TECHNICAL FIELD

The present invention relates to polyolefin composition for rotational molding.

BACKGROUND OF THE INVENTION

The manufacture of polyolefin hollow articles in particular the large articles such as ice container, box, water tanks, industrial tank, kayak, surfboard, etc. by injection molding, extrusion or blow molding may be not feasible due to high investment cost The suitable manufacturing process of such articles is rotational molding which comprised of 4 steps: Loading, Hearing, Cooling and Demolding.

The main raw materials for rotational molding comprise polymer resin, stabilizer, pigment and additives. Such components are blended together before rotational molding process. There are two ways for blending which are dry blending and melt blending. Melt blending is preferred for the process due to the better color consistency and better appearance of the molded articles. Currently, the most widely used polymer resin for rotational molding is polyethylene because it provides balance characteristics in terms of mechanical properties, processability, and costs.

Due to some limitations of polyethylene properties in rotomolded application, other polymers are incorporated for improvements the properties which are required. For example, multilayer rotomolded tank which outer layer is mostly made of polyethylene and inner layer is made of polar polymer e.g. Nylon or Ethylene-vinyl alcohol copolymer which has resistance to fuel permeation resulting in fuel emission reduction to environment In a certain application, rotomolded articles have to be filled inside their hollow cavity with polyurethane foam in order to provide necessary functions including weight reduction, buoyancy, dimensional integrity, thermal insulation and etc.

Polyethylene itself could not be bonded effectively with other polymers due to its inertness and non-polar nature. The poor bonding between them caused serious problems when the products experienced in some circumstances. In cooler/ice box applications, outer wall and inner layers are made from rotational molded polyethylene and the interlayer is filled with polyurethane insolation. When the box exposed to high temperature e.g. sunlight, heat welding, and/or flame treatment, the polyethylene outer wall will detach from the filled polyurethane foam resulting in losing their dimensional integrity which will affect the box appearance.

US 4,440,899 disclosed the surfboard consisted of polyethylene outer wall made by rotational molding and polyurethane foam which is filled inside thereof, hi case the surfboard is damaged at polyethylene outer wall, the poor adhesion between polyethylene outer wall and polyurethane foam could allow water penetration throughout the polyethylene and polyurethane interface resulting in the damage spread through the surfboard. However, if mere is a good adhesion between the polyethylene outer wall and polyurethane foam, the water penetration will occur only at the damaged spot.

To enhance the adhesion properties between polyethylene made by rotational molding and polyurethane foam, the chemical and physical techniques have been exploited. In a physical technique, coarse polyethylene powders are spreaded inside a hollow cavity of a rotomolded article as a second shot before cooling step. The coarse polyethylene powder attached onto a hot inner polyethylene wall provides a rougher inner surface, allowing a better adhesion to polyurethane foam. However, the adhesion depends strongly upon the distribution of coarse PE. Usually, a variation of a poor to very strong polyurethane foam adhesion has been attained by this technique. This technique is quite high cost process because high amount of coarse polyethylene have to be used, for example the amount of coarse polyethylene is about 20%weight based on weight of polyethylene in first shot In addition, the spreading step is also a labor intensive.

WO 2006000770 Al disclosed the improvement of adhesion between polyethylene wall and polyurethane foam by using plasma treated polyethylene powder for rotational molding process. The functional group on the polyethylene surface increases the adhesion between polyethylene and polyurethane foam. However, the plasma treatment is a high cost process and the quality of the plasma-treated polyethylene powder in terms of polyurethane foam adhesion depends upon polyethylene resin and its additive package as well as plasma treatment conditions.

US 4,440,899 disclosed a polyolefin composition for rotational molding having improved adhesion to polyurethane foam. Said composition comprises slightly stabilized polyolefin or unstabilized polyolefin and stabilized polyolefin in the presence of stabilizer such as 2 -Hydroxy-4-n-octyloxy-benzophenone and Octadecyl-3-(3,5-di-tert-butyl-4- hydroxyphenyl)propionate. Under the process condition, said slightly stabilized polyolefin or unstabilized polyolefin is capable of forming oxidized groups resulting in improved adhesion to polyurethane.

