KR100701011B1 - Monovinylaromtic polymer with improved stress crack resistance - Google Patents

Abstract

The process is disclosed to improve the environmental stress cracking and impact resistance, and other desirable properties of the elastomer-modified monovinylaromatic compounds, which process involves blending polymerization initiators, including perketal and peroxycarbonates. Take advantage.

Stress Cracking Resistance, Monovinylaromatic, Perketal, Peroxycarbonate

Description

Monovinylaromatic polymer exhibiting improved stress cracking resistance {MONOVINYLAROMTIC POLYMER WITH IMPROVED STRESS CRACK RESISTANCE}

The present invention relates to thermoplastic compositions utilizing polymers of monovinylaromatic compounds useful in the manufacture of articles modified with rubber to increase impact strength and in particular require increased environmental stress cracking resistance (ESCR). More specifically, the present invention typically discloses high impact polystyrene (HIPS) materials which are particularly advantageous for food containers against environmental stress cracking.

It is known that rubber reinforced polymers of monovinylaromatic compounds, such as styrene, alphamethyl styrene and ring-substituted styrene, are variously preferred to the user. More specifically, rubber-reinforced polymers of styrene, which contain discontinuous particles of polybutadiene, such as crosslinked rubbers (discontinuous particles of rubber are dispersed through a styrene polymer matrix) are used for refrigerator linings, packaging materials, furniture, kitchenware and toys. It can be used in a variety of applications, including. Common terms for such rubber reinforced polymers are "high impact polystyrene" or "HIPS". The physical and mechanical properties of HIPS depend on many factors, including the particle size of the crosslinked rubber particles. One of the most desirable features of HIPS materials is that they are decomposable or destructive by factors such as contact with food and cooking oil. In addition, other properties for retaining the article include flexural strength and tensile strength.

Stress cracking properties, or environmental stress cracking (ESCR) properties are particularly important for thermoplastic copolymers utilized in food containers. The food contents of the polymer container typically do not degrade the form of the polymeric material from which the container is made, but when the thermoplastic polymer is thermally formed from the extruded sheet material, residual stresses are inherent in the molded article. These stresses are released so that generally resistant polymers are susceptible to attack by the material. Such articles made from styrene polymers modified with rubber to increase impact strength are susceptible to stress cracking when contacted with conventional formulations found in organic foods such as fats and oils. Similarly, these products also create stress cracks when contacted with organic blowing agents such as halohydrocarbons containing fluorine and chlorine. Such polymers are commonly found in kitchen utensils such as refrigerator liners, and cracks can occur when the cavity is filled with polyurethane foam as a result of the blowing agent utilized in the foam.

In the past, environmental stress cracking has been prevented by a complex procedure where the intermediate protective layer of the polymer is located between the polystyrene layer and the blowing agent or fatty food material, typically involving the formation of a multilayer polymer. One layer of material utilized to insulate styrene from the formulation is a known terpolymer material such as ABS, or acrylonitrile-butadiene-styrene. By increasing the amount of rubber incorporated into the polymer, there has been other intention to improve the stress cracking resistance of high impact monovinylaromatic polymers. Unfortunately, the higher the rubber content, the lower the tensile and refractive strength of the final material. Another solution has been to tightly control process conditions to tightly control the particle size of the cross-linked rubber particles in the polystyrene matrix. One such patent, which discloses this technique, is assigned to the assignee of the present invention in US Pat. No. 4,777,210 on October 11, 1988, which is a method of producing, reliable and reproducible high impact polystyrene for particles of various sizes. Continuous flow processes have been disclosed. In the above patented process, a pre-inverting reactor was utilized to convert styrene, rubber (such as polybutadiene) and peroxide catalyst solutions into high impact polystyrene materials exhibiting high environmental resistance cracking.

Another effort to improve stress cracking was disclosed by Mittnacht in US Pat. No. 4,144,204, dated Mar. 13, 1979, wherein the amount of rubber dissolved in the monomer prior to polymerisation of the monovinylaromatic compound with rubber to increase ESCR and polymerize. The content of the soft component (gel phase) in the silver impact polymer was at least 28% by weight, and preferably at least 38% by weight, based on the weight of the impact polymer. It has been found that the upper limit of the content of the soft component is about 50 to 60% by weight and preferably 30 to 40% by weight is advantageous.

A third method commonly used to increase ESCR in HIPS is disclosed in British Patent No. 1,362,399, wherein liquid hydrocarbon telomers having unsaturated carbon chains are added to the HIPS material in an amount of 0.2 to 5 parts per 100 parts. do). Telomeres are previously defined in Webster's Non-Summary as a product of a chemical reaction involving the addition of a fragment of one molecule (eg, alcohol, acetal or chloroform) to the polymerization end of an olefin chain. In the British patent, the telomers employed exhibited a number average molecular weight of 1000 to 6000. Although experimental attempts have been made to utilize low molecular weight polybutadiene to produce ESCR-HIPS, the patented process utilizes butadiene mixed or blended with polystyrene rather than added during the polymerization reaction.

