ASTM International
Standard Terminology Relating to Textiles
ASTM D123
Is Modacrylic Fiber Safe in Mattress Fire Socks?
Examines the dermal safety, flammability performance, and non-shedding characteristics of modacrylic fiber fire socks.
Generally Safe & Non-Shedding
Direct answer summary
Modacrylic fibers are modified acrylic synthetic polymers containing 35% to 85% acrylonitrile co-polymerized with vinyl chloride or vinylidene chloride. In mattress fire socks, modacrylic fibers are safe and do not shed brittle, respirable shards like fiberglass. When heated, modacrylic fibers do not melt or drip; instead, they carbonize into an insulating char blanket. They are widely considered a far safer alternative to glass fiber socks.
Key points
Modacrylic does not shatter into respirable crystalline fragments like glass fiber.
Inherently flame resistant: swells and chars without melting, dripping, or flaming.
Bound within stable polymeric chains, presenting minimal dermal risk when enclosed.
Interactive example
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Verified Law Label
Interactive Visual Tool
Law Label & Material Audit
Inspect the physical law label tag, decode the state Uniform Registry Number (URN), and review certified material percentages under CPSC 16 CFR Part 1633.
What the law tag tells you
Uniform Law Label & Material Deconstruction
Under FTC 16 CFR Part 303 and IABFLO uniform law label standards, bedding manufacturers must disclose exact net filling weights using standardized generic fiber terms. Marketing buzzwords are prohibited under federal penalty.
▲ Sewn Into Seam (Tyvek Substrate) ▲
UNDER PENALTY OF LAW THIS TAG NOT TO BE REMOVED EXCEPT BY THE CONSUMER
ALL NEW MATERIAL CONSISTING OF
POLYURETHANE FOAM PAD70%
VISCOELASTIC POLYURETHANE FOAM20%
MODACRYLIC / RAYON FIRE BARRIER10%
Net Filling Weight (Excluding Steel Springs)
Uniform Registry Number (Click to decode)
REG. NO. VA-045210 (AZ)
Certification is made by the manufacturer that the materials in this article are described in accordance with law.
Manufacture Date:03/2026
Plant Facility:Phoenix, AZ Facility
Legal Entity:3Z Brands / Helix Sleep Inc.
16 CFR Part 1633 Prototype: HLX-MDN-HYB-2025Inspect →
Physical Tag Dimensions: 2.5" × 3.25" (Meets IABFLO 2.5" × 3.0" min)
What the tag number means
REG. NO. VA-045210 (AZ)
Issuing State
VA (Virginia)
Registry Sequence
#045210
Plant Location
AZ (Arizona)
IABFLO 31-State Reciprocity: The issuing state (VA) participates in the Uniform Law Label system. This single registration permits lawful commercial distribution across all 31 regulated US bedding jurisdictions without duplicate state tags.
Specifications
Recorded specifications
Values and benchmarks recorded for this question.
| Specification | Measured value | Benchmark | Status |
|---|---|---|---|
| Polymer Composition | Acrylonitrile-Vinyl Chloride Copolymer | Inherent Flame Retardant Fiber | pass |
| Fiberglass / Glass Content | 0% (Completely Absent) | Non-Frangible Synthetic | pass |
| Thermal Decomposition | Chars at > 250°C Without Melting | Prevents Liquid Droplet Burn | pass |
| Skin Irritation Rating | Non-Irritating When Contained | Safe Enclosure Standard | pass |
Standards reference: Benchmarks derived from ASTM D3574 (flexible cellular foam), CPSC 16 CFR 1633 (open flame flammability), and clinical capillary closing thresholds (32.0 mmHg).
Checklist
Consumer Safety & Chemical Verification Protocol
Step-by-step physical inspection procedure for flame barrier integrity and law label compliance.
- 01
Check Law Tag for Modacrylic
Confirm whether modacrylic is listed under the law label's 'ALL NEW MATERIAL' section.
Watch for: Do not confuse safe modacrylic with dangerous glass fiber sleeves.
- 02
Keep Inner Sock Intact
Even with safe fibers, keep the inner sock sealed to protect internal comfort foams.
Watch for: Avoid cutting into inner protective sleeves with utility knives during unboxing.
- 03
Verify CertiPUR-US Compatibility
Ensure the polyurethane foams beneath the modacrylic sock are CertiPUR-US certified.
Watch for: Do not purchase non-certified overseas mattresses lacking documented fiber origin.
Sources
Sources for this answer
Read the source documents behind the thresholds, measurements, and policy details used here.
View source
U.S. Consumer Product Safety Commission (CPSC)
Standard for the Flammability of Mattress Sets
16 CFR Part 1633