Strength Characteristics of Bonding Compounds made from Acrylic Resin and Three Different Fillers

Submitted in Partial Fulfillment of Requirements for the Residency in Prosthetics
Texas Scottish Rite Hospital for Children,
Orthotics & Prosthetics Department
by Martin Bailey

Abstract

One of the steps in fabricating a lower-extremity prosthesis is the attachment of a socket to the pylon to facilitate static and dynamic alignment. The strength of this bond is an important safety consideration in the alignment procedure. Bonding is typically achieved by using a putty, commonly called ‘gunk', which is made from acrylic or polyester resin mixed with a filler material. The filler thickens the liquid or resin paste to produce a putty with smooth and malleable consistency.

Various materials have been used for the filler component of the putty. Fine cellulose (Solka-Floc®) is a common filler, but talcum powder and cornstarch powder have also been used. No research or technical information could be found which quantified strength characteristics of putty made with any of these fillers.

Tensile strength and torsional strength testing was conducted on samples made from Orthocryl® resin paste using Solka-Floc®, talcum powder, and cornstarch powder as filler materials. Control samples were made from resin paste without filler. ANOVA comparisons show that samples made using talcum powder as a filler are significantly stronger (P>.001) than other fillers tested in both tensile and torsional strength. Samples with cornstarch are stronger than those with Solka-Floc© in torsional strength, but not significantly different (P= .05) in tensile strength. Possible reasons for these findings include the organophilic nature and high thermal conductivity of talc.

Introduction

To achieve static and dynamic alignment of the prosthesis, the socket must be temporarily attached to adjustable components. This allows the prosthetist to maintain the spatial relationship of the components that will later be transferred to the finished prosthesis. Attachment of the socket to the lower components for alignment purposes is typically achieved by using a bonding compound made from a polyester or acrylic resin mixed with a filler material. The filler thickens the resin to produce a putty with smooth and malleable consistency.

Various materials have been used for the filler component of the putty. Though recommendations for the use of fine cellulose (Solka-Floc®) and talcum powder are available (1), no data could be found that quantified the tensile strength or torsional strength of gunk made from either filler. Personal communications with product specialists at Otto Bock?, the company that supplies Orthocryl® resin, confirms that there is no published material strength data for this product in this type of application. Cornstarch powder is commonly used as filler at Scottish Rite Hospital for Children because it produces a putty with a smoother, more workable consistency than Solka-Floc® filler. However, no manufacturer's recommendation could be found for the use of cornstarch powder as a filler material.

Methods and Materials

It was determined that a 2:1 ratio of resin- to-cornstarch produced a putty of the preferred consistency used in practice at Scottish Rite Hospital. Penatrometer tests were then performed using the MTS Bionics 858 machine. These tests were used to determine the weight in grams of talc and of Solka-Floc® needed to formulate putty of equal consistency to that of the cornstarch mixture. The ratios of resin-to-filler were as follows:

Ingredients were pre-measured for each sample using a Denver Instrument XP-3000 scale. Measurements were accurate within ± 0.1 gram. Catalyst was added to the Orthocryl® resin paste at a rate of 3% by weight in accordance with the manufacturer's recommendations. All samples were allowed to cure for a minimum of 24 hours before testing.

The MTS Bionics 858 machine was used for all tensile and torsional strength testing. Tensile strength tests were conducted using the criteria defined in the ASTM D-638 standards (2). Torsional tests were performed following the ASTM E6 and E8 standards, as described in E143-02 (3). Samples were molded into standard dumbbell-shaped coupons as shown in the following diagram:

Twelve specimens each were molded from gunk using Orthocryl® resin paste with Solka-Floc®, with cornstarch powder, and with talcum powder, plus twelve control specimens of Orthocryl® resin paste alone. Test samples were milled to uniform thickness. Each sample was subjected to axial tension at the rate of .5 cm per minute until failure occurred.

For torsional testing, five specimens were molded for each of the three treatments listed plus five Orthocryl® control specimens. Test samples were molded into solid cylinders approximately 2.5cm by 12.7 cm. Samples were tested at a torsion rate of 6 degrees per minute (4).