US 3,974,114 disclosed polyolefin composition for rotational molding comprising polyolefin, stabilizer and C ₈ -C ₁₈ fatty acid salt selected from calcium, lithium, magnesium, barium, cadmium and aluminum salt Preferably, the stabilizers are 2,6-Di-tert-butyl-p-crcsol (DBPC) and Dilauryl thiopropionate (DLTDP).

US 8,357,324 B2 disclosed a process for the production of polyethylene hollow articles employing one or more NO-acyl hindered amine as a stabilizer to reduce the heating time in rotomolding resulting in good mechanical property of the hollow articles. Said process comprises charging the polyethylene having one or more NO-acyl hindered amine, filling this mixture into a mold, heating this mold in an oven to above 280° C , rotating the mold around at least 2 axes, cooling the mold while still rotating, opening it, and taking the resultant hollow article out

Although it has already been attempted to improve the properties of the composition fin- rotational molding and molding process as above mentioned, there still have some problems for example the quality control of adhesion with polyurethane foam, unsatisfied surface appearance such as surface pinhole and color consistency of the hollow articles in the case mat polyolefin composition comprising of stabilized polyolefin with pigment and unstabilized polyolefin without pigment In some case, more working step for matching color between stabilized and unstabilized polyolefin may be needed.

SUMMARY OF THE INVENTON

The present invention relates to a polyolefin composition for rotational molding comprising a stabilized polyolefin and an unstabilized polyolefin wherein the unstabilized polyolefin has a particle size in the range of 10 - 250 micron and a melt flow index in the range of 0.001 - 15.0 g/10 rnins (at 190°C, 5 kg) or 0.0002 - 3.0 g/10 mins (at 190°C, 2.16 kg). The polyolefin composition further comprises a metal stearate.

The object of the present invention is to provide a polyolefin composition for rotational molding having several technical advantages. The rotomolded product made from the polyolefin composition according to the present invention has good adhesion with other materials e.g. polyurethane foam, nylon etc. and good outer surface appearance mat is less color difference and pinhole on the surface. Furthermore, the temperature for rotational molding the polyolefin composition according to the invention is lower whereas the obtained rotomolded product still have comparable tensile adhesion properties comparing with the polyolefin composition comprising unstabilized polyolefin having the average particle size and melt flow index out of the defined range according to the present invention.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 shows ATR-FTER spectra of outer surface (the surface attaching to the mold) and inner surface (the surface attaching to the air inside the mold) of the rotomolded article made from polyolefin composition according to the present invention.

Figure 2 shows graph representing the relationship between the amount of unstabilized polyethylene in polyolefin composition and color difference value (Delta E) of the rotomolded article made from polyolefin composition according to the present invention.

DETAILED DESCRIPTION

The following details describe the specification of the invention, and are not intended to limit the scope of the invention in any way.

According to the present invention, a polyolefin composition for rotational molding comprises a stabilized polyolefin and an unstabilized polyolefin.

According to the present invention, the unstabilized polyolefin has an average particle size in the range of 10-250 micron and a melt flow index in the range of 0.001-15.0 g/10 mins (at 19°C, 5 kg) or 0.0002-3.0 g/10 mins (at 190°C, 2.16 kg)According to the present invention, preferably the unstabilized polyolefin has the average particle size in the range of 50-250 micron, more preferably 100-250 micron.

The technical advantages of the unstabilized polyolefin having the selected average particle size according to the present invention are no segregation between the stabilized and the unstabilized polyolefin when they are mixed and stored in powder form and the obtained rotomolded product has good appearance with very few surface pinhole or pinhole-free surface comparing with using the unstabilized polyolefin having the average particle size out of the defined range according to the present invention. In addition, the merit of this selected average particle size is that it circumvents the necessity of the color matching step for the unstabilized polyolefin and the stabilized polyolefin. According to this invention, the unstabilized polyolefin having no pigments or colorants can be mixed with the colored stabilized polyolefin and formed the rotomolded product which has good appearance without stone effect [The stone effect is the phenomena which the non-colored unstabilized polyolefin appears on the surface of the colored rotomolded product resulting in color non-umformity and poor appearance.] As me additional pigment or colorant for the unstabilized polyolefin is unnecessary, this inventive polyolefin composition provides a cost saving. From the test result, it was found that the average particle size of more than 250 micron of the unstabilized polyolefin can cause large pinhole, high variation of pinhole size and uneven distribution of pinhole on the surface. The stone effect is also observed in case of using the polyolefin composition comprising of the colored stabilized polyolefin and the unstabilized polyolefin having no pigments or colorants. Preferably, the unstabilized polyolefin should have the average particle size from 100 micron, but not larger than 250 micron to obtain the excellent rotomolded product

