US20150158997A1

US20150158997A1 - Gypsum composite modifiers

Info

Publication number: US20150158997A1
Application number: US14/561,242
Authority: US
Inventors: Mianxue Wu; Jeffrey T. Fields; Pierre Donaldson; Bill Bodiford
Original assignee: Georgia Pacific Gypsum LLC
Current assignee: Georgia Pacific Gypsum LLC
Priority date: 2013-12-06
Filing date: 2014-12-05
Publication date: 2015-06-11
Also published as: WO2015085133A1; US10280288B2; CN105874013B; CN106061919A; WO2015085139A1; US20150158735A1; CN105874013A

Abstract

Anhydrite calcium sulfate whiskers are prepared by combining alpha calcium sulfate hemihydrate and water to form a slurry, autoclaving the slurry to form alpha calcium sulfate hemihydrate whiskers in water, dewatering the alpha calcium sulfate hemihydrate whiskers, and heating the alpha calcium sulfate hemihydrate whiskers to form anhydrite calcium sulfate whiskers. A composite includes alpha-derived anhydrite calcium sulfate whiskers and a base material and is prepared by combining alpha-derived anhydrite calcium sulfate whiskers with a base material to form a composite.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Applications No. 61/912,609, filed Dec. 6, 2013, and 62/074,163, filed Nov. 3, 2014, the disclosures of which are each incorporated by reference herein.

BACKGROUND

The present disclosure relates generally to the field of calcium sulfate-based products and manufacturing, and more particularly to calcium sulfate whiskers and composites and methods for their use and manufacturing.
Various fillers are known for use as reinforcement, opacification and/or additives in paint, coatings, papermaking, and plastics. For example, fillers such as fiberglass, carbon black, calcium carbonate, silica, talc, kaolin, and aluminum hydroxide are currently used in these applications. However, fibrous fillers generally provide improved stiffness, strength, and thermal stability properties to composites, as compared to particulate and platelet shaped fillers.
Single crystal fibers, also known as “whiskers,” have been shown to provide improved properties in composites as compared to traditional fibers. For example, whiskers may provide improved surface quality and aesthetics for composites, because they are typically finer and smoother than fibers. Whiskers may also provide improved dimensional and thermal stability, increased strength and toughness, and higher fluidity (e.g., for improved mold casting). However, whiskers are difficult to economically manufacture, due to the slow growth rate of whiskers.
Accordingly, there is a need for improved whiskers and composites made therewith, as well as for methods for use of and manufacturing improved whiskers.

SUMMARY

In one aspect, a method of making anhydrite calcium sulfate whiskers is provided, including: (i) combining alpha calcium sulfate hemihydrate and water to form a slurry; (ii) autoclaving the slurry to form alpha calcium sulfate hemihydrate whiskers in water; (iii) dewatering the alpha calcium sulfate hemihydrate whiskers; and (iv) heating the alpha calcium sulfate hemihydrate whiskers to form anhydrite calcium sulfate whiskers. Anhydrite calcium sulfate whiskers made by this method are also provided.
In another aspect, a method of making a composite is provided, including combining alpha-derived anhydrite calcium sulfate whiskers with a base material to form a composite.
In yet another aspect, a composite is provided, including alpha-derived anhydrite calcium sulfate whiskers and a base material.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, which are meant to be exemplary and not limiting, and wherein like elements are numbered alike:

FIG. 1 is a micrograph showing alpha-derived anhydrite calcium sulfate whiskers prepared in accordance with one embodiment of the methods disclosed herein.

FIG. 2 is a micrograph showing beta-derived anhydrite calcium sulfate whiskers.

FIG. 3 is a graph showing the elastic modulus of composite samples prepared in accordance with one embodiment of the methods disclosed herein, as tested according to three measuring methods.

