Rapidly Crystallizing Polyethylene Terephthalate Blends

This invention lies in the field of rapidly crystallizing polyethylene terephthalate molding resin blends.

Various crystallizing and nucleating agents (including esters) are known for promoting the crystallization rate of polyethylene terephthalate from a melt phase. Such a rate increase in crystallization is desirable for purposes of shortening molding cycle times and lowering molding temperatures.

Bier et al in U.S. Pat. No. 4,223,113 teach combinations of polyalkylene terephthalate and "an oligomeric polyester having a minimum polymerization degree of 2 and a maximum number-average molecular weight of about 6,000, said oligomer being free of any substitutes which are substantially reactive with said high molecular weight polyalkylene terephthalate under injection molding conditions" (see, for example, claim 1 of Bier et al '113). The oligomers can be formed of sebacic or adipic acid with diols and monohydric alcohols.

Hecht et al in U.S. Pat. No. 4,338,243 and Deyrup in Re No. 32,334 teach the use of esters of aromatic carboxylic acid and alcohols as polyethylene terephthalate crystallization promoters.

However, so far as now known, no one has previously taught or suggested the use of medium number-average molecular weight aliphatic polyesters as reactive crystallization promoters for polyethylene terephthalate.

The object of the present invention is to provide polyalkylene terephthalate resin compositions which are rapidly crystalline.

Another object is to provide such compositions which have a high broad crystallization range and high heat distortion temperatures.

Another object is to provide such compositions which can be compounded with other materials to produce molding resin blends which have improved properties, such as, for example, blends with acceptable engineering resin performance characteristics.

Another object is to provide processes for making and using such compositions.

Another object is to provide articles made from such compositions.

Other and further objects, aims, purposes, features, advantages, embodiments, and the like will become apparent to those skilled in the art from the teachings of the present specification taken with the appended claims.

The present invention is directed to a new and very useful class of polyethylene terephthalate resin compositions which contain from about 0.1 to about 25 percent by weight of an aliphatic polyester having a number-average molecular weight in the range from about 7,500 to about 20,000 and which comprises a condensation reaction product of a dialkanoic acid containing from about 8 to about 12 carbon atoms per molecule and a dialkanol containing from 2 to about 5 carbon atoms per molecule.

Such compositions crystallize rapidly, have a broad crystallization temperature range, distortion temperature range, and display high heat distortion temperatures. The physical strength properties of such compositions are generally acceptable; that is, such properties are usually almost unchanged or are improved compared to the corresponding properties of polyethylene terephthalate compositions which do not contain such polyester.

When compounded with suitable other material, such resin compositions can be used to produce molding resin blends which have acceptable engineering resin performance characteristics.

The present invention is further directed to methods for making and using such resin compositions.

The polyethylene terephthalate employed herein has an inherent viscosity of at least about 0.25 and preferably at least about 0.4 as measured by ASTM D-2857. The polyethylene terephthalate preferably has an upper limit on inherent viscosity of about 1.2. Inherent viscosity is measured in a 3:1 volume ratio of methylene chloride and trifluoroacetic acid at 30.degree. C. The polyethylene terephthalate can optionally contain up to 50 percent by weight of other comonomers, such as a diethylene glycol, glutaric acid, polybutylene terephthalate, polyalkylene oxide, cyclohexane dimethanol, and other diols. Mixtures of polyethylene terephthalate can be used. Suitable polyethylene terephthalate polymers are commercially available.

The polyesters have a number-average molecular weight in the range of from about 7,500 to about 20,000, preferably about 8000 to about 10,000. Preferred dialkanoic acid comonomers for such polyesters contain about 8 to about 10 carbon atoms per molecule and preferred dialkanol comonomers for such polyesters contain about 3 to about 4 carbon atoms per molecule. One presently most preferred such polyester is a condensation product of sebacic acid with 1,2-propane diol. Characteristically, the polyester is in the physical form of a liquid at ambient conditions. It is believed that the polyester reacts with the resin matrix during extrusion processing conditions.

