The History of DIDP (Diisodecyl Phthalate) as the Industry Reference Material for Automotive VOC Fog Testing
Why One Plasticizer Became the Industry Benchmark—Despite Its Known Variability
In automotive interior materials testing, few reference materials have been used as consistently as Diisodecyl Phthalate (DIDP). For more than three decades, DIDP has served as the reference liquid for verifying the performance of fogging test equipment used to evaluate volatile organic compound (VOC) emissions from plastics, textiles, leather, foams, adhesives, coatings, and other interior materials.
At first glance, DIDP appears to be an imperfect standard. Laboratories often observe measurable variability in fogging results from one bottle to another, one supplier to another, and even between production lots. Yet despite this variability, DIDP remains the universally accepted reference material throughout the automotive industry because it fulfills a very specific purpose:
It verifies that the test system is functioning correctly—not that the reference material itself produces an exact numerical value.
This distinction is critical to understanding why DIDP continues to be specified in virtually every major automotive fogging standard.
Origins of Automotive Fog Testing
During the 1970s and 1980s, automobile manufacturers began encountering customer complaints involving:
- Windshield haze
- Interior film deposits
- Reduced driver visibility
- Oily residue on glass
- Unpleasant interior odors
Engineers eventually traced these problems to volatile plasticizers and other organic compounds evaporating from interior materials.
As vehicle cabins became more airtight and interior temperatures increased, manufacturers required laboratory methods capable of predicting these fogging tendencies before production.
This led to the development of standardized fogging methods including:
- DIN 75201
- SAE J1756
- PV 3015
- PV 3920
- ISO 6452
- ISO 17071
- Nissan MS300-54
Although these procedures differ slightly in specimen preparation and acceptance criteria, they all use essentially the same physical principle:
- Heat the material.
- Allow volatile compounds to evaporate.
- Condense them onto a cooled surface.
- Measure either:
- reflected light (reflectometric method), or
- condensate mass (gravimetric method). (ISO)
Why DIDP Was Selected
An ideal reference material needed several characteristics:
- chemically stable
- commercially available
- reproducible manufacturing quality
- representative of automotive plasticizers
- capable of generating measurable fog condensate
DIDP satisfied these requirements better than competing plasticizers.
As a high-molecular-weight phthalate ester, DIDP:
- evaporates slowly
- produces measurable fog
- remains chemically stable during storage
- is relatively easy to handle
- provides repeatable behavior under controlled laboratory conditions
Most importantly, it behaves similarly to many of the plasticizers historically used in automotive interior materials.
For these reasons, automotive OEMs gradually standardized on DIDP as the reference material used during equipment verification.
DIDP Is Not a Certified Reference Material
One of the biggest misconceptions surrounding DIDP is that it is intended to be a calibration standard.
It is not.
Unlike a certified reference material (CRM), DIDP is not supplied with a certified fog value.
Instead, laboratories use it as a performance verification material.
The goal is simply to confirm that:
- the heating block reaches the correct temperature,
- the cooling plate is functioning properly,
- specimen holders are sealing correctly,
- glass placement is correct,
- condensate forms normally,
- optical or gravimetric measurements respond appropriately.
In other words:
DIDP verifies the operation of the entire test system.
This aligns with general metrological principles for reference materials, where a material may be considered fit for its intended use without being a certified calibration standard. (NIST)
Why Results Vary
Laboratories frequently observe differences in DIDP fogging values.
These variations arise from several factors.
1. DIDP Is a Commercial Mixture
Unlike a pure chemical compound, DIDP consists of a mixture of C10 branched isomers.
Different manufacturing processes produce slightly different isomer distributions.
Those differences affect:
- vapor pressure
- volatility
- condensation behavior
without changing the material’s identity.
2. Supplier Differences
Different manufacturers may produce DIDP with:
- different catalyst systems
- different purification methods
- different impurity levels
- different isomer balances
Each can produce small shifts in fogging behavior.
3. Lot-to-Lot Differences
Even within one supplier:
- raw material sources change,
- process optimization changes,
- manufacturing tolerances exist.
Therefore one production lot may fog slightly differently than another.
4. Laboratory Variables
Even perfect DIDP cannot compensate for differences in:
- glass cleanliness
- cooling plate temperature
- oven uniformity
- specimen preparation
- weighing accuracy
- reflectometer calibration
- operator technique
These often contribute more variation than the DIDP itself.
Why the Variability Is Acceptable
At first glance, accepting a variable reference material seems contradictory.
However, the purpose of DIDP is verification, not calibration.
Automotive standards recognize that fog testing itself is inherently variable because it involves:
- evaporation,
- condensation,
- droplet formation,
- optical reflection,
- temperature gradients,
- surface chemistry.