DETAILED DESCRIPTION

Disclosed herein are calcium sulfate-based whiskers and composites and methods for their use and manufacturing. These whiskers, composites, and methods meet one or more of the above-described needs by providing single crystal fibers and composites made therewith, having improved properties. For example, the whiskers may have an improved aspect ratio compared to known fibers and/or a faster growth rate.
Whiskers and Methods of Making
In certain embodiments, whiskers are provided. As used herein, the term “whiskers” refers to single crystal fibers. Generally, the whiskers disclosed herein are made using calcium sulfate, which may also be referred to as gypsum or plaster. Calcium sulfate may exist as a hemihydrate (CaSO₄*1/2H₂O), a dihydrate (CaSO₄*2H₂O), or an anhydrite (CaSO₄). Generally, the beta calcium sulfate hemihydrate, which is obtained from the calcination of dihydrate calcium sulfate under an elevated temperature at ambient temperature, may be used to prepare calcium sulfate whiskers. However, it has been discovered that alpha calcium sulfate hemihydrate provides surprising advantages to whisker processing and properties. Alpha calcium sulfate hemihydrate, which has the same chemical composition as the beta form, has gone through a pressurized calcination process, which produces well-crystallized, prismatic particles. For example, the pressurized calcination process may be an autoclave process. It should be understood that while the present disclosure generally relates to alpha calcium sulfate whiskers, the use of other suitable materials is also envisioned and intended to fall within the scope of this disclosure. For example, other particles that have been subjected to a pressurized hydrothermal reaction process may also be used.
In certain embodiments, methods of making anhydrite calcium sulfate whiskers include: (i) combining alpha calcium sulfate hemihydrate and water to form a slurry; (ii) autoclaving the slurry to form alpha calcium sulfate hemihydrate whiskers in water; (iii) dewatering the alpha calcium sulfate hemihydrate whiskers; and (iv) heating the alpha calcium sulfate hemihydrate whiskers to form anhydrite calcium sulfate whiskers.
In one embodiment, the step of combining the alpha calcium sulfate hemihydrate and water includes combining the alpha calcium sulfate hemihydrate with the water such that the alpha calcium sulfate is present in the slurry in an amount from about 1 to about 30 percent by weight of the slurry. For example, the alpha calcium sulfate hemihydrate may be combined with the water such that the alpha calcium sulfate is present in the slurry in an amount from about 1 to about 20 percent by weight of the slurry, from about 2 to about 15 percent by weight of the slurry, or from about 2 to about 10 percent by weight of the slurry. In one embodiment, the alpha calcium sulfate hemihydrate may be combined with the water such that the alpha calcium sulfate is present in the slurry in an amount from about 5 to about 8 percent by weight of the slurry. Without being bound by a particular theory, it is believed that the combination of alpha calcium sulfate hemihydrate and water results in dihydrate crystals in the slurry.
In certain embodiments, the alpha calcium sulfate hemihydrate has a median particle size from about 1 micron to about 100 micron, such as from about 1 micron to about 20 micron. For example, the alpha calcium sulfate hemihydrate may have a median particle size from about 2 to about 5 micron. In certain embodiments, methods of making anhydrite calcium sulfate whiskers further include sizing the alpha calcium sulfate hemihydrate to a median particle size from about 1 micron to about 10 micron, prior to combining the alpha calcium sulfate hemihydrate and water to form a slurry. For example, the alpha calcium sulfate hemihydrate may be sized by jet milling or other suitable sizing or grinding processes known to those in the art.
In certain embodiments, the slurry is autoclaved to form alpha calcium sulfate hemihydrate whiskers in water. In one embodiment, the step of autoclaving the slurry includes subjecting the slurry to saturated steam at a pressure from about 0 psig to about 50 psig and a temperature from about 100° C. to about 150° C. for a duration from about 30 minutes to about 8 hours. For example, the step of autoclaving the slurry may include subjecting the slurry to saturated steam at a pressure from about 1 psig to about 30 psig and a temperature from about 101° C. to about 134° C. for a duration from about 30 minutes to about 8 hours. For example, such autoclave parameters may be applied in a small-scale production setting, such as in the lab. For example, the step of autoclaving the slurry may include subjecting the slurry to saturated steam at a pressure from about 30 psig to about 52 psig and a temperature from about 134° C. to about 150° C. for a duration from about 30 minutes to about 8 hours. For example, such autoclave parameters may be applied in an industrial-scale production setting. Without being bound by a particular theory, it is believed that the calcium sulfate dihydrate crystals dissolve during the autoclave process and reform as hemihydrate whiskers at the high pressure.
