PU incorporating surface-modified particles, fibers.
We use the term composites to describe combinations of polymer particles or fibers with thermoset polyurethane. This is because the different polymer domains remain clearly distinct. This move to combine existing materials in different ways is comparable to thermoplastic polymer blends and alloys, which has been the fastest growing segment of polymer materials in the past decade.
The key to combining polymer particles and fibers with polyurethanes is surface modification. Most thermoplastic materials have poor compatibility and bonding characteristics with the more polar family of polyurethanes. However. with appropriate chemical treatment of the surface of particles and fibers, significant improvements in wetting and adhesion are realized. Surface modification entails formation of new functional groups on the surface. which results in greater surface energy. It is believed that this enhanced polyurethane-particle adhesion is a result of greater van der Waals forces, more similar solubility parameters and even covalent bond formation.
The development of surface-modified particles and fibers has been facilitated by a family of reactive gas treatments. These treatments are rapid and cost effective, Futhermore, the resulting surfaces are permanent, and exhibit unsurpassed adhesion properties.
One can view the use of surface-modified polymer particles and fibers as a new dimension in polymer engineering in general. However, most of our work to date has focused on cast polyurethane as the continuous phase. There are several reasons for this. Since polyurethanes are relatively expensive there is opportunity for surface-modified fillers to reduce overall raw material costs. In comparison to thermoplastic resins, liquid polyurethane precursors can be readily mixed with particle/fiber fillers prior to molding. This makes it easier to perform development work and facilitates commercial implementation. The broad diversity of polyurethane physical properties enables on to fine-tune composite properties by making slight alterations to the polyurethane formulation. Finally, the specialty nature of many applications for cast polyurethane makes it a natural for using unique reinforcing fillers to enhance specific engineering properties.
In this article, we will review our results for some novel polyurethane composites. Often the physical properties of these materials are essentially equivalent to the weighted average properties of the respective polymer components. Synergisms sometimes result, giving unexpected property enhancements. Furthermore, with appropriate treatment conditions, a much deeper treated layer is formed, which can alter the effective bulk properties of particles or fibers, and, in turn, can alter composite properties.
The first reactive gas surface modification developed for polymer particles was fluorine-initiated oxidation using blends of fluoride (F2) and oxygen (02). This chemistry is based on the fact that elemental fluorine, even at ambient conditions, has an equilibrium concentration of dissociated fluorine radicals. When fluorine radicals interact with an organic substance, such as a polymer surface, a major reaction that occurs is hydrogen abstraction to form hydrogen fluoride (HF) and a carbon radical. These carbon radicals react with molecular oxygen (02) to form hydroperoxy radicals, which undergo further reactions/rearrangements to form various carbon-oxygen functionalities. These can include hydroxyls, carboxylates, aldehydes, ketones and esters.
The net result is high surface energy on the surface of the polymer particles. This polar surface is readily wetted by polar liquids, such as water and polyurethane precursors, and facilitates excellent dispersion. After the polyurethane phase has cured (cross-linked), the treated particles are tenaciously bonded to the polyurethane. It is believed that this bonding is primarily a result of van der Waals and hydrogen bonding. Some degree of reaction between isocyanates in polyurethane precursors and hydroxyls on the particles' surface is also possible. This, in effect, would be chemical grafting of the polyurethane to the particle surface.
Attributes of reactire gas modification include unsurpassed adhesion performance and permanence. Also, there is no need for operating under vacuum, filtering or product drying. Furthermore, the process is rapid and the treatment is uniform and cost effective. Potential alternate methods for treatment include plasma treatment, corona discharge, wet chemical treatment, primers and flame treatment.
Polar UHMW PE particles
The technical and commercial feasibility of reacrive gas surface-modification of polymer particles for polymer composites was first demonstrated with ultra high molecular weight polyethylene (UHMW PE). This material, which is simply polyethylene with very high molecular weight (3-5 million), is produced as a fine particle/powder. UHMW PE has high commercial value as a material with outstanding abrasion resistance and low coefficient of friction. However, a negative property of UHMW PE is its difficulty to be molded into end products. Its high molecular weight almost eliminates thermoplastic properties.
