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Tailoring the properties of hydrogenated styrenic block copolymers for medical tubing applications.

Kraton Corporation invented styrenic block copolymers (SBC) and commercialized them in 1959. The anionic polymerization method used for making SBCs leads to a very clean product with minimal catalyst residuals. As a result, Kraton SBCs have found extensive use in the medical device market in applications like medical films, bags, tubing, stoppers and more. Apart from being very clean, Kraton SBCs also provide high transparency and clarity with minimal haze. They are very soft and flexible, yet provide good mechanical strength. They can be sterilized with most commonly used sterilization methods. An important distinguishing feature of SBCs is they do not contain or need additional phthalate based plasticizers. Consequently, Kraton has successfully served the medical market for more than 30 years.

Kraton SBCs are made by copolymerization of styrene monomer forming the polystyrene block, and either isoprene or butadiene, or a mixture of isoprene, butadiene monomers, forming the rubber blocks. At room temperature, the polystyrene blocks are hard, with a glass transition temperature above 100[degrees]C, and the rubber blocks are soft, with a glass transition temperature below -55[degrees]C. The styrene and rubber blocks are highly incompatible, which results in a strong phase separation. This leads to styrene blocks forming domain structures which are dispersed throughout the rubber phase. The typical size of these styrene domains is on the order of 20 nm. As a result, the hard styrene blocks act as physical crosslinks in the soft rubber network, providing strength and elasticity to the material without the need for vulcanization.

The rubber midblock in Kraton SBCs can either be unsaturated (called USBC) or saturated by selectively hydrogenating the midblock (called HSBC). Hydrogenation of the midblock in HSBCs leads to greater stability of the material to heat and oxidation as compared to USBCs. The HSBCs can be blended with polyolefins, like polypropylene (PP), where varying the ratio of HSBC and PP will lead to compounds with a wide range of properties. A blend of HSBC/PP exhibits several advantages over flexible PVC used in medical applications. HSBC/PP blends are lighter. With no added plasticizers in HSBC/PP compounds, contamination issues due to the plasticizers can be avoided in drug applications. The barrier properties of HSBC/ PP compounds are higher as compared to PVC. They have greater temperature stability up to 250[degrees]C, where PVC starts degrading at 130[degrees]C. HSBC/PP blends also have excellent UV, ozone and chemical resistance. They pose no known risk to health and the environment during processing, and can be easily recycled. PVC, on the other hand, releases hazardous chemicals like dioxins at process temperatures, and can be difficult to recycle when blended with other polymers.

The properties of typical Kraton HSBC grades that are commonly used in medical applications are listed in table 1. FG1924 is a functionalized grade, where the rubber block is functionalized with maleic anhydride. G1645 is the most common grade used in the industry in medical applications for making tubing and films. In recent years, Kraton strived to improve the color, tack and processability of G1645. The result of that effort has culminated into making a new polymer, MD1646. In this article, we present the physical, mechanical and processing properties of MD1646 and MD1646/PP blends, and compare them to those of G1645 in film and tubing applications. Finally, we also present the results of work done towards developing another new polymer with improved kink resistance and solvent bonding that could potentially be an alternative for PVC in tubing applications.

Experimental

Mechanical properties and hardness

Mechanical properties were measured according to the tensile test method of ASTM D412 using a mini D-die dogbone sample. The tests were conducted on an Instron 3366 fitted with a 1 kN load cell. The gauge length of the sample was 25.4 mm and the test was carried out at an extension rate of 254 mm/minute. Durometer A hardness of all samples was measured using an automatic hardness tester. Three sheets of the same compound with 2 mm thickness were stacked, and the hardness was measured at four comers and the center. The hardness was noted 10 seconds after the durometer tip made contact with the material. The average value of the five measured hardness values was reported.

Dynamic mechanical analysis and ODT rheology

The glass transition temperature (Tg) of all polymer samples was measured by dynamic mechanical analysis using a TA Instruments DMA Q800. Temperature sweep experiments were conducted from -80[degrees]C to 120[degrees]C, where storage moduli (G'), loss moduli (G") and loss factors (tan [delta]) were obtained as a function of temperature. All experiments were done at a frequency of 1 Hz. Glass transition temperature has been reported as the temperature at the peak value of tan [delta].

Tests to determine ODT rheology were done on a Bohlin rheometer from Malvem Instruments. Temperature sweep experiments were conducted at two frequencies of 0.005 Hz and 0.2 Hz, where complex viscosity was measured. The ODT is reported as the temperature where the complex viscosity of the polymer is the same at both frequencies.

