United States Patent 4,233,361: Scope, Claims, Expiration, and Patent Landscape

United States Patent 4,233,361 protects a composite insulation panel made from low-density expanded thermoplastic particles embedded in a resole phenolic foam matrix, with expanded glass concentrated near the panel’s major surfaces. Dependent claims narrow the invention to styrene polymer particles, perlite, aluminum facings, and specified particle dimensions.

The patent is a materials and construction patent, not a pharmaceutical or drug patent. It does not create FDA exclusivity, Orange Book protection, Paragraph IV risk, biosimilar exposure, or drug-launch restrictions. Based on the pre-1995 patent term regime and the patent’s 1980 issue date, the patent expired in 1997.

What does United States Patent 4,233,361 protect?

The patent protects a multilayered structural foam panel with five required technical characteristics:

A composite synthetic resinous foam panel.
At least two generally opposed major surfaces.
Expanded multicellular synthetic thermoplastic particles with a density of approximately 0.5 to 2 pounds per cubic foot.
A resole resin foam matrix surrounding or encapsulating the particles, with interstitial bulk density of approximately 2 to 6 pounds per cubic foot.
Expanded glass particles encapsulated in portions of the resole foam adjacent to the panel’s major surfaces.

Claim 1 is the only independent claim. Claims 2 through 6 depend from claim 1 and add narrower material, dimensional, or facing limitations.

The central inventive combination is not merely a foam panel or a phenolic foam. It is the combination of:

Low-density expanded thermoplastic inclusions;
A resole resin foam matrix;
Expanded glass near the major surfaces; and
Defined density ranges for both the thermoplastic particles and the matrix foam.

What are the claim limitations of Patent 4,233,361?

Claim 1: Composite panel with expanded thermoplastic particles and expanded glass

Claim 1 requires each of the following elements:

Element	Claim requirement	Scope implication
Article	Composite synthetic resinous foam panel	Covers a panel, not an unrestricted foam composition
Geometry	First and second generally opposed major surfaces	Requires a panel-like structure with opposing principal faces
Internal particles	Expanded multicellular synthetic resinous thermoplastic particles	The particles must be expanded, multicellular, synthetic, resinous, and thermoplastic
Particle density	Approximately 0.5 to 2 lb/ft³	A numerical limitation that can restrict literal infringement
Matrix	Resole resin foam	The surrounding foam must be a resole-type resin foam
Encapsulation	Particles generally encapsulated within the matrix	Requires integration of the particles into the matrix rather than loose or merely laminated particles
Matrix density	Approximately 2 to 6 lb/ft³ in interstitial spaces	Density applies to the resin foam between the thermoplastic particles
Surface-region additive	Expanded glass in particulate form	Expanded glass must be present as particles
Location	Encapsulated within portions of the resole foam adjacent to major surfaces	Surface-adjacent placement is required, not merely random distribution throughout the panel

The claim does not expressly require aluminum, styrene, perlite, or a particular particle size. Those limitations appear only in dependent claims.

What does “generally encapsulated” mean?

The phrase indicates that the expanded thermoplastic particles are embedded or enclosed within the resole foam matrix. A structure in which particles are simply placed between separate foam layers would present a potential noninfringement position if the particles are not encapsulated by the matrix.

The term “generally” introduces some flexibility. It does not necessarily require every particle to be completely surrounded. The claim still requires the particles, viewed as a population, to be incorporated into the matrix rather than merely attached to a surface or mechanically retained.

What does the surface-adjacent expanded glass limitation require?

The expanded glass must be:

Particulate;
Encapsulated within the resole foam; and
Located in portions of the foam adjacent to the panel’s major surfaces.

A product containing perlite only in the center of the panel would not satisfy this limitation literally. A product with expanded glass uniformly distributed throughout the panel could raise a claim-construction issue, but the patentee would need to show that the glass is also present in foam portions adjacent to the major surfaces.

What do dependent claims 2 through 6 add?

Claim	Added limitation	Commercial significance
2	Expanded thermoplastic particles are styrene polymer	Narrows the particle composition to styrene-based material
3	Expanded glass is perlite	Narrows expanded glass to perlite
4	Aluminum sheet adhered to at least one major surface	Covers faced or clad panels
5	Thermoplastic particles are approximately 1/16 to 3/8 inch	Adds a particle-size range
6	Styrene polymer particles, perlite, and average particle diameter of approximately 1/16 to 3/8 inch	Combines the principal narrowing limitations

Claim 2: Styrene polymer particles

Claim 2 covers a panel satisfying claim 1 in which the expanded thermoplastic particles are styrene polymer. This would include expanded polystyrene-type particles if they also satisfy the claim’s density and encapsulation requirements.

The claim is narrower than claim 1 because it excludes non-styrene thermoplastics such as expanded polyethylene or expanded polypropylene.

Claim 3: Perlite

Claim 3 limits the expanded glass to perlite. Perlite is an expanded volcanic glass commonly used as a lightweight mineral filler and insulation component.

A panel using expanded glass other than perlite could still fall within claim 1, but not claim 3.