As described above, surface-treatment gives tremendous increase in bonding to the particles. Of greater significance, some very important property changes result from the incorporation of these particles in matrix materials. Stress-strain curves for composites made with 10% treated UHMW PE/ polyurethane, 10% non-treated UHMW PE/polyurethane and unfi1led polyurethane show that the treated particles give higher flex modulus and greater ultimate tensile strength than do the non-treated particles. This is a direct result from greater compatibility and bonding because of surface-treatment.
An example of unexpected synergism resulting from these novel polymer combinations, facilitated by surfacetreatment, is abrasion resistance. Composites formed with surface modified particles in polyurethane elastomers give abrasion resistance which is superior to that of either the pure polyurethane or UHMW PE.
Surface-modified UHMW PE fibers
Short, chopped fiber reinforced polymers are well known for their outstanding engineering properties. In order for high strength fibers to improve the strength and other properties of composites, there must be sufficient compatibility and bonding between the fibers and matrix to facilitate effective stress transfer. In fact, it is deficiencies in wetting and interfacial bonding that significantly limit the number of viable fiber-matrix resin combinations.
A fibrous form of UHMW PE, highly oriented through a proprietary gel protrusion process, is the strongest fiber materialknown on a weight basis. This material is sold under the tradenames of Spectra in the U.S. (Allied Signal Corp.) and Dyneema in Europe (DMS). With a tensile strength ten times that of steel, on a weight basis, there are numerous applications which could benefit from use of these fibers for reinforcement - if only they could be bonded to.
UHMW PE fibers, surface-modified by treatment with a F2/02 atmosphere, have significantly increased wetting and bonding properties. The efficacy of this treatment in compatibilizing these fibers with polar matrix systems was demonstrated with a cast polyurethane formulation. Treated UHMW PE fibers were readily wetted and uniformly dispersed in the liquid polyurethane precursor prior to curing. In comparison, the untreated fibers were not wetted nor could they be uniformly dispersed in liquid polyurethane precursor. Following curing of the polyurethane matrix, the samples made with 10% (by weight) treated, 1/4" long fibers were very stiff - a result of stress transfer to the fibers through good bonding. In comparison, the samples made with 10% (by weight) untreated fibers were much more flexible and showed stress whitening upon being flexed. Stress whitening is generally associated with void formation as a result of interfacial adhesion failure.
The enhanced bonding property of treated fibers is demonstrated more quantitatively through testing on an Instron testing machine. Stress-strain curves for samples of 10% treated fiber/polyurethane, 10% untreated fiber/polyurethane and untilled polyurethane are shown in figure 1. The stiffening effect of the surface treated fibers is readily apparent.
Scanning electron microscope (SEM) micrographs of the fracture surfaces of the samples discussed above confirm the conclusions of adhesion. The treated fibers appeared yielded and broken, whereas the untreated fibers were simply pulled out. The benefits of enhanced bonding to the UHMW PE fibers on fracture toughness were evaluated via a double edge-notched test (ref. 1). In this technique, test specimens with a progression of ligament lengths are prepared and then stretched on an Instron tensile testing apparatus. From the plot of the maximum load for each sample versus ligament length, the positive effects of fiber treatment are obvious (figure 2).
Surface-modified rubber particles
As families of materials, polyurethane and rubber frequently compete for the same applications. Both of these elastomers have outstanding engineering properties which can be varied over broad ranges through changes in formulation. For many applications, either material will meet the minimum performance requirements, and selection depends on price per part. In general, the raw material costs for polyurethane are higher than for rubber. On the other hand, the costs associated with molding end-products in polyurethane are usually lower than for rubber. Hence, the total costs per part for each material can be very competitive.
Surface-modified rubber particles have potential for having a significant impact on polyurethane technology and markets. With appropriate surface-treatment, rubber particles can be made to disperse more readily in, and bond tenaciously to polyurethane. In this way, the attributes of lower rubber costs and lower polyurethane molding costs can both be appreciated.