Kink resistance

A test setup for measuring kink resistance was internally developed. A picture of the setup is shown in figure 1. A tube is bent and placed between the grips with a distance of 100 mm. The tube is then bent further by downward movement of the crosshead until it kinks, and this distance, x mm, is noted. The apparent kink diameter is then calculated as: apparent kink diameter = (100-x) mm.

Kink resistance was also measured by a hand test method, where the tube was folded into a loop. The loop was then slowly closed, just until a kink appeared in the tube. The ends of the loop at kink were pinched, the loop opened, and the length between the pinch points was measured. This length is called kink diameter.

In both Instron testing and the hand test method, a smaller kink diameter indicates higher kink resistance.

Solvent bonding

Solvent bonding of extruded tubes was conducted with ABS connectors. The solvents used were the most commonly used solvents in the industry: cyclohexanone (CH), methyl ethyl ketone (MEK), tetrahydrofuran (THF), and two combinations of 50:50 CH in MEK, and 80:20 THF in CH. The tubes were dipped instantaneously in the solvent. As the connectors were tapered, the tube was inserted only two thirds of the length of the connector shaft. The assembly was allowed to dry for seven days at room temperature. The bond strengths were then measured on an Instron 3366, with 25.4 mm gauge length at an extension rate of 500 mm/minute.

The aging of the tube connector assemblies was done by subjecting them to a temperature and humidity cycle over a time period of 22 days.

Results and discussion

Comparison between Gl645 and MD1646

Both G1645 and MD1646 are HSBC polymers of the Enhanced Rubber Segment (ERS) family of Kraton polymers, where the microstructure of the rubber midblock has been modified. The ERS structure makes the polymers soft, provides better compatibility with a polyolefin like PP, and leads to higher melt flow. The ERS grades have relatively lower PSC, which makes them highly elastic with good recovery, low hysteresis and good kink resistance.

MD1646 is a low molecular weight version of G1645, but with similar polystyrene content (PSC) of 13%. As a result, MD 1646 has a higher melt flow rate (MFR) than G1645, which makes it amenable to processing at lower temperatures, and also easier to handle due to lower tack during processing. MD1646 can potentially be used in applications like medical tubing, medical film, as PVC alternatives in coated fabric or wire/cable applications, hot melt adhesives, impact modifiers for PP, and in TPE compounds.

A comparison of physical and mechanical properties between G1645 and MD1646 is presented in table 2. It can be seen that MD1646 has lower solution viscosity and significantly higher MFR as compared to G1645. However, the mechanical properties, specifically tensile strength and elongation at break, and the hardness of MD1646 and G1645, are the same. The mechanical properties were measured on films cast from a solution of polymer in toluene.

Properties ofMD1646 versus Gl645 in welt cast film applications

Melt cast films of M D 1646 and G1645 were prepared by blending with random copolymer PP (rcPP) at different ratios. The films were prepared at a thickness of 200 [micro]m. Tensile tests were conducted on the films in the machine and transverse direction (MD and TD, respectively) relative to the direction of film extrusion. The tensile strength of the films made with MD1646 and G1645 are shown in figure 2. It can be seen that the tensile strengths are very similar for films made with G1645 and MD1646 at all ratios of the blends with polypropylene. The difference between tensile strength in MD and TD for G1645 films is also very similar to that of films made from MD1646 compounds.

The ultimate elongation of the films made with G1645 and MD1646 is given in figure 3, which shows that films from PP blends of both polymers can be stretched to more than 700%, and the ultimate elongation is very similar.

Comparison of optical properties of G1645 and MD1646 A comparison of the Yellowness Index (YI) values and other optical properties like haze, clarity and transmittance between pure G1645 and MD1646 is shown in table 3. It can be seen that the optical properties of MD 1646 are much better than those of the G1645 polymer. A comparison of the color of the pellets, and color of a solution cast film is also shown in figure 4 for comparison.

Processability of MD1646 as compared to G1645

The order-disorder transition (ODT) temperature of an SBC is the temperature at which there is no phase separation in the polymer, which is in a single phase melt state. The ODT temperature indicates the lowest temperature at which the polymer can be melt processed. The ODT temperature analysis of MD1646 and G1645 is shown in figure 5. It can be seen that the ODT temperature of G1645 is higher at 240[degrees]C as compared to MD 1646, which has an ODT temperature of 220[degrees]C. This shows that MD1646 can be processed at a lower temperature than G1645.

The lower ODT also leads to a lower melt viscosity of MD1646 at a typical processing temperature of 230[degrees]C (above its ODT temperature) as compared to G1645. The melt viscosity as measured on a capillary rheometer is shown in figure 6.