Results

In the process of making the samples, one of the 20 torsional strength samples and five of the 48 tensile strength samples were broken. Data was obtained for 43 tensile strength samples and 19 torsional strength samples. Of these, eight tensile samples and one torsional sample failed in ways that did not meet our testing criteria or were deemed to be unrepresentative samples. Specifically, seven of the tensile samples failed in the wider radius area rather than the narrow central area of the coupon. This indicated that the failure was not from tensile forces, but rather from non-uniform pressure areas in the testing jig. The eighth tensile sample failed in an area where an air pocket transversed more than half of the width of the sample and it too was deemed to be unrepresentative. One torsional sample was eliminated because it ruptured when strain was released after withstanding a force of 415 inch-pounds at which point the wooden block used to affix the sample in the testing machine deformed. Since this was the only sample in which this occurred, it was deemed to be unrepresentative.

Of the 35 representative tensile strength samples (Figure 1), talc yielded a significantly stronger material than the other fillers (p= .001). In tensile strength, the cornstarch filler did not perform significantly better than resin paste control samples. Solka-Floc®, while stronger than resin paste (p=.002), was not significantly different from cornstarch in tensile strength.

Of the 18 samples used to test torsional strength (Figure 2), talc again proved substantially stronger than any other filler material evaluated (p= .05). Unlike the tensile strength data, samples made from cornstarch did show significantly higher torsional strength than the resin paste control samples (p=.01). Solka-Floc® was not significantly different from either resin paste control samples or cornstarch samples in torsional strength.

Figure 1. Tensile strength test results for gunk made from Orthocryl^® resin paste with three, cornstarch (CS, n=9), Sulka-floc^® (SF, n=9) and talcum powder (T, n=10) and control samples of Orthocryl^® resin paste alone (RP, n=7).

Figure 2. Torsional strength test results for gunk made from Orthocryl^® paste resin with three fillers, cornstarch (CST, n=5), Sulka-floc^® (SFT, n=5) and talcum powder (TT, n=4) and control samples of Orthocryl^® resin paste alone (RPT, n=5).

Discussion

Compared to Solka-Floc® or corn starch, the use of talc as a filler material substantially improves the strength of gunk. Solka-Floc® provides less than 60% of the tensile strength of talc. Cornstarch, though stronger under torsional stresses than Solka-Floc®, is still only 74% of the strength of talc.

There are several possible explanations of the superior strength of gunk made with talc as a filler. Talc is used in the production of both polypropylene plastic and rubber because it improves the strength of those materials (5). Polypropylene is a polymer with a structure composed of both amorphous and crystalline constituents (6, 7). The crystalline portion provides tensile and sheer strength while the amorphous fraction affords elasticity and flexibility. Talc particles act as nuclei for crystallization as the molten resin cools (8). This phenomenon increases the crystalline fraction, thereby increasing tensile strength of the plastic (5, 7, 8). However, the methyl methacrylate polymer found in Orthocryl® has the characteristic linear stress-strain curve of a brittle, cross-linked polyacrylic material with little if any crystalline structure (9). For this reason it is questionable whether the mechanism whereby talc increases the strength of Orthocryl® is a result of the nucleation process that occurs in polypropylene.

Talc is also used as a reinforcement agent in the manufacture of rubber products (5). Although talc is chemically inert, it is an organophilic mineral and therefore has a marked affinity for organic polymers. In the manufacture of rubber products, the organophilic nature of talc results in greater cohesion between the filler and the organic rubber thereby enhancing tensile strength. The affinity of organic molecules (e.g., the acrylic found in Orthocryl®) for talc particles may play a role in the superior strength of the talc samples observed in this study.

The relatively high thermal conductivity of talc may also affect the strength of gunk (7, 9). As the resin undergoes an exothermic reaction, this higher conductivity allows heat to be transmitted through the mixture at a faster rate and also leads to faster cooling of the resin. Scott & Peppas (9) indicate that there is a correlation between temperature and the amount of cross-linking in methyl acrylics. Although this mechanism could be a factor in increasing the strength of gunk, temperature data were not recorded for this study.

Though the strength characteristics of talc as a filler material are superior to other fillers tested, there are two areas to consider when using talc. First, talc is heavier than the other fillers studied. When the weights of the tensile strength coupons were compared, talc samples proved to be heavier than the other fillers by almost 20% (Figure 3). However, considering the relatively small amount of gunk used compared to the total weight of the prosthesis, the practitioner may find this an acceptable trade-off for the added strength obtained.

Of greater concern is the potential health risk associated with talc. Talc has been implicated as a possible carcinogen because of its structural similarity to asbestos. There are talc mines that contain asbestos as well as intermediate molecular species. Federal law has prohibited the use of talc from these sources for personal care products since 1973 (10). However, the potential roll of mineral talc itself as a carcinogen remains controversial. Although research has linked household grade talc use to ovarian cancer, the methods used and the conclusions drawn have been called into question by some medical researchers (11). At this time the National Toxicology Program has not listed talc as a carcinogen. However, in 2000, the NTP moved to defer their decision rather than rejecting talc as a carcinogen, indicating that the matter is still open to further investigation (11).