The alpha calcium sulfate hemihydrate whiskers may then be dewatered, i.e., the whiskers may be separated from water. In certain embodiments, the step of dewatering the alpha calcium sulfate hemihydrate whiskers includes filtering, vacuuming, centrifuging, or a combination thereof. For example, a screen filter may be used to dewater the whiskers. In one embodiment, the step of dewatering the alpha calcium sulfate hemihydrate whiskers includes filtering the alpha calcium sulfate hemihydrate whiskers
The alpha calcium sulfate hemihydrate whiskers may then be heated, or “dead burned,” to achieve a stable, insoluble anhydrite form. In certain embodiments, the step of heating the alpha calcium sulfate hemihydrate whiskers to form anhydrite calcium sulfate whiskers includes heating the alpha calcium sulfate hemihydrate whiskers at a temperature from about 500° C. to about 900° C. for a duration from about 1 to about 24 hours.
Anhydrite calcium sulfate whiskers may be produced by the methods disclosed herein. In certain embodiments, the whiskers have a Mohs hardness from about 3 to about 3.5. In certain embodiments, the whiskers are thermally stable up to at least 1450° C. In certain embodiments, the whiskers have a mean aspect ratio of at least 30. As used herein, the term “aspect ratio” refers to the ratio of the length of a calcium sulfate whisker to its diameter, and the “mean aspect ratio” is the ratio of the average whisker length to the average whisker diameter for a plurality of whiskers.
Composites and Methods of Making
In certain embodiments, composites including whiskers are also provided. For example, the whiskers described herein may be lightweight and have desirable properties for structural reinforcement, and thermal and acoustic insulation. For example, the whiskers may provide improved dimensional and thermal stability, increased strength and toughness, and higher fluidity (e.g., for improved mold casting). Moreover, the whiskers may provide improved surface quality and aesthetics for composites, because they are typically finer and smoother than fibers.
In certain embodiments, methods of making composites include combining alpha-derived anhydrite calcium sulfate whiskers with a base material to form a composite. In certain embodiments, composites include alpha-derived anhydrite calcium sulfate whiskers and a base material.
For example, the base material may include a material selected from the group consisting of paints, coatings, paper, paper precursors, films, plastics, resins, gypsum wallboard, facers (including fiberglass mat), cement and concrete, and combinations thereof. Generally, alpha-derived anhydrite calcium sulfate whiskers may be used in any applications where traditional fillers, opacification agents, or reinforcing additives, such as but not limited to fiberglass, carbon black, calcium carbonate, silica, talc, kaolin, or aluminum hydroxide, are currently used. For example, alpha-derived anhydrite calcium sulfate whiskers may be combined with paper materials, plastics, including elastomers, thermoplastics, and thermosets, and/or resins, including epoxies, and thermoset resins. In certain embodiments, the base material is polyethylene, polypropylene, polybutylene, polyester, polyethylene terephthalate (PET), nylon 6, nylon 66, polyurethane, epoxy, nitrile rubber, butyl rubber, silicone rubber, PTFE, or a phenolic.
In one embodiment, the step of combining the alpha-derived anhydrite calcium sulfate whiskers and the base material includes combining the alpha-derived anhydrite calcium sulfate whiskers with the base material such that the alpha-derived anhydrite calcium sulfate whiskers are present in the composite in an amount from about 1 to about 50 percent by weight of the composite, such as from about 3 to about 40 percent by weight of the composite, or from about 3 to about 10 percent by weight of the composite. In one embodiment, a composite includes from about 3 to about 10 percent by weight alpha-derived anhydrite calcium sulfate whiskers.
In one embodiment of a composite, the base material includes polypropylene, the composite includes from about 2 to about 20 percent by weight alpha-derived anhydrite calcium sulfate whiskers, such as from about 4 to about 10 percent by weight alpha-derived anhydrite calcium sulfate whiskers, and the composite has an elastic modulus of at least 1000 MPa.
The mechanical properties of whisker-reinforced polymer composites may be further improved by a proper treatment of the whisker surface before they are introduced into the base material. For example, silane coupling agents, or other additives capable of coupling or compatibilizing inorganic materials to organic resins, such as 3-methacryloxypropyltrimethoxysilane, may be used.

