Biodegradable Polycaprolactone Urethane Elastomers

Polycaprolactone polyols have received significant attention as the soft segment of biodegradable and biocompatible polyurethane elastomers.   There are a few key advantages formulators gain through using polycaprolactone polyols,  including the ability to engineer the specified mechanical properties, processing characteristics, and degradation kinetics that are required within various applications.

Polycaprolactone-based polyurethanes stand-alone in meeting a critical set of criteria for biodegradable polyurethane elastomeric materials (B-PURs), including:

• Containing polyurethane segments that are open to hydrolysis and producing degradation products that are non-toxic to esterase microorganisms.
• Having mechanical and thermal properties.
• Facilitating easy processing as a TPU, cast polyurethane elastomer, coating and adhesive, or porous film material.
• Showing a diversity of polyol structures to facilitate the engineering of properties and degradation characteristics.

In this article, we will address the engineering principles useful in the design of B-PURs based on polycaprolactone polyols.   A substantial level of relevant literature exists on tailoring polycaprolactone B-PURs in relation to resorbable biomedical materials used as “scaffolds” for tissue engineered devices like stents and bone and cartilage repair materials.  Environmental sustainability has been another driver of studies on the biodegradation and composting properties of polycaprolactone based B-PURs.

Markets for Biodegradable Polycaprolactone-Based Polyurethanes (B-PURs)

Polycaprolactone is one of the preferred polyols for manufacturing biodegradable polyurethane elastomers.  Adoption of polycaprolactone-based polyurethanes is driven by a unique performance profile that includes excellent mechanical properties, low temperature flexibility and high temperature capabilities, adhesion, abrasion resistance, durability, toughness, good chemical and solvent resistance, as well as UV resistance.  The main markets for polycaprolactone-based polyurethanes are in the CASE sector, environmentally-friendly packaging materials, agriculture, the oil and mining industry, and the medical sector (drug delivery and wound-care membranes). Global initiatives for biodegradable polymers have been a key factor for accelerating the growth of polycaprolactone-based B-PURs.

Design Criteria for B-PURs Based on Polycaprolactone Polyols

The performance and biodegradation kinetics of B-PURs based on polycaprolactone polyols are influenced by the polyol, the diisocyanate, and the selected chain extender .  However, the polycaprolactone polyol commands the greatest attention, because it’s the entity susceptible to microbial attack under the proper conditions.   Key factors known to control the biodegradation kinetics of the polycaprolactone segments, include the following:

• The molecular weight of the macro-diol.
• The extent of crystallinity in the soft-segment.
• The level of microphase separation between the soft and hard block segments.
• Hydrophobicity and hydrophilicity. 

Amorphous soft-block segments will biodegrade at a higher rate than crystalline segments. Studies have shown that the amorphous soft-block region in semi-crystalline polymers degrade prior to the crystalline regions in the PUR because they are more accessible to degrading microorganisms.  Crystalline domains consist of tightly-packed polymer chains, which are more resistant to hydrolysis and degradation by microorganisms.  With polycaprolactone-based PURs, it’s estimated that the upper limit of soft-block crystallinity is 50-60 percent.

The presence of the hard-block segment within the polycaprolactone phase has a strongly negative effect on microbial attack and biodegradation. The soft segment will always contain some levels of the urethane block segment due to incomplete phase separation.  Factors driving the hard-block separation include the molecular weight and degree of crystallinity of the soft-block, the hard segment concentration and segment lengths, as well as the processing conditions.  All of these considerations must be taken into account when designing biodegradable poly(caprolactone-b-urethanes).

One asset of the polycaprolactone family of polyols is the great diversity of chemical structures available, including semi-crystalline and amorphous polyols, varying degrees of hydrophobicity, and a wide range of molecular weights.  To understand this advantage, let’s look at the process technology used to manufacture polycaprolactone polyols.