Conventional fillers known to the art can be used. Examples of inorganic, non-fibrous fillers include clay, mica, talc, kaolin, calcium carbonate, barytes, alumina trihydrate, Wollastonite, glass spheres, calcium silicate, and the like. Examples of such organic fillers include wood and shell flours, starches, carbohydrate by-products, synthetic organics, such as polyvinylidene dichloride spheres, and the like. Examples of metal powders include aluminum, bronze, zinc, nickel, stainless steel, and the like. Examples of fibrous fillers include those comprised of aramid, carbon, glass, hybrids (such as aramid/carbon, aramid/carbon/glass, aramid/glass, carbon/glass, and the like), boron, ceramic, metal, mixtures thereof and the like.

Fibrous fillers (preferred) preferably have average cross-sectional thicknesses in the range from about 7 to about 15 microns and an average length in the range from about 1 to about 10 millimeters. Glass fibers which have an average cross-sectional thickness ranging from about 8 to about 10 microns and an average length in the range from about 2 to about 8 millimeters are presently most preferred particularly when present in an amount ranging from about 25 to 60 weight percent based on total composition weight.

In addition to the components discussed herein, the blends of the invention may contain other additives commonly employed (and in the quantities known to the art) with polyethylene terephthalate, such as, for example, colorants, mold release agents, tougheners, heat and ultraviolet light stabilizers (including phenols and phenol derivatives), nucleating agents, antioxidants, flame retardants, and the like. Usually, the total quantity of such other additives is not more than about 20 weight percent of a total resin blend although higher amounts could be useful if desired.

The blend compositions are prepared by blending together the components by any convenient means. For example, dry polyethylene terephthalate can be dry mixed in any suitable blender or tumbling means with the other components and the resulting mixture melt extruded. For example, an aliphatic polyester, such as is available from C. P. Hall as "Easy Mix Paraplex G-25", is obtained conveniently preliminarily premixed with a finely divided calcium silicate and then the resulting mixture is admixed with the polyethylene terephthalate before melt extrusion. If a filler is used, the polyester is conveniently preblended therewith and then the resulting mixture is mixed with the polyethylene terephthalate before melt-extrusion. A convenient melt extrusion temperature ranges from about 540.degree. to about 580.degree. F. (282.degree. to 304.degree. C.). The extrudate is preferably in a strand form which can be chopped into pellets or the like as desired.

The compositions of this invention are summarized by the following Table I:

The polyethylene terephthalate molding resin blend compositions of this invention are conventionally injection moldable; for example, using injection molding temperatures ranging from about 518.degree. to about 583.degree. F. (270.degree. to 305.degree. C.). When water cooled molds are used, the mold surface temperature ranges from about 85.degree. to about 110.degree. C. (185.degree. -235.degree. F.).

The compositional characteristics are illustrated, for example, in Table II below. Table III presents illustrative properties for (1) polyethylene terephthalate alone, (2) polyethylene terephthalate in combination with aliphatic polyester, (3) polyethylene terephthalate in combination with glass fibers, (4) polyethylene terephthalate in combination with glass fibers and with aliphatic polyester. Table III demonstrates (a) the rapid crystallization, (b) the broad crystallizing temperature range, and (c) the high heat distortion temperatures which are associated with compositions of this invention.

Available evidence suggests that a filler such as glass fibers generally acts as a mild nucleating agent for promoting crystallization in a polyethylene terephthalate resin matrix since it increases resin Tmc (the temperature at which a molten polymer starts to crystallize when cooled from the melt) and decreases resin Tcc (the temperature at which an amorphous polymer starts to crystallize when heated) in polyethylene terephthalate, and since it does not appear to alter either resin Tm (the melt temperature of a polymer at ambient pressure) or resin Tg (the glass transition temperature of a polymer).