EXAMPLES

Embodiments of the calcium sulfate whiskers and composites disclosed herein were manufactured and tested. The results are shown below and at FIGS. 1-3.

Example 1

Calcium sulfate whiskers were prepared using alpha and beta hemihydrate particles mined from the same location, according to the following method. Calcium sulfate hemihydrate was combined with water to form a slurry containing 5 weight percent calcium sulfate. The slurry was autoclaved at a pressure of 10 psig and a temperature of 115° C. for 2 hours to form whiskers. The resulting whiskers were dewatered by screen filtering for 5 minutes. The dewatered whiskers were heated at a temperature of 600° C. for 3 hours to form stable, insoluble calcium sulfate anhydrite whiskers. The resulting alpha and beta calcium sulfate anhydrite whiskers were then photographed.
FIG. 1 is a micrograph (600×400 microns) taken by optical microscopy showing the alpha-derived anhydrite calcium sulfate whiskers prepared according to the above method. FIG. 2 is a micrograph (600×400 microns) taken by optical microscopy showing the beta-derived anhydrite calcium sulfate whiskers prepared by the same method as the alpha whiskers. These micrographs show the distinct morphology (e.g., length, diameter) of the alpha versus beta whiskers. For example, the alpha whiskers are much longer than the beta whiskers, and have a greater aspect ratio than the beta whiskers. Furthermore, the micrographs reveal that the growth rate is much faster for alpha whiskers than beta whiskers.
Specifically, it has been determined that the alpha calcium sulfate has a crystal growth rate of at least 3-5 times that of beta calcium sulfate. Thus, the autoclave time needed to make whiskers of identical length is much shorter with the alpha calcium sulfate, reducing the necessary processing time. This is significant because the low growth rate of beta calcium sulfate whiskers led to high economical costs for manufacturing, making the practical applications of calcium sulfate whiskers cost-prohibitive. Without being bound by a particular theory, it is believe that due to its “pre-autoclaved” nature, alpha calcium sulfate is able to more quickly grow into elongated whiskers under the autoclave environment.

Example 2

Alpha and beta calcium sulfate whiskers were prepared in accordance with the method of Example 1. The whiskers were combined with polypropylene to form composites and the stiffness/flexural modulus of each composite was measured according to the Chord method and compared against the stiffness/flexural modulus of the polypropylene alone. The results are shown below in Table 1.
The whiskers were either premixed or side fed with the polypropylene. Premixed refers to the whiskers being combined with extruded polypropylene pellets prior to melting to form a composite. Side fed refers to the whiskers being fed into the extruder when the polypropylene is already molten.

TABLE 1

Stiffness/Flexural Modulus of Whisker-
Reinforced Polypropylene Composites

	Flexural Modulus of	Flexural
	Whisker-Reinforced	Modulus of	Increased
	Polypropylene	Polypropylene	Stiffness
Sample	(MPa)	(MPa)	(%)

Beta-derived	813	863	−5.8
whiskers: 10 wt. %
premixed
Beta-derived	1106	1085	1.9
whiskers: 10 wt. %
premixed
Alpha-derived	1060	754	40.6%
whiskers: 10.1 wt. %
premixed
Alpha-derived	1240	754	64.5%
whiskers: 4.39 wt. %
side fed
Alpha-derived	1630	834	95.4%
whiskers: 9.25 wt. %
side fed
Alpha-derived	1886	849	122.1%
whiskers: 9.49 wt. %
side fed

These results show that polypropylene composites with alpha whisker reinforcement display much better mechanical properties than composites with comparable beta whisker reinforcement. Without being bound by a particular theory, it is believed that the premixed process causes the whiskers to be ground up by the unmelted polypropylene pellets, which may result in a lessened reinforcing effect of the whiskers. However, the premixed alpha whiskers performed surprisingly well, showing a 40% increase in stiffness, as compared to the premixed beta whiskers, which showed negligible stiffness increases.
FIG. 3 shows the elastic modulus of polypropylene versus a composite of polypropylene and 4.39 wt. % alpha calcium sulfate whiskers, measured according to the Flexural Chord, Flexural Tangent, and Flexural Secant methods. As shown, the stiffness of the polypropylene composite increases significantly (up to 65%) with the addition of the alpha whiskers.

Example 3

The commercially obtained alpha calcium sulfate particles used in the foregoing examples had a median particle size of 15.4 micron and a maximum particle size of 120 micron. Micronized alpha calcium sulfate particles were also prepared and tested. Specifically, commercially obtained alpha calcium sulfate particles were ground in a jet mill to a median particle size of 3.06 micron and a maximum particle size of about 12 micron. Thus, the micronized particles were smaller and more uniform in size than the commercially obtained calcium sulfate particles, as shown in Table 2.