The Chemistry of Polycaprolactone Polyols Creates Their Uniqueness

The polycaprolactone polyols are available as diols, triols and tetraols, with both liquid and wax-like appearances and  molecular weights ranging from 310 to 4000. The polycaprolactone polyols are produced by a unique Ring Opening Polymerization (ROP) process in which a glycol initiator is reacted with a six-carbon cyclic-ester monomer, ɛ-caprolactone. A wide variety of glycol initiators are used, providing a significant custom design feature of this unique class of materials. See the figure below.

This chemistry accounts for the uniqueness of the polycaprolactone polyols, including their very low acid values (contributes to hydrolytic stability), low polydispersity (contributes to low viscosities), and perfect primary hydroxy end-functionalities (good stoichiometry control with reactivities that can be adjusted to meet your specific application requirements).  The symmetry of the caprolactone repeat unit enhances interchain packing in the soft-block segment of a polyurethane or polyurea, which increases flexibility and elastomeric characteristics.

Daicel Corporation produces a broad portfolio of Placcel® polycaprolactone polyols, including standard polycaprolactone diols with molecular weights from 530 to 4000, initiated with various glycols to achieve a variety of property enhancements.

Listed below are just a few of the “initiator” diols (R-[-OH]_f]) that formulators use and the key characteristics of the resulting polycaprolactone polyols in a polyurethane elastomer.

• Ethylene Glycol (EG):  General purpose initiator affording a good balance of properties, and some hydrophilicity with semi-crystalline polyol behavior.
• Diethylene Glycol (DEG):  Contributes some polyether performance characteristics, amorphous liquid diols, and more hydrophilicity.
• Neopentyl Glycol (NPG):  Reduced propensity to crystallize and better hydrolytic stability, along with greater hydrophobicity.
• Butanediol (BDO):  Semi-crystalline polyols with high hydrolytic stability, cut, tear, and  chunk resistance, high tensile properties, and increased hydrophobicity
• Hexanediol (HDO):   Semi-crystalline polyols with high tensile and thermal properties, as well as cut-chip resistance and increased hydrophobicity.
• Trimethylolpropane (TMP):  Affords trifunctional polyols for controlled crosslinking of a polyurethane elastomer.

Affecting the Biodegradation Kinetics of B-PUR by Variation of the Polycaprolactone Polyol

Key criteria that affect the biodegradation kinetics of a polycaprolactone based B-PURs include the molecular weight of the polyol,  the level of crystallinity, the hard-block concentration, and the level of hydrophilicity.

In published studies, the comparative microbial degradation rates have been investigated for polycaprolactone polyols of molecular weight of 530 (amorphous) through 1600 and above (semi-crystalline).   The more amorphous polyols had the fastest rate of microbial degradation.   Below MW 4000 polyols, the microbial degradation rates correlated inversely with the degree of crystallinity (i.e. fastest rates were associated with lower crystallinity).  Above 4000 MW, there were no differences in the degradation kinetics. (Agric., Biol. Chem., 42, 1071, 1978).

The soft-segment concentration varied from 55 to 90 percent by weight of the polyurethanes.  Microbial degradation studies showed intermingling of the hard-block segment into the amorphous sections of the soft-block phase had a strong negative effect on the degradation rates.  This dynamic was likely due to surface exclusion of microbes from access to the more degradable polycaprolactone phase. Improving hard-segment domain separation increased the rate of biodegradation in the soft-segment.  This goal can be accomplished using higher molecular weight polyols, higher hard-block concentrations, and longer urethane segment lengths.  

The figure below shows the relative rates of biodegradation of two B-PURs based on two different MW grades of polycaprolactone diol.  The differences are due to much higher hard-block domain separation achieved using a high MW soft block.  The high MW polyol exhibited crystallinity whereas the lower MW diol was amorphous demonstrating the greater effect of hard-block phase separation vs. polyol crystallinity in the final B-PUR. (Macromolecular Symposia 197(1):255-264, July 2003).