The melt viscosity of a blend of MD1646 and PP was also compared to a compound of G1645/PP at 190[degrees]C. The data are shown in figure 7. It was found that the MD1646 compound had a lower melt viscosity as compared to the G1645 one, indicating better processability of MD 1646 at a lower temperature.

Development of a new polymer for tubing applications

In recent years, work has been done on developing a new polymer for medical tubing applications that can be used either as a neat polymer or by compounding with other polyolefin polymers.

Use of the polymer in the neat form allows for a very clean product which is highly desirable in medical tubing applications. The aim of this development was to make a polymer that resulted in a tube with better kink resistance and solvent bond strength. This polymer belongs to the family of the Kraton A grade of polymers. In this grade line, the SBCs not only have end blocks that are polystyrene, but also have a controlled distribution of styrene in the rubber midblock. The new developmental polymers show interesting properties owing to a higher amount of total PSC in the end blocks and the midblock.

The following is a summary of the property set that is desired in a polymer for use in a neat or compounded form in medical tubing applications:

* Melt flow rate range of 3.0 to 6.0 dg/minute at PP conditions (230[degrees]C, 2.16 kg)

* Hardness range of 70-75 durometer A

* Transparent appearance so air bubbles in the fluid being transported are visible

* Low surface tack

* Low sensitivity to temperature changes in the range of 0-40[degrees]C

* Ability to undergo sterilization by the commonly used sterilization techniques

* Good kink resistance

* Solvent bondable with typically used solvents in the industry like cyclohexanone (CH), THF or MEK.

The physical properties of the new Kraton A polymers, named Kraton A-1 and Kraton A-2 are provided in table 4.

Kink resistance tests

Tubing of the dimensions 4/3 mm and 7/5 mm (OD/ID) was extruded using neat Kraton A-l and Kraton A-2. The 4/3 mm diameter tubing was also extruded with G1645/PP and MD 1646/ PP compounds with a 70/30 ratio of the SEBS to polypropylene. The results of the Instron test method on the 4/3 mm tube is shown in figure 8, along with kink resistance of the PVC tubing of the same dimensions.

It can be seen from the above figure that, while tubing made from new Kraton A-1 polymer shows kink resistance equivalent to that of the PVC reference, the kink resistance of tubing made with Kraton A-2 is even better than tubing made with PVC. The kink resistance of tubes made with the new polymers is significantly better than for those made from SEBS/PP compounds. A comparison of the kink diameter obtained with a hand test and Instron test methods is shown in figure 9. The tests were conducted on the same tubing with 4/3 mm dimensions. It can be seen that the trend for kink diameter by both tests is the same, where new Kraton A polymers have kink resistance comparable to the PVC reference.

Additionally, we conducted kink resistance tests on tubing of a larger size (7/5 mm), both by hand test and Instron test methods, and the data are shown in figure 10. Data of the PVC tubing are not included here due to unavailability of the tubing in this size. However, we again observe the same trend, validating that even with tubing of different sizes, the kink resistance of the new Kraton A polymers is better than for the typical SEBS/PP compounds.

Solvent bonding

The bond strength of tube-connector assemblies prepared with different solvent systems is shown in figure 11. The tests were done after drying the tube connector assemblies for seven days at room temperature. It can be seen that bond strength of the new Kraton A polymer is much higher than for the SEBS/PP compounds for all the solvents used. However, the bond strengths are lower as compared to PVC tubing.

It should be noted that the bond strength depends significantly on the connector design. This is exemplified by the data shown in figure 12, where ABS male luer connectors were used for preparing the tube connector assemblies.

It can be seen from figure 12 that the bond strength of the new Kraton A polymer is almost double the strength that was obtained with the female luer connectors. Moreover, even tubes with SEBS/PP compounds show similar bond strengths as the Kraton A polymer with the male luer connectors. The difference in bond strengths of PVC tube connector assemblies between male and female luer connectors is not very large.

Bond strengths were also measured after subjecting the tube connector assemblies to an aging cycle. The aging was conducted after drying the tube connector assembly for seven days at room temperature. Aging was done over a period of 15 days, where the samples were subjected to vaiying conditions of temperature and humidity. The comparison of bond strengths before and after aging is shown in figure 13 (a-c) for tubes made with SEBS/PP compounds, and Kraton A polymers using the solvents THF, CH and MEK.

It can be seen from figure 13 that the solvent-bonded tube connector assemblies withstand the aging cycle, and there is no significant change in bond strengths observed before and after aging for the solvents studied.