TABLE 2

Micronized Alpha Calcium Sulfate
Particle Dimension Distributions

	d₁₀Particle	D₅₀Particle	D₉₀Particle	d₁₀₀Particle
	Size: 10^th	Size: 50^th	Size: 90^th	Size: 100^th
	Percentile	Percentile	Percentile	Percentile
Time	(μm)	(μm)	(μm)	(μm)

Before	2.55	15.72	52.83	120.22
Micronizing
After	1.13	3.06	6.42	12.00
Micronizing

The particles were jet milled on a fluid energy mill employing compressed air to produce the smaller particles. High-speed rotation within the mill subjects the calcium sulfate particles to particle-on-particle impact. As shown in Table 2, the micronized particles displayed a maximum size 10 times smaller than the commercially obtained particles. Also, the micronized particles had a maximum particle size less than 4 times the average particle size.
Calcium sulfate whiskers were prepared by the method of Example 1, using the micronized alpha particles. Whiskers were also prepared using the commercially obtained alpha particles and the micronized alpha particles, but at a lower concentration of 2.5 wt. % in the slurry. Comparative samples of alpha whiskers made using commercially obtained calcium sulfate were made at a concentration of 5 wt. % in the slurry. Commercially available magnesium oxysulfate whiskers were also obtained for comparative purposes. The average length and diameter was measured by scanning electron microscopy for the resulting whiskers, and the aspect ratio was calculated. The results are shown in Table 3.

TABLE 3

Length, Diameter, and Aspect Ratio for Various Whisker Samples

	Average Length	Average Diameter	Aspect Ratio
Sample	(μm)	(μm)	(L/D)

Low Concentration	71.1	1.27	56.1
Whiskers
Standard Alpha	64.4	1.36	47.5
Whiskers
(Example 1)
Micronized Alpha	48.8	0.75	64.9
Whiskers
Micronized Alpha	52.6	0.48	109
and Low
Concentration
Whiskers