From the above discussion, it's possible to see how varying the initiator types in the polycaprolactone polyols and the MW can be used to fine-tune biodegradability.   Initiators like NPG and DEG reduce the crystallization propensity of the polycaprolactone segments.  Diethylene glycol is a particularly interesting initiator, since it will both increase hydrophobicity and reduce crystallinity in the soft-segment of the PUR. 

Adjusting the type and MW of the polyol and the ratio between the soft-segment and the hard-segment also influences the mechanical properties of the polyurethane, including hardness and flexibility, elasticity and toughness, strength and durability, and more.

Other Molecular Influences on Biodegradability

There are three primary components in polyurethanes:

• Polyol(s).
• Diisocyanate.
• Chain extender.

We have already covered the role of the polyol in PUR development above  For the diisocyanate, there are two classes:  aromatic isocyanates like MDI and aliphatic isocyanates like H₁₂MDI and HDI.  Both categories of isocyanates produce strong and durable elastomers with good overall property profiles.   However, it has been observed that aromatic-based polyurethanes are less susceptible to biodegradation versus their aliphatic-based B-PUR counterparts.  This difference is attributed to both the higher packing energy and density between aromatic urethane units, as well as the higher bond strengths in the aromatic urethane linkages.

The chain extenders, usually diols, also influence biodegradation, as well as the overall urethane performance.  1,4-butanediol (BDO) is the most-often cited chain extender.  BDO is a versatile liquid diol intermediate with reactive primary hydroxyl functionality and a linear structure that promotes phase separation and lends itself to formulating polyurethanes with a good balance of mechanical properties and processability. 

B-PURs have also been studied using a tertiary amine-diol, 2,2’-(methylamino)diethanol (MIDE).  Polycaprolactone-MDI prepolymers chain extended with MIDE are more hydrophilic and exhibit high elongations.  As expected, the MIDE chain-extended B-PUR showed higher microbial degradation rates versus BDO chain-extended B-PUR. (Int J Nanomedicine. 2011; 6: 2375–2384).

In other studies, the more hydrophilic polyether polyols were incorporated in the polyol composition of polycaprolactone-based polyurethane elastomers to increase water diffusion into the polymer matrix.  A 50:50 polyethylene glycol-polycaprolactone polyol-based co-polyurethane showed enhanced degradation under both soil burial conditions and water immersion.  In this case, the ester linkages in the polycaprolactone segments undergo increased hydrolase; the ether linkages will be much less susceptible to biodegradation.  Modifying the block ratios in a poly(ester-ether urethane) provides another option for engineering the degradation kinetics of a B-PUR. (Polymer Degradation and Stability 95(10):2013, October 2010).

Placcel® Polycaprolactone Polyol Grade Slate 

The Placcel 200 series diols range from amorphous liquids to pastes and hard waxes.

Increasing the functionality to three hydroxyl groups in each molecule affords polyurethanes with a greater crosslink density, higher stiffness and hardness, enhanced thermal and chemical resistance, and improvements in physical properties.

Conclusion

Polycaprolactone is one of the most attractive polyols for the production of biodegradable polyurethanes. Global initiatives for biodegradable polymers have accelerated the growth of polycaprolactone based B-PURs. Polycaprolactone B-PUR’s unique performance attributes include excellent mechanical properties, low temperature flexibility & high temperature capabilities, adhesion, abrasion resistance, durability, toughness, and good chemical & solvent resistance, and UV resistance.

This blog provides some of the key engineering principles useful in the design of B-PURs based on polycaprolactone polyols. The blog also summarizes how the broad compositional range of our polycaprolactone polyols can be used to fine tune the microbial degradation kinetics of a B-PUR. Conversely, the information can also be used to design caprolactone based polyurethanes that exhibit high resistance to biodegradation.

To explore how our biodegradable polycaprolactone polyols can address your unique polyurethane elastomer application, partner with the expert teams at Gantrade Corporation. Our teams, armed with a wealth of technical knowledge and expertise, can guide you to the best solutions for your applications. Contact Gantrade today to get started.  We offer a wide portfolio of products to achieve your high-performance polyurethane requirements.