The polyethylene terephthalate/aliphatic polyester combinations of this invention display (compared to untreated, or fiber glass filled, polyethylene terephthalate):

(1) A reduced or substantially shortened crystallization time, which may conveniently be defined, for example, as the time needed at some preset temperature which is substantially below Tcc to go from Tm to Tcc (such preset temperature can be chosen to be the upper surface temperature limit of a water cooled mold or about 110.degree. C.);

(2) An increased crystallizing temperature range, which may conveniently be defined, for example, as the difference between Tmc and Tcc;

(3) A generally unchanged or improved combination of physical strength characteristics; and

(4) A substantially higher heat distortion temperature.

This combination of effects is believed not to have been observed previously for a crystallization agent of the ester type in polyethylene terephthalate. The increased or elevated heat distortion temperature is theorized to be associated with a reaction occurring under extrusion conditions, and also under molding conditions, between the polyethylene terephthalate and the aliphatic polyester which increases the polyethylene terephthalate effective molecular weight. This result is contrary to the teachings of, for example, Bier et al. in the afore-referenced U.S. patent where it is stated that the reaction between polyethylene terephthalate and ester induces chain scission or an undesired deterioration in properties of polyethylene terephthalate. This result is also contrary to the teachings of Hecht et al. and Deyrup in the afore-referenced U.S. patents where aromatic esters are used to effect a type of steric hindrance which minimizes or obviates any chain scission reaction between ester additive and polyester terephthalate.

The following examples are presented in further illustration of the invention and are not to be considered as unduly limiting the scope of the invention.

The following examples illustrate molding resin blends of this invention in comparison with the prior art. Unless otherwise noted, the data presented is based on actual tests.

Examples 1 and 3 involve the prior art and are provided for comparison purposes. The material of Example 1 consists of polyethylene terephthalate. The composition of Example 3 consists of a mixture of polyethylene terephthalate and glass fibers.

Examples 2 and 4 illustrate the present invention. Example 2 consists of a blend of aliphatic polyester and polyethylene terephthalate. Example 4 exists of a blended mixture of polyethylene terephthalate, aliphatic polyester, and glass fibers.

The composition of all blends is shown on Table II below.

The starting polyethylene terephthalate was preliminarily dried at about 250.degree. F. (121.degree. C.) for about 16 hours in a vacuum oven.

Examples 2, 3 and 4 were prepared by tumble mixing the respective components together. The composition of each example was melt extruded through a 38 mm single screw extruder at a melt temperature of about 580.degree. F. (304.degree. C.) and passed through a stranding die. In each instance, the strand was cooled and chopped into pellets. The pellets were dried at about 250.degree. F. (121.degree. C.) for about 16 hours in a vacuum oven.

The dried chopped strands were molded in a 1.5 ounce injection molding machine at about 540.degree. F. (282.degree. C.) with a fast ram using a delayed injection time of 0.1 second, a hold time of 4 seconds, a cool time of 25 seconds, and an open time of 3 seconds. The mold cavity surface temperature was 235.degree. F. (113.degree. C.). The objects molded include an ASTM standard "dog bone" and an impact bar for Izod impact testing, heat distortion testing, and flexural property testing.

The properties of the resin of each example and objects molded therefrom are summarized in Table III below. These properties show that the compositions of this invention display rapid crystallization rates, broad crystallizing temperature range, and very high heat distortion temperatures.

For present purposes the crystallization time is defined as the time needed to go from Tm to Tcc (as these values are defined above in the above comments to Table III measured using a mold surface temperature of about 110.degree. C.). The aliphatic polyester surprisingly induces crystallization in less than about 30 seconds.

For present purposes, the crystallizing (or crystallization) temperature range is defined as the difference between Tmc and Tcc (as these values are defined above in the comments to Table III). As can be seen from Table III, the aliphatic polyester surprisingly induces an increase in the crystallizing temperature range.

For present purposes, heat distortion is directly measured, as shown in Table III (by the procedure defined in the above comments to Table III). As can be seen from Table III, the aliphatic polyester surprisingly induces a very substantial and unexpected increase in the heat distortion temperature.

While this invention has been described in detail for the purpose of illustration, it is not to be construed as limited thereby, but is intended to cover all changes and modifications within the spirit and scope thereof.