A reactive gas treatment, different from that used on polyethylene, had to be developed to affect greater bonding between rubber and polyurethane. It was discovered that chlorine (C12) must be a component of the treatment atmosphere. The effectiveness of this surface-modification in facilitating adhesion is demonstrated by comparing the bond strength of strips of rubber with polyurethane cast on them. T-peel test were used to develop appropriate treatments. The results from test specimens, where polyurethane was cast onto nontreated strips of rubber, is an adhesion strength of 3 Ib./inch. In analogous tests with surface-modified strips of rubber, bond strength exceeds 150 Ib./inch; the rubber tears before the adhesion bond fails.
Results from our laboratory evaluations are very encouraging. Incorporation of up to 40% by weight finely ground (180m) rubber particles, treated with the reactive gas atmosphere developed above, in cast polyurethane elastomers gives physical properties nearly identical to those of the unfilled polyurethane. The modest reduction in ultimate tensile strength is not considered of importance since few applications for elastomeric polyurethanes involve elongation greater than 100%.
Stress-strain curves are illustrative of the benefits of surface-modification of the rubber. In figure 3, the stress-strain curves are compared for untilled polyurethane; 15% surfacemodified rubber/85% polyurethane; and 15% untreated rubber/85% polyurethane. The significance of these data is that the flexural moduli of surface-modified rubber/polyurethane composite and of untilled polyurethane are virtually identical over the 0-100% elongation range, which is the useful working range of virtually all cast polyurethanes. In other words, the treated-filled and untilled parts will perform essentially identically. Uses of non-surface-treated rubber in the same polyurethane gives a much softer and more flexible part. Also, the Youngs modulus for untilled polyurethane and composites made with treated rubber particles are very similar. The Youngs modulus for composites made with non-treated rubber particles is considerably lower.
It has been discovered that the bulk properties of rubber particles can also be affected by the reaction gas treatment. When more pressing treatment conditions are used, such as longer treatment time, the chemical modifications occur deeper in particles. This is illustrated in the stress-strain curves of composite samples in which the rubber particles were treated for various lengths of time.
The significance of this phenomenon is that treatment of polymer particles facilitates another degree of control in custom tailoring engineering properties. With appropriate treatment, the modulus of the rubber particles can be increased. The ramifications of this for composites formed with polyurethane is that the modulus and hardness can be adjusted to match that of the matrix polyurethane.
It has been demonstrated that the dynamic hysteric heating of treated rubber-filled polyurethane composites is lower than for analogous non-treated rubber composites. It is hypothesized that this is a result of treated rubber particles having a flexural modulus more similar to that of the polyurethane. It is well established that in dynamic applications, heat is generated as the interface of materials with different moduli of elasticity.
The use of treated rubber particles in polyurethane formulations will give significant cost reduction and, in turn, can open new markets. The projected selling price of treated rubber particles is in the range of $0.40-$0.70/1b. In comparison, the raw material costs for polyurethane are in the range of $1.00 to $4.00/1b. Since the use of treated rubber particles as a filler in polyurethane results in essentially a pound for pound replacement, raw material savings of 10% to 70% or greater are possible. With considerably lower raw material costs, polyurethane will experience greater success in capturing applications/markets currently served by other materials.
It is noteworthy that the development of surface-modified rubber has been partially funded by the U.S. Department of Energy. This work has been focused on the development of value-added application for the rubber from scrap automotive tires. Reuse of scrap tire rubber via this surfacemodification technology, facilitating its replacement of virgin polymer resin, results in an average energy savings of 63,000 BTU/Ib. rubber reused. By contrast, burning scrap tire rubber for energy recovers only 14,000 BTU/Ib. rubber.
Surface-modified rubber has the potential for providing benefits beyond those shown above. Scrap tire rubber is a diverse mixture of different types of rubber. Utilization of a specific type of rubber formulation will probably provide further control of polyurethane composite properties.
Surface-modified polymer particles and fibers have been shown to dramatically improve the performance and/or reduce the cost of polyurethane systems. Widespread use of these materials has the potential to open sizable new markets for polyurethanes.
1. M.A. Williams, B.D. Bauman and D.A. Thomas, Polym. Eng. & Sci., 31 (13) 992-998 (1991).
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|Author:||Bauman, Bernard D.|
|Date:||Apr 1, 1993|
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