According to the present invention, the unstabilized polyolefin preferablyhas a melt flow index in the range of 0.001 - 5.0 g/10 mins (at 19°C, 5 kg) or 0.0002 - 1.0 g/10 mins (at 190°C, 2.16 kg), more preferably has a melt flow index in the range of 0.001 - 3.0 g/10 mins (at 190°C, 5 kg) or 0.0002 - 0.6 g/10 mins (at 190°C, 2.16 kg)

The technical advantage of the unstabilized polyolefin having the selected melt flow index is the obtained rotomolded product having good adhesion with other materials e.g. polyuremane foam, nylon, etc. This is because the unstabilized polyolefin having the melt flow index not more man 15.0 g/10 mins (at 190°C, 5 kg) or not more man 3.0 g/10 mins (at 190°C, 2.16 kg) provides the rougher inner surface (the surface attaching to other material) resulting in better adhesion with other materials comparing with the unstabilized polyolefin having the meh flow index higher than the selected melt flow index. The unstabilized polyolefin having melt flow index of less man 0.001 g/10 mins (at 190°C, 5 kg) or less man 0.0002 g/10 mins (at 190°C, 2.16 kg) will cause more pinhole on the surface of the rotomolded product. In addition, it is quite difficult to be supplied.

According to the present invention, the unstabilized polyolefin has a melting point at least 4°C higher than the stabilized polyolefin. This results in the inner surface (the surface attaching to other material) of the rotomolded articles mostly comprising of the unstabilized polyolefin and the outer surface (the surface attaching to the mold) mostly comprising of the stabilized polyolefin. Consequently, the obtained rotomolded product has strong adhesion with the target materials and also has high strength and durability resulting from stabilizer in the stabilized polyolefin.

According to the present invention, the unstabilized polyolefin is in an amount of 0.01 - 15% by weight, preferably 0.01 - 10% by weight, and more preferably 0.01 - 6% by weight based on total weight of the polyolefin composition. The rotomolded product with the selected amount of the unstabilized polyolefin has no or slight color difference comparing with mat of higher amount of the unstabilized polyolefin. Besides, higher amount of the unstabilized polyolefin will cause more pinhole, larger pinhole size as well as high variation of pinhole size distribution presented on the outer surface.

According to the present invention, the stabilized polyolefin comprises a polyolefin and a stabilizer.

According to the present invention, the stabilizer is organosulfides compound, phenolic compound, organophosphorus compound which is organophosphate, organophosphonite, hindered amine compound, or a mixture thereof.

According to the present invention, preferably stabilizer is phenolic compound, organophosphite, hindered amine compound, or a mixture thereof.

Examples of phenolic compound stabilizer according to the present invention are

Pentaerythritol tetrakis(3 -(3 ,5 -^-tert-butyM-hydroxvphenyl^nOpionate), Tris(3 , 5 -di-tert-butyl-

4-hydroxybenzyl)-isocyanurate, Tocopherol), etc.

Examples of organophosphorus compound stabilizer according to the present invention are Bis(2,4-di-tert-butyl-6-methylphenyl)ethyl phosphate or Tris(2,4-ditert- butylphenyl)phosphite.

Examples of hindered amine compound stabilizer according to the present invention are

Poly(4-hydroxy-2-2,6,6-tetramethyl-l-piperidine ethanol-alt-l,4-butanedioic acid), Poly[[6- [(1,1 ,3,3-tefcmnemylbutyl)an.ino]-s-^

hexamethylene-[(2^,6,6-tetramemyl-4-piperidyl)imm or chemical having CAS No. 192268- 64-7,etc.

According to the present invention, the stabilizer is in an amount of 0.005-02% by weight based on total weight of the stabilized polyolefin.

According to the present invention, the stabilized polyolefin further comprises a pigment Subjecting to rotational molding, polyolefin composition according to the present invention gives no or little color difference even using the unstabilized polyolefin without any pigment or colorants.