Even ISO 17071 notes that the reflectometric method exhibits relatively large inter-laboratory variation, while the gravimetric method generally demonstrates better reproducibility. (ISO)
Consequently, laboratories establish acceptable operating ranges for DIDP rather than expecting a single fixed result. If the measured value falls within the specified range, the equipment is considered to be functioning properly.
Consistent Industry Acceptance
One remarkable aspect of automotive fog testing is the consistency among international standards.
Although each OEM specification has unique acceptance requirements, they continue to rely on DIDP for system verification, including:
- ISO 6452
- ISO 17071
- SAE J1756
- Volkswagen PV 3015
- Volkswagen PV 3920
- Nissan MS300-54
This widespread adoption reflects decades of inter-laboratory experience and confidence in DIDP’s suitability as a practical verification material, even though no single absolute fog value is assigned to it. (ISO)
Why Hasn’t DIDP Been Replaced?
Several alternatives have been considered over the years.
However, none has gained industry acceptance.
Potential replacements generally introduce one or more disadvantages:
- excessive volatility,
- insufficient fog formation,
- poor long-term stability,
- limited commercial availability,
- incompatibility with historical data,
- lack of global acceptance.
Replacing DIDP would also require:
- extensive international round-robin testing,
- revisions to numerous OEM standards,
- re-establishment of historical performance databases,
- worldwide agreement among automotive manufacturers.
Given that DIDP continues to perform its intended verification function successfully, there has been little motivation to undertake such a major transition.
Lessons for Modern Test Laboratories
Rather than focusing on obtaining a single “perfect” DIDP value, laboratories should emphasize:
- purchasing high-quality DIDP from reputable suppliers,
- using consistent lots whenever practical,
- monitoring historical verification trends,
- maintaining calibrated temperature control,
- verifying cooling system performance,
- ensuring optical instrument calibration,
- following standardized specimen preparation procedures,
- participating in proficiency testing when available.
Stable trends over time are often more meaningful than isolated numerical values.
Conclusion
For decades, DIDP has served as the automotive industry’s benchmark reference material for VOC fog testing. While it is well understood that DIDP exhibits inherent lot-to-lot and supplier-to-supplier variability, this characteristic has not diminished its value. Instead, the industry has recognized that the purpose of DIDP is to confirm the proper operation of the complete fog testing system—not to provide an absolute calibration point.
The continued inclusion of DIDP in standards such as ISO 17071, ISO 6452, SAE J1756, PV 3015, PV 3920, and Nissan MS300-54 demonstrates broad international confidence in its role as a practical and reliable verification material. Until a demonstrably superior alternative is developed and universally accepted, DIDP remains the reference material against which automotive fog testing systems are verified.
For laboratories performing VOC fog testing, the focus should remain on consistent equipment performance, rigorous maintenance, and adherence to standardized procedures. When used in that context, DIDP continues to provide exactly what the industry requires: confidence that the test system is operating as intended.
DIDP Availability for Laboratory Verification
Because DIDP remains the recognized verification material specified or referenced throughout the major automotive fogging standards, maintaining a consistent supply is an important part of laboratory quality assurance. Laboratories performing routine verification should use high-purity DIDP intended specifically for fog testing and should document the supplier and lot number as part of their ISO/IEC 17025 quality records.
Schap Specialty Machine supplies DIDP as a laboratory consumable specifically for automotive VOC fog testing applications. The material is intended for use in equipment verification procedures supporting standards including:
- ISO 17071
- ISO 6452
- SAE J1756
- Volkswagen PV 3015
- Volkswagen PV 3920
- Nissan MS300-54
Although DIDP itself is not a certified reference material with a single certified fog value, using a consistent supply from a reputable source helps laboratories establish stable historical control data, detect changes in equipment performance, and support repeatable verification practices. As with any verification material, laboratories should trend results over time and investigate significant deviations from their established control limits rather than focusing solely on an individual test value. This approach is consistent with the intent of the automotive fogging standards, which are designed to verify the proper operation of the complete test system rather than to certify the absolute performance of the reference liquid. (Thermo Fisher Documents)
For laboratories using Schap Dry Fog Testers or other fogging equipment, DIDP is available directly from Schap Specialty Machine as part of its line of fog testing consumables and replacement supplies, allowing customers to obtain both the instrumentation and the verification materials from a single source. While laboratories should always establish their own internal acceptance ranges in accordance with applicable standards and quality procedures, using the same supplier over extended periods can help minimize unnecessary variability introduced by changes in material sourcing. (SD Atlas)
Need DIDP Reference Material or Fog Testing Supplies?
Schap Specialty Machine supplies DIDP reference liquid, glass plates, sealing rings, beakers, replacement components, and complete Dry Fog Testing systems to automotive OEMs, Tier suppliers, and independent testing laboratories worldwide. Contact Schap Specialty Machine to discuss your fog testing requirements or to request a quotation for DIDP and other consumables.