Another effort to improve the stress cracking resistance of HIPS materials can be found in British Patent GB 2,153,370A, where HIPS materials can be used for high molecular weight rubber materials having a stated molecular weight of at least 300,000 and a viscosity of at least 140 centipoise. Manufactured by utilizing; The resulting HIPS contains 7 to 10% by weight of the rubber and the polymerization is selected from alphamethylstyrene dimers or other various compounds of n-dodecylmercaptans, quaternized dodecylmercaptans, diphenyl 1,3 butadiene or mixtures thereof. to be. In addition, the process was carried out in the presence of at least 7% by weight of cyclohexane and ethylbenzene in total components. There was also a need for additives comprising monotriglylids of stearates and polyethylene waxes.

On the other hand, additives are used for reasons of ESCR improvement. Patent No. 3,506,740 to Dempsey et al teaches the use of low molecular weight polyolefins as internal lubricants for impact polystyrene compositions. Examples listed include polybutylene and polypropylene having a molecular weight in the range from 800 to 1600 (measured by a vapor osmosis meter).

The final properties of the polymerizable material such as its molecular weight can also be influenced by the form and amount of the polymerization initiator used. For example, U.S. Patent 5,266,603 discloses the preparation of expandable polystyrene in the form of beads with low benzene balance using perketal and / or monoperoxycarbonate initiators.

U.S. to Sosa. Patent 4,861,827 discloses a styrene polymerization process for the production of high impact polystyrene utilizing free radical initiators which decompose during the polymerization process, leaving only in the recycle stream to form fugitive decomposition by-products that do not inhibit further polymerization. Doing. Such free radical initiators include azo and peroxy compounds.

US Pat. No. 5,559,162 teaches a process for preparing polymeric peroxycarbonates and their use as initiators. US Pat. No. 4,433,099 teaches the use of organic peroxide initiators to prepare impact resistant polymers having M _v molecular weights in the range of 150,000-170,000. US Pat. No. 4,279,703 discloses the use of peroxycarbonate initiators to produce polystyrene in two thermally distinct polymerization stages, between about 70 ° C. and 150 ° C., with different initiators in each step.

The present invention overcomes the disadvantages of known high impact polystyrene materials by providing a polystyrene material (PS) containing polybutadiene or styrene-butadiene rubber (wherein the PS material provides improved ESCR properties over conventional PS initiators). The high impact polystyrene (HIPS) disclosed by the present invention exhibits high environmental stress crack resistance, high impact strength, good tensile strength and good refractive strength.

The present invention discloses a thermoplastic composition comprising a polymer of a monovinylaromatic compound modified with an elastomer to increase impact strength and environmental stress crack resistance, wherein the compound is monovinyl in the presence of a combination of a polymerization initiator and an elastomer. Obtained by polymerizing an aromatic material.

In the composition, the portion of the soft component in the polymer, which has been modified to increase the impact strength, is less than 28% by weight of the polymer, and the soft component increases the impact strength and results in the toluene insoluble component of the polymer which is free of any pigments that may be present. It is defined. The specialty rubbers used in the present invention may be one of several types, such as commercially available from Firestone, have a Mooney viscosity of approximately 55, a molecular weight of about 150,000, a weight average molecular weight of about 300,000, and gels. There is a type designated diene 55 having a Z molecular weight of about 500,000 measured by transmission techniques. Another type of advantageous rubber material includes high-Cis rubber.

If the above mentioned components are used, the high impact polymer can be prepared according to any conventional method. Normally, manufacturing processes include bulk polymerization and solution polymerization (as disclosed in U.S. Patent 2,694,692) or bulk suspension polymerization (as disclosed in U.S. Patent 2,862,906). If the processes are capable of using the aforementioned components, other manufacturing processes may also be used.

Suitable monovinylaromatic compounds utilizing the present invention include styrene, as well as styrene alkylated in the nucleus or side chain, such as alphamethyl styrene and vinyl toluene. Monovinylaromatic compounds may be used alone or in admixture. In one preferred embodiment, styrene is preferably a monovinylaromatic compound. High impact polystyrene (HIPS), prepared according to the present invention, is formed by the polymerization of monovinylaromatic compounds in the presence of rubber and by the new bonding of initiators including perketal and peroxycarbonate. The level of rubber used is preferably in the range of about 5 to 15% by weight in solution. The polymerization is conventionally carried out by mass polymerization, solution polymerization or polymerization in aqueous dispersion, in which the rubber and the solution initially dissolved in the polymerizable monomer are then polymerized in the presence of initiator bonds.

Suitable peroxycarbonate polymerization initiators include, for example, t-amyl 2-ethylhexyl peroxycarbonate (TAEC) and suitable perketal initiators include Elf Atochem North America, 2000 Market St., Philadelphia, PA Sold commercially and is referred to commercially as LUPERSOL 233. When using solution polymerization, the starting solution can be mixed up to about 10% by weight based on the monovinylaromatic compound used in the inert diluent. Preferred inactive diluents include aromatic hydrocarbons or mixtures of aromatic hydrocarbons such as toluene, ethylbenzene, xylene or mixtures of these compounds. Suitable chain transfer agents (eg, mercaptan or alphamethyl styrene dimer) can be added to control the polymer molecular weight and rubber particle size.

