of thermal conductivity, resin absorption and glass transition temperature, with the same cell diameter, are higher with respect to the foams of the known art.