Claim 4: Aluminum sheet

Claim 4 covers an aluminum sheet disposed on and adhered to at least one major surface. The claim does not require aluminum sheets on both sides.

The wording also does not specify:

Aluminum thickness;
Alloy;
Adhesive chemistry;
Whether the sheet covers the full surface;
Whether the sheet is structural or merely facing material.

A panel with an aluminum skin on one face could therefore satisfy claim 4 if all limitations of claim 1 are present.

Claim 5: Particle size

Claim 5 requires thermoplastic particles ranging from approximately 1/16 inch to 3/8 inch. Converted to metric dimensions, this is approximately 1.59 to 9.53 millimeters.

The claim does not state whether the range applies to:

Every particle;
A nominal production range;
A measured average; or
A distribution in which some particles fall outside the range.

That issue would matter in a literal infringement analysis.

Claim 6: Narrow commercial embodiment

Claim 6 combines:

Styrene polymer particles;
Perlite as the expanded glass; and
Average particle diameter of approximately 1/16 to 3/8 inch.

This is the narrowest claim in the set and most closely describes a commercial expanded-polystyrene/perlite phenolic panel with particles in the stated size range.

How strong is the patent estate for Patent 4,233,361?

The patent estate is no longer commercially enforceable because the patent expired in 1997. Its historical claim structure, however, was relatively concentrated.

Estate characteristic	Assessment
Independent claims	One
Core technology	Composite foam insulation panel
Main claim strategy	Composition and architecture of the panel
Material limitations	Expanded thermoplastic particles, resole foam, expanded glass
Numerical limitations	Particle and matrix density; dependent particle-size range
Product-facing coverage	Aluminum sheet in claim 4
Method claims	None in the supplied claims
Manufacturing claims	None in the supplied claims
Pharmaceutical claims	None
Current enforceability	None after expiration
Regulatory exclusivity	None

The estate’s historical strength came from the combination of structural and quantitative limitations. A competing panel had to avoid at least one required element to avoid literal infringement. The principal design-around options would have included:

Using a non-resole foam matrix;
Omitting expanded glass near the major surfaces;
Using thermoplastic particles outside the claimed density range;
Using a monolithic foam rather than embedded particles;
Locating the expanded glass away from the major surfaces;
Using a panel architecture in which the thermoplastic particles were not generally encapsulated.

The density limitations could also have created proof issues. Testing would need to distinguish the density of the expanded particles from the bulk density of the resole foam occupying the interstitial spaces.

When did United States Patent 4,233,361 lose exclusivity?

United States Patent 4,233,361 issued on November 11, 1980. Patents issued before the 1995 patent-term transition generally had a 17-year term from grant. On that basis, the patent expired on November 11, 1997.

Event	Date or status
Patent issued	November 11, 1980
Ordinary patent term	17 years from issue under the applicable pre-1995 regime
Expected expiration	November 11, 1997
Current status	Expired
Patent-term extension	Not applicable to this non-drug materials patent
Patent-term adjustment	Not generally applicable under the governing historical term framework
Current blocking right	None

The expiration date eliminates present-day infringement liability under the patent. It does not eliminate historical relevance for prior-art analysis, validity disputes involving later patents, or freedom-to-operate reviews of related continuation or foreign rights.

What is the Orange Book status of Patent 4,233,361?

Patent 4,233,361 has no Orange Book status. The patent does not claim a drug, active pharmaceutical ingredient, drug formulation, method of treatment, or drug-delivery system.

The following FDA concepts do not apply:

FDA or Hatch-Waxman concept	Applicability
Orange Book listing	Not applicable
New drug application	Not applicable
Paragraph IV certification	Not applicable
Small-molecule generic entry	Not applicable
Biologic license application	Not applicable
Biosimilar application	Not applicable
Pediatric exclusivity	Not applicable
New chemical entity exclusivity	Not applicable
Prescription-drug market exclusivity	Not applicable

The patent should be classified as a building-materials or insulation-panel patent rather than a drug patent.

Are there Paragraph IV challenges or biosimilar risks?

No. Paragraph IV certifications concern abbreviated new drug applications that challenge listed drug patents. Biosimilar litigation concerns patents associated with licensed biological products. Patent 4,233,361 concerns a composite foam panel and has no connection to either pathway.

There is no generic-drug launch scenario tied to this patent. Any commercial competition would involve insulation-panel manufacturers, construction-product suppliers, or licensees rather than pharmaceutical manufacturers.

What formulations and manufacturing methods are protected?

The supplied claims do not protect a manufacturing process. They protect the resulting panel.

The claims do not expressly require:

A particular mixing sequence;
A molding pressure;
A curing temperature;
A blowing agent;
A specific resole formulation;
A specified catalyst;
A particular adhesive;
A continuous or batch manufacturing process;
A specified aluminum-lamination process.

A manufacturer could therefore use a different production method and still potentially infringe if the resulting panel contains all elements of claim 1 or a dependent claim. Conversely, a process that produces a panel lacking one required structural limitation would not infringe merely because it resembles the disclosed manufacturing approach.