According to the present invention, the stabilized polyolefin is in an amount of 85-

99.99% by weight, preferably 90-99.99% by weight, and more preferably 94-99.99% by weight based on total weight of the polyolefin composition.

The use of stabilized polyolefin in the said amount gives the technical advantage as well as the use of unstabilized polyolefin amount as previously mentioned.

According to the present invention, the stabilized polyolefin and the unstabilized polyolefin can be the same or different polyolefin type, preferably is the same polyolefin type.

According to the present invention, the stabilized polyolefin and the unstabilized polyolefin are polyethylene, polypropylene or a mixture thereof.

The stabilized polyolefin according to the present invention can be prepared by melt blending. Mixing polyolefin, additives, pigment or colorant and stabilizer can be processed by twin-screw extruder.

Acxording to the present invention, the polyolefin composition for rotational molding further comprises a metal stearate. The metal stearate is zinc stearate, calcium stearate, magnesium stearate, aluminium stearate or a mixture thereof.

According to the present invention, the metal stearate is in an amount of 0.004-0.04% by weight based on total weight of the polyolefin composition.

Polyolefin composition in the presence of metal stearate according to the present invention could give technical advantage. When molding at a certain peak internal air temperature (PIAT), the molded product obtained from polyolefin composition in the presence of metal stearate shows fewer pinholes than that obtained from polyolefin composition in the absence of metal stearate. In other words, the molded product with the pinhole-free surface could be attained at lower PIAT by utilizing polyolefin composition in the presence of metal stearate.

According to the present invention, the polyolefin composition for rotational molding may be prepared by high-speed mixer with the speed of 500-1500 rpm for 2-8 minutes to obtain a good compatible polyolefin composition.

As all mentioned above, the polyolefin composition for rotational molding according to the present invention has the important characteristics such as components, an amount of the components, and others which gives technical advantage to rotomolded articles as mentioned above. However, the polyolefin composition for rotational molding is not limited to the scope of the invention. Indeed, various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description. Such modifications are also intended to fall within the scope of the present invention.

The followings are examples according to this invention without any intention to limit the scope of the invention.

Experiment 1 Analysis of oxidation of the rotomolded product during rotational molding

Polyolefin composition of this invention shows the good adhesion to polyurethane foam which results from the oxidation of unstabilized polyolefin during the molding process. The said oxidation can be characterized by Attenuated Total Reflectance Fourier Transform Infrared

Spectrometer (ATR-FTIR),

From the ATR-FTIR results, it was found that the oxidation mostly occur on the inner surface of the rotomolded product which exposed to the air. The polyethylene oxidation can be identified by observing the peak of carbonyl stretching at the wavenumber of 1715 cm ^'1 , as shown in Figure 1.

Experiment 2 Adhesion test and rotational molding time

The adhesion between the rotomolded product and polyurethane foam is tested by determining tensile strength according to ASTM D1623. The rotomolded samples can be prepared as follows;

Sample 1 is molded by using polyolefin composition comprising stabilized polyethylene powder with the average particle size of 300 micron and T _m at 124°C.

Sample 2 is molded by using polyolefin composition comprising 95% by weight of stabilized polyethylene powder with the average particle size of 300 micron and T _m at 124°C and 5% by weight of unstabilized polyethylene powder with the average particle size of 165 micron andT _m at l29°C.

Stabilized polyethylene powder in polyolefin composition in sample 1 and sample 2 comprising stabilizer mixture of 0.03% by weight of phenolic compound and 0.1% by weight of organophosphite based on total weight of said stabilized polyethylene powder.

An internal air temperature is the key processing parameter to track the heating cycle during rotomolding. Physical and chemical properties of resultant rotomolded parts are strongly related to a maximum temperature of the internal air or normally known as peak internal air temperature (PIAT).

According to the present invention, tensile strength is determined as a function of PIAT. It was found that for Sample 1, PIAT to attain the tensile strength more man 200 kPa is higher than 238°C. Sample 2, PIAT to attain the tensile strength more man 200 kPa is about 220 °G. These results suggest that the rotomolded sample using polyolefm composition according to the present invention reduces the energy consumption and heating time during rotomolding with an improved polyurethane foam adhesion. ExperimeHt 3 Effect of particle size of unstabilized polyolefin

To illustrate the technical advantage of particle size of unstabilized polyolefin according to the present invention, polyolefin composition samples with unstabilized polyolefin having various average particle size were prepared and studied.