As shown in Table 3, the micronized alpha whiskers surprisingly had a significantly higher aspect ratio than the standard alpha whiskers (almost 40% greater), with a slightly shorter length and a smaller diameter. The low concentration commercial particle-derived whiskers also showed a slightly higher aspect ratio than the standard whiskers, while the low concentration micronized particle-derived whiskers showed a significantly higher aspect ratio than all other samples. The magnesium sulfate whiskers are much shorter than the calcium sulfate whiskers, and have an even smaller diameter. Generally, an aspect ratio above 30 is desirable for reinforcement applications, with higher aspect ratios providing better structural reinforcement. Also, finer whiskers tend not to settle in the dispersion after autoclaving, such that they can avoid the crowdedness that often results in localized excessively high concentration of crystals and thus short whiskers. However, micronizing the calcium sulfate adds processing time and cost to the whisker manufacturing process. Overall, these examples show that alpha calcium sulfate whiskers can be manufactured to have application-specific dimensions.
Embodiments of the present disclosure further include any one or more of the following paragraphs:
1. A method of making anhydrite calcium sulfate whiskers, comprising:
combining alpha calcium sulfate hemihydrate and water to form a slurry;
autoclaving the slurry to form alpha calcium sulfate hemihydrate whiskers in water;
dewatering the alpha calcium sulfate hemihydrate whiskers; and
heating the alpha calcium sulfate hemihydrate whiskers to form anhydrite calcium sulfate whiskers.
2. The method of paragraph 1, wherein the step of combining the alpha calcium sulfate hemihydrate and water comprises combining the alpha calcium sulfate hemihydrate with the water such that the alpha calcium sulfate is present in the slurry in an amount from about 1 to about 30 percent by weight of the slurry.
3. The method of paragraph 1, wherein the step of combining the alpha calcium sulfate hemihydrate and water comprises combining the alpha calcium sulfate hemihydrate with the water such that the alpha calcium sulfate is present in the slurry in an amount from about 2 to about 15 percent by weight of the slurry.
4. The method of paragraphs 1 to 3, wherein the alpha calcium sulfate hemihydrate has a median particle size, D₅₀, from about 1 micron to about 20 micron.
5. The method of paragraphs 1 to 3, further comprising sizing the alpha calcium sulfate hemihydrate to a median particle size, D₅₀, from about 1 micron to about 10 micron, prior to combining the alpha calcium sulfate hemihydrate and water to form a slurry.
6. The method of paragraphs 1 to 5, wherein the step of autoclaving the slurry comprises subjecting the slurry to saturated steam at a pressure from about 5 psig to about 55 psig and a temperature from about 100° C. to about 150° C. for a duration from about 30 minutes to about 8 hours.
7. The method of paragraphs 1 to 6, wherein the step of dewatering the alpha calcium sulfate hemihydrate whiskers comprises filtering, vacuuming, centrifuging, or a combination thereof.
8. The method of paragraphs 1 to 6, wherein the step of dewatering the alpha calcium sulfate hemihydrate whiskers comprises filtering the alpha calcium sulfate hemihydrate whiskers for a duration from about 1 to about 10 minutes.
9. The method of paragraphs 1 to 8, wherein the step of heating the alpha calcium sulfate hemihydrate whiskers to form anhydrite calcium sulfate whiskers comprises heating the alpha calcium sulfate hemihydrate whiskers at a temperature from about 500° C. to about 900° C. for a duration from about 1 to about 24 hours.
10. Anhydrite calcium sulfate whiskers made by the method of paragraphs 1 to 9.
11. The anhydrite calcium sulfate whiskers of paragraph 10, wherein the whiskers have a Mohs hardness from about 3 to about 3.5.
12. The anhydrite calcium sulfate whiskers of paragraph 10 or 11, wherein the whiskers are thermally stable up to at least 1450° C.
13. The anhydrite calcium sulfate whiskers of paragraphs 10 to 12, wherein the whiskers have a mean aspect ratio of at least 30.
14. A method of making a composite, comprising combining alpha-derived anhydrite calcium sulfate whiskers with a base material to form a composite.
15. The method of paragraph 14, wherein the base material comprises a material selected from the group consisting of paints, coatings, paper, paper precursors, films, plastics, resins, gypsum wallboard, facers, cement and concrete, and combinations thereof.
16. The method of paragraph 14, wherein the base material is selected from the group consisting of polyethylene, polypropylene, polybutylene, polyester, polyethylene terephthalate (PET), nylon 6, nylon 66, polyurethane, epoxy, nitrile rubber, butyl rubber, silicone rubber, PTFE, phenolics, and combinations thereof.
17. The method of paragraphs 14 to 16, wherein the step of combining the alpha-derived anhydrite calcium sulfate whiskers and the base material comprises combining the alpha-derived anhydrite calcium sulfate whiskers with the base material such that the alpha-derived anhydrite calcium sulfate whiskers are present in the composite in an amount from about 1 to about 50 percent by weight of the composite.
18. A composite, comprising alpha-derived anhydrite calcium sulfate whiskers and a base material.
19. The composite of paragraph 18, wherein the base material comprises a material selected from the group consisting of paints, coatings, paper, paper precursors, films, plastics, resins, gypsum wallboard, facers, cement and concrete, and combinations thereof.
20. The composite of paragraph 18 or 19, comprising from about 1 to about 50 percent by weight alpha-derived anhydrite calcium sulfate whiskers.
21. The composite of paragraph 18, wherein:
the base material comprises polypropylene;
the composite comprises from about 2 to about 20 percent by weight alpha-derived anhydrite calcium sulfate whiskers; and
the composite has an elastic modulus of at least 1000 MPa.
22. The composite of paragraphs 18 to 21, wherein the alpha-derived anhydrite calcium sulfate whiskers have a mean aspect ratio of at least 30.
23. The composite of paragraph 18, 19, 20 or 22, wherein the base material is selected from the group consisting of thermoset plastics, thermoplastics, and combinations thereof.
24. The composite of paragraph 18, 19, 20, or 22, wherein the base material is selected from the group consisting of polyethylene, polypropylene, polybutylene, polyester, polyethylene terephthalate (PET), nylon 6, nylon 66, polyurethane, epoxy, nitrile rubber, butyl rubber, silicone rubber, PTFE, phenolics, and combinations thereof.
While the disclosure has been described with reference to a number of embodiments, it will be understood by those skilled in the art that the disclosure is not limited to such disclosed embodiments. Rather, the disclosed embodiments can be modified to incorporate any number of variations, alterations, substitutions, or equivalent arrangements not described herein, but which are commensurate with the spirit and scope of the disclosure. Additionally, while various features of the invention have been described, it is to be understood that aspects of the invention may include only some of the described features. Moreover, while features of the embodiments have been discussed separately, it should be understood that any of the features disclosed herein may be combined in composites, whiskers, and methods for making the same, with or without any other of the features or combinations of features. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.