What patent litigation affects Patent 4,233,361?

The supplied claims do not establish any litigation history, settlement agreement, licensing transaction, terminal disclaimer, reexamination, or reissue proceeding.

Because the patent expired in 1997, it presents no current enforcement risk. Any historical litigation would primarily matter for:

Claim construction;
Prosecution-history estoppel;
Validity analysis;
Prior-art treatment;
Interpretation of density and encapsulation limitations;
Ownership or licensing history.

No Paragraph IV, biosimilar, or FDA-related litigation can arise from this patent.

How does Patent 4,233,361 compare with pharmaceutical patent estates?

Issue	Patent 4,233,361	Pharmaceutical patent estate
Protected subject matter	Composite insulation panel	Drug substance, formulation, treatment method, or delivery system
Regulatory agency	None	FDA
Orange Book	No	Often relevant for approved small-molecule drugs
Paragraph IV	No	Relevant to ANDA filings
Biosimilar pathway	No	Relevant to biologics
Exclusivity	Patent term only	Patent term plus regulatory exclusivity
Formulation claims	Material composition only	Drug formulation and dosage-form claims
Revenue exposure	Construction-material sales	Product sales and reimbursement
Generic launch risk	None	Often central to valuation
Current status	Expired	Depends on patent and regulatory dates

What geographic coverage did the patent provide?

United States Patent 4,233,361 provided rights only in the United States. The U.S. patent did not automatically protect corresponding products in Canada, Europe, Japan, or other jurisdictions.

Foreign coverage would require separate national patent rights or regional rights. The supplied information does not establish any foreign counterparts, continuation applications, divisional applications, or related patents. The U.S. patent’s expiration did not itself determine the status of foreign rights, although any corresponding foreign patents from the same historical filing period would generally also be expired by now.

What generic or competitive entry risks existed?

During the patent term, competitors faced a product-architecture risk if they sold a panel containing all of the following:

Expanded thermoplastic particles within the claimed density range;
A resole foam matrix within the claimed interstitial density range;
Expanded glass near both or either major surface region;
General encapsulation of the particles by the matrix.

The most material competitive risks were product similarity and testing uncertainty. A panel could appear technically distinct while still satisfying the broad independent claim if it retained the same matrix, particle, density, and surface-region structure.

After expiration, the patent created no continuing entry barrier. Competitors are free to use the claimed combination, subject to other unexpired patents, trade secrets, trademarks, building codes, and contractual restrictions.

Key Takeaways

United States Patent 4,233,361 is a composite insulation-panel patent, not a drug patent.
Claim 1 requires expanded thermoplastic particles, a resole foam matrix, defined density ranges, and expanded glass near the major surfaces.
Claims 2 through 6 narrow the invention to styrene polymer, perlite, aluminum facings, and specified particle sizes.
The narrowest claim covers a styrene-polymer/perlite panel with particles averaging approximately 1/16 to 3/8 inch.
The patent issued on November 11, 1980, and its ordinary 17-year term ended on November 11, 1997.
It has no Orange Book listing, Paragraph IV relevance, biosimilar relevance, or FDA exclusivity.
The patent contains no supplied method-of-manufacture claims.
Current infringement risk under this patent is zero because the patent is expired.
Any present freedom-to-operate concern must come from later patents, foreign rights, contractual rights, or non-patent restrictions.

FAQs

Is Patent 4,233,361 a patent for expanded polystyrene?

Only dependent claim 2 and the narrower claim 6 expressly identify styrene polymer. Claim 1 is broader and covers expanded synthetic resinous thermoplastic particles generally, provided the other limitations are met.

Does the patent cover a panel with perlite but no aluminum sheet?

Yes. Claim 3 covers perlite without requiring aluminum. Aluminum is added only by claim 4. A panel may fall within claim 3 without falling within claim 4.

Does claim 1 require expanded glass on both major surfaces?

The claim requires expanded glass in resole foam portions adjacent to the major surfaces. It does not expressly state that glass must be present adjacent to both surfaces. The phrase should be interpreted in the context of the full claim and specification.

Can a panel avoid the patent by using polyurethane foam instead of resole foam?

A polyurethane matrix would not literally satisfy the express “resole resin foam” limitation. The expired status makes that distinction commercially irrelevant for current enforcement, but it could matter in historical infringement or later-patent analysis.

Does expiration of the U.S. patent place foreign versions in the public domain?

No. U.S. expiration ends U.S. patent rights. Foreign counterpart rights must be assessed separately, although patents from the same era would generally also have reached the end of their terms.

References

United States Patent No. 4,233,361. (1980, November 11). Composite synthetic resinous foam panel. United States Patent and Trademark Office.
United States Patent and Trademark Office. (n.d.). Patent term and adjustment information. U.S. Department of Commerce.
United States Food and Drug Administration. (n.d.). Approved drug products with therapeutic equivalence evaluations. U.S. Department of Health and Human Services.

Details for Patent: 4,233,361

Summary for Patent: 4,233,361