Effect of temperature on storage modulus

An important property for materials that are used in medical tubing applications is that hardness of the material should not undergo drastic changes in the typical application range of 0-40[degrees]C. This was characterized by the slope of the storage modulus curve as a function of temperature in the 0-40[degrees]C range. The data of storage modulus versus temperature are shown in figure 14 (a-d) for the SEBS/PP compounds, new Kraton A polymers, and for commercial Kraton A polymers.

It can be seen from figures 14 (a) and (b) that the Tg of commercial Kraton A polymer A1536 and that of the compound MD1646/PP is much below room temperature, and there is a sharp decrease in G' at Tg. Moreover, the slope of the G' curve in the region of 0-40[degrees]C is very high. On the other hand, Tg of the new Kraton A polymers, both A-l and A-2, is around room temperature. The storage modulus, G', drops very gradually near Tg, and the slope of the G' curve in the range of 0-40[degrees]C is much lower. This shows that the hardness of the tubing made with new Kraton A polymers will not change much when it is used to transport fluids at varying temperatures.

Sterilization

The new Kraton A polymers, along with G1645/ PP and MD1646/PP compounds, were subjected to ethylene oxide (EtO) and gamma sterilization, and mechanical and optical properties were measured before and after sterilization. The SEBS/PP compounds can also be sterilized by a steam sterilization technique, and data were collected for that also. The data have been summarized in table 5, where medical grade PVC has been included for comparison. Steam sterilization was conducted at 121 [degrees]C for 30 minutes, with 20 minutes of drying time. EtO sterilization was conducted at 55[degrees]C in a four hour cycle. Gamma sterilization on all samples was done at a dosage of 50 kGy.

It can be seen from the table that the change in color after gamma sterilization, as indicated by Yellowness Index, is much worse in PVC. The other properties are mostly unchanged for all polymers in different types of sterilization techniques.

Conclusions

In the first part of this article, we have discussed a new polymer, MD1646, created as an easier processing version of Kraton's workhorse polymer grade G1645, where MD1646 retains all key properties of G1645. In the second part of the article, we have discussed in detail a new polymer based on Kraton A technology of mixed rubber and styrene midblock. Properties of two polymers in this family have been shown as examples. Tubing made from these new Kraton A polymers shows excellent kink resistance, where kink resistance is comparable to PVC, better solvent bonding ability than SEBS/PP compounds, and more stable modulus behavior with temperature.

This article is based on a paper presented at the 194th Technical Meeting of the Rubber Division, ACS, October 2018.

by Aparajita Bhattacharya, Bing Yang, Richard Ma and Stefan Karis, Kraton

Caption: Figure 1--photo of test setup developed to measure kink resistance of tubes

Caption: Figure 2--tensile strength of melt cast films made from a blend of PP with MD1646 (left) and G1645 (right)

Caption: Figure 3--ultimate elongation of melt cast films made from a blend of PP with MD1646 (left) and G1645 (right)

Caption: Figure 4--visual comparison of G1645 and MD1646 pellets (top) and solution cast film (bottom)

Caption: Figure 5--ODT temperature analysis of MD1646 and G1645; the data were obtained by an oscillatory temperature sweep experiment at 0.005 Hz and 0.2 Hz

Caption: Figure 6--melt viscosity of MD1646 and G1645 measured by a capillary rheometer at 230[degrees]C

Caption: Figure 7--melt viscosity at 190[degrees]C of MD1646/PP and G1645/PP blends

Caption: Figure 8--apparent kink diameter of tubes made from SEBS/PP compounds and the neat developmental Kraton A polymers as compared to a PVC reference

Caption: Figure 9--comparison of apparent kink diameter of 4/3 mm tubing made with G1645/PP, new Kraton polymers and PVC by hand test method and Instron test methods

Caption: Figure 10--kink diameter of 7/5 mm tubing with hand test and Instron test method; the data for PVC tubing is not included due to unavailability of this size

Caption: Figure 11--bond strength of tube connector assemblies with ABS female luer connectors and polymers bonded to it by different solvent systems, as shown in the legend

Caption: Figure 12- bond strength of tube connector assemblies prepared by ABS male luer connectors

Caption: Figure 13--bond strength of tube connector assemblies before and after aging; data are shown for three types of assemblies made with solvents: (a) THF, (b) cyclohexanone and (c) MEK

Caption: Figure 14--G', G" and tan o as a function of temperature for (a) A1536 (commercial Kraton A), (b) MD1646/PP (70/30) blend, (c) new Kraton A-1 and (d) new Kraton A-2
Table 1 - HSBC grades for medical tubing and film applications