Samples of polyolefin composition (1-4) comprising of

(1) 95% by weight of stabilized polyethylene having melt flow index of 3.5 g/10 mins (at 190°C, 2.16 kg), density of 0.932 g/cm ³ , T _m at 124°C and average particle size of 300 micron.

Said stabilized polyethylene comprises polyethylene, pigment and stabilizer which comprises 0.1% by weight of hindered amine compound and 0.015% by weight of organophosphite based on total weight of stabilized polyethylene, and

(2) Unstabilized polyethylene having various average particle size as shown in Table 1.

Said unstabilized polyethylene has melt flow index of 0.030 g/10 mins (at 190°C, 5 kg) and T _m at 129°C. The PIAT during rotomolding was set at 180 °C.

From the results in Table 1, molded article from polyolefm composition comprising unstabilized polyethylene having the average particle size of 670 micron (Sample 4) showed stone effect on the outer surface of the sample.

Table 1 Effect of average particle size of unstabilized polyolefin on outer surface aesthetics of rotomolded samples

When considering pinhole on surface, the molded samples from polyolefin comprising unstabilized polyethylene having the average particle size more than 250 micron (Sample 3 and 4 which have the average particle size of 320 and 670 micron, respectively) showed the unacceptable surface appearance that is larger pinhole size as well as high variation of pinhole size distribution presented on the outer surface.

From the above results, it can be concluded that the average particle size of unstabilized polyethylene according to the present invention gives excellent and acceptable appearance.

Experiment 4 Effect of melt flow index (MFI) of unstabilized polyolefin

To illustrate the technical advantage of melt flow index of unstabilized polyolefin according to the present invention, two polyolefin composition samples with unstabilized polyolefin having various melt flow index were prepared and studied as follows,

Two samples of polyolefin composition contain 95% by weight of stabilized polyethylene which said stabilized polyethylene comprises polyethylene and stabilizer. The stabilizer is the mixture of 0.1% by weight of hindered amine compound and 0.015% by weight of organophosphite based on the total weight of stabilized polyethylene. Said stabilized polyethylene has melt flow index of 3.5 g/10 mins (at 190°C, 2.16 kg), density of 0.932 g/cm ³ , T _m at 124°C and average particle size of 300 micron.

For polyolefin composition sample 1, said stabilized polyethylene was mixed with 5% by weight of unstabilized polyethylene having an average particle size of 165 micron, melt flow index of 7.5 g/10 mins (at 190°C, 2.16 kg) and T _m at 131°C.

For polyolefin composition sample 2, said stabilized polyethylene was mixed with 5% by weight of unstabilized polyethylene having an average particle size of 165 micron, melt flow index of 0.03 g/10 mins (at 190°C, 5.0 kg) and T _m at 129°C.

Above mentioned polyolefin composition samples were mixed by using high-speed mixer.

Two samples of polyolefin composition were subjected to rotational molding at various PLAT. It was found that the lowest temperature that carbonyl functional group peak detected by ATR-FTIR technique of sample 1 is 4°C higher than that of sample 2. Besides, inner surface (which attaches to polyurethane foam) of sample 1 is smooth whereas inner surface of sample 2 is rough. It is known that rough surface allows a better adhesion with the substrate. Adhesion of the samples on polyurethane foam tested according to ASTM 1623 showed mat tensile strength of sample 1 is lower than that of sample 2.

From the results (as shown in table 2), it can be concluded mat polyolefin composition in the presence of unstabilized polyethylene having melt flow index according to the present invention gives advantages which are 1) reduction of the energy consumption and heating time (luring rotomolding and 2) the rotomolded article has an improved adhesion with polyurethane foam and good mechanical properties, comparing to the use of unstabilized polyethylene having higher melt flow index.

Table 2 Effect of melt flow index of unstabilized polyethylene on inner surface appearance of rotomolded articles and tensile strength between polyethylene wall and polyurethane foam.

* smooth appearance similar to the inner surface of the article made from the polyolefin composition in the absence of unstabilized polyolefin

"critical PIAT is the lowest PIAT which the carbonyl peak began to be detected by ATR-FTIR technique.