Although questions regarding talc as a carcinogen are ongoing, studies of long-term occupational exposure to talc have demonstrated that respiratory ailments such as pulmonary fibrosis and silicosis are potential health threats (5, 11, 12). These threats are dramatically reduced by the use of proper ventilation (12, 13). Regarding prosthetic fabrication, it should be recognized that the use of Orthocryl® resin paste (as well as many other chemicals used in lamination) requires an approved ventilation system (14). Such a system would adequately meet the precautionary measures necessary for safe handling of talc (15).

In conclusion, our research indicates that in applications requiring greater torsional and tensile strength, talc is a superior filler material when compared to cornstarch or Solka-Floc®. Although the added weight of talc and the potential occupational health concerns should be considered, the benefit of greater torsional and tensile strength is substantial.

Figure 3. Comparison of mean weight in grams of tensile strength samples using Orthocryl^® resin paste with three fillers, Cornstarch (CS, n=5), Sulka-floc^® (SF, n=12) talcum powder (T, n=6) and control samples of Orthocryl^® resin paste alone.

References

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2. ASTM D 638-02. (2004). Standard Test Method for Tensile Properties of Plastics. Annual Book of ASTM Standards. Vol. 08.01, 47-60.

3. ASTM E 143-02. (2004). Standard Test Method for Shear Modulus at Room Temperature. Annual Book of ASTM Standards. Vol. 03.01, 329-332.

4. Thebault, M-A., Moreau, S., Assad, M., Likibi, F., Rivard, G-H., Chernyshov, A., Leroux, M.A. (2001). Mechanical Testing of Porous Nitinol for Intervertebral Fusion Devices. Retrieved June 16, 2005 from Biorthex Web site: http://www.biorthex.com/pdf/new%20scientific%20publications/Abstract%20ORS%202004%20Mechanical%20REV0.pdf

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6. Lunsford, T. R. (1997). Strength and Materials. In: B. Goldberg & J.D. Hsu, (Eds.) Atlas of Orthoses and Assistive Devices, 3rd Edition (pp 15-66). Mosby-Yearbook, Inc.

7. Mondo Minerals OY. (n.d.) Talc in Plastics . Bulletin 1301, 8 pages. Retrieved July 15, 2005, From Mondo Minerals Web site: www.mondominerals.com/pdf/plastics.pdf

8. Bhattacharyya, A. R., Sreekumar, T.V., Tao Liu, Kumar, S. Ericson, L.M., Hauge, R.H., & Smally, R.E. (2003). Crystallization and Orientation Studies in Polypropylene/Single Wall Carbon Nanotube Composite. Polymer, 44, 2373-2377.

9. Scott, R. A. & Peppas, N.A., (1999). Compositional Effects on Network Structure of Highly Cross-Linked Copolymers on PEG-Containing Multiacrylates with Acrylic Acid. Macromolecules, 32, 6139-6148.

10. Talc use Linked to One Type of Ovarian Cancer. (2002). Retrieved July 14, 2005 from American Cancer Society News Center Web site: http://www.cancer.org/docroot/NWS/content/NWS_1_1x_Talc_Use_Linked_to_One_Type_of_Ovarian_Cancer.asp

11. Wehner, Alfred P. (2001). Cosmetic Talc Should Not Be Listed as a Carcinogen: Comments on NTP's Deliberations to List Talc as a Carcinogen. Regulatory Toxicology and Pharmacology ,36, 40-50.

12. Wild, P. (2000). Une etude epidemiologique de mortalite dans l'industrie productrice de talc.(A Mortality Study in the French Talc Producing Industry. Version 6 (final) Fr. P 1-73. Retrieved July 19, 2005 from Web site: www.ima-eu.org/en/wild.pdf

13. National Institute for Occupational Safety and Health.(1995) Worker Notification Program -- Talc Miners and Millers. Retrieved April 28, 2005, from CDC Web site: http://www.cdc.gov/niosh/pgms/worknotify/Talc.html

14. Otto Bock, Minneapolis, MN. (2003). Orthocryl Sealing Resin. Material Safety Data Sheet Number: 617H21. 8 pages.

15. Mallinckrodt Baker, Inc. (2002). Talc. Material Safety Data Sheet Number: T0026. 8 pages.