The present invention produces polystyrene using a pre-inversion reactor (polymerization of styrene, rubber and binding initiator is polymerized to a point below the conversion point and then introduced into a second stirred tank reactor). It can be used in a continuous flow process to. The viscosity of the solution in the preconversion and in the second stirred tank reactor is tightly adjusted to produce the desired HIPS. Special processes for preparing preferred embodiments are described in U.S. et al., Published on October 11, 1988, in Sosa et al. Patent 4,777,210.

The combination of ESCR-enhancing initiators of perketal and peroxycarbonate can preferably be added to the initial monolayer / rubber feed stream or at any point in the polymerization process (including the final polymerization reactor). Special initiators that have been found to result in unexpected increases in ESCR properties include t-amyl 2-ethylhexyl peroxycarbonate (TAEC), which is used for binding with the previously mentioned perketal initiator LUPERSOL 233. TAEC is used in various ratios, with weight ratios of about 150 to 800 PPM being preferred. LUPERSOL 233 is added in amounts ranging from around 200 PPM.

In the original implementation of the invention, a mixture of traditional butadiene rubber and styrene monomers, having a molecular weight corresponding to a Mooney viscosity of approximately 55, by means of a patented process utilizing the various levels of TAEC and perketal initiators mentioned above, high impact polystyrene Polymerized into a substance.

Table 1 below shows the improvement in ESCR that can be obtained using the initiator combination of the present invention compared to traditional methods.

For reference, HIPS materials for the use of food containers containing fats or oils are successful according to the following ESCR levels.

ESCR result

10% hasty stress break

20% lowest success

30% good resistance                     

40% excellent resistance

In an example of another embodiment, the present invention is used to maximize material flexibility (increase in gel level) in HIPS materials for cup making. Table II below shows two examples of cup-grade HIPS materials that use lower levels of rubber than food. One example used a traditional cup grade HIPS material and the other used the following invention.

conclusion

In Table 1, the combination of the perketal and peroxycarbonates of Examples C and D resulted in marked improvements in ESCR values (36.8 and 44.3) compared to traditional materials A and B (with ESCR levels of 22.5 and 22.8). Is clearly shown.

Table 2 shows that not only ESCR enhancement is obtained using the present invention. In non-food grade HIPS materials (ie CUP class) where ESCR is not fatal, the present invention shows an improvement in gel amount (12.5 vs. 9.7) and fusion level (184 vs. 130). Example B represents the use of the present invention and Example A represents a traditional process.

Thus, it can be clearly demonstrated from the results presented above that the combination of peroxycarbonate initiator and perketal initiator yields HIPS material with improved ESCR as well as improvements in other areas such as gel content and grafting levels.

Claims (9)

Introducing a monovinylaromatic monomer feed stream into the polymerization reactor; Introducing an elastomer feed stream into said polymerization reactor; Introducing a polymerization initiator compound comprising at least one perketal and at least one peroxycarbonate into the reactor; And Reacting the monomer, the initiator compound, and the elastomer to form an elastomer modified monovinylaromatic polymer having a high ESCR value. A method of improving environmental stress cracking resistance of an elastomer-modified monovinylaromatic polymer material comprising a. The method of claim 1 wherein perketal is added in an amount of about 200 parts by weight per million parts by weight and peroxycarbonate is added in an amount of about 150 to about 800 weight parts per million (PPM). The method of claim 1 wherein the perketal comprises Lupersol L-231 and the peroxycarbonate comprises t-Amyl 2-ethylhexyl peroxycarbonate. 4. The process of claim 3 wherein perketal is added in an amount of about 200 weight PPM and peroxycarbonate is added in an amount of about 400 weight PPM. 5. The method of claim 4, further comprising adding at least one chain transfer agent and at least one lubricant to the reactor, wherein the lubricant is selected from the group consisting of mineral oil and polyisobutylene. 6. The method of claim 5, wherein the chain transfer agent is mercaptan and is added in an amount of about 500 weight PPM. 6. The method of claim 5 wherein the lubricant added is mineral oil and polyisobutylene. Introducing a styrene monomer feed stream into the polymerization reactor; Introducing a reduced level of elastomer feed stream into the polymerization reactor together with the styrene monomer feed; Introducing an initiator compound comprising at least one perketal initiator and at least one peroxycarbonate initiator into the reactor; And Reacting the feedstream and initiator compound to produce impact resistant polystyrene Method for producing cup grade high impact polystyrene having a grafting level of improved gel content and reduced elastomer content comprising: 9. The method of claim 8, wherein perketal is Lupersol L-231 added in an amount of about 200 weight PPM and peroxycarbonate is TAEC in an amount of about 600 weight PPM.