Claims

What is claimed is:

1. A method of making anhydrite calcium sulfate whiskers, comprising:

combining alpha calcium sulfate hemihydrate and water to form a slurry;

autoclaving the slurry to form alpha calcium sulfate hemihydrate whiskers in water;

dewatering the alpha calcium sulfate hemihydrate whiskers; and

heating the alpha calcium sulfate hemihydrate whiskers to form anhydrite calcium sulfate whiskers.

2. The method of claim 1, wherein the step of combining the alpha calcium sulfate hemihydrate and water comprises combining the alpha calcium sulfate hemihydrate with the water such that the alpha calcium sulfate is present in the slurry in an amount from about 1 to about 30 percent by weight of the slurry.

3. The method of claim 1, wherein the step of combining the alpha calcium sulfate hemihydrate and water comprises combining the alpha calcium sulfate hemihydrate with the water such that the alpha calcium sulfate is present in the slurry in an amount from about 2 to about 15 percent by weight of the slurry.

4. The method of claim 1, wherein the alpha calcium sulfate hemihydrate has a median particle size, D₅₀, from about 1 micron to about 20 micron.

5. The method of claim 1, further comprising sizing the alpha calcium sulfate hemihydrate to a median particle size, D₅₀, from about 1 micron to about 10 micron, prior to combining the alpha calcium sulfate hemihydrate and water to form a slurry.

6. The method of claim 1, wherein the step of autoclaving the slurry comprises subjecting the slurry to saturated steam at a pressure from about 5 psig to about 55 psig and a temperature from about 100° C. to about 150° C. for a duration from about 30 minutes to about 8 hours.

7. The method of claim 1, wherein the step of heating the alpha calcium sulfate hemihydrate whiskers to form anhydrite calcium sulfate whiskers comprises heating the alpha calcium sulfate hemihydrate whiskers at a temperature from about 500° C. to about 900° C. for a duration from about 1 to about 24 hours.

8. Anhydrite calcium sulfate whiskers made by the method of claim 1.

9. The anhydrite calcium sulfate whiskers of claim 8, wherein the whiskers have a Mohs hardness from about 3 to about 3.5.

10. The anhydrite calcium sulfate whiskers of claim 8, wherein the whiskers are thermally stable up to at least 1450° C.

11. The anhydrite calcium sulfate whiskers of claim 8, wherein the whiskers have a mean aspect ratio of at least 30.

12. A method of making a composite, comprising combining alpha-derived anhydrite calcium sulfate whiskers with a base material to form a composite.

13. The method of claim 12, wherein the base material comprises a material selected from the group consisting of paints, coatings, paper, paper precursors, films, plastics, resins, gypsum wallboard, facers, cement and concrete, and combinations thereof.

14. The method of claim 12, wherein the base material is selected from the group consisting of polyethylene, polypropylene, polybutylene, polyester, polyethylene terephthalate (PET), nylon 6, nylon 66, polyurethane, epoxy, nitrile rubber, butyl rubber, silicone rubber, PTFE, phenolics, and combinations thereof.

15. The method of claim 12, wherein the step of combining the alpha-derived anhydrite calcium sulfate whiskers and the base material comprises combining the alpha-derived anhydrite calcium sulfate whiskers with the base material such that the alpha-derived anhydrite calcium sulfate whiskers are present in the composite in an amount from about 1 to about 50 percent by weight of the composite.

16. A composite, comprising alpha-derived anhydrite calcium sulfate whiskers and a base material.

17. The composite of claim 16, wherein the base material comprises a material selected from the group consisting of paints, coatings, paper, paper precursors, films, plastics, resins, gypsum wallboard, facers, cement and concrete, and combinations thereof.

18. The composite of claim 16, comprising from about 1 to about 50 percent by weight alpha-derived anhydrite calcium sulfate whiskers.

19. The composite of claim 16, wherein:

the base material comprises polypropylene;

the composite comprises from about 2 to about 20 percent by weight alpha-derived anhydrite calcium sulfate whiskers; and

the composite has an elastic modulus of at least 1000 MPa.

20. The composite of claim 16, wherein the base material is selected from the group consisting of polyethylene, polypropylene, polybutylene, polyester, polyethylene terephthalate (PET), nylon 6, nylon 66, polyurethane, epoxy, nitrile rubber, butyl rubber, silicone rubber, PTFE, phenolics, and combinations thereof.