Polymer                        G1645     G1657     G1643     G1730

Hardness (durometer A) (1)        35        47        52        61
PSC (%)                          13%       13%       19%       20%
Diblock content                   7%       29%        7%       <1%
MFR (g/10 minutes) at            3.5         9        19         4
  230[degrees]C, 2.16 kg
MFR (g/10 minutes) at             --        --        --        --
  230[degrees]C, 5 kg
Tensile strength (MPa) (2)        10        23        14        20
Elongation (%) (2)               600       750       600       800
HSBC type                        ERS      SEBS       ERS      SEPS
Product form                  Pellet    Pellet    Pellet    Pellet

Polymer                       G1652    G1650

Hardness (durometer A) (1)       69       72           49
PSC (%)                         30%      30%          13%
Diblock content                 <1%      <1%          30%
MFR (g/10 minutes) at            <1       <1           --
  230[degrees]C, 2.16 kg
MFR (g/10 minutes) at            --       --           40
  230[degrees]C, 5 kg
Tensile strength (MPa) (2)       31       35           23
Elongation (%) (2)              500      500          750
HSBC type                      SEBS     SEBS    SEBS-g-MA
Product form                  Crumb    Crumb       Pellet

(1) Typical values on polymer compression
molded at 200-230[degrees]C.

(2) Typical properties determined on
film cast from toluene solution.

Table 2 - comparison of properties between
G1645 and MD1646

Polymer                        MD1646VO (1)    G1645MO (2)

Solution viscosity (cP),                570            955
  25% at 25[degrees]C
MFR (g/10 minutes)                       13            3.5
  at 230[degrees]C, 2.16 kg
MFR (g/10 minutes)                       49             13
  at 230[degrees]C, 5 kg
PSC (%)                                  13             13
Hardness (durometer A)                   38             38
Tensile strength (MPa) *               10.4           11.9
Ultimate elongation (%) *             1,050          1,180
Product form                      Pellet **         Pellet

* Properties measured on film cast from toluene solution.
** MD1646VO is dusted with organic dust and is
available in 25 kg bag or 625 kg bulk box.

(1) Product from Mailiao, Taiwan location.

(2) Product from Belpre, OH location.

Table 3 - comparison of optical properties
between G1645 and MD1646

                      Y.I.    Haze    Clarity    Transmittance
                                 %          %                %

MD1646 VO              2.1      16         92               91
  (lot #11WXH1012)
G1645MO                4.6      24         65               91
  (lot #05RBL7132)

Table 4 - summary of physical properties of
new Kraton A polymers

                                            Kraton A-1    Kraton A-2

Total PSC                              %            54            58
ODT                           [degrees]C       210-220           280
Tg (tan [delta] peak)         [degrees]C            25            36
MFR, 230[degrees]C/2.16kg      dg/minute           6.3           5.6
Hardness                     Durometer A            71            76
M50                                  MPa           2.0           3.8
M100                                 MPa           2.9           4.1
M300                                 MPa           5.6           6.8
M500                                 MPa          12.7          14.5
Tensile strength                     MPa          17.6          17.8
Elongation at break                    %           580           580

Table 5 - summary of the effect of steam, EtO and
gamma sterilization on mechanical and optical
properties of SEBS/PP compounds, new Kraton A
polymers and medical grade PVC

                   Before sterilization

Property           Medical     G1645/rcPP         New
                       PVC            and    Kraton A
                              MD1646/rcPP

Kink resistance                                          Similar to PVC
Yellowness index                                         Slightly worse
Bond strength                                                  than PVC
Durometer A
  hardness
Tensile strength

                   After EtO sterilization

Property           Medical     G1645/rcPP         New
                       PVC            and    Kraton A
                              MD1646/rcPP

Kink resistance
Yellowness index
Bond strength
Durometer A
  hardness
Tensile strength

                   After gamma sterilization

Property           Medical     G1645/rcPP         New
                       PVC            and    Kraton A
                              MD1646/rcPP

Kink resistance
Yellowness index
Bond strength
Durometer A
  hardness
Tensile strength

                   After steam sterilization

Property           Medical     G1645/rcPP         New
                       PVC            and    Kraton A
                              MD1646/rcPP

Kink resistance                                   N/A
Yellowness index                                  N/A
Bond strength                                     N/A         No effect
Durometer A                                       N/A     Slight effect
  hardness                                                  Significant
Tensile strength                                  N/A            effect
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Author:Bhattacharya, Aparajita; Yang, Bing; Ma, Richard; Karis, Stefan
Publication:Rubber World
Date:Jun 1, 2019
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