^♦♦♦ average tensile strength at critical PIAT

Experiment 5 Effect of the amount of unstabilized polyolefin

To illustrate the technical advantage of the amount of unstabilized polyethylene in polyolefin composition according to the present invention, polyolefin composition sample with various amount of unstabilized polyolefin were prepared to study the effect on the appearance of the article e.g. surface pinhole, as follows,

Samples of polyolefin composition (1 -3) comprising

(1) 95% by weight of stabilized polyethylene having melt flow index of 3.5 g/10 mins (at 190°C, 2.16 kg), density of 0.932 g/cm ³ , T„, at 124°C and average particle size of 300 micron. Said stabilized polyethylene comprises polyethylene, pigment and stabilizer mixture which comprises 0.03% by weight of phenolic compound and 0.1% by weight of organophosphite based on total weight of stabilized polyethylene, and

(2) Unstabilized polyethylene having various amount as shown in Table 3. Said unstabilized polyethylene has average particle size of 165 micron, melt flow index of 0.030 g/10 mins (at 190°C, 5 kg) and T _m at 129°C.

The PIAT during rotomolding of polyolefin sample (1-3) was set at 190 °C. The results are shown in Table 3 and Figure 2.

Table 3 Effect of the amount of unstabilized polyethylene on outer surface appearance of rotomolded product

From the results (Table 3), it was found that the rotomolded article from polyolefin composition in the presence of 20% by weight of unstabilized polyethylene (Sample 3), which is out of scope of the present invention, showed unacceptable outer surface appearance that is larger pinhole size, a lot of number of pinhole as well as high variation of pinhole size distribution presented on the outer surface.

After that, the obtained rotomolded samples were subjected to test the effect of unstabilized polyolefin on sample color. The color difference (Delta E) at 2.3 was set to be an upper limit for the color tolerance.

According to the present invention, color difference can be determined by Benchtop Spectrophotometer, Hunterlab and can be calculated from the following equation;

wherein ΔΕ is color difference

ΔL is the difference of lightness Δ a is the difference of redness-greenness

Ab is the difference of yellowness-blueness

From the results (Figure 2), it was found that the rotomolded samples from polyolefm composition in the presence of more than 6% by weight of unstabilized poryetfaylene (sample 2 and 3 which have 10% and 20% by weight of unstabilized polyethylene, respectively) snowed color difference value more than 2.3. In other words, color difference of each piece of the sample can be observed.

As mentioned above, it can be concluded that the unstabilized polyethylene amount according to the present invention which is 0.01-15% by weight, gives acceptable appearance mat is few and small pinhole. Besides, at the suitable amount of unstabilized polyethylene, 0.01- 6% by weight allows the excellent and acceptable appearance due to low color difference value.

Experiment 6 Effect of metal stearate on pinhole on the surface of rotomolded product

To illustrate the technical advantage of metal stearate in polyotefin composition according to the present invention, the effect of metal stearate was studies as follows;

Samples of Polyolefm composition (1-3) comprises

(1) Stabilized polyethylene having melt flow index of 3.5 g/10 mins (at 190°C, 2.16 kg), density of 0.932 g/cm ³ , T _m of 124 °C and average particle size of 300 micron. Stabilized polyethylene comprises polyethylene, pigment and stabilizer which is the mixture of 0.03% by weight of phenolic compound and 0.1% by weight of organophosphate based on total weight of stabilized polyethylene.

(2) Various amount of unstablilized polyethylene as shown in Table 4. Such unstabilized poryemylene having melt flow index of 0.030 g/10 mins (at 190°C, 5:0 kg) and T _B of 129°C, and

(3) Optionally, 0.01% by weight of calcium stearate based on total weight of polyolefm composition as shown in Table 4.

From the results, the rotational molded product from polyethylene composition in the presence of calcium stearate shows lower PIAT and less pinhole when compared to the product from poryemylene composition in the absence of calcium stearate. This results in reduction of time and energy consumption in rotational molding process.

Table 4 component of polyolefm composition used for testing and PIAT that shows pinhole-free surface

calcium stearate is mixed with unstabilized polyethylene.

BEST MODE OF THE INVENTION

Best mode of the invention is as disclosed in the detailed description.